344 GENERAL BOTANY 



tetrad then develops into a microspore, or pollen grain. When 

 the pollen grains are ready to be shed, the cellular partition 

 separating the two microsporangia on each side of the anther 

 breaks down, and the two anther sacs are thus formed. The 

 anther wall then ruptures along the line of dehiscence and sheds 

 the microspores (consult Fig. 202, A, x). 



The young megaxporangia, or ovules, arise in the shepherd's 

 purse (Capsella) from two placentae, formed at the junction of 

 the two sporophylls in the ovary. Each megasporangium, when 

 young (Fig. 203, A), consists of a sporangium proper, called the 

 nucellus, and of two cellular outgrowths at the base of the 

 sporangium, which are the beginnings of the outer integument 

 and the inner integument. The funiculus is not perfectly 

 developed in the young ovule, but as the megasporangium 

 increases in size it grows more rapidly on one side than on the 

 other, which gives it a curved form (Fig. 203, j5). Coincident 

 with these changes in form a single cell of the sporangium 

 enlarges and becomes the mother cell of the future megaspore. 

 In Capsella this cell divides into a row of three cells which are 

 potential megaspores, and the lowest of the three then enlarges 

 ind forms a large megaspore, such as is shown in B, es. In many 

 angiosperms four cells arise from the megaspore mother cell 

 instead of three, as in Capsella, which indicates that these cells 

 constitute a spore tetrad. This process relates the formation 

 of the megaspore in the angiosperms to the usual process of 

 sporogenesis as it occurs in Selaginella and in the microspores 

 of angiosperms. The megaspore, when it has reached the size 

 shown in Fig. 203, B, germinates at once and forms a female 

 gametophyte (<7) exactly like that already described in the 

 mandrake. This germinated megaspore is called the embryo sac. 



GAMETOPHYTES AND FERTILIZATION 



The female gametophyte of Capsella (Fig. 203, C) corresponds 

 exactly to that of the mandrake (Fig. 86, 5) and the iris 

 (Fig. 204). It consists of the egg cell, or female gamete, the 

 synergidae (which are closely associated with the egg), the polar 



