364 STUDIES IN FOSSIL BOTANY 



in the interior of the primary wood are conveniently 

 termed mesarch ; those in the stem of Lyginodendron 

 may thus be described as mesarch collateral bundles. 

 This is a rare structure in recent plants, and, in fact, is 

 almost limited to the leaves and petioles of Cycadaceae. 

 The agreement between these foliar bundles and those 

 in the stem of Lyginodendron is wonderfully close, as is 



s.& 



Fig. 131. — Lyginodendron oldhamium. Part of transverse section of stem, showing a 

 primary xylem-strand, and adjacent tissues, /.r, protoxylem ; x, centripetal primary 

 wood ; Jtr 1 , centrifugal primary wood ; jr 2 , secondary wood ; r t medullary rays ; 

 p, pith; s.s., secretory sac. X 100. Phil. Trans., W. and S. Will. Coll. 1884. 



shown, for example, by the comparison of Fig. 132, 

 representing a leaf-trace of Lyginodendron, with Fig. 

 133, drawn from a petiole of the recent Stangeria. In 

 the stem of recent Cycads, however, the bundles are 

 normal (endarch), with the spiral elements on the inner 

 side of the wood. The occurrence of mesarch bundles 

 in the stem of Lyginodendron and some other fossil 

 plants, suggested a search for possible traces of the 



