Brenchley . — Organic Plant Poisons . /. 455 
of toxic or stimulant action. This indicates that if the HCN is hydrolysed 
into formic acid or its derivatives the highly toxic substance is replaced by 
another which is practically indifferent in its action except when it is 
present in relatively large quantities. It is evident, though, that the higher 
concentrations of prussic acid do not merely paralyse growth, but kill the 
plant so that there is no possibility of recovery if and when the poison is 
hydrolysed. 
In order to see whether the toxic action is due to the cyanogen radicle 
independent of the combination in which it is presented to the roots, 
a series of barley was grown with sodium cyanide replacing the HCN in 
equivalent concentration (1-815 sodium cyanide = 1 HCN). A parallel 
Gms. 
Fig. 3. Average dry weights of 8 series of barley plants grown in strong nutrient solutions 
in the presence of differing amounts of sodium cyanide. April 23-June 9, 1914. 
series with prussic acid was grown alongside, and it was found that the 
results were similar with both combinations of the cyanogen radicle, though 
growth was rather more seriously depressed with the 1-815/500,000 sodium 
cyanide than with the 1/500,000 HCN. This indicates that the toxic 
agent in prussic acid is the cyanogen radicle, and that it acts in the 
same poisonous way when presented in other combinations than that of 
HCN (Fig. 3). 
The experiments have shown that prussic acid and sodium cyanide are 
highly toxic to plants down to a weak concentration, the poisonous action 
still being evident at 1/4,000,000 strength and even lower. Although the 
dilution was carried down to 1/1,000,000,000 prussic acid no trace of 
stimulation was obtained in any case. In this respect HCN behaves in the 
