32 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
between them and join the broad band of chlorophyll.. Chlorophyll-bearing 
tissue on the inner face of the ring of hard bast connects the medullary 
rays. So that there are three rings of chlorenchyma in the cortex, namely, 
the chlorophyll band and two inner rings separated by hard bast. The 
medullary rays of the wood and of the pith in a‘stem 3 mm. in diameter 
contain small amounts of chlorophyll. ; 
With increased diameter certain changes in the distribution of the chlor- 
ophyll occur which should be noted. As usual, these modifications are 
most marked in the cortex. With lateral stretching, which accompanies 
the growth of the stem, the groups of hard bast become farther and farther 
apart and the intervening spaces become filled with a chlorophyll-bearing 
tissue, so that the relative and actual amount of chlorenchyma is much 
increased. 
In stems 1.4 cm. in diameter the formation of cork has begun. The 
phellogen appears to take its origin in the epidermis and shows little indi- 
cation of activity. Only a small amount of periderm was observed and 
almost no phelloderm. The periderm in the larger branches is colored a 
deep red and is broken through at intervals by lenticels. No chlorophyll 
is to be found in wood or pith, although in a stem 1.55 cm. in diameter and 
1.6 m. from the tip the cortical chlorophyll band was still to be seen. 
When it disappeared from the stem, if ever, was not learned. 
The following measurements were made: 
: : Width of | Depth of 
Diameter | Distance Width of . 
of branch. | fromtip. | cortex. sae a eierpny: My 
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