LEAF STRUCTURE. 



59 



eventually forms the differentiated stem. During the active 

 period of growth the apical portion of such a stem or root 



Fig, II. — (x 360). ' Cross section through the leaf of the beech [Fagus 

 sylvatica], ep, ep"\ epidermis of upper surface of leaf ; ejl' , epidermis of 

 under surface, bearing one stoma, st ; pi, palisade parenchyma, showing 

 the cells placed end on to the upper surface of the leaf and closely 

 packed ; sp>, sp'\ spongy parenchyma of lower portion of the substance of 

 the leaf, showing numerous large, irregular, intercellular spaces ; k, 

 cells containing crystals ; k', a cell containing a cluster-crystal. (From 

 Strasburger. ) 



