PLANT POISONS. 381 



deadly in their effects, are converted by substances present in 

 soil into insoluble and almost harmless compounds. Others 

 remain in a soluble form in the water of the soil for some time, 

 and therefore are very effective as destroyers of plants. 



Almost any soluble salt if applied in strong solution will 

 generally kill plants, probably by producing plasmolysis. 



Soluble sulphides, sulphocyanides, and sulphites are ex- 

 tremely powerful poisons and can be used as weed-killers. 

 Even strong brine is effective for this purpose. 



Coupin investigated the poisonous effect of a large number 

 of salts," and determined the minimum strength of a solution 

 which had an injurious effect in hindering (not preventing) the 

 growth of the roots of wheat during the first 15 days. 



The following are some of his results, the strengths given 

 being the weakest which had an apparent effect : 



Copper sulphate ... 1 in 700,000,000 



Mercuric chloride ... 1 in 30,000,000 



Cadmium chloride ... 1 in 10,000,000 



Silver nitrate ... ... lin 1,000,000 



Zinc sulphate ... ... lin 40,000 



Lithium chloride ... lin 12,000 



Calcium iodide ... lin 10,000 



Barium nitrate ... 1 in 4,200 



Borax ... ... 1 in 1,000 



Manganese chloride ... 1 in 1,000 



Calcium bromide ... 1 in 400 



Calcium chloride ... 1 in 260 



In all cases the salts are assumed to be anhydrous. The 

 very minute quantity of copper sulphate which produces a 

 poisonous effect is remarkable a quantity such as no ordinary 

 chemical test would detect. 



Compt, Rend. 1901, 645 ; J.C.S. 1901, abst. ii. 355 



