DIV. n 



SPERMATOPHYTA 



575 



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 



FIG. 556. Fertilisation of Lilium Martagon. 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. Lettering as in Fig. 

 557. (Diagrammatic.) 



FIG. 557. A, Embryo-sac of HtHanthiis 

 annuus (after NAWASCHIN). B, The male 

 nuclei more highly magnified, ps, Pollen- 

 tube ; sj, .2, synergidae ; spi, sp%, male 

 nuclei ; ov, egg-cell ; efc, nucleus of em- 

 bryo-sac ; a, antipodal cells. 



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- 

 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 ( 9 ). 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 



