250 
MORPHOLOGY 
The branches of a root are formed at the periphery of the vascular 
cylinder and push through the cortex, this endogenous origin being in 
sharp contrast with the method of origin of stem branches (fig. 558). 
LEAF 
The structure of an angiosperm leaf is in every essential the same as 
that of a pteridophyte leaf, and should be clear at this point. For those 
FIG. 559. Transverse section of lily leaf : beginning above, the regions are the 
upper epidermis (e) ; the palisade layer (/>) ; the region of spongy tissue (extending to the 
lower epidermis), with intercellular spaces and vascular strands (v), the cells containing 
chlorophyll; and the lower epidermis (e'), in which sections of three stomata are seen, 
each opening into a large intercellular chamber (i). After COULTER. 
unfamiliar with this structure, it may be pointed out that the essential 
features of an ordinary dorsiventral leaf are as follows : a layer of 
close-fitting or even interlocked epidermal cells above and below, in 
which stomata are developed (figs. 559-561); between the epidermal 
layers the mesophyll region, whose cells contain chloroplasts (fig. 559); 
