314 BRYACE^. 



plants densely tufted. Leaves usually more or less ovate in 

 outline, of a thin texture and very little hygroscopic, cells 

 smooth, rather large, more or less widely rhomboid or hexagonal- 

 rhomboid, rarely linear, the marginal often very narrow and 

 forming a more or less distinct, sometimes thickened border; 

 nerve usually excurrent. Calyptra narrow, cucullate, soon falling 

 off. Capsule on a long seta, usually more or less pendulous, 

 rarely horizontal, pyriform, but varying from sub-globose to 

 elongate-clavate, symmetrical or almost so, annulate, lid conical, 

 obtuse or apiculate, not rostellate nor rostrate. Outer peristome 

 of 16 long, lanceolate, undivided teeth, closely articulated, 

 especially in the lower half, externally trabeculate, sometimes 

 slightly perforate ; inner thin, pale, with a membrane about half 

 the length of the teeth, to which it sometimes adheres, with 16 

 lanceolate processes alternating with the teeth, perforate or split 

 and gaping along the keel or median line, usually with 1-3 

 intermediate cilia sometimes rudimentary, when perfect bearing 

 short transverse appendages at intervals. 



The genus Bryum is one of the largest among mosses, 

 consisting of more than 400 species, to which number additions 

 are frequently being made. It is the one, moreover, which both 

 to student and to systematist offers perhaps the greatest difficulty. 

 One reason for this difficulty lies in the fact that a very large 

 number of the species are separated by very slight and incon- 

 spicuous and yet apparently very constant characters, such as the 

 shape of the capsule, the structure of the peristome, and the form 

 of the leaf. Each of these presents, over all the extent of this 

 large genus, a much smaller range of variation than is found in 

 many far smaller genera, hence the difference they exhibit 

 between species and species can often be at the most extremely 

 slight, hard to detect, and no less hard to define. Another and 

 very important difficulty lies in the fact that many species are 

 almost invariably found in the barren state, and the absence of 

 fruit in a Bryum often almost prohibits its certain identification. 



These difficulties would be greatly minimised if the species 

 admitted of classification in groups or sub-genera on clearly 

 defined characters or such as are easily observed ; but unfor- 

 tunately the case is quite the reverse. There is so little 

 correlation between the different parts that any natural classifi- 

 cation based upon more than one organ or structure seems to be 

 out of the question ; while, on the other hand, a classification 

 based upon any single structure, such as the peristome, or the 

 length of the nerve in the leaf, proves entirely arbitrary and 



