i 3 4 STUDIES IN FOSSIL BOTANY 



parts of one and the same individual ; sometimes 

 (as in the larger stems of most species) the interior 

 of the vascular cylinder is occupied by a medulla, 

 around which the wood forms a continuous ring 

 (Figs. 56 and 58); sometimes the wood constitutes 

 a solid mass extending to the middle of the stele 

 (Figs. 59 and 60). In all cases, the wood of the stem, 

 whether solid or hollow, forms a perfectly continuous 

 cylinder or ring, and is not broken up, as in the higher 

 plants, into distinct vascular bundles. The xylem- 

 cylinder is surrounded by a ring of phloem (Figs. 57 

 and 60). From the outer border of the stele, numerous 

 leaf-trace bundles arise, passing out obliquely through 

 the cortex to the leaves, each of which receives a 

 single bundle (Figs. 56 and 59). The cortex is of 

 great thickness relatively to the vascular cylinder. Its 

 structure varies much according to the species, and 

 to the dimensions and age of the branch. 



The majority of the British species, in which the 

 anatomy is preserved, are known to have formed a 

 zone of secondary wood and bast, often of considerable 

 thickness, around the primary cylinder. In addition 

 to this growth of the stele, a still more extensive 

 development of secondary tissues went on in the 

 outer cortex, leading to the formation of a thick 

 periderm (Figs. 58 and 59), which seems to have 

 been produced in all the species, even in cases where 

 no secondary vascular tissues have so far been found. 

 The following is a list of the British forms, hitherto 

 described, of which the structure is known : — 



