798 MODIFICATIONS OF ASPECTS OF OEGANIC NATURE 



soil, whilst rain which falls in larger masses runs off and forms 

 torrents. The roots making up an interlacing fibrillar mass by 

 their multitudinous divisions, entangle and detain the moisture 

 which comes to them in capillary columns ; and from the loaded 

 sponge which they thus come to represent, they dole or issue out in 

 rations the supplies necessary for keeping springs and streams in 

 constant and perennial volume ^. 



It is, I must say, a considerable marvel, that upon a third func- 

 tion of that part of a tree which man can affect, either by his own 

 hands or through the intermediation of his domestic animals, with 

 the greatest results in the way of mischief at the least cost of 

 labour to himself, so much room for dispute and doubt should still 

 be left open by the botanists. Upon this third function of the 

 leaves, their power as evaporators, the most important perhaps of 

 all their functions, both as regards the tree's own economy and as 

 regards ours, it is little less than marvellous that a Professor of 

 Botany should have to write thus in 1875. Professor Koch, how- 

 ever (' Vorlesungen iiber die Dendrologie/ 1875, p. ^^84), following 

 Ebermayer, 1. c, p. 1 83, says : — 



' The question of the evaporation of water through the tissues of a plant is very 

 like the question in medicine of the treatment of diseases. The more there is 

 written about a disease, and the more we have so-called infallible remedies recom- 

 mended for it one after the other, the less do we get of any real knowledge of it. 

 There is scarcely a single point in the life of a plant on which so much, and indeed 

 often so much that is intrinsically self-contradictory, can be specified as having been 

 written, as this point of evaporation. Whilst Unger, and indeed certainly with right 

 on his side, owns that a surface of (so much ?) water gives off by evaporation tliree 

 times as much as (an equal surface of?) a tree, Schleiden says that on the contrary 

 the tree gives off three times as much as the open surface of water ^.' 



It is true that Professor Koch goes on to say that nevertheless, as 



* It is of course possible to exaggerate the preventive power of arboriculture, as of 

 other beneficial agencies. If a mountain is sufiiciently high, and can be blown upon 

 by sea breezes as yet undeprived of the full proportion of moisture which a warm 

 latitude can give them, you will have from time to time destructive torrents rushing 

 down their sides, however well wooded they may be. But what is an occasional 

 occurrence only in a well-wooded mountainous country, is a very common one in a 

 district where the charcoal-burner, the wood-merchant, and the goat have been 

 allowed to have their wasteful will unchecked. Homer's lines, II. xi. 492-495, show 

 that however striking the phenomenon he describes, it was nevertheless not so very 

 common as the complaints with which so many of the Reports I have referred to 

 prove it to be now in so many countries in Europe and elsewhere. 



^ The German words, which I have not attempted to translate quite literally, are 

 as follows: — 



* Mit der Verdunstung des Wassers durch die Pflanze geht es, wie in der Medizin 



