PERIODIC PHENOMENA OF VEGETABLE LIFE. 83 



well as the disturbance produced by a fall in temperature during 

 growth, is not the same for every plant ; we have already observed 

 that high Alpine plants suffer less from a sudden cooling than 

 those which grow lower down, and especially cultivated plants. 

 The commencement of vegetation takes place with more or less 

 heat according to the peculiar constitution of any given species ; 

 but it is also generally affected by the temperature of the pre- 

 ceding winters ; a mild season, for instance, may have so far 

 prepared the commencement of vegetation, that its development 

 in spring may take place at a lower temperature than usual. 

 Great heats may also act prejudicially on vegetation. Therefore, 

 the temperatures to be taken into account in respect of the 

 development of vegetation are included between higher and lower 

 limits, which vary according to the species of plant, and to the 

 stage of development at which it has arrived.* Temperatures 

 which have a direct influence on the development of plants appear 

 to be always above freezing point, but in many cases a temperature 

 of from + 1° to + 3° centigrade will remain without any 

 perceptible effect, if after a short time it be interrupted by greater 

 cold. This is particularly evident from the little alteration which 

 winter Cereals show before the advent of the warmer spring 

 months, although higher midday temperatures may have com- 

 menced earlier, and the snow may have partially disappeared. 

 Similar phenomena are observable, according to Alph. de Candolle,f 

 towards the northern limits of plants. On the other hand, in 

 many larger plants, trees for instance, there is no absolute stag- 

 nation I even during winter. It becomes, therefore, extremely 

 difficult to judge of the real amount of heat which a plant 

 experiences, either during its whole period of vegetation, or 

 during specific portions of it. It is not even expressed by the 

 sum total of all temperatures above freezing point, for sunshine, 

 distribution of temperature, and especially isolated extreme heats. 



* We may mention here, as an instance of a great range of temperature, 

 the process of germination, which will take place from 3° or 4° centigrade 

 to 40° or 50°. See Boussingault, Economic rwrcde, German edition, vol. ii. 

 p. 420. 



t Sur les Limites polaires des Especes : Bibliotheque Universelle de 

 Geneve, 1848. 



Z By the interesting experiments of Boucherie on the colouring of woods 

 by infiltration, it appears ihat winter is indeed the season of rest for the 

 sap, but that there is, nevertheless, no absolute stagnation. This is 

 especially the case with evergreen Coniferac. Annates de Chimie, Second 

 series, vol. liv. p. 134. 



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