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The Rural Texrt=-Book Series
Epitep By L. H. BAILEY

PLANT PHYSIOLOGY.

WITH SPECIAL REFERENCE TO

PLANT PRODUCTION

The Rural Text-Book Series

Lyon AND Fippin, THE PRINCIPLES OF SOIL
MANAGEMENT.

WARREN, ELEMENTS OF AGRICULTURE.

J. F. DUGGAR, SOUTHERN FIELD CROPS.

B. M. DuGGArR, PLANT PHYSIOLOGY.

Others in preparation.

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Coryrigut, 1911,
By THE MACMILLAN COMPANY.

Set up and electrotyped. Published May, rgrt.

; Norwood press
J.8. Cushing Co. — Berwick & Smith Co,
Norwood, Mass., U.S.A.

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PREFACE

In the preparation of this text and reference book, the
writer has attempted to consider both the student and
the general reader, interested alike in the fundamental
requirements of plants and in plant production. Through-
out biological study at the present time increased emphasis
is placed on the activities and responses of organisms.
It is instruction in this type of biological phenomena that
is rapidly becoming a part of the cultural side of education,
and the practical value of such knowledge is every day
being demonstrated, — notably in agriculture and medicine.
Plant physiology finds its practical applecation in plant
production, to which it stands in much the same relation
as does industrial chemistry to general manufacturing.

It is somewhat strange, therefore, to find that as a sepa-
rate course plant physiology is not yet offered in some of
the colleges whose purpose is primarily to train persons
for practical or rural pursuits. Such students require some
fundamental work, and few will become specialists. For
this general class of students, and for other readers as well,
there seems to be needed a text (1) that shall exhibit a con-
siderable range of material, rather than a few topics ex-
haustively treated; (2) that shall include both qualitative
and quantitative work; and (3) that shall keep in view,
as far as possible, the relations of the science to plant
production, drawing the illustrations, wherever convenient,

v

vi Preface

from plants which are familiar and directly useful. By
maintaining some direct contact with practical problems,
interest is aroused for further desirable fundamental prepa-
ration. “The idea that useful knowledge cannot be cultural
must be dismissed. ... Every possible application must
be made of each abstract principle.” (Eliot, “The Conflict
between Individualism and Collectivism in a Democracy,”
page 66.)

In the field of pure physiology, there are recent texts
and ‘guides embodying much of what is considered best
in the modern content and attitude of the science. An
elaboration of the methods of quantitative study is there
indicated, and stress is laid on the materials and energy
involved in plant activity. Such books will be consulted
with much profit.

In selecting from the great amount of available material
that which has seemed to be most suitable for the present
purpose, consideration has been given the fact that in many
colleges general courses are offered, not only in such dis-
tinctively plant lines as agronomy, horticulture, and breed-
ing, but likewise in fields overlapping physiology, or partially
included in this subject, such as soils, bacteriology, pathol-
ogy, and genetics. The subject-matter included is intended
to be sufficient for a course of one-half year involving two
recitations and two laboratory periods; but it may be
made the basis of a shorter course by suitable selection of |
material, or of a longer course by an extension of the col-
lateral work.

In the preparation of this text I have used freely any
available source of information. The subject-matter has
been presented at one time or another in class work. Iam
indebted to Mr. Lewis Knudson, Instructor in Plant Physi-

Preface vil

ology in Cornell University, for many suggestions and for
the form of certain sections of the laboratory notes. Some
of the illustrations were furnished by others, or borrowed,
as credited in the text. Certain of the drawings were pre-
pared by Miss Anna M. Keichline; others by Mrs. B. M.
Duggar, of whose constant assistance with manuscript and
proof I would express also my appreciation.

B. M. DUGGAR.

pl a

CONTENTS

CHAPTER «I

INTRODUCTION
PAGES

Permanent high production -— The relation of physiology to pro-
duction — Physiology and ecology — Physiological processes
— Environment — Crop ecology — Literature of Plant Physi-
ology — Physiology and other sciences — References . 1-14

CHAPTER. if
THe Puant CELL

The cell a physiological unit — Early use of the term “cell” — Meri-
stem or embryonic cells — Cytoplasm — The nucleus — Plas-
tids — The cell- wall — Cell-sap — Cell-forms — Parenchyma
Sclerenchyma— Trachez or vessels— Sieve tubes — Proto-
plasmic movement —Protoplasmic irritability and response
— Laboratory work—References .. . ; - - 15-34

CHAPTER III

THE WATER-CONTENT OF PLANTS AND THE GENERAL RELATIONS
oF Root SysTEMS

Hydrostatic rigidity — The water-content of plants — Variation
in water-content of organs— The water-absorbing system —
The root habit of crops— The production of root-hairs —
Root-hairs and the water-content of the soil — The root-cap
— Structure of the root-tip — Soil particles —Soil texture

ix

x Contents

PAGES
and water-holding capacity — Exceptional plants — Unavail-
able water — Leaves poorly fitted for water absorption —
Laboratory work — References ‘ ; : : : 35-63

CHAPTER IV

CONDITIONS AND PRINCIPLES OF ABSORPTION

Imbibition phenomena — Osmosis and diffusion— The demon-
stration of osmotic pressure— An explanation of osmotic
pressure — Plasmolysis and wilting — Variation in turgor —
Substances active in producing turgor — Osmosis and the
absorption of nutrient salts— Protoplasmic permeability —

The role of diffusion and osmotic pressure—Sap or root
pressure — Laboratory work — References . : : 64-83

CHAPTER V

TRANSPIRATION AND WATER MOVEMENT

Observations upon transpiration — Amount of transpiration —
The mechanism permitting transpiration — Distribution of
stomata — The control of water-loss by stomatal movement
— Modifications tending to check excessive transpiration —
Conditions affecting transpiration — Effects of excessive
evaporation — Guttation — Transpiration and evaporation —
Transpiration and growth — Water transport — Fibrovas-
cular bundles — Leaf venation — Rate of transport — Labora-
tory work — References : : ; ; ; . 84-115

CHAPTER VI

THE WaTER REQUIREMENTS OF CROPS AND OF VEGETATION

Relative requirements of a few crops— Precipitation and crop
growth — Irrigation — Potted plants and water-supply —
Ecological classification based upon the water relation —
Semi-xerophytism and hard-wheat production — Subsidiary
work — References . : : : : : : . 116-135

Contents xl

CHAPTER VII

MinerAL NUTRIENTS
ee PAGES
The ash content of plants— Composition of the ash — Effects of

conditions upon ash content — Ash content at different ages
— Translocation of mineral substances — Water cultures —
Nutrient solutions and water cultures— Strength of the
nutrient solution — The forms of the nutrient compounds —
Plant nutrients in rock — Soil fertility — Nutrients removed
by farm crops — Nutrients removed by fruit crops — Amount
of nutrients in soils — Availability of the nutrients — The
solvent action of roots — CO, excretion and the availability
of phosphorus— Another view of soil fertility — The paraf-
fined wire basket in nutrition studies — Laboratory work —
References. ; . ‘ . : . 136-168

CHAPTER VIL
SPECIAL FUNCTIONS AND RELATIONS OF MINERAL NuUTRIENTS

The roles of mineral nutrients: The nature of the special réles
— The role of phosphorus— The rdéle of potassium — The
role of magnesium — The role of calcium — Iron — Sodium
— Chlorine — Sulfur — Silicon — References . : - 169-183
Balanced solutions: The injurious action of certain basic nutri-
ents — The relation of calcium to magnesium — Other
nutrient bases and antitoxic action— Laboratory work —
References. : : ; : ; ; : . 184-194

CHAR TRER. 1x
THe INTAKE OF CARBON AND THE MAKING. OF ORGANIC Foop

The amount of carbon in the plant — Carbon dioxid the source
of carbon in green plants—Chlorophyllous plants — Re-
specting the distribution of chlorophyll — The nature and
properties of chlorophyll — The factors essential in photo-

xl

Contents

PAGES

synthesis — The course of photosynthesis — The demonstra-
tion of photosynthesis — The formation of sugar and starch
The diffusion process— The amount of carbon dioxid —
Light the source of energy — Efficiency of the food~making
apparatus — Light, intensity and quality — Temperature —
Organic matter, rate of production — Laboratory work —

References : Z ; ‘ : : : 7 . 195-2

CHAPTER X

THe RELATION TO NITROGEN

Combined nitrogen — The nitrogen content of plants — Synthesis

of nitrogenous bodies — Soil nitrogen — Nitrites — Nitrates
— Compounds of ammonium — The sources of soil nitrates
and ammonia — Ammonification — Nitrification — Nitrifying
organisms — Conditions favoring nitrification — Denitrifica-
tion — Nitrogen fixation — Organisms which fix free nitro-
gen — Bacteria of leguminous tubercles — Certain saprophytic
soil bacteria — Fungi — Mycorhizal fungi— General sources
of supply of nitrogen — Electric fixation of nitrogen —

25

Laboratory work — References , : ; : . 226-249

CHAPTER XI

Propucts oF METABOLISM; DiGEsTION AND TRANSLOCATION

Metabolism — Temporary foods, storage products, and perma-

nent structures— Annuals, biennials, and perennials — Car-
bohydrates — Sugars — Starches — Cellulose — Fats and oils
__ Proteins — Classes of proteins — Amides — Organic acids
—Tannins — Resins and turpentine — Digestion — Diges-
tion in different organisms — Enzymes and enzyme action —

Carbohydrate enzymes and their products — Protein enzymes |

— Conduction of digested foods — Ringing — Laboratory

work — References . : : ; : ; ; . 250-279

Contents Xlll

CHAPTER XII

RESPIRATION, AERATION, AND FERMENTATION
5 M : J PAGES
The term ‘‘respiration’’— An obvious result of respiration —

The demonstration of respiration — Respiratory phenomena
in aérobic respiration — Oxygen promotes catabolic processes
— The ratio of O2 absorption to COz production — Respira-
tory activity — Respiration of wounded plants — Heat re-
lease — The mechanism for gas exchange — Anaérobic
respiration — Fermentation — Lactic fermentation — Alco-
holic fermentation — Acetic fermentation — Laboratory work
— References . ; ‘ : : , : : . 280-504

CHAPTER XIII
GROWTH

The factors — Evidences of growth— Growth of the embryo—
Polarity — Elongation of roots — The stem apex — The for-
mation and exfoliation of leaves — The resting bud — Types
of stem elongation — Fruit buds and age of shoot — Persist-
ence of the rest period in temperate regions — Differentia-
tion of stem tissues — Secondary thickening — Growth of
the cell — Cell division — Nuclear division — Cell division
and respiration — The relation of pruning to growth — Bud-
ding and grafting—Scion propagation — Relation of stock
to scion — Forcing — Etherization — The effect of etheriza-
tion — Forcing by immersion in warm water — Transplanting
after wilting — Growth movements — Laboratory work —
References . , : : : : . : . 9805-346

CHAPTER XIV
REPRODUCTION

The seed habit and vegetative production — The flower: essential
structures — Pistil and stamen — Pollination and pollen-tube
penetration — Fertilization— Universality of fertilization —
Cross-fertilization and self-fertilization — Cross-fertilization

XIV Contents

PAGES
apparently the rule — Darwin’s conclusions — The need of

further work — Experiments with self-sterility in pear —
Self-sterility in other orchard trees — Parthenogenesis —
Xenia in corn — Indications of xenia— False xenia — Other
secondary effects of pollination — Parthenocarpic develop-
ment — Parthenocarpic formation in pomaceous fruits —
Seedlessness in the orange, grape, and banana — Nonsexual
reproduction — Thickened roots and tubers — Cuttings —
Precautions with cuttings — Vegetative reproduction and
running out— Relation of vegetation to fruiting — Labora-
tory work — References . : : : : : . 9347-380

CHAPTER XV
Tue SEED IN PLANT PRODUCTION

Habitat conditions of the parent plant — Localization of seed
production — Maturity — Conditions of harvesting and cur-
ing — Duration of vitality — Environmental conditions —
Buried seed — Delayed germination — Effect of weight and
size of seed upon vigor — Experiments with wheat — Ex-
periments with cotton — Experiments with tobacco — Lab-
oratory or supplementary work — References , . 381-399

CHAPTER XVI

Ture TEMPERATURE RELATION

Ciimatic extremes and introduced plants— Temperature and
production — Cardinal temperatures — Inhibition at high
temperatures — Heat units — Heat units and germination —

The date-palm — Control of temperature — The temperature
of the plant — Adjustment of structure — Irritable response
— Freezing — Buds — Laboratory work — References . 400-414

CHAPTER: XVII
THe LiGut RELATION

The adjustment of plant members — Light perception — Diverse
requirements — Light intensity — Injurious effects — Artifi- |

Contents XV

PAGES
cial light — Monochromatic light — Half-shade in plant
propagation — Crops responding to half-shade — Morpho-
genic effects — Half-shade and quality — The effect of shad-
ing upon other environmental factors— Laboratory work —
Hseterences~ ; : 3 : : ; : : . 415-455

CHAPTER XVIII
RELATION TO DELETERIOUS CHEMICAL AGENTS

General relations to poisons — Comparative resistance — Toxic
action and the substratum — Method of action — Inorganic
and organic acids — Alkalies— Salts of the heavy metals —
Formalin — Organic bodies — Root excretions — Unproduc-
tiveness — Relative toxicity of some organic compounds —
Illuminating gas — Stimulation by means of weak toxic
agents — Protection of crops by insecticides and fungicides
— Destruction of weeds by poisons — Deleterious substances
employed — Practicability of the chemical method — Labora-
tory work — References . . - ; ; ‘ . 436-462

CHAPT Hr aive
VARIATION AND HEREDITY

Variation: Individuals and species — Fluctuating variations —
Darwin’s theory of natural selection — Rate of increase —
Fluctuating variation and the origin of varieties — Pure lines
— Mutations— Mutation and crop improvement . . 463-476

Heredity : Nonsexual reproduction and heredity — Sexual repro-
duction and heredity — The early studies — Types of inherit-
ance — Recent studies— Mendel’s experiments— Purity of
the gametes— Results of segregation— Tomato characters
— Chromosome relations — Selection — Laboratory work —
Hererences ; A : 3 : : : . 476-493

CHABURn kek
GrowTtH MovEMENTS

Stimulus and response — Tropic curvatures — Geotropism — Thig-
motropism — Chemotropism — Nutation — Nastic curvatures
— Nyctitropism — Laboratory work — References . . 494-507

ANT PEYSEOkoG ¥

CHAPTER I
INTRODUCTION

THE relation of plant physiology to crop production and
vegetation requires no explanation except where physiology
and plant production are alike incompletely comprehended.
No one may thoroughly understand a modern agricultural
problem who has not learned the full significance and
scientific relations of the two eminently practical terms
“production”? and “‘conservation.’”? The basal field of
agriculture is plant production, for upon this animal pro-
duction is dependent; and throughout all agriculture con-
servation is necessarily the key to continuous development
and success. 7

Conservation in the broadest sense implies neither
waste of the product grown nor waste of the forces and
conditions which make high production possible. These
forces are the environment under which the crop is grown
and the inherent hereditary possibilities within the seed
or seed-material.

1. Permanent high production. — Plants form the nat-
ural covering of the surface of the earth, and if there is
at present no such covering, where the rock is sufficiently

B 1

2 Plant Physiology

decomposed to be termed soil, it indicates that something
is radically wrong with the soil or climate of that region
from the standpoint of the permanent occupation of it by
man or animal as well as by plants.

Of all object lessons in permanent occupation by plants,

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Fic. 1. Inthe Rainier National Forest, Washington. [Photograph from

the Forest Service. ]
that of the aged forest stands out supreme. Here a vigor-
ous growth may have endured for centuries, and except
for such accidents as those of floods and fires, or of fla-
grant devastation by man, it might continue for centuries
more. So far as may be seen, or measured by the short
space of agricultural record, at least, there is with the
greater growth of the forest an ever increasing fertility of
the land.

Introduction 3

From this lesson of production and conservation one
turns to another in discouraging contrast, — and that
other is this: much of this fertile forest land has been
cleared, and year by year fields which were once highly
productive are left untilled, or abandoned as of no longer
interest agriculturally. If lessened production is the cause
of abandonment or discouragement, the system which
leads to waste of this nature should have a speedy end.

All of the results of science and practice are needed to
assist in working a change in the conditions. Each science
may contribute something.

2. The relation of physiology to production. — Plant
physiology is anintimate part of scientific plant production.
It concerns itself with plant response and plant behavior
under all conditions; that is, with all relations and processes
readily evident or obscure, simple or complex, which have
to do with the maintenance, growth, and reproduction of
plants. It is then concerned with vegetation or crops,
with the relation of the plant as a whole, and with all
special responses or functions of any organ or cell. From
the standpoint of physiology one should be able to get
facts alike applicable in understanding or interpreting
the behavior or yield of plants of all description. The
principles of growth are learned by the same methods,
whether the plants are those constituting the vegetation of
the mill-pond or of the vast fields of cultivated grain; of
the greenhouse or of the weedy growth of the neglected
lot; of the sparse vegetation of the poor prairie or of the
primeval forest. Throughout all, the principles involved
are ultimately those of analyzing the complex stimuli and
the resulting growth, or maintenance, and reproduction.

4 Plant Physiology

Through physiological study it is possible to understand
better that which pertains to production, since increased
knowledge of plant response makes it more nearly possible
to modify opportunely and to improve upon current prac-
tices of production, and to develop progressive varieties
and strains. Obviously, production involves a variety of
nonphysiological conditions, but it also involves physio-
logical conditions, and little progress may be anticipated
without an intimate knowledge of the relation of the
growth of the plant or crop to the conditions under which
it is grown.

3. Physiology and ecology. — At the outset, moreover,
it is necessary to recognize two possible lines of study and
observation. The one is primarily concerned with the
isolated and controlled plant and the functions or responses
of its diverse organs and structures. This is generally
considered pure physiology. The other line of study deals
with plants or the crop in the field, or stated technically, in
a natural or seminatural habitat. This is field physiology
or ecology. There is, of course, no sharp line between
the two subdivisions indicated, and both are important in
production. It is necessary to know the plant, and it is
equally essential to know the environment, for that is the
sum of the conditions to which the plant responds.

4. Physiological processes. — The engineer who does
not understand his machine cannot expect to get effective
work out of it. He should know its intimate structure,
what work it can perform under all conditions, and how it
may be controlled. In the same way the plant producer
who knows the structure of the plant and its behavior is
provided with the means of interpreting the effects of con-

Introduction =

ditions upon the organism. The plant is a delicate physi-
cal, chemical, and living mechanism; and through the
work which is performed, often expressed in growth, or
change of some sort, it is responsive to practically all
manner of stimuli.

Under whatever conditions it may be able to grow and
reproduce itself there are many phenomena, or processes,
which are recognized as fundamentally physiological, al-
though the information respecting these may often be es-
sentially physical or chemical. All modern physiologists
are necessarily pursuing physico-chemical methods in in-
terpreting all the activities of plants. Among these pro-
cesses may be mentioned the absorption, movement, and
incorporation of water and of gases; the absorption and
disposition of nutrient salts; the manufacture of organic
food material; the accumulation, digestion, and assimila-
tion of foods or food materials; respiration; growth and
variability ; reproduction and heredity; and the special
growth or other changes as responses to environmental
factors. Much of the material presented in this book is
for the purpose of demonstrating simply some of the essen-
tial principles involved. Qualitative measurements are
frequently sufficient.

5. Environment. — The environment of any plant or
crop is a complex of factors or conditions as a resultant of
which there is the response of the plant in vigorous or
weakly production, and in diverse form or habit of growth.
Most of the important factors of the environment affecting
the agricultural plant are perfectly obvious. In consider-
ing these one almost unconsciously thinks of those factors
which operate above the soil and those which operate

6 Plant Physiology

through the soil. This division of the factors is not wholly
satisfactory, as will be seen. Agronomy is commonly sub-
divided nowadays into “‘soils”’ and “crops”; but these
do not exclude physiology, they make it, in its broadest
relations, more nearly indispensable.

Conditions acting above the soil are mainly sunshine,
heat, precipitation, humidity, evaporation, and air move-
ment and composition; while through the soil there is af-
forded fixity, mineral nutrients, and water, as well as heat
and air. There are, however, many other factors acting
above or below the soil which may affect vegetation directly
or indirectly, including the bacterial flora of the soil, and
injurious substances in the soil solution; fungous diseases,
insect pests, and higher animals; the constant factor grav-
ity ; and many special stimuli.

This complexity of environmental factors renders an
interpretation of the effects of any one factor in terms
of plant behavior peculiarly difficult. Progress in field
study, or experimental ecology, is nevertheless being made,
and much is due to the greater perfection in instrumenta-
tion as applied to the habitat.

In nature a species of plant thrives best where the re-
quirements of the particular form are most completely
met. In the case of the cultivated plant, however, while
the natural factors condition production, these alone are,
of course, insufficient to determine whether or not a par-
ticular crop should be grown in a particular locality, for
there are a host of economic considerations which must
have weight. Market, transportation, labor, machinery,
and many other factors must be taken into account.

From the point of view of the natural factors there are

Introduction i

again two lines of inquiry: (1) Is the crop suited to the
general conditions of the region? (2) Are the conditions
of environment and the cultural practices afforded the
crop such as to result in maximum yield? It is not ex-
pected that any considerable number of facts enabling one
better to answer the first question shall be presented in
this book; such facts, at any rate, may be only incidentally
touched upon. The fundamental facts developed should,
however, enable one to observe, test, and answer for himself
more completely any question which would fall in the sec-
ond group of inquiry.

6. Crop ecology. — Since ecological data may be in-
cluded only incidentally, a few words may be said here
concerning the general relations of plants as distributed
over the surface of the earth or as cultivated under special
conditions. In the steppes of northern Africa and portions
of Australia, in the dry prairies of the western United States
and southern Russia, or in the equivalent regions of west-
ern Brazil, the vegetation is similar in physiognomy. Here
tough and drought-resistant grasses thrive, and for the
most part these regions are treeless tracts where rains fall
infrequently or precipitation is poorly distributed, and
here, too, winds often exert their highest force. The great
permanent grass lands, such as these areas are, may be
considered as being ecologically most closely related to true
desert.

On the other hand, in tropical or temperate regions of
abundant, or at least sufficient, rainfall forests of one type
or another find a natural home. It is clear that upon the
discovery of North America, the region now included in the
United States was, practically speaking, a continuous forest

8 Plant Physiology

extending from the Atlantic westward to the region of little
precipitation.

Agriculture and commerce have already encroached to
an enormous extent upon the natural domain of both the
native forest and grazing lands; but in mountainous
regions and towards the northern limit of vigorous growth

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Fic. 2. Merriam’s Life Zones of the United States; Boreal (1), Transi-
tion (2), Upper and Middle Austral (3), Lower Austral (4), Gulf strip
of Lower Austral (5), and Tropical (6). Dotted parts of the Austral zones
indicate humid divisions. [After Cockerell, in Bailey’s Cyclo. Agr.]

most small crops become less profitable, and the forest will,
through many generations, at least, form the natural
boundary line separating agriculture from the Arctic zone.
In addition, of course, forests will continue to thrive in the
agricultural region where permitted by man.
Furthermore, in taking a bird’s-eye view of the forests
of the United States there is noticed a more or less striking
limitation in range, hence in general adaptability, of many

Introduction 9

well-known forest species. Thus the range of white pine
as a commercial crop is practically limited to a region ex-
tending westward from the New England States to Min-
nesota, while the long-leaf yellow pine is restricted to the
sandy coastal region of the Southern Atlantic and certain
Gulf States.

A similar relation of particular crops to one or more
factors of the environment is strikingly brought out by
those crops especially that are commonly associated with
southern climates. Cotton has a relatively long season of
growth, and it is restricted in the United States to a region
practically below the thirty-seventh parallel,—a region
which is, for about seven or eight months of the year, free
from frost and with a high mean temperature. Citrus
fruits are accustomed to an almost continuous growing
season, where frosts are few and severe freezing practically
unknown.

A large proportion of the varieties of rice, also restricted
to warm regions, may not be grown beyond those sections
in which irrigation is possible. Hard wheats gradually
lose the quality of ‘‘ hardness ” (high per cent of gluten)
when grown in moist regions. The potato is grown from
Canada to Texas and from Scotland to Italy. It is inter-
esting to note, however, that in the United States, usually,
the yield diminishes toward the South; and, except under
special conditions, the crop matures relatively early
throughout the United States. Under such conditions
average production is but about 85 bushels per acre.
With intensive culture 400 bushels is a maximum for some
of the most productive lands in the eastern United States,
although 1000 bushels have been reported under peculiar

10 Plant Physiology

conditions in the far West. In Scotland, with its more or
less continued cool climate, affording a long growth and
slow maturity of the potato, we find an average of nearly
250 bushels per acre; while a maximum of 1000 to
1200 bushels is commonly attained. At the well-known
seed farm of Lord Rosebery a yield at the rate of over 1700
bushels per acre was reported for a particular plot during
the past season. Such facts as these cannot fail to be sug-
gestive from the ecological standpoint.

If the more fundamental lines of general physiology
seem less a part of plant production or of practical agricul-
ture than the broader relations above referred to, it must be
that it is so partly because of the name which has been ap-
plied to this subject, and partly to the fact that the meth-
ods of instruction necessarily take the student or reader
to a far greater degree away from the cultivated field.
To a considerable extent this is necessary, for physiology
must remain one of the fundamental sciences, and the fun-
damental attitude should be kept prominent. It has been
considered, too often, a subject with merely laboratory
applicability. This erroneous view is vanishing as plant
producers become more and more interested in the causes
which produce results and not merely in the results them-
selves. Both horticultural and agronomic work have in
recent years extended more and more into the realm of
pure plant physiology, which should mean that they have
extended into that of accurate experimental study, with
the plant response as the central feature.

7. The literature of plant physiology. — The litera-
ture of this subject is extensive and scattered, as is that
of any other science. The student will do well to bear in

Introduction i Ui

mind that while both the brief and the extensive standard
works are important, the subject is one which, through its
diverse relationships, encourages breadth of preparation
and of application; so that frequently physiological texts
alone are insufficient. Any standard text is in large part a
logical arrangement and correlation of the facts of many
separate papers or monographs; and the detailed data
respecting any phenomenon should be sought in the spe-
cial paper.

The rapid strides which have been made in scientific
agriculture and horticulture, especially in plant chemistry,
soils, and intensified production, have developed a great
array of interesting phenomena. ‘This has given a decided
impetus to physiological study. Often, unfortunately, the
agriculturist has been compelled to go forward without due
knowledge of physiology in the interpretation of his results,
but this is no excuse for the neglect of the large amount of
valuable and sound work which has been done.

Again, it should be further emphasized that many phys-
iological phenomena are only properly understood when
they are viewed in the light of physical and chemical the-
ory, and it is frequently necessary that the student who is
encouraged to go further shall turn to the sources of in-
formation in these fundamental sciences.

8. Physiology and other sciences. — The aim of plant
physiology is a definite one, like that of other sciences; it
is ultimately to obtain precise information concerning all
those factors and forces which are operative within or
through the living plant. Facts are derived and laws es-
tablished in exactly the same manner as in other sciences.
It does not stand apart from physics and chemistry, but

12 Plant Physiology

utilizes and advances these or any other sciences which
may assist in deducing the facts of plant life. Just as
chemistry may utilize the plant as an indicator of chemical
reaction or chemical fact, so physiology may use or develop
chemical facts in analyzing the phenomena of plant life.

In general, a study of physiology must assume or in-
clude facts regarding the form and structure of plants;
that is, morphology and histology. The more elaborate
the morphology of an organism, as a rule, the more special-
ized and intricate are its reactions. These reactions are
those of its constituent units, and the cell is a convenient
and necessary unit of structure. The cell is likewise an im-
portant physiological unit, and as such requires special con-
sideration.

REFERENCE Books AND Texts!

Bai.ey, L. H. Cyclopedia of American Agriculture. 1: 618 pp.,
756 figs., 25 pls., 1907; 2: 699 pp., 907 figs., 19 pls., 1907.

Barnes, C. R. Physiology (in Coulter, Barnes, and Cowles,
College Botany, Part II), pp. 297-484, figs. 619-699, 1910.

CieMEnts, F. E. Research Methods in Ecology. 334 pp., 85
figs., 1905.

—— Plant Physiology and Ecology. 315 pp., 125 figs., 1997.

Curtis, C. C. Nature and Development of Plants. 471 pp.,
342 figs., 1907.

Darwin, F., and Acron, E. H. Practical Physiology of Plants.
321 pp., 43 figs., 1894.

Detmer, W. Practical Plant Physiology. (Transl. by S. A.
Moor.) 555 pp., 118 figs., 1898.

1 In this list of reference works it is intended to include some of the
more useful texts and general works which contain physiological infor-
mation along many lines. Other books of greater specialization are in-
cluded under the selected references for particular topics.

Introduction 13

Detmer, W. Das kleine Pflanzenphysiologische Prakticum.
(3d Ed.) 319 pp., 179 figs., 1909.

Ganone, W. F. Plant Physiology. (2d Ed.) 265 pp., 65 figs.,

1908.

The Teaching Botanist. 439 pp., 40 figs., 1910.

GoopaLe, G.L. Physiological Botany. 533 pp., 214 figs., 1885.

GREEN, J. R. An Introduction to Vegetable Physiology. 459
pp., 182 figs., 1907.

HaABERLANDT, G. Physiologische Pflanzenanatomie. (4th Ed.)
650 pp., 291 figs., 1909.

Hansen, A. Pflanzenphysiologie. Die Lebenserscheinungen
und Lebensbedingungen der Pflanzen. 314 pp., 160 figs.,
1898.

JOHNSON, S. W. How Crops Feed. 375 pp., 1904.

Jost, L. Plant Physiology. (Transl. by R.J.H. Gibson.) 564
pp., 172 figs., 1907.

MacDovaat, D. T. Practical Text-book of Plant Physiology.
352 pp., 159 figs., 1901.

OstERHOUT, W. J. V. - Experiments with Plants. 492 pp., 252

Swigs-, 1905.

Peirce, G. J. A Text-book of Plant Physiology. 291 pp., 23
jigs., 1903.

Prerrer, W. Physiology of Plants. (Transl. by A. J. Ewart.)
le Ga2 pp. 67 figs.; 1900; .2: 296 pp., 31 jfigs., 1903; 3: 451
pp., 70 figs., 1906.

Sacus, J. Lectures on the Physiology of Plants. (Transl. by
H. M. Ward.) 836 pp., 455 figs., 1887.

ScuimperR, A. F. W. Plant Geography on a Physiological Basis.
(Transl. by Groom and Balfour.) 839 pp., 502 figs., 4
maps, 1903.

SoravueEr, P. A Popular Treatise on the Physiology of Plants.
(Transl. by F. E. Weiss.) 256 pp., 33 figs., 1895.

Stevens, W. C. Plant Anatomy. 349 pp., 136 figs., 1907.

STRASBURGER, NOLL, SCHENK, and Karsten. A ‘lext-book of
Botany. (Uransl. by W: 4: Lane.) 746. pp: 779 figs:
1908. (Later German Ed. by Strasburger, Jost, Schenk,
and Karsten.) :

14 Plant Physiology

VERWORN, Max. General Physiology. (Transl. by F. S. Lee,
from 2d German Ed.; 5th German Ed., 1909.) 615 pp.,

285 figs., 1899.
Vines, S. H. Lectures on the Physiology of Plants. 710 pp.,

76 figs., 1886.

——A Student’s Text-book of Botany. 1:480 pp., 279 figs.,
1895; 2:431-821, 483 figs., 1895.

WarMinG, E. Ecology of Plants. (Transl. by Perey Groom
and Isaae Bayley Balfour.) 422 pp., 1909.

CHAPTER, it

THE PLANT CELL

CERTAIN aspects of the physiology of complex organisms
may be convincingly presented and perhaps adequately
understood without necessarily assuming any knowledge
of the minute structure of such organisms. In the same
way the demonstration of important chemical facts and
reactions may be measurably feasible and instructive
for those with little or no conception of the significance of
atoms and molecules. Nevertheless, in the same way that
a knowledge of the atom is indispensable in understanding
chemical theory, just so the minute structure and the re-
lations of cells is fundamental in order to gain a compre-
hensive view of the activities of a multicellular organism.

A century ago it became apparent to a few physiologists
that some fundamental physiological problems could find
more nearly complete solution only through experimental
studies upon the cell. In the time which has since elapsed
the relative importance of cell physiology has been more
and more appreciated. Advances in this field, however,
are necessarily associated with advances in morphology,
chemistry, and physics. The development of cell mor-
phology has been dependent largely upon the improvement
of the microscope, and of current methods of technique,
both of which have now reached a high state of per-
fection. Physical and chemical theory and method have

15

16 Plant Physiology

undergone profound changes, and the method of these
sciences is now the method applicable to a study of all
matter. In view, then, of the relationship of cell physi-
ology’ to morphology, on the one hand, and to physico-
chemical advances, on the other, a study of the cell has
become fundamental for any comprehensive view of gen-
eral physiology.

9. The cell a physiological unit. — Representing the pro-
toplasmic unit, the living cell is ultimately the seat of all
those complex chemical and physical changes, or diverse
energy transformations, of the living body. As a unicel-
lular organism the cell must act independently in a par-
ticular response; in a multicellular body it responds also
as a distinct unit, in unison, however, with many other cells
associated together as a tissue. In any case it has been by
an investigation of the cell that many of the principles of
absorption, digestion, assimilation, excretion, and respira-
tion have been demonstrated. The fundamental concep-
tions of growth and differentiation, of fertilization and
reproduction, were only possible through the development
of cell study. .

Every cell passes through a cycle of changes. Each is
a seat of many, if not of all, of the physiological processes
characterizing the organism as a whole. In the more
complex plants and animals diversity of labor among the
cells has developed to such an extent that certain cells are .
restricted, or specialized, with respect to their activities,
but all cells must perform certain fundamental functions
necessary to growth, development, and differentiation.

In almost any physiological process, or in the ultimate
effects of various stimuli upon the organism, the cell is

The Plant Cell 17,

“the important substratum of all vital activity.’ Refer-
ring to the cell-theory and the importance of it, Wilson
concludes: ‘‘ No other biological generalization, save only
the theory of organic evolution, has brought so many ap-
parently diverse phenomena under a common point of view
or has accomplished more for the unification of knowledge.
The cell-theory must therefore be placed beside the evolu-
tion-theory as one of the foundation stones of modern
biology.”

10. Early use of the term “cell.’’ — In the earlier studies
upon the cell, beginning in the latter part of the seven-
teenth century, the term was applied to the firm walls
alone, from their resemblance to the cells of the honey-comb.
When, however, protoplasm, or the living substance within,
was later discovered, and its significance as the important
morphological and physiological unit determined, the
same term was retained for this essential unit of living
substance. Nevertheless, with the obvious distinction in
mind, the term is still applied to the many cell-forms or
cell-cavities, from which all living matter has disappeared,
—such cell-forms of many types constituting the great
bulk of the conductive tissues of woody plants, and all
of the heart-wood, stony tissue, dry bark, and the like.

11. Meristem or embryonic cells. — Structurally or
physiologically the term “‘cell’’ is now employed to denote
the simplest unit into which the organism may be con-
veniently resolved. It consists essentially of a unit mass
of living protoplasm with certain inclusions or surround-
ing materials.

In plants the protoplasm is usually inclosed by firm

often box-like cell-walls. Plant cells are usually so diverse
C

18 Plant Physiology

that it is difficult accurately to speak of a typical cell.
Nevertheless, in the higher plants, those cells which make up
the meristem, or growing tissues, possess certain character-
istics in common, and they may be considered typical in
this restricted sense. All other tissues are derived from
the meristem, hence its
peculiar importance.
A vegetative cell of
the growing root apex
of corn (Fig. 3) is
more or less isodia-
metric in form, often
shown as a rectangle
or polygon in section.
The granular proto-
plast, or protoplasmic
body, differentiated, as
denoted later, may be
distinguished in such
cases with compara-
tive ease. It is closely
surrounded by the

firm cell-wall which is

Fic. 3. Cell of the meristem, from root in general more con-
apex of corn.

spicuous and _ serves
better than the protoplast to differentiate the limits of the
cell units. The protoplasm is further differentiated into a
dense, often rather centrally disposed, spheroidal body,
the nucleus, and a less dense but granular enveloping
cytoplasm. In the cytopiasm, when the meristem is
included within the green tissues, there may also be noted

The Plant Cell 19

certain small refractive protoplasmic bodies termed
chromatophores. Vacuoles and food-materials of various
sorts may also occur as inclusions within the general
protoplast.

An examination of rectangular or polyhedral cells at a
short distance back of the tip will reveal certain changes,
often denoting a passage from the formative to the non-
formative or adult type. In the latter the cell is larger,
the cytoplasm less abundant, and much of the cell-cavity
may become occupied by vacuoles filled with cell-sap.
As the vacuoles form the cell may show radiate or strand-
like cytoplasmic areas usually connecting the central with
the peripheral cytoplasm, but the nucleus may be still
more or less central, and is invariably imbedded in cyto-
plasm. As the change goes on the sap cavity enlarges and
all the protoplasm is drawn to the periphery, the nucleus
occupying the center of a marginal mass.

The preceding defines the general type of many of the
living cells of the plant body not undergoing rapid growth
and division. In the active parenchyma of certain roots,
tubers, and other organs to which there has fallen the
office of starch or other food-stuff accumulation, the cell
may become packed with such products; still, nucleus
and a thin layer of protoplasm remain, and these are later
essential in the digestion and transport of the stored food-
materials.

12. Cytoplasm. — When reference is made to the struc-
ture of protoplasm, it is usually the cytoplasm which is
considered. Fixing the attention upon the cytoplasm
alone, it is found that this feature of meristematic cells,
of germinating pollen grains, of the hyphe of black-mold

20 Plant Physiology

fungi, of slime molds, — indeed of all plants, — is much
alike. The cytoplasm is evidently a semiliquid trans-
lucent substance, and it contains granules, resulting usually
in a distinctly granular appearance. All protoplasm is
readily killed by a solution of iodine by which it is also
stained yellowish brown. This reagent is therefore con-
venient in demonstrating protoplasm in cells where it is
not readily visible. Moreover, the use of a strong salt
solution, causing contraction of the cytoplasm from the
cell-wall, is also important in demonstration. The outer
margin of the cytoplasm, or the margin bordering a vacu-
ole, possesses important physiological characters, and for
convenience it is called the plasma membrane. The
cytoplasm may inclose food-materials not easily distin-
guished from the usual protoplasmic granules.

The structure of protoplasm has received much con-
sideration, and three noteworthy conceptions of its form
have been advanced as follows: (1) the netted or reticu-
lum theory; (2) the fibrillar theory, and (8) the alveolar
or foam theory. The present tendency is to regard it as
unnecessary to assign a definite structure persistent under
all conditions, and physiologically it is logical to believe
that the structure is simply a manifestation of a type of
activity. Stains of various sorts have been most im-
portant in the study of cytoplasm as well as of nucleus.
The chemistry, movement, and special responses of pro-
toplasm in general are considered later.

13. The nucleus. — The nucleus presents the appear-
ance of a dense or refractive protoplasmic mass, and it
does contain denser bodies. It is often nearly spherical,
but in certain old or specialized cells it may be irregular

The Plant Cell pA |

in form. It is differentiated from the cytoplasm by a
distinct membrane, but the minute structure is only ap-
parent by the use of staining methods. It shows usually
a strong affinity for many stains, and its parts may react
in a differential manner. In the growing nucleus there is
usually a refractive reticulum staining rather lightly in
general, but deeply at certain points, or angles, where there
is or seems to be an aggregation of chromatic substance.
There is also present upon the reticulum at least one
nucleolus. This latter is most evident by staining, but
in the unstained nucleus it is strongly refractive, and often
serves to locate the nucleus.

Nucleus and cytoplasm are interdependent, and few
cells are long functional in which either of these parts is
killed, or from which either is removed.

14. Plastids. — In addition to cytoplasm and nucleus
the other protoplasmic organs of the cell requiring brief
mention at this point are dense bodies termed plastids,
usually disposed in the parietal protoplasm of the cell.
In the higher plants they are usually spheroidal or ellip-
soidal in form. Of these there are three types: (1) chlo-
roplasts, containing the pigment chlorophyll, to which is
due the green color of plants, essential, as shown later,
in the manufacture of organic food-material in the green
plant; (2) leucoplasts or amyloplasts, those starch-form-
ing plastids contained in subterranean or other organs of
the plant receiving no light, — plastids, nevertheless,
which are able, when exposed to light, to develop into chlo-
roplasts; and (3) chromoplasts, plastids of various colors,
generally yellowish to red, sometimes crystalline in form,
from the presence of albuminous crystals, or from the crys-

22 Plant Physiology

tallization of the pigment contained. The special signifi-
cance of these three types of bodies will require treatment
later.

15. The cell-wall. — The plant protoplast is commonly,
and in the vegetative organs of higher plants invariably,
invested by a firm cell-wall. When constituting a part
of a tissue-system, the cell-walls are throughout most of
their length in close contact, mutually supporting, and,
with the modifications subsequently noted, they form
together a complex circumcellular organic skeleton. Some
walls are also infiltrated with mineral matters; espe-
cially are the outer walls of grasses and the like silicified.

The form of the cell-wall is, of course, in all cases a per-
fect index of the form of the cell, although in some cases
after the death of the protoplasm the ceil may be some-
what modified in shape. The wall is formed by the pro-
toplasm and it is properly regarded as a product of pro-
toplasmic metabolism or secretion. It is commonly
composed of two or three distinct layers, three often occur-
ring when the wall is strongly thickened. In special cases
where successive layers are deposited, the wall may present
a laminated structure. In the formation of the cell-wall
in general a middle lamella is first laid down. This is the
primary layer, and upon it is deposited a secondary, and
finally a tertiary, layer. The second layer is, as a rule,
the thickest or most completely developed. In the looser
tissues of the body the middle lamella may split at the
angles between the cells, thus leaving intercellular spaces
of greater or less extent, the importance of which in gas
diffusion through the plant will subsequently receive
special consideration.

The Plant Cell Ze

The successive layers in the formation of the cell-wall
may be interrupted at certain points or along certain lines,
and there will thus result pores or pits of various types.
Again, the thickening may be confined to particular re-
gions, so that the peculiarities of the wall may be consid-
ered due to the deposition of the layers in very limited
areas, as in the annular or spiral vessels. In the tracheal
tissue these pores may occur in adjacent cells opposite to
one another, so that the cells at these points are in reality
separated merely by the primary layer. Such connections
are important in the transport of water and substances in
solution; but it is not the province of the present brief
description to make an examination of wall structure,
the aim being merely to indicate the mechanism of special
physiological interest.

In the case of the soft-rot of cabbage and other vege-
tables, the causal bacillus attacks and decomposes the
middle lamelle, so that the organization of the tissues is
promptly broken down. Gelatinization also of the cell-
wall may occur in seed-coats. Flax, mistletoe, and some
other plants exhibit this phenomenon when placed under
conditions favorable for germination.

16. Cell-sap.— The protoplasm is infiltrated with water,
and there are closely associated with it nutrient and other
substances in solution. Moreover, as already indicated,
there are generally present within the protoplasm some
definite ‘‘ vacuoles,’ also containing substances in solu-
tion; and such solutions are called cell-sap. These vacu-
oles are apparently of much the same nature as the large
central one ultimately formed in the great majority of
differentiated cells. The term “‘cell-sap,’’ at any rate, is

24 Plant Physiology

indiscriminately applied to the liquid contents occurring
in the vacuolate areas large and small.

The vacuoles are unquestionably of much physiological
significance, and certain materials diffuse into such spaces
and may to a considerable extent accumulate there.
The substances held in solution may include a variety of
organic or inorganic compounds, again referred to in the
- discussion of metabolic products. In some eases the color
of plants is due to coloring matters occurring in the cell-
sap alone. With respect to the other liquid cell contents
the vacuoles may have therefore a certain differential
character.

17. Cell-forms. — Young or growing cells in tissues are
often somewhat rectangular in outline. When, however,
the pressure of adjacent units is released, there is an ap-
parent tendency to assume a form more or less spherical.
This should not be confused with the fact that micro-
organisms possess a considerable diversity with respect to
their specific forms; indeed many unicellular organisms
of elongate forms which grow for a time in pairs or groups
become also more convex along the lines of attachment
upon being set free; thus rod-shaped bacteria may become
more rounded at the ends. In the meristem of the grow-
ing tip‘where the cells are closely united, and encompassed
by a variety of pressures, the typical form of the cell is
isodiametric or polyhedral. Back of the formative region,
under the influence of the growth pressures and various
other stimuli, there is a tendency for many cells or cell-
groups to take up an elongate form. The latter may
make possible, in time, other important modifications.

Under all circumstances the embryonic meristem cell

; The Plant Cell 2p

is capable of changing its form or of undergoing differen-
tiation when in a position where this response is called

forth. It must be un-
derstood that this dif-
ferentiation is in direct
or indirect response to
a variety of stimuli
which are normally
operative during the
growth of the plant.
It is found, therefore,
that while all the cells
of higher plants have
developed from an
original meristem of
the type indicated,
there is the greatest
diversity in the ulti-
mate form, as also in
the ultimate work, of
the cells differentiated
therefrom.

The different types
are, of course, associ-
ated with a specialized
form of labor, or func-

Fia. 4. Cell from a leaf-hair of squash,
showing vacuolate cytoplasm, nucleus,
and chloroplasts.

tion; therefore these cell-types are of peculiar physio-
logical importance, as well as of obvious anatomical and

evolutionary interest.

The following common types may

be briefly characterized for further reference : —
18. Parenchyma. — This type includes various forms

26 Plant Physiology

of relatively thin-walled cells which may have undergone
very little, though sometimes considerable, change in
shape. They are generally more or less rectangular or
polygonal in outline, and frequently exhibit large inter-
cellular spaces. In some cases, especially when the proto-
plasm has been lost, the walls may be infiltrated with
mineral matters. In situations where they may be
directly or indirectly exposed to the drying action of the
air, the walls may contain cutin or suberin, thus rendering
them less penetrable to the passage of water. If the
walls are thickened at the angles, as in the supporting
cells of the cortex, they are commonly termed collen
chyma. .

Parenchyma of some form almost invariably accom-
panies conductive tissues, but it is not particularly adapted
for the rapid movement of solutions, being in large part
dependent upon simple diffusion phenomena. It has been
found, however, that in the parenchyma there are com-
monly minute cytoplasmic connections between adjacent
protoplasts; that is to say, minute pores May occur in
walls separating cells, and through these pores cyto-
plasmic fibrils may extend, connecting therefore adjacent
protoplasts. These connections may be of great impor-
tance in the relations existing between the cells in paren- —
chymatous tissue.

19. Sclerenchyma. — This term is usually employed to
denote cells with considerably thickened walls. Thick-
ening may proceed to such extent that the protoplast dis-
appears and the lumen may be practically closed. The
grit cells of the fruit of pear are much thickened, and the
stone cells of the ‘‘ pits”’ of drupaceous fruits, or of the

ce

The Plant Cell Dh

shells of nuts, are extreme examples. Elongate cells with
thickened walls may also be included here, such as bast,
or those surrounding the bundles in Indian corn (Fig. 5).

Fic. 5. Some extreme cell-types: collenchyma (a) ; sclerotic cells from
pine needle (b) ; and vegetable ivory (d) ; unusual palisade cell (ec) ; bast
(e, f) ; prosenchyma (g) ; from cotton seed-coat (h) ; and stinging cell of
nettle hair (7).

The term “stereome’”’ has been applied to thick-walled
cells serving primarily for support.

20. Tracheids. — These are thick-walled cells, more or
less elongate, with walls often showing pitted, reticulate,
or spiral thickenings. They may possess a considerable
lumen, and the mature cell may show no trace of proto-
plast. They are usually lignified, and are important in
the conduction of water. In many plants (such as the
pine and other conifers) they constitute the sole water-
conducting system.

These tissues are likewise important in support, and this
fact emphasizes a point worthy of special note, and that is

28 Plant Physiology

this: a tissue may be primarily important for a specific
type of action, but differentiation is not commonly so
complete as to render it unserviceable in many correlated
activities.

21. Trachee or vessels.— The vessels are formed
from rows of elongate cells by the absorption of the inter-
vening walls coincident with the disappearance of the
protoplasm. Such cell-cavities, or ducts, may extend
continuously for several centimeters in length, and they
are especially important in the conduction of water in
angiosperms generally. These ducts also show usually
the annular or spiral thickenings previously referred to.
Both this and the preceding type are commonly associated
with at least a small amount of parenchyma, and it is
probable that their physiological properties depend to
some extent upon the latter.

22. Sieve tubes. — The sieve tubes are also elongate
cells, but they are peculiar in the fact that the protoplasm
in the adjacent members of the cell-row is continuous by
means of very distinct connecting pores through the
intervening walls. These walls are thickened and form
a so-called cell-plate, a perforate plate through which,
therefore, the protoplasm is continuous (Fig. 6). Cell-
plates may also occur at points of contact in more or less
vertical walls.

Another peculiar feature of such sieve cells is the fact
that the nuclei become disorganized, but the cytoplasm
remains. These cells, however, are in close contact with
certain cells which are typical parenchyma elements, the
companion cells, the latter containing both cytoplasm
and nuclei. The sieve tubes usually occur in the woody

The Plant Cell 29

bundles, and are easily identified in the outer part of
these (commonly in the bark, therefore, of dicotyledonous
plants). They are regarded as most important in the
conduction of the less diffusible organic materials.

The arrangement or association of certain of these types
of cells and further indications respecting their several
functions in the general physiology of the plant are again
referred to under growth and transport.

=| ta on
e rs
SS 2
> iK i 7 =i
PS SOS
cS ee OC RS
SS eS Ny
Se 2s
Pac = Recs A | i
Se Cs cA |
=|

y
ra

a

Fic. 6. Conducting cells of fibrovascular bundles, ducts, trachez (J);
pitted vessel (m) ; sieve tubes with companion cells, in longitudinal (n)
and cross section (0). [Adapted.]

23. Protoplasmic movement. — Naked protoplasmic
units or aggregates such as amcebex, or the plasmodia of
Myxomycetes, show a considerable power of locomotion
due to streaming movements in the cytoplasm. It is
also well established that within the protoplast of a variety
of cells invested with a firm cell-wall there is relatively

30 Plant Physiology

rapid movement. It is a phenomenon so common!
that it must be assumed to have physiological significance.
Moreover, it is often rapid in cells of large size, so that
it seems safe to say that it is not unimportant in facili-
tating diffusion. A study of this capacity for movement
gives an impressive mental picture of the protoplasm as
the seat of activity in the cell or member.

The various types of protoplasmic movement are com-
monly grouped in four categories: (1) simple streaming,
(2) circulation, (3) rotation, and (4) orientation.

Streaming movements are rather spasmodic in different
parts of the cell, first in one direction, and sooner or later
a reversal. Aside from the slime molds, the ccenocytie
filaments of the black molds show, among plants, pro-
nounced movements of this type.

Circulation consists in movement at any instant in
more than one direction in the cell. The motion may
occur in the peripheral cytoplasm, but this type is char-
acteristic of cells possessing cytoplasmic strands. In
fact, when the cytoplasmic strands are lost, the movement
may become rotary. Circulation may be conveniently
observed in the stamen hairs of Tradescantia and in the
stem and leaf hairs of various plants, especially cucurbits;
also in young root-hairs and other young cells.

Rotation, or the movement in a rather constant current
or direction around the cell, or in some area of the cell, is
the most striking type. It usually occurs in cells which

1 For a list of greenhouse material suitable for the study of movement
the following paper may be consulted: Bushee, Grace L., The Occur-
rence and Rate of Protoplasmic Streaming in Greenhouse Plants. Botan.
Gaz., 46: 50-53, 1908.

The Plant Cell al

have lost the cytoplasmic strands. It may be observed
in the water weed Elodea, and attains a maximum rate
and clearness in an inner parietal cytoplasmic layer of
the internodal and other ccenocytic segments of the alga
Nitella (stonewort). In this last-named plant it is not
uncommon to find, at a temperature of 28 to 30° C., a
rate of movement from 3 to 4 mm. per minute.

Movements of orientation result in a gradual, or scarcely
directly visible, shifting of a portion of the cytoplasm or of
other portions of the protoplast. By this means the nu-
cleus is able to change its position in the cell, and the plas-
tids (chloroplasts) show peculiarities of arrangement under
different intensities of light. Orientation is doubtless to
a considerable extent characteristic of all living cells, but
the result of the movement is more easily noted in those
green cells quickly responsive to changes of light.

24. Protoplasmic irritability and response. — The move-
ments previously referred to are indicative of a type of
activity. It is of interest to note to what extent this
activity may be affected by a change in the environment,
as, for instance, a change in temperature. If favorable
material of any kind (such as Nitella, Tradescantia,
Cucurbita) is carefully studied at different temperatures,
effected by a temperature stage, it will be found that with
respect to rate of movement there is, in general, a mini-
mum, an optimum, and a maximum temperature for move-
ment, so that the protoplasm is highly responsive to these
differences of the environment.

This change in rate of motion with the above-mentioned
minimum-optimum-maximum manifestation is obviously
an indirect effect. Moreover, since temperature changes

o2 Plant Physiology

are a constant environmental factor, it is safe to assume
that the organism is adjusted to this factor, so that it
manifests what is known as a tonic response.

Another case of response has already been noted: under
different intensities of light the orientation of the chloro-
plasts may be diverse. In the cells of the duckweed
(favorable for observation) the chloroplasts are distributed
in the upper portion, or dome, of the cell and also across
.the bottom, in diffused light; while in bright light they
lie at the sides and one above another. The position in
the dark is along the vertical walls, also the horizontal
wall when that does not abut upon the epidermis. This
type of response to light (in this instance) is generally
regarded as denoting protoplasmic irritability.

LABORATORY WORK

Living cells. — Remove with the forceps or scissors the fila-
mentous, purplish hairs from the stamens of any available
species of Tradescantia. Mount these, and note carefully the
form and size of the distinct cells. Distinguish cell-wall, pro-
toplasm, and colored vacuole, and observe each of these erit-
ically. Describe the peripheral and strand cytoplasm, also the
form and position of the nucleus, with nucleolus. Draw. Com-
pare the cell drawn with others both nearer the base and the
apex of the filament. Kall with tincture of jodine, and examine.

As in the preceding, study the cells of hairs clipped from a
petiole of a fairly young squash or pumpkin leaf. In this ease
note also the form and position of the plastids (chloroplasts).
Peel off a little of the epidermis of Cyclamen or Begonia;
mount, study, and describe these cells. For comparison study
and draw a stained preparation of a root-tip or bud-apex and
compare with the previous material.

The Plant Cell OD

Cell-forms.'— Study parenchyma in the young stem of
Indian corn, or in the pith of an herbaceous plant, also collen-
chyma in the peripheral portion of a stem of wild carrot or
squash. Sclerotic cells may be easily identified in the gritty
portion of the fruit of pear. Tracheids and tracherx may be
studied by means of longitudinal sections of almost any woody
plant, grape vine and squash showing, particularly,. ducts of
considerable size. These plants are also good for an examina-
tion of sieve tubes.

Cells such as the tracheids may be conveniently studied
after maceration. Place sections of the desired material in very
strong or concentrated chromic acid for about one minute, or
until thoroughly limp and easily teased apart; then wash and
tease out, or mount immediately, and separate the cells one
from another by gentle pressure upon the cover-glass.

Protoplasmic movement. — Following the general indications
in the text, study types of protoplasmic movement in the cells
of such plants as Nitella and Elodea; hairs of Tradescantia,
of a cucurbit, or of Gloxinia; and the young hyphe of any
common black mold, Mucor.

Use the most favorable material for further observation upon
the effects of temperature upon movement, employing a tem-
perature stage in determining the rate of movement at tem-
peratures varying from towards the minimum to an approximate
maximum. Plot a curve of the results.

REFERENCES

Burscuu, O. (Eng. Ed., transl. by E. A. Minchin.) Investi-
gations on Microscopic Foams and on Protoplasm. 379
pp., 23 figs., 12 pls., 1894.

1 Tt is considered important that students not qualified in anatomy or
histology should devote several laboratory periods to a study of the cell
and cell-forms. Reports based upon their own observations may be
supplemented by a more complete review of the subject as presented in
Stevens, Strasburger, Vines (‘‘ Text-book of Botany ’’), or other suitable
text.

D

o4 Plant Physiology

Ewart, A. J. On the Physics and Physiology of Protoplasmic
Streaming in Plants. 131 pp., 17 figs., 1903.

Fiscuer, A. Fixierung Farbung und Bau des Protoplasmas.
302 pp., 21 figs., 1 pl., 1899.

Hertwic, O. The Cell. 368 pp., 168 figs., 1895.

Lors, J. The Dynamics of Living Matter. 233 pp., 64 figs.,
1906.

Witson, E. B. The Cell in Development and Inheritance.
(2d Ed.) 483 pp., 194 figs., 1906.

Texts. Verworn, Jost, Pfeffer, Ganong, Strasburger.

CHAPTER III

hae WATER-CONTENT OF PLANTS AND
iat GHNERAL RELATIONS OF ROOT
SYSTEMS

THE life and special activities of the plant or animal
are at all times conditioned by the water-supply. Plant
growth and production may be more sharply limited
within countries, regions, or localities by the water-supply
than by any other factor of the ordinary physical environ-
ment. A soil which does not receive and deliver to the
. plant throughout the growing season a reasonably constant
supply is a sterile desert whatever may be the quality of
this soil with respect to latent mineral possibilities.

Water is often regarded as a crude food-stuff, because
it enters abundantly into the composition of living things.
It does, in fact, contribute elements to the making of or-
ganic food, as shown later; but for the moment it is most
important to consider water with respect to its solvent
action. All organic food-material presented to the living
cell must be in solution; likewise the mineral nutrients
and the gases which take part in metabolism. Ordinary
plants are constantly in contact with a water-supply,
during their growing period, by means of special absorb-
ing surfaces. It is to be expected, therefore, that the
forms and functions of plants are to a considerable degree

30

36 Plant Physiology

concerned with the use and distribution of water and of
substances in solution.

Properly to consider the use of water there arises the
necessity of learning or reviewing the structure of the
organs and the nature of the processes whereby water is
absorbed, conducted, and eliminated, as well as general

Fie. 7. Thrifty squash-plants as typical examples of the relation of
water-content to rigidity.

and special crop relations in which this factor plays an
important role.

25. Hydrostatic rigidity. — Under conditions favorable
for growth, it is obvious that the living cells of a plant are
commonly in a state of extension or hydrostatic rigidity.
Small and succulent stems are able under such circum-

Water-Content of Plants ot

stances to support a considerable load of branches, leaves,
and flowers. Any condition which deprives the plant of
water inaugurates, on the contrary, a state of flaccidity;
that is, a drooping or wilting. These phenomena will be
referred to again, but the fact may not be too strongly
emphasized that rigidity and abundant water-supply are
closely related, especially where the mechanical supporting
tissues do not reach the fullest development. Compare
the appearance and vigorous yield of well-watered lettuce,
squash, or tomatoes with those unattractive and miser-
able plants whose leaves or fruits fall limp upon them-
selves.

26. The water-content of plants. — The growing plant
contains invariably a high percentage of water. It is gen-
erally stated that an active, succulent plant or tissue, one
which contains relatively a small amount of fiber, shows
a water-content of 75 per cent or more. When the plant
contains a larger number of thick-walled cells, or woody
tissues, which may be required for protection, support,
or conduction, the percentage of water may be lessened.
In every case, it is probable that the active protoplast
requires a water-content of from 80 to 90 per cent or more.
The necessary water, must be obtained by absorption from
the environment, and in the case of the common agri-
cultural plants absorption is almost exclusively by means
of the root-system.

The amount of water contained in different plants, or,
in fact, in the same plant, is subject to considerable varia-
tion. Nevertheless, it is instructive to note the composi-
tion of a number of crop or useful plants with respect to
this factor. The following table will indicate approxi-

38 Plant Physiology

mately the average water-content of a number of familiar
plants or plant products: —

||
|

| WATER- | WATER-
Sa ess | CONTENT, | PLANT CONTENT,
| WEIGHT | WEIGHT
PER CENT) PER CENT
Apples, fruit . . . .| 83.2. ||Cucumbers . 5). >=)3aeee
Beets, mangel wurzels_ . 90.9 | Oats, cured grain .. . 11.0
iBects. Fed’ “Gch era es 88.5 Onions ee 87.6
Bects suear. + 20s -29 | 868 | Potatoes, Irish . . . .| 78.9
Bech: TONS) sh Ye eee es 87.0 || Potatoes, sweet. . . . ig ys
Cabbage so0F" i eae | Sas | Pumpkin, flesh .°. 9.34 93.4
Clover, red, green hay . 70.8 | Rice, grain a 12.6
Clover, white, green hay 78.2 | Spruce needles, old, in Oc-
Corn;.dry seed 1.2. e" 10.9 || tober’ >.>. eee 56.7
Corn fodder, green. . 79.8 Spruce needles, young, in
Corn silage’) ity -23ius ee 79.1 spring > Se 80.6
Cowpeas, green hay . . 83.6 || Timothy hay, cured . . 42.2

27. Variation in water-content of different organs. —
An examination of the various analyses reported by chem-
ists will indicate that the different products or organs of
the same plant may vary materially in the water-content,
as would be anticipated. This may be due in part to
differences in the amount of supporting or otherwise dif-
ferentiated tissues. Fruits may, however, contain much
more water than the growing shoots upon which they are
developed, or certain fruits may contain at maturity very
much less. This will all depend upon the nature of the
tissues in these parts, and upon the degree of maturity,
or the method by which maturity is accomplished.

During the ripening of seeds the water-content may be

Water-Content of Plants 39

reduced by several hundred per cent. This may go on
- simultaneously with a reduction in the water-content of
the plant as a whole, which is the case in cereals and many
other plants having a definite growth cycle. On the other
hand, the maturity of the seed in many annuals and per-
ennials which grow in an indefinite manner may be wholly
independent of any general ripening process of the entire
plant.

The seed within the body of the fruit may likewise differ
from the latter; thus the seed of watermelon or peach
shows, when the fruit is ripe, a water-content far less than
that of the pulp which surrounds it. The water of the
plant does not merely permeate all parts indiscriminately ;
it is accumulated within or withheld from organs by virtue
of complex histological, chemical, or physical relations.
The formation of a few layers of corky tissue may cut off
the water-supply of an organ, the storage of solid food-
materials may reduce the quantity of water, or the pres-
ence of certain compounds may increase or inhibit absorp-
tion.

28. The water-absorbing system. — The root-system
constitutes the mechanism whereby the water-supply
must be secured in practically all higher plants, including
the common agricultural plants. There is, moreover,
diversity in the form, texture, and distribution of the roots
of crop plants. The diversity in form and texture is not
necessarily coupled with great differences respecting the
water-supply furnished.

There are two general types of root complexes ordinarily
recognized. In the one there may be a central or main
root called the tap-root, the branches and sub-branches

40 Plant Physiology

of which arise in rather irregular order, but make up a
general root-system which may occupy a fusoidal, a coni-

cal, or an obconical soil volume.

Fic. 8. General appearance of the root-
system of corn at the time of tasseling.

roots which may be termed of

This type includes let-
tuce, parsnip, and a
great variety of common
plants. In the other
type there may be little
or no indication of a tap-
root, and instead few or
many lateral roots of
more or less equal size
may in a way take its
place. Corn, for in-
stance, possesses at the
beginning of germination
a distinct tap-root, but
very soon, under ordi-
nary circumstances, in
the soil the length of this
may be approached or
exceeded by laterals or
by whorls of secondary
roots originating consid-
erably later (Fig. 8). In
many of the small cere-
als there are produced
upon germination several
the first order, and the

direction of growth of these determine for many days the

general form of the system.

It is a part of the function of the root-system to fix the

Water-Content of Plants Al

plant in the soil, but of chief interest must be regarded the
relations of the root-system to the water and the nutrient
salts of the substratum.

From a casual examination of plants in the field it is
difficult to form a proper conception of the extent of the
root-system. Pull up a wheat plant or any fibrous rooted
grass, and the root-system may seem extensive. Proceed
in the same manner with a beet or with an herbaceous
plant like the sunflower. The root-systems which come
to view in these two cases would give an entirely erroneous
impression of the relative or actual extent of the roots.
A large proportion of roots and rootlets remain in the soil,
especially in the case of the fleshy plant.

Upon a careful examination it is noted invariably that
accompanying vigorous growth in the soil there is a sur-
prisingly extensive system of small rootlets, and these are
usually disregarded in rough estimates of root extent. The
only methods of determining approximately the root devel-
opment is either by excavating carefully, and then washing
away the soil while the roots are in some way effectively
supported, or by growing the plants in special root cham-
bers. 3

29. The rooting habits of crops. — Plants vary greatly
with respect to the distribution of roots in the soil. In the
same habitat, or under the same cultural conditions, one
plant may show the greater extent of its roots close to the
surface, while another may branch more freely at greater
depth. Freidenfeld has made a careful study of root-
habit in a variety of common plants.

A study of this distribution of roots under diverse con-
ditions is a matter of considerable importance. Upon it

42 Plant Physiology

may be based better practices in soil preparation and cul-
ture. Investigations upon root distribution have been
more extensive at some of the experiment stations in the
West, and there are very few data available for conditions
in the United States essentially different. Ten Eyck has
shown that the roots of the corn, wheat, oats, and other
cereals may reach a depth of from three to four and a half
feet, the small grains reaching the greater depth. His
method consists in supporting the cuboidal mass of soil
containing the roots in wire-netting cages, through the
meshes of which many steel rods are thrust horizontally.
When the soil is washed away, the roots do not break so
- readily as when unsupported. Nevertheless, this method
has many difficulties and involves special apparatus for
handling large quantities of earth.

Recently a method has been developed in Russia by
Rotmistrov whereby the difficulties experienced in han-
dling large quantities of soil are to a considerable extent
eliminated. Some new sources of error have been intro-
duced, but apparently the work has been as well controlled
as possible. The method consists in growing plants in the
natural top-soil and sub-soil compacted into extensive
narrow boxes sunken in the soil during the period of growth.
When placed in position, these boxes present a surface
1 inch wide, 20 to 40 inches long, and 20 to 40 inches or
more in depth. The roots eventually occupy a volume
of earth equivalent in form to that of a narrow slab or
broad board. After the desired period of growth it is
possible to obtain practically the exact form of the entire
root-system by the maripulation suggested. Practically
speaking, this consists in the inversion of the root-pene-

Water-Content of Plants 43

trated mass upon a support or screen closely studded with
nails. Upon this screen careful washing is subsequently
given, and the entire root-system is then readily transferred
to cardboard. Figure 9 shows a root-system of the potato
grown in this manner. About 30 plants were grown by
Rotmistrov in such boxes for varying periods of time.

As a control upon this method some plants were grown
in natural soil beds, and the development and extension of
the root-systems were studied by means of deep pits with
horizontal chinks or tunnels. The pits permitted a care-
ful record of the depth of root extension, and through the
small horizontal chinks observation could be made upon
lateral penetration.

Working with the pit-and-chink method indicated, it
was determined that even in seven days the roots of “‘a
ereat many cultivated plants extend beyond the limits of
the soil when tilled to a medium depth ”’ (8 inches). The
roots of winter grain often extend laterally and vertically
to a distance of over 40 inches. Winter rye was found to
extend to a greater depth, and winter wheat to a greater
extent laterally, than other small grains.

Among the roots of corn there may be distinguished,
according to Ten Eyck, primary vertical and primary
lateral organs. The latter in their course again give off
vertical roots. The main laterals grow several feet, and
as they reach those of neighboring hills, they also strike
downward. Under the rather dry conditions of the West
a majority of the lateral roots are within from three to
twelve inches of the surface. When the sub-soil is poor,
deep culture of corn may therefore kill the roots or prevent
their formation in the most fertile parts of the soil. On

44 Plant Physiology.

the other hand, it is especially necessary under the dry

Fie 9. Root-system of a potato grown to

maturity in a deep, narrow box.
Rotmistrov.]

[After

conditions which
prevail in that region
to have a deep surface
mulch. It is of inter-
est to note that under
these circumstances
shallow cultivation
early and deep culti-
vation later has
proved the most satis-
factory method.

Kafir corn and sor-
ghum produce roots
of the same general
type and distribution
as those of corn, but
the former are tough
and fibrous and the
laterals are beset with
numerous, fine, feed-
ing roots which are
to be found between
the main laterals and
the surface. The up-
per eighteen inches of
soil is very completely
filled with these fine
roots. Moreover, the
plant grows late in
the season, and the

Water-Content of Plants 45

mere presence of the numerous tough roots and crowns
are sufficient to leave the soil in bad physical condition.
This unfavorable condition restricts the absorption of
water by the soil during fall and winter, and discourages
the requisite preparation for the succeeding crop. Owing
to these facts, sorghum land commonly shows a defi-
ciency in moisture the following spring.

The rooting habit of the sugar-beet, according to Ten
Eyck, indicates that it is a deep “ feeder’ at least during
the late stages of growth. The roots of potato seem to
occupy the soil as completely as any crop, and a consider-
able number penetrate to a depth of two or three feet;
yet many of the deep-rooted individuals possess a surpris-
ingly small number of feeding roots, at least on plants
examined in the autumn.

Nobbe measured the root-system of a wheat plant about
one year old and found the aggregate length of the roots to
be 500-600 meters (545-655 yards), while that of a full-
grown pumpkin vine measured about 50 times as long,
or about 25 kilometers (15s miles). The rooting habits
of shade trees are particularly worthy of study, especially
in view of the difficulties experienced with trees in towns
and cities.

From the studies in narrow boxes made by Rotmi-
strov it has been shown that a large number of farm
crops penetrate both loamy and sandy soils to a depth of
one meter or more.

30. The production of root-hairs. — The roots and mi-
nute rootlets which in a complete root-system are read-
ily evident to the eye are, however, secondary with respect
to the relations existing between the plant and the soil

46 Plant Physiology

water, or the soil solution. When the plant is removed
from the soil, even most carefully, organs smaller than
the rootlets are not made evident.
There are, nevertheless, ‘numerous
minute, simple, and effective struc-
tures generally present in abun-
dance. These are the root-hairs
arising from the surfaces of all
yourg and growing roots.

If seeds of radish or squash are
placed in damp moss or germinated
between sheets of moist filter paper,
or in any of the germinators sub-
sequently described, the root-hairs
become evident (Fig. 10). As soon
as the root has attained a length of
an inch or more there are developed
at a short distance behind the tips
a large number of these structures.
They arise practically perpendicular
to the surface, and a microscopic
Radish seedling, examination indicates that they are

simple, siphonaceous cells consisting
of a rather resistant cell-wall within which .is contained
the granular protoplasm and cell-sap.

When grown in the manner indicated, the root-hairs
may be perfectly straight tubes. As they develop in the
soil, however, where the numerous sharp soil particles
obstruct their growth, they bend about and flatten out
against and around these particles, becoming, as a result,
contorted or deformed in appearance. It is evident that

Fie. 10.
showing root-hairs.

Water-Content of Plants A7

they come into the most intimate contact with the minute
soil particles, — so intimate, at times, that fine particles
actually stick into the walls (Fig. 11). They are, there-
fore, peculiarly fitted for the needs of absorption, as will
be later developed.

It will also be noted that those regions of the rootlet
clothed with root-hairs have ceased to elongate; that is,
so soon as the hairs are developed it is an indication that
this portion of the root is fixed in the soil; otherwise its
growth would crush such organs and
prevent their further efficiency. In
this connection it may, therefore, be
observed that the “ push” which is
needed to force the root forward in the
soil is concerned with a relatively short
axis, perhaps not more than a quarter
of an inch in length. The practical ad-
vantages of this mode of growth are
obvious upon a moment’s reflection.
If, for instance, one should attempt to
force into the soil a fine wire two feet
long, pushing from the upper end, it
would certainly bend. In fact, the
difficulties of such a mode of growth
in the soil practically precludes the pos-
sibility of its occurrence.

The root-hairs are relatively short-
lived upon the majority of plants.
Their activity may be embraced in a Fic. 11. Root-hairs
period of from a few days to a month =‘ grown in coarse

BN, ad We sand; cortex (b)
or two, and they are readily injured by and epidermis (c).

48 Plant Physiology

unfavorable conditions of the environment. The ability
of a plant to take water from the soil, moreover, will de-
pend in large measure upon the ex-
tent of these simple organs. It has
been estimated that the surface of
the system in corn is increased from
five to six times by favorable root-hair
production; in barley about twelve
times, and in Scindapsus about eight-
een times.

31. Root-hairs and the water-con-
tent of the soil. —It is only in ex-
ceptional cases that land plants de-
velop few or no root-hairs. In general,
where few are present, the plant will
wilt with a higher water-content of
the soil than when more are provided.
Root-hairs are considerably suppressed
in the case of corn, wheat, and other
crop plants when the soil is saturated.
Nevertheless, many plants continue to produce such
organs in water cultures, even though the number of these
organs or the individual length of each may be greatly
reduced. In the soil so long as the plant does not wilt,
it appears to be generally stimulated to most abundant
root-hair production at a moisture content somewhat
less than that which will afford the highest yield.

It is commonly assumed that darkness is an immediate
factor in the development of the root-hairs, but this as-
sumption does not appear to be sustained by experiment.
Relatively strong, diffused light with adequate water-

Fic. 12. Root-tip of
corn, diagrammatic.

—

Water-Content of Plants 49

supply does not inhibit root-hair production in the ordinary
crop plant; but with strong light it is more difficult to
maintain a high moisture content under laboratory con-
ditions. |

32. The root-cap. — A longitudinal section of the tip
ce

ron OTS

ie ;

bos Coyees

:

oP PETS? ;

6:0 oe

SLO i or Rag ee Ge ier
PAO We oe
SPAIIIS Peay x
6.

Fic. 13. Root-tip of corn, high-power study of formative region:
epidermis (e), cortex or periblem (c), plerome (py), and root-cap (7).
[After Curtis. ]

of the root (Fig. 12) shows a rather complex but interest-

Ing structure. Protecting the growing tip, there is inva-

riably found the well-known structure called the root-cap.

The root-cap consists of a mass of cells to which falls the

duty or office of a bumper organ. It is very effectively

developed by divisions in the epidermal (protodermal)

cells of the growing tip parallel to the surface. This mass
E

50 Plant Physiology

of cells is always resistant and compactly arranged within ;
but as the cells are pushed outward, becoming old and
sloughed-off by the continual addition of new cells, they
undergo gelatinization and decay. This latter process,
however, is important, for the gelatinization of these cells
doubtless acts as a constant lubricant to make easier the
course of the tip progressing through the soil.

33. Structure of the root-tip. — In the root-tip proper
there is a primordial meristem (Fig. 13) or region of rapid
cell division. It is often called the formative region, and
it is from this portion that the differentiation of tissues
proceeds. In some cases the meristem is relatively ex-
tensive, as in the pea and certain other legumes, while in
the example given (corn), barley, sunfiower, and others,
it is limited to very few cells, — differentiated tissues
extending practically to the morphological apex.

The central portion of the root-tip is occupied by the
plerome or central cylinder, a columnar mass of cells,
most of which elongate considerably at a short distance
back of the tip. In longitudinal section they appear as a
rectangular type of cell in all that portion of the root
unoccupied by root-hairs, but many of these cells are later
transformed into the primary woody bundles. The cen-
tral cylinder is surrounded by a cortical portion termed
the periblem, made up at first of rather isodiametric par-
enchymatous cells. The inner layer of this periblem is
commonly differentiated by thicker walls to form a more
or less definite sheath or endodermis. The external layer
of the root-tip is the epidermis, also composed, in the
immediate tip portion, of cells generally isodiametric.
It is from these epidermal cells that tubular outgrowths

Water-Content of Plants 51

develop as root-hairs. It is believed that factors operat-
ing to increase rapidly the longitudinal extension of these
cells will decrease the number and length of the root-hairs.

y A Sern 8 oe a {

Seeteteasene toon

A A Kx DAE
ye

A
a
&

A
.f 4
_a—-a~ )
Y =O 8. ce
Coes
QS Bre
LA TT

DOORS 4S
Narw<e.S
DARE) xy
EOE OOF]

ies r~
LAE PRT Oak
2X ASH

y
Ab
oy
S
as

@)
ry
(}
es

C4

Lp
ze
mos
Ss
\ IR
(o)

Ce)

VX

Ls

@
YOO.
-2OO®8
AY

:

LY

:
So-@
SSelse,
Cy
Ee.
2

we
a
st
@:
e
ic

aC ‘,
ver
y\

Fig. 14. Cross-section of a rootlet: epidermis and bases of root-hairs
(a), cortex (b), endodermis (c), and central cylinder (e) with xylem and

phloem elements.
It would seem that in some cases the retardation of growth
by soil particles has a beneficial effect upon hair produc-
tion. In a completely saturated air or in a high temper-
ature — in other words when the conditions are such as to

rapidly promote increase in length of the cortical cells of

52 Plant Physiology

the root — there is a tendency to suppress hair pro-

duction.

34. Soil particles. — The extent of the root-system
previously discussed enables the plant to come into con-

Fic. 15. Seedling from a
sand culture, showing
adherence of grains.

tact with an enormous quantity of
soil particles. Each particle of the
soil, even air-dried soil, is invested
with a film of water, and the amount
of water which the soil may contain
is, to a considerable extent, depend-
ent upon the degree of fineness of
these particles. It is also dependent
upon the amount of organic matter,
the so-called humus, as subsequently
indicated.

With respect to the sizes of the
particles, a soil may be designated
as sand, silt, or clay. These three
classes are subdivided into types
or grades, depending upon the mi-.
nuter differences in size of the parti-
cles. The following table is generally
adopted by soil experts for the pur-

pose of a mechanical classification : —

Coarse sand
Medium sand
Fine sand
Very fine sand
Silt

Fine silt .

Clay .

3-—2; inch diam.
11 inch diam

50 100 -
rio-z35 Inch diam.
si;-¢}5 inch diam.
Eb0-B000 inch diam.
3000-5000 inch diam.
xdvo-zs$00 Inch diam.

All soils contain, in addition to larger particles, certain

Water-Content of Plants De

proportions of these general constituents. In addition a
fertile soil must contain a considerable amount of humus
derived from the disintegration of plant and animal re-
mains.

35. Soil texture and water-holding capacity. — The
water-holding capacity and the capacity for delivering
water to the plant will depend, therefore, to a very con-
siderable extent upon the mechanical constitution. In
general, a fertile soil should consist of relatively fine par-
ticles, since the water-holding capacity and the amount
of food-materials will be, up to a certain point, a factor
of the fineness of the particles. On the other hand, fine-
ness of particles at or approaching the point of satura-
tion of the soil may cause exclusion of air to such an
extent that the finer the soil the less the amount of air
present. In this connection, however, it should be re- .
membered that under ordinary circumstances the pore
space of a clay soil may be 50 per cent of its volume, while
that of a coarse sand uniform in texture may not exceed
30 per cent.

One cubic foot of an ordinary loam would cover a rela-
tively enormous area if spread out as a single layer of
particles. It is calculated that the contents of one cubic
foot would cover about one acre. Some sort of mental
picture of the absorbing system and its possibilities may
be had by recalling, in conjunction with these indications
respecting soil particles, the extent of the root-system as
clothed with root-hairs. The root-system of a mature
sunflower may almost completely permeate a cubic yard
of soil.

It is necessary to make a comparison of some main soil

o4 Plant Physiology

types with respect to the water-holding capacity in order
properly to understand the relation of the plant to the
soil. The water-holding capacity of the soil as ordinarily
measured is the ratio of the weight of water, when the
soil is at the point of saturation, to water-free soil, or dry
weight of soil.

The following table is an indication of what may be
expected in this regard in certain general soil types : —

Ordtnary’ sand ~ su... (6-3 ws, tS 6
Rich sandy loam... .. . «<5 3) 2 SO
Rich clay loam ©. -. 9. 5. « Sob we.) 406
Very heavy clay... «© <0 6.0. \.5 (@0-—-@eee
Garden. soil, rich tm fitimus. ).%. << ae 70— 90 per cent
Homus Se ee Ae > eo

It is considered to be a general rule that a large number
of cultivated plants find most favorable conditions for
rapid growth when the soil contains from 40 to 50 per cent
of its maximum water capacity. In the case of a sand,
therefore, with a maximum capacity of 25 per cent the
optimum for plant growth would be 10 to 124 per cent.
This will depend somewhat, however, upon the other con-
ditions under which grown.

Under the conditions encountered in the laboratory and
greenhouse, where in general the soil employed has been
well stirred, or is constantly well aérated, the optimum
moisture content may be much higher. In some cases
this optimum may run as high as 70 to 75 per cent. Under
field conditions it is apparent that the amount of organic
matter and the aération of the soil are most important in
determining the optimum. When organic matter is abun-
dant in the soil, especially when the soil is compact, a

—

Water-Content of Plants 5d

tendency toward saturation apparently encourages a type
of bacterial action which may promptly result in great
harm to many agricultural plants.

36. Exceptional plants.— The preceding statements
relative to the optimum water-supply are to be understood
as applying to a great majority of cultivated plants; how-
ever, as an example of an exceptional crop the cranberry
may be cited. ‘This plant grows to the greatest advantage
in typical bog situations. As ordinarily cultivated,
drainage is given this crop in such manner that the sur-
face soil will not contain free water; yet under ordinary
circumstances the soil approaches saturation on account
of the low water-table, — at the same time bog conditions
are such as to retard oxidation. The cranberry and many
other bog plants are therefore adjusted to the peculiar
conditions of their habitat. These bog conditions are, in
fact, extremely interesting, but it is unnecessary to go into
this subject further at this point.

37. Unavailable water. — The plant is unable to with-
draw all of the film moisture in contact with the soil par-
ticles. If at any time the plant is unable to obtain from
the soil the water it requires, wilting will ensue. The
water then remaining in the soil is unavailable or non-
physiological. When this point is reached, the soil is dry to
the touch, yet an appreciable percentage of water remains.
For any plant the film water unavailable in a variety of
soils is proportional to the water-holding capacity of these
soils; that is, the greater the water-holding capacity the
greater the pull against the plant when the content is low.

Under ordinary agricultural conditions with loamy
soils, there will be from 5 to 12 per cent of water unavail-

56 Plant Physiology

able for most cultivated plants. This may be reduced to
less than 1 per cent in coarse sand, and may rise to more
than 50 per cent in typical New York muck. The tables
which follow are after Heinrich !; the first shows the rela-
tion existing between water sae hygroscopic water,
and unavailable water; the latter table gives a suggestion
as to the conduct of different crops on two types of soils : —

2 HyGro- WATER
WaATER-
SCOPIC UNAVAIL-
Soin CONTENT AT -
: WATER- ABLE TO
SATURATION
CONTENT PLANTS
Per cent Per cent Per cent
Coarse sand . . are eye Fhe 0.42 1.5
Medium fertile ton ail ear 43.9 1.68 4.6
Infertile sandy muck soe aka 41.4 .97 6.2
Sandy loam . . eo ea a 43.3 2.40 7.8
Very fertile ite soil | 38.3 3.65 9.8
Peat soil 274.0 | 20.60 49.7
MOoIsTUuRE CONTENT AT WHICH PLANTS
BEGIN TO WILT
PLANT
On Caleareous Soil On Peaty Soil
Per cent Per cent
ate Ss rao i eae 8.4 32.3
BREE lot eis oer 9.98 oa.0
Rye Spy oe, ree Fs 9.55 32.8
PREG MIOGOCE. ie MeFi Men ts 10.28 34.3
IRGtatOeS eee tae fete Pd pul NF 41.4

1 Cited by Cameron and Gallagher, Bureau of Soils, U. S. Dept. Agl.

Bul., 50: pp. 57-58.

Water-Content of Plants 57

It will be noticed that so soon as the amount of water
in ordinary soils becomes about three times the hygro-
scopic content it begins to assume physiological impor-
tance. A soil which contains merely hygroscopic moisture
is “ air-dry’; and if this amount only, or any amount
less than “‘ available,’ were present, the soil would actually
withdraw water from the plant, thus inducing drying-out
‘Independent of transpiration.

The following table, compiled from Hedgecock, includes
certain agricultural plants as well as species inhabiting
marshy and xerophytic conditions : —

PLANTS GROWN IN LOAM, UNDER SIMILAR GREENHOUSE CONDITIONS

ee UNAVAILABLE
WATER
Coleus (Coleus blumei Benth.) . . . EA te 3.0
Morning glory (Ipomea purpurea Roth. ye rat te 4.1
(eapbazer( brassica oleracea Ly.) 2. 5.8
SMMC COMICS TA) 62 3 Se ee ee eS 5.9
pusnepeet (be vulgaris lL.) .°. 2 oe ee 5.9
valharye (lymus canadensis I.) 2 os se es 5.9
Oats (Avena sativa L.) . . Ry eR ear ee 6.2
Asparagus (Asparagus Opicinatis i Seeks a teteho ete 7.0
Wetmee (Lactuca/sativa L.) 2 6 oe Se | 8.5
Cucumber (Cucumis sativus L.) LaPR uhttes: MAe BETS 10.8
Arrowhead (Sagittaria latifolia Wild) . . . . . 15.6
- Pondweed (Potamogeton americanus C. and 8.). . 24.8

It will be noted that plants normally inhabiting water or
swamp-land wilt first, and next to these are cucumber and
lettuce, both with high-water requirement and relatively
little structural protection against water-loss.

58 Plant Physiology

38. Leaves poorly fitted for water absorption. — In
general leaves are of little practical value in the absorption
of water. On a hot day a wilted plant recovers after a
shower, not because it absorbs water rapidly through the
leaf parts, but because (1) the atmospheric conditions then
generally reduce the amount of transpiration, and (2) the
roots are able promptly to get the water needed. Never-
theless, partially wilted lettuce or peach leaves will be
revived if the blades are dipped into water, even though
the cut ends of the petioles are exposed to the air. A
flax plant or a cabbage leaf would show no perceptible
effect from the immersion for a long time.

On the other hand some plants are capacitated for the
absorption of water through leaves or pitcher-like vegeta-
tive organs. Among these are certain of the Bromeli-
ace (a family to which the pineapple belongs) possessing
leaves the bases of which sheathe the stem so closely as
to form reservoirs for precipitation water. In this
family, moreover, the absorption of water is more abun-
dant by means of certain cells in the peculiar shield-
shaped scales, and Tillandsia usneoides, the Florida moss,
is an extreme form in this respect. It is an epiphyte,
consisting of thread-like stems and narrow leaves, very
common on trees in the far South. This plant is provided
with much the same type of water-absorbing hairs which
give the entire surface a glistening appearance. The
a€rial roots of orchids and some other tropical plants are
provided with velamen, a chambered epidermal tissue
which may absorb water like a sponge.

Water-Content of Plants 59

LABORATORY WORK

W ater-content.— Determine separately the water-content of
any convenient twigs and fruits, or of fruits and seeds ;—
apples and apple twigs, squash and squash seed are generally
available. If crude scales only are accessible, use from 50 to
100 grams of material; while if very delicate scales are em-
ployed, 10 grams are sufficient. The process may be as follows:

Fic. 16. A dry-heat sterilizer, serviceable also in determining water-
content.

(1) weigh and record weight of vessels to be employed, prefer-
ably small glass, aluminium, or tin dishes; (2) mince or break
up the material finely, using a quantity more or less than that
which is desired, into the proper vessel, weigh immediately, and
record ; (3) dry at from 100 to 110° C. to constant weight (if a con-
stant temperature dry-oven is at hand, leave in this until the

60 Plant Physiology

next period; otherwise dry in any oven used for dry steriliza-
tion during two successive laboratory periods), weigh, and
record. If not weighed immediately, leave the material in a
desiccator over fresh (dry) CaCl2 until determined. Calculate
percentage of water.

Determine the water-holding capacity of any rich garden
loam, sand, and powdered quartz which may be available for
later experiments. To completely saturate the material a sim-
ple method is to fill a small pot or wire basket with the moist
material, tamp down lightly, dip into (or pour on) water care-
fully until slightly more than saturated; then, as soon as the
drip has ceased, dump approximately the quantity desired into
the weighed vessel, and proceed as before. In calculating the
water-content of soils it is to be remembered that at present
the method more commonly employed is to determine the ratio
or percentage of water, with respect to dry weight of soil, a

= . Ww — W = °
calculation easily made as follows: r = ————,, in which w=
Ww

/ "?
—w
weight of saturated soil and vessel; w’, of dry soil and vessel;
w'', of vessel; r being the per cent of water.

Root-systems. —The distribution of roots in the soil may
be studied by careful excavation and washing out (ef. Ten
Eyck, Kansas Agl. Exp. Sta. Bul. 127). <A far better concep-
tion of the fine root-system (under fairly natural conditions only
when care is taken in the manipulation) may be obtained for
demonstration, according to the method of Rotmistrov, by
growing plants in flat zine, or zine-lined, boxes which for class
purposes are large enough if 1 inch across, about 15 to 18
inches wide, and 18 inches deep. The box should contain sur-
face-soil and sub-soil well compacted and arranged as in
some local soil type. During the period of growth the boxes
should be immersed to the surface in deep boxes of sand or soil.
Growth should be permitted to proceed for a month or more.
When the boxes are open, a side is removed, a fine mesh wire
sereen (about { inch) is laid on the soil, the whole being inverted ;
the soil is left upon the screen, and upon dipping this gradually
into water an almost perfect root-system may be obtained.

Water-Content of Plants 61

Root-hairs.—Germinate seeds of radish and barley in a
germinator! (or upon sawdust or moss) and in moist soil.

Fig. 17. Germinator for dry room: aérated chamber (A); saucer (B) ;
and flower-pot (C).

Compare under the microscope the length and appearance of the
root-hairs, — the plants from soil being carefully washed and
small rootlets mounted for observation.

1Germinators of a variety of types are employed in various physio-
logical or seed-testing laboratories. In general, any device which secures
constant moisture for the germinating seed is satisfactory, but in no case
should any materials be employed which encourage the growth of molds
or bacteria. Commonly it is sufficient to distribute the seed so that they
shall not be in close contact one with another upon the surface of moist
sphagnum moss, sawdust, or other moisture-holding materials of this
nature into which the roots will readily penetrate.

The type of vessel employed is of little consequence so long as there is

62 Plant Physiology

Root-cap.— Make a preparation of the whole root-tip of
barley and note with the low-power of the microscope the
general extent of the root-cap, the histology of which may only
be studied by adequate sections.

Root- tip. — Make longitudinal sections of the root-tip of corn
or sunfiower and compare with those of pea or vetch, distinguish-
ing and describing primordial meristem, central cylinder, ple-
rome, epidermis, and root-cap. Hand sections are sufficient,
if properly made, but prepared slides are also desirable.

Unavailable water. — Use young plants of cucumber, lettuce,
and barley or wheat grown under similar conditions in small
pots of rich or heavy loam and pure sand or quartz [preferably
those soils the water-content of which were determined earlier].
The pots should contain no moss or such material at the
bottom. When wanted for the experiment, the plants should
be barely supplied with adequate water, so that during the
laboratory period incipient wilting may begin. At the moment
of wilting of each plant weigh out a portion of the soil of the
pot and record the weight. These samples are retained and
subsequently dried to determine the content of unavailable
water in the different types.

Water absorbed by leaves.— Secure leaves of lettuce, peach,

good aération. Seed grown for physiological purposes should not be
turned over or shaken during germination, as curvature of the roots is
likely to result. In testing seeds for vitality, where numerous varieties
or sorts are employed, special boxes with ruled spaces, each space de-
signed for a given number of seed, are important. Information regard-
ing this matter niay be obtained from any bulletin on seed-testing.

In the laboratory it is often desirable to obtain root-hairs in full
development; in such cases flower-pots or special porous germinators
(covered saucers) may be employed. With small seed, such as lettuce,
mustard, etc., the procedure may be as follows: moisten the germinator
or pot and sling a few seed against the inner surfaces. They will adhere
to the surface at convenient distances apart, and then if provided with
sufficient moisture, absorbed through the walls of the vessel, a rapid and
vigorous germination will result, and the development of root-hairs will
be shown in a simple and striking manner.

Water-Content of Plants 63

bean, cabbage, cudweed, or other plants available. Some of
these should be leaves readily wetted, and others provided with
a bloom, or with hairs effectively preventing wetting. Permit
the leaves to wilt slightly, then weigh each kind accurately upon
a delicate balance. Next place the leaves in water in a dark
chamber, immersing all parts except the petioles (or, previous
to weighing, seal the petioles carefully with wax). After from
6 to 24 hours note any change in the rigidity in the leaves;
also remove all moisture from the surface with filter paper,
weigh carefully, as before, and compare the weights of each
kind.

REFERENCES

DanvdeEno, J. B. Effects of Water and Aqueous Solutions of
Some Inorganic Substances on Foliage Leaves. Trans.
Canad. Inst. 7: 238-350, 1901. ©

FREIDENFELT, T. Studien tiber die Wurzeln krautiger Pflanzen.
Flora 91: 115-208, 1902.

Hepcecock, G. G. The Relation of the Water-content of the
Soil to Certain Plants. Botan. Survey of Nebr. 6:79 pp.,
1902.

Hinearp, EK. W. Soils. Pp. 188-310, 1905.

Lyon, T. L., and Firpin, E.O. Soils. Pp. 136-165.

Rormistrov, V. Root-systems of Cultivated Plants of One
Year’s Growth. 57 pp., 22 figs. (Issued by the Experi-
ment Station, Odessa, Russia.)

Snow, L. M. The Development of Root-hairs. Bot. Gaz. 40:
12-48, pl. 1, 1905.

SpatpiInG, V. M. The Biological Relations of Certain Desert
Shrubs. I. Bot. Gaz. 38: 122-160, 7 figs., 1904; II. Ibid.
41 : 262-282, 1906.

Tren Kycx, A.M. The Roots of Plants. Kansas Agl. Exp. Sta.
Bul. 127: 199-252, 26 pls., 1904.

TEXTS. Goodale, Jost, Pfeffer, Sachs, Sorauer, Stevens.

CHAPTER IV

CONDITIONS AND PRINCIPLES OF AB-
SORPTION

Tue physical and chemical factors governing the ab-
sorption of water and of solutions have long been the ob-
ject of careful study. Many phases of such work have
been developed in connection with plant physiology. The
main facts, however, belong, in large part, to physics and
physical chemistry; yet an appreciation of the essential
principles is necessary to an adequate understanding of
the mechanism and work of absorbing organs.

39. Imbibition phenomena. — Organic bodies of the
most varied nature are able to take up water. Commonly,
where this water is held within the body by ecapillarity or
surface tension, and where there is produced also more or
less swelling, the combined phenomenon is recognized as
imbibition in a physiological sense. The hardest wood
may absorb water by imbibition, and the force exerted in
swelling or warping is capable of lifting or sustaining
heavy weights; — sometimes made use of in splitting solid
rock or stone. Dry seed-coats generally exhibit a high
degree of imbibition, although these may be infiltrated
with substances preventing the absorption of water, and
in that way germination may be delayed.

In the living plant imbibition phenomena are of im-

64

Conditions and Principles of Absorption 65

portance, and particularly important in the activities of
the nonliving cell-walls. The maintenance of the water-
current or water-content of the plant is conditioned by
imbibition, but under the conditions of growth the inflow
of water into the cells of the root surfaces is effected by the
force of osmosis, a mental picture of which is essential to
an understanding of absorption phenomena.

40. Osmosis and diffusion. — Osmosis and diffusion
as generally understood determine the inflow of water to
the root-hair, as well as that of the nutrients or other sub-
stances (solutes) in solution. These forces are likewise
important in the interrelations existing between cells.

The fact of diffusion is readily observed. If a crystal of
copper sulfate is placed in a tumbler of water, the salt goes
into solution, and in time the colored solution diffuses
itself equally throughout the water. This diffusion is
wholly independent of any convection currents due to
changes of temperature, and it is true for all such soluble
substances as sugar, common salt, and the like.

The movement of the particles of the dissolved sub-
stances from the region of greater concentration to that
of less implies a force, or pressure, which may be termed
osmosis, or diffusion tension.

41. The demonstration of osmotic pressure. — The
osmotic action of a solute, in water as a solvent, may be
conveniently demonstrated qualitatively by a simple
experiment in which it is made evident as hydrostatic
pressure. A factor not yet mentioned which is involved
and which is absolutely essential in this demonstration is
a semipermeable membrane, an imbibition membrane

which, in this case, permits water to pass through readily,
F

66 Plant Physiology

but is impermeable, or very slowly permeable, to the sub-

stance in solution. Semipermeable membranes are of

the most diverse sorts. A piece of

pig’s bladder, a commercial article

readily obtainable, is a very satis-

factory membrane. This, after be-

ing soaked in water, is tied tightly

over the bell end of a thistle-tube

(Fig. 18) with waxed thread. The

solution to be tested, preferably a

known strength of some solute, say

20 per cent sugar, is carefully

poured into the stem of the thistle-

tube (easily accomplished with a

guide-wire, or without when the

tube is clean and wet) until the

bell is filled with the sirup. The

tube is then lowered into a vessel

Fic. 18. Diffusion shell of water until both liquids, which
and thistle-tube for

demonstration osmo- Should have come to room tem-

scopes. perature, are at the same level, when

the tube is clamped to a support. It is well to add about

1 per cent of formalin to both liquids as a preservative.

If the experimental conditions are properly carried out,

the liquid in the thistle-tube will rise perceptibly in a few

hours, and an experiment to be continued a day or more

will require at the outset an extension of tubing. There

is, therefore, a major flow of water through the membrane

to the strong solution; thus there is manifest a pressure

which, if it could be completely measured by this column

of water in the apparatus, would be the osmotie pressure

Conditions and Principles of Absorption 67

of the 20 per cent sugar solution. If there were a less
concentrated sugar solution in the outer vessel, or any
solution of salts containing fewer solvent particles per
volume, the major flow of water would still be inward, and
the extent of this pressure would be in direct proportion to
the difference in number of solvent particles. .

Special diffusion shells (Fig. 18) are also prepared for
demonstration experiments, and these likewise give quick
results. In this connection it might be mentioned that
accurate measurement of osmotic pressures involves
special apparatus in which the natural membrane is re-
placed by an artificial precipitation membrane deposited
in the.interstices of a porous cup, the pressure being indi-
cated by a mercuric manometer. The original of this
apparatus, described by Pfeffer in 1877, has been greatly
improved in recent years by Morse.

42. An explanation of osmotic pressure.—In the
thistle-tube demonstration experiment it was suggested
that the molecules of sugar or other solute tend to diffuse,
to distribute themselves equally, but the semipermeable
membrane retards and almost prevents this outward
diffusion. The inflow of water is conceived to be in direct
response to this force, analogous to a pressure, and the
water would continue to flow inward until equilibrium
between the pressures were established. This commonly
accepted view of osmosis regards the solute as obeying
the laws of gases. At a constant temperature, therefore,
the osmotic pressure varies with the density or concentra-
tion; that is, with the number of particles of the solute.
The molecular weight of cane-sugar (a nonelectrolyte)
in grams, 342, dissolved in water to 1000 cc. (called a gram-

68 Plant Physiology

molecular solution, or M solution), would give, according
to this, an osmotic pressure of 22.4 atmospheres.! Ordi-
narily the osmotic pressure of an epidermal cell of a leaf
or of a meristem cell is somewhat more than 4 atmos-
pheres, or about .20 gram-molecular (7 per cent) of sugar
as determined by the method subsequently discussed.
The plant cell behaves very much as the simple os-
mometer above described. The root-hair, for example, is
a case in which the cell-sap is the strong solution, the limit-
ing layer or edge of cytoplasm is the plasmatic membrane
(the cell-wall in this instance furnishing support and pro-
tection), and the soil water is the weak solution. The
flow of water is into the cell; in fact, under such circum-

1A gram-molecular solution of such substances as potassium nitrate
or common salt (electrolytes) yields a pressure higher than 22.4 atmos-
pheres. This is explainable on the ground that the number of particles
in solution is increased by the partial or complete dissociation of the
molecules of such substances into their ions. Thus KNOs3, when partially
dissociated, yields in addition to KNOx3 the ions K* and NO3~-. Gram-
molecular solutions of this salt show a pressure of about 35 atmospheres.
For this reason cane-sugar and potassium nitrate are not osmotically
equal; that is, isosmotic at the same molecular strength. Before the
theory of dissociation was developed De Vries determined that organic
substances such as sugar have about two thirds the osmotic value of
monovalent salts; that is to say, that the ratio of their coefficients is as
2to3. This ratio and those developed by De Vries for dibasic or other
compounds are fairly satisfactory as indications of the isosmotic relations
at the strengths corresponding to the plasmolysis of higher plants.
Nevertheless, since the per cent of dissociation varies considerably
among the salts of the monobasic, dibasic, or other groups, it is essential
in any comparative quantitative work to know accurately the per cent of
dissociation of any electrolyte employed. This may be obtained from
physical chemical tables. The discussion of the relation between elec-
trolytes and nonelectrolytes and the formula for comparing the latter
with the former is developed in the work by Livingston, cited in the
literature.

Conditions and Principles of Absorption 69

stances, so long as an equilibrium has not been established,
and water is available, there is absorption of water, and
there is manifest always, with adequate water-supply
within the cell, an hydrostatic pressure known as turgor.
This turgor, existing throughout the plant, is, as already
indicated, the chief cause of the rigidity of leaves and suc-
culent shoots. Turgor is then the expression of the os-
motic pressure of the cell. This turgor may be measured
by simple experiments conducted as described below.

43. Plasmolysis and wilting. — If the root-hair or any
equivalent cell is placed in a solution stronger than the
cell-sap, the major current of water will be outward, so
that water will be withdrawn from the protoplasm, the
latter contracting from the cell-wall. This state of con-
traction is termed plasmolysis. Plasmolysis throughout a

py I
Whe M(( 4 =
ase wy Bp th T= >

= "Su: Ns y
a LY, L, ¢ ey
Y BR ik

Geko MR
Ce Dy

i ‘Aor RS : 3
oe at vit
NP a yf i,

Fig. 19. Lettuce plants in solutions: A, tap water; B, 2.9 per cent
sodium chlorid.

Pe ee eee I

70 Plant Physiology

tissue or organ results in flaccidity or wilting. In Figure
19 are shown two lettuce plants transferred from soil.
The roots of plant A were put into water, and those of B
into 2.9 per cent sodium chlorid (approximately .5 gram-
molecular). The latter has caused a prompt loss of
water by the plant, so that wilting has resulted.

If slices of beet or potato are placed in solutions similar
to those just mentioned, turgescence on the one hand and

Fic. 20. Successive stages in plasmolysis : epidermis of Tradescantia
(a) and cells of Spirogyra (b).

flaccidity on the other will result in the same manner.
The phenomenon of plasmolysis in the cell is sufficiently
important to be carefully studied. It is often more readily
observed in a cell with colored contents, or numerous
chloroplasts, so that the cells of filamentous alge, the
colored epidermis of certain begonias, or of Tradescantia
zebrina, also the stamen hairs of Tradescantia and Ana-
gallis, are convenient materials for demonstration. Figure
20 shows successive stages in the plasmolysis of the epi-
dermal cell of Tradescantia zebrina and of Spirogyra.
The concentration which will just cause the least trace of

Conditions and Principles of Absorption 71

plasmolysis is taken as the measure of the turgor of the
cell.

Plasmolysis may be regarded as a general phenomenon,
yet it should be observed that the cytoplasm may undergo
contraction, wholly independent of osmotic relations, under
the influence of certain stimuli. Greeley! ascertained
that low temperature may produce this effect, and under
certain circumstances high temperature may also cause
contraction. It has long been known that injurious sub-
stances may produce plasmolysis coincident with injury,
a fact to which also Osterhout ? has recently called atten-
tion.

44, Variation in turgor.— Considering plants as a
whole, thus including fungi, alge, and the higher _ plants,
there is great variability in turgor. The fungi show the
ereatest range and are therefore adapted to thrive in the
most diverse situations with respect to concentration.
Fresh-water algee and all the higher plants show, on the
whole, a less extensive range; yet even in the same plant
the cells of different parts, or tissues, may show no signifi-
cant variation. Commonly, as already suggested, the tur-
gor in many epidermal and parenchyma cells is equiva-
lent to about 7 per cent of sugar (4.5 atmospheres, but
often the range may be from 4 to 8 atmospheres). This
fairly close range in the higher plants is perhaps to be
anticipated, since it is not conceivable that the strength
_ of the soil solution, under the complex physical and chemi-
cal conditions, would show unusual extremes. It may be
readily shown by experiment, however, that in a strong

1 Greeley, A. W., Am. Journ. Physiol., 6:112—128, 1901.
2 Osterhout, W. J. V., On Plasmolysis. Bot. Gaz., 46: 53-55, 1908. —

72 Plant Physiology

solution plants develop a somewhat higher turgor than
when grown in an extremely weak solution.

According to the work of De Vries many plant cells are
just plasmolyzed at a concentration of about 1.2 to 1.4
per cent KNO,; (.12 to .14 gram-molecular solution),
which is equivalent to about 5 atmospheres. Active cells
of the cambium may require a concentration of from
4 to .56 gram-molecular solution; and in an investigation
of medullary ray cells in a common willow (Salix fragilis)
Kny finds that the different types of these cells are plas-
molyzed at concentrations varying from .10 to 8 M.
Moreover, turgor varies with age, nutrition, and with
environmental factors, such as heat and light.

In general, observations upon living cells render it
perfectly obvious that turgor is usually an essential attri-
bute of active cells. There is, unquestionably, an impor-
tant interrelation between turgor and growth; therefore,
conditions affecting turgescence affect ae
all growth processes.

45. Substances active in producing turgor. — No one
compound or group of compounds is responsible for turgor.
It may be due to dissolved substances both organic and
inorganic, and while, in some cases, it does not change
materially during the growth or other activities of the
cell, yet the composition of the sap may undergo a rela-
tively great change. According to Pfeffer, the turgescence
of cells in the root of the sugar-beet is produced largely
by cane-sugar, while in the sunflower more than 40 per
cent may be represented by potassium nitrate. The beet
contains relatively little sugar when young, and the sun-
flower little nitrate. Various other inorganic salts, glu-

Conditions and Principles of Absorption (Gs

cose, organic acids, and many other compounds are also
important in the osmotic strength of cells. The high
turgor of certain mold-fungi growing upon concentrated
solutions has been determined to be due to organic sub-
stances, which may be readily produced within the cell.

46. Osmosis and the absorption of nutrient salts. —
The water requirement is not the only one with which
osmosis is concerned, for the principles of osmosis and
diffusion govern also the absorption of nutrient salts;
likewise, of course, the absorption of any other substances
present in the soil solution. Moreover, the plasmatic
membrane is to a degree permeable to all the nutrients,
and to many other substances as well.

Each substance in the soil solution has its specific tend-
ency to diffuse, and it therefore tends to come to equilib-
rium with the tension of the same substance in the cell.
The cells which are active in absorption have in turn a
relation to those adjacent to them, and this relation,
emphasized or otherwise modified by cells especially ca-
pacitated for conduction, extends to all parts of the com-
plex organism. The root-hair, then, in so far as it is
permeable, absorbs each substance or solute particle inde-
pendently, and in accordance with a certain attraction for,
or use of, that substance in some way, as in the deposition
in an insoluble form —it may be in the building up of pro-
toplasm, or in the accumulation of complex food-materials.

One of the most remarkable facts respecting the osmotic
relation of the plant to the soil solution is that there is so
little exosmosis, or outward diffusion of substances from
the plant, — substances present in the plant but not in
the soil. Again, it is difficult to understand the absorp-

74 Piant Physiology

tion, transport, and final accumulation (often without
change) of certain substances in special organs. There
are a number of factors affecting such relations, but much
is yet unexplainable on a physical basis.

47. Protoplasmic permeability. — It is an obvious fact
that the plasma membrane is permeable to certain solutes,
else no growth could result. It is as clearly apparent that
this membrane is impenetrable to certain other solutes,
and this implies selective absorption. The fact of imper-
meability becomes evident from a simple observation upon
colored cell-sap, and especially so upon contemplation of
the phenomenon of plasmolysis in cells containing colored
sap. The colored cell-sap of a red beet or of a cell from
a stamen-hair of Tradescantia does not diffuse into the
surrounding water so long as the cell is uninjured. More-
over, when such cells are plasmolyzed, there is, with con-
tinued health, no noticeable exosmosis of the colored
material. From dead cells there is prompt diffusion of
the colored sap. In this connection it is also to be re-
membered that in many cases red or blue color in plant
cells is merely an indication of acid or basic substances,
and this color may be changed in the living cell if it is per-
meable respectively to basic or acid compounds.

Pfeffer has clearly demonstrated important facts regard-
ing permeability through his experiments upon the pene-
tration of dye stuffs. Methylene blue at a strength of
1 part to 100,000 of water yields a solution which is not
visibly blue unless observed in a layer several centimeters
thick. It would not therefore give an evident coloration
in a plant cell. It is found, however, that upon being
placed in such a solution certain root-hairs, Spirogyra,

Conditions and Principles of Absorption ZD

and other cells are quickly colored blue. It is evident that
there has been penetration, and further that there has
been accumulation of the dye. In some cases this accu-
mulation is particularly noticeable, due to the formation
of a granular precipitate, as in Spirogyra. These facts
give some faint idea of the complexity of the problems of
cell absorption.

In accordance with the foregoing statements it is pos-
sible to assume that when a mixed solution is presented
to a root-hair, certain substances, independent of their
concentration in the environment, may be absorbed, while
others, whether dilute or concentrated, will fail to enter.
Upon this ground the relatively abundant occurrence of
iodine in seaweeds may be explained. In seawater iodine
is present at very great dilution, about .000001; yet it
is accumulated in marine alge to such an extent that it
has yielded (and still yields upon the coasts of Japan) a
commercial source of this material. Similar and striking
examples may be found from a study of the ash content
of any plant; thus the content of potash, iron, phos-
phoric acid, etc., may be greater than the ratio of these
substances in the soil solution, whereas other substances
may be absorbed in relatively less quantity. The ash
content of plants, however, is discussed at greater length
later.

It is scarcely practicable to consider here some of the
factors which have been found to affect permeability and
selective absorption. It is necessary to observe, however,
that Overton has developed. an interesting theory of
absorption based upon certain facts. One of these facts
is that substances may be assembled into diverse groups

76 Plant Physiology

with respect to permeability, and there has seemed to be
some relation between the capacity to penetrate and the
solubility of the solute in cholesterin or other similar com-
pound. This has naturally led to the assumption that
some such substance constitutes an important part of the
plasma membrane. Nevertheless, there are many ex-
ceptional cases, and different plants frequently exhibit
marked specific peculiarities.

The differences in penetration referred to are charac-
teristic also of toxic or injurious compounds as well as of
nutrient or beneficial substances. This fact is frequently
of service in explaining the relative toxicity of different
reagents. Nowhere is this shown more clearly than in the
experiments of Brown, from which it is evident that the
seeds of barley may be placed for a considerable time in a
relatively strong solution of sulfuric acid without injury,
whereas mercuric bichloride rapidly effects an entrance
and kills the cells. Further details of this experiment
are cited later (section 263). The illuminating experi-
ments of Kahlenberg on osmosis demonstrate clearly that
the nature of the semipermeable membrane is a matter
of great importance in osmotic phenomena. Further-
more, it has been shown that external conditions, includ-
ing those of temperature, light, and nutrition, affect
permeability and selective absorption to a high degree.
The plasma membrane should be regarded as made up
in part of a variable and complex colloidal solution.

48. The réle of diffusion and osmotic pressure. — From
what has been brought forward respecting osmosis and
diffusion it can be said that these forces are conspicuous
in the work of the cell. The concentration of the cell-

Conditions and Principles of Absorption 77

sap above that of the soil solution, or other liquid environ-
ment, conditions a turgor, an expression of osmotic force.
This turgor is coexistent with growth. It likewise confers
upon cells or organs a substantial rigidity. The concen-
tration of osmotically active substances manifest through
the absorbing surfaces represents a constant pull upon the
environment for water, so that root-hairs are able to ab-
stract water from surfaces or solutions which do not repre-
sent a greater pull. The plasmatic membrane is extremely
complex with regard to permeability, and it may exhibit
marked powers of selective absorption. In simple (few-
celled) plants osmosis and diffusion may be all-sufficient
in what is practically the movement of solutions, but in
higher plants there are, in addition to these important
forces, also other factors affecting mass movement along
the special conducting paths of the fibrovascular system,
as noted later.

49. Sap or root pressure. — The absorptive capacity
of the root, conditioned by its osmotic relations, may give
rise to a pressure, termed root pressure or sap pressure,
which may be manifest within the plant whenever the
greater rapidity of transpiration does not create a nega-
tive tension.

Bleeding phenomena are evidences of this pressure.
During the spring, in particular, the maple, birch, grape,
potato, black nightshade, nettle, and a variety of other
woody and herbaceous plants bleed profusely. In some
cases bleeding is checked by drying-out, by the deposition
of solid or glutinous matter, and by growth processes (ty-
loses) filling up the vessels from adjacent cells. In other
instances corky layers may be formed sooner or later.

78 Plant Physiology

The amount of the exudation may vary from a few
drops to several liters per day. Large quantities have
been reported for a few plants, especially tropical or sub-
tropical forms; thus Humboldt reports for the Ameri¢an
aloe 7.5 liters per day, or about 1000 liters during the
entire period; while if the observations of Semler are
taken, Caryota wrens may produce 50 liters per day, the
maximum amount observed. Among agricultural plants
employed in demonstration work, the potato and tomato
are good for short observations, and the grape vine —

Eckerson finds that among common greenhouse species,
Fuchsia speciosa and Begonia coccinea are especially favor-
able for quantity. The pressure under which the exuda-
tion is produced necessarily bears no relation to quantity
of exudate. The following table, taken from the data of
Eckerson, indicates what may be expected of satisfactory
material in experimental studies : —

MEAN DvuRATION =
PRESSURE
PLANT QUANTITY | OF FLOw,
| IN ATMOS-
IN CC. Days
| PHERES
Begonia coccinea (Begonia) . . : 168 29 .858
Chrysanthemum frutescens (Margue aie ) | 40 9-21 1.014
Fuchsia speciosa (Fuchsia) . .. . 99 12-34 1.246
Helianthus annuus (Sunflower) . . | Sua) 16 1.276
Lycopersicum esculentum (Dwarf aitine |
OMALO Me 4 eV oe ek owt cl lg a 13 5 1.164
Pelargonium zonale (Horseshoe gera-
iSVit a) OL na ee ee eg, } 15.5 10 881

Conditions and Principles of Absorption 79

A demonstration of the quantity of liquid pro-
duced, and of the existence of root pressure, may
be made by comparatively simple methods. The
quantity is readily determined by cutting off the
plant an inch or two above the surface of the
ground and connecting the stump by rubber and
glass tubing with a measuring glass protected
against evaporation. For the proper demon-
stration of pressure a suitable manometer is re-
quired (Fig. 21).

LABORATORY WORK

Imbibition ; swelling of wood. —Use small blocks of
oak, basswood, and pine, practically cuboidal in form,
preferably cut so that tangential, radial, and longi- a
tudinal axes are represented. Mark opposite points
of each axis with a pencil and measure carefully with
the calipers provided. Then soak the
blocks in distilled water for from five
to ten days, changing water each day;
after which, remeasure each axis and
compute the percentage of change.

Heat of imbibition. — Reduce 100
grams of common starch to a uniform
powder, dry in an
oven at about 105° C.,
and at the same time,
for a control experi-
ment, dry 100 grams
of quartz flour or
graphite. Cool both
powders to room tem-
perature in a desic-

eater, and pour each Fic. 21. Ganong’s manometer. [After the
into a Dewar flask or Bausch and Lomb Optical Company. ]

eeroettitetoeee aoreee ad

80 Plant Physiology

small thermal bottle (a tumbler may be used when double-walled
vessels are unavailable). Take the temperature of each powder,
then add 100 ce. of water at the same temperature, stir promptly
with a clean wooden stirring rod (the starch mixes with water
less readily), observe the temperatures, compare, and discuss the
results.

Osmoscope. — Set up an osmoscope as indicated in section
41, using a thistle-tube and membrane, or a diffusion shell.
Different strengths of sugar solution, 20, 40, and 60 grams per
100 ce. of water, may be used to note differences in rate of flow
and total height of column, but no accurate quantitative results
are to be expected. Describe the results obtained.

Precipitation membrane. — Drop a crystal of copper sulfate
into a bottle containing 5 per cent potassium ferrocyanide, and
observe the formation of a semipermeable precipitation mem-
brane of copper ferrocyanide, and the prompt rise of an irregu-
lar column of solution inclosed by this, which grows and may
attain considerable proportions in fifteen minutes. More neatly,
the precipitation membrane may be studied by employing a
more dilute solution of potassium ferroeyanide (2 per cent) in a
dropper bottle into which is lowered cautiously to its position a
dropper tube with capillary outlet, containing a single drop of
strong copper sulfate. Note and describe the phenomena tak-
ing place.

Plasmolysis and wilting. — Prepare 250 ec. of .5 gram-
molecular (M.) solutions of potassium nitrate and of sodium
chlorid as stock solutions. From these solutions make dilu-
tions in small vials, capacity about 25 eec., to contain the fol-
lowing strengths of each of the above solutions, namely, .10, .20,
.o0, and .40 molecular (M.); also one ‘vial with distilled water
as a control. In each of the dilutions place a seedling of some
plant (root as nearly entire as possible) with delicate stems, or
leaf stalks, such as lettuce, radish, or mustard. Observe the
dilutions in which wilting occurs, and note the time required in
the solutions in which it occurs. Compare the equivalent
strengths of the two salts. The above experiment will illustrate
the withdrawal of water by strong solutions and will suggest

Conditions and Principles of Absorption 81

the progressive plasmolysis and wilting of the cells of the plant
through the root-system, but the osmotic strength of the eell-
sap may be more accurately studied through the next two
experiments.

Osmotic pressure of cell-sap; observation upon tissues. —From
the stock solutions used in the preceding experiment prepare in
stender dishes or Syracuse watch glasses dilutions which shall
contain the following strengths, .10, .12, .15,. 18, and .20 molec-
ular (M.). Split the apical portions of several flower stalks
of the dandelion (or other scape which has been found suitable)
each into four approximately equal parts. These strips will
curve outward, the epidermis being within or on the concave
side. Dip strips momentarily into water in which spirals will
be formed, then cut into distinct rings. Place one or two of
the rings in each of the above solutions and also in distilled
water. Follow and note the changes which occur. Further
curling of the strips indicates absorption of water, that is, the
solution is too weak; no change in the curvature indicates a
solution equal in osmotic strength to the eell-sap (isosmotie
with the cell-sap) ; and elongation or reverse curvature indicates
loss of water and plasmolysis. Intermediate dilutions may also
be made, and the threshold of plasmolysis more accurately
determined.

Osmotic pressure of cell-sap; direct observation upon plas-
molysis. — The osmotic pressure of the cell-sap may be deter-
mined fairly accurately by direct observation upon the
plasmolysis of the cell, employing as the plasmolytic agents
substances which penetrate the cell only very slowly. The
substances employed above, also other neutral salts, cane-sugar,
ete., may be used. Cells with protoplasts the limits of which
may be easily seen are best for preliminary study, especially
alge, such as Spirogyra, Pithyophora, ete. All precautions as
to the cleanliness of vessels, also purity of the reagents and
distilled water, should be observed.

From the stock solutions of the monovalent salts previously
used prepare for a preliminary test a small quantity of a .2 M.
solution. Mount in a drop of this solution one or two filaments

G

82 Plant Physiology

of the alga, observing under the microscope for ten minutes
whether there is or is not some plasmolysis. Then, according
to the result, prepare dilutions of
less or greater concentration, and
determine accurately the thresh-
old of plasmolysis. For accurate
work the hanging-drop culture
may be employed. Determine
the osmotic strength also in terms
of cane-sugar. Peel off some of
the lower epidermis (with colored
cell-sap) of Cyclamen or Trades-
cantia zebrina, or use leaf hairs of
a cucurbit, and determine the os-
motie strength of these cells.

With cells of any plant just
distinctly plasmolyzed determine
if turgor may be restored by
irrigation with tap or distilled
water.

Shrinkage. — With any of the
above plant material mounted in
water measure accurately with
the ocular micrometer a cell easily
located. Draw off the water and add successively stronger
salt solutions until approaching the point of plasmolysis; re-
measure; plasmolyze the cell, and again measure. Compare
the results with respect to shrinkage.

Protoplasmic permeability. — Into a solution of methylene
blue, 1 part to 100,000 parts of water, place a seedling of radish
or mustard with well-developed root-hairs; also filaments of
Spirogyra and a sprig of Elodea. In two hours examine the
root-hairs, the cells of Spirogyra, and the leaf cells of the Elodea
for penetration of the dye, and discuss the results.

Sap or root pressure. — Utilizing suitable plants in the open,
or potted specimens, determine the amount of water exuded
upon decapitation, and also the pressure of exudation in two

Fic. 22. Simple method of
demonstrating exudation
from a decapitated plant.

Conditions and Principles of Absorption 83

species of plants (see section 49). In determining the amount
of exudation, conduct in each case the liquid into a graduated
test-tube with foot, in which test-tube is placed a drop or two
of oil to prevent evaporation. In the pressure determination
employ a Ganong manometer, or one similar in principle im-
provised from materials at hand. Observe frequently, calculate
the pressures at the different intervals, and draw curve of
results.

REFERENCES

EcKerson, 8S. H. Root Pressure and Exudation. Bot. Gaz. 45:
50-54, 1908.

‘KAHLENBERG, L. On the Nature of the Process of Osmosis and
Osmotic Pressure. Journ. Phys. Chem. 10: 141—209, 1906.

Livineston, B. KE. The Role of Diffusion and Osmotic Pressure
in Plants. Dec. Publ. Univ. Chicago. 8: 149 pp., 1903.

NatHansoun, A. Ueber die Regulation der Aufnahme anor-
ganische Salze durch die Knollen von Dahlia. Jahrb. f. wiss.
Bot. 39: 607-644, 1904.

OveRTON, E. Ueber die osmotischen Eigenschaften der Zelle.
Festschrift Naturf. Gesellsch. Ziirich. 1896.

PrerrerR, W. Osmotische Untersuchungen. 236 pp., 1877.

UHLAND, W. Beitriige zur Kenntniss der Permeabilitaét der
Plasmahaut. Jahrb. f. wiss. Bot. 46: 1-54, 1909.

Vries, H. pe. Hine Methode zur Analyse der Turgorkraft.
Jahrb. f. wiss. Bot. 14: 427-601, 1884.

Van’t Horr, J. H. Die Rolle des osmotischen Druckes in der
Analogie zwischen Lésungen und Gasen. Zeitsch. f. phys.
Chemie. 1: 481-508, 1887.

Wacuter, W. Ueber den Austritt von Zucker aus den Zellen
der Speichorgane von Allium Cepaund Beta vulgaris. Jahrb.
f. wiss. Bot. 41: 165-220, 1905.

Texts. Barnes, Ganong, Jost, Pfeffer.

CHAPTER V

TRANSPIRATION AND WATER MOVEMENT

THE water-content of a plant is no index of the amount
which has been absorbed throughout its life by the root-
system. It is a thoroughly familar fact that water is

commonly eliminated from the plant as water-vapor.

This elimination, termed transpiration, is important and
should receive special consideration. A very large pro-
portion of the water absorbed by plants is transpired ;
that is, it passes into the atmosphere by diffusion through
the leaves and other delicate parts. This loss of water
may be very simply demonstrated by placing a potted plant
under a bell glass, taking the precaution to place a rubber
cloth over the pot and over all possible evaporating sur-
faces except the plant itself. In a short time a mistiness
upon the glass will indicate roughly the loss of water.

50. Observations upon transpiration. — The demon-
stration of water-loss may be made in a variety of ways,
best of all by loss of weight. Nevertheless, single leaves
and abscised branches or organs may be employed in
various potometers, by means of which there is measured
the water absorbed, this latter corresponding in the end,
of course, very closely to that which is given off. Interest-
ing experiments may be readily set up with single leaves
or shoots (Fig. 23). By another type of experiment

S4

Transpiration and Water Movement 85

individuals growing in the
field! may be made the
objects of observation and
comparative study.
Experiments made with
abscised branches may not

be typical, for the shoots are hs

in abnormal relations, lack-
ing the usual organs of absorption, as
well as the special soil conditions ; and
since there is, further, a certain re-
sponse to the injury received, the
results of experiments made with plant
parts do not, perhaps, represent the
loss under natural conditions. These
parts may be employed, nevertheless,
for demonstration and for determining
more or less accurately the relative
rate of loss under different conditions.
Transpiration may be most accu-
rately determined by using potted
plants, observing the precautions in-
dicated with respect to evaporating
surfaces, and weighing at successive
intervals. Special recording balances
have been constructed and used for
this purpose, but ordinarily such de-
vices are unnecessary to demonstrate
principles and limiting conditions.

1 Freeman, G. F., A Method for the Quan-
titative Determination of Transpiration in
Plants. Bot. Gaz., 46: 118-129, 1908.

Fic. 23. Burette po-
tometer ; shoot fitted
with rubber tissue.

86 Plant Physvology

On a large scale, a rapid loss of water from plants is
familiar to all in the process of hay-making. The differ-
ence in weight between green and dry hay is perfectly
obvious. There may, of course, be a slight loss of water
from the cut surfaces of the stems, but even should these
be sealed by paraffin or wax, wilting and loss of water will
proceed almost as rapidly as before. Practically all parts

SSE OP CRS SSE SSR SS SESS) SSRs SS SSSR S SSS .

o
TD,
Ss

Fic. 24. Potometer with shoot-chamber (A), small-bore record-tube
(B), water-reservoir (C), and stop-cock for refilling tube (D), sup-
ported by base (£). [Adapted from Ganong.]

of plants lose water to at least a slight extent. Apples or
potatoes stored in a fairly dry situation during a consider-
able period of time will show considerable loss, although
the normal surfaces of such parts are so constructed that
rapid drying-out is prevented. h

As soon as wilting takes place, sufficient practically
to close the stomata, the rate of loss will drop, and thus

Transpiration and Water Movement 87

the effect of closure of the stomata is made evident in the
otherwise more or less normal curve of evaporation.

51. Amount of transpiration. — According to Haber-
landt, a corn plant may transpire during a single growing
season 14 kg. of water, a hemp plant 27, and a sunflower
66.1 That is to say, a sunflower may transpire more than

Fic. 25. Portions of epidermis stripped from a leaf of Cyclamen: upper
epidermis to the left (no stomata), lower epidermis to the right
(6 stomata).

500 grams per day throughout its entire season, which would
mean a very much greater amount during a day of maxi-
mum loss. Estimated from the transpiration of a small
plant, an apple tree of, say, 30 years old might lose 250
pounds per day, possibly 36,000 pounds during a growing
season. ‘Therefore, one acre of 40 trees would represent a

1 The indications are that these figures are far too low for conditions
in the United States generally.

88 Plant Physiology

water elimination of about 600 tons. Land covered by
grass or clover may lose during the growing season from
500 to 750 tons of water, almost entirely through the sur-
faces of the growing plant. .

52. The mechanism permitting transpiration. — The
elimination of water from the surfaces of plants takes
place because of the fact that the leaves or other surfaces
are not wholly impermeable to water-vapor. In the case
of delicate, especially young, leaves or shoots there may be
some loss of water directly through the epidermis, which
is then relatively uncutinized, or otherwise unprotected
against water-loss. In many instances this amount is

¢ GG GG Eee
eee Ayes 2 aR cre : sis td P fa
EE: ee tia: 5

Fic. 26. Section of tomato leaf: epidermis (e), palisade tissue (p), paren=
chyma (g), vascular bundles (v), and stomata (s).
negligible, and just as the continuous cuticle commonly
absorbs practically no water, so it does not permit of elim-
ination. The epidermis, however, of one or of both sur-
faces of the leaf and of other delicate parts may be pro-
vided with numerous pores or stomata (Figs. 25 and 26)
which are the most important means of communication

between the internal tissues and the external air.

The stomata open and close in response to complex
internal conditions, and under certain circumstances
external factors may perhaps play at least a secondary

Transpiration and Water Movement 89

role, as later developed. They usually open into a sub-

stomatal cavity which, in turn, is in communication with
the intercellular spaces, or aériferous system. Since the
leaves are the organs commonly active in transpiration,
it is necessary to note the structure in a typical case.

In Figure 26 there is shown a cross-section of the leaf of
tomato. There is a single epidermal layer (e) on each
surface, a single palisade layer (p), and the mesophyll or
leaf parenchyma. Small veins, or fibrovascular bundles,
in cross and longitudinal section are also shown. In the
lower epidermis there are several stomata. Many leaves
show a multiple palisade, and there is considerable diver-
sity generally in the form and compactness of the tissues.

Each cell of the leaf is directly or indirectly in contact
with the air spaces, and ultimately with the substomatal
cavities, so that the mechanism is a physical system per-
mitting diffusion. The protoplasm of each cell is thor-
oughly penetrated with water; it is in contact with the
penetrable cell-wall, an imbibition membrane, which is
therefore moist. From such moist membranes water-
vapor passes into the, intercellular spaces, which have a
tendency to become saturated. Under external conditions
favorable for evaporation there is a high gradient with
respect to the external air, so that water-vapor diffuses
rapidly from the substomatal cavity through the stomata.

As a result of the work of Brown and Escombe it is clear
that the stomatal system in such a plant as the sunflower,
for example, constitutes an extremely efficient multiper-
forate septum, the form and distance apart of the stomata
commonly permitting, when they are open, a diffusion
almost as rapid as though there were open space. This

90 Plant Physiology

is a fact of peculiar interest. Furthermore, it has been
calculated that the capacity of the stomata in the sun-
flower, for example, is about six times as great as any
observed transpiration; that is, the stomata only one
sixth open would be sufficient to accommodate the most
rapid loss of water which has been observed.

The stomata exhibit a considerable range in size, but
according to Eckerson the average approximates 18 x 6p.
This minute size is scarcely appreciated until one com-
pares it with some visible perforation, such as a needle-
prick made with the smallest sewing needle, which is rela-
tively enormous, measuring about 600 ,» in diameter.
Nevertheless, the total maximal stomatal opening of an
average leaf is approximately one nineteenth of the
surface.

53. Distribution of stomata.— While stomata may
occur in the epidermis of any plant organs, they are com-
monly confined to the aérial surfaces, and especially to
the leaves, or to organs performing the functions of leaves.
As a general rule, in fact, it may be said that the under
surfaces of the leaves are the situations most important
with respect to stomatal occurrence. Eckerson has found
that only about two fifths of the common greenhouse
plants possess stomata on the upper surfaces. Weiss
and others have collected considerable data showing the
relative abundance of the stomata upon the different sur-
faces of dorsi-ventral leaves, from which the following
examples may be suggestive : —

Transpiration and Water Movement 91

LEAVES WITH No STOMATA ON THE UPPER SURFACES

STOMATA PER Sq. Mm.

PLANT LOWER SURFACE
Abies balsamea (balsamfir) . . aS ee 228
Acer pseudoplatanus (Norway maple) oi ae 400
Anemone nemorosa (wind anemone) .. . 67.
Begonia coccinea (red begonia) . . .. . 40
berberts vulgaris (barberry)). : .. . . 229
Pocusverasiica (rubber plant). . . . . . 145
Juglans mgra (black walnut) .... . 461
iamanepuoirerum (ily). . . . . «. - 62
Morus alba (white mulberry) .... .| 480
Fibesmeurcum. (red currant). ... . . . 145
Syringa vulgaris (lilac) J aR ie remees W RL 330
Tropeolum majus (nasturtium). . . . . | 130

LEAVES WITH STOMATA RELATIVELY SCARCE ON UPPER SURFACES

LOWER UPPER
PLANT

SURFACE SURFACE
Asclepias incarnata (milkweed) . . . . 191 | 67
Crcurowa Pepo (pumpkin). . . . . . 269 28
Lycopersicum esculentum (tomato) . . .| 130 12
iengsenigiseuigaris (bean) ... . « \. . 281 40
Lopes quai (poplar)... we 270 55
aE MUNCUIAT Ot. > 6 kw ee 263 60

LEAVES WITH STOMATA MORE NEARLY EQUAL ON BoTtTH SURFACES

LOWER UppER

PLANT SurracE | SuRFACE
) j 23 { 25
Avena sativa (oats) - - »- + - - = . lo7 148
Brassica oleracea (cabbage) .... . 301 219
Helianthus annuus (sunflower) . . . . 325 175
PIsesmioesirts, (DING) fs. kk a 71 50
Pisum sativum (garden pea) . ... . 216 101
Zea mays (corn) tie ue
(52)

leu St(G8)

92 Plant Physiology

LEAVES WITH More STOMATA ON UPPER SURFACES

PLANT LOWER UppER
Nymphea aiba (water lily). . . . . . 0 460
Pinus strobus (white pine). . ... . 0 142
Triticum sativum (wheat) . . . . «. 14 33

54. The effects of conditions upon stomatal production.
— It has been indicated that the preceding data are sug-
gestive. They do not, however, represent absolute rela-
tions, for the reason that the number of stomata is to a
certain extent a factor of complex environmental condi-
tions, varying with moisture-content of air or soil, light,
temperature, and other conditions. In this connection
some previously unpublished data! for corn and wheat
grown under conditions similar except as to moisture-con-
tent of the soil may serve as an illustration. The plants
were grown in tumblers of fine sand for seventeen days,
and the counts of stomata are averages for the microscopic
field employed (8 x ocular and 16 mm. objective).

CoRN WHEAT
= a No. Lvs. | Avg. Wt. .No. iAvg. Length, Avg. Wt. No.
in Sand | Pe Plant} of Tops Stomata of Tops of Tops Stomata
38 3110 ee 3.63 181 3.9 46 103
30 3.0 3.04 130 6.4 1.09 85
20 3.0 3.36 129 4.7 EDy/ 82
15 2.8 2.00 124 3.7 Bete 81

141 FOE A) 56 107 a7 AT 59

1 These data are the results of experiments made by Mr. F. M. Harris
in my laboratory.

Transpiration and Water Movement 93

The preceding table is sufficient to indicate that the
number of stomata in a given area is variable. Again,
there is no constant relation between the number in a
given area and the size of the leaf; for, in the highest and
lowest moisture-content with wheat, both length of
leaves and entire weight of tops are approximately
equal, yet in the low moisture-content there are only 57
per cent of the stomata found in the high moisture.

iN

Fic. 27. Stoma of Helleborus: position of guard cells open (darker
lines) and closed (lighter lines) ; cell contents shaded. [After Schwen-
dener and Strasburger.]

55. The control of water-loss by stomatal movement. —
The mechanism of stomatal movement has been abun-
dantly studied. Commonly, wilting releases the tension
which forces the guard cells apart, so that closure must be
effected. Turgor of the guard and other cells of the
stomatal region are therefore primarily important in
determining the extent of the opening. The relative
positions of the guard cells open and closed are shown in
Figure 27, after Schwendener.

Recent studies upon the relation of stomatal movement
to transpiration point to several suggestions and conclu-

94 Plant Physiology

sions of interest, although these studies also make it evi-
dent that there is yet much room for quantitative work
in this field. In general, it may be said that, contrary to
many early opinions, the stomata do not open and close in
direct response to the varying conditions of the atmosphere
which may inhibit or promote transpiration. When the
plant is provided with sufficient moisture, the stomata are
commonly open, but as Brown and Escombe and others
have shown, maximum transpiration does not necessarily
correspond with maximum opening. Wilting effects a
closing of the pores, but according to Lloyd there can be
no closing in anticipation of wilting. Again, the stomata
may remain open when the humidity is extremely low,
provided only sufficient water is available for the plant.

Many investigators have shown a primary relation be-
tween stomatal opening and the time of day; and it is
believed that possibly through the possession of chloro-
phyll, and the relation to organic food-materials, there
may be found in the turgor of these cells an important
factor in stomatal regulation.

56. Modifications tending to check excessive transpira-
tion. — Closure of the stomata is in all cases a means of
checking the excessive transpiration, as already discussed. -
However, this check may be insufficient in extreme cases.
It may prove also a menace to other activities of the plant.
In any event many plants exhibit a structure peculiarly
fitted to limit excessive transpiration. This is important
in the occupancy by plants of arid habitats, and it is
certain that many delicate species are unable to survive
under conditions necessitating the most excessive trans-
piration. This may be due in part to the incapacity on

Transpiration and Water Movement 95

the part of such plants to respond to these conditions by
the production of a protected surface or of growth-forms
tending further to reduce the water-loss.

When transpiration is excessive, leaves commonly droop.
This is an indication of wilting. It is usually regarded as
a further protection against water-loss. At all events, such
leaves obey an obvious physical law. The leaves of corn

Fic. 28. Stomatal apparatus in leaf of carnation.

and some other plants ‘‘ roll’ under the same conditions.
Here the tensions are different. It is often stated that in
corn this rolling results because the stomata occur on the
upper surface, but the data previously cited indicate that
corn may exhibit more stomata on the lower than on the
upper surface.

Plants which are commonly able to maintain themselves
to advantage in arid situations may be modified in one or
more of a variety of ways, some of which are as follows : —

96 Plant Physiology

(1) Reduced surfaces, such as in cactus, aloe, and many
desert plants.

(2) Reduction in number of stomata, as in many grasses
and sedges. :

(3) Sinking of stomata in special epidermal cavities as
in yucca and carnation.

(4) Thickened cuticle, as in carnation, pine, many desert
plants, and the like.

(5) Production of a waxy bloom upon the cuticle, as
in cabbage, sugar-cane, and wheat.

(6) The development of hairs upon the leaves, as in
mullein and numerous mountain plants.

(7) The possession of water-storage tissues, as in many
desert plants, begonia, ete. :

57. Conditions affecting transpiration. — The condi-
tions of the atmosphere greatly affect the evaporation from
a water surface or from any other surface. In dry, hot
weather, hay is quickly made and cured. The “ pull ”
of the atmosphere upon all
moist surfaces results, there-
fore, in a prompt loss of water.
In the same way transpira-
tion in a healthy plant is ob-
viously influenced by condi-
Fie. 29. From a stalk of sugar tions of the air, and it is toa

a vega eee and certain extent influenced by

[Atier De Bary} posit. conditions of the soil. In gen-

eral, the important air fac-
tors are humidity, temperature, wind velocity, and light.
Low humidity, high temperature, rapid movement of the
wind, and intense light commonly facilitate transpiration

5
~S—w

Transpiration and Water Movement 97

to a marked degree. In fact, if the water-supply is not
abundant, a combination of these conditions may promptly
result in wilting. The water-loss is not necessarily pro-
portional to changes in conditions, since when transpira-
tion becomes excessive, concentration of the cell-sap and
the closure of the stomata exert, in many cases, a most
important inhibiting effect, which is, in a way, protective.

The soil factors indirectly important in transpiration
are water-supply and the strength or composition of the
soil solution.

58. Effects of excessive evaporation. — The permanent
effects of an excessive loss of water vary with the type
of. plant. Herbaceous annuals might quickly wilt and
dry up. Deciduous perennials might be promptly de-
foliated. Many trees will show this during a summer
drought, and if later wet weather prevails, there may be
an entirely new season of growth. It is then comparable
in effect to cold. Doubtless the excessive shedding of
young flower-buds or “squares” of cotton is ‘due to
changes of the water relation. In general, reduced water-
supply has a tendency to ripen up all parts, to mature
seeds early, and often a considerable effect upon the com-
position of the product. Seeds ripened in this way are
‘said to show the effects of immaturity, as shown later.

59. Guttation.— The elimination of water as liquid
may occur in certain plants when absorption is promoted
and transpiration checked. It consists in the forcible
excretion through certain stomata of water which may
collect as drops on the edges of the leaves or may stream
down the leaf blades. It is conveniently observed upon
young corn, blackberry, canna, and other plants. It may

H

98 Plant Physiology

occur during a cool afternoon or evening of a warm day;
and quite commonly in the cool early morning after a hot day

<_<
z
*:
p
ee
ie
ie

[Photograph by Russell and Harding.]

Fig. 30. Destruction of cabbages by Pseudomonas campestris, a germ
entering through the water-pores.

which has served to heat up the soil toa considerable depth.
The continuous water connection between the tissues
and the external atmosphere established through guttation

ee

Transpiration and Water Movement 99

makes possible the entrance of the germ of the black-rot
disease to the cabbage and allied plants. This organism
is productive of one of the severest of the cabbage diseases
(Fig. 30).

60. Transpiration and evap-
oration. — Since transpiration
is an evaporation phenomenon,
it is possible to compare the
amount of evaporation in dif-
ferent habitats, and thus be able
better to determine or forecast
plant behavior in such habitats.
There are many difficulties in-
volved in employing as a meas-
ure the evaporation of water
from a freely exposed water-
surface. The simple evapo-
rimeter devised by Livingston
is extremely satisfactory for this
purpose (Fig. 31).

This instrument affords a
means of measuring the evap-
oration from a porous cup. It
consists merely of a
bottle, or mason jar,
through the well- SS
paraffined stopper of 3S
which passes a tube
connected by a rub- ~

ber stopper with some
BP Fic. 31. Simple evaporimeter, Livingston
type of porous cup Pane

100 Plant Physiology

or filter tube, the whole being filled with water. This
filter tube may be shellacked to a known surface, adopted
as a unit, and all other instruments may be standardized
with respect to this.' Under different conditions the
curve of water-loss from this instrument may not be
comparable to that from a free water-surface; but the
effects of conditions upon it are supposed to be more
nearly comparable to the effects upon plant surfaces.

The evaporimeter has also been serviceable in contrast-
ing transpiration and evaporation in unit areas, thus
relative transpiration may be taken as the ratio of trans-

Soe : R
piration to evaporation, conveniently expressed as B The

extent of variability respecting this ratio has been regarded
a fair indication of what is conveniently termed physio-
logical checking of transpiration. In studying this regu-
latory check upon transpiration Livingston finds that in
certain desert plants it is especially operative between

6.30 a.m. and 1 P.M., and especially pronounced at tem-.

peratures from 79 to 90° F.

A proper study of the relation of certain horticultural
plants to evaporation factors promises to yield much data
of practical value.

Various observers have compared the loss of water from
leaves with the loss from an equal surface area of soil.
Nobbe has shown that evaporation from the surface of
the soil may be 1.6 to 5 times the amount lost from an

equal leaf area. In these cases the ordinary crop plants

1Transeau (Bot. Gaz., 49:459, 1910) has recently constructed an
instrument which seems to possess some advantages in the way of sim-
plicity and rate of evaporation.

ee ———— ee

Transpiration and Water Movement 101

are considered. If, however, we should compare evapora-
tion with the loss of water in such a plant as prickly pear
(Opuntia), the former might be one hundred times as great.
On the other hand, it should be remembered that a crop
on a given plot may develop in leaf surface an area many
times that of the soil upon which it is growing. Practi-
cally all direct measurements of the relative water-content
of bare soil as contrasted with areas producing crops
indicate that the percentage of water-loss is greater where
a crop is grown. In other words, it is possible to conserve
water in the soil by fallowing. The use of a fallow, to-
gether with sufficient cultivation to keep a constant surface
mulch, is one of the first principles in dry-land farming.

King ! has cited a case showing the effects of fallowing
versus cropping, which is striking. Two plots which had
been almost identical in water-content were used in the
experiment. After the summer fallowing there was the
next spring in the upper surface foot 9.35 pounds per
square foot (or 203 tons per acre) more water than in the
soil cropped the previous season. A considerable differ-
ence was still manifest after both plots had been cropped
alike the succeeding season.

Practically, therefore, plants deprive the soil of moisture.
It is well known that willows or birches in a moist spot in
a yard or meadow keep the soil fresh and mellow. In
some cases trees or other vegetation may seem to increase
the soil moisture, but a closer examination will generally
reveal the fact that in such instances the vegetation pre-
vents rapid run-off and, therefore, appears to use the
smaller quantity.

1 King, F. H., ‘The Soil,’”’ pp. 291-292.

102 Plant Physiology

61. Transpiration and growth. — It has long been evi-
dent that there is, under certain circumstances, a relation
or fairly definite ratio between transpiration and growth.
As a result of various series of water cultures with wheat.
and other grasses, Livingston has attempted a further
analysis of this relationship. He finds that the transpira-
tion data are frequently as instructive as a comparison of
total increase in weight or growth. It is observed, then,
that transpiration and relative growth vary with weight
and area of the leaves. The amount of transpiration is
regarded as a simple function of the leaf surface, which
again varies directly with leaf weight, or, practically speak-
ing, with the weight of the entire tops. It follows, of
course, that total transpiration is a more or less accurate
measure of the total growth.

This relationship, however, is limited by several factors.
It is necessary to have conditions favorable for fairly rapid
transpiration and favorable for growth. Again, increasing
the salt content of the solution in which plants are grown
measurably affects transpiration and may not increase
growth materially, so that plants growing in diverse
concentrations may show extreme variations with respect
to the amount of water-loss. Reed has also recently
demonstrated that potassium in any combination exerts
a depressing effect upon transpiration, while a small
quantity of tannic acid facilitates it. In other words, the
relation applies to a relatively narrow set of conditions.

62. Water transport.— This is a convenient but
scarcely an accurate expression, since, except in the dif-
fusion of water-vapor and in the formation of ice-crystals,
there is, perhaps, within the plant no such thing as the

Transpiration and Water Movement 103

movement of pure water. In osmotic transfer water does
move independently of substances in solution, but it is
always associated with substances in solution.

The movement of water in the plant has been a line of
experimental inquiry since the dawn of plant physiology.
Many important facts have been clearly enunciated and
numerous interesting data accumulated, yet some of the
phenomena of movement observed find as yet no entirely
satisfactory explanation. A chief source of difficulty lies
in the complexity of the factors involved.

In general, however, diffusion is important, but the rise
and maintenance of water are complicated by such factors
as capillarity, the cohesive strength of water columns,
the lifting power of evaporation, the peculiar structure
of the conducting vessels, and, under certain conditions,
the existence of high root pressures.

It has been stated that in the root the region of hair
production is commonly characterized by a radial bundle
arrangement apparently permitting more readily the move-
ment of water from the parenchymal cells directly into the
woody portion of the bundle. There is, of course, no such
thing in plants as a true circulation, analogous to the circu-
lation of the blood in animals; yet it is possible to dis-
tinguish in a very general way two types of movement in
vascular plants, as indicated below.

(1) There is a “ transpiration stream,” from the absorb-
ing organs (of water containing some salts and com-
monly traces of organic substances) to the leaves. This
stream is directed mainly through vessels, the xylem part
of the woody bundles (Fig. 32). During this transfer
there is, moreover, general diffusion to all parts requiring

104 Plant Physiology

water, or possessing a sufficient tension therefor. From
what has been said of water-loss and of the principles of
diffusion it will be apparent that the type of movement

sa

Fic. 32. Cross-section of primary fibrovascular bundle of Ricinus:
phloém (P), showing sieve tubes (S), companion cells (AC), parenchyma
and sclerenchyma; cambium (C); and xylem (X), showing especially
vessels (V) and tracheids. [After Curtis.]

here discussed is more than diffusion. Moreover, rapid
movement is essential in order to supply the demands of
transpiration, and it is this transpiration stream which

Transpiration and Water Movement 105

effects, for one thing, perhaps, a rapid distribution of
absorbed mineral nutrients.

On the other hand there is (2) a more gradual movement
by diffusion of soluble organic materials, or “ elaborated”’
foods, along the paths provided by the plasmatically con-
nected sieve tubes (Fig. 33), from which general paths

\\\)

A

AW

MAifil
\

i
age

IM

1)

NNN

TANABE
AU AAUR

i
N

fh i]
N\

i h

}

AN

v

Fic. 33. Longitudinal section of a bundle similar to the preceding.
[After Curtis.]

organic substances pass, also by diffusion, into all cells
where growth and differentiation are proceeding. At
certain seasons, in many plants during the spring, the
movement of organic material in the xylem part of the
bundles is common. Usually, however, the distinction
may be made that the dead vessels or xylem elements
conduct a liquid which is more nearly the nutrient solu-
tion absorbed from the soil, whereas the sieve-tube part
of the bundle is primarily the path of diffusion for organic
materials. All cells of the body—parenchyma, cortex,

106 Plant Physiology

and the like — permit of diffusion, and in the end the
demands of each cell govern the flow toward that cell.

63. Fibrovascular bundles.—If freshly cut (under
water) shoots of the jewel-weed, sunflower, Indian corn,

canna, or other convenient rep-

resentatives of monocotylous and

dicotylous plants are placed in
a solution of a dye such as eosin
or fuchsin, the stain will pass
upward through the conducting
system of the plant, and the
paths of conduction may thus be
made evident, although there is
sometimes a slight lateral diffu-
sion tending to obscure the defi-
nite channels.

It will be recalled that mono-
cotylous plants are characterized
by stems in which the vascular
strands are commonly distributed
in irregular manner throughout a
eround tissue called
parenchyma, as in corn
or sorghum. A hand
section will show at a
glance this distribution
of the bundles, and it
Fie. 34. Vascular system of Clematis, is also strikingly

apical portion of the stem : longitudinal brought out by break-

view of stem and leaf trace bundles(A), ino -
and cross-section of internode (B). preps: dry corn stalk

[After De Bary and Nigeli.] through an internode

Transpiration and Water Movement 107

and noting the strands. Examined microscopically a
bundle exhibits in cross section the typical collateral
arrangement. In this the phloém or sieve-tube part
is outermost, the xylem therefore within, and both are gen-
erally inclosed by a sheath of stereome or mechanical
supporting tissue. There is, however, no meristem or
cambium within the bundle, signifying a closed type.
The irregular distribution of bundles in the stem usually
precludes the formation of a ring of wood, and there is,
moreover, no bark in the usual sense. These are matters
of much physiological significance, both from the stand-
point of growth and conduction.

In dicotylous plants the primary bundles are arranged
in a ring and are also commonly collateral. The inter-
position of secondary bundles (as subsequently discussed,
section 187) may result in the formation of a complete
wood-ring. Where no wood-ring is produced, the bundles
run parallel throughout much of the internode (Fig. 34),
divide and unite in a characteristic fashion at or near all
of the nodes, also sending off branches to the leaves at
each node.

When a wood-ring is produced in a dicotylous stem, the
meristem of the bundles forms a continuous growing layer,
the outer portion of the wood-ring then consists of phloém
and the inner portion of xylem. The cambium between
permits the addition of seasonal or growth rings of phloém
and xylem on the outer and inner sides respectively. In
plants which attain a considerable age the parenchyma
accompanying the bundles loses its protoplasm and the
bundles cease entirely to take part in conduction. The
number of rings of new wood which may be active in con-
duction varies greatly with different plants.

108 Plant Phystology

64. Leaf venation. — Each leaf receives a definite quota
of bundles, and these and their subdivisions continued into
petiole and lamina constitute the so-called venation sys-
tem. In the case of monocotylous plants the veins are
usually parallel from the leaf stalk, or from the mid-vein,
so that they are often designated parallel-veined plants.,

In the leaves of the dicotylous type the bundle systems
branch repeatedly, and also form a complete reticulum.

[Photograph by H. M. Benedict.}
Fic. 35. Minute venation of the leaf of Vitis riparia; leaves of differ-.
f ent ages.

In any event the leaves are well provided with fibrovascu-
lar tissue, easily demonstrated by macroscopic or micro-
scopic observation. As a matter of fact the bundles extend
to the most remote parts, and in dicotylous plants espe-
cially the leaf is divided up into a complete network, with
the areas between the vascular tissue being seldom larger
than 1-3 mm. in diameter (Fig. 35). The ultimate sub-
divisions of the bundles consist of tracheids and elongate
parenchyma cells (meristem). Sometimes the bundles
end abruptly or blindly. As the leaf grows each area
subtended by veinlets becomes larger, and this increase

a

Transpiration and Water Movement 109

in size may be followed by the laying down of new veinlets
of a lower order (at first fewer tracheids) from each side
of the original space. These may be at first procambial
in nature, but tracheids are rapidly differentiated within.
It is of special interest to note that the sieve tubes disap-
pear relatively early in the minute continuations of the
bundles.

65. Rate of transport. — The rate of transport of water
in the fibrovascular bundles may be determined with a
fair degree of accuracy by means of the rise of dyestuffs
as before noted, but more accurately in many cases by the
method of Sachs, wherein lithium nitrate is used in the
solution and its presence after intervals determined by
burning the tissues and examining the flame spectro-
scopically. According to Sachs the rate of water rise is
extremely diverse, and may vary from a few centimeters
per hour to one or more meters. Doubtless the extremes
are often greater than these indicated, but unquestionably
the difficulties of measurement are greater at the extremes.

LABORATORY WORK

Indication of transpiration. — Stahl’s cobalt test may be
employed to determine water-loss from a plant surface. In-
cidentally it determines roughly the presence or absence of
stomata, or the relative abundance upon the upper and lower
surfaces of the leaf. Soak filter paper in a 5 per cent solution
of cobalt chloride, dry in the oven or over a flame, and note the

_blue color. Breathe upon a small piece of this paper and note

that the absorption of moisture induces a change to pink.
Now cut out two pieces of the paper of equal size; place one
upon the upper and one upon the lower side of the leaf to be
tested, cover each with a piece of mica and cement the latter

110 Plant Physiology

to the leaf with plasticene or prepared wax. In this experiment
handle the paper with forceps, and preferably use a leaf attached
to the plant, or a shoot, the stem of which is immersed in water.
Note any change of color, and the time required to produce
change, in the two pieces. Experiment with several of the
plants mentioned in section 53, and contrast your data with the
indications regarding stomata there furnished.

Amount of transpiration, determined by weight. —'The actual
transpiration of potted plants may be carefully determined by
loss of weight, as already indicated. Employ plants of any
kind convenient, preferably one-stemmed plants with large,
relatively simple leaves; inclose the pot in soft rubber cloth, in
aluminium shells and rubber cloth, or in any manner convenient
to prevent evaporation from the pot and soil, the plant being
previously watered. Weigh carefully and repeat the weighing
after each of several intervals of not more than twelve hours.
If water is again applied, add approximately the quantity lost,
and weigh again. Plot the results. This experiment may be
extended through a considerable period of time, and different
types of plants may be contrasted. Ultimately, the area of
each plant must be taken into consideration or unit areas com-
pared, as indicated in later experiments.

The transpiration of plants grown in water cultures in par-
affined wire baskets, or in glazed pots covered with paraffin,
is also conveniently determined by weighing, as referred to in
subsequent sections of this book.

Measurement of leaf areas. — It would be difficult to deter-
mine directly the transpiration of a tree or of any vegetation
under natural conditions. For the laboratory experimental
work in contrasting different plants or plants under different
conditions, as well as for an indication helpful in estimating
water-loss in the field, it is desirable to have a quick method of
measuring leaf areas.

Many methods of determining leaf areas are now used. Ordi-
narily it is sufficient to trace the outline of the leaf upon codrdi-
nate paper, the area being determined by a count of spaces.
Another simple method is to trace the outlines of the leaves

nies Jee ie

Transpiration and Water Movement 111

employed (or of average leaves) upon paper of uniform thick-
ness, these outlines being subsequently cut out and weighed
accurately for comparison with the weight of a known area.
Prints may be made upon sensitized paper, or the planimeter
may be employed. The area of stems and petioles is generally
negligible, but may be roughly estimated when necessary. It is
well to express all transpiration data, as suggested by Ganong,
in grams per hour per square meter of surface, written g m2? h.

Amount of transpiration, determined by potometers. — Set up a
transpiration experiment, employing either the Ganong potom-
eter (Fig. 24), a burette potometer (Fig. 23), or some other
form equally satisfactory. The former may be employed for
short periods, contrasting the effect of conditions; while the
short burette potometer may be used for longer intervals, care
being taken, however, to keep the column of water in the burette
at a height approximately equal to that in the other arm.!

Effects of conditions upon transpiration. — It has been indi-
eated that temperature, humidity, air movement, ete., are
directly and indirectly important in varying transpiration quan-
tities. While the effect of light variation may be demonstrated,
more satisfactory experiments may be made with the other
factors.

A rough idea of the effects of temperature and humidity may
be obtained by simple transpiration experiments with simul-
taneous observations upon simple thermometers and hygrom-
eters, placing plants of more or less equivalent areas, even for
a few hours, under conditions determined to be diverse. With
more or less equal lighting, contrast the transpiration, for ex-
ample, in a moist basement room with that in a warm upper
room; or, at a uniform temperature, also, contrast a plant
exposed in a quiet room with one in the same room under a large
bell glass, the latter securing greater humidity. Diverse con-

1[In all transpiration or other experiments where the further ab-
sorption of water by excised shoots is required, the cutting of the shoot
should be done while it is bent under water, and the ends of the shoots
should be promptly immersed in water until used.

112 Plant Physiology

ditions may also be obtained in different greenhouses. Keep
an accurate record of the conditions of the experiments, and
accompany this with a record of the transpiration data, express-
ing the latter in terms of g m? h, as above explained.

A more accurate evaluation of the factors, and consequently
a better conception of the effects of external conditions upon
plants, may be obtained by means of experiments continued
several days, whilst utilizing, also, autographic recording in-
struments. Study the mechanism of the thermograph (Fig. 112)
and hygrograph (Fig. 36); also set up and standardize some
simple evaporimeters (Fig. 31) after the method of Livingston

[Illustration from Julien P. Friez.]

Fic. 36. Hygrograph.
(section 60) or of Transeau (Bot. Gaz., 49: 459, 1910). Then
set up in duplicate with the burette potometers a transpiration
experiment (preferably two, under two sets of conditions).
This is to be accompanied by the continuous record of tempera-
ture and humidity. Make observations upon w loss as
often as possible. The experiment may be continued Several
days if shoots with woody stems are chosen. Plot and discuss
the results.

ie a’

Transpiration and Water Movement 113

Under conditions otherwise similar place one plant (or potom-
eter) and a standardized evaporimeter in a current of air (an
electric fan may be employed), and a similar plant and instru-
ment in a quiet atmosphere. Contrast the water-loss after a
sufficient interval.

Guttation. — Water freely some potted plants of cabbage or
corn with warm water until the temperature of the soil is about
35°; then transfer the pots promptly to a cool room, cover
with bell glasses, and after a few hours describe any exudation
phenomena noted.

Leaf structure. — Make hand sections of a variety of leaves
(at least four types) and compare by microscopic study, es-
pecially the epidermis, palisade tissue, and intercellular spaces.
The following leaves are suggested: beech, cherry, or ivy;
rubber plant or rhododendron; snap-dragon or jewel-weed;
carnation or small cereal; pine or spruce. Draw in detail one
type.

Examine and compare, if possible, leaves of any variety of
small cereal grown under diverse water conditions. Note es-
pecially, the width and venation of the leaf, the amount of
bloom, and the number and distribution of the stomata.

Conduction of water. —Cut under water several shoots of
young sunflowers, castor-oil plants, jewel-weed, corn, and some
plants with light-colored flowers (hyacinth, phlox, or other her-
baceous plant convenient), and place the cut ends in vessels
containing a red dye. After the lapse of an*hour or two note
the course of the dye through the stem, also into the leaves and
petals. With long standing is there more general diffusion of
the dye? Describe the results.

Remove shoots which have been in the dye for a very short
period (15 minutes to 1 hour), wipe off the surplus dye from
the outside with filter paper, and with a sharp knife or razor
cut off the stem and examine promptly with the hand lens to
ascertain what portion of the bundle is colored. In the case
of the sunflower and castor-oil plant is the entire ring colored ?
Peel off the bark of the dicotylous plants and examine it for
the dye. Draw conclusions.

=

114 Plant Physiology

The rate of movement may be studied by leaving the shoots
from half an hour to one hour in the dye, then cutting off the
stems at successive intervals until the uppermost indication of
the stain is found, through examination with a hand lens. After
determining the rate of rise of the liquid at laboratory tempera-
ture, place some shoots under conditions favorable for rapid
transpiration and others under a bell glass, and contrast the
results. According to the directions in the next paragraph ! de-
eolorize a leaf of the grape, sunflower, or fuchsia, and under the
low power of the microscope study the minute ramifications of
the veins.

Place fresh tissue in equal parts of 95 per cent alcohol and
glacial acetic acid. After from 24 to 48 hours, take pieces and
hold them immersed in pure nitric acid until clear (usually a
matter of seconds), place on a slide, add glycerin, and boil
over flame until tissue becomes entirely transparent. Put on
cover glass and examine in the glycerin.

Ring small plants such as geranium, sunflower, Ricinus, or
other forms with definite bark, by removing a circle of bark
about one fifth of an inch long, extending completely around
the stem. The plants should not be in an atmosphere so ex-
treme as to cause rapid drying-out from the cut surface. Does
ringing interfere with the conduction of water to the leaves ?

REFERENCES

BurGERSTEIN, A. Die Transpiration der Pflanzen. 142 pp.,
14 pls., 1904.

Criapp, G. L. A Quantitative Study of Transpiration. Bot.
Gaz. 45: 254-267, 32 figs., 1908.

CiemeEnts, E. S. The Relation of Leaf Structure to Physical
Factors. Trans. Am. Mie. Soe. (1905) : 19-102, 9 pls.
CopELAND, E. B. The Rise of the Transpiration Stream. Bot.

Gaz. 34: 161-193; 260-283, 1902.

1A method suggested by Professor H. M. Benedict, University of
Cincinnati.

Transpiration and Water Movement 115

Darwin, F. Observations on Stomata. Phil. Trans. Roy. Soc.,
London. 190 B: 531-621, 1898.

irons HH. H., and Jory, J. On the Ascent of Sap. Phil.
Trans. Roy. Soc. 186: 563-576, 1895.

Ecxerson, S. H. The Number and Size of the Stomata.
Bot. Gaz. 46: 221-224, 1908.

Ewart, A.J. The Ascent of Water in Trees. ~ Phil. Trans. Roy.
Soc. 198 B: 41-85, 1906.

Harter, L. L. The Influence of Soluble Salts upon Leaf Struc-
ture and Transpiration of Wheat. Bur. of Plant Ind. Bul.
134 : 22 pp., 1908.

Livineston, B. E. The Relation of Desert Plants to Soil
Moisture and to Evaporation. Carnegie Institution Publ.
50: 78 pp., 16 figs., 1906.

Luoyp, F. E. The Physiology of Stomata. Carnegie Institu-
tion Publ. 28: 142 pp., 40 figs., 14 pls., 1908.

Reep, H.S. The Effects of Certain Chemical Agents upon the
Transpiration and Growth of Wheat Seedlings. Bot.
Gaz. 49: 81-109, 9 figs., 1910.

Texts. Barnes, Detmer, Ganong, Goodale, Jost, Pfeffer, Stevens.

CHAPTER VI

THE WATER REQUIREMENTS OF . CROPS
AND OF VEGETATION

From what has been said respecting the use of water by
plants, more especially transpiration, it is obvious that
the requirements of vegetation and of crops will be most
diverse, and that any particular crop or type of vegeta-
tion will show a modified use dependent upon temperature,
light intensity, strength of soil solution, texture of soil,
and the like.

66. Relative requirements of a few crops. — Lyon and
Fippin have compiled a statement of the water needs of
several crops which is suggestive. These crops were tested
by the different observers under dissimilar conditions, and
close agreement is not to be expected. Moreover, the
methods of controlling or estimating the evaporation of
water from the surface of the soil has not been the same
with the different observers, and this might easily lead to
important differences. See table on opposite page.

Taking 300 pounds of water as an average amount
transpired by crop plants, in order to produce 1 pound of
dry matter under conditions in England, Hall has prepared
a table giving the precipitation necessary to supply the
water used by certain crops. For conditions in the central

116

ae

Water Requirements 117

WATER TRANSPIRED BY GROWING PLANTS FOR ONE Part oF Dry
Matrer PRopucED

ESTIMATIONS MADE BY

eee ond Hellriegel, Wollny, King,
Gilbert, 3 ;
Germany Germany Wisconsin
England
Beans 214| Beans . |262| Maize . |233| Maize . | 272
Wheat 225| Wheat . | 359| Millet . | 416| Potatoes. | 423
Peas . 235| Peas . . | 292| Peas. . |479| Peas . . | 447
Red clover | 249 | Red clover | 330| Rape. . | 912) Red clover 453
Barley 262 | Barley . | 310] Barley . | 774] Barley . | 393
WAS: ...°.,,| 402): Oats..." .8)| 66a, @ats: .- 557
Buck- Buck-
wheat. |371| wheat. | 664
Lupine . | 373} Mustard. | 843
ey eeeh | 377 | Sunflower | 490
Average 237 341 | 608 424

United States it would be more nearly accurate to assume
an average requirement according to King’s results of
about 425 pounds of water for each pound of dry matter.
Modifying the data in accordance with this, the indications
are as follows : —

WT. or |

ca Wat oad Per Cent | Dry Mat-| Catc. WATER USED
ieee. | Or WATER TER AT DURING GROWTH
HARVEST
Tons per A. Tons per A. Tons per A.} In. of rain
Wheat 5 WRB Pee 18 2.05 | 922.5 9.13
isamley oi... 2.0 ald 1.66 747 9.39
\ 2.5 16 2.10 945 9.36
Meadow hay . es, 16 1.26 567 5.61
Clover hay . . 2.0 16 1.68 756 7.48
Swedes .. . 170) 88 2.04 918 9.09
Mangolds .. 30.0 88 3.60 1620 16.03
Potatoes’. «. >. 15 15 1.87 841.5 8.32

Beans . sek 2.0 17 MEG is TAL 7.41

118 Plant Physiology

In all cases the amount of water given is a considerable
part (averaging about one fifth in the central United
States) of the annual rainfall. Considering the run-off
and the evaporation from the soil, both during and outside
of the growing season, it is essential to study carefully
the water requirements of crops, even in regions where
the rainfall seems generally adequate.

An abundant or optimum supply of water in part
obviates the necessity of maximum cultivation, since
cultivation may be very considerably concerned with con-
servation of water. Nevertheless, there are factors of aéra-
tion, proper conditions for certain types of bacterial action,
texture of soil, and the like which require cultivation,
wholly aside from the water relation.

67. Precipitation and crop growth. — Under ordinary cir-
cumstances the greater part of the precipitation water
is not conserved by the soil. Of the total annual rainfall
only a certain percentage is available to vegetation or to
the crop. Some of the water is lost in the immediate
surface run-off, a small part may be lost by percolation,
and there is further a considerable amount represented by
evaporation. When the water-table is low, plants are, of
course, wholly dependent upon the water which is con-
served in relatively superficial strata.

Practically speaking, no section of Europe or of the
United States is wholly free from droughty periods. This
implies the well-recognized fact that precipitation during
the growing season is demanded by the great majority
of crops and types of vegetation. Nevertheless, when
proper measures are taken for the conservation of water
which may fall outside of (as well as during) the growing

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120 Plant Physiology

season, a relatively small precipitation — say 25 to 30
inches— may be sufficient for crop production. Early
maturing grains and other grasses require as little, perhaps,
as any other type of vegetation affording an equal yield.

A chief cause of the annual variation in yield of many
staple crops is to be found in the variation in rainfall.
Smith has prepared charts showing a remarkable agreement
between yield of corn and precipitation in the corn belt
of the United States for the chief growing months — June,
July, and August. In Figure 38 the dotted line gives ©
the average rainfall for the months mentioned, covering a
period of fifteen years, and the full line gives yield of grain
per acre for the same time. ‘The data are taken from
Ohio, Indiana, Illinois, lowa, Nebraska, Kansas, Missouri,
and Kentucky.

The chart (Fig. 37) shows a rainfall map of the United
States for a period including, in the main, the growing
season. From this, it is apparent that the rainfall west of
the hundredth meridian practically to the Coast Range
valleys of the Pacific is less than the usual requirement,
and so the number of crops which may be grown in this
general region without irrigation is extremely limited. In
fact, throughout a very large portion of western North
America, eastern and southern Europe, northern Africa,
and a large part of Asia and Australia, crop production is
limited much more by insufficient or ill-distributed rain-
fall than by all other factors combined.

It is believed that the great agricultural countries of
the world must be, in time, those of great area, such as
Australia, Brazil, China, India, Russia, and the United
States. In three of these, however (China, India, and

Water Requirements

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2 Plant Physiology

Russia), famines may be expected any year when the rain-
fall is but slightly less than usual, and without their fairly
well-developed systems of irrigation much larger areas of
these countries would remain in doubt with respect to pro-
duction. Where irrigation is not practiced, it is frequently
necessary to introduce systems of dry-land farming
whereby the principles of soil-moisture conservation are
effectively applied, and sometimes a single crop is grown
in two years, water being allowed to accumulate every sec-
ond year.

Precipitation has a maximum effect, of course, when all
conditions are favorable; that is, when the nature of the
soil and its depth, the type of sub-soil, the slope and ex-
posure of the ground, all combine to conserve moisture
and deliver it to the growing crop.

68. Irrigation. — Both in Europe and America (in
many sections where irrigation has not been considered
necessary) it has now been abundantly demonstrated that
the yield of most crops may be materially increased by a
rational use of water. In Wisconsin, for example, King?
has found that during a six-year period the yield of pota-
toes was increased from 217.3 bushels to 301.7 bushels
per acre. Again, with twelve inches of rainfall during a
growing season for corn, the yield of grain was increased
by means of irrigation from 30.14 to 65.3 bushels.

It has already been indicated that profitable crop pro-
duction is only possible in many regions when irrigation
practices supplement the effect of the normal rainfall. |
In every drainage basin, large or small, there are oppor-
tunities for conservation.

1 King, F. H., Wis. Agl. Exp. Sta. Report, 18: 195, 1901.

Water Requirements 123

ant Le Ae
TTT pi

oe K ue etre

ees Wp ee =

Fic. 39. Rice-field prior to drawing off water for harvesting,
Louisiana.

il t fice fl

mn fin Ly 7

a

Fruits. — In most sections of the Pacific coast region,
deciduous fruits are commonly irrigated when the rainfall
is less than twenty inches, and many believe that irriga-
tion may be desirable when the precipitation is equal to
or somewhat greater than this amount. Citrus fruits
grown on a commercial scale in that region are invariably
irrigated. In all cases the purpose of irrigation, as Wick-
son says, ‘‘ is a means of soil improvement to be employed,
like other means of improvement, when the soil needs
16?’

The following tables will suffice to indicate for the two
types of fruit mentioned the usual amount of water added
by irrigation to supplement the normal precipitation : —

124 Plant Physiology
Decipvuovus FRUvITS
RAINFALL No. Eacu ToTaL
IN IN. aa IrrRic. |IrRic. In.|IRRIG. IN.
Sacramento . 18-20 | June to October 3-18 1-1.25 | 3.25-18
Santa Clara . 12-20 | Spring, summer,; 1- 3 3-12 12-16
or winter
Fresno 8 Summer or win-| 1- 4 2.5-12 | 7.5-12
ter
Los Angeles . 12-20 | Spring or sum-| 1- 3 2-9 4-9
mer
CITRUS FRUITS
RAINFALL Tate Hane No. Eacu TOTAL
) IN IN. 7 Irric. |Irria. In.|IrR14, IN.
Fresno 8 April to October | 2-7 2 4-14
Los Angeles . 10-20 | March to No-| 3-7 1-9 3-27
vember
San Bernardino . 12 March to De- .
cember 4-8 1.5-6 6-36
Riverside . 7-12 | When needed, or
April to De-
cember 3-9 1.5-6 10-36
Orange 10-18 | When needed, or
March to De-
cember 3-8 2-5 10-40
Tulare 10 March to Octo- 5-10 2.6 | 12-60
| ber | |

Corn. — The period of growth of this plant is long,
and in temperate regions it extends throughout the

warmest season.

The water requirements are consider-

able; consequently in semiarid or dry regions it responds

abundantly to proper irrigation.

The most striking re-

ea

Water Requirements 125

sults have been secured at the Utah Experiment Station.
In the table below there are included the data respecting
yield, and also the effect upon protein content : —

Irric. WATER IN. YIELD OF GRAIN PER A., | PROTEIN IN WATER-FREE

APPLIED Bu. SUBSTANCE, PER CENT
38.00 82.71 12.99
36.53 69.28 12.05
19.98 77.00 13.00
19.97 49.28 12.65
15.00 46.28 1347
15.00 58.71 13.79
10.06 56.56 13.42
7.50 35.14 15.08
0.00 26.00 14.52

|

Wheat. — In comparison with the data given for corn
with different amounts of irrigation, it is of interest to
examine the results secured at the same station with wheat.
_ The accompanying table indicates not only the amount of

: PERCENT- YIELD (IN YIELD (IN
WATER PERCENT-
YIELD PER AGE OF POUNDS) PER |POUNDS) PER
APPLIED, AGE OF ASH
AcrRE, Bu. PROTEIN ACRE OF ACRE OF
IN. IN GRAIN =
IN GRAIN NITROGEN ASH
4.63 4.50 24.8 PAIS) 10.7 Gio
Olt: 3.83 Zoe 3.07 8.5 ZOO
8.73 10.33 19.9 2.54 19.7 15.74
8.89 ibs? 19.4 2.93 ae 19.72
10.30 14.66 18.4 2.34 25.9 20.24
12.09 eG Pile Boas 22.8 21.44
TEES 11.66 Dork 2.88 25.8 20.30
12.80 13.00 NZS Zoe, Diles 21.50
17.50 Taeoe ez PATE Ys: 23.64
Pa) Vai ka | UB 15.9 2.34 26.4 24.33
30.00 26.66 14.0 4.14 35.8 26.20
40.00 14.50 geal! Dee, 23.8 21°92

126 Plant Physiology

water supplied and the resulting yield, but also the pro-

tein and ash content of the grain and the total amount

of these components (protein as N) taken from the soil.
From the preceding table it is evident that, in general,

Fic. 40. Springs and reservoir for irrigation of date-palms, Figuig,
Morocco.

irrigation water up to thirty acre inches increased the yield
of grain and diminished the nitrogen content. The effect
of an increase of over thirty inches of water is greatly to
diminish the yield; but the percentage, the composition,
and the total removal of soil constituents per acre remain

Water Requirements 1 WArg

practically the same as when one third as much water
was supplied.

Date-palm. — In the Saharan region of northern Africa,
where the date-palm is most extensively grown, the pre-
cipitation is commonly less than ten inches. Moreover,
during the growing season the air is intensely dry, and
evaporation reaches a maximum. Under such conditions,
and assuming no subterranean water-supply, it has been
estimated ! that this plant (a tree of medium size) requires
a maximum of from 100 to 190 gallons of water per day
during at least four months, making a total of from 3+ to
At feet of irrigation-water annually.

69. Potted plants and water supply. — Potted plants
possess such diverse water requirements that it is often
difficult for the amateur grower to arrive at any satisfac-
tory principles for watering. First of all, it is clear that
the amount given should be in proportion to the water-
loss. Plants in a dry room or greenhouse may require
many times as much water as those in a shaded green-
house full of vegetation, with air fairly saturated. Most
potted plants are quickly injured or killed by constant
saturation, and the practice of saturating the pot and filling
the jardiniére around it is soon fatal; for with the usual
amount of organic matter in the soil the exclusion of air
to this extent is harmful both directly and indirectly.

Viewing this matter in the light of such experimental
work as has been undertaken, it seems that during the
growing season a constant, favorable supply of water from
below is most desirable. This, of course, is not always

1Swingle, W. T., “The Date-palm.” Bur. Plant Ind., U. 8S. Dept.
Agl., Bul. 53 (cf. pp. 47-48), 1900.

128 Plant Physiology

practicable, but it serves to emphasize the fact that alter-
nate flooding and drying is not necessarily ideal. The
latter is far better than stagnation. When in vigorous
growth, the plant suffers from drying-out, and some part
of the absorbing surface is killed every time the soil be-
comes air-dry.

In general, there is a certain relation between abundant
water-supply and vegetative growth, so that it may be
necessary to check watering somewhat to induce more
abundant flowering. Again, in the case of plants which
flower periodically, it may be desirable, or even imperative,
to permit the plant to pass into a resting or semidormant
condition. If the plant as a whole is to remain alive, water
may not be entirely withheld, but in. the case of many
bulbous and fleshy-rooted plants it may be highly desirable
that all other vegetative organs disappear, and coinci-
dently it may be desirable that all other conditions favor-
ing metabolism (such as high temperature) may be reduced.

The cultivation of plants whose peculiar growth-forms
are dependent upon dryness of habitat is a special case,
just as is the cultivation of water plants, and some of the
general relations of these types are subsequently treated.

70. Ecological classification based upon the water
relation. — In the previous paragraphs of this chapter
plants of the most diverse water relations have been dis-
cussed; those of the desert represent one extreme and
those of ponds and water-courses the other, between which
extremes falls the great majority of plants. The water
relation was recognized by Warming to be most important
in attempting a habitat or ecological classification of forms.
With respect to this factor he has made from the natural

Water Requirements 129

intergrading series of forms three primary groups which
are conveniently designated (1) xerophytes, (2) meso-
phytes, and (3) hydrophytes.

AX erophytes. — Plants adjusted to physiological dryness
are properly termed xerophytes. In the preceding pages
references have been made to
the fact that there are a large
number of plants both peren-
nial and annual which are able
to exist in typical desert situ-
ations. In general, such plants
are tough, often hairy, and
they usually possess reduced or
leathery leaves. Accompanying

; oh ; Fic. 41. Section of Begonia
these modifications there may leaf, showing colorless water-

be histological adjustments storage tissue adjacent to epi-

a dermis. [After Coulter.]
which may serve to check

water-loss during the more arid periods, and to accu-
-mulate or store water when it is more plentiful. Special
peculiarities of the epidermis, and of the plant in general,
as affecting transpiration, have been discussed. The
cactus, yucca, and sage-brush of the southwest are
plants possessing the capacity for types of modifications
which enable them to persist and to become the dominant
vegetation in much of that region.

Some of the most famous writing papers are those manu-
factured in Scotland and England from a widely distrib-
uted and much exploited African desert grass known as
alfa. This name refers particularly to Stipa tenacissima,
which occupies millions of acres in the steppes of northern
Algeria. It is a plant too tough even to furnish food for
; EK

Plant Physiology

130

[‘zqueys “T "Hy Aq ydeisoj0yg]
*(odBs-puvg) vyofyy visiwajipy A] eloodse ‘opvsojoH U194sve ‘sT]TY pues pLIBIUIES JO WOI}VIIGIA “SH OMT

Water Requirements 131

the camel, and it thrives under conditions which would
perhaps eliminate the great majority of the grass species
of the semiarid United States.

Mesophytes. —'The mesophytes occupy an intermediate
position with respect to water requirements. They con-
stitute therefore the chief elements of the terrestrial flora,
and in fact the main crops and herbaceous vegetation of
the earth. Likewise the species constituting the typical
forests of northern Europe and America, as well as most of
those of tropical regions, would be classed in this category.
In other words, we have in this group a great majority of
those plants which constitute crops in the usual sense of
this term. The relative abundance of plants requiring
an intermediate amount of water results in a tendency
to consider them as the normal plants, whereas others may
be regarded as abnormal, or as specially adjusted to per-
sistence under intensified conditions.

Hydrophytes. — All plants growing wholly or partially
submerged are denoted hydrophytes. Typical members
of this group, such as the water lilies, or water millfoil,
exhibit modifications of structure which are of much
interest. It is important to refer again to the fact that
soil water is not pure, and must of necessity contain sub-
stances in solution. The water of most streams, ponds,
and inland lakes contains relatively small quantities of
organic matter, and invariably small amounts of many
mineral compounds; such fresh waters support one type
of vegetation. Ponds which are common in the typical
bog regions of the northeastern United States and else-
Where may contain organic materials produced under
peculiar conditions, which apparently serve to make the

132 | Plant Physvology

water less valuable physiologically. Any tendency in
this direction results in a similar inclination to xerophyt-
ism in the flora which may occupy these waters. From
the standpoint of the relations of vegetation in general,
the water of the sea is to be regarded as almost unavailable
physiologically, on account of the large content of salt
which it contains. Flowering plants which grow in salt
marshes are often, therefore, typical xerophytes.

71. Semi-xerophytism and hard-wheat production. —
The hard-wheats are species or varieties which, for the
perfection of their particular economic qualities, require a
relatively small water-supply. They are the varieties now
cultivated in much of the Central West immediately west
of the hundredth meridian. In this section the precipi-
tation during the growing season is so inadequate as dis-
tinctly to shorten the growing period. ‘This is, moreover,
emphasized by the high temperature of the summer season.
Other factors may play a part, but in general the growing
season is determined by the conditions mentioned.

This shortening of the growing season is apparently
wholly comparable to incomplete maturity. The hard-
wheats have a tendency to produce high nitrogen content,
and immaturity accentuates the relative increase in pro-
tein material and sometimes seems even to augment the
total absorption of nitrogen. At any rate, in high nitrogen
content, or gluten, lies the advantage of these wheats for
semolina and other purposes (including bread-making)
to which they are put, so that there exists an interesting
relation of region to product.

The hard-wheats have, for the most part, originated ©
under conditions more or less similar to those prevailing

Water Requirements 133

in the West, and the introduction of these varieties has
_ greatly increased crop production and the possibilities of

Teasemsnarr es
ive

Fic. 43. Semiarid sandhills of eastern Colorado; Andropogon scoparius
(bunch grass) and Bouteloua hirsuta. [Photograph by H. L. Shantz.]

agriculture. These wheats are apparently adjusted struc-
turally to absorb more water, by increased root develop-
ment, and to conserve it better, by lessened transpiration.

eel Be eh eb ee:
= oe =<
= 5 .
-¥

134 Plant Physiology

They also mature early, ripening before the conditions are
such as to prevent development, and finally, they are able
to adjust themselves more or less to considerable changes
within the growing period.

The following summary, adapted from Lyon, may
therefore indicate the conditions under which hard-wheat
production may be maintained : —

(1) A relatively dry atmosphere which emphasizes the
drought conditions.

(2) A short growing period, which is equivalent to
early maturity and is unfavorable to starch-storage in the
later stages of growth.

(3) A favorable nitrogen supply in available form.

It may be readily inferred that if certain of these condi-
tions do not naturally obtain, or if they are artificially
changed, there may be a tendency to make softer wheats
of the hard varieties. From experiments in Washington
(Thatcher), it has been shown that the total precipitation
in the different counties of the state governs very closely
the composition of the kernel; therefore, as under irriga-
tion, there is here a tendency — with higher precipitation
— to produce the characters of soft-wheat where otherwise
a hard-wheat would be developed.

SUBSIDIARY WORK

Students not taking work directly along agricultural lines may
be required to prepare a report upon some phase of the water
requirements directly related to drainage, some aspect of irriga-
tion, or the water-relations of special crops, utilizing any ac-
cessible literature. Agricultural students, less likely to consider
adequately the ecological aspects of vegetation, may be given a

Water Requirements 135

topic requiring some careful study in Schimper and Warming,
or a critical analysis of any special articles available.

REFERENCES

Kine, F. H. Irrigation and Drainage. 502 pp., 163 figs., 1899.

Lyon, T. L., and Fierin, E.O. Soils. Pp. 133-136, 1909.

MacDonatp, W. Dry Farming. 290 pp., 24 figs., 1909.

MacDovuaeat, D. T. Delta and Desert Vegetation. Bot. Gaz.
38 : 44-63, 7 figs., 1904.

SHantz, H. L. A Study of the Vegetation of the Mesa Region
East of Pike’s Peak. Bot. Gaz. 42: 16-47, 7 figs.; 179-207,
6 figs., 1906.

Smiru, J. W. Relation of Precipitation to Yield of Corn.
Yearbook U.S. Dept. Agl. (1903) : 215-224.

Voukens, G. Flora der agyptisch-arabischen Wiiste.

Wicxkson, E.J. Irrigation, in Field and Garden. U.S. Dept. of
Agl., Farmers’ Bul. 138: 40 pp., 18 figs., 1901. (Also Off.
Exp. Stas. U.S. Dept. Agl. Bul. 108: , 113: etc.)

Wiptsog, J. A.,and McLaucuiin, W. W. The Right Way to
Irrigate. Utah Agl. Exp. Sta. Bul. 86: 101 pp., 12 pls., 1903.

Witcox, L. M. Irrigation Farming. 494 pp., 113 figs., 1902.

Wo.uuny, W. Untersuchungen itiber den Einfluss der Luftfeuch-
tigkeit auf das Wachsthum der Pflanzen. Wollny’s Forsch.
a. d. Geb. d. Agrikult.-Physik. 20: 397-437, 1898.

Texts. Clements, Jost, Schimper, Sorauer, Warming.

CHAPTER VII

MINERAL NUTRIENTS

In a physiological sense the common fertilizers, or “ arti-
ficial ”’ fertilizers, sold upon the markets of a large part
of the world are, with respect to plants, soil nutrients or
amendments. A study of fertilizers and of conditions and
factors governing the use of these under diverse field con-
ditions constitutes a special phase of agronomic or soils
work. At all points this field of work overlaps physiolog-
ical inquiry, for ultimately the plant response, or yield,
is the index to favorable or unfavorable soil condition.
But the most significant fact is that in his most important
work the agronomist usually deals with these problems
in such a complex form that it is not possible to analyze
the result in terms of direct plant response. Just as the
agronomist’s work is important, however, in securing such
general results, that of the physiologist is important in the
attempt to simplify conditions, to analyze factors, and
ultimately to determine the nature of the plant response.

72. The ash content of plants. —It has been noted
that water constitutes ordinarily about four fifths of the
weight of herbaceous plants. The remainder is solid
matter. When the latter is burned in an open fire, the
organic products are volatilized, and most of the mineral

136

Mineral Nutrients avy

constituents remain in the ashes (technically the ash).
Water, total solid matter, and ash are therefore readily
determined by simple methods.

The total ash seldom amounts to more than 2 or 3 per
cent of the green-weight, and any single mineral element
of this ash constitutes, as a rule, merely a fraction of 1 per
cent of the weight of the plant; yet every essential mineral
element is quite as important as any other factor in plant
production. The percentages of ash in some familiar
plants and plant products are given in the following
table : ! —

AsH, PER CENT pu ee as
PRopuctT . Torat Soups W ATER-FREE
TotTaL Propuwct | :
SUBSTANCE
Corn, green fodder. . 20.67 1.16 5.6
Corn, ripe grain. . 89.44 1.53 Ed
Sorghum, green fodder 20.60 1.09 5.3
Wheat, ripe grain. . 89.48 1.87 2.0
Timothy, green hay . 38.42 2.10 5.4
Red clover, green hay . 29.21 2.10 12
Red clover, cured hay . 84.76 6.15 io
Alfalfa, green hay . . 28.25 2.66 9.4
edpoeets Ls. 11.538 1.04 9.1
Sueanr beets 9... .0 SAO) 88 6.5
“LCC Si 9.54 .80 8.4
ucumbers,.. |. . 4.01 46 115
SEAIOOAMCS . ke es 9.48 1.40 14.8
MeiOGe: Aik ee ok 4.13 1.49 19.0
Ayoples: W.E.Gi ek 17:50 . .80 Pia
Strawberries, fruit . . 9.16 .60 6.5

1 Many of these are the average results of Jenkins and Winton (Com-
pilation of Analysis of American Feeding-stuffs, Off. Exp. Stas. U. S.
Dept. Agl., Bul. 11: 156 pp., 1902).

138 Plant Physiology

Knowledge of the ash content is of interest physiologi-
cally when related to plant behavior or work.

73. Composition of the ash. — A detailed analysis of
the constituents of the ash indicates that through absorp-
tion the plant obtains, as a rule, more or less of all of the
soluble mineral elements of the soil. Whatever occurs
in the soil solution is apt to be found in the plant to at
least a slight extent, although the plasmatic membranes
of the root-hairs show a certain definite selective absorp-
tion, as already indicated. Commonly eleven elements
are found in the ash, as follows: phosphorus, potassium,
calcium, Magnesium, sulfur, iron, sodium, chlorine, silicon,
manganese, and aluminium; and, generally speaking,
the soil is the only source of these elements. (Nitrogen,
likewise derived from the same source, is, of course, a part
of the volatile product.)

Chemical analysis cannot determine with any degree of
exactness what the plant actually requires from the soil;
but it is important because it gives a general indication of
the relation of plant to soil solution, it sheds some light
upon the general problem of nutrition, and it makes pos-
sible an exact computation of the amounts of mineral
nutrients which various crops remove from the soil. The
table on the opposite page compiled by Kedzie shows the
percentage composition of the ash of familiar crops.

From these data it is obvious that there are certain
general relations worthy of recollection, such as these:
the seed is relatively rich in phosphorus and magnesium,
and usually deficient in calcium; stems and leaves may
contain much calcium, and often a high per cent of silicon ;
while the fleshy roots here noted show the highest potas-

—*,

Mineral Nutrients 139

CoMPOSITION OF 100 Parts oF THE PURE ASH

K0 | Na,O| CaO MgO ‘Fe,0, P,0; | SO, | Sio, |} Cl
| | | |

_ Seeds |
Wheat . . . . (30.24) .65| 3.50/13.21| .60 |47.92) —| .73}| —
Bont. 2) 29:8 11.10 | 2.17|15.52| .76 |45.61| .78| 2.10] .91
Flax . ... . .|26.67|2.22| 9.61115.86] 1.11 |42.48 [|
Clover . . . .(|35.35| .95| 6.40/12.90] 1.70 |37.93| 2.40] 1.30] 1.23
Beans. . . . . |41.48/ 1.10] 4.99] 7.15] .46 |38.86/3.40| .65/ 1.80

Fodders | |

Soren. . - . |27.25|. .80'|29:26|.8.32| “57 |10.66| —— | 6.18] ——
Timothy. . . . |34.69| 1.83 | 8.05] 3.24] .83 |11.80| 2.80 |32.17] 5.20
Com)... . .. (27.18) .85 | 5.70/11.42| 285) 9:14) — |40.18} —

Straws |
Mies. 9. . . |84.07| 4.37 |24.81/15.04| 3.67 | 6.24] — | 6.70) —
Buckwheat . . . |46.60) 2.20 18.40! 3.60 1149) =") 5-50) =
iwnenie.. . . . (18.65) 1.38 | 5.76| 2.46] .61 | 4.81) —— |67.50| ——

Roots
Potatoes. . . . |60.00| 2.96 | 2.64| 4.93) 1.10 |16.86/ 6.50! 2.10] 3.40
Sugar-beets. . . (53.10| 8.92] 6.10] 7.86] 1.14 |12.20| 4.20| 2.28] 4.80
Turnips . . . . {45.40| 9.84 |10.60] 3.69] .81 |12.71| — | 1.80] 5.00

sium content. Different parts of the same plant may
exhibit great diversity in ash content, indicating an im-
portant selective absorption between cells and organs.

74. Effects of conditions upon ash content. — For any
particular plant or plant product produced under diverse
conditions the ash content is subject to considerable varia-
tion; and this variation, while most marked with respect
to total ash content, extends also to the ratio of the dif-
ferent elements of which the ash is composed. Official
analyses of the sugar-beet show an ash content, calculated
to dry-weight, of from 3.2 to 14.6 per cent. There may be
a difference with varieties of any plant, but even in the

140 Plant Physvology

same variety a marked difference will result when plants
grown in Michigan, for example, are compared with those
grown on alkali land under irrigation in Colorado. It is
to be expected, therefore, that there will occur considerable
variation in the ash under different conditions of soil
water, fertilization, temperature, and light, or under any
conditions affecting transpiration and growth.

75. Ash content at different ages. — It is of interest to
note that at different stages of growth the rate of absorp-
tion of mineral nutrients and nitrogen bears no constant
relation to body weight. Arendt,! Bretschneider,? and
others have shown that in general ash and nitrogen are
present in the young plant in relatively greater quantities
than in later stages of growth; while starch accumulates
relatively more rapidly in the maturing plant. Each of
the observers referred to employed oats, and they divided
the growing period into five intervals practically as fol-
lows: (1) as three to five leaves are unfolded, (2) some-
what previous to full heading, (3) plants in full blossom,
(4) beginning of ripening period, and (5) complete matur-
ity. In these experiments the roots were not taken
into consideration. The table on the opposite page
from Bretschneider shows the absorption of total ash
and of nitrogen during different stages. With respect to
the absorption of individual constituents, phosphoric acid
is obtained in relatively greater quantity during heading,
while potash is more rapidly absorbed during the early
stages, according to Arendt.

1 Arendt, ‘‘ Wachsthumverhialtnisse der Haferpflanze.” Jour. f. prakt.

Chem., 76: 193, 860.
2 Bretschneider, “Das Wachsthum der Haferpflanze.”’ Leipzig, 1859.

Mineral Nutrients 141

PERIOD | ASH NITROGEN
i | 8.57 3.59
2 5
a 5.96 2.79
4 5.33 2.78
5 5.40 2.43
76. Translocation of mineral substances. — It is well

known that as a plant begins to develop fruit or seed there
is generally a movement of certain elements or substances
to these parts. They may become the storage organs of
carbohydrates, proteins, and other organic compounds,
but there is also a selective absorption of mineral constit-
uents. Throughout the period of fruit formation phos-
phoric acid migrates toward the fruiting organs from leaves
and stems ; and magnesium is invariably translocated, espe-
cially from the lower leaves and stem to the younger organs
of the upper portion, or to the fruit. Potassium frequently
reaches a maximum in the fruiting organs at the time of
blossoming, and subsequently may be slightly replaced by

PuHospHoric Acip CONTENT AT VARIOUS STAGES OF GROWTH

PERIODS OF GROWTH
Part OF PLANT

u 2 | 3 4 5
Three lower joints, stem . AZ .20 a ee .20 19
Two middle joints, stem . — .39 St ee 46 18
Upper joint, stem... --- .66 if 33, || Bey .36
Threelowerleaves. . . 1.05 .70 .69 | ol a
Twoupperleaves .. . 1x 1.67 1.18 74 .o9
UL ETe a SUT Ala eh a = 2.36 5.36 10.67 1252

142 Plant Physiology

magnesium. Such substances as lime, silicon, and chlorine
do not seem to move appreciably. According to Arendt
1000 oat plants contained in the various periods of growth
the quantities of phosphoric acid given in the preceding
table, expressed in grams.

77. Water cultures. — From a study of the nutrient
requirements of plants in soils, or even in sand cultures, it
is not possible to arrive at a definite conclusion respecting
the elements needed by plants through the soil solution.
For this purpose water cultures are required, and such
cultures have been employed for more than half a century
in the study of plant nutrition and other physiological
relations. Relatively simple experiments afford the chief
fundamental facts. Many plants lend themselves to
water-culture experiments; in fact all cereals, peas, beans,
buckwheat, and many other crop plants may be employed,
in spite of the unusual conditions to which the roots are
subjected.

The seed represents a considerable accumulation of
necessary mineral nutrients as well as organic foodstufts,
and if so supported that the roots may grow in a vessel of
distilled water, this supply of nutrients alone may support
a strong growth for one or two weeks. If peas or beans are
employed and the cotyledons are cut off as soon as the plu-
mule is well developed, the growth in distilled water will be
very slight. Ordinary well water, or the seepage water from
a tile drain, used as a culture medium, and frequently re-
newed, affords a vigorous growth.

A water culture containing as soluble salts the elements
nitrogen, phosphorus, potassium, calcium, magnesium,
sulfur, and iron will afford more or less perfect growth.

MG s

143

Mineral Nutrients

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oTLEISIP

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JAZOO FNOYIIM puB

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lsoyd S

JIM se

Sse] pure ‘(G) UOIL sso ‘(g) WINIsoUsvUT Ss

soy ‘(7) ueso1z1U sso] ‘(¢) SJUSTIyNU Ie ‘(g) ope dt

od prey

‘yLOM UOLPLIYNU UL

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144 Plant Physiology

These seven elements, in addition to hydrogen, oxygen,
and carbon (this last supplied by the carbon dioxid of the
air, see Chapter IX), are those indispensable for green
plants generally ; and the absence of any one of the seven
in the nutrient solution will eventually result in the ces-
sation of growth.

In the preparation of the cultures it is convenient to
employ as culture vessels ordinary glass tumblers (Fig. 44)
covered with black paraffined paper, preferably doubled ;
a shell of black paper is also fitted over the remainder of
the tumbler, and wire guards to assist in supporting the
plants as they grow are attached with rubber bands.
Canada field peas (Pisum arvense) give a quick growth,
and are satisfactory for this work. They have the dis-
advantage of being unusually sensitive to a lack of calcium,
as discussed later. Wheat or oats may be used, and these
do well, especially when the solution is often renewed.
In general, the cereals in solution culture respond quickly
at the outset to potassium and to nitrogen, and, relatively
speaking, there is often a deficiency of these elements in
the seed.

Solution cultures in vessels of the size above noted are
important merely for those observations extending over
comparatively short periods. Large vessels of the nature
of battery jars, permitting the use of several liters of the
solutions, are required when it is desirable to bring the
plants to an advanced state of growth, or to maturity.

78. Nutrient solutions and water cultures. — The nu-
trient solution may be variously constituted. It must
contain the elements previously mentioned, and it may be
well to include also sodium and chlorine. It is probable

Mineral Nutrients 145

that there is no one ideal nutrient solution, since plants
vary considerably in their requirements. The solutions
given below have been much employed, and they are
among those that are generally satisfactory : —

1. PFEFFER’S SOLUTION .

mrounmmtirate .*. . . . 1. 2 ce a e Ss 4 grams
Pree ITAtO +. . . . «ss » ce «+.» 1 gram
Magnesium sulfate . . . aietie anne hats aie TTL
Potassium dihydrogen Ahosphate ee esr Pee ie Nie)
eee Monde . ).. wk rk ee yy SS 6 BD BAM
EMCMOMOFICG | 5. wk we ve eo trace
DeMCIEIUCE 5 tk ee wt ee ee ws we) tO" Siters

2. CRONE’S SOLUTION

Potassium nitrate reg ee ane * Cache eens ora em 0b Wears eets
Iron phosphate PE ohh des i a) SmI ott alee pax +) OR RES
DEMS), stew le ee 8 oe! | 25 PTAs
Seeee aS HO OS 8 el) ob a oo ele be |, 20 PTEMS
Distilled water eee a ce os ie tab. a eeelipee aieet tae ee Ont heES

The first solution has been more commonly employed.
For different plants it is particularly important to change
the ratio of calcium to magnesium. ‘This is conveniently
done by reducing the amount of calcium nitrate and adding
to the potassium nitrate. The Crone solution is reported
satisfactory for cereals; but it is more difficult to handle
on account of the relatively insoluble iron phosphate.

Water cultures for most seed-plants are preferably
slightly acid at the outset, especially where the solution is
constituted as in number one. This solution becomes
alkaline in time. In the preparation of these solutions for

L

146 Plant Physiology

careful work only the purest distilled water should be em-
ployed, and in no case should water from a copper still
be considered acceptable for water cultures. Water
double distilled from hard glass is preferably employed in
accurate work. A method in use at the Bureau of Soils —
and elsewhere for the treatment of distilled water, such as

Fic. 45. Tobaccoin continuous culture : Plat 9, ‘‘complete’’ fertilizer ;
Plat 10, unfertilized. [Photograph from the Ohio Agl. Exp. Sta.]

might be obtained from a tin-worm still, for instance, con-
sists in shaking up the water with carbon black, or with
iron hydrate to free it of any injurious substances.

79. Strength of the nutrient solution. — In the Pfeffer
and Crone solutions given above, the concentration is
about .1 per cent, or 1 part in 1000 parts of water.
This concentration is for many plants sufficiently favorable

Mineral Nutrients 147

in solution cultures. According to Nobbe, nutrient solu-
tions half this strength, on the one hand, or three times as
strong, on the other, failed to give the best results with a
majority of the plants tested. The higher concentration,
however, is far too weak to produce any immediately
recognizable osmotic disturbance. The plasmolyzing con-
centration of KNO., for example, would be for many plants
10 to 15 parts per thousand. Nevertheless, from the above
facts, it is evident that aside from all financial considera-
tions in the application of fertilizers there is a definite
physiological limit to the application of soluble commercial
manures.

Neglecting for the moment the effect of the soil upon
solubility, an extreme case may be taken: assume that
500 pounds per acre of fertilizer are applied to a loamy
soil, and that all of the fertilizer goes into solution. If
the water-holding capacity of the soil is 40 per cent and
the actual water-content, say, 15 per cent, there would be
in the upper 7 inches of soil about 315,000 pounds of water,
and the concentration of the KNOs alone would be 14 parts
per thousand. This calculation is, of course, far from
what would actually occur, for the soil is a strong absorp-
tive matrix, and by no means all of a soluble nutrient added
would be effective in the soil solution. Moreover, in most
cases, a relatively small quantity of the fertilizers added
remains in soluble form.

Commercial fertilizers applied in the drill or in contact
with the seed may readily be present in sufficient quantity
to be injurious to the germinating seedling. Claudel and
Crochetelle! find that solutions of 1 to 1000 of ammonium

1 Annales agron., 22: 131-142, 1896.

148 Plant Physiology

sulfate, sodium nitrate, and some other salts are injurious
when applied to seed in pure sand. Newman ! concluded
that on sandy soil 400 pounds of sodium nitrate is unfavor-
able to the germination of peas. Hicks,’ reporting upon
the germination of seeds as affected by diverse fertilizers,
states that, “commercial fertilizers should not be brought
into direct contact with germinating seed.”

80. The forms of the nutrient compounds. — Since the
mineral nutrients (including nitrogen) are available to the
plant usually only through the soil solution, it is a general
rule that any soluble inorganic salts which are not toxic,
or poisonous, may supply the nutrient or nutrients needed.
Nitrogen, for instance, in water cultures may be supplied
in the form of any of the nitrates. In the field it could not
be supplied either as calcium or magnesium nitrate, on
account of the greater expensiveness of these compounds.
It may be supplied as potassium nitrate, saltpetre; but
more extensively as sodium nitrate, or Chilian saltpetre,
a common fertilizer. Again, nitrogen is to a certain extent
supplied as ammonia compounds, the compound of prac-
tical importance being ammonium sulfate. Ammonium
compounds are further readily diffused through the soil,
and if not used directly, they are, by microdrganisms,
easily converted into nitrates; hence they may be con-
sidered, in general, as readily available forms of this the
most expensive of the nutrient elements. In this connec-
tion it is important to note that it is only after decompo-

1 Arkansas Agl. Exp. Sta., Bul. 34: 99-124.

2 Hicks, G. H., “The Germination of Seed as Affected by Certain
Chemical Fertilizers.”” Div. Bot. U.S. Dept. Agl., Bul. 24: 15 pp., 2 pls.,
1900.

Mineral Nutrients 149

sition and conversion into ammonia and nitrates that the
numerous important organic nitrogen fertilizers, such as
stable and green manures, dried blood, tankage, and the
like, are to any practical extent valuable for plants. De-
composition and nitrification processes, however, will be
discussed later.

é
z
“
=
*
4
Za
‘
ee
-

Fic. 46. Tobacco experiments: Plat 1, no fertilizer; Plat 2, acid phos-
phate. [Photograph from the Ohio Agl. Exp. Sta.]

The soluble phosphates of the various bases are all imme-
diately available and may be used in water cultures; but
phosphates are frequently applied to the soil in some
insoluble form, such as bone-meal or phosphatic rock,
which become gradually available by chemical changes in

150 Plant Physiology

the soil, and by root action. The common phosphatic
fertilizers are the four phosphates of lime, and the only one
of these which is soluble is the saturated, or superphos-
phate [Ca(H,PO,), + H,O], although the reverted or dical-
cic phosphate is also readily available.

Many soluble forms of potash might be used, but the
important commercial forms are the sulfate, chloride
(muriate), and carbonate. In quantity the chloride is
injurious to some crops. The chief sources besides ashes
are now the crude products of the German potassic
mines.

81. Plant nutrients in rock. — It is more particularly
the province of instruction in soils and economic geology
to consider the origin of the plant nutrients of the soil.
The geological history is, of course, of no physiological
significance ; it is information; so that it is here sufficient,
by way of reference, to note some few of the more impor-
tant facts.

The rocks of the earth’s crust from the oldest to the most
recent, from the hardest to the softest, whatever may have
been their origin, are made up of a variety of minerals,
some of the chemical constituents of which are the ele-
ments previously noted as necessary in the growth of
plants. Even the hardest granites, basalts, and lavas
contain, in general, a small percentage of potash, soda,
lime, magnesia, and iron. A single form of rock, such as
one of the red granites, may be deficient in magnesia ;
another, like a red, soil-forming basalt, may lack in potash ;
whilst a limestone may contain no iron. The plant nu-
trients form commonly a minor portion of the bulk of the
rock, the balance consisting often of silica and alumina.

~

Mineral Nutrients 151

The following table shows the distribution of the sub-
stances mentioned in a few types of rocks : ! —

Ware LimME- | LiMeE-
GRaA- BASAL- Saige STONE | STONE
NITIC TIC (\ rhe (Vir- | (Doxo-
vIUs)
GINIA) MITIC)
Geer (ew,) =. .,- - | 69.80 | 53.13 | 48.12 | 4.13 |
PermaimaecAl@,) . . . | 14.45 | 13.74 | 17.16 | 4.19 |
Ferric oxid (Fe,O;) . . | 2.62 1.08 | > |f 4:33 |
Ferrous oxid (FeO) . . 1.94 9.10 J a) Zid
meeaO)y. 5). . . . |. 1.84 9.47 | 9.84 | 44.79 | 53.4
Magnesia (MgO) . . . 49 8.58 3.99 0 | 44,2
parma). . . . | 3.91 2.30 BET Ws) ee |
ctigcl i dh @) ae 3.96 L034) > 224 16 |
Phosphoric acid (P,0;) . 10 40 —- 3.04 |
Carbonic acid (CO,) . . —. — —— 34.10 |
Ignition and loss . . . 89 1.17 | — |

Practically, no soil is made up of mineral constituents
in the same proportion as they occur in the original rock.
There are losses and gains of plant nutrients with respect
to any one type of rock, but residual soils, referable to the
decomposition of a particular rock, approach the rock
more nearly. In general, however, it is clear that, since
soils are formed by the grinding down and decomposition
of rocks, they may contain all the minerals: of the earth’s
crust. A soil is ordinarily of complex origin, and aside
from this, the chief qualities which make it a favorable
environment for the plant as contrasted with broken rock

1 For a comprehensive treatment of the composition of original rock
and residual soils derived therefrom the student should consult the
following: Lyon and Fippin, ‘‘Soils,’’ pp. 1-68; Bailey, ‘‘ Cyclopedia of
American Agriculture,” 1 (Chapter X) : pp. 323-371.

152 Plant Physiology

or coarse sand may be chiefly three: (1) comminution
and greater water-holding capacity, previously discussed;
(2) the addition of accumulated organic matter, and
(3) the presence of a variety of microdrganisms, gradually
transforming the organic matter. The fine state of divi-
sion of the soil particles also permits great freedom to
the further weathering influences of water and other
factors concerned in the rock disintegration which is con-
stantly in progress.

82. Soil fertility. — Fertile soils will generally contain
an abundance of the soil nutrients, sufficient to produce
crops for many successive years. This does not necessarily
imply that the nutrients are available in proper ratio..:
Intelligent growers, moreover, consider not merely the pres-
ent production of crops, but also the maintenance of high
fertility in the case of fertile soils, and the development
of fertility in unproductive soils. It is necessary, then,
to have in mind the supply and the source of supply of the
important elements and their relative abundance.

Sulfur and iron may be dismissed from further considera-
tion, since they are naturally abundant in soils, and are
used by plants in such limited quantities that a dearth
of these nutrients is not common. As would be expected
these two elements are only incidentally constituents of
commercial fertilizers. Magnesium is also ordinarily
present in sufficient quantities, and it may be present in
such excess as to be harmful, as noted later. The plant
producer is now certain that more attention must be paid
to lime, and especially to the relative abundance of lime
and magnesia. Furthermore, when liming is required
every few years, it is a good custom to determine for any

Mineral Nutrients 153

soil the value of using, about once in twelve or fifteen years,
a lime with high magnesia content. Finally, of all the
important elements furnished by the soil, nitrogen, phos-
phorus, and potassium are less abundant, relatively more
in demand by the growing crop, and accordingly to be
conserved and consistently restored.

83. Nutrients removed by farm crops. — In order to
appreciate properly the relation of cropping to the fertility
of the land it is necessary first to note the amounts of the
more important soil nutrients — nitrogen, phosphorus,
and potassium — which may be removed by various
crops annually. The table below is adapted from data

given by Hopkins : ! —

AMOUNT IN POUNDS REMOVED

Per A. \VALUE OF
CROPS YIELD ; Lees
Phos- ENTS PER
Nitrogen | phoric Potash | A.
Acid |
a Ee |
Mitaiadnay.. . . \...{| S5tons| 250.0 22.5 | 120.0 | 47.40
lover nay . ... . . 3 tons! 120.0 15.0 | 90.0 | 25.20
timothy hay i... . 2tons| 48.0 aor 0 10.76
Potatoes, tubers . . . |200 bu. 42.0 8.7 60.0 10.94
Sugar-beets, roots only .| 15tons| 80.0 1A) 125.6 21.26
Morciyeraini:. ©... .; .| 60 bu. 60.0 10.2 ip i ee
Born. stover . . . :|1.8tong| 28.8 3.6 31.2
Corn crop ee 88.8 13.8 42.6 | olioe2
ivneat.orain 2). . ./| 80 bw 42.6 7.2 7.8
muyvleat, straw 5 . . .|1.5tons| 15.0 2.4 21.0
“Wheat crop . Bis. 3 57.6 9.6 28. Se ae
Pepites poe. a: 6 S400 bu. les on 38.0 7.38

1 Hopkins, C. G., “‘The Fertility in Illinois Soils.”’ Ill. Agl. Exp., Bul.
123: p. 189.

154 Plant Physiology

In the preceding table it is shown significantly that
the amount of nitrogen taken up by the leguminous crops
reaches a figure averaging far above that of the others.
As indicated later, much of this nitrogen is derived from

Fic. 47. Fertilizer experiments with cereals and grass; to the right,
effect of a nitrate.

the air through the remarkable activity of the bacteria of
the root tubercles; and in reality it often represents, even
with the harvesting of the crop, a soil gain. It would
represent a large soil gain if the crep were returned to the

Mineral Nutrients 155

land. In general, for the crops included, the losses of
nitrogen and potash are fairly comparable, while the loss
of phosphorus is only about one fourth as great as either
of the other constituents. Frequently, however, much
of the potash-containing products, such as straw and
stover, are returned to the land.

84. Nutrients removed by fruit crops. — The following
table indicates the amounts of nitrogen, phosphoric acid,
potash, and lime removed by various fruits, the quantities
being determined for the fresh fruit per thousand pounds,
assuming that leaves, wood, etc., of the trees will be
eventually returned to the soil : —

QUANTITIES OF Sort INGREDIENTS WITHDRAWN BY VARIOUS Fruits!

AMOUNTS OF NUTRIENTS REMOVED, IN PouNDS
Fresu FRvuit

1000 Pounps ie Phos- | ie.

Total Ash) Nitrogen | phoric Potash | Lime
| Acid | |

Mimeddet.). . . 5 | 17.29 | 7.01, | 2.044) 9.95 1.04
Pere ke. | 264 | 1.05.) 5 BB | 1:40 ai
PRPPICOUS! 2. +c kt ee 508s ty COA Sa Ra eae |e tO. 16
Peimamasio ios on 6 cy. | 10.78 oF es live 41, 6.50 .10
POMETINES! > ek 4.82 2.29 ae Nee ak 1. .20
hespnuts? . 9.5)... . 9.52 GAD) |) obs 3.67 1.20
OER a Tn 7.81 2.38 .86 4.69 | 85
PE TIACS se aes fie hak 5.00 1-26 eal Dao | 25
Pipe Ceme uta ky | 13.50 5.60 1.25 9.11 | 2.43
Mines Se 4.32 1.83 iS Dat .97
Prunes, French Ae dt ae 4.86 1.82 .68 3.10 22
Bielnmts 2 me. os ., « [12.98 5.41 (ea 8.18 155

1 Data taken from Wickson, E. J., ‘California Fruits,” p. 157, 1900.
2 Tncluding hulls.

156

Plant Physiology

It is also further interesting to note the requirements
per tree and also per acre in the case of certain fruits, as
shown in the following tables reported by the New York
Experiment Station : —

IMPORTANT NUTRIENTS USED DURING A GROWING SEASON BY
Mature Fruit TreEEs!

AMOUNT REMOVED PER TREE, IN Pounpbs

Fruit wee
Nitrogen | phoric | Potash
Acid
Apple . a eee 1.47 .39 1.57
Peach . .62 15 .60
Pear .20 .06 2d
Plum ao .O7 oo
Quince 19 .06 .24

Lime

1.62
95
32
4
27

Magnesia

-66
29
.09
me
-08

NUTRIENTS USED PER ACRE BY DIFFERENT FRuIT TREES 2

AMOUNT REMOVED PER A., IN PouNnpDs

VARIETY TREES Phos-
PER A. | Nitrogen phorie Potash Lime

Acid
Apple . 35 a ie 14.0 55 57.0
Peach . 120 74.5 18.0 tip: 114.0
Pear 120 29.5 7.0 33 38.0
Plum 120 29.5 8.5 38 41.0
Quince 240 45.5 15.5 57 65.5

1N. Y. Agl. Exp. Sta., Bul. 265: p. 366.

2 [bid., p. 369.

Magnesia

23
35
11
13
19

Mineral Nutrients 157

AMOUNTS OF NUTRIENTS REMOVED PER ACRE BY THE FRuIT! ALONE

PuHos-

Mac-

a On Tame VARIETY OF Noa cne PHORIC | POTASH LIME li eee
Fruit TREE AcIp COI (ea) bee

(P,0;) ; eO?

Pounds | Pounds | Pounds | Pounds | Pounds

Fruit . . .| Apple 20.0 8.5 AGO 39 | 64
Fruit . . .| Peach 17.5 8.6 36.0 210 imme Nae |
Fruit. . .| Pear 9.0 3.2 20.2 2.2 | 2.6
Pra). ss | Plum 13.3 4.7 18.5 44 | 3.0
tiie. . . | Quince 20.0 10.0 44.4 cy. eae eae 8

Assuming that the leaves, dead twigs, etc., are annually
returned to the soil, fruits are ordinarily less exhausting
than field crops. In this connection it is entirely imma-
terial that a bushel of oats of the same variety, or a barrel
of Baldwins, will not always contain the same amounts of
nitrogen, phosphoric acid, and potash. Other analyses,
therefore, will not accord in detail with those given. The
fact that these crops are not exhausting is important in con-
templating the maintenance of fertility in intensive fruit
production. It would seem that in fruit production it
may easily be possible to realize a permanent system of
agriculture.

85. Amount of nutrients in soils. — Fertility is a matter
so complex — dependent upon such a variety of factors
— that a chemical analysis is important in two respects,
chiefly: (1) to indicate the total amounts of plant food,
for the time available or unavailable, and (2) to point out
unbalanced conditions, or to suggest lines of treatment.
Ultimately, experiments with the plant are invariably

1 [bid., p. 370.

158 Plant Physiology

required in order to determine what is, for any soil, the
most effective fertility.

Many analyses of tillable soils have been made through-
out the United States, and it is shown that the storage of
nutrient elements therein is most diverse. Calculated to
pounds per acre in the upper seven inches of soil many
complete analyses afford extremes as shown in the follow-
ing tabular summary : —

NUTRIENT AMOUNT PER A,

Pounds
Phosphamté atid S. oc Sones) <) el s eee 500 to 10,000
Potash 0,2. ee 3 ad eee oe 3000 to 100,000
Lime 3 ete FAN A tale SO SOD 9 gl aes 2000 to 200,000
Maonesian 05 te" G0, Ge ce he. a ae eee 1500 to 150,000

When the distribution of the roots and the availability
or lack of availability of the compounds are taken into
consideration, it is evident that with respect to the minima
one may speak of exhaustion or lack of nutrients, but with
respect to the maxima there may be sufficient, conserva-
tively used, for generations.

A very thorough study is being made of the soils of
Illinois, and the table on the opposite page gives the
average amount of plant food for a variety of soil types.

These represent, except in the last two cases, the total
plant food in 2,000,000 pounds of dry surface soil, this
weight being approximately that of 7 inches of ordinary
soil. In the case of sand, which is heavier, about
2,500,000 pounds are concerned in the same depth, and
in the case of the light peat only about 1,000,000 pounds.

Mineral Nutrients 159

eee: TOTAL | ToTaL
Soi Type PHOs- Porvas-
NITROGEN
PHORUS SIUM

a | i
orate lt keg) ec as 2,880 840 24,940
ea OAM. 2 «ek 5,030.) 1230 35,792
Pemeieariinlonm . . 2. ss. 2s T2280) GAT5S 1) 33.510
Pewee OAM 1 2 we we 2,016 884 | 33,901
Paommcaingy loam: 2°... 1 lel 3,070 S50)" |. - 26:700
ieowm bottom loam ... .. . . A,720-44, 1620) 5 -/)¢39:970
Odi S00). 6 SI nee re 1,440 | 820 |’ 30,880
erate. to .. s 6. | 84,880 5] 1960 | 2,930

The vast array of facts which have been developed with
respect to the amount of nutrients :n soils is, after all, of
somewhat limited application. This is due in large part
to fundamental difficulties in obtaining a satisfactory
basis for a computation of effectiveness. If, for ex-
ample, the total quantities of the nutrients contained
in the first few inches of soil are made a basis, then, to
modify the calculations of the amounts of the nutrients,
there are such conflicting factors as the following : —

(1) Roots are not commonly limited to the first 7 or
10 inches of soil (Fig. 9).

(2) To a certain extent there is in the soil a movement
of the soluble nutrients from higher to lower levels and
also the reverse. There is further a variable loss from
leaching.

(3) The roots do not actually come in contact with all
of the soil, and the special solvent action (discussed later)
is greatest in the immediate vicinity of these structures.

(4) No absorbing organs are able completely to “ex-
haust ’’ or remove all the nutrients from any soil, and the
amounts readily removable depend upon complex chemi-
cal and physical factors.

160 Plant Physiology

86. Availability of the nutrients. — Plant nutrients
exist in the soil in conditions most diverse with respect to
availability, and chemical analysis does not satisfactorily
distinguish between availability and nonavailability.
Potassium, for example, may be present in conjunction
with aluminium silicate, or it may be present in far more
soluble form; but there are at present very few data con-
cerning the nature of these compounds. If any element
is present in markedly unavailable form, that element
will be needed as a fertilizer, especially to hasten the early
- stages of growth.

Fertilizers are generally applied, not only to keep up
fertility, but to increase availability. In the latter case,
therefore, from the immediate standpoint of the plant,
fertilizers are supplied either (1) as directly available
nutrients; (2) as substances, effecting readjustments in
the soil, so that needed elements become more available
to the plant; or (3) in order to counteract the effects due
to some unbalanced condition of the nutrients (later dis-
cussed at length), injurious acidity, alkalinity, and the
like.

87. The solvent action of roots. —It is well known
that roots and root-hairs are able to render available a
certain amount of nutrient materials. There is a solvent
action of the roots. The case almost universally cited is
the corrosion of marble (limestone) by roots. The nature
of this solvent action has been much studied and discussed.
It is certain that the excretion of aqueous CO, is sufficient
to account for much, and probably for nearly all of this
action.

Kunze and others seem to have convincingly demon-

Mineral Nutrients 161

strated that there is no excretion of a mineral acid, and that
any organic acids present are beyond the sensitiveness of
litmus. Nevertheless, some investigators have found other
acids present under certain conditions. ‘These conditions
are mainly poor oxygen supply. Stoklasa and Ernst,
for example, have identified traces of acetic acid and
formic acid with poor oxygenation of the roots of corn and
barley. Under such circumstances these may be regarded
as evidence of unfavorable surroundings, and not as excre-
tions beneficial to the plant. Under similar circumstances
oxalic acid was identified in the case of the sugar-beet
and of the hyacinth. From the preceding it seems safe
to assume that in the case of cultivated plants normal
solvent action is due to CO, (an excrete product produced
by every living cell; cf. respiration).

It should be observed, however, that recent studies
by Schreiner and Reed call special attention to the
oxidizing action of roots. This seems to be brought
about by a peroxidase, and the process may be practically
important, since many cultural practices are designed
to promote oxidation.

88. CO, excretion and the availability of phosphorus.
—Stoklasa and Ernst have further given some data indi-
cating that the relative rate of excretion of CO, by the
roots per gram of dry weight of substance is directly im-
portant in determining the capacity of a plant to get phos-
phoric acid from the more insoluble substrata. The follow-
ing table exhibits side by side the excretion of COs,, as
shown by water cultures, and the absorption of phosphoric
acid when the same kinds of plants are grown in gneiss
and basalt : —

M

162 Plant Physiology

PLANTS GROWN 90 Days aT 20° C. PLANTS GROWN 77 Days

te eae | Substratum ee ae
er. : Dry Substance

Mg. per 24 hr.

oid eS oe a aia me! 74.6 | Gneiss .285

Basalt .297
Went) 2 itce sae 89.6 cars is
Ve: x? Pa Code poke 110.8 airs -
pig as Sas oto oe 118.9 Rien ee

\

89. Another view of soil fertility. — In the discussion
of fertility thus far it is accepted that soils may be rela-
tively deficient in nutrients, that removal of nutrients
by crops tends towards practical deficiency, and that the
addition of fertilizers, although it may also affect avail-
ability, or balance, is a considerable factor in maintaining
fertility. Some investigators advocate another view.
This is in part based upon a method of observation and
experiment yielding results which seem to point to an
unexpected uniformity in the constitution of the soil solu-
tion from diverse types of soil.

They conceive that the addition per acre of a few hun-
dred pounds of fertilizers to one or two million pounds of
earth (surface soil) is of no consequence in increasing
available nutrients, and they would ascribe the admitted
value of fertilizers to some more general effects upon the
soil, all of which are not understood. This view would
seem to demand that sodium chlorid would be, in general,

Mineral Nutrients 163

as valuable a fertilizer as potassium chlorid or sulfate.
We must regard as one of many types of facts opposed to
this view the vast amount of experimental work showing
the direct value of particular nutrients, and more espe-
cially, of particular nutrients at certain stages of the
growth of the crop. The view is of undoubted value
in suggesting lines of investigations. Associated with it,
usually, is the idea of toxic excreta from plant roots, which
is considered in another place.

90. The paraffined wire basket in nutrition studies. —
In determining through plant growth certain soil relations
by a quick laboratory method it has been the custom to
employ tumblers or other similar glazed vessels from which
there could be no loss of the materials employed, These
are not always satisfactory, since, if drying out proceeds,
spaces are left between the soil and the vessels, and under
unfavorable conditions, especially, it is in these spaces that
the roots grow, thus giving no exact indication of the soil
conditions.

The paraffined basket method is well demonstrated by
Figure 48, in which, from left to right, successive stages in
the preparation of the culture are shown. As described
by Schreiner, the basket is dipped top downward into hot
paraffin several times until a rim is made. It is then filled
with soil to the rim, and firmly packed near the gauze,
the surplus protruding soil being brushed off. The basket
is then dipped into the paraffin up to the rim several times.
The paraffin penetrates into the soil pores or capillaries,
and there is no line of cleavage, as with glazed vessels.
The surface of the soil may be covered with paraffined
paper, in which slits are made for placing the seedlings.

164 Plant Physiology

The method has been much used in connection with
Livingston’s plan of using transpiration as a measure of

Fic. 48. The paraffin-basket method. Upper illustration shows se-
quence of stages in preparing cultures, and the lower a comparison of
root growth in a basket (left) with a tumbler (right). [Photograph
from the Bureau of Soils, U. S. Dept. Agl.]

growth, but it has a much wider application, whatever
the indicator may be, in the general study of the mineral
nutrients of the soil, and many other soil conditions.
This method is unnecessary where the conditions of soil
moisture are constant, and with grades of coarse sand.

Mineral Nutrients 165

Again, such plants as corn and vigorous varieties of the
sunflower are able to force the roots through the paraffin,
especially in warm weather.

LABORATORY WORK.—SUGGESTED EXPERIMENTS

Solution cultures, essential nutrients. — Since the seed repre-
sents a considerable accumulation of the necessary food-materials
required by the growing plant, the absolute necessity of a particu-
lar nutrient may not be readily demonstrated except by growing
plants to maturity in relatively large vessels. The latter is com-
monly impracticable, and in simple experiments it suffices to
determine the comparative effects upon growth or green weight
of a full nutrient solution, along with other solutions lacking
each element in turn. While the method is open to criticism,
the student will find much use for the experience in manipulation ;
and after a study of balanced solutions, he may define his criti-
cism.

Materials needed: cheap tumblers covered and arranged as
suggested (section 77 and figure 44), or wide-mouth bottles with
flat corks notched to receive the seed; black paper shells for
darkening the cultures, and black paper circles or squares dipped
in hot paraffin for tumbler covers; chemicals required by the so-
lution; as many stock flasks, or bottles, as nutrients; distilled
water, graduates, rubber bands and labels; and germinating
seed.

Uniform seedlings should be employed, and these should be
grown on moist moss or sawdust, or upon a paraffined wire screen
floated on water by corks, but sufficiently weighted to keep the
seed moist. All vessels should be chemically clean (preferably
by the acid-dichromate method), and only the purest chemicals
and distilled water employed.

Prepare a stock solution of each main constituent of the Pfeffer
solution in the proportional quantity of water, thus for a 5000 ce.
solution, as follows : —

166 Plant Physiology

Calcium nitrate, 4 grams in distilled water 1000 ec.

Potassium nitrate, 1 gram in distilled water 1000 ce.

Magnesium nitrate, 1 gram in distilled water 1000 ee.

Potassium dihydrogen phosphate, 1 gram in distilled water 1000
ee.

Potassium chlorid, .5 gram in distilled water 1000 ce.

Taking 50 ec. of each of the preceding, cultures of the full nu-
trient solution lacking iron are prepared, the iron being added in
every culture where desired by afew drops of a 2 per cent solu-
tion of the salt indicated.

In omitting the several elements separately, substitutions are
made from solutions of other salts made up in the same propor-
tion, but taking cognizance, in each case, of the smaller quantity
desired, the following substitutions being reeommended : —

Less calcium, use NaNOs.

Less nitrogen, use CaCl, and KCl, respectively.

Less potassium, use NaNO;, NH2PO,, and NaCl, respectively.
Less phosphorus, use KCl.

Less magnesium, use NasSQO,.

Less sulfur, use MgCly.

Set up duplicate cultures with full nutrient solution, with so-
lutions lacking each element successively, with distilled water,
with tap water, with Crone’s solution. Also make for comparison
cultures of the Pfeffer solution both five times the strength and
one fifth the strength of that above used. Also set up two ad-
ditional tumblers (employing the full nutrient solution) of peas
to be employed in the last experiment.

If it is possible to include tests with several plants, Canada
field peas, oats or wheat, and buckwheat are important, for each
manifests special requirements in the early stages of growth de-
pending largely upon the composition of the seed.

Use ten plants in each culture, keep in a fairly moist place (or
invert a tumbler over each culture) for a day or two, then trans-
fer to greenhouse, if possible. Replace, as needed, by pipette

Mineral Nutrients 167

the water lost by transpiration. If the cultures are continued
longer than two weeks, renew the solutions. When necessary,
support the plants by the wire standard and ring (Fig. 44). Close
the experiment within four weeks, measuring tops, weighing roots
and tops separately, taking notes on general appearance, tabulat-
ing results, and representing graphically the green weight of tops
and of the whole plant.

Corrosion by roots. — Place the polished marble plates provided
in the small germinating plats (cigar boxes 2 inches deep answer-
ing well), cover with 2 inches of sand, sow seed of squash and bean,
and maintain under conditions favorable for growth. When the
seedlings have grown vigorously to a height of 5 or 6 inches, exam-
ine the marble plates for etched tracings.

Determination of acid excretion by roots. — When the two addi-
tional tumblers employed ina preceding experiment with nutrient
solution afford vigorous seedlings, set up the following experiment :
Boil one liter of tap water in a flask, cool, and aérate, make slightly
alkaline with potassium hydrate, and add a few drops of phenol-
phthalein to give distinct pink color. With this solution fill four
tumblers, two of which are to be covered with paraffined paper as
controls; to the other two transfer the covers and seedling peas
above indicated. Place both sets under similar conditions, and
after 24 hours note and compare the color in the two cases. If the
pink color has disappeared, the solution has become acid. In
that case pour the contents of one tumbler into an evaporating
dish, and bring toa boil. If the color reappears promptly, it in-
dicates carbonic acid.

REFERENCES

BENECKE, W. Die von der Cronesche Nahrsalzlosung. Zeitsch.
Bot. 1: 235-252.

BREAZEALE, J. F. Effect of the Concentration of the Nutrient
Solution upon Wheat Cultures. Science, N.S. 22: 146-
149, 1905.

Cameron, F. K.,and Beuu, J. M. The Mineral Constituents of

168 Plant Physiology

the Soil Solution. Bur. of Soils, U. S. Dept. Agl. Bul. 30:
70 pp., 1905. |

Fest, F. Ueber den zeitlichen Verlauf. der Nahrstoff-Auf-
nahme [ete.]. Journ. f. Landw. 56: 1-47, 1908.

Haut, A. D. Fertilizers and Manures. 384 pp., 1909.

—— The Book of the Rothamsted Experiments. 294 pp., 49 figs.

Hopkins, C. G. Soil Fertility and Permanent Agriculture.
653 pp., 1910.

Hiuearp, E. W. Soils. 593 pp., 89 figs., 1906.

Lyon, T. L., and Firrin, E.O. Soils. 531 pp., 157 figs., 1909.

Srokuasa, J., and Ernst, A. Beitrige zur Losung der Frage
der chemischen Natur des Wurzelsekretes. Jahrb. f. wiss.
Bot. 46: 55-102, 1908.

VoorHEES, E. B. Fertilizers. 335 pp., 1907.

WHEELER, H.J., and Apams, G. E. Concerning the Agricultural
Value of Sodium Salts. R. I. Agl. Exp. Sta. Bul. 106: 109-
153, 1905.

Wuitney, M. A Study of Crop Yields and Soil Composition in
Relation to Soil Productivity. Bur. of Soils, U.S. Dept. Ag].
Bul. 57: 127 pp., 24 jfigs., 1909.

Wixrartu. Rémer,u. Wimmer. Ueber die Nahrstoffaufnahme
der Pflanzen in verschiedenen Zeiten ihres Wachsthums.
Landw. Versuchsst. 63: 1-70, 1905.

Texts. Detmer, Johnson, Jost, Pfeffer, Sachs.

CHAPTER VIII

SPECIAL FUNCTIONS AND RELATIONS OF
MINERAL NUTRIENTS

THE ROLES OF MINERAL NUTRIENTS

Puant physiological literature contains many references
to the specific roles or effects of the various mineral nu-
trients. Some of the observations and results are of par-
ticular interest; but many of the suggestions are based
upon such slight evidence as to require no consideration
in this place. It is an interesting and important field of
work, but explanations of many of the effects which have
been noted are more easily formulated than proved, and
a satisfactory interpretation of the results is proving most
difficult.

The method of inquiry involves, on the one hand, a
study of the effects produced upon the plant or cell when
an element is, as far as possible, eliminated; or, on the
other hand, observations upon the results of supplying the
particular nutrient element under study when it has been
deficient. These are practically the only methods which
can be employed; but it must be admitted that the
absence of any nutrient may lead to unbalanced conditions
which may induce general pathological effects, so that the
particular primary rdle may be obscured. An analogous

169

170 Plant Physiology

criticism would be equally valid in many other lines of
investigation.

91. The nature of the special réles. — Certain soil ele-
ments are needed in the building up of the permanent pro-
teins of the living matter. Those which are known to
enter invariably into the composition of albuminoidal or
protein bodies are necessarily of first importance. Other
essential mineral elements play only doubtful roles in pro-
tein activities, yet they have evidently such important
functions to perform in connection with the activities of
the protoplasm and its products as to be indispensable.

Practically, as expressed by Reed, we may say that in
general essential elements appear to function in two ways:
(a) as component parts of necessary cell structures and
fluids; and (b) as agents indirectly essential, by causing
less understood physical or chemical reactions, — acting
as carriers of other ions, as specific antidoting agents, or
otherwise.

The first group includes, among the elements now under
discussion, nitrogen, phosphorus, and sulfur; while
potassium, calcium, magnesium, and iron fall apparently
in the second group. If the chemical work of the future
demonstrates fully the existence of the basic proteins,
now postulated, as noted later, it would then only, perhaps,
be safe to assume the incorporation of these elements into
the protoplasm itself. The latter elements (especially
potassium) may be important in the osmotic work of the
cell, requisite as carriers or accumulators of food atoms,
as catalytic agents, etc.; but with nitrogenous bodies
like proteins they seem to form at most only temporary
combinations.

Special Functions and Relations 17 t

92. The réle of phosphorus. — Phosphorus is indispens-
able primarily because it is a necessary constituent of the
nucleo-proteins of every living cell. It is accumulated in
relatively large amount in the seed, so in the younger
stages of growth, when practically all cells are embryonic,
it is relatively most abundant.

Many observers have commented upon the prompt
migration of the sum total of phosphorus compounds
from maturing stems and other older vegetative parts
to the growing tips or to the developing seeds. It has
been shown by Wilfarth and his associates that during
the ten days from June 17 to 27, as barley is maturing,
there is a striking change in the phosphorus relations,
the amount in the straw being reduced from 29.04 kilo-
grams to 9.59; while in the grain there is an increase in
the same time from 3.54 to 29.84 K. per hectare. At the
same time they give data which they interpret to mean
the movement of some phosphorus back into the soil.

Loew was the first to suggest important additional
functions of phosphorus. As a result of phosphorus
hunger the cells of Spirogyra soon cease to grow, but
starch is formed for a time. He also found that oily and
protein substances were not used, but in fact accumulated
in the cell. Owing to the phosphorus content of lecithin,
he explained the accumulation of fats by assuming that
such substances are changed into lecithin before becoming
assimilable by the protoplasm; thus phosphorus would be
essential in the assimilation of fats.

Overton assumes that lecithin and similar bodies are
important in the osmotic properties of the plasma mem-
brane, this view being largely based upon the penetrability

172 Plant Physiology

of the membrane to substances like alcohol. There are,
however, serious objections to this idea. Reed found,
among other pathological conditions attending an insuffi-
ciency of phosphorus, that starch was transformed into
unusual carbohydrate forms, and that cell-walls were
often thickened.

93. The réle of potassium. — Potassium is an essential
element, and the experiments which have been carefully
and accurately carried out make it possible to say that in
general there may be no fairly complete substitution of
potassium by means of the related metals, lithium, sodium,
rubidium, and cesium, and generally very slight partial
substitution among higher plants. It is, however, true
that, when potassium in sufficient quantity is not available,
the addition of sodium is almost invariably attended by
increased growth. The relation to sodium is discussed
more at length later.

Potassium in organic food formation. — Many investiga-
tors agree in assigning to potassium a peculiarly impor-
tant function in the formation of carbohydrates and pro-
teins. Loew and Reed have devoted special attention to
this point. When potassium fails, starch is not formed,
and even if sugar is furnished, proteins are not normally
produced. Cells in a condition to divide are also consider-
ably influenced by lack of potassium. Such cells might
elongate to twice their normal length, supposedly by a
process of stretching, but there would be no evidences of
cell or nuclear division. Loew regards the potassium as
a strong condensing agent (and he shows that in certain
cases potassium is able to effect changes which sodium
will not). Since condensation processes are probably

Special Functions and Relations 173

involved in carbohydrate, fat, and protein making, the
relation of potassium to general metabolism is deduced.

The potassium and protein relation. — The relation
between protein and potash in storage organs has also been
shown to be suggestive, at least to this extent: Seeds or
other organs rich in protein are generally relatively rich
in potash, although there is no definite ratio. Loew cites
certain analyses of Wolff which may be summarized in the
following table : —

Pro
POTASH
No No EIN
; PoTasH, PER PROTEIN, Ava.
PRopuctT Spre- | ANAL- Ava.
CEenT IN ASH PER CrFntT PER
CIES | YSES PER
CENT
CENT

Seeds of cereals 5 200 | 16.32 to 31.47} 9.8 to 11.0-| 23.07 | 10.2
Seeds of legumes | 6 64 | 29.84 to 44.01 | 22.7 to 35.3 | 39.21 | 29.0

The osmotic relation and winter injury. —It has been
generally held that another important rdle of potassium
may be found in its action as an osmotic agent. Some
plants contain relatively large quantities of potash in their
Juices, as K,SO,, KNO;, KH,PO,, and certain organic
salts. Other plants, however, have high osmotic coeffi-
cients on account of organic substances, and there seems
to be no sufficient reason why as an osmotic agent the
potassium is to be regarded as having a constant rdle.
In fact, the view seems to be justified that in so far as po-
tassium is necessary osmotically it may be replaced by
sodium. Certainly the high osmotic value of certain
fungi which may grow upon strong sugar or nutrient solu-'

174 Plant Physiology

tions is not due to the presence of K compounds, and this
fact has been abundantly demonstrated.

With this osmotic relation in view, it was natural that
there should exist also the belief that plants afforded an
abundance of potash are better able to withstand drought.
This is not yet sufficiently proven. Resistance to drought
may possibly be due in part merely to increased salt con-
tent of the plant; in which case, however, it would be
inferred that many soluble salts should have a similar
effect. The latter is not reported to be the case. The
experiments of Atkinson in Alabama on the prevention
of ‘‘ rust’ of cotton have been interpreted to mean that
potassic fertilizers are partially important in the water
relation of the plant, guaranteeing sufficient water, con-
sequently preventing the blight, which is a combination
of drought and fungous effects. Nevertheless, there is
apparently no evidence that desert plants possess any
particular relation to potassium. It is also claimed that
by virtue of relations to the water-content plants well
supplied with potash would be less injured by freezing.

Maturity, quality, and color.— The belief is current
that orchard trees well fertilized with potash ripen their
wood more thoroughly, and that as a partial but direct
consequence of this the shoots and buds are not so subject
to winter or early spring injury. In other words, the belief
indicates that potash content is a special factor in the har-
diness of perennials. Heightened color and quality in
apples has also been attributed to it, but a careful exami-
nation of this point indicates that there is no such relation.
It seems rather that such a deficiency of any element as
to check growth necessarily affects quality.

Special Functions and Relations 175

94. The réle of magnesium. — Magnesium is an ele-
ment concerning some of the functions of which practically
all physiologists seem to be agreed. It may be inferred
that it does not play a direct role in the formation of pro-
teins. It is, in general, more toxic to protoplasm than the
other mineral nutrients, and according to Loew its chief
function is probably to be found in the conveyance of
phosphoric acid for assimilation. Magnesium is more
abundant in those parts of the plant undergoing develop-
ment, as in growing tips and seeds. This would imply
that it acts indirectly to condition the formation of the
nucleo-proteins. Loew believes that ‘the same amount
of base can serve over and over again as the vehicle for
assimilation of phosphoric acid.” It is well known that
magnesium is migratory in the plant, so that maturing
organs are considerably depleted. Attention has been
called to the fact that oily seeds contain a larger proportion
of this element than do starchy seeds, and this is regarded
as a point strengthening the argument of Loew respecting
the function of this element, especially since lecithin is
formed in cells rich in oil. Reed has also found that there
is some definite connection between magnesium and phos-
phorus. He has demonstrated that oil globules are not
formed in Vaucheria when magnesium is lacking in the
nutrient solution, and he believes that there is an intimate
. relationship between magnesium and vegetable oils.

95. The réle of calcium. — The judicious use of lime
in plant production may be the determining factor in the
active fertility of a soil. It appears that the addition
of lime to soils is a practice which has shown more or
less alternation in various agricultural epochs. Wheeler

176 Plant Physiology

has suggested a cause of this use and disuse. When the
benefits from it at any time became known, this probably
led-to excessive use, causing injury, whereby the practice .
again fell into disfavor. In the United States a careful
study of the liming practice and of its effects has been
made in comparatively recent years.

Calcium has functions to perform which are strictly
physiological; that is, directly important in the metabolism
of the plant; it has other effects distinctly ecological,
affecting the plant through its action upon the physical
and chemical environment. It is not always possible to
distinguish the one form of effect from the other. From
an agricultural standpoint Wheeler has given in the “ Cyclo-
pedia of American Agriculture”’ a concise enumeration of
the effects. In this connection the physiological side re-
quires more particular consideration.

In vegetative organs. — There is generally a considerable
accumulation of lime in leaves and other vegetative organs,
and on this account it has been assumed to play an impor-
tant role in some of the functions associated with the
chlorophyll. Up to a certain point calcium hunger does
not affect starch formation, and the evidence points rather
to an inhibition of starch and other carbohydrate diges-
tion and transport. In fact, many fundamental experi-
ments have established a definite relation — whether
direct or indirect it is impossible to say — between calcium
content and starch digestion. The addition of soluble
carbohydrates is generally beneficial where plants lack
calcium. In this connection it is of interest to note that
calcium is apparently not required by fungi and some
of the lower alge, yet it is required by higher plants
forming no starch. —

Special Functions and Relations Les

Other investigators regard calcium as important, in
the main, in the neutralization of oxalic acid and acid
oxalates, assumed to be a factor in protein synthesis.
Neutralization is often effected in this way, for calcium
oxalate is of frequent occurrence; yet in some of the
higher plants there is no such accumulation of oxalates.

Boehm considered calcium essential in the formation of
the cell-wall, and while he erroneously interpreted this
to be similar to the use of calcium in bone formation, yet
the relation of an adequate calcium supply to the forma-
tion of cell-walls has been clearly brought out by many
investigators. This apparent function may be merely an
indication of imperfect use of carbohydrates as above
discussed. Moreover, the formation of complete cell-
walls in the various fungi without calcium is against any
supposition of its direct importance in modified cellulose
formation.

As early as 1880 it was ascertained that salts of magne-
sium are toxic when used alone, and that this toxicity dis-
appears when sufficient calcium is present in the nutrient
solution. In recent times the peculiar and antagonistic
relation which exists between calcium and magnesium, and
also between other nutrient elements to a less extent, has
been more completely developed. The work begun by
Von Raumer, and followed up by Loew, Loeb, Kearney,
and Osterhout, will be discussed at greater length under
Balanced Solutions. It is necessary, at this time, merely
to indicate that calcium is important in preventing the
injurious effects of an excess of magnesium.

Calcium in protein formation. — In studying this rela-

tion of the elements, Loew has developed an important
N

178 Plant Physvology

hypothesis respecting the réle of calcium in protein forma-
tion. According to him we must anticipate a calcium-
protein compound as important in the building up of the
nucleus and plastids of the cell. In the absence of suffi-
cient calcium he believes that magnesium takes its place,
and that this magnesium compound does not possess the
necessary capacity for imbibition phenomena required by
the cell structures. There are some important. objections
to be met in considering this hypothesis, in view of the

Fic. 49. Effect of liming in the production of alfalfa ; no fertilizer (1),
lime only (2), andlime with nitrogen (3). [Photograph from the Rhode
Island Exp. Sta.]

facts that magnesium salts are not toxic for the fungi and
for the lower alge, and, in the presence of small amounts
of calcium, relatively nontoxic also for the marine alge,
as well as for a few of the higher plants.

On the other hand it is true that plants grown in solu-
tions lacking calcium show, coincident with the expected
pathological conditions, an increase in the magnesium
content, whereas other pathological effects produced by
unfavorable conditions show a normal ratio of calcium
and magnesium.

Special Functions and Relations 179

Chemical effects. — Lime is almost as important through
its action in rendering the soil environment chemically
favorable as in its specific réles in cell metabolism. Soils
in which vegetation is growing have, in general, a tendency
to develop the condition fittingly termed acidity. When
the acidity increases beyond a certain point, it may become
extremely inhibitory to the proper growth of a variety of
agricultural plants, and lime, either as carbonate or as
slaked lime, is necessary in order to neutralize this condi-
tion. The carbonate of lime is less injurious and more
generally applicable in large quantities.

The ecological relation of plants to soils containing
much or little lime is particularly interesting, and has been
extensively studied from the standpoint of the adaptability
of crops and of the distribution of wild plants as well.
Upon the crop side Wheeler has contributed excellent data.
In general, the experiments indicate that when the soils
show a marked acid tendency, liming is beneficial.

Some of the plants to which the greatest benefit accrues
are such as lettuce, beet, onion, and cantaloup. Again,
crops such as cranberry, watermelon, red-top, cow-pea,
and others may be favorably influenced when the acidity
is considerable. The great majority of crops occupy an
intermediate position, many responding satisfactorily
under field conditions to moderate liming. Upon some
Rhode Island soils the yield of sugar-beets has been in-
creased by liming up to one hundred fold. Liming will
also affect, within the season, the character of the weeds
or native vegetation. It is of interest to note that closely
related plants are differently affected; thus the watermelon
and the muskmelon, or red-top and timothy, may be

180 Plant Physiology

contrasted, the last-named in each case enduring much
‘less acidity.

Lime is important in effecting a liberation of (by
rendering available) other nutrients, and on this account
it should be used cautiously, in order that waste by leach-
ing may not result.

It is important in maintaining phosphates in available
form, and in counteracting the injurious effects of many —
substances in the soils, including certain products of fer-
tilizers. In many ways it has an intimate relation to the
nitrogen supply of plants, for it promotes the formation
of nitrates from organic matter, diminishes the destruction
of these, and seems to be generally almost indispensable
for the proper development of the nitrogen-fixing root-
tubercle organisms.

The above effects may be considered those of most
intimate consequence for plants generally ; but in addition
it may improve (by flocculation) heavy soils, and it may
be important as an insecticide and a fungicide (although
it is favorable to potato scab and to root rot of tobacco).

96. Iron.— A certain amount of iron seems to be
necessary as one of the factors in the normal development
of leaf green, or chlorophyll, although itis not regarded
as a constituent of the organic bodies which make up this
substance. Lack of iron is one of the many conditions
leading to pathological chlorosis. It may be that the lack
of iron affects the protoplasmic structure (the plastid)
in which the chlorophyll is deposited, for the best evidence
points to the use of iron by every living cell, including,
therefore, those organisms which contain neither this pig-
ment nor any allied compounds.

Special Functions and Relations 181

In cases where iron is deficient in the soil, or held as
markedly insoluble compounds, beneficial results have
been obtained by the application of a soluble salt. Rich-
ards and Ono have shown that iron salts have a remarkably
stimulating effect upon filamentous fungi, increasing the
dry weight several fold over that obtained when the mini-
mum used is merely that which would occur as impurities
in the purest salts. Final proof of the relation of diverse
plants to iron is most difficult to obtain, owing to the pres-
ence of traces of this metal in many of the purest salts.

97. Sodium. — Sodium, a metal indispensable in ani-
mal nutrition, is not required by plants. It would seem
that it may at times prove beneficial, and in the field
relations of crops it is often indirectly serviceable by setting
free other requisite bases. Breazeale has shown, by experi-
ments interesting both with respect to method and result,
that more sodium is absorbed, and that it may be directly
beneficial, in the absence of sufficient potassium.

Wheeler has conducted extensive field experiments,
upon various aspects of the sodium problem. ‘This work
supports the views advanced, in a measure. It also indi-
cates that field applications of sodium may be beneficial
in subsequent years “‘in those cases where the previous
application of potassium salts had been large.’ He re-
gards this as ‘‘ due, in part at least, to the retention in the
soil of a part of the previous applications of potassium
salts, by virtue of extra soda having been taken up by the
preceding crops in the place of superfluous potash, whereby
the potash supply in the soil was really conserved.”

98. Chlorine. — Chlorine seems to be generally ines-
sential for the complete development of the higher plants.

182 Plant Physiology

Knop and other students of nutrition so regarded it, and
it is sometimes omitted from the nutrient solution. It is
an invariable constituent of the soil sclution, and either
on this account, or in the belief that it is generally some-
what advantageous, it is commonly added to the nutrient
ration as NaCl or KCl.

Nobbe and others have found KCl indispensable in the
proper maturity of buckwheat, which, deprived of it,
develops a pathological condition at or following the period
of flowering, resulting in a failure to form seed. A light
fertilization of special crops with sodium chlorid has not
infrequently resulted in increased yield; but in most cases
it is not certain that the action is direct, and even less clear
that it is the additional chlorine which is important in the
substance employed. This relation of plants to chlorine
is the second notable difference in the metabolism of
plants and animals.

99. Sulfur. — Sulfur is primarily important because of
the fact that it is contained in albuminoidal compounds.
It occurs in some of the by-products of protein production,
and also as sulfates of the bases — especially potassium
— occurring in the cell-sap. It is usually required in such
limited quantity that the seed may furnish all that is
needed for the normal growth of the plant through a con-
siderable period.

100. Silicon. — Silicon forms a predominant part of the
ash of many grasses and other plants. It accumulates in
old stems and caulms, and may constitute from 40 to
70 per cent of the ash of cereal straws and corn stover.
Nevertheless, corn may be grown without any further
addition than that furnished by the seed. One of the

Special Functions and Relations 183

species of the scouring rush (Equisetum) has an ash con-
tent of SiO,, amounting to from 70 to 80 per cent. The ac-
cumulation is chiefly in the cell-wall, where it is doubtless
important in support and protection. Silicon is regarded
as an inessential element because development proceeds
in its absence; but in the complex relations of plants in
the field it may determine the capacity of a plant to exist
in a particular habitat. Wolff regarded silicon as impor-
tant in furthering the migration of phosphoric acid com-
pounds from maturing leaves and stems to the forming
seeds.

REFERENCES

BrREAZEALE, J. F. The Relation of Sodium te Potassium in
Solution Cultures. Journ. Amer. Chem. Soe. 28: 1013-
1025, 1906.

Deeano, N. T. Etude sur le réle et la fonction des sels miné-
raux. 48 pp., 1907.

Lorzw, O. Ueber die physiologischen Functionen der Calcium
und Magnesiumsalze im Pflanzenorganismus. Flora. 75:
368-394, 1892.

— The Physiological Réle of Mineral Nutrients. Bur. Plant
Ind., U.S. Dept. Agl. Bul. 45: 70 pp., 1903.

Reep, H. S. The Value of Certain Nutritive Elements to the
Plant: Cell. Ann. Bot. 21: 501-543, 1907.

RauMER, v. Calcium und Magnesium in der Pflanze. Landw.
Versuchsst. 19: 253-280, 1883.

Woops, A. F The Relation of Nutrition to the Health of
Plants. Yearbook U.S. Dept. Agl. (1901) : 155-176, 7 pls.

Texts. Jost, Pfeffer.

184 Plant Physiology

BALANCED SOLUTIONS

Since the early studies upon the mineral nutrients of
plants, it has been more or less apparent that any one of
the nutrient salts employed singly may be injurious, or
may inhibit growth. The extent of this inhibition of
growth has in recent years been more extensively meas-
ured. Moreover, it has long been realized that in the
preparation of the nutrient solution a certain ratio of the
different salts is required, or may be favorable, for the best
results.

It is now known that there are certain interesting antag-
onistic relations between some of the nutrient and other
bases whereby the inhibitory effects of one may be in part
or entirely counterbalanced by the presence of another.
A solution in which the inhibitory or toxic action of one
substance is rather effectually eliminated by an “ antago-
nistic’’ compound is now generally termed a balanced solu-
tion. Some cases of alleged antagonism are apparently
complicated by factors of nutrition and exosmosis, but
at present it is not possible to evaluate the different
factors.

101. The injurious action of certain basic nutrients. —
Although toxic action in general is discussed at length
later, it is necessary here, in connection with balanced so-
lutions, to note the relations of some plants to some of the
several single nutrient compounds. The following table,
from data by Kearney and Harter, shows approximately
the limiting concentrations of two sodium and two mag-
nesium salts, endured for twenty-four hours by wheat,
lupine, and maize : —

Special Functions and Relations 185

WHEAT LUPINE | Maize
Parts | shee
Saur DG eese fee, {Par eroE ae bee Parts of Parts per
= 100,000 a 100,000
a Normal| 100,000 | Normal
: : : of Normal of
Solution of | Solution | , d : eT os :
it Solution | Solution | Solution
Solution | |
Magnesium sulfate .007 39 .00125 i AS) 1400
Magnesium chlorid .009 108 .0025 12 .O8 394
Sodium sulfate . . .043 302 .0075 53 05 | 353
Sodium chlorid . . .054 313 .02 ees 8 i .04 232

In general, the magnesium compounds are particularly
toxic to the higher plants. Corn is an apparent exception
to the rule, as are also most fungi and some alge. On
account of the fact, then, that magnesium compounds are
so generally harmful when alone, or in relative excess, it
is of special interest to note in some detail the relation of
this element to other bases.

102. The relation of calcium to magnesium. — The
toxic action of magnesium and the effect of calcium in
modifying it were established by Von Raumer in 1883;
but the most important work in outlining and directing
attention to this field with respect to plants was done by
Loew and his associates. There is at present a mass of
data available both on the plant and on the animal side.
As a general result of all the work on the higher plants it is
now clear that when magnesium is injurious, the presence
of calcium in a certain ratio (variable for the plant) de-
stroys this toxicity entirely. To explain this relation Loew
formulated his theory of the existence of a calcium-protein
body as previously outlined (section 95).

a

186 Plant Physiology

The lime-magnesia relation in the soil is, moreover, of
much practical importance, and it is certain that when
magnesia is relatively abundant in soils there is usually
need of liming. Under such circumstances it is obvious
that the application of a dolomitic limestone (rich in mag-
nesium) should be avoided. The evidence upon this
point is also extensive. There are practical difficulties,
however, in determining the proper ratio of CaO: MgO in
the soil, for the question of availability must be considered.
Water culture and pot experiments have suggested such
differences in the requirement of plants as shown by the
following favorable ratios : —

Buckwheat >... : 2 .s. ...' CaO to Ms0- --..... 3 eee
Cabbage. .- 2) 2.6.2 Se. CaO to MgO) i: 2° Sai
Oats ©. 2.) hewn’ 3 a0 te MEO 2.

Loew believes that the greater the leaf surface produced in
a given time, the greater the necessity for lime; that is,
the higher the ratio. In this connection it is shown that
with respect to the lime requirement the need of tobacco,
clover, grass, sugar-beet, and wheat form a decreasing
series. The cereals never show a ratio higher than 2: 1.
In solution cultures in the laboratory the effect of the omis-
sion of calcium from a solution containing magnesium,
or the inappropriate ratio of calcium, results in a markedly
decreased growth. In the following chart, giving the
effect of the calcitum-magnesium ratio upon the growth
of the Canada field pea, this fact is made clear,’ the

1 These data correspond to the series of cultures in Figure 51, and are
from careful laboratory experiments by graduate students.

Special Functions and Relations 187

single average plant being taken as the basis of the dia-
gram,—each major ordinate denoting .1 gm.:

Fic. 50. Antitoxic action of calcium and magnesium nitrate ; curves of
total green weight (unbroken line) and green weight of stem (broken
line).

In the cultures diagramed above the whole period of
growth is 17 days. The original quantity of the solution
was maintained by replacing with distilled water the loss
by transpiration; but after 14 days all solutions were

188 Plant Physiology

completely renewed. For such a period of growth the
seeds of the pea carry a fairly adequate supply of the other
nutrients. Again, the seed is relatively rich in magnesium,
so that the omission of this element affects growth very
slightly, while the same fact emphasizes the need of cal-
cium in the solution.

103. Other nutrient bases and antitoxic action. — The
neutralizing action of various bases upon one another
has been demonstrated by Loeb, Kearney, Osterhout,
and others. In this regard calcium is most important.
At suitable concentrations it reduces the toxicity of delete-
rious solutions containing either potassium, sodium, or
ammonium, as well as of certain nonnutrient bases.

The table on the opposite page includes data furnished
by McCool! from two distinct series of cultures with the
Canada field pea grown 30 days.

In the first series the concentration of sodium employed
shows no growth whatever, and the addition of one
fortieth as much calcium gives a very considerable growth ;
therefore, a marked antitoxic effect. The best growth
occurs where the stronger concentrations of calcium are
used with the sodium, so that there appears to be a slight
mutual antagonistic action with respect to peas. The
strong effect of calcium upon the relatively toxic ammo-
nium salt is apparent. “

In general, all of the nutrient bases show a series of rela-
tions with respect to toxic action upon plants. For each
base the relations may be different, and a certain varia-
bility is to be accounted for by differences in the composi-

1 The experiments of which these constitute a small part will be pub-
lished as a bulletin of the Cornell Experiment Station.

Special Functions and Relations 189
Antitoxic ACTION — CANADA FIELD PEAS
AVERAGE LENGTH OF | GREEN WT. IN GRAMS

10 PLANTS OF THE 10 PLANTS
SOLUTION CULTURES
CONTAIN
a al Zz: Goss Tops Roots
in cm, in cm.
ie oi 22 13 6.2 ary
1000
PM OSC 18 10 5.1 2.2
ek
—— CaCly 19 9.5 4.8 2-1
ra |
N_ naci no growth) no growth) no growth no growth
100
ane C.Cl, and —— NaCl 18 124 5.25 3.9
1000 7
-N_ Cach, and Naci 25 14 6.5 4.6
a 100
BNE CaCl, and = = Nacl 17 Ld 4.8 3.7
a
Distilled water . r 3 Qe 1.8
EN CaCl, 13 8 —— 2.85
00
Ny NH,C 4 no growth; —— _ j|no growth
2000
N NH,C 4 no growth; —— _/|no growth
3000
Jones SRG 8] AG 4 no growth} —— _§|no growth
4000
N Gal, and _ nu.cl 9 7 8s 2.0
100 1000
Be el and. 2 = NEC!) 14 8 ones 2.0
100 2000
each ani NH,C1.|. 10 8 a 2.54
00 3000
N N iiss
— CaCl, and —— NH,Cl 10 8 2.03
100 000
Distilled water . 7 3 —_—— 1.8

Plant Physiology

190

‘surpuodser
-109 SIOqUINU 94} ‘QE “By Ul ATTRoIVUIUTBISRIpP Polpoqute SoiNy[No oY} Jo ydeasojyoyd B WO

Tg ‘DIY

Special Functions and Relations 191

tion or absorptive action of the plants used as indicators.
Moreover there is diversity in the visible results of the
toxic action by the different nutrients; thus salts of
ammonium kill the roots before the shoots are noticeably
affected, while sodium salts kill the shoot promptly. It
is important that there is toxic action, and that there may
be antagonistic or mutually antagonistic action.

An entirely satisfactory explanation of these phenomena
is not at present available. Loew’s views on the calcium-
magnesium relation are strengthened by the opinions of
Loeb and others who postulate in organisms a number of
metal proteins, so that any solution containing only one
class must be ultimately toxic. Even this view is not suffi-
ciently broad to account for all of the known facts; for
example, the antagonism between inessential and essential
ions.

LABORATORY WORK.— SUGGESTED EXPERIMENTS

Injurious action of single nutrients.— After consulting the
_ literature of the subject determine the concentrations of MgCh,

CaCl., KCl, and NH,Cl, which when used separately will just
permit the growth of roots of wheat or peas. Employ ten
plants in tumblers or bottles, as in the nutrition studies.

Balanced Solutions. — Guided by the indications given under
nutrient solutions regarding manipulation, set up the experi-
ments outlined below, employing in each culture wheat or

Canada field peas. Prepare stock cultures of = Ca(NO:)s,

a5 Ma(NOz)s, =a NaCl, = CaCl, also double strength of nutri-

100
ent solution; from these prepare, by dilution, all the following
cultures, also employing distilled water as a check : —

10.

125 ce.

125 ce.
125 ec. —

125 ce.
50 ee.
200 ce.
25 ee.
225 CC.
125 ce:

(

ah

|

|

a

|

| 125 ce.
125 ce.

| 5O0'ce. —

|

o

|

75 ee.
125 ee.

25 ee.
100 ce.
125 ee.

125 ee.
{ 125 ce.

| 125. ee.
Distilled H20.

Plant Physiology

CaLciumM versus MAGNESIUM

USE

SF Ca(N Os)e.

distilled water.

N
Me(NOs2)s.
oe

distilled water.

N
— Me(NOs)z.
50 g( )

distilled water.

#4 Me(NOs)e.

distilled water.

N

50 Ca(N QOs)e.
N

— Me(NOs)e.
50 g( 3)

s S Ca(NOs)

ee
Me(NOs)2.
50) g( )e

distilled water.

N Ca(NOs)2.
50

N
=~ Mg(NOs)e.
50 g( 3)

distilled water.

N Ca(NOs)2.
50
nutrient sol.
N Meg(N 0Os)2.
50

nutrient sol.

SoLUTION CONTAINS

iN
100 Ca(NOs)e.

N
N_ Me NOs)o.
oo

N
Me(NOs)s.
ae

N
Mg(NOs)s.
500 g(NOs)

N_(a(NOs)s and N. Mg(NOs)s.

100 100

N N

—< NOs)2 N ;
; Ca(NOs)2 and oEp Meg(NOs)2

N N
= & 3)9 pao N :
100 Ca(NOs)2 and 500 Meg(NOs)2

N Ca(NOs)2 and nutrient sol.
100

7 Me(NOs)2 and nutrient sol.

Distilled water.

Special Functions and Relations 193

The above series should show toxicity of the magnesium salt
and antagonism.

In the same manner as for the preceding prepare solutions
to contain the following : —

hh tec ee CaCle.
25 N N N
ies. CaCl! ise e acl. CNe
ee 2500 om 2509
| 100
ls 3 CaCle aint Slee ig, = cack and = NaCl.
100 2500 2500
N N N
ig. Naciie 19. “e. cL
35 CaCle and —— 5500 a 25 CaCle and nutrient sol

Le Ny CaCle and — N INaCIZ se 20: N_waci and nutrient sol.

2500 100 100
Permit the plants to grow under favorable conditions four-
teen days, supplying distilled water as needed by transpiration.
Discuss the results, tabulate data on length and green weight ;
also plot curves of the total green weight in the two series.

RERERENCES

- Ducear, B. M. The Relation of Certain Marine Alge to Vari-
ous Salt Solutions. Trans. Acad. Sci., St. Louis. 16: 473-
489, 1906.

HANSTEEN, B. Ueber das Verhalten der Kulturpflanzen zu den
Bodensalzen. Jahrb. f. wiss. Bot. 47: pp. 289-377, 1 pl.,
19 figs., 1910.

Harter, L. L. The Variability of Wheat Varieties in Resist-
ance to Toxic Salts. Bur. Plant Ind., U.S. Dept. Agl. Bul.
79: 48 pp., 1905.

KEARNEY, T. H., and CAMERON, F. K. Some Mutual Relations
between Allkati Soils and Vegetation. U. S. Dept. Agl.
Report. 71:78 pp., 1902.

KEarRney,.T. H., and Harter, L. L. The Comparative Toler-
ance of Various Plants for the Salts Common in Alkali Soil.

0)

194 Plant Physiology

Bur. of Plant Ind., U. S. Dept. of Agl. Bul. 113: 22 pp.,
1907.

Lores, J. Toxic and Antitoxic Effect of Ions. Studies in Gen-
eral Physiology. Part II, Chap. 35, ete., 1905.

Loew, O.,and May, D. W. The Relation of Lime and Magnesia
to Plant Growth. Bur. Plant Ind., U.S. Dept. Agl. Bul. 1:
53 pp., 1901.
Loew, O. The Physiological Role of the Mineral Nutrients.
Bur. Plant Ind., U. 8S. Dept. Agl. Bul. 45: 70 pp., 1903.
OsterHOoUuT, W. J. V. The Importance of Physiologically Bal-
anced Solutions for Plants. Bot. Gaz. 42: 127-134, 1906;
Ibid. 44: 259-272, 7 figs., 1907.

— Die Schutzwirkung des Natriums fiir Pflanzen. Jahrb. f.
wiss. Bot. 46: 121-136, 3 figs., 1908.

CHAPTER IX

THE INTAKE OF CARBON AND THE MAKING
OF ORGANIC FOOD

ORGANIC matter constitutes the predominant part of the
solid constituents of plants. As organic matter so-called,
this element is linked chiefly with hydrogen; with hydro-
gen and oxygen; with hydrogen, oxygen, and nitrogen;
or with the preceding and one or more of the essential
mineral elements. Nevertheless, carbon may be regarded
as the significant element of organic compounds. The
number of such organic compounds in plants and animals
and their products is almost beyond count. It is the
special province of organic chemistry to deal chemically
with these carbon series, but some account of the origin,
nature, and role of certain of these substances in the living
organism is a most important part of physiology, whether
elementary or advanced.

104. The amount of carbon in the plant. — We have
noted the relatively small amount of mineral or ash ele-
ments in plants, constituting usually from 1 to 5 per cent
of the total weight of the water-free substance. Nitrogen
adds a further fraction,—seldom more than 5 per cent,
—and the remainder, that is, more than 90 per cent of
the dry matter, is made up of carbon, hydrogen, and oxy-

195

196 Plant Physiology

gen. A crude picture of the distribution and importance
of the carbon in plants is afforded by the well-known
process of charcoal making,—a burning without free
access of oxygen. As a result, the hydrogen, nitrogen,
and oxygen of the plant are set free, while only carbon
and the small proportion of ash remain. When burned
with abundant oxygen, the carbon combines with the
oxygen by an oxidation or hydroxylation process, and
gaseous carbon dioxid is ultimately formed. In this case
a perfectly definite amount of energy, as heat, is released.

105. Carbon dioxid the source of carbon in green
plants. — We are now concerned with the source from
which plants obtain their carbon supply, the conditions of
intake, and the method by which the carbon is incorpo-
rated into food for the plant cell. Carbon in inorganic
form, especially as carbonates of lime and magnesia, con-
stitutes no inconsiderable portion (about twice as much as
phosphorus) of the minerals of the earth’s crust. Yet, as
will be subsequently indicated at length, carbonates are
valueless as a source of the carbon from which to make
organic compounds for either plant or animal. Moreover,
both water and sand cultures abundantly demonstrate
that green plants are able to grow and to attain their full
development (necessitating the making of organic matter)
in nutrient solutions containing no carbonates and also
no organic matter whatsoever save that derived from the
seed.

It is easy to convince ourselves of the requirements of
common animals with respect to organic matter. They
feed upon plants or other animals, or upon the products
of these. Moreover, there are the countless fungi and

The Intake of Carbon — 197

bacteria (with marvelously few exceptions!) familiar
as molds, plant parasites, mushrooms, organisms of decay,
and of various fermentative processes; these all require
an intake — from without the body — of organic carbon
nutrients. Organisms which thus obtain their carbon as
organic matter, and which have no apparatus for making
it from carbon dioxid and water are in the end dependent.
Such organisms have also been termed heterotrophic.
Green plants are practically alone in being able to make
organic matter out of the raw materials, carbon dioxid and
water; they are independent. Conveying the idea that
they make organic food somewhere within the body, and
in the first instance for their own use, they have been called
autotrophic.

106. Chlorophyllous plants. — So far as is known, green
plants have always supplied the earth with organic matter,
including fuel. The leaf-green which they contain is the
strongest link binding living things to the sun, — the one
ultimate source of radiant energy available upon the earth.
The means whereby this making of organic food is accom-
plished is fundamentally important, and requires careful
consideration. All living processes and phenomena are
important, but since this stands out as the method whereby
the world’s supply of organic matter is made, the process
assumes an interest scarcely second to that of life itself.

The green or yellow-green color, sometimes partially
veiled, is practically universal among plants which we now
recognize as possessing the highest type of plant habit

~1The exceptions consist in a few species of bacteria, subsequently

discussed (section 128), whose paltry contribution to the stupendous
quantity of organic matter existent is such as to be wholly negligible.

198 Plant Physiology

and form. It characterizes also to a very high degree the
algee and mosses, but it is absent from all the fungi. Sey-

eral facts regarding the habi-
tats and distribution of green
plants afford us an indication
of some of the conditions req-
uisite for the proper work
of plants thus endowed. The
presence of the green color
referred to is universally indic-
ative of the possession of
chlorophyll, a mixed pigment,
imbedded in certain chlo-
roplasts, or chlorophyll con-
taining bodies which are differ-
entiated portions of the living
protoplasm. In particular, it
is apparent that plants con-
taining this substance are sun-
loving or at least light-loving
organisms. They may grow
in partial shadow at times, but
they are wholly absent from
all permanently dark or deeply
shaded places. The large sur-

Fie. 52. Cell of chlorenchyma
showing chloroplasts with starch
grains. [Adapted.]

faces of the leaves and the evident arrangement of these
and of the branches which bear them, with respect to light,
all indicate clearly a certain relation of green color with the
light factor. As the chief bearers of chlorophyll in seed-
plants the leaves command special attention, wholly aside
from their other functions or accessory work. Any agency

199

The Intake of Carbon

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edvis oy} Aq pojJIqIyxe sv oLesoUr Jeol V

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200 Plant Physiology

affecting their health injuriously, such as insect pests,
fungous diseases, general unfavorable environment, ac-

Fic. 54. Abutilon, side view, under greenhouse illumination.

cumulation of dust or soot thereon, means restriction of
their work in production.

107. Respecting the distribution of chlorophyll. — In
the higher plants the chlorophyll bodies may be disposed in

The Intake of Carbon 201

all exposed vegetative organs, but the leaves are primarily
the seats of their occurrence. The palisade and general
parenchyma cells of the leaf ordinarily contain many

Fic. 55. Abutilon ; looking down upon the plant shown in Fig. 54.

chloroplasts (Fig. 25). Such tissue is designated chlor-
enchyma. In stems or other thick organs the chloren-
chyma is comparatively near the surface; for, as a rule, the
formation of chlorophyll is directly or indirectly dependent

202 Plant Physiology

upon light. Notice the color of seedlings which have
grown in the dark and of grass beneath a board or pile of
leaves. The epidermal cells of seed-plants are commonly
colorless, yet the guard cells of the stomata are important
exceptions. Nevertheless, there are certain structures
which, supplied with a good food-supply, contain chloro-
phyll, even when produced in the dark; for example, the
cotyledons of pine.

The white or yellow areas of variegated leaves may
contain no chlorophyll; but leaves which are during
erowth brown, red, or otherwise highly colored contain
chlorophyll bodies, the color in such cases being veiled by
the presence of other pigments often present in the cell-
sap. The diverse pigments of many alge exhibit a
greater complexity. In the great majority of plants the
chlorophyll bodies are discoidal or button-like forms (often
lenticular or more nearly plano-convex), although in cer-
tain of the alge (Spirogyra, desmids, etc.) they may
possess unusual peculiarities of shape. The intimate
structure of the chloroplast is none too well known.
Briefly, it may be said that there is a cytoplasmic stroma,
and within this is contained the green pigment, somewhat
diversely deposited in different cases.

108. The nature and properties of chlorophyll. — By
means of alcohol, chlorophyll may be extracted from plants,
leaving the tissues practically white. Either ethyl,
methyl, or denatured alcohol may be employed, and the
process is greatly facilitated by carefully bringing the
alcohol to a temperature close to its boiling point over a
water-bath. Seedling plants of the horsebean, small ce-
reals and grasses, radish, or nasturtium afford as favorable -

The Intake of Carbon 203

solutions as may be conveniently obtained. The solution
is fluorescent by reflected light, and it is rapidly decom-
posed in strong light.

The chlorophyll pigment as extracted is a mixed sub-
stance. Two products which are constant and predomi-
nant permit of partial separation through their diverse
relations to solvents. Thus if benzole is added to the
alcoholic solution and the latter vigorously shaken, there
will result on standing a blue-green benzole layer and a
yellow alcohol layer. There are therefore two substances,
a blue-green one which has passed largely into the benzole,
and from the color it is usually called blue chlorophyll
or cyanophyll; while the yellow substance remaining in
the alcohol is mostly carotin.

Apparently cyanophyll does not exist alone in nature.
It is a complex molecule containing nitrogen, and is vari-
ously supposed to have phosphorus or magnesium associ-
ated or combined with it. Cyanophyll is closely related,
it would seem, to the hemoglobin of blood; and it yields
a variety of decomposition products, some of which col-
orimetrically and chemically seem to be identical with
certain products of hemoglobin.

Carotin is of common occurrence in a variety of colored
tissues, and in its crystalline form it is most conspicuous
in the root of the carrot and in the petals of certain orange
or yellow flowers. This pigment belongs to the group
often called xanthophyll. The term “etiolin’’ is also ap-
plied to it. The substance is present in etiolated organs,
and it may long persist in the chloroplasts of leaves during
the autumn.

The most important property of chlorophyll is its ca-

204 Plant Physvology

pacity to absorb light, that is, radiant energy to which the
retina of the eye is sensitive. The relation of the chloro-
phyll and of its main constituents to the absorption of
light of different wave lengths, as shown by a spectro-
scopic examination, is discussed later. The radiant energy
absorbed by the chlorophyll is the force operative in photo-
synthesis.

109. The factors essential in photosynthesis. — We
may now review briefly the essential features of the process
whereby chlorophyll-containing plants in the presence of
light are able to construct organic food-materials. The
process is termed photosynthesis. In order that photo-
synthesis may proceed in the cells of healthy plants, it is
necessary that light shall fall upon chlorophyll bodies in
the presence of aqueous carbon dioxid. ‘Temperature and
other factors are important,—the exact relation to tem-
perature being especially difficult to analyze, — and in
general the process is possible only within a certain range
of physiological conditions. Nevertheless, under ordinary
conditions of growth we may regard as the primarily
essential factors: (1) chlorophyll, (2) light, and (3) carbon
dioxid, the last two of which will receive further consider-
ation later.

110. The course of photosynthesis. — Briefly stated,
the gas exchange and the actual phases (several of which
are more or less simultaneous) of the process of photo-
synthesis as commonly conceived are as follows : —

1. Gas exchange between the green tissues and the
surrounding air, whereby carbon dioxid may be absorbed
by the cell-sap and reach the protoplasm.

2. The absorption of radiant energy, as light, by means
of the chlorophyll bodies.

The Intake of Carbon 205

3. The use of this kinetic energy in the decomposition
of carbon dioxid and water (H,O + CO,, or H,CO,), the
synthesis of an elementary organic product, and the con-
sequent storage of potential or latent energy.

4. The probable condensation of the synthate into a
carbohydrate of high food value, generally fruit sugar,
which is then often in part transformed into starch.

5. The elimination, by gas exchange, of O,, a by-product
of the process (some of which, however, may be used in
respiration, subsequently treated).

It is seen, therefore, that there is a physical mechanism
for gas exchange, a series of transformations of energy and
of compounds, and ultimately the deposition of food-ma-
terials, frequently starch. It is now necessary to consider
a method of demonstrating this process, and later there
will be required a further consideration of the course of
events, the factors involved, the energy transformations,
and some of the products resulting.

111. The demonstration of photosynthesis. —It is
possible to demonstrate photosynthesis in any plant more
or less completely by one or more of several methods, and
no single simple experiment will reveal all the facts desired.
With all other factors well controlled,! increase in weight,
or the accumulation of some organic product (especially
starch) are practicable demonstrations. Another type of
experiment involves, when accurate, an analysis of the
gas used, or that eliminated, or both; but such eudio-

1The student who may pursue this matter farther should examine
carefully the difficulties and beauties of well-controlled experiments,
consulting Ganong’s ‘‘Plant Physiology’”’ (2d Ed.), pp. 79-114; also the
earlier account in Sachs.

206 Plant Physiology

metric methods should be carried out in an accurate man-
ner by the use of special apparatus.

For demonstration purposes the evolution of oxygen
from cut stems of water plants (such as Elodea or Ca-
bomba) is the simplest indication of photosynthesis; but
this may not be applied to land plants. During photo-
synthesis gas escapes from the large air chambers through
the cut stems, and with vigorous action a slow stream of
bubbles may arise. It is then necessary to employ a
method whereby these bubbles may be caught so that the
gas may be simply identified.

A funnel may be inverted over a quantity of clean,
growing sprigs of Elodea, or water weed, in a deep vessel
or aquarium. Over the funnel is inverted a test-tube of
water (Fig. 58) for the collection of the oxygen. In order
that there may be free access of carbon dioxid, the funnel
should be much smaller than the vessel and should rest on
supports several inches above the bottom; while the
water should be spring or well-aérated tap water, or should .
contain a supply of CO, introduced from a generator.
The gas caught in the tube may be tested by proper ma-
nipulation (see Laboratory experiments, p. 221), with an
oxygen absorbent, preferably pyrogallate of potassium.

If the gas is collected under favorable conditions, it will
consist largely of oxygen, — about four fifths; the remainder
consisting of other gases formed in the plant and of nitro-
gen from the air, which must, of course, diffuse into the
air spaces. Having determined that oxygen is the chief
part of the gas given off as bubbles from water weeds,
during photosynthesis, the simple bubble-counting method
may be employed in determining relatively the rate of

The Intake of Carbon 207

photosynthesis with different intensities and quantities
of light, with varying quantities of CO, (up to the satura-
tion point of water), at various temperatures, etc.

112. The formation of sugar and starch. — With respect
to the formation of organic food-material it has been indi-
cated in the brief outline of the course of photosynthesis
that glucose is generally regarded as the first stable result.
The formation of glucose and free oxygen from carbon
dioxid and water constitutes a complex process, but the
reaction is commonly expressed in the following conven-
tional manner : —

6 CO; 6 HO =: C, F074 -6:O:.

Some years ago the view was advanced by Von Baeyer
that formaldehyde is an early step in the reduction of the
carbonic acid, and that then six molecules of the formalde-
hyde, H .COH, become linked together or condensed to
form a hexose sugar, C,;H,.O,. Recent work along many
lines strengthens this conception of the process, and it
seems to have been demonstrated (although there are some
criticisms of the method) that by artificial experiments
with the factors light, chlorophyil, and CO., formaldehyde
may be produced, although in small amount as compared
with the quantity which must result from photosynthesis.
The details of this work, however, and the criticisms
thereof do not require consideration.

It is difficult to picture simply all possible relations of
the glucose which may appear as the first stable product,
but the accumulation of this substance in the cell leads to
the formation of other sugars, especially bioses (Cy.Hs.01;)
and ultimately to starch, a complex molecule having the
general formula (C,H,)0;),.

208 Plant Physiology

Starch then is an accumulation product apparently
conditioned only when the tension of the sugar which has
been produced is considerable, ‘at least so considerable
that the cell is unable to use the surplus in building up the
permanent structures, or to remove it fast enough. Starch
is deposited within the chloroplasts in the form of small
eranules. During the growing season it normally accu-
mulates in most leaves through the day, or so long as the
leaves are exposed to strong light; while during the photo-
synthetic inactivity of the night much or all of this starch
may be removed. In most cases the leaf will be depleted
of starch if placed in the dark for a period of 12 hours, if
the leaf is not in itself a storage organ. The process of
starch removal and subsequent deposition, when that
occurs, invites special consideration later.

In those plants forming starch abundantly in the leaves
it is often desirable, and extremely convenient, to employ
the relative accumulation of starch as a rough qualitative
indication of photosynthetic activity. Leaves from which
chlorophyll has been extracted may be stained with a
weak alcoholic solution (tincture) of iodine, the leaves
being preferably placed on a white plate to be stained.
When added to a weak suspension of starch, or to a weak ~
starch paste, iodine yields an intense blue or blue-black
color. Starch in the leaf, or in other tissues, is, however,
considerably obscured, and it often gives a blue-brown
or even brown-black reaction. Plants of the iris, lily,
amaryllis, and orchid families form, as a rule, little or no
starch.

113. The diffusion process.—It has already been
shown that the leaf (or an analogous structure) is an ad-

The Intake of Carbon 209

mirable device to permit rapid diffusion with a minimum
direct exposure of delicate cells. Uncutinized surfaces are
moist and may absorb CO, directly, but the epidermis is
usually cutinized, and therefore it is through the stomata
largely or entirely that a constant gaseous diffusion takes
place between the air spaces of the leaf and the external
atmosphere. ‘The epidermis is an effective multiperforate
septum, which means that, with a difference of gradient
within and without, the relatively small stomatal areas
are far more efficient in diffusion than would be suggested
by their actual area. They are in fact sufficient to pro-
vide for the maximum diffusion of CO, which may take
place from a natural atmosphere into the plant.

The CO, which enters the air chambers of the leaf is
rapidly absorbed by the moist cell-walls within. These
cell-walls absorb the carbon dioxid just as would any mem-
brane moistened with water. The above capacity for
absorption is so great that there is during photosynthesis
practically no tension of CO, in the air spaces. The carbon
dioxid in solution is presented by the cell-sap to the chloro-
plast, and there is, of course, continuous absorption and
migration through diffusion in solution, so long as photo-
synthetic action proceeds. The CQO, absorbed does not
immigrate to any considerable distance before it is used.
This is easily demonstrated by the fact that in small
darkened areas no starch would be produced. It must be
transferred, in some leaves, however, as far as the upper
palisade layers, for in these there is usually abundant
starch-making. It is apparent that in general the sphere
of each stoma is more or less local. The intake of carbon

dioxid is greatest, usually. over the lower surface of the
P

210 Plant Physiology

leaf, and there the air chambers are most numerous; but
chlorenchyma is better developed toward the upper sur-
face. There are, however, so many factors which influ-
ence the structure of the leaf that the apparent inconsis-
tency of this arrangement must be regarded as an effective
compromise.

The same stomatal mechanism effects, of course, a
rapid elimination of the oxygen produced during photo-
synthesis, after this oxygen has diffused into the air spaces
from the moist membranes of the cells wherein it is pro-
duced.

114. The amount of carbon dioxid. — The amount of
carbon dioxid in the air seems almost infinitesimal when
we contemplate the results of its use. The air contains
normally about .028 to .03 per cent, although this amount
may be temporarily somewhat increased in the neighbor-
hood of cities, or of areas where manufacturing is a chief
industry. The limited amount of this gas suggests,
further, the necessity of broad surfaces and the thorough
distribution of chlorophyll.

It has been found that the normal supply of carbon
dioxid is often insufficient for the maximum work of the
leaf. Under ordinary conditions, as when the plant is
erowing in strong light at a temperature of from 20 to
25° C., and with a sufficient water-supply, a chief limiting
factor in growth is the minimum tension of carbon dioxid.

It has been shown experimentally that an increase in the
amount of this gas to such extent that the air will contain
from 1 to 10 per cent or more may be beneficial, provided
the other factors permit a maximum activity. The results
obtained by Godlewski and Kreusler are not entirely

The Intake of Carbon 211

concordant, but sufficiently so to indicate that the curve
representing photosynthetic activity rises rapidly as the
content of CO, is increased to an air content of from .1
to 1 per cent, and subsequently the rise, if continuous,
is slow to about 10 per cent, after which it may decline.

The amount of CO, in the present atmosphere of the
earth is sufficient for all the needs of plants throughout
imaginable time. It must be assumed, indeed, that this
amount will never be much more or much less than at
present, and that, practically speaking, the forces governing
supply and demand are ultimately somewhat regulatory ;
although there is geological evidence that atmospheric
CO, has not been constant. The result of all animal and
plant respiration (see Respiration, p. 280) is to return to
the air daily an enormous quantity, — an amount esti-
mated for mankind aione to be not less than 50,000,000
tons. The great present consumption of fuel— coal, wood,
oil, etc. — returns to the air several billion tons every year.
A moment’s consideration of the production of coal in
the United States alone during 1907 (400,000,000 tons
- yielding about 2 times this amount of carbon dioxid) is
alone sufficient to indicate the immensity of the quantities
which are involved in these exchanges. This coal repre-
sents in part, of course, the photosynthetic activity of
plants of the carboniferous age. In addition to these
sources of CO, there is also the disintegration of rock
carbonates.

With rapid circulation of air the CO, of the atmosphere
is evenly distributed throughout, and plants, tall and low,
are in situations equally favorable. When, however, the
atmosphere is quiet, there is, especially in rich ground,

o12 Plant Physiology

rapid diffusion of CO, from the soil, due largely to the activ-
ity of microérganisms of the soil. In consequence there
may be a stratum near the soil so much richer in CO, as
to be distinctly advantageous for low-lying or rosette-
forming plants.

115. Light the source of energy. — It has been indicated
that an important feature of the work of chlorophyll
is the absorption of light, or the taking over of energy.
If a beam of sunlight is dispersed by a suitable prism, it is
found to consist of groups of rays of different wave length,
refrangibility, and “ color”’; the beam is thus separated
into the well-known spectrum, the visible portion of which
presents a series of colors as follows: red, orange, yellow,
ereen, blue, indigo, and violet.

If in the path of light there is interposed a weak solution
of chlorophyll in a suitable glass vessel, certain definite
absorption bands appear in this spectrum. There are,
in fact, seven of these, four in the region of red to green
and three beyond the blue, generally rather indistinctly
demarcated, and in strong solutions cutting out the visible
rays beyond the blue (Fig. 56). The four bands in the »
red end of the spectrum are those of the blue-green solu-
tion, cyanophyll; and the most important absorption
band is in the red, corresponding to wave lengths of 650 wz
and thereabout.

A considerable amount of experimental work has been
done to determine the rate of photosynthesis under the
different monochromatic lights. Colored glass screens
and double-walled vessels containing colored liquids have
been much employed. Since such materials seldom afford
pure monochromatic lights, they give only a crude idea of

213

The Intake of Carbon

[poydepy]

‘qAydoroyyo jo uinijzoods uoydiosqy 9g ‘DIT

214 Plant Physiology

the relative effects of light quality. Experiments of this
nature are important, especially when the light employed
is analyzed as to its energy value. It can be shown
that bubbles of oxygen are more rapidly given off, or
starch is more rapidly formed, under red-orange screens
than under green or blue. Red light is therefore a chief
source of the energy used in food-making.

116. Efficiency of the food-making apparatus. — Pains-
taking and brilliant investigations have been made upon
the energy relations of leaves. The work of green plants
is truly remarkable, and it is impressive to consider these
organisms as the noiseless machines engaged in the manu-
facture of all that organic material upon which life depends
—the foremost conservators of the energy derived from

Fic. 57. Ganong’s simple light-screen and aérated box for showing the
necessity of light in starch-making.

The Intake of Carbon 215

sunlight. When, however, it is asked how economic or
efficient is this world-distributed apparatus with respect
to the energy received, one experiences at first a keen dis-
appointment to ascertain that the highest estimates indi-
cate an effectiveness of only 3 per cent, and, according to
other estimates, it may be as low as .5 per cent. Still,
the amount of light absorbed by the leaf is considerable,
and it is important to note the result of this.

In diffuse light the leaf may absorb 95 per cent falling
upon it, while in direct light only about one half is ab-
sorbed, or reflected. In either case much of this absorp-
tion is due to the chlorophyll bodies which have a capacity
of from 10 to 20 times or more the amount effective in
actual photosynthesis. The surplus energy absorbed is
in part operative in raising the temperature of the leaf,
which, according to Blackman, may be in direct sunlight
from 10 to 15° C. higher than that of the surrounding air.
This surplus, of course, induces a more intensive evapora-
tion. Perhaps if we knew more of the physical and chem-
ical changes involved in food-making, this efficiency would
be unchallenged.

116a. Light, intensity and quality. — The relation of
food manufacture to the intensity and quality of light is
most complex. Under favorable conditions of tempera-
ture the working capacity of many plants is proportional
to the increase in light intensity, at least up to the point
where the available CO, is not a limiting factor. Never-
theless, experiments made trom another standpoint indi-
cate that with respect to photosynthesis under field condi-
tions there are shade-loving plants — plants which seem
to be thoroughly attuned to a maximum capacity for

216 Plant Physiology

food-making in weaker light. In this connection, how-
ever, it is possible that an important. factor limiting high
production in intense light is the increased evaporation
then resulting, which would tend to dry out the plant
and induce a closure of the stomata, as well as otherwise
affect photosynthesis. The control of light intensity is
important in crop work, as more particularly discussed
under shading.

116b. Temperature. — Temperature is just as impor-
tant in food-making as in any other physiological process.
According to Blackman the best temperature for sustained
photosynthesis is generally about 25 to 30° C., and this
in spite of the fact that at a temperature of about 10° the
cell-sap may absorb and hold practically twice as much
CO, as at the former temperature. The effect of higher
temperatures upon respiration complicates the heat
relation. It may be expected that plants adapted to
diverse environmental conditions will not respond alike
to heat, especially under field conditions. High tempera-
ture is an important factor in the early maturity of wheat.
The grain then contains relatively little starch, and the
yield of straw is lessened. On the other hand, with ade-
quate soil moisture, corn requires a distinctly higher tem-
perature for abundant starch formation and maximum
yield. |
117. Organic matter, rate of production. — A vigorous
vine of the Concord or Niagara grape may expose to the
light about ro square meters or more of surface. Careful
experiments with other plants indicate that the produc-
tion (taking no account of respiration) per square meter
of surface may be about 1 gram of organic matter per hour,

The Intake of Carbon ZAK

which has been expressed 1 gm?h. This gram of sugar
involves the use of the carbon dioxid contained in 2.5
cubic meters. At the height of the growing season we
may count an average maximum of ten hours of work per
day; therefore, a grape-vine of the dimensions indicated
has the capacity of 10 X 10 = 100 grams per day, equiva-
lent to about 400 grams (about 14 oz.) of fresh substance.
To do this all the carbon dioxid would be taken from 250
cubic meters of air.

Looking at this from the standpoint of a crop per acre,
an impressive though hazy picture may be had of the at-
mospheric changes concerned in the making of organic
material. A yield of 300 bushels of potatoes on an acre
involves, including tops and roots, about 5400 pounds of
water-free substance. Estimating as for making fruit
sugar (2.5 cu. m. or 3.2 cu. yd. per gram) there would be
required all the CO, to a height of more than one and one
third miles over this acre, assuming no gain meanwhile.

LABORATORY WORK

Chloroplasts. — Study under the microscope the distribution
of the chloroplasts in one or more types of leaves available, such
as geranium, ivy, and tomato, using hand sections in all eases.
Contrast one of the above with the distribution in a species of
live-forever, purslane, or Begonia. In the best material study
earefully under high power of the microscope the forms of the
chlorophyll bodies and their cytological relations. In the young
leaves of moss, Elodea, or other convenient material determine
how multiplication of these bodies occurs. Study the form of
the chlorophyll in desmids, procurable either from an aquarium
or any pond containing alge.

Light and the formation of chlorophyll. — Germinate seed of
mustard, radish, or small grain upon moss or in small germi-

218 Plant Physiology

nators or pots, placing the vessels in complete darkness. After
a few days note the color of the cotyledons and leaves, then place
the seedlings in strong diffuse light protected by a bell glass.
Observe the greening and note the time required to develop a
noticeable green appearance. If convenient, some of the seed-
lings may be put in a cold room and some at a much higher tem-
perature, conditions of lighting being the same; or in the same
room the vessels may be circulated in one case with warm, and
in the other case with cold, water. Note the effect of tempera-
ture upon the rapidity of greening. _

Extraction of chlorophyll. — Make an alcoholic solution of
chlorophyll from young leaves of Vicia faba, grass, small cereal,
or radish. Put the leaves into a flask containing 95 per cent
aleohol and heat the flask carefully over a water-bath, the latter
being regulated to about the boiling point of aleohol (ethyl
alcohol, 95 per cent, boils at about 78° C.). When a fairly strong
extract is obtained, pour off the solution into one or more large
tubes and protect from the light until used; but fresh solutions
should be prepared for each period. Make a careful examina-
tion of the solution (1) by reflected light and (2) in diffused or
transmitted light. In the examination by reflected light con-
ecentrate the rays upon the surface of the solution by means of
a hand lens.

Decomposition of chlorophyll in alcoholic solution. — Prepare
four small test-tubes (1 inch in diameter) each with about 15 ee.
of chlorophyll solution recently boiled and cooled. Place one
under each of the following conditions: (1) in direet sunlight;
(2) similar to the preceding, but with the solution covered by a
thin layer of olive oil; (3) in complete darkness; (4) similar
to (2) except in complete darkness. After an hour or two note
any change in color; compare the tubes by holding them above
white paper, returning each to the same condition as before, and
observe after a lapse of about 24 hours.

Separation of the chief pigments of chlorophyll. — Into a test-
tube containing about 20 ec. of the fresh chlorophyll solution
reduced in alcoholic content to about 80 per cent add also 20 ee.
benzole. Note the position taken by the benzole, then shake

The Intake of Carbon 219

vigorously for several minutes, cork, and let stand until the two
areas are constant. Describe the separation phenomena.

If a spectroscope is to be employed, as suggested in the next
experiment, larger quantities of the materials may be used in the
bottle, and a more complete layering effected by the addition of
a small amount of water. In that case the two solutions are
separated by pipetting or by the burette; each is again washed
with the opposite solvent, again separated, and subsequently
employed for a determination of the absorption bands of each.

Spectroscopic examination of chlorophyll. —If practicable,
make, under standard conditions, a careful spectroscopic exami-
nation of a chlorophyll solution of different strengths; also of
the constituents dissolved respectively in benzole and alcohol,
resulting from the separation of the pigments above. Make a
comparison with the living leaf, the latter preferably exhausted
of air and injected with water, by being placed in a vessel of water
under the exhaust or filter pump.

Evolution of gas during photosynthesis. — Notice that when a
mass of alga or aquatic moss, Elodea, or other water-weed is
placed in spring water and exposed to the light, bubbles of gas
promptly accumulate, especially from cut surfaces of the larger
plants, and rise to the surface. No such evolution of bubbles
takes place with control plants in the dark, although a few bubbles
may, of course, gradually form on the walls of vessels or of sub-
mersed plants standing for some time exposed to temperature
changes.

Quantity and nature of gas released in photosynthesis. — Ma-
terials: fresh shoots of a water plant, Elodea or Cabomba;
a battery jar, at least 9 X 5 inches, filled with spring water, or
with water into which a small amount of CO, has been led; a
funnel not more than 3 inches in diameter, with short stem;
a 2-inch test-tube, preferably graduated, and two pieces of glass
tubing of same diameter, — one about 5 inches long, and the other
1 inch; black rubber tubing suitable for attachment to the pre-
ceding; 2 pinch-cocks; a ring stand with clamp; and a netted
wire basket 3 inches high, with cross rods at the top to support
the funnel, all metal being paraffined.

220

Plant Physiology

A

Set up the experi-
ment about as shown
in Figure 58. Ar-
range netted wire
support (C), funnel
(B), and plant in
position. (Why so
much distance be-
tween (B) and walls
of vessel (A)?) The
water in the battery
jar should more than
cover the stem of the
funnel. Arrange the
series of tubes with
rubber connections
and pinch-coecks as
shown, but fill the
series with water and
place the finger over
(D) before inversion
over the funnel.
Support the series
by the clamp. As
the bubbles of gas
come off they will be
caught in (F), dis-
placing water. The
experiment should be
set in fairly bright
light until about two
inches of water are
displaced.

When sufficient
gas is caught, note
the displacement, or

Fic. 58. Apparatus for the determination of mark accurately with

oxygen release in photosynthesis.

a label. The gas

The Intake of Carbon 2K

collected is to be tested as to oxygen content by means of potas-
sium pyrogallate.t Unfortunately this solution would also absorb
another gas, carbon dioxid, so it is necessary to test first for carbon
dioxid. This is accomplished as follows: Clamp pinch-cock (@),
remove the series of tubesfrom the support, and pour out water be-
low (@), invert and fill (#) with a weak solution of potassium hy-
drate (an absorbent of carbon dioxid) until the liquid rises in (D),
then clamp (H) securely, open (G@), and shake so that the COs. in
(F) willbe absorbed. Fill (D) with water, close with a finger, and
again open the series under water to liberate any tension from
absorption of CO,. If there is any change in the volume of gas
in (F), note this, or indicate by a new label. Next, to absorb
the oxygen, proceed in the same manner as before, except that
fresh potassium pyrogallate is to be substituted for the potas-
sium hydrate solution; also the shaking, in order to absorb all
the oxygen, should be continued for several minutes. When the
tension is again released by inversion over water, the difference
between the preceding and present displacement will show the
volume of O, caught during the experiment. Compare this with
the volume per cent of oxygen in the air. As a check on the
accuracy of manipulation, introduce ordinary air into the tube
series, and then determine the volume of oxygen. (If graduated
tube (F) is not available, accurate measurements may be made
after the experiment is complete by filling, from a graduated
pipette or burette, the tube to the points marked with labels
in order to determine the volumes indicated.)

In case the experiment is stopped for any purpose, as on

1The potassium pyrogallate solution now strongly recommended by
Ganong consists of ‘‘1 part pyrogallic acid to 5 parts caustic potash to
30 parts of water.’’ With this quantity of caustic potash 1 gram of the
pyrogallate may absorb + gram of oxygen; but for uncontrolled experi-
ments it is well to figure at the rate of not less than 1 gram of pyrogallic
acid to 7; gram of oxygen. The solution deteriorates rapidly, even in
diffuse light, and should be made up immediately preceding its use.
It is preferably made by using equal parts of two solutions, each con-

taining one of the constituents in double strength.

222 Plant Physiology

account of darkness, but is to be continued later, close pinch-
cock (G) and refill with water the (DE) end of the series, before
continuing the experiment.

Use of the bubble-counting method to show rate of photosynthesis.
—A general idea of the relative rate of photosynthesis under
different conditions may be obtained by counting the number
of bubbles of oxygen evolved in the same space of time.

a. Method. With a rubber band attach a freshly cut sprig
of Elodea or other water plant to a glass rod and submerse in a
large test-tube of water at laboratory temperature, or somewhat
above 20° C. Water from the tap generally contains sufficient
CO;, but in long-continued experiments it may be necessary to
lead in some CO, from a generator. Place the tube as above
prepared in a wire rack in direct sunlight, and after a few minutes
ascertain if the bubbles escape uniformly ; also the average num-
ber given off in a unit of time, say one minute. If the bubbles
do not come off with sufficient uniformity, try another shoot, or
seal the cut end of the stem with wax, and then pierce a hole
through the latter with a small needle.

b. Light intensity. When a eareful count of the bubbles
has been made in direct sunlight, remove the tube to light sue-
cessively weaker; note any change in the rate, and determine
where the evolution of gas ceases. If possible, contrast the light
intensity at this point with that of the open window, as a stand-
ard, using an ordinary photographic actinometer.

c. Temperature. After counting in direct. ight the number
of bubbles given off when employing water at laboratory temper-
ature, transfer the sprig promptly to water brought to a tem-
perature of from 2 to 3° C., but otherwise similar to the preceding.
After allowing a few minutes for adjustment, make observation
upon the rate of O. evolution promptly, before the sunlight has
had an opportunity to raise the temperature appreciably; then
warm the tube gradually to 20 or 25° C. and note the result.

d. If time permits, determine by the bubble-counting method
the rate of photosynthesis under blue and under orange-yellow
screens, employing apparatus described in Chapter XVII.

A simple test for starch. — Make a small quantity of a very

The Intake of Carbon 223

weak starch paste (using a piece of starch as large as a lupin
seed in 10 ce. of water, and boil a few minutes), then add to this
a few drops of an alcoholic solution of iodine and note the intense
blue color. This is a common test for starch. Use the iodine
test in determining if starch is present in leaves of nasturtium,
geranium, or potato, which have been in bright sunlight for a
few hours, as suggested in section 112. First extract the chlo-
rophyll by alcohol, then stain with the iodine solution.

Chlorophyll and photosynthesis (starch accumulation). — The
necessity of chlorophyll in starch-making may be simply shown
by using variegated leaves, white and green, of certain varieties
of Coleus, or other greenhouse plants of this nature conveniently
obtained. From a plant which has been exposed to sunlight
several hours select a leaf, outline the white and green areas, and
then test for starch as above suggested. Indicate the relation
between the occurrence of starch and the areas outlined. For
further proof place the plant in, the dark a few hours or over
night, so that all starch is removed from the leaves, then replace
the plant in light and determine if starch is deposited after a few
hours, and in what areas.

Light and photosynthesis (starch accumulation). —'The obser-
vation that leaves are depleted of starch in the dark is alone suffi-
cient to suggest that no starch has been formed; nevertheless,
it is instructive to determine if starch is formed in a darkened and
aérated area of a leaf, the remaining portion of which is exposed
to light. Employ a Ganong aérated box or light screen (Fig. 57)
or simpler devices similar in principle improvised for the
purpose. After exposing the leaf for a few hours, apply the
starch test and describe the conditions and results.

Obtain two small potted plants, such as Fuchsia, nasturtium,
sunflower, or jewel-weed, which shall have been determined to be
suitable for starch formation and starch removal in the leaves.
Place these in the dark for a few hours or over night, until a test
indicates no starch present. Place one in strong light and the
other in very weak light, with conditions otherwise as nearly
the same as possible. One may be placed under a bell glass and
the other under a bell glass covered with manila paper, or with

224. Plant Physiology

two or three folds of white cloth. Insert thermometers, and
equalize the temperatures as well as possible. At intervals
contrast the rate of starch accumulation in the two eases.

Carbon dioxid and photosynthesis (starch accumulation). —
In this experiment one plant (A) is exposed to a current of air
deprived of COz, and a control (B) to similar conditions, except
that the air is natural. Arrange the experiment preferably in
the greenhouse or in the open, but a south window is also a pos-
sible situation.

Place the plants (Fuchsia is desirable) in the dark over night
and keep them darkened until demanded. Each is covered by
a tubulated bell glass (and with (A) is included a dish of 10 per
cent potassium hydrate solution). Seal jar (A) to a ground
glass or metal base, and cover both with a 2-holed rubber stopper,
one hole serving for a connection with an aspirator or filter pump,
and the other (in A only) connected with potassium hydrate
wash bottles. When connections are tight, draw air through
(A) until a baryta-water wash bottle shows no further CO, in
the chamber. Then draw through both (A) and (B) acurrent of
air for several hours, or as long as the experiment may be con-
tinued in the light, and test the leaves from each plant for starch.

REFERENCES

BuackMaNn, F. F., and Marruer, G. L. C. Experimental Re-
searches in Vegetable Assimilation and Respiration. Proce.
Roy. Soe. 76 B: 402-460, 1905.

Brown, H. T., and Escomssr, F. Statice Diffusion of Gases and
Liquids in Relation to the Assimilation of Carbon [ete.].
Phil. Trans. Roy. Soe. 193 B: 223-291, 1900.

Brown, H. T.,and Witson, W. E. On the Thermal Emissivity
of a Green Leaf in Still and Moving Air. Proc. Roy. Soe.
76 B: 122-137, 1905.

Brown, H. T., and Escomse, F. Physiological Processes of
Green Leaves. Proce. Roy. Soe. 76 B: 29-111, 1905.

CzapEk, F. Die Ernihrungsphysiologie der Pflanzen seit 1896.
Progressus Rei Botanier. 1: 419-532 [468-477].

The Intake of Carbon 225

Hansen, A. Die Ernahrung der Pflanzen. 299 pp., 79 figs, 1898.

KrevusterR, U. Ueber die Methode zur Beobachtung der
Assimilation und Athmung der Pflanzen [ete.]. Land-
wirthsch. Jahrb. 14: 913-965, 1885; 16: 711-755, 1887;
17: 161-175, 1888; 19: 649-668, 1890.

MarcHuewski, L. Die Chemie der Chlorophyll. 187 pp.,
7 pls., 5 figs., 1909.

Scuuncxk, C. A. The Xanthophyll Group of Yellow Coloring
Matters. Proc. Roy. Soe. 72 : 165-176, 2 pls., 1903.
Senn, G. Die Gestalts- und Lageveranderung der Pflanzen-

Chromatophoren. 397 pp., figs., 9 pls., 1908.
TIMIRIAZEFF, C. The Cosmical Function of the Green Plant.
Proce. Roy. Soe. 72: 424-461, 1903.

Texts. Barnes, Ganong, Jost, Pfeffer.

CHAPTER X

THE RELATION TO NITROGEN

THE significance of the nitrogen content of soils has been
recognized deservedly as so important in crop production
that a stupendous number of investigations has been
directed toward securing the facts regarding the diverse
relations of this nutrient. Many of these investigations
have yielded data of surprising interest respecting the use
of nitrogen by higher plants and by microérganisms as
well. Furthermore, the results have been sufficient to
demonstrate most clearly the intimate relations existing
between soil bacteria and cultivated crops as regards the
nitrogen supply. With phosphoric acid and potash it
constitutes the trio of nutrients which experience has de-
manded as usually the most important fertilizers for crop
production.

118. Combined nitrogen.— The water-culture experi-
ments (sections 78-80) have demonstrated in a manner
sufficiently convincing that the nutrient solutions for
higher plants must contain nitrogen. As soon as the
supply of this element in the seed is fairly exhausted the
addition of combined nitrogen is required. About the
middle of the last century Boussingault showed conelu-
sively that the N, of the air, unlike the CO,, is not directly
serviceable as a source from which nitrogenous foods

226

The Relation to Nitrogen 224.

may be made by the green plant. The air contains about
78 per cent of “free” nitrogen, but this vast source is
wholly inert naturally except, as later indicated, either
through the intermediary of certain microdrganisms or
by means of electric discharges, whereby this free nitrogen
is combined. Nevertheless, since the original rocks seem
to contain no nitrogen, the air is the original source of all
that at present found in arable soils — constituting often
from .1 to .3 per cent of the dry weight of the soil.

119. The nitrogen content of plants. — Nitrogen
enters into a variety of organic compounds among which
the proteins are of the greatest importance, for these in
turn are apparently the main constituents of protoplasm,
whether living or dead. Some other compounds, occur-
ring in plants, which contain nitrogen are various amino
and amido acids, certain alkaloids, and also nitrates.
The protein content is greatest in seeds, storage organs,
and meristematic tissues. In beans and other legumi-
nous plants it may amount to 25 per cent of the dry weight,
while in wheat straw it constitutes only about 3.5 per cent.

120. Synthesis of nitrogenous bodies. — Animals ob-
tain nitrogen, for the most part, as protein foods, furnished,
of course, by the bodies or products of other animals or
plants. On the contrary, the rule is that green plants and
many fungi and bacteria are able ultimately to construct
amido compounds, proteins, and other nitrogenous bodies
from certain of the raw materials; that is, from some
of the mineral nutrients and photosynthates (carbohy-
drates).

Proteins represent an immense group of compounds
(sections 147-148) with a relatively enormous molecule.

228 Plant Physiology

They must contain carbon, hydrogen, oxygen, and nitro-
gen; many contain sulfur, certain forms phosphorus,
and others apparently mineral bases, the latter either
combined or as ash. Proteins may be regarded as per-
fect foods for protoplasm.

Lack of knowledge respecting the proteins renders
impossible a clear picture of their synthesis, although
much may be inferred from the decomposition products,
which have been extensively studied. Speaking generally,
the amides (containing the group NH,) seem to represent
products intermediate between certain of the raw mate-
rials (organic acids or carbohydrates and nitrates) and
the proteins. Glycin, for a simple example, is an amido-
acetic acid [CH.(NH.)* COOH] in which the amide group
replaces one H of the acid. It may be assumed that pro-
tein is constructed from amido compounds, especially
from those derived from carbohydrates, some of which
may be further modified by the incorporation of sulfur
from sulfates, and others by the introduction of phos-
phorus from phosphates. :

121. Soil nitrogen. — Some data regarding the nitro-
gen relations of higher plants have been exhibited in con-
nection with the discussion of mineral nutrients. On
account of the unusual importance of the nitrogen supply
in any permanent system of agriculture, special attention
should be given to the interesting transformations of ni-
trogenous materials in the soil, and likewise the building
up of nitrogenous bodies within the plant require some
consideration.

It is now a matter of common knowledge that nitro-
gen exists in the ordinary arable soils in a variety of com-

The Relation to Nitrogen 229

binations. Aside from the Ne of the air there may be
undecomposed organic matter, containing most of the com-
pounds of the plants and animals which it represents.
There is also the converted organic matter, that resulting
from decay,— commonly designated humus, — which may
consist, in fact, of a variety of substances. Finally there
is the inorganic nitrogen, including nitrates, often a small
proportion of nitrites, and compounds of ammonia. The
total nitrogen content of the soil is therefore most diverse,
but in productive agricultural soils there is invariably
a considerable nitrate content during the growing season.

It is probable that under exceptional conditions higher
plants may use to a certain extent organic nitrogen in the
form of amido compounds, but, practically speaking, it
seems certain that such organic bodies are not absorbed
or utilized in sufficient quantity to make this question one
of importance.

122. Nitrites. — The nitrites are commonly injurious,
and the presence of these in any quantity is a sure indica-
tion of unfavorable conditions. As will be shown sub-
sequently, they occur as temporary products during the
oxidation of ammonia to nitrates, but may be looked upon
under favorable conditions as merely transitory.

123. Nitrates. By means of water cultures it is
relatively a simple matter to determine that nitrates are
the most favorable source of nitrogen in water or sand
cultures under normal conditions, and more especially
under sterile conditions. It is certain that such com-
pounds are usually absorbed by the plant unchanged.
Nitrates of the various nontoxic bases are therefore val-
uable direct fertilizers, and the data furnished by the

230 Plant Physiology

extensive fertility experiments throughout the world lead
to the conclusion that the maintenance of an adequate
nitrate supply in the soil, or of conditions leading to the
transformation of nitrogenous matter into nitrates, is an
important principle in production.

The nitrates are readily soluble, and while this character
enhances the rapid action of such substances as fertilizers,
it is at the same time a quality making possible constant
loss through percolation and leaching. It is evident,
therefore, that unless the nitrate supply is maintained by
natural or artificial means, exhaustion of this requisite
element would sooner or later occur. Fortunately, there
are both natural means as well as conditions of cropping
which may suffice to maintain and to increase the nitrogen
content, as developed later.

124. Compounds of ammonium. — Prior to the latter
quarter of the last century the prevailing view was to the

Fic. 59. Fertilization of grass land, all plats given muriate of. potash
and acid phosphate ; also, from left to right, no nitrogen, } nitrogen
ration, and full nitrogen ration. [Photograph from the Rhode Island
Exp. Sta.]

effect that compounds of ammonium (such as the sulfate,
chlorid, and nitrate) should be considered the important
sources of nitrogen for plants, and the weight of Liebeg’s
opinion was upon it. At that time the cycle of changes
involving nitrogen was incompletely known. Unques-

The Relation to Nitrogen 251

tionably the addition of ammonium salts to the soil gave
increased yields, and the inference was that they were
directly beneficial; that is, that they were absorbed as
salts of ammonium.

Subsequently, when it was determined that, as a result
of nitrification, ammonium compounds may be oxidized,
ultimately to nitrates, the dominant view was to regard
nitrates as practically the only source of nitrogen for crops.
This is still held by many, but the relatively recent exper-
iments of Mazé,! Hutchinson and Miller,? and others
seem to indicate that salts of ammonium are directly
absorbed. In some cases nitrogen in the latter form
afforded growth equal to that where nitrates were em-
ployed, and the nitrogen content of peas is reported greater
when ammonia is the source of nitrogen. Nevertheless,
the salts of ammonium are more toxic than nitrates, this
toxicity exhibiting itself at the lower concentrations merely
in depressing the growth of roots.

125. The sources of soil nitrates and ammonia. —
Briefly stated, the supply of nitrates and ammonium com-
pounds in the soil annually removed by crops, by leach-
ing, and through denitrification (section 130) are or may
be renewed by the following means, most of which are
subsequently discussed : —

(1) By the decomposition or decay of organic matter,
accomplished by microérganisms.

(2) By means of the bacteria producing and inhabiting
the root-tubercles of leguminous plants, which bacteria
possess the power to “‘ fix’ atmospheric nitrogen.

1 Mazé, P., Ann. de l’inst. Pasteur, 14: 23-45, 1898.

2 Hutchinson, H. B., and Miller, N. H. J., Journ. Agl. Sci., 3: 179-
194, 1909.

232 Plant Physiology

(3) By the action of certain soil bacteria and fungi
which are also able to utilize atmospheric nitrogen.

(4) By the ammonia returned to the soil as a result of
rainfall; but since this, in general, is that which escapes
from the soil into the air, it is negligible.

(5) As a result of electrical discharges nitrous and nitric
acids may be produced in the air and through rains brought
to the soil, but this amount is relatively inconsiderable,
consisting, under the most favorable conditions (in the
moist tropics), of about five pounds per acre annually.

126. Ammonification.— The remains of plants and
animals are “‘ returned ”’ to the soil through processes of
decay and putrefaction brought about largely by means
of fungi and bacteria. Decay is a relative term usually
implying decomposition without the production of mal-
odorous compounds, and commonly taking place with
access of oxygen. In putrefaction ill-smelling compounds
result, usually from the decomposition of nitrogenous
substances, taking place, as a rule, with poor oxygenation.

A result of both of the above processes is that nitrog-
enous compounds are broken down into ammonia, carbon
dioxid, and other products. This reduction to ammonia
constitutes what is known as ammonification. Under
favorable conditions a large part of the ammonia is held
in the soil by entering into combinations with the soil
bases. During this decomposition many substances more
or less injurious may be at least temporarily set free in
the soil. :

Numerous species of bacteria and fungi affect decom-
position. Bacteria are particularly important in arable
soils, especially such species as Bacillus mycoides and B.

ih it el i ee le

dial tt ple

The Relation to Nitrogen 200

vulgaris, while many common molds, punks, and mush-
rooms are among the various fungi inducing decay in the
forest.

127. Nitrification. — Ammonification is the comple-
tion of the first stage in the cycle of changes whereby ni-
trogenous matter may be converted to nitrates, the suc-
ceeding transformations being as follows : —

(1) Oxidation of the salts of ammonium into nitrites.

(2) Oxidation of nitrites into nitrates.

The production of nitrates (ultimately) from organic
matter has been long known and practically carried out
by means of the niter beds so much employed a generation
or two ago. A process wholly similar in nature has given
rise to the natural deposits of niter and is constantly at
work in the best arable soils to develop nitrates.

128. Nitrifying organisms.—In 1877 it was first
determined (Schloesing and Miintz) that nitrification is
effected by bacterial agencies, and Winogradski, Waring-
ton, Godlewski, and others have laid bare many impor-
tant features affecting the action of the organisms involved.
These bacteria are widely distributed in soils and in drain-
age or run-off waters.

The organisms oxidizing ammonia to nitrites (nitrite
organisms) are small, oval, motile cells generally included
in the genus Nitrosomonas, while those oxidizing nitrites
(nitrate organisms) are considered nonmotile and included
in the genus Nitrobacter.

These two types of bacteria are commonly associated
in the soil, and all forms seem to exhibit the peculiar
physiological quality of being able to make their own
carbohydrate food from CO, and water. Unlike green

234 Plant Physiology

plants, they accomplish this in the absence of light (chemo-
synthetically).

In cultivated soils they are most abundant below the
surface mulch, and down to the limits of frequent culture.
The following table shows the distribution of nitrifying
bacteria and nitrates at different depths in a Dakota
soil 1 : —

COLONIES OF NITRIFYING NITRATES, PouNDs
Som Deprn BAcTERIA PER ACRE

3 in. 2300 415.9

6 in. 2300 415.9
12 in. 600 234.3
18 in. 200 196.6
24 in. 10 430.9
36 in. 0 674.9
48 in. 0 316.3
60 in. 0 395.3
72 in. 0 247.0
84 in. 0 293.9

129. Conditions favoring nitrification. — The general
conditions favorable for nitrification in soils are good
aération (oxygen and carbon dioxid), a medium water-
content, a soil temperature not to exceed 40° C., the
presence of a basic compound such as calcium carbonate,
and the absence of much soluble organic matter and free
ammonia. In general these conditions are those of good
sanitation and tilth, a judicious application of lime, and
such a rotation of crops as will produce and maintain the
best of soil conditions. The advantage of paying the

1 Ladd, E. F., North Dakota Agl. Exp. Sta., Bul. 47: pp. 685-704.

The Relation to Nitrogen 239

closest attention to those conditions resulting in the high-
est nitrification is obvious.

As a rule nitrate formation in the soil begins rapidly in
the spring and with most crops a maximum is reached
during the first half of the growing season; subsequently
there is a fall in the nitrate content which may approach
a minimum in the late fall, or with the maturity of the crop.
Recent studies upon the relation of crops to the nitrate
content have developed a number of interesting views which,
however, may not be discussed in this place.

130. Denitrification. — Almost the counterpart of nitri-
fication is the process exhibited by many micro-organisms
of reducing nitrates and nitrites, known as denitrification.
From an agricultural standpoint the most serious case is
that of the reduction of nitrates and nitrites with the forma-
tion of free nitrogen, and the consequent loss to the soil
of combined nitrogen. In many cases, however, the reduc-
tion is not carried so far as to form free nitrogen.

A large number of organisms are able to accomplish
nitrate reduction, both bacteria and fungi; but the condi-
tions under which denitrification occurs are not usually
those developed under the best agricultural practices.

Aside from the presence of necessary nitrates this reduc-
tion requires the presence of considerable soluble organic
matter and poor aération. Many of the organisms which
induce denitrification are aérobic forms which in the pres-
ence of sufficient oxygen show no tendency toward nitrate
reduction. It is apparent that saturation of the soil with
water after heavy manuring may actually result in nitro-
gen loss and frequently also in the production of in-
jurious compounds in the soil.

236 Plant Physiology

131. Nitrogen fixation. — Since by nitrification (in-
cluding ammonification) nitrogenous bodies are merely
transformed into inorganic nitrogen, this does not increase
the total nitrogen of the soil. Moreover, the nitrogen
brought to the soil as a result of electrical discharges is a
small amount. It is then apparent that the loss of com-
bined nitrogen over the surface of the earth through the
washing away of sewage, the leaching of soils, and the
liberation of free nitrogen in denitrification would mean
in time a nitrogen famine. There is, however, a more than
compensating process of nitrogen fixation.

132. Organisms which fix free nitrogen. — Since the
classical researches of Hellriegel and Wilfarth, there has
accumulated a vast array of facts and observations with
respect to the fixation of free nitrogen by micro-organisms.
The réle played by the bacteria of the leguminous tuber-
cles was the first to be clearly demonstrated, but the im-
portance of certain saprophytic soil bacteria in the process
of nitrogen accumulation was fully recognized a short
time later. Strikingly little has been said in agricultural
publications regarding the rdle which may be played by
fungi in this process. Nevertheless, as a result of a series
of observations and experiments, it is now commonly
held that certain fungi are likewise important in fixation,
and this view is regarded in the succeeding discussion,
although there is some doubt respecting the real impor-
tance of the fungi in this connection.

133. Bacteria of leguminous tubercles. — In recent

1 The evidence now commonly accepted is to the effect that certain
bacteria and fungi alone among all organisms possess the capacity for
nitrogen fixation.

The Relation to Nitrogen 251

Fic. 60. Roots of vetch with clusters of fan-shaped tubercles.

238 Plant Physiology

times no plant structures have, perhaps, attracted more
attention than the nodules, or tubercles, of the leguminous

KY.

YS
y,

SX
a aie
is
ff

a
CAI

OO LH A
yeaa

SS OO OS
Nees eGlaee
it ess

Fic. 61. Infection thread and abnormal tissue in a tubercle developing
upon the root of Vicia sativa.

At the right, infection thread in root-
hair. [After Geo. F. Atkinson.]

plants. These structures are abnormal growths resulting

from the attacks of parasitic bacteria, Figures 60 and 61.

The Relation to Nitrogen 239

Parasites generally make little or no return to the hosts
in which or upon which they live. These nodule bacteria,
Pseudomonas radicicola, are exceptions to this rule. The
tubercles are, in fact, root colonies of the micro-organisms.
The bacteria get their carbon, minerals, and water from
the host, yet ultimately they give to the host in return
combined nitrogen which has been acquired by the fixa-
tion of the free nitrogen of the air.

In the earliest days of historic agriculture, it was known
that leguminous plants benefit the land for succeeding
crops. The methods by which benefit results were, of
course, unknown. When it was ascertained that the chief
benefit is concerned with the accumulation of nitrogen,
it was assumed that these legumes and other plants might
themselves be able to assimilate atmospheric nitrogen.
Boussingault’s experiments clearly demonstrated the incor-
rectness of this view. His work was convincing, and
finally attention was directed to the tubercles of legumi-
nous plants as the cause of nitrogen accumulation.

Hellriegel and Wilfarth demonstrated that on sterile
soil no tubercles are present and no nitrogen is fixed.
The complete chemical and biological studies which sub-
sequently followed have led to a full confirmation of the
work of Hellriegel and his associate. It is now a simple
matter to determine that leguminous plants growing in
the absence of the bacteria are wholly dependent upon the
soil supply of combined nitrogen, whereas, in the presence
of the proper bacteria, such plants are able to reach normal
development with a deficient soil supply of nitrogen, and
even to give to such deficient soil, through root decay, an
increased nitrogen content.

240 Plant Physiology

The bacteria producing the tubercles seem to have
acquired at least racial specialization, so that no one form
of the organism will
infect all leguminous
plants. The introduc-
tion of a_ particular
legume into a region in
which that plant (or a
closely related species) -
has not been grown
may necessitate, for
best results, “‘ inocula-
tion ”’ of the soil or of

the seed employed.
Fic. 62. Bacterioids from legume tu- Formerly the organ-

bercles: Melilotus alba (1), Medicago . F

sativa (2, 3, and 5), and Vicia villosa (4). 18M Was introduced

[After Harrison and Barlow.] by importing soil
from a locality in which the legumes had been grown.
This method has many disadvantages, and at the present
time a very thorough test is being made of the practicabil-
ity of employing pure cultures of the organism desired.
Good results have been secured with pure cultures in many
cases, but in some particulars the method is still in the
experimental stage.

134. Certain saprophytic soil bacteria. — Evidence
that saprophytic soil bacteria are able to fix free nitrogen
was afforded when Berthelot found in 1885 that bare soil
with its normal population of micro-organisms may con-
siderably increase in nitrogen content over and above
that added through rainfall. At the same time, no increase
occurred upon eliminating micro-organisms by steaming.

The Relation to Nitrogen 241

Fic. 63. Crimson clover inoculated (background) arfd uninoculated
(foreground). [Photograph by J. F. Duggar.]

R

242 Plant Physiology

Ten years later Winogradski isolated Clostridium
Pasteurianum, an organism which proved to be capable
of fixing free nitrogen when grown in the absence of air
(oxygen), also similarly capacitated in the presence of air
when associated with other bacteria utilizing free oxygen.
Since that time much work has been done. Several other
species of soil bacteria having the power of fixation have
been isolated, these latter being included under the genus
Azotobacter. They are common and important in arable
soils containing a relatively small amount of combined
nitrogen; moreover, their activity is enhanced by the
presence of considerable lime and by general fertility as
regards the other mineral nutrients. From the majority
of experiments thus far reported it does not appear that
the addition to the soil of cultures of these organisms has
occasioned increased nitrogen fixation.

135. Fungi. — The investigation of nitrogen fixation
by fungi has yielded many data of interest, although there
is some conflicting evidence. Fixation of nitrogen by
fungi was reported as early as 1862, but the more impor-
tant work has been done since 1895. Among several
saprophytic and parasitic species employed, Saida secured
maximum fixation with cultures of Phoma Bete, a fungus
normally parasitic upon sugar-beets.

Several observers have reported fixation for a few of the
common molds of soils and decaying vegetation, including
Aspergillus niger and Penicillium glaucum. In all cases
the organisms were grown in pure cultures, either in the
absence of combined nitrogen, or in the presence of very
small quantities of such compounds. In no case does the
amount of nitrogen fixed amount to more than a few
milligrams.

———————e SS

The Relation to Nitrogen 243

136. Mycorhizal fungi.— Ternetz has isolated and
tested a fungus from the roots of certain heaths. The
association of root and fungus is known as mycorhiza.
This fungus proved to be a species of Phoma, and several
strains of it were found to show a high capacity for nitro-
gen fixation. In fact, while the total amount of nitrogen
fixed in a given period of time is relatively small, the fixa-

Fic. 64.. Mycorhiza of the orchid Corallorhiza, also cells showing the
hyphe. [After M. B. Thomas.]

tion per gram of dextrose used by the fungus indicates that
in pure cultures fixation is more economic than with any
other organisms yet investigated. The following table
affords a comparison of the efficiency of several strains of
this fungus as compared with certain soil bacteria : —
The Phoma discussed above develops an endophytic
mycorhiza, penetrating the cells. This endophytic form

244 Plant Physiology

|
|

DEXTROSE| NITRO- Nirsages
FIx. PER
TIME IN FUR- |GEN FIxa-
ORGANISM GRAM DEx-
Days NISHED, |TION, MIL-
TROSE, MILLI-
GRAMS | LIGRAMS
GRAMS
Clostridium Pasteurianum. . 20 40 53.6 1.34
Clostridium Pasteurianum. . 20 20 24.4 1.22
Clostridium Americanum . . 30 1:25 4.6 3.7
Clostridium Americanum . . 30 5 8.2 3.01
Azotobacter chro6dcoccum . . ahr 5 42.7 8.56
Azotobacter chrodcoccum . .|, 35 ee 127.9 10.66
Phoma radicis Oxycocci . . 28 rd 15.3 18.08
Phoma radicis Andromedze ; 28 7 4a 10.92
Phoma radicis Vaccinii ee 28 7 15.7 22.41

of mycorhiza occurs also in several orchids and in a few
other plants. Many common forest trees, such as beech
and pine, likewise exhibit mycorhiza. In the latter case
the fungus invests the root with a mycelial weft, the threads
merely coming in close contact with the cells (ectophytic).
These fungi are believed to be not only of importance to
the tree as absorbing organs for water and nutrient salts,
but possibly in the fixation of nitrogen. Nevertheless,
this point has not been established.

137. General sources of supply of nitrogen. — From
the data presented it is apparent that the nitrogen problem
in plant production is one of peculiar interest and diver-
sity. Commercial sources of nitrogen for plant production
may be natural supplies of nitrates, compounds of am-
monium (chiefly the sulfate, as a by-product of coke and
gas making), waste and prepared animal products, and
green-manure crops (especially legumes). In addition,
good conditions for fixation by bacteria and fungi and

The Relation to Nitrogen 245

rotation with legumes are supplementary means of nitro-
gen restoration. Finally, electric fixation is a source of
supply.

138. Electric fixation of nitrogen. — In recent years
several methods have been devised for the oxidation of
atmospheric nitrogen. These methods involve the use of
cheap power, since high-power electric currents are neces-
sary. Those which are now important in the production
of materials commercially valuable as fertilizers require
also a cheap source of lime. The two methods referred
to are known respectively as the Birkeland-Eyde process
and the calcium carbide process. By the former a basic
lime nitrate is produced, and by the latter a lime-nitrogen
consisting of calcium cyanamid and calcium cyanide. The
first mentioned product, which is much employed in north-
ern Germany, is a direct fertilizer, whereas the latter must
first undergo decomposition in the soil. The cyanamid
is more easily handled. the basic nitrate being strongly
hydroscopic. 3

LABORATORY WORK

Ammonification.1— The decomposition of protein with for-
mation of ammonia may be demonstrated by the action of certain
bacteria upon egg albumen. Prepare a solution containing about
2 grams of egg albumen in 50 ee. of water, and to prevent coagu-
lation add 50 ee. of .05 ferrous sulfate. Pour about 10 ee. of
the solution into each of several test-tubes, sterilize for 1 hour
at 100° C., cool, and inoculate some of these with a pure culture
of Bacillus mycoides, or some other organism reported to possess

1 For more experiments upon ammonification, nitrification, and
related phenomena the student should consult especially Percival’s
“* Agricultural Bacteriology,” 1910.

246 Plant Physiology

the power of ammonification. Place the cultures at a tempera-
ture of 28 to 30° C., and in two weeks test the inoculated and
uninoculated tubes for ammonia with Nessler’s solution.

The Nessler solution is prepared by dissolving 2 grams of
potassium iodide in 5 ee. of hot water, while warm add mereuriec
iodide to excess of solution, cool, dilute with water to 25 ee.,
shake, settle, filter, and then dilute the filtrate to 50 ce. with a
concentrated solution of caustic potash. This solution assumes
a yellow color when there is added to it a few drops of a solution
containing ammonia.

Nitrification. — A crude but simple demonstration of nitri-
fication phenomena, usually successfully carried out, may be
made with impure cultures as follows:

Prepare a solution containing —

Ammonium sulfate . gram
Dipotassium phosphate . gram
Sodium chlorid 2. gram
Magnesium sulfate 2 gram
Ferrous ‘sulfate... |\. —-s< ike cvar ol hot
W 8bOF fees se Beer) Gar See oe ee

Weigh out .5 gram of basic magnesium carbonate into each of
four small Erlenmeyer flasks, add to each 50 ce. of the above
salt solution, plug with cotton, and sterilize. When cool inocu-
late three of the flasks with a small quantity (about .1 gram)
of garden loam taken 5 or 6 inches below the surface of the soil.
Save the fourth flask as a control. Place all at a temperature
of 28 to 380° C. Once a week remove from the inoculated flasks
from 3 to 5 ee. of solution and make the following tests : —

1. For ammonia. Employ Nessler’s solution.

2. For nitrites. Acidulate with sulfuric acid about 2 ec. of
the solution, add a few drops of potassium iodide and starch
paste. If nitrates are present the starch is colored blue from the
reduced iodine.

3. For nitrates. When from (2) it is evident that nitrites
are no longer present, dissolve a erystal of diphenylamine in
about 1 ec. of sulfuric acid in an evaporating dish. The addi-

The Relation to Nitrogen 247

tion of a drop or two of the culture solution will give in the
presence of nitrates (no nitrites being present to give the same
reaction) a blue-violet color. At the close of the experiment
test thoroughly the control in order to determine if the ammo-
nium salt has remained unchanged.

Denitrification. —'To about 100 cc. of prepared nutrient
bouillon (consult any bacteriology) add 3 grams of sodium ni-
trate, and pour about 10 cc. of the solution into each of several
test-tubes. Inoculate the tubes from a pure culture of some
denitrifying organism, such as Bacillus denitrificans, or each
tube with about .5 gram of fresh cow manure. Place the tubes
at a temperature of from 30 to 35° C. Note all the changes in
appearance of the culture and test occasionally for nitrates as in
the preceding experiment. Discuss the results.

Root tubercles. — Study the root tubercles of several legumi-
nous plants, such as vetch, red clover, and alfalfa, with special
reference to the form and distribution of the nodules.

Examine prepared slides to determine the distribution of the
organism within the tissues. If prepared slides are not at hand,
make sections, stain in gentian violet, counterstain with orange
G., locate the band-like colonies of the organism, and note the
general conditions of the tissue modifications.

Crush a bit of tissue from the inner portion of the nodule on a
clean cover glass, dry, and stain several hours in dilute gentian
violet; then rinse the cover glass, dry, and mount in balsam.
This affords a satisfactory preparation for an examination of the
organism (Fig. 62).

If facilities are at hand, determine the necessity of the bacteria
for tubercle production and contrast the growth of certain leg-
umes in inoculated and uninoculated soil. The experiment can
only be relative unless much care is taken with sterilization
precautions. The materials needed are six pots of fairly poor
soil, a pure culture of the root tubercle bacteria, seed of the legume
host plant, and a .2 per cent solution of formaldehyde. Steam
the pots of soil 2 hours, disinfect the seed by soaking 1 hour in
the formaldehyde solution, then sow a few seed in each pot.
Inoculate three of the pots with the pure culture and leave three

248 Plant Physiology

as controls. Place the pots in the greenhouse upon fresh cinders,
separating the two lots and protecting the pots as well as practi- —
cable from contamination by dust or soil from other pots or beds.
Water with distilled water only. After the plants have grown
sufficiently, compare the two lots, as indicated, for nodule for-
mation and amount of growth.

REFERENCES

ATKINSON, G. F.* Contribution to the Biology of the Organism
causing Leguminous Tubercles. Bot. Gaz. 18: 157-166;
226-237 ; 257-266, 1893, 4 pls.

Atwater, W. O. On the Acquisition of Atmospherie Nitrogen
by Plants. Amer. Chem. Jr. 6: 365-388, 1885.

Beyerinck, M. W. Die Bakterien der Papilionaceen-Knéll-
chen. Bot. Zeit. 46: 723-735; 741-750; 757-771;
781-790; 797-804, 1 pl., 1888.

Harrison, F. C., and Bartow, B. The Nodule Organism of the
Leguminose. Its Isolation, Cultivation, Identification, and
Commercial Application. Cent. f. Bakt. II] Abt. 19:
264-272 ; 426-441, 9 pls., 1907.

HELuLRIEGEL, H., and Witrartn, H. Untersuchungen iiber die
Stickstoffnahrung der Gramineen und Leguminosen. - Zeit.
des Vereins fiir Riibenzucker Industrie. Beilageheft, 234 pp.,
1888. [Abs. in Biedermann’s Central. 18: 179-185, 1889.]

Kina, F. H., and Wuirson, A. R. Development and Distribu-
tion of Nitrates in Cultivated Soils. Wisconsin Agl. Exp.
Sta. Bul. 93: 39 pp., 6 figs., 1902.

Lipman, J. G. Report of the Soil Chemist and Bacteriologist.
N. J. Agl. Exp. Station Report for (1907) : 139-204 pp.,
3 pls., 1908.

Lurz, L. Les Microorganismes, Fixateurs d’azote Disserta-
tion. (1904): 187 pp., 19 figs., 1904.

Petrce, J. G. The Root-tuberecles of Bur Clover and of Some
Other Leguminous Plants. Proce. of the California Acad.
of Sciences. (3d Ser.) 2: 295-328, 1 pl., 1902.

PrazMowskI, ADAM. Die Wurzelkndéllchen der Erbse. Landw.

The Relation to Nitrogen 249

Versuchsstation. 37: 161-238, 2 pls., 1890; 38: 5-62,
1891.

Rossi, Gino pe’. Ueber die Mikroorganismen, welche die
Wurzelknollchen der Leguminosen erzeugen. Cent. f.
Bakt. IJ Abt. 18: 289-314; 481-489, 2 pls., 1907.

Saipa, K. Ueber Assimilation freien Stickstoffes durch Schim-
melpilze. Ber. d. deut. bot. Ges. 19: (107)—(115), 1901.

TERNETZ, CHARLOTTE. Uber die Assimilation des atmosphiar-
ischen Stickstoffes durch Pilze. Jahrb. f. wiss. Bot. 44:
353-408, 1907.

VoorHEsgs, E. B., and Lipman, J. G. A Review of Investiga-
tions in Soil Bacteriology. Office of Expt. Stations, U. S.
Dept. of Agl. Bul. 194: 108 pp., 1907.

Warp, H. M. On the Tubercular Swellings on the Roots of
Vicia Faba. Phil. Trans. Roy. Soe. Lond. 178: 539-562,
2 10a ote d

Woronin, M. Uber die bei der Schwarzerle (Alnus glutinosa)
und der gewohnlichen Gartenlupine (Lupinus mutabilis)
auftretenden Wurzelausschwellungen. Mem. de 1 Acad.
Imp. de St. Petersbourg. [7] 10: 1-13 pp., 2 pls., 1866.

Texts. Barnes, Jost, MacDougal, Pfeffer, Peirce.

CHAPTER XI

PRODUCTS OF METABOLISM; DIGESTION
AND TRANSLOCATION

From the discussion of the general relations of the green
plant to carbon and nitrogen it has been developed that
a variety of organic products are characteristic of the plant
cell and plant body. Beginning, in the typical case, with
those which may be regarded as the products of photo-
synthesis (photosynthates), on the one hand, and with
the elementary organic substances containing nitrogen,
on the other, there may be built up in various ways diverse
series of organic compounds, constituting the plant body
and including, of course, the protoplasm and the cell
walls. This shall not be taken, however, to indicate that
there is a normal sequence of products, or a continuous
building up, for, as will be shown subsequently, the build-
ing up may be interrupted at any point and breaking down
may occur. The food products are then utilized in a
manner dependent upon the specific nature of the sub-
stances and upon the chemical requirements of the cell.
Pfeffer has distinguished plastic and aplastic substances.
The former term is used by him to include substances
which may be used as food, which may be mobilized and
utilized in metabolism; while aplastic substances include
the solid and permanent constituents of the cell, — such
as the cell wall, — and certain by-products or waste ma-

250

Metabolism; Digestion and Translocation 251

terials. It is difficult, of course, to draw any line between
these two types of substances.

139. Metabolism. — All of those chemical changes
which take place within the body incident to growth and
development are commonly included under the term “ me-
tabolism.’”’ These changes may be constructive (or ana-
bolic) and destructive (or catabolic). Some brief indica-
tions have been given respecting the building up of a few of
the more important organic compounds, and it is necessary
to include now a somewhat comprehensive view of the
general relations of a few of the foods and by-products
and some characteristics of these materials.

140. Temporary foods, storage products, and perma-
nent structures. — It would seem that many substances
produced within the cell are temporary, that is, they may
be labile compounds readily used in the metabolism of
the active cell. If formaldehyde is a first product of photo-
synthesis, it is necessarily one of this nature. Naturally
the transient compounds are the lesser known as plant
constituents; but it seems certain that many simple car-
bohydrates, fatty acids, amides, and the like are distinctly
temporary. Nevertheless, a substance which is temporary
in one plant may be accumulated in another.

Whenever the food manufactured is in excess of that
used, it accumulates, and may be regarded as a storage
product. The chlorenchyma of higher plants is a tem-
porary storage structure, for starch or other substances
may accumulate in the cells of this tissue during photo-
synthesis. Specialized storage structures are extremely
common among higher plants, and to such organs the
food in diffusible form is transported.

202 Plant Physiology

It has been noted that certain mineral constituents
migrate from old organs to the seed. In an analogous
manner organic products are accumulated. Storage may
also occur in bulbs, tubers, or aérial stems, roots, leaves,
and fruits. It is, of course, such natural storage organs
that have been seized upon particularly as food for man and
feeding stuffs for animals, and many of the plants possess-
ing these have been wonderfully improved through selec-
tion and breeding.

A storage organ of available food-material has a phylo-
genetic reason for existence in the fact that it has to do
with subsequent growth and fruiting or with the propa-
gation of the species. Seeds, bulbs, tubers, and other such
structures are essentially propagative devices, and it is
not uncommon to find that they possess the capacity to
lie dormant for a period, or to withstand desiccation.

141. Annuals, biennials, and perennials. — These
terms are used to imply one or more seasons of growth.
When an annual plant, like the oat, reaches maturity by
gradual, natural means, a very large part of the mobi-
lizable carbohydrate and protein material is deposited or
accumulated in the fruit or seed. The work of the plant
as a whole is done, and there is no great wastefulness of
readily used substance in the dead tissues. In the ease
of a biennial plant, such as the sugar-beet, which has grown
for one season, the leaves have died, but the root has be-
come an organ for a great accumulation of sugar and other
food-materials. The next season by virtue of this stored
food the energies of the plant may be vigorously directed
toward the production and maturity of seed.

The potato uses up during the early part of the season

Metabolism; Digestion and Translocation 253
not only the starch of the “ seed tuber,” but also practi-
cally all of the starch which is made daily by the leaves, in
the production of stem and leaf structures — additional
starch-making surfaces. The growth of new tubers
develops slowly at first, but finally the energy of leafy
shoot “ vegetation ’’ wanes, and then a considerable sur-
plus of carbohydrate is accumulated as starch in the
tubers. In consequence, at maturity about 80 per cent
of the dry matter of the tuber is starch.

When a tree ceases to make food-material in the fall,
there may be little or there may be much starch already
accumulated. A peach tree, for example, heavily laden
with young fruit in July may make each day a considerable
quantity of starch. The latter may be found by the
usual test applied to the leaves. The starch, however, is
in considerable part used every day to furnish the carbo-
hydrate used in the building of wood, in the making of
fruit, and ultimately in respiration, so that only when the
fruit is becoming ripe and the development of new wood
is checked may there be a surplus of starch to accu-
mulate in trunk and branches. After the ripening of the
fruit much more starch may be made and accumulated
in the twigs as a reserve for the young growth of another
season. Such accumulations of food-material are indis-
pensable, for in the peach thousands of blossoms are
produced and the fruit set before the leaves are unfolded,
a result of using food-materials that represent the work
of the previous season.

Many varieties of apple do not ripen until after the
leaves fall, and it is possible that this holding and sustain-
ing of the fruit so long (meanwhile using stored food-

254 Plant Physiology

material) may be a factor in the apparent tendency to-
ward biennial fruit production, that is, to a greater
production in alternate years. It stands to reason
that a too heavy yield of fruit one season may have
some effect upon the crop of the next year, although
this tendency may be offset to a considerable extent by
good culture, fertilization, and favorable season.

142. Carbohydrates. — The carbohydrates include the
sugars, starch, cellulose, and many of the other compounds
containing carbon, hydrogen, and oxygen. They consti-
tute the bulk of food-materials in general. In these sub-
stances the molecule contains hydrogen and oxygen in
the proportion of 2:1, or as in water; and the number
of carbon atoms is usually six or a multiple of six, but in
some compounds five. The following are some important
classes of these compounds: —

(1) Monosaccharids (sugars), C,H ».O,, including glu-
cose, fructose, and galactose.

(2) Disaccharids (sugars), CyH»..0;,, including sucrose
(cane sugar), lactose (milk sugar), and maltose.

(3) Polysaccharids or amyloses, n (CsgH,)O;), including
such compounds as starch, inulin, dextrin, glycogen, and
cellulose. :

Briefly stated, the disaccharids and polysaccharids are
for the most part readily convertible into monosaccharids
through hydrolysis. This may be accomplished by boil-
ing with acids, and also through the action of certain other
compounds of metabolism, the enzymes, subsequently
discussed. The transformation of cane-sugar may be
represented as follows : —

Cy2H20y + H2O = CyHy205 + CeH20s,

)
|

—

a

Metabolism; Digestion and Translocation 255

that is, producing dextrose and levulose. The hydrolysis
of the polysaccharids may involve a series of changes in
which water is taken up and ultimately n molecules of
hexose sugar (or sugars) are split off.

143. Sugars. — The commoner forms of sugar found
in plants are sucrose (cane-sugar), glucose (dextrose),
and fructose (levulose). Glucose and fructose are ap-
parently important products in the metabolism of cells
usually, but these compounds are often promptly used
in general metabolism, especially in the building up of
other products. There may be no accumulation of them
in the plants where they are being constantly manufac-
tured. Accumulation does occur, however, particularly
in ripening fruits, such as the grape, peach, prune, and
date. In the raisin the characteristic brown nodules of
this sugar may be seen. Indirectly these sugars are of
much commercial value, for sweetness and flavor together
determine the prices paid for fruits, prices which are in
general far above their actual food value.

The monosaccharids are reducing sugars, precipitating
heavy metals from solutions of their salts upon heating.
Maltose also possesses this quality, but cane-sugar does
not. A standard test for reducing sugars is obtained by
Fehling’s solution (see Laboratory work), consisting of
copper sulfate in an alkaline solution of potassium sodium
tartrate. Upon boiling with this solution a brick red
precipitate of cuprous oxide is produced.

Sucrose is the form in which sugar commonly accumu-
lates in plant cells. From the stems of the sugar cane and
from the root of the sugar-beet there were extracted during
1909 nearly 15,000,000 tons of commercial sugar. The

956 Plant Physiology

juices of selected races and strains of the two plants indi-
cated may contain from 14 to 18 per cent of sugar, and the
history of the breeding of the highly productive races of
the sugar-beet is of special physiological interest. Sor-
ghum and.a few species of tropical palms also contain
cane-sugar in sufficient quantity to be of commercial im-
portance. Sugar maple (Acer saccharum) growing in
northern latitudes yields a sap which in the usual time of
cupping (late winter or early spring) may contain from
2 to 5 per cent of cane-sugar, besides other substances
imparting the peculiar flavor for which this sugar is prized.

144. Starches.— The great majority of green plants
produce starch (Fig. 65). There are some exceptions
among several orders of flowering plants, especially certain
monocotyledons (Sect. 112); and certain groups of alge
do not possess this capacity, notably the families of blue-
greens and browns.

The starch molecule is very complex and difficult of ex-
act study, but the occurrence and reaction of the starches
are well known. Starch occurs in the form of insoluble
grains with a characteristic general appearance, varying
considerably, however, in form, size, and markings in the
different plants in which produced. The grains may be
simple, semicompound, or compound. Very large grains,
often so large as to be visible to the unaided eye, are found
in the root-stock of Canna; those of potato are of medium
size; in rice the components of the compound grains are
small and numerous; while in spinach they may be ex-
tremely minute, and according to Nageli as many as 30,000
may be united together.

Starch grains are produced within plastids, — chloro-

Metabolism; Digestion and Translocation 257

plasts and amyloplasts (leucoplasts),— and the exact
method of formation is imperfectly understood. It is a
general belief that starch
is formed from glucose!

and under the influence of
one or more enzymes.
The latter are believed to
be active in starch pro-
duction, as a rule, when
carbohydrates exist in the
cell in considerable excess
of use.

Schimper considers the
starch grain to be a
spherite made up of a
multitude of needle-like
crystals radiating from
the center. The striated appearance commonly ex-
hibited seems to be due to difference in nutrition during
the formation, and the eccentric arrangement of the mor-
phological center may be due to the development of the
grain near the periphery of the plastid.

Starchy products constitute a very large portion of the
food of man and of domestic animals, so that many prod-
ucts are valuable chiefly from the high starch content.
In passing from colder to warmer regions some of the more
important starch-producing plants are the following:
the small cereals, buckwheat, corn, beans, potatoes, sweet
potatoes and yams, cassava, rice, yautia, arrow-root,

Starch grains of various
forms.

1 The view is also current that the starch molecule is split off from
protein material of the plastid.
Ss

258 Plant Physvology

sago, tapioca, bread-fruit, banana, and many other vege-
tables and fruits. Reckoned in per cent of dry weight,
the potato tuber contains a starch content of about 80
per cent, while corn and many cereals may contain 60
per cent or more. According to Konig starch contains on
the average about 15 per cent water, 1 per cent nitrogenous
bodies, and generally much less than 1 per cent of ash.

Upon hydrolysis starch yields, as subsequently shown,
first dextrins, then maltose, and this is ultimately trans-
formed to glucose, although some of the dextrin (about
one fifth) is more resistant to hydrolysis.

Inulin, an amylase less complex than starch, is char-
acteristic of the tuberous roots of Dahlia and of some other
composites, although occurring also in other plants. It
is dissolved in the cell-sap, but may be crystallized out as
spherites. These crystals are soluble in hot water. It
yields fructose on hydrolysis.

145. Cellulose. — The cell-walls consist in large part
of a substance which passes under the general name of
cellulose n(C,H,,O,). Cell-walls are frequently impreg-
nated with gummy, metallic, or other substances; this
is the usual case with epidermis, cork, wood, and the like.
Nevertheless, some form of cellulose forms a large per cent
of the walls of flowering plants.

The celluloses proper resist hydrolysis with weak acids,
and except at the time the cell-walls are being laid down
they are unimportant in metabolism. Hemicelluloses are
forms which are readily hydrolyzed, yielding monosac-
charids other than glucose. They constitute, in fact,
the reserve cellulose deposited upon the cell-walls in the
endosperm of many seed and some other storage organs,

Metabolism; Digestion and Translocation 259

especially in the seeds of palms. This reserve food be-
comes available during germination.

146. Fats and oils. — Fats and oils are far more com-
mon and important constituents of plants than is popu-
larly supposed. In Liliacee and a few other mono-
cotyledonous orders oils replace starch as the first visible
photosynthetic product. Oily bodies occur in active cells
often as small droplets in the cytoplasm. In a variety of
seeds the amount of fatty substances is considerable, a
part occurring as globules or as crystals.

Among the more important fats and oils may be men-
tioned those from corn, coconut, various palms, olives,
mustard, poppies, flax, castor-bean, Bergamot-orange,
carnation, Brazil-nut, cotton, etc. Thousands of tons of
palm and coconut oil are annually imported into Europe
and constitute an important item of trade. The value
of the cotton-seed oil produced during 1909 and 1910 is
estimated to have been upwards of $300,000,000.

Oils and fats may be identified by comparatively simple
tests and they are obtained for commercial purposes by
crushing and pressing, or by extraction.

147. Proteins. — The vegetable proteins are numerous,
and they vary greatly in physical and chemical character-
istics. ‘They may occur in solution in the cell vacuoles,
partially dissolved in intimate association with the pro-
toplasm, and as solid forms — crystals or granules. The
latter occur especially in storage organs or tissues with
reduced water-content, usually associated with carbohy-
drate storage products, oil, and other substances, as in
many legumes.

The vegetable proteins have been studied more particu-

260 Plant Physiology

larly in the seed, so that the storage forms are better
known. The aleurone grains of the endosperm of cereals
are familiar protein
bodies, and in this
case they are found
abundantly in the
outer layer of the
endosperm, while
starch is more abun-
dant within.

Fic. 66. Endosperm cell (A) of Ricinus in The gluten of

water; aleurone grains (B) in olive oil; wheat consists of a

protein crystal (k) and globoids. (g). [After variable mixture of
Strasburger. ]

proteins. Flour may
contain about 10 per cent of this material and about
70 per cent of starch. The gluten is readily separated
from the starch in a proximate manner by kneading the
flour under water in a thin cotton bag. In this manner
the flour is filtered out and the gummy nitrogenous sub-
stance remains in the bag. Hard wheats are particularly
valuable in the manufacture of products like macaroni
(alsoin bread-making) as a result of the relatively high
content and composition of the gluten.

148. Classes of proteins. — A system of classification
of the proteins in keeping with that of other chemical sub-
stances (the molecular structure of which is better known)
is now impracticable. Classification is based largely upon
solubility under certain standard conditions. Three
principal groups of proteins are recognized, namely, (1)
simple, (2) conjugate, and (3) derived.

The simple vegetable proteids include albumins, some

Metabolism; Digestion and Translocation 261

of which occur in seeds and in cell-sap, such as Jeucosin of
many cereals, legumelin of legumes, and ricin of the castor-
bean. Such proteins are generally soluble in water.
Globulins are soluble in salt solutions, but practically in-
soluble in pure water; such are legumin of many legumes,
amandin of nuts, tuberin of the potato, and many others.

The conjugate proteins are so named because of the
apparent association of two substances in the molecule
(nucleic acid and protein in the case of nucleo-proteins)
or of the ready separation of the molecule into these two
substances. The nucleo-proteins are of much importance
as constituents of nuclei.

Derived proteins, such as the proteoses and peptones,
are considered more particularly under digestion, and
may be regarded as the digested and diffusible products
demanded by the cell for direct use in assimilation.

149. Amides. — Amides are also well known in plant tis-
sues. Among these asparagin is of frequent occurrence.
As arule these compounds are not a storage form of nitro-
gen. ‘They are commonly produced, and may accumulate
to a considerable extent, during germination; from 10 to
30 per cent of the nitrogen in this form is not infrequent
at that time. Leguminose may contain these com-
pounds in exceptional amount, 75 per cent of the total
nitrogen in vetch during germination being thus reported
by Schultze. It may be regarded as a degradation prod-
uct of protein, a product which is readily diffused, and
again used.. |

150. Organic acids. — Organic acids are common con-
stituents of plant juices. They may occur free or com-
bined with mineral bases. As a result of the presence of

262 Plant Physiology

free acids and acid salts the cell-sap may be acid in
reaction.

These substances may be looked upon usually as the
by-products of metabolism, but they may be serviceable
and some may function further in general metabolism.
The group of fatty acids is well represented among the
compounds in plant tissues; but the acids of commoner
occurrence are the related oxalic, malic, tartaric, and citric,
all being oxidation products of glycols (dihydroxy-deriva-
tives of the paraffins).

Oxalic acid is of widespread occurrence, and it is most
familiar (in the form of calcium oxalate) as the raphides
or needle-shaped crystals so common in many vegetative
organs. Malic acid is well known in many unripe poma-
ceous and stone fruits, but it occurs far more commonly,
especially in ‘‘ fatty ’’ plants like the stone-crops; and it
is the substance found by Pfeffer and others to be chiefly
responsible for the “ attraction’ directing motile sperms
to the egg-cells of certain ferns. Tartaric acid is readily
extracted from the grape, in which fruit it occurs as acid
potassium tartrate. Citric acid may constitute from
6 to 7 per cent of the juice of lemon, and it is also more
abundant in the other species of Citrus than in higher
plants generally.

The production of acids is usually favored by the abun-
dance of soluble carbohydrates in the tissues. Submersed
in a solution of glucose, for instance, the leaves of Oxalis
rapidly increase in acidity. Among the lower plants,
fungi and bacteria, the production of organic acid is even
more common than with the higher plants, as again referred
to under fermentation phenomena.

Metabolism; Digestion and Translocation 263

151. Tannins.— The tannins are bitter, astringent,
water-soluble, amorphous substances widely distributed in
the leaves, bark, and fruits of plants. All of these sub-
stances, which are of commercial importance, may be em-
ployed in the process of tanning skins and hides, since they
form insoluble compounds with various nitrogenous bodies,
giving a toughness and durability to the skin which con-
stitutes the differences between leather and natural skin.

The tannins are alike in certain physical and chemical
properties, but there may be dissimilarity in chemical
composition. The tannin (glucoside) used in the making
of leather is usually derived from the bark of various trees,
including that of hemlock and oak, so extensively employed
in the United States. The bark of hemlock may yield
from 8 to 10 per cent of its dry weight of tannin and the
leaves of tea may contain as much as 15 per cent.

Tannin is also extensively used as a mordant in the
process of dyeing, for it produces colored products with
various dye-stuffs; and it has long been employed in the
manufacture of ink. The characteristic purplish brown
color of the trunk of the cork oak from which the bark
has been removed is due to this sudstance. The chief
source of tannic acid (digallic acid) is a gall-nut produced
upon an oak (Quercus infectoria), a product obtained for
the most part from Turkey. Tannic acid constitutes
more than one half of the dry weight of this gall. A
similar product is yielded by a gall upon the sumac, Rhus
semi-alata, which occurs in China.

Upon heating with sulfuric acid, tannic acid is
hydrolyzed, yielding two molecules of gallic acid, thus,
C,,.H,,0. + H,O = 2C,H,O,;. | This’ process is also ac-

264 Plant Physiology

complished naturally by means of a few fungi, especially
Aspergillus niger.

152. Resins and turpentine.—A great variety of
preducts of physiological interest and of commercial im-
portance are included in the groups commonly called
resins and turpentine. They are produced in the cortex
and young wood of a variety of plants generally charac-
terized by special ducts or canals formed in connection
with the conduction of these products.

The conifers furnish the chief commercial supply, and
they constitute an important economic item in many of
the coniferous forests of Europe and America. For a long
time the balsams, especially the Canada balsam, have been
a product of northern forests, whereas the turpentine in-
dustry has been best developed in the Southern States.
According to Mayr a cubic meter of the splint wood of the
standing tree contains approximately the amounts of
fresh resins named, of which turpentine oil constitutes a
considerable percentage, as follows : —

RESINS TURPENTINE OIL
FPanua ‘silvestris >. 7". Soa A ae | 60.0
Larix europea oir ete Mata oe 18.3 33.1
Piers excelag. io <". VG re.08 9.4 38.2

Pine alas os 4. Reo ape een wae 32.4

It is thus evident that the hemlock, which is poorest in
solid resins, contains a very large per cent of the product
as turpentine oil. The resins belong to the terpene series,
but they occur along with various acids and other com-

Metabolism; Digestion and Translocation

pounds. Turpentine
oil contains a large
per cent of various
volatile oils. The
common method of
turpentine orchard-
ing; has resulted in a
great loss of timber
due to the severe in-
jury to the tree from
the boxing and
chipping employed.
Methods have been
suggested whereby
this injury is now re-
duced to a minimum

Giiteen 6 7-).. The
crude products ob-
tained by the

method indicated are
distilled, the volatile
spirits being con-
.densed, constituting
turpentine, while the
nonvolatile products
are the solid rosins
of commerce.

153. Digestion. —
The seed and the
tuber are effective
propagative devices,

Fie.

67. Turpentine orcharding.
[After Forest Service.]

265

266 Plant Physiology

because of the fact that they are at the same time storage
structures. When, subsequently, conditions become
favorable for the growth of the seedling or of the sprout,
the seed or tuber is exhausted of its stored substances,
which again move to the growing organs.

The starch and many other reserve foods stored in the
tuber or in the endosperm of the seed are insoluble and
indiffusible. It has already been indicated that for storage
purposes solid or indiffusible forms may be necessary or
economical. Nevertheless, such reserve foods, or the sub-
stances from which they were formed, enter the storage
cells as diffusible products, and in such forms only can they
find exit. The process of rendering organic materials
soluble and diffusible, that they may be used in the cell
or transferred to other cells and organs, or used in the
building up of new substances, is digestion. It is a cata-
bolic or breaking-down process, and the special nature of
the changes involved is as diverse as the products acted
upon.

From the preceding it is evident that when starch or
any other insoluble food product is formed in the cells
which are actively engaged in photosynthetic work, these
products must undergo digestion before use or removal.

154. Digestion in different organisms. — Digestion in
any cell or organ, in the animal or in the plant, is the same
in principle. It is generally accomplished or accelerated
by means of certain nitrogenous bodies or enzymes (in-
cluded among catalytic agents) secreted by the protoplasm
of the storage cells or the cells in the vicinity. In the ver-
tebrate animal, digestion is effected through the secretion
of digestive enzymes which enter the alimentary tract,

Metabolism; Digestion and Translocation 267

for the food-stuffs must be made soluble prior to direct
absorption by the cells of the body. The parasitic fungus
or bacterium may be able to dissolve and to penetrate the
cell-wall. Then upon entering the cell the fungus may also
gradually ‘“ appropriate’ or digest the starch and other
foods, absorbing them in this case, as does the higher ani-
mal, after digestion. It matters not what the organism
may be which digests starch, the method is the same,
and it is dependent upon the ability of the organism under
the conditions to produce and often to secrete the starch-
digesting or the starch-splitting enzymes. The same
applies to other solid or indiffusible food substances, so that
in general it may be said the use of all such substances
as food is a factor of the specific digestive capacity of the
organism, however simple or elaborate the digestive
apparatus may be.

After all, the whole phenomenon of nutrition of even the
green plant is not essentially different from that of the
animal. The green plant makes its carbohydrate foods
in certain cells, and it builds up nitrogenous substances out
of these and inorganic nitrogen ; but once sugar and nitrog-
enous bodies are formed, nutrition follows a course com-
parable in the two.

155. Enzymes and enzyme action.— The enzymes
or soluble ferments are doubtless exceedingly numerous,
and possibly the digestive enzymes alone are almost as
many as the different kinds of reserve foods. So far as
the study of these substances has progressed, they seem
to be, for the most part, of protein character. At any
- rate, they are precipitated with proteins, yet certain
analyses of the purified products disclose no nitrogen con-

268 Plant Physiology

tent, and much doubt is entertained respecting their pre-
cise nature. In general, however, they are regarded as
protein. They show noteworthy differences among
themselves, with regard to solubility, conditions of precipi-
tation, and the like.

The enzymes are products of protoplasmic activity
and are not generally regarded as readily diffusible; that
is to say, the work of many of these is primarily within the
cell where they may be produced. These are the intra-
cellular enzymes. Nevertheless, in a number of cases
there may be specialized secretory cells of some impor-
tance; and in other cases the digestion, or partial diges-
tion, of products prior to absorption, is an indispensable
character, as in the case of fungi and higher animals gen-
erally. Those enzymes which are active in part without,
or beyond, the limits of cells producing them are termed
extracellular enzymes.

The hydrolysis or decomposition of organic bodies like
protein, starch, and fat under laboratory conditions (other
than by the use of enzymes) is effected only by means of
fairly strong acids, high temperatures, and other intensive
agents. Contrariwise, the enzymes effect hydration and
decomposition under the conditions of the plant cell or
body, although their activity frequently reaches a maxi-
mum at 40° C. or slightly above.

A great majority of the commoner enzymes act by
hydration; thus the effect of invertase upon cane-sugar
is as follows : —

CpyH2On + H:O = CgHi20¢6 + CeH20c.
Sucrose Dextrose Levulose

On the other hand, certain classes of enzymes, not here

Metabolism; Digestion and Translocation 269

particularly considered, are believed to act by oxidation,
and simple, molecular decomposition may also occur.
The products may be diverse, as is common, or alike, as
when maltose is transformed into two molecules of
dextrose.

From relatively recent work it has also become certain
that enzymes are important in synthetical processes as
well as in the analytical ways referred to. In the former
case they are said to possess a reversible action; that is,
for example, — contrary to the instance above cited, where
cane-sugar may be hydrated with the production of hex-
oses, — the hexose molecules may be built up by means of
an enzyme into the anhydride or disaccharid form. Re-
versible actions appear to be very common, but little
definite information is available at the present time.

Direct sunlight is promptly injurious to enzyme action.
Fermentation is also commonly weakened at temperatures
above 50° C. and ‘‘ death” may result above 70° C.
Most toxic agents are injurious to enzyme action at con-
centrations much above the normal death-point of the pro-
toplasm, but at considerable dilution acids or alkalies may
be stimulating. While a weak percentage of alcohol and
a saturated solution of chloroform may not be injurious,
strong alcohol may be fatal, so that in the precipitation of
enzymes by 95 per cent alcohol there may be danger of
losing the product.

156. Carbohydrate enzymes and their products. —
Of the many carbohydrate enzymes it is possible here
briefly to consider only a few. Chief in importance among
these are diastases (amylases), acting upon starch, the
hydrolysis and splitting of which yields a series of dextrins,

270 Plant Physiology

and finally maltose — which is subsequently converted
by the enzyme glucase into glucose.

The diastases are widespread, and two forms are dis-
tinguished accord-
ing to the type of
corrosion of the
starch — graim
Diastase of trans-
location occurs es-
pecially in the
chlorenchyma of
leaves, and it cor-
rodes the grain

almost evenly.

Fic. 68. Corrosion of starch grains by dias- ])jgstase of secre-
tase of secretion. [After Strasburger.]

tion, corroding the
grain irregularly (Fig. 68), is that which occurs in storage
organs generally, but especially in seeds. Apparently a
third form, takadiastase, is the product of Aspergillus
Oryze in its action upon wheat or rice starch. Allied to
diastase is inulase, converting inulin into fructose.

Cytase is a ferment often associated with diastase, as
in the endosperm. It may be important in the dissolution
of a certain “ shell”’ of the starch grain. It is, however,
best known from its action on the reserve cellulose, con-
verting this into hexose sugars. It is probable that
several enzymes are required in the decomposition of other
celluloses, about which very little is known.

It has been indicated incidentally that the disaccharids
sucrose and maltose are hydrolyzed and yield hexoses by
invertase and glucase respectively. These enzymes are

Metabolism; Digestion and Translocation 271

widely distributed in both higher and lower plants. Both
these and the diastases are most important in the produc-
tion of malt used in brewing and distilling. The diastases
are also important medicinally as an aid to amylose diges-
tion, and many patented forms are on the market.

Another enzyme of special interest requiring further
study is pectase, a form important in the hydrolysis of a
portion of the cell-wall, producing a jelly from the pectic
compounds.

157. Protein enzymes. — Protein enzymes were among
the first to receive attention, and they have been more
completely studied in the animal organism, where the
action of pepsin in the stomach and that of trypsin
received into the intestine from the pancreas are well under-
stood. These enzymes must occur in plants, or else
others which serve the same purpose.

Through ferments many proteins are converted into the
more diffusible proteoses and peptones; while tryptic
ferments may give a more complete digestion, reducing
the peptone to the readily diffusible amido and amino
acids, such as leucin and asparagin. Ferments which
have been regarded as tryptic have been known for some
time in plants, such as papain from the papaw, and bro-
melin from the pineapple. From recent work it appears
that some of the so-called tryptic ferments may be, in fact,
combinations of peptic and ereptic ferments. The last-
named class is found by Vines to be well distributed in
plants, and it decomposes the peptones with the pro-
duction of amido and amino acids.

Doubtless many of the protein enzymes in plants are
intracellular. The carnivorous plants, such as the sun-

272 Plant Physiology

dew and Nepenthes, secrete enzymes which act from with-
out the absorbing organs, and likewise the fungi produce
enzymes at least slowly diffusible. The exact method
of action of the protein enzymes is not yet clear, but it is
generally assumed to be hydrolytic. It is necessary that
protein enzymes be produced in plants in some quantity
at the time of germination to effect the movement and use
of stored products, for, as has been shown already, indif-
fusible protein compounds are common in such organs.
Moreover, at the maturity of the plant or of any vegeta-
tive organs of the plant much of the solid protein and pro-
toplasmic material is converted and accumulated in the
seed, and in the recovery of this from organs which have
ceased to grow there is, of course, much economy.

158. Conduction of digested foods. — From what has
been said regarding the action of digestivé enzymes, it is
apparent that there is required an effective means of trans-
location, as may be demanded, for digested materials to
and from the leaves, shoots, storage organs, and seeds.
Diffusible organic substances in complex plants seem to
require, then, for rapid diffusion to and from active organs
specialized paths or tissues. This demand is met through
the phloém of the vascular bundles, in which the sieve
tubes occur (Fig. 6,7,0). The large protoplasmic connec-
tions between the rows of sieve cells evidently permit a
movement more rapid than simple diffusion.

The sieve tubes are important in the movement of such
products as shown by direct and indirect evidence: thus,
by the fact that sieve tubes in particular contain a quan-
tity of simple organic substances; by the absence of such
products in so great a quantity in xylem, pith, or cortex;

Metabolism; Digestion and Translocation 273

by the interruption of conduction upon the removal of
the phloém; and by the failure of the loss of cortex to
affect directly this movement of organic substances.

159. Ringing. — In horticultural practice ringing is ap-
plied to the removal of a small band of bark encircling the
stem of dicotyledonous plants so as to include the cortex
and phloém. Sometimes there is made merely a circum-
ferential cut through to the wood ring, or a small wire is
bound tightly about a limb so as to cut into the bark,
but these latter may heal too quickly to effect the result
desired.

The principle is evident. Ringing interrupts the more
rapid movement of digested foods toward the roots or
basal parts to the detriment and often to the ultimate
death of these structures ; but there is an accumulation of
foods above the ring, and this may be favorable for the
developing fruit. This operation may result in a consider-
able increase in the size of the shoot, or in the production
of tumors, above the incision.

Ringing is reported a widespread practice in Europe
with grapes and apples, and it is employed to some extent
in the United States. It should be used with caution where
the plant is expected to serve future usefulness. It may,
however, increase or incite productivity, hasten ripening,
or enhance the size and quality of fruit. In the latitude
of New York the grape is generally ringed during late
June. The place of ringing should be between the chief
fruiting canes and the main vine, but the exact location
will be determined by the system of training. There
should be such a development of canes below the ring

as to fairly well nourish the main vine and root system.
T

274 Plant Physiology

LABORATORY WORK

Starch. — Examine under the microscope and describe the
starch grains from a variety of sources, such as Canna (root-
stock), potato (tuber), rice or oat kernel, milky juice of Euphor-
bia, and seed of beet. Leucoplasts associated with starch grains
may be best observed in fixed and stained material, but they
are also visible without staining in such favorable material as
the young shoots of Canna, or in the young root-stocks of various
monocotyledons.

Rub up about 1 gram of starch with a small quantity of water
in an evaporating dish, and when there are no more lumps dilute
to 50 ce. Is starch soluble in cold water? Heat the preceding
to the boiling point, and when a paste is formed, examine it
microscopically with respect to solubility.

With the paste above prepared, and with a weak alcoholic
solution of iodine, make a complete test of the iodine reaction.
In small test-tubes first use a few drops of a strong paste and
considerable iodine solution, then weaken the paste up toward
a dilution of one hundred times, using also less or weaker iodine.
Determine the effect of heating and recooling, also of a few drops
of strong caustic potash, upon the iodine reaction. Compare
the reactions of the starch paste toward iodine with that of a
suspension of starch in cold water. )

Study the distribution of starch in any plant available, em-
ploying sections, especially from fleshy roots, leaves, ete. Deter-
mine where in the resting twigs of apple, lilac, or maple the
storage of starch occurs. In order to stain starch occurring in
small quantities in the tissues, especially in the cells of leaves,
as of Elodea grown in weak light, or to bring out the stareh in
chlorophyll bodies, the material may be stained in a concentrated
solution of iodine in potassium iodide, when the grains stand out
black. Again, a dilute solution of iodine in potassium iodide
may be used, and after washing, the material may be laid in a
strong solution of chloral hydrate which dissolves most of the
cell-contents, swells the stained grains, and in time decomposes
these last also, so that a prompt examination must be given.

Metabolism; Digestion and Translocation 275

Inulin. — Make and examine sections (mounted in alcohol)
from small pieces of the tuberous roots of Dahlia which have
lain for a week or 10 days in strong alcohol. Describe the
spherites observed. Treat the sections with cold water and ex-
amine, then treat with hot water, and discuss solubility.

Glucose and other sugars. — The most decisive test for glucose,
other reducing sugars, and certain glucosides is the precipitation
of cuprous oxide in Fehling’s solution. The identification of the
different sugars or other substances may require other tests.

Prepare Fehling’s solution using two bottles as follows: A,
34.6 grams of pure crystals of copper sulfate dissolved in distilled
water and made up to 500 ce.; 8B, 173 grams of Rochelle salt
(potassium sodium tartrate) and 60 grams of sodium hydrate
dissolved in distilled water to make 500 ce. In employing this
test use always equal quantities of the two solutions.

Add to some Fehling’s solution in a test-tube a small granule
of glucose or a few drops of a strong solution. Boil the solution
for three minutes, and describe the reaction. In the same manner
test the juice of ripe grapes, or ripe plums or peaches. In this
case note the rapidity of the reaction, to contrast with a later
test of beet juice.

Use the Fehling’s solution with a few erystals of cane-sugar.
Is there any reduction? Boil the cane-sugar previously with a
few drops of hydrochloric acid, neutralize with KOH (to litmus),
and then repeat the Fehling’s test. Discuss. Press out some
juice of the sugar-beet, or grate up a small amount, and extract
with water; then test this juice (or extract) with Fehling’s
solution, being careful to heat gently, since violent heating will,
through other substances present, be alone sufficient to convert
cane-sugar. Compare the result with that obtained when grape
or peach is employed. Compare the reaction of crystals of cane-
sugar and granules of glucose in a few drops of concentrated
sulfurie acid.

Celluloses. — Crack a seed of date, make a section or shaving
of the endosperm, and study the preparation with respect to
the reserve cellulose deposited upon the eell-walls, describing
accurately the nature of the cells in which such deposits oceur.

276 Plant Physiology

Test the solubility of cellulose (cotton fibers) in concentrated
sulfuric acid and in cuprammonia. In the first case use small
quantities of the materials, rub up in a Syracuse watch glass,
when dissolved neutralize with KOH and test for reducing sugar
(glucose).

With half-concentrated sulfuric acid determine the length of
treatment required to yield a blue color with iodine.

Place cotton fibers, sections of a root-tip, ete., in a solution of
ehloriodide of zine (dissolve chloriodide of zine in less than its
weight of water and add metallic iodine until a bright cherry
color is produced). Place the material in the concentrated solu-
tion, examine under the microscope, and describe the character-
istic color reaction.

Fats and oils. — The fats and oils are generally soluble in
ether, chloroform, benzene, and other solvents of this nature,
and certain oils (castor oil) in absolute alcohol. Examine
sections of the endosperm of castor-bean and of the garden bean
in water; then after immersion for a few minutes in absolute
alcohol and ether, reéxamine. Stain similar sections from a few
minutes to half an hour with a 50 per cent alcoholic solution of
eyanin (or in a solution of aleanna in absolute alcohol, then
diluted to 50 per cent) and note the deep color of the oily bodies.

Proteins. — Proteins may be soluble in water, in salt solutions,
in alcohol, and in acids and alkalies. Make sections, or shave
off with the razor fragments from the endosperm of wheat,
mount in water and examine for ‘‘aleurone ”’ grains (not soluble
in water) in the outer layer particularly. In the same way
examine sections of the endosperm of castor-bean for protein
erystalloids and globoids, preferably after removing the oil (in
this case) by immersion for a few minutes in absolute alcohol.

Mix some wheat flour and water, place in a cloth bag and
knead under a stream of water at the faucet. The glutinous
dough resulting after the starch is washed out is the gluten of
wheat, consisting of a mixture of protein substances some of
which, in the living cells, are indistinguishable from the cyto-
plasm. Take a small portion of this gluten, rub it up with a
2 per cent salt solution, and save for later study. Test the sol-

Metabolism; Digestion and Translocation 277

ubility of another portion of the gluten in 70 per cent alcohol
and save the solution. Use a part of this solution for an obser-
vation upon coagulation by diluting the part taken to three times
the volume with distilled water.

Grind up three grams of beans extracted with 30 cc. of water
in a test-tube, shake occasionally for 10 minutes, filter, and
employ the filtrate along with the preceding solutions in some
of the tests given below. A portion of this filtrate, however,
may be tested as to coagulation in two ways: (a) acidulate a
small amount in the test-tube and apply heat; (6) add to a
few cubic centimeters in a test-tube four times the volume of
95 per cent alcohol.

The following are some reactions of proteins which are in
part distinctive : —

1. Brick or rose-red color with Millon’s reagent on standing,
or with gentle heat. [To prepare Millon’s reagent dissolve 1
gram of mercury in 2 grams of nitric acid (1.42 s. g.), and then
dilute with twice the volume of water.]

2. The Biuret reaction, a violet or purple color with copper
sulfate and sodium hydrate. To a very weak copper sulfate
solution add excess of potassium hydrate and apply heat, then
add a small amount of a protein solution and heat again.

3. Yellow color on boiling with nitric acid, the xanthoproteic
reaction. After boiling, cool under the faucet and add ammonia,
when the color will change to orange.

4. Violet color with acetic and sulfuric acid. Use 2 parts
glacial acetic and 1 part sulfuric acid, with a small amount of
protein material, and apply gentle heat.

Starch digestion. — Several forms of diastase may be obtained
as commercial products, but it is well to undertake the extraction
of one or more of these in the laboratory. Take 200 grams of
clean barley seed, soak in running water over night, and germi-
nate in a thick layer over moss until the roots are about 1 inch
long and the plumules well started. Dry at 40 or 50° C. for
about six hours. From this malt the diastase of secretion is to be
obtained. In the same way collect (preferably an hour or two
at least after sunset) about 200 grams of nasturtium, bean, or

278 Plant Physiology

potato leaves and dry in the same manner. These will yield
diastase of translocation (ungerminated seeds of barley likewise).
Grind the products separately (also powder the leaves) and ex-
tract for 24 hours with two times the weight of 20 per cent alco-
hol. Filter the alcoholic extract and precipitate the crude
diastase (along with some other proteins) by adding 22 parts
or more of 95 per cent alcohol. After a short time filter to collect
the precipitate. Dissolve the first (from barley seed) in 50 ce.
of water, and the second (from leaves) in 25 ec., adding to the
first 2 ec. chloroform, and to the latter 1 ece., to inhibit the
growth of bacteria.

Make about 200 cc. of 1 per cent starch paste (from rice
starch), and into each of three test-tubes pour 25 ec. of the
starch paste, labeling the tubes, A, B, and C.

To tube A add 5 ce. secretion diastase.

To tube B add 5 ce. translocation diastase.

To tube C add 5 ee. distilled water.

At intervals of } hour shake the tube, take 1 ec. samples, and
test with iodine, noting the changes of color (as dextrins are
produced) from blue through purple to wine-red, finally colorless.
When the starch reaction has disappeared, note the change in
appearance of the solution. Test with Fehling’s solution for
reducing sugar.

Drop a small quantity of potato starch into a few cubie centi-
meters of each of the two diastase solutions (translocation and
secretion) in two vials, slightly acidulate with weak HCl, and
after intervals of an hour or so study the types of corrosion.

If time permits, compare the effects of high and low tempera-
ture, bright and reduced light, and strong and weak acids upon
diastatic action.

Translocation. — Verify the previous indications respecting
the use of starch by the leaf or loss from the leaf when placed in
the dark. Employing a Fuchsia (as previously used, page 223),
geranium, or nasturtium, secure a plant which has been exposed
to bright light three hours or more, so that abundant starch oceurs
in the leaves. Select two or three healthy leaves and on one
side of the midrib, or middle, of each sever completely the main

Metabolism; Digestion and Translocation 279

nerves or veins. Place the plant in the dark for four hours,
or over night, then dissolve out the chlorophyll, apply the starch
test, and discuss the results with respect to translocation.

REFERENCES

ArmstTRonG, E. F. The Simple Carbohydrates and the Gluco-
sides. 112 pp., 1910.

Bayuiss, W. M. The Nature of Enzyme Action. 90 pp., 1908.

Cross, C. F., and Bevan, E. J. Researches on Cellulose, 1895-—
1900. 180 pp., 1901.

Czaprnk, F. Biochemie der Pflanzen. 1: 584 pp., 1905; 2: 1026
pp L905.

Dox, A. W. The Intracellular Enzymes of Penicillium and
Aspergillus. Bur. An. Ind. Bul. 120: 70 pp., 1910.

Errront, J. Enzymes and Their Application. Vol. 1. The
Enzymes of the Carbohydrates. (Transl. by S. C. Pres-
eott.) 322 pp., 1902.

Fiscuer, EK. Untersuchungen iiber Kohlenhydrate und Fer-
mente. 1884-1908.

GREEN, J. R. The Soluble Ferments and Fermentation. (2d
Hide) ol2 pp. 1901:

Heprick, U. P., and Weuiineton, R. Ringing Herbaceous
Plants. N. Y. Agl. Exp. Sta. Bul. 288:17 pp., 4 pls.
Meyer, A. Untersuchungen iiber die Staérkekorner. 318 pp.,

99 jigs., 9 pls., 1895.

Ossorne, T. B. The Vegetable Proteins. 125 pp., 1909.

Pappock, W. Experiments in Ringing Grape Vines. N. Y.
Agl. Exp Sta. Bul. 151: 8 pp., 3 pls., 1898.

Vines, S. H. The Proteoses of Plants. Ann. Bot. 18: 289-
318; 19:149-162; 19:171-188; 20:113-122; 22: 103-
114; 23: 1-18.

Wiesner, J. Die Rohstoffe des Pflanzenreichs. 1:795 pp.,
453 figs., 1900; 2:1071 pp., 297 figs., 1903.

ZIMMERMANN, A. Botanical Microtechnique. (Transl. by J. E.
Humphrey.) 296 pp., 63 figs., 1893.

Texts. Barnes, Czapek, Deherain, Green, Jost, MacDougal,

Pfeffer, Stevens.

CHAPTER XII

RESPIRATION, AERATION, AND FERMEN-
TATION

THE importance of air in the maintenance of animal
efficiency has been recognized as long as mankind has
existed. Shortly after the discovery of oxygen by La-
voisier and Priestly (1774), Scheele showed that air exhaled
by animals contains a smaller proportion of oxygen and
an increased content of carbon dioxid. ‘That marks the
beginning of our knowledge of one of the products of
respiration and of the extensive use of oxygen (O,) by
active living things. It was not, however, until later that
the relation of plants to oxygen was understood; and at
that time, of course, the fundamental nature of the cata-
bolic or destructive changes taking place with or without
free oxygen in both animal and plant cells could not be
suspected.

160. The term “ respiration.’’ Long before there was
any accurate knowledge respecting the nature of the chem-
ical changes (whether anabolic or catabolic) which may
proceed in the cell, the term “respiration ’’ was in use to
denote in animals “ breathing,’’ — this latter term never
aptly applying to any part of the process in plants.
With the progress of physiological study upon plants and

280

Respiration, Aération, and Fermentation 281

animals the “heat of respiration’’ and the “ energy of
respiration ’’ became well-known terms, so that respira-
tion acquired a significance far wider than at first. In
spite of this, the mechanism effecting gas exchange and
the production of certain of the more common products
of respiration (COz and water) long received consideration
as respiration. In more recent times the determination
that the fundamental changes involved are those taking
place in the living cells themselves has therefore stretched
further the use of this term. In both animals and
plants these changes — alike in kind — are the essential
features of respiration.

At present there is a clear distinction between the
usual oxygenating or aérating processes! and the en-
ergy-releasing changes occurring in the cell. These last
have been termed “ energesis,’? —a term differing from
catabolism chiefly in its more limited application and in
laying emphasis upon the current view of the chief effect
of respiration.

161. An obvious result of respiration. — If an animal is
given no food-materials, even for a single day, it must lose
weight, although it may be supplied with the normal
amount of water. A green plant placed in distilled water,
and deprived of light —the essential condition for the
making of organic food — will lose from day to day in
dry-weight of its substance. Similarly, seeds germinated
in darkness may, with requisite water, increase many times
in bulk; but the dry-weight will constantly decrease, — so
rapidly, in fact, that quickly germinating seeds may lose

1The suggestion that these alone should be called respiration seems
inadvisable.

282 Plant Physiology

in two weeks half their original substance, the expelled
products being mostly CO, and H,O.

This loss of organic substance is indicative of respiration,
a process .of catabolism and energy-release absolutely
essential for the maintenance of every active cell,— plant
or animal,—in both of which the essential process is
substantially the same.

162. The demonstration of respiration. —It is not at
all requisite that respiration shall have exactly the same
course in the cells of different organisms, nor is it necessary
that it shall be dependent upon a common type of mechan-
ism for the usually accompanying gas exchange.

Once properly understood, activity and growth are
themselves the best evidences of respiratory activity.
Nevertheless, there is a gas exchange commonly inseparable
from the process, and this gas exchange, since it is an
accompaniment of respiration, serves as a simple and
definite demonstration of the material end result, or of the
type of material change which goes on. This gas exchange
consists in the absorption and use of O, and the elimination
of CO,. Most important as a provisional experimental
demonstration of respiration is the evolution of COs.
This is, of course, positive indication that organic matter
is undergoing catabolic or dissimilatory transformations,
and the COs: set free may be made, usually, a satisfactory
qualitative or even quantitative measure of the com-
parative rate of the process, especially in the higher plants,
with which we are particularly concerned.

Proof of O, absorption and CO, production during the
activity of living tissues is most definitely shown by an
accurate analysis of the gases from a respiration chamber.

Respiration, Aération, and Fermentation 283

This may not be practicable for the present purpose, and
a simple suggestion of these gas relations is sebreuent out
a means of the often misused experi-
ment with germinating seed. In this
experiment the seed are placed in
two bottles or jars carefully corked
or sealed. After the lapse of a few
hours a lighted taper lowered into
one jar will be extinguished, and at
the same time a thick film of barium
carbonate will form on baryta water
in a dish introduced into the other
jar. (This film is far more pro-
nounced than that which would form
upon a similar test in a control jar
containing air.) :

The above experiment suggests
definitely two things: (1) that the
content of CO, has been increased,
and (2) that oxygen has disappeared ;
but neither this nor any other sim-
ple experiment, unfortunately, af-
Tors Convincing prook that MO qe 69.. Simple respiro-
other change takes place. scopes.

A far better demonstration of the evolution of carbon
dioxid is obtained by employing growing seeds in other
types of apparatus, two of which may be briefly referred
to: (1) by germinating seeds in a bottle or test-tube over
baryta water (Fig. 69), or in a chamber in which a dish
of baryta water is placed, and by comparing the amount
of the precipitate in this respiration chamber with that in

284 Plant Physiology

a control vessel lacking seed; (2) by drawing air deprived
of CO, through the respiration chamber, and catching
the CO, given off in a wash-bottle of baryta water. For
quantitative work it is absolutely necessary to study stand-
ard methods of CO, determination and to employ these
with all the chemical precautions required; especially
important is a gravimetric method in which the CQ, is
caught in potash bulbs.

The rapid inhibition of growth in the absence of oxygen
may be taken as indicative of the use of this atmospheric
constituent. The effect upon growth of an atmosphere
deprived of oxygen may be demonstrated by a compara-
tively simple experiment in which germinating seeds are
placed in a tube containing normal air, and this is compared
with another from which the O, (and CO,) are absorbed, as
explained in the laboratory instructions.

163. Respiratory phenomenain aérobic respiration. — In
the type of respiration thus far particularly considered
oxygen has free access to the respiring cells, and it is used
in promoting chemical changes. This is called aérobic
respiration, as distinguished from anaérobic respiration
(subsequently discussed), which proceeds in the absence
of free oxygen.

The following may be given as a concise summary of
the aérobic respiratory phenomena including the accom-
panying gas exchange in green plants : —

(1) Along with other gases, oxygen diffuses into the
tissues, it is absorbed by the cell-sap, and it reaches all
parts of the protoplasm of the cell.

(2) Oxygen promotes catabolic processes, and whether
through the protoplasm and its constituents directly, or

Respiration, Aération, and Fermentation 285

chiefly through foods associated with the protoplasm, it
takes:an active part in the chemical changes of the cell,
as a result of which the ultimate excrete products CO, and
H,O may be formed.

(3) Complex organic molecules are decomposed; thus
simpler products are produced, and kinetic energy is
released. <A part of this energy is in some manner utilized
by the plant in growth and other activities, while another
part, set free as heat, has little or no obvious value.

(4) Of the final excrete products of this series of changes,
the most significant is carbon dioxid, and it is eliminated
from the tissues by the general diffusion mechanism in-
volved in the entrance of gases.

164. Oxygen promotes catabolic processes. — The
studies which have been made upon the decomposition
and hydrolysis of protein and other foods and the iden-
tification of oxidizing and hydrolyzing enzymes as
of widespread occurrence in cells point clearly to decom-
position as the essential nature of the process. By some
the decomposition of the protoplasm (cf. Barnes and others)
is regarded as most important, while others view the pro-
cess as essentially, or at least in part, a decomposition of
foods, including carbohydrates and fats. Neither view is
at present entirely satisfactory, but it is not possible here
to review the evidence. It is certain, however, that the
presence of free oxygen involves ultimately the decom-
position of less material and a more economic energy-
release. Moreover, normal respiration, and consequently
the growth of most plants, is promptly checked by even
the temporary exclusion of free oxygen.

165. The ratio of O, absorption to CO, production. — By

286 Plant Physiology

many physiologists respiration has long been considered
to be a combustion process. If by combustion one under-
stands a direct union of the O, with C in such a manner
that CO, is a direct result, this comparison is unfortunate.
The combustion of any product results in a_ perfectly
definite amount of energy as heat; and this heat-energy
may be very simply determined, whether it involves the
combustion of coal, of proteins, of starch, or of cane-
sugar. There is, furthermore, in combustion (however
produced) a definite relation between the amount of oxy-
gen needed and the amount of carbon dioxid given off.
There is therefore a definite CO, /O, ratio; thus the
combustion of glucose would require 6 molecules of Os,
and 6 molecules of CO, would be produced, by the follow-
ing formula : —

C,H,,0. + 6 0, = 6 CO;+6 HO.

The combustion quotient in this case is unity. In respi-
ration the transformations are not necessarily complete,
and the respiratory quotient is seldom exactly unity, and
as a consequence there are by-products and products
less stable then CO,. The quotient is affected by tem-
perature and other environmental conditions; thus it is
possible to picture a more complex and less definite se-
quence, of changes in which protoplasm is involved. The
series of transformations may be of the same type in the
two cases, but they are not properly regarded as compa-
rable processes so far as may be determined at present.

166. Respiratory activity. — Respiratory activity is
greatest during periods of rapid growth and differentiation.
So soon as adequate water is absorbed by seeds previously

Respiration, Aération, and Fermentation 287

dried, active respiration begins. The curve of CO, ex-
cretion depends upon various external conditions, but a
maximum is generally obtained by the time shoot and root
are fairly elongated, after which there may be a rapid or
eradual decline until the seedling is well developed. The
curve shown (Fig. 70) is made up from data given by
Mayer, and it shows the results with seedlings of wheat
at a temperature of 23.8° C. |

This curve may be taken as merely a sample of the res-
piratory rate during germination, although the curves for
other seed may not be closely conformable. Frequently the
rate of respiration for germinating seeds is, with respect to
weight, about equivalent to that of man, generally given
as about 1 per cent of the body weight for 24 hours. Seeds
which germinate rapidly may, however, lose, under favor-
able conditions, one third of their dry-weight during a
period of 10 days, which is an average of about 3 per cent
per day. In such cases, therefore, the intensity of respira-
tion is relatively greater than for many warm-blooded
animals.

As the embryonic tissues in the plant are relatively
reduced, the respiratory ratio will fall, so that well-de-
veloped plants, or those growing slowly, will show, with
respect to body weight, an exceedingly low ratio. Plants
or organs passing into a resting stage will fall to a minimum
which, in the case of dry seeds and well-protected bulbs,
may approximate zero. On the other hand, opening
flower-buds may show a high respiratory activity, often
relatively greater than at the time of germination.
According to Pfeffer, rapidly growing bacteria may con-
sume oxygen at a rate 200 times as rapid as required by

Plant Physiology

25

21 28
[After Rischawi.]

ly)

17

11

re)

Germination of wheat ; curve of CO, excretion at 21° C.

Pre. 70.

12 i= le —| te) > ”
oD N N rm nm

Mgm,. CO,
per da

Das. 1

Respiration, Aération, and Fermentation

Normal tubers
Ede Oln295
9 :45-10:45 a.m.

10:45-11:45 ‘“

Wounded tubers
12:15-1:15 p.m.

1:15-2:15 *

2:15-8:15 *S

3:15-4:15 “

4:15-5:15 *
xo 1 a)

9 : 25-10 : 25 A.M.
HOR ols 25) -°
11 : 25-12 :25 p.m.
2250.“
2520-8220. *

8:25- 4:25 *

4:25- 5:25 *
cxilep 125, 90)
10:10-11:
11: 10-12
-12:10- 1

:25- 2

10 A.M.
:10 P.M.
aaa

*1:10- 3:10 ‘
3:10- 4:10 ‘*

xi. 13, °95

10: 00—11:00 a.m.
xi. 14, °95
9 :15-10:15 a.m.

xi. 15, 795
8:15- 9:15 a.m.
*Calculated to 1 hr.

U

Mem, CO,

per hr.

SEpceES

2

89

Hie

Ppt Bae wt

Eee B SESR5 BESee
2S S8 SSSR Sees Pease
S55 Seeen sanee

SS5ss beget rene: coneenenes ecesseess

S2eneS5
SEE EE eee GERBER EREEE
BBEEE SEEes BaeEe
SHEE SEEes a SBean
Titi T

[Data from Richards. |

Chart of CO, excretion, normal and wounded potato tubers.

alk

Fie.

290 Plant Physiology

man, and this fact is significant with respect to the im-
portance of these organisms in the general disintegration
of organic materials.

167. Respiration of wounded plants. — It has been re-
peatedly demonstrated that the respiratory activity is
increased by injuries to the tissues. Precise data upon this
point have been contributed by Richards. He employed
chiefly potato tubers, but the experiments with these tis-
sues were supplemented by carrot roots, also certain seed-
lings, leaves, and willow-twigs. In general, it is found
that following injury there is increased respiration for a
time. Usually after two days — under the conditions of
the experiments — the activity again declines to a rate
more nearly the normal. The chart (Fig. 71) shows the
CO2 developed from the respiration of 24 small potato
tubers (weighing 200 grams) before and after wounding,
the wounding consisting of slicing the potatoes length-
wise.

The ordinates represent milligrams of COs, and the
abscisse, time intervals of one hour each, on parts of
several succeeding days as indicated. The sudden rise
in CO, production after the injury of such solid tissues is
explained by the inclusion of CO., which is then rapidly
lost during the first two or three hours.

168. Heat release. — Exact determinations of the heat
release in plant respiration have not been possible with
the experiments as generally conducted. It has long
been shown, however, that the temperature inside of the
respiration chamber containing germinating seeds, open-
ing flower-buds, and other materials exhibiting vigorous
respiration may be 5 or 10° C. above that of the surrounding

Respiration, Aération, and Fermentation 291

air. In rough experiments, how-
ever, much of the heat is lost by
radiation, and such data are merely
suggestive. Complex calorimeters
are in use to measure the heat of
animal respiration, and recently an
advance has been made in simple
plant experimentation by the use
of the doubled-walled silvered
Dewar bulbs, or thermal bottles.
In preliminary experiments with-
out sterilization precautions, Peirce
secured a rise of temperature with
an unweighed quantity of peas (in
a silvered 250 cc. Dewar flask)
amounting to about 25° C. in

three days, with a makxi- :
mum rise of about 40° C.
reached on the seventh f

day. With sterilization
precautions and employ-
Ing in 250 ec. silvered
Dewar flasks, 300 seeds
of Canada field peas, a
maximum difference of
about 20° C. has been
observed in some

class tests.

This does not =
represent the ~-
actual differ- Fig. 72. Dewar bulb.

292

aN
&
a
CD

CEE
we
OW <

ES
e
=)
K
a

L)
ISS
iis

Fic. 73. Dewar bulb in cross-sec-
tion, showing also method of ex-
periment, — heat release.

Plant Physiology

ence, since heat is lost even
from the silvered flasks. It
is greatly to be desired that
an accurate calorimeter may
be perfected.

169. The mechanism for
gas exchange. — lo a con-
siderable extent, the histo-
logical mechanism permit-
ting the diffusion of oxygen
through the tissues of the
plant is the same as for
carbon dioxid used in photo-
synthesis. Nevertheless,
the cells active in photo-
synthesis are localized, in
higher plants, and the
mechanism which permits
aération, in general, is
more complex and may be
treated.

Since respiration is pro-
ceeding in every living cell,
the possibility of a fairly
rapid gas diffusion must ex-
tend to the remotest tissues.
In developing or mature
roots, stems, leaves, and
fruits, there may be found
considerable — intercellular
space providing for diffusion

‘4

+

Respiration, Aération, and Fermentation 293

of gases; sometimes indefinite air chambers form in a
The intercellular spaces are produced

variety of ways.
by the splitting
apart of the walls
separating mer-
istematic cells,
generally at the
angles. In this
way, small
spaces are pro-
duced into which
air diffuses, and
when such spaces
are numerous,
they form practi-
cally continuous
air chambers.
In some _ cases
these spaces oc-

-cupy far more

volume than the

cells themselves.
This is particu-
larly true in the
case of the mes-

Fic. 74. Experiment suggesting the efficient
aération of the leaf. [After Detmer.]

ophyll tissue of leaves. .
Very large air spaces are a characteristic feature of many
water plants, and they are often accompanied by a pecul-
iar distribution of the cells, or of irregular or stellate
out-growths from these into the cavities at a later period.
Again, during the process of growth, cavities may arise

294 Plant Physiology

through the more rapid growth of certain regions, and the
rupture of cells adjacent, as in the case of certain grasses,
and of many other plants generally possessing a loose
central pith during the early stages of growth. In the
more solid or woody stems, intercellular spaces constitute
some part of the structure, and the better aérated cortex
may be provided with lenticels, or special areas of loose
tissue permitting gas exchange. These lenticels are
interruptions of the more or less continuous corky enve-
lope constituting an essential part of the true bark of
many woody plants.

Roots may, in general, obtain some oxygen in solution,
but the cortical parts of these organs exhibit, as a rule,
rather large intercellular spaces, so it is evident that this
special type of diffusion mechanism for aération is impor-
tant likewise in subterranean organs. In fact, in many
roots there may be found special tissues, apparently
insuring a surplus of air, and such may be designated
air-storage tissues. Certain plants inhabiting stagnant
water are provided with special roots, or root branches,
which seem to be important in aération. To these organs
the term “‘hydathodes”’ has been applied.

The relation of leaf-stalk and blade to air or the con-
tinuity of the aérating tissues may be very well empha-
sized by the experiment shown in Figure 74, in which,
when suction is applied to the tube, air passes through the
leaf, and is given off in bubbles from the petioles below the
surfaces of the water in the bottle.

170. Anaérobic respiration. —It has been clearly
demonstrated that respiration may proceed for a time, in
most tissues and cells, when no free oxygen is available.

Respiration, Aération, and Fermentation 295

Considerable diversity may be manifest as to the extent of
this respiration, and in the case of germinating seeds, the
nature of reserve foods is an important factor in this
regard. In certain tissues, anaérobic respiration takes
place to such an extent as to be very readily recognized
by the usual demonstration of CO, production. Never-
theless, while CO, is commonly an end product of this
type of respiration, alcohol, lactic acid, hydrogen, and
other products may be identified with it. Since no free
oxygen is required, decomposition resulting in CO, and
the other products mentioned are obviously by rearrange-
ment of the atomic groups in the organic molecules.
This type of respiration is therefore truly anaérobic, or
without aération. The term “ intramolecular”’ has also
been employed in this connection.

Anaérobic respiration in the tissues of higher plants may
be experimentally studied by use of germinating seeds,
preferably starchy seeds, such as barley and buckwheat,
but slices of potato or other solid tissues of this nature are
also useful. The essential apparatus is the same as for
_aérobic respiration, but in this case it 1s necessary either
to exhaust all air with a first-class air pump, or else to
replace air with some gas which is physiologically inert.
Hydrogen was formerly used as a gas of this nature, but
as there is a possibility that it may be so injurious as to
introduce error, nitrogen may be substituted therefor.

Attention has been drawn already to the fact (section
87) that anaérobic respiration of roots is found to result
in the production of traces of some excrete organic acids,
whereas, in aérobic respiration, CO, alone is evolved. All
of the results point to the conclusion, therefore, that by-

296 Plant Physiology

products and wastes are far more abundant in anaérobic
respiration, owing to the lack of oxygen to stimulate the
changes resulting in the production of CO, and water.
With respect to micro-organisms which frequently demon-
strate anaérobic respiration, this subject is further dis-
cussed in succeeding sections upon fermentation.

171. Fermentation. — The original significance of this
term had reference to decomposition or change in or-
ganic substances, such as sugar solutions and cider, ac-
companied by the evolution of bubbles of gas, and
generally by the production of alcohol. After the work
of Pasteur and others, it was apparent that such changes
(although they may be due ultimately to enzyme action)
are in consequence of the activity of micro-organisms,
which are then defined as having the capacity to ferment
certain substances.

In more recent times the term has applied to a variety
of types of decomposition due to microscopic organisms,
and also to the action of a large class of enzymes (soluble
ferments), whether obtained from micro-organisms or
from higher plants. The “ferment” action of the great
majority of enzymes does not involve the liberation of COs.

In general, fermentation phenomena may be regarded
as representing the various steps in decay. The type of
fermentation in any particular case is usually given the
name of the chief product produced, although in some cases
it is the group of substances acted upon which stands
sponsor for the name. In this chapter, only a few of those
fermentation phenomena are discussed that are induced
by the growth of micro-organisms, and as a result of which
alcohol and certain acids are produced.

Respiration, Aération, and Fermentation 297

172. Lactic fermentation. —If no precautions are
taken to prevent the contamination of milk as it is drawn
from the udder, it normally undergoes lactic fermentation,
at the temperature of the dairy or living room. Among
the organisms usually finding access to the milk, are
bacteria (especially Bacterium lactis acidi) the action of
which produces a slight acidity or souring, and later a
marked effect exhibiting itself in the precipitation of the
casein (curdling). The acid developed is largely lactic,
and the course of the main changes referred to, involving
the milk sugar (lactose, which is first converted into
hexose sugars), is probably substantially thus : —

‘OF Be OF + H.O => (Orls ln ©): + C,Hiw.O; — 4(C3H,Os)

lactose glucose galactose lactic acid

One or more of a variety of organisms produce this end
result. In general, they utilize as food a portion of the
sugar, and they may produce, in small quantities, beside
lactic acid. and carbon dioxid, the following: one or more
of several organic acids, also hydrogen, nitrogen, and traces
of methane, depending somewhat upon environmental
conditions. The production of lactic acid is so rapid in
milk that the medium is soon sterilized with respect to the
presence of other organisms; but this is scarcely an ad-
vantage to the lactic forms, since this fermentation and the
activity of the lactic organism is usually brought to a
standstill when about .8 per cent of acid has been pro-
duced. According to certain investigators, a small amount
of lactic acid as an excrete product may be produced
during the anaérobic respiration of roots.

173. Alcoholic fermentation. — Alcoholic fermentation

298 Plant Physiology

is more commonly brought about by the growth of various
species of yeast in liquids or moist substrata containing
certain sugars. Some organisms show a marked specific
election with respect to the sugar fermented; but in
general, the hexose forms are most important, while trioses
and nonoses are sometimes used. When abundant Os,
is supplied, the yeasts may grow rapidly, utilizing the
sugars as foods, and effecting relatively little fermenta-
tion. In the absence of sufficient O, for rapid growth
the organisms grow slowly, but fermentation proceeds
vigorously. The decomposition of glucose yields ethyl
alcohol and carbon dioxid as chief products, expressed
conveniently by the following equation : —

CvH,20¢ = 2(C.H,O) = CO.

glucose ethyl alcohol

On account of this capacity to produce alcohol, yeasts
are the important organisms utilized in the production of
alcoholic beverages, and the proper regulation of growth
and fermentation is the essential factor in economic pro-
duction. Starch, cane-sugar, and other carbohydrates
transformable into the fermentable types are, of course,
ultimately used, if first hydrolyzed, as in the malting
process.

Yeasts are unusually resistant to the alcohol produced,
but the fermentative activity declines rapidly at 12 per
cent, and at 14 per cent there is usually complete inhibi-
tion. This concentration, however, is far greater than
that which the more resistant molds may endure, com-
monly from 4 to 5 per cent. It is well known that the pro-
duction of CO, bubbling through and held by the cohesive

w

Respiration, Aération, and Fermentation 299

gluten of wheat flour, is the important factor in light-
bread making. It is of interest to note that anaérobic
respiration in higher plants results in the production of a
small amount of alcohol, so that the two processes are
comparable.

174. Acetic fermentation. — Acetic fermentation in
nature generally follows the alcoholic, and it is brought
about by the so-called acetic bacteria. These organisms
effect the oxidation of ethyl alcohol in weak solution to
acetic acid, probably in two steps, with the following,
general result : —

C.H;OH + O: = CH;COOH + H:;0

ethyl alcohol acetic acid

At the same time some alcohol is utilized and CO, pro-
duced. In commercial vinegar-making, cider, weak wine,
and other products of this nature are utilized, and there is
a slow and a quick method of procedure depending upon
the aération.!

LABORATORY WORK

Loss of weight. — Select two lots, each of 25 seed, of peas or beans.
Determine the dry-weight of one lot and record. Soak and ger-
minate the other lot in the dark closet upon a plate containing
moist filter paper. When the seedlings have grown about as
long as they will from the food material of the seed, determine
the dry-weight and compare with those ungerminated.

Absorption of Oo and evolution of COs.— Into jars or wide
mouth bottles put some soaked or germinating seeds of peas or
beans, and cork tightly or seal. After a few hours, or next day,

1 Prescott, S. C., ‘‘ Wine, Cider, and Vinegar,” Bailey’s Cyclopedia of
American Agriculture, 2: 181-186.

300 Plant Physiology

test the air in one with a lighted wax taper and in another
with a small dish of baryta water, comparing with similar
tests carried out in a bottle containing only air. Explain the
results and indicate the limitations of the experiment.

Evolution of CO. — Pour some baryta water into a large test-
tube, then introduce a closely fitting perforated cork upon which
two or three germinating seeds may be placed; cork and seal
(Fig. 69). Set up a control experiment without the-seeds; in a
few hours compare and discuss the results with respect to the
baryta water.

Set up an apparatus as suggested in section 162 consisting of
a chamber with germinating seed, or other favorable material,
connected on the side toward the inflow of air with two wash
bottles of potassium hydrate, to take out the COs of the air,
and on the side toward an aspirator or filter pump with two
bottles of baryta water,’ for the demonstration of any COs
given off. Before connecting up with the baryta water bottle
draw air through the apparatus to remove the normal air.
Connect with the baryta bottle, darken the respiratory chamber
[Why ?], and draw air through the entire series for an hour or
two. Describe the result.

In quantitative work a standard method of CO, determination
should be employed, preferably a gravimetric method, in which
potash bulbs are used, protected as to acquisition and loss of
water by calcium chlorid drying tubes. In order to demonstrate
anaérobie respiration in seed or other material the most satis-
factory simple method is to replace the air in the apparatus with
hydrogen. For this purpose a hydrogen generator is required
in connection with the other apparatus described. There is
evidence, however, that nitrogen is preferable to hydrogen, but
in most laboratories it is not practicable to employ this gas.

Heat release during respiration. — Soak for about 12 hours

1 Make a concentrated solution of barium hydrate, with excess of the
barium salt, keep the bottle or flask well stoppered, and decant off or
pipette out the liquid, when needed, avoiding all unnecessary exposure
to the air.

Respiration, Aération, and Fermentation 301

somewhat more than 200 grams of
field peas or wheat, and after soaking
weigh out two lots of 100 grams each.
Kill one lot by immersing it in boil-
ing water for about 10 minutes, then
place both the killed and the living
seed in separate cheesecloth bags and
immerse each in formalin solution
(1 part to 600 parts of water) for
15 minutes. Take the bags from the
formalin and dip them into boiled
water with as little handling as pos-
sible (the water should be at room
temperature or at the temperature of
the incubator to be used). Have ===
thoroughly cleaned by the chromic » &
acid mixture two double-walled, vac-
uum, silvered flasks or Dewar bulbs
of 250 ee. capacity (Figs. 72 and 73).
Sterilize these also by rinsing with
the formalin solution above indicated.
Provide each fiask with a short vial
or small dish of KOH protected from
. tipping over by a wire cage lowered
by a string, also previously steri-
lized as to the exterior. Pour care-
fully each lot of seed into a separate
Dewar bulb, insert a standard ther-
mometer, previously dipped in forma-
lin and rinsed, and plug the flasks
with wool. Wrap the flasks well with
felt or woolen cloths and place them
at a temperature as constant as pos-
sible. Ten minutes later take the
temperature of each, and at intervals
of 12 hours for several days, or until
there is a rise of temperature in the Fy¢,75, Oxygen and growth.

tl
flat

N

Sy (

302 Plant Physiology

control flask (containing killed seed) which would indicate con-
tamination, necessitating the close of the experiment. Plot
curves of the temperature conditions in the two flasks.

Oxygen and growth. — After introducing seeds (Fig. 75)
attached to a cork into A (shown at the left), fill AB with
water recently boiled and recooled. Close stop-cock N, invert,
connect with C (a tube with air only) and with D (a tube con-
taining potassium pyrogallate (see p. 221). Shake the series
(stopcock O open) until the oxygen is absorbed, close O, im-
merse vertically the OD end of the series into boiled, cool
water, disconnecting D, and opening O temporarily, to relieve
(by inflow of water) the negative tension due to the absorption
of oxygen. Close O, remove the apparatus from the water,
open N, permitting the oxygen-free air to escape into A
displacing water. Then close N, detach the parts below the
latter, and place the AB part (as shown at the right) under
temperature conditions favorable for growth. Compare at
intervals the growth with seeds similarly placed in an open tube.

Fermentation of sugars. — Properly fill two fermentation tubes
(preferably the Kiihne form) with a 10 per cent sugar solution
(fresh) of each of the following: glucose, sucrose, and lactose.
In each tube insert a fragment of pressed yeast, plug the mouth
lightly with cotton, and in 24 hours or more compare the pro-
duction of gas caught in the closed arm. Insert in those showing
gas a stick of caustic potash and explain the results.

For experiments such as the above, continued so short a time,
it is unnecessary to supply mineral nutrients, nor are sterilization
precautions necessary.

Alcoholic fermentation. — Prepare 500 ec. of a modified Pasteur
solution to contain the following : —

Giucose™ <...0 2s 2 a
Ammonium tartrate yi nor eee a 5 grams
Potassium di-hydrogen phosphate . . . . . . 1gram
Caletum.chlorid . . .° 2°. 3% Ss) Ls) Se
Magnesium sulfate... ." ... <-. } .2 =e

Water <.9s0s) 2 oe 8 ny be ae ei ee

Respiration, Aération, and Fermentation 303

Pour this into a 1-liter Erlenmeyer flask and add 2 grams of
pressed yeast. Fit the flask with a cork through which passes
the short arm of a piece of glass tubing bent so that the long
arm may reach over through the cork of a wash bottle containing
baryta water. Do not connect with the baryta water, however,
until there is time for the air in the flask to have been driven
out by the gas which is being produced. (How may this be
determined, approximately ?) Describe the result.

When the fermentation is practically complete, or after one
week, the flask containing the fermented solution may be con-
nected with a condenser and distilled. Ata temperature of from
80-85° C. redistill, and when afew cc. have been caught note the
odor; then pour into a test-tube, add a erystal of iodine, heat
gently to 60° C., and maintain at this temperature while adding
a strong solution of caustic soda until the iodine dissolves. A
yellow precipitate of iodoform is indicative of the presence of
alcohol.

REFERENCES

Barnes, C. R. The Theory of Respiration. Bot. Gaz. 39:
81-98, 1905.

BuackMAN, F. F. Optima and Limiting Factors. Ann. of Bot.

19 : 281-295, 2 figs., 1905.

BucHNER. Ber. d. deut. chem. Gesellsch. 30:117—-124, 1896.

FunrMann, F. Vorlesungen tiber Bakterienenzyme. 136 pp.,
9 figs., 1907.

Kuockrr, A. Fermentation Organisms. (Transl. by G. E.
Allan and J. H. Millar.) 392 pp.

KostytscHew, S. Ueber die norm. u. die anaerob. Atmung bei
Abwesernheit von Zucker. Jahrb. f. wiss. Bot. 40: 563-592,
1904. [See also Ber. d. d. bot. Ges. 24: 436-441.

Prircre, G. J. A New Respiration Colorimeter. Bot. Gaz.
46: 193-202.

PuriewitTz. Physiolog. Unters. iiber Pflanzenatmung. Jahrb.
f. wiss. Bot. 35: 573-610, 1900.

304 Plant Physiology

Ricuarps, H. M. The Respiration of Wounded Plants. Ann.
of Bot. 10: 530-582, 2 figs., 1896.

Sroxuasa, J. and Cerny. Isolierung des die anaerob. Atmung
der Zelle d. héh. org. Pflanze u. Tiere Bewirk. Enzyme. Ber.
d. deut. chem. Gesellsch. 36: 622-634, 1903.

Texts. Barnes, Detmer, Ganong, Jost, Pfeffer.

CHAPTER XIII

GROWTH

A PROPER conception of growth and important growth
relations is fundamental in plant production. Growth
necessarily receives consideration at least indirectly
throughout every chapter, for it enters into any discussion
of the relation of the plant to factors of environment ; to
the making, use, and accumulation of food-materials ;
and to the phenomenon of reproduction as well. In gen-
eral, the practical measure of growth is yield. It is im-
portant, however, to examine somewhat more carefully
certain observations and fundamental facts regarding the
-mechanism of growth.

175. The factors. — Growth is conditioned by internal
and external factors. Among the internal factors must
be assumed vitality, not explainable, yet known as an
attribute of the living mechanism; heredity, operating to
reproduce specific form; and often a certain food-supply.
The external factors are many of the environmental con-
ditions previously enumerated (section 5); and essential
are moisture, a certain range of temperature, a source of
oxygen, the several nutrients and crude food-materials,
and (for continued growth in green plants) light. These
factors in relation to growth and development receive
special consideration as independent topics.

305

306 Plant Physiology

Klebs has in recent years developed important relations
between the continuance of growth and certain external
factors. For a few plants he has indicated the ‘conditions
tending to maintain vegetative growth and he has con-

Sad Ls
Lie >
=

a
ES

\
la
‘*

i

Fic. 76. Effect of conditions on the growth of pine needles: the short
needles were produced during the season of transplanting (poor water-

supply).
trasted these with the influences inducing flowering —
tending toward maturity. These are subsequently re-
ferred to (section 225), but it is important here to note
that most plants exhibit such complex relations as to
render the problem especially difficult.

Growth | 307

176. Evidences of growth. — Observed as a whole the
erowth of any crop from seed-sowing to harvest is an
obvious phenomenon. In general, the popular concep-
tion of growth in flowering plants is that’ conspicuous

Fic. 77. The effect of complex factors on the growth of corn.

form of increase in size and weight which may be noted
- as the seedling develops into the mature plant, as the
rapid exfoliation of leaves, or as the unfolding of the
flower-cluster. Growth is associated with the formation
and extension of living cells, and it may result in pro-
nounced changes in external form or in internal structure.

Growth involves at least two distinct phases. The one
is Increase in length, and often in size — extension; the

308 Plant Physiology

other is a change in internal structure, either within the
cell, or affecting groups of cells, resulting in differentiation.
Extension is evident, and differentiation may be obscure ;
when the flower is fully open, for example, growth processes
may go on within, which may or may not result in evident
increase in size or weight, but new and important structures
may be formed, and there is growth. Just so there is no

Fig. 78. <A potato sprouting in a dry, hot atmosphere, and in strong
light.

increase in the size of the incubating hen’s egg, but by
growth the little chick is soon developed from the simple
fertile egg cell, with its stored food-material.

177. Growth of the embryo. — Growth of the embryo
(Fig. 79) in plants has been nowhere more carefully studied
than in certain of the crucifers, notably in Capsella Bursa-
Pastoris, a classical example among dicotyledons, which

Growth 309

also may be taken as a type. After fertilization there is
developed a row or filament of cells, called the proembryo,
invested at the be-
ginning, as a_ rule,
with the endosperm.
The apical cell of the
proembryo divides
longitudinally, and
there are then cross
and longitudinal divi-
sions, either occurring
first, thus producing
a stage with 8 cells of
the embryo proper,
the remaining _ fila-
mentous portion being
designated suspensor.
The following com-
plete description of
the embryonic growth
from Coulter and
Chamberlain! indi-

eates the differentia- Fic. 79. Germination of zygote and em-
# : bryology of Lepidium : fertilized egg (A) ;
to es of the Important proembryo (B) with suspensor (S) and
Teglons of the young apical embryo cell (e) ; later stages (C, D,
plant. E, F) showing development of stem, root,
“k and cotyledonary parts. [After Curtis.]
Whether’ the

transverse division precedes or follows the second
longitudinal division, it separates the cotyledonary and

1Coulter and Chamberlain, “‘ Morphology of Angiosperms,”’ pp.
196-198.

310 Plant Physiology

hypocotyledonary regions of the embryo. In the octant
stage the dermatogen begins to be differentiated, the
periclinal divisions appearing first in the terminal oc-
tants and proceeding toward the root end of the embryo.
The differentiation, however, is almost simultaneous, so
that the dermatogen is soon completed, except that of the
root-tip, which is derived from the adjacent cell of the
suspensor, and appears comparatively late. The periblem
and plerome are differentiated early from the tissue within
the dermatogen. The stem-tip and cotyledons are de-
rived from the four apical octants, and the bulk of the
hypocotyl from the four basal octants. The root-tip,
however, is completed by the adjacent cell of the suspensor.
This cell divides transversely, the basal daughter-cell
taking no part in the formation of the embryo, but the
other daughter-cell (hypophysis of Hanstein) filling out
the periblem and dermatogen of the root-tip. The
hypophysis divides transversely, the daughter-cell next
the embryo completing the periblem of the root. The
other daughter-cell by two longitudinal divisions gives ~
rise to a plate of four cells, each of which divides trans-
versely, the plate of four cells toward the embryo com-
pleting the dermatogen of the root-tip, and the other plate
constituting the first layer of the root-cap.”’

178. Polarity. — This term denotes a differentiation of
the two poles of a growing cell or organ. Any part of
any seed-plant or member is therefore recognized as hay-
ing an apical pole and a basal pole, the apical being the
direction of growth of the shoot, and the basal the direc-
tion of growth of the root. From the preceding description
of the growth and differentiation of the embryo the sig-

Growth ait

nificance of this phenomenon may be seen. Polarity is
known merely through the behavior of organs.

179. Elongation of roots. — The nature and distribu-
tion of the tissues at the apex of the root axis have been
noted. The elongation of the root may be readily followed
by a simple experiment. The tip of a young seedling
should be divided into zones by a half dozen or more
- parallel marks about 2 to 3 mm. (about 3/5 inch) apart, the
first mark, however, being practically at the tip. The
marks can be made with India ink and a pen, or a very
fine brush.

Observations after from 6 to 24 hours, under favorable
conditions, will show that the region of elongation is con-
fined to a space covering usually only a few millimeters
back of the tip. The zone farthest away may have
already ceased to elongate, or practically so; while those
nearest the tip elongate at first slowly, then faster, to a
maximum, after which they decline. In fact, if new zones
were constantly marked off each would be seen to go
through a certain grand period of growth. The impor-
tance of the shortness of this region of growth with respect
to the ability of the root to penetrate the soil has been
pointed out in the discussion of the relation of the root to
water and to the soil (section 30).

New lateral roots form in the quiescent region behind
the root-tip; such lateral roots originate in the pericycle,
just within the endodermis, and as they push out they
literally break through the cortical tissues, which latter
are in part broken down and dissolved. Lateral roots are,
therefore, said to arise endogenously.

- The growth and branching of the roots of agricultural

ailee Plant Physiology

plants are in general increased by favorable food-supply and
a water-content of the soil which shall not ordinarily be
more than from 30 to 50 per cent of soil saturation, as
discussed elsewhere. Under favorable conditions the
rate of growth is rapid; for example, the roots of corn may
elongate at the rate of about 15 inches per day. In many
cases increase in size follows elongation, although it may
be coincident with this.

Increase in the size of roots commonly involves no
shortening of the root-axis, yet in the rooting of certain
bulbs, and following the germination of a few seed, shorten-
ing may occur after the roots are fairly fixed in the soil,
thus resulting in effectively burying or sinking the storage
organ. The practical advantage is evident.

180. The stem apex. — The stem apex of the flowering
plant shows, like the root apex, no single apical cell from
which growth proceeds; instead, there is within the
epidermis a group of cells rather indefinite in area which
constitute the primary meristem. In the apical cells of
this meristem divisions rapidly occur, and there is also a
rapid extension of those somewhat older, or farther from
the tip. This multiplication and elongation of cells is
the direct cause of the observed increments of growth.
The epidermal layer in order to accommodate this increase
in growth is extended by divisions perpendicular to the
surface (anticlinal divisions).

The cells of the meristem are gradually differentiated
posteriorly in two chief regions, — an outer, or periblem,
and an inner, plerome. It is primarily within the plerome
that vascular tissue in seed-plants is differentiated (sec-
tion 186).

Growth 313

The rate of growth extension in seedlings or other small
plants may be conveniently recorded by means of auxa-
nometers of various forms. A desirable type of this in-
strument produces a record by the following principle:
A cord attached to the growing organ passes over a small
wheel to a weight which takes up the slack induced by
erowth. The rotation of the small wheel carries with it a
larger wheel from which a cord is connected with a pointer
working against carbonized paper on a revolving drum.
In this manner a continuous record is secured of the incre-
ments of growth.

181. The formation and exfoliation of leaves. — Origi-
nating in the developing periblem just behind the apex the
young leaves arise, in spiral order, or verticillately disposed
about the stem. These are at first small protuberances
resulting from cell divisions parallel to the surface (peri-
clinal divisions) ; but in time they flatten and grow faster
than the stem apex. They curve over the stem apex
more or less to form a bud.

In many annuals, and in perennial herbs, but only
occasionally in trees, the leafy axis continues to elongate
throughout the so-called growing season. If the shoot
apex thus constantly elongates, each leaf in succession
remains a part of the bud for only a short space of time,
since by further growth, — more rapid on the upper sur-
face, — complete exfoliation is soon effected. The exact
point of origin of the leaf back of the stem apex depends
in general upon the type of leaf arrangement in the
species of plant. In the axil of each young leaf there
is commonly formed later a region of growth destined to
become a lateral bud. This bud also originates ordinarily

314 Plant Physiology

in the periblem in a manner very similar to the fundament
of the leaf.

182. The resting bud. — Aside from the method of more
or less continuous leaf development in the bud which has
been stated to be the rule during the growth period of
annuals, it is necessary to consider more particularly this
phenomenon with respect to certain trees. In a majority
of trees the shoot axis is terminated during the summer or
early autumn by a resting bud. This bud is merely a
very short leaf axis protected by bud scales, the latter
being structures homologous with leaves or leaf parts.
As a matter of fact, this terminal bud may be more or less
completely differentiated in the early portion of the
summer, but it does not necessarily present the appear-
ance of a resting bud until midsummer or later.

Growth within the so-called resting bud proceeds very
slowly, or may entirely cease, for several months during
the winter. The following spring, with the return of
favorable conditions for growth, there is generally a rapid
unfolding of the leafy shoot, or of the cluster of leaves or
flowers. It must not be understood, however, that this
resting bud cannot be forced into more or less immediate
erowth, or that it is necessarily formed so early in the
season. As a matter of fact, it appears that adventitious
buds may arise and develop shoots during a single season,
and that water shoots may form no terminal bud until
completing a season of growth more or less equivalent to
that of an annual.

183. Types of stem elongation. — Wholly apart from
the exceptional cases referred to, the method of elongation
of the shoot, or bud axis, in woody plants is diverse, and

Growth 15

the several types given below should not be regarded as
clearly distinguished one from another : —
(1) Growth of the bud into a shoot may consist of the

Fic. 80. Stages in the elongation of a shoot of pine.

rapid elongation of the parts which have been previously
laid down in the bud. In this case, the leaves which
appear on the leafy shoot are merely the full number which

316 Plant Physiology

existed as leaves in minute form in the winter bud. Re-
cent studies by Miss Moore indicate that a considerable
number of our north temperate deciduous trees are of this
type; and a number of observations suggest that it is the
method common among conifers. Excellent examples. of
this type are offered by the beech and pine. In the beech
the first sign of activity in the spring is that of gradual swell-
ing of the bud, and at first a rather general stretching of
the internodes. The bud quickly doubles its former length,
and by this time observations upon the method of elonga-
tion are most easily made. It will be found that the
growth increments in the basal internodes are at first
stronger, successively passing to others, and the terminal
internodes are the last to show rapid extension. Never-
theless, there is a distinct grand period of growth for each
internode in turn.

In the pine, on the other hand, there is this difference :
the shoot is unsegmented, and every portion of the bud
axis from base to apex becomes successively the region
of greatest extension; although in this case extension is
more nearly uniform throughout the whole shoot axis.
It appears that pomaceous fruits ordinarily follow the
plan of this general type, but peaches may frequently fall
into the next class.

(2) From the data available it would seem that the
lilac, willow, and some other trees may develop normally
during the summer a few more leaves than are ordinarily
contained in the resting bud. In this case there is, of
course, a formation of new nodes and internodes from the
young meristem as the bud is expanded, or at least dur-
ing the grand period of growth of the shoot.

Growth Bab

It is quite possible that many trees show types of de-
velopment from the resting bud dependent upon the con-
ditions. In the Carolina poplar the writer has found that
old trees may show an exfoliation of few if any more leaves
than are normally contained in the bud, while younger
trees in rapid growth may produce more than five times
as many as were thus preformed.

Frequently the leaves of the apple, pear, peach, and
other fruits seem to be produced in clusters upon short
branches or spurs; in those cases, as a rule, the internodes
are suppressed, and the axis is therefore greatly shortened.
This is particularly common on fruit spurs. It is impos-
sible here to consider the modifications in growth accom-
panying fruit production, the alternations of growth,
elongation, fruiting,! etc.

184. Fruit buds and age of shoot. — It is of importance
to consider briefly the relation of fruit buds to the age of
the wood in a few economic plants. In the grape, the
fruit is borne on canes produced during the current year.
Where a vine is left for a season unpruned, many buds
upon each cane push into new shoots, yet relatively few of
these will then bear fruit, or at least, large bunches of
fruit. As a rule, the better fruit-producing canes on
American grapes are developed from side canes (pruned
to a bud or two) on a leader which is not less than two
years old.

Peaches and almonds develop fruit on wood which is
one year old, and generally the fruit buds are more abun-

1 For extensive observations on branches, fruit spurs, and other mat-
ters of interest in this connection, the student should refer to Bailey’s
“Lessons with Plants,’’ pp. 1-69.

318 | Plant Physiology

dant toward the middle portion of the shoot. On the
other hand, apples are produced on wood which is two
years old, and the same is true of cherries and plums.
The pear belongs, in general, to the apple class, but on
old spurs the relations may be somewhat more complex.
It is evident, therefore, that the pruning practices with
respect to any particular fruit must take into consideration
not merely the effects upon general growth and form of the
tree, but must consider also a proper regulation of the
buds, shoots, or branches which are to produce fruit.

185. Persistence of the rest period in temperate
regions. — Owing to the periodic production of the ter-
minal bud, the shedding of the leaves, and the passage
of the plant into a definite state of rest each autumn,
it has been more or less assumed that this period is essen-
tial, and that it may only with great difficulty be short-
ened. It has been felt, particularly, that it is extremely
difficult to force perennials into new growth before their
normal resting period is completed. Klebs has shown,
however, that under favorable conditions for growth, a
number of plants require no winter rest period, and may
be cultivated more or less continuously.

More recently, Howard and others have demonstrated
that, even with those plants which are most persistent
in refusing to grow until the normal rest period is over,
etherization or other special forcing may be effective in
giving the necessary stimulus to early growth (as devel-
oped later, section 197). First, however, it is necessary
to indicate the effect of restoring, during the resting period,
favorable conditions for growth. It is shown that of
234 species of plants brought into the greenhouses at

Growth 319

Halle, Germany, from October 28 to November 4, more
than one half, or 125 species, began to make growth
promptly under greenhouse conditions. ‘Thus it is
apparent that a large number of deciduous trees merely
require favorable conditions for growth in order greatly
to reduce the normal rest period.

Observation upon orchard and forest trees fully confirms
this view. It happens frequently in temperate regions
visited by drought in the early summer that the resting
bud is formed early, and defoliation of many of the spring
leaves may occur by midsummer. In such cases, a return
of moist weather and favorable conditions in the late
summer or early fall may result in a flush of growth from
the resting bud of the same season. Sometimes this may
be accompanied by fall blossoming.

186. Differentiation of stem tissues.— A complete
study of internal anatomy, or histology, is not the pur-
pose of the following paragraphs. It is, however, essen-
tial to note the common method of growth or develop-
ment of some of the chief tissues and tissue systems
within the plerome of the growing tip. There are pro-
duced by division and differentiation of the tip meristem
strands of elongated cells known as the procambium.

In dicotyledons, these strands may be commonly 4 to
10; typically they are disposed as an interrupted ring in
the outer portion of the plerome, surrounded by cells of a
less differentiated meristem, often termed the ground or
fundamental tissue. The procambial strands become
by differentiation the primary fibrovascular bundles.
In developing the common type of bundle (collateral
type), the inner portion of the procambium becomes the

320 Plant Physiology

xylem, or wood, in which wood-cells, pitted and spiral
vessels occur. The outer portion develops the phloém,
or sleve-tube and soft-bast part of the bundle. Between
these two there remains a growing meristem, the cambium,
which is most important in secondary growth, or thicken-
ing, as subsequently stated. In woody plants this cam-
bium becomes a continuous ring, being formed between the
bundles by the differentiation of the ground meristem.

In the root, however, the procambial areas originating
the phloém and the xylem of the several bundles alternate
radially (radial arrangement); and when a ring of cam-
bium is formed, it is between phloém and xylem, as well
as on the periphery of the xylem, thus constituting an
irregular or lobulate ring.

187. Secondary thickening. —Secondary vascular growth
normally arises by the development of new bundles from
the ring of cambium. Ultimately so many of these second-
ary bundles may be interposed as to produce in woody
or semi-woody plants a complete wood-cylinder. The
bundles may be more or less completely separated one
from the other by thin bands of ground-tissue, known as
medullary rays. In perennials, the cambium_ differ-
entiates each year (or season) new xylem within; thus
the cambium is carried farther and farther from the
center. It develops new phloém without, so that both
wood and bark are annually or seasonally augmented.
However, the xylem vessels produced during the first
flush of growth, that is, in the season of greatest activity,
are larger and more important, and they have thinner
walls than those produced later, so that a definite ring
formation results.

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fall wood

Fic. 81. Oak branchin cross, tangential, and longitudinal section: cork
(ck), cork cambium (cc), parenchyma (cor. p.), stereome (str), bast (op),
sieve-plate (sp), sieve-tube (st), duct (d), tracheid (tr), wood paren-
chyma (wp), medullary ray (mr), and lenticel (/). [After Osterhout.]

xe

O22 Plant Physiology

In a monocotyledon, like the Indian corn, the procambial
strands are rather numerous, and generally irregularly
disposed. The differentiation of this tissue into the
mature elements is also complete, so that there remains
no growing tissue in the bundle, and there is no further
growth by secondary thickening. The thickening in the
stem which results between the young and mature stages
of the corn is very largely due to an increase of the size of
cells already laid down at an early stage.

The leaves of many dicotyledons are generally more
or less completely formed with respect to fundamental
tissue very shortly after they begin to unfold, although
there may be a subsequent growth and differentiation in
the veins and veinlets. On the other hand, in the case of
certain monocotyledons, especially, a growing zone, gen-
erally indefinite in extent, may be maintained for a con-
siderable time near the base of the leaf. A few plants,
such as the ferns, also elongate for a time by growth at, or
near, the apex.

188. Growth of the cell. —It has been abundantly
indicated that the growth of the organs of the plant, and
of the plant as a whole, are dependent upon the capacity
of the meristem or embryonic cells to extend or to divide.
Extension is commonly associated with differentiation
and maturity. It may result in a great relative reduc-
tion of the protoplasmic content, or as previously shown,
it may result in protoplasmic loss, and eventually in the
death of the cell, the firm cell-wall alone remaining.
This type of cell growth, therefore, usually produces a
specialized tissue, and the differentiation is to some
extent an immediate growth response, for the extent of

Growth 323

these tissues may be determined, in considerable measure,
by the conditions of growth.
Many meristematic cells, especially cells of the pri-

Fie. 82. Nuclear and cell division in the root of corn: cell with promi-
nent resting nucleus (A); prophases of nuclear division, spirem (B)
and chromosome (C) stages; bipolar spindle (D); early (£) and late
(F) anaphases ; telophases (G) and first evidence of cell-plate ; location
of cell-wall clearly defined (H#). [After Curtis.]

mordial meristem, are so situated and conditioned that
erowth or increase of the protoplasmic content takes
place, and at the same time the size of the cell may in-
crease; this condition, however brought about, usually

results in cell division.

324 Plant Physiology

189. Cell division. — Usually, in vegetative organs,
division results in such manner that any meristematic
cell, temporarily regarded as a primary (or often desig-
nated parent) cell, produces two more or less equal sec-
ondary (daughter) cells. Exceptionally, differentiation
may accompany division; and, in any case, the subse-
quent life history of the secondary cells may or may not
be similar. It is obvious that cell division must carry
with it the division of most of the essential organs of the
cell. There is, in fact, division of nucleus, cytoplasm,
and plastids. The nuclear division is of peculiar interest.

190. Nuclear division. — The nuclei of both plants and
animals seldom divide by a direct halving of the nuclear
substance, or direct division. Such a type of division
is, however, known. ‘The usual process is complex, char-
acterized by several distinct phases, all of which are
apparently important in securing the equal division of
certain chromosomes, or nuclear segments, which appear
during division. This indirect process is termed mitosis,
or karyokinesis. The observation of nuclear division
usually requires material which has been carefully fixed
(with respect to protoplasmic structure), sectioned, and
stained.

A meristematic cell from the root of Indian corn may
typify the usual phenomena (Fig. 82). During the
erowth of the cell, the nucleus exhibits toward certain
stains definite reactions, and these are, for the most part,
ereatly intensified during division. The nuclear retic-
ulum shows at first some small chromatic thickenings or
scattered areas taking the stain more deeply, whilst the
nucleolus is also deeply stained. When the reticulum-

Growth ayay

like nature of the nuclear substance gradually gives way
to elongate chromatic structures, or to a chromatic band,
the prophase of the nuclear division is well advanced.

Later there appear well-defined nuclear segments.
termed chromosomes, these resulting apparently from the
ageregation and growth of chromatic substance in a cer- ©
tain area. Coincidently, the nucieolus is less stainable
and may show an apparent degeneration, foretelling its
final disappearance. The chromosomes thicken, the nu-
clear membrane disappears, and out of the mass of fibrous
protoplasmic elements now present, there is oriented first
a multipolar, and later a bipolar, spindle with the chro-
mosomes arranged as an equatorial plate. In this stage,
the metaphase, spindle fibers are attached to either side
of each chromosome, and a longitudinal split is apparent.
The halves of each chromosome separate and the ‘‘ daugh-
ter’? groups move (anaphase) to opposite poles of the
cell, where the organization of the daughter nuclei pro-
ceeds (telophase). Here a new reticulum is ultimately
evolved and a nucleolus reappears, formed, doubtless, in
some manner from the nuclear material. Upon the re-
maining spindle fibers at the middle points thickenings
occur, and these gradually extend as a plate between the
two ‘‘daughter”’ cells. Thus the cell-space and the
cytoplasm also are divided, and the cell division is com-
plete.

Upon the reappearance of the chromosomes in every
successive mitotic vegetative division, the number of
these segments is constant; that is to say, there is a defi-
nite chromosome number for every species of plant, and
the same is true of animals.

326 Plant Physiology

191. Cell division and respiration. — It is obvious that
the growth of the embryonic cell in protoplasm often leads
to a climax of energy-release in the complex activities of
nuclear and cell division. When growth of the cell does
not lead to division, or multiplication of kind, it is usually
a progression towards differentiation, a process likewise
involving abundant metabolic changes and energy-release.
It is not strange that respiration in healthy organs is, in
general, a measure of growth intensity.

192. The relation of pruning to growth. — Pruning, as
applied to trees, shrubs, and vines, is a practice which has
as its chief ends a regulation of growth and fruiting, and
a shaping or training of plants. Either one or the other
of these ends may be purely incidental, but the process
is most important as a thinning of the fruit buds, and for
the regulation and distribution of new wood. The prac-
tice must vary with the species of plant, and with the local
ideas of proper size and shape. Properly performed, it is
physiologically rational, and the world-wide development
of the practice attests its effectiveness. Pruning should
not be regarded usually as a special form of forcing for
fruit production.

In trees the leaf buds often develop most abundantly at
the tips; that is, at the periphery of the entire tree, so that
the tree grows as a constantly enlarging shell. There are
many more buds produced on the periphery than could
possibly be developed profitably. Ordinarily, many more
begin to develop than could succeed. Pruning is needed
to suppress some buds, and to permit others to grow more
vigorously. It is also needed with certain fruits in order
to cut out and restrict large branches, so that light may

Growth ZOOL,

enter the central portion of the tree, for the encourage-
ment of fruit production throughout.

In the majority of cases, cutting out some of the old
wood or pruning off a portion of the young wood, incites
more vigorous growth in the parts remaining. A _ too
heavy pruning may be distinctly injurious. It may incite
a large sucker or water-shoot growth at the expense of
{

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(Ml i

ny OY HAS ee
. i HTT if ok YY
Ae eG NX Se MN ale i pee . AY

Acie Zs
ae eka

Fic. 83. Pear trees trained against a wall.

fruiting, produce a general weakening of the tree, espe-
cially by loss of organic food to the roots, and finally
become a source of danger through the unnecessary
wounds. In general, pruning is most common in order to
maintain a certain balance between vegetative growth and
fruiting. No plant can illustrate this relationship better
than the grape. A failure to prune during a single season
will be followed by the development of a large number of

328 Plant Physiology

canes, but the bunches of fruit will be small and poorly
filled.

Pruning at the time of transplanting is invariably
necessary in order to keep the balance between root and
shoot. Resetting or transplanting may result not only
in injury to the roots, but often in the death of all rootlets ;

Fic. 84. The healing of wounds, after cutting off a lateral branch ; first
formation of callus (cl), after which three seasons (rings) of growth
were required. [After Curtis.]

and while the latter are being developed the leaf surface
must be reduced.

Ordinarily, pruning is a late winter practice, and this is
desirable, in the first place, because there is no injury
from bleeding, and secondly, on account of the prompt
covering of wounds by growth in the spring. For the latter
reason, also, branches are cut close to the main branch or
stem, where practicable, and no large stubs are left.

The covering or healing of wounds by the growth of
tissues beginning about the margin of the wound is a

Growth 329

response or adjustment bringing with it most important
sanitary advantages. A wound long exposed is an almost
certain beginning of a heart-rot of some type. The
development of tissue covering the wound proceeds from
the cambium. A callus or cushion of vigorous meriste-
matic tissue is produced, and this is extended from all
sides, until the wound is completely closed, when new
wood-rings are laid down over it (Fig. 84).

193. Budding and grafting. — The growth processes
immediately involved in budding and grafting are well
understood, but all of the relations of stock and scion are
not so clearly defined. In both budding and grafting,
the important principle is to unite the cambium of stock
and scion. When held firmly in contact by grafting-wax
or raffia, the meristems of the two individuals thus united
develop a callus, effecting a
close union; and wood is
subsequently laid down
from each contributing part,
cementing this union com-
pletely. In general, a
union so close as to insure:
the life of the scion is only
possible when the plants
are closely related. If
stock and scion grow at a |
different rate, that is, if the
seasonal rings of one are
thicker than those of the
other, there will be a con-

: : ; : Fia. 85. Grafting : cambium of stock
siderable difference in size and of scion (on one side) in contact.

330 Plant Physiology

of trunk above and below the region of the graft, and
this difference becomes more pronounced with age. Im-
proper, or difficult, union, if it does not result in im-
mediate death, will inhibit the transfer of material be-
tween shoot and root, and may lead to an abnormal
swelling in the region of the union.

194. Scion propagation. — As referred to in the dis-
cussion of reproduction, vegetative propagation is often
desirable, and propagation by buds or scions possesses a
variety of advantages, some of the most important of
which are as follows: (1) for the maintenance of varietal
characters, especially when the plant is of uncertain or
hybrid origin, when a return to the seed would yield an
unknown progeny, (2) for the more rapid propagation of
desirable species and varieties, and (3) for certain advan-
tages of growth or hardiness which may result by placing
the scion on roots other than its own.

195. Relation of stock to scion. — Commonly there is
believed to be relatively little direct formative influence
of the stock upon the scion, and an analysis of the facts
thus far demonstrated makes it clear that, as a whole, the
relations between stock and scion are very complex. The
effect of the stock upon the total amount of growth is
most evident in dwarf varieties, such varieties of the pear,
for example, are obtained by grafting pear scions upon
quince stocks. The scion is then furnished, in all proba-
bility, by a root system less active in absorption, and the
effects of this are evident not only in diminished size, but
also in slight modifications of leaves and sometimes of
fruit. Waugh has called attention to certain differences
in vigor of growth as well as in size and serration of leaves

Growth see

caused by the use of different stocks with plums. The
Milton plum worked on Mariana stock is more vigorous,

oe

Fic. 86. Fasciated shoot of Fritillaria, apparently induced by rapid
forcing.

and develops larger leaves than when Wayland or ameri-
cana stocks are employed. In the former case, however,
the leaf serrations are finer.

302 Plant Physiology

In the case of some other fruits, greater hardiness or
resistance to cold is secured by grafting upon hardy stocks.
The sweet orange is now commonly grafted upon the
rough lemon and upon the sour orange, both in Florida
and California, although some believe that the quality of
the orange is thereby somewhat affected. Several species
of American grapes are notably resistant to Phylloxera
(especially Vitis rotundifolia and V. riparia), and these
vines are now commonly employed as stocks in certain
sections of southern Europe where this insect has done
ereat damage. Since the insect is mainly injurious upon
the root, there is a direct advantage in using American
stocks.

The transmission of certain diseases, or pathological
conditions, such as peach yellows, contagious chlorosis,
etc., may occur by grafting, but in general it is felt that
there is relatively little of what may be termed special
chemical influence of the stock upon the scion. Litera-
ture is full, however, of contradictions and strife regarding
the mutual influence of stock and scion. An hereditary
effect has been claimed, but the lack of definite work with
strains sufficiently pure, renders the whole matter proble-
matical.

196. Forcing. — This term is rather loosely employed.
It may signify merely the production of plants out of
season, generally under glass or other protection, such as
the growing of tomatoes in the winter; again, it may
suggest the growing of plants which, in a particular lati-
tude, require certain well-controlled conditions. These
applications of the term require no further consideration
physiologically. When, however, it is implied that fore-

Growth 333

Fic. 87. Leaves of rhubarb grown under diverse conditions: in the
open (A) ; forced in dark cellar after being frozen outside (B) ; forced
in well-lighted cellar (C).

334 Plant Physiology

ing involves production under abnormal, or what may be
termed intensified, conditions, that is, under conditions
stimulating rapid growth, then factors may enter in
which require special attention from the standpoint of
erowth-stimulation. High temperature, increased mois-
ture, and an abundant food-supply are the factors com-
monly involved in forcing. Under such conditions there is,
of course, up to a certain maximum, a stimulation of vege-
tation. High succulence and brittleness are characteris-
tics of forced crops.

For the production of salad crops, radishes, ete., forcing
may be continuous, while in other cases, forcing condi-
tions are employed to start resting plants or roots into
rapid and vigorous growth for early market, as in the case
of roots of rhubarb and asparagus brought in from the
open. Large roots of rhubarb grown in the open for three
of four years may be lifted in the late winter or early
spring, reset in loose soil in a special cellar, hot bed, or
greenhouse, and then forced into rapid leaf-stalk produc-
tion. Forcing may also be employed for bulbs, tubers,
and seed in the seed bed. The practice in general requires
special care with ventilation; it often demands subirriga-
tion, and it repays a constant watchfulness with respect
to sanitary surroundings. Otherwise, the conditions may
greatly encourage the growth and spread of fungous
diseases and the development of other pathological dis-
turbances.

A special phase of forcing has become important in
recent times. This consists in awakening activity in
dormant plants or organs by means of warm water or
anesthetics.

Growth aon

197. Etherization. — Etherization of plants and bulbs
is rapidly becoming a common forcing practice with
florists, and it is to some extent applicable in market gar-
den work. By means of a suitable incubation in an atmos-
phere of ether or chloroform, it is possible to furnish the
incitation for rapid growth, particularly in the case of
resting plants and dormant bulbs and roots. It is thus
possible to bring such plants into more rapid vegetation
and flowering, to meet the special demands of particular
seasons or occasions.

Stimulated by the many experiments of Johannsen, in
northern Europe the practice has been very successfully
employed in forcing lilacs for the cut-flower trade, while in
southern Europe it is usually applied to the ‘‘ mimosa,”
a species of Acacia. It is notably economical of time,
space, and heat, in forcing many bulbous flowering plants.

In general, a common method of etherization is as
follows: The plants are exposed from 24 to 48 hours
in a tight chamber or box to an atmosphere of ether vapor,
with an ether tension preferably from 30 to 40 grams per
100 liters of space (approximately 4 ounce per cubic foot).
The concentration and the length of exposure should,
however, vary to suit the material, the more delicate
material requiring the weaker treatment. After treat- .
ment the plants are ordinarily placed immediately under
conditions favorable for growth.

If employed relatively early during the period of winter
rest with the lilacs, marked contrast is shown between the
forced and the control plants. During the early winter,
in the latitude of New York, this plant can be brought
into flower after etherization in from three to six weeks,

336 Plant Physiology

whereas twice as much time would be required without
etherization. Later in the season the lilac is not so
readily forced, and there is no such marked contrast be-
tween the treated and the control plants. Material

Fic. 88. Lilies of the valley, etherized (A) and unetherized (B), then
grown under similar conditions for the same length of time.

capable of beginning growth immediately cannot be
forced in this way.

There is some contradictory evidence respecting the
etherization of bulbs, but in general, the practice has been
successfully employed with lilies of the valley, narcissus
and daffodils, and certain lilies and tulips; but the best

Growth Bon

results in the United States have been obtained with lilies
of the valley. In many cases, the treated bulbs have been
brought into perfect blossom two or three weeks earlier
than normal. :
Howard determined that etherization will incite to more
rapid bud activity a large number of common deciduous
trees. He employed in one interesting series of experi-
ments. shoots from 70 species of trees, including many

EXPERIMENTS WITH 70 SPECIES OF TREES AND SHRUBS

TIME IN TIME IN | Poole PER CENT
a eee Days To | Days To wich | WHOSE Bups
BEGIN FULLY UNFOLDED
GROWTH OPEN Bups Sarit COMPLETELY
(COMIC LE Tne ZACS a elon Wa 50815) 44.2
Etherized 48 hours. . . eel 20.3 62.8 50.0
Etherized 48+ 48 hours! . PATE 18.3 Ayo Bow
MEIC CEO AN nice Wis ws 20a 26.2 52.8 45.7
Wriedrordays: “A . . . 13.8 18.7 ook 32.8
Hrezenis days . 0 3. 18.3 DEES | er aT af 22.8
Erozen 14 -days :.: . . 16.4 23.8 14.2 11.4
Darkness 8 days .. . 22.8 29.1 00.4 34.2
Darkness 14 days . . 23.9 29.2 58.5 32.8
Frozen 8 days
RN ae te cick ih 15.5 31.4 i taare
Frozen 8 days
Etherized 72 hours Ly ee ae pas

Darkness 5 days
Darkness 8 days
Etherized 48 hours
Darkness 8 days
Etherized 72 hours

20.5 26.5 65.7 38.9

j
Frozen 8 days ae 17.5 25.0 Dial 20.0

18.2 25.8 48.5 35.7

1 Interrupted exposure.

308 Plant Physiology

species which are known to be difficult to force into
activity. In this test there were employed several species
of Acer, Alnus, Azalea, Castanea, Cornus, Cratzgus,
Fraxinus, Populus, Quercus, Tilia, and Ulmus, besides
many genera represented by a single species. Shoots from
these plants were brought into the greenhouse at Halle,
Germany, from December 8 to 23. The preceding table
indicates the result of the etherization processes, and also
compares this method of forcing with others involving
change of conditions.

198. The effect of etherization. — There are as yet no
such definite indications as will permit a competent expla-
nation of the effects of etherization upon the plant. By
some the treatment is assumed to cause a stimulation,
and no further suggestion is made. The view is also ad-
vanced that there is an indirect effect upon the stored
food. Again it is assumed that there is a loss of water
from the cells, equivalent to a considerable time factor in
the general maturity process. There is apparently no
experimental work to confirm this view, and no ordinary
method of desiccation is so promptly effective. It is
more probable that the permeability of the protoplasm is
directly influenced.

199. Forcing by immersion in warm water. — In order
to start into more rapid and certain growth dormant stock
for transplanting, it has long been the custom with some
gardeners to immerse the roots in warm water. Recently
Molisch has reported many interesting experiments based
upon a practice of forcing by means of warm water.
The method is applicable to most plants commonly ether-
ized, such as lilac, mimosa, Forsythia, and bulbs. He has

Growth 339

also found it possible to force in a similar way several
hardy shrubs.
The method consists, in general, in immersing the plant

HI

Fie. 89. Shoot of lilac: branches to the right forced by the hot-water
method of Molisch ; branches to the left, control.

340 Piant Physiology

or branch in water at a temperature of from 30 to 35° C.
for a period of from 9 to 12 hours. When potted plants
are employed, it is preferable to invert the pot and im-
merse the stem portion only, since the roots are generally
more sensitive to injury. This method has certain prac-
tical advantages over etherization, and if as generally
successful, it will doubtless become important. The
changes brought about by this treatment have not been
determined.

200. Transplanting after wilting. — Practical truck
growers are often met who are in the habit of wilting cer-
tain seedlings before transplanting, claiming that plants
thus wilted recover promptly and grow off more vigor-
ously than others not so treated. Experiment seems to
confirm the practice for the tomato, and it may be sug-
gested, provisionally, that the effect is indirect. A rather
rough removal of tomato seedling from the seed bed
results in some injury to the rootlets and root-hairs. If
wilted, these roots do not recover upon transplantation,
and vigorous new roots are promptly developed under
suitable conditions.

On the other hand, it appears that in the case of those
seedlings placed without wilting under more favorable
conditions for growth, the injured roots may recover slowly,
and generally new roots are not so promptly developed.
It may, perhaps, be inferred that any plants which do
not readily develop new roots, such as the lettuce, corn,
etc., would be greatly injured by the wilting process. It
seems certain that transplanting with so great a ball of
earth as not to injure the rootlets would be preferable in
all cases, except where the roots are so much entangled as

Growth o41

to require being set free. It is not at all evident why
wilting may be favorable to many cuttings, unless, perhaps,
there is a tendency to permit too many leaves to remain
on the cuttings, the vigorous activity of which is then
permanently checked or inhibited by the wilting.

201. Growth movements. — Growth movements of
the varied sorts known may be referred to two types.
These are (1) autonomic, or those resulting from internal
and generally unknown conditions, and (2) paratonic,
resulting as a response to external conditions or stimuli.
Such movements are discussed in the special chapter on
growth movements, also in those chapters dealing with the
relations of plants to single environmental factors. It is
sufficient here to note that there are various types of
growth movement.

LABORATORY WORK

Elongation of root and shoot. — Determine the growing region
of roots of the horse bean, bean, or field pea. Use germinating
seed in which the radicle has developed to the extent of from 1 to
li inches. With a fine thread dipped in India ink mark off parallel
lines at equidistant intervals, of from 1 to 2 mm., placing the
first mark in one or two cases as near the root-tip as possible and
in other specimens at a full interval from the tip. Make daily
observations and measurements and give a table or plot curve
of the results. In order that the marked seedlings may be kept
under suitable conditions, place each in the bell of a thistle tube
(containing a little moist moss) with the root extending into the
tube, the lower end of the latter resting in water. Favorable
conditions may also be secured by pinning the seed, with the
roots projecting vertically, to the bottom of a large cork to which
has also been fastened moist filter or blotting paper. The cork
is then fitted into a tumbler containing some water.

342 Plant Physiology

Mark off also convenient (about 2 mm.) intervals on several
of the younger internodes of plants of Phaseolus growing in soil
and determine the region of elongation of the stem. Determine
also the total growth in successive internodes of a mature plant
and develop a graph of the results.

Remove carefully the leaves from a node or two of half-grown
oats or rye, mark off parallel lines on the internodes both near
the basal and the upper parts, and describe the elongation phe-
nomena, preserving the plants, if possible, under moist con-

Fic. 90. Force of growth in the ostrich fern; leaves breaking through
concrete pavement. [After Stone.]

ditions. Study and prepare a curve of growth for the scape of
dandelion, using plants growing either in the field or under green-
house conditions.

Extension of leaves. — Study the rate of development of broad
leaves such as those of grape, squash, or bean, measuring on
successive days or periods both length and breadth.

Secure branches of one or more trees in winter condition, such
as lilac, beech, poplar, and apple. Determine the average
number of nodes produced by a season’s growth and compare this
with the number of nodes or leaves found by the dissection of
half a dozen buds.

Growth in tissues. — Dissect out the growing point of Elodea
or Hippuris, mount in water under a cover glass, and examine.

Growth 343

Describe the formation of leaves. From prepared slides study
and draw the growing tip in longitudinal section.

From hand sections or from prepared slides study the secondary
thickening in the stem of sunflower, castor-bean, or other plant
of similar texture. Make some sections near the apex of the
growing shoot and some farther distant in order to follow the
development of inter-fascicular cambium and secondary bundles.
From prepared slides measure the extent of variation in the
growth of the seasonal rings. .

Adventitious organs. — (Roots.) Follow the development of
. adventitious roots upon cuttings of tomato, geranium, or grape.
In the case of tomatoes in fairly dry soil this is also conveniently
studied by binding to the stem at a node a ball of moist moss.
Germinate sunflower seeds and as soon as the radicle has emerged
about 4 inch cut off the latter about 1 inch from the coty-
ledons, place the cotyledonary portions on moss in a moist cham-
ber, and note the method of origin of the roots.

(Buds.) Grow seedlings of flax in a saucer of sand or soil
until the hypocotyls have about reached full growth. Then cut
off the upper portion of the plant about 1 inch below the coty-
ledons and discard the leafy portion. Cover the rooted hypo-
cotyls with a bell glass or tumbler to prevent drying out and
follow and describe the development of buds. Study the fleshy
root of sweet potato to ascertain if preformed buds are present.
Halve the root, place it upon moist sand under a bell glass, and
observe the development of shoots. Examine the leaves of
Bryophyllum calycinum for the presence of buds in the indenta-
tions of the margin. If no buds are found, place the leaves on
moist sand and observe occasionally. Follow likewise the
development of buds from a leaf of Begonia Rex, placing the leaf
upon moist sand with the petiole or a small part of the leaf slightly
covered. Sever a few of the larger veins, and protect the leaf
from drying out.

Hot water forcing. — During early winter or midwinter im-
merse for from 6 to 12 hours twigs of lilac (generally good) and
apple in a water bath controlled at a temperature of 35° C. Re-
serve some twigs untreated, or immerse them in water at 20° for

044 Plant Physiology

control. Plan both sets under conditions favorable for growth,
and compare the time periods required for development. It
should be remembered that positive results are obtained by such

WW

Fic. 91. A convenient etherization chamber, sectional view, showing
also carrier (c), groove for melted paraffin (g), and method of intro-
ducing ether.

forcing only when the plants treated are not in condition normally
to show immediate growth. Potted plants may be inverted and
immersed to the edge of the pot.

EKtherization. — In a tight zine box such as shown in Fig. 91,

Growth 345

or in a chamber improvised from vessels at hand, etherize shoots
or small plants of lilac and resting bulbs of lily-of-the-valley.
Use about 40 grams of ether per 100 liters of space, and leave the
plants in the chamber about 24 hours. Then place the plants
with some untreated specimens under conditions favorable for
growth, and compare the results. Write a short report stating
your opinion of the extent to which such forcing may find
practical application.

REFERENCES

AsKxenasy, KE. Ueber die jahrliche Periode der Knospen. Bot.
Zeit. 35: 792-815 et seq., pl. 4, 1877.

BaiLey, L. H. Lessons with Plants. (Cf. pp. 1-69.)

The Pruning-Book. 537 pp., 33 figs., 1898.

Bussz, W. W. Beitrage zur Kenntniss der Morphologie und
Jahresperiode der Weisstanne (Abiesalb a Mill). Flora.
it ANS 175, pl. 3, 1893.

GorBEL, K. Einleitung in die Experimentelle Morphologie der
Pflanzen. 260 pp., 135 figs., 1908.

Gruss, J. Beitrige zur Biologie der Knospen. Jahrb. f. wiss.
Bot. 23: 636-703, 4 pls., 1892.

Hartic, R. Das Holz der deutschen Nadelwaldbiume. 147
pp., 6 figs., 1885.

HABERLANDT, F. Landwirthschaftliche Pflanzenbau. II Die
Pflanze und ihr Wachsthum, pp. 189-296, 1879.

Howarp, W. L. Untersuchungen ueber die Winter-ruheperiode
der Pflanzen. Inaugural-Dissertation, Halle, 111 pp.

— An Experimental Study of the Rest Period in Plants.
Missouri Agl. Exp. Sta. Research Bul. 1: 105 pp., 1910.

JOHANNSEN, W. Das Aether-Verfahren beim Friihtreiben. 65
pp., 1906. [Fischer.]

Koopman, K. Grundlehren des Obstbaumschnittes. Landw.
Jahrb. 25: 497-618, pls. 5-28.

Mouiscu, H. Das Warmbad als Mittel zum Treiben der
Pflanzen. 38 pp., 1909. [Fischer.]

346 Plant Physiology

Moore, EMMELINE. The Study of Winter Buds (ete.). Bul.
Torrey Bot. Club. 37:117—-145, pls. 9-11, 1909.

Wauau, F. A. TheGraft Union. Mass. Agl. Exp. Sta., Tech.
Bul. 2:16 pp., 10 figs., 1904. (Compare also, Report
21 : 174-192.)

Texts. Barnes, Detmer, Ganong, Jost, Pfeffer, Stevens, Stras-
burger.

CEA PAE 4 X1Vv.

REPRODUCTION

Tue production of new individuals by any method
whatsoever is reproduction in the broader sense. Physio-
logically it is a complex and peculiarly interesting pro-
cess. In the higher plants, — angiosperms and gymno-
sperms, — we are concerned with reproduction by seeds
and reproduction by vegetative parts.

Seeds are embryonic plants with a certain food-supply
and protective coverings, while vegetative parts may be
shoots or any portion of the old individual which, when
placed under favorable conditions, will develop shoot and
root. The one type is usually sexual; the other is invari-
ably nonsexual.

Vegetative reproduction generally implies (1) the ad-
ventitious development of roots, as in cuttings, bulbs,
the potato, etc., where buds are preformed; or (2) of both
root and bud, as in the sweet-potato, Dahlia, etc. Repro-
duction by seeds involves commonly a variety of phe-
nomena including the differentiation of new structures,
the fusion of cells (gametes), and the origination of a new
individual from a fertilized egg-cell.

202. The seed habit and vegetative reproduction. —
Reflection upon the general conditions prevailing among
cultivated and wild plants leads to the conclusion that the

347

348 Plant Physiology

Fic. 92. Apricot blossoms ;
growth from stored food.

production of seed is for most
plants of paramount importance.
Vegetative methods of repro-
duction may also occur in plants
possessed of the power of abun-
dant seed production; and, in-
deed, under favorable circum-
stances the former may propagate
individuals more rapidly. Sup-
plementary vegetative methods
of reproduction are therefore
common. Wild onions and lilies
may have their “sets” and
bulbs. Numerous plants develop
offshoots, root shoots, and natural
layers, and so perpetuate them-
selves in a variety of ways. A
few plants both wild and cul-
tivated, such as forms of the water
weed and the yam, have entirely
or practically lost the power of
seed-making. In general, how-
ever, the seed is the basis of plant
production, although vegetative
reproduction has been employed
far beyond its natural course, and
this in order to perpetuate a type,
to multiply individuals quickly,
and to grow plants under climatic
conditions rendering seeding un-
profitable or impossible.

Reproduction 349

203. The flower: essential structures. — Richness of
color or striking form and fragrance in flowers may serve
useful ends leading toward reproduction.
Moreover, in ornamental plants these
qualities often represent the crop value of
the plant. Beneath an apple tree in
spring the ground may be white, strewn
with discarded petals, representing much
energy of growth, that was, nevertheless,
serviceable. In seed production, however,
it is stamens and pistils which are directly
important, and the inconspicuous, unob-
trusive, or unattractive flowers of spinach,
lettuce, and corn are as effective as the
beautiful or gaudy structures of the
orchid and hollyhock.

204. Pistil and stamen. — The pistil
is commonly composed of one or more
earpels. Whether consisting of one or of
several carpels, it embraces in common
types (1) the ovule-sac, generally a
membranous or fleshy structure, contain-
ing at the time of flowering the relatively
small, seed-like ovules, or megasporangia ;
(2) a more or less well defined style, upon ~
the terminal portion or surface of which is Fic. 93. Flower
differentiated (3) the stigma. obarley.

The stamens consist in general of a stalk part or fila-
ment, supporting the anther, which latter contains the
anther sacs, or microsporangia, with their pollen-grains.
Stamens and pistil may be present in the same flower,

350 Plant Physiology

known as a perfect flower, of which the apple, cotton,
wheat, etc., are examples. These structures may oc-
cur in different flowers, termed staminate and pistillate
flowers, upon the same plant, that is, moncecious (one
household) plants, of which the corn and squash are
examples; but they may occur upon different individuals,
that is, staminate and pistillate, or dicecious (two house-
holds) plants, of which latter type the hemp, certain

Fic. 94. Carpels and stigmas (A) of orchard grass; also enlarged view
of stigmatic cells and pollen germination.

mulberries, and the date-palm are examples. In any case,
approximately at the time the flowers are open, or mature,
the anthers of healthy stamens may set free considerable
pollen. At about the same time the stigma or stigmatic
surface of the carpel is receptive; that is, generally, in a
condition to catch or affix pollen-grains, and to afford
special conditions for their germination. ,

205. Pollination and pollen-tube penetration. — Pol-
lination isa mechanical process. As it naturally occurs, it

Reproduction aol

is the mere dusting of the stigmatic surfaces with pollen.
This pollen may be derived from the anthers of the same
blossom, from different flowers, individuals, or species.

7
ee
= Ree >
: J

ae

= —— ss,
Toe eee
—_

—— ale

a . os {
SS R
SS ~" Se eo
(aa NS OS oe
ee ee Ss: D

m=
th 5
es rt
oS ~

~

=

= SS

Fic. 95. Flowers of date-palm : staminate cluster (A) and blossoms (a) ;
pistillate cluster (B) and blossoms (b). [After Swingle.]

If such pollen-grains do not germinate, or if no germ-tubes
penetrate the stigma, they may awaken no more response
than so much dust. If we may acknowledge faith in the
current belief regarding pollen as a factor in ‘‘ hay fever,”

a02 Plant Physiology

then the pollen grains of the ragweed, of timothy, and of
some other plants are among those forms which may

Fic. 96. Staminate and pistillate blossoms of Begonia.

as dust, or perhaps in some chemical manner, severely
irritate the mucous membranes of man. However, the

» ae

Reproduction 308

dusting of flowers with pollen would not be dignified with a

name unless it led toward fertilization and reproduction.
The stigmatic surfaces of the receptive flowers are

generally moist, and often provided with a perceptible

Fic. 97. Pollen grains and pollen germination ; corn (a, b), apple (c),
sweet-pea (d), and Althea (f).

secretion. Upon this surface the pollen may germinate, —
almost any pollen may germinate; yet it usually happens,
from a variety of circumstances, that the pollen most
abundant upon any stigma is the pollen of the same
species. This is really what is generally implied by effec-
tive pollination. Strasburger, however, has made the
interesting discovery that upon any particular stigma pollen
of a plant in an entirely different family may not only

germinate, but may even penetrate the style to some extent.
2A

304 Plant Physiology

Normal pollination takes place through the agency of
wind and insects, for the most part; and it may be inter-
fered with by rain or other climatic conditions not resulting
in the death of the flower. Such conditions may close the
flowers, preventing the transfer of pollen; or beating rains

Fic. 98. Pistillate blossom of squash, showing large stigmatic surfaces.

may wash the pollen from the stigma. In consequence,
orchards may fail to be productive through the effect of
climatic conditions upon pollination.

206. Fertilization. — Fertilization is the union and fu-
sion of two single gametic cells, or nuclei, which have
previously been differentiated by a special course of de-
velopment. In flowering plants the one gamete is derived

inl

Reproduction 25

from the pollen-grain, the other from the embryo-sac in
the ovule. In the fusion of these nuclei, usually derived
from different organisms or flowers, the characters of two
individuals are fused. Two lines of ancestry are brought
together in one cell, the fertilized egg, or zygote, which
will develop into the embryo of the seed. It is important
to bear in mind some further details regarding the fertili-
zation process.

The pollen, as has been noted, is a distinct phase of the
plant. It represents upon germination the complete
male gametophyte, whose reproductive function is the
production of a gamete. In most cases the pollen-grain
consists of merely two cells,—asmaller cell practically within
alarger. The larger produces through germination a vege-
tative tube, the germ tube, which (in angiosperms) grows
through a differentiated portion of the style, or stylar canal ;
thence it penetrates the ovule, commonly through the
micropyle, until it comes in contact with the egg-apparatus,
and ultimately with the egg-cell. The smaller cell of the
pollen-grain is largely nucleus. The latter divides by the
time the pollen-tube breaks or ruptures, and one of these
two gametic nuclei fuses with the nucleus of the egg-cell,
the other gamete; thus fertilization is effected. The
fertilized egg —this single cell, or zygote —is the be-
ginning of the new individual that is developed within
the protecting coats of the ovule (now the young seed), in
turn inclosed by the ovule-sac, and often by other parts
of the flower which may assist in the development of the
fruit.

207. Universality of fertilization. —It is remarkable
how universal is this phenomenon of fertilization. It

306 Plant Physiology

occurs throughout nearly all the phyla and classes of
organisms, and is of unquestioned importance. It is not
possible to consider the many interesting opinions regard-

nsiohs : He ae

ee ee
Siicia cd ce
Pe
e

,&
ay ave
AC Nhe
ale be
paral A pie
a th age
4 | ee a Se:
af nat My
fie Rape
stds (nnigeeeoiees
sunlli Nupaniese giole tee
Py) a + XA ®.
rot SR RDS AROROCE RD
ORI HAR) Just’ ONS
Ted | RNY RA PKS
sort : wt Od O

Fic. 99. Pistil of squash: cross-section of style (A), with region of pollen-
tube penetration (a); portion of preceding, enlarged (B); longitud-
inal section (C) corresponding to the preceding ; cross-section of ovule
case (D) with region of pollen-tube penetration (a).

Reproduction 307

ing the results and benefits of the process; but emphasis
should be laid upon the union of characters — amphimixis
— thus effected. Reference is made i.
later to the segregation of characters G)
which is conceded to take place. =~ i

Every ovule requires a pollen-grain ie
and a pollen-tube. In fact, for fer- q
tilization of all the ovules in any
plant, many more pollen-grains ger- Hi
minate and penetrate the style, since |
two or more tubes may be directed
toward the same ovule. The water-
melon may develop more than five
hundred seed, so that more than a
thousand pollen grains should fall upon
a single stigma to insure the maturity
of all the seed.

All the ovules may be fertilized,
yet this does not guarantee fruit devel-
opment of all. Frequently the plant
would be unable to support the weight,
or growth demands, resulting from the
development of every fruit.

Unfertilized blossoms are usually
the first to fall, but familiar examples
are evident on every hand of wild and
cultivated plants which shed many Fic. 100. Carpel of a
fertile blossoms. Correlative growth aoe pigs.
influences, which are little understood, _ time of fertilization.
or an unfavorable environment, may take heavy toll as
the young fruit develops; so that at maturity only a small

358 Plant Physiology

percentage of the flowers may have completed their
functions. According to Waite, apples in good season set

cent of fruit, and
of this small per-
centage much is
lost during fruit
development. If
we put this into
4 figures, we find
that 1000 apple
blossoms may
yield about 50
young fruits, of
which only some
few reach matu-
rity. In other
cases, practically
every ovule may

\ mature, if fertili-
Fic. 101. Blossoms of cotton ; showy, but often zation is effected.
self-pollinated.

This is particu-
larly true of plants producing fewer flowers or floral axes,
as the corn or strawberry.

208. Cross-fertilization and self-fertilization. — These
terms are used more or less loosely. Cross-fertilization
generally indicates a fusion of gametes derived from differ-
ent individuals, resulting, therefore, from pollination of a
stigma with pollen derived from a different plant. To be
consistent, self-fertilization would then indicate that both
gametes are derived from the same individual.

no more than 5 per

Reproduction 309

As a matter of fact, there are several grades of self-
fertilization; thus fertilization as a result of pollinating
the stigma with pollen from the same blossom, from a
different blossom upon the same plant, or from another
plant derived by bud or scion propagation from the same
“parent ” stock. In much the same way cross-fertiliza-
tion is a broad term, applying when the gametes are de-
rived from any two. individuals (grown from seed) within
the species; that is, whether the crossing is between indi-
viduals from pure lines, from merely mixed seeds, or from
distinctly different strains or races.

When reference is made merely to the dusting with
pollen, the terms self and cross pollination should be
employed, but many authors writing popularly fail to
make these distinctions.

209. Cross-fertilization apparently the rule. — Cross-
fertilization is a phenomenon of common occurrence with a
considerable number of ecologically well-established native
species of plants, and vigorous cultivated varieties as
well. It is evidently effective, but it is by no means uni-
versal among seed-plants. If we accept the analysis
which has thus far been made, it is, however, far the more
common method among flowering plants. Cross-fertiliza-
tion is, of course, dependent upon cross-pollination, and
both are commonly associated with the remarkable
developments in form, color, and other characteristics of
numerous familiar flowers to which popular attention has
been so much attracted. Nevertheless, it should not be
understood that these striking peculiarities of floral
structures are in strict correlation with cross-pollination.
Dates, mulberries, hops, and hemp are invariably cross-

360 Plant Physiology

pollinated and cross-fertilized when fruit is produced,
because in these species stamens and pistils are on distinct
individuals. In corn there is opportunity for self-fer-

Fic. 102. An ear from an isolated stalk of corn ; infertility from lack of
cross-pollination.

tilization, but crossing is the rule. In fact, isolated stalks
of corn seldom set more than scattering grains (Fig. 102).

Darwin’s observation respecting the necessity of cross-
pollination in red clover has become a familiar instance
among perfect flowers. He demonstrated that the heads
of this species protected from bees and other insects set
no seed. This may not be due, in the case of clover, to
the ineffectiveness of the pollen of each particular blossom

Reproduction . 361

upon the stigma of the same flower, but rather to struc-
tural difficulties preventing pollination; that is to say,
itappears that the flowers may be self-fertile; nevertheless,
the effect is that the best seed production requires insect
visits. In consequence, to produce clover seed economi-
cally, this crop should be permitted to flower only in the
season when the bumblebees are abundant and active.
The first crop is too early, so that it is commonly cut for
hay, and the second crop is permitted to develop seed.

210. Darwin’s conclusions. — A study of the remark-
able morphological devices in many flowers pollinated by
insects suggested to Darwin the importance of determin-
ing, with respect to the offspring, the comparative physio-
logical effects of cross and self fertilization. He made
fertility and constitutional vigor of the offspring his field
of investigation. As a result of his extensive experiments
with some familiar plants throughout a period of years,
Darwin concluded that ‘“‘ cross-fertilization is generally
beneficial, and self-fertilization injurious.” In drawing
these conclusions, he made careful comparisons of the
offspring with respect to height, weight, constitutional
vigor, and fertility.

Darwin recognized some general exceptions, in which,
for instance, self-fertilization was often more effective for
a generation or two (as in tobacco and Petunia) than
crossing with relatively closely related individuals.
Nevertheless, in the case of tobacco a cross with wholly
fresh stock was invariably more effective than self-fer-
tilization. The most notable exception to his general
statement, quoted above, occurred in a vigorous individual
of the morning glory ([pomea purpurea), Hero, whose

362 Plant Physiology
descendants ‘‘ varied from the common type, not only in
acquiring great power of growth and increased fertility
when subjected to self-fertilization, but in not profiting
from a cross with a distinct stock; and this latter fact, if
trustworthy, is a unique case, as far as I have observed
in all my experiments.”’

211. The need of further work. — The question of cross
and self fertilization (cross-breeding and in-breeding) is
now receiving renewed attention. The indications are
that with many plants in-breeding is for plant production
far less dangerous than has been supposed. It seems to
be conspicuously dangerous in the case of corn, some
reasons for which will be subsequently considered. Sha-
mel states, ‘‘ In the breeding of tobacco it is well known
that cross-pollination within the limits of a single strain
produces inferior offspring, and only self-fertilization gives
offspring of the highest degree of vigor, though hybrids
between distinct strains of tobacco often display a vigor
superior to that of either parental strain. Examples
could be continued indefinitely, but even one instance, in
which long-continued in-breeding results in no injurious
effects, would be sufficient to discredit the old hypothesis.”

It is evident that in breeding studies each crop must be
examined with respect to this important point. It is to
be expected that much new evidence on the general prob-
lems of self and cross fertilization will be available, for
the more certain methods in recent years with pure-line
ancestry, the conception of unit characters, the develop-
ment of biometry — all make possible far more definite
experimental conditions.

212. Experiments with self-sterility in pear. — Waite

es

Reproduction 363

established an important principle in fruit-growing when
he showed that many varieties of the pear are self-sterile.
In the case of some varieties the capacity to set fruit when
self-pollinated is wholly lacking ; in other cases when the
varieties are limited to their own pollen, normal fruits
may be developed, yet even then fruit production is con-

Fic. 103. Difference in size of pears self (a) and cross pollinated (6).
[After Waite. ]

sidered to be less certain and the size often slightly re-
duced. These results, and many others since reported by
various observers, are particularly interesting, since self-
pollination has reference not merely to the use of pollen
from the same plant, but also from other individuals within
the variety. We are dealing, in this case, with clonal
varieties, such varieties being maintained by budding and
grafting. Different individuals are looked upon, therefore,
as very closely related. In most instances all are de-

364 Plant Physiology

scended from some one ancestral or stock individual, and
there have been, so to speak, nothing but generations of
this original individual. Pollination between the different
trees ina block of Bartlett pears would, therefore, be con-
sidered self-pollination.

According to Waite, some of the common varieties of
pear generally self-sterile are the following: Angou, Bart-
lett, Clairgeau, and many others, 22 in all. The varieties
showing a capacity for self-fertilization were 14 in number;
among these being the Flemish Beauty, Keiffer, Le Conte,
and Seckel. Climatic conditions have been shown to be
of some importance with respect to the general problem
of self-sterility (cf. Fletcher).

213. Self-sterility in other orchard trees. — The apple,
plum, peach, and other fruits are similarly more or less
self-sterile. The investigation of self-sterility, which was
given a special impetus tnrough the work of Waite, has
resulted in a modification in orchard-practices of im-
mense economic value. It is certain that the grower
needs to consider an adequate distribution of varieties
so that pollination with effective pollen may be secured to
all. A particular variety may not be constantly self-sterile
under diverse conditions, and it is clear that many prob-
lems respecting pollination await careful investigation.

214. Parthenogenesis. — There are exceptions to the
rule that the egg cell must be fertilized in order to develop
the embryo of the seed. The maturation of a cell oceupy-
ing the position of an egg with or without the usual segre-
gation, or reduction, division and its development without
fertilization constitute parthenogenesis. This phenome-
non is reported to be characteristic of several forms of

Reproduction 365

dandelion; and there are similar instances among hawk-
weeds, meadow rue, Alchemilla, and several other genera
of flowering plants. In fact, if we examine the cases re-
ported for all plants, both higher and lower, we find that
parthenogenesis, or that which is here included under that
term, is not uncommon. Development without fertili-
zation is a well-established phenomenon in certain insects
and other lower animals. There are also some extremely
interesting cases of what has been termed artificial par-
' thenogenesis, reported by Loeb, Lefevre, and others.
In the latter the artificial development of the egg-gamete,
without fertilization, is induced by chemical or physical
stimuli. When there is no tendency toward natural
parthenogenesis, this artificial stimulation of development
has never gone so far as to produce adult individuals, but
larval stages have been successfully reared.

215. Xenia in corn.— The appearance of the seed
gives usually no indication of the pollen which was effective
in fertilization. In corn, however, the observation was
recorded nearly two centuries ago that certain colored
sorts planted together ‘“ will mix and interchange their
colors’; that is, in the seed of the first year there will be
mixed colors. Numerous carefully conducted experiments
in recent years by De Vries, Correns, Webber, and others
now clearly demonstrate the immediate effects of the poiien
in corn, and a physiological explanation is at hand.

2152. Indications of xenia. — We may first note the re-
sults of xenia. If pollen of the black Mexican or Cuzco
varieties, forms possessing a bluish-black aleurone layer,
are applied to the silks of white or yellow sorts, many of
the seed resulting show the blue-black color of the pollen

showing the immediate effect, through double

Fic. 104. Xenia in corn,
[Photograph from Bureau

fertilization, of the pollen-producing parent.
of Plant Industry.]

Reproduction 367

plant. Again, if the young ears of sweet corn are pol-
linated with a pollen from Dent and Flint varieties, there
result many seeds with smooth kernels and starchy endo-
sperm (Fig. 104). These results may be secured with pure-
line strains under control conditions, and the phenomenon
is recognized as xenia. Characters such as those above
noted alone exhibit true xenia; thus color or chemical
content, qualities which reside in the endosperm, are of
this nature, while qualities evident through the embryo
belong to another category. ‘The explanation of this en-
dosperm phenomenon has now been found in the process
of double-fertilization.

Generally the nucleus and cytoplasm of the embryo-sac
develop the endosperm, and only the pistil-bearing plant
is concerned with the qualities of this material. In the
lily, in corn, and in many other cases more recently made
known, the nucleus of the embryo-sac may fuse with the
second sperm nucleus from the pollen-tube, and thus the
endosperm may acquire, as well as the embryo, qualities
of the pollen-producing plant. It is evident that corn
which possesses color by virtue of a pigment in the pericarp
will not show this type of phenomenon, for the pericarp has
no means of becoming immediately endowed with the
qualities brought by the pollen. Cases of xenia, therefore,
should not be confused with ordinary speckled ears. The
latter may result in the second generation, or later, from
crosses in which color is one of the characters of one or
both parents, or from bud variation. Xenia is a question
of physiological interest in plant production, but it has
apparently very little practical bearing. When it exists,
it is an infallible sign of hybridization, and in some cases

368 Plant Physiology

it may serve as a valuable control suggestion in hybridi-
zation work.

216. False xenia. — Certain beans and peas show ex-
ternally in the color of the embryo the immediate effects
of the pollen. The seed-coats are more or less colorless,
and the characteristics of the embryo are, therefore, ap-
parent. Physiologically this is in no way comparable to
the previous phenomenon, and if the term “ xenia’’ is
used in this connection, it should be expressed false xenia.

217. Other secondary effects of pollination. — Under
xenia we have considered only those instances of the sub-
sidiary effects of the pollen which may be attributed
directly to fertilization, or double fertilization, and
manifest through effects produced in a readily explained
manner upon the embryo and endosperm. A certain
special stimulating action of pollination upon structures
outside of the ovule was long ago suggested by Focke and
others. The problem may, for the moment, be restricted
to cases in which there is fertilization. The question may
then be formulated as follows: In the normal production
of fertile seeds, is there any evidence that pollen from
different varieties or species will influence the form, color,
or quality of the fruit?

Beyond all question, the form of the fruit, and even the
quality of the fruit, will be affected when only a few ovules
are fertilized; for the reason that there will be incomplete
development of the fruit as a whole in the great majority
of plants, notably in many varieties of the tomato and
apple. Opinions differ regarding the important effect of
pollen from different varieties on the form or. color of fruit
when fertilization is complete. In various horticultural

Reproduction | 369

reports it has been stated that very often a definite effect
is due to the source of the pollen. An analysis of the data
seemed to indicate that the observations which have been
made neither positively confirm nor deny a direct and
specific stimulation of the pollen upon the fruit-production.
In the paper previously referred to, Waite draws this
conclusion: “There seemed to be, however, constant
differences between the Bartlett fruits crossed with dif-
ferent kinds of pollen. If these distinctions can be con-
firmed by future experiments, a question of considerable
importance will be settled.”’ Lewis and Vincent seem to
concur in the belief that there is an immediate effect of
the pollen, and they cite the deep red color in Spitzenberg
apples pollinated with Arkansas Black as compared with
the lighter red obtained when Jonathan is the pollenizer.
218. Parthenocarpic development. —It is considered
to be a general rule that lack of fertilization is followed by
more or less prompt shedding of the infertile blossoms.
There are, however, important exceptions to this course of
development. Seedless fruits of garden and orchard crops
are known. Seedlessness, or imperfect seed development,
is very properly associated with the failure of fertilization,
although it may happen that some ovules fail to develop
after fertilization. On the whole, it seems to be clearly
demonstrated that in some cases the ovary and attached
parts, technically the fruit, may develop as a purely vege-
tative organ, varying more or less, of course, in size from
its normal form when fertilization has taken place.
Among vegetables the cases of parthenocarpic develop-
ment best known are those of the English forcing cucumber

and certain varieties of the eggplant. Gardeners fre-
2B

BYAU Plant Physiology

quently prevent the pollination of the forcing cucumber,
especially when it is to be used for fancy market purposes.
Certain varieties normally show the development of seeds
toward the apical end of the fruit only, and this part is
then more or less abnormal in size, so that the general form
of the plant is injured. It would appear that in the case
of the eggplant a relatively small number of blossoms will,
under ordinary circumstances, develop fruit without seeds,
and Munson succeeded in obtaining fruits of normal size
and form. Many other observations might be cited of the
occasional appearance of seedless vegetables, but our chief
interest should be directed towards certain observations
upon. fruits.

219. Parthenocarpic formation in pomaceous fruits. —
Waite ascertained that in certain cases self-fertilized
pears “‘ are deficient in seeds, usually having only abortive
seeds, while the crosses are well supplied with sound
seeds.” In fact, there were two significant exceptions to
the rule requiring pollination and fertilization for fruit
development. Upon the Le Conte and MHeathcoate
varieties a few fruits were set without pollen. Still, in
those two instances, there was some doubt as to the com-
plete exclusion of pollination. A few isolated instances
of the occurrence of pears, apples, and other pomaceous
fruits without seed have been recorded, but the possibil-
ity of perfect fruit development in certain varieties, or
the development of varieties which may not require fertili-
zation, has only recently been carefully investigated.
Ewert for one has made this matter a subject of experi-
mental study, and it would appear that when pollination
is prevented, perfect fruits may result in the Cellini and

Reproduction 371

Charlamowski apples, and in six varieties of pears, of
which the most important is the Clairgeau. In the best
of the cases reported by him the fruits suffered neither
diminution in size nor change of quality, although parthen-
ocarpic development is commonly accompanied by de-
creased size. Sections of well-fertilized and seedless
fruits are shown in Figure 105.

Rant if ae
1

_ | | WG
/

——
—————
‘ ~

SS

Fie. 105. Section of fertile (seed-bearing) and seedless (parthenocarpic)
apples.

2192. Seedlessness in the orange, grape, and banana.
— Well-established varieties of citrus fruits, commercially
important, normally produce no seed. The California
navel oranges are of this type. In this instance pollina-
tion cannot lead to fertilization, since, according to the
reports available, the stigmatic surface apparently fails
to reach full development, or to become normally exposed,
and thus germination and the entrance of the pollen tubes
is precluded. Further observations upon this point are
needed.

Historically of more interest are certain cases of par-

B12 Plant Physiology

thenocarpy among grapes. The small seedless black or
currant grapes of Greece and the eastern Mediterranean
region furnish the dried cur-
rants of commerce. Other in-
stances are to be found in the
famous Sultana and a few other
raisins now propagated in Cali-
fornia as well as in the Medi-
terranean region. The percent-
Fig. 106. Seedless navel (par- age of seedlessness In one of

thenocarpic) and common the table grapes, Black Eagle,

(seed-bearing) orange. is likewise considerable. The
cause of seedlessness, or lack of fertilization, in these
cases does not seem to have received scientific attention.

Many varieties of the banana fail to set seed, although
it would seem that effective pollen is produced.

220. Nonsexual reproduction. — Multiplication by
vegetative parts is notably common among plants, wild
and cultivated. A single individual may in various ways
give rise to a colony or complete plantation of its own kind
without the production of seed. These may remain in
organic connection for a period of time, or they may be
promptly separated, one from another, by the death of
connecting parts. Wheat and other cereals and grasses
have the habit of stooling; that is, of multiplying by buds
from the lower submerged nodes. Blue-grass and John-
son-grass are among those plants which produce under-
ground stems, while the decumbent Bermuda and quack-
erasses are among those which regularly take root at the
joints. White-clover spreads in a way analogous to the
latter, and the strawberry develops runners which bud

Reproduction 373

and take root a few inches or more from the parent plant.
Advantage may be taken of all such nonsexual methods
in practical propagation. Moreover, in all such cases the
vegetative method enables the producer to be sure of the
variety or form, the propagation of which he is continuing,
since there is then, at least, no chance of mixing by hy-
bridization, or of change through segregation. As a rule,
plants that reproduce in a vegetative manner occupy the
land quickly. The method is, of course, of great service
when the plants are useful, but it may be most trying when
this habit is that possessed by a persistent weed.

221. Thickened roots and tubers. — Irish potatoes and
Madeira vines are types of plants propagated by tubers or
thickened stems, produced generally only by underground
buds. In the varieties of the potato commonly grown the
seed-ball is now seldom seen, so that there are varieties
the existence of which, in culture at least, is dependent
upon vegetative reproduction. Sweet potatoes, yams,
dahlias, and other familiar plants are propagated by
thickened roots. Many of these forms have in cultivation,
under ordinary conditions, lost the power of seed produc-
tion. The propagation of some edible and many floricul-
tural plants by bulbs and corms is so common among
liliaceous genera that the production of bulbs now repre-
sents a group of special industries. Holland is famous the
world over for this type of work, but doubtless the condi-
tions there afforded are practically duplicated in many
other places.

222. Cuttings. — A countless array of ornamental
herbaceous forms and some small and bush fruits are
regularly propagated by cuttings. In fact, carefully

ol4 Plant Physiology

handled, there are few herbaceous plants of indefinite
growth which may not be successfully multiplied in this
manner. In general, the growth relations are simple.
The propagator employs a small portion of a branch con-
taining one or more nodes. These nodes with active or
dormant buds are capable of developing shoots. The
essential response demanded of the plant is that under
conditions favorable for growth it shall be able to develop
adventitious roots. Such roots are by no means uncom-
mon in nature, and they develop apparently more or less
in correlation with the needs of the plant. The extent to
which plant parts are able to develop such roots is re-
markable. It has been ascertained that cotyledons and
leaves are in no small number of cases able to develop
these roots as efficiently as stems, and the leaves could,
therefore, be employed in propagation if there were also
the possibility of bud development. In fact, there are a
few types which have the habit of producing buds, and
such leaves are made use of in this manner. The leaves
of several species of Begonia, notably the varieties of
Begonia Rex, also certain forms of Bryophyllum and related
plants, are thus employed, as already indicated.

223. Precautions with cuttings. —It is evident that
branches or shoots used as cuttings require, in general,
careful treatment. Twigs or canes in a resting condition
may require no special consideration, so that currants,
grapes, and other fruits root readily under field or garden
conditions. Throughout the South, sugar cane is gener-
ally propagated by planting the whole stalks, and also by
using the lower parts of stools, —the rattoons. Shoots
in full leaf always require more attention, since the loss of

Reproduction 310

water (transpiration) may be excessive and death by dry-
ing-out is then a chief cause of failure. In all cases the
reduction of the transpiratory surface to a minimum is
required, and it is essential that the conditions of the cut-
ting bench shall be most favorable with respect to moisture,
drainage, light, and temperature. The exposed cut sur-
faces are also more subject to the attacks of hemi-parasitic
or damping-off fungi; therefore, the cutting bench re-
quires the same careful attention as the seed-bed. In
many cases a thorough knowledge of the growth habits of
the plants and the best skill of the gardener will be required
to determine the conditions needed. It may be neces-
sary by special means to induce root development in the
portion to be used for a cutting before the branch is sepa-
rated from the stalk, as by attaching a pot with moist soil
or moss. Special cases, however, are too numerous to
receive consideration.

224. Vegetative reproduction and running out. — Some
observers have held that vegetative reproduction repeated
through numerous generations results in deterioration ;
but many or all of the cases cited in substantiation of this
view are, in the opinions of others, wholly invalid. Never-
theless, in one sense varieties may ‘‘run out.’’ Thus bud
variation may be so great that in time the original form
may be entirely lost; this-is ‘‘ varying out.”’

It would seem that the yam (Dvoscorea sativa) has been
vegetatively propagated in China for two thousand years,
and there is no evidence that it is decadent. The sweet-
potato has apparently long lost the power of seed produc-
tion, and we cannot assume that it has lost in vegetative
vigor. The fig and the date have not been commonly

376 Plant Physiology

grown from seed for several centuries. The European
grape-vine (Vitis vinifera) has been grown more than 5000
years, and vegetative propagation has been the rule.
When native American vines were carried to Europe,
diseases new to Europe were introduced, and European
vines were so susceptible that enormous injury resulted.
Many thought that this weakness with respect to disease
was a result of the long vegetative culture ; but this seemed
to be disproved by the fact that seedling sorts of that
species of vine were equally susceptible.

Resistance or susceptibility to any disease appears to be,
in many cases, a character, or a complex of characters, and
may follow known laws of heredity, as in the case of other
characters subsequently discussed. Moreover, among the
more familiar molds and other fungi there are some notably
ubiquitous and vigorous forms which are not known to
possess sexual stages. Among the species of living things
generally, however, the frequency of gametic fusion, on
the one hand, and the complete loss of this process among
others constitutes a biological paradox.

225. Relation of vegetation to fruiting. — Plants exhibit
a remarkable diversity in the relations between vegetative
development and fruiting. With respect to annual,
biennial, and perennial habits this has been briefly con-
sidered. Generally speaking, fruiting is the climax of a
continuous or interrupted period of vegetative develop-
ment. The American Agave grows many years vegeta-
tively, and then through the formation of an enormous
flower-stalk and abundant fruits the leafy parts are drawn
upon to such extent that they are left exhausted and
incapable of recovery.

Reproduction ald

Under exceptional conditions fruiting of the oak or
apple may occur in the nursery stock. On the other
hand annuals may be induced to grow for a period of
years without flowering. With a careful selection of
conditions, and by employing vegetative propagation when
necessary, Klebs was able to induce continuous growth
and fruiting in Parietaria officinalis. Uninterrupted
erowth, without flowering, was obtained with Fragaria
lucida, Glechoma hederacea, Rumex acetosa, and other
species, — plants which normally produce blossoms in’
summer. These facts suffice to suggest the complexity
of the relations, and the importance of determining the
releasing stimuli, with respect to vegetation and fruiting.

LABORATORY WORK

The flower. — Review or study the morphology of the flower,
giving attention to moncecious and dicecious plants as well as to
those with perfect flowers.

Study more completely the floral mechanism in two or three
representatives of some one order, such as the Liliacez or Legu-
minosz. [Consult Church or some other convenient text.|

Anther and pollen. — Cut crosswise the large anthers of some
plant, such as lily or tomato, press out the pollen-forming areas,
and note the changes in the character of the contents of the anther
(pollen) sacs, or microsporangia as maturity proceeds.

Study and describe the pollen from plants in at least two
different orders, mounting it both dry and in water.

Set up germination experiments with pollen from several plants
which produce tubes readily (within a few hours) in water or
sugar solution. Germinating well in 3 per cent sugar solution
there are among the numerous monocotyledons which might be
used, several species of lily, also orchids, tulip, and Narcissus;
while cucumber, buttercups, willows, and Erica are among favor-
able dicotyledons.

378 Plant Physiology

The pollen grains may be sown in a drop of the sugar solution
on the slide, which is then placed in a moist chamber. Prefer-
ably, however, prepare hanging drop cultures (Fig. 107) made
of glass rings cemented to the slide with wax, over each of which
is inverted a cover-glass with a drop of the solution, in which the
grains are sown. In the bottom of the cell is placed the same
solution as employed in germination, and upon the upper rim
of the ring is a thin layer of petrolatum in order to afford a closed
chamber.

Pollen of corn and some other grasses, also many sedges and
rushes, germinate best in a moist atmosphere, and these may be
sown on a dry cover-glass inverted over a cell containing water.

Fic. 107. Hanging-drop cultures, used in the study of pollen germination.

Pistil and fertilization. — Follow the changes in the stigmatic
surfaces of several flowers as they open. Trace the canals or
modified tissue through which the pollen tube penetrates. In
the lily, squash, or cucumber, and many other plants the pollen
tubes are readily seen in longitudinal section.

If prepared slides are available, study the morphological evi-
denees of fertilization.

From the open buds of any plants convenient dissect out the
stamens (‘‘emasculate’’) before the pollen is matured, or the
stigmatic surfaces exposed ; inclose the flowers in paper bags, or
oiled paper, and after a week or more determine the effect upon
ovule development and seed production in comparison with
control plants. With a dicecious plant, such as Indian corn,
merely protect from pollination the pistillate axis or ear.

Fruit setting. — In the proper season make a careful count of
the number of blossoms produced by such plants as the apple
or peach, and later determine the percentage of fruit which may
be set.

Reproduction 379

Xenia. — Examine ears of corn from plots in which a starchy
dent variety has been grown alongside of, or together with, a sweet
corn. In the case where sweet corn was planted note particu-
larly the influence of the starchy variety in modifying endosperm
characters, and compare this with the amount of modification in
the other variety.

Parthenocarpy.— Examine the seedless fruits of any material
available, such as banana, grape, navel orange. Determine
the extent of ovular development. If English forcing cucumbers
(such as the Telegraph) are available, cut out the stamens before
the pollen is matured, bag the flower, and determine the effect
upon the development of fruit in comparison with that of a fruit
hand-pollinated at the proper time.

REFERENCES

Cuurcu, A. H. Types of Floral Mechanism. 1:211 pp., 39
pls., 1908.

Correns, C. Bastarde zwischen Maisrassen, mit bes. Beriick-
sichtigung der Xenien. Bibl. Bot. Orig.-Abh. a. d. Gesammt-
gebiet d. Bot. 53: 161 pp., 2 pls., 1901.

CouutTser, J. M., and CHamBeERLAIN, C. J. Morphology of
Angiosperms. 348 pp., 1903.

Darwin, CHarues. Cross and Self-fertilization in the Vege-
table Kingdom. 469 pp., 1885. [Appleton.]

Ewert, R. Die Parthenocarpie oder Jungfernfriichtigkeit der
Obstbiume. 57 pp., 18 figs., 1907.

FuetcHer, 8S. W. Pollination in Orchards. Cornell Agl. Exp.
Sta. Bul. 181: 335-364, figs. 65-86, 1900.

Kuess,G. Willkiirliche Entwickelungsanderungen bei Pflanzen.
166 pp., 28 figs., 1903.

—— Uber Variationen der Bliiten. Jahrb.f. wiss. Bot. 42:155-
320, 27 jfigs., 1 pl., 1905.

Lewis, C. I., and Vincent, C. C. Pollination of the Apple.
Oregon Agl. Exp. Sta. Bul. 104: 40 pp., 14 pls., 1909.
Liprorss, B. Weitere Beitrige zur Biologie des Pollens. Jahrb.

f. wiss. Bot. 32: 232-312, 1899.

380 Plant Physiology

Liprorss, B. Untersuchungen iiber die Reizbewegungen der
Pollenschlauche. I. Der Chemotropismus. Zeit. Bot.
1: 443-496, pl. 3, 1909.

Mosius, M. Beitrage zur Lehre von der Fortpflanzung der
Gewiichse. 212 pp., 1897. [Fischer.]

SHAMEL, A. D. The Improvement of Tobacco by Breeding and
Selection. Yearbook U. S. Dept. Agl. (1904) : 435-452.
Waitt, M. B. The Pollination of Pear Flowers. Div. Veg.
Path. and Phys., U. S. Dept. Agl. Bul. 5: 110 pp., 12 pls.,

1894.

Texts. Detmer, Jost, Pfeffer, Strasburger.

CHAPTER XV

Pe SHEED IN PLANT PRODUCTION

NECESSARILY the quality and potential vigor of the seed
are most important considerations in crop production.
Quality is to a very large extent based upon physiological
conditions. It does not seem entirely appropriate to dis-
cuss here such matters as “‘ true to type ”’ seed, impurities
and adulterants, the contamination of the seed by means
of fungous spores present, or of a dormant mycelium within
the tissues which may carry disease to the new crop. We
are concerned, however, with the adaptability of the strain
to the conditions under which it is to be grown, and with
the capacity of the seed to produce the most vigorous
plants of which the variety is capable. Quality of the
seed so far as vigor and adaptability are concerned will be
affected by conditions which permit of an arrangement in
the. following category : —

By the conditions under which the parent plant has been
erown.

By the conditions under which maturity has been attained.

By methods of harvesting and curing.

By the period and conditions of storage.

By size and weight.

226. Habitat conditions of the parent plant. — This is
properly an hereditary consideration, but it is conven-
381

382 Plant Physiology

iently treated here for special emphasis. It is now estab-
lished beyond reasonable doubt that the quality of seeds
will be modified in a single generation by the climate and
cultural conditions under which the crop has been grown.
This may have no reference whatsoever to the special
factors influencing curing, storage, and the like. For a
long time it has been clearly recognized that if early corn,
notably sweet corn, is grown from the same seed at points
North and South, there will result differences in the quality
of the seed produced, so far as earliness is concerned, so
that if seed from the two regions are sown side by side, that
from the North will mature earlier.

It is possible that in an indirect way the differences may
be in some cases ultimately referred to maturity; yet
these effects must be regarded, at present, as the imme-
diate effects of the environment. Many data respecting
the rapidity of the changes which may be induced under
different cultural and climatic conditions have been given in
agricultural literature. The table on page 340, from Lyon,
exhibits results with a variety of wheat grown several
years in different localities (with, of course, some oppor-
tunity for selection) and finally in adjacent plots in Ne-
braska, — these being strains which were, “ without
doubt, originally the same.”

The facts developed regarding corn are also true when
applied to spring wheat and oats, for it is agreed that in
planting spring wheat, seed obtained from farther north
will ripen earlier and give better yield, as well as quality,
than seed of the same strain introduced from a point
farther south. In the case of winter wheats, however, the
facts seem thoroughly to substantiate the general belief

The Seed in Plant Production 383

that the reverse condition is true; that is, seed from points
south give a better yield than northern grown seed of that
strain. In general, adjusted local varieties are best.

MopIFICATIONS INDUCED IN WHEAT By ITs ENVIRONMENT

aioe ace Iowa SEED Ou10 SEED

Date of sowing Sept. 9 Sept. 9 Sept. 9 Sept. 9
Kodged =>. . None None Badly Badly
US Very little Very little Much Much
Date of ripen-

TS Tanne a ae June 25 June 27 July 2 July 3
Yield of grain

DEL ACTE. 0. 29.1 bu. 27.5 bu. Docs Uk. 23-1 bus
Weight of grain

per bushel . 64.2 lb. 62.2 lb. 56.9 lb. 58.9 lb.

227. Localization of seed production. — In many cases

it is not possible to explain the localization which charac-
terizes commercial seed production, and it would appear
that seeds are grown in particular regions to-day, not only
because of some apparent advantages of the region with
respect to the maintenance or development of desirable
hereditary qualities, but also because of such factors as
(1) cheapness of production in the locality, (2) the effects
of conditions upon maturity and curing, referred to later,
or else (3) from merest accident. It would be well if
more general recognition were given to the two underlying
principles. On the one hand there are regions, or locali-
ties, especially favorable for the production of high quality
in seeds; on the other hand, it is often the case that
local seed production and seed selection would have

384 Plant Physiology

advantages outweighing all other considerations of special
habitat.

In the United States alfalfa seed mature well only in the
dry climate of states like Colorado and Utah. In the
South the potato matures so early that a long season of
storage, resulting in probable injury, would be required if
home-grown tubers were used in planting.

Some growers have expressed the opinion that there
is a marked physiological change more or less gradually
developed in strains of onions or radishes repeatedly grown
in certain sections of California. It is not possible at pres-
ent to determine if other factors have been overlooked,
but at any rate it is believed that seed from radishes which
have been grown for successive years in California will,
when planted in other sections of the country alongside of
the home-grown or recently imported seed of the same strain,
show clearly that the far western-grown product has under-
gone some marked change with respect to eastern conditions.

Similarly, onions grown from California seed are said
to be different in keeping quality from those bulbs grown
from seed produced in Michigan. This effect is said to
assert itself even when the most stringent methods of
selection are practiced. There is grave doubt if this is a
general rule, and we may well believe that varieties may
be developed which will not show this tendency.

The seed of cabbage, cauliflower, and some other crucifers
were first grown extensively in this country upon Long
Island, and the region became famous for the production
of these crops. More recently it has been found that a
similar favorable locality is the Puget Sound region.
Growers are so sure of the wholesome effects of these

The Seed in Plant Production — 385

localities upon the product that one will frequently hear
it stated that the failure to head properly is due to the fact
that the seed was not grown in either of these regions.

Tomato seed are grown extensively in Michigan, and they
have been successfully produced in many parts of the North
and of the central-West. On the other hand, the belief
is prevalent that tomatoes grown from seed produced
in the South rapidly deteriorate, and that in the course
of a few years the well-established and highly prized
varieties may revert to the common little-tomato type.
Here again there are no statistical data indicating that
these opinions have been formed as a result of any properly
controlled experiments. It is, for instance, quite possible
that by means of crossing between varieties, or by crossing
with the little-tomato type these reversions may be ac-
counted for.

Tracy has reported that beans are promptly modified
by soil conditions, and that in general seed should be grown
on. the type of soil for which they are intended. It. is,
furthermore, an interesting fact that German and French
growers importing seed are often careful respecting the
climatic and soil conditions under which the seed are
grown.

228. Maturity. — Quality may also be affected by the
conditions which maintain just at the time the seed is
maturing or during the state of maturity. Too much
moisture at the time the seed is approaching this state
precludes a proper gradual ripening, and the final effect
is usually manifest in decreased vitality; that is, lessened
capacity to germinate, and this is true even if the seed

is subsequently dried and stored. The reduced vitality
2

386 Plant Physiology

may be connected with the conditions in which the food
substances are stored in the seed, with the development of
injurious substances which lead to undesirable transfor-
mations, or with the continuance of activity after the
seed should have attained practically a dormant condi-
tion.

Moreover, immaturity has a tendency to lessen the
keeping quality of most seeds, and many of the shrunken
seeds upon the market, frequently met with in the case of
alfalfas and clovers, are due to their immaturity at the
time of harvesting. Apparently it is a general rule that
the sooner immature seeds are sown the more vigorous
will be the plants which they are able to produce. In
other words, a gradual deterioration takes place in stor-
age, but more promptly than in the case of well-matured
seeds. Extensive experiments in determining the effect of
maturity upon vitality as exhibited by germination tests
were carried out by Hellriegel. In the case of rye the
seeds were harvested at four different stages, and the
following table indicates the relative condition of ripeness
and the percentage of germination from such seeds, which
were subsequently treated alike with respect to drying
and storage: —

STAGE OF RIPENESS PERCENTAGE OF GERMINATION
Contents of kernel watery . . . ./| 4.5
17 LE Pegs So 2 eS er or i eS 5.0
Dough stage Seg)! Pe ees 9.5
Yellow ripe <2” 42 .on. se Ss 2 Se ete 36.0
Dry ae eine te be) cena ane ees 84.0

The Seed in Plant Production 387

The above data were secured from seeds immediately
separated from the parent stalk and then dried. When,
however, the seeds were allowed to remain attached to the
harvested stalks, notable gain in the vitality was shown
by those seeds harvested in early stages. In such experi-
ments as the last mentioned there is opportunity for con-
siderable ripening after the early harvesting, and the
results are not contrary to what might be expected.

According to the experience of some observers, a con-
tinued practice of selecting immature seeds may result
in the development of an earlier variety. This is some-
times, however, at the expense of size, quality, and vitality.

Kedzie has shown the effect of maturity upon vitality
of wheat, and his results are so striking that they may be
presented in detail : —

Maturity as Arrectine VitTauity (Kedzie)

DaTE OF HARVEST STAGE aa ENED Oo

PER ACRE PLUMULE
ieee ormncatin bee! . °. \ Malky jince +: 11 bu. 6.0 in.
peered oo Dougie so 25 bu. 9.0 in.
Nalyat@e eo. eo es. | Full yellow ripe 30 bu. 10.1 in.
Nien ee fe | Dead ripe. noe 28 bu. 11.0 in.

It should be said, however, that so far as ability to
grow is concerned, no very narrow restrictions may be
placed upon the stage of development of the seed, provided
adequate and suitable nourishment can be given the young
embryo. In a series of experiments recently carried out
by the writer, whereby the young embryos were trans-
ferred from the developing seeds to sterile nutrient solu-

388 Plant Physiology

tions, the results confirm the view that embryos thus
treated are able to maintain themselves and sometimes able
to develop mature plants. The vigor and strength of the
plant, however, was in direct proportion to the degree of
maturity of the transferred embryo. In view of these
facts, it is unlikely that the selection of immature seeds is
to be recommended as a means of securing earliness, unless,
of course, all other methods fail.

229. Conditions of harvesting and curing. — The con-
ditions of harvesting and curing form in a measure a con-
tinuation of the phenomenon of maturity, and a discussion
of these factors might be included in a broad interpretation
of the general process of maturity. However, it is a dis-
tinct phase of the subject and deserves full, independent
consideration in this place. Uniformly favorable con-
ditions for harvesting and curing a given crop, other factors
remaining fairly similar, may alone be sufficient to establish
seed production as an industry in a locality.

Among the most striking instances of localization which
are to be found is that of the Santa Clara Valley, California.
This region has won an enviable reputation for seed-
growing, and over numerous other equally tertile localities
it possesses the distinct advantage of relative certainty
in the prevalence of uninterrupted dry conditions during
late summer and far into the autumn, the time when most
seeds are harvested. Hundreds of acres of the sweet-pea
are grown for seed in California, yet the sweet-pea is
equally thrifty and vigorous in many other sections of
the country. Dry summers and autumns are particularly
important, moreover, in cases where, in order to harvest
the seed, the whole crop must be cut, and there results

The Seed in Plant Production 389

consequently a large bulk of material which must be cured
previous to threshing.

In harvesting and storing seed the unfortunate practice
often prevails of storing the product in bulk; this in spite
of the fact that no small proportion may be somewhat
immature. Asa common result, the material heats rapidly,
and in the end much loss may occur (see section 168).
This heating is due in many cases to respiration, yet a part
of the difficulty also lies in the fact that the growth of
microorganisms is much encouraged by the ‘sweating
process.”

230. Duration of vitality. — In recent years considerable
attention has been bestowed upon the problems of main-
tenance of seed vitality, and upon a determination of the
conditions which are injurious. Much new work and
valuable data are therefore available; but the problem
is not a new one, and much was done By De Candolle and
others fully eighty years ago.

Species differ in a decided manner with respect to the
length of time in which vitality is maintained, and this is
true whether the conditions to which they are subjected
are favorable or unfavorable. Among seeds readily killed
by storage for a relatively short period may be included
those of many Composite, Crucifere, and Graminee ;
avhile some of those far more resistant are Malvacee,
Solanacez, hard-seeded Leguminose, and in general those
with water or air-resistant seed-coats. It should not be
understood, however, that all species of the same genus
or family are even approximately alike in resistance.
Becquerel reports an age of about eighty years for several
species of legumes which were still capable of germination.

390 Plant Physiology

The following table from Duvel indicates the average loss
of vitality of thirteen kinds of seed sent to seven different
localities in the United States and to Porto Rico, and
kept under ordinary conditions of storage, the first test
covering a period averaging 128 days (February to June),
the second period averaging 251 days (February to Octo-
ber) : —

First TEst Seconp TEST
KInp or SEED
Deterioration in | Deterioration in
Vitality Vitality
Per cent Per cent
ACGMALOS «se Seep ag sone yearn ate Be 2.55 5.20
Pegi he, a te Vee ie Ae ee Oe eee 3.92 11.39
Corn, ‘sweets A ow ae ee 1.20 1Z.t@
Watermelotn toy aa ot ee oe 12:31
hettwee’s. kat, tee ne eae 1.96 15.77
Radish...) ci Si cate eee at Nal ee 11.02 22.67
Cor, sweet S352 les een Aes 12.47 26.10
BGR Seek ep ek ae ie ee 5.76 29.58
GHDDAGE .~ 5 te) Cbs Meee be a eae 7.22 43.56
Carrots: S06) ei a ee Pe ee 9.77 53.89
DION) vs Gk". od ee ee a Cee ae 15.26 74.10
sry 80. «ee faethe oe ee oe eee 38.33 84.90
Phlox Drummond...) «Sika es led: 34.97 85.85

The control seeds referred to in the table were kept in
a cool, dry closet in the botanical laboratory, Ann Arbor,
Mich., and these showed a remarkable vitality.

231. Environmental conditions. — Duvel determined
that under ordinary conditions moisture and temperature
are the more important factors. Rise of temperature
alone may not be injurious unless accompanied by in-

The Seed in Plant Production 391

creased moisture-content. The following table gives a
record of vitality as related to precipitation and tempera-
ture at the seven points in the United States where the
thirteen kinds of seed were stored : —

Toaatoe ANNUAL haa sradsces
eee ee ene SBEDS VITALITY PRECIPITA-
i awe oe rashid Mean Fahr. Maximum
Per cent Inches Degrees Degrees
atone AV ae 71.98 91.18 vile! 96
Baton Rouge, La. : 41.39 66.37 G22 98
Durham, N.H. Hts 39.58 48.20 a3 98
maou, Ala, 9... 33.91 62.61 64.4 98
jake Citys Fla: . 29.38 49.76 (333% 103
Wagoner, Ind. Ter. . 28.41 42.40 67.1 107
Ann Arbor, Mich. . Pgs ie 28.58 49.12 98

In general, it would seem that a further drying out of
thoroughly matured seeds may enhance the keeping ca-
pacity. Moreover, such mature seeds keep well at high
temperatures. Immature seeds, or those which may not
be thoroughly dried out, keep best in a cool, dry situation.
When moisture is present, it would seem that respiration
is rapid and may be regarded as an important factor in
reducing vitality. Under ordinary conditions “ the life
of a seed is undoubtedly dependent on many factors,
but the one important factor governing the longevity of
good seed is dryness.”’

232. Buried seed. — Duvel, Beal, and others have
shown that, in general, seeds which are buried deeply
- maintain their vitality for a long period. An instance

392 Plant Physiology

came to my attention in Columbia, Mo., of the germina-
tion of clover seed which had been buried for more than
thirty years. The conditions were these: A cut was made
in clay soil exposing the ground-level of a fill made more
than thirty years before of from two to four feet. A few
weeks after the exposure of the old ground-level a contin-
uous growth of white clover appeared along that line.
There could be no doubt of the age of those seed, and an ex-
amination of undisturbed soil farther in disclosed the fact
that there were present not only white and red clover
seed capable of germination, but also, in smaller quantity,
cocklebur, sonchus, and a species of sedge.

In general, it would seem that the burial of agricultural
seeds results in death far more promptly than in the case
of resistant weed seeds. Shallow burial of weed seeds,
however, affording moisture conditions favorable for
decay, may often result in their destruction.

233. Delayed germination.— The rest period of the
seed seems to be to a considerable extent, if not entirely,
due to the development of a structure, or device, during
the maturing process which may serve to exclude water
or air until acted upon by gradual processes of decay or
special agents. It is well known that the germination of
many seeds is quickened by soaking in strong sulfuric
acid, by cracking the tough seed-coats, and some even by
the action of the digestive juices of certain animals.

Nobbe and others have pointed out the relation -of
germination to certain structural devices. Recently
Crocker finds that the marked case of delayed germina-
tion in the seed of Abutilon is due to the fact that the
condition of the seed-coats precludes the possibility of

The Seed in Plant Production 393

water absorption. Again, in the case of a few seeds, it
‘seems to be established that the exclusion of oxygen is
the important factor. The resting spores of many fungi
are notably difficult to germinate until after a period of
rest, and it is quite probable that similar factors are con-
cerned here, especially through the deposition of some
resinous substance in the cell-wall.

234. Effect of weight and size of seed upon vigor. —
Since the weight and size of seed determine the amount
of food-material immediately available for the plantlet,
at the time of germination, it is to be inferred that these
factors might have some influence upon production.
Early experiments by Hellriegel, Wollny, Marek, and
others were favorable to the view that seed of greater
size and weight give generally more vigorous plants than
those smaller or lighter. Much additional experimental
work has been reported in recent years, and some of this
evidence should be considered with respect to a few crops.

The problem is not so simple as it seems. Viewing the
matter from the standpoint of the factors readily recog-
nized, the effect of the accumulated food-materials is cer-
tainly to start the seedling off vigorously. If the coty-
ledons of the bean or pea are removed even during the
late stages of germination, the plants thus deprived of a
portion of their resources fall behind in growth. It is to
be expected that the final effect of this loss would depend
much upon the conditions subsequently encountered. If
the season is bad, or the soil poor, the seedling with more
potentiality in itself should be able to become established
more safely and quickly, and the advantage secured
might persist. Hellriegel supports the view that differ-

394 Plant Physiology

ences at maturity between the product of heavy and light
seed are intensified when the conditions are unfavorable.
With all conditions favorable, differences at first evident

might, in time, disappear. In all cases, comparisons are

only fair within the variety.

Hicks and Dabney have made a test of the relative
effects of weight upon vigor, using many sorts of seeds.
They attempted to eliminate all unsound seed, conse-
quently the material was sieved and afterwards hand
selected. The results are as follows : —

EXPERIMENTS WITH HEAvy AND Licut SEEDS

NUMBER
Garett ieee Warawh NUMBER|OF PLANTS pbs WEIGHT
VARIETY IN Eacu OF SEEps! GorMi- | weraHen | °° Ex or Eee
; lay : NATED |1N Eaco | OF “* | uines
Lor PERIMENT
Grams Grams
Radish, early || [|4 1.770 | A 73 . A 49.5
long scarlet Bee B 1.037 | B 84 [me * B 31.5
A‘4.077 | A 48 || gy 15 || 4 33.0
We beby WAN GED ace 5 Ae Es RCO Pee B 18.0
Sweet pea, Herj| ,, |) 46.092 | 446 || 4, Bd A 58.0
Majesty . . | C 4.045 | C 47 || ‘c C 44.4
Cane Early || AP 411") AAS | A 23.5
: | 43

Amber . [ 109°)! 1.360 | B 48 40 || B 12.0
A 3.298 | A 90 |! A 22.0

Kafir C d. ,
foam ore Te 100 ie t74ad: ap ae oe a || B 13.0
Ry A ae go [41105 |445 | ye og {| 4345
Oe een Ht BAS A B4g || B 20.0

Noe koto. J
Oats, White || 59 ||41.298 | 450 || 49 93 || A 37.2
Wonder [|B .805 | B49 B 25.0

1 A, heavy; B, lighter than A; C, lighter than B.

a —-s

The Seed in Plant Production 395

From these results it seems just to conclude that, in gen-
eral, amore vigorous growth, and consequently a better stand
in the field, is secured by employing only the heavier seed.

235. Experiments with wheat. — The effect of size of
seed on production has been with no other plant so exten-
sively studied as with wheat. The evidence is most con-
tradictory. The majority of the results seem to favor
the view that large or heavy seed are preferable, especially
when among the small seed are included distinctly imma-
ture grains. With wheat the factors are complex, for
size may be considerably affected by plumpness, and the
latter may be due largely to starch and water content.
Additional starch in the grain may not affect the vigor
and yield of the plant secured from such seed. Again,
in the same variety, there may be different types or strains,
— some with larger grains, some with smaller, although
the yields may run practically the same. All these factors
may affect the experiments. The results of grading and
testing seed wheat are shown in subsequent tables.

In the first case reported by Zavitz, the seed were
selected from both winter and spring wheats, and the
experiments were continued five and eight years, respec-
tively, but each crop was grown from previously unse-
lected seed : —

YIELD PER ACRE IN BUSHELS
KIND OF SEED

Spring Wheat Winter Wheat
Lares. ToT ay 6 asa a en ea te 2AE 42.4
SHAT PE MONUINITD 2s es STN Se ete 18.0 34.8
SLT TOLET SST Aa nem ee 16.7 33.7

396 Plant Physiology

In the second case, reported by Hickman, to be con-
trasted with the preceding, ‘“ three grades were used:
first grade, the large grains; second grade, the best of the
grains passing through the sieve in screening out the first
grade; third, unscreened wheat as it came from the
thresher.’?’ The experiments were continued nine years,
and after the first year, the selections for each were made
from the same grade of the previous year : —

YIELD FOR 9 YEARS, Bu. PER ACRE AVERAGE WEIGHT PER BUSHEL
First Grade |Second Grade} Third Grade ae oo Feet
15.48 16.06 16.03 57.8 58.3 58.1
17.16 17.64 16.69 Pl oy 57.9 58.1
16.11 15.82 16.06 Dred 58.0 57.4
16.25 16.50 16.26 Sit | 58.1 57.9

There is also difference of experience with respect to
large and small grains from the same head or plant.

236. Experiments with cotton. — Comparative pro-
duction tests of the value of heavy cotton seed over the
usual farm product have been made at the United States
Department of Agriculture.! The heavy seed were sepa-
rated, in the case of those varieties the seed of which are
covered with fuzz, by special devices and methods. The
field tests were made with results as shown in the table
on the next page.

1 Webber, H. J., and Boykin, E. B., “ The Advantages of Planting
Heavy Cotton Seed.” U.S. Dept. Agl., Farmers’ Bul. 285:16 pp., 6
figs., 1907.

The Seed in Plant Production

397

VARIETY, HAWKINS, TEST AT

YIELD IN PoUNDS ON EquaL AREAS, EAcH

APPROXIMATELY ONE ACRE

Lamar, §.C.

First Second Third Total
Picking Picking Picking Yield
Heavy seed (20 rows) . 375 2532 419 10472
Unseparated seed (20 rows) . 3a0 228 3814 9442

VARIETY, JONES'S IMPROVED, TEST

AT HARTSVILLE, S.C.

Heavy seed (14 rows) . 2 1582 793 212% 11643
Unseparated seed (14 rows) . 139 ray 2213 10753

In these two cases, the gain from the use of heavy seed is

respectively 10.9 and 8.25 per cent.

This is by no means

a trifling gain when reckoned as additional profit per acre.
237. Experiments with tobacco. — Among practically
all varieties of tobacco there is great difference in the size

Fie. 108. Tobacco plants from seeds of different sizes; heavy, medium,

and light seeds respectively employed, from right to left.
from Bureau of Plant Industry.]

[Photograph

398 Plant Physiology

and weight of the seed from similar individuals. Trabut!
found it possible to effect a separation into heavy and
light sorts through the capacity of these two kinds, re-
spectively, to sink or float in water. It was found that the
heavy seed produced plants which were greener, more
vigorous, and of larger size. Shamel has made further
studies of this relation, separating, by means of a current
of air, the seeds into three categories — heavy, medium,
and light. Samples of these seed were germinated, and
the accompanying illustration shows the relative vigor of
the plants resulting from the different grades.

LABORATORY OR SUPPLEMENTARY WORK

Write a report upon the vitality of seed as affected by
methods of harvesting and storage, consulting the literature
accompanying this chapter, also such of that contained in re-
cent volumes of the Experiment Station Record as may be
readily available.

REFERENCES

Crocker, W. Role of Seed-coats in Delayed Germination.
Bot. Gaz. 42: 265-291, 1906.

Dr Canpo.ie, A. P. Physiologie végétale. 2:618, 1832.

Dermer, W. Vergleichende Keimungsphysiologie. 565 pp.,
1880.

Duvet, J. W. T. The Vitality and Germination of Seeds.
Bur. Plant Ind. U. S. Dept. Agl. Bul. 58:96 pp., 1904.
— The Vitality of Buried Seeds. Bur. Plant Ind. U. 8. Dept.

Agl. Bul. 83: 22 pp., 3 pls., 1905.
Hickman, J. Fremont. Field Experiments with Wheat. Ohio
Agl. Exp. Sta. Bul. 129: 27 pp., 1901.

1 Trabut, L., Bul. 17, Service Botanique de |’ Algérie.

The Seed in Plant Production 399

Hopkins, C. G. The Chemistry of the Corn Kernel. Ill. Agl.
Exp. Sta. Bul. 53: 1898.

Lyon, T. L. Improving the Quality of Wheat. Bur. of Plant
Ind., U. S. Dept. of Agl. Bul. 78: 120 pp., 1904.

—— Modifications in Cereal Crops induced by Changes in
Their Environment. Proc. Soc. Prom. Agl. Science. 28:
144-163, 1907.

Nosse, F. Handbuch der Samenkunde. 631 pp., 339 figs.,
1876.

Pietrers, A. J., and Brown, Epcar. Kentucky Blue Grass
Seed: Harvesting, Curing, and Cleaning. Bur. of Plant
Ind., U. S. Dept. of Agl. Bul. 19: 19 pp., 6 pls., 1902.

SHameL, A. D. The Improvement of Tobacco by Breeding and
Selection. Yearbook U. S. Dept. Agl. (1904), 435-452.
(Value of large and heavy seed, pp. 440—442.)

CHAPTER XVI

THE TEMPERATURE RELATION

A LARGE number of species of plants composing the
main vegetation of the earth are seldom, if ever, exposed
within their normal ranges to great extremes of tempera-
ture. There are many annuals which first appear after
the dangers of severe frosts are past, and they perfect
their fruits long before the growing season is closed. A
considerable number of perennials may be exposed to ex-
tremes only in a resting or semidormant condition. In
general, then, native plants have been long acted upon
by the particular climatic factors of the region, so that
they show in a telling manner the influence of a long line
of ancestry whose development and survival within the
region is at least relatively fixed.

238. Climatic extremes and introduced plants. — In-
troduced plants in any region are, generally speaking,
much more likely to suffer exposure to an injurious ex-
treme, especially cold; yet exceptional conditions may
bring disaster to any type of vegetation. The peach in
the South and Southwest is sometimes in blossom before
the winter is at an end, and the blossoms are not infre-:
quently caught by late frosts. The famous peach belt of
Michigan was visited in 1905 by an early frost in October,
and the result was the practical annihilation of the peach

400

The Temperature Relation 401

industry in that section, for the wood of the peach trees
was entirely ‘‘ unripened.”’

Throughout a large portion of the zone of its culture the
cotton plant on well-watered and rich land grows contin-
uously until killed by frosts. In the same way the nas-
turtium and the tomato may be in full growth when killed
by frost. To a less extent this is true for familiar native
plants of the field. In spite of these facts, the impression
should not prevail that the vegetative period of a plant is
so fixed by heredity and ancestral adjustment as to be
incapable of responding fairly rapidly to the new environ-
ment. Ina new region the growing season of a species or
variety may be changed noticeably within a very few years.
Corn from the far South with a growing period of six
months will, if at all able to maintain itself in the North,
modify its period of growth so that it will mature well
within the season. Relatively few crops, however, are
able to survive and propagate themselves if left to form
fruit and germinate in the open, and in the relation of
cultivated crops to temperature the question is more
complex than is generally assumed.

239. Temperature and production. — As one goes north-
ward in the United States or in Europe, a certain general
change of crops is evident, indicating the universal im-
portance of the temperature factor in modifying produc-
tion. Potatoes may be grown from Mexico to Maine, but
throughout this whole range the growing season is well
within the normal length of the Maine summer. In fact,
in the far South two crops may be grown during a single
season. Corn is produced in the same region, but certain
strains of field corn grown in the South might not reach

2D

402 Plant Physiology

maturity unless protected during the first season in New
England. The cotton and the cowpea disappear entirely
in a little more than half the range of corn, while timothy
and barley, almost unknown southward, approach their
prime near the northern limit of this area.

In any scheme of continental plant zones, temperature is
recognized as most important. In general, such zones are,
therefore, constructed with special reference to the annual
or seasonal isotherms. No scheme of regions based largely
upon a single factor is entirely satisfactory. It is better,
however, than no attempt at classification. Koeppen,
Schimper, and others have indicated, on a broad basis, the
plant zones of the earth, and Merriam has arranged for
North America a suggestive scheme of life and crop zones

(Fig. 2).
240. Cardinal temperatures. — Certain cardinal temper-
atures are recognized. ‘‘ Maximum ” and “ minimum ”

are terms referring respectively to the highest and lowest
temperatures at which the development of a particular
organism may occur. It is apparent, however, that there
may be separate maxima and minima for every process or
activity of the plant. The maximum temperature for
germination may be below that which will support con-
tinued growth in the developing plant. It is difficult, or
at least inconvenient, to determine the most favorable
temperature for any process or function; yet, within cer-
tain limits, such determinations are possible. The most
favorable temperature is designated the optimum. It is
also customary to employ the terms “ ultra-maximum”’ and
“ultra-minimum,’’ denoting respectively the death point at
high and at low temperature. The following tables from

The Temperature Relation 403

Haberlandt give a comparative view of the relation of
some familiar plants to these cardinal temperatures : —

CARDINAL TEMPERATURES FOR GROWTH, DEGREES C

MINIMUM Optimum | Maximum

LSU eye 0-4.8 20-31 37-44
MAEMO EeM VS, t,o oY ae A 0-4.8 37-44 44-50
203) Sh an ee 0-4.8 LES 31-37
Rye 5 2 Regn ape oa tet an een eac ee Een 0-4.8 25-31 Bil si7/
Eeyore 3) AEE le eee eae 0-4.8

OES By Et 2 0-4.8 P5951 31 337/
BS AMER ae Cle EN oi. eas Sie 4s 0-4.8 25-31 31-37
Flax 2 ol a em ea ea a 0-4.8

1253) 5 BUS gee ee en 0-4.8 25-31 31-37
SOLE O Se Ae 4.8-10.5 31-37 37-44
NABI CASS 2 EF Niagara CR 4.8-10.5 37-44 44-50
Parade sa thes Saas a) ts 10.5-15.6 37-44 44-50
MOAR CON ls sg 10.5-15.6

LMC SIGEEY A, oS a ni a aaa 15.6-18.5 31-37 44-50
(COUEUEED GYES Ber Weer a 15.6-18.5 31-37 44-50

CARDINAL TEMPERATURES FOR GERMINATION, DEGREES C.

PLANT MiInIMuM OPpTimMuM MaxIMuM
LER, UMTS AS Mid Atle eat a eS 9.4 34.0 46.2
Phaseolus multiflorus . ... . 9.4 34.0 46.2
GucurbitoyRepo, 6) ee ak at 14.0 34.0 46.2
rite: yalleare )o if. oo 5.0 29.0 42.5
POTOSI. Ta ee ee) la eS ee 5.0 29.0 ote

These figures should be considered as merely suggestive,
for it is apparent that differences in varieties, in local ad-

A()4 Plant Physiology

justment, and also in environmental factors will affect
the cardinal temperatures in any particular case.

Reference has been made already to the fact that pho-
tosynthesis, metabolism, and other processes or responses
of the plant are to a certain point rapidly accentuated
with increase of temperature. Blackman has shown very
clearly that maximum activity, especially for respiration
and photosynthesis, has commonly been placed too high,
since proper consideration of the time factor has not al-
Ways been given.

241. Inhibition at high temperatures. — From recent
work reported by Balls it would seem that the inhibition
of growth at high temperatures during a considerable pe-
riod of time is in all probability the result of an accumula-
tion in the cells of injurious metabolic products. The time
factor is most important. According to his views, some
of these deleterious products are produced at low tempera-
tures, but under such circumstances they are constantly
decomposed, whereas at high temperatures production is
more rapid, and consequently accumulation and injury
result. Upon this hypothesis the effect of high tem-
perature upon the protoplasm would be that of favoring
auto-intoxication. |

242. Heat units. — Considerable attention has been be-
stowed upon computations of the heat units (thermal con-
stants) required to mature certain crops. Such data are
not without interest, yet examination of the evidence thus
far accumulated indicates that there is practically no such
thing as a relatively invariable thermal constant for any
plant when factors other than temperature are inconstant
or uncontrolled. Assuming that every other factor of the

The Temperature Relation 405

environment is constant, there is a theoretical thermal con-
stant, and it is of sufficient importance to receive some
practical consideration.!

243. Heat units and germination. — If the number of
heat units required in order to bring a plant to maturity
were at all constant, then the number requisite for any
phase of growth should likewise be more or less constant.
Some interesting data are available respecting germination,
and in the following table the time intervals are given for
germination at the temperatures indicated, and the heat
units may be readily computed : —

Sinapis alba as Ente | sabe Zea Mays
ee Time | Pee Time pes Time (eae Time pros Time
0.0 408 1.8 816 16.9 222 5.7 | 240 9.2 |240—-288
1.9 384 19.4 68 9.2 144 12.9 |120—168
4.8 408 || 25.05 44 12.9 72 16.9 90
aah 96 a, 144 || 28.0 74.4]| 13.0 69 FR aN yy 4 By
9.2 84 40.6 94 17.05 | 62.4]|2 -05| 23-44
12.9 41 12.9 66 PALER 42 28.0 | 36-48
Mi? 41 25.05! 42
DAL T 22 W205 72 28.0 i2
25.05 36 les: 36 34.0 192
ZO deo 20.00 38
28.0) |\GO-72
34.0 192

1 There are several methods of computing heat units. In each case it
is necessary to know the period of growth in days and the daily mean
temperature during the growing period. With this data we may then
obtain the total heat units by multiplying the growth period by the daily
mean temperature. This method makes 0° C. or 32° F. the basis. In the

406

Plant Physiology

Sum oF Darity MEAN TEMPERATURES ABOVE 18° C. (64.4° F.) FoR
FRUITING PERIOD OF DATE-PALM FROM May 1 To Oct. 31

Sum or Daity
MEAN TEMP.

LOCALITY, ABOVE 18°C.
(64.4° F.)
Degrees Degrees) Meteor-
C. F. | ological
Algiers, Algeria 652") 1,174
Orleansville, Algeria . 788 | 1,418
Fresno, Cal. 1,054 | 1,897
Tucson, Ariz. 1,409 | 2,538 | Obs.
6 yrs.
Cairo, Egypt =... .°.<.) 1,593 | 2,868
Pheenix, Ariz. (Salt River | 1,677 | 3,019
Valley)
Biskra, Algeria 1,836 | 3,304
Ayata, Algeria (Oued Rirh| 1,906 | 3,431 | Temp.
region) ; 1891
Tougourt, Algeria 2,049 | 3,689
Bagdad, Mesopotamia 2,356 | 4,242
Indio, Cal. (Salton 2,237 | 4,027 | Obs.
Basin) 7 yrs.
Salton, Cal. 2,679 | 4,823 | Obs.
12 yrs.

REMARKS

Ripening

No dates ripen.

Very early sorts mature.

Sorts grown usually
fail to ripen.

Sorts now grown usu-
ally fail to ripen.

Dates ripen regularly.

Many sorts ripen reg-
ularly.

Date culture the lead-
ing industry. Even
Deglet Noor ripen.

Deglet Noor datesripen,
but not always well.

Do.

Many excellent varieties

ripen.

case of the F. scale it is necessary, of course, to subtract 32 from the daily
It would seem, however, that
the method of computation to be preferred is one whereby an approxi-

mean before multiplying for the product.

mate growth minimum is taken as the basis, and the difference between

this and the daily mean represents the daily efficiency during the grow-
ing period. An example in the latter case is as follows: assuming the
growth period of wheat to be 100 days, the minimum growth temperature
40° F., and the daily mean to be 70° F., we have 70 — 40 x 100 = 3000° F.

The Temperature Relation 407

244. The date-palm. — One of the most important ap-
plications of a study of the relation of plants to the heat
units of the region in which they are grown is that made by
Swingle respecting the date-palm. He has shown that as
the heat units increase, the general adaptability of arid
regions to date culture is advanced, and a certain minimum
may not be exceeded for any type of date. From such a
study it was considered possible to foretell with approxi-
mate accuracy what section of the Southwest might be
utilized in date culture.

245. Control of temperature. — It is obvious that limi-
tations of expense impose pronounced restrictions upon
the exercise of control over the temperature factor in the
open. With a few intensive crops, such as asparagus,
waste steam has been utilized to some extent in forcing in
open culture, but proper control of temperature for forcing
or for producing crops out of season is usually confined to
greenhouse and hot-bed culture.

In some sections of the United States the loss of the
entire peach, apple, or other fruit crop may occur in conse-
quence of one or two late frosts, when, as experience has
shown, the temperature may fall from 4 to 14° below
freezing. Recently a control or prevention of this loss
has been successfully accomplished by means of coal or
oil heaters. The general plan is to place from 60 to 100
small ovens or heaters per acre at appropriate distances
apart. Then, if by midnight the indications are that a
freezing temperature will be reached in the early hours of

1 Paddock, W., and Whipple, O. B., “ Fruit-Growingin Arid Regions.”’
(Frost Injuries, Secondary Bloom, and Frost Protection.) Chapter 19:
324-354, 1910.

408 Plant Physiology

morning, usually the coldest period of the day, the heaters
are lighted. It has been found possible at an average cost
of about $20 per acre to raise the temperature of the or-
chard as much as from 5 to 14° F. above that of the nor-
mal air, and this often in the face of considerable wind.
The practice has recently assumed unexpected impor-
tance, and seems to have superseded the relatively ineffec-
tive smudge methods.

246. The temperature of the plant. — The temperature
of the plant is in general the temperature of the environ-
ment. Twigs, branches, and even trunks of trees will show
during cold weather changes of temperature more or less
in accordance with that of the air. In the case of large
branches or trunks some time will be required in order that
the minimum of the air may be registered by the tree, and
there will be, therefore, a very definite temperature lag.

In the sunshine dark buds, branches, or trunks may ab-
sorb heat to such an extent that the internal temperature
will be greater than the external. In the same way, green
leaves exposed to sunlight show a temperature from two or
three to fifteen degrees higher than the air, depending upon
the intensity of the light. This latter point has received
careful attention by Blackman, who has employed in the
work very delicate electro-thermometric methods. The
ordinary method of wrapping the bulb of a thermometer
with one or more thicknesses of a leaf will not afford ac-
curate indications of the actual leaf temperature.

247. Adjustment of structure. — There are few or no
protective structures in plants which are of direct service
against injurious temperatures. As will be shown later,
both high and low temperatures act upon the plant cell to

The Temperature Relation 409

cause drying out, and the structures which are ordinarily
assumed to be protective against cold or heat are in reality
serviceable in preventing loss of water. The delicate
young buds of the peach or other deciduous trees may be
inclosed by bud-scales, hairs, and resins; nevertheless,
such buds promptly freeze solid when the temperature
falls below the freezing-point of the cell-sap, or the point of
supercooling. The trunk of the tree is, of course, pro-
tected in a way by thick bark, yet so far as the entrance of
cold or loss of heat is concerned this protection is insignifi-
cant.

248. Irritable response.— Through growth movements
toward or away from a source of heat, plants commonly
exhibit the capacity for irritable response (positive and
negative thermotropism) with respect to temperature ; but
this response is of TU

a2 SZ eo Tp

=p

little practical sig-
nificance, except as
further evidence of
the paratonic rela-
tions of the organism.
Thermonastic move-
ments also occur, but
this general class of
phenomena is dis-
cussed in section 306.

249. Freezing. —
Some of the results

of freezing deserve

; ; Fic. 109. Frozen stem of Fritillaria, show-
careful consideration. ing ice-masses (stippled). [After Miiller-
It is well known that = Thurgau]

FS

Y?

Cy

Sop

410 Plant Physiology

in the freezing of a plant cell under ordinary conditions,
the ice crystals are formed upon the surfaces of the cells.
In the case of tissues with intercellular spaces these crystals
form in the latter. In this way the protoplasm gives up its
water and the mechanical injuries of the ice crystal are not
ordinarily exhibited within the protoplast. In the case of
very rapid supercooling of large cells it is probable that ice
crystals develop within the cell; thus mechanical harm
may result. Similarly, in tissues mechanical injury may
sometimes result, and the bark of immature wood may be
ruptured when severely frozen. It has been found, how-
ever, that the diameter of a frozen twig is usually less than
normal.

In view of all the facts which have been presented by
various investigators, it would appear that the ability of a
plant to withstand cold is in
large part determined by the
capacity of the cells to give
up water without injury
during freezing. On the
other hand, according to the
views of Molisch, death from
cold commonly results dur-
ing the process of freezing.

atera, showing ice-masses (black). This refers particularly to

[After Muller-Thurgau.] active cells, or herbaceous
shoots, and is at variance with the popular impression that
frozen plants are less injured when thawed out gradually.

Many plants are injured at temperatures above the
freezing-point. This may be due to a simple disturbance
of the water relation, but it is more probable that there

The Temperature Relation All

are complex effects, the permeability of the protoplasm
being also affected.

250. Buds.— The relation of buds to cold has received
careful attention by Wiegand. He finds that ice may form in
a large number of species when the temperature falls as low

Fic. 111. Section of a bud of Populus nigra frozen at 5° F. sectioned and
photographed in the open; light areas are ice crystals. [After
Wiegand.]

as— 18°C. At this temperature it may be formed in large
quantities and is more abundant in cortical and paren-
chymatous tissues than in meristem. When absent at this
temperature, it may be assumed that the tissue is made up
of very small cells with thick walls and low water-content.
This is explained by the fact that “‘ the degree of cold nec-
essary to cause the separation of ice is proportional to the

412 Plant Physiology

force which holds water in the tissue. This, in turn, de-
pends upon the relative proportion of water to cell-wall
and protoplasm.’’ Measurements were made by Wiegand —
of seven species of trees frozen at a temperature of —18°
C. and of seven species which failed to freeze. The com-
parative data for two species in each group are presented
by the following table : —

CELL Diam.

IN MM. PER
CENT
TEXTURE
: or WALL ai!
Max. Min. WATER

Aver. Aver.
]

A. Ice abundant in leaves and growing
points at — 18° C.

Cratzgus punctata . . . . . .{| 0.040 | 0.012 thin 49.4

Prunus serotina.”..-4.. .. 2 <<") 2 O02 Ors thin 47.6
B. Ice not present at — 18° C.

Quercus: alba: 3.08. ov one Se CO ee thick Fae mE

Carya alba. \..- <a Sw «2. &) re 9 3) DSS oe very thick 31.4

LABORATORY WORK

Effect of heat and cold upon germination. — Expose selected
dry seed of barley or peas for 1 hour to dry-oven temperatures
of 50° C., 75° C., and 100° C. Place 25 of each lot in a ger-
minator together with an equal number of control seed and
determine the effect upon the percentage of germination. In
the same way employ seeds of the above plants in water at the
temperatures above given, and test similarly. For further
comparison it is also desirable to employ in both- experiments
a lot of seed which are just beginning to germinate. Discuss
the results.

The Temperature Relation 413

Soak some seed of barley or peas an hour or two in water
and keep another lot dry. Expose lots of 25, soaked and un-
soaked, to such low temperatures as may be conveniently
prepared by freezing mixtures of salt and ice. One lot may be
placed in test-tubes immersed in broken ice, another in similar
tubes at from 5 to 10° below zero, and another exposed to about
een irom — to — 10°C: may be obtamed im, a treez-

ean ee

“PSUUEN P.FRIEZ, BELFORT “7 OBSERVATORY, BALTIMORE,MD.
[Le ;
Fig. 112. Thermograph. [Illustration from Julien P. Friez.]

ing mixture of 10 parts common salt to 100 parts snow, while
— 20° C. requires 33 parts salt. Subsequently, test the ger-
mination and discuss the results.

Formation of ice crystals. — Place filaments of Spirogyra in a
drop of olive oil in a hanging-drop culture. Expose in a cham-
ber surrounded by a freezing mixture such that the tempera-
ture of the chamber is reduced to about — 10° C., then remove
the culture and examine promptly under the microscope to
locate position of any ice erystals formed.

On a day when the temperature of the air is about 0° C.,
or below, make sections of artificially or naturally frozen buds
and locate the ice erystals.

Effects upon foot elongation. — By means of the method em-
ployed in the study of growth, mark with parallel lines on the

A414 Plant Physiology

root-tips of germinating beans, and study the effect upon elonga-
tion of such different temperatures as may be obtained in the
incubators at hand, in the refrigerator, and at the room tem-
perature. In all cases the beans employed should be uniform,
and the experiment should be carried out in a moist atmosphere.
Temperature relations. —In ease time has not permitted, in
the appropriate place in connection with various phenomena
discussed, to experiment upon the effects of changes of tempera-
ture, such experiments should be included here, as far as pos-
sible; especially important are the effects of temperature upon
transpiration, photosynthesis, enzyme action, and growth.

REFERENCES

Batis, W. L. Temperature and Growth. Ann. Bot. 22:
557-592, 11 figs., 1908.

BiackMaN, F. F. Optima and Limiting Factors. Ann. of Bot.
19: pp. 281-295, 2 figs., 1905.

Davenport, C. B. Action of Heat upon Protoplasm. Experi-
mental Morphology. 1: pp. 219-273, 8 figs., 1897; ibid.
2: pp. 450-469, 5 figs., 1899.

HaBERLANDT, F. Die oberen und unteren Temperaturgrenzen
fiir die Keimung der wichtigeren landw. Simereien. Landw.
Versuchsstation. 17: 104-116, 1874.

Merriam, C. H. Life Zones and Crop Zones of the United
States. Div. Biol. Survey Bul. 10: 79 pp., 1 map, 1898.

Mouiscu, H. Untersuchungen ueber das Erfrieren der Pflanzen.
To pp:, 1897.

Mue.uer-TuurGavu, H. Ueber das Gefrieren und Erfrieren
der Pflanzen. Landw. Jahrb. (1880): pp. 133-189; ibid.
(1886): pp. 453-609.

SWINGLE, WALTER T. The Date Palm and Its Utilization in the
Southwestern States. U.S. Dept. of Agl. Bur. of Plant Ind.
Bul. 53: 155 pp., 22 pls., 10 figs., 1904.

WieGAND, K. M. Some Studies regarding the Biology of Buds
and Twigs in Winter. Bot. Gaz. 41: pp. 373-424. 8 figs.,
1906.

CHAPTER XVII

Pe LIGA ee ELAT IO N

ALL green plants exhibit direct relations to intensities
of light. The influence of light in the synthesis of the first
organic products, or photosynthates, has been considered ;
and it is now necessary merely to indicate some of the more
general ecological relations.

251. The adjustment of plant members. — No phenom-
enon of plant life is more familiar than the turning of leafy
shoots toward light or the orientation of leaves in a man-
ner to occupy a favorable exposure. Plants placed at the
window of a dark room promptly show the effects of the
light stimulus. The same relations may be observed in
the field. The capacity to show through growth curva-
tures an irritable response to light from one side is called
phototropism. We have to distinguish as main classes
of responding structures those axes which are parallelo-
tropic, curving in such manner that the tips point toward
or away from the source of light, and those which are
plagiotropic, or at some angle. Leaves are transversely
phototropic, and the response secures a favorable illumi-
nation of the chlorophyll bodies. As a result broad-
leaved plants develop commonly to form a more or less
perfect mosaic, no better examples of which can be found
than those of the grape-vine or Boston ivy. The adjust-

415

416 Plant Physiology

ment of the single shoot, of the plant as a whole, or of a
group of plants is of the same nature, ample considera-
tion being given to other forces which may be operative
at the same time. ‘Trees thickly branched, such as the
Norway maple and the linden, or the unpruned apple and
pear, will exhibit in a significant manner this effect, ex-
posing a complete shell of leaves. (See Chapter XX for
growth movements. )

When trees grow up close together in the forest the lower
branches are ultimately too much shaded, so that these are
killed and in time drop off. The leafy shoots are confined
to the uppermost parts, and this system of constant self-
pruning through the survival of those favorably placed
results in the characteristic long trunks of the forest trees
as compared with the shorter trunks and abundant
branches of isolated specimens in the lawn or meadow.
The tall trunks are, of course, most desirable from the
standpoint of the lumberman; but, at the same time, the
decayed branches or stumps offer favorable opportunity
for the entrance of destructive fungi which cause great
annual loss through the decay of sap or heart wood, and
thus artificial pruning possesses great advantages.

252. Light perception. — Phototropic organs may pos-
sess special perception regions, and these regions do not
necessarily correspond to those of curvature or bending.
The method of perception is not understood, but the sensi-
tiveness of the mechanism is almost incredible.

The perceptive mechanism resulting in leaf orientation
has received much attention. Haberlandt and others find
in the lens-shaped cells and cuticular thickenings of epi-
dermal cells the structures which they regard as indirectly

The Light Relation 417

important. In these cells the light may be focused in some
basal region of the pro-

toplasm, and through 1S Ga a
the unequalillumination a 7S—<\ ~~ Sisar
the stimulus to orienta- | | /
tion is supposed to be’

given. It has been dem-
onstrated photograph-

ically that these cells i Rosy

focus the rays, but-since pe SS a
such cells occur under a on)
variety of conditions

y ) (ae

and for ENERO other ieee Fic. 113. Epidermal modifications which
sons, they are not posi- focus light rays; Berberis (a), Rhodo-

Bor nadted. witty | ndion 0) and Prints Dawe coan
this form of irritability.

Important in the orientation is the direct or indirect
sensitiveness of the petiole. Wager believes ‘‘ that the
perception of light is bound up with its absorption by the
chlorophyll grains, in which case the palisade cells would
be the percipient cells.”’

253. Diverse requirements. — From casual observa-
tion of plant habitats, it may be noted that there is great
diversity in the light intensity under which different species
grow to maturity. Many plants reach perfection only
when exposed, but others develop more vigorously under
the partial shade of the forest or thicket. Exposed and
shaded situations usually differ with respect to other en-
vironmental factors, such as humidity and evaporation ;
and in a careful study of habitats it is necessary to measure
and to attempt an evaluation of all factors.

25 :

418 Plant Physiology

It has been reported that during a unit interval the even-
ing primrose utilizes in direct sunlight about three times
as much CO, as when in diffuse light, while the common
polypody works more effectively in the latter. Many
shade-loving plants ae reach maturity in light which
is reduced to about 3'5 the intensity of maximum sunlight.
Beyond a certain light intensity the plant gains little, for
the small amount of CO, in the air is then the limiting fae-
tor in growth. Temperature is a further limiting condi-
tion. In the warm forest of the tropies there may be a
vigorous forest-floor vegetation, but the cold shade of a
far northern forest affords only a scant undergrowth.

254. Light intensity. — Upon the surface of the earth
light intensity varies considerably both diurnally and
seasonally, depending, of course, with a clear sky, upon the
altitude of the sun. The possible daily maximum is at
sun-noon, June 22. If this intensity should be repre-
sented at the equator by 100, then with a growing season
in the north temperate zone approximately from March
21 to September 23, or its equivalent in the southern
hemisphere, the light intensity from 9 a.m. to 3 P.M.
would be represented approximately by 82 to 98 and the
noon variation by 93 to 98, as calculated by Clements for
Lincoln, Neb. From this it is apparent that the variation
in light intensity throughout agricultural regions with a
clear sky during the growing season is not considerable.
Range of intensity in the open is in general insufficient
seriously to affect vigorous growth, although it may mod-
ify form and chemical content. In some regions cloudi-
ness may be an important factor.

255. Injurious effects. —It has long been well cntaie

The Light Relation 419

lished that injury may result from continuous or lengthy
exposure to intense light. Many of the simpler green alge
may be killed by an exposure to brilliant sunlight of less

Fig. 114. Effect of light upon a plate culture of Pseudomonas campes-
tris; colonies have appeared only where the plate was protected by a
letter (W) screen. [After Russell and Harding.]

than one hour. It has been shown conclusively that strong
light inhibits the action of various enzymes. ‘The diastases
are notably affected, so that the conversion of starch in a

420 Plant Physiology

clear solution may be readily prevented by exposure to
light. The chlorophyll of shoots protects in a measure the
diastase from the injurious action of light during the day.
Nevertheless, from this and other causes starch conversion
in the leaf is reduced to a minimum during days of bright
sunshine.

Bacteria and other hyaline microscopic organisms are
killed by direct sunlight, and this fact is important in sani-
tation. The convincing demonstration of the effect of sun-
light upon bacteria was made by Ward. He prepared
cultures of the bacteria upon clear agar in Petri dishes, and
then exposed the dishes to sunlight. It was determined
that the organism causing anthrax, Bacillus anthracis,
may be killed in such cultures in direct sunlight by an ex-
posure of from a few minutes to several hours, depending
upon the intensity of the lhght. Striking results were
obtained by covering the dish to be exposed with a black
paper or metal stencil so that the contrast between exposed
and unexposed parts of the plate may be sharp. <A spec-
trum was also thrown upon prepared cultures and it was
determined that the blue-violet rays constitute the effective
killing portion of the spectrum. With the use of glass
covers or globes the injurious rays are to a considerable
extent excluded.

256. Artificial light. — Interesting studies have been
made upon the use of artificial light in greenhouse culture,
as in forcing lettuce, endive, radish, and certain flowers.
In such experiments the artificial light has been employed
usually at night, or supplementary to daylight. | Eco-
nomically, artificial light is probably a failure, owing to the
expensiveness of it; but the results of the experimental
work bring out some points of interest.

The Light Relation 421

By the use of the protected electric arc during half of the
night Bailey was able to hasten lettuce two weeks. The

WL

Fig. 115. Lettuce of the same age under normal sunlight (above) and
with electric arc a part of the night in addition (below). [After Bailey.]

naked electric are yields light distinctly injurious to the
majority of crops. This injury is due to the richness in
ultraviolet rays, which, as already shown, are destructive
to protoplasm. When screened by glass, clear or opales-

422 Plant Physiology

cent, the harmful rays are largely excluded. Continuous
night illumination with the electric arc may promote more
rapid growth in some
plants, but with others
there is a tendency to
run to seed.
The incandescent elec-
trie light, which is rela-
tively rich in red rays,
has been successfully em-
ployed by Rane in fore-
ing lettuce. By the use
of the acetylene light
Craig has found it possible
to force the growth of
radish, lettuce, and a few
other crops; but the best
results were with flowers,
\... Easter lilies especially giv-
ing increased production
in a shorter time.
257. Monochromatic
Fic. 116. Field peas grown for equal light. — Many experi
periods in white (a), blue (6), and ments have been made to
Seen) ee determine approximately
the effects of light of different wave lengths on the form
and structure of plants. In much of the work which
has been done pure screens were not employed, yet his
type of work is sufficiently important to justify careful
physical methods. In general, the dry weight of plants
grown for a considerable period under monochromatic

The Light Relation 423

screens is greatest in red, and least in violet; yet the
growth in red is not equal to that in white light. The vio-
let rays are also important in the production of bloom.

In the following table there are given, after Teodoresco,
the relative areas of leaves developed from the bud in dif-
ferent qualities of light during a period of about thirty
days. With each plant the leaves occupied equivalent
positions on the young shoot: —

Errect oF Wave LENGTH UPON AREA OF LEAVES, AREAS IN SQ. MM.

i Kinp or Licat

PLANT EMPLOYED

White | Red | Green Blue | Darkness
Ririaibal =. . |. (37, 4948.3 | 127.8 | 654.8| 52.8
Lupinusalous ... ... ihitdos 49.5| 38 62 8
Polygonum Fagopyrum Genty=
| Vero loy 0%) au 128 59 Bort G4 11
Ricinus Be es bot yledond) LTLOSEs | o03 200 | 600 ae

The next table indicates the thickness of the leaf under the
different conditions of illumination, and also the number of
stomata on equal areas of the lower surface : —

WHITE RED GREEN BLuE
PLANT EMPLOYED
Th 40 = The = Th.
é No. : No. No. No.
ee Stone Leaf ier aah Stom. po Stone
Waciastabs,.« .. | 671 4 37 7 297 12 332 7
Arachis hypogea .| 243 12 216 | 18 ZOT a 420 216 13
Ricinus sanguineus| 346 2 256 6 218 10 297 6
Lupinus albus . .| 352 14 PO) jy a3 195 38 7 Al 25

424 Plant Physiology

258. Half-shade in plant propagation. — In conserva-
tory and greenhouse production of certain tropical plants
whose habitats are naturally the moist, shady woods, some
form of shading has commonly been practiced. It is only
in this manner that many delicate ferns and succulent
species are grown successfully. The same purpose is ef-
fected by the cloth-covered tents or slat-covered sheds

4
Ij
Bares Ae
° ‘
BS)

one

\ ah©

Fic. 117. A coffee plantation in a Hawaiian forest. [After Van Leen-
hoff.]

often employed in southern climates, ostensibly to dimin-
ish the light, but also to insure higher humidity and to pro-
tect against wind and frost. As a result of such work it
has become apparent that partial shade may be advanta-
geous in many horticultural or even farm crop operations.
The demand for vegetable products out of season is an ad-

The Light Relation 425

ditional incentive to the
use of any device which
may regulate or control
the conditions of the en-
vironment.

259. Crops responding
to half-shade. — In gen-
eral half-shade increases
succulence and delicacy,
so that it is particularly
applicable to such crops
as asparagus, cauliflower,
celery, lettuce, and rad-
ish. It is employed in
forcing rhubarb, in the
cultivation of ginseng,
and has proved especially
important in pineapple
eulture in Florida.
Sumatra tobacco, grown
for wrapper purposes in
the Connecticut valley
and in other regions, has
been greatly improved by
half-shade conditions.
Half-shade is also neces-
sary in many cases to
the maintenance of
proper conditions for seed
beds, and it is essential in
the nursery propagation

Fic. 118. Peas grown 6 daysin dark-
ness (a), in about 3 light (b), and open
in greenhouse (c).

426 Plant Physiology

of many forest trees. A thorough study of the relations
of seedling trees to half-shade operations is greatly to be
desired in the advancement of forestry work generally.

Many bush fruits and other agricultural products are
commonly grown in the partial shade of other plants. It
is generally believed that currants are benefited by the
partial shade of grapes or certain tree-fruits, provided the
water-content of the soil is not seriously affected. Coffee
and tea are more profitably produced in subtropical regions
in forest glades, or when partially shielded by occasional
trees. In southern Algeria and other portions of the Sa-
hara shading is practiced on a large scale in the oasis cul-
tures. The protecting palms improve the conditions for
figs, peaches, and other fruits, under which, in turn, vege-
tables may be grown, provided only that the water-supply
is adequate. On the other hand, the production of grapes,
with higher sugar content for wine purposes May require a
selection of slope insuring best exposure to light during a
certain period of growth and maturity. Fruit trees are
grown on the southern sides of walls in England and
France, and it would appear that both the additional light
and heat thus obtained are advantageous.

In many parts of the United States, especially in the
Central West, lettuce and other salad crops become bitter
and undesirable for table use with the stronger light and
heat of the summer season. The shade tent, properly
employed, will permit the constant summer culture of
such crops. The strong flavor of radishes produced dur-
ing the summer are also modified and improved by partial
shade.

260. Morphogenic effects. — The comparative effects

The Light Relation 427

of light and darkness upon the form and growth of plants
has been much investigated. In most cases no adequate

Fic. 119. Sun print showing difference in opacity (thickness and chlo-
rophyll content) of celery leaves grown in half-shade (left) and in sun-
light (right).

consideration has been given to other factors than light,
but in general the response to light is so much more marked

428 Plant Physiology

than to other factors that the errors are perhaps negligible.
When the buds of ordinary herbaceous plants with central
axes are permitted to develop in the dark there is a marked
elongation of the internodes and a suppression of branches.
Shoots from tubers or tuberous roots affording constant
food-supply will show this characteristic in a striking man-
ner. The leaves of such herbaceous plants are usually

— z. 48
At sot fee
Trem ivticeay Sees -
Sia Ez. S'4 GAN ECR
SEA Eady F He

Fic. 120. Sumatra tobacco under cloth tent, Connecticut Valley.
[After Shamel.]

greatly reduced in size, and sometimes restricted to mere
scalelike structures. On the other hand, when the light in-
tensity is reduced to from 20 to 40 per cent of normal sun-
light, the leaves may be increased to twice their size in di-
rect sunlight, as demonstrated by many experiments upon
lettuce, tobacco, and other broad-leaved plants. When
produced in the dark, the radicle leaves of such plants as
the rhubarb develop in a short time petioles of unusual
extent and delicacy, while the leaf blade remains small.

The Light Relation 429

This effect is of much practical value in the forcing of rhu-
barb for early market (Fig. 87).

The leaves of tobacco produced in the shade tent are not
only larger, but thinner, and they possess relatively larger
air spaces and more spongy parenchyma; the fibrovascu-
lar bundles, or venation systems, are less prominent, and
the leaf is thereby improved for wrapper purposes. In the
fibrovascular bundles of half-shade plants the mechanical
supporting tissues are usually reduced, and this is a factor
in blanched celery. Self-shading to produce crispness and
tenderness may be practiced in some cases; thus in the
cultivation of Cose lettuce, romaine, and cauliflower,
the simple operation of bringing together and tying the
leaves in the form of a head may produce the effect
desired.

261. Half-shade and quality. — Plants grown in half-
shade commonly contain a higher per cent of moisture and
less ash. It has been ascertained that the apparent acid-
ity of strawberries is increased by shade. This apparent
increase is, however, due to lessened accumulation of
sugar in the berries.

The aromatic products of plants are not important as
animal nutrients, but they are physiologically essential,
and represent almost the sole value of many economic
plants used as condiments. In 1838, De Candolle called
attention to the diminished production of savors and odors
in shaded plants. It was found later that plants removed
from southern latitudes to the latitude of Scandinavia
during the two months of maximum sunshine in the latter
region, showed an increase in the development of aromatic
products. Indeed, it has long been suggested that many

430 Plant Physiology

fruit-bearing plants containing objectionable flavors might
be improved by reduced light.'
262. The effect of shading upon other environmental

pe. 7 NON Aa
B

Fig. 121. Bark of Acer Pseudo-platanus; epidermis (ep), periderm (pr),
primary cortex (pe) ; developed in white light (A) and in red light (B).
[After Teodoresco.]

factors. — From the preceding statements it has been noted
that half-shade may modify in a direct manner other condi-
tions of the environment. The factors commonly affected
are the following: (1) moisture conditions of the soil;
(2) rate of evaporation; (3) humidity; (4) temperature ;
(5) air movement; and (6) certain biological relations.
The tables on the next page indicate the effect of the
usual shade tent (made of unbleached cotton) upon soil
moisture and evaporation (first table after Whitney).
Much remains to be determined respecting the modifica-
tions in plants induced by shading, and likewise exten-
sive studies are required to evaluate the different factors
involved. From the indications already presented, it is

1Schuebeler, Bonnier, and Flahault have shown that in northern
climates flowers are more highly colored and plants commonly richer in
essential oils. It is also well known that plants rich in volatile oils and
other aromatic products are numerous in the Mediterranean region, a
region in which the rainy season is confined largely to the winter months,

and the summer is practically a continuous exposure to intense sunlight. ~

The Light Relation

431

obvious that many of the effects conveniently discussed
under “shading,” in the sense of “‘ half-shade,”’ are in
large part the results of changes in the water or mois-

ture relations.

diseases also deserves a more careful study.

The relation of shade plants to fungous

TABLE OF Sort Moisture INSIDE AND OUTSIDE OF SHADE

(CLotH) TENT

0-3 Ins. 0-9 Ins.
DaTE
Inside Outside | Inside Outside
aanlign 1. 1 epe2 123) Seb 13.6
July 2. 15.0 13.3 || 15.4 14.0
Any ke: eZ 1A Ie oes 14.3
ouly A: TBE?! ele 14.9 13.4
July 5. 12.8 9.9 | 13.5 120
Maly «Gi; 1p Dl ets 11.8
ty, f= 12.0 8.4 14.0 10.8
July 8. ieZ CSeo eis st 10.5
July 9. ud bats) 6.9 13.0 9.8
July 10. VE) 6.3 1 are 8.7
EVAPORIMETER READINGS, SHADE-TENT EXPERIMENTS,
ItHaca, N.Y., 1908
Cc. oF WaTER LOST By
STANDARDIZED EHVAPORIMETERS
DaTE

ese neo eepeet
Aug. 11-23 AED Na eth ews hae ec 8 96 163 263
JEN ES, PAZ) ISTE) 0 iran Cane aa ee Me cea 85 162 240
i es es Pap Soe mee a tke ONS 80 140 210
“Seca b> T= 1S Ea ee ae ep era io 110 170

432 Plant Physiology

LABORATORY WORK

Orientation. — Make and record observations in the open or
in the greenhouse upon the relations of shoots and leaves of
any plant to ight. Begonia, grape, or Norway maple may be
used; also note the relations of the compass plant (Lactuca
Scariola) if available.

Place a pot or water culture containing seedlings (several
centimeters high) in a chamber permitting one-sided illumina-
tion. The chamber may consist of a tight box, black on the
inside, arranged with a slit on one side through which rays of
light may be admitted. Place the plants as far as possible
from the source of light, and for some hours note the response
of the shoot (and also of the root if a water culture is employed).
Expose another plant which has been in complete darkness to
one-sided illumination for some moments and then return it to
a dark chamber. Note any subsequent response and discuss
the results.

Light perception. — Make hand sections of leaves of oats,
hyacinth, hepatica, Sazifraga Geum, or Garrya elliptica, and
describe the lens-shaped cells or epidermal modifications con-
sidered by Haberlandt and some others to be light-perceptive
organs. Consult the article cited by Wager, note his method
of photographing objects through cells, and read his conelusions
regarding light perception.

Wave length and rate of growth. — With bottles, test-tubes,
and corks prepare three pieces of apparatus as shown in Fig.
122. Prepare the solutions of (1) ammoniacal copper carbonate
and (2) naphthol yellow, so as to give practically pure colored
lights (spectroscopically tested, if possible), the one excluding
practically all except blue and blue violet rays, and the other
excluding all except the red end of the spectrum. Fill one
bottle: three fourths full with each of these solutions and one
with water. Place in each test-tube, on filter paper or moss, a
germinated seed of the field pea, and insert the tubes as shown
in the figure. Relative growth may be observed until the seed
have outgrown the chambers. With the apparatus commonly

The Light Relation 433

at hand, it is impracticable to at-
tempt to compute equal energy inten-
sities of the colored lights.

Light intensity. — Determine the
relative value of light in the open
and contrast it with the light inten-
sity in the greenhouse and in the
shade of vegetation or buildings. To
make this study use the ordinary
photographic actinometer, the device
employed by Clements,! or strips of
solio paper. If the latter are em-
ployed, it is simplest to determine the
length of exposure in seconds neces-
sary to bring the paper to a certain
standard shade of brown. This may
be done by previously following the
changes in the paper while contrast-
ing it with a brown color scheme,
choosing some shade of color in the
color scheme as a standard which is
invariably one of those attained by
the paper in the process of darkening.

Etiolation. — Place in a perfectly
dark chamber water cultures of peas,
potatoes, and onions sprouting on
moist moss, and any potted plants
available. Make accurate observa-
tions of the conditions of the plants
or buds when placed in the dark, and,
if possible, arrange control cultures
exposed to the light, but under sim-
ilar conditions of moisture and tem-
perature. After ten days or more,
make comparative observations, not-
ing the effect (1) upon structures

1 Physiology and Ecology, pp. 72-75.
2F

Fic. 122. Simple apparatus
for qualitative tests of the
effects of light of different
wave length.

434 Plant Physiology

developed in the dark; (2) upon structures formed previous to
placing the plants in the dark.

Secure the same variety of any plant grown in half-shade and
in an exposed situation;
compare the two with re-
spect to structural modifi-
cations, water-content, and
extent of root system.

Light and _ blossoms. —
Place over carnations, just
coming into blossom, aér-
ated bell glasses, one of the
bell glasses being covered
with manila paper or un-
bleached cotton. Follow
the effect of severe shading
upon the opening of flower-
buds of other plants which
were equally advanced at
the outset.

Killing effect of light. —

Fie. 123. Potato sprouting in a dark, Prepare in a Petri dish a

moist atmosphere. dilution culture of any
species of bacteria convenient, using the minimum quantity of
the clearest agar obtainable. When the agar is solidified, expose
the cultures about one hour to direct sunlight, protecting, how-
ever, a portion of the dish by means of darkened cardboard.
Replace the cover of the dish, incubate the cultures for several
days, and note the effect of the exposure to light. This experi-
ment cannot be carried out where laboratories are not equipped
for the cultivation of micro-organisms.

REFERENCES

Battery, L. H. Some Preliminary Studies of the Influence of the
Electric Are Lamp upon Greenhouse Plants. Cornell Agl.

The Light Relation A35

Exp. Sta. Bul. 30: 83-122, 1891; 42: 131-146, 1892; 55:
145-157, 1893.

Duaeear, B. M. Shading Plants. Cyclopedia of American Ag-
riculture. 1: 119-123, 1907.

HABERLANDT, G. Lichtsinnesorgane der Laubblatter, 1905.

Knigep, H., unp Minper, F. Ueber den Einfluss versch. Lichtes
a. d. Kohlensaure-assimilation. Zeit. Bot. 1: 619-650, 1909.

MacDouaau, D. T. The Influence of Light and Darkness upon
Growth and Development. MemoirsN.Y. Bot.Gard. 2:
319 pp., 176 figs., 1903.

Rane, F. W. Electro-Horticulture. W. Va. Agl. Exp. Sta.
Bul. 37 : 1894.

Sremens, C. W. On the Influence of Electric Light on Vegeta-
tion. Proc. Roy. Soc. 30: 210-219.

Stewart, J. B. The Production of Cigar-Wrapper Tobacco
under Shade in the Connecticut Valley. Bur. of Plant Ind.
WES) Dept. Acl. Bul? 138-31 pp:, 5 pls., 1908.

StonE, G. E. Response of Plants to Artificial Lights. Cyclo-
pedia of American Agriculture. 2: 22-27, 1907.

TroporeEsco, EK. Influence des différentes radiations lumi-
neuses sur la forme et la structure des plantes. Ann. d. Sci.
Nat. (Bot.). 10 (sér. 8) : 141-268, pls. 5-8, 1899.

Wacer, H. The Perception of Light in Plants. Ann. Bot.
23 : 459-489, 2 pls., 1909.

Watney, M. Growing Sumatra Tobacco under Shade. Bu-
reauasoils, U.S. Dept. Agl. Bul. 20:31 pp: 7 pls., 1902.

Texts. Clements, Jost, MacDougal, Pfeffer.

CHAPTER XVIII

RELATION TO DELETERIOUS CHEMICAL
AGENTS —

A LARGE number of water-soluble chemical substances
are injurious to all living protoplasm at concentrations
considerably below the osmotic equivalent of the cell-sap.
Such injurious substances are poisons, or toxie agents.
These may act directly or indirectly upon the protoplasm,
and the inference is that the action is ultimately chemical.
The dilution of a deleterious agent often results in stimu-
lation, whilst at still further dilution this effect also
disappears.

There is at present very incomplete knowledge of toxic
action; yet many advances have been made within the
past quarter-century. These advances have served to in-
crease knowledge generally, and in agricultural lines they
have been important in the study of soils, bacteriology,
plant pathology, and entomology. The results have been
utilized in the interpretation of experiments with fertiliz-
ers, in improving methods of disinfection or purification of
water-supplies, in the protection of plants against insect
pests and fungous diseases, and in various other ways to
which subsequently subsidiary reference may be made.

263. General relations to poisons. —Toxic agents may
be general or specific poisons. Specific poisons are as yet

436

Relation to Deletertous Chemical Agents 437

of minor importance in plant work. General poisons are
usually either strong (such as salts of mercury), or weak
(alcohol) for all organisms. Nevertheless, plants may
show some specific adjustment to poisons, and diversity
in effect may be due to one of the following causes : —

(1) A certain selective absorption may be shown, as in
the case of the nutrients, so that penetration will be rapid
in one case and practically prevented in another.

(2) Upon penetration the deleterious substance may be
‘converted into a relatively insoluble and nontoxic form,
before effecting serious injury to the protoplasmic organi-
zation.

(3) There may be specific differences in the effects upon
protoplasm, — peculiarities which it is at present impos-
sible to explain definitely.

One parasitic fungus may be killed by a dilute solution
of a copper compound, and another may germinate in a
relatively concentrated fungicide. Again, alkaloidal or
other toxic organic bodies may be produced within living
tissues, where they seem to set up no particular disturb-
ance; whereas they may serve as strong toxic agents
when placed in contact with other cells or organisms. In
the fermentation of fruit sugar the common yeast plant
produces alcohol, which soon prevents the growth of other
micro-organisms. Brown has demonstrated a marked
selective permeability in the coverings of seeds of a variety
of barley. These seeds take up water from a fairly strong
solution of sulphuric acid, and remain uninjured; but
mercuric bichlorid penetrates them with comparative ease.

264. Comparative resistance. — The fungi and bacteria
are commonly much more resistant to toxic agents than are

Growth, mm. per 24 hrs.

438 Plant Physiology

species of seed-plants; that is to say, many fungi and bac-
terla may grow in solutions which would inhibit root
growth. However, aérial surfaces of seed-plants do not,
as a rule, permit the rapid absorption of water or of chemi-
cal agents, so that for crop protection such surfaces may
be covered with strengths of toxic solutions prohibitive
to the germination and growth of fungi, as in the use of
fungicides and inseeticides.

16 SHEHSEESEEE
siiriatiiiil
ae
So ..88
\f --
paces exe
12 824 ise
an Po
SSS08 Seeeca eee
Sees eeeeeoas +H
HH
[Ti Ty
8 peess
Gsasesaues
6 PE
cosceeseesesees
GEGGcSSGeEGeneS
eH
4 soceeeneas
HEH BBEees
; HePsreasieafatatesosstsneat sevtitz
SEESECEEES peceeceeet
m/3500 m/3500 m/3500 m/3500 m/3500 m/3500
CuSO, CuSO, CuSO, CuSO, CuSO, CuSO,
~ ~ - - -
40 grams 80 grams 120 grams 160 grams 200 grams
sand sand sand sand sand

Fic. 124. Depression of toxicity with addition of sand. [After True
and Oglevee.]

439

Relation to Deleterious Chemical Agents

*(9) 19}8M poT]Astp

ydooxo UOI)N]OS yUOTIyNU

Ge

‘yoryMOO ! (G) OZ/U + (F) WOTJNTOS yUoTAyNU ! (Z) OJ /U ! (T) OF/U : 9 ‘ou
SULUIG

}U09 |]

BBPBSGS JUoIOHIp ut §(FOS)FV Jo worjow

Mnpun40949,)=_

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440 Plant Physiology

Many species of the lower alge are particularly sensi-
tive to certain toxic agents, such as the salts of copper and
other heavy metals. Zodspores of fungi and some species
of bacteria pathogenic in animals may be equally sensitive.

265. Toxic action and the substratum. — Much of the
literature of toxic action is confusing, owing to the fact that
the results are not comparable. Substances usually ex-
hibit their greatest toxicity in distilled water. Any nearly
neutral nutrient solution reduces toxic action even in cases
where molecular readjustments would not seem to be im-
portant. In the soil complex physical and chemical con-
ditions prevail, and these further modify toxic action.

Solid particles, such as pure sand, graphite, and filter
paper, may reduce toxic action to a considerable extent.
True and Oglevee found that twice as much sand as solu-
tion may reduce the toxic action of CuSO. for Lupinus
albus as much as thirty-two times (Fig. 124). The method
of reducing toxicity by solid particles is usually denoted ad-
sorption. It is a phenomenon explained upon the hypoth-
esis that many molecules or ions of the toxic substance are
physically held by the surfaces of the particles of the inert
material, and are, for the time, removed from the possi-
bility of chemical action. Another explanation is that the
solid substances offer obstacles to the free movement of
the solvent particles. Possibly both views are important.
Many of the so-called absorptive properties of soils both
respecting fertilizers and deleterious agents are in reality
adsorptive.!

1 The table from Jensen, on the opposite page, affords a comparison of
toxic action in sand and in solution cultures.

_ Relation to Deleterious Chemical Agents 441

From the data presented, it is evident that in defining
toxic concentrations it is necessary to speak in terms of the
substratum. When soil cultures are employed, the type
of soil and amount of organic matter are important. The
nutrient solution may modify the action of a poison by
forming with it chemical combinations less diffusible or
dissociated, and ultimately less injurious. Again, there
may be antitoxic action, as in the calcium-magnesium
relation. Mass action is also important, as suggested by
Dandeno; thus a seedling injured by 5 cc. of a toxic agent
may be killed by a greater quantity of the same concentra-
tion.

266. Method of action. — It is not possible at present
to state definitely the method of action of all deleterious
agents. Many metallic salts and other substances pre-
cipitate protein, and it is easy to picture the immediate
disturbance of protoplasmic organization effected by such

Sort CuLTURE (QUARTZ FLOUR SOLUTION CULTURE
WITH NUTRIENTS) (witH NUTRIENTS)
Toxic
AGENT Parts of an N/10,000,000 Solution
Inhibiting Stimulating Inhibiting Stimulating
Growth Growth Growth Growth
Ni(NOs). 70,000— 60,000; 5,000— 1,000 5,000— 2,500 4-2
ZnSO,. .| 300,000— 100,000} 3,000— 1,000 7,000— 6,000 none
AgNO, .| 300,000— 100,000) 90,000— 10,000 1,000-— 900 20-10
CuSO,. .| 300,000— 100,000) 10,000— 4,000 10,000— 5,000 none
FeCl, . .| 600,000— 400,000} 90,000— 20,000} 100,000- 98,0000} 4,000— 2,000
Pb(NOs3)o 500,000— 300,000) 90,000— 40,000} 400,000— 200,000|20,000—10,000
Phenol .{ 200,000—- 100,000) 8,000— 4,000} 200,000— 100,000! 8,000— 4,000
Alcohol _. |7,500,000—2,500,000)750,000—250,000)7,500,000—2,506,000|75,000—25,000

442 Plant Physiology

agents, yet it is impracticable to adopt a special grouping
based upon a similarity of action within the cell. Among
the deleterious agents known, those of economic signifi-
cance are of special interest. Of these the important groups
are inorganic and organic acids; caustic alkalies; salts of
the heavy metals; formalin; alcohol and anesthetics;

Se a
Ra eS A Vid
aw dang iy Ap et}

Fic. 126. Indications of the effects of the substratum upon the toxic
action of CuSO,; loam (L), sand (8S), graphite (G). [Photograph by
W. W. Bonns.]

various organic compounds, including decomposition and
hydration products of proteins and lecithins, alkaloids and
miscellaneous nitrogenous bodies, also many non-nitroge-
nous organic products of diverse composition; and cer-
tain deleterious gases of the carbon series.

267. Inorganic and organic acids. — Inorganic acids are
usually the most toxic of the acid substances for the higher

Relation to Deleterious Chemical Agents 443

plants. Some of the results secured by Kahlenberg and
True are given in the table below, where also a compari-
son may be made with acetic acid, the latter occupying
an intermediate position with respect to toxicity among
organic acids : —

PISUM SATIVUM | ZeA Mays LUPINUS ALBUS
AcIps l l
Gram Mol. | Parts Per ||Gram Mol.| Parts Per) Gram Mol.| Parts per
| Sol. Million! Sol. Million || Sol. Million
i |
HCL eee colle 1/ 12800 3 1/3200 11 || 1/6400 Syd
HSO, . . .|| 1/12800 3 1/3200 | 11 || 1/6400} 5.5
Ey Ore || “1/1 2800 3 1/3200 11 || 1/6400 55
CH.COOH oil IV@AOC 11 1/400 91 || 1/1600 Dope

In these experiments the roots of a few seedlings were
immersed in 300 ce. of solution (the acid in distilled water)
and the concentrations given are just sufficient to kill at
least 50 per cent of the roots after an exposure of 24 hours.
The toxicity of inorganic acids is strikingly reduced by the
presence in the solution of solid particles.

268. Alkalies. — Alkalies are in general less toxic to the
roots of seed plants than are equivalent concentrations of
acids or of salts of the heavy metals. In order to inhibit
root growth of seedlings in water cultures, it requires from
5 to 10 times as strong a solution of caustic alkali as ef a
mineral acid. Alkalinity (basicity) and acidity as applied
to field conditions are merely relative terms, since under
such conditions the usual methods of determining these
qualities are inaccurate. It is well known, however, as in-

+ Approximate.

Plant Physiology

44-4

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446 Plant Physiology

dicated under the discussion of lime, that many plants
thrive under basic conditions, while others yield best when
the substratum is acid.

269. Salts of the heavy metals. — The salts of the
heavy metals constitute a group of the most toxic agents
known. The various soluble inorganic salts of the same
metal are commonly of about equal toxic value. The
table given below is comparable to that given for inorganic
acids, the concentrations representing those which kill the
majority of the roots in 24 hours :—

|

|
‘Pisum sativum, Zea Mays’ |LUuUpPINUS ALBUS
| |

SUBSTANCES
Gram Molecular Solution
= ns
Cul. ty Tie ipa hat 1/51200 1/102400 1/25600
CS Oitate pt. ve” pala tel eee ee 1/51200 1/102400 1/25600
NSO ee LE. 0A eae gale aoe 1/51200 1/51200 1/25600
INTO as cc. Os cy ane ie oe 1/51200 1/51200 1/25600
OSU) ack: Soe Claret one 1/25600 1/6400 1/12800
COUNT) oty al eee gaan 1/25600 1/6400 1/12800
AV SCINIC RY Sol’ is Vca oe te ee iS 1/204800 | 1/204800 1/204800
A SO rc: it Seer PA Ga wee 1/204800 | = 1/204800 1/204800
Ee ch ee. aires at Bs) 1/204800 1/51200 1/12800
WG afi a ee a 1/12800 1/6400 1/6400
: |

Kanda employed pots holding two liters of soils in some
experiments with horse beans. These were in one case
watered daily with copper sulfate to such extent that at the
end of three weeks the pot contained 26.394 grams of the
salt. This amount caused only a slight reduction of root
growth, but stem growth was greater than in the control.

Copper compounds are extremely injurious to certain

Relation to Deletertous Chemical Agents 447

alge. They have been effectively employed by Moore
and Kellerman! for the eradication of such organisms in
ponds and water supplies. For this purpose copper sul-
fate is used at the rate of 1 part to 250,000—1,000,000
parts of water. A copper coin in a small dish of water
containing half a dozen threads of a green alga is sufficient
to cause death in a day or two.

270. Formalin. — Formalin is a penetrating toxic agent
for all plant cells. According to Clark it ranks close to
mercuric bichlorid and silver nitrate as a poison for fungi
in beet decoction. In agricultural practice formalin so-
lutions are important in the control of certain fungous
diseases by seed treatment. The seed do not absorb the
solution so rapidly as the spores, so that a short immersion
may serve to disinfect the former. Formalin is employed
for the prevention of bunt of wheat, loose smut of oats,
and potato scab.

271. Organic bodies. — The effects of various alkaloids
and other nitrogenous bodies upon the higher vertebrates
have long been a matter of experimentation. The toxic
products of disease-producing bacteria are of this nature.
Such substances are frequently more toxic to organisms
possessing complex nervous and circulatory systems; but
similar substances may be injurious to protoplasm in gen-
eral. Through the decomposition of animal or vegetable
matter in the soil, toxic bodies may be formed, and these
may at times play a recognizable réle in the relations of
vegetation.

272. Root excretions. — De Candolle made the sugges-
tion more than half a century ago that plants may influence

1 Bureau Plant Ind., U.S. Dept. Agl., Bul. 64 : 44 pp., 1904.

448 Plant Physiology

one another by means of substances derived from their
roots. This view was at first credited, but soon lost sup-
port. Rotation of crops is based largely upon the idea of
physical advantage, or disease suppression. In very
recent years some investigators have proposed that soils are
commonly unproductive on aecount of the presence in them
of toxic organic compounds. This view with some persons
implies that the injurious substances arise through the ex-
cretions of roots. The assumption of any general excre-
tion of toxic bodies by roots is at present scarcely justified,
although an oxidizing power of roots is now demonstrated.

273. Unproductiveness. — Interesting and _ valuable
data have been accumulated by Schreiner and his asso-
ciates, which throw much light upon the nature of the or-
ganic compounds which may be found in the soil, and like-
wise upon their toxicity. The decomposition of root-hairs
and cast-off portions of roots, of green manures, or of any
plant or animal remains in the soil give rise to temporary
products which may be injurious. Nevertheless, it is not
believed that the quantities of injurious organic bodies
set free in a well cultivated soil during the growth of a
staple crop, whether due to the decomposition of roots or
to direct excretion, are often sufficient to be of agricultural
importance. With the large number of bacteria ordinarily
present in the soil, and the amount of aération necessarily
given in cultivation, such toxic substances would seem to
be of merely temporary concern. In the case of bog soils,
or land where there is insufficient drainage and lack of
aération, the toxic factor may be permanently important.
It is certain that unproductiveness is not due to a single
factor of this type, and at present many lines of work are

Relation to Deleterious Chemical Agents 449

being directed toward a solution of the problems of in-
fertility.

274. Relative toxicity of some organic compounds. —
In the table below are given some of the interesting results
obtained by Schreiner and Reed respecting the effects of
various organic compounds upon wheat placed in water
cultures from 7 to 10 days; the concentrations indicated
are in parts per million (p.p.m.) in distilled water : —

LOWEST LOWEST
Conc. Conc.
SUBSTANCES CAUSING CAUSING
DEATH, INJURY,
P.P.M. P.P.M.
PMIAMINE es os) 500 Only roots injured at 500.
MevT@siie ss os 10
Meneine ae io No injurious action.
‘Clee linavsi 2 ai aa 500 Roots most affected.
Meumimen 3... -: 250 US,
Rretaimercae ahs. No injury.
‘GOGH. oe No injury.
Goamidine -.... . 100 1
Said) Ge 200 50 | Roots more injured.
ynigine +. . . 50

Among twenty-two nitrogen-containing compounds, two
were found which were injurious at the surprisingly low
concentration of less than 10 p.p.m. In the above ex-
periments water cultures were employed; but tests by
Bonns in my laboratory have shown that wheat in paraf-
fined pots containing rich garden loam is practically un-
affected by pyridine at the enormous rate of 8000 p.p.m.,
this solution being used to moisten the soil to 60 per cent
of its water-holding capacity. If this relationship should

2G

450 Plant Physiology

be found to hold good for the other organic substances
mentioned, it is apparent that the accumulation of such
bodies in the soil in amounts which might be toxic (wholly
neglecting the possibility of their immediate destruction)
would require long periods of irrational cropping.

275. Illuminating gas. — It has long been known that
illuminating gas is injurious to vegetation. Even small
leaks in gas pipes are fatal to the roots of trees in the vicin-
ity. Vegetation in cities suffers greatly from this cause.
The danger is greatly increased by the fact that gas diffuses
through the soil to considerable distances, particularly
when the surface of the ground is frozen or compact, as
when streets or roadways supervene. Many decorative
plants are reported to fail as house plants when illuminat-
ing gas is burned. This may be due to gas-escape at the
time of lighting burners (since, as will be shown subse-
quently, the amount of gas needed to cause injury is ex-
tremely small), or it may be due to incomplete combustion
of the gas.

Crocker and Knight have shown that ethylene, although
present in very minute quantities, is apparently the chief
toxic constituent of the illuminating gas with which they
worked. They employed as indicators flowers and buds
of the Carnation, — Boston Market and the pink Lawson.
After an exposure of three days the young buds of these
plants were dead, and bursting buds were prevented from
opening by a concentration of one part of gas in 40,000
parts of air; while after an exposure of twelve hours 1 part
to 80,000 caused the flowers that were already opened to
close. In ethylene of 1 part in 1,000,000 buds in which
the petals were just showing failed to open after an ex-

Relation to Deleterious Chemical Agents 461

posure of three days, and flowers closed after an exposure
of twelve hours to an atmosphere of only 1 part to 2,000,000.

276. Stimulation by means of weak toxic agents. — Small
quantities of acids and other substances may serve as
stimulants in several types of enzyme action, — they may
increase the velocities of the chemical reactions. The
transformation of starch by diastase and of certain pro-
teins by pepsin are both accelerated by traces of acid.
Richards and Ono have shown conclusively that the dry
weight of certain fungi in nutrient solutions may be in-
creased two or three times by the addition of a small
quantity of one of several metallic salts.1. In general,
zine has afforded the best results. Spore production is
diminished in the stimulated cultures. Furthermore, it
has been shown by subsequent work that stimulated plants
are able “to dispose more economically of the sugar
used, . . . thereby permitting a more rapid production of
dry substance in a given time.”

1 The data from some experiments (Richards) in which the fungus was
grown on a nutrient solution containing sugar are as follows : —

STIMULATION OF GROWTH IN ASPERGILLUS NIGER BY ZNSO4
(CULTURES KEPT AT 30° C. UNLESS OTHERWISE NOTED.)

WEIGHT IN MILLIGRAMS

PERIOD
SPECIAL CONDITIONS OF
OF CULTURE GROWTH

Days | _No_ | .002%| .004% | .008% | .016% | .083 %
ZnSO, | ZnSO,| ZnSO, | ZnSO, | ZnSO, | ZnSO,

335 730 760 765 770 715
300 670 700 680 650 610
560 970 980 770 390 250
1120 | 1490 | 1375 | 1515 | 1455 | 1280
650 630 680 650 645 610
200 335 300

Asparagin added .
Peptone added .
2% FeSO, added.
DAC C.

DDWMWA AN

|

Plant Physiology

452

The results of stimulation experiments in which seed-
plants have been employed are somewhat contradictory.
On the whole, a certain degree of stimulation seems pos-

sible, especially when some of the conditions of growth are

unfavorable.

From investigations conducted in Japan

rat

oo
oH bee
A
oo
Eu Hage ESE eae EEE

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HE ee i os
FR Faueees td ne SON :
St (HHH EES yee
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ed

0085 007 015% ZnSO,

002

-008

OL

004
Fic. 129. Stimulation of growth in Aspergillus by ZnSO

-0005

Control

% NaF!
(continuous

[Data from Richards.]

016

-002

Control

.033 - - - -

4

line) and NaF I (broken line).

there have been reported some benefits from the joint

action of iron and manganese, also with salts of iodin, so-

dium fluorid

yet some of the effects

and some other agents,

p]

are probably indirect.

277. Protection of crops by insecticides and fungi-
cides. — Practically all cultivated or exploited crops are

More than

twenty-five important species of insects are known to at-

subject to the attacks of insects and fungi.

Relation to Deleterious Chemical Agents 453

tack the grape-vine and there are at least half as many
fungous diseases of the same plant. Numerous instances
might be cited in which the number of fungous and insect
pests of a particular crop is as great as those indicated.!
On the other hand, there are cultivated crops which re-
quire very little consideration with respect to parasites of
any kind.

It is only in relatively recent times that spraying opera-
tions have developed, especially spraying to prevent fun-
gous diseases. This type of control has been in large part
due to a careful investigation of the relations between
plants and toxic solutions on the one hand, and to the de-
velopment of effective spraying devices on the other. In
nearly all cases protection against fungous diseases and
Insect pests is effected by covering the surfaces of fruit,
leaves, and stems with a poisonous substance, which
should, while relatively noninjurious to the host, prevent
the effective germination and penetration of the spores,
or kill the insects concerned.

In controlling insects, it is to be remembered that there
are two great classes with respect to the method of attack ;
these are : —

(1) Chewing insects which bite off and eat the vegeta-
tive parts of the plant; for example, cabbage worms, tent
caterpillars, and potato beetles, which would be killed by
poisons sprayed upon. the plant.

(2) Sucking insects, or those which get their food by in-

1Some estimates of the amount of damage annually sustained by the
crops of the United States have been made, and taking as a basis the
prices at which the crops actually sell, it seems to be demonstrated that
the vast sum of one billion dollars may be aggregated.

454 Plant Physiology

serting a beak directly into the tissues, for example, plant
lice and squash bugs.!

278. Destruction of weeds by poisons. — For years it
has been more or less customary to employ salt for the
destruction of weeds in the lawn or garden, or to suppress
all plants growing in walks and playgrounds. It is only
within recent years, however, that any special study has
been bestowed upon the use of toxic solutions in the form
of sprays as one of the recognized methods of weed control
in lawns and cultivated fields. It is not a method which
may be expected to replace the usual practices of clean
cultivation, rotation, or pasturing, nor is it one which
should lead the grower away from a close study of the root-
ing and reproductive habits of weeds.

1 The poisons or insecticides commonly employed for biting insects
are such as Paris green, arsenate of lead, arsenite of soda, arsenite of lime,
London purple, and hellebore. Of these, Paris green is by far the most im-
portant. The use of this substance attracted general attention between
1860 and 1870, when the Colorado potato beetle became an important
factor in potato production. Shortly afterwards, the same mixture was
employed throughout the South against the so-called army-worm of cot-
ton, and it has since been used to give protection against an endless num-
ber of biting insects.

In employing the usual means of control against sucking insects, such
substances as kerosene emulsion, miscible oils, whale oil soap, and lime-
sulphur wash may be used, as well as methods of fumigation. The first
three substances mentioned may be employed, with care, upon the foliage
and growing parts. Miscible oils, carbolic acid, and relatively strong
kerosene may be used only when the plant is in a dormant condition.
Fumigation with tobacco smoke is common. The highly toxic vapor of
hydrocyanic acid, prepared from potassium cyanide and sulfurie acid,
has also been employed in the fumigation of trees under tents and with
nursery stock in a dormant condition. It may also be used in the green-
house with care, but special instructions are needed in any particular case.

The effective use of chemical agents as protective measures against
fungous diseases, dates from the discovery of Bordeaux mixture by Millar-
det in France, 1883. Since that time, there has been organized through-
out the United States and in foreign countries extensive methods of con-
trolling these diseases.

Relation to Deleterious Chemical Agents 455

279. Deleterious substances employed. — Common salt
is only slightly toxic, yet it has been much employed on

Fig. 130. Greater ragweed in untreated field of wheat. [Photograph by
H. L. Bolley.]

account of its osmotic action. It may be used dry in
roadways and other situations, and it has sometimes been
effective in suppressing broad-leaved, delicate weeds in

456 Plant Physiology

lawns, where it may be applied at the rate of from 3 to 6
pounds per square rod. Crude carbolic acid possesses an

t

Fic. 131. Wheat in plat contiguous to that in Fig. 130, showing effect
of iron sulfate spray on ragweed. [Photograph by H. L. Bolley.]
objectionable odor, but it is very effective in killing
vegetation in walks or courts. It may be sprayed upon
the ground at a strength of 1 quart of the acid to 5 gallons

Relation to Deleterious Chemical Agents 457

of water. Waste formalin at the rate of 1 pound to 10 gal-
lons of water may be used under similar circumstances.
Copper sulfate and iron sulfate are, however, the two
compounds which may be commercially employed with
hand or power sprayers for the suppression of certain weeds
in fields of grain or flax, and in large lawns. Copper

Cees,
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Fig. 132. Wild mustard of size effectively reached and readily injured
by the spray.

sulfate is commonly used as a solution containing from 3
to 5 per cent of the salt, — 12 to 20 pounds to 50 gallons of
water. It is usually recommended to employ iron sulfate at
a strength of 13 to 2 pounds of the salt per gallon of water.

280. Practicability of the chemical method. — The
chemical method may be wisely employed under certain
circumstances, as follows : —

458 Plant Physiology

(1) Upon ground where no vegetation 1s desired — walks,
playgrounds, courtyards, ete.

rs
5
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Fic. 133. Natural growth of dandelions in an untreated lawn. [Photo-
graph by H. L. Bolley.]
(2) When particularly undesirable weeds are present in
small spots, and the temporary suppression of all growth
is not a serious objection...

— -

Relation to Deleterious Chemical Agents 459

(3) When it is desired to suppress weeds or to prevent
weed seeding during the maturity of a seed crop.

Fic. 134. Lawn with dandelions, similar to that in Fig. 133, but treated
with iron sulfate two weeks before blossoming. [Photograph by H. L.
Bolley.]

(4) When, during the growing season of the crop, a ma-
jority of the undesirable weeds are more sensitive than the

crop grown.

460 Plant Physiology

It is obvious that the use of chemical sprays for weed
eradication in the field is dependent upon the resistance of
the crop as compared with the weed and to the penetration
of the chemicals employed. This method has been found
especially applicable in growing cereal crops, grasses, flax,
and peas.!

The plants which are killed are those whose surfaces are
easily wet by the spray, but there are some plants, the
common plantain (Plantago major), for example, which,
although wet, is almost unaffected. Those which are not
wet generally possess smooth glaucous leaves, or are pro-
vided with a waxy bloom. In any plant the succulent
or rapidly growing portions are more easily killed. Thus
it follows that this means of eradication may be generally
employed for plants with an indefinite habit of growth.

LABORATORY WORK

Toxic action. — Determine the limiting concentrations of
CuSO, and H.SO, for inhibition and growth of roots of corn

and peas. Use the tumbler-culture methods employed in the.

study of mineral nutrients, or, if observations cover only a
short interval of time, the germinating seeds may be pinned to
the lower surfaces of corks covering the vessels employed, the
roots projecting into the solutions. Make decinormal stock
solutions of the toxic agents. With the CuSO, employ at least

1 From a considerable number of experiments, it has been found that
such plants as the following are more or less readily killed: bindweed,
Canada thistle, dock, great ragweed, lamb’s quarters, mustard or char-
lock, orange hawkweed, sow thistle, wild buckwheat, and wild radish.
Weeds which have not been successfully combated without injury to the
growing crop are such as bentgrass, bull-thistle, couch-grass, horse-tail,
pigweed, and others. With iron sulfate Bolley has been able to hold
the dandelion in check ; but on account of the perennial root, this plant
is one of the most difficult to eradicate.

Relation to Deleterious Chemical Agents 461

and ; while with
10000 300000

Besa, f N N
H.SO, prepare dilutions ranging from aa to aaa

Effect of insoluble particles. — After following carefully the
discussion in the text, determine, through cultures in tumblers,
the concentration of CuSO, and of H.SO, which will inhibit and
permit growth in (1) granulated quartz or infertile sand, and in
(2) a rich garden loam. In these experiments use, in each ease,
sufficient of the solution to moisten the substratum approxi-
mately, and in comparative experiments the same amount of
solution should be used. Permit the experiments to run only
one week, and watering will not be required.

Toxic agents and foliage. — With a hand spray or atomizer
treat the foliage of convenient plants in the greenhouse or field
-with 3 per cent copper sulfate, and 5 per cent iron sulfate.
Study the comparative effects. Cereals, grasses, carnations,
and onions may be taken as types of foliage not easily wetted,
while dandelions, mustard, beans, and peaches will furnish suit-
able contrasts.

four or five dilutions between

REFERENCES

Boutey, H. L. Weed Control by Means of Chemical Sprays.
N. Dak. Agl. Exp. Sta. Bul. 80 : 541-574, pl. 11-29,
1908.

Brown, A.J. The Selective Permeability of the Covering of the
Seeds of Hordeum vulgare. Proc. Roy. Soe. 81 B: 82-93,
1909.

Cuark, J. F. On the Toxic Effect of Deleterious Agents on the
Germination and Development of Certain Filamentous
Fungi. Bot. Gaz. 28: 289-327, 378-404, 1899.

Crocker, W., and Knicut, L. I. Effects of Illuminating Gas and
Ethylene upon Flowering Carnations. Bot. Gaz. 46: 259-
276, 1908. [Compare also, Knight, Rose, and Crocker,
Science, N.S. 31: 635-636.]

462 Plant Physiology

Davenport, C. B. Experimental Morphology. 1: 1-52.

Duaear, B. M. Fungous Diseases of Plants. Pp. 85-92, 1909.

Heap, F. D. On the Toxie Effect of Dilute Solutions of Acids
and Salts on Plants. Bot. Gaz. 22: 125-153, 1896.

JENSEN, C. H. Toxic Limits and Stimulation Effects of some
Salts and Poisons on Wheat. Bot.Gaz. 43: 11-44, 34 figs.,
1907.

Jones, L. R. Chemical Weed Killers or Herbicides. Bailey’s
Encyclopedia of American Agriculture. 2:115-118, 1908.
[Also Vt. Agl. Exp. Sta. Report. 12: 182-188, 1899.]

KAHLENBERG, L., and Trur, R. H. On the Toxie Action of Dis-
solved Salts and their Electrolytic Dissociation. Bot. Gaz.
22 : 81-124.

LopEMAN, E.G. The Spraying of Plants. 399 pp., 92 figs., 1896.

Loew, O. Ein natiirliches System der Gift-wirkungen. 1893.

On the treatment of Crops by Stimulating Compounds.

Bul. Coll. of Agl. Imp. Univ. Tokyo. 6: 161-175, 1904.

Ricuarps, H. M. Die Beeinflussung des Wachsthums einige
Pilze durch chemische Reize. Jahrb. f. wiss. Bot. 30:
665-688, 1897.

ScHREINER, O., and Reep, H. 8S. Certain Organic Constituents
of Soils in Relation to Soil Fertility. Bur. of Soils, U.S.
Dept. Agl. Bul. 47:52 pp., 5 pls., 1907.

Stone, J. L. Spraying for Wild Mustard. Cornell Agl. Exp.
Sta. Bul. 216: 107-110.

True, R. H., and Oatever, C. S. The Effects of the Presence
of Insoluble Substances on the Toxie Action of Poisons.
Bot. Gaz. 39: 1-21, 1905.

CHAPTER, X1X

VARIATION AND HEREDITY

IN organic variation, our interest centers on the
mechanism and forces concerned in the adjustment of
an organism to its environment. Variation signifies
change, and may be evidence of past and present influences ;
heredity gives a record of the past and certain promises
for the future. Both of these are important aspects of
evolution.

Many theories have been advanced in explanation of
the facts of variation and heredity. No single theory re-
ceives at present universal sanction. Every reasonable
hypothesis merits careful consideration, and the present
widespread interest in experimental evolution makes it
particularly desirable to view new facts in an unpreju-
diced light. The limited scope of this book makes it
possible to include only a brief presentation of some of the
important facts and views, as an introduction to the sub-
ject; the fuller theoretical treatment and application
must be sought in the literature.

VARIATION

The capacity for variation is a fundamental possession.
It is universal with living organisms. Though heredity
| 463

464 Plant Physiology

characteristics of parents are transmitted to the offspring,
yet not all individual characteristics of all ancestors are
transmitted. The offspring may exhibit modification.
This modification may be evident under constant condi-
tions, or it may occur in response to environmental changes.
It may be an acquirement manifest merely during the
life of the organism, or it may be innate and _ trans-
missible. Every organism possesses an individuality.

281. Individuals and species. — Individuals which re-
semble one another closely and which have a common
origin may collectively constitute what may be called a
race, variety, or species. Our ideas of such groups are
based upon a study of individuals (few or many, small or
large populations) and naturally center about average
examples. We recognize, however, certain extremes; in
fact, there are multitudinous variations, for there may be
as many extremes as characters, or character combinations.
These extremes, perhaps, have in many cases so insensibly
entered into other recognized varieties that opinions would
differ in determining to which variety a particular individual
should be attached. Some varieties, on the other hand,
may stand apart with sharply differentiated characters;
within these, individuals may also differ perceptibly
among themselves. In any case, a group of individuals,
such as a race, variety, or species, is in a measure a theo-
retical average with respect to characters, and is made
up of a series of individuals showing in the different
characters considerable fluctuation.1

1 All organisms resembling one another closely must look alike to
the inexperienced eye, — to the eye unfamiliar with the group. Upon
close inspection and measurement, however, relatively wide differences

:
:

Variation and Heredity 465

Fig. 135. Variation in the leaves from a single bud of sassafras.

2H

466 Plant Physiology

282. Fluctuating variation.— The minor differences
which all individuals of any population exhibit are com-
monly fluctuating, or continuous, variations. The ideal,
or type, which these individuals approach is an average
individual, with reference to a number of characters. If
many individuals be examined with respect to any one
character, the result may be given in the form of a curve of
variation. An examination, for instance, of a population
of the common field daisy would disclose a variation in
the number of ray flowers; thus there might be from 10 to
20. This variation in number represents the range, and
the numbers 10, 11, 12, 13, ete., constitute the variates or
classes. Perhaps the majority of the population would
have the same number of ray flowers, say 15, which class
would then represent the mode, or class of greatest fre-
quency. In anormal curve there would be a diminishing
frequency towards both higher and lower classes. Quete-
let has shown that this curve, in the main, corresponds
with the law of probabilities, or curve of frequency of
error.

invariably appear. The tomato is an excellent example of variability.
Little more than a century ago it was introduced as a vegetable. To-
day its varieties are numbered by the hundred, and there are a great
many well-defined types and forms of fruit, characters of leaf, size, ete.
We are thus sure that by one means or another variation has been effected,
and in a marked degree. Once several strains or varieties are developed,
hybridization is the greatest possible source of variation, or multiplica-
tion of forms.

The use of score cards in judging corn, apples, and other farm and
horticultural crops draws special attention in a practical way to standards
among economic plants, and at the same time to departure from the
standards, — to variation. The producer is likely to have in mind as
an ideal the best or most highly developed type of any variety or strain,
and thus in the work of selection he may constantly depart from the
old ideal in the direction of a new and improved strain.

~~ —_— Ss =

Variation and Heredity 467

The symmetrical curve shows the highest frequency, or
mode, in the center, but not infrequently the mode is
considerably shifted from one side to the other, giving
skew curves. Again, multimodal curves occur, and many
other subsidiary forms have been found to prevail in cer-
tain species or races.

283. Darwin’s theory of natural selection. — Through
his wide experience with living things Darwin was thor-
oughly conversant with the existence of fluctuations in
nature. It was apparent that growers select, isolate, and
breed desirable forms or individuals, excluding or destroy-
ing those undesirable. Such a process appears to have led
to the origination of new breeds and races. Darwin saw
in nature similar forces yielding similar results; viewing
the problem, therefore, in the uncontrolled or natural
environment, he formulated the following ideas: (1) any
individual variation, slight or considerable, which enables
the organism possessing it to succeed or maintain itself
better than its neighbor will have a strong chance of be-
coming perpetuated; (2) more seeds are produced than
can grow again unto seedage, more organisms enter upon
life than can be reared; (3) those less well equipped for
life’s struggle succumb, and there is manifest a powerful
process of Natural Selection.

He would seem to have maintained that the main line of
evolutionary progress and change lies in the natural or
artificial selection of relatively minute variations; that is,
natural selection lays hold upon fluctuating variations.
There is constant variation, hence there is constant change,
or evolution. Wide variations arising suddenly, termed
sports, or discontinuous variations, were apparently

468 Plant Physiology

regarded by Darwin as of less importance in evolutionary
progress (see section 287).

In most of the present-day discussions respecting evo-
lution, natural selection is recognized as a potent force :
but great diversity of opinion prevails with regard to the
magnitude of the variations by means of which progress in
selection is maintained.

284. Rate of increase.— A study of the theoretical
rate of increase of many organisms involves numbers which
are not easily grasped. A single tobacco plant may
produce from 500,000 to 1,000,000 seed. At the minimum
production mentioned the second year there would be
250 billion seed, and the product the third year would
be expressed by eighteen figures. At the rate of one seed
per square foot, this number would plant the surface of
the earth several hundred times over. <A _ vigorous
specimen of the common dandelion under observation
produced in a season about thirty flower-heads, each
averaging about 300 seed, or 9000 seed for the season.
In this case, there would result at the end of the fourth
season 6,561,000,000,000 seed.!

Assuming the capacity of an organism to vary, the power
of the environment to suppress and exterminate unfitness
makes it a very strong factor in determining the nature of

1Qne instance from the animal side may be cited (adapted from
Jordan and Kellogg, ‘‘Evolution and Animal Life,”’ p. 59), that of the
quinnat salmon of the Columbia River. This is a prolific fish whose
eggs and young are poorly protected and consequently devoured by
numerous greedy enemies. A female will ascend the river when four
years old and deposit about 4000 eggs, subsequently dying. Allowing
for 50 per cent of males and the normal period of maturity of females,
only five generations would be required, should all individuals survive,
for this fish to occupy far more than the volume of the sea.

Variation and Heredity 469

those creatures which survive. Since, moreover, the or-
ganisms which survive respond to the influence of the
environment, be it much or little, the stamp of environ-
ment is ultimately borne by every living thing. This
does not imply, however, that the environment stimulates
change in the direction of fitness for the particular en-
vironment, yet a strictly physico-chemical explanation
would perhaps demand this.

285. Fluctuating variation and the origin of varieties. —
The artificial selection of fluctuating variations has been
the basis of great improvement, or of the maintenance of
standards, in many cultivated crops. The sugar content
of good varieties of the beet has been increased from be-
tween 8 and 10 per cent to from 14 to 18 per cent. Many
deny permanence to this type of selection, and much experi-
mental work appears to be in progress, designed to throw
light upon the question.

Punnett says: “ The small fluctuating variations are not
the materials on which selection works. Such fluctuations
are often due to conditions of the environment, to nutri-
tion, correlation of organs, and the like. There is no in-
disputable evidence that they can be worked up and fixed
as a specific character.’’ Castle, speaking of the heredity
of fluctuations, says, ‘It is an exceedingly difficult and
slow process, and its results of questionable permanency.”

Physiological modifications in corn. — Unusually inter-
esting experiments in corn variation and breeding have
been conducted at the Illinois Experiment Station. In this
case variation in chemical content with respect to high
protein and low protein, also high oil and low oil, has been
made the subject of study, and the results for a ten-year

470 Plant Physiology

period have been reported. A white dent corn was used,
and to this the name Illinois has been given. In protein
content the 100 ears of seed corn used the first year varied
from 13.87 to 8.25 per cent. From the table in the note
below ! it will be seen that the ten years of selection sufficed
to bring the average of the plat (1906) in high protein
(14.26 % ) above the best earin the first crop (13.87 % ), and
the average in the low-protein plat (8.64%) was practi-
cally as low as the lowest ear (8.25%). The yearly
averages in the high-oil and low-oil breeding show a varia-
tion often more striking than in the case just cited, as
shown in the footnote.

1 Tren GENERATIONS OF BREEDING CORN FOR INCREASE AND DECREASE
OF PROTEIN

HIGH-PROTEIN PLOT, LOW-PROTEIN PLor, |
AVERAGE PER CENT AVERAGE PER CENT || DipFERENCE
PROTEIN PROTEIN | BETWEEN
YEAR !
CROPS,
In Seed In Crop In Seed In Crop | PER CENT
planted harvested planted harvested |
— — —
145)! ae aa Re 10.92 || —— 10.92 .00
by SRR Pm Sane S| 11.10 || 8.96 | 10.55 1 lime
SOS es Mowe 12.49 11.05! 9.06 10.55 .50
ISSO biog. cae hee 13.06 |. 11.46 8.45 9.86 1.60
UO oe pie dose » |} 42.32 8.08 9.34 2.98
POOR Sur. % 14.78% | SA 7.58 | 10.04 4.08
POOR Sc oes 15.389 | 12.354 8.15 8.22 4.12
$908 > Sods ole SO oe 6.93 | 8.62 4.42
PSU < Sin He,t 15.39 15.03 7.00 9.27 5.76
SAS 1a ph in ena tae 16.77 14.72 7.09 8.57 6.15
$906 7.52.5 16.30 14.26 7.21 8.64 8.62

The relatively high protein content in the product harvested in 1901 is
evidently the result of early maturity of the seed during that remarkably
dry season.

Vanation and Heredity A71

Summarizing the extreme variations in these qualities at
the beginning and close of the periods, the table on the
next page is suggestive.

286. Pure lines. — Johannsen has employed the term
“pure line ” to denote the offspring of a single individual
produced by self-fertilization (thus isolating a type,
genotype). With pure lines he has conducted extensive
selection experiments, and the results for quantitative
characters stand in contrast to those obtained by selec-
tion in an ordinary population. Selection within a pure
line, from modal individuals and from those showing the
greatest deviation, yield offspring the averages of which
are the same. According to this, selection within the

TEN GENERATIONS OF BREEDING CORN FOR INCREASE AND DECREASE

OF OIL
HieuH-o1t Piot, Low-o1L PLot, DIFFER-
AVERAGE PER CENT OIL ||AVERAGE PER CENT OIL || enor pE-
Sane TWEEN
In Seed In Crop In Seed In Crop (Crops, Par
planted harvested planted harvested CENT
Pe aone |

SOGWE es ase — 4.70 — 4.70 .OO
LOO <5 Asam 5.39 4.73 4.03 4.06 .67
OOS mari, oats). % 5.20 501145) 3.65 3.99 LG
1c15)0) Say ot eal a 6.15 5.64 3.47 3.82 1.82
HQ) 0) S35 Sia as 6.30 6.12 3.39 BIN 2239
TOONS oles cota ae aan 6.77 6.09 2.93 3.43 2.66
19) 023 S0 a ie Ne ea 6.95 6.41 3.00 3.02 3.39
719) 053 0 Sco ean a 6.73 6.50 2.62 2.97 3.53
NGO AE ea! 7.16 6.97 2.80 2.89 4.08
Es Fs ac. 7.88 7.29 2.67 2.58 4.71
Bete ifs. ss 7.86 ea 2.20 2.66 4.71

.

472 Plant Physiology

pure line cannot change the averages, or shift the mode.
These results are most suggestive and important, and
the principle has been confirmed by Jennings and others;
but many additional data will be required before this
type of behavior is recognized as of general significance.

TABLE SHOWING EXTENT OF VARIATION

Tyre oF BREEDING Best Ear EXTREME VARIATION
High protein per cent per cent

DS Uhey Ve os 3 Bae eee 13.87 5.62 original seed

1906 x" 4 eel 17.67 10.94 tenth year selection
Low protein

[S964 Sg ee eae 8.25

VOUG SCR 6.73
High oil |

1896 4 vA sete 6.02 2.18 original seed

1906 8.51 6.91 tenth year selection
Low oil

TS964 5. 4 Fe a 3.84

DQG OF RA Rok een 1.60

287. Mutations. — A sudden variation which is fully
transmissible has been called by De Vries a mutation. It
is a discontinuous variation or hereditary saltation.
Mutation phenomena have become prominent in variation
studies since the publication by De Vries of his Mutation
Theory. Already the term has been loosely employed, but
in general it has the special significance above indicated.
De Vries advanced the view that all evolutionary progress
is based upon the occurrence of mutations. His conclu-

473

Variation and Heredity

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OULBS OY} UO SOTONBA [BIN}[NOLIOY OAY JO orsiopovieyo spuors : Stdojorqdoyy Ul UOL}eLIVA png ‘ge, DI

474 Plant Physiology

sions were primarily the result of extensive studies upon
the evening primrose (Ginothera Lamarckiana), and upon
a general review of available information respecting both
the origin of domesticated varieties, and the behavior of
organisms in nature. Even though some hesitate to
accept all the conclusions arrived at for the Cnothera
mutants, the principle of mutation has been accepted by
many students of evolution as a working hypothesis;
and many are now endeavoring to determine the extent,
frequency, and behavior of such mutants.

According to the current view a mutation may be a
variation relatively great or small, involving a single unit
character or a group of such characters. The mutation
is often of greater, but may be of lesser, extent than the |
fluctuation, and the existence of the two types together
may lead to much confusion. Far more careful analytical
work will be required before it may be possible fairly to
estimate the respective value in evolution of mutation and
fluctuation, or indeed properly to distinguish types of
variation. There can be no doubt that striking cases
are on record of the occurrence of saltation; but it is
obvious that the extreme supporters of the mutation
principle, by the definition and explanation of the term,
actually exclude the possibility of any such phenomenon
as transmissible fluctuation.

Tower and Blaringhem working with beetles and with
corn respectively have reported some results particularly
interesting in this connection. After demonstrating the
effect of environment in producing continuous variation
in Chrysomelid beetles within the range of the species,
Tower reports a striking case of difference in behavior.

Variation and Heredity A475

The offspring of certain pairs of beetles showing precisely
the same variation in spot characters were compared.
The offspring of one pair transmitted the variation, while
those of other pairs were unable to do so, varying
toward the mean of the species. Blaringhem was able
through a variety of injuries to produce certain abnor-
malities of corn flowers, especially the production of
grains in the staminate inflorescence. This abnormality
was not generally transmissible, yet it was transmitted
in a few cases, and even the degree of transmission was
found to be variable.

288. Mutation and crop improvement. — The principle
of mutation appears to be particularly important in crop
improvement. Taken in conjunction with the facts of
alternative inheritance, subsequently discussed, it directs
attention to uncommon individuals and types, and to the
greater probability of securing permanent and immediate
improvement by the isolation and breeding of such forms.

No one has contributed more to the method and results
of selection work than Nilsson, the Swedish investigator,
whose work has been made a special study by De Vries.
Nilsson devoted particular attention to the cereals, and
his method of selection was founded upon the discovery
that “‘ a protean group of types was found to constitute
each so-called variety. These types were seen to be differ-
ent from one another in a previously unsuspected degree,
covering a range of variability adequate to comply with
almost all the needs of practice.” }

The practical success of the work of Burbank and others
seems to rest upon a careful search for variable forms;

1 De Vries, ‘‘ Plant Breeding,”’ p. 68.

476 Plant Physiology

the utilization of large numbers, in order that there may be
more chance for variation; and in the detection and iso-
lation of the unusual individual or type.

HEREDITY

In the production of plants there are two primary re-
quirements, — there must be (1) the seed or propagative
parts, and (2) certain favorable conditions for growth
and reproduction. The one is a biological mechanism
which has behind it ages of ancestors determining specifi-
eally or racially what type of plant there shall be; the other
is a complex of physical and chemical factors conditioning
what kind of individual there shall be. The embryo
plant possesses its particular hereditary possibilities, and
it is encompassed by an environment which sustains it or
subjects it. Heredity and environment are therefore
forces closely linked together in biological investigation.
Environment is important in molding heredity, and
heredity constantly affects the method of response to
environment. All biologists agree that either structural
or functional adjustments to environment may ultimately
become hereditary’; but a chief tenet of Weismannism is
that no change is hereditary which does not affect the
germ cells.

There is at present great activity in the study of hered-
ity, a manner of cell behavior which we-may now tenta-
tively define as being concerned with the transmission
through successive generations of racial and individual
characters. A fundamental study of transmission is
properly termed Genetic Physiology, or simply Genetics.

Variation and Heredity A477

It finds direct practical application in all the practices
of plant and animal breeding.

289. Nonsexual reproduction and heredity.— In non-
‘sexual reproduction a part of an individual reproduces a
new individual,<and the latter commonly resembles the
former as closely as environment or the conditions of its
growth will permit. In this case we may scarcely speak
of heredity or transmission in the usual sense; yet it is
important to recall that whatever the nature of the part
used for such vegetative multiplication, it retains the racial
or specific characteristics of the plant from which derived.
The propagative part or scion usually includes one or
more buds. Reduced to the lowest terms conceivable, it
might be a single vegetative cell. In at least two cases
among flowering plants an epidermal cell may develop a
bud, and this bud reproduces the plant. In any case it is
remarkable that a single cell, or even a group of meriste-
matic cells, should be able to reproduce so completely
all the qualities of a complex adult. Bud variations may
occur in plants propagated by nonsexual means, yet most
clonal varieties are said to be fairly constant.

290. Sexual reproduction and heredity. — In sexual
reproduction, individuals contribute characteristics through
the two gametes or uniting cells, and through this union
two lines of ancestry are united in one organism. As
already noted, in the angiosperms these gametes are a
nucleus, with little or no accompanying cytoplasm, from
the pollen tube and an egg cell in the ovule; these micro-
scopic, protoplasmic units must carry all the characteris-
tics entering into the organism. .This organism will not
commonly resemble in absolute detail either parent, nor

A78 Plant Physiology

will it be an exact mean between the two, as a rule, espe-
cially where the parents show contrasting characters. It
may show distinctive characteristics of both, perhaps some
characteristics evident only in more distant ancestors,
and others which may seem to have been modified, or
which may appear to be entirely new.

The problems respecting the method of transmission
are important, and the theories offered in explanation are
both interesting and valuable; but the physiological
picture is as yet more or less indefinite. The evidence
derived from a minute study of the cell, or cytology, may
be of assistance, but it is not to be expected, in general,
that any single characteristic of the organism will leave a
special morphological imprint upon the nuclear structure.

291. The early studies.— The early studies upon
heredity yielded many valuable observations, yet they
were disappointing with regard to definite results and to
the development of special methods for attacking the
general problem. K6lreuter, Knight, Gartner, and others
accumulated interesting data. Galton employed statis-
tical methods, and his studies of pedigree records led him
in 1897 to announce his famous “ law of ancestral heredity.”
By this hypothesis, assuming unity as the total hereditary
possession of any organism, he assigned diminishing values
in a geometrical series (the total approaching unity) to
the ancestors in preceding generations, from parents to
those more remote, averaging as follows: parents one half,
grand-parents one fourth, great-grand-parents one eighth,
ete. This conception may be regarded, perhaps, as an
expression of the practical results of complex hereditary
influences, through many generations ; but from it we seem

Variation and Heredity 479

to get no indication of a particular method in heredity,
and no possible analysis of the independent characters
concerned. As a matter of fact, in Mendelian inherit-
ance, subsequently discussed, it will be apparent that
certain ancestors may contribute nothing, or some few
characters only.

292. Types of inheritance. There are apparently
several distinct types of hybrid inheritance, such as
blended, intensified, mosaic, heterogenous, and alternative.
Three of these may be briefly characterized as follows : —

1. Blended inheritance. —'The crossing of forms dis-
tinct with respect to any character yields offspring pos-
sessing this character to a degree intermediate between
the parents. Some cases of apparent blends are ques-
tioned, and much more study of this type is required.

2. Intensified inheritance. — The crossing of forms dis-
tinct with respect to any character (e.g. size) yields off-
spring possessing that character more highly developed
than either parent. Certain plums produced by Burbank
are apparently of this type.

3. Alternative inheritance. —'The crossing of forms
distinct with respect to any character yields offspring
which resemble one parent only; but the hybrid nature of
this first generation is shown by its offspring. In the latter
there is segregation, as explained later, in such manner
that each character appears practically unchanged in a
part of the offspring.

293. Recent studies. — The newer studies upon heredity
practically began with the new century, and with the
rediscovery of work done nearly half a century earlier.
These investigations are in a large measure concerned

480 Plant Physiology

with alternative inheritance in hybrids, that is, with
offspring from parentage showing contrasting characters.
The work has resulted from a clear appreciation of certain
fundamental observations. A single case may serve to
typify the simplest form of the problem: Bearded wheat
is crossed with beardless. Will the progeny be bearded,
beardless, or intermediate? What will happen in sueceed-
ing generations? For the development of this line of
inquiry, we are indebted first of all to Gregor Johann
Mendel, — Priest and later Pralat of the Konigskloster
of Briinn — and to De Vries, Bateson, Castle, Correns,
Tschermak, and many others. Mendel’s important con-
tribution was published in 1866, but it attracted no atten-
tion and was practically lost to the scientific world until
rediscovered in 1900.!

294. Mendel’s experiments. — Mendel had followed
carefully the work of such predecessors in this line of in-
vestigation as Ko6lreuter, Knight, Gartner, and others.
He was particularly interested in what has been charac-
terized as the constant appearance of the same hybrid forms
when any two species are crossed. He sought to deter-
mine the number of such forms which may arise, the con-
duct of these in the succeeding generations, and the rela-
tions of the forms one to another from a numerical or
statistical point of view. He had an unusually clear idea
of the indications which should be possessed by the species
employed in crossing in order to demonstrate the points
in a definite manner. He declared that species to be

1 For a comprehensive, bibliographical sketch of Mendel, the student
should read the notice of him in Bateson’s ‘‘ Mendel’s Principles of Hered-
ity,’’ pp. 304-316.

Variation and Heredity 481

crossed should possess differentiating or contrasting
characters; that the hybrid offspring should offer the
possibility of being readily protected from foreign pollen ;
and that the offspring should be, with respect to fertility,
unaffected by the inbreeding process necessarily pursued.

He found in the common garden pea (Pisum sativum),
and other related forms, promising material for his work.
After testing for two years the constancy of thirty-four
varieties, proceeding with great care, he selected those
which showed well-defined contrasting characters, — in all,
seven combinations. We may consider three of the typical
cases with a single differential character-pair (later termed
simple allelomorph, or allelomorphic pair) as follows :—

(1) Difference in color of cotyledons; yellow vs. green.

(2) Difference in the color of the seed coats; white vs.
colored.

(3) Difference in size; tall vs. dwarf.

Like Darwin and others who were interested in a more or
less similar line at the same time, he recognized the neces-
sity of dealing with large numbers in order that individual
errors might be avoided as far as possible. From the
crosses obtained with the strains showing the contrasting
characters above mentioned, he found that with respect to
these characters all of the hybrids resembled one of the
parents; that is, there was no blending of these qualities.
The character which appeared in the hybrid of the first
generation, known as the F, generation, was termed the
dominant of the pair, and that character which was veiled
or latent in the F, was termed the recessive. In the three
cases above the dominant characters were yellow cotyle-
dons, colored seed coat, and tall habit. No transitional

Pat

Plant Physiology

482

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Variation and Heredity 483

forms were found in the hybrid generation, and in these
cases reciprocal crosses gave in the F, generation plants
entirely alike.

A summary of the results of the F2 generation is of
special interest. Upon planting the seed of the F, genera-
tion there resulted, in the first case, 258 individuals;
and these yielded in the Fs generation 8023 seeds, of which
6022 were yellow and 2001 green. In other words, the
relation of yellow to green was 3.01:1. Where color in
the seed coats was a contrasting character there were 929
plants in the F, generation, of which 705 produced colored
seed coats, correlated also with color of blossoms; and
224 produced white seed coats, correlated with white
flowers. In this instance the proportion was 3.15:1. In
the third case, there were in the F2 generation 1064 plants,
of which 787 were tall and 277 dwarf, or a ratio of 2.8: 1.

In each case one fourth of the individuals, showing the
recessive character, breed true in all subsequent generations,
that is, in F;, F,, ete. In analogous manner one fourth of
the whole number of F, individuals (one third of the ap-
parent dominants) breed true as dominants. The re-
mainder, one half, are hybrid dominants and break up in
the F; generation exactly as did the whole number in the
F, generation. This method pertains through successive
generations. It is therefore apparent that the F,
generation may be represented thus: D+2 D(R)+R,
in which D represents dominants, R recessives, and D(R)
hybrid or impure dominants, in which only the dominant
character is evident. The iatter are here indistinguishable
from dominants, except as they show segregation in the
next generation.

484 Plant Physiology

295. Purity of the gametes.— As a result of these
hybridization studies, Mendel naturally developed his
theory of the purity of the gametes, founded substantially
in this way: In the case of the hybrid between a tall (T)
and a dwarf (8S) pea, for example, the male gametes in
equal number will carry the character tallness or dwarf-
ness, never both; and so also with the egg cells. Assum-
ing large numbers, these gametes, uniting by the law of
chance (without selective fertilization), would yield, tall-
ness being dominant, —

Tall with tall or TT (homozygote)

Tall with dwarf or T(S) (heterozygote)

Dwarf with tall or (S)T=T(S) (heterozygote)

Dwarf with dwarf or SS (homozygote)
from which we get T+2 T(S)+S. The essential features
of Mendelism are dominance and segregation, and these
phenomena are sufficiently important to-day to receive
unusual attention. It has been well shown that many
Mendelian character pairs may be expressed conveniently
in terms of presence and absence of a single character.
Either presence or absence may be dominant. The
idea of the purity of the gametes requires modification,
at least with respect to certain characters.

296. Results of segregation. — In corn yellow kernels
are dominant over white. Indicating the yellow by Y,
the white by W, and the white in the hybrid, where it is
inevident, by (W), the following diagram indicates the
method of segregation in five generations, and the relative
number of pure dominants, recessives, and hybrid domi-
nants in the hypothetical case where the rate of increase
is fourfold : —

Variation and Heredity A485

<a DOMINANTS Dene RECESSIVES
Parent (dominant yellow) Y i W (tecessive white)
F, Se ke
re Y (W) +(W) Y (W) ¥ |
SEES TOC HR RRC Ae ee
Hor Y. 2Y (W) W

Pe 2 Ve Hee 2W 4W
| 4¥ (W) +4 (W) Y
Pia eee a

me 6X 4Y 8 ey Wy) Saw. 8 We Ww

or
eo ens? Y 16% 78 Y 16 Y (W) 8W 16W 32W 64W
120 Y 16 YOOW) 120 W

Upon the principle of the purity of the gametes and the
combination of these according to the law of chance, it
becomes a simple mathematical problem to determine the
number of combinations resulting from a cross in which
two or more character pairs are considered.

486 Plant Physiology

297. Tomato characters. — To accord with Mendelian
results every plant may be regarded as made up of a cer-

Fic. 138. Hybridization of tomatoes: parental types (top row), Honor
Bright and Yellow Pear; F; generation (2d row) quite uniform; F:
generation (3d and 4th rows) showing segregation. [Photograph by
H. L. Price.]

tain number of unit characters, which may be contrasted
in different forms or varieties. Price and Drinkard have
determined thirteen such alternative character pairs

Variation and Heredity 487

for the tomato. These may serve as a further example
of unit characters, and they are listed below, the dominant
unit of each pair being placed in the middle column: —

Fruit Shape: Spherical or Round . Pyriform
Two-celled . . . . Many-celled
Roundish-conie . . Roundish-compressed
iain Color: .- Red Fruit... . «. Pink Fruit
Redukniite . “ee.7,. vellow Hrunt
incite nt oe ello seiront
Yellow Fruit Skin. .. Transparent Fruit Skin
Hriourace: Smooth . . . . ... Pubescent
Foliage : Normal or Cut Leaf . Potato Leaf
Pimpinellifolium Leaf Normal Leaf
Green Leaf . . . . Yellow Leaf
Normal or Smooth
Leaf Surface . . . Rugous Leaf
Stature : Standard Stature . . Dwarf Stature

298. Chromosome relations.— A material basis for
certain types of inheritance, especially Mendelian phe-
nomena, seems to be found in the behavior of the chromo-
somes. The reduction division, occurring in seed plants
in the formation of the microspores (ultimately pollen)
and megaspores (ultimately embryo-sac) is the important
stage.. In this the 2x (somatic or diploid number of
chromosomes is reduced to the x (gametic or haploid)
number. It is generally held that the essential features
of this division are two: (1) close association in pairs
(bivalent chromosomes) of paternal and maternal chromo-

A488 Plant Physiology

somes, in which association, particularly, mutual character
influences may find explanation; and (2) separation of
the previously associated chromosomes (members of a
pair), one to each of two daughter nuclei, thus segregating

Fic. 139. Hybridization of tomatoes: same fruits as shown in preced-
ing figure, in section. [Photograph by H. L. Price.]

maternal and paternal characters. A comprehensive
discussion of cytological relations is entirely beyond the
scope of this work.

299. Selection. — Selection of forms for hybridization

Variation and Heredity 489

requires intelligent care. Hybridization is primarily
useful in order to effect the combination of desirable
characters and the elimination of undesirableones. Never-
theless, it is generally agreed that in some cases of alterna-
tive inheritance there may be left an influence of the cross,
so that the characters may not reappear in their original
purity. Moreover, as a result of hybridization latent or
reversionary characters may appear, and crossing has a
tendency to intensify variability.

A knowledge of Mendelian behavior has necessitated a
change in the methods of selecting hybrid offspring. In
alternative inheritance there can be no selection in the F,
generation. In the F, generation selection for the pure
recessives may be made. However, since homozygous
dominants usually distinguish themselves from hetero-
zygous dominants only in the absence of segregation in
subsequent generations, it is necessary to isolate individ-
uals and to test these in breeding plots. When several
character pairs are involved, all of which require considera-
tion, selection with respect to dominant characters may
become very complex and tedious.

LABORATORY WORK

Variation. — Utilizing convenient plants in the greenhouse or
in the field make a careful study of variation with respect to
single characters readily enumerated, or measured. The number
of ray flowers, or of bracts in certain composites, the number of
flowers in a cluster, the number of eyes upon potatoes, the num-
ber of leaflets in compound leaves, the difference in weight of
grains or corn, and many other similar characters may be em-
ployed. In such cases the counts or measurements should be
made within the single variety. Determine the classes of

490 Plant Physiology

variation, construct curves and prepare a report which shall
include the results of the study made, together with an abstract
of one or more of such papers as the following : —

Davenport, C. B. Statistical Methods. Pp. 19-41.
Davenport, E. Principles of Breeding. Pp. 681-703.
Harris, J. A. Amer. Nat. 43: pp. 350-355 ; ibid., 44: pp. 19-30.
Lupwic, F. Biometrika. 1: pp. 11-29.

Pearson, K. Grammar of Science. Pp. 381-402.

SHuut, G.H. Amer. Nat. 36: pp. 111-152.

Compare, if possible, the curves of variation with respect to any
character in two populations grown under dissimilar conditions.

Fic. 140. Dahlia flowers with bursting anthers. [Photograph from
Bureau Plant Industry.]

Heredity. — Laboratory work upon heredity covering only
one or two periods may be only suggestive, or may give opportu-
nity for the presentation of materials which may be subsequently
worked up as areport. Practical studies are preferably confined
to the following : —

Variation and Heredity 49]

1. A study of the methods and manipulation of crossing, con-
sulting such references as —

Baitey, L.H. Plant Breeding. Pp. 344-358.
OuiverR. Bureau of Plant Industry. Bul. 167: 39 pp.

aE

Fie. 141. Dahlia flowers after depollination by means of a stream of
water. [Photograph from Bureau Plant Industry.]

2. The results of crossing. In the latter case parents, to-
gether with first and second generation crosses, should be com-
pared with respect to characters, following the suggestions in
sections 294-297. Fresh material is most desirable, but if it
may not be had at the time desired, alcoholic or dried material
will serve adequately for the study of many characters.

REFERENCES

Baiuey, L. H. Plant Breeding. 4th Ed. 463 pp., 20 figs., 1907.
Bateson, W. The Progress of Genetics since the Rediscovery

492 Plant Physiology

of Mendel’s Papers. Progressus rei botanicae. 1:368-
418, 1907.

Bateson, W. Mendel’s Principles of Heredity. 396 pp., 6 pls.,
33 figs., 1909. (Contains an extensive bibliography up to
the date of publication.)

BLARINGHEM, L. Mutations et traumatismes. Bull. Sci. de la
France et de la Belgique (1907) : 248 pp., 8 pls.

CastLte, W. E. A Mendelian View of Sex Heredity. . Science,
N.S. 29:395—400.

Correns, C. Die Bestimmung und Vererbung des Geschlects.
81 pp., 1907.

Darwin, CHARLES. Variation of Animals and Plants under
Domestication. 1:473 pp.; 2:478 pp.

Davenport, EK. The Principles of Breeding. 727 pp., 52 figs.,
1907.

Davenport, C. B. Statistical Methods with Special Reference
to Biological Variation. 2d Ed. 223 pp., 1904.

East, E. M. The Relation of Certain Biological Principles to
Plant Breeding. Conn. Agl. Exp. Sta. Bul. 158:91 pp.,
1907.

Gatton, F. The Average Contribution of Each Several An-
eestor to the Total Heritage of the Offspring. Proce. Roy.
Soe. 61: 401-413, 1897.

JOHANNSEN, W. Uber Erblichkeit in Populationen und in
reinen Linien. 19038. Jena.

Lock, R. H. Recent Progress in the Study of Variation,
Heredity, and Evolution. 299 pp., 1906. New York.
Lotrsy, J. P. Vorlesungen iiber Deszendenztheorien mit be-
sonderer Beriicksichtigung der botanischen Seite der Frage.

1906. Jena.

MacDouaat, D. T. Mutants and Hybrids of the Ginotheras.
Carnegie Institution Publ. 24:57 pp., 13 figs., 1905.

Puiate, Lupwia. Selectionsprinzip und Probleme der Artbil-
dung. 493 pp., 60 figs. Leipzig, 1908.

Price, H. L., anp Drinkarp, A. W. Inheritance in Tomato
Hybrids. Virginia Agl. Exp. Sta. Bul. 177: 18-53, 10 pls.,
5 figs., 1908.

Variation and Heredity 493

Punnett, R.C. Mendelism. 85 pp., 1909. Cambridge.

Reip,G. A. The Laws of Heredity. 548 pp., 25 figs., 1910.

Suuuut, G. H. The Presence and Absence Hypothesis. Amer.
Nat. 43: 410-419, 1909.

—— The Inheritance of Sex in Lychnis. Bot. Gaz. 49: 110-
125, 1910.

Smitu, L.H. Ten Generations of Corn Breeding. Ill. Agl. Exp.
Sta. Bul. 128 : 453-475, 1908.

Varies, H. pr. The Mutation Theory. 1:582 pp., 112 figs.,

- 4col. pls., 1909; 2: 683 pp., 149 figs., 6 col. pls., 1910.

—— Species and Varieties. 847 pp., 1905.

Plant Breeding. 360 pp., 114 figs., 1907.

Witson, E. B. Recent Researches on the Heredity and De-
termination of Sex. Science, N.S. 29:53-71, 1909.

CHAPTER XX

GROWTH MOVEMENTS

Aut plants possess the power of movement to at least a
limited extent. The various types of movement and their
relations constitute a considerable part of plant physiol-
ogy as commonly presented. Here, however, it will be
possible merely to outline portions of the subject, which
may be further pursued in the special literature. It is
entirely beyond the present purpose to consider locomotory
movements, likewise the phenomenon of dehiscence, and
other effects due to swelling and contraction. Neverthe-
less, special movements of turgor are included on account
of their closer relationship.

Movement may occur within the protoplast, and may
be limited to the cell, or it may occur in such manner that
complex structures exhibit change of position or change
in the direction of growth. As popularly regarded, those
plants possessing roots are fixed in the soil or other sub-
stratum, and movement is only associated with such
striking changes as may be seen in the sensitive plant
(Mimosa pudica), the Venus’s-flytrap (Dionaea muscipula),
or certain climbers and twiners. As a matter of fact,
movement is almost inseparable from growth.

The elongation of root or shoot is a type of growth
movement. There is, furthermore, a remarkable variety

494

Growth Movements 495

of growth responses resulting in curvature or orientation of
members. As previously indicated, the movements of
plant members are now regarded as primarily of two types,
spontaneous (autonomic) and induced (paratonic). The
former are little understood. It is not always possible
to distinguish positively between the two types, or the
movement may be the result of conjoint (internal and
external) stimuli. In general, growth movement is a
fundamental requirement in the effective adjustment of
organisms to their environment. A study of the phenom-
ena is more important educationally in liberalizing our
views of plant relations than of any direct assistance in
special problems of plant production.

300. Stimulus and response. — The relations of or-
ganisms to growth factors have been considered, but it is
necessary to refer again to the environmental forces which
condition plant activity. When the environment is
favorable the plant is regarded as exhibiting the condition
of tone; and the effect of each factor or of the various
factors severally is a tonic influence. The factors con-
cerned are those essential in growth, especially oxygen, mois-
ture, food-supply, light, and heat. They are sometimes
known as the formal growth conditions. Some of these,
and likewise other environmental factors, may act not as
tonic influences but as special stimuli releasing growth
responses, that is, movements.

The plant is not merely a complex mechanism; it is,
when in a condition of tone, a source of readily releasable
energy, — growth energy. It requires a stimulus to make
this energy manifest, but it appears unnecessary that the
stimulus should impart force. As so often pointed out,

496 Plant Physiology

the stimulus required is analogous to the pressure of the
finger on the electric button which sets at work powerful
dynamos. ‘The pressure upon the button has no relation
to the amount of work which will be accomplished by the
machines thus released. When a stimulus affecting the
plant is external, its relations to response may be worked
out with a fair degree of success; but the methods of ac-
tion of internal stimuli are almost entirely unknown.
In the action of any stimulus upon a sensitive organ there
are to be distinguished primarily (1) perception, (2) trans-
mission, and (3) reaction or growth response. Commonly
the perceptive region is at no great distance from the motor
or responsive part; yet in certain cases the stimulus may
be transmitted through considerable intervening tissue.
Practically nothing is known regarding the mechanism of
transmission. Perception often resides in the terminal
portion of the organ, but not always in the formative
region.

In order to produce response a stimulus must act
usually for a certain interval of time, and this interval
(presentation time) depends upon temperature and other
growth conditions. The visible response to the stimulus
may be prompt, as in the case of many tendrils, or it may
be delayed for several hours. The interval between stimu-
lation and response (reaction time) is usually longer than
that of presentation.

301. Tropic curvatures.— Every plant exhibits a
normal form and habit, and its members are arranged in a
definite manner with respect to one another and to en-
vironmental forces. Tropic curvatures are commonly
the results of irritable growth responses manifest when the

Growth Movements 497

normal position of a sensitive structure is shifted, or when
this member comes under the influence of new or intensified
forces, or of forces acting from a new direction. The
curvature of horizontally placed roots toward the earth
and the bending of the hypocotyl of a seedling exposed to
one-sided illumination are familiar illustrations.

The importance of a means of orientation in order to
assume or restore the normal is obvious. If the reaction
of the growing organ results in orientation parallel to the
direction of the exciting force, the organ is parallelotropic,
while one assuming a position at an angle to the direction
of the stimulus is plagiotropic, — diatropic being at right
angles to the path of the stimulus. The tropic movements
here discussed are effected in growing structures, or those
in which growth may be initiated, and they cease with
incapacity for growth. Moreover, the stimulus remaining
the same, the rapidity of growth determines the prompt-
ness of the response, or reaction time.

Attention has been directed to the tropic responses of
plants to light (phototropism) and to heat (thermotropism).
Other stimuli inducing reactions are such as gravity (ge-
otropism), contact (thigmotropism), moisture (hydrotro-
pism), electricity (electrotropism), and certain chemical
agents (chemotropism).

302. Geotropism. — The vertical position of the main
axis of most plants is as apparent as the erect posture of
man. A seed may be planted in any position in the soil ;
but as soon as sufficient growth is made the parallelotropic
position of the axis is assumed, with the root directed
straight downwards (positively geotropic) and the stem
‘directly upwards (negatively geotropic). Germinating

2k

498 Plant Physiology

seed in a moist chamber pinned in a horizontal posi-
tion will respond to the stimulus of gravity by growth
curvatures in the same manner.

—— I Secondary roots and branches take

up plagiotropic positions, but in the
/ remote branches of the root little geo-
tropic response is manifest. Shoots
from a fallen trunk assume the ver-

tical position. If the terminal shoot
WM of spruce is cut off, one or more lateral

shoots of the first whorl may be
Z raised into the vertical position.
The erection of the jointed stems of
grasses is effected by curvatures in
the nodes, and these stems are par-

ticularly interesting for study.

Fic. 142. Geotropic ‘ J

curvature of root of @eotropic response 1s not a ques-

Vicia Faba; horizontal tion of weight, and this is shown by

position (I), after 7 hrs. the diverse reacts f “th .

(II), and after 23 hrs. the diverse reac ions oO e main

(III). [After Sachs and axis and branches. Furthermore,

hae there is no geotropie response when
eravity is eliminated, as by revolving seedlings in a ver-
tical plane on a klinostat geared to make one revolution
in about fifteen minutes. On a klinostat rotated hori-
zontally at a low rate of speed the usual stimulus of
eravity is felt; but when rotated at a higher rate of
speed the root grows outward and toward the horizontal,
and the shoot inward and toward the horizontal, de-
pending upon the rate of rotation.

In the case of the root the perceptive region is usually
confined to about one millimeter, or less, at the very tip,

Growth Movements 499

while curvature occurs in the
region of greatest growth.
The time required for per-
ception (presentation time)
varies from a few minutes to
several hours. Reaction time
is often several hours, and re-
sponse is evident even if
meanwhile the position of the
organ is again shifted. The
mechanism of geotropic per-
ception is not clearly under-
stood. An early mechanical
theory of Knight has found
new life in the statolith theory
of Némec, Haberlandt, and
others. By this hypothesis it
is assumed that in shifted
structures the sinking of cer-
tain cell products, especially
starch, to the bottom of the
cell produces a change of
pressure, and it is this change
which furnishes the excitation.

303. Thigmotropism. — The
capacity for thigmotropic re-
sponse, or growth curvature
induced by contact, is most
highly developed in tendrils.

Fic. 143. Diagrammatic view of geo-
tropic curvature in an etiolated hypo-
cotyl, horizontally placed. [After
Noll.]

>

500 Plant Physiology

Roots and leafy shoots appear to possess this power to a

Fic. 144. Demonstration klinostat,
after Ganong, vertical arrangement.

[Illustration from Bausch and Lomb
Optical Co.]

very limited extent.
Plants producing ten-
drils are particularly well
equipped to climb aloft,
supporting themselves by
the attachment of these
to any small supports,
especially to those hori-
zontally placed. By this
means such plants as the
grape vine, wild cucum-
ber, Passiflora, and many
others are enabled to climb
through trees and lay-
ered vegetation, whereas
twining plants commonly
require a support which
is more or less vertical.
The tendrils are com-
monly axillary or super-
numerary branches devoid
of leaves, or leaf-parts
entirely lacking blades.
Sometimes, however, pet-
ioles of normal leaves or
extended leaf tips may
function as _ tendrils.
These structures com-
monly exhibit — dorsi-
ventrality, and aright and

Growth Movements 501

left flank may be differentiated. They usually complete
their growth within a few days, so that the plant may be
attached to its supports almost as rapidly as the shoot
elongates.

The terminal part of the tendril is the more perceptive
region, and commonly the under surface exhibits greater
sensitiveness. Both surfaces and flanks may, however,
respond to contact stimuli. When the tendril is from
one-fourth to one-third grown, it exhibits marked auto-
nomic nutations, and the swinging of the tip through space
brings it into contact with any objects in the range of this
motion or of swaying movements caused by wind. Scrap-
ing the surface of the tendril against a suitable support
(especially repeated scraping) is followed by coiling and
close attachment around the object. The tendril is now
fixed at both ends, the prompt grasping of the support
being in part, apparently, due to turgor movements.
After attachment growth proceeds more rapidly on the
upper surface, and the tensions resulting throw the tendril
into a close coil, once or more reversed.

Fixation by means of tendrils affords not merely secure
support, but the attachment at many points affords a
general elasticity and freedom from severe shock well
known through the principle of vehicle and car springs.

304. Chemotropism. — The curvature and growth of
roots, pollen-tubes, or fungous hyphz in response to the
stimulus of chemical agents is chemotropism. At one
time it seemed that chemotropic response, especially
positive chemotropism, might commonly determine the
direction of growth in roots, penetration of parasitic fungi,
and other phenomena. Further study has developed the

502 Plant Physiology

probability that positive chemotropism is not a highly
developed response. It may occur in roots and pollen-
tubes, although the evidence is not entirely convincing;
while serious doubt has been thrown upon the existence
of positive chemotropism in fungous hyphe.

305. Nutation.— The tips of growing axes or other
plant members are not as a rule extended in a straight
line. Instead, they nod here and there or commonly trace
an irregular spiral, the projection of which yields a series
of more or less circular or elliptical figures. This type of
movement is called nutation (circumnutation). It was
extensively studied by Darwin, and the main effects to-
gether with some of the important relations were clearly
set forth at that time.

The type of curve varies with the growth relations. In
stems which are radially symmetrical nutation results
from unequal growth in the vertical segments. The
effects produced are accounted for by greater growth in
each segment successively around the stem. When
asymmetry occurs, and especially in flattened or dorsi-
ventral organs there is more likelihood that the movement
will tend toward narrow ellipses or even the back-and-forth
linear type. The extent of the movement depends upon
the unevenness and rapidity of growth. It is generally
greatest in organs growing rapidly, such as tendrils and
climbing shoots, and the whole of the growing region may
be involved. Nevertheless, the pronounced nutation of
twiners does not begin, as a rule, until after a fewinternodes
are produced. Tendrils, likewise, show little nutation
during the early stages of growth, and the movement
ceases in matured organs.

Growth Movements 503

‘All stages are shown between trifling and pronounced
nutation, according to the plant, to the stage of develop-
ment, and to the external conditions. The curves are not
always regular and similar, even when there is a pronounced
tendency to linear, elliptical, or circular nodding, as the
case may be. Even when the last named is most pro-
nounced it may temporarily alter into to-and-fro pendulum
movements.” !

The stimulus to nutation is in most cases primarily
internal and spontaneous, but it may be conditioned,
initiated, or in large part induced by other agencies, espe-
cially by gravity and light. The time required for the
completion of a single ellipse, or back and forward move-
ment, may be one or two hours or as many days; and when
there is a tendency toward the latter type of nutation,
the movement of the organ is least rapid near the point
of reversal.

306. Nastic curvatures.—In most of the types of
growth response already considered the stimulus is uni-
lateral and the curvature may occur inany plane. Fairly
well distinguished from the preceding are those cases in
which the structure of the organ is such that response is
usually limited to orientation in a single plane, whether
the stimulus is diffuse or unilateral. Bilateral or dorsi-
ventral members, such as leaves, floral leaves, and flat-
tened stems, are structures of the type above noted. The
bendings resulting in such organs are known as nastic
curvatures, and they may be distinguished by the same
prefixes as in the other cases to denote the type of stimu-
lus, thus photonasty, thermonasty.

1 Pfeffer (Ewart), Physiology, 3: p. 20.

504 Plant Physiology

Nastic curvatures, however, are not necessarily the
result of external stimuli, hence they may be either auto-
nomic or paratonic.

In the development of leaves (section 181) there is
usually a growth response whereby the under or dorsal
surface grows faster, yielding an upward curvature (hypo-
nasty). As a result of this each leaf in turn becomes a
part of the bud. Later the growth on the upper or ventral
surface is more rapid and there is outward bending (epi-
nasty) during exfoliation. There may be a recurrence of
epinastic and hyponastic curvature under the influence
of various stimuli until maturity of the leaf. Growth
upon the upper surface called forth by light is a paratonic
nastic bending, or photepinasty.

307. Nyctitropism. — The old idea of floral clocks was
founded on the observation that flowers of diverse species
open and close with different light and temperature
relations. There are some flowers which remain closed
during the night, opening in the early morning with in-
creased temperature or sunshine. Others are less readily
stimulated and remain closed until the conditions are
further intensified. Again, some blossom when the heat
of the day begins to decline, while the night-blooming
Cereus and certain other flowers bloom at night.

Movements of floral leaves have been shown to be typi-
cally nastic growth movements and they disappear as
soon as the power of growth is lost in these organs, unless
accompanied by special basal articulations which may
show turgor movements.

Quite as characteristic are the sleep movements of leaves
in a number of families, especially Leguminose and Mi-

Growth Movements 505

mosz. All plants possessing jointed leaves do not exhibit
the same behavior. Nyctitropic movements are com-
monly due to changes of turgidity, and growth is not
usually involved. The articulations are cushions in which
cortical tissue predominates. Under stimulation the
dorsal and ventral halves give osmotic changes unequal
in rapidity so that movement is brought about.

LABORATORY WORK

Geotropism. — For a few observations upon the geotropism of
roots fairly large seeds are desirable, such as those of peas or
beans. Germinate the seed in moss or on paraffined wire netting
over water. When germination has progressed to the extent of
a few centimeters, the roots may be marked off with India ink
as for determining the region of extension. The growth curva-
tures are then to be followed by placing the radicles in a hori-
zontal, or any other desired, position. If only a few seed are
used, they may be pinned to the lower side of large corks covering
jars or dishes partially filled with water.

For a larger number of seed and particularly for observations
réspecting the effects on side roots, the seedlings may be arranged
at various angles on two thicknesses of moistened carpet or felt
paper between plates of glass. The plates are clamped together
with wooden clothespins, and wads of filter-paper here and there
prevent crushing. Place the plates on edge in a moist greenhouse
or cover with wet cloths. Observe from time to time, note the
results, and shift the position of the plates through ninety degrees
after secondary roots are produced. Discuss the results.

Negative geotropism of young shoots may be followed by ob-
serving the behavior of bean or pea seedlings when the pots are
placed horizontally. Determine also the time of presentation and
of reaction for such seedlings grown in very small (2 inches)
pots. Compare the presentation time at 12 to 15° with the
interval at 25 to 30° C.

Secure shoots of Tradescantia or of oats ‘embracing several

506 Plant Physiology

nodes; pin the basal node to a cork or block of wood and follow
the process of erection.

With the special instructions given determine the behavior of
roots and shoots of seedlings when gravity is equalized through
vertical rotation upon the klinostat.

Chemotropism. — The existence of positive and negative chem-
otropism would seem to be established and some of the chemo-
tropic relations of pollen-tubes may be conveniently and easily
observed. Utilize pollen known to germinate freely, such as
that of Tradescantia virginica and Narcissus Tazetta and prepare
hanging-drop cultures as for pollen’ germination. When the
grains begin to germinate, introduce into the drops bits of the
stigma of the plant from which the pollen was taken. Ascer-
tain if these stigma bits or if particles of any vegetable proteins
(albumins and globulins) exert any influence on the direction of
growth of the tubes.

If time for more extensive study is available, consult the paper
by Lidforss (or follow special instructions), employ Pfeffer’s
capillary tube method, and install the necessary experiments.

Growth and movement of tendrils. — Utilizing any tendril-
bearing plant available in the greenhouse or field, select several
tendrils about one fourth grown, mark off into ten or twenty
spaces by means of India ink, and determine the region and
period of growth, also the daily percentage increase in the differ-
ent longitudinal segments.

Review in suitable literature the more extensive accounts of
tendril movements, and make an extended observation upon the
behavior of one type, presenting the results in the form of a-
report.

REFERENCES

Boss, J.C. Plant Response. 781 pp., 278 figs., 1906.

Darwin, CuHarues. The Power of Movement in Plants. 592
pp., 1885 [Appleton].

Darwin, CHARLES and Fr. The Movements and Habits of
Climbing Plants. (2d Ed.) 208 pp., 1884 [Appleton].

Growth Movements 507

Darwin, Fr. Lectures on the Physiology of Movement in Plants.
New Phytologist. 5 and 6: (Lectures I-VI), 1906, 1907.

Firtinc, H. Weitere Unters. z. Physiologie der Ranken.
Jahrb. f. wiss. Bot. 39 : 424-526, 21 figs., 1903.

Die Reizleitungsvorgange bei den Pflanzen. 157 pp., 1907.

Futton, H. R. Chemotropism of Fungi. Bot. Gaz. 41:81-—
108, 1906. :

HABERLANDT, G. Sinnesorgane im Pflanzenreich. 205 pp.,
1901.

—— Zur Statolithentheorie des Geotropismus. Jahrb. f. wiss.
Bot. 38: 447-500, 1903.

Kouu. Die Mechanic der Reizkriimmungen. 1894.

Liprorss, B. Unters. tiber Reizbewegung d. Pollenschliuche.
Zeit Bot. 1: 448-496, pl. 3, 1909.

MacDovuaat, D. T. The Mechanism of Curvature of Tendrils.
Ann. Bot. 10: 373-402, 1 pl., 1896.

Nemec, B. Die Reizleitung und die Reizleitende Strukturen.
153 pp., 3 pls.; 1901.

—— Studien tiber die Regeneration. 387 pp., 180 figs., 1905.

Newcomess, F. C., and Ruopes, AnNa L. Chemotropism of
Roots. Bot. Gaz. 37: 25-35, 1904.

Prince, G. J. A Contribution to the Physiology of the Genus
Cuscuta. Ann. Bot. 8: 53-118, 1 pl., 1894.

PrerreR, W. Die periodische Bewegungen der Blattorgane.

ike pp; 1375:

Irritability of Plants. Nature. 49: 586-587, 1894.

Poutuock, J. B. The Mechanism of Root Curvature. Bot.
Gaz. 19: 1-80, 1900.

RicuterR, O. Ueber das Zusammenwirken von Heliotropismus
und Geotropismus. Jahrb. f. wiss. Bot. 4: 481-502,
1909.

Scuenk, A. Beitrige z. Biologie und Anatomie der Lianen.
1892.

Spatpinc, V. M. The Traumatropic Curvature of Roots.
Ann. Bot. 8: 423-450, 1894.

Texts. Barnes, Detmer, Ganong, Jost, MacDougal, Pfeffer.

INDEX|

A

ABSORPTION, of carbon dioxid, 196;
of oxygen, 285, 299; of water by
leaves, 58, 62; principles of, 64.

Acids, toxic action of, 114.

Acton, E. H., 12.

Adams, G. E., 168.

Adventitious organs, 343.

Alkalies, toxic action of, 443.

Allelomorph, 481.

Amides, 261.

Ammonia compounds, 230.

Ammonification, 231, 245.

Amyloplasts, 21.

Annuals, food storage, 252.

Antitoxic action, 184.

Arendt, 140.

Armstrong, E. F., 279.

Ash, composition of, 138; content,
136, 140; effects of conditions
upon, 139.

Askenasy, E., 345.

Atkinson, G. F., 248.

Atwater, W. O., 248.

B

Bavey ot: He A2. 151.5302 317,
345, 434, 491.

Balanced solutions, 184, 191.

Balls, W. L., 414. ©

Barlow, B., 248.

iseinaes, (Ce I ae eB Is AOR 2a
249, 279, 346, 507.

Bateson, W., 480, 491, 492.

Bayliss, W. M., 279. .

Bell, J. M., 167.

Benecke, W., 167.

Benedict, H. M., 114.

Bevan, E. J., 279.

Beyerinck, M. N., 248.

Biennials, food storage, 252.

Blackman, F. F., 224, 303, 414.

Blaringhem, L., 492.

Bleeding, 77.

Bolley, H. L., 461.

Bose, J. C., 506.

Boykin, E. B.; 396.

Breazeale, J. F., 167, 183.

Bretschneider, 140.

Brown; Al WJ50 7675461.
Fae 89> 224s

Buchner, 303.

Buds, resting, 314.

Budding, 329.

E., 499;

| Burbank, L., 475.

Burgerstein, A., 114.
Bushee, G. L., 30.
Busse, W. W., 345.
Biitschli, O., 33.

C

Calcium, role of, 175.

Cameron, F. H., 56, 167, 193.
Candolle, A. P. de, 398.
Carbohydrates, 254.

Carbon content, 195.

Carbon dioxid, amount in air, 210;

from respiration, 285, 299; ex-
cretion by roots, 161.

Castle, W. E., 492.

Cell, 15; division, 324; embry-
onic, 17; forms, 24, 33; living,
32 sap; 23; theory, 17; wall,
is}, 2272,

Cellulose, 258, 275.
Central cylinder, 50.

509

510

Cerny, 304.

Chamberlain, C. J., 379.

Chemotropism, 497, 501, 506.

Chlorine, 181.

Chlorophyll, decomposition of, 218; |
distribution, 200; properties,
202; relation to light, 217; ex-
traction of, 218.

Chlorophyllous plants, 197.

Chloroplasts, 21, 217.

Chromoplasts, 21.

Chromosomes, 487.

Chureh,, A. H.,. 379,

Clapp, G. L., 114.

Clark, J. F., 461.

Claudel, 147.

Clements, E. S., 114;
1h ASe:;

Conduction, 272.

Conservation, 1.

Copeland, E. B., 114.

Correns, C., 379, 492.

Coulter, J. M., 12, 379.

Cowles, H. C., 12.

Crochetelle, 147.

Crocker, W., 461, 398.

Crone’s solution, 145.

¥. E., ¥2,

Crop, ecology, 7; growth, 118; |
improvement, 475; water re-|
quirements, 116; zones, 8.

Cross, C. F., 279.
Cwnsve.C;, 12.
Cuttings, 373.
Cytoplasm, 19.
Czapek, F., 224, 279.

~-—<

D

Dandeno, J. B., 63.

Darwin, Charles, 379, 492, 506; F.,
12, 115, 506, 507.

Davenport, C. B., 414, 462, 490,
492; E., 490, 492.

Déherain, 279.

Déléano, N. T., 183.

Deleterious agents, 436.

Index

Denitrification, 235, 247.

Detmer, W., 12, 13, 115, 168, 304,
346, 380, 398, 507.

Dewar flask, 291.

Diffusion, 65; rodle of, 76.

Digestion, 250, 265.

Dixon, BH. Hig

Dominant, 481.

Dox, A. W., 279.

| Drinkard, A. W., 492.

Ducts, 28.
Duggar, B. M., 193, 435, 462.
Duvel, J. W. T., 398.

E

East, E. M., 492.

Eckerson, S. H., 83, 115.

Ecology, 4; crop, 7.

Effront, J., 279.

Electrotropism, 497.

Endodermis, 50.

Environment, 5.

Enzymes, 267; carbohydrate, 269;
protein, 271.

Ernst, A., 168.

Escombe, F., 89, 224.

Etherization, 335, 344.

Evaporation, 99; excessive, 97.

Evaporimeter, 99.

Ewart, A. J., 34, 115, 503.

Ewert, R., 379.

F

Factors, environmental, 5.

Fats and oils, 259, 276.
Fermentation, 296; acetic, 299;
alcoholic, 297, 302; laetic, 297.

Fertility, 152, 162.
Fertilization, 354; cross and self,
358.

| Fest, F., 168.
| Fibrovascular bundles, 106.

Fippin, E. O., 68, 135, 151, 168.
Fischer, A., 34; E., 279.
Fitting, H., 507.

Index

Flaccidity, 37.

Fletcher, S. W., 379.

Flower, morphology, 349, 377.

Foods, temporary, 251.

Forcing, 332; by warm water, 338,
343.

Formalin, toxic action of, 447.

Formative region, 24, 50.

Freeman, G. F., 85.

Freidenfelt, T., 63.

Fruit buds, 317.

Fruit setting, 378.

Fruiting and vegetation, 376.

Fuhrmann, F., 303.

Hultonwbe kh... 507.

Fungicides, 452.

G

Gallagher, 56.

Galton, F., 492.

Ganone, W. F., 13, 34, 83, 115, 225,
304, 346, 507. ‘

Gartner, 478.

Gas exchange, 492.

Genetics, 476.

Geotropism, 492, 505.

Germinators, 61.

Goebel, K., 345.

Goodale, G. L., 13, 63, 115.

Grafting, 329.

Gram-molecular solution, 68.

Greeley, A. W., 71.

Green, J. R., 13, 279.

Growth, 305; cell, 322; embry-
onic, 308; evidences of, 307;
factors, 305; movements, 341,

494: tissues, 342.
Griiss, J., 345.
Guttation, 97, 113.

H

Haberlandt, G., 13, 435, 507; F.,
345, 414.
Half-shade, 424;

crops, 425; fac-

oll

tors, 480; morphogenic effects,

426; quality, 429.
Hall, A. D., 168.
Hansen, A., 13, 225.
Hansteen, B., 1938.
Hard wheat production, 132.
Harris, BS Me 92-07. A. 490%
Harrison, F. C., 248.
Harter, I. L., 115, 193.
Hartig, R., 345.
Heald, F. D., 462.
Heat-release, 290, 300.
Hedgecock, G. G., 63.
Hedrick, U. P., 279.
Heinrich, 56.
Hellriegel, H., 248.
Heredity, 463, 476, 490.
Hertwig, O., 34.
Hickman, J. F., 398.
Hicks, G. H., 148.
Hilgard, E. W., 63, 168.
Hopkins, C. G., 153, 168, 399.
Howard, W. L., 345.
Hydrophyte, 131.
Hydrostatic rigidity, 36.
Hydrotropism, 497.
Hygrograph, 112.

I

Illuminating gas, toxic action of,
450.

Imbibition, 64, 79.

Inheritance, types of, 479.

Insecticides, 452.

Inulin, 275.

Tron, 180.

Irrigation, 122; corn, 124; date-
palm, 127; fruits, 123; wheat,
125:

Jenkins, 137.

Jennings, H. S., 472.

Jensen, C. H., 462.
Johannsen, W., 303, 345, 492.

512

Johnson, 8. W., 13, 168.

Jones, L. R., 462.

Jordan, D. S., 468.

Jost, L., 13, 34, 63, 83, 135, 183,
225, 249, 279, 304, 346, 380, 507.

K

Kahlenberg, L., 83, 462.
Karsten, 13.

Kearney, T. H., 193.
Kellerman, K., 447.
Kellogg, 468.

King, F. H., 101, 122, 135, 248.
Klebs, G., 379.

Kloécker, A., 303.

Kniep, H., 435.

Knight, 478; L. I., 462.
Kny, 72.

Kohl, 507.

KG6lreuter, 478.
Koopman, K., 345.
Kostytschew, S., 303.
Kreusler, U., 225.

L

Leaf, areas, measurement of, 110:
extension, 342; structure, 113:
venation, 108.

Leaves, exfoliation of, 313;
absorption of, 58, 62.

Leguminous tubercles, 236.

Leucoplasts, 21.

Lewis, C. I., 379.

Lidforss, B., 379, 380, 507.

Life zones, 8.

Light, and blossoms, 434 ; artificial,
420; energy, 212; injury, 418,
434; intensity and quality, 215,
418, 433; monochromatic, 422:
perception, 416, 432; relations,
415; requirements, 417.

Lipman, J. G., 248, 249.

Livingston, B. E., 83, 115.

Lloyd, F. E., 115.

water

Index

| Lock, R. H., 49.

Lodeman, E. G., 462.

Loeb, J., 34, 194.

Loew, O., 183, 194, 462.

Lotsy, J. P., 492.

Ludwig, F., 490.

Lutz, L., 248.

Lyon, T. L., 63, 135, 151, 168, 399.

M

MacDonald, W., 135.

MacDougal, D. T., 13, 135, 249,
279, 435, 492, 507.

Magnesium, role of, 175: toxic re-
lations, 185.

Marchlewski, L., 225.

Matthei, G. L. C., 224.

May, D. W., 194.

McCool, M. M., 188.

McLaughlin, W. W., 135.

Mendel, 480.

Mendelian characters, tomato, 486.

Meristem, 17, 24.

Mesophyte, 131.

Merriam, C. H., 414.

Metabolism, products of, 250.

Meyer, A., 279.

Middle lamella, 22.

Minder, F., 435.

|Mineral nutrients, 136; avail-

| ability of, 160; forms of, 148;
injurious action, 184, 191; in
rock, 150; in soils, 157; removed
by crops, 153; réles of, 169;
translocation of, 141.

Mobius, M., 380.

| Molisch, H., 435, 414.

| Moore, E., 346; G. T., 447.

_Mueller-Thurgau, 414.

| Mutation theory, 472.

| Mycorhiza, 244.

N

Nastic curvatures, 503.
Nathansohn, A., 83.

Index

Natural selection, 467.
Nemec, B., 507.
Newcombe, F. C., 507.
Newman, 148.
Nitrates, 229.
Nitrification, 233, 246;
of, 234.
Nitrifying organisms, 233.
Nitrites, 229; soil, 233.

conditions

Nitrogen, content, 227; electric
fixation, 245- : fixation; 236;
fungi fixing, 242; organisms
fixing, 236; relation, 226; soil,
228; sources of, 244.

Nobbe, 45, 147, 399.

Noll, 13.

Nuclear division, 324.

Nucleus, 20.

Nutation, 502.

Nutrient solutions, 144; strength

of, 146.
Nyctitropism, 504.

O

Oglevee, C. S., 462.

Oliver, G. W., 491.

Organic acids, 261.

Organic, food, 195.

Organic matter, rate of production,
216.

Origin of varieties, 469.

@sborne. T,. B., 279.

Osmoscope, 80.

Osmosis, 65; explanation of, 67;
nutrient salts and, 73.

Osmotic pressure, 81; role of, 76.

Osterhout, W. J. V., 13, 71, 194.

Overton, E., 83.

Oxygen, in respiration, 285;
growth, 302.

and

ii

Paddock, W., 279, 407.
Paraffined basket, 163.
24

O13

Parenchyma, 25.

Parthenocarpy, 369, 379.

Parthenogenesis, 364.

Pearson, K., 490.

Peirce, G. J., 13, 248, 303, 507.

Percival, 245.

Perennials, food storage, 252.

Periblem, 50.

Picifer, Wi: 13; 34. 63. 63, 115.168:
183, 225, 249, 279, 346, 380, 435,
503, 507.

Pfeffer’s solution, 146.

Phosphorus, availability, 161; réle,
ieate

Photosynthesis, 219; course of,
204; demonstration of, 205; fac-
LOLs 2045) 22an 224es rabeyvolecle.

Phototropism, 497.

Pieters, A. J., 399.

Plasmolysis, 69, 80.

Plastids, 21.

Plate, L., 492.

Plerome, 50.

Poisons, 436.

Polarity, 310.

Pollination, 350; secondary effects
of, 368.

Pollock be o0m-

Population, 464.

Potassium, pyrogallate, 221;
Os WAZ

Potted plants, water supply, 127.

Prazmowski, A., 248.

Precipitation, 118; annual, 119.

Precipitation membrane, 80.

Prescott, S. C., 299.

Price, H. L., 492.

Proteins, 259; classes of, 260.

Protoplasm, 17; irritability of, 31;
movement of, 29, 33; per-
meability of, 74, 82, 276.

Pruning and growth, 326.

Punnett, R. C., 493.

Pure lines, 471.

Puriewitz, 303.

Purity of gametes, 484.

role

014

R

Rane, F. W., 435.

Raumer, v., 168, 183.

Recessive, 481.

Reed, H. S., 115, 183, 462.

Reid, G. A., 493.

Relation of Ca to Mg, 185, 192.

Reproduction, 347; and _ heredity,
477; non-sexual, 372.

Resins, occurrence, 264.

Respiration, 200; aerobic, 284;
anaerobic, 294; and cell divi-
sion, 326; demonstration of,
282; of wounded plants, 290;
result of, 281.

Respiratory activity, 286.

Rest period, 318.

Rhodes, A. L., 507.

Richards, H. M., 304, 462.

Richter, O., 507.

Ringing, 114, 273.

Root cap, 49, 62.

Root excretions, 447.

Root hairs, 45, 61.

Root pressure, 77, 82.

Root systems, 39, 60.

Root tip, structure of, 50, 62.

Root tubercles, 247.

Rooting habits, 41.

Roots, acid excretion from, 161,
167; corrosion by, 167; solvent
action of, 160.

Roots, elongation, 311, 341.

Rossi, G. de, 249.

Rotation, 30.

Rotmistrov, V., 63.

Ruhland, W., 83.

Running out, 375.

Ss

Sachs, .J., 13:63; 415 168:
Saida, K., 249.

Sand hills, vegetation of, 130.
Sap pressure, 77, 82.

Index

Schenck, A., 13, 507.

Schimper, A. F. W., 13, 135.

Schreiner, O., 462.

Schunck, C. A., 225.

Scion propagation, 330.

Sclerenchyma, 26.

Secondary thickening, 320.

Seed, buried, 391; delayed germi-
nation, 392; habit, 347;. har-
vesting, 388; maturity, 385;
production, 381; size and weight
of, 393; vitality, 389.

Seedlessness, 369.

Segregation, 484.

Selection, 488.

Selective absorption, 75.

Self-sterility, 362.

Semi-permeable membrane, 65.

Senn, G., 225.

Shamel, A. D., 380, 399.

Shantz, H. L., 135.

Shrinkage, 82.

Shull, G. H., 490, 493.

Siemens, C. W., 435.

Sieve tubes, 28.

Silicon, 182.

Smith, J. W., 135; Le 49a.

Snow, L. M., 63.

Sodium, 181.

Soil, bacteria, 240;
162; particles, 52;
water capacity, 53.

Solution cultures, 151, 165.

Sorauer, P., 13, 63, 135.

Spalding, V. M., 63, 507.

Species and varieties, 464.

Starch, 207, 256, 274; digestion,
277; test for, 222.

Stem apex, 313; elongation of, 314,
341.

Sterome, 27.

Stevens, W. C., 13, 63, 279, 346.

Stewart, J. B., 435.

Stimulation, 451.

Stimulus and response, 495.

Stock and scion, 330.

fertility, 152,
texture and

Index

Stoklasa, J., 168, 304.

Stomata, 90; production of, 92.
Stone, G. E., 485; W. L., 462.
Storage products, 251.
Strasburger, E., 13, 34, 346, 380.
Streaming, protoplasmic, 29.
Sugar, 207, 255, 275.

Sulfur, 182.

Swingle, W. T., 127, 414.
Synthesis, 227.

a

Tannins, 263.

Temperature, adjustment to, 408;
and buds, 411; and germination,
412: and photosynthesis, 216;
and production, 401; and root
elongation, 413; control of, 407 ;
inhibition by, 404; relations, 400,
414; response, 409; units, 405.

Temperatures, cardinal, 402; plant,
A408.

Tendrils, 506.

Ten Eyck, A. M., 63.

Teodoresco, E., 435.

Ternetz, C., 249.

Thermotropism, 497.

Thigmotropism, 497, 499.

Timiriazeff, C., 225.

Tissues, 16; differentiation of, 319.

Tomato, Mendelian characters of,
A486.

Tonic influence, 495.

Tower, W. L., 474.

Toxic agents, 436, 461; acids, 442;
alkalies, 443; effects of solids,
440, 461; effects of substratum,
440: formalin, 447; illuminat-
ing gas, 450; methods of action,
441: organic compounds, 449;
salts of heavy metals, 446.

Trabut, L., 398.

Trachee, 28.

Tracheids, 27.

Translocation, 141, 250, 278.

515

Transpiration, 84; amount of, 87,
110; and evaporation, 99; and
growth, 102; conditions affect-
ing, 96, 111; indications of, 109;
mechanism of, 88; modifications
affecting, 94.

Transeau, 100, 112.

Transplanting and wilting, 340.

Tropic curvatures, 496.

Girie, oH 462:

Tschermak, 480.

Turgor, 71.

Turpentine, occurrence, 264.

U

Unavailable water, 55, 62.
Unproductiveness, 448.

Vv

Van’t Hoff, J. H., 83.

Variation, 463, 489;
466.

Varieties and species, 464.

Vegetation and fruiting, 376.

Velamen, 58.

Verworn, M., 14, 34.

Vessels, 28.

Vaneent, ©. €., 379:

Vines, S. H., 14, 279.

Volkens, G., 135.

Voorhees, E. B., 168, 249.

Vries, H. de, 83, 493.

fluctuating,

WwW

Wachter, W., 83.
Wager, H., 435.

Waite, M. B., 380.
Ward, H. M., 249.
Warming, 14, 128, 135.

Water, absorption, 39; content,
35, 37, 59; conduction, 113;
transport, 102, 109; variation

in organs, 38.
Water cultures, 142.

516 Index

Water loss, control of, 93. Wilfarth, 168, 248.

Water requirements, 116. Wilson, E. B., 34, 498; W. E., 224.
Waugh, F. A., 346. Wilting, 37, 69, 80.

Webber, H. J., 396. Wimmer, 168.

Weeds, destruction by poisons, 454. | Winton, 137.

Wellington, R., 279. Wollny, W., 135.

Wheats, hard, 9. Woods, A. F., 183.

Wheeler, H. J., 168. Woronin, M., 249.

Whipple, O. B., 407.

Whitney, M., 168, 435. ».¢

Whitson, A. R., 248.
Wickson, E. J., 135, 155.
Widtsoe, J. A., 135.
Wiegand, K. M., 414. Z,
Wiesner, J., 279.

Wilcox, L. M., 135. Zimmermann, A., 279.

Xenia, 365, 379; false, 368.
Xerophytes, 129.

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I. P. Roberts’ The Farmer’s Business Handbook |
George T. Fairchild’s Rural Wealth and Welfare
S. E. Sparling’s Business Organization .
In the Citizen's Library. Includes a chapter on Farming
Kate V. St. Maur’s A Seif-supporting Home
Kate V. St. Maur’s The Earth’s Bounty ;
G. F. Warren and K. C. Livermore’s Exercises in Farm Man-
me Oi .
N. Ogden’s Rural Hygiene

Everything Agricultural

L. H. Bailey’s Cyclopedia of American Agriculture:
Vol. I. Farms, Climates, and Soils.
Vol. II. Farm Crops.
Vol. III. Farm Animals.
Vol. 1V. The Farm and the Community.

BHHAH HA

HNNH

50 net
25 net
oo net
60 net
oo net

go net

60 net
50 net
25 net
60 net
50 net

25 net
oo net
oo net

75 net

50 net
25 net
25 net
25 net
25 net
25 net

75 net
75 net

8o net
50 net

To be complete in four royal 8vo volumes, with over 2000 illustrations.

Price of sets: cloth, $20 net; halfmorocco, $32 net.

For further information as to any of the above,
address the publishers

PUBLISHED BY,

THE MACMILLAN COMPANY
64-66 Fifth Avenue, New York

FLEMENTS OF AGRICULTURE

BY

G. F. WARREN

Professor of Farm Management and Farm Crops, New York State College
of Agriculture, at Cornell University

Cloth, 12mo, price $1.10 net

_ This book is designed for use in high schools, academies, and
normal schools, and in colleges when only a short time can be
given to the subject. It is also useful to the farmer or general
reader who desires a brief survey of agriculture.

The purpose of the book is to make the teaching of agricul-
ture in the existing high schools comparable in extent and
thoroughness with the teaching of physics, mathematics, history,
and literature. In fact, the chemistry and botany should, if
possible, precede the agriculture as given in this book ; and the
pupil will be all the better prepared for the subject if he comes to
it with considerable other high-school training, for much of the
value of the work will be conditioned on the student’s maturity
and his experience with life. The subject is not one that can
be memorized, or even acquired in the ordinary method of
school study ; it must relate itself to the actual work and
business of the community in such a way as will develop the
student’s judgment of conditions and affairs.

IN PREPARATION
BY THE SAME AUTHOR A NEW BOOK ON

FARM MANAGEMENT

to be included in the well-known Rural Science Series, under the general
editorship of Prof. L. H. Bailey.

THE MACMILLAN COMPAR

64-66 Fifth Avenue, New York

The Teaching Botanist

By WILLIAM F. GANONG, Pu.D.

NEW EDITION

Illustrated Cloth 12mo STEEL O)
COMMENTS ON THE FIRST EDITION

Education: ‘Professor Ganong presents the educa-
tional world with a useful and suggestive manual of
information upon botanical instruction in this volume.
It is a book about the art of teaching the subject, not a
text-book of the subject itself. . . . No teacher of bot-
any can fail to be profited by its perusal. This is one
of the best books we have seen on the modern lines,
and it will be widely welcomed by teachers and mature
students of botany.”

The Outlook: “Self-made men have always had the
sense to use the inductive, the natural, method of ac-
quiring their knowledge, but when young minds are in
the most receptive and formative state, students are
subjected to excessive deductive work, the text-book
kind. This volume, in bringing together the best knowl-
edge concerning botanical teaching, lays special stress
upon the introduction of physiology and ecology as the
most marked characteristics of present progress in that
teaching.”

PUBLISHED BY

THE MACMILLAN COMPANY
64-66 Fifth Avenue, New York

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