THE GAMETOPHYTES OF AN ANGIOSPERM 381 



icture enters into the formation of the ovule case. Each 

 rale consists of a central region called the nucdlus (Figs. 303; 

 )6, A ; 309, A, ri), which becomes en- 

 sloped by two protective integuments 

 rigs. 303 ; 306, A ; 309, A, B, C, ii, oi) 

 it arise from its base and grow up 

 rand it, forming a small opening 



ibove termed the micropyle (meaning 

 btle gate). A cell in the interior of 

 le nucellus becomes the embryo sac 

 figs. 303, ; 306, A, ), which in 

 lost cases is the exact equivalent of 



a megaspore. This is proved by the 



fact that the embryo sac in such forms 



is one of a group of four cells, or tetrad 



(Fig. 304), and that the development 



of this group follows the same history 



as in pollen and spore formation. The 



nucellus is therefore a megasporangium. 



Certain forms of angiosperms, as the FIG. 304. A group of four 



lily, have given up the formation of megaspores (tetrad) in the 



tetrads, and the spore mother cell de- 

 velops directly into the embryo sac. 1 

 361. The gametophytes of an an- 



giosperm. The male gametophyte 



(contents of the pollen grain and tube) 



is clearly similar to that of the gymno- 



sperm; but the female gametophyte of the angiosperm is a 



very much more reduced structure than anything in the gym- 



nosperms. 



1 In these cases the first two nuclear divisions within the embryo sac 

 have the peculiarities of those in all spore mother cells. In the lily the 

 nuclei of the nucellus have 24 chromosomes, but the nuclei of the embryo 

 sac have 12. This shows that the two nuclear divisions characteristic of 

 spore formation have become a part of the gametophyte phase of the plant's 

 life history. 



nucellus of an ovule 

 (Canna) 



The upper three megaspores 

 of the group are breaking 

 down, while the lower is 

 rapidly enlarging to become 

 the embryo sac. After 

 Wiegand 



