(a) Stimulation in water cultures.
[True and Gies (1903)] suspended seedlings of Lupinus albus for 24–48 hours with their roots in solutions of zinc sulphate and calcium sulphate (m/256)[7], and found that while zinc sulphate alone at m/8192 retarded growth, yet with m/2048 ZnSO4 and m/256 calcium sulphate growth was more than twice as rapid as in controls grown in water, indicating a marked stimulation. The presence of the calcium exercised a definite ameliorating influence, reducing the toxicity of zinc to one-sixteenth at most. The hypothesis put forward is that interior physiological modifications are responsible for the observed differences in growth rate, the cell processes being so affected as to bring about different results on cellular growth—i.e. that where mixtures of salts are concerned growth rate represents the physiological sum of oppositely acting stimuli or of antagonistic protoplasmic changes.
[Kanda (1904)] found that peas were stimulated in dilute solutions of zinc sulphate in the absence of nutrients, the optimum concentration being between ·00000287% and ·000001435% (about 1 in 34,840,000 and 1 in 69,700,000), higher concentrations being poisonous when the solutions were changed every four days. [Jensen (1907)] stated that he obtained no stimulation at all with water cultures, even in a solution as dilute as n/100,000 (about 1 in 1,239,000), but he suggested that it was quite possible that in proper concentration the zinc sulphate might prove to be a stimulant.
[Javillier (1910)] grew wheat in nutritive solutions with quantities of zinc salts containing from 1/5,000,000–1/250,000 zinc, and found that the dry weight of the plant was increased in so far as the stems and leaves were concerned, though it remained uncertain whether a similar increase occurred in the grain.
A consideration of the [Rothamsted experiments] shows that up to the present time there is no conclusive evidence that zinc sulphate acts as a stimulant to barley grown in water cultures. As a general rule the growth of those plants with 1/5,000,000 ZnSO4 approximates closely to that of the controls. Beyond this the growth varies in different experiments. In some cases lower concentrations from 1/5,000,000 to 1/50,000,000 seem to cause some slight improvement in comparison with the normal, indicating a possible stimulus, but this improvement is not at all well marked. In other cases these great dilutions are apparently indifferent, neither a poisonous nor a stimulative action being exerted on the growth of the plant ([Fig. 6]). With peas some increase has been obtained with 1/20,000,000, and although the rise is only slight, yet it is possible that it may indicate the setting in of a stimulus which would make itself more strongly felt with still weaker concentrations ([Fig. 7]).
(b) Stimulation in sand cultures.
While [Jensen] denied stimulation in wheat grown in water cultures even when the solutions were as dilute as n/100,000 zinc sulphate, yet he found increase of growth with the same plant in artificial soil (quartz flour) to which much stronger solutions of zinc sulphate, from 5n/10,000–n/10,000, had been added.
(c) Increased growth in soil.
[Nakamura (1904)] dealt with a few plants of agricultural importance, adding ·01 gram anhydrous zinc sulphate to 2300 grams air-dried soil. The marked individuality in the response of the various plants to the poison is very striking. Allium showed signs of increased growth throughout; Pisum was apparently improved in the early stages of growth, but when the dry weights were taken at the end of the experiment no increase manifested itself in the weights of the plants treated with zinc; with Hordeum the same quantity of zinc exercised a consistently injurious action. These results with peas and barley corroborate those obtained in the Rothamsted experiments with water cultures in that zinc sulphate proved to be less toxic to peas than to barley.
[Kanda] found that both peas and beans when grown in soil as pot cultures were improved by larger quantities of zinc sulphate than when they were treated as water cultures—a result in full accordance with current knowledge.