138 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
temperature in 95 percent alcohol for ten minutes. The leaves had been 
examined first under a hand lens to make sure that through possible injury 
no resinous sap was on the surface. When placed in the finger bowl the 
sap was prevented from running down the pedicel from the cut end into the 
alcohol. The leaves when taken out of the alcohol had lost their gloss. 
The pale yellowish alcoholic solution remaining was concentrated by boiling 
in an open beaker. It was found to be non-toxic. It was not darkened by 
potassium hydroxide nor did it respond to other chemical tests for the 
poison. These results indicate that neither the plant trichomes nor their 
exudate are poisonous. 
The cork cells of the older stem were likewise found to be non-toxic 
either when the branch was rubbed on the skin or when an alcoholic solution 
was made of scrapings from the outermost cork cells of a branch as thick 
as a man's wrist. 
As no resin ducts were seen on a microscopical examination of the pith o^ a 
one-year-old stem of the poison oak nor in the woody stem, experiments were 
undertaken to determine their toxicity. The bark was carefully removed 
from the pith, a clean knife being used to shave off the outermost portions of 
the pith. The pith was then cut up in small portions and extracted in a 
Soxhlet apparatus with hot 95 percent alcohol. This alcoholic solution 
when concentrated gave neither a physiological test for the poison nor any 
of the chemical tests. 
A similar experiment carried on with the woody xylem gave correspond- 
ing results. 
Summary 
1. The fresh sap emulsion is the only part of the plant capable of pro- 
ducing dermatitis. 
2. Those portions of the plant that do not contain the resin canals do 
not normally have this kind of toxic effect. 
3. The non-toxic portions are the anthers, pollen, xylem, epidermis, 
cork cells, and trichomes. 
Liability to Poisoning Relative to Growth of Plant 
At just what stage in its life the Rhtis diver siloha plant first contains its 
irritant poison has not yet been determined. After the plant has become 
several years old, however, all parts except the xylem, cork cells, epidermis, 
and trichomes are toxic. Although many persons know the sap of the stems 
and leaves to be poisonous, yet there are some who do not consider the sap of 
the roots toxic. Such is the case, however, as is attested by persons who 
have come in accidental contact with the broken roots of the plant while 
digging out other botanical specimens (Kunze, 35; Stirling, 55). The 
poisonous action of the roots might be expected from their structure, as 
they have numerous vertical resin canals encircling the xylem (PI. II, C). 
