TRITICUM 



175 



ment of the epithelial layer occurs after this elongation has taken 

 place. (Fig. 79, F, G.) 



The differentiation of the primary root within the coleorhiza, 

 or root sheath, takes place at about the time that the coleoptile 

 begins to develop; and the histogens and root cap are clearly de- 

 fined before the embryo is a millimeter in length. A difference in 

 the growth rate of the cells of the root cap and those of the adjacent 

 tissues results in the formation of a schizogenous cavity around the 



— e 



Fig. 79. A-F, longisections of embryos in progressive stages of development; G, epi- 

 cotyledonary portion of embryo : a, upper portion of scutellum ; c, coleoptile ; e, epiblast ; 

 /, first foliage leaf ; f, ventral scale; at, region of intercalary elongation of scutellum. (Re- 

 drawn from Percival, The Wheat Plant, Duckworth and Co.) 



root apex. As the elongation of the embryonic root proceeds, this 

 cavity is extended so that in the mature embryo the root is separated 

 from the coleorhiza by a cylindrical cavity. 



The Endosperm. — After fertilization, the development of the 

 endosperm parallels that of the embryo; and, before the first divi- 

 sion of the zygote, the primary endosperm nucleus undergoes 

 division followed by a series of free nuclear divisions until con- 

 siderable numbers of nuclei are formed. These occupy a parietal 

 position in the embryo sac; and, by the time the embryo has 

 reached the 10- to 15-celled stage, there is a continuous layer of 

 endosperm cells lining the embryo sac. In six or seven days, the 

 sac is completely filled with endosperm tissue which has developed 



