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PLANTS OF THE SOTOL REGION 631 
comes white and broken by fissures on the old stems. Beneath 
the epidermis is a thick zone of cortical assimilation perenchyma 
with cells elongated radially in the manner of palisade (/. 9, as). 
Toward the vascular cylinder, which, because of the great thick- 
ness of the cortex, appears to have the central location as in roots, 
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OU = a Saat OY aE 
Fic. 9. Cross-section of young stem (1 yr.) of Euphorbia antisyphilitica, trans- 
versely through a stoma. wa, the heavy waxy secretion of the epidermis perforated at 
sp, by crater-like opening above the guard-cell apparatus, g,@ ; other lettering as before. 
the assimilation-cells gradually become more nearly isodiametric, 
thicker-walled and pitted storage-cells. Within the vascular cyl- 
inder is an ample pith-region in which the cells are branched and dis- 
connected so as to form almost a trabecular tissue (7 70, arm, z). 
Viewed from the surface, the stoma appears as a crater-like 
Opening through the waxy covering, the wax being elevated about 
the pores, volcano-like. On younger stems it is easily observed 
that the stomata are more or less grouped or run in diagonal or 
