20 



TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



and more of the surface of the stem becomes covered with cork the breadth 

 of the chlorophyll band decreases until in the older portions it likely quite 

 disappears. During- these processes very little phelloderm is organized, 

 and exactly how the formation of cork cuts off the chlorophyll band was 

 not determined. It is probable, however, that the secondary phellogen 

 strikes deeply into the cortex and cuts out segments of the band. As a 

 result of this kind of cork formation the bark has a characteristic scaly or 

 shaggy appearance. 



Chlorophyll disappears from the stem rather early. In a branch 1.5 cm. 

 in diameter it was barely demonstrable in the inner cortex, although the 

 cortex was only 0.75 mm. in thickness. 



The following measurements were made: 



KRAMERIA CANESCENS. (Plate I,A, and fig. 9.) 



k'ranicria occurs on the mesa and low hills as scattered individuals; rarely 

 are there several of them growing together in groups. The plant selected 

 for study is growing on the aerial mountain delta and below to the west of 

 the Laboratory. The shrub attains a height of about 50 cm.; the branches 

 spring from the base, which may be either the primitive stem or the bases 

 of older branches which have died. The newer growth is covered with hairs 

 and beneath the hairy covering by its green color the stem shows that it con- 

 tains chlor< >] >h vll . During dry seasons the branches are bare of foliage, but 

 in favoring times they are fairly well provided with small, narrow leaves. 



Branches of a plant about 50 cm. high were studied, and as usual sections 

 were taken from several at measured intervals from the tip. In cross- 

 section a branch 1.5 mm. in diameter and 5 cm. from the tip presents five 

 well-defined regions, namely, the epidermis with its rather thin cuticle, a 

 broad subepidermal chlorophyll band, the inner portion of the cortex with 

 its groups of hard bast and occasional crystal-bearing cells, and the wood 

 and the pith. Regarding the wood and the pith there is nothing note- 

 worthy from the present point of view. 



Chlorophyll is to be found from the epidermis to the pith in practically 

 all of the parenchyma. It occurs sparingly in the epidermis in stems 

 2.5 mm. in diameter, although it was not seen in one 1.5 mm. in diameter. 

 l\irkinsonia also has chlorophyll in the epidermis of the young branch; this 

 is an unusual condition in the desert perennials. The most important 



