CHAP. XVIII EFFECT OF NON-LIVING COVERING 73 



The reasons for this significance on the part of a snow-covering arc 

 the following : 



(a) The thermal conditions in snow play a part, though scarcely the 

 leading one. Snow is white because the spaces between its crystals 

 are occupied by air, which may be very considerable in quantity. And 

 it is this air that mainly renders snow a bad conductor of heat. Bv 

 reason of its feeble thermal conductivity snow keeps the soil warmer, 

 and the deeper one descends into snow the less cold is it, so that soil 

 lying under deep snow is exposed to less cold than is bare soil. But this 

 does not suffice to explain all the observed facts, for even under deej) 

 snow plants may be exposed to very extreme cold.^ Neither can it be 

 of very great importance that fluctuations in temperature are decreased so 

 that plants are not exposed to the alternate heat of the day and cold of 

 the night ; snow does serve to prevent too sudden thawing, which may 

 constitute a danger.^ 



Snow acts as a protection against those changes of volume in frozen 

 soil, occasioned by hoar-frost, which cause plants to be ruptured and 

 uprooted. 



{h) Of far greater importance is the significance of snow in regard 

 to the amount of water in the plant, as shown in succeeding para- 

 graphs . 



Snow acts as a defence against transpiration. It is to this action that 

 we must attribute the preservation of many species during winter, and, 

 as described by Kihlman, the death of twigs which project above the 

 snow. It is not low temperature that kills those twigs, but the great 

 atmospheric aridity prevailing in arctic countries, and the violent storms 

 which increase transpiration at a time when the roots are incapable of 

 absorbing water. Twigs and whole plants wilt through desiccation ' ; 

 the shapes of the shrubs serve to show how high the layer of snow stands 

 in winter. The aberrant, sometimes bent and contorted, shapes are 

 occasioned by the death of many twigs and the production of new ones 

 in abnormal positions. 



The topographical distribution of species is also affected by relations 

 in regard to water, namely, by the uneven distribution of water in the 

 soil that results from the uneven distribution of snow on its surface. 

 Depressions filled with snow remain moist for a longer time than do 

 more elevated spots bare of snow, in fact, they may be moist throughout 

 the vegetative season.^ 



In some places, for instance, in the steppes of Russia and North 

 America, snow, by reason of its depth, acquires importance as a reservoir 

 of water ; the greater or smaller the supply to the soil, the richer or 

 scantier will be the vegetation in the succeeding vegetative season. 



When a covering of snow has an injurious effect, for example, on a 

 dense vigorous winter-crop in depressed spots of fields, the cause may be 

 that it suffocates them by restricting the supply of air. 



Snow affects adjoining slopes by wetting them when it melts. As 

 mentioned on p. 39, in Greenland the northern slopes of a mountain 

 chain may be fresh and vivid green (rich in mosses) in summer, while 

 the southern slopes at the same time are dry and .scorched : this is becau.sc^ 



* Kjellman, 1884. ' See p. 23. ' Kililman. 1890. 



