CHAPTEU II. 



On the Quantity of Moisture Evaporated through the Stomates 

 OP Leaves. 



On results obtained by observation and calculation in regard to the 

 quantity of moisture evaporated through the stomates of leaves a 

 considerable difference of opinion exists. 



Eemarking on the results arrived at by observations of Schleiden, 

 Pfaff, and others, Marsh says, — " Allowing two hundred days for the 

 period of forest vital action, the wood must, according to Schleiden's 

 position, exhale a quantity of moisture equal to an inch and one-fifth 

 of precipitation per day ; and it is hardly conceivable that so large a 

 volume of aqueous vapour, in addition to the supply from other 

 sources, could be diffused through the ambient atmosphere without 

 manifesting its presence by ordinary hygrometrical tests much more 

 energetically than it has been proved to do ; and in fact the observa- 

 tions recorded by Ebermeyer show that though the relative humidity 

 of the atmosphere is considerably greater in the cooler atmosphere of 

 the wood, its absolute humidity does not sensibly differ from that of 

 the air in open ground. The daily discharge of a quantity of aqueous 

 vapour corresponding to a rainfall of one inch and a fifth into the cool 

 air of the forest would produce a perpetual shower, or at least a drizzle, 

 unless, indeed, we suppose a rapidity of absorption and condensation 

 by the ground, and of transmission through the soil to the rootlets 

 and through them and the vessels of the tree to the leaves, greater 

 than have beea shown by direct observation." 



Cezanne, also, writing in regard to the action of vegetables as 

 "veritable alembics, which distil into the air a certain quantity of 

 water which their roots have drawn up from the soil," says, — 

 " The Marshal Vaillaut put a branch of an oak like a bouquet into 

 a vase full of water ; he measured the water lost through its leaves 

 and he considered himself enabled to conclude that the tree from which 

 this branch had been detached would be emitting into the atmosphere 

 in twenty-four hours upwards of two thousand kilogrammes of water.* 



* " Revue des Eaux et Forets," July 1865 and June 1867. 



