246 



PLANT MORPHOLOGY 



The female gametophyte develops inside the megaspore and resembles 

 in a general way that of Selaginella (Fig. 204A). The megaspore is 

 uninucleate when shed. Free-nuclear division occurs, followed by wall 

 formati(Mi in the apical region. Then walls fill in the entire megaspore 



D ^-^ F 



Fig. 204. Female gametophyte and archegonia of Isoetes echinospora {A to D), sperm of 

 Isoetes malinveriana (E), and embryo of Isoetes lacustris (F). A, female gametophyte with 

 a mature arfhegonium, the megaspore wall removed, X140; B, young archegonium with 

 primary ne(-k cell, neck canal cell, and primary ventral cell, X400; C, later stage, showing 

 two tiers of ne(;k cells, neck canal cell, ventral canal cell, and egg, X400; D, archegonium 

 with mature egg, X4.30; E, sperm; F, embryo;/, foot; r, root; //', first leaf; Ip, second leaf; 

 Ig, ligule; Is, sheath of first leaf. (A to D, after Campbell; E, after Belajeff; F, after Liebig.) 



cavity, developing centripetally as in gymnosperms. The female game- 

 tophyte does not protrude, as it does in Selaginella, but a triradiate crack 

 develops in the megaspore wall along which one or several archegonia and 

 numerous rhizoids appear. In some species the rhizoids are few or want- 

 ing. The archegonium is completely embedded. It consists of four tiers 

 of neck cells, a single binucleate neck canal cell, a ventral canal cell, and 

 an egg (Fig. 204B-Z)). No basal cell is formed in development. The 

 archegonium of Isoetes, like that of Selaginella, represents an advanced 

 type. 



