188 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



ordinary hair-moss, is remarkable for its strong,, 

 wiry character, and is eminently adapted to resist 

 the wind. This kind of seta is characteristic of the 

 urn-mosses, many of which grow in very exposed 

 positions indeed. Their spores occur in the capsule 

 unaccompanied by elaters. Owing to the exposed 

 position of the plants, elaters are apparently not 

 required to disperse the spores. But it is otherwise 

 with the Jungermannke — Pellla, for example, which 

 grows in sheltered places, has the stalk thick, 

 succulent, weak, and in no way fitted to resist a 

 strong breeze. Accordingly, in the Hepaticw, the 

 spores are provided with numerous elaters ; these 

 being rendered necessary from the wind, in the 

 sheltered situations where these plants grow, being 

 insufficient to secure the dispersion of the spores. 

 In Nitella and Chara the shape of the plant-body is 

 such as to offer but little resistance to water cur- 

 rents. The former is strengthened by the deposition 

 of carbonate of lime in the walls of its cells, which 

 also happens in the Corallines before referred to. 

 The twisted cells forming the cortex of Chara are 

 of interest as illustrating a principle which is fre- 

 quently discovered in vegetable structures, viz., 

 strengthening by means of torsion. Darwin, in his 

 work on climbing-plants, considers that a stem 

 gains in rigidity by becoming twisted, on the 

 principle that a much-twisted rope is stiffer than a 

 slackly-twisted one. In accordance with this view, 

 the s^iiral thickening of the walls of cells and 

 vessels should be explained as a contrivance for 

 securing greater rigidity. 



II. Vascular Cryptogams. The more common British 

 Ferns grow in sheltered ravines or in rock crevices. 

 The leathery texture and flexibility of the Harts- 

 tongue may possibly be a comxDensation for the 

 undivided frond. The common Brake, growing on 

 exposed hill-sides, is admirably adapted to resist the 

 wind. The rigidity of the stipes is great in pro- 

 portion to its thickness, and the numerous sub- 



