304 The Philippine Journal of Science 1923 



indication of the formation of a resin cavity is a fissure in the 

 tissue, which results from the splitting of the middle lamellae 

 between the four cells (Plate 5, fig. 1) and the subsequent 

 separation of the elements. The process may continue tangen- 

 tially (same plate, figs. 2 and 3) until other parenchyma cells are 

 involved and cavities of considerable extent are formed, or the 

 mature duct may have but the four epithelial cells, in which case 

 it is usually isolated. The resin cavities of Parashorea measure 

 from 30 to 130 /* tangentially and are surrounded by from four 

 to twenty secreting cells. 



In cross section the epithelial cells bordering the cavities dif- 

 fer little from the surrounding parenchymatous tissue. They 

 are somewhat arched on the distal face owing to the absence of 

 contiguous cells on that side and a resultant lack of tissue tension 

 but, in general, retain the tabular form of the neighboring paren- 

 chyma. Strictly speaking, the epithelium consists of but a sin- 

 gle layer of cells which lines the resin cavity and is followed pe- 

 ripherally by one or more layers of parenchyma, the first of which 

 is usually more or less transitional. The disparity in form and 

 appearance which distinguishes the epithelium from the border- 

 ing tracheids in conifers is not evident in dipterocarp woods. 



When observed in the longitudinal plane the wood of Para- 

 shorea assumes a very different aspect, since the vertical ele- 

 ments are now seen from the side and features appear which 

 are not evident in the transverse section. These may now be 

 discussed to advantage and should serve, when considered in 

 connection with those of the cross section, to render compre- 

 hensible the microscopic structure of Parashorea wood. Two 

 sorts of longitudinal section are possible; namely, one parallel 

 to the wood rays, known as the radial section, and one at 

 right angles to the rays and tangent to the growth rings, the 

 tangential. 



Radial section.— (Plate 6.) In thin radial section the compos- 

 ite character of the vessels is readily apparent, since they appear 

 as interstices or, where the radial walls chance to be included, as 

 punctate fields in the tissue which are bounded laterally by articu- 

 lated walls and extend longitudinally in the wood. Constrictions 

 occur at intervals and delimit the various segments, which vary 

 appreciably in length not only in different vessels but also, for 

 unaccountable reasons, in the same duct. The shorter segments 

 are wider than long and are often barrel-shaped, while the other 

 extreme is represented by the long cylindrical form. All grada- 



