CHAP. VII ATMOSPHERIC PRECIPITATIONS 35 



the former countries hot and dry seasons coincide, and the vegetation 

 consequently has a xerophilous impress ; in districts with summer-rain, 

 where the same quantity of rain falls, the vegetation has a more meso- 

 philous impress. 



Against dry seasons plants may protect themselves by shedding the 

 strongly transpiring surfaces {defoliation). Other plants that do not shed 

 their foliage necessarily retain, also during the moist season, the features 

 required to protect them from dryness during the dry season. In the 

 tropics, where there is a prolonged rainless season, deciduous foliage is the 

 rule. In sub-tropical (warm-temperate) zones, evergreen trees and shrubs 

 predominate, while many herbs dry up during the dry season. In sub- 

 tropical districts with summer-rain, we may perhaps regard the reduced 

 evaporation during the cooler winter as being responsible for the evergreen 

 nature of the trees. In districts with winter-rain, trees and shrubs that 

 are leafless during winter subsist but badly, because the summer is too 

 dry. Nor are shrubs that shed their leaves in summer so common as 

 evergreen ones. In dry districts with a very short vegetative season 

 (steppes and deserts), the herbs dry up during the dry season, and the 

 shrubs, for the most part, shed their leaves. It should be noted that in 

 many steppes (e.g. South Russia and Hungary) the summer months 

 during which the vegetation is dried up, are the rainiest ; the rainfall in 

 summer is not sufficient to supply the amount of water required by plants, 

 as it is not great and is not sufficient to atone for the intense evaporation 

 during the hot summer months when the air is dry ; the rain falling in 

 spring, though still less in quantity is more efficient. In cold-temperate 

 zones winter is to be regarded as a ' physiologically dry season \^ because, 

 while low temperatures prevail the plants cannot absorb water from the 

 soil. The trees and shrubs are either deciduous or have perennial pro- 

 tection against drought. According to Grisebach,"^ deciduous trees have 

 effective protection against evaporation during winter, but the means 

 employed are not economical, because a not inconsiderable portion of the 

 vegetative season is consumed in the issue of foliage. Consequently, 

 evergreen Coniferae preponderate immediately that the length of the 

 vegetative season sinks below a certain minimum. According to Koppen ^ 

 the southern boundary of the predominant coniferous forests is parallel 

 with the lines denoting equal duration of the warm season. Herbs in the 

 cold temperate zone are mostly evergreen. During the frosty period 

 they find protection under cover of the snow. This is also true of the herbs 

 and dwarf-shrubs in the Arctic zone. 



It is clear that matters influencing the amount, distribution, and 

 other distinctive features of atmospheric precipitations, are of indirect 

 significance to oecological plant-geography. Such matters are especially 

 topographical, and include : relief of the earth's surface, altitude above 

 the sea-level, proximity to the sea, prevailing winds and their humidity. 



Heat and moisture may be the two weightiest factors determining the develop- 

 ment of vegetation. According to the different quantitative i)roportions in which 

 plants receive and are adapted to them, A. dc Candollc* has ranged plants into 

 the following six groups : 



I. Hydromegathermic : Plants making the greatest demands as regards 



' Schimper, 1898. * Grisebach, 1872. 



* Koppen, 1900. ' A. de CandoUe, 1874. 



D 2 



