800 MODIFICATIONS OF ASPECTS OF ORGANIC NATURE 



observable in an isolated mass of leaves, or in a single isolated tree, 

 does not give us a measure of the rate at which the same process 

 will go on in a wood when the exposed and evaporating surface is 

 relatively so much smaller. And this difficulty, which lies in the 

 geometrical nature of the case, may account for the great dis- 

 crepancies in the estimates which various writers have given of the 

 amount of watery vapour given off by masses of wood ^. 



It must, however, be allowed that the cases in which the cutting 

 down of trees, and the consequent putting into abeyance of the 

 functions of their leaves, have been followed by the drying up of 

 springs, are much more numerous, even if they are not better 

 established, than those in which the reverse effect has been recorded, 

 as by Mr. Abbay. The explanation of this apparently self-anta- 

 gonising or capricious operation of the same primary cause is not 

 far to seek. When a tree is cut down, the area once protected by 

 its leaves is exposed to the uncounteracted action of the summer 

 sun, and rainfall may run off it when thus hardened, just as it runs 

 off an imperfectly thawed surface in the spring, or may sink away 

 into chinks and fissures which that exposure may, and very often 

 does, produce, and in either case such rainfall is lost to the summer- 



^ Professor Pfaff, for example (cifc. Ebermayer, I.e. p. i86), gives us 120 kilo- 

 grammes as the entire amount evaporated by an oak with 700,000 leaves, each of a 

 square surface of 2325 mill, during the period from May 18 to October 24. 



Vaillant (cit. ibid.) gives the amount of watery vapour given off by an oak of 21 

 metres height and 2.63 metres girth at a height of i mfetre above the ground, as 2000 

 kilogrammes on a fine day. 



Hartig (cit. ibid.), the author of a ' Lehrbuch fiir Forster,' Stuttgart, i85i, calculates 

 that a German morgen (= 2.3895 acres), carrying a thousand trees of nine different 

 kinds of conifers and broad-leaved trees of twenty years' planting, exhales daily 

 during the period of vegetation 3000 pounds weight of water. 



Professor Prestwich, in his 'Water-bearing Strata,' 1851, p. 118, gives us as an 

 estimate for the amount of watery vapour given off by the leaves of *a tree of 

 average size ' two and a half gallons per diem. 



Mr. Lawes (cit. in loc), from ' Journal of Horticultural Society,' vol. v. pt. i,, 1850, 

 gives us as a foundation for an estimate of the relations between the amount of water 

 taken in by vegetable organisms, with the matters it held in solution, and the solid 

 residue thence extracted and retained by the plants for its uses or for ours, a state- 

 ment to the effect that three plants of wheat or barley gave off i| gallon, 250 grains 

 of water for every grain of solid residue in the adult plant. 



Hellriegel, on the other hand (cit. Ebermayer, 1. c. p. 187), gives us as his estimate 

 that for the production of i lb. of dry barleycorns, 700 lbs. of water, inclusive of the 

 water evaporated from the soil, are all that is necessary, and that other cerealia have 

 their demands limited within somewhat similar proportions. Intervalla vides humane 

 commoda. 



