40 MOSSES AND FERNS chap, 



of the young segments separate and form the beginnings of the 

 characteristic air-spaces. In R. hirta some of the dorsal cells 

 about the same time form short pointed papillae, the first indica- 

 tion of the pointed hairs characteristic of this species. As the 

 plant grows, new root-hairs are formed by the growing out of 

 ventral cells into papillae, which are cut off by a partition from 

 the mother cell. These first -formed root -hairs are always 

 smooth-walled, and it is only at a much later stage that the 

 other form develops, as well as the ventral lamellae, which are 

 quite absent from the young plant. 



Classification of the RicciacecE 



Besides the genus Riccia, which includes all but three species 

 of the family, there are two other genera, each represented by 

 a single species, which undoubtedly belong here. Of these 

 Ricciocai'pus natans is of almost world-wide distribution. It is 

 a floating form which, like Riccia fluitans, only fruits when 

 growing upon the earth. Leitgeb ^ has made a very careful 

 study of the structure and development of the thallus, which 

 differs a good deal from that of Riccia, in which genus this plant 

 was formerly placed. The apical growth is essentially the 

 same, and the differentiation of the tissues begins in the same 

 way, but the chlorophyll -bearing tissue is extraordinarily 

 developed. The air-spaces are formed in the same way as in 

 Riccia, but they become very deep, and at an early stage, while 

 still very narrow, are divided by cellular diaphragms into several 

 overlying chambers, which, narrow at first, later become very 

 wide, so that the dorsal part of the thallus is composed of a 

 series of large polyhedral air-chambers arranged in several 

 layers, and separated by walls but one cell thick. The upper 

 chambers communicate with the outside by pores, quite like 

 those of the Marchantiese. The ventral tissue and midrib are 

 rudimentary, and the very long pendent ventral lamellae are 

 produced separately in transverse rows, which however become 

 displaced by the later growth of the thallus, so that their original 

 arrangement can no longer be made out. Oil-bodies like those 

 found in the Marchantieae occur. The fruiting plant, which 

 grows on the margins of ponds, etc. where the floating form is 

 found, is much more richly branched and more vigorous than 



^ Leitgeb (7), vol. iv. 



