26 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
PROSOPIS VELUTINA. (Figs. 12, 13, and 14.) 
Prosopis velutina is the most characteristic tree of the river-bottoms, where 
in places it forms extensive forests. It varies in size from a small shrub 
to a well-formed and shapely tree 15 m. or more high. The difference in 
size depends mainly on the lack or the abundance of the water-supply. 
Leaves are formed in the spring and are shed in the autumn with a regu- 
larity characteristic of deciduous trees of more humid regions. 
Fic. 12.—Prosopis velutina: A, transverse section ofstem 4 mm. in diameter, 
to show distribution of chlorophyll; 8, segment from cortex of stem 7 mm. 
in diameter, to show relation of hard bast (2. 4.) tochlorophyll. 
The sections of branches studied wefe cut at nine separate intervals 
from 3 cm. to 1.23 m. from the tips, and were from 1.3 mm. to 1.5 cm. in 
diameter. 
In several particulars the distribution of chlorophyll in Prosopis recalls 
that in Parkinsonia. In the young stem chlorophyll may be found in prac- 
tically all of the ground-tissue both of cortex and of wood. With increase 
in size the chlorophyll distribution of the stem becomes greatly restricted 
and the topography of the chlorophyll apparatus becomes much changed. 
The general structural relations of the stem will be apparent from the suc- 
ceeding account of the distribution of the chlorophyll and will not require 
especial discussion. 
The epidermal cells of Prosopis do not contain chlorophyll—in this as 
well as certain other particulars Prosopis is unlike Parkinsonia. Prosopis 
