EXHALATION. 25 



to ascertain the quantity of watery vapor which they usually throw 

 off. 



Hales planted a sun-flower in an air-tight vessel, the top of which 

 was sealed hermetically by a leaden cover. This cover was pierced 

 by two holes : one for the passage of the stem of the plant, the other 

 for the introduction of the water necessary to its growth. For a 

 fortnight the apparatus was regularly w^eighed, and our ingenious 

 experimenter found that the green parts of the sun-flower threw off 

 on an average about twenty ounces of water in twelve hours of the 

 day. The evaporation was always increased during dry and warm 

 weather ; moist air lessened it ; during the night season, the evapo- 

 ration was sometimes no more than three ounces, and it occasional- 

 ly happened that it was nil. 



Vegetable life appears to be intimately connected with the pheno- 

 menon of evaporation. From the inquiries which I have myself 

 undertaken on this subject, so well deserving the attention of obser- 

 vers, it would appear that a plant grows only when it transpires, and 

 that in hindering this transpiration, we in fact arrest vegetation. 



We now associate with the phenomenon of exhalation the source 

 or accumulation of certain substances which are met with in con- 

 siderable quantity in the organization of plants, although scarcely a 

 trace of them can be detected in the water with which they are sup- 

 plied. The water evaporating, leaves these substances behind ; and 

 as the mass of liquid imbibed by the roots and exhaled by the green 

 parts is very considerable, it is easy to conceive how they should 

 finally come to be present in rather large quantity. 



A portion of the water which a plant in full vigor absorbs, must 

 necessarily enter into its constitution ; the water exhaled by the 

 leaves, therefore, cannot equal the whole of that which has been 

 absorbed by the roots. Sennebier endeavored to ascertain the rela- 

 tion which exists between the absorption and the exhalation, and he 

 found in the particular case which he observed, that about ^ of the 

 water absorbed was fixed, and became a constituent part of the 

 vegetable. 



§ II.— CHEMICAL PHENOMENA OF VEGETATION. 



The chemical phenomena of vegetation are accomplished by the 

 concurrence of the elements of the atmosphere, of water, and of 

 certain organic substances which exist as constituents of the soil. 



The action of the atmosphere upon plants presents two phases 

 perfectly distinct from one another ; germination, and vegeta' ion 

 properly so called, w'hich last comprises the development, the gro'vth, 

 and the mult?plication of species. 



3 



