26 WOOD AND FOREST. 



(3) Eunning radially (up and down in the picture) directly thru 

 the annual belts or rings are to be seen what looks like fibers. These 

 are the pith or medullary rays. They serve to transfer formative 

 material from one part of the stem to another and to bind the tree 

 together from pith to bark. 



(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 prac- 

 tically 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 bor- 

 dered, 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 inter- 

 woven 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. 



