SECT. x. 



MARSILEACEJE. 



371 



SECTION X. 



MARSILEACE^E, OR RHIZOSPERM^E. 



THE Marsileacese, a natural tribe of small perennial 

 aquatic herbs, have a filiform creeping rhizome with 

 alternate erect leaves, curled in vernation like those of 

 the ferns. The sporangia are enclosed in oval or 

 spherical leathery capsules, or receptacles, which contain 

 two dissimilar forms of reproductive organs sporangia 

 and antheridia, and are sessile, or nearly so, on the 

 rhizome at the base of the leaves, whence the general 

 name of Bhizospermae. The Pilularia globulifera, or 

 Pillwort, the only British species of its genus, may be 

 taken as a type. Its rhizome creeps over sand or mud, 

 at the margins of lakes and pools, where 

 it. is always submerged, or in sandy or 

 gravelly places, which are only occasion- 

 ally overflowed. At regular intervals the 

 rhizome sends off a tufb of roots and a 

 tuft of leaves opposite to them ; the leaves 

 are smooth, erect, and very slender, in 

 deep water almost hair like, varying from 

 four to five inches in height. The soli- 

 tary globular receptacles, brown, hairy, 

 and about the size of a small peppercorn, 

 spring from the axils of the leaves, sup- 

 ported on so short a stem that they ap- 

 pear to be sessile. Fig. 71 shows an Algerian species 

 of its natural size. The receptacles are divided by cross 

 partitions into two or more cells, and separate at ma- 



B B 2 



