PERIODIC PHENOMENA OF VEGETABLE LIFE. 79 



Under all the above influences, the periodic phenomena are not 

 less varied than the limits of plants. 



We should here point out that influences, which appear most 

 favourable on considering only the periodic phenomena of vegeta- 

 tion, do not always promote equally the general growth of the plant, 

 or any important rise in the upper limits of its range. This appears 

 more clearly in regard to vernal phenomena, and by a compari- 

 son of smaller plants with larger ones, as trees for example. The 

 effect of an open aspect, the influence of violent storms, the steep 

 declivity of the ground, &c, which are so prejudicial to the growth 

 of the latter, disappear, for the most part, in the case of smaller 

 plants. In the steeper parts of a declivity, the snow melts in 

 spring more rapidly thau in flatter situations ; and the re-appear- 

 ance of vegetation takes place earlier, although these very declivities 

 are unfavourable for the formation of close masses of vegetation, 

 and still more so for the establishment of trees. So also dry, 

 even stony localities, which are rapidly heated by sunshine, hasten 

 the development of spring vegetation, and the commencement of 

 flowering, far beyond what is seen in moist rich soils under the 

 same aspect ; whilst, later in the season, the plants in the former 

 case become very stunted. Also, an early commencement of 

 vegetation, and a rapid formation of shoots and flowers, become 

 very injurious to the plants on the occurrence of frequent night- 

 frosts ; whilst those which grow in situations where, owing to 

 less sunshine, to the composition of the soil, or other circumstances, 

 these periods are delayed, succeed far better, and in greater 

 numbers.* 



The consideration of the degree of humidity is more important 

 than that of the repartition of rain, because it depresses the tem- 

 perature much more in summer than in the other seasons, and 

 thereby often disturbs considerably certain stages of development. 

 Great droughts, which sometimes oppose as great hindrances to 

 vegetation as cold, are rather scarce in the Alps, as the absolute 

 quantity of rain is so great, that even a considerable diminution of 

 it is but little injurious ; much more frequently do moist summers 

 affect considerably by their cold the ripening of fruits. 



Amongst the local influences, must here be mentioned the 



in Siberia near Nerehinsk, where the soil ia constantly frozen at a certain 

 depth; in the middle of August, this is the case at seven feet below the 

 surface. See Kupffer in the Bulletins de V Academie de St. Petersbourg, 

 Classe Phys. Math. vol. iv. p. 67. 



* "We shall farther on have the opportunity of giving some examples of 

 these relations in the case of the Cheriy. 



