ON THE AQUEOUS EMANATION OP VASCULAR VEGETABLES. 



91 



want them ; and on the contrary, roots, grains, and in general all organs without 

 stomata, are only subjected to the insensible deperdition. If vegetables are com- 

 pared with each other, we arrive at the same general results : thus, fleshy leaves 

 that have few stomata exhale but little ; petals and fleshy fruits which have none, 

 appear only submitted to the insensible deperdition. 



All these facts are the results of experiments made by Guettard, St. Martin, 

 Bonnet, and Senebier, before even the existence of stomata was known. Mr. Knight 

 confirmed them, in showing that a leaf of the vine exhaled minute drops of water 

 only from the inferior side, that is to say, the side supplied with stomata, and not 

 the superior side. 



Those outward circumstances, which might have influence over this pheno- 

 menon, are ordinarily heat and light ; but heat, which seems to act sensibly upon 

 the deperdition, appears on the contrary to have very little upon the exhalation. 

 The very little augmentation in the weight lost by vegetables exposed to a tempe- 

 rature more or less great (provided that the degree of heat sufficient to disorganise 

 their tissue be out of the question) might indeed be attributed quite as well to 

 the acceleration of the insensible deperdition, as to that of exhalation. Light 

 appears to be, on the contrary, of all the exterior causes, that which acts most 

 intensely in exciting exhalation. Senebier has observed that when a plant is 

 placed in total obscurity, it ceases suddenly to transpire, although it continues 

 still to absorb for some time ; so that its weight augments a little. Hales had 

 already noticed that plants only transpired during the day, and that they 

 augmented a little in weight during the night. This phenomenon depends either 

 upon the cessation of exhalation ; or upon the external air becoming more humid 

 and depositing small drops of water on their surface ; or, finally, upon the green 

 parts absorbing a little oxygen during the night. Guettard and Senebier have 

 enclosed leafy branches in flasks, and have placed them, some in the light, the 

 others in obscurity ; the first have given out a much greater quantity of water 

 than the second. I have observed that the light of lamps produces the same 

 result as that of the sun, as regards their relative intensity. This effect of 

 light is indeed in proportion to the intensity of the light ; and the effect produced 

 by the interposition of a linen cloth or a sheet of paper is well known. Gardeners 

 and vendors of bouquets. know very well that they preserve their branches of trees 

 much longer fresh in an obscure situation than in a well-lighted one. If living 

 plants are kept too long from the influence of the solar rays, as they will cease to 

 transpire and continue to absorb, they will acquire by degrees a state of dropsy, 

 which disposes to the disarticulation of the leaves from the stalk, and announces 

 a state of feebleness, owing to the too great abundance of water. We will return, 

 elsewhere, to the subject of the influence of obscurity upon plants or the parts of 

 plants which develop themselves therein. I confine myself here to the rendering 

 evident the extreme influence of light in exciting transpiration. 



( To be continued.) 

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