ALLIUM CEPA 



101 



Stomata. — Stomata are numerous in the epidermis of the coty- 

 ledon less than a week after germination. In the development of 

 the stoma, a mother cell is cut off from one end of an epidermal cell 

 so that it is approximately square in surface view. This cell by 

 one longitudinal division forms the guard cells which subtend the 

 stoma. In the mature leaf, the guard cells are sunken beneath the 

 surface of the adjacent epidermal cells which partially overgrow 

 them and develop a thick, lamellated cuticle which may exceed 

 half the radial dimension of the cells. 



The Bulb. — The bulb consists of a short stem which bears a 

 series of thickened leaves and leaf bases, together with the cycles 

 of adventitious roots which have 

 been described as arising from the 

 cortical and pericyclic regions. 

 (Fig. 93.) The arrangement of 

 the leaves is concentric so that 

 the older ones surround the 

 younger, and the apical meristem 

 is located at the center of the 

 bulb. The blades of the leaves 

 comprising the bulb may be 

 photosynthetic or they may dry 

 down entirely. In a full-sized 

 mature bulb, some of the inner 

 leaves do not develop an elongated 

 blade, and the functional blades 

 of the outermost six or seven leaves become dry and break off so 

 that only the sheaths remain. These protect the fleshy storage 

 leaves which surround three or four leaves with short undevel- 

 oped blades; and centrad to the latter are successively younger 

 leaves with longer blades. 



In the spring, the youngest leaves continue development. The 

 storage leaves outside them also elongate, especially at the base, 

 but their green blades remain relatively small. As the inner leaves 

 are growing, the outer storage leaves dry progressively from the 

 top of the sheath to its base because of the withdrawal of food re- 

 serves and moisture. Preceding and accompanying this develop- 

 ment, numerous adventitious roots arise from the periphery of the 

 stem and penetrate the soil. These remain functional throughout 

 the life of the plant. 



Fig. 93. Diagrammatic transection of 

 young bulb showing relation of leaves to 

 each other : / b, leaf bases ; / s, leaf sheath ; 

 / /, leaf tips. 



