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CAMBRIDGE AGRICULTURAL MONOGRAPHS
INORGANIC PLANT POISONS AND STIMULANTS
CAMBRIDGE UNIVERSITY PRESS
C. F. CLAY, MANAGER
London: FETTER LANE, E.C.
Edinburgh: 100 PRINCES STREET
[Colophon]
London: H. K. LEWIS, 136 GOWER STREET, W.C.
London: WILLIAM WESLEY AND SON, 28 ESSEX STREET, STRAND
New York: G. P. PUTNAM’S SONS
Bombay and Calcutta: MACMILLAN AND CO., LTD.
Toronto: J. M. DENT AND SONS, LTD.
Tokyo: THE MARUZEN-KABUSHIKI-KAISHA
_All rights reserved_
INORGANIC PLANT POISONS
AND STIMULANTS
BY
WINIFRED E. BRENCHLEY, D.SC., F.L.S.
Fellow of University College, London
(Rothamsted Experimental Station)
Cambridge:
at the University Press
1914
Cambridge:
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
PREFACE
During the last century great and widespread changes have been made in
agricultural practice--changes largely associated with the increase in the
use of artificial fertilisers as supplements to the bulky organic manures
which had hitherto been used. The value of certain chemical compounds
as artificial manures is fully recognised, yet many attempts are being
made to prove the value of other substances for the same purpose, with
a view to increase in efficiency and decrease in cost. The interest in
the matter is naturally great, and agriculturists, botanists and chemists
have all approached the question from their different standpoints. In the
following pages an attempt is made to correlate the work that has been
done on a few inorganic substances which gave promise of proving useful
in agricultural practice. Much of the evidence put forward by different
workers is conflicting, and it is clear that no definite conclusions can
yet be reached. Nevertheless, examination of the evidence justifies the
hope that results of practical value will yet be obtained, and it is hoped
that the analysis and coordination of the available data put forward in
this book will aid in clearing the ground for those investigators who
are following up the problem from both the academic and the practical
standpoints.
W. E. B.
ROTHAMSTED.
_October 1914._
CONTENTS
CHAP. PAGE
I. INTRODUCTION 1
II. METHODS OF WORKING 7
I. Discussion of Methods 7
1. Water cultures 7
2. Sand cultures 8
3. Soil cultures in pots 9
4. Field experiments 9
II. Details of Methods 10
III. EFFECT OF COPPER COMPOUNDS 15
I. Presence of Copper in Plants 15
II. Effect of Copper on the Growth of Higher Plants 17
1. Toxic effect 17
(a) Toxic action of copper compounds alone in water
cultures 17
(b) Masking effect caused by addition of soluble
substances to solutions of copper salts 20
(c) Effect of adding insoluble substances to
solutions of copper salts 22
(d) Effect of copper on plant growth when present
in soils 24
(e) Mode of action of copper on plants 25
2. Effect of copper on germination 27
(a) Seeds 27
(b) Spores and pollen grains 28
3. Does copper stimulate higher plants? 28
4. Action of copper on organs other than roots 30
(a) Effect of copper sprays on leaves 30
(b) Effect of solutions of copper salts on leaves 32
III. Effect of Copper on Certain of the Lower Plants 33
Conclusion 35
IV. EFFECT OF ZINC COMPOUNDS 36
I. Presence of Zinc in Plants 36
II. Effect of Zinc on the Growth of Higher Plants 38
1. Toxic effect 38
(a) Toxic action of zinc salts alone in water
cultures 38
(b) Effect of soluble zinc salts in the presence of
nutrients 39
(c) Effect of zinc compounds on plant growth when
they are present in soils 41
(d) Mode of action of zinc on plants 43
2. Effect of zinc compounds on germination 43
3. Stimulation induced by zinc compounds 45
(a) Stimulation in water cultures 45
(b) Stimulation in sand cultures 46
(c) Increased growth in soil 46
4. Direct action of zinc salts on leaves 47
III. Effect of Zinc on Certain of the Lower Plants 48
Conclusion 50
V. EFFECT OF ARSENIC COMPOUNDS 51
I. Presence of Arsenic in Plants 51
II. Effect of Arsenic on the Growth of Higher Plants 52
1. Toxic effect 52
(a) Toxic action of arsenic compounds in water
cultures in the presence of nutrients 52
(b) Toxic effect of arsenic compounds in sand
cultures 57
(c) Toxic effect of arsenic when applied to soil
cultures 57
(d) Physiological considerations 59
2. Effect of arsenic compounds on germination 60
3. Do arsenic compounds stimulate higher plants? 61
III. Effect of Arsenic Compounds on Certain of the
Lower Plants 62
1. Algae 62
2. Fungi 63
Conclusion 64
VI. EFFECT OF BORON COMPOUNDS 65
I. Presence of Boron in Plants 65
II. Effect of Boron on the Growth of Higher Plants 67
1. Toxic effect 67
(a) Toxic action of boron compounds in water
cultures 67
(b) Toxic action of boron compounds in sand
cultures 70
(c) Toxic action of boron compounds in soil
experiments 71
2. Effect of boron compounds on germination 72
3. Does boron stimulate higher plants? 73
(a) Water cultures 73
(b) Sand cultures 73
(c) Soil cultures 74
III. Effect of Boron Compounds on Certain of the Lower
Plants 76
Conclusion 77
VII. EFFECT OF MANGANESE COMPOUNDS 78
I. Presence of Manganese in Plants 78
II. Effect of Manganese on the Growth of Higher Plants 81
1. Toxic effect 81
(a) Toxic action of manganese compounds in the
presence of soluble nutrients 81
(b) Toxic action of manganese compounds in sand
cultures 82
(c) Toxic action of manganese compounds in soil
cultures 82
2. Effect of manganese compounds on germination 84
3. Does manganese stimulate higher plants? 84
(a) Stimulation in water cultures 85
(b) Stimulation in soil cultures 86
III. Effect of Manganese Compounds on Certain of the
Lower Plants 90
IV. Physiological Considerations of Manganese
Stimulation 90
Conclusion 92
VIII. CONCLUSIONS 93
BIBLIOGRAPHY 97
INDEX OF PLANT-NAMES 107
GENERAL INDEX 109
LIST OF ILLUSTRATIONS x
LIST OF ILLUSTRATIONS
FIG. PAGE
1. Sketch illustrating water culture methods 12
2. Photograph. Barley grown with copper sulphate _To face_ 20
3. Curve. Ditto 21
4. Photograph. Peas grown with copper sulphate _To face_ 29
5. Curve. Ditto 29
6. Curve. Barley grown with zinc sulphate 40
7. Photograph. Peas grown with zinc sulphate _To face_ 40
8. Curve. Ditto 41
9. Photograph. Barley grown with arsenious acid _To face_ 54
10. Curve. Ditto 55
11. Curve. Peas grown with arsenious acid 55
12. Curve. Barley grown with arsenic acid 56
13. Curve. Barley grown with sodium arsenite 56
14. Curve. Peas grown with sodium arsenite 57
15. Curve. Barley grown with boric acid 69
16. Photograph. Peas grown with boric acid _To face_ 69
17. Curve. Barley grown with manganese sulphate 85
18. Photograph. Ditto _To face_ 86
19. Photograph. Peas grown with manganese sulphate 86
CHAPTER I
INTRODUCTION
Ever since the physiological side of botany began to emerge from
obscurity, the question of the relation between the nutrition and the
growth of the plant has occupied a foremost position. All kinds of
theories, both probable and improbable, have been held as to the way in
which plants obtain the various components of their foods. But quite early
in the history of the subject it was acknowledged that the soil was the
source of the mineral constituents of the plant food, and that the roots
were the organs by which they were received into the plant.
A new chapter in the history of science was begun when Liebig in 1840
first discussed the importance of inorganic or mineral substances in plant
nutrition. This discussion led to a vast amount of work dealing with the
problem of nutrition from many points of view, and the general result
has been the sorting out of the elements into three groups, nutritive,
indifferent, and toxic. Thus calcium, phosphorus, nitrogen and potassium
are classed as nutritive, arsenic, copper and boron as toxic, and many
others are regarded as indifferent.
Closer examination, however, shows that this division into three classes
is too rigid. Now that experiments are more refined it has become evident
that no such simple grouping is possible. It has been found that typical
nutrient salts are toxic when they are applied singly to the plant in
certain concentrations, the toxic power decreasing and the nutritive
function coming into play more fully on the addition of other nutrient
salts. For instance, Burlingham found that the typical nutrient magnesium
sulphate in concentrations above _m_/8192 (_m_ = molecular weight) is
toxic to most seedlings, the degree of toxicity varying with the type
of seedling and the conditions under which growth takes place. It will
be shown in the following pages that even such a typical poison as
boric acid may, under suitable conditions, increase plant growth just
as if it were a nutrient. A review of the whole subject leads one to
conclude that in general both favourable and unfavourable conditions of
nutrition are present side by side, and only when a balance is struck
in favour of the good conditions can satisfactory growth take place. As
indicated above, experiments have shown that the very substances that are
essential for plant food may be, in reality, poisonous in their action,
exercising a decidedly depressing or toxic influence on the plant when
they are presented singly to the roots. This toxic action of food salts
is decreased when they are mixed together, so that the addition of one
toxic food solution to another produces a mixture which is less toxic than
either of its constituents. Consequently a balanced solution can be made
in which the toxic effects of the various foods for a particular plant
are reduced to a minimum, enabling optimum growth to take place. Such
a mixture of plant foods occurs in the soil, the composition of course
varying with the soil.
While the earliest observations set forth the poisonous action of various
substances upon plants, it was not long before investigators found
that under certain conditions these very substances seemed to exert a
beneficial rather than an injurious action. The poisons were therefore
said to act as “stimulants” when they were presented to the plant in
sufficiently great dilution. This stimulation was noticed with various
plants and with several poisons, and a hypothesis was brought forward that
attempted to reconcile the new facts with the old conceptions. Any poison,
it was suggested, might act as a stimulant, if given in sufficiently small
doses. It will be seen in the following pages that this is not universally
true, such substances as copper, zinc, and arsenic failing to stimulate
certain plants even in the most minute quantities so far tested.
Of recent years investigators in animal physiology have brought into
prominence the striking effect of minute quantities of certain substances
in animal nutrition, as for example iodine in the thyroid gland (see
E. Baumann, 1895). This and other work has rendered it imperative to
re-examine the parallel problems in plant physiology.
The words “stimulant” and “stimulation” themselves need more precise
definition. As a matter of fact the “stimulation” noticed by one observer
is not necessarily held to be such by another. Stimulation may express
itself in various ways--the green weight and the general appearance of
the fresh plant may be improved, the dry weight may be increased, the
transpiration current may be hurried up, entailing increased absorption
of water and food substances by the roots, assimilation processes may be
encouraged. But these benefits are not of necessity correlated with one
another, e.g. a plant treated with a dilute solution of poison may look
much healthier and weigh far more in the green state than an untreated
plant, whereas the latter may prove the heavier in the dry state. To a
market gardener to whom size and appearance is so important, stimulation
means an improvement in his cabbages and lettuces in the green state, even
though the increased weight is chiefly due to additional water absorbed
under the encouragement of the stimulative agent, whereas to a scientific
observer, the dry weight may give a more accurate estimate of stimulation
in that it expresses more fully an increased activity in the vital
functions of the plant whereby the nutritive and assimilative processes
have gone on more rapidly, with a consequent increase in the deposition
of tissue.
While stimulation expresses itself in the ways detailed above poisoning
action also makes itself visible to the eye. Badly poisoned plants
either fail to grow at all or else make very little or weak growth. Even
when less badly affected the toxic action is well shown in some cases
by the flaccidity of the roots, and in others by the formation of a
“strangulation” near the crown of the root, which spreads to the stem,
making it into a thin thread, while the leaves usually wither and die. If
such plants as peas are able to make any shoot growth at all the roots
show signs of a desperate attempt to put forth laterals. The primary
root gets much thickened and then bursts down four sides, the tips of the
laterals all trying to force their way through in a bunch, but failing to
do so on coming in contact with the poison. Most curious malformations
of the root arise from this strong effort of the plant to fight against
adverse circumstances.
While all the inorganic substances examined in this monograph are
toxic in high concentrations, some lead to increased growth in lower
concentrations, while others apparently have no effect. In this sense all
substances could be classed as toxins, even the nutrients. Thus the old
distinction between toxin and nutrient has now lost its sharpness, but
it does not lose all its significance. The old “nutrients” had certain
definite characters in common, in that they were essential to plant
growth, the growth being in a great degree proportional to the supply,
a relatively large amount of the nutrients being not only tolerated but
necessary. The substances dealt with more particularly in this book have
none of these characters. Even those that cause increased growth do not
appear to be essential, at any rate not in the quantities that potassium,
phosphorus, nitrogen, &c., are essential, while there is no evidence that
growth is proportional to supply. The substances fall into two groups:
(1) Those that apparently become indifferent in high dilutions and never
produce any increase in plant growth.
(2) Those that cause a small, but quite distinct, increased growth when
applied in quantities sufficiently small.
The former group may be legitimately regarded as toxins; the latter
present more difficulty and even now their function is not settled. It
is not clear whether they stimulate the protoplasm or in some way hasten
the metabolic processes in the plant, whether they help the roots in
their absorbent work, or whether they are simple nutrients needed only
in infinitesimal quantities. The two groups, however, cannot be sharply
separated from one another. Indeed a substance may be put into one of
these classes on the basis of experiments made with one plant alone and
into another when a different plant is used, while it is quite conceivable
that further experiments with other plants may abolish the division
between the two groups altogether. It is even impossible to speak rigidly
of toxicity. The addition of the inorganic food salts to solutions of a
poison reduces the toxicity of the latter, so that the plant makes good
growth in the presence of far more poison than it can withstand in the
absence of the nutrients. This masking effect of the inorganic food salts
upon the toxicity of inorganic plant poisons is paralleled by a similar
action on organic toxic agents. Schreiner and Reed (1908) found that the
addition of a second solute to a solution decreases the toxicity of that
solution; further the plant itself may exercise a modifying influence upon
the toxic agent. Water culture experiments were made upon the toxicity
of certain organic compounds, with and without the addition of other
inorganic salts. Arbutin, vanillin, and cumarin were definitely toxic and
the toxicity decidedly fell off after the addition of sodium nitrate and
calcium carbonate, especially with the weaker solutions of the toxins.
Curiously enough, while weaker solutions of vanillin alone produced
stimulation, the stimulating effect of this toxic agent disappeared
entirely on the addition of the inorganic substances. The results showed
that the addition of certain inorganic salts to solutions of toxic organic
compounds was decidedly beneficial to the plant.
Another important problem has come to the front with regard to these
toxic substances--How do these substances get into the plant? Are they
all absorbed if they occur in the soil, or is there any discriminatory
power on the part of the root? In other words, do the roots perforce take
in everything that is presented to their surfaces, or have they the power
of making a selection, absorbing the useful and rejecting the useless and
harmful?
Daubeny (1833) described experiments in which various plants, as radish,
cabbage, _Vicia Faba_, hemp and barley were grown actually on sulphate
of strontium or on soils watered with nitrate of strontium. No strontium
could be detected in the ash of any of the plants save barley, and then
only the merest trace was found. Daubeny concluded that the roots _were_
able to reject strontium even when presented in the form of a solution.
“Upon the whole, then, I see nothing, so far as experiments have yet
gone, to invalidate the conclusion ... that the roots of plants do, to a
certain extent at least, possess a power of selection, and that the earthy
constituents which form the basis of their solid parts are determined as
to _quality_ by some primary law of nature, although their _amount_ may
depend upon the more or less abundant supply of the principles presented
to them from without.” Some years after, in 1862, Daubeny reverted to
the idea, stating “I should be inclined to infer that the spongioles of
the roots have residing in them some specific power of excluding those
constituents of the soil that are abnormal and, therefore, unsuitable
to the plant, but that they take up those which are normal in any
proportions in which they may chance to present themselves[1].” This,
however, was not held to apply to such corrosive substances as copper
sulphate. De Saussure had found that _Polygonum Persecaria_ took up copper
sulphate in large quantities, a circumstance which he attributed to the
poisonous and corrosive quality of this substance, owing to which the
texture of the cells became disorganised and the entrance of the solution
into the vegetable texture took place as freely, perhaps, as if the plants
had been actually severed asunder[2]. Daubeny concluded that a plant is
unable to exclude poisons of a corrosive nature, as this quality of the
substance destroys the vitality of the absorbing surface of the roots and
thus reduces it to the condition of a simple membrane which by endosmosis
absorbs whatever is presented to its external surfaces, so that whenever
abnormal substances are taken up by a living plant it is in consequence
of some interference with the vital functions of the roots caused in the
first instance by the deleterious influence of the agent employed.
[1] This idea of a selectivity of the roots has been recently
revived by Colin and Lavison (1910) who found that when peas
were grown in the presence of barium, strontium or calcium
salts no trace of barium could be found in the stem, strontium
only occurred in small quantities, while calcium was present in
abundance. They concluded that apparently salts of the two latter
alkaline metals could be absorbed by the roots and transferred
to the stem and other organs, but that this is not the case with
salts of barium. They obtained similar results with other plants,
beans, lentils, lupins, maize, wheat, hyacinth. Their proof is
not rigid, and exception could be taken to it on chemical grounds.
[2] Vide Daubeny, _Journ. Chem. Soc._ (1862), p. 210.
In spite of the enormous amount of work that has been done on this subject
of toxic action and stimulation it is yet too early to discuss the matter
in any real detail. A voluminous literature has arisen around the subject,
and in the present discussion some selection has been made with a view
to presenting ascertained facts as succinctly as possible. No attempt
has been made to notice all the papers; many have been omitted perforce;
it would have been impossible to deal with the matter within reasonable
length otherwise. A full and complete account would have demanded a
ponderous treatise. This widespread interest on the part of investigators
is fully justified, as the problems under discussion are not only of
the highest possible interest to the plant physiologist, but hold out
considerable promise for the practical agriculturist.