412 M. Be Candolk on the Geography of Plants. [June, 



highest mountains are covered ; and it is accordingly found that 

 the fertility of the sides of mountains depends very much upon 

 their being furnished with a regular supply of moisture. The 

 frequent rains which occur in all elevated situations, in some 

 measure supply the place of snow in mountains that are not 

 covered by it. Plants that grow at great heights must always 

 necessarily be watered by a fluid which is at a low temperature ; 

 and although we are not able exactly to learn the effect which 

 is produced upon plants by the temperature of the water which 

 is applied to them, yet we have many facts which would lead us 

 to conclude that it must have a considerable influence. Gar- 

 diners know that tropical plants will not bear very cold water ; 

 and, on the contrary, it is observed on the sides of hills, that if 

 we meet with a very cold spring, the plants that grow round it 

 are such as we generally find in a colder climate. An observa- 

 tion the reverse of this has been observed with respect to warm 

 springs, which are found to be surrounded by plants that are 

 natives of a lower situation, or more southern latitude. 



With respect to the question, Whether the rarity of the air, 

 which is the direct and immediate effect of absolute height, has 

 any influence upon vegetation, we may observe, that as plants 

 act chemically upon the air, especially upon the oxygenous 

 portion of it, it is necessary that the atmosphere be sufficiently 

 dense to supply them with the requisite quantity of oxygen. 

 We have, however, reason to suppose that the air of the highest 

 mountains is sufficiently dense for this purpose ; the absolute 

 quantity of oxygen absorbed by plants is not very considerable ; 

 it is further stated that they exhale as much oxygen as they 

 absorb, and although M. Theodore de Saussure has observed 

 that the succulent plants absorb the least oxygen, yet it is known 

 that plants of this kind are not those which flourish the best in 

 mountainous situations. M. Humboldt has suggested that a 

 rare atmosphere may act upon plants by augmenting the evapo- 

 ration, and perhaps the idea may be allowed to be correct ; but 

 the effect must be very inconsiderable, and is scarcely, if at all, 

 to be perceived. 



From the above remarks it appears that we may lay down six 

 laws or general principles respecting botanical geography. 



1. The degree of rarity of the atmospherical air, considered 

 independently of every other circumstance, between the level of 

 the sea and the limit of perpetual snow, does not appear to have 

 any very essential direct action upon the geography of plants. 



2. The geography of the plants of different regions is principally 

 determined by the mean temperature, and by its annual phases. 



3. As the mean temperature of a given place is determined by 

 the latitude, the absolute height, and the exposure, it follows 

 that the nearer we are to the equator, and consequently the 

 more important the latitude and the exposure are, the more 

 influence the absolute height has upon the habitation of plants, 



