In the foregoing chapters a very limited number of plant poisons have been considered, yet there is sufficient evidence to show that even these few differ considerably in their action upon plant-life. This action is most variable, and it is impossible to foretell the effect of any substance upon vegetative growth without experiments. The degree of toxicity of the different poisons is not the same, and also one and the same poison varies in the intensity and nature of its action on different species of plants. While certain compounds of copper, zinc and arsenic are exceedingly poisonous, compounds of manganese and boron are far less deleterious, so that a plant can withstand the presence of far more of the latter substances than of the former. Again, the tested compounds of copper, zinc and arsenic do not seem to stimulate growth, even when they are applied in the smallest quantities, whereas very dilute solutions of manganese and boron compounds decidedly increase growth. But, differentiation occurs even in this stimulative action, for while manganese is the more effective in stimulating barley, boric acid is far more potent for peas, the shoots being particularly improved.

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. It is probable that what has been found true with the few substances tested would prove to be similarly true over a much wider range of poisons, and at any rate the hypothesis must be dismissed in its universal application. A more accurate statement would be that some inorganic poisons act as stimulants when present in small amounts, the stimulating concentrations varying both with the poisons used and the plants on which they act.

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.

Thus it is evident that it is exceedingly difficult sharply to characterise either toxic or stimulant action. In neither case is the reaction simple—many factors may come into play and many processes are concerned, while the effect of a so-called poison may vary in respect of each of the functions and processes concerned. If the poison is presented in great strength the toxic action is dominant, and probably affects many functions in the same sense, so that the action is, so to speak, cumulative. Lower down in the scale of concentration differentiation of action may set in, and while some processes may still be hindered, others may be stimulated. If the two actions balance one another an apparent indifference may be manifested, so that it seems that such strengths of the poison have no effect on growth, either harmful or beneficial. At still lower concentrations, with certain plants and certain poisons, the stimulative action overpowers the toxic effect, so that in some respect or other improvement occurs in growth.

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.

The modus operandi of these stimulative agents is not yet fully understood. Perhaps at the present time two main theories hold the field: (1) that they act as catalytic agents, being valueless on their own account, but valuable in that they aid in the procuring of essential food substances; (2) that the stimulants themselves are of integral value for nutrition. The French school, with Bertrand at the head, hold strongly to the catalytic theory, maintaining that manganese and boron compounds are able to increase growth if they are present in small quantities, as they act as “carriers” whereby the various functions of the plant are expedited by the increased facility with which the essential nutritive elements are supplied. The manganese in laccase, for instance, is held to be an oxygen carrier, whereby the oxygen is first absorbed and then released for the benefit of the plant, the manganese being regarded as essential for the functioning of the enzyme. But, if these elements are essential, this theory seems to stop short of the truth. If certain functions are dependent for their very occurrence upon the presence of even minute traces of any element, then surely that element is as essentially a nutrient element, as vital to the well-being of the plant as is such an element as carbon or nitrogen or phosphorus, even though the latter occurs in far greater quantity. It is necessary that one should free one’s mind from the idea that the quantity of an element present in a plant is an index of its value to the plant. Naturally enough, in the early days of plant physiology, the most abundant elements first engaged the attention of investigators, and they were divided into essential and non-essential, ten elements being classed in the former category. More recent work is beginning to show that other elements are constantly present in plants, but in such small quantities that the older and cruder methods of analysis failed to reveal them, so that until lately they have been completely overlooked in work on plant nutrition. Even yet we do not know which of these other elements are essential and which are merely accidental. 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. If one takes a broad outlook, the two theories seem to be in reality only parts of one, the “nutrition” theory carrying matters a little farther than the “catalytic” idea, broadening its scope and extending its application.

It seems probable that all the experimental work that has been discussed will prove to be simply preliminary to a far greater practical application of the principle of stimulation or increased growth. While the physiologists have been feeling their way towards the conclusions put forth on this subject, the agriculturists have been discovering and extending the application of artificial manures, until at the present time such manuring is coming into its own and is receiving more of the widespread attention that it deserves. The possibility now exists that in some respects the two lines of work are converging and that the more purely scientific line will have a big contribution to make to the strictly practical line. Artificial manuring aims at improvement of the soil and crop by the addition of food substances that are needed in a particular soil, a result that used to be obtainable only by the use of the bulky farmyard manure, seaweed, &c. Apart from any other aspect of the matter the artificials, when intelligently used, are far more easy to handle and to regulate in supply, and they yield excellent results, especially in conjunction with a certain proportion of organic manures. The further prospect now opened up is the possibility of utilising some of these stimulating compounds as artificial manures. As only small traces are beneficial, larger amounts being poisonous, it is obvious that only small quantities would be needed, and, as the compounds are not usually very expensive, a considerable increase of crop for a relatively small outlay might be anticipated if no complicating factors intervened. Very much work will be required in the field to test the value of these substances, as their action may be influenced by the nature of the soil, climatic conditions, general conditions of manuring, and the crops grown. Some tests have already been made, especially in Japan, with boron and manganese, and these indicate a promising field for investigation.

Above all, it is most important to realise that one is approaching an entirely unexplored field, and that it is inevitable that the results of the initial experiments will be contradictory, at least in appearance, so that it is necessary to keep an open mind on the subject, being ready to modify one’s ideas as circumstances require, as improved experimental methods lead on to more accurate results.