516 



BOTANY 



secondary nucleus of the embryo-sac to form tin- EXDOSPEKM NUCLEUS (Figs. 

 502-504). Both the male nuclei an- often spirally curved like ;i cork-screw, and 

 N AW ASCII ix, who tirst demonstrated the behaviour of the second generative nucleus, 

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

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

 large number of nuclei lying in the protoplasm lining the wall of the embryo-sac 

 are derived. The endosperm arises by the formation of cell walls around these 

 nuclei and their proper surrounding protoplasm, and by the increase in number of 

 the cells thus formed (Fig. 95) to produce a massive tissue. The distinctive feature 

 of the development of the endosperm in Angiosperms from the prothallus of 



Fio. 501. Longitudinal section of -an ovary of 

 Juglans regia to show the chalazogamy. ps, 

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

 (Somewhat diagrammatic, x 6.) 



Flo. 502. Funkia nrntn. Apt-x of nucellus, 

 showing part of embryo-sac and egg-ap- 

 paratus. A, Before, B, during fertilisation ; 

 o, egg-cell; s, synergidii' ; t, pollen tube ; 

 n, nucellus. (x 390.) 



Gymnosperms lies in the interruption which occurs in the process in the case of 

 the endosperm. In the embryo-sac, when ready for fertilisation, only an indication 

 of the prothallus exists in the vegetative, antipodal cells. The true formation of 

 the endosperm is dependent on the further development of the embryo-sac, and 

 waste of material is thus guarded against. The starting-point of this endosperm 

 formation is given by the secondary nucleus of the embryo-sac, which needs to be 

 stimulated by fusion with the second generative nucleus (the so-called vegetative 

 fertilisation) to form the endosperm nucleus, before it enters on active division. 



From the fertilised ovum enclosed within its cell wall a PKO-EMBRYO consisting 

 of a row of cells is first developed ; the end cell of this row gives rise to the greater 

 part of the EMBRYO. The rest of the pro-embryo forms the SVSPEXSOU. Between 

 the embryo and suspensor is a cell known as the HYPOPHYSIS, which takes a small 



