166 
W. Watson. 
or spherical in shape. The thickening is often more apparent at 
the edges of the leaf, as in Orthotrichum diaphanum, etc. 
In Rhacomitrium, Andrecea petrophila , Jamesoniella Car ring - 
tonii, Anastrophyllum Donianum, and some species of Grimmia, the 
cells are sinuose, so that the absorbing and conducting surface is 
increased, as in the cells of Pinus, thus compensating for the 
greater difficulty in conduction owing to reduction of cell-diameter 
and increase in thickness of cell-wall. 
The cell-walls are also thickened in the liverworts Alicularia 
Breidleri, Lophozia alpestris , L. bicrenata, Sphenolobus minutus* 
Diplophyllum albicans and species of Gymnomitrium, but usually 
this thickening is more conspicuous at the angles so that triangular 
masses (trigones) appear to be present between the cells as in 
jf. Carringtonii, A. Donianum, L. porphryoleuca, S. saxicolus, 
Plagiochila punctata v. minuta, Chandonanthus setiformis, Bazzania 
triangularis , and in species of Targionia (epidermal cells), Frullania 
and Ptilidium. The presence of trigones seems to be quite as 
characteristic a xerophytic adaption for the liverworts as the 
occurrence of papillae in the mosses. 
The cells of most bryophytes have their greatest diameter 
across their middle portions, so that their superficial walls may be 
said to overlap their respective cells. 
(c) Doubling of the Lamina is a xerophytic device, which acts 
in a similar manner, but is of uncommon occurrence. Sometimes 
the entire lamina is doubled, but usually the thickening is confined 
to the margins of the upper portion, which is most likely to be 
exposed during drying. This bistratose character of the margin is 
found in Coscinodon, Campylostelium, Tortula marginata, T. angus- 
tata, Orthotrichum rupestre, Bryum Donianum, and many species of 
Grimmia. It is also present in some hydrophilous mosses (e.g., 
Cinclidotus) and then serves to protect the leaf from injury (25). 
The sclerotic cells scattered in the thallus of Preissia appear 
to be chiefly concerned in giving rigidity to the plant. 
(d) Midrib. The thickening and widening of the midrib, and 
consequent reduction of the lamina is of common occurrence in 
xerophilous mosses, the strong fibrous leaf being more resistant to 
drought. This structure is found in Andrecea crassinervia 
Dicranella heteromalla , Dicranodontium longirostre, Dicranum longi- 
folium, D. asperulum, D.uncinatum, species of Diphyscium, Swartzia, 
Polytrichum, Seligeria, and Campylopus. In Poly trichum the surface 
