8 



NATURE 



[May 5, 1892 



difficulty, and if there is water behind it, the water may 

 thu s be restrained from passing through the meshes. 



(6) MusHn bag hung in front of the lantern. Water 

 poured into the bag (a large spoonful) does not flow out ; 

 but when the muslin beneath the water is rubbed with a 

 rod, it becomes wetted, the surface-film passes to the 

 outside of the bag, and the water trickles through. 



There are many plants which take advantage of this 

 property of the surface-film of water, viz. that it will 

 not penetrate small spaces, in order to keep themselves 

 dry. You must have observed how the hairy grasses 

 repel water. The surface -film is unable to pass into the 

 fine space between the hairs, and accordingly the water 

 above the surface-film is kept from contact with the leaf. 

 This simple artifice is often employed by plants which 

 float at the surface of water. Here it is important that 

 they should keep dry, not only for the purpose of respira- 

 tion, but for another reason too. They commonly have 

 great power of righting themselves when accidentally 

 submerged, and this self-righting property depends upon 

 the fact that the under surface of each leaf is always wet, 

 while the upper surface is incapable of being wetted. 



Fig. I. — Duckweed {Leinna Minor), magnified. A, single frond; a, scar 

 of attachment to parent. A ridge extends from a to b across the upper 

 surface of the frond, gentl/ subsiding towards b. B, frond, budding-out 

 two new fronds, c, longitudinal section from a to b (a), showing 

 ascending capillary curves at a and b. D, transverse section, at right 

 angles to the last. The margins of the frond in this plane are level with 

 the surface of the water. N.B. The form of the fronds is somewhat 

 variable. Minor inequalities occur along the margin, but the principal 

 ascending curves, which are also centres of attraction, are at a, b, and c. 



The microscopic hairs which thickly cover the upper sur- 

 face are sufficient to exclude the water. A leaf of Pistia 

 is now submerged, and shown as an opaque object in the 

 lantern. You see by the gleaming of its surface that it is 

 overspread by a continuous flat bubble of air, which looks 

 like quicksilver beneath the water. I will next invert a 

 leaf of Pistia by means of a rotating lever. It is now 

 brought up beneath the surface of the water in an 

 inverted position, and you see that, notwithstanding its 

 buoyancy, it is unable to free itself and rise to the sur- 

 face, because of the air-bubble, which adheres both to 

 the leaf and to the disk at the end of the lever, and ties 

 both together. Complete separation of the leaf from the 

 disk would involve the division of the air-bubble into two 

 smaller bubbles, one adhering to the leaf and the other to 

 the disk. In this operation the surface-film would neces- 

 sarily be extended directly in opposition to its natural 

 tendency to contract. Several other water-plants exhibit 

 the same properties as Pistia. I will mention two of the 

 wdter-ferns— Salvinia and Azolla. Salvinia is found 

 floating on still water in the warmer parts of Europe, as 

 well as in other quarters of the globe. The leaves are 

 attached on opposite sides of a horizontal stem. Long 



hairy roots (or what look like roots, and really answer 

 the same purpose) hang down into the water. Salvinia 

 has in a remarkable 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 



Fig, 2. — Salvinia natans. a, combined surface-view and section of floating 

 leaf, modified 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. 



water or drenched with rain. Its nutrition, like that of 

 all green plants, depends largely upon substances ex- 

 tracted 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 



NO. 1175, VOL. 46] 



Fk;. 1. — Azolla caroliniana. A, stem with leave.s magnified; B, longi- 

 tudinal section through part of ditto, highly magnified. The air-cavities 

 of the leaves are shown, the narrow .spaces between the leaves, into 

 whi(h water cannot enter, the fine hairs of the upper surface, the sub- 

 merged leaf-lobes, and t e vascular bundles. 



upper surface are placed at regular distances a 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 



