Page:Popular Science Monthly Volume 29.djvu/450

434 of the species, and at the same time enable one to judge of the suitability of a particular wood for definite uses. 80 little has been done in this country in the microscopical study of the woods for engineering, architectural, or mechanical purposes, that but few are aware of the variety in form and structure of the wood cells, ducts, and special fibers which make up the woody tissue of the different species. An expert can readily determine whether a certain wood, used for rail-road-ties, will sustain the service of a trunk line, or is only suitable for a branch of limited traffic.

In the Coniferæ, which includes the pines, cedars, larches, red-woods, spruces, and firs, as a rule, each layer of growth only has two kinds of wood-cells called tracheids, one of thin walls and a large lumen, and the other of thick walls and a small lumen; when the former pre-dominates, making nearly all of the layer, the wood is generally soft, as in the white pine (Pinus strobus, L.), the cedars, redwoods, spruces, and firs. When the thick-walled cells form one fourth to one half of the layer, the wood is much harder, as in the long-leaf yellow pine (Pinus palustris, Mill), Pinus mitis, and the larches. On the thin-walled



cells of all the species of the Coniferæ are dome-like or lenticular markings, principally on the sides parallel to the medullary rays.

The thick-walled cells are often marked on the sides at right angles to the medullary rays. The Coniferæ have more or less resinous products, and the presence or absence of the upright resin-canals aid in distinguishing the genera, while the form and character of the medullary rays, the presence or absence of resin-ducts, the character of the cells, enable the species to be identified. In the alburnum or sap-wood, the starch is confined to the cells around the resin-canals and in the cells of the medullary rays.

The cellular structure of the oaks, chestnuts, hickories, ashes, walnuts, maples., beeches, birches, and magnolias is far more complex and