The Genus Riccardia in Chile. 179 



under these circumstances the thallus is three or more cells thick 

 except on the very edge, where an indistinct and interrupted row of 

 cells can be distinguished. In cross section (Fig. ii, C, D) the 

 axis is biconvex in outline, the median portion being usually ten 

 to twelve cells thick. The surface-cells average about 20 [x in 

 width and 30 /x in length ; toward the margin a gradual increase in 

 size is usually apparent and some of the marginal cells are 30 /x 

 wide. In the median portion the cells become gradually larger 

 toward the middle, the innermost cells averaging about 25 fx in 

 width and usually measuring 80-150 /x in length. In the marginal 

 portions, especially if these are pale and translucent, the contrast in 

 size between the superficial and interior cells is more abrupt. The 

 walls of the surface layer are thickened but pale; the walls of the 

 next two or three layers are still more thickened and are usually 

 more or less deeply pigmented ; the walls of the interior part of 

 the axis are thin and pale or colorless. There is thus formed a 

 narrow sclerotic zone around a delicate central core, but the 

 boundaries of the zone are poorly defined toward the sides, espec- 

 ially in the more flattened axes. 



At intervals of perhaps 0.5-1.5 mm. on each side the axis gives 

 off branches, which are exceedingly variable. The most charac- 

 teristic are those which develop into ascending or sviberect branch- 

 systems, photosynthetic in character and quickly limited in their 

 growth. Even these, on account of their irregularity, are difficult 

 to describe in a few words. They are mostly 5-10 mm. long and 

 may be as wide or even a little wider than the main axis, often 

 showing a tendency to broaden from a narrow base. They are 

 usually more strongly flattened than the main axis, the median por- 

 tion being perhaps eight cells thick. At close intervals (mostly 

 1.5 mm. or less) the primary photosynthetic branches bear branch- 

 rudiments or branches. The branch-rudiments vary in number and 

 are usually in the form of short rounded lobes with the apical cell 

 in the axil. The secondary branches are usually shorter and nar- 

 rower than the primary branch but may almost equal it in length; 

 they give rise in turn to tertiary branches (or branch-rudiments) and 

 these sometimes bear branches or branch-rudiments of a lower 

 order. Although most of the primary branches and many of the 

 secondary branches are broad at the apex, others become narrower, 

 their tips thus resembling branches of a higher order. Even the 



