i 4 8 -STUDIES IN FOSSIL BOTANY 



The outer cortex ends at the exterior of the stem in 

 the zone of the leaf-bases, which collectively cover 

 almost the whole surface. The tissue immediately 

 within the leaf-bases remained thin-walled, for it was 

 here that the phellogen arose. 



The leaf-trace bundles start, so far as their xylem 

 is concerned, directly from the angles of the primary 

 wood, and not between them, as in L. Harcourtii. 

 The leaf- traces are normally collateral, with xylem 

 directed inwards and phloem, consisting of elongated 

 narrow elements, outwards. The spiral elements, 

 where their position can be determined with certainty, 

 lie on the inner edge of the xylem, which was thus, as 

 a rule, endarch, another point of difference from L. 

 Harcourtii, where the structure in the corresponding 

 region was regularly mesarch} The soft bast is 

 bounded on the exterior by some elements with thicker 

 cell-walls, which may be most probably interpreted as 

 secretory sacs. The whole bundle is surrounded by a 

 well-marked sheath. 



Where the leaf-trace enters the denser outer cortex, 

 a large strand of delicate parenchyma — much larger 

 than the bundle itself — accompanies the latter on its 

 lower side, 2 and passes out with it into the leaf-base ; 

 here the parenchymatous strand forks into two, the 

 two branches diverging to the right and left of the 

 bundle. It is these strands of tissue which srive rise 



1 These are convenient terms for shortly characterising the development 

 of the wood of a vascular bundle. If the protoxylem lies on the inner side 

 the strand is endarch ; if in the middle of the xylem, mesarch ; if on its 

 outer side, exarch. 



2 Indicated by the gaps accompanying the leaf-trace bundles in Fig. 56, 

 A, from L. Harcourtii. 



