Xerophytic Adaptations of Bryophytes. 167 
of the midrib is covered by vertical plates of chlorophyllous cells, 
which compensate for the reduction of the ordinary assimilatory 
tissue, and these may be protected by having the row of cells 
terminating each plate widened ( P . commune) or papillose (P. 
alpinum, P. urnigerum) or almost without chlorophyll. A form of 
the dry heath-loving P. gracile found in a damp locality had the 
Iamminae so much larger, and the lamellae so much reduced, that it 
was originally described as a new species of Catharinea (C. Dixoui). 
Goebel (7) notes the fact that if Polytrichum be cultivated in water 
the old leaves blacken and die, whilst the new leaves have less tall 
lamellae. A similar method of compensation for reduced leaf-surface 
is found is some species of Tortilla (Fig. 3, 15-17). 
The presence of a nerve may in some sense be considered a 
character of a xerophytic nature, since as Haberlandt showed (9) it 
occurs in mosses inhabiting soil and is related to water conduction. 
The leaves of leafy liverworts are generally without midribs, the 
only foliose British species showing any sign of a midrib being 
Diplophyllum albicans, but this does not appear to be directly 
related to water conduction (Tansley and Chick, 18). 
(6). Stem Structure. 
There is comparatively little differentiation of the stem-cells in 
bryophytes, the central strand being the only indication of vascular 
tissue, and this seems to bear little relationship to xerophily, but is 
rather related to whether the plant lives on soil or not (Haberlandt, 
9). Where conduction is provided for by external capillary means, 
it is reduced or entirely absent, e.g., Thuidium. In Polytrichales 
there is some differentiation between the water-conducting and the 
other cells of the central strand (18), in the central strand 
is usually present, in Andreaeales it is absent, in Hepaticae it is 
absent or rudimentary. The cortical cells appear to have some 
relationship to the habitat, but even here no very definite statement 
can be made as to the amount of thickening which takes place, 
since it is modified by the degree to which the stem is exposed. 
Generally, it may be said, that a xerophilous bryophyte has a cortex 
of cells with thickened walls, whilst a plant of a moist habitat has 
a cortex of thinner and larger cells. In many mosses where parts 
of the stem are exposed, whilst other parts are covered by the leaf- 
bases, the cortical cells of the former are more incrassate and 
smaller than those of the latter. 
