BIOLOGY
LIBRARY
6
AN INTRODUCTION
TO
VEGETABLE PHYSIOLOGY
BY
J. EEYNOLDS GEEEN, Sc.D., F.E.S., F.L.S.
TRINITY COLLEGE, CAMBRIDGE
PBOFESSOB OP BOTANY TO THE PHABMACEUTICAL SOCIETY OF GBEAT BBITAIN
FOBMEBLY SENIOB DEMONSTBATOB IN PHYSIOLOGY IN THE
UNIVEBSITY OF CAMBBIDGE
LONDON
J. & A. CHUECHILL
7 GREAT MARLBOROUGH STREET
1900
GENERAL
K 1 (I
BIOLOGY
LIBRARY
G
PREFACE
ALTHOUGH during recent years considerable additions have
been made to our elementary botanical textbooks, not one
has appeared which deals solely, or at any length, with the
subject of vegetable physiology. This has been either
presented to the reader as a particular section in a com-
prehensive work, or treated of incidentally in connection
with anatomical detail. This is the more strange, as an
adequate and intelligent appreciation of the forms and
structure of vegetable organisms can only be gained by a
consideration of the work they have to carry out. It must
be evident to the student of Nature that the peculiarities
of external and internal form, of which any particular plant
has become possessed, have arisen necessarily in con-
nection with the need of mechanisms to do certain work,
to overcome particular disadvantages, and generally to
bring the organism into a satisfactory relationship with the
surroundings among which it finds itself.
I have been led by these considerations to endeavour to
fill this gap by writing an introduction to the subject,
which, while putting physiology into its proper prominence
among the branches of botanical study, shall serve to pave
the way of the student and of the general reader to the
more complete discussion of the subject which may be met
with in the advanced textbooks of Sachs, Vines, and
Pfeffer.
I O O f\ A Q
vi VEGETABLE PHYSIOLOGY
With this view I have endeavoured to present the
plant as a living organism, endowed with particular
properties and powers, realising certain needs, and meet-
ing definite dangers. I have attempted to show it to be
properly equipped to encounter such adverse conditions,
and to avail itself of all the advantages presented to it by
its environment.
I have also set before myself another purpose, which,
however, is naturally subordinate to the one just mentioned.
When we consider the origin of the different organisms
which we find around us, we are led irresistibly to the
conclusion that the classification of living beings into
animals and plants has been too strongly insisted upon in
the past, and that while much has been made of their differ-
ences, their points of resemblance have been minimised.
The fact that organisms exist, which it is difficult or
impossible to refer with certainty to either kingdom, points
to a fundamental unity of living substance. Protoplasm
in short is the same material, whether we call it animal or
vegetable. This being the case, its conditions of life and its
immediate necessities must be practically the same, what-
ever its degree of differentiation in either direction. I
have tried to bring out this identity of living substance
throughout the book, and to indicate that apparent differ-
ences of behaviour and structural arrangement are to be
traced rather to differences of environment and habit of
life, than to those of constitution. The correspondence of
the processes of respiration in animals and plants has long
been recognised ; many points of similarity in those of
nutrition have been observed. The idea is, however, still
prevalent that plants live upon inorganic materials ab-
sorbed from the air and from the soil. This seems to
indicate a fundamental difference between the modes of
nutrition of animal and vegetable protoplasm. I have
PREFACE vii
endeavoured to show that this view is erroneous and that
both are nourished similarly.
I have also tried to show that the sensitiveness of the
plant and the animal is alike in properties, though differ-
ences are apparent in the direction of its differentiation.
I have avoided as far as possible the discussion of con-
troverted points, feeling that this would be out of place in
a work intended to serve as an introduction to the subject.
Such matters are more properly treated of in the more
comprehensive works to which I have already alluded.
J. EEYNOLDS GREEK.
CAMBBIDGE: June 1900.
CONTENTS
CHAPTEE I
THE GENERAL STRUCTURE OF PLANTS
PAGE
Unicellular plants ; zoogonidia, yeasts, bacteria ; multicellular plants ;
the protoplast, its structure and arrangements ; characters of
protoplasm ; nuclei and nucleoli ; association of protoplasts in
colonies ; slime-fungi ; ccenocytes ; arrangements in multicellular
plants — Needs of protoplasm ; its relation to water ; formation of
vacuoles ; relation of water to the plant in general ; the aeration of
protoplasm — Connection of protoplasts with one another in the
body of the plant ;. . . . \ . •> ;-.-.-- . 1-16
CHAPTEE II
THE DIFFERENTIATION OF THE PLANT-BODY
Division of labour the clue to differentiation of structure — Formation
of protective tissues ; epidermis, cuticle, periderm, bark— System of
conducting tissues ; vascular bundles and their distribution —
Strengthening tissues ; collenchyma and sclerenchyma ; the
different arrangements of them which are met with — The stereome
of the plant — The metabolic tissues — The arrangements for the
aeration -of the interior ; stomata, lenticels 17-35
CHAPTEE III
THE SKELETON OF THE PLANT
Necessity of a skeleton to support the protoplasts ; varieties of the
skeleton — Development of the skeleton as the plant grows— Charac-
ters of the cell-wall ; cellulose, its properties and reactions ; pectose
and related substances — Arrangement of the solid matter and the
water of the cell-wall ; hypotheses of Naegeli, Strasburger, and
Wiesner — Differentiation of the substance of thickened cell- walls ;
VEGETABLE PHYSIOLOGY
stratification, middle lamella — Lignin and its reactions — Cutin —
Impregnation of cell-walls with various matters — Mucilage —
Differences between temporary and permanent portions of the
skeleton , 36-52
CHAPTEE IV
THE RELATION OF WATER TO THE PROTOPLASM OF
THE CELL
Dependence of the protoplasts on water ; function of the vacuole —
Eenewal of the water of the vacuole— Osmosis — Formation of the
vacuole as the protoplast develops — Regulation of osmosis by the
cell-protoplasm ; the plasmatic membranes— Movements of water
from cell to cell — Evaporation into the intercellular spaces—
Turgescence and its dependence on the protoplasm — Storage of
water . ........ 53-65
CHAPTEE V
THE TRANSPORT OF WATER IN THE PLANT
Varied needs of different plants in this respect — Transport in a ter-
restrial plant — The ascending sap — Condition of water in the soil ;
absorption of water a function of the root-hairs ; mechanics of
the root-hair — Path of the ascending stream ; forces causing the
movement — Evaporation of water from the interior — Influence of
the stream of water upon the development of the plant . . 66-77
CHAPTEE VI
THE TRANSPIRATION CURRENT. ROOT PRESSURE.
TRANSPIRATION
The ascending sap sometimes called the transpiration current — Its
path ; methods of demonstration— Bate of the transpiration
current — Causes of the upward flow ; root pressure ; transpira-
tion ; capillarity ; pumping action of living cells ; osmotic action
of the parenchyma of the leaves — Root pressure ; its nature and
mode of action ; bleeding of cut stems ; of entire plants ; measure-
ment of root pressure ; conditions of the activity of roots ; diurnal
, variations of root pressure — Transpiration ; methods of demonstra-
tion ; amount of water given off ; negative pressure in the wood
vessels ; character of the evaporation of transpiration ; regulation
by stomata, their mode of action ; variations in numbers of
stomata ; conditions affecting transpiration ; light, temperature,
CONTENTS xi
PAGK
moisture of air, rest — The Potometer — Suction of transpiration —
Osmotic action of the parenchyma of the leaves and its effect —
Eegulation of all these forces by the protoplasm . . . 78-102
CHAPTEE VII
THE AEEATION OF PLANTS
Necessity of admitting oxygen to the protoplasts — The intercellular
space system ; its origin and development ; condition in terrestrial
plants ; relative extent in roots, stems, leaves — Air reservoirs in
aquatic plants ; in Equisetum, grasses, rushes, &c. ; mode of
formation of the reservoirs — External orifices of the intercellular
space system ; stomata and lenticels — Relative dimensions of
cellular tissue and intercellular spaces — Movements of air in inter-
cellular space system — Composition of the air ... 103-117
CHAPTEE VIII
RESPIRATION
Dependence of the plant upon oxygen ; absorption of oxygen and
exhalation of carbon dioxide ; apparatus to demonstrate these
processes — Loss of weight during respiration — Variations in the
respiratory activity — Relation between the absorption of oxygen
and the exhalation of carbon dioxide ; the respiratory quotient —
Exhalation of water during respiration — Respiration a function of
protoplasm — Conditions affecting respiration ; temperature, light,
differences of gaseous pressure and of nutritive materials . 118-131
CHAPTEE IX
THE FOOD OF PLANTS. INTRODUCTORY
True nature of the food of plants — Materials absorbed by plants, and
their relationship to actual food — Differences between food and
food materials— Construction of food from the latter — Assimilation
of food — Intricacy of the metabolic processes of plants . . 132-139
CHAPTEE X
ABSORPTION OF FOOD MATERIALS BY A GREEN PLANT
Examination of substances absorbed from the soil ; water-culture ;
destructive analysis — Classification of materials absorbed — The
xii VEGETABLE PHYSIOLOGY
PAGKF.
ash of plants —Conditions of absorption of substances in the soil —
Absorption of nitrogen by leguminous plants — Insectivorous plants
and their behaviour — Absorption of metallic compounds ; silicon — .
Absorption of carbon dioxide from the air ; its mechanism . 140-154
CHAPTER XI
THE CHLOKOPHYLL APPARATUS
Formation of organic substances from the inorganic materials
absorbed — Chlorophyll— Structure of a chloroplast — Properties of
chlorophyll ; its absorption spectrum — Xanthophyll — Erythrophyll
— Composition of chlorophyll — Distribution of the chloroplasts — Ke-
lationship between the plastid and the colouring matter — Leucoplasts
— Conditions of formation of chlorophyll ; light, temperature, iron
— Formation of carbohydrates by chloroplasts ; conditions of their
activity — Theories of photosynthesis — Eelation of starch to the
process — Bays of light made use of in photosynthesis ; researches
of Engelmann, of Tmiriazeff — Inhibition of the chlorophyll appara-
tus— Formation of organic substance in its absence . . 155-172
CHAPTEE XII
THE CONSTRUCTION OF PROTEIDS
Complexity of the composition of proteid ; its percentage composition
— Classification of proteids ; albumins, globulins, albuminates, pro-
teoses, peptones, coagulated proteids, proteids soluble in alcohol —
Synthesis of proteids in plants ; various hypotheses — Locality of
proteid construction in the plant 173-183
CHAPTEE XIII
THE CONSTITUENTS OF THE ASH OF PLANTS
Nature and composition of the ash — Water-culture and the limitations
of its usefulness in the study of the ash — Classification of the
constituents of the ash — The selective power of plants — Sulphur
and phosphorus — Potassium, magnesium, calcium, iron — Sodium,
silicon, chlorine, bromine, iodine, manganese — Accidental con-
stituents of the ash — Eelation of nitrogen and potassium to herbage
plants ..; . . . . 184-195
CONTENTS xiii
CHAPTEE XIV
OTHEE METHODS OF OBTAINING FOOD
PAGE
Partial or entire absence of the constructive power — Nutrition of
saprophytes — Insectivorous plants — Utricularia — The pitcher-
plants— Lathraea — Drosophyllum — Pinguicula— Dionaea— Drosera
— Digestion of substances by Fungi — Commensalism — Symbiosis —
Mycorhiza — Hoot parasites — Parasitism among green plants and
Fungi . . ..,*;.. ',"• . '.^r'. , :,-'.• .' .. . . 196-220
CHAPTEE XV
TEANSLOCATION OF NUTEITIVE MATEEIALS
Conditions of the constructive processes ; surplus production and
storage — Necessity of circulation of food material in consequence
of localisation of construction, and intermittence of consumption —
Mode of transport of substances in the plant ; osmosis and diffusion ;
temporary storage — Translocatory and storage forms of food — The
so-called descending sap — The pathway of translocation . 221-234
CHAPTEE XVI
THE STOEAGE OF EESEEVE MATEEIALS
Connection between transport and storage ; forms in which food is
stored — Eeservoirs of storage ; stems, roots, floral organs— Storage
of carbohydrates ; starch grains and their formation by chloro-
plasts and leucoplasts, by the cytoplasm ; glycogen ; inulin ;
sugars ; cellulose and similar compounds — Storage of proteids ;
aleurone grains, their composition and mode of formation ; proteid
crystals ; antecedents of gluten — Storage of asparagin ; glucosides ;
fats and oils — Mode of formation of the last-named group . 235-255
CHAPTEE XVII
DIGESTION OF EESEEVE MATEEIALS
Nature of digestion — Its localisation in plants— Agents of digestion-
Secretion of enzymes — Conditions of their action — Zymogens —
Differentiation of glandular structures — Classification of enzymes
—Diastase and its action on starch— Mulase— Invertase— Glucase
— Cytase and cell-walls— Pectase—Proteoly tic enzymes— Rennet-
Enzymes which decompose glucosides — Lipase and its action on fats
— Zymase and the production of alcohol — Oxidases — Fermentative
activity of protoplasm — Assimilation ... . . 256-271
xiv VEGETABLE PHYSIOLOGY
f
CHAPTEE XVIII
METABOLISM
PAGK
Constructive and destructive processes ; anabolism and katabolism—
Constructive processes depending on katabolism — Secretion — Bye-
products — Secretion of enzymes — Formation of cell-walls, of starch
grains, of aleurone grains, of fat, of chlorophyll, of anthocyan —
Formation of resin, of alkaloids, of acids — Decomposition-products
of cellulose ; colouring matters ; nectar ; etherial oils . . 272-287
CHAPTER XIX
.THE ENEEGY OF THE PLANT
Preliminary considerations — The expenditure of energy in evaporation,
in constructive processes, in movements, in radiation, in light —
Source of the energy of plants the radiant energy of the sun ; its
absorption by chlorophyll ; absorption of heat rays — Fixation of
energy — Kinetic and potential energy — Distribution and liberation
of energy — Eelation of respiratory processes to local utilisation of
potential energy — Oxidative actions other than respiration — Intra-
molecular or anaerobic respiration — Fermentation — Anaerobic
plants ... . ' ... . . . ".-.. - - . . 288-304
CHAPTER XX
GROWTH
Relation of growth to constructive metabolism — Definition of growth
—Distribution of growth — Conditions necessary for growth ; plastic
materials, turgescence, temperature, oxygen— The grand period of
growth — Growth of a cell and of a multicellular organ — The region
of growth in the latter — -Daily period of growth in length— The
Auxanometer — Variations in growth ; hyponasty and epinasty ;
nutation and circumnutation — Tensions accompanying growth —
Eectipetality . • . . . . ' 305-319
CHAPTER XXI
TEMPERATURE AND ITS CONDITIONS
Range of temperature through which the vital processes proceed ;
photosynthesis, germination— Causes and effects of fluctuations of
CONTENTS xv
PAGE
temperature — Influence of the light rays on temperature ; impor-
tance of anthocyan — Absorption of heat by conduction — Dissipa-
tion of heat in evaporation of water — Kadiation — Nyctitropic
movements — Loss and gain of heat by conduction — Eegulation of
heat— Power of resistance to extremes of temperature . . 320-329
CHAPTEK XXII
INFLUENCE OF THE ENVIRONMENT ON PLANTS
Characters of aquatic plants ; influence of a watery environment on
structure — Xerophytes and their peculiarities —Alpine plants-
Epiphytes — Parasites — Insectivorous plants .... 330-344
CHAPTEK XXIII
THE PROPERTIES OF VEGETABLE PROTOPLASM
Adaptability of plants to their surroundings— Contractility— Ciliary
and amoaboid movement — Locomotion — Movements of rotation
and circulation— Turgor and its maintenance— Mobile condition of
protoplasm — Ehythm and its manifestations — Irritability and its
conditions — Tone — Phototonus — Thermotonus — Tonic influence of
light — Etiolation— Influence of too brilliant illumination ; para-
heliotropism, apostrophe and epistrophe — Photoepinasty — Regu-
lating action of light on growth— Conditions of health — Acclima-
tisation . 345-36
CHAPTEK XXIV
STIMULATION AND ITS RESULTS
Response of an organism to changes in its surroundings — Nature of
stimulation — Purposeful character of the response— Stimulation of
light — Nyctitropic movements, their conditions and purpose —
Mechanism of the movements — Effect of incidence of lateral light
— Heliotropism — Stimulus of gravitation ; geotropism — The Klino-
stat — Knight's wheel — Stimulus of contact — Behaviour of various
organs in relation to this form of stimulation — The root— Twining
stems and tendrils— Hydrotropism — Chemical stimuli — Chemotaxis
—Induced rhythm . . . 368-396
xvi VEGETABLE PHYSIOLOGY
CHAPTEK XXV
THE NERVOUS MECHANISM OF PLANTS
PAGE
The purposeful responses of plants to stimulation ; relation of stimulus
to effect— Nature of nervous mechanisms — Sense organs and their
differentiation — Motor mechanisms of plants — Contraction —
Eegulation of supply of water to the cell — Glandular organs — Con-
duction of impulses ; continuity of protoplasm — Co-ordination of
impulses — Latent period of stimulation — After-effects — Fatigue —
Anaesthetics — Comparison of nervous mechanisms of plants and
animals 397-410
CHAPTEE XXVI
REPRODUCTION
Distinction between the individual protoplast and the colony or plant
— Process of multiplication of protoplasts ; gemmation, karyo-
kinesis, formation of cell- walls ; free-cell formation — Vegetative
propagation — Formation of asexual reproductive cells, spores or
gonidia ; zoocoenocytes — Development of sexual cells or gametes :
planogametes and conjugation ; male and female cells ; anthero-
zoids and oospheres — Gametangia and their varieties — Fertilisation
— Alternation of generations — Gametophytes and sporophytes —
Heterospory and its consequences — The seed and its formation 411-436
CHAPTEE XXVII
REPRODUCTION (continued)
Pollination and its mechanisms —Advantages of cross-pollination —
Dichogamy, protandry, protogyny — Diclinism — Heterostylism or
dimorphism — Prepotency — Self-sterility — Self-pollination ; cleisto-
gamy — Mechanism of fertilisation ; the growth of the pollen-tube
— Hybridisation— Eesults of fertilisation ; formation and ripening
of fruits and seeds — Germination of the seed — Apospory ; apogamy ;
parthenogenesis . . •. • •-. -" ; '• ; • . •-.- --.,:- - . . 437-449
INDEX ..^".. . '. .;;.:.'.:-- ' • :; . -•• . 451
LIST OF ILLUSTRATIONS
1. Zoospore of Ulothrix 1
2. Yeast Plants 2
3. Bacteria . . .-.".. 3
4. Plasmodium of a Myxomycete . . ' . . . .4
5. Vegetable Cells (Young) . . .... . '. . . 5
6. Vegetable Cells (Adult) ......... 5
7. Cells exhibiting Kotation, from Elodea . . . . . . 7
8. Cells of Tradescantia, showing circulation . . . .7
9. Structure of the Nucleus 7
10. Colonies of Protococcus 9
11. Volvox Globator . . . . 10
12. Ccenocytic Suspensor of Orobus . - •* . . . . .11
13. Filaments of Nostoc . . . • ; . 12
14. Pediastrum '-- 12
15. Vegetable Cells (Young) . . , v 12
16. Vegetable Cells (Adult) . . .....''. . . .13
17. Continuity of Protoplasm in Seed . 16
18. Continuity of Protoplasm in Seaweed .... 16
19. Thallus of Pelvetia 18
20. Stem of Sphagnum . . . . . . . . . . .19
21. Stem of Common Moss 19
22. Section of Blade of Leaf . . 19
23. Cork Cells . . .- . - . .. . -. . .' . . 20
24. Bark of Oak . . ; \ . . , 20
25. Collenchyma . . . , . . . . . . 21
26. Exodermis of Boot 21
27. Diagram of Course of Vascular Bundles in a Dicotyledonous plant . 23
28. Venation of Leaf . . . ... . . . .23
29. Section of Bhizome of Fern 26
30. Section of Leaf of Pimts . . . > . . . . .27
31. Vascular Bundle of Monocotyledon ... . . . . 28
32. Different Arrangements of Stereome in Herbaceous Plants . . 29
33. Chloroplasts in Cell . .30
34. Section of Stem of Potamogeton 31
35. Cortex of Boot . . . . .-•"'. . . . . . 32
36. Section of Blade of Leaf . ... . . . . .32
37. Stomata on Lower Surface of Leaf . . . , . . . . 33
38. Section of Epidermis of Leaf . . . . . " . . .33
xviii VEGETABLE PHYSIOLOGY
FIG. 1'AGE
39. Section of a Lenticel . . . . ' . . ' .'..,'•.. 34
40. Section of Dicotyledonous Stems of two ages . . . . 38
41. Embryo of Orobus . . • ' . . . . . . . 39
42. Stratification in Cell-walls . • . . . . . . . . 44
43. Longitudinal Section of Vascular Bundle of Sunflower Stem . . 45
44. Wood-cells, showing Middle Lamella . ..,.". .45
45. Section of Epidermis of Leaf . . . . • • " • . . 48
46. Cork in Twig of Lime . . . . ._ . . . .49
47. Section of a Lenticel . . . . ' 49
48. Crystals in Wall of Cell of Bast . . . . . . .50
49. Cystolith of Ficus . . f . . .. . . . . . 50
50. Apparatus to show the process of Osmosis . . . • . .55
51. Young Vegetable Cells . • ' .• •• 56
52. Adult Vegetable Cells . . > . . . , . . 57
53. Cells undergoing Plasmolysis . ...... . . ' . . 59
54. Kootlets with Koot-hairs - . .68
55. Boot-hair in contact with Soil . .. , . ...... . . 69
56. Section of Young Boot 70
57. Diagram of Course of Vascular Bundles in a Dicotyledonous Plant 71
58. Veins of a Leaf 72
59. Ending of a Vascular Bundle in a Leaf . . . ... 73
60. Section of Blade of Leaf . . ' . 73
61. Stomata on Lower Surface of Leaf . 75
62. Apparatus for the Estimation of Root-pressure . . . .84
63. Apparatus to demonstrate Transpiration . . . . . . 89
64. Section of Blade of Leaf 90
65. Apparatus to show Dependence of Withering on Loss of Water . 91
66. Stomata on Lower Surface of Leaf . . . . . .93
67. Section of a Stoma . . ... . ' . .: . . 93
68. Darwin's Potometer . . ... • • ' V . . 98
69. Apparatus to show the Suction of Transpiration .... . . . 99
70. Ending of a Vascular Bundle in a Leaf . - . . • • 101
71. Formation of Intercellular Spaces . . .. ... . .105
72. Intercellular Spaces in Koot . ' . . . ' . • 105
73. Intercellular Spaces in Leaf ~. . . . . . , . • 106
74. Section of Leaf of Isoetes . . . .^ . . . .106
75. Section of Rhizome of Marsilea . . . . , . . > . • 107
76. Section of Stem of Potamogeton . -. V ~- : . . -. . 108
77. Section of Stem of Equisetum . . - . .... 109
78. Section of Stem of Juncus . . , .. • . . . .110
79. Section of a Lenticel .... '. " -.: :~ '. . . . . Ill
80. Apparatus to show Continuity of Intercellular Spaces in a Leaf . Ill
81. Section of Leaf of Heath . .... . . . . . . 112
82. Apparatus to show the Absorption of Oxygen by a Green Plant . 119
83. Apparatus to show Exhalation of Carbon Dioxide by Germinating
Seeds . . ...... 121
84. Root of a Leguminous Plant with its Tubercles .... 148
85. Section of Blade of Leaf . . 153
86. Absorption Spectra of Chlorophyll and Xanthophyll . . . 157
LIST OF ILLUSTEATIONS xix
FIfi. PAGE
87. Chloroplasts in Cell . . ... . . .. . . 159
88. .Section of Leaf of Beta . . . . , . . . .160
89. Section of Stem of Equisetum 161
90. Apparatus to show the Evolution of Oxygen by a Green Plant . 165
91. Plants of Buckwheat cultivated in various nutritive solutions . . 186
92. Aleurone Grains in Cell of Bicinus . . . .... 190
93. Bladderworts (Utricularia) . . 200
94. Traps of Utricularia 201
95. Pitcher of Sarracenia . . ' 202
96. Pitcher of Nepenthes . . 203
97. Shoot of Lathrva 204
98. Prison-chamber in Leaf of Lathrcea 205
99. Leaf of Drosera . . 207
100. Leaf of Dionaa 208
101. Section of Lichen . . . . 211
102. Mycorhiza on Beech Koot 213
103. Plant of Thesium . . 215
104. Sucker of Thesium 216
105. Plant infested with Dodder 217
106. Haustoria of Dodder ,« • . 218
107. Haustoria of Phytophthora . .- . . .- ' . . . 220
108. Starch Grains in Chloroplast ' . 228
109. Section of Stem of Bicinus . .... . ... 239
110. Section of Stem of Tilia three years old ... . . 240
111. Starch Grains in Chloroplast . 242
112. Starch Grains in Cell of Potato . . . . . . .243
113. Starch Grain of Potato . . . . 243
114. Compound and Semi-compound Starch Grains . . . . 244
115. Laticiferous Cell of Euphorbia . . . . . ... 245
116. Leucoplasts of Phajus . . . : ... . . 245
117. Inulin Sphasro-crystals . . . . . f . . . . 247
118. Aleurone Layer of Barley 248
119. Cells of Embryo of Pea . ' . . . . . . ,. . 249
120. Formation of Aleurone Grains ; -. . . . . . 249
121. Aleurone Grains of Lupin . . . . * . . . . 250
122. Aleurone Grains of Bicinus . . -. 250
123. Section of Oat Grain . , . . 260
124. Epithelium of Scutellum \ . '. . . 260
125. Aleurone Layer of Barley . . . , • . - . . 261
126. Gland of Drosera . . . . ... . . .261
127. Corrosion of Starch Grains by Diastase . . . . . . 263
128. Glandular Hairs of Primula 283
129. Glandular Hairs of Hop . . . . . , . 283
130. Oil Keservoirs of Hypericum ...... . 286
131. Bast-cell of Ephedra 286
132. Crystals of Oxalate of Calcium in Cells 287
133. Cystolith of Ficus 287
134. Absorption Spectra of Chlorophyll and Xanthophyll . • • . . 293
135. Longitudinal Section of Growing Point of Eoot . . . . 306
xx VEGETABLE PHYSIOLOGY
F!G- PAGE
136. Section of Blade of Leaf . . . . ' . . ... 307
137. Section of Stem of Eush . . j . . • . . . .308
138. Adult Vegetable Cells . . . - . ' 310
139. Kegion of Growth in Koot of Bean . . . . .31
140. Pfeffer's Auxanometer - . . . . . • . . . . . . 314
141. Air Passages in Potamogeton . .,' . . . . . 331
142. Leaf of Isoetes ,. '.•'-•. . . . 332
143. Petiole of Water-lily . . . .'.'.-. . . 333
144. Ehizome of Marsilea . . . * 335
145. Water-gland of Saxifraga . . .... . .337
146. Leaf of Heath ...'.-. . . . 339
147. Suckers of Thesium . . . . . - . ' . '.. . 342
148. Section of a Sucker . . . . . . ." . . 343
149. Zoospore of Ulothrix .« .. -. . .. . ... . 346
150. Plasmodium of a Myxomycete . . . . . . . . 347
151. Cells from Leaf of Elodea . . . . . . . .350
152. Cells from Hair of Tmdescantia . . ... . . . 350
153. Leaf of Telegraph Plant . - . . % . • \ . . . . 355
154. Pulvinus of Mimosa \ . . . . 356
155. Desmodium gyrans, day and night position ..... 373
156. Nicotiana glauca, day and night position 374
157. Pulvinus of Mimosa . . 376
158. Barwin's Klinostat . . 383
159. Section of Sucker of Thesium . . . . v . . . 389
160. Haustoria of Cuscuta . . . . . . - . . . 390
161. Leaf of Dionaa . -. . . . , . . f . .391
162. The same . . . - . . . 401
163. Continuity of Protoplasm through the Cell-wall . ' . - . • .404
164. Yeast Plants . . \ . 413
165. Stages in Karyokinetic Division of the Nucleus .... 414
166. Zoospore of Ulothrix ., . . . . . . . 418
167. Gonidangia of Achlya , . . . . . . 418
168. Mycelium of Mucor . , . . „ . . ... 419
169. Part of Hymenial Layer of Peziza . ...... . .419
170. Stylogonidia of Eurotium . . . • . . • . . . . 420
171. Filament of Ulothrix with Gametes escaping . ( . . . 421
172. Oogonium of Fucus . •: . . . . . • . . . 423
173. Oosphere and Antherozoids of Fucus . ' . . . . . 423
174. Procarpium of a Ked Seaweed . . . . - . . . . 424
175. Archegonium of Fern . . . . • . . . 425
176. Prothallium of Fern . : ^. • ,. *. . . . ..428
177. Germination of Microspores of Salvinia . . . . 429
178. Germination of Megaspore of Salvinia . . . . . . 430
179. Germination of Megaspore of Selaginella . . . '. . . . 431
180. Ovule of Pinus .' . . . .432
181. Ovule of an Angiosperm .... . . . . . 432
182. Antherozoids of Moss and Fern ... . ... 434
183. Antheridium of Fern . . . . . • > . . . .435
184. Archegonium of Fern ' .' •. . 435
VEGETABLE PHYSIOLOGY
CHAPTEE I
THE GENERAL STRUCTURE OF PLANTS
EXAMINATION of the body of every living organism shows
us that it is composed of different materials, which exhibit
a great deal of variety in the ways in which they are
arranged. These different materials fall very naturally
into two classes, which include respectively the living sub-
stance itself, and various constituents of the body which
have been constructed by it. The relative proportions in
which these two classes of materials exist vary very greatly
in different organisms ; in some of the simplest forms
indeed we can discern nothing structural except the living
substance itself. In others the materials constructed by
the latter are much the greatest in amount.
When we study the life history of the simplest or the
most complex plant with which we can become acquainted,
we find that at some time or other in
its existence it is found in the form of
a minute portion of jelly-like material
which is endowed with life. Some-
times this piece of living substance is
motile, and can swim freely about in
water by means of certain thread-like
appendages which it possesses (fig. 1).
Such structures occur almost exclu-
sively among the lowest forms of plants, particularly the
seaweeds. They are known as zoospores, or zoogonidia,
FIG. 1. — ZOOSPOBE OF
Ulothrix. x 500.
2 VEGETABLE PHYSIOLOGY
and are produced in large numbers. In other cases the
little mass of living substance is not capable of locomotion,
but may be found floating about in water, or enclosed in
particular cavities in its parent plant.
The jelly-like substance of which these bodies are com-
posed is living and capable of carrying out all the functions
necessary for its life, growth, and multiplication. It is
called protoplasm, and each portion of protoplasm which is
thus capable of independent existence is known as a vege-
table cell, or protoplast.
These free-swimming organisms are not protected by
any coating, but every part of their surface is in complete
contact with the water in which they live. This condition
FIG. '2. — SACCHABOMYCES CEBEVISLE. OB YEAST-PLANT, AS DEVELOPED
DUEING THE PROCESS OF FEBMENTATION. X 300.
a, 6, c, d, successive stages of cell-multiplication.
is, however, exceptional. Usually the protoplast is encased
in a colourless homogeneous membrane of extreme tenuity
which is known as its cell-wall. Examples of unicellular
organisms of this kind are found in great numbers among
the fungi, the Yeasts (fig. 2) and the Bacteria (fig. 3) being
exceptionally numerous. Such plants may be motile or non-
motile, a few of the bacteria being furnished with thread-
like appendages, known as cilia or flagella, which are
similar in most respects to those of the zoospores already
mentioned. These plants show a little more differentiation
than the others, the protoplasm being clothed by a kind
of exoskeleton, the cell-wall, which is at once supporting
and protective.
More complex organisms consist of two or more proto-
plasts united together in various ways. The number of
THE GENEEAL STEUCTURE OF PLANTS 3
these masses of protoplasm, or cells, may be as small as
two, or may be enormous, as is the case in such plants as
the gigantic seaweeds of the tropics or the tall terrestrial
trees which abound all over the surface of the globe.
Whether the plant is simple or complex, we find the
same fundamental arrangement of its parts : there is a
certain number of protoplasts, in close relationship with
each other, supported upon a framework or skeleton which
shows a wonderful variety of arrangement, its details
depending on the manner of life of the whole organism of
which it forms so large a part. In such an organism each
FIG. 3. — FIGUBES OF DIFFERENT BACTERIA. (After Cohn and Sachs.
Very highly magnified.)
1, Sarcina ; 2, Bacillus ; 3, Spirillum ; 4, Spirillum with flagellee ;
5, 6, 7, Micrococcus. (Single, in strings, and in groups.)
protoplast is usually found occupying a particular cavity
which is formed by its cell-walls, and communicating with
its neighbours on all sides' by delicate prolongations of
living substance which extend through the walls of con-
tiguous chambers. Each chamber is often called a cell.
In dealing with the physiology of the plant, it is the
living substance which should first engage our attention,
though the arrangements of the supporting structures or
skeleton exhibit the greatest variety. We have seen that
in the simplest forms of plants the living substance may
exist without any cell-membrane, and may be freely motile,
swimming in water by means of cilia. The absence of the
4 VEGETABLE PHYSIOLOGY
cell-membrane can also be observed in certain peculiar
fungi, which are to be found creeping over moist surfaces
without such appendages (fig. 4). These are known as
the slime-fungi or Myxomycetes. In many respects they
approach very near to one of the humblest animals, the
Amoeba. They have hardly any structure, appearing like
a lump of transparent jelly, the whole mass being called a
plasmodium. They have the power of extruding a certain
portion of their substance in the form of a blunt protrusion
FIG. 4. — PORTION OF A PLASMODIUM OF A Myxomycete. x 300.
(After De Bary.)
known as a pseudopodium, and by means of these pseudo-
podia they can creep slowly over the surface on which they
are lying. The naked condition is, however, exceptional in
plants. In most of those which are unicellular the
living substance is covered by a delicate membrane or
cell-wall, and it may either fill the space inside the latter,
or may have in its interior a cavity or vacuole, which is
filled with a watery fluid. In the multicellular plants each
chamber during life contains its own protoplast or little mass
of protoplasm, whichjis connected, as already mentioned,
THE GENEEAL STEUCTUEE OF PLANTS 5
with its neighbours on all sides. In such cells the proto-
plast when young usually occupies the whole of the interior
(fig. 5, A), but when they are adult it generally lies as a
FIG. 5. — VEGETABLE CELLS.
A, very young ; B, a little older, showing commencing formation of vacuole.
p, protoplasm ; n, nucleus ; v, a vacuole.
peripheral layer round the wall, to which it is closely
pressed, while a central vacuole occupies the greater space
of the cavity enclosed by the cell-walls (fig. 6). Sometimes
the vacuole is crossed by a number of bridles or strands
of protoplasm, which generally pass
from a somewhat central spot to
the periphery. The protoplasm is
transparent, but somewhat granular
in appearance, and is saturated with
water. Somewhere in its substance,
whether it fills the cell-cavity or
not, there exists a specially differen-
tiated portion called the nucleus.
Sometimes, but only in particular
cells, the protoplasm contains other
differentiated portions, distinct from
the rest of the substance, which are
known as plastids. The bulk of the
living substance, to distinguish it
from these specialised portions, is
usually called the cytoplasm. It is not of the same con-
sistency throughout, a generally firmer portion lying next to
the cell-wall being known as the ectoplasm. A similar firm
FIG. 6. — ADULT VEGETABLE
CELLS. x 500. (After
Sachs.)
h, cell- wall ; p, protoplasm ;
k Jc, nucleus, with nu-
cleoli ; s s', vacuoles.
6 VEGETABLE PHYSIOLOGY
layer may frequently be detected round the vacuole. Some-
times these limiting layers are spoken of as plasmatic
membranes on account of their specially dense character ;
they are not, however, to be confused with the cell-membrane
or cell-wall, being particular layers of the cytoplasm.
The exact chemical composition of protoplasm cannot
be ascertained, as analysis involves its death, and this is
attended by changes in its substance. It contains carbon,
hydrogen, oxygen, nitrogen, and probably sulphur and
phosphorus, but we are quite unable to say in what different
combinations they exist within it. Enclosed in it are
always varying quantities of organic substances such as
proteids, carbohydrates, and fats, and small quantities of
various inorganic and organic salts. The substance of the
protoplasm has been thought either to be arranged in the
form of a network, these various bodies occupying the meshes,
or to have a foamy structure much like that produced by
vigorously stirring a mixture of oil and water. The various
substances alluded to as occurring in close relationship
to it are connected with the nutritive and other vital
processes of the cell, or its metabolism, and hence differ
greatly in nature and amount from time to time.
In ihe case of the free-swimming protoplasts, with
which we began the study of protoplasm, we saw they were
in active motion. As the protoplast becomes enclosed in
cell-walls this motility is, of course, less and less obvious ;
indeed in most cells it cannot be distinguished at all.
There is reason to suppose, however, that protoplasm,
wherever existing, is in active,, though imperceptible,
motion. In many of the constituent cells of even some of
the higher plants this motility can be observed, particu-
larly where the protoplasm has a granular appearance.
In certain of the cells forming the leaves of many aquatic
plants, e.g. Vallisneria, Nitella, Elodea (fig. 7), and
others, a streaming movement of the granules the proto-
plasm contains can be detected under a high power of
the microscope. In other plants of terrestrial habit, e.g.
THE GENEEAL STEUCTUEE OF PLANTS
certain cells of Tradescantia and Chelidonium, a similar
streaming of the protoplasm is observable (fig. 8) ; such
FIG. 7. — CELLS FBOJI THE LEAF
OF Elodea. x 300.
, nucleus ; p, protoplasm, in which
are embedded numerous chloro-
plasts. The arrows show the
direction of the movement of the
protoplasm.
FIG. 8. — Two CELLS FROM A
STAMINAL HAIB OF Trades-
cantia. x 300.
The arrows show the direction
of the movement of the
protoplasm.
movements are spoken of as rotation when the current
flows uniformly round the cell, or as circulation when the
path has a more complicated course.
It has been mentioned that,
with very rare exceptions, all cells
contain a specially differentiated
portion of protoplasm, known as the
nucleus (figs. 6 and 9). This struc
ture does not occupy a very definite
position in the cell, but not infre-
quently is found almost in the FIG. 9.— NUCLEUS OF A CELL,
T0 ,, , , „ ,, . SHOWING ABBANGEMENT OF
Centre. If the whole Of the Space IS THE CHBOMATIN THBEADS.
not filled with protoplasm, the part x 1000'
. a, threads ; o, nucleolus.
in which the nucleus lies is con-
nected with the lining layer by means of strands or
bridles. In other cases the nucleus is embedded in
8 VEGETABLE PHYSIOLOGY
some part of the lining layer itself. This body has a
more definite structure than the rest of the cytoplasm ;
it is bounded at the surface by a delicate membrane,
which is thought, however, to be a denser layer of
the protoplasm of the cell, rather than to belong to the
nucleus itself. Within this nuclear membrane are found
two substances which differ from each other in their power
of staining with various reagents. The bulk of the
nucleus is composed of a semi-fluid material known as
nucleoplasm, in which is embedded a network of fibrils or a
long much-coiled thread. The fibrils, or the thread, are
composed of a hyaline substance in which lie, close to each
other, a number of granules which stain deeply with many
colouring matters. The threads contain these granules
in such large proportion, that, except with very high magni-
fication, the latter cannot be distinguished, and consequently
the whole fibril appears stained. The fibrils are generally
said to be composed of chroma tin, the name having
reference to nothing more than this reaction to stains.
One or more small deeply staining bodies, termed
nucleoli, are found in each nucleus, sometimes being very
prominent, and at other times hardly distinguishable from,
the nodes of the fibrillar network or the crossings of the
coiled-up thread (figs. 6, k k, and 9, b). Chemically the
nucleus resembles the rest of the protoplasm to a consider-
able extent. It contains, however, a material known as
nuclein, of which phosphorus is a constituent. It is not
known how the nuclein is related to the rest of the nuclear
substance, but it appears to be present in the thread or
fibrillar network and not in the general nucleoplasm.
It is of such protoplasts or aggregations of small
portions of living substance that all plants are built up.
There is, however, a wonderful variety in the relative
arrangements of these units of construction, a variety
which finds its expression in the multiplicity of existing
forms, and the difference of dimensions which various
organisms exhibit.
THE GENEEAL STEUCTUEE OP PLANTS 9
FIG. 10. — COLONIES OF Proto-
coccus. x 750.
The simplest plants, as we have seen, are unicellular,
and many remain in this condition throughout the whole
of their existence. When they have attained a certain
size the cell divides into two. Sometimes these two be-
come separated from each other, and we have two plants
where but one existed before. Plants with this habit
remain unicellular, and the
division of the cell is equiva-
lent to the reproduction of
the plant. The unicellular
condition in other cases is
transitory, and the plant soon
comes to consist of two, four,
or more cells, in consequence
of the products of each divi-
sion remaining attached to-
gether. We get in this way a small colony of cells, each
like the others both in structure and in function. When
the power of division is limited the resulting colony
consists of a limited number of cells, and is often found
surrounded by a common cell-wall or membrane. This
condition is seen in such plants as Chroococcus, Proto-
coccus, and other humble Algae (fig. 10). A colony of
somewhat higher type, though still of microscopic size, is
found in the form of a hollow sphere (fig. 11), the wall
of which is one cell thick (fig. 11, A). This organism,
known as Volvox, shows a little higher differentiation than
those last described, the cells being furnished with cilia by
means of which the little sphere can propel itself through
the water.
In other cases the association of a number of protoplasts
is not complicated by the formation of any cell-wall. Fig.
4, A shows an aggregation of a number of naked protoplasts
which have combined to form a plasmodium. These
organisms are found creeping about upon moist surfaces ;.
they form the group known as the Myxomycetes or slime-
fungi. One species, Mtlnalium, is found frequently among
10
VEGETABLE PHYSIOLOGY
the refuse of tanyards and is known as 'flowers of tan.'
These fungi pass the greater part of their life without
possessing any cell-walls, only forming them indeed in con-
nection with their processes of reproduction.
FIG. 11. — VOLVOX GLOBATOR. (After Kny.) x 120.
A, section of a portion of the wall of the sphere, x 1000.
A third mode of arrangement of a colony of proto-
plasts is found in the so-called Ccenocytes (fig. 12). In one
of these plants, which are represented by several very
important seaweeds and by a large number of fungi, we
THE GENEEAL STEUCTUEE OF PLANTS 11
m
have a number of protoplasts arranged together over
the inner surface of a common cell-wall. The separate
protoplasts are often in such close contact with each other
that their separate outlines cannot be detected. They
have the appearance of a mass of protoplasm lining the
wall of a hollow, generally tubular,
cavity, and having a large number
of nuclei embedded in the mass.
The presence of a number of nuclei
indicates that there are really as
many protoplasts, as we have seen
a nucleus is an essential part of one
of the latter. Moreover, a single
protoplast contains only a single
nucleus.
The difference between a colony
of this kind and one constructed
like Chroococcus or Volvox is the
absence of a cell-wall between the
protoplasts. They are a stage
higher than the Myxomycetes, as
the whole colony is protected by an
external membrane.
Other cosnocytes exist in which,
besides the limiting wall, certain
transverse walls exist, dividing up
the chamber into compartments.
This condition is intermediate
between the coenocyte already de- FIG. 12.— EMBRYO OF Orobus
AT THE BASE OF A LONG
Scribed and the Simple Colony Or SUSPENSOR. THE LATTER
,, -.,. ITT i SHOWS A CCENOCYTIC STRUC-
the multicellular plant. TUEE. (After Guignard.)
In most cases the division of
the cells goes on for a considerable time and may continue
almost indefinitely, the number of the constituent proto-
plasts becoming very great and the colony proportionately,
large. According to the direction of the divisions we
get filaments (fig. 13), plates (fig. 14), or masses of cells,
m
12
VEGETABLE PHYSIOLOGY
the latter undergoing much subsequent differentiation
according to their ultimate dimensions and the nature of
their habitat or environment.
The protoplasm being the living substance of the plant
is possessed of certain properties which are not shared bv
FIG. 13. — FILAMENTS OF Nostoc.
(After Luerssen.)
FIG. 14. — Pediastrum, CONSIST-
ING OF A PLATE OF CELLS.
the framework on which it rests. It is, indeed, the centre
of all the activities which the plant manifests. It assimi-
lates the food which the plant requires and carries out all
the chemical processes necessary for life. It constructs
FIG. 15. — VEGETABLE CELLS.
A, very young ; B, a little older, showing commencing formation of vacuole.
p, protoplasm ; n, nucleus ; v, a vacuole.
the framework of the plant by which it is itself supported.
It receives impressions from without and regulates the
responses which the plant as a whole makes to those
impressions, both by internal and external movements or
changes of position. It is only by its powers of responding
to such impressions that the whole organism is able to place
THE GENERAL STRUCTURE OF PLANTS 13
itself in harmony with its environment. Finally, it carries
out the processes of reproduction.
The primary needs of a plant are fairly simple. If we
study the life and the behaviour of one of the free-swim-
ming organisms of which we have already spoken, we see
that its first requirement is water. In this it lives ; from
this it draws its supplies of nutriment and into this it pours
forth its excreta. The arrangement of the protoplasm in the
cell in one of the higher plants points to a similar need. If
we regard the arrangement whether in the young or the
adult cell, we notice particularly the very close relation of
the protoplasm to water. -The young cell enclosed in its cell-
membrane speedily shows a tendency to accumulate water
in its interior, and gradually drops appear in its substance
which lead ultimately to the formation of a vacuole always
full of liquid (figs. 15, 16). This store of water in the
interior of a cell is of almost universal occurrence in the
lowly as well as the highly organised
plant. The constitution of proto-
plasm, so far as we know it, depends
upon this relation, for the appa-
rently structureless substance is
always saturated with it. It is only
while in such a condition that a cell
can live ; with very rare exceptions,
if a cell is once completely dried,
even at a low temperature, its life is
gone, and restoration of water fails
to enable it to recover.
FIG. 16.— ADULT VEGETABLE
The constancy of the occurrence CELLS, x 500. (After
Of the VaCUOle in the Cells Of the
vegetable organism is itself an evi-
dence that such cells are completely
dependent upon water for the maintenance of life. The
cell-wall, though usually permeable, yet presents a certain
obstacle to the absorption of water, and so even those
cells which are living in streams or ponds usually possess a
cell-wall : p, protoplasm ;
nu'
14 VEGETABLE PHYSIOLOGY
vacuole. Cells without a membrane, such as the zoospores,
already many times mentioned, can more readily absorb
water from without, and hence they are not vacuolated to
the same extent as the former ones ; indeed, many of
them have no vacuoles. Where the vacuole exists it
always contains water, so that the protoplasm of the cell
has ready access to it, as much so indeed as the cell which
possesses no wall. The vacuole contains a store which is
always available.
The advantages which water supplies to the plant are
many. In the first place, we have seen there is a very close
connection between it and the protoplasm, the life of the
latter being dependent upon its presence. The information
we have at present does not enable us to explain the nature
of this dependence. There are other features of the rela-
tionship, however, into which we can enter more fully. The
protoplasm derives its food from substances in solution in
the water ; the various waste products which are incident
to its life are excreted into it and so removed from the
sphere of its activity. The raw materials from which cer-
tain cells construct the food which is ultimately assimilated
are absorbed from the exterior in solution in water. More-
over, water is the ultimate medium through which gaseous
constituents necessary for life reach the protoplasm.
Passing from the consideration of the protoplasm in
particular, the plant as a whole shows a similar dependence
on water. Many parts owe their rigidity to the distension
of their cells by liquid ; growth of the different members is
dependent upon the same hydrostatic pressure. In many
cases communication between different parts of a plant is
brought about through the same instrumentality, and thus
the response of the plant to various forms of stimulation
is facilitated or indeed made possible.
Another primal necessity of the plant is air. Every
living organism, with the exception of a few of the very
lowly forms of microbes, is dependent on the access of
oxygen for the maintenance of life.
THE GENERAL STRUCTURE OF PLANTS 15
The oxygen is usually obtained by the plant through
the intervention of water. The aquatic plant, whether
free -swimming or stationary, unicellular or possessed of a
highly differentiated body, absorbs the needed supply from
the quantity which is dissolved in the water of the sea,
stream, or pool in which it lives. The higher plant conveys
it to the protoplasts in solution in the water with which its
tissues or its walls are saturated. In such an organism
there is need of a special mechanism by means of which
the gases of the exterior may obtain access to the living
cells in the interior of the mass.
A third requirement of the plant is food. Here
ultimately, again, its dependence is placed upon the water
it obtains. The food or the materials from which the food
is constructed are absorbed by the plant in solution in
water, whether the food material is solid, liquid, or gaseous
in the condition in which it is presented to it.
Another condition is imperative in the case of a plant
which is composed of a large number of protoplasts or cells.
Not only must each have its own needs supplied, but it
must be in a condition to influence others and be influenced
by them. In such a plant we have, in fact, a community of
individuals, situated differently with regard to the supply
of individual and collective needs, and the well-being of
the whole community must depend upon the co-operation
of all in carrying out the different processes of life. The
protoplasts of such a community must therefore be in
organic connection with each other, so that such co-opera-
tion can be secured. The connection between contiguous
protoplasts which are separated by cell-walls is not easy
to determine. Special methods of preparation, and the
application of particular staining reagents, will show, how-
ever, under very high magnification, that the living sub-
stance of one cell is continuous with that of its neighbour
by fine delicate fibrils which perforate the wall (fig. 17).
In a few cases, as in certain seaweeds, and in the sieve-
tubes of the flowering plants, the connecting strands are
16
VEGETABLE PHYSIOLOGY
sufficiently coarse to be visible under a comparatively low
power of the microscope, and to need hardly any special
preparation (fig. 18).
It will no doubt have been noticed that the term ' cell '
is somewhat loosely used. A typical cell of a multicellular
plant consists of three parts — the protoplast, the cell-wall,
and the vacuole (fig. 6) ; of these the first is the most im-
FIG. 17. — CONTINUITY OF THE PROTOPLASM
OF CONTIGUOUS CELLS OF THE ENDOSPERM
OF A PALM SEED (Bentinckia). Highly
magnified. (After Gardiner.)
«, contracted protoplasm of a cell ; b, a
group of delicate protoplasmic filaments
passing through a pit in the cell-wall.
FIG. 18. — SEMI-DIAGRAMMATIC LON-
GITUDINAL SECTION OF AN OLD
AND STOUT PORTION OF Cera-
mium rubrum, SHOWING CON-
TINUITY BETWEEN THE PROTO-
PLASMIC CONTENTS OF THE AXIAL
OR CENTRAL CELLS, a a, AT THEIR
ENDS, AND LATERALLY WITH THE
CORTICAL CELLS 6, BY MEANS OF
PROTOPLASMIC THREADS. (After
Hick.)
portant, being the living substance. A protoplast which
has no cell- wall and contains no vacuole is still called a cell.
The term is again often applied to a cavity which contains
no protoplast, as in the case of old wood or cork. In such
cases a protoplast once occupied the cavity, but it has been
removed by death. These cells are consequently only the
skeletons of dead protoplasts.
17
CHAPTEE II
THE DIFFERENTIATION OF THE PLANT-BODY
THE primary needs of a complex plant are the same as those
of a single protoplast, the greater size of the former involv-
ing however a more elaborate method of supplying them.
In multicellular plants we consequently meet with a consider-
able degree of differentiation of structure. Each protoplast,
which is one of the units of the colony, has originally the
same properties as the unicellular plant. With increase
of number in the plant- body, and with the consequent
increase of size, a certain division of labour soon makes
its appearance, and particular groups of cells develop one
property more than the others. A specialisation of powers
is very quickly apparent, and we can recognise masses of
cells devoted to the discharge of one function, others
to that of another, and so on. Such limitations of the powers
and properties of the individuals have for their object the
well-being of the community of which those individuals
are constituents.
Various groups of plants show this specialisation of
function or differentiation of structure in very different
degrees, any particular development having a special
reference to the habitat or the mode of life which is
characteristic of the community in question. A plant-body
which takes the form of a long filament or a plate of cells
shows little differentiation beyond the formation of a
vacuole in each protoplast. The setting apart of special
cells for purposes of reproduction is generally the first
specialisation which takes place.
As soon as the cells of the plant begin to divide in
c
18 VEGETABLE PHYSIOLOGY
three dimensions, so that a mass of protoplasts is formed,
the progress of differentiation becomes marked.
In such a mass the necessity of supplying water to all
the constituent units involves particular difficulties which
vary according to the environment of the plant under
observation. Those which live in water need much less
complex arrangements than those which are at home on
land, as they can absorb water from the exterior by their
general surface, and after absorption it can easily make its
way from cell to cell. Those which derive their supply of
water entirely from the soil, as is the case with nearly all
terrestrial plants, need a specialised mechanism for trans-
port of the water after it has been taken up.
On the other hand the supply of a suitable atmosphere
to the interior of the plant for the service of its more
deeply seated protoplasts is attended with more difficulty
in the case of an aquatic than a terrestrial plant.
In cell-masses, therefore, such as are found in all
plants possessing more than microscopic dimensions, we
meet with considerable differentiation of the plant-body.
The explanation of the details of such differentiation is to
be found in the division of labour which the size and the
mode of life of the particular plant demand.
The first indication of this differentiation in the vegeta-
tive body of the plant is a change in the character of the
exterior, which has for its object the
protection of the plant from external
injurious influences. This can be
seen even among the seaweeds, simple
as is generally the structure of mem-
bers of this group. Fucus and its
allies, which form part of the class
F THALL^PI OFE p^eivetia, of the brown Algae, have their external
JACENT CELLS, x 300. together, and generally much denser
than the rest of their tissue (fig. 19). In the group
of the Mosses certain arrangements of this kind can
THE DIFFERENTIATION OF THE ^PLANT-BODY 19
be seen. The common bog moss (Sphagnum) shows its
stem to have on the outside several layers of large empty
cells whose walls are marked with spiral thickenings.
FIG. 20. — TBANSVEBSE SECTION OF
STEM OF Sphagnum,
FIG. 21. — SECTION OF STEM OF Moss,
SHOWING CENTBAL STBAND OF
THIN-WALLED CELLS SUBBOTJNDED
BY COBTEX AND EPIDEBMIS. THE
WALLS OF THE OUTEB CELLS OF
THE COBTEX ABE CONSIDEBABLY
THICKENED. (After Sachs.)
Inside these a further protective layer of small cells with
uniformly thick walls is met with (fig. 20). In the smaller
mosses the outer layers of the cortex are thickened (fig. 21).
FIG. 22. — TBANSVEBSE SECTION OF THE BLADE OF A LEAF, SHOWING THF.
OUTEB WALLS OF THE EPIDEBMAL CELLS THICKENED AND CUTICU-
LABISED. x 100.
In the higher terrestrial plants we have evidence of
great specialisation for protective purposes, a special
tegumentary system being developed, which varies in
20
VEGETABLE PHYSIOLOGY
complexity in the different groups. In the smallest forms,
which are only herbaceous in habit, we find the protective
mechanism takes the shape of a thickening and cuticulari-
sation of the outer walls of the cells of the epidermis
(fig. 22). The protection secured is twofold ; evaporation
of water is prevented, and so
an economy of the supply is
secured, while the dangers
incident to cold or heat are
minimised.
In plants of sturdier habit
the protection afforded by
after a while by a more
complicated tegumentary sheath. Certain cells become
specialised and form layers of cork (fig. 23), which arise
FlG. 23. — OUTEB PORTION OF CORTEX
OF YOUNG TWIG OF LIME.
per, cork layer.
the epidermis is replaced
FIG. 24.— SECTION OF BARK OF Quercus sessiliflora. (After Kny.)
IH-, cork layers arising at different depths in the cortex.
successively at gradually increasing distances from the
exterior, and in the case of trees finally lead to the con-
struction of a bark (fig. 24). The corky formations are
THE DIFFEEENTIATION OF THE PLANT-BODY 21
FlG. 25.— COLLENCHYMA UNDEK
THE EPIDERMIS OF PETIOLE.
x 50.
supplemented by masses or sheaths of hardened or scleren-
chymatous parenchyma or even by sclerenchyma itself. In
forms which are intermediate
in requirements, such as the
petioles of leaves, layers of
collenchyma are developed below
the epidermis (fig. 25).
Sometimes sheaths or layers
of sclerenchyma are developed
instead of cork ; this condition
occurs especially among the
stouter Monocotyledons.
The protective mechanisms
developed by roots also show a
good deal of variety. There is
not at first a provision of an impervious membrane com-
posed of the outermost layer of cells ; this would be
inconvenient in view of the necessity for the existence of
root-hairs. The second layer later undergoes modification,
its cells becoming thickened in the peculiar manner
characteristic of the exodermis (fig.
26). Other sheathing layers are also
found more deeply seated, while
eventually the pericycle becomes the
place of formation of corky tissue.
The second prominent differentia- FIG. 26.— SECTION OF OUTER
,. , . , ., ,„ . ., ,, REGION OF BOOT, SHOW-
tion which presents itself is the forma- ING EXODERMIS, ex.
tion of a system of cells and vessels for
the transport of water through the plant and the circula-
tion of nutritive and other materials. We may speak of
this as the conducting system. A little reflection will
show us the necessity for the development of some such
system as this, which must be more extensive and complex
as the size of the plant increases. We find that the source
of water on which a terrestrial plant relies is the soil in
which its roots are embedded. Even when it is young
many of its protoplasts are placed at a considerable dis-
22 VEGETABLE PHYSIOLOGY
tance from such a source of supply, and in the absence of
a ready means of communication must die in consequence
of their position. These moreover are among the most
active of the protoplasts, discharging important duties in
connection with nutrition, and needing for their purpose
considerable quantities of the water from the soil with the
salts dissolved in it.
The conducting system is formed by the collections of
cells and vessels which are known as the vascular bundles.
These structures consist in most cases of two parts, the
wood, which is the path for the ascent of water from the
roots, and the bast, which is more concerned with the
transport of the elaborated products of the metabolism of
the cells.
The degree of development of this system varies very
much in different plants. In an ordinary herbaceous
Dicotyledon the bundles remain separate, and can be
traced separately from the root, through the stem to the
leaves (fig. 27) in which they form the branching network
known as the veins (fig. 28). With greater size, however,
more capacious channels are demanded, and we find more
and more bundles developed, until we reach the condition of
the oldest trees, nearly the whole of whose trunks are
formed of tissue which either is or has been devoted to
this service. In such trees the most actively living parts
are found at the extremities, by far the greatest number of
their protoplasts being situated in the twigs and leaves.
Indeed, the greater part of the wood of the trunk of many
trees is dead, and consequently functionless.
The same tissues serve for transport in the Monocotyle-
dons, and in the Vascular Cryptogams, though the mode of
arrangement of the elements is altogether different from
that of the Dicotyledons.
In those vascular plants which live in water, and
particularly in those which are totally submerged, there is
no need for so elaborate a transport system, as water can
be readily absorbed by the general surface. We find two
THE DIFFEKENTIATION OF THE PLANT-BODY 23
modifications of structure in such plants ; the epidermis is
hardly at all cuticularised, so that water can pass into its
cells ; while the vascular bundles are comparatively feebly
developed, the woody part of them being particularly
small.
A third requirement of a plant of considerable mass,
especially if it has a terrestrial habitat, is a power of resist-
ing such external forces as
would lead to its uprooting,
which must be combined with
a considerable degree of flexi-
bility, at any rate at the ex-
tremities of the body. This
FIG. 27. — DIAGRAM OF THE COURSE
OF THE VASCULAR BUNDLES IN
AN HERBACEOUS DICOTYLEDO-
NOUS PLANT.
FIG. 28. — DISTRIBUTION OF THE
VASCULAR BUNDLES OR VEINS
IN A FOLIAGE LEAF.
combination of rigidity and flexibility has been secured in
various ways, varieties of both the form and the structure
of the plant being concerned in it. In the simplest plants
but little differentiation of the body is needed ; such forms
as consist of single cells, or rows or plates of cells, living in
water, need hardly any rigidity, and in their cases the
unthickened cell-wall yields sufficient support to the proto-
plasm. Larger plants which grow in rapidly flowing water
usually possess flexible stems and much-divided leaves, which
consequently give way to the current, and escape damage.
Small terrestrial plants or parts of plants, which have but
24 VEGETABLE PHYSIOLOGY
a short life, resemble these aquatic forms in their general
characteristics, though they show much greater variety in
the forms of their leaves. The rigidity and flexibility of
both depend upon the distension of their cells with water.
We find this mechanism in succulent petioles, such as
those of the rhubarb, and in certain herbaceous stems
which contain little wood, such as those of the cabbage and
lettuce. Plants of terrestrial habit which attain very
large dimensions, such as the forest trees, need, however,
much greater modification. Being exposed to winds and
storms, they need a firm anchorage below the surface of
the ground, and a more or less massive axis to secure
stability when atmospheric disturbances are severe. For
the needs of their protoplasts, to secure the exposure of
the greatest possible number of them to the access of air,
warmth, and light, a great subdivision of this axis is
necessary, so that the form usually attained is that of a
relatively very large head resulting from the repeated sub-
divisions of the trunk, and ending in finely divided twigs.
The danger of too great rigidity in this portion soon
becomes apparent, as it presents a very considerable
surface to the wind. The rigidity needed for support must
be combined with sufficient flexibility to enable the body,
already helped by its fine subdivision, to give way before
the force of atmospheric currents, and so to prevent the
danger of uprooting the tree. In other forms a weak axis,
quite incapable of supporting any great development of the
plant-body, nrnst be capable of obtaining support by cling-
ing in various ways, and holding by various mechanisms
to other structures, such as the trunks of trees, rocks,
walls, &c.
In many cases the strength and prominence of the
tegumentary and conducting tissues supply the particular
need. In most forest trees the anchorage is afforded by
the strong much-branched root system, the centre of whose
members is composed of great developments of secondary
wood, forming part of the conducting system. The trunk
THE DIFFERENTIATION OF THE PLANT-BODY 25
and twigs are of similar composition, the former being
strengthened very materially also by its bark. But there
are many smaller trees and shrubby plants, as well as some
herbaceous forms, whose requirements are similar, but
which for various reasons have not a very great develop-
ment of either primary tegumentary tissue or of vascular
bundles. With no additional mechanism for support, they
would be in great danger of either collapsing or being
actually uprooted. In their cases we meet with a sub-
sidiary development of supporting tissue, which shows a
great variety in its arrangement and distribution.
We find that the tissue which most frequently subserves
this purpose is either collenchyma, sclerenchymatous paren-
chyma, or true sclerenchyma. In a few delicate stems
these tissues are much more prominent than the vascular
bundles. We can notice three regions of the stem or axis
where they may appear, and in these places they may take
the form of isolated cells, or strands of tissue, or complete
sheaths going round either the whole axis or separate parts
of it. The first of these regions is the layer underlying
the tegumentary tissue, which the new development sup-
plements and strengthens. Most moss plants show the
hypoderrnal cells of their axis thickened, while such a
development is very common in many petioles and leaf-
blades. The new development may occur in close relation-
ship with the vascular bundles which, in such cases, are
found among large-celled somewhat succulent parenchyma,
and are not generally very strongly developed. The scleren-
chyma by forming a separate sheath round each bundle
gives it a rigidity which it could not derive from its own
elements, and in addition prevents the whole stem from being
crushed. This is seen in the stems of many semi-succulent
monocotyledonous plants, such as those of the maize and the
asparagus (fig. 31). The sclerenchyma may also occur freely
in the ground tissue, at some distance from both tegumentary
and vascular structures. The bands of it which occur in
the rhizome of the bracken fern are good illustrations of
26 VEGETABLE PHYSIOLOGY
this mode of disposition. The two main ones form an
interrupted cylinder (fig. 29), so arranged as to protect the
delicate vascular tissue, which is in great part placed either
within this cylinder or in some similar relation to other
similar sclerenchymatous strands. In the case of a plant
of humbler type, the common hair-moss (Polytrichum), a
development of somewhat sclerotised cells forms a central
core passing down the stem. In many of the flowering plants
more complex distribution of sclerenchyma can be noticed,
strands in the middle of the cortical tissue, or in the pith
e/7—
sc — — ^L-
FIG. 29. — TRANSVERSE SECTION OF RHIZOME OF THE BHACKEN FERN.
x 10.
sc, bands of sclerenchyma ; hy, hypodermal sheath of sclerenchyma ;
st, steles ; ep, epidermis.
of the stem, being occasionally seen. Stems which are
angular in section are usually found to have their angles
strengthened in a similar way.
The arrangement of this sclerenchyma is generally such
as to supplement the bundles, and to secure the greatest
amount of solidity and sufficient flexibility, with the least
expenditure of material.
Instances of various methods of arrangement of
strengthening material may serve to illustrate -this par-
ticular differentiation (fig. 32). In the simplest cases the
sclerenchyma is developed in connection with only one of
THE DIFFEEENTIATION OF THE PLANT-BODY 27
the three regions already alluded to. The stem of Equise-
tum and the leaves of Conifers are furnished with a layer
of thick-walled cells immediately under the epidermis
(fig. 30) ; the vascular bundles of many Monocotyledons
are surrounded separately by a sheath of small cells of
similar character (fig. 31) ; in Pennisetum a sheath is
developed round the stem in the form of a hollow cylinder
which lies between the bundles and the epidermis.
More frequent instances occur in which two of the
FIG. 30.— LEAF OF Pinus (ONE OF THE CONIFERS).
ep, epidermis ; hy, layer of sclerenchyma ; et), endodermis ; v.b., vascular
bundle ; r.d., resin duct.
regions in question are strengthened simultaneously. In
the stems of Scirpus there is a development of sclerenchyma
round the periphery, and strands occur also in connection
with the bundles. Sometimes these are connected by
bands of sclerenchyma lying between them. In Fimbri-
stylis there is a ring of sclerenchyma in the cortex and
patches around the periphery, which in other cases are
joined like those of the former type. In the stems of
Typha a band of sclerenchyma lies at the back of each
bundle, and either a ring or some isolated strands may be
28
VEGETABLE PHYSIOLOGY
found in the cortex. The stem of Juncus shows these
two forms combined together.
Still more complicated cases show sclerenchyma arising
in all three regions, sometimes the bands being all
independent, sometimes united in various ways. In
Cladium Mariscus those of all the regions are united into
a continuous system which goes from the tegumentary
FIG. 31. -VASCULAR BUNDLE OF STEM OF MONOCOTYLEDONOUS PLANT.
(After Kny.)
pit, phloem ; x, xylem vessels ; p ph, protophloem. The bundle
is surrounded by a small-celled sheath of sclerenchyma.
region towards the interior of the stem, embracing the
vascular bundles and attaching them to each other.
Similar differentiation of the supporting system is found
in many leaves, in which it subserves the same purposes.
In many cases the veins afford sufficient protection against
tearing or rupture in consequence of violent winds. The
methods of their arrangement in many cases subserve
THE DIFFERENTIATION OF THE PLANT-BODY 29
this purpose very completely. In other leaves of tough
leathery habit the delicate tissue of the mesophyll is fre-
quently protected from crushing by isolated thick- walled
cells of curious shape which extend from one epidermis
to the other. Others show bands of sclerenchyma sup-
plementing the veins and not infrequently enclosing them
and reaching the epidermis on each side.
The supporting tissue is frequently known as the
FIG. 32. — DIAGRAM SHOWING THE CHIEF DISPOSITIONS OF THE SKELETAL
APPARATUS IN A STEM WITH FIVE COLLATERAL BUNDLES (IN TRANSVERSE
SECTION).
(The sclerenchyma is black ; the bast of the bundles is white ;
the wood is dotted.)
1, Type without accessory sclerenchyma ; 2, Equisetum ; 3, Bambusa ; 4,
Pennisetum; 5,Scirpus', 6, Erianthus ; 7, Fimbristylis ; 9,Typha; 10,
Juncus ; 14, Cladium. (After Van Tieghem.)
stereome of the plant. It forms, as we have seen, the
most prominent part of the endo-skeleton.
The cells of which the masses of sclerenchyma are
composed have been ascertained to possess almost as much
power of withstanding longitudinal strain as the finest
steel, and they are much more ductile than either this
metal or wrought iron. Their arrangement in the different
ways described has a very distinct relationship to the
character of the strain they have to resist. In such
30 VEGETABLE PHYSIOLOGY
structures as hollow stems where there is but little
substance of tissue, but where they are required to resist
lateral bending, the supporting tissue is situated near the
periphery of the stem, and the latter is often still further
strengthened by being furnished with ridges or flanges.
An instance of an almost converse character is afforded by
a young root. In its growth, while it must possess sufficient
rigidity to enable it to penetrate the soil, it must be capable
of frequent bending to enable it to avoid obstacles. This
is most advantageously provided for by a central core of
strong tissue, surrounded by more succulent material.
The transporting tissue of the centre is comparatively
little affected by the flexures of the structure, and its
function is not interfered with.
Another kind of differentiation in such a cell-mass as
we are dealing with, is the setting apart of particular
groups of cells for various metabolic
purposes. We have the formation of
glandular tissue, of the laticiferous
systems, and so on. This differentia-
tion may be marked also by the pro-
duction of definite organs in the proto-
plasts, such as are seen for instance in
the case of the chloroplasts of the leaves
(fig. 33) and other green parts of plants.
FIG. 88.— CHLOBOPLASTS The habit of life of a plant again
EMBEDDED IN THE PttO-
TOPLASM OF A CELL OF may influence its structure and the
THE PALISADE TISSUE , „ -..„. ,. ,. „ ., , ,
OF A LEAF. degree of differentiation of its body to a
very great extent. The great group of the
Fungi afford us an illustration of the degradation of structure
which accompanies a saprophytic or parasitic habit. Similar
instances of degradation are met with among the flowering
plants.
The needs of the cell-mass thus usually lead to the
differentiation in its substance of at least four physiologi-
cally different regions — the tegumentary, the conducting,
the supporting, and the metabolic. The latter includes all
THE DIFFEKENTIATION OF THE PLANT-BODY 31
the parts in which the protoplasts are comparatively little
changed, and consequently are most concerned in carrying
out the vital processes.
The needs of the protoplasts forming the community of
the plant embrace, however, as we have seen, something
more than the arrangements so far described serve to secure
for them. Each protoplast must be furnished with a certain
FIG. 34. — SECTION OF STEM OF Potamogeton, SHOWING AIR PASSAGES
IN THE CORTEX.
amount of air, or rather oxygen. Almost all living sub-
stance must carry on during life the process known as
respiration. The free-swimming zoospore to which we
have so often referred obtains a supply of oxygen from the
water in which it lives, the gas being dissolved therein.
Aquatic plants also obtain their oxygen from this -source,
but many of them are composed of a large number of cells;
manv of which are situated at some distance from the
32
VEGETABLE PHYSIOLOGY
exterior. In such plants large cavities or reservoirs are
constructed, in which a quantity of air is slowly accumulated
and into which the respiratory products can be discharged.
From such reservoirs the oxygen which the cells require
is obtained. The composition of the atmosphere in these
chambers or lacunae is not accurately known, but it pro-
bably differs somewhat from that of ordinary air.
These air passages
or reservoirs are very
conspicuous in the
stalks of floating
leaves such as those
of the water-lily, and
in the submerged
stems of most aquatic
plants (fig. 34).
A somewhat simi-
lar mechanism is
provided in the case
of terrestrial plants.
At the time of their first formation, all the cells are
in close approximation to each other at all points of
their surface. This condition is, however, only tempo-
FIG. 35. — CORTEX OF ROOT, SHOWING INTER-
CELLULAR PASSAGES BETWEEN THE CELLS.
FIG. 36. — SECTION OF LEAF SHOWING THE LARGE INTERCELLULAR SPACES
OF THE MESOPHYLL.
rary ; during the early stages of growth the cell-walls
split apart at particular places, usually the angles of the
cells. A system of intercellular spaces is thus formed
THE DIFFEKENTIATION OF THE PLANT-BODY 33
which, as growth proceeds, become continuous with each
other and form a system extending throughout the plant.
They can be detected in the root, in the cortex of which
FIG. 37. — PART OF LOWER SURFACE OF A LEAF, SHOWING THREE STOMATA -
IX DIFFERENT STAGES OF OPENING. X 300.
they are conspicuous (fig. 35) ; they may be traced through
all the ramifications of the stem, and are seen to form a
very prominent feature of the mesophyll of the leaves (fig.
36). They communicate with the exterior in all the green
parts of the plant,
especially the leaves.
In the -epidermis of
all such parts are
small openings known
as stomata (figs. 37,
38), which are pro-
vided with two guard-
rpll<t hv thp hpViavinnr FIG. 38.— SECTION OF LOWER EPIDERMIS OF
' ^ l A LEAF, SHOWING A STOMA. x 300.
of which the aper-
tures can be opened or closed. In those regions of the
axis where corky layers cut off the metabolic tissue of the
34 VEGETABLE PHYSIOLOGY
cortex from the exterior, certain other special apertures,
the lenticels, are present (fig. 39). The atmosphere conse-
quently enters the plant by these orifices and circulates
through the whole of the intercellular space system. As
nearly every protoplast abuts in part upon a channel of
this system, its necessary
aeration is secured. Each
protoplast is thus in a
somewhat intricate manner
in contact with the external
air, though really situated
perhaps deep in the tissues
FIG. 39.-SECTION OF A LENTICEL. °f a Plant of large dimen-
Z, lenticel ; per, cork layer. SlOnS.
Like the aquatic plant,
the terrestrial one thus possesses a reservoir containing an
atmosphere, which, though its composition may not be
exactly that of the exterior, yet contains oxygen for the
need of the protoplasts and serves as the medium by which
all surplus carbon dioxide is removed from. them.
This intercellular space system not only subserves the
purpose of the gaseous interchanges of respiration, but
ministers in two ways to the metabolic phenomena carried
out by the plant. It permits the access of the atmospheric
carbon dioxide to structures in the leaves which make it
available for the construction of food material. It further
is of great importance in helping to regulate the supply of
water to the cells. We have seen that a transport' system
is differentiated which carries the water to them. This
transport system does not, however, remove it from them
subsequently. The protoplast can only get rid of water by
the process of evaporation, and as it constantly needs a new
supply, it must continuously exhale watery vapour to make
room for the incoming stream. Such evaporation takes
place into the intercellular spaces through the delicate cell-
walls which abut upon them. The intercellular reservoir
contains, therefore, an atmosphere which is charged almost,
THE DIFFEBENTIATION OF THE PLANT-BODY 35
if not quite, to saturation by aqueous vapour, and under
ordinary atmospheric conditions this is being continually
exhaled as long as an excess of water is passing through
the plant. The regulation of the process of exhalation
depends mainly upon the condition of the guard-cells of
the stomata which can permit it to go on freely or can
check it by partially or entirely closing the apertures
according to various internal and external conditions (fig.
87).
D 2
36 VEGETABLE PHYSIOLOGY
CHAPTEK III
.THE SKELETON OF THE PLANT
IN the last chapter we discussed the differentiation of the
body of the plant, and examined the constitution of various
mechanisms which are associated with such differentia-
tion. If we. study the arrangements which are peculiar to
any plant, we shall find that almost all such differentiation
as exists involves a modification of the non-living part, and
particularly the walls of the supporting and conducting
tissues, the living protoplasts having fundamentally the
same structure or composition, whatever may be the
nature of their immediate support. All the various dis-
positions of the non-living elements or structures are
secondary in importance to the protoplasts.
We cannot, indeed, lay too great stress on the fact that
the needs and conditions of the protoplasts are primarily
the causes of the differentiation of the non-living structural
parts, and such differentiation is the expression of the fact
that division of labour has arisen among the protoplasts of
the community.
We have seen that a protoplast in its simplest condi-
tion is capable of an independent existence without any
form of mechanical support beyond that which it derives
from the slight difference of density between its external
layer and its interior. In most cases, however, this is
not sufficient for protection during its whole life, and a
membrane is subsequently formed around it. The mem-
brane itself is a secretion from the protoplast, which in
fact prepares its own defensive mechanism. In most cases
the protoplast is always clothed by a cell-wall, the forma-
THE SKELETON OF THE PLANT 37
tion of every new cell being completed at once by the
membrane which is formed as soon as the protoplast has
divided into two. This is particularly noticeable in cases
where a cell-complex or community forms the plant-body.
Each protoplast thus continually forms for' itself a chamber
to dwell in, the wralls of which at first, at any rate, are
probably all alike. We may thus recognise in the cell-wall
an exoskeleton for the individual protoplast, which may or
may not undergo subsequent modification.
Tn the case of a large plant consisting of innumerable
protoplasts, the cell-walls of the separate units are found
united together in different ways, and to a different extent
in different individuals. The resulting network constitutes
at first the skeleton of the whole plant. The modification
of the cell-wall which was unnecessary so long as the
protoplast was solitary, becomes imperative as soon as the
needs of a large community are established, and secondary
differentiations of such cell-walls result, the alterations
being due, like the original formation, to the activity of
the protoplasts. Not only are the walls changed in
substance and in thickness after they are formed, but the
protoplast itself frequently alters its shape, and conse-
quently the form of the cavity containing it, by irregulari-
ties of subsequent growth. The skeleton of the plant is
not therefore merely the hard tissues which will survive
maceration and desiccation, not merely those coarser
structures evidently set apart for protection and support,
but it includes all the delicate cell-walls which form the
cavities in which the protoplasts are living. We may
indeed discriminate between the skeleton of the individual
protoplast and that of the large community of which it
forms a part.
The skeleton of a large plant such as a tree increases
in complexity as its life continues. In such a plant
growth continually proceeds so long as life lasts. Every
year new branches or twigs with their associated leaves
are constantly produced. With such continuous increase
38
VEGETABLE PHYSIOLOGY
of size, new conducting tissue must be formed. The
skeleton of a young plant is consequently much smaller
FIG. 40. — SECTION OF DICOTYLEDONOUS STEM (Helianthus).
A, young condition, with the primary vascular bundles only developed ;
B, older, after secondary bundles have been developed between the
primary ones by the interfascicular cambium, forming a ring of wood.
than that of an old one. The difference between the con-
dition of a stem at two periods may be seen by comparing
fig. 40, A and B, the former of which shows the arrange-
THE SKELETON OF THE PLANT
39
ment of the supporting and conducting tissue at an early
stage of its life, and the latter indicates the condition
after several months, during which a large formation of
secondary vascular tissue has taken place.
The structure of a ccenocyte shows a similar mode of
formation of the skeleton to that of
a multicellular plant- community.
In this case, however, the several
protoplasts are not furnished with
separating walls. The only skele-
ton is the external memhrane which
limits the whole structure, and
which is formed by the conjoint
activity of them all. In compound
or septated ccenocytes we have in
addition certain transverse walls
crossing the interior and giving a
greater degree of strength to the
whole body. These separating
walls have a similar origin.
The primary cell -wall which
clothes the unicellular plant, and
which serves as the original sup-
porting membrane of the separate
protoplasts of a community or
colony, is, when first formed, a
clear, transparent, extensible, and
elastic membrane, which remains
long as the latter is living. Under
certain conditions it is capable of
.., ... , FIG. 41.— EMBRYO OF Orobus
ID. Contact With the protoplasm SO AT THE END OF A LONG
*££"? SaFLSR
?™oc™? STBUCTUBE.
(After Guignard.)
absorbing considerable quantities The rounded bodies in theseg-
of watpr and in mnqpnnpnpp swpll- ments of the CGenocytes are
6r' aiK the nuclei of the protoplasts.
ing to a greater or less extent.
Under ordinary conditions it is freely permeable by
water. It is usually said to be composed of a sub-
stance termed cellulose, whose chemical composition is
40 VEGETABLE PHYSIOLOGY
represented by the formula rc(C6H1003), the value of n
not yet having been accurately determined. This sub-
stance is related to such bodies as starch, sugar, &c.,
being a member of the group of carbohydrates. It is
capable, under the action of hydrating reagents, of being
converted into a form of sugar, and under certain circum-
stances it can yield nutritive material for the use of the
plant. Cellulose possesses the peculiar property of becom-
ing a deep blue in colour when treated with iodine in the
presence of sulphuric acid, chloride of zinc, or other hydrat-
ing reagent. It dissolves with readiness in a solution of
ammonio-cupric sulphate (Schweizer's reagent), but is not
soluble in dilute acids or alkalis. Strong mineral acids,
such as sulphuric or phosphoric, cause it to imbibe water
and swell up, ultimately becoming gelatinous and dissolv-
ing. Certain soluble ferments affect it similarly.
When the cell-wall is examined by polarised light it is
found to be doubly refractive.
When cellulose is oxidised with strong nitric acid it
yields oxalic acid.
Cellulose is capable of existing in more than one condi-
tion. We find some kinds of it which will stain blue with .
iodine without previous hydration. Examples of this
variety are found in the cell- walls of the bast of Lycopo-
dium, the endosperm of the Pseony, the cotyledons of some
of the Leguminosse, &c. The walls of the hyphae of the
fungi differ again, in that they will not give the blue colour
with iodine even after treatment with hydrating reagents.
Eecent observations suggest that this variety of cell-wall ap-
proaches in composition the chitin of the animal kingdom.
The celluloses which have been so far examined have
been divided into three categories, according to the ease with
which they can be made to undergo hydrolysis, and to yield
some variety of sugar by such treatment. The celluloses
of cotton fibres are perhaps the most resistent of all, and
maybe taken as representatives of the most refractory group.
The cellulose found in the main mass of the fundamental
THE SKELETON OF THE PLANT 41
tissue of the flowering plants is less resistent, giving very
easily the reactions which have been just described. A
third variety is hydrolysable with still greater readiness.
It is to a certain extent soluble in alkalis and is easily
decomposed by acids with formation of other carbohydrates
of low molecular weight. Such cellulose is represented in
the cell-walls of most seeds.
It is probable that cellulose is chemically combined
with a certain amount of water, and that the degree of such
hydration differs in the different varieties described.
Though, as . already stated, the cell-wall is commonly
said to be composed of cellulose, the latter material is always
associated with other constituents. Among the latter we
find various members of another group known as pectoses,
which differ in many ways from cellulose. This group
includes two series of bodies which vary among themselves
as to the degree of their solubility in water. One of these
series comprises bodies of a neutral reaction, while those
of the other are feeble acids. In each series there are
probably several members, which show among them every
stage of physical condition between absolute insolubility
and complete solubility in water, the intermediate bodies
exhibiting gelatinous stages, characterised by the power of
absorbing water in a greater or less degree.
Of the neutral series the two extremes are known as
pectose and pectine. The former is insoluble in water, and
is closely associated with cellulose in the substance of most
cell-walls ; the latter is soluble in water and forms a jelly
with more or less facility. Pectose has not yet been
obtained pure, in consequence of its close association with
cellulose and the readiness with which it undergoes change
in the process of extracting it. The reagents which
separate it from cellulose convert it into pectine, or into
pectic acid, the former being soluble in water, the latter in
alkalis. The cell-wall can be shown to contain the two
constituents by the action of Schweizer's reagent, which,
when used with proper precautions, dissolves out the
42 VEGETABLE PHYSIOLOGY
cellulose and leaves the framework of the cell apparently
unaltered ; it consists then, however, not of pure pectose,
but of a compound of pectic acid with some of the copper of
the reagent.
Pectine swells up and dissolves in water, forming a
viscous liquid which soon becomes a jelly. It exists in
considerable quantity in many ripe fruits and in some
mucilages. It gives no precipitate with the neutral acetate
of lead, but is thrown down by the basic acetate in the form
of white flocculi. If it is boiled for some hours in water, it
is converted into parapectine, which is precipitated by
neutral lead acetate. Further boiling with dilute acids
converts it into metapectine, which is precipitated by
barium chloride.
The acid series shows peculiarities similar to those of the
neutral one. Its most insoluble member is pectic acid, which
will not dissolve in water, alcohol, or acids ; it forms soluble
pectates with alkalis, and insoluble ones with the metals
of the alkaline earths, of which calcic pectate is the most
widely distributed. It dissolves in solutions of alkaline
salts, such as the carbonates of sodium and potassium,
alkaline phosphates and most organic ammoniacal salts,
forming with them double salts which gelatinise more or
less freely with water. Its solution in alkaline carbonates
is mucilaginous, but when ammonic oxalate is the solvent
it is perfectly limpid.
The member at the other end of the series is meta-
pectic acid, a body with an acid reaction, freely soluble in
water and forming soluble salts with all bases, especially
those of calcium and barium, which precipitate pectic acid.
Metapectates are coloured yellow when they are warmed
with an excess of alkali. This body and its compounds
are probably very prominent in the gums ; when acted
on by dilute sulphuric acid they split up, one of their
products being a crystallisable dextro-rotatory sugar which
is apparently arabinose. Metapectic acid does not form a
jelly, its solutions always being limpid.
THE SKELETON OF THE PLANT 43
The two series of pectic bodies are closely related to
each other, for by the action of heat, acids, and alkalis
the various members of both can be prepared from pectose.
The final product of the action of the reagents is the freely
soluble metapectic acid.
The cellulosic and pectic constituents of the cell-wall
show considerable differences of behaviour. The former
are soluble, the latter insoluble, in Schweizer's reagent ;
when oxidised with nitric acid the former yield oxalic, the
latter mucic acid. The celluloses when partially hydrated
stain blue with iodine ; the pectic bodies give no coloration
with this reagent. They behave differently also to staining
reagents and to dilute acids and alkalis.
Cellulose, as we have seen, is a member of the group of
carbohydrates. Various writers are not agreed as to the
relation of the pectic bodies to this group, some holding
that their reactions separate them from it entirely, while
others contend that they are closely connected with it, if
they do not actually belong to it. It has been suggested
that they are carbohydrates chemically combined with
acids. Like cellulose they yield some form of sugar when
hydrolysed with dilute mineral acids.
All unchanged cell-walls contain a varying quantity of
water, and various views have been advanced as to the way in
which the latter is held by the other constituents. It is
probably not in a state of chemical union, as the quantity
present can be easily increased or diminished.
Naegeli suggested that the wall contained particles of
solid matter or micellce, of crystalline form, the long axis
of the crystals being arranged at right angles to the sur-
face of the wall. He supposed each micella to be surrounded
by a thin film of water. Every cell- wall is thus under
some considerable internal strain, the micellae attracting
each other and tending to squeeze out the water. The
latter, on the other hand, tends to separate the micellae
According to Strasburger, the particles of solid matter
are arranged in a network, water occupying its meshes.
44
VEGETABLE PHYSIOLOGY
On either hypothesis the quantity of water is capable of
considerable increase or diminution, and the wall can be
made to swell up by causing it to imbibe more fluid. This
can be brought about by exposing it to the action of strong
mineral acids, such as sulphuric acid. The water is held,
however, by the solid particles with very great tenacity.
A different view of the composition of cell-wall has been
advanced in recent years by Wiesner. He holds that the
substance of cell-wall as it is first formed consists of rows
of granular bodies which he terms dermatosomes ; these are
connected together by protoplasm which surrounds them.
On this hypothesis the cell-wall
is living while young and grow-
ing. The protoplasm thus exists
between particles of solid matter
and holds water in its substance.
The thickening which always
supervenes to a greater or less
extent upon the first formation
of the cell -wall is brought about
by the protoplasm in a way
similar to that of its original
construction. Layers composed
like the original one are continually secreted by the proto-
plast and are deposited upon its exterior in apposition with
the wall already there. Hence walls which have a perceptible
thickness show a certain stratification, which is most easily
seen in transverse sections (fig. 42). When several such
layers can be distinguished it has been found that pectic
bodies are prominent in the layers furthest from the proto-
plasm and cellulose in those nearest the interior of the cell.
The action of the protoplast is frequently irregular, so that
the thickening layers are often seen as bands of various
form, giving the surface of the membrane particular patterns,
thin and thick places alternating in various ways (fig. 43) .
These are seen most conspicuously in the walls of the
vessels of the wood.
FIG. 42.— THICKENED CELLS OF
WOOD, SHOWING STBATIFICA-
TION. (After Sachs.)
THE SKELETON OF THE PLANT
45
In cell-walls which have undergone considerable thick-
ening the membrane shows a marked differentiation. The
centre of the wall is found to possess a chemical composi-
tion very unlike that of the thickening layers. It marks
£T IV' p W />
St St
FIG. 43.— LONGITUDINAL SECTION OP VASCULAR BUNDLE OF SUNFLOWER
STEM. (After Prantl.)
p', pith ; s, s', spiral vessels ; w', w, wood-cells ; p, p, pitted vessels ;
c, cambium; st, st, sieve-tubes;^/?, fibres; n, bundle sheath;
c, cortex.
off the limits of the cells, occupying the position of the
original thin membrane and looking as if it were the
basis on which the thickening
layers have been deposited. When
a piece of tissue is warmed gently
with a mixture of potassic chlorate
and strong nitric acid, this layer
dissolves and the cells become sepa-
rated from each other. It has by
certain writers been termed the
intercellular substance and by others
the middle lamella (fig. 44). Though
.",.-, , FIG. 44.— THICKENED WOOD-
it IS most easily Seen in thickened CELLS, SHOWING MIDDLE
cells, it is probably not confined to LiM
them, but exists in all cell-membranes, even when they are
46 VEGETABLE PHYSIOLOGY
very young. Treatment with the reagent mentioned will
disintegrate the tissue of even the growing points of stems
and roots, and will cause their cells to become isolated.
A thin layer of this nature must therefore exist even in
the primary cell-wall. It is added to materially, however,
during the growth in thickness of the walls, and in many
cases it can be seen easily under a comparatively low
magnification.
This middle lamella is composed of a material which
is very unlike that of the rest of the cell-wall. It dissolves
readily under the action of pofcassic chlorate and nitric acid
which do not affect the inner layers of the membrane. It
resists completely the action of sulphuric and other mineral
acids which cause the inner layers to swell and ultimately
to dissolve.
Eecent investigations have led to the view that it is
composed of a calcium salt of pectic acid.
Whether the primitive cell-wall is homogeneous or not
is uncertain. If it is, it must be regarded as being formed
of an intimate mixture or perhaps of a compound of cellu-
lose and pectose. At a very early period in its development
the middle lamella becomes differentiated, owing possibly to
the conversion of the pectose into pectic acid and the inter-
action of the latter with some salt of calcium derived from
the cell-sap which infiltrates the wall. The calcium pectate
becomes deposited in this way halfway between the con-
tiguous cells which are separated by the particular mem-
brane in which the change is taking place.
If the cell-wall is not at first homogeneous, we must
suppose that the original thin membrane is composed of
three layers, a central one of calcium pectate, on each face
of which is a layer composed of the mixture or compound
of cellulose and pectose. We never find, even at the
moment of cell division, that the membrane is formed of
calcium pectate only.
It is possible to explain the growth in thickness of the
middle lamella on either hypothesis. It is clear that the
THE SKELETON OF THE PLANT 47
wall is the seat of a considerable chemical change which
affects its whole substance, though the degree, and possibly
the character, of the change may vary in the different
layers of which the wall is built up.
Not infrequently it is noticeable that the intercellular
spaces contain small concretions of various form, which
consist of the same substance as the middle lamella. This
is scarcely to be wondered at, as, when the intercellular
spaces are formed by the splitting of the cell-wall, the
region of the middle lamella, which is the central part of
the membrane, must abut upon the space formed in the
rupture. The calcium pectate which is formed or deposited
in the central region, and which causes the thickening of
the middle lamella, may well exude to a certain extent into
the intercellular space that has been formed.
In such parts of the framework of a well-differentiated
plant-body as need considerable rigidity, a conversion of
cellulose into lignin takes place. This material is found
conspicuously in the walls of wood -cells and sclerenchyma.
It is formed in the substance of the cell-wall, and in par-
tially lignified membranes the lignin can be dissolved out by
appropriate reagents, leaving a cellulose basis. In its
chemical characters lignin differs remarkably from cellulose.
It does not stain blue with iodine and sulphuric acid, but
can be recognised by its property of becoming red when
treated with phloroglucin and a mineral acid, or yellow
with anilin chloride under the same conditions. Its physical
properties are also different, and bear a definite relation
to the function of the tissue as a medium for the transport
of water. It has no extensibility, nor can it absorb water
and swell as can unaltered cell-wall ; on the other hand,
it allows water to pass through it with great rapidity and
ease.
Lignin is probably not a definite chemical compound,
but a mixture of substances successively formed from the
cellulose.
Walls containing it subserve a double purpose. Its
48 VEGETABLE PHYSIOLOGY
physical properties render it particularly adapted to serve
as the material of which the tissues conducting the stream
of water are composed. Its lack of flexibility or extensi-
bility makes it suitable for the securing of rigidity in tissues
or structures needing considerable power of resistance to
winds or storms. It is thus a valuable material in the con-
struction of sclerenchyma.
The protective tissues show a different modification of
the original structure. In the simplest cases we have seen
that the degree of protection secured is slight, and evidently
only transitory. The epidermis is, in these cases, the seat
of the changes which may be observed. The cells show
their walls sometimes very materially thickened on the
exposed sides (fig. 45),
though the thickness
varies in different
cases. Layer after
layer of substance is
deposited upon the
original wall in these
regions, the other
FIG. 45. — SECTION THROUGH EPIDERMIS OF LEAF, t . .
SHOWING THE OUTER WALLS MATERIALLY pai'tS Of it 1'emaining
THICKENED. ... m, ,-1 • i
thin. The thickness
itself secures a certain amount of protection against cold,
but to prevent absorption or dissipation of water or of
gases by these membranes, a chemical change also is
brought about. The outer layers of the wall undergo a
process known as cuticularisation, which generally extends
about halfway through its thickness. This change in
the outer walls of numbers of contiguous cells renders
it possible to strip off from such a tissue a piece of
apparently structureless membrane which is technically
called the cuticle, and which consists of nothing more
than these altered layers of the outermost walls of the
contiguous cells. The alteration of the chemical character
of this membrane in forming the cuticle of the epidermis
is due to the transformation of its cellulose or pectose
THE SKELETON OF THE PLANT 49
constituents into a substance known as cutin. Its
properties are very different from those of the original
cell-wall ; it is but slightly permeable by water, and it is
not easy for gases to pass into or through it. This dif-
ference of physical property is accompanied by characteristic
reactions ; it stains yellow instead of blue when treated with
iodine and sulphuric acid, and becomes brown under the
action of strong alkalis, such as caustic potash.
More efficient and prolonged protection is afforded by
the formation of sheaths of cork, certain layers of cells
being differentiated as meristem tissue for the continued
production of this material. The walls of true cork cells
are thin, but the presence of cutin is a conspicuous feature
FIG. 46.— OUTER PORTION OF CORTEX
OF YOUNG TWIG OF LIME.
per, cork layer.
FIG. 47. — SECTION OF A LENTICEL.
I, lenticel ; per, cork layer.
in them. They are very regular in form, and are closely
arranged together without any intercellular spaces (fig. 46) .
Coming as they do between the exterior and the metabolic
tissue of the cortex of stems, thus cutting off the inter-
cellular space system of the latter from access to the air,
they are usually penetrated by special structures known as
lenticels, which are made up of corky cells very loosely
arranged, and which consequently set up the communica-
tion needed (fig. 47).
In the corky cell-wall the cutin is frequently associated
with a certain amount of lignin.
The thin corky walls possess almost exactly the same
physical properties as the thickened cuticle of the epidermis,
a fact which affords evidence that the primary meaning of
both is the same.
E
50
VEGETABLE PHYSIOLOGY
Like the substance of the middle lamella, both lignin
and cutin are soluble in warm nitric acid containing potassic
chlorate.
In some cases the cell-wall of the epidermal protoplasts
is impregnated with various matters that do not proceed
from its own disintegration. Among these are various
fatty bodies, while wax is sometimes very conspicuous.
The bloom of such fruits as the grape and the plum is
composed of very line waxy particles ; the impregnation
in their case having been so great that certain particles
have passed beyond the walls and formed a layer on the
FIG. 48.— CRYSTALS OP CALCIUM
OXALATE IN WALL OF CELL
OF THE BAST OF EpJiedra.
FIG. 49.— SECTION OF PORTION
OF LEAF OF Ficus, SHOW-
ING CYSTOLITH (cijs] IN
LARGE CELL OF THE THREE-
LAYERED EPIDERMIS (<?jt?\
outer surface. The leaves of the wax-palm show an even
denser deposit.
Mineral matters are also of frequent occurrence in the
cell-wall. The chief of these are salts of calcium, usually
the oxalate, but often the carbonate. Some cell-walls show
a copious deposit of regular crystals of one of these — such
are the cells of the bulb scales of the onion, the fibres of
the bast of Epkedra and others (fig. 48). In many plants
copious deposits of silica are formed in the cell-wall,
especially in the epidermal cells of the Equisetacea, and in
those of the cereal grasses. The value of this deposit to
the plant is not very evident ; it appears at first sight to be
THE SKELETON OF THE PLANT 51
an adaptation enabling the plant to remain upright, but it
is found that its absence does not render the grasses more
liable to fall.
Some cells of the epidermis of certain plants, especially
among the Nettle family, contain curious ingrowths of
cellulose, in which there is a very large deposition of
calcium carbonate. They are known as cystoliths (fig. 49).
The cell-walls of certain regions of particular plants
are transformed into mucilage. This material is especially
prominent in the large brown seaweeds, particularly the
FucacecB, where it forms the bulk of the internal tissue. It
occurs also in certain layers of the seed-coat of such seeds as
linseed, and in certain regions in the sporocarps of Marsilea.
It is of assistance in the dissemination of the spores of
this plant, and possibly has a similar value in the cases of
such seeds as contain it. It differs from cellulose by
absorbing water greedily, and swelling up considerably.
It gives a blue colour with iodine and sulphuric acid as
cellulose does, differing from the latter chiefly in the ease
with which the absorption of water is brought about. It
is not clear at present whether mucilage is derived from
cellulose only, or whether the pectoses take part in its com-
position, though the latter is probable. The gums are closely
related to mucilage, and seem to represent a further dis-
integration of the cell-wall in that direction. Many of the
gums yield derivatives much like those of pure pectic
bodies, which suggests that their affinities are rather with
the latter. In all probability, however, they are all mix-
tures of the two classes of constituents.
We see thus that in the construction of the skeleton of
a complex plant, while its basis -is the cell-membranes of
the several protoplasts, which at first form an almost
homogeneous tissue, not only does differentiation take
place in the direction indicated in the last chapter, but
this differentiation is accompanied by changes in chemical,
physical, and mechanical properties, which fit the definite'
tissues formed for the functions which fall to them.
E 2
52 VEGETABLE PHYSIOLOGY
Temporary structures possess a very different chemical
composition from permanent ones. The transitory cuticle
gives place to the more permanent cork, and this becomes
strengthened by the introduction of sclerenchymatous
elements as the cork formation becomes continuously more
deeply seated. The strengthening tissue varies similarly ;
the walls of collenchyma, though thickened in a particular
way, are not chemically changed in the same manner as those
of sclerenchyma or woody tissue, for their cellulose under-
goes no conversion into, or impregnation with, lignin.
The fibres of the bast differ from those of the wood in the
same particulars.
53
CHAPTER IV
THE KELATION OF WATER TO THE PROTOPLASM OF THE CELL
WHEN we regard the arrangement of protoplasm in the
cells of the plant, or observe the environment of the free-
swimming protoplast, we notice especially its very close
relation to water. The naked zoospore is naturally
saturated with the latter, being in the fullest contact with
it. Unicellular plants which are not actually immersed in
it are generally to be found in more or less moist situa-
tions, where they continually obtain supplies from dew or
rain. Indeed in times of drought when moisture is not
supplied to them they are seriously injured. The young
cell which is clothed with a cell -membrane speedily shows
a tendency to accumulate water in its interior ; gradually
drops appear, which lead ultimately to the formation of a
vacuole, which is always full of liquid. In a plant which
consists of a complex of cells, such a vacuole is found in
every adult cell so long as it is living. The healthy proto-
plasm is thus always in contact with water. Indeed the
molecular constitution of protoplasm, as far as we know it,
lends itself to this relation, for the apparently structureless
substance is always saturated with it. It is only while in
such a condition that active life can exist ; with very rare
exceptions, if a cell is once completely dried, even at a low
temperature, its life is gone, and restoration of water fails
to enable it to recover.
The constancy of the occurrence of the vacuole in the
cells of the vegetable organism is itself very strong evidence
that such cells are dependent upon water for the main-
tenance of life. The cell-wall, though usually permeable.
54 VEGETABLE PHYSIOLOGY
yet presents a certain obstacle to the absorption of water,
and so even those cells which are living in streams or
ponds usually possess a vacuole. Cells without a mem-
brane, such as the zoospores already many times men-
tioned, can more readily absorb water from without, and
hence they are not vacuolated to the same extent as are
those which possess a cell-wall ; indeed many of them
have no vacuole. This cavity when present being always
filled with liquid, the protoplasm of the cell has ready
access to water, as much so indeed as the protoplast which
possesses no cell-wall. The vacuole contains a store which
is always available.
The quantity of water which a vacuole can contain is
very small, and as the needs of the protoplasm are some-
what extensive, a need arises for the continual renewing of
its supply. This is evident when we consider that the
protoplasm draws its nutriment eventually from the water,
and that it must return to it such waste products as it
gives off. Its oxygen must be drawn from the same
source, for this gas can only pass into the interior of a cell
by entering into solution in the liquid which it contains.
In cells which are deep-seated this need can only be
supplied by a slow passage from cell to cell of the gas
which has been dissolved by those abutting upon a free
surface. Similar considerations apply to the elimination
of the carbon dioxide which accompanies the respiratory
processes.
The life of a plant is consequently very intimately con-
nected with the renewal of the water which the cells contain.
Fresh liquid must be taken in, and that which is already
there must be to a certain extent removed ; the plant
demands in fact a kind of circulation of water, and this
becomes the more imperative as the mass of the plant
increases, with the possible exception, however, of those
massive plants whose habitat is marine.
In examining the way in which this circulation is set
up and maintained, it is first necessary to inquire into the
RELATION OF WATER TO THE PROTOPLASM 55
nature of the process by which water makes its entry into
a cell. This is based upon a physical process which is
known as osmosis.
When two fluids of different densities, such as water and
syrup, are separated from each other by a homogeneous
permeable membrane, they will tend to pass through the
latter in both directions till there is a mixture of the two of
equal density on each side of it. WTe shall thus have a stream
of water passing through the membrane to the syrup, and a
stream of syrup similarly passing to the water. The rate
of flow of the two streams will not be the same however,
and the first result will be a considerable increase of the
volume of the liquid upon the side of
the membrane in contact with the syrup,
owing to the greater amount of water
that will have passed through.
A convenient form of apparatus to
exhibit this process of osmosis is repre-
sented in fig. 50. It consists of a
bladder fastened to the end of a narrow
tube which is immersed, as shown, in a
vessel of water. The bladder and part
of the tube are filled with syrup, and the
height at which the latter stands in the
tube is noted. After some time the
contents of the tube will be increased in
consequence of the entry of water being
greater than the escape of syrup, and
the liquid will stand at a higher level
in the tube. If the positions of the water and the syrup had
been reversed, the liquid would have fallen in the tube,
showing that the greater osmotic stream was in the opposite
direction.
The relative difference in the rate of the two streams
will vary with the concentration of the syrup.
Other substances than sugar have a similar power of
setting up osmotic currents, which indeed is especially pro-
FIG. 50. — APPARATUS
TO SHOW THE PRO-
CESS OF OSMOSIS.
56 VEGETABLE PHYSIOLOGY
minent in those which are crystalline in character, though
it is not confined to them. Solutions containing different
substances in equal degrees of concentration do not, however,
possess equal osmotic powers ; each one has its own special
ability which is often spoken of as its osmotic equivalent.
With any particular osmotic substance, however, the osmotic
efficiency varies with the concentration of the solution.
Though the process of osmosis as illustrated in the
experiment just described is far simpler than that which
we have reason to believe takes place in the vegetable cell,
we can apply it to explain the original formation of the
vacuole. Consider the case of a young non-cuticularised
B
FIG. 51.— VEGETABLE CELLS.
A, very young ; B, a little older, showing commencing formation of vacuole.
p, protoplasm ; n, nucleus ; v, a vacuole.
cell of the external layer of a plant which is immersed in
water. It is full of protoplasm, and limited or clothed by a
cell-membrane which is permeable more or less readily by
water. The protoplasm is saturated with water, but there
is no separate accumulation of the latter in its interior.
Part, at least, of the cell-wall is in contact with water on
the outside. The protoplasm is actively living, and in the
course of the chemical changes which are incident to vital
action certain substances are produced by it, which, like the
syrup in the experiment already described, have an affinity
for water, or, to use a more technical phrase, have a fairly
high osmotic equivalent. Water consequently passes into
the cell, at first only in such quantities as to distend it
somewhat. As the process goes on, more liquid is taken up
EELATION OF WATER TO THE PROTOPLASM 57
than can be stored in the molecular interstices of the proto-
plasm. Drops consequently appear, and these gradually
run together until a distinct though small vacuole, and
later a number of such vacuoles, are apparent in the proto-
plasm (fig. 51). These soon run together as the amount of
water still increases, and after a time the protoplasm forms
a layer round the cell-wall, enclosing a single large cavity
in which the surplus liquid is held
(fig. 52).
But, as has already been said, the
process is not a simple physical one.
Though the conditions of the first
experiment are approximated to, they
are not altogether realised. The
syrup in the bladder finds its repre-
sentative in the osmotic substances
formed by the protoplasm and dis-
solved in the water in its substance ;
the water outside the cell is much
the same as the water in the outer
vessel.
erice in the membrane. The bladder
of the experiment is replaced by a
film of cell-wall substance, which we may speak of in
general terms as cellulose, and this is lined by a delicate
coating of protoplasm. This again is not homogeneous,
but has on its surface, which is adpressed to the cell-
wall, a very thin dense layer which forms a kind of
membrane known as the plasmatic membrane. As soon
as the vacuole is recognisable its cavity becomes lined by
another similar membrane, and between the two lies the
nearly homogeneous protoplasm. These plasmatic mem-
branes are immediately formed as soon as the surface of
the protoplasm comes into contact with water. The mem-
brane of the cell, therefore, through which osmosis must
take place, is composed of four different layers. In the
experiment we have assumed that the outer liquid was pure
FIG. 52. — ADULT VEGETABLE
CELLS. x 500. (After
Sachs.)
But there is a great differ- &>ceu.waU; P, protoplasm;
cleu
k k,
cleol
nucleus, with nu-
s s', vacuoles.
58 VEGETABLE PHYSIOLOGY
water ; this is not, however, the case with the fluid in
which the plant is living, for all such water contains a
large number of various inorganic salts dissolved in it,
though of course the concentration of these salts is extremely
small. While all the layers of the cell's membrane are
permeable to water, they are not at all equally so to the
salts which it contains. In such a weak solution these can
pass freely through the cell-wall, but the plasmatic mem-
branes of the protoplasm offer a variable resistance to their
passage further. A further experiment will show a very
important modification of the process depending on this
property of the protoplasm, and demonstrating that the
entry of both water and its dissolved saline contents into the
cell is very largely under the regulation of the latter, when
what is practically a dilute saline solution is presented to it.
Take a cell of the cortex of a plant and put it into
contact with a liquid of higher osmotic power than that
which is contained in its own vacuole ; for instance, a
solution of common salt of about 10 per cent, concentra-
tion. Watch its action on a slide under the microscope,
and let the salt solution be coloured with some vegetable
dye which will not injure the living substance. As the
salt solution reaches the cell, the protoplasm of the latter
gradually retreats from the walls (fig. 53), at first at the
corners and then all round the sides, till it appears as a
rounded or irregular mass in the centre. The salt solution
has abstracted the water from the vacuole, and the proto-
plasm, relieved of the pressure outwards caused by the
liquid in the latter, has shrunk away from the walls. The
outward stream has been accompanied to a certain extent
by an inward one, as in the first experiment. The coloured
salt solution will be visible inside the cell-wall, between it
and the protoplasm. There has been an osmotic stream
therefore through the cell- wall inwards. But it will be
seen that the colour will not penetrate the protoplasm,
which in fact retreats before the coloured salt solution.
The latter has no power to pass the external plasmatic
EELATION OF WATEE TO THE PEOTOPLASM 59
membrane, even in the condition of dilution which must
result from its mixing with water which has been with-
drawn from the vacuole. If now the salt solution is
replaced by water, the latter is gradually attracted again,
of course osmotically, into the cell. It passes through
the whole thickness of the protoplasm, the vacuole is re-
established, and the protoplasm again comes to line the
cell-wall, pressed against it by the water.
The protoplasm by means of its plasmatic membranes
thus can oppose the passage through it of various osmotic
/,
FIG. 53.— CELLS OF PARENCHYMA UNDEBGOING PLASMOLYSIS
a, b, c, d represent successive stages. The dotted area in each cell
represents the protoplasm.
bodies with which it may be brought into contact, though
it allows the water in which they are dissolved to permeate
it freely. In the experiment just described the strong salt
solution failed to pass through the external plasmatic
membrane ; the re-entry of the water into the vacuole
showed that the internal one prevented the osmotic sub-
stances, originally present in the water which the cell
contained, from escaping in the issuing osmotic stream.
These substances must have been left behind, or there
would have been no osmotically active material to draw the
water back, when it was allowed to replace the salt solution
outside the cell.
60 VEGETABLE PHYSIOLOGY
That this behaviour is dependent on the vital activity
of the protoplasm can be shown by repeating the experi-
ment after killing the living substance by a short immer-
sion of the cell in alcohol. Then the process of osmosis
goes on exactly as in the first experiment described. The
salt solution penetrates into the vacuole as if only a cellulose
septum were present, the dead protoplasm exerting no regu-
lating influence.
We must not conclude from this experiment that inor-
ganic salts in all degrees of concentration are kept from en-
tering the cell by the protoplasmic membranes. If extremely
dilute solutions are employed, the protoplasm permits their
passage together with a certain appropriate amount of
water. Similarly, extremely dilute solutions of bodies
found in the fluid of the vacuoles, the so-called cell-sap,
can make their way out of the cells. The protoplasm
exerts a definite regulating influence, however, upon both
the entry and the escape of these different substances.
The modified osmosis which is thus the mode of entry
of water into a cell containing no vacuole, and which
causes the growth or completion of the vacuole after its
first appearance, continues after its formation is finished.
This can be studied most favourably in aggregations of
cells, such as we find in the cortex of a stem or the loose
mesophyll of a leaf, as in such cells there is a more evident
renewal of the water of the vacuoles than in those of
tissues which are surrounded by liquid. In such tissues as
those just mentioned we can demonstrate with ease what
is more difficult to detect in the others, that not only is
water admitted to the cells, but it is also given off from
them. This does not depend on osmosis in the stem or
leaf, but is due to evaporation, which takes place from the
surfaces of the cells abutting on the intercellular spaces,
whence the watery vapour is exhaled through the stomata,
or, in the case of a woody stem, through the lenticels. In a
cell surrounded by water such removal must depend upon
osmotic currents.
RELATION OF WATER TO THE PROTOPLASM 61
This removal of water occasions a need for a continuous
replenishment of the liquid in the vacuoles, which is
brought about by the same modified osmosis which has
been described. We can see that this process must be
continually taking place in a complex of succulent cells.
If we consider two which are contiguous and are separated
from each other by a common cell-wall, it is evident that
unless the proportion of water to osmotic substances in the
vacuoles of both is the same, osmotic currents will flow
from one to the other till this equilibrium is reached.
Any disturbance taking place in one cell of a complex will
hence spread from cell to cell until the composition of the
fluid contents of them all is uniform. When we consider
the differences, sometimes very slight, sometimes more
extensive, which are continually taking place in the meta-
bolic activities of the separate cells of a community, it is
evident that, so long as life lasts, osmotic currents of this
kind must be continually passing from cell to cell in various
directions, and frequently at very different rates.
Evaporation from a cell into an intercellular space
must lead to a certain increase of the concentration of the
solution of osmotically active substance in its vacuole.
This then attracts water from the contiguous cells, and
consequently, independently of metabolic changes affecting
the quantities of such osmotic bodies, evaporation itself
must help in causing movements of water from cell to cell.
The quantity of these osmotic substances which are
present in any particular cell will depend upon the
behaviour of the protoplasm from time to time. Such
substances are usually being continually produced in all
growing cells, and in most others in which chemical
changes are proceeding. Hence such cells are continually
absorbing water, and are consequently so full that a certain
stretching force is exerted on the cell-wall which bounds
them. Cells in such a condition are called turgid, and
the condition itself is known as turgor or turgescence. The
equilibrium which is attained by such a cell is reached
f>2 VEGETABLE PHYSIOLOGY
when the distension caused by the entering osmotic stream
is balanced by the elastic recoil of the extensible cellulose
wall. In some cases the tension set up in a tissue by the
turgescence of the cells is sufficient to force the water, by a
process of nitration, through the walls of the outermost
ones, so that it escapes in drops or in a slow stream. This
may often be seen on the edges or apices of blades of grass
in the early morning. It is of great use also in forcing-
water into the axial woody cylinder of roots, as will appeal-
later. Occasionally the turgescence becomes so great as to
lead to rupture of the cell-walls, as is the case in some
pollen grains, and sometimes in certain fleshy fruits.
That the condition of turgescence in cells is attended by
a stretching of the cell-walls can be seen by taking a piece
of a plant which is turgid, such as the stalk of a rhubarb
leaf, and, after carefully measuring its dimensions, steeping
it for some time in a ten per cent, solution of common salt.
On removing it, it will be found to have become flaccid,
and a remeasurement will show that both its length and
thickness have diminished. Turgescence is not, however,
due simply to physical causes ; the protoplasm which lines
the cells has a regulating influence over the passage of the
water into and out of them. When a turgid pulvinus of
such a plant as Robinia or Mimosa is stimulated by rough
handling of the leaf, the latter falls backwards from its ex-
panded position, and the fall is found to be due to the escape
of water from the cells of the lower side of the pulvinus.
The original state of equilibrium has been disturbed by the
shock to the protoplasm administered by the stimulation,
and the latter allows or compels the water to pass outwards.
The active influence of the protoplasm is seen also in
another class of phenomena. Certain structures known as
nectaries occur conspicuously in many flowers. They are
aggregations of cells of a particular kind which exude a
sugary fluid upon their surface. The liquid in the cells
contains a little sugar, and this weak solution is capable
of passing through the protoplasm, not by osmosis, but by
EELATION OF WATER TO THE PROTOPLASM 63
a kind of filtration. Its concentration is usually increased
by subsequent evaporation of the water in which it is dis-
solved, so that the secretion when collected has a distinctly
sweet taste. When the petals of certain flowers bearing
these nectaries are cut off, and their cut ends immersed in
water, the glands continue for some time to exude the
nectar. There can be no question here of a gross nitration
of water under pressure through the tissue, as there is no
such pressure acting on the base of the cut petal. The
protoplasm causes a stream of water to flow into the cells
of the gland by producing osmotic substances inside them,
in this case chiefly sugar. The turgescence thus set up in
the gland cells exerts a strong hydrostatic pressure on the
limiting membranes of these secreting cells, which ultimately
so stimulates the protoplasm as to cause the sugary
solution to exude upon their free surfaces. We can dis-
criminate between two forces at work in the excretion of
the nectar. The absorption of water by the gland cells is
due to osmosis ; the excretion from them on to the exterior
of the gland is more a question of filtration under pressure
from the turgid cell. This is shown by the fact that if the
surface of the gland is carefully dried, the exudation
shortly recommences. Osmosis is not possible under these
conditions. If the gland is killed by alcohol, the sugar
already there is retained in the cells, and no exudation of
nectar, or even of water, takes place.
The vital activity of the protoplasm is thus seen to be
intimately connected with the presence of water in its
substance. The importance of the ready access of the
latter is seen further from other considerations. We have
incidentally alluded more than once to the fact that the
liquid concerned in these osmotic currents is not pure
water only, but should rather be regarded as an extremely
dilute solution of various salts. Though the protoplasm
opposes the passage of anything like a strong solution of
inorganic salts, it allows very dilute ones to enter the cell;
much as it does pure water. In this way the slowly dif-
64 VEGETABLE PHYSIOLOGY
fusing stream brings to the protoplasm of each cell the
inorganic materials which are absorbed from the earth,
and enables the matters elaborated or formed from them
by the protoplasm to pass from cell to cell. The feeding
or nutrition of the various cells, together with the con-
struction of the substances which minister to that nutrition,
is thus dependent on the transit of fluid about the plant
in the way described. The access of various gases is
similarly made possible, for these are dissolved in the liquid
stream. The oxygen upon the presence of which life depends
is thus transported to each cell, and the carbon dioxide
of respiration is removed from the seats of its liberation.
The condition of turgescence is necessary also for growth,
and for various movements of different parts, enabling
them to adapt themselves to varying conditions of their
environment. Some plants, particularly those which are
aquatic in habit, and such parts of terrestrial plants as
contain but little woody tissue, are dependent on the
turgescence of their cells for the rigidity which enables
them to maintain their position in the medium in which
they live. The maintenance of the turgid condition of
the cells is further of the highest importance in enabling
the interchange of water between contiguous cells to take
place as freely as possible, and without intermission.
Flaccid cells do not effect such interchange with sufficient
readiness. Flaccidity of an organ is attended by a partial
collapse of the tissue, which involves a diminution of the
volume of its intercellular spaces, and hence often a serious
interference with its processes of gaseous interchange,
particularly respiration.
The importance of the water supply, and indeed its
necessity to the plant, explains the existence of certain
subsidiary mechanisms for its absorption and storage
which are occasionally met with. These will be considered
in detail in a subsequent chapter, but a few of such
adaptations may be noticed here. We frequently find
particular aggregations of cells set apart for storage of
RELATION OF WATER TO THE PROTOPLASM 65
water. The epidermis of certain parts frequently subserves
this purpose, and many plants possess a considerable
development of aqueous tissue, variously disposed, which
forms a similar storehouse. The cells of this tissue contain
little else than water, and thus serve to supplement the
vacuoles of the ordinary cells. In plants that inhabit dry
arid soils such as sandy deserts there are often other
adaptations relating to water storage. Such plants are
often covered with large bladder-like hairs which hold a
considerable quantity of liquid. Plants which are exposed
to conditions threatening too copious evaporation are gene-
rally furnished with a very prominent cuticle tending to
check undue escape.
66 VEGETABLE PHYSIOLOGY
CHAPTEE V
THE TKANSPOBT OF WATER IN THE PLANT
WE have seen that it is necessary for the life of a plant
that all its living cells shall be freely supplied with water.
According to the habit of life of plants the mode of supply
must necessarily vary. Those which are so constituted
that water finds free access to all the cells, such as the
unicellular or filamentous Algcz, which live in streams,
pools, &c., present no difficulty, as osmosis can go on freely
in each cell, water entering its vacuole from the exterior.
Sturdier plants of aquatic habit are almost equally easily
supplied ; the water enters by osmosis into the vacuoles of
the epidermal cells, the walls of which in these plants
are not cuticularised, and from them it can pass from
cell to cell all over the plant-body. No force in addition
to osmosis is necessary in these undifferentiated plants.
Others, which have a terrestrial habitat, from the nature of
their environment require a more elaborate mechanism
which is found, as we have already pointed out, in the
well-differentiated system of conducting tissue, composed
largely of lignified vessels, fibres, and cells. Throughout
all such plants a stream of water passes, entering at the
roots, passing along the woody axis, and so rising up the
stem into the leaves, where a very large part of it is
evaporated. This stream of water is often known as the
ascending sap. In addition to this comparatively rapid
stream, slow currents of diffusion from cell to cell are also
maintained, as in the plants of humbler type. These
diffusion currents, depending mainly on osmosis between
contiguous cells, have not the definite direction of the
THE TRANSPORT OF WATER IN THE PLANT 67
rapid current, and play quite a subordinate part in the
supply of the whole plant with water. They are, however,
supplementary to the ascending sap, and effect interchanges
in regions which the latter does not immediately reach.
The cortex of the axis of the plant is especially dependent
upon them, as various mechanisms exist in the different
regions of the stele to guard against too free an escape of
water from its tissues into the cortex.
Except in some special cases the water which passes
through the body of an ordinary terrestrial plant is
obtained from the soil in which its roots are embedded.
The soil itself is composed of minute particles of inorganic
matter of very different degrees of solubility, derived origi-
nally from the breaking down of rocks, together with decay-
ing animal or vegetable matter mixed with the inorganic
constituents. This organic matter is known as humus and
is of very varied composition. The soil thus consists of a
loose matrix of granular character, the interspaces of which
are normally filled with air. The air is in most cases
mixed with a certain quantity of carbon dioxide which is
being evolved from the humus constituents of the soil, and
which is slowly exhaled from the surface. The interspaces
are capable of containing varying quantities of water ;
indeed the soil may be so saturated with it that they are
all full. We find soils of all conditions in this respect,
from the dry sands of deserts to the mud of bogs. The
water may be held with greater or less tenacity, clays and
sandy soils affording instances of two extremes in that
particular. When the interspaces of the soil are filled
with water, the plants which it is supporting are very
unfavourably placed for absorbing the liquid. By the
excess of water their roots are deprived of the air which they
need for purposes of respiration ; their structure does not
enable the absorption of water to take place all over their
surfaces, as their external cells are more or less cuticular-
ised ; they are consequently hindered and not helped by the
superfluity of liquid. When a soil is properly drained, its
F 2
68
VEGETABLE PHYSIOLOGY
interspaces are filled with air, and a delicate film o
surrounds each of its particles and adheres closel
This water, often spoken of as hygroscopic water,
source of the plant's supply. The presence of ai
interspaces supplies the wants of the root and frees
the difficulties which have been pointed out.
The hygroscopic water adheres so closely
particles of the soil that it escapes ordinary obs:
when, however, soil that has been allowed to d
ordinary temperature till its interspaces are
empty, is exposed to a heat approaching that
water, a considerable quantity of vapour is gi
to the volatilising of the hygroscopic films.
The difficulty of the entry of the water in
of the outermost layers of the young roots
development of a specia
mechanism upon them. 1
form of a number of delica
of the external cells, whi <r
thin-walled hairs (fig. 54,
not distributed all over Wf ^Ux
the young rootlets, but 8
a particular region not f
apex. As the delicate b
root grow, the root-hairs g}
more being formed contr
the same distance from i
is thus a continual renew^
tion of hairs, which is mt
as the root system exten '• ^es
functional. The interspaces of the soil
are .penetrated by the young roots, the i> aner of whose
growth involves a very close approximation of their sub-
stance to the surface of the particles of w ^oh the soil con-
sists. The delicate hairs standing out at right angles to
the surface of the roots are consequently brought into very
close and intimate relations with these particles and with
FIG. 54. — ULTIMATE
BRANCHES OF A
ROOT, SHOWING PO-
SITION OF ROOT-
HAIBS.
7 ~E TEANSPOET OF WATEE IN THE PLANT 69
the Jm of hygroscopic water which surrounds them. In
so- Ceases the pressure between the two is so close that
t1 .rticles become embedded in the membrane (fig. 55).
rl . -^groscopic film of water is thus separated from the
; of the root-hair by a most delicate pellicle of cell-
bstance, lined by an almost equally delicate layer of
ism. The vacuole of the hair contains a somewhat
U.-sap, by virtue of which osmosis is set up; the
{equivalent of the acids of the
; considerable, the cell quickly
turgid and distended, such
-e continuing so long as the
remain favourable. The root-
^ry numerous, and their united
'jies a considerable accumula-
fr in the cortex of the root,
"_, into the cells of this region
^rough the base of the hair.
mtt ;.•:
wall
prot<
acid v
osmot
sap be ;
become' ,
turgea
conditi
hairs ar»
action c
tion of w
for it pas
by osmosi,
This, beinj; )0ne of the cells of the ex-
ternal layei. i impinges upon one or more
of the cortifa cells, which have a similar
reaction tc ;/at of the root-hair itself.
Osmotic CL v\nts are thus set up from
every hair, ^ a gradual accumulation
of water tak, - place in the cortex of the
young root, rmaking all its cells turgescent and causing
a considerably [^hydrostatic pressure in the tissue. This
turgescence ^(ith its consequent pressure extends all along
the axis of th< ^ung root, though it is originally set up only
by the region ^hich is clothed by the absorbing hairs.
The central portion of the axis of the root is occupied
by a cylindrie /, mass which extends throughout its whole
length, and wi,ieh is known as the stele (fig. 56). It is
generally mark-jd off sharply from the cortex, the cells of
whose innermost layer, the endodermis, are often peculiarly
thickened. This thickening is not, however, usually very
marked in the region of absorption. At certain places
FIG. 55. — ROOT-HAIR
IN CONTACT WITH
PABTICLES OF SOIL.
70 VEGETABLE PHYSIOLOGY
round the periphery of the stele of the root, the woody
strands (tig. 56, Sp) may be seen. These are in contact
with the succulent and turgid parenchyma which has been
tilled with water in the wray described, and consequently
the hydrostatic pressure which has thus been set up is
brought to bear upon the walls of the woody vessels which
constitute the greater part of those strands. These form
the lower portions of continuous open, or nearly open, tubes,
which extend from the roots to the leaves ; at the time
when the absorption of the root-hairs and cortex is greatest
these vessels are empty, or nearly so, and the effect of
FIG. 56. — SECTION OP BOOT, SHOWING ROOT-HAIRS ABUTTING ON THE PAREN-
CHYMA OP THE CORTEX, AND THE WOODY STRANDS, Sp, OF THE STELE. (After
Kny.)
the hydrostatic pressure on their walls is to force the
water from the turgid cortex into the walls and cavities of
the vessels. How the water is distributed is not fully
known ; we have seen that lignified cell-walls have a
certain power of taking up water, and of passing it on with
considerable rapidity, so that part of it may be expected to
remain in the walls. Part, however, passes through into
the cavities of the vessels, and in the early part of the
year, before the leaves of the plant expand, they thus
become filled with liquid. This filtration into the vessels
tends to relieve the pressure in the cortex, and additional
THE TEANSPORT OF WATER IN THE PLANT 71
water can then be absorbed from the soil as before. The
consequent increase of the turgescence is followed by further
filtration into the vessels, and these two factors continually
acting together, the water is made to rise gradually in the
axial stele. The root-hairs and the turgid cortex, in fact,
exert in this way a kind of continuous pumping action,
forcing it along the axis. The force, which is the expres-
sion of the elastic recoil of the cell-walls of the over-
distended cortical cells, and which is brought to bear upon
their fluid contents, squeezing a quantity of liquid through
the cell-walls into the vessels, is known as root-pressure,
and is one of the main factors in the transport of water
through the plant.
The turgescence not only leads to the rise of the
sap in the axial stele, but it
spreads throughout the whole of
the cortical tissue of the plant,
stem as well as root, reaching
indeed every cell into which
osmotic diffusion can take place.
The action of the root-hairs is
thus responsible not only for the
rapid ascent of the sap, but also
for the maintenance of turgidity
outside the region supplied by
the ascending stream.
The stele of the root is
directly continuous with that of
the stem, and though the dis-
position of the woody elements
is somewhat different in the two
regions, there is no doubt that
they also are continuous throughout (fig. 57). The stream
of water consequently passes up the woody tissue of the
stem so long as the cells are living. The stream in
young plants passes along the whole substance of the wood;
which in most cases forms a central mass of some size.
FlG. 57.— DlAGKAM SHOWING
COURSE OF THE VASCULAR
BUNDLES IN A DICOTYLE-
DONOUS PLANT.
VEGETABLE PHYSIOLOGY
In herbaceous plants the bundles do not usually form a
continuous cylinder, but are more or less isolated in their
course. In old trees the water-conducting area is limited
to the outer regions of the central woody mass, which are
known as the alburnum or sap-wood. The central portion
of the wood is dead, and the cell -walls are often very
much altered in chemical composition. This region is
known as the duramen or heart-wood ; it takes no part in
the conduction, the tissue always remaining dry.
The vascular bundles are seen to be continuous from
the axis to the leaves, where they are no longer found
arranged in a cylindrical manner, but are disposed in
various ways as a much-branched net-
work (fig. 58). The separate ramifica-
tions are known technically as veins,
and they are distributed in the various
ways known, largely through the method
of branching of the leaf axis. The
latter, however, with very rare excep-
tions, is flattened or winged throughout
the whole or part of its length, and the
wings or flattened portions are supplied
with veins continuous with those of
the branched or unbranched axis. The
vascular tissue, therefore, if traced from below upwards, is
seen to exhibit a separation of its constituent bundles, which
continually appear to subdivide until they form a series of
delicate ramifications of considerable tenuity which per-
meate the whole of the flattened portions of the leaves or
other parts. The tenuity of the ultimate endings of the
vascular bundles is attended with certain changes in the
character of the constituent cells, but they remain woody
and irregularly thickened as they are lower down in the axis.
In the leaves these endings of the bundles, which are some-
times free, and sometimes disposed in the form of an open
network, are surrounded by delicate parenchymatous tissue,
whose cells are in immediate contact with the woody ele-
FIG. 58. — VASCULAR
BUNDLES (VEINS)
OF LEAF.
THE TEANSPOET OF WATEE IN THE PLANT 73
ments, as they are in the root (fig. 59). These delicate cells
are also in contact with the special parenchyma of the leaf,
which is in part very loosely arranged and provided with a
great development of the intercellular space system (fig.
60), which we have seen to be characteristic of the whole
FIG. 59. — ENDING OF A FIBRO-VASCULAK BUNDLE IN THE
PABENCHYMA OF A LEAF.
of the tissue of the plant. The cells abutting on the
bundles are filled, like the root-hairs arid the cells of the
cortex, with a watery sap which contains substances possess-
ing a relatively high osmotic equivalent. The woody
FIG. 60. — TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING
THE INTERCELLULAR SPACES OF THE INTERIOR, x 100.
elements of the veins are not completely empty ; their
walls, at any rate, are saturated with the water ascending
from the roots. We have consequently here a resumption'
of the osmosis which we noticed to play so conspicuous a
74 VEGETABLE PHYSIOLOGY
part in the original absorption of water. The water is
drawn from the woody elements into the parenchyma of
the leaf, and as it passes from cell to cell the leaf tissue is
made turgescent. The turgescence is very largely due to
the ascending stream, whose progress we have traced ; at
the same time we must remember that the turgid cortex of
the root is continuous through that of the stem with the
soft tissues of the leaves, and hence the slow movement of
diffusion assists in its maintenance. In plants which have
but little woody tissue, such as the greater number of
herbaceous annuals, this slow movement plays relatively a
more important part than in those trees which have a
conspicuously woody trunk.
As we have seen, the turgid mesophyll tissue has a
great part of the surface of its cells abutting on the inter-
cellular spaces of the leaf. The cortical cells of the axis are
also similarly placed, though the spaces are much smaller
in that region. The intercellular spaces of the plant are
in communication throughout, and the cells which abut
upon them are in most places, and particularly in the
leaves, furnished with very delicate cell-walls which readily
allow a process of evaporation to take place, watery vapour
passing into the passages. The whole intercellular space
system thus becomes charged with vapour, the process of
evaporation from the cells being, however, much more
marked in the leaves, owing to the greater development of
the spaces there. At particular spots in the leaves and
other green portions of the plant, these intercellular spaces
or canals communicate with the external air by means of
small openings or crevices in the outer layer of cells, which
are known as stomata (fig. 61). Each stoma is surrounded
by two cells of peculiar shape, known as guard-cells, which,
by being approximated to each other to a greater or less
degree, enable the extent of the communication to be
varied from time to time according to the conditions of
the plant. The ultimate escape of the watery vapour from
the interior of the plant is subject by means of these
THE TRANSPORT OF WATER IN THE PLANT 75
stomata to a very delicate regulation. So long as the
apertures are open the watery vapour diffuses outwards
into the external air. We may thus have a copious
exhalation taking place from the surfaces of the leaves and
other green parts, which plays an important part in causing
the flow of water through the plant. This evaporation or
exhalation from the surface is known as transpiration ; it
will be discussed more fully in a subsequent chapter.
Little or no evaporation takes place from the surface
FIG. 61.— THREE STOMATA ON THE LOWEB SURFACE OF A LEAF, SHOWING
DIFFERENT DEGREES OF CLOSURE.
of the epidermal cells of the leaves, which have their outer
walls generally cuticularised to a greater or less extent, the
cuticle offering considerable resistance to the passage of
water or watery vapour through them in either direction.
The escape of watery vapour by transpiration is supple-
mented in some cases by an actual excretion of water in
the liquid form. This happens when the hydrostatic pres-
sure is very high at times in herbaceous plants, water being
forced out at the tips of the leaves. It is not infrequently
76 VEGETABLE PHYSIOLOGY
seen in the case of grasses, the edges or apices of whose
leaf-blades may show drops of liquid standing upon them
in the early morning. Similar drops are often to be seen
on the surfaces of the leaves of Alchemitta when they have
ceased to transpire during the night, while the absorption of
water by the root has continued actively. The escape of
liquid in this way is due to a filtration similar to that by
which the water is forced into the woody elements of the
stele of the root, as previously described.
A subsidiary mechanism allowing the escape of watery
vapour from the cortex of stems and roots is provided by
the lenticels. We have seen that these are loose aggrega-
tions of corky cells which are developed in connection with
the sheaths of cork that form part of the secondary
tegumentary protective tissue of a thickened axis. They
are not, however, so intimately connected with evaporation
as the stomata, probably being more concerned with the
aeration of the tissue (fig. 89).
The stream of water thus passing through the plant
has a very important influence upon its development. We
have seen how important a factor in its growth is the
maintenance of a condition of turgescence, which in turn
depends on the constant absorption of water to take the
place of that removed by evaporation. The quantity pass-
ing is correlated with the amount of leaf surface which the
plant possesses ; where there is a large leaf area there is
copious transpiration ; this necessitates a large path for the
ascending stream, and a consequent development of the
axial portions of the plant.
The greatest increase in the number of the proto-
plasts takes place at the so-called growing points which are
situated at the terminations of the twigs, and which give
rise continuously to additional leaves and branches. The
development of new material of this kind and of the new
protoplasts which they contain is largely dependent upon
another feature of the water supply to which attention
has already been called. A considerable part of the
THE TRANSPORT OF WATER IN THE PLANT 77
material from which the food of the plant is constructed
is absorbed from the soil in solution in the water, and
is transported by means of this stream to the regions of
cell-formation. The fact that the quantity of the nutri-
tive salts in the water is extremely small is a further
reason for the transport of such large quantities of
water as pass through the plant ; for by the gradual con-
centration of the solution in the cells of the leaf enough
new material can be obtained by the protoplasts for the
construction of the food necessary for their nutrition,
growth, and multiplication. Where there is a large flow
of water, as in a tree, there is a continuous formation of
new cells and of the various mechanisms their life demands ;
where the transpiration is but slight, as in a Cactus, or
where the supply of water is limited, as is the case with
such plants as grow in deserts or in rocky situations, there
is but little formation of new substance.
78 VEGETABLE PHYSIOLOGY
CHAPTER VI
THE TRANSPIRATION CURRENT. ROOT PRESSURE.
TRANSPIRATION
IN terrestrial plants, so long as circumstances are favour-
able to the vital activity of the organism, we have, as we
have seen, a stream of water passing from the roots
through the axis to the green twigs and leaves, where the
greater part of it is evaporated. This stream, which we
have spoken of as the ascending sap, is often called the
transpiration current. Its path through the axis of the
plant has been determined to be the^ylenrvessejs, which are
in complete continuity from the young rootlets to the veins
of the leaves.
In thick tree-trunks, in which the wood can be seen to
consist of alburnum and duramen, the stream is confined
to the former. Proof of this can be obtained in various
ways. If an incision is made all round the trunk of a tree
and a ring of tissue removed, everything being cut away
down to the outermost ring of wood, the leaves of the parts
above the wound continue to be turgid. If, on the other hand,
the woody cylinder is cut through, while the continuity of
the cortex and that of the pith are allowed to remain
intact, the leaves very speedily droop and become flaccid.
If a plant in a pot is watered with a solution of a dye
which has no noxious action on the protoplasts, the colour-
ing matter is absorbed in the liquid which the roots take
up, and its progress can be traced by a subsequent micro-
scopic examination of the various tissues of the axis. The
colouring matter will be found to have stained the wood
for a considerable distance ; in the case of a small plant,
THE TRANSPIRATION CURRENT 79
indeed, it will be coloured quite up to the veins of the leaves,
while the pith and cortical tissues will remain unstained.
An isolated branch can be taken as the subject of the experi-
ment, its cut surface being placed in a solution of the dye.
The dye in these cases passes with the current of water,
as may be seen by the difference in its rate of passage when
transpiration is vigorous, and when from severance of the
leaves of a branch it can penetrate only by diffusion. ^
A good deal of controversy has been excited with refer-
ence to the manner in which the transport of the water in
the wood takes place. ^Sachs^originally suggested that the
path was altogether the walls of the cells, and that their
cavities were empty. This view was based partly on the
fact that the vessels undoubtedly contain a quantity of air
during the period of active vegetation, and that this air is
at a less pressure than that of the atmosphere. Another
reason advanced for it was based on the nature of lignin
and its relation to water. While refusing to absorb much
water and swell as cellulose can be made to do, lignin can
contain a certain quantity, which it will part with very
easily. On this view the walls of the lignified vessels may be
regarded as a column of water held together by the mole-
cules or micellae of lignin. A very little water removed
from the top of such a column would be immediately
replaced from below so long as a supply existed there.
Such a remarkable conductivity, however, is probably
not possessed by the walls of the vessels. Many observa-
tions made in recent years tend to negative this view, and
to support the hypothesis that the water passes in the
cavities of the vessels. Sachs's opinion that these are
always free from water during active transpiration has
been shown not to be well founded, for various observers
have proved that their cavities are occupied by a chain of
water-columns and air-bubbles, the air having been
originally absorbed from the intercellular space system.
If the end of a transpiring branch is injected for a short
distance with a viscid fluid, which will penetrate the
80 VEGETABLE PHYSIOLOGY
cavities of the vessels, and subsequently solidify, these
passages can be occluded for a distance of a few centi-
metres. (Jelatin or paraffin can be used for the experi-
ment, being injected at a moderately low temperature such
as will not injure the vitality of the tissue. If after it has
solidified a fresh surface is made by a clean cut a very
short distance from the end, and the branch immersed in
water, the leaves very soon flag, even if some pressure
is applied to the water in contact with the cut surface. If
the path of the liquid were the cell-walls, no obstacle being
offered to the transfer of water to them, the upper portions
ought to remain turgid. The experiment shows that the
normal channels are blocked by the paraffin or gelatin
used, and flagging results.
A similar demonstration that the water passes by the
cavities or lumina of the cells is afforded by the experi-
ment of compressing the stem in a vice ; if the pressure
is carried so far as partially or entirely to obliterate their
cavities, the rate of flow is materially interfered with.
The progress of a dye injected into the surface of a cut
branch also points to the same conclusion. If such a stain
as fuchsin or eosin, which colour wood very rapidly, is
forced up into a stem and sections made almost immedi-
ately, the lignified walls will be found to be in process of
staining, and the colour will be seen to be deepest on the
side of the wall abutting on the lumen, often only penetrating
partly through the thickness. If the wall itself were the
path of the pigment solution, its thickness would be stained
uniformly as far as^the dye penetrated at all.
The rate at which the transpiration current naturally
flows varies a goo</ deal, plants showing differences among
themselves as to facilities of transport. In a fairly vigorous
tree it may be taken to be about 1-2 metres per hour,
though in some plants it has been observed to be three
times as rapid. In other cases as low a speed as -2 metre
per hour has been found. It is a little difficult to measure
in most cases ; the plan generally adopted has been to
THE TRANSPIRATION CURRENT 81
immerse the cut ends of branches in a solution of such a
dye as eosin, and notice how far the dye penetrates in
some unit of time. The objection to this method is that
very frequently the water of such a coloured solution will
travel faster than the dye dissolved in it. Sachs used
instead a solution of a salt of lithium, which he found was
free from this objection. He detected the rate of progress
of the lithium by means of spectroscopical examination,
ascertaining how far the metal could be traced in the stem
when pieces were cut out and burnt after a definite time,
during which absorption had proceeded.
The causes of the transpiration current are not fully
known, but there is no doubt that it is due to the co-
operation of many factors, not one of which by itself is
sufficient to account for it. Two of the main influences
which are at work have been incidentally alluded to, which
must now be discussed in greater detail. These are tjie
constant jumping action of the cortex of the root, giving
us the force known as root-pressure, and the modified
evaporation into the intercellular spaces, and its exhala-
tion from the surfaces of the green parts of the plant,
which we have spoken of as transpiration. Recent
investigations make it probable that we must add to these
the force of osmosis in the parenchyma of the leaves, which
apparently brings about the passage of the water from the
veins into the cells of the leaf -substance.
Besides these, other factors have been held to co-
operate, though much less certainly than they. - The walls
of the vessels having an extremely narrow calibre, capil-
larity has been suggested as playing a part. This cannot,
however, have much effect in a system of closed tracheids,
like those of the secondary wood of the Conifers, which,
nevertheless, conduct the water. It has been thought
that the living cells of the parenchyma, which abut upon
the woody tissue of the stele, may play a part similar to
the pumping action of the root. The medullary rays of
the stele in tall tree trunks have been held to play a
82 VEGETABLE PHYSIOLOGY
similar part. Against this theory we have the fact that, if
the transpiration current is made to contain substances
that are poisonous to the living cells, and the latter are
consequently killed, the current still goes on. Considerable
lengths of a stern have been killed by heating it to the
temperature of boiling water, and the dead part has proved
to be no obstacle to the transport. Nor do differences of
gaseous pressure within and without the plant, or at
different portions of the axis, explain the matter more
satisfactorily.
ROOT-PRESSURE. — We have seen how the absorption of
water osmotically from the soil by the root-hairs leads to a
great turgescence of the tissue of the cortex of the root, not
only in the regions of absorption but along the whole length
of the younger portions, which turgescence exerts consider-'
able pressure on the sides of the vessels and tracheids of
the xylem of the stele. By this means water with various
salts and other constituents in extremely small quantity
is forced into the nbro-vascular tissue. The process is not
a purely physical one of nitration under pressure, but is
regulated to some extent by the protoplasm of the cells which
abut upon the xylem. When these are distended to their
greatest capacity, their protoplasm appears to be stimulated,
perhaps by the very distension, and in consequence to
allow water to transude through its substance. This mode
of response to stimulation is not infrequent in vegetable
tissues ; indeed it appears to correspond to the response of
a muscle to stimulation by the process of contraction. We
must not push this comparison, too far, for the protoplasm
of the vegetable cell seems to respond not by contracting
but by modifying its permeability, so that the hydrostatic
pressure existing in the cell is able to force the water
through the living substance with greater facility than it
could before the stimulus was appreciated. By thus modify- /
ing the turgor of the cell, the protoplasm relieves itself of
the over-distension, and we get an intermittent pumping
action set up, which has a certain rhythm. By it large
EOOT-PEESSUEE 83
quantities of liquid are continually being forced into the
axial stele. This rhythm, which is comparatively rapid,
must not be confused with another rhythm which is much
more gradual, and which constitutes what is called the
periodicity of the root-pressure.
When transpiration is not taking place, the water may
accumulate in the vessels, and its presence can then very
readily be demonstrated, and the force of the root-pressure
measured. If a vine stem is cut through in the early
spring before its leaves have unfolded, a continuous escape
of water takes place from the cut surface, and the vine is
said to bleed. The phenomenon is not peculiar to the vine,
but is exhibited by most other terrestrial plants.
In plants which have a large woody system the accu-
mulation of water in the vessels can only be demonstrated
while the absence of leaves renders transpiration impossible.
Many herbaceous plants show a similar phenomenon daily,
owing to the intermission of transpiration during the
night. In these cases it is not necessary to cut the axis
at all ; the accumulation of water extends to the whole of
the plant. In the early morning the plants show a certain
exudation of water from the tips or apices of the leaves,
drops accumulating on their surfaces. Alchemilla and
Tropceolum especially display this phenomenon, which is
due to' the over-turgescence of their tissues, brought about
by the pumping action of their roots.
This phenomenon of setting up a hydrostatic pressure
causing an exudation of water is not confined to roots.
Whenever the active living cells of the stem, or even of
the leaves, force water into the vessels, the same exudation
can be noticed. It can be shown by burying the cut ends
of young stems of grasses in wet sand ; after a time drops
of water ooze out of their projecting upper ends. If the
leafy branches of some trees are immersed in water so that
only the cut ends project, the leaves can absorb water and
force it through the stem, so that an exudation after a time
can be noticed to take place from the cut surface which is
G 2
VEGETABLE PHYSIOLOGY
not immersed. A similar exudation can be caused to take
place from the hyphae of fungi and from the tissues of
mosses.
We must, however, be cautious not to attribute every
escape of water from a plant to this cause. When a tree
trunk is wounded or cut on a warm sunny day in winter,
there is frequently an exudation of water from the wound.
This is generally due to purely physical causes, being-
brought about by the expansion of
the air which is contained in the
vessels of the wood. It can be
artificially produced at any time
in winter by warming a freshly
cut piece of wood ; and its cause in
this case can be seen to be physical
by the fact that as the wood cools
the water in contact with the cut
surface is again absorbed, owing to
the contraction of the air, which
was expanded by the warming.
To measure the root-pressure
in a plant the apparatus shown in
fig. 62 may be used. It consists of
a T-piece of glass tubing (B), which
is fastened by indiarubber rings
FIG. 62.— APPARATUS FOR THE (r) to the top of a Cut stem, Such
ESTIMATION OF ROOT-PRES- ,, « TT i* n m xi -j
SL.BB. as that of Hehanthus. To the side
arm of the tube a manometer (q\,
with a capillary bore, is attached by a tightly fitting cork
(&), and the T-piece is filled with water from the upper end
(&'). Mercury is poured into the manometer till it stands at
a level a little below the cork k, and the aperture k' is
then tightly closed. As the root continues to take up water,
it forces it into the tube E, whence it overflows into the
proximal arm of the manometer, causing the mercury in
the two limbs to be at unequal levels. By the displacement
of the mercury thei-force of the root-pressure can be
EOOT-PEESSUEE 85
estimated. A variation of the apparatus can be used, in
which the manometer is replaced by a glass tube bent at
right angles. The water will be forced through this, and
can be collected in a suitable receiver, and its amount
ascertained.
In performing the experiment it is best to allow the
apparatus to stand for some time before closing the tube
at kf, as, if the plant is taken while transpiration is proceed-
ing, the vessels of the stem will contain air at a certain
negative pressure, and a certain amount of water will be
sucked back until the vessels are full. As soon as this
condition is reached, the pumping action of the roots will
become evident, and the root-pressure will make itself
obvious.
The root-pressure of various plants has been measured
by different observers ; an idea of its amount may be
gathered from the fact that a medium-sized Fuchsia in a
pot has been found able to send a column of water up a
tube of the same diameter as the stem to a height of
twenty-five feet.
The activity of the roots will depend upon various
conditions, of which temperature, both of the air and of
the soil, is one of the most important. The exudation of
water has been observed at temperatures as low as freezing
point, but most plants will not show it below about 5° C.,
and as the air becomes warmer the quantity of water given
off increases. Warming the soil of the pot in which is the
plant under observation also increases the flow.
Of other influences which exert an effect upon the
activity of the roots may be mentioned oxygen^ Like all
other vital actions, the absorptive power of the root-hairs
depends upon their being in a healthy condition, and this
cannot be maintained in any protoplast without the due
performance of respiration. The character of the soil
must also be considered. Without a due supply of moisture,
the process, of course, cannot go on, and a disturbance of
the normal constituents of the soil will lead to modifications
86 VEGETABLE PHYSIOLOGY
of the process. If there is too great a preponderance of
neutral salts such as sodium chloride or potassium nitrate,
so that the liquid presented to the roots is practically a
saline solution, the exudation will cease ; indeed, under such
circumstances water may actually be withdrawn from the
plant.
Root-pressure is continually at work while the trans-
mission of water is going on ; but it is not easily seen later
in the year when the development of the leaves has caused
an aptive transpiration to proceed. If the stem of the vine
is ciit in July instead of in Mar6h, no bleeding follows the
wound. This is not,, however, due to the absence of
activity in the roots, but to the fact that the copious
evaporation of transpiration prevents the necessary ac-
cumulation of water in the cavities of the woody elements.
In the experiment in the early spring the conditions were
different ; there were no expanded leaves, and the water
absoi*bed and sent upwards by the root consequently
remained in the vessels of the stem, escaping at once when
the latter was cut. In July the vessels have been emptied
by the transpiration, and there is no accumulation of
water to overflow. The apparatus described will show,
however, if the experiment with it is continued for some
time, that root-pressure is still at work, even though
transpiration is vigorous until the stem is severed.
The force of root-pressure must therefore be regarded
always as a factor in maintaining the transpiration current.
It is continually forcing water into the vessels of the axis,
and the fact that transpiration prevents an accumulation
there does not show that the influence of root-pressure is
done away with as soon as it ceases to be easily demefiT
strated.
The root-pressure, though always considerable, is not
the same at all times of the day and night. It can be
measured by observing the output of water in the second
form of the apparatus described above, measurements
being taken every hour, or in the first form of the
ROOT-PRESSURE 87
apparatus the manometer can be fitted with a float carrying
a pen, which can be made to trace a continuous line on a
slowly rotating recording surface. The line will be found
to describe a curve, showing points of activity varying from
maximum to minimum. The general features of the curve
will be the same for all plants, but all do not give the
maximum at the same time of the day. In the case of
Cucurbita Melopepo the minimum point occurs in the early
morning ; the curve rises slowly during the forenoon,
reaching its maximum soon after midday. From this
point it falls ; sometimes a second smaller rise takes place
towards evening, and then it sinks continuously all night.
The time of the occurrence of the maximum point varies
in different plants, but in all it appears to be during the
afternoon. In Prunus Laurocerasus it is much later than
in Cucurbita. The points of maximum and minimum
activity appear, however, to be about twelve hours apart,
so that there is a complete diurnal cycle.
There may be noticed in some trees also a variation
which suggests a yearly periodicity. The power of exud-
ing water is lost for a time during the winter, the loss
being noticeable at different times in different trees. Vitis
vinifera does not show any exudation usually in January ;
Acer platanoides is passive in November ; many plants will
not bleed at all during the winter.
The causes of these variations in the activity of the
absorbing mechanisms of the roots are still obscure. The
annual periodicity, when it exists, appears to be connected
with conditions which lead to the discontinuance of growth
during winter. The trees pass in fact into a state that may
be compared to hibernation. The daily periodicity does not
appear to depend upon variations in the surroundings of
the plant, but to be due to some cause or causes inherent
in its constitution. It has been suggested that it has been
induced in plants by long-continued variations of external
conditions, particularly those of illumination, involved as
these are in the alternation of day and night. This alter-
88 VEGETABLE PHYSIOLOGY
nation, affecting successive generations of plants through
an enormous length of time, may have impressed upon the
protoplasm a peculiar rhythm of greater and less general
activity, which has become ultimately automatic and inde-
pendent of the immediate surroundings. Of this the vary-
ing action of the roots may be a particular expression.
It is remarkable, however, that very young plants do
not exhibit this diurnal variation, but they gradually
acquire the power of doing so as they develop, subject as
they are under normal conditions to the alternation of light
and darkness. In many cases, again, the diurnal periodi-
city is not manifested at all.
The effect of the periodic alternation of light and dark-
ness cannot in any case have been originally appreciated by
the roots, as they are implanted in the soil and so escape
its influence. If it was originally due to such variations,
these must have been impressed upon the general orga-
nisation of the plant.
TRANSPIRATION. — The modified evaporation by which
the protoplasts get rid of water and enable the contents of
their vacuoles to be continually renewed takes place ulti-
mately from the surfaces of all the succulent parts of
plants, and to a less extent from portions of the exterior
which are covered by a layer of cork. Like the activity
of the absorbing organs of the root, it is essentially a vital
process and is regulated by the protoplasm of the cells
which take 'part in it. As we have seen, it .is usually spoken
of as transpiration.
It is easy to demonstrate the fact of its continuous
existence during daylight by enclosing a plant, or part of
one, in a dry glass vessel which can be closed so as to
admit no air. Very soon the surface of the glass becomes
covered by a fine dew, which is the condensed vapour that
has escaped from the plant. The same thing may be seen
when a vigorous plant is covered over by a bell -jar, the
water condensing copiously upon the sides of the latter.
A more elaborate method of demonstrating transpiration
TEANSPIBATION
89
consists in placing the end of a cut branch in a small glass
vessel, preferably a U-tube, filled with water, as shown in fig.
63. The branch passes through the cork of the vessel in
such a way as to prevent any escape or evaporation of
water at that point. Communicating with the other arm
of the U-tube is a side tube bent at right angles, which dips
into the water through a perforated cork. This tube is
also filled with water. As transpiration proceeds the water
is gradually drawn from the horizontal tube, and its pro-
gress can be noted by arranging a scale behind it. The
FIG. 63. — APPARATUS TO DEMONSTRATE TRANSPIRATION OF A BRANCH.
stem or branch should be kept with its cut end immersed
in water for several hours before being placed in the
apparatus, as its vessels contain air at a negative pressure
when it is cut, owing to the transpiration which has been
taking place from it before its separation from the plant.
The existence of this negative pressure will lead 4o an
immediate absorption of water, which might be mistaken
for an active transpiration.
The evaporation takes place to a certain extent through
all the epidermal cells of the transpiring organ, but not to
90
VEGETABLE PHYSIOLOGY
a very great one, the degree of the development of the
cuticle having considerable influence upon its amount. It
is carried out much more freely through the thin walls of
the cells abutting upon the intercellular spaces, which, as
we have seen, communicate with the external air by means
of the stomata and the lenticels. Very little watery vapour
is given off by the latter, so that by far the greater amount
that is exhaled passes through the stomata. Transpiration
is consequently most copious from the leaves, the structure
of the lower side of which, in dorsiventral forms, is espe-
cially favourable to it (fig. 64). If a leaf is taken which has
stomata upon its under surface only, and the rates of watery
FIG. 64. — TBANSVEKSE SECTION OF THE BLADE OF A LEAF, SHOWING
THE INTERCELLULAR SPACES OF THE INTERIOR.
exhalation from the two sides are compared, it will be found
that the stomatal gives off considerably more vapour than
the other surface.
A method first introduced by Stahl enables us to prove
with considerable facility that the escape of vapour through
the stomata is much greater than that through the cuticular
surface. It consists in applying to each side of a leaf which
has stomata only on the under surface, a piece of filter-
paper which has been impregnated with a solution of
cobalt chloride and dried. When dry this paper is blue in
colour, but it rapidly becomes pink when exposed to
moisture. A fresh dry leaf is taken and placed between
two pieces of the cobalt- paper and the whole put between
TEANSPIEATION
91
two dry sheets of glass of somewhat larger area. In a very
short time, often in less than a minute, the paper in con-
tact with the lower side of the leaf becomes pink, while the
other piece remains blue for a considerable time.
The amount of water given off by transpiration varies
in different plants. In the sunflower (Helianthus) the
amount has been stated to be y-^-g- cubic inch of water per
square inch of surface in twelve hours. V. Hohnel has
computed that a birch-tree with about 200,000 leaves may
transpire 60 to 80 gallons of
water during a very hot day.
Doubtless, however, individual
plants show a considerable variety
in the amount. This copious
evaporation readily explains why
the bleeding of plants from wounds
can seldom be observed when the
leaves are expanded and active.
When transpiration is exces-
sive the leaves and branches lose
their turgescence, become flaccid,
and droop. A branch which has
reached this condition may be
revived by forcing water into it,
which can be done by fastening
it into one arm of a U-tube con-
taining water (fig. 65), and pour-
ing mercury into the other. The restoration of the water
restores the turgescence of the tissues, and the branch
regains an erect position.
The exhalation of the water accumulated by root-
pressure in the closed system of the vessels leads to a
diminution of the pressure of the air which they contain
in addition to the water. Indeed it is by such a suction
that the air is originally enabled to enter the vessels, being
drawn into them from the intercellular spaces. Conse-
quently, while transpiration is active, there is a negative
FIG. 65. — APPARATUS TO SHOW
DEPENDENCE OF WITHERING
UPON Loss OF WATER.
92 VEGETABLE PHYSIOLOGY
gaseous pressure existing in the wood vessels. This con-
tinues after transpiration ceases, and no doubt, like the
evaporation itself, it is of assistance in maintaining the
upward flow, acting as it does in the same direction as the
turgid cortex, upon which it exerts a considerable suction.
It continues until the entry of water from the root causes
the pressure of the air in the vessels to be equal to the
atmospheric pressure. This negative pressure is of con-
siderable importance also in assisting the movements of
gases in the plants.
The exhalation of watery vapour from the surface of
the cells is not a process of simple evaporation. As in the
/other phenomena which we have examined, the proto-
plasm exercises a regulating influence upon the escape of
watery vapour from the cell. If the amount given off from
a measured area of leaf-surface is compared with the
quantity evaporated from an equal area of free water, the
latter is found to be much the greater. This area is
probably much less than the area of the cell-walls actually
involved, which abut upon the intercellular spaces opening
by the stomata included in the measured area. That this
difference is due to the life of the leaf, and consequently to
the protoplasm, is seen from the fact that a dead leaf gives'off
its water and dries up more rapidly than a surface of freely
exposed water. The cuticle of the living leaf and its cell-walls
are consequently not the causes of the differences observed.
The ultimate exhalation of watery vapour, we have seen,
is chiefly carried out through the stomata of the green
parts, at any rate in those plants which possess them.
Each stoma is situated above a somewhat conspicuous
intercellular space, to which it forms an outlet. The
stoma originates by the vertical division into two of one
of the cells of the epidermis which is usually somewhat
elaborately differentiated from the rest. The partition
which is formed between the two daughter cells thickens
slightly and splits so as to form an opening between them,
which does not, however, extend the whole length of the
TEANSPIRATION
93
wall, so that the two cells remain attached to each other by
their ends (fig. 66). The split constitutes the stoma, and
FIG. 66.— SUKFACE VIEW OF PART OF THE UNDER SURFACE OF A LEAF,
SHOWING THREE STOMATA IN DIFFERENT STAGES OF OPENING AND
CLOSING.
the two cells are known as the guard-cells. They are
commonly of a more or less semilunar form and contain
some chloroplastids, a point in which they differ from the
other cells of the epidermis in the higher plants. Their
walls become thick-
ened and cuticu-
larised, particularly
those which abut upon
the slit and upon the
intercellular space
(fig. 67) ; the wall
which is in contact
wifh thp nthpr Pr»i FIG. 67.— SECTION OF LOWER EPIDERMIS OF
el)1" A LEAF, SHOWING A STOMA. x 300.
dermal cells, however,
remains thin. When the guard-cells are full of water, their
form and mode of attachment cause them to become
94 VEGETABLE PHYSIOLOGY
curved so that the orifice is widely open. This is helped
by the thickening of the free edges, which makes it difficult
for them to swell in the direction of each other. When,
on the other hand, they lose their water, they relax, and
their edges coming into contact, the aperture between them
is more or less completely closed (fig. 66).
The number of the stomata varies very considerably.
The following table will give some idea of their abundance
in leaves, and it will be observed that the number of stomata
is usually greatest in those leaves from whose upper surface
they are entirely absent.
Stomata in One Square Inch of Surface
Upper surface Lower surface
Mezereon ,. . ' . . . none 4,000
Pfeony * . . ' .. . , % . • . none 13,790
Vine . . . ' , ." . . none 13,600
Olive .' .. . -"'.« ' . '". . none 57,600
Holly . .' . Y . . none 63,600
Laurustinus . . *; . . none 90,000
Cherry -laurel . . i . . none 90,000
Lilac . . . . . . none 160,000
Hydrangea . . . ' . . none 160,000
Mistletoe. '.. i -. V. . 200 200
Tradescantia .-. . ;, . . 2,000 2,000
House-leek . . . . . . . 10,710 6,000
Garden Flag . . . , • • . 11,500 11,500
Aloe . .' . . , ;,. • . 25,000 20,000
Yucca ... ... . 40,000 40,000
Clove Pink . . ... . 38,500 38,500
The modification of the turgescence of the guard-cells
is caused by the osmotic transference of water between
them and the other cells of the epidermis from which they
are separated by thin walls. The vapour which is in the
intercellular space below them does not penetrate them,
the walls abutting on the space being thick and cuti-
cularised. The osmosis alluded to may be associated with
the presence of the chloroplasts in the guard-cells, which
are instrumental in the production there of various sub-
stances, so that their contents have a higher osmotic
equivalent than those of the epidermal cells which are con-
TKANSPIKATION 95
tiguous to them. When, therefore, the epidermal cells are
charged with water, this is osmotically drawn into the
guard-cells, which become turgid, and consequently separate,
opening the aperture. When the contiguous epidermal cells
lose their water, the osmotic constituents of their contents
become more concentrated, as these do not leave the cells
with the water. The direction of the osmotic stream is
consequently reversed, the guard-cells lose some of their
turgidity, so that their edges fall together and partially or
wholly close the slit. Thus the escape of watery vapour
is accelerated or retarded by their action.
Transpiration is markedly increased by sunshine, rising
to many times its original amount when a plant is trans-
ported into it from a dim light. No doubt this is due in a
very large measure to the heat rays which then fall upon
the plant, and which would raise its temperature very
dangerously were they not applied to the evaporation of
the water. But it is not due entirely to them, nor to the
higher temperature of the air accompanying their passage.
The light has, indeed, an influence apart from the heat.
No doubt, so far as the visible rays of the spectrum are
converted into heat vibrations after absorption, they must
influence transpiration indirectly in this way. Besides
acting thus indirectly, light has a direct eftect upon the
process, for it influences, the size of the stomatal apertures.
These have been observed to be open during the day and
more or less completely closed during the night. The
gaseous interchanges which light induces, in causing the
decomposition of carbon dioxide and the evolution of
oxygen, on the whole favour the exhalation of watery
vapour* When green plants are exposed to light of various
colours the most marked increase of transpiration is caused
by the light of which the plants absorb most. This can
be observed not only in the green parts of plants, but in
those which are not green, as in the petals of the flowers.
The fact that the rays which are absorbed by chloro-
phyll are the most active in promoting the process has
96 VEGETABLE PHYSIOLOGY
some significance when it is remembered that the guard -
cells of the stomata contain this pigment. The nature of
the action of chlorophyll in this direction is not, however,
fully understood.
Apart from direct radiation, the temperature of the
air, and its hygrometric condition, are important factors
in causing an increase or a diminution of the watery
vapour exhaled. They act principally by exerting an
influence directly upon the evaporation from the cells,
but several indirect effects can also be noticed. The
general movements of water in the plant, as well as its
absorption, are influenced particularly by variations of
temperature, and the latter has also an effect upon the
width of the stomatal orifices. A rise of the external
temperature causes the saturated air in the intercellular
passages to expand, as the air acquires the new temperature
more rapidly than do the tissues of the plant. The escape
of vapour is consequently accelerated as the temperature
rises, even though the rate of evaporation from the cells
into the intercellular spaces is not at first affected.
The influence of the hygrometric condition of the air,
apart from changes of temperature, can be seen when a
plant which has been exposed to a dry atmosphere till its
leaves have become flaccid is transferred to one saturated
with moisture. After a short time the drooping leaves
again become turgid. This is not due to an absorption of
water in the form of vapour by the leaves, but to a
diminished loss by the checking of transpiration. The
return of turgidity is caused by the accumulation of the
store drawn from the earth by the roots. This can be
shown by comparing the behaviour of two plants treated in
the way described, one of which is allowed to remain
rooted in soil, while the other is taken up from the earth
and exposed in that condition to the saturated air. There
is in the latter case no recovery of turgescence.
The temperature of the soil in which the roots of a
plant are embedded has also an influence upon the exhala-
TEANSPIRATION 97
tion of watery vapour, which increases as the soil is
warmed and diminishes as it becomes cooler.
If the protoplasts of the cells of the turgid leaves of a
branch are stimulated by violently shaking it, the leaves
become flaccid. The protoplasm under the stimulus allows
more water to pass through it to the cell-walls, and hence
evaporation is promoted. The effect may be compared
with that which has already been mentioned as set up in
the cells of the cortex of the root by their over-distension
by the water which accumulates in them in consequence of
the continuous osmotic activity of the root-hairs. The
stimulus of this distension is responded to by the proto-
plasm by its becoming more permeable by the water of the
vacuoles of the cells. The response made by the protoplasts
of the leaves to the stimulus of shaking may help to explain
the flaccid condition observable in the foliage of certain
trees after the prevalence of a high wind. Besides this
effect upon the protoplasm, the continuous removal of the
air around the transpiring organs has, no doubt, a consider-
able influence upon the removal of the watery vapour from
their intercellular passages.
The effect of alteration of the external conditions upon
transpiration may be investigated by means of Darwin's
potometer, which enables approximately accurate determina-
tions of its amount to be made from time to time. This
instrument is shown in fig. 68. It consists of a glass tube
with a side arm which is bent upwards so as to be parallel
with the tube itself. A capillary tube of about -2 mm. bore
is fastened by an indiarubber cork into the lower opening
of the tube so as just to project beyond the cork. A con-
venient length of the capillary tube is about 20 cm. Its
lower end dips into a small vessel of water, arranged so as
to be easily withdrawn from the tube. The upper orifice
of the potometer is closed by a tightly fitting cork, and the
plant whose transpiration is to be observed is fitted into
the side arm by means of an indiarubber band or tube
which embraces the glass arm and the end of the cut
H
98
VEGETABLE PHYSIOLOGY
branch so as to make a water-tight connection. The whole
apparatus must be filled with water, and care must be
taken that no escape of liquid can take place at any of the
junctions. Any air that finds its way into the instrument
during the arrangement of the branch in its position can
be removed by causing it to collect at the upper portion
of the straight tube of
the potometer. To take an
observation . of the rate of
transpiration of the branch,
a bubble of air must be
admitted into the capil-
lary tube by momentarily
removing the vessel into
which it dips, and replac-
ing it as soon as the tran-
spiration has caused the
air to enter. The bubble
of air must be of uniform
size in successive readings,
to ensure that the latter
shall be strictly compar-
able with each other. The
bubble will rise in the
tube, and finally make its
way to the upper part of
the straight limb of the
instrument, the rate at
which it travels serving as
an index of the rate of the
transpiration. The capillary tube should be marked
by a transverse line a few millimetres from its lower end,
and by means of a stop-watch the time taken by the
bubble to rise from this mark to the free end of the tube
should be observed. The branch may be covered by a
bell-jar, so that the variations of temperature, moisture,
&c. of the air surrounding it can be controlled during a
FIG. 68. — THE POTOMETEB.
TRANSPIRATION
99
series of observations. Less accurate observations can be
made by substituting for the capillary tube a tube of wider
bore bent at right angles a little below the orifice of
the potometer, and affixing to it a scale by means of which
the rate of passage of the
column of water in the tube
can be observed (fig. 63).
According to the varia-
tions in the external condi-
tions of the plant, including
all the features already
alluded to, the amount of
watery vapour transpired
is continually changing.
The most favourable con-
ditions being afforded in
summer, it is not to be
wondered at that tran-
spiration attains an annual
maximum during that sea-
son. It does not, however,
entirely cease during the
winter, though it is reduced
to a minimum, especially
in the case of such trees
as shed their leaves in the
autumn.
Apart from such changes
in the external conditions,
transpiration appears to
show no independent pe-
riodicity, differing in this
respect conspicuously from
root-pressure. It is, how-
ever, very sensitive to only slight changes in the environ-,
ment.
It was mentioned in an earlier part of this chapter that
FIG. 69. — APPARATUS TO SHOW THE
SUCTION CAUSED BY TRANSPIRA-
TION. (After Detmer.)
100 VEGETABLE PHYSIOLOGY
the force of transpiration was of considerable assistance in
maintaining the upward flow of water from the roots. The
apparatus shown in fig. 69 enables this to be demonstrated.
The cut end of a branch is connected by an air-tight joint
with a glass tube filled with water, the lower end of which
dips into a vessel of mercury. As the water is transpired,
a certain quantity of mercury enters the tube, and is drawn
up for some considerable distance by the suction.
The evaporation from the cells takes place, as we have
seen, not immediately into the external air, but into the
intercellular passages of the plant. The force causing this
suction, so far as it is due to evaporation, is therefore localised
in the surface • film formed in the evaporating cell-walls.
Such an evaporation has been shown by Strasburger to be
capable of raising a current of water through pieces of dead
wood which have been soaked and injected with water.
There isj*eason to believe, however, that a third factor
in the ascent of tKe^stream is interposed between the forces
of root-pressure and the evaporation described. The water
is passed from the wood-vessels or conduits to the evaporat-
ing cells through a varying thickness of parenchyma (fig.
70), which is kept turgid during active transpiration. The
turgid condition of the cells is maintained by osmosis, just as
is the similar condition in the roots. The vessels abutting
on the parenchymatous cells are well supplied with water,
which is in their cavities and which saturates their walls.
The cells contain substances of an acid reaction which
possess a high osmotic equivalent. We cannot doubt that
osmosis takes place through the walls of the cells, and that
the turgidity of the tissue of the leaf is due to it as much
as is that of the cortex of the axis. Eesearches carried out
by Dixon show that this osmotic force plays a very im-
portant part in supplying the water to the evaporating
surfaces. If the end of a cut branch is immersed, in
any of the forms of apparatus described, in a solution of a
salt which will plasmolyse these cells by destroying their
turgescence, such as the sodium chloride which we have
TEANSPIEATION 101
already seen capable of doing so, the rate of transpiration
continues without much, if any, diminution till the salt can
be detected in the leaves, when it suddenly falls off. This
takes place though there is no interruption of the con-
tinuity of the fluid in the channels of the transpiration
current. From this point onward, instead of evaporation
sucking up water from the root, it gradually leads to a
drying of the leaf.. A similar result is brought about by
raising the temperature of the transpiring branch to such
a point as will kill the protoplasm of the cells. As these^
die the evaporation is unchecked at first, but gradually the
water is taken from their interior and no more is supplied.
FIG. 70. — ENDING OF A FIBBO-VASCULAB BUNDLE IN THE
PABENCHYMA OF A LEAF.
The cells rapidly become flaccid, the leaves droop, and the
total quantity of vapour exhaled is materially lessened, the
intercellular passages soon becoming partially obstructed
by the collapse of the cells abutting upon them. The
experiment does not interfere with the continuity of the
water- stream, but as soon as the cells are made unable to
retain their turgidity by the interference with osmosis
which follows the death of the protoplasm, the evaporation
empties the cells and no more water enters them to replace
what has been lost. As we have seen in other cases, the
death of the protoplasm is followed by the escape of the
osmotic substances, which do not leave the cells during
102 VEGETABLE PHYSIOLOGY
their life. The mechanical effects which follow the collapse
of the tissue are the consequence of the assumption of a
flaccid condition, and they intensify the check to the escape
of watery vapour from the affected organ.
The course of events in a normal leaf during active
transpiration appears to be, then, the setting up of a tension
in the parenchymatous cells of the leaf by evaporation
from their surfaces which tends to cause them to collapse
and become flaccid. This tendency is opposed and over-
come by a greater force excited by the turgescence of those
cells whose osmotic properties exert a traction upon the
water in the conduits or wood- vessels. Water is thus
supplied through the inner walls of the evaporating cells as
quickly as it is lost by evaporation from the surfaces which
abut upon the intercellular passages.
Dixon ascertained that the osmotic pressure in the
leaves of transpiring branches of the Laburnum amounted
to between six and eight atmospheres, a force which is
capable of raising a column of water to a height of more
than 200 feet.
Careful consideration of the facts recorded in this
chapter shows us that although we cannot fully explain the
ascent of the transpiration current, we can see that it
ultimately depends upon the behaviour of the protoplasm.
All the factors which aid its progress, j^otrjDrgfisJire, tran-
spiration, osmosis_in the cells of the leaves, are largely
underthe control of the living substance, and are particu-
larly influenced by the power it possesses of allowing more
or less water to pass through it, according to its condition.
Moreover all the external influences which we have ex-
amined, which are brought to bear upon these factors, are
mainly efficient in as far as they affect the protoplasm in
the exercise of this power.
303
CHAPTEE VII
THE AEBATION OF PLANTS
IN the study of the vital processes carried on by the
protoplast we have seen so far how entirely it is dependent
upon the free access of water. Another factor necessary
for its existence is a supply of air. With but few exceptions,
and those occurring among the lowliest plants, every living
organism carries out a series of gaseous interchanges, a
feature of which is the absorption of oxygen. In nearly all
cases a corresponding amount of carbon dioxide is exhaled.
In the case of many plants, of all, indeed, that are green,
another gaseous interchange takes place, carbon dioxide
being absorbed and oxygen simultaneously eliminated.
Every protoplast must consequently be afforded facilities
for carrying out gaseous interchanges, the nature and
extent of which vary according to its constitution. The
water with which it has such a close relationship serves as
the medium through which such interchanges take place,
for it is only in solution that gases are able to penetrate
into the living substance.
In the case of those protoplasts which live in a watery
environment, the latter supplies them with the gases they
absorb and receives those which they exhale. If all air is
withdrawn from the water in which they are living, death
speedily ensues. The gases enter the naked protoplasts by
diffusion through the film of water which is in contact with
their free surfaces. In the case of those which have a
cell-wall the same means are made use of. Gases in
solution can diffuse through the cell-wall, which, as we
have already seen, is saturated with water. If we turn to
104 VEGETABLE PHYSIOLOGY
those unicellular or filamentous plants which live on the
surfaces of rocks or tree-trunks, the process is only slightly
modified, for the gases of the atmosphere readily dissolve
in the water which the cell-walls contain and diffuse thence
into the interior of the cell.
In the cases of those more bulky plants which we have
especially been considering in the last chapter, a further
mechanism is necessary, as the external air cannot gain
access into the interior of a large mass of cells without
special arrangements for its admission. This is especially
the case with such plants as are possessed of protective
mechanisms like the corky layers of the bark, or the
strongly developed cuticle of the leaves. The arrange-
ments of the structural elements in these plants we have
seen to include a very complete system of intercellular
spaces, passages, or canals, by means of which almost all
the constituent cells are placed in nearly or quite complete
relation with the external air. The intercellular space
system has consequently a very important function to
discharge in this particular, as well as to serve as the
means of carrying off from the interior the aqueous vapour
exhaled from the cells.
The intercellular space system begins to appear at a
very early period in the development of the young plant.
While all its cells are merismatic, as is the case when it
begins to emerge from the seed, they are united together
entirely, a condition which persists at all the. growing
points of the plant as its age increases. During this
condition the aeration of the internal cells is provided for
by the slow diffusion of the gases from cell to cell, absorp-
tion from the exterior by the external cells being possible
so long as their walls are not cuticularised. Some of the
cells situated deep in the interior of the adult parts are
dependent upon a similar process, but the majority of the
protoplasts are provided with access to the air by the early
formation of spaces due to the splitting of certain of the
cell-walls, and the subsequent partial separation of the
THE AEEATION OF PLANTS 105
cells. Air makes its way into these spaces by a process of
diffusion outwards from the cells abutting upon them, and
very soon external orifices in the shape of stomata make
their appearance. The various constituents of the air
make their way into and out of the cell by a process of
diffusion, being dissolved in the water of the cell -wall or
escaping from such a moist mem-
brane according to the conditions
existing, and the relation between the
internal and external pressure of the
particular gas in question.
As soon as the differentiation of FIG. 71.— CELLS SPLITTING
,-,,.. ,1 • P AT THEIR ANGLES TO FORM
the tiSSUe in the growing part Of an INTERCELLULAR SPACES.
organ begins to take place, the forma-
tion of the intercellular spaces can be observed. In these
regions they begin by a splitting of the wall between two
contiguous cells or at the angles where three cells join
(fig. 71). The crevice soon extends and may make its
way for a considerable distance round any particular cell.
The cavities so come
into communication
among the cells, each
of the latter abutting
upon a single one or
upon several. While
the tissue is young
these are very narrow
and slit-like, or are
only visible at the
angles when the cells
are polyhedral. They FIG. 72. — CORTEX OF BOOT, SHOWING INTER-
. CELLULAR PASSAGES BETWEEN THE CELLS.
rapidly become larger
(fig. 72), and in some parts, particularly in the interior of
the lower strata of the mesophyll of dorsiventral leaves, they
may occupy more space than the cells themselves (fig. 73).
Light appears to influence their development somewhat,
though no definite relation can be shown to exist between
106
VEGETABLE PHYSIOLOGY
the degree of the illumination and the capacity of the
cavities formed. Light is, however, not the only factor,
FIG. 73. — SECTION OF LEAF SHOWING THE LARGE INTERCELLULAR
SPACES OF THE MESOPHYLL.
and probably not the most important one, in determining
their extent, for they are usually prominent in the cortex
of roots, which receive but little illumination. The
FIG. 74. — SECTION OF LEAF OF Isoetes.
a, lacunar cavities ; Z>, vascular bundle.
explanation of the relatively large development in this
region may lie in the fact that the intercellular cavities
there have very little communication with the outer air, as
THE AEEATION OF PLANTS
107
stomata do not exist upon roots. There is thus a necessity
for a larger reservoir of air than in parts where gaseous
interchange is more readily effected. '
Besides these comparatively narrow channels we find
cases where reservoirs of large size are specially developed.
Such structures occur in the leaves, rhizomes, and roots
of aquatic plants which are nearly or entirely submerged.
FIG. 75. — SECTION OF KHIZOME OF Marsilea.
co.la, lacunae in cortex.
Among them conspicuous examples are afforded by the
leaves of Salvinia and Isoetes (fig. 74), the rhizome of
Marsilea (fig. 75), and the leaf stalks of many of the aquatic
Phanerogams. These are developed in a similar manner to
those already described, and they are so prominent in the
structure that a section shows them separated from each
other by rows of cells not more than one cell thick (fig. 76) .
In some cases where large cavities of this kind occur
108 VEGETABLE PHYSIOLOGY
the mode of formation is different. A mass of tissue lying
in the position of the subsequent cavity does not keep
pace in its development with the growth of the cells sur-
rounding it, and consequently becomes ruptured, and the
cells of which it is composed are gradually destroyed, leav-
ing a cavity of some size. Instances of this mode of
formation are afforded by the stems of Equisetum (fig. 77),
FIG. 76. — SECTION OF STEM OP Potamogeton, SHOWING Am PASSAGES
IN THE COKTEX.
the haulms of grasses, and the hollow stems of the
Umbelliferae and other plants.
The occurrence of these large air-containing cavities in
partially submerged plants may be explained by a considera-
tion of their habitat. The plant is in contact with the air
by only a very small portion of its surface ; the leaf -stalk of
Nymphaea, for example, is always submerged, and only the
floating lamina can obtain a direct supply of air. The
THE AEKATION OF PLANTS
109
stomata are placed upon the upper surface, and afford its
only means of entrance. The stems and roots are also cut
off from air by being placed either in water or in mud. The
protoplasts of such a plant are almost entirely dependent
upon the reservoir of air which the body of the plant can
contain, a small quantity only entering by diffusion from
the water into its epidermal cells.
The air cavities which arise in the stems of terrestrial
FIG. 77. — PORTION OF AERIAL STEM OF Equisetum.
a, cortical lacuna ; 6, lacuna in vascular bundle ; c, chlorophyll-containing cells.
plants, such as the grasses, are probably not primarily
developed with a view to the aeration of the plant, but are
rather intended to economise the material used in construc-
tion. The hollow stems with a rigid periphery, strength-
ened at intervals by diaphragms, such as occur at the
nodes of these .organs, are especially adapted to maintain
110
VEGETABLE PHYSIOLOGY
an upright position with comparatively little expendi-
ture of material. . A somewhat similar mechanism is met
with in the stellate parenchyma of the stems of the Bushes
(fig. 78). There is little doubt, however, that these spaces
FIG. 78. — PORTION OF SECTION OF STEM OF RUSH, SHOWING STELLATE
TISSUE OF THE PITH, WITH LARGE INTERCELLULAR SPACES.
are of great assistance in promoting the aeration of the
whole structure.
As has been already mentioned, the external orifices of
the system of the intercellular spaces are the stomata of
the leaves. In woody and corky parts these are supple-
mented by the lenticels. The evidence for this statement
does not consist only of microscopic examination of the
THE AEEATION OF PLANTS
111
tissues. A direct proof can be afforded by a simple experi-
ment. If the lamina of a
leaf is immersed in water,
air can be driven through
it by subjecting the cut end
of the petiole to gaseous
pressure by means of an
air-pump, or even by the
effort of the lungs of the
observer, and can be seen
to emerge from the surface
of the leaf on which the stomatal apertures are situated.
If a petiole is passed into a glass bottle through a tightly
FIG. 79. — SECTION OF A LENTICEL.
I, lenticel ; per, cork layer.
FIG. 80. — APPARATUS TO SHOW CONTINUITY OF INTERCELLULAR
SPACES IN THE LEAF. (After Detmer.)
fitting cork, and covered with water, while the lamina
remains in the air outside (fig. 80), bubbles of gas can be
112 VEGETABLE PHYSIOLOGY
made to emerge from its cut surface in a continuous stream
by reducing the pressure above the water by means of an
air-pump.
The facility of the interchanges will largely depend
upon the number, size, and position of these orifices. A
lenticel will allow more gas to pass between its loosely
arranged cells than will a stoma, but their relative numbers
make the stomata much more important than the lenticels.
In most cases there is- a free passage through the stomatal
pore, but in others considerable difficulty is afforded by
the aperture being sunk in the epidermis or situated in a
Fia. 81. — TRANSVERSE SECTION OF KOLLED LEAF OF HEATH.
depression of the leaf. In the rolled leaves of heaths and
certain grasses this difficulty is frequently partially com-
pensated by the lacunar character of the parenchyma,
which is in the immediate neighbourhood of the stomata
(fig. 81).
It must be noted in this connection that the stomata
and the lenticels are passive with regard to the process of
aeration, and do not exert an active influence upon it.
The variations in the width of the stomatal apertures
which are of so much importance in the regulation of
transpiration must be regarded as bearing upon that
function alone, being caused by fluctuations in the amount
THE AERATION OF PLANTS 113
of water in the plant. They serve automatically to preserve
the plant from excessive loss of water, but they have
no direct regulating influence upon the interchange of
gases. Indeed, when, from flaccidity of the leaves or from
other causes, they close, the aeration of the plant is, to a
certain extent, interfered with, if not suspended — a con-
sideration which will help us to understand why a plant
needs to contain so large a reservoir of air as is afforded
by its intercellular spaces. The vol-ume of this reservoir
varies considerably in different plants, as has already been
shown. Unger has put on record measurements of the
relative volumes of air and cellular tissue in the leaves of
forty-one species of plants. These were found to range
from 77 : 1000 in Camphora officinalis, where it was least,
to 713:1000 in Pistia texensis, in which it was greatest.
The movements of the air in the intercellular space
systems of plants depend almost entirely upon the physical
processes of diffusion. The entrance and exit of air from
the exterior are generally possible, occasions when the
orifices are completely occluded being very rare. It does
not, however, at all follow that the atmosphere in the
spaces has the same percentage composition as the external
air. When we consider that it is the source of the supply
of the gases used in the metabolism of the plant, and the
recipient of those which are from various causes exhaled, it
becomes evident that this is not the case. Nor is its
composition uniform for even a short time, as the various
processes which subtract from or add to it take place in
different parts with very different rapidities. At the same
time there is a tendency for it to become uniform according
to the laws of the diffusion of gases.
The amount of nitrogen varies but little. This gas
has a certain feeble solubility in water, and a small
quantity goes into solution in the water which saturates
the cell-walls ; but as such nitrogen is not made use of in
the cells, its absorption very speedily ceases, the cell-sap
not being able to contain more than a trace of it. The
i
114 VEGETABLE PHYSIOLOGY
percentage of nitrogen in a volume of gas obtained from a
plant may not correspond with the percentage in an equal
volume of air, but this will result from an interference
with the amount of oxygen and carbon dioxide, and not be
due to an absorption or exhalation of nitrogen, neither of
which takes place to an appreciable extent.
The variations in composition which are noticeable are
due to two processes which are characteristic of the vital
processes of green plants. As we shall see in a subsequent
chapter, all the green parts of plants are during daylight
engaged in absorbing carbon dioxide from the air, and
exhaling oxygen into it. In such parts this interchange
takes place with considerable energy, and the composition
of the air in their intercellular spaces varies accordingly,
becoming relatively much richer in oxygen than it is in the
deeper parts which are not illuminated, and which contain
no green colouring matter. A change in the opposite
direction goes on continually wherever there is living
protoplasm, for this is always absorbing oxygen so long
as it lives, while a good deal of carbon dioxide is simul-
taneously exhaled. This process, unlike the other one, is
not confined to any particular part of the plant, nor is it
ever in abeyance. Thus the plant shows a continuous and
universal production of carbon dioxide, and a partial and
local consumption of this gas. At the same time it
exhibits a constant demand for oxygen everywhere, and a
temporary production of it in places. The composition of
the air in the intercellular spaces must therefore vary
from time to time, and from place to place, according to
the intensity and the localisation of these changes.
The process of diffusion, which is one of the phenomena
characteristic of gases, leads to a constant occurrence of
gaseous currents in plants. These currents may be influ-
enced by various properties of the gases concerned, and by
other factors, both internal and external. The rate at which
carbon dioxide is absorbed by the cell -wall is very different
from the rate of absorption of oxygen. If an atmosphere
THE AERATION OF PLANTS 115
containing a good deal of the former gas is in contact with
wet cell-walls, the result of the active absorption will be to
set up a stronger current to that spot than would be the
case if oxygen replaced it. Any cessation in the absorption
of carbon dioxide by the green cells owing to diminution of
light must be attended by a certain variation in the gaseous
stream. The wa}^s in which alterations in the absorption of
oxygen will affect the currents will also be readily apparent.
During bright sunlight, when both processes are proceeding
in the same and in different parts of the plant, local
positive pressures of either oxygen or carbon dioxide may
occur, and it is evident that the direction of the gaseous
currents will be very varied in consequence.
The structure of the plant has a certain influence on
the composition of its internal atmosphere. The epidermis
of most terrestrial plants is strongly cuticularised, while
there is but little cuticle to aquatics. The entry of gases
into the latter is accordingly easier than it is into the
former, penetration into which must take place through
the stomata. Moreover, the larger reservoirs in the
interior of aquatics serve to equalise the composition of
the internal atmosphere, and to cause it to resemble more
closely that of ordinary air.
Such plants again as contain no green colouring mat-
ter— for example the bulkier Fungi, which require provision
for the supply of air to their interior— have only the one
metabolic process in which the interchange of oxygen and
carbon dioxide is involved, the former being absorbed, and
the latter exhaled. To a corresponding extent, therefore,
the gaseous currents are simplified, though even in these
plants the direction and the amount are never constant for
long together, the metabolism continually varying.
In another important respect the internal air of plants
differs from that of the atmosphere. It is always charged
with aqueous vapour, frequently even to the saturation
point, as we have seen in connection with the process of
transpiration.
116 VEGETABLE PHYSIOLOGY
The external conditions to which a plant is exposed
have a considerable influence upon the gaseous currents.
The effect of light upon a green plant has already been
alluded to. The influence which it exerts is an indirect
one, affecting the consumption of carbon dioxide and the
liberation of oxygen. Nearly all the vital processes are
subject to modification by the various external conditions.
Transpiration we have seen to be very largely influenced
thereby, and the varying amounts of watery vapour exhaled
introduce variations in the amounts of the purely gaseous
interchanges. The influence which the variation of the
quantity of water in the plant exercises takes the form
especially of modifying the width of the stomatal apertures,
and hence of favouring or checking the entry and exit of
gases into and from the leaves.
Mechanical disturbances due to wind are of some
importance, generally increasing the gaseous interchanges.
Diminution of the turgidity of the tissues, amounting
sometimes to flaccidity, interferes at times to a serious
extent, the intercellular spaces becoming narrowed by the
falling together of the cell-walls, a phenomenon which is
noticeable also in. the partial or complete closure of the
stomatal orifices, due to the flaccidity of their guard-cells.
Variations of barometric pressure and of temperature
also influence to a considerable extent the process of diffu-
sion within the plant, as well as the interchange between
the interior and the external air.
The movements of the air in the plant are subject to
disturbance also by the setting up of the negative pressure
in the cavities of the vessels of the wood which we have
seen to be caused by active transpiration. This negative
pressure can be demonstrated with considerable ease in the
cases of woody stems, but it can be seen also in plants in
which the development of wood is only very slight, having
been observed in some cases in the elements of the central
cylinder of some of the stouter Mosses.
To demonstrate the existence of the negative pressure
THE AEEATION OF PLANTS 117
in the vessels of the stem, a young plant should be removed
from the soil and allowed to become flaccid. The stem
should then be partially immersed in mercury and cut
across below the surface of the latter. The mercury will
immediately rise to some distance in the vessels, being
drawn up by the suction exerted by the negative pressure
therein.
An actual positive pressure can under certain conditions
be observed in the intercellular air-reservoirs of particular
plants. This can be shown by cutting the stems of sub-
merged plants such as Myriophyllum, when, if they are
brightly illuminated, bubbles of gas may be seen to emerge
from the cut end. This positive pressure appears to be
due to a considerable production of oxygen by the green
parts of the plant under the conditions of illumination, as
it varies with the intensity of the latter, and ceases entirely
in darkness.
It is well that we should lay some stress upon the rela-
tion which the stomata show to the processes of gaseous
interchange. Though they are the chief means of the
entry of gases into and their exhalation from the plant,
it is misleading to speak of them as the organs of such
gaseous interchange. The actual processes of interchange
take place between the protoplasts and the air of the
intercellular reservoirs, so that the latter are the special
organs devoted to such functions. The stomata and the
lenticels are merely the openings by which the air of these
internal formations communicates with the outer atmo-
sphere. The true gaseous interchanges which subserve the
life of the protoplasts, and hence of the plant, take place
not at the stomatal orifices, but completely throughout the
interior of the substance of the plant.
118 VEGETABLE PHYSIOLOGY
CHAPTER VIII
RESPIRATION
OF the gaseous interchanges which were mentioned in the
last chapter as characteristic of living protoplasts, the
most widespread is that which is marked by the absorp-
tion of oxygen. With the exception of a few of the lowlier
organisms, all of which are members of the group of
Fungi, every living protoplast must be constantly absorb-
ing this gas in order not only that its vital activities may
continue to be discharged, but that its life itself may be
maintained. Withdrawal of oxygen from the environment
of the protoplast is after a longer or shorter interval
followed by its death. It is true that under certain con-
ditions which we shall discuss in a subsequent chapter the
interval may be prolonged, but death ultimately ensues.
This absorption of oxygen is in most cases associated
with an exhalation of carbon dioxide, which is generally
given off in a volume approximately equal to that of the
oxygen taken in.
The universality of this process is not always easy to
demonstrate. It can be ascertained without difficulty in
the case of almost all animal organisms, and of such of
the vegetable ones as possess no chlorophyll. In the case
of those plants which are green, however, there is, as we
have already mentioned, and as we shall discuss more fully
later, a converse gaseous interchange occurring so long as
the green parts are exposed to sunlight, carbon dioxide
being absorbed and decomposed, and an equal amount of
oxygen being exhaled. This interchange is usually more
vigorous than the first one, and the latter is therefore diffi-
RESPIRATION
119
cult of detection under conditions which allow both to take
place simultaneously.
The absorption of oxygen can be easily observed in the
case of a large fungus, such as a mushroom. If one of
these plants is placed in a closed receiver containing air,
and is left there for several hours, at the conclusion of the
experiment the mixture of gases in the receiver will be
found to be almost devoid of oxygen, that which was there
originally having disappeared. An almost equal amount
of carbon dioxide will be found to have replaced it, so that
the volume of gas in the receiver will be unaltered.
It is possible to devise an experiment which will show
that a green plant has the same absorbing power. If the
light is excluded from one placed in a similar vessel, no
evolution of oxygen will take place from it, and that the
oxygen present in the air at the commencement of the
observation will diminish to the
point of extinction can be made
evident, just as in the case of the
mushroom.
We have evidence, however,
that this is not caused by the
exclusion of the light, but that
the gaseous interchange in ques-
tion proceeds in the light as well
as in darkness. An apparatus
which was originally devised by
Garreau, and which can be easily
arranged to show the absorption
of oxygen, even when a green
plant is exposed to a bright sun-
light, is shown in fig. 82. It consists of a glass vessel
which can be closed by a cork through which a bent glass
tube of small calibre is passed. The tube is carried over and
made to dip into a small dish containing mercury. The
bottom of the vessel is covered with finely broken glass,
upon which is poured a strong solution of caustic potash.
FIG. 82.— APPARATUS TO SHOW
THE ABSORPTION OF OXYGEN
BY A GREEN PLANT.
120 VEGETABLE PHYSIOLOGY
Above the latter, supported by the glass so as not to be in
contact with the alkali, is placed the plant to be examined.
Watercress or any other herbaceous plant will answer
very well. The potash will absorb the carbon dioxide of
the atmosphere originally admitted as well as whatever
quantity of this gas is given off during the experiment.
As the experiment progresses the temperature must be
kept constant, when the mercury will be found slowly and
gradually to rise in the small glass tube, indicating a
diminution of the volume of the air in the flask. If the
experiment is continued till the mercury ceases to rise
in the tube, and the gas remaining in the vessel is
measured at the ordinary atmospheric pressure, and at
the temperature at which the experiment was started, it
will be found that its volume has been diminished by
about twenty per cent., and that what is left consists of
nitrogen. The oxygen will have been completely removed
by the green plant, even when the apparatus is left
exposed to the sunlight during the daytime. If the
caustic potash is examined, it will be found to have gained
considerably in weight, and to contain a quantity of car-
bonate of potassium, derived necessarily from the plant
during the experiment. The weight of this, will enable the
volume of the evolved carbon dioxide to be ascertained.
There will have been proceeding during the experiment an
absorption of oxygen, attended as before by an exhalation
of carbon dioxide, the latter having combined with the
potash.
The evolution of carbon dioxide by the plant can be
more easily demonstrated by the use of the apparatus
shown in fig. 83. The jar A in the centre contains the
plant to be examined, which may preferably be represented
by a number of germinating peas. It is closed by a cork,
which is perforated in two places. Into one hole a tube is
inserted which passes to the bottom of the jar, and serves
for the admission of air. An outlet tube passes through
the other hole from the upper part of the jar, and leads to
EESPIEATION
121
another jar, c, which is partially filled with baryta water.
The final outlet from c can be attached to an aspirator
by which a stream of air can be drawn through the
apparatus.
Before the incoming air reaches the jar A it is made to
pass through another jar, r, containing a solution of caustic
potash which frees it from all traces of carbon dioxide.
To ascertain that this is secured, it passes next through a
jar B which contains baryta water. A stream of air is
then passed slowly and continuously through the whole
apparatus, and as it bubbles through the baryta water in c
FIG. 83. — APPARATUS TO SHOW THE EXHALATION OF CARBON DIOXIDE BY
G-ERMINATING SEEDS. THE AIR ENTERS THROUGH THE TUBE ON THE LEFT ;
ITS CARBON DIOXIDE IS ABSORBED BY THE POTASH IN F. IT PASSES THROUGH
A, IN WHICH THE SEEDS ARE PLACED, AND THE CARBON DIOXIDE GENERATED
THERE IS CARRIED OVER INTO C, WHERE IT IS PRECIPITATED BY THE BARYTA
WATER.
it causes the formation of a white precipitate, which
analysis shows to be barium carbonate. The formation
of this body proves the evolution of carbon dioxide from
the seeds, as the entering air contains none. By using a
known strength of baryta water in c, the amount of the
gas evolved in a definite time may easily be ascertained.
These two processes, the absorption of oxygen and the
exhalation of carbon dioxide, are characteristic of what is
known as respiration. As already stated, it is a normal
process of the life of almost all protoplasm, and is con-
tinually going on so long as life lasts, although it is not
easily observed while the converse process, the absorption
122 VEGETABLE PHYSIOLOGY
and decomposition of carbon dioxide, is proceeding, accom-
panied by the evolution of oxygen. It is frequently said
that during daylight the process of the respiration of a
green plant is masked by that of carbon dioxide decom-
position. To put this statement into somewhat different
terms, the carbon dioxide which is liberated in the course
of respiration by the green plant, and which is in com-
paratively small amount, is reabsorbed by the green parts
of the cells, and undergoes the same decomposition as that
which is brought to the plant by the surrounding air. It
thus escapes observation unless special means, such as
those detailed, are adopted to bring it into evidence.
The respiratory processes are easily observed in the
case of all plants, and parts of plants, that are not green,
as there are in such cases no gaseous interchanges that
would interfere with their manifestation.
If a plant be carefully weighed at the commencement
and at the end of such an experiment as has been described,
it will be found to have lost weight during its stay in the
receiver, so that respiration is associated with a loss of
weight to the plant. This may readily be inferred from
the fact that the oxygen absorbed and the carbon dioxide
exhaled are approximately equal in volume, carbon dioxide
being perceptibly heavier than oxygen. Besides the
carbon dioxide, however, there is always also a certain
exhalation of watery vapour which takes place quite
independently of any supply from the root or the cut end
of the stem. The nature of the metabolism, or the vital
processes, is such that the living substance gives off both
water and carbon dioxide, while it coincidently absorbs
oxygen. This is quite independent of any constructive
processes, for it can be observed when no nutritive material
of any kind is supplied to the plant.
Though respiration is constantly proceeding wherever
living substance is found, the activity of the process is
by no means uniform. With care it can be detected in
such quiescent parts of plants as resting seeds, or buds
EESPIEATION 123
during their winter suspension of development, but in
such cases the gaseous interchange is reduced to a mini-
mum. In growing shoots or germinating seeds in which
vital processes such as the growth of protoplasm are going
on rapidly, and life is very active, it reaches a maximum.
In ordinary adult leaves and branches the activity of
respiration is intermediate between the other two condi-
tions. It is more intense, again, in the floral organs during
the time of their maturation. We may say in general
terms, wherever protoplasm is abundant, and the chemical
processes connected with the manifestation of its life are
going on most vigorously, there respiration is most active.
It is connected especially with the vital processes, and is
not associated directly with the presence of food materials.
A proof of this is afforded by an estimation of the activity
of respiration in seedlings, which, in the case of wheat, has
been found to increase steadily for about a fortnight, and
then to decline. Further evidence is afforded by the fact
that if seeds are thoroughly dried they do not respire. In
this condition the protoplasm is completely quiescent, so
far as we can ascertain. If, however, only a little water is
supplied to them, which, as we have seen in an earlier
chapter, is a condition necessary to set up changes in the
protoplasm, respiration commences, and increases as the
proportion of water present rises up to a certain limit.
When the respiratory processes are carefully measured
and compared with the weight of the organism, it is found
that under appropriate conditions they are more intense in
plants even than in warm-blooded animals. The respiratory
activity is as great in many seedlings as it is in the human
body, provided that both are maintained at the same
temperature. There is, however, a very great variability in
this respect, and the maximum activity is never maintained
very long in any particular plant. As maturity succeeds to
development its amount falls materially, being marked at
or near the original rate only in the regions of the active
meristems.
124 VEGETABLE PHYSIOLOGY
All seedlings, again, are not alike in the vigour with
which they carry on their respiratory processes.
We may pass on to inquire what is the relation between
the absorption of oxygen and the formation and elimination
of carbon dioxide and water. It is conceivable that the
oxygen may unite in the plant with carbon and with
hydrogen to produce at once the exhaled compounds. A
study of the living organism at work, howeverj soon shows
us that the process is not of this simple nature. We have
said, in the course of what has already b|een advanced, that
the amount of the carbon dioxide exhaled and that of the
oxygen absorbed are approximately equal. This, however,
is only true within certain limits ; if each is measured
accurately, they are not found to show an exact correspon-
dence. The ratio Co2:0 is usually spoken of as the
respiratory quotient. When the two processes are equal
the value of the respiratory quotient is unity ; when the
carbon dioxide is in excess it is greater, and when the
oxygen is in largest amount it is less, than unity. The
respiratory quotient has been found to vary to a greater or
less extent in different plants, and in the same plant under
different conditions. If its value is determined in the case
of germinating seeds, these differences are soon evident.
With starchy seeds the quotient is unity ; with oily seeds
it is much lower. That is, in the former case, the seeds
absorb a volume of oxygen equal to that of the carbon
dioxide they exhale ; in the latter case they take up more.
Various observers have shown that in certain cases
succulent leaves, such as those of the Agave or of particular
plants belonging to the Saxifragacece and the Crassulacece,
or again the phylloclades of Opuntia, one of the Cactacece,
are capable of absorbing oxygen without the simultaneous
evolution of carbon dioxide. » Nor is the oxygen absorbed
in these cases any more than it is in others without enter-
ing into some form of chemical combination/ for it cannot
be extracted by the air-pump. The latter also fails to
extract any carbon dioxide from the plants. The oxygen
EESPIEATION 125
enters the plant, and is in some way fixed or combined ;
the other process which usually accompanies this absorp-
tion does not take place, the carbon dioxide not only not
being exhaled, but apparently not even formed.
Conversely, carbon dioxide may be thrown off from a
plant without any simultaneous or even antecedent absorp-
tion of oxygen. When a seed is made to germinate in a
vacuum over a column of mercury, carbon dioxide is
found to be evolved. Kipe fruits have been found to give
off this gas in an atmosphere quite devoid of oxygen.
Too much stress must not, however, be laid upon these
latter observations, as we have certain evidence which
points to a different mode of formation of the carbon
dioxide in the presence and in the absence of oxygen
respectively.
Again, it is found that the respiratory quotient varies
according to the temperature at which the observations are
made. Evidently the two processes are not directly depen-
dent upon each other.
In making the estimation of the respiratory inter-
changes we are apt to lose sight of a fact to which atten-
tion has already been called, viz. that carbon dioxide is
not the only respiratory exhalation. The watery vapour
which accompanies it must also be accounted for. On the
hypothesis of the direct oxidation of carbon and hydrogen, if
the volume of carbon dioxide is equivalent to that of the
oxygen, there cannot have been the absorption of sufficient
of the latter to unite with hydrogen to form the water.
Even when the respiratory quotient is less than unity, the
same consideration has a certain value. The idea of
such direct oxidation cannot therefore be accepted.
It is evident from the foregoing considerations that the
vital activity of the protoplasts is somehow associated
with the two factors in the gaseous interchange. In the
absence of oxygen this vital activity gradually ceases, the
living substance being in fact slowly stifled or asphyxiated.
During its life one of the manifestations of its vitality is
126 VEGETABLE PHYSIOLOGY
the formation and exhalation of two fairly simple com-
pounds, carbon dioxide and water. To ascertain what is
the true relation of the two processes, it is necessary to
look closely at the nature of the chemical changes going
on in the protoplasm itself, or what is usually spoken of as
its metabolism.
Eespiration in the strict sense is therefore a process
going on in the living substance itself. The gaseous inter-
change observed is the expression of thl beginning and the
end of a series of complex changes in which the molecules
of the living substance are involved. The details of the
absorption of the oxygen of the plant from its environment,
and its presentation to the protoplasm, together with those
of the ultimate evolution of the carbon dioxide and water
from the plant-body, should be regarded rather as belonging
to the mechanism of respiration than to respiration itself,
which is a function of the living substance only. The former
corresponds to the entry of the oxygen into the lungs of
an air-breathing mammal, and its transport to the tissues,
together with the return of the carbon dioxide and water
therefrom ; the latter is strictly comparable to the changes
taking place in those tissues after the entry of the oxygen
into them.
The variation of the respiratory quotient which we
noticed in starchy and oily seeds respectively points to a
varied metabolism, according to the nature of the food
supplied to the living substance.
We see, then, that the two processes are not immediately
connected in the sense of the carbon dioxide and water
coming at once from the direct oxidation of carbon and
hydrogen, but that they are ultimately associated there can
be no doubt, though they are separated in time by a series
of chemical changes taking place in the living substance.
This ultimate association is shown by the fact that, if the
access of oxygen to a plant is prevented, after a longer or
shorter period the evolution of carbon dioxide ceases.
To get a true view of the nature of the process of
KESPIRATION 127
respiration we must therefore turn our attention to the
metabolic changes which are taking place normally in the
living substance. From the instability which we have
noticed in the protoplasmic material, we can infer that its
own molecules are in a constant state of decomposition
and reconstruction, new material being incorporated and
certain other substances cast off. Besides these, we are
probably not wrong in concluding that other changes also
take place in the various substances which are contained
in it, into which its own molecules do not enter. Pro-
cesses of slow oxidation and gradual reduction are taking
place there continually, excited, however, in all probability
by the changes in the protoplasm itself. Even these, how-
ever, are by no means simple, and the direct oxidation of
either carbon or hydrogen has probably no place among
them. An instance of them may be seen in the oxidation
of alcohol in the cells of Mycodermi aceti, a fungus which
converts alcohol into acetic acid. This process, into which
the molecule of protoplasm apparently does not enter, can
only go on in the living cell. Other similar instances
could be quoted.
The probable course of events is that the oxygen in
some way unites with the protoplasm, rendering it un-
stable, and initiating a series of decompositions which
result in the successive formation of many bodies of less
complex composition, each successive decomposition pro-
ducing simpler ones, till finally carbon dioxide and water are
formed. Simultaneously, reconstruction of the protoplasm
goes on, many of these residues being in whole or in great
part built up again into its substance, together with new
material supplied to it in the shape of food. If the
temperature is low, the breaking down of the protoplasm
proceeds but slowly, and reconstruction is rapid, so that
under these conditions the quantity of oxygen absorbed
or fixed as intramolecular oxygen by the protoplasm is
greater than the quantity of carbon dioxide formed by its
decomposition. At a higher temperature decomposition is
128 VEGETABLE PHYSIOLOGY
much more easily carried on, and its products are more
numerous and simpler. The decomposition and recompo-
sition go on side by side, simpler bodies being gradually
produced, either by their splitting from the protoplasm
directly, or by their being formed at the expense of the
more complex decomposition-products during processes of
slow oxidation in the substance of the protoplasm, till
finally a certain production of carbon dioxide and water is
arrived at. So long as the protoplasm remains alive the
amount of these is relatively small, reconstruction con-
tinually taking place. When, however, the protoplasm
dies, simple bodies, such as carbon dioxide, water, and
possibly ammonia, in addition, are produced abundantly
from the decomposition which attends its death.
It is possible that such processes as these are supple-
mented by others, in which the living substance is not
so intimately concerned. The protoplasm may combine
loosely with oxygen, and transfer it to other substances
present in it, which are consequently more directly oxi-
dised, the protoplasm in some way inducing the oxidation
of compounds which oxygen alone cannot attack. In this
way the protoplasm may act as an oxidising agent, without
itself undergoing the decompositions suggested.
The carbon dioxide is thus the final term in a series
of decompositions, of which the living substance is the seat
and into which it may actually enter, the decompositions
themselves being promoted by the access of oxygen. In
some cases, such as those of the succulent leaves of the
CrassulacecB and the tissues of the Cactus already alluded
to, this final term is not reached, no carbon dioxide being
exhaled. We have no reason to think that in these cases
a fundamentally different series of changes is set up. De
Saussure found that a piece of stem of Opuntia absorbed a
quantity of oxygen, which could not be extracted from it
by the air-pump. The fate of this oxygen must have been
similar to that which is absorbed by other plants ; it must
have entered into some form of combination, probably with
RESPIRATION 129
the living substance. The resulting decompositions,
though taking at first the same course as in other cases,
did not go so far. Instead of the liberation of carbon
dioxide, there was found a considerable increase in the
amount of certain organic acids, chiefly malic and oxalic
acids, which remained in the cells, and which probably
represented the ultimate products of the decomposi-
tions.
We have mentioned in the earlier part of this chapter
that there are a few lowly forms of plants which do not
require oxygen for their vital processes, and which do not
absorb it. Many of these so-called anaerobic plants are
killed by exposure to this gas. We must not, however,
necessarily conclude that their metabolism is of a totally
different kind from that of others, but rather that they set
up the decomposition and reconstruction of their proto-
plasm in a different way from those plants which need a
supply of oxygen to determine them. We shall return to
this point in a subsequent chapter, in which we shall
consider also the meaning of the complex metabolic
processes, \vhich we have so far discussed.
Though respiration is always proceeding wherever
there is living protoplasm, the activity of the process is
modified by different physical conditions. Of these,
temperature is one of the most important. There is a
lower limit, beyond which it appears to be suspended,
though life is not destroyed. This limit varies in different
plants, but is generally one or two degrees below the
freezing point of water. In a few cases, stwft as Conifers
and Lichens, it may% eVen be --10° C., but this is rare.
As the temperature rises from tnis minimum point, the
activity of respiration increases up to a certain optimum
point, which is usually not well defined, and which varies
considerably in different plants. If the temperature is
raised only a little higher than this, the living substance is
rapidly injured, and its respiration is checked. Variations
130 VEGETABLE PHYSIOLOGY
in temperature do not affect equally the absorption of
oxygen and the exhalation of carbon dioxide. At low
temperatures the latter is smaller than the former ; at high
ones the reverse is the case.
The effect of light upon respiration is not very marked,
and is probably indirect. Plants which grow in shady
spots usually manifest less respiratory activity than
similar ones growing in bright sunlight, but this may be
the result of the difference in the amount of nutritive
material they obtain, which is incident to the difference in
their situation. As we shall see in a subsequent chapter,
light has a very marked influence on the metabolic
processes, and • its indirect effects may be very far-
reaching.
Kespiration is considerably affected by variations in the
amount of oxygen which the environment of the plant
contains. The protoplasts can absorb even the last traces
of the gas which reach them, but a certain amount is
necessary for them to maintain a healthy condition. Great
variations are not usually met with, but on the summits of
high mountains there is much less available for them than
at the sea-level. If the amount of oxygen in the atmosphere
from any cause falls below about 5 per cent., respiration is
seriously impeded. Similarly plants cannot thrive in the
presence of too great an amount. When the pressure of the
gas attains the amount of twenty to thirty atmospheres,
respiration becomes very difficult and after a short time
ceases, and death ensues.
The process of respiration is also affected to a consider-
able extent by the nature of the substances which serve as
nutritive material for the reconstruction of the protoplasm.
It has already been pointed out that seeds containing oil
absorb more oxygen during germination than those which
contain principally starch. Fungi which are fed with car-
bon-compounds that contain relatively little oxygen,, give
off relatively less carbon dioxide than others which are
RESPIRATION 131
supplied with food containing a large percentage of this
constituent. Organs which contain much proteid matter
respire more copiously than others which contain but little.
The nature of the inorganic salts absorbed also influences
the process to a certain extent, though probably these only
act indirectly.
K 2
132 VEGETABLE PHYSIOLOGY
CHAPTEE IX
THE FOOD OF PLANTS. INTRODUCTORY
A GOOD deal of misconception exists as to the nature of the
food of plants. The character of their environment, and
the absence in most cases of any means provided in their
structure for the taking in of any material having a com-
position in any way approaching that of living substance,
have led to a not unnatural idea that they feed upon simple
inorganic compounds of comparatively very great simplicity.
This idea has found considerable support in the fact, which
is easily ascertained, that such bodies are those which are
absorbed in the first instance. By their roots when they
live fastened in the soil, or by their general surface when
they are inhabitants of water, comparatively simple inor-
ganic salts are found to enter them with the water which
they take up. By their green parts, and especially by
their leaves, carbon dioxide is absorbed, either from air or
water, according to their habitat. A study of the whole
vegetable kingdom, however, throws considerable doubt
upon the theory that these compounds are, in the strict
sense, to be called their food. Fungal and phanerogamic
parasites can make no use of such bodies as carbon di-
oxide, but draw elaborated products from the bodies of
their hosts. Similarly those fungi which are saprophytic
can only live when supplied with organic compounds of
some complexity, which they derive from decaying animal
or vegetable matter. We have no reason to suppose that
the living substance of these non-chlorophyllaceous plants
is so radically different from that of their green relations
that it has a totally distinct mode of nutrition.
THE FOOD OF PLANTS 133
In the flowering plants we find a stage of their life in
which the nutritive processes approximate very closely to
those of the group last mentioned. When the young sporo-
phyte first begins its independent life — when, that is, it
exists in the form of the embryo in the seed— its living
substance has no power to utilise the simple inorganic
compounds spoken of. Its nutritive pabulum is supplied
to it in the shape of certain complex organic substances
which have been stored in some part or other of the seed,
sometimes even in its own tissues, by the parent plant from
which it springs. When the tuber of a potato begins to
germinate, the shoots which it puts out derive their food
from the accumulated store of nutritive material which has
been laid up in the cells of its interior. Considerable growth
and development can take place without the access of any
of the inorganic substances which the parent plant was
continually absorbing. Fleshy roots, corms, bulbs, and all
bodies which are capable of renewed life after a period of
quiescence, show us the same thing ; the young shoots
emerging from any of them are not fed upon simple inor-
ganic bodies, but upon substances of considerable com-
plexity, which they derive from the tissues of the structures
from which they spring.
In adult plants of the most considerable complexity we
find instances of the same thing, though in these cases it
is generally rather more difficult to determine it ; the
living substance is nourished by materials which have
been constructed by it and stored at various places in its
tissues till their consumption has been called for.
What, then, are these substances which, in the strict
sense, constitute the food of plants ? We can ascertain
what are necessary by inquiring what are the materials
which are deposited in the seed for the nutrition of the
embryo during the process of germination. This process
is the most favourable for the elucidation of this point,
because, in its early stages at any rate, the nutrition of the
young plant is not complicated by any absorption from
134 VEGETABLE PHYSIOLOGY
the surrounding medium, such as sometimes rapidly
supervenes on the emergence of a shoot from a tuber or
a fleshy root. We find the seed contains in some part or
other of its substance, sometimes even in the embryo itself,
examples of great classes of food -stuffs which are the same
as those on which animal protoplasm is nourished, and
whose presence renders seeds such valuable material for
animal consumption. As these disappear during the
development of the young plant, which thus evidently
grows at their expense, we cannot doubt that they form
its food, and that vegetable protoplasm is essentially
identical with animal, at any rate so far as its methods of
nutrition are concerned. Proteids, carbohydrates, fats or
oils, together often with certain other bodies which are
less widely distributed, are the materials which, in various
forms, are met with.
If we study the protoplasm of a living, active, vegetable
cell, and treat it with appropriate solvents, we can extract
representatives of these, or of some of them, from its
substance, in the interior of which they are held some-
times in solid amorphous form, sometimes in fine
suspension or in actual solution. The nutrition of the
protoplasm can only take place when these substances are
brought into the most intimate relations with it ; from
them, no doubt, in ways not yet discovered, it builds itself
up, and by its own decompositions it reproduces many of
them. The details, however, of the interchange of matter
between the living substance and its food, the way in
which the latter is transformed into the former, are points
about which almost everything essential remains still to be
discovered.
But while we recognise that the ultimate nutrition of
protoplasm is dependent upon its receiving a supply of
such materials, we are face to face with the fact that, with
a few exceptions, the consideration of which may be
deferred, they are not furnished at all from the environ-
ment to the ordinary green plant, and often only partially
THE FOOD OF PLANTS 135
so to the saprophytic fungus, though they are freely
obtained from their host-plants by parasites. On the
contrary, we find the ordinary green plant taking in by
ordinary physical processes carbon dioxide from the air,
and water containing a variety of salts from the soil. The
saprophytic fungus may, and frequently does, obtain from
its surroundings certain compounds of ammonia, together
with some carbohydrate bodies, such as sugar. We can
ascertain that if these different compounds are supplied
under suitable conditions to the groups of plants mentioned,
the latter can flourish and develop. While we have the
strongest grounds for holding that the protoplasm is
essentially similar in all these cases, we see marked
differences between them with regard to the materials
which they absorb. The substances supplied to the green
plant are utterly unlike what we have seen to be the actual
food ; the saprophytic fungus can make use of the compounds
of ammonia, but absorbs carbohydrates as such, while the
parasite, whether fungus or phanerogam, obtains the
materials which we see are directly capable of feeding it.
If we say that the food of these various groups of
plants varies in the degree of its complexity, we must
carefully consider in what sense we use the term. food. In
the nutrition of the green plant there are clearly two very
different processes combined, which should be kept care-
fully distinct. We have the absorption of food materials
rather than of food in the true sense, and we have, follow-
ing such absorption, the expenditure of a considerable
amount of energy upon these food materials, with the
result that they are worked up into the complex compounds
which we find protoplasm can assimilate, and which are
those which are stored away in the substance of the plant
for the nutrition of vegetable substance and the develop-
ment of embryo, bud, or growing point.
In the case of the green plant this power of construct-
ing food extends to all the classes of food-stuffs ; in thai
of the saprophytic fungus it only applies to the proteids
136 VEGETABLE PHYSIOLOGY
and the fats, the carbohydrates needing to be supplied to
it as such, as we have seen.
This difference between food and the crude materials
from which it is constructed can be made clearer by
examining whether such simple inorganic bodies as the
green plant absorbs are capable of nourishing protoplasm
when freely supplied to it. If they are the true food,
plants everywhere should be able to make use of them.
But if we consider only one of them, the carbon dioxide of
the air, we find this is not the case. The plants which
are not green — that is, which contain no chloroplasts — can
do nothing with this gas. So long as a seed is in the
early stages of its germination, it is surrounded by carbon
dioxide, which is given off by its own protoplasm. But it
can make no use of it, and if the store of nourishment
provided for it in the endosperm or cotyledons is cut off, it
inevitably dies of starvation. A saprophytic fungus in
like manner is dependent for its life upon the absorption
of such a compound as sugar, and carbon dioxide cannot
aid at all in its nutrition.
Another fact throws a certain light upon the relation
of carbon dioxide to the feeding of a green plant. If such
an individual, in good health and endowed with ample
vigour, is removed from light to darkness, though this gas
be supplied in appropriate quantity, it can make no use of
it. The gas is evidently useless for immediate nutrition,
and its ultimate utility is dependent upon its being sub-
mitted to the action of some mechanism in the plant which
is called into play under certain conditions, of which ade-
quate illumination is one.
Similar considerations apply to other constituents of
the materials from which the true food of the living
substance is elaborated. They are absorbed in quantity,
but they do not become food until a considerable amount
of work has been done upon them by the plant itself.
In the strict sense, it thus appears that the ordinary
green plant does not absorb its food from without. It takes
THE FOOD OF PLANTS 137
in various raw materials from which it manufactures its
food in particular parts of its own tissues.
In connection with the nutrition of plants we have thus
to deal with the absorption of the crude food materials,
and to study the changes which they undergo after such
absorption. But this is not all ; the food which is manu-
factured from them is not merely prepared in answer to
the immediate requirements of the moment. A considerable
excess is usually constructed, and the surplus quantity is
stored in various parts of the plant's body for subsequent
consumption.
The food which is thus laid up in seeds, tubers, bulbs,
&c. is not deposited there in exactly the condition in which
the living substance requires it, so that there remains for
us to consider the processes of storage and the changes
which the stored materials subsequently undergo for the
purpose of feeding the living protoplasm.
The construction of food from the materials absorbed is
one of building up complex bodies from simple materials.
The utilisation of the stored surplus is comparable with
the digestion which is so marked a feature of animal
alimentation, and is one of breaking down of complex bodies
into simpler ones.
The actual nutrition of the protoplasm shows again two
distinct phases : the incorporation into its substance of the
ultimate constituents of the food, or its assimilation, is a
constructive process ; it is in turn associated with a
destructive one, by which, from the protoplasm itself, and
by its own activity, simpler bodies are produced.
The whole round of changes which embraces all these
operations is called metabolism, the constructive processes
being grouped together under the name of anabolism, the
destructive ones under that of katabolism.
The absence of well -differentiated organs set apart for
the discharge of these separate functions makes it rather
difficult at first to appreciate their independence. In most
animal organisms such a differentiation is easily seen, but
138 VEGETABLE PHYSIOLOGY
in. plants the cellular structure is so prominent, and the
life of the protoplasm is so closely related to its condition
in the cell, that attention needs to be specially directed to
the point. Each protoplast is dependent upon the contents
of its own vacuole, and the early constructive processes in
the metabolism, including the manufacture of food in such
cells as carry out this process, may take place in it side
by side with the digestive changes and at almost the same
time. True, a certain division of labour can be noted, but
it is not very clearly associated with particular organs.
Thus the leaf is especially concerned in the manufacture of
food, but it is mainly so by virtue of the chloroplasts which
its cells contain, These processes can go on perfectly well
in other parts than leaves ; indeed wherever there are
chloroplasts we know they do. Thus, though we associate
the leaf with this manufacture, it would be wrong to speak
of it as the organ to which this process must be referred.
We can say with greater accuracy that the cliloroplast is
the organ which conducts these preliminary constructive
processes, and that they take place wherever the chloro-
plasts are found. The wide distribution of the latter, how-
ever, shows us that there is no specially differentiated
member of the plant set apart to be an organ for this
function. In the same way the digestive process, or the
utilisation of stored products, goes on wherever there are
reservoirs of such bodies, and takes place in the cells of
which such reservoirs consist. There, and there only for
the most part, unorganised ferments or enzymes are found,
instead of being located in particular glands, as in the
animal body. These reservoirs, as we have already seen,
and shall see again later, are found in the most varied
regions of the plant's substance, regions moreover which
differ considerably in situation in different plants. We
cannot therefore speak of a differentiated organ of
digestion.
Starting, then, with the intricacy of the metabolic
processes placed before us, and with their relations to each
THE FOOD OF PLANTS 139
other, we may begin the consideration of them in detail
with an inquiry into the preliminary absorption of the
materials from which the food is ultimately made. Even
here we meet with some complexity, as the ordinary green
plant shows marked differences in behaviour from its
parasitic relative and from the great class of Fungi, which
possess no chlorophyll. We have already pointed out that
the construction of food does not follow exactly the same
course in green plants and saprophytic fungi, the chief
point of difference being seen in connection with the
carbohydrates. It will be best to consider first the
ordinary terrestrial green plant, noticing in passing
differences in behaviour shown by aquatic and epiphytic
forms.
140 VEGETABLE PHYSIOLOGY
CHAPTER X
THE ABSORPTION OF FOOD MATERIALS BY A GREEN PLANT
WE have seen that the materials which protoplasm is
eventually able to assimilate or incorporate into its own
substance, and which, therefore, constitute its food, are of
a similar nature to those deposited in seeds and other
storehouses of nutriment. We know further that these
are not the materials which an ordinary green plant takes
into itself from the environment in which it lives. We
know also that its structure prevents its taking in any-
thing in a solid form, and that everything entering it muse
either be in solution in the water which it is almost
constantly absorbing through its roots, or must become
dissolved in the liquid which permeates the walls of the
cells which line the intercellular passages. The only
substances that can be taken up under these conditions are
certain gaseous constituents of the air, and various inorganic
salts which are present in the soil. Between such raw
materials, and the complex products which are needful for
the nutrition of its substance, there is a great difference,
and the manufacture of the latter from the crude materials
absorbed constitutes a very important part of the metabolic
processes.
There are several ways in which we may proceed to
discover what a green plant absorbs from the soil, two of
which especially have been made use of by various observers.
The first is known as the method of water -culture. It
consists in cultivating plants with their roots inserted in
water containing various salts in solution, and observing
what effect upon their growth and development is pro-
ABSOEPTION OF FOOD MATERIALS 141
duced by the addition of certain compounds to the culture
fluid, or how the absence of any particular salt affects their
well-being.
In carrying out experiments in this way, it is usual to sow
some large seeds, such as those of the broad bean, in damp
sawdust, and allow them to germinate. When the radicle
of the seedling has elongated to the extent of about an
inch, the seed is placed upon a perforated cork inserted into
the neck of a bottle containing the liquid which is the
subject of the investigation. It is so arranged that the
radicle dips down through the cork into the liquid. As
growth proceeds the radicle develops a root-system in the
way appropriate to the particular plant used, which absorbs
from the liquid the salts which are required by it, so far as
these are present. At the same.the plumule grows upwards,
and soon a shoot appears, which develops pari passu with
the root.
By this method various plants can be cultivated with
different degrees of success ; in some cases not only leaves,
but flowers and even fruit can be produced. The progress
of the plant, and the readiness with which it will develop,
will depend upon the salts which are supplied to it in the
water, if it is maintained in normal conditions of light,
temperature, and aeration. In preparing the solution,
particular mixtures can be employed, and the most favour-
able one ascertained, while subsequent analysis of the
liquid will show to what extent the various constituents of
the culture fluid have been abstracted from it.
This method is, however, only of use in determining
particular points, such as the effect of the presence of
certain metals in particular combinations, or the influence
of different concentrations of particular substances. It
does not give an account of what is happening to a
plant with its roots embedded in the soil, for the com-
position of the latter cannot be compared with that of
a solution definitely made up for purposes of experiment.
The composition of the soil, as we have seen, is very far
142 VEGETABLE PHYSIOLOGY
from uniform, and the constituents which are within
the reach of the roots of two plants growing almost side
by side naturally may be materially different in their
proportions. This consideration makes it almost or quite
impossible to ascertain, by observation of the soil and the
plant growing in it, what are the substances which are
entering its roots.
The other method, which is of much more general
application, consists in making an analysis of the whole
body of the plant after its removal from the soil, and so
ascertaining what chemical elements it contains. A plant
gives off no solid excreta, and consequently whatever it
absorbs remains in its substance. The ultimate composi-
tion of the true nutritive matters, proteids, carbohydrates,
fats, &c., is known. Such an analysis having shown what
elements enter into the composition of a plant, and of the
food which it has stored in its tissues, it is easy to inquire
into the manner in which each is supplied to the plant
under examination, and into the work which is done upon
them in its cells.
As already noticed, the structure of the plant demands
that all the materials of a solid character shall be in such
a solution that they can enter its substance by means of
the physical process of osmosis taking place though the
cell-wall. This is equally true of gases, of which there
is considerable absorption by all plants, whatever may
be the nature of their habitat.
The details of absorption vary to some extent, however,
according to the environment of the plant. Aquatic plants
can absorb water, and whatever is dissolved in it, whether
of gaseous or solid character, by all parts of their surface.
Those which grow with their roots embedded in soil, and
their shoots exposed to the air, show a certain division of
labour in this respect. The mineral constituents obtained
from the soil are taken in by the root-hairs with the stream
of water ; those of a gaseous nature mainly find entry
through the leaves and other green parts.
ABSORPTION OF FOOD MATERIALS 143
If we examine the food-stuffs described as being essential,
we find that proteids contain carbon, hydrogen, oxygen,
nitrogen, sulphur, and perhaps phosphorus. Carbohydrates
and fats contain only the first three of these elements. To
make a destructive analysis of the plant, it must be dried
at 110°-120° C. to drive off the water it contains, and it
must then be carefully burnt, and the residue of the com-
bustion collected. The volatile products given off can also
be absorbed by appropriate methods, and their nature and
amount ascertained.
The incombustible residue, which is known as the ash,
is composed of several metals and some other elements,
which vary in nature and amount in different cases. An
analysis of this ash will reveal the nature of its constituents,
but it will not tell us in what condition or combination
they existed in the living plant, on account of the various
chemical changes which go on during the combustion.
The ash of plants when analysed is always found to
contain the four metals potassium, magnesium, calcium,
and iron. These are not present in the metallic condition,
_but are in combination with various acids, forming nitrates,
sulphates, chlorides, carbonates, phosphates, &c.
The presence of these nitrates, sulphates, &c. must not
lead us to infer that they have all been absorbed as such from
the soil and retained unaltered in the plant. Part no doubt
may be accounted for in this way, but much of the
nitrogen, sulphur, and phosphorus which formed part cf
the substance of the plant entered into combination with
the different metals and with oxygen during the combustion.
Some of the carbon of the carbonates found may have had
a similar origin.
Besides the four metals mentioned, various plants may
individually contain larger or smaller quantities of many
other elements variously combined. We find sodium very
generally present ; less frequently so, aluminium, copper,
zinc, manganese, silicon, bromine, iodine and others. All
of these are derived from compounds present in the soil,
144 VEGETABLE PHYSIOLOGY
or the water with which they are in contact ; indeed
the composition of the soil in which a plant grows deter-
mines to a very great extent what minerals enter it. If a
particular substance is soluble in the liquid which the root-
hairs absorb, and is capable of osmosis through their
membrane, a certain quantity will, by ordinary physical
processes, be taken up by them.
It does not, however, follow that, if the conditions
alluded to are realised, absorption of a particular salt will
go on indefinitely. The quantity of any substance which
a plant will absorb will depend upon whether it is made
use of in any way, or can be deposited in its tissues in an
insoluble form.- This can be seen most easily by studying
the behaviour of a single cell. If any substance which
enters the cell by osmosis is used in its metabolism, it will
be quickly removed from the sap in its vacuole, and more
will enter. If not, the cell-sap will soon have taken up as
much of it as it can contain, and the absorption of that
particular substance will cease. This is equally true of
such a complex of cells as constitutes a plant, though the
time of the absorption will be more prolonged. As soon as
all the cells of the complex attain a condition of equili-
brium with regard to the particular salt in question, no
more will be taken up. This follows from the nature of
the process of osmosis. If the substance under examina-
tion is withdrawn from the sap in any part of the plant,
and made use of for any purpose, or deposited in the cells
in an insoluble form, the condition of equilibrium will not
be attained so long as such a withdrawal at any point takes
place, and a stream of the substance will flow continuously
to the point in question, so that the process of absorption
will be continuous also.
Some of the materials found in the soil are readily
soluble in the water which surrounds its particles. We
have already seen that it is only this hygroscopic water
which finds its way into the root-hairs. Such salts dis-
solve in this water and can enter the plant without difii-
ABSOEPTION OF FOOD MATERIALS M5
culty if they are capable of passing through the plasmatic
membranes which are the limiting layers of the protoplasm
of the root-hair (page 57). The solution of the salts is
always very dilute, and, on account of the ready diffusion
that takes place, their concentration is approximately uni-
form in any particular soil. Other salts are insoluble in
pure water, and their absorption presents more difficulty.
Many are soluble in water which contains carbon dioxide,
and as considerable quantities of this gas are continually
being generated in the soil, the water there is charged
with it, and bodies, otherwise intractable, are thereby
brought into solution and absorbed.
The power of water containing carbon dioxide to effect
the absorption of such substances is capable of easy demon-
stration. One of these salts is calcium sulphate or gypsum.
If a plate of this substance is placed at the bottom of a
flower-pot and the pot then filled with moist earth, a plant
caused to grow in it till its root system is well developed will
have some of its roots closely adpressed to the gypsum
j)late. After a time, examination will show the surface of
the plate eaten away at all points except where the roots
have become adpressed to it, and the regions covered by
the latter will stand out in slight relief. The whole sur-
face will have been subjected to the action of the water
and the carbon dioxide it contains, except where it has
been covered by the roots, and the solvent action will con-
sequently be recorded.
A third factor which must be considered in the process
of absorption is the acid sapjvhich the root-hairs contain.
Not only does the acid cause water to enter the hair
osmotically, but a little of the sap exudes in the same way,
and this has a certain solvent action upon the particles to
which the root-hairs cling. Thus certain salts can be
absorbed, though they may be soluble neither in pure
water, nor in water containing carbon dioxide.
A similar experiment to the one just described will
demonstrate this property of the acid sap. If, instead of
L
146 VEGETABLE PHYSIOLOGY
gypsum, a polished plate of marble is inserted into the
flower-pot, after a certain time of growth of the plant con-
tained in it, the plate will exhibit a tracing of the course
of the roots which have come into contact with it, but,
instead of being in relief as in the former case, it will be
etched to a certain depth. The solvent influence can thus
be seen to come from the root itself, and not the water in
the soil. It will, in fact, be the acid sap which makes its
way out of the root -hairs.
Certain constituents of the soil can be absorbed which
are made available in neither of the ways mentioned.
Soils contain many constituents which cannot pass through
the plasmatic membranes of the protoplasm, but which, in
the presence of water, react with one another, producing
new compounds which are capable of such osmotic entry
and which are consequently absorbed.
The solutions taken in are excessively dilute. We
cannot make a plant take up a greater quantity of any
salt by bringing its roots into contact with a strong solu-
tion of it. There is a certain relation necessary between
the substance and the water, which has been the subject of
considerable investigation. Every salt is absorbed by a
particular plant in a certain strength of solution, or in other
words with each molecule of salt there is a certain invari-
able quantity of water taken in. The quantity is not the
same, however, for each salt.
The salts which different plants absorb, in like manner
vary in amount. If two species are growing in the same
soil, side by side, under exactly the same conditions,
the amounts of the several salts present in the soil which
are absorbed by the plants of the different species will not
be the same. In each case the quantity will vary accord-
ing to the use the plant can make of it. This is well
illustrated by the amounts of silica which can be taken up
by grasses and by leguminous plants respectively. In an
ordinary pasture there are always found several kinds of
grasses, together with clover and other allied plants. An
ABSORPTION OF FOOD MATERIALS 147
analysis of these will show that the ash of the grasses may
contain many times the percentage of silica that is found
in that of the leguminous plants. The grasses accumulate
silica in their epidermal cells, while the leguminous plants
do not. Hence the absorption of that substance soon
ceases in the latter case.
Again, if a particular soil contains several different
salts, a plant growing in it will not absorb them in equal
proportions, nor in those in which they exist in the soil.
An illustration of this fact is afforded also by marine Algae,
which accumulate in their tissues much greater amounts of
potassic than of sodic salts, though sea-water contains
much larger quantities of the latter than of the former.
This fact admits of a similar explanation to that given in
the case already mentioned. The absorption of a salt will
cease as soon as the cell-sap attains exactly the same degree
of concentration as the entering stream. In this case
there will be no further osmotic action as far as the salt
is concerned, though there may be a continuous entry of
water into the absorbing cells.
We have seen that the continuous absorption of water
by the root-hair will depend upon certain external condi-
tions, such as the temperature of the soil, the activity of
transpiration at the time, the degree of illumination the
plant receives, &c. These conditions affect also the absorp-
tion of the substances in solution.
The substances which are absorbed by the roots in this
way are naturally very varied. The most important of
them in the metabolism of the plant are the compounds of
nitrogen. In the soil these exist in the form of nitrates or
nitrites of the metals mentioned, and as compounds of
ammonia. Green plants take in little or none of the latter,
which are however made available for their use by the
action of certain bacteria which the soil contains. These
humble organisms have the power of converting the
ammonia compounds into nitrites, and the latter into
nitrates, in which form they are taken up. This process
L 2
148
VEGETABLE PHYSIOLOGY
of nitrification is the special property of two different
bacteria, one of which forms nitrites from the ammonia
compound, and the other transforms nitrites into nitrates.
Certain fungi differ in their behaviour from green plants,
absorbing ammonia compounds without such conversion.
It is in the way described that a normal green plant
absorbs all the nitrogen which it uses for the construction
of food substances. The nitrogen
of the air is made use of only in
very exceptional cases. Certain
lowly Algae are said to have the
power of using it, but the process
is not fully understood. Some of
the bacteria in the soil appear to
be able to cause the nitrogen of
the air to enter into some form
of combination, probably yielding
either nitrates or compounds of
ammonia. A few plants belong-
ing to the Natural Order Legumi-
noscB can also use atmospheric
nitrogen, but their power depends
upon the association with their
roots of certain fungi or bacteria
which infest the cortical tissues
and develop peculiar tubercular
structures upon the roots (fig. 84) .
The actual mode of absorption in
this case also is obscure ; the parts
Plo. 84. -ROOT OF A LEGUMI- played by the root and the fungus
NOUS PLANT, SHOWING THE Qr bacterium respectively are not
at all determined. The atmo-
spheric nitrogen apparently is
made to enter into some form of combination, and is then
absorbed by the root, probably through the tissue of the
fungus. It is not absorbed by the leaves of the plant.
Organic compounds of nitrogen are seldom presented to
TUBERCLES ATTACHED TO THE
MAIN ROOT AND TO ITS
BRANCHES.
ABSOEPTION OF FOOD MATEEIALS 149
the roots of plants, so that the amount of the element which
is absorbed in such a way is very small. Indeed it may
be said that such an absorption is almost entirely excep-
tional. It has been found that plants are able to utilise
urea and other amides when these are present in the soil.
In very rich soils, or those containing a large quantity of
humus, such compounds are to be met with, and there is a
probability that they are more easily worked up into actual
nutritive substance than the inorganic compounds which
have been spoken of.
A few plants obtain a more fully elaborated material in
a very different way. These are the so-called insectivorous
plants, which have the power of utilising proteid matters.
Among them may be mentioned the pitcher plants,
Nepenthes, Sarrdcenia, &c., and the fly-catching plants,
Drosera, Dioncea, and others. In the pitchers of Nepen-
thes, &c., which are specially modified foliar structures,
there is an accumulation of water, in which insects are
from time to time drowned. Their bodies decay, or are
digested by a peculiar secretion, which is prepared in the
tissue of the pitchers, and excreted into the water they
contain. The products of the decomposition or digestion
are absorbed by the tissue of the pitchers in the same way
as similar products are absorbed by the stomachs and
intestines of animals. Drosera and Dioncea bear certain
glandular structures on their leaves which pour out a fluid,
by which insects become surrounded after alighting on
the laminae. This secretion possesses digestive properties
resembling those of the gastric and pancreatic fluids of
animals, and by the action of this juice the bodies of the
captured insects are digested, and the nitrogenous material
is subsequently absorbed by the leaf surface. A fuller
discussion of these mechanisms will be found in a subse-
quent chapter. These plants are generally found growing
in such a situation that they are not brought into contact
with inorganic compounds of nitrogen, and hence are cut
off from the supplies which are afforded to the roots of
150 VEGETABLE PHYSIOLOGY
ordinary terrestrial plants. The mechanisms described
afford instances of special adaptations to particular environ-
ments, and will therefore be considered in more detail
later.
Besides compounds of nitrogen, the materials absorbed
by the roots of normal green plants include the constituents
of the ash. Of these the more prominent are the com-
pounds of potassium, sodium, magnesium, calcium, and
iron. The sulphur and phosphorus which enter into the
composition of the protoplasm are also taken in by the
roots, in combination with the metals mentioned, and
with others whose occurrence is not so general. The
sulphur is absorbed in the form of sulphates, and the
phosphorus in that of phosphates, of these metals.
Potassium is present in the soil in various combinations,
principally as the sulphate, phosphate, chloride, and
probably the silicate. After the nitrate the chloride
appears to be the salt which is the most advantageous to
plants. Calcium and magnesium exist in similar combi-
nations, all of which, except the chloride, appear to be
suitable for absorption. The chloride is, on the whole,
deleterious. Iron can be absorbed in almost any inorganic
combination. Sodium is absorbed in similar forms to
those of potassium, the nitrate being the most valuable.
Sodium chloride is frequently present in considerable
quantity in the plants which are found on the sea-shore.
Silicon is present in many plants, being especially
prominent in the grasses and the horsetails. It is taken
up from the soil in the form of soluble silicates, and
possibly to some extent in that of soluble silicic acid.
The other occasional constituents of the ash, which
have not so general a distribution as those already
mentioned, include a number of metals which play no part
in the nutritive processes. They are usually present in
very small amount, and appear to be of accidental occur-
rence, being absorbed by reason of the solubility of their
salts and their power of entering the root-hairs by the
ABSOEPTION OF FOOD MATEEIALS 151
ordinary process of osmosis. They are taken up in very
various combinations. Their presence is not generally
constant, and appears to depend entirely on the composi-
tion of the soil.
The water which the plants take up is the chief source
of the hydrogen and oxygen which enter into the compo-
sition of the substance of the plant. A little of both these
elements is taken in in the several combinations of the
metals ; phosphates contain both, nitrates and carbonates
contain oxygen. The amount of them absorbed in these
forms is, however, relatively small. As we shall see
later, the water plays a very prominent part in the con-
structive changes which take place in the plant.
The gases present in the water of the soil also make
their way into the root-hairs with the stream, but the
quantity is very slight compared with what is absorbed by
the sub-aerial parts. The gas carbon dioxide, which we
have seen to be present in the earth in considerable
quantity, is, however, not made use of in the constructive
processes. All of this particular food material is taken in
from the air. A little carbon is absorbed in the form of
carbonates. Many complex organic compounds of carbon
are taken in by those roots with which fungi, such as the
mycorhiza of certain trees, are living symbiotically, but
this is exceptional. The root-hairs are capable of absorb-
ing such organic compounds as sugar, but these materials
are rarely presented to them.
The absorption of gases from the air takes place in the
leaves and other green parts. They enter freely through
the stomata so long as these are open, and find their way
into the intercellular space system, the importance of
which we have already examined. These intercellular
spaces contain, as we have seen, a mixture of gases which,
though approximating to the composition of the atmo-
sphere, yet differs from it in the relative quantities of the
constituents. We have seen that the composition of this
mixture of gases tends to become uniform by the currents
152 VEGETABLE PHYSIOLOGY
which circulate in the intercellular cavities, and by the
slower processes of diffusion, which are set up in conse-
quence of local production or abstraction of particular
constituents. So long as the stomata and the lenticels are
open, the composition of the atmosphere within the plant
tends to become identical with that of the external air.
The actual absorption of the gases takes place almost
entirely from this internal reservoir, very little finding
entrance into the cells of the epidermis. A certain
amount is, however, taken in by the very young parts
which have not become modified by the development of a
cuticle.
The cells which abut upon the spaces in the leaves
and other green parts are those which are principally
concerned in the absorption of gases. Their walls are very
thin and delicate, and are saturated with water. The
different gases present dissolve in the outermost film of
this water, according to their degree of solubility, and
thence diffuse slowly through the membrane into the cell
sap, which saturates the protoplasm, and fills the vacuoles.
The quantity of each taken up depends, as in the case of
the metallic salts already discussed, upon the ability of the
protoplasts to make use of the gas, and thus to withdraw
it from the sap. If it can be combined in any way with
other bodies in the cell, or with the living substance itself,
it is thus withdrawn from the water, and room is made for
more to enter. If not, the limit of saturation of the sap is
soon reached.
The only gas which is absorbed from the air for the
purposes of food-construction is carbon dioxide. This
exists in the atmosphere in very small amount, only about
four parts in ten thousand being normally present. The
very large green surface which an ordinary terrestrial
plant possesses renders, however, a considerable amount
of absorption possible. If the general conditions are
favourable, the absorption is continuous, for carbon dioxide
is at once decomposed or made to enter into some form of
ABSOEPTION OF FOOD MATEEIALS
153
combination in the cells of the green tissues, and so a
stream is always entering.
Both nitrogen and oxygen are soluble in water, though
to a different extent. It has already been stated that
the nitrogen so taken in is not -used in the constructive
processes, and accordingly a mere trace is absorbed in this
way. A larger amount of oxygen enters, but experiments
have proved that it is not used for the manufacture of
nutritive substances, being applied to other purposes.
The absorption of carbon dioxide takes place usually
at the ordinary atmospheric pressure. In some parts of
FIG. 85. — TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING THE
DIFFERENT ARRANGEMENT OF THE MESOPHYLL ON THE TWO SIDES. X 100.
the internal reservoirs it exists at a slightly higher pressure,
in consequence of a local production in the tissues. Plants
can, however, absorb this gas when it is present in much
larger quantities than it is in air. Too much, however, is
possible, and then the cells are unable to take it in at all.
The continuous absorption of carbon dioxide is possible
only under certain conditions ; the cells which contain
chloroplasts are the only ones which can take it in in any
quantity, and they can only do so when they are exposed to
light, preferably that of bright sunshine, and when the plant
is maintained at an appropriate temperature. Its absorption-
is accompanied by the exhalation of a volume of oxygen
which is equal to the volume of the carbon dioxide absorbed,
and it is attended by a continuous increase in the weight
of the plant.
154 VEGETABLE PHYSIOLOGY
We have seen that most of the water absorbed by the
roots is conveyed regularly through the axis of the plant
until it reaches the leaves, in which, after traversing the
cells of the mesophyll, it is evaporated into the intercellular
spaces. Into these cells of the interior of the leaf, all the
materials used in the construction of food are thus at once
transported, both those entering the tissues from the soil,
and those absorbed from the air. These mesophyll cells
have generally a different arrangement on the two sides of
the leaf (fig. 85), but they all agree in containing chloro-
plasts. In these cells takes place the work of construction
of organic nutritive substance, such as the plant can
live upon — work which is carried out mainly through the
instrumentality of the chloroplasts.
155
CHAPTEE XI
THE CHLOROPHYLL APPARATUS
THE food materials whose absorption we have now discussed
are built up in the body of the plant into such substances
as are capable of being assimilated by the protoplasm, and
consequently of ministering to its nutrition. They undergo
a striking series of changes before they are capable of sub-
serving this purpose, and of becoming incorporated into
the plant-body. The great object to be attained is the con-
struction and growth of the living substance, which itself
subsequently produces the more permanent material that
we find stored in the shape of the masses of wood and bark
and the other substances which an adult plant contains.
The green plant contains a mechanism for the formation of
organic substance from these simple inorganic materials,
and it is to the activity of this mechanism that we owe
almost the whole of the organic matter which is found in
nature, whether exhibited by animal or by vegetable struc-
tures. This mechanism is known by the name of the
chlorophyll apparatus, and our attention must now be
turned to its nature and its mode of working.
Chlorophyll is a green colouring matter which is gene-
rally found associated with definite protoplasmic bodies
known as plastids. These are usually considered to possess
a reticulated structure, and the pigment, in some form of
solution, occupies the meshes of the network. From their
being coloured green by the pigment they are known as
chloroplastids or chloroplasts. The solvent of the pig-
ment which is in these bodies is of a fatty nature, and- is
probably some kind of oil. Alcohol, chloroform, ether,
156 VEGETABLE -PHYSIOLOGY
benzol, and a few other liquids can extract the chlorophyll
from the plastids and leave them colourless. The pigment
can be obtained from them also by treatment with dilute
alkalis, such as potash and soda. By whatever solvent it
is extracted, however, it appears to undergo decomposition,
so that the solution does not yield it up in the form in
which it exists in the vegetable cell.
A solution of chlorophyll in alcohol or chloroform shows
the curious property of fluorescence ; if regarded by trans-
mitted light it appears 'green, whatever may be the degree
of concentration of the solution ; if a strong solution is
looked at by reflected light, it has a blood-red coloration.
When a beam of white light is allowed to pass through
a prism, and is then made to impinge upon a screen of
white paper, it gives the appearance of a band in which all
the colours are represented in the following order : — red,
orange, yellow, green, blue, indigo, and violet. This is due
to the different degrees in which the rays which produce
the sensations of those colours are bent or deflected by
the prism. This coloured band is called the spectrum of
white light. In order to get it exhibited to the greatest
advantage, it is best to admit the beam of light to the
prism through a narrow slit. The spectrum may then be
regarded as a succession of images of the slit, each ray
giving its own image of the aperture and producing that
image in its appropriate colour. If a solution of chlorophyll
is placed in the path of the beam before it reaches the slit,
the resulting spectrum is found to be considerably modified.
Instead of showing a continuous band in which all the
colours are represented, it is interrupted by seven vertical
dark spaces. The rays which would have occupied these
spaces in the absence of the solution of chlorophyll have no
power to pass through the latter, and consequently their
images of the slit are represented by dark lines, which
together constitute the black bands. In other words,
chlorophyll absorbs these particular rays of light which are
missing.
THE CHLOROPHYLL APPARATUS
157
In fig, 86 is a representation of the spectrum which
such treatment produces and which is called, from the facts
just narrated, the absorption spectrum of chlorophyll. The
uppermost figure is that which is exhibited by an alcoholic
solution or extract of leaves ; the middle one is given by
chlorophyll dissolved in benzol. The first band on the left
is the darkest, and is found to be in the red part of the
spectrum. The three bands on the right are broader, but
are not so well defined. They cover nearly all the blue end.
The three thinner and lighter bands are in the yellow and
green parts of the spectrum. Chlorophyll therefore has
m w v
FIG. 86, — ABSORPTION SPECTBA OF CHLOROPHYLL AND
XANTHOPHYLL. (After Kraus.)
Vff
the power of absorbing a large number of red rays, a good
many blue and violet ones, and a few of the green and
yellow. The distinctness with which these absorption
bands are seen depends upon the strength of the solution,
those in the red and blue being, however, always promi-
nent. Careful experiments have proved that chlorophyll
is a single pigment and not a mixture of two, as has often
been stated. It is, however, easily decomposed, and the
products of its decomposition are generally found with it
in the chloroplast. One of these, Xantliophyll, which is
of a bright yellow colour, is always extracted with the
158 VEGETABLE PHYSIOLOGY
chlorophyll by alcohol. It can be separated from the
extract by appropriate means, and its solution yields the
absorption spectrum represented below those of chlorophyll
in fig. 86. Another pigment, Erythrophyll, is present in
those leaves which are found upon the trees in autumn.
Like xanthophyll, it appears to be a product of the decom-
position of chlorophyll, and it has a spectrum which differs
from both the others.
It is extremely difficult to say what is the chemical
composition of chlorophyll, on account of the readiness with
which it is decomposed. In all the processes which have
been adopted for its extraction it undergoes decomposition,
and consequently no definite conclusions as to its chemical
nature can at present be arrived at. It can be made to
yield definite crystals by appropriate methods of treatment
after extraction, but it is probable that these crystals are
a derivative of chlorophyll and not the pure pigment.
Analyses of the crystals have been made by Gautier and by
Hoppe-Seyler, who give them the following percentage
compositions :—
Gautier Hoppe-Seyler
C 73-97 73-34
H 9-8 9-72
N 4-15 5-68
0 10-33 9-54
Ash 1-75 1-72
According to Hoppe-Seyler the ash contains phosphorus
and magnesium.
From his analysis Gautier came to the conclusion
that chlorophyll is related to the colouring matter of the
bile ; Hoppe-Seyler considered, on the other hand, that it
is a fatty body allied to lecithin.
Except in the lowest unicellular plants, the chlorophyll
is always attached to some form of protoplasmic body
known as a plastid. These are small masses, of varying
size and shape, which are embedded in the general cyto-
THE CHLOEOPHYLL APPARATUS 159
plasm of the cell (fig. 87). Even in the lowly forms it is
apparently never uniformly distributed through the body
of the protoplast. The form, dimensions, and structure of
the chloroplast differ considerably in the different groups of
plants. In some of the filamentous green seaweeds it may
appear as variously shaped bands or plates. Spirogyra
shows it as a spiral band passing round the cell ; in Zyg-
nema it has the form of two star-shaped masses which are
attached to the cytoplasm by bridles ex-
tending to the cell- wall. In the brown
and red seaweeds the plastids are not
green, but have the appropriate colours
of the plants. These plastids contain
other pigments in addition to the chloro-
phyll, but the latter can be made ap-
parent by extracting the cells with cold
distilled' water, in which the other pig-
ments are soluble. In all plants higher in T°PLASM OF A CELL OF
THE PALISADE TISSUE
the scale than the Algae the chloroplasts OF A LEAF.
are found as round or oval bodies embedded in the cytoplasm.
They never occur in the vacuoles of the cells. Though
normally green, they can assume other colours, such as
yellow, brown, or red, but this is due to an alteration of
the pigment they contain. Examples of this change are
afforded by the assumption of the autumnal tints by
foliage leaves, and by the changes in colour which are
characteristic of ripening fruits.
In the Mosses the chloroplasts are found throughout
the cells of the leaves, in the outer parts of the sporo-
goniurn, and in certain cells of the axis. In the Ferns they
occur chiefly in the leaves, occupying the cells of the
epidermis as well as those of the mesophyll. In the
higher plants they are found most prominently in the
mesophyll of the leaves, the epidermis as a rule being free
from them. When the leaves are dorsiventral in structure,
the chloroplasts are more numerous in the palisade paren-
chyma which lies just below the upper epidermis than they
160
VEGETABLE PHYSIOLOGY
are in the spongy tissue which occupies the lower half of
the thickness of the leaf (fig. 88). The guard-cells of the
stomata, however, always contain them. The green cortex
of young stems and twigs also exhibits them. In such
plants as the Casiiarinas and the Equisetums (fig. 89), in
s.pa.
FIG. 88. — TRANSVERSE SECTION OF PORTION OF THE BLADE OF THE
LEAF OF Beta.
cu, cuticle ; ep, epidermis ; p.pa, palisade tissue ; s.pa, spongy tissue ;
v.b, vascular bundle; st, stoma ; i.s, intercellular space.
which the leaves are rudimentary, definite longitudinal
bands of cells- in the young stems contain them.
The structure of such a chloroplast as is characteristic
of one of the higher plants has not been very completely
investigated. There is undoubtedly a protoplasmic basis
with which the colouring matter is in some way associated.
THE CHLOKOPHYLL APPARATUS
161
As already stated, many botanists consider the protoplasm
to be arranged in a network, whose meshes are filled with
a solution of the pigment. Others consider the protoplasm
to be homogeneous, but honeycombed with vacuoles which
are filled with the solution of the chlorophyll. By the
action of dilute acids, or by treating the chloroplasts with
steam, the colouring matter may be made to exude from the
framework in viscid drops, leaving the latter colourless.
FIG. 89. — POBTION OF AEKIAL STEM OF Equisetum.
a, cortical lacuna; b, lacuna in vascular bundle; c, chlorophyll-containing cells.
It then appears to have a reticular structure, but how far
this condition is brought about by the action of the reagents
is uncertain. The chlorophyll, however, is certainly not
uniformly diffused through the body of the plastid.
In the process of the formation of the chloroplast
it is not difficult to see that its two constituents are not
M
162 VEGETABLE PHYSIOLOGY
inextricably connected. The plastids are not, as already
mentioned, differentiated out of the ordinary protoplasm of
the cell, but are formed by the division of other plastids.
In many cases they are found without the colouring
matter, as in underground organs such as the tubers of the
potato. They are then known as leucoplasts. Plants
which are grown in darkness have no green colouring matter
in their leaves, but the cells of the mesophyll contain the
plastids much as normal ones do. They are pale yellow
in colour, containing another pigment known as etiolin,
which appears to be an antecedent of chlorophyll, and to.
be transformed into the latter when brought into the
presence of sunlight. Exposure to light is almost uni-
versally a necessary condition for the formation of the green
pigment. Exceptions are known among the Ferns and the
Conifers, particularly the seedlings of Pinus ; also in the
seed of Euonymus europaus, the embryo of which is
green, though it is buried in the interior of the endosperm
and surrounded by a thick testa covered by an arillus.
If a green stem is withdrawn from the light, the chloro-
phyll slowly disappears, as is shown in the process of the
bleaching of celery. The disappearance is, however, very
gradual. It is probable that in the living chloroplast
the colouring matter is continually being decomposed and
reconstructed, and that the reason of the bleaching is that
the reconstruction cannot take place in darkness. Light of
too great intensity causes the destruction of the green colour.
Chlorophyll can be developed only when the temperature
rises above a certain point, which varies with different plants.
It is a matter of common observation that the leaves of young
Hyacinths which emerge from the soil in the early spring are
often colourless or pale yellow. The chloroplasts are found
to be present in such leaves, but they are yellow, owing to
the presence of etiolin instead of chlorophyll. The leaves
which are produced later, when the temperature of the air is
higher, have the normal green appearance.
Chlorophyll is not developed in a plant unless the latter
THE CHLOEOPHYLL APPAEATUS 163
is supplied with a certain quantity of iron, but the relation
of the latter to the pigment is not known. It does not
enter into its composition. The influence of the metal can
be ascertained by cultivating a seedling, by the method of
water-culture, in a solution which is free from iron. The
seedling assumes a sickly yellow appearance, not unlike that
presented by a plant grown in darkness. It is said to be
chlorotic. The addition of a very small quantity of an
iron salt to the culture-medium causes the appearance of
chlorophyll in the plastids. The presence of oxygen is
also necessary for the formation of the pigment.
The chlorophyll apparatus of a plant is primarily con-
cerned with the production of carbohydrate bodies, such as
the various sugars which the plant contains, and it is to
the formation of these that attention must first be given.
It carries out this constructive process only under particular
conditions, the most important of which is light. We have
seen that a certain degree of illumination is necessary for
the formation of the chlorophyll. The pigment once
formed may continue to exist for a time in darkness, but it
is quite incapable of exercising any constructive power
unless light be admitted to it. Consequently the formation
of carbohydrates is an intermittent process, being quite in
abeyance during the night. The effect of light is thus
twofold, its access causing the original formation and the
subsequent activity of the chlorophyll apparatus. The il-
lumination need not be very intense, though it is probable
that the greatest activity is manifested in direct sunlight.
Plants which grow in deep shade are however capable of
decomposing carbon dioxide. It must be remembered, too,
that the chloroplasts are situated some little distance within
the leaf or stem, at any rate in phanerogamic plants, and
there must be a certain loss of light as it penetrates through
the epidermis.
The activity of the chlorophyll apparatus is also con-
siderably influenced by variations of temperature. The
lower limit beyond which no carbohydrates are constructed
164 VEGETABLE PHYSIOLOGY
lies probably a little below the freezing point of water, at
which point, however, activity is not long maintained, and
then only by alpine forms. Jumelle has stated that in cer-
tain plants of hardy type it can proceed at as low a
temperature as — 40° C. Plants which normally live in hot
climates cannot manifest any power of action below about
4° C. The optimum temperature for the plants of tempe-
rate climates is from 15° C. to 25° C., above which activity
diminishes, though not very rapidly, ceasing when about
45° C. is reached. These high temperatures affect the
living substance of the chloroplasts very injuriously. The
activity of the chlorophyll apparatus is dependent also to
some extent upon certain of the mineral salts present in the
cells. According to Bokorny it cannot be called into play
in the absence of compounds of potassium.
As the activity of the chlorophyll apparatus is so essen-
tially dependent upon light, the process of construction of
carbohydrate substances from carbon dioxide and water,
which is its primary object, may appropriately be called
photosynthesis. This term has certain advantages over
the older expression, the assimilation of carbon dioxide, as
the term ' assimilation ' may preferably be reserved for the
process of the incorporation of food materials into the sub-
stance of the protoplasm.
Photosynthesis consists, then, in the formation of some
form of carbohydrate from the carbon dioxide which is
absorbed from the air, and the water which is present in
the cells. When these simple bodies are exposed to the
action of the chloroplast in presence of light and mode-
rate warmth, the carbon dioxide disappears, and a volume
of oxygen equal to that of the carbon dioxide is evolved.
The apparatus shown in fig. 90 will enable this inter-
change of gases to be seen. Into a glass jar is poured
some water containing carbon dioxide in solution. Some
watercress is put into the water and a funnel inserted above
it, the end of which rises into a burette filled with water
and closed by a stopcock. The whole apparatus being
THE CHLOEOPHYLL APPARATUS
165
placed in sunlight, bubbles of oxygen will be given off by
the leaves and will rise into the burette. If no carbon
dioxide is in the water, no oxygen will be given off.
There is nothing certainly known at present as to the
details of the changes which connect these two phenomena.
It has been suggested by Baeyer that the carbon dioxide is
decomposed with the formation of carbon monoxide and
oxygen, according to the equation 2C02 = 2CO + 02. At
the same time there is a decomposition of water, possibly
in the way denoted by the equation 2H20 = 2H2 + 02.
The oxygen is given off, the volume being found, when care-
fully measured, to be equal
to the volume of carbon
dioxide undergoing de-
composition* The carbon
monoxide and the hydro-
gen are then thought to
unite, producing formal-
dehyde, a body repre-
sented by the formula
CH20, or preferably
HCOH. This suggested
series of reactions agrees
fairly closely with the ob-
served facts, but it must
not be regarded as anything more than an hypothesis. Indeed
there are considerable difficulties in accepting it as it
stands. There is no evidence that carbon monoxide is
formed. Experiments have shown that this gas is quite
useless to a plant ; if it is supplied in the place of the
dioxide, the formation of carbohydrates does not take
place. Nor has any formation or liberation of hydrogen
ever been detected so long as the plant is maintained in
normal conditions.
The formation of formaldehyde, again, is very difficult
of proof. It very readily undergoes change, and therefore
is difficult to detect in a plant. If administered directly to a
FIG. 90. — APPARATUS TO SHOW THE EVOLU-
TION OF OXYGEN BY A GREEN PLANT IN
SUNLIGHT.
166 VEGETABLE PHYSIOLOGY
green cell such as one of a filament of Spirogyra, the plant
cannot make any use of it. It has been found, however,
that if Spirogyra is fed with a compound of formaldehyde
and sodium-hydrogen-sulphite, which slowly evolves the
former in the presence of water, a formation of carbo-
hydrates occurs. This cannot, however, be accepted as proof
that formaldehyde normally subserves this purpose.
If we concede that formaldehyde is very probably the
first stage in the photosynthetic process, a consideration of
the probable decomposition seems to lead us to the view
that the carbon dioxide and the water are made to interact
without the liberation of carbon monoxide, and that the
reaction may be -represented by the equation C02 + H20 =
HCOH + 02, which agrees equally well with the observed
facts.
The formaldehyde may give rise without much difficulty
to a form of sugar. It is a property of the aldehydes to
undergo readily what is known as polymerisation, or
condensation of several molecules. Such a condensa-
tion of formaldehyde would lead to the formation of sugar
thus : — 6HCOH = C6H120G. There are many sugars of
this composition in the plant, especially glucose or grape
sugar, and fructose or fruit sugar.
That some such process takes place is extremely probable,
for sugar is present in the mesophyll cells very speedily
after the absorption of the carbon dioxide and the begin-
ning of the exhalation of oxygen. Sugar of some kind
appears to be the first carbohydrate to be formed ; it is not
very readily detected, being freely soluble in the cell- sap.
Almost as quickly as the formation of sugar we have the
appearance of starch in the substance of the chloroplasts,
and as this is easily visible, it was long thought that starch
was the culminating product of the photosynthetic process.
We shall find reasons shortly for suggesting a wholly different
meaning to the appearance of the starch, that it is indeed
only a temporary store of carbohydrate in an insoluble
condition, due to the production of sugar being in excess
THE CHLOROPHYLL APPARATUS 167
of the quantity needed by the cell for immediate consump-
tion.
If we accept the view of the polymerisation of formalde-
hyde to give rise to the sugar, we cannot withdraw this
operation also from the activity of the chloroplast. Sugars
are what are called optically active compounds ; that is, they
possess the power of deflecting a ray of polarised light to
the right or to the left as the latter is made to pass through
either crystals or a solution of them. Formaldehyde has no
such power. There is no process known by which an
optically active compound is formed from an optically
inactive one without the intervention of living substance.
Consequently we must suppose that the polymerisation is
brought about by the chloroplast as certainly as is the
original decomposition of the carbon dioxide.
We have so far assumed that a sugar having the
formula CGH120(i, and known as a hexose, is the first carbo-
hydrate formed. This, however, is not certain. Some ex-
periments carried out in 1892 by Brown and Morris point
rather to cane-sugar as the first carbohydrate synthesised.
Cane-sugar is a more complex substance, and has the formula
C12H220U. This conclusion is based on repeated observations
that when leaves of Trop&olum were plucked and then ex-
posed to sunlight for twelve hours, there was a great accu-
mulation of this sugar in the leaf, while the simpler hexoses
did not increase in quantity. The severance of the leaves
from their stems prevented the transport of the sugars to
any other part of the plant, so that they accumulated at the
seat of their formation.
Further investigations on this point are, however,
necessary before a definite conclusion can be arrived at.
This theory of the processes of photosynthesis is by no
means the only one which has been advanced, though on
the whole it is that which has been received with most
favour. A modification of Baeyer's view was advanced by
Erlenmeyer, who suggested that the first interaction of
carbon dioxide and water leads to the formation of formic
168 VEGETABLE PHYSIOLOGY
acid and hydrogen peroxide, according to the equation
C02 + 2H20 = HCOOH + H202, and that then they are
decomposed, yielding formaldehyde and water, and giving
off oxygen, HCOOH + H202 = HCOH + H20 + 02.
An hypothesis of a different nature was put forward
by Crato many years later. He suggested that the carbon
dioxide after absorption becomes orthocarbonic acid,
C(OH)4, which exists in solution in the cell-sap. The
orthocarbonic acid has the structure of a closed benzene
ring in which six molecules are linked together. This
becomes decomposed, liberating six molecules of water
and six molecules of oxygen, and forming a hexavalent
phenol :— 6C(OH)4 = C6H6(OH)6 + 602 + 6H20. This new
body then undergoes a molecular rearrangement and
becomes glucose, C6H6(OH)6 = C6H1206.
This view is purely hypothetical, and cannot claim to
be based on experiment.
The most recent suggestion on this subject is that made
by Bach in 1893. He points out that when sulphurous
acid, H2S03, is exposed to light, it becomes transformed to
sulphuric acid, sulphur and water being split off, 3H_,S03 =
2H2S04 + H20 + S, and he argues that a similar process
analogous with this reaction takes place in a leaf. The
carbon dioxide unites with water and forms carbonic acid,
which is then split up in the same way as the sulphurous
acid, 3H2C03 = 2H2C04 + H20 + C. The carbon and
water are not set free separately, but in combination as
formaldehyde. The new acid, H2C04, splits up into carbon
dioxide and hydrogen peroxide, and the latter is decomposed
into water and free oxygen.
All these views must be regarded rather as ingenious
speculations than as sound hypotheses resting upon observa-
tion and experiment.
A theory of a totally different nature was advanced
some years ago by Vines. Starting with the observation
that a carbohydrate substance (cellulose) is produced or
secreted by protoplasm in the process of the thickening of
THE CHLOEOPHYLL APPARATUS 169
cell-walls, and noticing the formation of starch grains in
the chloroplast almost as soon as the photosynthesis
has been established, he argues that the carbohydrate is
not directly formed from the simple materials absorbed,
but appears as a secretion product of the chloroplast.
He suggests that a body possibly allied to formaldehyde
is first formed according to Baeyer's theory, and that this
is used in the construction of proteid, by combining with
the nitrogen and sulphur absorbed in the form of salts
from the soil, or with nitrogenous substances derived from
previous decompositions of proteid. This proteid then is
assimilated by the protoplasm of the chloroplast, and from
the latter the carbohydrate (starch) is secreted.
This view, while no doubt, in the main, accurate as far
as the mode 'of formation of starch is concerned, cannot be
regarded as explaining the formation of carbohydrates from
the simple compounds absorbed. The leucoplast of the
tuber, as well as the chloroplast itself under certain con-
ditions, can form starch grains when supplied with sugar
in the absence of carbon dioxide, and in all probability the
appearance of the starch is the result of the presence of an
excess of sugar in the leaf -cells. Regarded as an explana-
tion of the photosynthesis of carbohydrates, it, like the others,
must remain hypothetical. Moreover it is based upon the
assumption that starch is the highest term reached in the
plant in the series of carbohydrate bodies. This assump-
tion, however, is not supported by the evidence at our com-
mand, the construction of sugar and not starch being the
completion of the photosynthetic process of the chlorophyll
apparatus. Though starch is a very general accompaniment
to this process, it never appears till a certain amount of sugar
has been formed, and in many plants, particularly the
onion and certain other Monocotyledons, it is not produced
at all, however active photosynthesis may be. To this
point we shall return in a subsequent chapter.
Though the production of starch is apparently not the
ultimate aim of the photosynthetic processes, its ready
170 VEGETABLE PHYSIOLOGY
occurrence affords us an easy method of demonstrating the
activity of the chlorophyll apparatus. If a leaf is partially
covered by a piece of opaque material, such as tinfoil, and
is then exposed to the light, starch rapidly appears in the
illuminated portion. Its presence can be shown by bleach-
ing the leaf with boiling alcohol, and then immersing it in
iodine, which forms a blue colour with starch. The blue
tint only appears where the light has reached the chloro-
phyll apparatus.
These processes are carried out by the chlorophyll
apparatus under the conditions set forth. It is evident
that such changes as have been described cannot be accom-
plished without- the expenditure of a considerable amount
of energy. In this need we have the explanation of the
composite nature of the chloroplast. The chlorophyll
absorbs certain rays of light which fall upon it, and the
energy which is liberated by the extinction of their vibrations
is taken up by the protoplasm of the plastid and applied
by it to effect the decompositions that take place. A
very ingenious method of demonstrating that the energy is
derived from the rays of light absorbed by the pigment
was devised by Engelmann. He observed that certain
bacteria were excited to active movement only in the
presence of free oxygen. He placed a filament of a green
alga upon a glass slide in a fluid containing a number of
the bacteria, covered it with a glass cover-slip, and sealed it
with wax. He kept it in darkness till the microbes had
come to rest, and then by the aid of a microspectroscope he
threw a spectrum upon the filament and observed in what
parts of it the bacteria accumulated as soon as they began
to move. These places corresponded with the positions of the
absorption bands which we have seen to be characteristic
of the chlorophyll spectrum, the maximum effect being
produced by the deep band in the red region. These were
evidently the places at which the chlorophyll apparatus
of the filament was at work, the movements of the bacteria
showing that oxygen was liberated there. Tmiriazeff
THE CHLOROPHYLL APPARATUS 171
proved the same thing by throwing the spectrum of solar
light upon a darkened leaf, when he found that starch was
produced only in the positions of those same absorption
bands, indicating that those were the only places of photo-
synthetic activity.
The process of photosynthesis has been found to pro-
ceed under certain circumstances in light which is too
feeble in intensity to cause the development of chlorophyll.
It is effected in these cases by the etiolin, which we have
seen to be the antecedent of chlorophyll. The photo-
synthetic power of etiolin is, however, exceedingly small.
The percentage of carbon dioxide admitted to the
chloroplasts has some influence upon the activity of
the process. Normal air contains a mere trace of the gas,
about 4 parts 'in 10,000. A more copious supply is, how-
ever, distinctly advantageous, and the activity increases as
the percentage rises. The optimum quantity appears to be
about 10 per cent, with light of the ordinary intensity.
More than this gradually exerts a paralysing influence on
the chloroplasts, and sets up consequently an inhibition of
the apparatus. A similar inhibition can be caused by the
accumulation of the products of its activity, a concentration
of the sugar amounting to 8 per cent, being sufficient to
bring it about.
The mechanism is an exceedingly delicate one and can
be thrown out of gear by various external agencies.
Ewart has shown that it can be inhibited by heat, cold,
desiccation, partial asphyxiation, prolonged insolation, and
by the action of dilute alkalis or mineral acids.
We mentioned at the commencement of this chapter
that the chlorophyll apparatus is concerned in the manu-
facture of almost the whole of the organic material of the
globe. In a few humble organisms the construction of
such material can proceed without its help. These are
certain bacteria which can transform ammonia compounds
into salts of nitrous and nitric acids, growing and multi-
plying at the expense of the products they thus obtain
172 VEGETABLE PHYSIOLOGY
together with carbon dioxide. There are two kinds of
these bacteria, one of which oxidises ammonia to nitrous
acid and the other converts this into nitric acid. They
grow freely in the soil and multiply with considerable
rapidity, the result being the formation of certain quanti-
ties of organic substance. They cause the carbon dioxide to
enter into combination, this gas being normally the only
source of their supply of carbon. They possess no chlorophyll
and consequently cannot utilise directly the radiant energy
of the sun. Their energy is apparently derived from the
oxidation of the nitrogenous compounds which they attack.
Nothing is known at present of the steps by which the
synthesis of the organic matter takes place.
A process which at first appeared to involve a
mechanism resembling that of the chlorophyll apparatus
was discovered some years ago by Engelmann. Certain
bacteria which contain a purple pigment were found to
possess the power of photosynthesis. The pigment was
thought to be allied to chlorophyll and to possess the same
power of absorbing and utilising the radiant energy of
light. Recent researches make it probable that, like the
red seaweeds, these organisms contain a certain amount of
chlorophyll, together with the purple pigment.
Saprophytic and parasitic fungi, wrhich contain no
chlorophyll, have no power of photosynthesis. They are
compelled to absorb their carbohydrates from the medium
in which they grow, and they take them in chiefly in the
form of sugar. Parasitic phanerogams depend upon a
similar source of supply.
173
CHAPTEE XII
THE CONSTEUCTION OF PROTEIDS
THE simple compounds containing nitrogen which we have
seen to be absorbed by the roots of green plants, are as
unavailable for direct nutrition as the carbon dioxide taken
in from the air. The nitrogenous organic material which
is actually assimilated by the protoplasts always takes the
form of prote"id. With very few exceptions, and these
occurring only among micro-organisms, gelatin and similar
bodies cannot be made to support vegetable living sub-
stance, though they can be made use of by animals to
supplement, but not to replace, their proteid supplies.
In studying the story of the construction of proteids
from the nitrates and ammonia compounds taken into the
plant, we meet with even greater difficulties than those which
are presented by the photosynthesis of carbohydrates. These
difficulties are connected with the stages which occur in
the course of the construction, with the mechanism which
is concerned in the transformation, and with the condi-
tions under which the building up of proteid takes
place.
At the outset of the study we find ourselves in com-
plete ignorance as to the chemical nature of proteid. We
know that it is the most complex material found in the plant,
with the exception of the living substance itself, but we
know nothing about its molecular structure or the arrange-
ment or grouping of its constituent atoms. Destructive
analysis has revealed its percentage composition within
certain limits, although, as there are many kinds of pro-
teid and all of them are extremely difficult to prepare in
174 VEGETABLE PHYSIOLOGY
a pure condition, too much stress must not be laid upon
the results obtained. These, moreover, are, as we should
expect, not altogether concordant.
Analysis of a crystallised proteid prepared from the
seed of the hemp showed it to have the following percentage
composition, which may be taken, within somewhat wide
limits, to be fairly typical of all.
Carbon .; -'.' . . 51'58
Hydrogen . . . . 6-88
Nitrogen . . • . . . 18-8
Oxygen . •.•.-• • 21-65
Sulphur . . . . 1-09
Besides containing these essential constituents, many
proteids leave on ignition a certain amount of ash. This
consists of small amounts of the chlorides, phosphates, sul-
phates,' and carbonates of sodium and potassium, with traces
of the corresponding salts of calcium, magnesium, and iron.
It is not certain that these ash constituents are an integral
part of the proteid molecule in any case ; the balance of
evidence points rather to their being impurities which are
very difficult of removal.
Most of the proteids found in plants exist in an amor-
phous condition, and are very closely incorporated with
the protoplasm. In a few cases they are met with as
definite grains, and in certain reservoirs of food material
they occur as crystals. Some of them can be made to
crystallise after extraction from the organism, but many
forms exist which do not possess this property, so far as
we know at present. It is not certain, however, that the
crystals are always composed of pure proteid only.
The proteids vary very much among themselves as to
their solubility in water and other neutral fluids. Some
are soluble, others insoluble, in water ; some are soluble
in solutions of neutral salts of various degrees of concentra-
tion. Nearly all are insoluble in alcohol and ether ; they
all dissolve in strong mineral acids and in caustic alkalis,
THE CONSTEUCTION OF PEOTEIDS 175
but they are decomposed during the process. Their
solutions have generally a power of deflecting a ray of
polarised light to the left.
The best known groups into which proteids have been
divided are the following :—
1. ALBUMINS.— These are soluble in distilled water, and
if the solution is heated, the proteid is converted into a
peculiarly insoluble form, known as coagulated proteid, and
deposited as a granular or flocculent precipitate. As the
temperature rises the liquid becomes markedly opalescent
before the separation of the proteid. The change takes
place at a point which lies between 65° and 80° C., its
exact place depending upon the nature of the albumin and
the reaction and concentration of the proteid solution.
This point is' known as the coagulation temperature.
Albumins can be precipitated unchanged by saturating
their solutions with sodio-magnesic sulphate. They are
not of frequent occurrence in plants, but can be extracted
from certain roots.
2. GLOBULINS. — These differ from albumins in not
being soluble in distilled water. They can be dissolved by
adding a little neutral salt, such as sodium chloride. Their
solutions are coagulated on heating, but they show a con-
siderable variability as to the coagulation temperature,
which in the case of some is as low as 55° C. Most of
them, however, remain unchanged below 75°-80° C. They
can be precipitated by saturating their solutions with
magnesium sulphate. If sodium chloride is used instead
of the latter, an incomplete precipitation usually takes
place. Different members of the group show different
degrees of solubility in solutions of sodium chloride ; some
require only a trace of the salt ; others need 8-10 per cent.;
and a few are soluble only in saturated solutions.
The proteids found in plants belong chiefly to this
class. Globulins can be readily extracted from most seeds,
and probably this form of proteid is the one which occurs
in the green parts of plants.
176 VEGETABLE PHYSIOLOGY
3. ALBUMINATES, or DERIVED ALBUMINS. — These are
insoluble in distilled water or in solutions of neutral
salts. They are readily soluble in very dilute acids and
alkalis, and their solutions do not coagulate on boiling.
They are precipitated by carefully neutralising their solu-
tions, and when they are boiled in the resulting state of
suspension they are converted into coagulated proteid,
and will not re-dissolve on the addition of either dilute
acid or alkali.
They are readily prepared from either albumins or
globulins by warming them in the presence of a little acid
or alkali, preferably at about 60° C. Alkali-albumin may
be prepared by 'acting on albumin with fairly strong caustic
potash in the cold.
The derived albumins are not of frequent occurrence in
plants, but may be met with in certain seeds.
4. PROTEOSES or ALBUMOSES. — These are generally
soluble in distilled water, though some are less so than
others. They can be precipitated from their solutions by
saturating the latter with neutral ammonium sulphate.
They differ from the members of the first two classes by
not being converted into coagulated proteid on boiling.
Their characteristic reaction is that they give with nitric
acid, or with potassium ferrocyanide in the presence of acetic
acid, a precipitate which dissolves on warming the liquid
and reappears as it cools. Unlike any of those of the
preceding groups, they have the property of dialysing
through a parchment membrane, but only very slowly.
5. PEPTONES. — These are much like albumoses, but do
not give a precipitate with nitric acid or with potassium
ferrocyanide in the presence of acetic acid. They are not
precipitated by saturation of their solutions with ammonium
sulphate, nor are they coagulated on boiling. Their power
of dialysis is much greater than is that of the albumoses.
Neither peptones nor albumoses occur very plentifully
in plants, and they are probably formed in them only from
the decomposition of the more stable forms of globulin and
THE CONSTEUCTION OF PROTEIDS 177
albumin. There is no evidence at present that they are
stages in the constructive process.
Some of the albumoses occur in certain seeds in associa-
tion with some of the globulins. Both the albumins and
the globulins, and probably the albuminates as well, are
transformed into albumoses and peptones by the action of
hydrolysing agents, such as dilute mineral acids and certain
secretions of the protoplasm known as enzymes, whose
action will be treated of in a subsequent chapter.
6. COAGULATED PROTEIDS. — These are the products of
the action of heat on the members of groups 1, 2, and 3.
They may also be formed by the action of alcohol on the
same bodies. They are insoluble in water, but are dissolved
with some difficulty by strong acids and alkalis, probably
undergoing decomposition during the process.
Besides these classes of proteids, another occurs which
presents the curious peculiarity of being soluble in alcohol.
Proteids of this group have been extracted from the endo-
sperm of some of the cereal grasses. Examples of them
are found in the zein of maize ; and the gliadin and
glutenin of wheat flour. Zein differs from the crystallised
proteid of the hemp in its comparatively low content of
nitrogen, which amounts to only 16*13 per cent. It dissolves
easily in alcohol of about -820 specific gravity, but is
insoluble in absolute alcohol. It is insoluble also in water,
but in mixtures of alcohol and water it dissolves to a
greater or less extent, being most easily soluble in a mixture
containing about 90 per cent, of the spirit. In one of a
lower concentration than 50 per cent, it is very sparingly
soluble. Zein can be dissolved by glycerine if heated to
150° C. ; also by glacial acetic acid and by dilute solutions
of caustic potash. Like other proteids, it is converted into
peptone by pepsin and hydrochloric acid.
The original construction of proteid matter, like that
of carbohydrates, seems to be carried out only by
vegetable protoplasm. It does not, however, appear to be
dependent upon the same conditions as the process already
178 VEGETABLE PHYSIOLOGY
described. It cannot be classed with the latter as a process
of photosynthesis, and it is only indirectly dependent
upon the action of the chlorophyll apparatus. Unlike the
construction of carbohydrates, it is not confined to green
plants — indeed the fungi can commence the synthesis at a
lower stage than the latter, beginning the construction with
compounds of ammonia, which have to be converted into
nitrates before green plants can utilise them.
For the synthesis of proteids we have accordingly two
certain starting points, to which may be added another
which is confined to a small group of plants, if not indeed
to a single organism. We have already alluded to the fact
that certain of -the Leguminosae have the power of using
the nitrogen of the atmosphere for the purpose of construct-
ing organic food. This utilisation of it is, however, not
carried out by the green plant independently, but only
when its roots are associated symbiotically with a fungus
which forms peculiar tubercular outgrowths upon the root-
branches. It is apparently the latter organism which
effects the first fixation of the nitrogen. The leguminous
plant alone is as powerless in this direction as any other
green plant. How the fixation takes place, what part of it
is due to the direct metabolism of the fungus, and how far
the protoplasm of the green plant is concerned in the early
stages, are at present quite uncertain. It seems, however,
probable that the fixation is carried out by the fungus alone,
without any influence or aid derived from the green plant.
A few other similar organisms can under appropriate
conditions carry on a similar fixation without being in
symbiotic union with any green plant. If this view is
correct, the leguminous plant is supplied by the fungus
with a food material which has already been worked up
from the simple form in which the elements of it are
absorbed ; but how far the manufacture has proceeded — that
is to say, in what condition the nitrogenous material is
actually presented to, and absorbed by, the tissues of the
root — is at present uncertain.
THE CONSTEUCTION OF PEOTEIDS 179
The power of fixation of free nitrogen thus possessed
by the organisms mentioned has been stated by several
observers to be shared by certain lowly Algse, but the
evidence as to their activity in this direction is conflicting.
It may be that they are capable of a similar symbiotic
relationship with certain of the nitrogen-fixing bacteria of
the soil already mentioned, but it is more probable that
bacteria living simultaneously, but not symbiotically, in
the soil with them, carry it out.
When we turn to the method of construction of proteid
by a green plant we find ourselves in possession of very
little accurate information as to the stages which are
involved. We find that nitrates especially are absorbed
by the root-hairs- from the soil, and that a continuous
stream of them passes into the plants. This naturally
is associated with a transportation of the nitrates through
the root and stem. They can be detected in varying
quantities in these regions, but the amount seems to
diminish as the termination of the stem is approached, and
none can be found to be present in the leaves. It may be
inferred that a gradual decomposition takes place as they
pass along the axis, and that this is completed in the leaves.
A theory has been advanced to explain this disappear-
ance, which may be mentioned here. It is that the nitrates
are decomposed by the organic acids of the plant, and in
particular by oxalic acid. Simultaneously the sulphates
which are absorbed undergo a similar fate. The resulting
bodies, the nitric and sulphuric acids, unite with some form
of non-nitrogenous organic substance, possibly form-
aldehyde, or a fairly simple carbohydrate, to form proteid.
From what has already been advanced, however, it is evi-
dent that this scheme of construction is purely hypo-
thetical.
When we search for a form of nitrogen compound that
is nearer proteid in its composition than these simple salts,
it is natural to look at the products of the decomposition of
proteid material to see if these furnish any clue to a possible
N 2
180 VEGETABLE PHYSIOLOGY
constructive process. When proteids are digested in the
animal organism under the influence of the strong ferment
of the pancreatic secretion, we find that among the pro-
ducts of the decomposition certain nitrogenous compounds
occur which are crystalline and capable of diffusing through
animal and vegetable membranes. These substances, the
chief of which are tyrosin and leucin, with a little asparagin,
are known technically as amido-acids, owing to their con-
taining the group NH2 (amidogen), replacing one atom of
hydrogen in the hydrocarbon group of an organic acid. It
is extremely probable that these compounds are made use of
again in the subsequent reconstruction of proteids in the
cells. Many of these substances have been found to occur in
plants, and among them asparagin is extremely conspicu-
ous. It can be detected in seeds and seedlings, and in older
plants it is not infrequently present in the leaves. There is
considerable probability that these substances occur as a
stage in the original construction of proteids, though they
may no doubt also be formed during its digestion in the
vegetable as well as in the animal organism. This
probability is supported by the observation that green plants
are able to absorb from the soil and utilise many such
amido-acids when artificially supplied to them.
Another hypothesis of proteid construction has been
advanced which takes account of these substances as stages
in the process. We have seen that salts of ammonia are
converted into nitrates in the soil before being absorbed.
The first step in the construction is thought to be the recon-
version of the nitrates into ammonia, which interacts in
some way with formaldehyde or one of its polymerides to
form one or other of these amido-acids. This subsequently
combines with some kind of non-nitrogenous organic sub-
stance together with some compound of sulphur, to form
proteid.
This view of the construction, though of course only
hypothetical, associates certain processes which apparently
occur in nature. The formation of proteid seems to involve
THE CONSTBUCTION OF PEOTEIDS 181
the simultaneous presence in the cells of some amido-acid,
frequently asparagin, and some carbohydrate such as sugar.
If shoots of plants which exhibit no accumulation of
asparagin during normal growth are cut off and kept in
darkness for some time, a gradual accumulation of the
amido-acid can be observed. This in all probability is the
expression of the decomposition of proteid taking place during
the life of the shoot, and is presumably a normal occur-
rence. The reconstruction which would explain its non-
accumulation during illumination is prevented by the non-
formation of the needed carbohydrate in the darkness.
The probability of a combination or interaction of these
two classes of substance in the synthesis of proteids is
supported by the fact that at the active growing points, where
protoplasm is energetically formed, and where consequently
abundant supplies of proteids are needed, neither sugar nor
amido-acids can be detected, though they can be traced quite
readily up to a short distance below the place where this
active growth is proceeding. This fact is easily understood
if we admit that proteid is constructed there at the expense
of these two constituents, supplemented, of course, by the
necessary compound or compounds of sulphur. If either of
these supplies ceases to be available, the growth of the plant
at that point stops.
Though we have seen reasons for thinking that nitrates
and amido-acids form two stages in the normal process of
proteid construction, we must not conclude that they in-
variably do so. In one plant, Pangium edule, which was
examined by Treub in 1894, the nitrogen needed for pro-
teid construction appears to be supplied in the form of
hydrocyanic acid. In the shoots of this plant, cells occur
in the cortex which contain this acid. In those nearest the
apex the latter occurs alone, but as they grow older, a little
proteid is found to be mixed with it. In still older ones
the proteid preponderates, and at some distance behind the
seat of growth it occurs alone, the acid having disappeared.
Certain fungi can utilise nitrogen-containing derivatives of
182 VEGETABLE PHYSIOLOGY
methane or benzol for the same purpose. It is probable,
therefore, that more than one pathway to the proteid mole-
cule may yet be found in different plants.
Probably the construction of proteid is not confined to
any definite tissue or series of tissues in the plant. It is
certainly only connected indirectly with the chlorophyll
apparatus, and that in so far as carbohydrates are
necessary for its formation. At the same time, there is a
certain amount of evidence which points to its synthesis
being in the first place effected in the leaves. The fact that
nitrates can be traced towards these organs, and that they
nevertheless do not appear to be present in the mesophyll
cells, makes it probable that they are manufactured into
something else there. The occurrence of amido-acids in
the leaves is more in harmony with the view that they are
built up there, than with the assumption that they arise
from the decomposition of already existing proteids, though,
no doubt, the latter is the case in the tissues of seeds, and
possibly of seedlings, which are being nourished at the
expense of materials stored in the seed. The proportion of
proteid to dry weight of tissue has been stated to increase
gradually and progressively from the roots to the leaves,
in which it attains a maximum. Moreover, proteids are
continually being removed from the leaves. If, however,
the process does primarily go on in the leaves, it does not
take place under the same conditions as the construction
of carbohydrates. It goes on quite well in green cells in
darkness, so that it is not, as already mentioned, a process
of photosynthesis. It has recently been claimed that the
construction of proteid in certain plants is favoured by
light, and more particularly by the ultra-violet rays, though
the luminous ones have a certain feeble effect. Whether
or no the energy for the construction is derived therefrom
is not, however, certain.
Sachs held that the sieve-tubes of the fibro-vascular
bundles of the axis of the plant are also the seat of the
construction of proteid. Though this is possible, it seems
THE CONSTEUCTION OF PEOTEIDS 183
more likely that they are concerned in the transmission of
organic nitrogenous material from the leaves to other
organs. In whatever form proteid material travels about
the plant, which for the present we cannot discuss, it is
almost certain that it passes by the sieve-tubes, and it
may well be that too great an accumulation of the travel-
ling form may be attended by its conversion into an
insoluble condition, and its deposition in the cells. There
is no conclusive evidence pointing to the sieve-tubes as the
places where it is originally synthesised.
The same considerations apply to the various growing
points or zones. No doubt proteid is constructed there,
but it is equally certain that it is so built up from bodies
which have resulted from the digestion or decomposition of
proteid that has already been synthesised elsewhere, and
which has undergone such decomposition solely with a
view to transport or translocation.
We judge it probable on all these grounds that the
great seat of proteid construction in a green plant is its
leaves, and this not on account of the possession of the
chlorophyll apparatus, but because of a property inherent
in the cell-protoplasm. Whence the energy is derived is
not clear, but many writers hold it to be supplied by
accompanying chemical decompositions.
Subordinate synthetic formation of proteids may take
place in the roots, for nitrates appear to be used up there,
though in much less quantity than in the leaves.
The construction of proteid by fungi is an additional
proof that it is altogether independent of the chlorophyll
apparatus, if not that it is unconnected with the access of
light.
The third group of foods, the fats or oils, are probably
not directly synthesised in plants, but are products of the
decomposition of proteids, or perhaps of the living sub-
stance itself.
184 VEGETABLE PHYSIOLOGY
CHAPTEE XIII
THE CONSTITUENTS OF THE ASH OF PLANTS
WE have seen in a previous chapter that when a plant is
carefully burned and the residue collected, the latter, which
is known as the ash, is found to contain a number of
elements which vary in different cases and which always
include certain metals, as well as some non-metallic
elements. The occurrence of this ash being universal, we
can conclude without any difficulty that some of its con-
stituents at least must be of importance to the organism,
though it cannot be denied that our information is exceed-
ingly incomplete. In the study of the nutritive processes
of animals we meet with similar phenomena. How far
any of the constituents of the ash can be regarded as
actual food is uncertain, nor can we solve this question
until we know something more about the composition of
living substance. Whether any of these bodies actually
enter into such composition is doubtful, but several of
them appear to be necessary for the assimilation of the
food which is either manufactured or supplied, as well in
the case of the vegetable as in that of the animal
organism.
Many of them, again, while not serving as food or even
as materials for the formation of food, no doubt play
important parts in the general metabolism of the organ-
ism. At present we are not in a position to say definitely
how most of them are concerned in any or all of these
processes.
From the nature of the plant-body and the absence of
THE CONSTITUENTS OF THE ASH OF PLANTS 185
the localisation of different functions in particular organs
which is so much more clearly characteristic of the animal
organism, it becomes very difficult to ascertain the exact
nature of the part played by any of these ash constituents.
We can more easily determine what is the effect produced
by variations in the amount supplied or by the total
absence of any of them. This effect is usually, however,
only the general effect upon the plant, and the experiments
leave us still quite in the dark as to the way in which any
general effect is produced, whether directly, or indirectly
by affecting the health of the plant and thus leading to
•secondary changes in its tissues.
The experiments in question are preferably carried out
by means of water-culture, the general nature of which we
have already explained. Plants will grow very well in
water containing small quantities of various, inorganic salts,
and these can be varied at will for the purpose of definite
inquiries. The composition of such a culture-solution is
given by Pfeffer as under : —
Calcium nitrate . . 4 grms.
Potassium nitrate . . 1 grm.
Magnesium sulphate . 1 grm.
Potassium acid phosphate 1 grm.
Potassium chloride , . *5 grm.
Ferric chloride solution . a few drops
Water .... 7 litres
Or a convenient fluid may be prepared by dissolving
2O 5 grms. magnesium sulphate in 350 cc. of water, and
40 grms. calcium nitrate, 10 grms. potassium nitrate, and
10 grms. acid phosphate of potassium in another 350 cc. ;
100 cc. of each of these solutions should then be added
to 9'8 litres of water. This culture-medium will contain
•2 per cent, of salts, and will need only the further addition
of a few drops of ferric chloride solution.
This percentage is generally satisfactory, though the
concentration may be increased twofold without affecting
186
VEGETABLE PHYSIOLOGY
the plants injuriously. Too great a quantity of salts,
however, becomes deleterious.
The effect of omitting any particular constituent can
be examined by making up the culture-fluid as required.
FIG. 91.— PLANTS OF BUCKWHEAT CULTIVATED IN VABIOUS NUTRITIVE
SOLUTIONS.
1, normal solution containing all necessary salts ; 2, solution containing the
same salts as 1, except potassium compounds ; 3, solution of same composi-
tion as 1, except that sodium salts have been substituted for potassium
compounds ; 4, solution of same composition as 1, except that no calcium
salts are present ; 5, solution containing no compounds of nitrogen.
Fig. 91 shows in (1) a plant growing in such a fluid as is
described above ; (2) shows a similar plant cultivated in a
THE CONSTITUENTS OF THE ASH OF PLANTS 187
medium from which potassium is absent, (3) one in which
sodium is made to replace potassium, (4) one in which
there is no calcium. The general character of such
experiments can be seen by comparing the relative
development of the plants under these conditions, and it
is at once evident that the different metals and other
elements employed have a certain functional importance.
Deprivation of any of those mentioned affects all plants
injuriously, though in different degrees.
We can, however, say very little as to the way in which
the injurious effects are produced in different cases. We
can, as a rule, only guess at the functions of the different
ash constituents by studying the effects thus made evident.
In a very few cases we can associate an element with some
definite metabolic process. An instance is afforded by the
behaviour of iron, in the absence of which, as we have
seen, there is no development of chlorophyll in the chloro-
plasts. We cannot even here say very definitely how this
inhibition is caused. It seems unlikely that it directly causes
the failure of the etiolin to be converted into chlorophyll,
for all analyses of the latter show that iron does not enter
into its molecule. It is probably an indirect effect arrived
at through faulty nutrition set up in the absence of the
metal.
At first sight it seems as if the absence of inorganic
salts may be effective by interfering with the maintenance
of the turgid condition of the cells, as all the compounds
mentioned have osmotic properties. It is evident, how-
ever, that this cannot be the only or even the main cause
of the disturbance of nutrition, as the salts are not inter-
changeable, and a salt of sodium in concentration quite
sufficient to maintain the condition of turgor is unable
to replace the salts of potassium normally required. More-
over, turgescence can be maintained by organic acids in
the total absence of the normal constituents of the ash.
We can divide the latter into four groups which sub-
serve different purposes. Of these the members of the
188 VEGETABLE PHYSIOLOGY
first are essential,' because they enter into the constitution
of the living substance. They are sulphur and phosphorus.
All analyses of proteids show that sulphur is an essential
constituent of them, and as proteids are immediately applied
to the construction of protoplasm, there can hardly be any
doubt that sulphur is contained in living substance. Phos-
phorus does not seem to be present in the ordinary cytoplasm,
but it is undoubtedly associated with the nucleus. The nature
of the connection is not very clear, but all nuclei contain
a constituent which bears the name of nuclein. This can
be extracted from it by appropriate treatment, and analysis
shows that phosphorus enters into its molecule. Nuclein
occurs also in the substance of many cells, either as nucleic
acid, or associated with certain proteid bodies.
The second group comprises certain metals which are
essential to the development of a plant, but which appa-
rently do not ever form part of the living substance. There
is some little doubt about this, as the fact cannot be
ascertained by analysis. The members of this group are
potassium, magnesium, calcium, and iron.
The third group includes several elements which are
not absolutely essential, but which are useful in many
cases, and which are very widely distributed, although not
universally present. Among them are sodium, silicon,
manganese, chlorine, bromine, and iodine.
The fourth group includes many other elements which
are only occasionally present, and which probably play no
part in the metabolic processes. They appear to be
absorbed because they are present in the particular soil in
which the plant happens to be growing, and have the
power of osmosing through the walls of the root-hairs, and
passing their plasmatic membranes. Many of them have
only been found in a few plants. Among them may be
mentioned aluminium, zinc, copper, cobalt, nickel, zirco-
nium, fluorine, and lithium.
What is frequently spoken of as the selective power of
plants is often misunderstood. If a substance is present
THE CONSTITUENTS OF THE ASH OF PLANTS 189
in a soil, can be made soluble in the hygroscopic water
permeating that soil, and can dialyse through the semi-
permeable membrane of the root-hair, absorption of a
certain quantity of it will take place. How much is ul-
timately absorbed is a question of the power of the plant
to decompose or utilise it after absorption. Many sub-
stances which are useless or even deleterious to the
plant which takes them up are absorbed continuously
until a very large percentage of them is present, because
other constituents of the plant decompose them, or because
their power of dialysis is such that they are easily removed
from the absorbing cells. The possibility of the dialysis
by which they are originally taken up is perhaps a ques-
tion of relationship between the size of their molecules
and that of the meshes of the plasmatic precipitation
membranes which bound the cytoplasm on its two
faces, abutting on the cell-wall and the vacuolar cavity
respectively. This possibility of penetrating these mem-
branes, and the power of subsequently removing the sub-
stances from the absorbing cells, are the special features
of the so-called selective power of the plant, and it is
evident that this power is particularly associated with the
disposition of the materials after absorption, more than
with the absorption itself.
We may now turn to the consideration of these varied
constituents of the ash, and examine them in detail. The
first group, we have seen, is composed of sulphur and
phosphorus. Its importance lies in the fact that these
elements enter into very close relationship with protoplasm,
the former being in all probability a constituent of its
molecule.
Sulphur is only taken up by the higher plants in the
form of sulphates of some of the metals of the other groups
or of ammonia. Fungi can also utilise salts of sulphurous
and hydrosulphurous acids when they are presented in
dilute solutions.
Phosphorus is associated with the nucleus rather than
190 VEGETABLE PHYSIOLOGY
with the cell-protoplasm. It is contained in the substance
called nuclein to the extent of about 6 per cent. The
nuclein is apparently chiefly in the chromatin substance
of the nucleus. Phosphorus is also a constituent of some
proteids, and is probably present in the enzymes which
are concerned in the true digestive processes of the plant.
It occurs in chlorophyll also, according to Hoppe-Seyler,
whose analysis of this pigment has already been quoted
(page 158). In a few plants phos-
phorus is temporarily stored in the
seeds. Examples are presented by
the Brazil nut (Bertliolletia) and
the Castor-oil plant (Ricinus), whose
seeds contain stores of proteid
FIG. 92.— CELL OF EICIMJS material in the form of complex
SEED. CONTAINING FIVE • T ,-, , , P , ,
ALEUBONE GRAINS. grams. In the substance of these
grains there is a small, usually
round, accumulation of mineral matter composed of a
double phosphate of calcium and magnesium (fig. 92),
which lies side by side with a crystal-like proteid body.
Lecithin, a complex fatty body containing phosphorus, is
present in actively growing cells of many plants.
Phosphorus is absorbed by the plant usually, if not
entirely, in the form of soluble phosphates, most frequently
a phosphate of calcium. Besides being important as an
integral part of the living substance, certain observations
tend to show that it materially assists in the construction
of proteids.
The second group of ash constituents includes four
metals which are essential to all plants, viz. potassium,
magnesium, calcium, and iron. Probably these act only
indirectly in the constructive processes, though there is
some evidence that they may be integral constituents of
living substance. They do not enter into the composition
of proteids.
Potassium is absorbed in a variety of compounds, of
which the nitrate and the chloride are the most advan-
THE CONSTITUENTS OF THE ASH OF PLANTS 191
tageous. The part which it plays is not at all well
understood. It may enter into the composition of protoplasm,
for it is especially abundant in embryonic tissues. It has
been thought to be connected with the construction of
carbohydrates, but in what way it affects it is not known.
It occurs in greatest quantity in the organs in which the
formation and storage of these bodies are most actively
carried out, viz. leaves, tubers, seeds, &c.
Magnesium has a distribution much like that of potas-
sium, and, as well as calcium, is thought by some bota-
nists to enter into the composition of protoplasm. It may
be absorbed in various combinations, but the chloride is
the least advantageous. Calcium is essential to all green
plants, but fungi do not always require it. Little of it
relatively is found in young tissues, but greater amounts
are present in adult ones. Its function is not understood,
but it is useful in neutralising oxalic acid. It is promi-
nent in cell-walls, part of which even in the very young
state consist of calcic pectate. In older cells the middle
lamella appears to consist entirely of this substance until
lignification is complete. Calcium may be absorbed in
the same combinations as magnesium.
As has been already mentioned, the most evi-
dent function of iron is to assist in the formation of
chlorophyll. As it is not contained in the pigment, its
influence here can only be indirect. It may be associated
in some way with the protoplasmic basis of the plastic],
so that the latter in its absence is thrown into a patho-
logical condition and ceases to form the colouring matter.
The influence both of the metal and of light in this
particular may consequently be similar. That it is asso-
ciated with the plastid does not appear improbable in
view of some observations of Macallum's that iron is
always found in direct relationship with the chromatin of
the nucleus, of which it appears to be an integral part.
There is here evidence of a close association between the
metal and some forms of living substance.
192 VEGETABLE PHYSIOLOGY
Iron can be absorbed with advantage apparently in any
soluble combination.
The third group of elements contains several that are
of importance to particular plants but are not universally
necessary. Others usually included here are not known
to be functionally important at all, except that they have
a certain power of replacing to some extent the more
important metals which have been already spoken of.
Of the metals of this group, sodium is the most widely
distributed. It exists in all soils, and it is capable of
absorption in considerable quantities. Experiments by
means of water-culture show, however, that its beneficial
influence is extremely slight. It can be omitted from the
culture-fluid without entailing any harm to the plant, and
its presence in any quantity will not compensate for the
absence of potassium (fig. 89, 1 and 3). If compounds
of sodium and potassium are present together in sufficient
quantity, the latter is always absorbed in far the largest
amount, indeed almost exclusively by many plants. So-
dium seems able, however, to effect a certain economy
in the use of potassium. If a cereal plant is supplied
with too little potassium, and with a certain amount of
sodium, development is normal, and an examination of
the distribution of the two metals in its tissues shows
that the potassium is accumulated in the flowers and seeds,
while the sodium replaces it in the vegetative parts. It is
absorbed in the same combinations as potassium, but the
chloride is not, as in the latter case, a valuable salt.
Indeed, sodium chloride is generally deleterious, except,
perhaps, to the plants of the sea-shore, in which it brings
about their peculiar succulence.
If we compare the influence of potassium, sodium, and
calcium, on the development of a crop of herbage plants,
we find that the presence of potassium leads to a develop-
ment of stems, flowers, and fruit, or to what may be regarded
as the maturing of the plants, while in the absence of
sufficient potassium and the presence of calcium and sodium
THE CONSTITUENTS OF THE ASH OF PLANTS 193
vegetative growth is more directly favoured, but the crop
remains backward and immature.
There is a possibility that all these metals serve
another purpose as well as some particular functional one.
We have seen that the nitrogen which the plant obtains is
derived from the soil, being most favourably supplied by
the latter in the shape of nitrates. In the soil the nitric
acid is combined most frequently with the metals under
discussion, and a not inconsiderable quantity of the latter
may be taken up solely for the sake of the nitrogen which
they can thus carry into the plant. The varying amounts
of sodium and calcium" which plants contain have been
found to bear a certain relationship to the amounts of
their compounds which occur in the particular soils in
which the plants have been growing. When calcium and
sodium nitrates are taken up for the sake of the nitrogen,
they are probably decomposed by the organic acids formed
in the plant, and the nitrogen is made to enter into
further combination, leading to the construction, possibly
of amido-acids, and eventually of proteids.
Of the other elements which are included with sodium
in this group, silicon is one of the most prominent. It is
absorbed almost entirely in the form of silicates of potas-
sium and sodium, the latter combination being the prin-
cipal one. It is difficult to say what purpose it serves.
It is usually found deposited in the epidermal cell-walls,
and as the grasses and the horsetails contain it in greatest
abundance, it has been suggested that its utility consists in
its contributing to the rigidity of their weak stems, and
consequently to the maintenance of their vertical position.
This is, however, not the case ; their rigidity is dependent
on the degree of development of their harder tissues, and
the absence of silica makes but little difference to them.
Silicates, when added in quantity to the soil in which green
crops are growing, have no marked effect upon the amount
of silica which is subsequently present in the straw. It is
uncertain whether the silica enters into the metabolism of
o
194 VEGETABLE PHYSIOLOGY
the plant, or whether the silicates are decomposed at once
and the silica deposited in the cell-walls in which it is
prominent. As it is most readily taken up in combination
with sodium, this is unlikely, the sodium being, as we have
seen, of very little, if any, use. It has been said that oats
mature less fully and completely in the absence of silica,
so that in the case of that particular plant there is some
evidence of its aiding in metabolism, though no suggestion
has been made as to the way in which it exerts its
influence. It is possible that it may be of value also by
protecting the plant from the depredations of animals or
from the attacks of fungi, as it is mainly accumulated in
the epidermis.-
The other elements of this group include chlorine,
bromine, and iodine. A little of the former is of universal
occurrence, but it may be due to its being taken up in
conjunction with potassium. Water-culture experiments
show, however, that in many cases it cannot be omitted
altogether without injury to the plant. It has been asso-
ciated by some writers with the translocation of carbohy-
drates, particularly in the buckwheat, a view which is
based upon the observation that in its absence the chloro-
plasts become abnormally filled with granules of starch.
Bromine and iodine are chiefly found in marine plants,
but their function is unknown.
Manganese is a constituent of many plants. Till quite
recently nothing was known about its influence on meta-
bolism, but it now appears probable it plays a part in
various oxidative processes, which are carried out by a
somewhat widely spread enzyme known as Laccase, whose
normal function is, however, at present obscure.
The elements of the last group are numerous ; they
vary with the composition of the soil in which the plants
are growing, and appear to subserve no useful purpose.
Many of them in even moderately dilute solutions are
extremely poisonous, so that they must be absorbed in a
high state of tenuity. Their presence shows that the
THE CONSTITUENTS OF THE ASH OF PLANTS 195
selective power of plants is not necessarily connected with
the development of normal metabolic functions, but is
mainly physical and only to a slight extent physiological.
From what has already been advanced, it is evident
that the time is not ripe for a detailed discussion of the
parts played by the constituents of the ash of plants. Nor
will it be till we have ascertained much more fully how
the various metabolic processes are carried on. Certain
broad statements of a somewhat general character are all
that are at present justified, and these concern only some
of the mineral matters which are absorbed. The meta-
bolism not only depends on the presence of certain elements,
but is largely influenced by the relative quantities of each
which the active cells contain.
The vegetative activity of, at any rate, herbage plants
is associated with a plentiful supply of nitrogen. In the
absence of sufficient potassium vegetative luxuriance is
obtained, but the degree of development of the plant is
limited by such deficiency. In the event of sufficient
supplies of potassium being afforded, the relative abundance
of the nitrogen has an important influence on the forma-
tion of carbohydrates, which are then produced in greater
quantities. Coincidently the plants go on to maturity;
the luxuriance of the leafy parts becomes curtailed, arid
the development proceeds normally, leading to the formation
of the flowers and subsequently the seeds. Thus the com-
position of the supplies in the soil determines largely the
character of the development of the plants growing in it.
It has also considerable influence upon the variety of the
species of these plants, owing to the various ways in
which particular constituents may influence different
individuals.
In the absence or the deficiency of particular salts,
others may be absorbed in proportions very different from
those which would have been found had the missing ele-
ment or elements been present.
o 2
196 VEGETABLE PHYSIOLOGY
CHAPTEK XIV
OTHER METHODS OF OBTAINING FOOD
IN our introductory considerations of the true nature of
the food of plants, and of the manner in which they obtain
it, it was pointed out that there are stages in the life-
history of all plants during which it is imperative that they
shall be supplied with food in a form in which they can
assimilate it at once, constructive mechanisms either being
altogether absent from them or not having been developed
at the particular time under consideration. There is thus
in every plant a power of assimilating organic food so
supplied, a power which in some cases is permanently
relied upon, sometimes completely, sometimes only
partially, and which in other cases is laid aside as soon
as the chlorophyll apparatus becomes developed. The
need for the supply of the organic food is always felt by
every protoplast, and the latter cannot be nourished except
by it. We may contrast in this respect the individual
protoplast and the colony of which it is a member, the
latter being able through the co-operation of its individuals
to construct the organic food which must be provided for
the use of every member, even of those to which the work
of construction is allotted.
The constructive power may be partially or wholly lost
or undeveloped ; in such cases the loss must be com-
pensated for by the supply from outside of the material the
plant is not able to synthesise for itself.
Examples of plants possessing different powers of such
absorption are supplied by every class of the vegetable
kingdom. They are most conspicuous among the Fungi,
OTHEK METHODS OF OBTAINING FOOD 197
because in them there is no chlorophyll apparatus, and
hence constructive processes must be very rudimentary.
Distinct differences can be seen, however, in this group.
Certain lowly forms appear to be able to utilise very
different compounds of carbon and to synthesise carbo-
hydrates therefrom. Many can grow and multiply in
solutions of simple acids such as formic or acetic. A much
larger number need for their nutriment a supply of carbo-
hydrates in the form of sugar, and if this is given them,
together with certain relatively simple compounds of
ammonia, especially ammonium tartrate, they can con-
struct therefrom proteid and fatty bodies.
Others need the nitrogen to be supplied in the form of
amido-acids, as they have no power to utilise the simpler
ammonium salts ; others again need their proteids as well
as their carbohydrates to be supplied to them as such, for
they possess scarcely any constructive ability.
A similar power of utilising carbohydrates and allied
bodies is exhibited by many green plants. If their roots
are watered with a solution of sugar, they can take it up
and economise by its aid the sugars which the chlorophyll
apparatus is constructing. Various bodies also from which
sugar can be formed are absorbed when presented to
the roots, and serve as forerunners of sugar in the plant.
Among these may be mentioned Glycerine. The process
of the S}7nthesis of proteids also may be shortened by sup-
plying the roots with material such as asparagin, leucin,
or urea. Proteid as such can only be utilised by a few
flowering plants which possess special mechanisms for its
preliminary digestion.
Among what we must regard as these abnormal methods
of food supply we must include certain processes in which two
organisms are associated, for the well-being, in some cases,
of both, in others for that of only one. The two organisms
are brought into very intimate relationships with each
other, in some cases a very complete union of their tissues
being effected, so that transport of elaborated food materials
198 VEGETABLE PHYSIOLOGY
can readily take place between them. In those cases in
which this close association is of benefit to both the
organisms it is spoken of as symbiosis ; in those in which
one flourishes at the expense of the other, the relationship
is called parasitism. While there are many cases which
can be definitely referred to both these categories, they
seem to blend one into the other, cases being known in
which it is very difficult or impossible to say whether the
advantages are all on one side or not.
The plants which differ least from the normal habit
which we have described are those which are known as
Saprophytes, their characteristic feature being that they
derive at least part of their food from decaying animal or
vegetable matter, absorbing it in some cases as actual food-
stuffs, and in others as organic compounds which require
relatively little expenditure of energy to build them up
again into proteids or carbohydrates.
Numerically the fungi are the most prominent in this
group, but the green plants also afford many instances of
the habit. Among the mosses Splachnum grows upon
lumps of dung, and various species of Hypnum flourish in
water which contains various compounds derived from the
decomposition of once living matter. Among higher
plants still, the soil of woods and pastures affords many
examples of individuals which depend partly upon the humus
of the soil and partly on their own chlorophyll. Among
the ferns we have notably the moon-wort, Botrychium
Lunaria, and among the club-mosses some species of
Lycopodium, while numerous flowering plants show this
peculiarity.
The chlorophyll apparatus is found in nearly all of
them, though in some cases it is so reduced as to be
almost functionless. Some of our native Orchids are
remarkable in this respect, that they are almost, if not
altogether, dependent upon their saprophytism. Neottia,
the so-called bird's-nest orchis, has a flowering stem above
ground, on which are only a few rudimentary leaves. At
OTHEK METHODS OF OBTAINING FOOD 199
the base of the stem there is found a cluster of fairly stout
roots which intertwine with each other to form a mass
sometimes as large as a man's fist. These are developed
only in masses of humus, from which they absorb the
products of decay. These plants differ thus from normal
phanerogams by their method of absorbing food. Their
roots are not provided with the system of short-lived root-
hairs which are so characteristic of the ordinary roots.
They are not in need of such close contact with continually
fresh particles of soil as are the latter, lying as they do
embedded in a mass of humus. In some cases all their
external cells absorb material from this ; in others special
absorptive cells are present, but these are not localised like
the ordinary root-hairs, and they are not being continually
renewed, but remain active for long periods. Frequently
they are only found on the roots at the points where
contact with the humus is effected. Many of these
saprophytes have the cells of their cortex infested with the
hyphae of a fungus.
The food which is thus absorbed from the decaying
organic matter is not necessarily in a fit condition for
immediate assimilation by the protoplasts. It may, and
frequently does, require alteration before being available
for nutrition.
The plants of the next group which we must consider
differ from the saprophytes in an important particular. Like
them they are provided with a chlorophyll apparatus, and
are consequently capable of carrying on carbohydrate con-
struction. Indeed they are generally more active in this
respect than the members of the last group. As in the case
of the greater number of the latter, it is chiefly their nitro-
genous material that they obtain nearly or quite ready for
assimilation. They appear to need this nitrogenous food
in the form of proteids, and they obtain it by capturing
and killing various animal organisms whose putrefying
bodies yield them what they want.
The Utricularias, which are members of this group, are
200
VEGETABLE PHYSIOLOGY
plants which live floating in water (fig. 93) ; they have a
much -branched stem which bears a number of leaves, the
shapes of which differ in the case of different species ; they
possess no roots. Growing out of the stems are numerous
small bladder-like bodies, each with a small opening at its
OTHEE METHODS OF OBTAINING FOOD 201
apex. This orifice is guarded by a number of stiff tapering
bristles, and is closed by a sort of trapdoor which opens
inwards and shuts again with a kind of spring. A small
animal such as an aquatic insect can easily open it by press-
ing against it, and thus can enter the bladder. The trapdoor
immediately closes by virtue of its own elasticity, and
cannot be opened by pressure from within. The insect
accordingly finds egress, impossible, and after a short
FIG. 94. — TRAPS OF Utricularia neglecta. (After Kerner.)
a, a bladder magnified ( x 4) ; 6, section of a bladder ; c, absorption-cells
on the internal surface of the bladder ( x 250).
time, usually ranging from one to three days, it perishes
and its body decays, yielding to the plant the products
of its decomposition, which are absorbed by particular
cells growing from the internal wall of the bladder (fig. 94).
Some of the ascidia-bearing or pitcher-plants show a
somewhat similar mechanism and utilise corresponding
organic substances. The Sarracenias afford good examples.
202
VEGETABLE PHYSIOLOGY
These are marsh plants having their leaves arranged in
rosettes, which spring apparently from the surface of the
soil, and from the centre of which arises a single flower
stalk. Each leaf is modified to form a curious pitcher-like
body (fig. 95), furnished with a
kind of lid. The pitchers are
generally conspicuously coloured,
while the lid, which is the lamina
of the leaf, often bears hairs which
secrete honey to attract the prey.
The inner surface of the pitchers
is lined with slippery recurved hairs
which make it impossible for an
insect to climb out of it after once
entering. Insects are attracted by
the honey, and, venturing upon
these hairs, slip down to the bottom
of the pitcher, from which escape
is impossible. The pitcher contains
a quantity of water, due either to
the entrance of rain or possibly to
some extent secreted by the sur-
face of the pitcher. The insects
become drowned in this liquid and
undergo decomposition. Frequently
a pitcher will contain so many that
the products of their putrefaction
become offensive. They are ab-
sorbed by the cells of the interior.
Certain other pitcher-plants show a still further ad-
vance in their method of obtaining proteid supplies. They
possess similar means of attracting insects and alluring
them to their death, but they do not depend on the
slow process of putrefaction for the decomposition of
their prey. Instead of this, they secrete and pour out a
definite digestive fluid possessing properties like those of
the secretions of the stomach and pancreas of the higher
FIG. 95. — LEAF OF Sarracenia,
MODIFIED TO FORM A
PITCHER. (After Kerner.)
OTHER METHODS OF OBTAINING FOOD 203
animals, by the instrumentality of which the insoluble
proteids of their prey are converted into peptones, and
possibly partially into amido-acids, prior to actual absorp-
tion. Among these Nepenthes
and Ceplialotus may be men-
tioned.
The pitchers of Nepenthes
(fig. 96) are in the main
similar to those of Sarra-
cenia. They possess means of
attracting insects to them, of
seducing them into the in-
terior of the pitcher, and of
preventing their subsequent
escape, all of which are com-
parable to those already de-
scribed. The pitchers contain
a watery liquid, which is
secreted by their interior
surfaces, and which has a
faintly acid reaction. When
an animal is captured and
falls into the liquid, it sets
up a further secretion, which
is more strongly acid, and
which contains a peculiar
body known as an enzyme
or ferment, the properties of
which will be discussed in a
subsequent chapter. This ferment somewhat closely re-
sembles the active principles of the gastric and pancreatic
juices of the human body, and in the acid medium is capable
of converting the proteids of the prey into peptone, leucin,
and tyrosin, products which are all soluble and diffu-
sible. This secretion is prepared by special glands, which
are plentifully distributed over the lower portion of the
internal face of the pitcher.
FIG. 96.— MODIFIED LEAF ' PITCHES)
OF Nepenthes. (After Kerner.)
204 VEGETABLE PHYSIOLOGY
The mechanism of Cephalotus is essentially similar to
that of Nepenthes.
The members of another group of plants, represented
by Bartsia and by Lathrcea, effect the absorption of
proteids in rather a different manner. Lathraea is a plant
which, unlike those so far described, has no chlorophyll,
but lives for the most of its time parasitically under-
ground on the roots of certain trees, only sending up into the
air the shoots which bear its flowers. Its stems, which are
entirely subterranean, bear a number of thick fleshy leaves
closely set together (fig. 97), which are quite colourless.
FIG. 97. — a, PIECE OF AN UNDERGROUND LEAF-SHOOT OF
LathrtEa squamaria. b, LONGITUDINAL SECTION
THROUGH SAME, SHOWING THE CAVITIES IN THE LAMINA.
(After Kerner.)
They are curiously bent, so that the under surface is of
very small dimensions, and where the two surfaces join are
the openings of several cavities which are hollowed out in
the thickness of the lamina. These chambers are irre-
gular in shape (fig. 98), and on their surfaces are curious
stalked hairs which project slightly above the other cells
which bound the lacunar space. When an insect or other
small animal makes its way into one of these chambers and
comes into contact with the heads of these hairs, the latter
are stimulated, and they then send out, through openings in
their walls, a number of delicate protoplasmic filaments,
which seize upon the intruder, killing it and ultimately
digesting and absorbing its nitrogenous constituents. There
OTHER METHODS OF OBTAINING FOOD 205
are other stalked gland-like structures growing from the
walls, which probably serve to hinder the escape of the prey.
Bartsia possesses a somewhat similar mechanism, which
is associated with its subterranean buds.
FIG. 98. — LEAF OF Lathrcea ENLARGED, SHOWING THE CHARACTER OF THE
CAVITY OR PRISON-CHAMBER. X 60. 6, PLASMIC THREADS RADIATING
FROM THE CELLS OF THE LITTLE HEADS, x 540. (After Kerner.)
There are other plants which effect the capture and
digestion of insects in other ways. Drosophyllum, which
is found in part of the Mediterranean region, is furnished
with a number of long filiform leaves, which are closely set
with stalked glands. These pour out a peculiar mucila-
ginous secretion which forms a drop of very glistening
appearance round their swollen heads. There are other
sessile glands among them which exude an acid digestive
secretion resembling the gastric juice of the stomach, when
they come into contact with proteid animal matter. An
206 VEGETABLE PHYSIOLOGY
insect, attracted to the leaves by their glistening appearance,
is at once entangled in the viscid mucilage and is presently
suffocated. It is speedily digested by the secretion of the
sessile glands.
Pinguicula, the butter-wort, has a mechanism of a
somewhat similar nature. It bears, resting on the ground,
large fleshy green leaves, the edges of which are slightly
curled over towards the upper surface, forming a kind of
open trough. All over the upper surface are distributed
glands which pour out a viscid mucilage. On contact with
any small mass of proteid, or with an insect or other small
animal, these glands also pour out an increased amount of
mucilage, mixed- now with a digestive fluid similar to that
of Drosopliyllum. If an insect alights upon the margin of
the leaf, not only is the secretion poured out, but the edge
slowly curls over more strongly, either covering the intruder,
or pressing it towards the centre of the trough. Here it is
suffocated and digested as in other cases. Pinguicula is
peculiar in that its secretion has the power of curdling
milk in the same way as the gastric juice of animals.
In some cases a yet more elaborate mechanism is found
to effect the same purpose. We find associated with the
power of digesting and absorbing animal food, a mechanism
for the capture of the prey which involves a movement of
either the leaf-blade itself or of the glands which it pro-
duces. The former is exhibited by Dion&a, the Venus's
fly-trap ; the latter by the different species of Drosera (the
Sundews).
Drosera is a small plant which is found growing upon a
substratum of bog-moss (Sphagnum). Its dimensions are
small, the plant not being more than a few inches in height.
It bears a rosette of leaves, from the middle of which rises a
single scape of flowers. The leaves are covered with stalked
glands (fig. 99), which stand out from the surface. Each
gland has a somewhat substantial stalk, containing a rudi-
mentary vascular bundle. At the top of the stalk is a
rounded head which is always covered by a viscid secretion
OTHEB METHODS OF OBTAINING FOOD 207
that it pours out. From the shining appearance of the
glands with their drops of mucilage, the name of the plant,
sundew, is derived. When an insect alights upon the leaf
it is entangled in the secretion, and, struggling to be free, is
brought into contact with more and more of the drops, becom-
ing hopelessly captured. The stimulus of contact provokes a
CT* * v* .*^^^^-^
^JL n+T _, £. v %* »j^* WiT"£-s>»«^
FIG. 99. — LEAF OF Drosera, SHOWING THE GLANDULAB TENTACLES.
movement of the stalked glands, all of which slowly bend
over and bring their viscid heads to bear upon the strug-
gling insect. The same disturbance causes an outflow of
acid enzyme-containing secretion, which surrounds the
prey, and digestion and absorption follow as before. After
a time the glands unfold again and resume their normal
attitude, and the leaf is ready to receive another visitor.
208
VEGETABLE PHYSIOLOGY
Dionsea affords an instance in which the movement of
capture is effected with greater rapidity. Like most of the
insectivorous plants it possesses a rosette of leaves which
rest upon the ground, and from the centre of the rosette it
gives off a single inflorescence. The leaves are very differ-
ent from those of Drosera. They have a flat expanded
petiole, at the end of which the lamina is attached by
a sort of joint. The lamina is roundish and is divided
FIG. 100.— LEAF OF Dioncea muscipula.
1, open ; 2, closed ; 3, one of the sensitive spines ( x 50) ; 4, glands on
the surface of the leaf ( x 100).
into two almost exactly similar halves, which are separated
by the midrib (fig. 100). The edge of each half is furnished
with a number of rigid teeth, and when the two halves are
folded together on a hinge which the midrib forms, the
teeth interlock with each other and a closed cavity is pre-
pared. On the upper surface of each half of the leaf, about
in the centre, are three short spines which project out-
wards and upwards. When either of these is touched twice
in rapid succession, the two lobes of the lamina become
OTHEE METHODS OF OBTAINING FOOD 209
slightly concave and fold over quickly, the teeth interlock,
and the cavity is closed. If the contact has been made by
an insect, it is captured and imprisoned between the lobes.
The closing is fairly rapid, taking perhaps ten seconds.
All over the upper surface of the lamina secreting glands
are found, whose secretion is similar to that of Drosera.
If the leaf encloses nitrogenous digestible matter, such
as the body of an insect, the prison remains closed for
some considerable time, and the glands surround the prey
with the digestive fluid, the products of its decomposition
being absorbed by the gland-cells.
These mechanisms for the digestion and absorption of
proteid substances are seen to be extremely complex. Evi-
dence of such digestion and absorption is shown also by far
humbler plants without any differentiated structure. Many
Fungi and Bacteria when cultivated in solutions containing
native proteids, such as albumin or globulin, are able to
effect their digestion by the secretion of a similar enzyme to
those of the plants already described. They subsequently
absorb the peptone or the amido-acids which result from
such action. Nor is proteid material alone affected in this
way by these humbler plants. They derive their carbo-
hydrate supplies from their environment in the same way
as their proteid ones. Many of the filamentous fungi
possess the property of forming digestive enzymes, which
attack sometimes starch, sometimes inulin, sometimes vari-
ous sugars which are not immediately available for nutri-
tion, sometimes other more complex substances, all of
which undergo this external process of digestion, the result-
ing bodies being subsequently absorbed.
In the earlier pages of this chapter we drew attention
to the fact that it was not at all uncommon to find two
plants closely associated together, with different degrees of
completeness, with a view to their co-operation in carrying
out some of these abnormal processes of nutrition. We
may now study these relationships a little more fully.
The simplest cases of the dependence of one plant
p
210 VEGETABLE PHYSIOLOGY
upon another are afforded by the so-called epiphytes,
representatives of which are supplied by many members
of the OrchidacecB and the Bromeliacece which inhabit
tropical forests. The dependence in these cases is merely
one of situation. The epiphyte grows upon the external
surface of some supporting tree, to which it clings by
various arrangements, without penetrating into its tissues.
Frequently the long roots of the epiphyte are attached
closely to the crannies of the bark of the tree, and the dust
and debris which accumulate there are utilised for the
purpose of supplying it with nutriment. In other cases the
supporting plant does not give it even so much assistance.
An almost- equally simple relationship is seen in the
cases of Anthoceros and Azolla. Cavities in the tissues
of these plants are inhabited by numerous cells of an Alga
(Nostoc). Beyond affording them shelter and a certain
degree of protection, the higher plant does nothing for its
guests. The relationship is sometimes called commensalism.
A more complete association, attended by distinct
advantage to one or both of the plants taking part in it, is
known under the name of symbiosis. By some writers
this term is confined to such an association as is of
benefit to both organisms, and does not profit one at the
expense of the other. Where the latter is the case the
relationship is said to be one of more or less complete para-
sitism. Others speak of reciprocal and antagonistic sym-
biosis, to indicate these two different kinds of association.
One of the best known cases of symbiosis in the strict
sense is that of the Lichens. These are lowly organisms
which are epiphytic upon tree trunks, old walls, rocks, and
other supporting structures. They are composed always of
two distinct plants, an Alga and a Fungus, which are closely
united together to form a kind of thallus (fig. 101). The
relative modes of arrangement differ in different species,
and many algae and many fungi are found to be capable of
entering into such an association. The advantages which
result to the two constituents of the lichen are consider-
OTHEE METHODS OF OBTAINING FOOD 211
able. The alga, which possesses chlorophyll, is able to con-
struct carbohydrate materials by its instrumentality, and
these are shared after their formation by the fungus,
which has no such construc-
tive powers. The fungus is
able to condense aqueous
vapour, which is very neces-
sary in the dry situations
lichens occupy. It can thus
dissolve much of the dust
and other debris of its rest-
ing place, and so carry raw
material to the constructive
algal cells. It also attaches
the thallus to the substratum.
Both partners can no doubt
take, part in the construc-
tion of proteids. The rela-
tionship affords a further
advantage, for the compound
organism is much better
able than either of its separate constituents to resist
adverse conditions of temperature, drought, &c.
A similar symbiosis is met with in the so-called Jcephir
organism and others of the same kind. In these cases the
two constituents are a yeast and a bacterium, the former of
which is closely surrounded by chains of the latter, making
a fleshy mass of irregular shape, and sometimes of compara-
tively conspicuous dimensions. The parts played by the two
organisms are not very well understood, but there seems to
be no doubt that the association is mutually beneficial.
In a former chapter mention was made of a property
which is possessed under certain conditions by various
plants belonging to the Leguminosce — that of being able to
utilise the free nitrogen of the air in the construction of
proteid food-substances. The power was shown to be con-
nected with the formation of certain tubercular structures
p 2
FIG. 101.— SECTION OF A LICHEN SHOW-
ING ALGAL CELLS (g) IN THE MIDST
OF A NETWORK OF FUNGAL HYPH^E
(m). After Sachs.)
212 VEGETABLE PHYSIOLOGY
upon the roots of the leguminous plant. These tubercles
are swellings of the cortex of the root, the cells of which
are inhabited by a particular fungus, which breaks up in
their interior into curious bacterioid bodies. The exact
nature of the fungus has not been accurately determined.
The soil contains many of these bacterium-like bodies,
which make their way into the interior of the leguminous
plants by penetrating their root-hairs, and growing down
them into the cortex of the root. In the cells of the latter
the penetrating filaments bud off the bacterioid bodies in
great numbers. The stimulus resulting from the invasion
causes a considerable hypertrophy of the roots at the points
attacked, and -the tubercles are the result. The fungus
appears to have the power of fixing atmospheric nitrogen,
bringing it into some combination, the exact nature of
which is unknown, but which serves as the starting point
of proteid synthesis, either by the green plant or by the
intruder. The relationship is clearly of great advantage
to both organisms, the fungus obtaining its carbohydrate
supplies from the green plant, much as is the case in the
lichens already described.
Many of our forest trees, among which the members of
the Cupuliferce are conspicuous, exhibit another symbiosis
which is of the greatest interest and importance. The
roots of these plants grow down into soil which is infested
with the mycelia of different fungi, with which they become
entangled. The hyphge of the fungi continue to grow
together with the root, and form an investment over it,
which is in some cases met with in the form of an open
network, and in others in that of a dense feltwork (fig. 102).
The fungi in some cases perforate the external cells of the
roots and form a network in the interior. From the out-
side of the investing mantle hyphae grow out into the soil
in a similar way to the root-hairs of ordinary plants.
These take the place of the root-hairs, which cease to be
developed, and serve the purposes of the roots as absorbing
organs for the water and the salts of the soil. The fungus
OTHER METHODS OF OBTAINING FOOD 213
is benefited by drawing its own nutriment from the cells of
the root into which it has penetrated. The fungoid mantle
is known as a myco-
ft
rhiza ; it is present
not only on the roots
of the Cupuliferae, but
on those of Poplars,
and many Heaths and
Rhododendrons.
A curious case of
this kind of relation-
ship is shown by
Monotropa, a member
of the Heath family
which possesses no
* FIG. 102. — A, EPIPHYTIC MYCOBHIZA OF Fagus
Chlorophyll. Mono- sylvatica (x 2); B, TIP OF BOOT PABTIALLY
DENUDED OF THE INVESTING MANTLE ( X 30).
tropa possesses a (After Pfeffer.)
rhizome, from which
rise sub-aerial stems from ten to twenty centimetres high,
bearing succulent membranous leaves. From the rhizome
are given off crowded masses of roots which are covered
with a mycorhizal mycelium, arid are embedded in humus.
There being no chlorophyll apparatus, Monotropa is de-
pendent entirely on the mycorhiza for its nourishment.
The latter is entirely saprophytic. We have here a curious
case of the complete dependence of a higher plant upon a
more lowly one.
A complete symbiosis between two green plants is
occasionally met with. A good instance is afforded by the
Mistletoe and the plants upon which it grows, usually
either the Poplar, the Silver Fir, or the Apple-tree. The
seed of the Mistletoe is left by a bird upon a branch of
one of these trees, and under appropriate conditions it
germinates. The root of the seedling penetrates into the
bark of the tree and grows inwards till it reaches the wood.
It makes its way no further, but maintains its position
there, and as the branch gradually thickens by the activity
214 VEGETABLE PHYSIOLOGY
of its cambium, -the intruding root is by degrees impacted
in the secondary wood, its own growth preventing its being
cut off and buried by the latter. The root branches in the
substance of the tree, and the secondary roots make their
way along in the bast, growing parallel with the exterior.
These branches also put out small vertical outgrowths,
which make their way to the wood just as the primary
root did. A very complete fusion of the tissues of the two
plants is thus ultimately arrived at. The advantage of the
alliance is on the side of the Mistletoe, which derives a
great part of its nourishment from the host. It possesses
evergreen leaves, however, which serve for the construc-
tion of carbohydrates, and as it manufactures these during
the winter, when the host plant has no leaves, the latter is
able to benefit in its turn during that season.
Passing on to notice the association of two organisms
which is known by the name of antagonistic symbiosis or
parasitism, we find various degrees of completeness in the
dependence of one form, the parasite, upon the other, the
host. As in the case of the insectivorous plants, there are
members of this class which are provided with a chlorophyll
apparatus, and which are therefore indebted to their hosts for
proteid substances only, or perhaps also for certain of their
ash constituents. As these almost without exception fasten
themselves upon the roots of the host plant, they are
frequently spoken of as root-parasites. From their general
structure and their relationship to the host plant, they
evidently have much in common with the Mistletoe, and
it is not very easy to distinguish between their semi-
parasitism and the symbiosis of the latter with the trees
on which it lives. They are, however, usually herbaceous
forms, and can - therefore be of no use to the host plant
in the winter. Moreover, most of them ultimately destroy
the root on which they have fastened.
These root-parasites are mainly members of the Scroph-
ulariacece or the Santalacece. As a rule, they are
herbaceous annuals, though there are some perennial
OTHER METHODS OF OBTAINING FOOD 215
species. They grow from seed with fair rapidity, the root
of the seedling attaining a length of an inch in two or
three days. Shortly after penetrating the soil, the main
root puts out secondary branches, which make their way
parallel to the surface. As they grow chiefly in woods or
among herbage, they speedily encounter the roots of other
plants, and on contact being made between one of these
FIG. 103. — Thesium alpinum, SHOWING THE SUCKERS ON THE BOOTS.
(After Kerner.)
root-branches and a root of a suitable host, a curious sucker-
like body is developed at the point of contact (fig. 103).
This is a kind of parenchymatous cushion, which partly
surrounds the host, and from the inner side of its con-
cavity certain absorption-cells grow out and penetrate
into the former, pushing their way until they reach the
centre of the invaded root (fig. 104). These absorbing
organs are often erroneously spoken of as roots. They
cannot properly be so called, as they are developed from
the cortex of the rootlet, and not, as root-branches are,
from the tissue of the pericycle. They are best spoken of
as haustoria, a term which is purely physiological, and
carries with it no anatomical significance.
216
VEGETABLE PHYSIOLOGY
While the root is setting up this relationship with a
host plant, the shoot of the seedling is growing normally.
Its leaves and other sub-aerial parts are well developed
and discharge their appropriate functions. The plants
would not be recognised at all as in any way parasitic
without an examination of the subterranean parts. They
absorb certain nutritive materials from the roots on which
they fix themselves, and generally destroy them. The
damage is, however, local, and does not involve the death
of the host plant. In-
deed, many of these
root-parasites do so
little harm to the latter
that an affected host
is often not noticeably
different in appear-
ance from a neigh-
bouring plant of the
same species which is
not attacked.
The perennial
forms produce fewer
suckers or haustoria
which only function
for one year. The
rootlets usually bear
only one sucker each, and when it has ceased to act
as an absorbing organ it dies. The rootlet grows on, and
in the next year develops a new sucker, and makes a fresh
attachment.
Some of these root-parasites are also saprophytic in
their habit, bearing, besides the suckers, absorbing hairs
on their underground stems, which come into relationship
with the humus of the soil.
There are many other plants which are parasitic upon
roots, but they must be distinguished from those we have
just discussed, on account of the greater degree of their
FIG. 104. — Thesium alpinum. PIECE OF A
BOOT WITH SUCKER IN SECTION. X 35.
(After Kerner.)
OTHEE METHODS OF OBTAINING FOOD 217
parasitism. They in-
clude such forms as
Lathr&a and Oro-
banche, which are
members of the
British Flora. La-
thrsea has already
been mentioned as a
plant which draws
proteid material from
small animals which
it captures and kills.
It supplements this
method of obtaining
food by becoming pa-
rasitic on the roots of
trees, to which its
roots attach them-
selves by suckers,
much in the same way
as the semi-parasites
already described.
The host plant in this
case is drawn upon
for carbohydrates as
well as proteids, as
Lathrsea possesses no
chlorophyll.
Orobanche resem-
bles Lathraea in ex-
hibiting the same de-
gree of parasitism.
It shows certain differ-
ences of structure,
and it does not attach
itself exactly in the
same way. It derives
FIG. 105. — PLANT OF Melampyrum INFESTED
WITH Cuscuta.
218
VEGETABLE PHYSIOLOGY
its nutriment entirely from its host, which is frequently a
herbaceous plant. The different species of the genus infest
FIG. 106.-SECTION OF STEM OF DICOTYLEDONOUS PLANT ATTACKED BY
HAUSTOBIA OF Cuscuta.
different plants, each having only one suitable host. None
of the species are like Lathr*ea in being partially carnivorous.
OTHER METHODS OF OBTAINING FOOD 219
Some curious parasites which are met with in the
tropics show a very peculiar method of attaching them-
selves to their host plant. They constitute the natural
order Eafflesiacece. The embryo, after emerging from the
seed, penetrates the cortex of its host, usually a root, though
not always, and gradually forms a hollow cylinder surround-
ing its woody centre. This sheathing structure is com-
posed of rows of cells, and in appearance resembles the
mycelium of a fungus. Buds arise upon this investment,
which eventually burst the cortex above them, and protrude
through the host plant. These, in Bafflesia itself, develop
a single flower which, in some cases, is of enormous size.
The plant produces no outgrowths of any kind except the
buds described. Other genera show some modification of
this structure, but exhibit exactly similar physiological
peculiarities.
Certain other parasites which resemble these in many
respects differ in attacking only sub-aerial portions of their
hosts. The most easily observed of these is the Dodder
(Cuscuta), which often attacks the clover (fig. 105). The
seed when germinating puts out an embryo which bears no
cotyledons. Germination takes place on the ground, and
the embryo grows to a length of about an inch. Its apex
attaches itself to the ground, and the free portion moves
round, describing a sort of spiral in the air. If it comes in
contact with a suitable host, it twines round it after the
fashion of a tendril, and numerous suckers are developed
in rows at the points of contact. Haustoria spring from
these suckers and penetrate the host, extending inwards till
they reach the wood (fig. 106). The part below the attach-
ment dies shortly after this relationship has been esta-
blished, and the parasite is left attached to the host. In
its further growth it continues to twine around the latter,
putting out numerous branches, which also form similar
coils, so that the host is completely immeshed in the
twining stems of the parasite. The latter bears no leaves
and possesses no chlorophyll in any part, so that it derives
220
VEGETABLE PHYSIOLOGY
all its food in fully elaborated form from the tissues of the
host. Cuscuta produces numbers of flowers on its branches,
and from them are developed fruits and seeds. The para-
sitism is complete, and the relationship frequently leads to
the death of the host which has been
attacked.
Parasitic plants are very frequently
met with among the Fungi and the
Bacteria. The former penetrate the
living cells of the plant they infest,
or in a few cases ramify between
them, sending haustoria into the
interior of the cells between which
the mycelium grows (fig. 107). They
make use of the contents of the cells,
destroying and absorbing the living
FIG. 107.— CELLS OF POTATO J
PLANT INFESTED WITH substance as well as any formed
Phytophthora. • i i • i i T
, , materials which may be present. In
ft, hypha running between J
the ceils and sending many cases also they destroy the
haustoria (a) into their ,, ,, _ . .,. ,
interior. cell-walls, and utilise the carbo-
hydrate materials of which the latter
consist. Their ravages only cease with the death of the
organism.
The power of living plants to assimilate the food
manufactured by others is taken advantage of in the
processes of grafting and budding. In these operations a
slip of a particular plant is inserted into a wound made
in another nearly related one, and the two are closely
bound together. The graft or scion comes into such close
connection with the stock that the food which is contained
in the cells of the latter passes into the tissues of the graft,
which thus receive their nourishment. After a 'longer or
shorter time the two become so completely united that they
live subsequently as a single organism, and the processes
of carbohydrate and proteid construction proceed as in a
normal plant.
221
CHAPTER XV
TRANSLOCATION OF NUTRITIVE MATERIALS
WE have so far traced the ways in which plants receive
their food, and have examined the processes by which it is
appropriated. In some cases, indeed in the vast majority
of instances, it is constructed in the interior of the plant
by certain of the protoplasts from simple inorganic
materials which are absorbed from the environment. In
green plants this construction extends to all the substances
which can be termed food. In plants without a chlorophyll
apparatus this construction is partial only, never going so
far as the formation of carbohydrates, though, when these
are supplied together with inorganic compounds of nitrogen,
proteids and fats can be manufactured. In other cases the
constructive processes are supplemented by the absorption
of food in a suitable condition for nutritive purposes, while
in others, again, the last method is the only one observable,
all constructive power being absent.
There are other considerations, which must be briefly
stated, which have a bearing upon this subject. The con-
ditions of life of an ordinary green plant involve a great
extension of the original constructive process. It has no
definite and regular times at which it can take in a certain
quantity of food, which are regulated partly by the needs
of the organism and partly by the mysterious factor which
we call appetite. Its absorptive processes are much more
under the influence of natural phenomena, such as the
degree of illumination, the amount of warmth, moisture,
&c., which it is receiving. Periods of intermission of ir-
regular duration are caused by differences in these respects,
222 VEGETABLE PHYSIOLOGY
even during an ordinary day, and still more by the alterna-
tion of day and night ; in the case of perennial plants yet
greater disturbances are caused by the succession of the
seasons of the year, and the alterations these produce in
the amount of foliage which the plant preserves ; weather
and its vicissitudes form a series of disturbing influences.
We have thus the certainty of failure to survive in the
struggle for existence unless the initial absorptive and
constructive processes are supplemented by others, which
in some way shall make the organism indifferent to these
changes and intermissions of supply, and capable of carry-
ing out true nutritive work, though the initial stages of
such work are checked or suspended. In other words,
suitable conditions for the construction of food being
intermittent, the plant must accumulate a reserve store on
which it can subsist during the periods, short or prolonged,
when no such manufacture is possible.
We may view the matter from a slightly different
standpoint, and yet come to the same conclusion. The
processes of absorption in a plant depend, as we have seen,
almost entirely upon physical conditions. Given a certain
amount of carbon dioxide in the air, and a certain amount
of water in the plant to which that air has access, the
carbon dioxide will be dissolved according to the power of
the water to dissolve it, or— putting it more technically —
according to its coemcient of solubility. In the presence
of the chlorophyll apparatus, with the access of sunlight,
the other subsequent changes which we have discussed
lead to the continuation of the absorption of the gas.
This is the case again with the root and its relations to the
soil. The process of absorption of water with its dissolved
substances will proceed as long as certain physical condi-
tions obtain. Thus the plant is, on the whole, rather
passive than active in the initial stages of its own feeding,
exercising no inhibitory power, such as that which in an
animal is attendant upon a failure or cessation of appetite.
These considerations lead us to the conclusion that when
TRANSLOCATION OF NUTRITIVE MATERIALS 223
the absorption of food or food materials by a plant is
proceeding, the probabilities are decidedly in favour of
such an absorption being much greater than the immediate
need for direct consumption. The constructive process,
followed by the accumulation of its products, is certainly
the leading one in the history of the different members of
the vegetable kingdom. The increase of the framework
which attends upon the multiplication of the protoplasts,
which we commonly speak of as growth, proceeds for such
long periods, moreover, that there is stored up in such a
structure as a forest tree an enormous amount of material
and of potential energy.
But this latter form of storage, devoted especially to
the production and maintenance of a very large plant-body,
differs materially from the accumulation of a quantity of
food which is temporarily a surplus, but which is destined
for subsequent consumption by the protoplasts. This is
a feature of the life of all plants in varying degrees,
whether they form a large plant-body or not. We must
turn to examine this surplus production in more detail.
In an earlier chapter we alluded to the very marked
division of labour which we can observe in such a com-
munity of protoplasts as form a large plant. We have
since studied certain of the different processes which are
carried on by particular tissues or collections of protoplasts,
rendering them unable to perform other necessary duties.
It is evident that to enable them to discharge their
special functions they must be fed and nourished. It is
equally clear that they are not living under conditions
which enable them to construct food for themselves. We
see that it is consequently necessary for food to be trans-
ported to them from the seat of its construction.
There is in every green plant a localised, though fairly
widespread, region in which construction is taking place,
and there are other equally well-defined regions which
must be supplied with food transported from the seats of
its manufacture. The cell or protoplast, which contains a
224 VEGETABLE PHYSIOLOGY
portion of the chlorophyll apparatus, has thus not only to
provide for its own nutrition, but to prepare a part of the
nutritive material required by other protoplasts which are
set apart for the discharge of other work.
But this is not all. We find, from a study of plants,
that in almost all cases, so long as life lasts, growth is
proceeding. This may result in a continuous increase in
the dimensions of the plant-body, or may only lead to the
replacement of parts which have a brief existence, and
need to be renewed. This is the case, for instance, in
forest trees that have attained their full dimensions.
Growth in the vegetable organism is very definitely
localised. Growth in length takes place at or near the
apices of stems and roots ; it has a definite though vari-
able localisation in leaves of different kinds. Growth in
thickness is confined to sheaths or bands of cells in different
regions of the axis, such as the cambium, and the different
phellogens met with in the cortex.
Growth and nutrition differ in another respect : the
former is intermittent, the latter needs to be constant,
though the intensity of the requirements may vary.
These considerations show us that there must exist in
the plant a very complete mechanism by which the diffe-
rent food-stuffs can be circulated about its body. Each
protoplast must be in receipt of a continuous, though per-
haps small, supply of nutritive material ; the demands
of growth must be satisfied by the transport of considerable
quantities of formative material to the growing regions.
The intermittence of growth makes a further demand.
Consider one among many places at which a large con-
sumption of such formative material is proceeding : a stream
is travelling there to supply the need. Suppose that some
temporary check to the growth at that spot takes place.
The stream will be diverted elsewhere by the demands of
the other growing parts, and when the hindrance is
removed and growth should again proceed, there will be no
stream of constructive material, and much time will be lost
TEANSLOCATION OF NUTRITIVE MATERIALS 225
before it can be restored. To prevent this there should be
a storage of food close to the seat of its consumption, so
that, with the awakening need, the required supply may be
at hand.
This temporary storage of food must play an important
part in the metabolism of an organ whose vital processes
are subject to such numerous and often rapid checks as
befall young stems, leaves, and roots. Still more necessary
is it to the floral and fruiting organs during the time of
their maturing.
We have seen again that plants set apart particular
structures for periods of longer quiescence, especially in
connection with their reproductive processes. Seeds may
remain for several years without germinating, and they
generally do so at least for months. The embryo in the
seed is, however, ready to resume its growth as soon as all
conditions are favourable. It is evident that it is in a
practically helpless condition with regard to the manufac-
ture of food, and it must depend upon a previously stored
supply for the resumption of vital activity. The parent
plant must, therefore, store quantities of its manufactured
products in or about the embryo in the seed, stores with
which it will itself have no further concern, but which will
be altogether the property of the new organism. The same
thing is seen to be the case with tubers, bulbs, and other
organs of vegetative propagation.
A condition intermediate between the two we have so
far described is presented by the large fleshy roots and
rhizomes of biennial and perennial plants. For an illustra-
tion we may consider an ordinary carrot or beetroot.
Though these plants propagate themselves by the prepara-
tion of flowers, fruits, and seeds, they do not enter on this
task during the first year of their lives. During this time
they are in full foliage, and their constructive processes
are at their best. They store in their roots a large amount
of the food so prepared, and these towards the close of the
first year's vegetation become enormously swollen by the
Q
226 VEGETABLE PHYSIOLOGY
development of succulent parenchyma. During the second
year they have a much smaller foliar development, but each
sends up its flowering stem. The constructive activity is
much less than during the previous year ; the root gradu-
ally dwindles as the fruit and seeds develop, the store
deposited in the succulent parenchyma being applied to their
formation and maturity.
Based upon considerations such as these, we may make
a further classification of the nutritive substances which
exist in the body of the plant. We can speak of those
which are used in the cells where they are formed, and of
those which are removed therefrom for the feeding of the
other protoplasts. These, again, may be devoted to imme-
diate use, or may be stored as reserve materials for deferred
consumption. We can recognise in every plant some kinds
which are suitable for transport from cell to cell, and others
which are not able to pass through cell-walls, but must
remain in the position in which they are formed. These
two classes of circulating and stored food-stuffs have an
intimate relationship to each other, and must be mutually
interdependent, each being reinforced by the other accord-
ing to the needs of the particular moment.
If now we turn from these general considerations to
the course of the events that are normally taking place in
the cells which contain the chloroplasts, we can form some
definite idea of the course of the processes of construction
of the carbohydrates and removal of the products. In
such a cell we have seen that there is, during favourable
conditions, a manufacture of sugar which is continuous and
rapid. The cell itself needs a certain amount of such sugar
for its own nutrition, but only a very small part of what is
being made. The sap of its vacuole soon becomes saturated
with the sugar, and if nothing further transpired the pro-
cess of manufacture would stop. But the cell is in contact
with others, in many of which, perhaps in all, a similar
manufacture is taking place. The ordinary processes of
diffusion tend to equalise the amounts in any contiguous
TKANSLOCATION OF NUTEITIVE MATERIALS 227
cells, so that very soon the whole of the parenchyma of the
constructive region is saturated with the sugar. This par-
enchyma abuts however on other cells which contain no
chloroplasts, especially the sheaths and the bast of the
nbro-vascular bundles. Diffusion of sugar into these takes
place, and proceeds from cell to cell, especially among the
delicate bast tissue, so that a stream of sugar is soon diffusing
all along the bast. As a rule it does not penetrate very
far beyond this tissue, owing largely to the anatomical
arrangements of the parts and the great facility which the
structure of the bast affords for this diffusion. So long as
the manufacture goes on, therefore, there is an outflow of
the manufactured carbohydrates from the region of its forma-
tion, the ultimate and even the temporary direction of the
stream being determined by other factors which we shall
consider later.
This removal of sugar from the leaf can be proved by
several observations. We find but little of it in the meso-
phyll of the leaf, though we know it is being continually
produced there. We find it fairly easily in the bast of the
veins, and if a leaf is cut off from the stem while construc-
tion is going on, so that it cannot be transported away, it
can very soon be detected in the mesophyll cells as well.
This, however, is not all. The process of diffusion is a
slow one and does not serve to remove the sugar as fast
as it is formed. The excessive formation of sugar would
soon lead to such saturation of the sap as would at any
rate temporarily inhibit its construction, were it not for
another agency at work. The chloroplasts are endowed
with another property than that so far described, which is
now called into play. This is a peculiarity of the body of
the plastid, and is quite independent of the colouring
matter, being shared by other quite colourless plastids
which occur in other parts of the plant. These structures
have the power of converting sugar into starch, a power
which we must examine more fully in a subsequent chapter.
The transformation is apparently a process of secretion.
Q 2
223 VEGETABLE PHYSIOLOGY
Part of the sugar consequently gives rise to numerous
minute grains of starch, which the plastid forms within itself
and deposits in its own substance. This formation of a tem-
porary store not only relieves the over-saturation of the sap
in the cell, but supplies the need of the protoplasm when
the formation of sugar from carbon dioxide and water is
interrupted by the failure of the daylight. These minute
granules are of very small dimensions, three or four of
them being formed within each plastid. They have no
apparent structure, but can be detected by treating the cell
with a solution of iodine, which stains them blue. If a
chloroplast so treated is examined with a
* High power of the microscope, it presents the
D ^ appearance of fig. 108, the little grains of
~ H starch lying as blue specks in the green
substance. They can be seen more dis-
FIG. 108.— STARCH J ...
GRAINS IN THE tinctly if the leaf under examination is
BODIES OF CHLO- .. . , , , , . ,
ROPLASTS. x2so. bleached by warming it in alcohol, which
dissolves out the chlorophyll. A leaf so
treated turns blue wherever the light has had access to it,
not only showing the formation of the starch but allowing
its exact locality to be determined with absolute precision.
In fact this test may be applied to ascertain whether the
chlorophyll apparatus of a part is at any time active, the
deposition of the starch taking place within a few minutes
of the commencement of carbohydrate construction. This
rapidity of appearance led indeed to the old view that the
construction of starch rather than sugar was the immediate
object of the chlorophyll apparatus. The reasons we have
given lead us preferably to the view that the starch is the
expression of the superabundant supply, requiring that a
certain portion shall be deposited in an insoluble form as a
temporary reserve material, to allow the process of carbo-
hydrate construction to proceed without intermission so
long as the conditions are favourable. At the same time
we cannot but notice that the appearance of starch in the
chloroplasts is so rapid when the conditions of carbo-
TEANSLOCATION OF NUTKITIVE MATEEIALS 229
hydrate formation are realised, that it may be relied on as
a test for the absorption of carbon dioxide by the tissue in
which it appears.
In connection with the manufacture and fate of carbohy-
drates, we can now see that they may be met with in two
different conditions : the one suitable for retention in the
cell and hence capable of functioning as reserve, but not
immediately nutritive, material ; the other capable of
diffusion, and hence serving as a translocatory form, or
one in which it can pass from cell to cell, remaining all
the time in a suitable condition to minister to the nutrition
of any protoplast which it reaches.
The same considerations affect the manufacture, trans-
port, and storage of proteids. We have already seen
reason to believe that these, like the carbohydrates, are in
the first instance constructed in the leaves, if not by the
chloroplasts. Our information about them is, however,
very incomplete ; we do not know even what form of
proteid is first formed, nor which kind is needed for
assimilation by the protoplasm. Probably it is a soluble
and diffusible form, such as a peptone or a proteose, but
our only reason for thinking so is that such properties
characterise the travelling forms of carbohydrates. We
can, however, readily believe in the construction being
greatly in excess of the immediate need of the cell, and
hence in the chain of events being similar to that in
which the carbohydrates are concerned.
The different properties of the two classes of bodies
involve, however, some differences in their behaviour,
and we can therefore expect similarity only and not iden-
tity. The diffusibility of peptone even is very greatly
below that of sugar ; and we can hardly suppose therefore
that peptone is the translocatory form of proteid in the
plant. It seems on the whole more probable that
nitrogenous plastic material is transported in the form of
some amido-acid such as asparagin, and that the latter is
subsequently worked up into proteid at the place where it is
230 VEGETABLE PHYSIOLOGY
assimilated by the living substance. This view is sup-
ported by observations made upon the utilisation of the
reserve stores of proteids found in seeds, which have been
found to give rise to similar amido-acids before being
transported from the site of storage. To this point we
shall return in a subsequent chapter.
We cannot say either in what form proteids are tem-
porarily stored in the cells of their first formation. Pro-
bably, like starch, they are made indiffusible and so retained
in the cell. But whether they are thrown into a solid
form we do not know. If so, they are amorphous and are
hidden away hi the substance of the protoplasm. They may
be kept in solution in the sap which saturates it. Different
forms of globulin and albumin have been found in the
cells in different regions. It is possible again that the
manufacture of proteid may be only so great as to provide
for the needs of the cells in which such formation takes
place, together with the amount that can diffuse during
such manufacture, so that there may be no occasion for a
temporary storage there.
The translocation of food has no very determinate
direction. On leaving the cells which are the seats of its
formation, its path is dependent on physical processes
taking place in different parts of the plant. We can study
it most simply by taking a special case, which as before
may conveniently be that of sugar. We have seen that it
passes from cell to cell under the influence of osmotic
diffusion— that is, that when two contiguous cells contain
sap of different degrees of concentration, an interchange of
the liquids takes place till the sap of both cells becomes
uniform. The nbro-vascular bundles, the bast of which we
have seen forms its principal path, extend in complete con-
tinuity throughout the plant, so that any travelling com-
pound can be transported from the leaves to the growing
points of the stem and root. So long as it is being used
by the protoplasm in these regions, the sap of the cells of
the tissue there, which are using it in the construction of
TRANSLOCATION OF NUTRITIVE MATERIALS 231
living substance, becomes continually weaker in that con-
stituent, and hence more and more diffuses into them as
the construction goes on. The utilisation or consumption
of the sugar so acts as an attracting force, directing the
stream to the points where it is required. The same
principle applies to the consideration of the deposition of
the large reserves of carbohydrates in seeds, tubers, or
other organs. The withdrawal of it from the travelling
stream which is the result of the formation of the quantities
of starch or cellulose which those reservoirs contain, leads
to fresh quantities being transported slowly but con-
tinuously to those cells, owing to the physical process of
diffusion already described. The stream passes in fact in
both cases exactly in proportion as the consumption takes
place, whether the consumption takes the form of construc-
tion of new protoplasm, or the transformation of the travel-
ling carbohydrates into the insoluble resting forms.
This passage of the sugar about the plant need not
demand a coincident transport of water, so that the old
idea that there was an actual stream of fluid along the
bast, or in the old nomenclature a stream of descending
sap, need not have any foundation in fact. The principle
of diffusion alone will suffice to explain the passage of the
sugar. Disturbances of the fluid contents of the cells do
no doubt occur, as osmosis is continually taking place
in both directions between the contiguous cells. A defi-
nite flow of water need not, however, coincide in either
magnitude or direction with the passage of the stream
of sugar.
The translocation of the sugar, we see, thus varies in
direction and in magnitude according to the varying pro-
cesses which are from time to time proceeding. As the
variations in these processes, particularly those of growth
and nutrition, are often sudden and considerable, we find
the translocation is generally accompanied by changes of
the carbohydrate from the labile to the storage forms, and
vice versa. It is very usual to find temporary accumulations
232 VEGETABLE PHYSIOLOGY
of starch in the neighbourhood of a growing region.
Grains of starch are of frequent occurrence in different
parts of the bast, and particularly in the bundle- sheaths of
certain regions. The explanation of their appearance
there is simple ; they are generally indications of such an
interference with the supply and the demand as we have
described. A checking of the demand by a cessation of
the vigour of growth or nutrition is attended by an over-
accumulation of the sugar, which is speedily changed into
a storage form instead of being removed by the slow pro-
cess of diffusion.
The transport of proteids follows the same course ; the
amido-acids are the travelling forms, and are conducted by
the same forces to the growing points, or to reservoirs where
accumulation of proteids takes place. Their deposition in
storage forms along the pathway can also be detected,
though these are not so widespread as those of carbo-
hydrates. They can be observed generally in the sieve-
tubes of the bast, which contain a curious modification of
protoplasm in which proteid as such is present. It was
formerly held that the sieve-tubes conduct proteid as such
along the vascular bundles. Though there is not a very
great improbability that such bodies may pass* from cell
to cell of the sieve-tube, on account of the protoplasmic or
quasi-protoplasmic threads which extend throughout the
openings of the sieve-plates, yet this method of transport
must be necessarily very slow and subject to much
hindrance. It seems more probable that the proteids in
these vessels are constructed there from the amido-acids
which reach them, and are to be regarded as temporary
stores, like the starch grains already alluded to as being
formed in different parts of the translocatory tract.
We have spoken of the bast as forming the pathway of
the translocation of nutritive material or of the different
food-stuffs which have been manufactured. The process by
which they travel, we have seen, is mainly one of diffusion
through the cell-walls, the latter being saturated with the
TEANSLOCATION OF NUTRITIVE MATEEIALS 233
cell-sap. It must not be forgotten, however, that the cell-
membranes are all perforated by very delicate strands of
protoplasm which extend from one protoplast to another.
There is here a further means of transport which no doubt
facilitates the passage.
We may find proofs that the pathway lies along the
bast by experiments carried out on plants in which trans -
location is actively proceeding. If we cut a branch from a
vigorously growing tree and remove from near its free end
a ring of tissue extending inwards through the bark and
cortex to the cambium, and then place all the lower part in
water or moist earth, very marked effects follow. After
some time, perhaps a few weeks, adventitious roots will be
put out from near its end. Those which arise below the
missing ring will be few and of small size ; those from above
this region will be numerous and strong, and will continue
to elongate. Any buds that may be on the part below the
ring will not develop, while those above it will grow
normally or even more freely than on an uninjured branch.
The tissue immediately above the ring will become to some
extent hypertrophied and show a decided swelling.
The continuance of the growth shows that the water
supply has not been cut off, but the different behaviour of
the parts above and below the excised tissue tells us that
the supply of nutritive material to the latter region has
been interfered with, and the buds and adventitious roots it
bears gradually perish of inanition. The passage of any
food or nutritive material across the ring has become
impossible.
If a similar incision is made into another branch but is
not carried so far inwards — if, that is, the ring of tissue
removed consists only of the structures external to the
bast — these appearances do not accompany or follow the
wound. Evidently in this case the translocation path has
not been interfered with. We may safely conclude there-
fore that the transport of elaborated products, chiefly food,
is the principal function of the bast.
234 VEGETABLE PHYSIOLOGY
To a certain extent the cortex of the plant shares the
translocatory function. The contents of the cells include a
certain amount of carbohydrate matter, but their reaction
is distinctly acid, and this region is probably concerned
much more definitely with the transport of vegetable acids,
so far as it takes part in translocation at all. At the same
time it is impossible to localise the transport of food
exclusively in the bast.
Other parenchymatous tissues are sometimes the region
of transport. In many germinating seeds there is a trans-
ference of large quantities of nutritive substance across the
endosperm to the embryo, and in young seedlings similar
transport takes place through both cortex and pith.
The vessels of the wood, which we have seen are the
paths of the transpiration current, are probably not con-
cerned normally in the translocation of manufactured
products, though exceptionally they may contain certain
amounts of proteids, amido-acids, &c., in solution. Their
function in this respect is, however, unimportant, and the
presence of such bodies in them is mainly accidental.
It is doubtful how7 far the laticiferous systems which
are present in many plants may be regarded as channels
for translocation. No doubt latex contains many nutritive
products, both nitrogenous and non-nitrogenous, but there
is reason to think they are to be referred to the storage
•rather than to the transporting system.
235
CHAPTEE XVI
THE STORAGE OF RESERVE MATERIALS
WE have seen that the large amount of food which is
continually being manufactured by a normal green plant is
very greatly in excess of its immediate requirements, and
that there is a very extensive system of storage in such an
organism, by the aid of which it is enabled to survive
periods, often of some duration, in which the manufacturing
processes are entirely suspended. We have considered
further the mechanisms of transport, by which the various
nutritive substances are transferred from the seats of their
manufacture to the places in which they are laid up for
longer or shorter periods.
The questions of transport and of storage are very inti-
mately connected. Food once formed is not always moved
at once to some place where, after a period of storage, it
will be ultimately consumed. It is often transferred more
than once, and may occupy several places in succession
as the demand for it varies. Indeed, we may regard the
surplus manufactured food, that is the quantity which is
in excess of the immediate requirements of the construc-
tive cells, as a single store, part of which is travelling
about the plant, and part of which is from time to time
withdrawn from the travelling stream and laid down in
particular cells, either to rejoin the travelling current after
a longer or shorter time, or to be separated from the parent
plant, and serve as a starting point for the growth and
nutrition of its offspring.
A very-little consideration will show us that the forms
in which the various food -stuffs are packed away in the
236 VEGETABLE PHYSIOLOGY
storage reservoirs must be materially different from those
in which they travel. We have, already seen that one of
the conditions of the continuous formation of any one of
them is the removal of it from the seat of its construction
as soon as its amount exceeds a certain limit. If this is
not secured, the sap of the constructing cells becomes
saturated with the body in question, and then no more is
made. The removal is dependent upon the deposition of
the substance from the sap in some way which lessens the
concentration of its solution in the latter. We find
accordingly that the bulkier reserve materials are very
frequently deposited in solid forms, sometimes amorphous,
sometimes granular, and sometimes crystalline. Other
cases are known as well, in which they remain in solution
in the sap of particular cells, but in these cases they are
retained in such cells through the difficulty or impossibility
of diffusing through the plasmatic membranes of the cyto-
plasm. They are generally formed inside these cells from
some particular constituent of the travelling stream, much
as are those which become insoluble, and once formed, they
are unable to pass out of the vacuole.
In considering the forms which the various reserve
food materials assume in the reservoirs they occupy, we must
then remember that they are not a simple accumulation of
food pabulum in the form in which it is of immediate use.
Granted that the plant in the first instance forms certain
materials on which its living substance draws at the place
where it is originally constructed, then, so long as the
immediate needs are in excess of the amount prepared,
there is no alteration in such materials ; they are at once
utilised by the living substance in the processes of nutrition
and growth. But as soon as the supply exceeds the imme-
diate demand, the surplus is not simply retained unchanged
in the cell, nor does it overflow unchanged to contiguous cells
where demand exceeds supply, or where provision is made
for storage. The storage forms, whether retained in the
cells of construction or transferred to others, are different
THE STOKAGE OF RESERVE MATERIALS 237
from and more complex than the originally prepared ones,
and further energy has to be expended on them, either
where they are made, or in the place of storage itself.
As we shall see later, when they come to be utilised in
after time, a converse process takes place, which is com-
parable to the digestion which they undergo when, as so
frequently happens, they are eaten by an animal. The
surplus food of the plant exists thus in two conditions, the
one suitable for travelling, the other for storage. The
former is characterised by solubility and diffusibility, the
latter generally by insolubility in the cell-sap, and always
by an absence of the power to pass through the plasmatic
membranes. The former usually consists of such substances
as can at once be assimilated by the living material ; the
latter does not, but requires the digestive changes to take
place before it becomes so.
The places where these reserve materials are deposited
are more numerous than we are apt to suppose. Parts of
the plant or definite structures, which ultimately serve as
reproductive organs, readily occur to us as reservoirs which
are adapted for a somewhat prolonged storage. Seeds,
tubers, fleshy roots and branches, bulbs, corms, and
rhizomes are instances of these, and in the short-lived
plants which we group together roughly as herbaceous
in their habit these are necessarily the most important
reservoirs. But it is different with trees and shrubs which
live for many years, and which do not form fleshy
receptacles. We have in these forms stout stems or
trunks, with numerous branches ; large woody roots which
continue to grow year after year, keeping pace with the
parts aboveground. Though the primary use of these
members is not to store food products, yet they have work
of this kind to do. We have seen that in the cells which
are the original seats of carbohydrate construction there is
almost always an excess of such matter formed, which is
partly deposited in the chloroplasts in the form of small
granules of starch. These afford us an instance of a very
238 VEGETABLE PHYSIOLOGY
transitory store, for the starch deposited there during
exposure to sunlight is removed almost as soon as dark-
ness supervenes. A plant which has been vigorously
forming starch in its chloroplasts during a summer's day,
will show that at evening there is a considerable amount
accumulated there ; if the leaves are examined again early
next morning, the starch will be found to have disappeared.
This is not brought about by its having been used in the
metabolism of the cells during the night, for if the path of
removal is obliterated, as it may be by severing the petiole
in the evening, the leaf is found as full as ever in the
morning. If a- plant whose chloroplasts are charged with
starch grains is kept for a time in an atmosphere free from
carbon dioxide, the starch is gradually removed, whether it
is kept in light or darkness, so that the removal of the
starch can, and probably does, take place continuously,
though it cannot be easily detected so long as construction
is proceeding simultaneously.
The deposition of food in such other reservoirs in trees
and shrubs as are not connected with the reproduction of
the plant is generally of a transitory character, though not
so markedly so as in the case of the leaves. These
temporary storage places are found very widely distributed,
and the reason for their occurrence is in each case trace-
able with comparative ease. A tree that has a trunk
and a root which are growing in thickness is in need of a
constant rather than an intermittent supply of food placed
near the actively growing regions. The growth in thick-
ness of such a trunk or root is brought about by the activity
of a layer of delicate living cells, which are constantly
dividing to produce new wood and new bast, and which
appear quite early as u ring of cambium on the exterior
of the woody mass (fig. 109, b). The new cells need a
constant supply of nutritive material, at the expense of
which they develop into the peculiar elements of wood and
bast respectively. The cambium too is in continuous need
of food, or it is perforce obliged to cease dividing, and so the
THE STOEAGE OF BESEEVE MATEEIALS 239
growth in thickness of the trunk or root is stopped. Cell-
division is indeed the result of cell-growth. When a cell
of the cambium has attained its full size it divides into
two, each of which then grows to its appropriate adult
dimensions ; some divide again, like those from which
they sprang ; others become transformed into wood or bast
cells. In either case an immediate supply of food is
needed, and from the condition of things this must be
near at hand. The stream from the leaves is inter-
mittent, and hence it is important that a certain reserve
FIG. 109. — SECTION OF PAKT OF STEM OF Ricinus communis.
a, starch sheath ; at the extremities of the figure its cells are
represented as empty ; 6, cambium layer.
shall be deposited not far from the growing cells, so that
a slow continuous supply may be available. We find such
reserves laid down near the cambium, either in the cells
of definite sheaths surrounding the whole ring of new tissue
(fig. 109, a), or in the spaces called medullary rays, which
are found between the separate masses of wood and bast,
these rays (fig. 110) being composed of cells which differ
in shape from the typical forms of both wood and bast
cells.
In stems of smaller girth which have not developed
much wood, we find stores of food laid up in the region
240 VEGETABLE PHYSIOLOGY
just underneath the surface, which constitutes what is
called the cortex, and which gives place later on to the
complex formation that is familiar to us under the name
of bark.
The formation of the successive rings of cork deeper
and deeper in the cortex, which ultimately constitute I the
bark, is attended by the same need of a continuous instead
IX
FIG. 110. — SECTION OF THREE-YEAR-OLD STEM OF Tilia, SHOWING THE
MEDULLARY RAYS RUNNING THROUGH THE WOOD. X 50. (After Kny.)
of an intermittent supply of food. We find, therefore,
during the process of the construction of the bark, similar
provision of food-containing tissue, which is situated near
the cork layers. In some cases it takes the form of regular
sheaths ; in others the food is irregularly distributed through
the cortex, which is the seat of the appearance of the for-
mative layers of the cork.
THE STOEAGE OF EESEEVE MATEEIALS 241
As the trunk grows older similar stores of food may
be detected deeper in the wood. These generally occur in
the medullary rays, either those which are the continua-
tions of the primary ones, or others which are formed
apparently for the purpose under discussion. These stores
are especially for the nutrition of the more deeply placed
wood -cells, when the ordinary constructive processes are
in abeyance, as in the winter-time.
Transitory stores may also be detected near the grow-
ing points of the axis. These are due to intermission of
growth and a consequent sudden cessation of the demand
upon the translocation stream. The latter, instead of being
diverted at once from the region to which it had been
travelling, deposits in a suitably stable form the food
which would have been consumed had not the check in
the demand occurred. The supply is consequently ready
to hand as soon as growth sets in again.
Deposits of reserve materials can be observed near the
extremities of twigs as winter approaches. The output of
the young leaves in the spring is greatly facilitated by the
occurrence of such temporary storage. It is possible by
appropriate pruning to influence to a considerable extent
the locality and the extent of such deposition. This is of
very common occurrence in horticulture, the nature of
the pruning having in this way a very considerable
influence upon the development of floral or foliage
shoots.
Transitory deposits of food take place also in the floral
organs. In many flowers which have long succulent styles,
which must be perforated by the pollen tubes on their way
to the ovules, there may be observed very frequently a
deposition of food in the tissue of the style at the time
when the germination of the pollen grain takes place upon
the stigma. The food is then usually stored in the paren-
chymatous tissue which surrounds the vascular bundles of
the organ.
Many of these reservoirs show by their structure that
R
242 VEGETABLE PHYSIOLOGY
they are only intended to compensate for regular or acci-
dental intermittence in the translocatory stream to the
parts in question. The food is thrown down in the ordinary
parenchymatous cells or in the sheaths of the conducting
tissue, and no special arrangements are made to receive it.
It is often of accidental occurrence — deposited suddenly
and gradually or rapidly removed. Such deposition and
reabsorption form, indeed, one of the features of the
transporting mechanisms.
We may now pass to the consideration of the forms in
which the different foods present themselves in these
reservoirs of storage. It is not surprising that we find
here a great deal of variety, even in any particular class
of food. The more prolonged the stay in the reservoir,
the more complex usually is the structure which the
nutritive substance assumes.
We may deal in the first instance with the stores of
carbohydrates. We have already noticed that in the
great majority of cases these take the form of starch. In
the chloroplasts in the leaf-cells the starch grains are
laid down as minute bodies, showing hardly
^ any trace of structure and crowded together
in the substance of the plastid till they are
almost in contact with each other (fig. 111).
The deposition is due to the protoplasm or
F GB^TK A?S" stroma of the plastid, and does not depend
BODIES OF CHLO- in any way upon the colouring matter, the
KOPLASTS. X250.
presence of the latter influencing only the
other function of the chloroplast, the synthesis of sugar,
as we have already seen in a previous chapter. The
process is thus one of true secretion, and the deposition
of the starch originating at several centres in the plastid,
several granules are coincidently formed. The number,
however, is not constant.
In the more permanent reservoirs of starch it usually
happens that the cells are so charged with the grains that
they appear to contain nothing else. Fig. 112 shows a
THE STOKAGE OF EESEEVE MATEKIALS 243
cell taken from the interior of a potato tuber. These
grains of starch are much larger than those which occur
in the chloroplasts of the leaf, and they have a complicated
structure. Most of them are irregularly oval in shape,
and their surfaces are marked by nearly concentric lines
of striation, dividing them apparently into layers. The
centre of these layers is not usually the geometrical centre
of the grain, but lies near the small end, and the rings or
layers are much narrower at that end than at the other
(fig. 113).
In most cases the deposition of starch in these and
similar cells is brought about by the agency of small
protoplasmic corpuscles, which closely resemble the chloro-
plasts, except that they are colourless. They are known
for this reason as leucoplasts ; like the chloroplasts they
FIG. 112— CELL OF POTATO FIG. 113. — STARCH GRAIN OF
CONTAINING STARCH GRAINS. POTATO.
occur in considerable numbers in each cell, being situated
usually near the nucleus. Their relationship to chloro-
plasts is shown by the fact that they turn green when they
are exposed for a considerable time to light.
The leucoplasts behave very much like the chloroplasts.
When a solution of sugar reaches the cell in which they
lie, they absorb it as the chloroplasts do the excess of sugar
manufactured in the cells of the leaf. They then secrete
starch, which is at once deposited in their substance. If
the point of deposition is the centre of the leucoplast,
successive shells of starch are deposited concentrically upon
the first-formed portion, and a symmetrical grain is
produced which ultimately attains a relatively considerable
size. It remains, however, surrounded by the leucoplast,
which gradually becomes much stretched until there is
B 2
244 VEGETABLE PHYSIOLOGY
merely a thin film of it surrounding the striated grain. It
can frequently only be detected by delicate staining as the
starch grain grows. If the point of deposition is near the
side of the leucoplast, as is generally the case, the succes-
sive shells of starch are not of equal width, but are wider
on the side of the grain which is in relation with the
greater bulk of the plastid. The amount deposited on any
part of the first-formed portion is proportional to the
thickness of the plastid in contact with that part. An
eccentric shape, often approximating to that of an oyster-
shell, is consequently arrived at. Even the most eccentric
grains can be • shown by delicate staining to be covered
entirely by the leucoplast, even the small free end which
appears to protrude from the
latter being clothed by a thin
film of its substance.
Some grains of ten found in the
potato are not so simple in their
structure. These are represented
FIG. lu.-A, COMPOUND, B, SEMI- in fig. 114, A and B. The former
p0oMTA0ToNDSTABCHG ' arise by two or more grains
originating in the interior of a
leucoplast ; as each grows by deposition of new layers they
become closely pressed together, and constitute a compound
grain. Fig. 114, B, shows what is often called a semi-com-
pound grain. In such a formation the leucoplast commences
deposition at two points, one towards each side. As the
starch is deposited round each, the concentric grains come
into contact, and the bulk of the leucoplast is reduced to a
shell surrounding the mass. Its subsequent continued
activity then forms new sheaths overlying the whole. The
leucoplast, as in the first case, is gradually used up by its
own activity, and it is finally reduced to a film of extreme
tenuity, which surrounds the whole grain.
A very curious starch grain occurs in the latex of
certain species of Euphorbia, having the appearance of a
dumb-bell (fig. 115). This also is formed bv a leucoplast ;
THE STOEAGE OF EESEEVE MATEEIALS 245
the latter is an elongated structure, and at first forms a
rod of starch along its axis. As the deposition proceeds
the leucoplast becomes very much stretched longitudinally,
till its centre is reduced to a thin film round the rod of
starch, while what is left of its substance is accumulated at
the two ends. The further activity of these portions
results in the development of the two heads of the dumb-
bell, the thin film connecting them ceasing to deposit any
starch along the centre of
the rod.
It is not very easy to
see the leucoplasts in the
potato ; they can be detected,
FIG. 115. — LATICIFEBOUS CELL
FROM Euphorbia, CONTAINING
DUMB - BELL - SHAPED STARCH
GRAINS.
FIG. 116. — GROUP OF ROD-LIKE
LEUCOPLASTS, Z, EACH BEARING
A STARCH GRAIN, S, COLLECTED
ROUND THE NUCLEUS, H, OF A
CELL OP THE PSEUDO-BULB OF
AN ORCHID (Phajus grandi-
folius\ x500 (After Schimper.)
however, more easily in other plants. Fig. 116 shows a
group of them forming starch grains in a cell in one of the
orchids. The greater bulk of each lies on the outside of
the grain ; they are disc-like in shape and not round
as in the potato.
In the temporary reservoirs which we have already
noticed, such as pollen grains and tubes, the sheaths of
cells in various regions of the stem, the tissue of the style of
the lily, &c., the deposition of starch is not caused by leuco-
plasts but by the general protoplasm of the cell. In these
cases immense numbers of very small grains, hardly larger
246 VEGETABLE PHYSIOLOGY
than mere specks, make their appearance, while the highest
powers of the microscope fail to enable an observer to
detect the presence of any form of plastid before or during
the deposition. Instead, the minute granules can be seen
to arise in a homogeneous transparent hyaline protoplasm.
The same phenomenon occurs in connection with the
deposition of starch grains in the cells of young developing
embryos, in the early stages of the formation of the seed.
The protoplasm of the cells may be seen to have the form
of a coarse network with many small meshes, which are
empty spaces or contain only cell-sap. There is no leuco-
plast inside them, nor anything comparable to one. The
starch grains originate in these meshes at some point in
contact with the protoplasm and gradually increase in size
till they fill them. In some cases simple, in others com-
pound, grains of starch are thus developed.
In a large number of the Fungi which store up carbo-
hydrate reserve materials, these take the form of glycogen.
This is a substance which presents a somewhat close
resemblance to starch, being readily converted into sugar
in a manner almost, if not quite, identical with that
which is characteristic of starch. It is coloured brown by
iodine. It is usually deposited in amorphous form in the
interior of the fungal hyphse, or of particular cells of them.
In a few cases there are definite granules, which to a
certain extent resemble grains of starch, and which have
been stated to originate in certain corpuscular bodies
resembling leucoplasts. In most cases the deposition
appears to be effected by the protoplasm.
Another carbohydrate which shows a certain resem-
blance to starch, though perhaps not a very close one, is
inulin. The distribution of this material is much more
limited than that of starch, but it is known to occur in
several groups of plants, being conspicuous in many of the
Composite among the Dicotyledons, and in several species
of the LiliacecB, Amaryllidacea, and other allied orders
among the Monocotyledons. Like starch and glycogen, it
THE STORAGE OF RESERVE MATERIALS 247
is capable of transformation into a sugar, though not the
same sugar as in the other cases. It exists, in the plants
mentioned, in solution in the cell-sap, but it can readily be
made to crystallise out or to be precipitated in an amor-
phous condition by the application of alcohol (fig. 117).
We find many instances of the occurrence of various
sugars as reserve materials. Cane-sugar is present in large
quantities in the succulent parenchyma of the roots of the
Beet and the Mangel-wurzel, and of the stems of the Sugar-
cane ; grape-sugar is found in the leaves of the bulbs of
the Onion and allied plants ; small quantities of raffinose
FIG. 117. — SPH^BO-CBYSTALS or INULIN FBOM THE ABTICHOKE.
A, small crystals in the interior of cells treated with alcohol ; B, large crystals
extending through many cells.
are met with in the grains of barley and other cereals.
These are all present in solution in the cell- sap, as has
previously been mentioned.
In many cases carbohydrate reserve materials are found
to take the form of considerable thickening of the cell-
walls. That these are really deposited in seeds with a
view to subsequent utilisation is evident from a study of
the endosperm of many palms, the cells of which consist of
little else; the walls are so thick that the cavities are
almost obliterated, and the small space that is left between
the thickened walls contains apparently nothing but a
small amount of protoplasm with which some amorphous
proteid matter is mixed. Even the unthickened cell-walls
248
VEGETABLE PHYSIOLOGY
of most seeds must be looked upon as reserve food material,
as they are used up in nourishing the embryo during the
early stages of germination.
It is necessary, however, to mention that thickened
cell-walls must not always be regarded as stores of food.
In thickened sclerenchymatous tissue and in ordinary wood-
cells the deposit must' be looked upon as a permanent
strengthening of the skeleton of the plant.
These thickened cell-walls are not composed always of
true cellulose. Our knowledge of their composition is not
FIG. 118. — SECTION THROUGH EXTERNAL REGION or GRAIN OF BARLEY.
p, pericarp of fruit ; t, testa of seed ; al, layer of cells containing aleurone
grains; am, cells of endosperm; n, nucleus. (After Strasburger.)
at all complete, but it extends so far as to show that both
cellulose and pectic compounds may be present and in very
different proportions in different cases. Layers of muci-
lage also are of frequent occurrence.
Nitrogenous material, like carbohydrate, is stored up in
various places and in different forms. By far the com-
monest condition is that of some description of proteid.
The most abundant deposits are found in seeds, in the cells
of which they usually occur in the form of granules of
varying sizes and often of complex composition. In certain
cases, as in fleshy roots, the proteid may be dispersed in
amorphous form in the substance of the protoplasm.
THE STOEAGE OF EESEEVE MATEEIALS 249
When proteid is stored in the condition of granules
these are known as aleurone grains. Like starch grains
they may be deposited all through the substance of the
seed, or they may occupy definite layers, as they do in the
cereal grasses (fig. 118). They occur sometimes in the
same cells as do starch grains, as in the pea or bean (fig.
119). In other cases they are found associated with a
quantity of oil, as in the seed of the castor-oil plant.
An instance of the occurrence of aleurone grains of
some size but yet of fairly simple composition is afforded
by the Lupin, one of the Leguminoscz. This is of interest
FIG. 119. — CELLS OF EMBRYO OF
PEA. (After Sachs.)
a, aleurone grains ; st, starch grains.
FIG. 120. — CELLS OF SEED OF Lupi-
miS, SHOWING COMMENCING FORMA-
TION OF ALEURONE GRAINS. (After
Eendle.)
a, nucleus ; &, vacuole ; c, originating
aleurone grain.
especially because the origin of the grain can be observed
and its development traced. In this seed the aleurone
grains begin to be formed at a very early period of the
development, just as the growth of the embryo is suffi-
ciently advanced to swell out the seed-coat. The cells of
the embryo at that period show the protoplasm not sufficient
in amount to fill each cell, so that a number of spaces or
vacuoles occur, filled with sap. At certain places small
projections from the protoplasm may be noticed which are
of spherical or ovoid shape (fig. 120, c) ; these gradually
increase in size, growing inwards into the protoplasm as
well as outwards into the vacuole, till they can be seen to
be in the form of grains embedded in the protoplasm, which
250 VEGETABLE PHYSIOLOGY
in consequence of their development assumes the appear-
ance of a coarse network. As this process continues, the
original grains growing in size, and new ones being con-
stantly formed, the original vacuoles become obliterated
and the cell swollen out by its own deposits (fig. 121).
While this mechanical process is going on chemical changes
also take place in the material secreted. The protoplasm
forms proteid originally at the expense of the amido-acids,
sugars, &c., brought down to the cell, but the variety
originally constructed is not necessarily the same as that
subsequently stored. At first the grains are not soluble in
either 10 per cent., or saturated, solutions of common salt.
Later on they can be dissolved by both of these fluids.
FIG. 121.— CELL OF KIPE SEED OF FIG. 122.— CELL OF RICINUS SEED,
LupinUS, FILLED WITH ALEURONE CONTAINING FIVE ALEURONE
GRAINS. GRAINS.
The deposition of aleurone grains in the cell is thus,
like that of starch, a process of secretion carried out by the
protoplasm : a process, that is, of manufacture of the grain
by the latter, after it has been supplied with less highly
organised material. It is so constructed by the interven-
tion of the protoplasm itself, the grain growing at the
apparent expense of the substance of the latter.
There is no doubt that the amorphous deposits of
proteids in the cells of fleshy roots and stems are due to a
similar process of secretion.
In many seeds, among which may be mentioned those
of the Castor-oil plant and the Brazil nut, the aleurone
grains possess a more complicated structure. Fig. 122
shows a section of one of the cells of a seed of the castor-
THE STORAGE OF RESERVE MATERIALS 251
oil plant in which some of them are lying. The figure
represents the cell after treatment with alcohol, and subse-
quently with water. The alcohol removes the oil with
which the cells are filled, and which obscures the appear-
ance of the grains. The latter are of ovoid shape, and as
they lie their structure is not apparent, Water dissolves
part of the outer portion, leaving visible the ovoid body,
which becomes transparent. Embedded in it are a large
regular crystal of proteid matter, and a small rounded
irregular mass of minute crystals of mineral matter. These
two constituents are spoken of as the crystalloid and the
globoid respectively. The part of the matrix which is not
soluble in water will dissolve in a 10 per cent, solution of
common salt, while the crystalloid is soluble only in a
saturated solution. The globoid is a double phosphate of
magnesium and calcium.
Examination of these grains and their reactions shows
that several proteids can be detected in them. Those
soluble in water are albumoses, while the others which
dissolve only in salt solutions are globulins. In grains
met with in other plants, proteids occur which are albu-
minates and dissolve only in dilute alkalis.
Crystals of proteid occur in other places than seeds.
If we examine a young potato, we find, in certain cells
lying a little below the epidermis, some regular transparent
cubical crystals, which are composed apparently of the
same material as the crystalloids of the complex aleurone
grains described. They are soluble in saturated solutions
of common salt. Similar crystals are met with in the
tissues of certain seaweeds. Many of them can be made
to crystallise from the solvents which are used to extract
them.
The seeds of the cereal grasses contain two other very
curious reserve proteids, which give rise in the flour to a
peculiar sticky material which is generally known as
gluten. They do not appear to be present in the aleurone
grains of the seeds, but to occur in the starch-containing
252 VEGETABLE PHYSIOLOGY
cells. They have been called gliadin and glutenin;
occurring separately in the seed they interact with one
another in the presence of water and form the gluten of the
flour. Like the zein of maize, these proteids belong to the
peculiar class whose members are soluble in dilute alcohol.
In many cases the proteids of the reservoirs do not
remain unchanged during the resting period which follows
their deposition. This is especially the case with seeds, in
which such changes are characteristic of the process known
as ripening.
Proteids occur also in the temporary reservoirs to
which allusion' has been made. Fleshy roots and stems
contain them in amorphous form in their parenchyma ;
certain forms are met with in the sieve tubes, and are
coagulable on boiling like the globulins of the seeds. The
proteids which are constant constituents of latex are no
doubt in great part reserve food-stuffs.
In many cases amido-acids such as asparagin may be
detected in the sap of various cells. These may be
reserve materials temporarily retained where they are
found, or they may be only translocatory products. Their
occurrence in some resting seeds suggests the former
explanation of their presence. It is not easy to detect
them in the cells, as they are dissolved in the sap, but in
many cases they can be caused to crystallise by placing a
section of the tissue on a glass slip in glycerine.
A great many plants store quantities of complex sub-
stances known as glucosides. These are bodies which on
decomposition give rise to some kind of sugar and some
other compound or compounds, usually belonging to the
aromatic series of carbon compounds. Among them may
be mentioned amygdalin, which is found in the seed of the
bitter almond. During germination it splits up into
benzoyl aldehyde, hydrocyanic (prussic) acid, and grape-
sugar. Many such bodies are known, and they are some-
what widely distributed. Some occur in seeds, but they
are more frequently represented in the reservoirs con-
THE STOEAGE OF EESEEVE MATEEIALS 253
tained in fleshy roots and stems. Many plants belonging
to the Cruciferce and several allied orders are particularly
rich in reserve materials belonging to this group. Sini-
grin, or myronate of potash, is the principal glucoside
which they contain. It splits up into sulphocyanate of
allyl, grape-sugar, and hydrogen-potassium-sulphate.
The nutritive value of these bodies is partly due to the
sugar which they yield on decomposition. The evidence
that the other products can minister to nutrition is not
very complete, though it seems satisfactory in certain
cases.
Fats or oils are frequently stored as reserve food -stuffs
in different plants. The distribution of this material is
very varied, though, as in so many other cases, the seed is
the most general place of deposition. Many seeds, that
for instance of the castor-oil plant, contain as much as 60
per cent, of their dry weight of oil, which is non-volatile.
Others contain as little as 2 per cent., and between these
limits very varying amounts may be found. When the oil
is in great preponderance, it is usual for no other form of
carbonaceous reserve to be present ; in cases where but
little oil occurs starch is usually found as well, as in so
many of the Leguminosce. The Cruciferce as a group
often contain oil in fairly large quantity. As a rule nitro-
genous reserves in the shape of aleurone grains accompany
the oil.
In other places than seeds large deposits of oil often
occur, though their purpose is not so obvious. We have
them in large amount in the pericarps of certain fruits,
such as the olive ; in the petals of many flowers, e.g.
Funkia and Ornithogalum ; in the leaves of some of the
Agaves, the roots of Oncidium, &c. They can hardly be
regarded in some cases as truly reserve materials, being
perhaps more strictly connected with the mechanisms of
dispersion.
The mode of deposition of oil or fat is not at all well
known. It is generally found saturating the protoplasm of
254 VEGETABLE PHYSIOLOGY
the cell in which it lies, and not occupying a definite space
as do aleurone and starch grains. Whether it is secreted
from the substance of the protoplasm, or whether the
materials of which it is made are taken to the latter in a
state near the condition of the finished fat, is uncertain.
It is formed by the combination of a fatty acid with gly-
cerine. Both these bodies can be formed in the plant, but
how they are finally presented to us in the shape of oil is
still in need of elucidation. As the oil appears in the cell
it seems to point to a process of breaking down of the
protoplasm itself, and not to a direct combination of the
antecedents mentioned. Thus if we stain cells which are
forming fat with osmic acid, which colours fatty bodies
brown or black, we see in the protoplasm small specks of
fatty matter, which, while in the youngest cells mere
dots, are in older ones larger, and can be recognised as
droplets. In still older ones the blackness permeates the
whole protoplasm, indicating that the latter is saturated
with the oil, the droplets having run together in conse-
quence of their number and dimensions.
The appearances are, however, not inconsistent with
the view that the work of the protoplasm is only to effect
the ultimate changes or interactions of the glycerine and the
fatty acids which are transported separately to the cells or
perhaps formed there from some antecedent.
The deposition of fat in some cases, particularly in
leaves, has been stated to be effected by the agency of
certain plastids corresponding to the leucoplasts already
mentioned in connection with the formation of starch
grains. . These structures, which have been called ela'io-
plasts, are curious bodies of various shapes, sometimes
round or oval, sometimes irregular in contour, which lie
near the nucleus of the cell. Like the other plastids they
consist of a spongy protoplasmic framework, in the meshes
of which the oil is formed, much as it is in the protoplasm
of the seeds already described.
All these bodies, when acted upon by a process analo-
THE STORAGE OF RESERVE MATERIALS 255
gous to the digestion known in the animal kingdom, are
converted into materials which can directly nourish the
living substance, or can be transported easily about the
plant in a condition of which the latter can readily take
advantage, needing indeed very little constructive change
to fit them for actual assimilation.
256 VEGETABLE PHYSIOLOGY
CHAPTER XVII
DIGESTION
WE have noticed in studying the deposition of reserve food-
stuffs that the forms in which they exist in the reservoirs
differ in many respects from those which they assume for
purposes of transport or translocation. They are generally
insoluble in water or cell- sap, and almost always indiffu-
sible, whereas they travel in the form of soluble, diffusible
bodies. The removal of them from the seats of storage
takes place at times which are dependent on the resump-
tion of activity of growth or development ; and as such a
removal involves the resumption of the travelling forms,
they must undergo a process which, from analogy with
similar processes in the animal body, may be described as
digestion. Each must, after such treatment, be presented
to the protoplasm of the growing cells in much the same
form or condition as that in which it was first constructed
from the simple bodies which the plant absorbed from its
environment. This is necessary in all cases, because,
as we have already noticed, the storage forms are not
directly assimilable by the protoplasm, but have under-
gone a certain modification in the process of their deposi-
tion.
The process of digestion in plants is chiefly intra-
cellular, and takes place in all cells in which reserve
materials occur. It is only occasionally that we find it
taking place on the exterior of the plant— that is, not in
the interior of a cell. In a few cases we find it carried
on in connection with the absorption of nitrogenous or
proteid food, as has been already shown in a preceding
DIGESTION 257
chapter. Digestion, though most generally associated in
plants with the utilisation of reserve materials, may
thus occasionally be met with in connection with the
absorption of food from without, when it is a process pre-
cisely similar to the digestive processes of the higher
animals, though it is somewhat simpler in the details of
its mechanism.
The intra-cellular digestion of plants agrees very closely
with that of many of the humbler animals, and corresponds
also to such processes in the higher forms as the utilisa-
tion of the glycogen of the liver and the fat of various
regions.
We have seen that in a few rare cases proteid material
is absorbed into the plant-body through various leaves
or modified foliar organs. The insectivorous plants are
materially assisted in their growth by capturing and
digesting various insects, the products of the digestion
being absorbed by the surface of the leaf or other organ
concerned. We examined several of these mechanisms in
some detail in Chapter XIV.
Absorption of food from without after preliminary diges-
tion is much more frequently observed when we study the
nutritive processes of the Fungi. Not only proteid, but
also carbohydrate and fatty substances are thus digested
outside the body of the plant, and the products of the diges-
tion are subsequently absorbed.
We have then to inquire how these processes of diges-
tion, whether internal or external, are brought about.
The protoplasm of the cell, among its many properties,
no doubt has the power of setting up these decompositions,
and probably in many of the very lowly plants, in which
the whole organism consists of only a few protoplasts or
perhaps a single one, the work is altogether effected by
its instrumentality. The protoplast, in fact, carries out-
all the various processes of life by the interactions of its
own living substance with the materials absorbed by it,
aided in the constructive processes by the chlorophyll
258 VEGETABLE PHYSIOLOGY
apparatus, if it possesses one. In such a protoplast we
may observe at times the storage of such a reserve
material as starch, and its digestion at the appropriate
period.
Even in more complex plants it is certain that the
living substance of every protoplast is in a constant state
of change, initiating many decompositions in which its
own substance takes part, as well as others into the course
of which it does not itself enter. Among these decomposi-
tions we must include the various intra-cellular digestive
processes.
Though all protoplasm has this power, it is not usual
in plants, any more than in animals, to find it exclusively
relying on it. The work of digestion, at any rate, is
generally carried out by peculiar substances which it forms
or secretes for the purpose. We have in plants a large
number of these secretions, which are known as enzymes or
soluble ferments.
The action of these enzymes is not at all completely
understood. They appear not to enter into the composi-
tion of the substances which are formed by their activity,
and they seem to be capable of carrying out an almost in-
definite amount of such work without being used up in the
process. They are inactive at very low temperatures, but
effect the decompositions they set up freely at the ordinary
temperature of the plant. As the temperature at which
they are working is raised, their activity increases up to a
certain point, which varies slightly for each enzyme, and
is called its optimum point. This usually ranges between
30° and 45° C. If the temperature is raised above the
optimum point, the enzyme becomes less and less active as
it rises, and at about 60°-70° C. it is destroyed. The exact
point, however, varies a good deal in the cases of different-
enzymes.
Enzymes work most advantageously in darkness or in
a very subdued light ; if they are exposed to bright sun-
shine they are gradually decomposed, the violet and ultra-
DIGESTION 259
violet rays being apparently most powerful in effecting
their destruction. They are often injuriously affected by
neutral salts, alkalis, or acids, though in this respect there
exists considerable diversity throughout the group.
The enzymes are manufactured by the protoplasm of
the various cells in which they occur, being produced from
its own substance, in a manner somewhat similar to that
of the formation of the cell-wall. Usually their presence
is accompanied by a marked granularity of the protoplasm,
due to the formation in it of an antecedent substance, known
as a zymogen, which is readily converted into the enzyme.
This granularity does not, however, always occur, though
we have reason to suppose that the secretion of the
enzyme always takes place by successive stages. The
zymogen has not, however, been definitely detected in all
cases.
We find various degrees of completeness of differentia-
tion of the cells which produce these enzymes. In the
simplest cases, such as the mesophyll of the leaves of most
plants, or the great majority of seeds, or the tubers of the
potato, the enzyme is found in all the cells which contain
the reserve materials, so that a rapid transformation of
the latter is readily possible. In the Horse-radish and
many allied plants the cells which secrete the enzyme do
not themselves contain any reserve materials, but are
situated among those which do, so that the enzyme has to
pass from the seat of its formation to other cells in order
to discharge its function. This is a very slow and gradual
process, and is probably carried out through the agency of
the delicate filaments of protoplasm which extend through
the cell-walls, for enzymes are not capable of dialysing
through a membrane.
The occurrence of such cells, which are apparently set
apart especially for the secretion of an enzyme, gives us,
as it were, the starting-points of the special structures
known as glands, whose function is similar but whose
structure is more complex. In some of the plants belong-
s 2
260
VEGETABLE PHYSIOLOGY
ing to the natural orders Capparidacea and Tropaolacea,
the glandular cell divides several times to form a little mass
or nodule of secreting cells, which must be regarded as a
rudimentary gland, though it is not provided with any
definite outlet or duct.
In the seed of the cereal grasses there is a special
organ separating the embryo from the endosperm. This
structure, which is a modification of part of the cotyledon,
is known as the scutellum (fig. 123) ; its function is to effect
the absorption of the nutritive material of the endosperm,
and supply it to the growing embryo. This scutellum is
covered on it's outer face, which is in contact with the
FIG. 123. — SECTION OF OAT-GRAIN.
p, plumule ; r, radicle ; s, scutellum.
FIG. 124. — SECTION OF PORTION OF
SCUTELLUM OF BARLEY, SHOWING
THE SECRETING EPITHELIUM.
endosperm, by a layer of cylindrical cells, whose long axis
is at right angles to the surface (fig. 124). These cells' are
very granular in appearance, and form a very marked
secreting structure, producing two enzymes, which are sub-
sequently discharged into the endosperm to effect the diges-
tion which must precede absorption. The aleurone layer of
the same grain (fig. 125), which has already been described,
is also a secreting layer, resembling the outer layer of the
scutellum in several respects.
The tentacles of the leaves of Drosera, to which allusion
has already been made, are very definitely secreting
structures ; in addition to preparing an enzyme they pro-
DIGESTION
261
duce a ; weak acid, both of which are present in the glairy
material that they pour out over the captured insect.
These tentacles (fig. 126) and the secreting structures of
FlG. 125.T-SECTION THROUGH EXTERNAL REGION OF GRAIN OF BARLEY.
p, pericarp of fruit ; t, testa of seed ; al, layer of cells containing aleuron.$
grains ; am, cells of endosperm ; n, nucleus. (After Strasburger.)
the leaves of Dioncea and other plants, as well as the
similar bodies which occur in the lining of the pitcher of
Nepenthes, must be regarded as actual glands, comparable
to those of the alimentary canal of the
animal body, though less complex in
structure. Glandular hairs, which con-
sist of a few cells situated on a stalk, are
found in great numbers on other plants,
especially some species of Saxifraga.
There are many of these enzymes
present in different plants, the function
of some of which is still not understood.
Many, however, have been investigated
with some completeness. They are
usually classified according to the mate-
rials on which they work. We may
describe here four groups, the members
of which take part in the digestion of reserve materials,
as well as in the processes of external digestion. These
FIG. 126. — GLANDULAR
APEX OF A TENTACLE
OF Drosera.
262 VEGETABLE PHYSIOLOGY
decompose respectively carbohydrates, proteids, glucosides,
and fats or oils. In nearly every case the action of these
enzymes is one of hydration, the body acted upon being
generally made to take up water, and to undergo a subse-
quent decomposition. .
Of those which act upon carbohydrates we have two
varieties of diastase, which convert starch into maltose, or
malt-sugar ; inulase, which forms another sugar, levulose
or fructose, from inulin ; invertase, which converts cane-
sugar into glucose and fructose ; glucase or maltase, which
produces grape-sugar from maltose ; and cytase, which
hydrolyses cellulose. Another enzyme, which does not
appear to be concerned with digestion so directly as the
others, is known as pectase ; it forms vegetable jelly from
pectic substances occurring in the cell-wall.
The members of the second group act upon proteid
substances, and are technically known as proteolytic
enzymes. The principal members of this group are pepsin
and the various trypsins, which convert albumins and
globulins into peptones, the trypsins also decomposing
certain peptones into amido-acids. Allied to these is rennet,
which converts the caseinogen of milk into casein, the
characteristic proteid of cheese. It occurs in a great
many plants, but its function in vegetable metabolism
is unknown.
The enzymes which act upon glucosides are many ; the
best known are emulsin and myrosin ; others of less frequent
occurrence are erythrozym, rhamnase, and gaultherase.
Those which decompose fats have not been so fully inves-
tigated : they are known as Upases, but whether there
are many different varieties or not has not at present been
ascertained.
Diastase appears to exist in two varieties, distinguished
from each other by their mode of action on the starch
grain. One, called diastase of translocation, dissolves the
grain slowly from without inwards, without altering its
general appearance ; the other, diastase of secretion, dis-
DIGESTION 263
integrates it by a process of corrosion before dissolving it
(fig. 127). The first of the varieties has a very wide
distribution in plants, being present
almost everywhere. The second is /Q
the body formed by the glandular ™
covering or epithelium of the scutel- jjk <j^
lum of the grasses.
The great function of diastase in FlG- 127.— STARCH GRAINS
,, , . . - T IN PROCESS OF DlGES-
the plant is to transform starch (and TION. THE SUCCESSIVE
probably glycogen where it occurs)
into maltose or malt-sugar. Wher-
ever starch is formed, whether in the living leaf or in the
reservoir set apart for storage, it must be regarded as a
reserve material, and its removal from the seats of deposi-
tion is preceded by its conversion into this sugar. The
details of the transformation are not fully known at present,
but a good deal of information has been obtained through
the labours of many observers. Starch has a rather large
molecule, but its exact formula is not thoroughly known. For
a long time it was taken to be approximately w(C6H1005),
and the value of n was thought to be 5. More recently the
suggestion has been made that the molecule is much larger,
and may be more truly represented by 5[(C12H20010)20], the
view being based upon the formation of several complex
substances during its decomposition. The starch molecule
is possibly composed of four dextrin-like groups, each
(C]2H20010)2() arranged about a fifth. It has been suggested
that the first action of the diastase is the liberation of these
from one another ; and that four of them by successive
incorporations of water are converted, through a series of
complex substances called malto-dextrins, into maltose,
while the fifth withstands the action of the enzyme for a
considerable time. After the action of the diastase has
been proceeding for some time the resulting product is
found to be four parts maltose and one part dextrin.
How far this series of decompositions represents what
takes place in the plant is uncertain, but it is clear that the
264 VEGETABLE PHYSIOLOGY
starch, which is insoluble, is converted into sugar, which
can be removed to the parts of the plant where it is
required for building up the protoplasm.
Inulase occurs in the tubers and tuberous roots of
some of the Composite, in .the bulbs of certain Monocoty-
ledons, and in some of the Fungi. It converts inulin
ultimately into levulose or fructose, but the action is not
a very simple one, at least one intermediate body being
formed during the process.
Invertase has a much wider distribution. It is easily
extracted from the Yeast-plant, in which it is present in
relatively considerable quantity. Other fungi which con-
tain it are Fusarium and Aspergillus, besides certain
bacteria. In flowering plants it has been found in seeds,
buds, leaves, stems, roots, and pollen grains. Its action
is the hydrolysis of cane-sugar, which it splits up into
glucose and fructose, according to the equation
C12H220H + H20 = C6H1206 + C6H1206
Cane-sugar Water Glucose Fructose
It is a little difficult to understand why this decomposition
of cane-sugar is necessary, as it can diffuse through
membranes, and in many cases it has been found capable
of assimilation by the protoplasm. Probably, however,
each of the sugars concerned in the transformation has a
special part to play in the metabolism of the plant, and
neither can readily replace either of the others.
Glucase occurs in the grains of various cereals, being
especially prominent in the Maize. It is also fairly
abundant in the Yeast-plant. It has no action on cane-
sugar, but splits up maltose into glucose, one molecule of
the former taking up water and yielding at once two
molecules of the latter.
Other sugars of similar constitution to maltose and
cane-sugar are made to undergo similar transformations
by enzymes of less widespread distribution. The chief of
these are trehalase, melibiase, melizitase, and lactase.
There appear to be several varieties of cytase, which
DIGESTION 265
can be prepared from various seeds. The enzyme was
first discovered in the germinating grain of the barley, in
which it is located chiefly in the aleurone layer and to a
less extent in the epithelium of the scutellum, where it
exists side by side with diastase. It dissolves the walls of
the cells of the endosperm, detaching them from each other
and giving a curious mealy character to the grain. Its
presence was first suspected in the Date-palm, where large
reserves of cellulose are found in the hard cell-walls of the
endosperm. The embryo dissolves these walls and absorbs
their products, the work being effected by an epithelium
which covers the part of the cotyledon which remains in
the seed during the early processes of germination. This
epithelium is composed of elongated cells arranged in a
manner resembling that characteristic of those which form
the secreting layer of the scutellum. It has recently been
shown that cytase is formed in the embryo, probably in this
layer, and passes thence into the endosperm. The amount
of it that can be detected is very small, however, and the
process of the decomposition of the cellulose is very slow
and gradual. Cytase exists in considerable quantity in
some of the higher fungi and in certain bacteria.
Pectase has recently been found to be very widespread
in plants. Its function is not very clear, but it may assist
cytase in the swelling up of the cell-wall which is ante-
cedent to solution. It is recognised by its power of forming
vegetable jelly from the pectic substances of the cell-wall.
This jelly appears to be a compound of pectic acid and
calcium.
The enzymes which digest proteids are frequently on
that account spoken of as proteolytic enzymes. There are
two main classes of them known at present. The first,
represented by the pepsin of the stomach of the higher
animals, converts albumins, globulins, and certain insoluble
proteids into peptones, several intermediate bodies, known
as proteoses or albumoses, being formed during the process.
The second group, which may be represented by the
266 VEGETABLE PHYSIOLOGY
trypsin of the pancreas, carries the digestion further and
splits up certain peptones into amido-acids, of which the
chief that have been observed are leucin, tyrosin, and
asparagin.
It is not quite certain that representatives of the first
class are to be met with in plants. It is for the present
probable, however, that the enzyme of some insectivorous
plants is a pepsin. It acts only in the presence of a weak
acid, as does the pepsin of the stomach, but the products
which it forms have not been accurately investigated. It
is apparently only secreted when the gland has been
stimulated by the absorption of nitrogenous matter.
Several varieties of vegetable trypsin have been dis-
covered and their properties investigated. The earliest
known enzyme belonging to the group is the papa/in which
has been extracted from the Papau (Carica Papaya). It
appears to exist in greatest quantity in the pulp of the
fruit, but is present also in the sap which can be expressed
from the stem and leaves. It is apparently associated in
the juice with a peculiar proteose or albumose, and it is
most energetic in a neutral solution, though it can act
also in a faintly alkaline one. It is easily destroyed by a
very small trace of free acid.
Another trypsin, which has been named bromelin, has
been extracted from the fleshy pulp of the Pine-apple
(Ananassa sativa). Like papain it is associated writh a
proteose. It acts most energetically in neutral and faintly
acid solutions, alkalis in very small traces being preju-
dicial to it. Its activity varies a good deal according to the
acid which is present, and to some extent according to the
proteid which it is digesting.
Other vegetable trypsins have been extracted from the
germinating seeds of the Lupin, the seedlings of several
plants, the fruit of the Kachree gourd (Cucumis utilissimus),
the juice of the Fig-tree (Ficus carica), and the leaves of
certain species of Agave. How far these are identical, or
whether they present specific differences, appears at present
DIGESTION 267
uncertain. They are all active in faintly acid solutions,
but the most favourable concentration appears to vary.
The enzyme of the Kachree gourd is most effective when
the medium is faintly alkaline, whereas that of the lupin
seed is inoperative under these conditions. Too much
stress must not, however, be laid upon this point, as the
enzymes have not been prepared in any case in anything
like a pure condition.
The action of all these proteolytic enzymes is probably
one of hydrolysis, though it is difficult to prove it by
analysis.
Eennet occurs in many seeds, in some cases in the
germinating, and in others in the resting, condition. It
has also a wide distribution in the vegetative and floral
parts of various plants. Whether it is really proteolytic
in the vegetable organism it is hard to say, as the details
of its action are unknown. It is so in the animal body.
The enzymes which decompose glucosides, as we have
already seen, are numerous and varied in their distribution,
occurring in various fungi and lichens as well as in the
higher plants. Their action may be illustrated by the
behaviour of emulsin, which exists in quantity in the seeds
of the bitter Almond and in the vegetative parts of the
Cherry-laurel (Prunus Laurocerasus). It splits up the
glucoside amygdalin according to the equation
C20H27NOU -f 2H^O = C7H40 + HCN + 2(C6H1206)
Amygdalin Benzole Prussic Glucose
aldehyde acid
This is, as in other cases, a process of hydrolysis. Myrosin,
another of the group, is peculiar in that it effects its
characteristic decomposition without causing the incorpora-
tion of water during the process, thus :
CIOH,8NKS.20,0 = C.H.CNS + C6H, ,0, + KHS04
Sinigrin Sulpho-cyanate Glucose Potassium-
of allyl hydrogen
sulphate
Others, such as rhamnase, existing in the seeds of Rhamnus
infectorius, eryihrozym in the Madder, gaultherase in the
268 VEGETABLE PHYSIOLOGY
bark of Betula lento, , act on various glucosides, after the
manner of emulsin.
The digestion of the glucosides, we may notice, is
always accompanied by the appearance of sugar, which is
one of the products of their decomposition. The fate of
the other bodies into which they split is not well ascertained,
though there is some evidence that cyanogen compounds,
even such as hydrocyanic or prussic acid, are used for
nutritive purposes by certain plants.
The digestion of fat or oil has not been very fully
investigated, though certain facts are known concerning
its fate in germinating seeds. The digestion is generally
accompanied by the appearance of starch grains in cells
near the seat of digestion, and it was formerly considered
that the starch arose directly from the oil. It appears
now that the oil is split up by an enzyme, lipase, the result
being the formation of a free fatty acid and glycerine. The
fatty acid undergoes further decomposition, being oxidised
into simpler acid bodies, which are crystalline instead of
being viscid like the fatty acid first liberated. These pass
into the general body of the seedling. The glycerine in
its turn seems to be converted into some variety of sugar,
from which the plastids of the seedling construct the starch
which has been referred to, its formation indicating, as in
other cases, a temporary surplus of carbohydrate supplies.
Within the last two years it has been ascertained that
the production of alcohol from sugar is brought about by
another soluble enzyme which has been prepared from
yeast. Though the existence of this body has been long
suspected, it is only quite recently that it has been demon-
strated. Like the decomposition which is brought about
by myrosin, the splitting up of the sugar is apparently not
a process of hydrolysis. It may be expressed by the follow-
ing equation :
C6H1206 = 2C02 f 2CH3CH2OH
In the reaction the sugar is decomposed, alcohol is formed
and carbon dioxide given off.
DIGESTION 269
This enzyme, which has been called zymase, has been
proved to exist not only in yeast but in certain fruits, being
formed there when the fruits are kept in an atmosphere
which contains no oxygen.
The physiological explanation of this observation will
be discussed more fully in a subsequent chapter.
There are other enzymes with a more restricted distri-
bution, about whose value to the plant little or nothing is
known at present. The cells of a particular microscopic
organism, known as Micrococcus Urea, decompose urea
with the formation of ammonium carbonate, and an
enzyme, urease, having the same power, can be extracted
from them. Many enzymes can be prepared from bacteria,
which set up various changes in proteids, some resulting in
the formation of peptone, and others producing toxic sub-
stances. Many bacteria excrete a variety of diastase.
Another class of enzymes has recently been discovered
which do not apparently take any part in digestion, but
which may be briefly alluded to here. They set up a
process of oxidation in the substances they attack, and have
consequently been named oxidases. They are apparently
very widely distributed, and perform very various functions,
being often concerned in bringing about the presence of
particular colouring matters. They occur very prominently
in Fungi, but are by no means confined to them. They
have not at present been studied from the point of view of
their utility to the plants which secrete them.
The conversion of zymogens into enzymes is much
facilitated by a gentle warmth, particularly when a trace of
free acid is present. The red rays of light exercise a similar
influence in some cases.
The fermentative activity of protoplasm was alluded to
at the opening of this chapter. The living substance of
many cells is capable of setting up various fermentative
decompositions, apparently identical with those that have
been described. Various cells can convert starch into
sugar, can peptonise proteids, and carry out other digestive
270 VEGETABLE PHYSIOLOGY
processes, without the intervention of an enzyme. Though
this property can easily be proved in the case of cells of the
higher plants, it is especially prominent in many of the
more lowly organisms such as the Bacteria. The processes
of putrefaction generally depend on this property in the
organisms which bring it about. Till quite recently the
alcoholic fermentation of sugar was attributed to such an
action in the yeast-cell, and in the cells of certain ripe
fruits under particular conditions, the chief of which was
the deprivation of oxygen. Such an action leads to the
formation of acetic acid from alcohol by the microbe Myco-
derma or Bacterium aceti. Similar protoplasmic action is
responsible for the production of various acids in the cells
of the higher plants. The dependence of these fermenta-
tions on the vital activity of the protoplasm is evident from
the fact that no enzyme can be extracted from the cells
which can set up the particular changes in question.
It is not difficult to prepare the enzymes from the
tissues in which they work, but it would be extremely rash
to say that they are in anything like a pure condition when
obtained. Nor is it easy to say much about their purifica-
tion, as they are not known except in close connection with
the substances on which they act, or with the products of
the decompositions they initiate. There is therefore no
known test of their purity.
They can be extracted by treating the tissue, which
should be very finely divided or ground in a mortar, with
glycerine, or with a solution of common salt, or with water
containing a trace of an antiseptic. After a period of ten
or twelve hours the extract should be strained and subse-
quently filtered, when the enzyme may be precipitated from
the filtrate by adding strong alcohol. It is very evident
that this process will not yield it pure, for the solvents
employed will dissolve many constituents of the tissue
besides the enzymes, particularly proteids and sugars.
The former will be thrown down with the enzyme by the
alcohol.
DIGESTION 271
Any description of the process of digestion should
naturally be followed by an account of the subsequent one
of true assimilation or the construction of protoplasm from
the food which is supplied to it as the result of digestion.
Unfortunately but little can be said upon this subject, as such
problems remain almost entirely unsolved. If we study the
changes which take place in the growing points of plants,
where such assimilation must necessarily be most active, we
can find very little evidence of what is taking place. We can
trace, for instance, the progress of sugar along the stem for
a considerable distance, but just where it is assimilated our
methods fail us. Sugar can no longer be detected, but in
what way it has been incorporated into the living substance
is still a mystery. Similar acknowledgment must be made
in respect of the proteids. Amido-acids can be detected
along the translocatory paths almost up to the locality of
growth, but beyond that nothing can at present be said.
We are unable also to explain the manner in which the
food originally constructed ministers to the nutrition of the
protoplasts or cells in which it is formed.
272 VEGETABLE PHYSIOLOGY
CHAPTEE XVIII
METABOLISM
WE have seen that the object of all the processes of con-
struction and digestion that we have examined so far has
been to present • to the protoplasm materials which it can
incorporate into its own substance. If we consider the
processes which take place in a vegetable cell or protoplast,
we find that they can be divided into those which minister
to this construction or building up of the living substance,
and those which are connected with its breaking down. The
latter accompany or immediately follow the former, and the
two together may be considered as the manifestation of the
life of the protoplasm. The whole round of changes in which
the living substance is concerned is generally spoken of as
its metabolism. So many of the reactions as culminate in
the construction of protoplasm are described as anabolic,
while the changes which it initiates, or which are concerned
in its decomposition, are termed katabolic.
We have been occupied mainly so far in discussing the
anabolism of the protoplasts . The substances we have traced
to the cells in which growth and repair are vigorous con-
sist in far the greatest part of some form of sugar and of
organic nitrogenous substances, either proteids themselves
or the products of their decomposition, or substances
constructed from simple materials with a view to the for-
mation of proteids, such as the amido-acids asparagin or
leucin. In the anabolic processes the protoplasm is con-
tinually reconstructing itself at the expense of such nutri-
tive substances, which indeed constitute its food in the
strict sense of the term. What is true of such cells as are
METABOLISM 273
actively growing and multiplying, which are found as we
have seen in the special growing points or layers, is equally
true of all cells so long as they are living. In all cases,
though growth and division may not be evident, we have to
do with processes of repair of the inevitable wasting of the
living substance during the operations of its life. The
same kind of change is evident in all cells, though the
immediate results of such changes differ according to the
part any particular cell takes in promoting the well-being
of the whole organism.
If we turn from these anabolic processes we find we
have proceeding, side by side with them, a decomposition
of the protoplasm, involving a separation from its complex
molecule of various substances which are of less com-
plexity than the living material itself. These often,
in the first instance, include such carbohydrate and nitro-
genous substances as it made use of in building itself up.
These can again be used in reconstruction of the proto-
plasm, or can be further broken down into simpler substances
stilL So long as the protoplasm is living, it is continually
in a state of change or chemical activity, undergoing con-
stant reconstruction and decomposition.
Besides initiating those chemical changes in which it
takes this prominent part, it is also the seat of a large
number of others into which its own molecule does not
immediately enter. Processes of both oxidation and reduc-
tion are continually going on in its substance in which are
involved the various materials which are found there, either
in solution in the water which saturates it, or in amorphous
form ; substances which have been transported from other
cells, or have been formed in the processes of the self-
decomposition of the protoplasm.
These katabolic processes vary a great deal in the
extent to which they are carried out. They may sometimes
go on so far as to produce such simple bodies as carbon
dioxide and water, which are given off from the organism.
This is a very marked feature of the metabolism that may be
274 VEGETABLE PHYSIOLOGY
observed in every living cell. Other katabolic changes,
proceeding side by side with this very complete decomposi-
tion, are not so far-reaching, and a great accumulation
of their products remains in the plant. Prominent among
them we find such organic substances as woody or corky
tissue. These must not be confused with what we have
described as reserve materials, as the latter, unlike those
now under discussion, are intended for ultimate consumption.
These changes involve the manufacture of great
masses of material, whose construction, though ultimately
dependent upon anabolism, is essentially a mark of the
katabolic processes. The constructive processes indeed
are both anabolic and katabolic, the former culminating
in the formation of living substance, the latter marking
the fabrication of its products. The great extent to
which the constructive katabolic processes exceed such
decomposition of protoplasm as is marked by the forma-
tion of carbon dioxide and water, finds its expression in
the enormous bulk which many trees and other plants
attain. This increase of the size of the plant-body is very
much facilitated by the fact that the katabolic processes
in question are not attended by the excretion of any-
thing from the body of the organism. As a rule plants
have no excreta except the gaseous bodies whose elimina-
tion we have already described, and these result in the
main from the profounder decomposition of the living sub-
stance. Whatever a plant absorbs from the soil, except
water, it nearly always retains within its tissues, so that
increase of weight almost inevitably accompanies con-
tinuance of vitality.
It must not be inferred, however, that plants do not
produce, during their constructive katabolic processes, any
substances which are useless to them or which may even
be deleterious. There are numerous products which
come under this category, but from the mode of construc-
tion of the body of the plant they are not cast off as they
would be from the animal organism under similar conditions.
METABOLISM 275
Instead of being eliminated entirely they are only removed
to such localities as ensure their being withdrawn from
the spheres of vital activity. They are generally deposited
in such regions as leaves which are about to be shed, or
the bark of trees, which is a collection mainly of dead
matter ; or they may be stored away in special cells, or in
cell-walls, or intercellular passages, or elsewhere. These
bodies really correspond to excreta, and the processes of
their formation and deposition are called processes of
excretion.
Most of the katabolic constructive processes are directly
applied to the production of substances which are of great
use to the plant. Emanating as these do directly from the
protoplasm, their formation is generally termed secretion.
Though they originate however directly in and from
the living substance, the latter does not always present
them in the form in which they are found in the adult
plant-body, for various changes both of the nature of oxida-
tion and reduction may take place in them after they have
been secreted.
The processes included under the general term katabolism
are thus seen to be very varied. During the course of such
changes many substances are frequently formed which
seem to have no direct bearing on the vital processes, and
whose meaning is still obscure.. These are often spoken of
as the bye-products of metabolism. .
We may now pass to consider~~in some detail some of
the more prominent processes of secretion.
For many reasons the formation of such enzymes as are
used during digestion may be regarded as the most typical
of these. A cell which is about to secrete is generally
found to be filled with colourless hyaline protoplasm in
which certain vacuoles may be seen. Immediately before
secretion begins an increase of the amount of the protoplasm
can be observed, which is effected at the expense of various
nutritive products which are transported to it. During the
whole of the process, when this is prolonged, such a supply
T 2
276 VEGETABLE PHYSIOLOGY
of nutritive material takes place. If during the secretion this
supply is stopped, the process is rapidly suspended. This
can be detected easily in the case of the epithelium of the
scutellum of the barley grain, which we have seen produces
considerable quantities of diastase. The first stage of the
process is thus evidently anabolic. As soon as the nutrition
of the cell has reached a certain point the appearance of
the protoplasm undergoes a change. Minute granules
begin to be formed in its substance, which increase in
number until the hyaline character is replaced by a
marked uniform granularity, the cell substance becoming
somewhat like ground-glass in appearance. The growth
of the protoplasm and this subsequent formation of
/ granules lead to the obliteration of the vacuoles, till the
/ cell is completely filled. After a time as the secretion
leaves the cell the latter shrinks again ; the granules are
passed out in solution in the sap which is exuded, and the
protoplasm is seen to be less plentiful and to become
V hyaline and vacuolated as at first.
Following the anabolic changes we have thus the
breaking down of the protoplasm, attended by the appear-
ance of the granules to which it has given rise. There is
reason to believe that the granules consist of the zymogen
rather than the enzyme, and that the final transformation
of the former into the latter takes place just as the exuda-
tion of the sap occurs.
In glands in which the process of secretion is repeated
more than once, similar changes may be traced. The
secretion of the enzyme in these cases can be shown to
take place by successive stages. The preliminary hyaline
condition is followed by the granular one, and in this state
the cell can remain for some time before the enzyme is
discharged. When this has happened the hyaline condition
is resumed.
The formation of the cell-wall which separates the cells
is due to a similar activity of the protoplasm. The division
of cells or the development of new protoplasts will be more
METABOLISM 277
fully considered in a subsequent chapter ; it will suffice to
say here that in all ordinary growing points this division
of a protoplast into two is followed immediately by the
formation of a new supporting membrane between them.
The division of the cell is preceded by the division of
its nucleus, which is attended by a series of complicated
movements of particular constituents of its substance. The
two daughter-nuclei are situated at some little distance from
each other and are connected by a number of delicate fila-
ments which are gathered to a point at each end and
spread out in the centre, forming what is called the
nuclear spindle. This generally stretches completely
across the long diameter of the cell.
During these introductory changes the hyaline proto-
plasm becomes more granular, and the granules, technically
spoken of as microsomata, are attracted to the spindle
fibres. They pass along these fibrils from both regions of
the cell and form a plate of extreme tenuity across it,
midway between the two new nuclei. This plate soon
undergoes a transformation, the granules disappearing and
the membrane becoming translucent, and so forming the
ordinary substance of the cell-membrane, generally, though
perhaps not strictly accurately, known as cellulose. The
cell-wall is thus seen to be formed from the protoplasm, or
to be secreted by it, the granules or microsomata of which
it is at first composed being the result of decompositions
set up in the living substance.
When cell-walls are growing in thickness or in surface
a similar decomposition of the protoplasm can be observed.
The microsomata or granules are formed in the proto-
plasm and are gradually deposited, often in oblique rows,
upon the original membrane. They are subsequently
changed in appearance and become the first thickening
layer of cellulose. The occurrence of the rows of granules
frequently leads to the striated appearance which can be
noticed on the walls of many fibres, particularly those of
the bast of the fibro-vascular bundles.
278 VEGETABLE PHYSIOLOGY
In all cases therefore the formation of cellulose can be
traced to the self-decomposition of the protoplasm, though
whether the granules are actually cellulose or an inter-
mediate substance is still uncertain.
A very similar phenomenon is observable in the forma-
tion of starch grains. In this case, as we have seen, we may
either have to deal with the general protoplasm of the cell
or, as is usual in reservoirs and in ordinary leaf parenchyma,
with a definite plastid, either a chloroplast or a leucoplast.
These structures, however, may be regarded as specially
differentiated protoplasmic bodies. We have already
discussed their behaviour and the formation of the starch
grain by them. Building themselves up at the expense of
sugar and probably of various nitrogenous compounds,
either brought to them or remaining in their substance,
they break down again to a certain extent, splitting
off a quantity of starch, which is deposited in the interior
of the plastid, sometimes at one point, sometimes at
several. As the process goes on, successive laminae or
shells of starch are continually deposited round the original
grain or granule till the structure of the fully formed starch
grain is reached. In this case the process is somewhat
clearer than the corresponding one in that of cellulose, as
there is little doubt that each shell is composed of starch
at the moment of its deposition.
The formation of starch is in these cases a secretion by
the plastid, just as that of cellulose is a secretion by the
protoplasm of the cell. The formation of the small starch
grains by the general protoplasm of cells in which no
plastid is present is of a similar character, though it is not
so long continued and the formation of successive laminse
does not take place.
The method by which aleurone grains arise in the
meshes of the cytoplasm is of a precisely similar
character.
The formation of fat is due to similar behaviour on the
part of the protoplasm. It can be observed most easily in
METABOLISM 279
the case of certain fungi when they are living under such
conditions as prevent their being properly nourished.
The protoplasm in the hyphae diminishes in quantity, the
vacuolation becomes considerable, and their cavities are
found to contain large drops of oil. In the cells of seeds
such as those of the castor-oil plant, in which large quantities
of oil are stored as reserve materials, the deposition of fat
can be studied. Sections of the cells should be stained
with osmic acid, which colours fatty substances brown or
black according to the quantity of them which is present.
When fat is beginning to be formed the substance of the
protoplasm becomes faintly granular, but the granules are
more or less transparent and cannot be seen without
staining. On the application of osmic acid they display
their fatty character by becoming brown. In cells which
are a little older the granularity is more marked, as the
separate granules have increased in size and many have
run together, forming small droplets. The staining is
darker in these cells, the larger granules becoming quite
black. In still older cells the whole substance becomes
intensely black and the protoplasm can be seen to be
saturated with the oil. If the latter is removed by treat-
ment with ether, the living substance will be found to have
diminished in amount. There has been a formation of
fat by the protoplasm, and the latter has evidently pro-
duced it by a decomposition of its own substance, for it has
become reduced in bulk.
The elai'oplasts, to which reference has been made,
behave similarly, their substance diminishing at the same
time that the fat or oil makes its appearance.
The decomposition of the protoplasm in the formation
of fat is not accompanied by much reconstruction, so that it
is soon very greatly diminished in amount, while the fat,
the product of the katabolic processes, increases.
The appearance of fat in the two cases described seems
to demand two different explanations. In the cells of the
seeds and in the ela'ioplasts it is to be regarded as a
280 VEGETABLE PHYSIOLOGY
storage of reserve materials. In the starved hyphse of
the Fungus it appears to be due to the decomposition
of protoplasm under conditions of grave disturbance of
nutrition, if not of approaching death. In both cases, how-
ever, it is derived from the breaking down of the living
substance, though the decomposition of the latter is due
to such different causes in the two cases.
One of the most important of the secretions of plants is
the green colouring matter, chlorophyll, which we have
already seen is present in the form of a solution in the
meshes of the chloroplasts. The formation of chlorophyll
is a more specialised process than any of those which we
have' just been considering, and is dependent upon a
variety of conditions. It probably involves not only the
self-decomposition of the protoplasm, but also other pro-
cesses which take place within the substance of the chloro-
plast.
The special conditions necessary for the formation of
chlorophyll are— 1st, access of light; 2nd, a particular
range of temperature ; 3rd, the presence of a minute
quantity of iron in the plant ; 4th, access of oxygen. There
are a few exceptions to the rule that chlorophyll can be
formed only in light ; the embryo in the seed of Euonymus
europceus is green at the time the seed is ripe, though it is
surrounded by a thick red protecting coat which is opaque.
Seedlings of Pinus also are green when they are raised
from seeds in light which is insufficiently strong to enable
chlorophyll to be formed in seedlings of Dicotyledons grown
side by side with them. A few other cases also are known.
If an ordinary plant is cultivated from the seed in darkness,
the resulting seedling will not be green, but will have a
yellowish-white colour. When its tissues are examined
with a microscope, the plastids will be found in the cells,
but they will be tinged with a pale yellow pigment known
as etiolin. When the latter is exposed to light it will
rapidly become green, being in fact converted into chloro-
phyll. The etiolin is in the first instance secreted by the
METABOLISM 281
protoplasm of the plastid, and subsequent changes take
place about which very little is known, but which result in
its conversion into chlorophyll. If the temperature is kept
very low, the etiolin remains unchanged, even though light
is admitted. Hence the first leaves of plants which spring
up in winter or early spring are frequently yellow and not
green. This peculiarity may easily be observed in the case
of snowdrops and hyacinths which appear very early in the
year.
The function of the iron is not understood ; plants which
are cultivated in such a medium that this element is not
supplied to them have an appearance much like that
associated with etiolation. Their colour is even paler,
indeed they are almost colourless, though the plastids are
present. A supply of iron at once causes them to assume
the normal appearance. Plants so suffering from the
absence of iron are said to be clilorotic.
The influence , of a supply of oxygen is probably not
a direct one. The failure of plants to form chlorophyll in
its absence is most likely due to a pathological or unhealthy
condition of the protoplasm, all whose activities are dis-
turbed under such circumstances.
Another pigment which is of fairly widespread distri-
bution in plants is the red colouring matter known as
anthocyan. This is not associated with any plastids, but
occurs in solution in the cell-sap. It is found very com-
monly in young developing shoots, on the illuminated side
of leaves which appear during cold weather, on the petioles
and midribs of leaves which are put out on twigs of many
plants in sunny places, and in many tropical plants which
grow in deep shade. In seedlings which are developed in
spring or in cold weather, the anthocyan may appear some-
what irregularly in the leaves, but it is mainly found along
the veins and on the leaf -stalk.
The function of anthocyan is not well understood.
Many facts point to the probability that it aids in the
transformation of starch into sugar in the leaves in which
282 VEGETABLE PHYSIOLOGY
it occurs, rendering translocation more rapid. It has been
found that the red rays of the solar spectrum which it
allows to pass are instrumental in the formation of leaf-
diastase from its antecedent zymogen. The pigment, while
allowing these red rays to pass into the leaf, acts as a
screen preventing the passage of the violet ones which
have a very destructive effect upon this enzyme.
Other views as to the significance of this pigment have
been advanced. It has been suggested that it effects a
conversion of light rays into heating ones, so facilitating
the metabolic processes of the plant. Another hypothesis
regards it as a' protective screen to the chloroplasts and to
the protoplasm, preserving them from injury from too
intense light. Neither of these views can, however, be
regarded as entirely satisfactory.
In many cases it is beneficial by absorbing the dark
heat rays and so facilitating transpiration as well as general
metabolism.
Anthocyan appears to be a derivative of tannin, an
aromatic substance which . is very widely distributed in
the vegetable organism. This substance has not generally
been included among the secretions of plants, but rather
as a bye-product of metabolism. It is not impossible that
it may in some cases be a definite secretion for some
particular purpose.
The distinction between definite processes of secretion,
and such reactions as lead to the formation of the so-called
bye-products of metabolism, is not at all well defined. In
many cases substances are included in the latter category
because nothing is known as to their function, and the
classification can therefore be regarded only as provisional.
In many cases it cannot yet be determined whether
particular substances are formed by the direct decomposi-
tion of protoplasm, or by subsequent changes in the primary
products of such decomposition. Till quite recently the
formation of resin and allied bodies in the resin passages
of the Conifers and in many glandular hairs was con-
METABOLISM
283
sidered a true secretion, the aromatic substances being held
to arise in the cells. Eecent investigations tend to show that
this is not the mode of their origin at all, but that these
substances are formed by a peculiar process of degradation
of the cell-wall. The glandular hairs of Primula sinensis
(fig. 128), and the more complex ones of the Hop (fig. 129)
have long been known to form their resins in this way. It
seems probable that we must now regard the resin-secret-
ing organs of the Conifers as comparable with these.
The bye-products of metabolism are too numerous to
be discussed in detail in the present treatise. Though
they seem to be quite subordinate to the main products we
FIG. 128. — GLANDULAR HAIRS FROM
Primula sinensis.
a, young hair ; 6, hair showing secre-
tion formed in the cell- wall of the
terminal cell ; c, hair after dis-
charge of the secretion.
FIG. 129. — GLANDULAR HAIRS FROM
THE HOP.
A, young hair; B, mature hair;
s.c, secretion under the cuticle.
have noticed, and to be formed indeed by decompositions
which take place during the construction of the latter, we
should not be warranted in ignoring their possible utility
to the plant, nor the probability that many of them may
be of nutritive value. We have seen that in the decom-
position of amygdalin by its appropriate enzyme emulsin,
besides the undoubtedly nutritive sugar there is a produc-
tion of prussic acid and benzoic aldehyde. Some plants,
have been shown to be capable of utilising the former of
these, toxic as it is to animal life.
The bye-products include bodies of very varying degrees
284 VEGETABLE PHYSIOLOGY
of complexity, some nitrogenous and others not. Among
the former may be mentioned the great group of the
alkaloids, which have not so far been found able to
^minister" to the nutrition or growth of the plant, though
their nitrogen is in organic combination. If a plant is
supplied with them, but with no other form of combined
nitrogen, it is rapidly starved. Many physiologists consider
this group to belong rather to the definite excretions of the
plant than even to its bye-products. They are always
deposited in regions which are situated well away from the
seats of active life, such as the bark of trees, the pericarps
of fruits, &c.' It is apparently very difficult to draw
a distinct line of separation between excretions and bye-
products, just as it is to distinguish clearly between the
latter and secretions.
The amidated fatty acids, as we have seen, generally
occur in direct relation to nutrition. We have examined
the part played by leucin and asparagin in proteid con-
struction and metabolism. Several other related substances
are met with in various plants, but how far they are avail-
able for nutrition and how far they are merely bye-products
is uncertain. Such substances are xanthin and glycin,
which can be extracted from various cells. The latex of
plants frequently contains many of these substances. Caou-
tchouc is also a frequent constituent of latex.
Among the non-nitrogenous bye-products may be men-
tioned the great variety of vegetable acids. Conspicuous
among these are tartaric, malic, citric, and acetic acids.
They are usually regarded as arising~m the course~oTthe
katabolic processes, but it is at least possible that some of
them may be formed in the elaboration of food from the
raw materials absorbed, having thus their origin in
anabolism.
The bye-products include also a variety of aromatic
substances. Mention has already been made of tannin,
and its position discussed. In addition we may include
phloroglucin and a variety of aromatic acids, such as
METABOLISM 285
benzoic, salicylic, &c., but the nature of the processes
which give rise to them is not well ascertained.
Certain decomposition products of cellulose may also
be mentioned here. The lignin and suberin which are
characteristic of woody and corky cell-walls arise in this
way. During their formation, which takes place in the
substance of the cell-wall, they can be removed by appropriate
solvents, leaving the cellulose skeleton which they have
been gradually replacing. These differ from most of the
substances described in that they can be produced in the
walls of cells that have lost their protoplasm, so that their
formation is not directly dependent on metabolism.
We have again the odorous substances, and the colour-
ing matters other than those already mentioned. Many
colouring matters are products of the decomposition of
chlorophyll, especially certain of those to which the
autumnal tints of leaves are due. One of this group,
xanthophyll, is a bright yellow pigment which is always
associated with the chlorophyll, though in varying amount.
We have finally in connection with the metabolic pro-
cesses to touch upon the excretions of plants. The term
must be used in a wide sense to include all such sub-
stances as are undoubtedly withdrawn from the seats of
active life, whether thrown off from the plant-body or not.
The excreta which are completely eliminated are few ;
under normal conditions only the carbon dioxide and water
which are products of respiration can be specified. Under
abnormal conditions volatile compounds of ammonia or
ammonia itself may be added to these. But there are
certain other substances which are thrown off by a few
plants, and for them some of these subserve definite
purposes. Perhaps the most frequently occurring instance
of these is the sugary solution known as the nectar, which
is so common in flowers, and which is excreted usually to
serve as an attraction to insect visitors. Mineral matters
such as calcium carbonate are in some cases excreted on to
the surface of the leaf, sometimes by special glands as in
286
VEGETABLE PHYSIOLOGY
certain Saxifrages. In these it aids in the formation of a
subsidiary water-absorbing apparatus as will be mentioned
in a subsequent chapter.
In most cases the materials which we are discussing
are not thrown off from the plant, but are removed to parts
which are not concerned in the vital processes to any very
great extent. Etherial oils are found deposited in special
cavities in leaves, stems, and other parts (fig. 130).
Mineral matters are often deposited in the substance of
cell-walls. The oxalate of calcium occurs frequently in
FIG. 130.— DEVELOPMENT OF LYSIGENOUS GLAND IN
STEM OF Hypericum. THE FOUR FIGURES REPRE-
SENT SUCCESSIVE STAGES. X 250.
FIG. 131.— CRYS-
TALS OF CALCIUM
OXALATE IN
WALL OF CELL
OF THE BAST OF
Ephedra.
this situation (fig. 131). In other cases it is deposited in
special cells, where it forms clusters of crystals of cha-
racteristic shape (fig. 132, A, B). In these cases the cluster
of crystals is usually invested by a delicate skin derived
from the protoplasm, thus shutting it completely off from
any participation in the metabolism of the cell in which it
lies.
Carbonate of calcium may also be deposited in the
substance of the cell- wall, or of protrusions from it, as
in the cystoliths of Ficus, Urtica, and other plants (fig.
133).
METABOLISM
287
Silica again is accumulated in the epidermis of many
grasses, and of the horsetails (Equisetum).
Though many of these substances, both excretions and
bye-products, are of no value for nutrition, some of them
may play a very important part in the defence of plants
against their natural enemies, their nauseous smell or
flavour preventing their being eaten by animals, &c. Some
odours and the nectar found in flowers are doubtless of
great service in attracting insects, which assist in the
FIG. 132. — CBYSTALS OF OXALATE OF
CALCIUM. A, FROM BEET (Sphcera-
phides); B, FROM ABUM (Eaphides).
FIG. 133.— SECTION OF PORTION
OF LEAF OF Ficus, SHOW-
ING CYSTOLJTH (cys) IN
LARGE CELL OF THE THREE-
LAYERED EPIDERMIS (ep).
process of cross-pollination, to be discussed in a subsequent
chapter.
Though we cannot trace the formation of all these
various substances, both bye-products and excretions,
directly to the self-decomposition of the protoplasm, but
must regard them as formed partly by the processes of
oxidation and reduction, which we have seen are often
associated with its activity, and partly by subsequent
further decompositions of bodies originally thus formed,
we can trace them ultimately to protoplasmic activity, and
may consequently regard their formation as belonging to
the katabolic processes going on in the organism.
VEGETABLE PHYSIOLOGY
CHAPTEE XIX
THE ENERGY OF THE PLANT
THE various operations which we have seen are continually
going on in the body of the plant involve the execution of
a considerable- amount of work. This is very evident when
we observe only the enormous development of a large tree,
and compare it with the relatively small seed from which
it has sprung. Such a process of construction has involved
the preparation of a vast quantity of highly complex
material from very simple chemical substances. The pro-
cesses incident to life also, though they may not lead directly
to the formation of such substances, cannot be conducted
without involving a considerable amount of work, whether
the plant is a minute body consisting of a single protoplast,
or an organism of a much higher degree of complexity.
We must therefore turn our attention to the question
of the supply and utilisation of the energy at the expense
of which the various processes of life are carried out. At
the outset it will be well to consider what demands for
energy we find presented by the plant, or what are the
ways in which energy is expended or lost.
Some of these have been incidentally alluded to in the
preceding chapters, though we have not specially regarded
them from this point of view. We may refer especially to
the very great evaporation of water from the living cells
into the intercellular spaces, which we have seen is in
some cases supplemented by an evaporation from the
general external surface, when this is not covered by any
very distinct cuticle. It is evident that the great quantity
of water which is given off by the leaves of a sunflower, to
THE ENEEGY OF THE PLANT 289
which allusion has been made in an earlier chapter, cannot
be evaporated without the expenditure of a considerable
amount of energy, which presumably takes the form of
heat. It has been computed recently that 98 per cent, of
the energy of the rays of light which are absorbed by the
chlorophyll is expended in causing this transpiration.
The great accumulation of material which is so marked
a feature of the life of a plant is the result of work which
has been carried out in the plant on the simple substances
which are absorbed. We may distinguish here between
such products as are destined for immediate or ultimate
consumption, and those which become incorporated into
the actual substance of the plant. The accumulation of
the latter is permanent, and the energy which is used in
their construction is not subsequently made use of in the
working of the organism. That it is stored however is
evident from the fact that it can be re-converted into heat
if the substance is burned. As we shall see later the pro-
ducts which are ultimately consumed in the nutritive
processes may be regarded as stores of energy as well as
of nutritive material. In both cases however their con-
struction involves the expenditure of a considerable
amount of energy before they assume their recognisable
condition.
Closely allied to these constructive processes we have
the phenomena of- repair and of growth. As we have not
yet studied the latter process in detail, we may be content
with pointing out that there are involved in it many
changes of various substances, which call for the execution
of considerable amounts of work, which in turn demand
the expenditure of energy. Many organs carry out their
growth under conditions of pressure ; roots, for instance,
often penetrating through stiff soils. . Not only is energy
necessary to produce the growth itself, but the pressure
upon the growing organs must be counterbalanced by the
internal forces they exhibit.
Many of the humbler plants possess a considerable
u
290 VEGETABLE PHYSIOLOGY
power of active movement or locomotion. Zoospores of
many of the Algae and Fungi, and the antherozoids of most
of the Cryptogams effect this locomotion by means of cilia
which wave to and fro vigorously in the water in which
they find themselves. The proportionate amount of
energy which they expend in this way is very great com-
pared with the total amount which they possess. Other
movements which are not dependent upon ciliary action
are not uncommon. The amoeboid movements of the
Myxomycetes or slime fungi, the rotation and circulation
of the sap in many cells, the other internal movements of
protoplasm, the hitherto unexplained movements of diatoms
and the oscillations of certain filamentous Algae, illustrate
these. All alike are dependent upon a certain expenditure
of energy.
The so-called movements of the growing parts of plants
are frequently quoted in this connection. As we shall see
hereafter however these are usually changes of position
induced by variations in the processes of growth, and may
rather be referred to expenditure of energy in connection
with the latter than to actual movement. The movements
of adult organs are also effected by causes which corre-
spond in great measure to those which modify growth,
being generally brought about by such variations in the
turgescence of particular cells or groups of cells as those
upon which we shall see growth largely depends. In this
sense they are to be associated with modifications of the
hydrostatic tensions in the parts concerned. A certain
amount of expenditure of energy in the cells concerned is
however most probable, though it is uncertain how far
such changes as modify the resistance of the protoplasm to
the passage of water througli it involve the application of
energy.
We have instances of what we may call the passive
escape of energy in the shape of heat, and to a less extent
in the manifestation of the so-called phenomena of phos-
phorescence. Heat is lost to the plant in many ways, one
THE ENERGY OF THE PLANT 291
of which, the evaporation of the water of transpiration, has
already been mentioned. Another almost equally important
source of loss is radiation from the general surface. This
is greatest from flattened members of the plant, such as
leaves.
The temperature of the plant is very largely influenced
by that of the air, and no doubt interchanges of heat take
place in both directions. But it must not be concluded that
the temperature of the plant and that of the air always
vary together. On the contrary, radiation under some
conditions may go on until the plant is several degrees
colder than the surrounding air. This is probably the
explanation of the ready formation of dew and hoar frost
on the surfaces of leaves at certain seasons of the year.
It is quite of frequent occurrence again that a plant or
part of a plant may have a much higher temperature than
the air, and hence a copious radiation may take place.
During the processes of germination the temperature of the
seed may be as much as 20° C. above that of the air. The
opening of flower buds is also attended by the attainment
of a high temperature and a consequent escape of heat.
If we turn again to plants with a watery environment,
the loss of heat may be observed under appropriate condi
tions. It is well known that the processes of alcoholic fer-
mentation provoked by the yeast-plant are attended by the
liberation of heat, which is given off by the active cells,
and causes a considerable rise of temperature in the fer-
menting liquid.
We may infer also from a consideration of the various
processes we have studied, and from the fact that they are
carried out most advantageously within a certain relatively
small range of temperature, that the maintenance of such
a temperature is a great desideratum to the plant. There
is not a very complete mechanism in the plant to secure this,
object, for the organism generally becomes of about the
same temperature as the medium in which it grows, though
the process of adjustment is often very slow, the tissues
u 2
292 VEGETABLE PHYSIOLOGY
being generally very poor conductors of heat. Still it
seems not improbable that a certain amount of energy is
devoted to the attainment of the range which is most
suitable for the vital processes. Though the dominating
factor in the determination of the plant's temperature is to
be looked for in the environment, the occurrence of the
warmth of germination and of the expansion of the flower-
buds is an indication that it is not the only one.
A fuller consideration of the relations of the plant to
heat must be deferred to a subsequent chapter.
The evolution of light by plants is a comparatively rare
phenomenon, being probably confined to certain Fungi,
though it has been attributed also to a few species of
Algae. It must call, however, for a certain expenditure of
energy in such cases as have been authenticated.
If we turn now to consider the sources of this energy, it
is evident that they must be in the first instance of external
origin. The radiant energy of the sun indeed is the only
possible source which can supply it to normal green plants.
The question of the absorption of this energy has already
been incidentally alluded to when we discussed the chloro-
phyll apparatus, but it may now be examined more closely.
The rays which emanate from the sun are generally
alluded to as falling into three categories, those of the
visible spectrum, those of the infra-red, and those of the
ultra-violet. The second of these are frequently spoken
of as heat rays, and the last as chemical.
The greatest absorption of energy appears to take place
in consequence of the peculiarities of chlorophyll. As we
have seen, this substance, whether in the plant or when in
solution in various media, absorbs a large number of rays
in the red and in the blue and violet regions of the
spectrum, together with a few others in the yellow and the
green.
The solar spectrum after the light has passed through
a solution of chlorophyll is seen to be robbed of rays in
these regions, and hence to present the appearance of a
THE ENEEGY OF THE PLANT
293
band of the different colours crossed by several dark bands
(fig. 134). The greater part of the energy so obtained in
the cells which contain the chloroplasts is at once expended,
partly in constructing carbohydrate food materials and
partly in evaporating the water of transpiration. The latter
process is much the more expensive ; recent observations
have made it probable that 98 per cent, of the radiant
energy actually absorbed during bright sunshine is at once
devoted to this purpose.
When we speak of radiant energy we must remember
that the rays of the visible spectrum do not supply all the
400
FIG. 184. — ABSOBPTION SPECTBA OF CHLOBOPHYLL /;ND
XANTHOPHYLL. (After Kraus.i
energy which the plant obtains. It has been suggested by
several botanists with considerable plausibility that the
ultra-violet or chemical rays can be absorbed and utilised
by the protoplasm without the intervention of any pigment
such as chlorophyll. There is some evidence pointing to
this power in the cells of the higher plants. Certain
bacteria also construct organic material from simple com-
pounds of nitrogen and carbon dioxide, though it is not
probable that they utilise radiant energy directly.
Finally we have evidence of the power of plants to
avail themselves of the heat rays. The relations existing
294 VEGETABLE PHYSIOLOGY
between the organism and its environment have already
been mentioned. Not only can the air rob the plant of
heat by radiation, but when its own temperature is high it
can communicate heat to it in turn. Leaves have been
proved to absorb heat with great avidity, particularly those
which are succulent or fleshy, a difference of more than
20° C. having been noted between their temperature and
that of the air. The direct absorption of the rays of heat
from the sun has also been noted, apart from the tempera-
ture of the air through which the rays were passing.
The supply of radiant energy is very much in excess of
the amount which is needed for the internal work. Indeed
its absorption by the leaves would be a source of consider-
able danger to the plant were it not for the cooling effect
of transpiration, which we have seen dissipates 98 per cent,
of it during bright sunshine. No doubt this dissipation is
one of the chief benefits secured by transpiration. .
It is evident, however, that in the general economy
of the plant something further must be at work in connec-
tion with the supply of energy. The absorption of these
external forms must take place at the exterior of the plant,
while many of the processes of expenditure are carried out
in parts which are more or less deep-seated. We are obliged
to turn our attention, therefore, in this connection as in
that of the construction and utilisation of food, to processes
of accumulation, distribution, and economy.
We may ask ourselves what is the immediate fate of
the energy absorbed. It enters the plant in what is known
as the kinetic form. A very considerable part of the
kinetic energy of the sun's rays, we have already seen, is
devoted at once to the evaporation of the water of transpira-
tion, but some of it is employed by the chloroplasts to con-
struct some form of carbohydrate. The energy so applied
can be again set free by the decomposition of this formed
material. If the latter were burned its combustion would
be attended by the evolution of a certain definite amount
of heat. This heat would represent the energy that had
THE ENEEGY OF THE PLANT 295
been applied to the construction of the material so burned.
Any accumulation of material in the body of the plant
represents therefore not only a gain of weight or substance,
but a storage of energy. This has disappeared from
observation during the constructive processes, but can be
liberated again during their decomposition and applied to
other purposes. Energy which has thus been accumulated
and stored is known as potential energy, to distinguish it
from the actual or kinetic energy originally absorbed. The
formation of material in the plant therefore involves a
storage of energy in the potential form, and wherever such
material is found there is an accumulation also of energy
which can be liberated with a view to utilisation at any
point to which the material has been transferred. The
translocation of material therefore involves also a distribu-
tion of the energy which, originally absorbed as the kinetic
energy of light or heat, has been applied to constructive
processes, and has consequently been made potential.
From these considerations we may see that plants which
absorb elaborated food from their environment have a
source of energy therein. This food is a store of potential
energy which is absorbed as such, and rendered kinetic
subsequently.
It is this potential energy on which the plant depends
for the various processes which go on in such cells as are
not the recipients of external kinetic energy. Even in
the cells which absorb the latter a certain amount of
potential energy also is present, which represents what
has been stored by them in the constructive processes they
carry out, or has reached them in the shape of complex
materials formed originally in other cells.
It is mainly on this store of energy that not only the
whole organism but every cell depends for the execution of
its vital processes. Each cell is practically the seat of the
liberation of this potential energy, or its conversion into the
kinetic form, during the decompositions which take place
within it.
296 VEGETABLE PHYSIOLOGY
The protoplasm itself contains a store of such potential
energy. We have seen that it can only be constructed at
the expense of food supplied to it. The formation of the
protoplasm which follows the supply of food to the cell
involves work, and the energy so used is partly changed
from the kinetic to the potential condition. When the
protoplasm undergoes what we have called its self-
decomposition, which is continually going on, a certain
amount of this potential energy is liberated and can be
observed and measured in various ways. When destruc-
tive metabolism is active we have already noticed that
there is usually a rise of temperature, as in the pro-
cesses of the germination of seeds. A certain amount
of the liberated potential energy in this case manifests
itself in the form of heat. A vegetable cell which obtains
no direct radiant energy from without can consequently
obtain the energy it needs from within itself, by setting
up decomposition either of its own substance or of certain
materials which have been accumulated within it.
The supply of elaborated material to a cell and that of
available potential energy within it are not however exactly
equivalent. A certain part of the transported material
is devoted to the maintenance of the fabric of the cell.
The protoplasm in a growing cell is permanently increased ;
frequently its cell-wall is permanently thickened. In these
cases the whole of such material is not subjected to sub-
sequent decomposition, but much remains unchanged
during the plant's life. The cell is consequently never
found to be capable of giving up to the plant of which it
is a member the whole of the potential energy which
reaches it. If we consider the round of the metabolic
changes which take place in such a cell, we find that energy
is absorbed to construct its substance, and that as the latter
undergoes self -decomposition energy is again liberated.
But a certain part of what is supplied to it is permanently
retained in potential form, and hence every cell is depen-
dent for the maintenance of its energy upon a constant
THE ENEEGY OF THE PLANT 297
supply of complex material, whose subsequent decomposi-
tions will replace the amount of energy which has been
utilised in the permanent increase of its substance.
The translocation of constructed materials which we
have already considered must be regarded therefore not
only as furnishing the materials for nutrition and growth,
but as carrying or distributing throughout the plant-body
the kinetic energy absorbed as light or heat by the cells
to which these forces are originally supplied. The chloro-
phyll apparatus is an important piece of mechanism for
the accumulation of energy which is subsequently distri-
buted and utilised wherever need arises for it. This is
true also of all cells which have the power of absorbing
kinetic energy in any form.
The absorption and fixation of energy involved in the
photosynthetic processes carried out by the chlorophyll
apparatus can be easily observed, and the immediate fate
of such energy can be readily determined. The accumu-
lation of the energy of heat is not so easy to trace, but
there is no doubt that it proceeds along similar lines.
Part of it can travel as kinetic energy, as heat is slowly
conducted along the tissues of the plant, but ultimately
some at any rate becomes potential.
We see therefore that wherever any substance that
has been manufactured by the plant is stored in a cell,
that cell is thereby put in possession of a certain amount
of potential energy corresponding to the quantity of such
stored material. Even the living substance itself may be
looked upon as a further store of energy, as it can liberate
it by its own decomposition.
Each cell is thus supplied through the general activities
of the whole plant not only with the food it needs for its
nutrition, but also with the energy required for carrying
out its vital processes. The ultimate utilisation of the
stored energy is consequently a process which must be
studied by a close scrutiny of the internal work of the cell
itself.
298 VEGETABLE PHYSIOLOGY
The transformation of potential into kinetic energy is
associated with decomposition just as the converse process
is bound up with construction. Destructive metabolism
in the cell is then the means by which its energy is made
available. We have seen that the processes of this kata-
bolism go on in the interior of each cell. Each liberates
at least as much energy as it requires for the maintenance
of its life and the discharge of its particular functions.
The processes associated with the utilisation of the
stored energy are, then, chemical decompositions in which
various constituents of the cell are involved. We may
divide them into two series, in the first of which the
protoplasm itself takes part, and which comprise the pro-
cesses in which its own breaking down takes place. In the
second series it effects the splitting-up of other bodies
without a necessary disruption of its own molecules.
We have already discussed the first of these two series,
which involves the phenomena of respiration. We have
seen that during the life of the cell oxygen is continuously
absorbed by the protoplasm ; the immediate result is its
self -decomposition, attended by the formation of substances
of very different degrees of complexity, the ultimate simplest
ones being carbon dioxide and water. If the self-decom-
position of protoplasm involved such a splitting-up as would
lead to the formation of nothing but these, together with
an equally simple form of combined nitrogen, nearly all
the potential energy of the cell would be liberated. But
we have seen that this does not take place. Carbon
dioxide and water are accompanied by many other products
of a fairly complex character. There is not, therefore, an
evolution of nearly the possible amount of energy, and
what is set free is mainly employed in the reconstruction of
protoplasm from these products and the new food supplied.
As carbon dioxide and water are continually escaping from
the cell in appreciable quantities, it is evident that apart from
any increase of its fabric the store of its potential energy
is gradually diminished, and that it can only continue
THE ENEEGY OF THE PLANT 299
its life if new stores are supplied in the shape of further
decomposable material which is capable of taking part in
the renewed building-up of the living substance.
Respiration is thus to be looked upon as a process very
largely connected with the utilisation of the store of energy
which each cell possesses, and to be perhaps primarily
concerned in the transformation of that energy from the
potential to the kinetic form. The oxygen appears to be
necessary mainly for the purpose of exciting those decom-
positions of the protoplasm which are so dependent upon
its instability. It is not, however, certain that this is the
only part it plays. It is possible that some of the products
of the protoplasmic disruption are oxidisable substances,
and that to a certain extent a direct oxidation of them takes
place. There is undoubtedly some evidence pointing in
that direction.
We have, besides the respiratory processes, another series
of chemical decompositions going on in plants, in which
the self -decomposition of protoplasm is not necessarily
involved. We have seen already that many processes of
oxidation and reduction are probably always taking place
among the substances which are in solution in the wafer
with which the cytoplasm is saturated. Besides these, other
changes take place in which no oxidation is involved, and
this whether oxygen is present or not. If the access of oxygen
to a protoplast is interfered with, its normal respiration
soon ceases, but very frequently other 'changes supervene,
involving decompositions of a different character, which
yield, at any rate for a time, the energy required for life.
Turning from the question of respiration to study other
changes which subserve a similar purpose with regard to
the local supply of energy, we may first examine such
processes as are oxidative. In them all we cannot fail to
mark the activity of the protoplasm in carrying them out.
The living substance does not however act as a general
oxidising agent, but different protoplasts possess specific
powers. Certain micro-organisms can cause the oxidation
300 VEGETABLE PHYSIOLOGY
of ammonia and the consequent formation of a nitrite;
others can convert the nitrite into a nitrate, but neither
can do the work of the other. Others have not such
limited powers ; a certain bacterium can cause the oxidation
of alcohol to acetic acid, and after the exhaustion of what
alcohol may be present, can further oxidise the acetic acid
to carbon dioxide and water. The exact way in which the
protoplasm acts as a carrier of the oxygen without
apparently undergoing decomposition is very obscure. It
may perhaps combine with the oxygen and pass it on to
these oxidisable substances, acting as a carrier only.
It has recently been found that besides exerting a direct
oxidative power, protoplasm can secrete an enzyme, or
perhaps a variety of enzymes, each with a special peculiarity,
through whose instrumentality the oxidation is effected.
These enzymes have been termed oxidases, and they are
probably wide-spread in the vegetable kingdom. A dis-
cussion of their peculiarities would be beyond the scope of
this volume, but we may call attention to their general
features.
The first one discovered is known as laccase ; it has a
very wide distribution, occurring in the roots, stems, and
leaves of various plants, and in a very large number of
fungi. It appears to oxidise various constituents of plants,
but particularly the colouring matters. Another, known
as tyrosinase, occurs in other fungi, and oxidises chiefly
ty rosin. Others oxidise various colouring matters, together
with tannin.
Many very complex disturbances set in when a normally
respiring plant is cut off from a supply of oxygen. Death
does not immediately supervene as might almost be expected.
Instead, the partial asphyxiation or suffocation stimulates
the protoplasm to set up a new, and perhaps supplementary,
series of decompositions, resulting in the liberation of
energy as do those of the respiratory process. We have
already noticed that under such circumstances the exhaling
stream of carbon dioxide can still be observed. This led
THE ENEEGY OF THE PLANT 301
originally to the view that the protoplasm excited these
decompositions of some complex substance in the cell to
obtain oxygen from it, which should replace the oxygen
whose access had been stopped. The ultimate changes
were accordingly held to go on with but slight interruption,
but the source of the oxygen taking part in them was
different. On this account the process was termed intra-
molecular respiration. The term is rather an unfortunate
one, for as we have seen the study of the ordinary respira-
tory processes has shown that the molecule of the living
substance is the seat of the changes they involve, and hence
that all respiration is intra-molecular. Moreover if the
object of the decomposition is to provide oxygen to replace
that which has been cut off, these transformations precede
the actual respiration, which must then be set up as soon
as the oxygen is liberated as suggested. Many botanists
now prefer to speak of decompositions taking place in the
absence of a supply of free oxygen as anaerobic respiration.
They thus include as respiratory changes all the decomposi-
tions primarily intended to liberate energy, and divide them
into those which are aerobic or dependent on oxygen, and
those which are anaerobic. The latter need not involve
the co-operation of oxygen in the disruption of the molecule.
The object sought is energy and not oxygen.
It is uncertain how far the self-decomposition of proto-
plasm is concerned in these anaerobic respiratory processes.
Probably not to any great extent ; it is more likely that it
secures the decomposition of other substances without being
itself materially used up. Such a course would be much
the more economical, not involving the consumption of
much energy in reconstructive processes. - This cannot
however be regarded as finally established.
The substance which seems most readily available for
this purpose is sugar. Under the conditions mentioned it
becomes decomposed or broken up entirely, the resulting
products being carbon dioxide and alcohol. The process of
alcohol-formation which was for so long a time associated
302 VEGETABLE PHYSIOLOGY
exclusively with the word fermentation, was first observed
in connection with the life of the yeast-plant. It has how-
ever since been ascertained to be much more widespread,
and to be indeed the most common of the anaerobic
respiratory processes. In cases where the metabolic
activities are very great, as in germinating peas, we find
this process supplements the ordinary respiration, for
alcohol can be detected in their cells in small quantities.
The same thing has been noticed in the leaves of the vine.
We must suppose here that the amount of oxygen absorbed
is insufficient for their requirements, and that partial
asphyxiation results.
Till quite recently it was held that alcoholic fermenta-
tion was conducted exclusively by the activity of the proto-
plasm of the cells in which it was observed. It has been
ascertained however that it may also be caused by the
action of an enzyme, which is secreted under conditions of
incipient asphyxiation by many cells, and which is formed
in the yeast-plant even in the absence of such stimulus.
Though the term fermentation was originally applied
and confined to the formation of alcohol, it 'is now usual to
extend it far more widely. Many other processes of similar
nature have been discovered, nearly all of which at first
were found to be carried out through the agency of microbes
or higher fungi. Hence the meaning of the term has been
extended to include them, and the organisms themselves
have been called ferments. As however these processes have
come to be recognised as normal in many of the higher
plants, and to be carried out in them by the protoplasm of
particular cells, this peculiarity is seen not to be special to
the microbes and the fungi. The idea was soon transferred
to the protoplasm in general, and this property of setting
up anaerobic decomposition has become known as its
fermentative power. The very similar processes set up
through the enzymes which we have discussed in connection
with digestion, show us another manifestation of the same
fermentative power. All these processes can therefore be
THE ENERGY OF THE PLANT 303
classed under the one term fermentation. We have seen
that all the katabolic changes in which the self-decomposi-
tion of the protoplasm is not directly involved may be
carried out either by the intervention of the living substance
itself or an enzyme secreted by it. The oxidation of various
matters is in some cases confined to the substance of the
protoplasm itself, and is in others carried out in its vacuoles
by an oxidase ; alcoholic fermentation is in some cells a
matter initiated and carried on by their protoplasm, and
in others is due to the enzyme secreted by them. The
digestive changes can similarly be conducted by enzymes
or by the living substance without their intervention.
We must not however include all digestive fermenta-
tive changes among anaerobic respiratory phenomena, if
such inclusion involves the acceptance of the view that
this is their primary purpose. Though they do effect
the conversion of potential into kinetic energy, this is
wholly subsidiary to their function in connection with the
nutrition of the plant. We have seen that in the processes
of germination the energy they liberate is so far in excess
of the requirements of the cells that a large amount
escapes in the form of heat. For them to work indeed
there must be an initial supply of energy, which is pro-
bably supplied to them in a similar form, for at 0° C. they
are incapable of effecting any decompositions.
We must not suppose that anaerobic respiration is
capable permanently of taking the place of the normal
aerobic process. Though the stoppage of oxygen can be to
a certain extent compensated for, the vital mechanism
gradually becomes exhausted, and life ceases if the cessa-
tion of the supply is prolonged. In the higher plants
anaerobic is at the best only capable of supplementing
aerobic respiration, and that for but a limited period. The
commencing asphyxiation serves as a stimulus to the
protoplasm, which responds by setting up the anaerobic
changes, but like all stimulations the ultimate effect is
exhaustion and a failure to continue the response.
304 VEGETABLE PHYSIOLOGY
There are other plants however which are incapable
of carrying on their life in the presence of oxygen. They
are of a very humble type, and occur only among the
Bacteria and Fungi. An instance may be found in the
organisms which induce the formation of butyric acid from
sugar or lactic acid. If a few of these are sown in a
suitable liquid, and this is then enclosed in a hermetically
sealed flask from which free oxygen has been removed,
they multiply with extreme rapidity, until indeed either
their food supply is exhausted, or the waste products of
their metabolism accumulate to an inhibitory extent. If a
little free oxygen is admitted their activity ceases and
death ensues, or they pass into a resting condition, which
lasts as long as oxygen is present.
305
CHAPTER XX
GROWTH
IN studying the growth of plants we must bear in mind
the relation which it bears to the processes of metabolism
which we have already discussed. We have seen that the
constructive processes, partly anabolic and partly kata-
bolic, are much greater than those which lead to the
disappearance of material from the plant-body. The result
of this is that there is a conspicuous increase in the
substance of the plant, as well as an accumulation of
potential energy which can be made use of by the plant
through various decompositions which its protoplasm can
set up. The great permanent accumulation of material is
what we associate with the processes of growth. Here
however we must distinguish between the increase of the
living substance, which is essentially an anabolic process,
and that of the manufacture of the framework, the con-
struction of cellulose, wood, cork, and other products, which
is the result of katabolism.
The growth of the living substance is always the result
of constructive metabolism, and is attended by an increase
of bulk and weight. The growth of an organ sometimes
appears to be independent of such increase of weight :
indeed, a diminution of the weight of the whole structure
is sometimes noticeable. Thus in the case of a potato
tuber allowed to germinate under such conditions as
prevent the absorption of food materials from without, we
meet with a marked change of form, but, owing to the loss
of moisture by transpiration, and of carbon dioxide as a
consequence of its respiration or the katabolic processes
306
VEGETABLE PHYSIOLOGY
going on in its tissues, the resulting plant weighs much
less than the original potato.
This difference is however rather apparent than real.
We shall see that the actual growth, as well as the manufac-
ture of new cells, is confined to certain regions. In these
regions there is a conside-
rable increase in bulk and
weight, but as the materials
which are used for the pur-
poses of this local growth
are derived from substances
stored up in the body of
the tuber, the latter, the
greater part of which is
not at any time the seat
of the growth, diminishes
in weight and size to such
an extent as more than to
counterbalance the gain in
the growing regions. Hence
the whole plant weighs less
than the tuber, though con-
siderable growth may have
taken place.
Mere increase of weight
in an organ does not, on
the other hand, necessarily
imply any growth.. The
deposition of reserve mate-
FIG. 135. — LONGITUDINAL SECTION OF
YOUNG ROOT, SHOWING STRUCTURE OF rials in many Seeds does
GROWING POINT, x 20. , n -,
not take place to any great
1, zone of cell division ; 2, region of greatest , .
growth ; 3, region of complete differen- extent till their mature
tiation.
dimensions are reached,
and growth is therefore completed. It involves, however,
a considerable increase of weight.
Growth is in the strict sense, then, always associated
with the formation of new living substance, and
GEOWTH
307
generally accompanied or immediately followed by additions
to the framework of the growing cells or organs. It is in
nearly all cases attended by a permanent change of form.
This is perhaps not so evident in the case of axial organs
as it is in that of leaves and their modifications, though
even in them it can be detected to a certain extent. It is
much more conspicuous in the case of leaves, for the latter,
as they expand from the bud, have usually a different
shape from that of the adult ones, and the assumption of
the mature form is a gradual process, taking place as the
age of the leaf increases.
This change of form can be seen not only in the case
of an organ such as a leaf, but also in that of the indi-
FIG. 136. — SECTION OF BLADE OF LEAF, SHOWING THE IRREGULAR CELLS OF
THE SPONGY MESOPHYLL ABUTTING ON THE LOWER EPIDERMIS.
vidual cells of which a plant consists. In the apical
meristem of the root of a flowering plant the cells when
first formed are almost cubical (fig. 135) ; after a little
while we find many of them becoming elongated, and
ultimately prosenchymatous. Many other cases can be
noted, particularly the irregularly shaped cells of the spongy
parenchyma of leaves (fig. 136), the stellate cells of the
pith of certain rushes (fig. 137)* the laticiferous cells of the
Spurges, &c.
Growth may, in the light of the considerations just
advanced, be defined as permanent increase of bulk,
attended by permanent change of form. We must not
assume that every increase of bulk is necessarily growth ;
x 2
308 VEGETABLE PHYSIOLOGY
for as we shall see, in growing cells and members there is
a constant stretching of the cell or tissue by hydrostatic
pressure or turgidity, which can be distinguished from
growth by the fact that it can be removed, the result
being a certain diminution of the size of the part under
consideration.
FIG. 137.— PORTION OF SECTION OF STEM OF EUSH, SHOWING STELLATE
TISSUE OF THE PITH, WITH LARGE INTERCELLULAR SPACES. , ~~
Growth in the lowliest plants may be co-extensive with
the plant -body. In all plants of any considerable size
however it is localised in particular regions, and in them
it is associated with the formation of new protoplasts. We
have already mentioned that in the sporophytes of all
the higher plants there exist certain regions in which
GEOWTH 309
the cells are merismatic — that is, which have the power of
cell-multiplication by means of division. In such regions,
when a cell has reached a certain size, which varies with
the individual, it divides into two, each of which increases
to the original dimensions and then divides again. These
regions have been called growing points (fig. 135) ; they
may be apical or intercalary. In such stems and roots
as grow in thickness there are other growing regions, which
consist of cylindrical sheaths known as cambium layers or
phellogens. By the multiplication of the protoplasts in
these merismatic areas the substance of the plant is
increased. In other words, as these growing regions
consist of cells, the growth of the entire organ or plant will
depend on the behaviour of the cells or protoplasts of which
its merismatic tissues are composed.
The growth of such a cell will be found to depend
mainly upon five conditions : (1) There must be a supply of
nutritive or plastic materials, at the expense of which the
increase of its protoplasm can take place, and which supply
the needed potential energy. (2) There must be a supply
of water to such an extent as to set up a certain hydro-
static pressure in the cell. This condition we have already
considered in an earlier chapter, in which we discussed
the relation of protoplasm to water. (3) The supply of
water must be associated with the formation of osmotic
substances in the cell, or it cannot be made to enter it.
In the absence of the turgescence, which will be the result
of the last two conditions, no growth is possible for reasons
that will presently appear. (4) The cell must have a
certain temperature, for, the activity of a protoplast is
only possible within particular limits, which differ in the
cases of different plants. (5) There must be a supply of
oxygen to the growing cell, for, as we have seen, the
protoplast is dependent upon this gas for the performance
of its vital functions, and particularly for the liberation of
the energy which is demanded in the constructive processes.
This is evident also from the consideration that the growth
310 VEGETABLE PHYSIOLOGY
of the cells is attended by the growth in surface of the
cell-wall, and as the latter is a secretion from the proto-
plasm, a product, that is, of its katabolic activity, such a
decomposition cannot readily take place unless oxygen is
admitted to it.
Growth so far as it implies only the formation of
living substance is thus a constructive process. It is how-
ever intimately associated with destructive metabolism or
katabolism, the latter being involved in the construction of
the increased bulk of the framework of the cell or cells,
and being essential to supply the energy needed for the
constructive processes.
When the conditions mentioned are present, the course
of the growth of a cell appears to be the following : the
young cell, immediately it is cut off from its fellow, absorbs
water in consequence of the presence in it of osmotically
active substances. With the water
it takes in the various nutritive
substances which the former con-
tains in solution. There is set up at
once a certain hydrostatic pressure
due to the turgidity which ensues
upon such absorption, and the ex-
tensible cell-wall stretches, at first
in all directions. The growth of
the protoplasm at the expense of
the nutritive matter for a time keeps
FIG.138.-ADULTVEGETABLE Pace with the increased size of the
CELLS. x 500. (After QQ[\ ^^ by an(J ^y ft becomes VaCUO-
Sachs.) * J
h, cell-wall; p, protoplasm ; ^ted as more and more water is
Loii;Ts>acuoies. m~ attracted into the interior. Even-
tually the protoplasm usually forms
only a lining layer to the cell-wall, and a large vacuole
filled with cell-sap occupies the centre (fig. 138). The
growth of the protoplasm, though considerable, is therefore
not commensurate with the increase in the size of the cell.
The stretching of the cell-wall by the hydrostatic pressure is
GKOWTH 311
•
fixed by a secretion of new particles and their deposition
upon the original wall, which as it becomes slightly
thicker is capable of still greater extensibility, much in
the same way as a thick band of india-rubber is capable
of undergoing greater stretching than a thin one. The
increase in surface of the cell-wall is thus due firstly to
the stretching caused by turgidity, and secondly to the
formation and deposition of new substance upon the old.
The latter only is the growth of the cell-walls ; the former
can be removed by irrigating the cell with a solution of
a substance, such as common salt, which will rob it of
the water it contains. The constructive changes leading
to the formation of new protoplasm are attended in this
process by the katabolic formation of cell-wall and other
substances, such as the osmotic bodies which are necessary
to draw the water into the cell. The supply of oxygen
is needed to allow the protoplasm to undergo these kata-
bolic decompositions, enabling it thus to prepare the several
products spoken of, and to gain from such decompositions
the energy which must be expended upon the construction
and reconstruction of the living substance, and used in
the secondary chemical changes which supervene.
The process of the growth of a cell is limited in its
extent, though the limits vary very widely in different
cases. In some, cells grow only to a few times their original
dimensions, in others they may attain i a very considerable
size. In any case, however, we can notice that the rate of
growth varies regularly throughout the process ; it begins
slowly, increases to a maximum, and then becomes gradu-
ally slower till it stops. The time during which these
regular changes in the rate can be observed is generally
spoken of as the grand period of growth.
Changes in the shapes of cells arising during growth
depend upon two factors. The capacity of the cell to yield
to hydrostatic pressure may be affected differently in
different directions by the conditions of the cells which
surround it. In the merismatic tissue of a growing point
312 VEGETABLE PHYSIOLOGY
there is generally least resistance on the side of the free
apex of the organ, and hence an increased protrusion of
the latter results. Whatever may be the distribution of
such pressure the growth of the cell will be greatest in the
line of least resistance. If any internal cause should give
rise to differences in the uniformity of hydrostatic pressure
in all directions, the growth will be most extensive in the
line of the greatest. In the second place the extensibility
of the cell-wall may be locally modified by the protoplasm,
so that a uniform internal hydrostatic pressure may affect
one part more than another, and the growth consequently
will become irregular, giving rise in many cases to cells of
curious form.
If we consider the behaviour of a growing organ in the
light of these facts, we shall see that, like the cell, it must
show a grand period of growth. If we take the case of a
root, in which the changes can be traced most easily on
account of the simplicity of its structure, we find that just
behind the apex the cells are all in active division. Growth
is small and consists mainly in an increase of the quantity
of protoplasm, for the cells divide again as soon as they
have reached a certain size. As new cells are continually
formed in the merismatic mass, those which are farthest
from the apex gradually cease to divide and a different
process of growth takes place in them, which is associated
more particularly with the formation of the vacuoles and
consequently with the establishment of considerable hydro-
static pressure, thus causing the bulk of the cells to be greatly
enlarged, as we have described. Hence it is here that
the actual extension in length of the root goes on, and the
cells reach the maximum point of the grand period. They
then gradually lose the power of growth, the oldest ones or
those farthest from the apex parting with it first, and they
pass gradually over into the condition of the permanent
tissue (fig. 135). In this way each zone of the root which
may be distinguished goes through a grand period of
growth. At first when the cells are merismatic, growth is
GEOWTH 313
at a minimum, it gradually becomes accelerated, reaches
a maximum, and slowly ceases, exactly as did that of the
cell which we first considered. By careful examination of
a growing root it can be found that the growth is greatest
just behind the merismatic region. If a young root be
taken and marked out into zones by a series of short lines
at equal distances apart (fig. 139, A),
and then allowed to continue its
growth, it will be found that the
lines remain close together at the apex
and for a very short distance from
it. Then they become separated by
broader spaces (fig 139, B). Further
, -, ,.,, ,, . . , . . , , FIG. 139. — GERMINATING
back still the original intervals be- BEAN, SHOWING OBOWTH
tween the lines will again be found to OF THE BADICLE-
be almost unaltered. The second region corresponds to
the part where the cells are undergoing the enlargement
described. The total growth of the root is, of course, the
sum of the increments of all the zones so marked out.
The same order of events may be ascertained to take
place in the stem, but in this region it is complicated by
the occurrence of nodes and internodes. Growth is almost
confined to the latter, each of which passes through a
similar grand period. The growth of the stem is the
algebraical sum of the growth of the internodes, many of
which may be growing simultaneously and which will be at
any particular moment therefore at different parts of their
grand period. The region of growth in the stem is, as a
rule, much longer than that in the root.
The growth of the leaf shows a little variation. The
apical growth, as a rule, is not very long continued, and
the subsequent enlargement of the leaf is due to an inter-
calary growing region near the base. This area has the
merismatic cells at about its centre, and regions of greatest
growth are on both sides of it. This can be traced more
easily in the elongated leaves of Monocotyledons than in
those of Dicotyledons.
314
VEGETABLE PHYSIOLOGY
The grand period itself is not quite uniform, as the
rates of growth in the active region may and do vary witL
changes in external cbnditions, and with differences in
activity in the protoplasm from time to time. This can be
observed very favourably in the case of a growing stem,
which shows considerable differences in its rate of growth
during twenty-four hours. The growth is greatest during
the night and least during the day, and the variations in
the rate are fairly regular, the total growth during succes-
sive periods of twenty-four hours being on the whole
uniform. This regular variation of the rate constitutes
what is known- as the daily period of growth in length.
An instrument by which the progress of growth of such
a structure as a stem can be ascertained and registered is
known as an auxanometer.
A very convenient form
which registers the gradual
increase in length automati-
cally, has been constructed
by Pfeffer, and is repre-
sented in fig. 140. A thread
attached to the plant passes
over the small wheel x,
which is cemented on the
large wheel r, and accurately
centred about the same axis.
A thin lever z is attached
to another thread which is
passed over the large wheel,
and is made to write upon
the smoked surface of a
paper fastened round the
cylindrical drum t. The
string is kept tight by the
counterbalancing weight g.
drum is caused to rotate slowly upon its axis by clock-
work, so that the indicator traces a line along its surface.
FIG. 140.— PFEFFEK'S AUTOMATICALLY
REGISTERING AUXAXOMETER. (After
Detmer.)
GEOWTH 315
So long as no growth takes place this line is horizontal,
'but as the indicator is displaced downwards by the descent
of the small weight attached to the first cord, which is
attendant upon any elongation of the axis of the plant, the
line actually traced during growth is a spiral. The rate
of the drum's revolution being known, the amount of the
elongation of the axis per hour can easily be calculated.
The actual augmentation of the plant's axis is magnified
in the record, in a ratio dependent upon the ratio between
the radii of the large and small wheels r and x.
For the sake of simplicity of description it has been
assumed, in what has already been said, that the turgidity of
the cells in the growing member is uniform. This, however,
is far from being the case. There is generally a fairly
rhythmic variation in this turgidity in the different parts
of the elongating member. The simplest case which we
may consider is one which shows a difference in structure
on two sides ; such a member is described as dorsiventral.
The two sides will often show a difference of degree of tur-
gidity and consequently of rate of growth. If we consider
a leaf of the common Fern, we find that in its young con-
dition it is closely rolled up, the upper or ventral surface
being quite concealed. As it gets older it gradually unfolds
and expands into the adult form. This is due to the fact
that in the young condition the turgidity and consequent
growth are greater on the dorsal side of the leaf, so that it
becomes rolled up as described. As it gets older the maxi-
mum turgidity and growth change to the upper side and
so it becomes unfolded or expanded. These two conditions
are generally described under the names of hyponasty and
epinasty respectively.
These conditions are not confined to the leaves of ferns,
but may be detected in those of other plants, though to a
less conspicuous degree. It is in consequence of them that
the leaves of the bud always fold over the apex of the stem
from which they spring. The opening and closing of certain
flowers, such as the Crocus, depend upon similar variations.
316 VEGETABLE PHYSIOLOGY
Cylindrical organs may exhibit similar phenomena.
One side of a stem may be more turgid than another, and
the maximum turgidity with its consequent growth may
alternate between two opposite sides. The greater turgidity
of the cells is often accompanied by an increased extensi-
bility of the cell-walls of the turgid region. The growing
apex of such a stem will alternately incline first to one side
and then to the other, exhibiting a kind of nodding move-
ment in the two directions. This is known as nutation,
and is of very frequent occurrence, particularly in such
stems as are slightly flattened instead of being truly
cylindrical.
The region of maximum turgidity instead of occurring
alternately on two opposite sides may pass gradually round
the growing zone. The apex in this case will describe a
circle, or rather a spiral, as it is elongating all the time,
pointing to all points of the compass in succession. This
continuous change of position has been described by Darwin
as circumnutation, and has been said by him to be universal
in all growing organs. The passage of the maximum tur-
gidity round the stem may vary in rapidity at different
places, causing the circle to be replaced by an ellipse.
Indeed the simple nutation spoken of above may be regarded
as only an extreme instance of the latter.
Circumnutation is exhibited during growth also by the
hyphse of many fungi, some of which have a coenocytic
structure. In these cases the movement appears to be
due to a rhythmic variation in the extensibility of the
membrane, induced probably by the protoplasm. It cannot
be caused by differences of turgidity on the two sides of the
hypha as this contains only one cavity.
By these movements of the growing apices — movements
incident to growth, and proceeding altogether from internal
causes— many advantages are secured by the plant. In
the case of a climbing stem, the circumnutation enables it
to reach a support, round which it twines, so that with
but little expenditure of substance it can secure access to
GEOWTH 317
more light and air, than it could obtain in its absence.
Boots by the same method are enabled more easily to make
their way through the crevices of the soil. The axis of the
embryo shows in one or other of its parts strong hyponastic
curvature, forming an arch which enables it to leave the
seedcoats and make its way through the soil without
damage to the young delicate plumule, its progress being
helped by simultaneous circumnutation. On reaching the
surface epinastic growth causes it to assume an erect
position, the arch opening out till the axis is straightened.
Coincidently with this change, circumnutation of the apical
region replaces that of the portion of the axis which was at
first arched.
During the period of growth the young organ is
extremely sensitive to changes in its environment, respond-
ing to such stimulating influences by further modifications
of its behaviour. These will be considered in detail in a
subsequent chapter,
Besides the hydrostatic tension set up in the cells of
the growing regions, the processes of growth are accom-
panied by the development of other tensions in the
interior of the growing member. These appear to depend
upon differences between the turgidities of their several
tissue systems as these develop, and upon different rates of
growth of different internal parts. If a petiole of Ehubarb
is taken, and a thin strip is peeled from one side, it will
immediately curl outwards. If it is thjen placed in apposi-
tion with the part from which it was cut, it will be found
to be appreciably shorter than the rest of the petiole. If
the petiole is carefully measured, and then deprived of its
cortical covering by the separation of successive strips, the
central part will be found to be slightly longer than the
original petiole. In such a petiole the central part is
clearly compressed by the external portions, and when
these are removed it undergoes an extension which is the
expression of the amount of such compression. Similarly
the external parts are stretched longitudinally by the
318 VEGETABLE PHYSIOLOGY
central region, and when they are freed from it, the recoil
is accompanied by a diminution of their length. There is
thus a longitudinal tension in the petiole, due to the
greater turgescence of the central part, which stretches the
outer portions, and is itself compressed by their greater
rigidity resisting the hydrostatic extension. This tension
is not due to greater growth, but to more pronounced
turgidity, for if such a petiole is soaked for a time in salt
solution till the water is in great part removed from its
interior, and it has become flaccid, removal of the cortex is
not accompanied by the same changes of dimension. A
similar experiment may be performed on the hollow flower-
stalk of a Dandelion. If it is slit into two halves by a
vertical cut, the two parts curl outwards from each other,
showing a similar tension in the internal regions.
Transverse tensions in young growing axes can also be
demonstrated. The cortex is found to be strained outwards
by the central tissues, so that if a ring of it is cut out of
such an axis and split longitudinally, it shortens. If the
split ring is again put back in its original position, it will
not completely surround the stem. The central tissues
are in a state of compression, and the cortex in one of
extension, laterally as well as longitudinally, as in the
other case already quoted.
Transverse tensions of a similar kind are set up in the
course of the thickening of stems and roots by the activity
of the cambium layer, by the division of whose cells new
bast is formed behind, and new wood in front of it. The
bast and cortex are thus compressed outwards, and the
wood and pith inwards, on account of the formation of the
new material. The phellogens which form rings of cork at
various depths in the cortex, give rise to similar strains.
Sheaths of new cells are intercalated in the substance of
the delicate tissue, which thus becomes greatly thickened.
These tensions are due to growth, and not, like the others,
to turgidity of the tissues. They cannot consequently be
removed by treatment with salt solutions.
•
GEOWTH 319
These tensions are capable of demonstration all through
the life of such stems and roots as increase in thickness.
They give us a partial explanation of the structure of the
annual rings of wood which are exhibited by such stems and
roots, and of the ruptures that are generally noticeable in
the exterior of such parts.
In the absence of various external stimulating influences,
which will be discussed later, young growing members
show a tendency to elongate uniformly, so that the direc-
tion of their growth is a straight line. Though the apex
of any of them may continually show the movement of
circumnutation, the mature part generally takes up a
fixed position, growing vertically or horizontally as the
case may be. This position is, however, usually due to
the combined action of a number of external forces acting
upon the growing member. The inherent tendency just
spoken of can be satisfactorily seen when, by artificially
eliminating the action of such forces, the plant is not
exposed to their stimulating influences. Such a tendency
has been called JRectipetality . It becomes apparent also
in the case of a member which has become curved, owing
to the action of one or other of the stimulating influences
referred to. If it is removed from the influence of the
stimulus, it becomes straight again.
320 VEGETABLE PHYSIOLOGY
CHAPTEK XXI
TEMPERATURE AND ITS CONDITIONS
THE various processes which are characteristic of vegetable
life only take place so long as the plant is exposed to a
particular range of temperature, which lies between the
freezing point of water and about 50° C., a few exceptions
on both sides of that range however being met with. It
is consequently essential to the well-being of the organism
that its temperature shall be maintained within those
limits. While life is possible within this range it is not
equally well manifested at all the points which lie between
the limits ; each vital function indeed shows considerable
variation in this respect. There is a certain point, lying
generally near the freezing point, below which it cannot be
observed. There is another point near the upper limit,
beyond which it is not carried out, and somewhere between
them there is a point at which it is manifested most
advantageously. These three points are known respectively
as the minimum, maximum, and optimum temperatures for
that function. These temperatures vary for each function
which accompanies the life of any particular plant. They
are not, moreover, in the case of a particular function,
necessarily identical in different plants.
The process of photosynthesis, for instance, commences
in the grasses at about 2° C., while in the Potamogetons it
cannot be detected below 10° C. The absorption of water
by the roots of the Turnip and other cruciferous plants may
begin when the soil has a temperature but slightly above
the freezing point of water ; in the case of the Tobacco-
plant it must be at 12° C at least. The lowest temperature
TEMPEEATUEE AND ITS CONDITIONS 321
for the germination of the seed varies between 5° C. for the
Wheat and 13° C. for the Vegetable Marrow. The upper
limit for this function in the cases of these two plants has
been ascertained to be 37° C. for the former and 42° C.
for the latter. The optimum point for the growth of the
roots of a seedling of Maize is 27° C., while the correspond-
ing temperature for that of the Barley and Wheat is about
23° C. Eespiration seems to show similar limits, but very
few observations have been made upon it from this point
of view. The optimum appears to be a little over 30° C.>
and the maximum 25 degrees higher.
The temperature of a terrestrial plant is subject to
great and frequent fluctuations, and there is considerable
difficulty in securing for it for any length of time the
optimum temperature for any of its vital functions, and
indeed sometimes of maintaining it within the limits which
are essential. As a rule such a plant only secures a
general approximation to the optimum point. The difficulty
is due to the fact that there is a continual and yet vari-
able interchange of heat between itself and its environment.
During the daytime it is constantly receiving supplies of
radiant energy from the sun, and as the air surrounding it
becomes warmer, a certain amount is absorbed by conduc-
tion. It is further continually expending heat on the
maintenance of transpiration, losing it also from time to
time by radiation and conduction. In its own metabolic
processes it is sometimes rendering heat latent, and always
liberating it by the processes of respiration, fermentation,
&c. Naturally, its temperature relationships are continually
varying. On the whole, such a plant tends to approximate
its temperature to that of its environment, but an equalisa-
tion is seldom reached, as both are varying simultaneously,
and owing to the slowness of the conduction of heat along
vegetable tissues, the processes of adjustment only take
place with difficulty. The trunk of a tree is during the
day often cooler than the air and warmer than the latter
during the evening and night. The mean annual tempera-
Y
322 , VEGETABLE PHYSIOLOGY
ture of such a tree trunk is, however, about equal to that
of the air. Less bulky parts than the trunk, the leaves
for instance, are very often much cooler than the air.
This is made evident by the frequency with which dew or
even hoar-frost may be detected on their surfaces. A
thermometer placed upon grass often gives a much lower
reading than one suspended in the air a little above the
ground. This is, no doubt, due to the loss of heat by
radiation from the leaves. Boots are often cooler than the
air, losing heat by conduction to the soil, and by the
evaporation which takes place into their intercellular
spaces.
Aquatic plants are less subject to these disturbances
than terrestrial ones. The range of temperature of the
water surrounding them is smaller, and as they are
practically in contact with water within and without, the
internal changes of temperature incident to their meta-
bolism are much more readily equalised.
In discussing the changes of temperature in the body
of a plant we have to deal at the outset with the supplies
of heat which it receives. We have already examined
them from the point of view of the absorption of energy
from without, but we may pursue still further here the
question of the warming or cooling of the plant itself
during such absorption.
The chief source from which heat is derived is the
radiant energy of the sun. When bright sunlight falls
upon a leaf about a quarter of its radiant energy is
absorbed. A much larger relative amount is taken up
when the light is less bright ; in a strong diffuse light,
such as that from a clear northern sky, the absorption
amounts to about 96 per cent, of the incident energy. We
cannot at present discriminate with any accuracy between
the influence of the heat rays and that of those of the
other parts of the spectrum. No doubt the relative
proportions vary considerably during the year.
This radiant energy falling upon the leaf comes into
TEMPEEATUEE AND ITS CONDITIONS. 323
X
relationship with it. quite independently of the temperature
of the air through which the rays pass, though the latter
gradually rises also, particularly during bright sunshine.
As we have seen already, three-quarters of the radiant
energy during such conditions is not absorbed by the
plant. The temperature of the air round tjae leaves under
a diffuse illumination rises but slightly, as only about
four per cent, of the radiant energy falling upon them
remains unabsorbed by them.
In certain cases, particularly where the temperature of
the air remains low for considerable periods, as in high
latitudes and on mountains, many of the light rays appear
to be transformed into heat. These are the rays which
are most vigorously absorbed by chlorophyll and by
anthocyan, and which cause the fluorescence of those
pigments. The importance of anthocyan in this respect
especially may be noticed. It is of very frequent occur-
rence among plants which grow in deep shade, and which
receive accordingly but little radiant energy. It is usually
found on the underside only of foliage leaves, and in special
leaves produced in summer upon the stems of deciduous
shrubs which occur upon the outskirts of forests or in
shady spots in their interior. It is found also at certain
times on the upper sides of foliage leaves, particularly when
the ordinary sources of heat are deficient, as in the cold
weather of early spring. Many grasses which when in
the lowlands are only green in colour develop a great
amount of the red or purple anthocyan when they grow
near the snow-line.
In both shady and alpine habitats the function of the
anthocyan appears to be the same, to secure to the
plant a certain amount of heat by the transformation of
the light rays.
The absorption of heat from the environment by the
processes of conduction is particularly noticeable in the
case of aquatic plants. Indeed any alteration of the
temperature of either the plant or the surrounding water
324 VEGETABLE PHYSIOLOGY
is readily transmitted from the one to the other. Similar
transmission of heat from the soil to the roots can take
place and no doubt has a considerable effect in promoting
the well-being of the latter, which as we have seen con-
tinually lose heat by the evaporation of water from their
cells into their intercellular spaces. Here, however, as in
the last case, the conduction of heat varies in direction
according to the relative temperatures of soil and root.
The absorption of heat from the air in contact with the
general surface must play a part very frequently in the
heat interchanges. As in the other cases mentioned,
however, the ' direction of this interchange is not
constant.
While we can thus recognise these ultimate sources of
heat supply, we find, no less evident, certain ways in
which heat is given off by the plant in greater or less
amount.
Of these losses the first and most important is the ex-
penditure which is necessary in order to evaporate the water
of transpiration. There can be no doubt that the amount
of transpiration is very largely determined by the amount
of the sun's rays which the plant receives. Not only are
its stomata open widely in bright light, so that the vapour
can readily diffuse into the air, but the actual evaporation
from the cells into the intercellular passages is enormously
accelerated during the absorption of the radiant energy.
The amount of the latter which is taken up by a leaf has
been computed to be nearly fifty times the amount which
can be utilised in the process of photosynthesis ; if the
heat were allowed to accumulate in the leaf unchecked,
it has been calculated that its temperature would rise during
bright sunshine at the rate of more than 12° C. per minute,
with of course, very rapidly fatal results. What is not used
for photosynthesis is employed in the evaporation of the
water of transpiration, the leaf being thus kept cool. It
is noteworthy that whether the leaf is brightly or only
moderately illuminated the same relative proportions of the
TEMPEEATUEE AND ITS CONDITIONS 325
total energy absorbed are devoted to the purposes of tran-
spiration and photosynthesis.
When we review the phenomena of transpiration we
find two very important considerations presenting them-
selves to us. On the one hand the suggestion comes that
the enormous stream of water passing through a terrestrial
plant is necessary in order that a sufficient amount of
inorganic salts may be supplied to the leaves, and that the
process of transpiration is maintained so that such a supply
may be at the disposal of the protoplasts. The dilute
solutions which are absorbed naturally involve the trans-
port of a large amount of water with the salts. Transpira-
tion seems thus to be subordinate to food supply.
On the other hand, the temperature relations which we
have just examined appear to place transpiration upon
quite a different plane. Instead of being a subordinate
process, it appears to be imperative in order to prevent a
fatal rise of temperature in the metabolic protoplasts ; to
be concerned primarily, that is, in the regulation of the
conditions necessary for the maintenance of metabolism
and life, rather than in the supply of material for metabolic
purposes.
Which of these is the chief function of transpiration
probably depends upon circumstances. The process serves
the two purposes, sometimes one, sometimes the other,
being the more prominent.
Another cause of loss of heat is found in radiation,
which takes place to a very important extent from the
surfaces of flattened organs such as leaves. This radiation
is to a certain extent independent of the temperature of the
surrounding air, and leads in some cases to a leaf being
several degrees cooler than the latter. A thermometer
placed on the grass will frequently show a temperature
some nine or ten degrees lower than another one suspended
a few inches above the surface of the ground. Evidence
of the activity of radiation at night is afforded by the
constant appearance of dew or hoar-frost on the leaves.
326 VEGETABLE PHYSIOLOGY
The effects of excessive radiation often threaten to be
disastrous and have led to the development of many pro-
tective adaptations by various plants. The masses of
woolly hairs which are often found upon leaves, forming,
indeed, in some cases a thick mantle, must generally be
looked upon as such a defensive mechanism. The delicate
leaves of buds are often protected by thick scale leaves,
which in some cases are hairy, in others furnished with
resinous excretions, to serve the same purpose. No doubt
the thick cuticle of many leaves and twigs discharges a
similar function.
Some plants secure a protection from excessive radia-
tion from the upper surfaces of the leaves during the night,
by folding them in various ways, so as always to expose as
little surface as possible, and that surface the one which is
least susceptible of injury by cooling. This so-called sleep
or nyctitropic movement plays a most important part in
the retention of heat, leaves that are prevented from carry-
ing it out perishing very rapidly. The features of this
behaviour will be examined more fully in a subsequent
chapter.
Conduction of heat from the plant to its environment
is of constant occurrence, but it is exhibited most clearly
by plants that have an aquatic habit. The general inter-
changes that take place between a plant and the water in
which it lives range usually through only a few degrees of
temperature, and are so constantly going on that the
temperature of both tends to become readjusted after every
slight disturbance. In some cases however a very large
amount of heat is dissipated by these means, as we may
see in the fermentation of a saccharine solution by yeast.
The metabolic processes of the latter, incident upon its
nutrition and respiration, are so vigorous that a very large
amount of energy is liberated by and during the decom-
position of the sugar, and this takes very prominently the
form of heat and passes from the plant to the sugary liquid
m which it lives.
TEMPEEATUEE AND ITS CONDITIONS 327
Terrestrial plants show less direct evidence of the loss
of heat by conduction. Their roots however no doubt
give up a certain amount to the soil at different times, just
as at others they absorb heat from the latter.
When we compare approximately the amount of heat
absorbed by a green plant with that which is given off by
it, we find that in all cases there is a certain excess of the
former. Most plants thus show a certain gain of heat
from their environment. This does not however usually
manifest itself by a rise of temperature in the tissues.
There is no uniformity in the absorption either ; at times
when there is the greatest balance in favour of absorption
throughout the whole plant, parts of it may be giving off
considerable quantities and may be cooler than the average
temperature of the whole plant.
The gain of heat which is secured in this way is to be
largely regarded, as we have already seen, as supplying
energy to the plant. This is devoted at first to constructive
processes, and thus much of it is rendered potential, being
afterwards reconverted into the kinetic form and made to
reappear, when it once more largely takes the shape of heat,
and is subsequently devoted to purposes of growth, meta-
bolism, repair of cell-substance, &c., as we have already
seen. But we may now lay a certain stress on the fact that
at any rate a part of this liberated heat is devoted to a
raising of the temperature of the cells which are the seat
of its liberation.
We have thus an elementary though very incomplete
mechanism for the regulation of the temperature of the
plant. An excess of heat is absorbed : part is at once
applied to purposes of growth, metabolism, &c. ; part is
retained, and the store is as it were economised, being
liberated later with some reference to the temperature of
the parts concerned in the vital processes.
This regulation of heat however is very rudimentary
and imperfect. We do not find that an increased loss of
heat stimulates metabolism in such a way as to set up
328 VEGETABLE PHYSIOLOGY
destructive processes, which should liberate heat to com
pensate for the loss. On the contrary such increased
decompositions are promoted by a rise instead of a fall of
temperature. On the other hand again the processes of
growth, repair, and constructive metabolism are also in-
creased as the plant becomes warmer. We can rather speak
of what takes place as a tendency to economise and distri-
bute heat, than as a process of regulation. Even the
distribution of heat, whether on its first absorption or
after subsequent fixation and liberation, is so unequal
that different parts of a plant may differ considerably as
to their temperature.
As we have seen, life is possible within certain limits of
temperature only. The maintenance of a healthy life
depends upon the adequate discharge of various functions,
each of which needs again a certain range. The limits
within which life is possible do not necessarily coincide
with those which are appropriate to every function. Out-
side the latter however a plant becomes unhealthy and
eventually perishes, falling a victim to the attacks of
internal or external adverse influences.
We do not find that all plants, or indeed all parts of
plants, show the same amount of resistance to the extremes
of heat and cold. The injury which any part of a plant
experiences under such conditions, depends very much upon
the amount of water which it contains. If more than a
trace of the latter is present, the formation of ice which
takes place below 0°C. may lead to rupture of the cells, the
ice being usually deposited outside them. A considerable
disturbance of the osmotic equilibrium of the sap may
occur, setting up secondary injuries. The protoplasm
becomes disorganised also at the low temperature.
After the freezing of a tissue has taken place, a subse-
quent rise of temperature leads to a process of thawing.
This in many cases is more fatal to it than the freezing, but
the effect depends largely on the rapidity of the thawing. If
it is so gradual that the water can be re-absorbed into the
TEMPEEATUEE AND ITS CONDITIONS 329
cells, they may continue to live, but otherwise the organ is
killed. The cells become flaccid and the protoplasm at
once ceases to have the power of maintaining them in the
turgid condition.
The effect of the absence of moisture in enabling
vegetable organisms to resist cold has recently been
examined in the case of seeds. Several kinds of these
have been found to be capable of germinating after immer-
sion for several hours in liquid hydrogen, the temperature
of which is the lowest at present known.
A similar effect is found at the other end of the scale.
If seeds are heated very gradually some will withstand a
temperature of 98° C. The gradual loss of water is a
necessary condition for this immunity, for when the heating
is conducted so quickly that the water is not driven off at
a low or moderate temperature, the treatment is fatal in
all cases. Under conditions of gradual heating, their tem-
perature being maintained at 60° C. for twenty-four hours,
seeds have been found capable of germinating after a
subsequent exposure to 98° C. lasting for ten hours.
Spores of bacteria and of fungi have a great power of
resisting high temperatures, and this is probably also asso-
ciated with a considerable degree of dryness. They can
withstand boiling in water for some time, but it is probable
that the reason why they are not destroyed is that their
walls successfully resist the passage of water into their
interior.
We are unable at present to explain in detail the
causes of the death of protoplasm under the conditions of
extremes of temperature. We can only say that under
these conditions living substance ceases to carry out the
normal reactions which are characteristic of it so long as
it is what we call * living,' and that the power to resume
them after the disappearance of the adverse conditions is
not regained by it. The nature of life and the intimate
causes and features of death are still beyond our knowledge.
330 VEGETABLE PHYSIOLOGY
CHAPTER XXII
INFLUENCE OF THE ENVIRONMENT ON PLANTS
ACCORDING to the nature of their surroundings and the
consequent differences in their mode of life, we find in
many plants certain peculiarities of form and structure in
which they differ from most of those which we have
hitherto considered. Of these the vascular plants which
live in water may be first discussed, as the direct influence
of the environment is most conspicuous in their case.
These aquatic plants, most of which are Spermaphytes,
but which include a few of the Pteridophytes, may be
divided into two chief groups ; those which are altogether
submerged, and those which bear floating leaves as well as
or instead of submerged ones.
In the former case the plant-body may be attached by
roots to the bottom of the stream or pool in which it lives,
or it may be altogether floating. The stems are generally
long and slender, and easily swayed to and fro in the water.
Some have however very short stems which give rise to
numerous elongated ribbon like leaves. These flexible
stems depend for their support upon the nature of the
medium in which they live, and though they possess a cer-
tain rigidity, this is not associated with any great develop-
ment of woody tissue. Generally the latter is reduced to a
minimum ; the fibro-vascular bundles are usually few and
contain few lignified elements. Their substance is largely
parenchymatous and the cells have thin walls. The inter-
cellular space system is often very complex, large lacunae
filled with air occupying considerable space in the distribu-
tion of the tissues (fig. 141). Their rigidity is secured by
INFLUENCE OF ENVIEONMENT ON PLANTS 331
the turgescence of the parenchymatous cells, and buoyancy
is much assisted by the air in the lacunae.
The primary root is generally feebly developed, and, as
a rule, does not persist through the life of the plant. The
floating forms frequently have no roots, but in many cases
adventitious roots are given off in large numbers from the
various nodes of the stem. The root-hairs which are so
FIG. 141. — SECTION OF STEM OF Potamogeton, SHOWING AIR PASSAGES
IN THE CORTEX.
characteristic of terrestrial roots are usually either very
scanty or altogether absent.
The epidermis of both stem and root is not cuticularised,
and therefore the cells remain capable of absorbing the
water in which the plant is living. In the stem this
tissue very frequently contains chloroplasts.
The character of the leaves differs according to the
habitat. Those which grow in rapid streams are generally
332 VEGETABLE PHYSIOLOGY
either long and thin, or are very much, and finely, divided, so
that they offer, in either case, no resistance to the force of
the current. In more sluggish water they may be long
and ribbon-like, but are frequently broader, and sometimes
attain a considerable size. The cell-walls of the former
are often thickened, but in the latter the tissue .is always
very weak, the parenchyma of the mesophyll sometimes
being greatly reduced. In Ouvirandra as the leaf becomes
fully developed this tissue disappears, only the veins
FIG. 142. — SECTION OF LEAF OF Isoetes.
a, lacunar cavities ; 6, vascular bundle.
remaining, so that it presents the appearance of a coarse
grating or piece of lattice-work. The epidermis of a sub-
merged leaf is never cuticularised, and it contains no
stomata. In many cases large lacunae are formed in the
substance of the tissue, particularly when the lamina is
somewhat stout, as in Isoetes (fig. 142).
In plants with floating leaves the roots and stems are
similar in character to those of the first class. The leaves
however, which lie upon the top of the water, are usually
tough and thick, their undersides being sometimes deeply
INFLUENCE OF ENVIEONMENT ON PLANTS 333
rugose. They have not the much-divided outline character'
istic of submerged leaves, but are usually simple and some-
times of considerable size. Those of the Victoria regia
are often three feet in diameter, and are turned up at the
edges, forming a rim, which helps to preserve the upper
surface from being wetted. The upper epidermis of such
floating leaves is often either strongly cuticularised, or
FIG. 143. — SECTION OF PETIOLE OF WATER-LILY (Nymphcea alba).
a, c, vascular bundles ; b, d, air-chanuels.
impregnated with a waxy secretion serving the same pur-
pose. The leaves are consequently shiny in appearance,
and water will not adhere to them. These floating leaves
bear their stomata upon the upper surface only.
The petioles are long and flexible, and possess a pecu-
liar power of adapting themselves to varying depths of
water. Should the stream in which they live become
shallow, the leaves still remain floating, owing to the power
of the petiole to become curved ; should the water rise, the
334 VEGETABLE PHYSIOLOGY
petioles respond by resuming their growth, so as always to
keep pace with the increased depth. Their structure-
resembles that of the stem in that they are composed of
turgid parenchyma and have little or no development of
woody tissue. They also contain conspicuous lacunae or
air channels (fig. 143).
Vegetative reproduction is very common, branches
becoming detached from the plant, which speedily put out
adventitious roots of their own and form new plants.
Their watery environment explains the peculiarity of
their structure. From the nature of their surroundings
and their power of absorbing liquid through their epider-
mis we can easily explain the absence of the woody tissue,
which we have seen to be, when present, especially devoted
to the conducting of water from the roots throughout the
plant. Their absorbing tissue being their whole super-
ficial investment, such conduction is not called for for
nutritive purposes. Their transpiration moreover is re-
duced to a minimum, and there is therefore no need of a
provision for the rapid current of water which is so essential
to the well-being of a terrestrial plant, in which this
function is so prominent. Their food materials reach them
dissolved in the water in which they live, and hence they
have no need of the complicated root system with its
absorbent root-hairs, which is so characteristic of a plant
growing in ordinary soil. Gaseous absorption takes place
through the general surface to a large extent, but this
direct supply is insufficient for respiration. The ordi-
nary arrangements for aeration, consisting of a network
of intercellular spaces freely in communication with nu-
merous stomata, are not exhibited by plants surrounded by
water. We have seen that many of them have no stomata,
the leaves being quite submerged ; others have relatively
few on the upper surfaces of the floating leaves. The
gaseous interchange between the interior and the exterior
is consequently greatly impeded. The large intercellular
lacunae form a mechanism by which this difficulty is
INFLUENCE OF ENVIEONMENT ON PLANTS 335
surmounted, affording large reservoirs of air in the interior
of all parts that are submerged, so that the slow rate of
renewal of air does not impede the gaseous interchanges
which the protoplasts require. These intercellular reser-
voirs are not confined to the vertical stems, petioles and
leaves, but occur also in the more woody stems or rhizomes
which many of these plants possess (fig. 144).
FIG. 144. — SECTION or RHIZOME OF Marsilea.
co.la, lacunae in cortex.
The absence of the transpiration current appears to be
correlated with a comparatively small development of the
plant-body. The large quantities of inorganic salts which
the dilute solutions absorbed by the roots carry into the
plant, in cases where the total absorption is very great
owing to a large transpiration, lead to a large increase of
constructive activity. In the absence of such an enormous
absorption the plant-body does not receive the materials
336 VEGETABLE PHYSIOLOGY
necessary for the acquirement of a considerable bulk.
Aquatic vascular plants are consequently never very large.
The difference between the two groups of aquatic plants
spoken of may be well seen in such forms as Cabomba,
which bears both submerged and floating leaves. These
show respectively the characteristics described in each
case.
Some curious adaptations of the organism to its environ-
ments are exhibited by certain of these plants which live
in marshy surroundings, sometimes being nearly or wholly
submerged, and at others, owing to the drying up of the
water, growing upon the mud. When the latter fate befalls
them, such of their leaves as are adapted to an aquatic life
become dried up, and perish. The upper leaves which have
always been exposed to the air do not suffer. As growth
continues, all the foliage which is produced is of the terres-
trial type. On the other hand, when the plant-body is
submerged the new leaves are all of the aquatic type.
These plants are often spoken of as amphibious.
Some aquatic plants are saprophytic in their mode of
life, flourishing best in water which is contaminated with
sewage or with the products of putrefaction. They are
chiefly certain species of Algae or Fungi, but among them
may be included a few Mosses and Phanerogams.
Another class of plants which show a definite response
in their structure to the conditions in which they live is
that to which the term Xerophytes has been applied.
These inhabit different situations, all of which are charac-
terised by presenting to the plant a very small supply of
terrestrial water. ' Many grow in sandy deserts, exposed to
great heat, and frequently undergoing long periods of
drought. Others grow upon a rocky substratum, and their
roots are confined to the crannies and crevices which are
present in the rock. Others are found in more temperate
countries, occupying light sandy soils which cannot retain
any considerable quantity of water. Such xerophytic plants
as are woody in habit frequently show considerable ten-
INFLUENCE OF ENVIEONMENT ON PLANTS 337
dency to diminish their leaf-surface, probably to reduce
evaporation and conserve their stock of water. They often
have many of their branches transformed into thorns or
spjnes? and very frequently their leaves show_simjlar reduc-
tion. Others which contain little wood are succulent, and
their ^surfaces are covered by a very thick and tougQ
epidermis, which is strongly cuticularised. Many of those
which grow upon rocks have leaves which show special
structures for absorbing water from rain or dew. Several
species of Saxifrage possess a number of glandular struc-
tures upon the teeth of their thick narrow leaves. Each
FIG. 145. — LEAF OF Saxifraga incrustata, SHOWING ABSOBBING\)KGAN.
consists of a small mass of cells with delicate walls, which
lie immediately under the epidermis of a small depression
of the surface, and which communicate with the exterior
by a few fine pores which perforate the latter. The epi-
dermis of this depression is made up of cells with thin
non-cuticularised walls. Each so-called gland is in contact
with the end of a fibro-vascular bundle, whose sheath is
carried forward over the general mass of delicate cells
(fig. 145). The depression of the surface is filled with a
mass of carbonate of lime, which is originally excreted by
the leaf, and which is held in its place by a few papillae
which project from the epidermis. Such an arrangement
338
VEGETABLE PHYSIOLOGY
serves a double purpose ; any dew or rain which reaches
the surface of the leaf is absorbed by the carbonate of lime
and can make its way slowly into the gland, whence it
passes into the nbro-vascular system ; while, when the leaf
is dry the incrusting mineral matter serves as a plug to
the depression, and reduces transpiration.
Many plants which inhabit sandy deserts- possess
similar mechanisms ; some excrete carbonate of lime,
others crystalline accumulations of common salt. The
latter can not only absorb dew and rain but can also con-
dense and take up moisture from the air. They are found
occurring in such sandy wastes as are by the seashore or
near salt lakes.
Many trees which grow in temperate climates, in poor
sandy soil on the margin of streams, show a somewhat
similar mechanism, but the excretion from their leaves
takes the form of a kind of resinous varnish or balsam
which can be readily wetted and which can absorb water.
In some cases so-called glandular hairs discharge a similar
function.
The water which is absorbed in this way is rarely pure,
but contains traces of sulphuric acid and ammonia, which,
though trifling in amount, are no doubt of value in the
nutritive processes. The adaptation to their environ-
ment which these plants exhibit is thus chiefly in the
direction of economising a limited water supply.
The influence of the environment on the form of the
plant can be seen equally well in the case of such plants
as grow in Alpine regions, where the cold is usually intense,
and the atmosphere for long periods so humid that transpira-
tion is only occasionally possible, and where consequently
the absorption of food materials is much impeded. Similar
conditions mark the bleak moorlands of temperate climates.
These show very great differences between the extremes
of temperature which mark summer and winter respectively.
The water supply also shows very great variations at
different times of the year. The plants are generally of
INFLUENCE OF ENVIEONMENT ON PLANTS 339
comparatively small size, and bear thick, often rolled-up,
leaves which are evergreen. The thick exterior and the
general hardness of the leaf are a response to, and a defence
against, the cold. In the heaths, which may be regarded
as typical moorland plants, transpiration is reduced to a
minimum, large air-chambers in the leaf with only a few
stomata, and those situated in a deep groove, providing for
the aeration of the protoplasts. During the cold the closing
of these almost hidden stomata guards the plant from the
evaporation, which, if unchecked, would lead to a loss
of heat that would be fatal to it. The metabolism being
FIG. 146. — TBANSVEBSE SECTION OF ROLLED LEAF OF HEATH.
reduced by the low temperature, the contents of the air
reservoirs suffice for such interchanges of gases as are
imperative, and for the coincident exhalation of watery
vapour by the protoplasts, but as these contents are very
slowly renewed the total evaporation is but slight. When,
on the other hand, for a part of the year the temperature
is high, the spacious reservoirs provide for a very rapid
transpiration as soon as the stomata are open, a very
large spongy mesophyll abutting on them (fig. 146). The
evergreen leaves also are an expression of the struggle
against the difficulty of the absorption of food materials,
which in such atmospheric conditions is possible for only
z 2
340 VEGETABLE PHYSIOLOGY
a limited period of the year. By preserving its leaves
green the plant can take advantage not only of the light
of summer, but also of those bright sunny days which occur
occasionally during the cold season, and thus improve every
opportunity afforded it.
Some lowland plants show a similar response to their
environment, the form and structure of different individuals
of the same species varying to a certain extent, according
to their advantages or the reverse, in such conditions as
sunlight or shade, drought or moisture, exposure to or
protection from cold winds, &c.
show some conspicuous modifications
of their structure in consequence of their peculiar habit of
life. They usually live upon the surfaces of trees, to which
they cling by various means, but from which they derive
no nourishment except such as is afforded by accumulations
of debris, &c. upon the trunks. They are not parasitic,
but merely live upon the tree as other plants grow upon
rocks or cliffs. Mosses and Liverworts are very largely
epiphytic, as are certain species of Phanerogams ; the
latter are very specialised forms, and show most adapta-
tion of form and structure. Perhaps the most remarkable
feature about them is their aerial adventitious roots, which
are given off in some cases from every node of the stem,
so that each internode has its own supply. These are
often long cord-like structures, which are of some thickness,
often contain chloroplasts, and are either covered by a
special epidermal development, or give rise to dense masses
of root-hairs. In the first case, which is common among
epiphytic orchids, the epidermis is many cells thick, and
is known as the velamen. The cells are small trache'ids,
with curious reticulated or spiral thickenings, and are often
perforated. These peculiar trache'ids contain only air, and
the velamen has consequently a curious glistening greenish
appearance. The mass of trache'ids forms a kind oLspongy
cpjj3nng_Jto_th^^
absorbing aqueous vapour from the moist atmosphere which
INFLUENCE OF ENVIEONMENT ON PLANTS 341
usually surrounds it. At other times when the air is dry and
there is a danger of evaporation from the root, this velamen
acts as a protective membrane against loss of water in this
way. The second case is illustrated by many aroids, and
the dense plexus of root-hairs borne upon the aerial roots
serves the same purpose as the velamen of the orchids.
Besides these roots, thus adapted to absorb watery vapour
from the air, epiphytes frequently have others which are
closely applied to the surface of the bark on which they
are growing. These are often strap-shaped, and cling very
closely to the tree, absorbing from the bark the soluble
products of its decomposition and any mineral debris that
may be accidentally carried thither. The small amount
of such food-stuffs available will explain the relatively large
development of the root system, which is in much greater
proportion than in ordinary terrestrial plants.
Parasites are another class of plants that have under-
gone much modification of structure in consequence of
their mode of life. The parasitic habit is seen most com-
pletely in the group of Fungi, but it is by no means con-
fined to them. We find many cases of partial or complete
parasitism among flowering plants. In all cases we notice
that the parasitic habit is associated with a degeneration
of structure, which especially affects the vegetative organs.
The fungus which is parasitic in habit derives all its
nourishment from the plant or animal whose tissues it has
invaded. Other plants of the same group are not parasitic,
but live upon decomposing organic matter, being known as
saprophytes,. Their mode of nutrition is, however, essen-
tially the same. They have all lost the chlorophyll
apparatus characteristic of the green plant, and cannot
therefore work up the food materials that the latter
absorbs from the air. Instead, therefore, of absorbing their
carbon in the form of carbon dioxide, these plants take it
in in the form of an organic compound of some complexity,
which is usually some kind of sugar. Saprophytes can
absorb nitrogen in the same combinations as a green
342 VEGETABLE PHYSIOLOGY
plant, but they appear to utilise compounds of ammonia in
preference to nitrates. No doubt their protoplasm is ulti-
mately fed with the same substances as is that of the
higher plants, but they lack a great deal of the constructive
power of the latter.
The degradation of the structure of such plants is
associated with the absence of the constructive processes
which depend on the presence of chlorophyll. Their body
is usually composed chiefly of delicate hyphse, which
PIG. 147. — Thesium alpinum, SHOWING THE SUCKERS ON THE BOOTS.
(After Kerner.)
ramify in the nutrient either living or dead substratum,
and which absorb elaborated products of some complexity
freely by their whole surface. They have, therefore, no need
of differentiated absorbing or conducting tissues, which are
consequently not developed. A further consequence of the
ease with which they obtain their food is the readiness
with which vegetative and asexual reproduction is brought
about ; hence sexuality is in many cases non-existent among
them.
Phanerogams which are completely parasitic show a
INFLUENCE OF ENVIEONMENT ON PLANTS 343
similar degradation of structure. They possess no chloro-
plasts, their leaves are absent or reduced to the condition
of scales, while their stems are often thick and succulent.
Their roots are replaced by the so-called haustoria, which
penetrate into the tissues of their hosts, complete fusion of
the tissue of the host and the parasite frequently taking
place. We have representatives of such parasites in the
British flora in the Cuscutas and the Orobancliacece.
Many of the plants belonging to the Santalacece and
the Scrophulariacece show a partial parasitism of this
FIG. 148. — Thesium alpinurn. PIECE OF A
BOOT WITH SUCKER IN SECTION. X 35.
(After Kerner.)
kind. They have short stems which bear green functional
leaves, but are peculiar in that their roots become attached
by curious sucker-like bodies to the roots of other plants
growing near them (figs. 147, 148), and from these suckers
absorbing cells are developed which penetrate into the
substance of their hosts and draw nourishment from them-
They are generally described as root parasites. The
Mistletoe behaves similarly, striking its haustoria into the
tissue of the branches of the apple, oak, poplar, &c. The
parasitism is partly compensated by the fact that its leaves
344 VEGETABLE PHYSIOLOGY
remain green when the host has lost its foliage, and by
their activity they to some extent assist the tree on which
the mistletoe is growing. The relationship seems to be
almost one of symbiosis rather than of parasitism. Pro-
bably the relationship of the root-parasites and their hosts
is also one of mutual assistance rather than true para-
sitism.
The habit of capturing insects, which we have seen to be
characteristic of several plants of very different forms, may
also be looked upon as connected with their environment.
Many of them, e. g. D rose ra, grow upon a substratum which
is largely composed of plants of Sphagnum, and which yields
to them a very limited supply of nitrogenous compounds ;
others are found growing on the surface of rocky mountains,
into the chinks of the stones of which their roots penetrate ;
others again flourish in the sandy soil of deserts ; in all of
which situations compounds of nitrogen exist only in very
small amount. The organic substances yielded by the
decomposing bodies of the captured insects must therefore
form a valuable supplement to the ordinary sources of
nitrogen.
Besides these responses to influences of the environ-
ment, which are very far-reaching, and modify very largely
the shape and structure of the plants exposed to them,
ordinary terrestrial plants also show great power of react-
ing to the different external conditions which they meet.
These will be considered in subsequent chapters.
345
CHAPTEE XXIII
THE PKOPEKTIES OF VEGETABLE PROTOPLASM
THE influence of the environment upon the structure of
plants we have seen to be far-reaching. Different condi-
tions of the surroundings are followed by differences of
structure which are greater in proportion as the time
during which those conditions act is more and more pro-
longed. The living substance of the plant is clearly the part
influenced by the environment, for we have seen that the
skeleton and other non-living parts of the plant owe their
construction to its activity. We may therefore with advan-
tage pause at this point to examine a little more closely
the properties which are exhibited by vegetable proto-
plasm.
We have seen throughout all the foregoing chapters
that all the processes which conduce to the well-being of
the plant are, to a large extent, if not entirely, under the
control of the living substance. Though the absorption
of its food materials from the air and the soil is due to
physical processes, these are nevertheless largely regulated
by the behaviour of the protoplasm under all sorts of vary-
ing conditions. The manufacture of food from these crude
materials, and its subsequent distribution, the accumula-
tion and dissipation of energy, the processes of nutrition
and growth, are all subject to the same regulation.
But there are also other properties of protoplasm which
have not so far been more than incidentally referred
to. The plant exhibits particularly the power of appre-
ciating changes in its surroundings, and is capable of
adapting itself in various ways to such changed conditions.
346 VEGETABLE PHYSIOLOGY
In many cases the adaptation in question takes the form
of a spontaneous movement, in which the living substance
is concerned in a manner resembling the muscular con-
traction so characteristic of animal protoplasm. In others
the response to such changes presents itself to us as a
modification of the normal behaviour of the living substance
with regard to the vital processes we have examined, and
in particular to the entry of water into the vacuoles of the
cells or its transmission outwards.
When we examine the phenomena of movement we
find that though evidence of contractility is procurable,
this phenomenon is of somewhat rare occurrence in plants.
Certain plants at particular times
emit from their body small masses of
naked protoplasm which are furnished
with a varying number of long fila-
ments (fig. 149). These filaments,
which are protoplasmic also, are
ordinarily in a state of active vibra-
FIG. 149.— ZOOS'PORE OF tion, causing currents in the water
Ulothrix. x 500. . , . , , , , . 1-1/3 J.T
in which they live, which float them
quickly from place to place. Among these free-swimming
protoplasts may be mentioned the zoospores of the Algae
and Fungi, and the anther ozoids of these and higher plants.
The movement is a spontaneous one, the organisms being
endowed with the property of locomotion, which they exer-
cise in the discharge of their ordinary life-work. Though
put forth in the absence of any external stimulation, the
protoplasts are capable of receiving such impulses and
modifying the vibratile action accordingly.
The mechanism of the movement is probably the con-
traction of each side of the filament or cilium alternately,
or of the part of the cell just at the point of attachment.
The impulse leading to the movement must be sought in
some decomposition originating in the protoplasm itself,
and not excited by any stimulation from without. The
phenomenon is often spoken of as ciliary motion.
PROPEKTIES OF VEGETABLE PEOTOPLASM 347
Of a somewhat similar character is the curious creeping
movement of the Myxomycetous Fungi. In a few cases the
zoospores of these organisms are furnished with cilia or
flagella, resembling those of the zoospores already men-
tioned, but more generally each consists of a minute mass
of naked protoplasm, which makes its way over the
surface of its substratum by putting out blunt processes
of its own substance, known as pseudopodia (fig. 150).
After a while a number of these zoospores become fused
together to form a large jelly-like mass, known as a
plasmodium. This colony of protoplasts then makes its
FIG. 150. — STAGES IN CONSTBUCTION OF THE PLASMODIUM OF A
Myxomycete.
way slowly over its substratum by similar pseudopodial
movements. Each pseudopodium is a protrusion of the
ectoplasm, and the more fluid endoplasm is in some way
drawn into the different protrusions, so that the rest of the
cell or of the plasmodium follows the extension of the
pseudopodium and is dragged after it. Which part of
the operation corresponds to the act of contraction is
disputed, but it seems probable that it is the second, and
that the first protrusion is of the nature rather of relaxa-
tion. The movement, like that of ciliary action, is a
property of the organism, and is used by it in the ordinary
course of its life, even in the absence of stimulation.
Among the lowliest of the Algae or seaweeds some other
348 VEGETABLE PHYSIOLOGY
organisms are conspicuous by their power of locomotion.
These are the Diatoms which are so prominent in ponds
and sluggish streams. They are unicellular plants of very
minute size, each of which consists of a protoplast encased
in two silicified shells or valves which fit together very
tightly, one overlapping the other by its edges. The cell-
wall which forms each valve is strongly impregnated with
silica, the latter being deposited in patterns which are
often of great regularity and beauty. The plants are not
provided with cilia, nor so far as we know are the silicious
valves perforated in any way. Each diatom is however
capable of effecting a peculiar gliding and very rapid move-
ment through the water, the mechanism of which is at
present unexplained.
Certain filamentous Algae, known as the OsciHatoria,
also carry out a peculiar movement. They consist of long
chains of protoplasts, each separated from its neighbour
by a cell-wall, and the whole thread surrounded or coated
by a peculiar semi-gelatinous sheath. Each chain is
anchored to a substratum of stone or rock at one end, and
the free portion is in constant waving or twisting motion
to and fro, a movement which is quite independent of
currents in the water, being exhibited in the total absence
of such disturbance. The movement appears to resemble
that of the Diatoms, but its mechanism is at present un-
explained. Like the others so far discussed it is one of
the features of the life of the organisms, and is carried out
by their protoplasm without excitation by an external
stimulus.
In certain organisms of still humbler type another
manifestation of the power of contractility can be observed.
These are unicellular beings consisting of small unclothed
masses of protoplasm. In their substance at some point
there may be seen a clear space or vacuole which exhibits
a more or less regular pulsation, assuming slowly the
appearance of a nearly spherical cavity and then suddenly
disappearing, recalling the active contraction of animal
PEOPEETIES OF VEGETABLE PEOTOPLASM 349
protoplasm. These pulsating or contractile vacuoles can
be seen very well in Chlamydomonas.
The power of movement which is thus exhibited by
many of the lowlier plants may be distinguished however
from certain of the movements of portions of higher plants
which have already been alluded to, and which will be
discussed more fully subsequently. These movements
include the circumnutation of growing organs, the closing
of the leaves of Dioncea, the bending of the tentacles of
Drosera, and many others. These are brought about in
multicellular organs, and by a mechanism different from the
one now under discussion, the movement being secondary
and following indirectly on a change in the behaviour of
the protoplasm of certain of the cells, which, instead of con-
tracting, modifies its resistance to the escape of the water
which they contain. In one or two cases, as in the curving
of certain tendrils and in the drooping of certain leaves in
response to stimulation, the hydrostatic disturbance seems
to be attended by, and perhaps partly dependent upon, a
contraction of the protoplasm of certain cells. These
phenomena will be discussed in a subsequent chapter and
need only be alluded to here as possibly showing the
inherent power of contractility residing in the proto-
plasm.
"Though the power of locomotion, which we have seen
in many cases to exist, is an evidence of certain powers of
movement or contractility possessed by living substance,
it must not be inferred that only organisms which are free
to move are possessed of these or similar properties. Loco-
motion is impossible to the great majority of plants on
account of their relationship to their environment. There
is however a certain amount of evidence to show that the
instability which, in the cases discussed, finds its expression
in movement, is a property of living substance in general.
We find many cases in which movement of the living
substance can be observed in the interior of ordinary cells.
It can only be seen when the protoplasm is more or less
350
VEGETABLE PHYSIOLOGY
filled with granules, as in their absence it is so transparent
that it is impossible to say whether it is in motion or not.
In the leaf of Elodea we find a very good instance of this
movement. Each cell contains a considerable quantity of
water, so that the protoplasm for the main part is found as
a layer lining the cell-wall. This layer consists of two
parts, an outer one in which are situated the chloroplasts,
and an inner one in which are large numbers of fine
granules. It is this inner layer which exhibits the move-
FIG. 151. — CELLS FBOM THE LEAP
OF Elodea. x 300.
?i, nucleus ; p, protoplasm, in which
are embedded numerous chloro-
plasts. The arrows show the
direction of the movement of the
protoplasm.
FIG. 152. — Two CELLS FBOM A
STAMINAL HAIK OF Trades-
cant ia. x 300.
The arrows show the direction
of the movement of the
protoplasm.
ment, which can be seen as a streaming motion of the
granules, the whole layer flowing slowly round the cell
(fig. 151).
In other cases, particularly in long pollen-tubes, where
the distribution of the protoplasm is so far different that
bands or bridles of it cross the vacuole in various directions,
the movement has a more complicated course, streams of
granules passing along these bridles as well as along the
peripheral portions of the protoplasm. These two cases of
PEOPEETIES OF VEGETABLE PEOTOPLASM 351
streaming movements of protoplasm are spoken of as
rotation and circulation respectively. There is no differ-
ence apparently between them, except what is involved in
the different distribution of the protoplasm in the cells.
Other instances are met with in the staminal hairs of
Tradescantia (fig. 152), the leaves of Vallisneria, the
internodal cells of Chara and Nitella, and the unicellular
Desmids.
It is evident from the structure of most vegetable
organisms that the possession of a power of active con-
tractility, such as is possessed by most animals, would be
of comparatively little use to them. Though flexible to a
certain extent, they are possessed of a fair amount of
rigidity, which under ordinary conditions they do not
relax. We have seen that one of the most important
relations of their life is that which is maintained between
the protoplasm and water. Each cell or protoplast is so
organised as to contain its own appropriate store, upon the
possession and renewal of which its efficiency as a member
of the colony, if not its actual life, depends. The regula-
tion of this supply of water is of the first importance to the
plant, and it is not surprising therefore to find that such
a regulatory power is one of the properties of vegetable
protoplasm.
All healthy vegetable cells are during life in a condi-
tion which is known as turgor. The cell is overfull of
water, so that a certain internal hydrostatic pressure is
exerted on the whole surface of the limiting membrane,
which is stretched accordingly. As the membrane pos-
sesses elasticity, the wall in turn exerts a pressure upon
the fluid inside it, and during healthy life a certain
equilibrium exists between these two pressures. Such a
cell is called turgid, and the degree of its distension is
the measure of its turgidity. This . turgor can vary within
fairly wide limits, consistently with the health of the cell.
The turgor depends chiefly upon two factors, both of which
are capable of control. The water is caused to enter the
352 VEGETABLE PHYSIOLOGY
cell, as we have seen, by the formation of various osmoti-
cally active substances in its interior, which have an attrac-
tion for water, the quantity which enters depending upon
the amount of such substance present. We have seen
already that this regulation of osmotic material is controlled
by the protoplasm. But besides this another important
factor exists in the greater or less difficulty with which water
is enabled to pass through the plasmatic membranes. The
power of altering its permeability by water is a property
of protoplasm which is of the highest importance in the
mechanics of the cell. It takes the place practically
which is held- by the power of contractility in the
living substance of animals. No doubt it can be called
into play during life under constant conditions, but it
becomes much more marked when the plant is subjected to
particular kinds of stimulation. A ready instance of its
employment under the former conditions is afforded by
the variations of turgidity and subsequent growth which
we have already spoken of as inducing circumnutation
(p. 316). Instances of its following upon stimulation will
be discussed more appropriately in a later chapter.
The facts thus briefly narrated impress upon us the
belief that all protoplasm is the seat of active molecular
movement, the intensity or vigour of which, as well as the
forms of its manifestations, varies very greatly in different
cases. Indeed, the life of the protoplasm is intimately
bound up with such a motile condition. The manifesta-
tions are in all cases appropriate to the manner of life and
the surroundings of the organism under observation ; they
may take the form of locomotion, of contractility, or of
variation of permeability, leading to the regulation of
turgescence.
If we look back to the behaviour of the contractile
vacuole of Chlamydomonas, we are struck by the fact that
its pulsations occur with a certain definite mtermittence
so long as they are not interfered with by external condi-
tions. The vacuole dilates slowly, reaches a certain size,
PEOPEETIES OF VEGETABLE PKOTOPLASM 353
and suddenly disappears ; then is gradually formed again,
and the series of events is repeated. This regular inter-
mittence constitutes what is often spoken of as rhythm.
The rhythm which is so easily seen in the case of
pulsating vacuoles is characteristic also of those less obvious
changes in protoplasmic motility which lead to the varia-
tions of turgidity in different organs, particularly in those
which are growing. We have already seen that during
the growth in length of a symmetrical organ, such as a
stem or root, the apex points successively to all points of
the compass, the successive changes of position being
spoken of as circumnutation. This is the result of a
rhythmic variation of the turgidity 'of the cells of the
cortex. If we consider a longitudinal band of such cells,
we find that at a certain moment the cells are at their
point of maximum turgidity, and the growing apex is made
to bend over in a direction diametrically opposite to this
band. The turgidity of this band then gradually declines
to a minimum, and again increases slowly to a maximum.
If we conceive of the circumference of the organ as
divided into a number of such bands, we can gain an idea
of the changes in turgidity which cause the circumnutation.
Each band is in a particular phase of its rhythm at any
given moment, and the successive bands follow one another
through the phases of their rhythm in orderly sequence, so
that when one is at its maximum, another diametrically
opposite to it is at its minimum. The phases of maximum
and minimum turgidity thus pass rhythmically round the
organ, and the apex is consequently compelled to describe
a spiral line as it grows. If the stem or root is not
circular in section, but is flattened in any direction, the
steady sequence of the rhythmic changes will lead to the
projection of this spiral assuming the form of an ellipse
instead of a circle, and if the flattening is extreme the.
movement will be a backward and forward one.
Modifications of the distribution of maximum and
minimum turgescence in a radially symmetrical organ may
A A
354 VEGETABLE PHYSIOLOGY
lead to a similar nutation. It is not infrequent for^ the
rhythmic change in the turgescence to affect only two sides,
instead of passing regularly round it. The organ, though
radially symmetrical in structure, will thus behave as a
bilaterally symmetrical one, its organisation indeed being
bilaterally symmetrical. Its changes will thus resemble
those of a flattened organ which can only be made to
oscillate backwards and forwards.
A similar rhythm can be noticed in the variations of the
extensibility of the limiting membrane which characterise
the circumnutation of a coenocytic hypha. We must sup-
pose these variations to be due to the protoplasm covering
the wall, though we cannot explain the mechanism. The
protoplasm has the power to soften the cell-membrane.
Rhythmic changes of this kind affect other processes
than those of circumnutation. We have had occasion to
notice that the behaviour of a growing organ during its
grand period shows certain diurnal variations which we
have called the daily periodicity of growth. Though no
doubt we have to do here to a certain extent with
changes in the behaviour of the protoplasm induced by
the alternations of light and darkness, with coincident
variations in temperature, this daily periodicity of the
rhythm does not appear to be altogether dependent upon
exposure to such alternations, for they persist for a
longer or shorter time during continuous darkness. Their
cessation after exposure to a period of darkness need
not necessarily point to their dependence on the inter-
mittent access of light and warmth, for, as we shall see
later, prolonged deprivation of light leads to a peculiar
condition of rigidity of the protoplasm which eventually
causes its death. The cessation of the rhythm indeed
appears to be a pathological phenomenon. The rhythm
of the daily periodicity appears however to bear a certain
relationship to the alternation of day and night, for plants
which have been cultivated from seed in continuous dark-
ness do not exhibit it.
PEOPEETIES OP VEGETABLE PROTOPLASM 355
This rhythmic change in the protoplasm is not exhibited
by organs during growth only ; in many cases it persists
throughout their life. Very conspicuous instances of it are
afforded by certain movements often exhibited by the leaves
of particular plants. Perhaps the most familiar of these is
the so-called Telegraph plant, Desmodium or Hedysarum
gyrans. Its leaves are ternate, the terminal leaflet being
very large in comparison with the two lateral ones (fig. 153).
If the plant is watched while exposed to suitable tempera-
ture and illumination, the lateral leaflets are found to move
up and down on the rachis, sometimes passing through an
angle of 180°, and twisting slightly
as they move. They thus describe
a kind of ellipse, the duration of
the movement being about two
minutes. Many other instances of
a similar kind are known, the
Leguminosce furnishing many ex-
amples. All of them do not exhibit
the movements with the same ease,
as they are interfered with by
other changes in position which
result from external Stimulation. FIG. 153.— TERNATE LEAF OF
mi £± i i • j -L i THE TELEGRAPH PLANT
They can often be made evident by (Desmodium gyrans).
keeping the plant under constant
external conditions. Darkness however if too prolonged
causes their cessation, though in some cases they are
made evident by deprivation of light for a short time.
The mechanism of the movement in most of these cases is
the rhythmically varying turgescence of particular organs
known as pulvini, which are situated at the bases of the
stalks of the leaves or leaflets. As in the cases already
noticed the alternations in this turgescence are the expression
of rhythmic changes in the protoplasm of the cells of each .
pulvinus. As these pulvini play a considerable part in the
changes of position which are exhibited by many leaves
under various conditions, their structure may well attract
A A 2
356 VEGETABLE PHYSIOLOGY
our attention here. Fig. 154 represents a longitudinal
section through one of them, which occurs at the base of
a leaflet of Mimosa. The stalk of the leaflet shows a
swelling at the point of union with the rachis, the pro-
tuberance being greatest on the under side. Here there
is a cushion of cells which are capable of containing a
FIG. 154. — PULVINUS OF Mimosa.
a, 6, the succulent parenchyma of its upper and lower sides ; c, bud ;
d, parenchyma of rachis ; e, pith.
relatively considerable quantity of water. When turgid
they swell out and force the leaf into an erect, or almost
erect, position. When they part with their water and
become flaccid the stalk of the leaf loses its support and the
weight of the blade causes it to fall downwards. This is
rendered more easy by the fact that the vascular strand
PEOPEETIES OF VEGETABLE PEOTOPLASM 357
or bundle which passes from the stele of the stem through
the petiole is somewhat reduced, giving greater flexibility to
the stalk at that point. The cells upon the upper side of the
pulvinus in some cases play only a passive part in the
phenomenon, the rhythmic variations only affecting those
already described ; in other cases the upper side or both
sides may show the changes of turgidity.
The same tendency to rhythmic change is shown in what
is called the periodicity of the various vital functions. If,
for instance, the root-pressure of a plant is examined by the
aid of the apparatus already described, in which the water
taken up is made to support a column of mercury in a
manometer, when the mercury has reached what we may
call its mean or average height, it does not remain steady at
that point, but begins to oscillate. It rises in the morning
till about midday, then sinks somewhat, rises again during
the evening, and falls during the night. There is thus
a daily variation of the absorptive activity of the roots
which is scarcely affected by changes in the environment.
It is an instance of an automatic rhythm.
There is a similar daily variation in the bulk of a plant,
the diameter of its various organs diminishing from night
till some time in the afternoon, and increasing thenceforward
till dawn. These variations largely depend upon the dis-
tribution of the water which the plant contains, which is
regulated by the living substance in the way already
described. This rhythm is under ordinary circumstances
very much affected by variations in transpiration, which
we have seen is a process that is very soon modified by
variations in illumination and temperature.
It is difficult to explain the occurrence of these various
manifestations of rhythmic change in the protoplasm. Many
of them suggest that they are the result of the influence of
the alternations of light and darkness, and perhaps of the
changes of the seasons, to which plants are exposed. But
others are exhibited so regularly under constant conditions
of the environment that they cannot be thus explained.
358 VEGETABLE PHYSIOLOGY
They are now hereditary and to a certain extent indepen-
dent of the changing conditions which the plants encounter.
It may well be however that they have become impressed
upon the organisation of vegetable protoplasm by the con-
stant recurrence of these changes of the environment during
the long ages of the past. This does not appear unlikely
in the face of the fact that, as we shall see later, it is possible
under appropriate conditions to impress a new rhythm
upon particular organs. The manifestation of rhythmic
change has however become one of the vital properties of
protoplasm.
We saw in an earlier chapter that the peculiarities of
form and structure which different plants possess are to be
associated with the character of their environment. From
such facts as were there discussed it is evident that a plant
is capable of receiving impressions from without and
responding to them in various ways. If we examine any
plant which does not show such marked adaptation to its
surroundings as those which were then more particularly
under consideration, we can still find evidence of the pos-
session of a similar power of appreciating differences in the
external conditions in which it finds itself, and of modifying
certain of its vital processes in response. When certain
zoospores of some of the lower Algae which swim freely in
water are suddenly exposed to a brilliant light, they take
up at once a definite position with regard to the incident
rays. When a leaf of Mimosa pudica, the so-called sensi-
tive plant, is roughly handled, it falls from its normal
position and takes up a new one, while its leaflets become
folded together. When a filament of Mesocarpus is exposed
to an electric shock sent through the water in which it is
floating, it is found not infrequently that it splits up into
its constituent cells. This power of receiving impressions
from without, to which we have had frequently to refer in
discussing the phenomena of growth and rhythm, is another
property of vegetable protoplasm and can be observed
to belong, in a greater or less degree, to every vegetable
PEOPEETIES OF VEGETABLE PEOTOPLASM 359
organism. It is usually spoken of under the general term
irritability.
This property, which is the most important of all, as it
is particularly the one which keeps the organism in a
proper relationship to its environment, is not always to be
observed or demonstrated with equal ease. Indeed the
protoplasm must be in a healthy condition to manifest it
satisfactorily. It is easily injured if changes in the
environment are too sudden or too severe. Consequently
the adaptation of groups of plants to special environments
has been a slow and difficult process, any single individual
undergoing little change, but alterations of considerable
extent having been effected by the continuous influencing
of many generations.
The maintenance of the health of the individual is no
doubt the great object of this sensitiveness ; and conversely
it is only the healthy plant that manifests it in the greatest
fulness. Health may be spoken of as the condition in
which the reaction between an organism and its sur-
roundings is a perfect one. In the case of the ordinary
terrestrial plant these surroundings present especially three
features which are subject to considerable variation. These
are light, temperature, and moisture. A plant must exhibit
a proper relationship to each of these conditions at any
rate to be healthy. The condition in which the relationship
to each of these factors is satisfactory is generally spoken
of as one of tone, and the influence which each exerts when it
affects the plant uniformly is spoken of as a tonic influence.
When a dicotyledonous plant which has been growing under
ordinary atmospheric conditions, exposed to diffused day-
light, is removed into darkness and kept there for some
time, it becomes incapable of being impressed by its sur-
roundings. Nor is its irritability alone affected by the
absence of light, for many of its parts, particularly its leaves,
cease to grow under such conditions. The condition which
is induced by light, and upon which the manifestations of
irritability depend, is known as Phototonus.
360 VEGETABLE PHYSIOLOGY
A corresponding condition marking an appropriate
relationship between the plant and temperature may be
called Thermotonus. This condition also is necessary for
the manifestation of sensitiveness. If it is materially
interfered with, the vital functions and the processes of
growth and nutrition suffer seriously.
There must also be a satisfactory adjustment of the
relations between a plant and moisture, though this is less
restricted than the two already mentioned.
As the maintenance of health involves the continual
adjustment of the plant to the changes in its environment,
we must examine a little more closely the nature of the
influence which the latter, and particularly the two factors
of light and temperature, exert upon the organism. This
influence is spoken of as a tonic or par atonic influence and
leads to the establishment of a satisfactory condition of tone.
In order to study the tonic influence of light upon a
plant we may first consider the features which characterise
the growth of a plant in darkness. We find that such a
plant is much modified both in form and structure. If we
experiment with an ordinary dicotyledonous plant which
has numerous leaves of moderate or small size upon an
elongated stem, we find that these features become much
exaggerated. The stem is very much elongated and
remains slender ; it is more succulent than a normal stem,
and bears extremely small leaves which grow out from it
at a more acute angle than those which arise upon a
normally illuminated stem. Certain Monocotyledons which
have normally small stems and large broad leaves are
differently affected. The great change in this case is in
the leaves, which become much elongated and relatively
narrower than normal ones. Certain phylloclades, such
as those of some of the Cacti, become elongated and slender,
instead of remaining broad.
The structure of the various parts also is modified ; the
woody and sclerenchymatous elements are much reduced,
and the parenchyma of the cortex is increased in bulk. It
PEOPEETIES OF VEGETABLE PEOTOPLASM 361
becomes more succulent, and the reaction of its sap is much
more acid. The chloroplasts do not become green, the
pigment which they contain, known as etiolin, being a pale
yellow. In the leaves the differentiation of the mesophyll
into palisade and spongy parenchyma does not take place.
The parenchymatous cells of the ground tissue of the
elongated organs, whether they are stems or leaves, become
altered in shape, their longitudinal diameter being con-
siderably increased. Plants thus affected by darkness are
said to be etiolated.
That these differences are to be attributed to the absence
of the light can be seen by comparing two similar plants,
the first cultivated in darkness and the second under
ordinary conditions of illumination, the other conditions
being kept the same for both.
The explanation of these changes is somewhat difficult.
The absence of light is clearly the cause of the different
colour, for, as we have seen in a preceding chapter, under
such conditions the pigment chlorophyll is not formed, but
is replaced by the yellowish-white etiolin. When an
etiolated plant is exposed to light, the etiolin is soon
replaced by chlorophyll, and the plant becomes green.
Etiolin appears indeed to be the antecedent of chlorophyll.
The question of the non-development of the woody elements
and the generally increased succulence is more difficult to
explain, and many hypotheses have been advanced to
account for it. There is a disturbance of the normal
course of the metabolism evidently, as shown by the greater
production or accumulation of organic acids, to the osmotic
properties of which the increased succulence is partly due.
It is known that in plants possessing considerable succu-
lence the free organic acids which are produced during the
night undergo oxidation when light finds access to them.
The reason for the disturbance in question is however
not explained. Diminished transpiration may perhaps
account for a good deal, for as we have seen in the absence
of light the stomata remain shut and there is but little
362 VEGETABLE PHYSIOLOGY
output of watery vapour. The increased turgidity of the
tissues resulting from this factor may very probably upset
the normal course of metabolism.
It is significant in this connection that the parts which
show the excessive growth are in all cases those in which
water accumulates as transpiration becomes checked.
If however the effects are admitted to be due to the dis-
turbance of transpiration, this is no satisfactory explanation
of the phenomenon, for, as we have seen, the actual evapora-
tion of the water of transpiration into the intercellular
spaces is under the regulating influence of the protoplasm,
and the effect must therefore be traced back to some
interference with the latter, caused by the absence of
illumination. With the lowering of the tone which follows
the absence of the light we have a failure of the proto-
plasm to exhibit its normal degree of permeability, and we
find it retain in the cell more than the usual quantity of
water.
We cannot easily explain the effects which we have
seen are produced upon the structural elements of the
plant. We do not know why the usual development of the
woody and sclerenchymatous cells of the stem should be
interfered with, nor can we explain the effect of light upon
the degree of differentiation of the mesophyll of leaves.
We find that palisade tissue is developed more readily
under the influence of bright light ; a phenomenon which
may be easily ascertained by comparing the structure of
two leaves from the same tree, one taken from a brilliantly
lighted and the other from a deeply shaded part. Indeed,
the differentiation of the mesophyll into palisade and
spongy parenchyma may be traced to the difference of
illumination which the two faces of a leaf receive, for when
both are well lighted, palisade parenchyma appears upon
both sides, while etiolated leaves, as we have seen, do not
develop this tissue at all.
It is possible that this difference of structure on the
two sides may be connected with the possibility of damage
PEOPEETIES OF VEGETABLE PEOTOPLASM 363
to the chloroplasts if they are too brilliantly illuminated.
The arrangement of the palisade cells shields them to a
extent.
If we pass to consider the effects of too intense an
illumination we find that it is attended with considerable
danger to the well-being of the protoplasm. When the
leaves of certain plants, among which may be mentioned
Oxalis acetosella, are kept exposed to very strong sunlight,
and prevented from shading themselves as they normally
do by changes in their position, they rapidly die, the dura-
tion of their life being reduced from two or three months
to as many days. Bright sunlight has in other cases been
found to check the growth in length of seedlings, the effect of
different degrees of illumination having been compared by
direct measurement. We find various arrangements in
different plants which appear to be directed towards pro,-
tecting them from the effects of too brilliant an insolation.
Many which normally have their leaves so arranged as to
expose their upper surfaces to the incident rays are found
under bright sunlight to place them so that their edges
and not their surfaces receive the light. This phenomenon
has been called Paraheliotropism. It is exhibited normally
by the leaves of Oxalis which have just been alluded to.
Another phenomenon, having for its purpose the protec-
tion of the chlorophyll, can be seen in many ordinary dorsi-
ventral leaves. When brightly illuminated they are of a
lighter green colour than when shaded, and this has been
found to be due to a different arrangement of the chloro-
plasts in the two cases. In a leaf exposed to diffused light
these are collected on the upper and lower walls of the cells
just under the epidermis, and they present their broader
surfaces to the incident rays. When the light is cut off
altogether for a considerable time, and other conditions
are unfavourable, they collect on the lateral and lower
walls. When the leaf is brilliantly illuminated they place
themselves upon the lateral walls only, and rotate on their
long axis so as to present their edges instead of their
364 VEGETABLE PHYSIOLOGY
surfaces to the light. In the first case the chloroplasts lie
parallel to the surface of the leaf, and receive as much
light as they can ; in the last they lie at right angles to
the surface so as to receive as little as possible. These
two conditions are known as epistrophe and apostrophe
respectively. When the conditions of the incidence of the
light are altered, the chloroplasts change their positions
accordingly.
The Alga Mesocarpus exhibits the phenomenon in a
very striking manner. It consists of somewhat oblong or
slightly elongated cells arranged in a filament. Each cell
contains a single band-like chloroplast which lies nearly
parallel to the long axis of the cell. In ordinary daylight
it places itself so that the surface of the band is exposed to
the illuminating rays, but if the light becomes intense, it
revolves quickly upon its long axis, so that its edge is
presented to them.
A different effect of a strong light is manifested by
many dorsiventral structures, of which the thallus of
Marchantia affords a good example. Whichever side of
this organ is brilliantly illuminated, the dorsal or upper
surface shows accelerated growth, so that the thallus exhibits
epinasty. Some of the radially symmetrical structures
which have been mentioned as bilaterally organised (page
354) behave similarly. Such are runners of Polygonum
aviculare, and other plants of similar habit. This pheno-
menon has been called photo-epinasty, as the increased
growth of the dorsal side is due to the access of light.
These facts may perhaps give us some idea of the
influence of light upon protoplasm, and the condition of
tone, one of whose chief features is the proper regulation
of the permeability of the protoplasm by water. In dark-
ness metabolism and growth are greatly affected, the latter
being unduly accelerated. In the presence of too strong a
light, a deleterious influence is exerted. An intermediate
condition exists in which the vital processes of growth and
nutrition and the sensitiveness to external influences are
PEOPEETIES OF VEGETABLE PROTOPLASM 365
seen at their best. This is the condition of tone or photo-
tonus, and its maintenance may be spoken of as due to the
tonic influence of light. It is frequently said that light
retards growth, and the tonic influence is associated with
this retardation. This is however a somewhat incomplete
presentation of the case. Retardation of the growth is not
the only effect produced by the access of a proper degree
of illumination. It is rather to be regarded as regulatory
than retarding, and as it affects many other functions
than growth, it seems more appropriate to consider the
influence of the light as directed to the maintenance of
this tone, which is really one of the conditions of health.
How the actual effect upon the protoplasm is produced we
cannot say ; it may be that the niotility which is charac-
teristic of healthy protoplasm and its control of its own
permeability are adjusted to a particular relationship with
the environment, of which phototonus is one condition.
The rays of the spectrum which exert this influence on
the living substance appear to be those of high refrangi-
bility, the blue and the violet. To these rays the proto-
plasm seems to be excessively sensitive. We do not explain
their action when we say that they bring about a variation
in the turgidity of the cells, or that they set up a change
in the manner of their nutrition ; the facts which we have
called attention to can only be referred to the power of the
protoplasm to respond to their influence.
The question of the influence of temperature upon the
tone of the plant need not here be considered so fully, as
in a preceding chapter we have discussed the phenomena
of the general relations of temperature to the plant at
some length. We may however again point out that
plants are affected by variations in temperature in ways
very similar to those depending on changes in light. It is
not however always easy to ascertain the effects due to
changes in temperature alone, as other conditions, such as
light and moisture, usually vary at the same time as the
temperature changes.
366 VEGETABLE PHYSIOLOGY
As we have seen, the environment of the plant is partly
the soil and partly the atmosphere, and the temperature of
both may or may not vary simultaneously. We have
seen that for each metabolic process there is a temperature
at which it progresses to the greatest advantage. At lower
and at higher points the protoplasm is less active, and in
each case there is a point below which activity ceases, and
one above which also it does not go on. The same thing
we have seen is true of the processes of growth. We may
say that for each plant there is a particular temperature at
which it carries out the aggregate of its functions most
advantageously, and it is when exposed to this temperature
it is in a condition of the most complete thermotonus.
This point is not the same for every plant, indeed consider-
able differences exist in this respect. We may say more-
over that it is perhaps not so much a point as a range of
temperature, for small divergences from the actual optimum
point have but little effect upon the tone. Within this
range the constant round of activity, chemical and physical,
which is the expression of life, goes on most advantageously,
below it it is injuriously affected, and at a minimum point
it is suspended. At another point, higher in the scale,
spoken of as the maximum temperature, the death of the
protoplasm usually ensues.
We cannot explain the influence of temperature upon
the protoplasm any more satisfactorily than we can that of
light. All we know is that the two co-operate together to
keep the plant in the condition to which we have given the
name of health.
The tone of the plant depends very greatly upon a
proper adjustment of the relations between the protoplasts
and water. For the maintenance of health it is essential
that the normal turgidity of the cells shall not be disturbed.
A definite amount of hydrostatic pressure inside such cells is
necessary as we have seen for the due or efficient discharge
of the processes of life. We may regard the maintenance
of this relationship as one of the chief features of tone,
PEOPEETIES OF VEGETABLE PEOTOPLASM 367
for it involves a particular condition of the protoplasm
with regard to its permeability. This condition may be
regarded as a kind of effort, the living substance exerting
some active living influence comparable to the condition
of almost passive contraction, which is the normal condition
of various muscular structures in the animal body. If
this effort is increased, the existing hydrostatic pressure
causes an excessive escape of water, and the cells become
flaccid ; if it is relaxed, the normal interchange of water
between cells is diminished to their detriment, the permea-
bility of the protoplasm becoming lessened.
A further aspect of tone may be seen to depend upon a
constant and regular supply of oxygen. The function of
this gas in vegetable life has already been discussed at
some length in a preceding chapter. We have seen that
if its access is interfered with the whole organism is for a
time, if not permanently, upset, all the vital functions
being thrown into disorder. The power of appreciating
and responding to stimulation is also lost.
Another property which vegetable protoplasm possesses;
and which is of the highest importance in adapting the
organism to its environment, is what has been termed
acclimatisation. This is manifested by the fact that after
long continued applications of a particular stimulus the
organism ceases to respond to it. This is shown by the
fact that a plant, accustomed to live in light of but feeble
intensity, if made to grow in a brighter region, though
injuriously affected at first, will ultimately thrive in it as
well as it did before. Similar phenomena in connection
with temperature have been observed.
368 VEGETABLE PHYSIOLOGY
CHAPTEK XXIV
STIMULATION AND ITS RESULTS
WE may gather from what has just been said that there
may exist for every plant, at any rate theoretically, a con-
dition of adjustment when it is in absolute harmony with
its environment, and when, consequently, its life is being
regulated to the utmost advantage. We can see however
that such a condition can be only momentary in any case,
for the environment is in a constant state of change and
the protoplasm of the organism is also exhibiting continual
motility. For the maintenance of health or even of life
it is essential that variations in one shall be adequately
responded to by variations in the other, and the impossi-
bility of securing indefinitely such a continual adjustment
of relations is the cause of the cessation of life.
The responses which the organism makes to such
alterations in its surroundings may now be considered
in greater detail, and we may thereby form some acquaint-
ance with the causes which have led to such great diver-
sities in form, structure, and habit of life as we have
already seen to characterise large groups of plants.
Any change in the environment which provokes some
difference of behaviour on the part of a plant is spoken of
as a stimulus, and the change of behaviour is to be looked
upon as the result of stimulation. When we come to
define more narrowly what we understand by the terms
stimulus and stimulation we find it is not easy to restrict
them however to such changes in the surroundings as we
are able to observe and perhaps measure by even the most
delicate instruments at our disposal.
STIMULATION AND ITS KESULTS 369
Many changes take place in protoplasm which escape
our observation, originating perhaps in the condition of
the protoplasm itself, or being due to disturbances in the
interior of the plant. The normal course of metabolism
may undergo a marked change in consequence of varia-
tion in the amount of some particular constituent of the
food or of an alteration of the distribution or direction
of the translocatory stream. Injury to the body of the
plant may involve redistribution of energy or of material
within its interior, which may have far-reaching effects
upon the course of the vital processes. Variations in the
supply of food, which may range between absolute starva-
tion and over-engorgement, may produce very great changes
not only in the outer life of the plant, but in the substances
it produces in its metabolism and the energy which it
liberates. The lack of oxygen may provoke an almost
entirely new metabolism in connection with the produc-
tion of such energy. These internal changes have been
already discussed, and the effect of various factors at work
in the organism have been examined, so that it is not
necessary in the present connection to do more than
emphasise the fact that we have in such matters evidence
of stimulation and the response it provokes, evidence
which points to the sensitiveness or irritability of proto-
plasm, as much as do the results of those changes in the
environment which are purely external. The internal
stimuli just noticed are largely chemical in character, and
though chemical changes in the protoplasm are continu-
ously occurring, many of them are directly instigated by
such stimuli. Whether the automatic changes in organs
and cells which we have already studied are due to stimu-
lation is perhaps a little doubtful, but at any rate the
nature of any stimulus provoking them has so far eluded
investigation, and to all appearance they are not initiated
in that way, but are independent of all stimulation.
Stimulation which is directly due to the physical con-
ditions of the environment may be looked upon as the
B B
370 VEGETABLE PHYSIOLOGY
effect of any modification of the conditions which have
induced tone. We have seen, for instance, that a particular
degree or range of illumination sets up in a plant the con-
dition of phototonus, which is one constituent of the healthy
tone of the organism. Any modification of that illumination
is followed by certain effects, the extremes of which we
have already discussed. This alteration of the optimum
illumination becomes at once a stimulating action, and
we can speak of a stimulating influence of light, which is
really any change in what we have called its tonic action.
It can be in the direction of increase or decrease of the
latter, but as .it induces changes it must be regarded as
stimulating.
What is true of light is also true of the other factors
which combine to produce the healthy tone of the plant.
Changes of temperature bring the organism nearer to or
further from that optimum point at which it is in the most
complete state of thermotonus and are responded to in
various ways accordingly. Any alteration in the fluid
contents of a cell brings about a change in what we may
call the tonic tension of that cell, in which condition the
permeability of the protoplasm exists at its best, and again
an appropriate response is made.
In considering broadly the result of stimulation we
must notice at the outset that it provokes a purposeful
response. The living substance appears to have a definite
aim ; it may be to remove the stimulating cause if the
latter affects it prejudicially ; it may be to readjust its
manifold forces to the new conditions to which the environ-
ment is suddenly or gradually subjecting it.
The means which the plant avails itself of are seldom
abrupt and violent, like the manifestation of muscular con-
tractility, but more frequently take the form of the modifi-
cation of some rhythm which is characteristic of its
behaviour. A few cases of sudden and sharp change are
met with, as when the leaf of Mimosa droops on being
touched, or when that of Dionaa rapidly closes over its
STIMULATION AND ITS EESULTS 371
captured prey. Less conspicuously purposeful are those
changes in metabolism which are brought about in conse-
quence of interference with the supply of food or oxygen,
but even here evidence of purpose can be found if
sought for.
To understand the purposeful changes in the behaviour
of plants when they encounter modification of their sur-
rounding conditions, we may consider briefly the nature of
their environment. In the case of an ordinary terrestrial
plant we find it to be as follows. The root system is
embedded in the soil, among the particles of which the
young root branches ramify as they grow, and to them the
root-hairs become firmly attached ; the soil undergoes
usually only comparatively small changes of temperature,
but is subject to a great deal of variation with respect to
the amount of water it contains and the distribution of
that water ; it is composed of various materials, partly
organic, partly inorganic, many of which are eagerly sought
for by the plant, but others of them are of no use to it ;
of the former, some though valuable are not in a suitable
condition for absorption. The stem rises vertically into
the air and bears its branches and leaves ; the air sur-
rounding them contains a varying amount of aqueous
vapour, together with a fairly constant quantity of carbon
dioxide. The sub-aerial portion is subjected to the alter-
nation of day and night, involving almost constant changes
of degree of illumination, together with varying direction
of the incident rays. During these times it meets with
considerable variations of temperature and moisture as
well as light. The whole plant is constantly acted on by
the force of gravity. The subterranean portions are less
affected by light, but they nevertheless receive a certain
amount through the crevices between the particles of the
soil, which varies from time to time both in amount and
in direction. The environment, though to a certain extent
constant, is nevertheless continually varying in these
B' B 2
372 VEGETABLE PHYSIOLOGY
respects, so that no two plants are situated exactly simi-
larly though they may be growing side by side.
The surroundings of an aquatic plant, though in some
respects very different from those of a terrestrial one,
exhibit the same general features and are subject to almost
as frequent disturbances, though a watery environment is
more uniform than a sub -aerial one.
We have considered already the effects which are pro-
duced by extremes . of light and darkness upon the be-
haviour and the structure of plants. We have however
still to examine the rhythmic excitations to which plants
are subjected' by the variations of illumination which
accompany the alternation of day and night. These are not
accompanied in every case by conspicuous responses which
can be easily observed, but certain plants exhibit a some-
what curious behaviour under these conditions. This is
especially connected with the positions of their leaves, which
assume different positions during the day and the night.
This sensitiveness to the alternation of light and darkness
is not however confined to ordinary foliage leaves, but is
in many cases shared by cotyledons also. The degree of
sensitiveness varies greatly in different plants.
This form of irritability is manifested in a very marked
degree by many plants of the Leguminosce, the Oxalidacece,
and a few other Natural Orders. Mimosa pudica may be
mentioned as especially favourable for examination in this
particular. When this plant is removed from light to
darkness its leaflets droop, and the opposite pairs become
closely approximated to one another, so that their upper
surfaces are in contact. On being restored to light they
separate again and attain their former expanded condition,
but little time intervening before the change of position is
assumed in either case. Another very good instance is
afforded by Desmodium gyrans, the so-called Telegraph
plant, the rhythmic movements of whose lateral leaflets
have already been spoken of. During the day its leaves
are extended almost at right angles to the stem (fig. 155, A) ;
STIMULATION AND ITS RESULTS
373
as night draws on, the terminal leaflets droop till they
assume a position almost or quite parallel to the stem
(fig. 155, B). The leaves of many others take up still more
curious positions, in some cases becoming twisted on their
petioles, or folded together in various ways. In some, as
in Nicotiana glauca (fig. 156), they rise instead of falling
and become somewhat closely approximated to each
other.
These changes of position are generally spoken of as
nyctitropic or sleep movements, though the latter term is
FIG. 155. — Desmodium gyrans. (After Darwin.)
A, stem with leaves as seen during the day ; B, a similar stem with leaves in the
nocturnal position, pointing downwards.
misleading if it be interpreted to mean a sleep similar to
that of animals. The latter phenomenon is attended by a
temporary suspension of sensitiveness, which is not neces-
sarily the case with the movements which we are discussing.
It is not difficult to prove that these curious changes of
position are effected in response to the stimulation of the
alternation of light and darkness, or to a rhythmic differ-
ence in the amount of light which they receive. The
374
VEGETABLE PHYSIOLOGY
accompanying rhythmic variation of temperature no doubt
in some cases also plays a part in the stimulation.
If a plant which changes the position of its leaves
as described, is placed for a time under constant conditions
such as darkness, the periodic movement is soon very
much interfered with, even before the effect of darkness is
evident in the loss of tone. If the rhythmic stimulus
is not regularly applied the movement ultimately stops.
The cessation is not however abrupt, but with most plants
FIG. 156. — Nicotiana glauca. (After Darwin.)
A, shoots with leaves expanded during the day ; B, the same in the
nocturnal position.
the movements will continue for at least a day. The
rhythm of the nyctitropic movement is excited by the
stimulus, and is dependent for its permanency upon the
continuation of the stimulating changes. Plants which
are found in other countries to show this sensibility will,
when cultivated in England, perform the movements at
the normal hours, and not at times corresponding to the
occurrence of day and night in the countries from which
they come. Nor is it the mere alternation of day and
STIMULATION AND ITS EESULTS 375
night which they appreciate ; it is rather the difference
between the illumination they receive during the two
periods which constitutes the stimulus, for some of them
will not assume the nocturnal position unless they have
been brilliantly illuminated during the day. The degree
of sensitiveness in this case is not so great as in those
where the diurnal and nocturnal positions are always
regularly assumed.
The peculiar movements which the leaves perform in
response to this stimulus are brought about by different
mechanisms in different cases. In young leaves they are
attendant upon growth, and are brought about by varia-
tions of turgescence upon the two sides of the leaf or its
petiole, which are frequently followed by growth. We
have seen that during growth the internal turgescence
varies rhythmically, and leads to the curious movements
of nutation or circumnutation. The actual nyctitropic
movement is in these cases a modification of the extent of
the circumnutation, the original rhythm being affected by
the stimulus. The leaves which exhibit it can be seen
by careful observation to be circumnutating during the
day. When they assume their nocturnal position it
is generally effected by their describing a much longer
ellipse than that of their ordinary movement. In some
cases only a single ellipse is described during the twenty-
four hours ; in others two ellipses, the nyctitropic one
being much the greater in amplitude. In yet other cases,
several ellipses may be described in the same time.
Adult leaves which show this movement do so by
virtue of a special pulvinus, a kind of motile organ which
is developed at that part of the leaf-stalk which joins the
stem. This structure has special developments of paren-
chyma on its upper and lower sides (fig. 157), which
become alternately turgid, and cause the leaf to droop and
to rise accordingly. These leaves generally exhibit the
movement for a much longer period than those in which
it is brought about by variations of turgescence accom-
376 VEGETABLE PHYSIOLOGY
panying or preceding growth. This naturally follows from
the fact that the growth of leaves is not as a rule very
prolonged.
That these movements are essentially dependent on
the power of the protoplasm to receive impressions from
without, or in other words upon its possession of tone, can
FIG. 157.— PULVINUS OF Mimosa.
a, &, the succulent parenchyma of its upper and lower sides ; c, bud ;
d, parenchyma of Bachis ; e, pith.
be seen from a study of the conditions under which they
are performed. When the soil is too dry, or when from
any other cause the protoplasm in the cells is not supplied
with water in sufficient quantity, they cease. When the
temperature is too low they are interfered with. Violent
disturbance of the protoplasm by shaking the plant will
STIMULATION AND ITS EESULTS 377
in some cases prevent their occurrence for one or two
nights.
The purpose of the movement is probably to protect
the delicate leaves from excessive radiation, which affects
them very prejudicially. Their upper surfaces are
especially liable to be injured in this way, and it is
noteworthy that in all cases these surfaces are most
sheltered when they take up their nocturnal positions.
Often the upper surfaces of leaflets are then closely ap-
proximated together ; in Bauhinia the leaf folds itself
upon its mid-rib as an axis, so as to hide completely the
ventral face.
Movements which bear a striking superficial resem-
blance to the nyctitropic movements of leaves are those of
the opening and closing of certain flowers, which take
place with astonishing regularity and precision at certain
hours of the morning and evening. Though they seem
to be influenced by the alternation of light and darkness,
it is more probable that they are really stimulated by the
changes of temperature which accompany such alternation.
These variations, to be effective, must lie however within
the range already indicated as being necessary for the
manifestation of irritability at all. The movement is due
to rhythmically varying turgescence of the cells upon the
two faces of the growing zone of the floral leaves, which is
in these cases a narrow transverse band situated near
their bases. This change of the turgescence is followed
in many cases by actual growth, and as the latter is not of
prolonged duration the flower can only open and close a
few times while it is attaining its maturity.
Besides the general reactions of protoplasm to varia-
tions in those features of the environment which bring
about modifications of its general tone, and which thus
affect more or less the whole plant, we find instances of
special sensitiveness in various parts to influences which
are not appreciated by the whole of the living substance.
Of these the most prominent are lateral light, gravity,
378 VEGETABLE PHYSIOLOGY
contact with foreign bodies, moisture, and certain chemical
stimuli. One or two other cases of special sensitiveness
affecting only particular organisms may also be discussed.
LATERAL LIGHT.— The effect of the lateral incidence of
light may be studied very easily in the case of young
seedlings. When one of these is so placed that one side
of its stem is more brightly illuminated than the opposite,
a curvature soon appears in the part which is actively
growing. This is of such a nature, and takes place to such
an extent, as to cause the axis of the plant to take up a
position in which it is parallel to the direction of the
incident rays.- It manifests itself in some cases very
rapidly, in others more slowly. This response to the
stimulus of a lateral illumination is not confined to the
stems of seedlings, but may be seen to a greater or less
degree in many adult plants. It is a matter of common
observation that geraniums grown in a window all bend
their stems and petioles towards the illuminated side.
In other cases the same stimulus may produce an
opposite effect. When certain young roots are exposed to it,
they curve so as to place themselves in the same position with
regard to the incident rays, but with their growing apices in
the opposite direction. Stems are said accordingly to grow
towards, and roots away from, the light-source. This
behaviour is not, however, confined to roots, it is exhibited
by the tendrils of Bignonia capreolata, the peduncles of
Cyclamen persicum, and by many other organs.
Leaves in many cases show a similar sensitiveness, but
the position they assume is different again. They place
themselves so as to present their upper surfaces at right
angles to the incident rays.
These phenomena, thus associated with the incidence
of a lateral light, are spoken of as heliotropism, aphelio-
tropism, and diaheliotropism respectively. The advantages
thus secured are in some cases very obvious ; thus the
heliotropism of a stem places its leaves in the most favour-
able position for the action of the chlorophyll in the
STIMULATION AND ITS EESULTS 379
process of photosynthesis of carbohydrate material ; the
same object is secured by the diaheliotropism of such
leaves as exhibit it ; the apheliotropism of a root assists it
in penetrating into the crevices of the soil. The tendrils
of Bignonia are aided by it in coming into contact with a
support about which they can twine. The apheliotropism
of the peduncles of Cyclamen, which are bent downwards
in a hooked fashion, enables them to grow towards the
soil, into which they press the capsule, thus burying the
seeds.
The response to the stimulus varies sometimes with
the age of the organ. The hypocotyl of the Ivy is helio-
tropic when young, but becomes apheliotropic when old.
The degree of sensitiveness varies very greatly in
different organs. Some of the seedlings of Phalaris
examined by Darwin responded to a degree of illumination
so feeble that it was hardly sufficient to cast the shadow
of a pencil upon a piece of white paper held close behind
it. The rapidity of the response also varies, some organs
bending almost immediately, while others do so much more
slowly. To this point we shall return later. The move-
ment of apheliotropism is usually much slower than that
of heliotropism.
The bending is not caused by a direct interference of
the light with the part actually growing. It would seem
at first as if the retarding effect of light upon growth
might explain the bending of the organ towards the
light-source, the non-illuminated side continuing to grow
and the illuminated one being prevented from doing so.
This explanation is directly contradicted by the phenome-
non of apheliotropism. It is moreover proved to be an
insufficient explanation by the fact that the part which is
sensitive to the stimulus is not the part which actually
bends. Darwin showed this by preventing the access of
the light to a small region about one-tenth of an inch in
length close to the tip of the seedling, when he found that
the heliotropic curvature did not take place, although the
380 VEGETABLE PHYSIOLOGY
normally bending part was illuminated. Further, when
the region normally curving under the influence of the
stimulation is mechanically hindered from bending, the
curvature takes place at a part a little lower down, which
normally remains straight.
When the lateral light is fairly intense the resulting
movement takes place uninterruptedly; when it is only
weak the position is assumed by a series of zigzag move-
ments, indicating that the new movement is an exaggera-
tion of the ordinary circumnutation of the part. When
the final position is reached the organ is found to circum-
nutate about the new direction of the axis.
A somewhat similar, response to the influence of a
lateral light is exhibited by many unicellular organisms.
When these are exposed to oblique illumination they take
up a definite position with regard to the incident rays,
placing their long axis parallel to them if the light is weak
and at right angles to them if it is intense. This behaviour
is known as phototaxis ; it is exhibited by the zoospores of
many of the Algae and by certain Desmids.
Before leaving the subject of the effect of a lateral
light in inducing these movements we may point out that
the phenomena of heliotropism and apheliotropism must
be distinguished from those of photo-epinasty and photo-
hyponasty, which were alluded to in the last chapter (p.
364). The difference is easily seen, for in the latter cases
the result of the access of the light is the same, whatever
be the portion of the organ stimulated. The thallus of
Marchantia becomes convex on the dorsal and concave on
the ventral side, whether the light impinges on the one or
the other. In the case of a heliotropic curvature the side
which is stimulated always becomes concave ; in that of an
apheliotropic one the stimulated side becomes convex.
GRAVITATION. — The force of gravitation exerts an influ-
ence upon plants which somewhat resembles that of lateral
illumination. Most stems grow vertically upwards into the
air ; primary roots grow vertically downwards into the
STIMULATION AND ITS EESULTS 381
soil. A few organs, among which may be mentioned certain
rhizomes and the runners of many plants, grow at right
angles to the direction of gravity. When one of these is
placed at an angle from the position which it usually
assumes, a curvature of the growing organ results, which
lasts till the normal attitude is regained. Thus when a
young seedling is detached from the earth and laid upon
its side, the stem gradually curves through an angle of
90° and becomes erect, while the young root curves in the
opposite direction till it points vertically downwards.
Similarly when a runner is placed vertically, its apex is
slowly deflected till it again grows parallel with the soil.
These movements are termed apogeotropic, geotropic, and
diageotropic respectively.
To prove these movements to be responses to the
stimulus of gravitation it is necessary to eliminate the
action of the latter force, and to observe the direction of
growth under the new conditions. This can be done by
causing the plant to grow supported upon an apparatus
known as a Klinostat, one form of which, devised by
Mr. Horace Darwin, is shown in fig. 158. The plant,
growing in a flower-pot, is fixed in a wooden box B, which
is secured by a thumb-screw tli to the plate pi ; the box
is cubical in form and can be fixed either as shown in the
figure, or with the axis of the pot at right angles to the
spindle ~k of the klinostat. The plate pi is attached to
this spindle, which ends in a point turning in the upper
end of the left-hand support s. The spindle is also sup-
ported at g on the friction wheel fr. The spindle (with
the plant attached) is made to rotate by means of a band
of silk dr, passing round the wheel w, and also round a
pulley on one of the axles of an American watch-action
clock c, which is attached by means of the screw E to
the support s. By passing the driving-gear over the large
pulley TF, the spindle is made to rotate once in thirty
minutes. By arranging wheels of different sizes at this
point, the period of rotation can be made longer or shorter.
382 VEGETABLE PHYSIOLOGY
In using the apparatus care must be taken that the centre of
gravity of the plant and flower-pot shall coincide with the
spindle, or the clock will have varying amounts of work to
do in different parts of the rotation.
For ordinary purposes it is well to arrange the apparatus
so that a complete revolution of the spindle may be made
once in twenty minutes. When the plant is placed in a
horizontal position on the revolving plate, every face of its
axis comes successively under the influence of gravity, so
that all parts of it are affected equally and similarly. It
is then found that no curvature of the horizontal axis of
the plant occurs in any direction.
Another experiment, due to Knight, pointing to the
same conclusion, is that of growing a plant upon a rapidly
revolving wheel mounted on a vertical axis. When the
speed of the revolution is sufficiently great, though the
plant is exposed all the time to the action of gravitation,
the centrifugal force of the apparatus is so much greater
than the force of gravity that the plant does not respond
to the latter. Instead, it responds to the stimulus of the
rapid rotation or centrifugal force, and the root grows
outwards from the centre of the wheel while the stem
grows inwards towards it. The force acts much like that
of gravitation, and the plant responds to it in a similar
way, the root growing in the direction of the force and the
stem in one opposite to it. If the rotation is conducted at
less speed, so that the centrifugal force is about equal to
that of gravitation, the position assumed by the axis of the
plant is that of a resultant between the two forces, in which
it makes an angle of about 45° with the vertical.
As in the case of heliotropic curvature, the part which
receives, or is sensitive to, the stimulus is not the part
which curves. In the case of a root it has been demon-
strated by Darwi,n, and more recently by Pfeffer, that the
sensitive part is the tip, while the curvature takes place at
a point further back, where active growth is taking place.
384 VEGETABLE PHYSIOLOGY
This point will be discussed however more fully in a sub-
sequent chapter.
The movements of geotropism and apogeotropism are
not confined to growing organs. When the haulm of a
grass is placed horizontally on the ground, as" is the case
when a patch of wheat or other cereal is beaten down by
wind or storm, it after a time again becomes erect. The
new position is due to the renewal of growth on the under-
sides of the swollen nodes, which is excited by the stimulus
and proceeds till the stem is again vertical.
As in the case of heliotropism, the sensitiveness varies
very much in . different plants, and the response made is
not always the same throughout the life of an organ.
CONTACT WITH A FOREIGN BODY. — Many instances of
sensitiveness to this form of stimulus have been observed.
When a leaf of Mimosa pudica is handled, the leaflets all
droop downwards with great suddenness, and if the hand-
ling is very rough, all the leaves on the plant behave
similarly. When a stamen of Berberis is touched at a
point a little below the anther, the whole stamen bends
forward towards the pistil. The stigma of Mimulus, which
is composed of two lobes normally extending outwards from
each other, will, if either lobe is touched with a fine point,
close, so that the upper surfaces come into contact with
each other. When an insect alights on the surface of a
leaf of Drosera, the tentacles with which it is furnished
slowly curl over so that their terminal glands are brought
together at the exact point of irritation, and at the same
time the glands are excited to pour out a viscid slightly
acid secretion which is capable of digesting the proteids of
the insect's body. The leaf of Dionaa, the Venus's fly-
trap, which is normally widely expanded, closes with some
rapidity when a touch is applied to one of the six sensitive
hairs which spring from its upper surface. The leaf closes
as if the mid-rib were a hinge, bringing together the upper
surfaces on each side so as to imprison the body which
touches it.
STIMULATION AND ITS EESULTS 385
This form of sensitiveness is exhibited in a very striking
way by the growing apex of a young root. If a seedling
bean is taken, and its tip is stimulated by pressing it
lightly against some hard particle, or if a small piece of
cardboard is attached by a drop of gum to one side of its
apex, a curvature speedily results which causes the root to
bend away from the irritating body. If the movement
takes the sensitive part away from the latter the curvature
is slight, but if, as in the case of the attached cardboard,
the foreign body accompanies it in its displacement, the
curvature will continue until the root is coiled completely
round. The stimulus in the case of this movement must
be prolonged, differing thus from the cases already noted,
in which a mere touch is sufficient to bring it about.
The cause of this curvature must be the sensitiveness
of the protoplasm to the stimulus of contact. The part
which curves is some little distance from the apex, at which
the capacity for receiving the stimulus is located, and the
mechanism of the curvature is a modification of the distri-
bution of turgescence of the cells in the zone of growth.
It is only while that part is actively growing that the
curvature can be caused.
Another kind of curvature can be detected in the course
of the growth of young roots, which differs fundamentally
from the one just described, and the two must be carefully
distinguished from each other. If a young root comes
into contact with an obstacle such as a small stone, so that
the latter presses not upon the tip as in the case described,
but upon the region of the growing cells some little distance
further back, the resulting curvature causes the root to
bend toioards the obstacle instead of away from it. This
appears to be due to the contact injuriously affecting the
cells which are pressed upon, so that their growth is
retarded or stopped. The cells on the other side of the
root not being affected, a curvature results from their con-
tinued growth. These two capacities for curvature are of
great assistance to a root during its growth downwards
c c
386 VEGETABLE PHYSIOLOGY
into the soil. On coming into contact with a particle of
earth which is directly opposed to its progress, the tip
becomes first stimulated and the subsequent curvature
causes it to be deflected past the obstacle if it is not too
large. A little further elongation, followed by an apogeo-
tropic movement, brings the growing zone into contact
with the particle and the converse curvature follows, so
that the root grows round the obstacle, and then resumes its
normal direction downwards, under the stimulus of gravity.
Perhaps the best instance of sensitiveness to slight
contact is afforded by the behaviour of twining organs,
tendrils, petioles, and climbing stems, the twining of these
organs round their supports being altogether due to it.
Very great differences of irritability are met with, tendrils
generally possessing it in a very high degree, but climbing
stems often exhibiting it very feebly ; indeed some observers
deny that they possess this form of sensitiveness. In the
most sensitive cases a very slight touch is sufficient to
bring about a perceptible curvature in a very short space
of time. Darwin found that the contact of a small loop of
thread, weighing not more than -^ grain, with one of the
tendrils of Passiftora gracilis, caused it to bend, while a
mere touch with a hard substance induced it to assume the
form of a helix in about two minutes. This is perhaps the
most sensitive tendril known ; with others a stronger
stimulus is needed, and the time taken for the response is
longer, the irritability varying considerably. Slight rub-
bing is more effective than mere contact.
The behaviour of tendrils in twining is somewhat pecu-
liar. When young they are generally circumnutating, and
if in their movement they come into contact with any
foreign body, they begin to curve round it. If the contact
is not prolonged the tendril will curve for some time, but
will ultimately straighten itself and move as before, till it
touches something else. If, on the other hand, the body
first touched is one round which the tendril can twine, it
coils itself round it ; the stimulus thus persists and the
STIMULATION AND ITS EESULTS 387
resulting curvature increases it, bringing more and more of
the sensitive side into contact with the support, till the
latter is encircled many times by the sensitive twiner. The
coiling is seldom confined to the part of the tendril in
contact with the support, but the free part between the
latter and the axis of the plant also twists itself into a
kind of helix. If the two are not very close together this
helix usually shows two parts, the coils of which are in
opposite directions. This is however only because the
filamentous body is attached at both ends.
-A tendril, though thus sensitive to contact, does not
coil, according to Darwin, if its sensitive surface is struck
by drops of rain, nor, in the case of the Passiflora already
alluded to, if contact takes place between two tendrils.
The sensitive region varies in different tendrils, but it
cannot be so strictly localised as in the case of a growing
root. They are usually irritable on one side only, which
is slightly concave, though in some cases the sensitive-
ness extends all round them. The lower part of a tendril
is, as a rule, only sensitive to prolonged contact. Their
susceptibility further varies with their age, being greatest
when they are about three parts grown. The part which
first responds to the stimulus is usually the part touched,
but, as we have seen, the coiling also takes place nearer
their bases, so that we have an evident transmission of
the stimulus backwards, as in other cases noted. The
method of response is usually increased turgidity upon the
convex side, followed by greater growth. In many instances
careful measurements have shown that both the concave
and convex parts grow during the coiling, but in a few
cases the concave side either does not grow or becomes
actually shorter than before.
This sensitiveness to contact which is so markedly
shown by tendrils is possessed also, though to a much
smaller extent, by most climbing stems. These organs
show the movement of circumnutation very conspicuously,
the portion which takes part in the formation of the spiral
c c 2
388 VEGETABLE PHYSIOLOGY
being frequently of considerable length. This is of course
a great advantage in enabling the stem to find a support.
The continuation of the circumnutating movement after
contact with such support, has given rise to the view that
circumnutation alone will enable climbing to take place.
Consideration of the behaviour of various twining stems
with supports of various thickness has shown however
that this is supplemented by changes resulting from the
contact effected by circumnutation, and therefore from the
possession of the sensitiveness under consideration.
Twining stems show individual peculiarities in the
direction of their twisting, and in the nature and particu-
larly the thickness of the support they need. The stem
of the Hop twists in the direction taken by the hands of a
watch ; that of the Convolvulus in one diametrically oppo-
site. The direction of the twining is not however always
constant ; Darwin noticed that it was not so always even
in a single individual. In Scyphanthus elegans it was
reversed in successive internodes of the same stem. Many
of our ordinary climbers can twine up a support having
only the thickness of a piece of string ; other plants, par-
ticularly the climbers of tropical forests, need supports of
some inches in diameter.
The twining of stems is often accompanied by a torsion
of the stem, or a twisting round its own axis. This is not
however of universal occurrence.
The stimulus of contact is sometimes followed by an
outgrowth or hypertrophy of the part affected. This is
seen in the tendrils of Ampelopsis Veitclii, which on pro-
longed stimulation develop little adhesive discs, that are
closely adpressed to roughnesses in the surface of the
support and, becoming mechanically attached to them,
enable the plant to maintain a very strong hold upon the
wall or other support to which it is clinging. The roots of
Thesium show a similar property. When they come into
contact with other roots growing near them they develop
a swelling at the point of contact, from which certain cells
STIMULATION AND ITS EESULTS
389
grow out and penetrate the host, forming haustoria (fig. 159).
The parasite Cuscuta, often found growing on clover, is
affected in the same way, first twining round the clover
stem and then putting
out haustoria, which IMhii^i m in 7,
penetrate its tissues
(fig. 160).
Another form of
irritability is exhi-
bited by many grow-
ing shoots, which is
perhaps somewhat
akin to sensitiveness
to contact. If a shoot
is gently struck late-
rally several times
near its base, its apex
curves over towards
the side struck. If
the blows are given
near the apex, the resulting curvature is in the opposite
direction.
The mechanism whereby the response to the stimulus
of contact is brought about in growing organs we have seen
to be an increased turgidity on the convex side, followed
by growth. In those cases where the organ is mature it
is evident that growth can have nothing to do with the
movement. In these instances we have rather to do with
a modification of turgescence, involving a redistribution
of the water contained in the organ. The falling of the
leaflets and leaves of Mimosa is due to a sudden change
in the protoplasm of the cells on the lower sides of its
pulvini, in consequence of which water escapes from them
into the intercellular spaces between them. It is attended
by a change of colour, the pulvinus becoming of a deeper
green in consequence of the replacement of the air there
by water. If a leaf is cut off just above the pulvinus and
FIG. 159 — Thesium alpinum. PIECE OF A
BOOT WITH SUCKER IN SECTION. X 35.
(After Kerner.)
390
VEGETABLE PHYSIOLOGY
the plant allowed to recover from the effects of the injury,
subsequent stimulation of an adjacent leaf causes water to
FIG. 160. — SECTION OF STEM OF DICOTYLEDONOUS PLANT ATTACKED BY
HAUSTOKIA OF Cuscuta.
exude from the cut surface of the pulvinus. The cases
of the irritable stamens and stigmas are probably to be
STIMULATION AND ITS EESULTS
391
explained similarly. The closing of the leaf of Dioncea
(fig. 161) is due also to a redistribution of the water in
the cells, brought about by a rapid change in the proto-
plasm, perhaps akin to contraction. In Drosera the in-
flexion of the tentacles has been found to be preceded by a
peculiar churning movement of the protoplasm in the cells
upon the side which becomes concave. This movement,
which Darwin, who discovered it, called aggregation, is
attended by a loss of turgidity.
FIG. 161.— LEAF OF Dioncea muscipula.
1, open ; 2, closed ; 3, one of the sensitive spines ( x 50) ; 4, glands on
the surface of the leaf ( x 100).
MOISTUEE.— Sensibility to variations in the moisture of
the environment is not so widely distributed as are the
forms of irritability hitherto discussed. It is exhibited
among green plants chiefly by young roots and by the
rhizoids of the Hepaticce ; it also occurs in the hyphae of
certain Fungi. These organs tend to curve in the direc-
tion of a moist surface if they are growing near one.
When young seedlings are cultivated in a vessel which
392 VEGETABLE PHYSIOLOGY
contains moist sawdust or sand and is perforated so as to
allow the rootlets to protrude, these at first grow vertically
downwards, according to their geotropism. Soon after
they protrude they curve to a greater or less extent towards
the moist surface, as if seeking the moisture. This
behaviour can be seen more easily if the vessel is inclined
at an angle to the vertical. The phenomenon is known
as hydrotropi-sm. The root-tip, as in other cases, is the
sensitive part, while the curvature takes place further
back, where growth is most active. Negative hydrotropism
or aphydrotropism is very rare, being exhibited only by
some of the -Myxomycetes, which move away from
moisture.
The advantage of this form of sensibility is evident in
the case of the root, which by virtue of it is drawn towards
the moisture of the soil as it penetrates between its
particles.
A curious instance of appreciation of lack of moisture
is afforded by Porlieria hygrometrica, which under such
conditions closes its leaflets much as nyctitropic plants do
when light gives place to darkness.
CHEMICAL STIMULI. — We have already alluded to the
fact that the various metabolic phenomena of plants are
influenced very considerably by changes in the composition
of the sap which the cells contain ; that certain consti-
tuents stimulate the protoplasts to initiate or to alter
particular reactions in those cells. Besides these responses
to chemical stimuli there is evidence that vegetable
protoplasm can modify its normal behaviour in other
ways when exposed to similar influences. This form of
sensitiveness is less widely distributed than those which
we have just discussed, but instances of it are fairly abun-
dant, especially among the more lowly forms of plants.
A certain number of unicellular organisms are strongly
affected by the presence of free oxygen. The most inte-
resting case of this sensibility is that of Bacterium termo ;
when a number of these plants are placed in a drop of
STIMULATION AND ITS EESULTS 393
water upon a slip of glass and examined under the micro-
scope, they are found to collect at the edge of the cover-
glass. If a small green Alga is placed in the drop of
water with them, and the slide exposed to light of sufficient
intensity to enable the decomposition of carbon dioxide to
take place, the coincident evolution of oxygen attracts the
bacteria, which at once swarm round the Alga. So sensi-
tive are they to this attraction, that if the spectrum of
sunlight is thrown upon the Alga, the bacteria accumulate
at those parts which are illuminated by the red and blue
rays, which we have seen to be capable of effecting the
evolution of the oxygen. This response to the attraction
of oxygen is not confined to these bacteria ; it is exhibited
by many zoospores and also by the plasmodia of some of
the Myxomycetes.
When the necks of the archegonia of the Bryophyta
and PteridopJiyta open with a view to the fertilisation of
the oospheres which they contain, they discharge a certain
mucilaginous fluid, which attracts to the organ the free-
swimming antherozo'ids. Careful experiments have been
made in many cases to ascertain what is the nature of the
attraction, and it has been found that the mucilage contains
various substances which the antherozo'ids seek. In the
cases of the Ferns and some Selaginellas, it has been
determined that the attractive body is malic acid. When
a capillary tube containing a weak solution of this substance
is inserted into water containing some of the antherozoids,
they make their way very quickly to the orifice of the tube.
They are very sensitive to the presence of the acid, being
guided apparently in their movements by very slight
differences of concentration. When the acid exceeds a
certain strength they avoid it as earnestly as they seek it
when it is in greater dilution.
In the case of the Mosses the attractive substance is
cane-sugar. Alkalies in any degree of concentration repel
the antherozoids of both groups.
A similar sensitiveness to chemical stimulation marks
394 VEGETABLE PHYSIOLOGY
the plasmodia of the Myxomycetes. They move slowly
towards a watery extract of tan, but retreat from a solution
of sugar, glycerine, or certain neutral salts. The zoospores
of Saprolegnia are attracted by a solution of extract of
meat.
The sensitive tentacles of Drosera can respond not only
to contact, as already described, but also to various sub-
stances placed upon the leaf. They are easily induced to
bend by drops of liquid containing proteid matter, such
as solution of albumin, or milk. Certain inorganic salts,
especially carbonate of ammonia, produce the same effect.
A curious instance of this kind of irritability has been
put on record by Miyoshi. He cultivated certain fungi in
gelatin containing a small proportion of sugar. Under the
stratum in which the hyphse were ramifying, he placed
another containing a larger proportion of sugar, and between
the two arranged a membrane. The hyphae very soon grew
towards the stronger sugar solution, and to reach it pene-
trated the membrane.
Other instances of similar behaviour might be quoted.
To this form of sensitiveness the name of cliemotaxis has
been given.
A few other forms of irritability have been observed in
various plants. Certain plants growing in currents of water
take up a definite position with regard to the direction of
the current, some growing with it, others against it.
Certain plants appreciate small differences of temperature
and modify their growth accordingly. Almost all show a
peculiar relationship to their substratum, stems growing
out from it and roots into it in a direction at right angles
to the surface. This can be seen by cultivating them so
that they do not emerge in the normal direction but from
the side of a cube of earth. They do not long maintain
this direction, as they speedily feel the influence of light
and gravity. If however appropriate means are adopted to
eliminate these, the growth is always at right angles to the
surface of the soil in which they live.
STIMULATION AND ITS EESULTS 395
If we now return to the study of the rhythmic changes
which we have seen to be essentially characteristic of
vegetable protoplasm, we see that while rhythm is no doubt
inherent in plants it lends itself especially to such changes
as are caused by stimulation. It is indeed this feature
which is especially brought out by the various responses
made to changes in the environment. While it occurs with
some regularity when conditions are kept constant, it is
easily affected by external causes. The effect of continuous
darkness, or of too great cold, or other abnormal conditions,
is that the rhythmic movements are made irregular and
ultimately stop. In many cases differences in the degree of
illumination during the day affect the readiness with which
the nyctitropic movements of the leaves are brought about.
After a day of brilliant sunshine they set in more quickly
than after one of dull light.
These movements may show indeed a secondary in-
duced rhythm superposed upon a normal one. The move-
ments of heliotropism, geotropism, &c. may be looked
upon as instances of this. We have seen that they are
based upon the ordinary movement of circumnutation, and
are in fact exaggerations of it. As the latter is generally
a manifestation of a rhythm of turgidity in the cells affected
we have in them a case in point. In other cases the
tendency to rhythmic change can be demonstrated by the
production of an altogether artificial rhythm induced by
submitting the plant to intermittent stimulation. F. Darwin
and Pertz have described a very interesting experiment of
this nature. A plant was fixed to a spindle placed hori-
zontally, in a modification of the klinostat, and was by an
arrangement of clockwork made to undergo a semi-revolution
at intervals of thirty minutes. The force of gravity thus
exerted its effect upon alternate sides for this interval of
time, so that each side of the stem became slightly convex
apogeotropically in turn. After a period of exposure upon
the instrument the clockwork was stopped. Instead of the
side which was then undermost increasing its convexity
396 VEGETABLE PHYSIOLOGY
till the stem was vertical, the two sides continued to
Become alternately convex, as if the reversal of the instru-
ment was still taking place. There was, in fact, an artificially
induced rhythm manifested.
While the movements of heliotropisni show the super-
position of an induced rhythm upon a natural one, a conflict
between the two can be observed in many organs. The
heliotropic curvature is not brought about by a direct move-
ment of the bending organ, but by its describing a series of
ellipses. The organ at the time of the incidence of the light
stimulus is performing its ordinary circumnutation, the apex
describing a circle. The effect of the stimulus is to turn
that circle into an ellipse ; when the rhythmic impulse
coincides with the stimulus of the light, the movement is
accelerated and the resulting curve takes the direction of
the long axis of the ellipse ; when the two act in the opposite
direction to each other, the short curve of the same figure
is described. The same result is obtained under the stimulus
of geotropism when the stem or root has by any means
been inclined from the vertical. The ordinary rhythm of
circumnutation is resumed when the new position has been
assumed and the stimulus consequently no longer acts.
The slow response to the action of a stimulating force
may frequently be explained in the same way. Often how-
ever the long delay is due to peculiarities in the protoplasm
which will be discussed in the next chapter.
The various positions which are assumed by the various
sub-aerial organs of plants are evidently those in which
they can react most advantageously with their environment.
It must be borne in mind however that in every case
during natural life the plant is receiving coincidently several
kinds of stimulation, the effect of some being not infre-
quently antagonistic to that of others. It is not easy to
discriminate between these, nor to say how the influence of
each helps to determine the resultant response. This is the
more difficult as not only the stimuli themselves but their
relative potencies differ continually.
397
CHAPTEE XXV
THE NERVOUS MECHANISM OF PLANTS
IT is difficult to refrain from coming to the conclusion, from
a consideration of the facts which have been discussed in
the last two chapters, that the nervous system of the
animal kingdom is represented in the vegetable one. That
plants are sensitive to variations in the conditions sur-
rounding them, and that the responses they make to such
variations are purposeful and conduce to the well-being of
the organism, is abundantly evident. The response to any
external stimulus, moreover, has been seen to be depen-
dent upon the plant being in a condition of tone, that is of
health and vigour. If its well-being has been interfered
with by such disturbances as deprivation of light, or lack
of oxygen, or exposure to too high or too low a temperature,
no response is given, for its irritability is in abeyance or
destroyed. The lack of response is not due to a failure in
the motor mechanism by which the change is brought
about, but by an absence of power to realise the altered
conditions which would constitute a stimulus to an organism
in a condition of full health. The age of the organism
again has been seen to have an important influence upon
its power of receiving impressions and its behaviour in
responding to them.
We have already called attention to the fact that the
responses made to stimuli of different character afford
clear evidence of purpose. No reply is at all haphazard,
but is devoted especially to some definite object which is
closely related to the stimulus.
Another consideration which bears upon this question
398 VEGETABLE PHYSIOLOGY
is that an extremely small stimulus is able to bring about
a very considerable effect, and that there is no direct or
simple ratio between the intensity of the stimulus and the
extent of the response, whether this takes the form of move-
ment or chemical change. The tendrils of Passiflora,
already alluded to, can be caused to move by the contact
with them of a small piece of thread, weighing not more
than T!2- of a gram> and tne resulting movement will be of
considerable extent and prolonged for some time. The
sensitive hair of the leaf of Dio-naa needs only a touch to
cause a rapid movement of the whole leaf-blade ; the
pricking of the staminal filament of Berberis causes a
considerable movement of a relatively bulky body. The
seedlings of Phalaris bend with some speed towards a
light which is not sufficient to cause a visible shadow at
the distance at which they are placed from it.
It can hardly be imagined that such slight disturbances
can act mechanically upon the parts that move. This point
is illustrated by the observation made by Wiesner, that if a
part which responds only to the stimulus of lateral light is
exposed for some time to such an illumination, and then,
before the heliotropic curvature has begun, is removed
into darkness, it will slowly bend towards the side which
has been stimulated. The same observation has been
made by other observers in the case of the stimulus of
gravitation. There is no explanation possible other than
that the stimulus brings about changes in the protoplasm
of the cells of the moving part, which slowly modify their
relation to the water of their contents, so that a great
alteration of their turgidity results. Moreover, the separa-
tion of the part stimulated and the cells which are the
seat of the resulting action, implies that there must be in
the plant a means of rapidly conducting such external
impressions from one part to another.
If then we admit that there is even a rudimentary
nervous system in plants, we may proceed with an inquiry
into the degree of its differentiation, and tfre completeness
THE NERVOUS MECHANISM OF PLANTS 399
of the parallelism which it may be expected to show with the
corresponding system in the animal kingdom.
The latter, in the most completely organised beings,
can be shown to possess three distinct parts : one by means
of which external stimulation is received and appreciated ;
another whereby movements, &c. are caused ; and a third
which is a regulating and controlling part, and which can co-
ordinate the responses to stimulation, or can initiate move-
ments, &c. in its absence. There are also definite paths or
channels by which the three are brought into connection with
each other, generally by impulses passing along such paths
in definite directions. In the higher animals these are well
differentiated from each other ; we have the sense-organs,
each devoted to and fitted for the appreciation of particular
stimuli. We have various motor mechanisms, usually con-
sisting of muscles or glands which are thrown into activity
in consequence of the reception of impulses by sense-
organs. It may appear to be straining matters somewhat
to class these as part of the nervous system, but it does not
appear wrong to do so in the sense that they are the means
by which alone the working of the more particularly
nervous elements of that system can be detected. The
nervous and motor systems are indeed so closely connected
that for the purposes of this discussion no inconvenience
will result from classing them together. In the animal
we have nerve-cells occurring singly or in groups, forming
very large aggregations such as the brain, or smaller ones,
the nerve ganglia. All such aggregations, or even single
cells, are concerned in the task of co-ordinating stimuli and
responses, or regulating the general life of the organism.
Lastly, we have well-differentiated nerves which serve as
the means of communicating between the three other
factors already mentioned. Each nerve-fibre ends in a
sense-organ or a motor mechanism, such as a muscle or a
gland-cell, and in a nerve-cell belonging to the co-ordinating
apparatus.
We can easily recognise in plants certain structures
400 VEGETABLE PHYSIOLOGY
which may not inaptly be termed sense-organs, as we can
localise in them the power of perception of stimulating
influences. Darwin found that the seedlings of Phalaris
were not sensitive to the faint light employed in his experi-
ments, except at a small region extending about ~ inch
from the apex. If this part were covered by an opaque
screen in the shape of a little blackened cap of not sufficient
weight to cause any flexion of the stem, the seedlings no
longer bent towards the light. We have already noticed that
the root-tip at a very short distance from the apex is the
only part which is appreciative of contact with a foreign body
in such a way as to cause the growing part to set up a
curvature that will carry the tip away from the obstacle.
The same region has been shown to possess the power of
appreciating the stimulus of gravitation. This has been
shown by Pfeffer in a very ingenious manner. He caused
the roots of various seedlings, especially using Vicia faba,
to grow into small and light glass tubes, closed at one end,
and bent at a right angle about j-1^ inch from that end. The
cultivation was carried on on a klinostat for about twelve
hours, when the root had penetrated to the end of the tube,
and had consequently become sharply bent at a right angle
about jijr inch from the apex. Eoots so prepared were then
allowed to continue their growth after being placed in
various positions. When the terminal portion was vertical,
and the long part of the root consequently horizontal, the
root continued to grow without any curvature ; when these
conditions were reversed a geotropic curvature resulted, which
continued as long as the tip of the root was mechanically
prevented from becoming vertical. Other observers have
proved the same thing in different ways. Cisielski ampu-
tated the tips of certain rootlets, and laid them horizontally
on a support. They did not then show any sensitiveness
to gravitation, until they had recovered from the wound
and a new root-tip was developed upon each. As soon as
the new tip was formed, the rootlets showed a power of
reacting to the stimulus of gravitation, and the curvature
THE NEKVOUS MECHANISM OF PLANTS 401
resulted in the usual place. If we turn to the reaction of the
leaf of Dionaea to contact, we find that the whole leaf may
be somewhat roughly handled without closing, so long as
no contact is made with the hairs, three in number (fig. 162),
which arise upon a particular portion of the blade. So
soon, however, as one of these is touched, the leaf closes.
It is impossible to avoid the conclusion that we have to
do in these instances, which are only representative ones,
FIG. 162. — LEAP OF Dioncea muscipula.
1, open ; 2, closed ; 3, one of the sensitive spines ( x 50) ; 4, glands on
the surface of the leaf ( x 100).
with a localisation of sensitiveness, or the differentiation
of sense-organs. If we compare them with physiologically
corresponding regions in the animal we find a certain
agreement, though it must not be pressed too far. The
power of sight is very complete in the higher animals,
partly in consequence of the highly differentiated character
of the eye, but in the lower animals it becomes less and
less perfect, till in some it goes probably little further than
the power of appreciating light. This power we have seen
D D
402 VEGETABLE PHYSIOLOGY
to be possessed by certain parts of the young seedlings of
various plants in a very high degree, and by other organs
to a less extent. The sense of touch may be compared
with the power of responding to the stimulus of contact
shown by tendrils and by the tips of roots ; the muscular
sense, or power of appreciating weight, is perhaps com-
parable to the property of responding to the attraction of
gravitation, while the chemotactic behaviour of the organ-
isms described in the last chapter suggests a rudimentary
power of taste or smell, or both.
The differentiation of these mechanisms in plants is
anatomically very slight. Indeed, no dissection will ex-
hibit any special feature of the structure which can be
associated visibly with the perception of the stimulus. It
remains a property of the protoplasm of the cells in
question, but is only one among many properties that the
latter possesses. The direction of differentiation in vege-
table protoplasm is not anatomical. But such a differentia-
tion is very considerable physiologically. The degree of
sensitiveness which many of these organs possess is ex-
treme, as we have shown already by several examples.
Another somewhat remarkable fact, in view of the
peculiar character of the differentiation of these organs, is
that the same sense-organ is sensitive to many stimuli,
though in different degrees. We have noticed in the case
of the root that its tip appreciates contact, gravitation, and
differences in hygrometric condition. There is nothing
anatomical corresponding to this. If a sensitive organ is
acted upon at the same time by two stimuli which both
affect it, and which usually produce opposite movements,
the resulting position is always that which would be
caused by the stronger of the two. The organ is, in fact,
able to receive both stimulations simultaneously, and to
respond to each as if the other were not received.
If we turn to the second feature of the nervous system,
we find that the motor mechanism of the plant seems at
first to be entirely different from that of the animal.
THE NEEVOUS MECHANISM OF PLANTS 403
Closer consideration however lessens the difference con-
siderably. The motor mechanism of an animal is very
largely either muscular or glandular. The contractile
power is but little developed in vegetable protoplasm, and
when present it seems to be rather passive than active,
to produce frequently recoil, rather than true contraction.
Still, the latter is not entirely absent. We have seen that
it can be detected in the pulsation of vacuoles, in ciliary
motion, and in the cmwjing_jn^ej^
cetes. Its manifestation under an external stimulus seems
to be evident when a filament of Mesocarpus splits up
into its constituent cells as soon as an electric shock is sent
through the water in which the plant is floating.
Though the power of contraction is comparatively
seldom found, it has its representative in the power which
vegetable protoplasm possesses of resisting or assisting the
transit of water. The effect is really similar in both cases ;
in the one the disturbance to the protoplasm leads to a
contraction of its substance, in the other to its modifying
its resistance to the passage of water through it. Each
protoplasm responds in its own appropriate fashion, which
is based upon the need of the organism of which it is part.
The main requirement of most animals is freedom of
locomotion or rapid assumption by the body of new positions.
The most important duty of the plant is the regulation of
the water supply upon which its constituent protoplasts are
so dependent.
The effects of stimulation may be seen in glandular
organs in plants as well as animals. Both Drosera and
Dioncea are excited by contact to pour out on to the surface
of their leaves acid digestive secretions, which are the
result of changes in the activity of the gland-cells.
The conduction of the stimuli received is due in
animals to the existence of differentiated nerves. The way
in which it is carried out by plants has been much debated,
but since the discovery of the continuity of the proto-
plasm through the cell -walls there is little doubt that we
1> D 2
404 VEGETABLE PHYSIOLOGY
have here a similar mechanism. There is scarcely any
differentiation, but the power of the protoplasm to con-
duct disturbances from one part of the cell to another is a
matter of common observation. The connecting strands
between adjacent cells (fig. 163) will suffice to suggest how
impulses from the tip of the root may reach the growing
cells.
The co-ordination of these factors we have seen is one
of the most marked features of a highly differentiated
nervous system. In this respect we cannot note anything
in the plajat which in its elaboration or in its peculiar
FIG. 163. — CONTINUITY OF THE PROTOPLASM OF CONTIGUOUS CELLS
OF THE ENDOSPERM OF A PALM SEED (Bentinckia). Highly
magnified. (After Gardiner.)
ft, contracted protoplasm of a cell ; b, a group of delicate proto-
plasmic filaments passing through a pit in the cell-wall.
efficiency can be compared with the co-ordinating mechanism
of animals. Certain responses to stimulation can be effected,
but no definite regulation of any function shows any great
completeness. We have seen this particularly in the case
of the influence of temperature. Though a certain range
of temperature is imperative for the plant's well-being, it
has no power, or but little, to co-ordinate its own produc-
tion or expenditure of heat with the variations of tempera-
ture to which it is exposed.
Neither anatomically nor physiologically do we find
much differentiation in the direction of such co-ordination.
The plant shows an almost complete absence of the
THE NERVOUS MECHANISM OF PLANTS 405
differentiation which reaches its highest point in the nerve-
cell. There is apparently no co-ordinating mechanism
which receives the impulses from the sense-organs, and
initiates in consequence the resulting movement. One
case only has so far been put on record which even
suggests a complexity of this kind. Attention has been
called by Darwin to a peculiarity in the behaviour of the
tentacles ot Drosera, in which something of this nature is
seen. When one of these organs is stimulated, its actual
bending is preceded by a curious motility of the protoplasm
of the cells of its stalk which has been called aggregation.
If a tentacle on the surface of the leaf is excited, the
tentacles of the margin ar'e gradually inflected towards the
excited spot. If the cells of one of these marginal tentacles
are watched during the experiment, their contents are
found to undergo this aggregation, but those nearest its
apex manifest it first. If the aggregation were the direct
effect of the stimulus, those which it reached first, i.e. those
nearest the base of the tentacle, would respond first. The
stimulus, apparently, has to travel up the gland, and a
disturbance has to originate at its apex in response, this
disturbance travelling down the tentacle in the direction
of its base. Darwin has pointed out that this corre-
sponds in a measure to the reflex action of the animal
organism.
But though this co-ordinating power is very feebly
developed we cannot deny that there is a power or property
of protoplasm which represents it, even if in only rudi-
mentary form. We have already alluded to the purposeful
character of the responses to stimulation. There must be
some means by which an appreciation of the character of
the stimulus is communicated to the protoplasm, which
suggests a certain possibility of perception, which must be
the antecedent of co-ordination. We do not know whether
the fact that the response is localised depends upon the
possession of particular properties by the responding organ,
so that while the impulses set up in the sense-organ travel
406 VEGETABLE PHYSIOLOGY
in all directions through the plant, only certain cells can
be excited to change in response to them, or whether the
paths of the conduction of the impulses only take them
to the responding organ. But the fact remains that the
response bears a definite relationship to the stimulus, par-
ticularly to its locality, and to a less degree perhaps to its
intensity. If a root-tip is brought into contact with an
obstacle, the bending is invariably in such a direction as
to enable the root to pass it. When one is allowed to
impinge upon a small stone at right angles to its direction
of growth, the curvature continues till the root has turned
through a right angle, and can for a short distance, at any
rate, grow parallel to the opposing surface, till, passing it,
it can again respond to the influence of gravitation and
grow vertically downwards. The stimulus causing the
movement of hydrotropism serves to bring the root-hairs
into contact with the moist surface, thus enabling them to
discharge their appropriate function.
The behaviour of the tentacles of Drosera rotundifolia
is very interesting in this connection. The leaf is of some
size, and can therefore receive stimuli over a fairly large
area. When the tentacles bend over in response to the
alighting of an insect, they do not do so irregularly, but
always place their glandular apices directly upon the
spot which is the centre of the disturbance. This is very
definitely purposeful, the invader being captured and
digested wherever it alights, as all the tentacles are brought
to bear upon it.
The purposeful character of heliotropic and diahelio-
tropic curvatures is also very evident, the leaves being
always placed thereby in the position most favourable to
the discharge of their functions.
The very rudimentary differentiation of any mechanism
for co-ordination suggests a very immobile condition of the
co-ordinating protoplasm. There are several considerations
which support this view. In many cases the movement
of heliotropism does not commence till a considerable time
THE NEKVOUS MECHANISM OF PLANTS 407
after the access of the lateral light, the actual time varying
in different cases. Similarly the apogeotropic curvature
of a stem placed horizontally may not be observable till
the stimulus has lasted for more than an hour. We have
what is generally called a long latent period before the
manifestation of the irritability. The time is taken up
in bringing about the response to the stimulus and not in
appreciating it. The power of appreciation is generally
rapid, as we should imagine when we remember the great
degree of sensitiveness as measured by the smallness of
the stimulus which is necessary to produce an effect. The
sluggish nature of the co-ordinating mechanism can be
seen from the fact that the removal of a stimulus before
any response to it has become evident does not prevent
that response from subsequently appearing. If young
roots are laid upon their sides for about an hour and a
half, and their tips are then carefully amputated so that
they no longer perceive the stimulus of gravitation, they
will nevertheless curve after a while towards the side
which was downwards during the first exposure. The same
curvature will be seen if they are placed in a vertical
position after the amputation. The long delay in the
response may no doubt be attributed partly to the disturb-
ance set up by the amputation, but the fact that the response
to the stimulus does eventually take place shows that the
delay is due to slowness of changes in the responding pro-
toplasm and not in the part which is sensitive.
An even more striking instance of action after the
removal of the stimulus which has originated it — a so-
called after-effect — may be seen by allowing a stimulus
to operate for some time and then reversing its direction.
This can be done by fastening a root horizontally in a damp
atmosphere and, as soon as the curvature commences,
inverting it so that the side showing the slight convexity
is downwards. The curvature will continue in the original
direction for some time and will only slowly cease and be
replaced by one in the opposite direction.
408 VEGETABLE PHYSIOLOGY
We can distinguish between the general condition of
irritability or the state of tone, and these special forms of
sensitiveness which we have examined. So long as the
conditions remain favourable the general sensitiveness of
the plant is maintained, but the power of responding to
particular impressions may disappear from various causes
without any disturbance of its sensibility to others. The
power of appreciating differences in the environment varies
with the age of the plant, disappearing in some cases from
an organ while it still retains its power of circumnutating.
The effect of a prolonged stimulation is sometimes failure
to induce a movement. In the case of Dioncea this is
very marked. If a leaf is for a time mechanically pre-
vented from closing, repeated touching of one of the sensi-
tive hairs brings about an exhaustion of its power to receive
a stimulus, so that if the leaf is released a disturbance
of that particular hair evokes no response. At first it may
seem doubtful whether or no the interference with the
free response of the leaf may have so injured the motor
mechanism as to make it incapable of acting. The ex-
haustion however is shown to be that of the hair and not
of the blade by the fact that touching another of the hairs
at once causes closure.
The nervous sensitiveness is shown by this and many
other similar experiments to be capable of fatigue. A
similar suspension of power may be demonstrated by
exposing the sensitive parts to anaesthetics, such as the
vapour of chloroform or ether. The effect of these drugs
at once suggests an action similar to that which they have
on the nervous mechanism of an animal. When the effect
of the fatigue or the anaesthetic has passed off, the organ
again becomes capable of responding.
While we are able from these considerations to recog-
nise in the plant a nervous system in some way compa-
rable to that of an animal, we must clearly recognise the
limitations under which it exists. It can only be regarded
as rudimentary and as showing a very slight degree of
THE NEEVOUS MECHANISM OF PLANTS 409
differentiation. This we have seen is particularly notice-
able with regard to its co-ordinating power. Another
feature must be mentioned however before leaving the
subject. We do not find/in the plant any indication of
anything corresponding to the higher functions of the
nervous system of the higher animals. There is no evi-
dence of anything which we may compare to consciousness
or volition. Though many of the responses to stimulation
are eminently purposeful we cannot regard them as in any
way modified or held in check by any controlling power.
A stimulus will produce its due effect although the mani-
festation of that effect at the particular moment may be
followed by injurious consequences. The connection between
the sense-organ and the motor mechanism is apparently
a direct one, and there is no power to modify it possessed
by the organism.
Nor, so far as we know, have we in plants any power
of initiative. True, there are many movements and
changes which are set up by causes that have their origin
in some alteration of the protoplasm which we cannot
explain, but there is no evidence of purpose in their
origination. Even the locomotion of the Myxomycetes
and the Diatoms shows no definite purpose except when
it is clearly set up in response to some external stimulus.
Though there is no particular differentiation of an
anatomical character in any of the sense-organs of a
plant, there is nevertheless a differentiation of a physio-
logical nature in the direction of sensitiveness which will
equal if not surpass the powers of the sense-organs of an
animal. The tendril of Passiflora appreciates and responds
to a pressure which cannot be detected by even the human
tongue ; the seedlings of Phalaris readily obey the stimulus
of an amount of light which is hardly perceptible by the
human eye. Many plants readily detect and respond to
the ultra-violet rays of the spectrum, which are utterly
invisible to man.
The extent of the response to any stimulus is of course
410 VEGETABLE PHYSIOLOGY
much less than that exhibited by an animal ; but this, as
we have seen, depends upon the differences in the motor
mechanisms. In the vegetable protoplasm we have a
much slower response, as well as one of a different
kind, the effects taking as a rule longer before they are
fully manifested and lasting for a longer time after the
stimulus has been withdrawn. We have however, as in
the animal mechanism, a much better response to a cumu-
lative or prolonged stimulation than to one which is rapid
and transitory.
411
OHAPTEE XXVI
N^
BEPKODUCTION
THE phenomena we have hitherto been considering all
concern the life of the individual plant. As this, however,
at the best is comparatively limited in duration, we find
plants possessed of the power of giving rise to new in-
dividuals. The process of originating each new individual
from its parent or parents is known as reproduction.
We have seen that the life of the plant is essentially
bound up with the individuality of the protoplasts which
compose it. Many plants consist of but a single one of
these organisms : others are composed of many, some of a
very large number. We have seen reason to look upon
each of these aggregations of protoplasts as a large colony
whose members have become differentiated in various
ways to carry out to the greatest advantage the vital pro-
cesses of all. In the simplest forms, such as filaments of
protoplasts like Spirogyra or Ulothrix, each protoplast is
apparently independent in its behaviour, though mechani-
cally attracted to its neighbours. In more complex and
bulky forms this independence has been given up in favour
of complete co-operation for the general welfare.
As every plant then is composed of either one proto-
plast or many, we may in the latter case distinguish
between the colony and its constituents. The term
individual is usually associated with the former, and we
speak of reproduction as leading to the appearance of such
individuals without making any reference to the proto-
plasts of which it consists. In dealing with reproduction
however in the broad sense we must consider also the
412 VEGETABLE PHYSIOLOGY
development of the protoplasts of the colony as well as of
the appearance of new colonies or so-called individuals.
Indeed in the case of unicellular plants such production
of new protoplasts is the only form of reproduction
possible.
It is important however to bear in mind the different
individualities of the protoplast and of the colony of which
it is part. In a filament of Ulothrix or other thread-like
alga, each protoplast being like every other in all essential
points, we may regard the formation of new protoplasts in
the chain as a process of reproduction of the units ; as the
chain however- grows by means of such multiplication of
its constituent protoplasts, and has a distinct individuality
as a filament, we may also regard the process of multiplica-
tion of the units as one of growth in the length of the
chain. What is reproduction of the units of construction,
the protoplasts, is growth of the individual, the colony.
The same thing is seen in all plants which consist of more
than a single cell.
We may study the method of multiplication of the
protoplasts either in the cases in which they have an
independent existence or in those in which each is part of
a colony. In any case the process involves the division of
the protoplast into two or many parts, each of which
strictly resembles in all respects its progenitor. The cases
in which two new protoplasts result from the fission are
the most numerous, and they are classed together gene-
rally under the term cell-division. Of this there are
various degrees of simplicity ; the most primitive is illus-
trated by the behaviour of some of the lower fungi, such
as the Saccharomycetes or yeasts. Each cell, which is
rounded in form, puts out a lateral protuberance of small
size, which grows until it is of nearly the same dimensions
as the one from which it sprang, and is gradually cut off
by the constriction of the cell-walls at the point of out-
growth. The new cell or protoplast becomes thus separated
from its parent, which it resembles in all respects. This
EEPEODUCTION 413
is known as gemmation or budding. It may go on so
rapidly that the new cell in turn may put out a bud of its
own before it is cut off from its parent, and in that way
chains of cells may be produced (fig. 164).
A more general method of the division of the cell or
protoplast is of a highly complicated character, and is pre-
ceded by a division of its nucleus. This structure we
have seen consists essentially of a delicate network of
fibrils of chromatin embedded in a hyaline substance, the
whole being surrounded by a more or less well-defined out-
line derived from the cell-protoplasm, and known as the
nuclear membrane. Associated with it in some cases are
two small centrospheres. The process of division, which is
known as Karyokinesis, or Mitosis, begins by the network
FIG. 164. — SACCHABOMYCES CEBEVISLE, OB YEAST-PLANT, AS DEVELOPED
DUBING THE PBOCESS OF FEBMENTATION. X 300.
a, b, c, d, successive stages of cell-multiplication.
of fibrils becoming coarser and gradually separating to
form a long coiled fibre. The nucleoli disappear and the
nuclear membrane ceases to be distinguishable. At the
same time, in those cases in which centrospheres have
been seen, they shift their position and come to lie on
opposite sides of the nucleus at some little distance from
it. The long coiled fibre of chromatin breaks up into a
number of pieces, often V-shaped, which point towards the
centre of the nucleus. The number of these varies in
different cases, but is constant in the successive divisions
of an individual. These pieces of the fibre are known as
chromosomes. The chromatin in them is broken up into
thin discs which are separated from each other by smaller
discs of unstainable substance.
414
VEGETABLE PHYSIOLOGY
Threads of a delicate character may next be seen to
extend from one centrosphere to the other, forming a body
known as the nuclear spindle. The positions of the
centrospheres are called the poles of the nucleus. When
no centrospheres can be detected the threads of the spindle
nevertheless converge to two similarly situated poles.
Some of the spindle fibres stretch uninterruptedly from
pole to pole, while others become in some way attached to,
or entangled with, the chromosomes. The latter travel
along these threads, with which their points are in contact,
till they form a disc across the spindle (fig. 165, b). This
stage is constant in all cases of karyokinesis, though some
L— J
d
FIG. 165. — STAGES IN KARYOKINETIC DIVISION OF THE NUCLEUS.
a, resting nucleus ; &, stage of equatorial plate ; c, separation of the chromo-
somes; d, commencement of formation of cell- wall; e} extension of
nuclear spindle across the cell.
variations of the antecedent steps have been observed, the
details of the formation of the disc not being always iden-
tical. This body is sometimes called the equatorial plate.
After this stage is reached, or sometimes before it is quite
completed, each chromosome splits longitudinally into
two, and the bodies forming the equatorial plate separate
into two sets in such a way that half of each original
chromosome makes its way towards one pole, and the other
half towards the other. The two sets of chromosomes so
formed travel back along the spindle fibres, each going to
one of the two poles of the nucleus, their positions as they
go being such that their convex sides point towards the pole
which they are approaching (fig. 165, c). They thus collect
REPRODUCTION 415
into two places which are determined by the positions of
the poles of the nucleus, or of the centrosomes if the latter
are present, and they present there the appearance of two
somewhat star-shaped aggregations. This is known as the
diaster stage. The chromosomes at each pole next be-
come united by their ends, and constitute two new nuclei,
each gradually becoming well denned by the appearance of
a nuclear membrane ; the original appearance is com-
pleted by the development of nucleoli in each new nucleus.
The mechanism of the movement of the chromosomes
towards the poles is not fully understood at present, but it
is held by some observers to be due to a contraction of the
spindle fibres to which the chromosomes are attached.
This process of karyokinesis is followed in various ways
by the production of a cell-wall between the two nuclei,
which completes the division of the protoplast. In the
cases in which the latter does not contain a relatively
large vacuole and is of comparatively small diameter, the
spindle fibres become increased in number, and form a
barrel-shaped body whose short diameter stretches com-
pletely across the cell (fig. 165, d, e) till the spindle is in
contact with the lateral cell-walls. Granules which have
been floating in the cell-protoplasm are to be seen stream-
ing along the spindle fibres till they form a plate stretching
across the cell from wall to wall. From this plate the
septum of cellulose and its associated substances is formed.
If the protoplast has a large central vacuole, the spindle
often does not reach completely across the cell. It is then
at first in contact with one side only, and the new wall
begins to be formed there in the same way as in the case
described. It then detaches itself from the part of the
new-formed wall which is in contact with the old membrane
and moves gradually across to the opposite side of the cell,
the new wall being completed as it goes. The spindle then
disappears.
In some of the Thallophytes the new wall is formed
without the intervention of a spindle. After the two new
416 VEGETABLE PHYSIOLOGY
nuclei have taken up their positions, the new wall arises
midway between them as a ring-like outgrowth from the
original cell membrane, and gradually grows inwards till it
is complete.
In the divisions of the protoplasts which constitute a
coenocyte the nuclear divisions are not followed by the
construction of any cell-walls, so that the limits of each
protoplast are not well denned ; in some cases indeed they
are indistinguishable.
The reproduction of the protoplast is sometimes attended
by the production of not two but several, which appear
simultaneously. Such a case is illustrated by the forma-
tion of the endosperm in the embryo sac of the Phane-
rogams. It is, however, only a modification of the process
already described. The division of the original nucleus is
followed by the disappearance of the spindle ; the daughter
nuclei divide in turn, and the process is continued until a
large number of free nuclei lie embedded in the protoplasm
of the cell. These then become connected with each other
by the simultaneous development of connecting fibrils or
small spindles like the first, and cell plates, which later
become cell membranes, arise across them as in the case
described. The protoplasts so formed exhibit no differen-
tiation among themselves, but are all alike in appearance,
structure, and fate.
This modification of the process of reproduction of the
protoplast is known as free cell formation, and in many
cases it is attended by a specialisation of function, which
will be alluded to a little later.
In many cases of the reproduction of such plants as
consist of enormous numbers of protoplasts variously
arranged and differentiated, we have to recognise essentially
no other process than the multiplication of the protoplasts
by such means as we have just described. Generally in
these cases some part of the parent plant becomes detached
and grows at once into the new individual. We have seen
that this is the regular method of the multiplication of the
EEPEODUCTION 417
yeast-plant, where each division of the protoplasts brings
into being a new individual. The process can be noticed
through all the families of the vegetable kingdom, though
as we advance upwards in the scale, the separated body
becomes more and more complex. We have the gemmae
of certain Algae and Bryophyta, which are multicellular ;
we have in certain Mosses branches which become detached
by the dying off of the shoot behind them. Many Ferns
develop buds upon the pinnae of some of their leaves,
which when separated from the latter grow intb-complete
ferns. Among the Phanerogams we notice a great variety
of this method of reproduction, many structures being de-
veloped normally to secure it, while others can be made to
lead to it by artificial means. We have the propagation
of plants normally by the formation and separation of
tubers, buds, and corms ; by the young plants which are
developed from the nodes of runners and stolons. The
artificial method of bringing it about is illustrated by cut-
tings, which are pieces of the stem, bearing buds ; these
when detached and planted in suitable soil, put out adven-
titious roots from the base of the cutting and develop into
plants like the original one. Other instances are afforded
by the buds which many leaves, notably those of Bryo-
phyllum and certain Begonias, put out when wounded.
These also develop adventitious roots, and young plants
arise which become independent.
This method, in which we never meet with the prepara-
tion of cells which are specialised in the direction of
reproductive powers, is usually spoken of as vegetative
reproduction or propagation.
Some curious cases of it are known. In the embryo
sac of Cwlebogyne there is no fertilisation of a sexual cell
in the manner which will shortly be described, but still
one or more embryos arise. This is caused by a vegetative
budding of certain cells of the nucellus .of the ovule which
grow into the interior of the embryo sac, and develop into
embryos.
E E
418
VEGETABLE PHYSIOLOGY
FIG. 166. — ZOOSPOBE OF
Ulothrix. x 500.
A feature of vegetative propagation which may here be
.emphasised, is that the new individual is developed con-
tinuously after its origination. There
is no resting period, such as we find
in most cases to mark the behaviour
of the more specialised reproductive
cells to be discussed below.
Apart from cases of vegetative
propagation of the individual, we meet
with two other methods of reproduc-
tion, both of which involve the pre-
paration jjfjspecial cells_set apart fp^Jhis^Durpose! Tfte
nrst oT~th<Bse~~is cha7acterisean3yrtKeTact that each cell so
produced is able to grow, either at
once, or after a short period of rest,
into a new plant, which may or may
not be exactly like the one from
which the reproductive cell was
formed. In plants exhibiting the
simple organisation which we find
among the seaweeds and the fungi,
the parent and the offspring are in
most cases precisely similar. The
difference in this respect between
them and plants higher in the scale
will be discussed a little later. A
good example of this mode of repro-
duction, which was probably the primi-
tive form, is afforded by the common
nlamentous Alga Ulothrix. Any
protoplast of the filament can divide
into a numb<* of separate pieces,
each °f °v°id shape with a p°inted
and furnished there with four
(fig. 166). The,se new proto-
plasts swim about for a time in the water, then come to
rest, and after a time grow out into new filaments. Not
FIG. ley.-Two GONI-
OF Achiya.
escape of the zoogonidia end
from them. .
. cilia
BEPKODUCTION
419
only the Algae but the Fungi afford examples of the de-
velopment of such cells, conspicuous among them being
Saprolegnia and its allies (fig. 167). These free -swimming
protoplasts are known as zoogpores or tfbogonidia. Each
on coming to rest clothes itself Vith a cell-wall, and can
develop into a plant exactly like the one from which it
arose. These zoogonidia are developed by the protoplasm
of a single cell dividing up into a variable but often large
number of separate protoplasts, the process being known as
FlG. 168.— COZNOCYTE OF MuCOT, BEABING A
GONIDANGIUM, k. THIS IS MOBE HIGHLY
MAGNIFIED IN THE FIGURE TO THE BIGHT.
m, columella ; Z, gonidia.
FIG. 169. — ASCI, a, MIXED
WITH BARREN HAIRS OR
PARAPHYSESe,/; FROM HY-
MENIAL LAYER OF
x 250.
free cell formation. Each protoplast possesses a nucleus
derived from the original nucleus of the cell in which the
formation takes place, in the manner already alluded to.
In most cases where these reproductive cells are met
with they have not so simple a structure as those so far
described, but each is furnished with a cell-wall. They
are commonly called spores or gonidia, and arise in differ-
ent ways upon the plant, often, or indeed generally, being
developed in or on special organs, known as sporangia or
gonidangia.
E E 2
420 VEGETABLE PHYSIOLOGY
The yeast-plant gives us perhaps the simplest form of
this organ. Any cell can play this part ; its protoplasm
divides into a number of pieces, frequently four, each of
which becomes rounded off and clothed with a new cell-
wall. After a time the four new cells are liberated by the
breaking down of the original cell-wall. They are deve-
loped in more highly differentiated plants in special cells or
chambers named asci (figs. 168 and 169), in very variable
numbers, and are known as ascogonidia or ascospores. In
other cases they are produced by abstriction from a cellular
outgrowth of the thallus (fig. 170), and in these again the
number produced from a single cell
may vary within wide limits. These
are generally called stylogonidia or
stylospores. There is an almost in-
finite variety of these bodies to be
met with in different plants, but the
variety affects only the conditions of
their situation and does not indicate
any difference in their own structure.
They are unicellular bodies, or simple
protoplasts, each clothed with a deli-
cate cell-wall.
These asexual cells are usually
spoken of as gonidia when they arise
upon a gametophyte, and as spores
when the sporophy te gives them origin.
FIG. 170. — STYLOGONIDIA
OF Euro'tinm, PRODUCED The fact that they do not usually
BY ABSTBICTION FBOM • ,.,, f, . ,
STEBIGMATA. germinate till after a period of rest,
though this is often not very pro-
longed, suggests that they originated in consequence of the
plant needing certain cells which should possess the power
of passing through times of exposure to unfavourable condi-
tions without destruction. Such unfavourable conditions
would be likely to kill the more delicate vegetative repro-
ductive bodies. This view is supported by the fact that
many of the lower plants, particularly Yeast, do not pro-
EEPEODUCTION
421
duce gonidia when conditions are suitable for the life of
the ordinary individual, but can be made to do so by
cultivating them under adverse conditions of moisture,
food supply, &c.
A somewhat similar structure to the zoogonidia de-
scribed is put out by the coenocytic Alga Vaucheria. It
appears as a mass of protoplasm, which is separated from
the contents of a filament, and is set free by an opening
at the apex of the latter. It is composed of several proto-
plasts which are arranged together as in the rest of the
cosnocyte, but their individual outlines cannot be seen.
The fact that it is cosnocytic is shown by the presence of a
number of nuclei in the protoplasmic mass. A pair of
cilia are given off opposite to each nucleus, so that it
swims very readily in the water after its liberation. It
is sometimes called a Zooccenocyte. After a period of
motility it comes to rest, the cilia are withdrawn, and it
becomes clothed by a cell-wall. The resting period lasts
for a variable time, after which it develops into a new
Vaucheria filament.
Besides these asexual reproductive bodies other cells
are produced by the great majority of plants, which are
incapable of giving rise to new individuals, unless two of
them unite or fuse with one another. On account of this
peculiarity they are known
as sexual cells or gametes.
In the lowliest forms,
such as many filamentous
Algae, they are produced by
the same filament as the
asexual cells or gonidia. In
the case of Ulothrix we
find the first indication of
these sexual cells. Besides
the large zoogonidia with
their four cilia, other smaller
developed in certain cells of
FIG. 171.— PART OF A FILAMENT OF
Ulothrix, FROM WHICH THE GAMETES
g ARE ESCAPING.
free gamete ; <73, g5, gametes
conjugating.
free-swimming bodies are
the filament. They are
422 VEGETABLE PHYSIOLOGY
produced in larger numbers and have only two cilia each
(fig. 171). After they are set free into the water they
swim about for some time, and then they usually fuse
together in pairs, nucleus joining nucleus and protoplasm
uniting with protoplasm. The new body so formed is
known as a zygospore. After a period of rest it can give
rise to a new filament. As there is no difference between
the cells which unite to form this structure, they are
frequently called planogametes.
In the ZygnemecB and the Mesocarpece the gametes are
solitary and non-motile and do not escape from the cells
in which they are formed. Two filaments take part in
the fusion of the gametes ; these are found lying close
together in the water; from a cell of each filament a
protrusion grows out towards the other and the two come
into contact and join, the separating walls breaking down.
The contents of one cell pass over into the other through
the channel so formed, or the contents of both the cells
meet in the middle of the passage ; fusion of the two takes
place, and the new body, called as before the zygospore,
clothes itself with a cell-wall. It is liberated after a while
by the breaking down of the wall of the structure which
encloses it, and can then give rise to a new individual.
A similar process is characteristic of certain Fungi.
In all these cases, though the cells are sexual cells,
the differentiation of sex is so slight that it is difficult to
speak of male and female gametes. In the Zygnemese, in
which the formation of the zygospore takes place in the
cell of the filament, the gamete which passes through the
passage may perhaps be regarded as male and the more
passive one as female. This differentiation cannot be dis-
tinguished in the Mesocarpese, where both gametes meet
in the connecting passage.
In Ulothrix the differentiation of sex is even more
rudimentary, as it is not always necessary for the fusion
to take place. If any cell escapes fusion it may develop
into a new filament independently of this process. This
EEPEODUCTION 423
fact suggests that the sexual cells have been derived from
asexual ones, and are a later development, therefore, in the
history of the race.
The more complete differentiation of the gametes into
male and female can be observed among several of the
families of the Algae. In some species of Ectocarpus and
Cutleria the gametes are much like those of Ulothrix, but
some are smaller than the others. The larger ones come
to rest soonest, and lose their cilia ; one of the smaller
more motile ones then fuses with each of the larger. We
can in this case speak of the larger as female and the
smaller as male. The differentiation is still very rudi-
FIG. 172. — OOGONIUM OF Fiicus, CON- FIG. 173. — AN OOSPHERE OF Fucus
TAINING EIGHT OOSPHERES. (After SURROUNDED BY ANTHEROZOIDS.
Thuret.) (After Thuret.)
mentary, as in the event of no fusion taking place the
female cell can still develop into a new plant.
The most complete differentiation of the gametes can
be traced in the higher members of the Algae. The females
become larger and cease to develop cilia, the males remain
small and motile. The former are then called oospheres
and the latter antherozoids or spermatozoids. A good
example of this stage of differentiation is afforded by Fucus
(figs. 172 and 173).
The structures or organs in which the sexual cells of
these plants are formed are known as gametangia. When
the gametes are distinctly male and female the gametangia
424
VEGETABLE PHYSIOLOGY
in which they are developed are termed antheridia and
oogonia respectively.
In the group of Fungi similar differentiation of gametes
occurs, but motile antherozoids are very rare, confined
indeed to the genus Monoblepharis. In other cases they
are generally undifferentiated masses of protoplasm which
do not escape from their antheridia, but are conducted
directly from it into the female organ, where the process
of fusion takes place. In Pythium the oogonium is a
swelling at the end of a hypha, which is cut off from the
rest by a transverse wall. Its contents divide up into an
oosphere and a -certain amount of protoplasm, which sur-
rounds the latter. The antheridium is another hyphal
branch, which becomes closely pressed to the oogonium.
A tube is put out by the antheridium, which perforates the
wall of the oogonium, and the male cell, which is formed
in the same way as the female one,
passes over into the female organ and
fuses with the oosphere.
In some other Fungi a similar
arrangement of the organs is brought
about, but the male cell does not pass
over into the oogonium.
A curious variation is seen in the
red seaweeds, the Rhodophycece. The
female organ, known as a procarpium,
does not produce any differentiated
oosphere, but the contents of the
male cell pass by means of an elon-
gated structure called a trichogyne
(fig. 174) into its interior and appa-
rently fuse with the whole of its
protoplasm. The male cell in these
plants is not naked as in other
cases, but has a cell-wall. A somewhat similar condition
is met with among the Ascomycetes, though whether
fusion of the contents of the cells takes place is disputed.
FIG. 174.— PROCARPIUM OF
A RED SEAWEED.
tr, trichogyne.
KEPKODUCTION
425
The gametangia of the plants above the Thallophytes
are known as antheridia and archegonia respectively. An
archegonium is a more complex structure than an oogonium,
being composed of many cells and showing differentiation
into a venter and a neck (fig. 175). It contains only a
single oosphere.
The sexual cells differ from the great majority of
asexual ones in never possessing cell-walls. The only
cases in which they are clothed with them are those of the
Ehodophyceae and the Ascomycetes already alluded to. In
both these groups the male gametes are the only one's that
FIG. 175. — DEVELOPMENT or THE ARCHEGONIUM OF THE FERN.
have them ; the females, as we have seen, not being
differentiated.
The fusion of the gametes is known as conjugation
when they are alike, and as fertilisation when they are
distinctly male and female. The resulting body is termed
a zygote ; it is a zygospore when it is produced by conjuga-
tion, and an oospore when it is the result of fertilisation.
In the more lowly organised forms it generally happens
that both sexual and asexual reproductive cells may be
produced upon the same individual. An exception is
found in the Fucacece, the members of which do not
develop any asexual cells. While it is possible however
for many plants to produce both gonidia and gametes, it is
426 VEGETABLE PHYSIOLOGY
more usual for them to bear the former only. So for a
long series of individuals reproduction is brought about
asexually by gonidia. Then for some reason an individual
produces gametes, and the series is interrupted by the
occurrence of sexual reproduction. This is generally fol-
lowed by a further series like the'first. We have here an
instance of a kind of alternation of generations, which is
however irregular and intermittent. As all the members
of the series, whether producing gonidia or gametes, are
essentially similar or homologous, this is often spoken of as
homologous alternation of generations.
The forms which we have discussed appear all to be
capable of producing gametes if conditions require them.
They are accordingly termed gametophytes, and are dis-
tinguished as actual or potential as they do or do not give
rise to sexual cells.
In plants which are higher in the scale the production
of both sexual and asexual reproductive cells ceases to be
possible upon the same individual, and we find consequently
that the plant exhibits two phases in its life cycle, one of
which is characterised by the production of sexual and the
other of asexual cells. How this sharply marked separation
arose is still a matter of controversy which we need not
here enter into. The two forms however are not homolo-
gous, one being capable normally of producing only gametes,
the other of giving rise only to spores. A further develop-
ment also makes itself evident in that the zygote arising
on the gametophyte is only capable of originating a form
which bears spores, while the spore can only develop a
form on which sexual cells arise. The asexual form is from
this point upwards known as the sporophyte. The occur-
rence of them regularly in turn as described is known as
antithetic alternation of generations. It is of constant and
regular occurrence in all the groups of plants above the
Thallophytes.
The existence of a sporophyte, or form which is never
capable of bearing gametes, is still a matter of discussion
EEPEODUCTION 427
as far as the Thallophytes are concerned. There are indi-
cations of its origination in that group, but they are ex-
tremely rudimentary, and occur in families which are widely
separated from each other. The gametophyte was doubtless
the primitive form of the plant, and in some way or other
the sporophyte took its origin from it. Certain phenomena
which may represent stages in the process can still be
observed. In CEdogonium the fertilised cell does not grow
out into a new filament, but produces in its interior four
zoospores which escape from it, and after a period of rest
germinate and produce new plants. The fertilised cell here
may perhaps represent the sporophyte, reduced however to
a single sporangium. An even simpler stage of develop-
ment may perhaps be recognised in Spirogyra, where the
nucleus of the fertilised cell divides into four, though no
definite cells are formed. On germination of the zygote
however only one filament grows out. A more complex
structure is formed in Coleoclicete ; the zygote becomes
invested with a covering derived from the adjacent cells,
and after sinking to the bottom of the water, it germinates,
producing inside its coating a small mass of cells, each one
of which liberates a zoospore. Other complex structures
are found as the result of the growth and development set
up by fertilisation in the Ehodophycese. These are known
as cystocarps, and they have been held to represent the
sporophytes of those plants. It is important to notice how-
ever, both in their case and in that of Coleochaete, that only
part of the structure in most cases is derived from the con-
tents of the fertilised cells, the rest coming from other
cells of the tissue of the gametophyte. As we have seen
the sporophyte in the higher plants is entirely derived from
the zygote.
The antithetic alternation of generations is seen most
clearly in the groups of the Mosses and Ferns. In the
former the Moss plant is the gametophyte, the so-called
capsule or theca with its stalk is the sporophyte. In the
Ferns the sporophyte is the predominant form and takes
428
VEGETABLE PHYSIOLOGY
on the chief vegetative functions, while the gametophyte
is the small prothallium (fig. 176).
Even without going beyond the Ferns we can notice as
we pass through the several divisions of the vegetable
kingdom that the predominant form of the plant has
changed. In the Thallophyta it is always the gameto-
phyte ; the sporophyte is not universal there and is never
more than a small structure, which nearly always remains
attached to the gametophyte. In the Bryophyta the two
phases are more nearly alike in degree of development ;
the gametophyte is always the vegetative body, while the
FIG. 176. — PROTHALLIUM (GAMETOPHYTE) OF FERN.
sporophyte often shows the greater histological differentia-
tion. It is always parasitic upon the gametophyte and
never attains a higher degree of morphological value than
a thallus. In the Pteridophyta the predominance of the
sporophyte is very marked, and as higher and higher groups
of plants are reached it becomes still more pronounced,
the gametophyte ultimately being reduced to microscopic
dimensions.
We encounter for the first time in the group of the
Pteridophyta, the Ferns and their allies, a phenomenon
which becomes of constant occurrence in all groups above
EEPEODUCTION
429
them, and which leads to the production of the structure
known as the seed, the latter being a special body produced
by all members of the group of Phanerogams or flowering
plants, and marking them off clearly from all below them.
The phenomenon in question is known as heterospory.
Plants which exhibit it bear two kinds of spore, which differ
from each other mainly in their relative dimensions. Some
are produced in large numbers in a sporangium and have
usually the structure which has already been described.
Others are much larger than these and are developed either
singly or in small numbers, usually four in a sporangium.
They are spoken of as microspores and megaspores respec-
tively. In the Pteridophytes the megaspores when formed
differ from the microspores chiefly in size ; in the Phane-
rogams they are never liberated from the sporangium and
have consequently thin and delicate walls.
The phenomenon of heterospory involves the production
of two gametophytes to one sporophyte, as each of the
spores produces its appropriate prothallium. The gameto-
phyte arising from the
microspore gives rise only
to male gametes, that from
the megaspore only to
female ones. Such plants
show in their life cycle,
therefore, three forms, one
sporophyte and two game-
tophytes, the latter occur-
ring synchronously.
The gradual appearance
or development of the seed
can be examined by study-
ing a series of forms. The
earliest indication of it
which we can find is ex-
hibited by the Hydropteridece, of which Salvinia is a
characteristic type. Salvinia is a heterosporous form, each
FIG. 177. — GERMINATION OF A MASS OF
MICROSPOBES OF Salvinia. (After
Sachs.)
1, The mass protruding tubular prothalli
from different spores ; 2, a prothallus
more highly magnified, showing an
antheridium, a ; 3, antherozoids in
mother-cells ; 4, ruptured antheridium.
430
VEGETABLE PHYSIOLOGY
microspore of which gives rise to a very rudimentary pro-
thallium bearing only one antheridium with four anthero-
zoids (fig. 177). The megaspore, like the microspore, is
liberated from the sporangium and on germination it pro-
duces a prothallium, part of which remains in the spore
and part protrudes from it (fig. 178). The inclusion of
part of the gametophyte within the spore is the first step
towards the formation of the seed. The young sporophyte
arises upon this prothallium
upon the exposed portion,
originating as in other cases
from the zygote produced in
the archegonium in conse-
quence of fertilisation.
A more advanced stage is
seen in Selaginella, which also
is a member of the Pterido-
phyta, though not a fern.
The heterospory is just as
pronounced as in Salvinia.
When the megaspore is set
free from the sporangium
and its germination can be
observed, it is found that
more of the gametophyte re-
mains inside the spore (fig.
179). The process of germi-
nation begins while the spore
is still in the sporangium, and by the time the spore opens
the prothallium has reached a fair degree of development.
A still further advance is shown by Isoetes, in which
the prothallium is developed inside the spore, which only
opens a little at the apex when the archegonia are mature,
in order that fertilisation may be possible.
When we pass to the Phanerogams two further
advances may be seen. The spore never escapes from the
sporangium, and the prothallium does not emerge even in
FIG.
178.— GERMINATION OF MEGA-
SPOBE OF Salvinia.
pro, prothallium ; a, young sporo-
phyte. The thick wall of the spore
has been ruptured and part of the
prothallium is protruding.
EEPEODUCTION
431
part from the spore, which does not open. In these plants
the megaspore is represented by the cell known formerly
as the embryo-sac, the sporangium being the ovule.
Among the Phanerogams we have two types of prothallium
which are characteristic of the Gymnosperms and the
Angiosperms respectively. Fig. 180 shows the structure in
the former ; the spore or embryo-sac is filled with the pro-
thallium, formerly called the endosperm,' at the apex of
which are several archegonia each containing a female
gamete or oosphere. After fertilisation the resulting zygote
FIG. 179. — GERMINATION OF MEGASPORE OF Selaginella.
arch, archegonia ; oos, oospheres ; em', embryo. The spore has been ruptured
and the upper portion removed.
gives rise to a young sporophyte or embryo, which becomes
embedded in the endosperm. The structure thus formed,
consisting of the sporangium or ovule, with the solitary
spore it contains, the latter having in its interior the
embryo- surrounded by the prothallus, constitutes the
structure known as the seed. It becomes detached from
the parent sporophyte and disseminated in various ways.
In the Angiosperms the formation of the seed is in the
main similar to the process described, but it has certain
peculiar features. The embryo-sac or megaspore has the
432
VEGETABLE PHYSIOLOGY
same structure as in the Gymnosperms and remains
enclosed in the sporangium or ovule. The development
of the prothalliuni is different. The megaspore has a
single nucleus as in other cases. When germination begins
this divides into two, one of which travels to each end of the
ovoid spore. Each of these gives rise by two successive
divisions to a group of four nuclei, and a single nucleus from
each group returns to the centre of. the cell, where the two
fuse together. These are often termed the polar nuclei.
FIG. 180.— OVULE OF Pinus,
SHOWING THE PROTHAL-
LIUM, end, IN THE MEGA-
SPOBE, mac,
arch, archegonia.
FIG. 181. — OVULE or AN ANGIO-
SPERM SHOWING THE MEGASPORE,
mac, WITH ITS PROTHALLIUM ;
OOS, OOSPHERE.
At this stage the prothalliuni ceases to undergo any change
(fig. 181) ; it consists of a group of three nuclei at the apex,
known as the egg apparatus ; another group at the base,
termed the antipodal cells ; and the nucleus in the centre
which is the result of the fusion of the polar nuclei, and is
called the definitive nucleus of the embryo-sac. Each nucleus
is surrounded by protoplasm, the egg apparatus in parti-
cular showing three well-defined naked or primordial cells.
The antipodal cells become clothed with cell-walls. There
EEPEODUCTION 433
is a certain amount of protoplasm existing in the spore,
lying around the wall and forming bridles across it, con-
necting the peripheral substance with that in the centre in
which the definitive nucleus is resting.
There are no apparent archegonia ; the oosphere is one
of the three cells of the egg apparatus, the other two being
known as the synergidce. The oosphere is a product of
the last division of the original upper nucleus, the other
half being the polar nucleus which takes part in the fusion
described.
After the fertilisation of the oosphere an embryo is
developed from it, which remains enclosed in the spore.
At the same time a large development of tissue takes place
in consequence of repeated divisions of the definitive
nucleus, so that the spore contains a massive so-called
endosperm in addition to the embryo, the latter being
embedded usually in the former. This so-called endosperm
has as we have seen a different morphological value from
the endosperm of the gymnospermous plant.
One of the most remarkable features about the struc-
ture and behaviour of the seed is the fact that soon after
the embryo is formed it enters upon a period of rest, which
in some cases is very prolonged. During this period the
seed becomes detached from the parent plant. The
resumption of its growth and development is known as the
germination of the seed. This resting period does not
occur during the development of the sporophyte in the
Cryptogams.
The embryo frequently attains a considerable size
before its resting period commences. In this case it
absorbs the contents of the cells of a considerable part, or
sometimes the whole, of the endosperm, so that it fills
more or less completely the cavity of the spore.
The seed may thus be a very complex structure ; it may
consist of the following parts :
(1) The testa or skin, derived from the integuments
of the ovule.
F F
434 VEGETABLE PHYSIOLOGY
(2) The perisperm, or remains of the body of the
megasporangium.
(3) The embryo-sac or megaspore.
(4) The endosperm derived from the definitive nucleus.
(5) The embryo developed from the zygote.
The antipodal cells generally disappear during the
development. (2) and (4) may be absent, having been
absorbed by the megaspore or by the embryo respectively
during their development. If either or both are present
the seed is said to be albuminous, the term albumen
embracing both perisperm and endosperm.
In the seeds of the Gymnosperms the endosperm repre-
sents the prothallium or gametophyte.
The formation of the seed we have seen to depend
upon the fusion of the sexual cells or gametes. This
FIG. 182. — AXTHEROZOIDS OF Moss (A) AND FERN (B).
process is a very widespread one and is the starting point
of the development of the young sporophyte in all plants
above the Thallophytes. The mode of bringing the gametes
together varies with the habit of life of the plants. Where
the male gamete is a motile antherozoid it makes its way to
the oosphere by means of its cilia, which enable it to swim
freely in water. In those forms with a terrestrial habit,
such as the Bryophyta and Pteridophyta, in which the
antherozoid is ciliated (fig. 182), fertilisation can only be
brought about when the gametophytes are moistened, as is
the case from time to time. The antherozoids sometimes
:arise in antheridia upon the same gametophyte as the arche-
gonia with their oospheres, sometimes upon different ones.
In the heterosporous forms of course the latter is always
EEPEODUCTION
435
the case. After their liberation they are attracted to the
archegonia by some constituent of the mucilaginous matter
which is excreted from their necks when they open (fig.
184). In the Mosses this has been ascertained to be cane-
FIG. 183. — DEVELOPMENT OF THE ANTHERIDIUM IN THE FERN. (After Kny.)
sugar, in the Ferns it is malic acid or one of its salts. In
the Khodophycese and such Ascomycetes as exhibit sexual-
reproduction, the passive male gamete, often called a
spermatium, instead of an antherozoid, is floated to the
female organ or its trichogyne by currents in the water.
FIG. 184. — DEVELOPMENT OF THE ARCHEGONIUM OF THE FERN. (After Kny.)
In the Phanerogams, where the female gametophyte is
always attached to the parent sporophyte, such a means
of fertilisation is of course impossible. For fertilisation to
take place it is necessary that the two gametophytes shall
be produced in close propinquity to each other. This is
F F 2
436 VEGETABLE PHYSIOLOGY
effected by the bringing together of the two spores con-
cerned in developing them. The microspore or pollen
grain is carried by various means to the neighbourhood of
the megasporangium ; in the Gymnosperms it falls upon
the megasporangium itself ; in the Angiosperms upon the
stigma of the pistil in which the megasporangia are hidden.
When it germinates the prothallium or gametophyte takes
the form of a long tube, which makes its way through the
intervening tissues till it reaches the megaspore itself,
close to the archegonium in the first case, and to the oosphere
in the Angiosperms, where there is no archegonium. In
the Gymnosperms the tube, the so-called pollen tube,
contains a single antheridium, which produces two gametes,
which are generally undifferentiated portions of protoplasm,
but which in Ginkgo and in some species of Cycas have
been found to be ciliated antherozoids. In the Angio-
sperms there is no antheridium, but two gametes are pro-
duced, which show no differentiation. From the great
preponderance of the nuclear matter they contain they are
often spoken of as the generative nuclei.
Fusion of the latter, or of the antherozoid, with the
oosphere, becomes possible by a deliquescence of the
separating walls, and in all cases a single male gamete fuses
with an oosphere. Where several oospheres are found
upon the same prothallium, as in the Gymnosperms, more
than one may be fertilised by gametes from the same
pollen-tube. This occurs in certain of the Cupressinece ; it
is rendered possible by a multiplication of the male gametes,
which takes place by ordinary processes of division ex-
hibited by them as they pass down the tube. Several
embryos may thus arise in the seed. Usually however
only one of these undergoes a normal development.
In certain cases in the Angiosperms the second of the
generative nuclei has been observed to fuse with the two
polar nuclei or the definitive nucleus of the embryo-sac.
The extent to which this takes place has not yet been
determined and its interpretation is not at present easy.
437
CHAPTER XXVII
REPRODUCTION (CONTINUED)
WE have seen that the phenomena of fertilisation are
preceded in the Phanerogams by an arrangement through
which the two gametophytes which give rise respectively
to the male and female sexual cells, are developed in such
close proximity that they ultimately come into contact.
That which is produced as the result of the germination
of the microspore or pollen grain, is a tube of varying
length, which bores its way through the tissue of certain
parts of the sporophyte, being guided in some manner not
yet fully understood, until it reaches some part, usually the
apex, of the megaspore or embryo-sac, in which synchro-
nously the prothallium which bears the oosphere has been
developed. In the process of sexual reproduction in these
plants we have two phenomena presented, which have
often been treated of as if they were inseparably connected.
The first of these, which is known as pollination, involves
merely the transport of the pollen grain to an appropriate
position on some part of the megasporophyll or of the
megasporangium itself. The second, which may or may
not follow the former one, is the actual fusion of the
gametes which are produced upon the gametophytes to
which the spores give rise, and which therefore must be
considerably later in the time of its occurrence. This is
what we have already described as fertilisation.
It is necessary to insist on the distinction between
these two processes, as the phrase ' the fertilisation of the
flower ' is frequently somewhat loosely and erroneously
made use of when pollination is meant.
We have seen that cross -fertilisation is as a rule more
438 VEGETABLE PHYSIOLOGY
advantageous to a plant than the fusion of gametes which
are both produced by the same individual. In the same
way certain advantages are secured by the process of cross-
pollination or the application of the pollen of one flower to
the stigma of a different one of the same species. In the
case of flowering plants or any others which are hetero-
sporous, self-fertilisation is of course in the strictest sense
impossible, as the male and female cells which fuse
together are necessarily borne upon gametophytes which
originate from different spores and cannot thus be derived
immediately from the same individual. Self-pollination, or
the transference- of pollen from the stamens to the stigma
of the same flower, is however possible, and in many cases
occurs in the ordinary course of events. Cross-pollination,
or the bringing together of spores from different flowers
of the, same species, has been found to yield more and
better seeds than self-pollination.
Very many mechanisms have been developed in different
plants to secure this end. Pollen may be carried from
flower to flower by wind or water, or by the agency of
insects or other animals. From this point of view flowers
have been classed as anemophilous, or wind-pollinated,
hydrophilous, or water-pollinated, entomophilous, or insect-
pollinated, and zoophilous, or pollinated by other animals.
Of these methods of cross-pollination, the anemophilous
and the entomophilous are most widespread. The former
is the more primitive ; indeed, the latter has been gradually
supplanting it. We find cases now of nearly allied genera
which illustrate the transition from the one to the other.
Among the Eanunculaceae the flowers of the genus Thalic-
trum are pollinated by the wind, while those of the more
specialised genera Aconitum and Delphinium depend upon
insects. The Plantains also afford instances of the replace-
ment of the one method by the other.
Anemophilous flowers exhibit certain structural features
which are associated with their mode of transference of
the pollen. It is produced in such flowers in great abun-
EEPEODUCTION 439
dance, is extremely light and dry, and in some cases 'is
furnished with bladders to aid its transport. The receptive
organ is in some cases a bulky cone, the leaves of which are
separated from each other, and from the common axis, by
spaces into which the pollen may drop. In others it is
a much -divided or plumose stigma, often furnished with
hairs, so that pollen falling on it may be readily retained.
The method however is a wasteful one and involves the
production of a superabundance of pollen. On the other
hand anemophilous flowers are always inconspicuous and
of a comparatively humble type.
Flowers which are pollinated by insects are usually
much larger and more showy, not infrequently possessing
irregular corollas, and are often very highly coloured and
provided with characteristic odours. Their perianths, and
sometimes their sporophylls, are highly modified to adapt
them to the habits of their insect visitors. As a further
attraction to the latter they usually produce honey in some
part of the flower, in such a situation as will lead to the
removal of pollen by the insect in its search for the
attractive liquid. The markings on the coloured perianth
leaves are often arranged in such a way as to direct the
insect towards the spot where the honey is concealed. The
pollen itself also is often the object of the insect's visit.
Many special mechanisms to secure the removal of the
pollen from the microsporophyll and its deposition on the
stigma of another flower are to be met with ; indeed
almost every Natural Order shows some modification of the
structure of the flower in this direction. The consideration
of them in detail, however, is beyond our present purpose.
Something akin to cross-pollination occurs in one of the
Hydropteridese, a family of Ferns with an aquatic habit.
The plant in question, which is known as Azolla, is a small
floating organism, consisting of a horizontal rhizome, some-
times copiously branched, on which are borne numerous
very small leaves, which are partially submerged. It bears
two kinds of spore, each produced in sporangia, which occur
440 VEGETABLE PHYSIOLOGY
in definite groups or sori. There are numerous microspores
in each microsporangium, which, when mature, are agglu-
tinated together in masses. The contents of a sporangium
usually exhibit two to eight of such masses, each of them
being known as a massula. These are set free separately.
A delicate skin surrounds each massula, and in some species
this is furnished with a number of hairs bearing barbed pro-
cesses or glochidia at their free ends. The mesgasporan-
gium, which is solitary in its sorus, bears only a single
megaspore. It is liberated from the sporangium, and is then
found to be furnished on its- lower surface with large spongy
bodies which are -developed from its outer coat, and which
serve as floats, enabling it to drift about in the water. The
apex of the spore bears a number of delicate filaments
extending between the floats. Both spores germinate after
liberation, each producing its appropriate gametophyte.
The glochidia of a massula of microspores generally catch
in the filaments of a megaspore, which may have arisen on
a different plant, and the massula thus becomes anchored
to the megaspore. The gametophytes are thus brought
together, so that the gametes can come into close propin-
quity to each other.
The mechanical adaptations which have been described
are however not the only means we find to secure cross-
pollination. There are peculiarities connected with what
we may call the receptivity of the pistil for any particular
pollen. Of these the most generally occurring is dichogamy 9
or the maturing of the microsporophylls and the megasporo-
phylls of a flower at different times. Two varieties of the
condition are met with ; in the first, known as protandry,
the stamens with their pollen are mature while the stigma
is not sufficiently developed to be pollinated. Examples may
be found in the Gentianacece, Onagraceoe, Campanulacece
Composites, &c. In Parnassia the receptive surface of the
stigma is not even formed until the anthers have discharged
their pollen. The second condition is known as protogyny,
and is the converse of the first, the stigma withering before
BEPKODUCTION 441
the pollen is mature. This condition occurs in both anemo-
philous and entomophilous flowers ; certain of the Plantains
(Plantago) and some grasses (Anthoxanthum, &c.) show it
in the former group, as does Scrophularia among the latter.
Something corresponding to dichogamy is found among
the Ferns, where the antheridia and archegonia on a
prothallium do not mature simultaneously. Cross-fertilisa-
tion must consequently be the only form possible. The
same peculiarity may be observed among the Mosses.
Another means often observed to secure cross-pollination
is diclinism, or the production of the stamens and carpels
in different flowers. Diclinous plants may be monoecious,
where the staminate and pistillate flowers are on the same
plant ; dioecious where they are on different plants ; or poly-
gamous where a plant bears flowers with stamens and
carpels as well as others which contain only one or the
other kind of sporophyll.
The terms ' mono3cious ' and ' dioecious ' are sometimes
applied to the Cryptogams, when their sexual organs are
upon the same or upon different plants. They then refer,
of course, to the gametophytic and not to the sporophytic
phase of the life cycle as in the cases just quoted.
Some flowers exhibit a peculiarity of form, which is
an adaptation favouring cross-pollination. The plants
possess flowers of two kinds, which are specially related
to each other. The most familiar instance in our own
flora is the common Primrose, which has five stamens and
a club-shaped stigma. In some flowers the stigma is
placed just in the throat of the corolla, and the stamens
some little way down its tube. In the rest of the flowers
the positions are reversed. We have here an adaptation
to the visiting insect, for when it touches the stamens of a
short-styled form, it covers with pollen the part of its body
which will come into contact with the stigma of the next
long- styled flower it alights upon. Another portion of its
body will be dusted with the pollen from the latter, which
will be suitably placed to be deposited upon the stigma of the
442 VEGETABLE PHYSIOLOGY
next short-styled form it may visit. The best seeds are
produced when each stigma is supplied with pollen from
stamens occupying a corresponding position to itself. This
method of cross-pollination is not thoroughly effective, as
the insect after a short time will be carrying pollen from
stamens of both lengths, having visited several flowers of
both kinds. The size of the pollen grains in each case is,
however, correlated with the features of the corresponding
stigmatic surface, which helps to secure the most advan-
tageous result.
This arrangement is termed heterostylism or dimorphism,
of which however it is only one form. Ly thrum Salicaria
is trimorphic, bearing two sets of stamens of different
lengths, and a style which differs from both. There are
three modes of arrangement of these organs, and as in the
Primrose, the most serviceable pollination is that which
takes place when pollen from a stamen of a particular
length is applied to a stigma of the same length.
Other arrangements are physiological rather than struc-
tural. Of these the strangest is what is called, prepotency.
When a stigma of a flower exhibiting this property is
pollinated by pollen from its own stamens, and at the same
time by pollen taken from another flower, the latter is
always the originator of the gamete by which fertilisation
is effected. Some flowrers show self -sterility, that is
fertilisation never takes place if they are only pollinated
by pollen from their own stamens ; in some few cases their
own pollen acts as a poison to them.
Though cross-pollination is generally most advantageous
it is not universal. Self-pollination occurs in many plants ;
in some indeed, special means have been developed to
secure it, either exclusively, or in cases in which cross-
pollination fails to be effected. Only one of these need
here be alluded to ; this is cleistogamy, or the production
of special flowers which do not open, in addition to the
normal ones. The most conspicuous instances of this are
afforded by several species of the genus Viola. In one of
BEPEODUCTION 443
these flowers the pollen grains often put out their pollen
tubes while they are in the sporangia, and the tubes grow
towards the stigma, penetrating it and reaching the ovules
as in the case of the normal flower, fertilisation resulting
in the same way.
The process of pollination is followed in the ordinary
course of events by the germination of the microspore or
pollen grain. The facts that it grows upon the substratum
of the stigmatic surface and that the resulting gametophyte
or pollen tube is often of considerable length mark a great
difference between it and the gametophytes of the vascular
cryptogams. It becomes indeed a parasite feeding upon a
host plant during the greater part of its development.
The course of events in the germination of the pollen
grain appears to be the following. It at the outset absorbs
water from the moist surface of the stigma and swells, its
protoplasm becoming generally more granular. It almost
simultaneously absorbs such food material as the surface
of the stigma can supply, usually some kind of sugar.
Most pollen grains contain a certain amount of reserve
food material, frequently starch or sugar, or both. The
process of absorption is followed by the secretion of
enzymes, which can act upon these reserve materials, the
most prominent of which are diastase and invertase. The
former seems to be the most wide-spread, but the latter is
far from uncommon. In some cases both enzymes are
developed. The outer coat of the grain then bursts, and
the inner one begins to protrude, probably in consequence
of the hydrostatic pressure set up by the water that has
been absorbed. Usually only one such tube protrudes,
though occasionally several are developed. Intra-cellular
digestion of the reserve materials follows, and the tube
grows at their expense. The increased nutrition is fol-
lowed by a further increase of the enzymes, which is
sometimes preceded by a temporary diminution. This
however does not last long, and soon a considerable increase
can be observed. In some pollen tubes such as those of
444 VEGETABLE PHYSIOLOGY
the Lily, in whose pollen starch granules are abundant,
the process of the digestion of the starch can be observed
taking place as the granules move along the tube during
its elongation. Soon an excretion of the enzyme into the
tissues of the style takes place, and the reserve materials
which are stored in the style are gradually digested as the
tube advances, thus ministering to its nutrition, absorption
of the products of the digestion being effected by the tube.
The latter in most cases makes its way to the micropyle of
the ovule, and by this channel reaches the embryo-sac or
megaspore. At this period the latter contains its gameto-
phyte, or prothallium, at the apex of which the oosphere
or female gamete occurs. The tip of the pollen tube
comes in contact with the wall of the embryo- sac close to
the oosphere. It then contains two gametes, which are
undifferentiated masses of protoplasm, each with a very
large nucleus. The separating walls become deliquescent
and are absorbed, and one of the male gametes fuses with
the oosphere, forming as before a zygote. The fate of the
other male gamete is uncertain ; it has generally been
held to disintegrate and disappear, but in Lilium Martagon
it has recently been seen to fuse with one or both of the
polar nuclei which go to make up the definitive nucleus of
the embryo-sac. How far this phenomenon is of general
occurrence is not known.
In a few cases the pollen tube makes its way to the
. base of the embryo-sac and burrows through its contents,
reaching the oosphere from below. This has been observed
particularly in Casuarina and in certain of the forest trees.
A few variations of this process have been observed
among the Gymnosperms. The pollen grain in these plants
falls upon the micropyle of the ovule, there being no ovary
and consequently no stigma. The growth of the tube is slow,
sometimes extending over several months. Indeed in
some cases the sporangium is detached from the parent
plant before it has reached the embryo-sac, from which
it is separated by a bulky portion of the nucellus or body of
EEPEODUCTION 445
the sporangium. In Ginkgo and in a species of Cycas the
male gametes are definite antherozoids, furnished with
cilia. In most of the Gymnosperms however this degree
of differentiation has not been observed. The character of
the female gametophyte has been already described.
Though cross-fertilisation is seen to be most advan-
tageous throughout the vegetable kingdom, it is only
possible within certain limits. For a new individual to be
produced, the sexual cells taking part in the process must
have a certain degree of relationship ; thus the antherozoid
of a moss cannot fertilise the oosphere of a fern. The
most favourable degree of relationship is that the two
gametes shall be produced by different plants of the same
species. Such a union results in greater numbers of off-
spring and in the possession of greater vigour by them.
Plants not so closely related may however produce offspring ;
thus we may have the union of gametes of plants standing
to each other in the relation of varieties of the same
species, or very frequently of distinct species belonging to
the same genus, or even of species of different genera.
Such fertilisation is known as hybridisation.
Hybrids, the offspring of such fertilisation, generally
exhibit peculiarities of form and structure intermediate
between those of their parents ; they are generally fertile
with either of the parent species, but not usually so with
another hybrid, or to a much smaller extent. When
crossed with one of the parent forms the offspring tend to
revert to that form.
The immediate result both of pollination and of fertili-
sation is generally to stimulate the parts concerned to
increased growth. In some Orchids the ovules are not
formed in the ovary until the stigma is pollinated, and
seem to arise in consequence of that process. The stimulus
of fertilisation is still more marked. In the Mosses its
result is to cause not only the development of the sporophyte
from the oosphere, but a considerable additional growth
of the archegonium, forming the calyptra. The same
446 VEGETABLE PHYSIOLOGY
thing may be noted in those Khodophyceae which produce
a bulky cystocarp. The stimulus is however most easily
observed in the Angiosperms, where it produces effects in
several regions of both gametophyte and sporophyte. The
oospore is excited to growth, and after a series of cell-
divisions becomes the embryo, while the definitive nucleus
of the embryo- sac similarly inaugurates a series of divi-
sions, ultimately giving rise to the endosperm, and other
parts of the ovule undergo modification, so that the seed
can shortly be recognised. Parts of the flower also exhibit
renewed growth and further development, the carpels
especially, though not exclusively, showing an almost
coincident enlargement, which often attains considerable
dimensions, so that a bulky structure known as the fruit
is produced. The new tissue is usually ordinary paren-
chyma, and in most cases it becomes conspicuously succu-
lent and frequently strongly acid. The attainment of its
maximum development is followed by a process technically
known as ripening. This may take one of two directions ;
the tissue may become dry and woody, the cells losing
nearly all their water, and their walls becoming converted
into lignin. On the other hand the succulence may persist
and even increase ; in such cases the acidity frequently
becomes very much diminished and a considerable quan-
tity of sugar is formed. Other changes in the cells lead
to the appearance of various flavouring matters, and often
of substances that are aromatic. Fruits thus acquire
special characteristics of flavour and fragrance which
they do not possess while they are young. The chemical
changes which give rise to these peculiarities are very
diverse, and cannot be said to be fully understood at
present.
We have noticed that the asexual reproductive cell,
whether spore or gonidium, is generally found to remain
in a state of quiescence for some time after its formation.
The same thing is seen, though not so constantly, in the
case of the zygote. In the Thallophytes this resting period
REPRODUCTION 447
is sometimes a long one : in the higher Cryptogams it is
not so noticeable, and in the Phanerogams or Sperma-
phytes, where the zygote is always developed inside the
sporangium, it usually proceeds to active growth almost at
once. In the latter plants, however, a resting period takes
place later, after the seed is fully formed. The develop-
ment of the young sporophyte, in fact, takes place in two
stages, the one ending with what may be called the ripening
of the seed, and the other beginning with the process of
germination. Seeds when detached from the parent plant
preserve their vitality for a variable length of time, some-
times even for years, and are capable of germinating freely
when exposed to favourable conditions.
The germination of the dicotyledonous seed occurs in
one of two methods. In the first of these, the cotyledons,
which are thick and fleshy, remain underground. When
kept warm and moist the seed absorbs water and swells,
the radicle makes its way out of the micropyle, the testa
bursts, and the plumule makes its way upwards, the epi-
cotyl, or part between the cotyledons and the first foliage
leaf or leaves, circumnutating and emerging in the form
of an arch, owing to the greater growth of one side. After
reaching the air the growth changes, the greatest increase
passing to the opposite side, so that the epicotyl straightens
itself. During this time it subsists upon the nourishment
stored in the cotyledons in the shape of reserve materials.
We have already discussed the means whereby these
digestive and nutritive changes are brought about, the
agencies which effect them, and the various transformations
which are met with. As the cotyledons remain under-
ground this process is called Jiypogean germination. In
the other method — that of the so-called epigean germina-
tion— the cotyledons sooner or later rise above the ground
and become green, the hypocotyl behaving as does the
epicotyl in the first case. These are generally, though
not always, albuminous seeds, and the nutritive matter is
stored outside the embryo. In both cases the root makes
448 VEGETABLE PHYSIOLOGY
its way into the soil by virtue of its geotropism and aphe-
liotropism, aided by the movement of circumnutation, and
by the adhesion of the root-hairs to particles of the soil.
In some Monocotyledons the upper part of the single
cotyledon remains in the seed and absorbs the nutriment
from the endosperm, while its base elongates and thrusts
the young plant downwards.
Sometimes the usual alternation of sexual and asexual
reproduction in the higher plants is interfered with by the
substitution of the vegetative method for one of them. In
the phenomenon of apospory, noticeable in some Ferns, we
have small prothallia developed on the back of the leaves
in the place of spores. This is a case of the production
of a bud instead of an asexual cell. Apospory is also
known to occur among the Mosses.
In the Ferns, again, the sporophyte sometimes arises
as a bud or vegetative outgrowth upon the prothallium, a
phenomenon known as apogamy.
There is another kind of apogamy known, which is
generally termed parthenogenesis. It occurs among the
Fungi, where, as in Saprolegnia, oospheres are formed in
oogonia, which do not become fertilised, and yet have the
power of growing out into new plants. In some species of
Mucor which normally exhibit the fusion of particular hyphae
and the admixture of their contents, or gametes, a variation
of the process is observed which comes under this category.
Instead of two gametangia meeting, and their contents
fusing, to form the zygospore, these organs are de-
veloped singly and do not coalesce. In this case the
fertile cell, which should be a zygote, is produced parthe-
nogenetically in each, and is known as an azygospore.
Another variety of parthenogenesis, which resembles the
apogamy of the Ferns, occurs in Ccelebogyne, where an
embryo is produced in the embryo-sac but without pollina-
tion or fertilisation. No sexual cell is produced, but there
occurs a vegetative budding of one or more of the cells
of the nucellus of the ovule, the buds growing into the
REPRODUCTION 449
cavity of the megaspore and there developing into
embryos. This is not quite the same thing as the apo-
gamy of the Fern, as the new sporophyte arises as a bud
upon part of the sporangium —that is, upon the parent
sporophyte and not upon the gametophyte. It is really a
curious variation of vegetative propagation.
G G
INDEX
Abnormal methods of food supply,
197
Absorption, of water 22, 69, of
food materials 140, of salts 144,
of gases 151, of organic food
197 ; conditions of continuous,
144; facilitated by CO2 145, by
acid sap 145 ; strength of salts
absorbed, 146 ; varied amounts
of salts absorbed, 146, 147
Acclimatisation, 367
Acer, 87
AconUum, 438
jtEthalium, 9
After-effect of stimulation, 398, 407
Agave, 253
Aggregation, 391, 405
Air-chambers, in Isoetes, 106 ; in
Marsilea, 107 ; in submerged
plants, 108 ; in grasses, 109 ; in
leaf of heath, 112
Albumen, 434
Albumin, 175
Albuminates, 176
Albumoses, see Proteoses
Alburnum, 72
Alchemilla, 76, 83
Alcohol, 268, 301
Aleurone grains, 249, 251
Alkaloids, 284
Alternation of generations, 426
Aluminium, 143, 188
Amidated fatty acids, 284
Amido-acids, 180
Ampelopsis, 388
Amygdalin, 252, 267
Anabolism, 137, 272
Anaerobic plants, 129, 304
Anaesthetics, action of, 408
Analysis, destructive, 143
Ananassa, 266
Anemophilous flowers, 438
Antheridia, 424
Antherozoids, 423 ; motion of, 434 ;
attraction of, 435
Anthoceros, 210
Anthocyan, 281, 323
Anthoxanthum, 441
Antipodal cells, 432, 434
Apheliotropism, 378
Aphydrotropism, 392
Apogamy, 448
Apogeotropism, 381
Apospory, 448
Apostrophe, 364
Aquatic plants, structure of, 330
Arabinose, 42
Archegonia, 425
Ascending sap, 66
Ascidia, 201, 202, 203
Ascogonidia, 420
Ascomycetes, 424
Ascospores, 420
Asexual reproductive cells, 420
Ash of plants, composition of, 143,
184 ; effect of its constituents on
vegetation, 195
Ash of proteids, 174
Asparagin, 180 ; as a reserve
material, 252 ; as a product of
digestion, 266
Aspergilhis, 264
Asphyxiation, 300
Assimilation, 137, 271 ; (of carbon
dioxide, see Photosynthesis)
Auxanometer, 314
Azolla, 210, 439
Azygospore, 448
BACH on photosynthesis, 168
Bacteria, 2, 3
Bacterium termo, relation to oxygen,
170, 392
Bambusa, 29
Bark, 20, 240
Bartsia, 204
Bast, 22, 52 ; function of, 227, 230,
232
Bauhinia, 377
GG 2
452
VEGETABLE PHYSIOLOGY
BAYER on decomposition of carbon
dioxide, 165
Begonia, 417
Bentinckia, continuity of proto-
plasm in, 16
Benzol, 182
Berberis, 384, 398
Bertholletia, 190
Betula lenta, 268
Biennial plants, 225
Bignonia, 378
Bleeding (of stems), 83, 86
Bloom (of fruits), 50
BOKORNY on action of chlorophyll
apparatus, 164
Botrychium, 198
Bromeliacea, 210
Bromelin, 266
Bromine, 143, 188, 194
BROWN and MORRIS on photosyn-
thesis, 167
Bryophyllum, 417
Buckwheat, 186
Budding, 413
Buds, position of leaves in, 315
Bye-products, 275, 283
Cabomba, 336
Cactacece, 124
Cactus, 128
Calcium, in ash, 143, 188 ; mode of
absorption of, 191 ; effect of, on
herbage, 192
Calcium pectate, 46
Calcium, salts of, in cell-wall, 50,
286
Calyptra, 445
Cambium, 238, 309
Cane-sugar, as reserve material,
247 ; digestion of, 264 ; as attrac-
tion for antherozoids, 393
Capillarity, 81
Capparidacetp, 260
Carbohydrates, 40, 143 ; formation
of, 163 ; theories of construction
of, 165 ; course of process of con-
struction of, 226 ; translocation
of, 227 ; resting and travelling
forms of, 229 ; storage of, 242
Carbon dioxide, absorption of, 114,
152 ; exhalation of, 120 ; relation
to oxygen absorbed, 125 ; inca-
pable of nourishing protoplasm,
136 ; temperature at which
decomposition by chloroplast
begins, 164
Carbon monoxide as a stage in
photosynthesis, 165
Carica Papaya, 266
Castor oil plant, see Eicinus
Casuarina, 160, 444
Cell, 2, 3 ; various applications of
the term, 16
Cell-division, 412
Cell-wall, of protoplast, 2 ; properties
of, 39 ; of Fungi, 40 ; theories of
its composition, 43, 44 ; thicken-
ing of, 44 ; stratification of, 44 ;
formation of, 276, 415
Cellulose, 39 ; properties of, 40 ;
varieties of, 40 ; as reserve
material, 247, 248
Centrifugal force, influence on
direction of growth, 382
Centrospheres, 413
Cephalotus, 203, 204
Ceramium, 16
Chara, 351
Chelidonium, 7
Chemotaxis, 394
Chitin, 40
Chlamydomonas, 349, 352
Chlorine, 188, 194
Chlorophyll, 135 ; condition in
chloroplasts, 155 ; solvents of,
155 ; properties of, 156 ; spec-
trum of, 157 ; analyses of, 158 ;
conditions of formation of, 162 ;
action of, dependent on light, 163 ;
relation to iron, 187 ; secretion of,
280
Chlorophyll apparatus, 155 ; action
of, 163 ; absorption of energy by,
292
Chloroplasts, 138, 153, 155 ; dis-
tribution of, 159 ; structure of,
161 ; functions of the two com-
ponents of, 170 ; effect of varying
pressures of carbon dioxide on,
171 ; inhibition of, 171
I Chlorosis, 163
Chlorotic plants, 281
Chromatin, 413
Chromosomes, 413
Chroococciis, 9
1 Cilia, 1, 2, 346
Ciliary motion, 346
Circulating food-stuffs, 226
Circumnutation, 316 ; dependent on
rhythm, 353
: Cladium, 28, 29
Cleistogamy, 442
| Cobalt, 188
Cobalt chloride as test for watery
vapour, 90
Ccelebogyne, 417, 448
Coenocytes, structure of, 10, 11 ;
INDEX
453
skeleton of, 39 ; reproduction of,
416, 421
Cold, injurious effects of, 328
Coleochtste, 427
Collenchyma, 21, 52
Commensalism, 210
Conducting system, 21
Conjugation, 425
Constructive processes, anabolic,
272 ; katabolic, 274
Contact, stimulus of, 384
Convolvulus, 388
Copper, 143, 188
Cork, 20, 49
Cortex, 240
Crassulacece, 124, 128
CRATO on photosynthesis, 168
Crystalloids, 251
Crystals of proteid, 251
Cucumis, 266
Ciicurbita, 87
Cupuliferce, 212
Curvature (of stimulated roots), 385
Cuscuta, 219, 343, 389
Cuticle, 48 ; influence on transpira-
tion, 90
Cuticularisation, 20, 48
Cutin, 49, 50
Cutleries,, 423
Cycas (antherozoids of), 436, 445
Cyclamen (apheliotropism of pe-
duncles of), 378
Cystocarps, 427
Cystoliths, 51, 286
Cytase, 262, 264
Cytoplasm, 5
DARWIN, on localisation of sensitive-
ness, 379, 382 ; on tendrils, 386 ;
on twining stems, 388 ; on Drosera,
391
DARWIN, F., and PERTZ on induced
rhythm, 395
Day and night, influence of alterna-
tion of, 372
Delphinium, 438
Dermatosomes, 44
Descending sap, 231
Desmids, 380
Desmodium gyrans, 355, 372
Diageotropism, 381
Diaheliotropism, 378
Diastase, 262 ; function of, 263 ; in
pollen, 443
Diaster stage in mitosis, 415
Diatoms, 348
Dichogamy, 440
Diclinism, 441
Digestion, 137 ; by Nepenthes, 203,
260; by Lathrcea, 204; by
Drosophyttum, 205; by Pingui-
cula, 206 ; by Dioncea, 208, 260 ;
by Drosera, 207, 260 ; by Bacteria,
209 ; of reserve materials, 256 ;
by Fungi, 257
Dimorphism, 442
1 Dioecious plants, 441
Dioncea, 149, 206, 208, 261, 349,
370, 384, 391, 398,^400, 403, 408
' Dodder, see Cuscuta
Dorsiventral structures, 315
Drosera, 149, 207, 261, 349, 384,
391, 394, 403, 405, 406
Drosophyllum, 205
Duramen, 72
Ectocarpus, 423
Ectoplasm, 5
Egg apparatus, 432
Elaioplasts, 254, 279
Elodea, 6, 350
Embryo, nutrition of, 133
Embryo-sac, 431
Emulsin, 262, 267
Endodermis, 69
Endosperm, 431
Energy, expenditure of, in photosyn-
thesis, 170, 288; expenditure of,
in transpiration, 288, 294 ; in
constructive processes, 288, 294 ;
in growth, 289 ; in movement,
290 ; in production of heat 290, of
light 292 ; sources of, 292, 322 ;
potential and kinetic forms of,
295 ; liberation of, 296 ; distri-
bution of, 297
ENOBLMAKN, on evolution of oxygen
in different parts of the spectrum,
170 ; on purple bacteria, 172
Entomophilous flowers, 438
Environment, nature of, 371
Enzymes, 138, 258 ; preparation of,
270 ; secretion of, 275 ; in pollen,
443
Ephedra, 50
i Epidermis, characters of, 48
: Epinasty, 315
Epiphytes, 210, 340
Epistrophe, 364
I Equatorial plate, 414
Equisetacece, 50
Equisetum, sclerenchyma of, 27, 29 ;
air-spaces in, 108 ; chloroplasts
in, 160
Erianthus, 29
ERLENMEYER on photosynthesis, 167
454
VEGETABLE PHYSIOLOGY
Erythrophyll, 158
Erythrozym, 262, 267
Etiolation, 361
Etiolin, 162; in photosynthesis,
171, 187 ; antecedent of chloro-
phyll, 280, 361
Euonymus, 162, 280
Euphorbia, 245
EWART on inhibition of chlorophyll
apparatus, 171
Exhaustion, 408
Exodermis, 21
Fat, 143 ; origin of, 183 ; as reserve
material, 253 ; digestion of, 268 ;
secretion of, 279
Fatigue, 408
Fermentation, 302, 326
Fermentative power of protoplasm,
302
Ferments (soluble), see Enzymes ;
(microbic), 302
Fern, skeleton of, 25, 26
Fertilisation, 425, 437 ; stimulating
effect of, 445
Fimbristylis, 27, 29
Flaccidity, 64 ; removal by injecting
water 91, by checking trans-
piration 96
Flagella, 2
Flowers, opening and closing of,
315, 377
Fluorine, 188
Food, its nature, 132 ; formation of,
155 ; conditions of continuous
formation of, 236
Food materials, relation to food,
135 ; mode of absorption, 140,
142
Formaldehyde, as stage in photosyn-
thesis, 165, 168 ; in construction
of proteid, 179
Formic acid, 167
Free-cell formation, 416
Frost, action of, 328
Fruit, formation of, 446
Fiwacece, 425
Fungi, composition of cell-walls of,
40 ; constructive powers of, 197 ;
digestive powers of, 209 ; haus-
toria of, 220
Funkia, 253
Fusarium, 264
Gametangia, 423
Gametes, 421
Gametophyte, 426
GARREAU on absorption of oxygen
by plants, 119
Gaseous interchanges, 103
Gases, mode of absorption of, 103 ;
movements of, in plants, 113, 114 ;
currents of, affected by external
conditions, 116; absorption of,
151
Gaultherase, 262, 267
GAUTIER on chlorophyll, 158
Gelatin, 173
Gemmae, 417
Gemmation, 413
Geotropism, 381, 395
Germination, of pollen, 443 ; of
seeds, 447
Ginkgo, antherozoids of, 436, 445
Glands, 259
Gliadin, 177, 252
Globoids, 251
Globulin, 175
Glochidia, 440
Glucase, 262, 264
Glucosides as reserve material, 252
Gluten, 251
| Gluteniri, 177, 252
j Glycogen, 246
j Gonidangia, 419
Gonidia, 419
Grafting, 220
Grape-sugar as reserve material,
247
Grasses, construction of stem of,
109
! Growing-points, 76, 309
! Growth, 224, 305 ; localisation of,
306, 309, 313 ; conditions of, 309 ;
course of, 310 ; grand period of,
311, 312; of leaf, 313; daily
period of, 314, 354
Gums, 51
Haustoria, 215, 219, 220, 343
Health, 359, 366
Heat, liberation of, 290 ; absorption
of, 323; conduction of, 326;
regulation of, 327 ; resistance to,
329
Heaths, rolled leaves of, 339
•Hedysarum, 355 ; also see Desmo-
' . dium
Helianthus, 91
Heliotropism, 378, 395, 396
Hepatica, 391
Heterospory, 429
Heterostylism, 442
i Honey, 439
i Hop, 388
INDEX
455
HOPPE-SEYLER on chlorophyll, 158,
190
Humus, 67
Hybridisation, 445
Hydrocyanic acid, 181
Hydrogen peroxide in photosynthe-
sis, 168
Hydrophilous flowers, 438
Hydrotropism, 392
Hypnum, 198
Hyponasty, 315
Individual, 411
Insectivorous plants, 149 ; Utricu-
laria, 201 ; Sarracenia, 202 ;
Nepenthes, 203 ; Cephalotus, 203 ;
Bartsia, 204; Lathrcea, 204;
Drosophyllum, 205 ; Pinguicula,
206 ; Dioncea, 206, 208 ; Drosera,
207
Intercellular passages, 31, 32 ; for-
mation of, 32, 104 ; function of,
34, 104 ; watery vapour in, 74 ;
in Isoetes, 106, 332 ; in Marsilea,
107, 335 ; relation to stomata,
111; ratio to cellular tissue in
leaves, 113 ; composition of air
in, 113, 114; positive gaseous
pressure in, 117 ; in aquatic
plants, 330
Intercellular substance, 45
Inulase, 262, 264
Inulin, 246
Invertase, 262, 264, 443
Iodine, 143, 188, 194
Iron, in ash of plants, 143, 188;
combinations absorbed, 150, 192 ;
relation to chlorophyll, 163 ;
function of, 191
Irritability, 359, 377, 394
Isoetes, 106, 107, 332, 430
JUMELLE on decomposition of car-
bon dioxide, 164
Juncus, 28, 29
Kachree gourd, 266
Karyokinesis, 413
Katabolism, 137, 272, 298
Kephir, 211
Klinostat, 381
KNIGHT on action of centrifugal
force on the direction of growth,
382
Laccase, 194, 300
Lactase, 264
Latent period of response to stimu-
lation, 407
Lathrcea, 204, 217
Laticiferous systems, 234
Leaves, sclerenchyma of, 27 ; rolled,
112 ; irritability of, 372
Lecithin, 190
LeguminoscB, cotyledons of, 40 ;
absorption of nitrogen by, 148,
178, 211 ; irritability of leaves of,
372
Lenticels, 34, 76
Leucin, 180
Leucoplasts, 162, 243
Lichens, 210
Light, influence of, on transpiration,
95 ; absorption of, by chlorophyll,
156, 292 ; necessary for formation
of chlorophyll, 162 ; action in
photosynthesis, 163, 170 ; tonic
influence of, 360 ; effect of, on
differentiation of leaves, 362 ;
effect on growth, 365 ; lateral,
378
Lignin, 47, 50, 285
Lilium Martagon, fertilisation of,
444
Lipase, 262, 268
Lithium, 188
Locomotion, 290, 347, 348
Lycopodium, 40, 198
Lythrum, 442
MACALLUM on composition of
nucleus, 191
Magnesium, in ash, 143, 188 ; mode
of absorption of, 191
Malic acid, attraction for anthero-
zoids, 393
Maltase, 262
Malto-dextrin, 263
Manganese, 143, 194
Marchantia, photo-epinasty of, 364,
380
Marsilea, 51, 107
Massula of Azolla, 440
Medullary rays, influence of, on
transpiration current, 81 ; as store-
houses of reserve material, 239
Megaspores, 429
Melibiase, 264
Melizitase, 264
Mesocarpece, 422
Mesocarpus, 358, 364, 403
Metabolism, 6, 137, 272
Metapectic acid, 42
Metapectine, 42
456
VEGETABLE PHYSIOLOGY
Methane, 182
Micellae of cell-wall, 43
Mici'ococcus Urea, 269
Microspores, 429
Middle lamella, 45
Mimosa, 62, 356, 358, 370, 372, 384,
389
Mimulus, 384
Mistletoe, 213, 343
Mitosis, 413
MIYOSHI on chemotaxis, 394
Monoblepliaris, 424
Monoecious flowers, 441
Mwiotropa, 213
Moorland plants, 338
Movements, of protoplasm, 6, 351 ;
for capture, 206, 208; of multi-
cellular organs, 349
Mucilage, 51, 248
Mucor, 448
Mycoderma aceti, 270
Mycorhiza, 213
Myriophyllum, 117
Myronate of potash, see Sinigrin
Myrosin, 262, 267
Myxomycetes, 49, 347, 392, 393, 394
NAEGELI, theory of composition of
cell-wall, 43
Nectar, 285
Nectary, 62, 439
Negative pressure, in wood-vessels,
89, 91 ; influence of, on move-
ments of gases, 116
Neottia, 198
Nepenthes, 149, 203, 261
Nervous mechanisms, 397
Nickel, 188
Nicotiana, 373
Nitella, 6, 351
Nitrates, 143, 193
Nitrification, 148, 171, 172
Nitrogen, absorption of, 113, 147;
in proteids, 143; fixation of, by
leguminous plants 148, 178, 211,
by Algae 179
Nostoc, 12
Nuclear spindle, 414
Nuclein, 8, 188, 190
Nucleolus, 8
Nucleoplasm, 8
Nucleus, 5 ; position in cell, 7 ;
structure of, 8 ; definitive, 432 ;
generative, 436
Nutation, 316
Nyctitropic movements, 326, 373,
375, 377
Nymphcea, 108, 333
Oil, see Fat
Oncidium, 253
Oogonia, 424
Oospheres, 423
Oospore, 425
Opuntia, 124, 128
Orchidacece, 210
Ornithogahim, 253
Orobanchacecs, 343
Orobanche, 217
Orthocarbonic acid in photosyn-
thesis, 168
Osmosis, 55 ; in adult cells, 60 ;
from veins of leaf, 74 ; influence
of, on transpiration- current, 100,
102
Ouvirandra, 332
Ovule, 431
Oxalic acid, 129, 179
Oxalidacece, 372
Oxalis acetosella, 363
Oxidases, 269, 300
Oxidation, 299
Oxygen, mode of absorption by
plants, 15, 31, 115 ; influence of,
on action of roots, 85 ; proof of
absorption of, 119 ; relation of
absorption of, to exhalation of
carbon dioxide, 125 ; variation in
amount of, as affecting respiration,
130
j Pceonia, 40
I Pangium, 181
1 Papain, 266
| Paraheliotropism, 363
Parapectine, 42
Parasites, 135, 341
Parasitism, 198, 210, 214
Paratonic influence of light, 360
Parnassia, 440
Parthenogenesis, 448
Passiflora, 386, 398
| Pectase, 262, 265
i Pectic acid, 42 ; relation to middle
lamella, 46
Pectine, 41
Pectose, 41
Pediastrum, 12
Pelvetia, 18
Pennisetum, 27, 29
Pepsin, 262, 265, 266
j Peptone, 176
1 Perception, 405
i Periodicity of root-pressure, 87, 357
Perisperm, 434
i PFEFFEB, on water-culture, 185 ; on
localisation of sensitiveness, 382,
400
INDEX
457
Phajus, 245
Phalaris, 379, 398, 400
Phosphorus, in proteicls, 143, 188 ;
mode of absorption of, 150 ; asso-
ciated with nucleus, 189
Photo-epinasty, 364
Photosynthesis, 164 ; theories of,
165 ; not carried out by fungi, 172
Phototaxis, 380
Phototonus, 359
Pinguicula, 206
Pitcher-plants, see Insectivorous
plants
Planogametes, 422
Plantago, 441
Plasmatic membranes, 6, 57, 59
Plasmodium of Myxomycetes, 4, 9,
347
Plasmolysis, 59
Plastids, 5
Polar nuclei, 432, 444
Pollen grain, germination of, 443
Pollen tube, 436
Pollination, 437 ; methods of, 438
Polygamous plants, 441
Polygonum, photo-epinasty of, 364
Polymerisation of aldehydes, 166,
167
Polytrichum, 26
Porlieria, 392
Positive pressure of gases in inter-
cellular reservoirs, 117
Potamogeton, 31
Potassium, in ash, 143, 188 ; con-
dition of, in soil, 150 ; mode of
absorption of, 190; function of,
191, 192
Potometer, 97
Predominant form of plants in
different groups, 428
Prepotency, 442
Primula, 283
Procarpium, 424
Protandry, 440
Proteids, 173 ; composition of, 174 ;
crystals of, 174; reactions of, 174 ;
coagulated, 177; construction of,
178, 182; translocation of, 229,
232 ; storage of, 230
Proteolytic enzymes, 262, 265
Proteoses. 176
Protococcus, 9
Protogyny, 440
Protoplasm, composition of, 6 ;
movements of, 6, 351 ; properties
of, 12 ; continuity of, through
cell-walls, 15, 404 ; regulation of
osmosis by, 60, 101 ; relation of,
to respiration, 127; nutritive sub-
stances for, 134 ; fermentative
activity of, 257, 269, 302; per-
meability of, by water, 352, 403
Protoplasts, structure of, 2 ; arrange-
ment of, in multicellular plants, 3
Prunus, 87, 267
| Prussic acid, 252
i Pseudopodia of Myxomycetes, 4, 347
Pulvinus, 62, 355, 376, 389
Pythium, 424
V
Kadiation, 291, 325
Kaffinose, 247
Rafflesia, 219
Raphides, 287
Kectipetality, 319
Reflex action, 405
Rennet, 262, 267
, Reproduction, 411 ; relation to
growth, 412; vegetative, 417;
asexual, 418 ; sexual, 421
Reserve materials, 236
Reservoirs, of air, 106 ; of food, 138,
237
Resin, 282
Respiration, gaseous interchanges
of, 121 ; masked by photosyn-
thesis, 122 ; loss of weight in-
volved in, 122 ; intensity of, 123 ;
nature of the process of, 126 ;
relation of, to metabolism, 127 ;
influence of external conditions
on, 129 ; of seeds, 130 ; a source
of energy, 298 ; intramolecular or
anaerobic, 301
Respiratory quotient, 124
Rhamnase, 262, 267
Rhamnus, 267
Rhodophycece, 424, 427
Rhythm, of root-pressure, 83 ;
nature of, 353 ; affected by stimu-
lation, 395 ; induced, 895
Ricinus, 190, 239
Ripening, 446
Robinia, 62
Root-hairs, 68 ; action of, 69
Root-parasites, 214, 343
Root-pressure, 71, 81, 82; rhythm
of, 83, 87 ; method of measuring,
84, 87 ; amount of, 85 ; perio-
dicity of, 87
Rush, construction of stem of, 110
Saccharomyces, see Yeast
SACHS, on theory of transport of
water in wood, 79 ; on a method
of estimating rate of transpiration
current, 81
458
VEGETABLE PHYSIOLOGY
Salvinia, 107 ; germination of me-
gaspore of, 429
Santalacea, 214, 343
Saprolegnia, 394, 419, 448
Saprophytes, 135, 198, 341
Sarracenia, 149, 202
SAUSSURE, DE, on Opuntia, 128
Saxifraga, 261, 337
Saxifragacece, 124
Scirpus, 27, 29
Sclerenchyma, 21, 25, 29
Scrophularia, 441
Scrophulariacece, 214, 343
Scutellum, epithelium of, 260, 263
Scyphanthus, 388
Secretion, 275
Seed, 429 ; formation of, 431 ; struc-
ture of, 433, germination of, 447
Selaginella, prothallium of, 430
Selective power, 188, 195
Self-sterility, 442
Sense organs, 400
Sensitiveness, variations of, 379 ;
localisation of, 379, 382, 401 ;
delicacy of, 386, 409 ; mainte-
nance of, 408
Sex, differentiation of, 423
Sexual cells, 421
Sieve-tubes, 182, 232
Silica, 50, 287
Silicates-, 150, 193
Silicic acid, 150
Silicon, 143, 150, 188, 193
Sinigrin, 253, 267
Skeleton, of plastid, 2, 37 ; of plant,
25,37
Sleep movements, see Nyctitropic
movements
Slime fungi, see Myxomycetes
Sodium, 143, 150, 188, 192
Soil, composition of>f>7
Sorus, 440
Spermatium, 435
Spermatozoids, 423
Sphagnum, 19
Spirogyra, 159, 166, 411, 427
Splachnum, 198
Sporangia, 419
Spores, 426
Sporophyte, 426
STAHL on method of detecting escape
of watery vapour, 90
Starch, appearance of, during photo-
synthesis, 166 ; probable cause of
appearance of, in leaf, 169 ; forma-
tion of, as evidence of activity of
chlorophyll apparatus, 170 ; depo-
sition of, in chloroplasts, 228, 242 ;
removal of, from leaves, 238 ;
storage of, 242 ; deposition of, by
leucoplasts 243, by protoplasm
245 ; digestion of, 263 ; secretion
of, 279
Stele of root, 69
Stimulation, 368 ; nature of, 370 ;
purposeful nature of responses to,
370, 405, 406 ; localisation of, 379,
387 ; after-effects of, 398 ; percep-
tion of, 405 ; latent period of, 407
Stimulus, nature of, 368 ; instances
of rhythmic, 372, 374 ; of lateral
light, 378 ; of gravitation, 380 ; of
contact, 384, 389 ; of moisture,
391; of oxygen, 392; chemical,
392 ; relationship of response to,
406 ; of fertilisation and pollina-
tion, 445
Stomata, 33, 75, 92; number of,
in different plants, 94 ; mode of
action of, 94 ; regulating influence
of, on transpiration, 95 ; relation
to gaseous interchanges, 112, 117
Storage of food, 225, 238
STRASBURGER, on composition of cell-
wall, 43 ; on evaporation of water
from cell-walls, 100
Stratification of cell-wall, 44
Style, reserve materials in, 241
Stylogonidia, 420
Stylospores, 420
Suberin, 285
Sugar, formation of, 166, 169 ; fer-
mentation of, 301
Sulphur, in proteids, 143, 188 ; mode
of absorption of, 150
Sundew, see Drosera
Sunlight, influence of, on transpira-
tion, 95
Surplus food, construction of, 222
Symbiosis, 198, 210
Synergida?, 433
Tegumentary system, 19
Telegraph plant, see Desmodium
Temperature, influence of, on ix>ot-
pressure,85 ; on transpiration, 96 ;
on respiration, 129 ; range of, for
different functions, 320 ; fluctua-
tions of, 321
Tendrils, 386
Tensions, in hollow stems, 30; in
growing organs, 317
Testa of seed, 433
Thalictrum, 438
Thallophytes, 427
Theca, 427
Thermotonus, 360, 366
INDEX
459
Thesium, 215, 388
TMIRIAZEFF on relative values of
the rays of the spectrum in photo-
synthesis, 171
Tone, 359, 364, 367
Torsion of stems, 388
Tradescantia, 7, 351
Translocation, of food, 223 ; direc-
tion of, 230 ; path of, 233, 234
Transpiration, 75, 81, 88 ; methods
of demonstrating, 88 ; amount
of, 91 ; effect of excessive, 91 ;
influence of external conditions
on, 95 ; suction of, 100 ; functions
of, 324, 325 ; relation to etiola-
tion, 368
Transpiration current, 78, 80
Trees, conditions of life of, 24
Trehalase, 264
TREUB on construction of proteid
in Pangium, 181
Trichogyne, 424
Tropceolacece, 260
Tropczolum, 83, 167
Trypsin, 262, 265, 266
Turgescence, 61 ; importance of,
64; methods of restoring, in
flaccid tissue, 91 ; affected by
shaking branches, 97
Turgor, 351 ; also see Turgescence
Twigs, reserve materials in, 241
Twining organs, 386
Typha, 27, 29
Tyrosin, 180
Tyrosinase, 300
Ulothrix, 1, 411, 418, 421, 422
UNGER on relative volumes of air
and cellular tissue in leaves, 113
Unicellular plants, 2
Urea, 149
Urease, 269
Utricularia, 199
Vacuole, 4 ; formation of, 13, 56 ;
function of, 14 ; occasional ab-
sence of, 54 ; pulsating, 348, 352
Vallisneria, 6, 351
Vascular bundles, 22, 27
Vauclwria, 421
Vegetable acids, 284
Vegetative propagation, 417
Veins of leaf, 22, 72
Velamen, 340
Venus' s fly-trap, see Dioncea
Vicia, 400
Victoria regia, 333
VINES on photosynthesis, 168
Viola, 442
Vitis, 87
Volvox, 9, 10
Water, importance of, to proto-
plasm 13, 14, to plant in general
14 ; absorption of, by aquatic
plants, 22 ; function of, in cell,
54 ; circulation of, in plant, 54 ;
exhalation of vapour of, 60, 125 ;
hygroscopic, 68; absorption by
root-hairs, 69 ; transport in root
70, in stem 72 ; excretion of, 76 ;
mode of transport in wood, 79 ;
exudation of, under pressure, 83 ;
exudation affected by salts, 86
Water-culture, 140, 185
Water-glands, 337
Wax in cell-walls, 50
WIESNEB, on composition of cell-
wall, 44 ; on heliotropism, 398
Withering, 91
Wood, 22
Xanthophyll, 157, 285
Xerophytes, 336
Yeast, 2, 302, 412, 420
Zein, 177
Zinc, 143, 188
Zirconium, 188
Zoocoenocyte, 421
Zoogonidia, 419
Zoophilous flowers, 438
Zoospores, 1, 419
Zygnema, 159, 422
Zygospore, 422, 425
Zygote, 425
Zymase, 269
Zymogen, 259
Square, London.
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IAN 3
T 2 1 1937
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•f-n-hirnductio:! to
Ivegetational physiology
OCT 21 1937 Perry
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