May, 1921] 
MCNAIR — TRANSMISSION OF RHUS POISON 
247 
preparations of hairs were spread on the skin, not the sHghtest irritation 
appeared. 
The glycerine layer of one or more experiments was applied and dried on 
the uninjured skin of the under side of Rost's forearm. The results were 
negative. Rost was susceptible to the resinous sap of the same shrub. 
It seems evident, therefore, that the trichomes are non-toxic and are not a 
means of conveyance of the poison from plant to person. 
Hubbard (17) and Hadden (16) thought insects might carry Rhus 
poison from the plant in ways similar to those by which flies carry bacteria 
from place to place. This method of transmission seems hardly practicable 
in many cases. It should be borne in mind that the insect could not trans- 
mit the poison by coming in contact with the uninjured plant. 
Recently, Frost (15) believed the poison to be bacterial. This has 
been refuted (McNair, 32). 
The methods already discussed constitute all that have been suggested 
for the transmission of Rhus poison to a distance. As none of them prove 
very serviceable, we still have to consider the question as to how poisoning 
occurs without contact with the plant. 
It has been found in an examination of the sap that: (i) The unelabo- 
rated sap of the xylem is non-toxic; (2) the elaborated sap of the phloem is 
non-toxic; and (3) the resinous sap of the resin canals is poisonous. 
A further examination of the plant tissues shows that the xylem, epider- 
mis, and trichomes which do not contain the resin canals are non-toxic. 
When the flowers are examined, it is evident that resin canals do not 
extend more than half-way up the fully matured stamens, and so it would 
be expected that the pollen would be non-toxic. The flower of the female 
plant, on the other hand, contains resin canals in the pistil, and an abundance 
of resin canals surround the ovule. The ovule remains highly toxic until 
the seed has fully ripened. The poison, therefore, acts as a protection to 
the immature seed. This plant thus exemplifies the natural law developed 
by Kipling (23) that the female is more deadly than the male. It has also 
been shown (McNair) that the maximum number of cases of Rhus dermatitis 
recorded in the University of California Infirmary occurs previous to the 
opening of the flowers. 
It has long been known that fresh leaves are more likely to produce poi- 
soning than are dry or fallen leaves. This difference in malignancy has been 
attributed to a poisonous gas given off by the plant. 
Van Mons (45) was convinced by the large number of cases among 
persons of his acquiantance, that the evil effects of Rhus were produced by 
a gaseous substance which escaped from the living plant, because the dry 
leaves or fallen leaves never caused trouble. 
Professor Asa Gray also held this same opinion in 1872, as the following 
letter to Dr. J. C. White discloses: 
My personal knowledge that Rhus dried specimens are harmless amounts merely to 
