78 PERIODIC PHENOMENA OF VEGETABLE LIFE. 



on the duration of daylight, which here corresponds with intensity, 

 whilst there is little prohability that there the southern limits are 

 fixed by any excess of summer temperature, which is, moreover, 

 inconsiderable. 



Exposure to sunshine depends on the state of the sky as to 

 cloudiness ; this varies in different groups of Alps, in a manner 

 analogous to the repartition of rain ; but it also acts unequally on 

 the periods of vegetable life, at different elevations. As the greater 

 clouds do not generally reach the highest summits, and isolated 

 mists especially remain long in the valleys, so the highest plants 

 are thereby exposed to much greater excitement from light. 



The temperature of the soil produces great variations, as well 

 in the quantity as in the distribution of the heat which a plant 

 receives. As plants of the higher Alpine regions are mostly 

 perennial, and their roots generally have a great extent in com- 

 parison with their green parts, so is their connexion with the 

 temperature of the soil much increased. This temperature, in 

 the upper strata, is exposed to greater variations from sunshine 

 by day, and radiation by night, than the temperature of the air ; 

 and in these cases, the colouring, the degree of looseness, &c, of 

 the surface have considerable influence. But at some depth the 

 temperature of the soil is much less extreme, and the minima of 

 cold and maxima of heat occur later than in the atmosphere ; the 

 degree and the rapidity of these changes is much pi*omoted by the 

 degree of conductibility of heat which the soil possesses.* Plants 

 with deep, especially tap-roots, experience therefore, in their sub- 

 terraneous parts, a cooler temperature in summer, and a warmer 

 degree in winter, than that of the atmosphere. For larger plants, 

 where roots are small in proportion to their green parts, the total 

 heat they receive is greater than the mean annual temperature in 

 the shade. For smaller plants, with roots lying near the surface, 

 these conditions are still more favourable. In winter they are 

 entirely covered with snow, which, being so bad a conductor, protects 

 them from the chilling effects of radiation. f In summer, their 

 roots are but little colder than the temperature of the air; they 

 even sometimes, in their whole extent, partake of the heating of 

 the upper strata by exposure to sunshine. J 



* See the numerous experiments of Forbes in the neighbourhood of 

 Edinburgh. 



+ See Boussingault's experiment?, Economie rurale, vol. ii. p. 454. 



X A fine example of the conditions of growth of this class of plants is 

 afforded by Cereals, which depend so much on the temperature of the air 

 and of the upper strata of the soil ; thus, for example, lye and barley ripen 



