l68 THE MORPHOLOGY OF PTERIDOPHYTES 



monophyletic, however, then it is most probable that the 

 ancestral type had marginal rather than superficial sori, and 

 that the early Superficiales underwent a 'phyletic sUde' early 

 in their evolution, while the Marginales are proceeding more 

 slowly in the same direction. 



'Water Ferns' 



There are two interesting groups of leptosporangiate ferns 

 which, at one time, were classified together as the Hydro- 

 pterideae. Features which they show in common are hetero- 

 spory and a hydrophilous habit, but in other respects they 

 are so different as to warrant a much wider separation, 

 from each other and from the rest of the ferns. Accordingly, 

 their taxonomic status has been elevated to the Marsileales 

 and the Salviniales respectively. 



Marsileales 



Pilulariaceae Pilularia 

 Marsileaceae Marsilea, RegneUidium 



All the members of the Marsileales have creeping rhizomes, 

 bearing erect leaves at intervals, on alternate sides. The only 

 member of the group represented in the British flora is 

 Pilularia globuUf era ('Pillwort'). Like all species oi Pilularia, 

 its leaves are completely v/ithout any lamina (Fig. 26A). The 

 leaves of the monotypic Brazihan genus RegneUidium have 

 two reniform leaflets. Marsilea occurs in temperate and 

 tropical regions, many of its sixty-five species occurring in 

 Austraha. Its leaves have four leaflets and somewhat 

 resemble a 'four-leaved clover' (Fig. 26B). All have soleno- 

 stelic rhizomes but, in Pilularia, the vascular structure is 

 much reduced, and the internal endodermis may be missing. 

 The sporangia, in all three genera, are borne in hard bean- 

 hke sporocarps, attached either to the petiole, near its base, 

 or in its axil, either stalked or sessile. The morphological 

 nature of these sporocarps has been the subject of much dis- 



