454 
Brenchley. — Organic Plant Poisons . /. 
more growth (as shown by the total dry weight) than the plants grown in 
the weak food solutions with the same concentrations of prussic acid. This 
difference was evident throughout the life of the plants, though it was less 
obvious in the later stages. 
The experiment was repeated later in the year, April 25 to June 9, 
with similar results, except that as growth was more vigorous at the later 
date all the differences were magnified, so that the graph of the dry weights 
showed a much steeper curve. 
The delay in development followed by more or less normal growth 
may perhaps be due to the behaviour of the poison itself. Prussic acid in 
the presence of water tends to hydrolyse gradually, giving ammonium 
formate, which may possibly be assimilated by the plant. Although high 
concentrations of prussic acid are so toxic, it seems evident that at certain 
lower strengths the poison simply paralyses the tissues, preventing growth 
entirely without destroying vitality. As the HCN hydrolyses in the 
presence of water in the slightly acid food solution the paralysing effect 
slowly wears off and eventually the plant starts into growth, which is more 
or less normal in character according to the amount of permanent injury 
the HCN was able to inflict in addition to acting as a paralyser. It may 
well be that this paralysing action is simply a manifestation of a toxic 
action which shows itself by inhibiting growth without actually killing the 
tissues, so that when the inhibitor is removed the plant is in many cases 
able to exercise its inherent recuperative power and to make more or less 
growth according to circumstances. Possibly, too, this temporary inhibi- 
tion of growth without ultimate loss of vitality is analogous to Townsend’s 
results (quoted earlier in the paper) on the power of hydrocyanic acid 
gas to suspend the germination of damp seeds for considerable periods 
without destroying their power to start growing when the inhibitory agent 
is removed. 
In view of the possibility of the formation of ammonium formate in 
the solution a set of plants was grown with formic acid (H 2 C 0 2 ) as the 
poison in strengths corresponding to those used with prussic acid, 1-704 
formic acid being equivalent to 1 part prussic acid. 1-704/50,000 formic 
acid was very variable in its action, as some plants were killed by the 
poison while the rest started into growth after an initial check and 
eventually produced quite good plants, though the total weight remained 
behind that of the controls. Plants grown with the same strength of 
HCN were killed outright at the very beginning. With 1-704/100,000 
formic acid there was some delay in starting into growth, but the plants 
soon began to make headway and presented a striking contrast to those 
which were grown in the corresponding strength of HCN. Below this con- 
centration the formic acid played no part, as plants in 1-704/500,000 were 
as good or better than the controls, and lower strengths gave no indication 
