yo TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



has established that the rain water intercepted by the trees, and 

 evaporated before reaching the ground, varies between 6 "/o when 

 the leaf is off and ii 7o i^i summer. Yet, after ten years of 

 observations, it has been found that the forest soil receives 12 "/o 

 more water than the open country. The water which reaches 

 the forest soil is partly evaporated, partly transpired by the 

 trees, partly retained by the trees (but this counts for very little), 

 partly sinks in, and, in certain circumstances, in part flows over 

 the surface. Direct evaporation is reduced to 35 '% under the 

 forest, as compared with open land, when litter is wanting, and 

 to 12 7o when it exists, the ground being screened from wind and 

 sun, while the air is moister and cooler than outside, but these 

 figures will vary according to the degree of humidity of the soil. 

 Evaporation by the trees themselves (i.e. transpiration) varies 

 according to the species, the heat, the light, and the intervals 

 between the showers. It is immensely greater than direct 

 evaporation, as is seen when plantations dry up marshy ground. 

 According to Risler a field crop transpires 75 7o of the rain that 

 falls ; a forest transpires a great deal less, and a young forest 

 very much less than an old one ; but, on the other hand, the 

 forest goes far deeper for the water, and transpires throughout 

 the period of vegetation. Hohnel states that birch and lime 

 transpire 600 to 700 kilogrammes of water per kilo of leaves 

 weighed in a dry state; beech, 400 to 500 kilos; oak, 200 

 to 300 kilos ; and conifer, only 30 to 50 kilos. However, 

 we should note, in passing, that this can scarcely apply to the 

 spruce, for this species has a quite unusual power of drying up 

 the soil. The water level descends far lower under forest than 

 it does outside. M. Viney thinks that in the Seine basin above 

 Paris, where the rainfall varies from 20 to 32 inches, 20 centi- 

 metres (or nearly 8 inches) is a fair estimate of the amount of 

 water which reaches the rivers. The absorption of water varies 

 according to the nature and depth of the soil, to the amount of 

 water present in the soil at the time, to the slope, to the surface 

 condition (grassy, cultivated, frozen, etc.), and, finally, to the 

 duration and intensity of the rainfall. To study this phenomenon 

 we must consider two things, the retaining capacity of the soil, 

 and the rate at which it absorbs. The retaining capacity is 

 reached when for every addition of water an equal amount runs 

 away. After Meister, humus can take up 70 % of its own volume, 

 garden soil the same, peat 64 %, and a lime soil 54 %. After 



