even after they are withered, and when, the original function being 

 fulfilled, their immediate fall might be considered inevitable. In a 

 less rigorous season, such leaves may, perhaps, remain unchanged for 

 a longer period, and then fall before they are actually withered and 

 dry ; for we see that if early frost attacks the leaves of any tree, 

 while yet discharging their proper functions, they wither without fall- 

 ing, and the same happens when excessive drought occasions pre- 

 mature decay. 



We may derive some illustration of the cause of dehiscence from 

 the cryptogamic tribes, especially from the thecal of mosses. I shall 

 select two species of Hypnum for eXj|mples. On a wall near my resi- 

 dence I have had opportunities of watching attentively, for several 

 seasons, Hypnum velutinum and H. populeum, which ripen their 

 thecae at the same period (November), but the first loses its operculum 

 as early as the middle of February, when the -peristome expands, and 

 the seeds are dispersed in myriads: H. populeum retains the operculum 

 till the end of March, and even longer, the seeds being, meanwhile, 

 pent up within the theca. On taking a very thin longitudinal section 

 of the ripening theca, it will be seen that H. velutinum has a large 

 well-formed annulus interposed between the edge of the operculum 

 and the mouth of the theca, and partly lodged in grooves formed in 

 those parts. The section of the annulus is elliptical, with its longer 

 axis nearly upright, but inclined a little inwards, composed of two 

 parallel series of highly succulent cells, which, when dry, contract 

 very much in dimensions, while the contiguous walls of the operculum 

 and the mouth of the mature theca, being formed of small cells of a 

 harder texture, do not contract much in drying. Previous to maturity 

 they all seem to be equally succulent and contractile, and no dehis- 

 cence of the operculum can be attained from an unripe theca, except 

 by positive violence producing a rupture, although the line of dehis- 

 cence is at all times easily seen. The operculum generally falls on 

 the return of moist weather, succeeding a period of comparative 

 drought ; and it seems correct to infer, that during the dry interval 

 the cellules of the annulus, by their contraction, have separated from 

 the surrounding parts, in consequence of the refusal of the walls of 

 the operculum and theca to yield in the same proportion, and thus, on 

 the return of moisture, the annulus, and the teeth of the peristome 

 becoming turgid, a species of elasticity is induced sufficient to throw 

 off the operculum. The turgiscence of the moistened annulus, in 

 many species of moss, especially in Funaria hygroraetrica, is so great 

 as to cause it to bend outwards, the parts becoming quite inverted in 

 their relative positions. 



