NUTRITION OP VEGETABLES. 



53 



to decrease. But the most remaikalile instance 

 of rapid transi^iration yet observed, is that whith 

 is related by Guettard, who found that a small 

 sprig of the comeil tree transpired, in the ooui-se 

 of the day, one ounce and three drachms, a 

 quantity almost double its own weight; they 

 found also in general, that branches deprived of 

 their leaves afford but little transpired matter, 

 and that branches furnished with their leaves 

 afford a great deal ; hence it follows that the 

 leaves are the principal organs of transpiration. 



The substance thus transpired by the plant 

 may be obtained by enclosing a. bough in a glass 

 vessel of proper dimensions luted to the branch. 

 Its properties have not yet been very minutely 

 investigated. Hales and Guettard could discover 

 in it nothing different from common water, ex- 

 cept that in some cases it had the odour of the 

 plant. But Duhamel found that it becam* 

 sooner putrid than water. 



These experiments have since been repeated 

 by Desfontaines and Mirbel, who have again 

 found occasion to admire the accuracy and 

 sagacity of the English philosopher. Senebier 

 demonstrated, by numerous experiments, that 

 the quantity of water expired was to that absorbed 

 by the vegetable as two to three. This circum- 

 stance is an additional proof that a part of this 

 liquid is fixed or decomposed in the interior of 

 the plant. 



These facts incontestably prove : 1. That 

 vegetables transpire by their leaves ; in other 

 words, throw out a certain quantity of aqueous 

 iiu\ds. 2. That this transpiration is greater in 

 proportion to the heat and dryness of the atmos- 

 phere ; whereas in moist weather, and especially 

 at night, there is scarcely any. 3. That this 

 function is performed with greater activity, the 

 younger and more vigorous the plant is. 4. 

 That nutrition takes place more effectually the 

 more the transpiration is proportionate to the 

 absorption ; for, when one of these functions is 

 performed with more vigour than the other, the 

 plant languishes. This is observed in plants which, 

 on being exposed to the heat of the sun, fade 

 and lose their vigour, because their transpiration 

 is no longer proportionate to the absorption per- 

 formed by the roots. 



In newly transplanted vegetables the tran- 

 spiration by the leaves is so great as often to ex- 

 haust and destroy the plant before the roots have 

 so far recovered their action as to supply the 

 waste. Hence in all such cases an abundant 

 allowance of water is necessary to secure the 

 health and vigour of transplanted vegetables. 



Expiration. We have already shovra that 

 vegetables absorb or inspire a certain qxiantity of 

 ah- or of other aeriform flxiids, either directly or 

 mixed with sap, by means of theu- roots and 

 leaves, which operate simultaneously in produc- 

 ing this effect. The portion of these absorbed 



fluids, which has not been decomposed for the 

 pui-pose of supplying alimentary matter, is 

 ejected by expiration. Plants, Uke animals, are 

 therefore provided with a kind Of respiration, 

 which in the former, as in the latter, consists of 

 two phenomena, inspiration and expiration. 

 This function is very perceptible when we im- 

 mei-se a branch of a tree, or a young plant, in a 

 glass bell filled with water, and expose it to the 

 action of light. There is then seen rising from 

 its surface a great number of small bubbles, 

 which are formed of a very pure air, almost en- 

 tirely composed of oxygen gas. On the other 

 hand, let the experiment be made in a dark place, 

 and the leaves will expire carbonic acid and 

 nitrogen gass, but no oxygen. It must here be 

 carefully remarked, that all the other parts 

 of the vegetable which are not of a green colour, 

 such as the roots, the bark, the flowers, and the 

 fruits, when subjected to the same experiments, 

 always exhale carbonic acid gas, but never oxy- 

 gen. Consequently, the expiration of oxygen 

 gas does not depend solely upon the direct influ- 

 ence of the rays of light, but also upon the green 

 colom-ing of the parts. 



We know that vegetables, when exposed to 

 the action of the sun, absorb a great quantity of 

 carbonic acid, which they decompose in the in- 

 terior of their substance, and eject the greater 

 part of the oxygen which was combined with 

 the carbon. Now, this phenomenon is also a 

 true expiration. 



When a plant is dead or languishing, either 

 expiration ceases entirely, or the expired fluid 

 is always nitrogen gas. Some vegetables, even 

 when exposed to the influence of the sun's rays, 

 expire only azote. Of this kind are the sensi- 

 tive plant, the holly, the rose-laurel, and some 

 others. It seems difiicult to point out the true 

 cause of this anomaly. 



Excretion. The ejected matters of vegetables 

 are fluids of various degi-ees of thickness, some- 

 times capable of condensing and becoming solid. 

 They are of very diversified nature, being some- 

 times resins, wax, or volatile oils ; sometimes 

 saccharine substances, manna, fixed oils, &c. 

 All these substances are thro'ftTi out at the 

 surface by the power of vegetation. Thus the 

 fraxinus ornus, and some other species of ash, 

 in Calabria, exude a thick saccharine fluid, 

 which, under the action of the air, becomes con- 

 crete, and forms manna, pines, firs, and, in 

 general, all trees of the family of coniferae, furnish 

 large quantities of resinous matter. Many plants, 

 such as the Ceroxylon andicola, a superb species 

 of palm, described by Humboldt and Bonpland, 

 and the Myrica cerifera of North America, yield 

 a large quantity of wax, which is usefully em- 

 ployed in the countries to which these plants 

 are indigenous. 



What is generally called perceptible perspira- 



