132 Alexander W. Evans, 



ever, of being a prostrate plant, forming loose depressed mats or 

 creeping over other bryophytes without being closely adherent to 

 the substratum. Its color is yellowish or brownish, becoming 

 darker with age ; and the vegetative thallus, which is the only part 

 studied by the writer, exhibits considerable dififerentiation. The 

 main axis continues its growth indefinitely, the living portion being 

 usually 1-3 cm. long, although according to SchifTner and Stephani 

 the plants are sometimes 5 cm. long. When well developed it meas- 

 ures 0.7-1 mm. in width and about 0.3 mm. in thickness, the median 

 portion being from ten to twelve cells thick. From this thickened 

 region it thins out gradually toward the edges but does not form 

 wings, the cross section appearing biconvex and acutely pointed. 



The histological features differ in several important respects 

 from those of the preceding species, although a central core of 

 elongated cells is still present, surrounded by an outside layer of 

 short cells. The latter, however, which measure about 20 fx in 

 diameter, are thin-walled and distinctly smaller than the interior 

 cells, which average about 30 /x just within the surface layer and 

 40 /i, in the central portion of the axis. The elongated cells have 

 slightly thickened walls but give no indication of an external zone 

 with their walls strongly thickened and pigmented. On the whole 

 the cell-differentiation of the axis is slight, agreeing in this respect 

 with the majority of the species. 



On each side the axis gives off complicated photosynthetic 

 branch-systems, sometimes only 0.5 mm. apart and sometimes more 

 distant. The axis of such a system, beginning at the base, gives 

 rise to closely crowded secondary branches and these in turn to 

 tertiary branches, the branches tending to decrease in length as 

 the apical portion of the system is approached. When well devel- 

 oped the system has the appearance of an ovate mass 0.5-1 cm. 

 long, composed of overlapping and interwoven, strap-shaped and 

 often variously crispate or contorted branches. The systems are 

 often so close together that they touch. 



The ultimate branches yield some of the most important charac- 

 ters of the species. They are clearly differentiated into a narrow 

 median portion, three cells thick and only two cells wide, and two 

 broad wings, one cell thick and three or four cells wide. The cells 

 of the wings are hyaline and average about 45 x 35 jx, those along 

 the margin projecting as distinct crenulations. The cells of the 



