Xerophytic Adaptations of Bryophytes. 165 
(Fig. 3, 18), Mnium subglobosum, Cinclidium stygium, are found in 
wet or shaded situations, whilst exposed Alpine plants (species of 
Andrecea, Grimmia, and Leskea) have small cells with thick walls 
(Fig. 3, 19-21). 
In the Hepatica, which are a more hydrophilous class than the 
Musci, we usually find much larger cells (Fig. 2, 11), though some 
xerophilous liverworts have relatively small ones, e.g., Anastrepta 
orcadensis, chandonanthus setiformis, Bazzania triangularis, and 
species of Gymnomitrium (Fig. 3, 12-14). 
Mosses with nerveless leaves often have long cells so that 
conduction can take place in a longitudinal direction more readily. 
(5). Thickenings of the Leaf-cells. 
These thickenings not only strengthen the leaf, but at the 
same time, decrease the transpiring surface. 
(a) Papillosity. The papillosity of the leaf is the most 
important adaptive character of a structural nature in the mosses. 
The papillae arise as local thickenings of the cell-wall and appear as 
small warts on the surface of the leaf. They are found in many 
xerophytes, sometimes being so numerous that the cells are 
completely hidden beneath them, but in some families (e.g., 
Grimmiacea) they are almost entirely absent, as these mosses are 
sufficiently provided for in other ways. They are found in 
Hedwigici ciliata, Rhacomitrium canescens, Pseudoleskea catenulata, 
Myurella julacea v. scabrifolia, Pterigynandrium filiforme and many 
species of Barbula, Tortula, Encalypta, Trichostomum, Weisia, 
Zygodon, Anoinodon, Leskea and Thuidium. Under moist conditions 
some mosses which have papillose leaves become almost smooth. 
I have noticed this especially in Barbula fallax. 
Mammillce appear to be of a similar nature, but instead of 
being solid structures are only vesicular. They are present in 
Timmia and Philonotis, and Grebe (8) considers them to correspond 
to gemma, though he gives no evidence in proof of this. 
Some liverworts also have their leaves studded with similar 
small warts, e.g., Cololejunea Rossettiana, C. calcarea, Scapania aspera 
S. nimbosa, S. cequiloba. 
(b) Iucrassate “ Cell-nets .” The walls of the leaf-cells are often 
thickened so that they offer a greater resistance to desiccation. 
This peculiarity is seen in the leaves of xerophilous species of 
Andrecea, Zygodon, Orthotriclium, Barbula, Tortula, Rhacomitrium, 
Grimmia, Ulota (especially U, Hutchinsice), Weisia, etc., all of which 
have very small cells, less than 5 /x in diameter and usually cubical 
