60 
INFLUENCE OF CLIMATE ON PLANTS. 
in the general dispersion of plants, M. de Candolle has very appropriately sug- 
gested the following particulars for inquiry ; and we shall here follow the arrange- 
ment of that distinguished botanist. First, it is necessary to acquire a knowledge 
of the mean temperature of a country throughout the year. Secondly, the 
extremes of heat or cold to which it is subjected at any and what seasons. And 
thirdly, the degree of temperature prevalent in each of the different months. 
A knowledge of the mean temperature of any country is only valuable to the 
cultivators of its vegetable productions, when not deduced from contrary extremes. 
If the climate be liable to any very great depression or elevation of temperature 
at different periods of the year, the average, amalgamating these vicissitudes, 
would be quite useless, with regard to its guidance in cultivation. But, where a 
nearly equable degree of temperature is maintained throughout the entire year, 
the variations on either hand being trifling and unimportant, such data would be 
of the highest possible utility. For instance, in tropical regions the derangement 
of temperature is never great ; since, during the period at which the sun's rays 
are vertical, the density of the atmosphere is so much increased by evaporation, 
that a very slight difference of the thermometer is indicated, on its removal from 
a shaded to an exposed situation. We shall hereafter show, that on no other part 
of the earth the same uniformity of temperature exists ; but, even in this case, 
general information is insufficient for practical purposes j and nothing but minute 
and careful observation, from residence or sojourn in or near the particular locality 
wherein a plant is found, can furnish proper materials for its cultivation. 
So far, then, as those climates are concerned in which there is the nearest 
approximation to equability of temperature, vegetation is found to maintain in 
some degree a similar uniformity of habitude and aspect, and, with the exception 
of a brief period, exhibits a constant appearance of verdure and luxuriance. Peren- 
nial herbaceous plants, with soft and succulent stems, trees with evergreen foliage, 
and very generally destitute of mucilaginous or resinous juices, in short, nearly all 
the shrubby or arborescent forms, and a great proportion of the herbaceous species 
of monocotyledonous or endogenous plants, inhabit those districts possessing the 
above character. On the other hand, annual plants are, we believe, nowhere 
seen ; deciduous trees, likewise, and those which form thick and numerous con- 
centric layers of bark, are almost exclusively confined to regions which present a 
greater diversity of temperature. 
Valuable, however, as is a knowledge of the mean temperature of certain por- 
tions of our globe, with reference to its influence on vegetation, inasmuch as it 
teaches that the plants thereto indigenous cannot be cultivated with us unless 
some artificial means be employed for imitating their native climate ; correct 
information respecting the extremes of temperature to which any country is sub- 
jected is of far greater importance, since it embraces a much wider field of obser- 
vation, and affords more comprehensive and appreciable results. To know that 
certain tracts are periodically exposed to great alternations of temperature, is to 
