GRIMMIA. 129 



It is a curious fact, and one which a beginner will find 

 worth remembering, that with the exception of Rhacomitrium 

 lanuginosum and R. canescens, every British species is a rock 

 growing plant. 



The leaves of most of the species of Grimmiaceae, as con- 

 trasted with those of most Orders, are extremely hygroscopic, 

 that is to say while undergoing considerable change of form when 

 becoming dry, they very quickly recover their normal shape when 

 moistened, usually from being twisted or incurved rapidly becom- 

 ing recurved, and then returning to their normal, more or less 

 straight position. This character they hold in common with the 

 Orthotrichaceae, the Andreaeacese, and many of the Tortulaceae. 



29. GRIMMIA Ehrh. 



Mostly short-stemmed mosses, growing in close cushions or 

 tufts on rocks ; stems dichotomous. Leaves crowded, more or 

 less lanceolate, usually hyaline-pointed, margin frequently 

 thickened, almost always entire ; areolation short and often 

 opaque above. Seta arcuate or straight. Calyptra smooth; 

 capsule oval. Peristome teeth 16, cleft at apex or entire, lanceo- 

 late. 



The species of Grimmia are for the most part recognisable 

 as belonging to that genus at first sight, but are difficult of 

 determination specifically. When the fruit is present it forms a 

 material aid to identification, but it is frequently wanting, indeed 

 in some species it has not yet been discovered, and the determina- 

 tion then usually rests on minute differences of areolation, difficult 

 to define ; and a microscopical examination of specimens, in some 

 of the critical species, is almost imperative. 



It should be noted that the hyaline leaf-point is not, as in 

 Barbula, formed by the excurrence of the nerve, but by the 

 prolongation of the lamina ; the nerve being lost, in most of the 

 species, in the apex, at the base of the hyaline point. The leaves 

 are extremely hygroscopic, and in most cases when wetted 

 immediately become squarrose, then slowly take their normal, 

 more or less erecto-patent position. In examining the basal 

 areolation, mature leaves must be chosen, as in the young leaves 

 the cells often present a very different appearance, as do those of 

 the perichaetial bracts. 



The margin of the leaves is often recurved, and the presence 

 or absence of this character affords a useful and a fairly constant 

 distinction between species. It should be noted that in many, if 

 J 



