THE ARCHEGONIAT^E 



207 



larly marked areolation, as in Marchantia and Gonocephalus (Fega- 

 tella). 



Upon the lower surface of the thallus are usually two series of 

 delicate scales, often of a dark purple color. These sometimes are 

 provided with a glandular tip, which secretes a mucilaginous sub- 

 stance, and they are doubtless protective in their function, closely 

 investing the delicate growing apex of the shoot. 



FIG. 165. Riccia glauca. A, longitudinal section of the apex of the thallus, with 

 young archegonium, ar (X 525) ; I, I, ventral lamellae. B-F, development of the 

 archegonium, longitudinal sections. G, diagram showing the arrangement of the 

 primary divisions in the archegonium. 



Classification of Marchantiales 



The Marchantiales may be divided into three suborders Riccia- 

 ceae, Corsiniaceae, and Marchantiaceae. 



Suborder I. Ricciaceae 



The lowest of the order are the Ricciaceae, containing the two 

 genera, Riccia and Ricciocarpus (Fig. 163). Most of them are ter- 

 restrial forms, but Ricciocarpus and Riccia Jluitans are genuine 

 aquatics. 



Apical Growth. The thallus grows from an apical cell (or possibly more 

 than one), which is wedge-shaped, with segments cut off alternately from the 

 dorsal and ventral faces. Segments are also cut off from the lateral faces. The 

 greater part of the tissue of the thallus is derived from the dorsal segments. The 

 ventral segments develop only the lower epidermis, from which the root-hairs 



