THE
PHYSIOLOGY OF PLANTS
A TREATISE UPON THE
METABOLISM AND SOURCES OF ENERGY
IN PLANTS
BY
DR. W. PFEFFER
PROFESSOR OF BOTANY IN THE UNIVERSITY OF LEIPZIG
SECOND FULLY REVISED EDITION
TRANSLATED AND EDITED BY
ALFRED J. EWART, D.Sc, PH.D., F.L.S.
PROFESSOR OF BOTANY IN THE UNIVERSITY OF MELBOURNE
AND VICTORIAN GOVERNMENT BOTANIST
WITH MANY ILLUSTRATIONS
VOLUME III
OXFORD
AT THE CLARENDON PRESS
M D CCCCVI
HENRY FROWDE, M.A.
PUBLISHER TO THE UNIVERSITY OF OXFORD
LONDON, EDINBURGH
NEW YORK AND TORONTO
PREFACE TO VOLUME III
As in the previous volume a certain condensation has been effected
in the present one, in spite of a slight increase in the subject-matter.
All additions or interpolations are, however, enclosed in square brackets,
except in the sections dealing with tendril-climbers, with protoplasmic
streaming, and with the mechanics of water-transport, to which a few
explanatory figures have also been added. The appendix gives a summary
of the more important literature which has appeared since the issue of the
final part of the German edition, and notices of other recent works are
interpolated in the foot-notes.
In regard to terminology, it has been the aim throughout to avoid
the introduction of any new terms into the text of the English edition
unless the weightiest reasons existed for their adoption. The present
tendency to a redundant and overlapping phraseology in Plant Physiology,
if unchecked, will ultimately lead to confusion similar to that existing
in Taxonomy before the compilation of the Kew Index. The fact that
a worker of the eminence, profundity, and breadth of Charles Darwin
added only two or three terms to botanical terminology which could not
be understood by reference to a standard English dictionary should
make modern workers hesitate to encumber a developing science with
more or less temporary pseudo-classical terms of doubtful utility or of
none at all. Physieclexis and epitedeioperileipsis would have been poor
substitutes for ' Natural Selection } and the ' Survival of the Fittest,' and
the use of such terms would probably have considerably retarded popular
acceptance of the Darwinian theory.
With the issue of the third and last volume of Professor Pfeffer's
monumental work, a new point of departure has been gained by botanical
physiology. Only those engaged in research can realize how much labour
the preparation of these volumes, with their encyclopaedic compendium
of modern literature, must have involved, and the completion of the work
at so early a date in spite of a serious and almost fatal illness affords
sufficient evidence of the devotion with which the author has pursued
the stupendous task set before him to its conclusion. If the results of his
labours have lost nothing in assuming their English dress, the task of
the translator has been amply fulfilled.
ALFRED J. EWART.
BIRMINGHAM UNIVERSITY,
December, 1905.
CONTENTS
CHAPTER I
MOVEMENT
PAGE
§ I. The different forms of movement . . l
2. The causes of movement 4
3. The mechanism of movement . . I2
CHAPTER II
MOVEMENTS OF CURVATURE
PART I. AUTONOMIC MOVEMENTS
4. Occurrence and distribution
5. The causes of autonomic movement . . . . . • • 25
6. The influence of the external conditions 29
7. The mechanics of autonomic movement / • 31
PART II. TWINERS AND CLIMBERS
8. General ... 32
9. The twining of stems .
10. Twining plants (continued} .....••••• 3°
11. Tendril-climbers 42
12. The special irritability of tendril-climbers 5°
13. The influence of contact upon the growth and curvature of tendrils . . . 57
PART III. MOVEMENTS DUE TO MECHANICAL AND CHEMICAL STIMULI
14. Irritability to contact and to mechanical shocks . . . 61
15. „ „ „ (continued]
16. Movements produced by mechanical stimuli ....
17- „ » >, » (continued] ?8
1 8. Movements produced by contact-stimulation . . .
19. Curvatures produced by chemical stimuli ....
20. The propagation of mechanical and chemical stimuli 91
PART IV. PHOTONASTIC, THERMONASTIC, AND HYDRONASTIC CURVATURES
21. General
22. Instances of photonastic and diurnal movements
23. The origin of the daily photonastic periodicity . . .108
vi CONTENTS
PAGE
§ 24. Thermonastic curvatures 112
25. Hydronastic movements . . . . 116
26. Conjoint effects 119
27. „ „ (continued} . . . . . . . . . .123
28. The mechanics of nutation movements 128
29. „ variation movements 134
PART V. THE INFLUENCE OF THE EXTERNAL CONDITIONS UPON
AITIONASTIC CURVATURE
30. Special and general actions . .140
PART VI. DEHISCENCE AND DISPERSAL MOVEMENTS
31. Special and general . 146
CHAPTER III
TROPIC MOVEMENTS
PART I. INTRODUCTORY
32. General . 154
33. „ (continued}* * . 157
PART II. THE VARIOUS FORMS OF TROPIC CURVATURE
34. Geotropism * . 162
35. Methods of investigating geotropism 166
36. Heliotropism 170
37. The heliotropic action of rays of different wave-length 174
38. Thermotropism ............ 176
39. Chemotropism and osmotropism 178
40. Hydrotropism 182
41. Mechanotropism ............ 184
42. Galvanotropism * . .188
43. Autotropism and somatotropism . . . . ... . .189
PART III. THE CONDITIONS FOR AND CHARACTER OF TROPIC STIMULATION
44. Instances of the separate localization of perception and response . . . 192
45. Instances of autogenic and of aitiogenic changes of irritability . . . 202
46. Changes of irritable tone (continued} 206
47. Minimal stimuli and the latent periods of induction and reaction . . . 209
48. The relation between the intensity of stimulus and the resultant excitation . 212
49. The conditions for stimulation and its progress . . . . ^ . .. 216
50. Perception and response . . . . . . . . . . .219
51. Instances of specific tropic irritability 221
PART IV. THE MECHANISM OF TROPIC MOVEMENT
52. The progress and mode of movement . . . . . . . . 230
53. The mechanism of curvature 238
54. The internal causes of movement . 244
CONTENTS vii
PAGE
PART V
§ 55. Special cases . 248
56. The orientation of foliage-leaves 255
CHAPTER IV
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
PART I. THE CHARACTER AND MECHANISM OF MOVEMENT
57. General 262
58. Ciliary movement ............ 264
59. Gliding movements 270
60. Amoeboid movement 275
6 1. The mechanics of amoeboid movement 276
62. Protoplasmic streaming . 283
63. Pulsating vacuoles 293
64. Other protoplasmic movements . •. . . . . . . . 299
PART II. THE INFLUENCE OF THE EXTERNAL CONDITIONS UPON LOCOMOTION
AND UPON PROTOPLASMIC MOVEMENT
65 306
66. The forms of tactic response to tropic stimuli 308
67. The influence of temperature 313
68. The influence of illumination 318
69. The tropic action of light on freely motile organisms 321
70. The photic orientation of chloroplastids 327
71. The action of gravitational and centrifugal forces 334
72. Geotactic reactions 336
73. Diffuse chemical actions 338
74. Chemotaxis and osmotaxis 343
75. Chemotactic and osmotactic repulsion 350
76. The influence of water 355
77. Mechanical actions 357
78. Galvanotaxis 360
79. Cytotaxis 364
CHAPTER V
THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
PART I. THE PRODUCTION OF HEAT
80. General 3^6
81. The evolution of heat by aerobes . 372
82. The production of heat by anaerobic metabolism ... -377
83. The temperature of the plant under normal conditions 379
viii CONTENTS
PAGE
PART II. THE PRODUCTION OF LIGHT
§ 84. Instances and causes of luminosity 382
PART III. THE PRODUCTION OF ELECTRICAL TENSIONS IN THE PLANT
85. The origin and detection of electromotive changes 388
86. The influence of external agencies upon the production of electricity . . 394
CHAPTER VI
THE SOURCES AND TRANSFORMATIONS OF ENERGY IN THE PLANT
87. General view 399
88. The forms of physical energy used by plants 402
89. Chemical energy 405
90. Special cases . . . . 409
APPENDIX 415
INDEX ... .423
PHYSIOLOGY OF PLANTS
VOLUME III
CHAPTER I
MOVEMENT
SECTION I. The Different Forms of Movement.
No plant is entirely without the power of movement, for even in rooted
plants the growing parts move in space, and, since this continues until
death, rhizomes and runners may traverse a considerable distance during
their existence.
The tip of a growing organ usually does not follow a straight line, but
describes a complicated curve in space. In many cases, indeed, the rates
of growth on opposite sides are such that a pronounced curvature may be
produced, or the tip may move to and fro, or trace a spiral curve in space
as it elongates (circumnutation). These growth or nutation1 movements
naturally cease with the cessation of growth, although active movement
may still be possible in some cases. For instance, the pulvini of many
Leguminosae, and of other plants also, are organs specially adapted for
pronounced movement by elastic shortening and lengthening2. The fact
that in plastic shoots no movements occur after the cessation of growth
simply shows that in these parts the activity of the plant is unable to
produce any perceptible effect. If, however, growth is reawakened, as
in the nodes of Gramineae by geotropic stimulation, we again encounter
curvatures due to nutation.
In adult but still living parts which are externally rigid, an internal
power of movement is never entirely absent, and is indeed permanently
connected in every cell with metabolism and exchange, for in the proto-
plast itself movements and changes of shape continually occur.
In the absence of a cell-wall amoeboid movements and changes of
shape are possible, as is especially well shown by Myxomycetes. Swarm
1 This term was first used by Duhamel (Naturg. d. Baume, 1765, Bd. n, p. 115) and de Candolle
(Pflanzenphysiol., 1825, Bd. 11, p. 666), and subsequently restricted by Sachs to movements pro-
duced by growth (Sachs, Lehrbuch, 1873, 3- Aufl., p. 757), whether autonomic or aitionomic. Frank
(Beitrage zur Pflanzenphysiol., 1868, p. 51) uses the term 'nutation' for growth-movements due to
external stimuli, and distinguishes autonomic movements as ' inclination.'
2 Pfeffer, Die Reizbarkeit d. Pflanzen, 1898, p. 9. (Reprint from the Verh. d. Ges. deutscher
Naturforscher u. Aerzte, 1893.)
PFEFFER. Ill
2 MOVEMENT
cells, owing to the presence of special locomotory organs, cilia, or flagellae,
are able to swim about actively in water.
Among plants it is only in the case of small organisms that active
locomotion is possible, and frequently only during a particular stage of the
life history. Since the response due to a stimulus is always dependent upon
the character of the resulting movements, a freely motile plant may travel
towards a source of illumination, whereas a rooted plant responds in a less
degree by growing and curving towards the illuminated side. In spite of
this difference, the actual perception and stimulation may be identical in the
two cases.
The movements of free-swimming plants appear to have a more pur-
poseful nature, simply because they resemble the movements of animals.
As a matter of fact the power of perceiving and responding to stimuli is
equally developed in plants rooted to the soil. Free-swimming plants, it is
true, lend themselves more readily to experimental studies, because they
usually react more rapidly than plants which can respond only by a change
in the rate or character of growth. Since most plants fall in the latter
class, and since curvatures are usually produced by growth, we shall confine
ourselves at first mainly to movements of this character.
The fact that in large plants the power of growth and movement are
not strikingly evident has caused plants to be popularly regarded as ' still
life.' Hence the rapid movements of Mimosa pudica were regarded as
extraordinary for a plant, and the same applies to the spontaneous
movements performed by the lateral leaflets of Hedysarum gyrans ] . If
mankind from youth upwards were accustomed to view nature under
a magnification of 100 to 1,000 times, or to perceive the activities of weeks
or months performed in a minute, as is possible by the aid of a kine-
matograph, this erroneous idea would be entirely dispelled 2.
Movements serve a variety of aims and purposes, and need to be con-
sidered not only as regards the causes which produce them and the way in
which they are carried out, but also as regards their importance to the
plant. We are, however, less concerned with oecological explanations
than with the determination of causes and mechanism.
In every case a response to a stimulus indicates a specific irritability,
although the nature of the response will vary in different plants according
to their nature and properties. We can, however, distinguish between
autonomic, autogenic, or spontaneous stimuli on the one hand, and
aitiogenic, induced or paratonic stimuli on the other, and the same
applies to the movements resulting from internal or external stimula-
1 Pfeffer, Die Reizbarkeit d. Pflanzen, 1893, p. 9. (Reprint from Verb. d. Ges. deutscher
Naturforscher u. Aerzte, 1893.)
2 Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 738.
THE DIFFERENT FORMS OF MOVEMENT 3
tion l. To invent names for each variety of movement such as gamotropic,
carpotropic, and the like, as Hansgirg has done2, aids nothing in eluci-
dating the phenomena in question3.
Curvatures produced by diffused stimuli are aitionastic, those pro-
duced by unilateral stimuli are orienting or tropic. The latter to which
geotropism, heliotropism, and the other tropic movements belong, are the
result of a sense of discrimination and have this in common, that the
responding organ assumes a definite position in regard to the direction of
the stimulus 4.
Both radial and dorsiventral organs respond in this way, whereas
a curvature can be produced by a diffuse stimulus in dorsiventral (and
anisotropic) organs in which the opposite halves respond by unequal
growth and elongation.
The movements of Mimosa pudica produced by a blow, as well as the
contraction of stamens of Cynareae, also take place in a definite direction
predetermined by the structure of the organ. The curvatures produced
by diffuse stimuli are termed * nastic,1 while by epinasty, hyponasty, and
paranasty, the sides are indicated which elongate on stimulation.
These distinctions only apply to special types of movement between
which transitions occur, not only because the two forms of movement may
take place at the same time, but also because the same movement may be
regarded as * nastic ' or as tropic, according to the point of view adopted.
Thus a curvature which we term ' nastic ' is primarily the result of tropic
stimulation, which is awakened in spite of the homogeneous external
conditions, owing to the dissimilar physiological properties of the sides of
the curving organ. This would be the case if the formation of pigment or of
a more opaque cuticle hindered the penetration of light on one side, as well
as when one side was smeared with Indian ink. In the same way, a local
increase in the permeability of the cuticle might cause stronger transpira-
tion on that side, and thus produce hydrotropic stimulation and curvature.
Further, equal contact on all sides of a physiologically radial tendril will
produce a curvature if a protective layer is interposed on one side so that
the stimulation on that side is less pronounced.
Autogenic curvatures which are produced under constant and homo-
geneous external conditions may be termed autonastic, and the single or
1 Cf. A. P. de Candolle, Physiologic des plantes, a German translation by Roper, 1883, Bd. II,
p. 552 ; Dutrochet, Mem. anat. et physiol. d. vegetaux et d'animaux, Bruxelles, 1837, p. 225.
2 Physiol. Unters., 1893, p. 966.
3 Pfeffer, Die period. Bewegungen d. Blattorgane, 1875, p. 2.
* Oltmanns (Flora, 1892, p. 206) suggests the term 'Photometry' to indicate the power of the
plant to respond to light. [The term is inadvisable, since in correspondence with its physical usage
it would suggest that plants detect and respond to the intensity of the light alone and not to its
direction.]
B 3
4 MOVEMENT
periodic movements resulting from internal non-homogeneous stimuli ex-
changes may be called autotropic. An autonomic movement resulting
from a change of the geotropic irritability affords, however, an undoubted
instance of tropic stimulation and would not be shown on a klinostat.
Further, the induction of a labile dorsi ventral ity by unilateral illumina-
tion produces the conditions for a photonastic reaction, and it is by no
means easy to resolve the combination into its component factors. Indeed,
all geotropic or phototropic curvatures may be regarded as the result of
epinastic or hyponastic properties induced temporarily by the unilateral
action of gravity or light.
There is, however, no necessity for rigid restriction in discussing these
phenomena. Thus, the tropic movements of tendrils may be treated
together with other adaptations for climbing, while various 'nastic' re-
actions will be first mentioned in connexion with the tropic orienting
movements. Furthermore, the mechanisms for dehiscence and active
dispersal are of economic importance, but of special character, and often
not vital phenomena.
SECTION 2. The Causes of Movement.
All these movements are produced in response to stimuli of either
internal or external origin. The first indication of a motile irritability is
afforded by the realized movement, which forms at the same time an
evidence of a power of perception. No movement is possible, however, if
a block or gap occurs in the chain of processes intervening between
perception and response.
In plants whose sensory and motor parts are some distance apart, the
destruction of the sensory organ, or a break in the path of the stimulus,
prevents response. Similarly no movement is possible if the responding
organ has lost its power of movement, so that parts which have ceased
to grow lose their motility, if they are only capable of growth-curvatures.
A power of perceiving stimuli might, however, still be present, although the
power of reacting to them appears to be absent.
In many such cases the processes of perception and induction appear
to take place as before, as is evidenced by the fact that the rapidity and
amount of response decrease when growth is enfeebled, but stop usually
only when growth ceases and may again become perceptible if it is
reawakened.
The power of movement in adult organs depends largely upon their
structure and upon the properties of their cell-walls. Thus a fall of turgor
which produces a pronounced shortening in the stamens of Centaur e a or
a curvature in the pulvinus of Mimosa does not cause any perceptible
change of shape or curvature in the filament of a Spirogyra^ or in the
branch of a tree.
THE CAUSES OF MOVEMENT 5
We must, as far as possible, endeavour to distinguish the processes
of sensation, induction, and movement from one another, and to resolve
these further into their component factors. At present, this is possible
to a very limited extent and only in a few cases, for the best know-
ledge of the conditions for stimulation and reaction, as well as of the
position, shape, and structure of the percipient organs and of the conducting
paths, affords no insight into the processes which underlie perception
and induction. Since perception and induction are usually so closely
connected that they cannot be separately considered, we shall discuss both
at the same time.
If we restrict the term c perception ' to the first physiological inter-
action involved in stimulation, we must not apply the same term to the
whole series of sensory processes, when these cannot be resolved in detail.
Preparatory processes may take place which render possible or initiate
perception and response. This is the case, for instance, when cutting the
stem of Mimosa produces a movement of water which calls forth a response
in the pulvini, or when the sinking of dense particles in the cell acts as the
cause of a geotropic response. Further, the same would be the case when
light or some endosmosing substance produced a chemical change in which
one of the products acted as a stimulus.
Just as one speaks generally of the processes of induction, so also may
we group all these preparatory processes together as instances of indirect
stimulation — although they may be varied and complicated in character.
In any case the introduction of special terms for phenomena which are
not yet understood, and for facts which are largely unknown or uncertain,
is hardly to be recommended l.
The movement of zoospores towards light or away from it when
intense can be regarded as the result of tropic stimulation. Further, the
conversion of a positive into a negative heliotropic curvature involves
a change in the sensory processes alone when the change from weak to
strong light which produces this alteration acts on the sensitive tip of
a seedling leaf of Avena, but not on the responding basal portion. But
when light or any other agency directly stimulates the responding region,
a change in the response may result either from an alteration of irritability
or from some influence upon the course of the reaction. The sensory
processes themselves may be of greater or less complexity, and hence may
be influenced in a variety of ways so as to lead to changed responses.
In such cases a change in the final result affords no indication as to whether
it is due to a modification of the primary act of perception or of some one
of the resulting stages leading from perception to response.
1 Cf. Czapek, Jahrb. f. wiss. Bot, 1898, Bd. XXXII, pp. 214, 302; Centralbl. f. Physiol.,
1900, Bd. xin, p. 209.
6 MOVEMENT
It follows, therefore, that when two different organisms respond similarly
to the same stimulus the processes of perception and response may be alike
in both, but need not necessarily be so. We do not know, for instance,
whether the mode of perception of light and of other tropic stimuli is in all
cases the same. Even if this were the case the power of response may vary
indefinitely, and can never be the same in a rooted plant as it is in a free
swimming one. The former may respond by movements due to growth or
to changes of turgidity, whereas the latter is dependent entirely upon the
special mode of locomotion it possesses. The same stimulus may produce
different responses according to the properties of the responding organism,
and widely dissimilar stimuli produce the same type of movement in
a particular plant. It is only natural, for instance, that swarm-cells should
always use the locomotory organs they already possess in moving from one
place to another as the result of stimulation, and should not seek out
and utilize currents of water or other external mechanical agencies for
this purpose. Similarly, we may assume that the curvatures resulting
from heliotropic, geotropic, and chemotropic stimuli are in many cases,
at least, carried out in a similar manner in all rooted plants.
Every organ which has the power of responding to one or more tropic
stimuli, singly or conjointly, must possess a special power of perceiving
each such stimulus, and the power of perception as regards one stimulus
may be lost or destroyed without the general perception being affected.
This remains true, in spite of the fact that plants possess no special sense-
organs, and that we are unable to say how it is that of two apparently
similar protoplasts one may temporarily or permanently possess a different
irritability and power of response to the other. The response is always
dependent upon the existent condition of tone, which again depends upon
the stage of development and upon the existing and previous external
conditions1. Furthermore, the resultant action of two conjoint stimuli is
not necessarily the arithmetical sum of their individual actions. Supposing
that the power of movement remains unaltered, either two separate impulses
may be exercised upon it, or the two stimuli may fuse during perception
and act as a single excitation.
1 The terms ' tone ' and ' tonic condition ' may be used in the same sense as in animal physiology,
so that by ' tonic stimuli ' we may denote the conditions which render possible an irritable response.
Cf. Massart, Biol. Centralbl., 1902, Bd. xxxn, p. 41 ; Miehe, Jahrb. f. wiss. Bot, 1902, Bd. xxxvn,
p. 571. Miehe distinguishes between 'anatonic,' ' katatonic,' and ' metatonic ' stimuli, according to
whether the reaction is increased, decreased, or reversed by them. Engelmann's ' photokinesis ' is
merely an instance of light acting as a tonic stimulus, as when illumination excites the movement
of certain motile forms. (Engelmann, Pfliigers Archiv f. Physiol., 1882, Bd. xxx, p. 169.) On the
equally unnecessary term £ chemokinesis,' cf. Rothert, Flora, 1901, p. 374, and also Nagel, Bot. Ztg.,
1901, Ref., p. 298. Carrey (The Effects of Ions upon the aggregation of flagellate Infusoria, 1900,
p. 291) has used the term 'photokinesis ' in another sense, to indicate the changes of movement pro-
duced by sudden alterations of illumination.
THE CAUSES OF MOVEMENT 7
The resulting movement affords no indication as to the mode of
perception, and no movement at all may occur when two opposed stimuli
neutralize each other, or when the resulting attempts at movement are
similar and of opposite kinds. If, however, one stimulus preponderates,
and a movement results, the same amount of energy will be expended as
when a similar movement is produced by a single stimulus.
The resultant reaction due to conjoint stimuli is neither quantitatively
nor qualitatively the sum of their separate actions. This is still the case
when the stimuli are of like kind, for since the power of reaction is always
limited, the superposition of a supra-maximal stimulus upon a sub-maximal
one may produce little or no additional response. Hence also with stimuli
progressively increasing in intensity, the later responses do not increase in
proportion to the increases of excitation.
A satisfactory solution of problems of this kind is not at present
possible, although sufficient is known to show that the mechanism of
irritable perception and response is not always the same. It is clear
that a changed response to a particular stimulus must be due to some
change in the mode of perception if the responding mechanism is unaltered.
Even when the percipient organ is distinct from the responding region,
however, any agency which affects the former may cause modifying
influences to radiate from it to the responding mechanism. Hence a -tonic
stimulus which primarily acts on the percipient organ alone may indirectly
modify the character of the curving zone, so that the capacities of both
perception and response are altered.
Without doubt a change of irritability is in many cases largely or
entirely the result of alterations in the sensory and related processes.
Modifications of irritability very commonly take place during the life of
an organ, so that a particular tropic stimulus does not always produce
the same result. It is not however certain whether, for instance, the lack
of response to shaking in an etherized plant of Mimosa is the result of an
inhibition of the power of perception, of induction, or of motion.
Similarly it is difficult or impossible to say whether in a particular case
two simultaneous stimuli fuse in the act of perception, or whether they act
singly upon the motor mechanism. The former appears to be usually the
case when two dissimilar tropic stimuli act conjointly, whereas a fusion of
this kind does not appear to occur between tropic and photonastic or
contact stimuli. In all cases, however, it must be remembered that the
independence of the processes of sensation and response is only relative, and
that a modification of the one is certain to react upon the other.
A perceptible response is in all cases only produced when the stimulus
reaches a certain minimal intensity, while between stimulation and
response a latent period of variable duration always intervenes. The
resulting movement is nearly always gradually accelerated to a maximum,
8 MOVEMENT
beyond which the effect of the stimulus gradually diminishes and dis-
appears.
The duration of the latent period lies between a few seconds and a few
hours in the case of the slow movements usual in plants. A stimulus
impressed upon a plant continues to act for a time after it has been
removed, and the greater the difficulty of producing an irritable response
the longer will be the persistent after-effect. Hence a stimulus may
produce a response some time after it has ceased to act. Similar relation-
ships hold good for the rapid movements of the leaves of Mimosa pudica
and of the staminal filaments of Cynareae or of Berberis, although they
become more immediately perceptible when the duration of the reaction
is lengthened by low temperatures. The irritability of the plant, as well
as its power of response, are dependent upon the external conditions,
although naturally the most favourable external conditions cannot increase
the response beyond a certain limit.
When the reaction is rapid, all the phases of stimulation must be
passed through in a very short time, but when it is slow the delay may
occur either in the perception of the stimulus or in the responding
mechanism, or in both. It is presumably owing to the lessened power
of movement that the nutation of the older parts of stems and roots is
a little later in time, and also less pronounced than in the younger parts.
In many cases a stimulus is only gradually perceived, and frequently
a long time elapses before the motor-mechanism begins to be called into
action. This is especially well shown when a conducting zone intervenes
between the percipient and responding organs. A prolongation of the
latent period in an organ capable of rapid response is probably in most
cases the result of slow perception.
Except in the case of motile organisms, the movements of plants have
almost always the purpose of gradually bringing the organs into a definite
functional position, and it is only rarely that for special purposes a power of
rapid movement is developed. In such cases we are usually dealing with
transitory reactions produced by sudden changes, as, for example, when
a blow or a sudden change of transpiration causes the leaves of Mimosa
to close. Reactions of this kind may be termed temporary, transitory,
or shock effects, whereas the slower movements involve a condition of
permanent or stationary stimulation. Here a condition of equilibrium
is maintained so long as the external conditions and the properties of the
organism remain unaltered, and a response of this kind is possible not only
to orienting stimuli such as gravity or light, but also to diffuse ones such as
temperature. No sharp distinction can however be drawn, for often both
forms of stimulation act together, and it is in fact in this way that the
peculiar sequence observed in thermonastic and photonastic movements
is produced. The leaflets of Mimosa pudica return to their original
THE CAUSES OF MOVEMENT 9
position in spite of repeated stimulation by blows, whereas tendrils and
the leaflets of Oxalis^ for instance, on a repetition of the stimulation, take
up a new position of equilibrium. Similarly, if stimuli are repeated on
a muscle before relaxation has taken place, the muscle responds to each
and remains contracted in a condition of tetanus.
Furthermore, Mimosa is exceptional in that any shock-stimulus to which
the leaflets respond produces the maximal possible movement. Usually,
however, as for example in the leaflets of Oxalis, a single blow may act
as a sub-maximal stimulus, and the full sinking of the leaflets be produced
only by repeated shocks. The existence of a labile condition is not
essential for the realization of an irritable movement, and in fact in many
cases the latter may not involve an increase in the general activity of
growth, but merely its guidance and regulation. Naturally, however, the
accumulation of potential energy in the form of high tissue-strains and
the like is necessary for the performance of rapid movements.
Except in those cases where any operative stimulus produces the
maximal effect, increasing intensity of excitation produces increasing and
more rapid response. This applies to transitory as well as to intermittent
and continuous stimulation. Weak heliotropic, geotropic, or photonastic
stimulation, for instance, produces a less pronounced curvature than strong
stimulation. There is, however, no exact relation between the intensity of
the stimulus and the amount of response, or of the sensory excitation.
These physiological processes usually increase less rapidly than the stimulus
does, so that a greater increase in the intensity of the stimulus is required
in a strongly excited organ than in one under weak stimulation to produce
the same increase of excitation or response. This rule is well known in
animal physiology, and in addition, beyond a certain intensity of stimulus,
the response may alter, as when organisms swim towards diffuse light but
away from strong sunlight, and hence collect at a definite distance from
a local source of illumination. Rooted plants also curve towards a strong
source of illumination when far away from it, take on a diaheliotropic
position when nearer, and curve away from it when still nearer.
These effects are the result of a change of tone, which may often be
due to the fact that some of the factors involved in sensation are affected
more than others by increasing stimulation. This is shown especially well
when with increasing concentration a negative osmotropism overcomes
a positive chemotropism.
Every disturbance of equilibrium inducing curvature excites reactions
directed towards the restoration of equilibrium. Hence on the removal
of a tropic stimulus, the organ affected returns to the original position
assumed in virtue of its autotropism, so long as the power of movement
is retained. Even in adult organs which have ceased to grow, curvatures
may be removed if a power of potential growth resides at the nodes.
io MOVEMENT
The rapidity of the return movement depends on the prevailing condi-
tions, but it is usually much slower than that induced by the original
stimulation, as is strikingly shown by comparing the sudden closure of
the leaflets of Mimosa or of the leaf-lobes of Dionaea, induced by a blow
or by contact, with their subsequent gradual re-expansion.
Since autogenic factors are always in play, even the movement
resulting from a single external stimulus is as much the result of conjoint
stimuli as when two external stimuli act simultaneously. As the result
of the co-operation of these autogenic and aitiogenic factors and of the
reactions due to the movement itself, the final curvature assumed is usually
preceded by a series of oscillations. The movement of the mercury in the
gas-regulator. of a hot chamber when the temperature is raised to a new
level forms a suitable analogy, for here also the excessive movement excites
factors tending to its reduction, and to a rapid diminution in the amplitude
of the vibrations. Oscillations of this kind occur during tropic and nastic
movements, as well as during the return of stimulated leaves of Mimosa to
their original position. The persistent after-effects of the daily movements
are also the result of oscillations of this kind, although oscillations having
a purely internal origin may exist.
These general remarks apply not only to the higher and lower plants but also to
each individual protoplast, for in each case the functionally dissimilar parts and organs
are variously affected by stimuli and are unequally responsive and' active. We do
not, however, know either the organs of perception or by what changes the latter may
be modified. Just as particular powers and properties may appear and disappear
under particular conditions, so also may the power of perception not always be
present. Furthermore it is possible that in many cases the perception of a stimulus
may involve the simultaneous awakening of different processes, and that the inca-
pacity for any one of these may make the organism irresponsive.
Since the organs of the protoplast are capable of a variety of functions, it is
hardly to be expected that any of them should be capable of response to a single
stimulus only, or that special sense-organs capable only of limited excitation should
be developed. It is, however, possible that in particular cases the nucleus may
perceive the stimulus or act as a reflex centre, whereas in others it may take no part.
Thus in non-nucleated masses of cytoplasm functions such as streaming and ciliary
movement may continue and be affected by external stimuli, as is especially well
shown when non-nucleated fragments of Infusoria exhibit galvanotaxis. Even when
interaction with the nucleus is necessary for the performance of a response by the
cytoplasm, it does not follow that the nucleus perceives the stimulus. For instance,
the unicellular rhizoid of Marchantia or of a fern prothallium responds by a nega-
tively heliotropic curvature when the tip is exposed to light, although the nucleus is
at its base and is not directly exposed to the stimulus of light.
The different parts of the cytoplasm have without doubt different and change-
able powers, but even when a particular stimulus is perceived by the isolated cilia
of a motile organism, the ectoplasmic membrane and other parts may also be
THE CAUSES OF MOVEMENT u
sensitive to this stimulus. The ectoplasmic membrane may in fact be specially
sensitive to orienting stimuli, but it is uncertain whether stimuli inducing move-
ment in the chloroplastids are perceived in the chloroplast itself, and it is very
doubtful whether the eye-spot of zoospores is an organ specially adapted for the
perception of light.
The process of sensation is not revealed by the movements or changes in the
protoplast which result from or accompany stimulation. Thus the movement of
a swarm-spore towards light, or the local accumulation of the cytoplasm or chloro-
plastids produced by tropic stimuli, afford no insight into the processes of perception
and induction. In many cases local accumulations of the protoplasm form the purely
mechanical result of a realized curvature, but in others preparatory processes of this
nature may precede or accompany the actual perception of a stimulus.
Historical. From the beginning of the nineteenth century attempts have been
made to explain the causes and mechanism not only of the rapid movements of
Mimosa pudica, but also of heliotropic and other growth curvatures. It was naturally
only at a somewhat later date that the smaller and less known motile organisms were
also drawn into consideration. At first it was attempted to explain the movement
as being the direct mechanical result of the exciting stimulus. Thus the partial
etiolation of the shaded side of a stem, or the modification of the elasticity of the
cell-walls by the direct action of light, were considered to be the causes of heliotropic
curvature, while geotropism was supposed to result from the plastic curvature of the
root or of the growing apex under its own weight, or to the unequal distribution
of food-materials of different densities brought about by the action of gravity.
The true nature of these complicated manifestations of irritability was therefore
not recognized, although Dutrochet1 in 1824 expressed the opinion that light and
gravity were only the inducing causes of heliotropic and geotropic curvatures, and not
the direct mechanical agencies in producing them. This author, however, can hardly
have thoroughly comprehended the phenomena in question, since at a later date he
arrives at direct contradictions to his original principles2. Even in the brilliant
Experimental Physiology of S^chs 3 the mechanical explanation of the slower growth
movements retains the upper hand. Pfeffer in 1877 * pointed out that the move-
ments were in all cases the responses of irritable structures to stimuli, and brought
the subject up to our present standpoint. The researches of Darwin were of the
utmost value in this connexion since they showed that the processes of perception,
induction, and movement might take place some distance apart 5.
Darwin 6 considered all curvatures to be modified forms of circumnutation, but
1 Dutrochet, Rech. s. la structure intime d. animaux et d. vegetaux, 1824, pp. 107, 117,
130, &c.
3 Dutrochet, Me"m. anat. et physiol. d. vegetaux etc., 1837.
3 Sachs, Experimentalphysiologie, 1865.
4 Pfeffer, Osmot. Unters., 1877, p. 202; Pfeffer, Pflanzenphysiologie, 1881, Bd. I, p. 3; Bd. II,
pp. 117, 178, 286, 327 u. s. w. Sachs, Vorlesung iiber Pflanzenphysiologie, 1882, p. 71 7, then pointed
out the general character of irritability, but was wrong in supposing that for every irritable response
a labile condition is essential. Cf. also Pfeffer, Die Reizbarkeit d. Pflanzen, 1893, p. 10 (Reprint
from the Verb. d. Ges. detitscher Naturf. u. Aerzte zu Nurnberg).
5 Darwin, Insectivorous Plants, 1875 ; The Power of Movement in Plants, 1880.
6 Darwin, The Power of Movement in Plants. Darwin himself doubted whether the movements
12 MOVEMENT
this view leaves out of consideration the special forms of irritability which the plant
has developed for particular purposes. In the case of either a growing plant or
a motile zoospore, a curvature or change of direction is due to an external or internal
stimulus modifying the previous activity, but in the nodes of grasses when laid
horizontal the external stimulus of gravity first awakens growth and then determines
its direction. Aitiogenic and autogenic curvatures, although they may co-operate,
do not always occur together. Hence a plant showing active circumnutation may
only respond to external stimuli by a feeble curvature, while an active power of
response may be accompanied by very slight circumnutation. There are, indeed,
plants in which aitiogenic movements are carried out in a different manner to
autogenic ones.
SECTION 3. The Mechanism of Movement.
Amoeboid movement and the locomotion of zoospores are effected in
a different way to the growth curvatures resulting from modifications
of nutation, and these again are of different origin to the temporary
movements resulting from changes of turgidity coupled with the elastic
contraction and expansion of the cell-walls.
All active nutation curvature is the result of unequal growth on the
two sides of the cell or curving organ. If the more active growth occurs
first on one side and then on the other, the apex will move to and fro more
or less regularly, but if the zone of more active growth travels round the
growing region, the apex will describe an ascending spiral in space. The
latter is especially well shown in the case of climbing plants and these
may twine around a support with or without torsion of the stem 1.
Most plants only carry out movements of nutation, and in such cases
the power of curvature is lost with the cessation of growth, but is regained
with the resumption of growth, as in the geotropically stimulated nodes of
grasses. The absence of curvature may also be due to the fact that the
energy of growth is unable to overcome the mechanical rigidity of the
organ affected. The woody stems of Conifers, for instance, may be able
to curve as the result of cambial activity up to their second or even third
year, but not beyond this 2. Similarly the curvatures shown when a
herbaceous stem is split longitudinally give evidence of tissue-strains,
of the leaflets of Mimosa and of the tentacles of Drosera could be regarded as modified circum-
nutation. Cf. also Wiesner, Bewegungsvermcigen der Pflanzen, 1881, p. 202.
1 Nageli und Schwendener, Mikroskop, a. Aufl., 1877, p. 416 ; Schwendener und Krabbe,
Abhandlg. d. Berl. Akad., 1892, p. 56; Kolkwitz, Ber. d. bot. Ges., 1895, p. 495 ; and the literature
quoted in these works.
3 [Errera (Proc. British Ass., 1904) states that the trunks of tall adult trees may curve geo-
tropically at their bases. The curvatures observed were, however, undoubtedly produced when young,
for to bend an old stem upwards at its base, the developing wood-elements would have to overcome
a mechanical moment representing in them pressures of many hundred or thousand atmospheres.]
THE MECHANISM OF MOVEMENT 13
which if they existed on one side only would suffice to produce a curvature
of the entire stem, if its mechanical rigidity were not too great.
In organs adapted for temporary or variation movements the structure
is such as to give a considerable freedom of movement. Thus in the
pulvini of Leguminosae and other plants the relatively rigid and inelastic
vascular bundle is curved and surrounded by active tissue in which, owing
to the elasticity of the walls and the changes of shape in the cells,
considerable shortening and lengthening is possible l. The vascular bundle
at the middle of the pulvinus (Fig. i) lies in the neutral zone, and is but
little affected by the curvature produced by a rise of turgidity on the
lengthening side of the pulvinus or by a fall on the shortening one. In
the first case the shortening of the concave side is due to the cells being
compressed by the expansion of those on the upper convex side2, just as
happens when a pulvinus is moderately bent by applying an external force.
In the case of a nutation movement, however, the median axis undergoes
permanent elongation, and it de-
pends upon the mean activity of
growth, upon the degree of curva-
ture, and upon the thickness of the
organ whether the concave side
becomes longer, shorter, or retains
the same length as before 3. The
amount of curvature is naturally
dependent not only upon the re-
lative growth Of the antagonistic FIG. i. Pulvinus of Phaseplus vulgaris (magnified),
-1 c (a) longitudinal, and (6) transverse sections.
connected tissues, but also upon
the resistance offered by the vascular bundles and other inactive elements.
The importance of this resistance is shown by the fact that when a young
pulvinus still capable of growth is caused to curve the vascular cylinder
undergoes a slight permanent elongation.
The realized curvature affords no evidence as to whether one or both
zones are active, or in the latter case whether the response is of similar
character but unequal amount on the two sides, or of dissimilar character.
As a matter of fact various combinations occur. Thus the variation
movement of Mimosa pudica is produced by a fall of turgidity on the
concave side, the expansive energy of the unstimulated convex side then
1 Pfeffer, Die period. Bewegungen d. Blattorgane, 1875, pp. 3, 157. On the anatomy of pulvini
see also A. Rodrigue, Bull, de la Soc. bot. de France, 1894, T. 41, p. 128 ; Schwendener, Sitzungsb.
d. Berl. Akad., 1896, p. 535; 1897, p. 228; 1898, p. 176 ; M. Mobius, Festschrift fUr Schwendener,
1899, p. 37. — E. Pantanelli, Studii d'anatomia e fisiologia sui pulvini motori di Robinia et Porliera,
1901 ; Haberlandt, Physiol. Anat., 2. Aufl., 1896, p. 475.
2 Pfeffer, Physiol. Unters., 1873, p. 73.
3 Pfeffer, Die period. Bewegungen d. Blattorgane, 1875, p. 17.
14 MOVEMENT
producing the curvature. On the other hand, during photonastic curvatures
the energy of expansion increases or decreases in both halves of the
pulvinus, but more rapidly in one half than in the other, so that the
original curvature is in time partially or entirely eliminated. The move-
ments produced as the after-effect of the daily movements, and the
spontaneous movements of variation are produced by a rise of pressure
on the one side and a fall on the other. The same takes place when
a heliotropic or geotropic curvature is produced in a pulvinus.
All possible combinations may be involved in the different kinds of
nutation movements. Thus most geotropic and heliotropic curvatures
are produced by an acceleration of the growth upon the convex side,
and a retardation on the concave one, the mean growth of the median
axis being unaltered only slightly so. On the other hand, the curvatures
produced in tendrils by contact as well as the aitionastic nutation move-
ments of stems, involve a general acceleration of growth, but this is more
rapidly produced on one side than the other. It is possible but not certain
that some curvatures may be produced by an acceleration or retardation
of growth on one side only, or even by an active growth contraction on
one side. Active growth contractions do actually occur in roots, and
Kohl erroneously assumed that the tropic nutation movements were the
result of the shortening of the concave side. The curvatures produced
in split stems owing to the release of the tissue-strains may undergo
a secondary increase owing to the resumption of growth in the two halves,
and a tissue like the pith, which when isolated grows straight, experiences
a curvature in the split stem. Hence the curvature realized in an organ
depends upon the powers and activities of its inter-related cells and tissues.
Frequently inactive tissues are curved by the active ones, and it may
happen that the concave side is shortened and its cells compressed,
owing to the more rapid growth on the convex side, although both sides
strive to grow more rapidly than before but not equally so. Many
curvatures are produced as the direct result of the fact that certain
tissues grow and elongate more rapidly than others.
Observations made upon Thallophyta and unicellular trichomes show
that individual cells may curve owing to one side of the cell-wall elongating
more rapidly than the other. In multicellular organisms the curvature
may either be directly produced in the curving cells or tissue, or may result
from the antagonism between connected but unequally elongating parts.
In the former case we can speak of the photonasty or heliotropism of the
responding cell or tissue 1, whereas in the latter case the organ responds
more as a whole. A sharp distinction is impossible in many cases, for
often both types may act together, and the mechanical action of the realized
1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 414.
THE MECHANISM OF MOVEMENT 15
curvature may originate stimuli tending to the modification and correlation
of growth in the different zones. Some such regulation is necessary even
when. the curvature is produced by the activity of the different cells, for
unless they all act at the same time and in the same direction no curvature
could be produced in an organ having a moderate mechanical rigidity.
During tropic curvatures each lamella from the concave to the convex
side seems to grow more actively than the one before it, so that all the
lamellas tend to curve actively. On the other hand, many aitionastic and
autonastic curvatures seem to be produced by the antagonism of unequally
elongating tissues.
Even when a curvature can be ascribed to the distribution of the active
and passive zones and to their relative rates of growth, we have still to
determine the causes which induce the latter. Growth curvatures may
be produced in various ways, either by plastic growth, growth by
intussusception, or by changes of shape of the cells affected. Hence
similar curvatures need not necessarily be produced in the same way in
aH plants.
The intermittent elongation and the related nutation movement of
Oedogonium are due to its plastic mode of growth, and the same peculiarity
may be responsible for many nutation movements. The rate of growth
is usually not regulated by changes of turgidity, but in other ways, and in
fact the turgidity usually sinks slightly in the cells on the convex side
which are growing most rapidly. It is quite possible for the increased
growth which produces curvature to be the result of a rise of turgidity on
one side, but hitherto not a single instance has been established. The
positive conclusions of various authors are based upon uncertain facts, and
are in part derived from incorrect views as to the mode of growth in surface
extent of the cell-wall 1. A change of turgidity can hardly be responsible
for a curvature due to the unequal growth of the cell-wall on the opposite
sides, although a rise of turgor will aid in stretching a wall which has
become more extensible. There can, however, be little doubt that, as in
the case of the tissue-strains, turgidity forms an important factor in the
growth of the cell-wall, and in enabling the growing cells to react
mechanically upon other parts.
The expansions and contractions involved in variation movements
are usually the result of changes of turgidity which bring about elastic
expansion or contraction of the cell-wall. If the cell-wall is highly elastic
and but little stretched, a slight contraction will be sufficient to restore turgor
after a fall in the internal osmotic pressure, but if the cell-wall is considerably
1 Thus de Vries (Stir les mouvements auxotoniques des organes ve'getaux, 1880, Repr. from
Archives Neerlandaises, T. 15) considers nutation to be the result of changes of turgidity, and pro-
poses the term ' auxotonic ' for movements produced by a rise, and ' allassotonic * for movements due
to a fall of turgor. It is, however, difficult to see the need for these terms.
16 MOVEMENT
stretched it may undergo a pronounced decrease in size before turgidity is
restored. In most cases the cell-wall is so little stretched that the shortening
of the cell on plasmolysis is slight or hardly measurable. In cells of the
staminal filaments of Cynareae, however, the walls are stretched to such
an extent that a slight fall of turgor produces a pronounced contraction 1.
A curvature may be produced in a tissue by a fall of turgor even when
the individual cells do not undergo any active contraction. For instance,
if the turgidity and hence also the rigidity of the cells in the stimulated
half of a pulvinus of Mimosa pudica diminishes, these cells will be
compressed by the tendency to expansion of the cells in the upper half
until equilibrium is reached. In other cases, as in the variation movements
due to light and gravity, the turgidity decreases on one side of the pulvinus
and increases on the other.
Changes of turgor produced as physiological reactions act in exactly
the same way as changes due to plasmolytic action or to excessive
transpiration. The drooping movement of herbaceous parts is the direct
result of the diminished turgor with its correlated decrease of rigidity
in the stretched thin-walled cells. An artificial removal of turgor produces
no perceptible movement when the cells possess sufficiently thick and rigid
walls, and in such cases no fall of turgor resulting from stimuli can produce
any movement.
If the turgidity remains constant, an active variation movement can
only be produced by a change in the properties of the cell-wall, a decreased
elasticity resulting in increased stretching, while an increase of elasticity
diminishes the stretching due to the osmotic pressure. In addition an
alteration in the power of imbibition may produce an active change of
shape in the cell. It must be admitted that the protoplast is able to
produce temporary or permanent changes of this kind in the cell-wall,
but hitherto no pronounced reversible movement has been traced to this
cause.
Owing to the usual semi-fluid consistency of the protoplasm, the
pressure exercised upon the cell-wall is almost solely the result of the
osmotic concentration of the sap. Whenever the protoplasm attains
a high cohesion it may by its own changes of shape bring localized
pressure to bear against a resistance. This is evidenced by the movement
of cilia, and it is possible, especially in the case of minute organisms, that
the protoplast may be able to exert considerable pressure against the
cell-wall, or to antagonize a portion of the osmotic pressure exerted within
the cell. If the expansion or contraction were localized, curvature would
readily be produced in cells with equally distensible walls, whereas a
1 Cf. Pfeffer, Zur Kenn trass d. Plasmahaut u. d. Vacuolen, 1890, p. 325 ; Studien zur Energetik,
)2, pp. 216, 221, &c.
THE MECHANISM OF MOVEMENT 17
general rise or fall of the internal hydrostatic pressure can only produce
curvature when the opposed sides are unequally extensible, or are made
so by the action of the protoplast.
Even when a variation movement is found to be due to changes in
the osmotic pressure, the elasticity of the cell-walls remaining constant,
it still remains to be determined in which of a variety of ways the
alterations in the osmotic pressure are produced. For instance, a fall of
turgor may be produced by a precipitation of the dissolved osmotic
materials, by their conversion into larger molecules of less osmotic activity,
by the physiological combustion of the osmotic materials or by their
removal in other ways, as when they are allowed to diosmose out of the
cell. If in the latter case they remained dissolved in the imbibed fluid
saturating the cell-wall, the osmotic pressure against the cell-wall would be
diminished in exactly the same way as when a plasmolysing solution is
applied. By the reabsorption of the excreted materials, or by the produc-
tion of new ones, the original condition of turgor may be restored. Here,
as in other cases, the disturbance due to a reaction excites activities tending
towards the restoration of equilibrium.
A fall of turgor causes the cell to contract with an escape of water,
until the concentration of the sap again balances the decreased tension
in the cell-wall. If the fall of turgor is sudden, the cell readily permeable
to water, and if the latter is able to escape into the intercellular spaces,
then rapid movements may occur, as in the leaves of Mimosa and the
staminal filaments of Cynareae. That the cells are capable of rapid
filtration under pressure is shown by the rapidity with which they contract
or become plasmolysed when placed suddenly in strong solutions of salt.
In the case of nutation movements, we have primarily to determine whether
the curvature does or does not involve any change in the average rate of growth,
and whether the latter is accelerated on the concave side as well as on the convex
one. Even in the case of small objects this can be ascertained by the use of suitable
micrometers. In measuring short distances the chord of an arc may be taken as the
length of the curved surface of the arc \
A change of osmotic concentration can only be detected by plasmolytic methods
when it persists for some time, and is not as rapidly readjusted as it is in the pulvinus
of Mimosa. Furthermore, the contraction or compression of the cell will always
cause a rise of the internal osmotic pressure if only water escapes from it. In fact,
it does not follow that a rapid movement must always be produced by a change
of turgor affecting the elastic stretching of the cell-wall.
Especially in the case of movements of variation, measurements of rigidity afford
1 Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 293; Periodische Bewegungen, 1875, p. 15;
Physiol. Unters., 1873, p. 27. [If the object is strongly curved, the length of its curved surfaces can
be satisfactorily found by reconstructing the figure on paper from a series of measured chords, or by
measuring the curved surface by means of an opisometer.]
PFEFFER. Ill C
i8 MOVEMENT
some evidence as to the expansion or contraction in the antagonistic tissues. Thus
the rigidity will increase if the force of expansion becomes greater either in one or
in both halves of the pulvinus, but will be lessened if it falls in one or both halves,
while if the rigidity remains constant we have evidence to show that one side expands
and the other contracts in equal degree. Briicke l measured the rigidity by noting
the bending when the organ was held horizontally, firstly with the curvature upwards,
then with it facing downwards. The angular divergence was read off on an arc
having its centre at the median point of the pulvinus. In the same way an increased
rigidity is shown when a load produces less bending in an organ kept in the same
horizontal position 2.
During its geotropic curvature a root may lift a considerable weight, and by
finding the weight required to prevent curvature a measure may be obtained of the
energy of curvature 3. Slender plastic roots which are easily bent can naturally
exert no great pressure unless lateral displacement is prevented. Nutation move-
ments may also take place against considerable external resistance since they result
from irregular growth. In such cases the external resistance antagonizes a portion
of the osmotic pressure acting against the stretched cell-wall. Similarly in variation
movements, either a rise of turgor takes place or a fall enables the previous stretching
of the cell-wall to come into play.
If the antagonistic tissues are symmetrically displaced, as in a radial organ, no
curvature is shown until the organ is split in half. The energy of curvature is
greater when the active tissues are some distance from the neutral axis, since the
leverage or bending moment they exert is increased. The bending moment
therefore depends upon the energy of expansion and upon the distribution of the
active tissues 4. The problem is the same whether the curving zone is short or long,
and the curving zone may in fact be made extremely short by preventing the
attempted movement by means of bandages over the greater part of the length 8.
Thin organs can naturally bend more sharply and rapidly than thick ones, since
in the latter a considerable difference in length has to be produced between the
convex and concave sides6. The most pronounced curvature does not always
occur in the most actively growing zone, since the conditions for curvature are often
later in development. Curvature is influenced by external conditions in exactly the
same way as is growth in general. In certain cases, as in tendrils, it is favoured by
abundant supplies of water, whereas the movement of the pulvini of Mimosa is for
obvious reasons decreased or prevented when the intercellular spaces are injected
with water.
1 Briicke, Miiller's Archiv f. Physiol., 1848, p. 452. Cf. Pfeffer, Period. Bewegungen, 1875, p. 89.
2 Schwendener (1897), Gesammelte Abhandlg., Bd. n, p. 237.
3 The best form of apparatus is a very stiff spring which can be adjusted by a screw. Cf. Pfeffer,
Period. Bewegungen, 1875, pp. 9, 97 ; Druck- und Arbeitsleistungen, 1893, p. 389 ; Meischke, Jahrb. f.
wiss. Bot., 1899, Bd. xxxin, p. 345.
* Cf. Pfeffer, 1875, 1. c., p. 99 ; 1893, 1. c., p. 392.
5 Meischke, 1. c. , p. 348.
6 Cf. Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. vil, p. 173.
CHAPTER II
MOVEMENTS OF CURVATURE
PART I
AUTONOMIC MOVEMENTS
SECTION 4. Occurrence and Distribution.
SPONTANEOUS, autogenic, or autonomic movements would arise in the
normal course of development even if the external conditions could be kept
rigidly constant. Locomotory and streaming movements will, however, be
discussed in a subsequent chapter. Movements may either be periodical,
as when a shoot nutates or a leaf folds at night, or may be incapable of
repetition (ephemeral or climacteric), as when a bud unfolds or a capsule
dehisces 1. Periodic or nutation movements are shown by the growing apices
of both vascular and non-vascular plants, and in the latter by the growing
tips of single cells such as the branching mycelium of Mucor, and by fila-
ments formed by chains of cells such as those of Penicillium or Spirogyra 2.
This was first shown by Darwin 3, and Fritsch has repeated some of the
observations under conditions kept as constant as possible, and has found
that the autonomic movements still continue.
When the movements are pronounced, their independence of the
external conditions is easily seen. Thus the growing ends of the stems of
climbers sweep round in wide circles, as also do many tendrils ; while the
lateral movements of the peduncle of Ttdipa and Allium may cause the
flower to be bent downwards during development 4. Individual cells or
1 A. P. de Candolle (Memoires d. savants etrangers de 1'Institut de France, 1806, T. I, p. 338)
termed flowers opening once ephemeral, and those opening repeatedly equinoctial.
2 F.Darwin, Bot. Ztg., 1881, p. 474; Fritzsche, Ueber die Beeinflussung d. Circumnutation
durch verschiedene Factoren, Leipziger Dissertation, 1899, p. 9 (Phycomyces) ; Wortmann, Bot. Ztg.,
1 88 1, p. 384 (Mucor stolonifer).
3 Darwin, The Power of Movement in Plants, 1880. Darwin and, later, Fritzsche have shown
that a slight change in the external conditions may influence the movements. On Fungi cf. Rein-
hardt, Jahrb. f. wiss. Bot., 1892, Bd. xxin, p. 479 ; Sokolowa, Das Wachsthum d. Wurzelhaare und
Rhizoiden, 1897. In most of Darwin's experiments the attached indicator exercised a certain
disturbing action.
* Cf. Darwin, 1. c., and the works already quoted; also Hofmeister, Pflanzenzelle, 1867, P- 323 J
Lecoq, Bull, de la Soc. bot. de France, 1867, p. 153 (Leaf of Colocasid) ; F. Miiller, Jenaische Zeitschr.
f. Med. u. Naturw., 1870, Bd. v, p. 134 (Peduncle of Alisma)\ Sachs, Lehrbuch, 3. Aufl., 1873,
p. 827; Rodier, Compt. rend., 1877, T- LXXXiv, p. 961 (Ccratophyllum}\ Wiesner, Bewegungs-
vermogen, 1881 ; Vochting, Bewegungen d. Bliithen u. Friichte, 1882, p. 186, &c. ; Hansgirg,
Phytodynamische Unters., 1889; Beihefte £• Bot. Centralbl., 1902, Bd. xil, p. 248; Phycol. und
phytophysiol. Unters., 1893; Askenasy, Eer. d. bot. Ges., 1890, p. 77 (Root of Maize); A. Schulz,
C 2,
20
MOVEMENTS OF CURVATURE
chains of cells are also capable of spontaneous movement, as is shown by
the stolons of Mucor stolonifer, as well as by Spirogyra and other Con-
jugatae. The threads of Spirogyra may often curve into rings or spirals,
and subsequently straighten themselves. As in the case of growth in
length, periods of rest and of activity alternate, and during the latter, curva-
ture may appear in a few minutes, and a complete circle be formed in ten
minutes to half an hour *. In connexion with its peculiar mode of growth,
lateral bending may be produced in the filaments of Oedogoimim> pre-
sumably because the cell-wall splits and the ring
of plastic cellulose stretches sooner on one side
than on the other.
All stages are shown between trifling and
pronounced nutation, according to the plant, to
the stage of development, and to the external
conditions. The curves are not always regular
and similar, even when there is a pronounced
tendency to linear, elliptical, or circular nodding,
as the case may be. Even when the last named
is most pronounced it may temporarily alter into
to-and-fro pendulum movements. Slight circum-
FIG. 2. Nutation of the sheathing leaf of
the seedling of Zea Mays, from 8.30 a.m. on
Feb. 4 to 8 a.m. on Feb. 6. The movement
is magnified 25 times. (After Darwin.)
FlG. 3. Circumnutation of a cotyledon
inches long,
a.m. July 14.
arwin.)
FIG. 3. Circumnutation ol
of Lagenaria vulgaris i£ i
from 7.35 a.m. July 11 to 9-5 i
Magnified 8 times. (After D.
nutation may change to a single large circular or lateral movement,
although a very irregular curve is produced when the movement of the
Ber. d. hot. Ges., 1902, pp. 526, 580; Neubert, Jahrb. f. wiss. Bot, 1902, Bd. xxxvin, p. 149
(Allium) ; Richter, Ber. d. bot. Ges., 1903, p. 175 (Seedlings).
1 The movements of Zygnemaceae were known to Link, Grundlehren d. Anatom. und Physiol.,
1807, p. 263; Meyen, Pflanzenphysiol., 1839, Bd. m> P- 5^75 and were studied in detail by
Hofmeister, Jahreshefte d. Vereins f. vaterland. Naturkunde in Wiirttemberg, 1874, Bd. xxx, p. 211,
and Oltmanns, Flora, 1892, p. 199. That they occur under constant conditions has been shown
by Winkler, Kriimmungsbewegungen von Spirogyra, 1902, who also found that when suddenly
killed the curvatures were retained. To show the movement single threads may be observed in white
porcelain dishes.
OCCURRENCE AND DISTRIBUTION 21
growing tip is projected from above on to a plane surface l. In flattened
organs, for mechanical reasons, the movement takes place mainly in
a definite plane, and for physiological reasons the same applies to dorsi-
ventral organs, and also to variation movements which in general are more
regular than nutation movements. During linear nutation the rapidity of
the return movement increases to a maximum and then gradually diminishes
up to the point of reversal. Secondary oscillations always occur, however,
and these are sometimes very pronounced.
Pronounced circumnutation 2, such as is shown by twining plants and
by many tendrils, usually maintains a constant direction. Nevertheless, in
the case of certain tendrils, the shoots of some leaf-climbers, and even of
a few twining plants, a periodic reversal of the circumnutation has been
observed 3. In all cases the circumnutation results from the progression of
the more rapidly growing (epinastic) zone around the apical region. Hence
both the convex side and the front flank are continually changing, a
transverse section of the stem moving around the axis of revolution in the
same way that the earth would move around the sun if it had no daily
rotation. Under such circumstances no torsion is produced, but this is at
once shown if the same side always keeps in front. A hanging shoot
subjected to torsion will naturally show a revolving movement.
In both young erect twining plants, and in the stems of older ones projecting
beyond the support, the entire growing zone is capable of nutation. The growing
and nutating zone of the Hop is 20 to 30 cm. and of Hoy a carnosa up to 80 cm.
long 4. In the case of tendrils the period of circumnutation is limited, and it stops
when growth ceases. Further the nutation only begins when the tendril has expanded
and attained a fair length 5. Even in twining plants circumnutation does not begin
until the seedling has attained a certain size, the first one or more internodes showing
no circumnutation6.
Under favourable conditions a revolution is performed in one or two hours in
the case of Akebia quinata, Convolvulus septum and Phaseolus vulgaris, whereas
Lonicera brachypoda requires five to six hours, and Adhatoda cydoniaefolia 24 to
48 hours 7. The non-twining stems of Passiflora gracilis and the tendrils of Cobaea
1 Cf. Darwin, Climbing Plants, 1875, P- IJ35 Dutrochet, Ann. d. sci. nat, 1843, 2e ser.,T. xx,
p. 314 ; Fritzsche, 1. c.
2 Darwin (The Power of Movement in Plants, 1880, p. i) employed the term ' Circumnutation/
Dutrochet (Ann. d. sci. nat., 1844, 3° se"r., T. II, p. 157) that of * Revolutive (rotary) Nutation.'
3 Darwin, The Movements and Habits of Climbing Plants, 1875, p. 34 seq.; O. Miiller, Cohn's
Beitrage z. Biologic, 1887, Bd. iv, p. 103 ; Wortmann, Bot. Ztg., 1887, p. 65 ; Baranetzsky, Die kreis-
formige Nutation u. das Winden d. Stengel, 1883, p. n.
* Darwin, The Movements and Habits of Climbing Plants, 1875, p. 3.
5 Darwin, 1. c., p. 5; Wortmann, 1. c., p. 51 ; Fitting, Jahrb. f. wiss. Bot., 1903, Bd. xxxvm,
P- 547-
6 Mohl, Ranken- und Schlingpflanzen, 1827, p. 104; Darwin, I.e., pp. 4, 26, 33; Schenck,
Beitrage z. Biologic u. Anatomic d. Lianen, 1892, I, p. 128.
7 Darwin, 1. c., p. 26 ; Simons, Contrib. from the Bot. Lab. of Pennsylvania, 1898, Vol. II, p. 66.
22 MOVEMENTS OF CURVATURE
scandens circumnutate as rapidly as the best twining plants1. In all cases, however,
the rapidity of movement is subject to pronounced variations even under constant
external conditions.
Variation movements. Spontaneous variation movements appear to be
shown by all motile pulvini. These are very
slight in the leaflets of Acacia lophantha> more
perceptible in those of Mimosa pudica and
Phaseolm vulgaris, and very pronounced in
those of Oxalis acetosella and Trifolium pra-
tense. The leaflets of Oxalis swing to and fro
through an angle of 20° to 70° in from 45
minutes to 2 hours, those of Trifolium through
an angle of 45 to 150 degrees in i^ to 4 hours 2.
The basal leaflets of Desmodium gyrans de-
scribe an elliptic curve and require only 85
FIG. 4. Leaf of Desmodium gyrans. to OO SCCOnds for a single revolution at
Nat. size, s = paired basal leaflets. ° p 3
35 *»
The movements of Desmodium are very dependent upon temperature, for Kabsch
found that at 28° to 30° C. a revolution takes four minutes, and at 22°C. the move-
ment is reduced to a minimum. Apparently also the excentricity of the elliptic path
alters with the speed. The fact that the ascent takes longer than the descent is the
natural result of the extra amount of work done in raising the leaf4.
The column in the flower of Stylidium adnatum shows a distinct to-and-fro
movement. When it presses against the labellum a trigger arrangement on the latter
holds it until the attempted return movement has produced considerable strain, when
release is followed by sudden movement. Kabsch, observing that contact was able
to produce this movement, considered it to be a physiological response to stimulation,,
whereas Gad has shown that the rapid movement does not take place if the labellum
is removed, or if a piece of paper is laid upon it. The latter prevents the catch
arrangement from acting, so that the column leaves the labellum as soon as the
return movement begins. A similar rapid movement can be produced by retarding
1 Darwin, The Power of Movement in Plants, 1880, pp. 106, 153.
2 Pfeffer, Periodische Bewegungen, 1875, p. 133; Darwin, 1. c., p. 352.
3 Kabsch, Bot. Ztg., 1861, p. 355 ; Hofmeister, Pflanzenzelle, 1867, p. 332 ; Meyen, Pflanzen-
physiol., 1839, Bd. in, p. 553; Treviranus, Physiologic, 1838, Bd. n, p. 766. The older literature
on these long-known movements is given by these authors. Cf. also Stahl, Bot. Ztg., 1897, p. 98.
4 Cels, Sylvestre and Halle", Annal. d. Botanik von Usteri, 1796, Stuck 19, p. 63; Kabsch, 1. c.,
P. 355-
5 Gad, Bot. Ztg., 1880, p. 216 ; Schilling, Der Einflnss der Bewegungshemmung auf d. Arbeits-
leist. d. Blattgelenke v. Mimosa pudica, Habilitationsschrift, 1895, p. 18. According to Burns
(Flora, 1900, p. 344) we are dealing with a growth-movement. Haberlandt, Sinnesorgane im
Pflanzenreich, 1901, p. 73. Whether the movements of the labellum of Megadinium falcatum and
si Pterostylis observed by Lindley and Morren (Ann. d. sci. nat., 1843, 2e ser., T. xix, p. 91) are
growth or variation movements is not yet certain, or even whether these movements are really
spontaneous. Cf. the literature given by Hansgirg, Phycol. u. phytophysiol. Unters., 1893, p. 149.
OCCURRENCE AND DISTRIBUTION 23
the movement of the leaflets of Desmodium, Tnfoh'um, or Oxalis until a sufficient
pressure has been produced in the pulvinus, and then releasing the leaflet. Similar
strains are produced by the growth of the flower-buds of Genista and other
Papilionaceae, which finally lead to the sudden opening of the flower *.
Ephemeral movements. Under constant external conditions the whole
progress of development, including the formation of organs, consists of
a series of ephemeral movements. Such are all the movements involved in
the opening of foliage and flower buds, in the straightening of the arched
stems of embryoes, or of the flower stalk of the Poppy, and the fruit stalk
of Campanula. In fact, all movements due to autonomic changes in the
rate of growth of opposed sides of an organ are of this character.
In many cases the organ oscillates a few times before assuming a
constant position, so that no precise boundary can be laid down between
ephemeral and periodic movements. Each stamen
of Ruta graveolens (Fig. 5), for instance, after the
flower has opened, bends away from the ovary, then
applies itself to it, and then again bends outwards
towards the perianth. Similarly some flowers open
and close more than once, so that in both cases we
may term the movements periodic 2.
In many cases the older and younger zones
of a growing region exhibit dissimilar curvatures. Fl9-s- Rut* graveolens. The
anterior stamens and perianth
Thus a developing fern frond has a somewhat leaves have been removed The
A ° stamen (a) is pressed against the
S shape, owing to the fact that the circinately ^^^St-h^e'amheTn^f d":
coiled apex unrolls by epinastic growth and is ovary ksiaoSrt?irmfadgoasainSt the
carried beyond the position assumed by the adult
basal portions. Similarly the stems of etiolated seedlings viPisnm sativum
and Vicia sativa show wavy curvatures extending over a few internodes, and
lying in the same or in different planes. Wiesner terms this 'undulating
nutation/ and the lateral displacement of the internodes often produced
by the formation and development of lateral buds he calls 'interrupted
nutation V The latter may lead to the formation of sympodial stems, but
the curvatures themselves are the result of special ephemeral movements.
1 Cf. Ludwig, Biologic der Pflanzen, 1895, p. 472.
3 On ephemeral flowers see A. P. de Candolle, Memoires d. savants etrangers de 1'Institut de
France, 1806, T. I, p. 338 ; Dutrochet, Memoires, &c., Bruxelles, 1837, p. 238; Royer, Ann. d. sci.
nat, 1868, 5e ser., T. ix, p. 350; Hansgirg, Physiolog. u. phycophytolog. Unters., 1893, p. 163,
Beiheft z. Bot. CentralbL, 1902, Bd. xn, p. 268; Oltmanns, Bot. Ztg., 1895, p. 31 J Schulz, ibid.,
1902.
3 Wiesner, Die undulirende Nutation d. Internodien, 1876 (Sep. a. Sitzungsb. d. Wiener Akad.,
Bd. LXXVII, Abth. i); Bewegungsvermogen, 1881, p. 22; Sitzungsb. d. Wiener Akad., 1883,
Bd. LXXXVIII, Abth. i, p. 454. On similar peculiarities in Algae, cf. Nageli, Pflanzenphysiol.
Unters., 1855, Heft i, Taf. 5; Berthold, Jahrb. f. wiss. Bot., 1882, Bd. XHI, p. 638. See also
Goebel, Organography, 1905.
24 MOVEMENTS OF CURVATURE
The torsions of the stems of climbers and many forms of loose winding
are autonomic in origin. The same applies to the contortion of the flower-
bud of Convolvulus^ the bending of the apex of the peduncle of Cyclamen,
and of the labellum of Himantoglossum, the twisting of the internodes of
Char a and of the peristome of Barbula \ the coiling of the stalk of the
female flower of Vallisneria, of the pods of Medicago, and of tendrils which
have failed to reach a support.
Although many of the movements taking place during development are the result
of external stimuli, an equally large number are autogenic in origin. Among these
are most of the movements of the sexual organs, which ensure proper pollination.
In addition to Ruta, similar movements of the stamens are shown by Dictamnus,
Parnassia, and Saxifraga, of the style by Saxifraga and Nigella, and of the stigmas
by Mtmulus, Martynia, Epilobium, and Compositae 2.
Historical. The remarkable movements of the basal leaflets of Desmoditim
gyrans attracted attention two centuries ago, and Hales also mentioned a few
ephemeral nutation curvatures 3. At a later date the circumnutation of twiners was
investigated by Palm and by Mohl, that of tendrils by Dutrochet *. After a number
of pronounced periodic and ephemeral movements had been recognized, Darwin
showed that all growing organs perform spontaneous periodic movements, which in
many cases are only perceptible when magnified 5, but in others have been increased
in amplitude by adaptive modification. This applies to the circumnutation of twiners
and tendrils, while Darwin considers that the pronounced curving nutation of subter-
ranean stolons makes it easier for them to pass between obstacles in the soil. The
same result may, however, be produced by the mechanical displacement of the growing
tip, so that roots grow equally well through soil, although they usually perform only very
slight autonomic oscillations 6. The pronounced periodic curvatures of Zygnemaceae
may aid in movement, or in escaping from deposited layers of mud. This may also
apply to the movements of Oscillaria, but the exact importance of periodic variation
1 A few instances are given by Wichura, Flora, 1852, p. 39; Jahrb. f. wiss. Bot, 1860, Bd. n,
p. 201. On forced torsion cf. de Vries, Jahrb. f. wiss. Bot., 1892, Bd. xxni, p. 13; Dingier, Flora,
1897, Erg.-bd., p. 289.
2 Beyer, Die spontane Bewegung d. Staubgefasse u. Stempel, 1888; Hansgirg, 1895, I.e., and
the literature here quoted. — Cf. also A. P. de Candolle, Pflanzenphysiol., 1835, Bd. II, p. 71;
Schulz, I.e., 1902.
3 Meyen, Pflanzenphysiol., 1839, Bd. ill, p. 553.
* Palm, Ueber das Winden d. Pflanzen, 1827, p. 16 ; Mohl, Ueber den Bau u. d. Winden d.
Ranken- u. Schlingpfknzen, 1837, PP- IO5j IJ2 ; Dutrochet, Ann. d. sci. nat., 1844, 3e se"r., T. xn,
p. 156.
5 Darwin, The Power of Movement in Plants, 1880.
6 Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 362. [This may under normal conditions be
due to the geotropic irritability suppressing any pronounced oscillations. At any rate the radicles
of Maize, Pea and Bean show more pronounced oscillations (two to six times greater) when rotated
horizontally on a klinostat, than when at rest or rotated with the apex downwards. In each case
the observations were made every few hours by comparing the position of the radicle by means of
a horizontal microscope with a triangular framework of glass threads attached to the seed but not
touching the root. The rotation was too rapid (twelve revolutions per hour) to permit of the
result being due to the inductive action of gravity.]
THE CAUSES OF AUTONOMIC MOVEMENT 25
movements in general is uncertain. Stahl's suggestion that they serve mainly to
favour transpiration is in the highest degree improbable l.
Dutrochet2 was the first to distinguish between spontaneous (autogenic) and
induced (aitiogenic) movements, and also held correct views as to the importance of
the external conditions as regards growth and movement. It is, however, often
forgotten that a movement can still be considered autogenic when, by the activity of
the plant itself, an external agency is used for purposes of orientation 3.
Methods. A hemispherical glass vessel, or a plane sheet of glass, may be placed
over the plant, and the position of the growing apex, as seen vertically above, marked
on the glass with indian ink, or an oil pencil 4. This gives a projection of the move-
ment, but unless the growing apex and the glass are near together the error of
parallax becomes considerable. Photographs may also be taken in one or two planes
and afterwards compared 5. Slight movements may be measured by means of a vertical
microscope containing a micrometer ruled in squares6. The attachment of a glass
thread is inadvisable, since this may readily produce disturbances of growth 7.
SECTION 5. The Causes of Autonomic Movement.
It is often the case that a change in the properties of the organism
or in the irritability of certain portions may cause constant external con-
ditions to act as stimuli producing response. Thus if the geotropic
irritability of an organ alters from positive to negative, a corresponding
curvature will be produced in the growing zone. Diffuse stimuli may also
be utilized in the same way. Thus supposing that at a particular stage
of development one of two opposed tissues is excited to more active growth
by the existing temperature than the other, then we should have a curvature
produced although the temperature remained constant. Curvatures pro-
duced in this way under constant external conditions are spontaneous or
autogenic, those produced by variations in the external conditions are
aitiogenic. In the former case we have to decide whether a constant external
agency is made use of in the manner indicated for directive purposes,
or whether the stimulus is of purely internal origin 8. The importance of
autogenic and aitiogenic changes of irritability for tropic movement will
1 Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 362 ; also Stahl, Bot. Ztg., 1897, p. 98.
2 Cf. Sachs, Flora, 1863, p. 449.
3 [If this were strictly applied, all physiological movements would be autogenic, since without
the activity of the plant, only the direct mechanical action of physical agencies could produce
movement.]
4 Darwin, Climbing Plants, 1876, p. 86.
5 Dewevre and Bordage, Revue ge"n. d. Bot., 1892, T. iv, p. 65.
6 Fritzsche, Ueber die Beeinflussung d. Circumnutation durch verschiedene Factoren, 1899, p. 6.
7 On th'e methods of magnifying the movement cf. Darwin, The Power of Movement in Plants,
1880, p. 5 ; Wiesner, Bewegungsvermogen, 1881, p. 158.
8 Movements resulting from the accumulation of secreted products or from the growth of an
organ into a dissimilar medium may be classed as aitiogenic.
26 MOVEMENTS OF CURVATURE
be discussed later. At present we have merely to deal with the fact that
certain spontaneous movements are produced by the autogenic utilization
of external factors for directive purposes.
This applies to ephemeral as well as to periodic movements, which can
be produced in plants as well as in animals, although hardly in so striking
a form as the respiratory movements or the pulsation of the heart in mam-
mals l. All organic life is a repetition in the individual of the course of
development of the parents, and we have mechanical instances of rhythm
in clocks, and in the movements of planets under the action of constant
external conditions. Similarly the rhythmic beat of the interrupter of an
induction machine is dependent upon an external agency (gravity) when the
interrupter falls back by its own weight, but solely upon the inherent
properties of the mechanism when the break is effected by an elastic
spring.
Each motile organ possesses a considerable degree of independence
as regards the inception and performance of movement. Thus similar and
dissimilar organs of a plant may perform various movements simultaneously,
and even the leaflets on the same leaf of Oxalis, Trifolium^ or Mimosa may
move in different directions at the same time. This can be very strikingly
shown by shading the pulvini of some of the leaflets, while the remainder
are exposed to bright sunlight so that they fold up. On now exposing to
slightly weaker general illumination the expanded leaflets fold up, while the
folded ones partially re-expand 2. In addition, similar organs of a plant do
not always respond alike to the same stimulus, owing to autogenic modi-
fication of the responding organs. The resulting movement is in fact
always due to the conjoint action of external and internal factors, some-
times the latter and sometimes the former predominating 3.
It is often the case that an organ performs a spontaneous curvature
and assumes a new direction of growth as the result of a change of its
geotropic irritability, the external conditions remaining unaltered. The
part played by gravity in such cases is readily ascertained by the aid of
the klinostat, and in fact a large number but not all of the autogenic
tropic movements performed by plants require the aid of gravity. When
a factor such as light undergoes continual change as regards direction
and intensity, observations in nature often suffice to determine the part
it plays in a particular movement, but it is only under light of constant
intensity and direction that a satisfactory decision can be made as to
whether the stimulus of light is involved in a particular autogenic move-
ment. That periodic movements may occur under such conditions is
1 On the production of rhythm by periodic changes in the external conditions cf. Darwin and
Pertz, Annals of Botany, 1892, Vol. vi, p. 245.
a Ewart, The Effects of Tropical Insolation, Annals of Botany, Vol. XII, 1898, p. 448.
3 Cf. Pfeffer, Periodische Bewegungen, 1875, pp. 35, 153.
THE CAUSES OF AUTONOMIC MOVEMENT 27
shown by the movement of swarm-cells to and from a constant source
of illumination, owing to the alteration of • their phototactic irritability,
according to the intensity of illumination. Similar movements have also
been observed as the result of autogenic changes in both the phototactic
and chemotactic irritability of micro-organisms.
The curvatures of the peduncles of Papaver, Tussilago, and of many
other plants are due to changes of geotropic irritability, whereas the
autogenic movements of the pedicels of Asphodelus luteus and the peduncles
of Allium controversum appear to be produced without the aid of any
external agency1. Similarly the movements of many stamens, styles, and
stigmas appear to be purely autotropic in character, whereas in the flowers
of Dictamnus, Aesctttus, and Epilobium the movements of the sexual organs
are due to changes of geotropic irritability 2. The curvatures of the
hypocotyl of Helianthus and of the epicotyl of Faba and Pisum are
produced when the seedlings are revolved on a klinostat3, whereas the
bending of the apex of the stem of Ampelopsis and of other plants is due
to a geotropic reaction4, the straightening of the older growing zones
being the result of a change in the geotropic irritability co-operating with
the autogenic orthotropism of the stem.
Since the curvatures result from the joint action of gravity with internal
factors, it is hardly surprising that in some cases they should not entirely
disappear when gravity is eliminated. Plants grown on a revolving klinostat
in fact perform a variety of movements, and the torsions in the internodes
of Chara and in the peristome of Barbula are produced independently
of gravity. Whether the same applies to the coiling and uncoiling of the
peduncle of Vallisneria has yet to be determined.
The pronounced movements of the leaflets of Desmoditim gyrans and
of Trifolium, as well as those of the gynandrophore of Stylidium appear
to be independent of the action of gravity, but direct proof is wanting.
The movements of cilia continue, however, even when the rotation of a
1 Vochting, Bewegungen d. Bliithen u. Friichte, 1882, p. 192 ; Scholtz, Cohn's Beitrage z. Biol.,
1 893, Bd. vi, p. 306 ; Hansgirg, Photodynam. Unters., 1889, p. 250 (Repr. from Sitzungsb. d. bohm.
Ges. d. Wiss.); Physiolog. u. ph.ycopb.ytol. Unters., 1893, Neue Unters. Uber d. Gamo- u. Karpo-
tropismus, 1896 (Repr. from Sitzungsb. d. bohm. Ges. d. Wiss.). — According to Vochting (I.e.,
p. 137) the bending of the stem of Viola is due to geotropism, a statement which Schwendener u.
Krabbe (Gesamm. Abhandl. von Schwendener, 1892, Bd. n, p. 336) contradict.
2 Dufour, Arch. d. sci. phys. et nat., 1885, III, T. xiv, p. 418 ; Vochting, Jahrb. f. wiss. Bot.,
1886, Bd. xvir, p. 340; J. af Klercker, Die Bewegungserschein. der FmwzVa-Bliithen, 1892 (Repr.
from Bihang till Svenska Vet.-Akad. Handlingar, Bd. xvill).
3 Vochting, Bewegungen d. Bliithen u. Friichte, 1882, p. 186 ; Darwin, The Power of Movement
in Plants, 1880, pp. 45, 553 ; Sachs, Arbeit, d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 403-; Lehrbuch,
3. Aufl., p. 75. That these and other curvatures are not the direct result -of the weight supported, as
supposed, has been shown by Vochting, I.e., and Scholtz, Cohn's Beitrage z. Biologic, 1892, Bd. v,
p. 400. Cf. also Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. vn, p. 141.
4 Scholtz, 1. c., 1892, p. 401.
28 MOVEMENTS OF CURVATURE
motile organism diffuses the action of gravity. Similarly the feeble nutation
movements of most stems continue on a rotating klinostat \ whereas the
pronounced circumnutation of the shoots of twiners 2, of tendrils 3, and of
the stolons of Mucor stolonifer* cease sooner or later when the action
of gravity is eliminated, feeble and irregular nutation movements taking
their place.
Baranetzsky showed that when the stem of a climber is slowly rotated horizontally,
the curved growing zone straightens, and its circumnutation ceases. Gravity here is
utilized by the plant as a directive stimulus, the power of reaction progressively and
periodically altering on the different sides, so that the growing zone bends alternately
to all quarters of the compass as it performs its circumnutation. This power of
producing an autogenic change of irritability is a special adaptation, for the apices
of non-climbers do not exhibit any such pronounced circumnutation when bent from
the perpendicular 5, but usually show slight pendulum movements, owing to variations
in the growth of opposite sides. If the plant is very slowly revolved horizontally on
a klinostat, the growing apex circumnutates as the result of the curvatures produced
by gravity, and light will act in exactly the same way. This, however, results from
the orthotropism of the plant which causes the repeated elimination of the pro-
gressively changing geotropic or heliotropic curvature.
It is not certain whether autogenic changes of irritability take place when the
plant is rotated on a klinostat, or whether the geotropic curvature of the apex is
essential for their initiation. A fact worthy of note in this connexion is that a shoot
of Cuscuta not only ceases to circumnutate when rotated horizontally on a klinostat,
but also loses its irritability to contact 6. In many cases also the direction of circum-
nutation may change periodically, while a large number of plants only show circumnuta-
tion under special conditions, as for instance when the stems of Tropaeolum majus or
Polygonum Fagopyrum are etiolated 7. It is, however, uncertain whether this is due
to the greater power of response of the thin-walled cells of the etiolated tissues, or
whether an inherent tendency to circumnutation has been excited or allowed to
become manifest.
The apices of twining stems primarily curve out of the vertical as the result of
their klinogeotropism, whereas when longer their own weight produces a certain
1 Fritzsche, Ueber d. Beeinflussung d. Circnmnutation durch verschiedene Factoren, 1899, p. 1 6.
2 Baranetzsky, Die kreisformige Nutation u. d. Winden d. Stengel, 1883, p. 24; Ambronn,
Mechanik d. Windens, 1884, Th. i, p. 6; Wortmann, Bot. Ztg., 1886, p. 314. Cf. II, § 84.
3 Wortmann, Bot. Ztg., 1887, pp. 86, 97. — Darwin (Climbing Plants, 1875, p. 131) observed
that the circumnutation of a tendril of Echinocystis lobata almost ceased when it was bent down-
wards, but recommenced when it was placed in a horizontal position.
* Fritzsche, I.e., p. 21.
5 Id., I.e., p. 20. Baranetzsky (I.e., p. 14) states that the previously erect stem of a twiner
begins to circumnutate when bent out of the perpendicular.
6 Peirce, Annals of Botany, 1894, Vol. vm, pp. 86, 116. Ordinary tendrils remain irritable to
contact after prolonged rotation on a klinostat.
7 Noll, Bot. Ztg., 1885, p. 664. Cf. also M. Scholtz, Cohn's Beitrage z. Biologic, 1892, Bd. v,
P- 393-
THE INFLUENCE OF THE EXTERNAL CONDITIONS 29
amount of drooping curvature1. The existence of such mechanical curvatures is
easily shown by hanging the apex downwards or by laying it on a horizontal sheet
of paper and noting the remaining curvature. The straightening of the apex on a
klinostat shows that this curvature results from the antagonism of klinogeotropism
and orthotropism. The somewhat irregular character of the curve described is due
to the action of disturbing factors.
SECTION 6. The Influence of the External Conditions.
Autonomic movements like all vital phenomena are dependent upon
the external conditions, and are accelerated or retarded according to the
temperature and the supply of food, oxygen, or water. The stimulating
action of light or gravity may also form an essential or favouring condition
for movement. The effects of a change in the external conditions may
persist for a long time, and since a local or general stimulus may modify
or awaken various correlated activities, it is not always easy to distinguish
between autogenic and aitiogenic movements. Nor can any general rules
be laid down. For instance an increased rate of growth, or in general
a greater demand, results in a diminution of the autogenic movements in
some cases, whereas in others they increase2. It is, however, commonly
observed that any serious general disturbances are reflected in the autonomic
movements, with the result that when these are normally feeble they are
excited to greater activity.
A certain temperature is necessary for all autogenic movements,
the optimum in the case of the leaflets of Desmodium gyrans being as high
as 35° C., and the movements becoming slower as the temperature falls.
Dutrochet3 observed that the tendrils of the Pea circumnutated once in
9 to ii hours at 5 to 6° C, but in i hour 20 minutes at 24° C. Darwin4
observed that the pronounced circumnutation of the internodes and tendrils
of Eccremocarpus scaber ceased in a cool house, in which slow growth
continued. Fritzsche 5 found that any rise of temperature below the
optimum increased the rapidity and amplitude of the feeble nutation-
movements of the stems of seedlings. It is, however, also possible that
the rapidity of movement might increase while the amplitude decreased,
and in fact Darwin 6 observed a result of this kind when the leaflets of
Averrhoa bilimbi were subjected to rising temperatures.
1 Baranetzsky, 1. c., pp. 19, 48. A drooping shoot curves geotropically upwards at the apex,
where the static moment is least, and hence attains a p^ shape.
2 Askenasy (Ber. d. hot. Ges., 1890, p. 77) states that the nutation of roots decreases when
growth is active, whereas Fritzsche (1. c.) obtained in some cases exactly opposite results.
3 Dutrochet, Ann. d. sci. nat, 1843, 3° sen, T. XX, p. 312. Cf. also Simons and MacKenney,
Bot. Jahrb., 1898, 1, p. 594.
4 Darwin, Climbing Plants, 1875, pp. 72, 103.
5 Fritzsche, Die Beeinflussung der Circumnutation durch verschiedene Factoren, 1899, p. 23.
6 Darwin, The Power of Movement in Plants, 1880, pp. 331-5. A rapid oscillation is also
3o MOVEMENTS OF CURVATURE
LIGHT and DARKNESS exercise effects dependent largely upon the
duration of the exposure. The autonomic l variation and nutation 2
movements of plants in a condition of phototonus continue at first unaltered
in darkness. In the prolonged absence of light, however, the variation-
movements gradually decrease, and cease with the onset of dark-rigor3.
Nutation-movements, .on the other hand, continue as long as growth does,
becoming actually more pronounced in some plants, but decreasing in
others. For instance etiolated plants of Tropaeolum and Polygomim show
pronounced circum nutation, whereas circumnutation decreases so much
in etiolated shoots of Dioscorea Batatas and Mandevillea suaveolens 4 that
the plants are no longer able to twine. Etiolated shoots of Phaseolus
and Ipomoea purpurea, however, circumnutate actively and twine readily
in darkness5. Other special peculiarities have without doubt yet to be
discovered, and it is highly probable that changes of photonasty involve
alterations in the power of autonomic movement 6.
Autonomic movements are affected by the conditions of turgidity, by
the supply of food and by various chemical stimuli 7. Darwin found, for
example, that the absorption of a little ammonium carbonate excited active
oscillating movement in the two leaf-segments of Dionaea muscipula
and in the leaf-tentacles of Drosera 8. The action of shaking, in retarding
growth and in equalizing the tissue-strains, enables us to understand why
excited in the leaflets of the frond of Aspknitim trichomanes , according to Asa Gray and Loomis,
Bot. Gazette, 1880, pp. 27, 43 (quoted by Darwin, I.e., 1880, p. 257). Fritzsche (1. c., p. 15) con-
siders this to be due to the changes of temperature influencing the transpiration and hence the position
of the leaflets.
1 Pfeffer, Periodische Bewegungen, 1875, P- J55-
3 Darwin, Climbing Plants; The Power of Movement in Plants (Twiners); Fritzsche, I.e.,
p. 1 4. (Seedlings) ; Dewevre et Bordage, Revue ge"n. de Bot., 1892, T. iv, p. 73 (Coloured Light).
Rothert (Cohn's Beitrage z. Biologic, 1894, Bd. xxvi) states that the cotyledons of Avena and
Phalaris nutate somewhat more actively in darkness.
3 Pfeffer, I.e., p. 155. According to Maige (Ann. d. sci. nat, 1900, 8e se'r., T. xi, p. 331)
strong light diminishes the movements.
* Duchartre, Compt. rend., 1865, T. LXI, p. 1142. The torsion is also absent from these plants
in darkness. Stems of Dioscorea developed in light are able to twine in darkness, according to
de Vries, Arb. d. Bot. Inst. in Wiirzburg, 1873, Bd. I, p. 328.
5 Mohl, Ranken- u. Schlingpflanzen, 1827, pp. 122, 150; Sachs, Bot. Ztg., 1865, p. 119;
Fritzsche, 1. c.
6 Heckel (Du mouvement vegetal, 1875, p. 551) finds the movements of the stamens of Ruta
and Saxifraga to be slower in darkness. Carlet (Compt. rend., 1873, T. LXXVII, p. 538) states
that the stamens of Ruta do not move at all in darkness. Organs pressed against one another or
against a support may not be able to move (cf. Pfeffer, 1. c., p. 48). Stahl (Bot. Ztg., 1898, p. 103)
concludes that the autonomic movements decrease in darkness, in order not to disturb the night-
position, but teleological conclusions are valueless in comparison with empirical facts.
7 Cf. Fritzsche, Die Beeinflussung d. Circumnutation durch verschiedene Factoren, 1899. The
statement that weak electrical currents increase the movements of the leaflets of Desmodium gyrans
requires further proof. Cf. Kabsch, Bot. Ztg., 1861, p. 358; Meyen, Pflanzenphysiol., 1839, Bd. lir,
P- 557-
8 Darwin, The Power of Movement in Plants, 1880, pp. 237-9.
THE MECHANICS OF AUTONOMIC MOVEMENT 31
the circumnutating apices of many climbers become partially erect after
shaking \ Slight injuries do not affect the power of curvature, and this
may even be retained when the root-tip is cut off in such fashion as not
to induce any traumatic curvature2. Since severe injury to the root-
system does not perceptibly affect the nutation of the shoot, the lessened
circumnutation of the cut shoots of twiners is either due to the manipulative
disturbance, or to a diminution of turgidity 3.
SECTION 7. The Mechanics of Autonomic Movement.
It is not definitely known in a single case whether the average rate of
growth alters or not during spontaneous movement. Presumably, however,
the nutation curvatures of Oedogonium and of Zygnemaceae are attended by
an acceleration of growth, and it is not unlikely that the feebler nutation
movements are connected with the continual variations in the activity of
growth in length. It is, however, uncertain whether the circumnutation
of twiners involves an increase in the average rapidity of growth. In
any case it does not follow that the maximum curvature should take
place at the period of most active growth, so that during elliptical nutation
growth would be most active during the passage of the extremities of
the major axis of the ellipse of curvature 4.
Autonomic growth curvatures are certainly not always produced in
the same way. Plastic growth takes place in Oedogonium^ but it is not
known in a single case whether rises of turgor come into play, de Vries'
researches being inconclusive in this respect. The curvatures of shoots,
and of filaments of Spirogyra persist when the plant is suddenly killed
by immersion in hot water, so that the growth responsible for the curvature
takes place without any perceptible preparatory elastic stretching 5.
The movements of the leaflets of Trifolium pratense and of Oxalis
acetosella are produced by the expansion of one half of the pulvinus coupled
with a corresponding contraction in the other half6. This is shown by
the fact that the rigidity of the leaflet remains constant even during active
movement. If the tendency to expansion increased in both halves of the
pulvinus, but in one more than the other, the rigidity of the leaflet would
1 Baranetzsky, Die kreisformige Nutation und das Winden der Stengel, 1883, p. 20.
2 Darwin, 1. c., 1880, p. 540; Prantl, Arb. d. Bot. Inst. in Wiirzburg, 1874, Bd. I, pp. 548, 554;
Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 292 ; Fritzsche, 1. c., p. 31.
8 Baranetzsky, I.e., p. 6l.
* Cf. Wiesner, Die undulirende Nutation d. Internodien, 1878, p. 26 (Repr. from Sitzungsb. d.
Wien. Akad., Bd. LXXVII, Abth. i).
5 Frank (Beitrage zur Pflanzenphysiol., 1868, p. 62) showed that the nutation of peduncles was
due to growth. Hofmeister (Pflanzenzelle, 1867, p. 324) also concluded that growth was in part
responsible for the movements. Cf. also Winkler, Kriimmungsbewegungen bei Spirogyra, 1902.
6 Pfeffer, Periodische Bewegungen, 1875, pp. 88, 156.
32 MOVEMENTS OF CURVATURE
increase, and the same would also occur if one side only performed an
active contraction or expansion, the other being passively stretched or
compressed1. Pfeffer has shown that in the latter case the force of
expansion in an active half of the pulvinus of Trifolium would lie between
0-6 to 2 atmospheres.
PART II
TWINERS AND CLIMBERS
SECTION 8. General.
According to the mode of climbing we may distinguish between
(a) twiners, like the hop, which wind their slender stems around supports,
(b) tendril-climbers which use special coiling attaching organs for this
purpose, (c) root-climbers which attach themselves by means of aerial
roots, (d) scramblers, like the bramble or Goosegrass which support them-
selves by means of the asperities or hooks upon the stem, or by the
unfolding of the leaves after the stem has grown through a bush- No
hard and fast distinction can be drawn between the different groups, how-
ever, since the tendrils of the Virginian creeper, for instance, attach themselves
by means of sucking-disks, while the attaching roots of Vanilla are able
to coil around supports. Typical root-climbers are Hedera helix^ Ficus
stipulata, and Tecoma, which are able to attach themselves to walls or
to the trunks of trees.
In the case of scramblers no phenomena of special physiological
interest are shown, the stem grows upwards in virtue of its heliotropism2
so long as it receives support, while the unsupported ends trail downwards
owing to their own weight. The stems of root-climbers are negatively
heliotropic and negatively geotropic, so that they strive to grow erect
but avoid light, hence pressing themselves against walls or the trunks of
trees. In this way the required conditions for the formation of aerial
attaching roots are produced, namely shade, moisture, and possibly contact
also in many cases.
The twining stems of Cuscuta are not only irritable to contact like
tendrils, but also form parasitic roots, while the twining stems of Hoy a
develop attaching roots. Many hook-climbers possess hooks which grow
1 [The rigidity depends upon the magnitude of the opposing forces, and therefore is ultimately
dependent upon the hydrostatic pressure within the cells. An 'active' contraction produced by
a fall of the osmotic pressure allowing the stretched cell-walls to contract on the ' active ' side, will
allow the cell-walls on the ' passive ' convex side to be more expanded by their internal pressure.
The increase of volume involving an absorption of water results in a fall of osmotic pressure on this
side also. Hence the antagonizing forces decrease on both sides, and the rigidity does not increase
but diminishes.]
2 Schenck, 1. c., pp. 7, 134, 156 ; Danvin, 1. c.
TWINERS AND CLIMBERS 33
in strength and size when attached, and function as most efficient attaching
organs, lasting in some cases for several seasons. Certain Bignonias have
twining stems in addition to tendrils or coiling leaf-stalks, while in a few
species shoots are even formed which develop attaching roots1. The
tendrils themselves may also not only coil but attach themselves at their
tips by special disks produced under the stimulus of contact2. Certain
plants only develop the climbing habit under special conditions, for instance,
most twiners lose the power of twining when the action of gravity is
eliminated, while Polygonum aviculare and Galium Mollugo develop
scrambling stems in moist shady rich localities, but short erect or creeping
stems in dry exposed situations.
The climbing habit enables the plant to reach light and air without
spending a large amount of material in forming a stout erect stem. For
this reason their growth in length is especially rapid, the stem of a hop
for instance often becoming twelve metres in length during the summer.
The conducting tissues need to be especially well developed in the thin
stems of climbers. The wood-vessels for instance may be above one to
three metres in length, and over half a millimetre in diameter, in this
way the resistance to the unusually rapid flow of sap being reduced to
a minimum 3.
Twiners are specially adapted for climbing up single thin supports,
whereas tendril-climbers grow best when numerous points of attachment
are presented, as in bushes or hedges. Root-climbers again are adapted
to cling to rough erect surfaces such as walls, rocks or tree-trunks, which
is only possible to tendril-climbers possessing attaching disks such as
Ampelopsis hederacea, or claw-like grappling-hooks such as Bignonia unguis.
All climbers are not equally well adapted, and in general tendril -climbers
exhibit the most striking instances of special adaptation.
The young stem of a climber usually attains a certain length before
climbing begins. Thus a seedling scarlet-runner develops at first a stout
erect stem with a pair of simple foliage leaves, and only after a period
of nutritive preparation produces the slender actively circumnutating twining
stem with its trifoliate foliage leaves. In all cases if the stem fails to reach
a support, it grows prostrate along the surface of the ground, until by
accident, aided by movements arising spontaneously or produced by the
wind, it comes into contact with a support. Climbers have no power
of seeking out supports, and even the negative heliotropism of the ivy
only comes into play when the stem is already quite near to the wall.
Circumnutation naturally aids the plant in finding a support, and it is
1 Darwin, Climbing Plants, 1875, pp. 93, 101, 135.
2 Darwin, I.e. ; Cohn, Bot. Ztg., 1878, p. 27. Ewart, Ann. du Jard. bot. de Buitenzorg, 1898,
T. xv, p. 208 seq.
3 Cf. Ewart, On the ascent of water in Trees, Phil. Trans., 1904, p. 65. [The longest vessel,
564 cms., was found in Wistaria chinensis.}
PFEFFER. Ill D
34
MOVEMENTS IN CURVATURE
not only shown by twiners but also by the apices of many tendril-climbers
and by the tendrils themselves. Furthermore the periodic daily movements
of leaves bearing tendrils may aid the latter in reaching a support l.
SECTION 9. The Twining of Stems.
The obliquely ascending coils formed by a twining stem around a
support do not result from the tip being closely applied to it and growing
along a continuous spiral path. They are instead the result of complicated
movements of the free circumnutating tip, which is pressed closely against
the support at one part of its revolution but stands out away from it
in
FlG. 6. Twining stem of
Convolvulus arvensts.
FlG. 7 a b c. Twining stem-apex of Humulus Lupulus in
successive phases (a, b, c) of movement.
at another2 (Figs. 6 and 7). It appears that the coiling is not the result
•of a contact stimulus as in tendrils, but that the support merely acts as
1 As in the case of Mutisia Clematis quoted by Darwin (1. c., p. 90).
8 For details see Darwin, Climbing Plants; de Vries, Arb. d, bot. Inst. in Wiirzburg, 1873,
Bd. Ij p. 326 ; Schwendener (1881), Gesammelte bot. Mitth., Bd. I, p. 405 ; Baranetzsky, Die kreis-
.formige Nutation und das Winden der Stengel, 1883, p. 54; Schenck, Beitrage z. Biol. u. Anat. d.
.Lianen, 1892, p. 115.
THE TWINING OF STEMS
35
a mechanical obstacle determining the production and nature of the per-
manent coils formed by the growing circumnutating apex.
In the absence of distinct circumnutation no twining is possible and
hence the seedling stem of the scarlet-runner, the shoots developed from
the rhizome of the hop, and in general all the branches of twiners which
are unable to circumnutate sufficiently are also unable to twine1. Hence
also etiolated stems of Tropaeolum majus, and of Polygonum fagopyrum
gain the power of circumnutating and of twining at the same time.
Circumnutation, and with it twining, cease when a twiner is rotated
upon a klinostat so that the action of gravity is eliminated, while as the
result of the plant's orthotropism the coiled younger portions may untwine
and straighten *. It still remains possible, however, that gravity may act
directly upon the process of twining, as well as indirectly by influencing
circumnutation. At the same time it is evident that the contact with the
support exerts no stimulus capable of preventing the uncoiling of the young
shoot on a klinostat.
Under such circumstances, however, the stem of Cuscuta loses not only
its power of circumnutation, but also the contact irritability which it
exhibits under normal conditions. Rubbing the stems of Cuscuta and
Lophospermum scandens with a solid body suffices to produce an irritable
curvature, but not in the case of the stem of Phaseolus or of other twining
plants. The same negative result is also obtained when one side of the
shoot is repeatedly rubbed, or when permanent contact against an edge
of wood is assured (Darwin), or when the circumnutating shoot presses
against a rod attached to an appropriate turn-table (de Vries 2).
The normal symmetric circumnutation is not sufficient to produce
twining, for if it were, the horizontal or sloping free end when attached
to a support at its base would continue to circumnutate and coil around
a horizontal or inclined ideal axis (Baranetzsky's asymmetric circumnuta-
tion 3). By attaching a piece of india-rubber tube to a retort-stand it is
easy to show that in this way no twining about an erect support could
be produced. It is evident, therefore, that sure and regular coiling involves
a regulation of the growth and circumnutation of the growing apex.
According to Schwendener 4 this is produced by grasping movements,
1 Bowiea volubilis twines when horizontally rotated on a klinostat, according to W. Voss (Bot.
Ztg., 1902, Originale, p. 231), if illuminated from the apical side. This plant is more strongly
heliotropic than other climbers, and its circumnutation is dependent upon illumination.
3 H. Mohl (1. c., p. 112) considered twining to be the result of contact irritability, but Palm
(Ueber d. Winden d. Pflanzen, 1827, pp. 20, 97) of rotary nutation. Darwin (1. c., 1875, pp. 16, 39)
and later de Vries (1. c., p. 321) showed the absence of any contact irritability, which has, however,
again been brought forward by F. G. Kohl (Jahrb. f. wiss. Bot., 1884, Bd- xv, p. 327). Ambronn
(Zur Mechanik des Windens, 1884, 1, p. 32, Repr. from Sitzungsb. d. sachs. Ges. d. Wiss.) has shown
that Kohl's experiments are inconclusive.
8 Baranetzsky, 1. c., pp. n, 16, distinguishes between symmetric and asymmetric nutation.
4 Schwendener, Gesammelte bot. Mitth., Bd. I (1881), p. 405; (1886), p. 441.
D 1
MOVEMENTS OF CURVATURE
which occur every time the circumnutating apex presses itself against the
support and drags upon the portion of the stem below, tightening the
coils, and drawing the stem over and around the surface of the support.
A pull of this kind is actually exercised every time the apex is pressed
against the support, and the periodic changes of tension set up in the
coiled but still growing parts by the movements of the free apex act in
the same way. Although these factors must aid in twining, it is not
certain whether they are the only ones acting. Baranetzsky in fact found
that Dioscorea Batatas was able to twine in their absence. It is, however,
readily possible that regulatory actions come into play of such character
as always to cause the side of the stem which is not in contact to grow
more actively, so producing the coiling around the support 1. A regulation
of this kind is in fact essential to maintain a homodromous curvature of
the free apex, that is a curvature in the direction of twining 2, for as the
result of circumnutation the permanently concave anterior side is continually
changing. Even according to Schwendener's theory twining is a physio-
logical manifestation produced by the plant exercising
a definite power of movement.
We have no grounds for assuming that the factors inducing
circumnutation are quite unaffected by the commencement and
continuance of twining, and indeed the contrary is indicated by
the fact that circumnutation is not performed during all stages
of development. Similarly, it is uncertain whether the changes of
tone which produce circumnutation continue on a klinostat, on
which Cuscuta loses its contact irritability. The normal twining
of this plant indeed involves a periodic inhibition of the contact
irritability.
Sachs 8 observed that growth was often distinctly retarded
in the free ends hanging beyond the support, or in shoots
which were unable to climb. According to Raciborski 4, many
tropical climbers which fail to reach a support either throw
off their leaves or show a retarded development, followed
ultimately by the death of the growing point. Baranetzsky found
that the duration of growth was decreased in the twining portion
of the stem and the circumnutation diminished 6.
Unfavourable conditions, or the abscission or fixation
FIG. 8. Free coiling of of the free apex, tend to cause a production of free coils
HumulusLufiulus.
1 Baranetzsky, 1. c., p. 38 ; Noll, Sitzungsb. d. Niederrhein. Ges. f. Natur- u. Heilkunde, 4. Febr.
1895; Strasburger's Lehrb. d. Botanik, 1898, 3. Aufl., p. 225.
3 De Vries, 1. c., pp. 336, 341 ; Darwin, 1. c., p. 19 ; Baranetzsky, 1. c., pp. 16, 65 ; Schwendener,
1. c. (1882), p. 436 ; Kolkwitz, Ber. d. bot. Ges., 1895, p. 513.
3 Sachs, Lectures on Physiology.
4 Raciborski, Flora, 1900, p. 2. These shoots have no contact irritability.
8 Baranetzsky, 1. c., p. 61 seq.
THE TWINING OF STEMS 37
conditions by many plants, such as Akebia quinata, Menispermum canadense, and
Humulus Lupulus, which have no support or have grown beyond it. The
fixation of the shoot a little below the apex often causes free coiling, although
this may be but slight \ It is possible that this physiological tendency to coiling
may be directly or indirectly awakened by contact with a support, and may aid in
twining. The fact that the free coils are often straightened again shows nothing,
for the same happens when a stick is withdrawn from the coils just made around it.
The factors which determine the permanent homodromous curvature of the asym-
metrically nutating free apex are uncertain. It is possible that the epinastic growth
of the side becoming convex during winding is more pronounced as the internode
approaches the adult condition, hence causing the permanent winding of the stem.
Although the apex is curved, the stem remains physiologically radial, and the
curvature passes in turn from side to side of the stem as the latter circumnutates.
In the same way the contact line of an adult twiner follows usually a more or less
spiral path around its stem 2.
The homodromous curvature of the apex is probably partly due to autonomic
and aitionomic variations of tone in which the external world and the progress of
twining act as directive stimuli. Baranetzsky and Noll 3, on insufficient grounds,
assume the existence of a diageotropic irritability in the apex inducing paranasty.
Ambronn ascribes the homodromous curvature to the conjoint action of circum-
nutation and negative geotropism, a conclusion which Schwendener disputes 4. The
latter erroneously regards circumnutation and geotropism as factors of constant mag-
nitude, and forgets that the circumnutation and the klinotropic position of the shoot
caused by it are themselves the result of regulated geotropic reactions. De Vries
supposed the curvature to be due to the torsion produced by the weight of the free
portion of the apex, but this has been shown to be untrue by various investigators 5.
The causes of twining are therefore unknown, but the very fact that regular coils
sloping at a definite angle are produced in each individual case suffices to show that
the position, thickness, and resistance of the support act as directive agencies upon
coiling or regulate the internal tendencies responsible for twining. It is quite
possible that the coiling is in one case produced by grasping movements, but in
another by an active curvature of the internodes. The stems of Cuscuta and Lopho-
spermum are able to twine partly as the result of their irritability to contact, which
in tendrils is the main factor in producing coiling. Although most twiners seem to
1 De Vries, 1. c., pp. 324, 339 ; Baranetzsky, 1. c., p. 42 ; Sachs, 1. c., p. 707. Pfeffer has observed
in a culture of Phycomyces nitens that most of the sporangiophores were spirally twisted, and the
shoots of some varieties, as for instance Juncus e/usus, var. spiralis, always show a spiral coiling.
3 De Vries, 1. c., p. 329. Circumnutation is the result of a changeable or labile induction of
a physiological dorsiventrality.
8 Baranetzsky, 1. c., p. 38 ; Noll, Sitzungsb. d. Niederrhein. Ges. f. Natur- u. Heilkunde, 4. Febr.
1895 ; Strasburger, Lehrb. d. Botanik, 1898, 3. Aufl., p. 225 ; Noll, Sitzungsb. d. Niederrhein. Ges.,
8. Juli 1901. That centrifugal force should act similarly to gravity is not surprising.
* Ambronn, Zur Mechanik d. Windens, 1885, 2. Thl., pp. 19, 47 (Repr. from Sitzungsb. d. sachs.
Ges. d. Wiss.); Ber. d. bot. Ges., 1887, p. 105; Schwendener (1886), Gesammelte bot. Mitth., Bd.
I, p. 452.
5 De Vries, I.e., p. 337; Baranetzsky, I.e., p. 69; Schwendener, 1881, I.e., pp. 403, 416;
Ambronn, 1. c., 1885, P- 25-
38 MOVEMENTS OF CURVATURE
have no special contact irritability, it remains possible that the pressure against the
support, or the curvature which this maintains, ma}7 play a certain part in twining,
which like circumnutation appears to result from the co-operation of various
stimuli.
SECTION 10. Twining Plants (continued).
In certain climbers special long shoots are adapted for twining1,
while some of the less active climbers may only develop the twining
habit in moist shady situations where long weak stems are produced.
This applies to such occasional twiners as Solanum Dulcamara and
Cynanchum vincetoxictim 2, but the causes of the non-twining of Polygonum
convolvulus during certain seasons are uncertain 3. Darwin 4 mentions that
Ipomoea argyroides and two species of Ceropegia develop in England as
twiners, but not in the dry South African regions to which they are
indigenous. Furthermore various cultivated varieties of Phaseolus multi-
florus have lost the power of twining in correspondence with their dwarfed
habit. Twining is shown more especially by the aerial stems of flowering
plants, but in Lygodium scandens and Blechnum volubile we have instances
of twining leaves5. It is, however, uncertain whether the twining occa-
sionally shown by rhizomes and by roots in air, water, and even soil,
is produced by the aid of circumnutation or by contact stimulation 6.
The same applies to the filaments of Algae which sometimes twine around
supports 7, and to the rhizoids of Catharinea undulata which may coil
around each other 8.
In all the cases hitherto observed circumnutation and twining take
place in the same direction. Usually the direction is against that of the
hands of a watch, as for instance in the stems of Convolvulus (fig. 6, p. 34),
Phaseolus, Ipomoea purpurea, Menispermum canadense, Aristolochia sipho,
Periploca graeca. On the other hand the stems of Humulus Lupulus
(Fig. 7, p. 34), Polygonum convolvulus, Lonicera caprifolium, Testudinaria
elephantipes twine to the right in the opposite direction. Usually the
direction of twining is constant, but in the cases of Polygonum complexum,
Testudinaria sylvatica, and Solanum Dulcamara it sometimes happens that
1 For .details see Schenck, Beitr. z. Biol. u. Anat. der Lianen, 1892, p. 115 ; Goebel, Organo-
graphy, 1902 ; Darwin, Climbing Plants, 1875, p. 41 ; Voss, Bot. Ztg., 1902, p. 249 (Celastra-
ceae).
Darwin, 1. c. ; Schenck, 1. c., p. 128.
Palm, Ueber d. Winden d. Pflanzen, 1827, pp. 43, 94.
Darwin, 1. c.
Cf. Schenck, 1. c., p. 113.
Hochreutiner, Rev. gen. de Bot., 1896, T. viil, p. 92.
Palm, I.e., p. 44.
Schimper, Rech. s. 1. mousses, 1848, Plate iv, Figs. 15, 16. Groups of filaments of Spirogyra
projecting into moist air may show coiling. Hofmeister, Jahreshefte d. Vereins f. vaterland. Naturk.
in Wiirttemberg, 1847, Jahrg. 30, p. 226; Winkler, Krummungsbewegungen von Spirogyra, 1902.
TWINING PLANTS 39
different individuals, or different shoots on the same individual, twine in
opposite directions. In Loasa aurantiaca, Scyphantus elegans, Blumenbachia
lateritia, Tropaeolum tricolorum^ Ipomoea jucunda, and Hibbertia dentata
the twining may even be reversed on the same shoot1.
Unless the growth is considerably diminished during the reversal
of circumnutation the youngest coils may untwine, and if the reversal
occurred frequently no permanent twining would be possible, as is the case
when the plant is rotated horizontally on a klinostat. If, however, only
a portion of the coils are untwined permanent coiling may continue, but
more slowly than usual. Homodromous twining may even take place, if
the reversal of circumnutation is only temporary and ceases before any
permanent coils have been formed, This was actually observed by Darwin
to occur in Hibbertia dentata 2. Indeed in the normal progress of circum-
nutation the later coils may be partially unwound.
There is no definite relationship between the number of coils and
the number of circumnutations, the latter being performed more frequently
than the former are produced. Darwin 3 observed that Ceropegia circum-
nutated once in six hours, but only formed a coil in nine and a quarter
hours. The same was the case with Aristolochia gigas, except that a cir-
cumnutation was completed in five hours instead of six. Naturally twining
is only produced by definitely regulated circumnutation, and it can be
artificially induced by causing a growing apex to slowly follow a tropic
stimulus around a support.
If the support is of appropriate thickness the coils are closely applied
to it, but around thread, fine wire, or string, loose coils are often formed.
These are often subsequently closely pressed to the thinnest supports by
the elongation of the stem, for the same reason that the diameter of a spiral
spring decreases when the spring is considerably stretched 4. This elonga-
tion is in part autotropic, but is also due to the increased geotropism
of the stem inducing the younger coils to straighten more or less 5. This
tendency may cause the straightening of the younger coils formed around
a thick support if this is removed, whereas the older coils are permanent,
owing to the fact that the power of growth rapidly disappears after coiling.
Owing to the same tendency a considerable pressure may be exercised
upon the support, sufficient to crush in a hollow paper cylinder6, to
partially strangulate a soft fleshy stem, to compress a leaf or petal, or
1 For further details see Darwin, 1. c. ; Schenck, 1. c., p. 123, and the literature there quoted.
2 L. c., p. 47. 3 L. c., p. 13,
* De Vries, 1. c., p. 326; Baranetzsky, 1. c., p. 58 ; Schwendener, 1. c., 1881, p. 419; Ambronn,
1. c., I, p. 5 ; n, p. 35-
5 Baranetzsky, 1. c.
6 Mohl (Ranken- u. Schlingpflanzen, 1827, p. 118) deduced the existence of this pressure from
the curvatures produced in a string round which coiling occurs. De Vries (1. c., p. 327) found that
the coils at once narrow when the support is removed.
40 MOVEMENTS OF CURVATURE
to keep a flower of Convolvulus closed. In this way, aided by roughnesses
on the stem and support, and often by special climbing-hairs as in Humulus
and Phaseolus, the fixation is rendered more secure 1.
From what has already been said as to the mode of coiling, it is
hardly surprising that a good climber should form an extremely regular
spiral curve around a cylindrical support, and the slope of the coils is
in many cases not appreciably altered by moderate changes in the diameter
of the support. With thinner and thinner supports the coils become
steeper until a limit is reached which is about that which the coils show
when the stem partially straightens after loosely coiling upon a thin
support.
As the result of the circumnutation about a vertical axis, the stem is
unable to twine around horizontal, or nearly 'horizontal, supports, and the
younger coils may untwine when the support is placed in a horizontal
position. Mohl2 found that a string inclined at an angle of 20° to the
horizon was no longer twined round by the stem of Ipomoea purpurea^
and one at an angle of 40° by Phaseolus 3.
The thickness of support a twiner can grasp is determined mainly
by the length of the circumnutating apex, and when this attains a con-
siderable length, as in certain tropical Lianas, twining is possible around
supports up to 40 cms. in diameter. Scarlet-runners and Hops may
twine around sticks of 8 to 15 cms. in diameter, whereas Convolvulus
arvensis and Polygonum convolvulus are usually unable to coil around
stems thicker than 3-4 cms.4 Since the length of the circumnutating apex
and other conditions also may vary in the same plant, it is not surprising
that de Vries should find plants of Wistaria chinensis twining around
supports 1 6 cms. in diameter, whereas Darwin found potted specimens unable
to coil around supports of slightly less diameter.
It depends upon these relationships whether a stem twines around
a single or several supports when these are grouped near together. That
stems should be able to pass from one support to another, sometimes
reascending after hanging downwards, is hardly surprising, or that branches
may twine around each other. The form of the support is of some importance,
thin stems being able to apply themselves more closely to flat supports
than thicker stems can, but the material of the support is only of value
from a purely mechanical point of view, the older ideas as to the attraction
1 Cf. Schenck, I.e., p. 131. a L.c., p. 132.
3 Voss (Bot. Ztg., 1902, Orig., p. 231) finds that Bowiea volubilis will twine around much-
inclined supports if the strongly heliotropic shoots are illuminated on one side.
* See Mohl, I.e., p. 134; Darwin, I.e., p. 29; Baranetzsky, 1. c., p. 56; Schwendener, I.e.,
p. 418; Schenck, I.e., p. 121. When woody twiners coil around trees, the latter may be slowly
strangulated, the twining stem being often deeply imbedded in the secondary wood. Cf. Schenck,
I.e., p. 122.
TWINING PLANTS 41
exercised by the support having long ago been shown to be erroneous
by Mohl.
Torsion. Circumnutation does not involve torsion, but the latter
is usually shown very strongly by the older internodes of stems which
have not twined. The torsion is indicated by the twisting of ridges on
the stem and by the displacement of the phyllotaxis, and follows the same
direction as the circumnutation and twining. It arises, however, from
internal causes and hence persists when circumnutation and twining are
arrested by rotation on a klinostat 1.
Stems twined around a support usually show antidromous torsion
resulting from the twining, and which, owing to the fixation of the coils
to the support, has been incapable of removal by the plant's tendency to
homodromous torsion. If portions of the support are cut away the latter
comes into play over these regions and the antidromous torsion is wholly
or partly removed. The same takes place when the coils are loose or
unattached, and hence it is hardly surprising that the torsions observed
in a climbing stem should vary considerably, and even be in some cases
antidromous, in others homodromous 2.
Mohl supposed that circumnutation and twining were produced by the torsion
of the stem, but Palm and, more especially, Darwin and de Vries have shown that
this, was an error. The two latter authors recognized the dissimilar origins of anti-
dromous and homodromous torsions, and their mode of action. Schwendener, and
at a later date Baranetzsky, Ambronn, and Kolkwitz, showed in detail how the anti-
dromous torsion was the mechanical result of coiling. If an india-rubber tube bearing
a longitudinal stripe is coiled around a support without hindering its tendency to
twist around its own longitudinal axis, the spiral twisting of the stripe will show the
antidromous torsion resulting from coiling. To keep the stripe on the convex side
the tube must be twisted during coiling, and if the end is partially freed the tube
will tend to twist back to the original condition. In a stem capable of growth the
forcible torsion might become partially or entirely fixed s, just as is the antidromous
torsion produced by twining when tight coils are formed. The homodromous torsion
attempted in the attached coils has the effect of fixing the stem more firmly to the
support by tightening up loose coils 4.
Heliotropism and twining. According to Mohl, Dutrochet, Darwin, and
Baranetzsky 5 the circumnutating shoots of climbers are usually positively heliotropic,
but this irritability is so weak as merely to somewhat accelerate circumnutation when
1 Baranetzsky, 1. c., p. 31.
2 For details see Kolkwitz, Ber. d. hot. Ges., 1895, p. 497; Schwendener (1881), Gesammelte
bot. Mitth., p. 420; Ambronn, Zur Mechanik d. Windens, 1884, I ; 1885, II (Repr. from Sitzungsb.
d. sachs. Ges. d. Wiss.) ; Baranetzsky, Die kreisformige Nutation und das Winden d. Stengel, 1883,
p. 66; De Vries, Arb. d. bot. Inst. in Wtirzburg, 1873, Bd. I, p. 330; Darwin, Climbing Plants,
I875-
3 Cf. Kolkwitz, 1. c., p. 505. * Cf. Id., p. 512.
5 Darwin, The Power of Movement in Plants, 1880, p. 449.
42 MOVEMENTS OF CURVATURE
the stimulus is applied so as to aid the autonomic movement, and to slightly retard
the latter when acting against it. Baranetzsky found that during the symmetric
nutation of Ipomoea purpurea the half of the orbit towards the light was performed
in 45 minutes and that away from it in 55 minutes. Similar differences were
observed in Ipomoea sibirica and Polygonum Convolvulus. The heliotropic action is
weakened during symmetric nutation by the fact that the anterior side is continually
changing, whereas since the latter takes place to a less degree during asymmetric
circumnutation, the heliotropic action would naturally be somewhat stronger, being
more prolonged on the respective sides.
Baranetzsky observed that an asymmetrically nutating apex of Ipomoea sibirica
performed the half of the orbit towards the light and downwards in 35 minutes,
that away from the light in 75 to 85 minutes.
The positive heliotropism of the apex is unfavourable to twining rather than an
aid to it, but it is of some advantage that the coiled parts should become negatively
heliotropic, for this causes them to curve towards the shaded side and hence towards
the support. Baranetzsky * found this negative heliotropism to be very pronounced
in the shoots of Ipomoea purpurea, Polygonum Convolvulus, and Dioscorea smuata,
whereas it was weaker in the stems of Dioscorea Batatas, and was not developed at
all in the stems of Boussingaultia baselloidcs and Menispermum dahuricum 2.
SECTION u. Tendril-climbers.
Tendril-climbers show much more varied special adaptations than
twiners, and they may be classed in different groups according to the
type of irritable attaching organ they possess 3.
Under tendril-climbers we include all such plants as Bryonia and
other Cucurbitaceae, Passiflora, Pisum, Latkyrus, Cobaea scandens, Bignonia,
Eccremocarpus, Vitis, Cardiospermum Halicacabum which possess filamentous
coiling attaching organs which are irritable to contact (Fig. 9). The
tendrils are continually produced at the growing apex, and radiate and
attach themselves in all directions. The spiral coiling of the portion
between the stem and the support acts like a spring against the tearing
effect of violent shocks of wind or rain, and also draws the plant nearer
to the support and by the antagonistic action of different tendrils affords
more rigid support. The same applies when the tendril itself is branched,
1 Baranetzsky, 1. c., p. 21. Cf. also Wiesner, Die heliotropischen Erscheinungen, 1880, II, p. 38 ;
Voss, Bot. Ztg., 1902, Orig., p. 238.
3 Mohl, 1. c., p. 1 20, observed that certain twiners succeed in coiling around a support if they
meet it while growing away from the light, but not if they are growing towards it. It is, however,
not certain whether this is due to negative heliotropism.
9 For details see Darwin, Climbing Plants, 1875 ; Schenck, Beitrage z. Biol. u. Anat. d. Lianen,
1892, i, p. 135; Ludwig, Lehrb. d. Biol. d. Pflanzen, 1895, p. 126; Goebel, Organography, 1900.
The physiologist has to deal with the tendril as a functional organ without regard to its morphological
origin. Hence Schenck's classification has no value here.
TENDRIL-CLIMBERS
43
as mPisum or Lathyrus, or when, as in the case of Cobaea scandens^z. tips
of the branches are furnished with curved claws which aid not only in
maintaining contact until coiling has taken place, but also act as permanent
attaching organs.
FIG. 9. Bryonia dioica. a, young spirally coiled tendril ; £, expanded and irritable tendril ; c, tendril which
has grasped a support ; d, tendril which has not grasped a support, and has undergone the old-age coiling.
The last-named plants afford instances of the development of a portion
(Cobaea, Lathyrus\ or of the whole (Lathyrus aphaca\ of the leaf into
a typical tendril. In leaf-climbers the leaf or some portion of it acts as
the attaching organ without losing its general character. The petioles of
44 MOVEMENTS OF CURVATURE
Solanum jasminoides are, for instance, able to coil around a support (Fig. 13),
FlG. 10. Tendrils of Cobaea scandens.
with three pairs of leaflets, and a branched te
the claw-like ends are shown slightly magnified.
FIG. ii. Dalbergia linga. a, young stem and
leaves ; £, young leaf with the curved terminal
pair of pulvini ; £, older leaf (reduced) with one
pulvinus attached and its leaflets thrown off (after
Ewart).
A young branch (a) and an older one (e\ each bearing a leaf (b)
\ tendril ; (e) has grasped and (a) is grasping a support (c) ; at (d)
while in Fumaria officinalis, var. Wirt-
geni, the slender leaf-segments act in
the same way as irritable attaching
organs. The more or less tendril-
like leaf-tips of Corydalis claviculata,
Gloriosa superba, Flagellaria indica, and
Littonia form coiling attaching organs
and show a transition from typical leaf-
climbers to typical tendril-climbers, the
first-named plant possessing tendrils
which approximate in character to
those of Lathyrus.
Among petiole-climbers are Solanum
jasminoides •, Clematis vitalba, Atragene,
Tropaeolum and Lophospermum scan-
dens, in which the petiole bears a normal
leaf lamina, although it may have coiled
around a support. The tropical Dal-
bergia linga (Boerlage) is of interest
since in this plant the basal pulvini of
TENDRIL-CLIMBERS
45
the terminal pair of leaf-pinnae are long, backwardly curved, and irritable
to contact. After coiling, which begins in five minutes and is usually com-
pleted in a day, to the extent of one to four coils according to the thickness
FIG. 12. Gloriosa suferba. Two leaf -tips have grasped
a grass haulm.
FIG. 13. Solanumjasmtnoides (after
Darwin). The petiole (&) has twisted
around a support (s).
of the support, the wood-cylinder becomes within a week more than double
the thickness of that of an unattached pulvinus, while very often the terminal
pinnae with their leaflets are thrown off at
the articulation to the pulvinus so that the
tendril character is fully established 1.
Many tropical plants possess leafy
branches, or specially shaped ones which
act as irritable attaching organs. In
Europe only certain comparatively in-
efficient climbers belonging to the genus
Antirrhinum are branch- climbers, and
among these Cuscuta may be included,
since its stem not only twines but is also
sensitive to contact. Tropical countries
also possess in Uncaria (Fig. 14), ' Olax'
(Roue her ia), Artabotrys, and Strychnos2,
shrubby climbers provided with attaching
hooks or hook-like tendrils, which are either branch-thorns (Uncaria) or
inflorescence stalks (Artabotrys), and which in all cases undergo more
FlG. 14. Uncaria ovalifolia. Nat. sire
(after Treub). The hook (a) attached to a
support has thickened considerably.
1 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, i, p. 227. Most of the climbing
Dalbergias are branch-climbers. Cf. Schenck, 1. c.
3 Cf. Treub, Ann. du Jard. bot. de Buitenzorg, 1882, Vol. in, p. i ; Ewart, ibid., 1898, Vol.
xv, p. 187
46 MOVEMENTS OF CURVATURE
or less marked secondary thickening as the result of contact. In the case
of Uncaria and Artabotrys no coiling occurs, but a slight one in Roucheria
and Ancistrocladus, while the hook-like tendrils of Strychnos and Bauhinia
are able to form one or more complete coils around a suitable support1.
These latter forms show a transition to the watch-spring tendrils whose
coiled apices grasp supports and then twine around them.
A feeble contact irritability is shown by various organs. Thus the
aerial roots of Vanilla and of a few other plants are irritable enough to
function as root-tendrils, from three days to a week being, however, required
to produce a complete coil, and the coiling often not being completed
until after the lapse of three weeks2. Both terrestrial and aquatic roots,
as well as the rhizoids of Catharina, have been observed to coil around
foreign bodies, but it is not certain whether this is or is not the result
of contact stimulation. Presumably, however, certain special branches of
CystocloniumpurpurascenS) of Hypnea musciformis, QiNitophyllum uncinatum,
and of a few other marine algae, are able to coil like tendrils 3, which power
is also possessed according to Zopf4, by the curved hyphae of Arthrobotrys
oligospora, and by the hyphae of a few other fungi 5, In addition Wort-
mann 6 observed a strong sporophore of Phycomyces nitens coiling around
a weaker one.
The stimulus of contact not only hastens the coiling of a tendril but
also causes its strength to increase. In some cases, as in Bauhinia tomentosa
and Amphilobium mutisii the tendril undergoes a secondary increase in
thickness, such as is shown to a marked degree in the tendril-hooks of
Strychnos, Roucheria, and in the non-coiling hooks of Uncaria and Arta-
botrys'1. Similarly the petioles of leaf-climbers may double in thickness
at the point of contact (cf. Fig. 13, p. 45), while an attached twining
pulvinus of Dalbergia linga may attain double the diameter of an unattached
one, owing mainly to the very rapid growth of the wood-cylinder and
partly to the enlargement of the cortical cells 8. Similarly the secondary
growth of the wood in the petiole of Solanum jasminoides leads to the
1 Cf. Ewart, I.e., p. 239.
2 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. xv, p. 233. The attaching roots were
noticed by Mohl, Ranken- und Schlingpflanzen, 1827, p. 24; and Darwin, 1. c., p. 144. These and
other aerial roots apply themselves closely to the trunk and walls as the result of their negative
heliotropism, and become attached by their root-hairs. A root of Vanilla attached in this way to
the smooth surface of another leaf was able to support a weight of 250 grams (Ewart, 1. c., p. 234).
On the roots of the potato developed in moist air, see Sachs, Flora, 1893, p. 8.
3 Wille, Bot. Jahrb. f. System, u. Pflanzengeographie, 1886, Bd. vn, p. ai ; Nordhausen, Jahrb.
f. wiss. Bot., 1899, Bd. xxxiv, p. 236.
* Zopf, Nova Acta d. Leopold. Carolin. Akad., 1888, Bd. LIT, p. 325.
8 Boudier, Bull, de la Soc. bot. de France, 1894, p. 371 ; Ludwig, Bot. Centralbl., 1899,
Bd. xxxvn, p. 359.
6 Wortmann, Bot. Ztg., 1887, p. 806.
7 Ewart, 1. c., pp. 189, 208, 218, 222. 8 Ibid., p. 228.
TENDRIL-CLIMBERS 47
production of a complete cylinder of wood 1t while the stimulus acts as
far as the base of the tendril of Bauhinia causing its originally flattened
outline to become more or less circular 2.
The stimulus of contact causes the development of the disks by which
the tendrils of various species of Am-
pelopsis (Fig. 15), Bignonia capreolata,
B. littoralis, Hanburya mexicana, Cissus
pauliniaefolia attach themselves to rocks,
walls, or the bark of trees 3. These disks
may attain a considerable size, those of
Amphilobium mutisii often being 12 to
14 mm. in diameter, and 4 to 6 mm.
thick at the centre4. The tendrils of
this plant and &i Ampelopsis quinquefolia
may also twine around supports.
The disks or suckers are usually
formed by outgrowths from the epi- FlG. IS. Ampeiopn* quinqu*foiia. The tendril
dermal cells and subjacent parenchyma, jjkSfSH? ^^ disks> and has bec°me
but those of Amphilobium mutisii*
contain vascular tissue, and often also an annular air-space around the
margin of the disk. By the aid of a sticky secretion, or by growing into
the irregularities of the supporting surface, so firm an attachment is often
produced that the tendril breaks before the disk is torn away. In the
tendrils of Ampelopsis Veitchii, Vitis inconstan s, and Cissus paulinaefolia the
suckers are preformed structures present as small swellings at the tips of the
branched tendril and are simply excited to further development by contact,
but no such rudiments are present on the tendrils of Ampelopsis quinquefolia
and Amphilobium mutisii. The suckers may be formed. at various points on
the tendril, but in Ampelopsis usually, and in Amphilobium always, at the
tip of one or all of the branches if these are in contact with the support.
The three-armed tendril of Amphilobium is able to coil around a smooth
glass rod, but not to form suckers even where the tips of the branches
touch the glass6. When in contact with a rough surface the disks may
become perceptible in two or three days, but the full development of the
large disks of Amphilobium may take from one to two months. The
tendrils of most Cucurbitaceae show a certain proliferation of the epidermal
1 For details on petiole-climbers cf. Derschau, Einfluss von Contact u. Zug auf rankende Blatt-
stiele, Leipziger Dissert., 1893.
2 Ewart, I.e., p. 222.
8 Mohl, Ranken- u. Schlingpflanzen, 1827, p. 70; Darwin, Climbing Plants; Pfeffer, Arb. d.
bot. Inst. in Wurzburg, 1871, Bd. i, jp. 95 ; Lengerken, Bot. Ztg., 1885, P- 4°8 '•> Schenck, Beitrage
z. Biol. u. Anat. d. Lianen, 1892, I, p. 240.
* Ewart, 1. c., p. 219. 5 Id., I.e., pp. 219-20. 6 Id., 1. c.
MOVEMENTS OF CURVATURE
and cortical cells at the point of contact, and in Sicyos angulatus and
a few other Cucurbitaceae the fixation is aided by a viscid secretion 1.
The physiologically radial stems of Cuscuta europaea, C. epilinum, and
of Cassytha twine like typical climbers, and in addition coil and produce
haustoria as the result of contact2. When this has occurred and a few
close coils with haustoria have been formed, the acropetal portion of the
stem loses its contact irritability for a
time, and a few much steeper coils are
formed by circumnutatory coiling. These
coils are often loose and form no haustoria.
If, however, no support is found, the new
growths of the stem of Cuscuta remain
continually sensitive to contact, which
shows that it is the satisfaction of the
desire for contact which causes the periodic
inhibition of the contact irritability.
In addition the stimulus of gravity is
necessary to maintain the irritability of
Cuscuta^ for on a horizontally rotated
klinostat not only the circumnutation but
also the power of responding to contact
disappear, while after three days' rotation
the irritability only returns after twenty-
four hours' rest under normal conditions 3.
It is uncertain whether in other cases the
stimulus of gravity may be necessary to maintain
contact irritability, for typical tendrils as well as
the hyphae of Phycomyces appear to remain
irritable when rotated on a klinostat. Whether
this also applies to the feebly irritable stems of
the petiole-climber Lophospermum scandens,
which rarely coils in nature, is unknown4,
aith°ush many instances have been f°und in
which the sensitivity and power of reaction
are more or less dependent upon geotropic induction.
1 Miiller, Cohn's Beitrage z. Biol., 1887, Bd. iv, pp. 107, 123, &c. ; Schenck, 1. c., p. 200.
3 First observed by Mohl (Ranken- u. Schlingpflanzen, 1817, p. 131) ; farther studied by Koch
(Hanstein's bot. Abhandl., 1874, Bd. n, p. 121; Die Kleeseide, 1880), and fully explained by
Peirce (Annals of Botany, 1894, Vol. viu, p. 53).
3 Darwin's statement (1. c., p. 100) that the tendril of Echinocystis lobata becomes straight and
non-sensitive when there is danger of contact with its own shoot requires further proof. The power
of discrimination by which Darwin supposed certain tendrils to be able to avoid coiling around one
another does' not actually exist, the absence of such coiling being due to the slenderness, pliability,
and smoothness of the tendrils, combined with their circumnutation movements. (Cf. Ewart, 1. c.,
pp. 224-7 ; and Pfeffer, Unters. aus dem bot. Inst. zu Tubingen, 1885, Bd. I, p. 495.)
4 Darwin, Climbing Plants.
FIG. 16. Cuscuta epuinum on
TENDRIL-CLIMBERS 49
The periodic inhibition of the contact irritability at the apex of Cuscuta affords
a good instance of the influence of a realized activity upon subsequent development,
and it has the importance of allowing the plant to spread from one host to a
neighbouring one, and of enabling more rapid extension over a single host. The
persistence of the irritability in the absence of a support gives a better chance of
one being immediately utilized when reached by the circumnutation of the elongating
apex.
Since Cuscuta usually gains a support by the aid of its circumnutation, the
coiling follows the direction of circumnutation1, but it is uncertain whether the
contact irritability suffices by itself to produce definite coiling. Since coiling takes
place around a rod of moist gelatine which exercises no contact stimulation, it is
evident that circumnutation alone produces fairly good coiling. Cuscuta, like other
twiners, usually produces no further coils around a support laid horizontally. This
is owing to the fact that during the phase when the apex is non-sensitive to contact,
the terminal internodes free themselves from the support and strive to become
erect 2.
A rod of any material suffices to produce the coiling of Cuscuta and the forma-
tion of haustoria, which however only attain their full development when they penetrate
an appropriate host-plant. Since the production of haustoria is dependent upon the
stimulus of contact, they are only formed on the side pressed against the support,
although all sides of the stem are capable of producing them.
Heliotropism. The negative heliotropism of certain tendrils aids them in acquiring
a support. This applies to the tendrils of Viiis vinifera, Ampelopsis hederacea*,
Bignonia capreolata, Eccremocarpus sealer •*, as well as to the root-tendrils of Vanilla
planifolia 6. The tendril of Smilax aspera 6 possesses very weak negative helio.tropism,
which causes it to circumnutate somewhat more rapidly away from the light than
towards it. The reverse is the case in the feebly positively heliotropic tendrils of
Passiflora7, whereas Darwin could detect no heliotropism at all in the tendrils
of Pisutn*. Tendrils, like the stems of twiners, are therefore only feebly helio-
tropic. This also applies to the stems of Cuscuta, although when they have been
rotated horizontally for some time on a klinostat they become distinctly positively
1 Peirce (1. c.) observed no coiling in the opposite direction, but Koch (1. c., 1874, P- I24)
this to occur occasionally.
2 Peirce, I.e., p. 115. According to Koch (1. c., p. 124), Cuscuta is also able to twine around
a horizontal support.
3 Knight, Phil. Trans., 1812, p. 314; Mohl, Ranken- und Schlingpflanzen, 1827, p. 76;
Darwin, Climbing Plants, 1875, p. 144; Wiesner, Die heliotropischen Erscheinungen, 1880, Th. n,
p. 38.
* Darwin, I.e., pp. 86, 103. Beccari (Bot. Jahrb., 1884, I, p. 27) observed that the tendrils of
Cissus do not apply themselves to strips of mica, possibly because of the negatively heliotropic
action of the reflected light.
5 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. xv, p. 237.
6 Darwin, I.e., pp. 118, 184. 7 Id., I.e., p. 153.
8 Id., I.e., p. 112. Wiesner (I.e., p. 38) finds that the tendrils of Pisum are positively
heliotropic in weak light, negatively heliotropic in strong light. Derschau (Einfluss von Contact
und Zug auf rankende Blattstiele, 1893, p. 12) finds the petioles of Lophospermum scandens to be
fairly strongly positively heliotropic.
PFEFFER. HI E
5o MOVEMENTS OF CURVATURE
heliotropic *. It may also be remarked that the tendrils of Bryonia remain irritable
even when developed in darkness 2, and that the absence of light exerts no perceptible
influence upon the development of the haustoria of Cuscuta 3.
SECTION 12. The Special Irritability of Tendril-climbers.
The contact of a tendril against a support induces a greater activity
of growth on the free side, and hence produces coiling. Since the
irritability is only attained at a certain stage of development, and
gradually disappears as the tendril grows old and loses the power of
growth, the effect produced by a stimulus depends not only upon its
intensity but also upon the age of the tendril. Ordinary tendrils only
become sensitive to contact when fully unfolded, and either at or after
the commencement of circumnutation. The irritability usually persists
until growth has entirely ceased, which occurs after the circumnutation
has become imperceptible4.
All parts of the tendril are not equally irritable, the basal portion
in many cases responding feebly or not at all to contact. • Usually only
one side responds to contact, but the tendrils of Cobaea scandens, Cissus
discolor, Smilax aspera, Actinostemma paniculatum5, and the pulvinar
tendrils of Dalbergia linga* are able to coil around an object touching
any side. The tendril of the last-named plant has the proximal surface
concave when young, but when older one of the other surfaces becomes
convex, and the slightly greater irritability of the original concave side
is transferred to the new one7. A physiologically dorsiventral tendril
remains unstimulated, and moves away from a support which touches one
of its non-irritable flanks. In such tendrils the irritability usually appears
to decrease from the irritable flank towards the sides, which are however
usually sufficiently irritable to commence coiling, and then a slight twist
1 Peirce, I.e., pp. 87, 116. 2 Sachs, Bot. Ztg., 1863, Beilage, p. 12.
3 Peirce, 1. c., p. 88.
* For facts see Darwin, Climbing Plants, 1875; Wortmann, Bot. Ztg., 1887, p. 53; Schenck,
Beitr. z. Biol. u. Anat. d. Lianen, 1892, I, pp. 141, 154; Fitting, Jahrb. f. wiss. Bot., 1903,
Bd. xxxvin, p. 554. On leaf-tendrils and hooks cf. Schenck, 1. c. ; Derschau, Einfluss von Contact
und Zug auf rankende Blattstiele, 1893, p. 12 ; Ewart, Ann. du Jard. hot. de Buitenzorg, 1898,
Vol. XV, p. 1 88. On the distribution of growth in developing tendrils cf. Macdougal, Annals of
Botany, 1896, Vol. X, p. 379 ; Fitting, 1. c., p. 547. Mohl (Ranken- u. Schlingpflanzen , 1827, p. 65)
incorrectly supposed that the irritability only appeared when growth in length had ceased. In many
cases growth may be re-awakened and a curvature be produced after the tendril has ceased to
elongate. Cf. Fitting, Jahrb. f. wiss. Bot., I.e., p. 554.
5 Darwin, I.e.; Schenck, I.e., p. 141; Derschau, I.e., p. 13; Fitting, 1. c , p. 551. The
decision is made according to the presence or absence of a curvature after contact on each flank.
Even in dorsiventral tendrils, the side on which contact produces no response is actually sensitive in
a special way, for contact on this side may prevent simultaneous contact on the irritable side from
producing any response.
6 Buitenzorg garden name, not given in Kew Index. 7 Ewart, 1. c., p. 229
THE SPECIAL IRRITABILITY OF TENDRIL-CLIMBERS 51
commonly brings the most irritable side against the support. It is rarely
the case that a tendril is physiologically perfectly radial, and numerous
transitions occur between isotropic and anisotropic tendrils. Contact
applied to the convex surface of the hook-tendril of Strychnos causes,
for instance, a slight increase of thickening but no coiling 1. Kohl 2 found
tendrils of Pisum sativum to be occasionally irritable on all sides instead
of on one only as is usually the case. The branches of the tendril of
Bignonia venusta are anisotropic, but the peduncular portion is able to coil
towards any side 3.
Among leaf-climbers the tip of the leaf of Flagellaria indica is irritable
on the upper side, but in all others the under side is the sensitive one4.
Darwin found the petioles of leaf-climbers to be irritable on all sides,
but according -to Derschau 5 not to the same degree.
Usually only the concave side of an attaching hook is
pronouncedly irritable, the back and sides being less
so or almost insensitive to contact. In the case of
Artabotrys the median portion of the hook (£, Fig. 17)
is much more irritable than either the terminal or
basal joints 6. In the case of the tendril of A mpelopsis
Veitchii only a particular point at the tip of each
branch is irritable, whereas the stems of Cuscnta are
physiologically radial to contact stimuli. In most
cases anisotropic tendrils are morphologically and
anatomically dorsiventral, while isotropic tendrils
which undergo secondary growth may become very
pronouncedly bilateral as the result of contact stimuli.
The same stimulus may cause a flattened tendril to
become more or less circular in outline 7. Anatomical
and physiological dorsiventrality are not necessary
postulates of each other, and in fact various dorsi-
ventral petioles are irritable on all sides. The anatomical structure affords
no direct evidence as to the distribution of irritability, and hence requires
no discussion 8.
Ewart, 1. c., p. 212. a Mohl, 1. c., p. 65. 3 Schenck, 1. c., p. 189 ; Fitting, 1. c.
Schenck, 1. c., p. 179. The tendril-leaves of Adlumia cirrhosa are irritable on all sides.
Cf. Pfeffer, Unters. d. hot. Inst. zu Tubingen, 1885, Bd. I, p. 485.
L.c., p. 13.
Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, pp. 193, 202, 204, 242.
Ewart, I.e., pp. 218, 222.
On anatomical relationships cf. Worgitzky, Flora, 1887, p. 2 ; Leclerc du Sablon, Ann. sci.
nat., 1887, 7° sen, T. v, p. 5; Miiller, Cohn's Beitr. z. Biol., 1887, Bd. IV, p. 97; Derschau,
Einfluss von Contact und Zug auf rankende Blattstiele, 1893 ; Borzi, Rend. Acad. dei Lincei, 1901,
5* ser., T. x, p. 395 ; Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, p. 187 ; Fitting, 1. c.,
p. 600; Schenck, Beitr. z. Biol. u. Anat. d. Lianen, 1892, I, p. 146 ; Macdougal, Annals of Botany,
1896, Vol. x, p. 394; and the literature quoted by these authors.
E 2
FIG. 17. a, coiled and thickened
hook-tendril of Strychnos lau-
n'na \ b, attaching hook of A rta-
botrys Blumei. (After Ewart.)
52 MOVEMENTS OF CURVATURE
In all these cases the attaching organs are stimulated by contact with
or rubbing against any solid body. Contact with air or liquids such as
a stream of water or mercury produces no effect, whereas in Mimosa and
similar plants any shaking or disturbance may act as a stimulus if
sufficiently intense. This difference is due to the existence of a special
contact irritability in the attaching organs, which may also be termed
haptotropism or thigmotropism, and which is excited by differences of
pressure or variations of pressure in contiguous or neighbouring regions l.
Hence smearing a tendril with stiff gum-arabic exercises no stimulatory
effect, and similarly a glass rod covered with moist but solid 10 per cent,
gelatine produces no excitation even when strongly pressed and rubbed
against the most sensitive tendrils. Coated glass rods may therefore be
used to handle tendrils without stimulating them, or the tendril may
be placed upon a glass dish coated with the solidified gelatine. Naturally
contact with a rough body exerts a greater stimulus than contact with
a very smooth one. Hence smooth and slender tendrils, since they can
exert but little pressure on one another, and usually remain in contact for
a short time only, rarely coil around each other2. Stouter and stiffer
tendrils like those of Bauhinia and Smilax naturally respond to self-contact
more readily. The absence of any response to wind and rain is obviously
of great use to the plant.
The tendrils of Sicyos angulatus, Cydanthera pedata, and Passiflora
gracilis are especially sensitive, the tendril of the first-named plant being
perceptibly stimulated by the contact of a thread of cotton weighing
0-00025 of a milligram laid upon the tendril3. The tendril is therefore
more sensitive than the human skin, which receives no impression when
a thread of this weight moves gently upon it 4. A worsted thread of I to
10 mgm. weight stimulates the less sensitive tendrils as well as many
irritable petioles, but a stronger stimulus is required for the tendrils of
Vitis. A bamboo fibre i mm. diameter and weighing o-i gram is just
able to produce a curvature and slight but perceptible thickening in the
pulvinar tendril of Dalbergia linga and in the hook tendril of Strychnos,
1 Pfeffer, Unters. a. d. hot. Inst. zu Ttibingen, 1885-, Bd. I, p. 483. A detailed list of cases in
which contact irritability has been established is given here. This form of irritability was later
detected by Peirce (1. c., p. 66) in Cuscnta, and by Ewart (1. c., pp. 196, 203) in the irritable hooks
of tropical climbers, although in these the stresses and strains set up in the attached hook influence
the ultimate amount of thickening.
2 Pfeffer, I.e., p. 495.
3 Id., I.e., p. 506; Darwin, I.e., pp. no, 131, 405 ; Climbing Plants, 1875, pp. 153, 171, 197.
4 'Exact determination is difficult, since the excitation depends upon the extent of surface in
contact, the degree of roughness, and the rapidity of movement. .Cf. Frey u. Kiesow, Zeitschr. f.
Psychologic u. Physiologic der Sinnesorgane, 1899, Bd. xx, p. 153. Kemmler (Hermann's Handbuch
d. Physiologic, 1888, Bd. ill, Kap. 2, p. 325) states that the minimal stimulus for*sensitive skin is
that due to the gentle movement of a weight of 0-002 of a milligram.
THE SPECIAL IRRITABILITY OF TENDRIL-CLIMBERS 53
whereas a piece of wood less than 2 mm. diameter and o-i gram weight
acts as a sub-minimal stimulus to Bauhinia tomentosa. The irritable hooks
of Uncaria^ Artabotrys, and Roucheria require the attachment of weights
of 100 to 1,000 milligrams according to whether a rough bamboo fibre
or hard twine, or a smooth copper wire or glass thread is used1. The
most sensitive tendrils may curve five to twenty seconds after stimulation,
whereas less sensitive ones may take one or more hours to respond
perceptibly2. The tendrils of Dalbergia linga begin to curve in five
minutes, those of Vitis discolor in one hour, those of Strychnos in twelve
hours, the root-tendrils of Vanilla planifolia in twenty-four hours, whereas
no increase of thickness resulting from stimulation can be detected until
after the lapse of one or more days 3.
The stimulus usually needs to act for some time to produce a response,
but in very sensitive tendrils a single strong contact is sufficient to produce
a slight curvature. This as well as more pronounced curvature is followed
by a straightening due to orthotropism if the contact stimulus no longer
acts4. Since the tendril remains irritable, Darwin was able to stimulate
the tendril of Passiflora gracilis twenty-one times in fifty-four hours, each
time the tendril being allowed to straighten after forming a hook-like
curvature.
When a weak continuous stimulus is applied, the tendril first bends
beyond the ultimate curvature resulting from the antagonism between its
orthotropism and the applied stimulus5. Although we may say that the
tendril accommodates itself to the stimulus, it is not certain whether this
is due to the gradual awakening of opposing reactions, or to the decrease
of the excitability, or to a combination of factors. No decisive conclusion
can be made from the fact that the satisfaction of the contact irritability of
Cuscuta produces a periodic inhibition of this irritability.
The minimal stimulus needs to be surpassed in order to cause the tendril
to coil completely around the support, and to maintain the coiling until
growth has ceased and the coils are permanent. The stimulus exercised
by a support is usually sufficient for this, and in fact slender sensitive
tendrils are able to form close coils around a thin thread. The less
sensitive and thicker tendrils of Vitis * are, however, only able to form
loose coils around supports less than 2 or 3 mm. diameter, while the hooks
of tropical climbers are usually unable to become firmly attached to
1 Ewart, I.e., pp. 211, 223, 231.
2 Darwin, 1. c., p. 172 ; Pfeffer, 1. c., p. 486 ; Miiller, 1. c., p. 109.
3 Ewart, 1. c., pp. 209, 223, 229, 236.
* Darwin, I.e.; de Vries, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 306; Fitting, I.e.,
p. 6n. This straightening was first observed by Gray, Edinburgh New Phil. Journ., 1859, Vol. X,
P- 307.
5 Pfeffer, I.e., p. 507 ; Darwin, I.e., p. 132.
' Sachs, Lehrb. d. Bot., 4. Aufl., p. 872 ; de Vries, 1. c., p. 307.
54 MOVEMENTS OF CURVATURE
supports of less than 3 to 5 mm. diameter, and the hooks of Strychnos and
Roitcheria are unable to coil around supports more than 7 to 10 mm.
diameter 1. The less actively curving and less irritable tendrils are unable
to apply themselves closely to the sides of a flattened support, whereas
a thin sensitive tendril may come into close contact with both sides of
a thin strip of sheet zinc2. If the tendril undergoes secondary growth on
attachment, the coils of thick tendrils often become extremely closely
applied even to irregular supports 3.
Similarly, if a tendril strives to tighten its coils it may exert pressure
upon the support, and either roll up a leaf around which coiling has
occurred or diminish the diameter of a paper cylinder slit along one side 4.
Hence, on withdrawing a solid support, the coils usually tend to narrow,
and de Vries found that a tendril of the cucumber which had formed
five and a half coils around a support 6 mm. thick showed eight narrower
coils when the support was removed.
When a tendril is in contact at one point only, the main curvature is
produced here, but the stimulus is perceptibly propagated in both directions
to a distance of 5 to 10 millimetres5. Similarly, the secondary thickening
which tropical tendrils such as those of Bauhinia and Strychnos undergo
takes place mainly at the point of contact, the effect of the stimulus ceasing
to be perceptible at a distance of i to 3 cms.6 The continued curvature of
the tendril usually brings fresh acropetal surfaces in contact with the
support until the whole terminal portion has coiled. The same tendency to
coil takes place basipetally, but is prevented by the tension existing in the
free portion between the plant and the support. If the tendril is allowed
to coil around a light paper shell a few coils may be formed basipetally
from the original point of contact, and this causes the shell to be drawn
towards the plant.
Tendrils may not only coil around horizontal supports or loose objects,
but may coil in different directions, so that either left- or right-hand coiling
may be shown by the tendrils of the same plant 7. The coils are usually
somewhat inclined, and though near together are not superimposed.
Tendrils are unable to coil around thick supports, since, if the stimulated
part cannot form a sufficient curvature, it is drawn away by the old-age
coiling 8. By the aid of this coiling long tendrils may sometimes succeed
1 Ewart, 1. c., pp. 189, 214.
2 Mohl, Ranken- u. Schlingpflanzen, 1827, p. 82.
3 Ewart, 1. c.
* Mohl, 1. c., p. 63 ; de Vries, I.e., p. 307. Cf. Macdougal, Ber. d. hot. Ges., 1896, p. i^.
5 De Vries, Arb. d. bot. Inst. in Wiirzburg, 1873, Bd. I, p. 304; Pfeffer, Unters. a. d. bot. Inst.
zu Tubingen, 1885, Bd. I, p. 509; Fitting, I.e. On petiole-climbers cf. Derschau, Einfluss von
Contact u. Zug auf rankende Blattstiele, 1893, p. 13.
6 Ewart, I.e., pp. 208, 223.
7 Cf. de Vries, 1. c., p. 307. 8 Mohl, 1. c., pp. 80, 142.
THE SPECIAL IRRITABILITY OF TENDRIL-CLIMBERS 55
in attaching themselves to flat or irregular supports, of as much as 3 or
4 cms. diameter if these are in close proximity.
The contact not only produces the attachment to the support and
accelerates the coiling but also induces an increase in the strength of the
tendril, and in some cases the formation of special growths such as suckers.
It is, however, uncertain whether the increase of strength by lignification,
or by secondary growth where this occurs, is directly due to the stimulus of
contact or is the result of the mechanical demands made upon the attaching
organs. A decision is by no means easy, since an increase of pressure at
the point of contact not only increases the contact-stimulus but also the
mechanical demands made upon the organ, and, further, the stimulus of
contact may be transmitted some distance away from the directly stimulated
area. It seems indeed that both factors enter into play, for Ewart observed
a slight thickening in hook-tendrils allowed to pull against gelatine- covered
rods where little or no stimulus of contact could be exercised, and observed
in other cases a thickening caused by contact without any appreciable
strain being set up in the organ, and that where a tendril was in contact
with two supports the thickening was mainly shown at the points in contact
and not in the region between 1 ; similarly Derschau found that the petiole
of a leaf-climber exhibited a slight secondary thickening after temporary
contact with a support too light to exert any appreciable stress upon the
petiole 2.
Hegler's statement that tension in general increases the strength of
ordinary stems is incorrect, for Ball 3, under similar conditions, and in part
with the same plants as those used by Hegler, observed in no case any
perceptible increase in the tensile strength. It is possible that positive
results may be gained with other plants, but further experiment is necessary
to determine whether the increase in the tensile strength of attached tendrils
without any secondary growth is due to the stimulus of tension, of contact,
or to other causes.
The acceleration of the ultimate coiling of the tendril due to contact
is sometimes very pronounced. Thus Darwin found that an attached
tendril of Passiflora quadrangular is coiled as much in two days as an
unattached one in twelve. The tendrils of Vitis vinifera, Ampelopsis
hederacea (quinquefolia}, and of various species of Cissus, only coil when in
contact with a support 4. The same applies to the hook tendrils of Strychnos
and to the branches of the tendril of Amphilobium mutisii which are thrown
off in the absence of a contact-stimulus 5.
The coiling of a free tendril usually begins when growth is reduced to
1 Ewart, 1. c., pp. 193, 215, 222, 227. 3 Derschau, 1. c., p. 30.
3 Jahrb. f. wiss. Bot., 1903, Bd. XXXIX, p. 305.
* Darwin, I.e., p. 125 ; v. Lengerken, Bot. Ztg., 1885, p. 360; Schenck, 1. c., p. 145.
5 Ewart, I.e., pp. 208, 218.
56 MOVEMENTS OF CURVATURE
a minimum, so that the accelerating of coiling produced by contact may
be connected with the retardation of growth which usually ensues 1. This
is presumably the result of the correlative stimuli awakened by contact and
not of the mechanical tension exercised on the attached tendril. Tension
appears usually to slightly retard growth in length, but subsequently to
accelerate it. That a free tendril should coil all one way, but that the free
portion of an attached one should form two or more reversed spirals is the
natural result of the same attempt at coiling combined in the second case
with the fixation of the ends of the tendrils 2. Similar results may be
obtained when longitudinal strips of the peduncle of Taraxacum which tend
to coil spirally are held at both ends, or when a cord attached at both ends
is twisted in opposite directions at two points equidistant from its ends ?.
The production of the suckers of Ampelopsis and Amphilobium, of the
haustoria of Cuscuta^ as well as the thickening of certain tendrils and
attaching hooks and of the petioles of leaf-climbers, are undoubtedly due in
the first instance to the stimulus of contact. The thickening only attains its
full development when permanent contact is assured and when the attaching
organ is subjected to increasing tension. The increased pressure at first
increases the contact-stimulation but finally retards or inhibits the growth
on the applied surface, which usually becomes more or less flattened when
the pressure is considerable 4. The hooks of tropical climbers may attain
a considerable increase of strength, in this way their breaking strain often
increasing four- or ten-fold, so that they are. able to bear weights of
10 to 15 kilogrammes5. The same takes place in the tendrils of Amphilo-
bium and Bauhinia which undergo secondary thickening, while according
to Worgitzky 6 the attached lignified tendrils of Cucurbita and Passiflora
become from two to twelve times stronger than unattached ones.
It is uncertain whether it is the absence of a contact-stimulus or of tension
which is responsible for the smallness, shrivelling, death, or abscission of
the unattached tendrils of certain plants. This was observed by Darwin on
the tendril of Ampelopsis hederacea (quinquefolid) and Bignonia Tweediana>
by Muller on that of Cyclanthera pedata, by Leclerc du Sablon on leaf-tips
of Flagellaria indica, and by Ewart on the tendrils of Amphilobium mutisii'1.
1 Fitting, Jahrb. f. wiss. Bot., 1903, Bd. xxxviil, pp. 550, 608. This coiling is associated with
a single slight acceleration of growth.
2 Correctly interpreted by Mohl, 1. c., p. 79, and Darwin, 1. c., p. 127.
3 Noll, Flora, 1899, p. 388.
* Derschau, I.e., p. 33 ; Ewart, 1. c., pp. 140, 189. 6 Ewart, 1. c., pp. 194, 208.
6 Worgitzky, Flora, 1887, p. 40. On the tensions to which tendrils are exposed cf. Macdougal,
Ber. d. hot. Ges., 1896, p. 153.
7 Darwin, 1. c., pp. 69, 113, 355 ; v. Lengerken, 1. c., p. 360; Muller, Cohn's Beitr. z. Biologic,
1887, Bd. IV, p. 108; Ewart, I.e., p. 219; Leclerc du Sablon, Ann. sci. nat., 1887, 7" ser., T. v,
p. 28. The attachment of the coiling portion of the leaf of Nepenthes favours the development
of the pitcher according to Goebel, Pflanzenbiol. Schilderungen, 1891, n, p. 98.
THE INFLUENCE OF CONTACT UPON TENDRILS
57
If two of the branches of the trifid tendril of this plant become attached
the other one usually persists also, but remains thinner and slightly shorter
than the attached ones.
On the other hand the coiling of the long pulvinus of the terminal pair
of leaf-segments of Dalbergia linga around a support often leads to the
leaflets being thrown off, but this may also occur spontaneously without
apparent cause1.
SECTION 13. The Influence of Contact upon the Growth and
Curvature of Tendrils.
Since we are dealing with growth-curvatures it is only natural that
a response should only be possible in organisms still capable of growth,
or in which the stimulus reawakens the power of growth. This applies not
only to the curvature of tendrils but also to the haustoria, sucking-disks,
and the coiling part of a petiole-climber in which the stimulus of contact
excites renewed growth or awakens a special form of productive activity.
According to Fitting the growth of a curving tendril undergoes
a pronounced but temporary acceleration persisting during the reaction2.
This acceleration is especially great when the curvature is rapid, for the
median axis may elongate 20 to 100 times, and the convex side 40 to 200
Convex s
de
»rve
TZ&
-^ 6_i*-
— yvC^V^*
3K>T
10'
20'
30'
50'
60'
70'
80'
FIG. 18. Curves representing the growth of the convex and concave sides of the tendril of Pilogyne suavis
after stimulation at *. The curve for the median axis is taken as the mean between those for the concave and
convex sides. The horizontal distances give the times in minutes, the vertical distances (i division = 0-0121 mm.)
the growth as indicated by the divisions marked on the tendril previously to stimulation, and which had
remained the same distance apart during the previous 20 minutes. (After Fitting.)
times as rapidly as before stimulation, and also after its effect has passed
away. After transitory stimulation the concave side, which either retains
the same length or only slightly shortens, begins to grow more actively,
and since the convex side has now ceased to elongate, the tendril soon
straightens. Similar results were obtained by Fitting with rapidly and
1 Ewart, I.e., p. 228.
2 These studies, temporarily interrupted by the untimely death of Ockel, who began them at
Pfeffer's instigation, were completed by Fitting, Ber. d. bot. Ges., 1902, p. 373; Jahrb. f. wiss. Bot,
1903, Bd. xxxvui, p. 545.
58 MOVEMENTS OF CURVATURE
slowly growing tendrils as well as with those which are irritable on all sides
and on one side only1. In Fig. 18, curves representing the growth of the
different regions of a tendril of Pilogyne suavis are given which five minutes
after stimulation had curved into an arc of 5 mm. radius. Similar curves
were obtained by using marks placed on the sides of the tendrils to
determine the elongation of the convex side 2.
Although the exact mode of production of these changes in the rate of
growth is uncertain they are undoubtedly the result of the action of the
contact-stimulus, and this also applies to the subsequent acceleration of
growth in the concave side which causes the tendril to straighten after
temporary contact, although it is only an indirect result of the contact
stimulation. Fitting 3 found that the accelerations of growth and the
tendencies to curvature followed in the same order when curvature was
rendered mechanically impossible, so that a realized curvature is not
necessary for the production of the secondary acceleration of growth on
the concave side. The changes of the tissue-strains produced by the
attempted curvatures might, however, act as the exciting cause to the
secondary response, for if the tendril is kept straight the growth of
the convex side will tend to stretch the concave one. This is shown by
the fact that the tendril immediately curves when released, until the concave
side is slightly or not at all compressed.
A realized curvature does, however, excite a compensating reaction
tending to produce straightening, as is shown by the fact that a tendril to
which a plastic curvature is forcibly imparted, has its growth accelerated
on the concave side so that it gradually straightens again 4.
It is evident that a chain of reactions is necessary in both radial and
dorsiventral tendrils, since the primary acceleration of growth occurs not
on the stimulated but on the non-stimulated side. Furthermore, as Fitting 5
found, no curvature occurs if the tendril is rubbed equally strongly on oppo-
site sides or around a circular zone. This applies to both radial and dorsi-
ventral tendrils, neither a curvature nor any acceleration of growth being
shown. Contact applied to the convex surface of the tendril of Strychnos
and of Bauhinia is, however, unable to prevent coiling around a support in
contact with the concave surface6. When the opposed stimuli produce
no response it is evident that they are still perceived but mutually
antagonize so that no reaction is awakened. The convex surface of many
1 Trzebinski (Bull, de 1'Acad. de Cracovie, 1902, p. 123) observed that contact produced
disturbances in the rapidity of growth of the sporophore and sporangium of Phycomyces nitens, but
no details are given as to the mode of application of the contact stimulus.
2 [These observations of Fitting's corroborate the original views of Sachs, Textbook of Botany,
1875, P- 779-1
3 L.c., p. 588. 4 Id., pp. 557, 582. 5 L. c., p. 582.
6 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, pp. 208 seq.
THE INFLUENCE OF CONTACT UPON TENDRILS 59
dorsiventral tendrils, though unable when stimulated to produce a curvature,
is nevertheless sufficiently irritable to be able to inhibit a response to
contact on the concave side, and this action is awakened by contact with
a rough surface but not by contact with smooth moist gelatine.
Presumably the compensation begins during perception, so that no
attempt at curvature is ever awakened. It is also possible that the two
excitations might act simultaneously, but that the power of response might
be temporarily lost, or that some essential connecting link between percep-
tion and response should be suppressed. The former is however impro-
bable. Fitting1 observed that the curvatures produced by a change of
temperature or by removing the tip of the tendril are inhibited when the
back of a dorsiventral tendril is rubbed, and this fact may when further
investigated lead to an explanation of the phenomena mentioned. Since
this inhibitory action is largely localized, it is possible to keep a portion of
a tendril straight while the remainder is performing a thigmotropic, thermo-
nastic or traumotropic curvature.
Continuous contact causes complete and permanent coiling, the
continuation of the coiling involving exactly the same stimulatory reaction
as is produced by temporary contact. According to Fitting2 prolonged
contact rapidly induces a complete cessation of growth, so that the
acceleration of growth on the concave side which produces straightening
after temporary contact no longer occurs. Evidently, therefore, the reactions
leading to this secondary response are inhibited by continued contact.
This applies only when permanent contact is assured, and in fact even
sensitive tendrils only partially raise themselves from the support during
coiling, partly as the result of accommodation, of orthotropism or of irregular-
ities in the support. Since the free portions usually again come into contact
with the support, continue to coil and show an acceleration of growth,
they must retain the power of growth for some time. In this way aided by
the tendency to curvature of the uncoiled basal portion, the tendril is often
able to creep over the surface of a support and increase the number of coils,
as was first observed by Darwin 3.
Sachs concluded that changes in the rate of growth on the different sides of the
tendril were responsible for its curvature, and this has been confirmed by Fitting. The
curvature is therefore not due, as certain authors have assumed, to an active con-
traction of the concave side 4. The measurements made by de Vries 5, although not
extremely accurate, pointed against this conclusion, but since they were taken after
1 L. c., p. 562. 2 L. c., p. 609.
3 Climbing Plants, 1875.
* Id., 1875, p. 180; Macdougal, Ber. d. hot. Ges., 1896, p. 151; Annals of Botany, 1896,
Vol. X, p. 399; Torrey Botanical Club, 1898, Vol. XXV, p. 69. Cf. Fitting, I.e., p. 565; Sachs,
Textbook of Botany, 1875, p. 779.
5 De Vries, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 309.
60 MOVEMENTS OF CURVATURE
the completion of the curvature, they failed to reveal the acceleration of growth on
the convex side.
De Vries l erroneously supposed that contact stimulation produced a rise of
turgor in the side becoming convex, the cells of which experienced an elastic
stretching, which was subsequently made permanent by growth. The fact that
contact accelerates growth is readily shown in slowly-coiling tendrils like those of
Strychnos and Bauhinia, while attached tendrils of Amphilobium mutisii usually
become about one-sixth longer than unattached ones2. Hence the straightening
of the curvature produced when the tendril is placed in hot water, or in alcohol
and then in water, is not greater than that which other curved objects experience
when similarly treated, and it is due to the result of the liberation of the tissue-
strains 3. No straightening at all occurs when a curvature is slowly produced, and
sometimes not even when it rapidly follows contact 4.
De Vries erroneously assumed that the straightening of the tendril in strong
saline solutions afforded a complete proof of his theory. As a matter of fact the salt
solution penetrates so slowly that plasmolysis is only produced after some hours, and
in the meantime the continued growth of the tendril causes it to straighten in the
usual manner6.
It is not certain here, any more than in other cases, how the growth of the tendril
is produced during curvature. The fact that the cell-walls of tendrils are readily
stretched beyond their limit of elasticity affords no proof of their plastic growth 6. In
any case, however, the plastic stretching of the cell-walls would need to be preceded
by a preparatory softening physiological action, since the curvature ceases in the
absence of oxygen. Regulation would also be necessary if the contact induced a rise
of turgor, but the latter is not necessary and has not been proved to exist.
Historical. Our detailed knowledge of tendrils begins with Palm's work in 1827,
and also with that of Mohl, who detected the irritability to contact and observed the
acceleration of the coiling of the unattached portion produced by contact, but
erroneously regarded twining as being due to contact irritability. After Dutrochet7
had added a few facts our knowledge of climbing plants in general was greatly
extended by Darwin. Further additions were made by de Vries and by the other
authors mentioned, while Pfeffer explained the inherent character of the sense
perception underlying thigmotropic irritability. ' Sachs showed that the curvature of
tendrils was the result of growth, and the fact that the coiling of slowly growing
tendrils and. tendril-hooks was also the result of growth, and that contact stimulated
the growth in length of tendrils was shown by Ewart (1898), while Fitting (1903)
studied the mechanics of the growth-curvature of the more irritable tendrils in detail,
and determined the changes in the rate of growth which produce curvature and
straightening.
1 De Vries, Bot. Ztg., 1879, p. 835; Landw. Jahrb., 1880, p. 509. A similar conclusion is
given by Leclerc du Sablon, Ann. sci. nat., 1887, 7e sen, T. XXV, p. 38. De Vries attempted to
explain the changes in the rate of growth involved in other movements in the same way.
2 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, pp. 208, 218.
3 Fitting, 1. c., 1903, p. 598. * Ewart, 1. c., pp. 210, 219, 221, 229, 236.
5 Fitting, I.e., p. 595. 6 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1885, Bd. I, p. 489.
7 Dutrochet, Ann. sci. nat., 1844, 3° ser., T. II, p. 156.
IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 61
PART III
MOVEMENTS PRODUCED BY MECHANICAL AND CHEMICAL STIMULI
SECTION 14. The Irritability to Contact and to Mechanical Shocks.
Mechanical agencies, such as pressure, blows, or shaking, produce
movements in many cases, including the pronounced variation movements
FIG. 19. Stem and leaves of Mimosa pudica. The leaf A is fully expanded, whereas the leaf B has been stimu-
lated ; p = primary pulvinus, * = secondary pulvini ; the tertiary pulvini at the bases of the leaflets are not shown.
shown by plants possessing motile pulvini, as in the Papilionaceae,
Mimoseae, and Oxalidaceae. (Cf. Figs. 19-24.) The response to stimu-
lation is especially rapid in the leaves of Mimosa pudica, and of Desman-
thus plenus. The leaves of the first-named plant rapidly pass from the
unstimulated (Fig. 19, A) to the stimulated condition (Fig. 19, B) when
62
MOVEMENTS OF CURVATURE
the plant is shaken, the main petiole sinking, the secondary petioles
becoming less spreading, and the leaflets folding up in pairs. If the tip
of a single leaflet is cut off, the stimulus first affects its pulvinus, but
then spreads down the leaf-segments, the leaflets folding up in pairs, and
then to the other segments and to the main pulvinus until the whole
leaf is in the stimulated condition. The leaves of Biophytum sensitivum
also respond readily, whereas repeated strong shaking is necessary to
produce a complete sinking of the leaves of Oxalis acetosella (Fig. 20).
The leaflets of Robinia pseudacacia are still less sensitive, and the strongest
shaking only produces a slight movement in the leaflets of Acacia lo-
phantha, although in the Tropics the sensitiveness may approach that of
Mimosa 1.
The power of response varies much among the stamens of different
FlG. 20. Trifoliate leaf of Oxalis acetosella. A, unstimulated ;
Z?, after repeated strong shaking. The pulvini are shown at g.
FIG. 21. Flower of Centanrea jacea after
the removal of the corolla. The stamens are
shown at A in the unstimulated, at B in the
stimulated condition (magnified). c= corolla
tube ; J = filaments ; a = anther tube ; g= stigma.
Cynareae, those of Centaurea jacea and Cynara scolymus suddenly drawing
together when stimulated by contact and at the same time becoming
10 to 30 per cent, shorter. The similar movement of all five filaments
pulls down the anther tube in which the pollen lies and causes the style
to push out pollen and protrude at the apex. In this case stimulation
produces a shortening as in a muscle, but when the active tissue is
appropriately joined to inactive or elastic tissue a curvature may be
produced as in Mimosa. The active region need not always be swollen
like a pulvinus, and indeed the irritable stamens of Berberidaceae (Fig. 22),
of Cistaceae, and of Sparmannia, as well as the stigmas of Mimulus
Ewart, Annals of Botany, 1897, Vol. xi, p. 455.
IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 63
(Fig. 23), of Martynia and of Bignonia show no external structural sign
of their power of rapid movement on stimulation. Furthermore the
sudden closure of the leaf of Dionaea muscipula (Fig. 24) and of Aldro-
vanda is produced by the influence of contact acting on the midrib and
lamina.
In all these cases the responding organ is also the percipient one,
but in Masdevallia muscosa, according to Oliver, the movement of the
labellum is produced by touching the neighbouring part of the flower
and not by touching the motile zone1.
The above-named plants respond to any sufficiently intense mechanical
shock or disturbance, whether produced by wind, rain, contact with solid
bodies, or vibrations propagated through the soil. They may hence be
FIG. 22. A flower of Berberis vulgaris after the re- FIG. 23. Longitudinal sections of the flower of
moval of the anterior petals and stamens (magnified). Mimulus luteus. In A the stigmas (») are unstimu-
The stamen (a) is unstimulated, but contact has caused the lated, in B& touch has caused them to close together,
stamen (d) to curve over and apply itself to the stigma (g).
said to possess a seismonic irritability as distinguished from the sense of
touch (contact or thigmotropic irritability) shown by tendrils, by certain
algae and fungi, as well as by the tentacles of Drosera. In these cases
a response is produced only by contact with a solid body, whereas the
strongest bending or shaking caused by wind, water, or the impact of
a thread of mercury, as well as rubbing with a wet rod covered with
10 or 15 per cent, gelatine, fail to awaken any irritable response. At the
same time sensitive tendrils respond to the lightest contact with a solid
body, such as fails to awaken any response in the highly irritable leaflets
of Mimosa. The tentacles of Drosera are almost as sensitive as tendrils,
the head of the tentacle perceiving the stimulus to which the stalk responds
by bending 2.
1 Oliver, Annals of Botany, 1888, Vol. I, p. 244.
2 For details see Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1885, Bd. i, p. 483.
MOVEMENTS OF CURVATURE
By means of rubbing with a wet rod covered with gelatine, and with an
ordinary wooden one, it can easily be determined whether an organ shows
contact or seismonic irritability, for we are dealing here with as distinct
types of irritability as in the geotropism or heliotropism of a root or stem.
The distinction would still be justified even if subsequently the two forms
of irritability should be found to be closely related, for we are dealing
here with collective terms for types of response varying as regards their
character and mode of production. Since, however, the distinction is
primarily based upon the perception or non-perception of the exciting
agency, it is immaterial whether the response is rapid or slow, and whether
it takes the form of a curvature, of a secondary thickening, or of a pro-
duction of haustoria or other attaching organs.
Although the details of the mode of perception are still unexplained,
it is impossible to deny
that the sensation of con-
tact is produced under
similar conditions in both
plants and animals l. In
both cases a stimulation
is only exercised when
unequal pressure is ex-
erted at different points,
so that local variations of
pressure are produced.
It is not the statical,
pressure but the rubbing
against the solid body
which acts as a stimulus,
but changing local variations of pressure produced without lateral move-
ment may also act as an excitation, as in the hooks of tropical climbers,
or when a weighted cork stuck full of pins is allowed to rest upon the
skin and its centre of gravity laterally displaced. A tickling sensation
FlG. 24. Leaves of Dtonaea muscipula, A unstimulated and showing
the three sensitive hairs on each leaf-lobe, B stimulated leaf which has
closed and captured an earwig.
1 Cf. Pfeffer, 1. c., p. 499. On the sensation of contact in man cf. Tigerstedt, Physiologic d.
Menschen, 1898, Bd. n, p. 71 ; Frey u. Kiesow, Zeitechr. f. Psychologic und Physiol. d. Sinnes-
organe, 1899, Bd. XX, p. 126. In plants direct contact with the cell-wall is necessary, and hence no
stimulus is exercised when direct contact is prevented by the interposition of a layer of gelatine or
mucilage. Cf. Pfeffer, 1. c., p. 513.
[The anatomical studies of Haberlandt (Sinnesorgane im Pflanzenreich, 1901, p. 117) have
brought nothing essentially new to light. The statement (I.e., p. 122) that only a tangential
stretching of the ectoplasmic membrane of the protoplasm is capable of producing an excitation is
not supported by the facts. Thus sudden and pronounced curvatures produced by the aid of gelatine-
covered rods do not exercise any stimulating action on tendrils, whereas the gentle movement of
a thread weighing 0-00025 of a milligram does so and can obviously produce only a minimal amount
of tangential stretching. The fact that sharp local inward bending of the outwardly curving epi-
dermal walls may produce a stimulatory response has already been pointed out by Pfeffer.]
IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 65
is awakened in the epidermis of man and of tendrils by weak induction
shocks, and furthermore, rubbing against a rough body acts in both cases
as a stronger excitation than rubbing with similar pressure against a smooth
one. In general the intensity of the excitation depends upon the amount
of surface in contact, upon the magnitude of the local variations of pressure,
and upon the rapidity with which they alter. The determination of the
numerical relations between these factors and the strength of the excitation
affords, however, no explanation of the actual nature of the sensation of
contact.
In any case the deformations produced by varying local pressure in
the outer cell-walls of the epidermis create the conditions for an excitation
of the irritable protoplasm, which does not come into contact with the
object exercising pressure any more than in the case of the touch-corpuscles
in the skin of animals. The structure of the cell and cell-wall may therefore
aid considerably in the perception of the stimulus, although an excitation
is only possible when the protoplasm is endowed with this special form
of irritability. The pits which occur in the outer walls of the epidermis
in the tendrils of Cucurbitaceae and a few other plants undoubtedly act
in this way. Since, however, similar pits are present in the non-sensitive
portion of the tendril of Bryonia, it is evident that their presence does
not confer this special form of irritability upon the protoplasm of all cells
possessing them. Furthermore no pits are present in the epidermal walls
of the very sensitive tendrils of Passiflora and Cobaea \ and in some motile
organisms only a portion of the cilia are sensitive to contact, although
here the sensitive protoplasmic organs come into direct contact with foreign
bodies.
It is, however, uncertain whether differences of pressure in the proto-
plasm act as the exciting stimuli, and also whether the entire protoplasm
or only the peripheral membrane, or only portions of the latter are able
to perceive contact stimuli. Even in the latter case, however, it is hardly
to be expected that so high a differentiation should be reached as in
the Pacinian or touch- corpuscles of vertebrate animals. A knowledge
of the nature and position .of the percipient organs does not, however,
reveal the mode of perception of the stimulus.
An organ having seismonic irritability responds to every variation
of pressure if sufficiently intense, quite independently of its origin. Certain
highly sensitive plants even respond to sudden variations in the atmospheric
pressure, or to sudden changes of temperature, or to rapid alterati'ons
of transpiration and to the resulting water-currents 2. The stimulus may
1 Pfeffer, 1. c., p. 524. Haberlandt, Physiol. Anat., 2. Aufl., 1896, p. 478; Haberlandt, Sinnes-
organe im Pflanzenreich, 1901, p. 126 ; Strasburger, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 515.
3 Long known in the case of Mimosa pudica. Munk (Die elektrischen- und Bewegungserschein-
ungen am Blatte von Dionaea, 1876, p. 105) observed that a sudden increase of transpiration acted
PFEFFER. lit T?
66 MOVEMENTS OF CURVATURE
apparently be perceived in the internal living cells as well as in the
peripheral ones, and possibly many plants may exist in which the epidermal
cells are quite insensitive to seismonic stimulation. Even in the case
of tendrils it is uncertain whether the subjacent cortex is able to perceive
contact-stimuli as well as the epidermis, or whether the latter only has this
special form of irritability. A contact-stimulus may easily be localized in
the epidermal cells as regards its application, but a blow or shaking almost
unavoidably affects the cortical cells as well as the epidermis. In every
case the change of pressure must be rapid even though transitory, for
statical pressure as well as gradual changes of pressure or tension are
inoperative as stimuli. In this respect seismonic irritability agrees with
contact-irritability, which requires for its excitation special pressure relation-
ships. Hence it is hardly surprising that tendrils should not be stimulated
by the strongest bending or twisting, so long as the latter fail to produce
the localized pressure-gradients required for excitation.
Both seismonic and contact irritability may, like geotropism and helio-
tropism, be developed in the same organ, and this may possibly be the case
in the leaf of Dionaea muscipula.
The flaccidity and the transitory
disturbances of growth produced by
mechanical agencies may be re-
garded as the result of seismonic
stimulation, and in this sense this
, special form of irritability is pos-
FIG. 25. Epidermal cells from a longitudinal section of *
the tendril of Cucumis sativus, showing the pits in the SCSSed tO E limited degree by all
outer walls.
growing organs including tendrils.
It is difficult to decide whether Mimosa pudica has a feeble contact-
irritability, since every mechanical agency of any intensity excites the usual
seismonic response.
Mechanical agencies probably awaken more or less feeble reactions
in all plants, and it has already been mentioned that in addition to the
special seismonic irritability, other forms of sensitivity to mechanical
agencies may be developed. Indeed, all stimulation resulting from
movements of water, or from other forms of movement in the plant,
may be termed mechanical, while if geotropic irritability is awakened
by the changes in the position of the denser particles of the cell it becomes
closely related in character to a form of internal contact-irritability. The
manner in which currents of water exercise a rheotropic stimulus is quite
uncertain, but it also is probably akin to a form of contact-stimulation.
as a stimulus to the leaf of Dionaea muscipula. [The streaming-cells of Chara and Nitella possess
very pronounced seismonic irritability, although here the response is not a movement but a cessation
of movement. Less pronounced seismonic irritability is shown by streaming-cells in general. Cf.
Ewart, Protoplasmic Streaming in Plants, 1903, p. 72.]
IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 67
The shape and relationships of the cell and cell-wall, as well as the way in which
the cells are joined and arranged in the tissues, may render the perception of the
stimulus more readily possible at particular points, but do not produce this special
form of irritability. The production and activity of the response are, however,
dependent in a much greater degree upon the structure of the organ, but the primary
perception always takes place in the sensitive protoplasm. The impermeability of
the cell- wall or of an intervening tissue may render it difficult or impossible for a sub-
stance to exert any chemical stimulation, or may restrict its action largely or entirely
to those points where the substance is able to penetrate. Differences in the trans-
parency of the tissues must act in the same way in regard to light stimuli, and hence it
arises that a seedling performs a heliotropic curvature in diffuse light if one side is
covered with indian ink. Similarly the presence of thick walls, or of resistant tissues,
may render the sensitive cells beneath less responsive or not responsive at all to blows
or pressure. Furthermore, the arrangement of the tissues may be such that pressure
and tension exercise different stimulatory actions, or may cause contact at a particular
region to produce a response especially readily as in the case of the sensitive haiis of
Dionaea. This is probably because pressure at these points is more readily trans-
mitted to the sensitive cells beneath \ The best knowledge of the structure of an
irritable organ will not reveal the nature of irritability, and in fact organs with a pro-
nounced similarity of structure may possess widely dissimilar irritabilities, while the
same sensitivity may be shown by organs differing widely in structure. Furthermore,
various special irritabilities may reside in cells and tissues which differ in no anatomical
features from ordinary indifferent cells and tissues 2. It <is also easy to see that the
coarser anatomical structure can more readily favour the perception of mechanical
stimuli, than of thermal or photic stimuli ; and the observed facts bear out this con-
clusion. It must, however, be remembered that the mere enumeration of all the
observed cases in which the anatomical structure shows a biological adaptation for
the reception of stimuli leads one to attach undue importance to structure, and as
a matter of fact in most cases the structure shows no perceptible adaptation for
sensory perception. In any case physiology is only concerned with structure in so
far as it affects functional -activity 3.
SECTION 15 (continued).
Since the distinction between seismonic and contact irritability is
purely a matter of special sensitivity, it remains an open question whether
both forms of stimuli involve similar or dissimilar primary reactions.
Seismonic stimulation usually produces variation movements, but contact-
1 The first interactions may be purely physical or chemical, and may act as a preparation for the
subsequent physiological perception. When purely mechanical transmission is performed by hairs
or the like, Haberlandt (Sinnesorgane im Pflanzemeich, 1901, p. 9) terms the intermediary structures
4 stimulators.'
ta All cells and organs capable of perceiving stimuli may be termed sense-organs, whether they
show any special anatomical structure or not.
8 On problems of this kind see Haberlandt, 1. c., 1901.
68 MOVEMENTS OF CURVATURE
stimulation growth curvatures. The closure of the leaf of Dionaea, however,
due to seismonic stimulation is partly produced by growth l. On the other
hand, the disturbances of growth in growing shoots produced by shaking
are to be regarded as the result of seismonic stimulation. Although at
present only nutation curvatures are known to result from contact-stimula-
tion, it is hardly to be expected that the potential powers of the plant
should find expression in this direction alone, and in fact we have in the
secondary thickening of the hooks and tendrils of many tropical climbers
induced by contact a special response which may or may not be accom-
panied by curvature. Furthermore, the movements produced in the cilia
of certain organisms by contact-stimuli are not due to growth, but are the
result of contractility, just as the movements of an animal produced by
a tickling sensation are due to muscular contraction.
In regard to sensitivity, the duration of the latent period, and the
rapidity of the reaction, no definite line of demarcation' can be drawn
. between seismon c and contact-stimulatioa It is true that the latter never
produces so rapid a reaction as occurs in the leaf of Mimosa pudica^ in
which, under favourable conditions the latent period may be less than
a second, while the sinking of the primary petiole and the folding of
a pair of leaflets may be performed in two to five seconds. The stamens
of Centaurea jacea and the leaves of Dionaea muscipula move with about
the same rapidity. Burdon-Sanderson 2 found that at 20° C., when the
leaves of the latter plant are moderately responsive, the latent period after
mechanical stimulation was about one second, and the closure of the leaf-
lobes required five to six seconds. Sensitive tendrils may, however, begin
to curve five to twenty seconds after contact-stimulation, so that the
reaction is more rapid than the movements produced by seismonic stimu-
lation in less sensitive plants such as Robinia> Oxalis> and Acacia lophantha.
Since the sensitivity and power of reaction are largely dependent upon the
stage of development and upon the external conditions, their precise
determination is of subordinate interest and importance. It is however
worthy of note, that under special conditions Mimosa pudica may show
only a slow and feeble power of reaction, while when the plant has been
kept for some time at a low temperature, such as 5° to 10° C., it temporarily
or permanently loses the power of response to seismonic stimuli.
In the case of the leaves of Mimosa pzidica and the stamens of Cynareae
and Berberis every successful stimulation excites the full amplitude of
movement. This is however not always the case, for even the strongest
mechanical stimulation only produces a partial folding or drooping of the
1 How far the curvature of the pulvini of Mimosa pudica is a matter of growth is uncertain^
but the latter does appear to take part in the performance of many sleep movements.
a Burdon-Sanderson, Phil. Trans., 1882, Pt. I (p. 48 of reprint); Biol. Centralbl., 1882, Bd. II,
p. 497.
IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 69
leaves of Robinia or of Oxalis. A strong blow also acts as a submaximal
excitation upon the leaves of Mimosa pudica when their irritability has been
diminished by keeping the plant at a low temperature *.
It is obviously advantageous that the response should be more marked
when the stimulus is more intense or prolongedror is increased by repetition
and summation. This applies more especially to organs endowed with
contact-irritability, for in this way they are enabled to a certain extent to
so adapt their response as to perform their special function in the best
possible manner. A few touches usually suffice to produce a distinct
reaction, although in very sensitive tendrils a single contact, if sufficiently
intense, will produce a response, while three or four touches are required to
produce a curvature in the highly-sensitive tendrils of Drosera*. Even
a single contact, however, may represent a series of local variations of
pressure, and it cannot be denied that a sudden maximal explosive
movement is better attained by the release of strains previously prepared,
than by changes in the rate of growth.
As in other cases the result of a transitory mechanical stimulation may
gradually disappear, whereas when the stimulus is continuous the new
position of equilibrium assumed will depend upon the intensity of the
stimulus, upon the awakened counter-actions, and upon the accommodation
of the plant to the stimulus, which is mainly due to its depressed excitability.
So long as the plant maintains the position induced by stimulation and
reacts to a rise in the intensity of the same stimulus, no accommodation
other than that involved in a certain depression of the excitability can take
place. This latter appears to be of general occurrence ; and in many cases,
as, for example, in the leaves of Mimosa pudica> it goes so far that the
stimulated organ in spite of the continued application of mechanical or of
weak induction shocks returns to its original position and is no longer
responsive to mechanical excitation 3. If the return to the original position
has taken place during the continued application of gentle shaking, the
sensibility is only weakened and an increase in the intensity of the
mechanical shocks brings about the usual movement. It is owing to these
facts that some authors have found that continually-shaken -plants of
1 Pfeffer, Physiol. Unters., 1873, p. 69 ; Unters. a. d. bot. lust, zu Tubingen, 1885, Bd. I, p. 520 ;
Macfarlane, Biological kctures, 1894, P- I9°- According to G. Haberlandt (Ann. du Jard. bot. de
Buitenzorg, 1898, Suppl. n, p. 35) gentle rubbing excites a sub-maximal movement in the leaves of
Biophytum sensitivum. In such circumstances the movement may be produced by repeated stimu-
lation as in the case of tendrils, although single stimuli may be ineffective. According to Burdon-
Sanderson (Proceedings of the Royal Society, 1877, Vol. xxv, p. 411) the sudden maximal move-
ment of the leaves of Dionaea imiscipula may be excited by the summation of the action of repeated
gentle blows. Cf. also Darwin, Insectivorous Plants. Macfarlane's statement (1. c., p. 187) that at
least two blows are required to produce a response in Dionaea miiscipula appears only to apply
under special conditions.
3 Darwin, Insectivorous Plants, 1875, P- J9-
8 Pfeffer, Physiol. Unters., 1873, p. 56 ; Unters. a. d. bot. Inst. zu Tubingen, 1885, Bd. I, p. 521.
7o
MOVEMENTS OF CURVATURE
Mimosa were irresponsive to blows, whereas others found that they remained
sensitive l.
A single stimulation of -the pulvinus of Mimosa causes its irritability
to be transitorily suppressed during the return movement, and it is only
gradually restored after the jeturn-movement has been completed. Hence
the same stimulus induces at first a feeble, and later a pronounced response 2,
and if gentle blows are struck on the primary pulvinus at intervals of three
to five minutes, the irritability is sufficiently restored during the intervals
to enable each stimulus to produce a moderate response. During the
period of insensitivity following mechanical stimulation, the pulvinus of
Mimosa remains irritable to photonastic, heliotropic, and other stimuli, so
that the absence of a response to mechanical stimuli is due to the temporary
inhibition of the power of perceiving such4 stimuli, and not to the motor-
mechanism being temporarily ineffective. Nothing is, however, known as to
the way in which this special sensitivity is suppressed and restored.
It is hardly to be expected that all sensitive plants should react
in this respect in a precisely similar fashion to Mimosa, but in general
any sudden explosive stimulatory reaction appears often to be followed
by a more or less transitory diminution of excitability. This applies to
the stamens of Cynareae, although here the excitability soon returns, and
is partly restored before 'the stamens have re-expanded3. A complete
suppression of excitability does not always follow as the result of stimu-
lation, for Pfeffer4 has shown that the leaves of Oxalis remain excitable
during the return movement. In the same way the voluntary muscles
of animals can be kept permanently contracted in a condition of tetanus
by rapidly repeated stimuli.
On the other hand, Cuscuta affords an instance in which stimulation
induces a periodic inhibition of the contact-irritability. The tentacles of
Drosera^ however, remain permanently irritable, although the sensitivity
is so far decreased by stimulation, that a weak continuous stimulus is
unable to produce a permanent curvature, the tentacles gradually straighten-
ing again5. It is highly probable that further specific peculiarities will
be discovered, and investigations in this direction are likely to throw light
upon the phenomena of irritability in general.
Both the stimulatory and the return movements begin slowly, increase
to a maximum and then gradually cease, while not only in the case of
Mimosa, but also where the movement is slow, the response to stimulation
takes place more rapidly than the return movement. The occurrence of
1 The literature is given by Pfeffer, 1. c., 1873, p. 56.
3 Pfeffer, I.e., 1873, p. 60.
3 Cohn, Abhdlg. d. schles. Ges. f. vaterl. Cultur, 1861, Heft i, p. 16.
* Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1885, Bd. I, p. 521.
5 Cf. Pfeffer, 1. c., 1885, p. 514.
IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 71
oscillations during movement is partly the result of the nature of the
motor-mechanism, and is partly due to the induction of opposed reactions
by the realized movement. The extent of the maximal movement in
Mimosa is determined by the diminution of the energy of contraction
and the increase of the mechanical resistance as the curvature progresses.
Similarly, in tendrils under sub-maximal stimulation, the curvature ceases
as soon as the stimulation is balanced by the counter-actions, although
an additional curvature is possible when the stimulus is increased.
Few detailed observations upon the progress of movement have been made.
Bert l found, for instance, that the end of the primary petiole of Mimosa had sunk
22 mm. 7 seconds after stimulation, but that on the commencement of the return
movement it rose 4 mm. in the first minute, 4-5 mm. in the second, 3 mm. in each of
the third, fourth, and fifth minutes, 2 mm. in the sixth, i mm. in the eighth, and
0-5 mm. in the ninth minute.
Uses. The importance of the movements of tendrils for purposes of attachment
does not need to be emphasized. The movements of stamens and stigmas induced
by seismonic stimuli are usually for the purpose of ensuring the transference or
reception of pollen, while in carnivorous plants the responses to seismonic, chemical
and contact-stimuli are especially connected with the capture and digestion of insects.
The extremely readily induced movements of Mimosa pudica and similar plants pro-
bably aid in keeping off large browsing animals such as goats and camels, and may
also be of use in warding off the attacks of injurious insects. One can often see how
goats, after the first tug at a bush of Mimosa, seek less bewildering pasturage, and
how a surprised fly hastens from a leaf on which his descent has excited a move-
ment 2. The folded leaflets and drooping leaves of Mimosa pudica are less readily
injured by rain and hail, while the re-expansion on continued stimulation aids in
avoiding a prolonged derangement of the functional activity of the leaf.
It is uncertain whether the slow response of the leaflets of Oxalis to mechanical
stimuli has any biological utility, for the leaves are not more readily injured by
mechanical agencies than other non-irritable ones.
SECTION 16. Movements produced by Mechanical Stimuli.
The mechanism of movement has been studied most in the cases
of the stamens of Cynareae and the pulvini of Mimosa, and as far as we
know similar mechanisms are employed in other motile organs which
1 Bert, Mem. de 1'Acad. de Bordeaux, 1870, T. vn, p. 41. A similar progress was observed
by Cohn (Abhdlg. d. schles. Ges. f. vaterl. Cultur, 1861, Heft I, p. 13) in the stamens of Cynareae,
and by Burdon- Sanderson (Proc. of the Royal Society, 1877, Vol< xxv» P« 4*6; Pnil< Trans., 1882,
p. 48 of the reprint) in the leaf of Dionaea muscipula.
3 See Johow, Kosmos, 1884, Bd. II, p. 124; G. Haberlandt, Tropenreise, 1893, p. 36; Ewart,
Annals of Botany, 1897, Vol. xi, p. 339 (Protective movements of leaflets); Burgerstein, Wiener
illustrirte Gartenzeitung, Marz 1898.
72 MOVEMENTS OF CURVATURE
respond to mechanical stimuli. Actual experiment is required, however,
in each case before any final conclusion can be made, since similar move-
ments may be produced in various ways. Although the movement of
the leaf of Dionaea appears to be accompanied by growth, it is nevertheless
possible that the cell-mechanism may be the same as in the irritable stamens
of Cynareae and in the pulvini of Mimosa pudica. It is indeed possible
that every movement of the young pulvinus may be accompanied by
growth-changes, whereas when adult pure movements of variation may
take place. As was shown by Pfeffer \ the movements both of the leaves
of Mimosa pudica and of the stamens of Cynareae result from the fact
that stimulation induces a sudden fall of turgor, and hence a sudden
equilibration of the elastic stresses in the motile organ, which are gradually
reproduced as. the original turgor is restored. The phenomenon can best
be followed in the stamens of Cynareae, of which those of Centaur ea jacea
shorten by 10 to 30 per cent., and those of Cynara scolymus by 8 to 20
per cent, of their length when stimulated by a touch. The whole length
of the filament takes an equal part in this contraction, with the exception
of the two extremities where less shortening is shown. An isolated stamen
remains capable of contraction, and when stimulated performs lateral curva-
tures or convolutions.
The construction of the filament from longitudinal rows of cylindrical
cells symmetrically disposed around the central vascular bundles results
in a close correspondence between the degree of contraction of the individual
cells and of the whole filament. Direct measurements have established
the fact that the epidermal and neighbouring parenchyma cells do actually
shorten, but retain their original transverse diameter and experience no
lateral curvature. The fall of turgor in the cells by lessening the tangential
stretching compensates for and prevents the broadening which would other-
wise result from the shortening of the cell2. Hence when the filament
shortens by 20 per cent, of its length, the individual cells also become one-
fifth shorter and hence correspondingly decrease in volume. This involves
an escape of water from the cells into the intercellular spaces, the displaced
air streaming away through the communicating intercellular spaces so
that its compression is avoided. If the filament is injected with water,
a drop of liquid exudes from the cut end when the stamen shortens on
excitation, although the shortening is less than before. This water appears
to escape from the intercellular spaces of the parenchyma, and hence it
is easy to understand how the stamens of Centaurea jacea and of Cynara
scolymus are able to shorten without increasing in diameter3. It is,
1 Pfeffer, Physiol. Unters., 1873; Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 325.
3 Pfeffer, 1. c., 1873, p. 96.
3 Pfeffer, 1. c., 1873, p. 89. The methods of measurement of other authors and a criticism
of them is given in this work. The matter is in no wise altered by the apparently somewhat
MOVEMENTS PRODUCED BY MECHANICAL STIMULI
73
however, always possible that the contraction of other stamens may involve
a decrease or increase in diameter. These facts, together with the absence
of any active contraction of the protoplasts, and of any transitory increase
in the elasticity of the stretched cell-walls, suffice to show that the shortening
is due to a fall of turgor, and the subsequent re-expansion to its gradual
restoration. The energy of contraction as determined by comparing the
maximal load with the area of cross-section of the filament amounts to
as much as i or 3 atmospheres. Hence it cannot possibly be produced
by an active contraction of a viscous fluid like the protoplasm \ and the
diameters of the cells are too
great to enable changes in the
peripheral surface tension to have
much effect.
The filament when con-
tracted possesses the same elas-
ticity as when expanded and
rendered non-irritable by chloro-
form. Further, the same weight
which is required to stretch a
contracted filament to its original
length also suffices to prevent
any contraction. Hence it is
obvious that no increase in the
elasticity of the cell-walls occurs
during contraction, although by
raising the pressure exerted by
the cell-wall against the internal
osmotic pressure this might pro-
duce a contraction of the cell ac-
companied by an outward filtration of water under pressure 2. It is evident
therefore that changes in the osmotic pressure are solely responsible for
the contraction, although, since these are only temporary in character, they
cannot be detected by plasmolytic methods3.'
The reason for the pronounced contraction resulting from a fall of
turgor lies in the fact that the cell-walls are as extensible as india-rubber,
and when not under any permanent tension can be stretched to double
their length without their limit of elasticity being passed, that is, without
undergoing any permanent stretching. Even when fully turgid the cell-
FiG. 26. A portion of the longitudinal half of a filament of
Centaurea montana (magnified), g — vascular bundle, p =
parenchyma, e = epidermis, i = intercellular spaces, h = hairs.
careless experiments of Schenkemeyer, Ueber die Contraction der Filamente von Centaurea, Breslauer
Dissertation, 1877.
1 Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, pp. 326, 329.
8 Pfeffer, Physiol. Unters., 1873, pp. no, 117; also 1. c., 1890, p. 327.
3 Pfeffer, 1. c., 1890, p 327.
74 MOVEMENTS OF CURVATURE
walls are not stretched to this extent, and hence a chloroformed filament
undergoes a considerable elastic elongation when weights are attached
to it. On the other hand, when a contracted filament is suddenly killed
by dropping it into boiling water, it undergoes an additional shortening
of 10 to 40 per cent, of its length, owing to the fact that the previous
stimulation caused a fall of turgor but not its entire removal. This
naturally applies only when the filament is highly irritable and before
the fall of turgor which precedes death has begun. A shortening corre-
sponding to that produced by excitation results from the action of an
injected solution of 0-5 to i per cent, potassium nitrate, which diminishes
the osmotic pressure in the cell by 17 to 3-5 atmospheres.
These general considerations are not affected by the fact that the
realized movement . of the filament results from the interaction of dissimilar
cells, for the association of the active cells with passive ones merely acts
like an increase in the thickness of the cell-wall and not only diminishes
the extent of the contraction produced by the available energy, but also
lessens the elastic stretching produced when turgor is restored. Presumably
not only the parenchyma, but also the epidermal cells and possibly also
the living cells of the vascular bundles are all active agents in producing
the contraction l. If this were not the case and only a limited number
of cells were active, we could hardly have so pronounced an energy of
contraction per unit area as is actually shown. The fact that the epidermis
and vascular bundles are under tension both in the contracted and uncon-
tracted conditions is the direct result of the fact that only a diminution
and not a removal of turgor is involved. In fact a further fall or an entire
removal of turgor causes a longitudinal compression of the vascular bundle,
and allows the walls of the parenchyma cells to show wavy bulgings 2.
The fall of turgor allows the stretched cell- wall to contract until the
decreasing tension of the wall is balanced by the internal osmotic pressure,
which rises somewhat as water escapes and the sap becomes consequently
more concentrated. A renewed production of osmotic materials causes
the extruded water to be again absorbed and the cell to be once more
distended and ready to respond to excitation. The mechanism can there-
fore be compared to an india-rubber tube in whose walls a spiral wire
is imbedded, so that on forcing in water under pressure the tube is
distended longitudinally but not transversely, and shortens when some
of the water is allowed to escape. The cell- walls do actually permit of
the rapid filtration through them of water under pressure, required to allow
sudden contraction.
Since a perceptible diminution in the size of a cell can only be
1 PfefTer, 1. c., pp. 102, 112. An excitation is produced not only by touching the hairs, but also-
the epidermal cells free from hairs. See also Haberlandt, Sinnesorgane im Pflanzenreich, 1901, p. 35.
2 Pfeffer, I.e., p. 114.
MOVEMENTS PRODUCED BY MECHANICAL STIMULI 75
produced by a fall of turgor when the cell-wall was previously stretched
sufficiently, it is possible that in certain cases no response may be shown
although the cells react as in the filaments of Cynareae. This special
irritability is, however, certainly not a general phenomenon, and the stamens
of Helianthus annum, for instance, have no seismonic irritability although
the cell-walls undergo a considerable elastic stretching when the cells are
fully turgid1.
The protoplast remains closely pressed against the cell-wall of a
stimulated cell, and this is still the case, even when a stimulated staminal
filament is loaded with a weight sufficient to prevent any contraction.
The retraction of the protoplasm from the cell-wall, such as occurs during
rejuvenescence, necessitates that the centrally-directed pressure exercised
by the protoplasm should be greater than the osmotic pressure of the
cell-sap, which cannot therefore be very great. This must also be the
case when, as Schlitt and also Benecke found, the protoplast of a Diatom
subjected to mechanical and other stimuli contracts away from the cell-
wall2. It is possible that this stimulatory plasmolysis may be the result
of a sudden change of permeability in the plasmatic membranes allowing
the osmotic materials in the cell to escape.
Stimulation also causes a fall of turgor in the under half of the
dorsiventral primary pulvinus of Mimosa pudica. The change of inclination
of the petiole is so great as to need a pronounced curvature of the pulvinus.
This, though aided by the mechanical moment exercised by the leaf-
segments, is mainly produced by an active contraction of the cells in
the under stimulated side, which cells are compressed by the expansion
of the upper turgid half of the pulvinus until equilibrium is restored.
The original condition of turgor is then gradually reproduced in the lower
half of the pulvinus which expands, raising the leaf and producing the
compression of the upper half of the pulvinus which aids in the rapid
curvature of the stimulated pulvinus3.
After the upper half of the pulvinus has been carefully removed no
movement is produced by stimulation, whereas when the lower half is
1 Pfeffer, 1. c., p. 107.
2 Schiitt, Die Peridineen der Planktonexpedition, 1895, p. no; Beneclce, Jahrb. f. wiss. Bot,
1901, Bd. xxxv, p. 554. According to Nageli (Tflanzenphysiol. Unters., 1855, Heft i, p. 13)
mechanical pressure causes in Spirogyra, and according to Hofmeister (Pflanzenzelle, 1867, p. 303)
in Nitella, a withdrawal of the protoplasm from the cell-wall. It remains, however, to be seen
whether we are dealing here with stimulatory functions or with the results of mechanical injury, and
the observations of Schiitt and Benecke require further proof.
* For details concerning the structure and mechanics of the pulvinus of Mimosa see Pfeffer,
Physiol. Unters., 1873, p. 9; Haberlandt, Das reizleitende Gewebesystem der Sinnpflanze, 1890,
p. 23 ; Physiol. Anat., 2. Aufl., 1896, p. 475 ; Sinnesorgane im Pflanzenreich, 1901, p. 38 ; Schwen-
dener, 1897, Gesammelte Abhandlungen, Bd. II, p. 211. On the structure and mechanics of the
pulvini of the leaflets cf. Schwendener, 1. c., p. 236.
76 MOVEMENTS OF CURVATURE
absent a weakened power of movement is retained l. Since, however,
the operation undoubtedly affects the irritability, it is impossible to deter-
mine from such experiments the exact part played by the active contraction
of the lower half of the pulvinus. Nor is it certain whether all the different
cells and tissues of this zone are equally excitable. The parenchyma
cells around the vascular bundles appear in fact to be of primary im-
portance, but the epidermal cells may also take part in the contraction,
although their tangential tension is converted into a tangential pressure,
that is, they are compressed instead of being stretched where a strong
curvature is produced. The way in which stimuli may be conducted from
one pulvinus to another, as well as the fact that the pulvinus may per-
ceptibly react after the epidermis has been removed, suffice to show that
the cortical cells may be stimulated without the aid of the epidermis.
The latter may also receive an excitation2, and contact with the hairs
alone is able to excite a response in the pulvinus. The hairs probably
only act by readily transmitting the pressures to the cells beneath, and
hence behave as * stimulators ' in Haberlandt's sense of the term. The
fact that gentle direct contact on the under half of the pulvinus may
act as an excitation points to the direct excitability of the epidermal cells,
for a much greater pressure must be applied or a more violent blow struck
upon the upper epidermis of the pulvinus in order to produce an excitation
of the under half.
The contraction and diminution of volume of the pulvinar cells of
Mimosa cannot be directly observed, but they are indicated by the escape
of water from the reacting cells, as in the stamens of Cynareae. This
water partly fills the intercellular spaces and is partly conducted into
the neighbouring tissues of the stem and petiole 3, and possibly also a little
may pass into the vascular bundles. If the leaf-stalk is separated from
the pulvinus by a sharp cut, and the still attached pulvinus kept in moist
air, on stimulation water escapes from the cut surface, and at first from
the inner, but not from the inmost layers of parenchyma in the lower
half of the pulvinus. A little later some water also escapes from corre-
sponding cells in the upper half of the pulvinus.
This displacement of air and water causes the under half of a stimulated
pulvinus to increase in volume as determined by micrometer measurements,
1 The observations of Pfeffer and of other observers are given in full in Pfeffer's Physiol. Unters.,
1873-
2 The researches of Borzi (L'apparato di moto delle sensitive, 1899, p. 17, reprint from Rivista
di Scienze Biologiche, Vol. iv) fail to reveal the distribution of sensitivity in the tissues. Cf. Haber-
landt, 1. c., 1901, p. 79. The latter author (p. 88) concludes that in the case of Biophytum sensitivum
the hairs on the pulvini directly perceive stimuli.
3 Hence arises the fact that Bonnier (Revue generate de bot., 1892, T. iv, p. 512) observed
slight variations of the air pressure during a stimulatory movement, when a manometer was inserted
in the stem of Mimosa pudica near to the origin of the pulvinus.
MOVEMENTS PRODUCED BY MECHANICAL STIMULI 77
whereas the elongating upper half slightly decreases in volume1. The
displacement of the intercellular air by water is also shown by the sudden
darkening following stimulation, just as occurs when the pulvinus is injected
with water, and as is also shown in the under half of the pulvinus when
the movement is mechanically arrested2. The presence of intercellular
spaces in the inner layers of parenchyma facilitates the rapid extension
and removal of water, but nevertheless the outer layers may also give
off water with sufficient rapidity, although no system of communicating
air-spaces exists between them3. The anatomy of the tissues does not
therefore enable us to conclude that the outer layers of parenchyma are
inactive or less active than the inner layers.
Additional and important evidence to show that the movement is pro-
duced by a fall of turgor is given by the fact that the stimulated pulvinus
is more flaccid and less rigid than the unstimulated one. This can be
shown by determining in each case the deviation of the angles between the
stem and petiole in the normal and inverted positions. Briicke * observed
the angles of deviation in the stimulated pulvinus to be two or three
times greater than in unstimulated ones. Similar relationships were deter-
mined by Hofmeister 5 to exist in the case of stimulated and unstimulated
stamens of Cynareae. These facts show that the water is not pressed out
by an increase in the elasticity of the cell-wall increasing the pressure on
the cell-sap, for in that case the rigidity of the cells and tissues would be
increased. From the load required to prevent movement it can be cal-
culated that the energy of movement in the pulvinus represents a fall of
turgor of two to five atmospheres 6. Hence it is obvious that the movement
cannot be due to an active contraction of the protoplast.
The fact that the rigidity of a stimulated pulvinus of Oxalis acetosella
decreases 7 and that water escapes under favourable circumstances from the
stamens of Berberis milgaris when a curvature is produced by irritation 8,
1 Pfeffer, 1. c., p. 23,
3 Pfeffer, 1. c., p. 35. The fact that this change of coloration, first observed by Lindsay in 1 82 7,
should not always be distinctly shown probably depends upon the fact that the air which is always
only partially displaced may in some cases be displaced but little or not at all. It is therefore quite
possible that Schwendener (I.e., p. 212) worked with plants which did not show any change of
colour, but the latter has been recently observed by Macfarlane (Biological lectures, 1894, p. 205) in
various species of Mimosa^ and more especially in Mimosa stnsitiva.
8 Pfeffer, I.e., p. n ; Schwendener, 1. c., p. 212.
* Briicke, Miillers Archiv f. Physiologie, 1848, p. 40. It has not yet been determined why the
rigidity rises after chloroforming and also when the irritability is suppressed by repeated shaking.
Pfeffer, Physiol. Unters., 1873, p. 65.
5 Hofmeister, Pflanzenzelle, 1867, p. 311 ; Pfeffer, I.e., p. 145.
6 Pfeffer, Periodische Bewegungen, 1875, p. in.
7 Pfeffer, Physiol. Unters., 1873, p. 74.
8 Pfeffer, 1. c., p. 158. Intercellular spaces are usually present in the stamens of Berberis. Cf.
Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 326, footnote 2.
78 MOVEMENTS OF CURVATURE
seems to indicate that the same mechanism is involved as in the pulvini of
Mimosa and the stamens of Cynareae.
SECTION 17 (continued).
The mode in which the fall of turgor is produced in the cell-sap l is
uncertain and need not always be the same. The rapidity with which this
occurs affords little evidence as to its character, for a rapid fall of turgor can
be produced in various ways. The escape of water is the natural result of
the pressure exerted by the stretched cell- wall when allowed to contract,
combined with the permeability of the walls of the cells and tissues
concerned 2.
Hitherto no visible changes in the cells have been observed which
might throw light upon the stimulatory movement. Stimulation does not,
for instance, affect the protoplasmic streaming of the stamens of Cynareae,
whether the movement takes place or is mechanically prevented 3. In case
any visible reactions should be detected, it would still remain to be deter-
mined whether they were directly connected with this stimulatory response
or were due to some simultaneously awakened activity. The protoplasmic
aggregations shown in stimulated cells of Drosera and Dionaea are partly
or entirely connected with the induced secretory activity. Changes in the
shape of the protoplast and in the position of the chloroplastids may be
produced without any change of turgor, and hence can hardly be responsible
for its induction 4. The same is still the case even when stimulation causes
the protoplast to retract from the cell-wall 5.
1 For details see Pfeffer, I.e., 1890, p. 333.
2 Cf. Pfeffer, 1. c., 1890, p. 329. Vines (Arb. d. hot. List, in Wiirzburg, 1878, Bd. II, p. 146)
and Gardiner (Annals of Botany, 1887-8, Vol. I, p. 366) assumed that an active contraction of the
protoplasm was responsible for the movement, without bringing any real arguments forward, and
without explaining how the high energy of contraction could be developed in this way. Pfeffer
has further shown that the fall of turgor is not produced by any active pumping action, and that the
escape of water is not the result of a local tearing in the protoplasm, such as occurs in many contract-
ing vacuoles. It hardly needs to be mentioned that so long as no exosmosis of dissolved materials
occurs, an increase in the permeability of the protoplasm or cell-wall cannot produce any fall of
turgor.
3 Pfeffer, Physiol. Unters., 1873, p. 138 ; Bot. Ztg., 1875, p. 290, footnote.
* Borzi (L'apparato di moto delle sensitive, Rivista di Scienze Biologiche, 1899) does not pay
sufficient attention to the principles indicated here. The same applies to the studies of Chauveaud
(Compt. rend., 1894, T. CXIX, p. 103) and Heckel on the stamens of Berberis. Cf. the criticism of
this work in the Bot. Ztg., 1875, p. 289, and 1876, p. 9. Heckel has, in fact, in part regarded the
appearances produced by plasmolysis or death as being the result of stimulation.
5 Hitherto the changes in the electrical currents as well as in the production of heat have not
been used to throw light upon the phenomena of stimulation and response. Bert (Mem. de 1'Acad. de
Bordeaux, 1870, T. viil, p. 43 ; Compt. rend., 1889, T. LXIX, p. 895) observed by means of thermo-
electric needles that the primary pulvinus of Mimosa pudica is somewhat cooler than the petiole and
stem, and remains so in spite of the slight rise of temperature on stimulation. According to Kraus
(Wasservertheilung i. d. Pflanze, 1880, li, p. 68) the percentage of sugar increases in continually
MOVEMENTS PRODUCED BY MECHANICAL STIMULI 79
Although we may safely assume that the perception of the stimulus
takes place in the protoplasm, nothing further is known concerning it. We
may, however, in general conclude that a mechanical stimulus produces in
a sensitive plant some explosive disturbance in the protoplasm involving
a sudden release of energy, and that the gradual storage of energy required
for the restoration of the original labile condition of equilibrium takes place
independently of the processes of stimulation and perception. The latter is
shown by the fact that the organ returns to its original position even when
its irritability is permanently suppressed by chloroforming or continual
shaking. We do not, however, know whether the return of irritability is
due to the formation of a substance capable of explosive decomposition, or
is a matter of structural rearrangement in the protoplasm, or involves other
changes l. In many cases the power of movement may be retained, although
the irritability has been suppressed, and this appears to occur more readily
in the case of seismonic than of other forms of irritability 2.
Historical. The cellular mechanism of movement in the pulvini of Mimosa and
the stamens of Cynareae was revealed in the manner stated above by Pfeffer 3, for
although Briicke 4 in his historical researches recognized that the curvature of the
pulvinus of Mimosa pudica was connected with the flaccidity of the responsive half of
the pulvinus produced by the escape of water, he did not further investigate thetcell-
mechanism, and left it uncertain where the stimulation induced a change iri^the cell-
walls, in the protoplasm, or in the cell-sap. Cohn, and also linger 5, erroneously
assumed that the movement of the stamens of Cynareae is produced by a change in
shape of the cells of the filament without5 any escape of water 6. The former author
inclined to the conclusion that the movement was due to an active contraction of the
protoplasm, a view adopted at a later date by Vines and Gardiner, but one which is
totally incapable of explaining the high energy of contraction. Hofmeister's 7 con-
clusion that the cell-wall was the responsive part of the cell was also based upon
incorrect or nebulous arguments.
shaken growing shoots while the percentage of acid often decreases. Niklewski, however, working
at Pfeffer's instigation, found no increase in the percentage of sugar under these circumstances.
1 Cf. Pfeffer, Physiol. Unters., 1873, p. 143; Osmot. Unters., 1877, p. 192. An attempt to
stimulate the stamens of Cynareae by sound-waves was without success.
a Irritability is not regained by sections of the stamens of Cynareae or of the pulvinus of Mimosa
pudica.
s Pfeffer, Physiol. Unters., 1873 ; a few complementary details are given in the Osmot. Unters.,
1877, p. 1 88. The older view that the spiral vessels were the contractile parts is given in the former
work. Ray (Historia Plantarum, 1686, p. i) was perhaps the first who attempted a mechanical
explanation. A few experiments were also performed by Hooke (Micrographia, 1767, p. 119). Cf.
also Sach's History of Botany, 1890, p. 535.
* Briicke, Archiv f. Physiologic, 1848, p. 443.
5 Cohn, Abhandlg. d. schles. Ges. f. vaterl. Cultur, 1861, Heft i, p. 28. Cohn (Zeitschr. f. wiss.
Zoologie von Siebold u. Kolliker, 1863, Bd« XII> P- 3^6) at a later date compared the contractile
cells to muscle-fibres.
6 Unger, Bot. Ztg., 1862, p. 112; 1863, P- 35°-
7 Hofmeister, Pflanzenzelle, 1867, P- 3°°* Cf. also Flora, 1862, p. 502 and Pfeffer, Physiol.
Unters., 1873, P- 6, 128.
8o MOVEMENTS OF CURVATURE
Our knowledge as to how the movement of the pulvinus of Mimosa pudica is
produced by the antagonism of the upper and under halves has developed gradually l.
Lindsay in 1790 considered the fall of the petiole to be due to the expansion of the
upper half of the pulvinus, whereas Burnett and Mayo2 recognized that only the
under half of the pulvinus of Mimosa is irritable, but failed to gain a correct view of
the entire mechanism. After Dutrochet, Treviranus and Mohl had collected definite
facts in regard to the strains between the distended parenchyma and the vascular
bundles, Briicke definitely established the fact that the curvature is the result of
the under half of the pulvinus becoming flaccid 3.
The varying grades of irritability in the leaves of Mimoseae, Papilionaceae, and
Oxalidaceae have already been discussed4. Meyen5 observed that the leaves of
Gleditschia triacantha possessed a feeble seismonic irritability, and Mohl6 observed
the same in the leaves of Robinia pseudacacia, R. viscosa, and R. hispida. In many
cases even the cotyledons are irritable, as was shown by A. P. de Candolle 7 in the
case of Mimosa pudica, and by Darwin 8 in those of Oxalis sensitiva, Smithia sensitiva,
and a few species of the genus Cassia, Dionaea, and Aldrovanda. When the leaf of
Dionaea muscipula is stimulated the two halves of the leaf fold sharply together and
become at the same time somewhat concave, so that the marginal teeth interlock *
(cf. Fig. 57, p. 378, Vol. i). Apart from the marginal zone, the whole leaf seems to take
an active part in the movement. According to Batalin's measurements, the most pro-
nounced curvature takes place along a zone on each side parallel to the midrib, while
the midrib itself takes little or no part in the movement. Darwin 10, however, found
that a pronounced movement occurs along the midrib. Batalin11 considered the
movement to be mainly the result of growth, but it is not certain whether young and
old leaves behave alike in this respect. The observations and discussion of Darwin
and of Munk fail to definitely decide whether the movements of Dionaea are wholly
or partially due to a similar cell-mechanism as that which exists in the pulvinus of
Mimosa pudica.
1 Pfeffer, Physiol. Unters., p. 3.
2 Burnett and Mayo, Quarterly Journal of Science, Literature and Arts, 1827, Vol. xxiv, p. 79 ;
1828, Vol. xxv, p. 434.
8 Cunningham (Annals of the Royal Botanical Garden of Calcutta, 1895, Vol. VI, p. i) goes so
far as to doubt whether the movements of Mimosa pudica are irritable movements at all, but thi&
somewhat voluminous work is without value.
4 An enumeration of the sensitive plants is given by Hansgirg, Physiol. und phycophytolog.
Unters,, 1893, p. 118; Neue Unters. lib. d. Gamo- und Karpotropismus, 1896, p. 102 (reprint from
Sitzungsb. d. bohm. Ges. d. Wiss.). Numerous cases were given by Dassen, in Wiegmann's Archiv
f. Naturgeschichte, 1838, Bd. I, p. 347 ; Meyen, Physiologic, 1839, Bd. HI, p. 539.
5 Meyen, 1. c,, p. 540. 6 Mohl, Vermischte Schriften, 1845, p. 372.
7 A. P. de Candolle, Physiologic, a German translation by Roper, 1835, Bd. II, p. 647.
8 Darwin, The Power of Movement in Plants. 9 Ibid.
10 For details see Darwin, Insectivorous Plants ; Munk, Die elektrischen- undj Bewegungs-
erscheinungen am Blatte von Dionaea muscipula, 1876, p. 97 ; Batalin, Flora, 1877, p. 105; Burdon-
Sanderson, Proceedings of the Royal Society, 1877, Vol. xxv, p. 411 ; Phil. Trans., i882,rp. 48 of
the reprint; Goebel, Pflanzenbiol. Schilderungen, 1891, u, p. 68 ; 1893, ir, p. 201; Macfarlane,
Contributions from the Biological Laboratory of Pennsylvania, 1892, Vol. I, p. 7; Biological
Lectures, 1894, p. 187. See more especially Haberlandt, Sinnesorgane im Pflanzenreich, 1901,
p. 108. ll 1. c.
MOVEMENTS PRODUCED BY MECHANICAL STIMULI 81
The whole of the inner side of the leaf is irritable, but the three large hairs
found on the upper side of each half of the leaf are especially sensitive (cf. Fig. 24,
p. 64). This is, according to Munk, simply because, owing to the structural arrange-
ments, pressure applied to the hair is transmitted with increased intensity by leverage
to the irritable parenchyma cells at the base of the hair. The cells at the tip of
the hair appear to be insensitive, since they can be cut away without producing an
excitation \ but the cells at the base of the hair appear to become flaccid when the
leaf is stimulated. This is of importance in that it aids in the bending of the basal
joint of each hair, enabling it to lie flat against the leaf when the latter closes.
The leaf is not sufficiently sensitive to be excited by the impact of a single rain-
drop 2, but responds to that of a jet of water, and also when the irritable hairs are
touched with a moistened gelatine-covered rod 3. It is not, however, certain whether
the leaf possesses contact-irritability in addition to seismonic irritability.
The mechanism of movement is apparently similar in the leaf of Aldrovanda
vesiculosa to that in Dwnaea, and the hairs on the inner surface of the leaf appear to
produce an excitation with especial readiness when touched. The leaves of this plant
only open when the temperature is fairly high, and very feeble contact is then suffi-
cient to excite them *.
Stamens. All members of the Cynareae appear to possess more or less irritable
filaments, and the same is also the case with a few species from the other sub-orders
of the Compositae, such as Cichorium intybus and Telekia spectosa. On the other
hand, all stamens whose cell- walls are readily extensible are not capable of perceptible
irritable movements 5.
The mechanism of movement of the stamens of Berberis * and of Mahonia
appears to be similar in character to that of the pulvinus of Mimosa.
The movements of the stamens of other plants 7 which cause them to approach
or recede from the stigma appear to indicate a power of response to seismonic
stimuli 8. Apparently, it is owing to the anatomical structure and distribution of the
irritable tissues that the stamens of Helianthemum and of other Cistaceae, as well as of
Mesembryanthemum, always move in the same direction wherever they may be touched,
1 Munk, 1. c., p. 103.
3 Darwin, 1. c., p. 273.
8 Pfeffer, Unters. a. d. hot. Inst. zu TUbingen, 1885, Bd. I, p. 518.
* For details see Stein, Bot. Ztg., 1874, p. 389; Cohn, Beitragez. Biol., 1875, I, Heft 3, p. 71 ;
Darwin, Insectivorous Plants ; Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 70 ; Haberlandt,
Physiol. Pflanzenanat., 2. Aufl., 1896, p. 480; Biol. Centralbl., 1901, Bd. XXI, p. 375; Sinnesorgane
im Pflanzenreich, 1901, p. 103.
5 Cf. Pfeffer, Physiol. Unters., 1873, pp. 107, 151. A detailed enumeration is given by Hansgirg,
Physiol. u. Phycophytol. Unters., 1893, p. 141 ; Neue Unters. lib. d. Gamo-u. Karpotropismus, 1896,
p. 106 (reprint from Sitzungsb. d. bohna. Ges. d. Wiss.).
6 Pfeffer, Physiol. Unters., 1873, pp. 127, 158. At a later date (Zur Kenntniss d. Plasmahaut u.
d. Vacuolen, 1890, p. 326, footnote) Pfeffer showed that intercellular spaces are normally present in
the active tissues. A summary of the literature is given by Usteri, Bot. Centralbl., 1900, Bd. LXXXIV,
p. 228. According to Haberlandt (I.e. 1901, p. 24), the papillose part of the inner surface of the
stamens of Berberis and Mahonia is especially irritable.
7 Facts and literature are given by Hansgirg, 1. c., 1893 and 1896 ; Beihefte zum bot. Centralbl.,
1902, Bd. XII, p. 273 ; Haberlandt, 1. c., 1901, pp. 17, 21, 32, 46, 51.
8 Cf. also Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1885, Bd. I, p. 518.
PFEFFER. HI Q.
82 MOVEMENTS OF CURVATURE
whereas the stamens of Opuntia and Cereus, and to a less degree those of Sparmannia,
and certain members of the Tiliaceae and Portulaceae, always bend towards the side
stimulated.
Irritable "stigmas ; which close together when touched, are possessed by Mimulus,
Martynia, Bignonia^ and Goldfussia \ In addition the style of Glossostigma ela/moides"2,
and also of Arctotis*, curves in response to contact.
Except in the case of Berber is, it is not known in the case of a single one of
these irritable stamens, stigmas, and styles whether the movement is produced by
growth or by elastic contraction, and the same applies to the movement of the irritable
labellums of certain orchids4. Hence it is impossible to say whether the cell-
mechanism is the same as in the pulvini of Mimosa and the stamens of Cynareae.
The rapid movements of Stylidium and of other objects are, however, due to the
sudden release of a mechanical resistance to an attempted growth-curvature 5.
SECTION 18. Movements produced by Contact-stimulation.
No contact-irritability can be detected in ordinary growing stems and
leaves, and, according to Newcombe 6, the same applies to the radicles of
seedlings, for the feeble curvatures which Sachs 7 observed as the result of
rubbing the growing zone strongly were traumotropic in character. Since,
however, the roots of Vanilla, and of a few other plants, possess distinct
thigmotropic irritability, it is possible that other roots may be found to be
more or less irritable to contact.
Strong contact-irritability is shown by the sporangiophore of Phy-
comyces nitens, for when the tip is rubbed on one side a curvature of the
growing zone begins in a few minutes 8. Since no reaction is produced by
contact below the growing zone, either the power of perception is restricted
1 For additional instances see Hansgirg, I.e., 1893 and 1896. Cf. also W. Oliver, Ber. d. hot.
Ges., 1887, p. 112 ; Miyoshi, Journal of the College of Science, Japan, 1891, Vol. IV, p. 205 ;
Haberlandt, 1. c., 1901, pp. 55, 58. According to Burk (Bot. Centralbl., 1902, Bd. LXXXIX, p. 645)
the stigmas of Mimulus and of Torenia close when pollinated owing to the withdrawal of water
by the swelling pollen -grains.
Quoted by Hansgirg, I.e., 1893, p. 149.
Minden, Flora, 1901, p. 238; Haberlandt, I.e., 1901, p. 60.
For additional literature see Oliver and Hansgirg, 1. c., 1893, p. 150.
Many such instances are given by Hansgirg, 1. c., 1893, p. 149.
Newcombe, Beihefte zum bot. Centralbl., 1902, Bd. xn, p. 343.
Sachs, Arb. d. bot. Inst. in Wiirzburg, 1873, Bd. I, p. 437 ; Darwin, The Power of Movement
in Plants. Darwin (1. c., pp. 109-71) erroneously ascribed a power of contact-irritability to the root-
tip, which enabled the root to curve away from solid bodies, whereas the observed curvatures appear
to have been traumatic in origin.
8 Errera, Bot. Ztg., 1884, p. 653; Wortmann, Bot. Ztg., 1887, p. 803; Steyer, Reizkrummungen
bei Phycomyces nitens, 1901, p. 19. That only solid bodies act as stimuli was shown by Wortmann.
The sporangiophore of Phycomyces responds most actively during the period of stretching growth,
but the weak power of reaction present just before the formation of the sporangium disappears with
the cessation of growth.
MOVEMENTS PRODUCED BY CONTACT-STIMULATION 83
to this zone or, if all regions are capable of receiving a stimulus, little or no
power of transmitting stimuli is possessed by the protoplasm. The hyphae
of this fungus have indeed no perceptible contact-irritability, and the same
applies to Mucor mucedo and M. stolonifer, whose sporangiophores behave
similarly to those of Phycomyces. On the other hand, the conidiophores of
Aspergillus and Penicillitim, as well as pollen-tubes and apparently also root-
hairs, seem to be devoid of this form of irritability 1, for the partial enclosure
of particles of soil by the root-hairs appears to be produced in a purely
mechanical manner.
In all the above-mentioned plants the reaction only takes place at the
point stimulated, whereas the leaf-tentacles of various species of Drosera
afford good instances of the transmission of stimuli from the receptive to
the responding regions 2. Contact and also chemical stimuli are only
perceived by the head of the tentacle, whereas
the curvature occurs at the base and median portion
of the stalk. When an insect alights on the leaf and
adheres to it, both kinds of stimuli co-operate, but
similar results are produced when either acts
separately. Since, however, the chemical stimuli
are more active, a partial recovery from the original
curvature occurs more readily during prolonged
contact than during the continued application of
a chemical stimulus 3. Thus the presence of a frag-
ment of glass on the tentacles is only able to keep
them fully curved for a few hours to a day, whereas
the body of an insect may cause them to remain FlG Leaf of Drosera
curved for one or more weeks, that is until all the SSSi^tftlS^cJS
soluble proteids have been digested and absorbed as the result of stimulation,
so that further chemical stimulation ceases 4.
Darwin showed that a curvature was only produced when the head of
the tentacle was mechanically or chemically stimulated, and not when the
stimuli were directly applied to the stalk or to the lamina of the leaf.
Hence when the head of a tentacle is cut off the latter can be excited
to a curvature by the transmission of a stimulus from a neighbouring
tentacle, but not by direct excitation. The effect of strong chemical and
1 Kny, Sitzungsb. d. bot. Vereins v. Brandenburg, 12. Juni, 1881 ; Dietz, Unters. a. d. bot.
Inst. zu Tubingen, 1888, Bd. n, p. 482 ; Miyoshi, Flora, 1894, p. 86.
2 For details see Darwin, Insectivorous Plants, 1875; Pfeffer, Unters. a. d. bot. Inst. zu
Tiibingen, 1885, Bd. i, p. 511. For anatomical details see Haberlandt, Physiol. Anat., 2. Aufl.,
1896, p. 397; Rosenberg, Physiol.-Cytol. Unters. tiber Drosera rotundiflora, 1899, p. 42; Haber-
landt, Sinnesorgane im Pflanzenreich, 1901, p. 94.
3 Cf. Pfeffer, Unters. a. d. bot. Inst. zu Tiibingen, 1885, Bd. i, p. 514.
* Darwin, 1. c., pp. 13, 21, 92 seq., 22, 117; Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II,
p. 203.
G 2
84 MOVEMENTS OF CURVATURE
mechanical stimuli is not restricted to a single tentacle but spreads through
the leaf-lamina to neighbouring tentacles not directly excited.
The small tentacles at the centre can perceive stimuli and transmit
them to the larger marginal ones which curve towards the centre of the
leaf, whereas the small central tentacles themselves do not curve. The
dorsi-ventrality of the stalk of the tentacle leads to the curvature being
always to the centre of the leaf, but does not prevent a slight lateral bend-
ing when a tentacle at the side is radiating a strong orienting stimulus.
By varying the intensity and duration of the stimulus, an excitation may
either be confined to the stimulated tentacle, or may be caused to spread
to neighbouring ones or even to all the tentacles on a leaf. Strong
stimulation, especially if chemical, may cause the lamina of Drosera
rotundifolia to become more or less concave, or may lead to an inrolling
of the margin of the elongated leaf of Drosera longifolia and D. intermedia,
which may sometimes be so pronounced as to completely enclose a captured
insect.
The irritable movements of the tentacles and lamina of Drosera rotundifolia were
first noted by Roth1. Nitschke2 gave an account of the movements and their
propagation which was in the main correct, but our knowledge was greatly increased
by the historical researches of Darwin. Among other important points Darwin
showed that the power of perception was localized in the heads of the tentacles.
Pfeffer established definitely the fact that the tentacles possessed contact-irritability,
although Darwin had previously shown that drops of rain did not act as stimuli, and
that a solid body only acts as a stimulus when it is pushed through the slimy
excretion into direct contact with the head of the tentacle.
The sensitivity to contact-stimulation depends upon the stage of development
and other conditions, but under favourable circumstances is nearly as great as that
of the most sensitive tendrils, since Darwin found that a perceptible result was
produced by rubbing a hair weighing 0-000822 of a milligram upon the head of
a tentacle of Drosera rotundifolia. A single touch hardly produces any result,
whereas repeated strong contact causes a curvature to begin in 10 to 20 seconds,
and in 10 to 20 minutes the heads of the tentacles are pressed against the middle of
the leaf.
The lamina of the leaves of Pinguicula vulgarts, P. lusitanica, and P. alpina 5
rolls inwards when subjected to mechanical and chemical stimuli. The excitation
spreads to a certain distance from the point of application of the stimulus, but the
leaf is only moderately sensitive, and it has not been determined whether the
stimulus is perceived by the lamina or by the heads of the numerous small stalked
glands present on the under surface. .
The mechanics of the movement. The curvature of a tendril produced by contact
1 Roth, Beitrage z. Botanik, 1782, T. I, p. 60.
2 Nitschke, Bot. Ztg., 1860, p. 229 ; 1861, pp. 224, 234, 253.
3 Darwin, I.e., p. 374 ; Pfeffer, I.e., p. 516. Cf. also Klein, Cohn's Beitr. z. Biologic, 1883,
Bd. in, p. 163 ; Goebel, 1. c., p. 186.
MOVEMENTS PRODUCED BY CONTACT-STIMULATION 85
has already been shown to be connected with a transitory acceleration of growth,
and the bending of the sporangiophores of Mucorineae is also a growth-curvature.
Batalin's * measurements, though not fully satisfactory, indicate the same to be the
case for Drosera. Here also a transitory acceleration of growth appears to result
from stimulation, but further research is required to make this certain. The con-
clusion that we are dealing with a growth-curvature is supported by Corren's observa-
tion that the curvature* remains permanent when a stimulated tentacle of Drosera
is suddenly killed by immersal in boiling water 2.
As in the case ot tendrils, the causes inducing the changed rates of growth are
unknown. De Vries' 3 supposition that the curvature was in the first instance due to
an elastic stretching of the cell-wall by turgor is as inapplicable to the tentacles
of Drosera as to tendrils, and in any case the curvature of the unicellular sporangio-
phore of Phycomyces could hardly be due to a rise of turgor unless the cell-wall on
the convex side became at the same time more extensible.
SECTION 19. Curvatures produced by Chemical Stimuli.
We are here concerned primarily with the curvatures due to diffuse
chemical stimuli, such as must occur whenever the growth of the opposite
sides of a dorsiventral organ is unequally affected. Chemonastic reactions
of this kind, like thermonastic responses, are rarely pronounced in character,
but the action of chloroform causes a strong curvature in the pulvini of
Mimosa pudica*, as well as in tendrils 5, which also respond to treat-
ment with a dilute solution of iodine. In addition, the rarification of the
surrounding air causes, when pronounced, a certain alteration in the position
of the leaflets of Mimosa and of the stigmas of Mimulus 6.
This power of chemonastic movement is, however, especially well
developed in certain carnivorous plants, and it is shown in response to such
substances as proteids, salts of ammonium, and phosphates. The chemo-
nastic movement resembles that produced by contact-stimulation in the
case of the tentacles of Drosera and the leaves of Pinguicula. Contact,
however, causes a sudden closure of the leaflets of Dionaea^ whereas chemical
stimulation induces a gradual closure, which may take as long as a day
when the stimulus is a feeble one 7. In addition, after mechanical stimu-
lation the leaves remain hollow so that they enclose a cavity, whereas
1 Batalin, Flora, 1877, p. 39. 2 Correns, Flora, 1892, p. 126.
3 De Vries, Bot. Ztg., 1886, p. 5. * Pfeffer, Physiol. Unters., 1873, p. 64.
•5 Correns, Bot. Ztg., 1896, p. 16. This author also states that ammoniacal vapours induce
a certain curvature in tendrils.
6 Correns, Flora, 1892, pp. 97, 146, 148. On Mimosa cf. also Bonnier, Revue generate de
botanique, 1892, T. iv, p. 525.
7 Darwin, Insectivorous Plants, 1875, p. 397. The progress of the movement was also followed
in detail by Darwin (The Power of Movement in Plants, 1880, pp. 239, 241, 261).
86 MOVEMENTS OF CURVATURE
chemical stimulation causes them to press closely together, and hence also
against the body of a captured insect l.
Since proteids, ammonium salts, phosphates and other substances act as stimuli,
the products of the digestion of a captured insect or of a piece of meat or egg-
albumin always induce a chemical excitation. Darwin, found that ammonium
phosphate was more active than any other substance, for a drop of water containing
0-000423 of a milligram of this substance caused a curvature when placed upon
the head of the tentacle. The same result was produced by 0-0025 of a milligram
of ammonium nitrate and 0-0675 of a milligram of ammonium carbonate. Darwin
also found that phosphates, and to a less degree camphor, a few ethereal oils, and
in fact most varied substances acted as stimuli, but not certain alkaloids, so that
all substances containing nitrogen are not chemical excitants. Darwin observed
that the irritability of the tentacles of Drosera was suppressed by the application
of small amounts of potassium salts, and this has been confirmed by Correns 2. The
latter author also finds that distilled water produces a feeble curvature, so that
it remains an open question to what extent the response or lack of response to
particular substances is due to external circumstances or to the presence of traces
of potassium salts. It is possible that the stimulating action of distilled water is
due to its dissolving away or diluting substances present in the glandular excretion,
which by causing a difference of concentration in regard to the cell-sap might induce
an excitation. In addition it is not sufficiently certain whether the inactivity of
certain substances is or is not due to their non-absorption. Since, however, in
general absorption is a preliminary to excitation, one may follow Munk 3 in speaking
of absorption stimuli and digestive movements, although this must not be taken to
indicate that only nutritive substances act as stimuli.
The association of a mechanical with a chemical irritability is of
biological importance to carnivorous plants, although in other cases the
one form of irritability may be developed but not the other 4. Tendrils
and the stamens of Cynareae are hardly or not at all responsive to chemical
stimuli, but are readily excitable by mechanical ones. Chemical stimuli
appear to have a more intense and prolonged action than mechanical ones
in the case of the carnivorous plants, and in fact the mechanical excitability
is so feeble in Drosera binata that it was overlooked by Morren 5. It
1 Darwin, 1876, 1. c., p. 307; Batalin, Flora, 1877, p. 134.
2 Correns, Bot. Ztg., 1896, p. 25.
3 Munk, Die elektr.- u. Bewegungsersch. an Dionaea, Reichert und du Bois-Reymond's Archiv,
1876, p. 98.
4 Darwin (1875, I.e.) was the first to distinguish between mechanical and chemical excitation.
The prolonged closure over insects was observed earlier, but was either unexplained or ascribed to the
continuance of the mechanical excitation. This explanation was, in fact, given by Oudemans (Bot.
Ztg., 1860, p. 163) in the case of the leaf of Dionaea.
s Morren, Note sur le Drosera binata, 1875, p. 10 (reprint from Bull, de 1'Acad. royale de
Belgique, 2e seV., T. XL). The mechanical excitability was detected by Darwin, 1. c., p. 256, and by
Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 199. Darwin (1. c., p. 270) also found that the
leaf of Drosera was still excitable by proteids when almost inexcitable by mechanical stimuli.
CURVATURES PRODUCED BY CHEMICAL STIMULI 87
is owing to this fact that when all the products of digestion have been
absorbed the leaves of Drosera^ Dionaea^ and Aldrovanda re-expand,
although a mechanical excitation may still be exercised by the undigested
remains. Since, however, it is entirely a question of specific excitability
it is not surprising to find, as was shown by Darwin, that in the case of the
leaf of Pinguicula vulgaris a chemical excitation does not persist much
longer than a mechanical one. Conjoint excitation produces varying
results, and although the summation is usually positive, chemical stimulation
causes the leaf of Dionaea to be less responsive to mechanical stimuli 1.
Both mechanical and chemical stimuli are perceived by the heads
of the tentacles of Drosera^ whereas the hairs on the leaf of Dionaea are
especially responsive to mechanical stimuli and much less so to chemical
excitations. The small gland-hairs of Dionaea appear to be the special
receptive organs for chemical stimuli, but show little or no mechanical
excitability 2. The motor-mechanism excited by both forms of stimulation
may, however, be of similar character, and this applies even when, as in the
leaf of Dionaea^ the movements induced by mechanical excitations are more
rapid than those produced by chemical stimuli.
Every vital response produced as the result of the chemical quality
of an absorbed substance may be regarded as being due to a chemical
excitation. This applies to all chemonastic movements, whether induced
by the presentation of some special material or by a quantitative change
in the composition of a nutrient medium. The chemical excitation may
naturally be transitory in character and need not necessarily produce
a permanent alteration. Instances of the former are afforded by the
contraction of the stamens of Berberis and the closure of the stigmas of
Mimulus produced by sudden exposure to ammonia vapour, and also by
the contraction of the stamens of Berberis and Helianthemum induced by
a sufficiently rapid fall in the partial pressure of the oxygen in the sur-
rounding air 3. Since, however, in these cases as well as in that of Mimosa
the movement may be excited by various internal disturbances, it is
impossible to say whether a chemical excitant directly awakens a special
chemical irritability, or acts indirectly by inducing internal disturbances
which operate as the immediate exciting agencies.
The same stimulus may in many cases excite several dissimilar
responses simultaneously, and in carnivorous plants stimulation may not
only induce movement but may also awaken, modify, or accelerate the
1 Darwin, 1875, 1. c. ; Munk, 1. c., p. 99. 2 Cf. Darwin, 1. c., pp. 267, 295.
3 Correns, Flora, 1892, p. 151. It is worthy of note that it is not every plant which readily
responds to seismonic stimuli that can be stimulated in this way. As Correns showed, the excitation
is not due to the mechanical disturbances induced by the sudden evacuation of the air. It is further
to be expected that many substances may produce a response in Mimosa if only they penetrate with
sufficient rapidity to produce a ' shock-effect.'
88 MOVEMENTS OF CURVATURE
secretory activity of the digestive glands. The secretory activity of the
leaf of Dionaea is in fact only aroused by stimulation, whereas the glandular
heads of the tentacles of Drosera continually excrete mucilage and water,
but do so more rapidly when chemically and also when mechanically
stimulated. At the same time the excretion of acid begins, so that the
reaction of the digestive fluid alters much as it does in the stomach of
a carnivorous animal 1. Although the pitchers of Nepenthes and the leaves
of Drosophyllum lusitanictim have no power of independent movement, the
secretion of water, mucilage, and enzymes by the glands is awakened or
accelerated to a certain extent by chemical stimuli. Chemical excitation,
on the other hand, induces both movement and secretion in the case of the
leaf of Dionaea^ whereas mechanical excitation induces movement only 2.
It will without doubt ultimately be found possible to excite excretion
without movement even in those cases where both occur together normally ;
and in fact, according to Darwin, a mechanical stimulus induces movement
without secretion in the leaf of Pinguictila, whereas ammonium carbonate
produces secretion without movement. In certain cases, however, a separa-
tion of this kind may not be possible owing to the fact that the excitation
of the motor-mechanism may unavoidably awaken changes leading to
secretory activity and vice versa.
Various visible changes in the cell-contents are associated with the
secretory activity, and these are also shown by the gland-cells of the
pitchers of Nepenthes and the leaves of Drosophyllum^ which possess no
power of movement. These changes are therefore presumably of secretory
origin, although it is possible that other visible changes in the cells may be
associated with the response to stimulation by movement. Neither the
causes nor the genetic relationship of these changes are, however, satisfac-
torily known, although it appears as though the waxing and waning of the
amount of nuclear chromatin is the direct result of secretory rest and
secretory activity respectively.
The intracellular changes have been studied by Darwin on Drosera rotundifolia
and by Gardiner also on Drosera dichotoma (= Drosera linatd)*) in which a change
of coloration accompanies the aggregation beginning in the head and progressing
1 Darwin, Insectivorous Plants, 1875, p. 85. Facts in regard to other plants are also given by
Darwin.
8 According to Macfarlane (Contrib. from the Bot. Lab. of Pennsylvania, 1892, Vol. I, p. 37),
a certain secretory activity is awakened in the leaf of Dionaea by strong mechanical or electrical
excitation.
3 These changes were first observed by Darwin (1. c., p. 38), who did not, however, distinguish
between aggregation and precipitation (granulation), as did Schimper (Bot. Ztg., 1882, p. 231);
de Vries (ibid., 1886, p. i) ; and, at a later date, Gardiner (Proc. of the Royal Soc. 1886, Vol. xxxix,
p. 229). Huie, Quarterly Journal of Microscopical Science, 1896, Vol. xxxix, p. 387 ; 1899, Vol.
XLII, p. 203 ; Rosenberg, Physiol.-Cytol. Unters. iiber Drosera rotundifolia^ 1899; and Haberlandt,
Sinnesorgane, 1901, p. 94, have paid especial attention to the changes in the gland-cells.
CURVATURES PRODUCED BY CHEMICAL STIMULI 89
along the stalk of the tentacle \ These intracellular aggregations involve in the cells
of the tentacle-stalk, according to de Vries, Gardiner, and Schimper, an increase
in volume of the protoplasm and a decrease in volume of the cell-sap2. At the
same time active protoplasmic streaming is excited, and the vacuoles increase
in number. The shapes of the vacuoles also alter, and this naturally involves
changes of shape in the protoplasm, although Darwin attached undue importance to
the latter. A little later a precipitate, the granulation8, usually appears in the
cell-sap when strong chemical stimuli are applied, but it is usually absent after weak
mechanical or chemical excitation. The precipitate is usually due to the excretion
of tannin into the cell-sap, which finally forms rounded masses, often tinged red
owing to the absorption and accumulation by them of the red colouring-matter
in the cell-sap 4. It need not, however, always be tannin which separates out and is
precipitated, although similar precipitations are produced by the action of ammonium
carbonate, caffein, and other substances in the cell-sap of many other plants which
contain tannin.
Hence it is possible that the granulation resulting from stimulation is also due
to the production of substances which precipitate the tannin in the same way that
ammonium carbonate does when applied to the head of a tentacle. Since mechanical
stimulation may influence the secretory activity, it may also lead when sufficiently
intense to a sufficient production of the materials responsible for the precipitation.
The intracellular changes have not been followed any further in the living gland-
cells at the head of the tentacle of Drosera. According to the researches of Huie,
and also of Rosenberg, carried out on fixed material, these cells show a decrease
in the volume of the protoplasm and an increase in the volume of the cell-sap after
stimulation, that is exactly the reverse changes to those occurring in the cells of the
stalk. It remains to be seen, however, whether the difference is due to the fact that
the observations on the heads of the tentacles were made on fixed material, but those
on the stalk-cells upon living material. It is possible that the active excretion of
mucilage and other materials from the gland-cells may lead to a diminution in their
protoplasmic contents5, and in fact the amount of chromatin in the nuclei of the
gland-cells of animals appears to decrease greatly in amount during active secretion.
Both Huie and Rosenberg have found that, especially after long chemical excitation,
the nuclei of the gland- and stalk-cells decrease in volume, while their chromatin
threads assume an appearance and differentiation resembling the initial stages in
mitolic nuclear division.
1 Gardiner, 1. c.
2 On the changes of volume in the cell cf. Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 290.
According to de Vries (1. c., p. 30) the turgor of the cell as determined by plasmolysis is uninfluenced
by the aggregation, whereas Gardiner (1. c., p. 232) assumes on insufficient grounds that a fall of
turgor ensues.
3 The term was suggested by Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 198. Darwin
(1. c., p. 263) found that aggregation is produced in the leaf of Dionaea by chemical but not by
mechanical excitation.
4 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1886, Bd. II, p. 244 ; Flora, 1889, p. 52.
5 It is uncertain what the importance of Gardiner's ' rhabdoid ' may be (1. c., p. 230). This
structure occurs in the cells of Drosera and Dionaea and, according to Gardiner, increases in size as
the result of stimulation, whereas Macfarlane (1. c., p. 36) could detect no such increase of size in the
rhabdoid of Dionaea.
go MOVEMENTS OF CURVATURE
The aggregation and precipitation produced by stimulation gradually pass away
again, not only in the cells of intact tentacles, but also in sections cut from them.
In the latter case repeated washing accelerates the solution of the precipitated
materials by removing the soluble diosmosing products as fast as they are formed.
The precipitation resulting from stimulation can only be regarded as a chemical
reaction due to the production of a precipitating substance, whereas the protoplasmic
aggregation as well as the changes in shape and differentiation of the nucleus
are vital responses l. That the aggregation is associated with the secretory activity
is shown by the fact that it is produced by nitrogenous and other substances, not
only in the cells of the tentacle stalk of Drosera, but also in cells of the non-motile
glands of Sarracem'a, Nepenthes, and Drosophyllum* !, and can apparently be also
induced in many cells which have a somewhat analogous power of secretion. It is
also known that changes of shape are shown by the nucleus and cytoplasm of a few
plant-cells and various animal-cells 3 during active secretion. This is in accord with
the fact that many chemical agencies which act fatally when concentrated, induce
various changes in the shape and visible structure of the protoplasm when applied in
diluted form.
The conduction of the stimuli. The visible nature of the aggregation and precipita-
tion enables the progress of the excitation to be followed from cell to cell. It has,
however, not yet been determined by comparison whether the motory and aggregation
reactions involve the conduction of a single or of two separate stimulatory processes.
Darwin4 observed that when the head of a tentacle of Drosera was moderately
strongly stimulated, a neighbouring decapitated tentacle curved but showed no
aggregation, whereas aggregation appeared in the head of a neighbouring intact
tentacle, and spread in a reflected fashion downwards in the cells of the stalk.
Apparently, therefore, a stimulus inducing aggregation was propagated from the head
in the reverse direction to the primary stimulus inducing curvature and exciting
the head. When the stimulus is more intense, however, the aggregation is directly
propagated to a greater distance and spreads to neighbouring decapitated tentacles.
If the stimuli for each reaction are distinct, they are at least conducted along the
same path, that is in the parenchyma cells and in the living elements of the vascular
bundles. That the parenchyma cells are capable of conduction is shown by an
experiment of Darwin's in which the vascular bundles were cut across and yet
a stimulus was transferred through the tentacle. In addition the aggregation may
be propagated from cell to cell of the epidermis. According to Batalin 5, stimuli
travel more rapidly along the vascular bundles than through the parenchyma. This
is probably the result of the elongation of the living cells of the vascular bundles,
1 Precipitating substances are not present in all secreting cells, even of carnivorous plants, and
hence, according to Goebel (1. c.,p. 119, footnote), no granulation is produced in the secretory gland-
cells of Utricularia and Pinguicula as the result of stimulation.
2 Cf. Schimper, 1. c., p. 231 ; Goebel, 1. c., p. 199.
3 For the literature see Rosenberg, 1. c., p. 112.
4 Darwin, Insectivorous Plants, p. 242. [Darwin does not state that the decapitated tentacle
showed no aggregation, but that it showed less aggregation, and further, the aggregation disappears
first at the base of the tentacle and travels upwards.]
5 Batalin, Flora, 1877, p. 66. Cf. also Zeigler, Compt. rend., 1874, T. LXXVIII, p. 1417.
CURVATURES PRODUCED BY, CHEMICAL STIMULI 91
for Darwin found that stimuli travelled more rapidly parallel to the long axes of the
parenchyma cells than transversely to them. It is for these reasons that stimuli
radiate mainly centripetally from the marginal tentacles, and centrifugally from those
near the centre, but are only propagated slowly and feebly tangentially.
It appears probable that the conduction of stimuli, at least in Drosera, involves
a transference of stimulatory materials, either by the diffusion of an absorbed
substance or as the result of the formation of stimulatory materials in the secretory
cells which diffuse to neighbouring ones and excite aggregation in them. In this
case the transference of the stimulus would be merely a matter of translocation, aided
possibly by the fact that the secondarily excited cells themselves begin to produce
stimulatory materials. The transference of these might take place if they are
diffusible, without the aid of any interprotoplasmic connexions, and, in fact, aggrega-
tion and granulation may be produced in the cells of the tentacle of Drosera by the
direct application of ammonium carbonate.
Comparative investigations on other plants will, without doubt, aid in the
elucidation of these problems, but so far it is only known that the effects of
mechanical stimuli are propagated through the parenchyma of the leaf of Dionaea,
and more rapidly along the vascular bundles *. No aggregation is produced by
mechanical stimuli in Dionaea, but this change and the chemonastic excitation due
to absorbed proteids appear to follow the same path but to travel more slowly. The
rapid transference of mechanical stimuli in the leaf of Aldrovanda must, however,
take place through the parenchyma of the leaf, since in the leaf-lobes no vascular
bundles are present.
SECTION 20. The Propagation of Mechanical and Chemical Stimuli.
The influence of mechanical and chemical stimuli is often restricted to
the region immediately surrounding the point of application, or to the
pulvinus when this is the only irritable portion. On the other hand,
Mimosa pudica affords a well-known and striking instance of the trans-
mission of stimuli, for under favourable conditions burning or cutting off
the terminal leaflets of one of the segments of the leaf may cause all
the leaves and leaflets to be stimulated in succession. The stimulus
is conducted somewhat less readily in Biophytum sensitivum 2, while in
the trifoliate leaves of Oxalis acetosella the reaction is restricted to the
leaflet directly stimulated3.
In the case of the highly irritable stamens of Berberis and Centaurea
the stimulus is not transmitted from an excited stamen to neighbouring
1 Darwin, 1. c., p. 313 ; Batalin, 1. c., p. 147.
8 G. Haberlandt, Ann. du Jard. bot. de Buitenzorg, 1898, Suppl. II, p. 33; Sinnesorgane im
Pflanzenreich, 1901, p. 88. On Oxalis dendroides cf. Macfarlane, Biological Lectures, 1894, p. 194.
3 Cohn, Verhdlg. d. schles. Ges. f. vaterl. Cultur, 1859, p. 56; Pfeffer, Physiol. Unters, 1873,
p. 74.
92 MOVEMENTS OF CURVATURE
ones, and no response is aroused when the corolla is cut through above or
below the insertion of the stamens l. Stimulation of one stigma-lobe of
Martynia lutea^ M. proboscidea^ and Mimulus cardinalis causes the other to
move, but not in the case of the stigmas of Mimulus luteus 2. An excita-
tion is propagated with extreme rapidity from one leaf-lobe of Dionaea to
the other, while in the case of the fairly sensitive stamens of Sparmannia
africana 3 irritation of one stamen spreads to a limited extent to the
neighbouring ones. In the case of Phycomyces> on the other hand, no
conduction of stimuli appears to take place, whereas a contact-stimulus is
rapidly propagated to the outer side of a tendril and also to some extent
longitudinally. The leaf of Drosera again affords a specially good instance
of the conduction of stimuli, for as the result of stimulating the head of
a single tentacle all the tentacle-stalks on the leaf may be caused to curve
inwards. Here the receptive and motory zones are distinct, and, according
to Oliver, the same is the case in the labellum of Masdevallia muscosa,
which appears to possess seismonic irritability. In all the other cases that
have been investigated the motory zone seems also to be capable of
perception, for one can hardly ascribe a vital power of perceiving stimuli
to the leaf-laminas or stems of Mimosa simply because the movement of
water produced when they are cut, crushed, or burned acts as a stimulus
to the motile pulvini4.
The above instances suffice to show that a high sensitivity to contact
or seismonic stimuli does not necessarily involve a pronounced power of
conducting stimuli, and that the transference of the stimuli may either be
vital or purely mechanical. The latter is the case in all organs which
respond to seismonic stimuli, for the collapse of one stimulated cell excites
the next, this the next, and so on. In the case of Mimosa pudica the
stimulus is propagated by means of a movement of water or hydrostatic
pulsation which is able to travel through dead portions of the stem and
leaf, and which excites the pulvini on which it impinges. Since this can
only occur when a proper connexion is maintained between the conducting
channels and the responding organ, it is not surprising that the stamens of
Berberis and of Cynareae cannot be excited in this way.
1 Pfeffer, Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 317.
2 Oliver, Ber. d. bot. Ges., 1887, p. 167; Hansgirg, Physiol. u. Phycophytol. Unters., 1893,
P-47-
3 Morren, Rech. s. 1. mouvement d'etamines du Sparmannia, 1841, p. 23 (reprint from Me'm.
de 1'Acad. de Bruxelles, T. XIV).
4 The movements observed by Darwin (The Power of Movement in Plants, 1880, p. 127) when
the laminas of the cotyledons of Oxalis sensitiva, and of a few species of Cassia, were strongly rubbed
were probably the result of the ensuing movements of water stimulating the motile pulvini. Goebel
(Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 201, footnote) observed incidentally that in the case of
a feebly irritable leaf of Diotiaea one leaf-lobe could be excited by stimulation of the other leaf-lobe,
but not directly.
PROPAGATION OF MECHANICAL AND CHEMICAL STIMULI 93
It is, however, probable that the conduction of stimuli in the stigmas
of Martynia and Mimulus> in the stamens of Sparmannia, and possibly in
the labellum of Masdevallia takes place in some other way. Furthermore,
the transference of stimuli in organs sensitive to contact-stimuli cannot
possibly be due to movements of water, since these organs do not respond
to repeated bending with its attendant movements of water. In such cases
we may assume that we are dealing either with a vital transmission of
stimuli which can only take place through intervening protoplasm, or with
a transference of stimulating materials, or of an electrical excitation from
cell to cell, for which the existence of living interprotoplasmic connexions
is not essential. It is in fact not inconceivable that dissimilar modes of
conduction may be excited at the same time. A simple instance of this is
afforded when the disturbance due to the response of a single stimulated
cell serves for the propagation of the stimulus through the whole of the
irritable organs, but not through the intervening non-motile tissue to
neighbouring motile organs. In addition, a mechanical disturbance can be
transferred so as to excite the rapid closure of the leaf of Dionaea, but not
the slow movements resulting from chemical stimulation.
According to Oliver1, the transference of stimuli in the labellum of
Masdevallia is restricted to the vascular bundles, although it does not
appear to be due to a movement of water as in Mimosa. Even here,
however, a slow vital transmission of stimuli may also be possible, while
a transference of stimuli across active parenchyma tissue occurs both in the
pulvini of Mimosa and in irritable stamens. In addition, stimuli are
transferred mainly or entirely through parenchyma cells in the case of the
stigmas of Mimulus and Martynia, according to Oliver 2, when these respond
to seismonic stimulation, for the stimulation of one stigma may excite the
other after the intervening vascular bundles have been severed. Both
mechanical and chemical stimuli appear to be conducted through the
parenchyma cells of the tentacles of Drosera, but the rate of propagation
appears to be more rapid along the vascular bundles.
The transmission of stimuli is in most plants extremely slow, but in
Mimosa pudica a rate of propagation of 15 mm. per second has been
observed 3, and in the pulvinus itself, as well as in the stamens of Centaurea>
stimuli may travel still more rapidly. On the other hand, the impulse
radiating from the chemically or mechanically excited head of a tentacle
of Drosera does not appear to travel at a much greater rate than 10 mm.
1 Oliver, Annals of Botany, 1888, Vol. I, p. 249.
2 Oliver, Ber. d. hot. Ges., 1887, p. 168.
3 Dutrochet, Recherch. anat. et physiol., 1824, p. 80; Bert, Mem. de 1'Acad. de Bordeaux, 1870,
T. vin, p. 47; Pfeffer, Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 325 ; G. Haberlandt, Das reizleitende
Gewebesystem der Sinnpflanze, 1893, p. 69. On the slow rate of propagation of stimuli in
Biophytum sensitivum, cf. Haberlandt, Ann. du Jard. bot. de Buitenzorg, 1898, Suppl., p. 35.
94 MOVEMENTS OF CURVATURE
per minute, for ten or twenty seconds after the head of a tentacle has been
strongly stimulated a curvature may be shown in a region of the stalk,
distant 2 or 3 mm. from the head. These times, it is true, include the
latent periods of perception and reaction, which can only be eliminated by
comparing the times at which curvature is shown at varying distances from
the head. In most plants, however, stimuli travel still more slowly than
in Drosera, and in this way plants largely avoid the waste of energy which
would be involved in a continual attempt to adjust themselves to transitory
variations in the external conditions.
The exact determination of the velocity of propagation of stimuli
is difficult, and even when measured by the appearance of a reaction the
rate will depend not only upon the specific nature of the plant but also
upon the external conditions. A fall of temperature, a decrease of
turgidity, and the action of chloroform all lower the rate of transmission
of stimuli in Mimosa and in other plants, and ultimately produce a com-
plete cessation of conductivity l. A result of this kind may be due either
to a direct action upon the conductivity, or to an action upon the sensitivity,
excitability, or power of response, or may be due to a combination of these
factors. The importance of a close study of the influence of the external
conditions upon conductivity, excitability, and the power of response cannot
be overestimated, and the fact that stimuli may travel through etherized or
dead pieces of the stem of Mimosa shows that in this case the transmission
of seismonic stimuli is not vital in character.
Dutrochet 2 was the first to show that stimuli are conducted through the vascular
bundles of Mimosa pudica, and he also came to the correct conclusion that the
transmission was due to a pulsation of water. Pfeffer3 subsequently showed that
the stimulus was able to travel over chloroformed parts of the stem, and Haberlandt
found that dead regions of the stem and leaf retained their conductivity some time
after they had been killed*. We are, therefore, fully justified in ascribing the
transmission of the stimulus to the movements and changes of pressure of the water
in the vascular bundles, and when a cut is made in the stem, a stimulus is only
exercised when the knife penetrates the vascular bundles and allows the escape
of a drop of water. The stimulation of the neighbouring leaves at once follows, and,
as the stimulus spreads, all the leaves and leaflets may be in succession excited when
the plant is in a highly irritable condition. No stimulation or conduction takes
place, however, when the plant's turgidity is so low that no drop of water escapes
1 Cf. Pfeffer, 1. c., p. 326.
2 Dutrochet, Recherch. anat. et physiol., 1824, p. 69; Mem. p. servir a 1'histoire d. ve'ge'taux,
Bruxelles, 1837, p. 272-
3 Pfeffer, Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 308.
* G. Haberlandt (Das reizleitende Gewebesystem d. Sinnpflanze, 1890, p. 35) observed a pro-
pagation of the stimulus over a locm. length of dead stem ; Macdongal (Botanical Gazette, 1896,
Vol. xxn, p. 296) over as much as 30 cms. The mode of treatment and the maintenance of turgidity
are factors of considerable importance.
PROPAGATION OF MECHANICAL AND CHEMICAL STIMULI 95
when an incision is made in the vascular bundles. Similarly the transference of an
excitation from one pair of leaflets to another in the intact plant only takes place
when it is sufficiently turgid, and may hence safely be assumed to be due to the
hydrostatic pulsation aroused by the sudden escape of water from the directly-
stimulated cells, possibly aided by the sudden bending of the part of the vascular
bundles lying in the, pulvinus. Since this pulsation is usually comparatively feeble,
it is only natural that the abscission or burning of a leaflet should produce an effect
which, being more intense, is propagated to a greater distance than that due to
touching a single leaflet.
According to Haberlandt, the conduction of stimuli takes place in the tannin-
tubes l of the phloem, which transfer positive or negative pressure waves to the pul-
vini, and these mechanically excite the motile cells. Macdougal, however, denies that
the stimuli are transmitted by hydrostatic pulsation in this manner. Haberlandt's
conclusion is mainly based upon the fact that the drop of liquid which escapes from
an incised vascular bundle is, for the most part, derived from these tannin-sacs.
Transmission is, however, also possible in their absence, for Dutrochet 2 found a con-
duction of stimuli was still possible when incisions were made through all the tissues
excepting the wood. Haberlandt has also overlooked the fact that in a dead portion
of the stem the conditions for the transference of a pressure wave through the sap-
containing tannin-sacs are not fulfilled 3. In addition Borzi 4 has found that the con-
duction of stimuli in Aeschynomene indica and Neptunia oleracea takes place in tissues
which do not possess any continuous system of tannin-sacs.
Macdougal found that no stimulation was produced when as large a cut surface
as possible of the shoot was submerged in a solution of potassium nitrate, so that
a sudden fall of turgor was produced in the exposed tissues, including the tannin-sacs.
Negative results were also obtained when the pressure with which water was driven
into a cut surface of the stem was suddenly raised by three to eight atmospheres.
Macdougal found that the rise of pressure was rapidly transmitted in the xylem vessels,
and also in the tannin-sacs, to the furthermost shoots and leaves, so that it is evident
that not every movement of water or change of pressure is able to transmit a stimulus
to the pulvinus.
[These results of Macdougal's do not necessarily show that the transference of
the stimulus is due to a special stimulatory substance, and indeed do not afford con-
clusive proof that the transference is not due to a hydrostatic pulsation.
The cells of Char a and Nitella, for instance, respond to seismonic stimulation
1 For details concerning these tubes and their contents, cf. Haberlandt, 1890, 1. c., Physiol.
Anat, 2. Aufl., 1896, p. 482 ; Baccarini, Bot. Centralbl., 1893, Bd. LIV, p. 171 ; Borzi, L'apparato
di moto delle sensitive, 1899. (A reference is given in the Bot. Centralbl., 1899, Bd. LXXX, p. 351.)
Since these tubes occur in other plants, and are primarily together with their contents of metabolic
importance, they can only secondarily have developed a power of conducting stimuli in certain
plants.
2 Dutrochet, 1824, 1. c., p. 69. Confirmatory results have been obtained by Haberlandt, 1890,
1. c. ; Macdougal, 1. c.
3 The living portion of the tube shuts itself off from the injured portion, according to Haberlandt,
and without this no restoration of turgor would be possible in the tube.
* Borzi, I.e., p. 4.
96 MOVEMENTS OF CURVATURE
(sudden pressure, or the impact of a falling body) by a temporary stoppage of stream-
ing, and the stimulus may be transferred to a neighbouring cell by a hydrostatic
pulsation in the cell-sap. The pulsation must, however, be a sharp one, and changes
of pressure produced in the same way as in Macdougal's experiment are ineffective
as stimuli even when high pressures are used. A hydrostatic impulse produced by
a blow upon a piston-rod does, however, produce a sufficiently intense wave to act as
a stimulus to the cell, and to be capable of propagation to the next one l. It is
evident, therefore, that this question needs further investigation before a definite con-
clusion can be made. It is in any case by no means improbable that other changes
besides the hydrostatic pulsation may co-operate in the transmission of stimuli in
Mimosa, and it hardly needs to be mentioned that the structure of the pulvinus affords
no evidence as to the means by which the stimulus is transferred to the motile cells.
The mere existence of inter-protoplasmic communications 2 does not indicate whether
these are of fundamental importance in a particular case, and the manner in which
stimuli travel from one part to another is dependent upon the course and connexions
of the vascular bundles, whether the stimuli travel in the phloem or in the xylem3.]
Biophytum sensitivum also responds, according to Macdougal 4, to stimuli travel-
ling through a dead portion of the leaf axis, although, according to Haberlandt 5, this
is not the case. A peculiarity of the latter plant6 -lies in the fact that the removal of
a leaflet acts as a sub-maximal stimulus to the pulvini of the remaining leaflets, and
this incomplete movement is repeated several times without any further stimulus being
applied. Since this periodicity might be produced in various ways, further research
is necessary to reveal its mode of origin. Under appropriate periodic stimulation
a periodic movement may be induced in the leaves of Mimosa pudtca, owing to the
gradual recovery or increase of excitability, but it does not follow that the periodic
movements of the leaflets of Biophytum are produced in a similar way. If we are
actually dealing in this case with a prolonged stimulatory action, it can hardly be due
to a temporary hydrostatic pulsation or movement of water.
1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 72.
2 Haberlandt's statement (1890, 1. c., p. 25) that no inter protoplasmic communications exist
between the tannin-sacs, and between the collenchyma cells is incorrect according to Kienitz-Gerloff
(Bot. Ztg., 1891, p. 25), but the positive statement of this author may be accepted with some caution.
3 Cf. Pfeffer, 1. c., p. 318 ; Haberlandt, 1. c.
4 1. c., p. 296.
5 Haberlandt, Ann. du Jard. bot. de Buitenzorg, 1898, Suppl. II, p. 38. On Oxalis dendroides
cf. Macfarlane, Biological Lectures, 1894, p. 194.
6 Haberlandt, 1. c., p. 35.
MOVEMENTS OF CURVATURE 97
PART IV
PHOTONASTIC, THERMONASTIC, AND HYDRONASTIC CURVATURES
SECTION ai. General.
Since the growth of the different cells and tissues of an organ is
unequally affected by temperature, light, and the percentage of water,
physiologically dorsiventral organs are often caused to perform thermonastic,
photonastic, or hydronastic curvatures by variations in one of the above
factors. These curvatures, though often trifling in amount, may be in many
cases pronounced, as in the case of the daily movements l dependent upon
variations in the intensity of the light, or upon changes of temperature.
Instances of these movements are afforded by those flowers which open and
close at definite periods of the day, and by those leaves which perform sleep
movements at night when the light is feeble, or at midday when it becomes
intense. In such cases the organ assumes a position best suited to the
external conditions, and within certain limits the amount of movement
corresponds to the degree of change in the external conditions, such as
illumination, temperature, or supply of water. At low temperatures or
under feeble illumination the peduncles of certain plants curve downwards
instead of being erect, while in other cases the foliage or floral leaves remain
pressed together, so that the flower of such a plant under these conditions
becomes cleistogamous and never opens. Such flowers may be said to be
facultatively cleistogamic.
It often happens that during these aitionastic movements the curvature
is at first excessive, so that the ultimate position of equilibrium is only
attained after a few oscillations. Thus a sudden rise of temperature causes
the flowers of Crocus and Tulipa to open widely at first, and this is followed
by a gradual assumption of the less expanded position which they maintain
so long as the new conditions remain unaltered. The same progress of the
reaction can be traced when the temperature is lowered, if by removing
five of the perianth-segments the remaining one is allowed to perform its
full amplitude of movement. When all the segments are present they press
against one another, and so prevent any movement in excess of that required
to close the flower. Similar results are obtained by illuminating or darken-
ing flowers and foliage-leaves capable of photonastic reaction. Hence it may
happen especially in the cases of foliage-leaves that the change from light
1 Since the term « tropism ' is reserved for curvatures produced by unilateral stimuli, it becomes
jssaryto change the term ' nyctitropic * used by Darwin (The Power of Movement in Plants,
1880, p. 281) into that of ' nyctinastic.'
PFEFFER. Ill TT
g8 MOVEMENTS OF CURVATURE
to darkness may produce a pronounced temporary but no permanent
curvature, since the leaf gradually returns to approximately the same
position that it occupied when illuminated.
The ultimate position is naturally independent of the transitory
oscillations, which are due to the fact that the antagonistic tissues attain
their new positions of equilibrium in different ways, or at least with
unequal rapidity. Hence oscillations are absent when the change in the
external conditions takes place gradually, as also are the temporary
curvatures shown only when the temperature or illumination is suddenly
altered. These considerations have been shown experimentally to apply
to the thermonastically-reacting flowers of Crocus and Tulipa, and
to the photonastically-reacting leaves of Impatiens and Robinia. Nor
is it surprising that slowly reacting or comparatively insensitive organs
should gradually assume a new position in response to sudden and pro-
nounced changes of temperature or illumination without exhibiting any
perceptible transitory oscillations. A good analogy is afforded by two
metal rods riveted together, and one of them being surrounded by a non-
conductor, for when the system is suddenly warmed a transitory curvature
will be produced independently of whether the rods have the same or
dissimilar coefficients of expansion, that is independently of whether the
rods ultimately straighten again or remain permanently curved.
Obviously the relationships are not quite so simple in a living organism
as in this instance, for although the curvatures are primarily due to the
unequal growth or expansion of the opposed tissues, the causes which
induce these variations of growth, or which cause the tendency to elastic
expansion or contraction, are extremely complex in origin. In addition, the
realized curvature, like vital reactions in general, excites regulatory stimuli
and counteractions, so that the progress of the response and the ultimate
position assumed depend upon the conjoint action of these factors with the
original stimulus. It has already been mentioned that special conditions
may be introduced by accommodation, by changes of excitability, and
by alterations in the power of response during excitation, even when the
organ remains excitable during response.
Transitory disturbances may frequently be produced by sudden
changes as the result of shock. For instance, a sudden change of temperature
induces an acceleration of growth in the perianth-segments of Crocus and
Ttilipa, while a sudden change of illumination has the same effect upon
foliage and floral leaves capable of photonastic reaction. This is of
importance in so far as it increases the power and rapidity of response. In
addition, owing to the unequal responses of the inner and outer sides of the
perianth-segments, a sudden fall of temperature produces a rapid closure of
the flower of Crocus even when the temperature is so low that growth
ultimately almost entirely ceases. A similar transitory acceleration of
GENERAL 99
growth is produced in tendrils by contact-stimulation, and presumably it
would not be shown if all shock-effect was avoided by allowing the contact-
stimulation to increase gradually from a sub-minimal to an optimal intensity.
Sudden changes in the external conditions probably leave no organism
entirely unaffected, although no disturbance of growth or other pronounced
reaction may be perceptible. Sudden variations of temperature and of
illumination do, however, appear in general to excite feeble transitory
disturbances of growth, and these have become especially pronounced in
certain cases as the result of biological adaptation, so that, more especially
in photonastic plants, a transitory acceleration of growth is produced even
by a comparatively slow diminution in the illumination. All plants have
not the same power of response, and there are even organs which appear to
experience no shock-effect, although they change their position in response
to alterations of temperature or illumination. It is even possible that in
some cases a sudden change may produce a temporary depression of growth,
just as a shock-stimulus causes a transitory fall of turgor in one-half of the
pulvinus of Mimosa pudica, or a transitory cessation of streaming in a cell
of Chara or Nitella.
The constant daily repetition of the sleep-movements of photonastic
leaves induces a periodic rhythm which gradually disappears in darkness or
under constant illumination. Under natural conditions the movements in
the morning and evening result from the co-operation of the photonastic
reaction with the after-effect of the previous ones, the photonastic rhythm
being induced and not hereditary.
Aitionastic reactions do not always exert appreciable after-effects, for
these are absent from the thigmonastic movements of bilateral tendrils and of
the tentacles of Drosera. The same appears to apply to thermonastic move-
ments, such as the opening and closing of the flowers of Crocus and Tulipa.
Since, further, the daily variations of temperature are much more irregular
than the daily changes of illumination, it is not surprising that the daily
opening and closing of thermonastic flowers should be more irregular than
the periodic movements of photonastic organs.
Photonastic, thermonastic, and hydronastic movements are often associated
together under natural conditions, and may also be coupled with tropic responses due
to unilateral stimulation. It is naturally necessary at first to determine the nature of
each form of response before studying conjoint actions. Granted that an organ
possesses definite properties, the character of its response can largely be predicted
from what is known as to the general influence of the external conditions on growth.
For instance, an opening movement may be converted into a closing one by an
additional rise of temperature, if the temperature optimum for the previously epinastic
side is lower than for the opposite more slowly-growing one.
Under constant external conditions only autogenic movements are performed,
such as the expansion of the foliage and the opening of the foliage and flower-buds.
H 2
ioo MOVEMENTS OF CURVATURE
Flowers which periodically open and close behave like ephemeral ones under these
conditions and open once only. The duration of both ephemeral and periodic flowers
may vary considerably *, and in fact at low temperatures the life of an ephemeral
flower may be so prolonged that it is able to perform daily movements.
The flowers of Crocus do not open when the temperature is kept low, nor those
of Stellaria media when the illumination is feeble. This is owing to the fact that at
no period of development does the growth of the inner surface of the perianth-seg-
ments become active enough, as compared with that on the outer surfaces, to produce
a separation of the closely applied leaves. An opening movement is, however, in
part attempted during development, as is shown by the fact that if all the perianth
segments are removed but one, this may curve at first nearly at right angles to the
stalk, but subsequently straightens more or less. At still higher temperatures the
segment expands outwards, but the opening of the flower is slower and less pro-
nounced than at the optimal temperature. A sudden rise of temperature produces an
opening movement which is temporarily in excess of the ultimate position for this
temperature, and this may cause the temporary opening of a flower, when raised to
a temperature at which it finally closes again. The same general considerations also
apply to photonastic and hydronastic movements.
The uses of the movements. When feeble they are probably accessory reactions
without any special biological importance. Moth-pollinated flowers which close in
the daytime avoid the visits of useless insects, and economize scent, nectar, and
pollen. Flowers which close at night keep the sexual organs protected from dew,
and to a certain extent from injurious cooling2. The drooping of flower- and
inflorescence-stalks, which causes many flowers to be inverted during the night, may
be of use in the same way.
The sleep-movements of leaves and leaflets reduce the amount of surface exposed,
and hence lessen the radiation of heat during clear nights. Darwin3 showed that
less dew formed on such leaves than on ones which had been fixed in the expanded
condition. The latter suffered more than the normally sleeping leaves, and hence
Darwin concluded that the nyctinastic movements were for the purpose of lessening
temporary cooling during night as far as possible. Stahl 4, however, considers the
utility of these movements to lie in the fact that the lessened formation of dew avoids
the blocking of the stomata and the consequent hindrance to transpiration. If Stahl's
1 Cf. Oltmanns, Bot. Ztg., 1895, pp. 32, 52 ; Hansgirg, Physiologische u. Phycophytolog.
Unters., 1893, p. 15 ; Kerner (Natural History of Plants, 1895, Vol. n, p. 211). [Hansgirg (1. c.,
p. 10) suggests the terms thermo-, photo-, and hydrocleistogamy to indicate the main causes which
keep a facultatively cleistogamic flower permanently closed. Since the causation may vary at
different times, these terms are as unnecessary and superfluous, as it would be to use special terms
(mechano-cleistogamy, plaster-of-paris-cleistogamy) for the cleistogamy produced by tying-up a
flower or embedding it in plaster-of-paris.]
2 Hansgirg, I.e., p. 175; Kerner, 1. c., Bd. II, p. 112. Die Schutzmittel des Pollens, 1873.
Sprengel (Das entdeckte Geheimniss der Natur im Bau u. in d. Befruchtung d. Blumen, 1793, p. 13)
considers the closing movements to be for the protection of the nectar.
3 Darwin, The Power of Movement in Plants, 1880, pp. 286, 413; Bot. Centralbl., 1881,
Bd. vni, p. 77.
4 Stahl, Bot. Ztg., 1897, p. 81. A detailed discussion of the biological utility of these move-
ments is given by Stahl.
GENERAL
101
view is correct, the blocking of the stomata is more likely to be a serious matter by
preventing the assimilation of carbon dioxide during the early morning hours. Both
the avoidance of dew-formation and of cooling may be of importance, and possibly
the sleep-movements may have still other biological advantages.
In many cases organs are brought into positions which enable them to utilize the
light best or to avoid it when intense, and this may take place by the aid of photo-
nastic reactions, coupled with tropic movements. Although photonastic reactions
may often be feeble, there is no reason for restricting the term nyctinastic to pro-
nounced sleep-movements.
SECTION 22. Instances of Photonastic and Diurnal Movements.
Pronounced daily movements are performed by the foliage and floral
leaves of many plants, and these movements are in some cases induced by
A
FIG. 28. Leafy shoot of Desmodium gyrans. A in the day position, B in the night position (after Darwin).
the daily changes of illumination, but in other cases by alterations of
temperature. Frequently the power of photonastic response is accompanied
by a feeble or pronounced capacity for thermonastic reaction. In such
cases decreases of temperature or illumination usually induce similar
curvatures ; and, since the movements produced by increases are also alike,
both factors co-operate at morning and evening in producing the awakening
or sleep-movements as the case may be.
The photonastic sleep- movements are the result either of growth or of
variation curvatures, the latter of which are commonly performed wherever
motile pulvini are present. These movements are shown by the leaves of
102
MOVEMENTS OF CURVATURE
most Leguminosae and Oxalidaceae, as well as by the leaves of Marsilea,
Porliera, Portulaca, and Phyllanthus Niruri. It can be seen from Fig. 28
that Desmodium gyrans allows the leaf laminas to droop so that the stalks
rise up when night falls, and hence assumes quite a different appearance.
The three leaflets of Oxalis acetosella x droop downwards around and against
the petiole, whereas the leaflets of Trifolium fold together upwards, and then
the upper end of the petiole curves laterally or downwards. In the case of
the pinnate leaves of Amorpha and Robinia the leaflets droop downwards
at night and press their under-surfaces together (Fig. 29) whereas those of
Acacia lophantha and Mimosa pudica fold upwards in pairs with the dorsal
surfaces together 2. Since at the same time the main petiole of Mimosa
pudica sinks, the position
assumed resembles closely
that produced by a me-
chanical excitation (Fig.
19, p. 61). If the plant is
highly turgid the pulvinus
may when mechanically
excited during the early
hours of the tropical night
show an additional curva-
ture and may bend the
leaf backwards across the
stem and. support it for
a short time upside down
against the action of gravity .
This excessive movement
is not always shown, and
later on the normal droop-
ing position is again as-
sumed 3. It is only in cer-
tain cases that, in addition to its photonastic excitability, the leaf-pulvini
FIG. 29. Leaf of Amorfihafruticosa. A in day position, B in
night position.
1 The leaflets assume the same position as after mechanical excitation. Cf. Fig. 20, p. 62.
8 A summary of the plants showing sleep-movements is given by Hansgirg, Physiolog. u.
Phycophytolog. Unters., 1893; Neue Unters. iiber den Gamo- u. Karpotropismus, sowie iiber Reiz-
u. Schlafbewegungen, 1896 (Sitzungsb. d. bohmisch. Ges. d. Wiss.) ; Beihefte z. botan. Centralbl.,
1902, Bd. xii, pp. 267, 272. Cf. also Pfeffer, Periodische Bewegungen d. Blattorgane, 1875, p. 159,
and'the literature there quoted. Numerous facts and figures are given by Darwin, The Power of
Movement in Plants, 1880 ; Kerner, Natural History of Plants, 1895, p. 534. For additional facts
see Popow, Bot. Jahresb., 1880, p. 278 (Gleditschia) ; Bruckner, Bot Centralbl., 1882, Bd. XII,
p. 171 ; Vochting, Bot. Ztg., 1888, p. 519 (Malvaceae) ; F. W. Oliver, Bot. Centralbl., 1891, Bd.
XLV, p. 52 (Abrus) ; Paoletti, Nuov. giornal. hot. ital., 1892, T. xxiv, p. 65 (Porlierd} ; Mobius,
Bot. Centralbl., 1894, Bd. xv, p. 8 ; Jost, Bot. Ztg., 1897, p. 17 ; Jahrb. f. wiss. Bot., 1898, Bd. xxxi,
p. 345 ; Ewart,^Annals of Botany, 1897, Vol. XI, p. 439 ; Stahl, Bot. Ztg., 1897, p. 85 ; Linsbauer,
Ber. d. bot. Ges., 1903, p. 27.
3 Ewart, 1. c., p. 453.
INSTANCES OF PHOTONASTIC AND DIURNAL MOVEMENTS 103
possess seismonic (Mimosa) or contact-irritability (Dalbergia), in most cases
the pulvini being of value for the performance of sleep-movements.
The nyctinastic variation movements continue usually until the death
of the leaf, though often their amplitude decreases, whereas the nyctinastic
nutation movements are performed only by the aid of those regions of the
petiole and lamina which remain capable of growth. Hence these latter
movements are shown for a few days only or for a longer period, and
cease when the leaf becomes adult. Usually the daily movements of
growing dorsiventral organs are but trifling, but the growing leaves
of Impatiens noli-me-tangere, Impatiens parviflora, Sigesbeckia orientalis
(Pfeffer), Myriophyllum proserpinacoides (Stahl), and of Stellaria media
(Batalin) raise themselves upwards every evening, whereas those of Nico-
tiana rustica, Chenopodium album, and of Amaranthus curve distinctly
downwards 1.
The flowers of various Oxalidaceae, Mesembryanthemaceae, Nymphaea-
ceae, and Compositae2 perform photonastic sleep-movements. Among
the last-named the evening closure of the capitulum is due to the inward
curvature of the whole of the ligulate florets of Leontodon or Hieracium
(Fig. 30), but only by that of the ray-florets in Bellis. In the capitulum of
Chrysanthemum the spreading ray-florets bend back along the stalk at
night, while flowers pollinated by moths open in the evening and close when
morning dawns 3.
The leafy shoots of Mimulus Tilingii, which are obliquely or horizon-
tally expanded during the daytime, rise upwards in the evening 4 by the aid
of a photonastic reaction. It is possible also that the change in the
illumination aids in producing^the evening sinking of the inflorescences of
Daucus, Falcaria^ and Scabiosa, and of the flowers of Viola, although these
movements are mainly produced in response to the change of temperature 6.
According to Morren 6, the stamens of Sparmannia africana perform sleep-
1 For additional facts see the works of Hansgirg, Darwin, Pfeffer, Stahl, Jost, which have
already been quoted. See also Batalin, Flora, 1873, p. 437.
2 For additional instances see Hansgirg, Pfeffer, Jost, 1. c. Also Pfeffer, Physiol. Unters., 1873,
pp. 195, 210; Royer, Ann. d. sci. nat., 1868, ve ser., T. IX, p. 355 ; Kerner, Pflanzenleben, Bd. II,
p. 208 (Natural History of Plants, 1895, Vol. II, p. 215) ; Burgerstein. Ueber die nyctitrop. Beweg.
d. Perianthien, 1887; Oesterreich. Bot. Zeitschrift, 1901, Nr. 6; Oltmanns, Bot. Ztg., 1895, p. 31;
R. Scott, Annals of Botany, 1903, Vol. xvn, p. 761 (Sparmannia).
3 Cf. Hansgirg, 1. c., 1893, p. 12 ; Oltmanns, 1. c., 1895, p. 50.
* Vochting, Ber. d. bot. Ges., 1898, p. 39.
5 For the literature see Hansgirg, 1. c., 1893, p. 88 ; Vochting, 1. c., 1898, p. 42, and Jahrb. f.
wiss. Bot., 1890, Bd. xxi, p. 285 ; Pfeffer, Period. Beweg., 1875, p. 162 ; Wittrock, Bot. Centralbl.,
1883, Bd. xvi, p. 220; Kerner, Pflanzenleben, Bd. I, p. 494; Bd. n, p. 120 ; (Natural History of
Plants, Vol. I, p. 530; Vol. n, p. 118). According to Knoch (Bibl. Bot., 1899, Heft 47, p. 17) the
flower-buds of Victoria regia and Nymphaea blanda are submerged in the evening by the curvature
of the flower-stalk. [The fact that movements of this kind may still be performed in a hothouse at
nearly constant temperature points to their being photonastic in character, but direct experiment is
necessary to make this certain. The repeated evening closure and partial or complete submersal of
the flowers of the Water-lily was known to Linnaeus.] ' Cf, Pfeffer, 1. c., p. 162.
104 MOVEMENTS OF CURVATURE
movements, and it is possible that further research may reveal the existence
of such movements among vascular and even non-vascular Cryptogams.
Variation movements are performed by comparatively short pulvini, whereas
nutation movements may be derived from growing zones of considerable length.
Indeed, in some cases the greater portion of the leaf may be capable of curvature,
which may in the case of the floral leaves of Silene nutans go so far as to cause a rolling
up of the leaf1. As growth dies out, the growing and curving zone gradually
decreases, so that in Malva, for instance, the nyctinastic movements are at first
carried out by the whole petiole, but ultimately only by the basal pulvinus 3.
The movement usually takes place in a vertical or oblique plane, but in many
cases a complicated curve is traced, and in others a twisting occurs which goes so far
in the leaves of Phyllanthus Niruri and Cassia s as to lead the downwardly curving
leaflets to apply their dorsal surfaces to one another. These twistings, as well as the
feebler ones of Mimosa pudica, are determined by the structure of the motile organs,
but the twisting of the leaflets of Mimosa, and possibly also of Phyllanthus Niruri, are
not produced by a true torsion in the pulvini, but by
its curving along two intersecting planes *. As in
other cases, the movement may either be spasmodic
or regular, but the remarkable spasmodic movements
observed by Darwin on the leaves of Averrhoa
Ifilimoi may possibly be in part autonomic in origin5.
If the further movement of a leaf is prevented by
contact with the stem or with another leaf, the ten-
dency to curvature continues, so that the pressure
exerted reaches a maximum, and then dies away
again as the return movement begins. Hence a
,n. ir leaflet of Acacio^ophantha or of Mimosa curves to
a greater extent when the opposite leaflet with its
resistance to movement is removed. In this case, as with other free leaves, the return
movement begins soon after the attainment of the extreme night position, whereas
when the leaflets are in contact they remain for some time pressed together without
movement 6.
Not only are different leaves capable of varied movements, but in addition the
different parts of the same leaf may behave dissimilarly. Thus the petioles of the
leaves of Desmodium gyrans and of Phaseolus vulgaris rise up in the evening, whereas
the laminas sink downwards. Furthermore, the primary petiole of Mimosa pudica
and the leaflets move in opposite directions, while the palmate leaflets of some species
1 Cf. Hansgirg, 1. c., 1893, p. 13.
a Vochting, Bot. Ztg., 1888, p. 519.
3 Pfeffer, Periodische Bewegungen, 1875, p. 159; Darwin, The Power of Movement in Plants,
1880, pp. 387-8.
* Schwendener, Gesaimnelte bot. Mittheil., 1897, Bd. n, pp. 214, 242.
5 Darwin, 1. c., p. 330. Cf. also Dewevre and Bordage, Rev. gen. de bot., 1892, T. iv, p. 77.
6 Pfeffer, 1. c., pp. 48, 160.
INSTANCES OF PHOTONASTIC AND DIURNAL MOVEMENTS 105
of Lupinus become erect at night-time, and those of other species droop downwards.
Darwin also gives instances of plants in which the cotyledons perform different sleep-
movements to the foliage-leaves, and mentions that the young cotyledons of Trifolium
strictum, in addition to the pulvinar movements, show a torsion of the petioles *. In
addition, the position of the leaf of Bauhinia changes at night-time, while the two
halves of the leaf fold together along the midrib. (Cf. Fig. 31, p. 107.) In many
cases, though not always, the sleep position of the leaves resembles that which they
occupied during their early development 2.
Since these daily movements are the result of a photonastic reaction
coupled with the after-effect of periodic stimulation, every increase or
decrease of illumination produces a more or less pronounced movement
corresponding to that occurring at morning and evening respectively. In
both cases the movement surpasses the permanent position of equilibrium,
and that often to a considerable extent. Hence it arises that periodic
flowers are usually most widely open during the early morning hours.
Changes of illumination always affect the position of the leaves more
or less. The latter is more especially the case with the leaves and leaflets
of Phaseolus and Acacia lophantha, for when these are placed in darkness,
after performing a photonastic movement they return approximately to their
original position, and maintain this position, in part at least, for an indefinite
length of time in continued darkness. On the other hand, amongst others,
the leaves of Impatiens noli-me-tangere and of Chenopodium album when
placed in darkness during the daytime assume a position intermediate
between the day and night positions, and one which approaches the former
or latter more closely according to the species of plant. Flowers which
perform nyctinastic movements show similar peculiarities 3.
Photonastic changes of position are, however, also shown by organs
which do not exhibit any pronounced daily sleep- movements, either because
they do not react rapidly enough, or not in such fashion as to produce
a definite movement. Reactions of this kind take part in and often are
mainly responsible for the assumption of the permanent position of dorsi-
ventral organs. Heteronastic, tropic, and other reactions may, however,
also co-operate with the photonastic, thermonastic, and hydronastic responses.
The leaves of Taraxacum officinale, Plantago media, and Primula elatior
assume a more or less erect position in deeply shaded places, but under
strong diffuse illumination spread more or less horizontally, so that the
rosette of leaves is often closely applied to the ground. Light therefore
favours epinastic and darkness hyponastic growth in these leaves, and when
1 Darwin, 1. c., pp. 309-13.
a Cf. Pfeffer, 1. c.; Dietz, Flora, 1887, p. 577.
3 Cf. Pfeffer, 1. c., pp. 19, 38, 49. Autonomic changes of position are possible under constant
external conditions. Cf. Pfeffer, 1. c, p. 49, &c.; Jost, Jahrb. f. wiss. Bot, 1898, Bd. xxxi, p. 382.
io6 MOVEMENTS OF CURVATURE
the light is withdrawn all the leaves capable of growth gradually assume
a more erect position. On the other hand, darkness favours the epinastic
growth of the leaves of Impatiens, Helianthus annmis, Ceratophyllum, and
MyriophyttuWi and hence causes a more or less pronounced downward
curvature of the leaves 1.
Certain flowers which perform no evident sleep-movements respond to
the presence or absence of light. Thus the flowers of Gagea lutea, Gentiana
campestris, Stellaria media, Holosteum umbellatum, Veronica alpina, and
Drosera longifolia develop and fade without ever opening in darkness 2, and
are therefore ' photo-cleistogamic.' The flowers of Stellaria media require
a considerable intensity of light to induce their expansion, and hence
remain closed when grown behind a window facing north.
All these considerations apply only for moderate intensities of light,
and leave it an open question whether under sufficiently intense diffuse
illumination the reaction would be reversed. The cases in which movements
have been observed in the leaves of Acacia, Mimosa, Robinia, &c., in response
to strong sunlight falling on one side give no satisfactory answer, since these
are heliotropic curvatures towards the light performed by the motile
pulvini. In this way the blades of the leaflets are placed parallel to the
incident rays. This reaction, sometimes termed midday sleep, was called
paraheliotropism by Darwin 3, and is due to the unilateral illumination pro-
ducing a greater fall of turgor in the more strongly illuminated half of the
pulvinus than in the less strongly illuminated one. In this position the
chloroplastids are protected, and the transpiration is usually diminished 4. The
leaflets of Cassia montana, however, assume positions which tend to increase
transpiration, the stomatic ventral surfaces facing outwards or upwards, so
that the plant apparently risks a fatal loss of water in order to keep down
the insolation temperature 5. That the response in not due to the localized
warming of the exposed side of the pulvinus is shown by the fact that it
takes place when the pulvini are submerged under water, and, as in the case
1 Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870, p. 46; Detmer, Bot. Ztg.,
1882, p. 787 ; Wiesner, Bot. Ztg., 1884, p. 677 ; Vines, Annals of Botany, 1889, Vol. ill, p. 421 ;
Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 272 ; Mobius, Biolog. Centralbl., 1894, Bd. XV,
pp. 8, 14.
2 Vochting, Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 180; Hansgirg, Physiol. u. PhycophytoL,
Unters., 1893, pp. 27, 45, 53 ; Beihefte z. bot. Centralbl., 1902, Bd. XII, p. 271 ; Oltmanns, Bot. Ztg.,
l895> P- 31 5 Leclerc du Sablon, Rev. ge"n. de bot, 1900, T. xii, p. 305.
8 The Power of Movement in Plants, 1880, p. 445. Cf. also Pfeffer, Periodische Bewegungen,
1875, p. 62; Hansgirg, 1893, 1. c., p. 134; Oltmanns, Flora, 1892, p. 238; Wilson, Contributions
from the Botanical Laboratory of the University of Pennsylvania, 1892, Vol. I, p. 66; Ewart, Annals
of Botany, 1897, Vol. xi, p. 447; Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 385.
* Wiesner, Die natiirl. Einrichtungen zum Schutze des Chlorophylls, 1875, p. 62 ; Stahl,
Bot. Ztg., 1897, p. 91.
5 Ewart, 1. c., p. 456.
INSTANCES OF PHOTONASTIC AND DIURNAL MOVEMENTS 107
of other responses to light stimuli, it is mainly produced by the blue and
more refrangible rays 1.
[The assumption that the * paraheliotropic * position is the result of
a heliotropic response is hardly justified, any more than is the assumption
that the pulvini of the main and secondary petioles possess the same
irritability and mode of response as those of the leaflets. The main
pulvinus of Mimosa pudica, and to a less degree those of the secondary
petioles, are, for in-
stance, heliotropic and
curve or twist under
unilateral illumination
even when compara-
tively intense, so as to
place the general sur-
face of the leaf more
or less at right angles
to the incident rays.
The folding-up of the
leaflets in strong sun-
light is, however, per-
formed in whatever
position the leaf may
be, and takes place
also when the leaves
are illuminated from
beneath by a beam of
light thrown upon one
or more of the pulvini
of the leaflets, each
pulvinus reacting
separately. The re-
lationships are some-
what complicated by
the fact that the pul-
vini of the leaflets also
appear to possess a weak heliotropic irritability ; but sufficiently strong illu-
mination, whatever its direction, always causes the same response, the
leaflets folding together owing to the reversal of their previous photonastic
response. In regard to the leaves of other Leguminosae, both the nycti-
nastic and the paranastic (paraheliotropic) positions of the leaflets are
produced, not in response to the direction of the illumination, but to its
FlG. 31. Bauhinia tomentosa. A plant climbing by hook-tendrils, and
whose leaves show photonastic movements, (a) A leaf expanded owing to
the pulvinus being covered with tinfoil ; (b) the same but the pulvinus being
shaded by another leaf; (c) young leaf which has not yet unfolded: (a)
folded leaves with pulvini exposed to sunlight and showing that the folding
is independent of the direction of the incident light. (From a photograph.
After Ewart.)
1 Ewart, l.c., pp. 451, 480; Macfarlane, Bot. Centralbl., 1895, Bd. LXI, p. 136.
io8 MOVEMENTS OF CURVATURE
intensity. If the movement of the leaflets causes the pulvini to be shaded
it may cease when a certain inclination is reached, which sometimes gives the
appearance of a heliotropic curvature. In addition, the old leaves of
Bauhinia (Fig. 31) are unable to fold together as completely as do the
young leaves, owing to the increased rigidity of the tissues T.]
In any case sufficiently strong diffuse illumination produces a sinking
of the leaflets of Oxalis 2, while Ewart (1. c.) has shown that in the case of
a variety of leaves that perform variation movements a reversal of the
photonastic response is produced by an increase in the intensity of diffuse
light above the optimum. It is presumably also owing to a reversal of the
previous heteronastic growth that, as Oltmanns found3, the flowers of
Tragopogon brevirostris close not only when the illumination decreases to
a minimum but also when it increases beyond a certain intensity.
SECTION 23. The Origin of the Daily Photonastic Periodicity.
The photonastic reactions of responsive organs are enhanced by the
periodicity induced by repeated previous stimulation. Hence when the
plant is kept in continuous constant illumination or in darkness the daily
movements are still performed for a certain time, but with gradually decreas-
ing amplitude. The periodic movements are at first pronounced both in
constant light and in darkness in the case of the leaves of Acacia lophantha.
Mimosa pudica^ Impatiens noli-me-tangere, and Sigesbeckia orientalis, and
they continue to be perceptible until after the lapse of four to eight days.
On the other hand, the daily movements of the flowers of Oxalis rosea cease
after being for three to four days in darkness, and the same happens in the
capitulums of Bellis perennis after one or two days4.
After the cessation of the daily periodicity, the leaves assume positions
corresponding to the illumination and to other factors, while under constant
external conditions all movements cease except those of autonomic origin.
When the external conditions are favourable such leaves retain fully their
irritability and power of response, so that a plant whose daily periodicity
has been removed by continuous illumination retains its photonastic irrita-
bility, and responds by the usual sleep-movement when placed in darkness.
This was found by Pfeffer to take forty-five minutes to two hours for
completion in the case of the leaflets of Acacia lophantha and Trifolium
pratense, and of the terminal leaflets of Desmodium gyrans, whereas the
leaves of Impatiens noli~me-tangere sink considerably but do not attain
the full nyctitropic position under these circumstances. The leaves of
1 Cf. Ewart, 1. c., pp. 448-59, 480. 2 Pfeffer> x. c>> p> 6o>
3 Oltmanns, Bot. Ztg., 1895, p. 51.
* Pfeffer, Period. Bewegungen d. Blattorgane, 1875, p. 34seq.
THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY 109
Sigesbeckia orientalis do not droop through an angle of more than 10° to
30°, whereas when the normal daily movements are performed the leaves
droop vertically downwards at night, moving therefore through an angle
ofQO01.
Plants exposed to the normal daily changes of illumination also afford
a measure of the photonastic irritability and response, for darkening during
the daytime produces a slight photonastic curvature in Sigesbeckia^ but
a pronounced one in Acacia and the other plants named above. In general
the photonastic reactions produced by variation movements are more rapid
and pronounced than those due to nutation. The pulvini of Portulaca
sativa only react feebly, however, whereas the nutating leaves of Impatiens
noli-me-tangere and of 7. parviflora are strongly photonastic2. «In the
Tropics motile leaflets usually begin to assume the sleep position at about
5 p.m., and have completed the movement commonly by 5.30, that is half
an hour before the fall of night. Naturally, however, the times fluctuate
somewhat in different plants, and they are also affected by the clearness of
the sky and by the humidity of the soil and of the air 3.
When the periodicity has been removed by continuous illumination,
a photonastic reaction does not merely cause a single to and fro movement,
but also produces an after-effect which is naturally but slight when the
reaction is feeble as in Sigesbeckia. In this case the gradual return to the
full amplitude of movement can readily be traced as the result of the
co-operation of new rhythmically repeated photonastic reactions with the
after-effects of previous ones. Thus a plant of Sigesbeckia orientalis^ after
five days' continuous illumination had removed the daily periodicity, was
placed in darkness daily from 8 a.m. to 4 p.m. The first darkening produced
curvatures in the leaves of 10° to 30°, the second curvatures of 15° to 45°,
which had increased to ones of 40° to 80° on the fourth day, and of 70° to
100° on the fifth day. Five periodic repetitions were therefore required
to reproduce approximately the normal amplitude of movement in this
plant.
This induction and summation cannot of course be followed when the
first darkening produces the full or nearly the full nyctinastic movement,
as in the case of the leaflets of Acacia lophantha which fold together when
first darkened after prolonged previous constant illumination. Even in this
case, however, only two periodic movements are shown as the after-effect
1 Pfeffer, 1. c., p. 39. The plant termed Sigesbeckia flexuosa proves to be a form of Sigesbeckia
orientalis L.
2 Pfeffer, 1. c., pp. 15, 39-
3 Ewart, Annals of Botany, 1897, Vol. XI, p. 453 seq. [The midday sleep-movements of
Mimosa pudica and of similar plants do not appear to induce any distinct secondary periodicity,
although they might do so when regular and prolonged.]
no MOVEMENTS OF CURVATURE
of a single reaction, whereas when the daily periodicity is fully induced four
or five periodic movements may be shown under constant illumination.
It is evident, therefore, that the nyctinastic periodicity is induced by the
rhythmically-repeated photonastic reactions and their after-effects. The
daily periodicity of growth is produced in a similar way, and a photonastic
periodicity must always result from the rhythmic and regular repetition
of changes of illumination whenever these affect either growth or the energy
of expansion of motile tissues \ The after-effects of photonastic stimulation
enable a phototonic plant to perform movements of considerable amplitude,
although the primary movement directly due to the photonastic stimula-
tion may be comparatively feeble, and in addition a plant with a pronounced
periodicity of this kind will tend to be more regular in its daily movements
than if nearly the full movement was produced in response to a single
change of illumination.
An analogy is afforded by a pendulum whose amplitude of oscillation
is gradually increased up to a maximum by a series of rhythmically-repeated
impulses, and which then continues to oscillate with gradually decreasing
amplitude but without any appreciable change of period2. In the living
plant, although the cumulative after-effects of the previous rhythmic stimula-
tion may be phenomena of complex origin, we can nevertheless deal with
them as with other empirically established facts. Not all movements or
stimulatory reactions are able to exercise appreciable after-effects, and since
when they result from a particular reaction they may vary in character
according to the nature of the plant and its power of reaction, it is to be
expected that specific peculiarities should be shown in regard to the after-
effects of photonastic stimulation. In fact they may persist for a long time
in some plants but only for a day in others, even when they had been per-
forming pronounced sleep-movements every night during the whole of their
adult existence. In addition, no periodic after-affects appear to be produced
in photonastic flowers by the alternation of night and day, and little or no
after-effect appears to be exercised by the pronounced thermonastic opening
and closing movements of the flowers of Crocus and Tulipa 3.
The periodic after-affects when present follow approximately the same
rhythm as the nyctinastic movements which give rise to them, and hence
the one aids the other. The times of oscillation of a simple pendulum
swinging in still air vary, somewhat according to their amplitude, and the
successive after-effects of periodic stimulation are still less isochronous than
1 On the feeble periodicity induced by rhythmically-repeated geotropic or heliotropic stimuli, cf.
Darwin and Pertz, Annals of Botany, 1903, Vol. xvii, p. 93.
2 Pfeffer, 1. c. It is difficult to understand how Schwendener (1897, Gesammelte bot. Mittheil.,
Bd. II, p. 241) can be in any doubt as to the propriety of using this analogy with a pendulum as an
illustration of the nature of periodicity and of periodic phenomena.
3 Pfeffer, 1. c., p. 133 ; Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 349.
THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY in
the swings of a pendulum. The rhythm of a simple pendulum is constant
so long as its length and the force of gravity are unaltered. On the other
hand, the photonastic rhythm of a living organ can be made to follow
periods of more or of less than twenty-four hours by corresponding
alterations of the periods of illumination and darkness *.
It is worthy of note that the nyctinastic periodicity vanishes com-
paratively rapidly and hence never becomes hereditary, although it may
have been regularly repeated through countless generations. Only in
very few cases, in fact, is a hereditary transmission of a long induced
periodicity possible. This actually applies to the resting and flowering
periods of certain plants, for when transferred to other climates, a new
hereditary rhythm may be gradually induced which is appropriate to the
altered seasons.
The spontaneous movements of the leaflets of Oxalis, Trifolium, and
of the terminal leaflet of Desmodium gyrans retain the same rhythm of
forty-five minutes to four hours under continuous illumination, whereas the
periodic nyctinastic movements gradually cease. Hence the latter cannot
be derived by the regulation of the spontaneous movements 2, although in
other cases a particular rhythm may result from the regulation of an
inherent periodicity, as is in part the case with the yearly periodicity.
The power of photonastic response is not necessarily coupled with
a pronounced thermonastic irritability, and most photonastic organs are
irresponsive to mechanical stimuli such as produce pronounced movements
in the leaflets of Mimosa and Oxalisz. Pronounced spontaneous move-
ments are shown by certain leaves, but are absent from most organs
capable of sleep -movements such as the leaves of Acacia lophantha,
Impatiens, and Sigesbeckia, while the lateral leaflets of Desmodium gyrans
which show rapid spontaneous movements perform no sleep-movements4.
Historical The sleep-movements of certain plants were first noted by Pliny, and
by Albertus Magnus, but Linnaeus was the first to call attention to the common
occurrence of nyctinastic movements among leaves and flowers 5. The subsequent
researches, which were mainly concerned with the mechanics and causes of the
phenomenon, left it uncertain whether the daily rhythm was due to the periodic
1 Cf. Pfeffer, 1. c., pp. 39, 43, 53. The time of reaction naturally sets a limit to the possible
shortening of the rhythmic period.
2 Pfeffer, 1. c., pp. 35, 52.
3 Conversely mechanically irritable organs such as the stamens of Cynareae, various stigmas and
tendrils, perform no sleep-movements, and the same applies to the leaves of Dionata (Munk, Die
elektrischen u. Bewegungserscheinungen von Dionaea, 1876, p. 101), and of Drosera rotundifolia
(Kabsch, Bot. Ztg., 1860, p. 247).
* Darwin, The Power of Movement in Plants.
5 For details on the historical development of this subject see Pfeffer, Periodische Bewegungen,
l875» PP- 3°> l63-
H2 MOVEMENTS OF CURVATURE
changes of illumination or of temperature, or whether it was the result of the
regulation of a hereditary periodicity. De Candolle at first inclined to the former
view, but later appears to have assumed that the periodicity was hereditary1.
Dutrochet2, Sachs3, and Hofmeister4 all adopted the same view, and apparently
considered that the periodic illumination regulated the rhythm, but did not induce
it, while the continuance of the movements in darkness was ascribed either to a
hereditary periodicity or to the incomplete absence of light. Pfeffer then showed,
in 1876, the induced character of the periodicity, and pointed out that the daily
movements might be produced by thermonastic responses as well as by photo-
nastic ones, or by a combination of the two. Royer5 went, however, too far in
ascribing all sleep-movements of flowers to changes of temperature, while it is
evident that all daily movements are not the result of circumnutation as Darwin
supposed, nor is the daily periodicity capable of hereditary transmission.
Methods. Pfeffer employed the light from a couple of Argand burners, which was
passed through cold water to diminish the heating effect 6. Nowadays, incandescent
burners, arc lights, or Nernst lamps might be used in preference. The incandescent
electric light is less suitable for the reinduction of the photonastic periodicity, since
it contains relatively fewer of the blue rays, which exercise the greatest photonastic
action 7. It has been observed that certain plants cease to perform sleep-movements
during the continuous summer day of high northern latitudes, as in the north of
Norway8. By the aid of artificial illumination, the periodicity may be reversed,
so that the sleep-movements take place in the daytime, or it may be lengthened or
shortened 9. Experiments in darkness are only decisive when the absence of light
does not appreciably affect the power of reaction.
SECTION 24. Thermonastic Curvatures.
Apart from the general influence of temperature on growth, a special
power of thermonastic response has been developed by various flowers, in
which low temperatures produce closing movements and high temperatures
opening ones. The flowers of Crocus vernus and Crocus luteus are especially
responsive, as are also those of Tulipa Gesneriana, for these flowers per-
ceptibly respond to a change of temperature of half a degree centigrade,
1 A. P. de Candolle, Physiologic des Plantes, a German translation by Roper, 1835, Bd. n,
p. 640.
Dutrochet, Memoires p. serv. a 1'histoire etc., Bruxelles, 1837, P- 287-
Sachs, Flora, 1863, p. 469.
Hofmeister, Pflanzenzelle, 1867, p. 331.
Royer, Ann. de sci. nat., 1868, ve ser., T. ix, p. 355. Cf. Pfeffer, 1. c., p. 170.
Pfeffer, 1. c., p. 31. The experiments of other authors are discussed here.
Cf. Pfeffer, 1. c., p. 67.
8 Cf. Schubler, Die Pflanzenwelt Norwegens, 1873, p. 88 ; Bot. Jahresb., 1880, p. 262.
9 Pfeffer, 1. c., pp. 40, 55. On the registration of the movement see Baranetzsky, Ber. d. bot.
Ges., 1899, p. 190.
THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY 113
and which pass from the closed to the fully expanded condition in a few
minutes when the temperature rises from 12° to 22° C. (Fig. 32). The
flowers of Adonis vernalis, Ornithogalum umbellatum^ and Colchicum
autumnale react more slowly and less strongly, while those of Ranunculus
Ficaria^ Anemone nemorosa, and Malope trifida are still less sensitive,
although they respond to changes of temperature of 5° to 10° C. by a
distinct movement. On the
other hand, such changes of
temperature induce only a
slight thermonastic movement
in the flowers of Oxalis rosea,
Nymphaea alba> Leontodon,
and Taraxacum 1, and flowers
which open once only show
no distinct thermonastic re-
sponses, although their open-
ing is hastened by moderately
high temperatures and re-
tarded by low ones.
Foliage-leaves usually re-
act but feebly to changes of
temperature, although these induce perceptible thermonastic responses in
the pulvini of the leaflets of Oxalis acetosella^ Desmodium gyrans^ Averrhoa
Bilimbi) and Mimosa pudica 2. In addition, the bilobed leaf of Aldrovanda
only opens when the temperature is raised sufficiently, while either a rise
or a pronounced fall of temperature may produce a thermonastic curvature
in dorsiventral tendrils and this is similar in character to the thigmotropic
curvature 3.
FIG. 32. Flower of Crocus luteus. A closed, B expanded
owing to a rise of temperature.
1 Pfeffer, Physiol. Unters., 1873, p. 194; Periodische Bewegungen, 1875, p. 122. Crocus and
Tulipa react so rapidly that the movement may be demonstrated to a large audience by means of
a projection lantern. Cf. Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 731. The simple observa-
tion that warming hastened the opening of the flowers of an anemone was made by Cornutus (quoted
by Ray, Historia plantarum, 1686, Vol. I, p. 2). Hofmeister (Flora, 1862, p. 516) found that varia-
tions of temperature produced opening and closing movements in the flower of the garden tulip ;^and
Royer (Ann. sci. nat., 1868, ve ser., T. IX, p. 355) regarded changes of humidity and temperature
as being responsible for the opening and closing of flowers. The true relationships were f then
established by Pfeffer. Additional instances of thennonastic flowers are given by Hansgirg,
Physiolog. u. Phycophytolog. Unters., 1893, pp. 27, 64. According to Mikosch (Bot. Jahrb., 1878,
p. 219), changes of temperature induce the opening and closing of the anthers of Bulbocodiunt
vernum and of certain species of Alchemilla, but it is not certain whether the reaction is a true
thermonastic one.
8 Pfeffer, Physiol. Unters., 1873, pp. 65, 78 ; Periodische Bewegungen, 1875, p. 135 ; Darwin,
The Power of Movement in Plants ; Jost, Jahrb. f. wiss. Bot., 1898, Bd. XXXI, p. 376 ; Bot. Ztg.,
1 897» P- 35-
8 Correns, Bot. Ztg., 1896, p. 2 ; Macdougal, Bot. Centralbl., 1896, Bd. LXVI, p. 145; Fitting,
Jahrb. f. wiss. Bot., 1903, Bd. xxxvni, p. 562.
PFEFFER. Ill
1 14 MOVEMENTS OF CURVATURE
According to Vochting1, sufficient cooling causes certain shoots of
Mimulus Tilingii and the flower-stalks of Anemone stellata to assume
a drooping position ; and, according to Lidforss2, the same applies to the
shoots of such plants as Lamium purpureum^ Veronica chamaedrys, and
Chrysanthemum leucanthemum. In addition, the evening drooping of
certain flowers and inflorescences appears in part to be the result of a
thermonastic reaction, such as may also be responsible for the drooping of
the shoot and leaves of many plants under natural conditions when the
temperature falls nearly to the freezing-point. It has, however, yet to be
determined whether these latter effects are actually due to a thermonastic
reaction, or are merely the result of the tissues being flaccid. The phe-
nomenon may indeed be as complex in origin as is the downward curvature
and plagiotropic position shown by the shoots of various plants in autumn
and winter 3. It is worthy of note that the changed orientation takes
place slowly, and that the daily changes of illumination induce no marked
movement in these cases.
Rapidly reacting thermonastic organs usually pass at first beyond the
position which they ultimately assume when the changed temperature is
maintained for some time. This is especially well shown by the flowers of
Crocus luteus, for the perianth-segments become temporarily partially
reflexed when the temperature is suddenly raised (Fig. 32, p. 113), whereas
when the temperature is raised slowly they hardly pass beyond the
position maintained by them so long as the new temperature remains
constant.
After the removal of all the perianth-segments but one, exactly similar
reactions to sudden and to gradual falls of temperatures can be traced for
the closing movement as for that of opening. As the result of its thermo-
nastic properties the intact flower of Crocus may pass through its entire
period of development without ever opening if the temperature is kept
below 8° or even I2°C. 4 The inflorescences of Leontodon hastilis, Hiera-
cium vulgatum and the flowers of Oxalis rosea remain closed at i° to 3° C.
even in diffuse daylight, partially open at 8° to 10° C., but do not fully
1 Vochting, Ber. d. bot. Ges., 1898, p. 42 ; Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 285.
2 Lidforss, Bot. Centralbl., 1901, Bd. LXXXVI, p. 169; Jahrb. f. wiss. Bot., 1902, Bd. xxxvm,
p. 343. According to Vochting (Bot. Ztg., 1902, pp. 90, 107), a fall of temperature also causes
young potato-shoots to droop. We are here only dealing with the results of a change of temperature
under otherwise constant conditions, and leave it an open question as to how far the results observed
are due to induced changes of geotropic irritability.
3 Cf. Vochting, Ber. d. bot. Ges., 1898, p. 50 ; Warming, Oekologische Pflanzengeographie,
a German translation by Knoblauch, 1896, p. 26 ; Krasan, Engler's bot. Jahrb., 1882, p. 185 ;
Lidforss, 1. c.
* Pfeffer, Physiol. Unters., 1873, p. 189; Period. Bewegungen, 1875, p. 131; Jost, Jahrb. f.
wiss. Bot., 1898, Bd. xxxi, p. 352.
THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY 115
expand until the temperature is still more favourable l. In addition, the
flowers of Spergula salina, as well as those of Hordettm distickum, and of
a few other grasses, remain closed at low temperatures, while there are
presumably numerous plants whose flowers perform no pronounced opening
and closing movements but remain closed at low temperatures 2.
Many organs are capable of both thermonastic and photonastic move-
ment, although usually those organs which are highly thermonastic are
only feebly photonastic, and the converse is also true 3. Thus the daily
opening and closing of the feebly photonastic flowers of Crocus and Tulipa
are mainly determined by the changes of temperature ; and the rapid
opening usually produced by insolation is mainly the result of the heating
effect of the sun's rays. Even a small fall of temperature is sufficient to
produce the closure of the flower in spite of the feeble opposed photonastic
action produced by exposure to diffuse daylight.
The daily temperature-curve, and hence also that of the resulting
thermonastic movements, are much more irregular than the periodicity
dependent upon the changes of illumination. Hence the absence or
feebleness of any induced periodicity in the flowers of Crocus and Tulipa^
and in other thermonastic organs, enables them to assume positions directly
corresponding to the prevailing temperatures. In this way spring flowers,
among which most strongly thermonastic flowers are included, are able
to remain closed on cold days — a fact of considerable biological importance.
When an organ is capable of both thermonastic and photonastic
response, increases of temperature and of illumination usually produce
similarly directed movements, and the same applies to decreases. In general,
therefore, the changes of illumination and of temperature co-operate in pro-
ducing the sleep-movements. Curvature can be induced in dorsiventral
organs in various ways even when the general rate of growth is accelerated
by a moderate rise of temperature but slightly retarded by a concomitant
increase of illumination. In addition, a transitory acceleration of growth
may result from the shock due to a sudden change of temperature or of
illumination. Nor is it surprising that in certain cases the thermonastic
and photonastic responses should be dissimilar in character. Thus Vochting*
found that a decrease of illumination produced an upward curvature in
certain shoots of Mimulus Tilingii, and, according to Jost5, decreases of
temperature and of illumination produce opposed movements in the case
of the leaflets of Mimosa pudica. It is, however, not certain whether in
all cases a rise of temperature above the optimum might produce a reversal
1 Pfeffer, 1873, 1. c., p. 189.
2 For facts see Hansgirg, Physiolog. u. Phycophytologische Unters., 1893, pp. 30, 46, 64;
Fritsch, Bot. Ztg., 1852, p. 897. Further critical investigation of these facts is, however, requisite.
3 Pfeffer, Period. Bewegungen, 1875, P- I22«
4 Ber. d. bot. Ges., 1898, pp. 39, 45. 5 Bot. Ztg., 1897, p. 35.
I a
n6 MOVEMENTS OF CURVATURE
of the thermonastic response such as is shown by photonastic organs
exposed to increasing illumination. A reversal of this kind does actually
appear to occur in the flowers of Crocus 1, and possibly also in the leaves of
Oxalis when the temperature rises above the optimum. It is, in fact, not
impossible that thermonastic reactions of this kind may play a more or less
prominent part in the assumption of the midday sleep-positions of the
leaves of so many tropical plants.
SECTION 25. Hydronastic Movements.
As is well known, changes in the percentage of water in plants com-
monly cause disturbances of equilibrium leading to movement. Thus the
diminished rigidity due to a decrease of turgidity leads to the flaccid
drooping of shoots and leaves, and in the case of dorsiventral organs
changes of turgidity which affect the antagonistic tissues unequally may
cause curvature. In general, the percentage and supply of water form
physiological conditions whose modification affects the activity of growth
and the power of response in much the same way that changes of tem-
perature do. Granted an appropriate structure, changes in the hydric
relationships may even induce hydronastic curvatures, as physiological
stimulatory reactions which are widely distinct in character from the purely
physical movements mentioned above. It is of course always possible that
the same loss of turgidity which primarily produces a drooping movement
may also act as a stimulus to a physiological curvature of like or of unlike
kind. Furthermore, this curvature may either result from a modification of
growth or may be a variation movement due to appropriate changes of
turgor induced in response to stimulation.
The use of the term hydronasty to denote curvatures produced by
changes in the hydric relationships says nothing as to the nature of this
form of irritability or as to the mode of perception of stimuli. The hydric
relationships may, however, exercise various stimulatory actions on growth,
so that hydronastic responses may be of varied origin. Hitherto the
researches have mainly been confined to determining the existence of such
reactions, and frequently no proper discrimination has been made between
the physical and physiological responses. Hence only a general account of
the phenomena observed and their distribution is possible at present.
Indeed, it will always be difficult to determine whether, in a given case, the
actual excitation is due to a fall of turgor, to a movement of water with or
without transpiration, to changes of consistency, or to other factors 2.
1 Cf. Pfeffer, Physiol. Unters., 1873, p. 190; Jost, Jahrb. f. wiss. Bot, 1898, Bd. XXXI,
PP- 351* 358-
a Since these movements are not solely produced in response to changes of turgor, the general
HYDRONASTIC MOVEMENTS H7
It is possibly owing to a hydronastic response that the position of many
foliage and floral leaves alters when the plant is freely watered or is brought
from dry air into a moist chamber. The changed position is maintained
under the new conditions, and ultimately becomes permanent when the
adult leaf ceases to grow. Evidently we are dealing with a physiological
growth-reaction, and not with a movement due to a temporary change ot
turgor. Similarly, the changes in the position of the foliage and floral
leaves observed by Kraus, Wiesner, and Hansgirg \ as the result of
alterations of turgidity, appear largely to be hydronastic in character.
Unfortunately, the other external conditions were frequently not kept
constant during these observations, and, in addition, insufficient attention
has been paid to the physical movements resulting from the changes of
turgor and to the influence of the latter upon the power of physiological
response. Hence the observations are not altogether satisfactory, and fail
to indicate the extent to which hydronastic reactions are responsible for the
result observed. From Kraus's researches it does, however, appear as
though the hydronastic equilibrium of the foliage-leaves of a variety of
plants was considerably disturbed by pronounced rises or falls of turgidity,
and the experiments of Wiesner and Hansgirg seem to indicate the same
for floral leaves. Thus the flowers of Anagallis coerulea and Gentiana
amarella close or remain closed in air saturated with moisture, according
to Wiesner, even when exposed to optimal illumination ; and the same
applies to the flowers of Stellaria media and Holosteum meditim, according
to Hansgirg, when submerged under water 2.
, Since a variety of factors come into play under natural conditions, it is
not possible to say in what degree hydronastic actions may be responsible
for the assumption of different fixed positions by leaves on dry and moist
habitats 3. Hydronastic responses take little or no part in the daily move-
ments of leaves and of periodic flowers, for these are primarily induced by
term ' hydronasty ' for this phenomenon seems preferable to that of turgonasty employed by
Hansgirg (Physiol. und Phycophytol. Unters., 1893, p. 1 1). [No additional terms are likely to become
essential even when the subject is further studied, and there seems to be no valid reason for retaining
the term turgonasty to represent those instances in which changes of turgor act as the stimulus. In
any case the terminations f -nasty ' and ' -tropism ' must be restricted to physiological responses, and
no special terms are needed for physical movements induced by turgor, by hygroscopicity, or by
imbibition and swelling. To invent unnecessary special terms is merely to strew the path of know-
ledge with useless lumber which tends to acquire a fictitious value in the eyes of those forced
subsequently to struggle over these obstacles.]
1 C. Kraus, Flora, 1879, p. n ; Wiesner, Sitzungsb. d. Wiener Akad., 1882, Bd. LXXXVI,
Abth. i, p. 212 ; Hansgirg, Physiol. u. Phycophytol. Unters., 1893, pp. 32, 42, 48.
2 According to the authors named (cf. also Planchon, Bull, de la Soc. bot. de France, 1896,
T. XLIII) there are also flowers which close when their turgidity decreases, and it appears that
certain flowers which are expanded when the turgidity is normal perform a hydronastic closing
movement when the turgidity either rises or falls.
3 Cf. Stenstrb'm, Flora, 1895, p. 132.
n8 MOVEMENTS OF CURVATURE
changes of illumination and of temperature, and may still continue under
water, or in air saturated with moisture, in which the turgidity of the tissues
is maintained at the highest possible limit. The fact that such flowers and
leaves often perform sleep-movements when the sky becomes cloudy or
before the fall of rain is due to the induction of a photonastic or thermo-
nastic response, which is accelerated by the rise of turgor due to the
diminution of transpiration *. It has, however, yet to be shown that any
plants exist in which pronounced daily hydronastic sleep-movements are
produced by the normal daily changes of turgidity.
Kraus, Wiesner, and Hansgirg have all shown that in many cases a moderate
change of turgidity produces a pronounced physical curvature, resulting in the
sinking of leaves or the closure of flowers, quite apart from the usual drooping
due to a pronounced fall of turgor. These movements often have considerable
biological importance by reducing the exposure, and in the same way the rolling-up
or folding of certain leaves by reducing the surface exposed aids in rendering them
resistant to desiccation2. The daily changes of turgor due to transpiration may
naturally cause the periodic repetition of the associated physical movements. Naturally
also, oscillations are bound to occur when the changes of turgor due to the sudden
withdrawal or absorption of water are produced with unequal rapidity in the tissues
on opposite sides of an organ 8.
Physical movements of this kind are possible, not only in growing organs,
but also in adult pulvini, although in most cases little or no effect is produced
by a moderate loss of water. A readier response is, however, given by a certain
form of Porliera hygrometrica, in which a deficiency of water causes a more or less
complete folding of the leaflets 4. The contradictory observations upon the influence
of moisture upon the leaf movements of Porliera are partly due to the fact that all
forms are not equally sensitive, and that the removal of water was less pronounced
in some cases than in others. Paoletti and Pantanelli 5 have recently shown that the
daily sleep-movements of this plant are produced in the usual way by changes of
illumination.
The continuance of the daily movements under water shows that they are not
of hydronastic origin, although in time the movement and power of reaction
disappear from the submerged plant. This is, in part, due to the injurious action
exercised by the insufficient supply of oxygen, owing to the diminution or almost
complete cessation of the gaseous exchanges, and by the cessation of transpiration.
1 Cf. Pfeffer, Physiol. Unters., 1873, p. 188 ; Period. Bewegtmgen, 1875, p. 137, and the literature
quoted in these works. Kraus, 1. c., p. 35 ; F. W. Oliver, Bot. Centralbl., 1891, Bd. XLV, p. 52 ;
Hansgirg, 1. c., pp. 40, 122.
3 See Ludwig, Biologic d. Pflanzen, 1895, p. 194; Tschirch, Jahrb. f. wiss. Bot., 1882, Bd. xiii,
P- 544-
3 Cf. Pfeffer, Period. Bewegungen, 1875, p. 137.
* Darwin, The Power of Movement in Plants.
5 Paoletti, Nuovo giornale botanico italiano, 1892, T. xxiv, p. 65 ; Pantanelli, Studi d'anat. e
fisiolog. sui pulvini motori, 1901, p. 258.
HYDRONASTIC MOVEMENTS 119
The former action alone is sufficient to explain the decrease in or cessation of the
power of reaction as the result of injecting the intercellular spaces with water \
The growth and development of many plants are strongly affected by submersion in
water, and hence it is not surprising that, according to Hansgirg, certain flowers
which remain closed under water open when placed in air saturated with moisture,
although the turgidity remains at its maximal limit. In addition, the leaves of
Callitriche assume different positions in moist air to what they do in water 2.
SECTION 26. Conjoint Effects.
Changes of temperature and of turgor always influence to a greater or
less extent the progress and amplitude of the photonastic daily movements,
either owing to their influence upon the power of response, or to their
awakening special thermonastic or hydronastic reactions. In addition, the
induced after-effects may cause a periodic repetition of the movements, and
this tendency acts in the same way as that to a movement of autonomic origin.
The simplest series of combinations is given when only the illumination
varies, the other conditions remaining constant, so that the daily movements
are due to the co-operation of the photonastic responses to changes of
illumination with the periodic after-effects which, under normal conditions,
follow approximately the same rhythm.
The degree to which the directly induced closing or opening movements
exceed that due to the after-effect of previous stimulation will depend
upon the readiness of the plant's photonastic response, and upon the intensity
of the after-effect. Naturally, however, the full possible movement may
not be shown when the different responding organs press against one
another. Both the after-effect and the original photonastic response involve
oscillations about the ultimate position of equilibrium, and hence action
excites reaction. It depends upon the time period of the after-effect of the
photonastic reaction whether the maximum movement is attained imme-
diately after sunrise or sunfall, or later on in the day or night, and also
whether the opening and closing of flowers is rapidly or slowly induced.
On cloudy days the photonastic reactions are feebler than usual at
morning and evening, so that the amplitude of the daily movements is con-
siderably reduced when the after-effects are less active than the direct
photonastic reactions. Naturally also, a plant placed in darkness from
morning onwards will perform less pronounced sleep -movements, or will
take longer to produce them, than one illuminated during the day and
hence strongly stimulated by the failure of the light in the evening. These
and similar consequences follow naturally from the facts put forward by
1 Pfeffer, Physiol. Unters., 1873, pp. 75, 98, 188.
2 Frank, Cohn's Beitrage zur Biologic, 1872, Bd. I, Heft 2, p. 80.
120 MOVEMENTS OF CURVATURE
Pfeffer1, but sufficient attention has not always been paid to this by
Oltmanns 2 in his interpretation of the opening and closing movements of
flowers.
Since the power of reaction is always present, numerous and often pro-
nounced oscillations may occur as the result of variations of illumination
during the day, especially in the case of organs exhibiting strong photonastic
irritability. Darkening at midday produces, however, more effect than in
the morning, since in the first case the photonastic response is aided by the
incipient periodic after-effect. Hence the appearance of thunder-clouds at
midday may cause the leaves and flowers of many plants to perform sleep-
movements, whereas the same darkening during the early morning may only
induce a feeble closing movement3. If, however, the periodic after-effect is
strong, but the direct photonastic reaction feeble, darkening in the morning
may cause an only temporary retardation or reversal of the opening move-
ment, which is ultimately resumed and completed 4.
Illumination during the evening closure acts in the same manner, and
in strongly photonastic plants such as Mimosa and Acacia the leaves may
be brought back into the expanded position by illumination applied at the
close of a cloudy day6. If a plant is illuminated during the night and
darkened during the day, a rhythm corresponding to the altered conditions
will be more or less rapidly induced after a few irregularities, and the new
rhythm may be capable of persisting for more or for less than a day.
Since some time is required for the accommodation to the new conditions,
a previously darkened plant must be exposed to light for some time before renewed
darkening is able to produce a perceptible response. The leaves of Acacia lophantha,
and of Impatiens noli-me-tangere, are able to show a feeble photonastic response to
darkening after five to ten minutes' illumination, and after thirty minutes to an
hour's exposure they are capable of exhibiting a maximal photonastic response, which
undergoes no further increase, even after prolonged constant illumination 6. [The
photonastic response to intense illumination is much more rapidly produced, and in
this way its utility as a protection against temporary intense exposure is considerably
enhanced. Thus, in highly turgid plants of Mimosa pudica, the leaflets begin to fold
together a second or two after strong sunlight falls upon them, and in thirty seconds
to a minute become completely folded. The re-expansion in weak diffuse daylight
takes from one to three minutes after short exposure, but a longer time is required
when the exposure has been more prolonged. After midday the responses are
usually less rapid, but this appears to be due merely to the lessened turgidity.
Owing to the existence of a latent period, and an after-effect, the stimuli due to short
periods of exposure may be summated so as to produce a response, and for the same
1 Period. Bewegungen, 1875. a Bot. Ztg., 1895, p. 44.
3 Pfeffer, Physiol. Unters., 1873, p. aoi. 4 L. c., 1875, p. 71.
8 This opposed action was utilized by Pfeffer to produce a more rapid elimination of the daily
periodicity under continuous illumination (1. c., 1875, pp. 35, 71).
6 Pfeffer, 1. c., 1875, p. 57.
CONJOINT EFFECTS 121
reason leaflets folded in sunlight re-expand at first in darkness before they show
a nyctinastic response. The leaves of Acacia, Dalbergta, and Robinia require
at least two to three minutes to fold together in sunlight, and may fully expand after
being shaded for five to eight minutes. The leaves of Bauhinia, Albizzia, Calli-
andra, and Cassia respond still more slowly, the closure requiring five minutes
to half an hour, and the re-expansion ten minutes to two hours *.]
A thermonastic response is also only possible when the change of temperature
persists for some time and, since in all cases we are dealing with phenomena of
irritability, the extent of the reaction always depends upon the power of perception.
It is clear, however, that the time required to produce the maximal movement of
a pulvinus will depend upon the rapidity with which a change of turgor follows
a change of illumination. Furthermore, in cases where the movement is due to
growth, rapidly repeated intense stimulation may produce a certain fatigue effect,
such as appears to be shown by the flowers of Crocus after several responses to
thermonastic stimulation 2.
It is, however, not certain whether the power of photonastic reaction is affected
by the movement subsequently induced, for other extraneous demands often influence
the power of response. Nevertheless, the increased action of darkening after midday
appears to be due merely to the co-operation of the photonastic reaction with the
induced periodicity. In any case, however, the summation of dissimilar stimuli
involves more complex reactions than that due to the frequent repetition of the same
stimulus.
The latent period of stimulation is shorter in the case of parahelionastic
responses than of nyctinastic ones. The minimal difference of illumination required
to produce a perceptible response varies in different cases, the leaflets of Mimosa
pudica being especially sensitive. Increasing stimulation produces increasing
responses within certain limits, but the stimulation needs to increase in geometric
proportion to produce equal additional increments of response, quite apart from
the reversal of the reaction which ensues under intense illumination.
Further investigation is needed to determine whether increases of illumination
or temperature always produce the same amount of response as decreases, and whether
the response is equally rapid in both cases. As regards parahelionastic responses,
the closure of the leaflets is always more rapidly produced than the expansion under
diminished illumination 3. The influence of increases and decreases of temperature
and of illumination on growth are not equally pronounced, and exposure to light
produces a smaller rise of the leaves of Impatiens noli-me-tangerc than the subsequent
fall on darkening. It is, however, always possible that in such cases the leaf had an
inherent tendency to curve to one side, which would minimize the induced curvature
in the opposite direction. Especially in the case of transitory stimulation, increases
of illumination or temperature may exercise effects which differ quantitatively and
qualitatively from those produced by similar decreases. Correns* found, in fact,
1 Ewart, Annals of Botany, Vol. xi, 1897, p. 447 seq.
3 Pfeffer, Physiol. Unters., 1873, p. 182.
3 Cf. Ewart, Annals of Botany, Vol. xi, 1897, p. 447. * Bot. Ztg., 1896, p. 13.
122
MOVEMENTS OF CURVATURE
an increase of temperature produced a greater curvature in tendrils than a corre-
sponding decrease. In the case of thermonastic flowers, however, if the same
peculiarity were always shown, it should be possible, by repeated equal and slight
rises and falls of temperature, to make the flower become fully expanded at com-
paratively low temperatures.
The sensation we experience on passing from darkness into intense light is not
the same as is produced by the reverse procedure, and the same may apply to plants.
Indeed, certain micro-organisms show a different response to increases of illumina-
tion to that produced by decreases. In addition, many organisms are able to
withstand sudden increases in the concentration of the surrounding medium, whereas
corresponding decreases may cause them to burst. Finally, very many chemical
changes which are induced by rises of temperature or of illumination are not
reversible.
An analogy is afforded by two metal thermometers of which one responds
more rapidly than the other, and which are so arranged that, when warmed, contact
is made, and an electric bell-circuit completed when a certain temperature is reached,
whereas a fall to the same temperature produces no contact or electrical excitation.
In addition, a clock strikes when the hands are moved round in the normal way, but
not when they are turned in the opposite direction.
In the case of the mainly thermonastic flowers of Crocus and Tulipa
a slight rise of temperature is sufficient to overcome the tendency to closure
induced by darkness, whereas a pronounced fall of temperature is unable
to reverse the daily opening movement of the mainly photonastic flowers
of Nymphaea alba, Oxalis rosea, Leontodon has tills and Taraxacum offici-
nale ]. Similarly, many photonastic flowers do not open at low temperatures
such as i° C. to 3° C., or only experience a slight temporary or permanent
opening movement when illuminated under these conditions. Further,
many flowers whjch open early and only close in the evening may remain
open only for a short period of the day under special conditions 2. The
heads of Leontodon hastilis and of Taraxacum officinale may open little
or not at all during the day if kept during the day and previous night
at a temperature of 3° to 4° C., and may open in the evening m darkness
as the result of the inductive action of the previous illumination when
the temperature is raised to about 20° C.3
In addition to these factors the influence of the turgor upon the power
of reaction may cause the daily rhythm of the sleep-movements to ex-
perience certain modifications which may in some cases become extremely
pronounced, while the times of opening and closing of flowers may
1 Pfeffer, Physiol. Unters., 1873, pp. 195, 206; Period. Bewegungen, 1875, p. 133.
a Cf. Oltmanns, Bot. Ztg., 1895, pp. 31, 50. Oltmanns puts forward, however, a one-sided
interpretation of the origin of the early-closing movement.
3 Pfeffer, Physiol. Unters., 1873, p. 197 '
CONJOINT EFFECTS 123
fluctuate for similar reasons, and also in accordance with the . length of
the day1.
SECTION 27. Conjoint Effects (continued).
The simplest response involves such factors as the mechanical resistance
of the curving organ, as well as the stimulatory effects due to the mere
progress of the curvature and to the altered geotropic induction due to
the changes of position. The statical moment of a leaf alters when it
passes from the horizontally expanded position into a vertical one, and
this is bound to exercise a certain influence upon the progress of the
curvature, as does also the fact that more work is done when a leaf is
raised than when it sinks downwards.
The energy of movement is, however, usually so great that the
mechanical factors due to the weight of the leaf are of minor importance or
may be negligible 2. In the case of Mimosa pudica, however, the secondary
petioles move forwards at evening3, and the increased statical moment
of the leaf causes a pronounced sinking of the primary petiole, which only
rises above the position during the progress of the night, although darken-
ing during the day always causes it to perform an upward photonastic
movement 3. That this evening fall of the primary petiole results from
a photonastic reaction coupled with the increased moment exercised when
the secondary petioles come more into line with the main one is shown
by the fact that it gradually ceases when the change of position of the
secondary petioles is mechanically prevented. The evening movement
of the primary petiole then corresponds from the commencement with that
produced by darkening, as it does in other plants. When the secondary
petioles are released, a certain sinking of the main petiole ensues on the
following evening, and increases gradually until, after five or ten days, it
has reached its original amplitude. The evening fall is, therefore, due
to the co-operation of the evening photonastic action with the after-effects
of previous response and the mechanical actions resulting from the move-
ment of the secondary petioles. As the result of the induced after-effects,
the evening fall of the primary petiole only ceases a week or a fortnight
after the secondary petioles have been fixed.
1 On the opening and closing times of flowers and on floral clocks cf. Kerner, Pflanzenleben,
1891, Bd. II, p. 2ii (Natural History of Plants, 1895, Vol. n, p. 212); Burgerstein, Ueber die
nyctitropischen Bewegungen der Perianthien, 1887, p. 39 ; Oesterreich. Bot. Ztg., 1901, Nr. i.
a Cf. Pfeffer, Period. Bewegungen, 1875, p. 144. The mechanically stimulated leaf shown in
Fig' J9> P« 61, will serve also to show the evening position.
3 Pfeffer, Period. Bewegungen, 1875, p. 73. The normal progress of the daily movements was
described in detail by Millardet, Nouv. Recherches sur la pe"riodicite" de la tension, 1869 (reprinted
from M&n. de la Soc. d. sci. nat. de Strasbourg, T. vi).
124 MOVEMENTS OF CURVATURE
Although the dependence of the sinking of the primary petiole upon the
movement of the secondary petioles has been empirically determined, it does not
follow that the fall is directly due to their increased statical moment, which may
sometimes increase by as much as one-half. A suddenly increased load does
actually cause a perceptible fall of the primary petiole, but there can be no doubt
that we are here dealing with a complex physiological reaction. Indeed, in their
normal habitat, the primary pulvini of Mimosa are capable of response to mechanical
excitation, even when the petiole has reached its lowest nyctinastic position, and
when the plant is highly turgid the pulvinus may curve to such an extent as to
temporarily bend the leaf back across the stem in a partially inverted position, so that
the mechanical moment is considerably increased *. Pfeffer inclines to regard
the gradual increase of the mechanical moment as being the stimulus responsible
for the fall of the primary petiole, but it might also occur in indirect correlation with
the other movements without being directly due to them. In addition, this fall does
not always appear to occur 2, while Schilling has shown that, during the daytime,
a load causing an enforced curvature of the main pulvinus of Mimosa excites a
reaction tending to the restoration of the original position of equilibrium 8.
If leaves capable of sleep- movements are exposed during the day to light coming
from one side only, so that the plane of the leaf remains oblique during the day, next
morning they may again assume a similar position, even when in darkness4. This
after-effect may, however, be different in character to those resulting from realized
sleep-movements, which, even when mechanically prevented, may lead to after-effects
if the plant strives to produce them.
Photonastic and thermonastic curvatures are not only possible when
the required physiological dorsiventrality is of internal origin, but also
when it is due to the stable or labile induction of external factors. In
the last case, the power of aitionastic curvature is naturally only retained
as long as the induction persists, and the reaction is, therefore, rapidly
modified when an alteration in the external condition modifies the tone
of the organ. Certain negatively geotropic pulvini afford good instances
of the modification of the photonastic reaction by labile geotropic in-
1 Ewart, Annals of Botany, Vol. XI, 1898, p. 453. [The mechanical moment is less in the fully
drooping position than in any other. In addition, the mechanical moment may increase during the
assumption of the parahelionastic position by as much as it does at the commencement of the nycti-
nastic movement without producing any distinct fall of the primary petiole.]
3 Cunningham, Annals of the Royal Botanical Garden of Calcutta, 1895, Vol. vi, p. 135. [In
some cases the fall may take place without any movement of the secondary petioles, and the
temperature appears to have some effect. Plants of Mimosa pudica appear rarely to be capable
of the same rapidity of response in European hothouses as in their natural habitat, and in addition
readily fall into an irresponsive condition although the leaflets may remain green, normal, and capable
of photosynthesis.] The mechanical considerations put forward by Schwendener (Ges. bot. Mittheil.,
1897, Bd. n, p. 238) do not alter the facts in the least.
3 Schilling, Der Einfluss der Bewegungshemmungen auf die Arbeitsleistungen d. Blattgelenke
von Mimosa pudica, 1895.
* Darwin and Pertz, Proc. of the Phil. Soc., Cambridge, 1900, Vol. x, p. 259; Annals of Botany,
1903, Vol. xvu, p. 93.
CONJOINT EFFECTS
125
duction1. When such plants are inverted or rotated on a klinostat the
absence of the geotropic stimulus or its reversal causes the position of
the leaves to alter. This takes place with such rapidity in the case of
Phaseolus multiflortts and P. vulgaris that when the plant is inverted,
a leaf in the day position passes in the course of a few hours into a position
resembling that assumed during night (Fig. 33, a and b). The pulvini still
perform photonastic curvatures, but these now take place in the opposite
direction in regard to the plant.
Similar changes are shown by Desmodium gyrans, although in the
inverted position the
terminal leaflet does
not quite reach the
same angle as under
normal conditions. In
most pulvini, however,
the dorsiventrality is
fixed to such an ex-
tent that after inver-
sion the sleep-move-
ments retain the same
direction in regard to
the plant as before.
Fischer 2 has shown
that the same is the
case when the geotro-
pic action of gravity
is eliminated by rota-
tion on a klinostat.
Under these condi-
tions the sleep-move-
ments of Phaseolus
vulgaris, P. multi-
florus, and Lupinus
albus cease mainly
or entirely, so that the pulvini of these plants are physiologically radial
to photonastic stimuli in the absence of any geotropic induction. On the
other hand, in most plants such as Acacia lophantha, Trifolium pratense,
Amicia, and Biophytum sensitivum the photonastic reaction is mainly the
result of an inherent physiological dorsiventrality, since the sleep-move-
ments continue on a klinostat with considerable amplitude and in the same
direction as before.
FlG. 33. Inverted plant of Phaseolus multiflorus. The petioles of the first
pair of foliage- leaves are fixed by the wire d, so that only the pulvinus at the
base of the lamina is able to curve. The leaf a is in the day position, while b is
shown in the night position. The leaflets of the trifoliate leaf c are brought into
the normal light position by the curvature of the basal pulvinus, and hence
carry out the normal sleep-movements.
1 Pfeffer, Period. Bewegungen, 1875, P*
Fischer, Bot. Ztg., 1890, p. 672.
I26 MOVEMENTS OF CURVATURE
The amplitude of the daily movements of Cassia marylandica decreases
on the klinostat, while the geotropic induction seems to overcome the
inherent physiological dorsiventrality of Desmodium gyrans> since Fischer
found the sleep - movements of this plant continue in the usual direction
on a klinostat, and Pfeffer found that in the inverted position they were
reversed. It is, in fact, only natural that intermediate conditions should
exist between strictly autonyctinastic and strictly geonyctinastic plants \
Owing to the induced periodicity the daily movements do not at
once cease on a klinostat, but continue for some days with decreasing
amplitude, and under normal conditions slowly regain their original value.
When a plant of Phaseolus is inverted, however, the dominating influence
of the geotropic induction causes the sleep-movements to be reversed on
the very first day. Since the curvature of the pulvinus influences not
only the geotropic induction but also the photonastic tone, the progress
of the movement exerts a certain modifying influence upon its continuance,
quite apart from any geotropic or heliotropic action. Stahl2, in dis-
cussing the subject solely from a biological standpoint, has unfortunately
not properly distinguished the tropic orienting movements from the
aitionastic ones. Indeed, the movements of the leaves on vertical branches
may differ slightly from those on more horizontal ones for a variety of
reasons3. Hitherto experiments have been performed only upon the
variation movements of pulvini, but it seems probable that similar relation-
ships will be found to hold good for the daily movements due to growth4.
Dorsiventrality, whether morphological or physiological, usually in-
volves a more or less pronounced power of aitionastic reaction, and many
cases in which a labile or stable dorsiventrality is induced by unilateral
stimulation afford at the same time instances of the induction of photonasty,
thermonasty, and the like. Probably no tropic action leaves the power
of aitionastic reaction entirely unaffected, and Phaseohts affords a good
instance of the reversal or induction of the power of photonastic response
by geotropic action. Although in this case the induction is coupled with
a geotropic curvature, nevertheless in other cases pronounced structural
induction may take place without any special motile response. The
structure, however, affords no indication of the existence of a power of
aitionastic, tropic, or other irritability, and hence the photonastic irrita-
bility of the pulvinus of Phaseolus may be induced or reversed without
the dorsiventral structure of the pulvinus experiencing any perceptible
alteration.
1 A. Fischer, 1. c., p. 711. The term autonyctinastic is employed here in preference to that of
autonyctitropic. According to Fischer (1. c., p. 709), Mimosa pudica is also autonyctinastic, but it is
not stated whether the evening fall of the primary petiole continues on the klinostat.
• Stahl, Bot. Ztg., 1897, p. 86.
8 Darwin, The Power of Movement in Plants, 1880, p. 263.
4 Cf. Pfeffer, 1. c., p. 143.
CONJOINT EFFECTS 127
All aitionastic reactions dependent upon physiological reactions need
not, however, result in rapid or pronounced movement, for slow movements
may be of the utmost value in ensuring appropriate positions of the sub-
aerial organs more especially in regard to light. The rhizomes of Adoxa
moschatellina, of Circaea^ and of a few other plants show no power of
photonastic reaction when rotated on a klinostat, but do so when exposed
to the inductive action of gravity. When the rhizome has assumed
a transversely geotropic position in darkness, exposure to diffuse light
excites a downward curvature which increases to a certain maximum as
the illumination increases. Renewed darkening results in the assumption
of the original diageotropic position. The subaerial runners of certain
plants behave in the same way, for they become erect in darkness, and
curve to a horizontal position when exposed to sufficiently strong diffuse
illumination. Geotropic induction may indeed take place in several photo-
nastic responses, especially when the organ possesses a strong geotropic
irritability. According to Lidforss, the thermonastic reaction of the shoots
of Holosteum ttmbellattim, Lamium purpureum^ Veronica chamaedrys, and
Mimulus Tilingii depends upon geotropic induction, but not that of the
peduncles of Anemone nemorosa'1.
These curvatures are to be classed as photonastic, since under this
head we include all reactions due to changes in the intensity of the
diffuse illumination without specifying the detailed mode of perception
and response. The same would still be the case when the illumination
merely modified the geotropic irritability, and hence produced varying geo-
tropic curvatures according to its intensity. Indeed, if primary importance is
attached to the geotropic irritability, the illumination and temperature may
be regarded as modifying the geotropic tone, for, apart from all considera-
tions as to the internal physiological reactions, it remains the fact that
the same tropic action of gravity may produce varying degrees of
curvature according to whether the plant is strongly or feebly illuminated,
that is according to its phototonic condition.
The knowledge that a particular curvature is due to the co-operation
of light and gravity, the former altering while the latter remains constant,
does not reveal all that is to be learnt about the phenomenon. The
geotropic irritability might alter according to the intensity of the illumina-
tion ; or, the former remaining unaltered, the dorsiventrality induced by the
constant stimulus of gravity might co-operate with the variable photonastic
response. Other factors might also come into play, but it is clear that
in all cases the geotropic stimulus is as directive in character as when
the photonastic irritability is based upon an inherent dorsiventrality, and
1 Lidforss, Bot. Centralbl., 1901, Bd. LXXXVIII, p. 169; Jahrb. f. wiss. Bot., 1902, Bd. xxxvm,
P- 343-
128 MOVEMENTS OF CURVATURE
the position of equilibrium results from the co-operation of photonastic
and geotropic reactions. It is also evident that a physiologically radial
organ will no longer respond on a klinostat to changes of illumination,
independently of whether the action of gravity renders possible a photo-
nastic response by inducing a labile physiological dorsiventrality, or whether
the geotropic irritability alters according to the illumination.
No safe argument can be drawn by analogy, since the same result and
purpose may be obtained in various ways. Even if in a particular case
a labile induction, responsible for a photonastic reaction, persisted for a time
after the removal of the inducing external agent, its detection would not
show that the conjoint action of tropic and diffuse stimuli always takes
place in this way. It is, in fact, well known that both the geotropic and
phototropic irritabilities are capable not only of autonomic modification,
but may also be affected by various external factors.
The production of a power of photonastic response in the pulvini of
Phaseolus appears to be due to geotropic induction, for the photonastic
irritability is acquired or modified in conjunction with the performance of
a pronounced geotropic curvature, and for this reaction illumination is not
essential. Noll's1 view, according to which we are here dealing with a
modification of the geotropic irritability of illumination, is the result of
a biased and incomplete comprehension of the problem. In any case,
however, it is still necessary to determine whether the changed reaction is
due to a modification of the photonastic irritability or to an altered power
of movement in the antagonistic halves of the pulvinus. That the latter is
possible is shown by the fact that the geotropic curvature considerably
modifies the expansive energy of the opposed halves, that in the lower side
after reversal increasing, and that in the side now facing upwards decreasing.
Since in general the existent mechanical considerations influence the pro-
gress and in some cases the activity of the response, it is not inconceivable
that on darkening the increased expansion which produces the photonastic
curvature should always take place more rapidly in the less expanded half
of the pulvinus than in the more expanded one.
SECTION 28. The Mechanics of Nutation Movements.
We must confine ourselves to photonastic and thermonastic movements,
since no researches have as yet been performed upon the mechanics of
hydronastic curvature. It is evident that whenever a rise or fall of tempera-
ture or illumination affects the growth of the two sides of an organ unequally
a curvature will result, which will continue until a position of equilibrium
Noll, Die heterogene Induction, 1892, p. 12.
THE MECHANICS OF NUTATION MOVEMENTS 129
is reached. This depends upon the growth tendencies of the different
tissues, coupled with the mechanical and physiological reactions due to the
realized curvature. ,
It depends upon the properties of the organ, and upon the rapidity
of the change of temperature or illumination, whether the new position
is assumed directly, or after a number of oscillations. These may arise
either owing to the fact that the different tissues assume rates of growth
proportionate to the new conditions with unequal rapidity, or they may be
due to the fact that the shock-stimulus produces a transitory and unequal
acceleration or retardation of growth. These transitory oscillations must
be reduced and finally eliminated when the change of temperature or
other condition is brought about sufficiently slowly. Their production
has, however, no influence upon the ultimate position, which when once
attained is maintained so long as no internal or external change occurs.
An organ may, however, react in such a way that the change produces
pronounced oscillation, but no permanent alteration of the original
position.
It is impossible, therefore, to say whether any shock-effect comes into
play. The new rapidity of growth corresponding to changed conditions
of temperature or illumination is, however, usually assumed without any
perceptible transitory disturbances being shown, and hence more especially
the slower photonastic and thermonastic curvatures, and possibly also
certain typical sleep-movements, may be produced without any transitory
acceleration or retardation of growth due to the effect of shock. A shock-
stimulation is, however, exercised in many cases1 in which a fall or rise of
temperature or illumination produces a certain transitory acceleration
of growth. As in the case of tendrils this renders the reaction more rapid,
and enables a flower of Crocus to close rapidly when subjected to a fall of
temperature at which growth ultimately almost ceases.
This acceleration of growth is as pronounced in highly photonastic, or
thermonastic plants as in the case of tendrils. Thus the growth of the
middle lamella of the petiole of Impatiens noli-me-tangere may temporarily
attain about twenty times its previous rapidity when an energetic photo-
nastic reaction is produced by sudden darkening 2. In one experiment the
marks on the petiole covered 1 83-5 of the micrometer divisions after four
hours instead of the original 183, which indicated a growth in length of
O'2i per cent, per hour. After darkening the leaf curved strongly down-
wards in half an hour, and since the marks extended on the upper side from
1 84 to 192 micrometer divisions the growth in length was 8-68 per cent.
1 Pfeffer, Period. Bewegungen, 1875, PP- I3> I22i J?1* The -*ex^ *s based mainly on these
researches and upon those of Jost.
3 Pfeffer, 1. c., p. 21.
PFEFFER. Ill
I3o MOVEMENTS OF CURVATURE
per hour. The simultaneous measurement of the under side gave a shorten-
ing of 0-53 per cent., so that the growth of the middle lamella was 4-07 per
cent, or half the algebraic sum of the growth on the two sides. Similarly
in the flower of Crocus a sudden fall of temperature from 17° to 7° C. may
cause the average growth of the middle lamella of the active zone of the
perianth to increase transitorily from seven to ten times in rapidity, although
ultimately growth is strongly retarded at 7°C. Observations on Tulipa
also showed an increase of growth to eighteen times its previous rapidity
when the temperature was suddenly raised from 11° to i8°C. and, even
allowing for the permanent increase at the higher temperature, the transitory
rise is eight times greater.
As in the case of the curvature of tendrils, during these photonastic or
thermonastic responses the concave side retains the same length or ex-
periences a very slight shortening during curvature. During the return move-
ment by which the leaf of Impatiens nearly regains the day position after
being darkened, the previously accelerated side grows but little or not at all.
The recent researches of Wiedersheim carried out at Leipzig under Pfeffer's
direction show that the return movement is accompanied by a secondary
feebler acceleration of the growth of the middle lamella, as in the case of
tendrils. This secondary acceleration is shown by the flowers of Crocus and
Tulipa, but is comparatively feeble, since the return movement only takes
place to an extent sufficient to remove the excess of curvature.
As in the case of tendrils, a transitory change only produces a temporary
curvature, the organ returning to its original position when the previous
conditions of temperature or illumination are restored. In such cases the
secondary acceleration of growth during the return movement naturally
becomes more pronounced. Although the curvature of tendrils results
from a tropic stimulus, and those of thermonastic and photonastic organs
from diffuse stimulation, the growth-mechanisms involved are the same in
both cases. The entire active zone on both sides of the organ experiences
an acceleration of growth, which begins at a later time on the side which
becomes concave, but which, whenever the organ straightens again, ultimately
produces the same total growth in spite of its originally slower rate on the
concave side. It follows, therefore, that the production of a permanent
curvature involves either a partial inhibition of the slower but more
prolonged growth response on the concave side or the prolongation of the
growth period on the convex side.
Among the factors responsible for these reactions the stimulating effects
of shock and of the realized movement are to be included. It is not, how-
ever, certain whether the latter is directly responsible for the return move-
ment by which the original position may be partially or entirely restored.
Wiedersheim has, however, found that when a fixed leaf of Impatiens parvi-
flora is darkened two opposed successive accelerations of growth ensue just
THE MECHANICS OF NUTATION MOVEMENTS 131
as in the case of stimulated tendrils. That a secondary acceleration of
growth actually occurs on the concave side is shown by the tendency to
a return curvature in a fixed leaf, as well as by direct measurement. Fixed
perianth-segments of Crocus and Tulipa show a feebler secondary accelera-
tion of growth, owing to the fact that a permanent change of temperature
alters the position of equilibrium in such manner as to lessen the return
movement. It is, however, possible that the return movement, although
excited in the absence of any realized curvature, may result from the altered
tensions in the tissues. However this may be, there can be no doubt that,
in the case of variation movements, correlative influences, as apart from
mechanical ones, do travel between the closely related halves of motile
pulvini.
A direct or indirect regulation of the growth in the different parts is
essential to produce a definite reaction. The fact that the concave side may
retain approximately its original length during the curvature of tendrils as
well as of photonastic and thermonastic organs simply shows that the growth
acceleration lessens towards the concave side, for in the middle lamella
of this side the growth will be ten times accelerated when the growth of the
middle lamella of the entire organ is accelerated twenty times. The slight
shortening sometimes shown on the concave side is probably the result of
compression, and would be greater during curvature were it not for the
simultaneous awakening of an increased tendency to growth 1.
Jost2 erroneously supposed that the thermonastic or photonastic
stimulation directly accelerated the growth on one side and retarded it on
the other, and does not sufficiently distinguish between the transitory and
stationary reactions and their results. It is not, however, impossible that
in isolated cases some such antagonistic action may be exercised, or that as
the result of shock-stimulation particular cells may experience a temporary
retardation of growth followed by the usual acceleration. The new constant
conditions of temperature or illumination always, however, produce the
same qualitative effect on growth although not always the same quantitative
effect, and special peculiarities may be shown when the temperature
or the illumination rises above the optimal values. Apart from this the
formal effect of a rise of temperature or decrease of illumination is an
acceleration of growth, while a permanent fall of temperature or increase
of illumination produces a retardation of growth. As the result of shock,
however, a sudden rise or fall of temperature or illumination may produce
either a transitory acceleration or retardation of growth according to the
nature of the plant. True 3 observed that a sudden rise or fall of tempera-
ture produced a transitory retardation of growth in the radicle, but it is
1 Pfeffer, Period. -Bewegungen, 1875, p. 17.
3 Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 368.
3 True, Annals of Botany, 1895, Vol. IX, p. 365.
K 3
132 MOVEMENTS OF CURVATURE
also possible, thpugh hardly probable, that a decrease of temperature or
illumination might transitorily affect growth but not an increase. It is also
possible that in thermonastic flowers a temporary retardation of growth
may precede its acceleration, but may be too transient to be capable of
detection, or may merely antagonize the first tendency to increased growth,
thus increasing the latent period of response.
In any case when we remember the influence of the specific properties
and its variable tone upon its power of response, it is not surprising to find
that the results obtained do not in all cases precisely agree. Thus Pfeffer1
found that a fall of temperature produced a very pronounced acceleration
of growth in the perianth-segments of Crocus, but that a sudden rise
produced no perceptible acceleration in the growth of the middle lamella,
whereas Jost 2 observed in both cases a strong acceleration of growth in the
perianth of the Tulip.
It is uncertain to what extent sudden changes of illumination may
exercise shock-effects upon photonastic organs. During the daily move-
ments of the flowers of Leontodon hastilis 3 and of Taraxacum officinale 4
the average growth is accelerated, but here the effect of the direct stimula-
tion is coupled with the induced periodicity. Since this periodicity and
also the daily periodicity of growth in length are induced by periodic
changes of illumination, we may assume that every photonastic reaction is
coupled with a temporary acceleration of growth. Even when the opening
and closing movements assume a more rapid rhythm in constant darkness,
each periodic reaction involves a temporary acceleration of the average
growth.
A very pronounced movement and acceleration of growth is produced
by darkening the leaves of Impatiens noli-me-tangere and /. parviflora.
Illumination only produces a feeble movement, but it is not certain whether
the acceleration of the average growth is also feebler than when the leaf is
suddenly placed in darkness.
A transitory acceleration of growth may enable more rapid curvature,
but it is not essential, and probably is either absent or feeble in many thermo-
nastic and photonastic movements. In the latter case it is easily overlooked,
since the activity of growth is always liable to spontaneous fluctuations, and
since it assumes a different stationary value in response to the new con-
ditions. Pfeffer5 was, however, overcautious in refusing to accept the
general acceleration of growth shown by his measurements as being the
result of the shock-stimulus.
Special instances. The following results have been obtained by micrometric
1 Pfeffer, Period. Bewegungen, 1875, p. 122.
a Jost, Jahrb. f.vwiss. Bot., 1898, Bd. xxxi, p. 346.
3 Cf. Table 4, p. 134, and Pfeffer, 1. c., p. 26.
* Jost, 1. c., p. 354. 5 Pfeffer, 1. c.
THE MECHANICS OF NUTATION MOVEMENTS
measurements of the distances between exactly opposite pairs of marks on the two
sides of the active zone of the perianth. From these hourly measurements percentage
values have been calculated for the opposed sides. The half of the algebraic sum of
the two values gives the percentage growth of the actual or ideal middle lamella
(Tables 3 and 4). In Tables i and 2 only this average growth is given, but since in
these estimations with the flower of the Crocus the concave side remained of the same
length or shortened very slightly, twice the average growth gives that of the convex
side.
The values in Tables i and 2 1 are the averages of six separate estimations, those
of Table 3 2 of three, and those of Table 4 are obtained from three separate flowers 3.
The measurements were made on a single remaining perianth-segment of Crocus
and Tulipa, and on a single remaining floret of Leontodon. Table i shows that the
cooling of the flower of Crocus 4 produced a pronounced acceleration of growth in
fifteen minutes, which rapidly lessened and had almost ceased after half an hour.
A feeble transitory acceleration of growth also appears in Table 2, but is less evident
(1-51 as compared with 1-03). A pronounced transient acceleration is, however,
shown in Table 3, and after two hours a return curvature is shown, which is accom-
panied by an acceleration of growth on the outer side of the perianth, and a retardation
on the inner side.
TABLE i. Crocus sp. Percentage Growth of the Middle Lamella per hour.
Temperatui
Time of o
165-16! hours.
e 1 7-18° C.
Dservation.
3 hours.
Then
after 15-20
minutes.
at 7-7^° C. and me
25-30 minutes
later.
asured
3 hours to 3 hours
20 minutes later.
o-75
o-54
5-24
2.44
0-29
TABLE 2. Crocus sp. Percentage Growth of the Middle Lamella per hour.
Temperature 8-9° C.
Time of Observation.
3-6 hours.
Thei
after 20-45
minutes.
i at 20-21° C. and measi
40 minutes to 2 hours
20 minutes later.
ired
45 minutes to 2 hours
later.
0-24
i-5i
1.26
1.03
TABLE 3. Tulipa Gesneriana (Due van Toll). Percentage Growth per hour.
Temperati
5.30-9 a.m.
ire ii°C.
9-12 a.m.
12.40 a.m.
to 1.40 p.m.
Then at
1.40 p.m.
to 2.40 p.m.
i8°C.
2.40 p.m.
to 3.40 p.m.
3.40 p.m.
to 5.40 p.m.
Outer side . . .
Middle lamella .
Inner side . . .
0.16)
VO-22
0.29]
0-20 ]
[0-17
O.I5J
i-i )
3-76
6-43 J
5-79)
o r98
0.18 J
1.46]
h-75
2-05)
0.78 \
[0.48
O.I9 J
1 Pfeffer, Period. Bewegungen, p. 125, Tables XI b, and p. 127, Table XIII b.
a Jost, 1. c., p. 354.
3 Pfeffer, 1. c., p. 27, Tables VII b and VII c.
* A large white-flowered garden variety was used.
I34 MOVEMENTS OF CURVATURE
TABLE 4. Leontodon hastilis. Percentage Growth per hour.
Daylight.
1 1. 30 a.m. to
10.30 p.m.
Darkness.
10.30 p.m. to
6 a.m.
Day]
6 a.m. to
8.45 a.m.
ight.
8-45 a.m. to
4p.m.
Experiment i
(Outer side . .
] Middle lamella
(inner side . .
i-47]
h<M5
o-43)
0-17)
lo-82
I-47J
0.46)
f 2.03
3-6oj
2-37)
[1.67
0.97)
Experiment 2
(Outer side . .
\ Middle lamella
(inner side . .
0.47)
0.3
0-13)
o )
fo.66
i'33j
0-15)
2.23
4-32J
2.17|
M-47
0-77]
Experiment 3
(Outer side . .
\ Middle lamella
(inner side . .
?
o
o )
Lo-82
1-65
° 1
[1.92
3-84J
i-54)
[0.83
0-13]
Table 4 shows that after a day's illumination, mainly the outer side of the corolla
grew in length during the evening curvature. During the night the flower returned
halfway to the day position, owing to the growth of the inner side, and this growth
was accelerated by the light at 6 a.m., leading to the assumption of the full day posi-
tion by 8 a.m. At 4 p.m. the closing movement begins again, while between 6 and
8.45 a.m. the growth of the middle lamella was accelerated.
SECTION 29. The Mechanics of Variation Movements.
Most variation movements are photonastic in character, and show
a general resemblance to nutation movement except that the curvature
is produced by the unequal expansive energy of the turgid tissues instead
of by unequal growth. A decrease of illumination produces an increase
of the expansive energy in the antagonistic tissues, but this takes place
more actively in one-half of the pulvinus than in the other, the tissues
of the latter being therefore compressed. Owing to the continued increase
of the expansive energy in the compressed half of the pulvinus a partial
return to the original position occurs, the fall of illumination exciting
a movement in excess of the permanent position adapted to the new
constant conditions of illumination. In darkness or in diminished illumina-
tion growth in general is accelerated, while the expansive energy of the
motile tissues is increased, and to the same extent in both halves of the
pulvinus when the leaf returns to its original position. On returning to
the previous strong illumination the expansive energy assumes its original
value, and possibly a sudden rise of illumination may act as a transitory
stimulus and produce an excess of movement. The latter may, however,
not be as pronounced as when a fall of illumination occurs, and indeed
it may be imperceptible in most cases.
In constant darkness or illumination the periodic movements are
produced by opposed changes of the expansive force of the halves of the
pulvinus without any general rise being shown, which indeed is no more
THE MECHANICS OF VARIATION MOVEMENTS 135
essential for curvature than is a general acceleration of growth for a
nutation movement. The automatic variation movements are produced
in the same way by an increase of the expansive energy in one-half of
the pulvinus and a decrease in the other, for in both cases the rigidity
of the pulvinus is unaltered during the movement, whereas a fall or rise of
rigidity would inevitably ensue if the movement was due to an increase or
decrease of the expansive energy on one side only of the pulvinus.
These conclusions are mainly attained from estimations of the rigidity
of the intact pulvinus under different circumstances, for although no simple
relationship exists between the tissue-strains and the weight supported,
nevertheless a decreased rigidity indicates a fall, and an increased rigidity
a rise of the expansive energy of the active tissues. The original deter-
minations were made by Briicke, who noted the angular displacement in the
normal and inverted positions with or without the addition of loads. Since
the divergences may be from one and a half to two and a half times greater
in light than in darkness, it follows that darkness produces a permanent
rise of the expansive energy on both sides of the pulvinus 1.
In the pulvini of Phaseolus^ Trifolium, and Desmodium the maximal
rigidity is attained at or before the completion of the curvature induced by
the withdrawal of light, and since the rigidity is unaltered during the
return movement, the latter can only be due to a decrease in the energy of
expansion in the contracting half of the pulvinus. If this were not the case,
and if, for instance, the partial or complete elimination of the primary
curvature were due to a rise in the expansive energy of the compressed
half of the pulvinus, then an increase of rigidity would accompany the
return movement as well as the original curvature. If, however, the return
movement were due solely to a fall of the enhanced expansive energy in
the active half of the pulvinus, it would be accompanied by a perceptible
decrease of rigidity. Hence there can be no doubt that the expansive
energy of the active half of the pulvinus undergoes a transitory increase
beyond the stationary value 2, and the same probably applies even to
slowly reacting pulvini. The permanent rise of rigidity after the curvature
produced by darkness has been eliminated shows that a permanent rise ot
expansive energy is produced in the half of the pulvinus which is at first
compressed.
These facts do not, however, enable us to say whether the darkening
does not also produce a certain transitory decrease of expansive energy in the
compressed half of the pulvinus, for perceptible changes of rigidity are only
produced by pronounced alterations in the expansive energy of the pulvinar
1 Pfeffer, Period. Bewegungen, 1875, p. 88 seq.
2 [So that the excess curvature cannot be due to the momentum of the moving leaf. The
mechanics of the whole subject require further elucidation and investigation.]
136 MOVEMENTS OF CURVATURE
tissues, and a decreased expansive energy in the less responsive half of the
pulvinus might be masked as regards changes of rigidity by a corresponding
increase in the more rapidly reacting half. The behaviour of pulvini, from
which one-half has been removed, as well as the analogy with nutation
reactions, point against the occurrence of any such transitory decrease
of expansive energy 1.
The effective energy of expansion is considerable in the case of the
primordial leaves of Phaseolus vulgaris, for the pressure required to prevent
movement, as measured by a dynamometer or spring-balance, is such as
to show that the upper half of the pulvinus generates an energy of expan-
sion equalling two to five atmospheres. This is, however, merely the
excess pressure over that in the lower half of the pulvinus, so that the
pressure in the upper half must be at least from five to seven atmospheres 2.
A pronounced energy of movement is also developed in a mechanically-
stimulated leaf of Mimosa pudica, but in this case a pronounced fall of
rigidity takes place.
The tension exerted on the dynamometer shows that the progress
of an attempted curvature resembles that of a realized one, while the
same increase of expansion in darkness is ultimately shown in the more
slowly reacting half of the pulvinus of a fixed leaf, as when a curvature
can take place. It remains, however, possible that the realized curvature
may act as a retarding stimulus to the expansion of the compressed half
of the pulvinus. At the same time the structure of the pulvinus is such
that when an attempted curvature is prevented the increased energy of
expansion exerts no tension on the opposed half of the pulvinus. This does
occur, however, in growing organs, and indeed it is largely by tensions of
this kind that growth is regulated and the development of pronounced
strains avoided. Hence during nutation curvatures no pronounced rise
of rigidity is shown, nor can any pronounced pressure be exercised against
a resistance which prevents the attempted movement 3.
The behaviour of pulvini from which one of the antagonistic halves
has been removed supports the above conclusions 4. Under these circum-
stances the remaining half, whether the upper or under one, shows an
increased tendency to expansion when the illumination decreases, and
a decrease when it increases, so that in both cases a curvature is produced.
The inherent periodicity is shown by the changes in the separate halves of
the pulvinus being opposite in character. Hence the leaves of Phaseolus
fall at evening, whichever half of the pulvinus is present. If only the under
1 [The varying mechanical moment of the leaf in its different positions is a factor of the utmost
importance in this connexion.]
2 Pfeffer, Period. Bewegungen, 1875, P- 97 se(l- J Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxni,
P- 347-
3 Pfeffer, 1. c., pp. 92, ill. 4 Pfeffer, 1. c., pp. 7, 84.
THE MECHANICS OF VARIATION MOVEMENTS 137
half is present sudden darkening during the evening produces a reaction
opposed to the normal periodic one, whereas in the intact pulvinus the
photonastic and periodic reactions would coincide.
The general agreement of the facts observed points to the conclusion
that each half of the pulvinus when freed from its counterpart reacts in
the same way as it did in the intact pulvinus. The behaviour of pulvini
which have been operated upon does not, however, indicate with certainty
what goes out in the pulvinus as a whole, for it is well known that
mechanical or other injurious agencies often very strongly modify the power
of reaction. Hence, although after operation the remaining under half of
the pulvinus of Phaseohts shows a rapid increase of expansive energy, it
does not follow that this half of the pulvinus reacts equally rapidly in the
intact pulvinus.
Similar observations indicate that darkening also causes an expansion
in the halves of the pulvinus, but since it takes the same progress on both
sides no curvature results under normal conditions 1. The same applies to
the upper and under halves when the plant is rotated on a klinostat. The
primary similarity can be removed by exposure to the action of gravity,
and it depends upon the normal or inverted position of the plant whether
the dorsal or ventral half of the pulvinus is compressed when a curvature
follows darkening. This fact points to the conclusion that the photonastic
curvature of this plant involves a quantitatively but not a qualitatively
dissimilar reaction in the antagonistic halves of the pulvinus.
Historical. Dassen 2 distinguished between curvatures with and without pulvini,
but did not recognize that in the one case the movement is one of variation, and in
the other is due to growth. Pfeffer 3 showed that the opening and closing movements
of flowers were due to growth ; and the same was observed by Batalin 4 in a few foliage-
leaves, but this author erroneously supposed that the movements of pulvini were also
due to unequal growth. The true condition of affairs was revealed by Pfeffer's
investigations on periodic movement 5. Burgerstein's 6 statement that the opening of
flowers is not due to growth, but to stretching by turgor, is either based on error or
on an incorrect grasp of the facts. It is difficult to see how this author in his later
work is able to deny that growth is responsible for the movements of the perianth-
segments of Crocus and Tulipa, for growth always occurs when a permanent elonga-
tion takes place. How the growth is produced is naturally another matter.
A fact of great importance was that observed by Briicke 7, who found the rigidity
1 Pfeffer, Physiol. Unters., 1873, p. n.
2 Dassen, Wiegmann's Archiv f. Naturgeschichte, 1838, iv. Jahrg., Bd. I, p. 214 ; iv. 2, p. 159.
For additional literature see Pfeffer, Period. Bewegungen, 1875, p. 163.
8 Pfeffer, 1. c., p. 161. * Batalin, Flora, 1873, p. 456.
5 Pfeffer, Period. Bewegungen, 1875.
6 Burgerstein, Oesterreich. Bot. Zeitschrift, 1901, Nr. 6; Ueber die Bewegungserscheinungen
der Perigonblatter von Tulipa und Crocus, 1902.
7 Briicke, Miiller's Archiv f. Anatomic u. Physiologic, 1848, p. 440.
138 MOVEMENTS OF CURVATURE
of the pulvinus of Mimosa pudica increased in the evening, so that the sleep-move-
ments are not produced by one-half of the pulvinus becoming flaccid, as are those
following mechanical excitation. A natural result of this fact is that in the drooping
evening position the main pulvinus is still capable of a pronounced curvature in
response to mechanical excitation J. Long before Briicke's time Dutrochet 2 had
concluded, mainly from observations upon operated pulvini, that the sleep-movements
were due to opposite changes of the energy of expansion in the antagonistic halves of
the pulvinus. Dassen, Briicke, and Sachs 3 came into more or less accordance with
this view, whereas Millardet 4 and Bert 5 concluded that the changes of expansion
were alike in character in both halves, but differed quantitatively, and also in their
progress in time. The subject was then fully explained as in the text by Pfeffer's
researches. Previously to these researches the effects of the periodicity and of the
direct stimulation were not properly distinguished, with the result that the observa-
tions upon operated pulvini led to contradictory conclusions. The completeness of
the operation is also of great importance, for if the parenchyma is removed from the
upper half of a pulvinus of Phaseolus down to the upper surface of the vascular
cylinder only, a fall is produced by darkening just as in the intact pulvinus, owing to
the fact that the expansive energy of the remaining portion of the pulvinus is still
greater than that of the lower half. If, however, the parenchyma is removed down to
a plane passing through the middle of the vascular cylinder, the leaf rises in darkness,
showing that the expansive energy of the lower half of the pulvinus has increased 6.
It was probably owing to the incomplete removal of the upper half of the pulvinar
tissue that Schwendener and Jost7 obtained contrary results with Phaseolus, while
Schwendener observed in a few other cases a shortening of the remaining half of the
pulvinus on darkening, if this is the half which is compressed when intact. Panta-
nelli 8 has found recently that both halves of the operated pulvini of Robinta pseudacacia
and Porliera hygromelrica react similarly to darkening. Schwendener 9 also observed
that after operation the main pulvinus of Mimosa pudica carried out the same daily
movements as before, provided that the periodicity was not disturbed by any exces-
sive and abnormal photonastic reaction. This result confirms that obtained by
Pfeffer.
1 Ewart, Annals of Botany, Vol. XI, 1898, p. 453.
2 Dutrochet, Rech. anatom. et physiol. s. la structure inthne d. animaux et d. vege"taux, 1824,
p. 134. For the detailed literature see Pfeffer, Period. Bewegungen, 1875, pp. 6, 163 ; Physiol.
Unters., 1873, p. 3. Cf. also Schwendener (1896), Gesammelte Botanische Mittheilungen, Bd. n,
p. 219.
8 Sachs, Bot. Ztg., 1857, No. 46 a, 47.
* Millardet, Nouvelles recherches sur la pe'riodicite de la tension, 1869, PP- 31* 48-
5 Bert, Me"m. de la Soc. d. scienc. physiques et naturelles de Bordeaux, 1870, p. 51 of the
reprint. Cf. Pfeffer, 1. c., 1875, p. 7.
6 Giessler and Wiedersheim have repeatedly found that the completion of the removal of the
upper half of the pulvinus always results in the shortening of the lower half on darkening being
converted into a lengthening. These results therefore confirm the original ones by Pfeffer (1. c.).
7 Schwendener (1898), Gesammelte Bot. Mittheilungen, Bd. II, p. 246 ; Jost, Jahrb. f. wiss.
Bot., 1898, Bd. xxxi, p. 370.
8 Pantanelli, Studii d' anatomia e fisiologia sui pulvini motori, 1901, pp. 225, 230.
9 Schwendener, 1897, 1. c., p. 229.
THE MECHANICS OF VARIATION MOVEMENTS 139
Since transition stimuli may exercise various shock-effects, it is not impossible
that in certain plants darkening may excite opposed reactions in the two halves of the
pulvinus, or transitory changes may occur without producing any pronounced move-
ment or altered rigidity. A slight increase of rigidity appears to be shown by many
plants in darkness, but the results which Schwendener l obtained with chloroformed
plants are not altogether satisfactory, since the treatment with chloroform slightly
increases the rigidity and may exercise other effects as well2. The changes of
rigidity in Mimosa pudica may be readily followed by working at low temperatures,
when the sleep-movements are still performed, but the seismonic irritability is largely
suspended.
The acceleration of growth in darkness is naturally not always alike in all plants
or in all parts of these, and the increased activity of growth produced by the with-
drawal of light in the convex side of an organ which performs a pronounced nutation
curvature in darkness is not of necessity permanent in character, but is in fact usually
transitory. As the effect of the stimulus due to the change passes away, the growth
assumes the same somewhat enhanced rate in all parts so long as no autonomic
modifications ensue.
Internal factors. It is certain that the modifications of growth pro-
duced by light and temperature are not the direct result of changes of
turgor, and until the exact way in which these agencies influence growth is
known it is impossible to gain any insight into the mode of production of
photonastic and thermonastic nutation curvatures. Even in the case of
variation movements the increased expansive energy might result from
a change in the elasticity of the cell-wall as well as from a rise of turgor.
Hilburg 3 was unable to detect any change of turgor in the active pulvinar
tissues by plasmolytic methods during photonastic and thermonastic curva-
ture, but this might simply be because the changes of turgor are rapidly
produced, or are affected by the mode of preparation necessarily adopted.
The turgor of the active parenchyma cells sinks after prolonged immersal
in water, but not after lying in a solution of potassium nitrate and of a few
other salts. Whether this is a question of diffusion, selective absorption, or
of some stimulatory action is, however, uncertain, and no light is thrown
upon the mechanism of curvature. The geotropic and heliotropic curvatures
of pulvini are, however, accompanied by changes of turgor equivalent to about
i per cent, solutions of potassium nitrate, according to the same author, so
that there appears to be some difference in the mode of production of the
variation movement according to the character of the stimulus applied. Even
when different mechanisms are in play variation and nutation may co-operate
in producing the curvature of a pulvinus, just as geotropism and photonasty
may co-operate in certain stems.
1 Schwendener, Ges. Bot. Mittheil., p. 236.
2 Cf. Pfeffer, Physiol. Unters., 1873, p. 65.
3 Hilburg, Unters. a. d. bot. Inst. zu Tubingen, 1881, Bd. I, p. 23.
I4o MOVEMENTS OF CURVATURE
It is evident from the above that the movements of pulvini are not
produced in such a simple manner as Bert1 supposed. This author con-
cluded that they were the direct result of the changes of turgor due to the
accumulation of the glucose produced by photosynthesis during the day-
time, and its gradual removal at night. The mere facts that the daily
movements continue in air deprived of carbon dioxide, and that the
periodic movements are repeated several times in continued darkness, are
sufficient to disprove this supposition.
PART V
THE INFLUENCE OF THE EXTERNAL CONDITIONS UPON
AITIONASTIC CURVATURE
SECTION 30. Special and General Actions.
Since modifications of growth and of the tissue-strains are more obvious
when they find expression in curvature, reactions of this kind serve especially
well to demonstrate the influence of the external conditions. It is easy to
see, for instance, that the movements of Mimosa pudica and of the stamens
of Cynareae take place most rapidly and actively at a certain optimum
temperature, and cease at high and low temperatures, owing to the onset
of cold or heat rigor. Provided that the unfavourable temperature is not
too severe or too prolonged in duration, the power of reaction is more or
less rapidly regained at a favourable temperature as the inhibitory after-
effect of the previous exposure disappears. Similar results are produced
by the partial or complete withdrawal of oxygen, by the excessive loss of
water, and by the action of ether or chloroform. It is, however, worthy of
note that the excitation of the pronounced seismonic movements of Mimosa
pudica is not essential to its growth and normal development, while the
tone of this plant and of plants in general is not only affected by the
temperature but also by substances such as chloroform and ether, which
the plant never encounters under natural conditions.
It is in some cases possible by special treatment to inhibit certain
partial functions, and in this way to obtain some insight into the relationship
between the sensory and motory processes. Thus the repeated shaking of
Mimosa pudica causes the suspension of the seismonic irritability alone,
as also do low temperatures and anaesthetization, whereas the autonomic
1 Bert, Compt. rendus, 1878, T. LXXXVII, p. 421 ; also in Me"m. de la Soc. d. sci. phys. et
nat. de Bordeaux, 1870, T. viu, p. 53. Cf. also the reference in Bot. Ztg., 1879, p. 187. The
speculations of G. Kraus (Flora, 1877, p. 73} are of no importance.
SPECIAL AND GENERAL ACTIONS 141
and daily movements continue. The latter cease, however, before the power
of response to mechanical stimuli is lost, when the air surrounding the plant
is rarefied.
So long as the power of growth is retained, or in general, whenever the
motor mechanism remains capable of action, a cessation of the power of
response under particular conditions can only be due to their influence upon
the power of perception or upon the processes of induction. For instance,
when the air is gradually rarefied, first the heliotropic and later the geotropic
irritabilities disappear, whereas growth only ceases when a still lower partial
pressure of oxygen is reached. The fact that the leaf of Mimosa pttdica
returns to its original position when the recovery of the seismonic irritability
is prevented by chloroform, cold, or shaking, shows that the return movement
is not dependent upon the restoration of seismonic irritability. Since
the re-expansion of the active tissues takes place in the chloroformed
pulvinus, it is evident the anaesthetization affects some stage of sensation.
Temperature. The minimum temperature for the photonastic move-
ments of the flowers of Crocus, and for those of the flowers and leaves of
various indigenous plants, lies between o° C. and 4° C. The stamens of
Berberis also react to strong mechanical stimuli at comparatively low
temperatures, whereas no response is produced in the leaves of Mimosa
ptidica when the temperature falls below 15° C.1, although weakened
sleep-movements and autonomic movements continue. Sachs 2 found that
transitory heat-rigor was produced by exposure to 40° C. for an hour, at
45° C. in half an hour and at 49° C. to 50° C. in a very short time. In some
cases plants which were still irritable at 40° C. became transitorily rigid
when brought to a normal temperature, either as the after-effect of the
previous exposure, or owing to the shock-effect of the sudden change.
Light. Organs which are able to develop more or less normally in
darkness are also able to curve in response to stimuli in the absence of
light. Thus flowers of Crocus and Tulipa which have grown ^in darkness
react strongly to changes of temperature, while tendrils as well as the
stamens of flowers of Cynara scolymus which have expanded in darkness 3
are sensitive to mechanical stimuli. Indeed even the leaves of Mimosa
pudica acquire their seismonic and photonastic irritabilities when brought
by special treatment to develop strongly in darkness.
Exposure to light is essential for the continuance of the variation move-
ments of adult phototonic leaves. In darkness the pulvini gradually fall into
1 Sachs, Flora, 1863, p. 451. The older researches of Dutrochet are quoted by Sachs. A few
details on the dependence of various aitionastic movements upon the external conditions are given in
the previously quoted works of Kabsch and Morren. Cf. also Hansgirg, Physiol. u. phycophytol.
Unters., 1893, p. 62.
2 Sachs, I.e., p. 453.
3 Pfeffer, Period. Bewegungen, 1875, p. 64.
142 MOVEMENTS OF CURVATURE
an immotile condition l. This occurs in Mimosa ptidica after three to six
days' darkness, and the pulvini of other plants behave similarly. Very
feeble illumination induces rigor in the leaves of Mimosa pudica, but suffices
to keep the pulvini of the shade-loving Oxalis acetosella in a phototonic
condition. According to Jost, the seismonic irritability disappears first in
some cases, but usually the photonastic irritability is lost first, while the
rigor is usually, but not always, more rapidly induced at high temperatures 2.
The rigor of the pulvini of foliage-leaves is apparently the result of a patho-
logical condition induced by continued darkness, and ultimately leading to
death3. Since the leaves are also injured when exposed to light in air
deprived of carbon dioxide in which photosynthesis is reduced to a very
low ebb 4, the pathological condition induced by darkness is probably the
result of the leaf being unable to perform its normal function. The rigor
does not appear to be due to any deficiency of food or to the lack of any
autoassimilatory products, for it is produced without any fall of turgor 5,
and in some cases when the leaves are abundantly provided with food 6, as
also are accompanying pathological changes, such as the alteration in colour
of the chloroplastids and the temporary or permanent loss of the power of
photosynthesis 7. It is not surprising that a leaf developed in light may
be unable to accommodate itself to darkness, whereas under special circum-
stances a leaf may develop to a considerable size and acquire irritability
in continuous darkness.
The experiments with coloured light lack critical precision, but, as far
as they go, seem to indicate that phototonus is maintained by the more
refrangible as well as by the less refrangible halves of the spectrum8.
Although the blue and violet rays exercise a stronger photonastic action,
nevertheless the red and yellow rays are able to induce the sleep-movements
of leaves. The movements, however, begin earlier, and take place more
rapidly in blue light than in red, just as when the effects of strong and of
feeble illumination are compared. Similar differences are shown by the
nutation movements of chlorophyllous and non-chlorophyllous organs,
while flowers open less in red light or under feeble white light than when
exposed to the blue rays 9.
1 Sachs, Flora, 1863, p. 499, and the literature there given; Jost, Jahrb. f. wiss. Bot., 1895,
Bd. xxvn, p. 457.
3 Jost, 1. c., pp. 465, 469.
3 Pfeffer, Period. Bewegungen, 1875, p. 64 ; Jost, 1. c., p. 457.
Ewart, Journ. Linn. Soc., Vol. xxxi, 1897, p. 569.
Pfeffer, 1. c., p. 68.
Pfeffer, 1. c., p. 64.
Ewart, 1. c., pp. 568, 5 70.
Daubeny, Phil. Trans., 1836, I, p. 519; Bert, Mem. de 1'Acad. de Bordeaux, 1871, p. 28 of
reprint ; W. P. Wilson, Contrib. from the Bot. Lab. of Pennsylvania, 1892, Vol. I, p. 71 ; Macfarlane,
Bot. Centralbl., 1895, Bd. LXI, p. 136.
9 Hansgirg, Physiol. u. phycophytol. Unters., 1893, p. 60.
SPECIAL AND GENERAL ACTIONS 143
Oxygen. All aerobic organisms rapidly lose the power of movement
and curvature in the absence of oxygen \ but the rigor is not immediately
produced in the tentacles of Drosera, for Correns found that they remain
for a time responsive to mechanical and chemical stimuli, just as a muscle
does in the temporary absence of oxygen. Similarly the leaves of Mimosa
pudica may show a feeble power of seismonic movement immediately after
the oxygen pressure has been reduced almost to nil2. In any case, the
seismonic irritability of Mimosa ptidica is lost at a lower partial pressure
of oxygen than the photonastic irritability which, in Mimosa as well as in
other plants examined by Correns, disappears in air at a pressure of 15 to
35 millimetres of mercury3. Tendrils cease to react to contact in air
at a pressure of 15 to 30 mm. of mercury, but their growth appears
to be still possible, for an induced movement continues to a slight extent
at still lower pressures in which the power of perception is lost. Correns
was, however, unable to observe any such after-effect when seedling-stems
were brought after geotropic or heliotropic induction into air sufficiently
rarefied to suppress the power of perception of these stimuli. The
seedling-stem of Helianthus annuus^ however, continues to grow for a time
in the absence of air 4, and is able to perform a geotropic curvature in an
almost complete vacuum, whereas no heliotropic response is possible when
the air-pressure falls below 75 mm. of mercury. It has, however, not
been determined whether the power of aitionastic curvature in general
is lost sooner than the power of growth, or whether shock-stimuli become
ineffective at a higher pressure of oxygen than continuous stimuli.
The seedling-stem of Sinapis alba is capable of a geotropic curvature
at an air-pressure of 30 to 37-5 mm. of mercury, but is unable to per-
form a heliotropic reaction below an air-pressure of 45 mm. Hence
below this air-pressure the stem is capable of a geotropic but not
of a heliotropic response. After exposure to geotropic or heliotropic
induction in air too rarefied to permit of any response, no after-effect is
shown on the return to ordinary air. Heliotropic induction, and to a less
extent geotropic induction, are therefore suppressed by a fall of the air-pres-
sure to limits which permit of growth and geotropic curvature, whereas no
heliotropic curvature follows previous stimulatory induction in ordinary
air. It follows, therefore, that in rarefied air not only is the power of
perception of heliotropic stimuli lost, but also the process of induction is
suppressed.
The action of geotropic and heliotropic stimuli is little or not at all
1 Correns, Flora, 1892, p. 87; Sachs, Flora, 1863, p. 501; Kabsch, Bot. Ztg., 1862, p. 341 ;
Dutrochet, Memoires d. ve"getaux et d. animaux, Bruxelles, 1837, pp. 186, 259.
3 Correns, 1. c., pp. 96, 144.
3 Correns, I.e., p. 117.
* Cf. Nabokich, Beiheft z. Bot. Centralbl., 1902, Bd. xui, p. 272.
I44 MOVEMENTS OF CURVATURE
affected by the transference of the plant to pure oxygen at. atmospheric
pressure 1. The injurious action of oxygen observed by Kabsch, in a few
cases, is apparently due to the presence of poisonous impurities, oxygen
made from potassium chlorate usually containing traces of chlorine unless
carefully purified. Kabsch also found that the irritability was retained in
nitrous oxide, but Correns2 has shown that this is not the case with the
stamens of Berber is, while Borzi 3 found that Mimosa soon becomes rigid
in this gas. Borzi states that Mimosa regains its irritability and power of
movement after being for some time in an atmosphere of nitrous oxide, but
this is probably due to the presence of free oxygen in the nitrous oxide,
coupled with the gradual accommodation of the plant to a low partial
pressure of oxygen. Pure carbon dioxide is highly injurious and produces
a rapid suspension of irritability 4.
Ether and Chloroform. All poisonous substances affect the power of
movement when sufficiently concentrated, but the action of anaesthetics
is of especial interest, since by them the reactions may be analysed and
their character revealed to a greater or less extent 5.
It has long been known that ether and chloroform suspend the irritability
of the pulvini of Mimosa pudica and of the stamens of Berberis. The
same applies to the leaves of Dionaea 6, the stigmas of Bignonia and
Catalpa1, and the stamens of Cynareae. Moderate doses of chloroform
suspend the seismonic irritability of Mimosa, but not the daily and auto-
nomic movements of the leaves. It is not certain whether these latter
movements can in all plants be temporarily suspended by anaesthetization
without causing permanent injury. In fact a complete suspension of the
irritability of tendrils, of thermonastic and photonastic movement, and
of growth 8 in general seems only to be produced by anaesthetization which
seriously injures the plant when slightly more prolonged. Slight etheriza-
tion produces a temporary acceleration of growth, but it is not certain
whether such treatment also accelerates induced curvatures.
Darwin 9 observed no suppression of irritability in etherized tendrils,
1 Correns, Flora, 1892, pp. 109, 120, 150.
2 Correns, I.e., pp. 108, 150.
3 Borzi, Rivista di Scienze Biologiche, 1899, Fasc. IV; Bot. Centralbl., 1899, Bd- LXXX, p. 351,
4 Correns, I.e., pp. 109, 121, 130.
5 On the influence of different substances see Goppert, De acidi hydrocyanic! vi in plantas com-
mentatio, 1827 ; Marcet, Biblioth. universelle de Geneve, Archiv, 1848, Bd. LX, p. 204; Bert, Me"m.
de 1'Acad. de Bordeaux, 1866, p. 30; Bernard, Le9ons s. 1. phenomenes de la vie, 1885, 2e e"d., T. I,
p. 258 ; Tassi, Nuovo giornale botanico italiano, 1887, T- Ix> P- 3°; Krutickij, Bot. Centralbl., 1889,
Bd. xxxix, p. 379; Borzi, L'apparato di moto delle Sensitive, 1899; Paoletti, Nuovo giornale
botanico italiano, 1892, T. XXI v, p. 65.
6 Darwin, Insectivorous Plants.
7 Heckel, Compt. rend., 1874, T. LXXIX, p. 702.
8 Detmer, Landw. Jahrb., 1882, Bd. xi, p. 227; Townsend,1 Annals of Botany, 1897, Vol. xi,
P- 522.
9 Darwin, Climbing Plants.
SPECIAL AND GENERAL ACTIONS 145
but possibly more intense and prolonged etherization might have this
effect. The anaesthetized tentacles of Drosera become in some cases
inexcitable, but not always1. It is, however, not certain whether ether
and chloroform suppress the excitability of tendrils and of the tentacles
of Drosera before the power of growth is lost. Czapek 2 finds that the
power of geotropic curvature is sooner and more readily inhibited by
anaesthetics than the power of geotropic sensation. Hence by applying
appropriate concentrations of chloroform to the radicles of Vicia Faba and
Lupinus albus it is possible to subject them to geotropic induction, which
only finds visible expression when the chloroform is removed. Similar
results may be obtained by the use of carbon dioxide, caffein, and a few
other substances, as well as by low temperatures, not only with the radicles
but also with the sporangiophore of Phycomyces*. In the case of the
pulvini of Mimosa pudica, however, the loss of the power of response is due
to the interference of the anaesthetics with the process of sensation.
. Electricity. From the available but incomplete researches on the
general action of electricity on growth it may be concluded that the
varying kinds of curvature are not appreciably affected by weak constant
currents, and that stronger currents retard curvature and ultimately act
injuriously or fatally. It is, however, uncertain whether a particular inten-
sity of current may act as an excitation and awaken curvature.
Electrical discharges and induction-shocks act like mechanical exci-
tations. Hence the full amplitude of movement is produced in the
pulvinus of Mimosa pudica and in the stamens of Centaurea and Berberis
by a single make- or break-shock, whereas repeated induction-shocks are
required to produce a similar effect in the pulvini of leaflets of Oxalis
acetosella and other species of this genus 4. Continued induction-shocks
act in the same way as repeated blows upon the leaves of Mimosa
pudica, which in both cases become inexcitable and, accommodating them-
selves to the continued stimulation, re-expand and return to their original
position 5. Sensitive tendrils are stimulated to curvature by weak induction-
shocks 6, although Hofmeister7 was only able to obtain this result by using
strong induction-shocks. Nitschke8 obtained negative results with the
1 Darwin, Insectivorous Plants. Cf. also Heckel, Compt. rend., 1876, T. LXXXII, p. 525.
3 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 199.
3 Steyer, Reizkriimmungen bei Phycomyces, 1901, pp. 7, 25. Cf. also Correns, Flora, 1892,
P- 134.
* The older literature is given in the works already quoted of Treviranus, de Candolle, &c.
Cf. also Kabsch, Bot. Ztg., 1861, p. 358; Cohn, Abhandlg. d. schles. Ges. fur vaterl. Cultur, 1861,
Heft i, p. 21 (Stamens of Cynareae) ; Blondeau, Compt. rend., 1867, T. LXV, p. 304; Pfeffer, Unters.
a. d. bot. Inst. zu Tubingen, 1885, Bd. I, pp. 505, 521.
9 Cf. Pfeffer, 1. c., p. 521. • See Pfeffer, 1. c., p. 505.
7 Hofmeister, Pflanzenzelle, 1867, p. 313. * Bot. Ztg., 1860, p. 229.
PFEFFER. Ill
146 MOVEMENTS OF CURVATURE
tentacles of Drosera, but probably positive results could be gained by
properly graduated and applied induction-shocks.
According to Mohl J, constant electrical currents are without effect upon
tendrils, but it is not certain whether the tone, and hence the power of
response to various stimuli, may be modified by a continuous current,
or by the continued application of induction-shocks. According to Kabsch 2,
the lateral leaflets of Desmodium gyrans, which have become motionless
at 22° C., are caused to move by weak induction-shocks, but further investi-
gation of this phenomenon is required. It is not, however, surprising that
induction-shocks should act like a blow and excite the rapid movement of
the gynostemium of Stylidium, or the sudden dehiscence of the fruit of
Impatiens 3.
PART VI
DEHISCENCE AND DISPERSAL MOVEMENTS
SECTION 31. Special and General.
The modes of dehiscence of fruits, anthers, sporangia, the splitting
of the integuments of seeds and of the membranes of spores, as well as
the mechanisms of dispersal of seeds, spores and other reproductive bodies
are all of great biological importance4. In most cases, however, the
phenomena are physical in origin, but even here the development of the
requisite physical conditions is a physiological problem.
The hygroscopic movements of dry fruits, of the carpellary beaks of
Erodium, and of certain hairs are the result of unequal imbibition and
swelling, whereas in other cases the fall of turgor consequent upon the
death of certain cells may result in purposeful movements or may aid in the
rupture of tissues. Even without actual death, movements may result
from the liberation of strains set up by attempted growth. It is in this
way that the rapid movements of the stamens of Parietaria and the sudden
dehiscence of the fruit of Impatiens are brought about. In these cases
the active tissues remain living, whereas the sudden escape of the contents
of the dehiscing spore-sacs of certain Ascomycetes is connected with the
death of the sac. In neither case, however, can the process be repeated,
since even where: the active tissues remain living they are no longer
capable of reproducing the requisite tissue-strains. This does, however,
occur during the autonomic movement of the gynostemium of Stylidium,
which is able to perform repeated sudden movements.
In all such movements not only the strains but also the conditions for
1 Mohl, Ranken- und Schlingpflanzen, 1827, p. 70. 2 Kabsch, Bot. Ztg., 1861, p.^6i.
3 Kabsch, 1. c., p. 358.
* See the accounts given by Ludwig, Biologic d. Pflanzen, 1895, pp. 296, 326 ; Kerner, Pflanzen-
leben, 1891, Bd. I, u. 2 (Natural History of Plants, 1895, Vol. II, pp. 91, 140, 429, 833).
SPECIAL AND GENERAL 147
their release are prepared by the activity of the organism, either by so
raising the strains, loosening the tissues or weakening the cell-walls, that
the existent strain, or a slight mechanical excitation, serves to produce
the sudden dehiscence. The plant prepares in the same way for the abscission
of leaves, flowers, and fruits either by the provision of special abscission
layers, in which the cells readily separate, or by the death of intervening
tracts of tissue.
As soon as the required instability has been produced, mechanical
agencies of external or of internal origin may release the dehiscing
mechanism. Changes of turgor or of the tissue-strains may act in this way,
whether produced by transpiration or by some indirect stimulatory reaction
of light, heat, or of chemical substances. In some cases a localized stimulus
may act at a distance. Thus Darwin1 has shown that a touch upon the
antenna-like prolongation of the rostellum of the orchid Catasetum causes
the pollinia to be shot forth by the release of pre-existent strains. Direct
contact with the pollinia is ineffective, so that presumably the antenna
receives a contact or seismonic stimulus and transmits an excitation to the
pollinium, causing the hindrance to movement to be removed.
Apart from the above movements which take place in plants fully
supplied with water, movements and change of shape may be produced
by a fall or loss of turgor due to excessive transpiration or plasmolysis.
Phenomena of this kind, though physical in origin, are nevertheless of
considerable biological importance, as, for instance, when the drooping
of flaccid insolated leaves aids in shielding them from an excessive loss of
water. From a mechanical standpoint it is naturally immaterial whether
the loss of turgor is due to death, transpiration, or plasmolysis. Delicate
tissues shrivel when very much water is removed from them, but it is only
when all the free water has been displaced that further drying removes the
water of imbibition and produces changes of shape in the cell-walls which
may lead to hygroscopic movements and curvatures. Movements of this
kind take place in dead as well as in living tissues, although turgor can
only be restored in cells which have not been killed by drying.
Movements due to turgor or to the tissue-strains dependent on turgor.
An instance of sudden movement without any tearing of the tissues is
afforded by the stamens of Parietaria, Urtica, Pilea, Spinacia, A triplex
and a few other plants. The stamens of Urtica are inwardly curved and
fixed between the perianth and the ovary, or when the latter is absent
they are pressed against one another. As development progresses strains
arise which mainly find expression in the compression of the inner under
side of the filament. When this is sufficient to overcome the mechanical
1 Darwin, The various contrivances by which Orchids are fertilized; Haberlandt, Sinnesorgane
im Pflanzenreich, 1901, p. 62.
L 2
148 MOVEMENTS OF CURVATURE-
resistance, the filament suddenly straightens like a spring and scatters the
pollen from the dehiscing anthers. The movement takes place spon-
taneously, but may be accelerated by the action of pressure or contact
on the perianth or stamens. If the stamens are not quite ripe the removal
of the external resistance is not immediately followed by the straightening
of the filaments. Askenasy 1 has shown that this sudden dehiscence is due
to the filaments being pressed into the grooves between the anther-lobes,
and clinging to these with a certain energy.
The sudden protrusion of the sexual organs in the flowers of Saro-
thamnus and Genista tinctoria is due to resistance being overcome or
removed 2, and the same applies to the sudden opening of the flower of
Stanhopea oculata 3, which produces a perceptible sound, and to the repeated
rapid movements of the gynostemium of Stylidium adnatum.
In other cases the vital activity gradually provides for the rupture
of the tissues at definite points, and for the sudden release of the strains
produced by growth. It is in this way that spontaneously, or as the result
of a slight touch, the fruits of Impatiens noli-me-t anger e and /. balsaminea^
of Cardamine hirsuta and Cyclanthera suddenly dehisce, the valves of
the fruit rolling up with considerable force and the seeds being shot to
a greater or less distance away4. The separation of the elongated cells
of Zygnema and Mougeotia is effected in a similar fashion by the splitting
of the common wall. The sudden splitting of the cuticle, coupled with
the outward bulging of the end walls previously flattened by mutual
pressure, causes the cells to shoot apart, and the spores of Basidiomycetes
appear to be thrown off by the basidia in this way. Any agencies such as
induction-shocks, chloroform, or iodine, which aid in the rupture of the
cuticle, induce the sudden separation of the cells if applied when the
segmentation is completed5.
In the case of Momordica (Ecballium) elaterium the fruit-stalk forms
a plug at the base of the fruit, and becomes loosened when the latter is
ripe, so that the seeds together with a slimy liquid spurt out from the
interior. Dutrochet6 recognized that the required energy was derived
from the elastic distension of the walls of the fruit by the compressed
1 Askenasy, Verhandl. d. naturhist.-med. Vereins zu Heidelberg, 1879, N-F-> Bd- Ir» P- 274-
2 Cf. Ludwig, Biologic der Pflanzen, 1895, p. 472.
3 Pfitzer, Beobachtungen iiber Bau und Entwickelung d. Orchideen, 1877, p. 12. Reprint from
Verhandl. d. natur.-med. Vereins zu Heidelberg, Bd. u.
4 Dutrochet, Me"moires d. vegetaux et d. animaux, Bruxelles, 1837, P- 2295 Hildebrand,
Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 238 ; Eichholz, ibid., 1886, Bd. xvn, p. 543; Ludwig, 1. c.,
P- 332. Other mechanisms, including that by which the seeds of Oxalis are dispersed, are
discussed in these works. On the mechanism of Sphaerobolus stellatus see Zopf, 1. c., pp. 84, 374.
5 See Benecke, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 453. On the fragmentation of the
frond of Rhodomela see Tobler, Ber. d. bot. Ges., 1902, p. 361.
6 Dutrochet, 1. c., p. 229; Hildebrand, 1. c., p. 238 ; Roze, Journal de Botanique, 1894, T. VIII,
p. 308.
SPECIAL AND GENERAL 149
contents. When the latter have escaped the wall of the fruit contracts
considerably, and the escape is prepared for by the loosening of the
tissue round the top of the fruit-stalk. A similar spurting mechanism is
responsible for the escape of the spores from the spore-sacs of many
Discomycetes, Pyrenomycetes and Lichens, for the spores, together with
a portion of the unused contents of the sporangium, may be thrown out
sometimes to a distance of several centimetres *. After dehiscence the
wall of the ascus distended by turgor may contract to three-quarters or
two-thirds of its previous length, as when a short rubber-tube distended
with water is pricked and the contents allowed to escape. A similar con-
traction is naturally also shown by the ascus when its contents are
plasmolysed.
In certain Pyrenomycetes, previously to the dehiscence of the ascus,
its outer cuticular wall ruptures and the distensible inner wall elongates to
as much as twice its original length, so that the apex of the ascus reaches
to or protrudes beyond the narrow mouth of the fructification 2. (Fig. 34.)
In some cases the spores all collect at the apex and are thrown out
simultaneously, but in other cases they follow one another. Each blocks
the apex for a while until the turgor has risen sufficiently to throw it out,
when another blocks the narrow opening and, after a pause, is thrown out
in its turn, when the turgor is once more restored. The fact that the
dehiscence takes place at a definite point shows either that the membrane
has remained weaker here than elsewhere or else that the protoplasm
has produced a diminution of the cohesion of the wall at this point pre-
viously to dehiscence. In the former case a rise of turgor would be
required to produce dehiscence, but not necessarily in the latter. It is,
however, not surprising that shaking or changes in the moistness of the
air may excite or accelerate the dehiscence.
In many cases a pronounced swelling of the wall takes place, which
may aid in producing dehiscence and in narrowing the cavity of the ascus.
Prior to dehiscence the swelling is possibly prevented by the pressure
exerted by the contents on the wall. The rupture of cuticular membranes
is by no means uncommon and occurs normally whenever the inner walls
continue to grow, or when cuticularized gland-cells are actively excreting.
The threads which escape from the grandular hairs of Dipsacus under water
are probably extended through cracks in the cuticle. They appear to
be products of the metamorphosis of the cell-wall, and their peculiar move-
ments are probably similar in character to those shown during the formation
of myelin threads 3.
1 De Bary, Morphologic u. Biologie d. Pilze, 1884, p. 90 (Fungi, Mycetozoa and Bacteria) ;
Zopf, Die Pilze, 1890, p. 87 ; Ludwig, Biologie der Pflanzen, 1895, p. 328.
3 Pringsheim, Jahrb. f. wiss. Bot., 1858, Bd. I, p. 190.
3 Cohn, Bot. Ztg., 1878, p. 123; F. Darwin, Journal of Microscopical Science, 1877, Vol. xvn,
p. 245, and 1878, Vol. xvm, p. 73.
150
MOVEMENTS OF CURVATURE
A still more striking instance of the same mechanism is afforded by
the ripe sporangium of Pilobolus crystallinus, which, according to Coemans,
may be thrown to a height of 105 centimetres. [Relatively to size, this is
higher than a man can throw a cricket-ball, even neglecting the air-resistance
which, relatively to the masses, is several
hundred times greater in the case of the
sporangium. This energetic movement is
produced by the gradual swelling of the
basal membranous wall, which loosens the
union between the sporangium and the
swollen apex of the sporangiophore. The
latter then ruptures at the apex and the
jet of escaping liquid throws the sporan-
gium away 1. The beginnings of this special
mechanism are seen in Mucor, where the apex
of the sporangiophore (columella) bulges
into the sporangium and causes the rupture
of the brittle sporangial wall. The spores of
Empusa muscae and of various Basidio-
mycetes are jerked away in a similar fashion
by pressures created by turgidity. Sper-
matozoa and zoospores when not ejected by
the dehiscence of the antheridium or zoo-
sporangium make their own way out through
the P°int °f rUPtUre> and the ^ Z°°SP01'eS
of Vauckeria may be nipped in two during
their exit fr°m the narrow opening of the
zoosporangium 2.]
The hygroscopic movements of dead organs are often of great use 3.
Thus the fact that many dry fruits and anthers open in dry air but close
when moistened ensures that the seeds or pollen-grains shall not be
dispersed during wet weather. Similarly the peristomes of many mosses
close the mouth of the capsule when moist but expand and allow
1 Cf. de Bary, Morphologic u. Biologic der Pilze, 1884, pp. 77, 90; Zopf, Die Pilze, 1890,
p. 81. ,
a On the escape of zoospores see de Bary, 1. c., p. 87 ; Falkenberg, in Schenck's Handbuch der
Botanik, 1882, Bd. II, p. 195 ; Strasburger, Wirkung des Lichtes und der Warme auf Schwarm-
sporen, 1878, p. 14; Walz, Bot. Ztg., 1874, p. 689; Rothert, in Cohn's Beitrage z. Biologic, 1892,
Bd. V, p. 344; Klebs, Bot. Ztg., 1891, p. 859; Goebel, Ann. du Jard. bot. de Buitenzorg, 1898,
Suppl. ii, p. 65.
3 Ludwig, Biologic d. Pflanzen, 1895, pp. 327, 344; Kerner, Pflanzenleben, 1891, Bd. II, p. 421
(Natural History of Plants, 1895, Vol. II, p. 447); Haberlandt, Physiolog. Pflanzenanatomie, 1896,
2. Aufl., pp. 469, 488 ; Hildebrand, Jahrb. f. wiss. Bot., 1873-4, Bd. IX, p. 245 ; Steinbrinck, Unters.
iib. d. anat. Ursachen des Aufspringens d. Friichte, 1873, Bot. Ztg., 1878, p. 561 ; Geovanozzi, Nuovo
giornale botanico italiano, 1901, T. viu, p. 207.
SPECIAL AND GENERAL 151
the spores to escape when the air is dry. During moist weather the dead
involucral leaves of Carlina and Helichrysum bend inwards and prevent
the dispersal of the seeds, whereas when dry they bend backwards and
remove the hindrance to dispersal J. In addition the pappus of Compositae
expands during dry weather when dispersal is possible, and closes when
the air is moist, so that any soaring fruits overtaken by rain are soon
washed to the ground. The well-known Rose of Jericho (Anastatica
hierochunticd) affords a striking instance of drought causing the branches
to curl up into a ball enclosing the fruits. When the rains begin they
re- expand, the fruits dehisce and the seeds take root in the soil 2. Many dry
capsules are also capable of hygroscopic expansion and contraction.
Hygroscopic torsions are performed by the setas of Funaria and
other mosses 3, as well as by the conidiophores of Peronospora and a few
other fungi *. This is especially marked in the beaks of the carpels of
Er odium gruinum^ which is often used as a hygrometer ; and these move-
ments, like those of Stipa and Avena, help the fruit to bore into the soil5.
Changes of shape produced by the loss of water are only the result
of the removal of the imbibed water of the cell-wall when the cell contains
no free water. The collapse and wrinkling of the cell-walls of a dead
tissue when a portion of the water filling the cells is removed results,
according to Kamerling, Steinbrinck, and Schrodt 6 from the cohesion
and high breaking-stress of the diminishing volume of water, while its
adhesion to the cell-wall causes the latter to be drawn inwards and
crumpled. The aid of the external atmospheric pressure does not appear
to be necessary, since, according to Steinbrinck, the same phenomenon
is shown in a vacuum. When the water in the cells ruptures, air rapidly
penetrates the cell, according to Steinbrinck 7, so that the air-pressure
is rapidly equalized within the cell.
When dry organs are placed in moist air, no water appears in the
cavities of the cells so long as the formation of dew is avoided. Hence
1 Dutrochet, Memoires, &c., Bruxelles, 1837, p. 236; Detmer, Journal fiir Landw., 1879,
Bd. xxvil, p. in.
Ascherson, Ber. d. bot. Ges., 1892, p. 94.
Wichura, Jahrb. f. wiss. Bot., 1860, Bd. II, p. 198; Goebel, Flora, 1895, p. 483.
Cf. Zopf, Pilze, 1890, p. 86.
Hanstein, Bot. Ztg., 1869, p. 526 ; F. Darwin, Trans, of the Linnean Society, 1873, 2nd ser.,
, p. 149; Steinbrinck, Bot. Ztg., 1878, p. 580.
Vol.
6 Kamerling, Bot. Centralbl., 1897, Bd. LXXII, p. 53; ibid., 1898, Bel. LXXlii, p. 472 ; Flora,
1898, p. 152. See also the summary in Bot. Ztg., 1898, p. 330; Steinbrinck, Festschrift fiir
Schwendener, 1899, p. 165; Ber. d. bot. Ges., 1899, pp. 99, 325; ibid., 1900, pp. 48, 217, 275, 286.
Steinbrinck (I.e., 1900, p. 219) suggests the term ' Schrumpfeln ' for crumpling caused by the
cohesion-mechanism, but a special term is quite unnecessary.
7 Schrodt, Ber. d. bot. Ges., 1897, p. 100; Steinbrinck, I.e., 1900, pp. 275, 286. Cf. also
Claussen, Flora, 1901, p. 422.
152 MOVEMENTS OF CURVATURE
any movements performed can only be due to the imbibition and swelling
of the cell-walls. This applies to the hygroscopic movements of many
dry fruits as well as of the awns of Grasses and of Eroditim^ although
when the parts are still turgid the cohesion-mechanism may produce the
first movements. It is possible that both mechanisms may produce the
same kind of movement, so that Steinbrinck and Schwendener 1 may each
be partly right, although the former ascribes the opening and closing of the
anthers to the water-cohesion-mechanism, and the latter to imbibition
and swelling.
The movement naturally in all cases depends upon the properties of
the organ, upon the power of swelling of the walls, and upon their rigidity and
the arrangement of the cells and tissues 2. The power of imbibition varies
in the different layers of the cell-wall, so that the swelling may not be
equal in all directions. Since imbibition takes place with great energy,
movements due to the swelling of the cell-walls can overcome more resistance
than those due to the water-cohesion-mechanism, which is usually unable to
produce any distinct changes of shape in thick-walled cells. Both the
cohesion- mechanism and the decreased swelling of the cell-walls may
be responsible for the dehiscence of different fruits, and may produce in
many cases strains which when released cause sudden movement. In the
annulus of the sporangia of Polypodiaceae, as the water evaporates from
the cells they are more and more contracted and deformed, the thin
outer walls being drawn inwards. When the strain reaches a certain limit
the walls of the sporangium rupture at the loosened lip-cells. Immersal in
glycerine excites dehiscence by removing the water rapidly from the
annulus-cells, and after the water in the annulus-cells has ruptured the
recurved annulus straightens more or less.
Historical. The existence of movements due to death, or to changes in the
moistness of dead organs, was recognized by de Candolle 3, and these were distinguished
from movements due to vital activity by Dutrochet 4, who also gave explanations of
the movements of dehiscence and dispersal which were in the main correct.
The influence of the external conditions can be predicted in the case of dead
objects from purely physical considerations, although the external conditions may
also affect the course of the preparation for dehiscence and dispersal. Changes in
the percentage of water may, for instance, act both physiologically and physically,
1 Schwendener, Sitzungsb. d. Berl. Akad., 1899, p. 101 ; Steinbrinck, Ber. d. bot. Ges., 1901,
p. 552; 1902, p. 117; 1903, p. 217; Schrodt, Ber. d. bot. Ges., 1901, p. 483; Schwendener,
Sitzungsb. d. Berl. Akad., 1902, p. 1056; Ursprung, Jahrb. f. wiss. Bot., 1903, Bd. xxxvin, p. 635.
2 Cf. Haberlandt, Physiol. Pflanzenanat., 2. Aufl., 1896, p. 465, and the works quoted by
Haberlandt on p. 488.
3 A. P. de Candolle, Physiologic des Plantes, a German translation by Roper, 1833, Bd. I, p. 13.
4 Dutrochet, Memoires pour servir a 1'histoire d. vegetaux et d. animaux, Bruxelles, 1837,
PP- 225, 235.
SPECIAL AND GENERAL 153
while a deficiency of oxygen may render proper ripening difficult or impossible, so
that if all free oxygen is removed while the sporangia or zoospores are unripe no
dispersal or dehiscence takes place l.
Apart from the physical action of temperature upon imbibition and the like,
a physiological action is also exercised upon the development preparatory to
dehiscence and dispersal. Plants adapted to low temperatures are able to throw
off organs and to discharge their swarm-spores or other reproductive bodies at
temperatures approaching the freezing-point of water or even slightly below it,
especially in the case of Arctic marine Algae8. Certain observations of Thuret
seem to indicate that the escape of the zoospores is delayed at temperatures above
the optimum, while in some cases changes of temperature appear to accelerate the
escape. Thus, Dodel observed a premature birth of the zoospores of Ulothrix when
frozen filaments of this Alga were rapidly thawed.
Light appears to exercise little or no direct physical influence upon these
movements, for when it accelerates transpiration or induces the development of
reacting organs, or of a reacting condition, its action is as indirect as when
illumination causes movement by modifying the growth or turgor of responsive
cells3.
The dehiscence and dispersal movements of ripe organs may take place in
temporary darkness even when the organs are unable to develop or do not develop
normally in continued darkness. Illumination or changes of illumination do, however,
appear in certain cases to favour these movements. Thus the illumination of previously
darkened plants hastens the throwing off of the sporangia of Pilobolus crystallinus 4
and the ejaculation of the spores of A scobalus furfur aceus 5. In addition, light appears
to favour the escape of the swarm-spores of many Algae, and in darkness the
zoosporangia may not be as completely emptied, or their contents as well dispersed,
as when illuminated 6.
1 Cf. Rothert, Cohn's Beitrage z. Biologic, 1892, Bd. v, p. 344, and the literature quoted by him.
3 For instances see Kjellmann, Bot. Ztg., 1875, P- 774? G- Kraus, ibid., 1875, p. 774; Dodel,
ibid., 1876, p. 178 ; Strasburger, Wirkung des Lichts und der Warme auf Schwarmsporen, 1878, p. 44 ;
Klebs, Die Bedingungen der Fortpflanzung einiger Algen und Pilze, 1896.
3 [The implied suggestion that the physical action of light is always a direct one, and its
physiological action indirect, is somewhat misleading. Possibly the only direct physical action of
light is the mechanical pressure exercised upon an illuminated surface by the impinging light-rays.
The chemical, heating, and fluorescent effects of light are as much indirect actions as when illumina-
tion affects turgor or transpiration, and in each case the percentage of the light energy utilized
depends upon the properties of the material affected.]
4 According to Coemans and to Hofmeister, Pflanzenzelle, 1867, p. 290. G. Kraus (Bot. Ztg.,
1876, p. 507) states that the blue and violet rays are most effective.
5 Coemans, quoted by de Bary, Morphologic und Biologic der Pilze, 1884, p. 99.
6 For the literature see Braun, Verjiingung, 1851, p. 237 ; Thuret, Ann. sci. nat., 1850, 3° ser.,
T. xiv, p. 247; Strasburger, I.e., p. 15; Walz, Bot. Ztg., 1868, p. 497; Dodel-Port, ibid., 1876,
p. 177; Rostafinski u. Woronin, ibid., 1877, p. 667; Klebs, I.e.
CHAPTER III
TROPIC1 MOVEMENTS
PART I
INTRODUCTORY
SECTION 32. General.
IN order that the plant and its organs may attain situations adapted for
the performance of their different functions they must possess special
tropic l irritabilities. These determine the primary orientation of the main
axis, upon which the lateral organs have definite positions assured to them
when they merely follow their inherent autotropic tendencies. This applies
to hairs and to the finer rootlets, whereas runners, leaves, and lateral roots
of the first order assume positions mainly determined by external tropic
stimuli. The latter induce movements which result in the organ placing
itself at a definite angle to the direction of the exciting stimulus, and
naturally such responses are best studied when the other external conditions
are kept constant and are diffusely applied.
The terms geotropism1, heliotropism (phototropism), thermotropism,
chemotropism, osmotropism, hydrotropism, rheotropism, thigmotropism
(haptotropism), galvanotropism and autotropism, merely indicate the
exciting agency and say nothing as to the physiological response involved.
It was in this sense that the term heliotropism was used by de Candolle
and other early authors, so that Wiesner is neither historically correct nor
practically justified in restricting it to curvatures produced by growth2.
The curvatures may, in fact, either be produced by heterauxesis or by
variation movements, and the locomotory and orienting movements of free-
swimming organisms are produced in a variety of ways. In the latter case
it is permissible to use the terms phototaxis, chemotaxis, and the like,
although frequently no sharp line of demarcation can be drawn between
tropic and tactic movements 3. An organism which passes through motile
and fixed stages may show at one time tropic and at another tactic
responses, while the movements of the chloroplastids of plant-cells, though
usually more tactic in character, simulate tropic movements in the case of
1 Pronounced, tropic, tropism.
a Die heliotropischen Erscheinungen, 1880, Bd. II, p. 22.
3 Pfeffer, Druck- nnd Arbeitsleistungen, 1893, p. 414, footnote.
GENERAL 155
Mesocarpus. In such cases the character of the responding mechanism
determines the dissimilar modes of response, and hence the latter afford no
evidence as to whether the sensory processes are alike or dissimilar in
tactic and tropic organisms.
When the organism or reacting organ places its main axis parallel to
the direction of the exciting stimulus we may speak of parallelotropism, in
preference to the term orthotropism or to the longitudinal tropism of
Frank. Plagiotropism may be used in a general sense, when the main axis
is inclined to the direction of the exciting agency. Diatropism was used
by Darwin to indicate a tendency to place the main axis at right angles to
the orienting stimulus, and is preferable to the 'transversal tropism ' of Frank
or the ' homolotropism ' of Noll. The word klinotropism may be employed
when the angle between the main axis and the direction of the exciting
agency is less than a right angle but greater than zero. The term
heliotropism was first used by de Candolle \ while that of geotropism was
invented by Frank2. The terms negative and positive heliotropism were
introduced by Hofmeister 3, while various special terms were employed by
Darwin, Rothert, and Massart 4. Curvatures towards the exciting agency
may be denoted as positive instead of using the word ' protropic ' suggested
by Rothert, or * anatropic ' as employed by Massart. The reverse curvature
will naturally be negative, so that the ' apotropism ' of Darwin, and the
* katatropism ' of Massart are unnecessary. In the same way we may
speak of positive and negative klinotropism in preference to 'anaklinotropism'
and ' kataklinotropism,' and in certain circumstances the use of the following
signs may prevent misconception: f positive parallelotropism, j negative
parallelotropism ; |-> diatropism ; | /* positive klinotropism, |\^ negative
klinotropism.
A displaced parallelotropic organ returns to its original position either
by a positive curvature only (stem) or by a negative curvature only (root),
whereas a displaced plagiotropic organ may assume its normal orientation
either by a negative or positive curvature according to the direction of
displacement. Flattened organs like leaves may assume profile positions,
a phenomenon to which the term of paraheliotropism was given by
Darwin 5 ; and if the movement involves torsion Czapek speaks of ' stro-
phism ' (geostrophism, photostrophism), and Schwendener of ' tortism ' 6.
1 A. P. de Candolle, Physiologic des Plantes, a German translation by Roper, 1835, Bd- n>
p. 609.
2 Frank, Die natiirliche wagerechte Richtung, 1870.
3 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. Ill, p. 86.
4 Darwin, The Power of Movement in Plants, 1881, p. 4; Rothert, Conn's Beitrage z. Biologic,
1896, Bd. vii, p. 5; Massart, Biol. Centralbl., 1902, Bd. xxii, p. 70.
5 L. c., p. 357.
6 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p. 273; Schwendener and Krabbe, 1892,
Gesammelte Mittheil., Bd. n, p. 302. [Since the torsion is the result of growth it is difficult to see
156 TROPIC MOVEMENTS
Sachs l used the terms parallelotropic (orthotropic) and plagiotropic more
to distinguish between perpendicular and horizontal organs, but they may be
used in a general sense to indicate the mode of orientation to any directive
agency, and if necessary the latter can be indicated by the usual prefixes as
in the terms geoparallelotropic, photoplagiotropic, aitiotropic and autotropic.
The words orthotropic and campylotropic or skoliotropic have been used
to indicate whether an organ is straight or curved 2, and hence Sachs' use
of the term orthotropic seems inadvisable.
The natural positions of the different organs are not solely due to
tropic stimuli, and in fact many organs have no tropic irritability, while in
all cases the autotropic tendencies of the organs come more or less into
play. Tropic irritability is naturally most strongly developed in the
organs where it is of greatest importance, and may be mainly or solely
responsible for the orientation of various parts. Since the different tropic
irritabilities may occur singly as well as in combination, it is evident that
each involves a definite form of sense-perception. Hence one positively
geotropic organ may be also positively heliotropic, but another may show
negative or plagio-heliotropism, while yet another may be devoid of one form
of irritability, or may have it modified without affecting its other senses 3.
Even in non-cellular plants the different organs develop varying
irritabilities, and the strong heliotropic irritability of the sporangiophore of
Phycomyces is absent from the hyphae. Changes of tone of internal or
external origin may also modify the result obtained by stimulating reacting
organs, as when the absence of light causes a dia-geotropic organ to
assume a klinotropic or parallelotropic position. In addition a rise in the
intensity of the stimulus may alter the orientation, as when a sufficient
increase of illumination causes the positively parallelotropic position of the
filaments of Vaucheria, the sporangiophore of Phycomyces, and the young
shoot of various flowering plants to be replaced by a plagio-heliotropic one.
All plants do not show such pronounced reactions, but nevertheless in all
cases the existent and pre-existent conditions have a considerable influence
upon the irritable tone.
Many radial organs may react plagiotropically, for the filaments of
Vaucheria and Phycomyces, lateral roots of the first order, as well as the
rhizomes of Heleocharis, Sparganium, Scirpus, and Agropyrum, and the
runners of Lysimachia nummularia, Glechoma and Vinca are not only
any need for a special term in preference to the general one of ' tropism.' In the case of an organ
which partly twists and partly curves towards the light it might become necessary to say that it
possessed a positively paralleloheliotropocampylostrophismic (tortismic) irritability.]
1 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. n> P- 237-
3 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvil, p. 312.
3 A few instances in regard to geotropism and heliotropism are given by Frank, Beitrage zur
Pflanzen physiologic, 1868, p. 89.
GENERAL 157
morphologically but also physiologically radial. The latter is shown by
the fact that the same tropic position is assumed whichever side is turned
undermost, whereas in responsive dorsiventral organs a stable position
is only gained when it twists or curves until a particular side occupies
a definite position in regard to the orienting stimulus.
The plagiotropic position suits most dorsiventral organs best, and hence
the majority of such organs have developed a plagiotropic irritability,
although in some cases they are parallelotropic. The strongly dorsiventral
thallus ofMarchantia is, for instance, photo-plagiotropic, but in darkness is
parallelo-geotropic, while certain leaves assume a photo-parallelotropic
position in intense sunlight l. In addition, Bodo saltans and the zoospores of
certain Phaeophyceae, although dorsiventral in structure, show a parallelo-
tactic orientation to photic stimuli. Plagiotropic irritability does not indeed
involve either morphological or physiological dorsiventralities, though
favoured by their presence. Sachs was therefore in error in supposing
that all dorsiventral organs were plagiotropic.
Other tendencies may influence the position assumed in response to
a tropic reaction. Thus the mere weight of the organ may cause
a pronounced curvature, although in other cases such action is feeble or
imperceptible. In addition, the realization of any curvature awakens
a physiological and mechanical counteraction, and tropic stimulation may
excite other forms of curvature. Thus a negatively or even a positively
klinotropic position may result from the antagonism of epinasty and
negative geotropism. Photonastic, thermonastic, and hydronastic responses
may also often co-operate with tropic reactions when the organ possesses
these forms of irritability, for an increase in the intensity of the direct lateral
illumination, for instance, also involves an increase in the general diffuse
illumination.
SECTION 33 (continued).
Phototropic and geotropic reactions may result from a variety of
stimulatory actions exercised by the exciting agency, and in certain cases
the same agency may awaken two tropic actions simultaneously as, for
instance, when a solution exerts an osmotactic and a chemotactic action upon
the same organism. Since the osmotactic action is a function of osmotic
concentration, whereas the chemotactic action depends upon chemical
quality and is not exercised by all substances, it is easily possible to study
the two actions apart from one another as well as together. Light also
exercises two dissimilar stimulatory actions upon organs possessing both
1 A few additional instances are given by Noll, Jahrb. f. wiss. Bot., 1900, Bd. XXXIV, p. 478;
and by Goebel, Organography, 1900, p. 234.
3 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, p. 227.
158 TROPIC MOVEMENTS
phototropic and thermotropic irritabilities. In addition, the blue and red
rays may awaken two different tropic reactions, which co-operate in
producing the position assumed in mixed light.
An orienting stimulus exerts a double action when it induces
dorsiventrality in a radial tropic organ, and this induced dorsiventrality
may result ultimately in the assumption of a permanently plagiotropic
position, as in the thallus of Marchantia. The same thing applies to the
prothallus of the Fern, although here the unilateral illumination only
induces a labile dorsiventrality. Labile or stable hyponastic or epinastic
tendencies may also be induced in connexion with the labile or stable
dorsiventrality, and the appearance of the latter may awaken or modify
special tropic or nastic powers of response.
Even when no dorsiventrality is induced, a single agency may exert
two dissimilar tropic reactions, as, for instance, in the case of the radial
plagiotropic branches of trees, in which the action of gravity appears to
excite an epitropic tendency to curvature on the upper side, and a
hypotropic one on the under side. Both responses may be regarded as
geotropic curvatures, whether they are indirectly or directed excited, or
whether the epitropic response follows as a counter-action to the induced
tendency to hypotropic curvature. Differences in the times of reaction
and induction merely show that dissimilar stimulatory actions are involved,
and afford no argument against both being geotropic responses. Every
tropic reaction may indeed involve epinasty or hyponasty, unless we
elect to restrict these terms to curvatures produced by diffuse stimuli.
The hypotropic reaction of the branch does, in fact, appear and disappear
more rapidly than the epitropic one on the upper side, so that the existence
of the two dissimilar tendencies is readily detected, whereas this would be
impossible if the times of induction and the duration of the after-effect
were alike in both cases.
Naturally no curvature results if the antagonistic stimulatory actions
balance, and the same applies when the direction of the stimulus alters,
provided that the opposing reactions increase or decrease in the same
proportion. This must actually be the case in such branches as continue
to grow in a new direction forcibly impressed upon them ; for if their altered
position in regard to the perpendicular caused unequal geotropic responses
to be given by the upper and under sides, the natural result would be to
produce a curvature of the branch to its original line of growth where the
geotropic actions balanced. Dissimilar tropic agencies or reactions may
also antagonize each other, and in the case of an organism which is
positively chemotactic and negatively osmotactic to a particular substance,
a position of equilibrium is reached at a definite point in the zones of
diffusion, owing to the fact that the negative osmotactic action increases
more rapidly than the positively chemotactic action with increasing
GENERAL 159
concentration. Similar relationships may often be responsible for the
changed reaction produced by increasing intensity of stimulation.
There is no reason, however, for assuming that all tropic or more
especially all plagiotropic reactions involve the antagonism of two or
more dissimilar stimulatory actions and responses. A single action may
induce or modify movement in organisms as well as in machines. Thus
the continued turning of a steam-cock (increasing stimulation) may induce
first a forward and then a backward movement of a locomotive. The
admission of steam into the cylinders can, however, produce no movement
if the wheels are fixed ; and in the same way an organ may be non-geotropic
or non-heliotropic either because the motor mechanism or the perceptive .
mechanism is undeveloped or out of gear, or because the connecting links
between the two are incomplete.
Even in simple cases it is often difficult to determine whether a
particular plagiotropic position results from a tropic action alone or
involves other co-operating factors, and many instances of such conjoint
action are known. The parallelo-heliotropism or -the parallelo-geotropism
of an organ are easily determined separately, and hence it is possible to show
that the plagiotropic position assumed by certain organs under horizontal
illumination is the result of the co-operation of negative parallelo-geotropism,
and positive parallelo-heliotropism. In other cases the plagiotropism of
a shoot may be due to the interaction of its negative geotropism and
autogenic epinasty, the latter permanently preventing the assumption of
a parallelotropic position. When the stimulus of gravity is eliminated
on a klinostat, the epinastic curvature continues until the autogenic
campylotropism is fully satisfied. If gravity is once more allowed to
act the campylotropic curvature is decreased by the negatively geotropic
reaction, but is increased when the stem is inverted until the plagiotropic
position is once more assumed. Similar results may be obtained when
a growing branch is split longitudinally for a portion of its length, for
each of the outwardly curving halves shows an autogenic epinasty. If an
organ is placed so that the epinastic curvature takes place horizontally, the
geotropic reaction takes place at right angles to the curvature, so that an
obliquely ascending curve is performed.
A plagiotropic position can equally well result from the co-operation of
autogenic epinasty with plagio-geotropism, as is actually the case in many
foliage-leaves. The pronounced backward curvature which these often
show on a klinostat demonstrates their autogenic campylotropism, and
also shows the part played by gravity in their plagiotropic orientation ;
for when the stimulus of gravity again acts the leaves raise themselves
into a horizontal position. If the leaf is pointed vertically upward it
descends into the plagiotropic position, which results from klino-geotropism
and epinasty, not from negative parallelo-geotropism and epinasty.
160 TROPIC MOVEMENTS
A plagiotropic position may, however, also be attained without the aid
of any epinasty, as when a leaf, owing to the position of the stem, has to
curve beyond the epinastic position of equilibrium. In such cases the
epinasty is no longer essential, and may modify the position assumed
little or not at all if the leaf orients itself definitely in regard to gravity,
whether it has to overcome epinasty, photonasty, and the like, or not. The
fact that the angle the leaf makes with the stem may vary indefinitely
suffices to show that the orienting action of the stem is either absent or is
so weak as to be ineffective.
De Vries considered that tropic stimuli always produced a parallelo-
tropic reaction, so that a plagiotropic position could only result from
the combination of a tropic action with some other attempted curvature.
This view is, however, not supported by the facts, nor is it easy to see any
reason why a responding organ should not be able to directly set itself
at right angles to an orienting agency.
When the expansive tissues are symmetrically arranged, an autogenic
epinastic curvature may be prevented, but may take place when the organ
is split longitudinally, and may then cause the parallelo-geotropic halves
to assume plagiotropic positions. In the same way two leaves bound
together with their upper surfaces together form a symmetric arrangement,
and may in certain circumstances react parallelo-geotropically because the
opposed plagiotropic tendencies only equilibrate in a vertical plane.
Dorsi ventral organs are much more liable to nastic curvatures than
radial ones, and any dissimilarity in the sensitivity or power of reaction
of the upper and under surfaces is bound to affect the tropic responses.
Thus the physiological dorsiventrality of certain tendrils results in the fact
that a curvature is only produced when contact is applied to the sensitive
concave side. In addition, a stem cannot place itself parallel to the
incident rays of light when one side has a feebler heliotropic irritability
than the other, or when one side is smeared with indian ink. Hence
a plagio-phototropic orientation is to be expected when the structure is
such that light penetrates more readily on one side than on the other.
Under such circumstances a photonastic curvature might result in diffuse
daylight, although this is actually due to unequal phototropic stimulation.
Care is needed in the interpretation of such phenomena, as is well shown
in the case of dorsiventral tendrils ; for although contact on the convex side
does not excite a curvature, it is able to suppress one when the concave side
is also stimulated, so that both sides are irritable, though in unlike degree.
It is difficult to determine from the tropic reactions in what degree the
irritabilities of the upper and under sides differ in intensity or in quality.
By altering the incidence of the light a plagio-phototropic leaf may be
caused to assume its proper position of equilibrium either by a positively
or negatively directed movement, whereas illumination of the under side
GENERAL 161
always produces a positive curvature towards the light. This is, however,
the natural result of the altered orienting action, and fails to reveal the
distribution of irritability in the leaf, for the movement continues only
until the appropriate plagio-phototropic position is again assumed.
Historical. Numerous facts concerning orienting movements were noted by
Bonnet *, while Knight and de Candolle investigated the geotropic and heliotropic
responses more intimately. Dutrochet 2 then pointed out that light and gravity acted
as inducing stimuli, and showed that natural orientation is the result of the varied
co-operation of geotropism, heliotropism, autotropism, weight, and so forth. Our
special knowledge of the different modes of orientation is due mainly to the labours
of Hofmeister, Frank, Sachs, de Vries, Darwin and F. Darwin, Pfeffer, Wiesner,
Krabbe, and Vochting. Subsequent authors are quoted in the text concerned with
their special studies.
Frank 3 followed Dutrochet in his attempt to give a full account of the various
factors concerned in the orientation of the plant and its organs. Apart from a few
errors and certain hypotheses based on insufficient proof, such as the supposed
polarity of the cell-wall, Frank's work corresponds in its general outlines to our
modern views. This applies also to Frank's transverse heliotropism and geotropism,
although de Vries * erroneously concluded that the unilateral action of gravity and
light was only capable of inducing parallelotropic orientation, and hence considered
that all plagiotropic positions were due to the antagonism of parallelotropism with
other tendencies to curvature. The actual existence of a diaheliotropic irritability
has been shown by Darwin and F. Darwin, while Pfeffer on more general grounds
came to the same conclusion5. A variety of instances of plagiotropic orientation
due to the isolated action of a single tropic agency were then brought forward6.
Several authors have, however, unfortunately failed to distinguish clearly between
nastic and tropic curvatures.
Sachs adopted de Vries's view, and applied it to dorsiventral organs, incidentally
discovering several important facts, and more especially showing that the same agency
might simultaneously excite more than one tendency to curvature. Sachs7 sup-
posed that the thallus of Marchantia might be considered to consist of cylindrical
elements arranged at right angles to the surface, and showing parallelotropic orienta-
tion ; but the facts that unicellular organs may show various modes of orientation,
1 Bonnet,' Unters. iiber den Nutzen der Blatter, 1762.
7 Dutrochet, Recherches anatomiques et physiologiques, 1824, P- 92-
3 A. B. Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870 ; Bot. Ztg., 1873, p. 17.
* De Vries, Arb. d. hot. Inst. in Wiirzburg, 1872, Bd. I, p. 223. The supposition of Wiesner
(Die heliotropischen Erscheinungen, 1880, n, p. 50), that the fixed light-position of leaves is due to
the antagonism of their negative geotropism and negative heliotropism comes under the same
category.
5 Darwin, The Power of Movement in Plants, 1881, p. 374 ; F. Darwin, Linnean Society Journal,
1881, Vol. xvm, p. 420; Pfeffer, Pflanzenphysiologie, i. AufL, 1881, Bd. II, p. 291.
6 Vochting, Bot. Ztg., 1888, p. 200; Krabbe, Jahrb. f. wiss. Bot, 1889, Bd. xx, p. an;
Schwendener und Krabbe, 1892, Gesammelte Abhandlg., Bd. II, pp. 255 u. s. w. ; Czapek, Jahrb. f.
wiss. Bot., 1898, Bd. xxxii, p. 271.
7 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, pp. 226, 254.
162 TROPIC MOVEMENTS
and that an increase in the intensity of the stimulus may convert the parallelo-
tropic position into a plagiotropic one, suffice to show the useless character of
this hypothesis. It is of course always possible that a dorsiventral thallus might be
produced in this way, but it is incorrect to suppose that the plagio-geotropic position
of the lateral roots merely results from their feeble parallelo-geotropism.
Sachs also incorrectly supposed that all tropically reacting dorsiventral organs
showed a plagiotropic orientation, and that the union of such objects to form a radial
or bilateral structure must result in the acquirement of a parallelotropic power of
reaction. This is, however, not the case, for a diatropic rhizome yields when split
two klinotropic halves which form a plagiotropic organ when bound together again.
Naturally no curvature is possible when opposed sides have the same tendency
to curvature, and two plagio-geotropic leaves bound together may assume a parallelo-
geotropic position. The same result is to be expected when the plagiotropic thallus
of Marchantia or Peltigera is rolled into a cylinder. To what degree radial organs
are formed in this way is uncertain, for the same result might be obtained by
a change in the power of reaction. Noll1 concludes that this actually occurs when
the apothecium of Peltigera is formed and assumes a parallelotropic position, for the
edges of the apothecium begin to rise upwards before the cylindrical shape has been
assumed. According to Noll, all leaves do not react parallelo-tropically when cylin-
drically coiled in the bud, and they still perform a plagiotropic orienting movement
when they are prevented from unrolling by means of a thread.
PART II
THE VARIOUS FORMS OF TROPIC CURVATURE
SECTION 34. Geotropism.
The constantly perpendicular direction of the force of gravity and its
universal action render it of more importance as an orienting agent to
rooted plants than any other, since in response to it the different parts
of the plant are caused to place themselves in such positions as will
best enable them to carry on their different functional activities. Other
orienting actions also go on to a greater or less degree, and in the case
of the organs which grow above the soil that of light becomes of special
importance. Indeed it is often sufficiently powerful to determine the
proper position of the subaerial organs even when it has to act against
.their geotropic irritability. In other cases, again, the stimulus of light is
oised to produce movements which are not directed towards the better
utilization of the stimulating agent. This is the case in those attaching
roots and tendrils whose negative heliotropism aids them in fixing themselves
to .a support, and also in the strongly heliotropic sporangiophores of many
Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 478.
GEOTROPISM 163
Fungi, which are aided by this means to develop their spores in air where
dispersal is possible but which do not primarily need illumination.
The lateral roots of the third or fourth order, thorns, hairs, and the stems
of the Mistletoe, are nearly or entirely devoid of geotropic and heliotropic
irritability, and hence grow in all directions independently of the direction
of gravity and of the illumination. The presence of a geotropic irritability
in a fungus mycelium might even become injurious by causing it to pass
from a suitable medium to comparatively innutritive soil.
The fact that the perpendicularity of the main axis is determined
by gravity is at once shown when a seedling is laid horizontally, for the
growing zone of the root curves downwards, and of the stem upwards
(Fig. 35). The lateral parts of the first order possess a definite diageo-
tropism, since they assume much the same angle with the perpendicular
whether the main root is laid horizontally or is even placed upside down 1.
The same fact shows that they are radial organs, and that directive influences
radiating from the main root exercise little or no effect upon them. In
all experiments of this kind it is natu-
rally essential that the conditions
should be kept as constant as possible,
and in this case the geotropic response
of the lateral roots is dependent not
only upon the intensity of the stimulus
but also upon the external conditions
and the tone of the root. The lateral
roots arising from the hypocotyl and
base Of the main rOOt Often grOW after being laid horizontally.
more or less horizontally as the result of their diageotropism, whereas later
roots arising at the base may form angles of 80° to 60° or even of 45°
with the perpendicular. In order that the root-system may spread
thoroughly through the soil it is necessary that the geotropic irritability
of side roots of the second and third order should diminish ; and in fact,
according to Sachs, the roots of the second order of Zea Mays have only
a feeble, and those of Cucurbita Pepo no geotropic irritability. It does
not, however, follow that lateral axes are always less irritable geotropically
or heliotropically than the main axis, for we are dealing here with special
phenomena of accommodation.
1 Dutrochet (Rech. s. la structure d. animaux et d. vegetaux, 1824, p. 102) supposed the direction
of the lateral roots to be determined as the resultant of their geotropism and their tendency to set
themselves at right angles to the main root. The matter was more fully explained by Sachs, Arb.
d. bot. Inst. in Wiirzburg, 1874, Bd. I, p. 602. Cf. also Czapek, Sitzungsb. d. Wiener Akad., 1895,
Bd. civ, Abth. I, p. 1197; Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 328; 1898, Bd. xxxn, p. 247 ;
Schober, Bot. Ztg., 1898, p. I ; Guillen, Compt. rend., 1901, T. cxxxil, p. 589.
M 2
164 TROPIC MOVEMENTS
In accordance with their special function, the attaching roots of the Ivy,
of Aroids, and of Orchids show usually little or no geotropism, but are com-
monly provided with a distinct heliotropic irritability. The erect growth of
the breathing- roots (pneumatophores) of certain Mangrove -trees, on the other
hand, appears to be due to their negative geotropism1. The roots of many
plants such as Palms, Sugar-canes and others appear, however, above the
soil when the latter is kept wet 2, and it requires to be determined whether
this is due to aerotropism, to negative geotropism induced by the peculiar
conditions, or to other causes.
Most horizontally-growing rhizomes maintain their position by the
aid of their strong diageotropism, and the growing zones curve back to
the normal position when the rhizome is disturbed. This applies not only
to dorsiventral rhizomes but also to physiologically radial and more erect
ones, including the root-stocks of Heliocharis palustris> Sparganium
ramosum^ and Scirpus maritimus*. The subterranean runners of Adoxa
moschatellina, Trientalis europaea^ and Circaea lutetiana are physiologically
radial, but nevertheless assume a more or less horizontal position in darkness
or in the soil. Exposure to diffuse light, however, induces such an altera-
tion in their geotropic irritability as to cause them to assume a positively
klinotropic, or even under special circumstances a positively parallelotropic
direction of growth*.
The downwardly-growing rhizomes of Yucca and Cordyline seem to
possess positive geotropism 5, which appears also to be responsible for
the downward curvature of the peduncle of Papaver, which later becomes
negatively geotropic and straightens as the flower expands 6. An alteration
of irritability is sometimes, but not always, employed to produce the upward
growth of the foliage-bearing portion of a sympodial rhizome, and to
induce changes in the position of flower-buds, flowers, fruits, and even of
1 Karsten, Bibl. hot., 1891, Heft 22, pp. 49, 55; Schimper, Bot. Mitth. a. d. Tropen, 1891,
Heft 3, p. 37; Went, Ann. d. Jard. bot. de Buitenzorg, 1894, Vol. XII, p. 26; Goebel, Organo-
graphy, Part II. On the radicle of Trapa cf. Kerner, Pflanzenleben, 1887, Bd. i, p. 83. On
negatively geotropic aerial roots cf. Wiesner, Die heliotropischen Erscheinungen, 1880, II, p. 77.
2 Kerner, Natural History of Plants, 1895, Vol. I, p. 88. See also Sachs, Flora, 1893, p. 4.
According to Eriksson, Bot. Centralbl., 1895, Bd. LXI, p. 273, Carex arenaria and other sand-plants
possess upwardly-growing roots.
3 Elfving, Arb. d. bot. Inst. in Wiirzburg, 1880, Bd. II, p. 489; Czapek, Sitzungsb. d. Wien.
Akad., 1895, Bd. Civ, Abth. i, p. 1218. According to Barth (Die geotrop. Wachsthumskrummung
d. Knoten, 1894, p. 35), the subterranean runners of Triticum refiens show no perceptible geotropic
irritability.
* Stahl, Ber. d. bot. Ges., 1884, p. 385 ; Goebel, Bot. Ztg., 1880, p. 790; Czapek, 1. c., p. 1230 ;
Rimbach, Fiinfstiick's Beitr. z. wiss. Bot., 1899, Bd. in, p. 201.
5 See the literature given in Vol. II, p. 194.
6 The literature will be given later, and it will be shown that we are dealing with a true
geotropic curvature, and not with a mere mechanical drooping produced by the weight of the flower-
bud. Wiesner (Sitzungsb. d. Wien. Akad., 1902, Bd. cxi, Abth. i, p. 747) does not, however, now
consider the downward curvature of the peduncle of a Poppy to be geotropic in character.
GEOTROPISM 165
the floral organs although the latter are in most cases nearly or entirely
devoid of geotropic irritability l.
Geotropism is in some cases of great importance in determining the
positions of plagiotropic main and' side shoots, but in other cases takes
little or no part in the orientation. The shoots of Lysimachia nummularia,
A triplex latifolia, and of Polygonum aviculare react plagio-geotropically in
strong light, but almost or entirely parallelo-geotropically in darkness, and
high and low temperatures may exert a similar effect.
Foliage-leaves are very commonly plagio-geotropic, although in many
cases a special power of geotropic reaction is developed for particular
purposes. Thus in seedlings of Phoenix, Allium, and Yiicca the positive
geotropism of a portion of the cotyledon carries the radicle and axis of
the stem downwards into the ground 2. According to Copeland 3, the
hypocotyls of seedlings of Lupinus albus, Robinia psetid-acacia, Helianthus
annuus and Cucurbita Pepo act in the same way, owing to the fact that
their original positive geotropism soon becomes negative.
The sporangiophores of Phycomyces nitens and Mucor mucedo are
strongly negatively geotropic 4, whereas the mycelial hyphae of these fungi 5,
as well as the stolons of Mucor stolonifer* show no perceptible geotropism.
The rhizoids of Bryopsis muscosa and of Caulerpa prolifera are positively,
the shoots negatively geotropic 7. The same applies to Char a and Nitella 8
whose shoots show a fairly strong negatively geotropic reaction, as also do
the stalks of the perithecia of Xylaria carpophila, of Claviceps purpurea, and
the stalks of the sporophores of various of the larger Agaricineae 9. The
lamellae, tubes, or lobes of the hymenium are, however, positively geotropic 10.
Among Thallophyta in general, however, geotropism is less used for
1 See the literature already given, and Wiesner, Sitzungsb. d. Wien. Akad., 1902, Bd. CXI, Abth.
i, p. 760. The downward bending of the fertilized flowers of Trifolium subterraneum and of
Arachis hypogaea, which causes the ripening fruits to be pushed into the soil, appears to be the result
of a change in the geotropic irritability. See Darwin, The Power of Movement in Plants ; Ross,
Malpighia, 1892, Fasc. VII-IX ; Huth, Ueber pericarpe, amphicarpe und heterocarpe Pflanzen, 1890.
2 Sachs, Bot. Ztg., 1863, p. 59; 1862, p. 241 ; Copeland, Botanical Gazette, 1901, Vol. XXXI,
p. 410; Neubert, Jahrb. f. wiss. Bot., 1902, Bd. xxxvill, p. 119 (Allium).
3 Copeland, 1. c. The stimulus appears in this case to be perceived by the root-tip.
* Hofmeister, Pflanzenzelle, 1867, p. 286; Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II,
p. 222; Wortmann, Bot. Ztg., 1881, p. 368; Dietz, Unters. a. d. bot. Inst. zu Tubingen, 1888,
Bd. II, p. 482 ; Steyer, Reizkriimmungen bei Phycomyces nitens, Leipzig. Diss., 1901, p. 6.
5 Kny, Sitzungsb. d. bot. Vereins f. Brandenburg, 12. Juni, 1881 ; Steyer, I.e., p. 28. Kny (I.e.)
and Stammeroff (Flora, 1897, p. 148) found that pollen-tubes possess no geotropism. [They appear
also to be devoid of any heliotropic irritability.] * Wortmann, I. c., p. 384.
7 Noll, Arb. d. Wurzburger Inst., 1888, Bd. Ill, p. 467 ; Klemm, Flora, 1893, p. 472.
8 Hofmeister, 1. c., p. 286; Richter, Flora, 1894, p. 408.
9 J. Schmitz, Linnaea, 1843, Bd. xvn, p. 474; Zopf, Die Pilze, 1890, p. 208.
10 Sachs, Experimentalphysiologie, 1865, p. 93; Jahrb. f. wiss. Bot, 1863, Bd. in, p. 93. [The
stipes of Lentinus lepideus only become geotropic when the formation of a pileus has been induced
"by exposure to light. Buller, Ann. of Bot., 1905, Vol. XIX, p. 427.]
166 TROPIC MOVEMENTS
purposes of orientation l than in most terrestrial flowering plants, while the
Bryophyta and certain flowering aquatics occupy an intermediate position
in this respect 2.
SECTION 35. Methods of Investigating Geotropism.
The orienting action of gravity only began to be properly understood
when Knight showed3 that centrifugal force exercised a similar orienting
action upon seedlings. On a rapidly rotating vertical wheel, for instance,
Knight found that the radicle grew outwards, the plumule inwards, both
organs curving so as to place themselves parallel to the direction of the
orienting force. In this case the disturbing action of gravity is eliminated
by the vertical rotation of the wheel, but if the wheel is rotated horizontally
the forces of gravity and of centrifugal force act at right angles to one
another upon the seedlings, and the ultimate position of the axis is along
a resultant line which bisects the angles between the forces if they are
equal, but is nearer to the more powerful one when they are unequal.
When the wheel is rotated very rapidly the axes of the seedlings grow
almost horizontally 4.
If a seedling is slowly and steadily rotated in a horizontal or vertical
position on a klinostat so that a revolution is performed in three to forty
minutes, the position of the plant is continually altered before any inductive
stimulating action of gravity can be made manifest5. For most plants
two to three revolutions per hour are sufficient, for at this rate practically
no centrifugal action is exercised, while at the same time neither the shoot
nor root has time to make a curvature before its position is reversed. If
each rotation takes several hours the slight, continually changing curvature
results in the production of a kind of circumnutation 6.
1 Cf. Berthold, Jahrb. f. wiss. Bot., 1882, Bd. XII, p. 572.
2 The unicellular rhizoids of Marchantia are geotropic, but the thallus less so. Mirbel, Mem.
de 1'Acad. royale de Paris, 1835, T. XIII, p. 354; Pfeffer, Arb. d. bot. Inst. in Wurzburg, 1871, Bd.
I, p. 89. A few facts concerning the Jungermanniaceae are given by Hofmeister, Pflanzenzelle,
1867, p. 294; Frank, Die natiirliche wagerechte Richtung von Pflanzenth., 1870, p. 66. On the
Muscineae cf. Bastit, Rev. gen. de Bot., 1891, T. Ill, p. 380; Jonsson, Bot. Ztg., 1899, Referate,
p. 132.
3 Knight, Phil. Trans., 1806, I, p. 99. Knight used a water-wheel, and carried out experiments
on rotation in both vertical and horizontal planes. The older and newer literature has been collected
by Cisielski, Unters. iiber d. Abwartskriimmung d. Wurzel, Dissertation, 1870. The same work
without the review of the literature is given in Cohn's Beitragen z. Biologic, 1871, Bd. I, Heft 2.
Cf. also Sachs, Arb. d. bot. Inst. in Wurzburg, 1879, Bd. n, p. 209.
* Cf. Wigand, Bot. Unters., 1854, P- J49 '•> Hofmeister, Jahrb. f. wiss. Bot., 1863, B(*. ill, p. 141.
5 This term was given by Sachs (Arb. d. bot. Inst. in Wurzburg, 1879, Bd. II, p. 217), who
was the first to use this method to any great extent for the elimination of gravity, although Hunter
had used it long ago to a limited extent, and also Dutrochet and Wigand. Hunter, Trans. Soc.
Imp. med., 1800, Vol. II. See A. P. de Candolle, Pflanzenphysiol., 1835, Bd. II, p. 556. Cf. also
F. Darwin, Linnean Soc. Journal, 1881, xvin, p. 425.
6 Darwin and Pertz. Annals of Botany, 1892, Vol. vii, p. 245 ; 1903, Vol. XVII, p. 93.
• METHODS OF INVESTIGATING GEOTROPISM 167
The heliotropic curvature produced by unilateral illumination may
also be prevented by rotation on a klinostat, so that both the heliotropic
and geotropic action may be eliminated if the plant is rotated about a
horizontal axis at right angles to the direction of the illumination. A slight
phototropic action may, however, be produced if the shadow of the axis
of the klinostat — or of the slice of bread commonly used to grow mould
fungi — falls upon the plant for a sufficient length of time at each rotation.
If the axis of rotation is horizontal but parallel to the incidental rays of
light, the action of gravity is eliminated, but not that of light ; and the
same applies whether the plant is fixed so that its own axis is parallel
or at right angles to that of the klinostat.
Dorsiventral organs often perform aitionastic movements under the
influence of changes in the diffuse external conditions. A photonastic
curvature may in fact be produced when a dorsiventral organ is rotated on
a klinostat so that it is equally illuminated on all sides. The same applies
to other agencies, including gravity, although under natural conditions
the latter never acts equally on all sides, as diffuse light may do without
causing any tropic curvature.
Tropic stimulatory reactions appear to be suppressed on a klinostat
in the same way as when the exciting agent is equally distributed on all
sides, but it must be remembered that specific irritabilities and the power of
response to a particular excitation may be excited or modified by tropic
stimulation 1. On the other hand, opposed stimuli acting on different flanks
in rapid succession may antagonize each other without producing any
responsive curvature either way. Under simultaneous stimulation of this
kind a dorsiventral tendril does not perform any curvature, and the same
effect follows when the tendril is revolved on a klinostat so that the point
of contact passes rapidly round and round an excitable zone. If, however,
the intervals between the successive stimuli are sufficiently long, a tropic
curvature will be produced which the stimulatory actions on the opposed
sides may be unable to eliminate. Finally, if both sides are equally
responsive, successive stimulations may be expected to produce the same
result as continuous diffuse excitation.
These and other considerations show that the tropic reactions of
a dorsiventral organ are not always entirely eliminated on a klinostat,
although when the rotation is sufficiently rapid, the action of a unilateral
agency will usually be the same as when it is diffusely applied. Neverthe-
less something depends upon the point of application of the stimulus, as is
shown by the fact that the stimulus of gravity reawakens the growth
1 In this way Wiesner (Die heliotropischen Erscheinungen, 1878, I, p. 55 ; 1880, II, p. 76) and
also H. Miiller (Flora, 1876, p. 76) were able to obtain heliotropic reactions on feebly sensitive
plants, which show none so long as they are exposed to geotropic induction.
i68 TROPIC MOVEMENTS
of the nodes of grasses when the haulm is rotated horizontally so that
gravity acts at right angles to the stem and equally on all sides, but does
not exercise this action to any appreciable extent when the haulm is rotated
in a vertical plane so that it is horizontally inclined only for short periods
of time. Similarly when an organ is equally illuminated on all sides, the
direction of the light rays is by no means immaterial, since more penetrate
when they fall perpendicularly to the surface than when they fall obliquely.
Swarm-spores react phototactically in spite of their rapid revolution around
their longitudinal axes, and this fact is an indication that a special
distribution of irritability may be able to prevent a tropic action being
eliminated by revolution on a klinostat.
Neither Czapek nor Noll l has paid full attention to these considera-
tions, for the former concludes that all geotropic action can be eliminated by
sufficiently rapid revolution on a klinostat, while both authors often do
not sufficiently distinguish between tropic and nastic stimuli and reactions,
and ignore the possibility of changes of tone being produced by the
transition from diffuse to unilateral stimulation 2.
Seedlings should be kept in moist air when used for experiments, and the older
adult portions of the root may be covered with wet filter-paper with one end of the strip
in water. In order to observe roots or rhizomes in earth or sawdust, they should be
grown in wood or zinc troughs with sloping glass sides, and pierced with holes
beneath3. Cut branches and peduncles may be placed in moist sand heaped up
beneath a covered zinc or glass cylinder.
Any apparatus may be used as a klinostat which is capable of performing
regular rotation, but the form prepared by Albrecht of Tubingen under Pfeffer's
instructions is extremely exact and serviceable 4. (Fig. 36.)
The movement is produced by a strong spring regulated by a fan, the mechanism
being attached to the lid (6) of the heavy box (h). One of the three axes on the
upper surface of the lid is joined by the gimbal joint to the axis (c), which rotates on
the friction-wheels (0), and has a pot attached at (g). The longer axis (m) is used to
attach a cylinder (t) containing germinating seedlings (/). If the cylinder contains
1 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, pp. 189, 270; Ber. d. hot. Ges., 1901,
Generalvers., p. (129); Noll, Flora, 1893, p. 357; Jahrb. f. wiss. Bot, 1900, Bd. XXXIV, p. 459 •
Ber. d. bot. Ges., 1902, p. 409.
2 As a matter of fact it is only a question whether the same effect is produced on a klinostat as
when the exciting agency acts simultaneously on all sides, and from this point of view the impossi-
bility of rigidly separating tropic and nastic reactions is of no importance. Every light ray, and also
the most momentary illumination, exerts a stimulating phototropic action, and the absence of
a response simply shows that the opposed stimuli balance. Hence, even when a plant is rapidly
rotated on a klinostat, it is still subject to phototropic and geotropic stimulation so long as its
irritability is unaltered.
3 Sachs, Arb. d. bot. Inst. in WUrzburg, 1873, Bd. I, p. 387. On a geotropic chamber see
Sachs, Flora, 1895, p. 293.
* See Bot. Ztg., 1887, p. 27.
METHODS OF INVESTIGATING GEOTROPISM
169
a little water the seedlings are kept moist, and to ensure an even distribution of
moisture wet blotting-paper may be placed around the inside of the cylinder.
If the box (h] is closed and a wood or cork plate attached to one of the axes,
a pot may be placed on it and rotated around a vertical axis, or the cover (£) may be
inclined at various angles and fixed by the screw-clamp (n) so that by means of the
gimbal attachment the rod (c) is able to rotate around an oblique axis. A pulley-
wheel can also be attached instead of the rod (c), and by means of a cord a glass
plate may be rotated under water *. The apparatus is strong enough to rotate several
FlG. 36. Pfeffer's klinostat : A^ showing mode of use with a potted plant ; /?, with a cylinder
containing seedlings.
pots at the same time if they are properly attached 2, and provided that the system is
equilibrated by means of the adjustable weight (e) so that the same amount of work
is performed at each phase of rotation. Finally, the time of a rotation may be varied
from two minutes to as long as eight hours.
Fitting has recently constructed a special attachment which enables the plant to
be turned through an angle of 180° at a given time, or through a lesser angle. In
this way the side turned towards the light or to the ground may be suddenly placed
in the opposite position and the reversal repeated at regular intervals of time 3.
The various klinostats constructed by different authors do not appear to surpass
1 Cf. Richter, Flora, 1894, p. 409; Klemm, Flora, 1893, p. 476. For transmission a thick
circular rubber tube is best.
3 See A. Fischer, Bot. Ztg., 1890, p. 705.
8 A simpler intermittent klinostat was used by F. Darwin (Annals of Botany, 1892, Vol. VI,
p. 245). An intermittent electro-magnetic arrangement is easily made, and full details as to the
mode of use in such cases are given by Pfeffer in the Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 738.
I7o TROPIC MOVEMENTS
the above instrument in accuracy and convenience. A simple form suitable for
demonstration can easily be made from an ordinary American clock, either by fixing
a cork plate to the finger-axis so as to get horizontal rotation, or by attaching a glass
rod to it by means of stout rubber tubing, and so obtaining a horizontally-rotating
rod on which seedlings may be fixed with the aid of cork rings. If the clock is
fixed in a heavy frame so that it may be canted at various angles, the direction of
rotation may be given varying degrees of obliquity1.
Any rapidly rotating wheel may be used to demonstrate the action of centrifugal
force 2, and slight centrifugal actions may be obtained by means of a klinostat 3. In
this way the intensity of stimulation required to produce a geotropic curvature can be
determined, as well as the relationship between the intensity of stimulation and the
response. It must, however, be remembered that under the action of high centrifugal
forces purely physical mass-actions come in and cause the attempted curvatures to be
more or less overcome and replaced by mechanical bending.
SECTION 36. Heliotropism.
Under this heading we may conveniently include all orienting move-
ments produced by unilateral illumination ; but since variations in the
intensity of diffuse daylight may produce photonastic movements, or
may cause the tone of the organ to alter, it is not always easy to say
whether a particular curvature is heliotropic in character, or results from
a dissimilar form of stimulatory response, or is due to a combination of
factors. It must also be remembered that the position of heliotropic
equilibrium may vary according to the intensity of the light, and may in
some cases alter to such an extent that the direction of the curvature is
reversed.
A reversal of this kind is especially well shown by those swarm-spores
which react positively phototactically in weak light, but negatively photo-
1 Cf. F. Darwin, Linnean Soc., 1881, p. 449; Wortmann, Ber. d. hot. Ges., 1886, p. 245;
Klemm, Flora, 1893, p. 472 ; Hansen, Flora, 1897, Erg.-Bd., p. 352 ; W. Oels, Pflanzenphysiol.
Unters., 1893, p. 50. The mechanism used for rotating tables in shop-windows is easily made into
a klinostat by changing the escapement so that the rotation is slower. Where a room at constant
temperature is not available, the arrangement employed by Czapek (Ber. d. bot. Ges., 1900, p. 131)
may be used to avoid effects due to changes of temperature.
2 An apparatus driven by a water-motor, and which enables centrifugal forces up to 40 g. to be
produced is described in Unters. a. d. bot. Inst. zu Tiibingen, 1881, Bd. I, p. 57. At Leipzig the
apparatus used was driven by a one-horse-power gas-motor, and varying velocities obtained by the
use of axes of different sizes, and of conical axes. Cf. Jahrb. f. wiss. Bot., 1895, Bd. xxvir, p. 304.
The centrifugal force is determined by the formula - — — , where — - = a constant (4-024) ;
O O
r = radius in cms. ; / = time of a rotation in seconds. On a large wheel the centring need not be
so accurately performed as on a small one. On simpler forms of apparatus cf. Oels, 1. c., p. 51 ;
Detmer, Pflanzenphysiol. Practicum, 1895, 2. Aufl., p. 384; Hansen, Flora, 1893, Erg.-Bd., p. 352.
Mottier, Annals of Botany, 1899, Vol. XIII, p. 326. Pfeffer has more recently used a specially
constructed milk centrifuge to obtain centrifugal forces up to 4,000 g.
3 Cf. Czapek, 1. c., p. 305.
HELIOTROPISM 171
tactically when the light passes beyond a certain intensity. A similar
change is also shown by the radial organs of various plants, for the
filaments of Vaucheria and Phycomyces^ the seedling stems of Lepidium and
other plants grow towards the light when of moderate intensity, but as the
latter increases pass through positively plagiotropic, diatropic, and negatively
plagiotropic positions, finally assuming a negatively parallelotropic position
of equilibrium1. Changes of this kind appear in certain plants and in
swarm-spores when the light is of moderate intensity, but in other plants
only when the intensity is considerable, and they may not be shown if the
light has to be so concentrated that the plant is rapidly injured. This
applies to many plants, and in others the deviation from the positively
heliotropic position of equilibrium is only produced by light of an intensity
which is not reached under natural conditions. The tendrils of Vitis and
AmpelopsiS) on the other hand, react positively heliotropically, according to
Wiesner 2, only when the light is feeble, and negatively heliotropically even
when only moderately strongly illuminated on one side. It does not follow,
however, that every negatively heliotropic organ will show positive helio-
tropism when the light is weak enough. Nor is it surprising that the state-
ments as to phototropic reactions and the phototropic positions of equilibrium
should not always agree, for the tropic condition of tone varies according
to the stage of development and the other external conditions 3.
After it had been shown by N. J. C. Miiller 4 in the case of seedlings of Lepidium,
by Stahl 5 in that of Vaucheria, and by Berthold 6 in certain marine algae that the
positive heliotropic position was changed to a plagiotropic or negatively phototropic
one under strong illumination, Oltmanns carried out more extended researches on this
phenomenon, using at first sunlight 7 and later employing a strong arc- light as a source
of illumination 8. These experiments showed that the sporangiophore of Phycomyces
1 The positive movement is that towards the source of illumination, the negative the one away
from it. Oltmanns (Flora, 1897, p. 7) regards the transversal position as an indifferent one, but
there can be no doubt that it is as much the result of a stimulatory reaction as any other.
2 Wiesner, Die heliotropischen Erscheinungen, 1880, II, p. 38.
3 According to Oltmanns (1. c.), the young sporangiophores of Phycomyces react positively
heliotropically to light of an intensity that causes the old sporangiophores to assume a transverse
or negatively heliotropic position. In other words, the old sporangiophores are adapted to light of
feeble intensity.
* N. J. C. Miiller, Bot. Unters., 1872, Bd. I, p. 57.
5 Stahl, Bot. Ztg., 1880, p. 412; Bot. Centralbl., 1882, Bd. xn, p. 142. Cf. Oltmanns, Flora,
1892, p. 214.
6 Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xin, pp. 574 ff.
7 Oltmanns, Flora, 1892, p. 214. On the gradation of the intensity of the light by the inter-
position of cells containing diluted indian ink cf. Oltmanns, I.e., p. 183, and Jahrb. f. wiss. Bot.,
1892, Bd. xxin, p. 416.
* Oltmanns, Flora, 1897, p. i. For details of the methods cf. Oltmanns, 1. c. On the removal
of the heat-rays see also Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 711. On the use of lamps
and gas-flames cf. Wiesner, 1. c., 1878, I, p. 35. A heliotropic curvature is readily produced by
covering the plant with a black cover having a slit or hole on one side. Cf. Sachs, Flora, 1895,
p. 293-
172
TROPIC MOVEMENTS
nitens assumed a diaheliotropic position when exposed to light equivalent to 25,000
Hefner lamps, whereas an intensity of 500,000 to 600,000 Hefner lamps was required
to produce the same effect on seedlings of Lepidium sativum and of Hordeum 1.
Observations under natural conditions show that the radial organs of many plants
assume a positively parallelotropic position when feebly illuminated from one side, but
in direct sunlight assume a more or less plagio-phototropic position 2. The position
of many dorsiventral organs alters according to the intensity of the illumination, and
although the exact mode in which this altered reaction is produced is uncertain, there
can be no doubt that the light-position of leaves, of the prothallia of Ferns, of the
thallus of Marchantia, and of the plagiotropic shoots of the Ivy, are mainly the result
of a heliotropic reaction. Furthermore, the movement of the chlorophyll-plate of
Mesocarpus from the transverse to the profile position is produced as a direct response
to the stimulus of light.
The positively heliotropic reaction of most seedling-stems, and of
subaerial stems in general under normal conditions of illumination, is
obviously a purposeful biological adapta-
tion 3. For in this way the leaves are
brought into brighter light and, when
endowed with a photometric power of
reaction, set their surfaces at right angles
to the direction in which the strongest
diffuse light falls upon them. Positive
heliotropism is also shown by the seed-
ling-stems of twiners, whereas the older
twining stem, in accordance with its habit,
shows only a feeble negative or positive
phototropic reaction. Most tendrils are
also comparatively indifferent, although
a few are aided in approaching and
.applying themselves to a support by
their negative heliotropism.
There is also evidence of biological
adaption in the fact that attaching aerial
roots such as those of Aroids, Orchids, and Hartwegia are usually
FlG. 37. Seedling of Sinapis alba.
The hypocotyl shows a positive, the
root in water a negative heliotropic
curvature. The arrows show the direc-
tion of the incident rays of light.
1 The brightness of a Hefner- Altenach light corresponds to 1-162 German standard candles.
The spermaceti candle used by Wiesner (1. c.) is equivalent to a Hefner- Altenach lamp. Oltmanns,
1897, I.e., pp. 2, 20. The cessation of growth and of heliotropic curvature observed by Wiesner
with much feebler intensities of light is apparently the result of some accessory action of the gas-
flames employed. It must also be remembered that the greatest heliotropic action is exercised by
the more refrangible rays, so that the action of the light is not always proportional to its apparent
brightness. Cf. also Wiesner, Bot. Centralbl., 1897, Bd. LXIX, p. 305.
3 Cf. Oltmanns, Flora, 1892, p. 225.
8 A few facts concerning stems and other organs, as well as references to the literature, are given
by Wiesner, Die heliotropischen Erscheinungen im Pflanzenreich, I, 1878; II, 1880 (reprinted from
Denkschriften d. Wien. Akad., Bd. xxxix).
HELIOTROPISM 173
endowed with a negative or transversal heliotropism, whereas the longer
nutritive roots which descend into the soil show a lessened power of
heliotropic reaction1. Furthermore, the penetration of the root of Viscum
into a host plant is brought about by the negatively heliotropic curvature
of the hypocotyl 2, whereas the adult stem of Viscum^ in accordance with
its special habit, shows neither geotropic nor heliotropic irritability. In
much the same way the heliotropic irritability decreases as we pass
outwards from the main trunks of many trees and shrubs to the successive
lateral branches. If the heliotropic irritability of a branch increases when
its neighbours are removed, it is evident that the dormant irritability
was suppressed or partially inhibited by the correlative and autotropic
stimuli radiating from the surrounding organs. Very many subaerial
runners are almost devoid of heliotropic irritability, changes in their
direction of growth produced by alterations in the intensity of the illumina-
tion being due to the fact that their geotropic irritability is modified by
the action of light.
Roots which grow normally in the soil are either without any
heliotropic irritability or show feeble negative heliotropism, as in the cases
of Sinapis alba, Lepidium sativum, and Helianthus annuus. The roots of
Allium sativum and Hyacinthus orientalis are, however, feebly positively
heliotropic 3.
Numerous instances of heliotropism in non-chlorophyllous organs are
afforded by fungi. Thus the stalks of the fructifications of Coprinus
stercorarius*, of C. niveus5, and of Peziza fuckeliana6, the young stipes
of Lentinus lepideus, the perithecia of Sordaria fimiseda 7, and the stalks of
the perithecium-heads of Claviceps microcephala 8 are positively heliotropic.
The same applies to the sporangiophores of Phycomyces nitens, Mucor
mucedo, Pilobolus crystallinus, and various other Mucorineae 9, whereas the
1 Dutrochet, Ann. sci. nat., 1833, Bd. xxix, p. 413; Wiesner, I.e., 1880, u, p. 76; H. Muller,
Flora, 1876, p. 93 ; Schimper, Bot. Centralbl., 1884, Bd. xvn, p. 274; Die epiphytische Vegetation
Amerikas, 1888, p. 53 ; WTent, Ann. d. Jard. bot. de Buitenzorg, 1894, Vol. XII, p. 24; Massart, Sur
1'irritabilite d. plantes superieures, 1902, p. 60 (fuus).
2 Dutrochet, Rech. s. la structure intime, &c., 1824, p. 1 15 ; Wiesner, Sitzungsb. d. Wiener Akad.,
1894, Bd. cm, Abth. i, p. 436. Keeble, Trans, of the Linnean Soc., 1896, p. 112 (Loranthus],
3 For the literature and numerous observations see Wiesner, Die heliotropischen Erscheinungen,
1880, II, p. 79; also F. G. Kohl, Mechanik der Reizkriimmungen, 1894, p. 26.
4 Brefeld, Unters. iiber Schimmelpilze, 1877, Heft 3, p. 96.
8 Hofmeister, Pflanzenzelle, 1867, p. 289; Wiesner, I.e., 1880, II, p. 89.
6 Winter, Bot. Ztg., 1874, p. i.
7 De Bary and Woronin, Beitrage z. Morphol. u. Physiol. d. Pilze, 1870, 3. Reihe, p. 10.
8 G. Kraus, Bot. Ztg., 1876, p. 505 ; Duchartre, Compt. rend., 1870, T. LXX, p. 779.
9 Hofmeister, Pflanzenzelle, 1867, p. 289; Vines, Arb. d. bot. Inst. in Wurzburg, 1878, Bd. u,
p. 133; WTiesner, I.e., II, p. 85; K. Steyer, Reizkriimmungen bei Phycomyces nitens, 1901. Since
Pilobolus curves towards the light during development, its sporangia will be thrown in this direction,
and can be collected on a glass plate. Noll, Flora, 1893, p. 32. See also Sorokin, Bot. Jahresb.,
1874, p. 214; Fischer v. Waldheim, ibid., 1875, p. 779; Brefeld, Bot. Unters. iiber Schimmelpilze,
I74 TROPIC MOVEMENTS
mycelium here and in other fungi appears to possess but little heliotropic
irritability. The rhizoids of Marchantia l, of the prothallia of Ferns 2, and
of Equisetum 3 afford, however, instances of unicellular organs which show
a negatively heliotropic reaction even to weak illumination, while a similar
reaction is shown by Vaucheria and by the sporangiophores of Phycomyces
when the light is intense. The non-cellular fronds of Caulerpa and Bryopsis *,
as well as the internodes of Char a and Nitella 5, react in the same way as
Vaucheria, and show positive heliotropism in ordinary light. Algae in
general, which are not adapted to high intensities of illumination, show
orienting heliotropic movements of this character 6.
Without doubt the heliotropic irritability is more or less dependent
upon the stage of development and upon the general external conditions.
The peduncle of Linaria cymbalaria is, for instance, positively heliotropic
when the flower opens, but later becomes negatively heliotropic, and hence
curves so as to press the ripe capsule against the wall, or into a crevice of
the rock or wall on which the plant may be growing7. In addition, the
young internodes of Tropaeolum mafus8 and of other plants are either
positively heliotropic or indifferent, whereas the older internodes assume
a positive or negative klinotropic position. We may still term a reaction
heliotropic when the change of position is due to the induction of dorsi-
ventrality, or to a related modification produced by the unilateral illumina-
tion. The change to the klinotropic position of the older internodes of the
Ivy indicates, therefore, an alteration of heliotropic irritability ; but, since it
may also be produced by changes in other properties, direct experiment
is necessary to determine the exact causation of an altered power of
response. It is only in a few cases, however, that these requirements have
been properly fulfilled.
SECTION 37. The Heliotropic Action of Rays of Different Wave-length.
The more refrangible rays are not only more effective in inducing
heliotropic curvature, but also influence growth, formative activity, and
Hefte 3, 6, 7; Zopf, Pilze, 1890, p. 204; Elfving, Einwirkung d. Lichtes auf Pilze, 1890, p. 19;
Eidam, Cohn's Beitrage zur Biologic, 1886, Bd. IV, p. aia; Klebs, Jahrb. f. wiss. Bot., 1898,
Bd. xxxii, p. 55 (Sporodinia) ; Neger, Flora, 1902, p. 228 (Erysiphe).
Pfeffer, Arb. d. bot. Inst. in Wiirzburg, 1871, Bd. I, p. 88.
Leitgeb, Studien iiber d. Entwickelung d. Fame, 1879, p. 7 (reprint from Sitzungsb. d. Wien.
Ak»(
., Bd. LXXX, Abth. i) ; Prantl, Flora, 1879, P- 679-
Stahl, Ber. d. bot. Ges., 1885, p. 338 ; Buchtien, Bibliotheca botanica, 1887, Heft 8, p. 28.
Klemm, Flora, 1893, p. 472 ; Noll, Arb. d. bot. Inst. in Wiirzburg, 1888, p. 467.
Hofmeister, Pflanzenzelle, 1867, p. 289 ; J. Richter, Flora, 1894, p. 400.
Cf. Oltmanns, 1. c., and Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xn, pp. 573, 581 ; E. Winkler,
Kriimmungsbewegungen von Spirogyra, 1902, p. 20.
7 Hofmeister, 1. c., p. 292. According to Wiesner (Die heliotropischen Erscheinungen, n,
p. 72) the peduncles of Helianthemum vulgare behave similarly at flowering and fruiting.
8 Sachs, Experimentalphysiol., 1865, p. 41 ; Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. n> P- 27r>
HELIOTROPIC ACTION OF RA YS OF DIFFERENT WA VE-LENGTH 175
movement in general more than the less refrangible rays. Hence a
heliotropic curvature is performed almost as rapidly beneath an ammoniacal
solution of copper hydrate, which allows mainly the blue and violet rays to
pass through, as in ordinary light, whereas beneath a solution of potassium
bichromate, which only allows the less refrangible rays to pass, little or no
heliotropic action can usually be excited.
This applies to green and non-green cells, to cellular and non-cellular
or unicellular plants, and to positively and negatively heliotropic organs 1.
The relative efficiency of the different rays is not, however, the same in all
plants, and, according to G. Kraus 2, the positively heliotropic stalks of the
perithecial heads of Claviceps microcephala react almost as rapidly under
a solution of potassium bichromate as under one of cupr.-ammonia.
According to Brefeld3, Pilobolus microsporus behaves similarly, and the
mixture of yellow and red light exercises nearly as strong a heliotropic
action upon the sporangiophore of Pilobolus crystallinus 4 as that from the
more refrangible half of the spectrum. Specific differences of this kind are
known to exist in other forms of growth and movement, and the curves
showing the action of the different rays of the spectrum upon these forms
of vital activity need not necessarily coincide with the curve showing their
relative heliotropic action. Pilobolus and Coprinus stercorarius behave as
regards etiolation and formative activity similarly in yellowish-red light
and in darkness, whereas the yellowish-red rays are able to excite a strong
heliotropic response in them. On the other hand, both the more and
less refrangible halves of the spectrum exercise approximately the same
action upon the stalks of the perithecium-heads of Claviceps microcephala in
regard both to etiolation and heliotropism.
In most cases, according to Wiesner5, the maximum point on the
curve showing the heliotropic action of different rays is reached between
the violet and ultra-violet rays. The curve, as measured by the rapidity
of the heliotropic response, falls gradually towards the green, sinks to
nothing in the yellow 6, recommences in the orange, and rises to a small
1 On negatively heliotropic organs cf. Wolkoff, communicated by Hofmeister, Pflanzenzelle,
1867, P- 299 (aerial roots) ; Sachs, Lehrbuch, 4. Aufl., p. 810 (Ivy) ; Kraus, Bot. Ztg., 1876, p. 505
(aerial roots) ; Prantl, Bot. Ztg., 1879, p. 699 (rhizoids of fern prothalli) ; Wiesner, Die helio-
tropischen Erscheinungen im Pflanzenreich, 1878, I, p. 53. Sorokin's statement (Bot. Jahresb.,
1874, p. 214) that Mucor mucedo and a few other fungi are positively heliotropic in blue light
(cupr.-ammonia) and negatively heliotropic in yellow light (potassium bichromate) is incorrect
according to other observers. Cf. Wiesner, 1. c., II, p. 88'.
2 G. Kraus, Bot. Ztg., 1876, p. 505.
3 Brefeld, Unters. iiber Schimmelpilze, 1881, Heft 4, p. 77 ; Grantz, Ueber d. Einfluss d. Lichtes
auf d. Entwickelung einiger Pilze, 1898, p. 1 8.
4 Wiesner, 1. c., II, p. 88.
6 Wiesner, 1. c., I, p. 50.
6 It is worthy of note that the yellow rays exercise a certain influence on growth. Cf. Wiesner,
I.e., II, p. ii.
176 TROPIC MOVEMENTS
secondary maximum in the ultra-red J. It can, therefore, readily be under-
stood why, when the light is feeble, a perceptible reaction may only be
produced by the more refrangible rays. It is, however, possible that in
some cases only these rays are able to excite a heliotropic response.
Many researches have been performed by various authors on the heliotropic
action of different rays2. Guillemin worked with especial care and showed that,
owing to the varying absorption and dispersion of the different rays, the position
of the heliotropic maximum varied according to whether prisms of quartz, rock-salt,
or flint-glass were used. For these reasons it is easy to understand why the curve
obtained by Guillemin, Wiesner, and other authors do not always precisely agree.
The fact that Sachs could detect no heliotropic action under a solution of potassium
bichromate was probably the result of feeble intensity of the light used, or of the
special properties of the experimental material. Wiesner s found that the heliotropic
action of the red and orange rays was weakened by the admixture of yellow rays.
Gardner, Guillemin, and Wiesner all observed that the plants did not always set
themselves precisely parallel to the incident rays, but curved somewhat towards the
more active regions of the spectrum, a result only to be expected.
Polarized light acts, according to Guillemin and Askenasy, in the same way as
ordinary light 4. The non-luminous ultra-violet rays exercise a strong and the ultra-
red rays a feeble phototropic action. Rontgen rays appear to exert mainly injurious
actions6, for Schober was unable to detect any tropic action of these rays on
seedlings, although Joseph and Prowazek found that Paramoecium and Daphnia
showed a negatively tactic reaction. The Becquerel and radium rays exercise a
certain injurious action, but have no tropic influence, as far as is known 6.
SECTION 38. Thermotropism.
In addition to the action of the ultra-red rays which are associated
with the visible part of the spectrum, dark heat rays of still greater wave-
length as well as differences of temperature may produce a thermotropic
curvature in certain cases. As far as our present knowledge goes, however,
1 According to Wiesner (Die heliotropischen Erscheinungen, 1878, I, p. ^6\ the ultra-red rays
which pass through a solution of iodine in carbon bisulphide also act in this way.
" Poggioli (1817) ; Zantedeschi, Bot. Ztg., 1843, p. 620; Payer, Ann. d. sci. nat., 1844, 3° ser.,
T. II, p. 99; Dutrochet, ibid., 1843, 2e ser., T. XX, p. 329; Gardner, London, Edinburgh, and
Dublin Phil. Mag., 1844, Vol. xxiv, p. 7 ; Guillemin, Ann. d. sci. nat., 1857, 4" ser., T. vii, p. 154;
Sachs, Bot. Ztg., 1864, p. 361 ; N. J. C. Miiller, Bot. Unters., 1872, Bd. I, p. 57; G. Kraus, 1876,
I.e. ; Wiesner, Die heliotropischen Erscheinungen, 1878, I, p. 44; 1880, u, pp. 10, 87, 89. Wiesner
gives a full account of the literature and also of the methods.
3 L.c., II, p. 50.
4 Guillemin, 1. c., p. 172 ; Askenasy, Bot. Ztg., 1874, p. 237.
5 Cf. Seckt, Ber. d. bot. Ges., 1902, p. 87 ; Joseph and Prowazek, Zeitschrift f. allgem.
Physiologic, 1902, Bd. I, p. 143.
6 A summary of all that is known in regard to the physiological action of these rays is given by
K. Hoffmann, Die radioaktiven Stoffe, 1903, p. 21. See also Bohn, Compt. rend., 1903, T. cxxxvi,
p. 1012.
THERMOTROPISM 177
a pronounced thermotropic irritability is present only in a few plants, and it
is a natural result of the conditions of life of an ordinary plant that it should
make use of thermotropic reactions only in a minor degree for purposes of
orientation.
Wortmann1 observed that seedlings of Lepidimn sativum and Zea
Mays, as well as the sporangiophores of Phycomyces^ curved towards a hot
iron plate emitting dark heat-rays. Steyer 2 has, however, shown that the
sporangiophore of Phy corny ces has no power of thermotropic reaction, so
that the curvatures observed by Wortmann may have been due to accessory
causes or were possibly heliotropic in character. Wortmann observed that
the seedling-shoot of Zea Mays was positively but that of Lepidimn
negatively thermotropic, although the latter possesses a stronger heliotropic
irritability than the former. Steyer, however, found that both plants were
positively thermotropic.
Wortmann 3 has also investigated the radicles of seedlings by growing
them in boxes of sawdust, one side being kept hot, the other cold. The
roots of Ervum lens were found to be diathermotropic at 27° C., and
similarly those of Pismn sativum did not curve out of a vertical position
when at 33° to 33° C. On being placed nearer the hot side, however, the
roots curved away from it, but when near the cold side showed a positively
thermotropic curvature. According to Klercker4, however, some roots
only show a negatively thermotropic reaction, whereas a strong positive
thermotropism is shown, according to Vochting5, by the peduncle of
Anemone stellata.
The smallness of the difference in the temperature of the opposite
sides, as well as the fact that either a positive or negative curvature may
be produced, suffice to show that they are not due to the more rapid
growth of the side exposed to heat. According to Wortmann, decapitated
roots show the same reaction, and, since hydrotropic stimuli are only
perceived by the root-tip, the curvatures can hardly be due to variations
in the amount of moisture on the hot and cold sides. In moist sawdust
there can hardly be any appreciable difference in the rate of transpiration
from the two sides, whereas when an object is exposed on one side to
radiant heat-rays in ordinary air, the resulting differences in the rate of
transpiration might be responsible for the tropic stimulation. Apart from
this effect, it is not known whether radiated and conducted heat exercise
a similar thermotropic action. Hence there is no need at present to adopt
1 Wortmann, Bot. Ztg., 1883, p. 457.
3 Steyer, Reizkriimmungen bei Phycomyces nitens, 1901, pp. 10, 20.
3 Wortmann, Bot. Ztg., 1885, p. 193.
4 Klercker, Die caloritropischen Erscheinungen bei einigen Keimwurzeln, 1891. (Reprint from
Qfversigt af K. Vetenskaps-Akademiens Forhandlingar, Nr. 10.)
5 Vochting, Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 269.
PFEFFER. Ill TV
178 TROPIC MOVEMENTS
Klercker's l term of ' caloritropism ' to indicate curvatures produced by
conducted heat.
SECTION 39. Chemotropism and Osmotropism.
Chemical stimuli not only play an important part in the general vital
activity, but are often specially employed to produce tropic orienting
movements. This power of reaction has, however, been more especially
studied in connexion with freely motile organisms, and less is known in
regard to the production of. chemotropic curvatures. Among these are
included all movements produced by a substance in virtue of its chemical
constitution and varying distribution. When the movement takes place
towards the source of the diffusing substance, or where it is more abundant,
we may speak of positive chemotropism, and of negative when the curvature
is in the opposite direction. Transverse chemotropism might be due to
the absence of any power of response, but could only be the result of
a definite chemotropic orienting stimulus when it was attempted in spite
of the action of other directive agencies. Reversal is possible as in the
case of heliotropism, for although a negative reaction may be produced by
some substances when in extreme dilution, frequently a positive reaction
becomes negative or transversal beyond a certain concentration.
An increase of concentration also involves an enhanced osmotic action,
and when this acts as a tropic stimulus we have an osmotropic reaction
before us2. A special osmotropic irritability is often shown, although
comparatively high concentrations are required to excite it, and the response
hitherto observed has always been negative. It is, however, not impossible
that positive osmotropism may be detected in some cases 3.
Since osmotropic stimulation does not depend upon chemical quality
but upon osmotic action, all substances exercise the same osmotic stimulus
when in equivalent concentrations, so long as the power of perception or of
reaction remains unaffected 4. On the other hand, chemotropic stimulation
is primarily dependent upon the chemical nature of the stimulating
substance, and hence isosmotic solutions of different materials exercise
widely dissimilar chemotropic actions. Furthermore, the chemotropic
sense, like the sense of smell and taste in animals, is developed to widely
dissimilar degrees in different plants. Hence a substance may be strongly
chemotropic for one organism but not for another, and while a power of
1 Die caloritropischen Erscheinungen bei einigen Keimwurzeln, 1891, p. 767.
2 Rothert (Flora, 1901, p. 408, footnote) suggests the terms 'osmotropism' and ' osmotaxis/
which are preferable to Massart's * tonotaxis.' Since it is not merely a question of the attraction by
food, and since all food-substances are not chemotropically active, the term * trophotropism *
suggested by Stahl (Bot. Ztg., 1884, P- ^5) is highly unsuitable.
3 Cf. Rothert, 1. c., p. 403, footnote.
* Id., p. 41 3.
CHEMOTROPISM AND OSMOTROPISM 179
responding to oxygen is in many cases associated with a power of
responding to peptone and other substances, it need not always be so.
Furthermore, a particular organism may respond to one or a few substances,
whereas another may be chemotropically stimulated by a large number
of substances, though not all to the same extent. At the same time bodies
of similar constitution may exert widely dissimilar physiological actions,
while dissimilar substances may be comparatively alike from a chemotropic
point of view. Whenever the chemotropic action depends upon acid or
alkaline action it is only natural to expect that the influence of equi-
molecular solutions of neutral salts will partly depend upon the degree
of dissociation, as in the case of poisons. The dissociated ions as well as
the undissociated molecules may, quite apart from any acid or alkaline
character, exercise independent chemotropic actions *.
Either or both of these forms of irritability may be developed in the
same organism, and in the latter case the two stimuli may act conjointly
when a chemotropic substance is applied in considerable concentration, or
when a dilute chemotropic solution has a large quantity of an indifferent
soluble substance added to it. Since the stimulating chemotropic action
is not directly proportional to the concentration, and since conjoint stimuli
may induce changes of tone, it is not always possible to say whether the
conversion of a positive into a negative response by increasing concentration
is of chemotropic or osmotropic origin. That the change is a chemotropic
one is, however, obvious in the case of organisms which have no osmotropic
irritability, and the same applies when the tropic reversal is shown in
a concentration at which isosmotic solutions of non-chemotropic salts
exert no osmotropic repulsion. When a chemotropic action is only shown
with high concentrations it is always accompanied by an osmotropic
excitation if the organ possesses this latter form of irritability. In this
way it arises that isosmotic solutions of different substances exert more
or less dissimilar stimulating effect.
These forms of irritability are especially important in freely motile
organisms, and often serve to lead them to nutriment or to suitable
habitats, or aid them in avoiding injurious or unfavourable media. Of
equal advantage are the chemotropic and osmotropic curvatures performed
by the hyphae of mould and other fungi. Chemotropic stimuli also aid
in directing the pollen-tube to the ovule and in bringing the antheridial
1 [Massart (Biol. Centralbl., 1902, Bd. xxn, p. 22) proposes the terms 'alcalio-' and 'oxy-
tropism ' for the chemotropism induced by alkalies and acids, while for the attraction exercised by
oxygen the term of ' oxygen otropism ' is suggested by Herbst, Biol. Centralbl., 1894, p. 694, and of
c aerotropism ' by Molisch (Sitzungsb. d. Wiener Akad., 1884, Bd. xc, I, p. in). As a holiday
amusement the invention of special terms for detailed phenomena has its advantages, but for serious
scientific studies the unnecessary duplication of terms is strongly to be deprecated.]
N a
i8o TROPIC MOVEMENTS
hypha of Saprolegnia1 into contact with the oogonium. They probably
determine the direction of growth of the fertilizing filaments of Dudresnaya 2,
and aid in bringing about the formation and union of the conjugation tubes
of Conjugatae 3. They may also play a more or less important part in
determining the union of fungal hyphae to form pseudo-parenchyma or
sclerotic tissue, and also in producing and maintaining certain symbiotic
associations.
It is hardly surprising that subaerial organs, such as stems and leaves,
should appear usually to be devoid of any chemotropic or osmotropic
irritability, for the latter could hardly be of any appreciable use for
purposes of orientation in such organs. Roots, however, appear also to
have developed these forms of irritability only to a limited extent, for
hitherto only a certain aerotropism, or rather oxytropism, as well as a
power of curving away from injurious gases, has been observed in them,
while they are apparently not subject to chemotropic stimulation by
nutrient solutions, or to osmotropic repulsion by concentrated saline
solutions.
After Engelmann * had discovered that oxygen exerted a chemotactic action on
certain bacteria, Pfeffer8 studied the phenomenon and showed that a chemotactic
irritability was possessed by a variety of freely motile organisms. Stahl' then
showed the existence of a chemotropic irritability in the plasmodia of Myxomycetes,
while Massart7 established the fact that the repulsion exerted by concentrated solutions
independently of their chemical nature was the result of an osmotactic reaction.
A variety of researches then followed on the chemotaxis of freely motile animals and
plants. Molisch ascribed the curving of the pollen-tube to the stigma to a chemo-
tropic reaction8 and previously examined the aerotropic curvatures of roots9.
Miyoshi 10 then fully investigated the chemotropic curvatures of fungal hyphae and
of pollen-tubes.
Miyoshi sowed the spores of fungi or pollen-grains on the under-surfaces of
leaves which had been injected with water or with nutrient solutions, and then found
1 De Bary, Beitrage z. Morphol. u. Physiol. d. Pilze, 1881, 4. Reihe, pp. 85, 90. Cf. Pfeffer,
Unters. a. d. hot. List, zu Tubingen, 1884, Bd. I, p. 469; Miyoshi, Bot. Ztg., 1894, p. i.
2 Berthold, Protoplasmamechanik, 1886, p. 282.
3 Overton, Ber. d. bot. Ges., 1888, p. 68; Haberlandt, Sitzungsb. d. Wiener Akad., 1890,
Bd. XLIX, Abth. i, p. 390.
4 Engelmann, Bot. Ztg., 1881, p. 440; Pfliiger's Archiv f. Physiologic, 1881, Bd. xxv, p. 285;
1881, Bd. xxvi, p. 541.
5 Pfeffer, Ber. d. bot. Ges., 1883, p. 524; Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I,
p. 363 ; 1888, Bd. n, p. 582. In the second work (1884, Bd. I, pp. 365, 469) the facts are mentioned
which suggested the existence of a power of chemotropic curvature.
6 Stahl, Bot. Ztg., 1 884, p. 155.
7 Massart, Archiv d. Biologic, 1889, Bd. ix, p. 515.
8 Molisch, Sitzungsb. d. Wiener Akad., 1884, Abth. i, p. in.
9 Molisch, 1893, Bd. en, Abth. i, p. 423; a preliminary communication in Sitzungsanzeiger
d. Wiener Akad., January 17, 1889.
10 Miyoshi, Bot. Ztg., 1894, p. i ; Flora, 1894, p. 76.
CHEMOTROPISM AND OSMOTROPISM
181
that the germ-tubes were drawn in at the stomata when positively chemotactic
substances were present, but passed over the stomata along the surface of the
epidermis when they were absent. The same result is obtained when the clean
epidermis of an onion scale, or a thin plate of mica is bored with fine holes and laid
on a mass of gelatine containing the substance to be tested. If gelatine containing
a chemotropic substance is placed in a capillary tube which is brought near to
a filament growing in water, the filament, if irritable, will show a chemotropic
divergence towards the open end of the tube.
Miyoshi found that phosphates and ammonium-salts, and hence also meat-
extract, exert a strong attraction upon Penicillium glaucum, Aspergillus ntger, Mucor
mucedo, and Saprolegm'aferox, which is already perceptible in solutions of o-oi per cent,
strength. Cane-sugar, grape-sugar, and dextrin are less effective, especially in the
case of Saprolegnia, while such nutritive substances as glycerine and quinic acid exert
little or no chemotropic action.
In the case of pollen-tubes, how-
ever, Miyoshi found that cane-
sugar, grape-sugar, and dextrin
exerted an especially strong
chemotropic attraction, whereas
phosphate of ammonium, pep-
tone, and meat-extract excited
no positive chemotropism. It
is possible that this is not always
the case under all circumstances,
for Lidforss found that proteids
and diastase both produced
strong attraction \
The above-named sub-
stances act in general as
stimuli to bacteria, which are
also attracted by potassium
nitrate and sodium chloride,
although these salts exercise no chemotropic action on fungal hyphae or pollen-
tubes. In all cases, however, hydrochloric and other acids exercise a repellent
action even in considerable dilution, and the same action is exercised by all
substances when sufficiently concentrated.
The penetration of the hyphae of fungi through the cell-walls of a host-plant is
in part the result of chemotropic stimulation, but the whole problem of the relations
and interactions of parasites and their host is one of extreme intricacy 2. This also
applies to the conduction of pollen-tubes to the ovules, which, according to Miyoshi 3,
FIG. 38. A portion of the epidermis from the under side of
the leaf of 7*radescantia discolor which had been injected with
a solution of cane-sugar. The germ-tubes from the spores of
Penicillium glaucum are seen growing towards and partly
into the stomata.
1 Lidforss, Ber. d. bot. Ges., 1899, p. 236.
3 Cf also Nordhausen, Jahrb. f. wiss. Bot., 1898, Bd. xxxili, p. i ; Behrens, Centralbl. f. Bad.,
2. Abth., 1898, Bd. iv, p. 514. On the penetration of cell-walls by bacteria cf. Buller, Die Wirknng
von Bacterien auf todte Zellen, Leipzig. Dissert,, 1899.
3 Miyoshi, Flora, 1894, p. 76, and the literature there given. On the path of the pollen-tcbe
i82 TROPIC MOVEMENTS
is brought about in the following way. The first penetration of the stigma by the
pollen-tube is induced by chemotropic stimulation aided by the hydrotropism of
the pollen-tube, and possibly also by aerotropic and other stimuli. The growth of the
tube down the conducting tissue appears to take place independently of any chemo-
tropic action. The actual entry at the micropyle appears to be brought about by the
exudation of a stimulating material from the ovule, for the pollen-tubes penetrate
the micropyles of isolated ovules injected with sugar, but not when injected with
non-chemotropic solutions, or when the ovules and pollen-tubes are placed in a solu-
tion of sugar so that the action of the sugar exuding from the micropyle is masked.
Aerotropism. According to Celakovsky l, the hyphae of Dictyuchus monosporus
curve towards water richer in oxygen, but pollen-tubes towards water poorer in
oxygen, according to Molisch 2. Roots, on the other hand, were found by Molisch 3
to be positively oxytropic and to curve from air deficient in oxygen to air where it
was more abundant. According to the same author, the one-sided accumulation of
carbon dioxide, as well as the unilateral action of ether and camphor vapours, produces
a negatively tropic curvature both in normal and in decapitated roots 4. The reactions
are, however, feeble, and it has yet to be shown that they take a prominent part in
the orientation of roots in water and soil. It is also uncertain whether the upward
growth of roots in mud or in soil whose pores are clogged with water 5 is due to
oxytropism or to an alteration of the geotropic irritability produced by the deficiency
of oxygen 6.
SECTION 40. Hydrotropism.
Many plants show tropic curvatures either towards moisture (positive
hydrotropism), or away from it (negative hydrotropism). Both the main and
cf. Dalmer, Jenaische Zeitschr. f. Naturw., 1880, Bd. xiv, p. 39; Strasburger, Jahrb. f. wiss. Bot.,
1886, Bd. xvn, p. 50; Busse, Bot. Centralbl., 1900, Bd. LXXXIV, p. 209; Murbeck, Verhalten des
Pollenschlauchs bei Alchemilla u. d. Chalazogamie, 1901, p. 7 (reprint from Lunds Universitets
Arsskrift, Bd. xxxvin).
1 Celakovsky, Ueber d. Aerotropismus von Dictyuchus monosporus. Reprint, 1897, p. 8.
2 Molisch, Sitzungsb. d. Wien. Akad., 1893, Bd. en, Abth. I, p. 432 ; Miyoshi, Flora, 1894, p. 87.
3 Molisch, I.e., 1884, Bd. xc, I, p. 194. According to Steyer (Reizkriimmungen bei
Phy corny ces nitens, 1901) the unilateral accumulation of carbon dioxide induces no tropic curvature
in the sporangiophore of Phycomyces nitens. [Bennett (Botanical Gazette, 1904, Vol. xxxvn,
p. 241) has conclusively shown that the roots of Zea, Cucurbita, Rafhanus, Vicia, Pisum, and
Lupinus have no aerotropic irritability, and that the curvatures observed by Molisch were hydro-
tropic in character.]
* Molisch, 1. c., Vol. xc, pp. 172, 194. Cf. also Rothert, Flora, 1894, Ergzbd., p. 216.
5 Cf. Jost, Bot. Ztg., 1887, p. 169; Goebel, ibid., p. 717; Schenck, Jahrb. f. wiss. Bot., 1889,
Bd. XX, pp. 534, 564, 569; Wieler, ibid., 1898, Bd. xxxil, p. 503. On the curvatures of roots
produced by deoxygenated water cf. Ewart, Trans. Liverpool Biol. Soc., 1894, v°l' VIII» P- 24°-
[6 The absence of oxygen, or the presence of poisonous gases, produces disturbances of growth
often resulting in irregular curvatures, which are not always traumatropic in character. When the
curvature is towards the region less deficient in oxygen, growth will be more rapid, and in this way
a certain biological advantage may be gained by parts of the root system, or by some of the seedlings.
It appears, however, as though the avoidance by the roots of regions poor in oxygen is in part aided
by the suppression or reversal of the geotropic irritability, for on repeating the experiments described
HYDROTROPISM 183
lateral roots 1 are positively hydrotropic, and hence curve towards moister
soil or moister regions of the surrounding air. In this way the roots of
plants growing on the sides of cliffs keep themselves buried in the soil
or curve back towards it. The positive hydrotropism of the rhizoids of
Marchantia 2 is of equal importance when the plant is growing on the sides
of rocks, and the possession by the pollen-tube of this form of irritability
aids it in applying itself closely to the stigma 3.
On the other hand, the sporangiophores of Phycomyces and of other
Mucoriniae 4, as well as the stipe of Coprinus velaris, according to Molisch,
are negatively hydrotropic. According to Steyer, however, the sporangio-
phore of Phycomyces assumes a diatropic direction of growth at a certain
distance from a wet surface, whereas when further away it performs a slight
positive curvature towards the region where the percentage of moisture is
most to its liking. When the young sporangiophore first rises above the
medium, it is strongly negatively hydrotropic (hydrophobic), and hence
grows at right angles to the surface of the substance, since the moistness
of the subjacent air decreases regularly in successive upward layers.
The aerial organs appear to be devoid of any hydrotropic irritability,
for it is only in the case of the hypocotyl of Linum usitatissimum that
feeble negative hydrotropism is shown 5.
For demonstration purposes seeds may be germinated in sawdust on an
obliquely inclined sieve, or on the porous clay niters recommended by Molisch.
Since the roots do not curve to the moist surface when the air is saturated with
moisture, it is evident that differences in the percentage of moisture form the external
causes inducing curvature. In the case of Phycomyces the culture medium, such as
a slice of bread, may be covered with a sheet of mica having small holes bored
through it. The sporangiophores which grow through these holes may be used for
experimentation.
It is owing to their negative hydrotropism coupled with their transpiration that
in Trans. Liverpool Biol. Soc., 1896, Vol. x, p. 191, on a klinostat, I was unable to obtain any
constant and definite curvatures of the radicles away from the deoxygenated portion of the medium.
The whole subject, however, well merits further investigation.]
* Knight (Phil. Trans., 1811, p. 212) first made it certain that the curvature of the roots to
moister substrata was due to their hydrotropic irritability, which at a later date was studied in detail
by Sachs, Arb. d. bot. Inst. in Wiirzburg, 1872, Bd. I, p. 209; and Molisch, Sitzungsb. d. Wien.
Akad., 1883, Bd. LXXXVIII, Abth. I, p. 897. Further research is required on the influence of the
irregular distribution of moisture upon the development of roots in soil.
2 Molisch, 1. c., p. 932.
3 Miyoshi, Flora, 1894, p. 84.
4 Wortmann, Bot. Ztg., 1881, p. 368; Molisch, I.e., p. 935 ; Dietz, Unters. a. d. bot. Inst. zu
Tubingen, 1888, Bd. n, p. 478 ; Steyer, Reizkriimmungen bei Phycomyces^ 1901, p. 14. The negative
hydrotropism observed by Klebs (Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 55) in the sporangiophore
of Sporodinia is disputed by von Falck (Cohn's Beitrage z. Biologic, 1901, Bd. vm, p. 237). On
the fruit-stalk of Dictyostdium cf. Potts, Flora, 1902, Ergzbd., p. 319.
5 Molisch, I.e., p. 937; Dietz, I.e., p. 480. According to Vochting, Bot. Ztg., 1902, p. 98,
the shoots of potatoes are hydrotropic. Cf. Singer, Ber. d. bot. Ges., 1903, p. 175.
184 TROPIC MOVEMENTS
the sporangiophores diverge from one another when closely crowded. Frequently
they may curve towards or away from a rod of metal or other material fixed upright
in the culture medium l. According to Errera and Steyer, this is due to the rod
either condensing or evolving water vapour, or changing the degree of saturation
of the surrounding air by warming it 2. There is therefore no need to assume the
existence of any mysterious action at a distance, although in some circumstances
other stimulatory reactions may come into play.
Even should it be found that the dissimilar rates of transpiration due to the
variations in the percentage of moisture act as the stimuli to curvature there would
be no need to change the term ' hydrotropism/ and still less need to invent a new one.
SECTION 41. Mechanotropism.
Under this head we may include all orienting movements produced
in response to mechanical agencies. Thigmotropism, or haptotropism, has
already been fully discussed when dealing with the irritability to contact
of tendrils and certain other organs. The seismonic irritability shown in
response to mechanical disturbances is not made use of for the attainment
of any pronounced tropic curvatures. Nevertheless, certain curvatures due
to rubbing or striking on one side may be seismonic reactions.
RHEOTROPISM. This 'special form of irritability by means of which
plants are able to perform curvatures in response to the movement of the
water in which they are growing was discovered by Jonsson3, and is
possessed by a variety of roots. The radicle of Vicia sativa responds
especially well, according to Juel, in water moving with a rapidity of
0-3 mm. per second. For the radicle of Zea Mays> however, a rapidity
of about 3 mm. per second is required. The extent and rapidity of the
curvature is increased by a further moderate rise in the rate of flow, but is
retarded when it becomes too rapid. When the current is as rapid as
500 mm. per second, a portion of the roots of Vicia sativa curve in the
direction of the stream, probably owing to the mechanical action of the
latter. According to Berg, however, at low temperatures so pronounced
a change of tone takes place that the roots no longer respond positively
but give a negatively rheotropic reaction. Juel found that decapitated
roots also showed positive rheotropism, so that the stimulus cannot be
perceived solely by the root-tip. Newcombe 4 indeed finds that the whole
1 Elfving, Ueber physiologische Fernwirkung einiger Korper, Helsingfors, 1890 ; Zur Kenntniss
d. pflanzlichen Irritabilitat, 1893 (reprint from Ofversigt af Finska Vet.-Soc. Forhandlingar, xxxvi).
2 Errera, Annals of Botany, 1892, Vol. vi, p. 373 ; Steyer, I.e., pp. 16, 21.
3 Jonsson, Ber. d. hot. Ges., 1883, p. 518; Berg, Studien liber Rheotropismus bei den Keim-
wurzeln, 1889 (repr. from Lunds Universitets Arsskrift, Bd. xxxv) ; Juel, Jahrb. f. wiss. Bot., 1900,
Bd. xxxiv, p. 507.
* Newcombe, Botanical Gazette, 1902, Vol. xxxm, p. 177; Annals of Botany, 1902, Vol. xvi,
p. 429.
MECHANOTROPISM
185
of the growing zone, as well as the next zone which has just ceased to
grow, are able to perceive rheotropic stimuli. Among the roots examined
by Berg, only those of Soja hispida showed no power of rheotropic reaction,
whereas, according to Newcombe, a variety of roots are insensitive.
Rheotropism has also been detected in the hyphae of fungi, those of
Phycomyces and Mucor being negatively and those of Botrytis cinerea
being mainly positively rheotropic, according to Jonsson. The strip of
filter-paper on which the mycelium is growing has each end immersed in
a nutrient liquid, one of the vessels being slightly higher than the other.
The slow movement of water thus induced is sufficient to act as a rheotropic
stimulus to the hyphae.
FlG. 39. Radicle* of Vicia sativa undergoing rheotropic excitation. The arrow shows
the direction of rotation, the movement of water producing the curvatures shown at the
end of sixteen hours.
For purposes of demonstration the apparatus shown in Fig. 39 may be used,
the glass dish containing water being rotated on a klinostat, so that the speed of the
current to which the radicles are exposed will depend upon their distance from the
axis of rotation. The same effect is produced when the seedlings are rotated and
the vessel kept stationary, and Jonsson placed the radicles in a narrow straight stream
of running water. Berg also succeeded in showing that roots show a rheotropic
reaction when growing in soil. *
Traumatropism. Injury causes a wound-reaction which may exercise
a correlative effect upon the growth and movement of associated or remote
parts. Among these are included certain tropic curvatures which are
induced by local injury to the growing-points of aerial and subterranean
roots, due to incision or to cauterization by heat, alkali, acid, or lunar
caustic *. A few hours afterwards a curvature begins in the elongating
1 Darwin, The Power of Movement in Plants, 1880, p. 528 ; Spalding, Annals of Botany, 1894,
Vol. vin, p. 423 ; Pollock, Botanical Gazette, 1900, Vol. xxix, p. i.
i86
TROPIC MOVEMENTS
zone of the root, away from the injured side or injurious agency. This
negatively traumatropic curvature is about as rapidly produced as a geo-
tropic one, and is shown as the result of comparatively trifling injuries,
while severe injury may cause the growing apex to perform a complete
coil (Fig. 40).
Since we are here dealing with a tropic stimulus which is only per-
ceived at the root-apex, no reaction is shown when the tip of the root is
removed by a transverse cut, or when it is entirely killed by the injury.
The removal of an oblique slice from one side of the apex produces,
however, a corresponding traumatropic curvature,
while, according to Spalding *, the incision must
pass through the meristem below the root-cap in
order to be effective. MacDougal 2 regards the
periblem as being the irritable and responsive
region, but without bringing forward any conclu-
sive proof.
Naturally gentle rubbing has no effect, but
the energy of growth of the roots in soil is such
that when in contact with stones sufficient pressure
and friction might be exerted to produce a trau-
matropic curvature away from the hindrance.
The root-apex, except in the case of the roots of
Vanilla planifolia> does not appear to possess
any thigmotropic irritability, for the curvatures
observed by Darwin away from the side to which
pieces of paper, glass, or mica had been attached were apparently trau-
matropic in origin, and were due to the means of attachment employed.
Indeed the local application of alcohol or of a solution of shellac readily
produces a traumatropic curvature away from the point of application.
The traumatropic stimulation is not the result of the generally occurring
transitory reaction, but is due to the cessation of the correlative influences
which normally radiate from the injured zone. This produces asymmetric
disturbances which induce an acceleration of growth on the side opposed to
the injury. Spalding found, in fact, that if the root was embedded in
plaster-of-paris immediately after being injured, a traumatropic curvature
was shown as soon as it was set free eight days afterwards 3. During this
FIG. 40. Seedlings of Vicia
Faba. The radicles have curved,
as shown, sixteen hours after the
application of silver nitrate at c.
In B the injury and resulting cur-
vature are greater than in A.
1 Spalding, Annals of Botany, 1894, Vol. vm, p. 432.
2 MacDougal, Botanical Gazette, 1897, Vol. xxm, p. 307.
3 Cf. Spalding, 1. c., p. 426 ; Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 373. The curvature
observed by Nemec (Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 87) as the result of attaching particles
of plaster-of-paris to one side of a root was possibly traumatropic in character. It is also possible,
however, that a local retardation of growth might act as a tropic stimulus to the primary meristem,
or that the curvature might be more or less mechanically induced.
MECHANOTROPISM 187
time the general wound-reaction had mainly ceased, whereas the regeneration
of the injured region was prevented by the plaster cast.
The traumatropic curvature was discovered by Darwin, and was further in-
vestigated by Spalding, who showed that certain authors were incorrect in denying
the existence of any such curvature1. Naturally there is no question of a tropic
curvature when the injury is so pronounced as to lead to the partial or complete
death of the whole of the tissues on one side of the growing zone of a root or other
organ, for in this case the retardation or cessation of growth on one side, and its
continuance on the other, unavoidably results in a curvature. Nor is any trau-
matropic irritability in play when an injurious agency retards the growth of that side
of the organ to which it is applied. It was in this way that the curvatures of roots
were produced which Newcombe considered to be thigmotropic in character, and
possibly similar curvatures may be produced by the unilateral action of poisonous
gases. The true traumatropic curvatures, however, are shown by roots even when the
zones of perception and response are some distance apart. The tip of the seedling
leaf of Avena, however, which is sensitive to heliotropic stimuli does not appear to
have any traumatropic irritability.
The traumatropic curvature is independent of whether the defect to
which it is a response has been produced by mechanical, chemical, or
electrical means. The other two mechanotropic reactions differ in that
the rheotropic response is excited by a current of water, but the thigmo-
tropic only by contact with solid bodies. It is not impossible that rheo-
tropism, hydrotropism, and osmotropism may all be forms of the same
irritability, and that the primary processes of perception may be alike in
all three cases2. In the case of osmotropism and hydrotropism, the
stimulation might arise from differences of turgor on the opposed sides
of the irritable organ, produced in the first case by the differences in the
concentration of the surrounding medium, and in the second by the
different rates of transpiration in unequally moist air. No such differences
of turgor can be responsible for- the rheotropic excitation, although the
unequal pressure of the water on the front and back of the root might
lead to a movement of water through the tissues which might operate as
a stimulus.
As far as is known, however, these three forms of irritability by no
means always occur together, but are in most cases separately developed,
and hence it is more probable that they are integrally distinct manifestations
of irritability. Roots which are strongly hydrotropic do not appear to
be osmotropic, while the osmotropic hyphae of certain fungi also show
rheotropism but have no hydrotropic irritability 3. Roots are, it is true, both
1 See Spalding, Annals of Botany, 1894, Vol. vin, p. 440 ; Bot. Centralbl., 1883, Bd. xm, p. 180.
2 Cf. Juel, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, pp. 507, 533; Rothert, Flora, 1901, p. 415.
8 Steyer, Reizkriimmungen von Phy corny ces, 1901, p. 28. The sporangiophore of Phycomyces is,
however, strongly hydrotropic.
i88 TROPIC MOVEMENTS
rheotropic and hydrotropic, but hydrotropic stimuli are only perceived
at the root-apex, whereas rheotropic stimuli are also perceived in the zone
of stretching growth. Probably further researches, especially in connexion
with freely motile organisms, will reveal additional instances of the occur-
rence of these forms of irritability single or in combination. Nothing is
known as to the mode in which rheotropic, osmotropic, and hydrotropic
stimuli are perceived, but it is quite possible that osmotropic stimulation
may be the result of a very different form of excitation to that involved
in the production of rheotropic or hydrotropic responses. On this basis it
is easy to understand why transpiration is able to excite an increased
development of cuticle but not the withdrawal of water due to the osmotic
action of a saline solution. Elfving l found that no curvature was induced
in the strongly hydrotropic sporangiophore of Phycomyces by the impact
of a stream of saturated air, but this empirical fact permits of no conclusions
as to the nature of a hydrotropic excitation.
SECTION 42. Galvanotropism.
Since many freely motile organisms are strongly galvanotactic, it
might be expected that the organs of fixed plants would often be capable
of galvanotropic curvature. Hitherto, however, this form of irritability has
only been detected in the radicles of seedlings, which according to some
authors are positively, and according to others negatively galvanotropic 2.
Brunchhorst considers that these contradictory results are due to the fact
that when the current is weak the curvature is towards the kathode, but
when strong towards the anode. This latter positive curvature is, according
to Brunchhorst, traumatropic in character, being due to the injury of the
anodal side of the root by the strong current. Further researches are,
however, required to determine whether this is actually the case, and also to
elucidate more thoroughly the observed phenomena.
Additional investigation is also needed concerning the negatively
directed curvatures produced on the sporangiophore of Phycomyces^ accord-
ing to Hegler3, by the action of the Hertzian electrical waves, also
concerning the negative curvatures observed by Lepellier4 away from
1 Elfving, Zur Kenntniss d. pflanzlichen Irritabilitat, 1893, p. 4.
3 Elfving, Bot. Ztg., 1882, p. 257; Miiller-Hettlingen, Pfluger's Archiv f. Physiol., 1883,
Bd. XXXI, p. 201; Brunchhorst, Ber. d. bot. Ges., 1884, p. 204; Notizen iiber d. Galvanotropismus,
1889 (reprint from Bergens Museums Aarsberetning) ; Rischawi, Bot. Centralbl., 1885, Bd. xxu,
p. 121. [None of the methods used, even by Brunchhorst, is wholly satisfactory. See Ewart and
Bayliss, Proceedings of the Royal Society, Nov., 1905.]
3 Hegler, Ueber die physiologische Wirkung der Hertzischen Elektricitatswellen auf Pflanzen,
1891 (reprint from Verhandlg. d. Ges. deutscher Naturf. u. Aerzte in Halle).
* Letellier, Bull, de la Soc. bot. de France, 1899, T. vi, p. n. Steyer (Reizkriimmungen bei
Phycomyces, 1901, p. 17) obtained negative results with Phycomyces. On the action of statical
electricity cf. also Danilewsky, Die physiolog. Fernwirkungen der Elektricitat, 1902 ; Loeb, Pfluger's.
Archiv f. Physiol., 1897, Bd. LXVII, p. 483; Bd. LXIX, p. 99.
GALVANOTROPISM 189
regions of high electrical potential. Should these responses prove to be
tropic in character J it remains to be seen whether the action of electrical
waves corresponds to that of an electrical current 2, and also whether the
varying magnetic permeabilities of the different constituents of the cells
and tissues 3 may render magnetotropic responses possible in a sufficiently
strong magnetic field.
According to Brunchhorst 4, the curvature produced by a strong current
is shown when the root is decapitated, but not the true galvanotropic
curvature in the opposite direction produced by a weak current. The
latter is shown when only the tip of the root is submerged in water or
touches a wet flannel 5 through which the current is passing. Hence
only the tip of the root seems capable of the perception of a negatively
galvanotropic stimulus 6.
SECTION 43. Autotropism and Somatotropism.
It was long ago observed by Dutrochet 7 that the sporangiophores of
Mucory PhycomyceS) and Pilobolus, and of other fungi placed themselves
at right angles to the substratum from which they had emerged. The
phenomenon was further studied by Sachs and by Dietz 8. Tjjie latter
author concluded that the escape from the substratum was regulated by
thigmotropic excitation whereas Steyer ° denies the truth of this statement.
Sachs supposed that seedling-stems would, in the absence of any other
excitation, set themselves at right angles to a block of moist turf in which
they were germinated ; but Dietz has shown that this is not the case 10.
The position assumed by the sporangiophores of the fungi mentioned is
primarily the result of their negative hydrotropism, since their position of
equilibrium is reached when they are parallel to the direction of diffusion of
the water- vapour from the substratum. Negatively heliotropic organs would
assume similar positions around a strong centre of illumination. This
apparent action of the substratum causes the young sporangiophores to grow
at first vertically outwards from the sides of the piece of bread, whereas
when they grow longer their geotropic stimulation becomes relatively
1 [The true nature of these galvanogenic curvatures has been recently investigated by Ewart
and Bayliss, Proceedings of the Royal Society, Nov., 1905.]
3 Induction-shocks act on tendrils like mechanical stimuli (Pfeffer, Unters. a. d. bot. Inst. zu
Tubingen, 1885, Bd. I, p. 504), and in much the same way upon protoplasmic streaming. (Cf.
Ewart, Protoplasmic Streaming in Plants, 1902, p. 88.)
Ewart, 1. c., pp. 45-9. * Brunchhorst, Ber. d. bot. Ges., 1884, p. 204.
Miiller-Hettlingen, Pfliiger's Archiv f. Physiologic, 1883, Bd. xxxi, p. aoi.
Cf. Rothert, Flora, 1894, Erg.-bd., p. 213.
Dutrochet, Rech. anat. et physiol., 1824, p. 100.
Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, p. 221. Dietz, Unters. a. d. bot. Inst. zu
Tubingen, 1888, Bd. in, p. 478.
9 Steyer, Reizkriimmungen bei Phy corny ces, 1901, p. 27.
10 Dietz, 1. c., p. 480.
I9o TROPIC MOVEMENTS
stronger and causes them to curve upwards. This is due to the fact that
the intensity of the hydrotropic excitation diminishes rapidly as the distance
from the source of moisture increases. The young sporangiophores are
also geotropically excitable, and if they are subjected to strong centrifugal
action they curve outwards almost as soon as they emerge from the
substratum. Similarly, an upward curvature is at once shown if they
develop in saturated air so that they are geotropically but not hydrotro-
pically excited.
Orienting actions of this character may arise from living as well as
from dead parts, if these evolve moisture. Pollen-tubes and parasitic fungi
are attracted in this way into living tissues, and it is largely owing to
stimuli of this kind that the stem of the Mistletoe and the sporophores of
parasitic fungi set themselves in a definite position, which is usually nearly
at right angles to the surface of the stem upon which they are growing.
In all symbiotic associations not only formative but also directive
interactions are exercised by the symbionts upon each other. These
relationships are of the utmost complexity in the higher plants, for by
them are determined not only the development and point of origin of
shoots, roots, hairs and leaves, but also the tendency to a particular
direction of growth of each organ in regard to the main axis. This
autotropism1 naturally only finds full expression in the absence of all
'external directive factors, but even when these are in play the position
assumed is the result of their co-operation with the autotropic tendencies.
Organs may be either auto-orthotropic as in the case of the primary root and
stem, or auto-campylotropic as in the case of the leaves and other lateral
appendages. The term autotropism may be used in the general sense to
correspond with that of automorphosis, and this terminology renders the
use of the words rectipetality and curvipetality unnecessary. These terms
were indeed used by Vochting 2 more especially in connexion with flowers.
Every disturbance of equilibrium excites reactions which tend to its
restoration, and it is in this way that an organ is brought back into its
original position after temporary stimulation has induced movement.
Experiments illustrating this fact have been carried out by various investi-
gators 3, and more especially Baranetzsky has shown that the return
1 The term ' Eigenric1 tung ' was suggested by Pfeffer (Pflanzenphysiol., i. Aufl., 1881, Bd. II,
p. 286; Die Reizbarkeit der Pflanzen, 1893, p. 19), and may be translated by ' autotropism/
3 Vochting, Bewegungen der Bliithen und Friichte, 1882, pp. 31, 192. Cf. also Czapek, Jahrb.
f. wiss. Bot., 1895, Bd. xxvn, p. 313. The terms 'autonasty/ ' autoepinasty,' and the like are less
suitable, since the positions assumed are to be regarded as the result of the action of internal
directive stimuli. Noll used the word ' morphaesthesia ' to indicate the tendency to assume definite
relations of symmetry (Sitzungsb. der Niederrhein. Ges. fur Natur- und Heilkunde, 15. Jan. 1900),
but the term is a quite unnecessary one.
8 Vochting, 1. c., 1882, pp. 31, 182, 192 ; F. Darwin and Pertz, Annals of Botany, 1892, Vol. VI,
p. 247 ; Czapek, 1. c., 1895, p. 308 ; Kohl, Ber. d. bot. Ges., 1898, p. 169; Baranetzsky, Flora, 1901,
AUTOTROP1SM AND SOMATOTROPISM 191
movement may involve a few transitory oscillations. The return move-
ment can naturally only be performed when the power of growth or of
expansion is retained, but it is worthy pf note that the growing apex
of an auto-orthotropic shoot or root continues to grow in a straight line
even when the parts immediately behind are permanently curved or
forcibly bent, and the attempted autogenic straightening prevented. It
follows that the autotropic reaction is strictly localized to the part affected,
and hence it is not surprising to find that autotropic return curvatures may
be performed by decapitated roots l.
Autotropic stimuli may, however, affect parts a greater or less
distance away by the aid of the correlative mechanism, and indeed the
removal of an organ such as the terminal shoot of a Conifer may affect
the autotropism as well as the geotropic irritability of neighbouring
branches. It is owing to some autotropic action at a distance of this
character that the lateral branches and roots assume at first their auto-
tropic position, but are more affected by the geotropic stimulus as
they increase in length. The lateral roots always ultimately assume the
same plagio-geotropic position independently of the angle which they
assume in regard to the parent axis. Dutrochet was therefore in error
in assuming that the plagiotropic position of the lateral roots was the
resultant of their positive geotropism and their tendency to set themselves
at right angles to the main root. It is, however, quite possible that the
lateral roots may possess a feeble geotropic irritability as soon as they
emerge externally.
A lateral shoot will only return to its original position when capable
of an autotropic curvature. So long as no mechanical hindrances intervene,
this is the case with hairs and with the lateral roots of second, third,
and higher orders, for these have no geotropic irritability, and orient
themselves in regard to the main root at angles determined by their
autotropism. The same applies to the lateral roots of the first order
when developed on a rotating klinostat, for they then grow out for the
most part at right angles to the main root ; whereas under normal con-
ditions they usually form acute downwardly-facing angles with the per-
pendicular main root2.
The orienting actions radiating from living and dead substrata
were first recognized by Dutrochet 3, and were studied more fully by
Sachs4. Dutrochet erroneously concluded that the autotropic angle was
Erg.-Bd., p. 143. See also Bonnet, Nutzen d.Blatter, 1762, p. 170; Dntrochet, Me"moires, &c.,
Bruxelles, 1837, P- 32°5 Ann- d- sci- nat-> l844» 3C S(5r-> T- n» P- 98J Miller, Flora, 1876, p. 91 ;
Darwin, The Power of Movement in Plants.
1 Czapek, I.e., p. 322.
3 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1874, Bd. I, pp. 596, 615.
s Dutrochet, Rech. anat. et physiol., 1824, p. 101.
* Sachs, Arb. d. bot. Inst. in Wurzburg, 1874, Bd. I, p. 598; 1879, Bd. II, p. 217.
I92 TROPIC MOVEMENTS
always a right angle, and considered that the directive action of the
substratum was due to the mass attraction of the latter. Van Tieghem l
supported this view, but its incorrectness was shown by Sachs2, and the
whole subject was discussed in a manner according with our present views
in the first editions of Pfeffer's Physiology. Various authors then brought
forward instances of the elimination of curvatures by autotropic action.
No precise determination is, however, possible at present of the complex
factors involved in all autotropic responses, for the same problems are
involved as in growth and formative activity in general.
The fact that alterations in the .tissue-strains, as well as in the tension
of the plasmatic membranes, may affect growth affords no evidence as to
the origin of the autotropic curvatures, and hence it is impossible to follow
Noll3 in his attempt to ascribe these curvatures to the result of the
changed strains in the tissues and plasmatic membranes. Klercker assumed
that the removal of the curvature was 'the mechanical result of the con-
tinuance of equal growth on the opposed sides, but Czapek 4 has shown
the insufficiency of this view.
PART III
THE CONDITIONS FOR AND CHARACTER OF TROPIC STIMULATION
SECTION 44. Instances of the Separate Localization of Perception
and Response.
Usually the effect of tropic stimulation is strictly localized and con-
ducted to only a short distance from the directly excited region5. In
addition, separated organs, or even fragments of organs, may still remain
capable of tropic response; and hence the existence of a power of transmitting
tropic stimuli from the percipient organs to the motory zones was overlooked
until Darwin's researches were made 6.
In all tropic action at a distance the intervening ductory processes are
such as to regulate the curvature to the direction of incidence of the exciting
agency upon the percipient organ. This is still the case when the motory
zone is not directly excitable, and can only be indirectly stimulated
1 Van Tieghem, Bull, de la Soc. hot. de France, 1876, T. xxm, p. 56.
2 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1879, Bd. n, p. 224.
3 Noll (Biol. Centralbl., 1903, Bd. xxm, p. 403).
* Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 320.
5 Cf. Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 263 ; 1898, Bd. xxxil, p. 248 ; Kohl,
Mechanik der Reizkriimmungen, 1 894.
e Darwin, The Power of Movement in Plants, 1880, p. 523.
SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 193
through the percipient zone, as well as when the stimulus merely spreads
more or less from the directly excited responding zone to surrounding
responsive regions. If in the latter case a particular zone lose the power
of growth and response, it may still remain capable of receiving stimuli
and transmitting them to neighbouring active zones. A separation of
perception and response also occurs when a portion of a growing zone
loses the power of perception, or when the meristem-cells at a growing
apex develop a special irritability before their rapid stretching growth
begins, and lose it as soon as this rapid growth commences. This is
actually the .case in the growing apex of the root, for the power of
receiving geotropic stimuli is lost as soon as the tissue- differentiation begins.
In other cases, however, a special irritability is absent from the primary
meristem, and only appears as the tissues differentiate.
The importance of these relationships was pointed out by Pfeffer1,
but many observers denied the accuracy of
A £ c Darwin's investigations2. Although certain of
the experiments were not altogether satisfactory,
the correctness of Darwin's conclusions was estab-
lished by Czapek, and our knowledge of the
localization of the phototropic irritability was
considerably amplified and extended by Rothert8.
As in other cases, the division of labour is not
always complete, so that one zone may be more
perceptive, the other more responsive. In such
generalized organs direct and indirect tropic
stimuli may co-operate in producing a particular
response.
The heliotropic curvature of grass seedlings
is especially instructive, and was studied in detail
by Rothert. In the cases of Setaria viridis, Panicum miliaceum, and a few
other Paniceae only the cotyledons are perceptive, whereas the pronounced
curvature is produced in the hypocotyl which is not directly excitable. The
hypocotyl of Sorghum vulgare, however, possesses a feeble phototropic
irritability. The same applies to the subapical portion of the cotyledon 4 of
Avena sativa^ which performs the heliotropic curvature in this plant, mainly
in response to the indirect excitation arising from the highly irritable tip of
1 Pfeffer, Pflanzenphysiologie, 1881, Bd. n, p. 327.
* Cf. the literature given by Rothert, Flora, Ergzbd., 1894, p. 179; Czapek, Jahrb. f. wiss. Bot.,
1 895, Bd. xxvil, p. 244.
8 Rothert (Cohn's Beitrage zur Biologic, 1896, Bd. vil, p. 3).
4 The same terms are used as by Rothert, without expressing any view as to the still doubtful
morphological nature of these organs. The term coleoptile, or cotyledonary sheath, may be used
instead of cotyledon, and mesocotyl instead of hypocotyl. Cf. Goebel, Organography, Vol. II, 1905,
p. 408.
FIG. 41. Seedlings of Panicum
miliaceum. A unstimulated. B,
after shorter, C, after longer helio-
tropic stimulation from the right,
r, cotyledon. A, hypocotyl.
I94 TROPIC MOVEMENTS
the cotyledon. The tip of the hypocotyl of many cruciferous seedlings, or
that of the epicotyl of Vicia sativa, is more irritable than the basal
regions ; but in other seedlings, such as those of Tropaeolum, Solanum,
and Coriandrum, and the organs of very many adult plants, the helio-
tropic sensibility is fairly evenly distributed.
The above examples of localized perception are also instances of the
transmission of tropic stimuli, but the same is shown in the peduncle of
Brodiaea congesta, one of the Liliaceae, although the perceptive and
responsive zones are not separately localized. Thus a phototropic stimulus
radiates in three hours to a distance 6 cms. from a directly illuminated area.
A somewhat less pronounced transmission is shown by the stems of Linum
usitatissimum and Coleus, whereas most plant-organs have only a feeble
power of conducting heliotropic stimuli. The stem of Galium purpureum,
however, not only affords an instance of the ready transmission of stimuli,
but is also able to receive and transmit the latter even when the power
of response is lost. Thus the basal parts of the internodes which remain
longer capable of growth and curvature may be excited indirectly by
stimuli applied to the apical non-growing region which has lost the power
of curvature1.
Similarly, geotropic stimuli perceived by the root-tip are transmitted
to the actively growing zones behind, which are not directly excitable.
The tip of the root itself is, however, able to perform slight geotropic curva-
ture 2, and forms the percipient organ for hydrotropic, and possibly also for
negatively galvanotropic 3 and heliotropic stimuli. As regards the latter,
however, Rothert4 was unable to obtain sure results, nor do the experi-
ments of Darwin 5 and of Kohl 6 form sure proof of the localization
of the heliotropic irritability in the root-tip. Traumatropic curvatures
are also usually directed from the root-apex, although the parts behind
may be directly excited as well, and indeed all tropic irritability need
not of necessity be localized in the root-tip. Thermotropic, aerotropic,
rheotropic, and thigmotropic stimuli may, in fact, be perceived by the
curving regions, and these may often be the only parts capable of direct
excitation. The localization of the heliotropic irritability to the tip of the
cotyledon of certain Grasses does not, therefore, necessarily indicate that
the geotropic irritability will be similarly localized, although experiment
has shown that this is the case. The power of perception is retained by
the tip of the cotyledon after it has ceased to grow, whereas in the primary
meristem of roots the geotropic irritability disappears when stretching
growth commences.
1 Rothert, 1. c., p. 139. 2 Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxv, p. 361.
3 [On the true nature of this irritability see Ewart and Bayliss, Proc. Roy. Soc., Sep., 1905.]
4 Rothert, 1. c., p. 140 ; Flora, 1894, Ergzbd., p. 207.
5 Darwin, 1. c., p. 413. 6 Die Mechanik d. Reizkriimmungen, 1894, p. 26.
SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 195
When one considers that the power of tropic reaction has been
developed for the purpose of bringing the various organs of the plant into
different positions suitable to the performance of their special functions,
it is evident that the organs will not only have dissimilar irritabilities but
also that the area over which a stimulus may spread must be restricted.
Otherwise the tropic stimulation of a stem might spread to the root
and cause it to perform unsuitable curvatures. In general the purpose
of tropic curvature can be attained when the perceptive and active
zones are not separated. Hence it is only in special cases that any such
separation is shown, or that a pronounced power of transmitting tropic
stimuli is developed. The special heliotropic irritability of the apical
parts of various seedlings may be of use in rendering possible a curvature
towards the light as soon as the tip emerges above ground, the stimulus
spreading to and stimulating the parts below the ground. Similarly, it
is evidently a purposeful adaptation which leads to the tip of the root
receiving geotropic stimuli and regulating the growth of the region behind
so that it assumes a proper position. The importance of such localization
must, however, not be overestimated, since equally rapid and appropriate
orientation is possible when the power of perception is evenly distributed
over the whole of the active zone. Teleological considerations must, indeed,
never be pressed too far, and they would lead us to conclude that the move-
ment of the leaf-stalk into a phototropic position would be best induced
by the directive action of the lamina. As a matter of fact, the heliotropic
sensibility appears never to be restricted to the lamina, and its orientation
seems always to be due to the co-operation of a variety of factors.
The power of transmitting tropic stimuli across small distances which
may surpass the breadth of the organ affected must always be present,
for all the cells are not equally irritable, and yet growth activities must
be excited in the responsive tissues corresponding to the extent of the
induced curvature. In the case of dorsiventral tendrils in which the convex
surface is not directly excitable, the stimulus to increased growth must
be transmitted from the concave to the convex surface, and probably
the same applies to tendrils in general, since it is always the outer side
not in contact whose growth in length is accelerated. In addition, Mucor
and Caulerpa afford instances in which the different parts of a cell are
endowed with dissimilar tropic irritabilities, and Steyer1 has shown that
in the case of Phycomyces the heliotropic sensitivity is restricted to the
apex of the sporangiophore. Hence localized unilateral illumination
beneath the growing zone produces no heliotropic reaction, either because
this zone has no power of perception or because it is unable to transmit
the stimulus to the growing zone and so direct the growth of the latter.
1 K. Steyer, Reizkrummungen bei Phycomyces 1901, p. 6.
O 2
196 TROPIC MOVEMENTS
The localization of the heliotropic irritability is most readily determined, since
the direction and point of application of the light is easily controlled. Thus Darwin
and Rothert found that unilateral illumination of the seedling of Panicum produced
no curvature when the cotyledon was covered with tinfoil, but that the full curvature
of the hypocotyl took place when the cotyledon was exposed, but the hypocotyl
wrapped round with tinfoil. These experiments can be performed without injury and
without placing the plant under abnormal conditions, and Rothert has shown that
the normal power of reaction is not affected by the enclosure in tinfoil.
According to Vochting 1, illumination of the lamina of Malva verticillata is able
to operate as a directive stimulus to the darkened petiole, causing the upper pulvinar
portion to move so that the leaf is placed in a diaphototropic position. Since,
however, the petiole is also capable of a heliotropic response, under normal circum-
stances its curvature is the result of direct and indirect heliotropic excitation.
Czapek2 finds that darkening of the lamina of Cornus sangumea, Linaria cymba-
laria and Viola odorata prevents any phototropic orientation, whereas Rothert3 was
unable to detect any phototropic direction of the leaf-stalk by the lamina of
Tropaeolum minus, and the same was found by Krabbe4 to apply to the leaves
of Fuchsia and Phaseolus. Finally, Ewart 5 has shown that the folding together of
the leaflets of various Leguminosae in strong light takes place when the laminas are
darkened but the pulvini exposed, but not when the laminas are exposed to light
and the pulvini darkened. The various factors concerned in the orientation of leaves
are by no means clearly determined, and it is not certain whether Czapek 6 is correct
in ascribing to the laminas of certain leaves a power of perceiving geotropic stimuli
and transmitting them to the leaf-stalk.
The perception of geotropic stimuli by the apex of the root. Darwin 7 found that
decapitated roots lost the power of reaction, whereas a curvature took place when
the decapitation followed previous geotropic induction. Although the geotropic
irritability is temporarily suspended as the result of injury, the opposition to Darwin's
views was largely unjustified, and Czapek 8 showed conclusively that the same results
could be obtained in the absence of an injury. The growing apex was caused to
grow in a bent glass tube closed at one end so that the apical region was kept
permanently at right angles to the growing zones behind, the segments derived from
the apical meristem expanding backwardly out of the tube. The seedlings were at
first rotated on a klinostat, and then arranged so that the apical region pointed
1 Vochting, Bot. Ztg., 1888, p. 519.
2 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p! 274. Further research is needed in this
direction.
3 Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. vn, p. 121.
* Krabbe, Jahrb. f. wiss. Bot., 1889, Bd- x*» P- 25^.
5 Ewart, Annals of Botany, 1897, Vol. XI, p. 452 seq. The same was found by Oltmanns and
by Macfarlane (Flora, 1892, p. 234; Bot. Centralbl., 1895, i, p. 136) to apply to the pulvini of
Robinia pseudacacia.
6 Czapek, 1. c., p. 274.
7 Darwin, The Power of Movement in Plants, 1880, p. 523.
8 Czapek, 1. c., 1895, Bd. xxvil, p. 243. The lateral roots behave similarly (1. c., p. 263).
SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 197
vertically downwards, but the rest of the root was horizontal. No curvature followed,
but when the root was placed as in Fig. 42, A, within twenty-four hours a curvature had
taken place as at B, so that the tip pointed downwards. It follows, therefore, that
the growing zones behind the apex which perform the curvature are incapable of
directly perceiving geotropic stimuli.
To obtain successful results, the roots must be able to slip easily into the glass
tubes, since otherwise disturbances of growth ensue, such as prevented Wachtel
and Richter from obtaining any positive results *. The experiments when properly
performed are, however, fully satisfactory ; and Czapek 2 has shown that exactly the
same phenomena are shown after the removal of the tube if the apex of the root
remains permanently bent for a time.
The special geotropic irritability of the root-tip is also shown by the fact
that the active zone curves beyond the vertical when the apical part is kept
permanently horizontal 3. This method was used by F. Darwin 4 to show that the
cotyledon of a seedling of Panicum not only perceives heliotropic, but also geotropic
B
FlG. 42. Seedlings of Lufinus albus
(smaller size). The seedling (A) has been
removed from the klinostat after the apex
is fixed in the glass cap /£, and after
twenty-four hours has curved so as to
place itself parallel with the perpendicular
line shown oy the arrow.
FlG. 43. Seedlings of Setaria italica. The roots have been cut
away down to the rudiments w, the cotyledon fixed in the glass
tube a, and the seedling is then placed horizontally. In A the hypocotyl
has curved through 180°, and at B has formed a complete coil. (Twice
enlarged.)
stimuli (Fig. 43). This method is, however, unable to determine whether the power
of perception is totally absent from the responding zones, and it is not surprising that,
owing to the abnormal conditions, the plant is not always able to bring the irritable
region into the normal position of equilibrium 6.
The hydrotropic irritability was not conclusively shown by Darwin's 6 experiments
1 Cf. Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 312 ; and the reference to Wachtel's
work in the Bot. Ztg., 1899, p. 227 ; Richter, Zur Frage nach der Function der Wurzelspitze, 1902.
a Czapek, 1. c., p. 336.
8 F. Darwin, Proceedings of the Cambridge Philosophical Society, 1901, Vol. XI, p. 133;
Linnean Soc. Journal, 1902, Vol. xxxv, p. 266.
* F. Darwin, Annals of Botany, 1899, Vol. XIII, p. 568. The special geotropic irritability of
the tip of the cotyledon was suggested by certain observations of Rothert (Cohn's Beitrage z. Bio-
logic, 1896, Bd. vn, p. 189) and of Czapek (Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 254). Massart
(Sur rirritabilite" d. plantes superieures), 1902, has applied this method to various roots and shoots.
5 Cf. Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvn, p. 590.
* Darwin, The Power of Movement in Plants, 1880, p. 180. Cf. Rothert, Flora, 1894, Ergzbd.,
p. 208.
198 TROPIC MOVEMENTS
to be localized in the root-apex. Molisch l was, however, able to obtain a curvature
when the root was enveloped right up to the tip in moist tissue-paper, while Pfeffer 2
found that, if only the extreme tip was clothed in moist paper while the rest of the
root was exposed to hydrotropic stimulation no curvature followed. Hence the
power of perceiving hydrotropic stimuli is developed in the root-apex alone.
The heliotropic and geotropic irritabilities are not equally distributed throughout
the sensitive apex of the cotyledon of Gramineae, and presumably the geotropic
irritability of the root-apex gradually disappears in the differentiating tissues.
Czapek3 found that the length of the geotropically irritable zone in the roots of
Lupinus and Faba was about 1-5 millimetres. If a less zone than this is included
in the terminal limb of the glass cap, a curvature takes place when the apex is placed
vertically, since the horizontal region just behind is geotropically excitable. Hence
the power of perception cannot be restricted to the extreme tip of the growing-point
or to the calyptrogen layer. The conclusion of Fritsch and N£mec, that perception
is localized in the root-cap, is based partly upon faulty experiments and partly upon
incorrect ideas as to the process of stimulation 4. Czapek concludes that the whole
of the meristem and of the young tissues abutting upon it is capable of perceiving
geotropic stimuli. According to Wachtel 5, the geotropic irritability returns to
decapitated roots where the apical meristem is regenerated 6. The removal of the
epidermis from the cotyledons of Gramineae 7 does not prevent them from perceiving
geotropic stimuli.
It is difficult to determine by operation whether certain tissues are more highly
excitable than others, since the removal of the other tissues may not only affect the
power of response, but may also result in traumatropic curvature. Rothert 8 found the
removal of the tip of the cotyledon of Panicum or Avena caused the geotropic
and heliotropic irritabilities to be entirely suspended for a few hours. At the same
time, growth is retarded, but not to such an extent as to cause the cessation of
a curvature which had already begun or which had just been induced. Similarly,
transverse or longitudinal incisions or punctures in the root-apex inhibit the geotropic
irritability for a few hours or even a couple of days, although the percipient organ
is neither removed nor destroyed. It is not surprising that the irritability should
return sooner after such an incision has been made than when the root-apex is
entirely removed 9. Owing to the fact that the injury excites an energetic process
of regeneration, it is difficult or impossible by operative experiments to determine the
part played by different tissues in the perception of stimuli. It is hardly to be
1 Molisch, Sitzungsb. d. Wien. Akad., 1883, Bd. LXXXVHI, Abth. i, p. 897.
3 Cf. Rothert, 1. c., p. 212 ; Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 316.
3 Czapek, loc. cit., 1895, Bd. xxvn, p. 262; Ber. d. hot. Ges., 1901, Generalvers., p. 117.
4 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 230; Ber. d. bot. Ges., 1901, pp. 117, 119.
5 Cf. Czapek, 1. c., 1901, p. 118.
6 Cf. also N6mec, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 98; Fiinfstiick's Beitrage z. wiss.
Botanik, 1901, Bd. iv, p. 193.
7 Czapek, 1. c., 1898, p. 255.
8 Rothert, Cohn's Beitrage z. Biol., 1896, Bd. vn, pp. 191, 211.
* Czapek, 1. c., 1898, p. 202 ; 1. c., 1901, p. 118; N&nec, 1. c., p. 97.
SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 199
expected that the removal of the tip of the root and of the cotyledon of a Grass
should produce exactly the same effect, since in one case we are dealing with
undifferentiated meristem, and in the other with a nearly adult differentiated tissue.
Hence, any incision into the root-apex temporarily inhibits its irritability, whereas the
complete removal of the tip of the cotyledon of a Grass is required, according to
Rothert, to produce the same effect.
The traumatic inhibition of the heliotropic and geotropic sensibilities on the one
hand, and the retardation of growth on the other, are two distinct reactions to
the same external agency. It is only possible to demonstrate the conduction
of stimuli leading to both forms of response when the zone of action is directly
excitable, but nothing is known as to the inherent character of the phenomenon.
Nevertheless, the removal of the' apex of the cotyledon of A vena must either entirely
inhibit the power of perception of heliotropic stimuli or must prevent the awakened
sensation progressing to the induction of movement. According to Rothert1, the
inductory processes once begun are not stopped by the injury, but progress, and are
propagated to the active zones. After only short exposure to unilateral illumination,
a heliotropic after-effect is shown in spite of the removal of the tip of the cotyledon,
and leads to a curvature. In roots, however, prolonged induction is required before
any geotropic after-effect is shown, and in such cases the ductory processes might
already have reached and affected the active zones before the sensitive apex was
removed. Darwin 2, for instance, decapitated the roots after they had been kept for
one to one-and-a-half hours in a horizontal position. Czapek 3 has shown why this
after-effect cannot be used to demonstrate the localization of the geotropic irritability
in the root-apex, and has also found that short induction periods may produce
perceptible after-effects4. It is, however, always possible that the processes of
induction themselves may be affected by traumatic agencies, and hence probably
arose the fact that Czapek 5 was unable to detect any geotropic after-effect in the
roots of Lupinus. Nor is it surprising that a short period of induction may not
be able to overcome the existent tendencies and the effects of decapitation, and
hence may fail to produce any after-effect. Owing to the fact that N£mec 6 did not
consider this possibility, his experiments fail to determine whether the injury entirely
suppresses the geotropic excitability of the root, or whether the sensory processes are
still excited up to a certain point. Decapitated parts, even when in a condition of
traumatonus, are still capable of reaction, and may indeed be capable of certain tropic
responses.
The conduction of stimuli usually occurs over a short distance only, even when
the transference is from one organ to another, as from the cotyledon to the hypocotyl
of Panicum. Copeland 7 suggests that the positively geotropic curvature of certain
Rothert, 1. c., p. 200.
Darwin, The Power of Movement in Plants, 1880, p. 525.
Czapek, Jahrb. f. wiss. Bot., 1895, Bd. XXVII, p. 252.
Czapek, ibid., 1898, Bd. xxxii, p. 219.
Czapek, 1. c., 1895, p. 252.
Nfimec, Fiinfstiick's Beitrage z. wiss. Bot., 1901, Bd. iv, p. 186.
7 Copeland, Botanical Gazette, 1901, Vol. xxxi, p. 410.
200 TROPIC MOVEMENTS
hypocotyls and cotyledons is due to processes of induction transmitted from the
sensory region of the root-apex.
Tropic stimuli are only slowly conducted, as are most stimuli in plants.
Under favourable conditions a heliotropic stimulus may travel at a rate
of i mm. to i mm. in five minutes in the case of Avena and Brodiaea \
while the geotropic excitation may pass from the root-apex at a rate of
i mm. in five minutes 2. Stimuli must travel in sensitive tendrils over at
least 1 8 mm. in five minutes, as measured by the difference in time between
the application of a stimulus to the concave side and the commencement
of the acceleration of growth on the convex side and resultant curvature 3.
Presumably the stimulus may spread in all directions where conducting
tissue is available; but, according to Rothert4, heliotropic stimuli travel
mainly in the basipetal direction in the cotyledon of Avena. Since the
latter has only two longitudinal vascular bundles, it is easy to cut these
and show that the heliotropic stimulus is able to travel through the
fundamental parenchyma5. According to Czapek6, the same is true for
geotropic stimuli, although it does not follow that here and in other cases
the vascular bundles are devoid of all power of conducting stimuli. In
addition, the cortical tissue of roots is able to transmit geotropic7 and
traumatropic 8 stimuli, for curvatures can still be produced in the active
zone when only a strip of living cortex is left between the stimulated
apex and the growing zones behind. No geotropic reactions can, however,
be excited in a node of Tradescantia fluminensis by stimulation of the
next younger node if the continuity of the vascular bundles is broken 9.
Probably also the stimuli involved in the regulation of translocation mainly
travel through the vascular bundles.
Geotropic 10 and traumatropic n stimuli are still able to travel from the
apex of the root to the active zone and to produce a normal curvature
when a pair of incisions are made in the path of the stimulus on opposite
sides one above the other and past the median line. This shows that the
stimulus may have followed a curved path and may be capable of lateral
I Rothert, 1. c., pp. 137, 209.
3 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 219.
8 H. Fitting, Jahrb. f. wiss. Bot., 1903, Bd. xxxvin, p. 610.
* Rothert, 1. c., p. 52. 5 Rothert, 1. c., pp. 63, 209.
6 Czapek, 1. c., 1898, Bd. xxxn, p. 255.
7 Czapek, 1. c., p. 220.
8 Pollock, Botanical Gazette, 1900, Vol. xxix, p. 24.
9 Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvn, p. 527.
10 Czapek, 1. c., 1898, p. 220. Cf. also N6mec, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 96.
NBmec states in another paper (Fiinfstuck's Beitrage z. wiss. Bot., 1901, Bd. iv, p. 207) that the
stimulus does not travel beyond an incision in the active zone of the root. See also N&nec, Die
Reizleitung u. die reizleitenden Structuren, 1901, p. 134.
II Pollock, 1. c., p. 24.
SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 201
transference, but how this is produced is quite uncertain. We may,
however, conclude with reasonable certainty that the protoplasmic com-
munications play an important or even essential part in the conduction
of stimuli. The fact that stem and roots are incapable of any geotropic
reactions when plasmolysed does not afford conclusive proof1, since the
treatment probably acts by suppressing the growth reaction. The fact
that the influence of the external conditions upon the rapidity and readiness
of transmission of stimuli corresponds to their influence upon perception
and sensation indicates that the former also is a vital phenomenon 2. The
possibility of the transverse conduction of stimuli is probably owing to
the presence of interprotoplasmic communications on the side walls, their
distribution being such as to restrict the stimuli to particular paths3.
There appears, however, to be a certain time block at each passage
from cell to cell, and it is for this reason that longitudinal propagation
is always more rapid in tissues composed of elongated cells than trans-
verse propagation. The times usually given for the transference of
stimuli include the latent period of response, but by eliminating this
Ewart found that traumatic stimuli inducing streaming travelled at rates
of i mm. to 2 mm. per minute at 30° C.4 Within the long cells of Chara
and Nitella, a much more rapid prolongation of stimuli inhibiting streaming
is shown when the time of reaction is excluded, for they travel at a rate
of i mm. to 8 mm. per second at room temperatures 5.
The protoplasmic fibrillae which N£mec 6 considered to be the channels for the
transmission of tropic stimuli may favour the transmission in a special direction.
According to Ngmec, they become more strongly marked as the result of stimulation,
and, if so, this may explain why a continuous stimulation may spread further than
a single excitation. The fibrillae do not, however, form a continuous conducting
system, nor are they always present 7, while in the latter case stimuli may be trans-
mitted as rapidly, or even more rapidly, than when they are present8. Czapek9
found that reducing substances increased in amount in geotropically-excited root-
Strasburger, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 578.
Czapek, ibid., 1898, Bd. xxxn, p. 221.
Cf. Strasburger, 1. c., 1901, Bd. xxxvi, p. 493; Kienitz-Gerloff, Ber. d. bot. Ges., 1902,
P-93-
Ewart, The Physics and Physiology of Protoplasmic Streaming in Plants, 1903, p. 105.
Ewart, 1. c., p. 103.
6 N6mec, Die Reizleitung und die reizleitenden Structuren, 1901, p. 135 ; Biol. Centralbl., 1901,
Bd. xxxi, p. 529.
7 Haberlandt, Sinnesorgane im Pflanzenreich, 1901, p. 150; Biol. Centralbl., 1901, Bd. xxxi,
p. 369 ; Ber. d. bot. Ges., 1901, p. 569. On the conduction of stimuli in nerves cf. Verworn, Das
Neuron in Anatomic und Physiologic, 1900. See also the summary by Borattau, Zeitschr. f. allgem.
Physiol. von Verworn, 1901, Bd. i, p. 129.
8 Ewart, 1. c., 1903, p. 102.
9 Czapek, 1. c., p. 208 u. Ber. d. bot. Ges., 1901, Generalvers., p. 122.
202 TROPIC MOVEMENTS
apices, and that this effect spreads from the excitable zone. We are, however, pro-
bably dealing with a secondary reaction, resulting from the primary processes of
sensation and induction.
SECTION 45. Instances of Autogenic and Aitiogenic Changes
of Irritability.
The special irritabilities of stems, roots, and other organs cannot come
into being before the primordial rudiments are developed, and in many cases
may only appear when a certain stage of development has been reached.
Thus stems and leaves while in the bud, or when just escaping from it,
usually show no geotropic or heliotropic irritability. In addition, the nodes
of stems do not at first possess any geotropic irritability, while those of
Dianthiis bannaticus only develop this irritability when fully grown \ In
the case of Spirogyra, Bacteria, and other asomatophytes only embryonic
cells are available, while the geotropic perception and reaction of mould-
fungi is restricted to the embryonic growing apex of the hypha. Further-
more, the geotropic irritability of the apical meristem of a root is lost
in the elongating segment-cells,, whereas in other cases a tropic sense may
persist after the power of reaction has been lost. Automatic changes of
tropic irritabilities are also frequently used to produce curvatures under
constant external conditions, and periodic movements may be normally
induced by regular autogenic changes of tone.
In addition, changes of the external conditions may induce changes
of tone resulting in modifications in the character or rapidity of tropic
reactions. It has already been mentioned that the heliotropic reaction
of seedling-stems is suppressed by a partial pressure of oxygen which still
permits of geotropic stimulation and curvature, while in air rarefied enough
to suppress curvature no perception of a tropic stimulus or after-effect are
possible. According to Czapek2, however, the root of Lupinus is able
to perceive a geotropic stimulus in the entire absence of free oxygen. A
root kept in a horizontal position at o° to 2° C. for twenty-four hours in
oxygenless air showed on a klinostat a curvature due to the geotropic
induction on returning to ordinary air and room temperature.
Low temperatures retard geotropic reaction sooner than geotropic
sensation 3, so that roots of Lupimts placed horizontally for eighteen hours
at o° to 2° C. perform a geotropic curvature when returned to a more
favourable temperature as the after-effect of the previous induction. The
curvature is, however, not very pronounced, partly owing to the lowered
1 Earth, Die geotropische Wachsthumskrummung der Knoten, 1894, pp. 8, 28. The same
applies to the development of irritability in tendrils and in the pulvini of Mimosa and other plants.
8 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 377.
3 Czapek, 1. c., p. 272 ; Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 195.
AUTOGENIC AND AITIOGENIC CHANGES OF IRRITABILITY 203
irritability due to the low temperature, and partly owing to the fact that
the power of reaction may be temporarily depressed. Similar results were
obtained by Czapek by the sufficiently intense action of chloroform, carbon
dioxide and caffeine.
According to Czapek 1, the receptivity of geotropically-sensitive organs
continually rises as the temperature does, so that the relationship between
receptivity and temperature is represented by an ascending curve, as is that
between respiration and temperature, whereas the growth-curve falls beyond
a certain optimum temperature. The injury of the cotyledon of Avena^
and of the root-apices of a variety of plants, produces a transitory inhibi-
tion of the power of perception, but not always of the power of conducting
stimuli, while the power of perceiving tropic stimuli is still retained when
growth is mechanically prevented by embedding in a plaster cast.
The inhibition of the power of perception or reaction is an instance of
the aitiogenic modification of tropic properties ; but, in addition, changes
of tone may be induced which cause alterations in the position of equilibrium
under the same constant stimulus. Both diffuse and unilateral stimuli
may directly and indirectly produce changes of tone, and a particular
tone may either rapidly appear owing to the changed conditions or may
gradually result from the conditions prevailing during development. These
considerations apply to existent organs, although external influences may
also induce a formation of organs with specific powers of reaction. No
sharp distinction can, however, be drawn, since the modification of tone may
only appear in the portions of the organ developed under the new con-
ditions, or, in the case of a Bacterium, in the new individuals. Cultivated
plants often show varied powers of reaction under different conditions, and
Vochting 2 found that the flowers of Itnpatiens parviflora and the cleisto-
gamic flowers of Linaria spuria possessed no power of geotropic orientation
when developed in feeble light. The cultural conditions also apparently
exercise a pronounced effect upon the power of reaction of Bacteria and
other micro-organisms 3, while in certain cases races may be developed with
particular tactic or tropic properties.
Although injuries may cause a transitory depression or inhibition of
the geotropic and heliotropic irritabilities, pieces of stems and roots are
usually capable of tropic reaction. Nevertheless, the injury probably
may either affect the rapidity of reaction, or produce a correlative modifica-
tion or suppression of the position of equilibrium or of the power of tropic
reaction. In certain cases, however, the removal or prevention of growth
of an organ may produce profound changes of irritability in neighbouring
1 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxxn, pp. 198, 303.
a Vochting, ibid., 1893, Bd. xxv, pp. 179, 189.
9 Cf. Rothert, Flora, 1901, p. 416.
204 TROPIC MOVEMENTS
ones, causing tropic curvatures often directed towards the replacement of
the missing organs.
If the apex of Picea excelsa or of other Coniferae is embedded in a plaster cast,
one or more of the side-shoots bend upwards and more or less completely replace the
main axis 1. Chara behaves similarly 2, while in many other plants a certain lessen-
ing in the geotropic angle of the side-shoots is produced by the removal of the apical
shoot 3. According to Strasburger 4, the effect extends to lateral shoots of Picea
pungens grafted upon the main axis of Picea excelsa when the apex of the latter is
embedded in a plaster cast. Similar changes of position may be produced by the
infection of the axis with parasitic fungi 5. In many cases, however, in which sympo-
dial axes are normally produced by the non-development of the terminal bud, the
required directive actions probably result from self-regulation rather than from any
modification of the geotropic irritability 6. On the other hand, the upward curvature
of the previously horizontal apex of a rhizome to form an annual upright shoot seems to
result from a change of the original diageotropic irritability into a negatively geotropic
one, and this change is correlated with the conversion into a leafy and flowering
shoot7. A change of the geotropic irritability not only occurs in sympodial rhizomes,
but also in uniaxial ones, and is produced or hastened by the removal or bending of
the subaerial shoots8. No such change is, however, produced in the rhizome of
Adoxa moschatellina by the removal of the flowering axes 9.
The removal of the apex .of the main root also causes the lateral roots to grow
more directly downwards, owing to a change in their geotropic tone, without their
reaching a vertical position 10. Vochting n found that, when the apical portion of the
tap-root of a beet was transplanted into the position of a lateral root, it grew in
a plagiotropic position, whereas a lateral root transplanted into the cut end of the
main root assumed a positively parallelotropic position. Apparently the irritabilities
were reversed in these cases by the correlative influence of the new associations.
Ngmec12 also found that the removal of the terminal leaflet of a compound leaf
influenced the position of the lateral leaflets to a certain extent.
1 Kunze, Flora, 1851, p. 145 ; Sachs, Arb. d. hot. Inst. in Wiirzburg, 1879, Bd. n, p. 280;
Busse, Flora, 1893, p. 144.
Richter, Flora, 1894, p. 416.
Vochting, Organbildung, 1884, Bd. n, p. 32.
Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 588.
Darwin, The Power of Movement in Plants.
Cf. Goebel, Vergl. Entwickelungsgesch. d. Pflanzenorgane, 1883, p. 192.
Cf. Goebel, 1. c., p. 193 ; Organography, Vol. n, 1905, p. 25.
8 Cf. Goebel, Bot. Ztg., 1880, p. 818 ; Organography, Vol. 1, 1900, p. 172 ; Vol. II, 1905, p. 463
(Sparganiutn, Sagittaria, Circaea, Scirpus maritimus, &c.) ; Sachs, Arb. d. bot. Inst. in Wiirzburg,
1880, Bd. n, p. 484 (Cordyline, Yucca) ; Elfving, ibid., 1880, Bd. n, p. 489 ; C. Kraus, Flora, 1880,
p. 54; Vochting, Bot. Ztg., 1895, p. 95 (Potato).
9 Goebel, Bot. Ztg., 1880, p. 791.
10 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1874, Bd- J» P- 622J Darwin, The Power of Movement
in Plants, 1880, p. 187; Vochting, Organbildung, 1884, Bd.il, p. 35 ; Boirivant, Ann. sci. nat., 1898,
7" sen, T. vi, p. 315.
1 Vochting, Transplantationen am Pflanzenkorper, 1892, p. 34.
18 N6mec, Ueber die Folgen d. Symmetriestb'rung bei zusammengesetzten Blattem, 1902 (reprint
from Bull, internal, de 1'Acad. de Boheme).
AUTOGENIC AND AITIOGENIC CHANGES OF IRRITABILITY 205
The pronounced geotropic reaction of a node on the stem of Tradescantia
virginica is somewhat decreased when the internode between it and the next younger
node is severed, and is entirely suppressed in the case of Tradescantia fluminensis and
T. zebrina. According to Kohl \ this is due to the fact that, as in roots and the
cotyledons of grasses, the perceptive and reacting zones are separately localized, the
young node perceiving the geotropic stimulus and transmitting it to the next older
one. Miehe 2 has shown that a geotropic curvature is produced in the third horizontal
node of an intact plant when the next younger second node is placed vertically by
bending the internode. If the third node is placed vertically, no curvature results
in it, but instead a geotropic reaction is awakened in the horizontal younger second
node. Furthermore, the geotropic irritability of the third node is diminished when the
second node is placed in carbon dioxide or in a plaster cast, so that its growth and
functional activity are depressed or stopped. It follows, therefore, that the removal of
the younger node awakens positive geotropism in the next older node, and that the
same effect can be produced by placing the node in a vertical position ; and to produce
a complete change of tone the entire younger node with its bud must be removed.
The remaining portion of the internode then dies and is thrown off. The older node
loses its geotropic irritability when the continuity of the internodal vascular bundles
is broken, so that these must serve for the transmission of the correlative interactions
concerned. It is possible that the correlative stimulatory actions in question are
derived from the distribution and diffusion of certain of the products of metabolism.
Vochting s found that after removing the flower of a poppy, or the capitulum of
Tussilago Farfara, the temporary positively geotropic power of reaction of the peduncle
was arrested, whereas the negative geotropism and the autotropism were unaffected.
Since the same effect is produced by the removal of the ovary only of the poppy, the
correlative influences which modify the geotropic tone seem to have their origin in
this part of the flower. According to Wiesner *, moreover, the upward curvature of
a horizontally-placed inflorescence axis of Digitalis and other plants no longer occurs
when the flowers have been fertilized. The decapitated peduncle of a Poppy still
remains capable of growth, whereas, according to Scholtz 5, the removal of the flower
of Clematis cylindrica or of Dahlia variabilis causes the peduncle to lose the power of
growth, and hence also of geotropic reaction.
Changes of geotropic tone may also be responsible for the absence of torsion in
the internodes of Philadelphus and Deutzia when the pair of leaves at the upper end
of the internode are removed 6. Similarly, Noll 7 observed that the removal of the
apex of the inflorescence of an orchid resulted in the neighbouring ovaries undergoing
no torsion.
1 Kohl, Bot. Ztg., 1900, p. i.
2 Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvn, p. 527.
3 Vochting, Beweg. d. Bliithen vu Friichte, 1882, pp. 107, 126; Scholtz, Cohn's Beitrage z.
Biologic, 1892, Bd. v, p. 371.
4 Wiesner, Biol. Centralbl., 1901, Bd. xxi, p. 803.
5 Scholtz, 1. c., p. 387.
6 De Vries, Arb. d. bot. Inst. in Wurzburg, 1872, Bd. II, p. 273; Schwendener u.Krabbe, 1892,
Ges. bot. Mitth., Bd. II, p. 309.
T Noll, Arb. d. bot. Inst. in Wiirzburg, 1887, Bd. Ill, p. 368.
206 TROPIC MOVEMENTS
SECTION 46. Changes of Irritable Tone (continued).
The thermonastic and photonastic curvatures produced by changes
of illumination or temperature are either the result of indirect changes in
the geotropic tone or are due to the action of gravity in producing physio-
logical dorsiventrality in the responding organ. When the latter is the
case a response may be shown at first on the klinostat, but when none
is shown it still remains to be determined whether the actual curvature
involves a labile ephemeral induction or a modification of the geotropic
tone. Definite results may be obtained in the future, but it is worthy of note
that an increased reaction following a rise in the intensity of the directive
agency might merely be the result of its enhanced dorsiventral inductive
action. Probably both changes of tone and inductive actions are utilized
separately and in various combinations by different plants for special
purposes. The increase in the intensity of a diffuse stimulus may modify
the tropic action of the same agency. This occurs whenever an increase
in the intensity of diffuse illumination or in concentration so alters or weakens
the tropic sensitivity to unilateral illumination or to the unequal distribu-
tion of a chemical substance that a change of position results.
Instances of the influence of illumination upon the geotropic irritability are
afforded by the subterranean runners of Adoxa moschatellina, Trientalis europaea, and
Circaea lufetiana, which are diageotropic in darkness, but curve downwards when
illuminated, even if already embedded in the soil. The curvature is accelerated in
Adoxa by the fact that illumination hastens or awakens the growth of the previously
darkened runner *. It is also owing to a change of their geotropic irritability that the
runners and other shoots of a variety of plants become approximately vertical in
darkness, but assume plagiotropic to horizontal positions under diffuse illumination of
increasing intensity 2. Illumination also causes a certain geotropic downward curva-
ture of the lateral roots, causing the angle between them and the main root to
diminish by about 20° to so03. Czapek found that this reaction was no longer
shown when the apex was covered with tinfoil, so that the tonic stimulus of light is
only perceived by the growing apex.
The geotropic angle of the lateral roots is somewhat lessened by a rise of tem-
perature 4, which also affects the geotropic position of certain shoots and leaves.
In dorsiventral organs, however, aitionastic curvatures may complicate matters, and
it is always possible that changes of the heliotropic tone may be induced by alterations
in the diffuse external conditions. No researches have, however, been performed in
this direction, although it is certain that not only the phototropic, but also other tropic
positions of equilibrium may be more or less modified by the diffuse action of
1 Stahl, Ber. d. hot. Ges., 1884, p. 391.
3 Czapek, Sitzungsb. d. Wien. Akad., i895/Bd. CIV, Abth. i, p. 1234; Oltmanns, Flora, 1897,
p. 34 ; Goebel, Organography, Vol. I, 1900, p. 93 ; Maige, Ann. sci. nat., 1900, 8e se"r., T. XI, p. 248.
3 Czapek, 1. c., 1895, Bd. civ, Abth. i, p. 1245; Stahl, 1. c., 1884, p. 393.
4 Czapek, 1. c., p. 1251 ; Sachs, Arb. d. bot. Inst. in Wiirzburg, 1874, Bd. I, p. 624.
CHANGES OF IRRITABLE TONE 207
temperature illumination, nutrient and non-nutrient substances, as well as other
agencies l. The geotropic position of the lateral roots, and in some cases also of the
primary root, may change somewhat according to the cultural conditions, but this
result is probably of complex origin. When insufficiently supplied with water, how-
ever, certain radicles do not curve vertically downwards, but assume a more or less
plagio-geotropic position '2. According to Neljubow, the presence of the acetylene
and ethylene of coal-gas in the air around a seedling-stem of Pisum sativum causes it
also to assume a plagio-geotropic position 3.
The response produced by conjoint stimuli is rarely the sum of their
actions when applied singly, even when the power of response remains
unaltered, and assuming that both sensations are separately excited and
remain distinct until movement is excited. As a matter of fact, it is highly
probable that any kind of tropic stimulation affects the tone of the plant
and its power of response to other tropic stimuli. The power of response
to other stimuli naturally need not be suppressed, and in fact geotropically-
excited plants remain capable of response to heliotropic stimuli and vice
versa. The energetic response to particular stimuli might, however, render
the plant temporarily irresponsive to special tropic agencies, either owing to
a temporary suppression of excitability or of the responsive mechanism.
In other cases the conditions for the production of a particular irrita-
bility might involve preceding tropic excitation. This actually applies to
Cuscuta, which develops no contact- irritability when rotated on a klinostat,
since the required tone needs the inductive action of gravity for its
production.
A complete or nearly complete inhibition of one form of irritability by
the functional exercise of another has not hitherto been detected, although
intense stimulation usually depresses the excitability more or less. Changes
of tone may, however, be produced by the combined tropic action of two
dissimilar stimulatory substances. In addition, when a radial tendril is
touched on both sides the excitations extinguish each other and no response
is produced. Such actions may either affect the intermediate stages between
sensation and response, or the primary sensation, as in the case of Cuscuta.
Noll 4 considers changes of tone to be due to the former, and Czapek 5 to the
latter, but the arguments of both authors are inconclusive.
1 A few additional instances are given by Massart, Sur 1'irritabilite d. plantes supe'rieures, 1902,
p. 13 ; Klebs, Willkiirliche Entwickelungsanderungen beiPflanzen, 1903, p. 93.
3 Sachs, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 445 ; Elfving, Beitr. z. Kenntniss der
Einwirkung der Schwerkraft auf Pflanzen, 1880, p. 32; Czapek, 1. c., p. 1252; N6mec, Jahrb. f. wiss.
Bot, 1896, Bd. xxxvi, p. 91.
3 Neljubow, Beihefte z. bot. Centralbl., 1901, Bd. X, p. 128 ; Singer, Ber. d. bot. Ges., 1903,
P. 175-
4 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 495; Ueber heterogene Induction, 1892, p. 56.
6 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 246; Sitzungsb. d. Wien. Akad., 1895,
Bd. civ, Abth. i, p. 337.
208 TROPIC MOVEMENTS
The fact that geotropic excitation does not inhibit the heliotropic
irritability leaves it undetermined whether the two excitations fuse or
proceed to separate motory responses. Teleological considerations lead to
the conclusion that the excitations summate so that a single motory response
is produced, and positive evidence of this could readily be obtained if the
two stimulatory reactions had latent periods of response of very unequal
length. According to Miiller l the respiratory activity decreases during the
performance of a geotropic curvature, but the experiments are not altogether
satisfactory. The geotropic and heliotropic curvatures of growing organs
do, however, occur without any acceleration of the average rate of growth,
so that the respiratory activity need not increase. When, however, as in
the nodes of grasses, growth is induced by geotropic induction, not only
is the power of heliotropic curvature gained but also the respiratory activity
of the awakened nodal cells may be raised.
The co-operation of geotropic and heliotropic stimuli in orienting an organ was
first observed by Dutrochet and by Mohl, and their interaction was studied in detail by
Miiller-Thurgau and by Wiesner 2. Pfeffer 3 pointed out that during such co-operation
changes of tone might modify the results observed, and a variety of instances of such
action have been subsequently obtained. It is of course possible to invent special
terms to indicate the different ways in which changes of tone may be produced, but
such terms are quite unnecessary and afford no explanation of the phenomena
observed. This applies even to the term ' heterogeneous induction ' used by Noll 4,
who has unfortunately failed to recognize the general importance of tone and of the
changes of tone due to internal and external factors 5. The discussions of Herbst
and of Driesch 6 as to whether special terms are needed when the change of tone is
not due to the external conditions, or when it is connected with special responses, are
without value.
Exact determinations of the actual relationships are extremely difficult, and hence
it is not surprising that Czapek and Noll should have obtained opposite results with
seedlings 7. According to Czapek, geotropic induction does not affect the heliotropic
1 N. J. C. Miiller, Fiinfstiick's Beitr. z. wiss. Bot., 1898, Bd. II, p. 267; Arct. Fiinfstuck's Beitr.
z. wiss. Bot., 1903, Bd. v, p. 145,
2 Dutrochet, Recherches anat. et physiol., 1824, p. 92; Mohl, Vegetabilische Zelle, 1851, p. 140;
Miiller-Thurgau, Flora, 1876, p. 94; Wiesner, Die heliotropischen Erscheinungen im Pflanzenreich,
1878, I, pp. 55, 63.
8 Pfeffer, Pflanzenphysiologie, i. Aufl., 1881, Bd. II, p. 338.
* Noll, Heterogene Induction, 1892. Cf. also Noll, Jahrb. f. wiss. Bot, 1900, Bd. xxxiv,
p. 496.
5 Cf. Pfeffer, Die Reizbarkeit der Pflanzen, 1893, p. 22.
6 Herbst, Biolog. Centralbl., 1894, Bd. xiv, p. 733 ; Driesch, Die organischen Regulationen,
1901, p. 19, footnote.
7 Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, p. 372 : cf. also Czapek, Jahrb. f.
wiss. Bot., 1898, Bd. xxxn, p. 271 ; Noll, Heterogene Induction, 1892, p. 56; Jahrb. f. wiss. Bot.,
1900, Bd. xxxiv, p. 494.
CHANGES OF IRRITABLE TONE 209
irritability, nor heliotropic induction the geotropic irritability, whereas Noll states that
heliotropic excitation inhibits the geotropic irritability. Feeble lateral illumination
produces a complete, or nearly complete, assumption of the position of heliotropic
equilibrium in many organs, the geotropic tendency being easily overcome ; but this
may be merely the result of a strong development of the heliotropic irritability, coupled
with an inherently feeble geotropic irritability. Possibly, however, the geotropic
irritability may be partially or entirely suppressed by strong heliotropic excitation in
those organs which are especially dependent upon the assumption of appropriate light
positions. In any case various tropic responses of roots and other organs, such as
those due to hydrotropic and rheotropic actions, appear to take place unaffected by
gravity, since the latter may exercise little or no effect upon the position assumed, and
is also unable to prevent a traumatropic stimulus producing a complete coil at the
growing apex. In addition, J£lebs has shown that hydrotropic stimuli readily over-
come the heliotropic irritability of Sporodinia grandis l.
SECTION 47. Minimal Stimuli and the Latent Periods of Induction
and Reaction,
Owing to the varying degrees of irritability in different organs towards
the same and to different tropic stimuli, a feeble intensity may act as
an excitation in one case, whereas in others a response may be produced
only when the stimulus is intense. Zoospores afford instances of the
almost complete absence of any latent period, the response to stimuli
being shown almost instantaneously, whereas in the case of tropic curva-
tures the latent period is rarely less than a few minutes, and is often
from one-half to several hours in duration.
Once the curvature has begun it continues for a longer or shorter
time after the stimulus has ceased to act, and an after-effect may be
shown if the stimulus is removed just before the curvature has begun.
It follows, therefore, that a perceptible interval of time elapses between
perception and response, although it remains an open question whether
the delay lies in the progress of the sensory excitation or in the awakening
of the motory reaction. When the perceptive and responsive zones are
separately localized, the slow transmission of tropic stimuli interposes an
additional delay.
A response presupposes a sufficient intensity of excitation, and
naturally a stimulus of very short duration may fail to produce any
reaction. Since a summation of transient stimuli is possible when they
are repeated at definite intervals of time, it is evident that each is per-
ceived, and that its inductive action has not faded away before the next
stimulus comes. Wiesner2 found, for instance, that the hypocotyl of
1 Klebs, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 56.
* Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, pp. 25, 87.
PPEFFER. Ill
210 TROPIC MOVEMENTS
Lepidium sativum, when successively laterally illuminated for one second
and darkened for two seconds during a period of twenty-five minutes,
performed as strong a heliotropic curvature as when continuously illumi-
nated for the same time from the side. It will probably also be possible
by using super-optimal intensities of illumination to produce more rapid
curvature by intermittent than by continuous illumination. In Wiesner's
experiment the same result was obtained in both cases owing to the fact
that the reaction is only increased up to a certain limit by increasing
intensities of light. Naturally when the intervals between the successive
periods of stimulation are unduly prolonged no response may be shown,
although periods of one second of strong illumination and fifteen to thirty
seconds darkness ultimately prove effective. A striking instance of the
varying degrees of summation is afforded by the sensitive leaflets of various
Leguminosae. Thus the leaflets of Mimosa pudica fold together fully
when alternately exposed to strong sunlight for two seconds and shaded
for two seconds, although the movement is slower than under continuous
exposure. If for two seconds in sunlight and four seconds in the shade
in regular succession, the leaflets rise up through angles of 15° to 20° only,
while under alternating periods of one second exposure and ten seconds
shade the leaflets remain fully expanded1. If an opaque wheel with an
indented rim is rotated between the object and the source of illumination,
the alternating periods of exposure and darkness may be made excessively
short, but nevertheless a response is still shown if the light is sufficiently
intense, so that the shortest flash of light can be perceived by the plant.
Similar summation appears to be possible in all the tropic reactions
hitherto investigated. Noll2 found, for instance, that geotropic induction
lasting for five minutes produced no effect, but that a curvature was induced
when for three hours the seedling was placed alternately horizontally for
five minutes and vertically for twenty-five minutes. It can, indeed, hardly
be doubtful that a feeble continuous tropic stimulus which is unable to
produce any perceptible response is, nevertheless, perceived as a feeble
sensory excitation, which is incapable of overcoming the autotropic tenden-
cies and self-regulatory activities of the organism.
Minimal stimuli. The minimal intensities of light required to produce a helio-
tropic response have been investigated by Darwin, and subsequently by Wiesner and
Figdor 3, who placed the plant in a dark room at varying distances from a candle-
flame. Under favourable conditions Figdor found that the sensitive hypocotyls of
1 Ewart, The Effects of Tropical Insolation, Annals of Botany, 1897, Vol. xi, p. 449.
2 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 463. Cf. also Czapek, Jahrb. f. wiss. Bot,
1898, Bd. xxxn, p. 206 ; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, p. 1217 ; Darwin and
Pertz, Annals of Botany, 1892, Vol. vi, p. 245; 1903, Vol. xvn, p. 93; Jost, Biol. Centralbl., 1902,
Bd. xxn, p. 175.
3 Darwin, The Power of Movement in Plants ; Wiesner, Die heliotropischen Erscheinungen,
1878, Bd. i, p. 40; Figdor, Sitzungsb. d. Wien. Akad., 1893, Bd. en, i, p. 45.
MINIMAL STIMULI AND LATENT PERIODS 211
Lepidium sativum and Lunaria liennis responded heliotropically to light of intensity
equivalent to 0-0003 of a standard candle, those of Helianthus annum and Mirabilis
jalapa to an intensity of 0-016 of a standard candle, whereas the etiolated shoots of
Salix required an intensity of ioa6 units, and still stronger lateral illumination is
necessary to produce a perceptible heliotropic curvature in less sensitive plants.
It is, therefore, not impossible that plants may be capable of a heliotropic
response to bright moonlight *, and they are able to detect and react to differences of
illumination imperceptible to the human eye. The strongest action is exercised by
the blue and violet rays, as well as by the ultra-violet rays, so that in this respect also
the photic sensitiveness of the plant surpasses that of the human eye. In addition,
the most sensitive plants may show a heliotropic reaction under an intensity of illumina-
tion which produces no perceptible browning in a sensitive chloride of silver paper 2.
Wiesner has shown the importance of eliminating the action of gravity, and as well
as that the sensitivity varies according to the cultural conditions 3.
The geotropic irritability also varies greatly, as can be shown by substituting
varying centrifugal forces. In this way Czapek 4 found that sensitive radicles and
seedling-stems performed slight curvatures in response to a centrifugal force equivalent
to o-ooi g. The extreme sensitivity of certain tendrils to contact-stimuli has already
been discussed, and comparatively slow currents of water may excite a rheotropic
curvature. The power of many micro-organisms of responding to the presence of
the minutest traces of stimulatory substances is in part correlated with their minute
size, but it also indicates a high degree of sensitivity.
Reaction and induction periods. The most rapid tropic responses appear to be
shown by tendrils, for a curvature may become perceptible five to twenty seconds
after stimulation. The pulvini of Lourea vespertilionis 5, and of a few other plants,
show the commencement of a heliotropic reaction within one minute, and under
favourable conditions the sporangiophores of Phycomyces may begin to curve towards
the light in one to three minutes 6. Usually, however, the time required to produce
a heliotropic reaction is at least seven to fifteen minutes even in the case of very sen-
sitive objects such as the seedlings of Phalaris, Avena, and Sinapis, while more than
an hour is required by the strongly reacting seedling-stem of Vicia sativa11. The
time required for a heliotropic reaction appears, however, to be shorter, on the whole,
than that required for a geotropic reaction, which appears never to be less than twenty
to thirty minutes 8.
1 Musset, Compt. rend., 1890, T. ex, p. 201. Cf. Bay, Bot. Ztg., 1891, p. 178.
2 Wiesner, Sitzungsb. d. Wien. Akad., 1893, Bd. en, I, p. 347 ; Bot. Centralbl., 1897, Bd. LXIX,
p. 305.
3 Wiesner, Die heliotropischen Erscheimmgen, 1878, Bd. I, p. 54; cf. also Figdor, 1. c., p. 58;
Oltmanns, Flora, 1892, p. 231.
* Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvu, p. 307 ; 1898, Bd. xxxil, p. 190.
5 Cf. Pfeffer, Periodische Bewegungen, 1875, p. 63. The leaflets of Mimosa and other
Leguminosae may begin to fold up one or two seconds after strong sunlight has fallen upon them.
Ewart, The Effects of Tropical Insolation, Annals of Botany, 1897, Vol. xi, p. 449.
6 Cf. Oltmanns, Flora, 1897, p. n.
7 Darwin, The Power of Movement in Plants ; Wiesner, Die heliotropischen Erscheintmgen,
1878, Bd. i, p. 37 ; Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 185.
8 Cf. Czapek, 1. c., p. 184 ; Darwin, 1. c., p. 422 ; Sachs, Flora, 1873, p. 321.
P 2
212 TROPIC MOVEMENTS
The time required for induction is naturally shorter than that necessary for the
commencement of a reaction to constant stimulation 1, for, if the latter ceases before
the reaction begins, an after-effect resulting in a response is shown. Thus Czapek
found that the length of the geotropic induction period was twenty minutes at 25° C.
in the case of various radicles, whereas the time of reaction was thirty minutes 2.
Czapek found no shorter geotropic induction period than fifteen minutes, whereas the
heliotropic induction period of sensitive seedlings lies between seven and twenty
minutes, and in the case of the epicotyl of Phaseolus is as long as fifty minutes. The
relative lengths of the induction and reaction periods probably vary somewhat even
in the same plant according to the external conditions.
It is evident, therefore, that a sensory excitation begins the moment the stimulus
is applied, and reaches a maximal value in a longer or shorter time under continuous
stimulation. In addition, a curvature would be perceptible sooner were it not for the
delay in bringing the motor mechanism into play. It is mainly for this reason that
freely motile organisms are capable of rapid response, for here the excitation merely
modifies a pre-existent activity.
After-effects. If the stimulus acts longer than the minimal induction period, the
after-effect is naturally increased. Sachs 3 found that if a negatively geotropic stem
was placed horizontally until a curvature just began it continued to curve strongly
when placed vertically, and the after-effect lasted from one to three hours. Similar
results were obtained by Muller and Wiesner 4 by heliotropically stimulating seedlings
until curvature just began. An after-effect is probably never entirely absent, though
it is not always pronounced. Freely motile organisms, for instance, on the removal
of a phototactic stimulus progress for a moment in the original direction. It was
probably owing to the result of the mode of experimentation adopted that Sachs was
unable to obtain any after-effects in roots, for Czapek found that they showed after-
effects extremely well5. The amount of the after-effect is, however, not directly
proportional to the intensity and duration of the induction, although in general the
after-effect is increased by prolonged exposure in the case of objects showing marked
reactions 6. Various other after-effects are known, both periodic and non-periodic ;
but these are discussed in connexion with growth, daily periodicity, and heredity.
SECTION 48. The Relation between the Intensity of Stimulus
and the Resultant Excitation.
In general an increase in the intensity of the stimulus produces
a greater excitation, enlarging the amplitude of movement, and at the
same time shortening the times of induction and reaction. The relationship,
however, is by no means a simple one, and cannot be represented by
Cp. Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 183.
Czapek, 1. c., p. 184 ; in regard to the nodes of Tradescantia cf. Kohl, Bot. Ztg., 1900, p. 19.
Sachs, Flora, 1873, p. 325.
Muller, Flora, 1876, p. 89.
Wiesner, Die heliotropischen Erscheinungen, 1878, Bd. I, p. 61, and 1880, Bd. II, p. 87.
Sachs, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 472.
THE RELATION BETWEEN STIMULUS AND EXCITATION 213
any general formula. Apart from other considerations, this is bound to
result from the fact that increases of temperature, illumination, or of chemical
action may deaden or inhibit sensation and motility, and may finally
produce death. Even within moderate limits the intensity of the stimulus
may modify not only the sensitivity and power of reaction, but also the
time of reaction and the ultimate position of equilibrium. Thus intense
unilateral illumination causes the positive phototaxis of swarm-spores to
become negative, and varying intensities of light suffice to convert the
positively heliotropic reaction of many rooted plants, and even of their
radial organs, into a plagiotropic or negatively heliotropic one. Similar
changes of reaction are known in the case of thermotropic, chemotropic,
hydrotropic, and galvanotropic stimuli. In addition, increasing intensities
of centrifugal action produce a lessening of the geotropic angle of the
lateral roots 1, and cause in diageotropic rhizomes an inward curvature,
so that if the mass of the earth were suddenly increased they would curve
downwards2. These responses are physiological in character, although
intense centrifugal action may produce purely mechanical curvatures. An
already stimulated organ is less responsive than an unstimulated one,
and hence, to produce a perceptible increase in the reaction, the stimulus
must be increased by a greater amount than suffices in the first instance
for the primary reaction. This applies not only to tropic but to other
forms of irritability, and to animals as well as to plants. Weber's law is,
in fact, of general application, for in plants also a definite relation exists
between the intensity of an existent stimulus and the additional intensity
required to produce a perceptible reaction 3. For instance, man can detect
changes of illumination of not less than one-hundredth of the existing
intensity, while in the case of Phycomyces the change must be at least one-
fifth. Thus this fungus under diffuse illumination equivalent to five units will
show a heliotropic curvature when exposed to an increase of illumination
of one unit on one side, whereas in diffuse light of 100 units intensity an
increase on one side of twenty units will be necessary.
That the excitation increases less rapidly than the stimulus producing it was
shown by Sachs, Elfving, and Schwarz, in regard to geotropic, and by Wiesner
in regard to heliotropic stimuli 4. Pfeffer's 5 researches on chemotactic irritability then
1 Cf. Sachs, Arb. d. bot. Inst. in Wiirzburg, 1874, Bd. I, p. 607. Cf. also Pfeffer, Pflanzen-
physiologie, 1881, Bd. II, p. 334 ; Elfving, Beitrag zur Kenntniss d. Einwirkung der Schwerkraft auf
die Pflanzen, 1880, p. 33 (reprint from Acta Soc. Scient. Fennic., Bd. Xll); Schwarz, Unters. a. d.
bot. Inst. zu Tubingen, 1881, Bd. I, p. 80.
8 Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, p. 1233.
3 For details see Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 395.
* Wiesner, Die heliotropischen Erscheinungen im Pflanzenreiche, 1878, Bd. I, and 1880, Bd. II.
5 Pfeffer, Ber. d. bot. Ges., 1883, p. 524; Unters. a. d. bot. Inst. zu Tiibingen, 1884, Bd. I,
P- 395J 1888, Bd. n, p. 633.
2I4
TROPIC MOVEMENTS
established the application of Weber's law to plants, and its extension to the
chemotropism of fungi and pollen-tubes was shown by Miyoshi, to phototropism by
Massart, and to geotropism by Czapek l.
Pfeffer placed freely motile organisms in water or in solutions of stimulatory
materials, and determined the excess concentration required in capillary tubes to
produce a chemotactic attraction. In the case of the sperms of Ferns 2, the liquid
in the tube must contain thirty times as much malic acid as that outside, and in the
case of Bacterium termo 3, about three to four times as much meat-extract as in that
outside. Thus o-ooi per cent, of meat outside requires at least 0-003 per cent, inside,
and i per cent, outside needs 3 per cent, inside the tube to produce a chemotactic
attraction of the bacterium used. Miyoshi found that a five times greater concentra-
tion was required to attract pollen-tubes, and a ten times greater concentration to pro-
duce a chemotactic attraction in the case of Saprolegnia.
Massart4 placed the sporangiophores of Phy corny ces between two constant sources
of illumination, and determined at what relative distances from the two sources
a curvature was just produced. Since the intensity of the light is inversely pro-
portional to the square of the distance, it is easy to calculate how much more
strongly one side must be illuminated than the other to produce a heliotropic
curvature. A difference of illumination of one-fifth was found to be necessary;
so that plants are less sensitive than man, who is able to detect a difference of
illumination of one-hundredth. We are, however, only able by our sense of touch
to detect increases or decreases of weight of one-third, and similar relationships
hold good in regard to our sense of smell and of warmth. It must, however, be
remembered that in the case of the plant our only evidence of perception is an
actual response, and that a feeble stimulus might be perceived but not be able to
excite any curvature.
Not only may the diffuse action of light or of chemical substances weaken
the tropic irritability, but also the performance of a response may have the same
effect. This is shown by the fact that as a tropic stimulus increases in intensity
the time of reaction is at first rapidly but subsequently slowly shortened. Thus
Czapek 6 found that the time of reaction of a root of Lupinus exposed to centrifugal
action equivalent to o-ooi and to i g. fell from six hours to one-and-a-half hours, but
only decreased to forty-five minutes when the centrifugal force rose to 40 g. The
times of induction afford, in fact, an indication of the relationship between the
excitation and the intensity of the stimulus. Diffuse and tropic actions probably
do not influence the excitability in precisely the same way ; but no investigations have
1 Miyoshi, Bot. Ztg., 1894, p. 21 ; Flora, 1894, p. 81 ; Massart, La loi de Weber, etc. Bull,
de 1'Acad. royale de Belgique, 1888, 3° ser., T. xvi, No. 12 ; Czapek, Jahrb. f. wiss. Bot., 1898,
Bd. xxxu, p. 191 ; 1895, Bd. xxvn, p. 305.
2 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, p. 397.
3 Pfeffer, 1. c., 1888, Bd. n, p. 634. The fact that the stimulation of bacteria is due to phobo-
chemotaxis is immaterial.
* Id. Massart used the light reflected from a single lamp by a pair of mirrors at varying
distances.
5 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p. 191 ; 1895, Bd. xxvn, p. 305.
THE RELATION BETWEEN STIMULUS AND EXCITATION 215
been made in this direction, and it is often difficult to raise the intensity of a tropic
stimulus without increasing its diffuse action.
Similar relationships hold good for non-tropic stimuli, although in many cases
the diminished excitability under increasing intensity of stimulation is accompanied by
special peculiarities l. Growth and other functions, under rising temperatures, or increas-
ing aeration or nutrition, increase at first rapidly and then more slowly as the optimum
is approached. Precisely similar curves are given by the action of unnecessary or
poisonous substances, and, in fact, every agency when sufficiently intense produces
a lessened response or excitation. Similarly, movements which alter with increasing
stimulation may be represented by angular curves. The apex of the curve does not
correspond to the optimum point on a growth-temperature curve, since beyond
it the response is reversed instead of continuing of like kind but lessened quantity a.
Considering the complicated nature of the reactions involved, it is hardly
surprising to find that the relationship between the intensity of the stimulus and the
degree of excitation should show many divergences8 from Weber's law, according
to which the stimulus must increase in geometric procession to produce an arith-
metical progression of the excitation, or, in other words, that the excitation is
proportional to the logarithm of the stimulus 4. In accordance with the logarithmic
curve, the excitation at first increases rapidly when the minimal intensity of stimulation
is passed, but subsequently more slowly with equal increases of intensity. In regard
to plants, there can be no doubt that the phenomenon is a physiological one,
although Fechner considered it to be of psychic origin in the case of man. It is,
therefore, inadvisable to use the term ' psycho-physical law ' as was done by Fechner.
In spite of this, however, the comparative effects of the receipt of a shilling upon
a pauper and upon a millionaire may be used as an explanatory illustration.
As in other cases, the change of tone with increasing intensity of stimulation
is undoubtedly the result of a modification of the power of sensation, and if this has
no effect upon another stimulatory reaction, it is evident that the two stimuli act
upon different sensory mechanisms B. This applies more especially to chemotropic
excitations, and Rothert6 has, in fact, shown that the attractive actions exercised
upon Amylobacter by meat-extract and by ether involves different powers of sensation,
for the attractive action of meat-extract is unaffected by the presence of 1-6 per cent,
of ether inside and outside the capillary.
A change of tone in a particular irritability may, however, also arise from
stimulation involving an entirely dissimilar sensory perception, and hence direct
conclusions can only be made with caution from changes of tone. The chemotropic
action of malic acid upon the sperms of Ferns is weakened in solutions already
1 See PfefFer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd- l> PP- 4°6> 5°6, 521 ; Correns,
Flora, 1892, pp. 107, 150.
2 On Phobophototaxis cf. Rothert, Flora, 1901, p. 401.
3 According to Mendelssohn (Centralbl. f. Physiol., 1903, Bd. xvil, p. n), the thermotropic
excitation is proportional to the temperature.
* Cf. Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 401 seq., and 1888, Bd. II,
p. 638.
8 Cf. Pfeffer, 1. c., 1888, Bd. n, p. 648.
• Rothert, Flora, 1901, p. 387.
I
216 TROPIC MOVEMENTS
containing this acid or maleic acid, so that both probably effect the same sensation l.
The same conclusion applies in the case of bacteria, when the attractive action
of dextrin is equally lowered in solutions of dextrin and of meat-extract 2. Various
bacteria are attracted by potassium-salts, meat-extract, and other substances in
a similar manner, whereas the attractive action of oxygen is shown only in the case
of certain forms, and appears to depend upon the development of a special sensory
excitability. When different stimuli excite the same response, we must, in the first
instance, presuppose the existence of dissimilar sensory perceptions, which in other
cases may be singly developed.
SECTION 49. The Conditions for Stimulation and its Progress.
In parallelotropic and plagiotropic organs the conditions for stimulation
are given when the organ is displaced from its normal position. When
a parallelotropic organ is inverted, however, slight autotropic curvatures
cause one side to be more stimulated than the other, and the organ curves
more and more rapidly out of the labile inverted position of equilibrium
into a normal stable one. In all cases the tropic stimulation results from
the unequal application of the external agency, and none is exercised
when the latter is uniformly distributed or acts equally in all directions.
Hence a plant placed between and equidistant from two equal sources
of illumination would show no heliotropic curvature, and the same would
be the case in a geotropic root placed between two planets exercising
the same mass-attraction upon it.
Tropic irritability, therefore, depends upon a power of differential
sensation, that is a power of detecting differences in the intensity of the
exciting agency 3 or in its direction of application, although the detailed
mode of response may vary according to the irritability affected, and,
in fact, unilateral illumination may exercise more than one kind of orienting
action. Indeed, certain organisms may respond to differences in the
intensity of the illumination, others to the direction of the incidental rays,
while the action of gravity can only be of the latter character, since its
intensity is the same at all points inside and outside an organ.
Although the conditions are simpler in radial organs than in dorsi-
ventral ones, Loeb is incorrect in supposing that symmetrically disposed
points are exposed to equal intensities of the orienting agency when
a radial organ has assumed its proper orientation4. The assumption
of a new tropic position by an organ in response to displacement always
1 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, p. 397. 2 Id., 1888, p. 635.
3 Cf. Pfeffer, Pflanzenphysiol., i. Aufl., Bd. II, p. 329, u. Unters. a. d. bot. Inst. zu Tubingen,
1884, Bd- !> P- 477- Nagel (Bot. Ztg., Ref., 1901, p. 297) has no grounds for supposing that only
phobotactic organisms possess a discriminatory sense.
* Loeb, Pfliiger's Archiv f. Physiologic, 1897, Bd. LXVI, p. 441 ; Vergleichende Gehirn-
physiologie, 1899, p. 4.
THE CONDITIONS FOR STIMULATION AND ITS PROGRESS 217
involves a certain change of tone, which, however slight, must inevitably
result from the altered conditions. Noll's objections to this conclusion
are based upon a one-sided consideration of the external relationships1.
It is, in fact, true that a lowered heliotropic sensibility results either from
a general increase of illumination or from an increase of illumination
parallel to the long axis of a parallelotropic organ. As an instance of
such action it may be mentioned that Hering found a general retardation
of growth in length to occur in inverted plants or organs, and similarly
the growth excited in the node of a grass by the diffuse horizontal action
of gravity is inhibited by the parallelotropic action of gravity. Tropic
stimuli often exercise more than one effect, so that the resultant position
may be due to the co-operation of two or more activities, as, for instance,
in the plagiotropic prothallia of Ferns, where the continuance of the labile
dorsiventral induction affords at the same time an instance of the main-
tenance of a special tone appropriate to the position assumed. The
tropic excitation due to a change of position usually rapidly increases to
a certain limit, as the angle of divergence from the normal position increases.
In the case of parallelotropic organs the maximal angle of divergence
from the normal position is 180°, but in that of plagiotropic organs
not more than 90°. The maximal excitation in the case of certain
parallelotropic organs, and possibly of all, is not reached until the divergence
is greater than 90° C., but it is quite possible that in some cases the
tropic excitation may be greater when the organ is at right angles to
the orienting agency, as was, in fact, concluded to be the case by Sachs,
and also by Bateson and Darwin 2. This view is supported by Massart 3,
but Elfving, on the other hand, supposed that the maximal geotropic
excitation is exercised when the main root is inverted4. It is, however,
quite certain that the geotropic stimulus is not directly proportional to
the sine of the angle of divergence, i.e. to the component of the force of
gravity acting at right angles to the stem, although an approximate
correspondence may be shown when the divergences are small. Czapek5
found, however, that the maximal geotropic action was exercised when
all the parallelotropic organs examined by him were diverted from their
normal positions through angles of 140° to 160° C. The increase of
excitation was evidenced in the first instance by the rapidity of reaction.
1 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 487. Cf. Pfeffer, Die Reizbarkeit d. Pflanzen,
1893, p. 19 ; Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. I, p. 476; Czapek, Jahrb. f. wiss. Bot.,
1898, Bd. xxxn, p. 195 ; G. Haberlandt, Jahrb. f. wiss. Bot., 1903, Bd. xxxvm, p. 468 ; Noll, Ber.
d. bot. Ges., 1902, p. 416.
a Sachs, Arb. d. bot. Inst. in Wurzburg, 1879, Bc*. H, p. 240; Flora, 1873, p. 325; Bateson and
F. Darwin, Annals of Botany, 1888, Vol. II, p. 65.
8 Massart, Sur I'lrritabilite" d. plantes supe"rieures, 1902, p. 28.
* Elfving, Beitrage z. Kenntniss der Wirkung d. Schwerkraft auf Pflanzen, 1880, p. 32.
6 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvil, pp. 283, 297 ; 1898, Bd. XXXII, p. 193.
218 TROPIC MOVEMENTS
Thus roots slowly responded to a displacement of as little as 2° C., and
more rapidly to one of 20° C. An increased divergence beyond this did
not further accelerate the reaction, but nevertheless, after equally long
periods of geotropic induction, the most pronounced after-effects were
shown by roots placed at angles of 140° to 160° C. to their normal
positions. Using the same method, Pertz 1 was able to show that the
node of a grass-haulm experienced a negatively geotropic excitation
when the stem was inverted and reverted for equal lengths of time, while
maintaining the same angle with the horizontal. Czapek found that
beyond angles of 140° to 160° C. the excitation again decreased, until
a labile position of equilibrium was reached in a precisely inverted position,
so that when placed on a klinostat no geotropic after-effect was shown
if the root or stem had been prevented from diverging from the vertical
position during its exposure to the action of gravity2. Under natural
conditions an inverted root always makes slight autonomic curvatures
from the vertical, which render possible a geotropic excitation leading to
the return to the normal direction of growth.
If the apex of a shoot is fixed in a horizontal position, and the base
left free to move, the negatively geotropic reaction of the active zones
causes it to curve upwards, but no reaction is shown if the apex is bent
upwards into a vertical position. When the apex is fixed, however, in an
inverted vertical position, the circumnutation of the free portion renders
geotropic excitation possible, so that the free end bends upwards. If
the apical segment of a horizontally-placed shoot is fixed at the middle
of the active zones both the free ends curve upwards3. If, however,
the apex of a root is fixed in a normal vertical position, the free basal
portion performs no curvature since the apex alone is capable of perception,
whereas when the apex is fixed in a horizontal position the free portion
curves continually owing to the continuous excitation, just as when the
tip of a cotyledon of Panicum is held in a horizontal position.
Diageotropic rhizomes behave in a similar way, but respond more
rapidly to an upward displacement than to a downward one of similar
extent 4. The radial lateral roots of the first order behave similarly, and
hence if a lateral root is displaced and then slowly rotated, it assumes its
proper position, owing to the fact that it is more strongly excited during
the upper phase than during the lower one5. The excitation increases
1 Pertz, Annals of Botany, 1899, Vol. xm, p. 620.
2 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvii, p. 291 ; Ricome, Compt. rend., 1903,
T. cxxxvu, cciv.
3 Cf. Frank, Beitrage z. Pflanzenphysiologie, 1868, p. 80; Noll, Heterogene Induction, 1892,
p. 22 ; Hochreutiner, Actes du Congres Botanique de Paris, 1900, p. 39; Massart, 1. c., 1902, p. 31.
* Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, I, p. 1231.
5 Czapek, 1895, 1. c., p. 1213; Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 244. Cf. also Schober,
Bot. Ztg., 1897, p. 7.
THE CONDITIONS FOR STIMULATION AND ITS PROGRESS 219
steadily with progressive upward or downward displacement, and attains
a maximal value when the upward displacement reaches about 90° C.,
so that the angle with the perpendicular is one of 150° to 160° C., which
is about the same as that which produces the maximal geotropic excitation
of a parallelotropic main root. In vertical positions the lateral roots behave
similarly to diageotropic rhizomes, being in a condition of labile equilibrium
both when the apex points vertically upwards and when it is directed vertically
downwards. It does not, however, follow that all plagiotropic organs will
behave similarly. Dorsiventral organs also have only one position of
stable equilibrium, and it appears that the geotropic excitation does not
increase with equal rapidity when they are inclined upwardly and down-
wardly 1.
SECTION 50. Perception and Response.
Even if the geotropic excitation proves to be due to the sinking of
the denser particles in the cells, we should only have found the internal
stimulus and should be as far as ever from understanding the mode of physio-
logical perception. The same applies when galvanotropism is found to be
due to the electrolytic action of the current producing the conditions for
chemotropic excitation 2, or if the unilateral illumination were found to create
changes of surface-tension which acted as the immediate agencies in producing
a heliotropic curvature. Changes in the configuration of the protoplasm may
also be of importance in inducing a particular movement or in enabling it to
be performed, but they give no insight into the mode of perception. Local
accumulations of the protoplasm are also often merely the result of a
realized curvature, or are accessory to the reaction.
Kohl and Wortmann have actually observed accumulations of the
protoplasm on the concave sides of organs performing geotropic, heliotropic,
and thigmotropic curvatures 3. Elfving 4 has, however, shown that the
accumulation follows the curvature, and is also produced as the result of
forcible bending, so that it is possibly the mechanical result of the hindrance
interposed to the movement of the protoplasm. Wortmann5 assumed
that in multicellular organs performing tropic curvatures the protoplasm
travelled to the concave side and largely accumulated there, but Noll and
Kohl 6 have shown that this is not the case.
1 Czapek, Jahrb. f. \viss. Bot, 1898, Bd. xxxn, p. 195.
2 See Ewart and Bayliss, Phil. Trans., 1905.
3 Kohl, Bot Hefte von A. Wigand, 1885, Bd- l, P- 161; Wortmann, Bot. Ztg., 1887, p. 803;
1888, p. 469; 1889, p. 491.
* Elfving, Zur Kenntniss d. Kriimmungserscheinungen, 1888, Sep. a. Ofversigt af Finska Vet.
Soc. Forhandlingar, Bd. xxx ; Bullot, Ann. de la Soc. belg. de Microscopic, 1897, Bd. xxxi, p. 71 ;
Mitschka, Ber. der hot. Ges., 1897, p. 164. Cf. also Noll, Flora, i895,Ergzbd., p. 38; Haberlandt,
Oestreich. bot. Zeitschr., 1889, p. 5.
5 Cf. Godlewski, Bot. Centralbl., 1888, Bd. xxxiv, p. 83.
• Cf. Noll, 1. c., and Arb. d. bot. Inst. in Wiirzburg, 1888, Bd. I, p. 531 ; Kohl, Die Mechanik
der Reizkriimmungen, 1894, pp. 27, 35.
220 TROPIC MOVEMENTS
In certain cases at least a tropic excitation may be produced without
the direct co-operation of the nucleus, and presumably the ectoplasmic
membrane plays a prominent part in the perception of tropic stimuli1.
Streaming cells in which only the peripheral layer of protoplasm is at rest
may be capable of a tropic response, but this is not an entirely satisfactory
proof that the perception is solely due to the peripheral membrane, since an
altered configuration of the streaming protoplasm may be maintained by
the continued action of a tropic stimulus in spite of the regular change. The
chloroplastids in a streaming cell of Elodea are, indeed, capable of phototropic
orienting movements in spite of their circulation around the cell. Contact-
stimuli naturally primarily affect the ectoplasmic membrane, and cells are
capable of tropic response when the protoplasm is reduced to a thin layer
of ectoplasm, while cilia composed solely of ectoplasm perceive stimuli.
None of these facts, however, affords any conclusive proof of the localization
of irritability in the peripheral layer, and as a matter of fact the whole
of the cytoplasm is irritable and capable of reaction. Probably the
different parts, including the nucleus, commonly co-operate in perception
and response, or in the former alone. The ectoplasmic membrane is only
relatively a permanent structure, and its irritability is undoubtedly not
alike in all cases.
Both plagiotropic and parallelotropic orientation may result from
a single tropic perception, in spite of the assumption of Sachs and
de Vries that a plagiotropic response to a single orienting agency must
always be due to the antagonism of opposing tendencies to movement.
The fact that an autogenic or aitiogenic conversion of a positive into a
negative tropism is possible does not show that the cells and tissues contain
both negatively and positively reacting elements, as was supposed to be
the case by Wiesner 1. Even when the orientation is due to two separate
stimuli these may fuse to a single impulse and excite only one tendency to
movement. A change in the degree of sensitivity may or may not affect
the tropic position assumed in response to the conjoint action of two
orienting agencies. Czapek formerly considered the plagiotropism of
lateral roots and of diageotropic rhizomes to result from the co-operation
of their positive and transversal geotropism 2, but now considers that their
plagio-geotropic position is assumed in response to a single tropic excita-
tion. Czapek's arguments are mainly based upon the dissimilar behaviour
of roots bent upwards and downwards through equal angles, and upon the
increase of the positively geotropic movement on exposure to rising
1 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, p. 21. Cf. Pfeffer, Osmotische
Untersuchungen, 1877, p. 211.
* Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, i, p. 1257. Cf. Noll, Sinnesleben d.
Pflanzen, 1896, p. 86 (reprint from Ber. d. Senkenberger naturforsch. Ges. in Frankfurt).
PERCEPTION AND RESPONSE 221
intensities of centrifugal action. Sachs is evidently incorrect in supposing
the lateral roots to be only very feebly geotropic, since when bent down-
wards they soon curve back to their proper plagio-geotropic position x.
Our knowledge of the human eye or ear affords a good instance of
how the most intimate familiarity with the structure and localization of the
organs of perception fails to reveal the processes of sensation and perception.
Even if the electrical vibrations which we call light excited syntonic
electrical surgings in the rods and cones of the retina with whose length
their wave-length harmonizes, and even if the fibres of Corti's organ
resonated to the sound-waves travelling in the lymph of the inner ear,
we should still have advanced no further than when we found that the
curvature of a tendril was induced by the pressure of discrete particles upon
the sensitive epidermis. Hence, to speak of the heliotropic organs as
forming a field of heliotropic sense, and the geotropic ones as forming one
of geotropic sense, is simply to clothe facts already known in a new dress,
which does not conceal our ignorance concerning their intimate causation.
Noll's2 attempts to elaborate stimulatory fields in cells or tissues, which
would theoretically produce the results actually observed, are devoid of
scientific value, and are in the first instance based upon the untenable
assumption that the orientation of the organ is directly dependent upon
the position of the supposed stimulatory fields in regard to the direction
of the orienting agency. Discussions of this kind, based on supposed
physical analogies, are usually highly misleading. It is possible to make
mechanical arrangements which will assume definite positions of equilibrium
according to the direction of incidence of light, gravity, or of contact -
stimuli, and which will return to the same position when disturbed ; but
no direct conclusions can be made upon a basis of this kind as to the
mode of orientation in the living organism. In other words, mechanical
models may serve to direct attention to vital phenomena, but afford no
explanation of them in the absence of any proof of a similarity of
mechanism.
SECTION 51. Instances of Specific Tropic Irritability.
THIGMOTROPISM affords a very good instance of the localization of
irritability, since a gentle touch which is insufficient to produce any
perceptible deformation in the epidermal cells stimulates the peripheral
layer of protoplasm and creates an excitation which spreads to the opposite
side. Diffuse contact on all sides does not excite the transitory accelera-
1 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 248.
a Noll, Heterogene Induction, 1892, p. 18 ; cf. also Fitting, Jahrb. f. wiss. Bot., 1903,
Bd. xxxvin, p. 619.
222 TROPIC MOVEMENTS
tion of growth which accompanies curvature, and this applies not only to
radial but also to physiologically-dorsiventral tendrils. In the latter case,
therefore, both sides are sensitive to contact, but in different ways, for
only stimulation of the concave side is able to produce a curvature. Further
research is, however, necessary to elucidate this phenomenon and to deter-
mine wherein the difference between the two surfaces lies.
RHEOTROPISM, TRAUMATROPISM, and HYDROTROPISM have already
been shown to be special irritabilities involving distinct powers of perception.
GEOTROPISM. Gravity and centrifugal force probably act indirectly,
the changes of pressure or of the position of the parts in the cell due to
their altered direction acting as the stimulus exciting curvature. That the
pressures external to the cell are immaterial is shown by the fact that
unicellular organisms show geotropic responses, and that a root will curve
down into mercury against an upward pressure. It is, however, uncertain
whether the pressure of the fluid or of the solid contents of the cell acts
as a stimulus, and it does not follow that the relationships are the same in
all organisms, or that plants must behave in the same way as certain lower
animals whose perception of and orientation in regard to gravity appear
to be due to the pressure exercised by solid bodies such as statoliths and
otoliths in special * auditory ' sense-organs.
By the term geotropism we merely indicate the power of response to
a particular tropic stimulus, and hence the same term would still be used if
this form of irritability proved to be due to some kind of internal contact
stimulation l. In the same way the term magneto-tropism would be used
if a tropic response was produced by the action of a magnet upon internal
particles of iron or upon the substances of varying magnetic permeability
of which the plant-cell is composed 2. As a matter of fact, plants, like man,
seem to be devoid of any direct power of perception of gravitational forces.
Knight 3 was probably the first to suggest that geotropic curvatures
were caused by the distribution of materials of varying specific gravity in
the plant, although according to Treviranus the same idea was previously
put forward by Astruc 4. Knight, however, seems to have assumed that the
mass-attraction of gravity directly produced a downward plastic curvature
of the root, and was unaware that the root will grow downwards in mercury
or against considerable resistances. Negative geotropism Knight considered
to be the result of the denser nutrient sap collecting on the under side of the
horizontally-placed stem, causing this side to grow more rapidly and hence
producing an upward curvature of the apex. Hofmeister5 accepted this
1 Cf. Verworn, Allgemeine Physiologic, 1901, 3. Aufl., p. 467.
2 Cf. Ewart, On Protoplasmic Streaming in Plants, Clar. Press, 1903, p. 45.
8 Knight, Phil. Trans., 1806, Pt. I, p. 104.
4 Treviranus, Physiologic, 1838, Bd. II, p. 599.
5 Hofmeister, Allgemeine Morphologic, 1868, p. 629.
INSTANCES OF SPECIFIC TROPIC IRRITABILITY 223
view of Knight's, and considered that the positive geotropism of the root
was due to the less dense nutrient materials collecting on the upper side of
a horizontally-placed main root and favouring the growth of this side. On
this assumption it is difficult to see how the nutrient materials would reach
the apex of the root when vertical.
The theories of Traube and of Cisielski1 were mainly based upon
observations made on precipitation membranes. Their general trend was
that the tensions due to mass-attraction, and the thickening of the walls
due to more favourable nutrition, were responsible for both negative and
positive geotropism. Dutrochet2 endeavoured to explain the phenomena
as being due to the co-operation of endosmotic actions with the tissue-strains,
and with the distribution of nutrient materials resulting from anatomical
considerations and their relative densities. Mohl and Hofmeister 3 showed,
however, that anatomical structure has nothing to do with geotropic
irritability, but all these authors failed to recognize that gravity and also
light acted merely as exciting stimuli.
It is only necessary to clothe these mechanical views of Knight,
Dutrochet, and Hofmeister in a modern dress4 by supposing that the
moving materials act as stimuli instead of nutritively to arrive at the recent
hypotheses of Berthold, Noll, Nemec, and Haberlandt 5. These authors
agree in supposing that the physical sinking of the denser bodies in the
cells, and the changes of pressure thereby produced, act as the immediate
causes of the tropic excitation. If analogy is any guide, it seems, however,
more probable that the excitation is the result of an internal contact-
stimulus. Possibly the strong thigmotropic excitability of the ectoplasmic
membrane in the epidermal cells of tendrils is transferred to the endoplasmic
membrane on the side walls of the cells in parallelotropic organs, and to
the membrane on the end walls in plagiotropic ones. In the vertical and
horizontal positions the hydrostatic pressures on the end and side walls of
an elongated cell alter slightly, but it is not easy to see how these changes
could act as the stimulating actions regulating geotropic curvatures, nor how
they could mechanically affect growth as Sachs suggested 6. The maximal
differences of hydrostatic pressure in the longest root-cells are extremely
small, and in virtue of Weber's law they cannot possibly produce any
1 Traube, Bot. Ztg., 1875, p. 67: cf. Pfeffer, Osmot. Unters., 1877, p. 215 ; Cisielski, Cohn's
Beitrage z. Biologic, 1872, Bd. n, Heft 2, p. 23.
2 Dutrochet, Ann. sc. nat., 1833, ire ser., T. xxix, p. 413 ; Me"moires, etc., Bruxelles, 1837,
p. 292.
3 Hofmeister, Jahrb. f. wiss. Bot, 1863, Bd. in, p. 178.
* Pfeffer, Period. Bewegungen, 1875, p. 147.
5 Berthold, Protoplasmamechanik, 1886, p. 73; Noll, Heterogene Induction, 1892; Nemec,
Ber. d. bot. Ges., 1900, p. 241 ; 1901, p. 310 ; Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 80; Ber.d.
bot. Ges., 1902, p. 339 ; Haberlandt, Ber. d. bot. Ges., 1900, p. 261 ; 1902, p. 189 ; Jahrb. f. wiss. Bot.,
1903, Bd. xxxvm, p. 447. A summary is given by Jost, Biol. Centralbl., 1902, Bd. xxn, p. 161.
6 Pfeffer, Period. Bewegungen, 1875, p. 149.
224 TROPIC MOVEMENTS
perceptible excitation, since they merely add to or subtract from the
enormously greater internal osmotic pressure1. Noll has recently sug-
gested that the centrosphere with its centrosome may act as the percipient
organ for detecting the direction of gravitational stimuli, but the fact that
the centrosphere and centrosome do not appear to be permanent organs of
the cells of flowering plants suffices to show the danger of putting forward
hypotheses unsupported by experimental evidence 2.
Any local discrete pressure produced by the accumulation of the starch-
grains or other bodies might act as an internal stimulus, and the deforma-
tions and changes of configuration due to the rearrangement of the denser
and lighter particles might be equally effective. In the former case the
cell-mechanism of the plant would resemble that of the equilibratory organs
of certain animals 3. These possess statocysts or otocysts in which lie
dense particles, statoliths or otoliths, and the latter pressing on the under
inner surface of the otocyst excite sensory reactions directed towards the
maintenance of the normal position of the otocysts and of the organism.
Kreidl even found that the insertion of particles of iron in place of the usual
otoliths caused the organisms to orient themselves in regard to a magnet 4.
It is, however, not known whether the side walls of the otocyst are sensitive,
but not the ventral wall, or whether only the latter is irritable. In the former
case movement would follow until the otocyst experienced no excitation,
whereas in the latter case it would be directed towards the renewal of the
normal constant tonic stimulus radiating from the otocyst.
According to Nemec and Haberlandt, the excitation in plant-cells is
usually due to the starch -grains, although other bodies may become
effective in fungal hyphae and other organs possessing geotropic irritability
but devoid of starch-grains. No sure proof of such action has, however,
been brought forward as yet, for the fact that when a cell is reversed the
starch-grains fall from one end to the other with the required rapidity
merely shows that the supposed stimulatory action is not an impossible one.
Jost has, indeed, shown that none of the arguments put forward by Nemec
is conclusive, while Ne'mec's conclusion that the power of perception of
geotropic stimuli is restricted to the starch-bearing columella of the root-cap
is negatived by the fact that the excitable apical region is usually about
1-5 mm. long.
Haberlandt 5 found that the stems of certain plants which had become
1 Noll, Ber. d. hot. Ges., 1902, p. 425.
3 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 502 ; Ber. d. hot. Ges., 1902, p. 403.
3 Cf. Noll, Das Sinnesleben d. Pflanzen, 1896, p. 71 ; Bethe, Biol. Centralbl., 1894, Bd. xiv,
p. 95 ; Steiner, Centralbl. f. Physiol., 1898, Bd. xii, p. 775 ; Laudenbach, ibid., 1900, Bd. XIII,
p. 586; Ilyin, ibid., 1901, Bd. xiv, p. 361 ; Haberlandt, Ber. d. bot. Ges., 1902, p. 448.
* Kreidl, Sitzungsb. d. Wien. Akad., 1892, Bd. ci ; 1893, Bd. en.
5 Haberlandt, 1. c., 1902, p. 193 ; Jahrb. f. wiss. Bot., 1903, Bd. xxxvm, p. 447.
INSTANCES OF SPECIFIC TROPIC IRRITABILITY 225
free from starch after prolonged exposure to low temperatures also lost
their geotropic irritability, but regained it at favourable temperatures
simultaneously with the reappearance of the starch. Haberlandt supposes
that these observations afford definite proof of the function of starch-grains
as the agents for geotropic excitation, but it is quite possible that the
solution and regeneration of the starch might merely form accidental
accompaniments of the disappearance and restoration of the geotropic
irritability1. It has yet to be found whether the geotropic irritability of
starchless organs is similarly affected by low temperatures. The geotropic
irritability is modified by many factors, and Darwin found that the helio-
tropic reaction is also weakened at low temperatures, though to a less extent
than the geotropic one2. Irritability in general seems to be affected by
low temperatures, and it is quite possible that in certain cases a tropic
sensibility may only be fully restored some time after growth has been
resumed under renewed favourable conditions.
Haberlandt 3 found that the nodes of Trade scantia mrginica lost their
power of geotropic response when the cortex included the endodermis or
starch-layer, and concludes that the latter is the seat of geotropic perception.
The effect might, however, be the direct result of the injury inhibiting the
geotropic irritability, or removing tissue essential for the production of
a curvature 4. On the other hand, the fact that weak centrifugal action
incapable of producing any displacement of the starch-grains may act as
an excitation to curvature does not disprove Haberlandt's views, for the
starch may exert local pressure without being displaced 5. The short
period of presentation required during intermittent excitation to produce
a response affords no argument one way or the other. Gentle shaking,
which might be supposed to cause the starch-grains to exert a greater
contact stimulus, does actually accelerate the geotropic reaction6, but here
also other actions may be involved besides the apparent one. Naturally
also the ascent of air-bubbles or of oil-globules in the cell might act as an
excitation as well as the descent of the denser starch-grains.
A local accumulation of protoplasm such as might be responsible for
the geotropic excitation does not appear to be produced by the usual
intensity of gravity, or at least not in all plants. Nemec 7 observed that in
1 Additional arguments against Haberlandt's conclusions are given by Noll, Ber. d. bot. Ges.,
1902, p. 423.
2 F. Darwin, Proceedings of the Royal Society, 1903, Vol. LXXI, p. 362.
3 Haberlandt, Ber. d. bot. Ges., 1900, p. 269.
* See Jost, Biol. Centralbl., 1902, Bd. XXII, p. 174.
5 Cf. Jost, 1. c., 1902, Bd. xxn, p. 176; Haberlandt, Ber. d. bot. Ges., 1902, p. 191.
6 Haberlandt, 1. c., 1903, p. 489 ; Darwin, 1. c., 1903, p. 366.
7 Nemec, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 147 : cf. Jost, 1. c., p. 177. On the appear-
ance of certain minute bodies at the tips of the rhizoids of Char a cf. Giesenhagen, Ber. d. bot. Ges.,
1901, p. 227 ; Jost, 1. c., p. 173 ; Ne"mec, Ber. d. bot. Ges., 1902, p. 351.
PFEFFER. Ill O
226 TROPIC MOVEMENTS
the cells of a displaced root the protoplasm accumulated at the points from
which the starch-grains had moved, but this was probably the direct result
of the displacement of the starch.
Jensen l supposes that, in the case of freely motile organisms, their
geotactic irritability is the result of their response to the differences of
pressure at varying depths, which enable them to orient themselves in
regard to the perpendicular. Jensen forgets, however, that the maximal
differences of pressure capable of affecting the organism at a given time are
exceedingly small. On the other hand, the gravitational acceleration is
only constant so long as the organism is moving with uniform velocity
along a straight path, which is never the case. Every time the velocity
changes, or the direction of motion alters, the organism experiences an
increase or decrease of the geotropic stimulus. These changes, though
relatively feeble, might well act as directive stimuli.
Owing to the subordination of the individual cells in each tissue or
organ their potential powers of sensation and response are not always fully
represented in every response. Hence when the growth in length of a
curving radial organ is accelerated on the convex side, retarded on the
concave one, and unaffected in the middle lamella, this does not justify
Noll's conclusion that a corresponding distribution of sensibility is involved
in the responsive cells2. A precisely similar distribution of the growth-
activity is shown in curving unicellular organs, and a tissue composed of
such cellular organs would undoubtedly show the same differences of
growth, for a tendency to curvature on the part of the individual cells can
only find external expression when the rate of growth of convex and
concave sides undergoes appropriate alteration.
It is therefore impossible to follow Noll, or even Nemec and Haber-
landt, in ascribing the realized reaction to the unequal distribution of
irritability in the individual cells, or in their radial and longitudinal walls.
Nor does it follow that the different cells of a Pandorina possess dissimilar
irritabilities because they are at varying angles with the incident rays when
the colony is phototactically oriented. In plagiotropic positions the starch-
grains collect at the lower corners of the cells, but this does not afford any
explanation of the plagiotropic irritability, as Nemec supposes3. There
can be no doubt that, as in the case of tendrils, each organ responds as
a whole to geotropic excitation, but the regulation of the individual cells
is probably an extremely complex phenomenon. Czapek 4 has attempted
to explain this regulation as being due to the pressures and stresses which
1 Jensen, Bot. Centralbl., 1893, Bd. LVI, p. 21.
3 Noll, Heterogene Induction, 1892, p. 31. Cf. also Jost, Biol. Centralbl., 1902, Bd. XXII,
p. 169 ; Haberlandt, Ber. d. bot. Ges., 1903, p. 470 ; Nemec, Ber. d. bot. Ges., 1902, p. 359.
3 Nemec, 1. c., 1901, p. 310.
* Czapek, Jahrb. f. wiss. Bot, 1898, Bd. xxxn, p. 236; Ber. d. bot. Ges., 1901, p. 123.
INSTANCES OF SPECIFIC TROPIC IRRITABILITY 227
the cells exert upon each other in virtue of their weight, tendency to
growth, and mode of union, but in rejecting these conclusions Noll and Jost l
have forgotten that Czapek was merely attempting to give a comprehensible
means of arriving at the required regulation.
There can be little doubt that, as in all vital phenomena, not only
the motory but also the sensory processes are connected with chemical
changes, and Czapek has, in fact, found that such changes do occur as
the result of tropic stimulation. Since they begin before any reaction is
shown they appear to be more or less directly related to the process of sensa-
tion. The change is evidenced by an increased reducing action upon an
alkaline solution of silver in the geotropically stimulated root-apex, as well
as by the reduced oxidatory action upon readily oxidizable reagents such
as guiacum. The change is propagated from the sensitive apex to the
elongating zones behind, reaches its maximum about the time curvature
begins, and then dies slowly away again, so that by -the completion of
the curvature the tissues are once more normal. Czapek's later researches
appear to show that the silver reduction is due to homogentisinic acid,
and that the latter is produced by the oxidation of tyrosin. Normally
the acid appears to undergo further oxidation, which is, however, suspended
in the presence of antioxydase ferments 2. These are produced on
geotropic stimulation and are responsible for the accumulation of the
reducing substances in the cell. The latter might, however, equally well
be the result of an increased productive activity only indirectly connected
with the tropic stimulation.
Similar results have been obtained with hydrotropically stimulated
roots and also with the heliotropically stimulated seedling stems of a few
plants. The increased reducing action is not, however, produced by
diffuse illumination, or in roots from which the sensitive apex (i^ mm.) has
been removed, so that the result is due to tropic stimulation. It does not,
of course, follow that all plants will react in the same way, and to all
forms of stimulation ; but if these changes prove to be a constant accom-
paniment of tropic stimulation they may serve as indications of the
latter when the power of movement is absent, or when the stimulation
is not intense enough to excite it. Czapek found that the reducing
substances appeared in equal quantity on both convex and concave sides of
a curving root, so that the unequal distribution of growth appears to have
a different origin.
PHOTOTROPISM. Phototropic excitation is dependent not only upon
1 Cf. Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv,p. 465 ; Jost, Biol. Centralbl., 1902, Bd. XXXII,
p. 165.
2 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 208 ; Ber. d. bot. Ges., 1901, p. 122 ; 1902,
pp. 454, 464; 1903, pp. 229, 243. Cf. also Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 485.
3 On antiferments cf. Czapek, Ber. d. bot. Ges., 1903, p. 229.
Q a
228 TROPIC MOVEMENTS
a difference in the intensity of illumination, but also upon the direction
of the light rays, owing to the fact that it is only those rays of light which
penetrate the cells and tissues which operate as stimuli. Hence a beam
of light must exert a lessened stimulatory action when it falls at an
oblique angle to the surface of the plant, since less light will penetrate
and more be reflected. It is, however, impossible to say whether the
sensitive cells respond to the direction of the light rays or to their relative
intensities on different surfaces. In the case of tendrils the directive action
of the support is the result of the tendril's own activity in bringing fresh
surfaces into contact, while chemotropic, osmotropic, and possibly also
galvanotropie stimulation depend upon the distribution of differences of con-
centration rather than upon the direction of diffusion of stimulatory materials.
Sachs and Miiller1 concluded that the light rays acted as stimuli
in virtue of their direction, but without bringing any definite proof
forward2. The arguments of Darwin, Wiesner, and Oltmanns do not,
however, definitely show that only differences in the intensity of the
illumination act as stimuli 3. The results obtained by using angular prisms
filled with humic acid4, or indian ink and glycerine gelatine5, are incon-
clusive. A beam of light falling upon the plant after passing through
a prism so arranged that the intensity of the light is diminished at right-
angles to the direction of propagation has only to induce a slight curvature
of the plant to produce the same intensity of illumination on both sides6.
If the beam falls on the plant at an acute angle, a phototropic movement
occurs even when the prism is so placed that the plant must curve towards
the less bright portion of the beam. The same occurs in the case of
freely motile organisms, whose direction of locomotion is in fact determined
by the phototropic orientation of the body. Strasburger performed ex-
periments of this kind with zoospores, and Oltmanns with freely motile
and rooted plants. Similar results are obtained with organisms creeping
on a substratum and exposed to an oblique beam. Many motile lower
animals can also be induced in the same way to move towards regions
where the illumination is feebler if the beam is so arranged that the
feebler portion of the beam is towards its source 7. If a plant is directed
towards a strong source of illumination, feeble light falling at right angles
1 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1880, Bd. n, p. 487 ; H. Miiller, Flora, 1876, p. 92.
2 Cf. Pfeffer, Osmotische Untersuchungen, 1877, p. 213; Unters. a. d. hot. Inst. zu Tubingen,
1884, Bd. i, p. 478.
3 Darwin, The Power of Movement in Plants, 1881, p. 398 ; Wiesner, Bot. Ztg., 1880, p. 456 ;
Oltmanns, Flora, 1892, p. 183.
4 Cf. Strasburger, Wirkung d. Lichtes und der Warme auf Schwarmsporen, 1878, p. 35.
5 Oltmanns, Jahrb. f. wiss. Bot., 1892, Bd. xxm, p. 416; Flora, 1892, p. 183.
6 Cf. Pfeffer, Pflanzenphysiol., i. Aufl., 1881, Bd. n, p. 373; Elfving, Die photometr. Bewe-
gungen d. Pflanzen, 1901 (Ofvertryck af Finska Vet. Soc. Forhandlingar, Bd. XLIII).
7 Cf. Nagel, Bot. Ztg., 1901, Abth. ii, p. 289, and the literature there quoted.
INSTANCES OF SPECIFIC TROPIC IRRITABILITY 229
to it will here also produce a curvature towards the weaker light. Darwin l
found that when one side of a plant was smeared with indian ink the
plant curved away from that side in diffuse light owing to the fact that
more light penetrated on the unsmeared side. Even this experiment,
however, does not afford sure proof that the difference in the intensity,
and not the direction of the light-rays, acts as the orienting stimulus.
It is quite possible that light may induce chemical changes or
variations of surface-tension capable of acting as stimuli, but it is by no
means certain whether Loeb 2 is correct in ascribing the phototactic
movements of animal organisms to the direct action of changes of surface-
tension produced by light. Quincke 3 has recently observed that the pre-
cipitations produced by alkaline carbonates in solutions of calcium salts turn
towards the light, so that light may exercise a direct physical orienting action.
No protoplasmic aggregation or displacement has as yet been established as
a precedent to phototropic or phototactic response. Vines supposed that
light directly depressed the motility of the protoplasm, while Wiesner
supposed that it increased the power of stretching in the cell-walls of
the illuminated sides, but neither of these hypotheses has any value as
an explanation of heliotropism4. Similarly, historical interest alone
attaches to de Candolle's5 view that the curvature towards light is due
to the partial etiolation of the shaded side. Organs which are not etiolated
in darkness are, however, capable of heliotropic reaction, while negatively
heliotropic organs may grow more rapidly in darkness ; and in this case
it is the exposed side which grows more rapidly during heliotropic
curvature. Further, when the zones of perception and action are some
distance apart the curvature may take place when the active zone is not
illuminated at all. Wolkoff6 assumed that negative heliotropism was
produced by the refraction and concentration of the light-rays in the tissues
upon the shaded side, so that this side was the more strongly illuminated
one; but this quaint idea is totally incorrect. In any case phototropism
and phototaxis are simply general terms for orienting movements produced
by light, and it does not follow that precisely the same irritability and
mpde of response are involved in all cases. Yerkes7 has suggested the
term ' photopathy ' for orienting movements due to differences of illumina-
1 Darwin, The Power of Movement in Plants, 1881, p. 398.
2 Loeb, Einleitung in d. vergleichende Gehirnphysiologie, 1899, p. 128 : cf. Nagel, Bot. Ztg.,
1901, p. 294.
8 Quincke, Annal. d. Physik, 1902, Folge iv, Bd. vn, p. 742.
* Vines, Arb. d. bot. Inst. in Wurzburg, 1878, Bd. II, p. 145 ; Wiesner, Heliotropische Er-
scheimmgen im Pflanzenreiche, 1880, Bd. II, p. 21 : cf. also Godlewski, Bot. Ztg., 1879, ?• JI3'
5 A. P. de Candolle, Physiologic ve"getale, 1832, T. ill, p. 1083.
6 See Hofmeister, Pflanzenzelle, 1867, p. 293 ; Sachs, Lehrbuch d. Botanik, 1874, 4. Aufl.,
p. 810.
7 Cf. Nagel, Bot. Ztg., 1901, Abth. ii, pp. 291, 298.
230 TROPIC MOVEMENTS
tion, while ' photocliny ' might be used to indicate responses due to the
direction of the incident rays, but the terms are premature at present.
CHEMOTROPISM. Chemotropic stimulation is dependent upon the
direction of diffusion in so far as the latter produces the differences of
concentration to which the organism responds1. It is, however, uncertain
whether the stimulatory substance must actually penetrate, or whether
the mere contact with the ectoplasmic membrane produces the chemical
action, or modification of surface-tension, which forms the first stage of
perception. Many strong excitants do not appear to penetrate the proto-
plast, or at least do so with difficulty, but an apparent impermeability
may allow of the penetration of traces of the substance sufficient to excite
internal stimulation. Even when the substance readily penetrates, the
stimulation may occur either during or after absorption.
OSMOTROPISM. The maximal osmotic action is exercised by imper-
meable substances, which may also be expected to exert the greatest
osmotropic action. It is, however, uncertain whether the tropic stimulus
is due to the unequal withdrawal of water, to the movement of water
through the cell, to the osmotic pressures, or to surface-tension. It is
also possible that a readily penetrating substance might exercise a tropic
excitation, for any unequal distribution in the external medium will also
be produced in the cell. A variety of observations upon freely motile
organisms seem, however, to show that readily penetrating substances
exercise little or no osmotropic action. Since, however, osmotaxis may
arise in more than one way, it is possible that organisms may exist which
are especially responsible to readily penetrating substances.
PART IV
THE MECHANISM OF TROPIC MOVEMENT
SECTION 52. The Progress and Mode of Movement.
All tropic curvatures produced by the aid of growth naturally cease
to be performed when the power of growth is lost, whereas the presence
of pulvini capable of variation movements renders possible various tropic
responses in adult organs2. It is, however, not known whether pulvini
may possess other tropic irritabilities in addition to those of geotropism and
heliotropism, although no heliotropic variation curvatures appear to occur
1 For details see Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1888, Bd. II, p. 650; 1884, Bd. i,
P- 475-
2 Pfeffer, Periodische Bewegungen, 1875, p. 63. On the pulvini of Marantaceae cf. Schwen-
dener, 1896, Gesammelte Abhandlungen, Bd. II, pp. 203, 210; Debski, Anzeiger d. Akad. d.
Wiss. in Krakau, Juli 1895.
THE PROGRESS AND MODE OF MOVEMENT 231
in unicellular organs. Any awakening of growth will naturally restore
the power of nutation curvature, and it is for this reason that grass-stems,
which have ceased to grow while erect, perform an upward geotropic
curvature when placed horizontally1. That two reactions are involved
is shown by the fact that on the klinostat, when the action of gravity is
uniformly distributed, no geotropic curvature is produced, whereas the
awakening of growth2 enables a heliotropic response to be made to
unilateral illumination.
According to Barth3, the stem-nodes of Dianthus bannaticus behave
similarly, while Miehe4 found that the adult nodes of Tradescantia
fluminensis remained capable of geotropic response. In most cases, however,
the heliotropic and geotropic irritabilities appear to be lost with the normal
cessation of growth. Before this happens tropic stimulation may often
cause a more or less marked acceleration of growth, such as is also shown
when those parts of tendrils where growth has fallen to a minimum are
subjected to contact stimu-
lation. Leaves, especially
when they possess pulvini,
may, however, remain capable
of heliotropic and geotropic
response for weeks or months
after the leaf appears to be
fully grown 5. Preuss even
found that a leaf of Codiae-
um Wendlandi eight months
Old remained Capable Of re- FIG. 44. Portion of haulm of TrUicutn vulgare showing the
_ T , . , . or eotropic curvature produced twenty-four hours after it had been
action. Ultimately the power placed in a horizontal position.
of reaction is lost in all
cases, and even in the nodes of grasses the power of renewed growth is
not indefinitely retained. The total amount of growth is in all cases
limited, and hence a grass-node can only perform one or two geotropic
curvatures. The production of two successive curvatures in opposed
directions appears, however, to result in a greater total growth than when
the unilateral action of gravity is eliminated on the klinostat.
Usually the power of geotropic curvature is restricted to the normal
1 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1872, Bd. I, p. 204; de Vries, Landw. Jahrb., 1880,
Bd. ix, p. 473; Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 390; Barth, Die geotropische
Wachsthumskriimmung der Knoten, 1894, p. 30.
2 Elfving, Ueber das Verhalten d. Grasknoten am Klinostat, 1884 (Ofvertryck af Finska
Vetenskaps Societetens Forhandlingar, Bd. xxvi) ; Barth, 1. c., p. 33.
3 Barth, 1. c., p. 27. 4 Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvil, p. 532.
6 Mobius, Festschrift f. Schwendener, 1899, p. 40; Preuss, Die Beziehungen zwischen dem
anat. Bau und d. physiol. Function d. Blattstiele und Gelenkpolster, 1885. Cf. also Frank, Die
natiirl. wagerechte Richtung von Pflanzentheilen, 1870, p. 50.
232 TROPIC MOVEMENTS
growing zone l, the lengths of the growing and curving zones corresponding.
It is, however, possible that the power of curvature may be temporarily
retained by the zones which have just ceased to grow, although Kohl's2
experiments do not suffice to show that this is a common phenomenon.
Woody twigs of Aesculus^ Tilia^ and other plants of one or more years'
age, and which have long ceased to grow in length, may still remain
capable of slow geotropic curvature when displaced from their normal
position3. Apparently the geotropic excitation awakens a corresponding
tendency to growth in the cambium and younger tissues, the energy of
which is sufficient to produce a gradual bending of the inactive and woody
parts. The existence of any power of geotropic reaction in the adult
petioles of Hedera helix4" is disputed by Frank5, and doubt also attaches
to Hofmeister's statement6 that the adult petioles of Hedera and adult
portions of the roots of Ranunculus aquatilis are capable of heliotropic
curvature. Errera's 7 statement that the trunks of large trees may perform
geotropic curvatures does not require discussion, since to produce the
required bending moment the cambium would need to develop pressures
of several hundred atmospheres to compress and extend the inactive tissues.
Every tropic curvature naturally depends, not only upon the nature of
the excitation, but also upon the plant's power of reaction. Hence etiolated
stems are usually capable of more rapid geotropic curvature than normal
ones, owing to their more rapid rate of growth8. Hence also growth-
curvatures appear earlier in the more rapidly growing zones than in the
older ones even when the same degree of excitation is assured in both cases.
Other factors come into play, however, in determining the further progress
and final character of the curvature. Among these are included the
mechanical resistance to curvature and the counteraction excited by its
realization, as well as the altered orientation of the organ in its new position
and the changes of the power of reaction and response with the progress of
development. Thin organs will naturally curve more rapidly than thick
I Sachs, Flora, 1873, p. 324; H. Miiller, Flora, 1876, p. 65; Wiesner, Bewegungsvermogen der
Pflanzen, 1881, p. 45 ; Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. VII, p. 152. In the case of
fungi and rhizoids cf. Haberlandt, Oesterr. bot. Zeitschr., 1889, p. 3 of reprint ; Zacharias, Ber. d. hot.
Ges., 1890, Generalvers., p. 57; Flora, 1891, p. 489; Oltmanns, Flora, 1897, p. 9; Steyer, Reiz-
kriimmungen bei Phy corny ces, 1901, pp. 6, 25.
II Kohl, Mechanik d. Reizkriimmungen, 1894, p. 13. Cf. Rothert, Biol. Centralbl., 1895,
Bd. xv, p. 596.
3 Vochting, Organbildung im Pflanzenreiche, 1884, Bd. n, p. 85 ; Frank, Lehrbuch d. Botanik,
1892, Bd. i, p. 470 ; Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxm, p. 363, footnote ; Jost, Bot.
Ztg., 1901, p. 20 ; Baranetzsky, Flora, 190-1, Ergzbd., pp. 202, 213; Wiesner, Sitzungsb. d. Wien.
Akad., 1902, Bd. cxi, Abth. i, p. 796.
4 Hofmeister, Pflanzenzelle, 1867, p. 285; Bot. Ztg., 1869, p. 95.
6 Frank, Bot. Ztg., 1868, p. 644. e Hofmeister, Pflanzenzelle, 1867, P- 289.
7 Report of British Association. Cambridge, 1904.
8 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, p. 7 ; H. Miiller, Flora, 1876, p. 91 ;
Darwin, The Power of Movement in Plants, 1881, p. 493.
THE PROGRESS AND MODE OF MOVEMENT
233
ones, granted that the differences in the rate of growth on the opposed
sides are the same in both cases, while the weight of the organ will favour
or retard curvature according to the direction of the latter in regard to
gravitational attraction.
When a radial shoot is placed in a horizontal position its negatively
geotropic upward curvature l begins first in the more actively growing zone,
so that the curvature does not exactly follow the arc of a circle. As the
apex curves upwards it is more and more withdrawn from the stimulating
action of gravity, but nevertheless it curves beyond the vertical, partly
owing to the persistence of the geotropic induction and partly because the
lower zones are still inclined to the perpendicular and hence continue to
curve. The apical region then performs a return curvature by which it
becomes straightened after one or more oscillations 2. Ultimately only the
basal portion remains curved although the reaction began latest in this
region, and only pro-
gressed slowly in it. It
is, however, by no means
surprising that in many
cases the excess curva-
ture and resultant oscilla-
tion should not in all
cases be perceptible.
Changes of position
produced by torsion are a
readjusted in a similar
manner. Thus when
young leaves of Fraxi-
HUS. Robinia. and Other Fl.G> 45- Shoot of Impaiiens glanduligera showing phases of geo-
tropic curvature (a-e). From photographs.
plants are turned upside
down, the orienting torsion begins first at the tip of the leaf and then
progresses basally, so that the apex passes beyond the appropriate
position and is caused to perform a return torsion3. Since the torsion
and retorsion progress basally, the twisting curvature is ultimately
restricted to the basal zone, as can easily be seen in the leaves on droop-
ing branches of the Ash and Weeping Willow, which must curve through
1 80° in order that the upper and under surfaces may gain their appropriate
1 Cf. Sachs, Flora, 1873, p. 324 ; Arb. d. hot. Inst. in Wiirzburg, 1873, Bd. I, p. 453 ; Bd. Ill,
Plates; H. Miiller, Flora, 1876, p. 88 ; Kohl, Mechanik d. Reizkriimmungen, 1894, p. n; Rothert,
Cohn's Beitrage z. Biologic, 1896, Bd. vn, pp. 161, 210; Meischke, Jahrb. f. wiss. Bot., 1899,
Bd. xxxnr, p. 338. On the cinematographic representation of curvature see Pfeffer, Jahrb. f. wiss.
Bot., 1900, Bd. xxxv, p. 741.
2 Baranetzsky, Flora, 1901, Ergzbd., pp. 145, 159.
8 Schwendener and Krabbe, 1892 (Schwendener's gesammelte Abhandlungen, Bd. II, p. 288).
234
TROPIC MOVEMENTS
positions. A similar progress of geotropic torsion or curvature may be
shown by the stalks of flowers.
Although the curvature usually begins first in the more actively
growing zones exceptions may occur. Thus, when the tip only is irritable,
as in the cotyledons of Avena^ the curvature begins first in the regions
bordering upon it, and later in the further removed most actively growing
zones 1. This is, however, the natural result of the slow transmission of
the tropic stimulus, and similarly geotropic curvature begins first just
behind the percipient apex 2, although shortly afterwards the curvature
is most marked in the most actively growing zone a little further away
from the apex (Fig. 46, B). Later still, the curvature is transmitted basally,
while the zones 2 and 3 (Fig. 46, C) which
have elongated most have nearly become
straight again3. These facts were correctly
interpreted by Frank4, whereas Hofmeister5
erroneously concluded that no curvature took
place in the most actively growing zones.
Similar relationships were found by Sachs,
Miiller, and Rothert to exist in the case of
heliotropic organs, for here also the whole
growing zone appears to be capable of curva-
ture. According to Wiesner, the basal grow-
ing portion of seedling-stems does not react
heliotropically, but merely shows a mechanical
bending due to the weight of the curving
portion above 6. Rothert has, however, shown
FIG. 46. Seedlings of Lupinus albus r
showing geotropic curvature. The hod- that this is not the case, and that all the grow-
zontally-placed radicle in A has its ter-
minal ten millimetres marked, and after ing zones are capable of heliotropic response.
three hours has curved as in fft and after &
eight hours as in c Presumably the same applies to all forms of
tropic curvature, although further investigation is needed in this direction 7.
The power of tropic reaction is, however, not always localized in the
most actively growing zones, as is shown by the existence of variation-
movements, and by those nodes in which the awakening of growth is due
to the tropic stimulus. In addition, the amount of the reaction depends
1 Darwin, The Power of Movement in Plants, pp. 421, 477 ; Rothert, Cohn's Beitrage z. Biologic,
1896, Bd. vir, pp. 163, 2ii.
3 Cf. Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 361.
3 For details see Sachs, Arb. d. bot. Inst. in Wiirzburg, 1874, Bd. I, pp. 440, 454, 612;
Cisielski, Cohn's Beitrage z. Biologic, 1872, Bd. I, p, 4; N. J. C. Miiller, Bot. Ztg., 1869, p. 390.
* Frank, Beitrage z. Pflanzen physiologic, 1868, p. 10.
5 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 96.
6 Wiesner, Das Bewegungsvermogen d. Pflanzen, 1881, p. 45. Cf. Rothert, 1. c., pp. 141, 152.
7 On traumatropism cf. Pollock, Botanical Gazette, 1900, Vol. xxix, pp. 17, 50; on rheo-
tropism, Jnel, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 530.
THE PROGRESS AND MODE OF MOVEMENT 235
upon the degree of irritability, and the latter may not be fully developed
in the most actively growing zones, or may be entirely absent from them
even when the powers of perception and reaction are not localized. The
maximum irritability appears commonly to be attained by nodes after the
grand period of growth has been passed, and this has been definitely proved
to be the case in the nodes of Tradescantia by Earth and Kohl ], while the
nodes of Dianthus bannaticus, and of a few grasses, only acquire their
special geotropic irritability after their normal growth has ceased. It is
owing to changes in the distribution of the altered irritability that the
positively heliotropic curvature of the stem of Tropaeolum majus takes place
mainly in the zone of most active growth, whereas the negatively heliotropic
curvature is performed by the older but still growing regions. The fact
that in other cases the heliotropic curvature begins in the most actively
growing zone affords no evidence of the existence of two special kinds
of negative heliotropism as suggested by H. Miiller 2. Since tropic
irritability is always lost beyond a certain stage of development, it is
possible that in certain cases it may disappear before growth in length
has ceased, although in all the plants hitherto examined the whole growing-
zone remained irritable.
In the case of the haulms of grasses, two or more nodes co-operate in
producing the geotropic upward curvature of a horizontally-placed stem,
since the internodes are inactive, and a single node is unable to curve
sufficiently to make the stem erect. Other plants which possess motile
nodes behave similarly, the geotropic response being performed mainly
or entirely by the nodes. The special geotropic irritability of the nodes
of Mercurialis was observed by Bonnet 3 a century and a half ago, but the
general nature of the phenomenon was only established by the researches
of de Vries 4 and later authors, while Wiesner 5 has investigated the helio-
tropic irritability of the nodes of certain plants.
The rapidity of reaction. This is most pronounced in the case of
sensitive tendrils, for they may perform a considerable curvature in a few
minutes when thigmotropically excited. It takes one or more hours for
a thin actively growing stem to become erect when geotropically excited,
while thicker or less irritable stems may require one or more days to attain
1 Earth, Die geotropischen Wachsthumskriimmungen d. Knoten, 1894, p. 19; Kohl, Mechanik
d. Reizkriimmnngen, 1894, p. 21.
2 H. Miiller, Flora, 1876, pp. 70, 93.
3 Bonnet, Nutzen d. Blatter, 1762, p. 68.
* De Vries, Landw. Jahrb., 1880, Bd.ix,p.473 ; Riitzow, Bot. Centralbl., 1882, Bd. IX, p. 81 ;
Briquet, Monographic du Genre Galeopsis, 1893, p. 60; Barth, Die geotropischen Wachsthums-
krummungen der Knoten, 1894; Kohl, Bot. Ztg., 1900, p. i (Tradescantia) ; Westermaier, Ueber
gelenkartige Einrichtungen an Stammorganen, 1901 ; Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvil,
P- 527 (Tradescantia).
3 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, p. 32.
236 TROPIC MOVEMENTS
the same end 1. The reaction begins at first slowly, then attains a maximum
rapidity, and slowly decreases again. At the same time the apex extends
regularly or in jerks, and describes a simple or complicated curve in space
according to circumstances 2.
A tendency to curvature can naturally only find expression when it is
able to overcome the internal and external resistance. A rise of the
internal resistance due to the production of wood or sclerenchyma will
render the active tissues capable of only feeble curvature or of none at
all. Similarly, by determining the exact external resistance required to
prevent curvature a measure is obtained of the energy of movement 3. The
latter is considerable in all movements produced by heterauxesis, and hence
a horizontal shoot is able to overcome a considerable statical moment
in curving upwards. To prevent movement the statical moment due
to the organ's own weight usually needs to be increased from four to thirty
times 4, so that under normal conditions the plant works with a considerable
margin of safety. Usually also the rapidity of curvature is not affected by
fractional increases of the normal statical moment or even by doubling it 5.
Exactly the same applies to the influence of a resistance upon rectilinear
growth, and in both cases a relatively considerable increase of resistance is
required to lessen the rate of growth or curvature perceptibly. Similarly,
a man may climb a mountain as rapidly with a small load as with none
at all, whereas when heavily laden he must climb slowly in order to be
able to perform the greater work required.
No upward geotropic curvature is possible when a shoot is unable to
support its own weight, but nevertheless, as in the case of the hanging free
ends of the stems of climbers, the basal part bends mechanically downwards,
while the apex turns upwards. That is the natural result of the tendency
to upward curvature coupled with the fact that the statical moment at any
point is proportional to the length of free stem beyond it. Hence such
shoots assume a double curvature or S shape, such as may also be produced
in normally erect stems, when they are subjected to sufficiently intense
centrifugal forces 6.
The statical moment in the case of the basal growing zone of the peduncle of
the Hyacinth may amount to 6 kilograms, in that of the lowest nodes of a ripe stem
1 Cf. Sachs, Flora, 1873, p. 327 ; Darwin, The Power of Movement in Plants. On heliotropic
curvatures see H. Miiller, Flora, 1876, p. 88, and Wiesner, Die heliotropischen Erscheinungen, 1878,
Bd. I, p. 68.
9 Darwin, 1. c., pp. 495-512.
3 On dynamometers see Pfeffer, Period. Bewegungen, 1875, p. 9; Druck- und Arbeitsleistungen,
1893, p. 251 ; Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxm, p. 345.
4 Meischke, 1. c., p. 362. 5 Id., p. 364.
6 F. Schwarz, Unters. a. d. bot. Inst. zu Tubingen, 1881, Bd. I, p. 80. Cf. also Baranetzsky,
Flora, 1901, Ergzsbd., p. 186.
THE PROGRESS AND MODE OF MOVEMENT 237
of Barley to 5 kilograms, and to no less than 1 30 kilograms in the case of a Maize-
stem *. Meischke also measured the maximal pressure exercised by curving organs
against fixed resistances, and found that usually the basal nodes are able to take part
in the geotropic erection of the shoot. In the case of Avena, however, they only
begin to curve when the upward bending of the more apical portion has lessened the
statical moment exercised upon them. The internal resistance increases as the
curvature progresses, so that less external energy of movement is available, and
in the haulms of most Grasses complete erection requires the co-operation of
several nodes.
Thin tendrils naturally are incapable of exercising any pronounced pressure
when curving, and in the case of the stiffer tendrils of Bauhinia, Strychnos, Vanilla^
and other plants, a considerable internal resistance must be overcome before any
considerable external pressure can be exercised. Coiled hooks and tendrils which
undergo secondary thickening exercise sufficient pressure to strangulate the branches
they have clasped 2, and become extremely rigid, whereas relatively thin fruit- and
flower-stalks (Apple, Snowdrop, Fuchsia) are mechanically bent by the weight of the
organ they support 8. The peduncles of the Poppy are able to support the rather
heavy bud, and hence can perform active geotropic curvatures both negative and
positive in character. Most peduncles are, in fact, rigid enough to support the
flowers and flower-buds in any position, whereas the fruits, especially when succulent
and heavy, naturally tend to assume a more pendent character.
The downward curvature of the root is always an active one, although Knight 4,
Hofmeister 5, and more lately Saposchnikow 6 and Letellier 7, have considered it to be
a passive plastic bending produced by the root's own weight. This obsolete idea is,
however, sufficiently disproved by the fact that the root may curve against resistances
equivalent to more than its own weight8, and that it may curve downwards into
mercury against an upthrust of about ten times the weight of the part submerged 9.
A free root cannot exercise any great pressure owing to the readiness with which
it becomes laterally displaced, and because of the plastic properties of the growing
1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 395; Meischke, Jahrb. f. wiss. Bot., 1899,
Bd. xxxin, p. 337. On the capacity for work in geotropically excited pulvini cf. Pfeffer, Periodische
Bewegungen, 1875, p. 145.
2 Ewart, on Contact Irritability, Ann. du Jard. bot. de Buitenzorg, Vol. xv, 1898, p. 187.
3 Cf. Vochting, Die Bewegungen d. Bliithen u. Friichte, 1882, p. 192 ; Wiesner, Sitzungsb. d.
Wien. Akad., 1902, Bd. cxi, Abth. i, p. 744.
4 Knight, Phil. Trans., 1806, I, p. 104. Bazin appears, according to Duhamel, Naturgesch. d.
Baume, 1765, Bd. II, p. 109, to have attempted a similar explanation.
5 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. ill, p. 102 ; Bot. Ztg., 1868, p. 273, and 1869, p. 57.
Wigand suggested (Botan. Unters., 1854, P- 3) that the downward curvature of the part was due to
the distensive enlargement of the cells on the under side, but Hofmeister has shown that this is not
the case (Jahrb. f. wiss. Bot., Bd. Ill, p. 80).
6 Saposchnikow, Bot. Jahrb., 1887, Bd. I, p. 225.
7 Letellier, Essai de statique ve"ge"tale, 1893.
8 Johnson, Linnaea, 1830, Literaturberichte, p. 148; Frank, Beitrage z. Pflanzenphysiologie,
1868, pp. 21, 35 ; N. J. C. Mtiller, Bot. Ztg., 1871, p. 719 ; Sachs, Arb. d. bot. Inst. in Wtirzburg,
1873, Bd. I, p. 450; Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 271; Wachtel, Bot. Centralbl.,
1895, Bd. LXIII, p. 309 ; Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxin, p. 366.
9 Sachs, 1. c., pp. 431, 451.
238 TROPIC MOVEMENTS
zones. The radicle of Vicia Fala may, however, develop a pressure of 13 grams at
its apex when in perpendicular contact, and one of 1.5 to 2-2 grams when the con-
tact is oblique *. The maximal pressure is naturally only obtained after a certain
length of time, and if the resistance is suddenly removed a rapid curvature due to
the released strains is produced. This is more pronounced in the case of variation
than of growth movements, since in the case of the latter the plasticity of the tissues
and the regulation of growth prevent the attainment of any pronounced strain, so that
the attempted curvature is only completed some time after the removal of the resistance
to it2. Hofmeister supposed that negative heliotropism required high, and positive
heliotropism low, tissue-strains, but these conclusions are based upon incorrect ideas
as to the importance of the strains in the tissues for tropic curvature, and there is no
evidence in support of his conclusions s.
SECTION 53. The Mechanism of Curvature.
Since the rigidity of the pulvinus of Phaseolus remains constant
when a negatively geotropic curvature is performed as the result of the
reversal of the plant, it follows that the expansive energy of the compressed
ventral side which is now uppermost must decrease in exactly the same
degree that the expansive energy of the under side increases 4. If this were
not the case a pronounced decrease of rigidity must ensue, since the force
of curvature may amount to a pressure of one to three atmospheres.
The plasmolytic experiments of Hilburg 5 showed in fact that the osmotic
pressure does actually fall in the upper side of a reversed pulvinus and rises
in the under half, the observed differences approximating to i per cent, of
potassium nitrate, which is amply sufficient to produce the required energy
of movement. The same takes place, according to Hilburg, during the
heliotropic curvature of the pulvinus of Phaseolus.
Thigmotropic growth-curvatures involve a pronounced transitory
acceleration of the average rate of growth, whereas, according to Sachs
and M tiller, the mean growth appears in many cases to be somewhat
retarded during heliotropic and geotropic curvature. In the case of the
nodes of grasses and of other plants geotropic curvature involves a pronounced
acceleration of the mean rate of growth, but it has not been determined
whether the growth is also more rapid than in the case of nodes in which
growth but not curvature has been excited by rotation on a klinostat.
The same question has also to be answered in the case of those nodes
which retain the power of slow growth when the stem is vertical.
1 Cf. Pfeffer, 1. c., p. 270.
3 Sachs, Flora, 1873, p. 207; de Vries, Sur les causes des mouvements auxotoniques, 1880,
p. 14 (reprint from the Archives Neerlandaises, Vol. xv) ; Pfeffer, Druck- und Arbeitsleistungen,
1893, p. 402.
8 Cf. Pfeffer, 1. c., 1893, p. 426.
4 Pfeffer, Periodische Bewegungen, 1875, PP- I4°> I45-
5 Hilburg, Unters. a. d. hot. Inst. zu Tubingen, 1881, Bd. I, p. 31.
THE MECHANISM OF CURVATURE 239
It depends upon circumstances as to whether the total length of the
concave side increases or decreases during curvature. A shortening of the
concave side always occurs during the variation movements of pulvini, and
usually also during the nutation curvatures of thick and slowly growing
organs, whereas the concave side may in some cases actually lengthen
during the curvature of stems and roots capable of active growth. This is
due to the fact that during the relatively slow progress of the reaction
the general elongation of the curving zone is sufficient in amount to be per-
ceptible. Hence rapid curvatures might be expected to produce a shorten-
ing of the concave side, and this is absent or hardly perceptible in tendrils
because the thigmotropic excitation simultaneously awakens a pronounced
general acceleration of growth. On the other hand, during geotropic and
heliotropic curvature, the convex side grows more rapidly than normally
in spite of the general retardation of growth. It is, however, possible that
organs may exist in which stimulation produces a retardation of growth on
all sides, the convex side being merely that in which growth is least
retarded.
Hofmeister 1 attached both ends of a straight piece of stem to the
under side of a horizontal sheet of glass. The resultant geotropic curvature
caused the concave side to be raised away from the glass, showing that
elongation had taken place on both sides. The same applies to heliotropic
curvature. Another method is to cover the surface with indian ink, the
cracks which appear showing that the geotropic curvature of some stems
involves an elongation of both sides, whereas in a grass-node only the
convex side elongates 2.
Sachs 3 placed marks of indian ink 2 mm. apart on roots and grew
them in various positions in loose earth behind glass plates. By means of
protractor scales marked on mica-plates, the radius of curvature and the
length of the marked segments of the concave and convex sides can be
determined. In the case of a vertical radicle of Vicia Faba the terminal
8mm. increased by 10-5 mm. in fourteen hours, and when placed hori-
zontally the root curved through an arc of 135°, the concave side becoming
6-1 mm. longer, and the convex 108 mm., so that the growth of the middle
lamella was 8-4 mm. The geotropic curvature hence involved an accelera-
tion of growth of 0-3 on the convex side, and retardations of 4-4 and 2-1 mm.
on the concave side and in the middle lamella respectively.
In the case of stems Sachs4 measured the elongation by applying
1 Hofmeister, Jahrb. f. wiss. Bot., 1863, B(*. HI, p. 86.
3 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 408.
8 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1873, Bd. I, p. 463; Noll, ibid., 1888, Bd. Ill, p. 507;
Macdougal, Botanical Gazette, 1897, Vol. xxm, p. 361.
* Sachs, Flora, 1873, p. 324; Arb. d. bot. Inst. in Wiirzburg, 1872, Bd. II, p. 193. No
measurements have been made of the curvature of unicellular organs.
24o TROPIC MOVEMENTS
paper measures, and observed in many cases only a slight retardation of
the average rate of growth, while more especially in thick and slowly
growing stems and peduncles a more or less pronounced shortening took
place on the concave side. Similar results were obtained by Barth with
nodes which show growth previously to geotropic excitation 1, whereas the
nodes of grasses always shorten considerably on the concave side, which
undergoes compression. Sachs2 observed that the thick nodes of cinquantino
Maize shortened from 4-3 to 2-5 mm. on the concave side during geotropic
curvature, and lengthened from 4-1 to 9-0 mm. on the convex side. The
more slender nodes of other grasses shorten but little on the concave side
when the curvature is moderately pronounced, so that the neutral axis,
which neither elongates nor contracts, lies near to the concave surface.
No cell- division accompanies the awakened growth of the nodes of
grasses, the individual cells increasing in size by stretching growth 3. The
same is shown during the geotropic curvature of roots, so that, as Frank
first observed, the cells are longer on the convex than on the concave side 4,
and this holds good even when the curvature is accompanied by cell-divi-
sion. When the concave side is compressed, as in pulvini and grass-nodes,
the diameter of the cells will in general tend to increase, but not, or only to
a slight degree, when growth is retarded without any compression. Kohl 5,
however, observed that during the geotropic curvature of stems the cells of
the concave side, and Noll that those of the convex side, attained a rela-
tively greater diameter, so that individual peculiarities may occur. The
varying growth of strips of equal length marked on straight and curving
stems corresponds to what might be expected, that from the concave side
being shorter and from the convex side longer than that from a stem in
which growth was rectilinear 6. According to Miiller 7, the altered rates of
growth in positively heliotropic stems and in negatively heliotropic aerial
roots during curvature correspond to those observed during geotropic
curvature, so that the same considerations may possibly apply to all forms
of tropic curvature produced by growth.
Each lamella assumes during growth a rate of growth proportionate
1 Earth, Die geotropischen Wachsthumskriimmungen d. Knoten, 1894, p. n.
3 Sachs, Arb. d. bot. Inst. in Wurzburg, 1872, Bd. I, p. 206; Pfeffer, Druck- u. Arbeits-
leistungen, 1893, p. 393; Earth, 1. c., p. 31. Pfeffer and Barth used microscopes with micrometer
eyepieces.
3 Sachs, 1. c., p. 207.
* Frank, Beitrage z. Pflanzenphysiologie, 1868, p. 40 ; Cisielski, Cohn's Beitrage z. Biologic,
1872, Bd. I, Heft 4, p. 18 ; Sachs, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 466; Macdougal,
Botanical Gazette, 1897, Vol. xxin, p. 364.
5 Kohl, Mechanik der Reizkriimmung, 1894, P« 5°« Cf. also Sachs, 1. c., pp. 462, 469;
Cisielski, 1. c., p. 18.
6 Noll, Arb. d. bot. Inst. in Wiirzburg, 1888, Bd. ill, p. 526.
7 Sachs, Arb. d. bot. Inst. in Wurzburg, 1872, Bd. I, p. 193; Experimentalphysiologie, 1865,
p. 507. Cf. also Frank, 1. c., p. 67.
THE MECHANISM OF CURVATURE 241
to its relative position, as is especially well shown when a stimulated node
of grass is cut into a series of parallel horizontal slices, and the growth of
each followed. In this case the splitting releases no disturbing tissue-
strains1, but even when these come into play positive results may be
obtained. Thus Sachs2 found that when a root, split into two equal
longitudinal halves which remained in contact, performed a positive geo-
tropic curvature, the upper half elongated more than the lower. When an
erect stem is split, the two halves curve apart owing to the released tissue-
strains, and as the result of their changed position each performs a nega-
tively geotropic curvature during which the under side of each half grows
more rapidly than the upper inner one 3. Similar experiments have been
performed by Hofmeister 4 with the stalks of Agaricineae and by Copeland 5
with the stems of seedlings. The latter found that a horizontally placed
segment in which a negatively geotropic curvature was produced grew
more rapidly than a vertical one when the cut surface was upwards, and
less rapidly when it faced downwards. Further investigation appears,
however, to be needed in this direction.
In any case longitudinal halves of stem and roots are capable of
geotropic curvature when placed horizontally, and the curvatures always
take place in the same direction independently of which side is placed
downwards, so that the curvature may either take place towards or away
from the cut surface. It follows, therefore, that in the intact organ as in
unicellular ones, correlative relationships determine the relative rate of
growth of the different parts, and these must even influence the growth of
the collenchyma strands in the nodes of grasses, since the tensions brought
into play are incapable of directly stretching them 6.
The conditions are naturally rendered more complicated by the fact
that the cells in a tissue are not all equally active and responsive, and
that inactive elements may be present which, when comparatively rigid,
may partially arrest or completely prevent an attempted curvature. Even
a realized curvature may involve the compression of cells which strive to
expand, as well as the regulation of the growth of some and the plastic or
elastic stretching of others. Actions of this character, although they may
influence curvature, do not induce it. Kohl7 supposed that geotropic
curvature was due to an active contraction of the tissues on the concave
side, but Rothert and Noll8 have shown the incorrectness of this supposition,
1 H. Muller-Thurgau, Flora, 1876, pp. 69, 92.
2 De Vries, Landw. Jahrb., 1880, Bd. ix, p. 483 ; Pfeffer, Druck- und Arbeitsleistungen, 1893,
pp. 394, 408 ; Sachs, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 470.
Sachs, Flora, 1873, p. 330.
Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 93.
Copeland, Botanical Gazette, 1900, Vol. xxix, p. 189. • Pfeffer, 1. c., pp. 401, 426.
Kohl, Mechanik der Reizkriimmungen, 1894, pp. 4, 40, 87.
Rothert, Biol. Centralbl., 1895, Bd. 15, p. 593 ; Noll, Flora, 1895, Ergzbd., p. 44.
PFEFFER. Ill
242 TROPIC MOVEMENTS
which in any case could not apply to the curvatures of unicellular organs.
Kohl supposed that the shortening of the cells was due to a rise of turgor
in them, of which we have no positive evidence, although certain growth
movements are actually produced by active contraction. The activity of
the convex side of a curving node of a grass-haulm is well shown by the
thickening and bulging it undergoes when curvature is mechanically pre-
vented *. Indeed, if the haulm is closely fitted in a glass tube the growth of
the under side may be so active in spite of the rectilinear direction enforced
upon it as to tear the upper side of the node 2.
In the case of the nodes of Triticum, Secale, and many other Grasses,
only the sheathing-leaf portion responds geotropically, the central portion
being passively bent, whereas in Zea Mays, Saccharum officinarum, both
the leaf and stem portions of the node are capable of perceiving and
reacting to geotropic stimuli 3. In Polygonaceae and Commelinaceae,
however, the irritability of the leaf-sheath is either slight or absent, so that
it is passively bent during curvature. Such cases make clear the fact that
the different tissues may not all be equally excitable and responsive, but
experiments with isolated tissues leave it uncertain whether the result
observed represents the actual part played by the given tissue in the intact
organ. Not only may the operation alter or inhibit the irritability of the
tissue, but also the removal of the correlating influence of the neighbouring
parts may produce a pronounced change of tone. In addition, tissues
capable of reaction but not of perception must always appear irresponsive
when isolated.
The removal of the epidermis or of the cells bordering upon it, as well
as the removal of the pith and even of the ring of vascular bundles, does
not suspend the power of geotropic reaction, whereas the isolated pith is in
many cases incapable of any geotropic response 4. According to Sachs 5,
the pith taken from geotropically curving stems straightens itself, so that
the permanent curvature ultimately assumed by the pith may be passively
impressed upon it. There is, however, no certain proof in a single case that
the pith is incapable of perception, but is able to actively respond to
geotropic stimuli transmitted to it6. In many cases, however, the cortex
of stems and stem-nodes, or portions of it, appears to be especially per-
1 Cf. Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 396 ; Noll, Arb. d. hot. Inst. in Wiirzburg,
1888, Bd. ill, p. 509 ; de Vries, Landw. Jahrb., 1880, Bd. IX, p. 483.
3 Pfeffer, Ber. d. Sachs. Ges. d. Wiss., 1891, p. 642.
3 Barth, Die geotropischen Wachsthumskrummungen der Knoten, 1894; Pfeffer, 1. c.,
pp. 390, 409.
* Sachs, Flora, 1873, p. 330; Barth, 1. c., p. 36; Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil,
p. 248 ; Haberlandt, Ber. d. bot. Ges., 1901, p. 269; NSmec, ibid., 1902, p. 339.
5 Sachs, Experimentalphysiol., 1865, P- 5^7- Cf. also Frank, Beitrage zur Pflanzenphysiol.,
1868, p. 73.
• Cf. Haberlandt, 1. c., p. 2'69.
THE MECHANISM OF CURVATURE 243
ceptive and responsive. Similarly the geotropic stimuli perceived by the
apex of the root appear to produce the most active response in the cortical
tissues1, and these, owing to their peripheral position, are more readily
capable of producing curvature than centrally placed ones.
Sachs2 found that the middle lamella cut out of a stem performed
a negatively geotropic curvature when placed horizontally with the cut
surface perpendicular, whereas when the cut surfaces face downwards or
upwards the results obtained vary and are often negative. The absence of
any curvature might possibly be due to the insufficient leverage exerted by
the thin slice of the cortex. Czapek found, however, that a horizontal slice
of the middle lamella of the hypocotyl of Helianthus annuus performed
a negatively geotropic curvature when the section was prepared after an
hour's previous geotropic excitation. This isolated observation does not
necessarily prove that the horizontally placed lamella is always able to
perform a geotropic response, but not to perceive geotropic stimuli.
Both irritability and the power of response change during development,
and all tissues which have lost the power of growth can only experience
a passive curvature. In addition, the less active tissues may be compressed
or stretched in accordance with their position in regard to the more active
ones. The latter applies to the nodes of grasses in which the originally
active parenchyma tissue on the convex side is ultimately ruptured by the
continued growth of the collenchyma strands 3. In this way the previously
compressed parenchyma is stretched, while the stretched collenchyma
becomes subject to compression. Evidently, therefore, the strains in the
tissues do not afford direct evidence as to the part each tissue plays in cur-
vature. In addition, every nutation curvature, and the tissue-strains to which
it gives rise, may co-operate in modifying the original growth-tendencies.
Even when the pith has no active power of curvature, its compression
may aid in producing curvature when this is once initiated, but apart from
this mechanical action the detailed changes of the tissue-strains during
the progress of heliotropic and geotropic curvatures fail to reveal the
mechanism of curvature 4. The anatomical differences between negatively
and positively tropic organs postulated by Dutrochet 6 were shown long ago
by Mohl6 to be non-existent. Dutrochet also erroneously supposed that
1 Macdougal, Annals of Botany, 1897, Vol. xxiil, pp. 346, 364.
2 Sachs, Flora, 1873, p. 330; Arb. d. hot. Inst. in Wiirzburg, 1873, Bd. I, p. 470; Czapek,
Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 350; Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 467 ;
Haberlandt, Ber. d. bot. Ges., 1901, p. 270; Jahrb. f. wiss. Bot., 1903, Bd. XXXVIII, p. 470;
Nemec, Ber. d. bot. Ges., 1902, p. 353.
8 Cf. Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 407.
4 Sachs, 1. c. ; Frank, 1. c. ; Hofmeister, Pflanzenzelle, p. 293; Kraus, Bot. Ztg., 1867, p. 129;
Ratschinsky, Ann. sci. nat, 1858, 3* se"r., T. IX, p. 172; Johnson, ibid., 1835, a6 s^r" T. IV, p. 327;
Pollock, Bot. Gazette, 1900, Vol. XXIX, pp. 25, 48.
5 Dutrochet, M^moires, etc., Bruxelles, 1837, pp. 322, 327.
8 Mohl, Vegetabilische Zelle, 1851, p. 141.
R 2
244
TROPIC MOVEMENTS
heliotropic response was due solely to a tendency to curvature of the concave
side, and the same objection applies to Kohl's view that geotropic curvature
is due to active contraction of the concave side.
SECTION 54. The Internal Causes of Movement.
Neither the mode in which the changes of turgor responsible for varia-
tion movements, nor that in which the altered growth of nutation curvatures
is produced, is precisely known. It is, however, certain that the changed
rates of growth are not due to alterations of turgor, as de Vries x supposed,
for, apart from the fact that no curvature could be produced in this manner
in unicellular organs, plasmolytic researches have shown that no rise of
turgor takes place during geotropic curvature 2. Beit's unfounded supposi-
tion that positive heliotropism is the result of the decomposition of sugar
on the illuminated side requires no discussion. In the case of rapidly
curving organs, a slight fall of turgor may actually take place in the cells
of the concave side, which apparently results from the rapid increase of
volume, water being absorbed in greater amount than the self-regulatory
production of osmotic materials is able to compensate for immediately.
Even when a general or unilateral rise of turgor accompanies a tropic
reaction, its relationship to the induced irregularity of growth is accessory
and not causal. Kohl's observations 3 do not prove that a rise of turgor
takes place in the cells of the concave side during geotropic response, and
Noll 4 has shown that a rise of turgor will not cause the cells of the concave
side to shorten. The turgor of the nodal parenchyma of Hordeum vulgare
rises by the equivalent of about I to 2 per cent, of potassium nitrate when
the stem is fixed in a horizontal position 5, but this is not in itself sufficient
to directly cause the growth of the cells, while no such rise is shown by the
nodal cells of Triticum vulgare and T. spelta, which are capable of as ready
and rapid geotropic response as those of Hordeum vulgare. In the same
way a rise of turgor is shown by some plants, but not by all, when working
against external resistance ; and although such rises act as an aid to growth
they do not directly induce it.
Although the mechanism of growth need not always be the same,
the required expansion is usually produced by a plastic stretching of the
cell-wall. Evidence of this is afforded by the fact that during curvature
De Vries, Landw. Jahrb., 1880, Bd. ix, p. 502.
Wortmann, Ber. d. hot. Ges., 1887, p. 961 ; Bot. Ztg., 1889, P- 456 > Noll> Arb- dt bot- Inst-
in Wiirzburg, 1888, Bd. Ill, p. 511 ; Flora, 1895, Ergzbd., p. 36.
Kohl, Mechanik der Reizkriimmungen, 1894, p. 59.
Noll, Flora, 1895, Ergzbd., pp. 48, 54.
Pfeffer, Druck- und Arbeitsleistungen, 1893, pp. 399, 405.
THE INTERNAL CAUSES OF MOVEMENT 245
the thickness of the walls of the epidermal and collenchyma cells decreases \
and often to a considerable extent, while the walls of the same cells on
the concave side frequently become distinctly thicker. According to
Wortmann2, the cell-walls become very much thicker on the upper sides
of shoots placed horizontally, and prevented from curving upwards by an
attached weight. Elfving 3 found that a similar thickening was produced
in the cells of the convex side when a shoot was strongly bent and fixed in
this position. Since the same result is produced on a klinostat, it must
be the direct result of the altered strains, whereas in Wortmann' s experi-
ment it probably results from the inductive action of gravity. Evenly
distributed longitudinal strains do not appear to produce any increased
thickening of the cell- walls4, but where the strains are always unevenly
distributed, as in the curved hooks of many tropical climbers, a pronounced
effect may be produced 5.
The fact that the changes in the thickness of the cell-wall only appear
during the curvature shows that they are the result and not the cause of it,
as Wortmann supposes 6. Since normally the distension of the walls lies
within their limit of elasticity, the plastic growth of the cell-wall must
be preceded by a physiological diminution of the cohesion of the component
cellulose micellae. At the same time, the elasticity of the cell-walls on
the convex side appears to be so modified as to allow of an elastic
lengthening of the cells without any rise of turgor. The curvature produced
in this way is reversible by plasmolysis until it has been followed up and
fixed by growth. A combination of growth and variation movement is
also shown by the young growing pulvini of Phaseolus, which, when adult,
still remain capable of variation movements. A few days after a plant has
been inverted, and the pulvini have performed a geotropic variation
curvature, a certain amount of growth takes place in the inverted and
unusually elongated dorsal sides of the pulvini7. The result depends to
some extent, therefore, upon the nature and duration of the stimulus ; and,
according to Mobius 8, the heliotropic curvature of the pulvinus of Maran-
taceae is rapidly rendered permanent by growth.
Frank 9 showed that the persistence of a completed curvature when turgor was
1 Noll, Arb. d. bot. Inst. in Wtirzburg, 1888, Bd. ill, p. 526; Flora, Ergzbd., 1895, p. 73;
Wortmann, Ber. d. bot. Ges., 1887, P- 4^3; Bot. Ztg., 1887, p. 808 ; 1888, p. 469; Kohl, I.e.,
p. 36 ; Macdougal, Botanical Gazette, 1897, Vol. xxill, p. 364.
Wortmann, Bot. Ztg., 1837, p. 824.
Elfving, Zur Kenntniss d. Krtimmungserscheinungen, 1888 (Ofvertryck af Finska Vet. Soc.
Forhandlingar, Bd. xxx).
Ball, Jahrb. f. wiss. Bot., 1903, Bd. XXXIX, p. 305.
Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, p. 190 seq.
Cf. Noll, Flora, Ergzbd., 1895, p. 38.
Pfeffer, Periodische Bewegungen, 1875, p. 139.
Mobius, Festschrift fur Schwendener, 1 899, p. 60.
Frank, Beitrage z. Pflanzenphysiol., 1868, p. 97.
246 TROPIC MOVEMENTS
removed by plasmolysing solutions was a sure indication that the heliotropic and geo-
tropic curvatures were produced by unequal growth. De Vries l then found that
curvatures which had just begun could be partially removed by the action of 20 per
cent, solutions of salt, so that the primary curvature is due to elastic stretching, which
is rapidly followed up by growth. De Vries, however, erroneously assumed that this
was due to a rise of turgor on the convex side, whereas direct observation affords no
evidence of any such rise. According to Noll 2, the primary curvature is due to an
increased extensibility of the cells of the convex side, but at the same time it is
possible that the thickening of the wall on the concave side may render this part less
capable of extension although the energy of turgor may be the same throughout.
In parts which had undergone positively heliotropic curvature, Weinzierl3 found
that the epidermis of the concave side possessed a higher breaking strain and limit
of elasticity than the epidermis on the convex side, but it is uncertain whether this is
due to changes in the properties or to unequal thickening of the respective cell-walls.
Wiesner * supposed that positive heliotropism is due to a rise of elasticity on the
shaded side, and to an increase of ductility in the cell-walls, and of turgor in the cells of
the illuminated side. Hofmeister 5 also seems to have considered that changes of elastic
extensibility took part in the production of heliotropic and geotropic curvature, but
his conclusions are vitiated by physical errors and by his inability to discriminate
between the results of growth and of strain.
In some cases no straightening of the curvature can be produced in young organs,
and it is not known whether the same effect is given by suddenly killing the geo-
tropically or heliotropically curving organs as by plasmolysis. To produce the
latter often requires a considerable time, during which readjustment may occur 6.
Slowly curving hooks or tendrils, when suddenly killed during curvature, show no
perceptible straightening7. Both plasmolysis and death by heat or the action of
alcohol may cause a withdrawal of water from the cell-wall, and so produce contrac-
tion or compression which does not exist in the intact organ. Correns 8 found, in
fact, that when curving tendrils were dropped into alcohol the curvature increased,
whereas straightening was shown when the dead dehydrated tendril was returned to
water.
Noll 9 found that the same force produced a greater bending when applied to
a stem in the direction of an incipient geotropic curvature than when opposed to it,
1 De Vries, Landw. Jahrb., 1880, Bd. ix, p. 302. Cf. also Wiesner, Die heliotropischen
Erscheinungen, 1880, Bd. II, p. 3 ; Noll, Arb. d. hot. Inst. in Wiirzburg, 1888, Bd. in, p. 516 ; Flora,
1895, Ergzbd., p. 82 ; Barth, Die geotropischen Wachsthumskriimmungen der Knoten, 1894, p. 12 ;
Kohl, Mechanik der Reizkriimmungen, 1894, p. 67.
8 Noll, I.e., 1 888 and 1895.
8 Weinzierl, Sitzungsb. d. Wien. Akad., 1877, Bd. LXXVI, Abth. i, p. 434.
4 Wiesner (1. c., 1880, Bd. II, p. 20).
5 Hofmeister, Jahrb. f. wiss. Bot., 1860, Bd. II, p. 265; 1863, Bd. in, p. 88; Pflanzenzelle,
1867, p. 287.
6 Cf. Fitting, Ber. d. bot. Ges., 1902, p. 380.
7 Cf. Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. xv, pp. 210, 221.
8 Cf. Ewart, 1. c., p. 221.
9 Noll, 1. c., 1888, p. 514; 1825, p. 56. Cf. also Pfeffer, Druck- u. Arbeitsleistungen, 1893,
p. 417; Kohl, 1. c., p. 73.
THE INTERNAL CAUSES OF MOVEMENT 247
but this does not necessarily afford positive proof of an increase of extensibility in the
cell-walls of the convex side. In addition, it has still to be shown why a plasmo-
lysing solution often produces at first a slight increase of curvature \ and subsequently
a decrease. Possibly this result may be due to the continuance of the tropic induc-
tion, but it might also be due to the more rapid penetration on the concave side or to
other factors.
The special Metabolism connected with tropic reactions has been investigated by
Kraus 2. Kraus observed a rise in the percentage of reducing sugar and a diminution
of acidity, especially in the under-surface of an ageotropic shoot when placed horir
zontally, even before the upward curvature had begun. During the curvature the
total amount of sugar, and often also of free acid, decreased on the convex side. Thus
in an etiolated bean-shoot, two hours after being placed horizontally, the upper half
contained 0-2358 of a gram, the lower 0-2404 of a gram of reducing sugar, an
excess of 0-0046 of a gram. Three-quarters of an hour later, other similarly-treated
shoots of the same plant contained 0-2095 of a gram of reducing sugar in the upper
half of the stem, and 0-2074 of a gram in the lower half, a deficiency of 0-0021.
These changes are not produced in the absence of oxygen.
During and before the commencement of the geotropic curvature the percentage
of water in the lower half of the shoot increases, so that the density of the expressed
sap decreases. Kraus 3 found, for instance, that in a stem of Anthriscus sylvestris,
which had been kept in a horizontal position for twenty-four hours, but had only
slightly curved, the specific gravity of the expressed sap from the upper side was
1-0240, and that from the lower 1-0226, a difference of 0-0014. This coincides with
the fact that during curvature the turgor of the cells on the convex side decreases, as
measured by plasmolysis. At the same time, we have an interesting instance of the
fact that the distribution of the denser nutrient sap in the tissues is not directly
determined by gravity. • %
According to Kraus, the above changes begin before the commencement of cur-
vature, and are also shown in stems which are no longer capable of curvature. Hence
they do not appear to result from the performance of the bending, and, like the latter,
represent reactions due to the stimulating action of gravity. Whether any causal
relationship exists is, however, as uncertain as in the case of the increase of silver-
reducing substances due to the inductive action of gravity, and taking place before the
commencement of curvature. It is also uncertain whether changes in the respiratory
activity accompany tropic curvature, and whether the phenomena observed are asso-
ciated with all forms of tropic curvature, although Kraus * found similar differences
} Noll, Arb. d. hot. Inst. in Wurzburg, 1888, p. 517 ; 1895, p. 84. Cf. also Pfeffer, Studien zur
Energetik, 1893, p. 247.
3 G. Kraus, Ueber die Wasservertheilung in d. Pflanze, Bd. II, 1880, p. 38, and Bd. I, 1879, p. 23
(reprint from Abhandl. d. Naturforsch. Ges. in Halle). See also Bot. Ztg., 1877, p. 596 ; Ueber die
Wasservertheilung, Bd. IV, 1884, p. 59. [The reducing sugars were estimated in the expressed sap
by Fehling's method, which gives, of course, merely the total percentage of reducing substances.
The differences observed are small and almost within the limit of experimental error.]
3 Kraus, 1. c., Bd. II, p. 42 ; Wiesner (Die heliotropischen Erscheinungen, 1878, p. 65) and Thate
(Jahrb. f. wiss. Bot, 1882, Bd. xni, p. 718) were, however, unable to detect any such differences,
possibly owing to less 'exact experimentation.
* Kraus, 1. c., Bd. II, p. 41.
248 TROPIC MOVEMENTS
as the result of heliotropic stimulation. In the case of geotropically stimulated roots,
Kraus l found an increase in the percentage of water as usual on the convex side,
which is here the upper one, while in old non-curving roots geotropic induction pro-
duces the same rise in the percentage of water on the upper side, as is shown in the
lower side of non-curving old stems.
Similar changes are very rapidly produced by shaking, for Kraus2 found that
after shaking a growing defoliated shoot of Alliaria officinalis the amount of sugar
rose from 0-1463 to 0-1618 of a gram, while the side kept convex during shaking
contained sap of higher density and with a higher percentage of sugar.
PART V
SECTION 55. Special Cases.
Although the usual loss of the power of tropic curvature in adult
organs may involve a certain disadvantage, nevertheless it would need too
great an expenditure of energy and material to render the older parts of
a tree not only capable of supporting the other organs but also of per-
forming tropic movements. Hence the plant strives to adjust itself by
means of the new shoots, and allows the older organs to remain in positions
forced upon them.
ROOTS. The primary geotropic curvature of the main root may
be more or less modified by hydrotropic, rheotropic, heliotropic, trau-
matropic, and aerotropic stimuli. In addition, obstacles may cause the
plastic apex to diverge temporarily from its attempted line of growth,
but the influence of all these factors upon the growth and shape of the
root-system does not require detailed discussion3. The avoidance of
obstacles does not appear to be the result of any contact stimulation but
may in extreme cases partly result from traumatropic excitation. Roots
can exercise a considerable downward pressure when lateral displacement
is prevented, and the pointed growing apex has a high power of lateral
expansion, as have also the older parts of the root during secondary
growth. The weight of the seed, or of a thin covering of soil, gives usually
a sufficient fulcrum for the downward pressure exercised by the radicle
in penetrating an ordinary soil. In many cases the formation of mucilage
or the early production of root-hairs aid in fixing the seed4, while an
increased leverage may be assured by the curvature assumed by the
hypocotyl.
RHIZOMES are usually diageotropic, but certain forms may temporarily
1 Kraus, Ueber die Wasservertheilung, &c., 1880, Bd. II, p. a6. 2 Kraus, 1. c., p. 69.
8 Freidenfelt, Flora, 1902, Ergzbd., p. 115.
4 Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 362, 365, 369; and for the literature concerning
the escape of shoots from the soil, p. 383 ; also Areschoug, Beitr. z. Biol. d. geophilen Pflanzen,
1896.
SPECIAL CASES 249
become positively geotropic, owing to an aitiogenic or autogenic change of
tone. The rhizomes of Adoxa and Circaea are, for instance, positively geo-
tropic when illuminated, but become diageotropic as soon as their downward
curvature below the surface of the soil brings them into darkness. It is,
however, not known whether differences in the distribution of oxygen, carbon
dioxide, water, and temperature may produce changes of tone or tropic
reactions regulating the depth of the rhizome, or whether the distance between
the rhizome and the subaerial parts influences the geotropic tone. The latter
is, in fact, strongly affected in certain rhizomes by the partial or complete
removal of the subaerial organs. Correlative reactions of this kind are
often of predominant importance, according to Rimbach *, although rhizomes
of the same plant under similar conditions may vary in depth within wide
limits. Miiller 2, in fact, concludes that the depth of rhizomes is due solely
to extraneous circumstances such as the action of earth-worms and the
like. That such factors may influence the depth is certain, but it is hardly
possible in this way to explain all the phenomena observed 3. An instance
of correlation is afforded by those cereals in which, according to Schellenberg,
the illumination of the leaves influences the development at the nodes to
which they are attached4. The development of contractile roots which
draw bulbs and corms deeper into the soil is possibly also the result of
correlative influences, as would also be the cessation of the formation of
these roots when an appropriate depth is reached. Naturally other factors
may also come into play in determining the position assumed, among
which the peculiar downward transference of the corms of young seedlings
of Crocus is included.
AERIAL STEMS. Owing to the tonic and orienting actions of light, and
to the influence of such agencies as wind and moisture, the relationships
are here more complicated, while in addition the mere weight of the
organ may cause it to diverge more or less from the position which it
strives to assume. The erect position of the main axis is largely due
to its negative geotropism, while the lateral shoots either assume a plagio-
tropic position in virtue of their autotropism or are led into particular
positions by various aitiogenic influences. The latter applies more especially
to the leaves, and here the orienting action of light is naturally of primary
importance, although heliotropic stimuli may also influence stems to a
pronounced degree.
RUNNERS AND CREEPING SHOOTS 5. The horizontal or obliquely
1 Rimbach, Beitrage z. wiss. Bot. von Fiinfstiick, 1899, Bd. ill, p. 177.
8 P. E. Miiller, Bot. Centralbl., 1896, Bd. LXVI, p. 22.
3 Areschoug, Beitr. z. Biol. d. geophilen Pflanzen, 1896 ; Goebel, Organography, 1900, p. 224.
* Schellenberg, Unters. iiber d. Lage d. Bestockungsknoten beim Getreide, 1902, p. 21 (reprint
from Forschungen a. d. Gebiete d. Landw.).
5 Frank, Die natiirl. wagerechte Richtung, etc., 1870, p. 17; Bot. Ztg., 1873, p. 36; Czapek,
250 TROPIC MOVEMENTS
ascending position is in many cases due to diageotropism, which may either
persist unaltered in light and darkness, as in Fragaria vesca and F. grandi-
flora, or may change more or less completely into negative geotropism
in darkness, as in Lysimachia nummularia, Polygonum aviculare, Rubus
caesius, Vinca major, and S tacky s sylvatica, so that the shoots are nearly erect
in darkness or when growing in thick grass and obliquely ascending when
illuminated. Hence in sunny situations the shoots of these plants are
pressed against the ground even when this necessitates a downward
curvature.
The power of changing the geotropic tone varies according to the
degree of development and morphological rank of the organs, although a day
or two is sufficient under favourable conditions to produce a reversal in
the active growing zone. Thus in Glechoma hederacea the runners formed
in spring show a pronounced geotropic erection in darkness, whereas those
formed later in the season show none at all *. It was owing to this fact
that Czapek could detect no change of position in the runners of this plant
and also of Potentilla reptans in darkness, whereas Maige 2 found that the
last-named plant also became negatively geotropic in darkness.
In certain plants the geotropic tone may be modified by changes
of temperature, and it is for this reason that the ascending shoots of
Veronica chamaedrys and Lamium purpureum sink to a more or less
horizontal position when the temperature is kept low. The photic and
thermal changes of geotropic tone will be opposed when doubly responsive
plants are subjected to simultaneous rises of temperature and of illumination.
Frank was the first to recognize that permanently diatropic positions were
due to diageotropism, but supposed that the changes of position according
to the illumination were due to negative geotropism and variable negative
heliotropism. Czapek 3, however, showed that the same changes of position
took place in homogeneous diffuse light, but not when the plants were
rotated on a klinostat. When the action of gravity is eliminated in this
way the shoots of Lysimachia nummularia and other plants are able to
show feeble positive heliotropism4, whereas the creeping shoots of a few
other plants are feebly negatively heliotropic 5.
The runners and creeping shoots of most of these plants are originally
physiologically radial, and only acquire a temporary dorsiventrality after
Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, pp. 1234, I249J Oltmanns, Flora, 1897, p. 24;
Maige, Ann. d. sci. nat., 1900, 7° ser., T. xi, p. 334 ; Massart, L'initabilite" d. plantes superieures,
1902, p. 13.
1 Maige, 1. c. ; Oltmanns, 1. c., p. 25 ; Klebs, \Villkiirliche Entwickelungsanderungen bei Pflanzen,
1903. On the influence of external and internal conditions on the formation of runners cf. Maige,
1. c. ; Goebel, Organography, 1905, p. 459.
a Maige, I.e., p. 340.
3 Czapek, 1. c., p. 1235. Cf. also Oltmanns and Maige, 1. c. * Czapek, 1. c., p. 1236.
8 Maige, 1. c., p. 358.
SPECIAL CASES 251
remaining for some time in a plagiotropic position. If this acquired
dorsiventrality induces a certain tendency to epinastic curvature l, the latter
must play some part in the orientation, as must also the primitive positive
or negative heliotropism. Short shoots can raise themselves in spite of
the action of gravity, whereas long ones unavoidably droop downwards
more or less from their attempted position.
All plagiotropic orientation is not necessarily produced in this way,
for many foliage-leaves and other objects are klino-heliotropic and assume
their positions mainly in response to the incidence of the light rays.
Similarly, under natural conditions negative geotropism and heliotropism
may often co-operate in producing a plagiotropic orientation, as for instance
when a parallelo-geotropic organ is caused to perform a positively heliotropic
curvature by lateral illumination.
Marchantia2. As soon as a permanent dorsiventrality has been
induced by the action of light, the subsequent growths take up positions
like those assumed by dorsiventral leaves. Thus in strong light the thallus
becomes approximately perpendicular to the incident rays, and hence usually
assumes a plagiotropic position. This orientation is produced by light
independently of the action of gravity, so that illumination from beneath may
cause the thallus to become inverted. Hence if the apex is illuminated from
the front the thallus tends to bend downwards, but rises up when the light
comes from behind the apex 3. In addition the plagio-phototropic position
is assumed when the plants are rotated on a klinostat. Nevertheless the
thallus reacts geotropically, and becomes erect in darkness, but more and
more horizontal as the illumination increases and the predominant action
of light comes into play. It is, however, uncertain whether the thallus is
permanently weakly negatively geotropic or whether, as Czapek supposes,
illumination affects the geotropic tone as it does that of many runners
so that in light the thallus is plagio-geotropic as well as plagio-heliotropic.
Sachs found that the thallus of Marchantia under ordinary illumina-
tion grew at right angles to the direction of a centrifugal force of 34 g.,
but was inclined at an angle near the centre of the wheel where the
force was less. The exact causation of this result remains, however, uncertain
until the action of gravity upon a thallus illuminated equally on all sides
is known. Czapek4 supposed that the radial lobes of the thallus developed
on the klinostat were diageotropic, but his experiments are not conclusive.
Since the upper side of the thallus may become either concave or convex
in assuming a plagiotropic position, the latter is evidently not the result
1 Maige, Ann. d. sci. nat., 1900, 7* se>., T. XI, p. 340; Czapek, I.e., p. 1235; de Vries, Arb.
d. hot. Inst. in Wurzburg, 1872, Bd. I, p. 271.
8 Sachs, Arb. d. bot. Inst. in Wtirzburg, 1879, Bd. II, p. 229 ; Czapek, Jahrb. f. wiss. Bot, 1898,
Bd. xxxii, p. 260; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, i, p. 1238.
s Sachs, 1. c., p. 232. « Czapek, 1. c., 1898, p. 263.
252 TROPIC MOVEMENTS
of dorsiventral epinasty or hyponasty. It is not yet, however, certain
whether a thallus grown in darkness may not perform a photo-epinastic
curvature when exposed to light, the curvature increasing as the light
becomes more intense.
As was first observed by Mirbel \ illumination of the under side causes
this to become concave until the upper surface is exposed to the light, the
curvature being at first towards the light and then away from it. Sachs2
considered the plagiotropism of Marchantia to be due to the interaction
of negative geotropism with a positive heliotropism of the lower side, and
epinasty in the upper one, whereas Czapek 3 supposed it to result from the
co-operation of diaphototropism, photo-epinasty, and a diageotropism
varying according to the illumination. The stalks of the fructifications of
Marchantia are parallo-geotropic and parallelo-heliotropic, and owing to their
high heliotropic irritability Sachs (1. c.) found that they assume a position
nearly parallel to the incident rays when obliquely illuminated.
THE PROTHALLUS OF FERNS is also oriented mainly by its plagio-
heliotropism, and reacts in the same way as does Marchantia when
illuminated from beneath4. Since the induced dorsiventrality is labile,
however, the new growths soon have their dorsiventrality reversed, and the
orienting movement ceases or may never be shown if it is delayed too long.
HEDERA HELIX 5. Unilateral illumination induces labile dorsiventrality
in the stems of this plant, and so produces the plagiotropic position of the
shoot. Hence the ascending stems press themselves against a vertical wall
and curve over the top of it away from the light until the free ends bend
downwards by their own weight. The hypocotyl as well as the inflorescence
axes are, however, radial and ortho-geotropic 6.
When illuminated equally on all sides by rotation on a klinostat the
shoots remain radial, while the dorsiventrality may be reversed by illuminat-
ing the under-surface. Owing to the slowness of curvature and the
relatively rapid reversal of the dorsiventrality, an ivy-shoot when illuminated
from beneath curves only slightly towards the light and then curves away
from it 7. It is not known, however, whether geotropic stimuli play any part
in the orientation. The shoots of Hedera do actually react geotropically,
but according to Sachs 8 they are negatively geotropic, whereas according to
Czapek 9 they are diageotropic. Sachs states, however, that in a horizontal
1 Mirbel, Rech. anat. et physiol. sur le Marchantia, 1835 (reprint from Nouvell. Ann. du
Museum d'Histoire nat., T. l). Cf. also Czapek, 1898, 1. c., p. 262.
3 L.c., p. 239. * L. c., 1898.
* Leitgeb, Flora, 1877, p. 174; 1879, p. 317.
5 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, p. 257 ; Czapek, Jahrb. f. wiss. Bot., 1898,
Bd. xxxii, p. 258; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, i, p. 1236; Oltmanns, Flora, 1897,
p. 26.
6 Czapek, 1. c., 1895, p. 1236. 7 Sachs, 1. c., p. 267.
8 Id., p. 269. • Czapek, 1. c., 1898, p. 358.
SPECIAL CASES 253
position only slight epinastic curvature takes place, whereas vertical shoots
curve until they assume a horizontal position both in light and in darkness.
This certainly points to the existence of a diageotropic irritability which
is not modified by illumination ; but in any case the diageotropism is bound
up with the induced dorsiventrality, since the radial shoots appear to be
ortho-geotropic. It is possible that unilateral illumination may be capable
unaided of producing a diaphototropic orientation, although Sachs1 con-
sidered the plagiotropism to be due to negative heliotropism and geotropism,
whereas Czapek 2 supposes it to result from phototropism, diageotropism,
and photonasty.
THE PLAGIOTROPIC BRANCHES OF HERBS AND TREES. These appear
in the case of Cucurbita Pepo 3, Linaria cymbalaria 4, and Tropaeolum majus 5
to resemble the ivy, in that unilateral illumination induces dorsiventrality.
The latter is, however, so feeble in Tropaeolum that we may equally well
suppose the plagiotropic position to result from the opposed action of
diaheliotropism and negative geotropism. Permanently dorsiventral plagio-
tropic organs are, like radial and temporarily dorsiventral ones, unequally
responsive to light and gravity. Naturally the action of light becomes of
predominant importance in the case of photosynthetic organs or surfaces,
as well as in the stem when the position of the latter is mainly responsible
for that of the leaves. This applies to the plagiotropic shoots of Atropa
Belladonna^Pilea^Pellionia, Goldfussia anisophylla^ and Selaginella*, although
in part the influence of gravity which is exercised even upon foliage-leaves
may predominate. Considerable uncertainty exists, however, in many
cases. Thus it is not known whether the dorsiventrality and plagiotropism
of Polygonatum multiflorum 7 is due to light, to gravity, or to both.
Obliquely ascending radial or dorsiventral branches which bear leaves 8
capable of self-orientation usually show only feeble phototropic reactions,
lateral illumination producing little or no heliotropic curvature. The
geotropic irritability may, on the other hand, be mainly responsible for
the direction of growth assumed, this being always at a definite angle to the
perpendicular. The primary and secondary branches of many herbaceous
and woody plants may, however, show but feeble geotropic reactions, and
have little or no power of plagiotropic orientation. In such cases the
branches spread in all directions, and continue any direction of growth
Sachs, Arb. d. bot. Tnst. in Wiirzburg, 1879, Bd. II, p. 266. s L. c., p. 258.
Czapek, Flora, 1898, p. 427; Noll, Landw. Jahrb., 1901, Ergzbd., p. 425.
Oltmanns, Flora, 1897, p. 26.
Sachs, I.e., p. 271.
Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 265. Cf. also Wiesner, Ber. d. bot. Ges.,
1902, p. 321 ; Sitzungsb. d. Wien. Akad., 1902, Abth. i, Bd. CXI, p. 733.
7 Cf. Vochting, Bewegungen d. Bliithen u. Fruchte, 1882, p. 148; Frank, Die natiirl. wagerechte
Richtung von Pflanzentheilen, 1870, p. 21.
8 Cf. de Vries, Arb. d. bot. Inst. in Wurzburg, 1872, Bd. I, p. 271 ; Frank, 1. c.
254 TROPIC MOVEMENTS
impressed upon them. According to Baranetzsky *, however, these
branches are negatively geotropic, but show no geotropic curvature because
at the same time an equal and opposed epinastic curvature is excited.
Baranetzsky found that the apices of the branches of Prunus, Fraxinus, Tt'lia,
Ulmus, Philadelphus, and other woody plants always performed a curvature when
directed vertically upwards or downwards or when rotated on a klinostat, the original
upper side becoming convex. This epinasty is induced by the action of gravity in which-
ever side happens to be uppermost. The induction is transitory and reversible, the
curvature being automatically straightened again, while the shoots on a klinostat continue
to grow in any direction in which they may be placed. Since the epinastic curvature
is not shown in the normal plagiotropic position, it must be balanced by an opposed
tendency to negatively geotropic curvature *. The latter actually appears according
to Baranetzsky when a branch is laid flat, which was previously erect or had been
rotated on a klinostat for a long time. Hence the epinasty is apparently more
slowly induced, but persists longer when the exciting agency is removed than does
the hyponastic geotropic induction. Since the epinastic tendency is shown by
straight branches, it cannot result, as Baranetzsky supposed, from any realized
curvature, although the latter does actually awaken reactions directed towards its
removal.
This suppressal of the geotropic reaction is only possible when the epinastic and
hyponastic tendencies alter correspondingly as the inclination varies. An autogenic
epinasty may aid in balancing the negative geotropism, but it is impossible to follow
de Vries in ascribing all plagiotropism to the antagonism of autogenic epinasty and
negative geotropism 3. Wiesner considers that changes of position are due to varia-
tions of epinasty, the negatively geotropic action remaining constant. He also
concludes that the autogenic epinasty attains its maximal value with a medium rate
of growth, so that either a diminution or increase of the average rate of growth
increases the geotropic erection. Many of the objects in which Baranetzsky could
detect no autogenic epinasty appear to possess this power 4, but it does not follow
that relationships of the kind described exist in all cases, nor does their discovery
reveal the causes producing them.
Whatever its origin may be, we are dealing with a positively geotropic reaction
when a lateral shoot takes the place of the decapitated apex of a Pine, or when
without injury the shoots of certain other plants, as occasionally happens, assume an
erect position. The distribution of the buds and the factors which affect their
development naturally exercise a considerable influence on the type of branching 6.
In addition, all long slender branches droop downwards more or less as the result of
1 Baranetzsky, Flora, 1901, Ergzbd., p. 138; Frank, I.e.; de Vries, I.e.; Vochting, Organ-
bildung im Pflanzenreich, 1884, Bd. II, pp. 4, 93 ; Wiesner, Ber. d. bot. Ges., 1903, p. 321 ; Sitzungsb.
d. Wien. Akad., 1902, Bd. cxi, Abth. i, p. 733.
a [If this is so, the growth of the under side should presumably be more rapid in the normal
position than it is on a klinostat.]
8 Cf. Baranetzsky, 1. c., p. 141.
4 Wiesner, Sitzungsb. d. Wien. Akad., 1902, Bd. cxi, Abth. i, p. 733.
8 Cf. Goebel, Organography, 1900; Wiesner, I.e., p. 326.
SPECIAL CASES 255
their own weight. The growing apices are, however, usually not only strong enough
to bear their own weight, but also to curve vigorously upwards. In some cases the
parts which have become woody and ceased to elongate may perform an upward
curvature and so counteract the mechanical drooping of the branch. The young
shoots of the Pinus are at first more erect and then spread horizontally, but this is
not due to the influence of their own weight, as Baranetzsky supposed, for Wiesner
has shown l that to produce such a curvature a load of fifteen to thirty times the weight
of the branch is required in the case ofPtnus Laricio. Vochting and Baranetzsky have
shown, however, that in certain weeping varieties the branches droop owing to their
own weight, and the apices continue to grow in the same direction without attempting
to curve upwards.
Frank found that the branches of various trees returned to their original position
in both light and darkness after forcible displacement, whereas Baranetzsky observed
no such return. Further researches must determine whether the apparent contra-
diction is due to the existence of varying powers of reaction. Frank 2 also observed
orienting torsions in twigs of* Abies in which dorsiventrality had been previously
induced, and these can hardly be mechanical in origin as Baranetzsky * suggests.
SECTION 56. The Orientation of Foliage-leaves4.
The leaves of such plants as Erica, Dracophyllum, and Viscum orient
themselves in regard to the stem alone, and so may stand out at various
angles with the perpendicular. Dorsiventral photometric leaves, however,
strive usually to place their surfaces at right angles to the direction of the
strongest diffuse light, whereas certain other leaves place themselves parallel
to it. Other leaves, again, place themselves parallel to the light only when
it is so intense that protection against it is needed 5.
Many responsive leaves when displaced in darkness return approxi-
mately to their original position, and if necessary by the aid of torsion, so
that gravity as well as light may act as an orienting stimulus, and Dutrochet 6
1 Vochting, Organbildung im Pflanzenreiche, 1884, Bd. II, p. 90; Bot. Ztg., 1880, p. 595;
Baranetzsky, I.e., p. 216.
a Frank, 1. c., p. 22. See also Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 267.
1 L. c., p. 203.
4 Bonnet, Unters. iiber d. Nutzen d. Blatter, 1762, p. 45; Dutrochet, Rech. anat. et physiol.,
1824, p. 126; Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870; Bot. Ztg., 1873,
p. 72 ; de Vries, Arb. d. bot. Inst. in Wiirzburg, 1872, Bd. I, p. 223; Wiesner, Die heliotropischen
Erscheinungen, 1880, Bd. II, p. 39; Biol. Centralbl., 1899, Bd. xix, p. I ; Darwin, The Power of
Movement in Plants, 1880; F. Darwin, Linnean Society Journal, 1881, Vol. xviil, p. 420; Schmidt,
Das Zustaudekommen d. fixen Lichtlage blattartiger Organe, 1883 ; Noll, Arb. d. bot. Inst. in Wiirz-
bnrg, 1885-7, Bd. ill, pp. 189, 315 ; Flora, 1892, Ergzbd., p. 265 ; Vochting, Bot. Ztg., 1888, p. 501 ;
Krabbe, Jahrb. f. wiss. Bot., 1889, Bd. xx, p. 211; Schwendener und Krabbe, 1892 (Gesammelte
Abhandl. von Schwendener, Bd. n, p. 255) ; Oltmanns, Flora, 1892, p. 231 ; Czapek, Jahrb. f. wiss.
Bot., 1898, Bd. xxxn, p. 269; Flora, 1898, p. 429; Wiesner, Biol. Centralbl., 1903, Bd. XXIII,
p. 209 ; Ber. d. bot. Ges., 1902, Generalvers. (p. 84).
6 Cf. Ewart, Annals of Botany, 1897, Vol. XI, p. 447 ; Wiesner, Biol. Centralbl., 1899,
Bd. xix, p. i.
6 Dutrochet, Memoires, etc., Bruxelles, 1837, p. 312; Vochting, Bot. Ztg., 1888, p. 549.
256
TROPIC MOVEMENTS
showed that centrifugal force acted in the same way as gravity upon leaves.
Under normal conditions, however, the influence of light preponderates, so
that dorsiventral leaves when illuminated from beneath may bend so as
to face downwards. These movements take place independently of the
epinastic tendency, which the stimulus of light is in fact able to overcome.
The plagio-geotropism of the leaf is also able to overcome its epinasty, so
that a leaf which has attained its plagio-geotropic position usually needs
only to move slightly in order to become plagio-heliotropically oriented.
FlG. 47. Coleus sp. A. Plant in normal position. B. After a day's rotation on a klinostat.
In addition, the curvature of the stem is usually such as to aid in the
assumption of the proper position by the leaves. The movements of the
latter are usually performed by the petiole or in sessile leaves by the
lamina, and in most cases the power of movement is lost when growth
ceases. The latter, however, often persists for a long time in certain regions
of the leaf, so that a leaf may remain capable of orienting movements long
after it is fully adult. Leaves which possess motile pulvini usually retain
this power until death.
De Vries and also Wiesner have assumed that the plagiotropic -orientation of
leaves is due to negative geotropism and autogenic epinasty, whereas Frank, Darwin,
THE ORIENTATION OF FOLIAGE-LEAVES 257
Vochting, and Krabbe have shown that leaves are not only plagio-heliotropic but also
plagio-geotropic. Naturally other factors may influence the position assumed, and
among these autogenic epinasty is included, which is often extremely pronounced.
Evidence of its existence is afforded by the fact that the leaves often curve strongly
backwards when the action of gravity is eliminated on the klinostat (Fig. 47)*.
When such a plant is inverted, plagio-geotropism and epinasty co-operate so that
a very rapid curvature ensues, but if a stronger curvature is produced than in
Fig. 47 B, on placing the plant on a klinostat a certain hyponastic lessening of the
curvature ensues, in place of the original epinasty. The epinasty of certain
leaves appears to be increased by a rise of the intensity of diffuse illumination,
and possibly a photonastic action of this kind may be responsible for the rising up of
leaves in weak light or in darkness. Further evidence is, however, required, for many
leaves curve downwards instead of upwards in darkness. The ' radical ' leaves of
many plants which become more or less erect in darkness press themselves against
the soil in strong light, and may even curve downwards when the plant is raised
above the level of the soil 2.
The leaf in many cases droops more or less owing to its own weight, but
nevertheless the plagiotropic orientation will take place under water, in which an
upthrust is exercised on the leaf3. In many cases complicated bending or actual
torsion is required to return the leaf to its proper position, but since this also is
produced under water, it cannot be due to the mechanical action of the weight of the
leaf, as de Vries supposed to be the case 4. Any lateral curvature of the leaf may
tend to produce torsion, but nevertheless the energy of movement is sufficient to
overcome this action not only in the case of leaves but also of flower-stalks 5. It is
also certain that some of the torsions shown by branches are not mechanical in
origin, as Baranetzsky 6 supposed them all to be.
Orienting torsions are produced in darkness by gravity, but are still better shown
as the result of suitable lateral illumination 7, although in many cases only under the
conjoint action of a gravitational stimulus. Thus, on a klinostat the leaves of Viola
and Dahlia no longer react to lateral illumination, while those of Phaseolus, Sofa,
and Acacia orient themselves to the light by pronounced curvature without torsion 8.
The flowers of Viola orient themselves by torsion on a klinostat to lateral illumina-
tion, so that the co-operation of gravity is not always required for the production of
torsion '. It is, however, uncertain whether the orientation of the leaves of Malva
1 F. Darwin, Linn. Soc. Journ., 1881, Vol. XVIII, p. 426; Vochting, I.e., 1888, p. 534;
Krabbe, 1. c., 1889, p. 248 ; Schwendener und Krabbe, 1. c., 1892, p. 340.
8 Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870, p. 45 ; Darwin, Insecti-
vorous Plants, 1876, p. 343; Wiesner, 1. c., 1880, p. 43; F. Darwin, I.e., 1881, p. 430; Vochting,
Bewegungen d. Bliithen u. Friichte, 1882, p. 179; Neger, Flora, 1903, p. 371.
* Bonnet, 1. c., 1762, p. 61 ; Frank, Bot. Ztg., 1873, p. 55 ; Noll, I.e., p. 222.
4 De Vries, I.e., 1872, p. 266; Wiesner, I.e., 1882 ; O. Schmidt, I.e., 1883.
6 VOchting, 1. c., 1888, p. 552 ; Noll, 1. c., 1885-7, PP- 220, 337.
• Baranetzsky, Flora, Ergzbd., 1901, pp. an, 194.
7 [The suggested terms ' geotortism ' or ' geostrophism ' and * heliotortism * or ' heliostrophism '
are as unnecessary as would be ' helioturgotropism ' or ' geoheterauxecism.']
8 Krabbe, 1. c., 1889, p. 244; Schwendener u. Krabbe, 1. c., 1892, p. 339.
9 Schwendener u. Krabbe, 1. c., 1892, pp. 327, 335, 348.
PFEFFER. Ill S
258 TROPIC MOVEMENTS
neglecta l on a klinostat, and the similar instances observed by Darwin 2, afford true
cases of torsion. No orientation by torsion is produced by the action of light upon
most dorsiventral flowers, whereas gravity exercises this effect upon the flowers of
Aconiium, Delphinium, and Scrophularia 3.
Tropic orientation to a single agency may be performed by torsion as well as by
curvature, and since the former only requires the existence of a physiological dorsi-
ventrality, it is not surprising that Schwendener and Krabbe 4 should fail to detect in
the peduncle of Aconitum any visible signs of morphological dorsiventrality. On the
other hand, Vb'chting 6 found that the small flowers, which Impatiens develops in dark-
ness, act like radial organs. Noll6 assumed that the supporting axis radiated an
' exotropic ' influence upon the orientation of dorsiventral flowers and leaves, but
there is no evidence of any such action in the case of leaves, while the stalks of the
dorsiventral flowers of Aconitum place themselves at a definite angle with the perpen-
dicular, and hence with the axis of the inflorescence, owing to their geotropic irritability.
According to Czapek7 the pedicel of Aconitum performs its orienting torsion when the
flower is removed, but according to Meissner this is not the case 8.
It is probably owing to correlative influences that after the severance of the
inflorescence of Orchis the flower-buds near to the injury perform simple geotropic
curvatures in assuming their proper position instead of the normal torsion move-
ments 9. In addition, a realized torsion excites a counter-action, which is sufficient to
remove the torsion of a pulvinus of Phaseolus when the agency inducing it is removed.
Autogenic torsions may also 'occur, as, for instance, when the leaves of A Ilium
ursinum and Alstromeria change from the inverted position to the normal one as
they expand from the bud. These leaves are also capable of aitiogenic torsion 10.
Although the detailed mode of production of torsion is unknown it certainly is
not necessarily always the result of growth movements, although these usually
accompany it. Noll11 assumes that torsion is due to the co-operation of dissimilar
tendencies to curvature, which may possibly apply in certain cases in spite of
Schwendener and Krabbe's dictum to the contrary 12. The fact that certain torsions
cease when the stimulating action of gravity is eliminated shows that the combined
action of more than one stimulus may be necessary to produce them.
If a plant of Chenopodtum, Coleus, or Helianthus is inverted and the curvature of
the main axis prevented, the leaves at first sink slightly owing to their own weight.
An upward curvature then begins, due to the co-operation of epinasty and geotropism
or heliotropism, which continues in active leaves until the dorsal side again faces
upwards. In many cases this curvature is not completed, owing to the early or late
1892
Vochting, Bot. Ztg., 1888, p. 534. » F. Darwin, I.e., 1881, p. 426.
Noll, Arb. d. hot. Inst. in Wurzburg, 1885-7 J Schwendener u. Krabbe, 1. c., 1892.
L. c., p. 317. 6 Vochting, Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 179.
Noll, l.c., 1885-7, Bd- i"t P- 367; Flora, 1892, Ergzbd., p. 273; Schwendener u. Krabbe,
Gesammelte Abhandl., Bd. II, p. 255.
Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p. 379.
Meissner, Bot. Centralbl., 1894, p. 12. 9 Noll, 1. c., p. 329.
10 Czapek, Flora, 1898, p. 249. Cf. Goebel, Organography, 1900, p. 250.
11 Noll, 1. c., 1885-7 and 1892. Cf. also Meissner, Bot. Centralbl., 1894, Bd. LX, p. i.
ia Schwendener u. Krabbe, 1. c., 1892.
THE ORIENTATION OF FOLIAGE-LEAVES
259
commencement of an orienting torsion in the petiole. It is by a torsion of this kind
that the young leaves on hanging branches of Fraxinus^ Caragana, Salix, and Betula
assume their proper positions.
If the stem is placed in a horizontal position the lateral leaves perform an
epinastic backward curvature, and then by torsion and a forward movement come to
face upwards with the lamina parallel or obliquely inclined to the stem. The leaves
on the upper side may attain a suitable position by the primary epinastic and geotropic
backward curvature, but frequently they do not reach or retain this position, lateral
curvatures coupled with torsion bringing them into positions similar to those assumed
by the lateral leaves. The same applies to the leaves on the under side.
It is in this way that the leaves on plagiotropic shoots of Vinca, Gkchoma,
Lysimachia nummularia, Buxus, Acer, and Taxus assume a more or less complete
dorsiventral arrangement, whereas on erect shoots they are radially arranged1
FlG. 48. Euonyntus radicans. A^ a vertical shoot with decussatel eaves. B, a horizontal shoot.
(Fig. 48). The decussate leaves of Deutzia, Lonicera and Philadelphus, as well as
the spirally-arranged ones of Spiraea salicifolia and Kerria japonica, are caused to
assume an exact two-rowed arrangement in sloping and horizontal shoots by the
twisting of the internodes, so that the individual leaves need only twist slightly to place
themselves in a horizontal position. This torsiori only begins in each internode
when that in the precedent one is completed, so that unnecessary torsion is avoided f .
The torsion is not only produced by gravity, but also in erect shoots by unilateral
illumination s, and since the leaves then exercise no torsion moment on the stem, it is
1 Frank, Die natttrl. wagerechte Richtung von Pflanzentheilen, 1870, pp. 14, 37> 57» 64. See
also the figures in Kerner's Natural History of Plants, 1894, Vol. I, pp. 417-23.
a Frank, 1. c., p. 16. * Schwendener u. Krabbe, 1. c., p. 320.
S 2,
260 TROPIC MOVEMENTS
evident that de Vries * was incorrect in supposing that the twisting of the internodes
was due to the mechanical action exercised by the weight of the leaves. The
absence of torsion in the internode when the pair of leaves are removed may be due
to a change of tone, or to the cessation of the directive influences radiating from the
leaf. According to de Vries, the torsion of the internode of Philadelphus is inhibited
by the removal of the upper but not by that of the lower leaf. This requires further
investigation, however, as does also the absence of torsion in the defoliated branches
of Ulmus and Celtis 2, since Czapek 3 found that similarly treated branches of Taxus
and Picea do undergo torsion 4.
INTENSE LIGHT or direct sunlight causes many photometric leaves to
rise, sink, or twist in such fashion as to place their laminas or those of
the leaflets more or less parallel to the incident rays. This is especially
well shown by the compound leaves of Mimosa pudica in which the primary
pulvini set the plane of the leaf during the daytime in a plagiotropic position
FlG. 49. A horizontal shoot of Diervilla lonicera. From the edges of the stem it can be seen that the
torsion is completed in the internodes i, 2, and 3, while internode 4 is still straight.
which is the resultant of the diageotropic and diaheliotropic irritability. The
pulvini of the leaflets are, however, able to perform one movement only, and
this is photonastic in character. In ordinary light the leaflets are expanded,
in darkness and in intense light they close. The latter movement depends
1 See Noll, Arb. d. bot. Inst. in Wurzburg, 1885-7, Bd- in, p. 358; Schwendener u. Krabbe,
1. c., p. 320.
2 De Vries, 1. c., p. 272. » Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p. 288.
* On the orientation of Mosses and their protonomata see Coesfeld, Bot. Ztg., 1892, p. 192 ;
Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 265 ; Correns, Festschrift f. Schwendener, 1899,
p. 385- A summary of the orienting movements of flowers is given by Noll, Arb. d, bot. Inst. in
Wurzburg, 1885-7, Bd. ill, pp. 189, 315. See also Wiesner, Biol. Centralbl., 1901, Bd. XXI, p. 801 ;
the quoted works of Schwendener u. Krabbe, Oltmanns, Czapek, as well as Vochting, Jahrb. f. wiss.
Bot., 1886, Bd. xvn, p. 297; 1893, Bd. xxv, p. 179; Schaffner, Bot. Centralbl., 1898, Bd. LXXVI,
p. 22 (Helianthus) ; Meissner, Bot. Centralbl., 1894, Bd. LX, p. i.
THE ORIENTATION OF FOLIAGE-LEAVES 261
solely upon the intensity of the light rays independently of their direction
or heating effect, and hence the leaflets fold together when the sunlight is
reflected upon the pulvini from beneath, but expand when the pulvini are
shaded and the laminas fully exposed l. When the leaf is strongly illu-
minated from the side the main pulvinus twists into a more or less diahelio-
tropic position and the leaflets perform the same closure as before in
response to the intense light. We have, therefore, here an instance in which
the irritability in the pulvini of the same leaf varies according to their position
and the task they have to perform. All leaves provided with pulvini seem
able to respond to intense illumination, although it is not in all cases
certain whether the response is photonastic or heliotropic in character.
Photometric leaves which respond by growth-curvatures may, however,
also place themselves at varying angles with the direction of intense
illumination. It is, however, only rarely that they attain a profile position
as in Lactuca virosa, Silphium laciniatum^ and a few other plants, in which
the position is assumed by a torsion at the base of the leaf. Since this
orientation is mainly due to the intense midday sun, the leaves of these
so-called compass-plants set their laminas in exposed localities, mainly in
a perpendicular plane running north and south, whereas in shady situations
the leaves show neither this orientation nor do they assume the profile
position 2.
1 Cf. Ewart, Annals of Botany, 1897, Vol. XI, p. 448.
3 Stahl, Ueber sogenannte Compasspflanzen, 1881 (reprint from the Zeitschr. f. Naturwiss.,
Bd. XV); Oltmanns, Flora, 1892, p. 248; Bay, Botanical Gazette, 1894, Vol. XIX, p. 251. On the
branching system of Biota see Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 268.
CHAPTER IV
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
PART I
THE CHARACTER AND MECHANISM OF MOVEMENT
SECTION 57. General.
APART from the spermatozoids of vascular cryptogams and a few
Gymnosperms, no power of independent locomotion is shown by any
vascular plant. Many Fungi, and an even larger number of Algae, produce
motile zoospores, and in the case of many Volvocineae, Flagellatae, Bacteria,
Diatomaceae, and Myxomycetes the power of active locomotion is only
interrupted by certain resting stages, or during reproduction.
Motile organisms are usually free-swimming and possess special loco-
motory organs such as cilia or flagellae ; but others creep or glide over the
substratum, and others again show amoeboid movements over moist surfaces
or even under water. No plant or part of a plant is, however, able to propel
itself through the air, although spores and winged seeds may float in it
for some time. The different types of movement are not always sharply
distinguished, and the zoospores of Myxomycetes may perform alter-
nately amoeboid and ciliary locomotion. Indeed, transitions occur between
transitory pseudopodia and typical cilia, while certain Infusoria may either
swim freely or run over the substratum by the aid of their cilia. A swimming
movement will always become a gliding one when an organism is fixed
to the substratum by a mucilaginous layer, which is viscous enough to
prevent the upward escape of the organism but not its lateral movement.
Transitions also occur between the active movements of rooted plants
and of free-swimming organisms. Thus a swarm-spore attached at one end
performs nodding and bending movements like a rooted plant. In addition,
the movement of certain Desmids due to the excretion of a gelatinous stalk
may be compared with the movement of a growing apex produced by the
elongation of the zones beneath. Growth curvatures cause locomotory
movements in the free threads of Spirogyra, and may also cause them to
group together in bunches.
Dermatoplasts may remain capable of swimming and gliding move-
ments, whereas the production of a rigid cell-wall renders external amoeboid
movement impossible so long as no extra-cellular protoplasm is present.
GENERAL 263
Various internal amoeboid movements are still possible, as well as streaming,
and slow changes in the shape and position of the organs. Visible move-
ments are never entirely absent, though often extremely slow, so that
a slight change of position can be seen only after a considerable time.
Slow movements necessarily accompany the growth of the cell and the
conversion of a solid protoplast into a vacuolated one, while cellular and
nuclear division involve special grouping and separating movements. Active
growth does not, however, involve active movement, and protoplasmic stream-
ing is, for instance, absent from the cells of the primary meristem. Streaming
persists in many adult cells so long as they remain living, whereas in other
cells it is not aroused during the most active respiration and metabolism.
The ejection of seeds may be regarded as a passive movement even
when due to tensions created by vital activity. The same applies to the
rise of algal filaments owing to the adherence of bubbles of gas to them.
If the gas is oxygen produced by photosynthesis, the movement is
indirectly due to vital activity, just as when the air-spaces formed in shoots
cause them to ascend as soon as they have developed from the resting buds,
which sank the previous autumn owing to their higher specific gravity1.
Certain lower organisms possess gas vacuoles within the protoplasm, and
these may be used like the air-bladders of fishes to produce ascent and
descent in the water 2. It it, however, uncertain to what extent modifica-
tions in the specific gravity of the protoplasm and cell-sap may take part
in flotation 3.
For such movements not only the specific gravity but also the shape
and relative amount of surface are of importance. This is evidenced by
hairy and winged seeds, and by the transport of dried bacteria and other
micro-organisms, as dust particles in the air 4. In the same way slow
currents of water suffice to prevent the settling of minute particles denser
than the water, although in the case of plankton organisms active move-
ments may aid in producing the same result 5.
1 Cf. Goebel, Pflanzenbiol. Schilderungen, 1893, T. ii, p. 356.- On the work done in forming
intercellular spaces cf. Pfeffer, Energetik, 1892, p. 232.
8 On gas vacuoles see Engelmann, Pfliiger's Archiv f. Physiol., 1869, Bd. II, p. 307 ; Klebahn,
Flora, 1895, p. 241; Strodtmann, Biol. Centralbl., 1895, Bd. XV, p. 113; Celakovsky, Ueber den
Einfluss des Sauerstoffmangels auf die Bewegung einiger aeroben Organismen, 1898, p. 21 (reprint
from Bull. Internationale de 1' Academic de Boheme) ; Wille, Biol. Centralbl., 1902, Bd. xxn,
pp. 207, 257; Molisch, Bot. Ztg., 1903, p. 47; Hinze, Ber. d. bot. Ges., 1903, p. 394.
3 Cf. Brandt, Biol. Centralbl., 1895, Bd. XV, p. 855 ; Schiitt, Jahrb. f. wiss. Bot., 1899,
Bd. xxxm, p. 680.
* Nageli, Sitzungsb. d. Bayerisch. Akademie, 1879, p. 389; Ostwald, Biol. Centralbl., 1902,
Bd. xxii, p. 596.
5 On Brownian or ' molecular ' movements see Exner, Ann. d. Physik, 1901, n, 4, p. 843 ;
Lehmann, Molekularphysik, 1889, Bd. I, p. 264; Bd. II, p. 7. Seeds and spores which are not
wetted by water may be supported on the surface-tension film, and appear to float. Cf. Nageli,
Beitrage z. wiss. Bot., 1860, Heft ii, p. 105 ; Nageli u. Schwendener, Mikroskop, 1877* 2- Atifl.,
P- 377-
264 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
Mechanical factors of this kind are of the greatest biological impor-
tance1, for they aid in the dispersal of seeds, pollen-grains, spores, and
bacteria by wind and water. The same applies to many freely motile
organisms, which are only able to cover comparatively small distances by
the aid of their own activity, since their absolute velocity of movement is
small. Within these limits, however, the organisms are able to seek
out the regions where the best conditions for their nutrition and develop-
ment prevail.
SECTION 58. Ciliary Movement.
Most motile vegetable organisms possess fine hair-like protoplasmic
projections, which are termed cilia when small and numerous, flagellae when
long and few in numbers, although naturally transition forms occur. In
some cases the cilia are uniformly distributed, but in others are grouped in
one or more bundles, while the flagellae are usually restricted to a definite
point on the body 2.
Many of the gametes and zoospores of Algae, as well as the cells of
Chlamydomonas, have two flagellae attached at the germinal spot, while the
zoospores of Oedogonium have a group of large flagellae arranged around
the anterior hyaline end 3. In these radial objects the flagellae are placed at
the anterior end of the oval body, whereas in the dorsiventral Peridineae
and in the zoospores of Phaeophyceae they are laterally inserted. The
zoospores of Vaucheria have cilia over their whole surface 4, and the same
applies to the coenobia of Pandorina and Volvox, although the individual
cells have each a pair of cilia only. In another member of the Volvocineae,
Gonium, the individual cells are arranged to form a flat plate-like expansion
covered with cilia on one side.
Among Bacteria the cilia may either be distributed all over the body
or a tuft or a single cilium may be present at one or both ends. The latter
applies usually to Spirillum, which is spirally twisted like the sperms of
Ferns, although these have only the anterior end covered with a diffuse tuft
of cilia. The sperms of Mosses are rod-like in shape, and have only a pair
of cilia at the anterior end.
A few of the zoospores mentioned have cilia of unequal size, and in
many Flagellatae and Peridineae one of the flagellae is pointed in the
1 Cf. Ludwig, Biologic der Pflanzen, 1895.
8 Cf. Hertwig, Die Zelle und die Gewebe, 1893, p. 64.
3 See Hofmeister, Pflanzenzelle, 1867, p. 28 ; Falkenberg in Schenck's Handbuch d. Botanik,
1882, Bd. n, p. 194 (Algae); Zopf, Die Pilze, 1890, p. 61 seq. ; A. Fischer, Jahrb. f. wiss. Bot.,
1895, Bd. xxvil, p. 84 (Bacteria) ; Migula, System d. Bacterien, 1897, Bd. I, p. 97 ; Ellis, Centralbl.
f. Bact., 2. Abth., 1902, Bd. IX, p. 546. On animal organisms and certain lower Algae cf. Biitschli,
Die Protozoen, 1880-9. On Flagellatae and Peridineae cf. also A. Fischer, Jahrb. f. wiss. Bot., 1894,
Bd. xxvi, p. 330; Schiitt, Die Peridineen d. Planktonexpedition, 1895, p. in.
* Cf. Strasburger, Histologische Beitrage, 1900, Heft vi, p. 187.
CILIARY MOVEMENT 265
direction of locomotion, while the other trails behind like a rudder. Many
animals possess in addition to large motile cilia others which function as
organs of taste or touch, while the ciliated epithelium of Vertebrata no
longer serves for bodily locomotory but for other purposes.
All free-swimming forms possess cilia as locomotory organs, and these
either vibrate to and fro or, when large, perform a corkscrew-like action
through the water, drawing the organism after them. If the cilia or
flagellae are removed or thrown off, the movement of the organism ceases1.
In the case of minute bacteria, however, the movement of the cilia cannot
be directly followed. Even in the case of the swarm-spores of Myxomycetes
the free-swimming is due to the cilium and not to any amoeboid movement,
although this may be shown at the same time 2. Most zoospores, however,
even when naked, have no power of amoeboid movement, and there seems
to be no free-swimming organism devoid of cilia. The latter were recog-
nized as locomotory organs by linger 3, and Nageli's assumption that they
were only passively moved like the oars of a boat was shown by Siebold to
be incorrect4. The supposition that bacteria moved without the aid of
cilia was disproved by the detection of these organs by special methods of
fixing and staining5. Berthold6, however, assumes that the swarm-cells
of Erythrotrichia move without the aid of cilia, and it is not impossible
that locomotion might be produced by the backward ejection of water
absorbed laterally or anteriorly. That certain zoospores such as those of
Chromophyton rosanoffii'1 should be able to creep on the surface of the
water is not surprising, since the surface-tension film is capable of affording
the required resistance.
The forward movement is usually accompanied by one of rotation around
the organism's own axis, and the ciliated end is usually first 8. Under these
circumstances the cilia must draw the body onwards, whereas when they
are at the hinder end they must push it forwards. The latter is the case
in Chytridium vorax 9 and Polyphagus euglenae 10, and possibly it may be
1 Strong shaking often causes the cilia to be thrown off. Cf. Strasburger, Wirkung des Lichtes
u. d. Warme auf Schwarmsporen, 1878, p. 6. If a zoospore is nipped in two during its escape from
the zoosporangium, only the ciliated portion shows any free-swimming movement. Cf. Hofmeister,
1. c., p. 29.
* For instances see Plenge, Verhandl. d. naturh.-med. Vereins in Heidelberg, 1899, N. F.,
Bd. vi, p. 216; Kolkwitz, Bot. Centralbl., 1897, Bd. LXX, p. 186. On the mechanical distortions
of antherozoids cf. Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 394.
3 Die Pflanze im Momente der Thierwerdung, 1843, p. 93.
4 Nageli, Gattungen einzelliger Algen, 1849, p. 22 ; Siebold, Zeitschr. f. wiss. Zoologie, 1849,
I, p. 287.
5 Cf. A. Fischer, 1. c. ; also Migula, 1. c.
6 Berthold, Protoplasmamechanik, 1886, p. 125.
7 Woronin, Bot. Ztg., 1880, p. 630.
8 Nageli, Beitrage z. wiss. Bot., 1880, Heft 2, p. 96.
9 Strasburger, Die Wirkung des Lichtes u. d. Warme auf Schwarmsporen, 1878, p. 13.
10 Nowakowski, Cohn's Beitrage z. Biologic, 1877, Bd. n, p. 208.
266 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
of common occurrence, since this mode of progression is usually adopted
by the spermatozoa of animals1. In general, there is a definite relation
between the direction of movement and the direction of the main axis.
Thus in Volvox2 the vegetative or trophic pole goes first, while in the
ellipsoid zoospores of Vaucheria^ as well as in equipolar ellipsoid individuals
of Pandorina 3, the long axis is parallel to the direction of movement and in
the same line. In all these cases the continually active cilia must work
in harmony, since if they all acted in different directions, no definite
locomotion could be produced.
The same applies to the diffusely ciliated as well as to the bipolar
bacteria. Among the latter Spirillum undula is included, and it moves
alternately with one end first, and then with the other after a period of
rest 4. Intermittent movement is in fact shown by many motile organisms 6.
In the case of Spirillum it is not known whether the cilia at each end
undergo a periodic reversal in their mode of action, or whether only one
set acts at a time, and whether the two groups produce movement in
opposed directions. The organism may either follow a spiral path around
an ideal axis or may move along a straight or curved line parallel to
the long axis of the body. In the former case the ideal axis may either
be parallel or inclined to the long axis of the body 6.
The movements of the cilia are autogenic in character and either
pursue the same rhythm under constant external conditions or may be
subject to self-regulatory periodic inhibition or reversal7. As in the case
of other forms of movement, the external conditions may modify the
ciliary activity, and may under special circumstances produce a reversal of
the movement. The same result may on occasion be caused by an auto-
genic or aitiogenic modification of the orientation of the cilia in regard
to the body. Possibly it is in this way that Paramaecium is induced to
move in the opposite direction to the normal one when placed in 0-4
to 07 per cent, solutions of sodium chloride 8. According to Putter the
backward direction of movement continues until the organisms have accom-
modated themselves to the salt solution.
In many cases the impact against a foreign body causes the organism
1 Hertwig, Zelle u. Gewebe, 1893, p. 65.
3 Overton, Bot. Centralbl., 1889, Bd. xxxix, p. 68.
8 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, p. 443. The same applies to Gonium,
in which the long axis of the body is the shorter axis of the colony. On Stephanosphaera see Cohn,
Zeitschr. f. wiss. Zoologie, 1853, Bd. iv, p. 84.
4 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. II, p. 591 ; Migula, System d. Bacterien,
1897, Bd. I, p. 108.
5 Biitschli, Die Protozoen, 1880-9, p. 850.
6 Nageli, 1. c. Cf. also Biitschli, 1. c., p. 850. On the importance of the rotation of the body
in asymmetric organisms cf. Jennings, The American Naturalist, 1901, Vol. xxxv, p. 369.
7 Loeb, Pfliiger's Archiv f. Physiologic, 1897, Bd. LXVI, p. 533.
8 Putter, Arch. f. Anat. u. Physiol., physiol. Abth., Supplementband, 1900, p. 397.
CILIARY MOVEMENT 267
to withdraw somewhat while the rotation around its own axis is reversed.
The normal rotation and forward movement is then resumed and may again
produce an impact against an obstructing plate of glass l. At the same
time, the orientation is usually somewhat altered, so that on the next
forward movement the organism has a better chance of avoiding the
obstacle. In other cases the organism continues the normal rotation
around its own axis when the glass plate prevents any forward movement.
According to Nageli, organisms which normally move in a straight line
remain pressed against the same point of the glass, but perform circles
on the surface of the glass when they have a natural tendency to eccentric
or spiral movement. In other cases, as for instance when the organism
glides or creeps over a solid substratum, the former locomotion continues,
while the rotary movement ceases 2. The boat shape of the free-swimming
Bodo saltans causes the twisting movement of the cilia to produce a rocking
movement but no rotation 3.
Since both locomotion and rotation are due to ciliary activity, it is
not surprising that the same type of rotation should be retained so long
as the direction of movement is unaltered. The ciliary activity might,
however, easily be so modified as to reverse the. rotation without producing
any change in the direction of locomotion, but observations pointing to
this conclusion must be accepted with caution4. A reversal of this kind
does, however, appear to be satisfactorily established in the case of Gonium
pectorale 5.
The rapidity and duration of the movement are naturally very
dependent upon the external conditions. Antherozoids, as well as the
asexual zoospores of Algae and Fungi, come to rest after a definite period
of activity, which may be comparatively short. It is, however, possible
under special nutrient conditions to keep bacteria, the swarm-spores of
Myxomycetes, and possibly also many Flagellatae and Volvocineae, per-
manently motile, and to prevent the recurrence of any resting stage 6.
Even under favourable conditions the most active plant zoospores
do not attain the speed of movement of Infusoria, and progress but slowly
in absolute measure. The highest velocity does not appear to exceed
i mm. per second, and is often not above 0-05 mm. per second 7. A zoo-
1 Cf. Nageli, Beitrage zur wiss. Bot., 1880, Heft 2 ; Butschli, I.e., p. 854; Jennings, Centralbl.
f. Physiol., 1900, Bd. xiv, p. 106.
a See Nageli, 1. c., p. 101 ; Butschli, 1. c., p. 853; Schiitt, Die Peridineen d. Planktonexpedition,
1895, p. 117; Jennings, American Naturalist, 1901, Vol. xxxv, p. 372.
Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1888, Bd. n, p. 594.
Cf. also Butschli, I.e., p. 853.
Migula, Bot. Centralbl., 1890, Bd. XLIV, p. 104; Pfeffer, Unters. a. d. bot. Inst. zu Tubingen,
1884 Bd. I, p. 443. Cf. also Nageli, 1. c., p. 97. '
On the zoospores of Myxomycetes cf. Klebs, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 196.
According to Hofmeister, Pflanzenzelle, 1867, P- 3°> tne zoospores of Aethalium septicum
cover per second 0-7-0.9 mm., and those of Gonium pecloralt 0-046 mm. See also Nageli, I.e. ;
268 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
spore which moves at a speed of 03 mm. per second appears to move
very rapidly under a magnification of 300 diameters, since in one second
it appears to cover a distance of 60 mm., although it actually only traverses
720 mm. in the course of an hour. Relatively to their size, however, these
motile organisms are very active, for they may travel two or three times
their length in a second, whereas a man while walking may cover about
the half of his length in a second, an express-train may travel about one-
third of its length in a second, and the earth moves through a space of
about ?%-o- of its diameter per second as it rotates around the sun1.
A swallow may, however, cover TOO times, and a bee more than 1,000
times its length in a second, but here the movement is in a less resistant
medium. Many active fishes may cover their own length or several
times their length in a second, so that the zoospore is in this respect
inferior as a locomotory organism. Owing to the small size and relatively
large surface of the zoospore, it needs a greater expenditure of energy
per unit mass to give it the same velocity as a larger organism 2. Zoospores
may indeed drag with them adhering bodies greater than themselves.
Such forms as Chlamydomonas and Euglena respond at first by
a negatively geotropic movement when exposed to increasing centrifugal
action, and are only overcome by its mechanical action when its intensity
is eight times that of gravity. It follows that these organisms are able to
lift about eight times their own weight in water 3, and according to Jensen
Paramaecium may raise nine times its own weight. Owing to the small-
ness of the organism, however, about 600 would be required to raise one
milligram 4. To do this the two cilia of Chlamydomonas or the single one
of Euglena must develop as much energy as the cilia of ciliated epithelium 5.
The strength of these organisms, is, therefore, greater than that of a horse,
which is able to lift a load about its own weight, whereas an insect can
raise a load about sixty-seven times greater than its own weight6. In
any case it is only to be expected that the movements of cilia and of
Biitschli, 1. c. ; and Bd. II, § 143. On Bacteria see Lehmann, Centralbl. f. Bact., 1903, 2. Abth.,
Bd. x, p. 545.
Nageli, Beitrage zur wiss. Bot., 1880, p. 30.
Cf. Pfeffer, Studien zur Energetik, 1892, p. 255.
Schwarz, Ber. d. hot. Ges., 1884, p. 60.
Jensen, Centralbl. f. Physiol., 1893, Bd. VII, p. 568.
Cf. Engelmann in Hermann's Handbuch der Physiologic, Bd. I, p. 392.
See Jensen, 1. c. [These comparisons are without value, since in the one case the weight
lifted is in water, but in the other in air. The experiments on centrifugal action can only yield
accurate results when the relative densities of the Paramaecium and of the liquid are known.
A living Paramaecium is evidently not much denser than the liquid in which it lies, so that but little
more work is done when swimming upwards than when swimming downwards, and in any case the
actual lifting power is relatively trifling. A Paramaecium having a diameter of 0-2 mm. when
sphericalfwould have a volume of 7^ cub. mm., so that thirty living ones would be needed to lift
a mass of one cubic millimetre of inactive Paramaecia in water ; but the actual power of work cannot
be given.]
CILIARY MOVEMENT 269
zoospores should be considerably retarded in viscous media, and should
cease in moderately firm gelatine 1.
Cilia are living plasmatic organs which in some cases may protrude
through an investing cell-wall 2. They arise, therefore, in the same way
as pseudopodia, and like these may be retracted in certain cases 3. When
highly specialized, however, they are usually thrown off when injured,
but undergo the deformations characteristic of living protoplasm through-
out their whole substance. Whether cilia are connected with the nucleus
or with centrosomes or with special blepharoplasts (cilium formers, or
prominences bearing cilia), they possess a certain degree of autonomy
like other plasmatic organs. Hence ciliary movement may continue for
a time on separate non-nucleated fragments of a cell, or even on isolated
cilia4. Nevertheless attached cilia must be partially governed by the
cell to produce harmonious movement, although it is not certain whether
each cilium is isochronous or may vary its phases of movement within
certain limits. It is not necessary that the cilia should all be exactly
isochronous to produce the even harmony of movement in a colony of
Volvox 5 or Eudorina. According to Migula the cilia of the cells of Gonium
do not work as harmoniously and regularly as those of Volvox. Inter-
protoplasmic communications occur between the cells of Volvox 6, but have
not been detected in the case of Gonium^ and the existence of a dishar-
monic ciliary movement affords no proof of their absence. In ciliated
epithelium and in ciliated infusoria waves of action run over the cells or
body, each cilium bending over a little later than the one behind it, but
all retaining the same rhythm. The undulatory rhythm is maintained
by non-nucleated fragments of Infusoria, so that the regulation is due to
the ectoplasm7.
1 Pfeffer, Unters. a. d^bot.^Inst. zu Tubingen, 1884, Bd. I, p. 391.
a On the formation of cilia cf. Zimmermann, Beihefte z. bot. Centralbl., 1894, Bd. iv, p. 169 ;
A. Fischer, Jahrb. f. wiss. Bot., 1894, Bd. xxvi, p. 207; 1895, Bd. xxvn, pp. 34, 126; Strasburger,
Histologische Beitrage, 1900, Heft 6, p. 188; Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 520; Plenge,
Verhandl. d. naturh.-med. Vereins in Heidelberg, 1899, N. F., Bd. vi, p. 218; R. Hertwig, Archiv
f. Protistenkunde, 1902, Bd. I, p. 22 ; Maier, Archiv f. Protistenkunde, 1903, Bd. II, p. 73.
3 For instances see Strasburger, 1901, I.e., p. 521. According to Rothert (Ber. d. bot. Ges.,
1894, p. 277), the zoospores of Saprolegnia retract their cilia at the end of the first swarm-stage, but
not at the close of the second period of activity. On pseudopodia cf. Plenge, 1. c. ; Hertwig, Zelle
u. Gewebe, 1893, pp. 26, &c. ; Verworn, Allgem. Physiologic, 1901, 3. Aufl., p. 248. On the
pseudopodia of Amoeba radiosa, which vibrate like cilia, cf. Butschli, 1. c., p. 856.
* See A. Fischer, 1895, 1. c., p. 73 ; Plenge, 1. c., p. 261. On Infusoria cf. Verworn, Psycho-
physiol. Protistenstudien, 1889, p. 169; A. Fischer, I.e.; Jennings and Jamieson, Biological Bulletin,
i902,*Vol. ill, p. 225.
9 On Volvox cf. Klein, Jahrb. f. wiss. Bot., 1889, Bd. XX, p. 162 ; Migula, Bot. Centralbl., 1890,
Bd. XLIV, p. 104. .
6 Kohl, Beihefte z. bot. Centralbl., 1902, Bd. xu, p. 345; Klein, 1. c. ; Migula, I.e.; Goebel,
Organography, 1900, Vol. I, p. 28.
T Verworn, Psycho-physiol. Protistenstudien, 1889, p. 183. On ciliate epithelium cf. also Engel-
mann'in Hermann's Handbuch d. Physiologic, 1879, Bd. I, p. 385.
270 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
The mode of movement of cilia. In the case of typical ciliated epithelium
the cilia bend sharply over in one plane and then more slowly return to
their original position 1. Naturally rotation will in such cases only result
when the free-swimming body is appropriately shaped or the cilia specially
distributed. It is, however, not known to what extent this type of move-
ment occurs in the zoospores of plants. In most cases at least each
flagellum appears to curve in successive zones along its length in cork-
screw fashion, like a piece of string rotated at one end 2. Under the
microscope the movement appears to be more in one plane, and is carried
out either at the apical end or along the whole length of the flagellum.
If the movement is slowed by low temperatures or by the viscosity of
the medium, the spiral nature of the movement is more prominent, and the
photographs of the movement taken under high magnification by Marey 3
seem likely to be of great value. In some cases a flagellum may retain
a transitory or permanent spiral curvature, while others may describe
cone-like revolutions with or without a spiral curving along their lengths.
Among the Peridineae, according to Schiitt, one flagellum appears to perform
mainly cone- like revolutions, and the other to be thrown into spiral waves.
In some cases the character of movement is strongly affected by the
external conditions, but in what way the motor mechanism is affected
is uncertain.
When a flagellum is thrown into spiral waves the action is the same
as that of a screw fixed in the bow of a boat, a forward movement being
produced and the other component of the resolved force tending to produce
an axial twisting movement. In both cases, by reversing the motion, the
motile organ may push or draw the body onwards, just as in the case of
an ordinary screw-steamer, in which, however, the tendency to a rotary
movement is negligible. A slight contraction or spiral curvature of the
flagellum will not suffice to produce a forward movement, but will produce
a lateral one, especially if the flagellum is in contact with a solid body. In
this way a jerky locomotion may be produced in many swarm-spores.
SECTION 59. Gliding Movements.
These are shown by most Diatoms and Oscillarias and also by certain
Desmids, which possess no cilia and have no power of amoeboid movement.
1 Cf. Engelmann, and also Verworn, I.e.; Bergel, Centralbl. f. Physiol., 1900, Bd. xiv, p. 34.
On the spermatozoa of animals cf. Hensen in Hermann's Handbuch d. Physiologic, 1881, Bd. vi,
Abth. ii, p. 90.
a For details see Butschli, Die Protozoen, 1880-9, P- 850; Schutt, Die Peridineen d. Plankton-
expedition, 1895, p. 119 ; Kolkwitz, Bot. Centralbl., 1897, Bd. LXX, p. 185. Cf. also Pfeifer, Studien
zur Energetik, 1892, p. 255.
3 Marey, Compt. rend., 1892, T. cxiv, p. 989.
GLIDING MOVEMENTS 271
Diatoms and Oscillarias glide slowly over solid substrata or over moist
surfaces which serve as a fulcrum for movement. After a time the
direction of movement is usually reversed, the posterior end becoming
anterior. Since, however, the motion is usually along a more or less curved
path, the organism does not always regain its original position. If owing
to tropic stimulation the movement towards the light is more energetic
and lasts longer than that away from it, progression will on the whole be
made in a definite direction.
Usually Diatoms and Oscillarias glide along with one of their longer
surfaces lying mainly or entirely on the substratum, but they may some-
times raise themselves so far as to balance on one end. Except in the case
of Cylindrotheca and Nitzschiella1 Diatoms do not revolve during the
forward movement, whereas all Oscillarias show a rotation round the
longitudinal axis, which is genetically connected with the mode of locomotion,
and which is reversed when the direction of motion changes. Both in the
case of the rigid Diatoms and the flexible Oscillarias the movement takes
place without any appreciable bending of the body, although mechanical
curvatures are readily produced when a flexible Oscillaria comes into
contact with an obstacle or temporarily adheres to some fixed body. It is
in this way that the nodding, snaky, or jerky movements are produced
which are very pronouncedly shown by certain species 2. Some forms may,
however, be found to perform active autonomic curvatures due to heter-
auxesis, as do various species of Algae.
The locomotory energy can be shown to be developed on the outer
surface by the streaming movement of external protoplasm, which under-
goes a periodic reversal of direction. This is shown by the fact that
particles of sand or indigo adhering to the upper valve side of a fixed
Diatom are moved alternately backwards and forwards from one pole to the
other. We owe this observation to Siebold, and its confirmation to
Schultze 3. Although the detailed mode of locomotion of Oscillaria is
uncertain, here also adhering particles move on the outer wall, but in
correspondence with the rotation shown by filaments free to move, the
streaming protoplasm, as evidenced by the adhering particles, travels to and
fro in a spiral path around the filament.
Diatoms. Several authors 4 have considered the movement to be due to
1 Borscow, Die Susswasser-Bacillariaceen des siidwestlichen Russlands, 1873, p. 35.
2 Nageli, Beitrage z. wiss. Bot, 1860, Heft ii, p. 89; Correns, Ber. d. hot. Ges., 1897, p. 141;
Kolkwitz, ibid., 1897, p. 460.
3 Siebold, Zeitschr. f. wiss. Zool., 1849, Heft i, p. 284; Schultze, Archiv f. mikr. Anat, 1865,
Bd. I, p. 386.
* Nageli, Gattungen einzelliger Algen, 1849, P- 3O > Siebold, I.e. ; Dippel, Beitrage z. Kennt-
niss der in den Soolwassem von Kreuznach lebenden Diatomeen, 1870, p. 332; Borscow, I.e.;
Mereschkowsky, Bot. Ztg., 1880, p. 529.
272 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
the backward ejection of water, whereas Schultze, Pfitzer, and Engelmann l
concluded that it was the result of the movement of extracellular masses of
protoplasm, which by friction against the surrounding media produced
a forward movement in the opposite direction. The existence and mode of
action of the extracellular protoplasm was, however, first determined by
O. Miiller2, who showed that the protoplasm exudes through the polar
furrow on each of the valve sides, streams along the crevice of the raphe to
its termination at the median nodule, where each stream returns to the
interior, and travels back internally. Although most Diatoms usually lie
on one of the valve sides, some forms frequently lie on the girdle side
where the edges of the valves overlap, but even here sufficient energy is
developed to move the Diatoms along by the friction of the protoplasm
against the surrounding water 3. It is, however, only on the valve sides that
any movement of adhering particles by the streaming protoplasm can be
seen, and Schultze has shown that they are only moved as far as the end of
the valve and not around its edge 4.
The extracellular protoplasmic layer is extremely thin, but this does not
affect its frictional surface, and Mil Her 5 has shown that the rate of streaming
need not exceed 3 mm. per minute to produce a velocity of movement
of about i mm. per minute, which is approximately the maximal speed
shown by any Diatom. These values correspond very well with the
rates of streaming shown in large plant-cells, and Ewart has shown that
whereas the streaming protoplasm of a Diatom may perform 0-5 to 0-8 erg
of work per minute per gram of moving plasma, the streaming protoplasm
of large plant-cells only performs 0-18 to 0-22 erg per minute per gram of
streaming protoplasm 6.
Although Diatoms may adhere to a surface-tension film and creep
along it, the exceptional cases of free-swimming observed by Pfitzer have
been denied existence by Miiller7. It is, however, impossible to doubt that
a slowly-sinking Diatom would show lateral progression if its long axis
was more or less horizontal, and if the protoplasm was streaming in the
usual manner along the raphe. Under such circumstances a tendency
1 Schultze, Archiv f. mikr. Anat. 1865, P- 388 J Pfitzer, Unters. iiber Bau u. Entwickelung d.
Diatomeen, 1871, p. 176 (in Hanstein's Bot. Abhandl., Bd. i) ; Engelmann, Bot. Ztg., 1879, P- 54-
8 O. Miiller, Ber. d. hot. Ges., 1899, p. 445; 1897, p. 70; 1889, p. 169. Summaries are given
by Karsten, Die Diatomeen d. Kieler Bucht, 1899, p. 163 ; Klebhahn, Archiv f. Protistenkunde,
1902, Bd. i, p. 429. Lauterborn (Unters. iiber Diatomeen, 1896, p. 113) has recently adopted
Miiller's views.
* O. Miiller, Ber. d. bot. Ges., 1894, p. 143 ; Karsten, 1. c., p. 165 ; Benecke, Jahrb. f. wiss. Bot.,
1900, Bd. xxxv, p. 551.
* O. Miiller, 1. c., 1894, p. 143 ; M. Schultze, 1. c.
5 O. Miiller, I.e., 1897, p. 75. Muller (I.e., 1896, p. 121) observed velocities of 0-007 to
0-017 mm- per second. The colourless forms move most rapidly, according to Benecke (1. c.).
6 Ewart, Protoplasmic Streaming in Plants, 1903, p. 31.
7 Pfitzer, I.e., p. 176. Cf. O. Muller, Ber. d. bot. Ges., 1896, p. 128.
GLIDING MOVEMENTS 273
to a waltzing movement would be shown when, as frequently happens,
the protoplasm streams in opposite directions on the two valve sides. The
moment of the couple is not, however, great enough to produce rotation
around the short axis in forms lying on the substratum, the opposed forces
mutually antagonizing so that the movement ceases l. All Diatoms do not
possess an investing layer of mucilage, which is, therefore, not essential to
movement2. When present it is either set in motion by the streaming
protoplasm or the latter by friction against it gives the organism an
onward movement. In both cases a trail of mucilage is often left behind,
and this has in some cases given rise to the idea that Diatoms possessed
a motory flagellum, while the same appearance probably gave rise to the
theory of propulsion by a backwardly directed water-jet. In any case the
resulting trail, as in the case of Oscillarias, serves to indicate the path of
movement 3, but whether the movement may also be induced or aided by
the extrusion of masses of mucilage must remain at present an open
question4. The existence of attached Diatoms serves to indicate that all
the members of this group do not necessarily behave similarly or develop
the power of independent locomotion.
Oscillariaceae. The existence of a power of movement in these plants
has been known since the time of Adanson (i767)5, and the threads are
usually covered by a gelatinous sheath in which or with which they move 6.
Continual secretion keeps the apex covered with the mucilage, in spite of
that which is left behind along the path of movement. The locomotion is
not connected with any power of curving, since it is also shown when the
filaments remain perfectly straight, but no sure proof has as yet been
brought forward of the existence of extracellular protoplasm 7. It is in
fact uncertain whether the locomotion results from the exudation and
swelling of mucilage or from an appropriate development and utilization of
surface-tension energy. Hansgirg supposed that the ejection of water
produced the movement, but all the weight of evidence is against this
assumption, for if such action existed perceptible signs of it would be
detected upon minute neighbouring suspended particles. Cilia do not appear
1 M. Schultze, Archiv f. mikr. Anat., 1865, Bd. I, p. 386. Cf. Benecke, I.e., p. 553.
3 O. Miiller, I.e., 1897, p. 81.
s M. Schultze, 1. c., p. 399 ; O. Miiller, 1. c. ; Lauterborn, 1. c., &c.
* Cf. Schlitt, Jahrb. f. wiss. Bot., 1899, Bd. xxxin, pp. 645, 656 ; Ber. d. hot. Ges., 1903, p. 202 ;
O. Miiller, Ber. d. bot. Ges., 1899, p. 445 ; 1900, p. 481 ; 1901, p. 195.
6 Meyen, Pflanzenphysiol., 1839, Bd- In> P- 5^3 5 Mohl, Vegetabilische Zelle, 1851, p. 136;
Nageli, Beitrage z. wiss. Bot., 1860, Heft 2, p. 89; Correns, Ber. d. bot. Ges., 1897, p. 141 ; Kolk-
witz, Ber. d. bot. Ges., 1897, p. 460; Hansgirg, Bot. Ztg., 1883, p. 831 ; Physiol. u. Phycophytol.
Unters., 1893, p. 207 ; Brand, Beihefte z. bot. Centralbl., 1903, Bd. XV, p. 53.
6 Cf. Hunger, Biol. Centralbl., 1899, Bd. xix, p. 385; Schroder, Verhandl. d. naturh.-medic.
Vereins zu Heidelberg, 1902, Bd. vn, Heft 2, p. 187.
7 Cf. Engelmann, Bot. Ztg., 1879, P- 54-
PFEFFER. Ill T
274 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
to be present, although certain observers appear to have mistaken adhering
flagellate bacteria for them.
Although foreign particles adhering to or embedded in the mucilaginous
sheath may be moved along spiral lines around the filament, this is not
shown over the whole length of the filament, and according to Correns
a portion of the filament always adheres to the substratum during loco-
motion. The latter never exceeds 0004 of a mm. per second, a velocity
which does not suffice for free-swimming movement. Nageli 1 and Kolk-
witz 2 state, however, that free-swimming is shown on rare occasions, and
Kolkwitz also observed a creeping locomotion on the surface of the
water.
The radiating arrangement of Oscillaria colonies when growing on
moist substrata 3 probably results from the realized movements along the
path of least resistance. Similar groupings may be shown by the threads
of Spirogyra and by Diatoms4. Hansgirg concludes that stimulatory
reactions also come into play, but without bringing forward any definite
proof5.
Desmidiaceae. The slow movements of Desmids are due, according to
Klebs, to the excretion of mucilage, and it is in fact easy to see that
certain forms are actually raised to a certain height in water by the
formation of a gelatinous stalk 6. Progression would also be possible over
a substratum by the continued forcible excretion of mucilage from the
hinder end. Many Desmids progress with one end only resting on the
substratum, the body being inclined obliquely upwards, while the attached
end may change from time to time. This applies to Closterium moniliferum,
which shows a phototactic progression to light by turning repeatedly over
so that first one end and then the other is attached to the substratum as it
moves towards the light 7.
It does, however, seem probable that the locomotion is due to the
regulation of the excretion of mucilage, although it does not follow that
the same means of locomotion is used in all cases, and in fact many
Diatoms and a few Desmids adhere very firmly to stones and rocks. In
any case it is worthy of note that if a snake were reduced to the size of
Nageli, Beitrage z. wiss. Bot., 1860, Heft ii, p. 90. J L. c., p. 466.
Nageli, I.e., p. 91.
Schultze, Archiv f. mikr. Anat, 1865, Bd. I, p. 396.
Hansgirg, 1. c., 1893, p. 207.
Klebs, Biol. Centralbl., 1885, Bd. V, p. 353 ; Unters. a. d. hot. Inst. zu Tubingen, 1886, Bd. II,
p. 383 ; Stahl, Bot. Zeit., 1880, p. 397 ; Verhandl. d. phys.-med. Ges. in Wiirzburg, 1879, Bd. XIV ;
Aderhold, Jenaische Zeitschrift f. Natnrw., 1888, N. F., Bd. XV, p. 333. On the excretion of
mucilage by Desmids cf. Schiitt, Jahrb. f. wiss. Bot., 1899, Bd. xxxm, p. 676 ; Schroder, Verhandl.
d. naturh.-med. Vereins in Heidelberg, 1902, N. F., Bd. VII, p. 139; Lutkemiiller, Cohn's Beitrage
z. Biol., 1902, Bd. vin, p. 347.
7 Stahl, I.e.
GLIDING MOVEMENTS 275
a Desmid, it would be extremely difficult, even under the highest powers of
the microscope, to detect its mode of progression by moving the ventral
scales attached to the ribs.
SECTION 60. Amoeboid Movement.
Pronounced amoeboid movements are only shown among plants by
the plasmodia and swarm-spores of Myxomycetes, as well as by the
zoospores of a few Fungi, and the zobspores and tetraspores of a limited
number of Algae l. All other gymnoplasts (zoospores, ova, &c.) and also
plasmolysed protoplasts show no power of amoeboid movement, although
slow internal amoeboid movements may be possible, and do often in fact
cause alterations in the shape of the vacuoles. In addition, the reproductive
nuclei of the pollen-tubes of Phanerogams appear to be capable of slow
amoeboid change of shape. The same applies to the nuclei in the epidermal
hairs of Tradescantia and in the leaf-cells of E lode a canadensis, and the
movement appears to become more active under the action of asparagin 2.
The protrusion of the pseudopodia is often followed by retraction, but
progression is possible in a definite direction when a pseudopodium steadily
enlarges until the whole body has flowed into it3. The pseudopodia of
Rhizopoda are extremely fine and slender, whereas Myxomycetes, in
addition to forming short fine pseudopodia, also produce broader-lobed,
fan-like or netted expansions. In plasmodia the amoeboid activity appears
to undergo an autonomic alternation from one side to the other, which
causes a to-and-fro streaming of the fluid contents. The latter is always
directed towards the developing pseudopodia, but it is not shown in all
cases 4, as for instance in the zoospores of Myxomycetes. These are also
provided with cilia, and when swimming freely often perform leaping or
backward movements when the cilia collide with a resistant body 5.
Amoeboid locomotion is a form of vital activity which is not ex-
plained by saying that it is due to the expansion and contraction of the
protoplasm. To speak of the rounding off under strong excitation as being
due to a spherogenic activity, and the re-expansion as being due to
1 Berthold, Protoplasmamechanik, 1886, p. 94 ; de Bary, Morphologic u. Biologic d. Pilze,
1884, p. 174; Zopf, die Pilze, 1890, p. 102. On the amoeboid movements of Protozoa cf. Hertwig,
Die Zelle u. d. Gewebe, 1893, p. 55 ; Verworn, Allgemeine Physiologic, 1901, 3. Aufl., p. 244.
3 Kohl, Bot. Centralbl., 1897, Bd. LXXII, p. 168. Cf. also Mottier, Fecundation in Plants,
1904. Amoeboid movements are often shown by the nuclei of animals.
3 De Bary, Morphologic u. Biologic d. Pilze, 1884, p. 453; Zopf, in Schenk's Handb. d.
Botanik, 1887, Bd« in, 2. Halfte, p. i ; Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890,
p. 256. On Amoeba and other animals cf. Khumbler, Archiv f. Entwicklungsmechanik, 1898,
Bd. viil, p. 114; Jensen, Die Protoplasmabewegung, 1902, Sep. a. Ergebnisse der Physiol., I. Jahrg.
* Cf. Berthold, 1. c., p. 109 ; Jensen, 1903, 1. c., p. 14.
5 De Bary, I.e., p. 954; Fayod, Eot. Zt£., 1883, p. 171.
T 2
276 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
a cylindrogenic activity 1 is merely to play with useless terms. The
adherence of the body to the substratum which permits of the extrusion
of a pseudopodium does not necessarily require the extrusion of mucilage 2,
and the retraction of the hinder pseudopodia during onward movement
merely involves the overcoming of the adhesion of the pseudopodium to
the substratum. It is, however, not impossible that the degree of adhesion
is capable of autogenic modification and that a mode of progression some-
what resembling that of the foot of a snail is involved.
The movement is about as rapid as that of Diatoms and Oscillarias,
since under favourable circumstances it does not exceed 0-006 mm. per
second. Hofmeister 3 gives a velocity of 0-4 mm. per minute for Didymium
serpula, and one of 0-15 of a millimetre per minute for Stemonitis fusca.
In certain animals, however, the pseudopodia are rapidly protruded and
retracted. The fact that plasmodia may creep upwards over moist sub-
strata in air shows that the energy of movement is sufficient to support the
weight of the organism and even to raise a somewhat greater load. The
latter cannot, however, be very great owing to the feeble cohesion of
the protoplasm. Hence in soft gelatine locomotion is arrested, although
periodic attempts at amoeboid movement may be recognized 4. Small
amoebae as well as leucocytes may, however, be able in virtue of their
plasticity to worm their Way through minute pores, which they either find
at their disposal, or which they produce by a solvent action like that of
parasitic Fungi 5.
SECTION 61. The Mechanics of Amoeboid Movement.
It was assumed by Mohl and Nageli that protoplasm has the
properties of a viscous fluid, and no doubt now exists that this is true
in the great majority of cases. The views of certain authors that proto-
plasm is a colloidal solid are incorrect, although naturally no hard and
fast boundary exists between such solids and viscous liquids 6. Evidence
of the liquid nature of protoplasm is afforded by the spherical shape
assumed by isolated portions of protoplasm when suspended in a liquid
of the same density, as well as by the rounded shape of the vacuoles. The
existence of streaming movement 7, the drawing out of the protoplasm
Jensen, Die Protoplasmabewegung, 1903, p. 7.
De Bary, 1. c., p. 458 ; Rhumbler, 1. c., p. 158 ; Jensen, 1. c., p. 36.
Hofmeister, Pflanzenzelle, 1867, p. 23. Cf. also Jensen, I.e., p. 15.
Pfeffer, 1. c., p. 277. On the work done by the pseudopodia of animals cf. Jensen, I.e., p. 14.
On Plasmodiophora see Nawaschin, Flora, 1899, p. 404.
Hofmeister, Pflanzenzelle, 1867, p. I ; Berthold, Protoplasmamechanik, 1886, p. 85; Pfeffer,
Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 267; Biitschli, Unters. iiber mikroskop.
Schaume, 1902 ; Rhumbler, Archiv f. Entwickelungsmechanik, 1898, Bd. vii, p. 172 ; Zeitschrift f.
allgem. Physiologic, 1902, Bd. I, p. 279; 1903, Bd. II, p. 183.
7 Ewart, Protoplasmic Streaming in Plants, 1903, p. 16.
THE MECHANICS OF AMOEBOID MOVEMENT 277
into threads, and its general lack of resistance to mechanical agencies
including surface-tension all afford evidence of its liquid consistency. The
viscosity of the protoplasm, varies, however, apart from the direct physical
action of temperature upon it1, but it is usually viscous enough to prevent
the vibratory molecular movement of small particles embedded in it 2,
and to stop the locomotion of ingested bacteria and Volvocineae 3, whereas
both these movements continue in the cell-sap. The viscous nature of the
protoplasm renders the removal of protoplasmic aggregations slow in the
absence of streaming movement, or even in its presence 4, and also results
in the plasmolysed protoplasts of elongated cells breaking up less readily
into fragments as compared with similar threads of water. External
agencies may, however, increase or decrease the viscosity of the proto-
plasm either by a direct or indirect action5. It may ultimately be possible
to determine the actual viscosity of the protoplasm in streaming cells,
by measuring the amount of slip of minute oil particles of known
density under the action of gravity 6.
The same physical factors which regulate the spread of liquids over
solid surfaces, and the creeping movements in emulsions of oil and soap_
solution, are also involved in determining the shape and movement of the
more or less viscous protoplasm 7. Thus the spherical shape assumed by
a plasmolysed protoplast floating in a liquid of the same density is due to
its homogenous surface-tension. If the protoplast is very small it will assume
an approximately spherical shape even when resting on a solid substratum
just as do sufficiently small drops of mercury, and also drops of water while
in the spheroidal condition on a white-hot plate. A local decrease of surface-
tension will cause a prominence to appear at that point, and this will
continue until the lesser radius of the protrusion enables it to exercise the
same centrally directed pressure, as does the larger sphere with a higher
surface-tension. The same applies when a distended balloon bulges out at
a weak point. If the position of equilibrium is passed and the difference of
surface-tension is maintained, the entire mass will be pressed towards the side
of least surface-tension, and the impelling force will automatically increase
as the radius of curvature of the original body lessens. Drops of a non-
miscible fluid lying in a medium or on a substratum may be caused to
assume all varieties of form by appropriate local modifications of surface-
tension. The spreading movements of a drop of oil upon an alkaline
Ewart, Protoplasmic Streaming in Plants, 1903, pp. 20, 59.
Vellen, Flora, 1873, p. 120.
Celakovsky, Flora, 1892, Ergzbd., p. 223. * Ewart, 1. c., p. 9.
Ewart, 1. c., pp. 10-20, 36, 38. 6 Id., p. 23.
For details see textbooks of Physics, and Lehmann, Molecularphysik, 1888, Bd. I, p. 351.
Also Berthold, 1. c. ; Butschli, 1. c. ; Rhumbler, 1. c. ; Jensen, Pfliiger's Archiv f. Physiologic, 1901,
Bd. LXXXVII, p. 366, and the works of Quincke here quoted.
278 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
solution da in fact resemble the amoeboid movements of certain
organisms l.
Locomotion is possible in a definite direction without any pronounced
amoeboid changes of shape, as for instance when a drop of olive-oil, or
of paraffin-oil, lying in water is in contact with a soap-solution on one
side. The local diminution of surface-tension produced on this side causes
the drop to bulge towards the soap-solution, while the peripheral layer of oil
is drawn by the surface-tension film from the region of low to that of high
tension. In this way a definite circulation is maintained in the oil-drop, and
also in the neighbouring soap-solution, for as the film is drawn over the
drop the adhering soap-solution is diluted and the surface-tension raised 2.
At the same time the drop of oil progresses towards the soap-solution,
that is to the side of least surface-tension3. In the same way drops of
castor-oil floating in dilute alcohol move towards chloroform, or potassium
hydrate4,, and according to Bernstein5 drops of mercury move towards
potassium bicromate.
Similar effects are shown when a drop of a mixture of olive-oil and
potassium carbonate is placed in pure water, the lowering of surface-tension
being in this case produced by the soap, which rises to the surface of the
drop and spreads over its exterior 6. The protoplast can, therefore, always
create the physical conditions for a change of shape or for streaming by
appropriate metabolic activity. In the same way by the activity of the
nucleus, of the chloroplastids, or of the surrounding protoplasm, internal
changes of surface-tension may be produced capable of causing changes of
shape or internal streaming. The cytoplasm of streaming cells does
in fact appear to behave like an emulsion in which the surface-tension
changes on the individual drops are responsible for the movement, and in
which the whole energy of movement is liberated internally 7.
Changes of surface-tension inducing movement may also be produced
by electrical means. Thus Ermann observed in 1809 that if the positive
terminal of an electrical battery was connected with a drop of mercury
lying in dilute sulphuric acid, and the negative terminal was placed in
the acid, the mercury moved away from the positive pole. The principle
is in fact the same as that of Lippmann's capillary electrometer, and
a feeble current will produce relatively considerable movement.
The amoeboid movements of fluid masses of protoplasm can only be
Cf. Berthold, Protoplasmamechanik, 1886, p. 96.
Butschli, 1. c., p. 43. Additional instances are given by Berthold and Rhumbler.
Butschli, 1. c., p. 44.
Rhumbler, Physikalische Zeitschrift, 1899, Nr. 3.
Bernstein, Pfluger's Archiv f. Physiologic, 1900, Bd. LXXX, p. 628.
Cf. Butschli, 1. c., p. 33.
Cf. Ewart, Protoplasmic Streaming in Plants, 1903, pp. 113, 116.
THE MECHANICS OF AMOEBOID MOVEMENT 279
due to changes of surface-tension when the protoplast is freely suspended
in a liquid, but in other cases the cohesion of the peripheral layers may
more or less counteract the direct action of surface-tension. In Myxo-
mycetes, for instance, the ectoplasm appears to be more solid in consistency
than the central endoplasm, and it forms a layer of variable thickness
and properties. Changes of consistency may often play an important part
in amoeboid movement, as well as in the formation of cilia, and many
protoplasts appear able to raise their consistency when necessary *, or to
render themselves solid like gelatine by forming a slender framework of
solid substance in which the more fluid materials are embedded. The
cell-wall is in fapt a peripheral skeletal structure formed either by external
secretion or by protoplasmic metamorphosis. All stages of transition may
be shown between a viscous liquid and a colloidal solid, just as during the
solidification of melted gelatine.
Since the protoplast is able to dissolve away its cellulose investment
in case of need, it is not surprising that the increased consistency of the
protoplasm should be capable of decrease, as appears to be the case
in Myxomycetes. When this occurs, surface-tension again becomes of
predominant importance, and may be able to produce the retraction
of prominences formed in the more solid condition. In case the expansion
and contraction are produced by an antagonism of this kind, a pseudo-
podium may be produced without any change of surface-tension, whereas
otherwise a very strong depression of surface-tension would be required.
No definite conclusions can be made as yet, for the cohesion and viscosity
of the protoplasm cannot at present be accurately determined, and in
addition it is not known to what extent the protoplast may raise or lower
its general surface-tension.
Even when a particular mechanical action has been proved to be due
to surface-tension energy or some other agency, it still remains to be
determined how the supplies of energy are controlled and utilized, and
how the conditions for their action are produced. Apart from its physical
action surface-tension energy and like forces may act as stimuli and induce
special responses. It is easy to see how the firmer ectoplasm of the
plasmodia of Chondrioderma, Aethalium, and other Myxomycetes is
produced from fluid endoplasm, and may be reconverted into the latter 2.
The ectoplasm may be o-oi mm. thick, and is, therefore, more than a mere
surface-tension film, and is much thicker than the ectoplasmic membrane.
Its production is the direct result of its peripheral position, and similarly
the cell-wall and the ectoplasmic membrane are only formed on the surface
1 Cf. Pfeffer, Zur Kenntniss der Plasmahant u. d. Vacuolen, 1890, p. 255 ; Rhumbler, Zeitschrift
f. allgem. Physiologic, 1902, Bd. I, p. 281.
* Pfeffer, Zur Kenntniss der Plasmahaut u. d. Vacuolen, 1890, p. 256.
280 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
of the protoplasm. The ectoplasm may not only include the somewhat
thinner layer of hyaloplasm, but also a little of the neighbouring layers
of granuloplasm.
By using plasmodial threads of about 0-3 mm. in thickness, in which
the surface-tension effect is small, Pfeffer1 was able to determine that
the consistency was about that of a jelly, and the same is shown by the
way in which moving particles are repelled from the surface layers without
producing any perceptible deformation or inducing any streaming move-
ment. Similarly oil-drops and vacuoles passing through a tube of
ectoplasm are compressed and distorted without producing any bulging
in the tube. The appearance closely resembles that shown when fluid
gelatine containing suspended particles is passed through a fine glass tube
kept lined with a layer of solidified gelatine2. The existence of such
a condition of cohesion in the peripheral layers renders it impossible
that amoeboid movement can be directly and solely due to a modification
of surface-tension. In addition, the increased surface involves an increase
of the total amount of ectoplasm, and this is probably the result of the
same vital activity which yields the energy for movement. It is, however,
uncertain whether the retraction of a pseudopodium is due to an active
change of shape or to a softening of the ectoplasm allowing surface-tension
to come into play. Possibly both factors may co-operate. The streaming
movement of the endoplasm is probably the direct result of the successive
contractions and expansions, the direction of movement being towards
the region of expansion. Hence but little endoplasm escapes when a
plasmodial thread is severed 3, whereas when an internodal cell of Chara
is cut open a large quantity of the streaming endoplasm may escape4.
When the streaming is reversed in a thread the return movement begins
at the end nearest to where the expansion or contraction is most active 5.
It is always possible that the amoeboid movements of certain
organisms may be solely due to surface-tension 6, although the arguments
of the different authors supporting this view are mainly based upon
the assumption that the whole of the protoplasm is fluid. In many
Amoebae, however, the temporary presence of a firm ectoplasm has been
1 Pfeffer, Zur Kenntniss der Plasmahaut n. d. Vacuolen, 1890, p. 264.
3 Id., p. 263.
3 De Bary, Mycetozoen, 1864, p. 48.
' See Cord, Meyen, Pflanzenphysiologie, 1838, Bd. II, p. 218. [The shock always causes
a temporary stoppage of streaming, and the phenomenon is only shown properly if streaming is
resumed again before death ensues, which is not always the case.]
5 Cf. de Bary, I.e., p. 48 ; Biitschli, 1. c., p. 175.
* Berthold, Protoplasmamechanik, 1886, p. 85 ; Biitschli, Unters. liber mikroskopische Schanme,
1892, p. 172; Verworn, Die Bewegungen der lebendigen Substanz, 1892, p. 36; Rhumbler, Archiv
f. Entwickelungsmechanik, 1898, Bd vm, p. 171; Zeitschr. f. allgem. Physiologic, 1902, Bd. I,
p. 279 ; 1903, Bd. II, p. 183 ; Jensen, Pfliiger's Archiv f. Physiologic, 1901, Bd LXXXVII, p. 361.
THE MECHANICS OF AMOEBOID MOVEMENT 281
proved1, and the long and slender pseudopodia of many animals appear
always to have an axial rod of firmer material 2, which acts as a skeletal
framework and appears to be capable of apical growth by the reversible
solidification of protoplasm streaming on the surface by the aid of surface-
tension energy. Even rapid amoeboid movements may involve changes
of cohesion, and the fact that all strong stimuli cause a tendency to
the assumption of a spherical shape may point either way. The fact
that most cilia are incapable of retraction indicates that they have
differentiated into solid organs, and are not liquid protrusions maintained
by special conditions of surface-tension. The contractible myoid fibres
in the stalk and protoplasm of a Vorticella are also solid structures 3.
Rhumbler4 now adopts the view that amoeboid movements may be
aided by changes in the consistency of the ectoplasm, but the possible
complexity of the conditions in motile organisms is indicated by the fact
that Blochmann5 found the rapid locomotion of Pelomyxa produced
streaming in the surrounding water in the opposite direction to that
caused by the movement of a drop of oil towards a soap-solution.
According to Blochmann, this is due to the fact that a special streaming
movement takes place on the surface of the organism.
Whether surface-tension or other sources of energy are employed or
not, the causes which determine the changes of cohesion still remain to be
determined. In addition, surface-tension energy may be brought into
play in the interior of the protoplasmic emulsion wherever non-miscible
substances are in contact, and in this way much greater total manifesta-
tions of energy are possible than when only the external surface-tension
comes into play 6. It is of course always possible that the special surface
conditions may directly induce or affect the changes of cohesion in the
peripheral layers, and Quincke has shown that surface-tension does affect
the formation of precipitation membranes7.
The foam structure which appears to be characteristic of protoplasm 8
produces for physical reasons a maximum consistency with a minimum
of material 9, but does not cause the endoplasm to lose its fluid character.
The solidity of the mass increases as the emulsion becomes finer, but the
rise of cohesion in Myxomycetes does not appear to be produced in this
Rhumbler, Zeitschr. f. allgem. Physiol., 1898, p. 195. 9 Id., p. 114.
Cf. Verworn, Allgem. Physiologic, 1901, 3. Aufl., p. 252.
Rhumbler, I.e., 1903, Bd. II, p. 315.
Blochmann, Biol. Centralbl., 1894, Bd. XIV, p. 82.
Cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 112 seq.
Quincke, Annalen der Physik, 1902, Bd. vn, pp. 631, 701.
Cf. Butschli, Archiv f. Entwickelungsmechanik, 1901, Bd. xr, p. 499; Rhurabler, I.e., 1903,
Bd. n, p. 326.
9 Lehmann, Molecularphysik, 1884, Bd. I, p. 257; Quincke, Ann. d. Physik, 1894, Bd. LIU,
p. 616 ; 1902, Bd. xvn, p. 639.
282 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
way, and can only be due to an actual increase of cohesion in the walls of
the films or in their contents. In the more solid portions of the protoplasm
of Aethalium, for instance, the reticulum is as thick in relation to the
meshes between as in the fluid endoplasm. The altered cohesion might
either result from chemical or physical changes in the actual substance of
the reticulum, or might be due to modifications of the surface-tension
between the walls of the meshes and their contents. In any case, however
probable the foam structure may seem, it is not necessarily essential, and
the facts collected by Rhumbler1 fail to afford absolute proof of its
universal existence. The properties of colloids may in part be due
to their reticulate structure2, although surface-tension forces are less
prominent than in a froth of soap bubbles or in an emulsion of two
non-miscible substances.
Active changes in the shape of the walls of the chambers will naturally
produce corresponding alterations in the shape of the mass of protoplasm.
In addition, the contents of the chambers might be capable of expanding
and contracting by imbibing and extruding water. Internal osmotic
actions can, however, only come into play when the walls of the chambers
are semi-permeable, as are those of small vacuoles. Pulsating vacuoles
afford striking instances of rapid expansion and contraction, and when the
vacuoles are small the expansion takes place against a strong tendency
to contraction due to surface-tension. In the filaments of Cynareae the
individual cells are the actively contractile elements, and although the
energy of movement is here due to changes of turgor, it might also be
produced by changes of tension in the walls of the cells, or of the proto-
plasmic meshes. It is indeed not impossible that the mechanism of
movement in the stamens of Cynareae may resemble that of cilia, although
the latter has still to be determined
The power of movement of protoplasm was ascribed to its general power of con-
tractility8, until Hofmeister4 attempted to show that it was due to changes of imbibition
or swelling. Engelmann 6 also concluded that owing to Imbibition changes of shape
occurred in the molecular aggregations termed inotagmas by him. The importance
of the surface-tension of the superficial layer of protoplasm was first put forward by
Berthold, while Butschli showed that the surface-tension actions in an emulsion
were even more important. In finely divided emulsions enormous amounts of surface-
tension energy may be brought into play, which far more than suffice for the ordinary
protoplasmic movements 6, and may even be able to produce the whole of the energy
1902
Rhumbler, Zeitschr. f. allgem. Physiol., 1903, Bd. II, p. 327.
Cf. Posternak, Ann. de 1'Institut Pasteur, 1901, T. xv, p. 85; Pauli, Naturw. Rundschau,.
Bd. xvn, Nr. 25.
Cf. Butschli, Unters. iiber mikroslcopische Schaume, 1892, p. 173.
Hofmeister, Flora, 1865, p. 7; Pflanzenzelle, 1867, P- 63-
Engelmann, Handbuch d. Physiologic von Hermann, 1879, Bd. I, p. 374.
Cf. Ewart, Protoplasmic Streaming in Plants, 1903, pp. 26, 114, 116.
'•
THE MECHANICS OF AMOEBOID MOVEMENT 283
of contraction in muscle *. The exact part played by this internal surface-tension
energy in ciliary, amoeboid, and muscular movement is not yet fully established, but
in any case the minute subdivision into fibrillar chambers containing liquid sarco-
plasm gives muscle the properties of a soft solid, and not of a liquid, as Jensen
supposes 2.
In the present uncertainty it is impossible to say whether the changes of
surface-tension are produced by the excretion of metabolic products or in other ways.
Verworn3 assumed that amoeboid movements were due to the combination of oxygen
with the superficial biogens lowering the surface-tension, and that the use or
dissociation of the oxygen caused the surface-tension to be raised again.
Jensen 4 supposed that the increase in the size of the superficial molecules produced
by assimilation lowered the surface-tension, while the diminution in the size of the
molecules produced by dissimilation raised it. Neither hypothesis is, however,
capable of proof, although the fact that amoeboid movement and protoplasmic
streaming often continue for a long time in the absence of oxygen, and the existence
of motile ciliate anaerobic bacteria show that Verworn's hypothesis cannot possibly be
of general application. These theories also assume the predominant importance
of surface-tension and neglect the part often played by changes of consistency and
cohesion. The fact that external agencies when intense usually produce retraction
affords no conclusive evidence, and merely shows that under these circumstances
the conditions for expansion are suppressed.
SECTION 62. Protoplasmic Streaming.
The protoplasm of many dermatoplasts exhibits streaming movements,
which may either be confined to the layer enclosing the central vacuole
(rotation), or may also follow more or less irregular paths up and down the
bridles of protoplasm crossing the latter (circulation). These two types
are, however, merely the direct result of the protoplasmic configuration,
for no cell in which protoplasmic strands cross the vacuole shows regular
rotation. In addition, when the application of external stimuli causes the
protoplasmic strands to be retracted so that a single uninterrupted central
vacuole is present, the previous circulatory streaming passes into rotation 5.
At the same time the velocity of streaming increases owing to the
diminished internal friction, possibly aided by an increased liberation of
propulsive energy 6. In the adult leaf-cells of Vallisneria and Elodea and
in the internodal cells of Chara and Nitella the direction of streaming
1 Cf. Bernstein, Pfliiger's Archiv fur Physiol., 1901, Bd. LXXXV, p. 305 ; Naturwiss. Rundschau,
1901, Bd. xvi, Nos. 33-5.
Jensen, Pfliiger's Archiv fur Physiol., 1900, Bd. LXXX, p. 327.
Verworn, Allgem. Physiol., 1901, 3. ed., p. 595.
Jensen, Die Protoplasmabewegung, 1902, p. 29. (Reprint from Ergebnisse d. Physiologic,
Vol. r.)
Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 193.
Ewart, Protoplasmic Streaming in Plants, 1903, pp. 39, 35.
284 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
is constant, and is such as to be in opposite directions on the two sides
of the dividing-wall between each pair of contiguous cells. Occasionally,
however, as the result of injury or death to neighbouring cells, the direction
of streaming may be altered or reversed \ and during circulatory streaming
reversals or changes of direction may occur at longer or shorter periodic
intervals 2.
In comparison with the movement of zoospores, protoplasmic streaming
is slow, for the maximum rate observed in the plasmodium of Didymium
serpula is 10 mm. per minute, while in the cells of Vallisneria and of
Elodea the streaming protoplasm usually does not cover more than
i«5 mm. per minute3, and in the internodal cells of Chara and Nitella
rarely exceeds a to 3 mm. per minute4. Arthur5 observed a velocity
of 3-3 mm. per minute in the hyphae of Rhizopus nigricans. In the
case of Chara and Nitella, where the streaming endoplasm is comparatively
thick, the velocity varies in different parts, being most rapid in the layers
just outside the median line of the endoplasm, falling thence abruptly to
nil against the ectoplasm, and diminishing gradually towards the cell-sap,
the outer layers of which move with the protoplasm. In addition, gravity
affects to a very slight extent the speed of floating particles of varying
density, accelerating or retarding their velocity according to whether the
streaming is upwards or downwards 6. In regard to the size of the cell,
however, the movement is comparatively rapid, for four to six rotations
may be performed per minute by the streaming protoplasm in the cells
of Elodea and Vallisneria.
Although the protoplasm is never absolutely at rest, numerous cells
do not show any perceptible streaming, nor can any be awakened in
them. Frequently, however, rapid streaming can be excited in inactive
cells by injury or by treatment with various chemical substances. This
applies to the leaf-cells of Vallisneria and to the leaf-parenchyma cells
of Elodea, whereas in the leaf-hairs of Cucurbita and Urtica and in the
staminal hairs of Tradescantia as well as usually along the midrib of Elodea
streaming appears under normal conditions. Hauptfleisch attempts to
distinguish between the streaming normally present and that excited by
stimuli as 'primary' and 'secondary' streaming, but the distinction is
a purely artificial one and cannot be applied in all cases7. In any case
1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 34.
3 On the distribution and special peculiarities of streaming cf. Hofmeister, Pflanzenzelle, 1867,
p. 48 ; Velten, Bot. Ztg., 1872, p. 672 ; Wigand, Bot. Hefte, 1885, Heft i, p. 169 ; Berthold, Proto-
plasmamechanik, 1886, p. 119; Janse, Jahrb. f. wiss. Bot., 1890, Bd. XXI, p. 198 (Caukrpd)\
Ternetz, ibid., 1900, Bd. xxxv, p. 273 (Ascobolus}.
3 Hofmeister, 1. c., p. 48. * Cf. Ewart, 1. c., pp. 24, 25, 63, 65.
* Arthur, Annals of Botany, 1897, Bd. XI, p. 493.
6 Cf. Ewart, I.e., pp. 23, 113.
7 Cf. Ewart, pp. 4, 75 ; Hauptfleisch, Jahrb. f. wiss. Bot., 1892, xxiv, pp. 190-200.
PROTOPLASMIC STREAMING 285
streaming is not shown by the cells of the primary meristem, irregular
sliding movements appearing as the cells enlarge and vacuoles begin to
appear, and circulatory streaming being then established, which passes
into rotation if the protoplasm is restricted to the peripheral membrane.
The streaming, after attaining a maximum at a certain period of development,
then often persists until death1, and in the case of Chara and Nitella
cannot be stopped for any length of time without killing the cells. In
the cells of Elodea and Vallisneria the newly-awakened streaming may
die away again, and it is even possible by prolonged culture in strong
sugar-solution to render the protoplasm of Elodea permanently immotile 2.
That streaming is possible in the absence of well-defined vacuoles is shown
by the plasmodia of Myxomycetes, although here the streaming is pre-
sumably the direct result of the amoeboid expansion and contraction of
the peripheral layers. The non-vacuolated protoplasm of certain cells of
the primary meristem may also show slight sliding movements under special
conditions 3, and the existence of streaming in the threads crossing the cells
of Spirogyra shows that well-defined streaming may be shown by embryonic
cells containing a large vacuole.
Although distinct streaming movement does not appear to be a general
necessity of protoplasmic existence, there can be no doubt that it has in
most cases a definite purpose. Usually it appears to have as its function
the rapid transport of material from one part to another, and it is largely
for this reason that it only appears when the developing cells reach a certain
size, and becomes inextricably connected with vitality in the extremely
large internodal cells of Chara and Nitella. In the latter the protoplasm
is able to stream several times around the cell in the time required by
most dissolved substances for complete diffusion across its length, whereas
in small cells diffusion is more rapid than streaming4. The absence of
streaming from very small cells is, however, also partly due to the relatively
high internal resistance to flow 5. It is, in any case, always possible that the
streaming movement may be an accessory but unavoidable accompaniment
of some other form of vital activity. A certain advantage is probably
gained by the absence of streaming from the meristem cells in so far as
the grouping and arrangements preceding cell-division are undisturbed.
Cell-division in Amoeba and Spirogyra and nuclear division in Myxomycetes
x Cf. Wigand, Bot. Hefte, 1885, Heft i, p. 186 ; also Nageli, Beitrage z. wiss. Bot., 1860, Heft ii,
p. 61 seq. ; Vesque-Piittlingen, Bot. Ztg., 1876, p. 574; Braun, Ber. iiber die Verhandl. der Berl.
Akad., 1852, p. 214.
8 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 15, 58.
* Cf. Butschli, Archiv f. Entwickelungsmechanik, 1900, Bd. x, p. 52.
* This is owing to the fact that the time required for complete diffusion is proportional to the
square of the distance across which diffusion occurs. Cf. Ewart, On the Ascent of Water in Trees,
Phil. Trans., 1905, p. 40 (reprint).
6 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 26-30.
286 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
are, however, not affected injuriously by the existence of streaming move-
ment. Although streaming may favour nutrition and metabolism, it does
not necessarily make them especially pronounced ; and, on the other hand,
streaming may be inhibited by treatment with chloroform, which does not
stop and may even accelerate respiration, and it may continue under partial
pressures of oxygen which do not suffice for the formation of chlorophyll
in etiolated chloroplastids *.
With the exception of the peripheral layer, all the rest of the protoplasm
shows streaming or is capable of it. In the case of Chara and Nitella
the non-motile peripheral layer is thick and contains almost all the chloro-
plastids embedded in it, whereas in Elodea and Vallisneria when streaming
is active all the chloroplastids are carried with the stream and only the
extremely thin ectoplasmic membrane adhering to the cell-wall is at rest.
Hanstein was, however, incorrect in supposing that the entire protoplast
turned round within the cell 2. When the protoplast is plasmolysed
streaming may continue, but particles adhering to the ectoplasmic membrane
remain at rest. Hence the absence of movement in the peripheral layer
is not due to its contact with or adherence to the cell-wall. In certain
cases, however, plasmolysed portions connected by a thread appear to- show
a slight rolling movement, but this does not appear to be directly connected
with the streaming movement, and is possibly the result of the action of
surface-tension or gravitational forces 3. In the case of Chara and Nitella
it is easy to see that the peripheral, well-defined, and permanent layer of
ectoplasm acts like a gelatinous solid which is incapable of being set in
motion by the friction of the moving layers. Similarly the cohesion of
the protoplasm of streaming cells of Elodea and Vallisneria probably
increases towards the periphery, so that when the rapidity of streaming
rises, more of the ectoplasm is brought into motion until only the extreme
peripheral layer which has the properties of a fixed membrane remains at
rest. The fact that the vacuolar membrane moves with the plasma and
sets the cell-sap in motion shows that either the ectoplasmic membrane
is more solid in character or that it is thickened by the attachment of more
highly cohesive layers of ectoplasm4. In the case of many pseudopodia
and in that of the external plasma of Diatoms, the water and peripheral
layers of protoplasm appear to move in the same direction, so that it is
possible that cells may exist in which the ectoplasmic membrane of
dermatoplasts may be capable of streaming movement. Apart from the
1 Ewart, Journ. of Linn. Soc., 1897, Vol. xxxi, p. 566.
8 Hanstein, Bot. Abhdlg., 1880, Bd. iv, Heft ii, p. 15. Cf. Velten, Flora, 1873, p. 97 ; Hof-
meister, Pflanzenzelle, 1867, PP- 35» 45 J Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 269; Wigand,
I.e., p. 194; Hormann, Studien iiber die Protoplasmastromung bei d. Characeen, 1898, p. 24;
Ewart, Protoplasmic Streaming in Plants, 1903, p. 6 seq. 3 Ewart, 1. c., p. 9.
* Velten, I.e., p. 98; Hofmeister, I.e., p. 43; Berthold, I.e., p. 122; Rhumbler, Zeitschr. f.
allgem. Physiologic, 1902, Bd. I, p. 304.
PROTOPLASMIC STREAMING 287
fact that the thickness of the non-streaming ectoplasmic layer is variable
and that it is often extremely thin, its immotility is only relative, for it
undergoes changes of shape during plasmolysis and amoeboid movement.
In addition to the non-moving external layer, large or smaller portions
of the general protoplasm may be temporarily or permanently in relative
rest. A transitory period of quiescence occurs between each rhythmic
reversal of circulatory streaming, and in some cases streaming may be
shown only along isolated bands. Even when streaming is general, a
narrow or even a broad indifferent line of rest is interposed between the
ascending and descending streams, and this line is characterized in Chara
and Nitella by the absence of chloroplastids. The dividing line between
neighbouring streams may, however, be of merely theoretical dimensions
without any signs of mutual disturbance, obliquely moving chloroplastids
being repelled from the neutral line as though an invisible elastic limiting
membrane separated the two moving layers 1.
Local streaming or gliding movements will naturally be produced
whenever the motory energy is localized, or is insufficient in amount to
produce complete streaming. Temporary local sliding movements are in
fact shown when young cells are acquiring the full power of streaming,
and also when streaming is recommencing in anaesthetized or partially
disorganized cells 2. These movements have been termed digression move-
ments by Wigand 3, and they often consist merely of to-and-fro movements
of individual particles of protoplasm.
The streaming endoplasm may carry with it various inactive suspended
bodies such as crystals, starch-grains, oil-drops, and vacuoles, and even
the nucleus or nuclei and chloroplastids may be passively carried with the
stream. At the same time the latter may possess a feeble tendency to
slow locomotion, which is, however, imperceptible when they are rapidly
carried round the cell, but which becomes perceptible when they are
embedded in resting protoplasm. Apart from their slow amoeboid changes
of shape, it is doubtful whether the nucleus and chloroplastids possess any
well-defined powers of locomotion. Naturally when a non-motile body
is in contact with a stationary layer on one side it will move more slowly
and tend to acquire a rotary movement or may even temporarily move
in the opposite direction as compared with particles surrounded on all sides
by streaming protoplasm 4. Hormann 5 has suggested that the chloroplastids
1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 108.
2 Cf. Nageli, Pflanzenphysiol. Unters., 1855, Heft i» P- 495 Beitrage z. wiss. Bot., 1860, Heft ii,
pp. 10, 84; Velten, Eot. Ztg., 1872, p. 651.
3 Bot. Hefte 1885, Heft i, p. 180.
* Goppert und Cohn, Bot. Ztg., 1849, p. 698 ; Nageli, Beitr. z. wiss. Bot., 1860, Heft ii, p. 66 ;
Velten, Activ oder passiv? Oesterr. Bot. Zeitschrift, 1876, Nr. 3; Eerthold, 1. c., pp. 118, 150;
Wigand, I.e., p. 195.
5 Hormann, Studien iiber die Protoplasmastromung Lei d. Characeen, 1898, p. 24.
288 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
of Characeae in passing into the endoplasm become covered with a layer
of the motory protoplasm which he supposes to bound the endoplasm
externally, and so acquire a power of independent locomotion. All the
phenomena described may, however, be shown by dead bleached chloro-
plastids, and isolated chloroplastids never show any power of independent
locomotion or of orientation, however long they may remain living and
functionally active l.
Frequently the starch-grains or chloroplastids may ball together and
form an obstruction round which the protoplasm flows until it is swept
away2. In this way and also by partial plasmolysis, variations in the
contour of the vacuolar membrane may be produced, while as the result
of exposure to high temperatures partial coagulation may influence the
direction and manner of streaming 3, and by exposure to localized intense
light streaming may be restricted to the two unaffected ends of a cell
of Char a 4.
The Physics of Streaming Movement 5. In spite of the fluid character of the
endoplasm, gravity exercises relatively little action upon the speed of ascent and
descent of particles of varying density*. Whatever the motor mechanism may
be, it is such that no backward reaction is exercised upon either the cell-wall or
cell-sap. The total resistance to flow depends upon the viscosity of the moving
liquids and upon the diameter and length of the cell. Any factor which decreases
the viscosity, such as a rise of temperature or an increase in the percentage of water,
will decrease the resistance to flow and hence will tend to increase the velocity of
flow. The relative resistance to flow is proportional to the square of the radius of the
moving portion of the cell, so that in very small cells the resistance to flow becomes
disproportionately great, and in the case of the minute interprotoplasmic connexions
between contiguous parenchyma cells flow in mass becomes practically impossible.
The amount of energy actually consumed in the production of the streaming cannot
be determined, but the theoretical consumption based upon the assumption that the
protoplast is a perfect machine is exceedingly small. Thus the energy used by
a streaming cell of Nitella represents only a theoretical consumption of 2oo1ooQ
of a gram of cane-sugar per annum per gram of plasma moving at a rate of 2 mm.
per minute in a cell of 0-4 mm. radius. In the smaller cells of ordinary plants less
than a tenth of a per cent, of the energy of respiration appears to be consumed in
the production of streaming movement. In the large cells of Chara and Nitella the
normal rate of streaming is more rapid than in the smaller cells of Vallisneria and
Elodea of lesser radius, but this is not necessarily the direct result of the relatively
greater resistance, for it is hardly likely that in all cases the same proportion is
1 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 107, 108.
3 Meyen, Pflanzenphysiologie, 1838, Bd. II, p. 220; Nageli, I.e., p. 62; Hofmeister, Pflanzen-
zelle, 1867, p. 44; Rhumbler, Zeitschr. f. allgem. Physiol., 1902, Bd. I, p. 321.
5 Ewart, I.e., p. 59.
4 Pringsheim, Jahrb. f. wiss. Bot., 1882, Bd. XII, p. 326.
6 For fuller details see Ewart, 1. c., p. 6 seq. • Cf. Ewart, 1. c , p. 23.
PROTOPLASMIC STREAMING 289
maintained between the total energy of respiration and that used in streaming.
In fact, streaming is usually more rapid, or at least as rapid in the narrow cells along
the midrib of JElodea, as in the broader parenchyma cells.
Historical1. Streaming movements were first observed in the cells of plants by
Corti in 1774 2. These observations were amplified and extended by Fontana, Tre-
viranus 3, Amici 4, Slack 5, Meyen 6, Dutrochet 7, Schleiden 8, and Hassal 9, but it was
not until Von Mohl had established the fact that the protoplasm was the essential living
substance of the plant-cell that Schacht 10 showed the seat of active movement to be
in the protoplasm, and concluded that streaming was merely an outward and visible
sign of the activity of the latter.
Velten assumed that streaming was a general and normal phenomenon, whereas
Frank, Keller, and Hauptfleisch have shown that in many cases it is only awakened
by external stimulation n. De Vries and also Janse considered that streaming was
of primary importance for the rapid transport of food-materials, and the same con-
clusion has been made by Hormann 12. It is, however, only in very large cells that
this applies, for in ordinary plant-cells transference by diffusion is more rapid than
by streaming movement 13.
Theories of Streaming. Heidenhain and Kiihne 14 considered that waves of con-
traction passed round the cell, producing streaming in the same way as when the
finger is drawn round an india-rubber tube filled with water. A similar explanation
was originally put forward by Corti 16, but de Bary and others have shown that the
contour of the protoplasm towards the cell-sap does not alter in the way required by
the theory 16. In any case, the streaming in dermatoplasts can hardly be produced in
the same way as in gymnoplasts, in which it is passively produced by the contractile
activity of the peripheral layers. Yet another type of passive streaming has been
shown by Arthur17 to exist in the mycelial filaments of many Fungi when local
variations of osmotic pressure coupled with the excretion or absorption of water
See Ewart, Protoplasmic Streaming in Plants, 1903, p. i seq.
Osservazioni microscopiche sulla Tremella e sulla circolazione del fluido in una pianta
acqu iola, Lucca, 1774, p. 127.
Physiologia, 1807.
Mem. della Soc. Ital. delle Scienze in Modena, 1818, T. xvin, p. 182.
Ann. sci. nat., 1834, 2<i ser., T- l> PP- I93? 271.
Id., 1835, 2e s6r., T. iv, p. 257. 7 Id., 1838, 2e ser., T. ix, pp. 5, 65.
Principles of Botany (Eng. Trans.), 1849, P- 92-
British Freshwater Algae, Vol. I, p. 85. 10 Die Pflanzenzelle, 1852, p. 340.
11 Velten, Bot. Ztg., 1872, p. 147 ; Flora, 1873, p. 82 ; Frank, Pringsh. Jahrb., 1872, Bd. viir,
p. 220 ; Keller, Ueber Protoplasmastromung im Pflanzenreich, 1890, pp. 12, 40 ; Hauptfleisch, Jahrb.
f. wiss. Bot., 1892, Bd. xxiv.
18 De Vries, Bot. Ztg., 1885, NOS. i and 2, p. i ; Janse, Jahrb. f. wiss. Bot., 1890, Bd. xxi,
p. 163.
13 Ewart, The Ascent of Water in Trees, Phil. Trans., 1905, p. 80.
14 Heidenhain, Studien d. physiol. Inst. in Breslau, 1863, Bd. II, p. 60; Kiihne, Unters. iiber d.
Protoplasma, 1864, pp. 73, 91.
15 Quoted by Goppert and Cohn, Bot. Ztg., 1849, p. 666.
16 De Bary, Flora, 1862, p. 250 ; Schultze, Das Protoplasma d. Rhizopoden u. d. Pflanzenzellen,
1863, p. 40; Nageli und Schwendener, Mikroskop, 1877, 2. Aufl., p. 389.
17 Arthur, Annals of Botany, 1897, Vol. XI, p. 491.
PFEFFER. HI
290
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
causes to-and-fro movements in mass of the protoplasm. Ternetz l has also shown
that similar passive movements are produced in the filaments olAscobohu (Ascophanus)
carneus by the expansion and contraction of the vacuoles. Streaming movements
may also be produced by pressure, and physical streaming of this kind is only
secondarily dependent upon vital activity, whereas true circulatory streaming and
rotation appear to be directly connected with the latter. There is, however, naturally
more than one way in which streaming could be produced by vital activity 2.
Engelmann considered streaming to be due to changes of shape in his hypo-
thetical inotagmas 3, while Hofmeister 4 and also Sachs 5 suggested that progressive
changes of imbibition passing round the cell by causing the protoplasmic particles
alternately to absorb and extrude water would cause them to move in a definite
direction. This would, however, involve a movement of the water in the protoplasm
in the opposite direction, and as a matter of fact this
does not take place 6. Similarly the supposition of Briicke,
Hanstein,and Heidenhain that the movement is produced
by the contractile activity of a system of tubes or
fibrillar network hardly harmonizes with the fact that
the whole of the endoplasm is in motion 7. A forward
movement might be produced by an oblique ejection
or exudation of water, but if this took place internally
it would involve a movement of the cell-sap in the
opposite direction, while it could not take place externally
in cuticularized hairs exhibiting streaming.
Amici 8 concluded that the chloroplastids electrically
propelled the endoplasm, and a similar conclusion was
made by Dutrocnet and Becquerel 9. Velten 10 also con-
sidered that the movement had a direct electrical origin,
and was able to produce a circulation of dead floating
particles in a cell which was reversed on reversing the
FIG. 50. Sectional diagram of direction of the stronp- electrical currents used. The same
electro - magnetic streaming. The
small arrows show the direction of occurs when mercury placed over a strong electro-magnet
the electrical current and the large
ones the movement of the mercury. is traversed by an electrical current, so that presumably
the cell-wall is capable of acting as a magnetic mem-
brane n. (Fig. 50.) Such action involves a corresponding backward reaction upon
1 Ternetz, Jahrb. f. wiss. Bot, 1900, Bd. xxxv, p. 273.
2 See the literature quoted by Butschli, Unters. iiber mikr. Schaume, 1892, p. 173.
3 Engelmann, 1. c., p. 373. * Hofmeister, Pflanzenzelle, p. 63.
5 Sachs, Physiologie, 1865, P- 45 J-
6 Cf. Ewart, Protoplasmic Streaming in Plants, pp. 109, no.
7 Briicke, Unters. iiber das Protoplasma und die Contractilitat ; Sitzungsb. d. Wien. Akad.,
1862, Bd. XLVI, Abth. ii, p. 36; Hanstein, Protoplasma, Heidelberg, 1880; Heidenhain, Einiges
iiber die sog. Protoplasmastromungen, 1897 (reprint from Sitzungsb. d. physik.-medic. Ges. zu Wiirz-
burg). Cf. Ewart, L c., p. 108. * Cf. Dutrochet, Ann. d. sci. nat., 1838, 2e ser., T. IX, p. 78.
9 Dutrochet and Becquerel, 1. c., pp. 85-7.
10 Velten, Bot. Ztg., 1872, p. 147; Flora, 1873, p. 82; Sitzungsb. d. Wien. Akad., 1875,
Bd. LXXIII, Abth. i, p. 343.
11 On the paramagnetism of cellulose cf. Ewart, I.e., 1903, p. 47.
PROTOPLASMIC STREAMING
291
the cell-wall or magnet, and this does not appear to be exercised in the streaming
cell l. In addition, the direction and velocity of streaming are not directly affected
by the use of strong magnets 2, so that the motor mechanism in the living cell can
hardly be of electro-magnetic origin, for the retarding effect produced after prolonged
exposure to intense magnetic action is probably of secondary origin 8.
Berthold considered that amoeboid movement was directly due to changes of
surface-tension, the movement always taking place towards the side of least surface-
tension 4. The latter statement does not, however, apply to all cases, for a piece of
camphor floating on water moves towards the side where the surface-tension is greatest.
Streaming he considers to be due to changes of surface-tension in the vacuolar
FIG. 51. A. Diagram of section of Chara cell, showing rows of emulsion globules in endoplasm. The row of
arrows shows the relative velocities of different layers. B. Row of emulsion globules showing surface-tension
forces and resultant movement. (After Ewart.)
membrane, and in support of this conclusion adduces the fact that the velocity of
streaming decreases from the vacuolar membrane towards the ectoplasm 5. In cells
with a thick layer of endoplasm, by using minute floating particles of similar diameter
1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. no seq.
2 Becquerel, Compt. rend., 1837, T. v, p. 784 ; Dutrochet, Compt. rend., 1846, T. xxn, p. 619 ;
Reinke, Pfliiger's Archiv f. Physiol., 1882, Bd. xxvn, p. 140. [The orientation of suspended
streaming cells in a strong magnetic field, due mainly to the magnetic properties of the cell-wall,
would probably have led these observers to exactly the opposite conclusion had they not over-
looked it. Cf. Ewart, 1. c., p. 45 seq.] 8 Ewart, 1. c., p. 50.
* Berthold, Protoplasmamechanik, 1886, p. 115 seq. Cf. also Butschli, I.e., p. 210.
5 Berthold, I.e., p. 123; cf. also Wigand, I.e., p. 196.
U 2
292 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
as indicators, it can usually be distinctly seen that the velocity of streaming increases
slightly from the vacuolar membrane to a point lying a variable distance beneath
the ectoplasm, and thence rapidly decreases to nil outwardly *. (Cf. Fig. 51.) This
distribution of velocity affords definite proof that the energy of movement is liberated
not at the boundary of the cell-sap but throughout the substance of the streaming
endoplasm. If we assume that the bipolar paramagnetic and diamagnetic particles of
protoplasm in the endoplasmic emulsion are definitely arranged in regard to the para-
magnetic cell-membrane, it is easy to see how continuous rotation might be brought
about if electrical currents are produced by the differences of potential at the internal
and external boundaries of the feebly-conducting protoplasm2, and are maintained
by the chemical actions in the latter. For these currents traversing the endoplasm
and producing definite changes of surface-tension in the regularly-arranged particles
of the emulsion might in this way cause a movement of the whole protoplasm 3.
Where the regular arrangement is not maintained, circulatory movements, or a cessa-
tion of streaming, may ensue.
Although this hypothesis coincides more exactly with the facts observed in
dermatoplasts than that of Berthold, it may ultimately prove to be as far from the
truth as Quincke's conclusion that the movement was due to surface-tension actions
exercised by the non-moving ectoplasm 4.
The influence of the shape of the cell and of the union in tissues. The typical
rotation in elongated cells takes place parallel to the long axis of the cell 5, the
plane of rotation being parallel to the surface of the leaf in Vallisneria and at right
angles to the surface in the cortical cells of Chara 6. The plane of rotation can,
however, be altered by injuries, by the death of neighbouring cells and by exposure
to strong light after prolonged darkening 7. According to Velten, in rotating around
the longitudinal axis of the cell the plasma follows the path of least resistance 8. In
Chara, however, as was observed by Braun 9, spiral streaming appears when the inter-
nodes undergo torsion, and then Hermann 10 considers the streaming to be along the
path of absolutely greatest resistance, while, according to Rhumbler n, the arrangement
of the chloroplastids is due to the spiral streaming instead of inducing it. Neither
Velten nor Hormann brings forward any experimental evidence or theoretical calcula-
tion in support of his statements, and as a matter of fact the resistance to flow in
cylindrical cells with rounded ends is not affected by the direction of flow. Naturally
in cells showing circulation the total resistance to flow increases as the number of
threads increases and their diameter decreases, but the path of least resistance is
that in which the passage across a definite space requires the least expenditure of
Ewart, Protoplasmic Streaming in Plants, 1903, p. 113. 2 Id., p. 123.
Id., p. 116. * Pfeffer, Plasmahaut und Vacuolen, 1896, p. 277.
Nageli, Beitrage zur wiss. Bot., 1860, Heft ii, p. 62. See also A. Braun, Ber. liber d. Ver-
hand g. d. Berliner Akad., 1852, p. 214 ; Hofmeister, Pflanzenzelle, 1867, p. 36.
Berthold, 1. c., p. 122. 7 Ewart, 1. c., p. 34.
Velten, Flora, 1873, p. 86; Berthold, 1. c., p. 120.
A. Braun, I.e., p. 225. See also Berthold, I.e., p. 121; Meyen, Pfianzenphysiol., 1838,
Bd. n, p. 236 ; Velten, 1. c., p. 85.
10 Hormann, Studien ii. d. Protoplasmastromung b. d. Characeen, 1898, p. 16.
11 Rhnmbler, Zeitschrift f. allgem. Physiologic, 1902, Bd. I, p. 300.
PROTOPLASMIC STREAMING 293
energy. This will be along as straight or as uniformly curved a path as possible,
so that the tendency to eddy currents with their increased resistance to flow is
avoided. A spiral path around the long axis of the cell fulfils this condition best
when the cell is an elongated cylinder as in Chara and Nitella. When the ends of
the cell are rounded the direction of streaming may be parallel to the long axis
of the cell1.
The influence exercised by neighbouring cells is shown by the fact that a
stimulus awakening or accelerating streaming may radiate to some distance from an
injured region. In addition, the planes of streaming in the cortical and medullary
cells of the internodes of Chara z show definite relationships, which may possibly be
such as to favour translocation 3. According to Berthold 4, there is no constant relation-
ship between the direction of streaming in the cells of Elodea and Vallisneria, but as
a matter of fact, almost without exception, the direction of streaming is opposed on
the two sides of each dividing wall 5. In the deeper leaf-cells, especially near the
midrib, the planes of rotation may intersect at various angles owing to the oblique
points of contact of the cells, while in other cases the direction of streaming appears
to be primarily determined by the shape of the individual cell.
SECTION 63. Pulsating Vacuoles.
Vacuoles may show various changes of shape and volume, and fre-
quently fuse as the living cell grows older. When vacuoles periodically
diminish and re-enlarge, or disappear and reappear, we speak of contractile
or pulsating vacuoles, such as are especially well shown by Infusoria e and
by many other Protozoa. They also occur in various Thallophytes and
Protophytes, such as most Volvociniae and Flagellatae 7, a few Palmellaceae 8,
and also in the zoospores of Stigeoclonium, Chaetophora* > Ulothrix™,
Microspora n, and many other Algae, as well as in the zoospores of such
Fungi as Saprolegnia 12 and Cystopus 13, and in the zoospores and plasmodia
Phil.
Ewart, Protoplasmic Streaming in Plants, 1903, p. 35.
A. Braun, 1. c., p. 231. For other cases cf. Hofmeister, 1. c., p. 40.
Hormann, I.e., 1898, p. 13; cf. also Ewart, I.e., p. 34; and The Ascent of Sap in Trees,
Trans., 1905, p. 40.
Berthold, 1. c., p. 121. 5 Ewart, 1. c., 1903, p. 34.
Butschli, Protozoen, 1 880-8, p. 1411.
7 Butschli, I.e., p. 708 ; O. Hertwig, Zelle tu Gewebe, 1893, p. 69, and the literature here
quoted; Cohn, Beitr. z. Biol. d. Pflanzen, 1877, Bd. n, p. 117; Klebs, Unters. a. d. bot. Inst. zu
Tubingen, 1883, Bd. I, p. 246; Senn, in Engler's Natiirl. Pflanzenfamilien, 1900, T. I, Abth. i,
p. 101.
8 Cienkowski, Bot. Ztg., 1865, p. 22 ; 1876, p. 70. 9 Id., 1876, p. 70.
10 Strasburger, Zellbildung u. Zelltheilung, 1875, p. 157; Dodel, Bot. Ztg., 1876, p. 183.
11 Maupas, Compt. rend., 1876, T. LXXXII, p. 1,451. See also Falkenberg in Schenk's Hand-
buch d. Botanik, 1882, Bd. n, p. 194; Hofmeister, Pflanzenzelle, 1867, p. 12 ; Woronin, Bot. Ztg.,
1880, p. 628 (Chromophytori).
12 Rothert, Cohn's Beitr. z. Biol., 1892, Bd. v, p. 323.
13 De Bary, Ber. d. nat. Ges. zu Freiburg, 1860, p. 8.
294 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
of Myxomycetes 1. No typical pulsating vacuoles have hitherto been
detected in the cells of plants above the Thallophyta, but nevertheless
transition forms occur between normal and pulsating vacuoles. All vacuoles
are formed in the same way by the protoplasm, and many normal vacuoles
undergo pronounced but slow changes of volume 2, which are in some cases
mechanically produced by protoplasmic streaming or surface-tension action.
In fact there is probably no vacuole whose size and shape are constant,
while various external agencies may progressively reduce and ultimately
inhibit the rhythmic activity of pulsating vacuoles. Periodic changes of
concentration in the external medium involve corresponding periodic
changes of volume in the vacuole, so that the normal progress of
metabolism is bound to influence the size of the vacuole, although such
purely mechanical actions may be controlled and regulated by the
protoplast within certain limits. Thus a rise of concentration in the
external medium will exercise no influence upon the size of the vacuole, if
the osmotic concentration of the cell-sap is proportionately increased, and
when the vacuole is very small, a fall of the surface-tension of the vaciiolar
membrane would be almost equally effective in balancing the increased
external pressure.
In Closterium, and a few other Desmids, the vacuole occurring at each
pole becomes smaller when the direction of streaming of the protoplasm is
towards that end, and it regains its original size with the periodic reversal
of the stream3. In the hyphae of Ascobolus and other Fungi, periodic
alterations in the volume of the vacuoles produce to-and-fro streaming
movements in the protoplasm, while the periodic movements of the leaves and
leaflets of Desmodium and Trifolium and of other plants involve rhythmic
contraction and dilation of the cells and hence also of the vacuoles. The
same occurs during every stimulatory movement of the filaments of Cynareae
and the pulvini of Mimosa, for this involves a considerable escape of water
from the cell and its subsequent reabsorption.
Pulsating vacuoles are always small, they usually maintain the same
locus and commonly not more than one to three are present in plant-cells 4.
The plasmodia of Myxomycetes have, however, numerous pulsating vacuoles
which may be present not only in the ectoplasm but also in the streaming
endoplasm. Most Volvocineae have two pulsating vacuoles, but Volvox has
only one, and Chlorogonium has numerous contractile vacuoles 5. According
1 De Bary, Mycetozoen, 1864, pp. 41, 81 ; Cienkowski, Jahrb. f. wiss. Bot., 1863, Bd. Ill,
p. 329 ; Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, pp. 192, 219, 336. On Amoeba
cf. Biitschli, Protozoen, 1 880-8, p. 105 ; Rhumbler, Archiv f. Entwickelungsmechanik, 1878, Bd. VII,
p. 256.
3 Pfeffer, I.e., p. 257 ; Butschli, 1. c., pp. 1412, 1435; Rhumbler, L c.
3 De Bary, Unters. ii. d. Familie d. Conjugaten; Schumann, Flora, 1875, p. 66; A. Fischer,
Jahrb. f. wiss. Bot., 1884, Bd. xiv, p. 142.
4 Butschli, 1. c., p. 708. 5 Massart, Bull, de 1'Acad. royale de Belgique, 1901, p. 100.
PULSATING VACUOLES 295
to Massart, Paramaecium aurelia forms numerous vacuoles when warmed
to 30 or 35° C. Similar vacuolations in the protoplasm of various plant-
cells were observed by Klemm * after the application of injurious agencies,
so that the phenomenon is probably a general one.
In all cases the systolic contraction is very rapid, whereas the re-expan-
sion or diastole takes place much more slowly. The vacuole may reappear
at the same or another spot, and expands at first rapidly, but then more
slowly until it regains its original size, when it suddenly collapses again.
In the case of the plasmodia of Aethalium septicum and Chondrioderma the
maximal diameter of the contractile vacuoles varies from 0-004 to o-oi of
a millimetre 2, but when the vacuoles are large the systole, though rapid, can
be followed, and often does not lead to the entire disappearance of the
vacuole.
The pulsatile frequency varies according to the external conditions,
and attains a maximum at a somewhat variable optimal temperature.
Under favourable circumstances 12 to 15 seconds may elapse from one
systole to the next in the case of the zoospores of Vlothrix 3, and 26 to 60
seconds in the case of Gonium 4. The duration of each period, is however,
usually 60 to 90 seconds 5 in the case of the plasmodia of Aethalium and
Chondrioderma, and the vacuoles of these organisms which do not com-
pletely empty often pulsate still more slowly6. In the case of the
Infusorian Spirostomum teres the pulsatile frequency is given as 30 to 40
minutes7, so that vacuoles may exist in plants which pulsate so slowly
that hitherto their special character has not been detected.
Although the pulsation usually maintains the same frequency under
constant external conditions there are naturally exceptions to this rule.
Cienkowski8 observed a very variable frequency in certain Palmellaceae,
and the same applies to those vacuoles of plasmodia which undergo
imperfect systole9. In addition neighbouring vacuoles of plasmodia may
be in all stages of systole and diastole at the same moment, whereas when
two vacuoles only are present one is usually expanding while the other
collapses10. In many cases, as for instance in certain Palmellaceae, both
vacuoles contract at the same time.
1 Klemm, Desorganisations-Erscheinungen in pflanzlichen Zellen, Jahrb. f. wiss. Bot., Bd.
xxvin, 1895, P- 685-
Pfeffer, 1. c., p. 192. 3 Strasburger, 1. c. ; Dodel, 1. c.
Cohn, Nova Acta Acad. Caesar. Leopold., 1854, Bd. xxiv, i, p. 196; Biitschli, Protozoen,
1880-8, pp. 714, 1453, gives summaries of the pulsatile frequency in various Infusoria.
Cienkowski, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 329.
Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 192.
Biitschli, I.e., p. 1454. 8 Cienkowski, Bot. Ztg., 1865, p. 22.
In the individual cells of colonies of Gonium approximately the same rhythm may sometimes
be maintained.
10 SeeButschli, I.e., p. 713.
296 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
The systole of a pulsating vacuole may either drive out the contents into the
surrounding water or merely into the surrounding protoplasm. The former is
usually the case in such Infusoria as the Acinetarias and Vorticellidae, which have
special exit channels leading from the vacuole to the exterior \ The latter appear,
however, to be absent from all other animals, and from plants also if we except
the Flagellatae. Transition forms occur, however, for in Paramaecium and other
organisms the radiating channels from the vacuole do not always appear at the
same point, and do not lead to the exterior. In many Amoebae, again, the peripheral
vacuoles bulge out externally and rupture when the wall has become very thin, so
that the escape of their contents may take place at any point on the surface 2. The
emptying of a number of the peripheral non-pulsating as well as of the pulsating
vacuoles of Myxomycetes is effected in this way. The coalescence of small vacuoles
with one another or with a large one also involves a gradual approach of the vacuoles
and a thinning of the dividing membrane until the point of rupture is reached3.
Deep-seated vacuoles, however, can only empty their contents into the surrounding
protoplasm when they are not connected with any actual or potential channels to the
exterior. The extruded fluid may, however, either be imbibed by the protoplasm or
exude outwardly through it. In the latter case a corresponding diminution of the
total volume must ensue. The existence of organisms with a single vacuole or with
two synchronous ones shows that the vacuolar fluid is not always driven from one
vacuole to another.
The escape from deep-seated vacuoles without special affluent channels takes
place by nitration under pressure through the vacuolar membrane, since, owing to
the plastic nature of the vacuolar membrane and of the surrounding protoplasm the
former cannot be ruptured under the conditions existent in the cell. In all cases
the centrally-directed pressure exercised by the vacuolar membrane partially
antagonizes the internal osmotic pressure required for the maintenance of the vacuole,
and any change in either of these factors is bound to influence the size of the vacuole.
It is, however, only when the latter is very minute that the centrally-directed pressure
attains relatively high values4. Under ordinary circumstances the diminution or
collapse of the vacuole can only result from a decrease or removal of its internal
osmotic pressure, produced either by the exosmosis of the dissolved materials or
by their conversion into larger or insoluble molecules. According to Cohn, just
before the systole of the vacuole of Gonium pectorale the vacuolar fluid becomes
turbid, possibly owing to the precipitation of the dissolved materials 5, but it is also
possible that the phenomenon may have a different origin and not be directly
connected with the vacuolar contraction.
When the vacuole is small, very rapid nitration under pressure through its rela-
tively large surface is possible, so that the vacuole may disappear instantaneously. If
the protoplasm is not at once able to absorb all the extruded water, radiating channels
1 Cf. Biitschli, Protozoen, 1 880-8 ; Hertwig, Zelle und Gewebe, 1893.
8 See Rhumbler, Archiv f. Entwickehmgsmechanik, 1898, Bd. vn, p. 257.
3 Cf. Pfeffer, Aufnahme u. Ausgabe ungeloster Korper, 1890, p. 159.
4 Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 298.
5 Cohn, Nova Acta Acad. Caesar. Leopold., 1854, Bd. xxiv, i, p. 194.
PULSATING VACUOLES 297
filled with sap may appear around it, or the vacuole may appear to have undergone
fine fragmentation l.
No increase of external pressure could produce a complete collapse of the
vacuole so long as it retained its dissolved substances, for any diminution of size
involves a corresponding increase of concentration and of osmotic pressure. For
this reason moderate changes in the external pressure are readily balanced without
appreciably affecting the size of the vacuoles, and the same applies to the protoplasm
in general2. The fact that neighbouring vacuoles may expand and contract at
different times, and that isolated fragments with single vacuoles may show pulsation
for some time, afford sufficient evidence that the systole and diastole are not produced
by local or general changes of pressure in the protoplasm. Nor can the pulsation be
due to changes in the percentage of osmotic substances in the protoplasm.
It does not, however, follow that the mechanism is alike in all cases 3, and in fact
the position of the vacuole in various Amoebae may determine whether it bursts on
the surface or allows its contents to escape into the surrounding protoplasm by filtra-
tion under pressure4. The latter always occurs when only a diminution in size is
shown, for an actual rupture of the vacuolar membrane would presumably involve an
escape of the whole of its contents. Under special conditions the vacuoles of most
organisms do not empty completely5, but this does not necessarily show that the complete
collapse is also merely due to filtration under pressure, however probable this assump-
tion may be. Vacuoles of Myxomycetes which have absorbed aniline blue by passive
secretion retain it during partial pulsations 6, whereas the selective permeability of the
vacuolar membrane enables it to allow the diosmotic excretion of other dissolved
materials. The addition of non-exosmosing dissolved substances to a vacuole must
necessarily convert a previous total pulsation into a partial one, and possibly this is
why the union of a pulsating vacuole with a non-pulsating one produces in the
plasmodium of Chondrioderma only a feebly pulsating vacuole 7.
The continuance of rhythmic pulsation in isolated vacuoles shows that the thinnest
protoplasmic layers may develop the required self-regulatory activity. Although the
systole ensues when a definite size is reached, other inactive vacuoles may surpass
this size without ever pulsating. Hence the pulsation is the result of some specific
peculiarity, and this holds good even when pulsation may be induced under special
circumstances in previously inactive vacuoles8. It is not easy to say whether a
vacuole entirely disappears at the close of the systole or merely decreases to sub-
microscopic dimensions. In the former case the vacuolar membrane would be
reconverted into ordinary protoplasm, but it is also possible that special factors
might prevent this happening, in which case the potential walls at least of the new
vacuole would be retained. The reproduction of a new contractile vacuole would,
1 See Rhumbler, Archiv f. Entwickelungsmechanik, 1878, Bd. VII, p. 289 ; Biitschli, 1. c., &c.
2 See Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 337.
A summary of the views of different authors is given by Biitschli, I.e., pp. 1433, 1458, 1452.
Rhumbler, I.e., pp. 257, 271.
See Biitschli, 1. c., p. 1457 ; Cohn, 1. c., p. 200.
Pfeffer, I.e., 1890, pp. 219, 337. 7 Pfeffer, I.e., 1890, p. 219.
Cf. Rhumbler, 1. c., p. 263.
298 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
however, be no more remarkable than the formation of a non-contractile one \ and
this might still occur even though the vacuole always reappeared at the same spot.
A localization of the vacuole merely involves a localized production of the conditions
for its formation. The latter may or may not involve the coalescence of preformed
droplets, but in any case the degree of independence and of differentiation attained by
pulsating vacuoles is not in all cases certain 2.
External influences. The frequency attains a maximum at a certain optimal
temperature, and in general the responses resemble those for other forms of vital
activity8, although the vacuoles appear to have a higher resistant power. Thus,
according to Klebs, mechanical agencies, high temperatures, and strychnine stop the
general protoplasmic movements before the pulsation of the vacuoles ceases, and the
latter is the first to begin on returning to normal conditions. Indeed, according to
Klebs 4, irregular slow pulsations may continue for a time after the general mass of
the cytoplasm has been killed by heat or disorganized by pressure. Evidently,
therefore, the pulsation is independent of the nucleus, and it may also continue for
a time in non-nucleated masses of living cytoplasm. Rossbach found that induction-
shocks as well as certain alkaloids stopped the ciliary movement and locomotion of
Infusoria before the pulsation of the vacuoles had ceased. Dodel5, however, found
that the vacuolar pulsation and ciliary movement of the zoospores of Ulothrix ceased
simultaneously, and that in other zoospores the cilia continued to move after pulsation
had ceased, so that specific differences appear to occur according to the organism
examined 6.
Various agencies may cause an enlargement of the vacuole, and this change is in
some cases reversible and extremely pronounced. When thus swollen the vacuoles
may pulsate little or not at all. Klebs found that this effect was produced in the case
of Euglena by various neutral salts which, however, appear to be less effective in their
action upon Infusoria. Rossbach was indeed unable to detect any action at all upon
them, but Massart has shown that it takes place at a particular concentration 7. High
concentrations naturally produce a plasmolytic contraction, and in some cases a
complete collapse of the vacuoles8. Rossbach found that alkaloids and alkalies
caused an enlargement of the vacuoles of Infusoria, but Klebs was unable to detect
any distension when Euglena was exposed to the action of strychnine. Both Klebs
and Massart have, however, observed a gradual accommodation of the vacuoles to
concentrated solutions.
1 Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 223. Biitschli and Rhumbler also consider tha.t the
vacuoles are formed anew after each complete pulsation.
2 Cf. Pfeffer, I.e., 1890, p. 223.
3 Biitschli, Protozoen, 1880-8, pp. 715, 1454; Klebs, Unters. a. d. hot. Inst. zu Tubingen, 1883,
Bd. I, p. 248.
* Rossbach, Die rhythmischen Bewegungserscheinungen d. einfachsten Organismen, 1872, p. 56.
See also Biitschli, 1. c., p. 1455.
5 Dodel, Bot. Ztg., 1876, p. 185.
6 Cienkowski, Bot. Ztg., 1865, p. 23; Strasburger, Ueber Zellbildung u. Zelltheilung, 1875,
P- 157.
7 Massart, Archive de Biologic, 1889, T- IX> P- 55°-
8 Cohn, Nova Acta Acad. Caesar. Leopold., 1854, Bd. xxiv, i, p. 194 ; Klebs, 1. c. ; Massart, I.e.
PULSATING VACUOLES 299
Sudden changes presumably exercise the customary shock-effect upon pulsation,
and it is possibly owing to some such action that only a few observers have been
able to detect an increased frequency when oxygen is deficient or carbon dioxide
abundant \
Functional importance. It is generally assumed that the contractile vacuoles aid
in the absorption of oxygen and other food-materials as well as in the excretion of
carbon dioxide and other waste products8. Maupas3 has indeed calculated that
Infusoria may expel and reabsorb their own volume of water in two to forty-six
minutes. All Protozoa do not, however, possess pulsating vacuoles, nor do the cells
of Fungi or of primary meristems which also possess very active powers of respiration
and of metabolism. In tissues, however, the transit between the cell and the external
world becomes of greater importance and is slower than the entry into or escape from
particular cells. Hence the latter do not require special aids to absorption and
excretion, and in fact the excretion of water in the tissues of aerial organs, as occurs
when the filaments of Cynareae and irritable pulvini are stimulated, always involves
a certain hindrance to gaseous exchange external to the cell. It is possible also that
the contractile vacuoles may in some cases serve special purposes, and in the case of
Chilodon propellens each ejection of water causes a jerky movement of the organism in
the opposite direction 4.
SECTION 64. Other Protoplasmic Movements.
All these movements, including those involved in cell and nuclear
division, are the direct or indirect results of vital activity, although their
detailed origin is comparatively unknown. All parts possessing the
properties of a viscous liquid must be subject to the physical laws
already discussed which determine or modify their shape. The flattened
character of many nuclei is, for instance, probably often due to the existence
of lateral pressure upon it. Every active enlargement of the nucleus, as
well as the growth of starch-grains, necessarily produce corresponding
displacements in the protoplasm, and in fact the expansion and contraction
of vacuoles may originate definite streaming movements in the protoplasm.
The rounding of the viscous protoplasm on plasmolysis is the direct
result of the existence of a uniform surface-tension pressure at its external
boundary, but a bulging will always be produced at any point where
a lower surface-tension is maintained. An accumulation of the denser
constituents at one end of a cell produced by centrifugal action may,
however, take more than a week to be readjusted, whereas a rapid
1 Butschli, Protozoan, 1880-8, p. 1452.
2 Cohn, Beitrage z. Biologic, 1877. Bd. n, p. 118. For details see Biitschli, I.e., p. 1452.
8 Quoted by Butschli, I.e., p. 1455. According to Rhumbler (Archiv f. Entwickelungs-
mechanik, 1898, Bd. VII, p. 257), Amoeba proteus shows a pronounced contraction with every collapse
of the vacuole.
4 Engelmann, Zur Physiologic d. contractilen Vacuolen der Infusionsthiere, 1878.
300 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
redistribution takes place when the protoplasm exhibits streaming move-
ment. Even in this case, however, a local accumulation of protoplasm
or chloroplastids may pass several times around a cell before being
broken up.
When displacements are only slowly readjusted, the protoplast may
never reach a condition of stable equilibrium, owing to the continued
production of new dispositions before the adjustment to the older ones is
completed. It is, therefore, impossible to predict what the stationary
condition of equilibrium would be, and in fact the protoplast might
maintain permanently an unequal distribution of tension. In general,
however, the shape and arrangement of the protoplast and of its organs
correspond to what would be expected in a viscous emulsion.
Since the organs of the protoplast lie in an active living medium, local
changes of surface-tension are likely to be of common occurrence, quite
apart from the changes of imbibition and swelling produced in the interior
of the organs affected. In addition, the various organs as well as portions
of the cytoplasm or nucleoplasm may acquire temporarily or permanently
a more solid consistency, and may then be capable of active changes of
shape. The preservation of their shape by the chlorophyll-bands of
Spirogyra demands the existence of a fair degree of consistency in them,
and possibly changes of cohesion play a part in the changes of shape and
configuration of the chromosomes during nuclear division. It is, however,
uncertain in most cases how the various internal movements and changes
of shape are produced. Even in the much studied case of protoplasmic
streaming an element of doubt still attaches, for the evidence in favour
of its surface-tension origin is for the most part indirect in character1.
Although in certain cases the chloroplastids and nuclei are undoubtedly
passively carried by the streaming protoplasm, they may also be capable
of slow independent locomotion by amoeboid change of shape, or by the
maintenance of appropriate differences of surface-tension. In the case
of comparatively large plastids and nuclei embedded in viscous protoplasm
only very slow movements could be produced in this way ; but the smallest
force will produce movement, since the resistance to flow is kinetic and pro-
portional to the velocity. The total force available in such cases would
not, however, suffice to overcome the static resistance offered even by a very
attenuated colloid to an incipient movement. In any case, if the differences
of surface-tension were only produced by the metabolic activity of the
surrounding cytoplasm, it is doubtful whether we should be justified in
speaking of an active locomotion of the nucleus, although the motory
energy was actually liberated at the boundary of nucleus and cytoplasm.
Similarly, when a drop of oil comes into contact with a soap-solution the
1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, pp. 108-19.
OTHER PROTOPLASMIC MOVEMENTS 301
difference arises in the external medium, whereas a drop of a mixture of
oil and potassium carbonate shows movement when surrounded on all sides
by a homogeneous medium, water. Even if the locomotory energy is
actually supplied by the cytoplasm the nucleus might easily exercise
a directive influence upon it, and so determine the direction of movement.
It is, therefore, not surprising that doubt should exist as to whether
the slow translocatory movements of nuclei and chloroplastids are always
passive in character or not. The nucleus may be passively carried to any
point where an accumulation of protoplasm is produced either by traumatic,
chemical, or other agencies. Even without such accumulation a passive
movement of the nucleus is as readily possible as an active one. Some
authors assume the former to be the case, others the latter \ but no critical
experiments have as yet been performed. The occasional amoeboid or
gradual changes of shape of the nucleus appear, however, to be active in
character, but even here interaction with the surrounding cytoplasm may
aid in their production2. The same applies to the chloroplastids, whose
movements in response to illumination may either be active or produced
by a directive utilization of the motile energy of the cytoplasm. In the
same way it is impossible to say whether the movements of the chromo-
somes are active or passive, or are compounded of both.
The special elongated, lobed, twisted, or even spirally coiled shapes
sometimes assumed by nuclei can often be seen to be independent of the
shape of the cell, and not to be mechanically impressed upon the nucleus.
The nuclei of animals more often show amoeboid movements than those of
plants 3, but whether amoeboid activity plays a part in the passage of the
reproductive nuclei from the pollen-tube to the ovum and embryo-sac is
still uncertain4. The same applies when the nucleus passes from one cell
to a neighbouring one during cases of vegetative fusion5. It is quite
possible that the fibrillae appearing during cell-division, but which may
also be produced in various ways, may be capable of producing internal
movement by their supposed contractile activity. These structures are,
however, transitory in character, and their tendency to shorten is of similar
1 Cf. Hanstein, Mittheil. ii. d. Bewegungserscheinungen des Zellkerns, 1870, p. 224 (reprint
from Sitzungsb. d. Niederrh. Ges.) ; Berthold, Protoplasmamechanik, 1886, pp. 150, 164; Haber-
landt, Function u. Lage d. Zellkerns, 1887, p. 103 ; Behrens, Bot. Ztg., 1890, p. 100.
2 Cf. Molisch, Studien ii. d. Milchsaft u. Schleimsaft, 1901, pp. 87, 107 ; Bot. Ztg., 1899, P- X77 5
Haberlandt, 1. c., p. 124; v. Wasielewski, Jahrb. f. wiss. Bot., 1902, Bd. xxxvili, p. 415; Ewart,
Journ. Linn. Soc., Vol. xxxi, 1896, p. 448.
3 [The nuclei of such parasitic plants as Cuscuta, Lathraea> and Orobancht seem to show
amoeboid movement more commonly and markedly than those of ordinary plants, but whether this
is connected with the rich nitrogenous nutrition or the general activity of metabolism is uncertain.]
4 Cf. Mottier, Fecundation in Plants, 1904, p. 176.
5 Cf. Strasburger, Jahrb. f. wiss. Bot, 1901, Bd. xxxvi, p. 551; Koernicke, Sitzungsb. d.
Niederrh. Ges., March, 1901.
302 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
character to that of a thread drawn from a viscous liquid. Any secretion
of mucilage in the interior of the protoplasm would also produce a certain
amount of displacement reversible by the reabsorption of the mucilage.
Nuclear division may be accompanied or preceded by an increase in the total
amount of nuclear material, and may take place independently of the cytoplasm,
although in mitotic division nucleus and cytoplasm come into close relationship
during the process and the latter forms the threads of the ' nuclear ' spindle 1. It
is, however, uncertain what part is played by the centrosomes, which are in fact absent
from most plant-cells 2. The mechanics of amitotic, and still more of the remarkable
movements involved in mitotic, division are indeed quite unknown. It is, however,
certain that they may be produced in a variety of ways, so that experiments with
non-living materials do not afford definite evidence as to the nature of the physio-
logical processes involved. Biitschli3 was able to produce radiations resembling
those shown by dividing nuclei by the contraction of air-bubbles embedded in
solidifying gelatine and also in emulsions of oil and soap under special conditions 4.
Fischer 5 produced remarkable radiations arranged around the remains of the nucleus
as a focus by precipitating albumose in dead cells of Elder-pith. In this case the
nucleus acts merely as a centre of deposition, so that in the living cell it may also
play a passive part in the production of the radiations formed around it. The
grouping of particles of iron in a magnetic field yields similar configurations6, but
the magnetic properties of the cell constituents are incapable unaided of directly
producing any such grouping in the strongest magnetic fields available 7. No actual
facts are, however, known as to the mode of movement in any single phase of the
changes, and even if the motion of the chromosomes should prove to be due to the
tension or pressure exercised by the threads of the spindle 8 we have still to determine
the mode of action of these threads themselves.
The external conditions may influence the character and progress of cellular and
1 See R. Hertwig, p. 698; Strasburger, I.e., 1900, p. 118; Zimmermann, Morphologic u.
Physiol. d. pflanzlichen Zellkerns, 1896, p. 48.
3 Cf. Strasburger, Histologische Beitrage, Heft vi, 1900, p. 156 ; Ber. d. bot. Ges., 1901, p. 458 ;
R. Hertwig, Abhandlg. d. Bayrisch. Akad. d. Wiss., 1898, Bd. XIX, p. 690; Mottier, Fecundation in
Plants, 1904, p. 2.
3 Biitschli, Unters. iiber Structuren, 1898, p. 156.
* Biitschli, Unters. ii. mikroskopische Schaume, 1892, pp. 29, 159, 166. Cf. also Strasburger,
Bot. Zeitnng, Referate, 1900, p. 300; Zacharias, Ber. d. bot. Ges., 1902, p. 298.
5 A. Fischer, Fixirung, Farbung u. Bau d. Protoplasmas, 1899, p. 206.
6 Cf. Errera, Compt. rend, de la Soc. royale de botanique de Belgique, 1890, T. xxix, p. 17 ;
Biitschli, I.e., 1898, p. 169; Rhumbler, Archiv f. Entwickelungsmechanik, 1903, Bd. XVI, p. 476;
Seddig, Ann. d. Physik, 1903, Bd. II, p. 815.
7 Ewart, Protoplasmic Streaming in Plants, 1903, p. 45.
8 The theories concerning cell-division are mainly based upon preconceived hypotheses, so that
nothing is to be gained by their discussion. Cf. Biitschli, 1. c., 1892, p. 160 ; Ziegler, Verhandlg. d.
deutsch. zoologischen Ges., 1895, p. 62; R. Hertwig, Abhandlg. d. Bayr. Akad., 1898, p. 694;
Rhumbler, Archiv f. Entwickelungsmechanik, 1898, Bd. vn, p. 535; Ergebnisse d. Anatomic u.
Entwickelungsgeschichte, 1898, Bd. viil, p. 605; A. Fischer, I.e., pp. 224, 257; Bethe, Bot.
Centralbl., 1902, Ed. LXXXIX, p. 513; Hacker, Praxis u. Theorie d. Zellen- u. Befruchtungslehre,
1899, p. 73.
OTHER PROTOPLASMIC MOVEMENTS 303
of nuclear division to a greater or less degree. Thus, when a cell of Spirogyra which
normally divides by mitosis is caused to divide amitotically by the action of ether, we
have a change similar to the production of Mucor yeast occurring under special
conditions \ The shape of other plants is, however, relatively little affected by the
external conditions, and hence it is not surprising that in most cases the external
conditions exercise little effect upon the character of the mitotic nuclear division.
Certain abnormalities may often be produced, however 2, and in many cases changes
in the external or internal conditions may result in one or in numerous amitotic
divisions 3. On the other hand, in the case of many lower organisms in which the
nucleus normally divides by amitosis4, mitotic nuclear divisions may possibly be
produced under special circumstances. In any case transitions occur between typical
mitosis and amitosis 5, and all forms of amitotic nuclear division characterized by the
non-production of pronounced mitotic figures do not fall in the two categories pro-
posed by Wasielewski 6.
Furthermore various instances are known of temporary and reversible differentia-
tion in the protoplasm, and in fact the distinction between hyaloplasm and granulo-
plasm is one of this character. There is no positive evidence to support Strasburger's
use of the terms trophoplasm or alveolarplasm, and kinoplasm or reticuloplasm as
indicative of fixed structures 7. Changes in the relative percentage of each according
to the external conditions or the progress of development 8 are quite in accord with
a unity of origin for both. Both Hertwig and Zacharias have opposed this doctrine
of the existence of permanent organically distinct differentiation in the general
cytoplasm.
The doctrine that continued existence and reproduction is impossible in the
absence of mitotic nuclear division is, like the dogma as to the necessity of free
oxygen for life, founded upon hasty, incorrect generalization. Nor is there any
reason why full hereditary transmission should not be possible unless some of the
reproductive living units, biophore, or pangens, group themselves into large visible
chromatin-threads. It is quite possible, however, that such grouping previous to
1 Cf. Pfeffer, Sitzungsb. d. sachs. Ges. d. Wiss., 3. Juli, 1899.
2 Blazek, Bot. Centralbl., 1902, Bd. XC, p. 548 ; Van Wisselingh, Flora, 1900, p. 373 ; Geras-
simoff, Zeitschrift f. allgem. Physiol., 1902, Bd. I, p. 220; Strasburger, Histologische Peitrage,
Heft vi, 1900, p. 127. On the lower animals cf. Doflein, Zell- u. Protoplasmastudien, 1900, p. 42 ;
E. B. Wilson, Archiv f. Entwickehmgsmechanik, 1901, Bd. xin, p. 389 ; Wasilieff, Biol. Centralbl.,
1902, Bd. XXII, p. 758; Werner, Bot. Centralbl., 1902, Bd. xc, p. 521; Wallengren, Zeitschr. f.
allgem. Physiol., 1902, Bd. I, p. 67. R. Hertwig, Abhandlg. d. Bayr. Akad., 1898, Bd. XIX, p. 687 ;
Archiv f. Protistenkunde, 1902, Bd. I, pp. n, 16, gives instances of variations in the nuclear figures
at different stages of development.
3 Wasielewski produced amitosis in roots by the aid of chloral hydrate (Jahrb. f. wiss. Bot.,
1902, Bd. xxxvin, p. 377). See also Magnus, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 242 ;
Chodat, Actes du Congres international de Botanique, Paris, 1900, p. 23; Shibata, Jahrb. f. wiss.
Bot., 1902, Bd. XXXVII, p. 648 ; Schimkewitsch, Bot. Centralbl., 1902, Bd. XXII, p. 605.
4 R. Hertwig, Archiv f. Protistenkunde, 1902, Bd. I, p. 26.
5 R. Hertwig, I.e., p. 25. 6 L.c., p 401.
7 Strasburger, Histologische Beitrage, Heft vi, 1900, p. 144.
8 Strasburger, I.e., p. 144; R. Schrammen, Bot. Centralbl., 1902, Bd. XC, p. 551 ; R. Hertwig,
Abhandlg. d. Bayrisch. Akad., 1898, Bd. Xix, p. 690; Zacharias, Flora, 1895, Ergzbd., p. 259.
304 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
division might present certain advantages. The latest researches seem, however, to
show that the nucleus is absent from, or at least not yet differentiated in, certain lower
organisms1. It is still possible that the chromatin elements may be present, but
diffusely distributed 2, although it is to be remembered that the structures to which
this general term is given cannot be precisely identical in all organisms, but must
exhibit greater or smaller dissimilarities capable of hereditary transmission.
Protoplasmic fusion. Contact must naturally precede fusion, but does not
necessarily produce it, as for instance when similar or dissimilar organs of the cell,
or dissimilar protoplasts, come into contact. Thus the closest contact does not
produce fusion between the plasmodia of different species of Myxomycetes 3, whereas
plasmodia of the same species readily unite. Even when a fragment of a foreign
plasmodium is ingested by another species no fusion occurs between them 4. In the
case of the swarm-cells of Aethalium the capacity for fusion only appears at a certain
stage of development, and hence it is possible under suitable conditions to permanently
prevent the appearance of the fusion stage so that no plasmodium is formed 5. Actual
fusion does not occur in Dictyostelium and other Acrasiae, although the amoebae
come into close contact and form an aggregate plasmodium 6. Similarly, sperms do
not fuse with one another, but readily unite with appropriate ova, in which the fusion
of male and female pro-nuclei ultimately occurs. It is worthy of note that immediately
after the entry into the ovum changes take place at the surface which prevent the
penetration of additional sperms. Probably it was owing to the suppression of these
change's by the agency of chloral hydrate that Hertwig was able to cause the entry of
a number of spermatozoids into the egg of a sea-urchin 7. The production of hybrids
shows that the protoplasts of dissimilar species may unite, and it is possible that
successful grafting involves the fusion of the interprotoplasmic connexions in the
neighbouring cells of scion and stock8.
In addition to intimate contact at some point or other, fusion involves the
rupture of the intervening surface-tension films. This occurs naturally when the
whole of the intervening medium is displaced at any one point, for the existence of
the surface-tension film is dependent upon contact with a dissimilar non-wetting
medium. Hence the presence of impurities on the surface of drops of mercury hinders
their fusion greatly, and the same result will be attained whenever a thin layer of the
surrounding medium is maintained between two drops of similar liquid 9. It is owing
1 See especially in regard to bacteria, Hinze, Ber. d. bot. Ges., 1901, p. 369; Unters. ii. d. Bau
von Beggiatoa mirabilis, 1902; Schaudinn, Archiv f. Protistenkunde, 1902, Bd. I, p. 335; Ernst,
Centralbl. f. Bact., 1902, Bd. vm, Abth. ii, p. I ; Biitschli, Protozoen, 1880, p. 107.
R. Hertwig, 1. c., 1902, p. 6.
Cienkowski, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 337.
Celakovsky, Flora, 1892, Ergzbd., p. 215.
Klebs, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 196.
Cf. Zopf in Schenk's Handbuch d. Botanik, 1887, Bd. in, Abth. ii, p. 22 ; Potts, Flora,
1902, Ergzbd., p. 281.
7 O. Hertwig, Zelle u. Gewebe, 1893, p. 93.
8 Strasburger, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, pp. 586, 592; Meyer, Bot. Ztg., 1902,
P- 173-
9 Quincke, Pfliiger's Archiv f. Physiol., 1879, Bd- XIX> P- I29 '•> Berthold, Protoplasmamechanik,
1886, p. 107; Rhumbler, Biol. Centralbl., 1898, Bd. xviil, p. 115.
OTHER PROTOPLASMIC MOVEMENTS 305
to changes of surface-tension that the addition of alcohol to an emulsion of oil in
water favours the fusion of the oil-drops, while, partly owing to this cause and partly
owing to the solution of impurities, the addition of nitric acid to an emulsion of
partially oxidized mercury produces a sudden coalescence of the droplets.
A high cohesion of the peripheral layers may aid in preventing fusion, as may
also the secretion of gelatinous membranes ; but the causes which determine fusion
have not as yet been satisfactorily determined in a single case. Klebs * found that
the gametes of Protosiphon botryoides do not conjugate at 26° to 27°C., although they
develop and swarm at this temperature, but the causes of this behaviour, as well as
for the absence of any power of fusion between the swarm-cells of Aethalium when
first produced, are quite unknown. Townsend8 found that fusion often does not
occur between the fragments of the protoplast separated by plasmolysis, possibly
because of the de'bris formed between them by the disorganization of connecting
protoplasmic threads. When the latter remain intact fusion always occurs, since the
most minute local union suffices to produce ultimate total fusion. The union of the
plasmodia of Myxomycetes is not, however, prevented by the intervention of a thick
layer of foreign substances, since the pseudopodia bore through it and unite.
Similarly, the ectoplasm affords no obstacle to complete fusion, since its high cohesion
is lost when it becomes withdrawn internally. Indeed the protoplast may, when
necessary, dissolve away intervening cell-walls, while, on the other hand, the segmenta-
tion into separate protoplasts may take place without any production of dividing
walls.
The ingestion and excretion of solid bodies. The continued existence of symbiotic
algae in the cells of Hydra viridis and of certain Protozoa shows that special conditions
determine whether foreign bodies are retained or rejected3. A tendency to the
rejection of foreign bodies is shown even in dermatoplasts, as for instance when
excreta, such as calcium oxalate crystals, are thrown into the cell-sap. Usually the
excretion is aided by the existence of protoplasmic movement, whereas particles of
various substances lying against the non-motile peripheral layer of a plasmolysed
protoplast free from its investing cell-wall are usually not ingested. According to
Rhumbler4, differences of surface-tension and spreading tendencies are solely
responsible for the ingestion of foreign bodies, but this can hardly apply to all
cases. A solid body in contact with a drop of chloroform in water will be ingested
by it as the result of the chloroform spreading over it and surrounding it. In the
same way a glass fibre covered with shellac will be ingested by a drop of chloroform,
and expelled when the shellac has been dissolved away, since as soon as the tip of the
thread is exposed, the changed surface-tension and the tendency to spread causes
the chloroform to be driven away from the thread by the water 5.
It is, therefore, quite possible that the digestion within the protoplast of an
ingested body might produce the conditions for the excretion of indigestible remains.
1 Klebs, Bedingungen d. Fortpflanzung, 1896, p. 209.
2 Townsend, Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 495.
3 Pfeffer, Aufnahme u. Ausgabe ungeloster Korper, 1890, p. 174.
* Rhumbler, Archiv f. Entwickelungsmechanik, 1898, Bd. VII, p. 224.
5 Rhumbler, 1. c., p. 250.
PFEFFER. Ill X
306 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
The plasmodia of Myxomycetes may, however, with equal readiness ingest and expel
indigestible particles such as grains of sand or of vermilion. In addition, mere contact
with non-motile regions of the ectoplasm is insufficient to produce ingestion, which
takes place usually only at those regions where amoeboid activity is shown.
PART II
THE INFLUENCE OF THE EXTERNAL CONDITIONS ON LOCOMOTION
AND ON PROTOPLASMIC MOVEMENT
SECTION 65.
Under special external Conditions the power of active locomotion
may be inhibited without growth ceasing, and the contrary may also occur.
A. Fischer 1 found that various bacteria become immotile in concentrated
solutions in which they grow and develop motile cilia. The presence of
carbolic acid, and in general any agency which when more intense suppresses
growth, may produce the same effect. Temperatures lying near the
maximum may act in the same way, but Matzuschita 2 did not determine
to what degree the immotility was due to the production of non-ciliated
developmental forms. Prolonged cultivation on solid media has, for in-
stance, always this effect upon the motile aerobic forms of Bacterium termo
used for testing the evolution of oxygen 3. According to Ellis 4, the
immotility is often due to the production of mucilage which mechanically
prevents movement, while Ritter5 found that facultatively anaerobic
bacteria lost their motility in the continued absence of oxygen, but
immediately regained it when oxygen was admitted.
Most locomotory and protoplasmic movements take place in darkness
as well as in light, whereas the purple bacteria which develop normally in
darkness 6 only begin to move when exposed to light, and fall into a con-
dition of dark-rigor when it is withdrawn. In addition, other phototonic,
thermotonic, and chemotonic actions upon locomotory activity are known.
Many substances, such as ether and chloroform, which when concentrated
A. Fischer, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, pp. 48, 153.
Matzuschita, Centralbl. f. Bact., Abth. ii, 1901, Bd. vn, p. 209.
Ewart, Journ. Linn. Soc., 1896, Vol. xxxi, p. 364.
Ellis, Centralbl. fur Bact., 1902, Bd. ix, p. 546.
Ritter, Flora, 1899, p. 337.
[This appears to be an error. The purple chlorophyll-containing Bacterium photometricum
and Manas Okenii will develop in feeble light but not in continued absolute darkness, even when
sown on various solid and liquid nutrient media. The green bacteria (Bacillus virens, Bacterium
chlorinum, and Streptococcus varians) may, however, be grown in darkness on gelatine -sugar media,
but then lose their chlorophyll. Cf. Ewart, Journ. Linn. Soc., 1897, Vol. xxxili, p. 123, and Annals
of Botany, 1897, Vol. xi, p. 486.]
THE INFLUENCE OF THE EXTERNAL CONDITIONS 307
retard or inhibit movement may accelerate it when dilute. The fact that
light causes certain zoospores, and meat extract those of Saprolegnia^ to
come earlier to rest 1 is due to the shortening of the period of development
by these agencies. Whether the similar influence of magnetic forces is also
of this character is, however, uncertain 2.
The existence of a power of rapid locomotion permits the shock-effects
of sudden changes to become more readily perceptible. The sudden
application of fatally injurious conditions often causes specially active
irregular locomotion which reminds one of the spasmodic struggles of a
poisoned or asphyxiating animal. Naturally shock- reactions are not always
equally pronounced, and are not shown in all cases and with all agencies.
Changes of temperature, of illumination, and of concentration, injuries
and transitory anaesthetization, as well as many other agencies, may excite
or accelerate protoplasmic streaming, and in some cases when once aroused,
especially as the result of injury, it may persist until death. The direct
action of a sudden change upon existent streaming is usually to cause
a temporary retardation or even stoppage ; but in some cases, especially
with moderate rises of temperature, the velocity is temporarily accelerated
beyond the value it ultimately assumes. Injurious external agencies,
especially when suddenly applied, usually cause a contraction of amoeboid
protoplasts to the spheroidal shape, but may occasionally increase the
amoeboid activity.
Contact or the change to another medium causes the cilia of Chlamydo-
monas to straighten suddenly, and so produces a backward movement of the
organism into the homogeneous medium, in which the ciliary and locomotory
activity is resumed in one or more seconds 3. A similar shock-movement
is produced in Bacterium photometricum by sudden decreases of illumina-
tion, and this may cause it to move ten to twenty times its length backwards
when it comes to the edge of an illuminated area to which it is, therefore,
restricted. The transit from a concentrated to a more dilute solution
produces a similar backward movement in many Bacteria, Infusoria, and
Flagellatae, so that the organisms collect in the more concentrated medium.
It is, however, not known whether this shock-movement is accompanied by
a temporary cessation of the ciliary activity, although, according to Fischer,
sudden changes of concentration do actually cause a temporary inhibition
of the ciliary movement4. All motile organisms do not show shock-reactions
of this character, and an organism sensitive to one form of shock may be
insensitive to others. The shock-movement of Bacterium photometricum is
produced only by the transit from light to darkness, and not by the reverse,
1 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, p. 467 ; Rothert, Flora, 1901, p. 374.
2 Ewart, Protoplasmic Streaming in Plants, 1903, p. 52.
8 Pfeffer, 1. c., p. 444.
* A. Fischer, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 76.
X 2
3o8 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
whereas in Pelomyxa palustris the shock-movement is produced by sudden
illumination, and not by the sudden withdrawal of light. Similarly, many
bacteria show a shock-movement on passing from strong solutions to weak
ones, but not on passing from regions of lower to ones of higher concentra-
tion. In addition, a pronounced deformation of the protoplasm is shown on
transferring from an almost maximal temperature to a normal one, but not
on raising to the higher temperature. The deformation is also absent on
cooling to low temperatures, but it appears when the temperature is raised
again. In the case of the streaming cells of Chara and Nitella, however,
both the sudden application and the rapid removal of pressure, as well
as sudden rises or falls of temperature or concentration, may produce a
temporary shock- stoppage of streaming.
SECTION 66. The Forms of Tactic Response to Tropic Stimuli.
Whenever a tropic stimulus causes a definite orientation of the main
axis of a freely motile organism, locomotion in a definite direction is assured,
since in most cases the latter takes place along the main axis. Whether
the movement is positive or negative in character will depend upon the
direction of the anterior end in regard to the orienting stimulus. Local
accumulation may also result from the fact that the organisms freely move
towards regions of higher illumination or concentration, but experience
a backward shock-movement on approaching regions of lower concentration
or illumination. Passive accumulation also takes place when organisms
which swim freely continually adhere to a mucilaginous region, or are
suddenly killed on coming into contact with a poisonous area, or rapidly lose
the power of movement in a zone deficient in oxygen.
Shock-stimulation is responsible for the accumulation of Bacterium
photometricum in illuminated areas, as well as for the accumulation of various
Bacteria and Infusoria in concentrated solutions. Whereas the phototactic
movements of the zoospores of many Algae, and the chemotactic attraction
of many antherozoids, and of the zoospores of Saprolegnia and of many
Flagellatae, are the result of a tropic orientation of the body axis, as are
also the geotactic and galvanotactic movements of various organisms. In
both cases we are dealing with stimuli due to dishomogeneity in the
surroundings, but the stimuli act upon dissimilar forms of irritability. The
shock-stimulation is a temporary action repeated every time the required
change of conditions is produced by the movements of the organism,
whereas in the typical tropic orientation the inclination the organism
assumes is maintained so long as the tropic agency is unaltered, even if the
organism adheres to the same spot. Such organisms move with a definite
aim, whereas forms like Bacterium photometricum may be said to possess
a phobotactic irritability by which they avoid dark areas. Similarly, by
TACTIC RESPONSE TO TROPIC STIMULI 309
chemo-phobotaxis we may indicate an irritability by which an organism
is able to avoid or to remain in solutions of chemical substances owing to the
backward shock-movement produced on entering or leaving them as the
case may be 1. In many cases the exact nature of the response is uncertain,
and in others tropic and phobic actions may co-operate in producing the
result observed.
In the case of small and active organisms it is difficult to determine
whether a tactic or a phobic response is given, for during chemotactic
attraction the individuals do not all travel along straight paths to the
capillary containing the exciting substance, while at its mouth and within
it the forms move about in the same way as organisms attracted in a phobic
manner. Hence it was only after careful study and after using slowly
moving forms that Jennings and Crosby were able to show that the
attraction of Bacteria by chemical substances was the result of a phobic
action, although Engelmann had previously shown that the attraction of
Bacterium photometriciim to illuminated areas was produced in this manner2.
The phobic reaction and accumulation of various Infusoria and Flagellatae
were demonstrated by Jennings 3, and were confirmed by Garrey 4 before the
chemophobic responses of Bacteria were investigated.
It is possible that in many cases the same agency may excite a feeble
phobic and a strong tactic, or a strong phobic and a feeble tactic response.
This may explain the backward movement of the strongly chemotactic
antherozoids of Ferns when they attempt to enter a capillary filled with
a solution of malic acid. The phototactic zoospores of Botrydium also
appear to be weakly photophobic 5, and some species of Bacteria may
possess a strong power of chemotactic response in spite of Rothert's con-
clusions as to the general chemophobic reaction of Bacteria.
If a chemophobic action is always exercised when the organism
1 [There seems to be no reason for adopting the terms topotropism and topotaxis, as suggested
by Pfeffer, to indicate the typical orienting movements, since the term ' phobism ' put forward by
Massart, Centralbl., 1902, Bd. XXII, p. 49, suffices to distinguish these special forms of tropic and
tactic irritability from the more general case. It is still possible to use the term * tropism ' in the
general sense (cf. Bot. Ztg., 1902, Referate, p. 17) instead of restricting it in the way that Massart
(1. c., p. 49) and Nagel (Bot. Ztg., 1902, Ref., p. 24) do. Rothert's term ' apobatic' (Flora, 1901,
P- 393) is both uncouth and unnecessary, nor can his term of ' strophotaxis ' be adopted, since
' strophism ' has already been used in an equally superfluous way to indicate movements produced by
torsion. The error arises in supposing that a dissimilar response necessarily indicates a totally distinct
form of irritability, and hence needs a new term, or that phenomena are made simpler or more easy to
understand by giving them a classical terminology. The same applies to the use of the term ' argo-
taxis ' (apyos, passive) to indicate purely physical, passive movements due to surface-tension, like those
of a drop of oil in a soap-solution. In any case Nagel (Bot. Ztg., 1901, p. 297 ; 1902, Ref., p. 24)
is in error in considering that phobic reactions alone arise from a special discriminatory sense.]
3 Engelmann, Pfliiger's Archiv f. Physiologic, 1882, Bd. xxx, p. 95; Jennings and Crosby,
American Journal of Physiology, 1901, Vol. VI, p. 29; Rothert, Flora, 1901, Vol. VI, p. 29.
3 Jennings, American Journal of Physiology, 1899, Vol. II ; 1900, Vol. III.
* Garrey, Centralbl. f. Physiol., 1900, Bd. xiv, p. 105.
5 See the literature quoted by Rothert, 1. c., p. 386.
3io LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
attempts to pass to a more dilute zone, an accumulation will be shown,
however high the concentration may be. Phobic movements do not
necessarily only result from a passage to zones of different concentrations,
but may result from changes of position in regard to an orienting agency.
Thus, owing to the unequal distribution of irritability over the surface of
the organism, every displacement might exercise a shock-effect producing a
return to the original orientation. From this point of view the typical tropic
reactions of rapidly moving organisms might be regarded as phobic responses.
A reversal of both the tactic and phobic responses may take place with
increasing concentration. Thus in the latter case, beyond a certain strength
the phobic movement might be excited by the passage to regions of higher
instead of to ones of lower concentration. In both cases, therefore, the
organisms may collect at a definite distance from the mouth of the capillary
from which the concentrated exciting solution is diffusing. Zoospores
ciliated on one side only show this reversal of the tactic response especially
well, for as the result of it they pass beyond the position of equilibrium
and then turning round swim back again. If the base of such an organism
were fixed it would presumably bend to a definite position as in the case of
a rooted plant, and would assume a diatropic position at some intermediate
point between the regions of repellent and attractive concentration. Usually,
however, no diatropism can be detected in freely motile organisms, although,
according to Verworn1, the ciliated Infusorian Spirostomum ambiguum
places itself at right angles to the direction of an electrical current, while
Oxytrichia and other Infusoria, which creep about with their ciliated surface
on the substratum may be said to be diathigmotropic. Similarly, certain
Desmids as well as the chloroplastids of Mesocarpus assume diaphoto-
tropic positions in light of moderate intensity. Diatoms, on the other hand,
are ortho-phototactic, although they may be made to assume plagio-photo-
tropic positions by inclining the glass on which they glide at an angle
with the light-rays.
Diatoms and other equipolar organisms may reverse their movement
without turning round, and many such organisms which normally move
to and fro are attracted in a definite direction merely by the movement
to one side lasting longer than that towards the opposite one. In Amoebae
and in plasmodia, however, the tropic attraction is attained by the excita-
tion of amoeboid movement on one side. The backward shock- movement
does not appear to be accompanied by any reversal of the organism, even
when the latter is ciliated at one end only. At least no such reversal was
observed by Engelmann in the case of the" unipolarly-ciliated Bacterium
photometricum 2. If the impact against a glass plate alters the orientation
1 Verworn, Allgem. Physiol., 1901, 3. Aufl., p. 480.
2 Cf. Rothert, 1. c., p. 391 ; Jennings and Crosby, 1. c., p. 36.
TACTIC RESPONSE TO TROPIC STIMULI 311
of the body, the resultant shock-movement will naturally take place in the
new direction. Dorsiventral organisms like Paramoecium 1 assume definite
positions as the result of every shock- movement, but whether this also
applies to vegetable organisms is uncertain.
Since we are dealing with two distinct forms of irritability, one agency
may induce a tactic and another a phobic movement, while in some cases
the same stimulus may excite both forms of response. Many Infusoria are
galvanotactic, but chemophobic and osmophobic, while certain Volvocineae
are phototactic and also osmophobic 2. According to Garrey 3 Chilomonas
is chemophobic to the more active inorganic acids, and chemotactic to the
feebler organic acids. It is in fact possible that in many cases the chemo-
tactic attraction by weak solutions becomes a chemophobic repulsion when
they are more concentrated.
In spite of the generally useful adaptive character of these responses,
it is not surprising that in many cases a galvanotactic irritability should be
shown, although it cannot have any practical importance. Similarly,
although many organisms avoid injurious concentrations, others swim into
these or even into poisonous solutions where they are killed. The best
chemotactic agency can only attract or repel across relatively small
distances, although light and gravity are more extended in their action.
For biological purposes of attraction tactic stimulation is in general more
advantageous, for the spermatozoids of Ferns, for instance, could hardly be
drawn with certainty in any other way to the ovum. Phobic stimulation
is, however, ample to attract and retain bacteria to special loci, or to
prevent their penetration into injurious media.
Various orientations within the cell probably result from unilateral
stimulation, but hitherto only the phototactic movements of chloroplastids
and the traumatropic movements of the nucleus are known with certainty.
The protoplasmic aggregation which results from various stimuli may be
due either to a primary or secondary reaction — a distinction difficult to
determine under the complicated relationships prevailing within the cell.
The slow progress of the internal movements afford, however, strong
evidence that they are not the result of shock-stimulation. In general,
the power of movement is antecedent to tropic stimulation, and its rapidity
is not perceptibly modified by the latter, although many instances may
ultimately be found to exist in which a latent power of movement is first
awakened by the tropic stimulus. Nageli 4 found, however, that the photo-
tactic stimulation of Algal zoospores, and PfefTer5 that the chqmotactic
1 Jennings, Am. Journ. of Physiol., 1899, Vol. II. 2 Rothert, 1. c., p. 396.
3 Garrey, The effects of ions upon the aggregation of flagellated Infusoria, 1900.
* Nageli, Beitrage z. wiss. Bot., 1860, Heft ii, p. 103 ; Strasburger, Wirkung d. Lichts u. d.
Warme auf Schwarmsporen, 1878, p. 27.
3 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, p. 375.
312 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
stimulation of the sperms of Ferns produced no acceleration of movement.
Nevertheless, if an organism whose movement is retarded or inhibited by
a deficiency of oxygen or of a food-material is tropically stimulated by the
unilateral access of oxygen or of the food- material, an acceleration of
movement is bound to ensue at the same time that the tropic response
is given 1. Similarly, in the negatively photophobic Bacterium photometricum
the power of movement is first awakened by exposure to light. Various
tropic curvatures involve an acceleration or retardation of the average rate of
growth, and in the nodes of grasses the awakening of growth is due to the
geotropic stimulation which produces curvature.
Although certain tropic movements may have a purely physical origin,
the reactions of plasmodia and of Amoebae are undoubtedly physiological
responses, although Rhumbler and Verworn 2 consider those of the latter
to be directly due to changes of surface-tension. The latter may act as
stimuli and may also play an important part in the performance of move-
ment, but nevertheless the fact that the amoeboid activity is shown in
homogeneous media indicates that it is under the control of the organism.
Hence the tactic movements of zoospores are no more to be regarded
as the direct result of a modification of surface-tension by the external
agency, than is the flying of a moth towards a candle or the curvature of
a plant towards light.
The cilia of Chlamydomonas and of other forms appear thigmotropically
excitable, for a rapid general response is shown when only the tip of a
cilium is in contact with a foreign body. It is, however, uncertain whether
the cilia are the perceptive organs for chemotactic and other tactic stimuli.
Phototactic stimuli appear to be perceived neither by the cilia nor by the
eye-spot of Euglena^ but by its hyaline anterior end. In any case, the cilia
being protoplasmic organs are able to transmit stimuli, and in the case of
Chlamydomonas with considerable rapidity. Similarly, the latent period of
induction and the duration of the after-effect are exceedingly short in
rapidly motile zoospores. It is worthy of note that zoospores, even when
radial, are capable of phototactic, geotactic, and chemotactic reactions,
although, as the result of their continued rotation, they are in a similar
condition to a plant rotated on a klinostat. Hence a rotating vertici-basal
zoospore when it reacts to light must direct one end towards the source
of illumination so that the axis of rotation is at right angles to the light
rays. It is, however, also possible that the unequal stimulation of any
pair of opposite sides might suffice to produce a tactic response, although
none would be possible if the axis of rotation was at right angles to
the direction of the orienting agency and both ends of the organisms were
1 Pfeffer, 1. c., p. 463 ; 1888, Bd. n, p. 631.
8 Verworn, Bewegung d. lebendigen Substanz, 1892^.44; Rhumbler, Ergebnisse d. Anatomic
u. Entwickelungsgeschichte, 1899, Bd. viu, p. 584.
TACTIC RESPONSE TO TROPIC STIMULI 313
equally excitable. A phobic response would, however, still be possible, for
the time of a rotation is longer than the latent period of stimulation.
Individual differences appear to be of commoner occurrence among
lower than among higher organisms; and, although critical researches are
wanting, it appears that in the case of many Bacteria and Infusoria the
irritability may vary according to the cultural conditions, so that a
particular species may react at one time strongly, at another feebly or
not at all to a particular agency 1. It is even possible that races may be
bred which are devoid of an irritability possessed by the common stock.
SECTION 67. The Influence of Temperature.
The maxima and minima for locomotion and streaming approximate
to those for growth, although plants may be found able to grow at
temperatures which do not permit of streaming or locomotory activity.
Both forms of movement may, like growth, continue for a time at a supra-
maximal or supraminimal temperature which ultimately proves fatal.
Zopf 2 observed, for instance, that Bacterium vernicosum, whose maximal
temperature for growth is 45° to 46° C, continues to move for a time at
50° to 52° C. Streaming may still be present in the cells of Chara^ Nitella,
and Elodea after ten minutes' exposure to 50° C., and in Elodea after an
even longer exposure to 55° C. 3 The determination of the optimum
points is rendered difficult by the fact that even in the absence of any
shock-effect the velocity assumed at high but not fatal temperatures is
always more rapid than it becomes after prolonged exposure, as the cell
becomes accommodated or fatigued 4. On the other hand, after prolonged
exposure to low temperatures a moderate rise may take some time to
produce its full effect. In addition, the tone may be modified in other
ways. Thus, according to Josing 5, streaming ceases within two minutes
at 45° C., but not till after twenty minutes' exposure to this temperature in
water containing 0-25 per cent, of ether 6. Individual variations are also
shown, for Ewart found that in some leaf-cells of Vallisneria streaming
was retarded beyond 35° C., but in other cases not until 45° C. was reached,
and an equally low optimum was obtained when the temperature was very
gradually raised. Moderate rises of temperature influence the velocity
of streaming in two ways — either by lowering the viscosity of the endoplasm
or by increasing either the total amount of energy generated or the
1 Rothert, Flora, 1901, p. 417.
8 Zopf, Beitr. z. Physiol. u. Morphol. niederer Organismen, 1892, Bd. I, p. 66.
8 Ewart, Protoplasmic Streaming in Plants, 1903, p. 59. * Id., p. 62.
5 Josing, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 217.
6 [Some doubt attaches to these results, for streaming may also continue for twenty minutes in
the leaf-cells of Vallisneria spiralis at 45° C. in the absence of any ether. Cf. Ewart, 1. c., p. 65.]
3*4
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
fractional amount of it directed into this channel. The former cause alone
is sufficient to more than double the velocity of streaming when the
temperature is raised from 2° to 32° C. Nevertheless, the increases of
velocity within this range of temperature are much greater than this, and
are hence mainly due to an increase in the amount of energy utilized.
Above 30° C., however, the influence of the changes of viscosity becomes
more prominent, the sudden stoppage occurring at 55° to 60° C. being due
to the increase of viscosity which precedes coagulation l.
The apparently higher optimum temperature observed for streaming
as compared with that for growth is largely the result of the lesser duration
of the observations in the former case, for prolonged exposure to tempera-
tures of from 37° to 40° C. causes streaming to cease or become extremely
slow in all the plants examined. In addition, the viscosity of the proto-
plasm may permanently increase during prolonged exposure, while the
motor-mechanism may also be affected, a change of tempo ensuing.
The following cardinal points were obtained by various authors 2 : —
Author.
Plant.
Minimum.
Optimum.
Maximum.
Dutrochet
Charafragilis
o°to i°C.
—
45° C.
( Cucurbita Pepo .
10° to ii°C.)
Sachs .
\ Solanum lycopersicum .
12° C.
30° to 40° C.
40° to 50° C.
{ Tradescantia
I2°C.)
Cohn .
Nitella syncarpa
-2°C.
—
—
( Vallisneria spiralis
o°toi°C.
38-7° c.
45° C.
Velten .
\ Elodea canadensis
o°C.
36. 2° C.
38 7° C.
( Char a foetid a
o°C.
38.1° C.
42.8° C.
Klemm .
Trianea and Momordica
-2°C.
4^° to 48° C.
Hauptfleish .
Streaming cells in general
o°C.
37° to 38° C.
41° to 42° C.
The discrepancy in these results is partly the result of the varying
duration of the exposure, and is partly due to such factors as age, supply
of oxygen, and previous treatment. Thus Ewart 3 obtained values varying
only a degree or two from those of Hauptfleisch when the exposures were
prolonged, whereas with short exposures an optimum of 40° C. and a
maximum of 50° to 60° C. may frequently be obtained. In addition, the
optimal and maximal temperatures are lower in young cells of Chara and
Nitella than in old ones. Streaming may in fact continue during a short
exposure of the latter to a temperature which causes subsequent death.
Similarly, the absence of oxygen raises the optimum for short exposures,
but lowers the optimum and maximum when the exposure is prolonged 4.
1 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 20, 61.
* Dutrochet, Ann. sci.nat., 1838, pp. 25-7 ; Me"moires, 1837, T. I, p. 561 ; Sachs, Flora, 1863-4,
p. 39; Cohn, Bot. Ztg., 1871, p. 723 ; Velten, Flora, 1876, pp. 210, 214 ; Klemm, Jahrb. f. wiss.
Bot., 1895, Bd. xxvin, pp. 635-6 ; Hauptfleisch, ibid., 1892, Bd. xxiv. Corti was the first to
observe the increase of velocity with rising temperature. See also Klebs, Biol. Centralbl., 1881, Nos.
16, 17, 19.
8 L. c., p. 59. * Ewart, Protoplasmic Streaming in Plants, 1903, p. 68.
THE INFLUENCE OF TEMPERATURE
When experiments are performed in water whose temperature is
altered, as were those of Velten, lower optima and maxima are always
obtained than when the objects are heated in moist air. This is in part
due to the greater rapidity with which they gain the required temperature
in the former case, although the deficiency in the supply of oxygen aids
in prolonged exposures to lower the cardinal points. In any case, it is not
easy to see how it was that Nageli found streaming to increase in rapidity
in the cells of Nitella syncarpa up to 37° C., when it suddenly ceased,
unless the temperature was raised so rapidly as to exercise a shock-
effect 1. The existence of an optimum temperature is always shown more
or less clearly 2, especially when the exposure is prolonged, although
FlG. 52. Combined hot stage and gas-chamber. The three apertures lead to tubes projecting externally, and
are used to ensure the better diffusion of dense gases. Through the upper aperture electrodes insulated at their
bases may be inserted. (After Ewart.)
Schafer's 3 attempt to give the detailed progress of the curve is largely
futile owing to its variable character.
The zoospores of those Algae which grow at Spitzbergen at o° C. to
i -8° C. are presumably motile at this temperature. The zoospores of
Vaucheria clavata 4, Ulothrix zonata 5, and Haematococcus lacustris 8 are in
fact motile in water at o° C., whereas those of Botrydium granulatum 7 fall
into cold rigor at 6° C. According to Strasburger, the optimum for the
zoospores of Haematococcus lacustris lies between 30° and 40° C., the
1 Nageli, Beitr. z. wiss. Bot., 1860, Heft ii, p. 77. Cf. Velten, Flora, 1876, p. 177.
* Schultze, Das Protoplasma d. Rhizopoden u. Pflanzenzellen, 1863, p. 48; Sachs, Flora, 1864,
p. 65 ; Hofmeister, Pflanzenzelle, 1867, pp. 47, 53 ; Wigand, Botanische Hefte, 1885, I, p. 216;
Klemm, 1. c., p. 635. For observations on streaming in the plasmodia of Myxomycetes see Kiihne,
Unters. ii. d. Protoplasma, 1864, pp. 47, 53.
8 Schaefer, Flora, 1898, p. 135. Cf. Ewart, On the Physics and Physiology of Protoplasmic
Streaming in Plants, 1903, p. 59.
* Unger, Die Pflanze im Momente d. Thierwerdung, 1843, p. 57.
* Dodel, Jahrb. f. wiss. Bot., 1876, Bd. x, p. 484.
6 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 62.
7 Strasburger, 1. c.
3i6 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
maximum at 50° C. Probably motile thermo-bacteria may remain capable
of movement at 70° C. and over, and certain Oscillarias and Diatoms above
50° C. Most Bacteria, Infusoria, Vorticellae, and Rotifers cease to move,
sooner or later, at from 40° to 45° C. 1
A sudden change of temperature may awaken streaming in quiescent
cells 2, and may produce a more or less pronounced disturbance in cells
which already show streaming. This may be evidenced either by a
temporary acceleration or retardation or by a succession of both. A rapid
rise to a supraoptimal temperature commonly produces an irregular feverish
activity of streaming 3. A sufficiently sudden and pronounced rise or fall
of temperature always produces a temporary or permanent shock-stoppage
of streaming in cells which normally show this form of activity ; but if
the streaming has been induced by previous stimulation, the superimposed
effect of a sudden change of temperature is naturally less evident 4. It was
probably owing to this reason, and to the insufficient rapidity with which
the temperature altered^ that Velten5 was unable to detect any shock-
disturbance at all. The existence of a shock-effect is well shown by the
fact that localized cold or heat suddenly applied to one end of a cell
of Chara or Nitella causes a temporary stoppage of streaming over the
entire cell.
In all cases, however, the protoplasm rapidly accommodates itself to the
new conditions if their action is not of too great intensity. Thus when
streaming is resumed after the application of localized cold 6, it is slower
in the cold area largely owing to the higher viscosity of the endoplasm 7,
and this causes an accumulation of protoplasm at that point.
Schultze, Kiihne 8, and Sachs 9 observed that exposure to high or low
temperatures produced a pronounced deformation, fragmentation, or vacuo-
lation of the protoplasm, such as may also be caused by the action of
induction-shocks or poisons. Klemm 10 found that these changes only take
place when the cell is returned to a normal temperature, and that they may
be accompanied by spasmodic feverish streaming until the cell becomes
normal again. The return to a normal temperature appears, therefore,
1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 62. A few details concerning Oscillaria
are given by Meyen, Pflanzenphysiologie, 1832, Bd. Ill, p. 565 ; on antherozoids see Hofmeister,
Pflanzenzelle, 1867, p. 33, and Voegler, Bot. Ztg., 1891, p. 675.
3 Hauptfleisch, Jahrb. f. wiss. Bot, 1892, Bd. xxiv, p. 210.
3 Dutrochet, Ann. sci. nat, 1838, p. 27 ; Hofmeister, 1. c., p. 53 ; Kiihne, Unters. ii. d. Proto-
plasma, 1864, P- Io3 > de Vries, Materiaux p. la connaissance de 1'influence de la temperature s. 1.
plantes, 1870, p. 8 (reprint from Arch. Neerlan daises, T. v) ; Klemm, Jahrb. f. wiss. Bot., 1895,
Bd. xxvin, p. 640; Hermann, Studien ii. d. Protoplasmastromung bei d. Characeen, 1898, p. 45.
* Ewart, 1. c., p. 66. The same effect may be produced by feeble etherization. Josing, 1. c.,
p. 330.
1 Velten, Flora, 1876, p. 214.
6 Hermann, 1. c., p. 46 ; Ewart, 1. c. 7 Ewart, 1. c., p. 48.
8 Kiihne, 1. c., pp. 64, 87, 101. * Sachs, 1. c., pp. 39, 66. 10 L. c.
THE INFLUENCE OF TEMPERATURE 317
to exercise a different effect to exposure to either extreme, but whether
this applies generally is uncertain. Similar reactions are, however, shown
by the plasmodia of Myxomycetes, in which moderate changes of tempera-
ture induce a temporary tendency to assume a spheroidal shape l. Possibly
also sudden changes of temperature may produce shock-movements in
many plant-zoospores. At least when suddenly exposed to high tempera-
tures they dart actively in all directions, like ants disturbed in their nest 2.
THERMOTAXIS.
Paramoecium and other Infusoria are strongly thermotactic, being
positively so up to a certain temperature, beyond which they swim towards
the colder zones (negative thermotaxis) 3. De Wildeman 4 ascribes posi-
tive thermotaxis to Euglena, not only in water, but also when on wet sand,
and this irritability may possibly be possessed by many free-swimming
plant- organisms, although the evidence brought forward by Schenk5 is
unsatisfactory. Stahl6 has, however, shown that the plasmodium of
Aethalium septicum moves towards the warmer side, when resting on a strip
of wet filter-paper, one end of which lies in water at 30° C. and the other in
water at 7° C. According to Wortmann 7, the movement is reversed and
becomes negatively thermotactic when the temperature on one side rises
above 36° C.
In creeping organisms a reaction of this kind may be of great utility,
whereas small free-swimming plants are likely to have their thermotactic
tendencies overcome by the convection currents set up by the difference of
temperature. This is, however, not the case where it is the surface layers
which become warmer, so that a thermotactic irritability is most likely
to occur in strongly motile surface organisms found in ponds exposed to
full insolation. It is evident that the slow response of plasmodia cannot
be phobic in character, but this does not necessarily apply to free-swimming
organisms, which may be capable of either thermotactic or thermophobic
responses.
1 Ktihne, Unters. ii. d. Protoplasma, 1864, P- 87»
2 On the influence of temperature on pulsating vacuoles and nuclear division cf. Matruchot et
Molliard, Rev. ge"n. de Bot., 1903, T. XV, p. 193.
3 Mendelssohn, Pfliiger's Archiv f. Physiol., 1895, Bd. LX, p. i ; Zeitschrift f. allgem. Physiol.,
1902, Bd. II, p. 38.
4 De Wildeman, Bot. Centralbl., 1894, Bd- LX> P- J76.
5 Schenk, Centralbl. f. Bact, 1893, Bd. xiv, p. 37. Beyerinck (ibid., 1894, Bd. XV, p. 799)
observed that Bacterium Zopfii spread on gelatine to the warmer side, because growth and repro-
duction are more rapid in that direction. [Zikes, Centralbl. f. Bact., 1903, Abth. ii, Bd. XI, p. 59.]
8 Stahl, Bot. Ztg., 1884, p. 174. See also Clifford, Annals of Botany, 1897, Vol. xiv, p. 179.
7 Wortmann, Ber. d. bot. Ges., 1885, p. 117. A negatively thermotactic reaction was observed
by Verworn (Psycho-physiolog. Protistenstudien, 1889, P- *>3) ^n the case vl Amoeba.
3i8 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
SECTION 68. The Influence of Illumination.
Numerous plants and organs which develop in darkness also show
locomotion or streaming, while, even when normal development takes place
only in light, the power of movement is often retained for a long time,
or even until death ensues. This applies especially to streaming movements,
which usually appear in organs etiolated by development in darkness1.
The zoospores of Vaucheria and of other chlorophyllous plants are motile
even when formed in darkness 2, and the period of swarming of asexual
zoospores is frequently prolonged in the absence of light. Thus Strasburger 3
found that when developed in darkness the zoospores of Ulothrix zonata
remained motile for over three days, and those of Haematococcus lacustris
for more than two weeks, whereas in favourable illumination the latter
more especially come to rest in a few minutes. This peculiarity is not
always so pronounced, but it aids in enabling the fixed form to be developed
where a suitable photic ration is assured. Many of the zoospores, in fact,
die in continued darkness without ever coming to rest and germinating.
Apart from any transitory shock - effect, the activity of movement of
zoospores is not directly affected by the withdrawal of light, and the same
applies to streaming, when this is either normally present, or persists for
a long time when aroused by stimulation4. In all plants incapable of
indefinite existence in darkness, streaming is ultimately retarded more
or less, but only as the indirect result of the absence of light5, and the
same effect is shown among Oscillareae 6 and Volvocineae 7. According to
Engelmann8, movement is excited in purple bacteria when they are
exposed to light, whereas they come to rest again in darkness or in constant
illumination. Winogradsky9 observed, however, a continuance of the
movement in darkness, possibly as the result of racial or cultural peculi-
arities. According to Sorokin10, streaming ceases in the plasmodium of
Dictydium ambiguum in darkness, and is reawakened by illumination.
1 Dutrochet, Ann. sci. nat, 1838, 2* sen, T. IX, p. 30 ; Nageli, Beitr. z. wiss. Bot., 1860, Heft ii,
p. 78; Sachs, Bot. Ztg., Beilage, 1863, p. 3; Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv,
p. aio ; Ewart, Journ. Linn. Soc., Vol. xxxi, 1896, pp. 564, 573 ; Josing, Jahrb. f. wiss. Bot., 1901,
Bd. xxxvi, pp. 198, 210.
a Klebs, Die Bedingungen d. Fortpflanzung u. s. w., 1896, p. 19; Walz, Jahrb. f. wiss. Bot.,
1866-7, Bd. v, p. 132.
8 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, pp. 27, 53.
4 Nageli, 1. c., p. 102 ; Strasburger, 1. c., p. 27. On streaming cf. Hauptfleisch, 1. c. ; Josing,
1. c., p. 198.
5 Ewart, Protoplasmic Streaming in Plants, 1903, p. 71.
8 Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. vi, p. 31 ; Hansgirg, Bot. Centralbl., 1882, Bd.
XII, p. 361.
7 Oltmanns, Flora, 1892, p. 196.
8 Engelmann, Bot. Ztg., 1888, p. 663 ; Pfliiger's Archiv f. Physiologic, 1882, Bd. xxx, p. 103.
9 Winogradsky, Beitr. z. Morphol. u. Physiol. d. Bact, 1888, p. 90.
10 Sorokin, Bot. Jahresb., 1878, p. 471.
THE INFLUENCE OF ILLUMINATION 319
Further instances of such actions may ultimately be discovered, and much
depends upon the condition of tone of the organism, which largely depends
upon external circumstances.
According to Josing T, the action of ether or the withdrawal of carbon
dioxide causes streaming to cease in darkness and to recommence on
illumination, whereas under normal conditions it is about as rapid in dark-
ness as in light. Thus Josing states that in leaf- eel Is of Vallisneria spiralis
in water containing from 0-25 to i per cent, of ether, streaming ceases after
darkening for ten minutes to half an hour, and recommences thirty seconds
to five minutes after reilluminating. Chloroform acts in the same way,
but not alkaloids or alcohol2. Similar, but slower, reactions are shown
when hanging-drop preparations are made in a gas-chamber, the floor of
which is covered by caustic soda. If, however, a non- volatile acid is added
(i of phosphoric acid or of citric acid in 10,000, and 20,000 of water
respectively), the streaming persists in darkness as well as in light. Since
non-chlorophyllous objects react in the same way, the recommencement of
streaming on exposure to light cannot be due to the photo-synthetic
production of oxygen.
[It is doubtful whether the action of ether actually depends upon the condition
of phototonus. Very dilute solutions of ether may slightly accelerate streaming, but
solutions of the strength given retard it8. The rise of temperature produced in
a strongly illuminated gas-chamber will cause ether to pass into the air of the chamber,
and the hanging drop to contain less ether, whether the floor of the chamber is covered
with a similar solution of ether or not and whether the chamber is open or closed. In
this way the retarding action exercised in darkness would be lessened on illumination, and
might even be converted into an acceleration. Josing states, however, that it is the blue
rays, and not the red ones, which excite streaming in the etherized preparations. In any
case I am quite unable to confirm the statements of Josing in regard to the effects of
the withdrawal of carbon dioxide, streaming continuing on the average equally long in
similar preparations of Vallisneria kept in darkness, whether small amounts of carbon
dioxide were present or not, sometimes the one and sometimes the other coming to rest
first. Further, cells of Vallisneria frequently continued to show streaming for more
than a day in darkness, although the carbon dioxide was continually removed and no
external acidity was present. Any considerable accumulation of carbon dioxide
retards streaming both in light and darkness, and cells of Chara and Nitella continued
to show slow streaming, although the carbon dioxide was continually withdrawn and
the plants kept in darkness for as long as six weeks*. Finally, the o-oi per cent,
solution of phosphoric acid which, according to Josing, causes streaming to continue
in darkness, produces a stoppage of streaming in Chara and Nitella within an hour or
two, and in Elodea and Vallisneria within a day 5.]
1 Josing, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, pp. 198, 210. 2 Josing, 1. c., p. 214.
8 See Ewart, Protoplasmic Streaming in Plants, 1903, p. 86.
4 Ewart, 1. c., p. 42. 8 Cf. Ewart, 1. c., p. 77.
320 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
A sufficient increase in the intensity of the light always produces
a retardation of locomotion and streaming1. By localized action local
retardations or interruptions of streaming may be caused 2, as well as various
protoplasmic deformations or vacuolation 3. Owing to the fact that the
plasmodia of Myxomycetes are killed by exposure to light of moderate
intensity, even feeble illumination distinctly retards the amoeboid activity of
these organisms, and hence also the streaming of the endoplasm 4. Similarly,
after several hours' exposure to continuous direct sunlight, streaming ceases
or becomes extremely slow in Elodea and Chara^ but may become active
again in feeble light5. Other instances in which submaximal intensities
of illumination produce a retardation of movement will probably be
discovered.
A striking instance of shock stimulation is afforded by Bacterium
photometricum and other purple bacteria, which perform a pronounced
backward movement when the illumination suddenly decreases, but not
when it increases. No fatigue is shown in this case, however rapidly the
stimulation is repeated 6. According to Engelmann 7, Bacterium photo-
metricum, which Winogradsky 8 considers to be a small Chromatium, also
performs a shock - movement when the percentage of carbon dioxide
suddenly alters, but not when the air is suddenly replaced by hydrogen.
In the case of the Rhizopod Pelomyxa palustris Engelmann 9 found that
sudden illumination causes the pseudopodia to be rapidly withdrawn, and the
plasmodia of Myxomycetes seem to react much in the same way although
more feebly. A shock-movement is, on the other hand, produced in many
lower animals by the sudden withdrawal of light 10, and, to a slight extent,
also in the zoospores of Botrydium granulatum n. In the case of the zoo-
spores of Bryopsis plumosa, however, sudden illumination produces a tem-
porary irregularity of movement. In many other organisms and zoospores
Strasburger could detect no perceptible photic shock-effect, and sudden
1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 69; Annals of Botany, 1898, Vol. xil,
pp. 383-9°-
3 Pringsheim, Jahrb. f. wiss. Bot., 1879, Bd. XII, pp. 334, 367.
3 Klemm, ibid., 1895, Bd. xxvni, p. 647.
4 Baranetzsky, Me"m. de la Soc. d. sci. nat. de Cherbourg, 1876, T. xix, pp. 328, 340; Hof-
meister, Pflanzenzelle, 1867, p. 21. Cf. also Lister, Annals of Botany, 1888-9, Vol. ill, p. 13.
5 Ewart, Protoplasmic Streaming in Plants, 1903, p. 70.
6 Engelmann, Pfluger's Archiv f. Physiologic, 1882, Bd. XXX, p. 103 ; Bot. Ztg., 1888, p. 666 ;
Winogradsky, Beitrage z. Morphol. u. Physiol. d. Bacterien, 1888, p. 95.
7 Engelmann, 1882, 1. c., p. 112 ; 1888, 1. c., p. 689.
8 Winogradsky, Bot. Ztg., 1888, p. 90.
9 Engelmann, Pfluger's Archiv f. Physiologic, 1878, Bd.xix,p. 3; Blochmann, Biol. Centralbl.,
1894, Bd. xiv, p. 85.
10 See Loeb, Pfluger's Archiv f. Physiol., 1897, Bd. LXVI, p. 459; Nagel, Bot. Ztg., 1901,
Ref., p. 289.
11 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 25; Stahl, Bot.
Ztg., 1880, p. ^\o>\Eugknd}.
THE INFLUENCE OF ILLUMINATION 321
illumination or darkening exercises no apparent effect upon protoplasmic
streaming. If, however, preparations which have been kept in darkness for
some time are suddenly exposed to concentrated sunlight, a temporary
stoppage lasting from a few seconds to a minute or a distinct retardation
may often be seen. The latter may be followed by a slight acceleration, after
which streaming rapidly decreases and ultimately ceases if the exposure
is continued l. A shock-effect may possibly always be exercised when
a sudden change is made from prolonged darkness to sufficiently intense light,
but the reverse does not hold good, since intense light rapidly proves fatal.
Plants may, however, exist in which both sudden darkening and sudden
illumination produce the same shock-effect.
SECTION 69. The Tropic Action of Light on Freely
Motile Organisms.
As in the case of rooted plants, varying degrees and forms of irritability
are shown. More especially the sensitive and actively motile zoospores
place their long axes immediately parallel to the direction of illumination
and swim in a definite direction instead of all ways as they do in uniform
diffuse light. The anterior end is turned towards the source of illumination
when this is of moderate strength, but away from it when intense, and the
direction of movement follows suit. The velocity is little if at all altered,
and if the organism comes into contact with a glass plate or adheres by
its hinder end, it may still show the same tropic orientation as before. In
fact, under these circumstances. Stahl 2 found that the positively phototropic
orientation of Euglena viridis became negative in intense light. Oscillarias
and Diatoms also place their long axes parallel to the direction of the
light falling from one side only, and move towards or away from it according
to its intensity. Various Desmids behave similarly, and some forms are
plagio-phototropic in light of medium intensity.
An accumulation of Bacterium photometricum is, however, also possible
by means of the backward shock-movement experienced every time the
organism passes to a dark region. Engelmann 3 was unable to produce
any distinct local accumulation by unilateral illumination, so that a gradual
decrease in the intensity of the illumination does not appear to act as
a phobic stimulus. It is, therefore, uncertain whether the attraction
observed by Winogradsky, and the repellent action of light on Beggiatoa,
are phobic or tropic in origin 4.
1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 71.
3 Stahl, Bot. Ztg., 1880, p. 410.
3 Engelmann, Pfliiger's Archiv f. Physiol., 1882, Bd. xxx, p. 121.
* Winogradsky, Beitrage z. Morphol. u. Physiol. d. Bact., 1888, Heft i, p. 94; Bot. Ztg., 1887,
PFEFFKR. Ill
322 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
The negatively phototactic movement of the plasmodia of Myxomycetes
is possibly directly due to the retarding action of the strong illumination
upon the amoeboid activity of the exposed side, but it is uncertain whether
similar phototropic movements and aggregations may be produced within
cells covered by cell-walls, apart from those shown by the chloroplastids of
Mesocarpus and other plants.
In many cases a sufficient rise in the intensity of the illumination
causes the positive phototaxis to become negative, so that Weber's law can
only apply within certain limits. Strasburger * observed only an attraction
to the light in the case of the zoospores of Botrydium granulatum, and of
the Flagellate Infusorian Chilomonas curvata. This was possibly due to
the fact that the organisms were temporarily adapted to high intensities
of light, since Stahl 2 found that the zoospores of Botrydium granulatum
readily performed negatively phototactic movements. Moderately strong
sunlight is sufficient to produce this in most zoospores, but in others
comparatively feeble light suffices, as, for instance, in the case of most
Diatoms, while no positive phototactic action at all has been detected in
Myxomycetes.
Zoospores hence usually collect at a certain distance from the source
of illumination, but instead of coming to rest, continually cross and recross
the imaginary line of equilibrium, where, we may assume, they would take
on a diaphototropic position if incapable of locomotion. Owing to the
unequal irritability of different individuals, the position of equilibrium varies
even in the same species. In addition, periodic autogenic changes of tone
may occur, which, under constant conditions, may cause the zoospores to
swim at intervals from one side to the other of a drop of water in the path
of the light 3. In some cases this reversal is slowly produced, but in the
zoospores of Ulothrix zonata it may take place so rapidly, that the
zoospore, immediately it has reached one side, swims back to the other.
Another instance of autogenic reversal is afforded by the fact that
the Desmid Closterium moniliferum turns first the young end and then
the older one towards the light at intervals of six to thirty-five minutes 4.
Changes of tone also occur during development 5, and they may be in-
duced to a greater or less extent by alterations in the cultural and external
conditions. Thus it requires a stronger illumination to change positive into
negative phototaxis when zoospores are used which have developed in strong
light than when they have developed under feeble illumination 6. Similarly,
1 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 26.
3 Stahl, Einige Bemerkungen u. d. richtenden Einfluss d. Lichts auf Schwarmsporen, 1879.
Reprint from Verh. d. phys.-med. Ges. zu Wiirzburg, N. F., Bd. XIV.
8 Strasburger, 1. c., pp. 17, 38. * Stahl, Bot. Ztg., 1880, p. 396.
5 Cf. Strasburger, 1. c., p. 38 ; Oltmanns, Flora, 1892, p. 187.
c Strasburger, 1. c., p. 39 ; Oltmanns, 1. c., p. 191.
TROPIC ACTION OF LIGHT ON FREELY MOTILE ORGANISMS 323
a rise of temperature with constant illumination causes the zoospores to
move to the further side of the drop, and this, presumably, because their
photic irritability is raised *. In addition, Chromulina Woroniniana shows
at 5° C. a negatively, but at 20° C. a positively phototactic response to the
same intensity of light 2, while Strasburger found that a deficiency of
oxygen raises the phototactic tone. In the absence of oxygen the zoospores
of Algae retain their phototactic irritability so long as they remain capable
of movement 3, but it must be remembered that the exposure to light
provides not only the stimulus to movement, but also the energy for it
by the agency of photosynthesis. Although no thorough researches have
been performed upon the influence of chemical agencies, Elfving 4 has shown
that etherization raises the sensitivity of Chlamydomonas pulvisculus, and
also its phototactic tone. According to Elfving, chloroform inhibits the
phototactic irritability without suspending the power of movement, but
these results are not in entire agreement with those of Rothert6. It is
evident, however, that various combinations of factors may be responsible
for the appearance of organisms on the surface at certain times of the day
or year, whereas at others they sink to a greater or less depth below it.
Engelmann found that Euglena only responded to an incident ray of
light when it fell upon the clear hyaline anterior end, and that it did so before
the light reached the eye-spot 6. It does not follow that a similar localiza-
tion of irritability is shown in all cases, while the assumption as to the
function of the pigment-spot as an eye 7 is devoid of proof, and is merely
based upon the analogy with the pigmented ocelli and eye-spots of lower
animals. In fact, many zoospores are phototactic, although they have
no pigment-spot.
When the zoospores are exposed to strong light which has been passed
through a prism filled with diluted indian ink, so that the intensity
diminishes along a plane at right angles to its direction, they move towards
the feebler light and across the incident rays. Diatoms and Desmids
behave in the same way, but nevertheless, these observations fail to prove
1 Strasburger, 1. c., p. 56. Strasburger finds (1. c., p. 52) that a sudden fall of temperature pro-
duces a transitory backward movement.
8 Massart, Bull, de 1'Acad. royale de Belgique, 1891, 3° ser., T. xxn, p. 164.
3 Celakovsky, Ueber d. Einfluss d. Sauerstoffmangels auf die Bewegung einiger aeroben Organis-
men, 1898, pp. n, 28. Reprint from the Bull, de 1'Acad. d. sciences de Boheme.
4 Elfving, Ueber 'd. Einwirkung von Aether u. Chloroform auf Pflanzen, 1886, p. 13. Reprint
from the Ofversigt af Finska Vetensk. Soc. Forh., Bd. xxvin.
6 Rothert, Jahrb. f. wiss. Bot., 1903, Bd. xxxix, p. I.
6 Engelmann, Pfluger's Archiv f. Physiologic, 1882, Bd. xxix, p. 396.
7 Klebs, Unters. a. d. bot. Inst. zu Tubingen, 1883, Bd. I, p. 263; Overton, Bot. Centralbl.,
1889, Bd. xxxix, p. 114; Franze, ibid., 1894, Bd. LVII, p. 81 ; Schiitt, Peridineen, 1895, p. 98;
Zinrmermann, Beihefte z. Bot. Centralbl., 1894, Bd. iv, p. 161 ; Senn, in Engler u. Prantl, Natiirl.
Pflanzenfamilien, 1900, I. Th., Abth. i, p. 102 ; Kohl, Carotin, 1902, p. 15. On the structure of the
eye-spot cf. also Strasburger, Histologische Beitrage, 1900, Heft vi, p. 193.
Y 2
324 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
that it is the intensity of the light and not its direction which acts as the
orienting agency.
Zoospores. Various groupings were observed by Colomb and by Olivi *, as well
as by Nageli, Cohn, and Famintzin 2, but up to the time of Stahl 3 and Strasburger
insufficient attention was paid to the mechanical influence of currents in the water.
Sachs 4 has, in fact, shown that the slight warming due to unilateral illumination
causes currents sufficient to produce special grouping of non-motile drops of oil sus-
pended in a mixture of alcohol and water. The movements are, however, largely
due to the changes of surface-tension as the alcohol evaporates, and if pure water is
used and evaporation checked the streaming in the liquid is much feebler 6, and is
insufficient to prevent the normal phototactic orientation 6. Thus, in a mixture of
dissimilar zoospores, one kind may show a negative, the other a positive action, while
non-reacting or dead forms become uniformly distributed, or at least not definitely
grouped. Active living zoospores, however, assume a suitable position in one or more
minutes.
Most chlorophyllous zoospores such as those of Ulothrix zonata^ Ulva,
Enteromorpha, Bryopsis plumosa, Scytosiphon lomentarius, as well as Euglena
and other green Flagellatae, and the unicellular and colonial Volvocineae, show
various grades of phototactic irritability 7. The zoospores of Vaucheria 8, as
well as the small yellow zoospores of Bryopsis plumosa, but not the large
green ones, are irresponsive to light. According to Thuret, the zoospores of
Codium tomentosum and Ectocarpus firmus hardly show any phototactic
irritability, although the colourless zoospores of Chytridium vorax and Poly-
phagus euglenae are strongly phototactic 9, and the same applies to one
species at least of Bodo 10. A phototactic irritability will obviously aid the
zoospores of parasites in seeking out regions where their hosts live, but
chemotactic stimuli are even more effective, and hence the zoospores of
1 Usteri, Annal. d. Botanik, 1793, Stuck VI, p. 30.
8 For the literature see Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878,
p. i.
3 Stahl, Bot. Ztg., 1878, p. 715 ; Verhandlg. d. physik.-med. Ges. zuWUrzburg, 1879, Bd- XIV»
p. 7.
4 Sachs, Flora, 1876, p. 241.
6 Berthold, Protoplasmamechanik, 1886, p. 113.
8 Cf. Strasburger, 1. c., pp. 6-8.
7 Strasburger, I.e.; Stahl,!. c.; Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. vi, p. i; Woronin,
Bot. Ztg., 1880, 629 (Chromophytori) ; Berthold, Fauna u. Flora des Golfs von Neapel, 1882, p. n;
Pfeffer, Unters. a. d. bot Inst. zu Tubingen, 1884, Bd. i, p. 443; Overton, Bot. Centralbl., 1889,
Bd. xxxix, p. 68; Oltmanns, Flora, 1892, p. 187 (Volvox]\ Kolkwitz, Bot. Centralbl., 1897, Bd.
LXX, p. 187; Holmes, ibid., 1903, Bd. xcm, p. 18 (Volvox). According to Borzi (Bot. Jahresb.,
1883, Bd. i, p. 26), the zoospores of Enteromorpha compressa lose their phototactic irritability on
copulation.
8 Thuret, Ann. sci. nat., 1850, 3° se>., T. xiv, p. 246; Woronin, Bot. Ztg., 1869, p. 139;
Strasburger, 1. c., p. 42.
9 Strasburger, 1. c., p. 18. Cf. also Kolkwitz, 1. c., p. 187.
10 Rothert, Flora, 1901, p. 372.
TROPIC ACTION OF LIGHT ON FREELY MOTILE ORGANISMS 325
Saprolegnia and of many colourless Flagellatae are not phototactic a. The
same applies to the antherozoids of Ferns 2, whereas those of Sphaeroplea 3
and Fucus* respond readily to light, although Bordet5 obtained negative
results in the latter case.
All chlorophyllous Diatoms appear to be phototactic 6, but not the
colourless forms7. The orienting action is feeble, however, so that
the oscillating forms pursue irregular paths towards or from the light.
The negative phototaxis is shown with light of moderate intensity, and
commonly causes the Diatoms to creep into the mud. Chlorophyllous
Oscillariaceae place themselves parallel to the incident rays, and creep
towards the light even when it is moderately intense 8.
Desmids. Most motile forms show phototactic reactions, although
these are often feeble 9, and the irritability may vary in different individuals
of the same species. Hence Aderhold and Stahl found that intense
illumination caused them in most cases to show negative phototaxis, whereas
Klebs could only detect positive phototaxis even on re-examining the same
species. Such forms as Pleurotaenium, Micrasterias, and Penium respond
especially well and glide slowly to or from the light according to its inten-
sity. Such forms as Closterium moniliferum and other species of the same
genus which regularly turn over and attach the free end to the substratum
continue the same movement when exposed to light, but then progress to or
from the source of illumination as the case may be.
According to Stahl and Aderhold the long axis is approximately parallel
to the direction of the light during positive, but at right angles to it during
negative phototaxis. Klebs, however, doubts the existence of any such
orientation, and, according to Braun 10, the younger end of Penium curium is
always turned towards the light. Probably various grades of irritability
1 Strasburger, 1. c., p. 18; Cohn, Bot. Ztg., 1867, p. 178; A. Fischer, Jahrb. f. wiss. Bot, 1882,
Bd. xin, p. 297 ; Kolkwitz, 1. c. In regard to animals see J. J. Loeb, Der Heliotropismus d. Thiere,
1890; Verworn, Psych o-physiolog. Protistenstudien, 1889, p. 35; Herbst, Biol. Centralbl., 1894,
Bd. xiv, p. 659; Jourdan, Die Sinne u. die Sinnesorgane d. niederen Thiere, 1891; Nagel, Der
Lichtsinn augenloser Thiere, 1896.
3 Pfeffer, 1. c., p. 372. 3 Cohn, Ann. sci. nat., 1856, 4* se'r., T. v, p. 201.
* Thuret, Ann. sci. nat., 1854, 4° ser., T. n, p. 210.
5 Bordet, Bull, de 1'Acad. royale de Belgique, 1894, 3° se'r., T. xxvn, p. 894. Cf. Winkler,
Ber. d. bot. Ges., 1900, p. 304.
6 Cohn, Jahrb. d. schles. Ges. f. vaterl. Cultur, 1863, P- K^3 J Bot- Ztg-> l8^7, p. 171 ; Stahl,
Bot Ztg., 1880, p. 400; Verworn, 1. c., p. 46.
7 Benecke, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 554.
8 Dutrochet, Mem. d. vegetaux et d. animaux, Bruxelles, 1837, p. 340; Famintzin, Jahrb. f.
wiss. Bot, 1867-8, Bd. vi, p. 27 ; Hansgirg, Bot. Centralbl., 1882, Bd. xn, p. 361 ; Verworn, 1. c.
p. 50.
8 Stahl, Bot. Ztg., 1880, p. 392 ; Verhandlg. d. physik.-med. Ges. zu Wiirzburg, 1879, N* F-»
Bd. xiv ; Klebs, Biol. Centralbl., 1885, Bd. v, p. 353 ; Aderhold, Jenaische Zeitschrift f. Naturw.
1888, N. F., Bd. xv, p. 323.
10 Braun, Verjiingung in d. Natur, 1851, p. 217.
326 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
exist, for in certain cases clear and definite orienting responses seem to be
given.
Myxomycetes. Stahl and Baranetzsky only observed negative photo-
taxis in the plasmodia of Myxomycetes, and it is possible that they respond
to the intensity of the light rather than to its direction. It is, however,
doubtful whether very feeble light induces positive phototaxis, as Hofmeister
states 1. The plasmodia of Acrasieae appear also to be phototactic 2.
Strasburger 3 suggests that a change of phototactic tone may be responsible
for the upward movement during fruiting, but since this movement is also
shown in darkness other directive factors must enter into play which are
able to overcome the negative phototaxis induced by exposure to light.
Coloured light. Phototaxis, like phototropism, is mainly or entirely
excited by the more refrangible rays. Strasburger 4 was in fact unable to
detect any phototaxis in zoospores exposed to red or green light, whereas
the maximum action is shown in the indigo region of the spectrum. The
same applies to Euglena according to Engelmann 5, and to Diatoms accord-
ing to Verworn 6, whereas all the visible rays of the spectrum appear to act
as phototactic stimuli to Oscillaria. Since similar peculiarities exist in the
case of colourless organisms 7, it does not follow that the general response
of Oscillaria is due to the special absorptive activity of the phycocyanin.
Various instances are known among animals in which the more refrangible
rays are not the most active in phototaxis, and to our eyes the yellow rays
are brighter than the blue and red.
In the case of purple bacteria, however, the most pronounced phobic
action is exercised by the ultra red rays 8, and owing to the association of
bacterio-purpurin with the chlorophyll of these organisms, these are also
the rays which are most readily absorbed. On the other hand, the photo-
tactic movements of plasmodia 9 and of chloroplastids are mainly produced
by the more refrangible rays, and according to Josing it is the blue and
not the green or red rays which excite streaming in etherized cells10.
Similarly, the injurious action of intense light is mainly due to the more
refrangible rays, and the disorganization observed by BorsSow and Luerssen u
I Hofmeister, Pflanzenzelle, 1867, p. 20 ; Allgemeine Morphologic, 1868, p. 625 ; Baranetzsky,
Mem. de la Soc. d. sci. nat. de Cherbourg, 1876, T. xix, p. 328 ; Stahl, Bot. Ztg., 1884, p. 167.
3 Olive, Proceedings of the Boston Society of Natural History, 1902, Vol. xxx, p. 485.
3 Strasburger, Wirkung des Lichts u. d. Warme auf Schwarmsporen, 1878, p. 70.
4 Strasburger, 1. c., p. 44. Cf. also Cohn, Bot. Ztg., 1867, p. 171.
5 Engelmann, Pfliiger's Archiv f. Physiol., 1882, Bd. xxix, p. 398.
8 Verworn, Psycho-Physiologische Protistenstudien, 1889, p. 49. When oxygen is deficient the
organisms may collect in the red and yellow regions of the spectrum where photosynthesis is most
active. Cf. Engelmann, 1. c., p. 390.
7 Cf. Nagel, Bot. Ztg., 1901, Ref., p. 293.
8 Engelmann, Bot. Ztg., 1888, p. 677. 9 Baranetzsky, 1. c., p. 331.
10 Josing, Jahrb. f. wiss. Bot, 1901, Bd. XXXVI, p. 208.
II Borscow, Bull, de 1'Acad. de St. Petersbourg, 1868, T. xn, pp. an, 230; Luerssen, Einfluss d.
TROPIC ACTION OF LIGHT ON FREELY MOTILE ORGANISMS 327
in plant-cells kept in light which had passed through potassium bichromate
solution was probably due to some accessory heating or other effect.
Reinke and Kraus l were in fact unable to detect any such deformations in
the protoplasm of epidermal hairs under similar exposure.
SECTION 70. The Photic Orientation of Chloroplastids.
The movements produced by the action of light serve not only to bring
the chloroplastids into suitable functional positions, but also to withdraw
them from the action of intense light. Other agencies such as temperature,
chemical actions, and the withdrawal of water may also affect the position,
and autogenic alterations of the normal position are also possible2. In
addition, when active streaming is excited by an injury to the leaves of
Vallisneria and Elodeu^ the chloroplastids may be carried with the plasma
for a variable length of time until the resting condition is again assumed 3.
During normal streaming either none or only occasional chloroplastids are
carried with the streaming protoplasm. Pringsheim found 4 that the chloro-
plastids bleached by sunlight in cells of Nitella were carried away by the
streaming endoplasm, whereas in cells of Chara they
retain their original positions5, and the most varied
agencies fail to cause them to leave the ectoplasm in
Nitella*. Nevertheless, slight disturbances of position
are probably easily produced, and these are very pro-
nounced in such Diatoms as Rhipidophora and Striatella^
for mechanical vibrations cause their chloroplasts to
retract and become spherical 7.
A phototropic orientation is especially evident in
the chlorophyll plates of Mougeotia and Mesocarpus,
which under favourable conditions place themselves at
right angles to the incident rays (Fig. 53 A), but in strong £«£li^pP-Hla" *
light twist round until a profile position is assumed with
the flat surface parallel to the direction of the light8. In other cases,
rothen u. blauen Lichtes auf die Stromung d. Protoplasmas, 1868. Cf. also Velten, Die physikal.
Beschaffenheit d. pflanzl. Protoplasmas, 1876, p. 14 (reprint from the Sitzungsb. d. Wiener Akad.,
1876, Bd. LXXIII, Abth. i) ; Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. VI, p. 38.
1 Reinke, Bot. Ztg., 1871, p. 800; G. Kraus, Bot. Ztg., 1876, p. 584.
a See Fr. Schmitz, Die Chromatophoren, 1882 ; Schimper, Jahrb. f. wiss. Bot., 1885, Bd. XVI,
p. 203 ; Hauptfleisch, 1. c. ; Haberlandt, Physiol. Pflanzenanat., 2. Aufl., 1896, p. 232.
3 Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 192.
Pringsheim, Jahrb. f. wiss. Bot., 1879, Bd- XII> P- 333-
Ewart, Journ. Linn. Soc., 1897, Vol. xxxi, p. 574.
Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 723.
Liiders, Bot. Ztg., 1862, p. 42 ; Schmitz, Chromatophoren, 1882, p. 82 ; Schimper, I.e., p. 218.
Stahl, Bot. Ztg., 1880, p. 299; Moore, Journ. of Linn. Soc., 1888, Vol. XXiv, p. 366;
328
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
however, where the chloroplastids are numerous and usually lens-shaped they
move in the protoplasm lining the cell to the face or profile positions. It is
in this way that the chloroplastids in a filament of Vaucheria collect in two
parallel rows along its upper
and under-surface when feebly
illuminated from beneath, but
along its sides when the light
is intense and a profile posi-
tion is assumed. Similar re-
actions are shown in the leaves
of Mosses and of Elodea> as
well as in the fronds of Lemna
and in Fern-prothallia *.
Under moderate vertical illu-
mination the chlorophyll
bodies assume the face posi-
tion by placing themselves
upon the outer and inner
walls, whereas in intense light
they group themselves upon
the side walls (Fig. 54 B).
Owing to the special internal
relationships the chloroplas-
tids do not spread themselves
uniformly in darkness, but
place themselves upon the
inner and side walls (Fig.
540-
In very intense light
irregular aggregations and
groupings of the chloroplastids are often shown, not only in the simple
tissues mentioned, but also in the more complex ones of higher plants 2.
Aggregation is rapidly produced in the chloroplastids of Acetabularia
mediterranea 3, whereas in Vaucheria long exposure is necessary and in
Nitella no aggregation at all is shown 4. We are here dealing with internal
Oltmanns, Flora, 1892, p. 207 ; Lewis, Annals of Botany, 1898, Vol. xn, p. 418. Wittrock observed
(Stahl, 1. c.) that the chlorophyll plates of Gonotonema reacted similarly.
1 Frank, Jahrb. f. wiss. Bot., 1872, Bd. vin, p. 216; Schimper, Jahrb. f. wiss. Bot., 1885, Bd.
xvi, p. 203 ; Stahl, 1. c. ; Haberlandt, Ber. d. bot. Ges., 1886, p. 206 ; Moore, I.e. ; Oltmanns, 1. c.;
Kohl, Carotin, 1902, p. 103. According to Prillieux (Compt. rend., 1874, T. LXXVIII, p. 506),^ the
chlorophyllous plasma in certain leaf-cells of Selaginella Martensii forms masses which glide in' the
same way over the cell- wall. Cf. also Haberlandt, Physiol. Pflanzenanatom., 2. Aufl., 1896, p. 229.
a Bohm, Sitzungsb. d. Wiener Akad., 1856, Bd.xxu, p. 479; 1859, Bd- xxxvn, p. 453; Stahl,
Bot. Ztg., 1880, p. 340 ; Schimper, 1. c., p. 225.
8 de Bary, Bot. Ztg., 1877, P- 731- * Stahl, 1. c., p. 324.
FIG. 54. Transverse section through the leaf of Lemna trisulca
(after Stahl). A. surface position (day position). B, arrangement
of the chlorophyll grains in intensive light. C, position assumed
in darkness.
THE PHOTIC ORIENTATION OF CHLOROPLASTIDS 329
disturbances which have no relation to the direction of the light, and which
may also be produced by temperature extremes as well as by injuries and
various mechanical agencies 1. On the other hand, the orienting action of
ordinary light is well shown by the fact that oblique illumination may
cause the chloroplastids to collect at the opposite corners of the cells in the
unilamellar leaf of a Moss 2. In addition, the chloroplastids in Bryopsis
move towards the better illuminated portion of the cell 3.
Even when we are dealing with simple tissues and responsive chloro-
plastids, the positions assumed do not always correspond precisely to the
above rules, partly owing to the shape of the cells and their relationships to
neighbouring ones. The position of the chloroplastids in the palisade-cells
of leaves is but little or not at all influenced by light, and the same applies
to other cells as well. This may either be due to the absence of any
phototactic irritability or to the suppression of any aitiogenic response by
FlG. 55. Mesophyll-cells from the under-surface of the leaf of Oxalis acetosella seen from above: (a) face
position of chloroplastids in diffuse light; (b) profile position after short exposure to sunlight ; (c) after longer inso-
lation (after Stahl).
more powerful aitiogenic factors. In addition, the light- rays may be con-
centrated by refraction or reflection upon particular areas, as in the proto-
nema of Schizostega (Schistostegd), or the light may be so dispersed that a
cell or cells in the interior may be uniformly illuminated on all sides even
when the leaf is under unilateral external illumination.
In the tissues of many of the higher plants the choroplastids show
similar groupings in response to light as do those in the leaf-cells of Mosses.
Thus in the spongy mesophyll of Oxalis acetosella the chloroplastids arrange
themselves upon the walls parallel to the surface in diffuse light (Fig. 55 tf),
whereas in direct sunlight a profile position is assumed (Fig. 55 b\ and after
long exposure the balling together shown in Fig. 55*: takes place. In the
leaf-cells of Sempervivum and Sedumy according to Stahl (I.e.), the face
position is assumed in shade, an intermediate position in bright diffuse
light, and an aggregated position in sunlight.
1 Cf. Frank, 1. c., pp. 261, 295 ; G. Kraus, Bot. Ztg., 1874, p. 206; Haberlandt, Ueber d.
Einfluss d. Frostes auf die Chlorophyllkorner, 1876, p. 6 (reprint from the Oester. Bot. Zeitschrift) ;
Schimper, 1. c., pp. 166, 235 ; Moore, 1. c., pp. 206, 371.
3 Stahl, 1. c., p. 346.
8 Winkler, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 455.
330 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
As the result of the dispersal of the penetrating rays, a tropic orienta-
tion of the chloroplastids will be less useful in the interior of a tissue
than it is on the surface. Nevertheless it is at the surface where the
light is strongest that protective movements are of most importance,
while in feeble light the position the superficial chloroplastids assume
still further darkens the interior of the tissue, especially in thick leaves.
This is one of the reasons for the thinness of shade leaves, since in this way
they acquire greater adaptability to varying intensities of illumination. All
chloroplastids do not, however, show a power of photic orientation, even when
in superficial cells. This power is in fact absent from the cells of Cladophora,
Nitella> Spirogyra> and from most palisade parenchyma cells. In these the
accumulation of the chloroplastids at the upper ends of the cells would
hinder instead of favouring the utilization of the light rays, which is best
performed when they lie on the side walls. Stahl * and Haberlandt 2 have,
however, shown that light exercises a strong orienting action upon the
chloroplastids in the palisade-cells of many plants, which Haberlandt found
to be especially pronounced when the light was at right angles to the long
axis of a palisade-cell exposed by sectionizing.
A certain advantage is attained by the fact that the immovable
chloroplastids of palisade-cells bulge inwards during moderate illumination,
but flatten themselves against the wall when the light is intense. This was
first discovered by Micheli 3, while Stahl observed the following changes of
shape of the chloroplastids of Ricinus in shade and sunlight : —
Diameter of base parallel to cell-wall.
Height at right angles to cell-wall.
0.0063 mm.
o«oos;7 mm.
In sunlight ....
0-0083 mm«
0-0036 mm.
In this way the chloroplastids are less exposed to intense illumination,
but intercept more light when the latter is less intense. These and other
changes of shapes are not restricted to the chloroplastids of palisade-cells,
but are shown in others also, and may not only be produced by intense
illumination but also by various other agencies, such as continued darkness,
extremes of temperature, saline solutions, and chemical substances 4.
Neither the mode of stimulation nor the mechanism of movement of the chloro-
plastids is as yet known, apart from the fact that chloroplastids may be passively
carried by streaming protoplasm. It is even uncertain whether the chloroplastid
1 Stahl, Bot. Ztg., 1880, p. 377.
2 Haberlandt, Physiol. Pflanzenanat., 2. Aufl., 1896, p. 210.
3 Micheli, Arch. d. sci. de la Bibl. univers. de Geneve, 1876, T. xxix, p. 26; Stahl, 1. c., p. 357.
* Stahl, 1. c., 1880, pp. 303, 361; Schmitz, 1. c., 1882, p. 82 ; Berthold, Jahrb. f. wiss. Bot.,
1882, Bd. xin, p. 691 ; Klebs, Unters. a. d. bot. Inst. zu Tubingen, 1883, Bd. I, p. 268; 1886, Bd. n,
P- 557 J Schimper, 1. c., p. 240 ; Moore, 1. c., p. 643 ; Haberlandt, Flora, 1888, p. 296 ; de Vries,
Ber. d. bot. Ges., 1889, P- J95 Tswett, Bot. Centralbl., 1897, Bd. LXXII, p. 329; Kolkwitz, Fest-
schrift f. Schwendener, 1899, p. 271.
THE PHOTIC ORIENTATION OF CHLOROPLASTIDS 331
makes use of the locomotory energy of the surrounding cytoplasm or whether changes
of surface-tension of its own production are responsible for its movement. The
chloroplastids may indeed possess no power of photic reaction at all and may be
carried to or from the light by the cytoplasm, but this can hardly apply to the orien-
tation of the chlorophyll-bands of Mougeotia or of the chloroplastids of Vaucheria and
of Moss-leaves. The nucleus, for instance, does not accompany the movement of the
chloroplastids, and it is only in a few cases that a certain accumulation of protoplasm is
shown where they collect1. It remains, however, to be determined whether the phototac-
tic chloroplastids merely act as directive agencies or are directly responsible for their
own movement. The ' active ' view supported by Velten 2 and Stahl 3 is as devoid of
proof as is that according to which the movement is passive (Frank *, Moore 5, and
Oltmanns 6). In any case, isolated chloroplastids show no power of orientation how-
ever long they may remain living and capable of photosynthesis, and in whatever
media, apart from the cytoplasm, they may be placed. Nor do dead chloroplastids
show a'ny phototactic orientation within a living cell 7.
Although the orientation of the chlorophyll-band of Mougeotia and of the chloro-
plastids of Vaucheria and Mosses appears to be due to the direction of the light-rays,
it is possible that in other cases the movements may be produced in response to
changes in the general intensity of diffuse illumination. Haberlandt 8 considers the
latter to be the case in Moss-leaves and Fern-prothallia, but the evidence given above
points rather to the opposite conclusion. It must be remembered that rays falling
perpendicularly to the surface of a Moss-cell mostly penetrate, whereas those with an
oblique incidence are mostly reflected. Hence a directive action may be exercised
even in diffuse light or when the plant is rotated on a klinostat. The condition is the
same as when a cylinder containing zoospores is rotated in diffuse light, but has strips
of partially opaque paper pasted on its sides. A phototactic response will be ^shown
towards the better illuminated areas in feeble light, but away from them when the
light is intense.
Within the tissues where the light is dispersed in all directions, responses may be
more commonly produced by changes in the intensity of the illumination ; and in fact
when very intense illumination produces changes of position and a balling together of
the chloroplastids, this is not the result of any tropic stimulation. When moderately
strongly illuminated on one side the chloroplastids of Vaucheria, Moss-leaves, and
Fern-prothallia collect on the opposite sides of the cell. This cannot be due to their
possessing dissimilar powers of reaction, since they retain their original position when
the shaded under-surface of a prothallus is exposed to light 9. The chloroplastids
are, therefore, not dorsiventral, and in fact they appear to respond as a whole and
not individually to changes in the direction and intensity of the illumination. The
chlorophyll plate of Mougeotia is also isobilateral but diaphototropic.
Frank, Jahrb. f. wiss. Bot., 1872, Bd. VIII, p. 283.
Velten, Aktiv oder passiv? Oester. Bot. Zeitschr., 1876, No. 3.
Stahl, Bot. Ztg., 1880, p. 351. * Frank, 1. c., p. 282.
Moore, Journ. Linn. Soc., 1888, Vol. xxiv, pp. 203, 264. e Oltmanns, Flora, 1892, p, an.
Ewart, Protoplasmic Streaming in Plants, 1903, p. 108.
Haberlandt, Physiol. Pflanzenanat., 2. Ann1., 1896, p. 234. 9 Stahl, 1. c., p. 350.
332 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
The orientation cannot be determined by the assimilatory activity, since it is
produced almost solely by the more refrangible rays, and hardly at all, or not at
all, by the less refrangible ones which are most active in photosynthesis *. Accord-
ing to Frank, light which has passed through a solution of potassium bichromate
exercises a feeble action, but Borodin and Schmidt could detect no action at all,
possibly owing to the use of more concentrated solutions or thicker screens. Chromo-
plasts, leucoplasts 2, and etiolated chloroplastids * show no power of phototactic
orientation, but it must be remembered that we are dealing with a special irritability
which is not developed by all chloroplastids. Chromoplastids may, however, occur
in plants which are capable of phototactic responses, such as are shown by certain
non-chlorophyllous zoospores, and by the pigment cells of such animals as the
chameleon 4. According to Berthold 5, peculiar plasmatic plates occur in Chylocladia
which are capable of phototactic reaction like chloroplastids. No evidence as to the
mode of orientation is, however, afforded by the fact that it ceases in the absence of
oxygen 6, or as the result of etherization 7, and is also more or less influenced by the
external conditions in general.
An unequal distribution of the chloroplastids may either be produced purely
mechanically as the result of the shape of the cell, or by gravitational or surface-ten-
sion forces, or it may be due to the physiological polarity of the protoplasm, or to the
action of the external conditions. In darkness the chloroplastids in the leaf-cells of
a Moss collect upon the walls at right angles to the surface, while those in the interior
of tissues have a tendency to avoid the surfaces and to collect on the walls border-
ing the intercellular spaces where air is present 8. The utility of this arrangement
is obvious, but according to Haberlandt the chloroplastids have a general tendency
to avoid those walls through which continuous translocation occurs. Frequently
the chloroplastids leave the peripheral protoplasm and they often tend to collect around
the nucleus, so long as they contain no large starch-grains *.
Rapidity of the reaction. This is especially pronounced in the chlorophyll-bands
of Mesocarpus 10, but the chloroplastids of Funaria and other plants may pass in less
1 Borodin, Ueber die Wirkung d. Lichts auf die Vertheilung der Chlorophyllkorner, 1869, p. 58
(Melanges biologiques, Bd. vu) ; P. Schmidt, Ueber einige Wirkungen des Lichts auf Pflanzen, 1870,
p. 27; Frank, Bot. Ztg., 1871, p. 228.
3 Schimper, 1. c., p. 204.
3 According to observations by Senn.
4 O. Hertwig, Die Zelle u. d. Gewebe, 1893, p. 8r.
5 Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xin, p. 690.
6 Demoor, L'etude de la physiol. de la cellule, 1894, p. 54 (reprint from the Archives de
Biologic, Bd. XTII). Lewis (Annals of Botany, 1898, Vol. XII, p. 421) observed a phototactic reaction
of the chlorophyll plate of Mesocarpus in an atmosphere of hydrogen, but in this case oxygen is pro-
vided by the assimilation of the carbon dioxide produced within the cell.
7 Elfving, Ueber die Einwirkung von Aether u. Chloroform auf d. Pflanze, 1886, p. 16 (reprint
from the Ofversigt af Finska Vetensk. Soc. Forh., Bd. xxvin).
8 Stahl, Bot. Ztg., 1880, p. 332 ; Haberlandt, Ber. d. bot. Ges., 1886, p. 217.
9 Schimper, 1. c., p. 206; Berthold, Protoplasmamechanik, 1886, pp. 140, 169; Haberlandt,
Flora, 1888, p. 304.
10 See Stahl, 1. c., p. 301 ; Oltmanns, 1. c. ; Lewis, 1. c., p. 418. Lewis investigated the induction-
period and after-effect.
THE PHOTIC ORIENTATION OF CHLOROPLASTIDS 333
than an hour from the night to the day position, whereas in less readily responsive
plants a few hours may be required. As is generally the case, the new position
is assumed more rapidly than the original position is regained when the stimulus is
removed. In plants which react rapidly enough, the chloroplastids therefore undergo
daily changes of position. Such parts as the thallus of Marchantia, in which light
causes the chloroplastids to collect upon the outer walls, will assume a deeper green
when illuminated l. The continued paling shown by many plants in prolonged dark-
ness is, however, in. part the result of pathological changes in the chloroplastids,
coupled with a decomposition of the chlorophyll. The shadow figures produced by
Sachs 2 by partially covering leaves with tinfoil or black paper are not solely the result
of the primary photic reaction. Similarly, the paling of leaves in intense light*
though in the first instance partly due to movements of the chloroplastids, is mainly
the result of the partial decomposition of the chlorophyll4.
Historical. The changes of position of the chloroplastids in the leaves of
Crassulaceat, when exposed to sunlight, were discovered by Bohm B, and the details of
the process, as well as the change of the reaction with increasing intensity of illumina-
tion, were investigated by Famintzin, Borodin, and especially by Frank6. Frank
considered the changes of position to be due to diffuse phototonic stimulation, whereas
Stahl 7 considers them to be phototactic orienting responses to the direction of the
illumination. Many undoubted instances of phototactic or phototropic orientation are
given by Stahl, but diffuse actions may also be exercised, as was shown by Schimper
and Haberlandt 8. Frank 9 used the term * epistrophe ' to indicate the normal orienta-
tion of the chloroplastids in light, and * apostrophe ' for that assumed in darkness, or
owing to the action of other factors. Schimper 10, and also Moore ", used Frank's terms
in a slightly different sense, and distinguished the balling together of the chloro-
plastids as ' systrophe.' None of these terms are, however, really necessary, since
light-position, dark-position, superficial, lateral, and aggregated positions indicate all
the possible movements in the cell, and profile and face positions, flattened and con-
vex shapes, describe those of the chloroplastid.
1 See Stahl, Bot. Ztg., 1880, p. 329 ; Schimper, 1. c., p. 225 ; Moore, 1. c., p. 233. First
observed by Borodin and Frank.
Sachs, Sitzungsb. d. Sachs. Ges. d. Wiss., 1859, p. 226 ; Stahl, 1. c.
First observed by Marquart, Die Farben d. Bliithen, 1835, P* 47-
Pringsheim, Pringsh. Jahrb., 1879-81, Bd. xn, p. 374; Keeble, Annals of Botany, 1895,
Vol.
ix, p. 63; Ewart, Annals of Botany, 1898, Vol. xii, p. 384.
Bohm, Sitzungsb. d. Wiener Akad., 1856, Bd. xxn, p. 479 ; 1853, Bd. xxxvir, p. 453.
Famintzin, Jahrb. f. wiss. Bot., 1 867-8, Bd. VI, p. 45 ; Borodin, Bull, de 1'Acad. de St. Pe"ters-
bourg, 1867, T. iv, p. 482 ; Melanges biologiques de St. Petersbourg, 1869, Bd. vir, p. 50 ; Frank,
Jahrb. f. wiss. Bot, 1872, Bd. vm, p. 216.
7 Stahl, Bot. Ztg., 1880, p. 297.
8 Schimper, Jahrb. f. wiss. Bot., 1885, Bd. xvi, p. 203; Haberlandt, Ber. d. bot. Ges., 1886,
p. 209.
9 Frank, 1. c., p. 221. " Schimper, 1. c.
u Moore, Journ. Linn. Soc., 1888, Vol. XXIV, p. 200.
334 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
SECTION 71. The Action of Gravitational and Centrifugal Forces.
Mechanical Actions.
Small particles only respond slowly to the action of gravity in water,
and still less in more viscous liquids or ones of nearly the same density.
Hence very slight agitation suffices to keep particles suspended in a
liquid. Strong centrifugal forces may, however, effect rapid separation
such as gravity is unable to perform. Crystals and particles of precipitated
methyl blue1 sink rapidly in the cell-sap, and only slowly, or not at all,
in the protoplasm according to its viscosity. The nuclei and chloroplastids,
although denser than the cytoplasm, as well as the vacuoles and oil-drops
which are less dense, do not appear to have their position in the cell
influenced by gravity so long as they are embedded in cytoplasm. The
latter, even when thin, does not collect on the under side of the cell, although
denser than the cell-sap. In streaming endoplasm of low viscosity, however,
a feeble gravitational accelerating or retarding action upon floating particles
of less and greater density can frequently be observed2, and the denser
particles come perceptibly nearer to the periphery on the under side of
the cells of Chara and Nitella 3. When large crystals of calcium oxalate
or large starch-grains occur in the protoplasm they are commonly found
on the under side4, and the starch-grains carry with them the attached
chloroplastids or leucoplastids. The movements of the starch-grains are
shown very well in the starch-bearing endodermal cells, as well as in the cells
of the root-cap, and when inverted the change of position may begin at
favourable temperatures in a few minutes and be completed in from ten
to twenty minutes. At low temperatures the movement is slower 5 owing
to the higher viscosity of the protoplasm 6. The starch-grains in the cells
of such an organ when rotated once every half-hour or hour on a klinostat
in a warm room will, as Dehnecke found, be in continual movement, and
the changes of position of the starch-grains are now considered to aid
largely in the perception of geotropic stimuli7.
In cells subjected to strong centrifugal action starchless chloroplastids,
1 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1886, Bd. n, p. 189 ; Zimmermann, Beitrage z.
Morphol. u. Physiol. d. Pflanzenzelle, 1893, p. 68.
2 Ewart, Protoplasmic Streaming in Plants, 1903, p. 23.
3 Nageli, Beitr. z. wiss. Bot., 1860, Heft ii, pp. 67-74.
4 Dehnecke, Ueber nicht assimilirende Chlorophyllkorper, 1880, p. 10 ; Heine, Ber. d. bot. Ges.,
1885, p. 189, Landw. Versuchsst., 1888, Bd. XXXV, p. 170.
5 N£mec, 1. c., 1901, p. 129.
6 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 16-20 ; W. Ostwald, Zool. Jahrb., 1903,
Bd. xvm, p. 3.
Nemec, Jahrb. f. wiss. Bot, 1901, Bd. xxxvi, pp. 108, 127; Ber. d. bot. Ges., 1902, p. 342 ;
Haberlandt, Ber. d. bot. Ges., 1902, p. 190 ; Jahrb. f. wiss. Bot., 1903, Bd. xxxvin, p. 487.
ACTION OF GRAVITATIONAL AND CENTRIFUGAL FORCES 335
the nucleus and the greater part of the cytoplasm when a vacuole is
present, are driven to the outer end of the cell, only a very thin peripheral
film and fine threads remaining at the other end l (Fig. 56). The oil-
drops in chloroplastids of Vaucheria move with the chloroplastids, whereas
free drops or masses of oil move in the opposite direction. Displacements
of this kind were found by Mottier to be produced in most plant-cells by
centrifugal forces 1,900 times more powerful than that of gravity after expo-
sures of from half an hour to several hours. In a few thin filamentous Algae,
however, only slight changes of configuration were shown, probably owing
to the small diameter of the cell coupled with the properties and mode of
arrangement of the cell-contents 2. Similarly, owing to the packing of the
cell with grains of starch and aleurone, Andrews only observed slight dis-
placements in the cells of turgid cotyledons of Vicia
sativa and Pisum when exposed to a centrifugal force
of 4,400 g., whereas a pronounced displacement was
shown as soon as a portion of the reserve- materials had
been consumed. Usually, no apparent effect is pro-
duced by centrifugal forces of 100 g. strength, but in
a few cells relatively feeble gravitational forces may
produce pronounced displacement. The fact that a thin
film always remained adherent to the cell- wall is ex-
plained by the increased degree of cohesion as the
membrane becomes thinner, and by the fact that in
short cells the centrifugal action is unable to overcome c%y%/£a
the osmotic pressure which keeps the plasmatic mem- ^S?32io?S
brane pressed against the cell-wall. Similarly, in thin £jS[^>dKio?fiS
threads the surface-tension pressure becomes so great n^st^the'^opltm
as to render them relatively rigid. ftSS^S'SZZS.
The tearing away of the chloroplastids acts in- ofe)700' (After An-
juriously and even fatally upon Characeae, but Mottier
found that other plants remained living. This applies even to the cells
of Spirogyra in which the chlorophyll-band had been driven to one end
by centrifugal action. The original positions are restored in less than half
an hour in cells showing active streaming, whereas the original configuration
is not entirely resumed until after a few days in the case of Spirogyra^
and after one or more weeks in the case of Cladophora. The restoration
was found to be still slower by Andrews, in cells of the cotyledons of
Helianthus and Cucurbita, filled with reserve-materials, if growth was pre-
1 Mottier, Annals of Botany, 1899, v°l- XIII> P« 325J Andrews, Jahrb. f. wiss. Bot., 1902, Bd.
xxxvui, p. i ; Miehe, Flora, 1901, p. 109.
8 On the influence of these factors upon the resistance to movement within cells cf. Ewart,
Protoplasmic Streaming in Plants, 1903, pp. 16-33.
336 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
vented. When growth took place the removal of the reserve-materials
allowed more rapid readjustment to occur.
Dehnecke 1 found that chloroplastids containing starch when kept
in movement by long-continued rotation on a klinostat ultimately expelled
the contained starch-grains and became more or less deformed, but the
exact origin of this effect is uncertain. The displacements produced by
high centrifugal action cannot be supported indefinitely, and Andrews2
found that a disturbance of growth was produced even by short exposure.
It is evident from these experiments that the nucleus, and also the
cytoplasm, have a higher density than the cell-sap, while the nucleolus and
chromatin have a higher density than the rest of the nucleus. The latter
may, in some cases, be driven out of the nucleus by very high centrifugal
forces 3. According to Nemec4, the nuclei in the cells of the root-cap move
upwards when the root is inverted. Since this movement is in the opposite
direction to that expected from their density, we must at present ascribe
a geotactic irritability to the nucleus, although the phenomenon may result
merely from the downward movement of the starch-grains.
Starchless chloroplastids, and all chromatophores excepting those in
the petals of Caltha palustris 5, are denser than the cell-sap. The centri-
fugal movement of the oil-bodies of Hepaticae show that they do not
consist solely of oil, which, being less dense, moves centripetally 6. Latex
may be separated by centrifugal action in the same way as milk7. It
has not yet been determined whether a permanent displacement can be
produced without the plant being killed. Mottier was, however, able
to produce unequal cell-division as the result of the accumulation of the
protoplasm on one side8, while Miehe9 was successful in reversing the
polarity of the initial cell of a stoma by centrifugal action.
SECTION 72. Geotactic Reactions.
According to Schwarz 10, Euglena viridis and Chlamydomonas pulvis-
culus are negatively geotactic, and the same applies, according to Aderhold,
to Haematococcus lacustris, and in a less degree to the zoospores of
Ulothrix tennis. The experiments were performed in darkness, and partly
I Dehnecke, Ueber nicht assim. Chlorophyllkorper, 1880, p. n. 2 Andrews, 1. c., p. 21.
9 Andrews, 1. c., p. 36 ; Mottier, 1. c., p. 352.
4 Ngmec, Ber. d. bot. Ges., 1902, p. 344. Cf. Andrews, 1. c., p. 35.
5 Andrews, 1. c., p. 37. e Andrews, 1. c., p. 34.
7 Andrews, 1. c., p. 24. » Mottier, 1. c., pp. 331, 357.
9 Miehe, Flora, 1901, p. 109.
10 Fr. Schwarz, Ber. d. bot. Ges., 1884, p. 51.
II Aderhold, Jenaische Zeitschrift f. Naturwiss., 1888, N. F., Bd. xv, p. 321 ; Massart, Bull, de
1'Acad. royale de Belgique, 1891, 3' sen, T. xxn, p. 164; Jansen, Bot. Centralbl., 1893, Bd. LVI,
p. 20.
GEOTACTIC REACTIONS 337
in capillaries filled with water and open at both ends. Schwarz also
observed an ascent through wet sand. Massart1 found that Chromulina
Woroniniana behaves similarly at 15° to 20° C., but becomes positively
geotactic at 5° to 7° C. Changes of geotactic tone according to the
temperature may possibly be used by many motile organisms to enable
them to collect in zones at a suitable temperature, and changes of geotactic
tone do actually occur among Infusoria J. Aderhold found, however, that
Euglena viridis and Chlamydomonas pulvisculus remained negatively geo-
tactic at o° C.2, while Schwarz could detect no geotactic reaction below
5° or 6° C.3
No geotaxis appears to be shown by Oscillariae and Diatomaceae,
although certain Desmids may possess this irritability to a feeble degree 4.
Various Infusoria, including Polytoma uvella, afford instances of geotactic
non-chlorophyllous objects 5. Massart found one species of Spirillum to be
negatively, and another species under the same conditions was found to be
positively geotactic 6. Stahl7 considers that the plasmodia of Myxomycetes
have no geotactic irritability, the creeping up to the surface of the sub-
stratum before fruiting being due to the change of the previous positive
into negative hydrotropism. Rosanoff and Baranetzsky 8 had previously
assumed the existence of negative geotaxis in plasmodia, but Strasburger 9
threw doubt upon this view.
We can hardly speak of geotactic irritability when an organism rises
or sinks owing to autogenic changes of its specific gravity, or when the
position of the centre of gravity causes the axis of the organism and
the direction of movement to be parallel to the perpendicular 10. That the
geotactic responses are not produced in this way is shown by the fact that
many zoospores with excentric centres of gravity show no geotaxis, while
Chromulina^ according to its tone, is negatively or positively geotactic.
According to Jensen n, Euglena viridis, in virtue of the position of
its centre of gravity, would react positively, instead of being negatively
1 Massart, Bull, de 1'Acad. royale de Belgique, 1891/3° se"r., T. xxii, p. 164; Sosnowsky, Bot,
Centralbl., 1901, Bd. LXXXVIII, p. 199.
2 Aderhold, 1. c., p. 320.
3 [In all cases the possibility of the existence of passive movements due to convection or
thermo-diffusion currents needs to be considered.]
* Aderhold, 1. c., pp. 322, 359 ; Klebs, Biol. Centralbl., 1885, Bd. V, p. 360.
5 Massart, 1. c,, pp. 162, 166 ; Jensen, 1. c. ; Mendelsohn, Centralbl. f. Physiol., 1895, Bd. ix,
p. 374. In regard to other animals see Loeb, Centralbl. f. Physiol., 1891, p. 429 ; 1893, Bd. vn,
p. 304.
6 Zikes, Centralbl. f. Bact., Abth. ii, 1903, Bd. xi, p. 59.
7 Stahl, Bot. Ztg., 1884, p. 1 68.
8 Rosanoff, Me"m. de la Soc. de sci. nat. de Cherbourg, 1869, T- XIV> P- J49 J Baranetzsky, ibid.,
1876, Bd. xix, p. 322.
9 Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 71.
10 Cf. Verworn, Psycho-physiol. Protistenstudien, 1889, p. 122.
11 Jensen, Bot. Centralbl., 1893, Bd. LVI, p. 21.
PFEFFER. Ill 2,
338 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
geotactic as it actually is. Such forms as Euglena viridis and Chlamydo-
monas pulvisculus only have their negative geotaxis mechanically overcome
when they are subjected to centrifugal forces eight times greater than that
of gravity, while the limit of geotactic perception seems to lie between
02 to 0-3 g., according to Schwarz. It is, however, uncertain whether
the perception arises from the movements of the denser particles in the
organism or is aroused by differences of pressure in the medium.
SECTION 73. Diffuse Chemical Actions.
Locomotion and intercellular movement, like all forms of vital activity,
are dependent upon metabolism, and hence cease sooner or later when the
latter is partially or completely suppressed. The same is the case when
oxygen is removed from an aerobic organism, although growth and move-
ment may be maintained under relatively low partial pressures of oxygen.
On the other hand, the access of air to obligate anaerobes, as well as
a sufficient increase of the partial pressure of oxygen upon aerobes, retards
and ultimately inhibits their
powers of growth and move-
ment. Among certain facul-
tatively anaerobic bacteria,
FIG. 57. Median section of gas-chamber (reduced). HOWCVCr, the absence of OXygen
causes movement to cease
although growth continues *. The power of movement is, however, retained,
and is at once shown when oxygen is admitted. Possibly in other cases
the removal of oxygen may produce a stoppage of growth before move-
ment, and more especially protoplasmic streaming, have ceased.
The withdrawal of nutriment or even of a single essential constituent
must sooner or later retard or stop movement, although streaming may
continue in starving plants almost until death ensues2. This is the case
in cells of Chara and Nitella, whereas in those of Elodea and Vallisneria
a long period may elapse between the cessation of streaming and the
permanent loss of vitality. On the other hand, cells packed with food-
materials show no streaming, and the latter is not shown until the cells are
partially emptied 3. This in part arises from the decreased resistance
coupled with the great activity of the cell during translocation, and
naturally also the addition of food- materials accelerates streaming in
starved cells.
1 Ritter, Flora, 1899, p. 329.
3 Kiihne, Zeitschr. f. Biologic, 1898, N. F., Bd. xvm, p. 85 ; Ritter, 1. c., p. 355 ; Ewart.
Protoplasmic Streaming in Plants, 1903, p. 76; Wallengren, Zeitsch. f. allgem. Physiol., 1902, Bd. I,
p. 67.
3 Ewart, 1. c.
DIFFUSE CHEMICAL ACTIONS
339
Oxygen. Various forms of gas-chamber may be used to follow the
influence of the withdrawal of oxygen upon streaming, and the partial
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FlG. 58. Apparatus for obtaining pure hydrogen and pure carbon dioxide free from all oxygen,
connexions are submerged in water, and all the liquids are covered with a layer of liquid paraffin. (The g
one-tenth and the remaining apparatus one-sixth natural size. After Ewart.)
CAll the
generators
pressure of oxygen may either be reduced by the aid of an air-pump
attached to (c) Fig. 57, or by displacement with a neutral gas entering
at the other tube. In the former case, the cover-slip (#), to which the
hanging drop with the object is attached, must either be small or thick,
and must be sealed on by vaseline stiffened with colophonium or wax.
A still better attachment is given by melted shellac. To produce a complete
absence of oxygen evacuation and the passage of hydrogen must be
frequently repeated. A steady supply of pure oxygenless hydrogen can
be obtained by the apparatus shown in Fig. 58, in which all the connexions
are under water covered with liquid paraffin, and the gas, after passing
through purifying tubes, is deoxygenated by pyrogallol and caustic potash 1.
1 A better form of gas-chamber is that figured on p. 315 (Fig. 52), by which the influence of
Z 2
340 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
By adding a few motile aerobic bacteria to the hanging drop an
additional test of the absence of oxygen is afforded. Under certain
circumstances Recklinghausen's gas-chambers may be used, and the exit-
and entry-tubes sealed after evacuation.
Facultatively anaerobic bacteria move for a longer time in the absence
of oxygen when supplied with certain food -materials than with others.
Thus Ritter1 found that Spirillum Finkler-Prior continued to move for
ten minutes in a solution of peptone, but for thirty-five to forty minutes
in one to which sugar had been added. This may possibly be due to the
sugar being a highly oxidized compound readily capable of yielding
energy by anaerobic decomposition. On the other hand, obligate anaerobes
cease to move within thirty minutes to an hour after the entry of oxygen 2.
In various chlorophyllous and non-chlorophyllous objects locomotion
ceases rapidly in some cases, but in others not for a long time after all
free oxygen has been removed 3. Celakovsky found, for instance, that in
darkness and in the absence of oxygen Pandorina morum ceased to move
in eleven hours, Euglena viridis in forty-four, and Pelomyxa palustris in
seventy-two hours.
The necessity of free oxygen for streaming was shown first by Corti 4,
although streaming does not always cease when the cell is placed in oil 5,
as in the experiments performed by Corti. Kuhne and Hofmeister 6
showed the necessity of oxygen in all the cases examined by them, and
Clark found that streaming usually ceased in plasmodia and in ordinary
cells a few minutes after the oxygen had been removed, but in a few cases
not until after four hours 7. Ewart 8 found that preparations of Chara
ringed with vaseline continued to show streaming for five weeks in darkness,
but that when the preparations were submerged in deoxygenated water and
oxygen and temperature, ether or electricity can be simultaneously investigated. See also Zimmer-
mann, Das Mikroskop, 1895, pp. 220, 223 ; Bot. Ztg., 1887, p. 31 ; Clark, Ber. d. bot. Ges., 1888,
p. 274.
1 Ritter, Flora, 1899, P- 329-
2 Beyerinck, Centralbl. f. Bact., 1893, Bd. xiv, p. 841 ; Ritter, 1. c., p. 345.
3 Clark, Ber. d. bot. Ges., 1888, p. 278; Celakovsky, Ueber d. Einfluss d. Sauerstoffmangels
auf d. Bewegung einiger aeroben Organismen, 1898 (reprint from the Bull, internal, de 1'Acad. de
Boheme).
4 Corti, 1772 (Meyen, Pflanzenphysiol., Bd. n, p. 224).
5 Goebel, Ueber die Durchlassigkeit d. Cuticula, 1903, p. 14.
6 Kuhne, Unters. ii. d. Protoplasma, 1864, pp. 88, 105 ; Hofmeister, Pflanzenzelle, 1867, p. 49.
7 Clark, 1. c.; Kuhne, Zeitschr. f. Biol., 1898, N. F., Bd. xvm, p. i ; Lopriore, Jahrb. f. wiss.
Bot., 1895, Bd. xxvm, p. 571 ; Bot. Centralbl., 1902, Bd. LXXXIX, p. 118 ; Demoor, Contribut. a
l'e"tude de la physiol. de la cellule, 1894 (reprint from the Arch, de Biologic, T. 13); Samassa,
Ueber d. Einwirkung von Gasen auf Pflanzen, 1898 (reprint from the Verh. d. naturhist. Vereins zu
Heidelberg, N. F., Bd. vi) ; Ritter, 1. c., p. 347 ; Josing, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi,
p. 221.
8 Ewart, Linnean Society, 1897, Vol. xxxm, p. 146. See also Farmer, Annals of Botany, 1896,
Vol. x, p. 288.
DIFFUSE CHEMICAL ACTIONS
kept in darkness, the streaming ceased in two or three days. This is,
however, simply because of the accumulation of the injurious products
of respiration, for Kuhne and Ritter1 found that streaming might continue
for as long as nineteen days in the absence of oxygen, and by the aid of
the apparatus shown in Fig. 59, which was sealed and kept under water,
Ewart 2 was able to demonstrate the continuance of streaming in darkness,
and in the absence of oxygen, for from six to eight weeks. This applies to
Char a foetida, Nitella translucens^ and N. flexilis, other species being less
pronounced facultative anaerobes. In general, Clark found that streaming
recommenced in aerobic plants under a pressure of from i to 7 mm. of
oxygen, which is below the pressure required for normal aerobic respiration.
The contradictory results of certain observers are partly due to the
presence of oxygen or of poisonous impurities in the hydrogen employed, and
partly to individual and cultural differ-
ences in the material used. Lopriore 3
stated that streaming never ceased in
hairs of Tradescantia and of Cucurbita
either in hydrogen or carbon dioxide,
but this was undoubtedly due to the
presence of oxygen in the gases used 4.
Lopriore 5 obtained different results in
the morning to those yielded in the
evening, possibly owing to changes of
tone, and Josing 6 found that etheriza-
tion causes streaming to cease sooner
in the absence of oxygen. Carbon
dioxide in all cases ultimately acts
injuriously7, but cuticularized hairs Aik'Ryroaaiioi
Which Seem tO be naturally aCCOmmO- FlG. 5Q. Apparatus for testing the anaerobism of
dated to high internal percentages of chara and Nitella (tw°-lhirds natttral size>-
carbon dioxide may continue to show streaming in a mixture of 80 per cent,
carbon dioxide and 20 per cent, oxygen. As in other cases, a temporary
shock-stoppage may be produced by the sudden change from air to carbon
dioxide 8. In no case, except in the very doubtful one of Pelomyxa, does
pure hydrogen exercise any direct injurious action, apart from that due to
its displacing the oxygen required for respiration. The same applies to
nitrogen and carbon monoxide in the case of plants, and as regards
Mercury
Valve
Kuhne, Zeitschr. f. Biol., 1898, N. F., Bd. xvm, p. 30; Ritter, 1. c., p. 351.
Protoplasmic Streaming in Plants, 1903, p. 42.
Lopriore, 1. c., 1895, p. 28. * Samassa, 1. c., p. 2 ; Ewart, 1. c., p. 38.
Lopriore, 1. c., 1902, p. 118. 6 Josing, 1. c., p. 221.
Cf. Samassa, 1. c., p. 2 ; Klemm, 1. c., p. 36 ; Kuhne, 1. c., 1864, p. 106; Ewart, 1. c., p. 78.
Cf. Ewart, 1. c., p. 79.
342 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
nitrous oxide, Demoor's l statement that aerobic plants continued to show
streaming in this gas can be regarded as a proof that the gas used con-
tained free oxygen2.
Kiihne 3 considered the long continuance of anaerobic streaming in
Characeae to be due to the presence of a store of occluded oxygen, but
Ewart 4 has shown that this is not the case.
Poisons. Every substance which influences metabolism may also
influence streaming5, either directly or as an after-effect, and apart from
any retardation or stoppage produced as the effect of the shock of a sudden
change. Many substances when dilute, such as alcohol, glycerine, and
various poisons, may distinctly accelerate streaming, for a time at least,
even when all shock-effect is avoided by gradual change. The effect of
shock is commonly to produce a retardation or stoppage followed by a
subsequent acceleration, but all these responses are manifestations of
irritability and bear no relation to the changes of viscosity directly due
to the presence of the exciting substance. Strong solutions of neutral
substances, however, retard streaming largely owing to the rise of viscosity
consequent on the withdrawal of water.
Transitory chloroforming6, or treatment with solutions of poisonous
or nutrient substances, may awaken streaming in quiescent cells, such as
those of Vallisneria, and similarly all substances which act as chemical
stimuli to the tentacles of Drosera also act as excitants to streaming in the
responsive cells of this plant. Anaesthetics, such as ether, chloroform, and
chloral hydrate, as well as such alkaloids as caffein, antipyrin, muscarin,
atropin, eserin, veratrin, and curare readily retard and ultimately stop
streaming and locomotory movement even when dilute. Muscarin, atropin,
and veratrin, however, which are deadly poisons to higher animals, exercise
relatively little effect upon plants7, and may indeed be used as a food-material
by such Fungi as Penicillium*. Anaesthetics appear commonly to stop
1 Demoor, Contrib. a 1'etude de la Physiol. de la cellule, 1894, p. 35.
2 Samassa, 1. c., p. 2 ; Kauffman, Einwirkung der Anaesthetica auf Pflanzen, 1899, p. 16.
8 Kiihne, 1. c., p. 92 : cf. Ritter, 1. c., p. 358.
* Ewart, 1. c., p. 350. Ritter (1. c., p. 350) found that after the prolonged absence of oxygen
this gas was immediately evolved on exposure to weak light, whereas the presence of absorbent
substances should retard its appearance for some time. Pringsheim (Sitzungsb. d, Berl. Akad., 1887,
p. 769) did actually find that Chara became incapable of evolving oxygen after remaining for a long
time in darkness, but this was due to the induction of a condition of assimilatory inhibition in the
chloroplastids. Cf. Ewart, Journ. Linn. Soc., 1896, p. 418.
5 Demoor, 1. c., p. 72 ; Lopriore, 1. c., 1895, pp. 573, 621 ; Klemm, Jahrb. f. wiss. Bot., 1895,
Bd. xxvill, p. 680; Samassa, 1. c., p. 2; Kiihne, 1. c., 1898, p. 36; Farmer u. Waller, Bot.
Centralbl., 1898, Bd. LXXIV, p. 377; Kauffmann, 1. c., p. 10; Josing, 1. c., p. 223. For the detailed
action of various chemicals see Ewart, Protoplasmic Streaming in Plants, 1903, p. 76.
6 Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. XXIV, p. 220.
7 Ewart, 1. c., p. 86.
8 Ewart, 1. c., p. 81.
DIFFUSE CHEMICAL ACTIONS
343
streaming in concentrations too dilute to cause a cessation of growth,
whereas in many cases the chemotactic or phototactic irritability is sup-
pressed before the power of movement l.
Acids, even when in considerable dilution, such as from o-oi to 0-05
per cent, in the case of most
mineral acids, cause a rapid
cessation of streaming 2. The
feebler organic acids are
naturally less effective, but
i-per-cent. solutions of tar-
taric acid produce a stoppage
of streaming within one or
more hours in all the plants
hitherto examined. Dutro-
chet 3 not only observed a
similar retarding action in the
case of alkalies, but also found
that the repeated change from
acid to alkali was more in-
jurious than remaining1 for the
FlG. 60. Young root-hair of Trianea bogotensis. A before,
Same length Of time in One and B one hour after, treatment with very dilute ammonia. (Magn.
' 1000.)
medium. Both acids and
alkalies induce protoplasmic deformation, which is evidenced by the pro-
nounced vacuolation assumed as the result of treatment with alkali 4
(Fig. 60). Methyl violet, Bismarck brown 5, as well as caffein and other
alkaloids 6, may produce pronounced deformation without causing streaming
to cease. In all cases the timely removal of the reagent is followed by the
recovery of the protoplasm, which reassumes its normal configuration, but
poisonous reagents which combine with the protoplasm usually act fatally
before they can be removed.
SECTION 74. Chemotaxis and Osmotaxis.
The usual method of showing chemotaxis is to place a capillary tube
open at one end and containing a solution of the exciting substance in a drop
of liquid containing the motile organisms. If the latter are positively
1 Rothert, Jahrb. f. wiss. Bot., 1903, Bd. xxxix, p. i, gives full details of actions of this character.
2 Dutrochet, Ann. sci. nat., 1838, 2e se"r., T. ix, p. 67 ; Klemm, Jahrb. f. wiss. Bot., 1895, Bd.
xxvin, p. 685.
3 Dutrochet, 1. c., p. 66: cf. also Ewart, 1. c., p. 80; Jurgensen, Studien d. physiol. Inst in
Breslau, 1861, Bd. I, p. 107.
4 Klemm, 1. c., p. 658.
5 Pfeffer, Unters. a. d. bot. Inst. zu Tiibingen, 1886, Bd. II, pp. 250, 262, 264.
6 Klemm, 1. c., p. 665.
344
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
chemotactic they collect at the mouth of the tube, and in covered and
ringed preparations aerobic organisms will ascend the tube in search of
oxygen as the latter is exhausted outside (Fig. 61 C). If the organisms
are negatively chemotactic or are osmotactic they will collect at some
distance from the mouth of the capillary, and if they are positively chemo-
tactic to dilute solutions they will gradually approach the tube as the
substance diffuses from it. Convection currents due to differences of specific
gravity may be avoided by placing the tube open end upwards on the tilted
stage of the microscope l.
Similar actions are exercised by the substances diffusing from a frag-
ment of solid, or from a piece of meat or the leg of a fly. In addition,
a drop of liquid may be tested in the manner shown in Fig. 60 2. Plas-
modia may be grown on wet filter-paper having one end in water and the
•'•'.:; •;'•.'•. '-'•' ' '-'V. :.;••";•'-••'' FiG. 62. Drops of distilled water («)
joined to drops of sea-water (o) containing
FlG. 61. Capillary tubes containing meat-extract and surrounded Spirillum, forms. The latter collect where
by bacteria. A. showing attraction ; B, showing attraction exer- the water is richest in salts, and hence ap-
cised by the air-bubble in the tube ; C, showing repulsion produced pear to be repelled by the distilled water,
by acidified meat-extract. (Magn.)
other in the substance to be tested 3. Currents of water must, however, be
avoided, since these may excite a rheotropic response.
Antherozoids. The strongly chemotactic antherozoids of Ferns4
escape in enormous numbers when small dry ripe prothallia are rapidly
washed and placed in a drop of water. They are strongly attracted to
a capillary containing o-oi per cent, of sodium malate, and a feeble
attraction is even exercised by solutions of oooi per cent, strength. Maleic
acid is much less attractive 5, although more active and varied in its combin-
1 Pfeffer, Ber. d. hot. Ges., 1883, p. 524; Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. i,
PP. 367, 451 5 I888» Bd- ". PP- 585> 627.
3 Jennings, Journal of Physiology, 1897, Bd. xxr, p. 264; Massart, Bull, de 1'Acad. royale
de Belgique, 1891, 3" ser., T. xxn, p. 152; Carrey, American Journal of Physiology, 1900, Vol. ill,
p. 295.
3 Stahl, Bot. Ztg., 1884, p. 156.
4 Pfeffer, Ber. d. hot. Ges., 1883, p. 524; Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I,
p. 367 ; Voegler, Bot. Ztg., 1891, p. 641 ; Buller, Annals of Botany, 1900, Vol. XIV, p. 543.
6 Pfeffer, 1. c., 1884.
CHEMOTAXIS AND OSMOTAXIS 345
ing powers than malic acid. Potassium nitrate and ammonium phosphate
exert a still feebler attractive action, which is easily overlooked, but Buller
was unable to detect any chemotactic response to sodium chloride, ammonium
nitrate, calcium chloride, sugars, asparagus, and glycerine l.
Inactive malic acid acts similarly to the active form, while the free
acid and its neutral salts seem to have the same excitatory value 2. On
the other hand, the diethylester of malic acid, in which the acid is not
present as an ion, exercises no chemotactic action 3. Malic acid exerts a
repelling action when concentrated, but not its salts, and Buller4 (I.e., p. 560)
has shown that the chief attractive substance in the archegonium may be
a salt of malic acid, possibly potassium malate, but cannot be free malic
acid. Since the repulsion may be produced by citric and other acids,
we have probably before us a reaction dependent upon the mere increase of
acidity, i. e. upon the relative number of hydrogen ions 5. Strong alkaline
solutions, and sufficiently concentrated solutions in general, exercise a certain
repulsion, which is often only shown at first, and which does not prevent the
gradual entry of large numbers of the antherozoids into the capillary
tubes, in which they soon become motipnless and die6. They have, there-
fore, not the power of avoiding all injurious liquids, and are readily attracted
to their death by introducing a tube containing malic acid mixed with
a little mercuric chloride 7. The chemotaxis of these antherozoids is the
result of a typical tactic reaction 8, and the same appears to apply to the
negative chemotaxis produced by free acids. It is, however, not yet certain
whether the osmotactic repulsion produced by concentrated solutions is
a tactic or a phobic reaction.
SPECIAL CASES. A salt of malic acid is probably also the chief
attractive stimulus for the sperms of Selaginella, and possibly cane-sugar
for those of Mosses, since the latter suffices to produce a perceptible
attraction when diluted down to o-ooi per cent, strength 9. The sperms of
Hepaticae, of Sphagnum, and of Marsilia seem to be attracted into the
archegonium in the same way, but the attractive substances have yet to be
found 10.
Cf. Buller, Annals of Botany, 1900, Vol. xiv, pp. 548, 571.
Pfeffer, 1. c., Vol. II, pp. 381, 654; Voegler, 1. c., p. 659.
Pfeffer, 1. c., p. 371. * Buller, 1. c., p. 560.
Pfeffer, 1. c., p. 387; Buller, 1. c., p. 567.
Pfeffer, 1. c., p. 385. Buller (1. c., p. 555) observed only weak repulsion or none at all, and
was unable to detect the transitory repulsion. Much depends upon the manner in which the experi-
ment is performed.
7 Pfeffer, 1. c., p, 388. 8 Cf, Rothert, Flora, 1901, p. 388.
9 Pfeffer, 1. c. 1884, pp. 422, 430. Other substances may also exert a slight action.
10 Pfeffer, 1. c., 1884, Bd. I, pp. 434, 435; 1888, Bd. II, p. 655. On the process of fertilization
in Hepaticae cf. Strasburger, Jahrb. f. wiss. Bot., 1869-70, Bd. vn, p. 402; Leitgeb, Flora, 1885,
P- 330.
346 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
It is not yet certain whether the antherozoids of Fucaceae adhere to
the ovum simply as the result of a contact-stimulus, as Bordet supposes, or
whether chemotactic influences come into play 1. Further, it is questionable
whether the sperms of Chara experience any chemotactic attraction, and
the latter does not seem to be responsible for the conjugation of the zoospores
of Chlamydomonas or of Ulothrix sonata 2, which seems to be left to chance.
It is, however, hardly surprising that, as in the case of pollination, various
modes should be used to bring motile sexual cells together 3. In all cases,
however, the ultimate fusion is determined by the properties of the proto-
plasts ; so that, although the sperms of other species may be attracted into
the archegonium and come into close contact with the ovum, no fusion
occurs. The special attraction exerted by malic acid and its salts as well
as their actual presence in the prothallus indicate their importance as agents
for inducing fertilization in Ferns 4.
Bacteria. Motile* Bacteria show all grades of sensibility, and both the
chemotactic and osmotactic reactions are carried out in a phobotactic
manner. Very sensitive forms react positively to most substances, but the
less sensitive forms give little or np reaction when feebly stimulating sub-
stances are used 5. Peptone and potassium salts are especially active, and
are responsible for the high attractive power of meat extract. Sodium
and calcium salts, asparagin and urea, are less active as stimuli, and while
glycerine appears to produce no attraction at all, oxygen appears to influence
all bacteria strongly.
Bacterium termo* and Spirillum undula appear to be especially sensitive,
for a response is produced when the liquid in the capillary contains o-coi per
cent, of peptone, potassium chloride, or of meat-extract. Spirillum serpens,
S. volutans. Bacillus subtilis, and especially Spirillum Finkler-Prior, are
much less sensitive. Dextrin attracts Bacterium termo strongly, but
Spirillum undula very feebly 7, while only a few bacteria are chemotactically
affected by ether8. Sulphuretted hydrogen attracts Chromatium Weissii
1 Thuret, Ann. d. sci. nat., 1854, 4° s^r-> T. II, p. 17 ; Bordet, Bull, de TAcad. royale de
Belgique, 1894, 3* ser., T. xxvn, p. 889; Farmer and Williams, Phil. Trans., 1898, Vol. cxc,
PP- 633, 643; Buller, Quarterly Journal of Microscopical Science, 1902, Vol. XLVI, p. 148.
3 Pfeffer, 1. c., 1884, pp. 438, 441.
3 Cf. Pfeffer, 1. c. 1884, p. 447. Chemotactic sensibility appears to be absent from the sperms
of Rana (Massart, Bull, de 1'Acad. royale de Belgique, 1888, 3e ser., T. XV, Nr. 5, und 1889, Nr. 8)
and of Echinodtrmata (Buller, Quarterly Journal of Microscopical Science, 1902, Vol. XLVI, p. 151),
but is shown by those of the rat (Otto Low, Sitzungsb. d. Wien. Akad., 1902, Bd. cxi, Abth. iii,
p. 118).
Pfeffer, 1. c., p. 884 ; Buller, 1. c., 1900, p. 570.
For details see Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. II, p. 582.
Pfeffer (1. c., p. 590) mentions what is included under the term 'Bacterium termo?
Pfeffer, 1. c., p. 606.
Rothert, Flora, 1901, p. 380.
CHEMOTAXIS AND OSMOTAXIS 347
and probably other sulphur-bacteria as well 1, while the comparatively insen-
sitive typhus- and cholera-bacilli 2 are strongly attracted by potato-sap.
Increasing concentrations of neutral salts exert hardly any perceptible
chemotactic or osmotactic repulsion 3 upon Bacterium termo, but a strong
one upon Spirilhim undula, while the action upon other forms lies between
these extremes. Free acids produce repulsion in the case of Spirillum
undula even when very dilute, but alkalies only when somewhat more con-
centrated. Ether and alcohol4 may in certain cases produce repulsion,
while the presence of oxygen dissolved at ordinary pressure from air is
sufficient to repel Spirillum undula and 6\ serpens. Sulphur bacteria and
other anaerobic bacteria react still more readily to oxygen, and in some
cases are so sensitive that the merest trace of oxygen produces repulsion,
although most bacteria are capable of positive chemotaxis in regard to
oxygen5. Since Bacterium termo has hardly any negative osmotaxis or
chemotaxis, motile forms penetrate concentrated solutions of sugar or
potassium chloride in abundance, whereas a slight concentration exercises
an osmotactic repulsion upon Spirillum undula.
Flagellatae and Volvocineae. Many colourless Flagellatae react chemo-
tactically and osmotactically to various substances, and in general the
reactions resemble those of Bacteria 6. Thus Bodo saltans, Trepomonas
agilis, and Hexamitus rostratus have about the same positively chemotactic
sensitivity as the most sensitive bacteria, whereas Hexamitus intestinalis
only reacts weakly, and Astasia proteus and Tetramitus restrains not at
all. So far as is known, the green Flagellatae show no positive chemotaxis,
apart from their aerotaxis 7, whereas some of the Volvocineae are able to
respond with moderate activity to the chemical substances already
mentioned.
Many substances which produce a phobotactic action upon Bacteria
induce a typical chemotactic reaction when presented to the above-
named Flagellatae8. The zoospores of Saprolegnia* behave similarly
1 Miyoshi, Journal of the College of Science, University of Tokyo, 1897, Vol. x, p. 169.
3 Pfeffer, 1. c., p. 615 ; A. Cohen, Centralbl. f. Bact., 1890, Bd. vin, p. 164.
3 Pfeffer, 1. c., p. 621.
* Rothert, 1. c., p. 380.
5 On ' Aerotaxis' or ' Oxygenotaxis,' cf. Bd. II, p. 582 footnote, and also Engelmann, Pfliiger's
Archiv, 1881, Bd. xxvi, p. 541 ; Beyerinck, Centralbl. f. Bact., 1893, Bd. XIV, p. 835; 1895,
Abth. ii, Bd. I, p. in ; Rothert, 1. c., p. 377. Oxygen exerts no chemotactic action on the sperms
of Ferns (Pfeffer, 1. c., 1884, p. 372) or upon the zoospores of Saprokgnia (Rothert, Cohn's Beitrage
z. Biol., 1892, Bd. v, p. 341 ; Stange, Bot. Ztg., 1893, p. 139).
6 Pfeffer, 1. c., 1888, pp. 595, 615, 625.
7 On the stimulating action of oxygen upon Euglena viridis see Aderhold, Jenaische
Zeitschr. f. Naturwiss., 1888, Bd. XXII, p. 314.
8 Cf. Rothert, Flora, 1901, p. 388.
9 Rothert, 1. c., p. 388. For substances acting as stimuli see Stange, Bot. Ztg., 1890, p. 124.
348 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
and for them phosphates are especially attractive. The hyphae of Fungi
respond in general to the same substances as Bacteria and Flagellatae, and
their positive and negative chemotropism has the character of a tropic
movement. The different grades of repulsion exerted by weak acids on
various Flagellatae as well as on fungal hyphae and on Bacteria are of
similar character 1. It remains, however, to be seen whether all Flagellatae
are capable of a tropic reaction, and to what extent the osmotropic reactions2,
which are as well developed in these organisms as in Bacteria, are carried
out in a tropic or a phobic manner. It may be mentioned that the Infusoria3
are usually not chemotactically stimulated by the substances mentioned,
but that certain species at least are capable of a positive phobotactic
response towards dilute acids including carbonicacid, and of a negative
one away from more concentrated solutions.
Myxomycetes. According to Stahl 4 the plasmodia show a positively
chemotactic amoeboid movement towards an extract of tan, and Stange 5
has found that various substances act as stimuli. Stange has also shown
that the zoospores of Aethalium and Chondrioderma are attracted by
various substances, more especially by lactic, butyric, and malic acids.
Concentrated solutions, or ones with a strong acid reaction, exert a repulsive
action upon the zoospores as well as upon the plasmodia. It may inciden-
tally be mentioned that the amoeboid leucocytes of animals are chemo-
tactically stimulated by a variety of substances, and that by reactions of
this kind various definite and physiologically important movements may
be produced within the body6.
THE USES OF CHEMOTAXIS AND ITS EXCITANTS.
By means of their chemotactic irritability organisms may be attracted
to regions where food-material is abundant, or where their function is
According to Rothert (1. c., p. 375) only the second zoospore stage responds chemotactically. The
substances which attract the zoospores of Chytridiaceae are not known. Cf. Pfeffer, 1. c., 1888, p. 643.
According to W. Benecke (Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 554), Diatomaceae are capable of
both chemotactic and aerotactic responses.
1 Pfeffer, 1. c., 1888, p. 625. According to Carrey (American Journal of Physiology, 1900,
Vol. Ill, p. 291), Chilomonas exhibits a normal tactic reaction to dilute acids, and a phobic one to
concentrated acids and other substances.
3 Cf. Massart, Archives de Biologic, 1889, T. IX, p. 531 ; Bull, de 1'Acad. royale de Belgique,
1891, 3e ser., T, xxn, p. 148.
3 Jennings, Journal of Physiology, 1897, Vol. XXI, p. 320; American Journal of Physiology,
1900, Vol. in.
4 Stahl, Bot. Ztg., 1884, p. 155. Olive (Proc. of the Boston Soc. of Natural History, 1902,
Vol. XXX, p. 463) could detect no chemotaxis in Acraseae.
5 Stange, Bot. Ztg., 1890, p. 155.
8 Cf. Verworn, Allgem. Physiologic, 3. Aufl., 1901, p 451.
THE USES OF CHEMOTAXIS AND ITS EXCITANTS 349
fulfilled, as when the sperm is attracted to the ovum 1. This latter is better
fulfilled by normal chemotaxis, although phobic reactions may also produce
movement to particular points. It is not always readily possible to dis-
tinguish between osmotactic and chemotactic irritability, especially when
repulsion only occurs with high concentration. If, however, an organism
responds only to very few substances, or to those in very great dilution, the
response is clearly due to the chemical properties of the exciting substance,
and not to any osmotactic action. The attraction of the antherozoids of
Ferns by malic acid, of Bacteria and flagellate Infusoria by peptone and
potassium salts, is undoubtedly a positively chemotactic response.
Although many substances may stimulate a particular organism, a
special substance may exert a preponderating action, and may overpower
all others. In this sense malic acid or its salts may be regarded as the
special stimulating substance for the antherozoids of Ferns, and probably
cane-sugar for those of Mosses. The antherozoids of Hepaticae, Sphagna-
ceae and of Marsilia are either devoid of any chemotactic irritability or are
only very feebly sensitive, since no certain attraction has as yet been
observed with any substance or mixture of substances. It is worthy of note
that if such reacting organisms as Bacteria, Flagellatae, Volvocineae, the
zoospores of Saprolegnia and fungal hyphae are tabulated in descending
order according to the stimulating action of a substance upon them,
the order will on the whole follow approximately the same course when
another substance of similar constitution is used .
The high sensitiveness to malic acid or to cane-sugar shown by the
antherozoids of Ferns and Mosses respectively does not involve any special
sensitiveness to peptone or potassium salts. These are in general the
strongest stimulatory substances for Bacteria, which respond but feebly to
malic acid and cane-sugar. Even in the case of Bacteria great differences
are shown, for certain forms are attracted by sulphuretted hydrogen, and one
species responds readily to dextrin which is usually but feebly chemotactic.
Similarly the antherozoids of Ferns are neither attracted nor repelled,
whereas very many bacteria, though not all, show pronounced chemotactic
response to this gas. Infusoria and Euglena are also aerotactic, although
they react but little (Infusoria) or not at all (Euglena) to other substances.
It seems unlikely that the mode of perception of different substances
by a particular organism is in all cases the same ; that, for instance, the
primary reactions involved in a chemotactic response to acids or oxygen
are the same as when the response is due to the presence of peptone or
potassium salts. It may, however, with safety be concluded that the
development of a chemotactic irritability adapted to the perception of
1 Since all nutrient materials do not act chemotropically, it is hardly advisable to follow Stahl
(Bot. Ztg., 1884, p. 165) and use the terms ' trophototropism ' and ' trophotaxis.'
350 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
a particular substance might unavoidably bring about a sensitivity to more
or less closely allied substances. In this way we can understand how it is
that, for instance, Bacteria are able to react to salts of rubidium or to
aniline blue, and the spermatozoa of Ferns1 to salts of rubidium and
caesium, substances to which under natural conditions they are never called
upon to respond.
The physiological action of a substance is of course dependent upon its
chemical properties, but we are unfortunately unable to deduce from these
chemical properties why one substance should be especially active but
another less so. In the case of compounds which dissociate in watery solu-
tions it needs to be determined how far the stimulation is due to the free ions,
and how far to the undissociated molecules. The antherozoids of Ferns
respond equally well to free malic acid and to its salts, whereas sodium and
ammonium chlorides as well as the non-dissociating diethylester of malic
acid are inactive. Hence the stimulating action is due to the malic acid
ions. In the same way it can be determined that the repelling action of
acids is due to the hydrogen ions, and that the intensity of action is pro-
portional to the degree of dissociation 2.
SECTION 75. Chemotactic and Osmotactic Repulsion.
Chemotactic attraction is due to the chemical properties of the
stimulating substance, but it is also possible that certain organisms may
possess a power of positive osmotactic response to differences of osmotic
concentration in the surrounding medium. Massart ascribes to this cause
the passage of certain marine Bacteria and Flagellatae from very dilute solu-
tions to sea-water, and similarly Stahl has observed that the plasmodia of
Myxomycetes may creep from a dilute to a more concentrated solution
of sugar3.
Many organisms show negative osmotaxis with high concentrations,
and hence whenever increasing concentration produces repulsion it needs
to be determined whether this is due to negative osmotaxis or chemotaxis,
or to their conjoint action. In some cases no repulsion appears to occur,
as for example in the case of Bacterium termo 4, a marine Spirillum 5,
Polytoma uvella, Euglena viridis, and various flagellate and ciliate In-
fusoria6. In all such cases the organisms swim without any check into
1 Buller, Annals of Botany, 1900, Vol. xiv, pp. 571 and 572.
2 Cf. Buller, Annals of Botany, 1900, Vol. XIV, p. 543.
3 Massart, Bull, de 1'Acad. royale de Belgique, 1891, 3° ser., T. xxii, p. 152; Stahl, Bot. Ztg.,
1884, *p. 166. The proof of the absence of chemotaxis is by no means sure in either case.
4 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. II, p. 626.
5 Massart, 1. c., p. 153.
6 Massart, Archives de Biologic, 1889, T. IX, p. 560. The power of reaction is not always fully
CHEMOTACTIC AND OSMOTACTIC REPULSION 351
the capillary filled with the chemotactic solution, however concentrated this
may be. On the other hand, Spirillum undula and Bodo saltans are
repelled by solutions of neutral salts having an osmotic concentration
equivalent to from 0-5 to i per cent, of potassium nitrate. To produce
the same repulsion in the case of Trepomonas agilis and Spirillum volutans
requires a somewhat higher concentration, the organisms being less
sensitive *.
Repulsion produced by very dilute solutions, as for example by acids,
can only be due to negative chemotaxis. Thus the presence of o-i per
cent, of citric acid is sufficient to overcome the chemotactic attraction
exerted by 0-19 per cent, of potassium chloride upon Spirillum imdtda,
and 02 per cent, of citric acid neutralizes the attraction of o-oi per cent,
of malic acid upon the antherozoids of Ferns 2. Similarly the repulsion
exerted by dilute solutions of potassium cyanide, and of calcium nitrate
are really chemotactic in character 3. The attraction of Spirillum by low
partial pressures of oxygen and its repulsion by high ones is obviously
a chemotactic phenomenon, and also affords a good instance of a reversal
of the reaction by increasing concentration 4.
A repulsion of Fern antherozoids is produced only by increasing
concentrations of free malic acid, and not by its salts. The effect produced
is, therefore, the resultant of the attraction exercised by the molecules
of malic acid and the repulsion due to the free hydrogen ions. This is
coupled with the fact that the full attraction of malic acid is produced
by very dilute solutions, whereas that of the hydrogen ions increases pro-
gressively up to a high limit. It does not, however, follow that every
chemotactic substance should produce repulsion when concentrated, or
that every negatively chemotactic substance should produce attraction
when sufficiently diluted. Thus free citric and hydrochloric acids always
repel Spirillum undula, and the antherozoids of Ferns. Similarly, the
smallest pressure of oxygen appears to produce repulsion in certain motile
anaerobic Bacteria. On the other hand, even 15 per cent, solutions of
cane-sugar do not repel the antherozoids of Mosses5. Presumably in
cases where the positive chemotaxis persists, the repulsion is due to the
fact that the negative osmotaxis increases more rapidly with rising con-
centration.
developed. Thus cultures of Spirillum undula are sometimes found to be almost non-sensitive, and
Pfeffer (1. c., p. 614) observed distinct repulsion in the case of Polytoma uvella, although Massart
found this organism to be non-sensitive.
1 Cf. Pfeffer, l.'c., pp. 601, 614, 626 ; Massart, 1. c.
2 Pfeffer, 1. c., 1888, p. 627 ; 1. c., 1884, p. 387.
* Massart, 1. c., 1889, p. 525.
* Cf. for this and the following, Pfeffer, 1. c., 1888, p. 621 seq.
5 Pfeffer, 1. c., 1884, p. 432.
352 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
Positive chemotaxis may so delay the osmotactic repulsion that it
can only become manifest with concentrated solutions, but when the
substance induces negative chemotaxis when dilute, repulsion may be
produced by solutions of less osmotic value than the cell-sap. Hence the
actual result may differ considerably from that which would be produced
by the osmotactic stimulus alone. It is, therefore, hardly surprising that
the concentrations of various substances required to produce perceptible
repulsion upon Bacteria and Flagellatae are not exactly isosmotic. That
other factors may come into play is shown by the fact that glycerine
usually exerts no action upon osmotactic organs1. Furthermore, the
power of osmotic response may itself be influenced indirectly by chemo-
tactic stimulation.
After Engelmann 2 had recognized the repulsion exerted by oxygen Pfeffer found
that various substances were able to produce the same result3, and concluded that
the result was either due to negative chemotaxis or directly to the concentration.
Massart4 then observed that a variety of substances in isosmotic concentration produce
about the same degree of repulsion. These results have only Jbeen obtained with
Spirillum undula and Bacillus megatherium 5, but nevertheless they appear to apply to
other motile forms. The stronger repulsion produced by potassium cyanide, calcium
nitrate, &c., is due to their exerting in addition a strong negative chemotaxis. The
lessened repulsion exercised by saccharose and dextrose, and the inefficiency of gly-
cerine, are ascribed by Massart to their rapid penetration of the protoplasm preventing
the depression of turgor which operates as the exciting stimulus 6. Although several
facts point to this conclusion, it remains to be seen whether all substances which
rapidly penetrate the protoplast are unable to exert any repulsive action.
Experimental evidence is necessary to determine in what way the diminution or
cessation of repulsion is produced. Phobotactic reactions may in fact be excited and
1 Pfeffer, 1. c., 1888, p. 626 ; Massart, 1. c., 1891, pp. 528, 559.
* Engelmann, Pfliiger's Archiv f. Physiologic, 1881 ; Bd. xxvi, p. 541 ; Bot. Ztg., 1881, p. 442.
3 Pfeffer, Ber. d. hot. Ges., 1883, p. 524 ; Unters. a. d. hot. Inst. zu Tubingen, 1884, pp. 385,
453; ibid., 1888, Bd. n, p. 621. Stahl (Bot. Ztg., 1884, p. 166) considers the repellent action of
sugar-solution upon the plasmodium of Atthalium to be directly due to the withdrawal of water.
4 Massart, Arch, de Biologic, 1889, Bd. ix, p. 529. The Bacteria used responded chemotacti-
cally to most of the substances used, and hence would have shown the antagonism between attraction
and repulsion without the addition of the potassium carbonate used by Massart.
5 Repulsion was attained by solutions isosmotic with a solution of from 0-005 to 0-006 of a gram-
molecule (I to | of a gram) of KNO3 per litre. The Spirillum undula used by Massart is apparently
slightly different to that used by Pfeffer. Cf. Rothert, Flora, 1901, p. 413 footnote.
6 Massart, 1. c., p. 528 ; Rothert, Flora, 1901, p. 409. According to.Miyoshi (Bot. Ztg., 1894,
P- I7)> glycerine appears to exert no repulsion upon the hyphae of Fungi. [Assuming that the ecto-
plasmic membrane were the percipient organ for osmotactic stimuli, it could only be stimulated
when its inner and outer surfaces were exposed to differences of osmotic concentration, which could
only be maintained by non-penetrating or slowly-penetrating substances. It is difficult to see how
a general fall of turgor, operating equally on all sides could act as a directive stimulus. The neutral
action of glycerine is certainly not due to its exerting a positive chemotaxis and negative osmotaxis
which balance at all concentrations.]
CHEMOTAXIS AND OSMOTAXIS 353
also inhibited in various ways. Possibly the unequal distribution of the materials in
the cell may act as a stimulus, which will be maintained so long as a difference of
concentration exists on the two sides.
Many bacterial protoplasts re-expand rapidly or slowly in plasmolysing solutions,
but others not at all1. These properties are not constant, however, and specific
peculiarities are often shown in regard to particular substances. The two bacteria
used by Massart behaved similarly on the whole, although asparagin repelled Bacillus
megatherium as strongly as potassium nitrate, but Spirillum undula not at all in the
concentrations used. This may be due to the especially rapid penetration of Bacillus
megatherium by asparagin ; but, for the reasons given, it is difficult to form a final
judgement.
Many organisms show no negative osmotaxis when placed in solutions which
strongly plasmolyse them2. On the other hand, Massart found that Tetramitus
rostratus showed negative chemotaxis, although it has the power of rapidly accommo-
dating itself to concentrated solutions without its power of movement being affected.
Hence Fischer is hardly justified in concluding that the production or non-production of
plasmolysis indicates the presence or absence of a capacity for osmotactic reaction.
The causes which overcome or antagonize repulsion are not necessarily always the
same, and repulsion, like chemotaxis, may often be produced by solutions in which the
organism is capable of continued existence. It depends upon the properties and
power of accommodation of the organism whether the transference to a concentrated
solution hinders or inhibits the power of movement, and whether death ensues rapidly
or gradually.
As the result of the attraction and repulsion, organisms of different sensibility
collect in zones at variable distances around the mouth of the capillary tube from
which the concentrated solution is diffusing. The gradual dilution caused by diffu-
sion, together with the accommodation of the organisms and the consumption of oxygen,
may cause the organisms in two zones to change places, or may induce the exit from
the capillary of forms which had previously penetrated it3. Excreted products of
metabolism may also produce attraction, and, according to Jennings 4, the crowding
together of Paramaecium is due to the chemotaxis exerted by the excreted carbon
dioxide.
Since these reactions may be produced either in a tropic or in a phobic
manner, direct experiment is necessary to determine the detailed character
of the reaction. The antherozoids of Ferns, Mosses, and Selaginella,
certain Flagellatae, and the zoospores of Saprolegnia, show positive and
apparently also negative chemotaxis ; but it is not certain whether the
1 Cf. A. Fischer, Vorlesungen ii. Bacterien, 1903, and Ed., pp. 24, 116. On the regulation of
turgor see H. v. Mayenburg, Jahrb. f. wiss. Bot, 1901, Bd. xxxvi, p. 381.
a Fischer, 1. c., p. 116.
3 For details see Pfeffer, 1. c., 1888, p. 639 ; 1884, p. 472. Also Massart, Bull, de 1'Acad. royale
de Belgique, 1891, 3° se>., T. xxii, p. 157; Beyerinck, Centralbl. f. Bact., 1893, Bd. xiv, p. 827 ;
Abth. ii, 1895, Bd. I, p. in ; 1897, Bd. in, p. i ; Yegounow, Arch. d. sci. biol. de 1'Inst. imper.
de medecine de St. P&ersbourg, 1895, T. in, p. 381 ; Centralbl. f. Bacteriologie, 1898, Abth. ii,
Bd. IV, p. 97.
* Jennings, Journal of Physiology, 1897, Vol. xxxi, p. 318. Cf. also Pfeffer, 1. c., 1888, p. 619.
PFEFFER. Ill
354 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
osmotactic reactions of these organisms are of phobic or tropic nature.
The bacteria hitherto examined are stimulated by the same substances
as the Flagellatae mentioned, but carry out phobic reactions alone. The
curvatures of fungal hyphae have, however, a normal tropic character,
and the same appears to apply to the positively chemotactic movements
of Plasmodia, although the precise nature of their negatively chemotactic
and osmotactic movements is doubtful.
It has been seen that it is often doubtful whether a substance exercises
one or more stimulatory actions, and still less is known of the mode of percep-
tion and the conditions for it. The osmotropic and hydrotropic irritabilities
might possibly be based upon similar sensibilities, although in some cases
at least this does not appear to be the case. A few facts are, however,
known concerning the minimal stimuli for response, the effect of the
intensity of the stimulus, and the power of discrimination.
In the case of sensitive organisms a very small amount of a good
stimulatory material suffices to produce a perceptible reaction. Anthero-
zoids and Bacteria are attracted to a capillary tube containing a hundred-
millionth of a milligram (0-00,000,000,001 gram) of malic acid or of
peptone1 respectively, and this although only a small fraction actually
comes into contact with each excitable organism. These quantities are,
however, not as small relatively as they appear, since an antherozoid
is about o-ooo,ooo,ooo,o2'5, and a Bacterium ttrmo 0-000,000,000,002 of
a gram., i.e. the material in the tube weighs five times as much as the
Bacterium termo, and has ^ the weight of the antherozoid. Negative
osmotaxis, on the other hand, is only exhibited in the presence of solutions
whose concentrations are equivalent to at least 0-5 per cent, potassium
nitrate solution.
It appears that the chemotactic and osmotactic sensitivities of certain
micro-organisms are extremely changeable. Thus Massart2 found that
Spirillum undula after gradual accommodation to saline solutions required
a salt solution of from five to eight times the previous concentration to
produce perceptible repulsion.
The sensitivity may be lowered by unfavourable conditions, and
Voegler3 has shown that at low temperatures the antherozoids of Ferns
require stronger solutions to produce a perceptible reaction than they do
at ordinary temperatures. It remains, however, to be seen whether the
sensitivity is lost sooner at low temperatures or in the absence of oxygen
1 Pfeffer, Unters. u. d. bot. Inst. zu Tubingen, 1884, p. 382 ; ibid., 1888, p. 628. [A trace of
oxygen may suffice for the movement of aerobic bacteria without being able to produce any perceptible
aerotaxis. This is well shown when the bacterium method is used to detect photosynthesis in isolated
chloroplastids.]
2 Massart, 1. c., 1889, p. 548.
8 Voegler, Bot. Ztg., 1891, p. 673. Cf. also Stange, Bot. Ztg., 1890, p. 139, in regard to the
zoospores of Saprolegnia.
CHEMOTAXIS AND OSMOTAXIS 355
than the power of movement. Rothert l appears to have obtained this
result by means of ether, which suppresses first the osmotactic and then
the chemotactic reactions, when applied in increasing concentrations. The
fact that a rise of concentration may convert attraction into repulsion is
also an instance of change of tone produced by demand. If the repulsion
is the result of the antagonism of positive chemotaxis and negative
osmotaxis, or of two opposed chemotactic actions as when malic acid
acts on Fern antherozoids, we have in both cases instances of the same
substance exercising two different stimulatory actions, of which one in-
creases more rapidly with concentration than the other.
The lessened effect of increasing stimuli follows in approximate
accordance with the so-called Weber's law both in the case of the typical
chemotactic and the phobo-chernotactic reactions of bacteria and of anthero-
zoids. Probably also similar relationships will hold good for negative
osmotaxis. Furthermore, in the presence of two chemotactic substances
an organism may either be affected by each separately, or the two stimuli
may fuse to a single perception.
SECTION 76. The Influence of Water.
GENERAL ACTIONS. The power of movement, like that of growth, is
dependent upon the supply of water, and organisms become immotile or
sluggish in concentrated solutions without necessarily being killed 2. Thus
Bacteria grow and form cilia in concentrated solutions, but these develop
no power of movement3. Similarly there must be a certain optimal
concentration for those forms which are unable to exist in dilute solutions 4.
Pure water is, indeed, injurious to many forms. On the other hand,
streaming may continue in plasmolysed cells although more or less
retarded 6.
Sudden transference from dilute to concentrated solutions usually
causes disturbances of the power of movement, as for instance a temporary
cessation of the motion of cilia, or a partial stoppage of amoeboid move-
ment6. Sudden plasrnolysis may produce a temporary stoppage of
1 Jahrb. f. wiss. Bot., 1903, Bd. xxxix, p. i.
2 It has already been mentioned that salt- solutions repel Paramaecium.
3 On ciliated epithelium see Engelmann, in Hermann's Handbuch fur Physiologic, Bd. I, p. 398.
* The statements of Velten (Bot. Ztg., 1872, p. 649) and Dehnecke (Flora, 1881, p. 8) on the
optimal turgor for streaming have no value, since the other factors at work were insufficiently
considered.
5 Dutrochet, Ann. d. sci. nat, 1838, 2e sen, T. IX, p. 73 ; A. Braun, Verhandlg. d. Berl. Akad.,
1852, p. 225; Nageli, Beitrage z. wiss. Bot., 1860, Heft ii, p. 75; M. Schultze, Protoplasma d.
Rhizopoden u. Pflanzenzellen, 1863, p. 41 ; Hofmeister, Pflanzenzelle, 1867, p. 52 ; Ewart, Proto-
plasmic Streaming in Plants, 1903, pp. 8-9.
6 Stahl, Bot. Ztg., 1884, p. 166.
A a 2
356
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
streaming as the result of shock l ; while, on the other hand, streaming
may be excited or accelerated by a
diminution in the percentage of water 2.
TROPIC ACTIONS. A RHEOTACTIC
IRRITABILITY3 has only hitherto been
detected in Myxomycetes, which creep on
wet filter-paper or other media against the
stream of water. To produce this move-
ment a slow stream is sufficient, as when
a plasmodium is developed upon a strip
of filter -paper placed with one end in
a beaker of water, and the other hanging
over the edge of the beaker. Since
freely motile organisms are carried along
mechanically even by a feeble current, it
is hardly likely that they should develop
any special rheotactic irritability. Hence
Roth's statement that certain Bacteria
do actually swim against currents of
water requires further proof4.
HYDROTAXIS is also shown only by
the plasmodia of Myxomycetes 5, and in
virtue of this irritability the plasmodium
creeps into a moist substratum. Towards
the time of fruiting, however, the positive
hydrotaxis becomes negative and the
plasmodium creeps on to the surface of
the substratum, and up the developing sporangial stalks away from the
moisture.
FIG. 63. Cell from a staminal hair of Trade-
scantia virginica : A fresh in water, B the same
with ball and clumps of plasma c, in the zone
a-b exposed to induction - shocks. Magn. 400.
(After Kuhne.)
1 Hofmeister, Pflanzenzelle, 1867, pp. 27, 53; Hermann, Studien ii. d. Protoplasmastromung bei
Characeen, 1898, p. 48 ; M. Tswett, Bot. Centralbl., 1897, Bd. LXXII, p. 329.
8 Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 214. [The evidence as to any such
stimulating action of water on streaming is very unsatisfactory. On the physical action of the per-
centage of water cf. Ewart, 1. c., p. 12.]
3 B. Jonsson, Ber. d. hot. Ges., 1883, p. 515 ; Stahl, Bot. Ztg., 1884, p. 147; J. B. Clifford,
Annals of Botany, 1897, Vol. xi, p. 180. According to Strasburger (Wirkung d. Lichts u. d. Warme
auf Schwarmsporen, 1878, p. 71) this action was first observed by Schleicher.
* Roth, Centralbl. f. Bact., 1893, Bd. xin, p. 755. Aderhold (Jenaische Zeitschr. f. Naturwiss.,
1888, N. F., Bd. XV, p. 314) could detect no rheotaxis in Euglena viridis.
5 Stahl, 1. c., p. 149. Whether the Myxamoebae of Acrasieae (cf. Fayod, Bot. Ztg., 1883, p. 172 ;
Olive, Proceedings of the Boston Society of Natural History, 1902, Vol. xxx, p. 486), and also
Diatoms and Oscillarias, react hydrotropically is uncertain.
MECHANICAL ACTIONS 357
SECTION 77. Mechanical Actions.
Pressure exercises in the first place a purely mechanical action, but
if suddenly applied produces a certain shock-effect. As might be expected,
the movement of swarm -spores is much retarded in viscous media such
as solutions of gum-arabic or gelatine1, and ceases like the movements
of plasmodia2 in solidified 2 to 5 per cent, gelatine, although Oscillaria
is still able to move slowly in this medium.
Gravity and still more powerful centrifugal forces are able to produce
accumulations of the denser constituents at one end of the cell in a purely
mechanical manner. The protoplast, indeed, is able in virtue of its
plasticity to undergo very pronounced deformation or may even be broken
up into pieces without death ensuing. Deformations may result from
rapid changes of temperature, from the action of certain chemicals, from
severe pressure, as well as from the action of weak induction-shocks, which
are especially well adapted to produce localized effects 3 (Fig. 63).
Streaming may continue in the internodal cells of Nitella^ and in
root-hairs of Hydrocharis 4 even when these are sharply bent, and similarly
plants may be strongly shaken without any pronounced effect on streaming.
In some cases, however, a transitory slowing or cessation of streaming5
may result, and this may be followed by a temporary acceleration. Shaking
and all mechanical shocks produce a distinct effect if sufficiently intense
and suddenly applied, although all cells are not equally sensitive. Cells
in which the streaming is permanent or has been a long time aroused
(Chara, Nitella, Eloded) are always more sensitive than ones in which
the streaming is only temporary and has been recently excited by stimuli
(Elodea, Vallisneria) 6. The plasmodia of Myxomycetes exhibit a tendency
to assume a rounded shape when subjected to mechanical stimuli, and
strong shaking as well as the action of electrical discharges causes a
1 See Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, pp. 390, 420.
3 Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 277.
8 Kiihne, Unters. ii. das Protoplasma, 1864, pp. 74, 94; Klemm, Jahrb. f. wiss. Bot., 1895,
Bd. xxvni, p. 647, and the literature here given. On methods see these works and also Nageli u.
Schwendener, Mikroskop, 2. Aufl., 1877, p. 462 ; Zimmermann, Mikroskop, 1895, p. 231.
4 Dutrochet, Ann. d. sci. nat, 1838, 2e s^r., T. ix, p. 32 ; Meyen, Pflanzenphysiologie, 1838,
Bd. n, p. 210; Hofmeister, Pflanzenzelle, 1867, p. 50. On the influence of injuries on the direction
of streaming in Caulerpa, cf. Janse, Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 206, and in other plants,
Ewart, Protoplasmic Streaming in Plants, 1903, p. 34 seq.
5 Dutrochet, 1. c., p. 32 ; Hofmeister, I.e., p. 50; Borscow, Bull, de l'Acad.de St. Petersbourg,
1868, T. xn, p. 213 ; Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 217 ; Hermann, Studien
ii. d. Protoplasmastromung b. d. Characeen, 1898, p. 39; Rhumbler, Zeitschr. f. allgem. Physiol.,
1902, Bd. I, p. 305.
6 Ewart, 1. c., 1903, p. 72. The detailed action of momentum, impact, and of pressure-waves
is given here.
358 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
temporary retardation of the movement of swarm-spores of Diatoms and
of Oscillaria1.
Contact and other mechanical agencies produce a transitory stoppage
of the cilia of Chlamydomonas pulvisculus*, and the cilia of many
locomotory organisms seem to possess a certain contact irritability,
such as appears to be exhibited by Stylonychia and other Infusoria which
run about over the substratum 3. According to Bordet 4, the antherozoids
of Fucus have a * thigmotaxis ' or ' haptotaxis * of this kind, and a similar
but feeble irritability is supposed by Massart to be shown by Spirillum
undula. Whether, as in the case of tendrils, the solid substratum directly
exercises a contact -stimulus is not perfectly certain, since the Infusoria
mentioned may also creep on the surface of the water5. The stoppage
of movement in the cilia of Chlamydomonas produced by mechanical shocks
is, however, comparable with the shock- movements of the leaves of Mimosa
pudica in so far as both are irritable responses to stimuli, but whether
still other special irritabilities may exist among these lower forms is an
open question.
Wounding and injuries, however produced, always exert a certain
influence on movement, and frequently an injury excites or causes an
acceleration of protoplasmic streaming, and may also produce various
traumatic aggregations of the cell-contents.
After a few observations by Frank and Velten, Keller and Hauptfleisch estab-
lished the fact that the active streaming shown in sections often does not exist in the
intact plant, but is produced, or accelerated when pre-existent, by the injury, and in
part also by other stimuli 6. Streaming is, for instance, absent from the intact leaves
1 Unger, Die Pflanze im Momenta d. Thierwerdung, 1843, p. 67; Strasburger, Wirkung d.
Lichts und d. Warme auf Schwarmsporen, 1878, p. 6 ; Engelmann, Bot. Ztg., 1879, p. 55 footnote.
a Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 444; Massart, La sensibilite tactile
chez les organismes inferieures, 1900 (extract from the Journal public par la Soc. royale d. sci. med.
et nat. de Bruxelles).
3 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. II, p. 618; Verworn, Psycho-physiolog.
Protistenstudien, 1889, p. 90; Massart, 1. c., 1900; Jennings, Journal of Physiology, 1897, Vol. xxi,
p. 298 ; American Naturalist., 1901, Vol. xxxv, p. 372 ; Putter, Archiv f. Anatomic u. Physiologic,
physiolog. Abth., Supplement, 1900, p. 243.
* Bordet, Bull, de 1'Acad. royale de Belgique, 1894, 3° se"r., T. xxvil, p. 889. On the thigmo-
taxis of certain animal spermatozoids see Dewitz, Pfliiger's Archiv f. Physiologic, 1886, Bd. xxxvni,
p. 358 ; Centralbl. f. Physiol., 1903, Bd. xvil, p. 89 ; Massart, Bull, de 1'Acad. royale de Belgique,
1888, 3e se>., T. XV, Nr. 5 ; Buller, Quarterly Journal of Microscopical Science, 1902, Vol. XLVI,
P- MS-
5 Massart, 1. c., p. 7. Massart concludes that the surface-tension film may act as a solid, and
considers the accumulation of Chromulina Woroniniana at the surface to be the result of tactic
stimulation. Even a very slight accumulation of minute solid or liquid floating particles at the
surface would be able to exercise tactic stimulation.
6 Frank, Jahrb. f. wiss. Bot., 1872, Bd. vin, pp. 220, 292 ; Velten, Bot. Ztg., 1872, p. 672 ;
I. Keller, Ueber Protoplasmastromung im Pflanzenreich, 1890; P. Hauptfleisch, Jahrb. f. wiss. Bot.,
1892, Bd. xxiv, p. 190; De Vries (Bot. Ztg., 1885, p. i) from his observations on sections con-
cluded that streaming was a much more common and normal occurrence than it actually is. The act
MECHANICAL ACTIONS 359
of Vallisneria spirah's, but soon appears in the leaf-cells when a section is watched
under the microscope. It appears first near the point injured, and spreads for a short
distance through the parenchyma, but for a longer distance when the vascular bundle
is also affected. In such cases it may spread over the entire leaf, or even over all the
leaves on the plant *. The leaf of Elodea canadensis responds similarly, except that
streaming is often present in the intact leaf in the cells along the midrib, and here the
streaming may be so accelerated that the chloroplastids are drawn into it and circu-
late round the cells. Usually the effect of the stimulus gradually passes away and the
plant returns to its original condition. The same applies to the increased respiration
and production of heat which, together with streaming, are all signs of the increased
activity produced by an injury. Streaming is absent from certain cells under all con-
ditions and whatever stimuli be applied, while in other cases streaming begins without
any special external stimulus being required, and may then continue as in Chara and
Nitella for the whole life of the cell 2.
Frank observed that even in the absence of streaming an injury might cause
a marked change in the position of the chlorophyll bodies, and Tangl, Nestler, Ne'mec,
and Miehe 3 have shown that a cut or puncture causes, in a great variety of plants,
a more or less pronounced movement and collection of the protoplasm and nucleus
on the wall facing the injury. The time of reaction depends upon the plant and on
the external conditions, but in roots it may, according to Ne'mec, be shown in from
a quarter to several hours, and spreads with decreasing intensity from 0-5 to 0-7 mm.,
according to Nestler, and even up to 1.3 mm., according to Ne'mec, from the point of
injury.
After the maximum reaction has been reached in a few hours or a few days, the
aggregation is gradually redistributed. According to Ne'mec, this takes place so
rapidly in roots that the reaction has already ceased near to the injury by the time it
has reached its maximum in the furthermost zones affected. A secondary change,
consisting of the enlarging and fusion of the vacuoles, was then observed by Ne'mec,
but this spreads to a less distance than the primary reaction. Interesting as these
movements are, however, they simply form another indication of the wound reaction,
and do not give any insight into the causes of it.
of preparation usually does not inhibit streaming or does so only temporarily. Kienitz-Gerloff s
discussions (Bot. Ztg., 1893, p. 36) show an ignorance of the nature of irritability, and the same
applies to I. Keller (1. c., p. 8), who considers streaming to be a purely pathological phenomenon.
1 Hauptfleisch, 1. c., p. 196 ; Kretschmar, Jahrb. f. wiss. Bot., 1903, Bd. xxxix, p. 275 ; Ewart,
Protoplasmic Streaming in Plants, 1903, p. 104. [All three authors observed a more rapid propa-
gation longitudinally than transversely. Kretschmar observed a maximal rate of propagation in
Vallisneria of 3 cm. per minute, whereas the average rate of propagation observed by Ewart (I.e., p. 105)
in the leaf of Elodea was 1-3 mm. per minute at 30° C., and was barely more rapid in Vallisneria.']
2 [On the reasons for the absence of streaming in certain cells, cf. Ewart, 1. c., p. 29seq. In
small plant-cells diffusion from end to end is more rapid than streaming, whereas in large cells like
those of Chara and Nitella in which streaming is an essential factor in continued life the protoplasm
may stream several times round the cell during the time required for complete diffusion. Hence the
importance of streaming in large cells : cf. Ewart, On the Ascent of Sap in Trees, Phil. Trans, of the
Royal Society, 1905, p. 40.]
3 Tangl, Sitzungsb. d. Wien. Akad., 1884, Bd. xc, Abth. i, p. 10; Nestler, Sitzungsb. d. Wien.
Akad., 1898, Bd. evil, Abth. i, p. 708 ; NSmec, Die Reizleitung u. d. reizleitenden Structuren, 1901,
p. 8 ; Miehe, Flora, 1901, p. 127.
360 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
SECTION 78. Galvanotaxis.
Electrical currents of no greater strength than those which normally
circulate through plants do not influence streaming or locomotory move-
ments. Since, however, strong currents act injuriously or even fatally, ones
of moderate intensity might be expected to produce some physiological
effect, such as the galvanotropic reaction of many motile organisms.
Furthermore, a sudden increase or decrease in intensity, as on making or
breaking the current, acts as a shock-stimulus, like a blow or sudden
pressure. A single make or break shock is sufficient to stimulate the
pulvinus of Mimosa pudica, whereas a series of successive shocks are
required to produce a complete reaction in tendrils or in the leaflets of
Oxalis. Electrical shocks produce the same effect in the plasmodia of
Myxomycetes, and in cell-protoplasts, as do mechanical ones *. According
to the properties of the organism and the intensity, frequency, and character
of the stimulus, either an acceleration of retardation of movement, or
a slight change of shape, or pronounced deformation may ensue. Further-
more, either electrical or mechanical shocks may cause cilia to contract or
may decrease or accelerate their movement.
Electrical stimuli are especially of value in that their intensity and
duration can be exactly measured, and in that they can be locally applied 2.
Since, however, their action in plants is merely that of shock-stimuli, no
special detailed description of their mode of action is required 3. Certain
peculiarities, as compared with mechanical stimuli, are due to the fact
that the action is strongly polar, and that local electrolytic actions due
to the separation, sorting, or interaction of ions, may arise at every
point where the current passes from one medium to a dissimilar one4.
The physical differences between the make and break shocks naturally
induce differences in their physiological action 5. It is, however, possible
that induction-shocks may exercise some special electrical influence in
addition to their more mechanical action.
1 This similarity has been recently pointed out by E. Roesele (Zeitschrift f. allgem. Physiologic,
1902, Bd. II, p. 162) and by Ewart (On the Physics and Physiology of Protoplasmic Streaming in
Plants, 1903, p. 88). For facts see Kiihne, Untersuch. ii. d. Protoplasma, 1864, 1874, 1894;
Engelmann, Handbuch d. Physiologic von Hermann, 1879, Bd. I, pp. 366, 403; Verworn, Psycho-
physiologische Protistenstudien, 1889, p. no; Allgem. Physiologic, 3. Aufl., 1901, p. 431 ; Klemm,
Jahrb. f. wiss. Bot., 1895, Bd. xxvui, p. 647 ; G. Hermann, Studien ii. d. Protoplasmastromung bei
d. Characeen, 1898, p. 60 ; and the literature quoted in these works.
2 For methods see Nageli u. Schwendener, Mikroskop, 2. Aufl., 1877, p. 462 ; Zimmermann,
Mikroskop, 1895, p. 231; Roesele, I.e., p. 143. Also Biedermann, Elektrophysiologie, 1895;
L. Hermann, Physiolog. Practicum, 1898.
3 [The existence of a power of galvanotropic response to constant currents in plants hardly
coincides with this assumption.]
4 Cf. Ewart, I.e., pp. 95, 99, 123 ; Ewart and Bayliss, Proc. of Royal Society, Nov., 1905, p. 63.
5 Cf. Verworn, Allgem. Physiologic, 3. Aufl., 1901, p. 431 ; Fr. Schenck, Pfliiger's Archiv f.
Physiologic, 1897, Bd. LXVI, pp. 257, &c. The action of make and break shocks is given here,
as well as their relation to Pfliiger's law, and the deviations from it.
GALVANOTAXIS 361
The influence of constant currents on streaming shows no features of
especial importance, apart from the fact that the direction of the current
in no wise influences the direction of streaming, and produces no effect
upon the relative velocity of opposed streams l. There is, however, a special
physiological reaction, galvanotaxis, which may be termed positive or
anodic, and negative or kathodic, according to whether the responding
motile organisms wander towards the negative or positive electrodes.
Galvanotaxis appears to be shown chiefly by Infusoria, Flagellatae, and
Bacteria2, and negative galvanotaxis appears to be commoner than
positive. In some cases, a rise in the intensity of the currents converts
a positive galvanotaxis into a negative one, while some forms exhibit
transverse galvanotaxis.
Negative galvanotaxis is shown by Paramaecium aurelta, P. Zwrsaria, Coleps
hirtus, and all the ciliate Infusoria examined, with the exception of Opalina ranarum,
which shows positive galvanotaxis with weak currents, but negative with stronger ones,
according to Wallengren. Among the Flagellatae, Verworn found Trachelomonas hispida,
Peridinium tabulatum to show negative, and Polytoma uvella, Cryptomonas ovata positive
galvanotaxis, while Chilomonas paramaedum behaves like Opalina ranarum 3. Volvox
aureus shows negative galvanotaxis, according to Carlgren4, and possibly other
Volvocineae as well, although Verworn could detect no such irritability in Euglena
•viridis*. Certain bacteria do, however, appear to have a power of galvanotactic
response 6, and, according to Verworn 7, Amoebae show negative galvanotaxis at
about 25°C., while Schenck8 has shown that at lower and higher temperatures the
galvanotaxis becomes positive.
Transversal galvanotaxis is shown by the Infusorian Spirostomum ambiguum 9
and by Oxytrichia and Stylonychia while creeping on the substratum, whereas free-
swimming individuals show negative galvanotaxis 10.
The chamber shown in Fig. 64 may be used to contain the organisms to be
1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 100, and the works mentioned there.
3 L. Hermann (Pfliiger's Archiv f. Physiologic, 1885, Bd. xxxvu, p. 457 ; 1886, Bd. xxxix,
p. 41 4) first observed galvanotaxis in tadpoles, and the same irritability was detected in Infusoria and
Flagellata by Verworn (Pfluger's Archiv f. Physiologic, 1889, Bd. XLV, p. 27; 1889, Bd. XLVI,
p. 268; Psycho-physiologische Protistenstudien, 1889, p. 115; Allgemeine Physiologic, 3. Aufl.,
1901, p. 476). See also V. Ludloff, Pfluger's Archiv f. Physiologic, 1895, Bd- LIX» P- 525 > J- J- Loeb>
ibid., 1896, Bd. LXV, p. 518; Jennings, Journal of Physiology, 1897, Vol. xxi, p. 305; Putter,
Archiv f. Anatom. u. Physiologic, physiol. Abth., Supplementband, 1900, p. 243; Wallengren,
Zeitsch. f. allgem. Physiol., 1902, Bd. n, p. 341 ; 1903, Bd. ill, p. 22.
3 Cf. also Wallengren, 1. c., p. 377.
* Carlgren, Centralbl. f. Physiol., 1900, Bd. XIV, p. 35.
5 Verworn, 1. c., 1889, p. 290. Diatoms have not yet been investigated.
6 Verworn, 1. c., 1889, p. 291 ; Chauveau, Compt. rend., 1896, T. cxxi, p. 892.
7 Verworn, 1. c., 1889, p. 272 ; Pfluger's Archiv f. Physiol., 1896, Bd. LXV, p. 47.
8 Fr. Schenck, Pfliiger's Archiv f. Physiol., 1897, Bd. LXVI, p. 253.
9 Verworn, 1. c., 1901, p. 480; Pfluger's Archiv, 1896, Bd. LXII.
10 Putter, 1. c., p. 275.
362
LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
tested. The current should be led in by non-polarizable electrodes *, the brushes
soaked in normal saline solution touching the porous sides of the cell. By reversing
the commutator and changing the direction of the current, the reacting organisms will
be caused to collect at the opposite electrode, and this may be repeated many times.
The striking reaction shown by Paramaecium may be made visible to a large audience
by means of a projection lantern 2. If a current of from five to twenty volts is used,
the best current strength is readily reached by adjusting the resistance. According
to Ludloff, this lies between o«i to 0-6 of a milliampere in the case of Paramaecium^
whereas with 0-06 of a milliampere the reaction is barely perceptible 8.
Little doubt can exist that we are here dealing with a physiological
movement due to stimulation, and that the movement is not the direct
result of the kata-
phoric action of the
electrical current.
That strong cur-
rents may exercise
such an action is
certain, but never-
theless, it is not
possible to agree
with BirukofY and
Pearl in ascribing
the movements to
the direct action of
the current4. For
instance, using a
mixture of species,
the positively gal-
vanotactic forms
will move towards
the anode, the negatively galvanotactic forms towards the kathode, while
dead individuals do not move at all.
Infusoria and Flagellatae show a typical tactic reaction, that is, they
orient themselves in regard to the direction of the current by presenting the
front end towards either the kathode or anode, and always swimming with
the same end first. At the same time, the movement may either be
FIG. 64. Apparatus to show galvanotaxis. The non-polarizable electrodes c are
applied to the bars of porous porcelain (' biscuit '). These are joined at b b by bars
of wax-colophonium mixture. The water in the cell contains the negatively
galvanotactic Paramaecium aurelia which moves towards the kathode.
1 On non-polarizable electrodes cf. Biedermann, 1. c., p. 150; Hermann, 1. c., p. 29; Cyon,
Methoden d. physiolog. Experimente, 1876, p. 386.
2 Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 719.
3 [The current density within the cell, which in the case of a uniform conductor depends upon the
area of cross-section and upon the current strength, requires consideration.]
4 Birukoff, Pfliiger's Archiv f. Physiologic, 1899, Bd. LXXVII, p. 555 ; Pearl, American Journal
of Physiology, 1900, Vol. IV, p. 96. See also Putter, Archiv f. Anatomic u. Physiologic, 1900^
Supplementband, p. 299.
GALVANOTAXIS 363
accelerated or retarded by a direct or indirect action of the electric current1.
Since the orientation is unaltered so long as the current remains constant, it
is evidently due to the stimulus exercised by the current, and not to any
transitory shock-effect. Possibly, however, organisms may be found to
show orienting movements under the action of induction-shocks, and it has
still to be determined whether certain Bacteria and other organisms can
show a phobo-galvanic response.
The conversion of positive into negative galvanotaxis produced by an
increase in the strength of the current is due to a reversal of the polar
orientation, just as in the reversal of phototaxis by increasing intensity
of light. In both cases the tropic stimulus so modifies the movement of
the cilia as to cause the organisms to turn in a particular direction, and
then to swim continually in this direction. This applies whether the
organism has a single flagellum, or a tuft of cilia at one end, or whether
it is covered all over by numerous similar or dissimilar cilia. According to
Ludloff 2, the galvanotactic orientation of Paramaecium is correlated with
a dissimilar action upon the ciliary movement at the two ends. Similarly,
Wallengren 3 concludes that it is owing to the cilia on different regions of
the body being unequally affected that Opalina ranarum shows negative
instead of positive galvanotaxis when the current increases beyond a certain
intensity. Paramaecium aurelia swims hinder end first in a 0-4 to
0-7 per cent, solution of sodium chloride ; and, according to Loeb, this
causes the organism to show positive instead of negative galvanotaxis,
although the body is oriented in the same way as in water 4.
These observations leave it uncertain whether the cilia are directly
or indirectly affected 5, nor do they give any insight of the mode of percep-
tion. Separate ciliated fragments of Infusoria move in the same way as the
intact organism, and in the case of Bursaria truncatella, show the same
galvanotactic responses 6. Hence it appears that individual cilia and groups
of cilia have a considerable degree of independence, and are in themselves
individually responsive to galvanotropic stimuli. It is not, however, certain
whether the galvanotactic movement of Amoebae carried out by the
protrusion and retraction of pseudopodia is a physiological reaction or is
1 Verworn, Pfliiger's Archiv f. Physiologic, 1889, Bd. XLVI, p. 280; Ludloff, I.e., p. 544;
Wallengren, 1. c., p. 369.
a Ludloff, 1. c., p. 552.
8 H. Wallengren, 1. c., pp. 375, 381. More varied results may be obtained when the organism
possesses dissimilar cilia which react differently.
* J. J. Loeb, Pfliiger's Archiv f. Physiologic, 1897, Bd. LXVI, p. 352. Putter (1. c., p. 297) finds
that the backward movement ceases as the organisms become accommodated to the salt-solution.
5 On the unequal sensitivity of dissimilar cilia cf. Verworn, Piitter, and Wallengren. E. Roesele
(Zeitschr. f. allgem. Physiologic, 1902, Bd. II, p. 164) states that the mouth opening near to the
basis of the cilia possesses the greatest sensitivity to induction-shocks and to mechanical stimuli in
Stentor and Vorticella.
6 Verworn, 1889, 1. c., p. 293.
364 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS
merely due to the fact that the polar electrolytic action of the electrical
current causes the surface-tension to be lowered on the side towards
which movement occurs, or raises it on the opposite side 1.
Theoretical. It seems probable that the first stage in perception is due
to the electrolytic decomposition, and the sorting of the ions set up by the
electrical current. If the organisms are mpure water2, changes of this kind
can only go on internally, whereas in saline media all the conditions for
chemotropic stimulation will be produced. The protoplast may possibly
not be permeable to all ions, so that local accumulations of them might be
produced 3. It is not, however, possible to say whether the separated anions
and kations may act like externally applied chemicals4, or whether the
partial or unequal dissociation at different points in the protoplast may act
as a tropic stimulus.
Loeb 5 concludes that the galvanotropic stimulus is directly due to the
impact of the negative and positive ions on the organism as they travel
to anode and kathode. Loeb finds that the local action of acids and
alkalies produces similar deformations to those caused by electric currents,
but forgets that the stimulating action of a reagent does not necessarily
remain the same when it is applied in concentrated form. Furthermore,
Piitter6 has shown that the action of a strong galvanic current is not the
same as that of acids and alkalies.
SECTION 79. Cytotaxis.
By negative cytotaxis is denoted the tendency of organisms or parts
of organisms to separate from each other, by positive cytotaxis their
tendency to approach7, but the terms give no direct indication of the
ways and means by which such phenomena are brought about. In some
cases tropic stimuli come into play, as when an excreted substance exerts
a chemotropic action, such as is shown during the attraction of certain
antherozoids to the ova. Individuals of the same species of Infusoria and
also of Bacteria may exert tropic stimuli on each other by means of their
excreta. The attraction of aerotactic Bacteria to an assimilating algal cell
1 Cf. Verworn, Pfliiger's Archiv f. Physiol., 1889, Bd. XLVI; Schenck, ibid., 1897, Bd- LXVI.
2 [Practically an impossibility owing to exudation from the organisms. The resistance of pure
water is so high (3.4 x io5 ohms, per c.c. at 11° C.) that a considerable increase of voltage would be
necessary, and the water would rapidly become overheated.]
3 That electrolysis may cause the culture-fluid to become poisonous is well known.
* Cf. Nernst, Nachricht. d. Ges. d. Wiss. zu Gottingen, 1899, p. 104 ; Ewart and Bayliss, Proc.
of the Royal Society, 1905, p. 63.
5 Loeb, Pfliiger's Archiv f. Physiol., 1897, Bd. LXV, p. 518. See also H. H. Dale, Centralbl. f.
Physiol., 1901, Bd. XV, p. 303.
6 Putter, Archiv f. Anat. u. Physiol., Supplementband, 1900, p. 294.
7 Roux, Archiv f. Entwickelungsmechanik, 1894, Bd. I, pp. 57, 200; Programm und Forschungs-
methoden d. Entwickelungsmechanik, 1897, p. IO.
CYTOTAXIS 365
is also an instance of chemotropic cytotaxis, and if sensitive Spirillum forms
are used these collect a little distance away from the cell. Chemotropic
cytotaxis is probably also involved in the attraction of the pollen-tube to
the embryo-sac, as well as in the penetration of a host by the hyphae of
a parasitic fungus, and in the formation of Lichens.
In some cases osmotropic, thigmotropic, and even also hydrotropic
stimuli may be used for purposes of physiological interaction, whereas
thermal, galvanic and photic stimuli are of little or no value in this respect \
Thus few plants are luminous, and the electric currents and differences of
temperature due to vital activity are so trifling as to be unable to exert
any appreciable tropic stimulation. Reflected rays, or local heating due to
external radiation, may, of course, exercise some effect, but these are not
within the control of the plant. Thigmotropic reactions, on the other hand,
are responsible for the attraction and fusion of the sperm and ovum of
Fucus, as well as for the coiling of tendrils round each other.
The stimuli may either act across short distances, or only when the cells
are in contact — in the former case attraction being ensured, while in the latter
case accidental contact is made permanent. Small objects may be brought
together by surface-tension forces, and also repelled from one another
without their possessing any special motile organs. In such cases we have
a purely physical movement produced in the same way as when an oil-drop
comes into contact with a soap-solution on one side. The movement is
only physiological in the sense that metabolism causes the production of
the substances responsible for the modifications of surface-tension. According
to Roux 2, it is by a physiochemical action of this kind that the separated
fragments of a frog's egg creep together again. The plasmodial aggregation
of the Myxamoebae of Acrasieae may be brought about in the same way ;
but, since the Myxamoebae have a power of independent movement, it
seems more probable that we have here another instance of physiological
chemotaxis. The fact that the aggregation ceases under certain conditions
shows nothing, for it might be due to a cessation of the secretory activity
on which the changes of surface-tension and the chemotaxis might alike be
dependent.
1 A regular arrangement may also arise from purely mechanical causes.
2 Roux, Archiv f. Entwickelungsmechanik, 1894, p. 43; Rhumbler, Biolog. Centralbl., 1898,
Bd. xvm, p. 22; Ergebnisse d. Anat. u. Entwick. von Merkel und Bonnet, 1898, Bd. vm, p. 587.
CHAPTER V
THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
PART I
THE PRODUCTION OF HEAT
SECTION 80. General.
PLANTS are poikilothermic organisms which assume approximately the
temperature of the surrounding medium owing to their relatively feeble
powers of heat-production and their excessive loss of heat by radiation and
transpiration. According to whether the former or latter preponderate, the
temperature will be slightly above or slightly below that of the surrounding
medium.
Since the heat is produced by respiration, less will be formed by the
more feeble intramolecular respiration than by normal aerobic respiration.
In the case of anaerobes, however, the pronounced decompositions they excite
may be accompanied by a considerable liberation of chemical energy in the
form of heat. Many plants respire actively, and Fungi and Bacteria have
four to one hundred times the respiratory activity of mammals, so that
such organisms may produce relatively more heat even than birds.
Owing to their extensive surface area, and the usual presence of so much
dead tissue, most plants, even when transpiration is reduced to a minimum,
become hardly at all or only 0-3° C. warmer than the surrounding saturated
air. In dry air the transpiration usually keeps the temperature of the plant
slightly below that of the air. In fleshy actively transpiring bodies such as
the spadix of Aroids a pronounced rise of temperature is shown, whereas in
tubers, in the trunks of trees and in most thick organs respiration is relatively
feeble and the rise of temperature is usually less than in thinner but more
actively respiring organs. Most Fungi and Bacteria expose a large surface
to the air, and if grown under water the heat produced is naturally conveyed
away still more rapidly l.
1 [The sporophores QiAgaricus, Boletus, and Lycoperdon (Scleroderma) also form good material,
the thermometer being placed in a hole bored in the sporophore while young, and the whole as well
as the control thermometer being enclosed in cotton-wool. Similarly, vigorous broth-cultures of
bacteria, if aerated and then corked after the introduction of a thermometer, show a temperature
from o- 1 to 0.4 C. higher than that registered by a similar thermometer placed in a corresponding tube
containing sterile broth, both tubes being surrounded by cotton-wool. Since the specific heat of
water is high, this slight rise of temperature represents a considerable production of heat.]
GENERAL 367
Although metabolism may involve exothermal as well as endothermal
chemical changes, these appear to balance approximately, the heat produced
being derived almost solely from respiration. The swelling of dry seeds
does, however, produce a distinct temporary rise of temperature, and the
rapid commencement of respiration in the moistened seeds produces a
secondary rise *,
Even when the living cells respire actively, organs containing a large
amount of dead tissue can never be much warmed. In the cell itself only
the protoplasm is active, and the production of heat in it must often be
great enough to produce an injurious or even fatal rise of temperature, were
it not for the rapid removal of heat by the surrounding water2. Similarly
the temperature of the most actively respiring Bacteria cannot be appreciably
higher than that of a fluid medium in which it is growing.
The curves of respiration and heat-production are approximately parallel
when plants are exposed to varying conditions in saturated air. For instance,
after an injury respiration and heat-production attain a maximum at about
the same time. Changes of temperature affect respiration and heat-production
in corresponding degree, and as far as is known continued rises of tempera-
ture produce increasing differences between the temperature of the plant
and that of the surrounding medium until the fatal limit is reached. Other-
wise the temperature of the plant closely follows that of the surrounding
medium, and hence plants appear to have no power of regulating their
temperature like mammals by either increasing the production of heat or
diminishing the loss of it. The cooling effect of transpiration may prevent
the plant from being excessively heated by insolation, but this is a purely
accessory physical effect, and as far as any physiological regulation comes
into play this is concerned solely in preventing a fatal loss of water. Hence
the transpiration from an exposed leaf may be checked just when its cooling
effect is most needed, the plant sacrificing the exposed organ rather than
risk its whole existence. The protective movements of certain leaves do
actually involve a temporary increase of transpiration, but usually the
movement is such as to reduce not only the transpiration but also the
exposure to the radiant energy of the sun 3.
Poikilotherms have this advantage over homoiotherms, that their body
temperature may vary within wide limits without danger to life. On the
other hand, homoiotherms, if well nourished, may remain active at low
temperatures which suppress the activity of poikilotherms more or less
completely. The latter, however, avoid the waste of energy involved in
maintaining a high body-temperature.
1 Wiesner, Versuchsstationen, 1872, Bd. xv, p. 138.
8 Engelmann, Bot. Ztg., 1888, p. 713.
3 Cf. Ewart, The Effects of Tropical Insolation, Annals of Botany, Vol. n, 1897, pp. 450,
457, 459-
368 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
Plants are able to grow when their temperature is, owing to transpira-
tion, kept permanently below that of the surrounding medium, and their
growth is not appreciably affected by their own production of heat. The
latter is therefore merely an accessory result of metabolism, and has no
special economic value, but merely indicates a more or less pronounced
preponderance of exothermic chemical changes. The heat-vibrations pro-
duced in this way in the interior of the protoplast may, however, have quite
a different value and importance to the heat derived from without.
In certain cases a rise of temperature may be of definite advantage to
the plant. For instance, the warmth of the spadix of an Aroid may aid in
the rapid development of the pollen, and also in attracting pollen-carrying
insects l. Furthermore, the production of heat enables feeble transpiration
to continue in air saturated with moisture, and hence may aid in preventing
the injection of the intercellular spaces with sap. No protection against
frost is possible, however, since respiration and the production of heat
entirely or almost entirely cease as the temperature sinks below zero centi-
grade 2. On the other hand, the increasing production of heat with rising
temperature instead of being advantageous may cause the plant to be more
rapidly fatally affected.
Owing to their relatively less surface, fleshy or bulky organs are
appreciably warmed by a smaller production of heat than thin membranous
ones. The aggregation of different parts as well as the provision of hairy
or non-conducting coverings by lessening the loss of heat will cause the
temperature to rise. In fact a heap of living respiring plants in saturated
air will always show a temperature appreciably higher than that of the
surrounding air. Furthermore, the rise of temperature in the interior will
excite more active respiration if the aeration is sufficiently rapid. The
heating of the heaped grass cut from a lawn, as well as of imperfectly dried
hay in hay-ricks, is in the first instance due to the plants' own warmth,
although the subsequent more pronounced heating is largely due to the
rapid development of micro-organisms at the raised temperature.
The actual amount of heat produced by a plant can only be determined
by calorimetric measurement, but such estimations give no idea as to the
exothermic and endothermic chemical changes which may go in the plant 3.
Even when such substances as starch or sugar form the main material con-
1 Cf. Ludwig, Biologic, 1896, p. 261 ; G. Kraus, Die Bluthenwarme bei Arum italicunt, 1882,
p. 20 (reprint from Abhandlg. d. naturf. Ges. zu Halle, Bd. 16) ; Ann. d. Jard. hot. de Buitenzorg,
1896, T. xni, p. 271).
3 Seignette (Revue generate de Bot., 1889, T. I, p. 614) observed in the case of bulbs and tubers
a greater difference of temperature at — 6°C. than at 3°C. and n°C., but this was probably due to
special causes. Cf. H. Dixon, Transact, of the Irish Academy, 1903, T. xxxil, Part III, p. 145.
3 Cf. Pfeffer, Studien zur Energetik, 1892, p. 189 ; Ostwald, Lehrb. d. allgem. Chemie, 2. Aufl.,
1893, Bd. n, p. i.
GENERAL 369
sumed in respiration other substances may also be oxidized, and in addition
to carbonic dioxide and water other substances may be formed in variable
amount with different or in some cases unknown caloric equivalents. The
production of carbon dioxide and absorption of oxygen does not, therefore,
form a sure guide as to the amount of chemical energy liberated, and hence
we are unable to decide how much of this energy is set free in the form of
heat and how much appears as mechanical work.
Rodewald1 found that in such resting organs as ripe apples and the
swollen stems of the cabbage turnip (Kohlrabi) the amount of heat produced
represented practically the whole of the energy of respiration, as determined
from the production of carbon dioxide and absorption of oxygen, and
.assuming that these represented so much completely oxidized carbohydrate
material. According to Bonnier2, however, seedlings of the Pea liberate
more heat than is represented by their respiratory activity. During later
stages of development the difference is lessened, and during flowering the
actual liberation of heat becomes less than the theoretical values.
Bonnier suggests that during germination, in addition to respiration,
other chemical changes and dissociations of exothermic character occur in
abundance, while at a later date, especially during the storage of reserve-
materials, endothermic condensations and polymerizations take place which
involve an absorption and storage of heat. The respiratory quotient (-^r^)
is actually less than unity during germination, especially in the case of oily
seeds, but the subject is worthy of further investigation. In any case the
difference between the actual and estimated production of heat is not due
to the work done during growth, since the excess of the actual production of
heat over the theoretical amount is greatest during the period when growth
is most active. Furthermore the mechanical equivalent of heat is very high,
so that a small absorption of heat would represent an enormous amount of
work. Ewart has, for instance, shown that the work done in maintaining
streaming in a large cell of Nitella for a year represents the heat produced
by the complete combustion of ^Winnr of a gram of cane-sugar, the work
done being 252 ergs per day. In smaller cells more energy is consumed in
streaming, but even then the work done is insignificant compared with the
heat produced by respiration 3.
1 Rodewald, Jahrb. f. wiss. Bot., 1888, Bd. XIX, p. 291 ; 1887, Bd. XVIII, p. 342.
2 Bonnier, Ann. d. sci. nat., 1893, 7' se*r., T. xvni, p. i ; Bull, de la Soc. hot. de France, 1880,
T. xxvii, p. 141.
8 Ewart, Protoplasmic Streaming in Plants, 1903, p. 27. [i gram-calorie is the amount of heat
required to raise a gram of water i° C. in temperature, a kilogram-calorie the amount needed to raise
a kilogram i° C. If the expansion of water were uniform the value of the calorie would be the same
at all temperatures, and this is practically the case between 4°C. and ioo°C. As regards the
mechanical equivalent of heat i gram-calorie represents 42,350 gram-centimetres, or 4.17 x io7 ergs
PFEFFER. Ill
370 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
In an experiment with germinating Barley carried out at i6°C., each kilogram
weight produced per hour 3-72 kilogram-calories1. The respiratory quotient was
0-65, and the heat-production estimated from the liberation of carbon dioxide was
1.74 kg.-cal., from the absorption of oxygen 2-7 kg.-cal. For the ripening ears the
respiratory quotient was 1-05, the heat produced 0-24 kg.-cal., and the estimated
amount both from the consumption of oxygen and the evolution of carbon dioxide
0-3 kg.-cal.
Both the respiratory activity and the production of heat attain a maximum
during germination, and subsequently gradually fall. An adult man produces per
kilogram per hour about 1-4 kg.-cal., but a puppy as much as 6-4 — so that the
production of heat in seedling plants is quite comparable with that in animals.
Few experimental researches on heat -production have been performed,,
but these suffice to show its dependence upon the respiratory activity,
and in fact the more readily followed changes in the heat-production
can be used to trace the wound-reaction in place of the respiratory changes.
Under constant conditions as regards the external loss of heat a change
of temperature must always indicate an alteration in the vital activity, involv-
ing an increase or decrease in the exothermic or endothermic metabolism.
Methods. The warmth of the spadix of certain Aroids can either be felt or
shown by pressing a thermometer with a small bulb against the object. Germinat-
ing seeds, or flowers of Rhubarb or Chamomile, may be placed in a vessel as in
Fig. 65, through which a stream of air or of hydrogen saturated with moisture can
be drawn2. In the absence of oxygen the rise of temperature decreases to a
minimum 3. For comparison a similar vessel should be used containing seeds killed
by steaming. If a large mass of germinating seeds is merely placed in a large
beaker and covered with a bell-jar, a rise of temperature of a few degrees may be
shown, and if the experiment is carried on in a large calorimeter, or if the whole
vessel is surrounded by cotton-wool, the temperature may be over io°C. higher than
that of the control. In some cases the rise is so high as to kill the seedlings, and
if in that case Aspergillus fumigatus or other thermophile organisms develop the
temperature may rise to over 60° C.4
To detect the slight warming of single organs thermo-electric methods were em-
ployed by van Beek and Bergsma, by Dutrochet, and by various subsequent authors 5.
of work. The heat of combustion of fats is greater than that of carbo-hydrates. Thus i gram of
glycerine produces 4,200 gram-calories; starch and cellulose, 4,100; cane-sugar, 4,000; lactose,
3,900; dextrose, 3,700; albumen, 5,000 to 6,000; fat, about 9,000 calories, when burnt into carbon
dioxide and water. Cf. Landolt and Bornstein, Physikalisch-chemische Tabellen.]
1 Bonnier's results are given in terms of one hour.
2 This method of heaping seedlings, &c., together to show the evolution of heat was first used
by Goppert, Ueber Warmeentwickelung in den lebenden Pflanzen, 1832, p. 10.
3 See Eriksson, Unters. a. d. bot Inst. zu Tubingen, 1881, Bd. i, p. 105.
4 Cohn, Schlesische Ges. fur vaterland. Cultur, 1888, p. 150; Ber. d. bot. Ges., 1893, General-
vers., p. 66.
5 Van Beek und Bergsma, Observations thermo-electriques s. 1'elevation de la temperature des
fleurs de Colocasia, 1838; Dutrochet, Ann. d. sci. nat., 1839, 2e s^r'» T.xn, p. 77; 1840, 2e sen,
GENERAL
371
A copper or German-silver wire (<?, Fig. 66) is smelted to two iron wires (m, n).
The tips, covered with shellac, are embedded, one in the living shoots, the other in
a dead one (d) held up by a thread (s). The whole is covered with a bell-jar, and
the current measured by means of a reflecting galvanometer. In this way a difference
of temperature of -5 JVC. can be detected and the temperature at different points on
the same plant can be compared.
For quantitative experiments various forms of calorimeters may be used, of
which several have been especially adapted for animal physiology1. Rodewald
FlG. 65. Apparatus for showing the
influence of oxygen upon the production
of heat : (a) contains the germinating
seeds ; (b) thermometer; gases can be drawn
through at (c).
FIG. 66. Apparatus for thermo-electric
measurement of temperature : (c) living,
(cf) dead shoot ; (e) German-silver wire ;
(MI and n) iron wire ; at o and e the thermo-
electric junctions are inserted in the shoots.
determined the absolute temperature thermo-electrically, and then estimated from this
the amount of heat required to balance radiation and transpiration. This method
is, however, liable to lead to serious error owing to the difficulty of control.
The difference between the heat of combustion of the seed and of the dried
seedling grown in darkness gives approximately the amount of heat liberated during
T. xiil, p. 5 ; Rodewald, 1. c., 1887, p. 276 ; 1888, p. 221 ; Seignette, Rev. gen. de Bot., 1889, T. I,
p. 574; Richards, Annals of Botany, 1897, Vol. xi, p. 31. On methods see also Cyon, Methoden
der physiologischen Experimente, 1876, p. 484; Hermann, Handbuch d. Physiologic, 1882, Bd. iv,
T. II, p. 305. The bolometric method has not yet been used for determining temperature in plant
physiology.
1 See Rubner, Die calorimetrische Methodik, 1891; Traite de physique biologique, public p
d'Arsonval, &c., 1901, p. 804.
B b 2
372 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
development ', but the method is naturally not a particularly accurate one even for
comparative experiments. The amount of transpiration in saturated air forms a still
more unsatisfactory measure of the production of heat.
SECTION 81. The Evolution of Heat by Aerobes.
Lamarck was the first to observe the production of heat by the spadix
of Arum italicum 2, and in the case of this plant as well as in that of Arum
maculatum a delicate thermometer may show a temperature 6° or io°C.
above the surrounding air when applied to the upper free sterile portion of
the spadix. A difference of I7-6°C. has even been observed in the case
of Arum italicum^ and G. Kraus3 obtained a rise of temperature of 27° C.
by grouping five spadices around a thermometer, and a rise of 35-9° C.,
when the whole was covered by a cloth. In the last case the temperature
of the air was 15-4° C., and of the spadix 5i-3°C., a temperature which
could hardly be sustained for any length of time without injury. A similar
rise of temperature was observed long ago by Huber4 in the spadix of
Arum cordifolium (Colocasia odora\ and apparently all spadices are able
to raise their temperatures to a greater or less extent.
Apparently this specially active production of heat is shown only during flower-
ing. In the case of Arum italicum and A. maculatum it begins during the. opening
of the spathe, increases for three or four hours, remains constant for the next one
or two hours, and then decreases to a minimum in the course of a few hours. In
other plants and aroids the rise of temperature is repeated at intervals, and the single
period of the two plants mentioned may begin at any time of the day, but usually
attains a maximum between 6 and 9 p.m., since the spathe commonly opens in the
afternoon or early evening.
Dutrochet observed a rise of temperature in the sterile portion of 8-2o°C., in the-
region of the male flowers of 4-9° C., and in the zone of female flowers of 1-4° C.6
1 Cf. Wilsing, Jahresb. d. Agrikulturchemie, 1884, p. 118.
a Lamarck, Flore frar^aise, 1778, T. in, p. 538; Senebier, Physiol. vegetale, 1800, T. in,
p. 314 ; Huber, Journal de physique, 1804, T. LIX, p. 281 ; Goppert, Ueber Warmeentwickelung i. d.
lebenden Pflanzen, 1832, p. 25 ; Vrolik and de Vriese, Ann. d. sci. nat., 1836, 2« sen, T. v, p. 142 ;
1839, 2e ser., T. xi, p. 77; van Beek and Bergsma, Observations thermo-e"lectriques sur l'e"levation de
la temper, des flours de Colocasia odora, 1838; Dutrochet, Ann. d. sci. nat., 1840, 2e ser.,T. xnr,
p. 65; Brongniart, Nouv. Ann. du Musee d'histoire nat., 1843, T. Ill, p. 153; Garreau, Ann. de
sci. nat, 1851, 3" ser., T. xvi, p. 255 ; Romer, Mittheil. d. naturwiss. Vereins von Neu-Vorpommern
u. Riigen, 1870, p. 51 ; Hoppe, Nova Acta d. Leopold. Carol. Akad., 1879-80, Bd. XLI, p. 199; G.
Kraus, Ueber die BHithenwarme bei Arum italicum, Bd. 1, 1882 ; Bd. II, 1884 (reprint from Abhandl.
d. naturf. Ges. zu Halle, Bd. xvi) ; Ann. du Jard. bot. de Buitenzorg, 1896, T. xin, p. 217 ; Passerini,
Nuov. giornale bot. italiano, 1901, T. vin, p. 64.
3 G. Kraus, 1. c., 1882, p. 12 ; 1884, P- 79-
4 Huber, 1. c. Cf. G. Kraus, 1. c., 1882, p. 12.
5 Kraus could detect no rise of temperature in the female flowers by means of a thermometer,
but Dutrochet succeeded in this by using a thermo-electric needle. Kraus denies the recurrence of
warming observed in Arum italicum by a few observers, and also shows that there is no evidence
to indicate whether the central or peripheral tissues produce most heat.
THE EVOLUTION OF HEAT BY AEROBES 373
Other aroids may show differences, and the fact that the maximum temperature is
not attained in all parts at the same time makes certain divergences in the observa-
tions of different workers comprehensible.
A rise of temperature of 5° or even of 10° C. has been observed in open
air on the inflorescences of a few Cycads l and Palms 2, as well as in the
flowers of Nelumbo nucifera 3 and Victoria regia 4. This applies more
especially to the stamens in the latter case, which in general appear to
become warmer than the carpels. Thus Saussure 5 observed a rise of tem-
perature of from 4° to 5° C. in the male flowers of Cucurbita, but of only
3°C. in the female ones. In open slender flowers and inflorescences the
rise of temperature is usually trifling, but is often more pronounced than
in the foliage-leaves. Flowers commonly respire relatively more actively
than foliage-leaves, and at the period of opening both the respiration and
the production of heat increase 6.
Vegetative organs rarely show any pronounced production of heat.
Thus Dutrochet 7 observed a maximal rise of temperature of 0-34° C. (shoots
of Euphorbia lathyris) under the most favourable conditions, while the
rise was usually below o-i°C. In many shoots, rhizomes, ripe fruits, and
other organs, no rise of temperature at all could be directly detected 8. The
fact that when heaped together all plant organs show a rise of temperature
shows that all living parts are able to produce heat. When transpiration
was allowed Dutrochet often observed a fall of temperature of 05° C. below
that of the surrounding air, while when the shoot was killed the fall was
at first still more pronounced, owing to the fact that the immediate effect
of death upon a suddenly-killed turgid organ is to hasten the rate of
transpiration.
In aerobes almost the whole of the heat is derived from aerobic
1 G. Kraus, Abhandl. der naturf. Ges. zu Halle, 1896, p. 218. The earlier observations are
quoted here.
3 G. Kraus, 1. c., 1896, p. 251.
3 K. Miyake, Physiological observations on Nelumbo nucifera^ 1898, p. 18 (reprint from the
Botanical Magazine, Tokyo, Vol. xn).
* Caspary, Flora, 1856, p. 218; E. Knoch, Bibliotheca botanica, 1899, Heft Ixxvii, p. 44.
Bory de St. Vincent (Journal de physique, 1804, p. 289) states that the flowers of Pandanus utilis
and of Cannaceae become in some cases warm enough to melt cocoa-butter.
5 Saussure, Ann. de chim. et de phys., 1822, Bd. xxi, p. 296. The temperature of these and
other flowers was measured by a kind of air-thermometer. A few observations on the flowers of
Cactus and Pancratium are given by C. H. Schulz, Die Natur d. lebendigen Pflanze, 1828, p. 185.
* Dutrochet (1. c., 1840, p. 81) observed a rise of temperature when the thermo-electric needle
was plunged in the ovary of the Rose, Papaver somniferum and Paeonia officinalis, when flower-
buds were examined in saturated air.
7 Dutrochet, Ann. d. sci. nat., 1840, 2e ser., T. xni, p. 44. Dutrochet and MacNab (Bot. Ztg.,
l873> P- S^o) give observations on Agaricus, Boletus ; and Lycoperdon.
* A slight rise of temperature was observed in tubers by Seignette, Rev.ge"n. de Bot., 1889, T. I,
P- 573- See also Dixon, Trans, of the Irish Academy, 1903, Vol. xxxil, iii, p. 145.
374 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
respiration, and Vrolik and de Vries, as well as other authors \ state that it
ceased when the plants were placed in nitrogen or hydrogen. The intra-
molecular respiration of aerobes sets free so little heat that special methods
are required to detect it2. Saussure3 indeed observed that the spadix of an
aroid absorbs oxygen most rapidly when it is producing most heat, and
that the spathe which barely warms at all consumes but little oxygen.
Saussure4, Dutrochet, and Wiesner5 have also shown
that in general the most active respiration occurs
during the period of most active heat-production, and
Bonnier's 6 quantitative estimations of the respiration
and heat-production of seedlings lead to the same
conclusion. An exact correspondence between the
curves showing the respiratory activity and the pro-
duction of heat is hardly to be expected, and the
divergences will be still greater between the respira-
tory curve and that showing the excess of tempera-
ture above the surrounding medium, since the amount
of excess is influenced by various factors.
The table given on p. 375 was compiled by Garreau from
observations upon Arum iialicum. The spadix was enclosed
in a narrow graduated cylinder, and the thermometer laid
against it surrounded by muslin (d). The inner walls of
the bell-jar were smeared with potash, the rise of water in
the bell-jar giving the consumption of oxygen. During the
first six hours, with an air temperature of i6°C., 470 c.c. of
oxygen were consumed, but only 300 during the following
eighteen hours, when the spadix was producing but little
heat. Kraus7 has shown that during this period of active
respiration the dry-weight may decrease by seventy-five per cent, in a few
hours.
FIG. 67. Garreau's appa-
ratus to show the relation
between the respiration and
the production of heat by
the spadix of Arum itali-
cum: (a) bell-jar, (b) ther-
mometer, (c) support for
spadix.
1 Vrolik u. de Vries, Ann. d. sci. nat., 1839, 2<! s^r-» T. XI, p. 79. A cessation of the produc-
tion of heat by the inflorescence of Colocasia odora was observed by Huber (Journal de physique,
1804, T. LIX, p. 284) after smearing it with oil, and similar observations were made by G. Kraus,
1. c., 1884, p. 60.
2 J. Eriksson, Unters. a. d. bot. Inst. zu Tiibingen, 1881, Bd. I, p. 105; G. Kraus, I.e., 1884,
p. 61.
3 Saussure, Ann. de chim.et de phys., 1822, T. xxi, p. 283. Dutrochet (I.e., 1840, p. 6) also
considered the heat to be produced by respiration. See also Garreau, Ann. d. sci. nat., 1851, 3* ser.,
T. xvi, p. 250.
4 Saussure, Memoires de Geneve, 1833, T. vi, pp. 251, 558.
5 Wiesner, Versuchsstationen, 1872, Bd. xv, p. 155.
6 Bonnier, Ann. d. sci. nat., 1893, 7e se'r., T. xvin, p. 33.
7 G. Kraus, Abhandl. der naturf. Ges. zu Halle, 1884, pp. 9, 67; 1. c., 1896, p. 271. See also
Knoch, 1. c., 1899, p. 52.
THE EVOLUTION OF HEAT BY AEROBES
375
Time.
Excess of temperature
of spadix.
Average excess
per hour.
Oxygen
consumed
in c.cm.
Oxygen consumed,
measured in volumes
of the spadix.
4 p.m.
2-5°C.
3-5 °C.
45
IO-O
5 P-m-
4-5 »
6.1 „
70
15-5
6 p.m.
7-7 »
8.6 „
95
21- 1
7 p.m.
9-5 »
JO-5 »
140
3T.I
8 p.m.
n-5 »
10-0 „
85
18.9
9 p.m.
8-5 »»
5-7 „
35
7-7
10 p.m.
3-o ,,
Presumably every influence acting on respiration will be reflected
in the production of heat, and it has in fact been observed that at lower
temperatures the excess of temperature over that of the surrounding air
decreases l. Bonnier also found that seedlings of Triticum produced per
kilogram per hour 2-T kilogram -calories at 15-8° C., and 0-18 kilogram-
calorie at 5-7° C. It appears further that the production of heat, like the
respiratory activity, rises with increasing temperature until death ensues ;
.and in fact thespadices of Aroids as well as masses of other plants may heat
themselves up to the fatal limit when heaped together and supplied with air 2.
No detailed research on the influence of the pressure of oxygen has
been carried out. Vrolik and de Vries, however, state that the temperature
of the inflorescence of Colocasia odor a rises, and J. Schmitz that that of the
buds of Aesculus hippocastanum does the same when the surrounding air
is replaced by oxygen 3. This is probably the result of an increase in
the activity of respiration.
Injuries increase the activity of respiration and also the production
of heat. Richards4 was able to show this by means of a thermometer
in plants massed together, and in single organs by thermo-electric means.
The feverish rise of temperature in the potato spread not more than 20 mm.
from the injury and in a particular case amounted to 0-05° C. 15 mm. away;
and to 0-21° C. immediately beneath the cut surface. An onion, on the other
hand, showed a rise of 0-28° C. beneath the cut surface and as much as 0-17° C.
at a distance of 45 mm. Injured onions when heaped together showed a
temperature higher by one or more degrees centigrade than uninjured ones.
Richards inserted thermo-electric needles into sound potatoes, and when the
1 Cf. Saussure, Me"moires de Geneve, 1833, T. vi, p. 251 ; J. Schmitz, Ueber die Eigenwarme d.
Pflanze, 187, p. 220.
2 Saussure (1. c., 1822, p. 298) stated that the production of heat in the flower of Cucurbita
decreases above 15 to 20° C., while Vrolik and de Vries (1. c., 1836, p. 140, cf. also Caspary, Flora,
1856, p. 219) observed that above 30° C. the spadix of Colocasia produces less heat. Possibly this was
partly due to a rise in the rate of transpiration and partly to some indirect action on respiration.
3 Vrolik and de Vries, 1. c.? p. 77 ; J. Schmitz, 1. c., p. 51.
4 Richards, Annals of Botany, 1897, Vol. xi, p. 29.
376 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
temperature became constant made an incision near one of the needles. In a par-
ticular case the temperature near to the fresh injury was higher than that at the point
of insertion of the other needle by 0-09° C. after 2 hours, 0-19° C. after 4^ hours,
0-31° C. after 8^ hours, 0-2 i°C. after 12^ hours, and 0-02° C. after 40 hours, while
towards the end of the fourth day the difference of temperature was imperceptible.
. The curves of respiration and heat-production are in this case very similar, the flatter
character of the latter curve being due to the rapid loss of heat by conduction and
radiation. The uninjured potato was 0-16° C. warmer than the surrounding air, so
that the temperature in the injured region underwent a twofold increase. The pro-
duction of heat depends upon the respiration, and hence on cutting an average potato
into quarters the production of heat increases approximately tenfold. The whole
increase takes place in the tissues immediately bordering the injury, so that these
must respire with remarkable activity.
Little is known as to the detailed course of the grand period of heat-
production and whether it exhibits secondary maxima or oscillations is
unknown. Daily variations of the excess of temperature do occur in
plants showing a marked production of heat, and Dutrochet observed slight
oscillations in shoots and fruits 1.
The existence of a daily periodicity in the warming of the spadix of Aroids has
been shown by Kraus and by the authors already quoted. Kraus has shown that the
same applies to the inflorescences of Cycads and Palms, and Knock to the flowers of
Victoria rcgia*.
Observations in the open seem to show a periodicity in the production of heat
independently of the air-temperature, and the same was shown under fairly constant
conditions by a plant of Colocasia odora (Arum cordifolium) kept in a room at 1 7° C. by
van Beek and Bergsma s. Thus in the selected zone of sterile male flowers the rise of
temperature above the surrounding air was io-6°C. at 2p.m., i4'7°C. at 5 p.m. of the
next day, 20-2° C. at 2.30 p.m. of the third day, and n-i°C. at 2 p.m. on the fourth
day. Each morning the excess-temperature lay between 1.300. and 5°C., it rose to<
a maximum during the day and fell to the morning temperature at evening.
The maximum may be earlier or later on some days than on others, and
although it usually occurs during the daytime may also appear early in the morning or
during the evening hours. The spadices of Arum maculatum and A. italicum show
only a single pronounced period of heat-production, but, according to Dutrochet 4, both,
before and after this a feebler daily periodicity is shown. Here, as in the case of most
of the shoots and fruits used by Dutrochet, the. excess temperature observed was
usually less than 0*3° C.5 The maximal excess temperature observed in air saturated
1 Dutrochet, Ann. de sci. nat., 1840, 2° se"r., T. xin, p. 41.
3 G. Kraus, Ann. du Jard. hot. de Buitenzorg, 1896, T. xin, p. 217; Knoch, ibid.
3 The irregularities observed by Hoppe (1. c., p. 239) in the rise of temperature in the spadix of
Arum were due to changes of temperature in the surrounding air. These may also exercise a stimu-
lating action, for Kraus (1. c., 1884, p. 52) found that the spadix is very sensitive to external agencies.
* Dutrochet, 1. c., 1840, p. 66. G. Kraus (1. c., 1884, p. 81 ; 1882, p. i) used an ordinary
thermometer, and hence was unable to detect any of these small oscillations.
5 According to J. Schmitz (Ueber die Eigenwarme der Pflanze, 1870, p. 20) the buds of Aesculu
hippocastanum show a daily periodicity of heat- production.
THE EVOLUTION OF HEAT BY AEROBES 377
with moisture was shown between 10 a.m. and 3 p.m., and usually became imper-
ceptible towards evening and remained so until morning.
Dutrochet has shown that the daily periodicity continues for a few days in
darkness with decreasing amplitude, and is reinduced on re-exposure to periodic
illumination, the other conditions remaining constant. This periodicity, therefore,
closely corresponds to the periodicity of growth and movement induced by the
intermittent daily illumination. It is, however, uncertain whether the periodicity of
heat-production in the spadices of Aroids is produced in this way or not, nor has it
been determined whether the rise of temperature may not be due to a decreased loss
of heat as well as to an increased production of heat. No daily periodicity in respira-
tion has yet been determined *, although in fleshy plants the daily accumulation of
organic acids in the tissues indicates a periodic diurnal alteration of respiration.
The spadices of Aroids have but little chlorophyll, and Hymenomycetes have
none, so that the daily periodicity of heat-production in these forms can hardly be
dependent upon photo-synthesis. Nor is it due to the fact that the lessened growth
in the daytime consumes less of the energy of respiration, for the spadices of
Aroids have ceased to grow when the production of heat is most active ; and a daily
periodicity of heat-production is shown, according to Dutrochet, by adult Cactus
stems 2. The rise of temperature is certainly not the direct result of the absorption
of heat from the radiant light-rays, although the latter by favouring transpiration
may cause an increased loss of heat, and hence lower the temperature. A periodicity
of temperature is also shown by man, the maximum at evening being about i«2°C.
higher than in the morning.
SECTION 82. The Production of Heat by Anaerobic Metabolism.
Anaerobic metabolism probably always involves a liberation of heat,
and alcoholic fermentation, in the absence of free oxygen, always produces
a distinct rise of temperature. No detailed researches on the production
of heat by anaerobic metabolism have been performed 3 ; and although
the heating of dung and of fermenting fluids is mainly due to anaerobic
metabolism, it is not impossible that anaerobic organisms may exist
whose normal metabolism involves an absorption of heat, just as that
of green plants involves an absorption of light. In such cases the
temperature of the plant will be continually below that of the surround-
ing medium. An absorption and extinction of the dark heat-rays does
actually occur in the purple bacteria, the energy of these rays being used in
photo-synthesis ; but in this case the supply of energy precedes the endo-
thermic chemical change, whereas in the other the endothermic chemical
change is supposed to take place first, the subsequent inflow of heat from
without following as a natural consequence of the fall of temperature 4. It
1 Cf. Kolkwitz, Jahrb. f. wiss. Bot., 1898, Bd. xxxni, p. 128.
2 G. Kraus, I.e., 1884, P- J7-
3 Popoff (Bot. Jahrb., 1875, p. 286) observed a slight warming during marsh-gas fermentation ;
Rubner, Hygienische Rundschau, 1903, Bd. xin, p. 753.
4 Cf. Pfeffer, Studien zur Energetik, 1892, p. 189.
378 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
is worthy of note that certain anaerobes are able to work with the utmost
economy. The anaerobic respiration of obligate aerobes is too feeble to
maintain life, but nevertheless it produces sufficient heat to raise the tem-
perature of plants massed together from 01 to 0-3° C. above that of similar
masses of dead plants, whereas in the presence of oxygen a difference of
temperature of from 5° to i6°C would be shown.
Eriksson1 in obtaining these results took ample precautions to ensure the
absence of all free oxygen, and also the absence of micro-organisms, by washing
and by rapid observation2. During alcoholic fermentation the fermenting liquid
may rise io°C. in temperature3. The decomposition of a gram-molecule (160
grams) of dextrose into alcohol and carbon dioxide yields 33 kilogram-calories,
i.e. sufficient heat to warm a litre of water from 4° C. to 37° C.4 The complete com-
bustion of dextrose yields, however, twenty times more heat, namely 673-7 kg.-calories.
During alcoholic fermentation traces of other substances are formed in addition to
alcohol and carbon dioxide. Hence the theoretical and the observed amount of
heat produced will not necessarily correspond. Bouffard found that 180 grams of
dextrose when fermented only produced 23-3 kg.-calories instead of the estimated
32-07, but this may possibly have been due to the difficulty of preventing the loss of
heat by radiation and conduction.
Alcohol and carbon dioxide are also produced during the intramolecular re-
spiration of aerobes ; but, since other decompositions occur simultaneously, no
theoretical calculation can be made of the amount of heat produced from the
quantity of alcohol and of carbon dioxide formed. It is possible, however, that
investigations of this kind may throw light upon the phenomena of intramolecular
respiration. The liberation of carbon dioxide from an oxidized compound may be
an endothermic change (decomposition of carbonates) or only a feebly exothermic
one (fermentation of alcohol), so that even when equal quantities of carbon dioxide
were produced the anaerobic production of heat would be considerably less than the
aerobic one, in which the process is practically one of complete combustion, and
the respiratory materials contain relatively little or no combined oxygen.
The decomposition of i gram-molecule of dextrose into 2 gram-molecules of
lactic acid liberates 14-7 kg.-calories, and its splitting into i gram-molecule
of butyric acid and 2 gram-molecules of hydrogen sets free 10-9 kg.-calories ;>.
1 Eriksson, Unters. a. d. hot. Inst. zu Tubingen, 1881, Bd. i, p. 105.
2 Pasteur (Compt. rend., 1872, Bd. LXXV, p. 1056, Etude s. la biere, 1876, p. 261) observed
a marked rise of temperature in fruits and fleshy roots in the absence of oxygen, but this was
probably diie to the development of anaerobic bacteria.
3 Cf. Dubrunfaut, Journ. f. pract. Chemie, 1856, Bd. LXIX, p. 444; Fitz, Ber. d. chem. Ges.,
l873« P- 57; Brefeld, Landw. Jahrb., 1876, Bd. v, p. 300; Eriksson, I.e.; Nageli, Theorie d.
Gahrung, 1879, P* 58 5 Bouffard, Compt. rend., 1895, T. cxxi, p. 136.
4 Cf. Bouffard, 1. c. ; E. Duclaux, Traite de Microbiologie, 1898, Bd. n, pp. 77, 739. The
heats of solution are allowed for in the above value, but otherwise it would be reduced to
22.3 kg.-calories, the difference between the heat of combustion of dextrose (i gram-molecule =
673*7 kg.-cal.) and of alcohol (2 gram-molecules = 651-4 kg.-cal.).
5 Cf. R. O. Hertzog, Zeitschr. f. physiol. Chemie, 1903, Bd. xxxvn, p. 383, and textbooks of
Physical Chemistry.
THE PRODUCTION OF HEAT BY ANAEROBIC METABOLISM 379
Hence the lactic and butyric fermentations yield heat, although the observed amount
is less than that theoretically calculated unless the heats of solution are taken into
.account. In addition the by-products of fermentation will influence the liberation
of heat according to their character and properties. Most metabolic processes are
attended by a trifling production of heat, and in fact it is even possible that metabolism
may in some cases be attended by an absorption of heat \
SECTION 83. The Temperature of the Plant under Normal Conditions.
External and internal radiation, the conduction and production of
heat, the temperature of the surrounding medium, and the activity of
transpiration are among the factors regulating the temperature of the
plant, and they do not affect the different organs of the plant alike.
Hence the temperature of a root or of a shaded organ is usually different
to that of the stem or of an insolated organ. In such cases a slow
transference of heat may occur from the hotter to the colder organ by
conduction, or by convection or transpiration currents of water.
Small or slender organs rapidly assume the temperature of the surround-
ing medium, but hours may elapse before the full effect of a change of
temperature in the external medium is shown at the centre of a tree-
trunk or of a large tuber. Neighbouring regions may indeed be at widely
different temperatures, if one part is insolated but the other not, or if
one part projects above water but the other is submerged. Plants are
able to grow in spite of these local and general variations of temperature,
if they are not too pronounced.
It must be remembered that under constant external conditions the
activity of transpiration may undergo autogenic modification, and that
its cooling effect will alter correspondingly. Gaseous exchanges may be
modified in the same way, but these have a much feebler influence upon the
body-temperature. Fleshy objects often become hotter in sunlight than the
exposed bulb of a thermometer. Thus Askenasy 2 observed a temperature
of 52° C. when the thermometer-bulb was inserted between the resetted
leaves of Sempervivum alpinum (shade temperature = a8-i°C.), whereas
a thermometer pressed against the thinner leaves of Gentiana cruciata
or between the tufted leaves of Aubrietia deltoidea showed a temperature
1 Cf. Pfeffer, Studien zur Energetik, 1892, p. 189. Nageli incorrectly supposed that all enzyme
action was accompanied by an absorption of heat, and proposed to make this a distinction from
* vital ' fermentation accompanied by a production of heat. Cf. Hertzog, 1. c.
3 Askenasy, Bot. Ztg., 1875, P- 441- Cf- also Haberlandt, Sitzungsb. d. Wiener Akad., 1892,
Bd. ci, Abth. i, p. 787 ; Passerini, Nuovo giornale bot. italiano, 1901, vol. VIII, p. 69. Rameaux
(Ann. sci. nat., 1843, 2* s^r-> T- XIX> P- 2I) observed 33° C. registered by a thermometer whose bulb
was inserted in a thin insolated branch, whereas with the bulb in sunlight 24° C. was shown.
Becquerel (Compt. rend., 1858, T. XLVII, p. 717) observed 37° C. at the centre of a thick stem
exposed to sunlight. For the older literature see Goppert, Die Warmeentwickelung i. d. Pflanze,
1830.
380 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
of 35° C. In the tropics fleshy leaves are quite commonly raised above
50° C. by prolonged insolation, and may be injuriously or even fatally
affected, for the cooling effect of the attendant rise of transpiration is limited
by the supply of water. Hence arises the common practice in tropical
plantations, especially when young (Coffee, Cocoa, &c.), of planting rapidly
growing shade-trees at intervals to ward off the midday sun ; and the
same effect is produced by the usual mode of planting Vanilla, whose
fleshy leaves are still more sensitive than those of such a plant as Hoya
carnosa^. Thin leaves, on the other hand, have relatively more surface
for radiation and transpiration, and in addition their gaseous exchanges
are much more rapid than those of fleshy leaves.
Colour, hairiness, and position are all factors of considerable importance
in determining the rise of temperature due to insolation. Green and other
coloured leaves may absorb from fifty to ninety per cent, of the sun's rays
falling upon them, while when a leaf places its lamina at right angles
to the incident rays the absorption of heat is naturally greatly increased.
A thick covering of hairs largely intercepts the incident rays, and only
a small fraction of the heat absorbed by the hairs is transferred by con-
duction to the body of the leaf. The excretion of ethereal oils by lowering
the diathermanicity of the surrounding air may help to cut off the heating
effect of the sun's rays to a certain extent, but the amounts excreted even
by the most active plant are not sufficient to have much effect, in spite
of the extreme efficiency of these vapours for that purpose 2.
Cork and bark are bad conductors of heat, but a thin layer of cork
can be penetrated by sufficient radiant rays to raise the temperature of
a young branch considerably. Even when a thick layer of cork is present
the heat conducted inwards may make the sunny side of a tree 20° C.
warmer than the shaded one, and the centre of a tree exposed for some
time may rise to over 40° C. in temperature 3.
Contact with cold water naturally removes heat more rapidly than
contact with equally cold air, and in both cases the movement of the
medium accelerates the loss of heat, to which the effect of air-currents
in accelerating transpiration is to be added. Similarly, during cold clear
nights the uninterrupted radiation makes the plant colder than when the
sky is covered by clouds which hinder radiation. The temperature among
the grass of a meadow may be 6° or 8° C. lower than that of the air above
during night-time 4, and in fact it is even possible for plants to be killed by
1 Cf. Ewart, On the Effects of Tropical Insolation, Annals of Botany, 1897, Vol. xi, p. 444.
8 Detto, Flora, 1903, p. 161 ; Volkens, Sitzungsb. d. Berlin. Akad., 1886, p. 78.
3 Cf. Ihne, Bot. Centralbl., 1883, Bd. xv, p. 231 ; Miiller-Thurgau, Landw. Jahrb., 1886, Bd. xv,
p. 531 ; R. Hartig, Forstl. naturwiss. Zeitschrift, i892,Heft iii, pp. 10, 12 ; Prinz, Bot. Jahresb., 1894,
Bd. I, p. 226; Biisgen, Bau tmd Leben d. Waldbaume, 1897.
* Boussingault, Agronom., China, agricoleet Physiol., 1861, T. n, p. 380; Tyndall, Fragments of
Science, 1879, Vo1- x» P- 9° 5 Muller-Thurgau, 1. c., 1886, p. 557 ; Th. Hormen, Bot. Ztg., 1894, p. 277.
TEMPERATURE OF PLANT UNDER NORMAL CONDITIONS 381
frost when a thermometer in air does not sink below zero. This fall of
temperature induces the deposition of dew upon the plant, the latent heat
of the condensed vapour exercising a pronounced warming action 1.
The water ascending the trunk of a tree usually exercises a more or
less pronounced cooling effect, according to the rate of ascent and the
coldness of the water. Rameatix 2 found that the centre of a transpiring
tree-trunk was io°C. colder than that of a non -transpiring dead tree when
both were exposed to the sun, but when the branches were cut off the living
tree the temperature in both trunks became approximately the same.
Convection currents within the cells will aid in transferring heat
upwards in elongated cells, but not downwards, and it is owing to the
conduction through the elongated wood-elements that heat is able to pass
more rapidly longitudinally than transversely through wood 3. During winter
the centre of a stem is usually warmer than the surrounding air, owing
to the upward conduction of heat from the warmer water in the soil,
coupled with the action of the cork jacket in retaining heat, and the absence
of transpiration. In the higher portions of the stem this heating effect
is slight owing to the poor conduction of heat by the wood, while in the
smaller branches it is negligible. It is partly owing to the slow inward
conduction of heat that Hartig4 observed that the maximal temperature
was reached 4 cm. deep in an oak stem at 6 p.m., and 20 cms. deep not
until towards midnight after a day's insolation.
The thinner plant-organs are subjected to greater extremes of tempera-
ture than the centre of a thick stem, which responds but slowly to changes of
temperature ; but the latter is subject to greater daily and yearly variations
of temperature than the root-system, owing to the more constant tempera-
ture of the soil 5. Hartig found, for instance, that the interior of a tree-
trunk sank to — i3°C. during a winter when the air was frequently at
— I5°C. to — 22° C., in spite of the upward flow of heat from the warmer
roots.
1 Cf. Jamin, Naturforscher, 1879, p. 140; Wollny, Forschung. a. d. Gebiete d. Agricultur-
physik, 1892, Bd. xv.
9 Rameaux, I.e., p. 23. Hartig observed (Bot. Jahresb., 1874, p. 760) that the temperature in
the interior of a stem sinks when the buds unfold and transpiration becomes active.
3 Researches on the conductivity of wood to heat were carried out by de Candolle, Ann. d.
Physik u. Chemie, 1828, Bd. xiv, p. 590; Knoblauch (ibid., 1858, Bd. cv, p. 623) ; Wiesner, Die
Rohstoffe des Pflanzenreichs, 1873, p. 292 ; Sowinsky, Bot. Jahresb., 1875, p. 773. Sowinsky found
the ratios between the transverse and longitudinal conductivities of wood to be as i : 1-15 (Quercus
robur) and i : 1.43 (Carpinus betulus}. Sowinsky found that some woods conducted better when
dry, others when moist.
* Hartig, Bot. Jahresb., 1873, p. 508. See also Goppert, Die Warmeentwickelung i. d. Pflanzen,
1830, p. 160. Cf. also Miiller-Thurgau, 1. c. ; Ihne, 1. c.
5 On the temperature of subterranean tubers see Seignette, Rev. ge"n. de Bot., 1889, T. i, p. 573.
382 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
PART II
THE PRODUCTION OF LIGHT
SECTION 84. Instances and Causes of Luminosity.
Like many animals1, certain plants are self-luminous, such as many
Bacteria, Fungi, and the chlorophyllous Ceratium tripos 2. Among Fungi,
it is usually the fruit body of Hymenomycetes or Ascomycetes that is
luminous, but the mycelium may also be luminous and cause, for instance,
the luminosity of wood attacked by Fungi. The so-called phosphorescence
of fish and of meat is due to the activity of Bacteria.
The glow is usually feeble like that of moist phosphorus in darkness,
but Gardner states that in Brazil a few specimens ofAgaricus Gardner* gave
out sufficient light to read print 3. The rays from luminous Bacteria are
able to produce heliotropic curvatures or even the production of chloro-
phyll 4, and to enable the organisms to photograph themselves on a sensitive
plate 5.
Bacteria. Pfluger showed that the phosphorescence of meat was due to Bacteria,
and Bacterium phosphorescens, B. Pfliigeri, B. indicum, and B. luminosum are all
strong!/ luminous species6. Certain forms are always present in sea-water, and
hence the readiness with which moist fish, or moistened smoked haddock, becomes
luminous when hung up in a fairly cool room. In the same way a piece of flesh
partially immersed in a saline solution will usually become luminous owing to the
1 Cf. Dubois, Le?ons de Physiol., 1898, p. 301 ; Verworn, Allgem. Physiol., 3. Anfl., 1901,
p. 263; de Kerville, Die leuchtenden Thiere u. Pflanzen, German translation by Marshall, 1893.
2 J. Reinke, Wiss. Meeresunters. d. deutschen Meere, 1898, N. F., Bd. in, p. 39.
3 Gardner, Flora, 1847, p. 756. Good cultures of phosphorescent bacteria give out sufficient
light to enable one to tell the time by a watch at night.
4 Molisch, Sitzungsb. d. Wiener Akad., 1902, Bd. cxi, Abth. i, p. 141 ; Isatschenko, Chloro-
phyllbildung im Bacterienlicht, Centralbl. f. Bact., 1903, Abth. ii, Bd. x, p. 498.
5 Forster, Centralbl. f. Bact., 1887, Bd. II, p. 338; B. Fischer, ibid., 1888, Bd. in, p. 140;
Molisch, Sitzungsb. d. Wiener Akad., 1903, Abth. i, Bd. cxir, p. 297.
6 Pfliiger, Archiv f. Physiol., 1875, Bd- x» P- 2755 Bd- XI> P- 223- cf- Molisch, Bot. Ztg.,
Orig., 1903, p. i. For nomenclature see Migula, System d. Bacterien, 1897, Bd. I, p. 336;
B. Fischer, Zeitschr. f. Hygiene, 1887, Bd. n, p. 54; Centralbl. f. Bact., 1888, Bd. in, pp. 105, 137 ;
1888, Bd. IV, p. 89; Beyerinck, Archives Neerlandaises, 1889, T. xxm, pp. 104, 367, 416; 1891,
Bd. XXIV, p. 369 ; Koninklijke Akad. v. Wetenschappen te Amsterdam, Proceedings of the Meeting,
27. Oktob., 1900, p. 359; Lehmann, Centralbl. f. Bact., 1889, Bd. v, p. 785 ; Kutscher, ibid., 1890,
Bd. VIII, p. 124; Katz, ibid., 1891, Bd. IX, p. 157; C. Eijkmann, ibid., 1892, Bd. xil, p. 656;
Suchsland, ibid., 2. Abth., 1898, Bd. IV, p. 713 ; Tarchanoff, Compt. rend., 1900, T. cxxxi, p. 246 ;
McKenney, Obs. on the cond. of light production in Bacteria, 1902, reprint from Proc. of the Biol.
Soc. of Washington, Vol. xv, p. 213 ; Barnard and Macfadyen, Annals of Botany, 1902, Vol. XVI,
p. 387. A summary is given by Migula, System der Bacterien, 1897, Bd. I, p. 336 ; Fliigge, Mikro-
organismen, 3. Aufl., 1896, Ed. I, p. 166.
INSTANCES AND CAUSES OF LUMINOSITY 383
wide distribution of the germs1. The germs grow well in a decoction of fish to
which one or two per cent, of peptone, of sodium and magnesium chlorides, and if
necessary of glucose, have been added. By the addition of gelatine or agar solid
media may be produced.
Meyen observed luminous masses containing numerous colourless Oscillaria
filaments in the Atlantic, and Ehrenberg states that the Diatoms Chaetoceras and
Discoplea are self-luminous2, but it is possible that the light was produced by
adherent Bacteria3.
Fungi. Retzius and von Humboldt 4 showed that the long-known luminosity
of wood was due to parasitic Fungi. The sclerotium (Rhizomorphd) of Agaricus
melleus, and the finer mycelium of Xylaria hypoxylon are commonly responsible for the
peculiarity. It is usually sufficient to keep wood destroyed by Fungi, especially that
of the root, in a damp chamber for it to become luminous5. The mycelium of
Agaricus melleus when grown in a fluid nutrient medium gives out a considerable
amount of light6, and the mycelia and fruit bodies of both Ascomycetes and of
Hymenomycetes may become luminous when grown on artificial media.
The gill lamellae of Agaricus olearius 7 which grow on old olives in S. Europe,
phosphoresce strongly, as does also the remainder of the sporophore, but less
strongly. In the tropics many forms seem to be strongly luminous, such as
Agaricus Gardneri* (Brazil), A. igneus* (Amboina), A. noctilucens™ (Manila).
The older observations upon the production of flashes of light by leaves, flowers,
and so forth are probably the result of optical illusions11, but the St. Elmo's fire
produced by electrical radiation may occur on plants. According to Mornay and
Martius, certain Euphorbias have luminous latex, the latter possibly undergoing
oxidatory photo-chemical changes on exposure to air, or becoming impregnated with
I Molisch, Bot. Ztg., 1903, p. 17, always found Micrococcus phosphoreus (Syn. == Bac t. phospho-
rescens Beyerinck).
3 Meyen, Physiol., 1838, Bd. n, p. 202. Cf. Ludwig, Centralbl. f. Bact., 1887, Bd. n, p. 402 ;
Ehrenberg, Die das Funkeln u. Aufblitzen des Mittelmeeres bevvirkenden kleinen Lebensformen,
1874, p. 3 (reprint from Festschr. d. Ges. naturf. Freunde zu Berlin).
3 Dubois (Le9ons de Physiologic, 1898, p. 451) has given up his earlier statement that the
phosphorescence of Pholas dactylus (the rock-boring mollusc) was due to a symbiotic Bacterium.
4 See Agardh, Allgem. Biol. d. Pflanzen, 1832, p. 179; de Candolle, Pflanzenphysiologie, 1835,
Bd. II, p. 680, footnote; P. Heinrich, Phosphorescenz der Korper, 1811.
5 For literature and facts see Ludwig, Ueber d. Phosphorescenz d. Pilze u. d. Holzes, 1874 J
Lehrbuch d. niederen Cryptogamen, 1892, p. 525. Hitherto no luminous bacteria have been found
to cause the luminosity of wood.
6 Brefeld, Bot. Unters. ii. Schimmelpilze, 1877, Heft iii, p. 170.
7 Fabre, Ann. sci. nat., 1855, 4e ser., T. iv, p. 179 ; Tulasne, ibid., 1848, 3° se>., T. IX, p. 541.
Cf. also Ludwig, I.e., 1874, p. 9. The light is evolved before the development of the hymenium,
and it ceases before the collapse of the tissues. Cut surfaces may also be luminous.
8 Gardner, quoted by Ludwig, 1. c., 1874, p. 9.
9 Rumph, Herbarium amboinense, 1750, Bd. vi, p. 130.
10 Gaudichaud, quoted by Ludwig, I.e., 1874, P- 9-
II For literature see Fries, Flora, 1859, P- J78 5 Meyen, Pflanzenphysiologie, 1838, Bd. u, p. 200 ;
Ludwig, I.e., 1874, p. 5; Crie", Compt. rend., 1881, Bd. xcrii, p. 853; Ascherson, Naturwiss.
Wochenschrift, 1901, p. 106. Senebier (Physiol. vegetal., 1800, T. in, p. 315) states that the
spadix of Arum maciilatum phosphoresces when placed in oxygen.
384 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
luminous Bacteria \ There is also always a possibility that the phenomenon is one
of fluorescence, light rays absorbed during the daytime being emitted at night. The
glimmering of the protonema of Schizostega, on the other hand, is simply due to the
collection of the feeble rays of light by the lens-shaped cells of the protonema 2.
Uses. Although it is possible that the luminous sporophore of a Fungus may
attract night-flying insects, and so aid in the dissemination of the spores, it is not
easy to see what use the luminosity of Bacteria or of a mycelium in wood could have.
The presence of luminous Bacteria on a dead fish may attract fishes which swallow
the Copepoda and other forms which devour the Bacteria, and in the dark depths of
the ocean the luminous properties of an organism acquire greater importance.
The production of light is a physiological process dependent upon
respiration, and, like the latter, it continues in darkness 3. We are dealing,
therefore, with a chemical production of light 4, and not with a fluorescent
emission of light rays previously absorbed. The production of light usually
begins in Fungi at a certain stage of development and then spreads to all
parts. Similarly, McKenney states that Bacteria become luminous only
at the end of the period of active locomotion. Under unfavourable conditions
the luminosity vanishes and it attains an optimum under definite conditions
as regards temperature, concentration, and food-supply. Luminosity, like
growth, decreases above the optimum temperature, and does not like re-
spiration and the production of heat increase up to the maximum
temperature.
Luminosity, like locomotion and the production of pigments or
poisons, may be suppressed without fatal injury to the organism. Many
luminous forms have been grown at temperatures at which they produce
no light, and Beyerinck 5 has in fact found that certain forms appear to
become temporarily luminous under special conditions.
When the conditions are favourable the light is emitted continuously,
1 For literature see Meyen, 1. c., p. 203.
a Unger, Flora, 1834, p. 33; Noll, Arbeit, d. hot. Inst. in Wiirzburg, 1888, Bd. ill, p. 477.
Ingenhousz (Versuche mit Pflanzen, German ed. by Scherer, 1786, Bd. I, p. 191) observed that the
vapours of ethereal oil excreted by the inflorescence of Dictamnus albus burst into flame when
a lighted match was brought near.
3 Moderate light appears to exercise no effect upon the luminosity of Rhizomorpha. Cf.
Ludwig, I.e., 1874, p. 26. Pfliiger and also McKenney (I.e., p. 222) obtained similar results with
Bacteria, but strong light, owing to its germicidal action, retards or inhibits the appearance of
luminosity. Cf. Tarchanoff, Compt. rend., 1900, T. cxxxi, p. 247; Suchsland, Centralbl. f. Bact.,
2. Abth., 1898, Bd. iv, p. 714.
4 Wiedemann, Ann. d. Physik u. Chem., 1889, N. F., Bd. xxxvn, p. 180; 1889, N. F.,
Bd. xxxvin, p. 485 ; Wiedemann und Schmidt, Zeitschr. f. physik. Chemie, 1895, Bd. xvill, p. 528 ;
Roloff, ibid., 1898, Bd. xxvi, p. 354; Winkelmann, Handbuch d. Physik, 1894, Bd. n, Abth. i,
p. 486. [Sudden crystallization may cause a liquid to glow with light, as, for instance, when salt is
precipitated in darkness by adding alcohol or concentrated hydrochloric acid to strong brine. This
is, however, hardly likely to be responsible for any appreciable production of light in the living
plant.]
5 Beyerinck, Koninklijke Akad. v. Wetenschappen te Amsterdam, Oct. 1900, p. 359.
INSTANCES AND CAUSES OF LUMINOSITY 385
no special stimulation being necessary as it is in the case of Noctiluca l. In
this organism and in Ceratium tripos the luminosity appears as the result
of a shock-stimulus. It is possible that sudden changes of temperature or
of concentration may temporarily increase or diminish the production of
light in Fungi and Bacteria. These changes are usually but slight in
amount, the organism rapidly adjusting itself to the new conditions. In
some cases spontaneous increases and decreases in the intensity of the
illumination are shown, but the causes of these are unknown.
Certain resistant organisms may continue to produce light at tem-
peratures or in concentrations which ultimately cause a cessation of the
luminosity or even death. In some cases gradual accommodation is possible,
so that the organism or its descendants become luminous at temperatures
which at first inhibited the production of light. Since variations in the
production of light are readily perceptible, they may be used as indications
of the vital activity upon which they are dependent. Beyerinck has in
fact used luminous Bacteria as a test for the evolution of oxygen, and by
means of his auxanographic method has determined the value of different
nutrient materials or of metabolic products for the production of light.
Owing to the after-effects already mentioned and to other physiological
peculiarities care is, however, needed in interpreting the results.
The influence of temperature. All observers agree as to the existence of an
optimum temperature for luminosity. The optimum lies between 25° and 30° C. in
the case of Rhizomorpha 2, the minimum between i° and 3° C. Similarly only approxi-
mate values have been obtained for Bacteria, and the divergences between the results
of different authors are due partly to incorrect naming, and partly to the influence of
dissimilar nutrient and cultural conditions 8. Various Bacteria are still luminous at
o° C. to 5° C., whereas McKenney found that Photobacterium indicum (Beyerinck), the
Bacillus phosphorescent of B. Fischer, ceases to emit light at 1 5° C., the optimum lying
between 22° to 28° C., the maximum between 30° and 35° C. In the case of Photo-
lacterium (Microspird) luminosum the cardinal temperatures are io°C., i5°C., and
22°C. respectively. Here and in other cases also the maximum temperature for the
production of light lay 5° to io°C. below that for growth. McKenney found that
the minimum temperatures for growth and for luminosity were the same, but the
results of other observers show that this is not always the case.
Various workers have observed that light continues to be given off for a certain
time after the luminous organism has been cooled below zero or even to —12° C.4
1 Biitschli, Protozoen, 1883-7, 2- Abth., p. 1088; Kruckenberg, Centralbl. f. Physiologic, 1887,
Bd. I, p. 689; Massart, Bull, scientifiqne de la France et de la Belgique, 1893, T. xxv, p. 76.
3 Ludwig, 1. c., p. 35 ; Brefeld, 1. c., p. 4. Wood has been observed to emit light at o° C. by the
older observers. Cf. Ludwig, I.e., p. 25. Fabre (l.c., p. 187) finds that Agaricus olearius emits
light only above 3° or 4° C.
3 B.Fischer, I.e., 1887, p. 78; 1888, pp. 89, 139; Lehmann, I.e., 1889, p. 789; Beyerinck,
1. c., 1891, pp. 8, 66; Eijkmann, I.e., 1892, p. 656; McKenney, I.e., p. 219.
* B. Fischer, 1. c. ; Lehmann, 1. c. ; Tarchanoff, 1. c., p. 247. Suchsland (1. c., p. 80) found that
after cooling to — 80° C. the luminosity of certain resistant Bacteria returned on warming.
PFEFFER. IH Q £
386 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
McKenney, however, observed that the luminosity rapidly disappeared below the
minimum temperature and above the maximum, while the continued cultivation of
Photobacterium indicum at the highest possible temperature raised the maximum for
the production of light from 30° to 35°C. According to the same author sudden
changes produce a shock-effect on the luminosity, whereas other authors have observed
slight transitory disturbances of the luminosity to result, especially in bacteria.
According to Ludwig1, Rhizomorpha becomes temporarily non-luminous when
suddenly cooled from 40° to io°C.
Chemical effects. .Insufficient nutriment naturally produces a cessation of the
luminosity more or less rapidly, but the presence of ether or alcohol, as well as
changes of composition or concentration of the medium, may allow the Bacteria to
grow but not become luminous2. Thus in all forms examined hitherto, not only
must organic food be supplied, but also inorganic salts. Thus McKenney 3 found
that when sodium chloride, sodium nitrate or other salts of sodium, or magnesium
chloride formed the only salt present, growth and luminosity were both shown, but
that both were suppressed when only a single salt of potassium, rubidium, lithium,
ammonium, or calcium was present. The addition of magnesium chloride to the
sodium chloride seems to favour luminosity, and hence the ready growth of these
organisms in sea-water. The amount of salt may vary between i and 4 per cent,
without growth and the evolution of light being perceptibly affected.
All luminous Bacteria appear to require peptone, while Photobacterium phospho-
rescens and P. Pflilgeri seem also to need a suitable carbohydrate, although for
Photobacterium luminosum and P. indicum peptone alone suffices. The presence of
a large amount of glucose diminishes the luminosity, and Photobacterium luminosum
is so sensitive that it ceases to be luminous in the presence of i per cent., and to
grow in the presence of 3 to 5 per cent, of glucose.
Beyerinck worked largely by the auxanographic method, and the slight divergences
between his results and those of McKenney are probably the result of dissimilar
cultural conditions. In all cases a slight acidity or a somewhat stronger alkalinity is
sufficient to inhibit luminosity, and subsequently growth also. Hence at the electrodes
in an electrolysed medium containing luminous Bacteria no luminosity is shown if the
acid and alkali are set free at the anode and kathode in sufficient amount 4. Since
McKenney found that the luminosity is only shown after movement has ceased, and
since it is possible by maintaining the original composition of the medium to keep
the organisms permanently motile, it would presumably be possible to grow them as
non-luminous forms 5.
The production of light is dependent upon aerobic respiration and ceases
in the absence of oxygen. This applies not only to aerobic fungi6 and
1 Ludwig, 1. c., p. 25.
a McKenney, 1. c., p. 223; Tarchanoff, I.e., p. 247. Cf. also the works quoted of B. Fischer,
Beyerinck, Lehmann, and Katz.
3 McKenney, 1. c., p. 226. 4 Suchsland, 1. c., 1898, p. 715. 5 Cf. McKenney, 1. c., p. 229.
6 Fabre, Ann. sci. nat., 1855, 4* sen, T. iv, p. 190; Nees von Esenbeck, Noggerath u. Bischoff,
Nova Acta d. Leopold. Acad., 1823, Bd. xi, Th. ii, pp. 667, 694. Boyle showed that oxygen was
necessary for the luminescence of wood. Cf. Dessaignes, Journ. de physique et de chimie, 1809,
T. LIX, p. 29, and Heinrich, Die Phosphorescenz d. Korper, 1811, p. 334.
INSTANCES AND CAUSES OF LUMINOSITY 387
Bacteria ' but also to the facultatively anaerobic Bacterium phosphor escens, Beyerinck,
which is able to develop but not to luminesce in the absence of oxygen. It is,
however, quite possible that facultative anaerobes may exist which are capable of
emitting light in the absence of oxygen.
The luminescence is decreased or suppressed when the partial pressure of the
oxygen is much increased or diminished, but no definite numerical results have been
obtained. According to Lehmann2, however, compressed air under a pressure of
six atmospheres, or pure oxygen under a pressure of an atmosphere, exerts no
perceptible effect upon the luminescence of meat or wood, whereas Fabre3 finds
that the emission of light by Agaricus olearius increases in pure oxygen. The fact
that the luminescence of certain Bacteria only gradually disappears in the absence of
oxygen does not afford satisfactory evidence that these organisms store up occluded
oxygen. •
The emission of light is not the result of intense respiration, for the
latter continually increases up to the maximal temperature, whereas the
former rapidly ceases above a rather lower optimum temperature. In
addition the luminous Fungi and Bacteria do not respire with especial
activity 4, while the spadix of an Aroid evolves no light during its most
active period of respiration and heat-production. Luminous organisms may
indeed evolve light when their production of heat is so slight that their
temperature is below that of the surrounding medium.
Certain substances evolve light during slow oxidation without any
perceptible production of heat 5, and hence it is possible that during either
the metabolism, or more especially the respiratory katabolism of luminous
organisms, materials may be produced whose slow oxidation gives rise to
light. According to Dubois 6, two substances, luciferin and luciferase, may
be isolated from Pholas dactylus. These evolve light when brought into
contact and therefore presumably are responsible for the emission of light by
1 Pfliiger, I.e., p. 223; B. Fischer, I.e., 1887, p. 37; Lehmann, I.e., 1889, p. 788; Beyerinck,
I.e., 1889; Katz, I.e., 1891, p. 314; Eijkmann, I.e., 1892, p. 657.
3 K. B. Lehmann, Einfluss des comprimirten Sauerstoffs auf d. Lebensprocesse, Zurich, 1883,
p. 87. [Dessaignes (1. c., p. 29) also observed no increased luminosity of wood in pure oxygen,
whereas Nees, Noggerath, and Bischoff (1. c., p. 693) state that it increased ; and Heinrich (1. c.,
p. 332) found that it increased in air at a pressure of two atmospheres, but not in pure oxygen.
These varying results are probably due to the influence of fatigue and of accommodation upon the
visual judgement of the intensity of a feeble source of illumination, a striking instance of which is
afforded by the statements of different observers in regard to Blondhlot's ' n rays.']
3 Fabre, I.e., p. 191.
* Fabre (I.e., p. 193) found that Agaricus olearius respired most actively during the luminous
condition.
5 Radziszewski (Ann. d. Chemie, 1880, Bd. CCIH, p. 330; Ber. d. chem. Ges., 1877, p. 321;
l883> P- 597) states that lophin dissolved in alkali, and liver oil dissolved in toluol containing a few
drops of cholin or neurin solution, emits light at as low a temperature as io°C. Dubois (Compt.
rend., 1901, T. cxxxu, p. 431) has shown that aesculin dissolved in alcoholic potash phosphoresces.
6 Dubois, Lefons de Physiologic, 1898, p. 524 ; Compt. rend., 1896, T. cxxm, p. 653. Dubois
formerly had expressed the opinion that the emission of light was produced by the conversion of
colloids into crystalloids.
C C 2
388 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
this organism. Even in this case the production, interaction, and oxidation
of these substances are processes of which the former is more especially
under physiological control, and so long as the substances in question are
kept separate no light would be produced. According to whether the
contact took place inside or outside the cell, we should have an intracellular
or extracellular production of light l. It is uncertain to what degree the
latter occurs, and it must be remembered that the emission of light by
excreted slime or mucilage may be due to the presence of luminous Bacteria.
Whatever the physiological action may be, we have in each case a pro-
duction of light by a transformation of chemical energy not involving any
appreciable production of heat 2. Hence light is produced here much more
economically than when a body is heated to incandescence by physical or
chemical action s. The actual expenditure of energy by the organism in
producing the luminous substance is uncertain, but it is of interest to note
that energy may be radiated from a cold body to a warmer one which
absorbs the emitted light4.
The composition 6 of the light is often that of white light, but in many
cases the light has a bluish or greenish tinge. The composition appears,
however, to vary according to the cultural conditions, as is shown by
spectroscopic examination. No rays resembling the Rontgen or Becquerel
rays appear to be present <6.
PART III
THE PRODUCTION OF ELECTRICAL TENSIONS IN THE PLANT
SECTION 85. The Origin and Detection of Electro-motive Changes.
No plants are able like electrical fishes to give perceptible electrical
shocks, but nevertheless slight differences of potential capable of maintaining
1 Noctiluca phosphoresces internally, especially at certain points, and this continues for a little
time after the organism has been crushed. Biitschli, Protozoen, 1883-7, Abth. ii, p. 1092. Whether
the luminescence of wood is produced in the fungal hyphae or outside of them is uncertain. Cf.
Ludwig, Lehrb. d. niederen Cryptogamen, 1892, p. 530. Lehmann (1. c., 1889, p. 789) and Beyerinck
(I.e., 1891, p. 52) are wrong in supposing that the cessation of the light on death disproves the
existence of a special luminous substance. Beyerinck's supposition that the production of light is
connected with the assimilation of peptone has no sure foundation.
2 [The light might still have a purely physical origin in certain cases without involving any
production of special luminous substances. When present these might undergo radiatory atomic
disintegration, or might shorten the wave-length of the heat vibrations due to respiration sufficiently
to produce visible light rays.]
8 Langley u. Very, Beibl. z. d. Ann. d. Physik u. Chemie, 1890, Bd. xiv, p. 1096; Dubois,
I.e., 1898, p. 376.
* Wiedemann, Ann. d. Physik u. Chem., 1889, N. F., Bd. xxxvin, p. 485.
5 Ludwig, Zeitschr. f. wiss. Mikroskopie, 1884, Bd. i, p. 181 ; 1. c., 1892, pp. 78, 537 ; Lehmann,
Centralbl. f. Bact., 1889, Bd. v, p. 787 ; Dubois, 1. c., 1898, p. 510.
6 Suchsland, I.e., 1898, p. 715; Barnard and Macfadyen, Annals of Botany, 1902, Vol. xvir
p. 587; Molisch, Sitzungsb. d. Wiener Akad., 1903, Bd. cxn, Abth. i, pp. 305, 310.
ORIGIN AND DETECTION OF ELECTRO-MOTIVE CHANGES 389
currents are of common occurrence, although to detect them delicate
measurements are usually required. These are usually made by laying
two non-polarizable electrodes on the regions to be examined, and placing
them in circuit with a galvanometer whose deflection indicates the passage
of a current from the region of higher potential to that of lower potential.
A positive result gives, however, no indication as to how the potential
differences are produced.
The continuance of the current in the external circuit shown by the
permanent deflection of the galvanometer indicates that the difference is
continually maintained, and that a return current flows in the plant in such
a direction as to form a complete internal and external circuit. A break in
the external circuit must exert some influence upon the internal circuit, but
the internal currents are hardly likely to cease as when a battery circuit is
broken, for a difference of potential between two points must always produce
a current of electricity if the resistance of the intervening medium is not too
high. The direction and intensity of these currents will largely depend upon
the shape, arrangement, and conductivity of the intervening tissues, which
may be such as to permit of the continual circulation of electrical currents in
plants. The only evidence at our disposal is, however, derived from observa-
tions made upon the currents led off and measured in circuits external to
the plant.
A variety of factors may induce variations of electrical potential in
plants, and if the sum of the processes producing a rise of potential is equal
to that of those tending to diminish the potential no external modification
will be made manifest. In general the visible differences of potential appear
to result directly or indirectly from metabolism, and to a very much less
degree from the imbibition of water and its passage through capillary tubes.
All chemical changes in which ions take part involve also electrical
changes. In a galvanic cell, for instance, the electrical charges imparted
to the plates maintain the difference of potential, which induces flow in
the external circuit. When oxidation or reduction takes place in tissues
separated by an intervening conducting space, it is usually possible to lead
off an external current, and the same is even possible when two reacting
substances are brought into contact by diffusion l. In plants, therefore, we
have all the conditions for the production of electrical currents.
It is possible that the protoplasmic membranes may allow some ions to
pass but not others, and in this way, or even by retarding the speed of
certain ions, a difference of potential may be produced capable of giving rise
1 Haake (Flora, 1892, p. 465) observed a pronounced deflection of a galvanometer connected
with the ends of a strip of filter-paper at the moment when copper sulphate and ferrocyanide of
potassium met by diffusion and interacted. Cf. also Dubois, Centralbl. f. Physiol., 1901, Bd. xiv,
p. 32.
390 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
to an external current l. The processes of diosmosis and of diffusion may,
therefore, give rise to electrical currents, which may indeed be produced in
various ways by differences of concentration. All these are factors within
the control of the organism, and capable of alteration by appropriate
metabolic activity.
In all cases it depends upon circumstances whether any current
perceptible externally is produced. For instance if a zinc-copper couple
is completely immersed in dilute sulphuric acid, the whole of the liberated
chemical energy is ultimately transformed into heat, no external current
being perceptible. In the same way no differences of potential need exist
on the surface of a cell when internal electrical currents circulate in the
cytoplasm or cell-sap, and these are impossible to demonstrate in small
plant-cells. It is, however, possible to demonstrate the existence of
differences of electrical potential between the different parts of the inter-
nodal cells of Nitella.
Bernstein 2 considers that in muscle-tissue the differences of electrical
potential are produced by the action of temperature, and not by chemical
changes, a rise of temperature increasing the electro- motive force in a chain
of rising concentration. In the case of plants, however, other factors may
come into play, and most animal physiologists regard the electro-motive
force as being directly derived from chemical changes 3.
The occasional existence of externally perceptible electrical currents in
dead organs is hardly surprising when we consider that the metabolic
products may be at first unequally distributed and that by their diffusion and
chemical interaction differences of potential may be produced *. Naturally
also, such currents gradually diminish and disappear, and the disappearance
may be so rapid that in the dead organ no currents can be detected. These
post-mortem currents may in part represent actions which go on during life,,
but in all cases the cessation of metabolism immediately influences the
1 Cf. Ostwald, Zeitschr. f. physik. Chem., 1890, Bd. vi, p. 69; Walden, ibid., 1892, Bd. x,
p. 718; Oker-Blom, Pfluger's Archiv f. Physiol., 1901, Bd. LXXXIV, p. 191. On concentration
chains cf. Ostwald, Lehrb. d. allgem. Chemie, 1. c., p. 824 ; Grundriss, 1. c., p. 442.
3 Bernstein, Pfliiger's Archiv f. Physiologic, 1902, Bd. xcil, p: 521. Here and in Winkelmann's
Elandbuch d. Physik, 1903, p. 420, full details are given as to the required temperatures.
* Biedermann (Elektrophysiologie, 1895, p. 300), L. Hermann, and E. Hering (Lotos, 1889,
N. F., Bd. IX, p. 56) all ascribe animal electricity to chemical processes. Du Bois-Reymond con-
sidered the phenomena to result from the special arrangements of bipolar molecules, but left the
sources of energy an open question.
4 Ranke (Sitzungsb. d. Bayrischen Akad., 1892, p. 181) and Munk (Die elektrischen u. Bewe-
gungserscheinungen im Blatte von Dionaea, 1876, p. 43) observed a gradual disappearance of the
electrical currents from dead organs. B. Velten (Bot. Ztg., 1876, p. 296) and O. Haake (Flora,
1892, p. 467, footnote) found that currents persisted for a time after sudden killing by steam or hot
water. Haake states that the current disappears rapidly from a dead stem of Pisum in moist air,
but reappears on laying in water, probably because of the differences of concentration produced by
the outward diffusion. According to Waller (Centralbl. f. Physiol., 1901, Bd. xv, p. 480) deaths
by cold is accompanied by a sudden ' explosive ' production of electricity.
ORIGIN AND DETECTION OF ELECTRO-MOTIVE CHANGES 391
electrical conditions. On the other hand, it requires a special arrangement
of the parts to enable the chemical actions involved in metabolism to
produce differences of potential sufficient to maintain perceptible external
currents. Changes of metabolic activity will naturally affect these currents 1,
but might conceivably take place in such fashion as to leave the difference
of potential unaffected. The absence of oxygen, changes of temperature,
the action of chloroform and ether all produce a distinct effect upon the
external currents led off from a plant, not only because of their general
action on metabolism, but also when locally applied. The local application
of anaesthetics or of poisons, as well as local injuries, may not only influence
a pre-existent electrical current, but may also cause difference of electrical
potential to appear in regions which were previously isoelectric 2.
Similar phenomena are shown by adult organs in which, when kept
under otherwise constant conditions and in air saturated with moisture, the
removal of oxygen or a change of temperature mainly affects the metabolic
activity. When, however, movement takes place, as in a stimulated leaf of
Dionaea, the resulting movements of water and of the tissues as a whole
may produce a certain amount of electricity. Under similar conditions
symmetric points on a leaf or stem are usually isoelectric, and the same
may even apply to organs which are morphologically and functionally
dissimilar. The reversal of the normal current of action during life or
under special conditions shows that the polarity of the organ does not
involve any fixed electrical polarity. Currents can usually be obtained
between any two points after appropriate treatment, provided that the
surfaces are not covered by non-conducting cork layers. Hence the pro-
duction of electricity, like the production of heat, is a property common to
all living organisms, and not one possessed by a few, as is for instance the
property of luminosity.
The difference of potential between different surfaces on an intact or
injured organ is usually less than o-i to 0-14 of a volt3, which is the same as
exists in resting muscle between the longitudinal and transverse surfaces.
The total amount of electricity produced is quite uncertain, and even when
the conductivity of the different tissues and of the different parts of the
1 The relationships here are the same as when growth and movement are affected by external
stimuli, and hence no sharp distinction can be drawn between currents of rest and currents of action.
Cf. Biedermann, I.e., p. 331.
3 [Waller (Journ. Linn. Soc., 1904, Vol. xxxvn, p. 32) finds that as the result of electrical
stimulation a 'blaze' current, lasting a few minutes or longer, is produced in the adult tissues of
most plants. The direction of this current may be the same or opposite to that of the exciting
current, and it is in some cases of quite appreciable intensity, the difference of potential produced
amounting to -^ volt. In some cases where a compensating current was used to balance the action
current or injury current of the object tested, the 'blaze' current obtained was simply due to
a decrease of resistance allowing the compensating current to produce a deflection of the galvano-
meter.]
3 Cf., in addition to the works already quoted, Biedermann, Elektrophysiologie, 1895, p. 441.
392 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
cells are better known, we shall still be unable to accurately determine the
magnitude and current-density of the internal streams of electricity. The
tissues of plants have in general, like the tissues of animals, a very low
electrical conductivity, their resistance being often one or two million times
greater than that of mercury 1. The resistance will naturally depend upon
the substances present in the cell, upon the arrangement of the cells,
upon the nature of the imbibed solutions, upon secretory activity, and upon
the presence of air or of sap in the intercellular spaces. Hence it is hardly
surprising to find that the transitory passage of a strong electrical current
through a tissue may cause a diminution of its electrical resistance 2. A fall
of resistance between two points will tend to lower the difference of
potential between them, for if the resistance between them was nil no
perceptible difference of potential could be maintained.
Ewart3 has shown that in plant-cells the protoplasm offers a greater
resistance to the passage of an electrical current than the cell-sap, or even
than the cell-wall when the latter is saturated with sap. The resistance
decreases considerably as the temperature rises, and in egg-albumin, which
appears to conduct in much the same way that protoplasm does, the
resistance of 501 ohms per centimetre cube at 16° C. sinks to one of 188 ohms
at 85° C., the coagulation of the albumin exercising no effect upon its con-
ductivity. Young highly protoplasmic organs have a very low conductivity,
which is presumably due to their deficiency in electrolytes 3.
Apart from the electrical fishes which use their special powers for
attack and defence, we know of no definite cases in which the production
of electricity is of use to the organism. Very possibly the production of
electricity is largely an accidental accompaniment of metabolism, although
the weak currents circulating in plants may exert stimulating or orienting
actions4 on the protoplast, or may aid by the transport of ions in the
conveyance of food and other materials from one part to another. No
conclusions can, however, be drawn from the galvanotactic responses of
certain organisms, and the facts known as to the influence of external
1 Cf. Biedermann, Electrophysiologie, 1895, p. 704; Kunkel, Arb. d. bot. Inst. in Wiirzburg,
1879, Bd. II, p. 333; Wjasemsky, Ueber den Einfluss d. elektrischen Strome auf d. Leitungswider-
stand der Pflanzengewebe, 1901; Galeotti, Zeitschr. f. Biologic, 1902, Bd. XLIII, p. 289. On the
conductivity of wet and dry wood cf. Villari, Ann. d. Physik u. Chemie, 1868, Bd. cxxxill,
p. 418; Mazotto, Bot. Jahresb., 1897, p. 92.
a Wjasemsky (1. c., p. 20) concludes that the fall of resistance is due to the passage of water
inwards from the moist electrodes, through the cuticle. Waller has shown, however (Journ. Linn.
Soc., Vol. xxxvii, 1904, p. 46), that the same fall of resistance is shown in peas after the skin has
been removed, and suggests that the action of the original current is to cause an increase in the number
of conducting electrolytes, which appear to be deficient in young highly protoplasmic organs. Ewart
(On Protoplasmic Streaming in Plants, 1903, pp. 96, 123) observed a fall of resistance in the proto-
plasm on death, and ascribes this to the same cause, since coagulation exercises no effect on
conductivity (1. c., p. 124).
3 Ewart, 1. c. * Cf. Ewart, 1. c., p. 116.
ORIGIN AND DETECTION OF ELECTRO-MOTIVE CHANGES 393
currents upon vital activity afford no safe guide as to the action of the
weak internal currents1. These may, however, exert a distinct stimulating
action, and if prolonged may represent a considerable total expenditure of
energy.
Variations of the electrical current in an external circuit indicate auto-
genie or aitiogenic changes within the plant, but unfortunately the origin of
the change of potential or of the altered resistance producing the modified
current is usually unknown and is in all cases difficult to determine. Never-
theless the ease and exactness with which the external currents can be
measured render them of great value as indicators of internal changes.
Even in animal physiology, however, where much work has been done in
this direction, but little is known as to the function of the electrical currents
observed 2. Hence we need only discuss the electrical currents and changes
of potential so far as is necessary to show their general character and their
relationship with other vital processes. The action of stimuli upon the
FlG. 68. Testing-apparatus for electrical currents. The glass tube (a) can be separated into two halves at (c).
Cases can be led into and out of the two halves by the tube at (/). The electrodes (e) are attached by air-tight
india-rubber caps (A) to the side tubes (6, b).
production of electricity agrees as regards the influence of summation,
intensity, and conjoint action with that of stimuli in general. Variations
of the strength of the current due to polarization effects or to changes
of resistance are naturally of less importance than those produced by an
increased difference of potential due to greater electrical activity.
Methods. These have already been developed very fully in animal physiology 3.
The brushes of the non-polarizable electrodes (Fig. 68) are moistened with a dilute
solution (0-05 per cent.) of sodium chloride, or with spring-water, care being taken to
wash away any traces of zinc sulphate that may diffuse through from the tube con-
taining the carbon electrode. Haake 4 used a tube (Fig. 68) which could be separated
1 Cf. Euler, Meddelanden fran Stockholms Hogskolas Botaniska Institut, 1899, Bd. II ;
Lemstrom, Electricity in Agriculture, 1904; Ewart, 1. c., pp. 88-93.
3 Biedermann, I.e., p. 273.
3 See Hermann, Physiolog. Practicum, 1898, p. 75; Biedermann, Elektrophysiologie, 1895;
Burdon-Sanderson, Kunkel, and Haake, 1. c. The addition of a drop of water, or the mere appli-
cation of the electrode may produce a transitory current. Hence careful control is required.
4 Haake, Flora, 1892, p. 461.
394 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
into two halves by an air-tight partition at <r, so that one-half of the plant might be
in hydrogen, the other in air, or if the plant was surrounded by cotton-wool at c, one-
half could be kept at a higher temperature than the other. At the same time the
whole plant, or stem, can be kept in air saturated with moisture.
To measure the current in the external circuit either a sensitive reflecting
galvanometer or a Lippmann's capillary electrometer may be used. The latter
instrument consists of a capillary tube containing mercury, whose open end is
immersed in dilute sulphuric acid which also fills the tube up to the mercury. On
passing a current from the mercury to the sulphuric acid the capillary constant alters
and a corresponding movement of the mercury ensues, which when read off by means
of a horizontal microscope may enable changes of potential of less than 0-0005 °f
a volt to be detected by means of a delicate instrument \ Rapid changes in the
intensity of the current may be detected and measured by causing the thread of
mercury to throw a strong shadow upon a slowly-moving photographic plate 2. The
electrical potential may be determined either by means of a compensator which is
adjusted until the mercury regains its original position, or by determining the actual
pressure required to drive the mercury back to its original position while the current
is still passing.
SECTION 86. The Influence of External Agencies on the
Production of Electricity.
The existence of externally perceptible electrical currents was discovered
by Becquerel 3 on injured plants, and on the uninjured leaves of Dionaea by
Burdon-Sanderson and Munk, while Kunkel, Mtiller-Hetlingen, Haake, and
others subsequently extended these observations to a variety of uninjured
plants4. In fact there does not appear to be a single plant of any size in
which differences of potential cannot be detected between points on its
1 See Hermann, Physiol. Practicum, 1898, p. 93; .Ostwald, Hand- u. Hilfsbuch f. physiko-
chemische Messungen, 1893, p. 247; Hermann and Gildemeister, Pfliiger's Archiv f. Physiologic,
1900, Bd. LXXXI, p. 491.
2 See Langendorff, Physiol. Graphik, 1891, p. 90; Garten, Abhandl. d. math.-physisch. Klasse
d. Sachs. Ges. d. Wiss., 1901 , Bd. xxvi, and textbooks of animal physiology.
3 Becquerel, Ann. de chim. et de physique, 1851, 3° sen, T. xxxi, p. 40; Wartmann, Bot. Ztg.r
1851, p. 308; Buff, Ann. d. Chem. u. Pharm., 1854, Bd. LXXXIX, p. 76; Heidenhain, Studien d.
physiol. Inst. zu Breslau, 1861, Heft i, p. 104; Hermann, Pfltiger's Archiv f. Physiologic, 1871,,
Bd. iv, p. 155 ; Ranke, Sitzungsb. d. bayrisch. Akad., 1872, p. 181 ; Velten, Bot. Ztg., 1876, p. 273.
4 Burdon-Sanderson, Proc. of the Royal Soc., 1876-7, Vol. xxv, p. 411 ; Phil. Trans., 1882,
Parti; 1888, Vol. CLXXIX, p. 417; Biol. Centralbl., 1882, Bd. II, p. 481; 1889, Bd. ix, p. i;
Munk, Die elektrischen u. Bewegungserscheinungen am Blatte von Dionaea, 1876 ; Kunkel, Pfliiger's
Archiv f. Physiol., 1881, Bd. xxv, p. 342 ; Arb. d. bot. Inst. in Wiirzburg, 1878, Bd. n, pp. i, 333;.
Muller-Hettlingen, Pfliiger's Archiv f. Physiol., 1883, Bd. xxxi, p. 193 ; Haake, Flora, 1892, p. 455 ;
B.Klein, Ber. d. bot. Ges., 1898, p. 335; Dubois, Centralbl. f. Physiol., 1899, Bd. xin, p. 699 -r
Waller, Proc. of the Physiol. Soc., 30. Jnni, 1900, und 9. Nov., 1901 ; Proc. of the Royal Soc., 1900,
Vol. LXVII, p. 129 ; Centralbl. f. Physiol., 1901, Bd. xv, p. 480 ; Tompa, Beihefte z. Bot. Centralbl.,
1902, Bd. xii, p. 99; Querton, Institut Solvay, Travaux du Laboratoire d. Physiol., 1902, T. V,
Fasc. 2, p. 81 ; Bot. Centralbl., 1903, Bd. xcn, p. 145 ; Plowmann, Bot. Centralbl., 1903, Bd. xcm,
p. 61. The rather fantastic discussions of R. Keller (Reibungselektrische Untersuch. an pflanzlichen.
Geschlechtsorganen, 1902) hardly need comment.
THE INFLUENCE OF EXTERNAL AGENCIES 395
surface. Haake even found this to be the case in the internodal cells of
Nitella and the same may apply to large non-cellular Algae like Caulcrpar
although, to judge from the absence of ' blaze ' currents from most Algae,
they are comparatively incapable of electrical response1.
There is, however, no constant rule for the distribution of the surface
potential even under homogenous and regular external conditions. Sym-
metric points on a leaf or stem are usually isopotential 2, while judging
from the direction of the current in the external circuit the midrib is
positive to the lamina. Nevertheless exceptions occur 3, as is also the
case when the potentials of old and growing zones are contrasted, while
in the latter case changes commonly occur during development. Although
electrical disturbances were known to occur during the rapid closure of the
leaf-lobes of Dionaea, and as the result of injury and of changes of tempera-
ture, Haake was the first to show that they always take place when the
metabolism is sufficiently modified by changes in the external conditions.
The removal of oxygen* always causes a certain electrical disturbance.
When the entire object is in hydrogen the galvanometer deflection is
usually lessened and is sometimes reversed, whereas the local absence
of oxygen produces an increased deflection, independently of whether the
negative or positive region is placed in the hydrogen. Although deviations
are often shown 5, the results indicate the prominent part played by aerobic
respiration in the production of electricity, although the latter can still be
formed by the intramolecular respiration occurring when oxygen is absent.
No definite causal relationships are revealed by these facts, and the com-
plicated nature of respiration in general renders it hardly surprising that
on the return of a still living plant to air, the original distribution of
potential may not be restored, and that in the continued absence of oxygen
the galvanometer may show a varying deflection. In both cases the
transition to the new conditions produces pronounced temporary deflections
of the galvanometer.
Temperature. The changes of current produced by rises or falls of
temperature in objects kept in air saturated with moisture are, in part
at least, due to quantitative and possibly qualitative alterations of respira-
tion and metabolism, although alterations of resistance and other factors.
1 Waller, Journ. of Linn. Soc., 1904, Vol. xxxvn, pp. 32, 40.
a On the isopotentials of leaves cf. Kunkel, 1, c. ; Haake, 1. c., p. 483 ; Munk, 1. c., p. 37.
3 Cf. Kunkel, 1. c., 1878, p. 2 ; Haake, 1. c., p. 458 ; Klein, 1. c., p. 336.
* Haake, 1. c., p. 467. On some researches on the effect of the removal of oxygen on animals
cf. Biedermann, I.e., p. 402. The changes of potential are not due to the gaseous movements due
to production and consumption.
5 Haake (1. c., p. 470) observed an increased deflection when the seedling of Vicia Faba was
placed in hydrogen, possibly because during the intramolecular respiration of this plant as much, or
in the case of the cotyledons even more, carbon dioxide is produced than during normal oxygen
respiration.
396 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
may come into play. Haake * found that in general the warmer half
of a shoot or leaf experienced an increase of positive potential, which
caused either an increased or a decreased deflection according to whether
the previous difference of potential was increased or diminished thereby.
Photosynthesis. The assimilation of carbon dioxide involves alterations
of potential, which hence become perceptible when a leaf is exposed to
changes of illumination, as was first shown by Haake and by Klein. Waller
and also Querton have shown that the current in the external circuit
moves from the shaded to the illuminated half of a leaf, but Tropaeolum
and Matthiola show exceptions to this rule. The most pronounced effect
is produced behind a solution of potassium bichromate which lets through
mainly the rays most effective in photosynthesis.
Anaesthetics such as chloroform and ether2 also produce changes of
potential, probably largely owing to their influence upon metabolism.
The movements of water in the cell-membranes and in capillary
spaces exert a purely physical electromotive action in both living and
dead objects 3, but in the living plant the electricity produced in this
way is but slight in amount. Haake4 could detect no difference in the
galvanometric deflection obtained from a leaf or stem on permitting and
then preventing transpiration with its attendant movement of water. Even
when a flaccid plant was suddenly made turgid by forcing in water under
pressure, only a slight variation of the current in the external circuit was
shown.
Since the normal differences of potential on plants are shown also in
air saturated with moisture, Kunkel's supposition 5 can hardly be correct,
for according to this author all the electrical currents in plants are derived
from the mechanical energy of the movements of water. Kunkel attaches
especial importance to the fact, corroborated by Haake, that the current
passing from the midrib to the mesophyll of a leaf undergoes a transitory
reversal when a drop of water is placed upon the mesophyll. The same
result is, however, produced when the leaf is saturated with water so that
no absorption occurs, and it remains an open question whether the varia-
tions in the electrical currents produced by rapidly bending a shoot are
due to movements of water as Kunkel supposes, or are produced in other
ways6. Since the effect is produced at once it cannot be the result
1 Haake, Flora, 1892, p. 476.
2 Waller, Proc. of Royal Soc., 1900, p. 134; Querton, I.e., p. no.
3 Querton, I.e., 1902, p. 119; Haake, 1. c., p. 480. For a few observations on the influence of
-chemical agents upon the production of electricity by animals cf. Biedermann, I.e., pp. 302, 408.
* On stream currents, and on electrical endosmosis, cf. Winkelmann, Handbuch d. Physik, 1893,
Bd. ill, i, pp. 493, 504 ; G. Bredig, Zeitschr. f. Elektrochemie, 1903, Bd. ix, p. 738.
5 Kunkel, I.e., 1878, 1881 ; cf. Haake, I.e., p. 457.
6 Whether changes of concentration or of resistance take part in the phenomenon is uncertain.
Protoplasmic streaming may be inhibited without the production of electricity being appreciably
THE INFLUENCE OF EXTERNAL AGENCIES 397
of an internal injury, for the wound-reaction is only manifested after
a certain latent period. It is not impossible that the sudden bending might
modify metabolism, although in the case of the leaf of Dionaea the varia-
tions in the current produced by excitation begin before the leaf closes.
The resulting movements of water may, however, then aid in maintaining
differences of potential1.
Dionaea muscipula. The leaf of this plant shows in the resting condition
a similar distribution of potential to that of an ordinary leaf, whereas after stimulation
pronounced disturbances occur according to Burdon-Sanderson and to Munk. The
former, using extremely delicate modes of investigation, found that a variation of the
current in the external circuit took place 0-04 of a second after the application of
a single weak induction-shock, whereas the resulting movement began only after
a latent period of a second, the closure of the leaf-lobes requiring 5 to 6 seconds for
completion at 2o°C. If the shock is extremely weak, the electrical variation may be
produced without any movement resulting. Presumably the electrical response is an
indication of the commencement of chemical or other changes which, when completed ,
lead to a movement. That this stimulatory action spreads rapidly is shown by the
speed of propagation of the electrical variation, for this occurs only 0-05 of a second
later at a point 10 mm. away, the velocity of propagation being therefore 200 mm,
per second. The progress and character of the electrical variation strongly resembles
that shown on animal objects 2.
A pulvinar thorn on the leaf of Mimosa pudica is strongly positive to the upper
surface of the pulvinus, and according to Kunkel 3 a marked electrical variation ensues
when the leaf is stimulated.
Injuries produce pronounced electrical variations, and these possibly
initiate or at least indicate the commencement of the disturbances leading
to the wound-reaction. According to Hermann, Ranke, Velten, and
Kunkel the injured region usually becomes negative or more negative
towards the uninjured part. Hence an injured stem may yield a current
although none was shown when intact 4. Indeed the current of injury
was the first one observed, for a considerable difference of potential often
exists between the injured and uninjured surfaces 5.
Not only is the injured surface of a previously quiescent stem negative to
uninjured regions, but also points on the latter near to the injury are negative
affected. Cf. Velten, 1. c., p. 295 ; Haake, 1. c., p. 480 ; Hermann, Studien ii. d. Protoplasma-
stromung bei den Characeen, 1898, p. 72.
1 A. Tompa (l.c., p. 116) denies Waller's statement (Proc. of the Physiol. Soc., 9. Nov., 1901 ;
Centralbl. f. Physiol., 1901, Bd. xv, p. 480) that local blows produce current-variations, but his
experiments are not conclusive. Cf. also Bose, Journ. of Linn. Soc. Botany, 1902, Vol. xxxv, p. 275.
a A complete summary is given by Biedermann, 1. c., p. 455.
3 Kunkel, I.e., 1878, p. n ; Dnbois, Centralbl. f. Physiol., 1899, Bd. xin, p. 699.
* According to Tompa (Beiheft z. bot. Centralbl., 1902, Bd. xn, p. 117) a current of injury i&
perceptible on injured air -dried seeds.
5 Internal currents may be present in a plant-organ although no external current can be led offr
whereas a resting muscle shows no internal currents. Cf. Biedermann, 1. c., p. 288.
398 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY
to those further away. The reaction extends for a limited distance, for Kunkel1
could observe no difference of potential on a previously isoelectric stem when the
electrodes were placed 5 and 6 cms. away from the injury. When a piece is cut out
of a stem, the two cut surfaces are isopotential and the greatest difference of potential
exists between the median point of the stem and either cut surface. Right and
left of the median line isopotential zones exist, the connexion of which, by an
external circuit, produces no current. The distribution of potential is therefore
exactly the same as on an isolated cylindrical muscle, in which it has been more
deeply studied 2.
If the epidermis is removed from the segment of the stem, the current is
immediately reversed according to Ranke and Velten 8, flowing from the transverse
section to the longitudinal surface in the external circuit. A similar reversal may
take place, according to Hermann, on pieces of stem in which the epidermis remains
uninjured, and which showed at first similar currents to those in muscle.
There are exceptions to these rules, and possibly more will be found in the
future. Thus Ranke * found that on the petiole and peduncle of Nymphaea alba
the current passed from the transverse surface to the longitudinal epidermal one both
before and after the epidermis had been removed, while Velten6 found that the
current directed from the longitudinal surface to the transverse one persisted after
the removal of the epidermis.
The current of injury is produced instantly, so that if the electrodes are laid on
the stem and an incision made near to one of them, an immediate deflection is
produced in the galvanometer. It is, however, uncertain whether the reaction is
purely one of physical chemistry or is due to a vital action such as that which leads
to the closure of the leaflets of Dtonaea. In ordinary tissues the electrical changes
might form the first indication of the physiological reaction leading to an increased
activity of respiration and an enhanced production of heat. It is also unknown to
what extent the electrical changes are connected with the gradual progress and
development of the wound-reaction. The electrical variations are produced when
neither electrode touches the cut surface, as well as when it is at once washed with
water. Kunkel 6 found that local bending also produced a negative variation at the
part affected, and it has yet to be determined whether this variation and the variation
due to injury are produced in the same way. If so, then the injury and death of
cells would not form an essential condition for the production of the ' injury ' current.
In all cases it must be remembered that the removal of the epidermis decreases the
previous electrical resistance, and furthermore that electrical stimulation may con-
siderably increase the conductivity more especially of young and highly protoplasmic
tissues such as the cambium and apical meristem.
1 Kunkel, Arb. d. bot. Inst. in Wurzburg, 1878, Bd. II, p. 6.
a Cf. Biedermann, I.e., p. 275.
3 Ranke (1. c.) calls the current led off from an uninjured epidermal surface the false, and that
from the injured epidermal surface the true plant-current.
4 L.C., p. 197. s L. c., p. 291.
6L.c., 1878, p. 7.
CHAPTER VI
THE SOURCES AND TRANSFORMATIONS OF ENERGY IN THE PLANT
SECTION 87. General View.
THE fact that all vital activity is bound up with a liberation of chemical
energy by respiration gives no indication of the mode in which the energy
is utilized, nor does this energy necessarily become immediately manifested
externally as movement, heat, light, or electricity. It may be stored as
potential energy in the form of food- materials, or as osmotic energy which,
together with surface-tension energy, form two physical factors of the
utmost importance to plants.
During photosynthesis the plant stores up food-materials and energy
for future or immediate use, and the energy thus obtained may never enter
directly into metabolism. For instance, many substances present in the
plant exert a considerable osmotic action without ever being drawn into
metabolism, being absorbed directly from the soil and accumulated in the
cells by a purely physical process of selective absorption and passive secre-
tion. Transpiration affords another instance of the creation of a difference
of potential which aids the ascent of water in trees, and hence is of consider-
able importance in the vital economy without being a purely vital function.
The action of any form of energy in the plant is largely dependent
upon the structural arrangement and physical properties of the cells and
tissues, so that the same form of energy may produce widely different
results in different plants, or in different parts of the same plant, or in
the same part at different times. Every physiological action is coupled
with a transformation of energy, and for a complete causal explanation
of any such action not only must the sources and transformations of
energy be known, but also the metabolic changes connected with them.
Locomotion, growth, translocation, the production of heat, light and
electricity, and constructive and destructive metabolism in general, all
involve transformations of energy which may become perceptible internally
or externally, and which are to be regarded as manifestations of vital
activity.
Apart from the locomotory movements which are absent from most
plants, as many external manifestations of energy are shown in the
vegetable kingdom as among animals. A growing plant, for instance,
may exert considerable pressure against a resistance. The internal mani-
festations of energy during growth are probably very similar in both
400 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
animals and plants, but must acquire much greater intensity in an actively
vegetating bacterium than in a slowly growing animal. The respiratory
activity and the production of heat are in fact much greater in rapidly
growing Fungi and Bacteria even than in warm-blooded animals. The
luminescence of a few plants and animals, as well as the feeble production
of electricity, represent relatively little energy. As comp ared with ordinary
animals and Fungi, chlorophyllous plants and animals have the advantage
of being able to convert a portion of the radiant energy of the sun into-
potential chemical energy.
Although vital activity is in the first instance based upon chemical
energy, nevertheless all the natural forms of energy may take part in one
or other of the detailed reactions in the plant. Since electrical currents da
actually circulate in plants, and since every current radiates magnetic lines
of force while the different constituents of the cell have varying magnetic
permeabilities, it is impossible to deny that even magnetic forces may take
part in certain vital phenomena, more especially where a directive or
sorting action is necessary. Thus the direction and maintenance of regular
streaming in a constant direction may involve some action dependent upon
the paramagnetic properties of the cell -wall and the varying magnetic
permeabilities of the remaining cell-contents1. In this connexion it is
interesting to notice that a constant direction of streaming is only main-
tained in cells provided with a cell-wall, or in cells containing a single
large central vacuole ; so that the streaming protoplasm is near to and
remains approximately equidistant from the cell-wall at all points. In
naked cells, and in cells crossed by strands of streaming protoplasm, the
direction of streaming is more or less variable and capable of reversal.
Osmotic energy is of the utmost importance in plants, and is a form
of energy dependent upon the number of particles in unit volume and
their kinetic energy. It is, therefore, comparable with gaseous pressure,
and both osmotic pressure and gaseous pressure are related to diffusion,
since all three involve the existence of movement among the molecular or
ionic particles 2. Surface-tension energy is involved in the phenomena of
capillarity, imbibition, swelling, and also absorption so far as no chemical
reaction takes place, for we may include under this form of energy all
energetic manifestations shown between solid and fluid bodies independ-
ently of whether these take place between visible or invisible and external
or internal component particles.
The energy of crystallization or of precipitation may be used in
1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, pp. 33, 45, 116.
2 On the different forms of energy cf. Ostwald, Gnmdriss d. allgem. Chemie, 3. Aufl., 1899,.
p. 247; Lehrb. d. allgem. Chemie, 2. Aufl., 1893, Bd. II, Th. i, p. n. In regard to plants cf.
Pfeffer, Studien zur Energetik d. Pflanzen, 1892, p. 159, in which work the subject is discussed fully
for the first time.
GENERAL VIEW 401
certain cases, although when the freezing of water produces frost-cracks,
the liberation of energy is excessive and beyond the plant's power of
control. According to the conditions and to the point of view energy
produced by the precipitation of a solid or by crystallization may be
regarded as a manifestation of volume energy, of chemical energy, or of
surface-tension energy 1.
Work is done during all movements in overcoming the resistance
of surrounding media and in displacing internal parts. In the latter case
the work done may be stored up in the form of potential energy capable
of sudden liberation, as in the tissue-tensions, in the pulvinus of Mimosa,
the leaf of Dionaea, and various suddenly-dehiscing fruits. The upright
growth of a shoot involves the storage of a certain amount of potential
energy, which is manifested as kinetic energy when the trunk is sawn
through, and is transformed mainly into heat when the trunk falls upon hard
ground. The total amount of energy involved here is, however, trifling as
compared with that represented by the raising of water during transpira-
tion, and by the kinetic resistance which the ascending stream has to
overcome.
The law of the conservation of energy and of mass holds good during
all the transformations of energy in the plant. The energy stored up
during life is ultimately set free on death either by decomposition, com-
bustion, or by being drawn into the metabolism of some other organisms.
There is no reason for assuming the existence of any special form of vital
energy, since the same form of energy may produce the most varied results
according to the mechanism on which it acts. The capacity of the organism
for continued and automatically regulated growth and the hereditary
tendencies of the germ -cells enable the offspring to employ the energy and
food-materials in the same manner as the ancestors. Hence the species
may remain unaltered although the descendants may contain not a single
atom or a single trace of the energy represented in the primitive stock.
These considerations also apply to all stimulatory actions, for although
the response may be altogether disproportionate to the stimulus, nevertheless
the latter represents a certain amount of energy, independently of whether
the exciting agent is a stimulatory substance or is physical in character. It
has already been mentioned that by the aid of the regulatory mechanism
gradual and continuous as well as sudden and transitory transformations of
energy may be produced, and that a local inhibition of a particular energetic
manifestation is possible.
FFEFFBR. Ill
1 Pfeffer, Studien zur Energetik, 1892, p. 163,
Dd
402 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
SECTION 88. The Forms of Physical Energy used by Plants.
OSMOTIC ENERGY. This special form of volume energy comes into play
whenever a soluble substance is unevenly distributed. The diffusion thereby
produced takes place in the same way as when different gases are mixed
together, and if the particles of the dissolved solid or gas are unable to pass
through a separating membrane, they bring to bear a pressure upon it which
is dependent upon the number of the molecules in unit volume and upon
their average kinetic energy. The latter, again, is constant for any large
number of molecules at a given temperature and is equal to half the product
of the mass of each molecule multiplied by the square of its average velocity.
A rise of temperature which increases the velocity of movement of the
molecules causes a slight rise of osmotic pressure, and also of the pressure of
a gas kept at constant volume.
A purely physical diffusion movement must take place whenever any
difference of concentration is produced in parts separated by permeable
partition-walls. If, however, the walls are semipermeable a permanent
osmotic pressure can be maintained, such as is commonly used in plants
for various mechanical purposes. A naked cell or gymnoplast would be
indefinitely stretched or burst by a high internal osmotic pressure, whereas
in dermatoplasts covered by a cell-wall a comparatively high pressure is
often required to render them fully distended and active. When death or
plasmolysis allows the cell-wall to contract, the potential energy latent in
it when stretched is manifested. Sudden decreases of turgor produced by
a physiological reaction are responsible for the rapid movements of the
stamens of Cynareae and of the leaves of Mimosa, which can be repeated
as soon as the original turgor has been restored. The pulsation of certain
vacuoles is, in some cases at least, produced by automatic variations of turgor.
In order* to maintain the turgor in a growing and enlarging cell,
a regulated production of osmotically active materials is necessary. During
plastic growth the mechanical work involved in the stretching of the cell-
wall is carried out by the previously accumulated osmotic energy. If,
however, the growing organ encounters a resistance, the tension in the
apposed cell-wall gradually decreases until nearly the whole of the osmotic
pressure is acting against the resistance.
By a similar counteraction of active and passive tissues, tensions and
pressures are produced which when released may lead to sudden move-
ments, as during the dehiscence of the fruits of Impatiens or of Momordica.
In these cases the potential energy is stored up by a definite physiological
activity, whereas a purely physical action on a given mechanism is involved
when the dry valves of the fruits of Leguminosae twist on drying
and untwist on moistening, or when leaves droop for want of water and
THE FORMS OF PHYSICAL ENERGY USED BY PLANTS 403
re-expand when supplied with it. The distinction is really one of little
value, since in both cases the responding mechanism is a product of vital
activity, and physical responses of this character are often capable of
frequent repetition and may take place against considerable resistance l.
The osmotic energy of the cell bears no definite or constant relation-
ship to the energy consumed in the production and accumulation of the
osmotic materials 2. The former is entirely dependent upon the number of
molecules, and remains the same whether energy is absorbed or liberated
during their production. Furthermore, an osmotic substance may be directly
absorbed from without and accumulated in the cell by passive secretion, but
it exercises precisely the same osmotic action as if it were a product of
anabolic metabolism or of katabolic respiration. The hydrolysis of insoluble
starch by an enzyme produces osmotically active sugars, and the osmotic
action is doubled when the large molecule of cane-sugar is converted by
invertase or by dilute sulphuric acid into two molecules of grape-sugar.
Similarly the reverse process, or the conversion of a soluble into an
insoluble substance, will lower the osmotic pressure.
As in the case of a compressed gas, the presence of a dissolved substance
in a cell only enables a limited amount of external work to be done, for
with the increase in volume of the growing cell, the solution is diluted
and the number of molecules per unit volume decreased, so that here, as in
the case of an expanding gas, the pressure falls 3. Hence the maintenance of
growth involves a continued production of osmotic materials.
When a gas does work in expanding its temperature falls, and in exactly
the same way when work is done by osmotic energy, as is the case when a
cell grows by plastic stretching, the osmotic pressure falls. In both cases
the work done is due to the energy of the moving molecules, and except in
so far as the temperature affects the velocity of the molecules, and hence
also the osmotic pressure they exert, it is immaterial to the plant whether
its temperature is kept higher by respiration than that of the surrounding
medium or whether it is kept permanently lower by transpiration. During
transpiration itself the heat absorbed from without does work in altering the
water from the liquid to the gaseous state, and this work is externally
manifested when water is raised up a vertical stem by the suction of the
leaves.
Apart from its chemical quality the value of a substance as a source of
1 Pfeffer, Studien zur Energetik, 1892, p. 236. 2 j^ pp> zyO) I73
3 On the work done during the expansion of gases see the textbooks of Physics. Rodewald
(Ber. d. hot. Ges., 1892, p. 83) erroneously assumes that the mechanical equivalent of the heat of
combustion] of a substance must always be greater than its power of doing work by its osmotic
action, and that bodies not produced in the cell can do no osmotic work. The latter statement
hardly coincides with the fact that a passive or active absorption from without and an accumulation
in the cell of soluble substances is possible.
D da
404 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
energy is not measured solely by its heat of combustion, nor is the series
of transformations it may undergo in the service of the organism immaterial.
The osmotic action depends solely upon the number of the molecules and
their kinetic energy, and not upon their potential chemical energy as
measured by their heat of combustion. Hence substances may exert a
powerful osmotic action when in solution, although completely oxidized
compounds. A substance which first exercises an osmotic function and is
consumed at a later date in respiration is more important physiologically
than one utilized for one function only. If the product of oxidation is to
retain an osmotic function and yet yield energy during its production, it is
far better when substances like organic acids, having a low heat of com-
bustion, are produced by respiration in place of the volatile carbon dioxide.
Thus when a molecule of glucose is oxidized to three molecules of oxalic
acid not only is the osmotic action trebled but also the greater part of the
available chemical energy is set free in the form of heat l.
SURFACE-TENSION determines the shape of drops of liquid, but it is
not yet certain to what degree amoeboid movements are the results of
spontaneous and induced changes of surface-tension coupled with alterations
in the cohesion of the outer layers. The same applies to pulsating vacuoles,
while protoplasmic streaming has been suggested to be due to the pro-
duction of differences of surface-tension in the regularly arranged bipolar
particles of the protoplasmic emulsion by the action of inwardly- or
outwardly-directed electrical currents. It is, however, uncertain how far
autogenic changes of surface-tension are responsible for the changes of
shape of the nucleus, of plastids, or of the reproductive cells of flowering
plants. Surface-tension may also take part in determining the fusion or
non-fusion of gametes, and the movements of cilia 2.
When a solid is finely divided the surface-tension of its component
particles becomes of increasing importance, since the inwardly-directed
pressure exerted by it on a spherical particle is inversely proportional to
the radius of the particle. The force with which particles of water or of
other fluids are able to penetrate between the molecules or micellae of
substances capable of imbibition and of swelling is the result of molecular
forces akin to that of surface-tension. Absorption phenomena of this kind
form a part of physical chemistry, and indeed the absorption of certain
substances involves a loose chemical union, so that the process may be
regarded as a physical or as a chemical one according to the point of view.
Furthermore, many kinds of imbibition are produced in much the same
way as the so-called solid solutions, as when two metals are placed in.
contact and the particles of one penetrate the other.
1 Pfeffer, I.e., pp. 173, 197 ; Rodewald, I.e.
2 For theories of streaming cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 108.
THE FORMS OF PHYSICAL ENERGY USED BY PLANTS 405
Imbibition and surface-tension energy are probably of as great im-
portance in vital economy as osmotic energy, and all these forms of energy,
but especially the first named, may produce pronounced external mani-
festations, as during the swelling of wood, of seeds, or of starch, or during
the imbibition movements of the awns of certain seeds which are repeated
with each drying and moistening. The energy manifested in movements
of this kind naturally bears no relation to the consumption of energy
involved in the production of the reacting mechanism, since the latter
merely directs the operation of the heat-energy derived from without. The
same applies to movements due to changes of surface-tension resulting from
diffusion, evaporation, or the action of electrical currents.
SECTION 89. Chemical Energy.
The production and accumulation of various substances enables the
plant to utilize osmotic and surface-tension forces, and metabolism may
.also produce electrical currents, though it is uncertain whether these are
of much value in the vital economy. By means of the former forces,
however, the plant is able to convert heat into work, whereas the direct pro*
duction of heat by respiration serves no such useful purpose as does the
fire in a steam-engine, and it is almost entirely dissipated by radiation,
conduction, and evaporation. In other words, the protoplast is neither
a thermodynamic nor an electro-dynamic machine.
It is by no means certain to what extent chemical energy may be
•directly utilized for chemical purposes, either within the plant or outside
of it. In any case, mechanical work is done when a chemical action
involves the dissociation and recombination or rearrangement of molecules.
The same applies whenever a chemical action involves an increase in
volume which takes place against the atmospheric pressure, or when
a substance is crystallized or precipitated in a colloid medium whose
resistance has to be overcome. In the latter case, however, if the pro-
duction and separation of the substance are distinct phenomena, the
separation may be regarded as a physical manifestation of volume energy
independently of whether it is produced by crystallization, by absorption,
or by the removal of a solvent.
Phenomena of this kind play a prominent part in all vital actions, for
the growth of the protoplasm by intussusception involves the intercalation
of new particles between pre-existent ones. Chemical changes, surface-
tension energy, the chemical affinities of the various materials, and imbibi-
tion may all take part in this process and determine whether the new material
shall be tacked on to particular micellae or placed between them. During the
growth by intussusception of the cell-wall, the influences radiating from the
pre-existent particles take an important part in determining the character
406 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
of growth, and the penetration of the new particles, whether produced by
chemical or physical attraction, may take place against considerable ex-
ternal resistance, and hence may render the plant capable of performing
pronounced external work.
The production of heat by an ordinary plant has no direct importance
in metabolism, for this is not appreciably affected by a rise of a fraction
of a degree, and plants develop normally when kept slightly cooler by
transpiration than the surrounding medium. Although aerobic respiration
is always connected with a production of heat, this need not always be
essential, and it is not impossible that anaerobic organisms may exist
whose metabolism involves a lowering of temperature. It is in fact, as far
as our present knowledge goes, impossible to affirm that no existence is
possible without the production of heat by metabolism. In the case of all
physiological actions due to a chemical product, it is immaterial whether the
product is the 'result of an endothermic or exothermic reaction. Chemical
actions involving a liberation of heat are more readily induced than
exothermic^ones, and hence may be preferably employed by the organism l.
Vital activity is inseparable from metabolism, and even adult organs
which have ceased to grow must respire as long as they live. The plant
may be compared to a factory in which all work ceases when the fire is
drawn, although the capacity for work may be retained during short
periods when the energy of the steam-engine is put to other purposes than
driving the different bench-machines. If the energy of the steam is mainly
employed in overcoming frictional resistance in the different mechanisms,
practically the whole of the chemical energy of the coal may be manifested
as heat.
It has already been mentioned that the accumulation of waste products
has to be avoided as far as possible, and it is of interest to notice that for the
most part carbo-hydrates capable of oxidation into carbon dioxide and water
are used in metabolism. The nitrate and nitrite bacteria, which oxidize
ammonia and nitrous acid, as well as the sulphur bacteria which oxidize
sulphuretted hydrogen, derive in this way energy for the synthesis of
organic food and probably for the whole of their vital activity as well.
If this is so, and these organisms use carbon compounds solely as building-
materials and directly utilize the energy obtained by the oxidation of
inorganic compounds in place of ordinary respiration, then a close study of
these organisms should throw much light upon the nature of life 2.
It might also be possible to determine whether proteid molecules are
continually decomposed and regenerated during respiration, or whether
1 Cf. Pfeffer, Studien zur Energetik, 1892, p. 174.
a Nathansohn, Mittheil. a. d. zool. Station zu Neapel, 1902, Bd. XV, p. 655, finds that no
carbon dioxide is produced during the respiration of certain aerobic Bacteria, which cany out chemo-
synthetic assimilation by the aid of the energy derived from the oxidation of thiosulphates.
CHEMICAL ENERGY 407
the respiratory materials, such as sugar or oil, are able to be directly
oxidized owing to their fine subdivision and intimate association with proto-
plasmic molecules J. The fact that during the anaerobiosis of yeast and of
butyric bacteria sufficient energy is obtained by the intracellular fermenta-
tion of sugar produced by an enzyme capable of isolation 2 points to the
fact that sugar present in the cell can be decomposed without its being
chemically united with the protoplasm.
The exact extent to which chemical, surface-tension, or osmotic energy
is used for the different forms of work carried out by the organism is
uncertain, but even if the chemical energy is not directly utilized, its trans-
formation still forms the essential accompaniment of all vital activity.
Similarly, the work done by all the machines in a factory is derived from
the chemical energy of the coal consumed, even when the energy of
expansion of the heated steam is used to drive an electric motor and the
electric energy transmitted to the different machines 3. The special advan-
tage attached to the use of chemical energy in both manufactories and in
organisms is due to its forming a specially concentrated form of potential
energy which is readily rendered kinetic.
The difference between the heat of partial or complete combustion of
the substance used in respiration and the actual production of heat indicates
how much of the realized chemical energy is converted into work, but says
nothing as to the details of the processes involved. Calorimetric investiga-
tions are nevertheless of great importance in the study of special questions,
although the utmost care is required even to obtain approximately accurate
results. In many cases, for instance, the quality and quantity of the
substances consumed and the degree of oxidation they undergo cannot be
exactly determined, so the amount of oxygen absorbed and of carbon
dioxide exhaled form unsafe guides as to the total heat of combustion 4.
Nor are we able to determine the heat equivalent of the mechanical
and other activities of the plant with sufficient accuracy. Owing to the
high mechanical equivalent of heat a considerable amount of work might
be done without the amount of heat liberated being appreciably lowered.
Furthermore, all energy used in overcoming friction or viscosity, or in
producing swelling, ultimately appears in the form of heat. Hence it
is not surprising to find that Rodewald observed that in the case of apples
and kohlrabi all the chemical energy of respiration appeared in the form of
heat, the estimated and observed values practically balancing. The differ-
ences observed by Bonnier between the calculated and actual amounts of
1 Cf. Nathansohn, Mittheil. a.d. zool. Station zu Neapel, 1902, Bd. xv, p. 655.
8 See Buchner, Die Zymasegahrung, 1903, where all the latest literature is collected.
* The production of differences of potential enables particular partial functions to continue for
a time in the absence of oxygen.
* Cf. Pfeffer, I.e., 1892, p. 201.
408 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
heat in the case of seedlings are probably due to the fact that the respiratory
oxidation- is not in all cases complete, and that endothermic changes pre-
ponderate in growth and constructive metabolism.
The whole of the energy transformed into heat is gradually lost by
the plant, although it may aid in maintaining transpiration when the
temperature of the plant is lower than that of its surroundings. In aquatic
plants, however, and in general in the absence of transpiration, the
production of heat serves no useful purpose apart from its biological
significance in Aroids. Poikilothermic organisms like plants lose much
less energy in the form of heat than megatherms like mammals and birds
whose body-temperature must be kept approximately constant, but in
general the modes of utilization of the liberated chemical energy for
mechanical purposes are approximately the same in plants and animals l.
In plants, however, the chemical energy liberated by metabolism
seems to appear almost entirely in the form of heat, and hence very little
can be used for mechanical purposes. In some cases, however, a reaction
may be performed very economically as, for instance, during the contraction
of a muscle or of the stamens of Cynareae, in which less heat is produced
relatively to the work done than in a steam-engine or gas-motor2. In
a muscle performing maximal work one-half of the liberated chemical
energy may appear in the form of work, and one-half as heat, whereas it
requires a good engine to utilize more than ten per cent, of the energy
of the coal consumed ; and during the protoplasmic streaming of an ordinary
plant-cell not more than T^TTTT °f the energy of respiration is consumed
in this form of work3. The plant may work more economically under
certain conditions than under others. Thus respiration increases con-
tinuously up to the lethal temperature, whereas growth and other
manifestations of energy are retarded beyond the optimum. Hence in
general the plant works more economically at moderate temperatures
than at high ones. It must further be remembered that chemical composi-
tion is a more important factor in a living organism than in a machine,
and that the economic coefficient, that is the ratio between the food
absorbed and the increase of body-weight, may vary according to the
prevailing conditions. Since the nutritive value of a substance depends
upon its chemical constitution, its heat of combustion forms no sure guide
as to its nutritive value4, although when different materials are consumed
1 Plants consume their food more completely than animals, which excrete combustible end-
products of metabolism.
2 Cf. textbooks of Physics and Animal Physiology, as well as the Traitd de physique biologique
published by d'Arsonval, 1901, T. I, p. 982.
3 Ewart, Protoplasmic Streaming in Plants, 1903, p. 29.
4 Cf. Pfeffer, Jahrb. f. wiss. Bot., 1895, Bd. xxvni, p. 258.
CHEMICAL ENERGY 409
in respiration each liberates exactly the same amount of energy as when
similarly decomposed or oxidized outside the plant J.
SECTION 90. Special Cases.
A few special instances may be discussed to illustrate the application
of the foregoing principles to concrete cases in which energy is consumed
in overcoming resistance, independently of whether the energy used is at
once dissipated or is in part stored up again for future use.
ABSORPTION AND TRANSLOCATION 2. Any unequal distribution,
however produced, tends to set up purely physical diffusion-currents which
ultimately restore equilibrium. It is immaterial whether the unequal distri-
bution is produced by the organism with or without a consumption of energy
by solvent enzyme action, or by the absorption or separation of soluble
constituents of the cell-sap. Since diffusion movements are extremely
slow, mechanical mixing and streaming movements become of great
importance in ensuring the rapid transference of substances from one
place to another3. Plants fixed to the soil are in part dependent upon
the movements of the surrounding air or water for a rapid supply of food-
materials.
Diffusion and currents of wind carry carbon dioxide to the summit
of a tree, and the carbon accumulated there represents stored potential
energy without the tree having raised any portion of it to this height. The
same is the case when a dissolved substance diffuses upwards from the
roots, and even although the upward passage may be aided by mixing
or bending movements, by thermo-diffusion, or by convection currents, and
by upward streams of water produced by transpiration, none of these
necessarily involves any consumption of energy on the part of the plant.
In other cases, again, streaming and mixing movements resulting from
protoplasmic activity may aid in translocation without being essential,
although the translocation of dead or living materials through the pores
of sieve-tubes and through the inter-protoplasmic connexions of ordinary
cells could hardly take place without the aid of the protoplasm*. In
1 See Rubner, Die Gesetze des Energieverbrauchs bei der Ernahrung, 1902. Cf. also F. Mares,
Biol. Centralbl., 1902, Bd. xxii, p. 282.
2 Cf. Pfeffer, Studien zur Energetik, 1892, p. 268.
3 [The rate of diffusion is more rapid than is usually supposed, especially when chemical
fixation aids in maintaining a high gradient of concentration along the path of the diffusion currents.
Even without this, less time is required for the complete diffusion of a dissolved salt through an
ordinary plant-cell than the protoplasm takes to stream around it when streaming is active. The
transference of a substance across a broad band of tissue by diffusion alone would, however, still be an
extremely slow process. Cf. Ewart, On the Ascent of Water in Trees, Phil. Trans., 1905, p. 40 of
reprint.]
4 [The inter-protoplasmic connexions of ordinary cells are of no importance in translocation.
Thus under normal conditions it would take 100 years for the transference of i cub. mm. of the cell-
4TO SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
discussions of this kind the activity of the living organism and the
existence of the potential differences it produces are taken for granted,
and even during absorption, especially when preceded by digestion,
activities may be involved which are directly due to the living plant.
THE MOVEMENTS OF WATER. The loss of water by transpiration, or any
production of osmotic substances in a cell capable of further distension, will
tend to draw water to the region affected from surrounding parts richer in
water. This action is capable of exerting suction over a greater or less dis-
tance, according to whether the suction is exerted upon vessels filled with
water or containing chains of water-columns and air-bubbles. In the former
case the resistance to flow is directly proportional to the viscosity of the
liquid, to the internal surface of the tube through which flow occurs,
and to the velocity of flow. In the case of circular tubes with smooth
walls the volume passing is greater than with any other shape of bore,
and the rate of flow under equal pressures in such tubes is inversely pro-
portional to the square of the radius and the length of the tube. The
total resistance due to the viscosity of the water flowing through the vessels
is less than the height of the tree when the vessels are filled with water, but
when they contain alternating columns of water and air another resistance
is introduced which is due to the adhesion of the surface-tension films at
the ends of the air-bubbles to the inward projections or perforate par-
titions where the segments of the vessel join. This resistance is inversely
proportional to the diameters of the vessels or pores, and to the difference
in convexity between the ends of the bubbles, and it is usually sufficient
to produce a total resistance equivalent to a head of water many times
the height of the tree.
Ewart 1 has, in fact, calculated that the total resistance to an average
rate of flow in the trunks of the tallest trees may be equivalent to pressures
of as much as 100 atmospheres, suction-pressures which are not only
incapable of being generated by transpiration and osmotic action in the
leaves, but which also cannot be transmitted through the wood-vessels
to the roots. The maximal strain which a water-column free from air-
bubbles is able to withstand appears to be about five atmospheres, and
in the presence of air the greatest negative pressure produced in the wood-
vessels is usually not more than half an atmosphere.
Hence it appears that a continuous adjustment equivalent to a stair-case
pumping action must go on in the trunks of tall transpiring trees, and Ewart l
contents from one cell to another through 3,000 threads of TV /* diameter, and the surface-tension
pressure exerted at the end of the thread, if in air, is as much as 34 atmospheres. In 50 cm.
length of the cribral system of Cucurbita, however, a pressure of only | an atmosphere would
suffice to produce an approximate rate of flow of 5 mm. per minute. See Ewart, On Protoplasmic
Streaming in Plants, 1903, pp. 29-30.]
1 Ewart, On the Ascent of Water in Trees, Phil. Trans., 1905, p. 15 of reprint
SPECIAL CASES
411
has suggested that surface-tension actions of this character may be
exercised by the wood-parenchyma cells along the path of the current.
This has still to be proved, however, and also whether the breaking
strain for continuous water-columns is the same in such tubes as the
tracheae and tracheides as in glass tubes of larger bore.
The exact causation of bleeding is by no means clear, and in
fact it is quite possible that in some cases it
may be produced in the same way as the plasmo-
lytic excretion of water from nectaries, with the
exception that the osmotic substances which have
drawn water into the vessels may be reabsorbed in
their upward passage l.
During the plasmolytic excretion of water
from nectaries the plant provides for the external
deposition of the sugar, which draws out water
from the turgid cells beneath and so produces
nectar. This physical action takes place whatever
the source of the sugar, and in this respect it is
immaterial whether the sugar is produced by
a metamorphosis of the cell-wall, or is formed in
the cell, and excreted externally.
GROWTH. During plastic growth the stretching
of the cell-wall is due to the osmotic pressure in
the cell, whereas when the cell-wall grows by intus-
susception, growth may take place against the
osmotic pressure, as during the internal thickening
of cell-walls. When a growing cell encounters a
resistance, the tension of the cell-wall is gradually
counteracted until the full osmotic pressure is
acting against the resistance. In some cases the
mechanical retardation of growth produces a rise
of osmotic pressure, but the latter determines in
all cases the maximal pressure which a thin- walled films' (After Ewart)
growing organ can exert. Thick-walled organs, however, so long as the
cells grow by intussusception, can exert greater pressures than those corre-
sponding to the osmotic pressures of the component cells 2.
If the resistance is not too great growth is resumed as soon as the
organ exercises a pressure greater than the resistance, and if the latter is
pushed in front of the growing organ the work done is equal to the product
of the force applied and the distance its point of application is moved.
With moderate resistances or loads the original activity of growth is soon
FlG<69. Diagrammatic longi-
ends
1 Ewart, Phil. Trans., 1905, p. 42.
a Cf. Pfeffer, Druck- und Arbeitsleistungen, 1893.
412 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
nearly or entirely resumed, but when the resistance increases progressively
growth is naturally more and more retarded. In the former case, how-
ever, more work is done than when growth is entirely unrestricted, just as
in the case of a man walking at the same rate in still air and against
a strong wind.
The external work done must reach a maximal value with a definite
resistance, since no work at all is done at either extreme, that is, when no
growth is possible or when no resistance is applied. The actual amount
of external work done affords, however, no criterion as to the internal work
involved in growth, and the latter may remain unaltered when the plant
is caused to do considerable external work against a resistance l.
Similar considerations apply to tissues 2, for in them the slowly grow-
ing or inactive cells constitute the resistance which is subjected to tension
by the elongation of the more rapidly growing cells. The conditions are,
however, somewhat more complicated, since, in part at least, we are dealing
with cells capable of growth-responses, and if the stretched tissues continue
to grow when the organ is enclosed in a rigid plaster cast, their tension
disappears and the compressed tissues act against the cast. This occurs
in the nodes of grass-haulms, whereas in many cases the tissue-tensions do
not entirely disappear even after prolonged enclosure in a plaster cast. On
setting free the organ the original tissue-strains are gradually restored, and
the same applies to the individual cells whose walls are again stretched by
the internal osmotic pressure. Since, however, the cohesion of the cell-wall
is unaltered, no ruptures occur however suddenly the external resistance is
removed.
Curving organs are often able to perform considerable external work.
Thus a horizontally-placed shoot may still be capable of a geotropic
curvature although it is forced to counteract from four to thirty times the
normal mechanical moment, and the curvature is only slightly retarded with
a moderate load, although much retarded by a considerable one. Even a
slender root may exert a considerable thrust if lateral displacement is
prevented, and in the same way shoots are able to break through stiff soil.
The bursting of the hard coats of seeds, the splitting of the bark by
the growth of the wood, and the strangulation of trees by lianes, are all
instances in which a considerable external force is exercised as the result
of physiological activity.
The rapid movements of the stamens of Parietaria, the sudden de-
hiscence of the fruits of Impatiens and Momordica, are produced by the
sudden release of tensions gradually built up during growth. The twisting
and untwisting of the fruit-valves of Leguminosae produced by imbibi-
tion and drying, as well as the similar movements of the awns of certain
1 Pfeffer, Druck- und Arbeitsleistungen, 1893, pp. 330, 419. a Id., 1893, pp. 379, 400, 426.
SPECIAL CASES 413
grasses, are purely physical in origin. Whereas in the first-named cases
not only does the plant see to the storage of the required potential energy,
but also so regulates matters that the tensions are released spontaneously
or by an external agency at a definite time. Since the contracting tissues
are never perfectly elastic, the full amount of the stored energy can never
be used in propelling the seeds '.
Mechanisms of this kind are only capable of a single response, whereas
the regeneration of the tissue-tensions in the stamens of Cynareae, and in
the pulvini of Mimosa^ renders frequent repetition possible. A sudden fall
of turgor allows the elastic walls to contract, the restoration of turgor
redistends them, but whether changes in the elasticity of the cell-wall may
also occur is uncertain. From a mechanical point of view the mode in
which turgidity is restored is immaterial, and the escape of water is the
result of the fall of turgor, so that sudden contraction can only take place
when a rapid filtration of water under pressure through the cell-wall is
possible.
The energy of contraction is as great in these motile tissues as in
animal muscle, in which it may be from i to 10 kilograms per sq. cm., while
a load of 5 kilograms per sq. cm. is required to prevent a staminal filament
of one of the Cynareae from contracting. In both cases most work is done
when the load is such that contraction is just possible, and to get the full
contraction the load must be steadily decreased as contraction continues 2.
Otherwise an excessive load at any phase of contraction prevents the
shortening and hence also prevents work from being done. In precisely
the same way the maximal work is done during the subsequent elonga-
tion of the filament, if a resistance is interposed of sufficient intensity to
prevent movement until turgor is fully restored, and if the filament is then
allowed to elongate to its full extent by gradually removing the resistance.
A growing organ, on the other hand, which exerts a constant pressure upon
a resistance pushed in front of it, performs the same amount of work in
unit time so long as the rate of growth remains the same.
1 Pfeffer, Studien zur Energetik, 1892, p. 239.
3 Pfeffer, 1. c., pp. 236, 238. The same applies to the work done during the expansion of a com-
pressed gas, or the contraction of a rarefied one. It is uncertain whether the slight increase in the
production of heat in the pulvinus of Mimosa pudica during a movement produced by stimulation
is due to a chemical reaction, or to the internal friction produced as the water escapes through the
cell-sap. [The latter is hardly probable. Suppose a total of 5 gram, centimetres of work were
done, a high estimate, this would represent — -t of a calorie. If the region warmed corresponded
to 5 mg. of water, it would be raised only ^° C. in temperature, even if all the heat was retained
during the whole time of contraction. It must further be remembered that in cells bounding
intercellular spaces the surfaces of the cell are, owing to evaporation, colder than the cell-sap,
which is entirely enclosed by the heat-producing layer of protoplasm. Hence a thermo-electric
needle lying in an intercellular space or in a pierced and collapsed cell will show a rise of
temperature as the warmer cell-sap exudes from the surrounding cells.]
414 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY
The high energy of expansion and contraction in the stamens of
Cynareae and in the leaves of Mimosa enables these organs to raise a con-
siderable weight in addition to their own. According to Schilling1, a
stimulated leaf of Mimosa pudica returns to its original position after the
statical moment exercised upon the primary pulvinus has been increased
from two to four times by the attachment of weights. This is probably
due to the fact that the change of position produced by stimulation awakens
reactions directed towards the restoration of the original position ; and hence
a leaf slowly rises up again after it has been merely bent somewhat down-
wards by the addition of a weight without being stimulated. The exact
causal relationship of these phenomena is, however, not satisfactorily deter-
mined by Schilling's experiments, which also leave it uncertain to what
degree a similar power of reaction is possessed by other pulvini. In any
case, however, the mechanical factors concerned in the movements of
irritable pulvini and of the stamens of Cynareae have been more fully
explained than those involved in muscular movement.
1 A. J. Schilling, Der Einfluss von Bewegungshemmtmgen auf die Arbeitsleistungen d. Blatt-
gelenke von Mimosa pudica, 1895, p. n.
APPENDIX
IN the following pages some important facts not mentioned in the first two
volumes are given, and also a summary of the more recent literature, especially that
connected with the present volume.
Action of Rontgen and Radium Rays. Koernicke (Ber. d. D. hot. Ges., Bd. xxu,
1904, pp. 148-55) finds that the Rontgen and radium rays slightly accelerate the ger-
mination of dry seeds, and retard growth if the exposure is sufficiently intense and
prolonged. The retarding effect may remain latent for a time, and may be preceded
by a temporary acceleration.
Correlation and Growth. Lindemuth (Ber. d. D. bot. Ges., Bd. xxn, 1 904, p. 1 7 1)
states that the leaves of Begonia rex, of Iresine Lindeni, and of other plants on rooting
increase in size, and concludes that this is due to the enlargement of the individual
cells. The latter can hardly apply to the non-living cells of the leaf so that internal
distorsions or ruptures should ensue in such leaves.
Ewart (Annals of Botany, Jan., 1906) has shown that by an early removal of all
the buds but one, the leaves of the Lime (Tilia europaea) may be caused to develop
to twice or thrice their normal size, and that this increase in size is due to an increase
in the number of cells in each leaf, their sizes being approximately constant. No
growth could be awakened in leaves which had ceased to grow.
The Phloroglucin Reaction is due to the presence of Hadromal^ an aromatic
aldehyde which is destroyed by potassium permanganate or hydroxylamine.
Wood sections treated with hydroxylamine no longer give the phloroglucin
reaction, but if treated for five minutes with i per cent. KMnO4, washed with HC1,
and then exposed to NHS vapour they turn red, giving a true 'lignin' reaction.
Hadromal occurs in many non-lignified cells (hard bast of certain plants, endodermal
cells and cork cells). These give no reaction with KMnO4, except in the case of
the endodermal cell-walls of Iris, which appear to be truly lignified.
Cleistoganiy. On the causes and occurrence of cleistogamy, see K. Goebel,
Biol. Centralbl., 1904, Bd. xxiv, p. 673 seq. See also Klebs, ibid., p. 545 (Ueber
Probleme der Entwickelung).
The Mechanical Properties of the Red and White Wood of Conifers have been in-
vestigated by Sonntag (Jahrb. f. wiss. Bot., Bd. xxxix, p. 71), with the following results :
Length
Incrusting materials
Tensile strength of walls ....
Resistance to pressure per unit area of wood
Comparing walls alone ....
Capacity for stretching ....
But Modulus of elasticity ....
Red wood tracheides.
80* %
I
(walls thicker) higher
slightly higher
1.5 to 2-5 % of length
i
White wood tracheides.
60* %
3
lower
slightly lower
the same
2 (Hartig).
Branches are as strong in the inverted as in the normal positions, until the
limit of elasticity is reached with increasing loads, when the normal position is best.
The red wood is more strongly lignified and swells less. It appears on the
4i6 APPENDIX
pressure side, in all cases, even when this is the side in the erect position com-
pressed by the wind. The response, hence, does not appear to be due to the
stimulatory action of gravity, but Sonntag concludes that heliotropic influences take
part in inducing the differentiation.
Observations by Ewart and Mason- Jones (Annals of Botany, 1906) upon the
formation of red wood in lateral and main axes of Cupressus and Pinus bent
forcibly into a circular form, show that the red wood mainly appears upon the under-
surfaces whether these are stretched or compressed, and thence spreads into the
neighbouring vertical or nearly vertical regions of the curved axis. It appears
therefore to be the result of a gravitational stimulus, which spreads along the same
side from the point directly stimulated. It is difficult to see how differences in
the intensities of illumination on the upper- and under-surfaces could act in the
way Sonntag supposes upon the living cambium of old stems covered by thick
opaque layers of bark. Nor can differences of temperature come into play since
otherwise red wood would appear on the sunny side of an erect stem.
Formative and Inductive Action of Light. According to observations made by
Dr. Buller at Birmingham, one half to one hour's exposure to light is sufficient to
induce the formation of a pileus upon the branching strands of Polyporus squamosus
developed in darkness. After several weeks' continuous darkness, however, small
patches of spore-bearing tubes are formed at intervals on the under-surfaces of the
strands, but some were even formed upon the upper-surface of an imperfectly
developed pileus. In normal healthy pilei, the hymenial tubes are positively geo-
tropic in both light and darkness.
Autonomic Movements. Molisch (Ber. d. D. bot. Ges., Bd. xxn, 1904, p. 372) ob-
served very rapid autonomic movements in Oxalis hedysaroides, H. B. K., the leaflets
falling suddenly or in jerks through i to i^cm. in 1-12 seconds.
Photonastic and Thermonastic Movements. W. Wiedersheim (Jahrb. f. wiss. Bot.r
1904, Bd. XL, p. 230) finds that the photonastic and thermonastic movements of flowers
and leaves involve a general acceleration of the average rate of growth, which is shown
even when movement is prevented. This is produced first on the concave side, and
later on the convex, which latter results in a more or less pronounced return curva-
ture. Since a forced mechanical curvature does not cause any such return curvature
when the leaf is released, it can hardly be due to an autonomic orthotropic response,
as Schwendener and Jost suppose it to be. In other words, both sides of the leaf or
halves of the pulvinus react in the same way, but one responds more rapidly than the
other. The fact that Impatiens parviflora performs its sleep-movements on a klino-
stat, and hence is ' autonyctitropic/ was first observed by Fischer (Bot. Ztg., Bd. XLVIII,
1890, No. 42).
Dispersal Movements. The spores of Agaricus, Polyporus, Boletus, Coprinus, and
other Hymenomycetes, do not merely fall off, but owing to the sudden rupture
of the stretched cuticle are jerked off with sufficient energy to clear the hymenium,
without striking the opposite gill-lamella or pore-wall. The vertical arrangement
allows them to fall clear, but, owing to their small size, they descend in still air, in
accordance with Stokes' formula with a constant velocity of from a few mm. to cms.
per second. (Observations by Buller at Birmingham.)
APPENDIX 417
Geotropism. Figdor (Ber. d. D. bot. Ges., 1905, Bd. xxm, p. 182) finds that the
leaf-sheaths of various grasses perceive and respond to geotropic and heliotropic stimuli,
whereas the laminas (Rothert) are irresponsive.
H. Fitting (Jahrb. f. wiss. Bot., 1905, p. 221) finds that in all cases the maximal
excitation is produced in the horizontal position, and not at an angular deviation of
135° from the equilibrium position, as is usually supposed. With angles less than
30° the excitation decreases somewhat more rapidly than the size of the angle of
divergence, but otherwise they are approximately proportional. Plants rotated
obliquely once every second show a geotropic curvature as the result of the sum-
mation of successive geotropic stimuli.
Large angles of divergence must differ more to produce unequal results than
when the angles are small (Weber-Fechner's law).
The presentation periods for epicotyls of Vicia Faba and Phaseolus are from
6 to 7 minutes, and for hypocotyls of Helianthus from 5 to 6 minutes.
Rapid intermittent stimulation is not more active than constant stimulation, and
the duration of the individual periods of stimulation is almost immaterial. If they
bear a ratio of i : 5 to the periods of rest, the response is nearly as rapid as
with continuous stimulation.
The length of the induction-period and the time of response afford no criterion as
to the geotropic irritability, which, in sensitive organs, involves a discriminatory power
equal to that possessed by heliotropic organs.
Newcombe (Annals of Botany, 1905, Vol. xix, p. 313) now considers that
orthotropic stems and roots are equally stimulated geotropically by similar angular
divergences above and below the horizontal, whereas lateral roots when dis-
placed curve more readily downwards than upwards.
Luxburg (Jahrb. f. wiss. Bot., 1905, p. 399) states that in shoots with apical
growth (excepting Hippuris] the average rate of growth is slightly retarded on
a klinostat, whereas in plants with nodes it is accelerated, and in Tradescantia flumi-
nensis more than when the nodes are placed horizontally so that they are under the
continuous unilateral action of gravity. In T. virginica, however, growth is unaltered
on the klinostat, but accelerated in the horizontal position.
The action of unilateral geotropic stimuli is, therefore, twofold, involving :
1. A change in the general rate of growth,
2. A change in the distribution of the rate of growth ;
but the exact relationship between them is uncertain.
Hering (Jahrb. f. wiss. Bot., Bd. XL, Heft 4) found that the growth of parallelo-
geotropic organs was directly retarded in the inverse position.
Portheim (Sitzungsb. d. k. Akad. der Wiss. Wien, October, 1904) discusses the
influence of gravity upon the orientation of flowers.
The Slatolith Theory of Geotropism. Tondera (Anz. d. Akad. d. Wiss. in
Krakau, 1903, p. 512) states that the youngest geotropic internodes of Cucurbitaceae
possess no movable starch-grains; but Jost (Bot. Ztg., 1904, p. 277) was unable
to confirm this observation.
F. Darwin (Proc. Royal Soc., 1903, Vol. LXXI, p. 362) confirms Haberlandt's obser-
vation that in plants kept at high temperatures, as the starch disappears, the geotropic
PFEFFER. HI
4i8 APPENDIX
irritability diminishes, but finds that the same applies to the heliotropic irritability,
and concludes that the general decrease of irritability is due to the direct action
of the high temperature.
He also finds that shaking favours geotropic responses as found by Haberlandt,
but does not appreciably affect heliotropic reactions. This affords no proof of the
starch statolith theory, since not only will all other dense particles be affected, but
also intermittent variations of hydrostatic pressure in the cell will be produced by
every up and down movement. Jost found that roots subjected to centrifugal forces
of 0-02 to 0-05 g. showed geotropic curvatures before any appreciable unilateral
accumulation of starch had taken place, and found movable starch in tertiary non-
geotropic roots. Darwin and Pertz (Proc. Royal Soc., 1904, Vol. LXXIII, p. 477) have
shown, however, that these roots become geotropic when the others are removed,
and they were unable to find any geotropic response without a movement of the
starch-grains, probably because less sensitive plants and longer exposures were
used.
Nemec (Beih. z. bot. Centralbl., 1904, Bd. xvn,p. 45) states that after the removal
of the starch-bearing columella of Lupinus roots, a geotropic curvature takes twenty
hours, and by this time movable starch-grains have reappeared.
The geotropic flowers of Clivia nobilis have motile starch-grains, which are
absent from the non-geotropic flowers of Clivia miniata. Many non-geotropic
organs have motile starch-grains, however.
Fitting (Jahrb. f. wiss. Bot., 1905, p. 331) has shown that the collection and
movement of the starch-grains are of no importance in geotropic perception, for the
response may be as rapid when the starch-grains do not move as when they do,
independently of whether they are regularly or irregularly distributed.
In a strong magnetic field the diamagnetic starch-grains would tend to be
repelled from the neighbourhood of either pole-piece, and the paramagnetic con-
stituents of the protoplasm to be attracted. Hence if the starch statolith theory were
correct, roots of Ptsum placed horizontally above and below one pole of an
extremely powerful electromagnet should show stronger and more rapid curvatures
in the lower than in the upper series. The reverse is, however, the case, according
to the observations of Bayliss and Ewart, so that the gravitational and magnetic
forces appear to directly stimulate the protoplasm. The exposures were for periods
of four to six hours in a room heated to 23° C. The possibility of a thermotropic
action of the heated magnet upon the radicles was avoided by enclosing them in
moist cotton wool, or by interposing layers of wet blotting-paper. Short exposures
appear merely to exercise a disturbing action on the roots, and even with
prolonged exposures in so intense a field as that used for these experiments, the
magnetic action is feeble as compared with that of gravity. Vertically placed roots
showed no perceptible tendency to curve towards or away from either pole of the
magnet, either during the exposure or when subsequently rotated on a klinostat.
The Localization of the Geotropic Irritability of the Root-tip. Piccard (Jahrb. f.
wiss. Bot., 1904, Bd. XL, p. 94) rotated kidney-bean roots 30 to 40 times per second,
arranged obliquely to the axis of rotation which passed just behind the sensitive
apex (Fig. 70). Hence the apical and growing zones were subjected to opposed
APPENDIX
419
centrifugal components, with the result that an S-shaped curvature was formed.
Piccard concludes that each part of the root is able to perceive and respond to
geotropic stimuli. In this case
Czapek's results would be due to
the root-apex suppressing or over-
coming the stimuli generated in
the regions behind. Piccard's cur-
vatures might, however, possibly be
plastic in origin; and ten experiments
failed out of twenty-four tried.
T>- , , j i r j ,1 . FIG. 70. Diagram showing position of axis of rotation
JPlCCard alSO found that a rOOt between a a and££ in regard to >the obliquely-inclined apex
curved towards a wire kept at high oftheroot-
potential, and that when root and wire were at still higher but like potentials the root
curved away. The first effect is undoubtedly a more or less ' galvanotropic ' one due
to the silent discharge from the electrified wire. The other may be of the nature of a
' geotropic ' repulsion or rather of a ' magneto-tropic ' response, produced owing to the
varying magnetic permeabilities of the cell-constituents. Piccard supposes that it is
due to the direct repulsion acting on the surface-layers, and hence concludes that the
'geotropic' irritability is localized in the superficial layers. This assumption is,
however, not justified ; and judgement must be suspended until details of the experi-
ments are given, or confirmation obtained. The roots were, however, often injured
by sparking and ozone. In fact, since leaking is always occurring, and since the root
has a high resistance and may undergo polarization, it will always be at a lower
potential than the wire when both are attached to the same terminal of the electrical
machine. It is also worthy of note that the root and wire were only 3 mm. apart.
Richter (Zur Frage nach der Function der Wurzelspitze, Wien, 1902 ; Inaug.-Diss.
Freiburg) failed to obtain Czapek's results, but F. Darwin (Linn. Soc. Journal Bot.,
1904, Vol. xxxiv, pp. 266-74) and Massart (Me'm. par 1'Acad. de Belgique, 1902) con-
firm Czapek's work by other methods. See also Czapek, Jahrb. f. wiss. Bot., Bd. xxxv.
Newcombe (Bot. Gazette, 1902, Vol. xxxn,-p. 177) finds that the non-growing
zones of roots as well as the apex are sensitive to rheotropic stimuli.
The Light Position of Leaves. G. Haberlandt (Ber. d. D. bot. Ges., Bd. xxn,
1904, p. 105) finds that in various species of Tropaeolum the lamina is able to perceive
light stimuli and transmit them to the upper part of the darkened pulvinus ; but the
response is slower than normal, and the full position is not always assumed. He
concludes that the petiole is responsible for the coarse, and the lamina for the fine
adjustment of the leaf. In Begonia discolor the influence of the lamina predominates,
and the same applies to Monstera deliciosa, which possesses large pulvini.
Vochting (Bot. Ztg., 1888) found that on the removal of the laminas of Malva
•verticillata, the petioles performed no orienting curvatures.
Krabbe (Jahrb. f. wiss. Bot., Bd. xx, 1889) stated that the leaves of Phaseolus and
Fuchsia assumed fixed proper light positions when the lamina was covered with dark
paper, but not when the pulvinus was darkened.
Rothert and Darwin also found that the petiole of Tropaeolum minus was
heliotropic, even when the lamina was darkened.
E e a
420 APPENDIX
Haberlandt confirms all the above results — leaves differing from one another,
and falling into three types.
1. The lamina predominates. Begonia discolor.
2. The lamina and petiole co-operate, the fine adjustment being regulated
by the lamina. Tropaeolum and Malva verticillata.
3. The pulvinus alone is the responsive and percipient organ. Phaseolus.
Haberlandt suggests that since the pressure of sunlight amounts to 0-5 mg.
per sq. metre, according to Maxwell, it is possible that the heliotropic perception may
involve a power of discrimination to light pressure on the part of the plasmatic
membrane. It must, however, be remembered that the pressure exercised by the
incident light upon the peripheral surface of the transparent plasmatic membrane
is only a very minute fraction of the total pressure exercised upon an opaque surface.
Phototaxis. Th. Frank (Bot. Ztg., 1904, orig., p. 162) finds that the zoospores
of Chlamydomonas tingens, which are negatively phototactic in strong light, and
positively so in feeble light, come slowly to rest in darkness without losing their vitality.
Light stimuli easily overcome their feeble chemotactic irritability.
Thermotropism. Ganong (Annals of Botany, 1904, Vol. xvm, p. 631) finds that
leafless shoots in winter move upwards and inwards until March, and then outwards.
The latter movement is shown to a less degree whenever the temperature rises during
winter. Young shoots show the movement best, and he concludes that it results
from a rise of turgidity in the cells due to the direct or stimulating action
of temperature. It is, however, possible that the wood on the upper and lower
surfaces may differ anatomically and in power of swelling, as was shown by Hartig to
be the case in the pine.
Lidforss, however (Jahrb. f. wiss. Bot., 1902, Bd. xxxvui, pp. 343-76) concludes
that the raising of the shoots of certain herbs in warm weather is due to negative
geotropism, and their horizontal position to diageotropism, their rapid fall at low
temperatures being due to epinasty.
Chemotropism. Lilienfeld (Ber. d. D. bot. Ges., Bd. xxxm, 1905, p. 91) found
that the radicles of Lupinus albus curved in gelatine towards phosphates and car-
bonates, but away from salt and poisons, and were indifferent to KNO3. It may be
noted that alkaline (Na2HPO4) or acid (KH2PO4) phosphates and alkaline carbonates
were used, so that the curvatures may be due to the action of the acidity and alkalinity.
The poisons might accelerate growth on the exposed side ; and, experimenting in
a somewhat different manner, this author, and also Newcombe and Rhodes (Bot. Gaz.,
Bd. xxxvn, 1904, p. 23) found that the roots of the same plant grew into slabs of
gelatine containing strong solutions of the feebly alkaline phosphate of sodium
(Na2HPO4) and were killed.
The local application of dilute acid or alkali does induce curvatures in roots, and
since these curvatures do not necessarily involve any injury, they cannot be trauma-
tropic in origin (Ewart and Bayliss, Proc. of Royal Soc., 1905, Vol. LXXVII, p. 64).
Chemotaxis. According to Senn (Schweiz. naturf. Ges., July, 1904), the chloro-
plastids of Funaria are positively chemotactic to CO2 and sulphates, organic acids,
and certain assimilatory products, but not to cane-sugar. They are negatively chemo-
tropic to nitrates and phosphates, and this feeble chemotropism is responsible for the
APPENDIX 421
normal position on the contact-walls assumed in darkness, but is readily overcome by
their more powerful phototropism on exposure to diffuse or strong light.
Frank (Bot. Ztg., 1904, orig., p. 162) finds that the zoospores of Chlamydomonas
tingens are attracted by nitrates, phosphates, nitric and carbonic acids, whereas sul-
phuric and hydrochloric acids, salts of ammonium and metals, cane-sugar, glycerine,
asparagin, and peptone are indifferent. Strong acids and alkali repel, and strong
meat-extract (0-3 to 2-0 per cent.) attracts.
Shibata has investigated the chemotaxis of Isoetes spermatozoids (Jahrb. f.
wiss. Bot., Bd. XLI, 1905, p. 561 ; Ber. d. D. Bot. Ges., Bd. xxn, 1905, p. 478). Malic
acid and its salts attract in a concentration of 0-00067 Per cent., but free acid repels
in one of 0-026 per cent. A few organic acids of similar constitution, such as fumaric
acid, act similarly, but more feebly, whereas its stereoisomer, maleic acid, has no
attractive action. H, HO, and acid ions repel, as is shown by the dependence of the
repellent action on dissociation and concentration.
Dissociating salts of Ag, and to a less degree of Hg, Cu, Zn, Ni, and Co,
exercise a very strong repellent action, but not poisonous alkaloids. Anaesthetics
suspend the irritability while locomotion continues. The repulsion is phobic, the
attraction tactic. On the chemotaxis of the sperms of Equisetum and of Salvinia see
Shibata, Bot. Magazine, Tokyo, Vol. xix, 1905, pp. 39, 51, 79.
Lidforss (Jahrb. f. wiss. Bot., 1905, Bd. XLI, pp. 65-87) finds that the antherozoids
ofMarchantia are positively chemotactic to albumins, globulins, nucleo-albumins, and
other proteids, the minimum dilution being 0-0005 per cent., while 5 per cent, solu-
tions repel by negative chemotaxis, since the organisms have no osmotactic irrita-
bility. They are, however, feebly aerotactic, and are attracted by an extract of the
archegoniate heads made in the same way as diastase is extracted from leaves. The
proteids mentioned above also attract pollen-tubes (Lidforss, Ber. d. D. bot. Ges.,
1899, Bd. xvii, p. 236).
Chemotaxis (infl. of anaesthetics). According to Rothert (Jahrb. f. wiss. Bot., 1904,
Bd. xxxix, p. i), chloroform and ether immediately suppress the chemotaxis, aerotaxis,
and osmotaxis of Bacterium termo forms, the chemotaxis of Spirillum tenue and Bacillus
Solmsii, the aerotaxis of Beggiatoa alba, the chemotaxis of Trepomonas agilis, the
chemo- and osmotaxis of Saprolegnia zoospores, and the phototaxis of Euglena
viridis, Chlamydomonas, Gonium pectorale, Pandorina morum. In some cases the
locomotion is as active as before, in others more or less retarded.
Weak chloroforming changes the negative phototaxis of Chlamydomonas and
Gonium to positive, i.e. renders them less responsive to the higher intensity of light,
whereas ether exercises no such effect even in fatal concentrations.
Elfving, Ueber die Einwirkung von Aether und Chloroform auf die Pflanzen,
Ofversigt af Finska Vetenskaps Soc. Forhandlingar xxvm, 1886, found that ether
produced this effect, but not chloroform.
All these reactions are instantaneous, and are independent of the duration of the
anaesthetization ; whereas slight doses, which at first affect neither locomotion nor
irritability, gradually retard the movement and may exert an ultimately fatal effect
before locomotion has ceased. In all cases individual differences are shown, some
forms being more sensitive than others. In the case of Gonium, after anaesthetiza-
422 APPENDIX
tion the organisms appear temporarily to be more sensitive to light, and show
a stronger tendency to negative phototaxis.
Galvanotropism. Ewart and Bayliss (Proc. Royal Soc., 1905, Vol. LXXVII, B.,
p. 63) have shown that the supposed positive parallelo-galvanotropism of roots does
not exist, and that this galvanogenic curvature is due to the stimulatory chemotropic
action of the products of electrolysis. When one electrode only is on the irritable
zone the curvature always takes place towards the stimulated side whether it becomes
acid or alkaline, but when a current of about o-oooooi of an ampere is led trans-
versely through the irritable zone the curvature takes place towards the acid side.
A similar curvature without injury is produced by the direct application of decinormal
acid and alkali on opposite sides of the root, and the application of an excised elec-
trolysed region of a root to another one produces in it a curvature towards the acid
side. Neither this ' positive ' curvature nor the ' negative ' one is traumatropic in
origin, since they are not necessarily accompanied by injury even to the superficial
tissues of the root.
The negative curvatures are only shown when the direct action of the travelling
ions is not overpowered by the action of the accumulated products of electrolytic
decomposition in or around the root. If roots are imbedded in 3 per cent, gelatine,
in which the deficiency of oxygen almost entirely suppresses the geotropic
irritability, negative curvatures appear in the median region of the gelatine two or
more hours after passing through a four-volt current at right angles to the roots.
This is owing to the acid ions coming from the negative electrode exercising a
greater stimulating action than the alkaline ones repelled from the positive electrode.
Roots near to either electrode curve strongly towards them as the direct result of
the action of the accumulated acid and alkali at these points. Hence three
types of response are possible to the same current according to the position of the
roots, and all may be shown without injury.
Transference of Stimuli. Kretschmar (Jahrb. f. wiss. Bot., 1904, Bd. xxxix,
p. 273) finds that an injury-stimulus causing streaming travels in the vascular bundles of
Vallisneria through distances of from 0-6 to 1-5 centimetres per minute, and more
rapidly towards older parts than acropetally.
Phosphorescence. According to Molisch (Bot. Ztg., 1903, p. i), the best
luminous Bacterium is Micrococcus phosphoreus, which is readily obtained by laying
meat in 3 per cent, salt solution and keeping it moist at from 9° to 1 2° C.
INDEX
Abies, torsion of twigs of, 255.
Abrus, sleep-movements of, 102.
Absorption, energy of, 409.
Acacia, 257 ; A. lophantha, 102 ; influence
of gravity on sleep-movements of, 1 25 ;
of light on daily movements of, 108,
109, no, in; latent period of, 68;
photonastic movements of, 105 ; varia-
tion movements of, 22.
Accommodation to stimulation, 9.
Acer, 259.
Acetabularia mediterranea, orientation of
chloroplastids of, 328.
Acid, changes in production of, during
curvature, 247 ; chemotactic action of,
345 ; influence of, on streaming, 343 ;
repellent action of, 351; tropic and
tactic action of, 420, 421.
Aconitum, orientation of flower of, 258.
Acrasieae, cytotaxis of, 365 ; fusion of,
365 ; phototaxis of, 326.
Adanson, 273.
Aderhold, 274, 325, 347 ; on geotaxis, 336,
337 ; on rheotaxis, 356.
Adhatoda cydonaefolia, rate of revolution
in, 21.
Adonis vernalis, thermonastic flower of,
US-
Adoxa moschatellina, changes of tone, 205,
206; influence of gravity on photo-
nasty of, 127 ; tropism of runners of,
104.
Aeration, influence of, on geotropism, 182.
Aerial stems, orientation of, 249.
Aerobes, evolution of heat by, 372, 373-7.
Aerotaxis, 347.
Aerotropism, 179, 182.
Aeschynomene indica, 95.
Aesculus, geotropism of twigs of, 232;
origin of movement in flower of, 27 ;
A. hippocastanum, 375.
Aethalium, 304 ; A. septicum, 317 ; chemo-
taxis of, 348, 352 ; consistency of, 279,
282 ; pulsating vacuoles of, 295.
Agardh, 383.
Agaricineae, geotropism of, 165.
Agaricus, 416; production of heat in, 366,
373 ; A. Gardneri, luminosity of, 382,
383; A. tgneus, 383; A. melleus,
383 ; A. noctilucens, 383 ; A. olearius,
luminosity of, 383, 385, 387.
Aggregation, 88, 89; recovery from, 90;
in Drosera, 78.
Agropyrum, plagiotropism of radial rhizome
of, 156, 157.
Air-pressure, influence of, on movement,
85, 87 ; pulvinar, 76.
Aitiogenic stimuli, definition of, 2.
Akebia quinata, free coiling of, 37 ; rate of
revolution in, 21.
Albumin, electrical conductivity of, 392.
Alcaliotropism, 179.
Alchemilla, 113, 182.
Aldrovanda, influence of temperature on
leaf of, 113; re-expansion of leaf of,
87 ; seismonic irritability of cotyledon
of, 80 ; transference of stimuli in, 91 ;
A. vesiculosa, irritability of, 81.
Algae, coiling of, 46 ; irregular curvatures
of, 23 ; production of electricity by, 395.
Altsma, movements of, 19.
Alkalies, influence of, on pulsating vacuoles,
298 ; on streaming, 342, 343.
Allassotonic, definition of, 15.
Alliaria officinalis, influence of shaking on
percentage of sugar in, 248.
A Ilium, geotropism of cotyledon of, 165 ;
heliotropism of roots of, 173; move-
ments of, 20; — of peduncle of, 19.
A. controversum, curvature of peduncle of,
27 ; A. ursinum, 258.
Alstromeria, 258.
Alveolarplasm, 303.
Amaranthus, sleep-movements of, 103.
Ambronn, 28, 39; absence of contact on
irritability in twiners, 35 ; origin of
homodromous curvature, 37 ; of torsion,
41.
Amici, on streaming, 289, 290.
Amicia, influence of gravity on sleep-move-
ments of, 125.
Amitosis, types of, 303.
Ammonia, action of, on motile stamens, 87.
Ammonium carbonate, influence of, on
movement, 30 ; salts, chemonastic
action of, 85 ; chemotropic action of,
181.
Amoeba, 275, 317; influence of streaming
on division of, 285 ; A. proteus, 299 ;
A. radiosa, 269.
Amoeboid movement, 275 ; influence of
light on, 320; mechanics of, 276-81;
origin of, 282, 283 ; rapidity of, 276.
Amorpha fruticosa, sleep-movements of,
102, Fig. 29.
Ampelopsis, 47, Fig. 15, heliotropic tendrils
424
INDEX
of, 171 ; origin of apical curvature of,
27 ; A. hederacea, climbing of, 33 ;
A. qninquefolia, disks of, 47.
Amphilobium Mutisii, disks of, 47 ; thicken-
ing of tendril of, 46.
Amylobacter,povtzr of discrimination in, 21 5.
Anaerobes, production of heat by, 377, 378.
Anaerobiosis, influence of food on, 340 ; in
Characeae, 341, Fig. 59, 342.
Anaesthetics, influence of, on chemotaxis,
421 ; — on production of electricity,
397 ; - - on streaming, 342 ; - - on
transference of stimuli, 94.
Anagallis amarella, hydronastic flowers of,
117 ; A. coerulea, 117.
Anaklinotropism, definition of, 155.
Anastatica hierochuntica, dispersal of, 151.
Anatomic stimuli, definition of, 6.
Anatomy of tropic organs, 243, 244.
Andrews, on influence of centrifugal force
on cells, 335, 336.
Anemone nemorosa, thermonastic flowers
of, 113 ; influence of gravity on, 127 ;
A. stellata, thermonasty of, 114;
thermotropism of, 177.
Antagonistic tissues, action of, 14, 18.
Antherozoids, chemotaxis and osmotaxis
of, 344, 345? 349 5 periodicity of swarm-
ing of, 267 ; phototaxis in, 325.
Anthers, influence of temperature on open-
ing of, 113.
Anthriscus sylvestris, changes in density
of sap of, 247.
Antidromous torsion, 41.
Antiferments, 227.
Antirrhinum, climbing of, 45.
Apobatic, 309.
Apostrophe, 333.
Apotropism, definition of, 155.
Arachis hypogaea, geotropic peduncles of,
165.
Arctotis, irritable stigma of, 82.
Areschoug, 248, 249.
Argotaxis, 309.
Aristolochia sipho, direction of twining of,
38.
Aroids, heat-production of, 370-7 ; tropic
irritability of aerial roots of, 164, 172,
173,
Artabotrys, 45, 46, 51, Fig. 17.
Arthur, on streaming, 289 ; in fungi, 284.
Arum cordifolium, heat-production of,
372 ; A. italicum, 368 ; heat-produc-
tion of, 372, 374, Fig. 67, 376; A.
maculatum, heat-production of, 372,
376 ; luminosity of, 383.
Ascherson, 151, 383.
Asci, dehiscence of, 149, 150, Fig. 34.
Ascobalus, 294 ; A . purpureus, influence of
light on dehiscence of, 153.
. Ascomycetes, mode of spore dispersal in,
146.
Ash, torsion of, 233.
Askenasy, 19, 29, 379; on action of
polarized light, 176; on dehiscence of
stamens, 148.
Asparagin, influence of, on amoeboid move-
ment of nuclei, 275 ; repellent action
of, 353-
Aspergillus, indifference of, to contact, 83 ;
A.fumigatus, heat production of, 370 ;
A. niger, chemotropism of, 181.
Asphodelus luteus, curvature of peduncle
of, 27.
Asplenium trichomanes, movements of, 30.
Assimilation, influence of, on surface-ten-
sion, 283.
Astasiaproteus, chemotaxis of, 347.
Astruc, 222.
Atragene, 44.
A triplex, movements of stamens of, 147 ;
A. latifolid, geotropism of, 165.
Atropa belladonna, 253.
Aubrietia deltoidea, 379.
Autogenic stimuli, definition of, 2.
Autonomic stimuli, definition of, 2.
Autonyctitropism, 416.
Autotropism, 189-92.
Auxanographic method, 386.
Auxotonic, definition of, 15.
Avena, 187, 234; change of irritability in,
5; hygroscopic awns of, 151; in-
fluence of darkness on nutation of, 30 ;
irritability of cotyledon of, 198, 199 ;
time of reaction of, 211 ; transference
of stimuli in, 200 ; upward curvature
of, 237 ; A. sativa, heliotropism of
cotyledons of, 193, 194, 198.
Averrhoa Bilimbi, influence of temperature
on circumnutation of, 29 ; spontaneous
movements of, 104 ; thermonastic
leaves of, 113.
Baccarini, 95.
Bacillus megatherium, osmotaxis of, 352,
353; B. phosphorescent, 385, 386; B.
Solmsii, 420 ; B. subtilis, chemotaxis
of, 346 ; B. virens, 306.
Bacteria, chemotaxis of, 180, 346, 347;
galvanotaxis of, 361 ; luminous, 382,
383 ; production of heat by, 366.
Bacterium chlorinum, 306; B. indicum,
382 ; B. luminosum, 382 ; B. Pfliigeri,
382 ; B. phosphorescens, 382, 383, 387 ;
B. photometricum, phobic movements
of, 306, 307, 308, 309, 310, 312, 320 ;
phototaxis of, 321 ; B. termo, 306,
421 ; chemotaxis of, 346, 347, 350 ;
discriminatory sense of, 214; sensi-
tivity of, 354 ; B. 'uernicosum, 313 ;
B. Zopfi, 317.
Ball, 245.
Baranetzsky, 21, 29, 31, 34, 39, 40, 112,
232, 233, 236, 320, 326, 337 ; on asym-
metric circumnutation, 35 ; diageo-
tropism of twiners, 37 ; influence of
INDEX
425
gravity on circumnutation, 28 ; helio-
tropism of climbers, 42 ; mode of
climbing, 36 ; origin of torsion, 41,
257; orthotropism, 190; plagiotropism,
254, 255 ; influence of twining on
growth, 36.
Barbula, torsion of peristome of, 24, 27.
Barley, production of heat by, 370 ; statical
moment of horizontal stem of, 237.
Barnard, 382, 388.
Barth, 164, 202, 240, 242, 246 ; on geo-
tropism of nodes, 231, 235.
Bastit, 166.
Batalin, 86, 91 ; on closure of leaf of
Dionaea, 80 ; on curvature oiDrosera,
85 ; growth curvatures, 137 ; path of
stimuli in Drosera, 90 ; sleep-move-
ments, 103.
Bateson, on optimal geotropic angle, 217.
Bauhinia, folding of leaves of, 108 ; pres-
sure of tendrils of, 237 ; sleep-move-
ments of, 105, 107, Fig. 31 ; B. tomen-
tosa, growth of tendril of, 46.
Bay, 211.
Bayliss, 418, 420, 422.
Bazin, 237.
Becquerel, 290, 291, 379, 394.
Becquerel rays, non-tropic action of, 176.
Beggiatoa, phototaxis of, 321 ; B. alba,
421 ; B. mirabilis, 304.
Begonia discolor, 419 ; B. rex, 415.
Behrens, 181, 301.
Beit, on cause of heliotropism, 244.
Bellis, closure of capitulum of, 103.
Benecke, 148, 272, 325 ; on chemotaxis of
Diatoms, 348 ; stimulatory plasmo-
lysis, 75-
Bennett, on aerotropism, 182.
Berberis vulgaris, irritable stamens of, 8,
63, Fig. 22, 68 ; action of ammonia and of
evacuation on, 87 ; escape of waterfrom,
77 ; excitation of, 92 ; influence of
induction shocks on, 145 ; — of nitrous
oxide and ether, 144; mechanism of,
8 1 ; transference of stimuli in, 91, 92.
Berg, on rheotropism, 184, 185.
Bergsma, 370, 372.
Bernard, 144.
Bernstein, 390 ; on surface-tension move-
ments, 278, 283.
Bert, 93; on pulvinar movements, 71, 138,
140 ; influence of light on, 142 ; of
poison on, 144 ; temperature of pul-
vini, 78.
Berthold, 23, 166, 180, 324, 330, 332 ; on
non-ciliary locomotion, 265 ; proto-
plasmic movement, 275, 276, 277, 280,
284, 286, 287, 291, 292, 293, 301, 304 ;
on tropism, 223 ; on reversal of helio-
tropism, 171, 174.
Bethe, 224, 302.
Betula, 259.
Beyer, 24.
Beyerinck, 317,34°, 347,353, 382, 385, 386,
387, 388.
Biedermann, 360, 362, 390, 391, 392, 393,
396, 397, 398.
Bignonia, 42 ; disks of, 47 ; irritable stigma
of, 82 ; influence of anaesthetics on — ,
144 ; B. unguis, grappling-hooks of, 33.
Biophytum sensitivum, 91, 93 ; influence of
gravity on sleep-movements of, 125 ;
response of, to contact, 69 ; sensitive
hairs of, 76 ; transference of stimuli in,
96.
Biota, 261.
Birukoff, 362.
BischofF, 386, 387.
Blaze currents, 395.
Blazek, 303.
Blechnum volubile, twining stems of, 38.
Blepharoplasts, 269.
Blochmann, 281, 320.
Blondeau, 145.
Blumenbachia lateritia, reversal of twining
in, 39-
BodOy 324; B. saltans, chemotaxis of, 347,
351; movements of, 267; tactic re-
sponses of, 157.
Bohm, 328, 333.
Bohn, 176.
Boirivant, 204.
Boletus, 416 ; production of heat by, 366, 373.
Bonnet, 191, 235, 255, 257 ; on orientation,
161.
Bonnier, 76, 85, 407 ; on production of
heat, 369, 374.
Bordage, 25, 104.
Bordet, 325, 346, 358.
Borodin, 332, 333.
Borscow, 271, 326, 357.
Boruttau, 201.
Borzi, 78, 144, 324 ; on conduction of
stimuli, 95 ; on distribution of sensi-
tivity in pulvini, 76.
Bose, 397.
Botrydiumgranulatum,y>q, 315 ; influence
of light on zoospores of, 320, 322.
Botrytis cinerea, rheotropism of, 185.
Boudier, 46.
Bouffard, 378.
Boussingault, 380.
Boussingaultia baselloides, negative helio-
tropism of, 42.
Bowiea volubilis, twining of, around hori-
zontal supports, jo ; on a klinostat, 35.
Boyle, 386.
Branches, woody, curvature of, 232.
Brand, 273.
Brandt, 263.
Brassica nigra, geotropic curvature of, 163,
Fig. 35-
Braun, 153, 285, 292, 293, 325, 355.
Bredig, 396.
Brefeld, 173, 175, 378, 383 ; on luminosity,
385.
426
INDEX
Briquet, 235.
Brodiaea congesta, localization of sensation
in, 194 ; transference of stimuli in, 200.
Brongniart, 372.
Brownian movements, 263.
Briicke, 290; method of, 135, 137, 138;
on pulvini of Mimosa, 79, 80 ; rigidity
of—, 1 8, 77.
Bruckner, 102.
Brunchhorst, on galvanotropism, 188, 189.
Bryonia, 42 ; B. dioica, tendrils of, 43,
Fig. 9.
Bryophyta, geotropism of, 166.
Bryopsis, 329 ; B. muscosa, geotropism of,
165; heliotropism of, 174; B. plu-
mosa, 324 ; influence of light on zoo-
spores of, 320.
Buchner, 407.
Buchtien, 174.
Buds, opening of, 23.
Buff, 394.
Bulbocodium vcrnum, 113.
Buller, 181, 358, 416; on chemotaxis, 344,
345, 346, 350.
Bullot, 219.
Burdon- Sanderson, 69, 71 ; on latent period
of Dionaea, 68; production of elec-
tricity, 393, 394.
Burgerstein, 71, 103, 123 ; on opening of
flowers, 137.
Burk, 82,
Burnett, on pulvinar mechanism, 80.
Burns, 22.
Bursaria truncatella, galvanotaxis of, 363.
Biisgen, 380.
Busse, 182, 204.
Biitschli, 264, 266, 267, 269, 270 ; on lu-
minosity, 385, 388 ; on protoplasmic
movement, 276, 277, 280, 282, 285 ;
physics of, 291, 293, 294, 296, 297, 298,
299> 3°2, 304.
Buxus, 259.
Calcium nitrate, repellent action of, 351, 352.
Caloritropism, 178.
Caltha palustris, 336.
Calypogeia trichomanis, centrifuged leaf-
cell of, 335, Fig. 56.
Calyptrogen, localization of irritability in,
198.
Campanula, 23.
Camphor, tropic action of, 182.
Cane-sugar, chemotactic action of, 345.
Capillary tubes, use of, for chemotaxis, 344,
Fig. 61.
Caragana, 259.
Carbon dioxide, chemotactic action, 353 ;
influence of, on pulsating vacuoles,
299; shock movements due to, 320;
tropic action of, 182.
Cardamine hirsuta, dehiscence of, 148.
Cardiospermum Halicacabum, 42.
Car ex arenaria, ascending roots of, 164.
Carlet, 30.
Carlgren, 361.
Car Una, dispersal of, 151.
Carpotropic, definition of, 3.
Caspary, 373, 375.
Cassia, irritable cotyledons of, 92 ; seis-
monic irritability of cotyledons of, 80 ;
sleep-movements of, 104 ; C. montana,
midday sleep of, 106.
Cassytha, twining of, 48.
Catalpa, influence of anaesthetics on stigma
of, 144.
Catasetum, irritability of, 147.
Catharinea undulata, twining rhizoids of,
38, 46.
Caulerpa, electrical currents in, 395 ; locali-
zation of irritability in, 195 ; streaming
in, 357 ; C. prolifera, geotropism of,
165 ; heliotropism of, 174.
Celakovsky, 263, 277, 304, 340; on aero-
tropism, 182 ; on phototaxis, 323.
Celastraceae, twining of, 38.
Cells, changes in shape and size of, during
curvature, 240; in reducing action of — ,
227 ; curvature of, 14 ; influence of
size of, on streaming, 284, 285, 288 ;
localization of irritability in, 195 ;
transference of stimuli in, 200, 201.
Cell-division, influence of centrifugal force
on, 335, 336; — of streaming on, 285.
Cell-wall, stretching and growth of, during
curvature, 15, 16, 244, 245, 246; in-
fluence of light on, 229.
Cels, 22.
Celtis, 260.
Centaurea, 62, Fig. 21 ; influence of induc-
tion-shocks on stamens of, 145; of
turgor, 4 ; transference of stimuli in,
91 ; C.jacea, latent period in, 68 ; mode
of movement of, 72 ; C. montana, fila-
ment of, 73, Fig. 26.
Centrifugal force, influence of, on orienta-
tion of Marchantia, 25 1 ; mechanical
action of, 334, 335; mechanism for,
166, 170; separating action of, 366;
stimulating action of, 1 66.
Centrosphere and centrosome, function of,
224.
Ceratium tripos, 382, 385.
Ceratophyllum, influence of darkness on
leaves of, 106; movements of, 19.
Ceropegia, abnormal twining of, 38.
Chaetoceras, 383.
Chaetophora, 293.
Chara, 24, 225, 308, 327, 334 ; anaerobiosis
of, 341, Fig. 59, 342 ; changes of tone
in, 204 ; chemotaxis of sperms of, 346 ;
escape of streaming endoplasm from,
280 ; independence of torsion of, on
gravity, 27 ; geotropism of, 165; helio-
tropism of, 174; seismonic irritability
of, 66 ; streaming in, 338, 357, 358 ;
— diagram of, 291, Fig. 51 ; — direc-
INDEX
427
tion of, 283, 292, 293 ; — duration of,
285 ; —influence of light and acids on,
319, 320; — of oxygen on, 341 ; — of
shocks on, 99 ; — of temperature on,
313? 3*45 — localization of, 286, 287,
288 ; — rate of, 284, 288 ; transference
of stimuli in, 95, 201 ; C. foetida,
anaerobism of, 341.
Chauveau, 361.
Chauveaud, 78.
Chemical actions on motile organisms, 338 ;
— energy, uses of, 405-8.
Chemokinesis, 6.
Chemonastic reactions, 85.
Chemotaxis, 343, 344, 353, 420 ; detailed
character of, 354, 355; substances
active in, 349 ; negative, 352.
Chemotropic tone, changes of, 215, 216.
Chemotropism, 178, 420; complex character
of, 179 ; discrimination by, 214 ; nature
of, 230; uses of, 179, 180, 181, 182.
Chenopodium, 258; C. album, photonastic
movements of, 105 ; sleep-movements
of, 103.
Chilodon propellens, 299.
Chilomonas, chemotaxis of, 348 ; C. curvata,
phototaxis of, 322; C. paramoedum,
galvanotaxis of, 361.
ChlamydomonaS) 307, 312, 421 ; energy of
cilia of, 268 ; flagellae of, 264 ; geo-
tactism of, 268; C. pulvisculus, chemo-
taxis of, 346; contact-irritability of,
358 ; geotaxis of, 336, 337, 338 ; photo-
taxis of, 323 ; C. tingenS) 420, 421.
Chloral hydrate, influence of, on fertiliza-
tion, 304 ; on nuclear division, 303.
Chloroform, influence of, on chemotaxis,
421; -- irritability, 144; — move-
ment, 85 ; streaming, 286.
Chlorogonium, pulsating vacuoles of, 294.
Chloroplastids, changes of shape of, 330;
mode of reaction of, 331 ; orientation
of, 220, 327, 328, 329, 331, 420 ; passive
movements of, 286, 287, 288; perception
in, 1 1 ; rapidity of reaction of, 332, 333.
Chodat, 303.
Chondrioderma, 279 ; chemotaxis of, 348 ;
pulsating vacuoles of, 295, 297.
Chromatin, density of, 336; influence of
stimuli on, 89.
Chromatium, 320; C. Weissii, chemotaxis
of, 346.
Chromatophores, density of, 336.
Chromophyton, 293, 324 ; C. rosanoffii,
creeping zoospores of, 265.
Chromoplastids, photic influence of, 332.
Chromosomes, origin of movements of, 302.
Chromulina Woroniniana, contact- irrita-
bility of, 358 ; geotaxis of, 337 ; photo-
taxis of, 323.
Chrysanthemum, closure of capitulum of,
103 ; C. leucanthemum, thermonasty
of, 14.
Chylocladia, photic cell-plates of, 332.
Chytridium vorax, 324 ; locomotion of,
265.
Cichorium intybus, irritable stamens of, 8l.
Cienkowski, 293, 294, 295, 298, 304.
Cilia, absence of, in Oscillariaceae, 273,
274 ; contact-irritability of, 358 ; dis-
tribution of, 264 ; influence of changes
of concentration on, 307 ; of gravity
on, 27, 28 ; mode of movement of,
265, 270 ; nature of, 269 ; reversal of
action of, 266, 267 ; thigmotropism of,
312.
Cinematograph, use of, 233.
Circaea, influence of gravity on photonasty
of, 127 ; C. lutetiana, changes of tone
in, 204, 206; tropism of runners of,
164.
Circulation, 283.
Circumnutation, characteristics of, 21 ;
general importance of, 1 1 ; in twining,
35, 39 ; influence of geotropic induc-
tion on, 28 ; periodic reversal of, 28,
.39-.
Cisielski, 166, 240 ; on tropism, 223.
Cissus paulinaefolia, disks of, 47.
Cistaceae, mode of movement of stamens
of, 81.
Cladophora, 335.
Clark, on influence of oxygen on streaming,
, 340.
Claussen, 151.
Claviceps microcephala, heliotropism of,
J73> 175 ; C. purpurea, geotropism of,
165.
Cleistogamy, 415 ; facultative, 97.
Clematis cylindrica^ changes of tone in
205 ; C. vitalba, 44.
Clifford, 317, 356.
Climbers, 32.
Climbing, uses and peculiarities of, 33.
Clivia nobilis, 418 ; C. miniata, 418.
Closterium moniliferum, phototaxis of, 274,
322, 325.
Coal gas, influence of, on irritability, 207.
Cobaea, absence of pits in tendrils of, 65 ;
C. scandens, circumnutation of, 21,
22 ; tendrils of, 42 ; claws of, 43, 44,
Fig. 10.
Cocoa, mode of planting of, 380.
Codiacum Wendlandi, tropism of leaf of,
231.
Codium tomentosum, 324.
Coemans, on dispersal of Pilobolus, 1 50.
Coesfeld, 260.
Coffee, mode of planting of, 380.
Cohen, 347.
Cohesion, influence of changes of, on amoe-
boid movement, 279, 280, 282 ; — on
shape of protoplasmic organs, 300.
Cohesion mechanism, 151, 152.
Cohn, 70, 71, 81, 91, 145, 149, 287, 314,
324, 325, 370; on movements of
428
INDEX
Cynareae, 79 ; on pulsating vacuoles,
293, 296, 297, 298, 299.
Coiling, influence of support on, 40; rate
and character of, 39 ; free, 36, 37 ;
homodromousj 37.
Colchicum autumnale, thermonastic flower
of, 113-
Coleoptile, 193.
Coleps hirtus, galvanotaxis of, 361.
Coleus, 258 ; orientation of leaves of, on
klinostat, 256, Fig. 47 ; transference
of stimuli in, 194.
Colocasia odora, production of heat by,
37o, 372, 374) 375> 376; movements
of, 19.
Colomb, 324.
Colour, influence of, on temperature of
plant, 380.
Coloured light, phototactic action of, 326.
Columella, geotropic irritability of, 224.
Commelinaceae, geotropism of nodes of,
242.
Compass-plants, 261.
Compositae, closure of capitula of, 103 ;
movements of stigmas of, 24.
Concentration, influence of, on osmotaxis,
350, 351 ; on pulsating vacuoles, 294,
296, 297, 298.
Conifers, curving of woody stems of, 12 ;
influence of decapitation on, 191 ; red
and white wood of, 414.
Conjoint effects, 119-28.
Conjoint stimulation, general action of,
6,7.
Conjugation, influence of temperature on,
305-
Contact, influence of, on direction of stream-
ing, 292, 293 ; on growth, 45, 46 ; on
formation of suckers, 47 ; on twining,
Contact-irritability, influence of ether on,
144 ; of geotropic induction on, 28 ;
persistence of, on a klinostat, 48 ; in
twiners, 34, 35, 36.
Contact-stimuli, mode of perception of, 65,
66, 67 ; response of Drosera to, 83,
Fig. 27 ; of fungi to, 82, 83 ; of stamens
to, 8 1 ; of stigmas to, 82 ; summation
of, 69.
Contractile vacuoles. See Vacuoles.
Contractility, of streaming protoplasm, 290.
Contraction, of coils of twiners, 39; in
curving organs, 14 ; during curvature,
239, 240 ; energy of, in irritable fila-
ments, 73, 74 ; influence of, on curva-
- ture, 241, 242.
Convection currents, influence of, on body
temperature, 381.
Convolvulus, 40 ; contortion of flower-bud
of, 24 ; direction of twining of, 38 ;
C. arvensis, limiting diameter for twin-
ing of, 40 ; twining stem of, 34, Fig. 6 ;
• C. sepium, rate of revolution in, 21.
Copeland, 165 ; on geotropic curvature,
241 ; — perception, 199.
Coprinus, 416 ; C. nivens, heliotropism of,
173 ; C. stercorarius, heliotropism of,
I73> 17S 5 C. velaris, hydrotropism of,
183.
Cordyline, 204 ; geotropism of rhizome of,
164.
Coriandrum, heliotropism in, 194.
Cork, influence of, on temperature, 380.
Cornutus, 113.
Correlation, 415 ; influence of, on curvature,
241, 242.
Correns, 87, 113, 145, 260, 271, 273; on
curvatures of tendrils, 246 ; on in-
fluence of oxygen on curvature, 143,
144 ; mechanism of curvature, 85.
Cortex, geotropism of, 242, 243.
Corti, 280, 314, 344 ; on streaming, 289.
Corydalis claviculata, grasping leaf-tips of,
44.
Cotyledon, heliotropic irritability of, 193,
194 ; localization of, 199, 200 ; seis-
monic irritability of, 80 ; sleep-move-
ments of, 105.
Crassulaceae, orientation of chloroplastids
of, 333-
Grid, 383.
Crocus, 1 10 ; downward growth of seedlings
of, 249 ; opening of flowers of, 97, 98,
99, ico ; thermonastic movements of,
129, 130, 131, 132, 133, 137; influence
of external conditions on, 141 ; C.
luteus, photonasty of, 122; thermonasty
of, 112, 113, Fig. 32, 114, 116; C.
vernus, thermonastic movements of,
112.
Crosby, on phobism, 309, 310.
Cryptomonas ovata, galvanotaxis of, 361.
Crystallization, production of light by, 384.
Crystals, distribution of, in cell, 334.
Cucumis sativus, pits in epidermis of, 66,
Fig. 25.
Cucurbita, 182, 335; excitation of streaming
in, 284 ; influence of Oxygen on — in,
341 ; C. Pepo, 253, 314 ; non-geotropic
lateral roots of, 163, — hypocotyl of,
165.
Cucurbitaceae, fixation of tendrils of, 47,
48 ; touch-corpuscles of, 65.
Cunningham, 80, 124.
Curvature, energy of, 1 8 ; internal cause
of, 244 ; measurement of, 17.
Curvipetality, 190.
Cuscuta, attachment of, 32 ; changes of
tone in, 207 ; coiling of, 37, 48, Fig. 16 ;
influence of gravity on circumnutation
of, 28 ; on twining of, 35 ; of stimuli on
excitability of, 70 ; normal twining of,
36 ; nuclear movements of, 301 ; twin-
ing of, 48, Fig. 1 6.
Cuticle, splitting of, 148, 159, 160.
Cyclamen, 24.
INDEX
429
Cyclanthera, dehiscence of, 148.
Cylindrogenic activity, 276.
Cylindrotheca, movements of, 271.
Cynanchum TJincetoxicum^ twining of, 38.
Cynara scolymus, influence of darkness on
stamens of, 141 ; mode of movement of,
72.
Cynareae, 8 ; absence of sleep-movements
in stamens of, in ; escape of water
from filaments of, 76 ; excitability of
stamens of, 81, 86, 92 ; influence of
anaesthetics on, 144 ; of stimuli on
streaming in, 78 ; mechanism of move-
ment in, 72, 79 ; rigidity of, 77 ; stretch-
ing of cell- walls of filaments of, 16.
Cyon, 362, 371.
Cystodonium purpurascens, coiling of, 46.
Cystopus, 293.
Cytoplasm, density of, 336 ; independence
,of, 10.
Cytotaxis, 364, 365.
Czapek, 5, 31, 106, 145, 190, 192, 220, 234,
419 ; on changes of tone, 207, 208,
209 ; on geotropism, 163, 164, 168, 170,
242, 243, 249, 250, 251, 252, 253, 255 ;
on changes of, 213, 214; influence of
anaesthetics on, 145 ; on optimal angle
for, 217, 218, 219 ; on minimal stimuli
for, 211, 212; theory of, 226, 227;
influence of oxygen on irritability, 202,
203 ; on localization of sensation, 190,
194, 196, 197, 198, 199, 200, 201, 206 ;
on tropic after-effects, 212 ; onstroph-
ism, 155 ; on torsion, 258, 260, 261.
Dahlia, 257 ; D. variabilis, changes of tone
in, 205.
Dalbergia, 103; D. linga, pulvinar tendril
of, 44, Fig. II; — growth of, on con-
tact, 45 ; — thickening of, 46.
Dale, 364.
Dalmer, 182.
Danilewsky, 188.
d'Arsonval, 408.
Darwin, 25, 27, 30, 31, 32, 36, 42, 46, 47,
48, 81, 82, 85, 87, 91, 92, 97, 102, 103,
105, no, in, 113, 118, 126, 145, 155,
204, 232, 236, 255, 256, 257, 419; on
abnormal twining, 38 ; on aggregation,
89 ; on attaching roots and disks, 33 ;
on autonomic movements, 19, 24; on
chemonasty, 86 ; circumnutation, 21,
28 ; climbing plants, 34 ; conduction of
stimuli, 90 ; contact-irritability, 35, 69 ;
curvature of Dionaea, 80 ; discrimina-
tory power of tendrils, 48; indepen-
dence of twining and circumnutation,
22 ; influence of ether on irritability,
144 ; — of temperature on circumnuta-
tion, 29 ; irritability of Catasetum, 147 ;
of Drosera, 83, 84 ; seismonic — of coty-
ledons, 80 ; localization of sensation,
191, 192, 193, 196, 199 ; on traumatrop-
ism, 185, 186 ; midday sleep, 105, 106 ;
— uses of, 100 ; minimal heliotropic
stimuli, 210, 2il ; movements of A ver-
rhoa, 104; — of Mimosa and Drosera,
12; twining, 35, 38, 40; origin of
torsion, 41 ; perception and response,
1 1 ; reversal of circumnutation, 39 ;
tropism, 161, 165.
Darwin, F., 26, 124, 149, 151, 190, 210,255,
258, 419 ; on directive action of I ight,
228, 229 ; influence of temperature on
tropism, 225 ; localized perception,
197 ; maximal geotropic angle, 217 ;
plagiotropism, 257 ; statolith theory,
417; tropism, 161, 166, 169, 170.
Dassen, on forms of curvature, 137, 138.
Daubeny, 142.
Daucus, sleep-movements of, 103.
Death, influence of, on curvature, 246 ; on
protoplasmic movement, 292, 298.
de Bary, 149, 150, 173, 180, 275, 276, 280,
289, 293, 294, 328.
Debski, 230.
de Candolle, A. P., I, 3, 19, 24, 80, 112,
152, 155, 166, 381, 383; on ephe-
meral flowers, 23 ; on heliotropism,
161, 229.
Decapitation, influence of, on autotropism,
191 ; — on galvanotropism, 189; — on
traumatropism, 186.
Deformation of protoplasm, 343, Fig. 10.
Dehiscence, 146; influence of light and
temperature on, 153.
Dehnecke, 334, 336, 355.
Delphinium , 258.
Demoor, 332, 340, 342.
Dermatoplasts, movements of, 262.
Derschau, on petiole-climbers, 47.
Desmidiaceae, movements of, 274, 275 ;
phototaxis of, 325 ; pulsating vacuoles
of, 293, 294.
Desmodium, 294 ; changes of rigidity in
pulvinus of, 135 ; production of sudden
movement in, 23 ; D. gyrans, 24 ; in-
fluence of electricity on movements of,
30; of gravity, 27, 126, of induction-
shocks, 146, of light, 108-11 ; opti-
mum temperature for, 29 ; sleep-move-
ments of, 101, Fig. 28, 102, 104; ther-
monasty of, 113 ; variation movements
of, 22, Fig. 4.
Dessaignes, 386.
Detmer, 106, 144, 151, 170.
Detto, 380.
Deutzia, 259 ; changes of tone in, 205.
de Vries, 24, 34, 36, 39, 85, 205, 220, 231,
235, 241, 242, 251, 253, 254, 255, 256,
316, 330; on aggregation, 89 ; on con-
traction of coils of twiners, 39 ; on
influence of turgor on curvature, 15,
31, 244, 246 ; on nature of twining, 35 ;
origin of coiling, 37 ; of torsion, 41,
257, 260 ; on streaming, 289, 358 ; tro-
430
INDEX
pic reactions, 160, 161 ; twining in
darkness, 30, of Wistaria, 40.
Dewevre, 25, 104.
de Wildeman, 317.
Dewitz, 358.
Diageotropism, in twiners, 37.
Diameter, influence of, on curvature, 18.
Dianthus bannaticus, geotropism of adult
nodes of, 202, 231, 235.
Diastase, chemotropic action of, 181.
Diastole of vacuoles, 295.
Diathermatropism, 177.
Diatoms, grouping of, 274; mode of move-
ment of, 270, 271; energy and speed
of, 272 ; phototaxis of, 325 ; stimu-
latory plasmolysis of, 75.
Diatropism, definition of, 155.
DictamnuS) origin of movement in flowers
of, 27, in stamens of, 24; D. albus,
flaring of, 384.
Dictydium ambiguum, influence of light on
streaming of, 318.
Dictyostelium, 183, 304.
Dictyuchus monosporus, oxytropism of,
182.
Didymium serpula, rate of movement of,
276, 284.
Diervilla lonicera, torsion of, 260, Fig. 49.
Dietz, 83, 105, 165, 183, 188.
Diffusion, relation of, to streaming, 285,
359-
Digitalis, changes of tone in, 205.
Digression movements, 287.
Dingier, 24.
Dionaea, 64, Fig. 24, 85, 86, in ; action of
hairs of, 67 ; closure of leaf of, 68 ;
electrical currents in, 394, 395, 397,
398 ; influence of anaesthetics on, 144,
of chemical excitation on, 88 ; irritability
of, 8 1 ; seismonic, of cotyledons of, 80 ;
perceptive organs of, 87 ; propagation
of stimuli in, 91, 92, 93; protoplasmic
aggregation in, 78 ; stimulation of, by
transpiration, 65, 66 ; D. mustipula,
influence of ammonium carbonate on,
30 ; latent period of, 68 ; summation of
stimuli in, 69.
Dioscorea batatas, heliotropism of, 42 ;
twining of, 36 ; influence of etiolation
on, —of, 30 ; D. sinuata, 42.
Dippel, 271.
Dipsacus, protoplasmic extrusions of, 149.
Discomycetes, dispersal mechanism of, 149.
Discoplea, 383.
Disks, influence of contact on, 47.
Dispersal, 416 ; — movements, 146 ; in-
fluence of light and temperature on, 153.
Dissociation, influence of, on chemotaxis,
350,420.
Diurnal movements, instances of, 101 ;
uses of, loo.
Dixon, 368, 373.
Dodel, 153, 293, 295, 298, 315.
Doflein, 303.
Dorsiventrality, induction of, in branches,
253, in Marchantia, 251, in prothalli,
252, in runners, 250, 251.
Dracophyllum, orientation of leaves of, 255.
Driesch, 208.
Drosera, 99; chemonasty of, 86; conduc-
tion of stimuli in, 90, 91 ; influence
of absence of oxygen on tentacles of,
143, of ammonium carbonate, 30, of
anaesthetics, 145, of chemical excita-
tion, 88, 90, of induction-shocks, 146,
of stimuli on excitability of, 70 ; irrita-
bility of tentacles of, 69, 84, 85, 87 ;
propagation of stimuli in, 92, 93 ; pro-
toplasmic aggregation in, 78 ; rate of
transference of stimuli in, 94; streaming
in, 342 ; D. binata, irritability of, 86 ;
D. longifolia, influence of light on
flowers of, 106 ; D. rotundifolia, ill ;
stimulated leaf, 83, Fig. 27.
Drosophyllum, aggregation in, 90 ; D. lusi-
tanicum, chemical excitation of, 88.
Dubois, 382, 383 ; on luminosity, 387,
388 ; on production of electricity, 389,
394, 397-
du Bois-Reymond, 390.
Dubrunfaut, 378.
Duchartre, 30, 173.
Duclaux, 378.
Dudresnaya, chemotropism of, 180.
Dufour, 27.
Duhamel, I.
Dutrochet, 3, 21, 80, 93, 95, 189, 191, 208,
255, 325 ; on anatomy of tropic organs,
243, 244 ; on causation of tropism,
223 ; circumnutation of tendrils, 24 ;
conduction of stimuli in Mimosa, 94;
curvature of pulvini, 138, 141, 143 ;
daily periodicity, 112 ; dehiscence of
fruits, 148, 151, 152; ephemeral
flowers, 23 ; heliotropism of climbers,
4 1 ; influence of acid and alkali on stream-
ing, 343 ; of external conditions on,
314, 316; of temperature on circum-
nutation, 29 ; on production of heat,
370, 372, 373, 374, 376 ; on streaming,
289, 290, 291, 355, 357; on spon-
taneous and induced movements, 25 ;
on tropic stimuli, 161, 163, 166, 173,
176.
Ecballium, dehiscence of, 148.
EccremocarpuS) 42 ; E. scaber, influence
of temperature on circumnutation of,
29.
Echinocystis lobata, discriminatory power
of tendril of, 48 ; immotility of hori-
zontal tendril of, 28.
Ectocarpusfirmus, 324.
Ectoplasm, regulation of ciliary movement
by, 269; retrogressive changes in,
279.
INDEX
Ectoplasmic membrane, irritability of, n.
Ehrenberg, 383.
Eicholz, 148.
Eijkmann, 382, 385, 387.
Elasticity, changes of, during curvature,
245, 246.
Electrical actions in plants, 400; — con-
ductivity, 392 ; — currents, influence
of, on movement, 145, of Desmodium,
30 ; — on streaming, 290, 292 ; — on
surface-tension, 278 ; shock-action of,
360 ; tactic action of, 361-4; — poten-
tial, differences of, 391 ; tropic action
of, 419-
Electricity in plants, detection of, 393, Fig.
68, 394 ; influence of anaesthetics on,
396, of injuries on, 397, of oxygen on,
395, of photosynthesis on, 396, of tem-
perature on, 395, 396, of water move-
ments on, 396 ; sources of, 388, 389,
390, 391 ; uses of, 392.
Electricity, static, tropic action of, 188, 189.
Electrolysis, curvatures due to, 422 ; move-
ments due to, 360.
Electromagnetic streaming, 290, Fig. 50.
Elfving, 164, 174, 204, 207, 228, 231, 245,
332, 421 ; on hydrotropism and gal-
vanotropism, 188 ; on maximal geo-
tropic angle, 217 ; on phototaxis, 323;
on tropic aggregation of protoplasm,
219 ; on Weber's law, 213.
Ellis, 264 ; on immotility, 306.
Elodea, orientation of chloroplastids of,
220, 327, 328, 338 ; streaming in, direc-
tion of, 283, 293 ; distribution of, 286 ;
duration of, 285 ; influence of light on,
319, 320 ; of temperature on, 313 ; rate
of, 284, 288 ; C. canadensis, nuclear
movements of, 275 ; streaming in, 357,
359-
Empusa muscae, dispersal of, 150.
Emulsions, importance of, 281 ; in stream-
ing protoplasm, 291, Fig. 51 ; surface-
tension forces in, 282, 283.
Energy, chemical, 405 ; consumption of,
by streaming protoplasm, 288 ; osmotic,
402 ; sources and transformations of,
399, 400 ; surface-tension, 404.
Engelmann, 6, 208, 263, 269, 270, 272,
273, 282, 290, 299, 309, 318, 347, 352,
355> 358> 36o, 367; on chemotaxis,
1 80; on phobic movements, 320, 321,
322, 326.
Enteromorpha comfiressa, 324.
Ephemeral flowers, 19 ; — movements, 23.
Epicotyl, curvature of, on klinostat, 27 ;
heliotropism of, 194.
Epidermis, influence of, on geotropism,
242 ; — on production of electricity, 398.
Epilobium, movements of stigma of, 24 ;
origin of movements in flower of, 27.
Epinasty, definition of, 3 ; in horizontal
branches, 254, 255 ; influence of, on
curvature, 23 ; — of light on, 257 ;
importance of, in orientation, 159 ; in
ivy, 253 ; in leaves, 256.
Epistrophe, 333.
Equinoctial flowers, 19.
Equisetum, heliotropic rhizoids of, 174.
Erica, orientation of leaves of, 255.
Eriksson, 164, 370, 374, 378.
Ermann, 278.
Ernst, 304.
Erodium gruinum, hygroscopic awns of,
146, 151, 152.
Errera, 82, 184, 302 ; on curvature of stems
of trees, 12, 232.
Ervitm lens, thermotropism of, 177.
Erythrotrichia, locomotion of, 265.
Esenbeck, 386.
Ether, influence of, on chemotaxis, 421 ;
— on irritability, 144; — on phototaxis,
421 ; — on phototonus, 319, 320; — on
streaming, 313,319,341 ; tropic action
of, 182.
Ethereal oils, influence of, on absorption of
heat, 380.
Etiolation, influence of, on circumnutation,
28, 30 ; — on curvature, 23 ; — on
heliotropism, 229; — on twining, 30,
33, 35-
Euglena, 312, 317, 320, 324, 326; energy
of cilia of, 268 ; function of eye-spot
of, 323 ; pulsating vacuoles of, 298 ;
E. viridis, 421 ; geotaxis of, 336, 337,
338 ; influence of oxygen on, 347, 349,
351; non-gal vanotaxis of, 361; non-
rheotaxis of, 356; phototaxis of, 321.
Euler, 393.
Euonymus radicans, orientation of leaves
of, 259, Fig. 48.
Euphorbia lathyris, rise of temperature in,
373-
Ewart, 26, 124, 272, 306, 307, 367, 400,
404, 408, 420 ; on ascent of water,
410, 41 1 ; on attaching disks, 33, 47 ;
on coiling roots, 46 ; on consumption
of energy, 369 ; on correlation of
growth, 415 ; on decomposition of
chlorophyll by light, 333 ; on diffusion,
285, 359, 409, 410 ; on discriminatory
power of tendrils, 48 ; on electrical
conductivity of cell-constituents, 392,
393 ; on formation of red wood, 416 ;
on galvanotropism, 422 ; on heat pro-
tection, 380 ; on hook- and petiole-
climbers, 45, 237 ; on influence of
darkness on phototonus, 142 ; — of
density and viscosity on movement
in cell, 334, 335 ; — of strains on thick-
ening of cell-wall, 245 ; — of light on
streaming, 321 ; on length of vessels
in climbers, 33 ; on localized percep-
tion, 196 ; on magnetotropism, 189,
222; on nuclear movements, 301, 302;
on orientation of chloroplastids, 327,
432
INDEX
331 ; on paranastic photometry of
leaves, 255, 260, 261 ; on protective
movements, 71 ; on protoplasmic move-
ment and streaming, 276, 277, 278,
283, 284, 285, 286, 287, 355, 35.6, 357,
360, 361 ; on changes in direction of,
292, 293; influence of light on, 318,
319, 320; — of oxygen on, 338, 340,
341, 342 ; — of temperature on, 313,
314, 315, 316 ; — surface-tension on,
281, 282; - - physics of, 288; -
theories of, 300 ; on transference of
stimuli, 95, 96, 201, 359 ; on root-cur-
vatures in deoxygenated water, 182,
183 ; on seismonic irritability, 66 ; on
sleep-movements, 102, 106, 107, 108,
109; on statolith theory, 418 ; on sum-
mation and induction, 210, 211.
Excitation, effect of increases of, 9.
Excretion, conditions for, 305, 306 ; influ-
ence of pulsating vacuoles on, 299.
Exner, 263.
Exothermic changes, in heat-production,
368.
Eye-spot, functions of, 323 ; use of, n*
Faba, 417. See Vicia Faba.
Fabre, 383 ; on luminosity, 385, 386, 387.
Falcaria, sleep-movements of, 103.
Falck, 183.
Falkenberg, 150, 264, 293.
Famintzin, 318, 324, 325, 327, 333.
Farmer, 340, 342, 346.
Fayod, 275, 356.
Fechner, 215.
Fermentation, production of heat by, 378.
Fertilization, influence of, on geotropism,
205.
Ficus stipulata, climbing of, 32.
Figdor, 210, 211, 417.
Fischer, A., 169, 264, 265, 269, 294, 302,
325 ; on influence of gravity on sleep-
movements, 125, 126; on immotility,
306, 307 ; on osmotaxis, 353.
Fischer, B., 382, 416 ; on luminosity, 385,
386, 387.
Fischer v. Waldheim, 173.
Fitting, 21, 113, 200, 221, 246, 418; on
geotropism, 416; on klinostat, 169.
Fitz, 378.
Flagellae, occurrence of, 264; mode of
movement of, 270.
Flagdlaria indica, coiling leaf-tips of, 44.
Flagellatae, chemotaxis of, 347, 348 ; gal-
vanotaxis of, 361.
Floral clocks, 123.
Flowers, ephemeral, 23 ; influence of light
on, i oo, 1 06, — of temperature on, 97,
98, 99 ; — of turgidity on opening of,
116, 117, 118; modes of opening of,
147, 148 ; opening and closing move-
ments of, 129-34; orientation of, 258,
260 ; sleep-movements of, 103 ; tem-
perature of, 373, and production of
heat by, 376.
Fliigge, 382.
Foam structure, 281, 282.
Forster, 382.
Fortuna, 289.
Fragaria grandiflora, diageotropism of,
250 ; F. vexa, 250.
Fragmentation, mode of, 148.
Frank, i, 119, 155, 166, 218, 231, 237, 240,
242, 249, 253-7, 259,420,421 ; on cur-
vature of adult petioles, 232 ; — of
roots, 234; on nutation and growth,
31; on orientation of chloroplastids,
327, 328, 329, 330-3 ; on persistence of
curvature, 245, 246; on protoplasmic
streaming, 358 ; on tropism, 161, 166.
Franzd, 323.
Fraxinus,2$c)\ epinasty of, 254 ; orienting
torsion of leaf of, 233.
Freidenfelt, 248.
Fries, 383.
Fritzsche, 25, 28, 30, 31, 115; on auto-
nomic movements, 19 ; influence of
temperature on nutation, 29 ; localiza-
tion of geotropic sensation, 198.
Fruits, active dehiscence of, 148, 149.
Fucaceae, chemotaxis of sperms of, 346.
Fuchsia, 419.
Fucus, phototaxis of, 325.
Fumaria officinalis, var. Wirtgeni) irritable
leaf-segments of, 44.
Funarta, 420; hygroscopic torsion of, 151 ;
orientation of chloroplastids of, 332,
3.33-
Fungi, chemotropism of, 180, 181 ; coiling
of, 46; hydrotropism of, 183, 190;
production of heat by, 360, of light by,
383, 384 ; rheotropism of, 185.
Fusion, conditions for, 305 ; of nuclei,
301, 303 ; of protoplasts, 304.
Gad, on movements of column of Sty-
lidium, 22.
Gagea lutea, influence of light on flower of,
106.
Galeotti, 392.
Galium mollugo, twining of, 33 ; G. pur-
pureum, transference of stimuli in,
194.
Galvanogenic curvatures, 188, 189.
Galvanotaxis, character of, 362, Fig. 64;
independence of, on nucleus, 10 ; in-
stances of, 361 ; mode of performance
of, 363 ; origin of, 364.
Galvanotropism, 188, 421 ; of Infusoria,
310-
Gamotropic, definition of, 3.
Ganong, 420.
Gardiner, on aggregation, 89; on proto-
plasmic contraction, 78, 79.
Gardner, 382, 383.
INDEX
433
Garreau, 372, 374.
Carrey, 309, 311 ; on chemotaxis, 344, 348 ;
on phototaxis, 6.
Garten, 394.
Gas chamber, 315, Fig, 52, 338, Fig. 57 ;
— vacuoles, uses of, 263.
Gaudichaud, 383.
Genista, opening of flower-buds of, 23 ;
G. tinctoria, opening of flowers of, 148.
Gentiana campestris, influence of light on
flower of, 1 06 ; G. cruciata, 379.
Geoheterauxecism, 257.
Geostrophism, 257.
Geotactic irritability, 268, 336 ; changes of,
337 ; limit of, 338 ; nature of, 226.
Geotortism, 257.
Geotropic curvature, changes of turgor
during, 139 ; influence of anaesthetics
on,,i45 ; — of oxygen, 143 ; production
of reducing substances during, 227.
Geotropic induction, influence of, on growth
and respiration, 208 ; — on nutation, 28 ;
optimal angle for, 217, 218; time of,
210.
Geotropic induction and response, separa-
tion of, 145.
Geotropic irritability, localization of, 196,
197,242,243.
Geotropic sense, 221, 223,224; sensitivity,
211.
Geotropic stimuli, channels for, 201, 202 ;
rate of transference of, 200.
Geotropism, definition of, 154, 155, 162,
416, 417 ; influence of aeration on, 182 ;
— of light on, 249 ; of lateral roots, 163 ;
of leaves of grasses, 416 ; of rhizomes,
164 ; of seedlings and sporangiophores,
165 ; of twiners, 35, 36 ; localization of,
418 ; nature of, 5, 220, 221, 222, 223,
224, 225, 226.
Geovanozzi, 150.
Gerasimoff, 303.
Germination, influence of Rontgen and
radium rays on, 415 ; production of
heat during, 369, 370.
Giesenhagen, 225.
Giessler, on curvature of operated pulvini,
138.
Gildemeister, 394.
Glechoma, 259 ; plagiotropism of runner of,
156, 157 ; G. hederacea, seasonal varia-
tion of geotropism in, 250.
Gleditschia, sleep-movements of, 102 ; G.
triacantha, seismpnic irritability of, 80.
Gloriosa superba, coiling leaf-tips of, 44, 45,
Fig. 12.
Glossostigma elatinoides, irritable stigma
of, 82.
Glucose, influence of, on luminosity, 386.
Glycerine, action of, on motile organisms,
352 ; chemotropic action of, 181.
Godlewski, 229.
Goebel, 23, 42, 80, 81, 83,90, 150, 151, 164,
182, 193, 204, 206, 249, 254, 258, 263,
269, 340, 415 ; on indirect excitation
of Dionaea, on twining shoots, 38.
Goldfussia, irritable stigmas of, 82 ; G.
anisophylla, 253.
Goniitm, 295 ; ciliation of, 264 ; working of
cilia of, 269 ; G. perforate, 296, 421 ;
reversal of movement of, 267.
Goosegrass, mode of climbing of, 32.
Goppert, 144, 287, 370, 372, 379, 381.
Gramineae, curvature of nodes of, i.
Grantz, 175.
Granulation, 89 ; removal of, 90.
Graphic representation of tropic reactions,
215.
Grass, self-heating of, 368.
Grass-haulm, geotropic summation in, 218.
Gravity, influence of, on tropic tone, 158,
*59 j — on autonomic movement, 26 ;
— on circumnutation, 28 ; on contact
irritability, 48; — on curvatures of
peduncles, 27 ; — on distribution of
sap, 247 ; — on growth, 245 ; — on
hyponasty, 257 ; — on photonasty, 127;
— on oscillations of radicle, 24 ; — on
reduction of time of reaction, 214 ; on
revolution of leaflets of Desmodium,
22 ; — on sleep-movements, 124, 125,
126; — on streaming movement, 284,
288; — on-thermonasty, 127; — on
torsion, 257, 258, 259 ; — on turgor,
244 ; — on twining, 33, 35.
Gray, Asa, 30.
Growing zones, influence of length of, on
curvature, 18.
Growth, awakening of, in leaves, 415 ; ener-
getics of, 41 1 ; importance of, in move-
ments of Dionaea^ 80; influence of
arrest of, on irritability, 203, 204 ; —
of attachment of threads on, 25 ; — of
attachment in climbers, 36; — of
curvature on rate of, 15, 208, 238, 239,
240, 241 ; — of gravity on, 231, 232 ;
— of movements of sensitive plants on,
68, 72 ; — of, on production of heat,
377 ; rate of, in inverted organs, 217,
417 ; relation of, to circumnutation, 29,
31 ; — to movement, 13, 14, 15, 16 ;
secondary influence of contact on, 45,
46 ; stimulatory action of temperature
and light on, 129-34 ; — causes of,
139-40 ; — influence of external con-
ditions on, 141-4.
Guillemin, on heliotropic action of rays of
different wave-length, 176.
Guillon, 163.
Haake, on production of electricity, 389,
390, 394, 395, 396.
Haberlandt, 13, 22, 65, 69, 71, 74, 75, 80,
81, 82, 83, 91, 93, 147, 150, 152, 180,
201, 217, 232, 242, 243, 334, 379, 417 ;
on geotropic causation, 223, 224 ; on
PFEFFER. Ill
Ff
434
INDEX
nuclear movements, 301 ; on orienta-
tion of chloroplastids, 327, 328, 330,
331, 332; — of leaves, 419; on stimu-
lators, 67, 76 ; on stimuli in dead stems
of Mimosa, 94, 95 ; on statolith theory,
225, 226 ; on transference of stimuli in
Biophytum, 96.
Hacker, 302.
Hadromal, 415.
Haematococcus lacustris, 315 ; geotaxis of,
336 ; influence of light on zoospores of,
318-
Hairs, climbing, 40 ; influence of, on tem-
perature of plant, 380; sensitive, 76.
Hales, on ephemeral movements, 24.
Halle-, 22.
Hanburya mexicana, disks of, 47.
Hansen, 170.
Hansgirg, 19, 22, 23, 24, 27, 81, 82, loo,
102, 103, 104, 106, 141, 142, 318, 325 ;
on forms of movement, 3 ; — hydro-
nastic, 116, 117, 118, 119; — ther-
monastic, 113, 115; — of Oscillaria,
273, 274.
Hanstein, 286, 290, 301.
Haptotaxis, 358.
Hartig, 380, 381.
Hartwegia, heliotropism of aerial roots of,
172.
Hassal, 283.
Hauptfleisch, 327, 342 ; on streaming, 283,
284, 289, 356, 357, 358, 359; — in-
fluence of external conditions on, 314,
316, 318-
Hay, self-heating of, 368.
Heat-production, 369; by aerobes, 366,
372, 373-7; influence of oxygen on,
371, Fig. 65 ; by anaerobic metabolism,
377, 378, 379 ; influence of changes of
• temperature on, 367 ; measurement of,
371 ; uses of, 368.
Heat of combustion, 369, 378.
Heckel, 78, 144 ; on influence of darkness
on movement, 30.
Hedera helix, climbing of, 32 ; geotropism
of petioles of, 232 ; orientation of, 252.
Hedysarum gyrans, movements of, 2.
Hegler, 188.
Heidenhain, 290, 394 ; on streaming, 289.
Heine, 334.
Heinrich, 383, 386, 387.
Helianthemum, irritable stamens of, 81 ;
— barometric movements of, 87.
Helianthus, 25 3, 335, 417; curvature of
hypocotyl of, 27 ; Htannuus, curvature
of split hypocotyl of, 243 ; geotropic
hypocotyl of, 165 ; heliotropism of
roots of, 173 ; influence of absence of
oxygen on growth of, 143 ; — of dark-
ness on leaves of, 106; minimal helio-
tropic stimuli for, 211 ; stretching of
filaments of, 75.
Helichrysum, 151.
Heliocharis, plagiotropism of, 156, 157;
H.palustris, tropism of root-»stock of,
164.
Heliostrophism, 257.
Heliotortism, 257.
Heliotropic curvature, changes of turgor
during, 139; influence of oxygen on,
143 ; - - irritability, distribution of,
193, 194 ; — responses, discriminatory
power in, 214; latent periods for, 210,
211 ; minimal stimuli for, 213; -
sense, 221 ; — stimuli, channels for,
201, 202; rate of transference of, 200.
Heliotropism, definition of, 154; indepen-
dence of, on nucleus, 10; relation of,
to twining, 41, 42; reversal of, 171,
172; of roots, 173 ; of scramblers, 32 ;
of seedlings and tendrils, 171 ; of
twiners, 35 ; use of, in climbing, 32.
Helioturgotropism, 257.
Hensen, 270.
Herbst, 179, 208, 325.
Hering, 390, 417 ; on retardation of growth
by reversal, 217.
Hermann, 360, 361, 362, 371, 390, 393, 394.
Hertwig, 264, 266, 269, 275, 302, 303, 304,
332 ; on pulsating vacuoles, 293.
Hertzian waves, tropic action of, 188.
Herzog, 378, 379.
Heterogeneous induction, 208.
Hexamitus rostratus, H.intestinalis, chemo-
taxis of, 347.
Hibbertia dentata, reversal of twining in, 39.
Hieracium, closure of capitulum of, 103 ;
H. pilosella, sleep-movements of, 104,
Fig. 30 ; H. vulgatum, thermonasty of,
114.
Hilburg, on turgor in stimulated pulvini,
139, 238.
Hildebrand, 148, 150.
Himantoglossum, 24.
Hinze, 304.
Hippuris, 417.
Histology, influence of, on perception of
stimuli, 67.
Hochreutiner, 38, 218.
Hoffmann, 176.
Hofmeister, 19, 20, 22, 31, 75, "3, HS> *53,
155, 229, 237, 241, 243, 264, 265, 267,
315, 3!6, 320, 326, 340; on coiling of
Spirogyra, 38; on contractile me-
chanism, 79 ; on curvature of adult
petioles, 232 ; — of roots, 234 ; on
daily periodicity, 112; on geotropism,
222, 223 ; on growth curvatures, 239 ;
on heliotropism, 238 ; on influence of
induction-shocks on tendrils, 145 ; on
rigidity of stimulated tendrils, 77 j on
streaming, 276, 282, 284, 288, 290, 292,
293, 355, 356, 357; on tropism, 161,
165, 166, 173, 174, 175, 241, 246.
Holmes, 324.
Holosteum medium, thermonastic flower of,
INDEX
435
117 ; H. umbellatum, influence of
gravity on thermonasty of, 127 ; — of
light, 1 06.
Homodrompus curvature, origin of, 37 ;
— torsion, 41.
Homogentisinic acid, in root-apices, 227.
Homoiotherms, 366, 367.
Homolotropism, definition of, 155.
Hook-climbers, 32, 33, 45, 46.
Hook-tendrils, secondary growth of, 46.
Hooke, 79.
Hop, growth of, 33 ; limiting diameter for
twining of, 40; nutation of, 21.
Hoppe, 372, 376.
Hordeum, heliotropism of, 172; H. disti-
chum, thermonastic flowers of, 115.
Hermann, 286, 316, 356, 357, 360, 397; on
streaming, 289, 292, 293.
Horme'n, 380.
Hot stage and gas-chamber, 315, Fig. 52.
Hoya, mode of climbing of, 32 ; H. carnosa,
nutation of, 21.
Huber, 372, 374.
Huie, on cellular changes in Drosera, 89.
Humic acid, use of, 228.
Humulus, climbing-hairs of, 40 ; H. lupu-
lus, direction of twining of, 38 ; free
coiling of, 36, Fig. 8, 37 ; twining-stem
of, 34, Fig. 7. See also Hop.
Hunger, 273.
Hunter, invention of klinostat by, 166.
Huth, 165.
Hyacinthus, statical moment of, 236 ; H.
orientaltS) heliotropic roots of, 173.
Hydra viridis, 305.
Hydrocleistogamy, 100.
Hydrogen, apparatus for production of, 339,
Fig. 58 ; influence of, on Pelomyxa, 341.
Hydronastic movements, 97, 116-19; uses
of, 1 1 8.
Hydrostatic pressure, influence of, on geo-
tropism, 223, 224.
Hydrotaxis, 356.
Hydrotropic irritability, 182; localization
of, 197, 198; nature of, 184, 187; of
rhizoids and sporangiophores, 183.
Hygroscopic movements, 150, 151, 152.
Hymenium, geotropism of, 165.
Hymenomycetes, dispersal of spores of, 416.
Hyphae, indifference of, to contact, 83.
Hypnea musciformis, coiling of, 46.
Hypocotyl, curvature of, on a klinostat, 27 ;
geotropism of, 165 ; heliotropism of,
J73» J93 5 hydrotropism of, 183 ; locali-
zation of irritability in, 194, 200 ; sensi-
tivity of, to light, 211.
Hyponasty, definition of, 3 ; influence of,
on geotropism, 254 ; of gravity on, 257.
Ihne, 380, 381.
Illumination, changes of response to, 9.
See also Light.
Ilyin, 224.
Imbibition, influence of, on streaming, 282-
Immotility, origin of, 306.
Impatient^ influence of induction-shocks
on fruit of, 146 ; -- of darkness on
leaves of, 106 ; photonastic leaves of,
98; I.balsaminea, 148; I.glanduligera,
geotropic curvature of, 233, Fig. 45 ;
/. noli-me-tangere, dehiscence of, 148 ;
influence of darkness on growth of,
129, 132 ; — of light on daily move-
mentsof, 105, 108-11; sleep-movements
of, 103 ; Lparviflora. 48, Fig. 16, 130,
416; influence of light on irritability
of, 203 ; sleep-movements of, 103.
Indicator, influence of, on movement, 19.
Indifferent line, 287.
Induction-period, 211 ; after-effect of, 212.
Induction-shocks, mode of action of, 356,
Fig- 63, 360.
Inflorescences, production of heat by, 372,
373, 3?6.
Infusoria, galvanptaxis of, 361, 362, 363.
Ingestion, conditions for, 305, 306.
Injuries, influence of, on autonomic curva-
ture, 31 ; — on production of heat,
375, 397, 398.
Inotagma, 282, 290.
Interprotoplasmic connexions, translocatory
inutility of, 91. See also Protoplasm.
Ions, influence of, on chemotaxis, 345 ; on
galvanotaxis, 364 ; on galvanotropism,
421, 422.
Ipomoea argyroides, abnormal twining of,
38 ; /. jucunda, reversal of twining in,
39 ; /. purpurea, angle of twining of,
4.0 ; — direction of, 38 ; influence of
light on circumnutation of, 42 ; twining
of, in darkness, 30 ; 7. sibirica, 42.
Iresine Lindeni, 415.
Iris, 415.
Irritability, changes of, 156, 157, 202, 206;
distribution of, in tissues, 226 ; in-
fluence of ether and chloroform on,
144, 145; — of injuries on, 198, 199;
— of light on, 141, 142 ; — of oxygen
on, 143 ; — of temperature on, 141,
225 ; mode of restoration of, 79 ; nature
of, ii.
Isatschenko, 382.
Isoetes, '420.
Ivy, geotropism of aerial roots of, 164.
Jamieson, 269.
Jamin, 381.
Janse, on streaming, 284, 289, 357.
Jennings, 266, 267, 269; on chemotaxis
and osmotaxis, 344, 348, 353, 358 ; on
galvanotaxis, 361 ; on phobism, 309,
Jensen, 268; on geotaxis, 336, 337; on
protoplasmic movement, 275, 276,
277, 280, 283.
Johnson, 237, 243.
Ff 2,
43^
INDEX
Johow, 71.
Jonsson, 166, 356 ; on rheotropism, 184,
185.
Joseph, on tactic action of Rontgen rays,
176.
Josing, 313, 316, 318, 319, 326, 340.
Jost, 102, 103, 1 10, 113, 114, 182, 210, 223,
224, 225, 226, 227, 417, 418 ;;; on in-
fluence of darkness on irritability, 142 ;
on origin of nastic curvature, 131, 132,
133-
Jourdan, 325.
Juel,234; on rheotropism, 184, 187.
Juncus effusus, var. spiralis, coiling of, 37.
Jurgensen, 343.
Kabsch, 30, in, 141, 145, 146; on influ-
ence of oxygen on curvature, 143, 144;
— of temperature on spontaneous
movements, 22.
Kamerling, on cohesion mechanism, 151.
Karsten, 164, 272.
Karyokinetic figures, artificial production
of, 302.
Kataklinotropism, definition of, 155.
Kataphoric action, 362.
Katatonic stimuli, definition of, 6.
Katatropism, definition of, 155.
Katz, 382, 386, 387.
Kauffman, 342.
Keeble, 173, 333-
Keller, I., 358, 359 ; on streaming, 289.
Keller, R., 394.
Kerner, 100, 102, 103, 123, 146, 150, 164,
259.
Kerria japonica, 259.
Kerville, 382. .
Kienitz-Gerloff, 96, 201, 359.
Kinematograph, use of, 2.
Kinoplasm, 303.
Kjellmann, 153.
Klebahn, 263, 272.
Klein, 84, 269, 394, 395-
Klebs, 150, 153, 174, 183, 207, 250, 267,
274, 304, 318, 330, 337, 415 ; on con-
jugation, 305 ; on hydrotropism, 209 ;
on phototaxis, 323, 325 ; on pulsating
vacuoles, 293, 298.
Klemm, 165, 170, 174, 295, 316, 341, 342,
343 ; on streaming, 357, 360 ; — influ-
ence of external conditions on, 314,
315,320.
Klercker, 27, 192 ; on thermotropism, 177,
178.
Klinogeotropism, of apices of twiners, 28.
See also Geotropism.
Klinostat, forms of, 168, 169, Fig. 36 ; in-
fluence of, on growth of nodes, 231 ;
production of torsion of, 41 ; use of,
26, 166.
Klinotropism, definition of, 155.
Knight, 237 ; on geotropism, 161, 166,
222 ; on hydrotropism, 183.
Knoblauch, 381.
Knoch, 103, 373, 374, 376.
Kny, 83, 165.
Koch, 48.
Koernicke, 301 ; on action of Rontgen and
radium rays, 415.
Kohl, 190, 192, 212, 233, 235, 269, 275,
323, 327 ; on cellular changes during
curvature, 240; on curvature of non-
growing zones, 232 ; on irritability of
twiners, 35 ; on localization of irrita-
bility, 205 ; on nutation, 14 ; on tropic
aggregation, 219 ; — curvature, 241,
242, 244, 245, 246.
Kolkwitz, 12, 36, 265, 270, 271, 273, 324,
325, 330, 377; on origin of torsion, 41.
Krabbe, 27, 155, 233, 255, 259, 419; on
localized perception, 196, 205 ; on
plagiotropism, 257, 258 ; on tropism,
161.
Krasan, 114.
Kraus, 140, 153, 173, 175, 176, 204, 243,
327, 329 ; on heat-production, 368,
372, 373, 374, 376, 377 J on hydronastic
movements, 117, 118; on metabolic
changes in curving organs, 247, 248;
on percentage of sugar in shaken
shoots, 78, 79.
Kreidl, on organs of equilibrium, 224.
Kretschmar, 359, 422.
Kruckenberg, 385.
Krutickij, 144.
Kiihne, on streaming, 289, 315, 316, 317,
357, 360 ; influence of oxygen on, 338,
340, 341.
Kunkel, on production of electricity, 392,
394, 395, 397, 398.
Kuntze, 204.
Kutscher, 382.
Lactuca virosa, photic orientation of, 261.
Lagenaria vulgaris, circumnutation of coty-
ledon of, 20, Fig. 3.
Lamarck, 372.
L,amhtm purpureum, orientation of, 250 ;
thermonasty of, 114; — influence of
gravity on, 127.
Langendorff, 394.
Langley, 388.
Latent period, 7, 8, 209 ; in sensitive plants,
68.
Latex, influence of centrifugal force on,
336 ; luminosity of, 383.
Lathraea, nuclear movements of, 301.
Lathyrus, 42 ; tendrils of, 43.
Laudenbach, 224.
Lauterborn, 272 ; on movement of Dia-
toms, 273.
Leaf-sheath, geotropism of, 242.
Leaves, influence of illumination of, on
development of nodes, 249 ; — of light
on position of, 101, 102, 104-8 ; -
floral, 103, 104 ; localization of irrita-
INDEX
437
bility in, 196, 197 ; opposed move-
ments of, 26 ; orientation of, 255, 256,
419,420; twining, 38; variation move-
ments of, 22.
Leclerc du Sablon, 106.
Leguminosae, motile pulvini of, I ; —
structure of, 13.
Lehmann, 263, 268, 277, 281, 382, 385,
386, 387, 388.
Leitgeb, 174, 252, 345.
Lemna trisulca, orientation of chloroplas-
tids of, 328, Fig. 54.
Lemstrom, 393.
Lengerken, 47.
Leontodon, closure of capitulum of, 103 ;
L. hastilis, growth movements of, 132,
133, 134 ; photonasty of, 122 ; ther-
monasty of, 1 14.
Lepidium, heliotropism of, 171, 172, 173;
L. sativum, minimal heliotropic stimuli
for, 211 ; thermotropism of, 177.
Letellier, 188, 237.
Lewis, 332.
Lianas, limiting diameter for twining of, 40.
Lichens, dispersal mechanism of, 149.
Lidforss, 420, 421 ; on chemotropism, 181 ;
on thermonasty, 114; — influence of
gravity on, 127.
Light, coloured, heliotropic action of, 174,
I75> J76; intense, orienting action of,
on leaves, 260, 261.
Light, formative action of, 416 ; influence
of, on autonomic movement, 30 ; — of
changes of, on position of leaves, 105 ;
— on circumnutation, 42 ; — on daily
periodicity of leaves, 108-11 ; — on
dehiscence and dispersal, 153; — on
development of nodes, 249 ; — of
runners, 250 ; — on direction of
streaming, 292 ; — on epinasty, 257 ;
— on geotropism, 249-52 ; — on irrit-
able tone, 141, 142, 203, 206; — on
locomotion, 306, 318 ; — on opening of
flowers, I oo ; — on orientation, 4 19, 420;
— of branches, 253, of chloroplastids,
327-33, of leaves, 255, of Marchantia,
251, of prothalli, 252 ; — on stream-
ing and amoeboid movement, 288,
319, 320; — on twining, 35, 40, 41 ;
— on periodic movement, 26, 27 ; —
on position of leaves, 105-8 ; — on
torsion, 257, 258, 259 ; — on tropic
irritability, 1 58 ; minimal intensity of,
for tropic response, 210, 211, 212, 213;
nature of action of, 228, 229, 230 ;
phobic responses to, 320; summative
effect of, 209, 210 ; tropic and tactic
action of, 321 ; value of various sources
of, 112.
Light-production, 382 ; composition of rays
of, 388 ; energy consumed in, 400 ; -
— gained from, 399 ; influence of
chemical substances on, 386 ; — of
temperature on, 385 ; uses of, 384,
by Bacteria, 382, by Fungi, 383.
Lignin reaction, 415.
Lilienfeld, 420.
Linaria, heliotropism of, 174 ; L. cymba-
laria, 253 ; L. spuria, influence of
light on geotropism of flower of, 203.
Lindemuth, 415.
Lindley, 22.
Lindsay, 77 ; on pulvinar mechanism, 80.
Link, 20.
Linnaeus, in.
Linsbauer, 102.
Linum iisitdtissimum, hydrotropism of,
183 ; transference of stimuli in, 194.
Lippmann's capillary electrometer, principle
of, 278.
Lister, 320.
Littonia, coiling leaf-tips of, 44.
Loasa aurantiaca, reversal of twining in, 39.
Locomotion, influence of pulsating vacuoles
on, 299.
Loeb, 188, 266, 320, 325, 337, 363, 364;
on phototaxis, 229 ; on symmetric
orientation, 216.
Lonicera, 259 ; Z. brachypoda, rate of
revolution in, 21 ; L. caprifolium,
direction of twining in, 38.
Loomis, 30.
Lophospermum, coiling of, 37 ; L. scandens,
irritability of, 35; climbing of, 48;
twining petioles of, 44.
Lopriore, 340, 342.
Lourea 'vespertilionis^ 211.
Low, 346.
Luciferase, 387.
Luciferin, 387.
Ludloff, 361, 363.
Ludwig, 23, 42, 46, 118, 146, 148, i49> *S°>
264, 383, 384 ; on heat-production,
368 ; on luminosity, 305, 388.
Luerssen, 326.
Luminosity, occurrence of, 382, 383; energy
of, 400; influence of chemical sub-
stances on, 386 ; — of temperature on,
385 ; nature and uses of, 384.
Lunaria btennis, 211.
Lupinus, 104, 182, 214, 418; length of
irritable zone in, 198 ; localization of
irritability in root-apex of, 197, Fig. 42 ;
Z. albus, 420 ; .geotropism of hypocotyl
of, 165, — of radicle of, 234, Fig. 46 ;
— influence of chloroform on, 145 ; —
of gravity on sleep-movements of, 125 ;
- of injury on irritability of, 199 ; -
of oxygen, 202.
Liitkemiiller, 274.
Luxburg, on geotropism and growth, 417.
LycopcrdoH) production of heat by, 366,
373*
Lygodium scandens, twining leaves of, 38.
Lysimachia nummularia, 259 ; geotropism
of, 165 ; — influence of light on, 250 ;
438
INDEX
plagiotropism of radial runner of, 156,
157-
Macdougal, 113, 186, 239, 240, 243, 245 ;
on transference of stimuli \nJ3iophytum,
96 ; — in Mimosa, 94, 95.
Macfadyen, 382, 388.
Macfarlane, 69, 77, 80, 91, 96, 142.
MacNab, 373.
Magnetic forces in plants, 400.
Magnetotropism, 189, 222, 418, 419.
Magnets, influence of, on streaming, 291 ;
orienting action of, 222, 224.
Magnus, Albertus, in, 303. •
Mahonia, irritable stamens of, 81.
Maier, 269.
Maige, 206, 251 ; on influence of light on
geotropism, 250 ; on light rigor, 30.
Maize, autonomic movements of root of,
19 ; statical moment of horizontal
stem of, 237; — cinquantino, growth
of curving nodes of, 240.
Malates, and malic acid, chemotactic action
of, 345> 354, 421.
Malope trifida, thermonastic flower of,
Malva, 104; M. neglecta, 257, 258; M.
verticillata, 420.
Malvaceae, sleep-movements of, 102.
Mandevillea suaveolens, influence of etiola-
tion on twining of, 30.
Mangrove, tropism of breathing-roots of,
164.
Marcet, 144.
Marchantia, 183, 421 ; changes of tropic
irritability in, 157, 158 ; geotropism of
rhizoid of, 166 ; — heliotropism of, 10,
172, 174; structure of, 161, 162; in-
fluence of light on colour of, 333 ;
orientation of, 251, 252.
Mares, 409.
Marey, 270.
Marquart, 333.
Marsilia, chemotaxis of sperms of, 345,
349 ; sleep-movements of, 102.
Martynia, irritable stigmas of, 24, 82 ; pro-
pagation of stimuli in, 92, 93.
Masdevallia muscosa, propagation of
stimuli in, 92, 93.
Mason-Jones, 416.
Massart, 155, 173, 197, 207, 294, 298, 385,
419 ; on alcaliotropism, 179 ; on chemo-
and osmotaxis, 178, 180, 344, 348, 350,
35i> 352, 3S3> 354, 35^5 on geotaxis,
336, 337 ; on maximal geotropic angle,
217, 218 ; on phobism, 309 ; on photo-
taxis, 323 ; on tonic stimuli, 6 ; on
Weber's Law, 214.
Matruchot, 317.
Matthiola, production of electrical currents
in, 396.
Matzuschita, 306.
Maupas, 293.
Maxwell, 420.
Mayenburg, 353.
Mayo, on pulvinar mechanism, 80.
Mazotto, 392.
McKenney, 29, 382, 384 ; on luminosity^
385, 386.
Meat extract, chemotropic action of, 181.
Mechanical efficiency, 407, 408.
— factors and stimuli, influence of, on move-
ment, 71.
Mechanocleistogamy, 100.
Mechanotropism, 184.
Medicago, twisting of pod of, 24.
Megaclinumfalcatum, movements of label-
lum of, 22.
Meischke, 18, 136, 232, 233, 236, 237.
Meissner, 258, 260.
Mendelssohn, 215, 317, 337.
Menispermum canadense, direction of
twining of, 38; free coiling of, 37.
M. dahuricum, negative heliotropism
of, 42.
Mercurialis, geotropism of nodes of, 235.
Mereschkowsky, 271.
Mesembryanthemum, 103 ; movement of
stamens of, 81.
Mesocarpus, 310; heliotropism of, *72 ;
orientation of chloroplastids of, 327,
Fig. 53, 332.
Mesocotyl, 193.
Metabolism, changes of, during curvature,
247, 248.
Metatonic stimuli, 6.
Meyen, 20, 22, 24, 30, 80, 280, 288, 289, 292,
3J6, 3^3, 384 ; on movements of Ostil-
laria, 273 ; on streaming, 357.
Meyer, 304.
Micheli, 330.
Micrasterias, phototaxis of, 325.
Micrococcus phosphoreus, 383, 422.
Microspira luminosum, 385.
Microspora, 293.
Miehe, 235, 335, 336, 359; on geotropism
of nodes, 231 ; on localization of irrita-
bility, 197, 200, 205 ; on tonic stimuli,
6.
Migula, 264, 265, 266, 267, 269, 382.
Mikosch, 113.
Millardet, 123 ; on curvature of pulvini,
138.
Mimosa, 5, 103, 294 ; escape of water from
pulvinus of, 17, 76 ; influence of
etherization on, 7 ; — of injection with
water on, 18 ; mode of action of pulvini
of, 13, 14, 75, 79; recovery of, from
stimulation, 10 ; M.pudica> 2, 4,8, u,
61, Fig. 19, 91, 99, 358 ; accommoda-
tion of, 9, 69 ; action of induction-shocks
on, 145, 360; chemonastic reaction of,
85 ; energy of expansion in pulvinus of,
136, 137, 138 ; — changes of rigidity in,
139; history of knowledge of, 79, 80;
influence of anaesthetics on pulvinus of,
INDEX
439
144, 145 ; — of light, 141, 142 ; — of
mechanical factors, 71 ; — of oxygen,
143 ; — of temperature, 140, 141 ; — of
turgidity, 4, 1 6 ; — of water currents,
65 ; latent period of, 68 ; midday sleep
of, 106, 107 ; movements of, 2, 22, 26,
102, 120, 121, 123 ; — use of, 71 ; —
work done by, 413, 414 ; orientation of,
260, 261 ; propagation of stimuli in, 92,
94,95 ; production of electricity in, 397 ;
summation of stimuli in, 210; sup-
pression of irritability in, 69, 70 ; tem-
perature of pulvinus of, 79 ; thermo-
nasty of, 113, 115 ; torsion of, 104; M.
sensitive 77.
Mimulus, 63, Fig. 23 ; irritable stigma of,
24, 82 ; influence of ammonia on, 87 ;
- of air-pressure, 85 ; propagation of
stimuli in, 92, 93; M. Tilingit, sleep-
movements of, 103 ; thermonasty of,
114,115 ; — influence of gravity on, 127.
Minden, 82.
Mirabilis jalapa, minimal heliotropic sti-
muli for, 211.
Mirbel, 166 ; on heliotropism of Mar-
chantia, 252.
Misletoe, tropic irritability of, 162, 173.
Mitosis, reduction of, 303.
Mitschka, 219.
Miyake, 373.
Miyoshi, 82, 83 ; on chemotaxis, 347, 352 ;
on chemotropism, 180, 181, 182; on
hydrotropism, 183 ; on Weber's law,
214.
Mobius, 13, 102, 106, 231 ; on fixation of
pulvinar curvature, 245.
Modulus of elasticity, in wood, 415.
Mohl, 21, 30, 40, 46, 47, 48, 80, 208, 273,
276, 289 ; on anatomy of tropic organs,
243, 244; on attraction of twiners to
supports, 41 ; circumnutation of
twiners, 24 ; influence of electricity on
tendrils, 146; — of light on twiners,
42 ; on mode of twining, 35 ; origin
of torsion, 41 ; on pressure of coiling,
39-
Molecular movements, 263.
Molisch, 263, 301, 382, 383, 416, 422; on
chemotropism, 180, 182; on hydro-
tropism, 183; on localized perception,
198 ; on luminosity, 388.
Molliard, 317.
Momordica, 314 ; M. elaterium, dehiscence
of, 148.
Monas Okenii, 306.
Monster a deliciosa, 419.
Moore, on orientation of chloroplastids, 327,
329, 331, 333-
Morphaesthesia, 190.
Morren, 22, 92, 141 ; on irritability of
Drosera, 86 ; on sleep-movements of
stamens, 103.
Mottier, 170, 275 ; on centrifugal actions,
33S» 33^; on nuclear movements, 301,
302.
Mougeotia, fragmentation of, 148 ; orienta-
tion of chloroplastids of, 327, 331.
Movement, I, 3, 4, n ; energy of, 77, 412,
413 ; influence of air-pressure on, 76 ;
— of turgor on, 17 ; — of, on lumi-
nosity, 387 ; - on rigidity, 77 ;
mechanics of, 12, 84; relation of, to
circumnutation, 12.
— amoeboid, 275 ; autonomic, 19, 416 ;
causes of, 25 ; history of, 24 ; influence
of external conditions on, 29 ; measure-
ment of, 25 ; mechanics of, 31.
— of Desmids, 274, 275 ; — of Diatoms
272, 273 ; ciliary, 264, 266, 267, 268 ;
ephemeral, 23 ; gliding, 270 ; grasping,
35, .36 ; locomotory, 262, 263 ; photo-
nastic and thermonastic, 416 ; stream-
ing, 283 ; variation, 22.
Mucilage, influence of, on movement of
Desmids, 274, 275 ; of Diatoms, 273 ;
of Oscillaria, 273, 274 ; of pseudopodia,
276.
Mucor, 150, 303 ; growth-movements of,
19 ; localization of irritability in, 195 ;
M. mucedo, autotropism of, 189 ;
chemotropism of, 181 ; contact irrita-
bility of, 83 ; geotropism of, 165 ;
heliotropism of, 173, 175; rheotropism
of, 189 ; M. stolonifer, autonomic
movements of, 19, 20 ; geotropism of,
165 ; influence of gravity on circum-
nutation of, 28.
Mucorineae, irritability of sporangiophores
of, 85.
Miiller, 48, 191 ; on tropic after-effects, 212.
F. Miiller, 19. H. — , 167, 173, 232,
233, 234, 238 ; on directive action on
light, 228, 235, 236; on growth curva-
tures, 240. N. J. C. — , 176, 234, 237 ;
on heliotropic reversal, 171 ; on re-
spiration during curvature, 208. O. — ,
21 ; on movements of Diatoms, 272,
273. P. E. — on sinking of rhizomes,
249.
Miiller- Hettlingen, 394; on galvanotropism,
188, 189.
Miiller-Thurgau, 208, 241, 380, 381.
Munk, 87, 1 1 1 ; on digestive movements, 86 ;
on hairs oiDionaea, 81 ; on mechanism
of — , 80 ; on stimulation of — , 65 ;
on production of electricity, 390, 394,
395-
Murbeck, 182.
Muscle, character of, 283.
Musset, 211.
Mutisia clematis, climbing of, 34.
Myoid fibres, 281.
Myriophyllum, influence of darkness on
leaves of, 106 ; M. proserpinacoides,
sleep-movements of, 103.
Myxomycetes, action of induction-shocks
440
INDEX
on, 360 ; chemotaxis of, 180, 348 ; con-
sistency of, 279/281, 282 ; movements
of swarm-spores of, 275 ; phototaxis of,
326 ; pulsating vacuoles of, 293, 294 ;
rheotaxis of, 356.
Nabokich, 143.
Nagel, 6, 216, 228, 229, 309, 320, 325, 326.
Nageli, 12, 23, 75, 263, 271, 273, 274, 282,
311, 315, 318, 324, 334, 355, 357, 360 ;
on cilia, 265, 267, 268 ; on production*
of heat, 378, 379 ; on protoplasmic
movement, 276, 285, 287, 288.
Nastic movements, definition of, 3.
Nathansohn, 406, 407.
Nawaschin, 276.
Nees, 386, 387.
Neger, 174, 257.
Neljubow, 207.
Nelnmbo nucifera, production of heat by,
373-
Nemec, 186, 207, 242, 243, 334, 336, 359,
418; on causation of tropism, 223,
224 ; on changes in geotropically excited
cells, 225 ; on localization of percep-
tion, 198, 199 ; on transference of
stimuli, 200, 201, 204.
Nepenthes, aggregation in, 90 ; influence
of chemical excitation on, 88.
Neptunia oleracea, 95.
Nernst, 364.
Nestler, 359.
Neubert, 20, 165.
Newcombe, 419 ; on indifference of radicles
to contact, 82; on rheotropism, 184,
185.
Nicotiana rustica, sleep-movements of, 103.
Nigella, movements of style of, 24.
Niklewski, 79.
Nitella, 308, 327, 328, 334; electrical cur-
rents in, 395 ; streaming in, 338, 357,
358 ; — direction of, 283, 293 ; — dura-
tion of, 285, 286 ; energy of, 288, 369 ;
— influence of temperature on, 313,
3U, 3I5> 3i6; — of light, and acids,
319 ; — localization of, 287 ; — rate of,
284, 288 ; seismonic irritability of,
66, 75, 99 ; — transference of stimuli
in, 95, 201 ; N.flexilis, anaerobism of,
341 ; geotropism of, 165 ; heliotropism
of, 174 ; N. translucens, anaerobism
of, 341.
Nitophyllum uncinatum, coiling of, 46.
Nitrous oxide, influence of, on irritability,
144-
Nitschiella, movements of, 271.
Nitschke, 84.
Noctiluca, 385, 388.
Nodes, curvature of, 242 ; geotropism of,
200, 205, 231, 232, 235, 242 ; growth
of, during curvature, 240 ; ruptures due
to, 243.
Noggerath, 386, 387.
Noll, 37, 192, 205, 218, 219, 220, 221, 225,
226, 227, 239, 257, 260 ; on causation
of geotropism, 223, 224 ; on cellular
changes during curvature, 240 ; on
changes of tone, 207, 208, 217 ; on
conjoint stimuli, 209 ; on diageotropism
of twiners, 37 ; on exotropy, 258 ; on
influence of gravity on sleep-move-
ments, 128; --of etiolation on cir-
cumnutation, 28 ; on summation, 210 ;
on tropic curvature, 161, 165, 168,
173, 174, 241-7, 255.
Nordhausen, 46, 181.
Nowakowski, 265.
Nuclear division, 302 ; importance of, 303 ;
influence of streaming on, 285 ; of
temperature on, 317.
Nucleolus, density of, 336.
Nucleus, influence of stimulation on, 89 ;
— on pulsating vacuoles, 298 ; move-
ments of, 275, 287, 301, 359 ; density
of, 336 ; as reflex centre, 10.
Nutation, definition of, I ; movements,
influence of gravity on, 28 ; mechanics
of, 128; nature of, 12, 13; special in-
stances of, 21 ; undulating, 23.
Nutrition, influence of, on streaming, 338.
Nyctinastic, definition of, 97 ; movements,
108-12.
Nyctitropic, definition of, 97.
Nymphaea alba, photonasty of, 122; pro-
duction of electricity by, 398 ; ther-
monasty of, 113; N. blanda, sleep-
movements of, 103.
Oedogonium, autonomic movements of, 20 ;
ciliation of zoospore of, 264 ; growth
and nutation of, 31 ; origin of move-
ments of, 15.
Oels, 170.
Oker-Blom, 390.
Olax, 45.
Olive, 326, 348, 356.
Oliver (F. W.), 82, 118 ; on propagation of
stimuli in Masdevallia, 92, 93 ; on
sleep-movements, 102.
Olivi, 324.
Oltmanns, 20, 23, 100, 103, 106, 174, 206,
211, 232, 250, 252, 253, 255, 260, 261,
318 ; on closure of Tragopogon in
strong light, 108; on directive action
of light, 228 ; on movements of
flowers, 120; on photometry, 3; on
phototaxis, 323, 324, 331, 332; on
reversal of heliotropism, 171, 172.
Onions, production of heat by, 375.
Opalina ranarum, galvanotaxis of, 361,
363-
Operations, influence of, on irritability, 198,
199, 200, 203, 204, 205.
Opuntia, movement of stamens of, 82.
Orchids, changes of tone in, 205.
Orchis, 258.
INDEX
441
Ornithogaluw, umbellatum, thermonastic
flower of, 113.
Orobanche, nuclear movements of, 301.
Oscillaria^ 316, 383 ; movements of, 24,
270, 271, 272, 273, 274 ; — in gelatine,
357 ; phototaxis of, 326.
Osmotaxis, 178, 343, 344, 350, 351, 352;
detailed character of, 353, 354, 355;
nature of, 230.
Osmotic energy, physical nature of, 400 ;
uses of, 402, 403.
Osmotic pressure, influence of, on move-
ment, 73, 74 ; — on pulsating vacuoles,
294, 296.
Osmotropism, 178 ; nature of, 187, 230.
Ostwald, 263, 368, 390, 394, 400.
Otocysts, 224.
Otoliths, 224.
Ova, reunion of, 365.
Overton, 180, 266, 323, 324.
Oxalidaceae, sleeping flowers of, 103.
Oxalis, 23, 26, 62, Fig. 20 ; action of in-
duction-shocks on, 360 ; continued ex-
citability of, 9, 70; dispersal of, 148; in-
fluence of strong light on leaflets of, 108 ;
latent period of, 68, 69 ; use of move-
ments of, 71 ; O.acetosella, influence of
feeble light on pulvini of, 142 ; — of
induction-shocks on, 145 ; — of stimuli
on rigidity of, 77 ; non-conduction of
stimuli in, 91 ; orientation of chloro-
plastids of, 329, Fig. 55, 330; sleep-
movements of, 102; thermonasty of,
113 ; variation movements of, 22 ; O.
dendroides, 91, 96 ; O. hedysaroides,
416; O. rosea, influence of light on
daily movements of, 108-11; photo-
nasty of, 122; thermonasty of, 113-15,
122 ; O. sensitiva, irritable cotyledons
of, 80, 92.
Oxygen, attractive action of, 180, 182 ; in-
fluence of absence of, on irritability, 140,
143, 144; — on chemotaxis, 354, 355 ;
— on heat-production, 371, Fig. 65, 372,
375) 377? 378 ; — on irritable tone, 202 ;
- on luminosity, 383, 387; — on
movement, 338; — on production of
electricity, 395 ; — on pulsating va-
cuoles, 299 ; — on streaming, 314, 315,
339> 340 ; — on surface-tension, 283 ;
repellent action of, 351, 352; stimu-
lating action of, 347.
Oxygenotaxis, 347.
Oxygenotropism, 179.
Oxytrichia, 310 ; galvanotaxis of, 361.
Oxytropism, 179, 182.
Paeonia officinalis, rise of temperature in,
373-
Palm, 38 ; on circumnutation, 24 ; on mode
of twining, 35 ; on origin of torsion, 41.
Palmellaceae, pulsating vacuoles of, 293,
295.
Palms, altered geotropism in roots of, 164.
Pandanus utilis, rise of temperature in,
373-
Pandortna, ciliation of, 264 ; irritability of,
226 ; locomotion of, 266 ; P. morum,
421 ; influence of oxygen on movement
of, 340.
Panicum, 218 ; P. miliaceum, heliotropism
of, 193, Fig. 41 ; transference of stimuli
in, 199.
Pantanelli, 13, 138; on sleep-movements of
P or Her a, 118.
Paoletti, 102, 144; on sleep-movements of
Porliera, 118.
Papaver, curvature of peduncle of, 27, 164 ;
P. somniferum, rise of temperature in,
373-
)ili(
Papilionaceae, opening of flower-buds of,
23, 3«-
Paraheliotropism, 106, 107, 108 ; defini-
tion of, 155.
Parallelotropism, artificial production of,
162; definition of, 155, 156 ; origin of,
1 60.
Pararnaetium, chemo- and osmotaxis of,
353> 3555 lifting-power of, 268; pul-
sating vacuoles of, 296; reversal of
movements in, 266 ; thermotaxis of,
317; P. aurelia, galvanotaxis of, 361,
362, 363 ; pulsating vacuoles of, 295 ;
P. bursaria, galvanotaxis of, 361.
Paranasty, definition of, 3 ; in apices of
twiners, 37.
Parietaria, movements of stamens of, 146,
147.
Parnassia, movements of stamens of, 24.
Passerini, 372, 379.
Passiflora, 42 ; absence of pits in tendrils
of, 65 ; P.graciliS) circumnutation of, 21.
Pasteur, 378.
Pauli, 282.
Payer, 176.
Pea, influence of temperature on circumnu-
tation of, 29.
Pearl, 362.
Peduncles, geotropism of, 164, 165 ; helio-
tropism of, 174.
Peirce, 48 ; influence of gravity on contact-
irritability, 28.
Pellionia, 253.
Pelomyxa, locomotion of, 281 ; P. palu-
striS) influence of oxygen on, 340 ; -
of hydrogen on, 341 ; photophobism of,
308, 320.
Peltigera, changes of irritability in, 162.
Penicillium, growth-movements of, 19 ; in-
difference of, to contact, 83 ; influence
of alkaloids on, 342; P. glaucum,
chemotropism of, 181, Fig. 38.
Penium^ phototaxis of, 325.
Peptone, chemotactic action of, 354, —
chemotropic, 181 ; influence of, on
luminosity, 386.
442
INDEX
Perception, definition of, 5 ; localization of,
192 ; of stimuli, 219; • — and response,
relation between, 8.
Periblem, traumatropism of, 186.
Peridinium tab^^latum, galvanotaxis of, 361.
Periodicity, in heat-production, 372, 376,
377 ; photonastic origin of, 108.
Periploca graeca, twining of, 38.
Permeability, influence of, on osmotropism,
230.
Peronospora, hygroscopic torsion of, 151.
Pertz, 26, no, 124, 166, 190,210, 218, 418.
Petiole, curvature of, 232.
Petiole-climbers, 43, 44, 45.
Peztza fuckeliana, heliotropism of, 173.
Pfeffer, I, 2, 3, 13, 14, 17, 18, 24, 25, 26,30,
31, 47, 48, 73, 74, 76, 81, 85, 89, 91,92,
93, 102, 104, 105, 106, 118, 119, 145,
154, 211, 220, 223, 233, 236, 237, 238,
239, 240, 263, 265, 266, 267, 268, 269,
270, 275, 276, 286, 292, 303, 325, 343,
358, 362,368,400; on changes of tone,
208; on chemotaxis, 180, 311, 312,
344, 345, 346, 347, 348, 350, 351, 352,
353, 354, 357 J on chemot.ropism, 230,
231 ; .on coiling of Phycomyces, 37; on
conjoint excitation, 216 ; on consis-
tency of protoplasm, 279, 280 ; on con-
tact - irritability, 83 ; on contractile
mechanism, 78, 79, 80 ; on daily move-
ments of plants, 119-26 ; on daily
periodicity, 1 12 ; on density of particles
in the cell, 334 ; on depression of ex-
citability, 69 ; on excitability of pulvini
of Oxalis, 70 ; on expansive energy in
pulvinus, 32 ; on influence of anaes-
thetics on irritability, 94 ; on irritability
tADrosera, 84 ; on localization of sensa-
tion, 193, 198 ; on mechanism of move-
ment, 72 ; — pulvinar, 75, 77, 79 ; —
variation and nutation, 129, 131, 132,
133, 135-9 > — influence of light and
temperature on, 141, 142 ; on motility,
3°5, 3°7, 3°9 J on movements of
Cynareae, 16; on nature of irritability,
1 1 ; on orientation of chloroplastids,
327 ; on pits in tendrils, 65 ; on pro-
duction of heat, 377, 379 ; on pulsating
vacuoles, 294-7 ; on sleep-movements,
103, 108-11 ; — thermonastic, 113-16 ;
on transformations of energy, 401, 403,
406, 407, 408, 411, 412, 413 ; on tropic
responses, 241-8 ; — on a klinostat,
161, 166, 169, 171, 174; on Weber's
Law, 213, 214, 215.
Pfitzer, 148, 272.
Pfltiger, 382, 384, 387.
Phalaris, influence of darkness on nutation
of, 30 ; time of reaction of, 211.
Phaseolus, 257, 258,417,419; changes of
turgor and rigidity in pulvinus of, 135,
238 ; climbing-hairs of, 40 ; curvature
of young pulvinus of, 245 ; direction of
twining of, 38 ; — limiting angle for,
40 ; insensitiveness to contact of, 35 ;
photonastic movements of, 105 ; time
of reaction of, 212 ; twining of, in dark-
ness, 30; P. multiflorus, influence of
gravity on sleep-movements of, 125,
Fig. 33, 126; loss of twining by, 38 ;
/*. vulgariS) energy of expansion in
pulvinus of, 136, 137,138 ; pulvinus of,
13, Fig. I ; rate of revolution in, 21 ;.
sleep-movements of, 104 ; influence
of gravity on — , 125 ; variation move-
ments of, 22.
Philadelphus, 259 ; changes of tone in,
205 ; epinasty of, 254 ; torsion of, 260.
Phloroglucin reaction, 415.
Phobism, 307, 309, 310 ; accumulation due
to, 311.
Phobophototaxis, 215.
Phoenix, geotropism of cotyledon of, 165.
Pholas dactylus, 383, 387.
Phosphates, chemotropic and chemotactic
action of, 181, 420, 421.
Phosphorescence, 421.
Phosphoric acid, influence of, on streaming,
3i9-
Photobacterium indicum, 385, 386; P.
luminosum, 386 ; P. phosphorescent?
385, 386.
Photocleistogamy, 100, 106.
Photokinesis, definition of, 6.
Photometric leaves, 260, 261.
Photometry, definition of, 3.
Photonasty, 97 ; instances of, 101 ; uses of,.
100 ; in pulvini, 14.
Photosynthesis, influence of, on movements
of chloroplastids, 332 ; — on produc-
tion of electricity, 396.
Phototaxis, 321, 419; changes of, 32 1,323 ;
character of, 322; of Diatoms and
Desmids, 325 ; of Myxomycetes, 326 ;
of zoospores, 324.
Phototonus, influence of, on autonomic
movement, 30 ; — of different rays on,
142 ; — of external agencies on, 319.
Phototropism, nature of, 227.
Phycomyces^ 92, 177, 182, 187, 188 ; auto-
nomic movements of, 19 ; autotropism
of, 189 ; contact-irritability of, 83 \
discriminatory sense of, 213; hydro-
tropism of, 183 ; influence of anaes-
thetics on geotropism of, 145 ; irrita-
bility of, on a klinostat, 48 ; localized
perception in, 195 ; parallelotropism of,
156; rheotropism of, 185; time of
heliotropic induction in, 211 ; P.
nitens, coiling of, 37, 46, 82 ; geotropism
of, 165 ; heliotropism of, 171, 173.
Phyllanthus Niruri, sleep-movements ofr
102, 104.
Piccard, 418, 419.
Picea, 260 ; P. excelsa, changes of tone in,
204.
INDEX
443
Pilea, 253 ; movements of stamens of,
147.
Pilobolus, autotropism of, 189; P. crystal-
linus, dispersal of, 150, 153; helio-
tropism of, 173, 175.
Pilogyne suains, growth of curving tendril
of, 57, Fig. 18.
Pinguicula, 85 ; absence of granulation in,
90 ; irritability of, 84 ; P. vulgaris,
irritability of, 87.
Pinus, orientation of branches of, 254, 255.
Pisum, 182, 335, 418 ; curvature of epicotyl
of, 27 ; electrical currents in, 390 ;
tendrils of, 42, 43 ; P. sativum,
changes of tone in, 207 ; curvature of
etiolated seedlings of, 23 ; thermo-
tropism of, 177.
Pith, geotropism of, 243 ; influence of, on
curvature, 243.
Plagiotropic shoots, orientation of leaves
on, 259, 260.
Plagiotropism, definition of, 155 ; origin of,
I58> 159? 160 ; in Hedera, 252; in
leaves, 255, 256 ; in Marchantia, 251 ;
in rhizomes, 249 ; in runners, 250 ; in
trees, 253.
Planch on, 117.
Plantago media, influence of light on
leaves of, 105.
Plasmodia, influence of changes of tempera-
ture on, 317.
Plasmodiophora, 276.
Plasmolysis, influence of, on curvature, 246,
247 ; — on irritability, 74, 201 ; — on
streaming, 355 ; — stimulatory, 75.
Plaster-of-paris cleistogamy, ico.
Plenge, 265, 269.
Pleospora scirpicola, 150, Fig. 34.
Pleurotaenium, phototaxis of, 325.
Pliny, in.
Plowmann, 394.
Pneumatophores, 164.
Poggioli, 176.
Poikilotherms, 366, 367.
Poisons, influence of, on streaming, 342.
Polarity, influence of centrifugal forces on,
336-
Polarized light, heliotropic action of, 170.
Pollen-tubes, aerotropism of, 182 ; chemo-
tropism of, 180, 181 ; discriminatory
power of, 214 ; indifference of, to con-
tact, 83 ; penetration of, 181, 182, 190.
Pollock, 200, 234, 243 ; on traumatropism,
185.
Polygonaceae, geotropism of nodes of,
242.
Polygonatum multiflorum, 253.
Polygonum, influence of etiolation on nuta-
tion of, 30 ; P. aviculare, geotropism
of, 165 ; orientation of, 250 ; twining
of, 33 ; P. complexum, 38 ; P. convol-
vulus, twining of, 38, 40 ; influence of
light on circumnutation of, 42 ; P.
Fagopyrum, influence of etiolation on
nutation of, 28 ; — on twining of, 35.
Polyphagus euglenae, 324 ; locomotion of,
265.
Polypodiaceae, dehiscence of, 152.
Polyporus squamosus, influence of light on
formation of pileus of, 416.
Polytoma uvella, chemotaxis of, 351; gal-
vanotaxis of, 361 ; geotaxis of, 337.
Popoff, 377.
PopOW, 102.
Poppy, changes of tone in, 205 ; geotropism
of peduncle of, 164.
Porliera, 13 ; sleep-movements of, 102 ; P.
hygrometrica, hydronastic movements
of, 118 ; reaction of operated pulvinus
of, 138.
Portheim, 417.
Portulaca, sleep-movements of, 102 ; P.
sativa, photonastic pulvini of, 109.
Portulaceae, motile stamens in, 82.
Posternak, 282.
Potassium salts, chemotactic and tropic
action of, 181, 349; repellent action of,
351) 352» 3535 suppression of irrita-
bility by, 86.
Potatoes, production of heat by, 375, 376.
Potentilla reptans, orientation of, 250.
Potts, 183, 304.
Prantl,3l, 174, 175.
Precipitation, influence of light on, 229.
Precipitation membranes, influence of sur-
face-tension on, 281.
Presentation period, 209, 211, 417.
Pressure, influence of, on pulsating vacuoles,,
297.
Preuss, on tropism of old leaves, 231.
Prillieux, 327.
Primula elatior, influence of light on, 105.
Pringsheim, 149, 288, 320, 327, 333, 342.
Prinz, 380.
Prisms, use of, 228.
Prothallus, orientation of, 252.
Protoplasm, accumulation of, during curva-
ture, 219; — on injury, 359; — on stimu-
lation, 1 1 ; consistency of, 276, 277 ;
— changes in, 279 ; deformation of, by
changes of temperature, &c., 308, 316,
317 ; — due to light, 320 ; extracellular,
272, 273 ; influence of cohesion of, on
movement, 16 ; — of light, 229.
Protoplasmic connexions, action of, 269;
resistance to flow in, 288 ; use of, for
transference of stimuli, 93 ; for trans-
location, 91.
Protoplasmic fibrillae, function of, 201.
Protoplasmic streaming, 283; diagram of,
291, Fig. 51 ; duration of, 283-4 ; excita-
tion of, 284; history of, 289 ; importance
of, 285 ; influence of alkaloids and
poisons on, 342, of acid and alkalies,
343, — of oxygen, 340, 341, of shape of
cell on, 292 ; — of, on pulsating vacuoles,
INDEX
294 ; localization of, 286, 287 ; physics
of, 288 ; rate of, 284 ; theories of, 289,
290, 291 ; types of, 284.
Protosiphon botryoides, 305.
Prowazek, 176.
Prunus, epinasty of, 254.
Pseudopodia, forms of, 275, 281.
Pterostylis, movements of labellum of, 22.
Pulsating vacuoles, 290, 293 ; action of, 296,
297 ; character of, 294 ; frequency of,
295 ; function of, 299 ; influence of
external agencies on, 298 ; — of tem-
perature, 317.
Pulvini, changes of turgor in, 139 ; ex-
pansive energy of, 32, 136; influence
of absence of oxygen on, 143 ; — of
chloroform and ether on, 144 ; — of in-
duction-shocks on, 145 ; - - of light
on, 141, 142 ; — of operations on, 76,
77, 136, 137, 138; — of shaking on,
140 ; — of temperature on, 141 ; growth
of, when inverted, 245 ; localization of
irritability in, 196 ; mechanism of, 13,
3i> 75> 76, 134; rigidityof, 18, 77, 135 ;
twisting of, 104.
Purple bacteria, 306.
Putter, 266, 358 ; on galvanotaxis,36i, 362,
363, 364-
Pyrenomycetes, dispersal mechanism of,
192.
Querton, 394, 396.
Quincke, 277, 281, 292, 299, 304.
Quinic acid, chemotropic action of, 181.
Raciborski, 36.
Radius of cell, influence of, on resistance to
streaming, 288.
Radius of curvature, influence of, on surface-
tension pressure, 277.
Radium rays, influence of, on germination,'
415 ; non- tropic action of, 176.
Radziszewski, 387.
Rameaux, 379, 381.
Ranke, 390, 394, 398.
Ranunculus aquatilis, heliotropism of roots
of, 232: R. Ficaria, thermonastic
flower of, 113.
Raphanus, 182.
Ratschinsky, 242.
Ray, 79, 113.
Reaction periods, 211.
Rectipetality, 190.
Reinke, 291, 327, 382.
Resistance, influence of, on curvature, 234,
236, 237 ; path of least, in streaming
cells, 292, 293.
Respiration, influence of curvature on, 208 ;
relation of, to heat-production, 369,370;
— to streaming, 286; intramolecular,
production of heat by, 374, 378.
Keticuloplasm, 303.
Revolutive nutation, 21.
Rhabdoid, 89.
Rheotaxis, 356.
Rheotropism, 184, 185.
Rhipidophora, 327.
Rhizomes, geotropism of, 164, 219; position
of, in soil, 248, 249.
Rhizomorpha, luminosity of, 383, 384, 385,
386.
Rhizopus nigricans, streaming in, 284.
Rhodomela, fragmentation of, 148.
Rhumbler, 305, 307, 312, 357, 365 ; on pro-
toplasmic movement, 276, 277, 279-82,
286, 288 ; — physics of, 292, 294, 296-
9, 302, 304.
Richards, on production of heat, 371, 375.
Richter, 20, 165, 169, 174, 204, 419 ; on irri-
tability of root-apex, 197.
Ricinus, 330.
Ricome, 218.
Rigidity of pulvini, 31, 32.
Rimbach, 164 ; on depth of rhizomes, 249.
Rischawi, 188.
Ritter, 306; on influence of oxygen on
streaming, 338, 340, 341, 342.
Robinia, 13 ; latent period and movement
of, 68, 69; orienting torsion of, 233,
photonasty of, 98 ; sleep-movements
of, 102 ; /?. hispida, 80 ; R. pseud-
acacia, geotropism of hypocotyl of, 165 ;
reaction of operated pulvinus of, 138 ;
seismonic irritability of, 80, and of R.
viscosa, 80.
Rodewald, 403, 404, 407 ; on production of
heat, 369, 371.
Rodier, 19.
Rodrigue, 13.
Roesele, 360, 363.
Romer, 372.
Rontgen rays, influence of, on germination,
415 ; tropic action of, 176.
Root, aerotropism of, 180, 182 ; curvature
of, 232, 237, 248 ; — measurement of
growth during, 239, 240 ; — resistance
overcome by, 238 ; galvanotropism of,
1 88 ; geotropism of, 163, 164 ; helio-
tropism of, 173 ; rheotropism of, 184 ;
thermotropism of, 177 ; traumatropism
of, 185, 1 86.
Root-apex, localization of irritability in, 196,
197, 198, 418, 419.
Root-hairs, response of, to contact, 83.
Root-tendrils, 32.
Rosanoff, 337.
Rosenberg, 83, 90 ; on cellular changes in
Drosera, 89.
Ross, 165.
Rostafinski, 153.
Rotation, 283 ; during free-swimming, 265,
266, 267.
Roth, 84, 356.
Rothert, 6, 18, 27, 150, 153, 155, 182, 187,
189, 232, 233, 234, 269, 293, 307, 309,
310, 311, 313, 355, 416, 419, 42i; on
INDEX
445
chemotaxis,2i5,343, 345, 346, 347, 348,
352 ; on geotropic curvature, 241 ; on
influence of darkness on nutation, 30 ;
— of external conditions on irritability,
203 ; on localization of sensation, 193,
194, 196, 197, 198, 199, 200; on osmo-
tropism, 178 ; on phototaxis, 323, 324.
Roucheria, 45 ; thickening of hooks of, 46.
Roux, 364, 365.
Royer, 113 ; on ephemeral flowers, 23 ; on
sleep-movements, 112.
Roz£, 148.
Rubidium salts, chemotactic action of, 350.
Rubner, 371, 377, 409.
Rubus caesius, orientation of, 250.
Rumph, 383.
Runners, geotropism of, 164; orientation
of, 249, 250.
Ruta graveolens, movements of stamens of,
23, Fig. 5 ; — influence of darkness
on, 30.
Riitzow, 235.
Saccharum offitinarum, geotropism of, 242.
Sachs, i, 19, 25, 27, 30, 37, 46, 188, 191,
192, 204, 206, 207, 211, 220, 223, 232,
233, 234, 236, 237 ; on curvatures due
to rubbing, 82 ; on daily periodicity,
112; on directive action of light, 228,
229 ; on geotropic curvatures of split
roots, 241, 242, 243; on growth of
hanging shoots, 36 ; on hydrotropism,
183 ; on influence of centrifugal force
on Marchantia, 251 ; — of external
conditions on streaming, 314, 315, 316,
318 ; — of light on Marchantia and
Ivy, 252, 253 ; — of temperature on
pulvini, 141, 142 ; on movements of
zoospores, 324 ; on nature of nutation
movements, n ; on optimal angle,
217; on streaming, 290; on shadow
figures, 333; on tropism, 161, 163-7,
168, 171, 174-6; — after-effects, 212;
on rate of growth during curvature,
238, 239, 240; on Weber's Law, 213.
Sagittaria, 204.
Salix, 259 ; minimal heliotropic stimulus
for, 211.
Salts, influence of, on pulsating vacuoles,
298.
Samassa, 340, 341, 342.
Saposchnikow, 237.
Saprolegnia, 269, 293, 307, 308, 325, 421 ;
chemotaxis of, 214, 347, 349, 353;
chemotropism of, 180, 181.
Sarothamnus, 148.
Sarracenia, aggregation in, 90.
Saussure, 373, 374, 375-
Saxifraga, staminal movements of, 24;
influence of darkness on, 30; move-
ments of style, 24.
Scabiosci) sleep-movements of, 103.
Scarlet-runner, early development of, 33 ;
limiting diameter for twining of, 40.
Schaefer, 315.
SchafFner, 260.
Schaudinn, 304.
Schellenberg, 249.
Schenck, 21, 32, 34, 39, 40, 42, 47, 48, 182,
317, 360, 364 ; on twining, 38.
Schenkemeyer, 73.
Schilling, 22, 414; on response to forced
curvatures, 124.
Schimkewitsch, 303.
Schimper, 38,90, 1 73; on orientation of chloro-
plastids, 327, 328, 329, 330, 332, 333.
Schizostega (Schistostegd), 329, 384.
Schleicher, 356.
Schleiden, 289.
Schmidt, O., 25 5, 257.
- P., 332, 384.
Schmitz, J., 165.
— Fr., 327, 330, 375, 376.
Schober, 163, 218 ; on action of Rontgen
rays, 176.
Scholtz, 27, 28, 205.
Schroder, 273, 274.
Schrodt, on cohesion mechanism, 151, 152.
Schiibler, 112.
Schultze, 315 ; on movements of Diatoms,
271, 272, 273; — of Oscillaria, 274;
on protoplasmic streaming, 355.
Schulz, 19, 23, 24, 373.
Schuman, 294.
Schiitt, 263, 264, 270, 274, 323 ; on move-
ments of Diatoms, 273 ; on stimulatory
plasmolysis, 75.
Schwarz, 236, 268 ; on geotaxis, 336, 337,
338 ; on Weber's Law, 213.
Schwendener, 12, 13, 18, 27, 34, 39, 40, 75,
77, 104, no, 124, 152, 205, 230, 233,
255, 258, 259, 260, 263, 357, 360; on
curvature of chloroformed pulvini, 139,
— operated, 13,8; on dorsiventrality,
258; on mode of twining, 35; on origin
of homodromous coiling, 37; on torsion,
41 ; tortism, 155.
Scirpus maritimus, 204 ; plagiotropism of
rhizome of, 156, 157, 164.
Scleroderma, production of heat by, 366.
Scramblers, 32.
Scrophularia, 258.
Scyphanthus elegans, reversal of twining
in» 39-
Scytosiphon lomentarius> 324.
Season, influence of, on orientation, 250.
Secale, geotropic response of, 242.
Seckt, 176.
Secretion, use of, for attachment, 48.
Seddig, 302.
Sedum, orientation of chloroplastids of, 329.
Seedlings, autonomic movements of, 20;
commencement of circumnutation in,
21 ; — influence of temperature on,
29; geotropism of, 165.
446
INDEX
Seeds, dispersal of, 151.
Seignette, 368, 371, 373, 3$i>
Seismonic irritability, absence of, in sta-
mens of Helianthus, 75 ; character of,
65, 66, 68, 140; influence of anaes-
thetics on, 144; uses of, 71.
Seismonic stimuli, influence of, on excita-
bility, 70.
Selaginella, 253 ; chemotaxis of sperms of,
345, 353 5 S.Martensii, 327.
Sempervivum, orientation of chloroplastids
of, 329; S. alpinum, temperature of,
379-
Senebier, 372, 383.
Senn, 293, 323, 332.
Sensation, nature of, 5.
Setaria italica, heliotropic seedling of, 197,
Fig. 43 ; S. viridis, heliotropism of, 193.
Shadow figures, 333.
Shibata, 303, 421.
Shock reactions, 87, 307 ; nature of, 8 ;
influence of, on streaming, 357, 358.
Sicyos angulatus, viscid secretion of, 48.
Siebold, on cilia, 265 ; on movements of
Diatoms, 271.
Sigesbeckia orientalis, influence of light on
daily movements of, 108-11 ; sleep-
movements of, 103.
Silene nutans, 104.
Silphhtm laciniatum, photic orientation of,
261.
Simons, 21, 29.
Sinapis alba, heliotropism of, 172, Fig. 37,
173 ; influence of absence of oxygen
on, 143; time of reaction of, 211.
Singer, 183, 207.
Slack, on streaming, 289.
Sleep-movements, 102, 103, 104; uses of,
100, 101.
Smithia sensitiva, seismonic irritability of,
80.
Sodium chloride, chemotropic action of,
181.
Soja, 257 ; S. hispida, 185.
Sokolowa, 19.
Solanum, heliotropism of, 194 ; S. dulca-
mara, twining of, 38 ; S. jasminoides,
44; twining petiole of, 45, Fig. 13;
— thickening of, 46, 47; S. lyco-
persicum, 314.
Somatotropism, 189-92.
Sonntag, on red and white wood, 415.
Sordariafimiseda, heliotropism of, 173.
Sorghum vulgar e, heliotropism of, 193.
Sorokin, 173, 175, 318.
Sosnowsky, 337.
Sound-waves, influence of, 79.
Sowinsky, 381.
Spadix, production of heat by, 370, 371,
374, Fig. 67 ; use of, 368.
Spalding, on traumatropism, 185, 186, 187.
Sparganium, 204 ; plagiotropism of, 146,
147 ; — of S. ramosum, 164.
Sparmannia, movement of stamens of, 82 ;
S. africana, 92; sleep-movements of
stamens of, 103.
Spergula salina, 115.
Sperms, discriminatory sense of, 214.
Sphaeria scirpi, dispersal of, 150, Fig. 34.
Sphaerobolus stellatus, 148.
Sphaeroplea, 325.
Sphagnum, chemotaxis of sperms of, 345.
Spherogenic activity, 275.
Spmacea, movements of stamens of, 147.
Spiraea salicifolia, 259.
Spirillum, aerotaxis of, 365 ; chemotaxis
of, 344, Fig. 62, 346, 350; cilia of,
264; locomotion of, 266; S. Fmkler-
Prior, 340, 346 ; S. serpens, 346, 347 ;
S. tenue, 421 ; S. undula, 346, 347,
35 r, 352; chemotaxis of, 346; osmo-
taxis of, 353, 354; S. -volutans, 346,
351-
Spirogyra, 4, 202, 262, 274, 300, 303;
autonomic movements of, 20; coiling
°f> 38 ; growth-movements of, 19 ; in-
fluence of centrifugal force on, 335 ;
persistence of curvature in, 31 ; re-
traction of, 75 ; streaming in, 285.
Spirostomum ambigimm, galvanotaxis of,
310,361; S.teres,^.
Sporangia, dehiscence of, 150, 151, 152;
— influence of light on, 153.
Sporangiophores, discriminatory power of,
214; autotropism of, 189; electro-
tropism of, 188 ; geotropism of, 165 ;
heliotropism of, 173, 195 ; hydro-
tropism of, 183.
Spores, dispersal of, 149, 150, 151, 416;
influence of light on, 153.
Sporodinia, 183 ; S. grandis, 209.
Sporophores, curvature of, when split, 241 ;
rise of temperature in, 366.
Sprengel, 100.
S tacky s sylvatica, 250.
Stahl, 22, 30, 102, 106, 174, 206, 261, 274,
337, 344, 348, 349, 35°- 352; on auto-
nomic movements, 25; on chemo-
tropism, 180 ; on orientation of chloro-
plastids, 327. 328, 329, 330, 331, 332,
333 ; on phototaxis, 320, 321, 322, 324,
325, 326; on protoplasmic streaming,
355 ; on reversal of heliotropism, 171 ;
on rheotaxis, 356 ; on sleep-movements,
103, 126; — uses of, 100; on thermo-
taxis, 317; on tropism, 164.
Stamens, dehiscence of, 147, 148 ; irritable,
24, 8 1 ; — influence of darkness on
movements of, 30 ; mechanism of, 72,
73; of Ruta, 23, Fig. 5.
Stammeroff, 105.
Stange, 354.
Stanhopea oculata, opening of flower of,
148.
Starch-grains, influence of gravity on, 334,
336 ; — of centrifugal force, 335 ;
INDEX
447
statolith theory of, 223, 224; — dis-
proof of, 418.
Statocysts, 224.
Statolith, 224 ; theory, 223, 417, 418.
Steinbrinck, 150; on cohesion -mechanism,
151,152.
Stem, 8l.
Steiner, 224.
StellaHa media, hydronastic flower of, 117 ;
- influence of light on, 100, 106 ;
sleep-movements of, 103.
Stemonitis fusca, rate of movement of, 276.
Stems, adult, curvature of, 12; split, cur-
vature of, 241 ; twining of, 34.
Stenstrom, 117.
Stentor, irritable zones in, 363.
Steyer, 82, 145, 165, 173, 177, 188, 189,
232 ; on aerotropism, 182 ; on hydro-
tropism, 183, 184, 187 ; on localization
of sensation, 195.
Stigeoclonium^ 293.
Stigmas, movements of, 82 ; — autonomic,
24.
Stimulation, influence of, on temperature,
78 ; recovery from, 9, 10.
Stimulators, 67, 76.
Stimuli, conditions for action of, 216, 217 ;
conjoint action of, 1 58, 1 59, 208 ;
mechanical propagation of, 91 ; —
- path of, 94, 95 ; - - rate of, 93 ;
minimal, 209, 210 ; path of, 201, 202 ;
perception of, 193, 194, 195, 196 ; re-
lation of intensity of, to response,
213-16 ; transference'of, 199, 200, 422 ;
types of, 2 ; — chemical, 90, 91, rate
o(> 93) path of, 94, 95 ; — tonic, defi-
nition of, 6.
Stipa, hygroscopic awns of, 151.
Stolons, importance of nutation of, 24.
Stoma, influence of centrifugal force on
initial cell of, 336.
Strangulation, by twiners, 40.
Strasburger, 37, 65, 150, 153, 182, 264, 265,
269, 293, 294, 298, 311, 337, 345, 356,
358 ; on directive action of light, 228 ;
on influence of arrest on irritability,
204; --of light on zoospores, 318,
320; - of temperature, 315; on
nuclear movements, 301, 302, 303,
304 ; on phototaxis, 322, 323, 324, 325,
326 ; on reaction of plasmolysed roots,
201.
Streaming, in Diatoms, 271 ; — uses of,
271, 272 ; influence of contraction on,
in stamens of Cynareae, 78 ; — of in-
duction-shocks on, 356, Fig. 63 ; — of
injuries on, 359; — of light on, 319,
320; of mechanical shocks on, 357,
358 ; — on phototropic orientation,
220; — of plasmolysis on, 355 ; — of
temperature on, 313, 314.
Streptococcus varians, 306.
Striatella, 327.
Strodtmann, 263.
Strophism, 309 ; definition of, 155.
Strophotaxis, 309.
Strychnine, influence of, on protoplasmic
movement, 298.
Strychnos, 45, 46 ; secondary growth in
hook-tendrils of, 46 ; — pressure due
to, 237.
Style, autonomic movements of, 24.
Stylidium adnatum, movements of gyno-
stemium of, 22, 82 ; influence of
gravity on, 148 ; — of induction-shocks,
146.
Stylonychia, contact-irritability in, 358 ;
galvanotaxis of, 361.
Suchsland, 382, 384, 385, 386, 388.
Sugar, changes in percentage of, during
curvature, 247 ; — during shaking, 78,
248 ; chemotactic action of, 420, 421 ;
chemotropic action of, 181 ; influence
of, on fertilization, 182; repellent
action of, 352.
Sugar-cane, changes of geotropic tone in,
164.
Sulphuretted hydrogen, chemotactic action
of, 349.
Summation of stimuli, 209, 210.
Surface, influence of, on suspension, 263.
Surface-tension, influence of, on amoeboid
movement, 277, 278, 279-83; — of,
on chemo- and osmotropism, 230 ; —
of electricity on, 278 ; — of, on fusion,
3°4) 365 ; — on ingestion and excre-
tion, 305 ; — on movement, 300,
301 ; of chloroplastids, 331 ; — on
phototropism, 229 ; — on precipitation
membranes, 281 ; — on pulsating
vacuoles, 294 ; — on shape of proto-
plast, 299 ; — of size of molecules on,
283 ; - - on streaming, 291 ; — on
tactic movements, 312; physical move-
ments due to, 278 ; uses of, 404, 405.
Surface-tension film, creeping of Diatoms
on, 272 ; — of zoospores, 265.
Swarm-cells, fusion of, 304.
Sylvestre, 22.
Sympodial stems, origin of, 23.
Systole of vacuoles, 295.
Systrophe, 333.
Tactic responses, 308 ; nature of, 309 ;
origin and uses of, 310, 311.
Tangl, 359.
Tannin-tubes, as paths for stimuli, 95.
Taraxacum officinale, growth-movements
of, 132 ; influence of light on leaves of,
105 ; thermonasty and photonasty of,
122.
Tarchanoff, 382, 384, 385, 386.
Tassi, 144.
Taxis, definition of, 154.
Taxus, 259, 260.
Tecoma, climbing of, 32.
448
INDEX
Telekia speciosa, irritable stamens of, 81.
Temperature, causes of rise of, in active
pulvinus, 413 ; influence of, on auto-
genie movement, 29 ; - - on chemo-
taxis, 354; -- on ciliary movement,
271 ; — on conjugation, 305 ; — on
dehiscence and dispersal, 153; — on
electrical conductivity, 392 ; — on
excitability, 69 ; — on formation of
vacuoles, 295 ; — on geotropism, 250 ;
— on irritability, 203, 206 ; — on move-
ments of zoospores, 315, — of pul-
sating vacuoles, 298, 317 ; — on power
of movement, 140, 141 ; — on produc-
tion of electricity, 395, 396 ; — of heat,
367, 368, 375 ; - on protoplasmic
streaming, 288, 313; — on protoplasm,
308, 316, 317 ; — on rheotropism,
184 ; — on spontaneous movements of
Desmodium, 22; — on thermonastic
flowers, 98 ; — on transference of
stimuli, 94 ; — on tropic irritability, 225 ;
movements due to changes of, 112-16 ;
thermoelectric measurement of, 371,
Fig. 66 ; uses of rises of, 368. See also
Heat.
Temperature of plants, 379; influence of
conduction on, 381 ; — of radiation
on, 380 ; — of stimulation on, 78 ; —
of transparency on, 366.
Tendril-climbers, 42 ; disks of, 47 ; influ-
ence of gravity on, 48.
Tendrils, chemonastic responses of, 85 ;
influence of absence of oxygen on, 143;
— of darkness, 141, of ether, 144;
— of gravity on nutation of, 28 ; — of
induction-shocks on, 145 ; — of tem-
perature on nutation of, 29 ; latent
period of, 68 ; pits in, 65, 66, Fig. 25 ;
spiral coiling of, 42; thermonasty of,
113; uses of, 71.
Terminology, fictitious value of, 117.
Ternetz, 284 ; on streaming, 290.
Testudinaria elephantzpes, twining of, 38 ;
T. sylvatica, 38.
Tetramitus rostratus, chemotaxis of, 347,
Thallophyta, mode of curvature in, 14.
Thate, 247.
Thermocleistogamy, ico.
Thermoelectric measurement, 371, Fig. 64,
376.
Thermonastic movements, 97, 112; uses
of, ico.
Thermotaxis, 317.
Thermotonus, influence of various factors
on, 314, 3I5> 3i6.
Thermotropism, 176, 420.
Thigmotaxis, 358.
Thuret, 324, 325, 346; on influence of tem-
perature on escape of zoospores, 153.
Tilia, epinasty of, 254; geotropic twigs,
232 ; T. europaea, 415.
Tiliaceae, motile stamens in, 82.
Tissues, distribution of irritability in, 226 ;
influence of, on direction of streaming,
292.
Tissue-strains, action of, in dehiscence, 147 ;
curvatures due to, 12 ; importance of,
for rapid movement, 9; influence of,
on curvature, 226, 241 ; — on helio-
tropism, 227, 238 ; — on thickening of
cell-wall, 245.
Tompa, 394, 397.
Tondera, 417.
Tone, changes of, 202, 206 ; — in photo-
taxis, 322, 323 ; definition of, 6 ; influ-
ence of chloroform and injury on, 203,
205 ; — of oxygen on, 202 ; — of, on
chemotactism, 354.
Tonotaxis, 178.
Torenia, closure of stigma of, 82.
Torsion, absence of, in circumnutation, 21 ;
influence of, on direction of streaming,
292 ; origin of, 24, 257-60 ; in twiners,
41.
Tortism, definition of, 155.
Touch-corpuscles in plants, 65.
Townsend, 144, 305.
Trachelomonas hispida, galvanotaxis of, 361 .
Tradescantia^iT., 314, 341 ; nuclear move-
ments of, 275 ; rate of streaming in,
284; 7. discolor, epidermis of, 181,
Fig. 38 ; T.fluminensis, 417 ; geotropic
nodes of, 231 ; transference of stimuli
in, 200, 205, and in 7. zebrina, 205 ;
T. virginica, inductionized cell of, 356,
Fig. 63 ; localized geotropism of, 225,
235 ; transference of stimuli in, 205.
Translocation, energetics of, 409.
Transpiration, influence of autonomic move-
ment on, 25 ; — on opening of flowers,
118; — on orientation of chloro-
plastids, 332 ; — on production of
heat, 366, 368; — on temperature,
372> 373 J stimulatory action of, 65, 66.
Transplantation, influence of, on irritability,
204.
Trapa, 164.
Traube, on tropism, 223.
Traumatropism, 185 ; excitation of, 186 ;
nature of, 187, 188.
Trees, supposed geotropic curvature of, 12 ;
temperature of, 381.
Trepomonas agilts, 421 ; chemotaxis of,
347, 351-
Treub, on hook-climbers, 45.
Treviranus, 22, 80, 222, 289.
Trianea, 314; 71 bogotensis, action of
ammonia on, 343, Fig. 60.
Trientah 's europaea, changes of tone in, 206 ;
tropism of runners of, 164.
Trtfolium, 294 ; changes of rigidity in
pulvinus of, 135 ; — expansive energy
of, 32 ; leaf-movements of, 23, 26 ; -
influence of gravity on, 27 ; 7. pra-
INDEX
449
tense, influence of gravity on sleep-
movements of, 125 ; — of light on daily
movements of, 108-1 1 ; origin of auto-
nomic movements of, 31 ; variation
movements of, 22 ; T. strictum, sleep-
movements of cotyledon of, 105 ; T.
subterraneum, geotropic peduncles of,
165.
Triticum, geotropic response of, 242 ; pro-
duction of heat by, 375 ; T. repens,
geotropism of runners of, 164 ; T.
vulgare, geotropic curvature of, 231,
Fig. 44.
Tropaeolum, 44, 49 ; heliotropism of, 194 ;
influence of etiolation on nutation of,
30 ; production of electricity by, 396 ;
T. majus, 253 ; etiolation and twining
of, 35 ; heliotropism of, 174, 235 ; in-
fluence of etiolation on nutation of, 28 ;
T. tricolorum, reversal of twining in,
39-
Trophoplasm, 303.
Trophotaxis, 349.
Trophotropism, 178, 349.
Tropic movements, 154; influence of re-
sistance on, 232, 236 ; localization of,
234, 235 ; measurement of turgor
during, 238, — of growth, 239, 240 ;
mechanism of, 230; progress of, 233,
Fig. 45 ; rapidity of, 235, 236.
Tropic tone, changes of, 215.
Tropism, history of study of, 1 6 1.
True, 131.
Tschirch, 118.
Tswett, 330, 356.
Tulipa, no; movements of peduncle of,
19 ; opening of flower of, 97, 98, 99 ;
thermonastic movements of, 115, 129-
33) 137 5 — influence of external con-
ditions on, 141, 144 ; T. Gesneriana,
112.
Turgor, changes of, during curvature, 15,
16, 72, 76, 77, 238, 239, 242, 244, 247 ;
influence of, on transference of stimuli,
94; — on dehiscence, 147; mode of
producing changes of, 17.
Tussilago, curvature of peduncle of, 27 ;
7". Farfara, changes of tone in, 205.
Twiners, 32; influence of gravity on cir-
cumnutation of, 28 ; pressure exerted
by, 39-
Twining, causes of, 37 ; direction of, 39 ;
independence of, on circumnutation,2i,
22 ; influence of etiolation on, 30 ; na-
ture of, 34 ; rate of, 39.
Tyndall, 380.
Ulmus, 260; epinasty of, 254.
Ulothrix, pulsating vacuoles of, 293, 295,
298 ; U. tenuis, geotaxis of, 336 ; U.
zonata, 315; chemotaxis of, 346; in-
fluence of light on zoospores of, 318;
phototaxis of, 322, 324.
PFEFFER. Ill C,
Ultra-violet rays, heliotropic action of, 175.
Ulva, 324.
Uncaria ovalifolia, hooks of, 45, Fig. 14, 46.
Unger, 315, 358, 384; on cilia, 265; on
movements of Cynareae, 79.
Unicellular organisms, curvature of, 239.
Ursprung, 152.
Urtica, movements of stamens of, 147 ; rate
of streaming in, 284.
Usteri, 81, 324.
Utricularia, absence of granulation in, 90.
Vacuolar membrane, movement of, in stream-
ing cells, 286.
Vacuolation, origin of, 295.
Vacuoles, fusion of, due to injury, 359 ; in-
fluence of, on streaming, 283, 285.
Vallisneria spiralis, 327; streaming in,
338, 342, 357, 359 5 — direction of, 283,
292, 293 ; — distribution of, 286 ; du-
ration of, 285 ; — of light and ether on,
319; — of temperature, 313 ; — rate
of, 284, 288 ; coiling of peduncle of,
24, 27.
Van Beek, 370, 372.
Van Tieghem, 192.
Van Wisselingh, 303.
Vanilla, 380 ; climbing of, 32 ; contact-
irritability of roots of, 46, 82, 237 ;
V.planifolia, 186.
Variation movements, 22 ; mechanics of,
134.
Vascular bundles, influence of, on geotro-
pism, 242 ; transference of stimuli in,
200 ; — cylinder, influence of curvature
on, 13.
Vaucheria, 324 ; orientation of chloro-
plastid of, 328, 331 ; — of oil-drops in,
335; parallelotropism of, 156; zoo-
spores of, 318 ; — ciliation of, 264 ; -
escape of, 150 ; — heliotropism of, 171,
174 ; — locomotion of, 266; V. clavata,
3I5-
Velten, 327, 330 ; on production of elec-
tricity, 390, 394, 397, 398 ; on proto-
plasmic streaming, 277, 284, 286, 287,
289, 290, 292, 355, 358; — influence
of external conditions on, 314, 315,
316.
Veronica, movements of flowers of, 27 ; V.
alpina, influence of light on flower of,
1 06 ; V. chamaedrys, influence of
gravity on, 127 ; — of temperature on
orientation of, 250; thermonasty of,
114.
Verworn, 201, 222, 269, 270, 275, 280, 281,
283, 312, 317, 325, 326, 337, 348, 358,
360, 361, 363, 364, 382 ; on galvano-
taxis, 310.
Very, 388.
Vesque-Piittlingen, 285.
Vessels, physics of, 411, Fig. 67.
Vicia Faba, curvature of epicotyl of, 27 ;
450
INDEX
growth of, during curvature, 239 ; in-
fluence of chloroform on geotropism of,
145 ; length of irritable zone in, 198 ;
pressure exerted by radicle of, 238 ;
production of electricity in, 395 ; trau-
matropism of, 1 86, Fig. 40 ; V. sativa,
182, 335 ; curvature of etiolated seed-
lings of, 23 ; heliotropism of epicotyl
of, 194; rheotropism of root of, 184,
185, Fig. 39; time of reaction of, 211.
Victoria regta, sleep-movements of, 103 ;
temperature of flower of, 373, 376.
Villari, 392.
Vinca, 259 ; plagiotropism of radial runner
of, 156, 157; V. major, orientation of,
250.
Vines, 106, 173 ; on heliotropism, 229 ; on
protoplasmic contraction, 78, 79.
Viola, 257 ; origin of peduncular curvature
in, 27 ; sleep-movements of, 103.
Viscosity, of protoplasm, 277 ; — influence
of, on streaming, 288, 300, 313, 314,
316, 342 ; — on movement in cell, 334.
Viscum, orientation of, 173, 255.
Vitality, relation of, to streaming, 285.
VitiSy 42 ; heliotropic tendrils of, 171 ; V.
inconstans, disks of, 47.
Vochting, 19, 27, 102, 103, 104, 106, 183,
232,. 237, 253, 254, 255, 258, 260, 419 ;
on influence of light on irritability, 203,
204, — of transplantation, 205 ; on
localization of sensation, 196 ; on
plagiotropism, 257 ; on rectipetality,
190; on thermonastic movements, 114,
115 ; on tropism, 161.
Voegler, 316, 344, 345, 354-
Volkens, 380.
Volvocineae, chemotaxis of, 347, 348 ; pul-
sating vacuoles of, 293, 294.
Volvox, 294, 324 ; ciliation of, 264 ; har-
monious working of, 269 ; locomotion
of, 266.
Vorticella, 281 ; irritable zones in, 363.
Voss, on twining of Bowiea, 35, 48.
Vriese, 372, 374, 375.
Vrolik, 372, 374.
Wachtel, 237 ; on irritability of root-apex,
197, 198.
Walden, 390.
Wallengren, 303 ; on galvanotaxis, 361, 363.
Waller, 342 ; on production of electricity,
391, 392, 394-7-
Walz, 150, 153, 318.
Warming, 114.
Wartman, 394.
Wasielewski, on amitosis, 303.
Water, escape of, from pulsating vacuoles,
296, — from stimulated cells, 17, —
pulvini, 76, 77, 78, — during curvature,
282, 283 ; influence of, on curvature,
I^> 355 5 — of movements of, on pro-
duction of electricity, 396 ; — of, on
streaming, 356; movements of, 410,
- in climbers, 33 ; -- in streaming
cells, 290, 291.
Weber's Law, 213, 214, 215; application
of, to chemotaxis, 355, — to photo-
taxis, 322.
Weeping Willow, torsion of, 233.
Weinzierl, on influence of curvature on
elasticity, 246.
Went, 164, 173.
Werner, 303.
Westermaier, 235.
Wichura, 24, 151.
Wiedermann, 384, 388.
Wiedersheim, on curvature of operated
pulvini, 138; on nastic movements,
416 ; on return curvatures, 130.
Wieler, 182.
Wiesner, 12, 19, 25, 31, 42, 1 06, 208, 220,
232-5, 237, 253-7, 260, 289, 367, 374,
381 ; on causes of heliotropism, 246,
247 ; on directive action of light, 228,
236; on hydronastic movements, 117,
118; on heliotropic action of different
rays, 176; on heliotropism of tendrils,
171, 172, 173, 175 ; on influence of
fertilization on irritability, 205 ; on
minimal heliotropic stimuli, 210, 211 ;
on tropism, 161, 164, 165, 167 ; on
tropic after-effects, 212 ; on undulating
nutation, 23 ; on Weber's Law, 213.
Wigand, 166, 237, 291, 315; on streaming,
284-7-
Wille, 46, 263.
Williams, 346.
Wilsing, 372.
Wilson, E. B., 303.
- W. P., 106, 142.
Winkelmann, 384, 390, 396.
Winkler, 20, 31, 174, 325, 329; on coiling
of Spirogyra, 38.
Winogradsky, 318, 320, 321.
Winter, 173.
Wistaria chinensis, length of vessels in,
33 ; limiting diameter for twining of,
40.
Wittrock, 103.
Wjasemsky, 392.
Wolkoff, 175 ; on heliotropism, 229.
Wollny, 381.
Wood, red and white, mechanical properties
of, 414 ; production of, 415.
Wood vessel, diagram of, 411, Fig. 69;
length of, in climbers, 33.
Work, energy of, in Diatoms, 272 ; — done
by plants, 401, 412, 413.
Woronin, 153, 173, 265, 293, 324.
Wortmann, 21, 28, 82, 165, 170, 244; on
coiling of Phycomyces, 46 ; on hydro-
tropism, 183 ; on mechanism of curva-
ture, 245 ; on thermotaxis, 317 ; on
thermotropism, 177 ; on tropic aggre-
gation, 219.
INDEX
451
Wound-reactions, 359, 375, 376, 397, 398.
Wound-stimuli, rate of propagation of, 359.
Xylaria carpophila> geotropism of, 165.
Yegounow, 353.
Yerkes, on photopathy, 229.
Yucca, 204; geotropism of rhizomes of,
164, — of cotyledon of, 165.
Zacharias, 232, 303.
Zantedeschi, 176.
Zea Mays, 182 ; geotropism of, 242 ; — of
lateral roots, 163 ; nutation of, 20,
Fig. 2 ; rheotropism of, 184 ; thermo-
tropism of, 177.
Ziegler, 302.
Zikes, 317, 337.
Zimmermann, 302, 323, 334, 340, 357, 360.
Zoospores, amoeboid movements of, 275 ;
cilia of, 264 ; influence of light on,
318, 320; — of light and temperature
on escape of, 153 ; — of temperature
on movements of, 315, 316; photo-
taxis of, 322, 323, 324 ; speed of, 268 ;
swarming-period of, 267.
Zopf, 46, 148, 149, 151, 165, 264, 275, 303,
313 ; on coiling of fungal hyphae, 46.
Zygnema, fragmentation of, 148.
Zygnemaceae, growth and nutation of, 31 ;
importance of autonomic curvatures
in, 24 ; movements of, 20.
OXFORD
PRINTED AT THE CLARENDON PRESS
BY HORACE HART, M.A.
PRINTER TO THE UNIVERSITY
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