36 PHENOMENA OF VEGETATION. 



trees or not ; but the water is strained out and saved by the forest 

 foliage." He says, " I have watched with much interest for the com- 

 mencement of dripping, in reference to the pre-eminence of certain 

 kinds of foliage in powers of condensation. The pine trees invariably 

 begin first, the rough brush-like clusters of leaves being well adapted 

 to intercept the smallest particles of moisture. The yield of water 

 from this source is very great. The laurels extract water plentifully 

 from mists which are more sensibly damp." 



Dr Graham further states that, — " In one of the Canary Islands, 

 the people show the place where, at the head of a deep valley, stood 

 a fine solitary til tree, which daily used to strain a large quantity of 

 water from the humid mist, conveyed inland by the sea breeze. 

 The tree is mentioned by Cordeyro and subsequent writers. But 

 both the spring of water and the tree are now gone ; and the mists, 

 though they still remain, pass over unstrained of their moisture." 



The difference noted by Dr Graham in the effect of different kinds 

 of trees may be attributed in some cases — in others if not in those 

 mentioned by him — in some measure. First, to differences in the 

 number of stomates through which the evaporation from the leaves 

 of the tree itself is effected. These vary in different plants from a few 

 to 160,000 or more in the square inch of surface. The number in 

 the leaves of the cherry-laurel is 90,000 in every square inch on the 

 back of the leaf. In such state of atmosphere as is specified by 

 Dr Graham, every stomate would probably be distended, an atmos- 

 phere of dense moisture would surround the tree and this precipitated 

 by a fall of temperature might in part create the supply. 



The difference may be attributable, in some measure, Secondly, to 

 diff'erences in the extent of the radiating surface, supplied by the 

 leaves, whereby the temperature might be effected. This is much 

 greater in the foliage of a pine tree than in the herbage of a laurel. 



And the difference between the eff"ect produced by the laurel and 

 other trees may be attributable, in some measure, to diff'erence in 

 the attraction of adhesion between water and the leaf. From the 

 glossy surface of the laurel leaf the moisture deposited might run off" 

 quickly, while to the leaves of other trees it might cleave, as if loathe 

 to shake itself free, and adhere till again evaporated, or till it fell in 

 drops undistinguishable from the rain, when this began to drizzle. 



In all such cases as have been mentioned, whether fog be visible 

 or it be otherwise, the precipitation and consequent dripping is 

 attributable to the temperature of the atmosphere surrounding the 

 tree being below the dew-point, and the moisture contained in it 



