74 PTERIDOSPERMEAE [CH. 



of Lyginopteris is replaced by a solid xylem-cylinder consisting 

 mainly of groups of large tracheids, reaching -3 mm. in diameter, 

 with multiseriate bordered pits (fig. 411, D) embedded in an 

 anastomosing parenchymatous tissue-system. In the stele repro- 

 duced in fig. 411, B, which with the exception of a very narrow 

 zone of secondary xylem, x 2 , consists entirely of primary xylem, 

 x l , the parenchyma is represented by a darker reticulum (cf. 

 fig. 415, B) dividing the metaxylem into islands as in Gleichenia. 

 In the peripheral portion of the xylem the tracheids are rather 

 narrower and arranged in more definite groups in many of which 

 is a single strand of narrow spiral elements (fig. 411, A', px) close 

 to the outer margin. These peripheral primary bundles in which 

 protoxylem is recognisable may be described as leaf-traces of 

 mesarch structure consisting of centripetal xylem and, to a much 

 less extent, of smaller centrifugal elements for the most part with 

 dense spiral bands in place of the multiseriate pits of the rest of 

 the metaxylem. The structure of these leaf-traces is practically 

 identical with that of the primary bundles of Lyginopteris. There 

 is, however, a difference to which attention is drawn by Williamson 

 and Scott. While in Lyginopteris in any transverse section the 

 primary bundles in the stele are equal in number to the leaf- 

 traces in the pericycle and cortex, in Heterangium the peripheral 

 groups in the stele may be as many as twenty, a number consider- 

 ably in excess of the leaf-traces beyond the limits of the primary 

 xylem of the stele. It may be that the leaf-trace of each leaf, 

 which joins the stele at a distance of 6 10 internodes below its 

 entrance into the cortex from the leaf-stalk, may branch in its 

 descent in the axial region, or some of the primary groups of 

 xylem may be confined to the axial region and independent of 

 the leaf -traces. Portions of the peripheral region of the stele 

 may be occupied by metaxylem groups without protoxylem and 

 identical with those which make up the bulk of the metaxylem. 

 Scott 1 has recently published a note in which he states that 

 most of the British Coal Measures Heterangiums were polydesmic. 

 Two bundles, and not a single strand as in the Scotch H. Grievii, 

 leave the stele for each leaf, and these divide into four, in some 

 cases at least, before entering the petiole. 



1 Scott (15). 



