The inner structure of these dicotyledonous trunks is seen when we examine the cross section cut of a felled tree. In the centre is the heartwood, the durable wood of commercial value, the cells of which are hard and dry; next it the soft sapwood, the living part of the tree containing cells filled with sap; then the cambium layer, the zone of growing cells, and outside of this the bark. Each year new cells are formed in the cambium layer, the inner ones making new wood, the outer ones new bark, and by counting these annual rings of growth the approximate age of the tree is found. In young trees there is a conspicuous central portion of pith which remains after the tree matures, as long as the heartwood is sound. The lines radiating from the centre to the circumference are called medullary or pith rays and form the “silver grain” of the wood. As the size of the trunk increases, the bark unable to expand, cracks in fissures or peels in layers, and is pushed off by the tremendous growing power from within. The heartwood is not a living part of the structure and often trees live for years without it,—hollow shells with a normal amount of vitality so long as the roots, the cambium layer and the buds are not injured. Branches grow from the axillary or lateral buds on the stem, continuing their growth every year by the development and unfolding of new buds, both terminal and lateral. When the growth is carried on by the terminal buds, the tree is more apt to be regular in outline than when these are injured or killed and lateral buds develop the growth instead. Branches vary in showing an upright, drooping, or horizontal habit of growth, as we see them in the Lombardy poplar, weeping willow and tupelo, and within these divisions there are other contrasts of rigidity and flexibility, with differences of color and texture as well.