22,3 Reyes: Woods of the Philippine Dipterocarps 303 



wood. As viewed in cross section they appear as small, thick- 

 walled elements of varying size - and narrow lumina, and are 

 grouped compactly in the areas not restricted to conduction and 

 storage. Libriform fibers arise from the same fusiform cambial 

 initials as vessel segments and tracheids but, as they mature, 

 they elongate and push in between one another, and thicken 

 their walls until only a narrow lumen remains. Owing to the 

 pressure resulting from their elongation, they assume an oblong, 

 rounded, or polygonal form in cross section. Here and there 

 a radial row consists of units of uniform size, indicating their 

 common origin from one initial in the lateral meristem. 



In places the intervals between wood rays are solidly banked 

 with libriform fibers and form extensive tracts of tissue whose 

 function is purely mechanical, but often the continuity is broken 

 by the pores with their envelope of tracheids and parenchyma 

 or by parenchyma alone. In the latter case the cells occur in 

 the midst of the libriform fibers as isolated units 23 which may 

 be identified by their thin walls, or in short tangential strings 

 that extend out laterally from the wood rays. Here and there 

 a cell bordering a ray enlarges somewhat and becomes modified 

 into an idioblast, which is conspicuous owing to the angular 

 crystal that it contains. The parenchyma of Parashorea is, in 

 the main, paratracheal, but a tendency toward the diffuse con- 

 dition is expressed through the invasion of the libriform tissue 

 by individual cells or tangential strings. 



As previously noted under microscopic features, the resin 

 ducts are borne in tangential rows, which occur at intervals 

 varying from a few millimeters to several centimeters, more 

 rarely solitary. At higher magnifications an affinity with the 

 larger pores is expressed, which is as yet unexplained since the 

 rows occur in areas where the vessels are numerous and bound 

 the resin cavities on either side. The resin cysts are inter- 

 cellular spaces of schizogenous origin, as in the Coniferae, and 

 are embedded in strings of parenchyma connecting the wood 

 rays. They originate in the cambium as areas of parenchyma- 

 tous cells, the innermost of which separate at the middle lamella 

 and pull apart as the tissue matures. In Parashorea the first 



22 The varying size is to be explained by the fact that the fibers are long- 

 attenuate and are cut at different heights in the cross section. 



-It follows that a living cell could not exist if totally surrounded 

 by prosenchyma (dead) tissue. Parenchyma cells, which m cross section 

 appear to be surrounded by libriform fibers, are in contact with ray pa- 

 renchyma above or below the plane of section. 



