﻿Evans: The genus Plagiochasma 263 



Plagiochasma in Leitgeb's group Operculatae (or its equivalent) of 

 the Marchantiaceae, It represents the most simply organized 

 member of the group, but there is a strong tendency at the 

 present time to consider it reduced rather than primitive. 



The species of Plagiochasma grow in depressed and intricate 

 mats. The thallus is remarkably uniform in structure throughout 

 the genus and shows clearly the usual differentiation into an 

 epidermis, a photosynthetic layer with intercellular spaces, and a 

 compact ventral layer, from the lower surface of which arise the 

 ventral scales and the rhizoids. The thallus is sometimes con- 

 tinuous in its growth, maintaining the same width, and sometimes 

 seems to be divided into a succession of joints, each joint springing 

 from the apex of an older joint and rapidly increasing in width. 

 According to Leitgeb this condition is not due to apical innovations 

 but simply to the fact that the growth of the thallus is subject to 

 interruptions, the new portions being narrower when the growth 

 is first resumed. According to Kashyap, however, true apical 

 innovations occur in P. articulatum. In addition to the usual 

 branching by dichotomy, in which both branches are alike, the 

 thallus often bears ventral adventive branches, which broaden out 

 rapidly from a stalk-like base. The authors of the Synopsis 

 Hepaticarum (ii, pp. 51 1-52 1) supposed that these branches 

 were never produced by species in which the thallus was jointed 

 and they therefore divided the genus into two sections, as follows : 

 § I. Frons ex apice cordato articulatim innovans; §2. Frons e 

 ventre innovans, apice saepe subcontinua. Leitgeb showed, how- 

 ever, that P. intermedium of § i often developed ventral branches, 

 that P. appendiculatum of § 2 often appeared jointed, and that 

 the distinction relied upon in the Synopsis was therefore inconstant 

 and to be used with caution. 



The epidermis, as in most genera of the Marchantiaceae, 

 consists of a single layer of pale or colorless cells, although the cell- 

 walls are sometimes more or less pigmented with purple. The 

 walls are usually firm but thin and this is true even of the cuticle. 

 The latter, however, sometimes appears thickened, roughened, and 

 slightly opaque, owing to the deposition of a waxy substance on 

 the surface. When this is present the thallus is bluish or glaucous 

 green instead of bright green. Trigones can always be demon- 



