33 
INFLUENCE OF CLIMATE ON PLANTS. 
SOLAR HEAT. 
In tlie last volume of this Magazine, some practical remarks were inserted on 
the agency of solar light. As we have reason to believe that they have been in 
some measure instrumental in promoting a rational inquiry into the influence 
exerted by that most vitally important agent, and the extent of modification 
required to suit the constitutions of different plants, we now request the attention 
of our readers while we attempt to fulfil our original design by submitting a few 
observations on the effects of a still more essential principle — heat. 
It would scarcely accord with the character of this work, and the design of 
the present article, to discuss the question of the nature of heat. Whether it be in- 
deed a substance, or merely a mode of existence, — a particular variety of form, or the 
vibration of rudimental atoms, — is far from being accurately determined. Believing 
that the majority of men of science incline to the opinion that it is an impalpable 
substance, and this hypothesis being likewise much more tangible and intelligible, 
we have adopted the mode of language which treats of it as such. Nor do we 
intend to institute any inquiry into latent heat, or the precise means of its develop- 
ment. We write on the subject for practical purposes, and solely with reference 
to botanical art and science. In this dissertation on solar heat, we shall therefore 
consider it as an active principle, noting the effects of its presence or absence upon 
vegetation, with applicatory deductions and illustrations ; and the few remarks we 
may deem it necessary to make on its general laws, or the mode of its agency, will 
be as deferential and cursory as possible. 
Heat — solar heat — is that great, undefined, and undefinable natural agent, 
which is the principal source of all the changes undergone by plants and vegetables, 
from the first germination of their seeds, to the period of their actual disorganiza- 
tion and decomposition. Being universally diffused, it pervades all their parts, 
and incites and invigorates their various functions when in health ; but the moment 
disease and decay seize them, its apparent mode of operation is reversed, and it has 
a direct tendency to hasten their dissolution. In either health or decay, heat 
fulfils most important ends in vegetable economy ; for, while in the one case it is 
the means of infusing life and vigour into what would necessarily be inert structures 
without its agency, in the other it prepares materials for the nourishment and 
support of succeeding generations, which are destined to flourish on the ruins of 
their progenitors and predecessors. 
These diverse effects of heat, although apparently attributable to two distinct 
and discordant properties, result fi'om precisely the same cause, and are realized by 
a similar process ; since, in either instance, the substance of plants is dilated and 
rarefied. Heat expands all things ; and when it appears to act contrariwise, such 
VOL. VI. NO. LXII. F 
