148 THE MORPHOLOGY OF PTERIDOPHYTES 



thick stalk; the majority have a vertical row of thick-walled 

 cells, constituting the annulus, while some have an obhque 

 row and others merely a group of thick-walled cells ; some 

 have a high spore output, while in most species it is thirty- 

 two or sixty-four. 



Most commonly the prothallus is either cordate or 

 butterfly-shaped ranging in size from a few mm to i cm or 

 more across. There is a midrib several cells thick, but the 

 wings of the prothallus are only one cell thick. It is surface- 

 living, green and photosynthetic, and there are rhizoids on the 

 underside, among which antheridia and archegonia are borne ; 

 the archegonia are usually concentrated near the growing 

 point, or 'apical notch'. Departures from this typical form 

 occur in certain famihes, e.g. some have filamentous pro- 

 thalli, resembhng an algal filament, while even subterranean 

 prothalli are known, but this habit is extremely rare. 



Stages in the development of the archegonium are illus- 

 trated in Figs. 22K-N, the only variations being in the 

 number of tiers of neck cells at maturity. The structure of the 

 antheridium is also fairly constant throughout the Filicales. 

 Figs. 22B-H represent the various stages in the development 

 of the commonest type. The way in which successive cross 

 walls bulge upwards or downwards is peculiar and is res- 

 ponsible for the formation of the characteristic ring-shaped 

 cells of which the mature antheridium wall is constructed.^^ 

 At maturity, the cap cell is pushed off (Fig. 22I) to release 

 the antherozoids (usually thirty-two in number) (Fig. 22J). 

 Some famihes have a slightly more massive antheridium, 

 composed of a greater number of wall cells and containing 

 more antherozoids ; these are beheved to be more primitive 

 than the rest. 



The embryology of the leptosporangiate ferns is Hkewise 

 very constant throughout. The first cross-wall is almost 

 invariably longitudinal and the second transverse. Thus, the 

 zygote is divided at a very early stage into four quadrants, 

 two directed towards the apical notch of the gametophyte 



