Winifred Elsie Brenchley

“ No one can have any idea of the composition of the soil even in a single field, as it may vary, sometimes very considerably, at every step. Further, no one knows the complicated action that may or may not occur in the soil on the addition of extraneous substances such as manures or poisons. Altogether, one is working quite in the dark as to knowledge of what is going on round the plant roots. It is impossible to attribute the results obtained to the direct action of the poison applied. ”

“ While we do know that the ten essential elements (C, H, O, N, S, P, K, Mg, Fe, Ca) are necessary for the well-being of all plants, it is conceivable that these other substances which only occur in very small quantities may be more individual in their action, and that while a trace of a certain element may be absolutely essential to one plant, that same element may be quite indifferent for another species. ”

“ 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 [3] 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. ”

“ A solution may be of such a nature and strength that at first growth is seriously checked, though later on some recovery may be made, while it is also possible that a concentration which is apparently indifferent at first may prove more or less toxic or stimulant at a later date, according to circumstances. Consequently too much stress must not be laid upon the results of the short time experiments with regard to the ultimate effect of a poison upon a particular plant. ”

“ The conditions of plant life appertaining to soil and water cultures are totally different, so different that it is impossible to assume that a result obtained by one of the experimental methods must of necessity hold good in respect of the other method. A certain similarity does exist, and where parallel investigations have been carried out this becomes evident, but it seems to be more or less individual, the plant, the poison and the cultural conditions each playing a part in determining the matter. ”

“ With a complex mixture consisting of five salts—copper sulphate and salts of sodium, magnesium, calcium and potassium, all except calcium being present in concentrations strong enough to interfere with growth if used alone—it was shown that “as a result of their presence together, not only is there no addition of poisonous effects, but a neutralisation of toxicity to such degree as to permit in the mixed solutions a growth-rate equal to or greater than that seen in the check culture.” ”

“ These experiments may possibly give some indication as to the action of copper salts on plant roots. So long as the solution of copper salt is dilute enough, the absorption layer of the root, acting as a semi-permeable membrane and upheld by the resistant protoplasm, is able to keep the copper out of the plant and to check its toxicity. As soon as a certain limit is reached the copper exercises a corrosive influence upon the outer layer of the root whereby its functions are impaired, so that it is no longer able efficiently to resist the entry of the poison. ”

“ While the toxic action of zinc on the higher plants is so obvious that it forced itself upon the attention of investigators at an early date, the question of possible stimulus is so much more subtle that it has only come into prominence during the last twelve years, during which time an extraordinary amount of experimental work has been done with regard to it. ”

“ It is quite possible for a stimulation in one respect to be correlated with a retardation in another. In the Rothamsted experiments on the action of manganese sulphate on barley the weaker concentrations of the salt improved the vegetative growth, as was shown by the increase in the dry weights, but with the same strengths of the poison the ripening of the grain was retarded, so that, while certain of the physiological functions were expedited, others were hindered by the action of the poison. ”

“ The only cases in which some spores or pollen grains were more or less harmed were those in which the water had stood over copper for more than two weeks, and even so the deleterious effect was chiefly noticeable when the pollen itself was old or derived from flowers in which the anther formation was nearly at an end. As a rule germination was better in the presence of copper, whether in pure water or food solution, the stimulus being indicated both by the greater number of germinated grains and by the regular and rapid growth of the pollen tubes. ”

“ As matters stand at the present day, it appears that it is still uncertain whether higher plants grown in water cultures are susceptible to stimulation by zinc salts. If a stimulus does exist, it must be at exceedingly great dilutions, but further evidence is needed. In soil cultures, however, the fact of increased growth seems to be more firmly established, certain species responding to zinc salts when used as manure, though no increase has been obtained with other species. It must always be remembered that the action may be an indirect one. ”

“ A consideration of the experimental work that has been done on this subject of poisoning and stimulation leads one to the inevitable conclusion that it is not true to maintain the hypothesis that all inorganic plant poisons act as stimulants when they are present in very small quantities, for while some poisons do increase plant growth under such conditions, others fail to do so in any circumstances. ”

“ 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.” ”

“ Manganese exerts a toxic influence upon the higher plants, if it is presented in high concentration, but, in the absence of great excess of the manganese compounds, the poisoning effect is overshadowed by a definite stimulation. As is the case with boron, manganese stimulates some species more than others, the action on barley being more evident than that on peas. It seems probable that manganese may prove to be an element essential to the economy of plant life, even though the quantity usually found in plants is very small. ”

“ It is quite conceivable, however, that some poisons are truly indifferent in weak concentrations, as no stimulation makes itself evident under any circumstances. In these cases one is inclined to suspect that the action is somewhat more simple, in that the toxic effects gradually diminish until no poisonous action is manifest at very weak concentrations, and as no stimulation is present to bring the growth above the normal with these very weak concentrations the plant is similar to those grown without any addition of the poison. ”

“ A review of the [2] 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. ”

“ No food is available in the water supply from the roots, so the plant is entirely dependent on the stores laid up in the seed and on the carbon it can derive from the air by photo-synthesis carried on by the green leaves. The result of the root checking in these particular cases seems to be so to stimulate the shoots by some physiological action or other, that this process of photo-synthesis is hastened, more carbon being converted into carbo-hydrates, so that the shoot development is increased, yielding a greater weight of dry matter. ”

“ 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. ”

“ Apparently the species of the plants concerned seems to be of less importance for their copper content than is the copper content of the soil. The deposition of copper (in wheat, buckwheat and paprika) is chiefly in the stems and leaves, little being conveyed to the fruits and seeds, so that a high content of copper in the soil does not necessarily imply the presence of much copper in the grain and seed. The metal is variously distributed among the tissues, the bark of the wood being the richest of the aerial parts in that substance. ”

“ Recent experiments at Rothamsted have shown that zinc sulphate is very toxic to barley, though the plant is able to make some slight amount of growth even in the presence of a solution of the anhydrous salt ZnSO4 as strong as 1/5000, rapid improvement occurring as the concentration decreases to 1/2,500,000 or less (Fig. 6) . On the whole the higher strengths of zinc sulphate are less poisonous to peas than they are to barley. ”

“ He indicated that the rôle of certain mineral salts in the plant economy is to enter into combination with the existing organic compounds, the resulting product of the reaction aiding in the formation of yet other purely organic compounds which ordinarily require for their formation either a very high temperature or a long time—in other words, such a mineral salt acts as a kind of accelerator. ”

“ No definite evidence of stimulation has yet been obtained with any arsenic compound, however great the dilution at which it is applied. With certain algae a stimulus may occur, and it is possible that arsenic acid is capable of replacing phosphoric acid to some extent under certain conditions. With fungi the toxic effect of great concentrations is marked with certain species, but there are others which are capable of living happily on arsenical compounds and of liberating highly poisonous arsenic gas. ”

“ While peas grew very well in it, they showed some slight signs of over-nutrition. A weaker solution is being tested which gives very good results. Peas grow very strongly in it and it also seems to be sufficiently concentrated to allow barley to carry on its growth long enough for the purposes of experiment. The solution is as follows:
The latter solution was made up so that the quantity of phosphoric acid and potash approximated more or less to the amount of those substances found by analysis in an extract made from a good soil. ”

“ Cleanliness of apparatus and purity of reagents are absolutely indispensable, as the failure of a set of cultures can often be traced to a slight irregularity in one of these two directions. Purity of distilled water is perhaps the greatest essential of all. Plant roots are extraordinarily sensitive to the presence of small traces of deleterious matter in the distilled water, especially when they are grown in the absence of food salts. ”

“ Do arsenic compounds stimulate higher plants?
The question of stimulation due to arsenic does not seem to have engaged the attention of investigators to any extent. Water culture experiments at Rothamsted have so far yielded negative results, and no stimulation has yet been obtained with any plant, with the possible exception of white lupin with sodium arsenite. In a single series a stimulus was suggested, beginning to make itself felt at 1/500,000, rising to an optimum at 1/10,000,000. ”