Mar., 1921] 
MCNAIR A STUDY OF RHUS DIVERSILOBA 
The resinous sap of the stems and roots retains its toxicity probably 
without much variation in amount or in the degree of virulency throughout 
the year. This is evinced not only by citations from literature (White, 61 ; 
Beringer, 3) and by statistics (table i), but also by experiments conducted 
with the sap by the writer. 
The virulency (the liability to cause poisoning) of the plant varies with 
the different seasons of the year in accordance with the stage of growth of 
the leaves, stems, and flowers. When the first leaves of the plant are 
unfolding in the spring they are very turgescent and easily injured. Analo- 
gously, the growing stems are less resistant than the mature stems. The 
mature leaves of the plant are not nearly as easily injured. Of the mature 
leaves, those that grow in the shade have a weaker structure than those 
which develop in the sun. From this fact one might expect the shade 
leaves to be less resistant to injury. 
The amount of poison in the plant varies with the capacity of its resin 
canals. Of this variation in amount, that of the stems and leaves is most 
commonly effective in the index of virulency. The leaf area undoubtedly 
makes its greatest increase in spring between the time when the leaves 
begin to unfold and the time when the flowers open. From this latter 
time the leaf area of the plant is nearly constant until the leaves begin to 
fall in autumn. Four weeks are generally required for the full development 
of a leaf. The flower and leaf buds begin to expand simultaneously, but 
the leaves soon expand more rapidly and reach maturity before the flowers 
open (PI . 1 1 , ^) . The staminate and pistillate plants begin to bloom at about 
the same time. At Berkeley, California, but few of the flowers were open 
April 4, 1915. The next spring the plants near the Greek Theatre, at 
Berkeley, bloomed mostly between March 22 and May i. Either the 
amount or the virulence of the poison in the autumn leaves is less than that 
of the normal mature leaves. Of the autumnal leaves the red are less toxic 
than the yellow, and when the leaves have finally withered and fallen they 
are non-toxic (McNair, 40). 
Inui (30) has noticed that the amount of secretion of R. vernicifera is 
influenced by the conditions of light and atmospheric humidity. In 
potted plants the secretion lessened when carbon assimilation was hindered. 
Similarly, secretion was greater in damp than in dry air. This secretion 
therefore seems to bear a relation to transpiration and hence to turgor. 
As the degree of turgor varies indirectly with the amount of transpiration, 
other factors being equal, secretion would be least when transpiration is 
greatest. Turgor, too, is a necessary accompaniment of growth; flaccid 
tissues do not grow larger. If those influences which affect R. vernicifera 
have a similar action on R. diversiloba, then secretion, and consequently 
the plant conditions for poisoning, would be greatest during that time of 
the year when the growth of the plant is most active and the tissues least 
resistant, namely, in the spring. Obviously enough, when the plant is in 
