CRYPTOGAMS 



407 



The Water-Ferns are divided into two families, Marsiliaceae and 



Salviniaceae, each of which includes two genera. 



Marsiliaceae. — Of the two genera belonging to this family the 



more important is the genus Marsilia, com- 

 prising about fifty species, of which M. 



quadrifoliata (Fig. 340) may be taken as an 



example. This species grows in marshy 



meadows, and has a slender, creeping, 



branched axis, bearing at intervals single 



leaves. The young leaves are coiled at the 



tip (circinate) ; in this respect the leaves of 



the Marsiliaceae exhibit the same mode of 



growth as those of the Ferns. Each leaf 



has a long erect petiole, surmounted by a 



compound lamina composed of two pair of 



leaflets inserted in close proximity. The 



stalked oval sporocarps (s) are formed in 



pairs above the base of the leaf-stalk, or in 



other species they are more numerous ; they 



represent a fertile leaf-segment correspond- 

 ing to the biju- 

 gate sterile leaf 

 lamina. The 

 sporangia, 

 united in sori, 

 are enclosed 

 within the cap- 

 sule, disposed 

 in two rows in 



correspondingly arranged cavities ; in the 

 young fruit each chamber opens outwards 

 on the ventral side by means of a narrow 

 canal, which eventually becomes closed. 

 The sporangia are developed originally, as 

 in the Fern, from superficial cells, but 

 become arched over by the surrounding 

 tissue, and thus subsequently appear as if 

 formed in internal chambers. 



Pilularia, the second genus included 

 in this family, grows also in bogs and 

 marshes. It differs from Marsilia in its 

 simple linear leaves, at the base of which 

 occur the spherical sporocarps, which 



arise singly from the base of each sterile leaf -segment (Fig. 341). 



Salviniaceae. — This family contains only free - floating aquatic 



plants belonging to the two genera Salvinia and Azolla. In Salvinia 



Fig. 340. — Marsilia quadrifoliata. 

 a, Young leaf ; s, sporocarps. 

 (After Bischoff, reduced.) 



Fig. 341. — Pilularia globidifera. s. 

 Sporocarp. (After Bischoff, re- 

 duced.) 



