(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.
Fig. 18.