DESCRIPTION OF WOODY TISSUES. 151 



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 nun., the average diameter of the tracheids in radial direction is 35 to 48 p. 



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, tlie diameter is quite con- 

 stant throughout the spriugwood, 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 I'l. XXI; also fig. 18, 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 adjoin- 

 ing. 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" 

 summerwood, on 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 

 ar Mind 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 snnunei 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 

 fibers 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 

 tiacheids in the summerwood as in the springwood. 



The walls of the cells are generally about 3 to 3 /u thick in the springwood, while in the 

 summerwood they are G to 7 /.i thick on the tangential side and 8 to 11 n 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 alfect 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- 

 formed 2 or 3 fibers of normal rings, are thinner, so that in these cases the wood is lighter in 

 color and weight not only because there is relatively less summerwood, but also because the 

 fibers of this summeiwood have thinner walls. (See fig. 18, A and B.) In very stunted trees, 

 where the rings are all very narrow, the reduced thickness of the walls is counterbalanced by 

 the smaller size of the cells. 



All tracheids communicate with each other by means of the characteristic "bordered" pits. 

 the structure of which is shown in fig. 10. These pits occur only on the radial walls of the fibers, 

 They are most abundant near the ends of each fiber, fewest in the middle, form broken rows, 

 single or occasionally double. (PI. XXIII, 0.) As in other pines the pits of the summerwood differ 

 in appearance from those of the springwood. In the latter the pit appears in the cell lumen 

 (radial view) as, a perforated saucer-like eminence; in the former as a mere cleft, elongated in the 

 direction of the longer axis of the fiber. (See PI. XXI, B, d and e; PI. XXIV, D, d and E, a.) In 

 both the essential part ot the pit is similar, a circular or oval cavity resembling a double convex 

 lens, with a thin membrane dividing it into two equal plano-convex parts. (This membrane is 

 shown only in the drawings, PI. XXIV, D and E.) In keeping with the small radial diameter of 



