Effect of Zinc Compounds 45 



3. Stimulation induced by zinc compounds. 



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. One investigator, Gustavson, was somewhat in advance of 

 his time, for as long ago as 1881 he hinted at the possibility that zinc, 

 aluminium and other substances might act as stimulants or rather as 

 accelerators. He indicated that the r61e 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. 



This work was apparently not followed up immediately, but it 

 evidently contains the germ of the " catalytic " hypothesis of which so 

 much has been made during recent years. 



The work dealing with zinc as a stimulant to plant growth has 

 yielded such various and apparently contradictory results that the 

 question cannot yet be regarded as settled — it is even still more or less 

 uncertain whether zinc compounds act as stimulants, or whether they 

 are merely indifferent at concentrations below the toxic doses. 



(a) Stimulation in water cultures. 



True and Gies (1903) suspended seedlings of Lupinus alius for 24 — 48 

 hours with their roots in solutions of zinc sulphate and calcium sulphate 

 (m/256)', and found that while zinc sulphate alone at m/8192 retarded 

 growth, yet with m/2048 ZnS04 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 modifi- 

 cations 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. 



' m probably = gram molecular weight. 



