I90 THE STRUCTURE OF ECONOMIC PLANTS 



The Structure of the Cotyledon. — In the growth and devel- 

 opment of the cotyledon, Hoffman observed that there is "appar- 

 ently no localized meristematic region in any part of the cotyledon, 

 cell divisions occurring throughout its entire length, although the 

 cells at each end reach their full size later than the others." As 

 this general growth takes place, the cells become vacuolate, en- 

 larging to six or eight times their embryonic length; and, subse- 

 quently, they may divide two or three times. He found no mitotic 

 figures during the first five days of development, and concluded that 

 growth up to this point is mainly the result of cell enlargement. A 

 few days later, mitoses occur in large numbers in all tissues, the 

 last region to initiate nuclear and cell division being that part of 

 the cotyledon adjacent to the seed. At the end of two weeks, 

 mitotic figures are again lacking. 



The cotyledon above the lateral opening has a single, centrally 

 located vascular strand extending to its extreme tip. This is 

 surrounded by several rows of cortical parenchyma which are in 

 turn bounded by the epidermis. The tip of the cotyledon is haus- 

 torial in function, and the epidermis at this point is without a 

 cuticle. (Fig. 83, H.) 



The vascular anatomy of the cotyledon and its relation to that of 

 the root has been described by Chauveaud (x) and Hoffman (7), the 

 account of the latter differing from the former chiefly with respect 

 to the ultimate disposition of the protoxylem elements. The 

 stelar organization of the hypocotyl, up to the cotyledonary plate, 

 resembles that of the primary root, and the two protoxylem points 

 of the primary xylem strand are alternate with the two primary 

 phloem groups. In the center of the stele are a few large paren- 

 chymatous cells which later mature to form the two or more large 

 metaxylem vessels. (Fig. 86, /4.) 



The differentiation of the xylem of the cotyledonary bundle 

 begins at a level slightly above the slit and proceeds from this point 

 upward and downward at about equal rates. As first noted by 

 Sachs (15), the xylem of the cotyledonary bundle is continuous 

 with one of the exarch xylem strands of the hypocotyl, and the 

 phloem of this bundle is joined with both of the phloem groups of 

 the hypocotyl. The difl^erentiation of the other primary xylem 

 strand of the stele is retarded at the point where the vascular system 

 of the first foliage leaf later differentiates and anastomoses with it. 

 At the base of the cotyledon, the vascular portion of the axis is 



