382 



BOTANY. 



fruits are one-celled in SalmniMem, and several-celled in MarMiaceoe. 

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

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

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

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

 at first). In Marsilia and FUularia the two kinds of spores occur in 

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



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

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

 Ma/)'siliacecE,Tirhich includes Marsilia and PUularia. The whole num- 

 ber of species is sixty-four, of which forty belong to Marsilia, the 

 others being unequally 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. Half a dozen 

 species occur in the Unifed States. 



Bhizocarps have been found as fossils in the Secondary (Jurassic) audi 

 Tertiary strata. 



§ III. Class Lycopodin^. * 



491. — The x)]ant-body of the asexual generation consists 

 of a solid, diehotomously 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 diehotomously 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 Lycopodinffl 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 Pliylloglossum) 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 Dichotomm, but as long as we have the 

 Equisetinm and MUcincB.vie may, for the sake of uniformity, retain the 

 old name given above. 



