500 ORGANOLOGY. 



damp atmosphere, and especially in the case of plants that have 

 already exhaled very much, a taking up of moisture, especially 

 through their green parts ; but our observations on this fact have 

 been too little accurate and purposeless to admit of a precise ex- 

 planation of the process. 



The study of vegetable exhalation in general requires a repetition and 

 improvement of the experiments made upon it. We need a set of 

 experiments which should show, with the greatest exactitude, the dif- 

 ference between the quantity of water absorbed and exhaled, from which 

 we might decide the quantity used for the nourishment of the plant. If 

 the amount of oxygen exhaled with the water was also obtained, we 

 should probably be able to arrive at conclusions respecting the nature of 

 the chemical processes carried on within the plant. We have yet to 

 ascertain the relation of the exhalation of the wall to its absorption. The 

 fact of its absorption (by other means than the root) has been established 

 by Hales, but we are still quite in the dark as to the manner. An accurate 

 knowledge of these relations is so much the more to be desired, as the 

 evaporation and absorption of water with the tension of the vapour must 

 exert an influence upon the absorption or exhalation of the several kinds 

 of gases. Yet in the experiments made upon the so-called respiration of 

 plants, this has been lost sight of. 



We know nothing of the organs through which exhalation is effected. 

 To myself it appears improbable that the living epidermis should be per- 

 meable to water and the vapour of water, except through the stomates 

 (see 36.). 



It is an established fact, that all evaporating water carries with it some 

 portion of the matter which it held in solution. This is seen in the 

 vapour of the ocean. It is probable that no water exhaled from plants 

 is absolutely pure. But no accurate analyses have been made on this 

 point. 



The natural consequence of this exhalation of water from the green 

 parts of plants which are exposed to the air, is the continual concentration 

 of the juices in the cells which lie next the evaporating surfaces. By 

 this, the endosmose of the cells which do not exhale undergoes a change, 

 of which we shall have to speak hereafter. 



The information which we possess respecting the exhalation of plants 

 is chiefly found in the experiments of Hales *, Guettard f, Sennebier J, 

 Schiibler, and Neuffer. 



The strange tendency always to attribute to vitality something different 

 from the physical powers, has introduced into the doctrine of the trans- 

 piration of plants a distinction between evaporation and exhalation ; the 

 first being supposed to take place in dead plants and the last in living ones. 

 I can find no distinction in this case in the facts, but merely in the 

 words. 



I will here add some facts upon the quantity of water exhaled by 

 plants. 



According to Hales ||, a sunflower evaporated daily 1*25 Ib. of water: 



* Vegetable Statics. 



f Memoires de 1'Acad. des Sc. de Par. Ann. 1784. p. 419, et seq. 

 \ Physiologic vegetale, vol. iv. p. 56. 



Untersuchung iiber die Temperatur der Vegetabilien und verschiedene dainit in 

 Verbindung stehende Gegenstande. Tubingen, 1829. 

 || Vegetable Statics. 



