22,3 Reyes: Woods of the Philippine Dipterocarps 305 



tions between these types occur, since the rule covering cell 

 dimensions likewise applies in this instance: vessel-segment 

 width is inversely proportional to vessel-segment length. 

 Where the section is not median to the duct, cross walls appear 

 to separate segments owing to the constriction of the latter in 

 the region of the pores, an illusion which is readily apparent, 

 however, when thicker sections are prepared. 



As previously pointed out, the light areas that bound the 

 vessels laterally consist of tracheids and parenchyma or paren- 

 chyma alone. The characters of these two types of cells are 

 now seen to better advantage, and they can be separated with- 

 out any trouble. The parenchyma, which is always associated 

 with the vessels, appears as rectangular cells several times 

 longer than wide, but much shorter than the neighboring vessel 

 segment. The tracheids, where present, are contiguous to the 

 duct, are much longer than the parenchyma cells, and are 

 characterized by rounded ends. Evidence of their origin from 

 the same cambial initials as vessel segments is to be found in 

 the fact that they approximate the latter in length, and transi- 

 tion forms between the two types are not uncommonly seen 

 at this magnification. As in the case of the vessels, the walls 

 appear punctate from the numerous bordered pits. 



Crossing the vessels at right angles are the wood rays, which 

 appear as broad bands of muriform parenchyma consisting of 

 many tiers of superposed cells whose lumina contain reddish 

 brown organic material of a gummy nature or, occasionally, 

 crystals. In Parashorea the rays are of the type designated as 

 heterogeneous, since the last series of the upper and the lower 

 margins are higher and shorter than the others, although ex- 

 hibiting similar pitting and content. In addition, rows of high 

 cells are occasionally interspersed in the ray, but as a rule the 

 interior appears quite homogeneous, consisting of seried ranks 

 of quadrangular cells. 



Owing to the nature of secondary thickening in trees the rays 

 pursue in general a radial course in the wood but, as previously 

 pointed out, are deflected more or less at the vessels, particularly 

 the uniseriate rays. Consequently, in sections that are strictly 

 radial and of necessity median here and there to ducts, the rays 

 are interrupted by the vessel cavities but continue again on the 

 other side. This results in a series of ray flecks extending across 

 the grain of the wood which have originated from one ray. 

 Furthermore, since new rays arise in response to necessity as 



