CHAP. XIII TEMPERATURE OF SOIL 51 



Asperula longifiora, and others. Cold soil would appear to give rise to 

 glaucous shoots, an abbreviation of the vegetative season, and other 

 characters. 



The main sources of the soWs heat are two in number : (i) heat from 

 the sun ; (2) chemical processes (especially decomposition) in the soil. 

 These processes acquire particular importance in cold countries. 



The heating or coohng of soil, and consequently plant-life, is obviously 

 greatly influenced not only by those factors (radiation, evaporation, 

 heat-conduction, and so forth) that promote or retard coohng, but also 

 by other factors which we may now consider briefly. Of these the first 

 three to be considered concern the sun's heat, and the remainder relate 

 to the soil itself. 



1. Availability of the sun's heat. Particularly in Polar countries direct 

 sunlight plays a leading part, as is clearly shown by the arrangement of the 

 plant-communities over the landscape. In determining this a greater 

 part is played by the heat of the soil than by the heat of the atmosphere.* 



2. Angle of incidence of the sun's rays. The nearer this is to a right 

 angle the greater is the heating power of the rays (their power being 

 proportional to the cosine of the angle of incidence). Latitude, slope, 

 and exposure of the land, all affect the result. In northern latitudes, 

 south-west, south, and south-east slopes are warmest, while north-east, 

 north, and north-west slopes are coldest. 



The relationships indicated in the two preceding paragraphs evoke 

 great differences in the distribution of plant-communities in all latitudes. 

 We see not only in Greenland, for example, that the southern slopes of a 

 mountain-chain may have an open xerophytic vegetation appearing as 

 if burnt up, while the northern slopes are at the same time covered with 

 a dense fresh green, mossy carpet, in which flowering plants are scattered 

 and which in summer are moistened by the slowly-melting snow ; ^ but, 

 also in north-temperate latitudes we note, for instance, that the different 

 faces of thatched roofs support different vegetation ; and again, in 

 Mediterranean countries we observe on the southern mountain slopes, 

 and ascending high up them, xerophilous Mediterranean vegetation with 

 its characteristic forms and its early flowering season, whereas the northern 

 and cooler slopes are stamped with the impress of Central European 

 vegetation, with its more tardy development.^ Even close to the equator, 

 for example in Venezuela (less than 10 N.), we observe most marked 

 distinctions between northern and southern slopes ; near Caracas, stretch- 

 ing from east to west, there are shallow erosion-valleys or folds in the 

 land which, on their southern slopes, are so poor in vegetation that the 

 red clay almost entirely determines the colour of the land, but on their 

 northern slopes are clad with denser and taller vegetation. In lower 

 latitudes (South Europe and the tropics) it must be remembered that 

 prevailing north winds convey more moisture to northern than to southern 

 slopes. This circumstance is perhaps of greater import than is the 

 exposure to the sun's rays, because when the sun is overhead a steep 

 southern declivity is exposed to less in.solation than is a more gently 

 ascending northern slope : the lower the sun stands the more dependent 

 is the intensity of insolation upon exposure. 



' See Chapter VI. * See Warming, 1887. 



' See FlahauU, 1893. 

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