J 9° Alexander W. Evans, 



mostly 2-3 cm. long and 2-3 mm. wide, thus exceeding in width all 

 the preceding species with the possible exception of R. Spcgassi- 

 niana. The thickness, however, is only about 0.15 mm. Both 

 surfaces are plane or nearly so (Fig. 13, B) and the rounded mar- 

 gins (Fig. 13, C) are only slightly thinner than the median region. 

 According to Stephani the thallus is everywhere four cells thick, 

 and this is true for the greater part of its extent, but the median 

 portion is usually five cells thick and the marginal portion only 

 three. In older parts of the thallus, moreover, the cells of the sur- 

 face layer are often divided by delicate periclinal walls, so that 

 under these circumstances the thallus becomes four to six cells 

 thick. The cells of the surface layer, taking it in its whole extent, 

 measure 80-120 /x in length by 20-30 /x in width, while the interior 

 cells are mostly 100-140 // long and about 40 /a wide. The cross 

 section, therefore, shows a rather marked contrast in size when the 

 superficial and interior cells are compared. The cells of the dorsal 

 surface layer have bulging and thickened outer walls, the thick- 

 enings extending down the vertical walls and gradually thinning- 

 out. The cells of the ventral layer have slightly thickened and 

 almost plane walls. Otherwise the walls are thin throughout. 



Along the edges of the axis, at intervals of 1-3 mm. on each side 

 (Fig. 12, A) branch-rudiments or branches are produced. The 

 branch-rudiments are in the form of short bilobed projections with 

 the apical cells in the indentations between the lobes. Most of 

 the branches are short and blunt, showing one or more apical cells 

 in shallow indentations. An occasional branch, however, shows 

 unlimited growth and is essentially like the main axis, although 

 tending to be a little narrower. The branches lie in the same plane 

 as the axis and none are differentiated as stolons. As a rule the 

 marginal cells of the thallus are not differentiated, but sometimes 

 on a short branch a distinct row of hyaline marginal cells can 1)e 

 distinguished, especially in the vicinity of apical cells. 



Only the female i)lant oi R. pallidevirens is known at the present 

 time. The female Ijranches (Fig. 13, A) arise without regularity 

 either on the main axis or on an axis-like l)ranch. They are very 

 short and at first resemble ordinary branch-rudiments, consisting 

 of two rounded IoIjcs witli one or two archegonia between them, 

 the lobes being confluent ventrally but separated dorsally. When 

 well developed the lobes grow out into a shallow cup, open dorsally. 



