CEETAIN STRUCTUEES IN THE GENUS GNETUM. 323 



2. Gnetum africanum. 



A. The youngest embryo-sac found contained four free nuclei. 



B. In a later stagie the free nuclei become somewhat crowded in the clialazal end 



of the sac. 



C. About this time the " pavement tissue " appeal^ in the nucellus below th 



embryo-sac. 



D. When the last nuclear division in the free-nucleate sac is completed, the lowe 



end of the sac becomes septate by the formation of division-walls of 

 form ; each compartment so enclosed is multinncleate. 



o 



E. Meanwhile the micropylar half of the sac remains unseptate and its nuclei free 



and unchanged in appearance. So far as is known, the nuclei throughout the 

 sac at this stage are of the same generation; they are alike in all visible 



characters. 



E. All the nuclei in the multinucleate compartments enter into fusion ; the fusion 



is of precisely the same character as that described for Welioitschia. Each 

 multinucleate compartment thus becomes a uninucleate cell — a cell of the 



F 



primary endosperm. 



G. The fusion is not simultaneous in all the compartments. 



H. The primary endosperm so constituted undergoes cell-division and growth. 



As in WelwUscJda, the cells of the later stages of the endosperm are of various 

 sizes. The number of chromosomes appearing in the division of the primary 

 endosperm cell is larger than that characteristic of the sporophyte (fig. 24). 



I. According to Lotsy, fertilization occurs soon after the constitution of the 



primary endosperm is complete (^/. Lotsy [1899] fig. 34). 



E. Discussion. 



Although there are some important gaps in the preceding account of the origin of the 

 endosperm of Gnetum africanum, it is probably safe to assume that the endosperm in this 

 species at least is homologous with that of Welwitschia. Taking into account the previous 

 statements of Karsten and Lotsy regarding the development of the endosperm of the 

 species studied by them, there can be little doubt that, except perhaps in the time of 

 its formation, the endosperm of Gnetum as a whole resembles that of Welicltachia, and 

 that both differ in the particulars described from that of Ephedra. Is any morphological 

 importance to be attached to these differences, or not ? 



In the account previously given of the Welwitschia endosperm *, its morphological 

 character was discussed. The conclusion arrived at was that, inasmuch as the nuclei of 

 the primary endosperm cells were formed by the fusion of nuclei of the ar-gcncration 

 which were potentially sexual, snd lost their sexuality only as a result of fusion, the 

 endosperm was not a prothallus, but a new structure recognized as a definite morphological 

 entity under the name of " Trophophyte." It was further pointed out that the endosperm 

 of the Angiosperm might perhaps prove to be a more highly specialized form of the 



• Pearson, 1909. 



