14 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



somewhat changed, since it has been extended to include the medullary rays 

 of both wood and cortex. The organization of the phelloderm, which may 

 be observed in stems 4 mm. in diameter, also modifies the chlorophyll 

 distribution. The phclloj>vn extends from the outer part of the subepider- 

 mal palisade layer to about the layer of cells which is next to the inner 

 chlorophyll-bearing cells. In the outer cells the outer ends are converted 



FIG. 6. Eplicdra antisyphilitica: A, cross-section of green branch, a detail of 

 which is shown in B. B. portion of stem to show structure of chlorophyll band 

 of cortex. I'onyitieria splendens: C, transverse section of stem 5 mm. in diameter; 

 bases of stout spines which entirely encircle stem are not shown. J), section of 

 older stem in which the chlorophyll band has become discontinuous as a result of 

 the stem's growth (see text), ch.b.. cortical chlorophyll band; earn., cambium. 



by transverse walls into phello.^'en; in the inner cells, however, which have 

 the lon.y axis parallel to the surface in stems of this size, the division walls 

 forming the phcUo^cn are parallel to the long diameter of the cells. The 

 bark thus oriffinatin.tr is lens-shaped. It therefore happens that a cross- 

 section of a stem 4 mm. in diameter shows a portion of the primitive cortex 

 with chlorophyll-bearing cells and a portion of it converted into cork which 



