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. [Illustration: 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. [Illustration: 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.