METHODS 15 



shoot, and finally almost completely disappear, so that the oldest part of 

 the shoot is almost perfectly cylindrical. The surface of the wings is 

 smooth and the margin undulate, but not developing lobes or teeth, in this 

 respect differing from P. Zollingeri. 



The ventral surface of the midrib is completely covered by short 

 papillate hairs, like the true rhizoids, outgrowths of single superficial cells. 

 These papillae become longer in the older parts of the thallus, and 

 gradually give place to the long, dark purple-red rhizoids, which in the 

 basal region form a dense mass. These conspicuous rhizoids (Fig. 7, 

 D, E) are composed of a single cell, but develop a number of short 

 branches at the apex. As in P. Zollingeri ventral, apparently adven- 

 titious branches are developed which much resemble the main shoots, but 

 are somewhat smaller. It is highly propable, however, that under favor- 

 able circumstances these develop into perfectly normal plants. Their 

 origin was not investigated, but it is probable that they arise in much the 

 same way as the similar branches in P. Zollingeri. 



Quite frequently the wings of the thallus are attenuated anteriorly 

 as well as posteriorly, and as a result the apex of the shoot is somewhat 

 pointed. In form and size the male and female plants are very similar. 



The antheridia form a row on each side of the midrib, and are 

 arranged in elongated groups separated by sterile areas (Fig. 7, A). 

 They are attached to the side of the midrib and project laterally, but are 

 directed slightly forward. They are covered by broad scales, one cell in 

 thickness, which arise at intervals behind an antheridium and, passing 

 above it, extend along the side of the midrib as a shelf-like projection. 

 These scales are deeply incised, scalloped, and sometimes toothed, vary- 

 ing much in size and shape. The scales may subtend a single antheridium 

 or a small group. Sometimes no antheridium is formed under a scale 

 (Fig. 7, C). The antheridia arise in strictly acropetal succession, and 

 the scales near the apex cover immature antheridia only. 



The female plants (Fig. 8, E) closely resemble the males, except for 

 the different appearance of the reproductive organs. The archegonial 

 receptacles resemble those of P. Zollingeri, but are much larger. 

 They arise at various points upon the dorsal surface of the midrib, but 

 there is no connection between their formation and the branching of the 

 thallus. They possess the conspicuous perianth and involucre character- 

 istic of the other members of the genus. The archegonia are much more 

 numerous than in either of the other species examined. The involucre, 

 also, is more conspicuous, having a deeply incised spreading margin, 

 which is strongly revolute. 



