WOOD OF SOUTHERN PINES. 115 



formed 2 or 3 fibers of normal rings, are thinner, so tbat 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 summerwood have thinner walls. (See fig. 22, 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 v pits, 

 the structure of which is shown in fig. 20. 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. (PL XXII, C.) 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 PL XX, B, d and e; PL XXIII, D, d and E, a.) In 

 both the essential part of 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, PL XXIII, D, and E.) In keeping with, the small radial diameter of 

 the fibers of the summerwood, these pits are much smaller in the summerwood than springwood, 

 and usually are very mucli fewer in number. 



The simple pits are in sets and occur only at the points where the fiber touches the cells of a 

 medullary ray. (See iig. 21, also PL XXIII, E, sp., and other figures of this plate and PL XXIY.) 

 Above and below these simple pits occur very small bordered pits, communicating with, those of 

 the short tranverse fibers or tracheids which form part of all medullary rays. (See PL XXI, 

 D, b. p.) 



As in all pines, the medullary or pith rays are of two kinds, the one small, 1 cell wide, and 1 

 to 10 — in large averages 5 to 7 — cells Mgh; the other large, and each containing in the middle part 

 a transverse resin duct. (See Pis. XXI, XXIII, XXIV, and XXVI.) Of the former there occur 

 about 21 to 27 on each square millimeter (about 15,000 per square inch) of tangential section. The 

 second class are much less abundant and scattered very irregularly, so that sometimes areas of 

 several square millimeters are found without any of these rays. Generally about one of these 

 rays occurs to every 1.5 or 2 square millimeters, or about 300 to 400 per square inch of tangential 

 section. In all rays the cell rows forming the upper and lower edge (see PL XXI) are composed 

 of short fibers or tracheids (transverse tracheids), while the inner rows contain only parenchyma 

 cells. Occasionally small rays occur which are composed of tracheids only. (See PL XXI, C.) 

 Frequently the rows of parenchyma are separated by one, rarely by two, series of tracheids (see 

 PL XXIII, D, and PL XXIV, D), giving rise to u double" or "triple*' rays. 



The number of cell rows in each medullary or pith ray varies from 2 to 10, on an average from 

 5 to 7, and of these the rows of tracheids or fibers form more than half. (See PL XXV, where 

 the outer cells or tracheids are marked with dots.) 



The tracheids of the rays have thick walls covered with point-and-bar-like projections, the 

 boldest of which are on the upper and lower walls and surround the bordered pits. (See Pis. 

 XXI and XXII.) These short tracheids communicate with the common wood fibers, with each 

 other, as well as with the parenchyma cells, by means of small bordered pits, which in this last 

 case are bordered on one side (side of the tracheid) and simple on the other (half-bordered pits). 

 The parenchyma cells occupying the inner rows of each ray communicate in the springwood part 

 of the ring with each neighboring tracheid by 3 to 6, commonly 4 to 5, simple elliptical pits, in 

 the summerwood by a single narrow, elongated slit-like pit (see Pis. XXI and XXII), and with 

 each other by small, irregular, scattered simple pits. 



The walls of these cells are generally smooth, but local thickenings, especially on the upper 

 and lower walls, and surrounding the pits, occur quite frequently, though not regularly. 



The parenchyma cellslof the rays are usually somewhat broader and higher than the fibers, 

 the average height for both being about 21 to 27 ja, the average width about 20 /*, while the length 

 of each cell and fiber, greater in springwood and least in the summerwood, is from two to ten times 

 as great as the height. Assuming 25 p and 20 jjl to represent the average height and width, and 

 allowing 25 rays of 6 cell rows each to each square millimeter of tangential section, then the rays 

 form about 7.5 per cent of the total volume and weight of the wood of these species. An attempt 



