THE FOOD OF VE«ETABLES. 



113 



I 



a stiiull scale. If a plant is placed under a glass 

 to which no new supply of air has access, it 

 soon begins to languisli, and at length withers 

 and dies ; but particularly if it is placed under 

 the exhausted receiver of an air pump, as might 

 indeed be expected from the failure of the ger- 

 mination of the seed in similar circumstances. 

 According to the experiments of Saussurc, plants 

 of peas, though completely developed and fur- 

 nished with their leaves, died in the space of three 

 days, when put into the exhausted receiver of 

 an air-pump, whether in the shade or the sun. 

 But plants with thick and succulent leaves seem 

 capable of supporting vegetation in vacuo, at 

 least if exposed to the sun. A plant of the 

 cactus op?intia, lived more than a month in this 

 state without showing any symptoms of decay, 

 except that the epidermis seemed dry, which 

 again recovered its freshness, however, in the 

 atmospheric air. And though plants with thin 

 leaves generally died under the experiment, yet 

 there were exceptions even among them. A 

 plant of the poli/gonum persicaria lived for six 

 months in the vacuum of an air-pump, and was 

 at the end of the experiment as fresh and vigor- 

 ous as at the beginning, with the exception of 

 two or three leaves near the root, which were 

 withered. The same was the case also with 

 plants of the epihhium molle, cpilohium hir- 

 sutum, Ij/thrtim salicaria, and inula dj/scnterica. 

 They were placed in the light, but not so as to 

 receive the direct rays of the sun, to which when 

 tliey were exposed they withered, even though 

 the rays were but feeble. 



In these experiments of Saussure, however, it 

 is doubtful whether the inclosed plants did not 

 create an atmosphere for themselves out of their 

 superfluous moisture, in the same way as is 

 illustrated in the recent experiments of enclosing 

 plants in an air-tight bell glass, or glass frame, 

 when they will continue to vegetate for years; 

 the moisture being restrained from passing off 

 by evaporation, and tlie vital energies of the 

 plant being adequate to reproduce the gases 

 necessary for its existence by the decomposition 

 of water. 



The gases, therefore, seem essential to vegeta- 

 ble existence, though we are not yet to conclude 

 that water and the gases are the sole ingredients 

 which are necessary for the growth and vigour 

 of plants. 



It must be admitted no doubt, that plants of 

 elow growth and tenacious of life, such as many 

 of the mosses, and some of the succulent plants, 

 do indeed effect the development of their parts, 

 without the aid of any other nourishment be- 

 yond what they derive from the atmosphere. 

 But plants of rapid growth, such as annuals, can 

 never effect that developement without the aid 

 of nourishment derived from the soil. Saussure 

 tried the experiment upon beans, peas, and 



cresses, by placing them in horbo-hair, or in pure 

 sand, and moistening them with distilled water. 

 They grew indeed, and some of them even 

 flowered, but never produced perfect seeds. It 

 is plain, therefore, that some essential principle 

 of nourishment was wanting, which is furnished 

 by the soil ; and that atmospheric air and water 

 are not the only principles constituting the food 

 of plants. 



But as in germination so also in the progress 

 of vegetation, it is part only of the component 

 principle of the atmospheric air that are adapted 

 to the purpose of vegetable nutrition, and selected 

 by the plant as a food. Let us take them in the 

 order of their reversed proportions. 



Carbonic acid gas. In the process of the ger- 

 mination of the seed, the effect of the application 

 of carbonic acid gas was found to be altogether 

 prejudicial. But in the process of subsequent 

 vegetation its application has been found, on the 

 contrary, to be extremely beneficial. Plants 

 win not indeed vegetate in an atmosphere of 

 pure carbonic acid, as was first ascertained by 

 Dr Priestley', who found that sprigs of mint 

 growing in water, and placed over wort in a 

 state of fermentation, generally became quite 

 dead in the space of a day, and did not even 

 recover when put into an atmosphere of common 

 air. 



But Dr Percival, of Manchester, observed that 

 a plant of mint, immersed in water by the root, 

 and exposed to a current of atmospheric air 

 mixed with carbonic acid gas, was more vigorous 

 and luxuriant than a plant of the same species 

 similarly situated and exposed to a current of 

 pure atmospheric air. 



Improving upon this hint, Saussure made 

 some experiments with a view to determine the 

 dose of carbonic acid gas, which, being mixed 

 with atmospheric air, is the most favourable to 

 vegetation. Having made some peas to germin- 

 ate in water till they acquired the height of four 

 inches, and weighed about twenty grains, he 

 then placed a number of them in glasses filled 

 with water by threes, so as that the roots only 

 were immersed, and introduced them into re- 

 ceivers filled with different mixtures of common 

 air and carbonic acid gas. They were situated 

 so as to receive the direct rays of the sun, 

 moderated when too intense. The mean aug- 

 mentation in weight of such as were placed in 

 pure atmospheric air, and exposed during ten 

 days to the sun, was eight grains to each plant. 

 Such as were exposed to the sun, in an atmo- 

 sphere of pure carbonic acid gas, faded and 

 withered away without any further develop- 

 ment. In an atmosphere containing three- 

 foiirths or two-thirds of its volume of carbonic 

 acid gas, they withered also ; but in an atmo- 

 sphere containing only one half of its volume of 

 carbonic acid they lived seven daj's. And in an 

 p 



