22,3 Reyes: Woods of the Philippine Dipterocarps 315 



Tracheids are present in the majority of the dipterocarp 

 woods and exhibit the same appearance and disposition as in 

 Parashorea. They vary in number from several to a dozen or 

 more and are always confined to the immediate vicinity of the 

 vessels where, in conjunction with the vasicentric parenchyma, 

 they form areas of lighter tissue about the pores. 



Among the species examined tracheids are most numerous 

 in Parashorea malaanonan and Isoptera borneensis, where as 

 many as thirty are sometimes present in a group. The opposite 

 extreme is found in Vatica, Balanocarpus, and species of 

 Hopea, in which tracheids are conspicuous by their absence. 

 The same applies to Anisoptera and Dipterocarpus, but in these 

 genera the tracheids are replaced by an intermediate type of 

 element, the fiber tracheid (see footnote 32). The transition 

 from tracheids to libriform fibers is always abrupt, since ver- 

 tical parenchyma usually intervenes; fiber tracheids, on the 

 contrary, usually grade into libriform fibers gradually. 



The libriform fibers make up the background of the wood; 

 they represent the ultimate development of mechanical tissue 

 and consist of thick-walled, long-attenuate cells (sixty to eighty 

 times as long as wide) which occur en masse and take very 

 little, if any, part in the movement of solutes in the tree. Ex- 

 tremes of variation are found in Hopea plagata (15 by 1,300 /x), 

 Vatica (18 by 1,340 fi), and Balanocarpus (18 by 1,380 *0, 

 which represent the minimum, in contrast to Dipterocarpus 

 vernicifluus (27 by 1,790 l x), Shorea teysmanniana (31 by 1,370 

 p.) , and other species of Shorea, Pentacme, and Parashorea. In 

 addition a curious relation was found to exist between fibers 

 and pores; in small-pored woods the fibers were invariably of 

 more-restricted dimensions than in woods of coarser texture. 



Libriform fibers fluctuate not only in dimensions-over-all in 

 the various species, but likewise in the thickness of the cell 

 wall. It follows obviously that the latter varies in inverse 

 ratio to the thickness of the cell lumen and that the density 

 of the wood is directly proportional. The relatively light and 

 soft species of Shorea, Pentacme, and Parashorea are character- 

 ized by thin-walled libriform tissue, while the reverse applies 

 in such hard and heavy species as Shorea balangeran, Hopea 

 plagata, and Vatica mangachapoi. 



The vertical parenchyma of dipterocarp wood is variable in 

 the different genera and species, both in amount and in dis- 

 tribution. Two types are prevailingly present in all members 

 of the family, the vasicentric and the diffuse. The former, 



