74 EFFECTS OF FORESTS ON MARSHES. 



disposed of, and the effect of forests in drying up marsbes, viewed as 

 an isolated fact, may be proved as satisfactorily as any of the effects 

 of forests which have been under consideration. 



I have already alluded to this effect in stating that the allegation 

 of the South African farmer, that the trees stole the water from the 

 water leadings, by the sides of which they were planted, was not 

 altogether without foundation, though the phenomenon on which his 

 opinion was founded was one which did not prove this, but proved 

 something very different. 



We have had under consideration the fact that very extensive pro- 

 vision has been made for the evaporation of water from the leaves of 

 trees : in the case of one tree, the Lilac, so many as 1 60 thousand 

 stomates being found on every square inch of the under surface of 

 the leaf. Whence is the moisture to be so evaporated obtained? 

 Mainly and almost exclusively, if not entirely so, from the ground. 

 It seems to follow that what the atmosphere thus gains the earth 

 must first lose, even though a portion of it may be subsequently 

 restored. And the quantity thus raised from the ground and given 

 forth to the atmosphere is very great. 



Mr Marsh, whom I have already quoted elsewhere, states : — " The 

 present estimates of some eminent vegetable physiologists in regard 

 to the quantity of aqueous vapour exhaled by trees and taken up by 

 the atmosphere are much greater than those of former inquirers. 

 Direct and satisfactory experiments on this point are wanting, and it 

 is not easy to imagine how they could be made on a sufficiently 

 extensive and comprehensive scale. Our conclusions must therefore 

 be drawn from observations on small plants, or separate branches of 

 trees, and of course ar6 subject to much uncertainty. Nevertheless, 

 Schleiden, arguing from such analogies, comes to the surprising result 

 that a wood evaporates ten times as much water as it receives from 

 atmospheric precipitation. In the Northern and Eastern States of 

 the Union [America] the mean precipitation during the period of 

 forest growth — that is, from the swelling of the buds in spring to 

 the ripening of the fruit, the hardening of the young shrubs, and the 

 full perfection of the other annual products of the tree — exceeds, on 

 the average, 24 inches. Taking this estimate, the evaporation from 

 the forest would be equal to a precipitation of 240 inches, or very 

 nearly 150 standard gallons, to the square foot of surface." 



Startled by this, he questions the correctness of the conclusion at 

 which Schleiden had arrived ; but he states in a foot-note : — " Pfaff, 

 too, experimenting on branches of a living oak, weighed immediately 



