i68 FORMATIONS AND GUILDS [Part II 



The yield is excellent, but summer fruits cannot be cultivated ; the soil dries up 

 so completely even in May that it is impossible for the plants to flower. In this 

 district there are forests with large, tall-stemmed trees. The supply of water 

 that is collected in winter suffices to cover the evaporation during summer \' 



The greater the amount of water in the soil, whether it is derived from 

 rain or from percolating water telluric in origin, the greater in general is 

 the height of the trees and the richer their foliage. However, the tallest 

 known trees, such as the Sequoia of California and the Eucalyptus of 

 Australia, are not inhabitants of a specially moist soil ; here specific 

 characters are largely involved. With a decreasing supply of water in 

 the soil, the height of trees and the surface of their foliage generally 

 diminish, yet many lofty trees are still found on dry soil, for examp e 

 in tropical savannahs. The driest districts possess only stunted trees. 

 Trcc-growtli is entirely prci'cntcd only by such a degree of drought as 

 excludes all kinds of plants ivit/i the exception of the loiver cryptogams. 



The amount of water necessary for the well-being of hygrophilous 

 trees obviously increases with the temperature. In the temperate zones, 

 hygrophilous trees- thrive with a rainfall that in the tropics would satisfy 

 only xerophilous trees. This matter is considered more in detail under 

 the special climatic headings dealing with the individual zones. Here 

 however it may be mentioned, that in the tropics — with the obvious 

 exception of the banks of sheets of water — hygrophilous trees require an 

 annual rainfall of at least 150 cm., whereas in the cool regions of the tem- 

 perate zones 60 cm. are sufficient. The occurrence of lofty xerophilous 

 trees depends less on the amount of rain than on specific characteristics. 



Another important factor in relation to the growth is the aviount of 

 aqueous vapour in the atmosphere, in which of course it is not the absolute 

 but the relative vapour tension that is of significance. In this respect 

 trees are less favourably situated than plants of less height, for their 

 transpiring surfaces are situated in higher and therefore drier and more 

 agitated strata of the atmosphere. The larger h\-grophilous trees, when 

 in leaf, require an average relative humidity of about f^'o /^, descending 

 to 60"/^ for a few hours only during the day. Less atmospheric humidity 

 suflices for xerophilous trees, and some species, even when in leaf, appear 

 able to endure without injury a relative humidit)' of 30 7^, lasting for some 

 time. 



As has been explained in an earlier chapter', it is of the greatest 

 importance in relation to tree-groivtii whether the surrounding atnu^spheric 

 strata are visually at rest or in motion, as the wind causes a vast increase 

 in transpiration. Dry winds during frosty weather determine the polar 

 limit of the grozvth of trees. Before the final proof of the fact was 



V. oeikof, I, p. 243. 2 Ti-ggs hygrophilous in the vegetative season. 



^ See p. 77. 



