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CHARLES V. PIPER, CONSULTING EDITOB

THE CHEMISTRY
OF PLANT LIFE

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THE CHEMISTRY
OF PLANT LIFE

ROSCOE W. THATCHER, M.A., D.Aon.

DEAN OF THE DEPARTMENT OF AGRICULTURE
AND DIRECTOR OF THE AGRIQULTURAL EXPERIMENT STATIONS,

UNIVERSITY OF MINNESOTA
(FORMERLY PROFESSOR OF PLANT CHEMISTRY. UNIVERSITY OF MINNESOTA)

FIRST EDITION

McGRAW-HILL BOOK COMPANY, INC.

NEW YORK: 370 SEVENTH AVENUE

LONDON: 6 & 8 BOUVERIE ST., E. C. 4

1921

L.B,

PREFACE

THE author has had in mind a two-fold purpose in the prep-
aration of this book. First, it is hoped that it may serve as a
text or reference book for collegiate students of plant science
who are seeking a proper foundation upon which to build a scien-
tific knowledge of how plants grow. The late Dr. Charles E.
Bessey, to whom I owe the beginning of my interest in plant life,
once said to me: " The trouble with our present knowledge of
plant science is that we have had very few chemists who knew
any botany, and no botanists who knew any chemistry." This
may have been a slightly exaggerated statement, even when it
was made, several years ago. But it indicated a very clear rec-
ognition by this eminent student of plants of the need for a better
knowledge of the chemistry of plant cell activities as a proper
foundation for a satisfactory knowledge of the course and results
of plant protoplasmic activities. It is hoped that the present
work may contribute something toward this desired end.

Second, the purpose of the writer will not have been fully
accomplished unless the book shall serve also as a stimulus to
further study in a fascinating field. Even the most casual perusal
of many of its chapters cannot fail to make clear how incomplete
is our present knowledge of the chemical changes by which the
plant cell performs many of the processes which result in the pro-
duction of so many substances which are vital to the comfort and
pleasure of human life. Studies of the chemistry of animal life
have resulted in many discoveries of utmost importance to human
life and health. It requires no great stretch of the imagination
to conceive that similar studies of plant life might result in
similar or even greater benefit to human lif e, or society, since it
is upon the results of plant growth that we are dependent for
most of our food, clothing, and fuel, as well as for many of the
luxuries of life.

The material presented in the book has been developed from a

4344G:

vi PREFACE

series of lecture-notes which was used in connection with a course
in " Phyto-chemistry " which was offered for several years to
the students of the Plant Science Group of the 'University of
Minnesota. In the preparation of these notes, extensive use was
made of the material presented in such general reference works as
Abderhalden's " Biochemische Handlexicon" and "Handbuch
der Biochemischen Arbeitsmethoden," Oppenheimer's "Hand-
buch der Biochemie des Menschen und der Tiere," Czapek's
"Biochemie der Pflanzen," Rohmann's "Biochemie/' Frankel's
"Descriptive Biochemie," and "Dynamische Biochemie," Euler's
"Pflanzenchemie," and Haas and Hill's "Chemistry of Plant
Products"; as well as of the most excellent series of "Monographs
on Biochemistry," edited by Plimmer, several numbers of which
appeared in print prior to and during the period covered by the
preparation of these lectures. Frequent use was made also of
the many special treatises on individual groups of compounds
which are mentioned in the lists of references appended to each
chapter, as well as of articles which appeared from time to time in
various scientific journals.

Hence, no claim is made of originality for the statements
presented herein, except in an insignificant number of studies
of enzyme action, and of the possible physiological functions of
certain specific compounds. The only contributions which the
writer has felt qualified to make to this general subject are those
of an intense personal interest in the chemistry of plant processes
and a viewpoint with reference to the relation of chemical processes
to vital phenomena which will be apparent as the various subjects
are presented.

The text has been prepared upon the assumption that the stu-
dents who will use it will have had some previous training in ele-
mentary inorganic and organic chemistry. A systematic labora-
tory course in organic preparations, such as is required of students
who are preparing to become professional chemists, is not at all a
necessary requisite to the understanding of the chemistry of the
different groups of plant compounds as here presented; but it is
assumed that the student will have had such previous training as
is now commonly given in a one-year collegiate course in "Gen-
eral Chemistry," or a year's work in general inorganic chemistry
followed by a brief course in "Types of Carbon Compounds" or
"Elements of Organic Chemistry," such as is usually required of

PREFACE vii

students who are preparing for advanced work in agricultural
science, in animal or human nutrition, etc.

An attempt has been made to arrange the material in such a
way as to proceed from simpler chemical principles and sub-
stances to those of more complex structures. This results in an
arrangement of the groups to be studied in an order which is quite
different than their biological significance might suggest. It is
believed, however, that in the end a more systematic under-
standing and a more orderly procedure is obtained in this way
than would result from the treatment of the groups in the order
of their relative biological importance.

CONTENTS

INTRODUCTION PAGE

Development of biological science; characteristics of protoplasm;
plant and animal life, similarities and differences; protoplasmic
activity essentially chemical changes; objects of study of the
chemistry of plant lif e xui-xvi

^CHAPTER I— PLANT NUTRIENTS

Definitions; the plant food elements; available and unavailable
forms; the value of the different soil elements as plant foods;
functions of the different plant food elements in plant growth;
inorganic plant toxins and stimulants; references 1-15

CHAPTER II — ORGANIC COMPONENTS OF PLANTS
Plants as synthetic agents; types of changes involved in plant
growth; groups of organic compounds found in plants; physio-
logical use and biological significance defined; physiological
uses of organic groups 1&-20

CHAPTER III— PHOTOSYNTHESIS

Definitions; physiological steps in photosynthesis; formaldehyde,
the simplest carbohydrate structure; the condensation of
formaldehyde into sugars; theories concerning photosynthesis;
the production of starches and sugars; references 21-29

CHAPTER IV— CARBOHYDRATES

Importance, nomenclature, and classification; groups of carbo-
hydrates; isomeric forms of monosaccharides; chemical con-
stitution of monosaccharides; characteristic reactions of
hexoses; the occurrence and properties of monosaccharides;
disaccharides; trisaccharides; tetrasaccharides; the relation
of molecular configuration to biochemical properties; poly-
saccharides, dextrosans, levulosans, mannosans. and galac-
tosans; physiological uses and biological significance of carbo-
hydrates; references 30-66

CHAPTER V — GUMS, PECTINS, AND CELLULOSES
Relation to carbohydrates; groups; the natural gums and pento-
sans; mucilages; pectins; celluloses; physiological uses of

celluloses; references 67-75

x CONTENTS

CHAPTER VI— GLUCOSIDES PAGES

Definition; general structure; hydrolysis of the natural glucosides;
general properties; the phenol glucosides; the alcohol glucosides;
the aldehyde glucosides; the oxycumarin glucosides; the cyan-
ophoric glucosides; the mustard-oil glucosides; the pigment
glucosides; the digitalis glucosides; the saponins; physiological
uses; biological significance; references 76-93

CHAPTER VII— TANNINS

General properties; occurrence; chemical constitution; classes;
some common tannins; physiological uses; biological signifi-
cance of tannins in fruits; references 94-101

CHAPTER VIII— PIGMENTS

Types and classes; the chlorophylls, chemical constitution, simi-
larity of chlorophyll and haemoglobin, properties of the chloro-
phylls; the carotinoids. carotin, xanthophyll, lycopersicin, and
fucoxanthin; phycoerythrin and phycophasin; the anthocyans;
the anthoxan thins; the production of ornamental pigments in
flowers, etc.; the functions of pigments; references 102-123

CHAPTER IX — ORGANIC ACIDS, ACID SALTS, AND ESTERS
Chemical constitution; some common organic acids; physiological
uses of organic acids; biological significance of fruit acids and
esters 124-128

CHAPTER X— FATS AND OILS, WAXES, AND LIPOIDS
General composition; fats and oils, occurrence, chemical constitu-
tion, acids which occur in natural fats, alcohols which occur in
natural fats, hydrolysis and synthesis of fats, extraction of oils
from plant tissues, identification of fats and oils, physiological
use; the waxes; the lipoids, lecithin, other plant phosphatides,
plant cerebrosides, physiological uses of lipoids; references. . . . 129-145

CHAPTER XI— ESSENTIAL OILS AND RESINS
Definitions, classes, occurrence; the essential oils; the resins;
physiological uses and biological significance of essential oils;
references 146-150

CHAPTER XII— THE VEGETABLE BASES

Composition and groups; the plant amines; alkaloids; the purine
bases; the pyrimidines; the nucleic acids, composition and
uses; references 151-163

CHAPTER XIII— PROTEINS

Importance; general composition; amino-acids and peptid units;
individual amino-acids; composition of the plant proteins;
general properties of proteins; classification ; differences between
plant and animal proteins; extraction of proteins from plant
tissues; synthesis in plants; physiological uses; references 164-180

CONTENTS xi

CHAPTER XIV— ENZYMES PAGES
Reaction velocities; enzymes as catalysts; general properties;
extracellular and intracellular enzymes; chemical nature;
nomenclature and classification; occurrence and preparation;
general and individual enzymes; nature of enzyme action;
accelerators and inhibitors; coenzymes and antienzymes;
zymogens; physiological uses; further studies needed; refer-
ences 181-201

CHAPTER XV — THE COLLOIDAL CONDITION
"Colloids" and " crystalloids "; the colloidal condition a dispersion
phenomenon; nomenclature and classification; conditions
necessary to the formation of sols; gel-formation; general
properties of colloidal solutions; suspensoids and emulsoids;
adsorption; catalysis affected by the colloidal condition;
industrial applications of colloidal phenomena; natural colloidal
phenomena; references 202-2^0

CHAPTER XVI— THE PHYSICAL CHEMISTRY OF PROTOPLASM
Heterogeneous structure of protoplasm; protoplasm a colloidal gel;
water; salts; osmotic pressure; surface boundary phenomena;
electrical phenomena; acidity and alkalinity; summary; vital
phenomena as chemical and physical changes; references 221-238

CHAPTER XVII — HORMONES, AUXIMONES, VITAMINES, AND TOXINS
External and internal stimulants; hormones; vitamines; auxi-

mones, toxins 239-248

CHAPTER XVIII— ADAPTATIONS

General discussion; adaptations, accommodations, and adjustments;
chromatic adaptations; morphological adaptations; accommoda-
tions; concluding statements 249-258

INDEX . . . 259-268

INTRODUCTION

THE history of biological science shows that the conceptions
which men have held concerning the nature of plant and animal
growth have undergone a series of revolutionary changes as the
technique of, and facilities for, scientific study have developed
and improved. For a long time, it was thought that life processes
were essentially different in character than those which take place
in inanimate matter, and that the physical sciences had nothing
to do with living changes. Then, too, earlier students had only
vague notions of the actual structure of a living organism. Begin-
ning with the earliest idea that a plant or an animal exists as a
unit organism, to be studied as such, biological science progressed,
first to the recognition and study of the individual organs which
are contained within the organism ; then to the tissues which make
up these organs; then (with the coming into use of the microscope
as an aid to these investigations) to the cells of which the tissues
are composed; then to the protoplasm which constitutes the
cell contents; and finally to the doctrine of organic evolution as the
explanation of the genealogy of plants and animals, and the study
of the relation of the principles of the physical sciences to the
evolutionary process. The ultimate material into which organisms
are resolved by this process of biological analysis is the cell proto-
plasm. But protoplasm is itself made up of a complex system of
definite chemical compounds, which react and interact according
to the laws of physical science. Hence, any study of the chem-
istry of plant growth is essentially a study of the chemical and
physical changes which take place in the cell protoplasm.

Protoplasm differs from non-living matter in three respects.
These are (1) its chemical composition; (2) its power of waste
and repair and of growth; and (3) its reproductive power. From
the standpoint of chemical composition, protoplasm is the most
complex material in the universe. It not only contains a greater
variety of chemical elements, united into molecules of enormous

xiv INTRODUCTION

size and complexity, but also a greater variety of definite chemical
compounds than exist in any other known mixture, either mineral
or organic in type. One of the first problems in the study of
protoplasm is, therefore, to bring this great variety of complex
compounds into some orderly classification and to become -familiar
with their compositions and properties. Again, living matter is
continually undergoing a process of breaking down as a result of
its energetic activities and of simultaneously making good this
loss by the manufacture of new protoplasm out of simple food
materials. It also has the power of growth by the production of
surplus protoplasm which fills new cells, which in turn produce
new tissues and so increase the size and weight of individual
organs and of the organism as a whole. Hence, a second field of
study includes the chemical changes whereby new protoplasm
and new tissue-building material are elaborated. Finally, living
material not only repairs its own waste and produces new material
of like character to it, but it also produces new masses of living
matter, which when detached from the parent mass, eventually
begin a separate existence and growth. Furthermore, the plant
organism has acquired, by the process of evolution, the ability
not only to produce an embryo for a successive generation but also
to store up, in the tissues adjacent to it, reserve food material for
the use of the young seedling until it shall have developed the
ability to absorb and make use of its own external sources of food
material. So that, finally, every study of plant chemistry must
take into consideration the stored food material and the germina-
tive process whereby this becomes available to the new organism
of the next generation. Also, the chemistry of fertilization of the
ovum, so that a new embryo will be produced, and the other
stimuli which serve to induce the growth phenomena, must be
brought under observation and study.

A further step in the development of biological science has
been to separate the study of living things into the two sciences of
botany and zoology. From the standpoint of the chemistry of
the processes involved this segregation is unfortunate. It has
resulted in the devotion of most of the study which has been given
to life processes and living things to animal chemistry, or " physi-
ological chemistry." As a consequence, biochemistry, which
deals with the living processes of both plants and animals, is yet
in its infancy; while phytochemistry is almost a new science,

INTRODUCTION xv

yet its relation to the study of plants can scarcely be less vital
than is that of physiological chemistry to studies of animal life.

The common conception that plant life and animal life are
antithetical or complementary to each other has much to justify it.
Animals breathe in oxygen and exhale carbon dioxide; while
plants use the carbon dioxide of the air as a part of the raw mate-
rial for photosynthesis and exhale oxygen. Plants absorb simple
gases and mineral compounds as raw food materials and build
these up into complex carbohydrates, proteins, fats, etc.; while
animals use these complex compounds of plant origin as food,
transforming parts of them into various other forms of structural
material, but in the end breaking them down again into the
simple gases and mineral compounds, which are expelled from the
body through the excretory organs. Thus it would seem that the
study of the chemistry of plant life and of animal life must neces-
sarily deal with opposite types of phenomena.

But one cannot advance far into the study of the biochemistry
of plants and animals before he discovers marked similarities
in the chemical principles involved. Many of the compounds are
identical in structure, undergo similar changes, and are acted upon
by similar catalysts. Plant cells exhibit respiratory activities,
using oxygen and giving off carbon dioxide, in exactly the same
way that animal organisms do. The constructive photosynthetic
processes of green plants are regulated and controlled by a pig-
ment, chlorophyll, which is almost identical with the blood pig-
ment, haematin, which regulates the vital activities in the animal
organism, differing from the latter only in the mineral element
which links the characteristic structural units together in the
molecule. Many other points of similarity in the chemistry of
the life processes of plants and animals will become apparent as
the study progresses. It is sufficient now to call, attention to
the fact that these vital processes, in either plants or animals,
are essentially chemical in character, and subject to study by the
usual methods of biochemical investigations.

The protoplasm of the cell is the laboratory in which all the
changes which constitute the vital activities of the plant take
place. All of the processes which constitute these activities —
assimilation, translocation, metabolism, and respiration — involve
definite chemical changes. In so far as it is possible to study each
of these activities independently of the others, they have been

xvi INTRODUCTION

found to obey the ordinary laws of chemical reactions. Thus,
the effect of the variations in intensity of light upon photosyn-
thesis causes increase in the rate of this activity which may be
represented by the ordinary responses of reaction velocities to
external stimuli. Similarly, the effect of rises in temperature upon
the rate of assimilation and upon respiration are precisely the same
as their effect upon the velocity of any ordinary chemical reaction.
Within certain definite ranges of temperature, the same statement
holds true with reference to the rate of growth of the plant, although
the range of temperature within which protoplasm lives and main-
tains its delicate adjustment to the four vital processes of life is
limited; beyond a certain point, further rise in temperature does
not produce more growth but rather throws the protoplasmic
adjustment out of balance and growth either slows up markedly
or stops altogether.

Hence, we may say that the methods by which the plant
machine (protoplasm) accomplishes its results are essentially and
definitely chemical in character and may be studied purely from
the standpoint of chemical reactions, but the maintenance of the
machine itself in proper working order is a vital phenomenon
which is largely dependent upon the external environmental
conditions under which the plant exists. A study of the phe-
nomena resulting from the colloidal condition of matter is throw-
ing a flood of light upon the mechanism by which protoplasm
accomplishes its control of vital activities. But we are, as yet,
a long way from a complete understanding of how colloidal proto-
plasm acquires and maintains its unique ability of self-regulation
of the conditions necessary to preserve its colloidal properties and
of how it elaborates the enzymes which control the velocity of the
chemical reactions which take place within the protoplasm itself
and which constitute the various processes of vital activity.

The object of this study of the chemistry of plant growth is
to acquire a knowledge of the constitution of the compounds
involved and of the conditions under which they will undergo the
chemical changes which, taken all together, constitute the vital
processes of cell protoplasm.

CHEMISTRY OF PLANT LIFE

CHAPTER I

•

PLANT NUTRIENTS

THERE is some confusion in the use of the terms "nutrient,"
" plant food/7 etc., as applied to the nutrition and growth of
plants. Strictly speaking, these terms ought probably to be lim-
ited in their application to the organized compounds within the
plant which it uses as sources of energy and of metabolizable
material for the development of new cells and organs during its
growth. Botanists quite commonly use the terms in this way.
But students of the problems involved in the relation of soil
elements to the growth of plants, including such practical ques-
tions as are involved in the maintenance of soil productivity
and the use of commercial fertilizers for the growing of economic
plants, or crops, are accustomed to use the terms " plant foods,"
or " mineral nutrients," to designate the chemical elements and
simple gaseous compounds which are supplied to the plant as the
raw material from which its food and tissue-building materials
are synthetized. Common usage limits these terms to the soil
elements; but there is no logical reason for segregating the raw
materials derived from the soil from those derived from the atmos-
phere.

The essential difference between these raw materials for plant
syntheses and the organic compounds which are produced within
the plants and used by them, and by animals, as food, is that the
former are inorganic and can furnish only materials but no energy
to the organism; while the latter are organic and supply both
materials and potential energy. It would probably be the best
practice to confine the use of the word " food " to materials of the
latter type, and several attempts have been made to limit its use

2 -CHEMISTRY OF PLANT LIFE

in this way and to apply some such term as " intake " to the sim-
ple raw materials which are taken into the organism and utilized
by it in its synthetic processes. But the custom of using the
words " food," or " nutrient," to represent anything that is taken
into the organism and in any way utilized by it for its nourish-
ment has been followed so long and the newer terms are them-
selves so subject to criticism that they have not yet generally
supplanted the loosely used word " food."

If such use is permitted, however, it is necessary to recognize
that only the green parts of green plants can use this inorganic
" food," and that the colorless plants must have organic food.

To avoid this confusion, the suggestion has recently been
made that all of the intake of plants and animals shall be con-
sidered as food, but that those forms which supply both materials
and potential energy to the organism shall be designated as
synergic foods, while those which contain no potential energy shall
be known as anergic foods. On this basis, practically all of the
food of animals, excepting the mineral salts and water, and all of
the organic compounds which are synthetized by plants and later
used by them for further metabolic changes, are synergic foods;
while practically all of the intake of green plants is anergic food.

It is with the latter type of food materials that this chapter
is to deal; while the following and all subsequent chapters deal
with the organic compounds which are synthetized by plants and
contain potential energy and are, therefore, capable of use as syn-
ergic food by either the plants themselves or by animals. It will
be understood, therefore, that in this chapter the word " food "
is used to mean the anergic food materials which are taken into
and used by green plants as the raw materials for the synthesis
of organic compounds, with the aid of solar energy, or that of pre-
viously produced synergic foods. In all later chapters, the term
" food " will be used to mean the organic compounds which serve
as the synergic food for the green parts of green plants and as the
sole supply of nutrient material for the colorless parts of green plants
and for parasitic or saprophytic forms (see page 16).

PLANT FOOD ELEMENTS

The raw materials from which the food and tissue-building
compounds of plants are synthetized include carbon dioxide,

PLANT .NUTRIENTS 3

oxygen, water, nitrogen, phosphorus, sulfur, potassium, calcium,
magnesium, and iron. The two gases first mentioned are derived
directly from the air, through the respiratory organs of the plant.
Water is taken into the plant chiefly from the soil, through its
fibrous roots. All the other elements in the list are taken from the
soil, nitrogen being derived from decaying organic matter (the
original source of the nitrogen is, however, the atmosphere, from
which the initial supply of nitrogen is obtained by direct assimila-
tion by certain bacteria and perhaps other low forms of plant life),
and the remaining ones from the mineral compounds of the soil.

Carbon dioxide and oxygen, being derived from the air, are
always available to the leaves and stems of growing plants hi
unlimited supply; but the supply available to a seed when ger-
minating in the soil, or to the roots of a growing farm crop, may
sometimes become inadequate, especially in soils of a very com-
pact texture, or " water-logged " soils. In such cases, the defi-
ciency of these gaseous food elements may become a limiting
factor in plant growth.

Water is often a limiting factor in plant growth. Experiments
which have been repeated many times and under widely varying
conditions show that when water is supplied to a plant in varying
amounts, by increasing the percentage of water in the soil in which
the plant is growing by regular increments up to the saturation
point, the growth of the plant, or yield of the crop, increases up
to a certain point and then falls off because the excess of water
reduces the supply of air which is available to the plant roots.
Hence, abundance of water is, in general, a most essential factor
in plant growth.

Under normal conditions of SLIT and moisture supply, however,
the plant food elements which may be considered to be the lim-
iting factors in the nutrition and growth of plants are the chemi-
cal elements mentioned in the list above.

AVAILABLE AND UNAVAILABLE FORMS

The plant food materials which are taken from the soil by a
growing plant must enter it by osmosis through the semi-permeable
membranes which constitute the epidermis of the root-hairs, and
circulate through the plant either carried in solution in the sap or
by osmosis from cell to cell. Hence, they must be in water-soluble

4 CHEMISTRY OF PLANT LIFE

form before they can be utilized by plants. Obviously, therefore,
only those compounds of these elements in the soil which are sol-
uble in the soil water are available as plant food. The greater
proportion of the soil elements are present there in the form of
compounds which are so slightly soluble in water as to be unavail-
able to plants. The processes by which these practically insoluble
compounds become gradually changed into soluble forms are
chiefly the " weathering " action of air and water (particularly
if the latter contains carbonic acid) and the action of the organic
acids resulting from decaying animal or vegetable matter or
secreted by living plants.

THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD

Analyses of the tissues of plants show that they contain all of
the elements that are to be found in the soil on which they grew.
Any of these elements which are present in the soil in soluble form
are carried into the plants with the soil water in which they are
dissolved, whether they are needed by the plant for its nutrition
or not. But in the case of those elements which are not taken
out of the sap to be used by the plant cells in their activities, the
total amount taken from the soil is much less than is that of the
elements which are used in the synthetic processes of the plant.
Hence, much larger proportions of some elements than of others
are taken from the soil by plants. The proportions of the dif-
ferent elements which are used by plants as raw materials for the
manufacture of the products needed for their growth varies with
the different species; but a certain amount of each of the so-called
" essential elements " (see below) is necessary to every plant,
because each such element has a definite role which it performs
in the plant's growth. A plant cannot grow to maturity unless
a sufficient supply of each essential element comes to it from the
soil.

From the standpoint of their relative value as raw materials
for plant food, the elements which are present in the soil may be
divided into three classes; namely, the non-essential, the essential
and abundant, and the critical elements.

The first class includes silicon, aluminium, sodium, manganese,
and certain other rarer elements which sometimes are found in
soils of some special type, or unusual origin These elements seem

PLANT NUTRIENTS 3

to have no role to play in the nutrition of plants; although silicon
is always present in plant ash and sodium salts are found in small
quantities in all parts of practically all plants. Nearly all species
of plants can be grown to full maturity in the entire absence of
these elements from their culture medium. Occasional excep-
tions to this statement in the case of special types of plants are
known, and are of interest in special studies of plant adaptations,
but need not be considered here.

The second group includes iron, calcium, magnesium, and,
generally, sulfur. All of these elements are essential for plant
growth, but are usually present in the soil in ample quantities
to insure a sufficient supply in available form for all plant needs.
Recent investigations have shown, however, that there are many
soils in which sulfur is present in such limited quantities that
many agricultural crops, when grown on these soils, respond
favorably to the application of sulfur-containing fertilizers. In
such cases, sulfur is a " critical " element.

The " critical " elements are those which are essential to the
growth of all plants and which are present in most soils in rela-
tively small proportions and any one may, therefore, be the limiting
factor in plant growth so far as plant food is concerned. These
are nitrogen, phosphorus, potassium, and (possibly) sulfur.

ROLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH

The use which a plant makes of the elements which come to it
from the soil has been studied with great persistency and care by
many plant physiologists and chemists. Many of the reactions
which take place in a plant cell are extremely complicated, and the
relation of the different chemical elements to these is not easily
ascertained. It is probable that the same element may play a
somewhat different role in different species of plants, in different
organs of the same plant, or at different stages of the plant's
development. But the usual and most important offices of each
element are now fairly well understood, and are briefly summarized
in the following paragraphs. It should be understood that a
thorough and detailed discussion of these matters, such as would
be included in an advanced study of plant nutrition, would reveal
other functions than those which are presented here and would
require a more careful and more exact method of statement than

6 CHEMISTRY OF PLANT LIFE

is suitable here. However, the general principles of the utilization
of soil elements by plants for their, nutrition and growth may be
fairly well understood from the following statements.

Nitrogen is a constituent of all proteins (see Chapter XIII).
Proteins are apparently the active chemical components of proto-
plasm. Since it is in the protoplasm of the green portions, usually
foliage, of plants that the photo-synthesis of carbohydrates and
the synthesis of most, or all, of the other tissue-building materials
and reserve food substances of the plant takes place, the impor-
tance of nitrogen as a plant food can hardly be over-emphasized.
Nitrogen starvation produces marked changes in the growth of a
plant. Leaves are stunted in growth and a marked yellowing
of the entire foliage takes place; in fact, the whole plant takes on a
stunted or starved appearance. Abundance of nitrogen, on the
other hand, produces a rank growth of foliage of a deep rich color
and a luxuriant development of tissue, and retards the ripening
process. In the early stages of growth, the nitrogen is present
most largely in the leaves; but when the seeds develop, rapid
translocation of protein material into the seeds takes place, until
finally a large proportion of the total supply is deposited in them.

Nitrates are the normal form of nitrogen in the soil which is
available to plants. During germination and early growth, the
young seedling uses amino-acids, etc., derived from the proteins
stored in the seed, as its source of nitrogen; and experiments have
shown that similar forms of soluble organic nitrogen compounds
can be successfully fed to the seedling as an external food supply.
Soluble ammonium salts can be utilized as sources of nitrogen by
most plants during later periods of growth, particularly by the
legumes. But for most, if not all, of the common farm crops
whose possibilities in these respects have been studied, it has been
found that a unit of nitrogen taken up as a nitrate is very much
more effective in promoting growth, etc., than is the same unit of
nitrogen in the form of ammonium salts.

While the proteins are finally stored up largely in the seeds,
or other storage organs, they are actively at work during the grow-
ing period in the cells of the foliage parts of the plant. Hence,
the popular statement that " nitrogen makes foliage " is a fairly
accurate expression of its role. Inordinate production of straw
in cereal crops and of leaves in root crops often results from liberal
supplies of available nitrogen in the soil early in the growing sea-

PLANT NUTRIENTS T

son. If the crops develop to normal maturity, this excessive foli-
age growth has no harmful results, as the surplus material which
has been elaborated is properly translocated into the desired stor-
age organs; but, unfortunately, the retarding effect of the surplus
nitrogen supply upon the date of maturing of the crop is often asso-
ciated with premature ripening of the plants from other causes, with
the consequence that too large a proportion of the valuable food
material is left in the refuse foliage material of the crop. Crops
which are grown solely for their leaves, such as hay crops, lettuce,
cabbage, etc., profit greatly by abundant supplies of available
nitrogen; although when foliage growth is stimulated in this way
the tissue is likely to be thin- walled and soft rather than firm and
solid.

Phosphorus is likewise an extremely important element in
plant nutrition. But phosphorus starvation produces no such
striking visible effects upon the growth of the plant as does lack
of nitrogen. Abundance of available phosphorus early in the
plant's life greatly stimulates root growth, and later on it undoubt-
edly hastens the ripening process; hence, this element seems to
act as the exact antithesis of nitrogen.

The role of phosphorus, or of phosphates, in the physiological
processes of the cell seems to be difficult to discover. The element
itself is a constituent of some protein complexes and of the lecithin-
like bodies (see page 141) which are supposed by some inves-
tigators to play an important part in determining the rate of
chemical changes which take place in the cell and the movement
of materials into and out of it. It is an essential constituent of
the nucleus, and a meager supply of phosphorus retards, or inhibits,
mitotic cell-division. Photosynthesis of sugars and the condensing
of these into starch or cellulose takes place in plants in the absence
of available phosphorus; but the change of these insoluble carbo-
hydrates back again into soluble and available sugar foods does
not.

Phosphorus is taken from the soil by plants in the form of
phosphates. Much study has been given to the problem of the
proper supply of available soil phosphates for economic crop pro-
duction. Any discussion of soil fertility and fertilization which
did not devote large attention to the conditions under which
phosphates become available as plant food would be wholly inad-
equate; but such a discussion would be out of place here.

8 CHEMISTRY OF PLANT LIFE

The final result of an ample supply of phosphates in hastening
the ripening process and stimulating seed production, as con-
trasted with that of an over-supply of nitrogen, has led to the
popular statement that " phosphates make seeds." This state-
ment, while not strictly accurate, is a fairly good summary of the
combined results of the role of phosphorus in the plant economy.
Large amounts of phosphorus are stored in the seeds. The two
facts that large amounts of these compounds are thus available
to the young seedling and that relatively large proportions of
phosphates are taken from the soil by the plant during its early
stages of growth are undoubtedly connected with the need for
rapid cell-division at these periods in the plant's life.

Potassium. — The popular expression that " potash makes
sugars and starch " is a surprisingly accurate description of the
role of this element in plant metabolism. Either the photo-
synthesis of starch, or the changes necessary to its translocation
(it is not yet certain which) is so dependent upon the presence of
potassium in the cell sap that the whole process stops at once if
an insufficient supply is present. The production and storage of
sugar, or starch, in such root crops as beets, potatoes, etc., dimin-
ishes in direct proportion with a decreasing supply of potassium
as plant food. The grains of the cereal crops become shrunken
as a result of potassium starvation; and are plump and well
filled with starch in the endosperm when sufficient potassium is
available for the crop's needs.

The general tone and vigor of growth of the plant is largely
dependent upon an ample potassium supply; potash-hungry
plants, like those which have been weakened by any other unfav-
orable conditions, have been found to be more susceptible to
injury by disease, than those which are well nourished with this
food element. But potassium-starvation does not produce any
pathological condition of the cell contents; its absence simply
prevents the possibility of the development of the necessary car-
bohydrates for vigorous growth.

There is no known difference in the availability, or effective-
ness, of potassium from the different forms of compounds con-
taining it which may be present in the soil. Apparently, the only
essential is that the compound shall be soluble so that it can be
absorbed into the plant through the root-hairs. Of course, the
acid radical to which the basic potassium ion is attached may, in

PLANT NUTRIENTS 9

itself, have some beneficial or deleterious influence which gives to
the compound as a whole some important effect in one case, which
might not follow in the case of another type of compound; but
the relative efficiency as plant food of a given unit of potassium
seems to be the same regardless of the nature of the compound
in which it is present.

Calcium is an essential plant food element but its physiological
use has not yet been definitely established. It seems to stimulate
root-development, and certainly gives vigor and tone to the whole
plant. It is commonly believed that calcium is hi some way con-
nected with the development of cell-wall material. It has been
reported that the stems of grasses and cereal plants become stiffer
in the presence of ample calcium, but this may be due to greater
turgidity rather than to strengthened cell-walls. Calcium remains
in the leaves or stem as the plant ripens, but it is not clear that this
has anything to do with the stiffness or weakness of the stem, or
straw, of the plant. Experiments with algae have shown that in
the absence of calcium salts mitotic cell division takes place,
showing that the nucleus functions properly, but the formation of
the new transverse cell-wall is retarded. This is the only direct
evidence that has been reported that calcium has any connection
with cell-wall formation.

Certain species of plants, notably many legumes, require such
large amounts of calcium salts for their growth as to give to them
the popular appellation of " lime-loving plants." Other plants,
known as " calciphiles," while not actually showing abnormally
large percentages of calcium in their ash, flourish best on soils rich
in lime. On the other hand, certain other species, known as
" calcifuges," will not grow on soils which are even moderately
rich in lime; in what respect these differ in their vital processes
from others which demand large amounts of calcium, or those
which flourish on soils rich in lime, has not been determined,
however.

The beneficial effect of alkaline calcium compounds in the soil,
in correcting injurious acidity, in improving the texture of clay
soils, and in promoting the proper conditions for bacterial growth,
is well known; but this has no direct connection with the role of
calcium as plant food. Furthermore, calcium salts in the soil
have a powerful influence in overcoming the harmful, or toxic,
effects of excessive amounts of soluble salts of magnesium, sodium,

10 CHEMISTRY OF PLANT LIFE

or potassium, in the so-called " alkali soils " (i.e., those which con-
tain excessive amounts of water-soluble salts). The probable
explanation for this fact is pointed out in a later paragraph of this
chapter (see page 14) ; but this property of calcium probably has
no connection with its physiological uses as plant food.

Magnesium, like phosphorus, is finally stored up mostly in
the seeds, not remaining in the leaves and stems, as do calcium
and potassium. This fact, together with other evidence obtained
from experiments in growing plants in culture solutions containing
varying amounts of this element, has led certain investigators to
the conclusion that the role of magnesium is to aid in the trans-
port of phosphorus, particularly from older to more rapidly grow-
ing parts of the plant. More recent investigations have shown,
however, that magnesium has other roles which are probably
more specific and more important that this one. It is now known
that magnesium is a definite constituent of the chlorophyll molecule
serving, as will be shown (see Chapter VIII), as the means of
linkage between its essential component organic groups. Because
of this fact, magnesium-starvation produces etiolated plants,
which cannot function normally. Further, magnesium seems to
be necessary for the formation of fats, apparently standing in a
similar relation to fat-formation to that of potassium to carbo-
hydrate-formation. This view is supported by the observations
that when algae are grown in magnesium-free solutions they con-
tain no fat globules and that oily seeds are richer in magnesium
than are those which store up starch as their reserve food material.
Observers of the second of these phenomena have failed to note,
however, that oily seeds are likewise richer in phosphorus than
are starchy ones, and that the presence of larger proportions of
magnesium in such seeds may, perhaps, be related to phosphorus-
translocation rather than to fat-formation.

Whatever relation magnesium may have to fat-formation, or to
the translocation of phosphorus, it is evident that these are roles
quite apart from its use as a constituent element in chlorophyll.
As yet, no explanation of how it aids in these other synthetic
processes has been advanced.

On the other hand, an excess of soluble magnesium salts in the
soil produces definite toxic effects upon plants, magnesium com-
pounds being known to be among the most destructive of the
" alkali soil " salts. Calcium salts are remarkably efficient in

PLANT NUTRIENTS 11

overcoming these harmful effects of magnesium salts. On this
account, a large amount of experimental study has been given to
the question of the calcium-magnesium ratio in plants. Numer-
ous analyses of plant ashes have established the fact that there is a
fairly definite ratio of this kind, which ratio, however, varies with
the species of plant and is not correlated with the ratio of these
elements present in the soil on which the plant grows, as was for-
merly believed. Cereal plants, as a rule, contain approximately
twice as much lime as magnesia; while leafy plants (tobacco,
cabbage, etc.) usually contain about four times as much calcium
oxide as magnesium oxide.

Iron is essential to chlorophyll-formation. It is not a con-
stituent of the chlorophyll molecule, as is magnesium; but in the
absence of iron from the culture solution, a plant fails to produce
chlorophyll and a green plant which is deprived of a supply of iron
rapidly becomes etiolated. The way in which iron is related to
chlorophyll-formation is not known.

Iron is taken from the soil by plants in the smallest propor-
tions of any of the essential elements. Only soluble ferric com-
pounds seem to serve as a suitable source of supply of the element;
ferrous compounds being usually highly toxic to plants.

Sulfur is an essential element of plant food. The amounts
required by plants were supposed, until recently, to be relatively
small. This was due to the fact that earlier studies took account
only of the sulfur which, on analysis, appeared 'as sulfates in the
ash. Improved methods, of analysis, which insure that the sulfur
which is present in the plant tissue in organic combinations is
oxidized under such conditions that it is not lost by volatilization
during the combustion of the material, have shown that the total
sulfur which is present in many plants approaches the quantity of
phosphorus which is present in the same tissue. Furthermore,
recent field and pot experiments have shown that at least a con-
siderable part of the beneficial effects of many fertilizers, which
has previously been attributed to the calcium, potassium, or
phosphorus which they contain, is actually due to the sulfur
present as sulfates in the fertilizers used.

Sulfur occurs in the organic compounds of plants, associated
with phosphorus. It seems probable that its physiological uses
are similar to those of the latter element; but there is as yet no
experimental evidence to establish its exact role in the economy

12 CHEMISTRY OF PLANT LIFE

of plant growth. It appears to be needed in largest proportion
by plants which contain high percentages of nitrogen in their
foliage, such as the legumes. There is some evidence that sulfur
has a particular role in promoting the growth of bacteria, and it
may be that the high percentages of total sulfur which are found
in the tissues of legumes are due to the presence of the symbiotic
nitrogen-gathering bacteria in the nodules on the roots of these
plants. This point has not yet been investigated, however.

Sodium is probably not essential to plant growth, although
it is present in small proportions in the ash from practically all
plants. In cases of insufficient supply of potassium, sodium can
apparently perform at least a part of the role of the former ele-
ment; but this seems not to be a normal relationship or use.

Chlorine is found in small amounts in the sap and in the ash
of nearly all plants. However, it does not appear to be essential
to the growth of a plant, except possibly in the case of certain
species, such as asparagus, buckwheat, and, perhaps, turnips
and some other root crops. Whether the benefit which these crops
derive from the application of common salt to the soil in which
they are growing is due to the direct food value of either the
chlorine, or the sodium, or to some indirect effect, is not yet
known. The presence of chlorine in the sap of plants is undoubt-
edly due to the inevitable absorption of soluble chlorides from the
soil and apparently has no connection with the nutritional needs
of the plant.

Silicon is always considered as a non-essential element, although
it occurs in such large proportions in some plants as to indicate
that it cannot be wholly useless. It accumulates in the stems
of plants, chiefly in the cell-wall, and has sometimes been sup-
posed to aid in giving stiffness to the stems. But large numbers of
analyses have failed to show any direct correlation between the
stiffness of straw of cereal plants and the percentage of silicon
which they contain. Further, plants will grow to full maturity
and with erect stems when no silicon is present in the mineral
nutrients which are furnished to them. On the other hand, cer-
tain experiments appear to indicate that silicon can perform some
of the functions of phosphorus, if soluble silicates are supplied to
phosphorus-starved plants. But under normal conditions of
plant nutrition, it seems to have no such function.

PLANT NUTRIENTS 13

INORGANIC PLANT TOXINS AND STIMULANTS

Much study has been given during recent years to the ques-
tion of the supposed poisonous, or toxic, effects upon plants of
various soil constituents. There seems to be no doubt that certain
organic compounds which are injurious to plant life are often pres-
ent in the soil, either as the normal excretions of plant roots or as
products of the decomposition of preceding plant growths. A con-
sideration of these supposedly toxic organic substances would be
out of place in this discussion of mineral soil nutrients. But there
seems to be no doubt that there may also be mineral substances
in the soil which may sometimes exert deleterious influences
upon plant growth. In fact, most metallic salts, except those
of the few metals which are required for plant nutrition, appear
to be toxic to plants. The exact nature of the physiological effects
which are produced by these mineral toxins is not clearly under-
stood; indeed, it is probably different in the case of different metals.
Further, it is certain that both the stimulating and the toxic
effect of metallic compounds upon low forms of plants is quite
different from the effects of the same substances upon the more
complex tissues of higher plants, a fact which is utilized to advan-
tage in the application of fungicides for the control of parasitic
growths on common farm crops.

Among the elements whose physiological effects upon higher
plants, such as the cereal crops, etc., when their soluble compounds
are present in the soil, have been carefully studied, there are three
fairly distinct types of injurious mineral elements. The first of
these, represented by copper, zinc, and arsenic, apparently exert
their toxic effect regardless of the proportion in which they are
present in the nutrient solution which is presented to the plant;
although the degree of injury varies with the amount of injurious
substance present, of course. The second type, of which boron
and manganese are representatives, apparently exerts a definite
stimulating effect upon plants when supplied to them in concen-
trations below certain clearly defined limits; but are toxic in con-
centrations above these. The third includes many soluble salts
of magnesium, sodium, potassium, etc., which while either innocu-
ous or else definite sources of essential plant foods when in lower
concentrations, become highly toxic, or corrosive, when present
in the soil solution in concentrations above the limits of " tolera-

14 CHEMISTRY OF PLANT LIFE

tion " of individual plants for these soluble salts. The tolerance
shown by the different species of plants toward these soluble salts
(the so-called " alkali " in soils) varies widely; indeed, there
seems to be considerable variation in the resistance of different
individual plants of the same species to injury from this cause.

With reference to the toxic effect of the third type of substances,
i.e., the common soluble salts, it is known that single salts of
potassium, magnesium, sodium, or calcium, in certain concen-
trations, are toxic to plants, while mixtures of the same salts in
the same concentrations are not. Thus, solutions of sodium
chloride, magnesium sulfate, potassium chloride, and calcium
chloride which, when used singly, killed plants whose roots were im-
mersed in them for only a few minutes, formed when mixed together
a nutrient solution in which the same plants grew normally. The
remarkable remedial effect of calcium salts in overcoming the
injurious effects of other soluble salts has already been men-
tioned. One explanation of these relationships between mineral
soil constituents and the living plant is that the life phenomena
depend upon a balanced adjustment between the compounds of
these different mineral elements with the proteins (producing
the so-called " metal proteids ") which constitute the active
material of the cell protoplasm. According to this theory, any
excess or deficiency of any one or more of these elements in the
plant juices which surround a given cell will, of course, cause an
interchange with* the mineral components of the supposed " metal
proteids " which upsets the assumed essential balance between
them, with disastrous results. A more recent, and much more
satisfactory, explanation of the " antagonism " between mineral
elements in their toxic effects upon plants, which has both theo-
retical and experimental confirmation, is that single salts disturb
the colloidal condition (see Chapter XV) of the protoplasm of the
plant cells in such a way as to destroy its permeability to nutrient
substances, while mixtures of salts restore the proper state of
colloidal dispersion and permit the normal functioning of the
protoplasm.

It is apparent from the above brief discussions that the role
of the different soil elements as plant food, and their relations
to the complex processes which constitute plant growth, afford
an interesting and promising field for further study.

PLANT NUTRIENTS 15

References

BRENCHLEY, WINIFRED E. — "Inorganic Plant Poisons and Stimulants,"
106 pages, 18 figs., Cambridge, 1914.

HALL, A. D. — "Fertilizers and Manures," 384 pages, 7 plates, London, 1909.

HALL, A. D. — "The Book of the Rothamsted Experiments," 294 pages, 49.
figs., 8 plates, London, 1905.

HOPKINS, C. G. — "Soil Fertility and Permanent Agriculture," 653 pages,
Chicago, 1910.

HILGARD, E. W.— "Soils," 593 pages, 89 figs., New York, 1906.

LOEW, O— "The Physiological Role of Mineral Nutrients," U. S. Depart-
ment of Agriculture, Bureau of Plant Industry, Bulletin No. 45, 70 pages,
Washington, D. C., 1903.

RUSSELL, E. J— "Soil Conditions and Plant Growth," 243 pages, 13 figs.,
Monographs on Biochemistry, London, 1917. (3d ed.)

WHITNEY, M. — "A Study of Crop Yields and Soil Composition in Relation
to Soil Productivity," U. S. Department of Agriculture, Bureau of Soils,
Buttetin No. 57, 127 pages, 24 figs,, Washington, D, C., 1909.

CHAPTER II
THE ORGANIC COMPONENTS OF PLANTS

FROM the standpoint of their ability to synthetize synergic
foods (see page 2) from inorganic raw materials, plants may
be divided into two types; namely, the autotrophic, or self-nour-
ishing, plants, and the heterotrophic plants.

Strictly speaking, only those plants whose every cell contains
chlorophyll are entirely self -nourishing; and some parts, or
organs, of almost any autotrophic plant are dependent upon the
active green cells of other parts of the plant for their synergic food.
Furthermore, if the term is used in a very wide sense, green plants
are more than self-nourishing, they really nourish all living things.
But the general significance of the term " autotrophic plants " is
apparent.

" Heterotrophic plants " must, of necessity, get food, either
directly or indirectly, from some other plant which can synthetize
synergic foods or, in a few cases, from animal organic matter. If
they do this by feeding upon the organic compounds of other living
organisms, they are known as " parasites "; while if they secure
their organic food from the tissues or debris of dead organisms,
they are called " saprophytes." The heterotrophic plants are
chiefly the bacteria and fungi; although a few seed-plants are
devoid of chlorophyll or have nutritive habits similar to those of
the non-green plants, and a few species are semi-parasitic or semi-
saprophytic.

It is obvious that the metabolic processes of the autotrophic
plants are very different from those of the heterotrophic type of
plants. These differences constitute a most interesting field of
study for plant physiologists. But the nature of the chemical
compounds themselves and of the chemical changes involved in
their transformations is not radically different in the two types of
plants, the essential difference being in the preponderance of one
kind of activities, or chemical reactions, over another in bringing
about the metabolic processes which are characteristic of each

THE ORGANIC COMPONENTS OF PLANTS 17

particular species. Hence, it does not seem necessary, or desir-
able, in this study of the chemistry of plant growth, to present
as detailed a consideration of the differences in metabolic activity
of the different types of plants as complete accuracy of statement
in all cases might demand. We will, instead, discuss the organic
chemical components of plant tissues and the reactions which
they undergo, using the more common type of autotrophic plants
as the illustrative material in most cases.

Hence, it will be understood that in all the following dis-
cussions of plant activities, except where specific exceptions are
definitely mentioned, it is the green, or autotrophic, plants to
which reference is made in each case.

From the standpoint of the sum total of its activities, a green
plant is essentially an absorber of solar energy and a synthetizer
of organic substances. Each individual autotrophic plant takes
up certain amounts of the anergic foods which are discussed
in the preceding chapter and manufactures from them a great
variety of complex organic compounds, using the energy of the
sun's rays, absorbed by chlorophyll, as the source for the energy
necessary to accomplish these synthetic reactions. The ultimate
object of these processes is to produce seeds, each containing an
embryo and a sufficient supply of food for the young plant of the
next generation to use until it has developed its own synthetic
organs; or (in the case of perennials) to store up reserve food
materials with which to start off new growth after a period of
rest and often of defoliation. To be sure, animals and men
often interfere with the completion of the life cycle of the plant,
and utilize the seeds or stored food material for their own nutri-
tion, but this is a biological relation which has no influence upon
the nature of the plant's own activities.

Since all of these synthetic reactions must go on at ordinary
temperatures, active catalyzers are necessary. These the plant
provides in the form of enzymes (see Chapter XIV) which are
always present in active plant protoplasm. Proper conditions
for rapid chemical action are further assured by the colloidal
nature (see Chapter XV) of the protoplasm itself.

TYPES OF CHEMICAL CHANGES INVOLVED IN PLANT GROWTH

The whole cycle of chemical changes which is involved in plant
growth represents the net result of two opposite processes; the

18 CHEMISTRY OF PLANT LIFE

first of these is a constructive one which has at least three different
phases: namely, a synthesis of complex organic compounds, the
translocation of this synthetized material to the centers of growth,
and the building up of this food material into tissues or reserve
supplies; and the second is a destructive process of respiration
whereby carbohydrate material is broken down, potential energy
is released, and carbon dioxide is excreted.

The synthetic processes which take place in plants are of two
types; namely, photosynthesis, in which sugars are produced,
and another, which has no specific name, whereby proteins are
elaborated. The translocation of the synthetized material
involves the change of insoluble compounds into soluble ones,
effected by the aid of enzymes. For storage purposes, the soluble
forms are usually, though not always, condensed again into more
complex forms, these latter changes requiring much less energy
than do the original syntheses from raw materials.

The destructive process, respiration, is characteristic of all
living matter, either plant or animal organisms. It takes place
continuously throughout the whole life of a plant. During rapid
growth it is overshadowed by the results of the synthetic process,
but during the ripening period in which the seed is matured, and
during the germination of the seed itself, growth is practically
at a standstill and the respiratory, destructive action predominates,
so that the plant actually loses weight.

GROUPS OF ORGANIC COMPOUNDS FOUND IN PLANTS

As a result of their various synthetic and metabolic activities,
a great variety of organic compounds is produced by plants.
Certain types of these compounds, such as the carbohydrates and
proteins, are necessary to all plants and are elaborated by all
species of auto trophic plants. Other types of compounds are
produced by many, but not all, species of plants; while still others
are found in only a few species. It is fairly easy to classify all
of these compounds into a few, well-defined groups, based upon
similarity of chemical composition. These groups are known,
respectively, as the carbohydrates and their derivatives, the glu-
cosides and tannins; the fats and waxes; the essential oils and
resins; organic acids and their salts; the proteins; the vegetable
bases and alkaloids; and the pigments. A consideration of these

THE ORGANIC COMPONENTS OF PLANTS 19

groups of compounds, as they are synthetized by plants, consti-
tutes the major portion of the study of the chemistry of plant
life as presented in this book. Following the discussion of
the compounds themselves, the chapters dealing with enzymes,
with the colloidal nature of protoplasm, and with the supposed
accessory stimulating agencies, aim to show how the manufac-
turing machine known as the plant cell accomplishes its remark-
able results, so far as the process is now understood.

PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE

In connection with the discussion of each of the above-men-
tioned groups of organic components of plants, an attempt will be
made to point out what significance these particular compounds
have in the plant's life and growth. Certain terms will be used
to designate different roles, which it is probably necessary to
define.

There may be two possible explanations of, or reasons for, the
presence of any given type of compound in the tissues of any partic-
ular species of plant. First, it may be supposed that this partic-
ular type of compounds is elaborated by the plant to satisfy its
own physiological needs, or for the purpose of storing it up in the
seeds as synergic food for the growth of the embryo, in order to
reproduce the species. For this role of the various organic food
materials, etc., we will employ the term " physiological use."
On the other hand, it is often conceivable that certain types of
compounds, which have properties that make them markedly
attractive (or repellent) as a food for animals and men, or which
are strongly antiseptic in character, or which have some other
definite relationship to other living organisms, have had much
to do with the survival of the particular species which elaborates:
them, in the competitive struggle for existence; or have been
developed in the plant by the evolutionary process of " natural
selection." For this relation of the compound to the plant's
vital needs, we will use the term " biological significance." Such
a segregation of the roles which the different compounds play in
the plant's economy may be more or less arbitrary in many cases ;
but it will be clear that when physiological uses are discussed, refer-
ence is being made to the plant's own internal needs; while the
phrase biological significance will be understood to refer to the
relation of the plant to other living organisms.

20 CHEMISTRY OF PLANT LIFE

PHYSIOLOGICAL USES OF THE ORGANIC COMPONENT GROUPS

From the standpoint of the role which each plays in the plant
economy, the several groups of organic compounds may be
roughly divided into three classes. These are: (a) the frame-
work materials, including gums, pectins, and celluloses; (6) syn-
ergic foods, including carbohydrates, fats, and proteins; and (c)
the secretions, including the glucosides, volatile oils, alkaloids,
pigments, and enzymes.

The framework material, as the name indicates, constitutes
the cell-wall and other skeleton substances of the plant. It is
made up of carbohydrate complexes, produced by the cell proto-
plasm from the simpler carbohydrates.

The synergic foods, or " reserve foods " as they are sometimes
called, produced by the excess of synthetized material over that
needed for the immediate use of the plant, are accumulated either
in the various storage organs, to be available for future use by
the plant itself or by its vegetative offspring, or in the seed, to
be available to the young seedling of the next generation. Pro-
teins not only serve as reserve food materials but also make up
the body of the living organism itself. Carbohydrates and fats
serve as synergic and reserve foods.

The secretions may be produced either in ordinary cells and
found in their vacuoles, or in special secretory cells and stored in
cavities in the secreting glands (as in the leaves of mints, skin of
oranges, etc.), or in special ducts (as in pines, milkweeds, etc.)
or on the epidermis (as the " bloom " of plums, cabbages, etc.,
the resinous coating of many leaves, etc.). As a general rule,
the glucosides, pigments, and enzymes are the products of unspe-
cialized cells and have some definite connection with the metabolic
processes of the plant; while the volatile oils and the alkaloids
are usually secreted by special cells and have no known role in
metabolism.

CHAPTER III
PHOTOSYNTHESIS

PHOTOSYNTHESIS is the process whereby chlorophyll-containing
plants, in the presence of sunlight, synthetize organic compounds
from water and carbon dioxide. The end-product of photosyn-
thesis is always a carbohydrate. Chemical compounds belonging
to other groups, mentioned in the preceding chapter, are synthe-
tized by plants from the carbohydrates and simple raw materials;
but in such cases the energy used is not solar energy and the process
is not photosynthesis.

Under the ordinary conditions of temperature, moisture supply,
etc., necessary to plant growth, photosynthesis will take place if
the three essential factors, chlorophyll, light,, and carbon dioxide
are available.

PHYSIOLOGICAL STEPS IN PHOTOSYNTHESIS

There are five successive and mutually dependent steps in the
process of photosynthesis, as follows:

(1) There must be a gas exchange between the plant tissue
and the surrounding air, by means of which the carbon dioxide of
the air may reach the protoplasm of the chlorophyll-containing
cells.

(2) Radiant energy must be absorbed, normally that of sun-
light, although photosynthesis can be brought about by the
energy from certain forms of artificial light.

(3) Carbon dioxide and water must be decomposed by the
energy thus absorbed, and the nascent gases thus produced com-
bined into some synthetic organic compound, with a resultant
storage of potential energy.

(4) This first organic synthate must be condensed into some
carbohydrate suitable for translocation and storage as reserve
food.

(5) The oxygen, which is a by-product from the decomposition

22 CHEMISTRY OF PLANT LIFE

of the water and carbon dioxide and the resultant synthetic process,
must be returned to the air by a gas exchange.

Of the five steps in this process, the first two and the last are
essentially purely physical phenomena, the chemical changes
involved being those of the third and fourth steps. Hence, it is
only these two parts of the process which need be taken into
account in a consideration of the chemistry of photosynthesis.

FORMALDEHYDE, THE SIMPLEST CARBOHYDRATE STRUCTURE

The simplest carbohydrates known to occur commonly in
plant tissues are the hexoses (see Chapter IV) having the formula
CeHi2O6, which is just six times that of formaldehyde, CH2O.
Also, it is known that formaldehyde easily, and even sponta-
neously, polymerizes into more complex forms having the general
formula (CH2O)n; trioxymethylene, CsHeOs, being a well-known
example. Further, both trioxymethylene and formaldehyde
itself can easily be condensed into hexoses, by simple treatment
with lime water as a catalytic agent. Hence, it is commonly
believed that formaldehyde is the first synthetic product resulting
from photosynthesis, that this is immediately condensed into
hexose sugars, and that these in turn are united into the more
complex carbohydrate groups which are commonly found in plants
(see Chapter IV).

There is considerable experimental confirmation of the sound-
ness of this view. The whole photosynthetic process takes place
in chlorophyll-containing plant tissues with astonishing rapidity,
sugars, and even starch, appearing in the tissues almost imme-
diately after their exposure to light in the presence of carbon
dioxide. Hence, any intermediate product, such as formaldehyde,
is present in the cell for only very brief periods and in very small
amounts. But small amounts of formaldehyde can often be
detected in fresh green plant tissues and, as will be pointed out
below, the whole process of photosynthesis, proceeding through
formaldehyde as an intermediate product, can be successfully
duplicated in vitro in the laboratory.

Assuming, then, that formaldehyde is the first photosynthetic
product in the process of the production of carbohydrates from
water and carbon dioxide, the simple empirical equation for this
transformation would be

H2O+C02 = CH2O+02.

PHOTOSYNTHESIS 23

It is apparent, however, that the process is not so simple as
this hypothetical reaction would indicate, as water and carbon
dioxide can hardly be conceived to react together in any such
simple way as this. Various theories as to the exact nature of the
steps through which the chemical combinations proceed have been
advanced. A discussion of the experimental evidence upon which
these are based and of the conclusions which seem to be justified
from these experimental studies is presented below. The only
value which may be attached to the empirical equation just
presented is that it does accurately represent the facts that a
volume of oxygen, equal to that of the carbon dioxide consumed
in the process, is liberated and that formaldehyde is the synthetical
product of the reactions involved.

It should be noted, in this connection, that formaldehyde
is a powerful plant poison and that few, if any, plant tissues
can withstand the toxic effect of this substance when it is present
in any considerable concentration. Hence, it is necessary to this
whole conception of the relation of formaldehyde to the photo-
synthetic process, to assume that, however rapidly the formalde-
hyde may be produced in the cell, it is immediately converted
into harmless carbohydrate forms.

THE CONDENSATION OF FORMALDEHYDE INTO SUGARS

As has been mentioned, it is easily possible to cause either
formaldehyde, or trioxymethylene, to condense into CeH^Oe,
using milk of lime as a catalyst. Of course, no such condition
as this prevails in the plant cell, and the mechanics of the proto-
plasmic process may be altogether different from those of the
artificial syntheses. Furthermore, the hexose produced by the
artificial condensation of these simpler compounds is, in every
case, a non-optically active compound, while all natural sugars
are optically active (see Chapter IV). Emil Fischer has suc-
ceeded, however, by a long and round-about process which need
not be discussed in detail here, in converting the artificial hexose
into glucose and fructose, the optically-active sugars which occur
naturally in plant tissues. The condensation of formaldehyde
directly into glucose and fructose in the plant cell is brought
about by some process the nature of which is not yet understood.
Probably synthetic enzymes (see Chapter XIV), whose nature

24 CHEMISTRY OF PLANT LIFE

and action have not yet been discovered, come into play. It is a
noteworthy fact, however, that the mechanics of this apparently
simple chemical change, upon which the whole nutrition of the
plant depends, and which furnishes the whole animal kingdom,
including the human race, with so large a proportion of its food
supplies, is as yet wholly unknown.

It is the common practice to represent the whole results
of the photosynthetic action by the empirical equation

6H20+6C02 = C6Hi206+602;

but here again the only value to be attached to such an algebraic
expression is that it accurately represents the gaseous exchange of
carbon dioxide and oxygen involved in the process. Certainly,
it throws no light upon the nature of the process itself.

THEORIES CONCERNING PHOTOSYNTHESIS

The many theories which have been advanced concerning the
nature of the chemical changes which are involved in photosyn-
thesis have served as the basis for much experimental study of the
problem. The following brief summary will serve to point out
the general trend of these investigations and the present state of
knowledge concerning the chemistry of photosynthesis.

Von Baeyer, in 1870, advanced the hypothesis that the first
step in the process is the breaking down of carbon dioxide into
carbon monoxide and oxygen and of water into hydrogen and
oxygen; that the carbon monoxide and hydrogen then unite to
produce formaldehyde, which is immediately polymerized to form
a hexose. These theoretical changes may be represented by the
following equations:

|CO2 =CO+0
• \H2O =H2+O

2. H2+CO = CH2O

3. 6(CH20) = C6H1206

In the investigations and discussions of this hypothesis, it
has been ascertained: first, that carbon monoxide has never been
found in the free form in plant tissues; second, that when Tropaeo-
lum plants were surrounded with an atmosphere in which there

PHOTOSYNTHESIS 25

was no carbon dioxide, but which contained sufficient carbon
monoxide to give a concentration of this gas in the cell-sap equiva-
lent to that hi which CC>2 is normally present, the plants grew
normally and apparently elaborated starch; third, other and more
extensive experiments indicated, however, that green plants in
general cannot make use of carbon monoxide gas for photosyn-
thesis, although this does not prove that von Baeyer's idea that
CO is a step in the process is necessarily erroneous; and finally
it was shown that carbon monoxide, in sufficient concentration
to produce the results with Tropaeolum mentioned above, usually
acts as a powerful anaesthetic towards most other plants. While
these considerations do not positively prove that von Baeyer's
hypothesis is incorrect, they render it so improbable that it has
generally been abandoned in favor of others which are described
below.

Erlenmeyer, even before the experimental work mentioned
in the preceding paragraph had been reported, suggested that in-
stead of assuming a separate breaking down of the carbon dioxide
and water, it is easier to conceive that they are united in the
cell-sap into carbonic acid and that this is reduced by the
chlorophyll-containing protoplasm into formic acid and then to
formaldehyde, as indicated by the following equations:

2. H2CO2 = CH20+0

Like von Baeyer's hypothesis, this assumes that formaldehyde
and oxygen are the first products of photosynthesis.

Proceeding upon this assumption, many investigators have
studied the question as to whether formaldehyde actually is
present in green leaves. Several workers have reported successful
identification of formaldehyde in the distillate from green leaves;
while others have criticized these results and have maintained
that formaldehyde can likewise be obtained by distilling decoc-
tions of dry hay, etc., in which the photosynthetic process could
not possibly be conceived to be at work. Other investigators,
notably Bach and Palacci, reported that they had succeeded in
artificially producing formaldehyde from water and carbon diox-
ide, in the presence of a suitable catalyzer or sensitizer. Euler,

26 CHEMISTRY OF PLANT LIFE

however, later showed conclusively that under the conditions
described by these investigators, formaldehyde can be obtained
even if no carbon dioxide is present, being apparently produced by
the action of water upon the organic sensitizer which was used.

These conflicting reports led Usher and Priestley, in a series of
studies reported between 1906 and 1911, to submit the whole
matter to a critical review. Briefly, these investigators showed
that the photolysis of carbon dioxide and water results in the for-
mation of formaldehyde and hydrogen peroxide, as represented
by the equation

C02+3H2O = CH20+2H2O2.

The formaldehyde is then condensed by the protoplasm into
sugars, while the hydrogen peroxide is decomposed, by an enzyme
in the plant cell, into water and oxygen. If the formaldehyde is
not used up rapidly enough by the protoplasm, it kills the enzyme
and the undecomposed hydrogen peroxide destroys the chloro-
phyll, which stops the whole photosynthetic process. Usher and
Priestley were able to cause the photolysis of carbon dioxide and
water into formaldehyde outside of a green plant, in the presence
of a suitable catalyzing agent which continually destroys the
hydrogen peroxide as fast as it is formed; to show the actual
bleaching effect of an excess of hydrogen peroxide in plant tissues
which had been treated in such a way as to prevent the enzyme
from decomposing it; and, finally, to demonstrate the condensa-
tion of formaldehyde into starch by the action of protoplasm which
contained no chlorophyll.

In the meantime, Fenton, in 1907, found that in the presence
of magnesium as a catalyst (it will be shown in Chapter VIII that
magnesium is a constituent of the chlorophyll molecule) formalde-
hyde may be obtained from a solution of carbon dioxide in water,
especially if weak bases are present.

Further, Usher and Priestley's later results showed that
radium emanations, acting upon a solution of carbon dioxide in
water, produce hydrogen peroxide and formaldehyde, and the
latter polymerizes but not up to the point represented by the
hexose sugars; also,, that the ultra-violet rays from a mercury
vapor lamp are very effective in bringing about the production of
hydrogen peroxide and formaldehyde from a saturated aqueous

PHOTOSYNTHESIS 27

solution of carbon dioxide, the reaction taking place even in the
absence of any " sensitizer," but much more readily if some
" optical " or " chemical " sensitizer is present. Finally, these
investigators were able to duplicate all their results, using green
plant tissues, and to show that the temperature changes which
take place in a film of chlorophyll when it is exposed to an atmos-
phere of moist carbon dioxide in the sunlight are such as would be
required by the formation of formaldehyde and hydrogen peroxide
from carbonic acid.

More recently, Ewart has showed that formaldehyde can com-
bine chemically with chlorophyll; from which fact, Schryver
deduces the theory that if for any reason the condensation of
formaldehyde into carbohydrates by the cell protoplasm does not
proceed as rapidly as the formaldehyde is produced by photo-
synthesis, the excess of the latter enters into combination with the
chlorophyll, and that if condensation into sugar uses up all the free
formaldehyde which is present in the active protoplasm, the com-
pound of formaldehyde with chlorophyll is broken down setting
free an additional supply for further sugar manufacture. Accord-
ing to this conception there are, in the chlorophyll-bearing proto-
plasm, not only the agencies for the production of formaldehyde
from carbon dioxide and water and for the condensation of this
into carbohydrates, but also a chemical mechanism by means of
which the amount of free formaldehyde in the reacting mass may
be regulated so that at no time will it reach the concentration
which would be injurious to the cell protoplasm or fall below the
proper proportions for sugar-formation. This explanation affords
a satisfactory solution of the difficulty which formerly confronted
the students of photosynthesis, namely, the fact that free formal-
dehyde is powerfully toxic to cell protoplasm. Without some such
conception, it was difficult to imagine how the presence of formal-
dehyde in the cell contents, even as a transitory intermediate
product, could be otherwise than injurious.

As a result of these studies, the nature of the chemical changes
which result in the production of formaldehyde as the first product
of photosynthesis, with the liberation of a volume of oxygen
equal to that of the carbon dioxide consumed, seems to be fairly
well established.

28 CHEMISTRY OF PLANT LIFE

THE PRODUCTION OF SUGARS AND STARCHES

The next step in the process, the conversion of formaldehyde
into sugars and starches, is not necessarily a p/iofosynthetic one,
as it can be brought about by protoplasm which contains no
chlorophyll or other energy-absorbing pigment. It is, however,
a characteristic synthetic activity of living protoplasm. There
is little definite knowledge as to how the cell protoplasm accom-
plishes this important task. As has been pointed out, the polym-
erization of formaldehyde into a sugar-like hexose, known as
" acrose," can be easily accomplished by ordinary laboratory
reactions, and acrose can be converted into glucose or fructose by
a long and difficult series of transformations. But such processes
as are employed in the laboratory to accomplish these artificial
synthesis of optically-active sugars from formaldehyde can have
no relation whatever to the methods of condensation which are
used by cell protoplasm in its easy, almost instantaneous, and
nearly continuous accomplishment of this transformation. Fur-
thermore, these simple hexoses are by no means the final products
of cell synthesis, even of carbohydrates alone. In many plants,
starch appears as the final, if not the first, product of formalde-
hyde condensation. At least, the transformation of the simple
sugars, which may be supposed to be the first products, into starch
is effected so nearly instantaneously that it is impossible to detect
measurable quantities of these sugars in the photosynthetically
active cells of such plants. Other species of plants always show
considerable quantities of simple sugars in the vegetative tissues,
and some even store up their reserve carbohydrate food material
in the form of glucose or sucrose. Attempts have been made to
associate the type of carbohydrate formed in cell synthesis with
the botanical families to which the plants belong, but with no
very great success. For each individual species, however, the
form of carbohydrate produced is always the same, at least under
normal conditions of growth. For example, the sugar beet always
stores up sucrose in its roots, although under abnormal conditions
considerable quantities of raffinose are developed. Similarly,
potatoes always store up starch, but with abnormally low tem-
peratures considerable quantities of this may be converted into
sugar, which becomes starch again with the return to normal con-
ditions.

PHOTOSYNTHESIS 29

While it is impossible, with our present knowledge, to even
guess at the mechanism by which protoplasm condenses formalde-
hyde into sugars and these, in turn, into more complex carbo-
hydrates, the structure and relationships to each other of the final
products of photosynthesis are well known, and are discussed at
length in the following chapter.

References

BARNES, C. R.— "Physiology" (Part II of Coulter, Barnes and Cowles' "Text-
book of Botany"), 187 pages, 18 figs., Chicago, 1910.

GANONG, W. F.— "Plant Physiology," 265 pages, 65 figs., New York, 1908
(2ded.).

JOST, L., trans, by GIBSON, R. J. H — "Plant Physiology," 564 pages, 172 figs.,
Oxford, 1907.

MARCHLEWSKI, L. — "Die Chemie der Chlorophyll," 187 pages, 5 figs., 7 plates,
Berlin. -1909.

PARKIN, JOHN. — "The Carbohydrates of the Foliage Leaf of the Snowdrop
(Galanthus nivalis L.) and their Bearing on the First Sugar of Photosyn-
thesis," in Biochemical Journal, Vol. 6, pages 1 to 47, 1912.

PFEFFER, W., trans, by EWART, A. J.— "Physiology of Plants." Vol. I, 632
pages, 70 figs., Oxford, 1900.

CHAPTER IV
CARBOHYDRATES

THESE substances comprise an exceedingly important group of
compounds, the members of which constitute the major proportion
of the dry matter of plants. The name " carbohydrate " indi-
cates the fact that these compounds contain only carbon, hydro-
gen, and oxygen, the last two elements usually being present in
the same proportions as in water. As a rule, natural carbo-
hydrates contain six, or some multiple of six, carbon atoms and
the same number of oxygen atoms less one for each additional
group of six carbons above the first one; e.g., CeH^Oe, Ci2H220n,
Ci8H32Oi6, etc.

Carbohydrates are classed as open-chain compounds, that is,
they may be regarded as derivatives of the aliphatic hydrocarbons.
From the standpoint of the characteristic groups which they
contain, they are aldehyde-alcohols. In common with many other
poly-atomic open-chain alcohols, they generally possess a charac-
teristic sweet, or mildly sweetish, taste. In the case of the more
complex and less soluble forms, this sweetish taste is scarcely
noticeable and these compounds are commonly called the
" starches," as contrasted with the more soluble and sweeter forms,
known as " sugars."

The characteristic ending ose is added to the names of the
members of this group. As systematic names, the Latin numeral
indicating the number of carbon atoms in the molecule is com-
bined with this ending; e.g., CsHioOs, pentose, CeH^Oe, hexose,
etc.

In recent years, as a matter of scientific interest, many sugar-
like substances which contain from two to nine carbon atoms com-
bined with the proper number of hydrogen and oxygen atoms to
be equivalent to the same number of molecules of water in each
case, have been artificially prepared in the laboratory and desig-
nated as dioses, trioses, tetroses, pentoses, hexoses, heptoses,

CARBOHYDRATES 31

octoses, and nonoses, respectively. Substances corresponding in
composition and properties with the artificial tetroses and one or
two derivatives of heptoses are occasionally found in plant tissues,
and a considerable number of pentoses and their condensation
products are common constituents of plant gums, etc.; but the
great majority of the natural carbohydrates are hexoses and their
derivatives.

GROUPS OF CARBOHYDRATES

Since the simpler carbohydrates are sugars, i.e., they possess
the characteristic sweet taste, the name " saccharide " is used as a
basis for the classification of the entire group. The simplest
natural sugars, the hexoses, CeH^Oe, are known as mono-sacchar-
ides. The group of next greater complexity, those which have the^^,
formula Ci2H22On and may be regarded as derived from the
combination of two molecules of a hexose with the dropping out
of one molecule of water at the point of union, are known as
di-saccharides. Compounds having the formula CisH^Oie (i.e.,
three molecules of CeH^Oe minus two molecules of H20) are
tri-saccharides; and the still more complex groups, having the
general formula (CeHioOs)^, are called the poly-saccharides.
The mono-, di-, and tri-saccharides are generally easily soluble in
water, have a more or less pronouncedly sweet taste, and are
known as the sugars; while the polysaccharides are generally
insoluble in water and of a neutral taste, and are called starches.
As will be seen later, there are many natural plant carbohydrates
belonging to each of these groups.

In addition to these saccharide groups, there are other types,
or groups, of compounds which resemble the true carbohydrates in
their chemical composition and properties and are often considered
as a part of this general group. These are the pentoses,
and their condensation products, the pentosans (
and their methyl derivatives, CeH^Os; certain polyhydric alco-
hols having the formula C6Hg(OH)6; pectose and its derivatives,
pectin and pectic acid; and lignose substances of complex compo-
sition. It is doubtful whether these compounds are actual
products of photosynthesis in plants, or have the same physiological
uses as the carbohydrates and it has seemed wise to consider
them in a separate and later chapter.

32 CHEMISTRY OF PLANT LIFE

ISOMERIC FORMS OF MONOSACCHARIDES

Four sugars having the formula CoH^Oe, namely, glucose,
fructose, mannose, and galactose, occur very commonly and
widely distributed in plants. In addition to these, thirteen others
having the same percentage composition have been artificially
prepared, while seven additional forms are theoretically possible.
In other words, twenty-four different compounds, all having the
same empirical formula and similar sugar-like properties are
theoretically possible. In order to arrive at a conception of this
multiplicity of isomeric forms, it is necessary to understand the
two types of isomerism which are involved. One of these is
structural isomerism, and the other is space- or sfereo-isomerism.

Structural Isomerism. — This refers to an actual difference
in the characteristic groups which are present in the molecule.
As has been said, all carbohydrates, from the standpoint of the
characteristic groups which they contain, are aldehyde-alcohols.
The hexoses all contain five alcoholic groups and one primary
aldehyde, or one secondary aldehyde (ketone), group. If the
aldehyde oxygen is attached to the carbon atom which is at the
end of the six-membered chain, the structural arrangement is

that of an aldehyde, C = 0, and the sugar is of the type known

as "aldoses"; whereas, if the oxygen is attached to any other

carbon in the chain, the ketone arrangement, C = O, results and

the sugar is a " ketose." This difference is illustrated in the
Fischer open-chain formulas for glucose (an aldose) and fructose
(a ketose) as follows:

Glucose Fructose

CH2OH CH2OH

CHOH CHOH

CHOH C = 0

CHO CH2OH

CARBOHYDRATES

Stereo-isomerism, or space isomerism, as its name indicates,
depends upon the different arrangement of the atoms or groups in
the molecule in space, and not upon any difference in the character
of the constituent groups. This possibility depends upon the
existence in the molecule of the substance in question of one or
more asymmetric carbon atoms and manifests itself in differences
in the optical activity of the compound.* Thus, in the formula
for glucose shown above there appear four asymmetric carbon
atoms, namely, those of the four secondary alcohol groups (hi
the terminal, or primary alcohol, group, carbon is united to hydro-
gen by two bonds, and in the aldehyde group it is united to oxygen
by two bonds). Similarly, fructose contains three asymmetric
carbon atoms.

As an example of how the presence of these asymmetric carbon
atoms results in the possibility of many different space relation-
ships, the following graphic illustrations of the supposed differ-
ences between dextro-glucose and levo-glucose, and between
dextro- and levo-galactose, may be cited, f

d-glucose

Z-glucose

d-galactose

Z-galactose

CH2OH

R—C— OH

H— C— OH

HO— €— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

CHO

* It is assumed that the reader, or student, is familiar with the theoretical
and experimental evidence in support of the existence of the so-called " asym-
metric " carbon atom and its relation to the effect of the compound which
contains it, when in solution, hi rotating the plane of polarized light. For
purposes of review, or of study of this most interesting and important phe-
nomenon, the reader is referred to any standard text-book on Organic Chem-
istry.

t Attention should be called, at this point, to the fact that such formulas
as these cannot possibly accurately represent the actual arrangement of the
constituent groups of a carbohydrate molecule around an asymmetric carbon
atom. The limitations of a plane-surface formula prevent any illustration of
the three-dimension relationships in space. Furthermore, there are certain
facts in connection with the birotation phenomenon and the relation of the
molecular configuration to biochemical properties (which see) that cannot be

34 CHEMISTRY OF PLANT LIFE

Comparisons of the above formulas will show that the differ-
ence between the formulas for d- and Z-glucose lies in the arrange-
ment of the H atoms and the OH groups around the two asymmet-
ric carbon atoms next the aldehyde end of the chain; while the
d- and Z-galactoses differ in that this arrangement is in the reverse
order around all four of the asymmetric carbons. By similar
variations in the grouping around the four asymmetric atoms, it
is possible to produce the sixteen different space arrangements
shown on page 37 for the groups of an aldohexose. Sugars
corresponding to fourteen of these different forms have been dis-
covered, three of which are of common occurrence in plants, either
as single mono-saccharides or as constituent groups in the more
complex carbohydrates; the remaining two forms have only
theoretical interest.

explained on the basis of the open-chain arrangement represented by the
Fischer formulas used here. A closed-ring arrangement, showing the alde-
hyde oxygen as linked by its two bonds to the first and the fourth carbon atoms
of the chain, thus forming a closed-ring of four carbon and one oxygen atoms,
instead of being attached by both bonds to a single carbon atom, as in the
above formulas, is undoubtedly a more nearly accurate representation of the
actual linkage in the molecule than are the open-chain formulas used above.

The differences in conception embodied by these two types of formulas may
be shown by the following formulas for glucose:

CH2OH CH2OH
CHOH CHOH
CHOH CH— CHOH i O

ivy i
__ _._ _ H- CHOH- CHOH. CHOH

!H-C]

CHOH CH— CHOH
CHO OH

Fischer's Closed-ring formulas

formula

It will be observed that in the closed-ring formula there are five asymmetric
carbon atoms, and the asymmetry of the terminal one forms the basis for the
explanation of the existence of the so-called a and /3 modification of d-glucose
(see page 46). However, the ordinary aldehyde reactions of the sugars are
more clearly indicated by the open-chain formula. Some investigators are
inclined to believe that these sugars actually exist in the open-chain arrange-
ment when in aqueous solution, and in the closed-ring arrangement when in
alcoholic solution. The closed-ring formulas will be used in this text in the
discussions of the birotation phenomena and of biochemical properties, but
for the explanations of the stereo-isomeric forms and similar phenomena, the
open-chain formulas are just as useful in conveying an idea of the possibilities
of different space relationships, and are so much simpler in appearance and in
mechanical preparation, that it seems desirable to use these rather than the
more accurate closed-ring formulas.

CARBOHYDRATES 35

Similarly, for a ketohexose, which contains three asymmetric
carbon atoms, there are eight possible arrangements. Three
sugars of this type are known, only one (fructose) being common in
plants; the others are of only theoretical interest.

CHEMICAL CONSTITUTION OF MONOSACCHARIDES

The term " monosaccharides," as commonly used, refers to
hexoses. It applies equally well, however, to any other sugar-like
substance which either occurs naturally or results from the decom-
position of more complex carbohydrates, and which cannot be
further broken down without destroying its characteristic alde-
hyde-alcohol groups and sugar-like properties.

All such monosaccharides, being alcohol-aldehydes, can easily
be reduced to the corresponding polyatomic alcohols, containing
the same number of carbon atoms as the original monosaccharides,
each with one OH group attached to it. All aldose monosac-
charides are converted, by gentle oxidation, into the corresponding
monobasic acid, having a COOH group in the place of the original
CHO group. Further ' oxidation either changes the alcoholic
groups into COOH groups, producing polybasic acids, or breaks
up the chain. When ketose monosaccharides are submitted to
similar oxidation processes, they are broken down into shorter
chain compounds.

The various monosaccharides which have thus far been found
as constituents of plant tissues, or as parts of other more complex
compounds which occur in plants, are shown in the following table:

Trioses (C3H603) Tetroses (C4H8O4)

Aldose — Glyceric aldehyde, Aldoses — d- and Z-Erythrose,

or glycerose Z-Threose

Ketose — Dioxyacetone

Pentoses (C5Hi005) Methyl Pentoses (C6Hi205)

Aldoses — d- and Z-Arabinose Aldoses — Rhamnose
d- and Z-Xylose Fucose

/-Ribose Rhodeose

Z-Lyxose Chinovose

36 CHEMISTRY OF PLANT LIFE

Hexoses

Mannitol series Dulcitol series

Aldoses — d- and Z-Glucose d- and /-Galactose

d- and Z-Mannose d- and Z-Talose

d- and Z-Gulose
d- and Z-Idose

d-Altrose
d-Allose

Ketoses — d-Fructose d-Tagatose

d-Sorbose

Heptoses (C7Hi4O7) Octoses (C8Hi6O8) Nonoses (C9Hi809)

Glucoheptose Gluco-octose Glucononose

Mannoheptose Manno-octose Mannononose

Galactoheptose Galacto-octose

Persuelose

Sedoheptose

The hexoses are by far the most important group of monosac-
charides. They are undoubtedly the first products of photo-
synthesis, and all the other carbohydrates may be considered to
be derived from them by condensation. Because of their bio-
chemical significance and their immense importance as the fun-
damental substances for all plant and animal energy-producing
materials, the following detailed studies of their chemical compo-
sition and molecular configuration are fully warranted.

That all the hexoses contain five alcoholic groups is proved
by the experimental evidence that each one forms a penta-ester,
by uniting with five acid radicals, when treated with mineral or
organic acids under proper conditions. Thus, glucose penta-
acetate, penta-nitrate, penta-benzoate, etc., have all been pre-
pared. The presence of the aldehyde group is proved by the fact
that all aldohexoses have been converted, by gentle oxidation,
into pentaoxy-monobasic acids, and the ketohexoses broken down
into shorter chain compounds by similar gentle oxidations; these
reactions being characteristic of compounds containing an alde-
hyde and a ketone group respectively. This experimental evi-
dence establishes the nature of the characteristic groups in the
molecule, in each case.

The molecular configurations illustrated in the following table
are those suggested by Emil Fischer, as a result of his exhaustive

CARBOHYDRATES

studies of the chemical constitution of the various carbohydrates.
There is, of course, no thought that the printed formulas here pre-
sented accurately represent the actual relationships in space of the
different groups; but there is fairly conclusive evidence that the
variations in special groupings in the different sugars are properly
referable to the particular asymmetric carbon atoms as indicated
in the several formulas as presented.

1. Aldohexoses of the mannitol series:

d-Glucose
CH2OH

Z-Glucose
CH2OH

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

CHO

d-Gulose

Z-Gulose

CH2OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

CHO

d-Altrose

Z-Altrose
(unknown)

CH2OH

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

HO— C— H

H— C— OH

CHO

d-Mannose
CH2OH

H— C— OH

/-Mannose
CH2OH

HO— C— H
H— C— OH HO— C— H
HO— C— H H— C— OH
HO— C— H

CHO

d-Idose
CH2OH

H— C— OH

CHO

Wdose
CH2OH

HO— C— H H— C— OH

H— C— OH HO— C— H
HO— C— H

H— C— OH
CHO

d-Allose
CH2OH

H— C— OH

H— C— OH
HO— C— H

CHO

Z-AUose
(unknown)

CH2OH
HO— C— H

H— C— OH HO— C— H

4-H

H— C— OH HO
H— C— OH HO— C— H
CHO CHO

CHEMISTRY OF PLANT LIFE

2. Aldohexoses of the dulcitol series:

d-Galactose
CH2OH

H— C— OH

J-Galactose
CH2OH

HO— C— H

H— C— OH

HO— C— JHL

H— C— OH

HO— C— H

CHO

d-Talose
CH2OH

H— C— OH
HO— C— H
HO— C— H
HO— C— H
CHO

Z-Talose
CH2OH

HO— C— H
H— C— OH
H— C— OH
H— C— OH
CHO

3. Ketohexoses:

d-Fructose
CH2OH

H— C— OH
H— C— OH
HO— C— H
C=0
CH2OH

d-Sorbose
CH2OH

HO— C— H

H— C— OH

d-Tagatose
CH2OH

H— C— OH

HO— C— H

H2OH

CH2OH

Reference will be made in subsequent paragraphs to the prob-
able chemical constitution of the monosaccharides other than
hexoses; but the above discussion of the structure of the hexoses
will serve as a sufficient introduction to the study of the compo-
sition of the common carbohydrates.

CHARACTERISTIC REACTIONS OF HEXOSES

Specific Rotatory Power. — All soluble carbohydrates, since
they contain asymmetric carbon atoms, with the consequent
larger groups on one side of the molecule than the other, rotate
the plane of polarized light when it passes through a solution of
the carbohydrate in question. The amount of the rotation depends
upon the nature of the carbohydrate, the concentration of the
solution, and the length of the column of solution through which
the ray of polarized light passes. But the same definite amount

CARBOHYDRATES 39

of the same sugar, dissolved in the same volume of water, and
placed in a tube of the same length, will always cause the same
angular deviation, or rotation, of the plane in which the polarized
light which passes through it is vibrated. In other words, the
same number of molecules of the optically active substance in
solution will always produce the same rotatory effect. This is
called the specific rotatory power of the substance in question.
It is expressed as the number of degrees of angular deviation of the
plane of polarized light caused by a column of the solution exactly
200 mm. in length, the concentration of the solution being 100
grams of substance in 100 cc. at a temperature of 20° C. Actual
determinations of specific rotatory power are usually made with
solutions more dilute than this standard, and the observed devia-
tion multiplied by the proper factor to determine the effect which
would be produced by the solution of standard concentration. If
the direction of the deviation is to the right (i.e., in the direction in
which the hands of the clock move) it is spoken of as " dextro "
rotation and is indicated by the sign -f, or the letter d; while
if in the opposite direction, it is called " levo " rotation and indi-
cated by the sign — , or the letter I. For example, the specific
rotation of ordinary glucose is +52.7°; of fructose, —92°; of
sucrose, +66.5°.

Reducing Action. — All of the hexose sugars are active reducing
agents. This is because of the aldehyde group which they con-
tain. Many of the common heavy metals, when in alkaline solu-
tions, are strongly reduced when boiled with solutions of the hexose
sugars. Alkaline copper solutions yield a precipitate of red
cuprous oxide; ammoniacal silver solutions give silver mirrors;
alkaline solutions of mercury salts are reduced to metallic mer-
cury, etc. Any sugar which contains a potentially active alde-
hyde group will exhibit this reducing effect and is known as a
" reducing sugar." In some of the di- and tri-saccharides, the
linkage of the hexose components together is through the aldehyde
group, in such a way that it loses its reducing effect; such sugars
are known as " non-reducing." Advantage is taken of this prop-
erty for both the detection and quantitative determination of the
" reducing sugars." A standard alkaline copper solution of definite
strength, known as " Fehling's solution," is added to the solution
of the sugar to be tested and the mixture boiled, when the char-
acteristic brick-red precipitate appears. If certain standard

40 CHEMISTRY OF PLANT LIFE

conditions of volume of solutions used, length of time of boiling,
etc., are observed, the quantity" of cuprous oxide precipitated bears
a definite ratio to the amount of sugar which is present, so that
if the precipitate be filtered off and weighed under proper condi-
tions, the weight of sugar present in the original solution can be
calculated. The proper conditions for carrying on such a deter-
mination and tables showing the amounts of the various " reducing
sugars " which correspond to the weight of cuprous oxide found,
are given in all standard text-books dealing with the analysis
of organic compounds.

Fermentability. — The common hexoses are all easily fermented
by yeast, forming alcohol and carbon dioxide, according to the
equation

C6Hi206 = 2C2H5OH+2C02.

The importance and biochemical significance of this reaction will
be considered in detail in connection with the discussions of the
relation of molecular configuration to biochemical properties
(see page 56) and the nature of enzyme action (see page 194).

Formation of Hydrazones and Osazones. — Another property of
the hexoses which is due to the presence of an aldehyde group in
the molecule, is that of forming addition products with phenyl
hydrazine, known as " hydrazones " and " osazones." For exam-
ple, glucose reacts with phenyl hydrazine in acetic acid solution, in
two stages. The first, which takes place even in a cold solution
may be represented by the equation

: N-NH.C6H5+H20.

Glucose Phenyl-hydrazine uGlucose-hydrazone

The' structural relationships involved may be represented as
follows:

CHO H2N-NH CH— N - NH

I /\ I

(CHOH)4 + = (CHOH)4

CH2OH \/ CH2OH

The hydrazones of the common sugars, with the exception of the
one from mannose, are colorless compounds, easily soluble in

CARBOHYDRATES 41

water. Hence, they do not serve for the separation or identifica-
tion of the individual sugars. But if the solution in which they
are formed contains an excess of phenyl hydrazine and is heated
to the temperature of boiling water for some time, the alcoholic
group next to the aldehyde group (the terminal alcohol group in
ketoses) is first oxidized to an aldehyde and then a second molecule
of phenyl hydrazine is added on, as illustrated above, forming a
di-addition-product, known as an " osazone." The osazones are
generally more or less soluble in hot water, but on cooling they
crystallize out in yellow crystalline masses of definite melting
point and characteristic forms. All sugars which have active
aldehyde groups in the molecule form osazones. These afford
excellent means of identification of unknown sugars, or of dis-
tinguishing between sugars of different origin and type.

Glucose, mannose, and fructose all form identical osazones.
This is because the structure of these three sugars is identical
except for the arrangement within the two groups at the aldehyde
end of the molecule (see formulas on page 44). Since it is to
these two groups that the phenyl hydrazine residue attaches itself,
it follows that the resulting osazones must be identical in structure
and properties. All other reducing sugars yield osazones of differ-
ent physical properties.

When an osazone is decomposed by boiling with strong acids,
the phenyl hydrazine groups break off, leaving a compound con-
taining both an aldehyde and a ketone group. Such compounds
are known as " osones." The osones from glucose, mannose, and
fructose are identical. By carefully controlled reduction, either one
of the C = 0 groups of the osone may be changed to an alcoholic
group, producing thereby one of the original sugars again. Hence,
it is possible to start with one of these sugars, convert it into the
osone and then reduce this to another sugar, thereby accomplishing
the transformation of one sugar into another isomeric sugar.

Formation of Glucosides. — By treatment with a considerable
variety of different types of compounds, under proper conditions,
it is possible to replace one of the hydrogen atoms of the terminal
alcoholic group of the hexose sugars with the characteristic group
of the other substance, forming compounds known, respectively,
as glucosides, fructosides, galactosides, etc. The structural
relation of methyl glucoside to glucose, for example, may be illus-
trated as follows :

42 CHEMISTRY OF PLANT LIFE

Glucose (CeHiaOe) Methyl Glucoside (C7Hi4Oc)
CHO CHO

(CHOH)4 (CHOH)4

H2OH CHOH

A general formula for glucosides is R- (CHOH) 5- CHO; and the R
may represent a great variety of different organic radicals (see
the chapters dealing with Glucosides and with Tannins). When
the glucosides are hydrolyzed, they yield glucose and the hydroxyl
compound of the radical with which it is united. All the state-
ments which have been made with reference to glucosides, apply
equally well with reference to fructosides, galactosides, manno-
sides, etc.

It is possible, by various laboratory processes, to replace
additional hydrogen atoms in the glucose molecule with the same or
other organic radicals, thus producing glucosides containing two or
more R groups; but most of the natural glucosides contain only
one other characteristic group.

Oxidations. — When the hexoses are oxidized they give rise to
three different types of acids, depending upon the conditions of
the oxidation and the kind of oxidizing agent used. With glucose,
for example, the relationships involved may be illustrated as
follows :
CHO COOH CHO COOH

(CHOH)4 (CHOH)4 (CHOH)4 (CHOH)4

CH2OH COOH C<

CH2OH CH2OH COOH COOH

Glucose Gluconic acid Glucuronic acid Saccharic acid

An important property of the acids of the gluconic type is that
when heated with pyridine or quinoline to 130°-150° they undergo
a molecular rearrangement whereby the acid corresponding to
an isomeric sugar is produced. For example, gluconic acid, under
these conditions, becomes mannonic acid, which can be reduced to
mannose. The process is reversible; mannose can be converted
to mannonic acid, thence to gluconic acid, thence to glucose.
Similarly, galactonic acid can be converted into talonic acid, and
this to talose, and this process is reversible. These facts afford
another means of conversion of one sugar into another.

CARBOHYDRATES

From the standpoint of physiological processes, glucuronic acid
is the most interesting and important oxidation product of glucose.
It is often found in the urine of animals, as the result of the partial
oxidation of glucose in the animal tissues. Normally, glucose is
oxidized in the body to its final oxidation products, carbon dioxide
and water. But when many difficultly oxidizable substances,
such as chloral, camphor, turpentine oil, aniline, etc., are intro-
duced into the body, the organism has the power of combining
these with glucose to form glucosides. These so-called " paired "
compounds are then oxidized to the corresponding glucuronic
acid derivatives and eliminated from the body in the urine. No
phenomenon similar to this occurs in plants, however, and glu-
curonic acid has never been found in plant tissues.

Synthesis and Degradation of Hexoses. — Monosaccharides of
any desired number of carbon atoms can be produced from aldoses
having one less carbon atoms, by way of the familiar " nitrile "
reaction. Aldoses, like all other aldehydes, combine directly with
hydrocyanic acid, forming compounds known as nitriles, which
contain one more carbon atom than was present in the original
aldehyde; the cyanogen group can easily be converted into a
COOH group ; and this, in turn, reduced to an aldehyde, thus pro-
ducing an aldose with one more carbon atom than was present
in the initial sugar. These changes may be illustrated by the
folio whig equations:
(1) CHO + HCN

:ci

(2)

(CHOH)3
(CH2OH

Aldopentose

(CHOH)4

H2OH

Nitrile

COOH
(CHOH),

CH2OH

Acid

+ H20

- 0

CHOH-CN
(CHOH)3 or
CH2OH

Nitrile

COOH +
(CHOH)4

CN
(CHOH)<
CH2OH
NH3

CH2OH

Acid

CHO

= (CHOH)4
CH2OH

Aldohexose

It is possible, by this process, to advance step by step from
formaldehyde to higher sugars, Emil Fischer and his students

CHEMISTRY OF PLANT LIFE

having carried the process as far as the production of glucodecose
(CioH2oOio). It usually happens, however, that two stereo-
isomers result from the " step-up " by way of the nitrile reaction;
thus, arabinose yields a mixture of glucose and mannose, glucose
yeilds glucoheptose and mannoheptose, etc.

The reverse process, or the so-called " degradation " of a sugar
into another containing fewer carbon atoms, may be readily
accomplished in either one of two ways. In Wohl's process, the
aldehyde group of the sugar is first converted into an oxime, by
treatment with ammonia; the oxime, on being heated with con-
centrated sodium hydroxide solution, splits off water and becomes
the corresponding nitrile; this, on further heating, splits off HCN
and yields an aldose having one less carbon atom than the original
sugar. This process is the exact reverse of the nitrile synthesis,
described above. The second method of degradation, suggested
by Ruff, makes use of Fenton's method of oxidizing aldehyde
sugars to the corresponding monobasic acid, using hydrogen
peroxide and ferrous sulfate as the oxidizing mixture; the aldonic
acid thus formed is then converted into its calcium salt, which,
when further oxidized, splits off its carboxyl group and one of the
hydrogens of the adjacent alcoholic group, leaving an aldose having
one less carbon atom than the original aldose sugar.

Enolic Forms. — A final avenue for the interconversion of glu-
cose, mannose, and fructose into one another, is through the
spontaneous transformations which these undergo when dissolved
in water containing sodium hydroxide or potassium hydroxide.
This change is due to the conversion of the sugar, in the alkaline
solution, into an enol} which is identical for all three sugars, and
which may subsequently be reconverted into any one of the three
isomeric hexoses. The relationships involved are illustrated in
the following formulas:

CHO CHO CH2OH CHOH

— C

H— C— OH HO— C— H

C-OH

HO— C— H
H— C— OH
H— C— OH
CH2OH

Glucose

HO— C— H HO— C— H
H— C— OH H— C— OH

H— C— OH

?H2OH

Mannose

H— C— OH
CH2OH

Fructose

H— C— OH
CH2OH

Enolic Form

CARBOHYDRATES 45

The preceding technical discussion of the chemical consti-
tution and reactions of the hexoses has been presented, not because
it has any direct connection with the occurrence or functions of
these compounds in plant tissues, but for the purpose of giving to
the student a graphic conception of the structure and properties
of these simple carbohydrates, as a basis for the understanding of
the nature, properties, possible chemical reactions, syntheses,
etc., of the more complex types of carbohydrates, which, along
with these simple monosaccharides, constitute the most important
single group of organic components of plants.

THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES

Only two monosaccharides occur as such in plants. These are
glucose and fructose. All the other hexoses, whose structure is
shown on pages 37 and 38, occur in plants only as constituents of
the more complex saccharides, in glucoside-formations, or as
the corresponding polyatomic alcohols.

The aldo-hexoses which occur most commonly in plants, either
free or in combination, are d-glucose, d-mannose, and d-galactose;
while d-fructose and d-sorbose are the common keto-hexoses.

Glucose (often called also dextrose, fruit sugar, or grape sugar)
occurs widely distributed in plants, most commonly in the juices of
ripening fruits, where it is usually associated with fructose and
sucrose, the two hexoses being easily derived from sucrose by
hydrolysis. Glucose is also produced by the hydrolysis of many
of the more complex carbohydrates, by the action either of enzymes
or of dilute acids; lactose, maltose, raffinose, starch, and cellulose,
as well as many glucosides all yielding glucose as one of the products
of their hydrolysis. In all such cases, it is d-glucose which is
obtained.

Glucose is a crystalline solid (although it does not form such
sharply denned crystals as does sucrose, or " granulated sugar"),
which is easily soluble in water. It usually appears on the market
in the form of thick syrups, which are produced commercially by
the hydrolysis of starch with dilute sulfuric acid, removal of the
acid after the hydrolysis is complete, and evaporation of the
resulting solution to the desired syrupy consistency. (Since
corn starch is commonly used as the raw material for this process,
these syrups are often spoken of as " corn syrup.") The sweet-
ness of glucose is about three-fifths that of ordinary cane sugar.

46 CHEMISTRY OF PLANT LIFE

Glucose exhibits all the properties of hexoses which have been
described in general terms above. It is a reducing-sugar, and is
easily fermented. The specific rotatory power of d-glucose is
+52.7°. But when glucose is dissolved in water, it exhibits in a
marked degree the phenomenon known as " mutarotation" ; that
is, freshly made solutions exhibit a certain definite rotatory power,
but this changes rapidly until it finally reaches another definite
specific rotation. Jn other words, glucose is " birotatory," or
possesses two distinct specific rotatory powers, and the changing
rotation effect in aqueous solutions is due to the change from one
form to the other. When dissolved in alcohol, it does not exhibit
this change in rotatory power. In order to explain this phenom-
enon, it is necessary to assume that there are two modifications of
d-glucose, which have been designated respectively as the a and ft
forms. The possibility of the existence of these two forms is
explained by the assumption of the closed-ring arrangement of
the glucose molecule, as indicated in the following formulas
which represent the two possible isomeric arrangements:

HO— C— H H— C-OH

H2OH CH2OH

o-Glucose /3-Glucose

It is assumed that the a modification (with its specific rotatory
power of +105°) is the normal form for crystalline glucose, but
that when dissolved in water it is changed into an aldehydrol, i.e.,
a compound containing two additional OH groups, which later
breaks down again, into the /3 modification (with its specific rota-
tory power of +22°). When dissolved in alcohol, this change does
not take place because of the absence of the excess of water neces-
sary to produce the intermediate aldehydrol form.

CARBOHYDRATES 47

There are other examples of the existence of the a and 0
modification of glucose. For example, a-methyl-glucoside and
/3-methyl-glucoside (specific rotatory powers, +157° and —33°,
respectively) are both known, as well as several other similar
glucoside arrangements.

Mannose. — This sugar does not occur as such in plants; but
complex compounds which yield d-mannose when hydrolyzed,
known as " mannosans," are found in a number of tropical plant
forms. The mannose which is obtained from these by hydrolysis
is very similar to glucose in its properties, forms the same osazones
as do glucose and fructose, exhibits mutarotation, etc. Mannose
may also be obtained by oxidizing mannitol, a hexatomic alcohol,
known as " mannite," which occurs in many plants, especially in
the manna-ash (Fraxinus omus)y the dried sap from which is
known as " manna."

Galactose occurs in the animal kingdom as one of the constit-
uents of lactose, or milk-sugar. It is also one of the constituents
of raffinose, a trisaccharide sugar found in plants, and occurs
as " galactans " in many gums and sea-weeds. The d-galactose,
obtained by the hydrolysis of any of these compounds, is a faintly
sweet substance which resembles glucose in many of its properties;
having one characteristic difference, however, in that it forms
mucic acid instead of saccharic acid when oxidized by concen-
trated nitric acid. These oxidation products are very different
in their physical properties and this difference serves to dis-
tinguish between the two sugars from which they are derived.

Fructose (levulose, honey sugar, or " diabetic " sugar) occurs
along with glucose in the juices of many fruits, etc. It is a con-
stituent of sucrose, of raffinose, and of the polysaccharide inulin,
from which it may be obtained by hydrolysis. It is a ketose sugar,
reduces Fehling's solution, forms the same osazone as glucose, and
is easily fermentable by yeast. Its sweetness is slightly greater
than that of ordinary cane sugar, rf-fructose (the ordinary form)
is easily soluble in water, and is strongly levorotatory, its specific
rotatory power at 20° C. being —92.5°; it is unique in the very
large effect which is produced in its rotatory power by increasing
the temperature of the solution; at 82° its specific rotatory power
is reduced to —52.7°, exactly equal to but in the opposite direction
of the effect of glucose; hence, invert sugar, which is a mixture of
an equal number of molecules of glucose and fructose, and which

48 CHEMISTRY OF PLANT LIFE

has a specific rotatory power of — 19.4° at 20° C., becomes optically
inactive at 82° C.

Sorbose is the only other ketohexose which has any importance
in plant chemistry. It does not occur free in plants, but is the
first oxidation product from the hexatomic alcohol, sorbitol, which
is present in the juice of the berries of the mountain-ash. Sorbose
is a crystalline solid, which is not fermentable by yeast, but which
otherwise closely resembles fructose.

DISACCHARIDES

The disaccharides, having the formula Ci2H220n, may be
regarded as derived from the monosaccharides by the linking
together of two hexose groups with the dropping out of a molecule
of water, in the same way that many other organic compounds
form such linkages. That this is a perfectly correct conception, is
shown by the fact that, when hydrolyzed, the disaccharides break
down into two hexose sugars, thus

With all known disaccharides, at least one of the hexoses obtained
by hydrolysis is glucose; hence all disaccharides may be regarded
as glucosides (CeH^Os-R) in which the R is another hexose
group.

Since hexoses have both alcoholic and aldehyde groups, and
since either of these types of groups may function in the linkage
of the two hexoses to form a disaccharide, it is possible for two
hexoses, both of which are reducing sugars to be linked together
in three different ways: (1) through an alcoholic group of each
hexose, (2) through an alcoholic group of one and the aldehyde
group of the other, and (3) through the aldehyde group of each
hexose. Disaccharides linked in either of the first two ways will
be reducing sugars, since they still contain a potentially active
aldehyde group; but those of the third type will not be reducing
sugars, since the linkage through the aldehyde groups destroys
their power of acting as reducing agents. Examples of each of
these three types of linkage are found among the common disac-
charides, as will be pointed out below.

The following table shows the general characteristics of the
common disaccharides.

CARBOHYDRATES 49

Type 1. — Aldehyde group potentially active, reducing sugars:

Sugar Components

Maltose Glucose and glucose

Gentiobiose Glucose and glucose

Lactose Glucose and galactose

Melibiose Glucose and galactose

Turanose Glucose and fructose

Type 2. — Non-reducing sugars:

Sucrose Glucose and fructose

. Trehalose Glucose and glucose

The disaccharides of Type 1 reduce Fehling's solution and form
hydrazones and osazones, although somewhat less readily than
do the hexoses. They all show mutarotation and exist in two
modifications, indicating that the component groups have the
closed-ring arrangement.

The disaccharides of Type 2, since they contain no potentially
active aldehyde group, do not reduce Fehling's solution, nor form
osazones; neither do they exhibit mutarotation. The only
disaccharides which occur as such in plants are of this type. Di-
saccharides of Type 1 may be obtained by the hydrolysis of other,
more complex, carbohydrates.

All disaccharides are easily hydrolyzed into mixtures of their
component hexoses, by boiling with dilute mineral acids, or by
treatment with certain specific enzymes which are adapted tp the
particular disaccharide in each case (see pages 55, also Chapter
XIV).

Sucrose (cane sugar, beet sugar, maple sugar) is the ordinary
" granulated sugar " of commerce. It occurs widely distributed
in plants, where it serves as reserve food material. It is found in
largest proportions in the stalks of sugar cane, in the roots of cer-
tain varieties of beets, and in the spring sap of maple trees, all of
which serve as industrial sources for the sugar. In the sugar cane,
and beet-roots, it constitutes from 12 to 20 per cent of the green
weight of the tissue and from 75 to 90 per cent of the soluble solids
in the juice which can be expressed from it. Its universal use
as a sweetening agent is due to the combined facts that it crys-
tallizes readily out of concentrated solutions and, hence, can be
easily manufactured in solid form, and that it is sweeter than any
other of the common sugars except fructose.

50 CHEMISTRY OF PLANT LIFE

Sucrose is a non-reducing sugar, forms no osazone, and is
not directly fermentable by yeast, although most species of yeasts
contain an enzyme which will hydrolyze sucrose into its component
hexoses, which then readily ferment.

When hydrolyzed by acids, or by the enzyme " invertase,"
it yields a mixture of equal quantities of glucose and fructose.
Sucrose is dextrorotatory, but since fructose has a greater specific
rotatory action to the left than glucose has to the right, the
mixture resulting from the hydrolysis of sucrose is levorotatory.
Since the hydrolysis of sucrose changes the rotatory effect of the
solution from the right to the left, the process is usually called the
" inversion " of sucrose, and the resultant mixture of equal parts
of glucose and fructose is called " invert sugar." As has been
pointed out, solutions of invert sugar become optically inactive
when heated to 82 C°., because of the reduction in the rotatory
power of fructose due to the higher temperature.

The probable linkage of the two hexoses to form sucrose, in
such a way as to produce a non-reducing sugar, is illustrated in
the following formula:

i O r

CH2OH - CHOH • CH - CHOH - CHOH - CH

CH2OH - CHOH - CHOH • CH • C - CH2OH

Trehalose seems to serve as the reserve food for fungi in much
the same way that sucrose does for higher plants. It is composed
of two molecules of glucose linked together through the aldehyde
group of each, as trehalose is a non-reducing sugar. This linkage
is illustrated in the following formula:

O »

CH2OH • CHOH • CH • CHOH • CHOH - CH

O
CH2OH • CHOH - CH - CHOH • CHOH - CH

I o !

CARBOHYDRATES 51

Trehalose may be hydrolyzed into glucose by dilute acids
and by the enzyme " trehalase," which is contained in many
yeasts and in several species of fungi. It is strongly dextro-
rotatory (specific rotatory power, +199°). It is not fermentable
by yeast.

Trehalose appears to replace sucrose in those plants which con-
tain no chlorophyll and do not elaborate starch. The quantity of
trehalose in such plants reaches a maximum just before spore-
formation begins. Since it is manufactured in the absence of
chlorophyll, its formation must be accomplished by some other
means than photosynthesis, yet it is composed wholly of glucose —
a natural photosynthetic product.

Maltose rarely occurs as such in plants, although its presence
in the cell-sap of leaves has sometimes been reported. It is pro-
duced in large quantities by the hydrolysis of starch during the
germination of barley and other grams. This hydrolysis is brought
about by the enzyme " diastase," which is present in the sprouting
grain.

Maltose is easily soluble in water, and crystallizes in masses of
slender needles. It is a reducing sugar; readily forms a charac-
teristic osazone; is strongly dextrorotatory (specific rotatory
power +137°); and is readily fermented by ordinary brewer's
yeast, which contains both " maltase " (the enzyme which hydro-
lyzes maltose to glucose) and " zymase " (the alcohol-producing
enzyme). When hydrolyzed, either by dilute acids or by maltase,
one molecule of maltose yields two molecules of glucose. Its com-
ponent hexoses are, therefore, the same as those of trehalose, a
non-reducing sugar, this difference in properties being due to the
difference in the point of linkage between the two glucose molecules,
that for maltose being such as to leave one of the aldehyde groups
potentially active, as shown in the following formula,

CH2OH - CHOH - CH • CHOH • CHOH - CH

O
CHOH • CHOH • CHOH • CH • CHOH • CH2

L j

52 CHEMISTRY OF PLANT LIFE

Isomaltose is a synthetic sugar, obtained by Fischer, by con-
densing two molecules of glucose. Its properties are quite similar
to those of maltose, but it yields a slightly different osazone and is
not fermentable by yeast. These differences are explained by the
assumption that this sugar is a glucose-/3-glucoside, while normal
maltose is a glucose-a-glucoside.

Gentiobiose is a disaccharide which results from the partial
hydrolysis of the trisaccharide gentianose (see page 53). It is
very similar in its general properties to isomaltose. Cellobiose is a
disaccharide which results from the hydrolysis of cellulose. It is a
reducing sugar, forms an osazone, and resembles maltose.

Maltose, isomaltose, gentiobiose, and cellobiose, are all glu-
cose-glucosides, the difference between them being undoubtedly
due to linkage being between different alcoholic groups in the glu-
cose molecules.

The disaccharide lactose is a glucose-galactoside. It is the
sugar which is present in the milk of all mammals. It has never
been found in plants. Melibiose, which is the corresponding vege-
table glucose-galactoside, may be obtained by the partial hydrolysis
of the trisaccharide raffinose (see below). It is a reducing sugar;
forms a characteristic osazone ; and exhibits mutarotation. It is
not fermented by ordinary top-yeasts, but is first hydrolyzed and
then fermented by the enzymes present in bottom-yeasts.

TRISACCHARIDES

Trisaccharides, as the name indicates, consist of three hexoses
(or monosaccharides) linked together by the dropping out of two
molecules of water. Their formula is CisH^OiB. When com-
pletely hydrolyzed, they yield three molecules of monosaccharides;
when partially hydrolyzed, one each of a disaccharide and a mono-
saccharide.

One trisaccharide of the reducing sugar type, namely rhamnose,
exists in plants as a constituent of the glucoside xanthorhamnin.
It is composed of one molecule of glucose united to two molecules
of rhamnose (methyl pentose, CeH^Os). It is of interest only in
connection with the properties of the glucoside in which it is present
(see page 84).

Three trisaccharides whicn are non-reducing sugars are found
in plants; namely, raffinose, gentianose, and melizitose.

CARBOHYDRATES 53

Raffinose occurs normally in cotton seeds, in barley grains, and
in manna; also, in small quantities in the beet root, associated
with sucrose. It is more soluble in water than is sucrose and
hence remains in solution in the molasses from beet-sugar manu-
facture, which constitutes the commercial source for this sugar.
Raffinose crystallizes out of concentrated solutions, with five
molecules of water of crystallization, in clusters of glistening prisms.
It is strongly dextrorotatory, the anhydrous sugar having a specific
rotatory power of +185°, and the crystalline form, CisH32Oi6,
showing a specific rotation of +104.5°. It does not reduce Feh-
ling's solution, nor form an osazone, and in its other properties it
closely resembles sucrose.

The hydrolysis of raffinose presents several interesting pos-
sibilities. If its structure is represented as follows:

Fructose Glucose Galactose

Sucrose Melibiose

it is apparent that it may break down by hydrolysis in three dif-
ferent ways: (1) into sucrose and galactose, (2) into fructose and
melibiose, and (3) into fructose, glucose, and galactose. As a
matter of fact, it does actually break down in these three different
ways, under the influence of different catalysts; invertase or
dilute acids break it down into fructose and melibiose, emulsin
hydrolyzes it to sucrose and galactose, while strong acids .or the
enzymes of bottom-yeasts break it down into the three hexoses.

Gentianose, a trisaccharide found in the roots of yellow gentian
(Gentian alutea), is a non-reducing sugar, which when hydrolyzed
yields either fructose and gentiobiose, or fructose and two mole-
cules of glucose.

Melizitose, a trisaccharide which, in crystallized form, has
the formula, CisIfeOie -2H2O, occurs in the sap of Larix europea
and in Persian manna, and has recently been found in considerable
quantities in the manna which collects on the twigs of Douglas
fir and other conifers. When hydrolyzed, it yields one molecule
of fructose and one of turanose, a disaccharide containing fructose
and glucose linked together in a slightly different way than they
are in sucrose. Turanose itself is a reducing sugar, but when
linked with fructose to form melizitose its reducing properties are
destroyed. Melizitose is a very sweet sugar.

54 CHEMISTRY OF PLANT LIFE

TETRASACCHARIDES

A complex saccharide, known as stachyose, which is found in
the tubers of Stachys tuberifera, is said by some investigators
to be a tetrasaccharide and by others to have the formula
C36H62Osi-7H2O (i.e., a hexasaccharide) . It is a crystalline solid,
with a faintly sweetish taste, and a specific rotatory power of
4-148.° When hydrolyzed it yields glucose, fructose, and two
(or more) molecules of galactose.

THE RELATION OF THE MOLECULAR CONFIGURATION OF SUGARS
TO THEIR BIOCHEMICAL PROPERTIES

As will be pointed out later (see Chapter XIV), all chemical
reactions which are involved in vital phenomena, including those
of plant growth and metabolism, are controlled by enzymes. The
biochemical reactions which the soluble carbohydrates undergo
afford such excellent illustrations of the relation of the molecular
configuration of an organic compound to the possibility of the
action of an enzyme upon it, that it seems desirable to discuss this
relationship at this point, rather than to postpone it until after
the nature of enzyme action has been considered. Undoubtedly,
the student, after he has studied the nature of enzymes and their
mode of action, as presented in Chapter XIV, will find it profitable
to return to this section and review the facts here presented, as
illustrating the principles and mechanism of enzyme action.
But a consideration, at this time, of the relation of the molecular
configuration of the sugars to their biochemical reactions cannot
fail to add interest to the study of these matters from the chemical
and biological standpoints.

It has been known for a long time that the dextro- and levo-
isomers of a compound which contains one or more asymmetric
carbon atoms are affected differently by biological agents, such as
yeasts, moulds, bacteria, etc. Pasteur, as early as 1850, showed
that the green mould, Penidllium glaucum, when growing in solu-
tions of racemic acid (a mixture of equal molecules of d- and
Z-tartaric acids) uses up only the d-acid, leaving the I- form abso-
lutely untouched. Later, it was found that the same green mould
attacks Z-mandelic acid in preference to the d- form; whereas the

CARBOHYDRATES

yeast, Saccharomyces ellipsoideus, exhibits the opposite preference
for these acids.

These observations upon some of the earlier known forms of
optically active organic acids led the way to a general study of
this phenomenon as exhibited by the optically active soluble car-
bohydrates. The results of these studies may be considered in
connection with the several different types of reactions which
these sugars undergo, as follows:

Glucoside Hydrolysis. — As was pointed out in connection with
the discussion of the mutarotation of glucose, this sugar may exist
in either the a or the |8 modification. Glucosides of both a and 0
glucose are of common occurrence. The difference in molecular
configuration, in such cases, may be represented by the following
formulas:

C— H
H— C— OH
CH2OH

o-Glucoside

H— C— R

H— C— OH \

HO— C— H

0-Glucoside

The radical represented by the R may be either a common
alkyl radical (as CHs, C2Hs, etc.), another saccharide group (as
in the case of the disaccharides, trisaccharides, etc.), or some other
complex organic group (as in the case of the natural glucosides
described in Chapter VI). But, in every case, the glucoside is
easily hydrolyzed by the enzyme maltose (or a-glucase) if the
molecular arrangement is that represented by the a-attachment,
^or by the enzyme emulsin (or /3-glucase) if the glucoside is of the
/3 type; but emulsin is absolutely without effect upon a-glucosides,
and maltase does not produce the slightest change in ^-glucosides.
These statements hold true regardless of the nature of the group
which is represented by the R in the formulas above. Hence, the

56 CHEMISTRY OF PLANT LIFE

bichemical properties of the glucosides, so far as their hydrolysis
by the enzymes which are present in many biological agents is
concerned, depends wholly upon the molecular configuration of
the glucose itself. Furthermore, neither the mannosides, which
differ from glucosides only in the arrangement of the H and OH
groups attached to one of the asymmetric carbon atoms in the
hexose, nor galactosides in which two such arrangements are dif-
ferent (see configuration formulas on page 57), are attacked by
either maltase or emulsin. But other enzymes specifically attack
other dissacharides, or polysaccharides, or glucoside-like complexes.
For example, lactose acts energetically upon ordinary lactose and
all other 0-galactosides; but not upon any glucoside, mannoside,
etc.

Again, neither a- nor 0-xylosides, which correspond with the
above-described glucosides in every particular except that the
HCOH group next the terminal CH^OH group is missing, are
hydrolyzed by either emulsin or maltase.

These instances, selected from among many similar observa-
tions, clearly prove that not only the number and kind of groups in
the molecule, but also the arrangement of the constituent groups
in space, must be identical hi order that the compound may be
acted upon by any given enzyme acting as a biological hydrolytic
agent.

Fermentability. — The enzyme zymase, present in all yeasts,
promotes the fermentation of the natural d- forms of the three
hexoses, glucose, mannose, and fructose, but is without effect upon
the artificial I- forms of the same sugars. The uniform action of
zymase upon these hexoses is easily explained upon the basis of
the same assumption which was used to account for the formation
of identical osazones from these sugars and their easy transforma-
tion into each other; namely, their easy transformation into an
enolic form which is identical for all three.

Further, galactose is fermented by some yeasts (although not
by all), but much less readily than are the other sugars, and the
temperature reaction is quite different with galactose than with
the others. Talose and tagatose .are entirely unfermentable. A
study of the configuration formulas for these several sugars shows
the explanation for these observed facts. These formulas are as
follows;

CARBOHYDRATES

CHO CHO

H— C— OH HO— C— H

HO— C— H HO— C— H

H— C— OH

CH2OH

Glucose

CH2OH

HO--C— H

CHOH
4-OH
HO— C— H

H— C— OH

CH2OH

Mannose

Fructose

Enol

CHO

CH2OH

H— C— OH

HO— C— H

C=0

HO— C— H

H— C— OH

CHoOH

Galactoae

CH2OH

Talose

CH2OH

Tagatose

It will be noted that in the case of glucose, mannose, and fructose,
the configuration is identical at every point except at the aldehyde
end of the chain, and that here the two groups readily arrange
themselves into the same enolic form for the three sugars. Galac-
tose differs from these three sugars only in the arrangement of the
H and OH groups attached to one of the other carbon atoms (the
third from the alcoholic end); the difficulty of its fermentation
indicates that some molecular rearrangement to bring this group
into its proper configuration must precede the fermentation process.
The fact that it is the third HCOH group which thus undergoes
rearrangement is significant because of the participation of these
parts of molecules in groups of threes in many biological processes,
as will be mentioned elsewhere. Talose is unfermentable, even
though the arrangement of its upper three groups is the same as in
the galactose and the lower three the same as in mannose.

If further proof that fermentability depends upon molecular
configuration were needed, it is furnished by the fact that no
pentose is fermentible, even though the stereo-arrangement of

58 CHEMISTRY OF PLANT LIFE

each of the four alcoholic groups in the molecule is identical with
the corresponding groups in a fermentible hexose.

Oxidation by Bacteria. — The bacillus Bacterium xylinum con-
tains an enzyme, or enzymes, which promote the oxidation of the
aldehyde group of an aldose sugar to COOH, or of one alcoholic
CHOH group next the terminal CEbOH group of a hexatomic
alcohol to C = O. But these oxidizing enzymes affect only those
compounds in which the OH groups are on the same side of the
two asymmetric carbon atoms next the end of the molecule where
the oxidation takes place, as indicated in the following groupings.

H— C— OH H— C— OH H— C— OH H— C— OH

II II

H— C— OH or H— C— OH but not HO— C— H or HO— C— H

CHO CH2OH CH2OH CHO

The configuration of the remainder of the molecule is
immaterial to action by these oxidizing bacteria; hence, the
enzymes in this case are apparently concerned only with the con-
figuration arrangement of a portion of the molecule, instead of
with the whole hexose grouping, as in the cases of the other reac-
tions which have been thus far considered.

It is apparent from these illustrations, and from many more
which might be cited, that there is a very definite relation between
the molecular configuration of a carbohydrate and its biochemical
properties, as represented by the possibilities of the action of
enzymes upon it. The probable nature of this relationship will be
better understood after the general questions involved in the mode
of enzyme action have been considered (see chapter XIV). But
for the present, it will be sufficient to note that it seems to be
necessary that the enzyme shall actually fit the molecular arrange-
ment of the compound at all points, in the same way that a key
fits its appropriate lock; or a still better illustration is that of the
fitting of a glove to the hand. On the basis of the latter illus-
tration, it is just as impossible for a dextro-enzyme to affect a
levo-sugar, or for a-glucase to affect a /3-glucoside, as it is to fit
a right-hand glove upon a left hand. Further attention will be
given to these matters in later chapters.

CARBOHYDRATES 59

POLYSACCHARIDES

The polysaccharides which, like the simpler saccharides, or
sugars, which have thus far been studied, undoubtedly serve as
reserve food for plants, are known under the general name of
" starches." They are substances of high molecular weight, whose
constitution is represented by the general formula (CeHioOs)^
It should be noted that an exactly accurate formula should be
(C6)re(Hi206)»-i ; but since the value of n is very high, the simpler
formula is approximately correct. The value of n has not been
accurately determined for any of the individual members of the
group, but is probably never less than 30 and may often be 200 or
more. The fact that these compounds are insoluble in most of the
solvents which can be used for molecular weight determinations
makes it difficult to determine their actual molecular constitution.

When completely hydrolyzed, the polysaccharides yield only
hexoses. They are, therefore, technically known as " hexosans."
Each individual polysaccharide which has been studied thus far
yields only a single hexose, although the particular hexose obtained
varies in different cases. In fact, the polysaccharides are often
classified according to the hexoses which they yield on hydrolysis,
into the following groups : the dextrosans, which yield glucose, and
include starch, dextrin, glycogen, lichenin, etc.; the levulosans,
which yield fructose, and include inulin, graminin, triticin, etc.;
the mannans; and the galactans. The more common representa-
tives of each of these groups are discussed below.

(A) THE DEXTROSANS

These are by far the most common type of polysaccharides to
be found in plants.

Starch. — It is probable that no other single organic compound
is so widely distributed in plants as is ordinary starch. It is pro-
duced in large quantities in green leaves as the temporary storage
form of photosynthetic products. As a permanent reserve food
material, it occurs in seeds, in fruits, in tubers, in the pith, medul-
lary rays and cortex of the stems of perennials, etc. It constitutes
from 50 to 65 per cent of the dry weight of seeds of cereals, and as
high as 80 per cent of the dry matter of potato tubers.

60 CHEMISTRY OF PLANT LIFE

Starch occurs in plant tissues in the form of microscopic gran-
ules, composed of concentric layers, there being apparently alter-
nate layers of two types of carbohydrate material, which have
been distinguished from each other by several different pairs of
names used by different authors: thus, Nageli uses the terms
"granulose" and " amylocellulose" ; Meyer, "a and /3 amylose";
Wolff, " amylo-cellulose " and " amylo-pectin" ; while Kramer
asserts that the layers are alternate lamella of crystalline and col-
loidal starch. Many theories as to the nature of these concentric
layers and their mode of deposition have been advanced, but it
would not be profitable to discuss them in detail here.

For purposes of study, starch may be prepared from the ground
meal of cereals, potatoes, etc., by kneading the meal in a bag or
sieve of fine-meshed muslin or silk, under a slow stream of water.
The starch granules, being microscopic in size, readily pass through
the cloth with the water, and may be caught in any suitable con-
tainer. The starch is then allowed to settle to the bottom, the
water poured off and the starch collected and dried.

Starch is insoluble in water; but if boiled in water, the granules
burst and a slimy opalescent mass, known as " starch paste," is
obtained. This is undoubtedly a colloidal suspension of the
starch in water. By various processes, such as boiling with very
dilute acids, treatment with acetone, etc., starch is converted into
" soluble starch " which dissolves in water to a clear solution.
Soluble starch is precipitated out of solution by alcohol, or by
lead subacetate solution.

Air-dried starch contains from 15 to 20 per cent of water; but
this can be completely removed, without altering the starch in
any way, by heating for some time at 100° C.

The starch granules from different sources vary considerably
in size and shape, and can generally be identified by observation
under the microscope.

The most characteristic reaction of starch is the blue color
which it gives with iodine. The reaction is most marked with
starch paste or soluble starch, but even dry starch granules are
colored blue when moistened with a solution of iodine in water con-
taining potassium iodide, or with tincture of iodine.

When hydrolyzed, either by boiling with dilute acids or under
the influence of enzymes, starch undergoes a series of decomposi-
tions, yielding first dextrins, then maltose, and finally glucose.

CARBOHYDRATES 61

'These transformations can be traced by the iodine color reaction,
as starch will show its characteristic blue, dextrins purple or rose-
red, and maltose and glucose no color with iodine.

Dextrins may occur in plants as transition products in the
transformation of starch into sugars, or vice versa. Most com-
monly, however, they are artificial products resulting from the
partial hydrolysis of starch hi the laboratory or factory. They
are amorphous substances, which are readily soluble in water,
forming sticky solutions which are often used as adhesives ("library
paste " is a common example of a very concentrated preparation
of this kind). They are precipitated from solution by alcohol,
but not by lead subacetate (distinction from starch). They are
strongly dextrorotatory (specific rotatory power 4-192° to +196°);
are not fermented by yeast alone, but readily undergo hydrolysis
to glucose which does ferment. There are several different mod-
ifications, or forms, of dextrins, depending upon the extent to
which the simplification of the starch molecule by hydrolysis is
carried. Three fairly definite forms are generally recognized, as
follows: amylo-dextrin, or soluble starch, slightly soluble in cold
water, readily so in hot water, giving a blue color with iodine;
erythro-dextrin easily soluble in water, neutral taste, red color with
iodine; and achroo-dextrin, easily soluble hi water, sweetish taste,
no color with iodine.

Commercial dextrin, which is much used in the preparation
of mucilages and adhesive pastes, is prepared by heating dry
starch to about 250° C. It is composed chiefly of achroo-dextrin,
mixed with varying quantities of erythro-dextrin and glucose.

Glycogen, or " animal starch/' is one of the most widely dis-
tributed reserve foods of the animal body; in fact, it is the only
known form of carbohydrate-reserve in animal tissues. But it is
present only rarely in plants. It occurs in certain fungi, par-
ticularly in yeasts. In the animal body, glycogen is found in all
growing cells; also in the muscles and blood; but most largely in
the liver, where it is stored in large quantities. The glycogen
found in yeasts is identical with that found in animal tissues. The
quantity of glycogen in a yeast cell increases rapidly as the yeast
grows during the fermentation process.

Glycogen is a white, amorphous compound, readily soluble in
hot water, forming an opalescent solution similar in appearance
to the solutions of soluble starch. It is strongly dextrorotatory

62 CHEMISTRY OF PLANT LIFE

(specific rotatory power +190°), is colored brown by iodine, and is
hydrolyzed to dextrin and maltose, and finally to glucose.

Lichenin, para dextran, and para isodextran are dextrosans
which have been isolated from various lower plants. They all
yield glucose when completely hydrolyzed. They resemble
starch in chemical properties, but differ from it in physical form,
etc.

(B) LEVULOSANS

Inulin replaces starch as the reserve food carbohydrate in a
considerable number of natural orders of plants, particularly in
the Compositae. It is the carbohydrate of the tubers of the
dahlia and artichoke and of the fleshy roots of chicory. It is often
found associated with starch in monocotyledonous plants, such as
many species of Iris, Hyadnthus, and Muscari. Among the mono-
cotyledons, starch seems to be the characteristic carbohydrate
reserve of aquatic, or moisture-loving, species, while inulin is
more common among those which prefer dry situations.

Inulin may be prepared from the tubers of dahlias or arti-
chokes, by boiling the crushed tubers with water containing a little
chalk (to precipitate mineral salts, albumins, etc.) filtering and
cooling the filtrate practically to the freezing point, which precipi-
tates the inulin.

Inulin is a white, tasteless, semi-crystalline powder, which is
soluble in hot water, from which it may be precipitated by alcohol
or by freezing. It forms no paste like that of starch or dextrin,
and gives no color with iodine. It is levorotatory, and when
hydrolyzed by acids or by the enzyme inulinase yields fructose;
in fact, inulin bears the same relation to fructose that starch does
to glucose.

Graminin, irisin, phlein, sinistrin, and triticin are all inulin-
like polysaccharides, which have been found in the plants after
which they are named. Their solutions are, as a rule, sticky or
gummy in consistency, which suggests that these compounds
bear the same relation to inulin that dextrins do to starch.

Mannan bears the same relation to mannose that starch does to
glucose and inulin to fructose. It occurs as a reserve food sub-

CARBOHYDRATES 63

stance in many plants. It has been reported as present in moulds,
and in ergot; in the roots of asparagus, chicory, etc.; in the leaves
and wood of many trees, such as the chestnut, apple, mulberry,
and many conifers; also as a part of the so-called " hemi-cellu-
loses " which are present in the seeds of many plants, notably the
palms, the elders, cedar, larch, etc.

It is a white, amorphous powder, which is difficultly soluble
in water, is strongly dextrorotatory (specific rotatory power
+285°), and when hydrolyzed yields mannose.

Secalan (or carubin) is a substance which is found in the seeds
of barley, rye, etc., which is similar to mannan, but is optically
inactive.

(D) GALACTANS

These bear the same relation to galactose that the preceding
dextrosans do to their constituent hexoses. Four different galac-
tans have been isolated from plant tissues; they are all white,
amorphous solids which dissolve with difficulty in water, forming
gummy solutions.

Both galactans and mannans commonly occur associated with
cellulose and hemi-celluloses in the seeds or other storage organs of
plants. They are practically indigestible by animals, as the proper
enzymes to hydrolyze them are not present in the digestive tract;
hence, they are commonly classed with the indigestible cellulose
as the " crude fiber " of plants which are to be used as food by
animals.

PHYSIOLOGICAL USE AND BIOLOGICAL SIGNIFICANCE OF
CARBOHYDRATES

If the organic compounds produced by plants be classified with
reference to their uses in metabolism into the three groups known,
respectively, as temporary foods, storage products, and perma-
nent structures, it is clear that the carbohydrates which have been
discussed in this chapter may fall into either one of the first two of
these classes. There can be no doubt that the first products of
photosynthesis, whichever ones they may be in different plants,
may be directly used as temporary foods, to furnish the energy
and material for the building up of permanent structures. Also,
there can be no doubt that these same carbohydrates are trans-

64 CHEMISTRY OF PLANT LIFE

located to the storage organs and accumulated for later use by
the same plant (as, for example, in the case of the perennials), or
by the next generation of the plant (when the storage is in the
endosperm adjoining the embryo of the seed).

There is no known explanation as to why different species of
plants make use of different carbohydrates for these purposes;
or why certain species elaborate starch out of the same raw mate-
rials from which other species produce sugars, inulin, or glyco-
gen, etc.

In general, starch is the final product of photosynthesis in
most green plants; but there are many exceptions to this. The
polysaccharides, which are generally insoluble, must be broken
down into the simpler soluble sugars before they can be trans-
located to other organs of the plant for immediate, or future, use.
When they reach the storage organs, they may be recondensed
into insoluble polysaccharides, or stored as soluble sugars. Exam-
ples of the latter type of storage are, sucrose in beet roots, glucose
in onion bulbs, etc. Sometimes, this habit of storage seems to be a
species characteristic; as potatoes store starch, while beets, grow-
ing in the same soil and under exactly the same environment, store
sugar. But in other cases, the nature of the carbohydrate ^tored
undoubtedly is correlated with the external temperatures at the
time of storage. It has been shown that cold, which tends to
physiological dryness, very frequently favors the storage of sugars
instead of starches. Thus, in temperate zones, among aquatic,
or moisture-loving plants, those species which hibernate during
the winter at the bottom of lakes or ponds and are killed by tem-
peratures below freezing, store starch and no sugar; while in the
same ponds, the species whose storage organs pass the winter above
the level of the water and can withstand temperatures as low as
— 7° C. contain sugar during the winter months, even if they con-
tain starch during warmer periods. Similarly, sugars often appear
in the leaves and stems of conifers during the winter months, only
to disappear, or be replaced by starch, when spring approaches.
This same phenomenon is noticeable in arctic plants, which gen-
erally contain but small proportions of starch and relatively large
amounts of sugars.

Similarly, the phenomenon of the turning sweet of potatoes when
exposed to low temperatures has often been noted. The change of
the starch in potato tubers to sugar is most rapid at the tempera-

CARBOHYDRATES 65

ture of 0° C., and ceases at 7°, or above. Also, if potatoes in which
the maximum amount of sugar is present (not over one-sixth
of the total starch can be converted into sugar) are exposed to a
higher temperature the sugar soon disappears.

In general, however, it may be said that each particular species
of plant has its own particular preference for a specific carbohy-
drate as its reserve food material, and elaborates the proper enzymes
to make it possible to utilize this particular carbohydrate for its
metabolic needs.

Again, the question as to whether the storage of energy-
producing materials for the use of the next generation shall be in
the form of carbohydrates or of fats seems to be definitely con-
nected with the size of the seed, and the consequent available
storage space (see page 138). Animals habitually use the space-
conserving form of fats for their energy-storage, while plants more
commonly use carbohydrates for this purpose, except in the case
of those small seeds in which sufficient energy cannot be stored in
carbohydrate form to develop the young seedling to the point where
it can manufacture its own food. As a general rule, nuts, which
contain the embryo of slow-growing seedlings, and need large
proportions of energy reserve, are characteristically oily instead
of starchy in type.

But, aside from temperature reactions and space requirements,
there is no law which has yet been discovered which determines the
character of the energy-storage compound which any given species
of plant will elaborate. The process of photosynthesis would
seem to be identical in all cases, at least up to the point of the
production of the first hexose sugar; but the transformation of
glucose into other monosaccharides, disaccharides, and polysac-
charides seems to be a matter which obeys no rule or law.

Finally, there remains to be considered the occurrence and
uses of sugars in the fleshy tissues of fruits. These tissues have,
of course, no direct function in the life history of the plant. They
surround the seed, but they must decay or be destroyed before the
seed can come into the proper environment for germination and
growth. In most fruits, starch is the form in which the carbo-
hydrate material is first deposited in the green tissue, but as the
fruit ripens the starch rapidly changes into sugars, with the result
that the fruit takes on a flavor which makes it much more attractive
as a food for men and animals. This purely biological significance

66 CHEMISTRY OF PLANT LIFE

of the presence of sugars (and of the other substances which give
desirable flavors 'to fruits, vegetables, etc.), can have no possible
relation to the physiological needs of the individual plant, how-
ever.

It is apparent that the production of these immense stores
of reserve food by plants makes them useful as food for animals,
and it is, of course, the storage parts of the plants which are most
useful for this purpose. This biological relationship needs no
further emphasis.

REFERENCES

ABDERHALDEN, E. — " Biochemisches Handlexikon, Band 2 ... Die Ein-

fachen Zuckerarten, Inuline, Cellulosen, . . .," 729 pages, Berlin, 1911,

and "Band 8 — 1 Erganzungsband (same title as Band 2) — " 507 pages;

Berlin, 1914.
ARMSTRONG, E. F. — "The Simple Carbohydrates and Glucosides," 233 pages.

Monographs on Biochemistry, London, 1919 (3d ed.).
FISCHER, E. — " Untersuchung ueber Kohlenhydrate und Fermente, 1884-

1908," 912 pages, Berlin, 1909.
MACKENSIE, J. E. — "The Sugars and their Simple Derivatives," 242 pages,

17 figs., London, 1913.
TOLLENS, B. — "Kurzes Handbuch der Kohlenhydrate, 816 pages, 29 figs.,

Leipzig, 1914 (3d ed.).

CHAPTER V
GUMS, PECTINS, AND CELLULOSES

THESE substances constitute a group of compounds which are
very similar to the polysaccharide carbohydrates in composition
and constitution, but which serve entirely different purposes in
the plant. As a class, they are condensation products of pentoses,
known as pentosans and having the formula (CsHgO^n, or hex-
osans having the formula (CeHioOs):!, or combined pentosan-
hexosans.

In general, these compounds make up the skeleton, or struc-
tural framework material, of the plant, in contrast with the proto-
plasmic materials or food substances for which most of the other
types of organic compounds (discussed in other chapters of this
book) serve. They are the principal constituents of " woody
fiber," of cell-walls, and of the " middle lamella " which fills up
the spaces between the plant cells. They are, therefore, found
in largest proportions in the stems of woody plants; but they are
also present in every other organ of plants, as the cell-wall or
other structural material.

For purposes of study, these compounds may conveniently be
divided into three groups; namely, the natural gums and pen-
tosans, the pectins and mucilages, and the celluloses. The segre-
gation into these three groups is not sharply defined. The dis-
tinction between the groups is based upon the solubility of the
compounds in water. The gums and pentosans readily dissolve
in water; the pectins form colloidal solutions which are easily
converted into " jellies"; the mucilages do not dissolve but form
slimy masses; while the celluloses are insoluble in and unaltered
by water. Some authors add a fourth group, known as " humins" ;
but as these are the products of decay (usually in the soil) of these
structural compounds, rather than of growth and development,
they need not be taken into consideration in a study of the chem-
istry of plant growth.

68 CHEMISTRY OF PLANT LIFE

THE NATURAL GUMS AND PENTOSANS

The natural gums, when hydrolyzed, yield large proportions
of sugars, but most of them also contain a complex organic acid
nucleus, by means of which they form salts with calcium, mag-
nesium, etc. Some of them, such as cherry gum and those which
are found in the woody stems of plants (wood gum, and those
found in corn stalks, the straw of cereals, etc.) yield practically pure
pentoses. These are known as pentosans. They bear the same
relation to the pentose sugars as do the dextrosans to glucose, etc.
The wound gums, for example, yield arabinose, and the wood
gums yield xylose. But most of the natural gums yield a mixture
of galactose, some pentose, and some complex organic acid.

The gums are translucent, amorphous substances, whose solu-
tions in water are levorotatory. They are precipitated out of
solution by alcohol and by lead subacetate solution.

Gums are extremely difficult to hydrolyze, the laboratory process
of hydrolysis usually requiring from eighteen to twenty-four hours
of continuous boiling with acids for its completion. Because
of this difficulty of hydrolysis, gums are practically indigestible
by animals and of little use as food.

The following common examples will serve to illustrate the
general nature of these compounds.

Gum arabic, found in the exudate from the stems of various
species of Acacia, is a mixture of the calcium, magnesium, and
potassium salts of a diaraban-tetragalactan-arabic acid. Arabic
acid has the formula C23H3sO22, and one molecule of this acid
serves as the nucleus for the union of eight galactose and four
arabinose groups, linked together in some unknown way. The
formula for the compound, exclusive of the metallic elements with
which it is loosely united is CgiHisoOrs. This gives some idea of
its complexity.

When boiled with nitric acid, it is oxidized to mucic, saccharic,
and oxalic acids. It gives characteristic reactions with alum,
basic lead acetate, and other common reagents.

Gum arabic comes on the market as a brittle, glassy mass,
which is used in the preparation of mucilages, and as a carrier for
essential oils, etc., in certain toilet preparations.

Recent investigations have shown that the so-called " meta-
pectic acid/' which is often found in sugar beets and interferes

GUMS, PECTINS, AND CELLULOSES 69

with the process of sugar manufacture, is identical with gum arabic
in composition and properties.

Gum tragacanth is the soluble portion of the natural gum which
is found in several species of Astragalus. It constitutes only 8 to
10 per cent of the total gum-like material which is present, the
remainder being composed of insoluble gummy substances of
unknown composition. The soluble gum consists of calcium,
potassium, and magnesium salts of an acid which, when hydrolyzed,
yields several molecules of arabinose, six of galactose, and one of
geddic acid (an isomer of arabic acid). It is said to be produced
by the metamorphosis of the medullary rays under unfavorable
conditions of growth. It comes on the market in globular masses
of amorphous material, and is used in the manufacture of
cosmetics, etc.

Wound gum is frequently found in the tracheae of plants, and
near surface wounds, which it stanches. It is secreted by the cells
surrounding the injured part. It responds to the reactions of
other gums and to some of those of woody fiber. Its exact com-
position is not known, but probably lies between that of the true
gums and that of cellulose.

These gums are generally considered to be decomposition
products of celluloses, resulting from the action of some hydrolytic
ferment, usually stimulated by some unfavorable condition of
growth, some injury, or some morbid condition.

The pentosans, araban and xylan, occur normally in the stems
and outer seed coats of many common plants. They constitute
a considerable proportion of these tissues, as indicated by the
following results of typical analyses: Wheat bran, 22 to 25 per
cent; clover hay, 8 to 10 per cent; oat straw, 16 to 20 per cent;
wheat straw, 26 to 27 per cent; corn bran, 38 to 43 per cent; jute
fiber, 13 to 15 per cent; various wood gums, 60 to 92 per cent.

They are white, fluffy solids, which are difficultly soluble in
cold water, more readily in hot water. They are very difficult
to hydrolyze, and indigestible by animals. When finally hydro-
lyzed, they yield arabinose and xylose, respectively. The pith
of dry corn stalks is a good illustration of their general char-
acter.

70 CHEMISTRY OF PLANT LIFE

MUCILAGES

These are characterized by forming slimy masses when moist-
ened with water. They are secreted by hairs on the skin of many
plants, so that the external walls of the leaves, fruit, and seeds
are often mucilaginous when damp. This is particularly true of
aquatic plants. The chemical composition of the mucilages is
unknown. When hydrolyzed, they yield arabinose and a hexose;
the latter is sometimes galactose and sometimes mannose.

When present on the surface of plant tissues, the mucilages
probably serve to prevent the too rapid diffusion of materials
through the skin, in the case of the aquatic plants, and too rapid
transpiration, in the case of young vegetative tissues or in other
plants when growing under extremely dry conditions. When
found in tubers, or other storage organs, it has been supposed that
they may serve as reserve food materials, but it seems that such
difficultly hydrolyzable compounds as these can hardly function
as normal reserve foods.

PECTINS

Many fruits, such as currants, gooseberries, apples, pears,
etc., and many fleshy roots of vegetables, such as carrots, parsnips,
etc., contain substances known as pectins. These are readily
soluble in water, and when dissolved in concentrated solutions in
hot water, they set into " jellies " when the solution is cooled.
These jellies carry with them the soluble sugars and flavors which
are present in the fruits, and constitute a familiar article of diet.

There are undoubtedly several different modifications of
the pectins, to which the names " meta-pectin," " para-pectin,"
" pectic acid," " meta-pectic acid," and " para-pectic acid,"
have been applied. These all seem to be products of hydrolysis
of a mother substance known as " pectose," which constitutes the
middle lamella of unripe fruit, etc. As the fruit ripens, the pectose
is hydrolyzed into the various semi-acid, or acid, bodies mentioned
above. The intermediate products of the hydrolysis are the
pectins, which swell up in water and readily form jellies; while the
final meta-pectic acid is easily soluble in water and resembles
the true gums in its properties. When the middle lamella reaches
the pectic acid stage, the fruit becomes soft and "mushy" in
texture.

GUMS, PECTINS, AND CELLULOSES 71

The pectins more nearly approach to the composition, proper-
ties, and functions of the celluloses than do any of the other groups
of organic compounds. They have been extensively studied in
connection with the parasitism of certain fungous diseases which
cause the soft rots of fruits and vegetables. These parasites
usually penetrate the tissues of the host plant by dissolving out
the middle lamella material, which may sometimes serve as food
material for the fungus; but more often the parasite secures its
food supply from the protoplasm of the cell contents. In such
cases, the parasite secretes both a pectose-dissolving enzyme,
known as " pectase " and a " cellulase " which attacks the cell-
wall material in order to provide for the entry of the fungus into
the cells. Other enzymes, known as " pectinases," which coagu-
late the soluble pectins or pectic acids into insoluble jellies in the
tissues of the plants seem to aid the plant in resisting the pene-
tration by the parasite.

CELLULOSES

Used in its general sense, this term includes all those substances
which are elaborated by protoplasm to constitute the cell-wall
material. Cellulose proper is a definite chemical compound, whose
properties are well established. In plants, however, this true
cellulose is nearly always contaminated by various encrusting
materials; and in the process of wood-formation, the cell-wall
material continually thickens by the conversion of the cellulose
into ligno-cellulose and the protoplasm of the cell as continuously
diminishes in volume. Thus the protoplasm of the cell produces a
number of different kinds of material which are deposited in the
walls of the cell. All of these, taken together, constitute the
general group known as the celluloses.

These may be divided into three classes: namely, (1) the hemi-
celluloses, (2) the normal celluloses, and (3) the compound cellu-
loses.

The hemi-celluloses (pseudo-, or reserve celluloses) include a
series of complex polysaccharides which occur in. the cell- walls
of the seeds of various plants. They are found in the shells of
nuts, rinds of cocoanuts, shells of stony fruits, etc., and in the seed-
coats of beans, peas and other legumes. They are much more
easily hydrolyzed than the other members of this group, and when

72 CHEMISTRY OF PLANT LIFE

hydrolyzed. yield various sugars, chiefly galactose, mannose, and
the pentoses. They bear the same relation to these sugars that
starch does to glucose, and are generally supposed to serve as
reserve food material, although it is difficult to conceive how the
shells, etc., in which they appear can be utilized by a growing seed-
ling. They differ in structure from the fibrous celluloses and are
probably not cell-wall building material. They appear to be a
form of reserve carbohydrates, which differ from the glucose-
polysaccharides in being condensed in, or as a part of, the external
structural material rather than in the internal storage organs.
They are soluble in water and exhibit the properties of gums, and
are often classified with the gums and described under the names
" galactans," " mannosans," " pentosans," etc.

The normal celluloses, of which the fibers obtained from cotton,
flax, hemp, etc., are typical examples, are widely distributed in
plants and form the commercial sources for all textile fibers of
vegetable origin. Ordinary cotton fiber contains 91 per cent of
cellulose, about 7.5 per cent of water, 0.4 per cent of wax and fat,
0.55 per cent of pectose derivatives, and 0.25 per cent of mineral
matter; or a total of only 1.2 per cent of non-cellulose solids.
Filter paper is practically pure cellulose.

Pure cellulose is a white, hygroscopic substance, which is insol-
uble in water and in most other solvents. If heated with water
under pressure to about 260° C., it dissolves completely without
decomposition. If boiled with a strong solution of zinc chloride,
or treated in the cold with zinc chloride and concentrated hydro-
chloric acid, or with an ammoniacal solution of copper hydroxide
(Schweitzer's reagent), it dissolves to a clear solution from which
it may be reprecipitated without chemical change by neutralizing
or diluting the solution.

Cellulose has the formula (C6Hi2O5)n. When hydrolyzed
under the influence of the enzyme cytase, it breaks down, first into
cellobiose, an isomer of maltose, and then into glucose. It is,
therefore, chemically like, but not identical with, starch; and
structurally it is arranged in fibrous form instead of in granules.
Under the action of fermentative enzymes, as when vegetable
matter decays under stagnant water, in swamps, etc., cellulose
breaks down into carbon dioxide and marsh gas, according to the
equation

GUMS, PECTINS, AND CELLULOSES 73

Cellulose is acted upon by caustic alkalies in a variety of
ways. When fused with a mixture of dry sodium and potassium
hydroxides, it is decomposed into oxalic and acetic acids. When
heated with a 10 to 15 per cent solution of caustic soda, cellulose
fibers thicken and become translucent, thus resembling silk fibers.
This process, known as " Mercerizing," is largely used for the
production of commercial fabrics.

Acids also act on cellulose in a variety of ways. When heated
with nitric acid (sp. gr. 1.25), it is converted into oxy cellulose;
while dilute sulfuric acid, under similar conditions, yields hydro-
cellulose , a substance having the formula C^IfeOii, which
retains the fibrous structure of the original cellulose but which,
when dry, may be rubbed up into a fine powder. Concentrated
nitric acid, or better, a mixture of concentrated nitric and sulfuric
acids, acts upon cellulose, converting it into various nitro-deriva-
tives, several of which have great industrial value. The number
of NOs groups which unite with the cellulose molecule under these
conditions depends upon the temperature, pressure, etc., employed
during the nitration process; di-, tri-, tetra-, penta-, and hexani-
trates are all known. Pyroxylin, or collodion, is a mixture of the
tetra- and penta-nitrates, which is soluble in alcohol and is used
in surgery, in photography, and in the manufacture of celluloid,
which is a mixture of collodion and camphor. The hexanitrate,
Ci2Hi4(NO3)6O4, is the violent explosive known as gun-cotton.

Gentler oxidizing agents, such as " bleaching powder," etc..
have no effect upon cellulose, and hence are extensively used in
the treatment of cotton and other vegetable fibers, in preparation
for their use in the manufacture of textiles, paper, etc.

Cellulose is indigestible in the alimentary tract of annuals, but
the putrefactive bacteria which are generally present there ferment
it, with the production of acids of the " fatty acid " series, carbon
dioxide, methane, and hydrogen. Excessive fermentations of
this kind are responsible for the distressing phenomenon known as
" bloat."

The compound celluloses comprise the larger proportion of the
material of the woody stems of plants. They consist of a base
of true cellulose, which is either encrusted with or chemically com-
bined with some non-cellulose constituent. Depending upon the
nature of the non-cellulose component, the compound celluloses
are divided into three main groups, known respectively as (1)

74 CHEMISTRY OF PLANT LIFE

ligno-celluloses, (2) pecto-celluloses, and (3) adipo-, or cuto-
celluloses. As the names indicate, the non-cellulose component
in the first group is lignin; in the second, pectic substances; and
in the third, fats or waxes.

Ligno-celluloses. — In the young plant cell, the cell-walls
consist of practically pure cellulose; but as the plant grows older,
this becomes permeated with lignin, or woody fiber, until in the
stem of a tree, for example, the proportion of cellulose in the tissue
is only 50 to 60 per cent. In the preparation of wood pulp for
the manufacture of paper, the lignin materials are dissolved off by
means of various chemical reagents, leaving the cellulose fibers
in nearly pure form for use as paper. The lignin material gen-
erally consists of two types of substances, one of which contains a
closed-ring nucleus of unknown composition and the other is
probably a pentosan. These materials are so extremely difficult
to hydrolyze that their composition has not yet been definitely
determined.

Pecto-celluloses are found in various species of flowering plants;
those which are present in the stems and roots being true pecto-
celluloses, while those which are found in fruits and seeds contain
mucilages rather than pectose derivatives, and are generally
designated as " muco-celluloses." The exceedingly inert char-
acter of these compounds makes their study difficult and their
functions uncertain.

The term cuto-celluloses is applied to the group of substances,
including suberin and cutin, which constitute waterproof cell-
walls. These were formerly supposed to consist of true cellulose
impregnated with fatty or wax-like materials. Recent investiga-
tions seem to indicate, however, that there is really no cellulose
nucleus in such walls as these, but that they are compound glyceryl
esters resembling the true fats (see chapter X) in composition. If
this view should finally be established as a fact, this sub-group of
supposed compound celluloses should be dropped from consider-
ation as such.

PHYSIOLOGICAL USE OF CELLULOSES

There seems to be no question that the sole use of celluloses is
to serve as structure-building materials. They are undoubtedly
elaborated from the carbohydrates as the cell grows. In only

GUMS, PECTINS, AND CELLULOSES 75

rare cases, however, is there any evidence that they can be recon-
verted into carbohydrates to serve as food material. Certain
bacteria can make use of cellulose as food, and secrete an enzyme,
cytase, which aids in the hydrolysis of cellulose to sugars for this
purpose. But this enzyme seems rarely, if at all, to be present
in the tissues of higher plants. It has been reported that
some cellulose is hydrolyzed during the malting of barley, indi-
cating that this might have some food use for the growing seedling;
but this observation has not been confirmed and later investigations
seem to throw doubt upon its accuracy.

Bacteria of decay also act upon cellulose materials, con-
verting them chiefly into gaseous products; but this seems to be
a provision of nature for the destruction of the cell-wall material
of dead plants, rather than an arrangement for the constructive
use of it as food for the bacterium. When fibrous plant residues
decay in the soil, the cellulose compounds are first converted into
a series of complex organic acids, known as " humins," which
undoubtedly have a significant effect upon the chemical and
physical properties of the soil, but these have little interest or
significance in a study of the chemistry of plant growth.

REFERENCES

ABDERHALDEN, E. — " Biochemisches Handlexikon, Band 2, Gummisub-
stanzen, Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin,
1911; ?nd "Band 8 — 1 Erganzungsband (same title as Band 2) — ,"
507 pages, Berlin, 1914.

SCHWALBE, C. G. — "Die Chemie der Cellulose," 665 pages, Berlin, 1911.

CHAPTER VI
GLUCOSIDES

STRICTLY speaking, the term glucoside should be applied only
to such compounds as contain glucose as the characteristic basic
group. But in common usage, it refers to any compound which,
when hydrolyzed, yields a sugar as one of the products of the
hydrolysis. In all the natural glucosides which occur in plant
tissues, the other organic constituent, which is represented by the
R in the formula for glucosides (R-C6Hn05, or R- (CHOH)5CHO)
is some aromatic group, or closed-ring benzene derivative.* The
different organic constituents of glucosides are of a great variety
of types, such as phenols, alcohols, aldehydes, acids, oxyflavone
derivatives, mustard oils, etc. It is noteworthy, however, that no
nitrogenous groups of the protein type have been found combined
with sugars in glucosides.

Some glucosides contain more than one saccnaride group, possi-
bly as di- or trisaccharides. Under proper conditions of hydroly-
sis, one or more of the saccharide groups can be removed from such
compounds, resulting in glucosides of simpler structure.

H
C

HC CH

* The structural formula for benzene, C6H6, I is one which it is

HC CH

H
difficult and inconvenient to reproduce in type. On that account, it is

/\
customary to indicate this formula by a plane hexagon, thus

It is understood, in all such cases, that the figure represents six carbon atoms
arranged in a closed ring, with alternate double and single bonds, and with a
hydrogen atom attached to each carbon. The printing of some other group
as OH, CH3, adjacent to an angle of the hexagon means that this group
replaces the H atom in the compound which is being illustrated.

GLUCOSIDES 77

Most of the common glucosides are derived from d-glucose.
Some are known, however, which are derivatives of galactose or
rhamnose; while in some cases the exact nature of the sugar which
is present has not vet been determined

HYDROLYSIS OF THE NATURAL GLUCOSIDES

All natural glucosides are hydrolyzed into a sugar and another
organic residue by boiling with mineral acids; although they vary
widely in the ease with which this hydrolysis is brought about.

In most cases, the glucoside is easily hydrolyzed by an enzyme
which occurs in the same plant tissue, but in different cells than
those which contain the glucoside. Injury to the tissues, germina-
tion processes, and perhaps other physiological activities of the
cells, result in bringing the enzyme in contact with the glucoside
and the hydrolysis of the latter takes place. A large number of
such enzymes have been found in plants, many of which hydrolyze
only a single glucoside. Howe.ver, two enzymes, namely, the
emulsin of almond kernels, and myrosin of black mustard seeds,
each hydrolyze a considerable number of glucosides. In general,
emulsin will aid in the hydrolysis of any glucoside which is a
derivative of 0-glucose, and myrosin will help to split up any sulfur-
containing glucoside. Glucosides which are derivatives of rham-
nose require a special enzyme, known as rhamnase, for their
hydrolysis.

The following reactions for the hydrolysis of arbutin and of
amygdalin are typical of this action, and will serve to illustrate
the general structure of these compounds:

CH2OH-CHOH-CH-CHOH-CHOH-CH'O-C6H4OH + H2O

Arbutin

= C6Hi2O6 -h HOCeKiOH

Glucose Hydroquinone

C6H5

(a) C6HiiO5-O.C6Hio04-O-CH + H2O

Amygdalin

+ C6Hi206

Mandelo-nitrile I Glucose

glucoside

78 CHEMISTRY OF PLANT LIFE

C6H5
(6) CeHnOs-O-CH + H20 = C6H5'CHOH.CN -I- C6Hi206

Mandelo-nitrile I Mandelo-nitril- Glucose

glucoside

Mandelo-nitrile Benzaldehyde Hydrocyanic

acid

GENERAL PROPERTIES OF GLUCOSIDES

As a rule, glucosides are easily soluble in water. They are
generally extracted from plant tissues by digestion with water or
alcohol. In most cases, the enzyme which is present in other cells
of the same tissue must be killed by heating the material, in a
moist condition, to the temperature of boiling water, before the
extraction is begun, as otherwise the glucoside will be hydro-
lyzed as rapidly as it is extracted from its parent cell. Macera-
tion or otherwise bruising the tissue, after the enzyme has been
destroyed, facilitates the extraction. The glucosides, after extrac-
tion and purification by recrystallization, are generally colorless,
crystalline solids, having a bitter taste and levorotatory optical
activity. This latter property is remarkable, as most of them are
compounds of the strongly dextrorotatory d-glucose.

Many of the natural glucosides have marked therapeutic
properties and are largely used as medicines; others are the mother-
substances for brilliant dyes; for example, indican, from which
indigo is obtained, and the alizarin glucosides.

Several hundred different glucosides have been isolated from
plant tissues, and their properties described, and this number is
being added to constantly, as the methods of isolation and study
are improved. They may be classified into groups, according to
the nature of the organic compound other than sugars which they
yield when hydrolyzed. The following descriptions of the occur-
rence, constitution, products of hydrolysis, and special properties
of typical members of each of the several different classes of glu-
cosides will serve to illustrate their general relationship to plant
growth.

GLUCOS1DES 79

THE PHENOL GLUCOSIDES

Arbutin, C^HieOj, is obtained from the leaves of the bear
berry (Arctostaphylos uva-ursi), a small evergreen shrub. When
hydrolyzed by mineral acids or emulsin, it yields glucose and
hydroquinone.

Hydroquinone has strongly antiseptic properties. Arbutin is
both an antiseptic and a diuretic, and is used in medicine.

Phloridzin, C2iH24Oio, is found in the bark of apple, pear,
cherry, plum, and similar trees. Mineral acids (but not emulsin)
hydrolyze it to glucose and phloretin (CisHuOs), according to the
equation

CH3

I
C2iH24Oi 0+H20 = C6Hi206+ (OH)3C6H2 - CO - CH - C6H4OH.

It is used in medicine as a remedy for malaria, having marked
anti-periodic properties.

Glycyphyllin, C2iH24Og, found in leaves of Smilax, yields
rhamnose and phloretin, when hydrolyzed.

Iridin, C24H26Oi3 (glucose and irigenin), found in root stocks
of Iris, is used in medicine as a cathartic and diuretic.

Baptisin, C26H32Oi4-9H20 (two rhamnose and baptigenin),
found in roots of wild indigo (Baptisia) , has strong purgative
properties.

Hesperidin, CsoHeoCb? (one rhamnose+two glucose+hes-
peritin) , is found in the pulp of lemons and oranges.

The characteristic phenol group which is present in these
glucosides has the following structural formula, in each case, the
X indicating the H atom which is replaced by the sugar molecule
to form the glucoside:

Phloretin

/\ - C— CH

II I
HOI JOH 0 CH3

80 CHEMISTRY OF PLANT LIFE

Irigenin

:Zo_/\o-CH3
^o-I^Jox

THE ALCOHOL GLUCOSIDES

Salicin, CisHigOT (glucose -f saligenin, or o-oxy benzyl alco-
hol) is found in the bark, leaves, and flowers of most species of
willow, the proportion present depending upon the season of the
year, and the sex of the tree. It is used as a remedy against
fevers and rheumatism, causing less digestive disturbances than
the salicylic acid which is the oxidation product of saligenin and
which is sometimes used as a remedy for rheumatism.

Coniferin, CielfeOg (glucose and coniferyl alcohol), is found
in the bark of fir trees. The coniferyl alcohol obtained from
coniferin by hydrolysis can be easily oxidized to vanillin, and is,
therefore, the source for the artificial flavoring extract used as a
substitute for the true extract of the vanilla bean.

Populin, C2oH220g (glucose -f- saligenin -f benzoic acid), found
in the bark of poplar trees, is used in medicine as an antipyretic.
It can be hydrolyzed, by a special enzyme, into salicin and benzoic
acid.

The structure of the two typical closed-ring alcohols which are
present in these glucosides is indicated by the following formulas;

Coniferyl alcohol
Saligenin CH = CH - CH2OH

|//X|CH2OH |/Xj

I/OX I JOCH3

GLUCOSIDES 81

THE ALDEHYDE GLUCOSIDES

Salinigrin, CisHieOy (glucose and ra-oxy benzaldehyde), is
found in the bark of one species of willow (Salix discolor). Its
isomer, known as helicin (glucose and o-oxy benzaldehyde, or
salicylic aldehyde), does not occur naturally in any plant, but is
easily produced artificially by the gentle oxidation of salicin.
Their relationships are shown on the following formulas;

Salicin Helicin Salinigrin

lCH2OH

JOX

Amygdalin, also contains a benzaldehyde group, but there is
linked with it a hydrocyanic acid group; hence, this glucoside
is usually classed with the cyanophoric glucosides (see page 86).

THE ACID GLUCOSIDES

The most common example of this group is gaultherin,Ci4Hi8O8,
which is found in the bark of the black birch and is a combination
of glucose with methyl salicylate. Both the glucoside itself and
the methyl salicylate (" oil of wintergreen ") which is derived
from it are used as remedies for rheumatism.

Jalapin, C44Hs6Oi6 (glucose and jalapinic acid), and con-
volvulin, C54Hg6O27 (glucose +rhodeose+ con volvulinic acid), are
glucosides of very complex organic acids, found in jalap resin,
which are used in medicine as cathartics or purgatives.

THE OXY-CUMARIN GLUCOSIDES

Cumarin itself is widely distributed in plants. No glucoside
containing cumarin as such has yet been isolated; but several
glucosides of its oxy-derivatives are known. The following are
common ones:

Skimmin, CisHieOg (glucose and skimmetin), is found in
Skimmia japonica; aesculin, CisHieOg (glucose and sesculetin),
is found in the bark of the horse-chestnut, dEsculus hippocastanum,
and its isomer, daphnin (glucose and daphnetin), in several species

82 CHEMISTRY OF PLANT LIFE

of Daphne; and fraxin, CieHigOio (glucose and fraxetin), is
found in the bark of several species of ash.

The structural arrangement of the oxy-cumarin groups which
are found in these glucosides is shown in the following formulas.
It is not known to which OH group the sugar is attached, in each
case.

Skimmetin ^Esculetin
CH=CH-CO CH=CH-CO
/\ n_l /\ rvJ

Daphnetin Fraxetin

CH-CO CH = CH CO

HO/N 0—

HO(N

OCH3

Scopolin, C22H2gOi4, found in Scopolia japonica, contains
two glucose molecules united to a monomethyl ether of aesculin;
while limettin, found in certain citrus trees, is the dimethyl ether
of sesculin.

THE PIGMENT GLUCOSIDES

Many, if not all, of the red, yellow, violet, and blue pigments
of plants either exist as, or are derived from, glucosides. These
are of three types: the madder, or alizarin, reds are derivatives of
various oxy-anthraquinones; most of the soluble yellow pigments
are glucosides derived from flavones or xanthones; and the
soluble red, blue, and violet pigments of the cell-sap of plants
are mostly anthocyan derivatives. The four basic groups, or
nuclei, which are present in these different types of compounds are
complex groups consisting essentially of two benzene rings linked
together through a third ring in which there are either two oxygen
atoms in the ring, or one oxygen in the ring and a second attached
to the opposite carbon in the (C = 0) arrangement, as shown by the
following diagrammatic formulas:

GLUCOSIDES

Anthraquinone

Flavone

1 ;

Anthocyan
O 5' 4/

!6 '' 2'

The red dyes which were formerly obtained from madder, the
powdered roots of Rubia tindoria, but are now almost wholly
artificially synthetized, consist of at least four different glucosides,
the organic group of which, in each case, is an hydroxy-deriva-
tive of anthraquinone. The most important of these is rubery-
thric acid, composed of two molecules of glucose linked with one of
alizarin (1, 2, dioxyanthraquinone). Xanihopurpurin contains
1, 3, dioxyanthraquinone, which is isomeric with alizarin; and
rubiadin is a monomethyl (the CHs being in the 4 position), deriva-
tive of this compound. Purpurin is a glucoside of 1, 2, 4, trioxy-
anthraquinone.

The soluble yellow pigments are generally glucosides of
hydroxy-derivatives of xanthone or flavone, known as oxyxan-
thones or oxyflavones. The sugars which are united to these
nuclei vary greatly, so that there are a great variety of yellow,
white, or colorless flavone or xanthone pigment compounds.
These compounds are almost universally present in plants. For
example, one typical set of examinations of the wood, bark, leaves,
and flowers of over 240 different species of tropical plants showed
that flavone derivatives were present in every sample which was
tested, the pigments being usually located in the powdery coating
of the epidermis of the tissues.

The following typical examples will serve to illustrate the com-
position and properties of the glucosides of this type.

84 CHEMISTRY OF PLANT LIFE

Quercitrin, C2iH2oOn, is found in oak bark, in the leaves of
horse-chestnut, and in many other plants, often associated with
other pigments. It is a brilliant yellow crystalline powder.
Industrially, it ranks next to indigo and alizarin in importance as a
natural dye stuff. It is a glucoside of rhamnose with 1, 3, 3', 4',
tetraoxyflavonol (i.e., the flavone nucleus with five OH groups
replacing the hydrogens in the 1, 3, 5, 3', and 4' positions). Quer-
cetin, CisHioOr, which is the tetraoxyflavonol itself, without any
sugar in combination with it, is found in the leaves of several
species of tropical plants and in the bark of others. Isoquercitrin,
C2iC2oOi2, is derived from the same flavone, but contains glucose
instead of rhamnose, as the sugar constituent of the glucoside.

Apiin, C26H2oOg, the yellow glucoside found in the
leaves of parsley, celery, etc., contains apiose (a pentose
sugar of very unusual structure, represented by the formula,
CH2OHV

y€OH-CHOH-CHO), and apigenin, which is a 1, 3, 4',

CH2oir

trioxyflavone.

Xanthorhamnin, Cs4H4202o, is a very complex glucoside
containing two rhamnose and one galactose groups, united with
rhamnetin, which is quercitin with the H of the OH in either the
1, or 3, position replaced by a methyl group. There are several
similar pigments which differ from xanthorhamnin only in the
number or position of the methoxy groups (i.e., the OH groups
with a CHs replacing the H), or in the nature of the sugar which is
present in the compound. Rhamnetin itself is found in the fruits
of certain species of Rhamnus, and is used in dyeing cotton.

The structural arrangement of the characteristic groups of
these flavone pigments will be dealt with more in detail in the
chapter dealing with Pigments (Chapter VIII).

The best-known yellow pigment which is a xanthone derivative
is euxanthic acid, known as " Indian yellow," which is a "paired"
compound of glucuronic acid (see page 42) and euxanthone. The
latter is a 2, 3', dioxyxanthone. • The pigment is found in the urine
of cattle which have been fed on mango leaves.

The soluble red, blue, and violet pigments are glucosides of
various hydroxy-derivatives of the anthocyan nucleus. Their
constitution and properties will be discussed in detail in the
chapter dealing with the Pigments. These compounds are iso-

GLUCOSIDES 85

meric with similar flavone and xanthone derivatives, and the
transition from one color to the other in plants takes place very
easily under the action of oxidizing or reducing enzymes. This
accounts for the change of reds and blues to yellows and browns,
and vice versa, under changing temperature conditions.

The following red or blue plant pigments, which are anthocyan
glucosides, have been isolated and studied (for the structural
arrangement of the characteristic groups, see pages 116): from
cornflower and roses, cyanin, C28HsiOi6Cl (2 molecules glucose
-fcyanidin); from cranberries, idain, C2iH2iOioCl (galactose
-fcyanidin); from geranium, pelargonin, C27H3oOi5Cl (2 mole-
cules glucose -f pelargonidin); from pseony, pceonin, C2sH33Oi6Cl
(2 molecules glucose +paeonidin, a monomethyl cyanidin); from
blue grapes, oenin, C23H25Oi2Cl (glucose -foenidin) ; from whortle
berry, myrtillin, C22H23Oi2Cl (glucose -f myrtillidin) ; from lark-
spur, delphinin, C^HsgC^iCl (2 molecules glucose +2 molecules
p-oxybenzoic acid+delphinidin); and from mallow, malvin,
C29H3sOi7Cl (2 molecules glucose +malvidin).

The blue dye, indigo, is derived from a glucoside of an entirely
different type, known as indican. Indican is readily extracted
from the leaves of various species of indigo plants. When hydro-
lyzed, it yields glucose and indoxyl (colorless). Indoxyl is easily
oxidized to indigotin (the deep blue dye known as " indigo ")•
The equations illustrating these changes are as follows:

(a) Ci4Hi706N + H20 = C6Hi2O6 + C8H7ON

Indican Glucose Indoxyl

(6) 2C8H7ON + O2 = Ci6Hi0O2N + 2H2O

Indoxyl Indigotin

The structural relationships of indoxyl and indigotin may be
illustrated by the following formulas:

Indigotin

o o

i i x I j

>N/ \N/ \/

I I

H H

86 CHEMISTRY OF PLANT LIFE

Natural indigo dye is prepared by fermentation of indigo leaves,
the decay of the cell-walls liberating the enzymes in the tissues,
which bring about the chemical changes illustrated in the above
equations.

THE CYANOPHORE GLUCOSIDES

Several glucosides which yield hydrocyanic acid as one of the
products of their hydrolysis are of common occurrence in plants.
These are generally spoken of as the " cyanogenetic " glucosides;
but as they do not actually produce cyanogen compounds, but only
liberate them when hydrolyzed, the recently suggested term "cy-
anophore" undoubtedly more correctly indicates their properties.

The best known and most widely distributed of these is
amygdalin. Amygdalin was first discovered in 1830, and was one
of the first substances to be recognized as a glucoside. It is found
in large quantities in bitter almonds and in the kernels of apricots,
peaches, and plums; also in the seeds of apples, etc., in fact in
practically all the seeds of plants of the Rose family. It is the
mother substance for " oil of bitter almonds," which is widely
used as a flavoring extract.

Amygdalin has been the object of very extensive studies, and
even yet the exact nature of the linkage between its constituent
groups is not certainly known. When completely hydrolyzed, it
yields two molecules of glucose and one each of benzaldehyde and
hydrocyanic acid. Recent studies indicate that the two sugar
molecules are separately united to the other constituents, rather
than united with each other in the disaccharide relationship. In
other words, amygdalin is a true glucoside rather than a maltoside.
This is indicated by the fact that when submitted to the action of
all known hydrolyzing agents which affect it, it has never been
found to yield maltose as one of the products of hydrolysis.
Furthermore, the rate of hydrolysis of amygdalin is not affected
by the presence of maltose; and the segregation of the two glu-
cose molecules is accomplished by enzymes other than maltase,
which is the only enzyme which is known to break up a maltose
molecule. Since the exact nature of the linkage is not known, it is
customary and convenient to indicate the unit groups as linked
together in the following order :

C6HiiO5— O— C6Hio04— 0— CGH5 • CH— C=N

(1) (2) (3) (4)

GLUCOSIDES 87

A study of the hydrolysis reactions of amydalin shows that
there are three different linkages in the molecule which may be
broken by the simple interpolation of a single molecule of water
and a fourth which may be split by a different type of hydrolysis,
namely, the C=N linkage. These are indicated by the numbers
below the corresponding portion of the formula above. Most
hydrolyzing agents break the molecule first at (1), yielding one
molecule of glucose and one of mandelo nitrile glucoside (see page
77). The next step usually breaks the latter at the point indi-
cated by (2), yielding glucose and benzaldehyde cyanhydrin, or
mandelo nitrile. The latter in turn breaks down at (3) into ben-
zaldehyde and HCN. But when amygdalin is boiled with con-
centrated hydrochloric acid, the first change is the splitting off
at (4) of the nitrogen in the form of ammonia and the consequent
conversion of the CN group into a COOH group, producing amyg-
dalinic acid. On further hydrolysis, this breaks up in the same
order as before. Similarly, it is possible to convert mandelo
nitrile into mandelic acid by splitting off the nitrogen to form a
COOH group, instead of splitting off the HCN group leaving
benzaldehyde.

The mandelo nitrile glucoside contains an asymmetric carbon
atom which is wholly outside its glucose group, thus CeHioOs — O —
CeHo-CH-CN. Hence, it may exist in dextro, levo, and racemic
forms. In the amygdalin molecule, it exists in the dextro form,
which has been named " prunasin." The levo form, known as
" sambunigrin," has been obtained by hydrolysis of a compound
isomeric with amygdalin, wrhose composition has not been def-
initely worked out; while the racemic form, known as " pru-
laurasin," has been prepared from isoamygdalin, by the action of
alkalies. Hence, all the possible compounds indicated by the
presence of the asymmetric carbon have been found and identified.

The crude enzyme preparation which is obtained from almond
seeds, known as " emulsin," contains two enzymes, amygdalase,
which breaks the amygdalin molecule at linkage (1), and prunase,
which breaks it at (2). The action of amygdalase must always
precede that of prunase. In other words, it is never possible to
break off a disaccharide sugar from the molecule, either by the
action of prunase alone, or by means of any other hydrolytic agent.

Dhurrin, Ci-iHiTOyN, is another glucoside of fairly general
occurrence in plants, which yields HCN as one of the products of

88 CHEMISTRY OF PLANT LIFE

its hydrolysis. It is found in the leaves and stems of several
species of millets and sorghums. Frequent cases of poisoning of
cattle from eating of these plants as forage have been reported.
On hydrolysis, dhurrin first yields glucose and paraoxy-mandelo
nitrile; the latter then breaks down into paraoxy-benzaldehyde
and HCN.

Vicianin, CioH^OioN, is a cyanophoric glucoside, found in
the seeds of wild vetch, etc. On hydrolysis, it yields glucose,
arabinose, and d-mandelo nitrile. It is, therefore, similar to
amygdalin, except that one glucose molecule is replaced by ara-
binose.

THE MUSTARD OIL GLUCOSIDES

The seeds of several species of plants of the Cruciferse or mus-
tard family contain glucosides in which the other characteristic
group is a sulfur-containing compound. These glucosides yield
" mustard oils " when they are hydrolyzed by the enzyme myrosin,
which accompanies them in the plant. The following glucosides,
found in the seeds of white and black mustard, are the best-known
representatives of this class.

Sinigrin, CioHi60gNS2K, found in black mustard seeds, when
hydrolyzed yields glucose, acid potassium sulfate, and allyl iso-
sulfocyanide (mustard oil), as indicated by the equation.

The acid potassium sulfate group separates first and most
readily, leaving a compound known as merosinigrin, for which the
following formula has been suggested :

V/ |

CH2OH • CHOH • CH - CHOH - CH • CH

:N=C3H5

^^.

C=N=(

This compound usually breaks down into glucose and mustard
oil; but by special treatment it is possible to obtain from it thio-
glucose, CeHnOs-SH. This indicates that in the original glu-
coside the glucose is linked with the mustard oil through the sulfur
atom.

GLUCOSIDES 89

Sinalbin, C3oH420i5N2S2, from white mustard seeds, when
hydrolyzed by myrosin, yields glucose, sinalbin mustard oil (a
paraoxybenzyl derivative of allyl isosulfocyanide) and sinapin
acid sulfate; according to the equation

Sinalbin Glucose Sinalbin mustard oil

Sinapin acid sulfate

The sinalbin mustard oil may be represented by the formula
HO^ yCH^NCS. Hydrolysis of the sinapin acid sulfate con-
verts it into sinapinic acid, C6H2OH-(OCH3)2-CH = CH-COOH,
choline, N(CH3)4C2H4OH (see page 152), and H2S04. It is,
therefore, a very complex glucoside.

THE DIGITALIS GLUCOSIDES

The five, or more, glucosides which are present in the leaves
and seeds of the foxglove (Digitalis purpurea) have been exten-
sively studied, as they are the active principles in the various digi-
talis extracts which are used in medicine as a heart stimulant.

Digitoxin, C34Hs4Oii, which is the most active of these glu-
cosides in its physiological effects, when hydrolyzed, yields digit-
oxigenin, C22Hs204, and a sugar having the formula C6Hi2O4,
which is known as " digitoxose " and is supposed to be a" dimethyl
tetrdse.

Digitalin, CssHseOu, is also strongly active. When hydro-
lyzed, it yields digitaligenin, C22Hio03, glucose, and digitoxose.

Digitonin, C54HQ2028, constitutes about one-half of the total
glucosides in the extract which is obtained from most species of
the digitalis plants. It is much less active than the others. It is a
saponin (see page 90) in type. On hydrolysis, it yields 2 mole-
cules of glucose, 2 of galactose, and one of digitogenin.

Gitonin, C49Hso023, containing 3 molecules of galactose, one
of a pentose sugar, and one of gitogenin; and gitalin, C^H^gOio,
containing digitoxose and gitaligenin, have also been isolated from
digitalis extracts.

The structural arrangement of the characteristic groups in
these glucosides has not yet been definitely worked out.

90 CHEMISTRY OF PLANT LIFE

Cymarin, the active principle of Indian hemp (Apocynum can-
nabinum), is similar in type to the digitalis glucosides. When
hydrolyzed, it yields a sugar known as "cymarose," CyHuOy,
which seems to be a monomethyl derivative of digitoxose, and
cymarigenin, C23H3oOs, a compound which is either identical or
isomeric with the organic residue obtained from other members of
this group.

THE SAPONINS

The saponins constitute a group of glucosides which are widely
distributed in plants, whose properties have been known since
early Grecian times. They have been found in over four hundred
different species of plants, belonging to more than forty different
orders.

The most characteristic property of saponins is that they
form colloidal solutions in water which produce a soapy foam when
agitated, and are peculiarly toxic, especially to frogs and fishes.
In dry form, they have a very bitter, acrid taste, and their dust is
very irritating to the mucous membranes of the eye, nose, and
throat.

On hydrolysis, the saponins yield a variety of sugars, — glu-
cose, galactose, arabinose, and sometimes fructose, and even other
pentoses — and a group of physiologically active substances, known
as " sapogenins."

The more toxic forms of these glucosides are known as " sapo-
toxins."

The chemical composition of the saponins varies so widely
that it is scarcely possible to cite typical individuals. Sarsaparilla,
.the dried root of smilax plants, contains a mixture of non-poi-
sonous saponins, from which at least four individual glucosides
have been isolated and studied. Corn cockle contains a highly
poisonous sapotoxin which, on hydrolysis, yields four molecules of
a sugar and one of sapogenin, CioHieC^. Other sapotoxins are
obtained from the roots of soapwort and from several species of
Gypsophila. Digitonin and digito-saponin are glucosides of this
type which are found in the extracts from various species of
Digitalis.

GLUCOSIDES 91

THE PHYSIOLOGICAL USES OF GLUCOSIDES

It is scarcely conceivable that substances which vary so widely
in composition as do the different types of glucosides can possibly
all have similar physiological uses in plants. The cyanophoric
glucosides, the pigment glucosides, the mustard oil glucosides, and
the saponins, for example, can hardly be assumed to have the same
definite relationships to the metabolism and growth of the plant.
To be sure, they are alike in that they all contain one or more sugar
molecules, and it is probable that the carbohydrates which are
held in this form may serve as reserve food material, especially
when the glucoside is stored in the seeds; but it is obvious that the
simpler and more normal form of such stored food is that of the
polysaccharides which contain no other groups than those of the
carbohydrates. It seems much more probable that the physiolog-
ical uses of glucosides depend upon their ability to form temporarily
inactive " pairs " with a great variety of different types of organic
compounds which are elaborated by plants for a variety of pur-
poses.

It has been noted that in most, if not all, instances, the glu-
cosides are accompanied in the same plant tissue (although in
separate cells) by the appropriate enzyme to bring about their
hydrolysis and so set free both the sugar and the other charac-
teristic component whenever the conditions are such as to permit
the enzyme to come in contact with the glucoside. This occurs
whenever the tissue is injured by wound or disease, and also during
the germination process.

Injury to the plant tissue seems to be a necessary preliminary
to the functioning of the active components of the glucoside, except
in the case of the seeds. This leads naturally to the supposition
that at least some of these glucosides are protective or curative
agents in the plant tissues. This conception is further sup-
ported by the facts that many of the non-sugar components of
glucosides are bactericidal in character and that the glucosides
commonly occur in parts of the plant organism which are other-
wise best suited to serve as media for the growth of bacteria.
Thus, it is known that in the almond, as soon as the tissue is
punctured, amygdalin is hydrolyzed and all bacterial action is
inhibited. Similarly, the almost universal presence of glucosides
containing bactericidal constituents in the bark of trees insures

92 CHEMISTRY OF PLANT LIFE

natural antiseptic conditions for all wounds of the outer surfaces
of the stem of the plant. In fact, it is easily conceivable that at
least one of the reasons for the failure of the processes of decay of
plant tissues to set in until after the death of the cells, is that dur-
ing living, respiratory activity these antiseptic glucosides are so
generally present in the tissues.

Further, it has been fairly well established that the " chro-
mogens," or mother-substances of the pigments, which, under the
influence of oxidase enzymes, serve to regulate the respiratory
activities of the plant are essentially glucosidic in character.
This, and other, functions of the pigments, most of which are glu-
cosides, will be discussed at some length in the chapter dealing
with the Pigments (Chapter VIII).

Many gaseous anaesthetics are known to have a marked effect
in stimulating plant growth. In a number of cases, it has been
shown that the contact of plant tissues with these anaesthetics
brings about an interaction of the enzyme and glucoside which
are present in the tissue, with the consequent hydrolysis of the
latter, setting free its characteristic components. This observa-
tion has led to the supposition that many of the organic constit-
uents of glucosides are definite plant stimulants, to which the
name " hormones " has been applied. There is considerable
experimental evidence to support this conception that glucosides
may be the source of stimulating hormone substances, which will
be discussed more in detail in the chapter dealing with these plant
stimulants (Chapter XVII).

Glucosides may also serve as the mechanism for putting out
of action of harmful products which may appear in the tissues as
the result of abnormal conditions. These harmful substances
may be rendered soluble by combination with sugars and so trans-
posed by osmosis to some other part of the plant. The abnor-
mally large percentages of glucosides which are present in certain
species of plants during unfavorable climatic conditions lends
some support to this view.

Finally, it may be assumed that easily oxidizable substances,
such as aldehydes and acids, are possibly protected against too rapid,
or premature, oxidation by being transformed into glucosides.

In general, it may be said that the glucosides seem to serve as
the regulatory, protective, and sanatory agencies of the plant
mechanism.

GLUCOSIDES 93

BIOLOGICAL SIGNIFICANCE OF GLUCOSIDES

The bitter taste of glucosides and their almost universal
presence in the bark of plants undoubtedly helps to prevent the
destructive gnawing of the bark by animals.

Glucosides having either a strong bitter taste, or pronouncedly
poisonous properties, likewise undoubtedly serve to protect such
important organs of plants as the seeds and fruits from being
prematurely eaten by birds and animals. The common disap-
pearance of these bitter substances as the seed or fruit ripens
adds to the attractiveness of the material for food for animals at
the proper stage of ripeness to provide for wider distribution of the
seeds for further propagation. Further, the very general occur-
rence of these protective glucosides in many of the vegetative
parts of plants during the early stages of growth, followed by their
disappearance after the seeds of the plant have been formed,
certainly serves to protect these plants from consumption as forage
by animals before they have been able to develop their reproductive
bodies. The lack of palatability, and even the production of
digestive disorders resulting from the eating of unripe fruit may
be due, in part at least, to the presence of protective glucosides in
unripe fruits and vegetables.

On the other hand, the almost universal presence of the bril-
liant pigment glucosides in the external parts of flowers undoubt-
edly serves to attract the insects which are biologically adapted to
provide for the transportation of pollen from one blossom to
another and so to insure the cross-fertilization which is so impor-
tant in maintaining the vigor of many species of plants.

It is apparent that this important group of compounds, with
its exceedingly varied and complex constituent groups, may play
a variety of significant roles in plant growth.

REFERENCES.

ARMSTRONG, E. F. — "The Simple Carbohydrates and Glucosides," 239 pages,

Monographs on Biochemistry, London, 1919 (3d ed.).
VAN RUN, J. J. L.— "Die Glykoside," 511 pages, Berlin, 1900.

CHAPTER VII
TANNINS

USING the term in its general application to a group of sub-
stances having similar chemical and physical properties, rather
than in its limited application to a single definite chemical com-
pound known commercially as " tannin," the tannins are a special
group of plant substances, mostly glucosides, which have the
following characteristic properties. First, they are non-crystal-
line * substances, which form colloidal solutions with water,
which have an acid reaction and a sharp astringent taste. Second,
they form insoluble compounds with gelatine-containing tissues,
as shown by the conversion of hide into leather. Third, they form
soluble, dark-blue or greenish-black compounds with ferric salts,
the common inks. Fourth, they are precipitated from their solutions
by many metallic salts, such as lead acetate, stannous chloride,
potassium bichromate, etc. Fifth, they precipitate out of solu-
tion albumins, alkaloids, and basic organic coloring matters.
Finally, most tannins, in alkaline solutions, absorb oxygen from
the air and become dark brown or black in color.

OCCURRENCE

Tannins occur widely distributed in plants. Practically every
group of plants, from the fungi up to the flowering plants, contains
many species of plants which show tannin in some of their tissues.
Among the higher plants, tannins occur in a great variety of
organs. Thus, they are found in the roots of several species of
tropical plants; in the stems, both bark and wood, of oaks, pines,
hemlock, etc.; in the leaves of sumac, rhododendron, etc.; in
many fruits, especially in the green, or immature, stages; and in

*The needle-like forms, in which commercial "tannin" comes on the
market, are not true crystals, but are broken fragments of the threads into
which the colloidal tannin is " spun-out " from the syrupy extracts of nut-
galls, etc.

TANNINS 95

the seeds of several species, either before or after germination.
Tannins are also found in certain special structures, such as gland
cells, cells of the pulvini, laticiferous tissues, etc. Further, they
are especially abundant in the pathological growths known as
galls, which often contain from 40 to 75 per cent of tannin and con-
stitute the most important commercial source for these materials.
The principal commercial sources of tannin, which is used in
the manufacture of inks, in the tanning of leather, in certain dyeing
operations, etc., are oak-galls, the bark and wood of oak, hemlock,
acacia, and eucalyptus, the bark of the mangrove, the roots of
canaigre, and the leaves of several species of sumac.

CHEMICAL CONSTITUTION

Tannins are either free phenol-acids or, more commonly, glu-
cosides of these acids. Common " tannin," when hydrolyzed,
yields from 7 to 8 per cent of glucose, which indicates that it is a
penta-acid ester of glucose, i.e., each glucose molecule has five
acid groups attached to it. The formula for such a tannin is,
therefore, as follows,

-0 ,

CH2OR - CHOR • CH - CHOR - CHOR - CHOR

in which the R represents a complex phenol-acid like tannic acid,
or digallic acid. These acids are derivatives of the common
phenols, whose constitution will be brought to mind by the follow-
ing series of formulas :

Ordinary phenol Pyrocatechol Resorcinol Hydroqu'none

Pyrogallol Oxyhydroquinone Phloroglucinol

OH OH OH

i i

HOIJOJ

96 CHEMISTRY OF PLANT LIFE

These phenols themselves do not occur as constituents of tannins,
although they are often found in other glucosides, gums, etc.
The following mono-carboxyl acid derivatives of these phenols
are, however, found both free and in glucoside formation as con-
stituents of many of the common tannins.

Pyrocatechuic add, derived from pyrocatechol, represented by
the formula,

lOH

COOH

Gallic add. derived from pyrogallol, and represented by the
formula,

HO/\

In most of the common tannins, however, the characteristic acids
are oxy-derivatives of the so-called " tannon " group, represented
by the formula, CeH^-CO-O-CeHs. For example, digallic add,
which is a constituent of many common tannins, is a tetra-oxy,
mono-carboxyl derivative of this group, having the structural
formula,

O - O

OH O

Ellagic add, which is an hydrolysis product of many of the pyro-
gallol tannins (see below) and which produces the characteristic
" bloom " on leather tanned by this type of tannins, has the fol-
lowing formula,

^ CO-

T4NNINS 97

CLASSES OF TANNINS

The tannins are divided into two general classes, known
respectively as the pyrogallol tannins and the catechol tannins.
These differ in their characteristic reactions as follows:

Pyrogallol variety Catechol variety

Ferric salts Dark blue Greenish black

Bromine water No precipitate Yellow or brown precipitate

Leather Produce a " bloom " No " bloom "

Cone, sulfuric acid Yellow or brown Red or pink

Lime water Gray or blue ppte. Pink to brown ppte.

Pyrogallol tannins contain approximately 52 per cent of carbon;
while the catechol tannins usually contain 59 per cent to 60 per
cent, the difference being due to the absence of glucose from the
molecule in the latter types.

The two types are distributed in plants as follows: pyrogallol
tannins in oak-galls, oak wood, sumac, chestnut, divi-divi, and
algaro billa; catechol tannins in the barks of pines, hemlocks,
oaks, acacias, mimosas, cassia, and mangrove, in quebracho wood,
canaigre roots, cutch and gambier. The so-called " pseudo-
tannins " (i.e., compounds which do not tan leather but possess
other properties like tannins) are found in hops, tea, wine, fruits,
etc.

SOME COMMON TANNINS

Ordinary commercial " tannin," or " tannic add," is a com-
pound of one molecule of glucose with five of digallic acid. It is
found in many plants, and is prepared commercially from the
Turkish oak-galls and the Chinese sumac-galls. It exhibits all
the characteristic properties which have been listed above for
tannins in general and responds to all the characteristic reactions
of a pyrogallol tannin. It is extensively used for the manufacture
of blue-black ink, and in many technical processes.

Catechu tannin and catechin are compounds of the catechol
tannin type. The latter is obtained from acacia wood, mahogany
wood, mimosa wood, etc. It is not a true tannin, since it does not
convert hide into leather; but when heated to 120° or above, it is
easily dehydrated, forming catechu tannin which is identical with
that which is obtained directly from gambier and Bombay cutch
(products made by evaporating water extracts from the bark of

98 CHEMISTRY OF PLANT LIFE

various tropical trees). This latter is a true tannin, which is
much used in dyeing and other technical processes.

" Quercitannic acid," obtained from oak bark, etc., is likewise
a catechol tannin. It yields no glucose on hydrolysis.

A great many other tannins are known, and their possibilities
for technical use in tanning, dyeing, etc., have generally been
investigated; but so little has been learned about their composi-
tion and relation to the plant's own needs, that it seems unneces-
sary to discuss them in detail here.

PHYSIOLOGICAL USES OF TANNINS

Tannins are probably not direct products of photosynthesis.
They are, however, elaborated in the green leaves of plants and
translocated from there to the stems, roots, etc. Their close
association with the photosynthetic carbohydrates has led many
investigators to seek to establish for them some significant func-
tion as food materials, or as plastic substances in cell metabolism.
Many conflicting views have been advanced, but a careful review
of these leads inevitably to the conclusion that tannins probably
do not serve in any significant way as food material. The glu-
cose which is generally present in the tannin molecule may, of
course, serve as reserve food material, but it seems probable that it
functions as a constituent of the tannins only to assist in making
them more soluble and hence more easily translocated through the
plant tissues.

Some fungi, and perhaps other plants as well, can actually
utilize tannins as food material under suitable conditions and in
the absence of a proper supply of carbohydrates. But this does
not prove that tannins can normally replace carbohydrates as
food material for these species of plants.

There seems to be ample evidence that tannins are elaborated
where intense metabolism is in progress, such as occurs in green
leaves during the early growing season; in the rapid tissue forma-
tion which takes place after the stings of certain insects, producing
galls, etc. ; during germination, and as a result of any other unusual
stimulation of metabolism. It may be, therefore, that tannins
serve as safety accumulations of excessive condensations of formal-
dehyde, or other photosynthetic products, under such conditions.
It seems certain that in all such cases tannins are the result of,

TANNINS 99

and not (as some investigators have supposed) the causative agents
for, the abnormally rapid metabolism.

It seems to be fairly well demonstrated that tannins are inter-
mediate products for the formation of cork tissue. This may
account for their common occurrence in the wood and bark of
trees. Indeed, it has been shown that gallic and tannic acids
are present in considerable proportions in those parts of the plant
where cork is being formed. Further, that they bear direct rela-
tion to cork-formation has been demonstrated in two different
ways. First, cork-like substances have been artificially produced
by passing a stream of carbon dioxide through mixtures of for-
maldehyde with various tannic acids. Second, by various treat-
ments of cork, decomposition compounds showing tannin-like
properties may be obtained.

Some investigators have held that not only cork tissue but
also other lignose, or cell-wall material, may be developed from
tannins. Certain observations with Spirogyra seem to indicate
that tannin may play an important part in the formation of new
cell walls during conjugation, as cells which are ready to conjugate
are rich in tannin, which gradually diminishes in quantity until it
is practically absent at the time of spore-formation. There seems
to be no evidence that tannins perform any such function as this
in higher plants, however.

Again, tannins may play a very important part in pigment-
formation. They are very similar in structure to the anthocyanin
pigments, both being made up of practically identical decomposi-
tion units, the phenolic bodies. The disappearance of tannins
during the process of ripening of fruits may be connected, in part
at least, with the development of the brilliant red, blue, and yellow
pigments which give such rich colors to the thoroughly ripe fruits.

Finally, certain of the tannins undoubtedly serve as protective
agents to prevent the growth of parasitic fungi in fruits, etc.
Recent investigations show that at least some of the varieties of
fruits which are resistant to the attacks of certain parasitic diseases
utilize tannins for this purpose. This protective effect may be
accomplished in two different ways. Either the tannin actually
serves as an antiseptic to prevent the growth of the parasitic
fungus within the tissues of the host plant, or it assists in the
development of a corky layer which " walls-off " the infected area
and so prevents further spread of the disease through the tissue.

100 CHEMISTRY OF PLANT LIFE

Examples of both types of protective action have recently been
reported.

It is obvious that the different forms of tannins may play
different roles in plant life, and the same tannin substance may
possibly serve different purposes under different conditions.

BIOLOGICAL SIGNIFICANCE OF TANNINS IN FRUITS

The presence of tannins in fruits and the changes which they
undergo during the ripening process cannot fail to attract atten-
tion to their biological significance in serving to protect the fruit
from premature consumption as food by animals.

Tannins are of frequent occurrence in green fruits, imparting
to them their characteristic astringent taste. They nearly
always disappear as the fruit ripens. The fact that during the
ripening process both sugars and fruit esters, as well as attractive
surface pigments, are developed has led certain investigators to
the conclusion that tannins serve as mother-substances for these
materials in the green fruits and are converted into these attractive
agencies during ripening. There is nothing in the chemical com-
position of tannins which indicates, however, that they are pre-
cursors of sugars or fruit esters, although (as has been pointed out)
they may give rise to anthocyan pigments.

Further, recent researches concerning the tannin of persim-
mons (the best-known and most striking example of the phenomena
under discussion) clearly show that the tannin is not actually used
up during the ripening process; that instead it remains in the ripe
fruit in practically undiminished quantity; but that when the
fruit is ripe, the tannin is enclosed in certain special large cells or
sacs, which are surrounded by an insoluble membrane, so that
when the fruit is eaten by animals the astringent tannin, enveloped
in these insoluble sacs, passes by the organs of taste of the animal
without causing any disagreeable effects. This walling-off of the
astringent tannins can be stimulated in partially ripe fruits by
treating them with several different chemical agents, the simplest
method being that of placing the unripe fruit in an atmosphere of
carbon dioxide gas for a short period. The artificial " processing "
of persimmons to render them edible for a longer period before
they become naturally fully ripe and subject to decay is now a
commercial enterprise. This process is of interest because of

TANNINS 101

its possible connection with the conversion of tannins into cork,
under the influence of carbon dioxide gas, as mentioned in a pre-
ceding paragraph.

From these facts, it is apparent that in persimmons, and prob-
ably in other tannin-containing fruits, the process of natural selec-
tion has developed a mechanism for the secretion of tannin in
green fruits, followed by a process for walling it off in harmless con-
dition when the fruit is ripe, which serves most admirably to
protect the fruit from consumption by animals before the enclosed
seeds have fully developed their reproductive powers.

REFERENCES.

ABDERHALDEX, E. — " Biochemisches Handlexikon, Band 7, Gerbstoffe,
Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze,
Kautschuk," 822 pages, Berlin, 1912.

ALLEN'S Commercial Organic Analysis, Vol. 5, "Tannins, Dyes and Coloring
Matters, Inks," 704 pages, 6 figs., Philadelphia, 1911 (4th ed.).

COOK, M. T. and TAUBENHAUS, J. J. — "The Toxicity of Tannin," Delaware
College Agricultural Experiment Station Bulletin No. 91, 77 pages, 43
figs., Newark, Del., 1911.

DEKKER, J. — "Die Gerbstoffe," 636 pages, 3 figs., Berlin, 1913.

GORE, H. C. — "Experiments on the Processing of Persimmons to Render
them Nonastringent," U. S. Department of Agriculture, Bureau of
Chemistry Bulletin No. 141, 31 pages, 3 plates, 1911; and No. 155, 20
pages, 1912.

LLOYD, F. E. — "The Tannin-Colloid Complexes in the Fruit of the Persim-
mon, Diospyrus," in Biochemical Bulletin, Vol. 1, No. 1, pages 7 to 41,
34 figs., New York, 1911.

CHAPTER VIII
PIGMENTS

PRACTICALLY all plant structures contain pigments. These may
be considered as of two types : (a) the vegetative pigments, which
have a definite energy-absorbing role in the metabolic processes
of the tissues which contain them, and (6) the ornamental pig-
ments. It is probable that the same chemical compound may
serve in either one of these capacities under different conditions,
but, in general, it is possible to assign either a definite vegetative,
or physiological, use, or else a simple ornamental, or biological,
significance to each of the common pigments. The first type is
found widely distributed through the protoplasm, or cell-sap,
of the plant structures ; while the ornamental pigments are located
chiefly in the epidermal cells, especially of flowers.

With respect to their colors, the plant pigments may be
grouped as follows:

Green — the chlorophylls.

Yellow — the carotinoids, flavones, and xanthones.

Red — phycoerythrin, lycopersicin, anthocyanin.

Blue — anthocyan derivatives.

Brown — phycophaein, fucoxanthin".

Of these, the chlorophylls, the carotinoids, phycoerythrin (in
red sea-weeds) and phycophsein (in brown sea-weeds) are gener-
ally vegetative pigments; while the others form the basis for most
of the ornamental pigments, although they may have a definite
energy-absorbing effect, in some cases.

THE CHLOROPHYLLS

The importance of the green coloring matter in plants has
been understood for more than a century, its connection with

102

PIGMENTS

103

photosynthesis having been known as far back as 1819. But
definite knowledge as to its chemical constitution is of very recent
origin. As recently as 1908, it was asserted that chlorophyll is a
lecithin-like body, yielding choline and glycero-phosphoric acid
on hydrolysis. It is now known, however, that chlorophyll con-
tains neither choline nor phosphorus, the earlier observations being
due to mixtures of various other materials with the true chloro-
phyll in the extracts which were examined. Beginning with 1912,
Willstatter and his collaborators, in a series of classic papers
which were finally collected in book form, clearly demonstrated
the chemical constitution of the green pigments of plants, which
had been previously designated under the single name " chloro-
phyll." In 1912, Willstatter and Isler first showed that the green
coloring matter which is extracted from plants by alcohol, ether,
etc., is made up of two definite chemical compounds, to which
they assigned the names " chlorophyll a " and " chlorophyll &,"
associated with two yellow pigments, carotin and xanthophyll, and,
in some cases, with the reddish-brown fucoxanthin. The per-
centages of total pigment materials, and the relative proportions
of the five different pigments, in several types of plants, are as
follows :

Land

Brown

Green

Plants,

Seaweeds,

Algae,

Per Cent.

Total pigment in the dry matter

0.99

0.29

0 21

Proportion of:

Chlorophyll a

Chlorophyll b

Carotin

Xanthophyll

Fucoxanthin

The two chlorophylls have the following formulas; chloropnyll
a, C55H72O5N4Mg, and chlorophyll 6, CooHroOe^Mg. Hence,
they differ only in having two hydrogen atoms in the one replaced
by one oxygen atom in the other. Both are amorphous powders,
from which crystalline chlorophyll (see below) can be obtained by
hydrolysis. ChloroplMl a is blue-black, is easily soluble in most
organic solvents, and when saponified by alcoholic potash gives a

104 CHEMISTRY OF PLANT LIFE

transient pure yellow color. Chlorophyll b is dark green, is some-
what less soluble than the other form, and when saponified by
potash gives a transient brilliant red.

Amorphous and Crystalline Chlorophyll. — When the chloro-
phyll of plants is extracted by alcohol and the alcoholic extract
evaporated nearly to dryness, beautiful dark green crystals are
obtained. Willstatter has shown, however, that in these crys-
tallized forms the ethyl group (from the ethyl alcohol used) has
replaced the phytyl group (see below) which is present in the pig-
ments as they exist in the plant tissues; and that, when
extracted by other solvents than alcohol, the pigments may
be obtained in the amorphous forms in which they exist in
the plant.

This change from amorphous to crystalline compounds may be
understood from the preliminary statement that the chlorophylls
are esters of tri-basic acids, in which one acid hydrogen is replaced
by the methyl (CHs) group and a second by the phytyl (C2oHs9,
from phytol, or phytyl alcohol, C2oHs9OH) group. When treated
with ethyl alcohol (C2HsOH) for the purpose of extracting the
pigments, the ethyl (C2Hs) group replaces the phytyl group, thus
yielding a methyl-ethyl ester, and these esters are the crystalline
forms of the chlorophylls. This replacement is made possible
through the action on the original pigment in the tissues of an
enzyme, chlorophyllase, which is also present in the tissues, which
splits off the phytyl group, forming phytyl alcohol, and leaving a
free COOH group in the pigment, with which the alcohol used
in the extraction forms the ethyl ester (see Chapter IX for a dis-
cussion of the formation and hydrolysis of esters).

While the chlorophylls are tri-basic acids, only two of the
acid COOH groups actually function in ester-formation. The
third acid group seems not to exist as a free acid group; but in
chlorophyll a, it is in what is known as the " lactam " arrangement,
represented by the — CONH — group, and in chlorophyll 6, it is
probably in the " lactone " arrangement, represented by the
— COO — group; the two bonds in each case being attached to
different structural units in the molecule (see page 106).

The change from amorphous to crystalline forms may be repre-
sented by the following formulas, in which the R represents the
whole of the complex group to which the acid ester groups are
united:

PIGMENTS

105

/COO-CHs
R/

XJOO- C2oH39

Amorphous chlorophyll

methyl-phytyl chloro-
phyllide

/COO-CHs
\COO- C2H5

Crystalline chlorophyll

methyl-ethyl chlorophyl-
lide

CH
H

Chlorophyllin

" Chlorophyllin," the compound in which the ester groups
have been converted into free acid groups, as indicated above,
may be obtained from either amorphous or crystalline chlorophyll
by treatment with caustic potash dissolved in methyl alcohol.

Phytol. — This alcohol, which furnishes the characteristic ester
group in the chlorophyll of plants, is a compound of very unusual
composition, which has never been found in any other form or in
any other type of compound which is present in either plant or
animal tissues. Careful studies of its addition and oxidation
products prove that it has the following structural arrangement:

HHHHHHHH H

H-C-C— C— C— C— C— C— C — ' H '
H CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3

As this formula indicates, the compound contains one unsaturated,
double-bond linkage, one primary alcohol group, and eleven
methyl groups. As has been said, this alcohol occurs nowhere
else in nature, and its presence and function in the chlorophyll
molecule are, as yet, wholly unexplainable. Phytol itself is a
colorless, oily liquid, with a high boiling point (145° in vacuo,
204° at 10 mm. pressure).

THE CONSTITUTION OF THE CHLOROPHYLLS

As has been mentioned, chlorophyll a differs from chlorophyll b
by having one more oxygen and two less hydrogen atoms in the
molecule, and in having one of its nitrogen atoms in the " lactam "
arrangement. These differences in structure are represented by
the following formulas which are commonly used to represent the
two compounds, but which do not show the arrangements of the
major groups of the complex molecules:

106 CHEMISTRY OF PLANT LIFE

/COO • C2oH39 /COO • C2oH39

MgC3iH29N3^-COO - CH3 MgC32H2802N4-C00 ' CH3

NH-CO Chlorophyll 6

Chlorophyll a

The chlorophylls are unstable compounds, readily acted upon
by acids or alkalies, and by the enzyme chlorophyllase, which
splits off the phytyl alcohol group. The progressive action of
acids and of alkalies in breaking down the molecule, and the prod-
ucts of its oxidation and reduction, have served to establish the
chemical composition of the compound in each case. Because
of the importance of these pigments in the whole metabolic proc-
esses of the plant, it seems to be desirable to consider the nature
of these reactions in some detail, as follows :

Decomposition of the Chlorophylls by Alkalies. — The first
action of dilute alkalies on the chlorophylls is to split off, by
hydrolysis, the alcoholic groups of the esters, producing the crys-
talline tri-basic acids, or chlorophyllins a and b. Each of these
chlorophyllins exists in two forms, the normal and the iso, in which
the attachment of the COOH groups to the other groups in the
molecule is in different positions. Hence, chlorophyll a yields
chlorophyllin a and isochlorophyllin a, and chlorophyll b yields
chlorophyllin b and isochlorophyllin b, all four of which are tri-
basic acids.

These compounds, when heated with alkalies, split off carbon
dioxide in successive stages, losing one COOH group at each step,
thus yielding a series of simpler compounds of the following types:
First, di-basic acids; second, monobasic acids; and finally,
cetiophyllin, a compound in which no COOH group is present.
In all of these compounds, derived from chlorophylls by the action
of alkalies, the Mg remains in the molecule, and all the Mg-con-
taining derivatives from the chlorophylls are known as " phyllins."
At the stage at which only one COOH group remains in the
molecule, only one group arrangement is possible, and the
derivatives from chlorophyllin a and isochlorophyllin 6, and those
from chlorophyllin b and isochlorophyllin a, are identical. At
the final stage, the derivatives from all four forms are identical.
This may be graphically illustrated by the following diagram indi-
cating the progressive decomposition of the two chlorophylls
under the action of alkalies:

PIGMENTS

107

"Esters

Chlorophyll a

Chlorophyll b

Tribasic Isochlorophyllina Isochlorophyllini Chlorophyllina Chlorophyllini

:ids ^

MgC31H29N3-

COOH

Dibasic Cyanophyllin Kubiphyllin Glaucophyllin

adds ErytharSphylhn Kfrrf, TI _M COOH Rhodophyllin

Monobasic Phyllophyllin

acids MgC31H33N.4-COOH

Neutral

aetiophyllin

Decomposition of Chlorophylls by Acids. — The first action of
dilute acids upon chlorophylls is to remove the magnesium,
without otherwise changing the molecule. Two hydrogens go
in in the place of the magnesium. Dilute acids act in precisely
the same way upon each of the " phyllins " shown in the above
scheme. In this way, a whole series of compounds, corresponding
to each of the chlorophylls and their alkali-decomposition products,
but with the magnesium lacking in each case, has been prepared.
Thus,

Chlorophyll a,

/COO-C2oH39
^-COO-CHs,
\(NHCO)

becomes Phseopnytin a,

/COO-C2oH39
^-COO-C
\(NHCO)

108 CHEMISTRY OF PLANT LIFE

/COO
Chlorophyll 6, MgC3iH2902N4<

\COO- CH3,

/COO • C2()

becomes Phseophytin b} C^Hsc^^^

XCOO-CH3

Similarly,

Isochlorophyllin a, becomes Phytochlorin e,
Chlorophyllin a, becomes Phytochlorin /, and g,

/COOH

C32H320N4<

\COOH

Isochlorophyllin b, becomes Phytorhodin g
Chlorophyllin b, becomes Phytorhodin i and k,

/COOH
C32H3o02N4<

XCOOH

And bodies known as " porphyrins " are similarly derived from
all the other known phyllins.

For example: cyanophyllin, MgC3iH32N4(COOH)2, becomes
cyanoporphyrin, C3iH34N4(COOH)2; a?tiophyllin, MgC3iH34N4,
becomes setioporphyrin, CsiH36N4, etc.

Phytochlorin e and phytorhodin g are the chief products of the
decomposition by acids of the chlorophylls. Indeed, it was the
production of these compounds which led to the discovery of the
existence of the two chlorophylls. When treated with alkalies,
they lose their carboxyl groups and become setioporphyrin.

Decomposition of the Chlorophylls by Oxidation and Reduc-
tion.— When acted upon by oxidizing agents, such as chromic acid,
the porphyrins yield two chief oxidation products, which are
pyrrole derivatives having the following formulas,

CH3— C— COV CH3— C— COV

II >NH || >NH

CH3— CH2— C— CO/ HOOC— CH2— CH2— C— CO/

Methylethylmalein imide Hsematinic acid imide

PIGMENTS 109

By reduction, there have been obtained from the chlorophylls
and the various porphyrins, three isomeric pyrrole derivatives
having the following formulas,

CH3 H CH3

C2H5— C=Cv C2H5— C=C\ C2H5— G=

>NH >NH I >NH

^ — Q/ PIT p — P/

CH3— C=CX
?H3 CH3 H

Phyllopyrrole Hsemopyrrole Isohaemopyrrole

As a result of the study of these decomposition units, Will-
statter has suggested the following formulas for the structural
arrangement of setiophyllin and setioporphyrin, the compounds
which result from the removal of all of the acid groups and finally
of the magnesium from the chlorophylls,

H HC=CH H HOCH

CH*?-°vw ^-c:

H5-C-CT

CH-C-C -CH C2H5-C-C

C2H5-C=C/ \/ \>C-C2H5 C2H5-C=C/ V=C-C2H5

I >-NV< I > NV I

cqrQ-cr >prC-CHi cH3-c=c/ Nc=c-cH3

CH3 CH3 CH3 CH3

aetiophyllin setioporphyrin

The COOH groups which are attached to these compounds to
form the various phyllins and porphyrins, as well as the original
chlorophylls, are supposed to be attached to the C2Hs groups in
the above formulas, the different modifications, or compounds,
depending upon the position in which one or more of these attach-
ments are made.

SIMILARITY OF CHLOROPHYLL AND HEMOGLOBIN

It seems to be desirable, at this point, to call attention to the
remarkable similarity in the chemical composition of chlorophyll,
the most important pigment of plants, and haemoglobin, the all-
important respiration-regulating pigment in the blood of animals.

110 CHEMISTRY OF PLANT LIFE

Hsemoglobin is a complex compound, consisting of about 96 per
cent of albumin (a protein, see Chapter XIII) united with about
4 per cent of hcematin, a brilliant red pigment which has the
formula FeClC32H3204N4. When treated with acids, the iron
(and its accompanying Cl) is removed, and hsematoporphyrin,
C32H3eO4N4, is obtained. When either haematin, or hsemato-
porphyrin is oxidized, hsematinic acid imide identical with that
obtained from aetioporphyrin is obtained. Also, when hsema-
toporphyrin is reduced, haemopyrrole identical with that from
setioporphryin is obtained. Thus, it would appear that the unit
structural groups in hsematin and in chlorophyll are identical;
although chlorophyll may exhibit more variations in isomeric
arrangement of these structural units than have been found in
haematin. Hence, it is apparent that the only essential difference
in composition between chlorophyll and hsematin is that in the
former the structural units are linked together by iron, while in
the latter, the same units are united through magnesium as the
linking element. Further, it is known that while iron is not a
constituent element in the chlorophyll molecule, it is, in some
unknown way, absolutely essential to the production of chloro-
phyll in plants; plants furnished with an iron-free nutrient solu-
tion rapidly become etiolated and photosynthesis stops.

The following skeleton formulas have been suggested to indi-
cate the way in which these elements are linked between the struc-
tural units in their respective compounds.

— C,

>N N/ >N N«

— c/ \ / X3— — <y \

Mg Fe

\ /C-

N< >N Cl N

Chlorophyll Haematin

It is understood, of course, that the mineral element does not
furnish the definite means of holding the structural units together
as otherwise it would not be possible to remove the iron, or mag-
nesium, without breaking down the molecule, as is done in the
case of the porphyrins. The actual binding linkage is undoubtedly
between carbon atoms, as indicated in Willstatter's formulas for
setiophyllin and setioporphyrin (see page 109). The attach-

PIGMENTS 111

ment of the magnesium to each one of the four nitrogen atoms in
the skeleton formula assumes the existence of subsidiary valences
of 2-4 for magnesium (and of 3-5 for iron), or of possible oscillating
valences similar to those supposed to be exhibited by carbon
in its closed-ring arrangements.

PROPERTIES OF THE CHLOROPHYLLS

The phytyl esters, or natural chlorophylls, are amorphous
solids; while the methylethyl esters (chlorophyllins) and the free
acids (phyllins) are crystalline compounds. All of these com-
pounds are easily soluble in ether and alcohol, but insoluble in
water. The chlorophylls and chlorophyllins are practically insol-
uble in petroleum ether and chloroform; but the monobasic acids
(pyrrophyllin and phyllophyllin) and the neutral aetiophyllin
dissolve easily in chloroform.

Solutions of the chlorophylls are fluorescent, being green by
transmitted, and red by reflected, light.

Chlorophyll a is a . blue-black solid, which gives dark green
solutions in all of its solvents. Chlorophyll b is a dark-green solid,
which yields brilliant green solutions. Solutions in ether of glau-
cophyllin and of cyanophyllin are blue; of rhodophyllin, deep
violet; of rubiphyllin, light violet; of erythrophyllin, red; and
of pyrrophyllin and phyllophyllin, bluish-red. Solutions of the
porphyrins are all red, the di-basic ones being usually a bluish-
red, and the simpler ones a brilliant red to deep brownish-red in
color.

The several chlorophyll derivatives are further distinguished
by characteristic differences in their absorption spectra. These
differences have been pictured by Willstatter in his book dealing
with the results of his investigations concerning the chlorophylls,
and reproduced in one or two other texts which treat in detail
with the physical-chemical properties of these pigments, but need
not be presented in such detail here.

THE CAROTINOIDS

The characteristic brilliant green of healthy plant tissues is
due to the fact that there are always associated with the dark
bluish-green chlorophylls two (or more) yellow pigments. These

112 CHEMISTRY OF PLANT LIFE

are known as the " carotinoids." This group includes the two
brilliant yellow pigments, carotin and xanthophyll, and the reddish
brown fucoxanthin and the brilliant red lycopersicin, which are
similar in their chemical composition. The first two are found
universally distributed in plants, associated with the chlorophylls,
and may be regarded as vegetative pigments, although the char-
acteristic ornamental yellow and orange colors of many flowers
and fruits, as well as that of the roots of carrots, etc.. are due to
these pigments

Carotin. — This pigment occurs in various forms in plants, both
amorphous and crystalline. It crystallizes out of solution in
flat plates, which are orange-red by transmitted light, and green-
ish-blue by reflected light, and have a melting point of 174°.
Carotin is insoluble in water, only very slightly soluble in acetone
or cold alcohol, readily soluble in petroleum ether, ether, chloro-
form, and carbon disulfide. Its solutions are strongly fluorescent.

Its molecular formula is C^Hso. It is, therefore, a hydro-
carbon of a very high degree of unsaturation. On exposure to
dry air, it absorbs 34.3 per cent of its own weight of oxygen,
which corresponds to 11J atoms of oxygen, computed on the basis
o£ the molecular formula C4oH56, and would indicate a formula of
(C4oHs6)2023 for the oxygenated compound; this being three oxy-
gen atoms less than would be required to bring the compound to
the theoretical stage of saturation represented by the unimolecular
formula CnH2ra+2. In moist air, two more oxygen atoms are
absorbed, probably forming two OH groups in the molecule.
Moreover, carotin absorbs iodine. When the calculated amount of
iodine is used, a definite compound having the formula C4oHs6l2
is produced; but in the presence of an excess of iodine
another compound having the apparent formula C4oH56l3 (or
2C4oH56l2+l2) is obtained. (Note that 2 atoms of iodine plus
12 atoms of oxygen, or 3 of iodine plus 11 J of oxygen, produce the
degree of saturation required by the formula CraH2w+2.) It is
evident from these experimental data, that a part of the unsat-
urated linkage in the carotin molecule is of a type which can easily
be saturated by direct addition of oxygen, while the remainder
may be saturated by iodine.

The reaction of carotin toward bromine is peculiar. With
this element, it forms a compound having the formula
indicating the direct addition of two atoms of bro-

PIGMENTS 113

mine and the substitution of twenty atoms of this element for the
same number of hydrogen atoms.

The oxygenated carotins are colorless substances, while the
iodide crystallizes in beautiful dark-violet prisms, having a cop-
pery red fluorescence.

Xanthophyll is closely related to carotin. It has the molecular
formula C4oH5eO2. It absorbs 36.55 per cent of oxygen (corre-
sponding to 13 atoms, which would indicate the formation of two
OH groups an addition to the saturation required by the CnH2n+2
formula) ; and an iodine addition product having the formula
C4oH5oO2l2, which crystallizes in dark- violet needles.

Xanthophyll differs markedly from carotin in its solubilities,
being insoluble in petroleum ether and only sparingly soluble in
carbon disulfide. It may be fairly easily reduced to carotin.
This transformation is reversible, and suggests a similarity to the
change from haemoglobin to oxyhaBmoglobin, and the reverse,
in the blood of animals, as a part of their respiration process.

Separation of the Chlorophylls, Carotin, and Xanthophyll.—
These pigments, which exist together in most plant tissues, may
easily be separated from each other by taking advantage of the
differences in their solubilities, according to the following pro-
cedure. Grind up a small quantity of the fresh tissue (leaves of
the stinging nettle furnish a conveniently large supply of each of
these pigments) with fine sand in a mortar. Cover with acetone,
let stand a few moments and then filter on a Buchner funnel.
Pour the filtrate into a separatory funnel, add an equal volume of
ether and two volumes of water. Shake up once and then allow
the ether layer to separate; the pigments will be in this layer.
Drain off the water-acetone layer. Now to the etherial solution,
add about half its volume of a concentrated solution of potassium
hydroxide in methyl alcohol. Shake well and allow to stand until
the mixture becomes permanently green. Now add an equal
volume of water and a little more ether, until the mixture separates
sharply into two layers. The chlorophylls will now be in the
lower dilute alcohol layer, and the carotinoids in the upper ether,
and may be separated by draining off each layer separately.
To separate the carotin from xanthophyll, place the ether solu-
tion in a small open dish and evaporate to a small volume. Now
add about ten volumes of petroleum spirit and an equal volume
of methyl alcohol, stir up well, transfer to a separatory funnel and

114 CHEMISTRY OF PLANT LIFE

allow the two layers to separate. The carotin will now be in
the upper layer of petroleum ether, and the xanthophyll in the
lower alcohol layer; these layers may be drained off separately
and the solvents evaporated in order to recover the pigments in
dry form.

Lycopersicin (or lycopin) is a hydrocarbon pigment having
the same formula as carotin. It is, however, brilliantly red in
color, and crystallizes in a different form and has a different
absorption spectrum from carotin. It is the characteristic pig-
ment of red tomatoes, and is found also in red peppers. Yellow
tomatoes have only carotin as their skin-pigment, while lyco-
persicin is usually present in the flesh of the ripe fruits of all vari-
eties and in the skin of red ones. It has been shown, however,
that if varieties of tomatoes which are normally red when ripe,
are ripened at high temperatures, 90° F. or above, their skins
will be yellow instead of red when fully ripe. Hence, the occur-
rence of carotin, or of lycopersicin, as the skin pigment is deter-
mined in part by the varietal character (being different in different
varieties when ripened at normal temperatures) and in part by the
temperature at which the fruit ripens. The two pigments are,
of course, isomers; but the difference in their structural arrange-
ment is not known.

Fucoxanthin, C^H^Oe, is a brownish-red pigment, found in
fresh brown algae, and in some brown sea-weeds. Its formula
indicates that it is an oxidized carotin. With iodine, it forms a
compound having the formula C^H^OeLi. It is unlike carotin
and xanthophyll in that it has basic properties, forming salts
with acids, which are blue in color.

PHYCOERYTHRIN AND PHYCOPttflEIN

These are the principal pigments of red and brown seaweeds,
respectively. Their most characteristic difference from the
pigments of non-aquatic plants is that they are easily soluble in
water, and insoluble in most organic solvents, such as alcohol,
ether, etc. At first thought, this would appear to be impossible,
since the plants grow in water and it would seem that their water-
soluble pigments would be continuously dissolved out of the tis-
sues. The reason why this does not occur lies in the fact that
these pigments exist in the cells of the seaweeds in colloidal

PIGMENTS 115

form (see Chapter XV), and, hence, cannot diffuse out through
the cell-walls. The only way in which they can be extracted
from the tissues is by rupturing the cells, by grinding with sharp
sand, etc., after which the pigments can readily be dissolved out
by water.

Phycoerythrin is the red pigment. It is a colloidal, nitrog-
enous substance, allied to the proteins (see Chapter XIII) but
not a true protein compound. Hydrolysis by acids indicates
that it contains leucin and tyrosin, two amino-acids which are
constituents of proteins, along with other bodies of unknown com-
position.

The colloidal solution of phycoerythrin in .water has a bril-
liant rose-red color, with an orange fluorescence. It readily sets
to a gel (see Chapter XV), so that the solution is almost impossible
to filter. On this account, purified solutions of this pigment are
very difficult to secure, and no satisfactory analysis to indicate its
composition has yet been obtained.

Actinically, it is a complementary pigment to chlorophyll,
that is, it absorbs the blue and green rays and permits the passage
of light which is of the wave length that is absorbed by chlorophyll.

Phycophaein. — Still less is known of the composition of this
pigment than of that of phycoerythrin. It is the characteristic
pigment of brown seaweeds. It is supposed to exist in the cells
of algae, chiefly as a colorless chromogen, which becomes first
yellow and then brown on exposure to air. Associated with it are
other pigments, which have been variously reported as carotin,
phycoxanthin, etc.

THE ANTHOCYANS

These are a group of pigments of red, blue, or violet color,
which occur in the flowers, fruits, or leaves of many species of
plants. They are essentially ornamental pigments, and consti-
tute a large proportion of the brilliant colors of flowers, etc.
They occur not only dissolved in the cell-sap, but also as deposits
of definite crystals or amorphous compounds in the cell proto-
plasm.

They are all glucosides. When the anthocyans are hydrolyzed,
the sugar molecules are split off and the characteristic hydroxy-
derivatives of the three-ring anthocyan nucleus (figured on page

116

CHEMISTRY OF PLANT LIFE

83), known as "anthocyanidins," remain. These anthocyanidins
are themselves pigments. They have been shown to be all deriva-
tives of the anthocyan nucleus. The oxygen atom in this nucleus
is very strongly basic and exhibits its quadrivalent property by
forming stable salts by direct addition of acid radicles. The
variation of color of the anthocyanins has been explained by
Willstatter, as follows; the red is the acid salt, the blue is a neu-
tral metallic salt, and the violet is the anhydride of the antho-
cyanidin in question, thus

Cl Cl

Violet

All of the natural anthocyanin pigments appear to contain a
chlorine atom attached directly to the ring oxygen, as shown in the
above partial formulas. In addition, they have four, five, or six
hydroxyl (OH), or methoxy (OCHs), groups attached at various
points around the three rings. The following formula for cenidin,
one of the most complex of these anthocyanidins, will illustrate
their structural arrangement.

Delphinidin is the corresponding compound without the two
CHs groups; while cyanidin contains only five OH groups; and
pelargonidin, only four OH groups.

PIGMENTS 117

The anthocyanin pigments are soluble in water, alcohol, and
ether, the solutions being red or blue in color according to the
acidity or alkalinity of the medium. Their presence in many
species of plants is hereditable, as these plants come true to color
from seed, as in the case of red beets, red cabbage, several species
of blue berries, etc. In other cases, the anthocyanin development
depends largely upon the conditions of growth, particularly those
which prevail during the later stages of development; as in the
case of apples, where the amount of red color in the skin depends
to a large extent upon the conditions under which the fruit ripens.

Anthocyanin pigments often make their appearance late in the
season; in fruits, etc., as the result of the normal ripening process,
but in leaves as the result of shorter daylight illumination accent-
uated also by sharp frosts.

THE ANTHOXANTHINS

The yellow plant pigments, other than the carotinoids, are
almost without exception glucosides having a xanthone or flavone
nucleus. These typical nuclei are illustrated on page 83. In
these nuclei, as hi the anthocyan one, the oxygen atom is strongly
basic and combines with mineral acids to form salts (the oxygen
becoming quadrivalent) and the color of the pigment depending
upon the nature of the combination formed in this way.

The anthoxanthin pigments are yellow, crystalline solids,
which are only slightly soluble in water. They dissolve readily
in dilute acids and alkalies, giving yellow or red solutions which are
of the same color in either acid or alkaline media. They are
extensively used as yellow dyes.

Many of the common members of this group have been men-
tioned in the chapter dealing with the glucosides. The charac-
teristic pigment nucleus of several of these is as follows:

Chrysin, found in various species of poplar and mallows,

118 CHEMISTRY OF PLANT LIFE

Apigeninj found in parsley and celery, as the glucoside apiin,

Campferol, found in Java indigo, as the glucoside campferitrin,

Fisetin, found in quebracho wood and fiset wood,

JOH

Quercitrin, found in oak bark, horse-chestnut flowers, and in
the skin of onions,

OH
™\

PIGMENTS 119

Morin, found in yellow wood (Moras tinctoria),

o H0

VCI>OH

JOH

Gentisin, found in yellow gentian (Gentiana lutea),

v/°\

As a rule, the most brilliant of these yellow pigments are found
in the largest quantities in the bark and wood of various species of
tropical plants; although they are also present, in smaller amounts,
in the blossoms of species growing in temperate zones.

The anthoxanthins are easily converted into anthocyanins,
and vice versa, by the action of oxidizing and reducing enzymes
which are commonly present in the tissues of the plants which
develop the pigments.

THE PRODUCTION OF ORNAMENTAL PIGMENTS IN FLOWERS, ETC.

The breeding of flowering plants having blossoms of almost
any desired color has become a commercial enterprise of large
importance. The results which have been obtained, in many
cases, have been made the object of scientific study of the genetics
of color inheritance. These studies have developed certain inter-
esting facts with reference to the chemistry of the development of
these ornamental pigments, which may be briefly mentioned here.

In many of the plants which have been studied, the color of
the flowers depends upon several different factors, as follows:

C, a chromogen (or color-producing substance) which is gen-
erally a flavone or xanthone glucoside, and which may be either
yellow or colorless.

120 CHEMISTRY OF PLANT LIFE

E, an enzyme which acts upon C, to produce a red pigment.

e, another enzyme which acts upon the red pigment, changing it
to some other anthocyanin color.

A, an antioxidase, or antienzyme, which prevents the action
of E.

R, an enzyme which changes reds to yellows.

Thus, if a plant whose flower contains only the factor C be
crossed with one which contains the factor E, a red blossom will
result, or if it contains the factor e more intense pigments are
developed. But if either A or R are present, no change in the
color of the original parents will result from the crossing.

THE PHYSIOLOGICAL USES OF PIGMENTS

The vegetative pigments undoubtedly serve as agencies for
regulating the rate of metabolic processes. At the same time, it
is extremely difficult to determine whether the presence of a pig-
ment in any given case is the cause or the effect of the changes in
the plant's activities which result from changes in its external
environment.

The chlorophylls are, of course, the regulator of photosyn-
thesis, absorbing solar energy with which the photosynthetic
process may be brought about. The simultaneous presence of
carotinoids in varying amounts undoubtedly serves to modify the
amount and character of the radiant energy absorbed, as these
pigments absorb a different part of the spectrum of light and hence
undoubtedly produce a different chemical activity or " actinic
effect " of the absorbed energy. The variations in depth of color
of foliage during different growing conditions, from a pale yellow
when conditions are unfavorable and growth is slow to the rich
dark green of more favorable conditions, is a familiar phenomenon.
\Wiether this change in pigmentation is the result of an adjust-
ment of the plant protoplasm, so that it can absorb a more highly
actinic portion of the light, or is a direct effect of the lack of con-
ditions favorable to chlorophyll-production and active photosyn-
thesis, has not yet been determined.

But there must be some influence other than response to en-
vironmental conditions which controls the vegetative color in
plants, since shrubs, or trees, which have green, yellow, red, and
purple leaves, respectively, will grow normally, side by side, under

PIGMENTS 121

identical external conditions of sunlight, moisture supply, etc.
The hereditary influence must completely overshadow the appar-
ent normal self -adjustment of pigment to energy-absorbing needs,
in all such cases.

Again, it appears that there is some definite connection
between pigment content and respiration. It is known, of course,
that the gaseous exchanges involved in animal respiration are
accomplished through the reversible change of haemoglobin to
oxyhaemoglotrin, these being the characteristic blood pigments.
The easy change of carotin, C4oH56, to xanthophyll, C^HseC^,
and vice versa, and the reversible changes of the yellow anthoxan-
thins to the red anthocyanins, under the influence of the oxidizing
and reducing enzymes which are universally present in plants,
would indicate the possibility of the service of these pigments as
carriers of oxygen for respiratory activities in plants in a way
similar to that in which the blood pigments serve this purpose in
the animal body. The fact, which has been observed in con-
nection with the experimental studies of the development of the
lycopersicin, that tomatoes which normally would become red
remain yellow in the absence of oxygen, indicates that this pig-
mentation, at least, is definitely connected with oxygen supply;
and the further fact that the development of lycopersicin in red
tomatoes, red peppers, etc., is dependent upon the temperature
at which the fruit ripens, may indicate a definite connection of
this pigment with the need for more oxygen (or for more heat, as
suggested in the following paragraph) at these lower tempera-
tures.

Again, many investigators have concluded that at least one
function of the anthocyanin pigments is to absorb heat rays and so
to increase transpiration and other chemical changes. In support
of this view,' there may be cited the general presence of such pig-
ments in arctic plants, their appearance in the leaves of many
deciduous trees after a frost in the fall, etc. Indeed, there is
much to support the view that the autumnal changes in foliage
pigments have the physiological function of absorbing heat in
order to hasten the metabolic processes of ripening and prepara-
tion for winter defoliation. The rapid and brilliant changes in
foliage coloring after a sharp frost which kills the tissues and makes
rapid translocation of the food material of the leaves to the
storage organs immediately necessary, have been explained as the

122 CHEMISTRY OF PLANT LIFE

response of the pigmentation of the leaves to the need for increased
heat-absorption. On the other hand, the red pigments of the
beet-root, etc., which seem to be identical in composition with the
other anthocyanin pigments, can have no such function as those
which have just been described. Furthermore, the fact that the
pigment often varies in color from red to yellow or brown, depend-
ing upon the temperature under which the tissue is ripening,
makes it an open question whether the pigment is the regulating
agency or whether its nature is the result of the environmental
conditions. Or, in other words, it is a question whether these
changes in color are a mechanism by which the plant cell adjusts
its absorptive powers, or whether they are only the inevitable
result of the changes in temperature upon a pigment material
which is present in the cell for an entirely different use.

A very interesting side-light upon the color changes which
many species of plants undergo when the external temperature
falls has been shown by the investigations of the relation of the
sugar content of the plant tissues to their pigmentation. It is
a well-known fact that not only do many species of deciduous
plants show the characteristic reddening of their leaves after
frost in the autumn but also many evergreens (Ligustrum, Hedera,
Mahonia, etc.) exhibit a marked reddening, or purpling, of their
foliage during the winter months, with a return to the normal
green color in the spring. Earlier investigations, which have
been confirmed by several repetitions, showed that the red or
purple leaves always contain higher percentages of sugar than
do green ones of similar types. More recent studies have shown
that artificial feeding of some species of plants with abnormally
large portions of soluble sugars produces a reddening of the foliage
tissues which is apparently identical with that which these tissues
undergo as the result of low temperatures. Thus, the connection
between the natural winter reddening of foliage and the develop-
ment of sugar in the tissues during periods of low temperatures
(see page 64) seems to be clearly demonstrated. It appears
that at least a part of tl\e seasonal changes in color of plants is
either the cause of, or the effect of, variations in sugar content
of the tissues of the plants, accompanying the changes in external
temperatures.

Oftentimes, the anthocyanin pigments seem to be associated
with sugar production, as contrasted with the chlorophylls, which

PIGMENTS 123

seem to be more favorable to the production of starch. But, in
this case also, it is impossible to say whether the pigment is the
direct causative agent in the type of carbohydrate production, or
whether it is the effect of the same external factors which deter-
mine, or modify, the character of the carbohydrate condensation.

BIOLOGICAL SIGNIFICANCE OF ORNAMENTAL PIGMENTS

The ornamental pigments undoubtedly have definite biological
significance. When present in the storage roots, such as beet-
roots, carrots, etc., or in the above-ground parts of plants, they
may have served to protect these organs against herbivorous ani-
mals which were accustomed to consume green foods.

In flowers, the brilliant ornamental pigments undoubtedly
serve to attract the insects which visit these blossoms in search of
nectar, and in so doing promote cross-fertilization. Recent
experiments have demonstrated that colors are much more efficient
than odors in attracting insects.

Taken altogether, it is apparent that the pigments may have
a variety of important roles in plants. At the same time, some of
them may be waste products, with no definite use in the plant
economy.

REFERENCES

ABDERHALDEN, E. — "Biochemisches Handlexikon, Band 6, Farbstoffe der
Pflanzen- und der Tierwelt," 390 pages, Berlin, 1911.

PERKIN, A. G. and EVEREST, A. E. — "The Natural Organic Colouring Mat-
ters," 655 pages, London, 1918.

WAKEMEN, NELLIE A. — " Pigments of Flowering Plants," in Transactions of
the Wisconsin Academy of Sciences, Arts, and Letters, Vol. XIX, Part
II, pages 767-S06, Madison, Wise., 1919.

WATSON, E. R. — "Colour in Relation to Chemical Constitution," 197 pages,
65 figs., 4 plates, London, 1918.

WHELDALE, M. — "The Anthocyan Pigments of Plants," 304 pages, Cam-
bridge, 1916.

WILLSTTATER, R. and STOLL, A. — "Untersuchung iiber Chlorophyllen,
Methoden und Ergebnisse," 432 pages, 16 figs., Berlin, 1913.

CHAPTER IX
ORGANIC ACIDS, ACID SALTS, AND ESTERS

ORGANIC acids, either in free form, or partially neutralized with
calcium, potassium, or sodium, forming acid salts, or combined
with various alcohols in the form of esters, are widely distributed
in plants. They occur in largest proportions in the fleshy tissues
of fruits and vegetables, where they are largely responsible for the
flavors which make these products attractive as food for men and
animals. But organic acids and their salts are also found in the
sap of all plants, and undoubtedly play an important and definite
part in the vital processes of metabolism and growth.

CHEMICAL CONSTITUTION

All organic acids contain one (or more) of the characteristic
acid group, — COOH, or — C^f , known as " carboxyl." This

NOH

group is monovalent, and in the simplest organic acid, formic acid
(H2C02), it is attached to a single hydrogen atom, thus, H-COOH.
In all other monobasic acids, it is attached to some other monova-
lent group, usually an alkyl radical, i.e., a radical derived from an
alcohol and containing only carbon and hydrogen (as methyl,
CH3, ethyl, C2H5, butyl, C^g, acryl, C2H3, etc.). Hence, the
general formula for all monobasic organic acids is R • COOH, the R
representing any monovalent radical. In the simplest dibasic
acid, oxalic (H2C2O4), two carboxyl groups are united to each
other, thus, HOOC- COOH; but in the higher members of the
series, the two characteristic acid groups are united through
one or more — CH2 — groups, or their oxy-derivatives (as
HOOC • CH2 • COOH, malonic acid ; HOOC • CH2 • CH2 • CH2 • COOH,
glutaric acid; HOOC -CHOH-CH2- COOH, malic acid, etc.).
Polybasic acids, containing three or more carboxyl groups,

124

ORGANIC ACIDS, ACID SALTS, AND ESTERS 125

linked together through one or more alkyl carbon atoms, are
also possible, and a few typical ones (as

CDOH
I
HOOC-CH2-COH-CH2-COOH, citric acid)

are found in fruits and other plant tissues.

The H atom of the COOH group may be replaced by metals, in
exactly the same way as it is replaceable in inorganic acids, pro-
ducing either neutral or acid salts, depending upon whether all or
only a part of the acid H atoms are replaced by the basic element.

Thus, with sulfuric acid:

<H yONa

(H2S04) + NaOH = S02< (NaHS04)+H20
H XOH

Sulfuric acid Acid sodium sulf ate

/OH /ONa

or, S02< (H2S04)+2NaOH = S02< (Na2S04) +2H2O

2
X)Na

Sulfuric acid Neutral sodium sulfate

Similarly, with oxalic acid;

COOH COOK

(H2C204)+KOH = | +H20

COOH COOH

Oxalic acid Acid potassium oxalate

or, COOH COOK

(H2C2O4) +2KOH = | +2H2O
COOH COOK

Oxalic acid Neutral potassium oxalate

Similarly, the acid H atom of either an organic or an inorganic
acid may be replaced by the alkyl group of an alcohol, producing
" ethereal salts," or " esters."

Thus, with nitric acid;

N02OH(HN03) -fC2H5OH = N02OC2H5(C2H5N03) +H2O

Nitric acid Ethyl alcohol Ethyl nitrate

And, with acetic acid;
CH3 • COOH(H4C202) +C2H5OH = CH3 • COOC2H5+H20

Acetic acid Ethyl acetate

With dibasic or polybasic acids, either one or more of the
carboxyl H atoms may be replaced with an alcohol radical, so that

126 CHEMISTRY OF PLANT LIFE

both acid and neutral esters of all such acids are possible. Exam-
ples of all of these different types of derivatives of organic acids
are frequently found in plant tissues.

The occurrence, properties, and functions of a particular type
of glycerol, and other esters of organic acids, which are known as
fats and waxes, are not taken into consideration in the following
discussions, but reserved for a subsequent chapter dealing specially
with them.

SOME COMMON ORGANIC ACIDS

Free organic acids, or their mineral salts or volatile esters,
sometimes occur as separate and characteristic individual com-
pounds in particular species of plants, or fruits; but much more
commonly, two, three, or even more acids or their derivatives, are
associated together.

Formic acid, H-COOH QEfeCCfe), occurs in free form and in
considerable proportions in the leaves of several species of nettle,
where it is responsible for the unpleasant effects of the " sting."
It may be detected in small amounts in the vegetative parts of
many, if not all, plants, especially during periods of rapid growth,
and is probably one of the intermediate products in the photo-
synthesis of carbohydrates (see Chapter III).

Higher members of the formic acid series (as acetic, CHs • COOH ;
propionic, C2H5-COOH; butyric, C3H5-COOH; etc.) are often
found in small quantities in the leaves of many plants and seem to
be characteristically present in certain species. They are easily
produced from carbohydrates by bacterial action and, hence, are
always present in fermenting tissues, such as silage, sauerkraut,
etc. Furthermore, the glycerol esters of higher members of this
and other monobasic acid series are constituents of all natural fats
and oils (see Chapter X).

Oxalic acid, HOOC-COOH (H2C2O4), is found in small
amounts in nearly all plants and in relatively large proportions
in those of Oxalis, rhubarb, etc. It occurs both as the free acid
and as neutral, or acid, oxalates of calcium, potassium, and, per-
haps, of magnesium and sodium. Solid crystals of insoluble
calcium oxalate are often found in plant cells, and it has been
shown that when so deposited the calcium cannot become again
available for metabolic uses. It is stated, further, that such

ORGANIC ACIDS, ACID SALTS, AND ESTERS 127

crystals form only when calcium is in excess in the plant sap;
hence, the deposition of crystallized calcium oxalate seems to be a
device for the avoidance of excessive calcium rather than excessive
oxalic acid, in the plant juices.

Succinic acid, HOOC • CH2 • CH2 • COOH (H6C404), occurs in
many fruits and vegetables, and is also found in some animal tis-
sues. In fruits, it is usually associated with its derivatives, malic
and tartaric acids.

Malic acid, HOOC • CH2 • CHOH • COOH (H6C4O5), occurs in
apples and in many small fruits, and in many vegetables. Acid
calcium malate is now produced commercially as a bye-product
from the manufacture of syrups from fruit juices, and is used as a
substitute for " cream of tartar " in the manufacture of baking
powders.

Tartaric acid, HOOC • CHOH • CHOH - COOH (H6C4O6), is
found in many fruits, but most characteristically in the grape,
where it occurs as the mono-potassium salt. During the fermen-
tation of grape juice into wine, this salt is deposited in considerable
quantities in the bottom of the wine-casks. This crude product is
collected and sold under the name " argols." From these argols,
pure acid potassium tartrate is obtained by decolorization and
recrystallization, and constitutes the " cream of tartar " of
commerce.

COOH
I

Citric acid, HOOC - CH2 - COH • CH2 • COOH (H8C607), occurs
in large proportions in lemons, and associated with malic acid in
strawberries, cherries, currants, etc. It is also found in small
quantities in the seeds of the common leguminous vegetables, beans,
peas, etc. '

Tannic acid occurs widely distributed in the plant kingdom as
a constituent of the special type of glucosides known as tannins,
whose properties and functions have already been discussed (see
Chapter VII).

PHYSIOLOGICAL USES OF ORGANIC ACIDS

No conclusive evidence concerning the role of organic acids in
plant, or animal, growth, has yet been produced. There can be
no doubt that the hypothetical carbonic acid and its acid and nor-

128 CHEMISTRY OF PLANT LIFE

mal salts have a significant effect in regulating the acidity or alka-
linity of plant juices, or body fluids, and so determining the nature
of the enzymic activities and colloidal conditions of the biological
systems (see Chapters XIV and XV). It is probable that other
organic acids, such as formic, acetic, oxalic, and succinic acids,
in plants and sarco-lactic acid, in animal tissues, perform similar
regulatory roles; but there seems as yet to be no indication as to
why different acids should be used for this purpose by different
species, or organisms ; or as to the methods by which they perform
their specific functions, whatever these may be.

In plants, the organic acids are usually in solution in the sap.
When the plant ripens, they generally disappear, either being neu-
tralized by calcium, or other bases, and deposited as crystals in
the leaves or stems, or else used up in the synthesis of other organic
compounds. Small proportions of these acids are usually present
in mature seeds, and the percentage increases materially during
germination, indicating that they play an important role in insur-
ing the proper conditions for the conversion of the reserve food of
the seed into soluble materials available for the nutrition of the
young growing plant.

BIOLOGICAL SIGNIFICANCE OF FRUIT ACIDS, ETC.

The occurrence of organic acids, or their derivatives, which
have pronounced odors or flavors, in the flesh surrounding the
seeds of fruits, in the endosperm of vegetable seeds, or in the tubers,
etc., of perennial plants, thus making them attractive as food for
animals and men, undoubtedly serves to insure a wider distribu-
tion of the reproductive organs of these plants; a fact which has
unquestionably had a marked influence upon the survival of spe-
cies in the competitive struggle for existence during past eras and
in the development and cultivation of different species by man.
Indirect evidence that the proportion of these attractive com-
pounds present in certain species may have been considerably
increased by the processes of " natural selection " in the past is
furnished by the many successful attempts to increase the per-
centage of such desirable constituents in fruits or vegetables by
means of artificial selection of parent stocks by skillful plant
breeders.

CHAPTER X
FATS AND OILS, WAXES, AND LIPOIDS

INCLUDED in this group are several different kinds of compounds
which have similar physical properties, and which, in general,
belong to the type of organic compounds known as esters, i.e.,
alcoholic salts of organic acids. The terms " oil," " fat," and
" wax," are generally applied more or less indiscriminately to any
substance which has a greasy feeling to the touch and which does
not mix with, but floats on, water. There are many oils which are
of mineral origin which are entirely different in composition from
natural fats. These have no relation to plant life and will not be
considered here.

The natural fats, vegetable oils, and plant waxes are all esters.
There is no essential difference between a fat and an oil, the latter
term being usually applied to a fat which is liquid at ordinary
temperatures. The waxes, however, are different in chemical
composition from the fats and oils, being esters of monohydric
alcohols of high molecular weight, such as cetyl alcohol, CieHaaOH,
myristic alcohol, CaoHeiOH, and cholesterol, C2-H45OH; whereas
the fats and oils are all esters of the trihydric alcohol glycerol,
CsH5(OH)3. Lipoids are much more complex esters, having
some nitrogenous, or phosphorus-containing, group and some-
times a sugar in combination with the fatty acids and glycerol
which make up the characteristic part of their structure. •

In general, waxes and lipoids have a harder consistency than
fats: but this is not always the case, since " wool-fat " and sper-
maceti, both of which are true waxes in composition, are so nearly
liquid in form as to be commonly called fats; while certain true fats,
like " Japan wax," are so hard as to be commonly designated as
waxes. It is plain that physical properties alone cannot be relied
upon in the classification of these bodies. In fact, there is no
single definite property by which members of this group can be
accurately identified. There are many other types of substances

129

130 CHEMISTRY OF PLANT LIFE

belonging to entirely different chemical groups, which have oily,
or fat-like, properties.

A. FATS AND OILS
OCCURRENCE

Fats and oils are widely distributed in plants. They occur
very commonly in the reproductive organs, both spores and seeds,
as reserve food material. In fungi, oils are often found in the
spores, but sometimes also in sclerotia, mycelia, or filaments. For
example, the sclerotia of ergot have been found to contain as much
as 60 per cent of oil. In higher plants, many seeds contain high
percentages of oil, so as to make them commercial sources for
edible or lubricating oils, such as olive oil, rape-seed oil, cotton-
seed oil, castor oil, corn oil, sunflower-seed oil, etc., etc. Nuts
often contain large proportions of oil, the kernel of the Brazil
nut, for example, sometimes contains as high as 70 per cent of oil,
while an oil content of 50 per cent, or more, is common in almonds,
walnuts, etc.

Oils also occur as reserve food material in other storage organs
of plants, such as the tubers of certain flowering plants, and the
roots of many species of orchids. Sometimes the appearance of
oils in the stems of trees, or the winter leaves of evergreens, seems
to be only temporary and to occur only during periods of very
low temperatures.

Much less frequently, fats or oils are found in the vegetative
organs of plants, as in the leaves of evergreens. Their appear-
ance and functions in these organs seem to be much less certain
than in the other cases cited above; although in rare cases a con-
siderable proportion of oily material has been found to exist in
definite association with the chloroplasts.

The vegetable fats and oils have many important industrial uses.
Some of them, such as olive oil, cottonseed oil, cocoanut oil, etc., are
largely used as human food. Others, as castor oil, are used as
lubricants. The so-called " drying oils " (see page 132), such as
linseed oil, etc., are used in the manufacture of paints and var-
nishes. Some cheap vegetable oils are used as the basis for the
manufacture of soaps, etc. Hence, industrial plants and processes

FATS AND OILS, WAXES, AND LIPOIDS 131

for the extraction of oils from plant tissues are of very great
economic importance.

CHEMICAL CONSTITUTION

The fats (of either plant or animal origin) are glycerides, i.e.,
glycerol esters of organic acids. As has been pointed out, esters
are derived from organic acids and alcohols in exactly the same way
that mineral salts are derived from inorganic acids and metallic
bases.

Thus,

Base Acid Salt

and, C2H5|OH + HjOOC • H = C2H5OOC - H+H2O

Alcohol Acid Ester

or, R- OH + H|OOC-R = R-OOC-R+H2O

Any alcohol Any acid Any ester

Glycerol is, however, a trihydric alcohol, i.e., it contains three
replaceable (OH) groups. Its formula is C3H5(OH)3, or
CH2OH-CHOH-CH2OH. Hence, three molecules of a mono-
basic acid are required to replace all of its (OH) groups.

For example,

CH2OH + HOOC-Ci7H35 = CH2OOC • Ci7H35

CHOH + HOOC-Ci7H35 = CHOOC-Ci7H35+3H2O

I I

CH2OH + HOOC-Ci7H35 = CH2OOC-Ci7H35

Glycerol 3 mols. stearic acid Stearin — a fat

It is theoretically possible, of course, to replace either one, two,
or three of the (OH) groups in the glycerol with acid radicals, thus
producing either mono-, di-, or triglycerides. If the primary
alcohol groups in the glycerine molecule are designated by (1)

(1) (2) (1)

and the secondary one by (2), thus, CH2OH • CHOH • CH2OH, it is
conceivable that there may be either (1) or (2) monoglycerides,
either (1, 1) or (1, 2) diglycerides, or a triglyceride, depending upon
which of the (OH) groups are replaced. Compounds of all of
these types have been produced by combinations of glycerol with
varying proportions of organic acids under carefully controlled
conditions; and all of them found to possess fat-like properties,

132 CHEMISTRY OF PLANT LIFE

All natural fats are triglycerides, however. Most natural fats
are mixtures of several different triglycerides in each of which the
three (OH) groups of the glycerol has been replaced by the same
organic acid radical, as in the example of stearin shown above.
But recent investigations have shown that some of the common
animal fats, and perhaps some plant oils, may be made up of mixed
glycerides, i.e., those in which the different (OH) groups have been
replaced by different acid groups, as oleo-stearin, oleo-stearo-
palmitin, etc,

THE ACIDS WHICH OCCUR IN NATURAL FATS

The acids which, when combined with glycerol, produce fats
are of two general types. The first of these are the so-called
"fatty acids" having the general formula CnH2n+i-COOH.
These are the " saturated " acids, i.e., they contain only single-
bond linkages in the radical which is united to the • COOH group ;
hence, they cannot take up hydrogen, oxygen, etc., by direct
addition. The second type are the " unsaturated " acids belong-
ing to several different groups, as discussed below, but all having
one or more double-linkages between the carbon atoms of the alkyl
radical which they contain. Because of these double linkages,
they are all able to take on oxygen, hydrogen, or the halogen ele-
ments, by direct addition. When exposed to the air, for example,
these " unsaturated " acids, or the oils derived from them, take
up oxygen, increasing in weight, and becoming solid or hard and
stiff. Hence, natural oils which contain considerable proportions
of glycerides of these " unsaturated " acids are known as " drying
oils " and are largely used in the manufacture of paints, varnishes,
linoleums, etc.; while oils which contain little of these glycerides
are known as " non-drying," and are used for food, for lubrication,
or for other technical purposes in which it is essential that they
remain in unchanged fluid condition when exposed to the air.

The following are some of the more important of the acids which
occur as glycerides in natural fats :
Saturated Acids:

(a) Acetic, or stearic, acid series — general formula,
CnH2n+i-COOH.

(1) Formic acid, H • COOH, occurs free in nettles, ants,
etc,

FATS AND OILS, WAXES, AND LIPOIDS 133

(2) Acetic acid, CH3 • COOH, occurs free in vinegar.

(3) Butyric acid, C3H7-COOH, in butter fat.

(4) Capric acid, CgHig-COOH, in butter fat and cocoa-

nut oil.

(5) Myristic acid, Ci3H27*COOH, in cocoanut oil and

spermaceti.

(6) Palmitic acid, CisHai-COOH, in palm oil and many

fats.

(7) Stearic acid, Ci yHas • COOH, in most fats and oils.

Intervening members of this series, such as caprylic acid,
CyHis-COOH, and lauric acid, CnH23-COOH, are also found in
smaller quantities in cocoanut and palm nut oils, in butter fat,
and in spermaceti; while higher members of the series, as arachidic
acid, CigHsg-COOH, and lignoceric acid, C23H47-COOH, are
found in peanut oil; and cerotic acid, C25H51 -COOH, and melissic
acid, C2gH59 • COOH, in beeswax and carnauba wax.
Unsaturated Acids:

(b) Oleic acid series — general formula, CJEbn-i-COOH.

(1) Crotonic acid, CsHs-COOH, occurs in croton oil.

(2) Oleic acid, Ci 7^3- COOH, occurs in many fats and

oils.

(3) Brassic acid, C2iH4i • COOH, occurs in rape-seed oil.

(4) Ricinoleic acid, Ci7H32OH-COOH, occurs in castor

oil.

(1) Linoleic acid, CiyHai-COOH, occurs in linseed and
other drying oils.

(d) Linolenic acid series — general formula, C»H2»-5 • COOH.

(1) Linolenic acid, Ci 7^9- COOH, occurs in many
drying oils.

It will be observed that all of these acids contain a multiple
of two total carbon atoms. No acid containing an uneven number
of carbon atoms has been found in a natural fat. Furthermore,
the acids which occur most commonly in natural fats are those
which contain eighteen carbon atoms; in fact, more than 80 per
cent of the glycerides which compose all animal and vegetable
fats are those of the Cig acids. This fact, in addition to the one
that the sugars and starches all contain multiples of six carbon

134 CHEMISTRY OF PLANT LIFE

atoms in their molecules, indicates a very great biological sig-
nificance of the chain of six carbon atoms. This has been alluded
to in connection with the discussion of the biological significance
of molecular configuration (see page 57) and will be mentioned
again in other connections.

THE ALCOHOLS WHICH OCCUR IN NATURAL FATS

Glycerol, as has been pointed out, is by far the most common
alcoholic constituent of natural fats and oils. This substance,
which is familiar to everyone under its common name " glycerine,"
is a colorless, viscid liquid having a sweetish taste. It is a very
heavy liquid (specific gravity 1.27) which mixes with water in all
proportions and when in concentrated form is very hygroscopic.

Glycerine is made from fats and oils by commercial processes
which clearly prove that the constitution of fats is as described
above. The fat is boiled with a solution of caustic soda and is
decomposed, the sodium of the alkali taking the place of the
glyceryl (CsHs) group, the latter combining with three (OH)
groups from the three molecules of alkali necessary to decompose
the fat. A sodium salt of the organic acid, or soap, and glycerol
are thus produced, and are separated by saturating the hot solu-
tion with common salt, which causes the soap to separate out as a
layer on the surface of the liquid, which, on cooling, solidifies
into a solid cake, which is then cut and pressed into the familiar
bars of commercial soap. From the remaining solution, the
glycerine is recovered by evaporation and distillation under reduced
pressure. Taking stearin, a common fat, as the example, the
reaction which takes place in the above process may be expressed
by the following equation:

COO)3+3NaOH = 3Ci

Stearin Sodium stearate — a soap Glycerol

This process, since it yields soap as one of its products, is
called " saponification." All fats, when saponified, yield soaps
and either glycerol or (more rarely) some of the other alcohols
which are described below.

Glycerine is also prepared from fats by hydrolysis with super-
heated steam. Using olein, a glyceride which is present in olive

FATS AND OILS, WAXES, AND LIPOIDS 135

oil and many common fats, as the example in this case, the equa-
tion for the reaction is :

• COO)3 +3H2O = 3Ci 7H33 • COOH+C3H5(OH)3

Olein Steam Oleic acid Glycerol

In this case the free fatty acid, instead of a soap, is the product
which is obtained in addition to glycerol.

In the equations presented above, a single glyceride has been
used as the example in each case. In the saponification, or hydroly-
sis, of natural fats and oils which, as has been shown, are mixtures
of many glycerides, the resultant soaps, or fatty acids, are mix-
tures of as many compounds as there were individual glycerides
of the original fat, but the glycerol is identical in every case.

When glycerol is heated with dehydrating agents, it is easily
converted into acrolein, an unsaturated aldehyde having a peculiar
characteristic pungent odor. Hence, the presence of glycerol, or
glycerides, in any substance may usually be detected by mixing
the material with anhydrous acid potassium sulfate and heating
the mixture in a test tube, when the characteristic odor of acrolein
will appear.

Glycerol possesses all the characteristic properties of an alco-
hol, forming alcoholates with alkalies, esters with acids, etc. It
is an active reducing agent, being itself easily oxidized to a variety
of different products depending upon the strength of the oxidizing
agent used and the conditions of the experiment. Microorganisms
affect it in a variety of ways, either converting it into simple fatty
acids, or condensing it into longer-chain compounds.

Open-chain monohydric alcohols, higher members of the ethyl
alcohol series, such as cetyl, Ci6H33OH, carnaubyl, C24H49OH,
ceryl, C2eH53OH, and melissyl, CsoHeiOH, are found in the esters
which constitute the major proportion of the common waxes.

Cholesterol and phytosterol are empirical names for certain
closed-ring, monohydric alcohols which are found in relatively
small amounts in all fats, the former term designating those found
in animal fats and the latter those of plant origin. Then- compo-
sition has not yet been definitely established. They are known
to contain two, or three, closed rings, probably of the phenan-
threne type; to form dichlor- and dibrom- addition products,
showing that they contain one side-chain double linkage; and
to yield ketones when oxidized, indicating that they are secondary

136 CHEMISTRY OF PLANT LIFE

alcohols. They form acetyl esters, or acetates, which can be
separated from each other and identified by their crystal forms
and melting points. Because of this fact and of the further fact
that they are present in detectable quantities in practically all
fats and oils, they afford a qualitative means of distinguishing
between fats of animal and of plant origin. This possibility is
the most interesting fact known concerning these complex alco-
hols; although their presence as esters in all plant and animal
fats indicates that they must have some biological function.

Phytosterol is not a single alcohol, but a mixture of at least two,
which have been separated and studied as sitosterol, C2?H43OH,
and stigmasterol, CsoH^gOH. As has been said, these are found
in small proportions in all vegetable fats, being present in largest
amounts in oily seeds, especially those of the legumes.

The saponification of esters of cholesterol and phytosterol is a
difficult and unsatisfactory process; but since this affords the only
known means to distinguish between fats of plants and of animal
origin, its technique has been fairly well worked out, and the
process used in the study of the changes which take place in plant
fats when they are used by animals as food.

HYDROLYSIS AND SYNTHESIS OF FATS

The reaction for the hydrolysis of fats has been discussed in
connection with the process for the manufacture of glycerine.
This reaction takes place very slowly with cold water alone, can
be easily brought about by the action of superheated steam,
and much more easily and rapidly in the presence of some catalyst
(sulfuric acid is an especially effective catalyst for this purpose).

Fats can be artificially synthetized by heating mixtures of
glycerol and fatty acids, under considerable pressure, for some time
at temperatures of 200° to 240° C. ; or by heating a mixture of the
disulfuric ester of glycerol with a fatty acid dissolved in sulfuric
acid. Recently, fatty acids have been prepared from carbohy-
drates, by first breaking the hexoses down into three-carbon
compounds, then carefully oxidizing these to pyruvic acid,
CHs • CO • COOH, which can then be condensed into acids having
longer chains. The violent reagents and long-continued processes
which must be employed for the artificial hydrolysis or synthesis
of the fats are in sharp contrast with the easy and rapid transition

FATS AND OILS, WAXES, AND LIPOIDS 137

of carbohydrates to fats, and vice versa, which take place in both
plant and animal nutrition.

THE EXTRACTION OF OILS FROM PLANT TISSUES

There are three types of methods which are employed for the
extraction of oil from oil-bearing seeds, etc., either as a com-
mercial industry or for the purposes of scientific study. These
are (1) by pressure; (2) extraction with volatile solvents; and (3)
boiling the crushed seeds or fruits with water.

By the first method, the seeds are first cleaned, then " decor-
ticated " (hulls removed), crushed or ground, then subjected to
intense pressure in an hydraulic press. In the commercial process,
the ground seeds are first pressed at ordinary temperature, which
yields " cold-drawn " oil, then the press cake is heated and pressed
again, whereby " hot-drawn " oil is obtained. The crude oil is
refined by heating it to coagulate any albumin which it may
contain, and is sometimes bleached by different processes before
it is marketed. The press cake from many seeds, such as flax-
seed (Unseed), cottonseed, etc., is ground up and sold for use as
stock feed.

In the second method, the finely crushed seeds are treated with
solvents such as gasoline or carbon bisulfide, in an apparatus which
is so arranged that the fresh material is treated first with solvent
which has already passed through various successive lots of
material and has become highly charged with the oil, followed
by other portions which contain less oil, and finally by fresh sol-
vent, whereby the last traces of oil are removed from the material.
The saturated solvent is transferred to suitable boilers and the
solvent distilled off and condensed for repeated use, leaving the oil
in the boiler in very pure form.

Extraction by boiling with water is sometimes used in the
preparation of castor oil and olive oil. In such cases, the crushed
seeds are boiled with water and the oil skimmed off as fast as it
rises to the surface.

IDENTIFICATION OF FATS AND OILS

Fats and oils are identified by determinations of their physical
properties, such as specific gravity, melting point, refractive

138 CHEMISTRY OF PLANT LIFE

index, etc., and by certain special color reactions for particular
oils; or by measurements of certain chemical constants, such as
the percentage of free fatty acids which they contain, the sapon-
ification value (i.e., the number of milligrams of KOH required to
completely saponify one gram of the fat), the iodine number (per-
centage by weight of iodine which is absorbed by the unsaturated
fatty acids present in the fat), percentage of water-insoluble fatty
acids obtained after saponification and acidifying the resultant
soap, etc., etc. Most of these tests must be carried out under
carefully controlled conditions in order to insure reliable identi-
fications, and need not be discussed in detail here. Full directions
for making such tests, together with tables of standard values for
all common fats and oils, may be found in any reference book on
oil analysis.

PHYSIOLOGICAL USE OF FATS AND OILS

In animal organisms, fats are the one important form of
energy storage. They also form one of the most important sup-
plies of energy reserve material in plants. Carbohydrates com-
monly serve this purpose in those plants whose storage reservoirs
are in the stems, tubers, etc. ; but in most small seeds the reserve
supply of energy is largely in the form of oil, and even in those
seeds which have large endosperm storage of starch, the embryo
is always supplied with oil which seems to furnish the energy
necessary for the first germinative processes.

Fats are the most concentrated form of potential energy of all
the different types of organic compounds which are elaborated by
plants. This is because they contain more carbon and hydrogen
and less oxygen in the molecule than any other group of sub-
stances of vegetable (or animal) origin. It has been pointed out
that a quantity of fat capable of yielding 100 large calories of heat
will occupy only about 12 cc. of space, whereas from 125 to 225
cc. of space in the same tissue would be required for the amount of
starch of glycogen necessary to yield the same amount of heat, or
energy, when oxidized.

The fats undoubtedly catabolize first by hydrolysis into giycerol
and fatty acids, and then by oxidation possibly first into carbo-
hydrates and then finally into the end-products of oxidation,
namely, carbon dioxide and water. The following hypothetical

FATS AND OILS, WAXES, AND LIPOIDS 139

equation to represent the oxidation of oleic acid into starch, sug-
gested by Detmer, is interesting as a suggestion of how much
oxygen is required and how much heat would be liberated by such a
transformation :

Complete oxidation of oleic acid to the final end-products,
carbon dioxide and water, would require much more oxygen, thus:

Ci8H34O2+510 = 18CO2+17H2O.

Hence, Detmer's reaction would yield only approximately
one-half the total energy available in the acid; but it does indicate
the possibility of redevelopment of fatty acids or fats from the
unoxidized carbohydrate material which remains in the equation.
Moreover, there is abundant evidence to show that, in both animal
and plant tissues, energy changes are brought about chiefly by
the transformation of fats into carbohydrates and vice versa.

Many different hypotheses have been put forward concerning
the mode of transformation of fats into carbohydrates, and the
changes which take place in oily seeds during their germination
have been carefully studied by many investigators. The follow-
ing seem to be fairly well established facts. First, that fats as
such may be translocated from cell to cell, since cell-walls and cell
protoplasm seem to be permeable to oil if it is a sufficiently fine
emulsion; or they may be hydrolyzed into glycerol and fatty
acids and translocated from cell to cell in these forms and recom-
bined into fats in the new location. Second, that fats are formed
from glucose in some plants, from sucrose and from starch in
others, and from mannite and similar compounds in still other
species. Third, that in germination the fatty acids are used up
in the order of their degree of unsaturation, those which contain
the largest number of double-bond linkages being used first, and
the saturated acids last of all. Fourth, that the sugar produced
by the oxidation of fats is derived either from the glycerol or from
the fatty acids of the fat, depending upon the nature of the latter.
If the fat is saturated, the glycerine is converted into sugar while
the fatty acids are oxidized; but if the fat contains large propor-
tions of unsaturated acids, these contribute to the formation
of sugar.

140 CHEMISTRY OF PLANT LIFE

Recent studies seem to show that in the animal body fats serve
an important function in connection with the production of anti-
bodies to disease germs. But there is as yet no evidence to show
that fats and oils have any similar function in plant tissues. The
fact that they are found almost wholly in the storage organs of
plants seems to indicate that their use as food reserve material is
their principal, if not their sole, function in the plant economy.

B. THE WAXES

Waxes are most commonly found in or on the skin of leaves or
fruits. They are similar to fats in chemical composition, except
that, instead of being glycerides, they are esters of monohydric
alcohols of high atomic weight. The term wax, when used in the
chemical sense, has reference to this particular type of esters
rather than to any special physical properties which the compound
possesses, and both solid and liquid waxes are known.

Carnauba wax, found on the leaves of the wax-palm (Coper-
nicia cerifera) contains ceryl alcohol (C23H5sOH) and myricyl
alcohol (CsoHoiOH) esters of cerotic acid (X^sHsi-COOH) and
carnaubic acid (C23H47-COOH). It is the best known vegetable
wax. Poppy wax is composed chiefly of the ceryl ester of palmitic
acid (Ci7H35-COOH).

Since waxes contain no glycerol, they give no odor of acrolein
when heated with dehydrating agents, do not become rancid,
and are less easily hydrolyzed than the fats. They are soluble in
the same solvents as the fats, but generally to a less degree.

The facts that waxes are impervious to water and usually
occur on the surfaces of plant tissues have led to the conclusion
that their chief function is to provide against the too-rapid loss of
water by evaporation from these tissues. This seems to be borne
out by the common experience that many fresh fruits and vege-
tables will keep longer without shriveling if their waxy coating is
undisturbed. No other function than that of regulation of water
losses has been suggested for the plant waxes.

C. THE LIPOIDS

The lipoids, or " lipins," as some authors prefer to call them,
are substances of a fat-like nature which are found in small quan-
tities in nearly all plant and animal tissues and in considerable

FATS AND OILS, WAXES, AND LIPOIDS 141

proportions in nerve and brain substance, in egg yolk, etc., and in
the seeds of plants. When hydrolyzed, they yield fatty acids or
derivatives of fatty acids and some other group containing either
nitrogen only or both nitrogen and phosphorus. The facts that
they are extracted from tissues by the same solvents which extract
fats and that they yield fatty acids when hydrolyzed account for
the name " lipoid," which comes from the Greek word meaning
fat. Some writers, who object to the word " lipoid " as a group
name, prefer to call these substances the " fat-like bodies."

The first group of lipoids to be studied were those which occur
in the brain; and the name cerebroside was given to those lipoids
which, when hydrolyzed, yield fatty acids, a carbohydrate and a
nitrogen-containing compound but no phosphoric acid; while
those lipoids which contain both nitrogen and phosphorus were
called phosphatides. Substances which correspond in composition
to both these types are found in plant tissues and the same class
names are applied in a general way to lipoids of either plant or
animal origin.

Plant lipoids have not been studied to nearly the same extent
as have those which occur in the animal body; and certain observ-
ers believe that there are significant differences between the lipoids
of plants and those of animal origin. However, most investigators
use the same methods of study and the same systems of nomen-
clature for these fat-like substances, regardless of their origin.

LECITHIN

This phosphatide is by far the best-known lipoid. It occurs in
the brain, the heart, the liver, and in the yolk of the eggs of many
animals; and either lecithin or a substance so nearly like it in
character as to be regarded by most investigators as identical with
it, is present in small, but constant, quantities in nearly all seeds,
especially those of leguminous plants. In many legume seeds, it
constitutes from 50 to 60 per cent of the " ether extract," or
" crude fat," which can be extracted from the crushed seeds, using
ether as the solvent.

Lecithin is a glyceride. Only two of the (OH) groups of the
glycerol are replaced .by fatty acids, however; the third being
replaced by phosphoric acid, HsPO^ or PO(OH)s, which, in turn,
has one of its hydrogen atoms replaced by the base choline. Cho-
line is a nitrogenous base, or amine, which may be regarded as

142 CHEMISTRY OF PLANT LIFE

ammonium hydroxide with three of its hydrogen atoms replaced
by methyl groups and the fourth by the ethoxyl group, the latter
being the ethyl group with an OH in place of one of its hydrogens.
Thus,

Ammonium hydroxide Choline

H CH3 C2H4OH

- 3

H/ X)H CH3/ X)H

Without the choline, lecithin would be a di-f atty acid derivative
of glycero-phosphoric acid. These relations may be seen in the
following formulas:

Fatty acid
HOOC-R

Glycerol

Glycero-phosphoric acid

CH2OH

CH2OH OH

CHOH

CH— O— P^O

1 \

CH2OH

CH2OH OH

Choline

Lecithin

HOC2H4\

(CH3)3=N
HO/

CH2OOC • R /OH

i \ /

CH2OOC-R X/

C2H4\

>NEE(CH3)3
HO/

/fatty acid

Or, glycerol^-fatty acid + choline = lecithin + H2O

\phosphoric acid

There are many different possible linkages of the constituent
groups which make up the lecithin molecule. In the first place, if
the (OH) groups of the glycerol molecule be numbered (1) and (2),
thus,

CH2OH (1)

CHOH (2)

CH2OH (1)

the fatty acid radicals may be attached either in one (1) position
and one (2) position, or in the two (1) positions; hence, two forms
of glycero-phosphoric acid are possible, thus

/fatty acid /fatty acid

(A) glycerol^-fatty acid (B) glycerol^-phosphoric acid

\phosphoric acid Matty acid

FATS AND OILS, WAXES, AND LIPOIDS 143

Again, the choline may be attached to the phosphoric acid either
through its alcoholic (OH) group or through its basic (OH) group,
thus

OH /OH

— P^O

\0-N=(CH3)3
or,

The facts that in the arrangement (B) the central carbon atom of
the glycerol would be asymmetric, and that both lecithin and the
glycero-phosphoric acid derived from it by hydrolysis are optically
active, prove that formula (B) correctly represents the arrange-
ment of that part of the lecithin molecule; and there is ample
theoretical and experimental evidence to prove that the choline
linkage is through the alcoholic (OH) group. Hence the formula
for lecithin indicating the linkage as shown above is the correct one.
The fatty acids in the lecithin molecule may be different in
lecithins from different sources, just as they are different hi fats
from different sources. Both oleic acid and a solid fatty acid have
been found in the hydrolysis products of lecithin from leguminous
seeds. In certain lupine seed, the fatty acids present in the lec-
ithin appear to be palmitic and stearic.

OTHER PLANT PHOSPHATIDES

Phosphatides other than lecithin are common in plants. In
these, various sugars replace part or all of the glycerol as the alco-
holic part of the ester. Percentages of sugar varying from mere
traces up to 17 per cent of the weight of material taken, have been
found in the products of hydrolysis of phosphatides prepared
from vetch seeds, potato tubers, plant pollens, and whole wheat
meal.

Furthermore, betaine

(tri-methyl glycocoU, OC/ N>N=(CH3)3)

^ O /

and perhaps other vegetable amines (see Chapter XII) sometimes
replace choline as the basic group in the phosphatides.

144 CHEMISTRY OF PLANT LIFE

PLANT CEREBROSIDES

Bodies similar to the animal cerebrosides seem to occur in
many plant tissues, since plant lipoids which yield no phosphorus
when hydrolyzed have often been isolated. The sugar which con-
stitutes the alcoholic portion of their structure appears to be
galactose in every case which has been reported. Beyond this,
little is known of the structure of these plant cerebrosides, as they
are very difficult to prepare in pure form and not easily hydrolyzed.

PHYSIOLOGICAL USES OF LIPOIDS

Lipoids are so universally present in plant and animal tissues
and so commonly found in those parts of the organism in which
vital phenomena are most pronounced (brain, heart, embryo of
egg, embryo of seeds, etc.), that it is evident that they must play
some important role in the activity of living protoplasm. There
is, as yet, however, no definite and certain knowledge of what this
role is. Various theories concerning the matter have been put
forward in recent years. For example, Overton, in 1901, pre-
sented the idea that every living cell is surrounded by a semi-
permeable membrane consisting of lipoid material, which regu-
lates the passage into and out of the cell of substances necessary
to its metabolism and growth. Recent investigations by Oster-
hout and others indicate, however, that Overton's hypothetical
lipoid membrane is not essential to a proper explanation of the
migration into and out of the cell protoplasm of nutritive materials,
etc. Other investigators have cited results which appear to indi-
cate that lipoids play an important, but as yet unknown, part in
the process of fat metabolism. Others go even further than this,
and argue that since the extraordinary rapidity of the chemical
changes which take place in plant protoplasm indicates the
necessity of the presence there of exceedingly labile substances, and
since both fats and proteins are relatively stable compounds, it is
possible that the lipoids, which contain both nitrogenous and fatty
acid groups, play an exceedingly important part in the metabolism
processes. Bang, in particular, has pointed out (in 1911) that the
lipoids are probably the most labile of all the components which
constitute the colloidal system known as plant protoplasm. The
importance of such considerations will be more apparent after the

FATS AND OILS, WAXES, AND LIPOIDS 145

relation of colloidal phenomena to the activities of plant cell con-
tents has been more fully discussed (see Chapter XVI).

Experimental studies of the physiological uses of lipoids have
thus far been devoted almost exclusively to those of animal tis-
sues. They have been seriously hampered by the difficulty of
securing properly purified extracts of lecithin and similar lipoids.
The same labile character which apparently makes them so
important in the chemical changes in the cell makes them equally
unstable compounds to work with hi attempting to secure pure
preparations for the purposes of experimental study. On this
account, there is, as yet, no certain knowledge concerning their
actual physiological uses. It is evident, however, that they
have some really important role to play, which opens up a prom-
ising field for further study.

REFERENCES

ABDERHALDEN, E. — " Biochemisches Handlexikon, Band 3, Fette, Wachse,
Phosphatide, Cerebroside, . . . "340 pages, Berlin, 1911.

HOPKINS, E.— "The Oil-Chemist's Handbook," 72 pages, New York, 1902.

LEATHES, J. B. — "The Fats," 138 pages, Monographs on Biochemistry, Lon-
don, 1913.

LEWKOWITSCH, J. — "Chemical Technology and Analysis of Oils, Fats, and
Waxes," Vol. I, 542 pages, 54 figs.; Vol. II, 816 pages, 20 figs.; and Vol.
III., 406 pages, 28 figs., London, 1909.

MACLEAN, H. — "Lecithin and Allied Substances," 206 pages, Monographs on
Biochemistry, London, 1913.

SOUTHCOMBE, J. E. — "Chemistry of the Oil Industries," 204 pages, 13 figs.,
London, 1918.

CHAPTER XI
ESSENTIAL OILS AND RESINS

INCLUDED in this group are all those substances to which the
characteristic odors of plants are due, along with others similar
in structure and possessing characteristic resinous properties.
They have no such uniformity in composition as is exhibited by
the oils which are included among the fats and waxes; but belong
to several widely different chemical groups. Furthermore, there
is no sharp dividing line between the essential oils and certain
esters of organic acids on the one hand and the fats on the other.
For example, if an aromatic fluid essence is a light fluid, non-viscid,
and easily volatile, it is usually classed with the organic esters;
denser liquid substances, of oily or waxy consistency, and with
comparatively slight odor and taste are usually fats, while oils of
similar physical properties but possessing strong characteristic
odors are classed as essential oils, regardless of their chemical
composition.

Included in this general class are compounds having a great
variety of chemical structures; e.g., hydrocarbons, alcohols,
phenols, organic sulfides and sulfocyanides, etc. Many of these
compounds are crystalline solids at ordinary temperatures, but
melt to oily fluids at higher temperatures. The characteristic
property which assigns any given plant extract to this group is
that it has a strikingly characteristic odor or taste, often accom-
panied by some definite physiological effect, or medicinal property.

These compounds may be either secretions or excretions of
plants, sometimes normally present in the healthy tissue, and
sometimes produced as the result of injury or disease.

The essential oils and the resins often occur associated together
in the plant; or, the resins may develop from the oily juice
of the plant after exposure to the air.

146

ESSENTIAL &IL AND RESINS 147

THE ESSENTIAL OILS

These may be divided, according to their chemical composition,
into two major groups; (1) the hydrocarbon oils, or terpenes, and
(2) the oxygenated and sulphuretted oils.

The terpenes are of three different types, namely: (a) the hemi-
terpenes, CsHg, unsaturated compounds of the valerylene series, of
which isoprene (found in crude rubber) is the best-known example;
(6) the terpenes proper, CioHie, which constitute the major pro-
portion of the whole group; and (c) the polyterpenes (CsHg)^ of
which colophene and caoutchouc are the most common examples.

Eleven different terpenes having the formula CioHie have
been isolated from various plant juices, and their molecular arrange-
ment carefully worked out. The following three examples will
serve as typical of the general structural arrangement of these
hydrocarbons :

Limonene Camphene PIn^ne

CH3 H |

H3C- <f C1U

CHf

CH3

A discussion of the evidence which supports these formulas as
properly represented the molecular arrangements of the various
isomeric forms would be out of place here, as its only particular
interest is in connection with the medicinal effects of the different
compounds. It is clear, however, that they are six-membered
hydrocarbon rings, with additional hydrocarbon groups attached
to one or more of the carbon atoms in the ring.

Different modifications, or varieties, of the terpenes constitute
the main proportions of the oils of turpentine, bergamot, lemon,
fir needles, eucalyptus, fennel, pennyroyal, etc.

The oxygenated essential oils may be either alcohols, aldehydes,
ketones, acids, esters, or phenols, derived from either five-mem-
bered or six-membered closed-ring hydrocarbons. They are
usually present in the plant oil in mixtures with each other or
with a terpene. Since most of them have pronounced physiological

148 CHEMISTRY OF PLANT LIFE

or medicinal properties, their structure has been well worked out, in
most cases; but it seems to be hardly worth while to present these
matters in detail here, as they are of interest chiefly on account of
their medicinal properties rather than their botanical functions.

Borneol, CioHiyOH, and menthol, CioHigOH, are typical
alcohols. The latter is a crystalline substance, which melts at
42°, which is present in peppermint oil, both as the free alcohol
and as an ester of acetic acid.

Amyl acetate, CHs-COOCsHn, and linalyl acetate,
CHs-COOCioHir, the latter occurring in the oils of lavender and
bergamot, are typical esters classed as essential oils.

As examples of the aldehyde oils, benzoic aldehyde,
"oil of bitter almonds," and cinnamic aldehyde,
CHCHO, found in the oils of cinnamon and cassia, may be cited.

Camphor, CioHieO, is a ketone, having the following structural
formula:

There are a considerable number of essential oils which are
phenols. Thymol, C6H3 • (CH3) • (C3H7) • OH, in oil of thyme, and
carvacrol, its isomer, in oil of hops, are familiar examples.

o— c=o

Coumarin, the anhydride of cinnamic acid, CelLtX ,

HC=CH

is an example of an acid substance which is classed as an essential
oil, even though it is a solid at ordinary temperatures. It has an
odor and flavor similar to that of vanillin, the essential flavoring
material of the vanilla bean, and is often used as a substitute for
the latter in the preparation of artificial flavoring extracts.

Of the essential oils containing sulfur, there are two common
examples; oil of mustard, allyl isosulfocyanide, CsH^NCS, and
oil of garlic, allyl sulfide (C3H5)2S. The latter is present in
onions, garlic, water cress, radishes, etc., the difference in flavor of
these vegetables being due to the fact that the allyl sulfide is
united with other different groups in the glucoside arrangement,

ESSENTIAL OIL AND RESINS 149

in the different plants. Similarly, mustard oil is not present in
mustard seeds as such, but as a glucoside which, when hydrolyzed
by the enzyme myrosin which is always present in other cells of
the same seeds, yields C3H5NCS, KHS04, and C6Hi2O6.

THE RESINS

The resins were formerly supposed to be the mother sub-
stances from which the terpenes are derived. It is now known,
however, that they are the oxidation products of the terpenes.
Their exact structure is still a matter of some uncertainty, as their
peculiar " resinous " character makes them very difficult to study
by the usual methods of chemical investigations.

Resins are divided into two classes: (a) the balsams, and (6)
the solid or hard resins. Canada balsam and crude turpentine
are familiar examples of the first class. They consist of resinous
substances, dissolved in or mixed with fluid terpenes. Ordinary
resin, or colophony, consists chiefly of a monobasic acid having the
empirical formula C2oHsoO2, known as sylvinic acid, whose exact
structure is not known. Its sodium salt is used as the basis for
cheap soaps.

The hard resins are amorphous substances of vitreous charac-
ter, which consist of very complex aromatic acids, alcohols, or
esters, combined with other complicated structures, known as
rescues, whose definite chemical nature is not yet known. Among
the hard resins are many substances which are extensively used in
the manufacture of varnishes, such as copal, amber, dammar,
sandarach, etc.

There are also resinous substances, such as asafcetida, myrrh,
gamboge, etc., which are mixtures of gums (see Chapter VI) and
true resins. Some of these have considerable commercial value
for medicinal or technical uses,

PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF
ESSENTIAL OILS

No theory has yet been advanced concerning the possibility of
the use of essential oils and resins by plants in their normal meta-
bolic processes. The very great diversity in their chemical nature
makes it impossible that they should all be considered as having

150 CHEMISTRY OF PLANT LIFE

the same physiological function, if indeed any of them actually
have any such function.

It is evident that those aromatic compounds which occur as
normal secretions of plants and which give to the plants their
characteristic odors may act either as an attraction to animals
which might utilize the plants as food and so serve to distribute
the seed forms, or as a repellent to prevent the too rapid destruc-
tion of the leaves, stems, or seeds of certain species of plants whose
slow-growing habits require the long-continued growth of these
portions of the plant for the perpetuation of the species. The
presence of these compounds in larger proportions in those species
of conifers, etc., which grow in tropical regions, in competition
with other rapid-growing vegetation, suggests the latter possibility.
It must be admitted, however, that their presence in such cases
may be the result of climatic conditions, as indicated by the fact
that most spice plants are tropical in habit, rather than the result
of their protective influence in the struggle for survival during past
ages.

Many of the oils and resins which are secreted as the result of
injury by disease or wounds have marked antiseptic properties
and undoubtedly serve to prevent the entrance into the injured
tissue of destructive organisms.

But apart from these possible protective influences which
may have had an important effect upon the preservation and
perpetuation of the species of plants which secrete them, there is
no known biological necessity for the presence of these aromatic
substances in plants.

REFERENCES

ABDERHALDEN, E. — " Biochemisches Handlexikon, Band 7, Gerbstoffe,
Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze
Kautschuk," 822 pages, Berlin, 1912.

ALLEN'S Commercial Organic Analysis, Vol. IV, "Resins, Rubber, Gutta-
percha, and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th
ed.).

HEUSLER, F. trans by POND, F. J.— "The Chemistry of the Terpenes," 457
pages, Philadelphia, 1902.

PARRY, E. J. — "The Chemistry of Essential Oils and Perfumes," 401 pages,
20 figs., London, 1899.

CHAPTER XII
THE VEGETABLE BASES

WE come, now, to the consideration of the characteristically
nitrogenous compounds of plants. None of the groups of com-
pounds which have been considered thus far have, as a group,
contained the element nitrogen. This element is present in the
chlorophylls and in certain other pigments, but not as the char-
acteristic constituent of the molecular structure of the group of
compounds, nor do these compounds serve as the source of supply
of nitrogen for the plant's needs.

The characteristic nitrogen-containing compounds may all be
regarded as derived from ammonia, or ammonium hydroxide, by
the replacement of one or more hydrogen atoms with organic
radicals of varying type and complexity. If the group, or groups,
which be considered as having replaced a hydrogen atom in ammo-
nia, in such compounds, is an alkyl group, the compound is strongly
basic in character and is known as an amine; whereas if the
replacing group is an acid radical, the resulting compound may be
neutral (known as add amides), or weakly acid (known as amino-
atids) in type. Compounds of the first type constitute the vege-
table bases; while those of the second type are the proteins.

The vegetable bases may be divided into three groups. These
are (a) the plant amines, which are simple open-chain amines; (6)
the alkaloids, which are comparatively simple closed-ring amines,
containing only one nitrogen atom in any single ring; and (c)
the purine bases, which are complex compounds containing a
nucleus with four carbon atoms and four nitrogen atoms arranged
alternately to form a double-ring group.

THE PLANT AMINES

The simple amines bear the relation to ammonia, or ammonium
hydroxide, represented by the following formulas, in which the
R indicates any simple alkyl radical:

151

152

CHEMISTRY OF PLANT LIFE

/R
N— H

H>/g

N\H

v _LJ_

N\l

N\R

\OH

H/ \OH

Ammonia

Primary

Secondary

Tertiary

Quaternary

Ammonium

amine

hydroxide

The simple amines which occur in animal tissues are known as
" ptomaines " and " leucomaines." The ptomaines are all decom-
position products resulting from the putrefactive decay of proteins
caused by moulds or bacteria. Some of these are highly toxic,
producing the so-called "ptomaine-poisoning"; while others are
wholly innocuous. They are all simple amines. Putrescine, di-
amino butane, NH2 • CH2 • CH2 • CH2 • CH2 • NH2, and cadaverine,
di-amino pentane, HN2 • (CH2) 5 • NH2, are common non-toxic
ptomaines, resulting from the decay of meat. Neurine, tri-
methyl-ethylene ammonium hydroxide, (CH3)3(C2H3) -NOH, is a
violently poisonous ptomaine produced in the decay of fish.
Amines of similar structure to these are occasionally found in living
animal tissues. Such compounds are known as leucomaines, to
distinguish them from the ptomaines, which are found only in dead
material.

Corresponding in structure and properties to these amines of
animal origin, there is a series of basic substances, found in many
plants, known as the plant amines. The following are common
examples :

Trimethyl amine, (CH3)3N, is a very volatile compound, found
in the flowers of several species of the Rose family, the leaves of
certain weeds, etc. When crushed, these tissues give off a very
fetid odor, which is due to this amine.

Choline, muscarine, and betaine are plant amines which are
closely related to each other and to neurine (the toxic ptomaine) in
composition and structure, as shown in the following formulas:

CH2CH2OH /CH2CHO

/
(CH3)3=N<

X)H N>H

Choline Muscarine

/CH2CO /CH=CH2

(CH3)3=N< / (CH3)3EEN<

XO X)H

Betaine Neurine

THE VEGETABLE BASES 153

Choline and betaine are non-toxic; while muscarine and neu-
rine are violent poisons.

Choline and muscarine occur in certain toadstools. Betaine
and choline often occur together in the germs of many plants.
Betaine is found in the beet root and the tubers of Jerusalem
artichoke. Choline occurs alone in the seeds and fruits of many
plants, sometimes as the free amine, but more often as a constit-
uent of lecithin (see page 141).

Phenyl derivatives of simple amines are sometimes found in

plants. Hydroxyphenylethyl amine, HO<^ yCH2 • CH2 • NH2,

found in ergot, and hordeine, H0<^ ^>CH2-CH2 -N-CCHs^,

found in barley, are examples. The former has marked medicinal
properties,.

There is no known physiological use for these simple amines in
plants. By some investigators, they are regarded as intermediate
products in the synthesis or decomposition of proteins; but it
would seem that if this were a normal procedure, these amines
would occur in varying proportions in all plants, under different
conditions of metabolism, instead of in practically constant propor-
tions in only a few species, as they do.

ALKALOIDS

These are a group of strong vegetable bases whose nitrogen
atom is a part of a closed-ring arrangement.

As a rule, alkaloids are colorless, crystalline solids, although a
few are liquids at ordinary temperatures. They are generally
insoluble in water, but easily soluble in organic solvents. Being
strong bases, they readily form salts with acids, and these salts are
usually readily soluble in water.

Alkaloids are usually odorless; although nicotine, coniine, and
a few others, have strong, characteristic odors. Most of them
have a bitter taste, and many of them have marked physiological
effects upon animal organisms, so that they are extensively used
as narcotics, stimulants, or for other medicinal purposes.

Most of the alkaloids contain asymmetric carbon atoms and
are, therefore, optically active, usually levorotatory, although a
few are dextrorotatory.

154

CHEMISTRY OF PLANT LIFE

The alkaloids are precipitated out of their solutions by various
solutions of chemical compounds, known as the " alkaloidal
reagents": iodine dissolved in potassium iodide solution gives a
chocolate-brown precipitate; tannic acid, phosphotungstic acid,
phosphomolybdic acid, and mercuric iodide solutions give color-
less, amorphous precipitates; while gold chloride and platinic
chloride solutions give crystalline precipitates, many of which have
sharp melting points and can be used for the identification of
individual alkaloids. There are a great many specific color
reactions for individual alkaloids, which are important to toxi-
cologists and pharmacists, but which it would not be desirable to
consider in detail here.

The alkaloids are conveniently divided into groups, according
to the characteristic closed-ring arrangements which they contain.
The several closed-ring arrangements which are found in common
alkaloids, and upon which their grouping is based, may be illus-
trated by the following formulas;

H2C— CH2
H2C CH2

Pyrrolidine, C4H»N

HC ; CH

I / II

HC CH

Pyrridine, CsHgN

H2C CH2

Piperidine, CsHnN

H2C-

-C CH2

NH CH2

H2C

Tropane,

THE VEQETABLE BASES 155

H H H H

A A 4 i

/\ /\ /\ /\

HC C CH HC C CH

HC C CH HC C N

Y Y Y Y

! I I

H H H

Quinoline, CsH8N Isoquinoline,

The common alkaloids are distributed in the several groups
as follows:

Pyrridine — piperidine group; piperine, coniine, nicotine.
Pyrrolidine group; hygrine and stachydrine.
Tropane group; atropine, hyoscine, cocaine, lupinine.
Quinoline group; quinine, cinchonine, strychnine, brucine.
Isoquinoline group; papaverine, hydrastine, morphine, codeine,
berberine.

The composition and properties of the individual alkaloids
have been extensively studied, because of then* medicinal uses.
As they have no known metabolic use to the plants which elaborate
them, it will not be worth while to consider all of these investiga-
tions hi detail here. The following facts with reference to certain
typical members of each group will serve to illustrate the general
constitution and properties of the alkaloids.

Piperine, CiyHigOs, is found in black peppers. Its constitu-
tion is represented by the following formula, the group which is
united to the piperidine ring, in this case, being piperic acid:

H2C CH2

>CH2
CH=CH-CH=CH-

156 CHEMISTRY OF PLANT LIFE

Coniine, CsHiyN, is found in the umbelliferous plant, Conium
maculatum. Structurally, it is a propyl-piperidine, represented
by the following formula:

H2C CH2

I I
H2C CH— C3H7

Nicotine, CioHi4N2, is the alkaloid of tobacco leaves. It is an
extremely poisonous, oily liquid, with a strong odor and a burning
taste. Its structural formula shows it to contain both a pyrridine
ring and a pyrrolidine ring, linked together thus

C H2C -- CH2

/\ i !

HC C - HC CH2

il I V

HC CH N

\/- I

N CH3

Hygrine, C?Hi3NO, from coca leaves, is an acetic acid salt of
pyrrolidine, represented by the following formula:

H2C -- CH— OC-CH3
H2C CH2

CH3

Atropine and hyoscyamine, Ci7H23N03, are optical isomers.
Atropine is an extremely poisonous, white crystalline compound,
which is obtained from deadly nightshade and henbane, and used
in medicine, in minute doses, as an agent for reducing temperature
in acute cases of fevers. Structurally, it is a tropic acid ester of
tropane, represented by the following formula:

THE VEGETABLE BASES 157

N— CH3 CHOOC— CH
H2C- CH CH2 CH2OH

Cocaine, CiyBkiNCU, is found in coca leaves. It is a white
crystalline solid, which is largely used as a local anaesthetic for
minor surgical operations. Its structural formula is

H2C -- CH— -HC— OOC • CH3

N— CH3 HC— OOC-C6H5

I I

H2C - CH - CH2

It is, therefore, a di-ester of acetic and benzoic acids with tropane.
Cinchonine, Ci9H22N2O, and quinine, C2oH24N2O2, are alka-
loids found in cinchona bark. They are white crystalline solids,
which are extensively used in medicine. They have been shown
to contain a quinoline group combined with modified piperidine
groups, as represented in the following formulas:

[2C

H2C HCH CH— CH=CH2

I I I
HOH— HC HCH CH2

Cinchonine

H2C CH — CH — CH2

HOH— HC CH2

Quinine

Strychnine, C2iH22N202, brucine, C2iH20(OCH3)N2O2, and
curarine are three alkaloids which are present in the seeds of several

158 CHEMISTRY OF PLANT LIFE

species of Strychnos. They are all highly poisonous. Beyond
the fact that when they are hydrolyzed they yield quinoline and
indole, their composition is unknown.

Morphine, CirHigNOs, is the chief alkaloid of opium, which is
the dried juice of young pods of the poppy. Both the alcoholic
solution of opium (known as " laudanum ") and morphine itself
are extensively used in medicine as narcotics to deaden pain.
Morphine has an exceedingly complex structure, being a combina-
tion of an isoquinoline and a phenanthrene nucleus, which is
probably correctly represented by the following formula:

H H2

HOC C CH2
C C N— CH3

HC CH3
HY

A Jin

Codeine, Ci7Hi8(OCH3)N02, which is also found in opium, is
a methyl derivative of morphine. Papaverine, laudanosine, nar-
cotine, and narceine are four other alkaloids found in opium.
They each contain an isoquinoline nucleus, combined by one
bond to a benzene ring, with one or more methyl groups and three
or more methoxy (OCH3) groups attached at various points
around the three characteristic rings. The following formula for
laudanosine will illustrate their structure:

OCH3
N— CH3

THE VEGETABLE BASES 159

The above discussions of the composition of typical alkaloids
clearly indicate the extreme complexity of then* molecular^tructure.
It is generally supposed that they are formed by the decomposition
of proteins. But they are developed in only a few particular
species of plants and are always present in these plants in fairly
constant quantities. Hence, it appears that, in these species, the
production of alkaloids is in some way definitely connected with
protein metabolism; but it is certain that this is not a common
relationship, as it is manifested by such a limited number of species
of plants, and there is absolutely no knowledge as to its character
and functions. Some authorities prefer to regard the alkaloids
as waste-products of protein metabolism; but here, again, it is
difficult to understand why such products should result in certain
species of plants and not in others.

THE PURINE BASES

This is a group of compounds, widely distributed in both plant
and animal tissues, all of which are derivatives of the compound
known as purine, CsH4N4. All of the naturally occurring com-
pounds of this group may be regarded as derived from purine,
either by the addition of oxygen atoms, or by the replacing of one
or more of its hydrogen atoms with a methyl (CHs) group or an
amino (NH^) group. The following structural formula repre-
sents the arrangement of the purine nucleus, the numbers being
used to designate the nitrogen or carbon atoms to which the addi-
tional atoms, or groups, are attached in the more complex com-
pounds of the group. In purine itself, the four hydrogen atoms are
attached in the 2, 6, 7, and 8 positions.

The double bonds, in each case except those between the 4 and 5
carbon atoms, are easily broken apart and readjusted, so that other
atoms or groups can be attached to any atom in the nucleus except
the 4 and 5 carbon atoms. In all of the statements with refer-
ence to the structure of the purine bases, the term "-oxy " is used

160 CHEMISTRY OF PLANT LIFE

to mean an oxygen atom attached by both its bonds to one of the
carbons in the nucleus, instead of its customary use to mean the
monovalent OH group replacing a hydrogen, as in the case of all
other nomenclature of organic compounds. With this under-
standing, reference to the numbered' nucleus formula above
will make plain the structure of all of the purine bases which are
included in the following list:

Hypoxanthine, CsH^N-iO, = 6-monoxypurine.

Xan thine, CsH4N4O2 = 2, 6-dioxypurine.

Uric acid, CsH^N^s, = 2, 6, 8-trioxypurine.

Adenine, CsHs^NH^, = 6-aminopurine.

Guanine, CsHs^ONH^, = 2-amino-6-oxypurine.

Theobromine, C5H2N402(CH3)2 = 3, 7-dimethyl-2, 6-dioxy-
purine, or dimethyl xanthine.

Theophylline, CsI^N-iCMCHa^l, 3-dimethyl-2, 6-dioxypu-
rine.

Caffeine, C5HN402(CH3)3 = 1, 3, 7-trimethyl-2, 6-dioxypurine,
or trimethyl xanthine.

In order to make these structural relationships quite clear,
the following formulas for uric acid and for caffeine are presented
as typical examples:

0=C C— N^H

I I >c=0
HN— C— N/H CH3— N

Uric acid Caffeine

Uric acid is found in the excrement of all animals; in the urine
of mammals, and in the solid excrement of birds and reptiles.
It is not known to occur in plants.

Xanthine and hypoxanthine occur in animal urine, and also in
the tissues of both plants and animals.

Adenine and gaunine are constituents of all nucleic avoids (see
below) and, hence, are found in all plant and animal tissues.
Guanine is the chief constituent of the excrement of spiders, and
is found also in Peruvian guano. It is also a constituent of the
scales of fishes.

Caffeine, theophylline, and theobromine are not found in animal
tissues, but are fairly widely distributed in plants. Caffeine and
theobromine are the active constituents of tea leaves and coffee

THE VEGETABLE BASES 161

seeds and are found also in cacao beans and kola nuts. The use
of these three compounds in the metabolism of the plants
which elaborate them is wholly unknown. They are not so
directly related to protein metabolism as are the other purine
bases.

The purine bases, other than the three mentioned in the pre-
ceding paragraph, are undoubtedly intermediate products in
protein metabolism. In animals, they constitute a large propor-
tion of the waste-products from the use of proteins in the body.
It is not clear that there are similar waste-products in plant
metabolism, however. In both plants and animals, the purine
bases which are a part of the nucleic acids undoubtedly play an
important and essential part in growth, since they form the major
proportion of the nucleus, from which all cell-division proceeds.

THE PYRIMIDINE BASES

These compounds do not occur free in plants; but since they
are constituent groups in the plant nucleic acids (see below), a
brief explanation of their composition is desirable. They are
nitrogenous bases, similar to, but somewhat simpler than, the
purine bases. Their general composition and structural relation-
ships are illustrated by the following typical formulas :

N=C— H H— N— C=0

C— H O=C

H— C C— H O=C C— H

H— N— C— H

Fyrimidine Uracil

C<H4N2O2
2, 6-dioxypyrimidine

II II

N— C— ]

N=C— NH2 H— N— C=0

HI I I

O=C C— H O=C C— CH3

H— N— C— H H— N— C— H

Cytosine Thymine

2, oxy-6-amino- 2, 6-dioxy-5-methyl-

pyrimidine pyrimidine

162 CHEMISTRY OF PLANT LIFE

THE NUCLEIC ACIDS

The nuclei of cells are composed almost wholly of complex
organic salts, in which proteins constitute the basic part and
nucleic acids the acid part. These salts, or esters, are known under
the general name " nucleoproteins." The composition of the
proteins is discussed in detail in the following chapter, and it seems
desirable to present a brief discussion of the constitution of the
nucleic acids here; although they are essentially acids rather than
vegetable bases.

The nucleic acids are complex compounds consisting of a
carbohydrate, phosphoric acid, two purine bases, and two pyri-
midine bases. So far as is known, all animal nucleic acids are
identical and all plant nucleic acids are identical; but those of
plant origin differ from those found in animal cells in the character
of the carbohydrate and that of one of the pyrimidine bases which
are present in the molecule, as shown in the following tabulation
ojr their composition:

Animal nucleic acid Plant nucleic acid

Phosphoric acid Phosphoric acid

Hexose (levulose) Pentose (d-ribose)

Guanine Guanine

Adenine Adenine

Cytosine Cytosine

Thymine Uracil

The structure of the plant nucleic acid may be represented
by the following formula :

=P — O — carbohydrate-guanire group
O

' — O — carbohydrate-adenine group

s=P — O — carbohydrate-uracil group

O
I
' — O — carbohydrate-cytosine group

THE VEGETABLE BASES 163

That this is probably a correct representation of the general
arrangement in this compound, is indicated by the fact that by
different methods of hydrolysis it is possible to split off either the
purine and pyrimidine bases, leaving a carbohydrate ester of
phosphoric acid; or the phosphoric acid, leaving carbohydrate
combinations with the nitrogenous bases.

Nucleic acid, prepared from animal glands which contain large
proportions of it, is a white powder, which is insoluble in water,
but when moistened forms a slimy mass. It is almost insoluble
in alcohol, but dissolves readily in alkaline solutions, forming a
colloidal solution which readily gelatinizes (see chapter on Col-
loids). Solutions of nucleic acids are optically active, probably
because of the carbohydrate constituents.

From their structure and properties, it is apparent that
nucleic acids are on the border line between carbohydrates, plant
amines, and proteins. They undoubtedly play an important
part, both in cell-growth and in the synthesis of proteins from car-
bohydrates and ammonium compounds.

References

BARGEE, GEO. — "The Simpler Natural Bases," 215 pages, Monographs on

Biochemistry, London, 1914.
FISCHER, E.— " Untersuchungen in der Puringruppe, 1882-1906," 608 pages,

Berlin, 1907.

HENRY, T. A.— "The Plant Alkaloids," 466 pages, Philadelphia, 1913.
JONES, W. — "The Nucleic Acids," 118 pages, Monographs on Biochemistry,

London, 1914.
PICTET, A. — "La Constitution Chimique des Alcaloides Vegetaux," 421 pages,

Paris, 1897 (2d ed.).
VAUGHAN, V. C. and NOVY, F. G. — "Ptomaines, Leucomaines, Toxins and

Antitoxins," 604 pages, Philadelphia, 1896, (3d ed.).
WINTERSTEIN, E. and TRIER, G.— "Die Alkaloide," 340 pages, Berlin, 1910.

CHAPTER XIII
PROTEINS

THE proteins are the most important group of organic com-
ponents of plants. They constitute the active material of pro-
toplasm, in which all of the chemical changes which go to make up
the vital phenomena take place. Combined with the nucleic
acids, they comprise the nucleus of the cell, which is the seat of
the power of cell-division and, hence, of the growth of the organism.
Germ-cells are composed almost exclusively of protein material.
Hence, it is not an over-statement to say that proteins furnish the
material in which the vital powers of growth and repair and of
reproduction are located. A recognition of their importance is
reflected in the use of the name " protein/' which comes from
a Greek word meaning "pre-eminence," or "of first importance."

In addition to the proteins which constitute the active proto-
plasm, plants also contain large amounts of reserve, or stored,
proteins, especially in the seeds. In the early stages of growth,
the proteins are present in largest proportions in the vegetative
portions of the plant; but as maturity approaches, a considerable
proportion of the protein material is transferred to the seeds.

GENERAL COMPOSITION OF PROTEINS

The plant proteins are fairly uniform in their percentage
composition. The analyses of some sixteen different plant pro-
teins show the following maximum limits of percentages of the
different chemical elements which they contain: Carbon, 50.72-
54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03; oxygen,
20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more
widely, both in percentage composition and in properties, than
do those of plant origin.

164

PROTEINS 165

Protein molecules are very large and, in the case of the so-
called " conjugated proteins " in particular, their structure is very
complex. The molecular weight of some of the proteins has been
determined directly, in the case of those particular ones which can
be prepared in proper form for the usual determination of molecular
weight by the osmotic pressure method; and has been computed
for various others, from the percentage of sulfur found on analysis,
or (in the case of the haemoglobin of the blood) from the propor-
tion by weight of oxygen absorbed. From these determinations
and computations, the following formulas for certain typical
proteins have been calculated: for zein (from Indian corn),
C736Hn6iNi84O208S3; for gliadin (from wheat), CessHioGsNige-
021185; for casein (from milk), C708Hn3oNi8oO224S4P4; for egg-
albumin, C69oHn25Ni75O22oS8. These few examples will serve
to illustrate the enormous size and complexity of the protein
molecule. The conjugated proteins are still more complex than
the simple proteins whose formulas are here presented.

Fortunately for the purposes of the study of the chemistry of
the proteins, however, it has been found that most of the common
plant proteins, known as the " simple proteins," can easily be
hydrolyzed into their constituent unit groups, which are the com-
paratively simple amino-acids, whose composition and properties
are well understood. A study of the results of the hydrolysis of
some twenty common plant proteins has shown that it is rarely
possible to recover the amino-acids in sufficient quantities to
account for a full 100 per cent of the material used, the actual
percentage of amino-acids recovered usually totaling from 60 to
80 per cent. The remaining material is supposed to be also com-
posed of amino-acids which are linked together in some arrange-
ment which is not broken apart by any method of hydrolysis
which has yet been devised. This view is borne out by the fact
that substances which exhibit all the characteristic properties of
proteins have been artificially synthetized, by using only amino-
acid compounds. Animal proteins often show a much larger pro-
portion of unhydrolyzable material than do plant proteins.

166 CHEMISTRY OF PLANT LIFE

AMINO-ACIDS AND PEPTID UNITS

The products of hydrolysis of the common simple proteins are
all amino-acids. These are ordinary organic acids with one (or
more) of the hydrogen atoms of the alkyl group replaced by a
— NH2 (or sometimes by a — NH — ) group. They may be
regarded as ammonia, NHa, with one of its hydrogen atoms
replaced by an acid radical ; or as the acid with one of its hydrogens
replaced by the NH2 group. For example, an amino-a^id derived
from acetic acid, CHa-COOH, is glycine, or amino-acetic acid,
CH2NH2-COOH; from propionic acid, CH3-CH2-COOH, there
may be obtained either a-amino-propionic acid, CHa-CHNH-
COOH, or /3-amino-propionic acid, CH2NH2 • CH2 • COOH, etc.

All of the amino-acids which result from the hydrolysis of pro-
teins are a-amino-acids, that is to say, the NH2 group is attached
to the a-carbon atom, i.e., the one nearest to the COOH group.
Hence, the general formula for all the amino-acids which are
found in plants is R-CHNH2-COOH.

These amino-acids contain both the basic NH2 group and the
acid COOH group. For this reason, they very easily unite
together, in the same way that all acids and bases unite, to form
larger molecules, the linkage taking place between the basic NH2
group of one molecule and the acid COOH group of the other, as
indicated by the following equation:

R R

HOOC - C - N— |H +• HO|OC - C • NH2

I I !

HH H

R R

= HOOC-C-N— OC-C-NH2+H2O
H

It is obvious that the compound thus formed still contains a free
NH2 group and a free COOH group, and is, therefore, capable of
linking to another amino-acid molecule in exactly the same way;
and so on indefinitely. In actual laboratory experiments, as

PROTEINS 167

many as eighteen of these amino-acid units have been caused to
unite together in this way, and the resulting compounds thus
artificially prepared have been found to possess the characteristic
properties of natural proteins.

These artificially prepared, protein-like, substances have been
called " polypeptides," and the individual amino-acids which
unite together to form them are called " peptides." Thus, a
compound which contains three such units linked together is
called a " tripeptid "; one which contains four, a " tetrapeptid."
The use of the term " peptid " was suggested by the fact that these
amino-acids are produced from the hydrolysis of proteins by the
digestive enzyme pepsin.

The peptid units of any such complex as those which have been
referred to in the preceding paragraphs may be linked together in a
great variety of ways. Thus, in a tetrapeptid containing units
which may be designated by the letters a, 6, c, and d, the arrange-
ment may be in the orders abed, bacd, acbd, dbca, etc., etc. Sim-
ilarly, the same peptid unit may appear in the molecule in two or
more different places. Hence, the number of possible combina-
tions of amino-acids into protein molecules is very great. Further,
it is possible that the peptid units in natural proteins may be
united together through other linkages than the one illustrated
above, as they often contain alcoholic OH groups in addition to
the basic NH2 groups, and these OH groups may form ester-
linkages with the acid (COOH) groups of other units. Still other
acid and basic groups are present in some of the amino-acids
which have been found in natural proteins, so that the possibility
of variation in the polypeptid linkages is almost limitless.

INDIVIDUAL AMINO-ACIDS FROM PROTEINS

About twenty different amino-acids have been isolated from
the products of hydrolysis of natural proteins, and this number is
being added to from time to time, as the methods of isolation and
identification of these compounds are improved. Many of these
same amino-acids have been found in free form in plant tissues,
particularly in rapidly growing buds, or shoots, or in germinating
seeds, where they undoubtedly exist as intermediate products in
the transformation of proteins into other types of compounds.

,168 CHEMISTRY OF PLANT LIFE

These amino-acids, grouped according to the characteristic
groups which they contain, are as follows:

A. Monoamino-monocarboxylic acids :

Glycine, C2H5NO2, CH2NH2-COOH, amino-acetic acid.
Alanine, C3H7N02, CH3 • CHNH2 - COOH, amino-propionic

acid.
Serine, C3H7N03, CH2OH - CHNH2 - COOH, oxy-amino-pro-

pionic acid.

K
Valine, C5HnN02, >CH - CHNH2 - COOH, amino-iso-

CH-/
valerianic acid.

X
Leucine, C6Hi3N02, >CH • CH2 - CHNH2 - COOH,

CH-/
amino-isocaproic acid.

CH3x
Isoleucine, C6Hi3N02, >CH - CHNH2 - COOH,

C2H5/
amino-methylethyl-propionic acid.

Phenylalanine, C9HnNO2, (" X|CH2 • CHNH2 • COOH,

Uphenyl-amino-pro-
pionic acid.

Tyrosine, C9HnNO3, /Nc^-CHNH^COOH, paraoxy-

phenylalanine.

OHV

Cystine, C6Hi2N2O4S2, HOOC • CHNH2 - CH2S— SH2C -
CHNH2-COOH, di(thio-amino-propionic acid).

B. Monoamino-dicarboxylic acids:

Aspartic acid, C4H7NO4, HOOC • CH2 • CHNH2 • COOH,

amino-succinic acid.
Glutamic acid, C5H9N04, HOOC - CH2 - CH2 - CHNH2 -

COOH, amino-glutaric acid.

PROTEINS 169

C. Diamino-monocarboxylic acids:

_ Ornithine, C5Hi2N202, H2N - CH2 - CH2 - CH2 - CHNH2 -

COOH, di-amino-valerianic acid.
Lysine, C6Hi4N2O2, H2N • CH2 - CH2 • CH2 • CH2 • CHNH2 •

COOH, di-amino-caproic acid.
Arginine, CoHi4N4O2,

/NH2
HN=C<

\NH • CH2 • CH2 • CH2 • CHNH2 • COOH,

guanidine-amino-valerianic acid.
Di-amino-oxysebacic acid, CnHi2N2O3.
Di-amino-trioxydodecanic acid, Ci2H2eN203.

D. Monoimido-monocarboxylic acids:

Proline, C5H9N02, H2C CH2

H2C CH - COOH, pyrrolidine-carboxy-
\/ lie acid

Oxyproline, CsHgNOs, proline with one (OH) group/

E. Monoimido-monoamino-monocarboxylic acids.

Histidine, C6H9N302, HC=C— CH2 - CHNH2 - COOH,

imidazole-amino-
<v , propionic acid.

Tryptophane, CnHi2N202,

— CH2-CHNH2-COOH, indole-amino-propionic acid.

As has been said, other amino-acids are being found, from time
to time, as additional proteins are examined, or as better methods
of examination of the cleavage products of the natural proteins are
devised.

170

CHEMISTRY OF PLANT LIFE

COMPOSITION OF PLANT PROTEINS

The distribution of the different ammo-acids in some of the
different plant proteins which have been examined in this way is
shown in the following table :

Hor-

Glob-

Ama-

Gliadin

dein

Zein

Legu-

Edestin

ulin

dinn

(wheat).

(bar-
ley).

(corn).

min

(vetch).

(hemp).

(squash
seed) .

(al-
monds).

Glycine

0.02

o.oo

0.00

0.39

3.80

0.57

0.51

Alanine

2.00

0.43

9.79

1.15

3.60

1.92

1.40

Valine

0.21

0.13

1.88

1.36

6.20

0.26

0.16

Leucine

5.61

5.67

19.55

8.80

14.50

7.32

4.45

Proline

7.06

13.73

9.04

4.04

4.10

2.82

2.44

Phenylalanine. . .

2.35

5.03

6.55

2.87

3.09

3.32

2.53

Aspartic acid. . . .

0.58

1.71

3.21

4.50

3.30

5.42

Glutamic acid. . .

42.98

43.19

26.17

18.30

18.84

12.35

23.14

Serine

0.13

1.02

0.33

Cystine

0.45

1.00

0.23

Tyrosine

1.20

1.67

3.55

2.42

2.13

3.07

1.12

Arginine

3.16

2.16

1.55

11.06

14.17

14 . 44 "

11.85

Hystidine

0.61

1.28

0.43

2.94

2.19

2.63

1.58

Lysine

3.99

1.65

1.99

0.70

Tryptophane. . . .

present

absent

present

• Ammonia

5.11

4.87

3.64

2.12

2.28

1.55

3.70

71.46

78.16

85.27

62.65

82.38

55.77

59.00

At the time when these analyses were made, a method for the
quantitative estimation of tryptophane had not been devised,
although one is now available. The addition of the percentages of
tryptophane and of other amino-acids for which methods of deter-
mination are not yet known, would bring the total, in each case,
more nearly up to the full 100 per cent. These data will serve to
show how widely the different plant proteins vary in the propor-
tions of the different amino-acids which they contain. Animal
proteins have been found to be still more variable in composition.

In the use of the proteins as food for animals, it appears that
the different amino-acids are in some way connected with the

PROTEINS 171

different physiological functions which the proteins have to per-
form in the animal body: thus, tryptophane is absolutely essential
to the maintenance of life, but does not promote growth; lysine,
on the other hand, definitely promotes growth, so that animals
which have been maintained without any increase in weight for
many months immediately begin to grow when furnished with a
diet in which lysine is a constituent; while arginine seems to be
definitely associated with the reproductive function; and cystine,
with the growth of hair, feathers, etc. It is not known whether
there is any similar relation of amino-acids to the functions of
different proteins in plant metabolism.

The separation of the individual amino-acids from the mix-
ture which results from the hydrolysis of any given protein is a
long and tedious process and, at best, yields only moderately
satisfactory results. For that reason, it has recently been almost
entirely abandoned in favor of the separation devised by Van
Slyke, which divides the total nitrogenous matter in the mixture
resulting from the hydrolysis of a protein into the following groups ;
ammonia N, humin (or melanin) N, cystine N, arginine N, histidine
N, lysine N, amino N of the filtrate, and non-amino N of the fil-
trate. These groups can be conveniently and fairly accurately
separated out of the hydrolysis mixture, by means of various
precipitating agents, and the quantity of N in the several precip-
itates determined by the usual Kjeldahl method. The actual
process for these separations need not be discussed here, as it is
given in detail in all standard text-books dealing with the methods
of biochemical analysis. The distribution of the nitrogen in any
given protein into these various groups is characteristic for that
particular protein, and the process serves both as a means of
identification of individual proteins and a method for tracing their
changes through various vital, or biochemical, transformations.

GENERAL PROPERTIES OF THE PROTEINS

Individual proteins differ slightly in their characteristics,
but in general they are all alike in the following physical and chem-
ical properties.*

* Since the proteins are essentially colloidal in nature, many of the terms
used in the discussions of their properties, and these properties themselves,
will be better understood after the chapter dealing with the colloidal condi-

172 CHEMISTRY OF PLANT LIFE

Physical Properties. — (1) The proteins are all colloidal in
character, that is, they form solutions in water, out of which they
cannot be dialyzed through parchment, or other similar mem-
branes. (2) All natural proteins, when in colloidal solution, may be
coagulated, forming a semi-solid gel, which cannot again be ren-
dered soluble except by decomposition. The most familiar exam-
ple of this type of coagulation is that of egg-albumin, when eggs
are cooked. This coagulation may be produced by heat, by the
action of certain enzymes, or by the addition of alcohol to the
solution. (3) All solutions of plant proteins are optically active,
rotating the plane of polarized light to the left, in every case.
(4) Proteins are precipitated out of their solutions, without change
in the composition of the protein, by saturating the solution with
various neutral salts of the alkali, or alkaline earth, metals, such
as sodium chloride, ammonium sulfate, magnesium sulfate,
etc. This is only another way of saying that the proteins are
insoluble in strong salt solutions. Separation from solution by
the addition of salts is different from coagulation by heat, etc., as
in this case simple dilution of the salt solution will cause the pro-
tein to redissolve, whereas a coagulated protein cannot be redis-
solved without some change in its composition.

Chemical Properties. (1) Precipitation reactions. — The pro-
teins have both acid and basic properties (due to the presence in
their molecules of both free NEb groups and free COOH groups).
Bodies of this kind are known as " amphoteric electrolytes," since
they yield both positive and negative ions, if dissociated. The
proteins readily form salts, which are generally insoluble in water,
with strong acids. For this reason, they are generally precipitated
out of solution by the addition of the common mineral acids. They
are also precipitated by many of the " alkaloidal reagents," to
tion of matter has been studied. A more logical arrangement so far as the
systematic study of these properties is concerned would be to take up chapter
XV before undertaking the study of the proteins (this order is actually fol-
lowed in some texts on Physiological Chemistry). But from the standpoint
of the consideration of the various groups of organic components of plants,
it seems a better arrangement to consider these groups in sequence, and then
to discuss the various physical-chemical phenomena which govern their activ-
ity. However, it is recommended that the student refer at once to Chapter
XV for an explanation of any terms used here, which may not be familiar to
him; and that after the study of Chapter XV, he return to this chapter dealing
with the proteins for an illustrative study of the applications of the principles
presented there.

PROTEINS 173

which reference has been made in the preceding chapter, namely,
phosphotungstic, phosphomolybdic, tannic, picric, ferrocyanic,
and trichloracetic acids, the double iodide of potassium, mercuric
iodide, etc. The precipitates produced by strong mineral acids
are often soluble in excess of the acid, with the formation of certain
so-called " derived proteins," which are probably products of
the partial hydrolysis of the protein.

The proteins are also precipitated out of solution by the addi-
tion of small amounts of salts of various heavy metals, such as the
chlorides, sulfates, and acetates of iron, copper, mercury, lead, etc.
This precipitation is different than that caused by the saturation
of the solution with the salts of the alkali metals, as in this case the
metal unites with the protein to form definite, insoluble salts,
which cannot be redissolved except by treatment with some
reagent which removes the metal from its combination with the
protein (hydrogen sulfide is commonly used for this purpose).

(2) Color reactions. — Certain specific groups which are present
in most proteins give definite color reactions with various reagents.
It is apparent that any individual protein will respond to a par-
ticular color reaction, or will not do so, depending upon whether
the particular group which is responsible for the color in question
is present in that particular protein. Color ' reactions to which
most of the common plant proteins respond are the following ones :

(a) Biuret Reaction. — Solutions of copper sulfate, added to an
alkaline solution of a protein, give a bluish-violet color if the
substance contains two, or more, — CONH — groups united
together through carbon, nitrogen, or sulfur atoms. Inasmuch as
most natural proteins contain several such groups, the biuret
reaction is a very general test for proteins.

(6) M illon's ^Reaction. — A solution of mercuric nitrate con-
taining some free nitrous acid (Millon's reagent) produces a pre-
cipitate which turns pink or red, whenever it is added to a solution
which contains tyrosin, or a tyrosin-containing protein.

(c) Xanthoproteic Add Reaction. — This is the familiar yellow
coloration which is produced whenever nitric acid comes in con-
tact with animal flesh. It is caused by the action of nitric acid
on tyrosin. The color is intensified by heating, and is changed to
orange-red by the addition of ammonia.

(d) Adamkiewicz' s Reaction. — If concentrated sulfuric acid
be added to a solution of a protein to which some acetic acid (or

174 CHEMISTRY OF PLANT LIFE

better, glyoxylic acid) has previously been added, a violet color is
produced. This color will appear as a ring at the juncture of the
two liquids, if the sulfuric acid is poured carefully down the sides
of the tube, or throughout the mixture if it is shaken up. It
depends upon the interaction of the glyoxylic acid (which is gen-
erally present as an impurity in acetic acid) upon the tryptophane
group, and is therefore given by all proteins which contain tryp-
tophane.

(e) Molisch's reaction for furfural will be shown by those pro-
teins which contain a carbohydrate group. In applying this test,
the solution to be tested is first treated with a few drops of an alco-
holic solution of a-naphthol, and then concentrated sulfuric acid
is poured carefully down the sides of the test-tube. If carbo-
hydrates are present, either free or as a part of a protein molecule,
a red-violet ring forms at the juncture of the two liquids.

(/) Sulfur Test. — If a drop of a solution of lead acetate be
added to a solution containing a protein, followed by sufficient
sodium hydroxide solution to dissolve the precipitate which forms,
and the mixture is heated to boiling, a black or brown coloration
will be produced if the protein contains cystine, the sulfur-con-
taining amino-acid.

THE CLASSIFICATION OF THE PROTEINS

Formerly, the classification of proteins was based almost
wholly upon their solubility and coagulation reactions. More
recently, since their products of hydrolysis have been extensively
studied, their classification has been modified, in attempts to make
it correspond as closely as possible to their chemical constitution
and physical properties. As knowledge of these matters progresses,
the schemes of classification change. On that account, no one
definite scheme is universally used. For example, the English
system varies considerably from the one commonly used by
American biochemists, which is the one presented below.

The proteins are divided into three main classes, as follows :

(1) Simple proteins, which yield only amino-acids when
hydrolyzed.

(2) Conjugated proteins, compounds of proteins with some
other non-protein group.

PROTEINS 173

(3) Derived proteins, decomposition products of simple pro-
teins.

The first two of these classes comprise all the natural pro-
teins; while the third includes the artificial polypeptides and pro-
teins which have been modified by reagents.

These major classes are further subdivided into the following
sub-classes, which depend in part upon the solubilities of the
individual proteins, and in part upon the nature of their products
of hydrolysis:

1. The Simple Proteins

A. Albumins — soluble in water and dilute salt solutions, coagulated by

heat.

B. Globulins — insoluble in water, soluble in dilute salt solutions, coag-

ulated by heat.

C. Glutelins — insoluble in water or dilute salt solutions, soluble in dilute

acids or alkalies, coagulated by heat.

D. Prolamins — insoluble in water, etc., soluble in 80 per cent alcohol.

E. Histones — soluble in water, insoluble in ammonia, not coagulated by

heat.

F. Protamines — soluble in water and ammonia, not coagulated by heat,

yielding large proportions of diamino-acids on hydrolysis.

G. Albuminoids — insoluble in water, salt solutions, acids, or alkalies.

2. Conjugated Proteins

A. Chromoproteins — compounds of proteins with pigments.

B. Glucoproteins — compounds of proteins with carbohydrates.

C. Phosphoproteins — proteins of the cytoplasm, containing phosphoric

acid.

D. Nucleoproteins — proteins of the nucleus, containing nucleic acids.

E. Lecithoproteins — compounds of proteins with phospholipins.

F. Lipoproteins — compounds of proteins with fats, existence in nature

doubtful, artificial forms easily prepared.

3. Derived Proteins

A. Primary protein derivatives.

a. Proteans — first products of hydrolysis, insoluble in water.

b. Metaproteins — result from further action of acids or alkalies,

soluble in weak acids and alkalies, but insoluble in dilute salt
solutions.

c. Coagulated proteins — insoluble forms produced by the action

of heat or alcohol.

B. Secondary protein derivatives.

a. Proteases — products of hydrolysis, soluble in water, not coag-
ulated by heat, precipitated by saturation of solution with
ammonium sulfate.

176 CHEMISTRY OF PLANT LIFE

b. Peptones — products of further hydrolysis soluble in water, not

coagulated by heat, not precipitated by ammonium sulfate,
give biuret reaction.

c. Peptides — individual amino-acids, or poly-peptides, may or

may not give biuret reaction.

The plant proteins which have been investigated, thus far,
fall into these groups as follows:

1A. Albumins

Leucosin, found in the seeds of wheat, rye, and barley.
Legumelin, " pea, horse-bean, vetch, soy-bean, len-

til, cowpea, adzuki-bean.
Phaselin, " kidney-bean.

Ricin, castor-bean.

IB. Globulins

Legumin, found in the seeds of pea, horse-bean, lentil and vetch.
Vignin, cowpea.

Glycinin, soy-bean.

Phaseolin, beans (Phaseolus spp.)

Conglutin, " lupines.

Vicilin, pea, horse-bean, lentil.

Corylin, hazel nut.

Amandin, nuts of almond and peach.

Juglansin seeds of walnut and butternut.

Excelsin, " Brazil nut.

Edestin, hemp seed.

Avenalin, oats.

Maysin, corn.

Castanin, the seeds of European chestnut.

Tuberin, potato tubers.

And, crystalline globulins found in the seeds of flax, squash, castor-
bean, sesame, cotton, sunflower, radish, rape, mustard, and in cocoa-
nuts, candlenuts, and peanuts.

1C. Glutelins

Glutenin, found in the seeds of wheat.
Oryzenin, " " rice.

ID. Prolamins

Gliadin, found in the seeds of rye, wheat, with glutenin forms "gluten."
Hordein, " barley

Zein, " " corn.

1E-1G. Histories, Protamines and Albuminoids. — So far as is
now known, no representatives of these classes are found in plants.

2. Conjugated Proteins. — There is no conclusive evidence of
the existence in plants of any of the conjugated proteins, other

PROTEINS 177

than the nucleoproteins and the chromoproteins, the composition
and properties of which have been discussed in previous chapters.
The nucleoproteins undoubtedly occur in the embryos of many, if
not all, seeds.

3. Derived Proteins. — Representatives of the various types of
derived proteins are undoubtedly found as temporary inter-
mediate products in plants, both as products of hydrolysis pro-
duced during the germination of seeds and as intermediate forms
in the synthesis of proteins. So far as is known, however, they
do not occur as permanent forms in any plant tissues. They have
been prepared in large numbers and quantities, by the hydrolysis
of the natural proteins and the artificial synthesis of polypeptides.

In the present state of our knowledge concerning the func-
tioning of the proteins, no significance in the physiology of plant
life, or metabolism, is to be attached to the particular type of
protein material which it contains, at least so far as the simple
proteins of the cytoplasm are concernedi

DIFFERENCES BETWEEN PLANT AND ANIMAL PROTEINS

A much larger variety of protein materials is found in animal
tissues than in plants. This is undoubtedly because different
animal organs perform so much more varied physiological func-
tions than do those of plants. Three groups of simple proteins,
the histones, the protamines, and the albuminoids, which are quite
common in animal tissues, are entirely unknown in plants. Fur-
ther, conjugated proteins of greater complexity and more varied
structure are found in animal tissues, especially in the brain,
nerve-cells, etc., than in plants.

Plant proteins, in general, usually contain larger proportions
of proline and of glutamic acid than are found in animal proteins;
also more arginine than is found in any of the animal proteins
except the protamines, which contain as high as 85 per cent of this
amino-acid.

Of the twenty-five plant proteins which have thus far been
hydrolyzed and studied from this standpoint, all contained leucine,
proline, phenylalanine, aspartic acid, glutamic acid, tyrosine,
histidine, and arginine; two gave no glycine; two others, no
alanine; four contained no lysine; and one, no tryptophane.
Zein, the principal protein of corn contains no glycine, lysine, or

178 CHEMISTRY OF PLANT LIFE

tryptophane. It is not sufficient to support animal life and
promote growth, if used as an exclusive source for protein for food.

THE EXTRACTION OF PROTEINS FROM PLANT TISSUES

Since proteins are indiffusible, it is essential that the cell-
walls of the tissue shall be thoroughly ruptured as the first step in
any process for the extraction of these compounds from plant tis-
sues. This is usually accomplished by grinding the material as
finely as possible, preferably with the addition of sharp quartz
sand, or broken glass, to aid in the tearing of the cell-wall material.

The solvent to be used in extracting the proteins from this
finely ground material depends upon the nature and solubility
of the proteins which are present, and also upon whether it is
desired to separate the proteins which may be present in the plant,
during the process of the extraction. A glance at the scheme of
classification of the proteins will show the following solubilities
which serve as a guide to the procedure to be followed: (a) pro-
teoses, albumins, and some globulins may be extracted with water;
(6) globulins and most of the water-soluble proteins may be
extracted by using a 10 per cent solution of common salt; (c)
prolamines are extracted by 70-90 per cent alcohol; glutelins and
prolamins dissolve in dilute acids or dilute alkali.

A common procedure is to extract groups (a) and (6), using a
10 per cent salt solution as the solvent, and then to separate the
albumins, globulins, etc., from this solution by suitable precip-
itants; then to treat the material with 80 per cent alcohol, to
extract the prolamines; and finally with dilute alkali, to extract
the glutelins. The dissolved proteins in each extract can be sub-
sequently purified by dialysis, precipitation, etc. The insoluble
proteins can be studied only after removing the other materials
associated with them in the tissue, by suitable mechanical or chem-
ical means,

THE SYNTHESIS OF PROTEINS IN PLANTS

The synthesis of proteins in plants is not a process of photo-
synthesis, as it can take place in the dark and in the absence of
chlorophyll, or any other energy-absorbing pigment. However,
protein-formation normally takes place in conjunction with car-

PROTEINS 179

bohydrate-formation. The carbon, hydrogen, and oxygen neces-
sary for protein synthesis are undoubtedly obtained from carbo-
hydrates. The nitrogen and sulfur come from the salts absorbed
from the soil through the roots and brought to the active cells in
the sap. Atmospheric nitrogen cannot be used by plants for this
purpose, except in the case of certain bacteria and other low plants,
notably the bacteria which live in symbiosis with the legumes in
the nodules on the roots of the host plants. In general, the sulfur
must come in the form of sulf ates and the nitrogen in the form of
nitrates; although many plants can make use of ammonia for
protein-formation. Presumably, the nitrate nitrogen must be
reduced in the plant to nitrites, and then to ammonia form, in
order to enter the amino-arrangement required for the greater
proportion of the protein nitrogen.

The mechanism by which ammonia nitrogen becomes amino-
acids in the plant is not understood. Artificial syntheses of
armno-acids, by the action of ammonia upon glyoxylic acid and
sorbic acid, both of which occur in plants and may be obtained
by the oxidation of simple sugars, have been accomplished, and it
seems probable that similar reactions in the plant protoplasm
may give rise to the various ammo-acids which unite together to
form proteins. Nothing is known, however, of the process by
which the more complicated closed-ring amino-acid compounds,
such as proline, histidine, or tryptophane, are synthetized.

The condensation of amino-acids into proteins, or the reverse
decomposition, is very readily accomplished in all living proto-
plasm, under the influence of special protein-attacking enzymes,
which are almost universally present in the cytoplasm. These
reactions in connection with the proteins are similar to the easy
transformation of sugars to starches, and vice versa, under the
action of the corresponding carbohydrate-attacking enzymes.

PHYSIOLOGICAL USES OF PROTEINS

There can be no doubt that the all-important role of pro-
teins, in either plant or animal tissue, is to furnish the colloidal
protoplasmic material in which the vital phenomena take place.
Their occurrence in seeds, and other storage organs, is, of course,
in order to provide the protoplasm-forming material for the young
seedling plant.

180 CHEMISTRY OF PLANT LIFE

They are, moreover, the source for the material which goes into
some of the secretion groups of organic compounds; as they are
easily broken down by various agents of decomposition into
nitrogen-free alcohols, aldehydes, and acids, which produce the
essential oils, pigments, etc.

Much, if not all, of their physiological activity is due to their
colloidal nature, the importance and effects of which will be more
apparent after the chapters dealing with the colloidal condition of
matter and with the physical chemistry of protoplasm have been
studied.

References

ABDERHALDEN, E. — "Neuere Ergebnisse auf dem Gebiete der Speziellen

Eiweisschemie," 128 pages, Jena, 1909.
FISCHER, E. — " Untersuchungen iiber Aminosauren, Polypeptide, und Pro-

teine, 1899-1906," 770 pages, Berlin, 1906.

MANN, G— " Chemistry of the Proteids," 606 pages, London, 1906.
OSBORNE, T. B. — "The Vegetable Proteins," 138 pages, Monographs on Bio-
chemistry, London, 1909.
PLIMMER, R. H. A. — "The Chemical Constitution of the Proteins, Part I,

Analysis," 188 pages; and "Part II, Synthesis, etc." 107 pages, Mono-
graphs on Biochemistry, London, 1917. (3d ed.).
ROBERTSON, T. B. — "The Physical Chemistry of the Proteins," 477 pages,

New York, 1918.
SCHRYBER, S. B.— "The General Characters of the Proteins," 86 pages,

Monographs on Biochemistry, London, 1909.
UNDERBILL, F. P. — "The Physiology of the Amino-acids," 169 pages, 13 figs.

1 plate. Yale University Press, 1915.

CHAPTER XIV
ENZYMES AND THEIR ACTION

THE characteristic difference between the reactions of inorganic
compounds and those of organic substances lies in the rapidity, or
velocity, of the chemical changes involved. Speaking generally,
chemical reactions take place between substances which are in
solution, so that they may come into sufficiently intimate contact
that chemical action between them can take place. There are, of
course, occasional examples of reactions between dry solids, such
as the explosion of gunpowder, etc., but the general rule is that
reacting materials must be in either colloidal or true solutions.

Inorganic materials, when dissolved in water, usually ionize
very readily. That is, they are not only disintegrated into indi-
vidual molecules, but a considerable proportion of these molecules
separate into their constituent ions. When solutions containing
ionized compounds are brought together, conditions for chemical
interaction are ideal, and the reaction proceeds with such tre-
mendous rapidity as to be completed almost instantaneously, in
most cases.

Organic compounds, on the other hand, ionize only very slowly,
if at all. Hence, reactions between organic compounds, even
when they are in solution, proceed very slowly unless carried on at
high temperatures, under^ncreased pressure, or under the influ-
ence of some catalytic agent. Even under the stimulation of these
reaction-accelerating agencies, most chemical changes in organic
compounds when carried on in the laboratory, require several
hours or even days and sometimes weeks, for then* completion.
But when similar reactions take place in living organisms, they
proceed with velocities which resemble those of inorganic com-
pounds in the laboratory. This difference between the velocity
of organic reactions when caried on under artificial conditions
in the laboratory (often spoken of as " in vitro ") as compared
with that of the same reactions when they take place in a living

181

182 CHEMISTRY OF PLANT LIFE

organism ("in vivo "), is due to the universal presence in the
living protoplasm of certain organic catalysts, known as enzymes.

ENZYMES AS CATALYSTS

The phenomenon known as " catalysis " is of common occur-
rence in both inorganic and organic chemistry. The effect of a
small amount of manganese dioxide in aiding in the liberation of
oxygen from potassium chlorate is an example which is familiar
to all students of elementary chemistry. Similarly, spongy
platinum accelerates the oxidation of sulfur dioxide to sulfur
trioxide, in the commercial manufacture of sulfuric acid. Again,
the hydrolysis of sucrose into fructose and glucose proceeds very
slowly in the presence of water alone, but if a little hydrochloric
acid or sulfuric acid be added to the solution, the velocity of the
hydrolysis is enormously accelerated. Many other examples of
the accelerating effect of various chemicals upon reactions into
which they do not themselves enter, might be cited.

The essential features of all such catalytic actions are: (1)
the velocity of the reaction is greatly altered, usually accelerated;
(2) the catalytic agent does not appear as one of the initial sub-
stances, or end-products, of the reaction, and is not itself altered
by the chemical change which is taking place; (3) the accelerating
effect is directly proportional to the amount of the catalyst which
is present; (4) relatively small amounts of the catalyst produce
very large results in the reacting mixture; and (5) the catalysts
cannot themselves initiate reactions, but only influence the velocity
of reactions which would otherwise take place at a different rate
(usually much more slowly) in the absence of any catalytic
agent.

Enzymes conform to all of these properties of catalysts, and
are commonly defined as the " catalysts of living matter." They
are almost universally present in living organs of every kind, and
perform exceedingly important functions, both in the building-up
of synthetic materials and in the rendering soluble of the food of
both plants and animals, so that it can be translocated from place
to place through the tissues of the organism.

Enzymes differ from inorganic catalysts in being destroyed by
heat, in not always carrying the reaction to the same stage as does
the inorganic catalyst which may accelerate the same reaction, and

ENZYMES A$D THEIR ACTION 183

in producing different changes in the same substance by different
enzymes.

The name " enzyme " comes from Greek words meaning
" in yeast," as the nature and effect of the enzyme involved in the
alcoholic fermentation of sugars by yeast were those which were
first recognized and understood. It was at first thought, by Pas-
teur and his students, that fermentation is the direct result of the
life activities of the yeast plant. Later, it was found that water
extracts from sprouted barley, from almond seeds, and from the
stomach, pancreas, etc., were able to bring about the decompo-
sition of starch, of amygdalin, and of proteins, respectively, in a
way which seemed to be quite comparable to the fermentative
action of yeasts. Hence, it was thought that there were two
varieties of active agents of this kind, one composed of living cells
and the other non-living chemical compounds, and these were
called the " organized ferments" and the " unorganized ferments,"
respectively. However, in 1897, Biichner found that by grinding
yeast cells with sharp sand until they were completely disinte-
grated and then submitting the mass to hydraulic pressure, he
could obtain a clear liquid, entirely free from living cells, which was
just as active in producing fermentation as was the yeast itself.
This discovery paved the way for a long series of investigations,
which have conclusively demonstrated that there is no distinction
between " organized " and " unorganized " ferments, that all
living organisms perform their characteristic functions by means
of the enzymes which they contain, and that these enzymes can
bring about their characteristic catalytic effects outside the cell,
or tissue which elaborates them, just as well as within it, provided
only that the conditions of temperature, acidity or alkalinity of
the medium, etc., are suitable for the particular enzyme action
which is under consideration.

GENERAL PROPERTIES OF ENZYMES

Since enzymes are catalysts, it is plain that an accurate de-
scription of their activity should, in each case, refer to the influ7
ence which they exert upon some definite reaction velocity. But
since the phrases necessary to describe such an effect are cumber-
some and inconvenient, and since most of the reactions which are
accelerated by the catalytic action of enzymes are either simple

184 CHEMISTRY OF PLANT LIFE

hydrolyses, changes in oxygen content, or other simple decompo-
sitions or condensations, which will otherwise proceed so slowly
as to be practically negligible, it is customary to speak of the
enzyme as " acting upon " the material in question. It should be
understood, however, that this is a misstatement, as the enzyme
cannot actually initiate a reaction, or " act upon " any sub-
stance; it only acts as a catalyzer to accelerate the action of
water, oxygen, etc., upon the material in question.

Generally speaking, most enzymes are colloidal in form and,
hence, do not diffuse through membranes such as the cell- walls.
Some of them perform their characteristic functions only within
the cell, or organ, which elaborates them, and can be obtained
outside these tissues for purposes of study only by first rupturing
the cell-wall or other membrane with which they are surrounded.
Such enzymes are known as " intracellular." Others are regu-
larly secreted by glands which discharge them into other organs,
as the stomach or intestines of animals, where they perform their
useful functions; or, as in the case of germinating seeds, they
move to other parts of the organ, and can be extracted from the
tissue by simple treatment jvith water. These are known as the
" extracellular " enzymes.

Enzymes are specific in their action. Any given enzyme
affects only a single reaction; or at most acts only upon a single
group of compounds which have similar molecular configuration.
Usually it is only a single compound whose decomposition is accel-
erated by the action of a particular enzyme; but there are a few
enzymes, such as maltase (which acts on all a-glucosides) and
emulsin (which acts on all 0-glucosides) which act catalytically
upon groups of considerable numbers of similar compounds.

Enzymes, like all other catalysts, act more energetically at
increased temperatures; but for each particular enzyme there is an
" optimum temperature," (usually between 40° and 65°) above
which the destructive effect of the temperature upon the enzyme
itself more than offsets the accelerating influence of the increased
temperature. At still higher temperatures (usually 80° to 100°) the
enzymes are " killed," i.e., rendered permanently inactive. All
enzymes are " killed " by boiling the solutions in which they are
contained. Dry preparations of enzyme material can withstand
somewhat higher temperatures, for somewhat longer periods of
time, than can the same enzyme in moist condition or in solution.

ENZYMES AND THEIR ACTION 185

When an enzyme has once been inactivated by heating, or " killed,"
it can never be restored to activity again.

Enzymes are extremely sensitive to acids, bases, or salts, their
activity being often enormously enhanced or, in other cases,
entirely inhibited, by the presence in the reacting medium of very
small amounts of free acids, or bases, or even of certain neutral
salts. For example, pepsin, the enzyme of the stomach will act
only in the presence of a slightly acid medium and is wholly inactive
in a mixture which contains even the slightest amount of free
alkaline material; while trypsin, the similar enzyme of the intes-
tine, acts only under alkaline conditions. Practically all enzymes
are rendered inactive, but not destroyed, by the presence of either
acid or alkali in excess of N/10 strength. Many will act only in
the presence of small quantities of certain specific neutral salts;
while, on the other hand, other salts are powerful inhibitors of
enzyme action. Enzymes often differ from the protoplasm which
secretes them in their response to antiseptics, such as toluene,
xylene, etc., which inhibit the activity or growth of the cell, but
have no effect upon the activity of the enzymes which it contains.

THE CHEMICAL NATURE OF ENZYMES

Nothing is known with certainty concerning the chemical
nature of enzymes. Being colloidal in nature, they adsorb carbo-
hydrates, proteins, fats, etc., so that active enzyme preparations
often respond to the characteristic tests for these groups of sub-
stances; and many investigators have reported what has, at first,
seemed to be conclusive evidence that some particular enzyme
which they have studied is either a carbohydrate, a protein, or
some other type of organic compound. Later investigations have
always shown, however, that if the preparation in question be
submitted to the digestive action of the enzymes which hydrolyze
the particular type of substances to which it is supposed to belong,
the material will lose its characteristic protein, or carbohydrate,
etc., properties, without losing its specific activity, thus clearly
indicating that the substance which responds to the characteristic
tests for some well-known type of organic compounds is present as
an impurity and is not the enzyme itself.

The present state of knowledge concerning the nature of
enzymes seems to indicate that, like the inorganic catalysts, they

186 CHEMISTRY OF PLANT LIFE

may vary widely in chemical composition; and that their tre-
mendous catalytic effects are due, in part at least, to their colloidal
nature. This will be better understood and appreciated after the
phenomena associated with the colloidal condition have been
considered (see the following chapter).

NOMENCLATURE AND CLASSIFICATION

Since nothing is known of the chemical composition of enzymes,
they can only be studied by considering the effects which they
produce. This is reflected in the systems which have been adopted
for their nomenclature and classification.

As they were first supposed to be proteins, the earlier repre-
sentatives of the group were given characteristic names ending
with the suffix in, similar to that of the proteins. Since this idea
has been found to be incorrect, however, a system of nomen-
clature has been adopted which assigns to each enzyme the name
of the material upon which it acts, followed by the suffix ase.
Thus, cellulase is the enzyme which accelerates the hydyrolysis
of cellulose; glucase, that acting upon glucose; amylase, that
acting upon starch (amylum), etc.

The substance upon which the enzyme acts (or, strictly speak-
ing, the substance whose hydrolysis, oxidation, or other chemical
change, is catalytically affected by the enzyme) is called the
substrate.

Most enzymes are catalysts for hydrolysis reactions and are,
hence, classed as hydrolytic in their action, and may be spoken
of as " hydrolases." Those which accelerate oxidation are
called " oxidases"; while those that stimulate reduction reactions
are " reductases"; those that aid in the splitting off of ammonia,
or amino-acid groups, are " deaminases"; and those that aid in
the splitting off of CCb from COOH groups are " carboxylases,"
etc.

The hydrolytic enzymes are further sub-divided into the
sucroclastic (sugar-splitting), or sucrases; the lipoclastic (fat-
splitting), or lipases; the esterases (ester-splitting); proteoclastic
(protein-splitting), or proteases; etc.

ENZYMES AND THEIR ACTION 187

OCCURRENCE AND PREPARATION FOR STUDY

Enzymes are present in all living matter. In animal tissues,
they occur in the largest amounts in those glands or organs where
active vital processes take place, as hi the brain, the digestive
tract, blood, etc. In plants, they may be found in all living cells,
and are especially abundant in the seeds, where they serve to render
soluble and available to the young plant the stored food materials.
The enzymes of moulds, and other parasitic plants, are usually
extracellular in type, being secreted for the purpose of making the
material of the host plant available to the parasite. Extracellular
enzymes are also developed in seeds during germination, in order
that the stored food material of the endosperm may be rendered
soluble and translocated into the tissues of the growing seedling.
But most other plant enzymes are intracellular in type. Hence,
in all preparations of plant enzymes for study, or for commercial
use, the first step in the process is, necessarily, a thorough ruptur-
ing of the cell-walls of the plant material.

The rupturing of the cells may be accomplished in a variety of
ways, as follows: (1) mechanical disintegration, as by grinding in a
mortar with sharp sand; (2) freezing the material, by treatment
with liquid air, then grinding; (3) killing the cells by drying, by
treatment with alcohol or acetone, then grinding the mass in a
paint mill with toluene ; (4) killing the cells by chemicals (sulf uric
acid, 0.5 to 1.0 per cent, or other suitable agents) followed by
extraction with water; (5) autolysis, or self -digestion, in which the
cells are mixed with toluene or some other antiseptic which kills
the cells without injuring the enzymes, then the material is minced
or ground up and suspended in water containing the antiseptic,
until the enzymes dissolve the cell-walls and so escape into the
liquid — this process being especially adapted to the preparation
of active extracts from yeasts, which contain the necessary cell-
wall dissolving enzymes to facilitate autolysis.

Enzymes may be separated out of the aqueous extract obtained
from cells ruptured by any of the above methods, by precipitation
with alcohol, acetone, or ether, in which they are insoluble; but
if this is done, the precipitate must be at once filtered off and rapidly
washed and dried, as prolonged contact with these precipitating
agents greatly diminishes the activity of most enzymes. Or, they
may be adsorbed out of solution on gelatinous, or colloidal, mate-

188 CHEMISTRY OF PLANT LIFE

rials, like aluminium hydroxide, or various hydrated clays. If
the dry preparations obtained in any of these ways are contam-
inated by carbohydrates, proteins, etc., these may be removed by
treatment with suitable digesting enzymes obtained from the
saliva, gastric, and pancreatic juices, and the digested impurities
washed out with 60 to 80 per cent alcohol, leaving the enzyme
preparation in a purified but still active form.

In any study of the " strength," or possible catalytic effects,
of an enzyme preparation, it is necessary, first, to determine what
particular reaction it affects, by qualitative tests with various
substrate materials, such as starch, sugars, glucosides, proteins,
etc., and then to determine quantitatively its accelerating effect
upon the reaction in question. The latter may be done by
measuring either the time required to carry a unit quantity of the
substrate material through any determined stage of chemical
change, or the quantity of the substrate which is changed in a unit
period of time. It would not be profitable to go into a detailed
discussion here of the methods of making these quantitative
measurements of enzyme activity. Such discussions must neces-
sarily be left to special treatises on methods of study of enzyme
action. It may be said, however, that generally both the quali-
tative tests for, and the quantitative measurements of, the accel-
erating influence of enzymes depend upon the observation of some
change in the physical properties of the substrate material, such
as the optical activity, electrical conductivity, or viscosity, of its
solution. In some cases, it is convenient to make an actual quan-
titative determination of the amount of end-products produced
in a given time, as in the inversion of cane sugar, the hydrolysis of
maltose, etc., but such determinations necessarily involve the
removal of some of the reaction mixture for the purposes of the
determinations, and are not, therefore, suitable for the study of
the progressive development of the reaction which is being studied.

Enzymes are found in all parts of the animal organism and
those which are active in the digestion of food, the metabolism of
digested material, the coagulation of blood, etc., have been exten-
sively studied. A discussion of these animal enzymes would be
out of place in such a text as this, however, and the following list
includes only enzymes which are known to occur in plant tissues.
These well-known enzymes will serve as examples of the several
general types which have thus far been isolated and studied.

ENZYMES AND THEIR ACTION

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ENZYMES AND THEIR ACTION 191

The above list includes only the more common and best-known
plant enzymes. It seems reasonable to suppose that for every
individual type of organic compound which may occur in general
plant groups, or even in single species, there is a corresponding
enzyme available to affect its physiological alterations. Indeed,
new preparations of active enzymes from special types of plants
and new evidences of the existence of enzymes in various plant
organisms are continuously being reported.

A few of the most common specific representatives of individual
groups of enzymes may be briefly described, as follows:

Amylase (or diastase, as it was first named and is still corn-
'monly called) is probably the most widely distributed enzyme of
plants. It is found in practically all bacteria and fungi; in prac-
tically all seeds (it has been found in active form in seeds which
were known to be over fifty years old); in all roots and tubers;
and in practically all leaves, where it is located in the stroma of the
chloroplasts.

It appears to exist in two modifications, known, respectively,
as (a) translocation diastase and (6) diastase of secretion. The
first form is found in the cells of ungerminated seeds, in leaves,
shoots, etc. It remains in the cells where reserve starch is stored
and aids in the transformation of starch into soluble materials for
translocation from cell to cell. It is active at a lower temperature
than the second form, its optimum temperature being 45° to 50°.
The second form is secreted by the scutellum, and perhaps by the
aleurone cells, of germinating seeds, being produced by special
glandular tissue. It aids in the hydrolysis of the starch for the
use of the growing embryo. Its optimum temperature is 50° to 55°.

The activity of amylase is accelerated by the presence of
small quantities of neutral salts, especially by sodium chloride and
disodium phosphate. It acts best in neutral solutions, its activity
being inhibited, although the enzyme itself is not destroyed, by the
presence of more than minute traces of free mineral acid or alkali.

Sucrase (or invertase) is present in almost all species of yeasts,
where it serves to convert unfermentable sucrose into glucose and
fructose, which are readily fermentable. Invertase is also present
in moulds and other microorganisms; and in the buds, leaves,
flowers, and rootlets of those higher order plants which store their
carbohydrate reserves in the form of sucrose. It appears that
sucrose, while easily soluble, is not readily translocated, or utilized,

192 CHEMISTRY OF PLANT LIFE

by plants until after it has been hydrolyzed into its constituent
hexoses.

The optimum temperature for invertase is 50° to 54°; it is
killed if heated, in the moist condition, to 70°. Its activity is
increased by the presence of small amounts of free acids; but is
inhibited by free alkalies.

Zymase is the active alcoholic fermentation enzyme of yeasts.
It accelerates the well-known reaction for the conversion of hexose
sugars into alcohol and carbon dixoide, namely,

Because of its scientific interest and industrial importance in the
fermentation industries, its action has been extensively studied.
It acts only in the presence of soluble phosphates and of a coen-
zyme (see below) which is dialyzable and not destroyed, which is
probably an organic ester of phosphoric acid. The significance
of the molecular configuration of the hexose sugars in their sus-
ceptibility to action by zymase has already been discussed in detail
(see page 56).

The optimum temperature for zymase action is 28° to 30°.
The enzyme is killed by heating to 45° to 50° in solution, or to
85° if in dry preparation.

Proteases of the erepsin type, i.e., those which break proteins
down to amino-acids instead of only to the proteose or peptone
stage, as is characteristic of the enzymes of the trypsin type, are
widely distributed in plants. Except in the case of the two which
occur in large amounts in certain special fruits (papain in papaws,
and bromelin in pineapples), they are very difficult to prepare in
pure form for study. In general, all proteolytic actions, even when
accelerated by active enzymes, proceed much more slowly than
do the hydrolyses of carbohydrates or fats. It seems that meta-
bolic changes of the complex protein molecules are much more
difficult to bring about and take place much more slowly than do
those of the energy-producing types of compounds.

The presence of proteolytic enzymes in most vegetative cells,
and in seeds, may be demonstrated, however, by studying the action
of extracts of these tissues upon soluble proteins. The best-
known example of this type of enzymes is the protease of yeast;
but similar ones may be found in germinating seeds. These

ENZYMES AND THEIR ACTION 193

vegetable proteases are usually most active in neutral or only
faintly alkaline solutions, and their activity is nearly always
inhibited by even traces of free acids.

Most laboratory studies of proteolytic enzymes are carried on
with preparations of the powerful members of this class of enzymes
which are found in the digestive tract of animals, namely, the pep-
sin of the gastric juice, which acts in the acid medium. in the stom-
ach, and the trypsin of the pancreatic ju'ce, which acts in the
alkaline medium of the intestinal tract. But even these powerful
proteases require several hours for the transformation of an amount
of soluble albumin into its ammo-acid constituents which is
equivalent to the amount of starch which is.hydrolyzed to maltose
by diastase in a very few minutes.

Enzymes which govern oxidative changes, known respectively,
as catalases and oxidases, are almost universally present in plants.
Catalase decomposes peroxides, with the liberation of free oxygen.
It is, therefore, necessary to the final step in the process of photo-
synthesis, as elucidated by Usher and Priestley (see page 26),
and serves to prevent the destructive action of hydrogen peroxide
upon chlorophyll. The almost universal presence of oxidases
in plant tissues has been repeatedly demonstrated. They are
present in especially large amounts in tissues which are being
acted upon by parasitic fungi or are combating unfavorable
conditions of growth. The oxidases, in such cases, seem to
be the agents by which the plant is able to stimulate its metabolic
activities to overcome the unfavorable environment for its normal
development.

In vegetables and fruits, the common browning, or blackening,
of the tissues when cut surfaces are exposed to the air has been
demonstrated to be due to the catalytic oxidation of the tannins
or of certain amino-acids, especially tyrosin, under the influence
of the oxidases which are present in the tissues. In fact, most
pigmentation phenomena are due to changes in the oxygen con-
tent of the chromogens of the cells of the plant, under the influence
of the oxidases which are present in the protoplasm of the cells in
question. Hence, the oxidases may be said to be the controlling
agencies for both the energy-absorbing activities and for respiration
in plants.

194 CHEMISTRY OF PLANT LIFE

THE NATURE OF ENZYME ACTION

The mechanism by which an enzyme accomplishes its catalytic
effects has been the object of extensive studies during recent years,
especially since the discovery by Blichner that enzymes could be
isolated in solutions entirely free from the disturbing influence of
growing cells. Several theories concerning the mode of this
catalytic action have been advanced. The earliest and simplest
of these was that the enzyme simply creates an environment favor-
able for the particular chemical reaction to take place, as by
exposing large surfaces of the substance in question to the action
of the hydrolytic, or other effective, agent, by means of surface
adsorption of the substrate material on the colloidal enzyme.

However, more recent investigations clearly indicate that there
is an actual combination between the substrate material and the
enzyme, which combination then breaks down with a resultant
change in the substrate material and a freeing of the enzyme
for repeated recombination with additional substrate, with the
net result that the chemical change in the substrate material is
enormously accelerated. That such a combination between sub-
strate and enzyme actually exists has been demonstrated in two
different ways: (a) experimentally, by mixing together solutions
of an enzyme and of its substrate, each of which is filterable
through paper or through a porous clay filter, with the result that
the active material in the combined solutions will not pass through
these same filters; and (6) mathematically, by a study of the curves
representing the reaction velocities of typical reactions which are
proceeding under the influence of an enzyme, which show that so
long as there is a large excess of substrate material present, the
accelerating influence of the catalyst is uniform over given suc-
cessive periods of time, but that when the quantity of substrate
material becomes smaller than that which permits the maximum
combining power of the enzyme to be exercised, the reaction
velocity immediately slows up.

Again, the fact that the specificity of the action of an enzyme,
i.e., the limitation of the action of that enzyme to a specific single
compound or group of similar compounds, is definitely related to
the molecular configuration of the molecule of the substrate, as has
been found to be true in all those cases where the molecular con-
figuration of the substrate material has been established (see

ENZYMES AND THEIR ACTION 195

pages 56 to 58), is an added indication that there is some kind
of a union between the enzyme and the substrate as a first step
in the catalytic process.

As to the nature of this supposed combination of substrate and
enzyme, two theories are held. The first is that this union is in the
form of an actual molecular combination, or chemical compound,
and the other is that it is a purely physical, or colloidal, complex.
The latter view has by far the greater weight of theoretical and
experimental evidence in its support. The relation of electrolytes
to the catalytic effect of enzymes, the appearance of the reacting
masses under the ultra-microscope, and the effect of heat Upon the
reacting mixtures, all point to the conclusion that the phenomenon
is colloidal rather than molecular in character. This view also
makes the remarkable catalytic effects which take place in living
protoplasm, wrhich undoubtedly exists in the colloidal condition,
much more easily understood. This phase of the matter will
be much more apparent after the chapter dealing with the physical
chemistry of the protoplasm has been studied.

A further indication that the mechanism of enzyme activity is
colloidal in character lies in the fact that, so far as is known, all
reactions which are catalyzed by specific enzymes are reversible
and the same enzyme will accelerate the velocity of the reaction
in either direction, the direction in which the reaction goes being
determined by the conditions surrounding the reacting material
at the time. It was formerly supposed that enzymes catalyze
only decomposition reactions and that the synthetic reactions of
living tissues are produced by means of some other force or agency.
This view supported the idea of a chemical union of the enzyme
with the substrate which, when it breaks down, breaks the mole-
cule of the substrate material into some simpler form, or forms.
But it is now known that the reaction which is influenced by the
enzyme will be catalyzed in either direction by the specific enzyme
which " fits " the particular substrate material at every point of
its molecular configuration, as the glove fits the hand. The con-
trast between this fitting of the enzyme to the entire configuration
of the molecule, and the union at a single point or group which is
characteristic of chemical linkages, is apparent. As examples of
the synthetic action of the same enzyme which, under other con-
ditions, accelerates the decomposition of the same material, there
may be cited the demonstrated synthesis of isomaltose from glu-

196 CHEMISTRY OF PLANT LIFE

cose by maltase; the production of ethyl butyrate from alcohol
and butyric acid; and the synthetic production of artificial fats, by
the aid of the pancreatic lipase; and the apparent synthesis of a
protein from the same amino-acids which may be obtained from it
by hydrolysis under the influence of the same protease, but under
different environmental conditions.

^ ACTIVATORS AND INHIBITORS

The activity of enzymes is strongly influenced by the presence
in the solution of other bodies, usually, although not always, elec-
trolytes. This is probably due, in most cases at least, to the action
of the electrolyte upon the colloidal condition of the enzyme. All
enzymes do not respond alike to the action of the same electrolyte,
however. The activity of certain enzymes is enormously increased
by the presence of a small amount of acid; while the action of
another may be absolutely inhibited by the same acid in the same
concentration. Thus, the activity of the amylase found in the
endosperm of many seeds is instantly stopped by adding to the
solution enough sulfuric acid to make it two-hundredth normal in
strength; while the same concentration of acid actually accel-
erates the activity of some of the proteases.

Formaldehyde, hydrocyanic acid, and soluble fluorides usually
inhibit both the activity of a cell and of the enzymes which it
contains; while other antiseptics, such as toluene, xylene, etc.,
prevent the growth of the cell, or organism, without interfering
with the activity of the enzymes which may be present. By the
use of this latter type of antiseptics, it is possible to distinguish
between chemical changes which are involved in the actual devel-
opment of a cell and those which can be brought about in other
media by means of the enzymes which are contained in the
cell.

Any substance which increases the catalytic activity of an
enzyme is known as an " accelerator," or " activator"; while one
which prevents this activity is called an " inhibitor," or " para-
lyzer."

A type of accelerating influence quite different from that of
electrolytes is found in the effect of certain amino-acids upon
enzyme action. The influence of small amounts of asparagine in
enormously increasing the hydrolytic effect of amylase is an exam-

ENZYMES AMD THEIR ACTION 197

pie. There is no known explanation for this type of activation of
the enzyme.

The influence of activators, or inhibitors, in providing favorable
or unfavorable conditions for the action of an enzyme, should not
be confused with the relation to the enzyme itself of what are
known as " coenzymes " and " antienzymes," discussed in the fol-
lowing paragraph.

COENZYMES AND ANTIENZYMES

In the cases of many enzymes of animal tissues, it has been
found that they are absolutely inactive unless accompanied by
some other substance which is normally present in the gland, or
protoplasm, which secretes them. Thus, the bile salts are abso-
lutely necessary to the activity of trypsin, in its characteristic
protein-splitting action. Such substances are known as " coen-
zymes." They can usually be separated from their corresponding
enzymes by dialysis, the coenzyme passing through the parch-
ment membrane. Such coenzymes are not killed by boiling
the dialyzate, and the activity of the enzyme is restored by adding
the boiled dialyzate to the liquid which remains within the
dialyzer.

The best known example of a coenzyme in plant tissues is in
connection with the activity of the zymase of yeast cells. If
yeast juice be filtered through a gelatin filter, the colloidal enzymes
which are left behind are entirely inactive in producing fermenta-
tion, but may be restored to activity again by mixing with the
filtrate. An examination of this filtrate, which contains the coen-
zyme for zymase, shows that it contains soluble phosphates and
some other substance whose exact nature has not yet been deter-
mined, both of which are necessary to the activity of the zymase.
The phosphates seem to enter into some definite chemical combina-
tion with the substrate sugars, while the other coenzyme seems to
be necessary in order to make possible the final breaking down of
the sugar-phosphate complex by the zymase. This phenomenon
of coenzyme relationship is not very frequently observed in plant
enzyme studies, probably because the coenzyme (if there be such,
in the case which is under observation) usually accompanies the
enzyme itself through the various processes of extraction and
purification of the material for study. However, care must be

198 CHEMISTRY OF PLANT LIFE

taken in all cases when dialysis is employed, to see that a possible
coenzyme is not separated from an otherwise active preparation.

An entirely different type of phenomenon is that exhibited by
" antienzymes." These are found in the various intestinal worms
which live in the digestive tracts of animals; and prevent the diges-
tive action of the enzymes of the stomach and intestines upon these
worms. Probably similar " antienzymes " are located in the mu-
cous linings of the intestinal tract itself, and serve to prevent the
auto-digestion of these organs by the active enzymes with which
they are almost continually in contact.

The difference between an antienzyme, which protects material
which would otherwise be subject to the attack of an enzyme, and
an inhibitor, which renders the enzyme itself inactive, is apparent.

So far as is known, however, no such substances as antienzymes
are present in plant tissues; although the question as to why the
proteoclastic enzymes which are elaborated by a given mass of
protoplasm do not attack the protoplasm itself, might well be
raised.

ZYMOGENS

It is apparent that, since enzymes are produced by protoplasm
for the special needs of any given moment or stage of development,
there must be a preliminary stage, or condition, in which they do
not exert their characteristic catalytic effect. When in this
stage, the compound is known as " proenzyme," or " zymogen."
In this stage, it is inactive, but can be made to exhibit its catalytic
effect, usually by bringing it into contact with a suitable activator.
When once so activated, however, it cannot be returned again to
the inactive state.

This phenomenon has been studied in connection with the
zymogens of the digestive proteases, pepsin and trypsin. Tryp-
sixiogen may be rendered active by contact with either calcium
salts or with another substance (apparently itself an enzyme)
known as enterokinase, which is secreted in the intestinal tract.

Similarly, proenzymes have been reported as occurring in
numerous plant tissues. These proenzymes are believed to be
present in the plant cells in the form of definite characteristic
granules, which may be observed under the microscope, and which
disappear when the enzyme becomes active. Thus, " proinu-

ENZYMES AND THEIR ACTION 199

lase " has been reported as occurring in artichoke tubers; " pro-
lipase," in castor, beans; " proinvertase," in several species of
fungi; and, probably, " prooxidase," in tobacco leaves. » In the
case of the last-named zymogen, it has been observed that after
the zymogen has been once activated, as in response to the need
for increased activity due to the entrance of the germs of certain
leaf-diseases, it can once again produce a second supply of the
enzyme, but the process cannot again be repeated.

Calcium salts, or very dilute acids, are usually energetic acti-
vators of proenzymes.

PHYSIOLOGICAL USES OF ENZYMES

There can be no doubt that enzymes exert a tremendously
important influence in vital phenomena, by determining the rate
at which the chemical changes which are involved in these phe-
nomena shall proceed. S nee they do not initiate reactions, and
since they may catalyze reversible reactions in either direction, it
cannot be said that they determine the type of reactions which
will take place in any given mass of protoplasm; but, undoubtedly,
they do exert a determining influence upon the rate at which the
reaction will proceed, after the protoplasmic activity has deter-
mined the direction in which it shall go.

Without the intervention of these catalyzing agents, it would be
impossible for reactions between these non-ionized organic com-
ponents of the cell contents to come to completion with anything
like the marvelous rapidity with which these changes must take
place in order to permit the organism to grew, to perform its neces-
sary vital functions, or to adjust itself to the changes in its environ-
mental conditions.

Since the number of different reactions which take place
within a living cell is very great, and since these chemical changes
are extremely variable in type, it follows that the number of dif-
ferent enzymes which must exist in either a plant or an animal
organism is likewise very large. For example, fourteen different
enzymes have been isolated from the digestive system, and at
least sixteen from the liver, of animals. They are universally
present in living protoplasm of every land, from the most minute
bacterium to the largest forest trees, in the plant kingdom; and
from the amoeba to the whale, in animals.

200 CHEMISTRY OF PLANT LIFE

While there is a great variety of enzymes which may be pro-
duced by a single individual organism, the same enzyme may be
found in the greatest variety of organisms; as, for example, the
protease trypsin, which has been found in several species of bac-
teria, in the carnivorous plant known as " Venus' Fly Trap,"
and in the human pancreas, as well as that of all other animals.

FURTHER STUDIES NEEDED

From the discussions which have been presented in this chap-
ter, it is apparent that the enzymes play a tremendously important
part in vital phenomena, by controlling the rate at which the bio-
chemical reactions take place in the cells of the living organism.

The means by which the protoplasm elaborates these all-
important chemical compounds are as yet absolutely unknown.
Even the nature of the enzymes themselves is still a matter of
speculation and study. Much intensive study is needed and
should be given to these matters, for the purpose of elucidating
the methods by which the enzymes accomplish their remarkable
catalytic effects, and, if possible, the actual chemical nature of the
enzymes themselves. It is conceivable, of course, that if the latter
object of these studies should ever be reached, it might be possible
to synthetize enzymes artificially, and so to develop a means for
the artificial duplication of the synthesis of organic compounds
with the same velocity that this is done in the plant cells. Such a
result would have a scientific interest fully as great as did Wohler's
artificial synthesis of urea, which proved that there is no essential
difference in character between the compounds which are the
products of living organisms and those which are produced in the
laboratory; and, at the same time, might have an immensely
more important practical bearing, since it would lead the way to
the artificial production of the carbohydrates, proteins, fats, etc.,
for which we are now dependent upon plant growth as the source
of these materials for use as human food.

References

BAYLISS, W. M. — "The Nature of Enzyme Action," 186 pages, Monographs

on Biochemistry, London, 1919 (4th ed.).
EULER, H., trans, by POPE, T. H. — "General Chemistry of the Enzymes,"

319 pages, 7 figs., New York, 1912.

ENZYMES AND THEIR ACTION 201

EFFRONT, J. trans by PRESCOTT, S. C. — "Enzymes and their Application,-1-
Enzymes of the Carbohydrates," 335 pages, New York, 1902.

EFFRONT, J., trans by PRESCOTT, S. C. — "Biochemical Catalysts in Life and
Industry — Proteolytic Enzymes," 763 pages, New York, 1917.

GREEN, J. R. — "The Soluble Ferments and Fermentation," 512 pages, Cam-
bridge, 1901, (2ded.).

GRUS. J. — "Biologic und Kapillaranalyse der Enzyme," 227 pages, 58 figs.,
3 plates, Berlin, 1912.

HARDEN, A. — "Alcoholic Fermentation," 156 pages, 8 figs., Monographs on
Biochemistry, London, 1914.

PLIMMER, R. H. A. — "The Chemical Changes and Products Resulting from
Fermentations," 184 pages, London, 1903.

OPPENHEIMER, C., trans, by Mitchell, C. A. — "Ferments and their Actions,"
343 pages, London, 1901.

CHAPTER XV
THE COLLOIDAL CONDITION

REFERENCE has frequently been made, in preceding chapters, to
the fact that proteins, enzymes, lipoids, etc., exist in the protoplasm
of plants and animals in the colloidal condition. The properties
and uses of these compounds by plants depend so much upon this
fact that, before proceeding to the consideration of the actual
physical chemistry of protoplasm itself, it will be appropriate
and profitable to give some attention to the nature and sig-
nificance of the colloidal condition of matter and of some of the
phenomena which grow out of it.

Every discussion of the colloidal condition in general properly
begins with reference to the work of the English physicist, Thomas
Graham, who carried on his investigations of the so-called " col-
loids " through a period of forty years, beginning with 1851.
His most important results were published, however, from 1861 to
1864. Graham studied the diffusibility of substances in solution
through the parchment membrane of a simple dialyzer. As a
result of his earlier investigations, he divided all the chemical
compounds which were known to him into two groups, which he
called " crystalloids " and " colloids," respectively, the first
including those substances which readily diffused through the
parchment membrane and the second those which diffused only
very slowly or not at all. He at first thought that crystalloids are
always inorganic compounds, while colloids are of organic origin.
He soon learned, however, that this distinction in behavior is not
always related to the organic or inorganic nature of the com-
pound. He further discovered that the same individual chemical
element or compound may exist sometimes in crystalloidal, and
sometimes in colloidal, form.- This latter discovery led to the
conclusion that diffusibility depends upon the condition, rather
than upon the nature, of the material under observation.

As a result of the long series of investigations which were
stimulated by Graham's work, the modern conception is that dif-

202""

THE COLLOIDAL CONDITION 203

fusibility is a condition of matter when in minute subdivision, or in
solution, in some liquid, as contrasted with its state, or condition,
when existing alone. That is, the state of a substance may be
either gaseous, liquid, or solid; and its condition when in solution
may be either crystalloidal or colloidal. Substances which are in
crystalloidal form, in true solution, exist there in molecular or
ionized condition; but, as will be pointed out below, when in the
colloidal condition they exist in aggregates which are somewhat
larger than molecules, but not large enough to be visible as indi-
vidual particles under the ordinary microscope, even under the
highest magnification "which has yet been obtained. Colloidal
particles are, however, generally visible under the Zigmondy
" ultramicroscope." (See below.)

The use of the word " colloid " as a noun, or as the name for a
substance which is in the colloidal condition, is of the same nature
as the use of the words " gas," " liquid," and " solid," in such
statements as " ice is a solid," " water is a liquid," or " steam is a
gas," etc. ; i.e., the noun represents a state or condition rather than
an actual object or thing. Hence, the expression " enzymes are
colloids," means only that enzymes exist in the colloidal condition,
and not that enzymes represent a definite type of substances having
the group name " colloids."

THE COLLOIDAL CONDITION A DISPERSION PHENOMENON

When one substance is distributed through the mass of another
substance, the mixture is said to be a " two-phase system," com-
posed of the dispersed phase, or substance, and the dispersion
medium, or continuous phase, through which the other substance is
distributed. The following examples illustrate the possibilities
of such two-phase systems :

(1) Dispersion medium a gas.

(a) Disperse phase a liquid — mist in the air.

(b) Disperse phase a solid — smoke or dust in air.

(2) Dispersion medium a liquid.

(a) Disperse phase a gas — foams.

(6) Disperse phase a liquid — emulsions.

(3) Dispersion medium a solid.

(a) Disperse phase a gas — solid foams, pumice stone, etc.

(b) Disperse phase a liquid — liquid inclusions in minerals.

204

CHEMISTRY OF PLANT LIFE

Although the same general principles of physical chemistry
apply to all two-phase systems, the term " colloidal condition " is
commonly used only in connection with a particular type of dis-
persions, in which the dispersion medium is a liquid and the dis-
persed material is either a solid or a liquid.

Thorough and careful studies have shown that when a solid or a
liquid is introduced into another liquid, and becomes dispersed or
distributed through it, the mixture may be either a true solution, a
colloidal solution, or a mechanical suspension. The characteristic
differences between these three conditions may be tabulated as
follows : although the significance of some of the phrases used will
not be apparent until the phenomena in question have been con-
sidered in some detail.

True Solutions.

Colloidal Solutions.

Suspensions.

(a) Particles of the disperse

phase are:

In molecular subdi-

In colloidal subdivision

In mechanical subdi-

vision

Invisible

Visible under " ultra-

Visible under micro-

scope "

scope or to naked eye

Less than 1/z/i in diam-

I/*/* to 1> in diameter

Greater than 1^ in diam-

.eter1

eter

Pass through niters

Pass through filters but

Do not pass through

and parchment mem-

not through parchment

filters or parchment

branes

In molecular motion

In Brownian movement

In gravitational move-

ment

(6) The system exhibits :

itt

High osmotic pressure

Low osmotic pressure

No osmotic pressure

Transparency

" Tyndall phenomenon"

Is generally opaque

No gel-formation

Forms gels

No gel-formation

1 In is one-thousandth of a millimeter; 1/x/z is one-thousandth of a
one millionth of a millimeter.

It is recognized by all students of these matters that it is not
possible to draw a sharp dividing line between these three types of
conditions, and that they shade into each other, in many cases;
but in general it may be said that a colloidal solution is one in
which the dispersed particles are usually between 5juju and 200juM in
diameter, are difficultly or not at all diffusible through the mem-

THE COLLOIDAL CONDITION 205

brane of a simple dialyzer, cannot be filtered out of solution, do not
settle out under the action of gravitation, and are visible only
under the " ultramicroscope" ; and one which has certain peculiar
optical, osmotic, and other physical and chemical properties.
Since colloidal particles are very minute in size, they possess very
large relative surface areas as compared with their total mass or
volume, very high surface tension, and a relatively high surface
energy as compared with their total, or molecular, energy. These
properties bring into play, in a substance which is in the colloidal
condition, in a remarkable degree, all the phenomena which are
associated with surface boundaries between solids and liquids,
liquids and gases, etc.

The properties arising out of the colloidal condition are of such
tremendous importance in connection with the vital phenomena
exhibited by cell protoplasm that it is necessary to give some
detailed consideration to them here. Many large volumes dealing
with this condition of matter have been written, and it is very dif-
ficult to condense even the most important facts concerning it
into a few pages, but an attempt has been made to present in this
brief summary the most essential facts and principles involved
in the colloidal phenomena.

NOMENCLATURE AND CLASSIFICATION

Colloidal mixtures may exist in two different forms: one, in
which the mixture is fluid and mobile, like a true solution, is known
as a " sol"; and the other, which is a semi-solid, or jelly-like, form,
is known as a " gel." Sols may be easily converted (or " set ")
into gels, by Changes of temperature or of the'-felectrolyte content,
or by^changes in the concentration of the mixture, etc., and in
most cases gels can be converted again into sols. In some cases, /u x
however, gel-formation is irreversible, the gels are permanent and
cannot be changed back again into sols by any known change in
environmental conditions.

Depending upon whether the liquid dispersion medium is JL.. V
water, alcohol, ether, etc., sols are known as " hydrosols," " alco-
sols," " ethersols," etc.; and gels as " hydrogels," " alcogels," etc.

Sols in which the disperse phase is a solid are known as " sus-
pensoids"; while those in which it is a liquid are " emulsoids."
Thus, sols of most inorganic compounds, of dextrin, gelatin, and

206 CHEMISTRY OF PLANT LIFE

(probably) of casein, etc., are suspensoids; while sols of egg-
albumin, of oils, etc., are emulsoids. The classification of these
substances into suspensoids and emulsoids is, however, more a
matter of convenience than of real difference in composition, since
it is practically impossible to say whether many of the organic
substances which normally exist in colloidal form are themselves
liquids or solids, when in the non-dispersed form.

CONDITIONS NECESSARY TO THE FORMATION OF SOLS

Suspensoids differ from mechanical suspension of solids in a
liquid in that in the latter the solid particles settle toward the
bottom of the mixture, because of the effect of the attraction of
gravity upon them. The rate at which such particles settle
depends upon the size and density of the particle and the vis-
cosity of the liquid, and can be roughly calculated from the formula
for Stokes' law for the rate of falling of a spherical body in a liquid.
This formula is

V= velocity of the falling body, in millimeters per second;

r = radius of the particle, in millimeters;

s = specific gravity of the solid;

s' = specific gravity of the liquid;

g =the attraction of gravity, in dynes;

n =the viscosity of the liquid.

For example, if this formula be applied to determine the rate at
which the particles of gold of the size of those in a red gold sol
would settle, if they were in mechanical suspension in water
(r=10MM, or one-ten-thousandth of a millimeter; s=19.3; s' = l;
0 = 980, and n = 0.01), it will be found that such particles will
settle at the rate of approximately 0.0146 millimeter per hour, or
a little over 10 mm. (0.4 inch) per month. Hence, the settling of
such particles, if in mechanical suspension, would be measurable,
although very slow. Shaking up the suspension would cause the
particles to rise through the liquid again. But in a gold sol, or
suspensoid, which contains particles of gold of the size used in this
calculation, the gold particles do not settle, even at the slow rate as
calculated above. They remain uniformly distributed through-

THE COLLOIDAL CONDITION 207

out the liquid for an indefinite period of time. The reason for
this phenomenon undoubtedly lies in the fact that these minute
particles carry an electric charge, which is of the same sign for all
of the particles and results in a repellent action which keeps the
particles in constant motion. This constant motion may easily
be conceived to keep the particles uniformly distributed through-
out the liquid, just as constant shaking would keep those of a
mechanical suspension uniformly distributed through the mixture.

The sign of the electric charge on the particles of a sol may be
either negative or positive, depending upon the chemical nature
and dialectric constants of the two phases of the system. The
proportion of the total electric charge of the system which is of
the opposite sign to that borne by the dispersed particles is, of
course, borne by the liquid which constitutes the other phase.
The origin of this electric charge on the colloidal particles is, as yet,
not known with certainty; but it seems probable that it is due to a
partial ionization of these small particles, similar to, but not so
complete as, that which takes place when compounds which are
soluble go into true solution in water, or other solvents which
bring about the dissociation of dissolved substances.

The conditions necessary to bring a solid substance into a "
colloidal mixture with some liquid, or, in other words, to produce a
suspensoid sol, require that the proportion of liquid to solid shall
be large and some means of disintegrating the material which is to
be dispersed into very fine particles. Many common chemical
reactions, if carried out in very dilute solutions, result in the pro-
duction of sols, especially if a small amount of some emulsoid is
present in the reacting mixture; sols produced in this way are very-
stable, and the emulsoid which is used in stabilizing the sol is
known as a " protective colloid." Direct methods of disintegra-
tion; such as reduction by chemical agents, discharge of a strong
electrical current through the substance which is to be dispersed
while it is submerged in the liquid, alternate treatment of finely
ground material with alkali and acid so as to frequently change
the electric charge, etc., are utilized for bringing inorganic com-
pounds into the colloidal state.

Suspensoids usually contain less than 1 per cent of the solid
dispersed through the liquid. In fact, extreme dilution is one
of the necessary conditions for suspensoid-formation.

Emulsoids are much more easily produced than are suspensoids.

208 CHEMISTRY OF PLANT LIFE

The property of forming an emulsoid seems to be much more
definitely a characteristic of the substance in question than does
the formation of sols from solids which, under other conditions,
may form true solutions. This difference may be due to the fact
that the liquids which easily form emulsoids (usually those of
organic origin) have very large molecules, so that the transfer
from molecular to colloidal condition involves much less change
in such cases than it does in the case of solid (inorganic) substances
of relatively low molecular weight. This view of the matter is
further borne out by the fact that solids which have very large
molecules (generally of organic origin) take on the colloidal form
much more readily than do those of small molecular size.

At the same time, a given liquid may form a true emulsoid
when introduced into one other liquid and a true solution when
introduced into another. Thus, soaps form emulsoids with water
(true hydrosols) ; but dissolve in alcohol to true solutions, in which
they affect the osmotic pressure, the boiling point of the liquid,
etc., in exactly the same way that the dissolving of other crystal-
loids in water affects the properties of true aqueous solutions.
Again, ordinary " tannin," when dissolved in water, produces a sol,
which froths easily, is non-diffusible, etc. ; but when dissolved in
glacial acetic acid, it produces a true solution.

The concentration of the disperse phase may be much greater
in the case of emulsoids than it can be in suspensoids. This is
probably because the dispersed particles do not carry so large
an electric charge and are not in such violent motion.

GEL-FORMATION

The one property which most sharply distinguishes sols from
true solutions is their ability to " set " into a jelly-like, or gela-
tinous semi-solid, mass, known as a "gel," without any change in
chemical composition, or proportions, of the two components of
the system. In the gel, the two components are still present in the
same proportions as in the original sol; but the mixture becomes
semi-solid instead of fluid in character. Thus, an agar-agar sol
containing 98 per cent of water sets into a stiff gel; while many
other gels which contain 90 to 95 per cent of water can be cut into
chunks with a knife and no water will ooze from them. The
water is not in chemical union with the solid matter in the form of

THE COLLOIDAL CONDITION 209

definite chemical hydration, however, as the same gel is formed
with all possible variations in the water content.

Gelsjnay be eitherjigid, as in the case of those of silicic acid,
etc., or elastic, as are those of gelatin, egg-albumin, agar-agar, etc.
The latter are the common type of gels among organic colloids.
They can be easily changed in shape, or form, without any change
in total volume.

In gel-formation, the two phases of the system take on a dif- j
ferent relationship to each other. The disperse, or solid, phase
becomes associated into a membrane-like, or film, structure, sur-
rounding the liquid phase in a cell-like arrangement. That is, the
whole mass takes on a structure similar to a honeycomb (except
that the cells are roughly dodecahedral in shape, instead of the
hexagonal cylinders in which the bees arrange their comb cells),
in which the original disperse phase constitutes the cell-walls and
the original liquid, or continuous phase, represents the cell-contents.
The cells of an elastic gel resemble closely the cells of a plant tissue
in many of their physical properties. They are roughly twelve-
sided in shape, as this is the form into which elastic spherical
bodies are shaped when they are compressed into the least pos-
sible space.

Imbibition and Swelling of Gels. — When substances which are
natural gels, such as gelatin, agar-agar, various gums, etc., are
submerged in water, they imbibe considerable quantities of the
liquid and the cells become distended so that the mass of the
material swells up very considerably. This swelling will take
place even against enormous pressures. For example, it has been
found that the dry gel from sea-weeds will swell to 330 per cent of
its dry volume, if immersed in water under ordinary atmospheric
pressure; but that it will increase by 16 per cent of its own volume
when moistened, if under a pressure of 42 atmospheres.

During the swelling of gels by imbibition of water, the total
volume of the system (i.e., that of the original dry gel plus that
of the water absorbed) becomes less. For example, a mixture
of gelatin and water will, after the gelatin has swelled to
its utmost limit, occupy 2 per cent less space than the total
volume of the original gelatin and water. It has been computed
that a pressure equivalent to that of 400 atmospheres would be
necessary to compress the water to an extent representing this
shrinkage in volume.

210 CHEMISTRY OF PLANT LIFE

On the other hand, gels when exposed to the air lose water by
evaporation, shrink in volume, and finally become hard inelastic
solids, as in the case of the familiar forms of glue, gelatin, agar-
agar, gum arabic, etc.

The difference in the relation of gels and that of non-colloidal
solids to water may be illustrated by the different action of peas,
beans, etc., and of a common brick, when immersed in water. Each
of these substances, under these conditions, absorbs, or " imbibes,"
water; but the peas and beans swell to more than twice their
original size and become soft and elastic, while the brick under-
goes no change in size, elasticity, or ductility. In all cases of col-
loidal swelling, the swollen body possesses much less cohesion,
and greater ductility, than it had before swelling. The essential
difference in the two types of imbibition is that in the case of
the non-swelling substances the cohesion, or internal attraction
of the molecules of the material, is too great to permit them to
be forced apart by the water; while in colloidal swelling, the
particles are forced apart to such an extent as to make the tissue
soft and elastic. ' It is possible, of course, to make this separation
go still further, until there is an actual segregation of the mole-
cules, when a true solution is produced; for example, gum arabic
•when first treated with water swells into a stiff gel, then into
a soft gel, and finally completely dissolves into a true
solution.

Reversibujty of Gel-formation. — In some cases, the change of
a sol to a gel is an easily reversible one. Glue, gelatin, various
fruit jellies, etc., " melt " to a fluid sol at slightly increased
temperatures and " set " again to a gel on cooling, and the
change can be repeated an indefinite number of times. On the
other hand, many gels cannot be reconverted into sols; that
is, the " gelation " process is irreversible. For example, egg-
albumin which has been coagulated by heat cannot be recon-
verted into a sol; casein of milk when once " clotted " by
acid cannot again be converted into its former condition, etc.
Irreversible gelation is usually spoken of as " coagulation." Some
coagulated gels, by proper treatment with various electrolytes,,
etc., can be converted into sols, the process being known as
" peptization" ; but in such " peptized " hydrosols, the material
usually exists in a different form than originally, having under-
gone some chemical change during the peptization, and the coag-

THE COLLOIDAL CONDITION 211

ulation and peptization cannot be repeated, that is, the process
is not a definitely reversible one.

Importance of Gel-formation. — From the physiological point
of view, gel-formation is undoubtedly the most important aspect
of colloidal phenomena. In the first place, the ability to absorb /
and hold as much as 80 to 90 per cent of water in a semi-solid
structure is of immense physiological importance. In no other
condition can so large a proportion of water, with its consequent
effect upon chemical reactivity, be held in a structural, or semi-
solid, mass. But a vastly more significant feature of the condi-
tions supplied by the gel lies hi the fact that the non-water phase,
or phases, of the system are spread out in a thin film, or mem-
brane, thus giving it enormous surface as compared with its
total volume. This effect is easily apparent if one thinks of the
enormous surface which is exposed when a tiny portion of colloidal
soap is blown out into a " soap-bubble " several inches in diam-
eter. This condition brings into play all the phenomena resulting
from surface boundaries between solids and liquids, liquids and
liquids, liquids and gases, etc., from surface tension, surface
energy, etc. Among these effects may be cited those of adsorp-
tion, increased chemical reactivity due to enlarged areas of contact,
permeability and diffusion, etc., the importance of which in the
vital phenomena of cell-protoplasm will be discussed in detail
in the following chapter,

GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS

Non-diffusibility. — The most characteristic property of all
sols is the failure of the suspended particles to pass through a
parchment, or any similar dialyzing membrane.

Visibility under the " Ultramicroscope." — The particles of
a sol, in contrast with the molecules of a true solution, are visible
as bright scintillating points under the ultramicroscope. This
is a modification of the type of dark-field illumination of the
ordinary microscope, as applied to microscopic studies, in which
the solution to be studied is contained in a small tube or box of
clear glass which is mounted on the stage of an ordinary micro-
scope and instead of being illuminated from below by transmitted
light is illuminated by focusing upon it the image of the sun,
or of some other brilliant source of light such as an electric arc,

212 CHEMISTRY OF PLANT LIFE

by passing the rays from the source of light through a series of
condensing lenses which are adjusted at the proper distance and
angles to bring the image of the illuminating body within the tube
containing the substance which is to be examined and in the line of
vision of the microscope. Obviously, this results in intense illu-
mination of any particles in the solution which come within this
brilliant image of the sun, or arc, and therefore renders visible
particles which are of less diameter than the wave-length of
ordinary light (450/z/i, to TGO^M for the visible spectrum) and,
hence, are not visible by the ordinary means of illumination in the
direct line of vision. It will be apparent that what is seen in the
field of the ultramicroscope is not the particles themselves, but
rather the image of the sun (or other illuminating body) falling
upon the particles which come within the image, just as one does
not see the paper but only the image of the sun when the rays from
the sun are brought to a focus upon a sheet of paper through any
ordinary convex lens, or " burning glass." Hence, the ultra-
microscope gives no idea of the shape, color, or size of the par-
ticles upon which the image falls; but it does permit the counting
of the number of particles within a given area, and a study of
their movements, from which it is possible, by mathematical com-
putations, to calculate the relative size of the particles themselves.
Repeated studies have shown that particles of the sizes between
SUP and 250/iM in diameter, which are visible under the ultra-
microscope, are sufficiently small to bring about the surface
phenomena which are known as properties of colloidal solutions.
Further, the ultramicroscope permits the 'observation of the
growth, or disintegration, under various chemical reagents, of the
individual colloidal particles, which appear as scintillating points
in the field of the microscope; ana the study of changes in rela-
tionships during gel-formation, peptization, etc.

The " Tyndall Phenomenon." — Colloidal solutions exhibit this
phenomenon; that is, if a bright beam of light be passed through a
sol which is contained in a clear glass vessel having parallel
vertical sides, and the solution be viewed from the side, it appears
turbid and often has a more or less bluish sheen. This effect is
due to the small particles in the sol, of polarizing the light which is
reflected from them, the blue rays being bent more than are those
in the other part of the spectrum. The Tyndall phenomenon is
similar in its effect in making the tiny particles of the sol visible

THE COLLOIDAL CONDITION 213

to the illumination of the dust particles in the air of a darkened
room when a ray or narrow beam of light passes through it. In a
true molecular solution, the particles are too small to be visible by
this mode of illumination.

Other Optical Properties. — Sols are generally translucent and
opalescent; many of them are highly colored, some of the sols of
gold, platinum and other heavy metals possessing particularly
brilliant colors. In general, metallic suspensoids are red, violet,
or some other brilliant color; while inorganic suspensoids are bluish
white, and emulsoids generally blue to bluish white.

Formation of Froth, or Foam. — Colloidal solutions, especially
those of the natural proteins, fats, glucosides, gums, and the
artificial soaps, have a strong tendency to produce froth, or
foam, when shaken; this being due to the enormous surface ten-
sion resulting from the finely divided condition of the dispersed
material.

Low Osmotic Pressure. — All colloidal solutions exhibit a very
low osmotic pressure; the freezing point of the dispersion medium
is lowered only very slightly and its boiling point is only very
slightly raised by the presence of the dispersed particles in it.

Precipitation by Electrolytes. — Sols of all kinds are precip-
itated, or caused to form gels, by the addition of electrolytes^
since these cause a disturbance of the electric charge on the
dispersed particles, to which the colloidal condition is due. In
the case of most emulsoids and of a few of the suspensoids, this
change converts the mass into a stiff gel; but in that of many
of the metallic suspensoids, the dispersed particles are gathered
together into larger aggregates, which settle out of the liquid in
the form of a gelatinous precipitate. In the latter case, the effect
is usually spoken of as " precipitation " by electrolytes; while in
the former, it is called " coagulation," or " gelation."

The effectiveness of the various electrolytes in bringing about
this change is proportional to their valency; bivalent ions are from
70 to 80 times, and trivalent ions about 600 times as effective as
monovalent ions.

Further, all sols in which the dispersed particles carry a
charge of the opposite sign likewise precipitate both suspensoids
and emulsoids.

A demonstration of the presence of an electric charge on the
particles of a sol and a determination of its sign can be made by

214 CHEMISTRY OF PLANT LIFE

placing the solution in a U tube, with a layer of distilled water
above the sol in each arm of the tube, and then passing an elec-
tric current through the contents of the tube, keeping the elec-
trodes in the distilled water, so that the migration of the particles
toward one pole or the other can be observed by their appearance
in the clear water at that end of the tube; or by passing an electric
current through the observation chamber of an ultramicroscope,
in which the solution under examination has been placed, and
observing the migration of the particles across the field toward
either one or the other (positive or negative) electrode.

Emulsoids and suspensoids differ in their properties in the
following respects. * Suspensoids are always very dilute, con-
taining less than 1 per cent of the dispersed solid; while emulsoids
may be prepared with widely varying proportions of the two com-
ponent liquids. ^Suspensoids have a viscosity which is only slightly
greater than that of the liquid phase when it exists alone, and their
viscosity varies with the proportion of dispersed solid which is
present in the sol; while emulsoids have a very high viscosity in
all cases. ^ Emulsoids usually form stiff gels when treated with
electrolytes; while suspensoids more commonly yield gelatinous
precipitates under the same conditions.

Suspensoids and emulsoids which carry electric charges of
opposite sign mutually precipitate each other. But emulsoids
often protect suspensoids from precipitation by electrolytes, by
forming a protective film around the particles of the suspensoids,
which prevents the aggregation of the particles into the precipitate
form.

ADSORPTION

If a sol be precipitated or coagulated by the action of an
electrolyte, substances which may be present in solution in the
liquid of the sol are carried out of solution and appear in the gel or
precipitate. This phenomenon is known as " adsorption," which
means the accumulation of one substance or body upon the surface
of another body, as contrasted with " absorption," which means the
accumulation of one substance within the interior of another.
Since substances which are in the colloidal form have very large
relative surface areas, it follows that the opportunity for surface
adsorption on colloidal materials is very great.

THE COLLOIDAL CONDITION 215

Surface adsorption is a common phenomenon. It was exten-
sively studied by the physicist, Willard Gibbs, who showed
that adsorption will take place whenever the surface tension
of the adsorbing body will be lowered by the concentration in
its surface layer of the material which is available in the solution
or other surrounding medium.

As applied to colloidal phenomena, adsorption may be exhib-
ited in either one of four different ways, as follows: (1) A crystal-
loidal substance which is in solution may be adsorbed on the col-
loidal particles of a hydrosol, so that if the mixture be dialyzed, or
filtered through a so-called " ultrafilter " (i.e., a filter with pores so
small that it will retain colloidal particles) the dissolved crystalloid
will remain with the separated colloidal particles, or the dis-
solved crystalloid will not react chemically as it would in a free
solution. For example, if to a solution of methylene blue, which
dyes wool readily, there be added a small quantity of albumin (a
colloidal substance), the dye is adsorbed by the albumin and will
no longer color wool with anything like the same readiness. (2)
During gel-formation, electrolytes and other soluble substances
which may be present in solution in the liquid may be adsorbed
out of the solution and appear in the gel. For example, a pre-
cipitate of aluminium hydroxide, or of silicic acid, is nearly always
contaminated with the soluble salts which are present in the
solution, and can be prepared in pure form only by repeated filter-
ing, redissolving, and reprecipitating. (3) Colloidal substances
may be removed from sols by being adsorbed upon porous mate-
rials like charcoal, fuller's earth, hydrated silicates, etc. For
example, animal charcoal (or bone black) is used commercially
for the clarification of sugar solutions, because it adsorbs out of
these solutions the colloidal proteins, coloring matters, etc., with
which they are contaminated. (4) Finally, colloids mutually
adsorb each other, as in the case of the " protective colloids "
previously referred to.

Certain characteristics of adsorption phenomena are of interest
and importance from both the physiological and the industrial
point of view. The following may be mentioned: (a) Amount of
adsorption. Relatively more material is adsorbed out of dilute
solutions than out of more concentrated ones. An increase of ten
times in the concentration of the dissolved material results in only
four times as much adsorption by the colloidal substance which

216 CHEMISTRY OF PLANT LIFE

may be introduced into the two solutions. In this, adsorption
differs from chemical action, as the latter is proportional to the
concentration of the reacting material which is present in the solu-
tion. (6) Adsorption out of different liquids, by the same adsorb-
ing body, is different in amount. It is usually greatest out of
water. Hence, many dyes may be adsorbed out of water by char-
coal, porous clay, etc., and if the latter be then introduced into
alcohol, or ether, the dye goes back into solution in these latter
liquids. This process is often used industrially and in the labora-
tory for the purification of such substances when they are present
in impure form in aqueous solutions, (c) Selective adsorption.
Different substances are not adsorbed out of the same solvent
to the same extent by the same adsorbing agent. Advantage is
taken of this fact when filter paper is used in the so-called " capil-
lary analysis " to separate different dyes, or other colloidal mate-
rials which have been stained different colors, into alternate layer
by reason of the different rate at which the paper adsorbs the dif-
ferent materials out of the solution in which they are present
together, (d) Similar relative adsorption by different adsorbing
agents. Although different adsorbing agents may possess varying
active surfaces and hence, variable adsorbing power, or rates of
adsorption, they adsorb the same relative amounts of different
materials; i.e., if substance A adsorbs more of X than it does of Z
out of any given solution, substance B will likewise adsorb more of
X than of Z out of the same solution; although the actual amounts
adsorbed by A may be quite different from those adsorbed by B.

CATALYSIS AFFECTED BY THE COLLOIDAL CONDITION

The velocity of a chemical reaction is the net result of opposing
influences. It is directly proportional to the chemical affinity of
the reacting bodies and inversely proportional to the so-called
" chemical resistance." The first factor, chemical affinity, is not
easily measured, as it depends upon both the mass of the reacting
molecules, atoms, or ions, and their attraction for each other.
But if, as the result of chemical affinity, a reaction takes place, it is
evident that the time required for its completion (which measures
the velocity of the reaction) is made up of two separate periods.
The first is the time required for the reacting molecules to come into
contact; and the second is that required for the molecular rear-

THE COLLOIDAL CONDITION 217

rangement which constitutes the reaction. Clearly, the time
required for the substances to come into molecular contact will be
greatly diminished if they are mutually adsorbed in large quan-
tities on the extended surface area of some colloidal catalyst which
is present in the mixture rather than scattered throughout its
entire volume. The application of this principle to the catalysis of
hydrolytic reactions is not apparent, if it is considered that the
H20 molecules which cause the hydrolysis are those of the solvent
itself; but is clear on the assumption (which is discussed in the
following chapter) that the water which enters into a colloidal
complex is in multimolecular form, represented by the formula
(H2O)n, in which the oxygen atoms are quadrivalent and, hence,
much more active chemically than as illustrated in the simple
solvent action of water.

Hence, the surface adsorption of reacting bodies by a colloidal
catalyst may have a very important influence in decreasing the
time required to bring the reacting molecules into intimate con-
tact, and so increasing the velocity of the reaction.

But the colloidal condition of the catalyst may also aid in
decreasing the " chemical resistance " which tends to slow up the
reaction. Chemical resistance may be understood to be the inter-
nal molecular friction of the densely packed atoms within the
reacting molecule, which tends to prevent the molecular rearrange-
ment and so to prolong the second period of the reaction time. To
overcome this friction and so decrease the reaction time, some form
of energy is necessary. If there be present in the solution in
which the reaction is taking place some colloidal catalyst, and if
the reacting bodies are concentrated at the surface boundaries
between the two phases of the colloidal system, they may be con-
ceived to be within the sphere of influence of the surf ace energy
of the dispersed particles of the catalyst, so that this may furnish
the energy necessary to overcome the chemical resistance of the
reacting bodies, and so to speed up the second portion of the
reaction time.

From these considerations, it would appear that the colloidal
condition of such catalysts as enzymes, etc., has much to do with
their ability to increase reaction velocities, both by reducing the
time necessary for the reacting bodies to come into molecular
contact and by furnishing the energy to overcome the chemical
resistance to the molecular rearrangement which constitutes the

218 CHEMISTRY OF PLANT LIFE

reaction itself. Evidence in favor of the accuracy of this view of
the nature of the catalytic action of colloidal substances is afforded
by the facts that catalysts accelerate the velocity of reversible
reactions in either direction and that they do not change the point
of final equilibrium, in any case; that is, they do not affect the
nature or direction of the reaction, but only accelerate a chemical
change which would otherwise take place more slowly because of
the stability (or chemical resistance) of the molecules involved, or
their inability to come quickly into intimate molecular contact.

These facts and principles have been clearly established in
many studies of the nature of enzyme action (enzymes are typical
colloidal catalysts) and probably apply equally well to the action
of other types of colloidal catalysts. On the other hand, the
catalytic action of certain inorganic and non-colloidal substances,
such as the action of acids in accelerating the hydrolysis of carbo-
hydrates, etc., may be conceived to be due to chemical influences
upon the internal molecular resistance, which are similar in their
effects, but entirely different in their mechanism, from the physical
effects of the surface boundary phenomena of the colloidal cata-
lysts.

INDUSTRIAL APPLICATIONS OF COLLOIDAL PHENOMENA

Large numbers of industrial processes are based upon colloidal
phenomena. Many of these processes were known and practiced
long before the nature of the phenomenon itself was understood.
But with the coming of the knowledge of the nature, causes, and
possibilities of the control, of the colloidal condition of the mate-
rials involved, immense improvements in the economy of the
process, or the quality of the end-products, have been worked out,
in many cases. Many volumes of treatises concerning the indus-
trial applications of colloidal phenomena have been written. Any
discussion of these would be out of place here; but the following
list of examples will serve to illustrate the immense importance
of these matters both in industry and to the needs of everyday
life: the tanning of leather; the dyeing of fabrics; vulcanizing
rubber; mercerizing cotton; sizing textile fabrics; manufacture of
mucilages and glues; manufacture of hardened casein goods;
manufacture of celluloid; production of colloidal graphite for
lubrication; the prevention of the smoke nuisance by electric

THE COLLOIDAL CONDITION 219

deposition; the purification of sewage; the manufacture of soaps;
the manufacture of butter, cheese, and ice cream; fruit jellies,
salad dressings, etc. This list could be extended to great length,
but is already long enough to emphasize the very great importance
and practical value of colloidal Dhenomena in daily life.

NATURAL COLLOIDAL PHENOMENA

Many of the phenomena of nature are colloidal in character.
These may be observed in the mineral, the animal, and the
vegetable kingdoms. Here, again, a lengthy discussion of the nature
of these phenomena would be out of place in this connection, and a
few typical examples will serve to illustrate the general importance
in nature of this property of matter.

In the soil, the following properties are easily recognizable as
definite colloidal phenomena: water-holding capacity of clays,
silts, loams, etc.; adsorption (or " fixation ") of soluble plant foods
so that they are not readily leached out of the soil by drainage;
flocculation and deflocculation of clay, etc.

In the animal body; the contraction of muscles, the convey-
ance of nerve stimuli, etc., are undoubtedly accomplished by col-
loidal changes; and the existence of insoluble casein and fat in
colloidal form in milk insures the proper nourishment of the young
of nearly all species of animals.

In both plants and animals, as will be pointed out in the fol-
lowing chapter, practically all the vital activities of the cell pro-
toplasm are definite manifestations of colloidal phenomena.
Enzymes perform their catalytic functions by reason of their col-
loidal form. Proteins exist in colloidal form and are the seat of all
vital functions. The regulation of the passage of materials into
and out of the cell is governed by minute changes in the elec-
trolyte concentration, etc., which produce enormous changes in
the colloidal character of the protoplasm.

It is apparent, therefore, that the study of the colloidal con-
dition of matter and of the properties arising out of it is of immense
importance to the biochemist. No other single field is capable
of yielding more fruitful results to the plant physiologist, in his
studies of the response of plants to changes in their environment, or
of the mechanism by which plants perform their internal func-
tions.

220 CHEMISTRY OF PLANT LIFE

References

BECHHOLD, H., trans, by BULLOWA, J. G. M. — "Colloids in Biology and

Medicine," 463 pages, 54 figs., New York, 1919.
BURTON, E. F. — "The Physical Properties of Colloidal Solutions," 200 pages,

18 figs., London, 1916.
CASSUTO, L. — "Der Kolloide Zustand der Materie," 252 pages, 18 figs.,

Dresden and Leipsig, 1913.
LEISEGANG, R. E. — "Beitrage zu einer Kolloidchemie des Lebens," 144 pages,

Dresden, 1909.
OSTWALD, W., trans, by Fischer, M. H. — "Theoretical and Applied Colloid

Chemistry," 218 pages, 43 figs., New York, 1911.

OSTWALD, W., trans, by FISCHER, M. H. — "A Handbook of Colloid-Chem-
istry," 278 pages, 60 figs., Philadelphia, 1915.
TAYLOR, W. W.— "The Chemistry of Colloids," 328 pages, 22 figs., New York,

1915.
ZIGMONDY, R., trans, by ALEXANDER, J. — "Colloids and the Ultramicroscope,"

238 pages, 2 plates, New York, 1909.
ZIGMONDY, R., trans, by SPEAR, E. B. — "The Chemistry of Colloids," 274

pages, 39 figs., New York, 1917.

CHAPTER XVI
THE PHYSICAL CHEMISTRY OF PROTOPLASM

THUS far, we have considered the chemical nature of the various
groups of compounds which are found in the tissues of living
organisms, laying emphasis upon those which are of plant origin.
These compounds constitute the material, or machinery, of the
cell, and then* various transformations furnish the energy for its
operation. We come now to a study of the mode of its operation,
or the processes of vital phenomena.

Our knowledge of these matters is not yet far enough advanced
to permit a definite statement as to whether there is any difference
between the protoplasm of plant tissues and that of animal origin
in their modes of action, or in the physical-chemical changes which
constitute the vital phenomena in the two groups of living organ-
isms. Thus far, no such differences have been discovered. Hence,
in the following discussions, no attempt is made to differentiate
between animal and plant protoplasm. Most of the facts and
principles which are here presented have been developed as the
result of the study of the physiological chemistry of animal life.
No similar careful study of plant chemistry has yet been carried
out; but preliminary studies seem to indicate that the same gen-
eral principles apply to all protoplasm, regardless of whether it is
of plant or of animal origin. It is possible, of course, that further
studies of plant protoplasm will render necessary some modifica-
tions of some of these views as applied to the growth of plants;
but they are believed to represent the best which is now known of
the physical chemistry of the plant-cell activities.

HETEROGENEOUS STRUCTURE OF THE CELL

Examination of cell protoplasm under the microscope reveals
that it is not a simple homogeneous mass. In the first place, it

221

222 CHEMISTRY OF PLANT LIFE

has a definite structure, composed of (a) a nucleus; (6) numerous
granular bodies of different sizes and kinds; and (c) a clear mass of
colloidal material, which (if observed under the ultra-microscope,
or photographed by ultra-violet light) is apparently made up of
very minute particles of many different types of materials;
the whole mass, in the case of plant protoplasm, being generally
surrounded by (d) a differentiated layer known as the cell-wall.
The actual internal structural arrangement of the clear colloidal
mass is uncertain; but its properties indicate that it may be con-
sidered to be like a mass of foam (resembling a compact mass
of soap-bubbles) the compartments of the foam being, of course,
very minute and the films themselves almost infinitely thin, the
contents of each compartment being probably liquid, and the
whole composing a typical colloidal gel of complex composition.

This conception may not be accurate in every detail, but it
seems to fit very closely the conditions and reactions of cell proto-
plasm. Furthermore, it is obvious that the definite structure, or
form, of the cell is essential to its life; since, if the structure be
destroyed by any kind of mechanical injury (freezing of the cell
contents, resulting in the puncturing of the membranes by ice
crystals; rupturing of the films, or cell-walls, by grinding with
sharp sand, etc.) so as to bring about an intermingling of the parts
which are segregated from each other in the organized structure,
there results an immediate exhibition of abnormal chemical actions,
accompanied by the liberation of carbon dioxide, and the death of
the cell.

A proper mental picture of the organization of the cell structure
and of the interrelation of all its working parts is suggested by the
figure of a well-organized chemical factory, with the different
chemical transformations which are involved in the whole process
being carried on in different portions, or rooms, of the factory,
with the various intermediate and final products regularly and
systematically transported from one room to another as they are
needed to keep each individual step in the whole process going at
the proper rate, and with the different parts of the whole factory
working in smooth coordination with each other. Any disturb-
ance of the mechanism in any particular room, or any abnormal
condition which breaks down the coordination or results in the
mixing of the reagents or processes of adjoining rooms in improper
order or proportions, produces instant destruction of the normal

THE PHYSICAL CHEMISTRY OF PROTOPLASM 223

process, abnormal reactions take place, and the factory output is
interrupted.

No other conception than this one of a definite structure and
coordination of the different working parts of a cell can adequately
account for the great variety of chemical changes which are
constantly going on in any given cell. It is wholly inconceivable
that a homogeneous mass of all the varying chemical compounds
which are contained in any given quantity of protoplasm could
either exist or produce any regular sequence of chemical reactions.
Structure, or organization of the cell-contents into separate col-
loidal compartments, and the segregation of cell-contents into
masses having different functions, is essential to any reasonable
conception of how the cell performs its various activities.

The best understanding of the structural arrangement is
afforded by the conception that protoplasm consists of a colloidal
gel, or sometimes a very viscid sol, containing water, salts, carbo-
hydrates, fats, proteins, and enzymes. Evidence in favor of this
conception is afforded by the appearance of protoplasm under a
high-power microscope, and by the close resemblance of the
processes which go on in it, and its responses to external stimuli,
to those of an artificial gel of similar chemical composition.

Two different conceptions of the form in which the chemical
components exist in this mass have been advanced. One is that
they are in true molecular unions, known as " biogens," and that
the reactions which take place in the mass may, therefore, be
studied from the same basis as are reactions between similar sub-
stances when they take place in a beaker or test tube in the
laboratory. It would seem, however, that the constantly varying
proportions of the materials themselves, and the lack of homo-
geneity of cell contents, afford insurmountable difficulties to this
conception as a basis for the study of cell activities. The other,
and seemingly more reasonable, conception is that these bodies
exist hi the form of colloidal complexes, whose composition
may vary within wide limits and whose reactions are responsive to
the usual phenomena incident to the colloidal condition of matter.

According to the latter conception, vital activities of cell
protoplasm may be due to changes in water content, to electrical
disturbances, to the phenomena resulting from the conditions
brought about by surface boundaries between the different phases
of the gel, to varying osmotic pressure, to changes in chemical

224 CHEMISTRY OF PLANT LIFE

reaction, etc., and may be controlled by various stimuli of chemical,
physical, or mechanical nature. This conception seems, 'there-
fore, to fit most closely the actual conditions under which the
protoplasm exists and carries on its vital functions.

With this conception in mind, we may now proceed to a con-
sideration of how the various components of the complex organic
colloidal system, and their specific properties, can affect its chem-
ical activities.

The components of the system are, of course, water, salts, and
the various organic compounds (fats, proteins, carbohydrates, and
enzymes in all cells; and other groups, such as essential oils,
tannins, pigments, etc., in cells which have certain special func-
tions to perform) which constitute the solid phase of the colloidal
mixture. In addition to the definite chemical properties of each
of these component groups, 'which have been studied in detail in
preceding chapters, there are many physical, or physical-chemical,
properties of the system as a whole, and of its component parts,
which are of the utmost importance in the physiological activities
of the protoplasm. These we may now proceed to consider in some
detail.

WATER

Water constitutes the largest proportion of the weight of active
protoplasm. In living cell contents (except those of such bodies
as resting seeds, etc.), water comprises from 70 to 95 per cent of
the total weight of the substance; the average proportion being
usually between 85 and 90 per cent. The fact that protoplasmic
material can exist in turgid form with such high percentages of
water as these is due, as has been pointed out, to its existence as a
colloidal gel. It is because of this condition that increases in the
proportion of water generally increase the turgidity, or turgor, of
the protoplasm; instead of, as in all other cases, rendering the
mixture less solid and more labile. Losses of water from the
protoplasmic gel decrease its " swollen " condition and so render
the tissue soft and flabby; while increases in water content swell
the gel and make the tissue stiff and turgid. No other condition
than that of a colloidal gel could respond in this way to changes in
water content.

The formula which is commonly assigned to water is the sim-

THE PHYSICAL CHEMISTRY OF PROTOPLASM 225

plest possible one; namely, H2O. But if the water molecule were
really as simple as this, the compound would boil at a very low
temperature, would have a very low surface tension, etc.; whereas
its actual boiling point, surface tension, etc., are much higher
than those of other compounds having a higher molecular weight
than is indicated by the formula H2O. Actual measurements of
the physical properties of water indicate that at the temperature
at which water is a vapor its formula is at least (H20)2; while
at lower temperatures, at which it exists as a liquid, its formula
may be (H2O)3, or (H20)4, or even more complex still. The
cause for this association of the compound into multiple molecules
undoubtedly lies in the extra valences of the oxygen. In many
organic compounds oxygen is undoubtedly tetravalent, and it
may be easily conceived that in these complex molecular groupings
in the water it exhibits this same property; the possible molecular
arrangements being represented by the formulas

H H

Hv /H H— O— O— H

>O=:0< and etc.

W \H H— O— 0-H

^ v

Such molecules may be conceived to break down very easily,
leaving the extra valences of the oxygen available to form linkages
with other atoms or molecules. This may constitute one of the
ways in which water exerts its remarkable effects both as a solvent
and as an accelerator of all kinds of chemical reactions. Other
organic compounds which contain tetravalent oxygen are exceed-
ingly active chemically, and there seems to be much to commend
this view of the chemical structure of the water molecule.

Probably the most remarkable property of water is its power
of solution. No other liquid surpasses water as a solvent. This
power, as has been pointed out, is supposed to be due to, or in
some way correlated with, the extra valences of the oxygen atoms,
which may perhaps unite with similar extra valences of other sub-
stances with which the water is brought into contact, and so cause
the latter to enter into solution. All kinds of substances dissolve
in water, and when in solution, or even when only moistened, are
much more active chemically than when dry. This property of

226 CHEMISTRY OF PLANT LIFE

water contributes greatly to the possibilities of the chemical
reactions which constitute life processes.

Water, likewise, has a higher dialectric constant than any other
common liquid. This means that it does not readily conduct
electricity, or readily permit electric equilibrium to be established
in it; or, in other words, that it is a good insulator. This prop-
erty permits the existence in it simultaneously of materials having
opposite electric charges, or the so-called ionization phenomena;
hence, water is the best-known ionizing medium, and ionization
favors chemical reactivity.

Again, water has a very high specific heat, a fact which is of
the utmost biological importance. It takes more heat to raise the
temperature of one gram of water through one degree than is
required to produce the same result in any other known sub-
stance; or, stated the other way around, a given amount of heat
will cause less change in temperature of water than of any other
known substance. .Further, the latent heat of liquefaction and of
vaporization (i.e., the amount of heat required to change the sub-
stance from solid to liquid and from liquid to gaseous state,
respectively) is greater for water than for any other common sub-
stance. These facts are of very great importance in cell-proto-
plasm. The high specific heat of water provides that the heat
liberated by the chemical reactions which take place in the proto-
plasm can be absorbed by the water of the cell contents, and given
off again to other reactions, with very slight effect upon the tem-
perature of the protoplasm itself. Hence, violent changes in
temperature, which might be disastrous to the life of the cell, are
prevented by the high specific heat of the water which it contains.
Similarly, the high latent heat of liquefaction of water, resulting
in the giving up of large quantities of heat before it can become
solid, or " freeze," tends to prevent freezing and thawing of the
cell contents with sudden changes of external temperatures at or
near the .freezing temperature of water.

As a result of its physical properties, as just briefly described,
water accelerates all kinds of chemical reactions in protoplasm,
both by solution and by ionization of such substances as undergo
electric dissociation; and serves to regulate the temperature of the
protoplasmic mass. Furthermore, in organic tissues, most of the
important chemical reactions of the protoplasm are reversible
hydrolyses; i.e., water actually enters into the reaction or is lib-

THE PHYSICAL CHEMISTRY OF PROTOPLASM 227

erated by it, and the equilibrium point of the reaction is changed
by the proportions of water which are present in the reacting mass.
Hence, the presence of large proportions of water in the colloidal
complex known as protoplasm has a very important influence upon
its possibilities of biological reactions.

SALTS

Active protoplasm contains mineral salts in solution. These
are of the same general nature as those found in sea-water, which
is the original habitat of the earlier evolutionary forms of living
matter. Or, it might be said that both plants and sea-water derive
their mineral salts from the same source, namely the soluble salts
of the soil. Recent investigations have shown that the propor-
tions of sodium ions to calcium ions in sea-water are precisely
those which maintain fats, proteins, etc., in a true colloidal emul-
sion; and that comparatively small variations in the ratio of
these two cations produce very marked effects upon the colloidal
conditions of these substances in an artificial colloidal preparation,
which resemble very closely the changes which apparently take
place in cell protoplasm under the influence of narcotics, or nerve
stimulants, in blood-coagulation, in the parthogenetic develop-
ment of germ cells, in cancerous growth of tissues, etc. In other
words, in so far as it has been studied hi this respect, cell plasma
exhibits exactly the same responses to variations in the propor-
tions of salts (electrolytes) in solution, that artificial emulsions of
oils (fats) in water do; and the normal, or critical, equilibrium
proportion of these electrolytes for all colloidal complexes is that
in which they occur in sea-water. It must be admitted that
there is as yet no definite evidence that the observations which
have been made upon the protoplasm of animal tissues will apply
equally well to plant cell protoplasm. But many of the phenomena
which have been studied in animal tissues have what are appar-
ently similar, if not identical, effects in plant tissues, and it seems
reasonable to suppose that these conclusions apply generally to
protoplasm of either animal or plant origin.

The effects which salts produce in protoplasm are undoubtedly
due to the fact that, when in solution, they readily ionize and
conduct the electric current. A discussion of the nature and
importance of the theory of dissociation of electrolytes in solution,

228 CHEMISTRY OF PLANT LIFE

or the so-called " ionization theory/' which has done so much to
clear up otherwise unexplainable properties of solutions, would be
out of place here. But it may be noted that the ionized condition
of salts in solution accounts for the avidity, or " strength," of
acids and bases; for the increased osmotic pressure of such solu-
tions; for the conduction of the electric current through solutions;
and for the effects of these dissolved electrolytes upon the col-
loidal condition of many substances, since this is due to the elec-
tric charge on the dispersed particles.

Hence, the presence of salts in solution in the water of the
protoplasm has a tremendous influence upon the osmotic pressure
(which governs the movement of dissolved materials into and out
of the cell protoplasm); upon the colloidal condition of the cell
contents (which controls all the effects due to the surface boundary
phenomena which are discussed below and which are responsible
for a large part of the remarkable chemical activity of the proto-
plasm) ; upon the electrical phenomena (which constitute many of
the stimulations which the protoplasm receives); and upon the
acidity or alkalinity of the cell contents (which determine the
nature of the respiratory, or oxidation, reactions of the protoplasm
and, indirectly, its life or death).

The general nature of these physical-chemical properties of
the protoplasm and of the relation of electrolytes in solution to
them may now be considered in some detail.

OSMOTIC PRESSURE

Osmotic pressure is one of the chief factors in controlling the
amount of water in the protoplasm. As 1& well known, the
phenomenon known as " osmosis " is the passage of solvents, or of
dissolved substances, into or out of any tissue, or substance,
through the membrane which surrounds it. In the case of a cell,
the membrane in question may be either the cell-wall or the internal
colloidal films which are distributed throughout the entire mass of
the cell contents.

From the standpoint of their relation to osmosis, membranes
may be either impermeable, in which case neither solvent nor dis-
solved materials can pass through them; semi-permeable, which
permit the passage of the solvent, but not that of dissolved crys-
talloidal substances; or permeable, which permit the free passage

THE PHYSICAL CHEMISTRY OF PROTOPLASM 229

through them of both solvents and solutes. The first and last of
these types of membranes have no effect upon osmotic pressure;
but osmotic pressure is at once set up whenever a semi-permeable
membrane is interposed between solutions of different concentra-
tions. It is due to the molecular motion of both the liquid and the
dissolved solids, as a result of which a greater number of molecules
are " bombarding," or pressing upon the membrane from the side
of the more concentrated solution. This sets up an unequal
pressure upon the two sides of the membrane, and if the latter be
semi-permeable there will result a passage of the liquid through the
membrane toward the denser solution so as to equalize the pres-
sure. The resultant tendency is for the solutions on the two
sides of the membranes to become equal in concentration by
movement of the liquid from the less dense to the more dense
portion, instead of by movement of the dissolved materials toward
the less dense part of the solution as in the case of diffusion when
solutions of different concentrations are brought in contact with
no membrane to interfere with free diffusion.

Osmotic pressure tends, therefore, to force the movement of
solvents through semi-permeable membranes from more dilute
toward more concentrated solutions. Protoplasm acts in general
as an approximately semi-permeable membrane or material.
For example, if the concentration of sugar in any given mass of
protoplasm becomes greater, by reason of the photosynthetic
activity, osmotic pressure is set up and water enters the mass,
thus preventing loss of turgidity due to increased concentration.
Similarly, any other increase in concentration of synthetic products
is compensated for by entrance of water because of increased
osmotic pressure, unless the products are insoluble and, therefore,
incapable of effecting the osmotic pressure.

Hence, osmotic pressure provides for the movement of water
into and out of protoplasm and so tends to keep the proportion of
water uniform throughout the entire tissue. It will at once occur
to the reader, however, that if the statements in the preceding
paragraph were unqualifiedly true, and if the protoplasmic mass
were absolutely semi-permeable in character, there would be no
possibility of the passage of dissolved solids into or out of the cell;
i.e., if the protoplasm acted as an ideally semi-permeable mem-
brane, only water could pass into or out of it. But we know that
mineral salts from the soil must pass into any cell before the syn-

230 CHEMISTRY OF PLANT LIFE

thesis of proteins, etc., can proceed, and that the fats, carbohy-
drates, proteins, etc., which are synthetized in vegetative cells
pass from these to other organs of the plant for use or storage.
The obvious explanation for this condition of things in the plant
is that protoplasm (and, indeed, this is equally true for prac-
tically all known membranes) is not absolutely impermeable to
dissolved crystalloids; or, in other words, semi-permeability
generally means only that the solvent passes through the mem-
brane more readily and more rapidly than do the dissolved mate-
rials in it. Even colloidal materials will diffuse through most
common membranes, although at so slow a rate that the process
is scarcely observable by ordinary methods of study. Hence, the
actual permeability of the protoplasm permits the movement of
both water and dissolved solids from one part of the organism to
amother; but its approximation of semi-permeability produces
osmotic pressure and induces freer movement of water than of dis-
solved substances, and so provides for turgidity of the cells and for
equalization of the water content of different portions of the pro-
toplasmic mass.

It is clear, therefore,. that osmotic pressure plays an important
part in the physical mechanism of cell activities and in the reg-
ulation of the proportion of water contained in the protoplasm,
with its consequent effects upon the chemical reactions which may
go on in the cell.

Actual measurements of the osmotic pressure of plant cell have
been made. The results are more or less uncertain, because, as
has been pointed out, a plant cell is not a definite quantity of
uniform protoplasm surrounded by an ideal semi-permeable mem-
brane, but is instead a mass of living matter which is approxi-
mately semi-permeable throughout its entire volume and is in a
constantly changing condition because of the anabolic and cata-
bolic activities which are going on in it; but values have been
obtained which show a normal osmotic pressure as high as four-
teen atmospheres in the cells of very turgid plants, such as those
of some of the green algae. Animal cells probably have an osmotic
pressure similar to that of the blood which circulates around them,
which is approximate that of seven atmospheres.

THE PHYSICAL CHEMISTRY OF PROTOPLASM 231

SURFACE BOUNDARY PHENOMENA

In the preceding chapter, a brief consideration of the phenom-
ena arising at surface boundaries was presented. It was pointed
out that when any substance exists in the colloidal, or dispersed,
condition, it has relatively enormous surface area and that, con-
sequently, enormous surface boundaries between the dispersed
phase and the dispersion medium exist in all colloidal mixtures.
Since protoplasm is conceived to exist in the form of a colloidal
gel, having a foam-like structure, it is apparent that it has these
enormous surface boundaries between the different phases of the
system, and that the phenomena arising from this condition are of
great importance in its biological activities. The following neces-
sarily brief discussion will serve to give some indication of the
physiological importance of the surface boundaries in such a
system.

It is easy to see that the molecules which are in the surface
layers at the interface, where two phases of a colloidal system are in
contact, are under the influence of forces quite different from those
which are acting upon the molecules in the interior of either phase.
It is apparent that the molecules in the surface layer 'are exposed
on the inner side to the attraction and influence of similar mole-
cules, while on the opposite, or outer, side they are exposed to the
influence of molecules of an entirely different kind. This results
in a state of tension, known as " surface tension," with the devel-
opment of resultant forces and energy which profoundly affect
the chemical reactivity of the molecules which are present in this
surface layer. The so-called " surface energy," which results
from this surface tension, produces marked increases in the pos-
sibility of chemical reaction between the materials which are
present at the surface boundaries. In colloidal gels, this effect
is so pronounced, in many cases, as to completely overshadow
other types of influences upon reaction velocities. Also, the sur-
face layer of a liquid is compressed by its surface tension, to such
an extent that the solubility of substances in this surface layer is
greatly increased over that of the same substances in the interior
of the liquid, which results in greatly increased concentration of
dissolved substances in the surface layer, and so increases the rate
of chemical changes which take place there, as contrasted with the
rate of the same reactions going on in the interior of the solution.

232 CHEMISTRY OF PLANT LIFE

This latter consideration seems to be the factor of largest influence
in colloidal catalysis.

But in addition to the increased rate of reaction in the surface
layer due to the increased energy available there and to the
increased concentration of dissolved substances, there is the pos-
sibility that the act of concentration itself bring into play molec-
ular forces which give rise to a resultant increase in chemical
potential, or chemical affinity, of the reacting materials, such as
has been observed to result in other concentrated solutions. A
discussion of the theoretical and mathematical considerations
upon which this conception is based would be out of plaoe here,
but there is ample experimental evidence to indicate its soundness.

Further, as has been pointed out, colloidal phenomena are
essentially due, in large part at least, to the electric charges on
the dispersed particles. Electric charges accumulate at the
surface of any charged body. Hence, the surface layers in any
colloidal system carry its electric charges in highest concentration,
and all of the chemical changes which are stimulated by electrical
phenomena are most strongly influenced at the surface boundaries
between the different phases of the system. This latter considera-
tion affords a satisfactory explanation of the well-known depressing,
or stimulating, action of electrolytes, especially acids and bases,
upon the enzymic catalysis of protoplasmic reactions.

These few, brief statements are sufficient to indicate how
extensively the chemical activities of colloidal protoplasm are
influenced by the phenomena arising from the surface boundaries
between different materials, which are present in such enormous
extent in a colloidal gel. Surface boundary phenomena in a
heterogeneous system, such as we have seen protoplasm to be,
provide the possibilities for many reactions which would other-
wise take place very slowly, if at all. Mere subdivision of the
protoplasmic materials into the film, or foam, structure brings into
play energies which may predominate over all other types of
energy in the system. Here, too, effects may be extraordinarily
modified by slight changes in environment, which effects could
not be explained by any considerations which govern ordinary
chemical reactions. Here, we deal with adsorption and other
colloidal phenomena, rather than with ordinary stoichiometric
combinations.

Indeed, it is not too much to say that the differences between

THE PHYSICAL CHEMISTRY OF PROTOPLASM 233

the chemical phenomena which are called " vital " and those
which take place in ordinary laboratory reactions are due to the
fact that the former are manifestations of the interchanges of
energy between the different phases of a heterogeneous colloidal
system, while the latter are governed by the laws of ordinary
stoichiometric combinations.

ELECTRICAL PHENOMENA OF PROTOPLASM

The investigations of this phase of the physical chemistry
of protoplasm have dealt almost exclusively with animal tissues
and reactions, and have included the study of such phenomena as
nerve impulses, muscular contractions, heart-beats, glandular
secretions, etc. Tissues which respond to nerve, or brain, con-
trol are, of course, not found in plants. But there is plenty of
experimental evidence to show that plant protoplasm carries
electrical charges and exhibits electrical phenomena which are
similar in character to those of animal tissues. In fact, it has been
shown that the contraction of the lobes of the Venus7 fly trap,
when they close over an imprisoned insect, are accompanied by
electrical phenomena in the leaf tissues which are precisely similar
to those which take place in an animal muscle when it contracts.
It seems probable that many of the observations and conclusions
which have been derived from the study of the electrical dis-
turbances in animal tissues may later be found to have definite
applications to the vital phenomena of plant cells. Hence, it
seems proper to give some brief consideration to these matters
here.

The statement has been made that " every active living cell is
essentially an electric battery," and it is believed that every activity
of living matter, such as the rhythmic contraction of the heart,
the passage of a nerve impulse, etc., is accompanied by an electric
disturbance in the protoplasm of the tissues in question. Experi-
mental proof of this electrical disturbance has been repeatedly
obtained, by connecting a delicate galvanometer in a circuit
through the living tissue which is undergoing different activities and
obtaining widely varying readings of the instrument as the different
phenomena are in progress, or by connecting the instrument with
muscular tissue and observing its fluctuations with either the

234 CHEMISTRY OF PLANT LIFE

irregular contractions of a voluntary muscle or with the rhythmic
contractions of a heart muscle.

By means of such investigations as those just mentioned, it
has been found that the part of the protoplasm which is most
active is always electro-negative to the part which is less so; that
is, the electric current flows from the more active to the less active
portion of the protoplasm.

Many different explanations of the origin of the electric
current which develops when the protoplasm is stimulated into
activity have been suggested; but none of them have, as yet, any
experimental confirmation. The most that can be said is that
whenever any stimulus excites the protoplasm into activity, there
is instantly developed in it an electrical disturbance, which con-
tinues as long as the action is in progress. Recent investigations,
which have shown that there is a direct relation between many of
the vital processes of protoplasm and the ratio of the electrolytes
which it contains, particularly the ratio of sodium and potassium
to calcium, would seem to indicate that the development of the
electrical disturbance is a direct result of variations in the pro-
portions of the salts of these metals, either brought about by, or
themselves causing, changes in the permeability of the protoplasm,
following the stimulus which determines the nature of the activity
which it is to undergo. But there is as yet no indication concern-
ing the mechanism by which this stimulation, with its resultant
electrical phenomena, is transmitted to the protoplasm and accom-
plishes its characteristic effects.

ACIDITY OR ALKALINITY OF PROTOPLASM

The preceding sections of this chapter have dealt almost
exclusively with the physical properties of protoplasm; including
the phenomena of solution, ionization, surface boundary effects,
and electrical disturbances, and their probable effects upon the
chemical reactions which constitute its biological activities. It is
necessary now to consider another phase of the physical chemistry
of protoplasm, namely, its chemical reaction, whether acid, alka-
line, or neutral, the effects of variation of this condition upon the
activity of the protoplasm, and the mechanism by which it tends
to preserve its own proper reaction in this respect.

The earlier methods of investigation of the chemical reac-

THE PHYSICAL CHEMISTRY OF PROTOPLASM 239

tion of protoplasm were all based upon its color reactions to various
staining agents. These sometimes led to erroneous conclusions,
because of the effects of the staining agent itself upon the tissue;
some stains are poisonous and result in the death of the protoplasm,
others do not easily penetrate the semi-permeable colloidal mass,
others are themselves changed by the oxidizing or reducing action
of the protoplasm, etc. Again, colloidal adsorption effects often
lead to the so-called " capillary segregation " of added staining
materials. So that this method of study must be used with great
care, or wholly erroneous conclusions will be reached, and many of
the earlier reports have subsequently been found to be incorrect.

The recent improvements in the apparatus and methods for
the determination of hydrogen-ion concentration have afforded a
much more trustworthy method of determining the actual acidity
or alkalinity of such materials than is obtained by color reactions,
and this method is now being extensively used in the study of the
reaction of active protoplasm.

It must be kept in mind that protoplasm is an heterogeneous
mass and not an homogeneous solution, so that it is not always
possible to determine the actual conditions as to neutrality of dif-
ferent parts of the protoplasm of a single cell, for example. Hence,
one of the best methods of determining the reaction which is favor-
able to the life and activity of any given type of protoplasm is to
investigate the reaction of a liquid medium in which the cells
live and grow; this plan being based upon the assumption that a
cell is not likely to have a reaction different from that of the
medium which is favorable to its growth.

The results of all of the many investigations which have
dealt with this problem point to the conclusion that the normal
reaction for living protoplasm is either neutral or very faintly
alkaline; but that it becomes acid when the cell is working in the
absence of sufficient oxygen, and after the death of the cell.

The first effect of a change in the reaction toward acidity of the
protoplasm is a decrease in the rate of respiration of the tissue,
while increased alkalinity stimulates respiratory activity. When
carried to the point of actual acidity, the respiratory coefficient
becomes negative, and the cell actually gives off carbon dioxide
because of the stoppage of the synthetic processes.

A second effect of change in reaction of protoplasm is to alter
the enzymic activity of the cell. As has been pointed out, enzymes

236 CHEMISTRY OF PLANT LIFE

are extraordinarily sensitive to minute changes in the reaction of
the medium in which they are working. A change toward acidity
in protoplasm immediately results in the stimulating of carbo-
hydrate-splitting enzymes, which increases the supply of easily
oxidizable simple carbohydrates, thereby tending to compensate
for the decrease in respiratory activity. Further, increase in
acidity increases proteolysis, thereby liberating alkaline ammonia-
derivatives which tend to neutralize the rising acidity and so to
restore normal neutrality or alkalinity. Thus it will be seen that in
the very great sensitivity of its enzyme catalysts to slight changes
in the reaction of the medium, the protoplasm possesses a very
efficient mechanism for regulating changes and restoring equi-
librium, if the latter be disturbed by any abnormal conditions.
It should also be noted, at this point, that the almost universal
presence in protoplasm of salts of carbonic and phosphoric acids
acts as an additional " buffer " against pronounced changes in
reaction of the material; the bicarbonates acting by means
of their ready release or absorption of carbon dioxide, and the phos-
phates by their easy change from mono-sodium phosphate to di-
sodium phosphate, and vice versa, the former being slightly acid
and the latter slightly alkaline in reaction.

A third effect of increasing acidity is that it induces increased
imbibition of water by the colloidal gel and causes swelling of the
tissue. After death, when the reaction of the protoplasm becomes
pronouncedly acid, this swelling often proceeds to the point of
rupturing of the cell-wall, or internal membranes of the proto-
plasm, thus permitting the entrance of the putrefactive bacteria
and hastening the decay of the tissue.

Finally, comparatively slight variations in the reaction of the
protoplasm produce enormous changes in its colloidal condition,
affecting in a very marked degree its permeability, its power of
adsorption, etc.

It is clear, therefore, that variations in the chemical reaction of
protoplasm profoundly affect its colloidal condition, its enzymic
activity, and its respiratory processes. This necessarily brief
survey is sufficient to indicate how important to the activity of the
protoplasm is the chemical reaction of the material, and the
mechanism with which it is provided for maintaining the favorable
condition of neutrality or slight alkalinity.

THE PHYSICAL CHEMISTRY OF PROTOPLASM 237

SUMMARY

It is evident that, within the limits of a single chapter, it has
been possible to give only a very brief and incomplete discussion of
some of the most important applications of the principles of phys-
ical chemistry to the properties and activities of protoplasm.
Therefore, it may be profitable to summarize briefly these into a
series of definite statements which may serve as a review of the
principles which have been discussed in the preceding chapters, as
applied to the activities of protoplasm.

Protoplasm is a complex hydrogel, composed of an heterogeneous
mixture of proteins, fats, and carbohydrates, arranged in a foam-
Jike structure, the compartments of the gel being filled with an
aqueous solution of the soluble organic products of synthesis and
of varying proportions of mineral salts which are of the same gen-
eral nature as those of sea-water.

The gel is not uniform throughout the volume of any given cell,
but is differentiated in different parts into what are known as the
nucleus, the chloroplasts, the plasma of the cell, etc.

The vital activities of the cell consist in chemical reactions
which are controlled by comparatively slight changes in the elec-
trolyte distribution, or other environmental changes which affect
the colloidal condition of the mass and, generally speaking, result
in changes of the water content of the plasma, most such chemical
changes being essentially reversible hydrolytic reactions.

The components of active protoplasm are in a condition most
favorable to chemical reactions by reason of the enormous surface
area of the colloidal material, resulting in abundance of available
energy, intimate contact of the reacting materials, and the nearest
possible approach to the condition of true solution which can be
obtained without the loss of stable form and structure.

The reactions which take place in cell protoplasm, as a result
of the action of either physical or chemical stimuli, are accom-
panied by electrical disturbances, which may be either caused by,
or the result of, changes in the electrical charges of the mineral
salts which are present in the gel. Such changes, like the chem-
ical reactions which they accompany, may be regarded as. rever-
sible and mutually self-regulatory; so that the protoplasm has
not only the possibilities of enormous chemical reactivity, but also
the mechanism for self-regulation of its actions, the products or

238 CHEMISTRY OF PLANT LIFE

results from any given series of changes generally tending to
reverse the process by which they are proceeding and so to restore
the condition of normal equilibrium.

Finally, the most characteristic difference between the reac-
tions which go to make up the vital activities of a living cell and
those of the same chemical substances when in inanimate form in
the laboratory lies in the presence in the colloidal mass of the accel-
erating catalysts known as enzymes, which are produced by the
protoplasm itself in some way which is as yet wholly unknown;
and which not only add to the possibilities of rapid chemical
change which are afforded by the colloidal nature of the material,
but also, because of their extreme sensitiveness to minute changes
in environmental conditions, serve to govern both the rate and
the direction of the individual chemical reactions which constitute
the vital activities of the protoplasmic mass. These enzymes are
not distributed uniformly through any given cell, or organism, but
are localized in different parts of the cell or tissue and so give
to its different parts the ability to perform their various dif-
ferent functions.

References

ATKINS, W. R. G. — "Some Recent Researches in Plant Physiology," 328
pages, 28 figs., London, 1916.

CZAPEK, F. — "Chemical Phenomena of Life," 152 pages, New York, 1911.

CZAPEK, F. — "Ueber eine Methode zur direkten Bestimmung der Oberflach-
enspannung der Plasmahaut von Pflanzen," 86 pages, 3 figs., Jena, 1912.

HOBER, M. R. — "Physikalische Chemie der Zelle und der Gewebe," 671
pages, 55 figs., Leipzig, 1911.

LIVINGSTON, B. E. — "The Role of Diffusion and Osmotic Pressure in Plants,"
149 pages, Chicago, 1903.

McCLENDON, J. F. — "Physical Chemistry of Vital Phenomena," 248 pages,
Princeton University Press, 1917.

MACDOUGAL, D. T. — "Hydration and Growth," Publication No. 297,
Carnegie Institution of Washington, 176 pages, 52 figs., Washington,
D. C., 1920

SPEIGEL, L. trans, by LUEDEKING, C. and BOYLSTON, A. C. — "Chemical Con-
stitution and Physiological Action," 155 pages, New York, 1915.

THOMPSON, D'A. W. — "On Growth and Form," 793 pages, 408 figs., Cam-
bridge, 1917.

WILLOWS, R. S. and HATSCHEK, E. — "Surface Tension and Surface Energy
and their Influence on Chemical Phenomena," 116 pages, 21 figs., New
York, 1919, (2d ed.).

CHAPTER XVII
HORMONES, AUXIMONES, VITAMINES, AND TOXINS

REFERENCE has frequently been made, in preceding chapters, to
the effect of various stimulating or inhibiting agencies upon the
physiological activities of plant protoplasm. In the main, these
agencies are external to the plant and are either physical, such
as changes of temperature, amount of light received, etc.; or
chemical, such as variations in the salts received from the soil,
or common anaesthetics applied to the plants by man. A plant
grows normally under certain conditions to which it has become
adjusted by hereditary acquirements. When these conditions
are altered, the effect upon the functioning of the plant proto-
plasm may be either stimulating or depressing. Extreme changes
in environmental conditions generally result in the death of the
plant; but changes which do not result in the lethal condition
affect the plant by either stimulating it to more rapid physiological
activity or by depressing its normal growth or functions. As
has been pointed out, the same external influence, either chemical
or physical, which acts as a stimulant if it differs only slightly
from normal conditions, may become depressing, or positively
toxic, if present to a larger extent.

There is also the possibility of the elaboration by the plant
itself of internal agents, or substances, which may have a definite
stimulating or inhibitory effect upon its metabolism and growth.
The study which has been given to these matters has practically
all been carried on within very recent years and is still in prog-
ress. Most of it is still in the experimental stage, in which no
definite conclusions are as yet possible. Hence, the most that can
be done at present is to give a brief review of the suggestions which
have been made thus far, as indicative of the uncertainty of
our present knowledge of these matters and of the general trend of
the investigations which are now in progress.

Substances which are elaborated by plants and which are sup-
posed to have a definite stimulating or beneficial effect upon

239

240 CHEMISTRY OF PLANT LIFE

the activities of the plant which produces them, or to influence the
physiological activities of other plants with which these substances
come in contact through either the parasitic or the symbiotic
relation, have been variously discussed under the names
"hormones," "auximones," and "vitamines"; while injurious
substances are generally known as " toxins. " Whether these
different terms actually represent different definite types of sub-
stances, or whether there are actually different groups of stimu-
lating or inhibitory agents produced in plants, is uncertain; but
the following brief statements will serve to indicate the general
nature of the suggestions which have been put forward and of
the experimental work which is now in progress.

HORMONES

The term " hormone " was first used to designate certain
stimulating substances which are supposed to exist in the
intestinal tracts of animals and to cause the glands to elaborate
and secrete their characteristic enzymes. The supposed " hor-
mones " are not themselves active in performing the digestive
functions of the glandular secretions, but are the exciting, or stim-
ulating, agents which cause the glands to secrete their active'
enzymes.

The same term has been used, by certain plant physiologists,
to designate any agency, either external or internal, which stimu-
lates plant protoplasm to abnormal activity. It has been pointed
out that there are a variety of substances, which are themselves
chemically neutral, that are powerful stimulants of vital activity
if used in only minute proportions, but are powerful poisons if
present in larger amounts. Many of the alkaloids act in this
way upon the animal organism; while chloroform, toluene, and
even some of the more complex hydrocarbons, act similarly upon
the tissues of plants, and ether vapor is known to be a powerful
stimulant in accelerating the flowering of plants and the ripening
of fruits. It has been shown that the vapors of all such sub-
stances readily penetrate the protoplasm of leaves, seeds, etc.,
even when the same parts are impermeable to most mineral salts,
sugars, etc. ; and that upon entrance to the protoplasm of a leaf,
or a seed, they tremendously stimulate its metabolic activity.

HORMONES, AUXIMONES, VI T AMINES, AND TOXINS 241

These hormones, as a class, are chemical substances which have
very little attraction for, or power of combination with, water;
and it has been suggested that the ease with which they penetrate
the protoplasm is due to the fact that they are not held at the sur-
face by combination with the active water molecules which are
present in the surface layer.

The principal effect which is supposed to be produced by
these " hormones " is the stimulation of the enzymic activity,
particularly that of the degenerative processes which take place
late in the plant's life, at the flowering or ripening periods.
Many of the changes which take place normally at ripening time,
such as the change in color from green to yellow or red and finally
to brown or black, when the fruit or vegetable is fully ripe, can be
greatly accelerated by treatment with these substances. Hormones
are similar in type to the ethereal salts, or esters, which constitute
the natural essential oils that develop in many plants at this
stage of their growth. Hence, it seems probable that these
changes in plants which are maturing naturally may be hastened
by the hormone action of the esters and similar bodies which are
developed in largest quantities at that stage. It has been pointed
out that the characteristic group which is present in many natural
glucosides is of the same general type as the " hormone " sub-
stances which are used in the artificial stimulation of the flowering
or ripening changes. This fact, together with the possibility of
the liberation of greater percentages of these aromatic compounds
from their glucoside combinations at the later periods of plant
growth, is assumed, by some plant physiologists, to account for
the change from synthetic to degenerative processes at this stage
of the plant's development.

Further, it has been suggested that the autumnal coloration of
leaves, and their dropping from the stems of the plant, as well as
the ripening of seeds, is probably determined by the liberation in
the plant, at that stage of its growth, or as a result of changed
climatic conditions at that particular season of the year, of the
hormones which either initiate or hasten the special enzymic
changes which distinguish the degenerative from the synthetic
processes of the plant.

Similarly, it has been suggested that parasitic fungi are able
to penetrate the host plant by first excreting " hormones " which
bring about degenerative changes in the tissues of the host plant

242 CHEMISTRY OF PLANT LIFE

and so make it more easily penetrable by the hyphae of the par-
asite.

It will be seen that, in general, " hormones " are a type of
substances (possibly often present in plants in the form of glu-
cosides) which are supposed to stimulate the degenerative (or
katabolic) vital processes in contrast to the synthetic (or anabolic)
changes. It has been suggested that they do this in either one of
two ways; namely, by favoring the introduction of water into the
protoplasm and so diluting the cell contents, changing the osmotic
pressure, etc.; or by bringing about a separation of the colloidal
layers, or films, of the protoplasmic complex, producing a result
similar to that produced by freezing the tissues. These ideas
have been suggested by studies of the changes in the protoplasmic
equilibrium of protoplasm when foreign substances are introduced
into it. These studies have not as yet been brought to the stage
of final conclusions, and the ideas presented must be considered
as suggestive rather than as conclusive.

VITAMINES

" Vitamines," as contrasted with " hormones," are supposed
stimulants of synthetic metabolic processes, or accelerators of
growth, rather than of degenerative processes.

The term " vitamine " was first used to designate the sub-
stance, or substances, which must be present in the diet of animals
in order that the animal organism may grow. Absence of these
substances from the food of the animal results in the stoppage
of growth of young animals and in various so-called " deficiency
diseases " (such as beri-beri, scurvy, polyneuritis, etc.) of adults.
This means that the animal organism is altogether unable to elab-
orate its own vitamines, and extended investigations have indi-
cated that the vitamines necessary for animal uses are wholly of
plant origin. The name " vitamine " was first used because it
was supposed that these substances are chemical compounds of the
amine type and, since they are necessary to normal life processes
of animals, the name " vitamine " seemed to represent both their
chemical character and their functions. Later investigations
have caused, doubt as to the accuracy of the first belief as to their
chemical nature, and various other names have been suggested
for the general group of substances which have the observed bene-

HORMONES, AUXIMONES, VIT AMINES, AND TOXINS 243

ficial effects; while such specific names as "fat-soluble A,"
" water-soluble B," etc., have been used to designate individual
types of these accessory food substances. However, the term
vitamine is such a convenient one and is so generally recognized
and accepted that it will probably continue to be used, at least
until some more definite knowledge of the nature and composition
of these growth-promoting, disease-preventing, and reproduction-
stimulating food constituents is obtained.

The following definition of the term " vitamines " gives a
satisfactory conception of the nature and functions of these
substances, so far as they are yet known. " Vitamines con-
stitute a class of substances the individuals of which are necessary
to the normal metabolism of certain living organisms, but which
do not contribute to the mineral, nitrogen, or energy factors of the
nutrition of those organisms." As sub-groups of the vitamines,
there have already been recognized the growth-promoting, fat-
soluble A; the antineuritic B, and the antiscorbutic C.

Until very recently, the investigations of vitamines have dealt
exclusively with their relation to human nutrition; although
it has been generally believed that the vitamines themselves are
elaborated only by plants. It was generally recognized, however,
that those plants, or parts of plants, which are capable of very
rapid growth or metabolic changes, such as germs, spores, leaves,
etc., are generally the richest source for vitamines for animal needs.
Hence, there seemed to be considerable basis for the assumption
that the elaboration of these substances by plants is definitely
connected with their own metabolic needs. Recently, inves-
tigations of the functions of vitamines in the growth of plants
have been begun. These are still in progress, but the following
conclusions seem to be justified at the present time: (a) Potato
tubers appear to contain growth-promoting substances which
are essential to the proper growth of the sprouts. Whether these
are the same substances which are efficient in the prevention of
scurvy in men has not yet been investigated, (b) Baker's yeast is
probably dependent upon a supply of vitamines in the medium
in which it is to grow. Yeast itself, after having grown in barley
wort, is one of the most important sources of vitamines for animal
uses or for purposes of investigations of vitamine activity. But
it has been reported that a yeast cell will not grow in an artificial
medium which contains all the essential nutrients for yeast but

244 CHEMISTRY OF PLANT LIFE

has no vitamines of other plant origin in it. The addition of
barley wort, containing the vitamines from barley germs, or any
other similar supply of vitamines, induces rapid growth and the
storage of vitamines in the growing yeast masses, (c) The growth
of many bacteria is either wholly dependent upon or greatly stim-
ulated by the presence of vitamine-like substances in the medium
upon which the micro-organisms grow, (d) Sclerotinia cinerea,
the brown rot fungus of peaches and plums, will grow only in a
medium which contains, in addition to the essential sugar, salts,
and nitrogenous material, vitamines derived from either the
natural host plant tissues or other plant sources. These may be
of two types (namely, a vegetative factor and a reproductive
factor) or two different manifestations of activity of the same
vitamine substance. But both of these factors must be pro-
vided before the fungus can make its characteristic growth.

There is, as yet, no conclusive evidence on many of the matters
concerning the relation of vitamines to plant growth. But it
seems that these substances are of almost universal occurrence in
the organic world; that they are not of the same general type as
other substances which are essential to the nutrition of plants or
animals, but have specific stimulating or regulating effects upon
the physiological activities of the organism; that the vitamines
which are essential to animal life are elaborated by plant tissues,
but that in the case of the bacilli of certain human diseases there
seems to be some indication that the affected tissues of the animal
host produce vitamines which are essential, or favorable, to the
growth of the parasitic organism. There seems, therefore, to be
evidence of a mutual relation between plants and animals with
respect to their nutritional needs for the so-called " vitamines."
But the evidence concerning the function of these substances in
the tissues of the organism which elaborates them is, as yet, inad-
equate to provide any clear conception of the reason for their
development or of the mechanism by which they are elaborated.
Neither is there, as yet, any conclusive evidence concerning the
chemical nature of the substances themselves.

AUXIMONES

Certain investigations have indicated that bacteria, at least,
develop exogenous vitamines which are beneficial to the growth of

HORMONES, AUXIMONES, V IT AMINES, AND TOXINS 245

other plants. These are the so-called " auximones." For
example, bacterized peat seems to contain auximones which may
be isolated from the peat and exert a beneficial effect upon the
growth of various seed-plants, including common farm crops.
Neither the original experimental data, nor the theories which
have been advanced to account for the observed beneficial effects
of the supposed " auximones " have, as yet, sufficient confirmatory
evidence definitely to establish their soundness. But it seems
that there is a probability that some plants, at least, do elaborate
vitamines, or auximones, which are useful to other plants.

TOXINS

Toxins are substances which affect injuriously the normal
activities of the organism. As has been pointed out, they may be
the same substances which, in lesser concentrations, exert a
stimulating effect upon the same organism. Hence, it is
probably inaccurate to discuss the toxins as a distinct group of
substances.

There are, however, a large number of water-soluble chemical
substances which are injurious to all living protoplasm, even at
concentrations considerably less than the point of osmotic equi-
librium in the juices of the protoplasm. These substances may
act either directly or indirectly upon the protoplasm, but at cer-
tain concentrations they always affect it injuriously. In the main,
these toxins are external agents of other than plant origin; although
chemical substances developed by one plant may be toxic to other
plants, or even to other organs of the same plant than those in
which they are elaborated.

Toxins may be either general (i.e., injurious to all types of
plants), or specific (i.e., injurious to only certain species) in their
action. Examples of specific toxicity are of only minor importance
in plant studies. They seem to be generally explainable on the
basis of some unusual lack of resistance or failure of the suscep-
tible plants to be able to exclude the entrance of these injurious
substances into the protoplasm by " selective adsorption,"
or to convert the injurious substances into insoluble and non-
injurious forms, as is done by other plants which are not sus-
ceptible to injury by these " specific " poisons. Hence, particular
attention need not be given to this type of toxins.

246 CHEMISTRY OF PLANT LIFE

Toxic substances may act injuriously upon plant tissues in a
variety of ways. Many electrolytes, especially the salts of the
heavy metals of high valency, coagulate protein material and the
entrance of such substances into the protoplasm causes disturb-
ances in the colloidal condition which cannot be otherwise than
injurious to its normal activities. Similarly, formaldehyde and
many other organic compounds may affect the colloidal properties
of the protoplasmic gel in such a way as to injure the plant tissues.

The same substance is sometimes much more injurious to the
tissues of one part of a plant than it is to those of another part
of the same plant. Thus, the rootlets of a young growing plant are
much more susceptible to injury by many mineral salts than are
the vegetative parts of the same plants; while anaesthetics of
various kinds generally exhibit their greatest injurious effects upon
the leaves, or synthetizing cells. Again, the mycelia of fungi are
much more easily killed by toxic agents used as fungicides than, are
the spores of the same fungi. Some of these observed differences
in toxicity may be due to differences in the physiological effect of
the substance upon the protoplasm of the tissues which it enters,
and others may be due to differences in the resistance of the pro-
toplasm, or of its protective coverings, to penetration by the toxic
material. Indeed, the possibilities of different types of toxic
action, and of resistance to it by individual plants and species,
are so varied that it is not possible to divide toxic agents into spe-
cific groups according to the nature of their injurious action upon
the plant cell. They are, therefore, more commonly grouped
into classes according to their chemical nature and economic
significance as fungicides, as follows: inorganic and organic acids;
caustic alkalies; salts of the heavy metals; hydro-carbon gases;
formaldehyde; alcohols and anaesthetics; nitrogenous organic
compounds; and miscellaneous decomposition productions of
organic origin. The following brief review of some of the results
of the experimental studies of the toxicity of different compounds
belonging to these several groups will serve to indicate the general
trend of the investigations of these matters which have thus far
been made.

Acids. — The common inorganic acids (hydrochloric, nitric, and
sulfuric) kill the rootlets of common farm crops when the latter
are immersed for twenty to twenty-four hours in solutions of these
acids containing from three to five parts per million of free acid.

HORMONES, AUXIMONES, V IT AMINES, AND TOXINS 247

Acetic acid must be about five times as concentrated as this, and
other organic acids may be much more concentrated still before
they produce the same injurious effects. The toxic effect of all
these acids is greatly reduced in soil cultures, or if particles of sand,
graphite, clay, filter paper, etc., are suspended in the solutions
containing the acids, the reduction in toxic effect being probably
due to the adsorption of the acids upon the solid particles. Hence,
the concentrations which limit the toxic effects of these acids in
water solutions cannot be taken as representing the condition
with which the same plant will have to contend when growing
under normal cultural conditions.

Alkalies. — The caustic alkalies must usually be present in
from five to ten times as great concentrations as those of the
mineral acids, in order to produce the same injurious effects upon
the rootlets of common plants. The so-called " alkali " of soils is
not alkali at all, but is neutral soluble salts present in sufficient
concentration to exert a toxic effect.

Salts of the heavy metals are especially toxic to rootlets of
plants. Salts of copper, mercury, and silver, have been found to kill
the roots of seedlings immersed in them for twenty-four hours when
present in proportions of less than three parts per ten million,
while salts of many other heavy metals are toxic when present in
concentrations of less than one part per million. The salts of
the alkali metals are considerable less injurious than are those
of the heavy metals, but even these exert their familiar injurious
effect if present in concentrations which, measured by the ordinary
standards, would still be regarded as very dilute solutions.

Illuminating gas, and similar hydrocarbon gases, kill plants
when present in the atmosphere in as little as one part per million.
Leaves, buds, and roots are all alike sensitive to this toxic effect,
the nature of which is not yet understood.

Formalin, or formaldehyde, is a penetrating toxic agent for
nearly all plant cells, and is commonly used as a fungicide for the
destruction of parasitic fungi. It probably affects the colloidal
condition in some way similar to its hardening effect upon gela-
tin, etc.

The toxic effect of many different organic compounds is so
varied in its nature and extent that it is impossible to give any
satisfactory brief review of its manifestations. Recent investiga-
tions appear to indicate that organic products of decomposition

248 CHEMISTRY OF PLANT LIFE

of plant residues in the soil may exert powerfully toxic effects
upon succeeding generations of the same, or of different, plants
growing on the land. But the experimental data and conclusions
concerning these matters are not yet accepted without question
by all students of plant science or of the problems of the productiv-
ity of the soil. In fact, it is yet an open question whether toxic
soil constituents are really an important factor in the so-called
" unproductivity " of certain soils.

Alkaloids, and even the amino-acids which are produced in the
tissues of some species of plants, while not toxic to the plants or
organs which elaborate them, sometimes exhibit strikingly toxic
action upon other plant organs with which they are brought into
contact. There is, as yet, no satisfactory explanation of this
difference in behavior between plant tissues toward various organic
toxic substances.

In fact, the whole subject of the toxic action of various sub-
stances upon plants needs much more study before it is brought to
the point where it will afford definite knowledge of either the
physiological problems involved or of their practical applications
in questions of soil productivity, etc.

CHAPTER XVIII
ADAPTATIONS

MOST of the discussions which have been presented in the pre-
ceding chapters have dealt with the types of compounds, the kinds
of reactions, and the mechanism for the control of these, which are
exhibited by plants under their normal conditions for development.
The results of the evolutionary process have produced hi the dif-
ferent species of plants certain fixed habits of growth and metab-
olism. So definitely fixed are these that hi each particular species
of plants each individual differs from other individuals, which
are of the same age and have had the same nutritional advantages
and environmental opportunities for growth, by scarcely percep-
tible variations, if at all. Indeed, this fixed habit of development
makes possible the classification of plants into genera, species, etc.
While different species of plants, given the same conditions of nutri-
tion and environment, produce organs of the widest conceivable
variety in form, color, and function; within the same species, the
form and size of leaves, the position and branching of the stem,
the color, size, and shape of the flower, the coloration and markings
of the fruit, etc., are relatively constant and subject to only very
slight modifications.

It is unnecessary to say that the mechanism, or the impulses,
which govern the morphological characters of the tissues which any
given species of plants will elaborate out of the crude food material
which it receives from the soil and atmosphere, are wholly unknown
to science. It is the commonly accepted assumption that the fixed
habit of growth of the species is transmitted from generation to
generation through the chromosomes of the germ cells. But the
nature of the elements, or substances, which may be present in
the chromosomes, which influence the character of the organs
which will develop months later, after the plant which grows from
the germ cell has gone through its various stages of vegetative
growth, is still altogether unknown. There can be no question,

249

250 CHEMISTRY OF PLANT LIFE

however, that some influence produces a fixity of habit of growth
and development which is almost inevitable in its operation.

But while this unvarying habit of growth is one of the fixed
laws of plant life, there are occasional deviations from it. A
plant which, under normal conditions of growth, develops in a
certain fixed way, when exposed to unusual environmental condi-
tions, may, and often does, alter its habit of growth in what may
metaphorically be said to be an attempt to adjust itself to the new
conditions. Numerous examples of this phenomenon might be
cited. Certain algae, which grow normally hi water at a tem-
perature of 20° to 30° and which are killed if the temperature rises
above 45°, have been grown for successive generations in water
the temperature of which has been gradually raised, until they
produce apparently normal growth in water the temperature of
which is as high as 78° ; also, certain types of algae normally grow
in the water of. hot springs at temperatures of 85° to 90°, and
others in arctic sea-water the temperature of which sometimes
falls to —1.8° and never rises above 0° C. This phenomenon of
the adjustment of a species of plants to new conditions, which in
the case of farm crops is sometimes called " acclimatization," is of
common occurrence and is often utilized to economic advantage
in the introduction of new strains of crops into new agricultural
districts. Again, the normal development of plants may be
altered as the result of injury or mutilation. Thus, if the ear is
removed from the stalk of Indian corn, at any time after flowering,
there always results an abnormal storage of sucrose in the stalk,
instead of the normal storage of starch in the kernels. Similarly,
midsummer pruning of fruit trees generally results in the produc-
tion of abnormally large number of fruit buds on the remaining
limbs. Many other familiar examples of alteration of normal
development in response to, or as the result of, abnormal condi-
tions of growth might be cited.

TYPES OF ADAPTATIONS

To designate these different alterations of normal growth,
several different terms have been used. Among these, " adapta-
tion," " accommodation," and " adjustment " have been com-
monly used by different biologists. Sometimes these are used
interchangeably, and sometimes different terms are used to desig-

ADAPTATIONS 251

nate different types of response to altered conditions of growth.
Inasmuch as there seems to be no generally accepted usage of these
different terms, only one of them, namely, the word "adaptation"
will be used here; and different manifestations of this phenomenon
will be distinguished by using appropriate adjectives, as " phys-
iological adaptations," " chromatic adaptations," " morphological
adaptations," etc.

Two markedly different types of responses to altered conditions,
or of adjustment to environment, may be recognized. In the first
of these, for which we will use the term " physiological adaptation,"
the species of plant simply acquires the ability to exist and grow
normally under conditions which formerly inhibited its growth.
Thus, we may speak of the phenomena mentioned above as
" acclimitization " as the physiological adaptation of the crop to
the new conditions of growth. In general, physiological adapta-
tions include such variations in the characters or habits of growth
of plants as results in differences in resistance to heat or to cold,
relations to water, aggressiveness in competition with other plants,
etc. In such cases, no modification of the morphological charac-
ters of the plant can be observed, the changes which take place in
the structure of the plant (if, indeed, there be any such changes)
must be only minor adjustments of the protoplasm to meet the
new environmental needs.

In the second type of adaptations, for which we will use the
term " morphological adaptations," the structure, or color, or
some other morphological character of the plant is actually
changed in some easily recognizable way, in order that the plant
may be better adjusted to its environment. As examples of
morphological adaptations, there may be cited the change in color of
sea-weeds with increasing depth in the sea, and other examples of
chromatic adaptation which are discussed below; the development
of fewer, or a larger number, of buds on the above-ground stems of
plants, hi response to decreases, or increases, in the available
supply of food; the alteration in the size and shape of the leaves
of many plants when they are grown hi shade; the dwarfing of
plants at high altitudes, or under conditions of severe drought;
the development of underground storage organs for certain species
of shrubs and trees which grow in regions that are subject to
periodical burning-over, in such a way as to destroy the above-
ground storage stems, etc.

252 CHEMISTRY OF PLANT LIFE

Hence, the two terms, as we will use them here, may be defined
as follows: morphological adaptation is a change in the structural
character of the species in order that it may be better fitted to meet
the needs of the new conditions of growth; while physiological
adaptation is an acquired power to survive and develop under
abnormal conditions, which is not accompanied by any visible
change in the characteristic structure of the species.

Both of these types of adjustment may be either hereditary
(or evolutionary), or spontaneous in their origin and development.
Changes which are evolutionary are fixed by heredity and become
definite habits of growth in the species. Their origin may be
explained in either one of two ways; namely, the so-called " in-
crease by use," and " the survival of the fittest." The hypothesis
of " increase by use," as an explanation of adaptations, is based
upon the well-known observation that, in animals, muscles and
other organs increase in volume as they are extensively used; and
the assumption of the application of this principle to the phe-
nomenon of adaptation supposes that the modification of any
given structure or composition is the result of the hereditary
accumulations of increased size resulting from use, or of atrophy
from disuse. The " survival of the fittest " theory supposes that
individuals of a species differ from each other by spontaneous vari-
ations, and that in the competitive struggle for existence those
forms which are best adapted to the environmental conditions
survive while the others perish. The contrast between these two
views is that the first holds that adaptation proceeds by develop-
ment, and the second that it proceeds by variation and elimination;
the first presupposes the existence in the organism of a mechanism
for response to changing conditions, and the second assumes that
there are chance variations followed by the death through compe-
tition of the forms which are not able to meet the needs of the
environment.

Confusion arises whenever an attempt is made to apply either
of these theories to all kinds of adaptations. The idea of increase
by use can be applied with some satisfaction to certain morpholog-
ical adaptations in animal structure; and to such phenomena as the
increase in strength of the branches of fruit trees, either with or
without corresponding increase in size, as the load of fruit increases.
But it certainly cannot apply to color change in surface pigmenta-
tion of either animals or plants, which is one of the most common

ADAPTATIONS 253

forms of adaptation. Furthermore, it is difficult to conceive the
general application of this idea to alterations of habits of growth
of plants, since a plant cannot have any such thing as a volun-
tary control over the amount of " use " which it makes of its dif-
ferent organs in response to changes of environment. The com-
mon form of statement that a plant develops an organ, or a process
to meet a certain need, or modifies its habits of growth to meet a
change of environment are, of course, purely metaphorical, and
can only be taken to mean that such processes are mechanical
responses to changes in external conditions.

The nature of the mechanism by which these responses are
accomplished is, as yet, wholly unknown. There is accumulating
a large mass of experimental evidence which goes to show that,
while both temperature and light are very important factors in
determining the type of changes which will take place in a living
organism, the so-called " photochemical action of light " is by far
the most potent of all the climatic factors which influence the
course of development of a plant. But we have, as yet, no inkling
of how the protoplasm of the plant adjusts or controls its responses
to variations in any of these external factors.

With these general considerations in mind, we may now pro-
ceed to the consideration of certain particular types of adaptations.

CHROMATIC ADAPTATIONS

Adaptations have been observed in both the energy-absorbing
pigments of the general tissues and in the ornamental epidermis
pigments of plants. The former are by far the most important
from the physiological point of view; while the latter may have
interesting biological significance.

Under nearly all conditions of growth of land plants, the supply
of the chlorophylls and their associated pigments provides for
the absorption of solar energy far in excess of the amount
necessary for the photosynthetic assimilation of all the carbon
dioxide which is available to the plant. It has been shown that
an active green leaf, on an August day, can absorb eight times as
much radiant energy as would be required to assimilate all the
carbon dioxide present in the air over its surface. No land plant,
under normal conditions, develops suppplementary pigments in

254 CHEMISTRY OF PLANT LIFE

order to utilize other than the parts of the spectrum which are
absorbed by chlorophyll and its associated pigments.

But deep-sea plants show quite a different phenomenon of
pigment development. Water is a blue liquid. At depths of
40 feet or more, the light which penetrates is devoid of red rays,
feeble in yellow, and is characteristically green or blue in color.
Now, the red rays of the spectrum are the ones which are most
efficient for photosynthesis. Sea weeds which grow at these
depths are brilliantly red in color, at intermediate depths they are
brown, and at the surface they are green, in the same latitudes.
While it is possible that the temperature of the water at these dif-
ferent depths may have something to do with the chemical syn-
thesis of the pigments, it appears plain that this color change at
increasing depths is a definite adaptation to provide for the absorp-
tion of the solar energy which is available at these depths. It has
been shown that these pigments of deep-sea plants are additional
to, and not substitutes for, the chlorophylls, etc. The latter pig-
ments are present in normal amounts, but are supplemented by
those which absorb the green and blue portion of the spectrum.
Hence, this type of adaptation might be conceived to be a " sur-
vival of the fittest," resulting in the " natural selection " of indi-
viduals of the highest total pigmentation. But, on the other hand,
there is experimental evidence to show that plants possess some
means of varying their pigmentation in response to the character
of the light which comes to them. For, it has been found that a
complete change in color of certain highly colored plants can be
produced in a single generation, by growing the plants in boxes or
chambers whose walls are composed entirely of differently colored
glass, so that the plants within receive light of only a particular part
of the spectrum. In such cases, the plant, starting with an initial
" natural " color, changes through a succession of colors until it
finally reaches equilibrium at one which provides for the proper
absorption of the right kind of light from the new supply which is
available to it. Hence, it seems proper to conclude that chromatic
adaptation is not a process of " natural selection," but a definite
result of an actual mechanism for adaptation to changed environ-
mental conditions of supply of radiant energy.

ADAPTATIONS 255

STRUCTURAL ADAPTATIONS

Changes in structure to meet special conditions of growth may
be of several different types.

One of these, which is often cited as an example of adaptation
(in this case, the term is used with a significance quite different
than that hi which it is being used here) is that of the development
of unusual and often fantastic shapes of flowers, which are so
related to the anatomy of certain species of insects that visit these
flowers hi search of nectar, that provision for the cross-fertiliza-
tion of the plants is insured, in that the pollen from the anthers
of one flower becomes lodged on the body of the insect as it is
withdrawing from the flower in such a way that it comes in con-
tact with the pistil of a second flower as the insect enters it. Such
flowers often have such peculiar shapes and lengths of nectar tubes,
etc., that only a single species of insect, whose anatomical shape is
" adapted " to that particular blossom shape can enter the flower
in its search for nectar. It is clear that this form of " morpho-
logical adaptation " is a highly specialized one, which can only be
the result of a long process of evolutionary development. It is
obvious that the plant cannot possibly possess a mechanism, or
ability, to alter its flower form hi order to make it conform to the
shape and length of the proboscis, or other body parts, of a par-
ticular species of insect. Either the bisect or the plant, or both,
must go through a process of evolutionary development in order
to arrive at this form of mutual " adaptation."

A form of true morphological adaptation (in the sense in
which we have been using the term) is exhibited by many species
of plants, which are provided with many more buds, or growing
points, than ever actually begin to grow. For example, the single
plumule which develops from a germinating wheat embryo has at
its upper end a hundred or more tiny growing points. At the
proper stage of its growth, several of these tiny buds begin to
grow into individual separate stems, and the new wheat plant thus
produces several stems from one seed and root system, a process
known as the " stooling." The number of stems in a single
" stool " depends upon the number of the potential growing points
which are stimulated into growth. It varies from only two or
three up to as many as thirty or forty, and is apparently con-
trolled by the favorable or unfavorable conditions of climate or

256 CHEMISTRY OF PLANT LIFE

nutrition at the time when the " stooling " takes place. The
plant is thus provided with a mechanism for adapting its possi-
bilities of growth to the supply of growth-promoting material
which is available to it.

Many other plants produce far more buds than ever develop
into growing tissues, and buds which, under normal conditions,
remain dormant, under altered conditions start into growth and so
provide for an " adaptation " of the total mass of the growing
plant to correspond with the altered conditions of growth. The
actual means by which certain buds are stimulated into growth
while others remain dormant, or are inhibited from growing, are
as yet unknown. Two theories have been advanced. One is that
the growing buds absorb all available nutrition and the others
remain dormant by reason of lack of growth-promoting material.
The other is that the vegetating (growing) tissue elaborates and
sends to other parts of the organism one or more substances, which
actually inhibit growth of the other parts, as dormant buds, etc.
The experimental evidence which has been presented thus far is
inconclusive, but seems to favor the distribution of nutritional
material as the governing factor, although there is some evidence
which seems to indicate that a supposed growth-inhibiting sub-
stance is actually translocated from rapidly-vegetating tissues
to other parts of the plant. There is, however, no explanation
of how the buds, or other tissues, which do grow get their initial
stimulus, while the dormant buds do not. After growth has once
started, the changes in osmotic pressure due to the accumulation
and translocation of synthetized materials can account for the
movement of new nutritional material for the synthetic processes
into the growing organ; but this would not account for the selective
stimulation of only a part of the buds, or possible growing points,
of a plant, or for an adaptational development of others under
altered conditions of growth.

The form of morphological adaptation which has been dis-
covered in the course of the study of the native vegetation of the
campos of Brazil (which have a very dry season and have been
regularly burned over by the natives for many generations) in
which the papilionaceous shrubs have developed underground
trunks, or stems, and seem actually to profit in luxuriance of
growth when the rainy season comes on by reason of this mor-
phological adaptation to the unusual environmental conditions,

ADAPTATIONS 257

is wholly inexplicable by any present knowledge of the science of
plant growth.

PHYSIOLOGICAL ADAPTATIONS

The type of adjustment to environmental conditions which
does not result in any recognizable alteration in the structure of the
plant, but simply permits it to grow under new conditions, man-
ifests itself in many ways. These adjustments are usually asso-
ciated with differences in temperature during the growing season,
and for this reason, most such examples of adaptation have been
studied in connection with possible temperature reactions upon the
growing organism.

However, recent investigations seem to point strongly to the
conclusion that the amount of light rather than the temperature
of the new surroundings is the most important influence in deter-
mining the physiological processes known as the " acclimatiza-
tion " of plants. For example, a very elaborate series of inves-
tigations has shown that the flowering stage in the development of
plants is determined by the length of the daylight period per day,
irrespective of the actual amount of vegetative growth which the
plant has made. Thus, tobacco plants, which during a period of
long days grow to the height of 8 or 10 feet before blossoming, if
grown at the same temperature in periods of short days (or if kept
in the dark during a portion of the longer days) will blossom when
less than 3 feet in height and when the total mass of vegetative
material which has been produced is less than one-third of that of
the " gigantic " plants of the same variety grown with longer
periods of illumination per day. This same principle has been
found to hold good for many widely different types of plants.
In some species, however, flowering is favored by long days, and
vegetative growth by short daylight illumination. But in all
species which have been studied, there seems to be a direct relation
between the length of day, or the total illumination per day,
and the normal or abnormal functioning of the plant. It is
apparent that at least the physiological function of sexual repro-
duction (flowering and seed-production) is determined by the
length of daylight illumination. The duration of daylight per day
which is necessary to induce the blossoming of the plants varies
for different species, but it is constant for individuals of the same

258 CHEMISTRY OF PLANT LIFE

species. This adaptation of stage of growth to duration of daily
illumination must, therefore, be an evolutionary character of the
species.

Hence, it appears that in many cases physiological adaptation
may be a direct response of the life-processes of the plant to the
daily length of photochemical stimulation which it receives from
solar light. But there is, as yet, no explanation of how this (or
any other) influence actually changes the vital processes of the
plant protoplasm so as to bring about either a morphological adap-
tation of structure or a physiological adaptation of functions to
altered conditions of growth.

CONCLUDING STATEMENTS

Enough has been said to show how very inconclusive and
unsatisfactory is our knowledge of the phenomena known as
" adaptation." Even the nomenclature used by different scientists
to describe its various manifestations is confused and misleading.
For example, certain crops are said to be " adapted " ("i.e., suited)
to certain types of soils, and vice versa; crops are said to be
" adapted " to given agricultural districts, etc.

In this chapter, an attempt has been made to arrange in some
semblance of order some of the known manifestations of alteration
of fixed habits of growth of plants in response to changes of environ-
ment, and to point out some of the suggestions of possible explana-
tions of these phenomena which have been presented by different
investigators.

This presentation cannot be considered as anything other than
an introduction to a field of study which is as yet almost entirely
unexplored, and, like all other unexplored territory, is full of
mysteries. If the study of this chapter serves to stimulate interest
in these mysteries and wonders of plant life, its purpose will
have been accomplished.

INDEX

Bold-face figures indicate main references

Accelerators, 196.
Accessory substances, 19.
Achroo-dextrin, 61.
Acid, acetic, 125, 126, 128, 132, 133,
166.

arabic, 68.

arachidic, 133.

aspartic, 168, 177.

brassic, 133.

butyric, 126, 133.

capric, 133.

caprylic, 133.

carnaubic, 140.

cerotic, 133, 140.

citric, 125, 127.

convolvulinic, 81.

crotonic, 133.

diamino-oxysebacic, 169.

diamino-trioxydodecanic, 169.

digallic, 96.

ellagic, 96.

euxanthic, 84.

formic, 25, 126, 128, 132.

galactonic, 42.

gallic, 96.

geddic, 69.

gluconic, 42.

glucuronic, 42, 43.

glutamic, 168, 177.

glycero-phosphoric, 142.

hydrocyanic, 77.

jalapinic, 81.

lauric, 133.

lignoceric, 133.

linoleic, 133.

linolenic, 133.

malic, 124, 127.

Acid, malonic, 124.

mannonic, 42.

melissic, 133.

metapectic, 68, 70.

mucic, 68.

myristic, 133.

nitric, 125.

nucleic, 162.

oleic, 133.

oxalic, 68, 124, 125, 126, 128.

palmitic, 133, 140.

parapectic, 70.

pectic, 31, 70.

phosphoric, 141, 142, 162.

propionic, 126, 166.

pyrocatechuic, 96.

quercitannic, 98.

racemic, 54.

ricinoleic, 133.

ruberythric, 83.

saccharic, 42, 68.

salicylic, 81.

sarco-lactic, 128.

stearic, 131, 133.

succinic, 127, 128

sulfuric, 125.

sylvinic, 149.

talonic, 42.

tannic, 97, 127.

tartaric, 127.

uric, 160.

xanthoproteic, 173.
Acid amides, 151.
Acidity of protoplasm, 234.
Acid glucosides, 81.
Acid potassium oxalate, 125.
Acid potassium sulfate, 88.

259

260

INDEX

Acid salts, 124.
Acids as toxins, 246.
Acid sodium sulfate, 125.
Acrolein, 135.
Acrose, 28.
Activators, 196.
Adamkiewicz's reaction, 173.
Adaptations, 249.
Adenase, 190.
Adenine, 160, 162.
Adipo-celluloses, 74.
Adsorption, 214.
^Esculetin, 81, 82.
.Esculin, 81, 82.
^Etiophyllin, 106, 107, 109.
^Etioporphyrin, 108, 109, 110.
Alanine, 168, 177.
Albumins, 175, 176.
Albuminoids, 175, 176.
Alcogel, 205.
Alcohol, ethyl, 40, 125.

benzyl, 80.

carnaubyl, 135.

ceryl, 135, 140.

cetyl, 129, 135.

coniferyl, 80.

mellisyl, 135.

myriscyl, 129, 140.

phytyl, 104, 105.

polyhydric, 31.
Alcohol glucosides, 80.
Alcosol, 205.
Aldehyde, benzoic, 148.
cinnamic, 148.
formic (see formaldehyde) .
glyceric, 35.

Aldehyde glucosides, 80.
Aldehydrol, 46.
Aldonic acids, 42, 44.
Aldose, 32.

Alizarin glucosides, 78.
Alkalinity of protoplasm, 234.
Alkalies as toxins, 247.
"Alkali salts," 10,247.
"Alkali soils," 10, 14.
Alkaloidal reagents, 154, 172.
Alkaloids, 18, 20, 151, 153, 248.
Allose, 36, 37.

Allyl isosulfocyanide, 88, 89, 148.

Allyl sulfide, 148.

a-glucose, 46.

a-glucosides, 55.

a-methyl glucoside, 47.

Altrose, 36, 37.

Aluminium, 4.

Amandin, 170, 176.

Amines, 151.

Amino-acids, 6, 151, 166, 179, 248.

Ammonia, 152.

Ammonium hydroxide, 142, 152.

Ammonium salts, 6.

Amorphous chlorophyll, 104, 105

Amphoteric electrolytes, 172.

Amygdalase, 87.

Amygdalin, 81, 86.

Amyl acetate, 148.

Amylase, 186, 189, 191.

Amylo-cellulose, 60.

Amylo-dextrin, 61.

Amylo-pectin, 60.

Amylose, 60.

Anergic food, 2, 17.

Animal nucleic acids, 162.

Antagonism, 14.

Anthocyans, 83, 102, 115, 121.

Anthocyanidins, 116.

Anthocyanin, 102.

Anthoxanthins, 117.

Anthraquinone, 83.

Antienzymes, 120, 197, 198.

Antioxidase, 120.

Antiscorbutic C, 243.

Apigenin, 84, 118.

Apiin, 84.

Apiose, 84.

Araban, 69.

Arabinose, 35, 44, 68, 69, 88.

Arabinosides, 56.

Arbutin, 77, 79.

Arginine, 169, 171, 177.

Arsenic, 13.

Asymetric carbon atom, 33.

Atropine, 155, 156.

Autotrophic plants, 16, 18.

Auximones, 239, 240, 244.

Available plant food, 4.

INDEX

261

Avenalin, 176.

Baptigenin, 79.
Baptism, 79.
Beeswax, 133.
Beet sugar (see sucrose).
Berberine, 155.
Betaine, 152.
/3-glucase, 55.
/3-glucose, 46.
/3-glucosides, 55.
/3-methyl glucoside, 47.
Biogens, 223.

Biological significance, 19.
Biuret reaction, 173.
Borneol, 148.
Boron, 13.
Bromelin, 189.
Brucine, 155, 157.
Buffers, 236.
Butter fat, 133.
Butyric acid ferment, 190.

Cadaverine, 152.

Caffeine, 160.

Calcifuges, 9.

Calciphiles, 9.

Calcium, 3, 5, 9, 10, 14, 68.

Calcium oxalate, 126.

Campferitirin. 118.

Campferol, 118.

Camphene, 147.

Camphor, 148.

Cane sugar (see sucrose).

Caoutchouc, 147.

Capillary segregation, 235.

Carbohydrases, 189.

Carbohydrates, 18, 20, 21, 30, 163,

234.
Carbon dioxide, 2, 3, 18, 21, 22, 23

24, 40, 222.
Carbonic acid, 227.
Carbon monoxide, 24.
Carboxyl, 124.
Carboxylases, 186, 190.
Carnauba wax, 133, 140.
Carotin, 112, 113, 121.
Carotinoids, 102. 111.

Carvacrol, 148.

Casein, 165.

Castanin, 176.

Castor oil, 130.

Catalases, 190, 193.

Catalysis, 182.

Catalysts, 17, 25, 183.

Catechol tannins, 97.

Catechin, 97.

Catechu tannins, 97.

Cellobiose, 52.

Cell structure, 221.

Cellulose, 71, 186, 189.

Celluloid, 73.

Cellulose, 20, 45, 63, 67, 72.

Cell-wall, 9, 12, 222.

Cerebrosides, 141, 144.

Chemical resistance, 216.

Cherry gum, 68.

Chinovose, 35.

Chlorine, 12.

Chlorophyll, 10, 11, 21, 27, 102, 105,

110, 111, 113, 122, 254.
Chlorophyll a, 103, 106, 107, 111.
Chlorophyll 6, 103, 106, 108, 11L
Chlorophyllase, 104.
Chlorophyllin a, 106, 107.
Chlorophyllin 6, 106, 107.
Cholesterol, 129, 136.
Choline, 89, 103, 141, 142, 152.
Chromatic adaptations, 251, 258.
Chromogens, 92, 119.
Chromo-proteins, 175.
Chrysin, 117.
Cinchonine, 155, 157.
Coagulated proteins, 175.
Coagulation enzymes, 190.
Cocaine, 155, 157.
Cocoanut oil, 133.
Codeine, 155, 158.
Coenzymes, 197.
" Cold-drawn oils," 137.
Collodion, 73.

Colloidal phenomena, 17, 202.
Colloidal solutions, 204.
Colloids, 202.
Colophene, 147.
Colophony, 149.

262

INDEX

Compound celluloses, 71, 73.

Conglutin, 176.

Coniferin, 80.

Coniine, 155, 156.

Conjugated proteins, 165, 174, 175.

Continuous phase, 203.

Convolvulin, 81.

Copper, 13, 247.

Cork tissue, 99, 101.

Corn oil, 130.

Corylin, 176.

Cottonseed oil, 130.

Critical elements, 4.

"Crude fat," 141.

Crystalline chlorophyll, 104, 105.

Crystalloids, 202.

Cumarin, 81, 148.

Curarin, 157.

Cuto-celluloses, 74.

Cyanidin, 85, 116.

Cyanin, 85.

Cyanophore glucosides, 86.

Cyanophyllin, 107, 108.

Cyanoporphyrin, 108.

Cymarigenin, 90.

Cymarin, 90.

Cymarose, 90.

Cystine, 168, 171.

Cytase, 72, 189.

Cytosine, 161, 162.

Daphnetin, 81, 82.

Daphnin, 81.

Deaminases, 186, 190.

Delphinidin, 85, 116.

Delphinin, 85.

Derived proteins, 173, 175, 177.

Dextrin, 59, 61.

Dextrinase, 189.

rf-galactose, 33.

d-glucose, 33.

Dextrosans, 59.

Dextrose (see glucose).

Dhurrin, 87.

Diastase (see amylase).

Diastase of secretion, 191.

Digitaligenin, 89.

Digitalin, 89.

Digitogenin, 89.
Digitonin, 89, 90.
Digito-saponin, 90.
Digitoxigenin, 89.
Digitoxin, 89.
Digitoxose, 89.
Diglycerides, 131.
Diose, 30.
Dioxyacetone, 35.
Dipeptides, 167.
Disaccharides, 31, 48.
Dispersed phase, 203.
Dispersion medium, 203.
Dispersion phenomena, 203.
Drying oils, 132.
Dulcitol series, 36.

Edestin, 170, 176.
Egg-albumin, 165.
Electrical phenomena of protoplasm,

233.

Electrolytes, 213, 227.
Emulsoids, 206, 214.
Emulsions, 206.
Emulsin, 55, 77, 87, 184, 189.
Enol, 44, 56.
Enzymes, 17, 18, 19, 20, 23, 26, 120,

121, 181, 183, 194, 199, 224.
Erepsin, 189.
Erythro-dextrin, 61.
Erythrophyllin, 107.
Erythrose, 35.
Essential elements, 4.
Essential oils, 18, 146, 147, 224.
Esterases, 186, 189.
Esters, 124, 125, 129.
" Ether extract," 141.
Etherial salts (see esters).
Ethersol, 205.
Ethyl acetate, 125.
Ethyl nitrate, 125.
Excelsin, 176.
Extracellular enzymes, 184.

Fats, 18, 20, 129, 224, 227.
Fat-soluble A, 243.
Fatty acids, 132, 142.
Fehling's solution, 39, 47.

INDEX

263

Fermentability, 40.

Ferments (see enzymes).

Ferric salts, 11.

Ferrous salts, 11.

Fisetin, 118.

Flavone, 82, 83, 102.

Flavonol, 84.

Food, 1.

Formaldehyde, 22, 23, 25, 26, 27, 247.

Frame- work material, 20, 67.

Fraxetin, 82.

Fraxin, 82.

Fructose, 23, 28, 32, 36, 38, 41, 44,

45, 47, 57, 162.
Fructosides, 41, 42.
Fruit sugar (see fructose).
Fucose, 35.
Fucoxanthin, 102, 112, 114.

Galactans, 47, 59, 63, 72.
Galactoheptose, 36.
Galactooctose, 36.
Galactose, 32, 36, 38, 45, 47, 57, 72,

77.

Galactosides, 41, 42.
Gaultherin, 81.
Gel, 172, 205, 208.
Gelation, 210.
Gel-formation, 208, 211.
Gentianose, 52, 53.
Gentiobiose, 49, 52, 53.
Gentisin, 119.
Gitaligenin, 89.
Gitalin, 89.
Gitogenin, 89.
Gitonin, 89.
Glaucophyllin, 107.
Gliadin, 165, 170, 176.
Globulins, 170, 175, 176.
Glucase, 186.
Glucodecose, 44.
Glucoheptose, 36, 44.
Glucononose, 36.
Glucooctose, 36.
Glucoproteins, 175.
Glucose, 23, 28, 32, 36, 37, 40, 41, 42,

43, 44, 45, 46, 57, 77.
Glucosidases, 189.

Glucosides, 18, 20, 41, 48, 55, 76, 91,

93.

Glue, 210.
Glutelins, 175, 176.
Glutenin, 176.
Glycerine (see glycerol).
Glycerol, 129, 131, 134, 142.
Glycine, 166, 168, 177.
Glycinin, 176.
Glycogen, 59, 61.
Glycyphyllin, 79.
Graminin, 59, 62.
Granulose, 60.
Grape sugar (see glucose).
Guanase, 190.
Guanine, 160, 162.
Gulose, 36, 37.
Gum arabic, 68.
Gums, 62, 67, 68.
Gum tragacanth, 69.
Gun-cotton, 73.

Haematin, 110.
Hematinic acid imide, 109.
Hematoporphyrin, 110.
Hemoglobin, 110.
Hemopyrrole, 109.
Helicin, 81.

Hemi-celluloses, 63, 71.
Hemi-terpenes, 147.
Heptoses, 30.
Hesperidin, 79.
Hesperitin, 79. 80.
Heterotrophic plants, 16.
Hexosans, 59, 67.
Hexoses, 22, 28, 30.
Histidine, 169, 177.
Histones, 175, 176.
Honey sugar (see fructose).
Hordein, 153, 170, 176.
Hormones, 92, 239, 240.
"Hot-drawn oils," 137.
Humins, 67,
Hydrastine, 155.
Hydrazones, 40, 49.
Hydrocellulose, 73.
Hydrogen peroxide, 26, 27, 190.
Hydrogel, 205.

264

INDEX

Hydrolases, 186, 189.
Hydroquinone, 77, 79.
Hydrosol, 205.

Hydroxy-phenyl ethyl amine, 153.
Hygrine, 155, 156.
Hyoscine, 155.
Hyoscyamine, 156.
Hypoxanthine, 160.

Idain, 85.

Idose, 36, 37.

Illuminating gas as a toxin, 247.

Imbibition, 209.

Impermeable membranes, 228.

Indian yellow, 84.

Indican, 78, 85.

Indigo, 78, 84.

Indigotin, 85.

Indole, 158.

Indoxyl, 85.

Inhibitors, 196.

Intracellular enzymes, 184.

Inulin, 59.

Inulinase, 62, 189.

Invertase, 50, 189, 191

Invert sugar, 47, 50.

Iodine number, 138.

lonization phenomena, 226.

Iridin, 79.

Irigenin, 79, 80.

Iron, 3, 5, 11, 110.

Isochlorophyllin a, 106, 107, 108.

Isochlorophyllin 6, 106, 107, 108.

Isohsemopyrrole, 109.

Isoleucine, 168.

Isomaltose, 51.

Isomerism, 32.

Isoprene, 147.

Isoquercitrin, 84.

Isoquinoline, 155.

Jalapin, 81.
Japan wax, 129.
Juglansin, 176.

Ketose, 32.
Lactam, 104.

Lactase, 56.

Lactic acid ferment, 190.
Lactone, 104.
Lactose, 45, 49, 52.
Laudanosine, 158.
Laudanum, 158.
Lecithin, 7, 141, 142, 143.
Lecithoproteins, 175.
Legumelin, 176.
Legumin, 170, 176.
Leucine, 115, 168, 177.
Leucomaines, 152.
Leu cosine, 176.
Z-galactose, 33.
Z-glucose, 33.
Levulosans, 59, 62.
Levulose (see fructose).
Lichenin, 62.
Light, 21, 253, 257.
Lignocelluloses, 74.
Lignose, 31.
Limettin, 82.
Limonene, 147.
Linayl acetate, 148.
Linseed oil, 133.
Lipases, 186, 189.
Lipins (see lipoids).
Lipoids, 129, 140.
Lipoproteins, 175.
Lupenine, 155.
Lycopersicin, 102, 122, 114.
Lysine, 169, 171, 177.
Lyxose, 35.

Magnesium, 3, 5, 9, 10, 11, 13, 14, 68.

Maltase, 55, 184, 189.
Maltose, 45, 49, 51, 52.
Malvidin, 85.
Malvin, 85.

Mandelo-nitrile, 78, 88.
Mandelo-nitrile glucoside, 77, 87.
Manganese, 4, 13.
"Manna," 47.
Mannans, 59, 62, 63, 72.
Mannite, 47.
Mannitol, 47.
Mannitol series, 36.
Mannoheptose, 36, 44.

INDEX

265

Mannononose, 36.
Mannooctose, 36.
Mannosans (see mannans).
Mannose, 32, 36, 37, 41, 44, 45, 47,

57, 72.

Mannosides, 42.
Maple sugar (see sucrose).
Maysin, 176.
Meiibiose, 49, 52.
Melizitose, 52.
Menthol, 148.
"Mercerizing" cotton, 73.
Mercury, 247.
Merosinigrin, 88.

Metallic salts, 13, 224, 227, 237, 247.
"Metal proteids," 14.
Meta-pectin, 70.
Metaproteins, 175.
Methylethylmallein imide, 108.
Methyl glucosides, 42.
Methyl pentoses, 35.
Methyl salicylate, 81.
Middle lamella, 67, 70.
Millon's reaction, 173.
Molisch's reaction, 174.
Monoglycerides, 131.
Monohydric alcohols, 135.
Monosaccharides, 31, 35, 45.
Morin, 119.
Morphine, 155, 158.
Morphological adaptations, 251, 252,

255.

Mucilages, 67, 70.
Muco-celluloses, 74.
Muscarine, 152.
Mustard oils, 88, 148.
Mustard oil glucosides, 88.
Mutarotation, 46, 49.
Mryosin, 77, 88, 149, 189.
Myrtillidin, 85.
Myrtillin, 85.

Narceine, 158.
Narcotine, 158.
"Natural selection," 254.
Neurine, 152.
Nicotine, 155, 156.
Nitrates, 6.

Nitrile reaction, 43.
Nitriles, 43, 44.
Nitrogen, 3, 5, 6, 151, 164.
Non-drying oils, 132.
Non-essential elements, 4.
Non-reducing sugars, 39, 49.
Nonoses, 31.
Normal celluloses, 72.
Nuclease, 189.
Nucleoproteins, 162, 175.
Nutrients, 1.

Octoses, 31.

(Enidin, 85, 116.

GEnin, 85.

Oils, 129.

Oil of bergamot, 148.

Oil of bitter almonds, 86, 148.

Oil of cassia, 148.

Oil of cinnamon, 148.

Oil of garlic, 148.

Oil of lavendar, 148.

Oil of mustard, 148.

Olive oil, 130.

Opium, 158.

Organic acids, 18, 124, 248.

Organised ferments, 183.

Ornamental pigments, 102, 123.

Ornithine, 169.

Oryzenin, 176.

Osazones, 40, 41, 49.

Osmotic pressure, 213, 228

Osones, 41.

Oxidases, 186, 190, 193.

Oxime, 44.

Oxycellulose, 73.

Oxycumarin glucosides, 81.

Oxygenated oils, 147.

Oxy-hydroquinone, 95.

Oxyproline, 169.

Paeonidin, 85.
Paeonin, 85.
Palm oil, 133.
Papain, 189.
Papaverine, 155, 158.
Para-dextran, 62.
Para-isodextran, 62.

266

INDEX

Paralyzers, 196.

Para-pectin, 70.

Parasites, 16.

Peanut oil, 133.

Pectase, 71.

Pectinase, 189.

Pectins, 20, 31, 67, 70.

Pecto-celluloses, 74.

Pectose, 31, 70.

Pelargonidin, 85, 116.

Pelargonin, 85.

Pentosans, 31, 67, 68, 72.

Pentoses, 30, 162.

Pepsin, 167.

Peptids, 166, 167, 176.

Peptones, 176.

Permeable membranes, 228.

Peroxidases, 190.

Persimmons, 100.

Persuelose, 36.

Phaeophytin, 107, 108.

Phaselin, 176.

Phaseolin, 176.

Phenol, 95.

Phenol glucosides, 79.

Phenyl alanine, 168, 177.

Phenyl hydrazine, 40.

Phlein, 62.

Phloretin, 79.

Phloridzin, 79.

Phloroglucinol, 95.

Phosphates, 7.

Phosphatides, 141, 143.

Phosphoproteins, 175.

Phosphorus, 3, 5, 7.

Photo-chemical action of light, 253,

257.

Photolysis, 26.
Photosynthesis, 7, 8, 18, 21, 22, 24,

254.

Phycoerythrin, 102, 115.
Phycophaein, 102, 115.
Phyllins, 106, 107.
Phyllophyllin, 107.
Phyllopyrrole, 109.
Physiological adaptations, 252, 257.
Physiological use, 19.
Phytase, 189.

Phytochlorin, 108.

Phytorhodin, 108.

Pigment glucosides-, 82.

Pigments, IS, 102, 224, 254.

Pinene, 147.

Piperidine, 154.

Piperine, 155.

Plant amines, 151, 152, 163.

Plant food, 1.

Plant nucleic acids, 162.

Polybasic acids, 124.

Polyhydric alcohols, 31.

Polypeptides, 167.

Polysaccharides, 59.

Polyterpenes, 147.

Poppy wax, 140.

Populin, 80.

Porphyrins, 108.

Potassium, 3, 5, 8, 10, 13, 14.

Primary amines, 152.

Proenzymes, 198.

Proinulase, 199.

Proinvertase, 199.

Prolamins, 175, 176.

Proline, 169, 177.

Prolipase, 199.

Prooxidase, 199.

Protamines, 175, 176.

Proteans, 175.

Proteases, 186, 189, 102.

Protective colloids, 209.

Proteins, 7, 18, 20, 151, 162, 163, 164,

224.

Proteoses, 175.
Protoplasm, 17, 26, 221.
Prulaurasin, 87.
Prunase, 87.
Prunasin, 87.
Ptomaines, 152.
Purine, 159.

Purine bases, 151, 159, 162.
Purpurin, 83.
Putrescine, 152.
Pyrimidine, 161.
Pyrimidine bases, 161, 162.
Pyrocatechol, 95.
Pyrogallol, 95.
Pyrogallol tannins, 97.

INDEX

267

Pyroxylin, 73.
Pyrrophyllin, 107.
Pyrridine, 154.
Pyrrolidine, 154.

Quaternary amines, 152,
Quercetin, 84, 118.
Quercitrin, 84.
Quinine, 155, 157.
Quinoline, 155, 158.

Radiant energy, 19.
Raffinose, 45, 52, 53.
Rape-seed oil, 130.
Reducing sugars, 39, 49.
Reductases, 186, 190.
Reserve food, 21.
Resenes, 149.
Resins, 18, 146, 149.
Resorcinol, 95.

Respiration, 18, 121, 235, 236.
Rhamnase, 77, 189.
Rhamnetin, 84.
Rhamnose, 35, 52, 77, 79.
Rhodeose, 35, 81.
Rhodophyllin, 107.
Ribose, 35.
Ricin, 176.
Rubiadin, 83.
Rubiphyllin, 107.

Saccharide, 31.
Salicin, 80.
Saligenin, 80.
Salinigrin, 81.
Salts, 224, 227, 237.
Sambunigrin, 87.
Sapogenins, 90.
Saponification, 134.
Saponification value, 138.
Saponins, 90.
Sapotoxins, 90.
Saprophytes, 16.
"Saturated" acids, 132.
Scopolin, 82.
Secalin, 63.

Secondary amines, 152.
Secretions, 20.

Sedoheptose, 36.

Semipermeable membranes, 228.

Sensitizers, 27.

Serine, 168.

Silicates, 12.

Silicon, 4, 12.

Silver, 247.

Simple proteins, 165, 174, 175.

Sinalbin, 89.

Sinalbin mustard oil, 89.

Sinapin acid sulfate, 89.

Sinigrin, 88.

Sinistrin, 62.

Sitosterol, 136.

Skimmetin, 81, 82.

Skimmin, 81.

Soaps, 134, 2018.

Sodium, 4, 9, 12, 13, 14.

Sodium stearate, 133.

Sol, 205.

"Soluble starch," 60.

Sorbitol, 48.

Sorbose, 36, 38, 45, 48.

Specific rotatory power, 38, 39.

, of fructose, 39, 47.

, of galactose, 49.

, of glucose, 39, 47.

, of maltose, 51.

, of ramnose, 53.

, of sucrose, 39.
Spermaceti, 129, 133.
Stachydrme, 155.
Stachyose, 54.

Starch, 8, 22, 28, 30, 31, 45, 59, 64.
"Starch paste," 60.
Stearin, 131, 134.
Stereo-isomerism, 32.
Stigmasterol, 136.
Structural adaptations, 255.
Structural isomerism, 32.
Strychnine, 155, 157.
Substrate, 186.
Sucrase (see invertase).
Sucrases, 186.
Sucrose, 28, 49, 64.
Sugars, 8, 18, 22, 28, 30, 31.
Sulfur, 3, 5, 11, 148.
Sulfuretted oils, 147, 148.

268

INDEX

Sulfur test, 174.

Sunflower-seed oil, 130.

Surface boundary phenomena, 231.

Surface energy, 231.

Surface tension, 231.

"Survival of the fittest," 254.

Suspensoids, 206, 214.

Suspensions, 206.

Synergic foods, 2, 20.

Synthesis, 18.

Tagatose, 36, 38, 57.
'Talose, 36, 38, 42, 57.
Tannins, 18, 94, 97, 99, 100, 127, 208,

224.

Tannon group, 96.
Terpenes, 147.
Tertiary amines, 152.
Tetrapeptides, 167.
Tetrasaccharides, 54.
Tetrose, 30, 35.
Theobromine, 160.
Theophylline, 160.
Thioglucose, 88.
Threose, 35.
Thymine, 161, 162.
Thymol, 148.
Toxins, 13, 239, 240, 245.
Translocation diastase, 191.
Trehalase, 51.
Trehalose, 49, 50.
Triglycerides, 131.
Trimethyl amine, 152.
Trimethyl glycocoll, 143.
Triose, 30, 35.
Trioxymethylene, 22, 23.
Tripeptides, 167.
Trisaccharides, 31, 52.
Triticin, 59, 62.
Tryptophane, 169, 171, 177.
Tuberin, 176.
Turanose, 49, 53.

Tyndall phenomena, 212.
Tyrosine, 115, 168, 177.

Unavailable plant food, 4.
Ultrafilter, 215.

Ultramicroscope, 203, 204, 205, 211.
Unorganized ferments, 183.
"Unsaturated" acids, 132, 138.
Uracil, 161, 162.
Urease, 190.

Valine, 168.
Vanillin, 80, 148.
Vegetable bases, 18, 151.
Vicianin, 88.
Vicilin, 176.
Vignin, 176.

Vitamines, 239, 240, 242.
Volatile oils, 20, 147.

Water, 3, 21, 22, 23, 224.
Water-soluble B, 243.
Waxes, 18, 129, 140.
Weathering, 4.
Wood gum, 68.
Wool fat, 129.
Wound gum, 68, 69.

Xanthine, 160.
Xrnthone, 82, 83, 102.
Xanthophyll, 112, 113, 121.
Xanthopurpurin, 83.
Xanthorhamnin, 52, 84.
Xylan, 69.
Xylose, 35, 68, 69.
Xylosides, 56.

Yeast, 61.

Zein, 165, 170, 176, 177.
Zinc, 13.

Zymase, 51, 56, 190, 192.
Zymogens, 198.

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