PSILOTOPSIDA 47 



sporophyte with three rhizomatous portions and a young 

 aerial shoot, the whole plant being still attached to the 

 gametophyte. This kind of embryology, where the shoot- 

 forming apical cell is directed outwards through the neck of 



Fig. 7 



Psilotum nudum: a, gametophyte; b-h, stages in development of 

 archegonium. Tmesipteris tannensis: i-m, stages in developing 

 antheridium; n-q, stages in developing sporophyte; r, young 

 sporophyte attached to prothallus ; s, spermatozoids 



(a, q, s, after Lawson; b-h, Bierhorst; i-p, r, HoUoway) 



the archegonium, is described as *exoscopic'. While relatively 

 unusual in pteridophytes, it is nevertheless universal in 

 mosses and hverworts. Indeed, the young sporophyte of the 

 liverwort Anthoceros is very similar indeed to that of 

 Tmesipteris, at least up to the stage illustrated in Fig. 7Q, 

 even in such details as the lobed haustorial foot, and some 

 morphologists have gone so far as to suggest some sort of 



