EMBRYOGENESIS IN GYMNOSPERMS 219 



cleavage embryos of conifers. Hence it is understandable that the pre- 

 dominance of one particular embryo may only be established at a much 

 later stage. Polyembryony, at least during the early stages, is a prevalent 

 condition in Gnetum. 



The segmentation pattern shown in Fig. 52b-e has been observed in 

 the early development of the embryonic cell. The young definitive 

 embryo in G. gnemon has an apical cell which may apparently persist 

 for some considerable part of the further development. In this species 

 also, concomitantly with the development of the two cotyledons, a 

 lateral outgrowth appears on the side of the hypocotyl lying next to the 

 ground. This protuberance, which becomes a cyUndrical suctorial 

 organ, appears to be of both exogenous and endogenous origin, for, 

 according to Johansen, tissues of the hypocotyl epidermis, cortex, 

 vascular tissue and pith all participate in its formation. At a later stage 

 the embryo becomes cylindrical and has a somewhat massive many-celled 

 apical region, with elongating cells behind. The single embryo which 

 comes to maturity has two cotyledons, a long hypocotyl and a massive 

 lateral suctorial, or haustorial organ, described as a foot (Campbell, 



1940). 



Several of the pre- and post-fertilisation developments in the 

 Gnetales are strongly reminiscent of the corresponding developments 

 in angiosperms. It is unlikely, however, that the two groups constitute 

 a sequence within a single phyletic line, but rather that parallel evolu- 

 tion is involved. 



welwitschiaceae 



The very remarkable features shown in the vegetative development 

 and mode of life of the single species Welwitschia mirabilis are matched 

 by features in its fertilisation and embryonic development. In the 

 ovulate flower, the nucellus is encased within an integument which 

 becomes extended acropetally into a long micropylar tube, this in turn 

 being enclosed within a sheath consisting of two broad perianth scales 

 conjoined at their margins (Chamberlain, 1935; Campbell, 1940). The 

 development of the gametophyte, or embryo sac, which exhibits some 

 very peculiar features, is still rather imperfectly known. The work of 

 Pearson (1909, 1910) suggests that only one embryo sac is formed. 

 Within this enlarging cell there is abundant free nuclear division, so 

 that eventually a thousand or more (approx. 1024) free nuclei are 

 distributed through the enlarged ellipsoidal gametophyte. A majority 

 of these are subsequently found towards the basal end. Cell walls now 

 begin to appear, these dividing the embryo sac into large multinucleate 

 cells. Archegonia are not formed. The multinucleate cells show a 

 number of curious developments: the nuclei may divide, but more 



