Effect of Zinc Compounds 43 



was confirmed by Jensch later on, and also by Nobbe, Baessler and 

 Will (1884), who state that both lead and zinc compounds work 

 disadvantageously to vegetation even when they are present in such 

 small quantities that the plants are outwardly sound, the harmful 

 action appearing in the decrease of dry weight. Contrary to Bau- 

 mann's opinion, zinc carbonate is said to be one of the salts that 

 exercises this insidious poisonous action. Storp (1883) noticed that 

 the direct poisonous action of zinc compounds is largely destroyed 

 by their admixture with soil, but he suggests that a secondary cause of 

 harm is introduced by the accumulation of insoluble zinc salts, so that 

 the fertility of the soil is impaired to the detriment of the vegetation. 

 Ehrenberg (1908) throws out a suggestion that zinc is specially 

 harmful to plant life when it occurs in conjunction with ammonia, but 

 no further evidence has come to light. 



(d) Mode of action of zinc on plcmts. 



The reason for the toxicity of zinc salts when present in soil forced 

 itself upon the attention of some of the early investigators in this field. 

 Freytag (1868) put forward the hjrpothesis that the zinc oxide is partly 

 or exclusively absorbed by the roots on account of the cell walls of the 

 root being corroded by the very thin layer of zinc salts lying in contact 

 with it — the same theory as has been held with regard to copper. He 

 stated also that the quantity of zinc oxide taken up by the plant through 

 its roots is strictly limited, not beiug proportional to the quantity 

 occurring in the soil, but varying between narrow limits. Krauch 

 (1882) found himself unable to accept another hypothesis which at one 

 time found favour, i.e. that the zinc salts kill the plants by coagulating 

 the protoplasm. If this were so, he argued, no plants at all could grow 

 upon soils containing zinc, and he was content to leave the cause as one 

 yet to be explained. Even at the present time, thirty years after, we 

 know very little more about the physiological cause of the toxicity of 

 zinc. 



2. Effect of zinc compounds on germination. 



In the course of his investigations on the influence of zinc on 

 vegetation Frejrtag just touched upon the question of seed germination. 

 According to his statement the presence of zinc oxide iu the soil does 

 not exercise much influence upon germination and the growth processes 

 of plants. Little zinc is stored up in seeds and on this account seeds 

 originating from plants containing zinc germinate quite normally and 



