INFLUENCE OF CLIMATE ON PLANTS. 
35 
Neither is the submersion of any dead vegetable substance in water at all effectual 
in preventing its dissolution, unless, as is the case in all natural bodies of water, by 
this fluid being considerably colder than the atmosphere. The fallacy of either of 
the above notions is easily demonstrated by placing any part of a plant in a close 
vessel, filled with boiling water. If the water be kept boiling, and the supply 
replenished as it evaporates, the vegetable tissue will ultimately be destroyed, and 
its succulent portion, together with the water, be completely volatilized. This 
simple experiment proves incontestibly that heat and moisture — when the former 
is sufficiently intense, and the latter proportionately supplied — will speedily reduce 
all vegetable matters to a state of gaseous fluidity. 
Heat performs an essential office in generating those tiny mosses which first 
appear on newly-formed islands, mountain ranges, and architectural ruins ; and, 
by the action of the same principle, these are converted into soil, successive tribes 
are produced in superior gradation, which likewise decay in their turns, and thus 
the surface of these barren districts is eventually clothed with both soil and 
vegetation, and rendered competent to sustain both animal and human life. As 
already hinted, all the food of plants, which is supplied to them either naturally 
or artificially, in the form of decayed vegetable or animal substances, is rendered 
soluble and qualified for absorption through the agency of heat. Aqueous aliment, 
which is a much more important article of vegetable sustenance, — -whether 
absorbed in a pure state by the leaves, or percolated through soil, — is also primarily 
produced entirely by solar heat. The rains and dews which distil upon plants, 
and on which they ever depend for support, have their origin in the watery 
vapours constantly exhaled by the heat of the sun, which, passing into a cooler 
atmosphere, are condensed into the various modifications of dew and rain, and 
being thus increased in density and weight, descend, by the law of gravitation, 
again on the earth. 
The most remarkable properties of heat are those of a diffusive and impon- 
derous character. By the latter of these it is enabled to pass unrestrainedly 
through the atmosphere, and permeate the densest substances. To tlie former we 
are indebted for that equability of temperature which is caused by what is termed 
the radiation of heat. This process is continually going on from all parts of the 
earth's surface, as well as from all substances, both solid and fluid, with wiiicli it 
is overspread. Hence, when, from the natural obliquity of the sun's rays, or from 
their total obscuration, the atmosphere is at a much lower temperature than the 
surface of the ground, heat is radiated from the latter, and from vegetation, to 
such an extent, that not only does the atmospheric vapour condense, but congeal 
upon them. Perhaps, also, the trifling evaporation caused by radiation from 
such bodies, may increase the quantity of hoar frost which is engendered upon 
humid ones during the spring and autumnal months, these being preceded in the 
one case, and succeeded in the other, by more intense and durable congelations. 
It is under similar circumstances, and entirely owing to the abstraction of heat by 
radiation, that ice is formed on the surface of water ; while the incrustation thus 
