u MUSCINE^—HEPATICJE—MARCHANTIALES 39 



now on is essentially the same. The growth of the cells in the 

 forward part of the dorsal surface of the young thallus is more 

 active than that of the ventral side, so that they project over 

 the growing point (Fig. 9), and as the outer cells of the lateral 

 segments of the apical cell (or cells) also increase rapidly in 

 size as they recede from the growing point, the forward margin 

 of the thallus, seen from below, is deeply indented, and the 

 forward part of the thallus is thus occupied by a deep cavity, at 

 the bottom of which, toward the ventral side, lies the growing 

 point. This cavity is the beginning of the groove or furrow 

 found in the older thallus. 



At first the cells of the young thallus are without inter- 

 cellular spaces, but at an early period (Fig. 9, C) the outer cells 

 of the young segments separate and form the beginnings of the 

 characteristic air-spaces. In R. trichocarpa some of the dorsal 

 cells about the same time form short pointed papillae, the first 

 indication of the pointed hairs characteristic of this species. 

 As the plant grows, new rhizoids are formed by the growing 

 out of ventral cells into papillae, which are cut of? by a partition 

 from the mother cell. These first-formed rhizoids 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 RicciACEiE 



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 

 Ricciocarpus natans is of almost world-wide distribution. It 

 is a floating form, like Riccia fluitans. Leitgeb ( (7), vol. iv.) 

 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 cel- 

 lular diaphragms into several overlying chambers, which, nar- 

 row at first, later become very wide, so that the dorsal part of 



