504 THIRSK NATURAL HISTORY SOCIETY. [/uly, 



like the corolla in flowering plants, its divisions are useful only 

 as affording specific characters/ 



"Thirty-four species of Orthotrichura are described in the 

 ' Bryologia Europsea ;' of these, twenty-three are given in Wil- 

 son's work as natives of Britain, and one has been found in 

 this country since its publication, so that twelve additions have 

 been made to this genus alone since the publication of Hooker's 

 ' British Flora ' in 1833. Little is known at present of the 

 laws which regulate variation. The species, of any Order, which 

 have the widest range of distribution always seem the most liable 

 to change their characters ; and this rule holds good with refe- 

 rence to the Orthotrichea. Alpine species, like 0. Ludwigii, and 

 species confined to peculiar habitats, like 0. Sprucei and strami- 

 neum, are usually constant ; but more commonly distributed 

 species, like 0. affine, found indifferently upon trees, walls, and 

 rocks, are subject to considerable variations. It becomes the 

 bryologist therefore not merely to acquaint himself with the 

 plant in its different stages of growth, but also with its appear- 

 ance in certain soils, and in humid, or dry and exposed localities. 

 The usual habitat of O. affine is near the roots of trees, or nest- 

 ling in some shady place where the direct rays of the sun sel- 

 dom reach it. In such places the leaves are dark green, broad 

 and undulated, the cellular tissue well developed, the capsule 

 narrow and pallid, and the calyptra light green. On hedges and 

 trees exposed to the sun, especially in dry subalpine countries, 

 another form is not uncommon : the leaves are more acute than 

 in the other, more rigid and of lighter colour, the capsule more 

 developed, stronger, and browner, and the calyptra yellowish- 

 brown. This constitutes the 0. fastigiatum of BryoL Eur., and 

 every shade of difference may be found between the two. There 

 is yet a third well-marked variety, found occasionally on trees 

 by streams, which approaches 0. rivulare in habit, and may ' 

 readily be mistaken for that species. The leaves are very obtuse, 

 and of more succulent texture than in the ordinary form, in co- 

 lour lurid-greeu ; the capsule is exserted, but pale and leptoder- 

 mous, and the calyptra of a lurid-green tint. From its resem- 

 blance to rivulare we have called this variety rivale." 



