GAMETOPHYTE AND SPOROPHYTE. 



327 



phyllum) and through the ovules when they are quite young, we 

 shall find that the ovule has a structure like that shown in fig. 389. 

 At m is a cell much larger than the surrounding ones. This is 

 called the macrospore. The tissue surrounding it is called here the 

 nucellus, but because it contains the macrospore it must be the 

 macrosporangium. The two coats or integuments of the ovule are 

 yet short and have not grown out over the end of the nucellus. 

 This macrospore increases in size, forming first a cavity or sac 

 in the nucellus, the embryo sac. The nucleus divides several 



Fig. 389. 



Young ovule (macrosporangium) of podophyllum. n, nucellus containing the one- 

 celled stage of the macrospore; i.int, inner integument; o.int, outer integument. 



times until eight are formed, four in the micropylar end of the 

 embryo sac and four in the opposite end. In some plants it 

 has been found that one nucleus from each group of four moves 

 toward the middle of the embryo sac. Here they fuse together 

 to form one nucleus, the endosperm nucleus or definitive nucleus 

 shown in fig. 390. One of the nuclei at the micropylar end is 

 the egg, while the two smaller ones nearer the end are the syner- 



