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. [7] _m_ probably = gram molecular weight. 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._