44 Effect of Zinc Compounds 



do not seem to be affected by the peculiar nutritive conditions of the 

 parent plants. 



In certain cases light seems to have something to do with the harm 

 zinc compounds work on plants. Storp found that when clover seeds 

 were germinated in the dark on filter paper moistened with water 

 containing '025 gm. ZnO per litre (added in the form of zinc sulphate) 

 no deleterious action was observed. Barley seeds were soaked for four 

 days in (a) distilled water, (6) water with '9 gm. ZnO per litre, which 

 was frequently changed. These seeds were then placed in the dark on 

 filter papers soaked respectively with water and with the solution con- 

 taining ZnO. So long as no light was admitted, for a period of eleven 

 days, germination was uniform in both sets, but directly the covers were 

 removed the growth of the seeds with zinc ceased almost entirely, and 

 they did not assume the green colour taken on by the unpoisoned 

 seedlings. With maize the germination was retarded by zinc even in 

 the dark, but the harmful action of light on the plants with zinc was 

 again established. These results seem to indicate that the formation and 

 activity of chlorophyll is impaired by the toxic agent, and this hypo- 

 thesis is borne out by the fact that in many fungi and non-assimilating 

 higher plants the toxic action of zinc is not evident. 



Micheels (1906) approached the matter from a totally different 

 standpoint, seeking to discover what influence the valency of a metal 

 has upon the toxicity of its salts. In each of a series of experiments 

 1000 c.c. of | decinormal solution of sodium chloride in pure distilled 

 water were used, with the addition of varying strengths of calcium 

 sulphate. Grains of wheat, which previously had been soaked in distilled 

 water, were placed in the solutions, and it was found that the stronger 

 the calcium sulphate solution (up to ^ normal the limit of experi- 

 ment), the better the growth. The calcium sulphate was then replaced 

 by salts of other bivalent metals, as zinc, lead and barium, with analogous 

 results, the quantity necessary to obtain the maximum development 

 varying with one and another; with zinc, n/128 gave the maximum. In 

 this case the toxic action of both sodium chloride and zinc sulphate on 

 germination were considerably reduced by their mutual presence a 

 result which fits in perfectly with what is known as to the masking 

 effect of soluble substances upon toxic action. The same fact obtains 

 in the animal kingdom, where Loeb and others have found that the 

 toxicity of solutions of sodium chloride for marine animals is reduced 

 by the introduction of salts of the bivalent metals. 



