52 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



It may be added that the snow-line may be at very different levels 

 on the north and south sides of a mountain, and that the altitudes reached 

 by many plants depend upon exposure. Selecting as an example the 

 beech in the Alps, according to Sendtner,i the maximum altitude attained 

 by it in South Bavaria is greatest on the south-eastern, and least on the 

 north-eastern side. In the northern hemisphere, species ascend far higher 

 up the southern side of mountains than up the northern (in the Pyrenees, 

 for example, according to Bonnier). The statements above will suffice to 

 show to what an extent heat in this case the soil's heat (though atmo- 

 spheric heat and radiation cannot be dissociated from this) depends 

 upon the relationships enumerated. 



3. Duration of radiation. In this duration the tropics and Polar lands 

 are very different, at least as regards the distribution of light according 

 to seasons of the year. 



4. The specific heat of soil varies with the mineralogical composition. 

 The most easy to heat is quartz-sand, and the most difficult peat ; between 

 these extremes stand calcareous sand, clay, and others. The specific 

 heat of quartz-sand is 0-2, that of peat about 0-5 (water = i). The 

 amount of humus in soil is of special importance in this relation. 



5. Colour of soil. Darker soil is more readily and strongly heated 

 than is that of lighter colour, other conditions remaining constant. 

 Humboldt found that black basalt-sand on the island of Graciosa attained 

 a temperature of 51-2 C, whilst white quartz-sand in the same circum- 

 stances attained only 40 C. In radiation the conditions are reversed ; 

 darker soil cools more rapidly at night-time than does lighter-coloured, 

 but does not become colder than the latter. 



6. Porosity of soil. A very porous, gravelly soil absorbs the sun's 

 heat rapidly and becomes intensely heated at its surface, but the heat 

 absorbed is equally readily lost by radiation. Soil rich in air conducts 

 heat slowly, the more slowly the greater is the amount of air, because air 

 is a bad conductor of heat. In a rock substratum the conductivity of 

 heat is greater, and varies in velocity with the nature of the rock. Karst 

 limestone, for example, is an excellent conductor of heat, because of 

 its uniform density and its dryness. Granite, basalt, and other crystalline 

 rocks are likewise good conductors. 



7. The amount of water in soil is, of all the factors, the one that perhaps 

 has the greatest influence on the temperature of soil, because heat is 

 consumed in the heating and evaporation of the water. Water has a 

 specific heat far greater than that of any kind of soil. The more abundant 

 the water the colder is the soil ; dry soil is more easily heated than wet 

 soil, but soil containing much water, on the other hand, retains heat 

 longer than does dry soil, and for this reason is warmer than the latter 

 in autumn. Sandy soils are ' warm ' because they rapidly lose water 

 and become heated ; clay soils are ' cold '. Soil containing abundant 

 water conducts heat to the subsoil better than dry soil. All these rela- 

 tionships are of profound significance to the development of vegetation 

 in spring for instance. A rock soil is the warmest of all kinds of soil, 

 because no heat is expended in the evaporation of water. Heat penetrates 

 rapidly and deeply into a rock soil, because this is a good conductor of 



* See Sendtner, 1854 ; see C. Schroter, 1904-8. 



