382 



THE SPERMATOPHYTES 



The male gametophyte. As in the gymnosperms, the male 

 gametophyte begins its history in the pollen grain before the lat- 

 ter has been shed. The first division forms a tube nucleus and a 

 generative cell (Fig. 302, C). The nucleus of the generative cell 

 divides sooner or later to form two sperm nuclei. These three 

 nuclei, with the rest of the protoplasm, constitute all there is of 



the male gametophyte (Fig. 305). 

 The pollen grains germinate on 

 the stigma of the pistil, finding 

 there suitable fluids to start their 

 growth. Each puts forth a tube 

 (Fig. 156, A, B, C) which pene- 

 trates the stigma and grows down- 

 ward toward the ovule case 

 (ovary). The tube nucleus and the 

 generative cell (or the two sperm 



nuclei if already formed) enter 

 FIG. 305. Pollen grain of the elder the tube and? passing to the tip> 



accompany its growth (Fig. 156, 



(Sambucus) 



The two sperm cells s and the tube ^ ^ 

 nucleus t, with the remaining pro- ' ' /' 

 toplasm, constitute the entire male The pollen tubes grow through 



the tissues of the stigma and 



style (if present) frequently over definite paths and enter the 

 micropyles of the ovules. This behavior resembles the way in 

 which parasitic fungi grow through the tissues of their hosts, 

 and it is clear that the pollen tubes live largely or wholly para- 

 sitically on the sporophyte. On entering an ovule the pollen 

 tube penetrates the nucellus and grows toward the embryo 

 sac, which by this time has developed the female gameto- 

 phyte. 



The female gametophyte. The mature female gametophyte 

 of an angiosperm (Fig. 306, B) contains only eight nuclei, the 

 products of three nuclear divisions in the embryo sac. These 

 are distributed as follows : There is a group of three nuclei at 

 the micropylar end of the embryo sac (Fig. 306, B, m), forming the 



