THE ANGIOSPERMAE 



1419 



one or two and the nucleus has then lost nearly all its chromatinicity. This 

 condition persists until nuclear fusion occurs. There is no evidence of 

 diffusion of the nucleic acid and what becomes of it is not known. 



The Polar Nuclei. The fourth nucleus at each end of the embryo sac 

 migrates towards the centre of the sac and together they form a contiguous 

 pair. One of the pair may sometimes be larger than the other, but the 

 difference does not seem to be significant, for in the great majority of cases 

 they are indistinguishable. These nuclei unite, either before, during or after 

 the entry of the pollen tube into the sac, and the product of their union is 

 called, variously, the secondary embryo sac nucleus, the central nucleus or 

 the primary endosperm nucleus, the last being perhaps the best description, 

 since this nucleus and the cytoplasmic mass around it, which is the general 

 cytoplasm of the embryo sac, may be regarded as the primordium of the 

 endosperm. 



This nuclear union is often delayed or suppressed, the latter in diploid, 

 apomictic embryo sacs or in those cases where endosperm formation is 

 itself suppressed. In the great majority of Angiosperms fusion of the polar 

 nuclei takes place before fertilization (Fig. 131 1). 



li\;'-' VL'''--ir'. -•'■^L-v. ■■:. 



'I 

 '•I 



1- : .,.1 



^'> 



' " B 



'•.■ - -.'y^.X. I ■■'■■■. 







-v "■.' 







Fig. 131 1. — Impatiem liltimhtlineia (I. roylei). Successive stages 

 in the fusion of the t\% o polar nuclei. {After Steffen.) 



