It is a circumstance that has long been suspected; but late experiments have decided beyond a doubt that vegetable heat is considerably above that of unorganised matter in winter, and below it in summer. The wood of a tree is about sixty degrees, when the thermometer is seventy or eighty degrees. And the bark, though so much exposed, is seldom below forty in winter.
It is from the sap, after it has been elaborated by the leaves, that vegetables derive their nourishment; in its progress through the plant from the leaves to the roots, it deposits in the several sets of vessels with which it communicates, the materials on which the growth and nourishment of each plant depends. It is thus that the various peculiar juices, saccharine, oily, mucous, acid, and colouring, are formed; as also the more solid parts, fecula, woody fibre, tannin, resins, concrete salts; in a word, all the immediate materials of vegetables, as well as the organised parts of plants, which latter, besides the power of secreting these from the sap for the general purpose of the plant, have also that of applying them to their own particular nourishment.
EMILY.
But why should the process of vegetation take place only at one season of the year, whilst a total inaction prevails during the other?
MRS. B.
Heat is such an important chemical agent, that its effect, as such, might perhaps alone account for the impulse which the spring gives to vegetation. But, in order to explain the mechanism of that operation, it has been supposed that the warmth of the spring dilates the vessels of plants, and produces a kind of vacuum, into which the sap (which had remained in a state of inaction in the trunk during the winter) rises: this is followed by the ascent of the sap contained in the roots, and room is thus made for fresh sap, which the roots, in their turn, pump up from the soil. This process goes on till the plant blossoms and bears fruit, which terminates its summer career: but when the cold weather sets in, the fibres and vessels contract, the leaves wither, and are no longer able to perform their office of transpiration; and, as this secretion stops, the roots cease to absorb sap from the soil. If the plant be an annual, its life then terminates; if not, it remains in a state of torpid inaction during the winter; or the only internal motion that takes place is that of a small quantity of resinous juice, which slowly rises from the stem into the branches, and enlarges their buds during the winter.
CAROLINE.
Yet, in evergreens, vegetation must continue throughout the year.
MRS. B.
Yes; but in winter it goes on in a very imperfect manner, compared to the vegetation of spring and summer.