504 STUDIES IN FOSSIL BOTANY 



Poroxylon. Each leaf-trace, on entering the stem from 

 a leaf, runs down through thirteen internodes, before 

 joining the trace of a leaf vertically below. Hence, in 

 any transverse section of the stem, the traces of thirteen 

 successive leaves are met with (see A, Fig. 181). 



The internodes are long, and as the leaf-trace tra- 

 verses the cortex in about half the length of an internode, 

 it follows that in any transverse section of the stem 

 not more than one outgoing trace (if any) is shown 

 (see Fig. 1 8 1 , A and B). Each leaf-trace, in the 

 upper part of its course, consists of a double bundle. 

 It will be remembered that in Lyginodendron also the 

 leaf-trace is a double one. In that genus, however, 

 the two strands of the trace unite, as we follow them 

 inwards through the peri cycle (see Fig. 129, p. 35 9); 

 in Calamopitys they unite in the same region (Fig. 

 176), whereas in Poroxylon they remain distinct for 

 some distance below their entry into the interior of 

 the stele. Thus, the two strands of primary wood 

 shown in Fig. 182 both belong to the same leaf- 

 trace, which (proceeding from the second leaf above) 

 has already taken up its position at the margin of the 

 pith. 



This figure also serves to illustrate the important 

 point that the development of the primary xylem of 

 the bundles was centripetal. At the level of the section 

 each of the twin-bundles has two protoxylem-groups 

 (j>x), separated by parenchyma from the secondary 

 wood on their { exterior side. The centripetal xylem 

 of each strand forms an arc, abutting, at its ends, on 

 the secondary wood. The whole structure is strikingly 

 like that in Lyginodendron (cf. Fig. 131, p. 364), except 



