AGE OF TREES. Plants, like animals, are subject to the lava of mortality, and seem mostly to have a limited period for their existence.
It is chiefly to annual and biennial plants that what may be called a precise period of duration is fixed ; a period determined by the productiou of their fruit, and not capable of being prolonged beyond that event, except by artificial means. Plants that live for a long Ulna belong either to the class of Endogens or Exagens.
To the first of these classes belongs the Palm Trite, and some other tropical trees. There is scarcely any well-attested evidence of these plants ever acquiring any considerable age. It has indeed been supposed, that certain Brazilian cocoa-nut palms may be from 600 to 700 years old, and that others probably attain to the age of something more than 300 years. But the method of computing the age of ',alma, which is either by the number of rings externally visible upon their rind between the base and summit of tho stem, or by comparing the oldest specimens, the age of which is unknown, with young trees of a known age, is entirely conjectural, and not founded upon sound physiological considerations ; besides which, the date-palm which is best known to Europeans, does not at all justify the opinion that palms attain a great age; the Arabs do not assign it a greater longevity than from two to three centuries. Independently of this, the mode of growth of such endogenous trees as palms seems to preclude the possibility of their existing beyond a definite period of no great extent. The diameter to which their trunks filially attain is very nearly gained before they begin to lengthen, and afterwards all the new woody matter, which every successive leaf necessarily producea during its development, is insinuated into the centre. The consequence of this is, that the woody matter previously existing in the centre is displaced and forced outwards towards the circumference. As this action is constantly in progress, the circumference, which in the beginning was soft, becomes gradually harder and harder, by the pressure from within outwards, till at last it is not susceptible of any further compression. After this has occurred, the central parts will gradually solidify by the incessant production of new wood, which thrusts outwards the older wood, till at last the whole stein must become equally hard, and no longer capable of giving way for the reception of new matter ; for what has once been formed always remains, and is never absorbed by surrounding parts. It is probable,
for this reason, that endogenous trees, suelt•as 'alma attain no considerable age, and that the duration of their existence must be absolutely fixed in each species by the power they may respectively have of permitting the descent of woody matter down their centre.
In exogenous trees it is quite the reverse, and to their existence no limited duration can be assigned. In consequence, first, of the new woody matter which is constantly formed beneath the bark near the circumference of their trunk, and, secondly, of the bark itself being eapable of indefinite distention, no compremion is exercised by the new parts upon those previously formed ; on the contrary, the bark is incessmitly giving way to make room for the wood beneath it, while the latter is, in consequence, only glued, as it were, to what succeeds It, without its own vital powers being in any degree impaired by compression. It is in the newly-formed wood that the greatest degree of 'vitality resides : in the old wood near the centre life in time becomes extinct ; but as each successive layer possesses an existence iu a great degree independent of that which preceded it, the death of the central part of an exogenous tree is by no means connected with any diminution of vitality in the circumference. Hence it is that hollow trees are often so healthy ; and that trees in the most vigorous state are often found decayed at the heart without any external sign, as timber-merchants frequently discover to their cost. Of the many remarkable cases upon record of aged trees the following are among the more interesting :— At Ellerslie, the birth-place of Wallace, three miles to the south west of Paisley, stands an oak, in the branches of which tradition relates that on one occasion that chieftain concealed himself with three hundred of his followers. However improbable the latter circumstance may be, it is at least certain that the tree may well have been a remarkable object even at the period assigned to it by tradition, namely, in the beginning of the 14th century ; and if so, this individual must be at least 700 years old. Its branches sre said to have once covered a Scotch acre of ground ; but its histOrical interest has rendered it a prey to the curiosity of the stranger, and its limbs have gradually disappeared till little remains except its trunk.