34 
INFLUENCE OP CLIMATE ON PLANTS. 
appearance is illusory. There may be only heat sufficient to evaporate the more 
soluble portions, and the residue may contract ; but, in every case, there is an 
expansion and diffusion of invisible particles, this causing the visible ones to become 
more closely compressed ; and if the heat were rendered more intense, the latter would 
probably be dissolved likewise. Heat evolves from the soil, rarefies, and then returns 
the fluid gaseous aliment of plants, which subsequently passes into, or is absorbed by, 
the minute orifices of the spongelets of their roots. Still influenced by caloric, this 
aliment is impelled and circulated throughout all their continuous and ramified 
arteries, depositing, in its passage through stem, leaf, or flower, its grosser elements, 
according to chemical affinity and assimilation, which deposite, by accumulation, is 
its actual growth or increase, while the rarer elements are constantly evaporated 
(by the continued influence of heat) through the innumerable pores. 
There is thus, in a healthy plant, an unintermitting supply, appropriation, and 
expenditure ; and as the supply exceeds the expenditure, so much is the appro- 
priation, and so much the growth. If the supply be suspended, or the plant 
unable to absorb it, or the circulation in any manner interrupted, and the plant 
debarred from appropriation ; in either case the influence of heat is continued 
upon the plant's substance, expanding and volatilizing into the atmosphere its more 
fluid matters, while the earthy portion subsides, and eventually mingles with the 
surrounding soil. Any derangement of the channels of circulation, — such as may 
be produced by a close ligature, by twisting the plant, by severing its stem, or a 
more inscrutable injury, — of course causes the gases which would have entered 
them to be dispersed abroad, and thus diverts the supply ; while evaporation being 
unceasing, and its progress in fact facilitated as decomposition advances, the plant 
is speedily reduced to its component elements. 
That the action of heat is alike indispensable to the growth and decay of 
plants, is further evinced by the fact, that, in a temperature below the freezing 
point, these processes are mutually arrested. Moisture, however, assists in pro- 
moting the decomposition of vegetable substances ; but water is only a consequence 
of heat, at least in the form in which it is supplied to the earth. The progress of 
volatilization is accelerated or retarded, according to the degree of uniformity in 
which heat and moisture exist, either collaterally or independently, and the extent 
of alternation to which they are subjected. In a temperature uniformly high, with 
a commensurate supply of moisture, plants decay most rapidly ; at a similar tem- 
perature, but in the absence of all humidity, the solid particles retain their texture 
for a great length of time ; and in a lower one, under the same circumstances, still 
longer. In the latter case, alternations of moisture and drought will greatly facili- 
tate the disorganization of a plant, but in the former they only impede its 
progress. 
Of the popular opinion that exposure to the atmosphere is necessary to induce 
vegetable decomposition, we may observe that it is to a great extent erroneous. 
Air can have no effect whatever in either causing or hastening this process, except 
in so far as it is the medium through which solar heat and moisture are conveyed. 
