142 TIMBER PINES OF THE SOLTHEKN UNITED STATES. 



thf fibers of the snnuneiwood, these pits are much smaller in the siiiiiiiierwood than sjtrintjwood, 

 anil usually are very nuieli fewer in number. 



The simple pits are in sets and oceur only at the points where the iiber touches the cells 

 of a medullary ray. (See lig. 17, also PI. XXIV, E, .sji., and other figures of this plate and 

 PI. XXV.) Above and below these simple pits oceur very small bordered pits, communicatin}^ 

 with those of the short transverse fibers or tracheids whicli form ])art of all medullary rays. 

 (See PI. XXII. I). />.;>.) 



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

 1 to 10 — in large averages ~> to 7 — cells high; the other large, and each containing in the middle 

 part a transverse resin duct. (See Pis. XXII, XXIII, XXV, and XXVII.) Of the former there 

 occur about 21 to 27 on each si|uare millimeter (about 1.5,000 per square inch) of tangential 

 section. The second class are much less abuiidant 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 

 I'l. XXIII) 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 PI. XXII, C.) Frequently the rows of i)arenchyma are separated by one, rarely by two, 

 series of tracheids (see PI. XXIV, D, and PI. XXV, D), giving rise to "double" or "triple" rays. 



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

 5 to 7, and of these the rows of tracheids or fibers form more than lialf. (See PI. XXVI. 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. XXII 

 and XXIII.) 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 ])art of 

 the ring with each neighboring tracheid by 3 to 0, commonly 4 to .5, simple elliptical pits, in the 

 summerwood by a single narrow, elongated slit like pit (see Pis. XXII and XXIII), and with each 

 other by small, irregular, scattered simple pits. 



The walls of these (;ells are generally smooth, but local thickenings, especially on the upper 

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



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

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

 of each cell and fib(u-. greater in springwood and least in the summerwood, is from two to ten times 

 as great as the height. Assuming 25 f.i and 20 n to represent the average height and width, and 

 allowing 25 rays of 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 

 to utilize for purposes of identification the difference in the number, size, and distribution of 

 these rays, or the jtroportion between the number of rows of tracheids and those of parenchyma 

 cells, as was done by Dr. J. Schroeder,' has not betiu successful, and ajqiears of little i)romise. 



The large rays with transverse resin ducts resemble the smaller rays described. On PI. XXV 

 at A such a ray is seen both in radial and tangential section. Series of tiansverse tracheids 

 occupy the upper and lower edge, but the interior, unlike that of common rays, is several 

 cells wide, and contains an open duct in its widest portion. (See PI. XXVll, r.fh) This duct 

 is commonly more or less filled with resin (see PI. XXVII, E); it is .surrounded by thin- walled 

 secreting cells, and, in the heart wood, often divided or filled up by thylosis, i. e., by very thin 

 walled, much puffed out cells, growing out of the surrounding secreting cells before the latter 

 perish. 



The walls of the secreting cells are quite thin, those of the remainder of the jjarenchyma 

 vary to some extent in the different species. In the Longleaf and I^oblolly Pines the walls of the 

 Ijarenchyma composing the jirincipal part of the ray arc generally quite thick (see PI. XXVll, A-E), 



' Dr. J alius Schroeder, Das Holz der Coniferen, Dresden, 1872. 



