512 



BOTANY 



PART II 



known. Treub first showed in Casuarinn that the pollen-tube entered the ovule 

 by way of the chalaza, and thus reached the peculiar sporogenous tissue, which in 

 this case develops a number of macrospores or embryo-sacs. Chalazogamy, as this 

 mode of fertilisation is termed in contrast to porogamy, has no special systematic 

 significance, since it is found to occur in numerous plants. Nawaschin especially 

 has established an almost complete series of forms which exhibit parallel transition 

 from chalazogamy to porogamy (Casuarinaceae, Juglandaceae, Betulaceae, TJlmaceae) 

 (Figs. 473-475). Wettstein has more recently supported this view and sought to 

 explain it on biological grounds. That higher series of Angiosperms sometimes 



Fia. 475.— Longitudinal section of an ovary of 

 Juglans regia to show the chalazogamy. ps, 

 Pollen-tube ; e, embryo-sac ; cha, chalaza. 

 (Somewhat diagrammatic, x 6.) 



Fio. 476. — Funkia ovata. Apex of nucellus, 

 showing part of embryo -sac and egg- 

 apparatus. A, Before, B, during fertilisa- 

 tion ; 0, egg-cell ; s, synergidte ; t, pollen- 

 tube ; n, nucellus. (x 300.) 



revert to chalazogamy is to be compared with the loss of other characters in angio- 

 spermic flowers, and does not by itself discredit the developmental series recognised 

 above. 



When the pollen-tube, containing the two generative cells, has reached the 

 embryo-sac, its contents escape and pass by way of one of the synergidaj to the 

 ovum ; the corresponding synergida then dies. One of the two genei'ative nuclei 

 fuses with the nucleus of the ovum, which then becomes surrounded by a cellulose 

 wall. The second generative nucleus passes the ovum and unites with the large 

 secondary nucleus of the embryo-sac to form the endosi'KRM nucleus (Figs. 

 476-478). Both the male nuclei are often spirally curved like a corkscrew, and 

 Nawaschin, who first demonstrated the behaviour of the second generative nucleus, 

 compares them to the spermatozoids of the Pteridophyta. The further develop- 

 meut usually commences by the division of the endosperm nucleus, from which a 



n 



