512 



BOTANY 



PART II 



Order 2. Hydropterideae (Water-Ferns) 



The Water-Ferns include only a few genera, which are more or less aquatic in 

 habit, growing either in water or marshy places. They are all heterosporous. The 

 macro- and micro-sporangia do not develop, like those of the Filices, on the under 

 side of the leaves, but are enclosed in special receptacles at their base, constituting 

 sporangial fructifications or sporocarps. The wall of the sporangium, which consists 

 of a single layer of cells, has no annulus. The spores are surrounded by a 

 specially developed perisporium. 



FIG. 477. A, Marsilia quailrifolia ; a, young leaf; s, sporocarps. B, Pilularia globulifera ; 

 s, sporocarp. (After BISCHOFF, reduced.) 



The "Water-Ferns are divided into two families : Marsittaceae, including three 

 genera, and Salviniaceae, with two genera. 



To the Marsiliaceae belongs the genus Marsilia, of which the European M. 

 quadrifolia (Fig. 477-4) may be taken as an example. This species has a slender, 

 creeping, branched axis, bearing at intervals single leaves. Each leaf has a long 

 erect petiole, surmounted by a compound lamina composed of two pairs 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. Each 

 of them corresponds in development to the quadripinnate sterile lamina, but is not 

 divided into pinnae. The young leaves, as in the Filices, are circinate. 



Pilularia also grows in bogs and marshes. P. globulifera is found in 

 Britain. 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 ; the sporocarp corresponds to a segment of the leaf (Fig. 477 B}. 



