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 largely 

 responsible for the green coloring- of the older parts of the plant. 

 The following measurements were taken: 



Franseria dumosa. (Plate 3, b, and fig 7.) 



Franscria 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 reg-ard, 

 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 var.\- 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. AVith 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 Foiiquieria and others, must be in its branches. 



The general stnictural characteristics of the branches, particularly of the 

 cortex, are indicated by the accompanying sketches and may be outlined 



