1818.] M. De Candolle on the Geography of Plants. 409 



therefore, to understand it in all its relations, it is necessary to 

 examine separately all the external agents of vegetation. Height 

 may act upon vegetables either mediately or immediately ; height 

 influences the temperature of the atmosphere and its humidity, 

 and also the intensity of the solar light ; but temperature, moist- 

 ure, and light, all affect vegetation ; therefore, in this way, 

 height will act mediately on vegetables. Height has likewise the 

 immediate effect of diminishing the density of the atmosphere, 

 and it becomes a curious subject of inquiry, whether this dimi- 

 nished density can affect vegetation. All these different points 

 wall be considered in succession ; and we shall begin with the 

 effect of absolute height as operating by a change of temper- 

 ature. 



It is well known, that as we ascend into the air, the mean 

 temperature diminishes according to a regular gradation, which 

 is so well ascertained, that if we make the proper allowance for 

 the influence of local and incidental circumstances, we can 

 determine beforehand the mean temperature of a given place, 

 by knowing its latitude and its absolute height. Vegetation, 

 consequently, which depends so much upon temperature, must 

 be much influenced by the latitude and the absolute height ; and 

 all the facts of which we are in possession tend to confirm this 

 conclusion. 



1. Every one knows that there are many plants which are 

 only found at certain elevations ; and it has been proved by 

 M. Humboldt that this limitation of plants to definite heights 

 above the level of the sea is the more constant the nearer we 

 approach to the equator. This fact affords a strong presump- 

 tion that the effect depends upon the temperature and not upon 

 the rarity of the air. 



2. Most of the plants of France which are indifferent to 

 temperature are so likewise to absolute height, and grow from 

 the borders of the sea to the summits of the mountains. In 

 France there are about 700 wild plants, which are found natu- 

 rally at veiy different elevations ; of these the following may 

 be cited as well known examples. The erica vulgaris and the 

 erica tetralix, which cover many parts of the western coast, grow 

 on the summit of Mount Calm, one of the Pyrenees, about 3,000 

 metres high. The statice armeria is found in Holland, in 

 districts that are below the level of the sea, and among the Alps, 

 on the " Col du Bon-Homme," at an elevation of about 2,500 

 metres. The tussilago farfara and the lotus corniculatus, which 

 are common at the level of the sea in France, are also found on 

 Mount Jovet at an elevation of about 2,400 metres. The same 

 kind of observations have been made with respect to cochliarea 

 officinalis, thymus seipyllum, and vulgaris, digitalis purpurea, 

 nardus stricta, potentilla verna, fritillaria meleagris, asplenium 

 viride, anthoxanthum odoratum, and phleum pratense. Several 

 species of the carices, scirpi, and junci are found in the same 

 diversity of situations ; and, m short, the number of plants to 



