EXHALATIOT^ OF VAPOR BY TREES. 187 



observations recorded by Ebermayer show that though the relo' 

 tive humidity of the atmosphere is considerably greater in the 

 cooler temperature of the wood, its ahsolute humidity does not 

 sensibly differ from that of the air in open ground.* The daily 

 discharge of a quantity of aqueous vapor corresponding to a rain- 

 fall of one inch and a fifth into the cool air of the forest would 

 produce a perpetual shower, or at least drizzle, unless, indeed, we 

 suppose a rapidity of absorption and condensation by the ground, 

 and of transmission through the soil to the roots and, through 

 them and the vessels of the tree, to the leaves, much greater than 

 has been shown by du-ect observation. ISTotwithstauding the high 

 authority of Schleiden, therefore, it seems impossible to reconcile 

 his estimates with facts commonly observed and well estabhshed 

 by competent investigators. Hence the important question of the 

 supply, demand and expenditure of water by forest vegetation 

 must remain undecided, until it can be determined by some- 

 thing approacliing to satisfactory direct experiment.f Valuable 

 observations, by Risler, on the evaporation from cultivated soUs, 

 and the exhalations and exudations of humidity by field plants 

 and forest trees, will be found in the Archwes des Sciences, Bih- 

 lioiheque Universelle de Geneve. His general conclusion, p. 

 263, is, that forests evaporate less than an equal extent of pastur- 

 age, and that if we suppose a mean precipitation of two and a 

 half millimetres per day, of which two milhmetres penetrate into 

 the soil, the forest takes up less than one-half of this supply, the 



* Ebermayer, Die Physikalisehen Mnwirkungen des Waldes, i., pp. 150 et 

 seq. It may be well here to guard my readers against the common error 

 ■which supposes that a humid condition of the air is necessarily indicated by 

 the presence of fog or visible vapor. The air is rendered humid by containing 

 invisibU vapor, and it becomes drier by the condensation of such vapor into 

 fog, composed of solid globules or of hollow vesicles of water — for it is a dis- 

 puted point whether the particles of fog are solid or vesicular. Hence, though 

 the ambient atmosphere may hold in suspension, in the form of fog, water 

 enough to obscure its transparency, and to produce the sensation of moisture 

 on the skin, the air, in which the finely-divided water floats, may be charged 

 with even less than an average proportion of humidity. 



f According to Cezanne, Surell, Stude sur les Torrents, 2* edition, ii., p. 

 100, experiments reported in the Bevue des Eaux et Forets for August, 1868, 

 showed the evaporation from a living tree to be "almost insignificant." De 

 tails are not given. 



