196 THE STRUCTURE OF ECONOMIC PLANTS 



elongation, while the base of the sheath remains meristematic 

 longer than any other part. Thus, 



"in the earliest stage of development the entire leaf is meristematic; 

 at a later date, only the sheath and base of the blade; and still later, 



Fig. 90. A, longisection through growing point of axis shown in B ; a and d are opposite 

 sides of same leaf which overgrow central region, bee ; B, earliest stage in leaf differentiation ; 

 C, cell detail of region, x in A and / (plane of cell division marks boundary between 

 regions e and c); ^ ^"d E, one side, c, has outgrown e, and the other side, b, is beginning to 

 overgrow it ; F and G, both sides, b and c, overgrowing axis, e, which is beginning to elon- 

 gate ; H and /, more rapidly growing side, c, is forming blade of leaf while b forms the sheath 

 or bulb scale ; /, diagram of vascular system in mature leaf at orifice : a, adaxial side of blade ; 

 ^, branches of larger bundles ; c, cross-connections ; ^, edge of orifice ; »«, continuous bundle 

 of sheath and blade; /;, pith; j, outer side of sheath ; ^, tip of younger leaf ; i^, bundles end- 

 ing blindly under orifice. (The small letters in A-H correspond to A', B' , C , D' in 

 Z; while a and d in 1 represent blade and sheath sides of a younger leaf. (After Hoffman.) 



