vegetable life, that it is difficult to abstract one part without impairing the efficiency of the remainder.

56. The office of the stem is, to convey the crude fluid obtained by the roots from the soil, and called sap, into the leaves for elaboration, and then to receive it back again. Sap is, originally, water containing various gases, earths, and salts, in solution: but, as soon as it enters the stem, it dissolves the vegetable mucilage it finds there, and becomes denser than it was before; it is further changed by the decomposition of a part of its water, acquires a saccharine character, and, rising upwards through the alburnum, takes up any soluble matter it passes through. Its specific gravity keeps thus increasing till it reaches the summit of the branches; and, by degrees, it is all distributed among the leaves. In the leaves it is altered, and then returned into the stem; not, however, into the alburnum, where it would meet the ascending current, but into the bark, through which it falls, passing off horizontally through the medullary rays into the interior of the stem, and fixing itself in the interior of the bark, especially of the root. It may be said, that, in trees, the alburnum and liber have each two equally important offices to perform: the al burnum giving strength and solidity to the stem, and conveying sap upwards; the liber not only conveying sap downwards, but covering over the alburnum, protecting it from the air, and enabling it to form

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without interruption. It is, therefore, indispensable to the healthy condition of plants, that neither the alburnum nor the liber should be injured. The central wood is of little consequence, and may be

destroyed, as it constantly is in hollow trees; and the rind is of comparatively small importance, for it is continually perishing under the influence of the atmosphere: but the liber and alburnum are naturally in a state of constant renovation, and cannot be permanently injured without injury to the plant.

57. But although, under ordinary circumstances, the sap of exogens rises through the alburnum and descends through the liber, yet the simplicity of structure in plants is such, that, together with the permeability of their tissue, it enables them, in cases of emergency, to alter their functions, and to propel their fluids by lateral instead of longitudinal communications. The trunk of a tree has been sawed through beyond the pith in four opposite directions; namely, from north to south, from west to east, from south to north, and from east to west, at intervals of a foot, so as completely to cut off all longitudinal communication between the upper and lower parts of the stem, as effectually as if those two parts had been dissevered; and yet the propulsion of the sap from the roots into the head of the tree went on as before: which could only have been effected by a lateral transmission of

this fluid through, or between, the sides of the woody tissue. So when "ringing" is practised, and the alburnum is partially destroyed, the ascending fluid diverges into the stratum of wood beneath the annulation; and, when it has passed by, it again returns into its accustomed channels; at the same time, it is probable, although not proved, that some portion of the descending sap forces its way laterally below the wound, out of the bark into the alburnum, using the latter as a means of communicating with the bark below the ring. Some curious experiments upon this subject

were contrived by Mr. N. Niven (Gardener's Magazine, vol. xiv.). In one case, he divested the stem of a tree of a deep ring of bark, and of the first twelve layers of wood below it (fig. 5.); nevertheless the tree continued to live and be healthy From the exposed surface of the wood no sap made its appearance, except from a cut which had been inadvertently made with

the saw on one side, to the depth of, perhaps, five or six layers of wood beyond the twelve actually removed. From that cut a flow of sap took place, and continued to run during the whole of the season in which the operation was performed. In this case, the sap must have ascended exclusively by the alburnum.

In another case, by making four deep and wide incisions into the trunk of a tree (fig. 6.), and re

num; and the tree upon which the operation was performed continued to live for two years, after which it was not observed. In the latter instance, no doubt can be entertained that the whole of the ascending sap was directed into the four pillars of alburnum, which were allowed to remain.

58. The cause of the flow of the sap appears to be the attraction of it by the leaves, which continually diminish its quantity; and the necessity that the sap abstracted should be replaced by a further supply sent upwards from the roots. The consequence of this is, that sap always begins to flow at the ends of branches, a circumstance which has led to the erroneous idea that it proceeds from above downwards through the alburnum. The flow of the sap must not, however, be confounded with the motion of the sap, which takes place in the winter as well as in the summer, and is a mere impletion of the system, caused by the attraction of the roots, unaffected by the exhalation of the leaves.