456 Brenchley. — Organic Plant Poisons . /. 
same way as the various inorganic poisons, zinc, arsenic, and copper, which 
were tested in earlier work. 
Summary. 
1. Prussic acid is very toxic to peas and barley. All strengths up 
to and including 1/100,000 killed peas outright, either immediately or 
after a short interval of poor growth. All strong concentrations kill 
barley, but with 1/100,000 a period elapses during which no growth occurs, 
after which a little progress is made, though the plants never attain 
any size. 
2. The peas killed by prussic acid shrivel from the cotyledons upwards 
and the roots contract so intensely that they are often completely withdrawn 
from the nutrient solution. Barley roots decline to enter strong solutions 
at all, but often put out laterals which stop short at the surface of the solution 
and form the characteristic bunchy root so often seen with this plant when 
in the presence of poison. 
3. Formic acid is comparatively harmless to barley, except in very 
strong concentrations, whereas sodium cyanide is quite as toxic as prussic 
acid, indicating that the cyanogen radicle is the toxic agent in the cyano- 
genetic compounds. 
4. No trace of stimulation in peas or barley has been obtained with 
any of the compounds tested. 
