272 BOTANICAL GAZETTE [MaRCH 
fallen they are non-toxic. During that period of the year when the 
plant is leafless the risk of its producing poisoning is least. 
This theoretical consideration of the liability of Rhus poisoning 
from a botanical point of view has its counterpart in clinical statis- 
tics. The latter lend analogous evidence to the conclusion that 
spring has the greatest number of cases (see frequency polygons) (9). 
The number of cases of dermatitis from R. diversiloba is in- 
fluenced, not only by the condition of the plant, but also by those 
conditions which tend to make individuals come in contact with it 
or with substances coated with its poisonous sap. ROBERT LOUIS 
STEVENSON (6) describes a tramp in California woods as follows: 
We struggled toughly upward, canted to and fro by the roughness of the 
trail, and continually switched across the face by sprays of leaf or blossom. 
The last is no inconvenience at home; but here in California it is a matter of 
some moment. For in all woods and by every wayside there prospers an 
abominable shrub or weed, called poison-oak (Rhus diversiloba). 
Many low plants seek the shelter of the Rhus diversiloba shrubs, 
and some of our loveliest flowers, such as Clarkias, Godetias, Col- 
linsias, Brodiaeas, and Larkspurs, seem to realize that immunity 
from human marauders is to be had within its safe retreat. JOHN 
Murr (4) “oftentimes found a curious twining lily (Stropholirion 
californicum) climbing its branches, showing no fear but rather con- 
genial companionship.” The desire to gather spring wild flowers — 
is often greater than the fear of Rhus diversiloba. Circumstances 
thus combine to bring victim and culprit together at the time when 
the culprit is capable of doing the most harm. It may truthfully 
be said in regard to this poisonous plant, as is said of the Scotch 
thistle, “no man provokes it without fear of punishment.” 
Summary 
1. The intercellular secretory canals of Rhus diversiloba T. and 
G. are found in the roots, stem, leaves, and fruit in the phloem of 
the primary vascular bundles. There are other secretory canals 
situated in the secondary bast of the stem. They are found also 
in the phloem of the mesocarp of the fruit and in the hypocotyl 
and cotyledons of the embryo. 
2. Their formation may possibly be schizolysigenous. In the 
beginning they are clearly schizogenous. 
