Page:Timber and Timber Trees, Native and Foreign.djvu/27

I.] necessary, for many purposes in carpentry, to regard this arrangement of medullary rays, to insure that the work shall remain, when finished, free from, warp or twist upon the surface. The timber should be cut as nearly as possible in the direction of these rays, the shrinkage in seasoning being, for the most part, angular to them. Workmen in general, and modellers in wood in particular, endeavour to embrace the greatest length of medullary figure in their work to guard against warping, well knowing that if they do so it will stand satisfactorily the test of time and wear. Others, who are engaged in the cleaving of posts, rails, or palings for park and other fences, know that they can only successfully do this by rending the piece in the direction of these rays. It is by a careful study of this that we obtain our best and most beautifully figured wainscot from the slow-growing Oaks found in die North of Europe, Austria, Asia Minor, and in some districts of North America.

By the contact of these medullary rays with the annual layers, and chiefly in the newly-formed wood, a means is afforded for the ascent of sap from the root. The sap is believed to be drawn upwards every spring by capillary attraction, and continues for a time to flow through the pores and fibres of the tree until it reaches the upper side of the leaves; thence it returns, by the under side of the leaf downwards, between the outer circle or zone of ligneous layers and the bark, permeating in its course the whole body of the tree, and contributing to form annually a new layer. During this progress the sap undergoes some very important chemical change, and, becoming gradually elaborated, tends to the formation of a substance called cambium, between the liber, or bark, and the alburnum. The stem is thus enlarged by a new layer on the outside of