BooK I. DISTRIBUTION OF VEGETABLES. 199 



918. Hence it is that jilants of high latitudes live on the mountains of such as are much lower, 

 and thus the plants of Greenland and Lapland are found on the Alps and Pyrenees. At 

 the foot of Mount Ararat (Jg. 67.), Tournefort met with plants peculiar to Armenia j 



above these he met with plants which are found also in France ; at a still greater height 

 he found himself surrounded with such as grow in Sweden ; and at the summit with such 

 as vegetate in the polar regions. This accounts for the great variety of plants which are 

 often found in a Flora of no great extent ; and it may be laid down as a botanical axiom, 

 that the more diversified the surface of the country, the richer will its Flora be, at least in 

 the same latitudes. It accounts also, in some cases, for the want of correspondence be- 

 tween plants of different countries though placed in the same latitudes ; because the 

 mountains or ridges of mountains, which may be found in the one and not in the other, 

 will produce the greatest possible difference in the character of their Floras. And to this 

 cause may generally be ascribed the diversity that often actually exists between plants grow- 

 ing in the same latitudes, as between those of the north-west and north-east coasts of North 

 America, as also of the south-west and south-east coasts ; the former being more moun- 

 tainous, the latter more flat. Sometimes the same sort of difference takes place between 

 the plants of an island and those of the neighbouring continent ; that is, if the one is 

 mountainous and the other flat ; but if they are alike in their geographical delineation, 

 then they are generally alike in their vegetable productions. 



919. Cold and lofty situations are the favorite habitations of most cryptogamic plants of the 

 terrestrial class, especially the fungi, algae, and mosses ; as also of plants of the class 

 Tetradynamia, and of the Umbellatee and Syngenesian tribes ; whereas trees and shrubs, 

 ferns, parasitic plants, lilies, and aromatic plants, are most abundant in warm climates ; 

 only this is not to be understood merely of geographical climates, because, as we have 

 seen, the physical climate depends upon altitude. In consequence of which, combined 

 with the ridges and directions of the mountains, America and Asia are much colder in 

 the same degrees of northern latitude than Europe. American plants, vegetating at forty- 

 two degrees of northern latitude, will vegetate very well at fifty-two degrees in Europe ; 

 the same, or nearly so, may be said of Asia ; which, in the former case, is perhaps owing 

 to the immense tracts of woods and marshes covering the surface, and in the latter, to the 

 more elevated and mountainous situation of the country affecting the degree of temper- 

 ature. So also Africa is much hotter under the tropics than America ; because in the 

 latter the temperature is lowered by immense chains of mountains traversing the equa- 

 torial regions, while in the former it is increased by means of the hot and burning sands 

 that cover the greater part of its surface. 



920. Elevation uifluences the habits of plants in various ways ; by exposing them to the 

 wind ; to be watered by a very fresh and pure water from the melting of adjoining 

 snow ; and to be covered in winter by a thick layer of snow, which protects them from 

 severe frosts. Hence many alpine plants become frozen during winter in the plains, and 

 in gardens which are naturally warmer than their natural stations. In great elevations, 

 the diminution of the density of the air may also have some influence on vegetation. The 

 rarity of the atmosphere admits a more free passage for the rays of light, which, being in 

 consequence more active, ought to produce a more active vegetation. Experience seems 

 to prove this in high mountains ; and the same effect is produced in high latitudes by 

 the length of the day. On the other hand, vegetables require to absorb a certain quantity 

 of oxygen gas from the air during the night ; and as they find less of that in the rarefied 

 air of the mountains, they ought to be proportionably feeble and languishing. According 

 to experiments made by Theodore de Saussure, plants which grow best in the high Alps 

 are those which require to absorb least oxygen during the night ; and, in this point of 

 view, the shortness of the nights near the poles correspond. These causes, however, are 

 obviously very weak, compared to the powerful action of temperature. 



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