DEVELOPMENT OF PLAXTS 



357 



groups, called the polar nuclei, now approach each other and fuse, 

 forming a single large nucleus that is usually called the endo- 

 sperm nucleus. This rudimentary growth represents the female 

 gametophyte. The three outer or micropylar cells are not pro- 



Fig. 270. Germination of the megaspore : A, first division of the mega- 

 spore. B, second division of the nuclei. C, final division of the nuclei. 



vided with walls and consist of a rather lower-lying gamete and 

 two nourishing cells, the synergids or helpers (Fig. 271). The 

 inner group or antipodal cells usually have walls and they are 

 either soon disorganized and absorbed by the enlarging gameto- 

 phyte or they may remain as permanent features of the gametophyte 

 for a long time and even increase greatly in number, serving to 

 nourish the gametophyte by absorbing food from the sporangium. 

 The endosperm nucleus plays a very important role in the devel- 

 opment of the sporophyte, for as soon as fertilization has been 

 effected it forms by repeated division a mass of cells that com- 

 pletely fill the entire space within the enlarging spore, thus pro- 

 viding food for the nourishment of the sporophyte. It would 

 appear impossible to compare the various cells of this peculiar 

 gametophyte with the tissues of the female gametophyte of the 

 gymnosperms or ferns. It has been suggested that the female 

 gamete and the svnergids are the remains of three archegonia, 



