188 RETROSPECT 



tion. Now, the passage of carbonic acid from the air into the 

 organism of the plant is effected by means of the leaves ; but 

 the absorption of carbonic acid cannot take place without the 

 contact of its particles with the surface of the leaf, or of a part 

 of the plant capable of absorbing it. Hence, in a given fffiie 

 the quantity of carbonic acid absorbed must stand in exact pro- 

 portion to the surface of the leaves, and to the amount of it exist- 

 ing in the air. 



Two plants of the same kind, with equal surfaces of leaves 

 (l. e. surfaces of absorption), will take, during the same time, and 

 under like conditions, the same amount of carbon. And if the 

 air contains double the quantity of carbonic acid tha* it does at 

 another time, the plants, under like conditions, will absorb double 

 the quantity of carbon.* A plant with only half the surfaces 

 of the leaves of another plant will absorb quite as much carbon 

 as the latter, if the air supplied to the former contains twice the 

 amount of carbonic acid. 



These considerations point out to us the cause of the favorable 

 action exerted on cultivated plants by humus, and by all decaying 

 organic substances. 



Young plants, when dependent on the air alone, can cnlv 

 increase their amount of carbon according to their absorbing 

 surfaces. But it is obvious, if their roots receive, by means of 

 humus, three times the amount ot carbonic acid absorbed by 

 their leaves in the same time, their increase in weight will be 

 fourfold, on the assumption of the existence of all the conditions 

 for the assimilation of the carbon. Hence, four times the quan- 

 tity of stems, leaves, and buds, must be formed ; and, by the 

 increased surface thus obtained, the plants will receive in the 

 same degree an increased power of absorbing food from the air ; 

 and this power remains in activity long after the supply of carbon 

 to the roots has ceased. 



But the use of humus as a source of carbonic acid, in arable 

 land, is not only to increase the amount of carbon in the plant ; 



* Boussingault remarked that leaves of a vine inclosed in a globe re- 

 moved completely from the air all the carbonic acid contained in it, how- 

 ever rapidly the stream of air was made to pass. (Dumas : Lectures, 

 p. 23.) 



