374 APPENDIX 



An attempt has been made to measure the comparative importance of evaporation 

 beneath the trees and outside of them by ascertaining the quantity of liquid lost from 

 receptacles full of water placed under cover and in the open fields. Under these condi- 

 tions two to five times, in certain cases eight times, but on an average three times, 

 more water is evaporated in the open country than under the trees. But these experi- 

 ments are of little value even when the receptacles of water are replaced by impene- 

 trable chests full of earth; the conditions under which the experiments are made being 

 too far removed from natural conditions. 32 



It remains for us now to compare the forest with land under cultivation from the 

 point of view of the quantity of water drawn off from the soil by the vegetation. 



To tell the truth, we are absolutely ignorant of the quantity of water necessary to 

 the production of agricultural or forest crops. One observer, Wollny, 33 undertook in 

 1879 and 1880 direct measurements of the quantities of water consumed by various 

 plants (barley, oats, red clover, grass, rye, etc.) which he had sown in especially pre- 

 pared boxes without drainage. At the beginning of the experiment he had ascertained 

 the quantity of water contained in the soil of the boxes; by adding to this the same 

 quantity of water as would be furnished under natural conditions lasting over a similar 

 period of time, either by rain or by dew, and by removing from the bottom of the boxes 

 all that filtered through the earth, and which he carefully collected, the amount of 

 water consumed was obtained. In reality the quantities measured are superior to 

 this consumption for they include, in addition, that which has been lost by evaporation 

 from the soil, or by evaporation of the water remaining adherent to leaves and stalks. 

 The experiments of Wollny were extended over 105 to 155 days of the season of growth. 



The consumption of water was on an average 38 million pounds to the acre, the maxi- 

 mum figure being furnished by the clover which reached 47 million. These figures 

 represent an average consumption per acre per day during the growing season of about 

 18 to 19 cubic yards. 



In 1870 and 1871 an older writer, Risler, 34 discovered that the average daily con- 

 sumption per acre during the season of growth was 27 cubic yards for Luzern and 

 fields generally, 23 for oats, 12 for rye, etc., and on an average 17 for cultivated vege- 

 tables, while it would only be 4.2 cubic yards for the fir tree and 3.1[for the oak. It is 

 much to be regretted that we have no means of judging of the value of these figures, 

 as we do not know how they were obtained. 



M. Ney, by combining the figures of Wollny and of Risler, calculates M that field 

 vegetables in general consume 2,093 cubic yards of water per acre during the growing 

 season. 



An Austrian experimenter, V. Hohnel, has directly measured the quantity of aqueous 

 vapor emitted by the leaves of different trees from June 1 to October 1. During that 

 period he found that the leaves of the several species emitted the following percentages 

 of their own weight in aqueous vapor: Birch, 68 per cent; ash, 57; hornbeam, 56; beech, 

 47; oak, 28; spruce pine, 6; Scotch pine, 6; fir tree, 3. 



With these data for basis, M. Ney x calculates that the consumption of water per 



32 For the French experiments see M. de Bpuville, op. cit., pp. 25 et seq. For those 

 carried out in Switzerland consult the " Mitteilungen " of the Research Station of Zurich. 

 For the German observations see the official accounts published by M. Miittrich on the 

 work of the Research Stations; a resume of the results is to be found reproduced by 

 M. Weber in the "Encyclopedic Forestiere de Lorey." 



"Forschungen auf dem Gebiete der Agricultur Physik, Vol. XII, p. 27. 



34 The experiments of Risler are only known by the quotations made by Wollny in 

 the work mentioned previously, and we are ignorant of the methods pursued by this 

 experimenter. 



35 "Der Wald und die Quellen ," p. 74. 

 * Op. cit., p. 75. 



