EXHALATION AND THE RISE OF THE SAP. 179 



thirty times as much as the wood of the same species ; although 

 the leaves contain the deposit of a single season only, while the 

 heart-wood is loaded with the accumulations of successive years.* 



313. Exhalation from the Leaves, The quantity of water exhaled 

 from the leaves during active vegetation is very great. In one of 

 the well-known experiments of Hales, a Sunflower three and a 

 half feet high, with a surface of 5,616 square inches exposed to 

 the air, was found to perspire at the rate of twenty to thirty ounces 

 avoirdupois every twelve hours, or seventeen times more than a 

 man. A vine, with twelve square feet of foliage, exhaled at the 

 rate of five or six ounces a day ; and a seedling Apple-tree, with 

 eleven square feet of foliage, lost nine ounces a day. The amount 

 varies with the degree of warmth and dryness of the air, and of ex- 

 posure to light ; and is also very different in different species, some 

 exhaling more copiously even than the Sunflower. But when we 

 consider the vast perspiring surface presented by a large tree in full 

 leaf, it is evident that the quantity of watery vapor it exhales must 

 be immense. This exhalation is dependent on the capacity of the 

 air for moisture at the time, and upon the presence of the sun ; 

 often it is scarcely perceptible during the night. The Sunflower, 

 in the experiment of Hales, lost only three ounces in a warm, dry 

 night, and underwent no diminution during a dewy night. 



314. Rise of the Sap, Now this exhalation by the leaves requires 

 a corresponding absorption by the roots. The one is the measure 

 of the other. If the leaves exhale more in a given time than the 

 roots can restore by absorption from the soil, the foliage droops ; 

 as we see in a hot and dry summer afternoon, when the drain by 

 exhalation is very great, while a further supply of moisture can 

 hardly be extorted from the parched soil ; as we observe also in 

 a leafy plant newly transplanted, where the injured rootlets are not 

 immediately in a fit condition for absorption. Ordinarily, how- 

 ever, exhalation by the leaves and absorption by the roots are in 

 direct ratio to each other, and the loss sustained by the leaves is 



* The dried leaves of the Elm contain more than eleven per cent, of ashes,, 

 while the wood contains less than two per cent. ; those of the Willow, more 

 than eight per cent., while the wood has only 0.45 ; those of the Beech, 6.69 r 

 the wood only 0.36; those of the (European) Oak, 4.05, the wood only 0.21 ; 

 those of the Pitch-Pine, 3.15, the wood only 0.25 per cent. Hence the decay- 

 ing foliage in our forests restores to the soil a large proportion of the inorganic 

 matter which the trees from year to year take from it. 



