194 THE STRUCTURE OF ECONOMIC PLANTS 



In longisection, the stem appears somewhat heart-shaped, since 

 the successively formed parts have increasing diameters and grow 

 more rapidly than the terminal meristem. The thick, parenchym- 

 atous cortex is traversed by numerous traces of adventitious roots 

 which form an almost continuous zone surrounding the stele; 

 and the leaf traces are separated from them by a layer of paren- 

 chymatous cells so that there is no direct connection between the 

 two vascular systems. The basal stem bundles are usually amphi- 

 vasal, with a layer or two of parenchymatous cells separating the 

 phloem from the surrounding xylem; but higher in the stem, the 

 amount of xylem on the outer face of the vascular strand diminishes 

 and the bundles are collateral. 



Ontogeny of the Leaf. — Each leaf develops as an upright, 

 hollow cylinder, one side of which grows more rapidly than the 

 other forming the blade. The leaf primordium arises as a dome- 

 shaped mass of cells which grows more rapidly than the center of 

 the axis and begins to partially enclose the growing point. (Fig. 

 90, A-G.^ As its eccentric growth continues and one side becomes 

 higher, the entire periphery of the growing region forms a fold 

 which grows upward and around the axis to constitute the basal 

 sheath of the young leaf. While this is taking place, another 

 primordium arises within the first, and its blade grows at the same 

 rate as the more slowly developing portion of the periphery of the 

 enclosing leaf. (Fig. 90, H, /.) In this manner, the vertically 

 oriented, cylindrical base of each young leaf completely surrounds 

 younger leaves which in turn enclose the apical meristem. 



As the differentiation of the sheath and blade proceeds, there 

 is a rapid increase in the diameter of the growing region of the axis, 

 and the base of the newly formed leaf is pushed farther away from 

 the center of the stem. By the time the next younger leaf is dif- 

 ferentiated, no more periclinal walls are formed, and further in- 

 creases in the thickness of the sheath, which becomes the fleshy 

 bulb scale, are due to cell growth and to the formation and enlarge- 

 ment of intercellular spaces. 



The orifice of the sheath is surrounded by a thin membrane ex- 

 tending around its upper edge, including the side bearing the blade. 

 It is an outgrowth of the blade, five or six cells in thickness, which 

 is usually devoid of vascular tissue. In the first six or eight leaves, 

 elongation is greater than in the later formed ones; and the open- 

 ings become much stretched longitudinally so that their margins 



