12 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
The chlorophyll band is made up of cuboid and elongated cells, of which 
the latter have the long axis placed parallel to the surface of the stem. In 
addition to the outer chlorophyll band, chlorophyll is found sparingly in 
the medullary rays both of the cortex and of the wood and in the pith also. 
With increase in diameter the stem exhibits certain changes in its general 
structure, of which the most important in the present connection are to be 
found in the cortex. Cork is organized early and is superficial. The 
phelloderm is in direct contact with the chlorophyll band and probably con- 
tributes chlorophyll-bearing cells to the latter, although this was not 
definitely determined. The other changes in the cortex do not affect the 
distribution of the chlorophyll and may be neglected. 
Fi. 5.—Covillea tridentata: A, transverse section of stem 5 mm. in diam- 
eter, showing character of spongy tissue of chlorophyll band; BZ, transverse 
section of leaf, showing palisade character of subepidermal chlorenchyma; 
C, cross-section of stem 1 mm. in diameter, to show general distribution of 
chlorophyll. Lettered as in fig. 3. 
The order of disappearance of chlorophyll from the stem was not followed. 
In a stem 1.5 mm. in diameter chlorophyll was observed in the pith, in the 
medullary rays of wood and of cortex, and in the chlorophyll band. Ina 
stem 3 mm. in diameter, however, it had practically disappeared from all 
tissues deeper in the stem than the chlorophyll band; in a stem 9.5 mm. in 
diameter no traces of chlorophyll were to be detected outside of this band. 
No chlorophyll was found in another branch 2 cm. in diameter and 145 cm. 
from the tip, although the primitive chlorophyll band, but without chloro- 
phyll, was still present. 
