CHAP. XXI EXPOSURE 8i 



but little rain, whilst the more raised ones receive heavier atmospheric 

 j precipitations and have more luxuriant vegetation. In miniature the 

 conditions of the surface may produce an effect ; for instance Blytt ^ 

 ! mentions that steep walls of rock facing south place the vegetation 

 j concerned under unusual conditions as regards temperature ; beneath 

 I lofty walls of rock at Christiania a vegetation occurs which is rich and 

 ! varied, and includes a number of southern species ; intense heat prevails 

 here on sunny days. 



Steepness of slope (angle or inclination to the horizon). This decides 

 whether the products of weathering and the humus-substances can 

 remain in situ or are carried away, the rapidity with which water 

 t]ows away from the surface, the extent to which the surface is soaked 

 with water, the density and height of the vegetation, and the intensity 

 with which the sun's rays can heat the soil.^ 



Exposure of slope. This largely determines the kind of vegetation 

 present. A slope exposed to sun and wind bears vegetation entirely 

 different from that on one less exposed to either. In addition 

 to what has been stated on p. 51, it may be noted that in the Russian 

 east sea-provinces the south-western slopes bear a more mesophilous, 

 and the north-eastern slopes a more xerophilous vegetation, because 

 the south-west winds bring humidity, and the north-east winds aridity .^ 

 Even in very small concerns, exposure may affect vegetation, for instance 

 on dunes ; Giltay * has made some observations, showing the differences 

 that can exist in temperature and atmospheric humidity only a few 

 paces apart on the northern and southern slopes of sand-dunes in Holland.^ 

 In like manner the vegetation on the opposite sides of a cutting or the 

 embankments of a railway may be very different, as Stenstrom has 

 pointed out. On the southern side of slopes in the east of North Germany 

 the flora of the sunny (Pontic) hills is especially characterized by the 

 development of plants belonging to a Continental climate. 



Di^erences in geognostic structure, for instance in the inclination of 

 the strata, evoke distinctions in vegetation. Inchnation of the strata 

 acts on the course taken by water, on the emergence of springs, and 

 therefore on vegetation. Moreover, the nature of the surface itself may 

 be entirely different, according as to whether it forms an angle with the 

 dip of the strata or runs approximately parallel with this ; in the former 

 case the surface may be steep and gravelly as well as dry, so that only 

 scanty and stunted vegetation can develop, while in the latter case it slopes 

 gradually, is richer in water, and consequently bears dense and vigorous 

 vegetation. Examples illustrating this are to be met with in many 

 districts with slate mountains.' 



' Blytt, 1893. ' See Chap. XIII. ' Klingc, 1890. 



* Giltay, 1886. ' Warming, 1907 (1909). * Stenstrom. 1905. 



' The study of oecology will be much promoted by the preparation of maps in 

 which the type vegetation is denoted by a special colour, and by a comparison 

 with maps showing the geognostic surface. Excellent detailed studies of the kind 

 have been made by Woodhead (1906) and W. G. Smith (1903-5). Flahault (1894, 

 1897, 1901), and Drude (1902, 1908), have pubhshcd vegetation-maps dealing with 

 more extensive areas. Clements's (1905) work may also be consulted. 



WARMING 



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