THE ENDOSPERM 



169 



tropa unifora are of interest. In this ease the polar nuclei may 

 fuse in the absence of pollination, but the fusion may be hastened 

 or regulated by pollination. In normal cases fusion of polar 

 nuclei occurs about five days after pollination, but when pollina- 

 tion is prevented the interval may be prolonged to ten days or 

 even longer. Development of the endosperm was also induced 

 experimentally in the absence of fertilization. When pollination 

 is prevented, many of the ovules die within two or three weeks, 

 1 nit in others the sac enlarges and becomes filled with endosperm. 

 This development of en- 

 dosperm was observed in 

 from three to five per cent 

 of the ovules, but at a tem- 

 perature of 28° C, or by 

 using osmotic solutions, en- 

 dosperm was developed by 

 from six to twelve per cent 

 of the seeds. 



If a fusion nucleus is 

 formed, as is certainly gen- 

 erally the case, it usually 

 begins to divide before the 

 fertilized egg and with 

 much greater rapidity. 

 After fertilization, the egg 

 usually seems to rest for a 

 period while free endo- 

 sperm nuclei are being 

 formed. For example, 

 among the Ranunculaceae 

 (Guignard 43 ) and in As- 

 clepias (Frye" c ) free en- 

 dosperm nuclei are scattered through the sac before the egg 

 divides. But there is every gradation from an approximately 

 simultaneous division of primary endosperm nucleus and fer- 

 tilized egg, as usually in Sagittaria (SchafTner 18 ), Lilium 

 (Coulter 10 ), Nelumbo (Lyon * 5 ), 8arcod.es (Oliver n ), Senecio 

 (Mottier 15 ), and Erigeron (Land 32 ) (Fig. 77), in which last 

 case sometimes the egg and sometimes the primary endosperm 

 nucleus divides first, to a sac almost or even completely filled with 



Fio. 77. — Erigeron philadelpMms. Longitudinal 

 sections of embryo-sac after fertilization. J, 

 fertilized egg dividing before primary endo- 

 sperm nucleus; J3, primary endosperm nu- 

 cleus dividing before egg; x 550. — After 

 Land. 32 



