80 PERIODIC PHENOMENA OF VEGETABLE LIFE. 



configuration of the ground. In very narrow valleys and ravines, 

 the duration of sunshine is much shortened, or, in a few rare 

 instances, almost entirely cut off. This diminution occurs chiefly 

 in winter ; partially, also, in spring and autumn. Under the high 

 steep mountains which often rise by the side of such valleys, many 

 points are entirely deprived of the direct rays of the sun for several 

 weeks in winter ; whilst in the others the duration of sunshine is 

 exceedingly short. Even in summer, it lasts in most valleys of the 

 Alps, except in the broader basins, some hours less than in plains. * 

 Many periods of vegetable life, the ripening of fruit for example, 

 is thus remarkably delayed in shady valleys. The same circum- 

 stances may be readily observed under northern aspects, in 

 comparison with southern declivities. 



Considerable accidental variations may be observed on com- 

 paring isolated phenomena and years, in consequence of irregu- 

 larities in certain plants. Age, or artificial transplantation, bring 

 out great differences ; single individuals are earlier than others 

 without our being able to attribute it to any external causes ; also 

 a degree of development given to a plant by the temperatui'e of 

 the preceding year, especially of autumn, is often perceptible in 

 its effects the following year.f Differences in the degree of con- 

 sistency or in the composition of the soil, slight variations in 

 moisture or irrigation, will also produce rather striking differences 

 in the period of development, in plants growing in close proximity 

 to each other. 



Although the influence of the temperature of the air in the 

 shade on the periodic phenomena of vegetable life is not extensive, 

 we may, nevertheless, make use of it for some general comparisons ; 

 partly because its influence is always great ; partly because it 

 depends so essentially upon many of the other modifications of 

 climate, that, within certain limits, it gives a fair estimate of 

 their mean variations. The points to be ascertained in this 

 respect are, at what degree of temperature a given phenomenon 

 takes place at different altitudes, and what is the temperature 

 which prevails during the whole period of development. 



The mean temperature by day, at which a given phenomenon 

 takes place in spring, remains generally nearly the same ; j although 



* The influence of the shade is also perceptible in the curves of daily 

 temperature. 



t See Quetelet, Climat de la Belgique, p. 35. 



J This is shown also by Que"telet's comparison of Brussels, Climat de la 

 Belgique, p. 19, with the carefully chosen means collected by Gasparin for 

 Western Europe, in his Cours d' Agriculture, vol. ii. p. 94. 



