22,3 Reyes: Woods of the Philippine Dipterocarps 299 



cells appear as isolated rectangular or triangular elements, or in 

 short series of three or four with tapering terminals. In the 

 latter case the rows resemble superficially the septate fibers, 

 but differ in their simple pitting and in the nature of their 

 cross walls. 13 Their fibrous shape is indicative of the origin 

 of the series in the cambium from primordial cells of the same 

 type as give rise to vessel segments and tracheids. The features 

 that separate the vertical parenchyma and prosenchyma arise 

 subsequent to cell division in the lateral meristem and are a 

 result of normal development in the maturation of the elements. 



Both the end and the lateral walls of the parenchyma are 

 abundantly provided with simple pits which vary in contour, 

 depending on the nature of the neighboring elements, from 

 round to ovoid or deltoid. The individual cells measure, re- 

 spectively, 56 to 216 ix (average, 124) in length and 12 to 36 y. 

 (average, 24) in width. As in the case of the vessel segments, 

 it follows that the greater the diameter of the individual ele- 

 ments, the shorter the length. In Parashorea the lumina are 

 commonly filled with gummy infiltration products. 



Epithelial cells.— (Plate 2, figs. 4 and 5.) The epithelial cells 

 are found in immediate proximity to the resin cavities and, as in 

 the case of coniferous trees, are undoubtedly intimately con- 

 cerned in the production of resin. In macerated material they 

 usually occur in aggregates of a half dozen or more and appear 

 as quadrate or rectangular cells which grade off into typical 

 wood parenchyma. Their relation to the vertical parenchyma 

 is still further accentuated by the similarity of their walls in 

 thickening, pitting, and in their chemical nature. In Parashorea 

 they measure from 10 to 30 ,* in width and from 10 to 43 M m 

 length. 



Ray parenchyma.— (Plate 2, fig. 3.) Ray parenchyma, as the 

 term implies, is found in the wood rays and makes up the total 

 volume of these structures, as in all hardwoods. The ray- 

 parenchyma cells are very similar to the vertical parenchyma 

 in shape, and in the nature of their cell walls and in macerated 

 material are very difficult to distinguish from the latter. Only 

 in the case of the marginal ray cells, which assume the shape 

 of a triangular prism, can identification be positive » although 



walls, while those of the parenchyma are comparable 

 i of the vertical rows of parenchyma are pyramidal. 



