﻿266 Evans: The genus Plagiochasma 



The photosynthetic tissue contains several layers of inter- 

 cellular spaces, separated from one another by plates of green cells 

 a single cell in thickness. These spaces, which vary greatly in 

 size in different species, are connected with one another by means 

 of holes through the green partitions, and some of them com- 

 municate directly with the outside air by means of the epidermal 

 pores. According to Leitgeb (i8, p. 64) the thallus first forms 

 primary intercellular spaces or air chambers, each of which has its 

 pore, and these chambers afterwards become divided up by 

 secondary plates of cells which grow out from the primary parti- 

 tions. Barnes and Land (2, pp. 210-213, /. i'j-22) admit that 

 such a partitioning of chambers in Plagiochasma may perhaps 

 occur to a limited extent, but they show clearly that some of the 

 air-spaces without epidermal pores are formed directly, in very 

 young segments, by the splitting apart of cells originally united. 



The compact ventral tissue is not highly developed. In the 

 median portion of the thallus it attains a thickness of perhaps ten 

 to twenty cells, but it thins out rapidly on each side until it 

 becomes only one cell thick and forms the ventral walls of the 

 air-chambers. It is composed of thin-walled cells, a few of which 

 contain oil-bodies, but it shows no signs of the slime cavities or 

 the fibrous cells which occur in some of the more complicated 

 Marchantiaceae. Sometimes, but not always, mycorrhiza is 

 present in the ventral tissue, occupying cells in the median portion 

 just below the green tissue. The cells containing the mycorrhiza 

 sometimes have their walls pigmented with purple, but this is not 

 invariably the case. 



The rhizoids are of the two types usually found in the Mar- 

 chantiales and require no especial comment. The ventral scales, 

 however, which are in two regular rows, are unusually well 

 developed and exhibit considerable variety in form and structure, 

 sometimes even in a single species (see Figs. 2, 3). They show 

 the characteristic division into basal portion and appendage or 

 appendages. The basal portion is ovate to lunulate in outline and 

 is attached by an oblique line extending forward from the median 

 region half way to the margin or beyond. In the apical part it 

 narrows gradually or abruptly into the appendage or appendages, 

 the number of these structures often varying from one to three on 



