PERIODIC PHENOMENA OF VEGETABLE LIFE. 77 



any constant differences. They cross each other, on the contrary, 

 repeatedly, and are of very dissimilar forms for different months. 

 In one and the same place there may he a delay in one phenomenon, 

 and a considerable acceleration in another. The vernal phenomena 

 and those of autumn show the greatest differences ; which depend, 

 more especially, on whether it is a continental or a maritime 

 climate which prevails, as in the latter case, besides the lower 

 degree of summer heat, the frequent cloudiness, and the con- 

 sequent deficiency of sunshine, delay much the ripening of fruits. 

 For the Alps, these synchronistic lines take somewhat more regular 

 forms, and the enclosed spaces indicate, in general, more uniform 

 variations ; although here, also, very considerable divergences 

 occur, according to the configuration of the ground in particular 

 stations, according to geographical longitude and latitude, and 

 according to the form of the groups of mountains. 



For the daily periods, the variations of temperature depend chiefly 

 on aspect ; for the periods of the year, positions in the bottom of 

 a valley, as we have already seen, act favourably, as in their case, 

 with the same mean annual temperature, the differences between 

 particular months are generally greater than on the sides of the 

 mountains. It is not all plants that are equally affected by these 

 conditions; — they relate chiefly to plants generally cultivated in 

 freer aspects and on sunny slopes, and where long stalks enable 

 them to take a greater part in the temperature of the air. The 

 temperature to which forests are exposed is very nearly that of the 

 thermometer in the shade ; they are, moreover, affected by that of 

 the deeper strata of the soil. For larger plants, also, the aerial 

 temperature is not everywhere the same ; for their lower parts are 

 exposed to greater heat in the sunshine, and to greater cold by noc- 

 turnal radiation* in calm, cold weather, than their upper portions. 



Together with the greater heat under sunshine, there arises also 

 greater excitement from light, which, it is known, has so important 

 an influence on the functions of vegetable life, and on the assimi- 

 lation of nutritive matter. The intensity of this excitement, 

 depending on the transparency of the air, increases with elevation, 

 and is certainly not without influence on the periods of vegetation, 

 and on the lower limits of Alpine plants. Grisebach has also 

 shown that the southern limits of northern plants in North-west 

 Germany,! depend very essentially on the excitement from light, 



* See Glaisher, Transactions of the Royal Society of London, 1847, vol. ii. 

 t Grisebach, The Lines of Vegetation in North-west Germany. Gottinger, 

 Studien, 1847, p. 476. 



