BRITISH MOSSES. 37 



leaf there are, as it were, two sheets or plates instead 

 of one. Various theories of the homology of this part 

 of the leaf have been suggested. By some it has been 

 thought to result from a vertical splitting of the leaf; but 

 each of the two plates where they are doubled is of an 

 equal thickness to the rest of the leaf. Some have 

 suggested that the double portion is alone the true leaf 

 and the rest an outgrowth, but this seems a violent 

 assumption. Others, again, have suggested that the 

 additional lobe is a stipule arising on the opposite side of 

 the stem, which has become adnate with the leaf. Some 

 of these suggestions carry conviction with them. 



In some cases the leaf is produced into a long thread or 

 beak, devoid of chlorophyll, and often with indented or 

 toothed edges. This structure is found chiefly in Mosses 

 living on stones and rocks, and in dry situations, such as 

 Grimmia and Racomitrium, and the presence of these long 

 white threads or beaks gives a grey tint to the whole Moss, 

 and in places where the Moss is predominant (as, for 

 instance, some parts of Dartmoor and North Wales, where 

 Eacomitrium abounds) a grey tint to the whole landscape. 

 These long hairs and prominences, especially when armed 

 with lateral teeth, no doubt retain the moisture which is 

 necessary not only for the vegetative life of the Moss, but 

 also for the process of reproduction by archegones and 

 antherids ; hence it probably is that this form of leaf 

 prevails in Mosses living in dry situations, just as the 

 thick leaves of succulent plants are found in similar 

 situations. 



The capsule. Of all the organs of a Moss plant, the 



