26 WOOD AND FOKEST. 



(3) Eunning radiallj' (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 ]iart of the stem to another and to Innd tlie tree 

 together from pith to bark. 



(4) Scattered licie and thci'e 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 wldte 

 pine. Fig. 18.) They are not cells, hut openings between cells, in 

 which the resin, an e.xci'etion 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 noti(.-c is the straightness of the long cells 

 and their overlapping where tliey meet endwise, like the ends of two 

 chisels laid together. Fig. 11. 



(6) On tlie walls of tlie 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 I'cmain thin and delicate. 

 The pit in a cell wall always coincides with the pit in an adjoining 

 cell, there Ijeing only a thin membrane between, so that there is prac- 

 tically free connnnnication of fluids between the two cells. In a 

 cross-section the pit appears as a canal, tlie length of which depends 

 upon the thickness of the walls. In some cells, the thiclcening around 

 the pits becomes elevated, forming a border, perforated in the center. 

 Such pits are called bordered ])its. These pits, both sini]ile and bor- 

 dered, are waterways between the different cells. They are helps in 

 carrying the sap up the tree. 



(7) The pith rays arc also to lie seen running across and inter- 

 woven in the other cells. It is to he noticed that they consist of 

 several cells, one above another. 



In the tangential section. Fig. IS: 



(8) The straightness and overhqiping of the cells is to be seen 

 again, and 



(9) '^I'lie numerous ends of the ]iith ravs appear. 



In a word, tlie structure of coniferous wood is very regular and 

 simple, consisting mainly of cells of <uie sort, the pitli rays being 

 comparatively iinnoticeable. This nnifonuitv is wbnt makes the wood 

 of conifers technicallv valnalile. 



