204 
INFLUENCE OF CLIMATE ON PLANTS.* 
SOLAR HEAT. 
While describing the manner in which water lowers the temperature of any 
tract in its vicinity, we must not omit to mention, in connexion therewith, the similar 
effects of forests or other extensive groups of trees. Wherever the superior kinds 
of vegetation abound, to that spot a large quantity of moisture is attracted, for the 
purpose of administering to their necessities. This is an essential feature in the 
economy of nature ; and such fluid, accumulating both in the soil and plants, can- 
not pass off so rapidly as if the surface of the ground were completely open to the 
atmosphere. A more than usual stagnation, proportionate to the limits and close- 
ness of the forest, is the consequence. 
We have previously stated, that whenever the atmosphere is highly charged 
with moisture, it is always at an inferior rate of temperature than when less 
humid. Taken in conjunction with the above observations, this will explain the 
reason why woods contribute to render any country colder. 
North America presents an excellent illustration of the influence of both these 
peculiarities. Traversed throughout by huge rivers and lakes, which are subdivided 
towards their origin into an innumerable quantity of lesser streams ; and covered in 
its less cultivated parts with dense and boundless forests ; its climate, in the same 
latitudes as those of Britain, or any of the more southerly countries of Europe, is 
uniformly less genial. 
There can, moreover, be no doubt that the burning deserts of Africa derive an 
inconceivable degree of the intensity of heat to which they are subjected, from the 
almost total absence of vegetation. We do not here allude to the grateful shade 
which trees would furnish to the traveller, for this could at the utmost be merely 
temporary ; but assume that masses of trees, if even growing at considerable 
distances, would, from the causes before pointed out, so modify the general tempe- 
rature as to make it far more easily supportable. 
Fully persuaded of the accuracy of these declarations, we may, by applying 
them practically, go on to exhibit the fallibility of such practices as are sanctioned 
only by general adoption, and the hallucinations into which persons may be led, 
by adhering to universally admitted rules, without inquiring whether they can be 
supported by reason and philosophy. Perhaps there is not a gardener but will 
assert, as a fundamental and indisputable dogma, that any site which may be 
chosen for the cultivation of plants that are not thoroughly hardy, and which may 
happen to lie particularly exposed to the quarters from whence cold winds are 
usually expected, should be defended by a plantation of trees. And where that 
plantation can be kept at a proper distance, such a system merits nothing but 
approval. But when such a species of shelter is brought within a few feet or yards 
* Concluded from page 184. 
