INTRODUCTION 5 



region, nearly or quite destitute of a lamina, and upright branches which 

 grow from the rhizome. In these upright shoots, the lamina is broadly 

 expanded and repeatedly forked. These dichotomously branched shoots 

 resemble very closely the leaves of certain small ferns, especially the Hy- 

 menophyllaceae (Fig. I, A). 



Of the three species to be discussed in the present paper, two, P. radi- 

 culosa and P. Levieri, belong to the section Eupallavicinia, the third, P. 

 Zollingeri, to Mittenia. 



The general structure of the thallus is much the same in all species. 

 Except for the rhizome-like portion occurring in some species, the thallus 

 is differentiated into a very distinct midrib, several cells thick, and a deli- 

 cate lamina, or wing, on each side of the midrib, these marginal wings 

 being always in Eupallavicinia but a single cell in thickness ; but in Mitte- 

 nia they merge more or less gradually into the midrib. The midrib is 

 traversed throughout its extent by a strand of conducting tissue composed 

 of cells which are narrowed and greatly elongated, pointed at the ends, 

 and with thick pitted walls. 



Branching is of two types. In the first there is an apparent dichot- 

 omy, the equal branches having their midribs and conducting tissues con- 

 tinuous with those of the main shoot. The second type of branching 

 has the appearance, superficially, of exogenous adventitious branches 

 occurring on the ventral surface of the midrib. In these ventral shoots, 

 the conducting tissue is not connected with the midrib of the main shoot 

 (Fig. 2, A, B). 



Rhizoids occur more or less abundantly on tihe lower surface of the 

 thallus where it is in contact with the substratum. 



All of the species are dioecious, the reproductive organs being borne 

 upon the dorsal surface of the thallus. The antheridia, which are pro- 

 tected by scales, occur either upon the midrib or close to it. The arche- 

 gonia are in definite groups whidh are surrounded by a double envelope, 

 the outer one forming the "involucre," the inner envelope a more or less 

 tubular sheath, the "perianth." 



