EXHALATION FROM LEAVES. 79 



by lightning. It is difficult to refuse to believe that these people 

 were not more exposed to danger in such a situation than if they had 

 remained in the open field." 



These statements I give as the views advanced by Marshal Vaillant. 

 There are points on which my own views are somewhat different; but 

 with regard to the grand fact, that the emission of moisture by the 

 leaves of the forest is very great, we are at one. 



With what is known of the affinity of humus and of clay for mois- 

 ture, it may be considered probable that not a little of the moisture 

 evaporated by the stomates may be re-absorbed by the soil, though 

 never precipitated as rain. But even supposing that this should have 

 happened, not once, but times innumerable in the course of a specified 

 period — and thus much of the moisture evaporated should be sub- 

 stantially the same moisture, passing again and again and again 

 through the circuit, evaporated, passed through the stomates, ab- 

 sorbed by the air, thence absorbed by the soil, and thence absorbed 

 by the rootlets to be passed on by endosmosic action again to the 

 leaves, to go through the same round again and yet again — it is not 

 unreasonable to suppose that a large portion, a very large portion, 

 must be taken from the soil. 



The experiments founded on by Pfaff, like that of Marshal Vaillant's, 

 give the result of evaporation which could not have been sustained in 

 this way, for the branch was severed from the tree. I have no means 

 of verifying his calculation resulting in the conclusion that the quantity 

 of moisture passing out by the stomates in the time specified must have 

 been equal in quantity to eight and a-half times the whole quantity of 

 rain which fell on the spot in the course of the period specified ; but 

 with what I happen to know of vegetable physiology his conclusion in 

 no way startles me. 



By Dr Asa Gray it is remarked : — " 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 5,616 square inches exposed to the air, was 

 found to perspire at the rate of twenty or thirty ounces avordupois 

 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 



