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.” 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. 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.
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.