The Genus Riccardia in Chile. 193 



the dried specimens do not readily regain their original condition, 

 even if the water is boiled, but treatment with caustic potash gives 

 more satisfactory preparations. At the same time the study of 

 fresh or carefully preserved material is much to be desired. 



The thallus is prostrate or perhaps ascending, and the main axis 

 is sparingly and irregularly branched (Fig. 13, E). The vegeta- 

 tive branches are essentially like the axis, except that they are often 

 narrower, but in many cases their growth is limited while they are 

 still very short. The main axis with its unlimited growth is usually 

 1-2.5 cm. long, 1-2 mm. wide (3-4 mm. according to Stephani), 

 and 0.1 -0.1 5 mm. thick. The median portion (Fig. 13, F) is 

 usually six or seven cells thick and the rounded edges are scarcely 

 thinner. These edges are f recjuently more or less involute, giving 

 the upper surface a canaliculate appearance. No indication of 

 a wing is ever present. At the apex the involute edges become 

 continuous and form a cucullate apical pocket, in which a cluster 

 of ventral papillae with thickened walls can be distinguished. In 

 the older parts of the thallus groups of rhizoids. many of which 

 are branched at the tips, are sometimes present and seem to be 

 largely confined to the vicinity of the margins. 



The most remarkable structural features of the thallus are found 

 in the cells of the superficial layer. On the upper surface these 

 cells are mostly 120-160 /x long and average about 25 /x in width, 

 while on the lower surface they are of about the same length but 

 considerably broader, averaging about 40 jx in width. Their walls 

 are all thickened but on the free walls the thickenings are in the 

 form of irregular truncate projections (Fig. 13, G), sometimes in 

 the form of transverse, anastomosing bands (Fig. 13, H) but 

 usually far more indefinite. These thickenings are laid down close 

 to the growing point in the form of a continuous layer, which on 

 the ventral surface soon becomes several times thicker than the 

 cavities of the young- cells. The layer soon attains its full thick- 

 ness and extent and, when the cells grow to their full size, becomes 

 ruptured in all directions, the transverse breaks being perhaps the 

 most numerous. In this way the irregular projections described 

 above are formed. Since these are lower than the thickenings on 

 the younger cells, it is evident that their tips easily become broken 

 off. On the dorsal surface the thickenings are much less con- 

 spicuous than on the ventral surface. When the margins of the 



