Xerophytic Adaptations of Bryophytes. 
151 
nevertheless able to live in habitats where they are cut off from a 
constant supply of water, though vigorous vegetative development 
and sexual reproduction can only be carried out when external 
water is available. 
As the criteria of “ mesophytism ” in bryophytes are even 
vaguer than in vascular plants we must first obtain a clear 
conception as to what is really meant by xerophily in this group 
of plants. Warnstorf (20) considered the chief criterion for a 
xerophyte to be its dependence on atmospheric humidity, but 
Grebe (8) rightly criticises this definition, since it would include 
many mosses which grow on the bark and bases of trees in the 
shade of woods and yet are not xerophytes, because the atmospheric 
humidity is sufficiently great to allow the physiological functions to 
continue through a dry period. On the other hand, this definition 
would exclude a number of earth-mosses (some of which Warnstorf 
placed in a “ semi-xerophytic ” group (21)) which are truly xero¬ 
phytic, and appear to be dead during a dry season. Grebe defines 
a xerophytic moss as one which is capable of surviving long periods 
of drought, is able to dispense with the soil-moisture, and at the 
same time shows some xerophytic structure (8). 
During drought the vital activity of the plant may be suspended 
to such a degree that it appears to be dead, but when moistened 
it quickly revives and resumes its normal functions. Grebe’s 
definition of xerophily in the bryophytes would embrace the 
inhabitants of exposed rock-surfaces (species of Grimmia, Tcrtula, 
Weisia, Trichostomum,Encalypta, Gymnomitrium,Frullania,Lejeunea, 
etc.), of dry heaths (e.g., Polytrichum piliferum, P. juniperinum , P. 
gracile, Campylopus pyriformis, Rhacomitrlum lanuginosum, forms of 
Gymnocolea infiat a), of dry banks or waste places (e.g., Bryum 
argenteum, Weisia viridula , W. microstoma), of trees growing in the 
open or in half-shade, as on trees with crowns which admit a good 
deal of light, such as the oak (e.g., Ulota crispa, Ortliotrichum 
diaphauum, Leucodon sciuroides, Grimmia Hartmani, Camptothecium 
sericeum, Cryphcca heteromalla, Frullania dilatata, F. Tamarisci ), 
and of dry sand-dunes (e.g., Brachythecium albicans, Camptothecium 
lutescens, Scapauia aspera). It would include many plants which 
have their rhizoids fixed in the soil, whilst it would exclude some 
plants which are entirely dependent on atmospheric humidity, but 
which, owing to their enjoyment of constant shade, are kept 
sufficiently moist to continue their living activity through a period 
of dryness. 
