6S4 PATTERNS AND PROBLEMS OF DEVELOPMENT 



that of dicotyledonous forms (Fig. 211, A-C); but in most members of 

 this group the single cotyledon develops from the terminal region, and 

 the stem tip from cells lying laterally (s of Fig. 211, C). Presumably the 

 side of the embryo on which the stem tip develops is determined by some 

 factor in the embryonic environment, but thus far no consideration of this 

 point has been found in the botanical literature consulted. 



A B C 



Fig. 210, A-C. — Three stages in early development of embryo and suspensor of a dicotyle- 

 donous angiosperm, Capsella biirsa-pastoris. A, two-cell stage of embryo; B, one side of eight- 

 cell stage; C, later stage (after Coulter and Chamberlain, Morphology of Angiosperms, 1903). 



Polyembryony occurs in many spermatophytes with development of 

 embryos, not only from single early cells of the primary embryo but from 

 other cells of the gametophyte and even from cells of the nucellus, which 

 is sporophyte tissue surrounding the gametophyte. In various gymno- 

 sperms polyembryony is normal or usual. In the pine, for example, each 

 of the first four embryonic cells, products of two vertical divisions, has 

 a suspensor cell, and each may form an embryo; or a single embryo may 

 develop from all four. In other conifers also, single cells of early embry- 



