74 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



ct 



the northern slopes are kept moist by, inter alia, the slowly melting snow, 

 which rapidly disappears from the southern slopes. 



A covering of snow shortens the vegetative season by preventing the 

 soil-temperature from rising above freezing-point in spring-time, thus 

 hindering plants from awakening into activity as early as on snowless 

 spots. This has a profound effect upon the economy and distribution 

 of plants ; certain species are excluded from places where the snow is 

 wont to lie for a long time, because the vegetative season is too short \ 

 or the soil too cold ; other species are actually favoured by these conditions. 

 Blytt records that on Norwegian mountains, around accumulations of 

 snow which melt to some extent during summer but scarcely ever 

 entirely disappear, the flora is high-alpine in nature on account of the 

 short vegetative season, and corresponds to an altitude above sea-level 

 that is greater than those of the places in question. Even in places where 

 the snow melts only in extremely warm summers we can find vegetation. 

 This must have rested under the snow for several years before awakening. 

 Obviously there are many spots where the snow lies so long that vegetation 

 is absolutely excluded. 



It is easy to see that orographic and other relations such as slope 

 and exposure of the soil, nature of the wind, specific heat of the soil, and 

 the like that influence the melting of the covering of the snow, thereby 

 acquire a phyto-geographical significance.^ 



{b) Fallen Foliage and Withered Grass 



The other kind of dead covering to the soil is made by fallen foliage 

 or withered grass ; fallen foliage is met with especially in forest (both 

 deciduous and evergreen), withered grass on meadow and savannah. 



These coverings have the same physical action as snow, diminishing 

 the extremes of temperature, keeping the soil moister, and so forth. 

 In the forest many plants can scarcely continue to exist without such 

 protection against desiccation that the protection is not merely against 

 cold, is even more evident than when the covering is composed of snow.^ 



A covering of leaves over the soil in beech-forest and similar forests, 

 where it is very thick, has a great influence upon vegetation on the ground 

 inasmuch as it suppresses mosses and sundry other plants. 



A covering of leaves powerfully affects the production of humus in 

 soil, which it thus improves, and is, further, of deep significance to animal 

 life in forest soil, for it concerns moisture, and provides food for animals 

 living on forest soil, among which earthworms seem to be the most 

 important.^ Both circumstances prevent the humus soil of forest from 

 changing into raw humus, and check all those modifications in the soil- 

 covering that would accompany such a change, and would gravely interfere 

 with the whole economy of forest.^ 



In this connexion may be mentioned the utility of their old dead 



' For further information on the significance of snow see Woikof, 1887, 1889. 

 ^ Concerning the characters of the various coverings on forest soil see Ramann, 

 1890, 1893, 1905. ' See Chapter XX. 



* See P. E. Miiller, 1878, 1894 ; Ramann, loc. cit. 



