THE GAMETOPHYTES OF AX ANGIOSPERM 



381 



structure enters into the formation of the ovule case. Each 

 ovule consists of a central region called the nucellus (Figs. 303; 

 306, A ; 309, A, n), which becomes en- 

 veloped by two protective integuments 

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

 that arise from its base and grow up 

 around it, forming a small opening 

 above termed the micropyle (meaning 

 little gate). A cell in the interior of 

 the nucellus becomes the embryo sac 

 (Figs. 303, e\ 306, A y B\ which in 

 most 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 megasporangiurn. 

 Certain forms of angiosperms, as the 

 lily, have given up the formation of 

 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 embiyo 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. 



FIG. 304. A group of four 

 rnegaspores (tetrad) in the 

 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 

 Wiegaud 



