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Transactions of the Royal Society of South Africa. 
the period separating the beginning of spore-germination and the consum- 
mation of fertilisation — an adaptation to zerophytic conditions.* On this 
view the embryo- sac of Welwitschia in its development up to the stage of 
fertilisation is highly specialised. Strasburger thought that no great weight 
was to be attached to the occurrence of embryo-sac-tubes in Welwitschia. 
But the relationships between G-netum and Welwitschia would be established 
on a sounder basis if any indications of a tendency to form such structures 
were recognised in G-netum. So far there is no reliable evidence of such a. 
tendency. On the analogy of the pollen- tube, it might be anticipated that 
the embryo-sac-tube in its original form was haustorial in function. Tubular 
prolongations, presumably haustorial, from the micropylar end of the sac 
are described for Gnetum funiculare and G. verrucosum,f but since these are 
formed very early in the development of the sac and are destroyed X long 
before its septation, there is little or no justification for regarding these as 
in any way related to the embryo-sac-tubes of Welwitschia. Lotsy § has 
recently adopted the view that structures in the embryo-sac of G. TJla, which 
he first regarded as apogametic embryos, || are more probably the homologues 
of the embryo-sac-tubes of Welwitschia. But, owing to the imperfections 
of his material, the life-history of this species is only very partially known,, 
and the value of the suggestion is consequently doubtful. But at least it 
directs attention to the possibility of gaining further knowledge of the origin 
of these remarkable structures and, incidentally, of the relationships between 
Gnetum and Welwitschia, from species of Grnetum of which at present little 
or nothing is known save the fact of their existence. 
The phylogeny of the G-netum type of embryo- sac is at present obscure. 
Prominent among the facts which justify the conclusion that it was imme- 
diately derived from that of the lower seed-bearing plants are the existence 
of Ephedra, which retains the intrasporic prothallus, and the characters of 
the nucellus, of the micropyle and of the pollen-grain in G-netum and 
Welwitschia. It has been shown (p. 236) that the pollen-grain, both in the 
course of its development from the microspore and in its later germination, 
exhibits unmistakable signs of afiinity with the gymnosperm pollen-grain. 
It is probable that the products of the germination of the macrospore are 
equally related to those which are typical of the lower gymnosperm. There 
are no grounds for seeking elsewhere for the origin of the G-netum sac. 
The fundamental differences between the sac of the lower G-ymnosperm 
and that of G-netum are : (1) The presence of vegetative tissue of the 
gametophyte in the former and its complete absence from the latter ;. 
* Strasburger, 1872, p. 295 ; Pearson, 1909, p. 387. 
t Karsten, 1893, pp. 354, 355, Taf. 9, figs. 45, 46. 
X Loc. cit., p. 354. 
§ Lotsy, 1911, pp. 350-352. 
II Lotsy, 1903. 
