The general structure of the stem may be understood by making a series of cross-sections through branches of different ages. In all, three regions are distinguishable; viz., an outer region (bark or cortex) (Fig. 76, _A_, _c_), composed in part of green cells, and, if the section has been made with a sharp knife, showing a circle of little openings, from each of which oozes a clear drop of resin. These are large resin ducts (_r_). The centre is occupied by a soft white tissue (pith), and the space between the pith and bark is filled by a mass of woody tissue. Traversing the wood are numerous radiating lines, some of which run from the bark to the pith, others only part way. These are called the medullary rays. While in sections from branches of any age these three regions are recognizable, their relative size varies extremely. In a section of a twig of the present year the bark and pith make up a considerable part of the section; but as older branches are examined, we find a rapid increase in the quantity of wood, while the thickness of the bark increases but slowly, and the pith scarcely at all. In the wood, too, each year's growth is marked by a distinct ring (_A_ i, ii). As the branches grow in diameter the outer bark becomes split and irregular, and portions die, becoming brown and hard. The tree has a very perfect root system, but different from that of any pteridophytes. The first root of the embryo persists as the main or "tap" root of the full-grown tree, and from it branch off the secondary roots, which in turn give rise to others. The sporangia are borne on special scale-like leaves, and arranged very much as in certain pteridophytes, notably the club mosses; but instead of large and small spores being produced near together, the two kinds are borne on special branches, or even on distinct trees (_e.g._ red cedar). In the Scotch pine the microspores are ripe about the end of May. The leaves bearing them are aggregated in small cones ("flowers"), crowded about the base of a growing shoot terminating the branches (Fig. 77, _A_ ♂). The individual leaves (sporophylls) are nearly triangular in shape, and attached by the smaller end. On the lower side of each are borne two sporangia (pollen sacs) (_C_, _sp._), opening by a longitudinal slit, and filled with innumerable yellow microspores (pollen spores), which fall out as a shower of yellow dust if the branch is shaken. The macrosporangia (ovules) are borne on similar leaves, known as carpels, and, like the pollen sacs, borne in pairs, but on the upper side of the sporophyll instead of the lower. The female flowers appear when the pollen is ripe. The leaves of which they are composed are thicker than those of the male flowers, and of a pinkish color. At the base on the upper side are borne the two ovules (macrosporangia) (Fig. 77, _E_, _o_), and running through the centre is a ridge that ends in a little spine or point.