174 PLANT MORPHOLOGY 



scales but bears numerous colorless rhizoids of the smooth-walled type. 

 Growth of the thallus results from the activity of a cuueate apical cell. 



In general appearance Riella is unlike any other liverwort. It is a sub- 

 merged aquatic, usually growing erect in standing water. It has a stem- 

 Uke axis that bears a dorsal leaf-like wing or, in the Algerian Riella bialata, 

 two wings. The wing is mostly one layer of cells thick. It is frequently 

 undulate and sometimes spirally twisted. The axis is commonly several 



times dichotomous. It produces 

 rhizoids near the base. 



Sex Organs. The antheridia and 

 archegonia are borne directly on the 

 thallus, each enclosed in a special 

 involucre that is open above. They 

 arise in acropetal succession from 

 dorsal segments of the apical cell. 

 V^^^^ In Riella both kinds of sex organs 



''^ B may occur on the same plant, al- 



FiG. 141. Female (A) and male (B) though generally, as in ^Sp/merocarpMS 

 gametophytes of Sphaerocarpus caii- ^ Qeothallus, they are borne on 



fornicus, X6. {From Gilbert M . Smith.) > '' 



separate plants. 

 In Sphaerocarpus the sex organs are closely crowded on the dorsal sur- 

 face of the thallus. The male plants, often purplish, are minute and 

 much smaller than the female plants (Fig. 141). The antheridial involu- 

 cres are flask-shaped and each contains an ovoid short-stalked antherid- 

 ium. In development, two transverse walls appear in the outer cell 

 arising from a transverse division of the papillate initial (Fig. 142 A, B). 

 In the upper two segments vertical walls are formed at right angles to 

 each other and then periclinal walls cut off a layer of outer sterile cells 

 from a central group of spermatogenous cells (Fig. 142(7-^). Further 

 development takes place as in the Marchantiales. The archegonial invo- 

 lucres, each enclosing an archegonium, are tubular or nearly spherical. 

 The archegonium develops as in the Marchantiales (Fig. 143). It has 

 two to four neck canal cells, its neck showing six cells in cross section. 

 Following fertilization, the calyptra becomes two-layered. It is soon 

 ruptured by the sporophyte. 



In Geothallus the sex organs are borne and develop as in Sphaerocarpus, 

 but are much less numerous and the male plants are only slightly smaller 

 than the female. In Riella the antheridia occur in a series along the mar- 

 gin of the wing, each enclosed in a pocket. The archegonia are arranged 

 serially on the axis, each surrounded by a flask-shaped involucre. 



Sporophyte. The sporophyte of the Sphaerocarpales is more advanced 

 than that of Riccia but simpler than that of nearly all the other Marchan- 

 tiales. It has a foot, capsule, and very short seta (Fig. 144.4). In 



