SPECIMENS OF FOSSIL WOODS IN NATAL MUSEUM. 378 



verse section seems to be caused by the cross-sections of the 

 longitudinal fibrous elements. 



With crossed prisms (fig. 1-5, b.) the section remains fairly 

 bright. In many parts it is traversed by narrow light and 

 dark bands which run across the remains of the medullary 

 rays, but not necessarily at right angles to them. The direc- 

 tion, however, tends to be more or less radial. 



We will now trace these changes in the tangential section. 



PI. XXVI, fig. 16 represents the least altered wood, Avhere 

 the cavities of the wood fibres are seen to be more or less 

 occupied by the yellow resin-like substance which appears in 

 the form of yellowish brown streaks (y). The outlines of the 

 wood fil)res are indistinct, except iu the case of the especially 

 silicified fibres, as at d.f. The thylose cells {fJi.) and the 

 wood parenchyma {icp.) are little altered. The cavities of the 

 medullary cells are partially filled. In fig. 17 the condition 

 is shown under greater magnification. Here the outlines of the 

 wood fibres are distinct, and an especially silicified fibre ('?./.) 

 is shown. In fig. 18 the medullary cells have become com- 

 pletely filled, and the outlines of the wood fibres have almost 

 completely disappeared. These changes have continued further 

 in fig. 19, where the yellow matter lias extruded out of the 

 oi-iginal cavities into the general matrix {m.) formed by the 

 coalescence of the fibre walls. A well-defined gland is shown 

 in the medullary ray at g. The condition is seen under greater 

 magnification at fig. 20. A tendency for a vertical striation 

 is now observable in the general matrix. In fig. 21 the yellow 

 substance is becoming dispersed in the general matrix which 

 as a whole is now becoming yellower and less distinct from 

 the yellowish brown matter which originally filled the 

 cavities of the elements. The vertical striation is more marked 

 than before, but occasional silicified fibres can be seen. 



In fig. 22 the medullary i-ays {r.m.r.) are becoming less 

 distinct, and the final result of the transformation is shoAvn 

 in fig. 23, A. Here the vertical striation is very evident. Micro- 

 scopic cracks {m.c.) and narrow lenticular cavities {I.e.) are 

 present. With crossed prisms (fig. 23, b) the vertical stria- 



