2.04 THE STRUCTURE OF ECONOMIC PLANTS 



of vascular bundles. This region is bounded centripetally by a 

 band of small, compact, parenchymatous cells which form a com- 

 plete ring. Within this zone are the large, loosely organized cells 

 which surround the inner cycle of vascular bundles. At maturity, 

 the central region breaks down schizogenously, and a large lacuna 

 is formed similar to that in the blade of the leaf. The collateral 

 bundles consist of primary sieve tubes and companion cells, and 

 primary xylem elements that are annular, spiral, and reticulate. 



Ontogeny of the Flower. — Jones and Boswell (9) have in- 

 vestigated the formation of flower primordia and the relation of 

 the time of planting onion sets or young bulbs to their differentia- 

 tion. Examination of the bulbs for the development of flower 

 primordia indicates that in general the flower primordia are dif- 

 ferentiated early in the spring under conditions prevailing in Col- 

 lege Park, Maryland. This occurs regardless of "whether or not 

 the bulbs are planted in the fall or early in the following spring, 

 having been kept in storage over winter." It was also observed 

 that "fall planted bulbs, which made a luxuriant foliage develop- 

 ment before the differentiation of flower primordia, produced uni- 

 formly taller and heavier seed-stalks with a heavier set of blossoms, 

 than was obtained from the spring planted bulbs." Under Cali- 

 fornia conditions, bulbs planted in December differentiate the floral 

 axes in February. 



Jones and Emsweller (10) have worked out the floral development, 

 and the following account is based in part upon their investiga- 

 tions. After the differentiation of the involucral bract, numerous 

 membranous bracts arise on the meristematic surface of the tip of 

 the floral axis, and these enclose the clusters of flower primordia 

 in their early stages. (Fig. 96, H, Z.) Each of these areas is 

 somewhat elevated and kidney-shaped, and it is from them that 

 the individual flowers arise. 



The flower primordium develops as a slight projection on the 

 meristematic surface, which becomes globose and later elongates 

 slightly so that it is circular in transection and convex at its apex. 

 (Fig. 97, B.) As growth proceeds, the primordium becomes tri- 

 angular in transection as a result of the differentiation of the outer 

 perianth segments and the outer stamen whorl which is the first 

 to be formed. (Fig. 97, C) At each angle of the triangular pri- 

 mordium, there is differentiated a perianth segment; and in the 

 axil of each is a primordium of one of the stamens of the outer 



