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Page:Popular Science Monthly Volume 24.djvu/64

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54
THE POPULAR SCIENCE MONTHLY.

at the point of fracture that caries into the trunk which will ultimately reduce it to a mere shell, similar to one of those bull-oaks wherein the bull loves to hide himself. These causes of disease and decay can never be absent, since they evidently belong to the permanent order of nature.

Again, De Candolle accounts with great probability for the diminished rate of tree-growth after a certain period by such considerations as the greater distance of the roots from the air, their coming into contact with the roots of other trees, or with a rocky or otherwise unsuitable substratum, or the diminished elasticity of the bark; and though it is possible that trees might continue to grow in their fifth century at the same rate as in their first, if the conditions remained equally favorable, yet, since the proviso can never be insured, a further difficulty, amounting to insuperability, occurs, to prevent such an hypothesis from being brought to the test of either observation or experiment.

Whether, therefore, a tree might possibly continue living and growing forever is a question of less entertainment than the question of its possible duration in the common state of nature and under the irreversible conditions of climate, soil, and the elements. What age may we ascribe to some of our largest specimens, either still existing or recorded in trustworthy history? Is the period of one thousand years, the favorite figure of tradition, a common or probable period of arboreal longevity, or have our proudest forest giants attained their present size in half the time that is commonly claimed for them?

In the discussion of this question we have but few known data to guide us, since statistics of the rate of growth, as afforded by careful measurement, date only from about the beginning of the eighteenth century. Of such statistics we may dismiss at once measurement of height or of the spread of a tree's boughs, the measurement of girth being far easier and more conclusive. But it is unfortunate that no standard of distance from the ground has yet been adopted for measurement, so that the needless perplexity might be avoided which is derived from giving the circumference now at the ground and now at two, or three, or six feet above it.

The counting of the rings added by exogenous trees every year to their circumferences can only, without risk of great error, be applied to trees cut down in their prime, and hence is useless for the older trees which are hollow and decayed. Trees, moreover, often develop themselves so unequally from their center that, as in the case of a specimen in the museum at Kew, there may be about two hundred and fifty rings on one side to fifty on the other. Perhaps the largest number of rings that has ever been counted was in the case of an oak felled in 1812, where they amounted to seven hundred and ten; but De Candolle, who mentions this, adds that three hundred years were added to this number as probably covering the remaining rings which it was no