90' 



ON THE AQUEOUS EMANATION OF VASCULAR VEGETABLES. 



near the air. Its exact appreciation is rendered complicated by the small 

 quantity of carbon, which, in certain cases, as we shall see presently, the oxygen 

 of the air draws from the vegetable tissue. In the cases to which I here allude, 

 the parenchymatous parts are surrounded by a cuticle, or an epidermis, without 

 stomata or evaporatory pores, which being but very slightly permeable, retain the 

 humidity, and only suffer a very minute portion to escape. 



2nd. If the organs, or vegetables, deprived of their true cuticle, be exposed 

 to the fresh air, such as the leaves of plants immersed in water, or those of 

 cellular vegetables, the deperdition of water is seen to be very variable in its 

 intensity, according to the kinds selected for experiment. The leaves of plants 

 which live habitually in water, lose in general, with great rapidity, that which 

 their parenchyma contains ; which M. Ad. Brongniart attributes to the absence 

 of the cuticle. This phenomenon presents itself even in the greater part of the 

 cryptogamic aquatics ; but in some of these, and in many cryptogamic aerial 

 plants, the deperdition is extremely slow, as is seen in the mosses, in certain 

 sea-weeds, in the leathery mushrooms, and more particularly in the lichens. 

 This slowness of deperdition, notwithstanding the absence of all real cuticle, 

 appears to be caused either by the cellules of cryptogamous plants being more 

 intimately joined together than those of the parenchyma of ordinary leaves, 

 and thus allowing a less free passage for the water to evaporate ; or, by the 

 exterior layers in many cases being arranged sufficiently close to perform the 

 office of the cuticle or epidermis ; or, finally, by some hygrologic disposition of 

 the tissue. 



3rd. Finally — If leaves are placed in the same circumstances, or, in general, 

 those organs invested with a cuticle more or less furnished with stomata, then 

 more active phenomena are observed ; that is to say, the exhalation of an 

 immense quantity of water in a very short time. It is this function which 

 exercises itself evidently by the stomata, which I designate under the name of 

 aqueous exhalation, in order to distinguish it from the preceding. It is very 

 probable that the parts of the foliaceous organs which have stomata, are also 

 susceptible of the insensible deperdition, and that consequently, we confound the 

 results of these two causes in the experiments cited ; but the insensible deperdition 

 enters into the phenomenon by so small a fraction, that it may be neglected 

 without inconvenience. We will now confine ourselves to the examination of the 

 circumstances of this important function, in order to deduce from it its physio- 

 logical action. 



All the parts of a plant do not exhale water in the same quantity, and every 

 experiment tends to prove that (except the facts which relate to the two preced- 

 ing cases, and everything besides being equal) the emanation of each part is in 

 direct accordance with the number of its stomata. Thus the surface of the leaves 

 which are furnished with stomata, exhale more than those which are deprived of 

 them ; the green bark which is furnished with stomata more than those which 



