CHAP, v.] Soil and Situation 1 1 1 



elevation, yet quality of the soil, aspect, and local climatic 

 conditions modify these demands in the most marked degree. It 

 is more particularly the amount of warmth during the period of 

 active vegetation which determines whether or not the different 

 species of trees can thrive and regenerate themselves naturally 

 in any given locality. If, between the time of their flowering 

 and the natural term of their ripening the fruit, there has not 

 been a sufficient development of warmth, accompanied by an 

 absence of destructive frost, to enable the seed to attain 

 maturity and germinative capacity, then the natural conditions 

 for thriving and regeneration are not offered to any given kind 

 of tree. The further northwards deciduous broad-leaved trees 

 occur, the later is usually the time at which they break into 

 leaf, as the amount of warmth requisite for stimulating active 

 vegetation is only obtainable later in spring than at similar 

 elevations situated further southwards. 



Light is also a factor which is undoubtedly of immense 

 importance with regard to the thriving of woodland crops. A 

 careful study of their demands with respect to light, and their 

 capacity for enduring shade, will quite logically explain many of 

 the concrete conditions in which woodland growth exhibits 

 itself, as was first pointed out by Gustav Heyer 1 (see also 



P- 54)- 



The Relative Humidity of the atmosphere is governed by 

 the atmospheric temperature ; for, as regards any given quantity 

 of moisture, the saturation-point will be much sooner reached 

 when the air has only a low temperature, than when its 

 temperature is high 2 . But, whilst the absolute humidity of 

 the air varies inversely to the latitude above the equator, no 

 such rule obtains regarding its relative humidity, as local 

 circumstances determine the supplies of moisture available for 

 evaporation. Thus, in Britain, the soft, mild, westerly winds 

 which reach this island only after sweeping over the Atlantic 



1 Das Verhalten der Waldbaume gegen Licht und Schatten, 1852. 



2 See Roscoe's Elementary Text-book of Chemistry, 1888, pp. 55, 56. 



