a Rhodora [JANUARY 
Haven (G. E. Nichols). The species has been reported also from the 
following localities: Closter, New Jersey (Austin), where the original 
material was collected; near Hull, Quebee (Macoun); Ottawa, 
Ontario (Macoun); near Richmond, Staten Island, New York 
(Howe); Highlands, New Jersey (Miss Haynes); Lehigh Mountain, 
Pennsylvania (Rau); Georgetown, District of Columbia (Coville). 
The color of R. arvensis is a dull grayish green, sometimes pigmented 
with purple along the margin. The older parts of the thallus often 
show a yellowish hue. Although the species sometimes forms rosettes 
0.75-1 cm. in diameter, the plants frequently grow scattered or in 
irregular mats. Under unfavorable conditions the thallus is minute 
and unbranched, and specimens with mature capsules have been 
observed in which the entire plant was only 1 mm. long and 0.5 mm. 
wide. It is much more usual, however, for the thallus to fork from 
one to three times, and the branches when well developed measure 
1.5-2 mm. in width and 2-3 mm. in length. The terminal branches 
are very bluntly pointed, rounded, or subemarginate at their extremi- 
ties and bear on the upper surface a shallow median groove, occupy- 
ing about one third the width of the thallus. This groove contracts 
abruptly at the apical end into a very narrow sulcus; toward the 
base of the branch it usually becomes more or less obliterated, especi- 
ally after capsules are developed, the upper surface thereby appearing 
plane or nearly so. In some cases, however, the groove can be demon- 
strated, more or less clearly, almost to the base of the plant. From 
the sides of the groove the flanks of the thallus gently curve away, 
and the branches are bounded laterally by rounded ridges. The 
ventral scales do not extend beyond the margin; they are usually 
hyaline and inconspicuous but are sometimes tinged with purple. 
In most cases cilia are absent altogether. When present they are 
commonly scanty and poorly developed, being scarcely more than 
slightly projecting cells with rounded ends. In cross section the 
lower surface of the thallus is seen to be plane or a little convex in the 
median region, the sides rising obliquely or abruptly and meeting 
the upper surface at an angle of 90 degrees or less. The section is 
therefore approximately a rectangle or trapezoid. "The thickness of 
the thallus is from one third to one half as great as the width. The 
epidermal cells are thin walled, rounded, and hyaline; they soon 
collapse and disappear more or less completely, the cells of the next 
layer then losing their chlorophyll and functioning as an epidermis. 
