24 MOSSES AND FERNS chap. 



cant even in the higher forms, and has the foot and stalk poorly 

 developed. While the Marchantiales grow for the most part 

 in moist situations, and some of them, e.g., Marchantia poly- 

 morpha, are very quickly killed by drying, some species, e.g., 

 Riccia trichocarpa, a common California species, grow by pref- 

 erence in exposed rocky places exposed to the full force of the 

 sun. This latter species as well as several others of the same 

 region, e.g., Fimbriaria Calif ornica, Targionia hypophylla, do 

 not die at the end of the rainy season, but become completely 

 dried up, in which condition they remain dormant until the 

 autumn rains begin, when they absorb water and begin to grow 

 again at once. In these cases usually only the ends of the 

 branches remain alive, so that each growing tip becomes the 

 beginning of a new plant. 



The Ricciace^ 



As a type of the simplest of the Marchantiacese, we may 

 take the genus Riccia, represented, according to Schiffner 

 ((i), p. 14), by 107 species, distributed over the whole earth. 

 Most of them are small terrestrial plants forming rosettes upon 

 clay soil or sometimes in drier and more exposed places. A 

 few species, e.g., R. Huitans, are in their sterile condition sub- 

 mersed aquatics, but only fruit when by the evaporation of the 

 water they come in contact with the mud at the bottom. 



The dichotomously branched thallus shows a thickened 

 midrib, which is traversed upon the dorsal surface by a longi- 

 tudinal furrow which in front becomes very deep. At the 

 bottom of this furrow, at the apex of the thallus, lies the grow- 

 ing point. A vertical section through this shows a nearly 

 triangular apical cell which lies much nearer the ventral than 

 the dorsal surface (Fig. 2, x). From this are cut off succes- 

 sively dorsal and ventral segments. Each segment next 

 divides into an inner and an outer cell. From the outer cells 

 of the dorsal segments the sexual organs arise, and from those 

 of the ventral segments the overlapping lamellae upon the lower 

 surface of the thallus, and also the rhizoids. The rapid 

 division of the inner cells of the segments, especially those of 

 the dorsal ones, causes the thallus to become rapidly thicker 

 back of the apex. Sections made parallel to the surface of the 

 thallus, and passing through the growing point (Fig. 3), show 



