SKOT. II 



PHANEROGAMIA 



513 



large number of nuclei lying in the protoplasm lining the wall of the enibrjo-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. 96) to produce a massive tissue. 



The distinctive feature of the development of the endosperm in Angiosperms 

 from the prothallus of Gymnosperms lies in the interruption whicli occurs in the 

 process in the case of the endosperm. In the embryo-sac, when ready for fertilisa- 

 tion, only an indication of the prothallus exists in the vegetative, antipodal cells. 

 The true formation of the endosjierm is dependent on the further development of the 



on 



Fio. 477. — Fertilisation of Lilium Martagoii. One 

 of the male nuclei is close to the nucleus of 

 the ovum, the other is in contact with the 

 nuclei of the embryo -sac. (Diagrammatic.) 



Fig. 478. —.-1, Embryo-sac of HcUanthus 

 a (DIM us (after Nawaschin). B, The male 

 nuclei more highly magnified, ps, Pollen- 

 tube ; Sj, $2, synergidie ; spi, sp2, male 

 nuclei ; ov, egg-cell ; ek, nucleus of era- 

 bryo-sac ; a, antipodal cells. 



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 (^''^) 

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



From the fertilised ovum enclosed within its cell wall a pro-embkyo 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 suspensor. Between 

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

 part in the formation of the lower end of the embryo. The segmentation of 

 the embryo presents differences according to whether the plant belongs to the 

 Monocotyledons or Dicotyledons. iN the latter two cotyledons are formed 



AT THE END OF THE GROWING EMBRYO (Fig. 479), AND THE GROWING POINT OF 



2 L 



