Organic Plant Poisons. 
I. Hydrocyanic Acid. 
BY 
WINIFRED E. BRENCHLEY, D.Sc., 
Rothamsted Experimental Station. 
With three Figures in the Text. 
T HE study of organic plant poisons presents one fundamental difference 
from that of inorganic poisons. Inorganic compounds containing the 
toxic elements must of necessity be present in the soil or nutrient medium 
in which the plant is growing, must be absorbed by the roots, and must be 
translocated within the plant either in the same form or as some other com- 
pound determined by the metabolism of the particular species concerned. 
In no case can an inorganic substance be developed within the tissues unless 
the essential basis has been supplied by absorption. The degree of 
virulence of such toxic elements is frequently determined by the chemical 
form in which they are presented to the roots, some compounds of an 
element such as copper being far more toxic than others. Organic poisons, 
on the other hand, are frequently developed within the plant in the ordinary 
course of metabolism. When this occurs they do not appear to function 
as poisons, but enter in some way into the economy of the plant. If, how- 
ever, these substances be presented to the roots for absorption, even in very 
low concentration, they may be toxic to a high degree. Many of these 
organic compounds are of considerable commercial or economic interest, so 
that much research has been done on their presence in various species 
of plants, giving rise to a great bulk of literature, largely pharmaceutical in 
character. The action of these same substances when applied to the roots 
has hitherto been largely overlooked, and little work has been published 
except for a very few compounds. This is in striking contrast to the vast 
literature dealing with inorganic poisons, and may be largely due to the 
fact that our knowledge of the organic constituents of the soil is very 
incomplete, owing to the great difficulty of identifying and isolating them, 
whereas the determination of inorganic constituents is less difficult and has 
been vigorously pursued for the last hundred years at least. 
It has long been known that various poisons act as stimulants to the 
Annals of Botany, Vol. XXXI. Nos. CXXIII. and CXXIV. July and October, 1917.] 
