hKCONPARY KFFKCTS OF TREES. L'S;) 



counted for, and the horizontal growth of roots of trees in a forest, 

 when such phenomena are seen; and by this downward growth of the 

 roots of planted trees may the descent of water to the subsoil be 

 promoted, there to be reserved against a day of need, converting the 

 subsoil into a reservoir of water, which, raised by evaporation by heat, 

 would moisten the superincumbent soil, promote vegetation, and 

 pass again through the stomates of the leaves into the atmosphere, — 

 again to be precipitated and absorbed by the soil instead of flowing 

 off to the sea, to be lost in that abyss which never says it is 

 enough ! — the springs and fountains and river sources being to us 

 significant of the greatness of the quantity of water which is thus 

 retained. 



In view of all that has been advanced, it appears that trees 

 by their growth, the extent of their rootage, and the decay and de- 

 composition of their foliage and debris, tend to increase the proportion 

 of the rainfall which is preserved by infiltration, and to utilise for the 

 promotion of vegetation, a larger portion of what is thus retained 

 than otherwise would be so utilized. 



The depth at which evaporation ceases must vary with the 

 temperature, humidity of the atmosphere and character of the soil. 

 Some interesting observatisns on this point are recorded by Johnstrus, 

 in a work entitled Om Fugtighedens Bevaegelese i den naturlige Jord- 

 bund, published in Copenhagen in 1866. He found in the neighbour- 

 hood of Copenhagen that there, at the depth of a mfetre and a half 

 (59 inches) the effects of rain and evaporation were almost impercep- 

 tible, and became completely so at a depth of from two to three 

 metres. (6| to 10 feet. 



Sect. V. — Secondary Effects of Trees in Arresting the Flow and 



Escape of the Rainfall, and thus equalizing to some extent 



the Flow of Rivers. 



While rivers in their sources indicate to us that the quantity of 

 water retained by the earth after a copious fall of rain, by the infil- 

 tration of a portion of this into the soil, must be very great — even 

 these being composed of what is only the drainage of the quantity in 

 excess of what the ground can retain — rivers in their flood indicate 

 that the portion of the rainfall carried away by ruissdlement, or 

 by flowing away over the surface of the ground, must also be great. 



It may be noted that of what is retained by infiltration a great 

 deal may be utilized by vegetation ; and that of what is in excess of 



2a 



