34 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



is very scanty, and finally of the great temperate forest-zones. The rain- 

 fall and its distribution during the seasons determine the great regional 

 distribution of types of vegetation, while differences in the water- 

 capacity of the various kinds of soil and the various conditions controlling 

 the course of water above ground determine the finer topographical shades 

 of distinction. On high mountains the regions are correlated with the 

 distribution of the rainfall. There are often three regions : a lower one 

 with scanty rainfall; a middle one, the cloudy region, with much mist 

 and rain, and consequently clothed with forest ; an upper dry one above 

 the clouds (e. g. Tian Shan, Madeira, Teneriffe). Mountains often show 

 a dry lee-side and luxuriant rainy weather-side. The coast mountains 

 of a country may arrest the rain so that in the interior, steppe, savannah, 

 and the like, develop on the drier soil, whilst the coast-land yields a rich 

 forest-vegetation (e. g. the coast of Brazil and the campos of the interior). 



The distribution of atmospheric precipitations. With the same rainfall 

 there is a very great difference according as the rain falls uniformly 

 throughout a long period (Central Europe), or falls for only a very short 

 time in the form of heavy storms ; the number of rainy days is, in so far, 

 therefore, of greater import than is the amount of rain. In the former 

 case the rain is capable of being much more beneficial to the vegetation ; 

 in the latter case the parched soil is not in a condition to absorb all the 

 water, most of which, flooding and denuding the soil, flows away over its 

 surface or percolates to its depths. Under the former conditions we find 

 growth-forms and plant-communities quite different (mesophilous) from 

 those under the latter, which are more extreme.^ 



/* It is remarkable that even in smaller districts relatively slight differ- 

 ences in the amount of atmospheric precipitations are capable of evoking 

 'great distinctions in the vegetation. Thus, in the rainier parts of North 

 Germany, especially in the north-west, heath dominates, and in its 

 company grow a whole series of typical Atlantic plants that are wanting 

 in the less rainy east. In this latter there is, consequently, a flora much 

 richer in species which prefer dryness and which (also in cultivation) show 

 themselves very sensitive to great humidity, especially in spring and 

 autumn.^ 



Small quantities of rain are of little or no use to vegetation, because 

 evaporation is rapid, and the water evaporates before it has time to sink 

 into the soil. 



The time of the atmospheric precipitations (according to the season of 

 the year) is of very great importance. Where in the tropics a heavy 

 rainfall is distributed throughout the whole year, evergreen rain-forest 

 prevails ; where the rainfall is likewise very heavy but is confined to a few 

 months in the year, whilst the rest of the year is dry, high-forest may be 

 present but it will consist of deciduous trees. Rain that is essentially 

 winter-rain, as in Mediterranean countries or in South- West Australia ^, 

 obviously favours a type of vegetation entirely different from that favoured 

 by rain falling in summer. The Mediterranean district and South-West 

 Australia are consequently poor in forest, rich in steppe and bushland ; 

 whilst the vegetation of districts with summer-rain, for example East 

 AustraHa, is characterized by rain-forest, and savannah rich in trees. In 



* Woikof, 1887 ; Koppen, 1900. '^ Grabner, 1895, 1901. 



* Diels, 1906. 



