﻿Evans: The genus Plagiochasma 265 



periclinal wall (with respect to the pore), and the opening under 

 such circumstances may be described as surrounded by two more 

 or less complete concentric series of cells. The cells surrounding 

 the openings are not elevated and are usually much like the other 

 epidermal cells. When the radial walls between them are slightly 

 or not at all thickened, as is sometimes the case in certain species, 

 the entire pore-structure is very inconspicuous. 



In pores of the second type (FiGS. 5-8, A), although the 

 opening itself is not necessarily larger than in pores of the first 

 type, the whole structure is much more evident because it is more 

 or less raised above the general level of the epidermis and because 

 the cells surrounding the opening are in two or more concentric 

 series with six to eight or even more cells in each series. It is 

 convenient to describe these cells, which differ considerably in 

 form from the other epidermal cells, as being arranged in radiating 

 series with two or more cells in each series. In many cases the 

 walls separating these are distinctly thickened. Immediately 

 bounding the opening a circular ridge is present which appears 

 to be formed by the thickened and coalescent inner walls of the 

 surrounding cells. Deutsch (6, p. 494, /. g), however, has recently 

 shown that in the very similar pores of Targionia hypophylla L. 

 the ridge consists of a circular row of dead and collapsed cells with 

 thickened and highly cutinized walls. Apparently his explanation 

 would apply equally well to the pores of the second type in 

 Plagiochasma, and it is sometimes possible to demonstrate the 

 radial walls between the collapsed cells. The presence of this 

 circular ridge in the second type of pore and its absence from the 

 first type is perhaps the most important difference between them. 



It will be noted when the pores of Plagiochasma are compared 

 with those of other genera of the Marchantiaceae that those of the 

 first type are essentially like the pores found in Oxymitra and in 

 the various genera of the Astroporae, while those of the second 

 type find their counterparts in the other genera of the Operculatae, 

 as well as in certain genera of other groups. Both types stand in 

 marked contrast to the complex pores of Marchantia, Preissia, 

 and Bucegia, where the cells surrounding the opening are in 

 several layers. Even in Plagiochasma, however, complex pores 

 of the marchantiaceous type are present on the carpocephala. 



