138 THE STRUCTURE OF ECONOMIC PLANTS 



tion, the meristems of the primary root and epicotyl are differen- 

 tiated, the scutellum enlarges, and there is also considerable 

 growth of the suspensor. In the ensuing week, the primordia of 

 the coleoptile and first foliage leaves arise, which usually number 

 five by the time the embryo is mature. The suspensor ceases to 

 enlarge after about twenty days. (Fig. 57.) 



The growth of the embryo is rapid during the next three weeks; 

 and, at the end of about forty-five days, it is morphologically 

 mature. The coleoptile is differentiated as a closed sheath about 



cot (scu) 



Fig. 57. Diagrams i-6, steps in the development of the embryo ; 7, longisection of nearly 

 mature embryo, reduced one-half : w/^, coleoptile ; co/*-, coleorhiza ; co/, cotyledon or scutel- 

 lum; /, leaf; r c, root cap; rt, root; su, suspensor. (Redrawn after Weatherwax, The 

 Story of Maize, Univ. of Chicago Press.) 



the young foliage leaves; and the portion of the axis between the 

 suspensor and the cotyledonary node forms the primary root within 

 the surrounding tissue of the coleorhiza. The differentiation of 

 two or more seminal roots just above the cotyledonary node 

 completes the ontogeny of the embryo. 



The Endosperm. — According to Randolph, the primary endo- 

 sperm nucleus undergoes division prior to that of the zygote, and 

 free nuclear divisions occur so that three days after pollination 

 approximately ii8 free nuclei may be present. Shortly after this 

 time, wall formation is initiated in the endosperm adjacent to the 

 embryo and it becomes almost completely cellular by the end of 

 four days, except in the antipodal region. The size and shape of 

 the endosperm changes rapidly thereafter, and the dimensions of the 



