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 phellogen 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 
Fic. 6.—Ephedra antisyphilitica: A, cross-section of green branch, a detail of 
which is shown in &. 8B, portion of stem to show structure of chlorophyll band 
of cortex. Fouguieria 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 chiorophyll band has become discontinuous as a result of 
the stem’s growth (see text). ch.6., cortical chlorophyll band; cav., cambium. 
by transverse walls into phellogen; in the inner cells, however, which have 
the long axis parallel to the surface in stems of this size, the division walls 
forming the phellogen are parallel to the long diameter of the cells. The 
bark thus originating 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 
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