Page:Popular Science Monthly Volume 29.djvu/451

 more highly differentiated than that of the conifers; beside the wood-cells, there are ducts, vessels, and special cells containing starch in the alburnum or sap-wood. In nearly all the species of the first five orders mentioned, the ducts grow in concentric rows, in the first of the season's growth; those which, form later may be inclined through the layer of wood-cells, becoming smaller as they approach the outer portion. In the live oak, the ducts run radially through the ring, and the small fibers are nearly solid, giving the wood great hardness, making it so valuable for ship-building.

In the maples, beeches, birches, and magnolias the ducts are well interspersed through the entire ring, and are nearly of the same size. In the alburnum of these woods there are a great many cells which are filled with starch as reserve material, like the medullary rays in this portion of the wood. During active growth the starch is transformed and withdrawn. In the duramen but little starch remains, other products taking its place.

In the hard woods, all or portions of the annular rings are made up of hard and nearly solid fibers, while in the softer woods the walls are not so thick. In many of the species each layer of growth is not



of uniform thickness or quality, some having but comparatively few of the dense, hard fibers, the growth of these depending upon certain climatic conditions which may not yearly occur.

In the thick forests, under quite uniform conditions of growth, the thickness of the annular ring largely depends upon the leaf-area, which remaining practically the same in the older trees, the wood-cells forming upon a larger diameter, the rings as a rule are not so thick or dense as those grown when the tree is much younger.

Formerly, in lumbering, the trees were felled in the winter, cut into logs, sledded on the snow to the streams, and driven down in rafts in the spring to the mills. Now, with the log-railroads, they are independent of the snow, and in many camps lumbering is carried on