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: 



