EXHALATION AND THE RISE OF THE SAP. 175 



a lamp, and examining the aslies by the microscope ; which will be 

 found very perfectly to exhibit the form of the cells. The ashes 

 which remain when a leaf or other vegetable substance is burned 

 in the open air, represent the earthy materials which it has accu- 

 mulated. A vernal leaf leaves only a small quantity of ashes ; an 

 autumnal leaf yields a very large proportion, — from ten to 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 5,61G square inches exposed, to the air, 

 was found to perspire at the rate of twenty to thirty ounces avoirdu- 

 pois 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 exposure 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 ; 

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



* 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 45 ; those of the Beech, 6.69, 

 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 decaying 

 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. 



