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themselves according to the exposure, the direction of the wind, or 

 the proximity of separate and extensive masses of glacier ; but these 

 influences are for the most part merely local, and the general varia- 

 tions of the limit of vegetation dependent on the massiveuess of dif- 

 ferent groups of Alps are but little affected thereby. A comparison 

 of the annual isotherms with the limits of vegetation proves that the 

 different groups of vegetation do not always terminate at the same 

 annual isotherm. With the exception of the beech, he showed that 

 up to the height of Coniferae, these limits in the Northern Alps are 

 reached at warmer isotherms than in the Central Alps ; and a some- 

 what lower mean temperature is observed on corresponding points of 

 the group of Monte Rosa and Mont Blanc. This is immediately de- 

 pendent on the fact that the growth of plants is not determined alone 

 by the mean temperature of the year, but also by that of the seasons 

 and of the months. The warmth of the summer is in this view of pe- 

 culiar influence ; the greater this is in connexion with the same mean 

 temperature of the whole year, the higher plants ascend, and the 

 colder are the annual isotherms which mark their limits. A review 

 of all the meteorological observations made in the district of the Alps 

 shows that in the Central Alps and in the group of Mont Blanc and 

 Monte Rosa, the summer warmth is greater and the climate conse- 

 quently more extreme than in the lower chains of the Northern Alps; 

 by which means the relation of the limits of vegetation to the annual 

 isotherms in these different mountain-groups is explained. 



He further stated that his and his brother's investigation of the pe- 

 riodical development of the vegetation at heights of from 1500 to 

 8000 Paris feet showed among other things that the retardation of the 

 development by the elevation is in general less during the flowering 

 than during the ripening of the fruit ; it amounts in the Alps during 

 the former period to ten days, during the latter to twelve and a half, 

 and on the average of the whole period of vegetation to eleven days. 

 The mean temperature is diminished in general about 2° of Celsius 

 for the same difference of height, during the period of the develop- 

 ment of vegetation. From their own observations on the influence of 

 height on the growth of Coniferae, he concluded that in Pinus Larix, 

 P. Abies, P. sylvestris and P. Cembra, an evident diminution in the 

 thickness of the annual rings takes place at greater elevations. A 

 regular diminution, however, must not be expected for each degree of 

 elevation. Not only the variations in the temperature of the air, of 

 the soil, and in the climate generally (which concur to disturb the 

 Conifer£e at greater heights) produce a diminution of their yearly 



