298 The Philippine Journal of Science w* 



segment Here and there the radial walls are latticelike or 

 reticulate in appearance, owing to an aggregation of the larger, 

 semibordered pits where a wood ray came in contact with the 

 vessel. The ends of the segments frequently exhibit more 

 abundant pitting, because the tips of neighboring cells (tracheids 

 of the length of the segment) interlace in such a manner as to 

 bring the maximum number in contact with the wall of the 

 vessel. 



Tracheids.— (Plate 2, figs. 10 and 11.) Tracheids are prosen- 

 chymatous elements whose primary function, like that of the ves- 

 sels, is conduction. When isolated they appear as slender, 

 elongated, generally twisted cells with rounded ends and lateral 

 walls, which are abundantly equipped with numerous, small, bor- 

 dered pits leading to neighboring tracheids or vessel segments. 

 Occasionally the larger, semibordered pits similar to those of ves- 

 sel segments are found on tracheid walls where the latter are 

 contiguous to vertical parenchyma of wood rays. The tracheids 

 of Parashorea measure from 352 to 800 ^ (average, 420) in 

 length and 14 to 30 /x (average, 24) in width and, as will be 

 pointed out subsequently, arise from the same cambial initials 

 as the vessel segments. In contrast to the latter, however, their 

 walls are about half as thick and are comparable to those of 

 the parenchyma cells. 



Libriform or wood fibers.— (Plate 2, fig. 1.) The final type 

 of prosenchyma in the wood of Parashorea, the libriform 

 fibers, 12 are longer, thicker walled, and more attenuated than 

 the tracheids and are to be regarded as typical mechanical 

 elements. They measure from 1,130 to 2,390 /* (average, 1,690) 

 in length and 14 to 31 ^ (average, 22) in width, with walls 3.5 

 to 7.5 jx in thickness, and taper gradually from the center. As 

 a rule, the ends are smooth but occasionally become serrated or 

 forked where the fibers are pressed together. Owing to the 

 mechanical nature of these cells, the pits on the vertical walls 

 have degenerated into slitlike openings, which extend vertically 

 in the cell wall and appear in the mature elements as more or 

 less vestigial structures. Gummy deposits sometimes leach 

 through such pits from contiguous parenchyma and partially 

 occlude the lumina. 



Vertical parenchyma (wood parenchyma proper). — (Plate 2, 

 figs. 6 and 7.) In macerated material the vertical parenchyma 



