1892.] 



on the Surface-Film of Water, &c. 



643 



upon a branching stem of minute size. There are a few hairs upon 

 the uj^per surface, and between the leaves are narrow clefts, connected 

 with globular cavities, which occupy the centre of every leaf. These 

 cavities, which are often closed, and never possess more than an 

 outlet of extreme minuteness, are always filled with air ; so are the 



Fig. 2. 



Azolla caroUniana. A, stem with leaves, magnified ; B, longitudiual section 

 through part of ditto, highly magnified. The air-cavities of the leaves are shown, 

 the narrow spaces between the leaves, into which water cannot enter, the fine 

 hairs of the upper surface, thi submerged leaf-lobes, and the vascular bundles. 



clefts between the leaves. No water can lodge on the upper surface, 

 apparently because the surface-film is stretched from the raised edge 

 of one leaf to that of the next ; and thus buoyancy, self-righting, and 

 repulsion of water are efiicieutly secured. 



Many j)lauts which ordinarily float on the surface of the water 

 (Salvinia, Azolla, Duckweed, Potamogeton natans, &c.) sink on the 

 approach of winter. At this time it is very curious to see how com- 

 pletely they lose both their buoyancy and their power of repelling 

 water. I do not know how this change is brought about, but the 

 result is one of obvious advantage. The leaves, or in some cases the 

 entire plants, sink to the bottom, and hibernate there, out of the reach 

 of frost. Mauy perish ; some are broken up by decay into isolated 

 buds. When spring returns, the few survivors float up, and soon 

 cover the surface with leaves. It would be interesting to know 

 something of the mechanism by which these seasonal changes are 

 effected. 



One of the commonest objects in Nature, which is apt to escape 



