February 18.
Revivification of Trees.
Mr. Arthur Aikin, in his “Natural History of the Year,” narrates the first vital function in trees on the conclusion of winter. This is the ascent of the sap after the frost is moderated, and the earth sufficiently thawed. The absorbent vessels composing the inner bark reach to the extremity of the fibres of the roots, and thus, through the roots, imbibe water, which, mixing there with a quantity of saccharine matter, forms sap, and is from thence abundantly distributed through the trunk and branches to every individual bud. The birch tree in spring, on being tapped, yields its sap, which is fermented into wine. The palm tree in the tropics of the same season yields its sap by the same method, which is made into palm wine, and the sap of the sugar maple in North America being boiled, yields the maple sugar.
“This great accession of nourishment (the sap) causes the bud to swell, to break through its covering, and to spread into blossoms, or lengthen into a shoot bearing leaves. This is the first process, and, properly speaking, is all that belongs to the springing or elongation of trees; and in many plants, that is, all those which are annual or deciduous, there is no other process; the plant absorbs juices from the earth, and in proportion to the quantity of these juices increases in size: it expands its blossoms, perfects its fruit, and when the ground is incapable by drought or frost of yielding any more moisture, or when the vessels of the plant are not able to draw it up, the plant perishes. But in trees, though the beginning and end of the first process is exactly similar to what takes places in vegetables, yet there is a second process, which at the same time that it adds to their bulk, enables them to endure and go on increasing through a long series of years.
“The second process begins soon after the first, in this way. At the base of the footstalk of each leaf a small bud is gradually formed; but the absorbent vessels of the leaf having exhausted themselves in the formation of the bud, are unable to bring it nearer to maturity: in this state it exactly resembles a seed, containing within it the rudiments of vegetation, but destitute of absorbent vessels to nourish and evolve the embryo. Being surrounded, however, by sap, like a seed in moist earth, it is in a proper situation for growing; the influence of the sun sets in motion the juices of the bud and of the seed, and the first operation in both of them is to send down roots a certain depth into the ground for the purpose of obtaining the necessary moisture. The bud accordingly shoots down its roots upon the inner bark of the tree, till they reach the part covered by the earth. Winter now arriving, the cold and defect of moisture, owing to the clogged condition of the absorbent vessels, cause the fruit and leaves to fall, so that, except the provision of buds with roots, the remainder of the tree, like an annual plant, is entirely dead: the leaves, the flowers, and fruit are gone, and what was the inner bark, is no longer organized, while the roots of the buds form a new inner bark; and thus the buds with their roots contain all that remains alive of the whole tree. It is owing to this annual renovation of the inner bark, that the tree increases in bulk; and a new coating being added every year, we are hence furnished with an easy and exact method of ascertaining the age of a tree by counting the number of concentric circles of which the trunk is composed. A tree, therefore, properly speaking, is rather a congeries of a multitude of annual plants, than a perennial individual.
“The sap in trees always rises as soon as the frost is abated, that when the stimulus of the warm weather in the early spring acts upon the bud, there should be at hand a supply of food for its nourishment; and if by any means the sap is prevented from ascending at the proper time, the tree infallibly perishes. Of this a remarkable instance occurred in London, during the spring succeeding the hard winter of the year 1794. The snow and ice collecting in the streets so as to become very inconvenient, they were cleared, and many cartloads were placed in the vacant quarters of Moorfields; several of these heaps of snow and frozen rubbish were piled round some of the elm-trees that grow there. At the return of spring, those of the trees that were not surrounded with the snow expanded their leaves as usual, while the others, being still girt with a large frozen mass, continued quite bare; for the fact was, the absorbents in the lower part of the stem, and the earth in which the trees stood, were still exposed to a freezing cold. In some weeks, however, the snow was thawed, but the greater number of the trees were dead, and those few that did produce any leaves were very sickly, and continued in a languishing state all summer, and then died.”