W. Watson. 
150 
occurs and the type of habitat from which the supposed species 
has been previously recorded may direct his attention to some 
easily-overlooked structural peculiarities known to occur in a plant 
of a certain habitat and not found in an otherwise similar plant 
from another habitat. Due care must be exercised in this respect, 
as a plant which is normally found in one habitat may occasionally 
be found in a quite different environment, and it is sometimes a 
matter of difficulty to decide to what species it actually belongs; 
thus a plant of half-shade may be a habitat-form derived from a 
shade plant, or it may be derived from a species which normally 
occurs in the open. This difficulty is often very real in dealing 
with species and their varieties ; for example, Alicularia Geoscyphus, 
a liverwort of open ground, has a variety suberecta of shady or moist 
ground, and it is often impossible to refer a plant of half-shade 
either to the type or the variety, as characters pertaining to both 
may occur in it. 
Edaphic factors, such as water-content, presence or absence 
of lime or other mineral substance, etc., often alter the appearance 
of the plant, and it is of interest to note that some mosses which 
have been discoloured by the urine of animals have had varietal 
names given to them. Here I may also mention an instance where 
the presence of a certain moss led me to examine the habitat more 
closely and definitely. I obtained a Barbula from a rock which 
appeared to be sandstone; on examination, the moss proved to be 
B. tophacea, a calciphilous species, and when the rock was tested 
with dilute acid the resulting effervescence distinctly showed the 
presence of a considerable amount of calcium carbonate. The 
occurrence of the moss thus proved to be a better test of the 
calcareous nature of the substratum, than perfunctory examination 
with the eye. 
Amongst the bryophytes examples of hydrophytes, mesophytes 
and xerophytes are met with, but the ordinary meaning of the term 
mesophyte (as applied to the vascular plants) is quite inadequate 
when used for bryophytes. Various species which grow in habitats 
characteristic of phanerogamic mesophytes exhibit characters of 
a xerophytic nature chiefly because they have no roots which, 
penetrating to some depth in the soil, can obtain a constant water 
supply. 
The hydrophytic ancestry of the bryophytes is clearly shown 
by the fact that they exhibit less complete structural adaptations 
to life on land than vascular plants do 4 but many of them are 
