360 BOTANY: PRINCIPLES AND PROBLEMS 



alighting directly upon the micropyle as it does in the gymno- 

 sperms, but instead is received upon a special projection of the 

 pistil, the stigma. Here it germinates and sends down a pollen 

 tube which ultimately reaches an ovule (Fig. 228). 



The male gametophyte has practically disappeared, for the 

 nucleus of the microspore now divides only into the tube nucleus 

 and the generative cell, and the latter produces two non-motile 

 male gametes. The female gametophyte is also greatly reduced 

 (Figs. 229 and 230). The megaspore (the successful one of four 

 originally produced in the nucellus) becomes much enlarged. 

 Its nucleus divides into two, and one of these migrates to the 

 end of the young embryo-sac next the micropyle of the ovule and 

 the other to the basal or antipodal end. Here each nucleus 

 undergoes two further divisions, so that at each end of the sac 

 there are now four nuclei. One from each set then moves toward 

 the middle and these two there fuse to iormtheeridosperm nucleus. 

 The three remaining at the micropylar end of the sac become 

 definite cells, one of them the egg cell and the other two the 

 synergids, probably the remains of an archegonium. The three 

 antipodal cells represent all that remains of the abundant vegeta- 

 tive tissue of the sac as it is found in the gymnosperms. The 

 gametophyte now consists of seven cells (or six cells and a naked 

 nucleus) and is ready for fertilization. The two male cells pass 

 down the pollen tube and enter the ovule. One fertihzes the egg 

 nucleus, as usual, and from this union the embryo results. 

 The other, however, instead of being eliminated, unites with the 

 endosperm nucleus, and from the union of these two nuclei (one 

 of which, it will be remembered, is already a product of fusion) 

 develops the endosperm of the seed. The endosperm thus differs 

 markedly in its history from that of gymnosperms. This phe- 

 nomenon of double fertilization is a distinctive feature of reproduc- 

 tion in all angiosperms. The development of the embryo and 

 the ripening of the seed go on much as among the lower seed 

 plants. 



The angiosperms are divided into two clearly marked subdivi- 

 sions, the dicotyledons and the monocotyledons. 



Dicotyledoneae or Dicotyledons. — As their name implies, 

 these plants are characterized by the presence of two cotyledons 

 in the embryo of the seed; as opposed to the single one of 

 monocotyledons, and they also display several other distinctive 

 features (Figs. 231 and 232). These are: (1) The presence of 



