The Genus Riccardia in Chile. 141 



The main axis is usually 3-5 cm. long and continues its growth 

 indefinitely. It is strongly flattened, being 1-1.5 mm. in width and 

 only 0.4-0.5 mm. in thickness. In cross section (Fig. 3, G) it pre- 

 sents a biconvex outline, bulging more ventrally than dorsally, and 

 the ends are rounded or subobtuse, no indications of a wing being 

 present. The cell-differentiation is similar to that of R. Spegaz- 

 ziniana but presents a number of interesting differences. The 

 superficial small-celled layer, for example, instead of being one cell 

 thick throughout, is two or three cells thick in the median portion 

 and one cell thick toward the margins. The cells measure about 

 30yu,in length and 14-16 /x in width, no ventral band of narrower 

 cells being differentiated. There is, nevertheless, a gradual 

 increase in width toward the edge, some of the marginal cells 

 measuring 35 x 30 fx. The outer walls of the surface layer are 

 sometimes slightly but distinctly thickened, the thickenings 

 extending along the radial walls ; otherwise the small cells are thin- 

 walled. The interior cells of the thallus average about 38 /x in 

 diameter and are usually 80-150 /x long. There is therefore a sharp 

 contrast in size between the outside cells and the interior cells, 

 although the difference is less marked than in R. Spegazziniana. 

 Most of the interior cells are thin-walled, but in one or two layers 

 just within the small-celled layers the walls are slightly and 

 uniformly thickened and may even be pigmented. 



Aside from the rare axial branches the thallus of R. spectahilis 

 is regularly pinnate, the primary branches arising on each side of 

 the main axis at intervals of 1-3 mm. They spread obliquely to 

 widely and usually attain a width of 0.5-1 mm. and a length of 3-4 

 mm., their growth being soon brought to the end. These branches 

 are flattened, at least at the base, and bear from one to three 

 secondary branches on each side, sometimes close together and 

 sometimes farther apart. The secondary branches, which are 

 mostly 0.5-1.5 mm. long and 0.3-0.5 mm. wide, are usually simple 

 but sometimes bear one or two very short tertiary branches. Any 

 branch of whatever rank may taper into a subterete apical portion, 

 but the primary branches are usually flat throughout. 



In their histological features the branches resemble the main 

 axis, except that their cell-differentiation is less pronounced. The 

 ultimate branches (Fig. 3, H) are about six cells thick in the 

 median portion, and the surface-cells run smaller than the interior 



