The true estimate, therefore, seems to lie between the determinations of De Candolle and Dr Lowe respectively. The latter writer admits that De Candolle’s rule is fairly sound when applied to young trees with undecayed centres, but he stipulates that the tree must be cut down and proved to have not more than one centre. He refers to the case of a yew in Kyre Park, Worcestershire, which possesses two huge trunks, united below, and he proceeds to argue that, if we suppose the tops to have been broken off underneath the junction, and young shoots to have sprung up from the base, the trees would have been deemed as old as our most noted specimens in the British Isles. Such a case, however, would rarely be encountered in actual experience.
Fig. 69. Transverse section of yew, showing annual rings. Longest diameter, 5½´´; shortest, 4⅛´´; number of rings, 51. It will be noticed that the section is excentric; this is due to irregular growth. The light coloured, outer zone, is the alburnum, or sap-wood; the dark, inner zone, the duramen, or heart-wood. The large, radial cracks are the results of shrinkage. These cracks run along the lines of the medullary rays, though the actual rays are much too fine to be seen without a good lens. (For an excellent description of such a section, see G. S. Boulger, Wood, 2nd edition, 1908, p. 301.)
We will now examine De Candolle’s “ring method” a little more closely. First, what is the botanical theory respecting the formation of the rings in a tree? In our climate trees make little or no growth during the winter season, hence the new spring wood, with its wide, thin-walled vessels, is rather sharply defined against the narrower, compact, dense-walled vessels which were formed in the preceding autumn or late summer. The successive concentric cylinders of new wood, therefore, when seen in cross-section, appear as zones, or “annual rings.” These rings are well shown in the transverse section of yew ([Fig. 69]). In vertical section the rings appear as parallel strips, forming what is popularly called the “grain” of the wood ([Fig. 70]). It is true, in general, that one ring represents one year’s growth. The rule must, however, be applied under slight reserve. Dr D. H. Scott clamped the branch of an elm in June, thus increasing the pressure on the cells. The result was that wood was formed similar to that which is usually associated with autumn. After six weeks, during which the nutrition had been practically uniform, the clamp was removed, and another ring was produced.
Fig. 70. Longitudinal, tangential section of yew. In this section, taken with the “grain,” the annual rings appear as alternate parallel strips of dark and light wood. The tracheids, or elongated thick-walled cells, are invisible to the unaided eye.
A branch is seen emerging on the right, forming a “pin,” which would be obnoxious to bowyers.
The conditions just described were artificial and abnormal, but varying temperatures, if extreme, might act in a similar manner. Sequoias and red-woods may naturally form several concentric wood rings in a year, a result probably due to alternations of heat and cold. Against the danger of over-calculation of age from neglecting such factors, may be set the consideration that, owing to damage by frost, or to seasonal peculiarities, no ring may be formed within the year[955]. In normal circumstances, “one ring, one year” is a trustworthy maxim. Unfortunately, to verify this rule, should its accuracy be challenged, the tree must be cut down—a most undesirable proceeding. Moreover, if the tree be aged, counting the rings is a matter of some difficulty; one must use a lens and a pair of needles, moving these “counters” as if scoring at a game of cribbage. Now Dr Lowe, while doubting whether the yew may not form more than one ring per year—a contingency that would seriously upset all computations based on this system—feels confident that in this country young trees produce one only. He is of opinion that the ring test consequently holds good for uninjured trees up to 200 or 250 years. Beyond that period, he asserts confidently that the yew sustains injuries from storm or disease sufficient to invalidate the rule. To the extent, let us say, of 250 years, Dr Lowe’s assumption runs parallel with that of De Candolle, who believed that there is practically no limit to the age of the yew, except disease. After the third century, Dr Lowe, as we shall soon see, actually claims a more rapid rate of growth, at least, intermittently.