42 



PISTILS AND STAMENS 



developed pollen tube. This feature is shown in Figure Jfi. The 

 nucleus at the end of the tube and known as tvhe nucleus directs 

 the growth of the tube and disappears 

 soon after reaching the embryo sac. 

 The two nuclei following closely be- 

 hind the tube nucleus are the s.'perms 

 or male nuclei, the structures which 

 join with the egg and primary endo- 

 sperm nucleus in fertilization. The 

 pollen tube is a passage way through 

 which the sperms pass to the embryo 

 sac. 



Fertilization. — After the two sperms 

 reach the embryo sac, one approaches 

 the egg and fuses with its nucleus, while 



Fig. 49. — Pollen grains in different stages 

 preparatory to fertilization. A, surface view 

 of a pollen grain; B, section through pollen 

 grain in uni-nucleate stage; C, section through 

 pollen grain showing the nucleus divided into 

 the generative (g) and tube nucleus (0; D, 

 pollen tube forming into which the two nuclei 

 have passed; B, tube more developed and 

 generative nucleus divided into two sperms 

 {g). Much enlarged. 



Fig. 50. — A diagram of a length- 

 wise section through the pistil of Red 

 Clover, showing pollen tubes trav- 

 ersing the stigma and style. Two 

 pollen tubes have reached the em- 

 bryo sacs, p, pollen grains develop- 

 ing tubes; st, stigma; ■p.i, pollen 

 tubes; o, ovules; e, egg; era, en- 

 dosperm nucleus; s, sperms. Much 

 enlarged. 



the other approaches the primary endosperm nucleus and fuses with 

 it. This process of fusion is called /erfiZizafion. Since there are two 



