ON THE DEVELOPMENT AND STRUCTURE OF MOSSES. 17 



frequently they are arranged in spirals of five or eight, and in 

 some cases the disposition is still more complicated. In a 

 very few instances they are unequal in size, or accompanied 

 by stipules. 



Though, however, the stems are almost universally destitute 

 of true stipules, their place is supplied in several Hypnei by 

 multitudes of irregular appendages (paraphylla) scattered with- 

 out order over the surface. These are sometimes foliaceous, 

 though often very delicate and jagged, but occasionally they 

 are so repeatedly dichotomous or irregularly divided, that they 

 approach in appearance to radicles, though evidently from their 

 nature and development more nearly related to leaves. They 

 answer in all probability the same physiological purposes as 

 the rootlets, protecting the stem from burning heat, and help- 

 ing to retain moisture for its sustenance. The genus Thui- 

 dium (Plate 8,, fig. 4) affords excellent examples. 



In point of colour, the leaves vary from bright or glaucous 

 green to various shades of brown red or purple. In some 

 species, from defect of chlorophyll, the leaves are nearly white, 

 an appearance which is sometimes due to the cells, like the su- 

 perficial cells of the aerial roots of Orchids, at length containing 

 air rather than moisture, in which case a slight green tinge is 

 communicated from adjoining or imbedded more minute and 

 slender cells containing- chlorophyll. In most instances they 

 revive perfectly on the application of water, though apparently 

 quite dry and parched, and crumbling beneath the fingers ; 

 but this is not always so strikingly the case where they are 

 very membranous. Stomates do not occur, I believe, upon 

 the leaves, though they are not uncommon on the surface of 

 the sporangia, in which case they resemble very closely those 

 of Phsenogams. In one or two Mosses, as Pottia cavifolia 

 (Plate 23, fig. 1 e), there is a little bag on the upper surface 



c 



