The Genus Riccardia in Chile. 129 



ually curves away from the substratum, becomes broader and 

 more and more flattened, and soon gives rise to a crowded series 

 of flattened branches, gradually decreasing in length toward the 

 apex of the main thallus, the latter becoming narrower and nar- 

 rower and soon being limited in growth. In typical cases the lower 

 branches of the system thus formed are pinnate or even bipinnate 

 to a slight extent, the secondary branches being crowded and 

 exhibiting in their turn a gradual decrease in length. As the apex 

 of the system is approached the primary branches, with their 

 decrease in length, give off fewer and fewer secondary branches, 

 and the primary branches in the vicinity of the apex are quite 

 undivided. \Mien well developed a photosynthetic branch-system 

 is broadly ovate in outline, 1-1.5 cm. long and 0.75-1 cm. wide, the 

 prostrate portion of the thallus being somewhat shorter. The main 

 axis in its basal part is mostly 0.6-0.7 mm. in diameter, becoming 

 perhaps i mm. wide above, and the ultimate photosynthetic 

 branches are mostly 0.2-0.3 mm. wide. It will be noted that the 

 photosynthetic branch-systems of R. Thaxtcri are terminal struc- 

 tures involving the main axes of thalli ; in R. fiiegieiisis, R. pre- 

 hciisilis and R. Saivticri the photosynthetic systems are lateral, and 

 the main axis of a thallus continues its growth indefinitely. 



The histological features of R. Thaxtcri may now be considered. 

 In the basal portion (Fig. 2, C) the thallus is about twenty cells 

 thick and is bounded by a single layer of short cells about 30 jx wide. 

 These cells have slightly thickened walls and are flat on the 

 outside. The interior is made up entirely of elongated cells. A 

 zone two or three cells thick, just within the surface layer, is com- 

 posed of cells with more strongly thickened and usually pigmented 

 walls ; the remainder is made up of cells with thin and colorless 

 walls. These cells vary but little in diameter although the cells of 

 the thick- walled zone are a trifle smaller than those of the outside 

 layer. In its widest part (Fig. 2. D) the main axis is perhaps 

 thirty-five cells wide and twelve cells thick. In this region it is 

 narrowly fusiform in section, thinning out gradually on the sides 

 and often bounded by a very narrow wing one cell thick and one or 

 two cells wide. The elongated thick-walled cells (Fig. 2, F) are 

 less pronounced than in the basal portion and often form an 

 interrupted zone, more marked ventrally than dorsally ; otherwise 

 the structure is very similar. The ultimate photosynthetic branches 



