194 Rhodora [OcroBER 
from Greenland. It is also widely distributed in Europe and Asia, 
especially in the north. 
The plants as a rule do not form rosettes but branch only once or 
twice and are irregularly scattered over the substratum. The branches 
of the thallus are usually from 3 to 4 mm. long and from 0.5 to 1 mm: 
wide and taper toward the apex. The upper surface is grayish green, 
sometimes tinged with reddish, and shows a narrow median groove in 
the apical region. In the older parts of the thallus the groove be- 
comes more or less flattened out. The postical scales are colorless 
and inconspicuous, and there are no marginal hairs. In transverse 
section the thallus is a little broader than thick, and its sides are 
almost vertical, although they flare somewhat above and thus form 
an acute angle with the upper edge. 
The photosynthetic tissue, as is characteristic for the entire genus, 
consists of cells in vertical rows separated by narrow air-canals. 
In sections cut near the apex the uppermost cell of each row is thin- 
walled and strongly inflated. Soon, however, the lower part of the 
wall becomes thickened and the thin upper part disappears. In this 
way there is nothing left of the cell except a shallow cup-shaped 
portion, which appears U-shaped in section. "The cells below these 
uppermost cells soon acquire thick walls also, but here the thickening 
is more or less uniform throughout and the cells remain intact, as- 
suming the function of an epidermis. In some cases, especially 
toward the edge of the thallus, the thickening affects three or four 
cells of a row, so that the functional epidermis may be two or even 
three cells thick. The thick-walled cells are usually colorless and 
stand in marked contrast to the green cells below them. The peculi- 
arities just noted were first adequately described by Heeg! and are 
clearly shown in a figure recently published by Müller? "They yield 
some of the most distinctive characters of the species. 
The capsules of R. sorocarpa are irregularly scattered in the thallus. 
The mature spores measure 70-95 u in diameter and are dark brown. 
The convex faces of the spores are finely and regularly reticulate, 
the ridges bounding the meshes being about 4 u high. The three 
triangular faces commonly show irregular and minute thickenings 
but may be almost smooth. At the junction between the convex 
VBot. Not. 1898: 19. N 
? Rabenhorst’s Kryptogamen-Flora 6: 145. /.101d. 1907. 
