THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



605 



root, but there is apparently a rotation during development so that the foot 

 comes to lie beneath, while the root-cotyledon axis lies horizontally above 

 (Fig 616). At this stage the ligule of the cotyledon begins to grow from a 

 single large cell and is about half as long as the cotyledon by the time the 



Fig. 6i5.-Isoetes echhwspora. Development of female gametophyte and archegomum 

 A, Mother cell of archegonium. B, Separation of neck cell, canal ,<^^" ^"'i ^"^P'^,^^^ 

 mother cell. C, Further differentiation of neck cells and ventral canal cell. D, Mature 

 archegonium. E, Section through the megaspore contammg the mature female 

 prothallus with archegonium. {After Campbell.) 



