﻿290 Evans: The genus Plagiochasma 



Stephani's treatment of this large and distinct species is 

 difficult to understand. He states that the thallus always inno- 

 vates at the apex, thus implying that dichotomy never occurs, and 

 he adds among other details that the epidermal pores are large 

 and surrounded by eight or nine radiating series of cells with four 

 cells in each series, that the appendages of the ventral scales are 

 ovate-oblong or broadly linear with shortly acuminate apices, and 

 that the apex of the carpocephalum is plano-convex. It would 

 almost seem as if he had not seen the true P. crenulatum at all 

 but had drawn his description from some other species. Gottsche's 

 original account is much more accurate, and Leitgeb (i6, p. 64, 

 pi. i,f. 6) has given an excellent description, with a figure, of the 

 epidermal pores. He calls attention to the fact that the con- 

 centric rings of cells surrounding the pore appear as if partially 

 shoved under one another. The pore-opening thus acquires the 

 form of a canal, although the cells all belong to the same epidermal 

 cell-layer. In the material studied by the writer the pores are 

 surrounded by from four to seven radiating series of cells, six being 

 the usual number (Fig. 5, A), and each series is composed of two or 

 three cells. The radial walls are thickened but not very strikingly 

 so. The circular ridge immediately surrounding the opening is 

 distinct in P. crenulatum and in all the following species. 



The ventral scales develop large and characteristic appendages, 

 usually singly but occasionally in pairs (FiG. 5, C, D). The 

 scales themselves, as in the other members of the genus, are 

 lunulate or ovate in form and overlap slightly. Their cells di- 

 minish in size toward the margin and become exceedingly irregular 

 in form and in the direction of their long axes. Some of the 

 marginal cells project and bear vestiges of slime papillae. The 

 appendages are strongly contracted at their junction with the 

 scales and vary considerably in size. Their form, in general, is 

 broadly ovate to orbicular and they narrow abruptly into an 

 apiculate or shortly acute apex, which is tipped with a single cell 

 or with a row of two or three cells. The cell-structure is very 

 similar to that in the scale itself and the border, composed of 

 irregular and contorted cells, is very different from what is found 

 in the narrower scale-appendages of P. rupestre, where the cell- 

 structure of the appendages is practically uniform throughout. 



