CERTAIN STEUCTUEES IN THE GENUS GNETUM. 



321 



be found compartments still containing free nuclei side by side with those which by 

 the completion of the nuclear fusion have become uninucleate cells (PL 32. figs. 19, 22). 

 Fig. 22 shows the completion of what may be described as the primary endosperm. 

 Pollination usually occurs between the stages of figs. 18-22, while the nuclei of 



micropylar end of the sac appear to be in exactly the 



condition as they 



when the chalazal end became septate preparatory to nuclear fusion in that region 



{cf. PI. 31. figs. 11, 14; PI. 32. figs. 16, 20, 21). 



It appears then, that while the nucellar " pavement tissue " described by Coulter in 

 G. Gnemon is present in G. afrlca?ium, this species also shows the fertilization antipodal 

 tissue of the embryo-sac, such as was found by Lotsy in G. Gnemon, the existence of 



which has been denied by Coulter 



This "antipodal 



>j • 



in the embryo-sac is tl 



primary endosperm, consisting of cells whose nuclei are formed by fusion 



ed 



m 



this 



vestigation has not permitted the development of the endosp 



The material 



jrm of 



G. Gnemon to be followed throughout. It does, however, show stages which leave 



alike, and 



little doubt that in this respect' G. Gnemon and G. africanum are exactly alii 

 that Lotsy's account in respect of the endosperm is perfectly accurate. 



Lotsy has followed the growth of the pollen-tubes down to the embryo 

 G. Gnemon, and although his account of fertilization is far from complete, there can be 



in 



little doubt that any of the free 



of the upper 



ptate part of the sac are 



ther words, are crametes 



pable of being fertilized 



For the present it may be safely assumed that the same 



true for G, ah 



If so, we are in a position to establish a comparison between the unfertilized embry 



of Gnetum and Welwitschia. In the latter 



we have a sac which, before septation 



and nuclear fusion, contains many nuclei apparently equol 



and potentially 



equivalent in function 



So far as is known, those of the upper or micropylar region of 



the sac are all of them capable of uniting with sperm 



produce pro-embry 



Since in the lower part of the 



some may escape fusion, and in this case travel up 



nucellus in embryo-sac tubes f, it app 



that all the nuclei of this 



o 



of the same quality, and lose their sexuality only when they fuse together to form the 



le nuclei of the primary endosperm cells 



So far as can be seen, Gnetum africanum 



entirely with Welwitschia, save only that in Gnetum there is no septation in the 



micropylar half of the sac. In Welwitschia 



ptation is, perhaps, merely of tl 



nature of an adaptation to the peculiar foi-m of siphonogamy which prevails 

 genus. It appears, then, that in 



this 



Gnet 



W 



free, potentially fertile nuclei 



the sac before septation 

 after the septation of the lower end of the 



the nuclei here lose their sexuality by fusion, while those of the upper end, which 



Therefore the primary endosperm of Gnetum is in all 



remain unfused, retain it. 



respects homologous with the primary endosperm of Welwitschia 



The cells of the primary endosperm of Gnetum soon ui ' 

 them immediately after the completion of the fusion, with the result that the endosperm 



de 



cell-division, some of 



* Coulter, 1908; Lotsy, 1899. 



t Tearson, 1909, text-fig. p. 3G5 ; see also p. 373. 



3a2 



