(4) Scattered here and there among the regular cells, are to be seen irregular gray or yellow dots which disturb the regularity of the arrangement. These are _resin ducts_. (See cross-section of white pine, Fig. 18.) They are not cells, but openings between cells, in which the resin, an excretion of the tree, accumulates, oozing out when the tree is injured. At least one function of resin is to protect the tree from attacks of fungi. Looking now at the radial section, Fig. 18: (5) The first thing to notice is the straightness of the long cells and their overlapping where they meet endwise, like the ends of two chisels laid together, Fig. 11. (6) On the walls of the cells can be seen round spots called "pits." These are due to the fact that as the cell grows, the cell walls thicken, except in these small spots, where the walls remain thin and delicate. The pit in a cell wall always coincides with the pit in an adjoining cell, there being only a thin membrane between, so that there is practically free communication of fluids between the two cells. In a cross-section the pit appears as a canal, the length of which depends upon the thickness of the walls. In some cells, the thickening around the pits becomes elevated, forming a border, perforated in the center. Such pits are called bordered pits. These pits, both simple and bordered, are waterways between the different cells. They are helps in carrying the sap up the tree. (7) The pith rays are also to be seen running across and interwoven in the other cells. It is to be noticed that they consist of several cells, one above another. In the tangential section, Fig. 18: (8) The straightness and overlapping of the cells is to be seen again, and (9) The numerous ends of the pith rays appear. In a word, the structure of coniferous wood is very regular and simple, consisting mainly of cells of one sort, the pith rays being comparatively unnoticeable. This uniformity is what makes the wood of conifers technically valuable. [Illustration: Fig. 18.] The cells of conifers are called tracheids, meaning "like _tracheæ_." They are cells in which the end walls persist, that is, are not absorbed and broken down when they meet end to end. In other words, conifers do not have continuous pores or vessels or "_tracheæ_," and hence are called "non-porous" woods.