302 



PLANT MORPHOLOGY 



effecting fertilization (Fig. 335). Although distinct male cells may be 

 present in the pollen tube, or even in the embryo sac, there is evidence 

 indicating that, in most angiosperms, only a male nucleus enters the cyto- 

 plasm of the egg. Immediately after fertilization, the egg becomes sur- 

 rounded by a cell wall. 



The second male nucleus entering the embryo sac now unites with the 

 nucleus resulting from the fusion of the two polar nuclei, thus forming the 

 primary endosperm nucleus. This unique behavior, which has been called 



"double fertilization," has been observed in 

 so many angiosperms that it must be regarded 

 as characteristic of the group as a whole. 

 Usually one of the synergids is destroyed by 

 the entrance of the pollen tube, while the other 

 synergid, as well as the antipodal cells, gen- 

 erally disappear soon after fertilization has 

 taken place. 



Although ordinarily the pollen tube enters 

 the ovule through the micropyle (porogamy) , 

 in some of the more primitive Archichla- 

 mydeae it may penetrate the lower end of the 

 ovule. This behavior, known as chalazogamy , 

 has been observed in the Casuarinaceae, 

 Juglandaceae, Corylaceae, Urticaceae, and 

 Euphorbiaceae. In certain other angiosperms 

 the pollen tube may follow an intermediate 

 route, entering the ovvile through the integu- 

 ment (mesogamy). 

 The behavior of the polar nuclei is variable, depending on the species. 

 Generally they unite before the pollen tube enters the embryo sac, forming 

 the fusion nucleus (Fig. 331). To this the male nucleus later is added. 

 Sometimes, as in Fritillaria and Lilium, the fusion of the polar nuclei is 

 delayed until the male nucleus has joined them, all three then fusing 

 simultaneously (Fig. 335). Sometimes the polar nuclei remain at oppo- 

 site ends of the embryo sac until the pollen tube has entered. Then the 

 male nucleus fuses with the micropylar polar nucleus, the other one join- 

 ing them later. 



Because typically a male nucleus unites with two haploid polar nuclei, 

 the primary endosperm nucleus, of course, is triploid. In some forms, 

 however, as in the Onagraceae, it is diploid, being formed by a union 

 between the male nucleus and one polar nucleus; while in such genera as 

 Peperomia, Fritillaria, Lilium, Plumbago, and Penaea various degrees of 

 polyploidy are attained, the primary endosperm nucleus arising from the 



Fig. 335. Fertilization in 

 Fritillaria bifiora, X 250. One 

 naale nucleus is in contact 

 with the egg nucleus, while 

 the other has joined the two 

 polar nuclei. 



