228 J. H. MAIDEN. 



France and Germany, and shows the inconclusiveness of 

 the results. Croumbie Brown in his " Forests and Rainfall" 

 also gives a full account of these researches, which cannot 

 be further alluded to in detail here. 



Let me however point out that the humidity of a forest 

 is not entirely a matter of rain-gauge measurements. I 

 think in order to thoroughly test this aspect of the question 

 the hygrometric state of the atmosphere in various places, 

 whether carrying forests or other vegetation and whether 

 denuded by the hand of man or not, should be ascertained 

 and compared for a series of years. The districts should 

 be as numerous as possible, but we should not limit the 

 observations to rain which can be measured in a rain-gauge. 

 It will be found that much of the moisture which goes to 

 assist plant growth and to modify climate is not measured 

 by such a crude instrument as that referred to. 



Those who desire further information on the subject are 

 recommended to read the chapter "Rainfall in, above and 

 near forests" by M. W. Harrington, at p. 106 of the work 

 "Forest Influences " already quoted. 



VII. Physiological Action of Trees — Transpiration. 

 Another phase of the conservation of water by trees is 

 the question of transpiration. This is the technical word 

 for what may be described as the perspiration of plants. 

 The tree absorbs moisture by its roots, which is utilized 

 to continue the functions of the plant, and a portion of it 

 is exhaled in the form of vapour by each leaf, and passes 

 into the atmosphere. The effect of a single tree is a very 

 large multiple of that of a single leaf, and that of a forest 

 is similarly greater than that of a single tree. This 

 emission of vapour by plants is more or less fully dealt 

 with in all works on vegetable physiology. In this way a 

 forest has an appreciable effect on the humidity of the 

 atmosphere, and this is one of the reasons why, on the 



