Page:Timber and Timber Trees, Native and Foreign.djvu/52

32 layers, and probably arises from the sap failing to possess some of the elements necessary for uniting the new layer to that of the previous year's growth. This deficiency of the cohesive matter between the woody layers is supposed to result from sudden changes of temperature, from the roots passing through a peculiar vein of soil, and even from frosts; violent and sudden gusts of wind and storms may also help to produce it.

It will sometimes happen that only a portion of a layer is detached, making the segment of a cup; at other times, a small part of several layers; and again, in some instances, we find that the disjunction is not complete, owing to there being a few fibres remaining to connect the two layers. When, however, it assumes its worst shape, that is, when the ring or cup is perfect, it will in all probability be found to pervade the greater part, if not the whole tree, evidence of it being frequently traceable in the remotest branches.

Experience has shown that with only a segmental cavity open, there is not much to fear, as it seldom extends far up from the root; any log, therefore, not having more cup-defect than this, may without hesitation be converted into plank, board, or scantling; but if one or more complete cups be present, especially if they are large, it could not safely be sawn longitudinally down the middle, as the centre or cup part would drop out, leaving in each half a deeply-grooved channel, equal to the semi-diameter of the cup-defect. The log in this case could, therefore, only be used advantageously by appropriating it to some purpose, where the full growth might be employed.

The cup-defect occurs in perfectly sound and healthy-looking trees, and there is not anything to indicate its