BRYOPHYTA 173 



with air chambers, and a compact, colorless ventral region. The gameto- 

 phyte reaches an extreme of complexity in forms with compound air 

 pores and air chambers having green filaments. It bears both smooth 

 and tuberculate rhizoids. In the lower members the sex organs are borne 

 directly on the thallus, sunken in the dorsal surface, but throughout the 

 group there is a marked tendency to restrict and specialize the regions 

 producing sex organs, resulting in the development of complex receptacles. 

 The female receptacles, and sometimes the male as well, are stalked. 

 The antheridia, when mature, are elongated organs lying in a deep 

 chamber. Their early development is characterized by a series of trans- 

 verse divisions. The neck of the archegonium shows six cells in cross 

 section. 



In the lower members the sporophyte is a spherical, undifferentiated 

 spore case, all the inner cells forming spores. In the higher members the 

 sporophyte is elongated and differentiated into a foot, seta, and capsule. 

 The capsule contains both spores and sterile cells, the latter practically 

 always developed as elaters. Thus, throughout the group, there is a 

 marked tendency to divert potentially sporogenous tissue to functions 

 other than spore production. The seta is comparatively short. The cap- 

 sule is spherical or nearly so, its wall being composed of a single layer of 

 cells (the apex usually thicker). Dehiscence, lacking in Riccia, nearly 

 always occurs by irregular clefts or an apical lid. The Marchantiales are 

 a group in which a complex gametophyte is combined with a relatively 

 simple sporophyte. 



2. Sphaerocarpales 



The Sphaerocarpales comprise a small order of 3 genera and 25 species. 

 Sphaerocarpus is a widely distributed genus but Geothallus, represented 

 by a single species, has been found only near San Diego, California. Both 

 of these forms grow on moist earth. Riella is an aquatic form occurring 

 in Europe, Africa, California, and western Texas. 



Gametophyte. The gametophyte of the Sphaerocarpales displays none 

 of the internal differentiation seen in the Marchantiales. It consists of 

 a simple plate-like thallus that differs somewhat among the three genera. 

 In Sphaerocarpus the thallus is small, flat, and often orbicular, with an 

 entire or more or less lobed margin. It has a broad indistinct midrib, 

 several layers of cells in thickness, that merges gradually into the one- 

 layered wings (Fig. 141). In Geothallus the thallus is larger and consists 

 of an elongated thickened axis giving rise to crowded leaf-like outgrowths 

 on either side, these mostly one layer of cells thick. A large portion of the 

 axis becomes converted into a fleshy tuber that lives over into the next 

 growing season. In both Sphaerocarpus and Geothallus the thallus may 

 be either simple or dichotomously branched. The lower surface lacks 



