178 STUDIES IN FOSSIL BOTANY 



foliar spiral', are seen in transverse section. The structure 

 of these bundles, which is sometimes admirably preserved, 

 is normally collateral. The smallest spiral tracheae lie 

 at about the middle of the xylem-strand, as in Lepido- 

 dendron Harcourtii. The xylem is surrounded by a 

 layer of delicate parenchyma, which on the outer side 

 separates it from the phloem. After passing through 

 the inner cortex, the whole bundle is further surrounded 

 by a parenchymatous sheath, continuous with the inner 

 cortical tissue. In this condition it passes through the 

 middle zone of the cortex, where the leaf-trace bundles, 

 with their sheaths, are usually the only tissues pre- 

 served. 



In Lepidostrobus Brownii, where the middle cortex 

 is partly preserved, the leaf-trace bundles are connected 

 with the surrounding tissues by trabeculae, analogous 

 to those radiating from the steles in Selaginella. The 

 leaf-traces of Lepidostrobus closely resemble those of 

 living Lycopods in structure. 1 



On reaching the outer cortex, where the bundles 

 gradually assume a more horizontal course, the same 

 structure is preserved. Here the sheath, immediately 

 surrounding the vascular strand, has less thick walls 

 than the enveloping cortical tissue. In this region a 

 gap in the tissue constantly appears below each bundle, 

 and accompanies it outwards into the sporophyll. 

 There can be no doubt that this gap was in nature 

 occupied by a delicate parenchyma (sometimes pre- 

 served in other species), continuous with that of the middle 

 cortex, and that this tissue, accompanying the leaf-trace, 



1 See Bower, " On the Structure of the Axis of Lepidostrobtis Brownii" 

 Annals of Botany, vol. vii. 1893. 



