382 



BOTANY. 



fruits are one-celled in Salviniaeea, and several-celled in Mardliacew. 

 In Salmnia (Fig. 273) the inicrosporangia are small and numerous, and 

 are contained iu separate fruits from the macrosporangia, 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 Marsilia and Pttularia the two kinds of spores occur in 

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



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

 families, the Salviniacece, which includes fsalvinia and Azolla, and the 

 Marsiliacece, which includes Marsilia and Pttularia. 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 genera. 

 All the species are 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 



491. The plant-body of the asexual generation consists 

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

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

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

 usually bear a considerable resemblance to those of Mosses. 

 The roots are mostly slender and dichotomously branched. 



The Lycopodinae are for the most part terrestrial peren- 

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

 height of 15 or 20 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 (Lycopodiiun, 

 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 

 microspores. 



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 this class the Dichotomy, but as long as we have the 

 EquisetincB and Filicince, we may, for the sake of uniformity, retain the 

 old name given above. 



