Book I. DISTRIBUTION OF VEGETABLES. 2(77 



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

 that die 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 

 1 iet ween 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, 

 \\ ill produce the greatest possible difference in the character of their Floras. To this 

 cause may generally be ascribed the diversity which often actually exists between plants 

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

 of North America, and also between those of the south-west and south-east coasts; 

 the former being more mountainous, 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 die one is mountainous and the other flat ; but if they are alike 

 in their geographical delineation, then they are generally alike in their vegetable pro 

 ductions. 



173-1. Cold and lofty situations are the favourite habitations of most cri/ptogamic plants of 

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

 Tetradynamia, and of the Umbelliferous and Syngenesious tribes ; whereas trees and 

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

 climates : but 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, vege- 

 tating 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 temperature. So, also, Africa is much hotter under the tropics than America; 

 because in the latter, the temperature is lowered by immense chains of mountains travers- 

 ing the equatorial regions, while in the former it is increased by means of the hot and 

 burning sands which cover the greater part of its surface. 



1735. Elevation influences the habits of plants in various ways : hy exposing them to the 

 wind ; by causing them 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 pro- 

 tects them from severe frosts. Hence many alpine plants become frozen during winter 

 in the plains, and in gardens which are naturally warmer than their proper 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 vege- 

 tation. Experience seems to prove this on high mountains ; and the same effect is pro- 

 duced 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 corresponds. These 

 causes, however, are obviously very- weak, compared to the powerful action of temper- 

 ature. 



1736. Great anomalies are found in the comparative height at which the same plant 

 will grow in different circumstances. In countries situated under the equator, the two 

 sides of the mountain are of the same temperature, which is solely determined by ele- 

 vation ; but in countries distant from it, the warmest side is that towards the south, and 

 the zones of plants, instead of forming lines parallel to the horizon, incline towards the 

 north. The reason, in both cases, is sufficiently obvious. In die temperate zone we 

 find the same plants frequently on low and elevated situations, but this is never the case 

 between the tropics. 



1737. Altitude influences the habits of aquatics : thus some aquatics float always on the 

 surface of the water, as Z.emna, while others are either partially or wholly immersed. 

 Such aquatics as grow in the depths of the sea are not influenced by climate ; but such as 

 are near the surface are influenced by climate, and have their habitations affected by it. 



173S. The moisture, or mode of watering, natural to vegetables, is a circumstance which 

 has a powerful influence on the facility with which plants grow in any given soil. The 

 quantity of water absolutely necessary for the nourishment of plants, varies according to 

 their tissue : some are immersed, others float on its surface ; some grow on the margin 

 of waters, with their roots always moistened or soaked in it; others, again, live in soil 

 slightly humid or ahnost dry. Vegetables which resist extreme drought most easily 

 are, 1 Trees and herbs with deep roots; because they penetrate to, anil derive sufficient 

 moisture from, some distance below the surface ; 2. Plants, which, being furnished with 



