i8o 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
The Leaf 
The glossy, dark green leaflets are deepest in color when in the sun, pale 
underneath, generally 3 in number, although sometimes 5, orbicular to 
ovate or oblong-ovate, undulate or plane, entire or variously lobed, seg- 
mented or toothed, i to 4 inches long. The 5-leaflet variety, according to 
Brandegee (3) is quite common on the Santa Barbara Islands. Leaves 
having 5 leaflets are also found on plants which have a majority of the 
3-leaflet kind. Leaflets are singularly variable in size, outline, and seg- 
mentation, even on the same plant. This fact constitutes one of the most 
remarkable features of the plant and is the principal basis for its differen- 
tiation from Rhus Toxicodendron L. Leaf tracings (21) made from mature 
leaves collected by the writer at Berkeley, California, on September 27, 
1916, were taken from plants within a radius of 100 feet, all of which were 
enjoying the same soil and exposure and had no apparent cause for such 
marked differences in leaf shape. 
Leaves in the sun differ from those in the shade, not only as regards 
color but also in several structural details. The young leaves are covered 
with hairs, which dry out and fall off as the leaves become fully matured. 
These hairs are apparently more frequent on leaves exposed to the sun 
than on those in the shade. Other differences will be described later. 
In autumn, as in spring and summer, the plant is singularly attractive, 
its leaves turning many shades of red, yellow, and brown. This color 
change may be induced in mature leaves in midsummer by certain insect 
injuries, by attacks of fungi, or by an interference with the flow of sap 
caused by twisting the stem. There is no apparent difference between 
the leaves of male and female plants in this respect. Some plants, however, 
particularly those in the shade, may have all their leaves yellow. Con- 
versely, red leaves seem to be peculiar to plants of sunny exposure, although 
there are many exceptions; far more frequently the leaves are mixtures of 
all three colors. The oldest leaves often assume autumnal tints first. 
The petiole in transverse section (21) has in form nearly a semi-circle 
for its dorsal side and a small concave arc as a ventral surface. Under the 
epidermis lie two or three layers of collenchyma cells. The vascular bundles, 
of which there are more than 18, are arranged in a flattened circle parallel 
to the outer surface of the petiole. The pith consists of large, thin-walled 
cells with very small triangular intercellular spaces. The vascular bundles 
are separated from each other by broad medullary rays. Large resin ducts 
are found in the phloem. The primary cortex is bordered internally by a 
starch sheath. The cells of the xylem have thick and lignified walls. The 
pith is enclosed by bast fibers and xylem and takes up the largest part of 
the section.- There are no resin ducts in the pith or in the primary cortex. 
The leaf in transverse section exhibits palisade parenchyma occupying 
about one third of the entire thickness of the mesophyl (PI. Ill, fig. 4). 
The spongy parenchyma occupies about five layers of cells. Cells with 
