PERIODIC PHENOMENA OF VEGETABLE LIFE. 75 



mencernent of vegetation iu spring and the hay-harvest, thirteen 

 days and a half for every 1000 feet (100 metres = 4"25 days). 

 Although these numbers are the result of only very short observa- 

 tions, they seem to show that in the Jura, as in the mean results 

 in the Alps, there is a greater delay in the development of 

 vegetation at the time of harvest, and especially in autumn, than 

 during the spring months. 



I should here mention, that the delay is generally greater in 

 the Jura than in the Alps ; this proceeds from the higher stations 

 in the former chain of mountains being in exceedingly exposed 

 situations, where the much more restricted cultivation of Cereals 

 and the want of fruit-trees already indicate a more rigorous 

 climate. 



Influence of CUmateric Relations. — Amongst the climateric 

 conditions influencing the periodic development of vegetation, the 

 following appear to be the most important : the mean temperature 

 of the air in the shade and iu sunshine ; the form of the curves of 

 temperatures during short periods ; and the magnitude of isolated 

 extremes in consequence of exposure to sunshine and radiation ; 

 the heat of the soil and its variations ; the moisture of the air and 

 of the soil ; the accumulation of clouds and repartition of rain ; 

 the pressure of the air, whether moist or dry ; winds, and in some 

 measure, electricity, &c. These groups of influences have all this 

 common character, that they depend essentially on changes in geo- 

 graphical position, — that is, in altitude, latitude, and longitude. 

 Aspect, configuration of the ground, composition, and especially 

 physical properties of the soil, are more local, but also of great 

 importance : add to this many irregularities peculiar to certain 

 plants, which, however, disappear in mean results, obtaiued from 

 the consideration of different plants. 



From the manner in which the phenomena of vegetable life in 

 any given year depend on the conditions of temperature, it will be 

 readily seen how important, among the above-mentioned influences, 

 is that of temperature. These phenomena always commence 

 earlier under the action of greater heat, always later when the 

 heat is less, than the ordinary means ; although the other cli- 

 materic conditions should not exercise their iufluence in the same 

 direction on the temperature. This constant relation of the non- 

 periodic changes of temperature points out to us what the 

 investigations of Dove have proved, that heat is generally the most 

 essential condition for the phenomena of vegetation. The great 

 influence of temperature, which acts directly as well as by pro- 



