542 Professor L. C. Miall [March 4, 



degree the power of rising when submerged, of always rising with its 

 leaves up and its roots down, and of rising with the upper surface of 

 its leaves perfectly dry. It is obvious that these qualities are most 

 useful to a plant which may be pressed under water or drenched with 

 rain. Its nutrition, like that of all green plants, depends largely 

 upon substances extracted from the air ; and to be overspread with 

 water, which disappeared only by a slow process of evaporation, 

 would be disadvantageous, especially if the water were not absolutely 

 clean. Every leaf of Salvinia is, to begin with, excavated by a 

 double layer of air-spaces, which lodge so much air as to give it great 

 buoyancy. On the upper surface are placed at regular distances a 



Fig. 1. 



Salvinia natans. A, combined surface-view and section of floating leaf, modi- 

 fied from a figure in Sachs's ' Botany,' showing the air-cavities, the submerged 

 hairs of the lower surface, and the groups of stiff hairs on the upper surface. 

 These latter inclose spaces into which water cannot enter, even when the leaf is 

 completely submerged. B, one group of hairs from the upper surface, seen from 

 above. 



number of prominences, each surmounted by a group of about four 

 stiff, spreading hairs, which keep the water from reaching the surface 

 of the leaf. "When forcibly depressed, the Salvinia takes down with 

 it a layer of air, which forms a flat bubble over the leaf, and of course 

 gives great power of self-righting, for the specific gravity of the 

 upper side is greatly reduced, while the lower side is weighted, as 

 before, by the long, water-logged roots. Once restored to the surface, 

 the bubble bursts, and the little drops into which it is instantly 

 resolved roll off like drops of quicksilver. Azolla, which is found in 

 most hot countries, and is often grown in hotliouses, behaves in a very 

 similar way. Here the leaves are far smaller, and crowded together 



