METHODS 19 



measure only about 2 cm. in length and 4-5 mm. in width. The female 

 plants (Fig. 9, B) are about 10 cm. in length and 7 mm. wide. 



The wings of the thallus are relatively very wide, and as usual but 

 one cell thick. They are waved slightly on the margin, which is entire 

 except for an occasional cell which projects slightly so as to form an incon- 

 spicuous tooth. Anteriorly the lamina of the antheridial plant narrows 

 abruptly to the apex of the shoot, which is indented. The midrib projects 

 strongly on the ventral side. The rhizoids are much less numerous than 

 in P. radiculosa, and quite different in color, being a rather light brown, 

 instead of the deep purple-red found in P. radiculosa. The conducting 

 strand of the midrib is much like that of P. radiculosa. 



From the sides of the midrib in the male plant extend a series of 

 scales which form a more or less continuous shelf-like structure. These 

 scales have their free margins deeply lobed and toothed, and underneath 

 the shelf formed by these confluent scales the antheridia occur, either 

 singly or in groups of several together ( Fig. 9, A ) . The antheridia arise 

 in acropetal succession, and are arranged in a somewhat broken row along 

 each side of the midrib. They are more completely covered by the sub- 

 tending scales than is the case in P. radiculosa, and are not so evidently 

 divided into groups separated by sterile areas. 



The antheridia, to judge from a somewhat cursory examination, 

 appear to agree in all essential details of structure with those of P. radi- 

 culosa. 



The apical cell of the thallus is not so deeply placed as in P. radicu- 

 losa, but a study of horizontal and vertical sections shows that it has the 

 same form. On the ventral surface of the midrib in P. Levieri, as in P. 

 radiculosa, there may occasionally be found small groups of meristematic 

 cells, which appear dormant, and do not show a definite apical cell. The 

 smaller groups are slightly sunken ; the larger ones, perhaps having 

 resumed activity, form hemispherical protuberances. The origin and 

 development of these groups of cells is apparently the same as the similar 

 ones in P. Zollingeri, and probably under favorable conditions these give 

 rise to normal branches. 



The archegonial plants of P. Levieri (Fig. 9, B), besides being much 

 larger than the male plants, show also a very different appearance at the 

 apex of the shoot. There is no indentation at this point, but the thallus is 

 prolonged ino a nearly cylindrical process of some length, in which the 

 wings are quite suppressed. It is possible that an examination of a larger 

 number of individuals might show that this difference in apices of the 

 male and female plants is not constant. The rhizoids of the female plants 



