OF THE GROWTH OF ROOTS. 163 



when the growth of the roots was retarded by want of moisture, the con- 

 tiguity of water, in the adjoining mould, though not apparently in actual 

 contact with them, operated beneficially : but I had reason to suspect 

 that the growth of roots was, under these circumstances, promoted by 

 actual contact with the detached and fugitive particles of the decomposing 

 body, and of the evaporating water. 



The growth and forms assumed by the roots of trees, of every species, 

 are to a great extent, dependent upon the quantity of motion, which their 

 stems and branches receive from winds ; for the effects of motion upon 

 the growth of the root, and of the trunk and branches, which I have 

 described in a former memoir, are perfectly similar*. Whatever part of 

 a root is moved and bent by winds, or other causes, an increased deposi- 

 tion of alburnous matter upon that part soon takes place, and conse- 

 quently the roots which immediately adjoin the trunk of an insulated tree, 

 in an exposed situation, become strong and rigid ; whilst they diminish 

 rapidly in bulk, as they recede from the trunk, and descend into the 

 ground. By this sudden diminution of the bulk of the roots, the passage 

 of the descending sap, through their bark, is obstructed ; and it in con- 

 sequence generates, and passes into many lateral roots ; and these, if the 

 tree be still much agitated by winds, assume a similar form, and conse- 

 quently divide into many others. A kind of net- work composed of thick 

 and strong roots is thus formed, and the tree is secured from the dangers 

 to which its situation would otherwise expose it. 



In a sheltered valley, on the contrary, where a tree is surrounded and 

 protected by others, and is rarely agitated by winds, the roots grow long 

 and slender, like the stem and branches, and comparatively much less of 

 the circulating fluid is expended in the deposition of alburnum beneath 

 the ground ; and hence it not unfrequently happens, that a tree, in the 

 most sheltered part of a valley, is uprooted ; whilst the exposed and 

 insulated tree, upon the adjoining mountain, remains uninjured by the 

 fury of the storm. 



In all the preceding arrangements, the wisdom of nature, and the 

 admirable simplicity of the means it employs, are conspicuously displayed ; 

 but I am wholly unable to trace the existence of anything like sensation or 

 intellect in the plants : and I therefore venture to conclude, that their roots 

 are influenced by the immediate operation and contact of surrounding 

 bodies, and not by any degrees of sensation and passion analogous to those 

 of animal life ; and I reject the latter hypothesis, not only because it is 

 founded upon assumptions which cannot be granted, but because it is 



* Above, p. 9. 



M2 



