OF LOWER GREENSAND PLANTS. 97 



with blackened contents, which suggest resin. Some of these 

 have rather thicker walls than the rest of the tissue, and may 

 represent somewhat specialised " resin -fcracheids." 



The size and frequency of the medullary rays are both rather 

 greater than the average in Coniferous woods (PI. IV). lu 

 transverse section, the tangential diameter of the ray-cells is 

 often greater than that of the adjacent tracheids, and as these 

 cells are largely filled by blackened contents they are very con- 

 spicuous. The rays are all uniseriate (PL IV, fig. 1), except 

 those which contain the transversely running resin-canal (PI. V, 

 fig. 1, r.c.). 



No evidence of Irancldng, or of the exit of short shoot traces, 

 is found in the sections. 



DETAILS OF ELEMENTS. Secondary wood only is present. The 

 tracheids average about 24 x 20 ^i to 30 x 25 /u in diameter. 

 The walls are somewhat rounded off, adjacent rows of 

 tracheids not lying strictly on the same tangent, but slightly 

 alternating, so as to fit the rounded walls into each other. 

 The walls are not excessively thickened, and there is no 

 tendency for the lumen to be obliterated, even in the outer 

 zones of autumn wood. In transverse section the pitting is 

 not a salient feature, though some of the autumn tracheids 

 show tangential pits. In radial section the bordered pits 

 are large (as wide as the tracheid), circular, and generally 

 isolated by a distance equal to or greater than their own 

 diameter (text-fig. 23). In a few cases, faint remains of 

 " Sanio's rims " are visible. 



Wood-parenchyma forms noticeable patches round the resin- 

 canals. Some of the isolated elements containing blackened 

 contents scattered in the main body of the wood may be 

 parenchyma, but the petrifaction makes it a little uncertain. 

 Most of these elements are certainly thick-walled. The par- 

 enchyma elements round the resin-canals are larger in diameter 

 than the average tracheid (PI. IV, fig. 2, r.c.a.) ; their walls, as 

 well as their contents, are often blackened. I have not been 

 able to detect any specialised pitting. The cells are irregular 

 in outline, and are grouped round the resin-canals so as com- 

 pletely to connect those near each other. The cells are 

 elongated in a vertical direction, and their transverse walls 

 are at right angles to the vertical walls. 



H 



