32 CYCADALES [CH. 



wood of the cauline bundle. The term mesarch has in recent 

 years been applied to the cycadean type of bundle. A mesarch 

 bundle is, however, one in which centripetal and centrifugal 

 xylem are alike in origin, both being primary structures derived 

 from a desmogen strand. Typical mesarch bundles occur in 

 several recent ferns ; in the stele of the Osmundaceae, Gleichenia, 

 and other genera ; but in these plants the xylem is all produced 

 directly from one primary desmogen region and there is no 

 question of 'primary' and 'secondary' as in the two portions of 

 the xylem of a cycadean bundle. Recent researches into the 

 development of cycadean foliar bundles show that they do not 

 conform to the mesarch type as generally understood. A leaf- 

 trace at the base of a petiole (fig. 399, A) comprises centrifugal 

 xylem only, and this consists of regular rows of tracheids separated 

 by medullary rays ; in the lower part of the petiole the structure 

 is gradually modified, the centrifugal xylem is reduced and 

 the formation of centripetal xylem is initiated. At a higher 

 level (fig. 399, B) the centripetal xylem is in excess of the 

 centrifugal and the latter, for a time connected with the former, 

 eventually becomes separated by a few parenchymatous cells 

 from the protoxylem and persists as a small strand or arc of 

 tracheids. Fig. 399 illustrates stages in the transformation of 

 a typical collateral bundle, at the base of a Stangeria petiole, 

 into one in which the xylem is almost wholly centripetal at a 

 higher level in the axis of the frond. A cambium is present in 

 all : in B the centrifugal xylem is more or less clearly differentiated 

 into two portions, loosely arranged tracheids near the phloem, 

 and the more compact groups abutting on the centripetal xylem : 

 figs. C E show a further reduction in the centrifugal tracheids. 

 The conclusion drawn from developmental study is that the two 

 xylem portions of the bundle are independent in origin 1 . Marsh 

 has, however, shown that in Stangeria bundles near the base of 

 the petiole the centrifugal xylem consists of rows of secondary 

 tracheids and an inner portion not in rows which connects the 

 centrifugal with the centripetal elements ; this connecting portion, 

 he adds, is 'probably primary and connects up the Cycadean foliar 

 bundle with the truly mesarch bundle of the Cycadofilices.' 



1 Le Goc (14); Marsh (14). 



