383 



BOTANY. 



fruits are one-celled in Salviniaceai, aud several-celled in Marsiliacem. 

 In Salviida (Fig. 273) the microsporangia are small and numerous, and 

 are contained in separate fruits from the macroeporangia, which are few 

 in number ; each of the former contains many microspores, and the 

 latter a single macrospore (by the abortion of three, as four are formed 

 at first). In Manilla and Pilularia the two kinds of spores occur in 

 tliB same fruit, and in tbe former in the same sporangium. 



Pour genera are known ; these are arranged under two suborders or 

 families, the Salmniacex, which includes Salvinia and Azolla, and the 

 M'm-siUaeece,w\nch includes MarsUia and Pilularia. The whole num- 

 ber of species is about fifty, of which thirty-seven belong to Marsilia, 

 the others being about equally divided between the remaining geuera. 

 All the species aje of small size, rarely exceeding a few centimetres in 

 height; they grow in ditches and other wet places. Three or four 

 species occur in the United States. 



Rhizocarps have been found as fossils in the Secondary (Jurassic) and 

 Tertiary strata. 



§ III. Class Lycopodiu"^. * 



491. — The plant-body of the asexual generation consists 

 of a solid, dichotomously branched, leafy, aud generally erect 

 stem. The leaves, which have a central fibre- vascular bundle, 

 or midrib, are small, simple, sessile, aud imbricated, and 

 usually bear a considerable resemblance to those of Mosses. 

 The roots are mostly slender and dichotomously branched. 



The Lycopodinse are for the most part terrestrial peren- 

 nials. They are usually of small size, rarely exceeding a 

 height of 15 or 30 centimetres (6 or 8 inches). 



492. — The spores of the Lycopodinae are produced in spo- 

 rangia which are generally (if not always) axillary appen- 

 dages of the leaves. In four of the genera {Lycopodium, 

 Psilotum, Tmesipteris, and Phylloglossum) the spores are 

 of one kind ; while in the two remaining genera {Selaginella 

 and Isoetes) they are of two kinds, the macrospores and the 

 micrbspores. 



493. — The prothallium or sexual generation is scarcely 

 known in the isosporous genera ; it appears, however, to be 

 a thickish mass of tissue, which develops underground, and 



* Sachs calls tliis class the Dichotomy, but as long iis w(! have the 

 Eqiiisetiiim and Filicince, we may, for tlie sake of uniformity, retain the 

 old name given above. 



