204 DRAINAGE BY EOOTS OF TREES. 



foliage of the pine continues to exhale through the winter a suf- 

 ficient quantity of moisture to account for tlie drying up of the 

 soil. This explanation is improbable, and I know nothing in 

 American experience of the forest which accords with the al- 

 leged facts. It is true that the pines, the firs, the hemlock, and 

 all the spike-leaved evergreens prefer a diy soil, but it has not 

 been observed that such soils become less dry after the felling of 

 their trees. The cedars and other trees of allied famihes grow 

 naturally in moist ground, and the white cedar of the Northern 

 States, Thuya occidentalism is chiefly found in swamps. The roots 

 of this tree do not penetrate deeply into the earth, but are spread 

 out neai* the surface, and, of course, do not carry off the waters of 

 the swamps by perpendicular conduction. On the contrary, by 

 their shade, the trees prevent the evaporation of the superficial 

 water ; but when the cedars are felled, the swamp — which some- 

 times rather resembles a pool fiUed with aquatic trees than a grove 

 upon sohd ground — often dries up so completely as to be fit for 

 cultivation without any other artificial drainage than, in the ordi- 

 nary course of cultivation, is given to other new soils.* 



* A special desiccative influence has long been ascribed to the maritime pine, 

 which has been extensively planted on the dunes and sand-plains of western 

 France, and it is weU established that, under certain conditions, all trees, 

 whether evergreen or deciduous, exercise this function, but there is no con- 

 vincing proof that in the cases now referred to there is any difference in the 

 mode of action of the two classes of trees. An article by D'Arbois de Jubain- 

 ville in the Bevue des Eaux et Forets for April, 1869, ascribing the same action 

 to the Pinus sylvestris, has excited much attention in Europe, and the facts 

 stated by this writer constitute the strongest evidence known to me in support 

 of the alleged influence of evergreen trees, as distinguished from the draining 

 by downward conduction, which is a function exercised by all trees, under 

 ordinary circumstances, in proportion to their penetration of a bibulous sub- 

 soil by tap or other descending roots. The question has been ably discussed 

 by Beraud in the Eevue des Deux Mondes for April, 1870, the result being that 

 the drying of the soil by trees is due simply to superficial conduction by the 

 roots, whatever may be the foliage of the tree. See post : Influence of the 

 Forest on Flow of Springs. 



It is however certain, I believe, that evergreens exhale more moisture in win- 

 ter than leafless deciduous trees, and consequently some weight is to be ascribed 

 to this element. 



The extraordinary rapidity of the growth of the eucalyptus has been ascribed 

 to the supposed exceptional power of absorption of water from the earth by the 

 roots of the tree, and the consequently increased supply of elements of vege 

 table nutrition contained in the water thus absorbed. It is affirmed, indeed^ 



