314 The Philippine Journal of Science i^s 



While dipterocarp woods are diffuse, and diffuse in the sense 

 of being homogeneous in that the annual zones of extra-tropical 

 woods are lacking, they nevertheless exhibit minor variations 

 in pore alignment. For example, the vessels of Anisoptera, 

 Dipterocarpus, and Vatica are predominately solitary and show 

 little tendency toward grouping, while the majority of the mem- 

 bers of this family are characterized by groups of three or 

 more (as high as fifteen in certain species) which are con- 

 tiguous in short rows, or string out obliquely to the wood rays, 

 a condition that is typified by Parashorea. 



Tylosic ingrowths are always present and arise in the same 

 manner as do those of temperate-region woods; namely, from 

 pits leading to parenchyma proximate to the vessels. They 

 vary in prominence and are most copiously developed in the 

 harder and heavier species where they often completely occlude 

 the pores. Shorea balangeran, Isoptera bomeensis, Hopea 

 plagata, and Vatica are striking examples. On the other hand 

 tyloses are usually sparsely developed in the softer, lighter 

 species of the genera Shorea, Pentacme, Parashorea, Diptero- 

 carpus, and Anisoptera, the woods of which are among the most 

 perishable of the dipterocarps. 33 



The structure of the individual tyloses varies somewhat in 

 the different species and deserves mention here. In the ma- 

 jority of cases it takes the form of a thin-walled, more or less 

 globose cyst, which at low magnifications is somewhat irides- 

 cent. Where several such cysts originate about the wall of 

 the vessel at nearly the same height crowding ensues, resulting 

 in forms of irregular shape, and the vessel becomes solidly 

 packed at that point. As a rule the wall of the tylosis is thin 

 and cellulosic in character but may, in some species, as Vatica 

 mangachapoi Blanco, become more or less thickened, lignified, 

 and provided with simple pits. As has been pointed out by 

 other workers, the presence of tyloses is indicative of increased 

 durability, since they inhibit the movement of air and moisture 

 in the wood, thus restricting fungal growth. 34 



"In species of Dipterocarpus although tyloses frequently occur in the 

 pores they do not completely block the passage of air and liquids, as may 

 be seen by blowing smoke through a section of wood. This fact is un- 

 doubtedly responsible for the facility with which wood of Dipterocarpus 

 species takes preservative treatment. 



•* This is true of the majority of dipterocarps, but it does not seem to 

 hold good in the case of Pentacme contorta and Dipterocarpus grandiflorus. 

 Pentacme has a more highly developed tylosis, but Dipterocarpus is the 

 more durable of the two. 



