GROUP III. PTERIDOPHYTA : FILICIN^. 373 



D. HETEROSPOROUS LEPTOSPORANGIAT.E. 



(Hydropteridese or Rhizocarpae.) 



This group includes the four genera, Salvinia, Azolla, Marsilea, 

 Pilularia ; of these the two former constitute the order Salviniacese, 

 the two latter the order Marsileacese. They are all more or less 

 aquatic in habit, Salviuia and Azolla being free-floating fugacious- 

 plants, whilst Marsilea aud Pilularia are perennials growing in 

 bogs and marshes. 



SPOROPHYTE. The stem is a horizontal dorsiventral rhizome. 

 It generally bears foliage-leaves in alternating longitudinal rows 

 (four rows in Salvinia ; two rows in the other genera) on the dorsal 

 (superior) surface ; and roots in one (Marsileaceae) or two (Azolla) 

 longitudinal rows on the ventral (inferior) surface. In Salvinia, 

 however, there are no roots, but the stem bears in place of 

 them two rows of submerged leaves on its ventral surface. The 

 lateral branches, sometimes very numerous, are borne on the 

 flanks. 



The foliage-leaf presents a considerable variety of form. In 

 Salvinia it is broad and flat, sessile and entire, with a well- 

 marked midrib ; in Azolla the leaf is small and two-lobed, the 

 lower lobe being submerged, whilst the upper floats on the surface 

 of the water : in Marsilea the leaf has a long erect petiole bearing 

 a paripinnate bijugate compound lamina of four leaflets: in 

 Pilularia the leaf is cylindrical and erect. 



Circinate vernation obtains in the Marsileacese, but not in the 

 Salviniaceae : in Salvinia the vernation is conduplicate, and in 

 Azolla the lamina is expanded from the first. 



In Salvinia the leaves are borne in a whorl of three at a node, 

 two being a pair of opposite foliage-leaves, and the third a sub- 

 merged leaf (p. 17) ; in the other genera the phyllotaxis is alternate. 



The suljmcrged leaf of Salvinia consists of a number of long 

 filamentous branches, springing from a short petiole, and densely 

 covered with multicellular hairs. 



The sporangia are of two kinds, microsporangia and macro- 

 sporangia : they are bsrne in sori enclosed in structures termed 

 sporocarps. The morphology of the sporocarp is, however, 

 altogether different in the two orders, and the same term ought 

 not to be applied to both : it would be well to restrict the term 

 " sporocarp " to the more complex fructifications of the 

 Marsileacese. 



