1850.] Linnean Society. 103 



warmth of the summer is in this view of peculiar 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 meteor- 

 ological 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 consequently 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 

 periodical 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 j)eriod 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 Comfer<E, he concluded that in Pinus 

 Larix, P. Abies, P. sj/lvest)-is 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 ce at greater heights) produce a diminution 

 of their yearly growth ; but the different nature of the soil has also 

 great influence on their growth. The mass of well-decomposed 

 earth, the presence of boulders or firm rock, the exposure of the 

 locality, the humidity of the soil, and in some degree also its inclina- 

 tion, have so great an influence on the growth of the tree, and are 

 moreover especially in the lower regions so irregularly distributed, 

 that the influence of elevation, which should be most closely con- 

 nected with the changes of climate, may be and is partially oblite- 

 rated. Very frequently indeed in investigations of the geography 

 of plants, a similar concurrence and a mutual correlation of the 

 various causes by which the changes of vegetation are produced, are 

 to be recognized. The observation of the progress from year to 

 year shows that very frequently considerable variations occur in the 

 amount of growth i.'i separate stems. These are not, however, con- 

 nected with deflnite years of the development, but irregularly dis- 



