Qn Some Stages in the Life History of Gnetum. 
Sporophyte. Sporophyte. 
251 
Macrospore. 
Prothallus (much 
reduced and 
evanescent). 
Antheridium 
(much reduced). 
the length of the sac*; (2) in (xiietum fertilisation occurs within the sac 
before septation is completed or even begun ; in Welwitschia fertilisation is 
effected after the completion of septation and not in the sac, but in the 
nucellar cap into which the female gametes are conducted by tubular pro- 
longations of the compartments in which they are enclosed — embryo- sac 
tubes. t A reasonable view of the relations between these forms, as indicated 
by the differences disclosed in the development of their sacs, is that Grnetum 
is the more primitive. + The formation of primary endosperm-cells by 
septation and nuclear fusion occurs throughout the sac in both cases. But 
these processes are completed more rapidly in Welwitschia than in G-netum, 
and to this is due the fact that the functional gametes are enclosed in a 
compartment in the former case, free in the sac in the latter. The speeding 
up of these occurrences in Welwitschia may perhaps be correlated with the 
extreme form of siphonogamy which prevails in this genus, and this may 
quite well be of the nature of an adaptation to the necessity for shortening 
Sperm-cell, 
Sporophyte. 
Trophophyte. 
* Pearson, 1909, p. 351. 
t Loc. cit., p. 349. 
X See p. 240. 
