THE FEMALE GAMETOPHYTE 



85 



in the latter the two outer begin the formation of embryo-sacs ; 

 and in Eriobotrya Guignard 12 found that while ordinarily the 

 innermost megaspore of three functions, the middle or the outer 

 one may form the embryo-sac, and even all three may bea;in its 

 formation. The same author 11 also reports great irregularity 

 in Acacia, in some species the innermost of four niegaspores 

 functioning, in others the next outer one, and in still others 

 the middle one of a row of three. In 

 Loranthus also, Treub 13 finds that the 

 outermost megaspore of three persist- 

 ently functions. Among the Aralia- 

 ceae (Ducamp 112 ) usually the inner- 

 most of four niegaspores functions, hut 

 occasionally one of the middle cells 

 may become the embryo-sac. Such 

 cases serve to emphasize the megaspore 

 character of all the cells of the row. 



Among the Sympetalae, the only 

 well-established exception is that of 

 Trapella, in which Oliver 21 finds that 

 the outermost of four niegaspores func- 

 tions, and in one case the next cell, 

 while the innermost megaspore devel- 

 ops the remarkable haustorium (Fig. 

 32). In Asclepias tuberosa, although 

 the innermost of the row of four niega- 

 spores ordinarily functions, Frye 118 

 has observed cases in which the outer- 

 most megaspore functions, and others 

 in which the two innermost develop 

 together ; while Vescjue 4 reports that 

 in Salvia pratensis the outermost of 

 the four niegaspores functions. In Crucianella (Lloyd 105 ) 

 all four niegaspores, which in this case are not separated by 

 cell-walls, may begin to germinate (Fig. 33). Guignard 12 

 also includes Pyreihrum as among the forms whose outer- 

 most megaspore functions, but it needs further investigation. 

 It should be noted in this connection that when a row of four 

 niegaspores is to be formed, the nucleus nearest the chalaza al- 

 most invariably shows a more advanced stage in mitosis than 



Fig. 32. — Trapella sinensis. 

 Ovule some time after fer- 

 tilization : m, micropyle ; s, 

 synergids ; sp, suspensor ; </, 

 embryo; f, endosperm; w, 

 vascular bundle ; c, two long 

 cells resulting from the lon- 

 gitudinal division of the low- 

 ermost megaspore of a row 

 of four; x 100.— After Oli- 

 ver." 



