22,8 Reyes: Woods of the Philippine Dipterocarps 301 



by the encirling walls. As previously noted, they are either soli- 

 tary and assume a more or less circular contour, or in sequence 

 and contiguous in strings of two to five, and somewhat flattened at 

 the points of contact. 16 The diameter varies from 72 to 317 * 

 but as seen in mass the impression is that of uniform size since it 

 is only in the groups or in an occasional solitary cyst that the 

 dimensions are appreciably restricted. The coarse texture of 

 the wood of Parashorea malaanonan is traceable to the prevailing 

 large size of the vessels, which make up about 32 per cent of the 

 wood volume. 17 



Here and there the pores are occluded with tyloses which are 

 infiltrated with organic material and appear as vesiculate plugs 

 neighboring wood rays. As pointed out by other investigators, 

 these have their origin in the large pits that separate the ray 

 parenchyma from the vessels, and have an important bear- 

 ing on the durable qualities of the wood. 18 In Parashorea they 

 arise, as a rule, from the uniseriate rays, but are not sufficiently 

 abundant to be a conspicuous feature. 



Tyloses usually occur where wood rays are contiguous to 

 vessels and, although a conspicuous feature of heartwood, are 

 formed while the elements are still a part of the sapwood. They 

 are of great practical significance in the utilization of wood, 

 since they inhibit the movement of air and moisture in the 

 tissue, both of which are necessary for fungal growth. Hopea 

 plagata (Blanco) Vidal, Isoptera borneensis Scheffler, and 

 various species of Shorea have highly developed tyloses and are 

 among the most-durable dipterocarp woods, while the reverse 

 applies to Shorea eximia Scheffler and species of Pentacme and 

 Anisoptera.™ 



Coursing between the pores in a radial direction and making 

 up from 11 to 17 per cent of the wood volume 20 are bands 



"The tendency toward grouping in oblique lines, which is to be noted 

 with a pocket lens, is obscure at higher magnifications. 



17 The volumetric data were obtained from photomicrographs by cutting 

 out the elements concerned and comparing their weight to the total weight. 



18 The causes underlying the formation of tyloses are not well understood, 

 but it is generally conceded that their formation is due to heightened 

 osmotic pressure in the parenchyma bordering vessel segments which 

 results in the "enlargement" of the middle lamella. Cystlike structures 

 result in the vessel cavity, which may continue their growth until the lumen 

 is completely occluded. 



"See Gerry, Eloise, Tyloses; their occurrence and practical significance 

 in some American woods, Journ. Agr. Res. 1 (1914) 445-469. 



10 The ray volume varies directly as the density. See Myer, J. E., The 

 ray volumes of the commercial woods of the United States and their 

 significance, Journ. of Forestry, Washington, D. C. 20 (1922) 337-571. 



