EMBRYOGENESIS IN GYMNOSPERMS 215 



a line of some considerable antiquity. The fossil evidence, however, is 

 conspicuous by its absence. 



EPHEDRACEAE 



There is only one genus in this family; some 32-35 species have 

 been described. Strasburger (1876), Land (1904, 1907), Berridge and 

 Sanday (1907), Berridge (1909), Khan (1943) and Johansen (1950) have 

 contributed to our knowledge of the gametophytic and embryonic 

 development. The ovule, typically with two archegonia, is a complex 

 structure with two integuments and a deep pollen chamber. Initially 

 the young archegonium has a neck cell and a central cell. The neck 

 cell gives rise to a neck of four, or even eight regular tiers, and after the 

 anticlinal walls have been formed the neck may consist of some 32 

 small cells somewhat irregularly disposed. The nucleus of the central 

 cell divides but no wall separates the ventral canal cell nucleus from the 

 egg nucleus. The ventral canal nucleus may remain near the neck or 

 may move down into the centre with the egg nucleus (Land, 1904; 

 1907; Chamberlain, 1935). 



Johansen (1950) has extended the account of the embryogeny of 

 Ephedra tr if urea as given by Land (1907). Eight free nuclei, described 

 as being somewhat unequal in size, are formed from the zygote nucleus. 

 These nuclei do not move to the basal region of the archegonium but 

 are unevenly distributed in the protoplasm. Three to five of the nuclei 

 become individually enclosed in somewhat irregular walls which later 

 become globular. Each of these globular cells is an embryo (or pro- 

 embryo); i.e. there is polyembryony without the cleavage phase that 

 is characteristic of so many gymnosperms. The nuclei which are still 

 free become arranged in a row down the centre of the archegonium, or 

 become scattered within it. The lower walled cells usually form embryos 

 successfully, and those near the micropylar end may also give rise to 

 embryos (Johansen, 1950). 



In the globular proembryo the nucleus divides, the more basal of 

 the daughter nuclei becoming larger than the other. Fig. 52k. The 

 two nuclei now move towards the basal end of the cell and in proximity 

 to each nucleus a small protuberance develops. The protuberance 

 opposite the larger nucleus grows out into a tubular structure- 

 described as the suspensor tube— while that opposite the smaller 

 nucleus disappears. If the embryo cell is at the base of the archegonium, 

 this suspensor tube will penetrate downwards into the prothallus: if 

 the cell is higher up, the tube may grow outwards and then downwards. 

 Both nuclei now move into the tube and a transverse wall is laid down, 

 Fig. 52l. The upper nucleus moves backwards in the tube and dis- 

 integrates. A considerable elongation of the suspensor tube now takes 



