5IO THE STRUCTURE OF ECONOMIC PLANTS 



young stem. The mesophyll is very compact at first, and there is 

 little differentiation into spongy and palisade regions; but, later 

 in ontogeny, the spongy tissue becomes loosely organized and large 



— g s 



r- — SC 



■msl 



--mv 



Fig. 168. Development of foliage leaf; A, transection of young leaf primordium near 

 base showing procambial strands and region of meristematic activity ; B, transection of 

 same primordium at slightly higher level showing development of lamina ; C, transection of 

 young leaf showing orientation of lamina with respect to midvein : g s, glandular scale; 

 msl, mesophyll ; 'fnv, midvein ; ps, procambial strand ; sc, secretory cell. 



intercellular spaces are formed. The palisade region consists of 

 one to three layers of palisade parenchyma which are occasionally 

 interrupted by groups of transversely divided parenchyma. In 

 the young leaf, secretory cells develop between the palisade and 

 spongy tissue, but these are less conspicuous as maturation proceeds. 



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