EMBRYOGENESIS IN THE PSILOTALES AND EQUISETALES 87 



the young sporophyte is initially leafless, but as it enlarges small 

 scale-like leaves begin to appear and normal leaves are subsequently 

 formed. There is thus an ontogenetic delay in the formation of the 

 first leaves, this possibly affording an example of what Haldane (1932) 

 has described as neoteny or paedogenesis. Because of their similarity 



Fig. 20. Psilotum triquetrum 



A-D, Stages in the embryonic development as seen in longitudinal section; I-I, 



first partition wall; a, neck of archegonium; s, shoot segment; /, foot segment; 



aw, apical meristem. (A-C, x 137; D, x 50; after Holloway). 



to the Psilophytales, e.g. the rootless, leafless fossils, Rhynia and 

 Psilophytoih it has been generally accepted that the living Psilotales, of 

 which no immediate or direct fossil ancestors are known, are simple 

 and primitive plant organisations which have remained relatively 

 unchanged over vast periods of time; i.e. Psilotum is rootless and 

 leafless because in its ancestors these organs never had been evolved. 

 As noted above, however, Psilotum does possess small scale leaves, 



