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THE LIBRARY

OF

THE UNIVERSITY

OF CALIFORNIA

DAVIS

GIFT OF
FRED N. BRIGGS



UNIVERSITY OF CALIFORNIA LIBRARY



THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW




5m-10,'22



AN INTRODUCTION TO THE
CHEMISTRY OF PLANT PRODUCTS



AN INTRODUCTION TO THE
CHEMISTRY OF PLANT PRODUCTS

By PAUL HAAS, D.Sc., Ph.D., University Reader
in Plant Chemistry at University College, etc. ;
and T. G. HILL, A.R.C.S., F.L.S., Reader in
Vegetable Physiology in the University of London,
University College. With Diagrams. 2 vols. 8vo.

Vol. I. ON THE NATURE AND SIGNIFICANCE OF
THE COMMONER ORGANIC COMPOUNDS OF
PLANTS. i6s. net.

Vol. II. METABOLIC PROCESSES.



LONGMANS, GREEN AND CO.
London, New York, Toronto, Bombay, Calcutta, Madras



AN INTRODUCTION

TO THE

CHEMISTRY OF
PLANT PRODUCTS

VOL. II. METABOLIC PROCESSES



BY

PAUL HAAS

D.Sc., PH.D.

READER IN PLANT CHEMISTRY IN THE UNIVERSITY OF LONDON, UNIVERSITY COLLEGE

AND

T. G. HILL

A.R.C.S., F.L.S.

READER IN VEGETABLE PHYSIOLOGY IN THE UNIVERSITY OF LONDON, UNIVERSITY COLLEGE



WITH DIAGRAMS



LONGMANS, GREEN AND CO.

39 PATERNOSTER ROW, LONDON, E.G. 4

NEW YORK, TORONTO

BOMBAY, CALCUTTA AND MADRAS

1922



Made in Great Britain



PREFACE.

IN the preparation of the present volume on the Meta-
bolic Processes of Plants two alternatives were presented;
the one to give as full an account as possible of the
literature, the other to give such an account as would
form a basis for further study. The latter, and more
difficult task, was chosen ; for, valuable though a digest
of the relevant literature would be, it would tend to
confuse rather than to assist the student. For this
reason we do not profess to have mentioned all research
on the subject matter ; indeed in some instances, the
chapter on Growth for example, some critics will say
that too much has been omitted ; we trust, however, that
no work of outstanding importance and requisite for our
treatment has been omitted. Details regarding methods
of experiment have been omitted since this aspect of the
subject more properly belongs to a practical treatise.

P. H.
T. G. H.

July, 1922.



CONTENTS.

PAGE

PREFACE v

CHAP.

I. INTRODUCTION: THE LIVING PLANT i

The hydrogen ion concentration 4

II. THE SYNTHESIS OF FATS 10

III. THE SYNTHESIS OF CARBOHYDRATES 14

The factors 15

External factors 17

Raw materials 17

Water 17

Carbon dioxide 17

Temperature 23

Illumination 25

Internal factors 32

Chlorophyll 32

The unknown factor 33

The products of carbon assimilation 35

The organic products of carbon assimilation .... 36
Hypotheses concerning the synthesis of carbohydrates by the

green plant 38

IV. THE SYNTHESIS OF PROTEINS 50

Hydrolysis of proteins on germination 58

V. RESPIRATION 61

The mechanism of oxidation 64

Intensity 74

Stimulation 78

The action of anaesthetics ........ 79

The conditioning factors 81

Temperature 81

Food 86

Water 89

Salts 91

Acidity 92

Light 93

The mechanism of respiration 94

vii



viii CONTENTS

CHAP. PAGE

VI. GROWTH 107

The conditioning factors 121

Temperature 121

Light .124

Water 128

Nutrition 129

Auximones I3 1

Hormones r 3 2

Vitamins *34

INDEX 137



CHAPTER I.
INTRODUCTION: THE LIVING PLANT.

THE study of plant life which in its fundamentals is physico-
chemical, is in its broad aspect the study of the origin of
life, since the plant arrived before the animal had its being.
This attitude is frankly mechanistic, to some minds grossly
materialistic, but more progress may be anticipated by follow-
ing mechanistic hypotheses than by the pursuit of theses based
on foundations the stability of which is not yet agreed on.
The present state of knowledge, however, does not permit
a full physico-chemical explanation even of phenomena ap-
parently purely chemical, and sooner or later a stage is
reached when agencies of a vitalistic nature have to be
offered in explanation.

The problems immediately involved may best be ordered
and formulated by a consideration, intentionally elementary,
of the history of a seed planted in good ground. The period
of rest completed, a period which varies much in duration
and in different species,* a sowed seed begins its germination
by the imbibition of water, provided the conditions, chiefly
of moisture, temperature and aeration, are suitable. When
the seed coat is saturated, water is absorbed by the under-
lying structures both by imbibition and by osmosis, for the
seed coat, although it may be impermeable to certain sub-
stances,! is permeable to water. Considerable swelling com-
monly results so that the volume of the seed is much increased

* The period of dormancy and consequently the beginning of germination
are conditioned by such factors as the degree of permeability of the seed coat
to water, the mechanical restraint imposed upon the embryo and associated
structures by the rigidity of the testa, the necessity of an after-ripening process
for the embryo, the degree of humidity, the amount of carbon dioxide, the
presence or absence of light, and the degree of temperature. See Crocker :
"Amer. Journ. Bot.," 1916, 3, 99; Kiihn : " Ber. deut. bot. Gesells.," 1916,
34, 369 ; and Crocker and Harrison : " Journ. Agric. Res.," 1918, 15, 137.

tSee Adrian Brown: "Ann. Bot.," 1907, 21, 790; " Proc. Roy. Soc.,"
Lend., B. 1909, 8z, 82.
VOL. II. I



2 THE LIVING PLANT

and in this swelling a relatively great force is exerted : Stephen
Hales in his classical experiment found that the force exerted
by swelling peas was sufficient to raise a weight of 184
pounds.*

The second phase in germination is now initiated, growth
starts : but growth is impossible without food to supply the
wherewithal for new structures and to make good the waste,
for vital activity requires energy which is obtained by various
oxidative processes. Thus aerobic respiration, the ordinary
catabolic process of green plants, is a marked feature con-
current with growth and may be sufficiently intense to cause
an obvious rise in temperature. The required food, chiefly
fats, carbohydrates and proteins, are stored in the embryo
itself or in special tissues, endosperm and perisperm : and
since the food is stored in a form mostly insoluble and
non-assimilable, water is the first essential and appropriate
enzymes the second, for not before it is hydrolized can food
be translocated from its storage cells and passed by osmotic
processes to the active tissues. The enzymes may be
elaborated in the cells or tissues containing the food, or
may be secreted by specialized structures, the scutellum for
example. Often the products of hydrolysis may be recognized
by simple means, sugar, for instance, in germinating barley.
In other cases their assimilation may be so rapid that
identification is difficult; indeed, sometimes their presence
can only be inferred from the results of carefully-controlled
test-tube experiments, glycerol, for example, in germinating
Ricinus. The embryo thus presented with appropriate food,
grows and develops. Growth is to a certain degree an
understandable problem which, on the present elementary
occasion, can be sufficiently indicated in a few words. Of
necessity must a cell be nourished through its surface and
growth will take place if assimilation be greater than waste
by oxidative and kindred processes. But growth means
increase, and as this increase in bulk takes place the surface
area of the cell is proportionally lessened. A stage ultimately
will be reached when the area of the surface is so limited
in proportion to the volume of the cell as to permit the
entry of only sufficient food to make good the losses ; thus

* Hales: "Vegetable Staticks," 3rd Edition, London, 1738, p. 102.



VARIOUS ACTIVITIES 3

the surface area is a limiting factor. One of three things
now is possible : the cell may remain as it is, a permanent
tissue element ; it may develop further, using up its own
contents either entirely or in part in fitting itself for another
function, water transport for instance; or it may divide and
by so doing increase its surface area in relation to its volume,
in which case the cycle may restart. Growth thus can be
interpreted in terms of physical chemistry : the first possibility
mentioned hardly requires contemplation, since nothing is
easier to do than nothing. The third proposition is less
easy to understand ; the second is a mystery. For instance,
why should the daughter of a merismatic cell develop into a
phlcem element if it be cut off on the one side of its parent
and into a xylem element if it be born on the other? Is
it due to some subtle influence or stimulus which has its
origin in the adjacent structural elements; or is it due to
some quality in the cell itself, an heredital predetermination ;
or is it due to some obscure colloidal property comparable
to the Liesegang phenomenon ? *

To these questions there are no real answers ; the facts
must be accepted, their explanation must be left to the future.

The embryo grows and develops into the autotrophic
organism of a form and structure determined by its conditions
of life and by its ancestry and exhibiting those actions and
reactions commonly associated with the higher plants. The
shoots and roots circumnutate and respond to various stimuli,
gravity and light being the most obvious. With respect to
circumnutative and other autonomous movements, these may
be explained by such conceptions as rectipetality and associ-
ated engrams ; whilst in explanation of tropisms various
mechanistic hypotheses have been formulated ; some chemical,
Czapek's explanation of gravitational stimulus of roots,f for
instance ; others physical, the statolith theory, for example.

The highly organized root system by means of its root
hairs takes up raw material by osmosis in the form of water
and its dissolved salts; in special cases, possibly in all, the
osmotic strength of the cell sap of the root hairs is continu-
ously adapted and is nicely adjusted to the osmotic strength
of the soil water. From the root hairs water is passed on
* See Vol. I., p. 301. f Vol. I., p. 361.

I *



4 THE LIVING PLANT

through the cortex to the water-conducting elements of the
vascular cylinder, and this supplies the shoot system. The
shoot system, no less highly organized, is, in the first instance,
concerned with the manufacture of food, carbohydrate, fat and
protein. In this connection the leaf, a marvel of organization
with its chlorophyll apparatus supported by the network
of veins which also are the conduits for the conveyance of fluid
raw materials and for the elaborated products, and with its
mechanism for the regulation of gaseous interchange is the
great synthetic factory, building up food apparently with the
greatest ease and certainly with remarkable rapidity.

In due season reproduction takes place. Of the problems
here involved the secretion of nectar, when it obtains ; the facts
of fertilization and the stimuli which invoke the segmentation
of the egg ; the transmission of hereditary characters ; the
reconstruction of the food destined for the use of the off-
spring ; and the mechanisms of dispersal, are of fundamental
importance.

Of the various aspects of the life of the plant outlined in
the foregoing rather breathless account, it is appropriate on
the present occasion to consider those associated with meta-
bolism, the making of food and the procurement of energy.
The various laws and conceptions involved have either been
explained in the first volume of this work or are considered
as circumstances demand in the following chapters. To this
there is one exception : the determination of the concentration
of the hydrogen ion is a very delicate measure of the reaction,
acid or alkali, of a fluid and is invaluable in investigations
where exact comparisons are required. An explanation of
the principles involved is given here rather than on the occasion
of its first mention in the following pages where it would
unduly interrupt the narrative.

THE HYDROGEN ION CONCENTRATION.

A normal solution of any acid or salt is defined as one
containing one gram of hydrogen or its equivalent dissolved
in one litre of water. According to this definition, the weights
of hydrochloric, nitric, acetic and any other monobasic acid
contained in a litre of normal acid would be the respective
molecular weights in grams, namely HC1 = 36-5, HNO 3 = 63,



HYDROGEN ION CONCENTRATION 5

CH 3 COOH = 60. In the case of a dibasic or tribasic acid, it
would be the molecular weight divided by two or by three ;
H 2 SO 4 H 3 PO 4

-^ 4 = 49, -* = 32-6.

From this reasoning it follows that whilst normal solutions of
all these acids contain in the litre different quantities of acid,
they all contain the same quantity of hydrogen, namely, I gram
per litre. Whilst, however, they all contain potentially the
same amount of hydrogen, it does not follow that the whole of
this quantity is ionized ; and inasmuch as the actual acidity of
a solution at any given moment is measured by the proportion
of ionized hydrogen atoms it contains, it follows that the actual
acidity of equinormal solutions of these various acids may be
very different. This does not mean that treir actual titratable
value, as measured by their power of neutralizing alkali, will
be different Thus for the complete neutralization of the I
gram of hydrogen contained in I litre of each of the above
mentioned normal solutions, exactly the same quantity of
caustic soda will be required, namely 40 grams.

HCl + NaOH = NaCl + H 2 O
36-5 40

CH 3 COOH + NaOH = CH 3 COONa + H 2 O.
60 40

As a matter of fact, hydrochloric acid, which is a strong acid,
is almost entirely ionized in dilute solutions, whilst acetic acid
in solutions of equivalent strengths is ionized to a much smaller
degree. In actual figures, about 97 per cent of the hydrogen
in a -ooiN solution of hydrochloric acid is ionized and only
about 84 per cent in a O'lN solution, whilst in -ooiN acetic
acid not more than 13-6 per cent of the hydrogen is ionized.
Thus in the case of these two acids of the same normality,
although the total amount of titratable hydrogen, as determined
by the alkali-neutralizing power, is found to be the same, the
actual percentage of ionized hydrogen is seven times as great
in the case of the hydrochloric as in that of the acetic acid.

That the comparatively feebly ionized acetic acid ultimately
requires the same amount of alkali for neutralization as the
more strongly ionized hydrochloric, is due to the fact that as
the ionized hydrogens in the acetic acid are neutralized a fresh
quantity of previously un-ionized hydrogens become ionized to



6 THE LIVING PLANT

take the place of those which have been neutralized, and so on
until all have been satisfied.

In practice it is the ionized hydrogen only which is respons-
ible for the acidity of a solution at any given moment and so
it comes about that the hydrogen ion concentration for a solution
is, for biochemical purposes, a much more valuable criterion of
the actual conditions prevailing in any given circumstances than
is the potential alkali neutralizing power.

The concentration of hydrogen ions may be expressed as
follows. In a decinormal solution of hydrochloric acid there
would be O'l gram in 1000 c.c., presuming it to be completely
ionized. In actual fact, however, a decinormal solution of
hydrochloric acid is only ionized to the extent of 97 per cent.,
consequently the concentration is only cr I x 97 or 97 x io~ 2 .
This concentration is more conveniently expressed as a
logarithm: Iog 10 97 = -9868, wherefore 97 x io~ 2 = io' 9868 ~ 2
= icr 1 ' 01 . It has been agreed to express hydrogen ion con-
centration as the exponent to the base 10 of the concentration
with the negative sign omitted, and this is represented by the
symbol P H . Hence the hydrogen ion concentration of the
above N/io hydrochloric acid would be P H = I'Oi ; if com-
pletely ionized it would be P H = I.

On this principle the following are synonymous methods
of expression :

N = . , = P o - ?! - = N x 10-6 = P 6

io u 1,000,000

= N x lo- 1 = P H i - 5 - = N x io-7 = p 7
10 10,000,000

= N x io- 2 = P 2 - - - = N x io- 8 = P 8

100 H 100,000,000 H

= N x io-3 = p 3 - N -

1000 H J 1,000,000,000

N x io- 4 = P 4 - - - = N x io- 10 = P io

^



10,000 H^ 10,000,000,000

N



= N x io-



100,000

It will be seen from the above that the greater the value of
PH, the lower is the actual hydrion concentration. Moreover,
it is an established fact that the product of the concentrations
of the hydrogen and hydroxl ions in any given solution, is a
constant, namely

C H X COH = 10- 14 ' 14



INDICATORS 7

and consequently at exact neutrality, when the concentrations
of the two are exactly equal, C H = icr 7 ' 07 and CQH = io~ 7 ' 07
whence it follows that for absolute neutrality, in which the
concentration of hydrogen ions is exactly equal to that of the
hydroxyl ions P H = 7*07.

There is no need to determine the OH ion concentration
since it is easily found from the difference between 14*14 and
the hydrion concentration.

Thus for P H i the hydroxyl ion concentration would be
P OH I3'I4, and for PHIO it would be P OH 4'I4.

Since for Pn7'O7 there is exact equality between H and
OH ions, it follows that on either side of this value one or
other will be in excess. Thus values of P H below 7 '07 indicate
acid solutions, while values of P H above 7-07 are alkaline.

The most accurate method of determining PH is the
electrical method depending upon conductivity determinations.
For practical purposes, however, a colorimetric method has
been devised depending upon the fact that a series of indicators
have been found whose colours depend upon the prevailing
P H and which are sensitive to changes in P H within certain well
defined limits.

Taking for example the commonly used indicators, the range
for methyl orange is from P H 3 "I to Pn4'4 red to yellow,
litmus PnS'4 to P H 7'8 red to blue,

phenolphthalein Pn8'3 to P H IO colourless to red.

It will be seen from this table that owing to the fact that
these indicators each have their clearly defined range of
sensitiveness, it follows that one and the same liquid, such as
urine, with a P H 5 may have an alkaline reaction to methyl
orange and yet be acid to litmus or phenol phthalein, and for
the same reason a solution which is neutral to litmus may still
be acid to phenolphthalein. This is well illustrated by the
fact that many media which require to be neutralized previous
to use require more alkali for neutralization if phenol-phthalein
is used as indicator than if litmus be employed.

Within recent years the importance of hydrogen ion
concentration to the well being and growth of plants has been
more and more recognized.

In most living organisms provision is made for securing
that the P H of the medium shall not be easily disturbed ; this



8 THE LIVING PLANT

is effected by the presence of certain salts such as the
phosphates of the alkali metals or sodium bicarbonate, etc.
These salts exert what is known as a Buffer action in counter-
acting any considerable increase in P H on the introduction into
the solution of a small quantity of acid. This principle may
be illustrated as follows. If a single drop of dilute hydrochloric
acid is added to a quantity of distilled water, the P H of this
water, which should be 7*07, may be very considerably altered,
and the same would apply if instead of pure water, a dilute
solution of sodium chloride had been used. If, however, the
water had contained, in the place of the sodium chloride, an
equivalent amount of sodium phosphate, the effect of the
addition of the hydrochloric acid would merely have been to
displace a corresponding amount of feebly ionized phosphoric
acid whereby the P H would have been hardly altered at all.
This may be expressed by saying that sodium chloride has no
buffer action whereas sodium phosphate and the salts of other
feeble acids, such as boric, citric and amino acids, have strong
buffer action.

The blood, as a typical physiological fluid, is provided with
a complex system of sodium phosphate and bicarbonate which
has a most efficient buffer action preventing the fluid from
having its P H appreciably altered in the event of the sudden
abnormal development of acid.

Acting upon this principle, standard solutions of known P H
are best made from suitable concentrations of salts of known
marked buffer action ; such solutions may be kept without
fear of alteration through contamination with atmospheric car-
bon dioxide or alkali from the glass bottle, whereas solutions
made from salts with little or no buffer action would rapidly
alter and be useless.

In practise it is found convenient to keep a number of such
standard buffer solutions of known P H for the purpose of
determining the P H of a given liquid by comparison of the
colours given with the same indicator. For this purpose a
small quantity of the liquid under examination is treated with
a few drops of the appropriate indicator and its colour is
matched against that buffer solution which gives the closest
approximation to its own with the same indicator. It should
be noted that the indicators employed in this work are sensitive






SENSITIVITY OF INDICATORS 9

only over a certain range of P H , say from Pn2*8 to Pn4'6 for
bromphenol blue and from Pn4'4 to PH^'O for methyl red,
and from P H 6 to P H 7 '6 for bromthymol blue and so on ; hence
if no match was obtained with one indicator, the P H of the
solution lies.outside that range and another indicator has to be
employed until the correct one has been found and the P H
fixed with the greatest possible degree of accuracy. With a
little practise it becomes possible to detect differences of O 1 ! in
the value of the P H .

The applications of the above principles to the problems
of soil chemistry will be obvious, and it will be realised that
some of the soil constituents have marked buffer action.



CHAPTER II.
THE SYNTHESIS OF FATS.

IN the plant fats are commonly associated with the reserve
food in seeds, spores, and vegetative perennating organs. As
a food they have considerable value in that on physiological
combustion they yield more energy weight for weight than
either protein or carbohydrate, the relative energy producing
values being roughly 5:3:2 respectively.

A consideration of the salient features regarding the physio-
logical significance of fats has been given in the first volume,*
in which place the consideration was retained for the sake of
a closer association of the characterization and other features
of fats with the physiological problems involved. For this
reason a general survey only, with such departures into detail
as necessity demands, will be given on the present occasion.

The fat characteristic of a plant makes its appearance in
the storage organ as that member approaches maturity. The
problem immediately presented is the origin of the fat. There
can be but little doubt that it is formed in situ from materials
in the immediate neighbourhood, a fact which is emphasized
by many observations on the development of fats in seeds
isolated from the plant whilst in an immature condition and
originally containing little or no fat.

Within the storage organ, the fat may have its origin in
the products of the hydrolysis of fat synthesized in some other
organ, the leaf for example, in much the same way as the
starch in the potato tuber has its ultimate origin in the carbo-
hydrate produced in the leaf. Possibly the best ascertained
instance of the normal occurrence of fat, or fat-like substances,
in green assimilating organs is that provided by Vaucheria.

With regard to the higher plants, the facts relating to the
occurrence of fats and kindred substances in the leaves do not

* Vol. I., p. 36. See also Terroine : " Ann. Sci. Nat. Bot.," 1919, se"r. x. I, i.

10



ORIGIN OF FATS



II



permit the drawing of a definite conclusion in regard to the
present problem, and, in fact, there would appear to be no
evidence to warrant the conclusion that fats have a synthetic
origin from raw materials in the same sense as carbohydrates.
For this reason their origin must be sought out elsewhere :
the proteins, the carbohydrates, and glycerol and fatty acid
are amongst the more obvious sources.

With regard to the proteins, evidence is not wanting that
they, under certain conditions, may be converted into fats;
but the evidence relates to the animal rather than to the plant.

WALNUT.


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