250 



MORPHOLOGY 



The branches of a root arc 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 (p); 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 (c'). m which sections of three stomata are seen, 

 each opening into a large intercellular chamber (*'). — 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); 



