16 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
phyll band opposite the center of each sclerenchyma mass, due perhaps to 
the fact that the covering of the chlorophyll band at that point is heavy and 
opaque, so that the chlorophyll in the older stems occurs opposite the newer 
external tissue only (fig. 6, D). This circumstance, together with the 
translucent condition of the newer portion of the external covering, is largély 
responsible for the green coloring of the older parts of the plant. 
The following measurements were taken: 
; Width of | Depth of 
Diameter ri } 
exterior |chlorophyll 
ofstem. | covering. jin the stem. 
mm. Be i 
5 498 818 
8 500 832 
30 1162 894 
FRANSERIA DUMOSA. (Plate 3, B, and fig 7.) 
Franseria is a globoid shrub about 50 cm. high which is growing in some 
abundance on the north slopes of Tumamoc Hill and on the aerial moun- 
tain-deltas in the western portion of the Laboratory reservation. It is 
characterized by numerous slender branches of approximately equal length, 
which spring either from the short main stem or from the bases of the 
older branches. It thus happens that new branches may replace dead ones 
and maintain the usual form of the plant when the latter fall away. The 
triangulate leaves are sage-colored, and for the most part are borne near 
the tips of the branches, although there is great variation in this regard, 
depending apparently on the adequacy of the water-supply. In times of 
extreme drought only small leaves remain on the very tips of the branches. 
The external tissues of the branch vary with its age and presumably 
with the conditions under which growth took place. The most recent por- 
tions are green and dark purple in color. The surface has a shining or waxy 
appearance, due to secretions from hairs, certain of which are provided with 
chlorophyll (fig. 7, c). Below the younger portions the branch is rough- 
ened by narrow longitudinal furrows and ridges, the latter of which are 
continuations of the epidermis. This condition marks the first appearance 
of bark. Towards the base of the stem the furrows widen, the ridges 
disappear, and the entire surface becomes black and of,a shaggy character. 
As will be shown below, chlorophyll occurs in the cortex up to the last con- 
dition of the bark given. With scarcely an exception chlorophyll is found 
in the cortex within 2 to 5 cm. of the bases of the secondary branches, from 
which it follows that a very large percentage of the entire carbon assimi- 
lative area of this plant, as Fouguieria and others, must be in its branches. 
The general structural characteristics of the branches, particularly of the 
cortex, are indicated by the accompanying sketches and may be outlined 
