FFXCTION.] EPIDERMIS. 179 



which incloses them under all circumstances except those 

 mentioned at p. 132, &c. The function of this membrane is 

 fourfold. It acts as a guard against injury, especially when, 

 as in succulent plants, it acquires much thickness, in which 

 cases, it may be regarded as a true vegetable hide. It is a 

 perspiring organ, carrying off the superfluous water intro- 

 duced into the system ; being enabled to do this by the 

 stimulating effect of light, independently of the action of 

 dry air, or of its own vital force. It is a respiratory organ, 

 allowing gaseous matters to pass freely through it in either 

 direction. And finally, it is an organ of absorption, readily 

 taking up water, whether as fluid or vapour, whenever it 

 is presented to it. Dr. I). P. Gardner has made some 

 experiments to prove its porosity, which will be found in a 

 note.* 



* The object in this place is to show, that the epidermis is not merely capable 

 of transmitting particular gases, but that it obeys all the laws of a porous 

 system. If this be found true for the bounding membrane, it will necessarily 



be true for the internal cellular structure. Experiments were planned for 



the purpose of determining whether carbonic acid would penetrate into a vessel 

 containing common air through a barrier of vegetable epidermis ; and secondly, 

 whether an inclosed mixture of gases of a theoretical composition would solicit 

 the passage of both carbonic acid and oxygen, and at the same time evolve 



nitrogen. A tube, five inches long and a third of an inch in bore, with a 



flattened and ground end, was closed by a piece of epidermis obtained from the 

 leaf of the Madeira vine (Basel la lucida). The tube was then immersed in a 

 mercurial trough, and filled to within an inch of the membrane ; on suspending 

 it by a wire it did not leak, though there was a pressure of three inches of 

 mercury. Clear lime-water was admitted to displace the mercury, and the 

 arrangement covered by a small bell-jar containing atmospheric air with 10 per 

 cent, carbonic acid. In five hours the lime water exhibited a distinct pellicle of 

 carbonate of lime. The same result was obtained in more or less time with the 

 epidermis of the cabbage, Alanthus alata, Chenopodium album, and several 

 species of Sedum. Some specimens, as that from the balsam, leaked so fast as 

 not to sustain any mercurial column, whilst others maintained four inches for 

 thirty hours and more. A similar tube was closed with epidermis, and con- 

 tained an atmosphere of nitrogen 87, oxygen 13 per cent, over mercury, and 

 was covered with a bell-jar as above. The included volume increased during 

 nine hours from 100 to 433 measures, and on analysis consisted of N 76, 017, 

 CO, 2 7 per cent. Hence the membrane comported itself as a simple porous 

 tissue, allowing nitrogen to pass out and admitting oxygen and carbonic acid. 

 This experiment was also repeated with the foregoing specimens of epidermis, 

 and no absolute variation perceived. (Philosophical Magazine, xxviii. 425.) 



N 2 



