114 



FORESTRY INVESTIGATIONS U S. DEPARTMENT OF AGRICULTURE. 



As usual in couifers, the tracheids are largest iu the roots and smallest in the limbs. In these 

 piiies, especially iu longleaf pine, they are larger in well-grown wood than in that of extremely 

 stunted trees, though very narrow rings in otherwise normal trees do not share this diminutive .size 

 of the tracheids. (See fig. 22, A and B, where a few very narrow rings are made up of elements of 

 normal size.) 



The following average figures illustrate the difference between wood from very stunted trees 

 and that of normal trees in longleaf pine, of which we give an average from aa extensive series 

 examined : 



As soon as the average width of the annual rings gets above 0.5 mm. the dimensions of the 

 elements approach the normal. Thus, in trees Nos. 1 and 2, with average width of annual rings 

 0.5 to 0.6 mm., the average diameter of the tracheids in radial direction is 35 to 48 /.i. 



Normally, the diameter in radial direction is greatest in the first-formed or inner part of any 

 ring, and decreases even before the summerwood is reached. In narrow rings with an abrupt 

 beginning of the summerwood, so common in these Southern pines, the diameter is quite constant 

 throughout the springwood, but changes, together with the thickness of the wall, quite suddenly 

 with the beginning of the summerwood, thus adding to the sharpness of the outlines of the two 

 parts. (See PI. XX; also fig. 22, B.) In nearly all sections there is an additional marked decrease 

 in radial diameter in the last 3 to 5 cells of each row, which helps to emphasize the limits of the 

 ring. In the so-called "false" rings, mentioned before, the cells of the false summerwood part 

 resemble those of the normal summerwood. The recognition of the false ring as such rests upon 

 the difference in shape and dimensions of the last cell rows in comparison with those adjoining. 

 In the true summerwood the last cells are much flattened, with small lumen and somewhat reduced 

 walls making a sharp definition toward the springwood of the next ring, which is still further 

 accentuated by the wide lumen and thin wall of the cells of the latter. In the "false" summer- 

 wood, 011 the contrary, the end cells are not flattened, and the cells of the light-colored adjoining 

 zone of wood have but a moderately wide lumen and comparatively thick walls. The fact that 

 the outline is less regular and 'commonly incomplete i. e., it does not extend around the entire 

 section also aids in recognizing the false rings. In the "lunes" of both limb and stem referred to 

 above the fibers are smaller, more rounded in cross section, and commonly exhibit conspicuous 

 intercellular spaces between them. The walls of these are often much thicker than those of the 

 summerwood of the same ring at this point. Since the radial diameter of the fibers of the summer- 

 wood is only about half as great as that of the springwood, it is clear that the number of fibers 

 of the summerwood forms a much greater per cent of the total number of libers than is indicated 

 in the per cent of summerwood given above and based upon its relative width. Thus, in wood 

 having 50 per cent of summerwood there are, in number, twice as many tracheids in the summer- 

 wood as in the springwood. 



The walls of the cells are generally about 3 to 3| ^ thick in the springwood, while in the 

 summerwood they are to 7 /< thick on the tangential side and 8 to 11 ft thick on the radial side 

 of the fiber. Generally it may be said that the thickness varies inversely as the extent of the 

 wall, i. e., the greater any diameter the thinner the walls- parallel to this diameter, which gives 

 the impression that each cell is furnished an equal quantum of material out of which to construct 

 its house and had the tendency of giving an equal amount to each of its four or six sides, 



Generally the absolute width of the ring does not affect the thickness of the cell walls, the 

 fibers of wide rings having no thicker walls than those of narrow rings; but when the growth of 

 a tree is unusually suppressed, so that the rings are less than 0.5 mm. (0.02 inch) wide and each 

 row consists of only a few fibers, the walls of the fibers of the summerwood, like those of the last- 



