230 



MORPHOLOG Y. 



apple (podophyllum) and through the ovules when they are 

 quite young, we will find that the ovule has a structure like that 

 shown in fig. 301. At m is a cell much larger than the surround- 

 ing ones. This is 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 



Fig. 301. 



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

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



several 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 to- 

 gether to form one nucleus, the endosperm nucleus or definitive 

 nucleus shown in fig. 302. One of the nuclei at the micropylar 

 end is the egg, while the t Vo smaller ones nearer the end are the 



