366 STUDIES IN FOSSIL BOTANY 



same structure in the stems of recent Cycads, and the 

 search was successful, for though no mesarch bundles 

 have as yet been found in the vegetative stems, yet 

 they occur distinctly in the peduncles of the cones in 

 several Cycads, of the genera Stangeria, Bowenia, Zamia, 

 Ceratozamia, and Dioon. In Stangeria and Dioon espe- 

 cially, the centripetal part of the xylem in these organs 

 is still well developed, though in the others it is more 

 reduced. 1 In these cases, then, the floral axis appears 

 to have retained primitive characters, as is also shown 

 in the simple course of its vascular bundles. 



The secondary wood of Lyginodendron oldhamium 

 forms a continuous ring, except where it is interrupted 

 by the outward passage of a leaf-trace (Fig. 129). 

 The fascicular and interfascicular cambium must have 

 sprung into activity almost simultaneously (Fig. 135). 

 The medullary rays vary much, both in height and 

 width ; they are sufficiently developed to have given 

 the wood a somewhat soft parenchymatous character, 

 approaching, that of recent Cycadaceous woods. The 

 tracheides are pitted on their radial walls only ; the 

 pits are multiseriate and bordered (cf. Fig. 157, p. 406, 

 from Heterangiuvi). Beyond the cambium (Fig. 134, cb), 

 we come to the secondary phloem {ph^). Here the 

 medullary rays become more or less dilated, breaking 

 up the phloem proper into narrow strands, each of 

 which is made up of alternate tangential bands of large 

 and small elements ; the former were probably the 

 sieve-tubes (Fig. 134). Where a leaf-trace bundle 



1 Scott, "The Anatomical Characters presented by the Peduncle of 

 Cycadaceae," Annals of Botany, vol. xi. 1897, p. 399. F. W. South and 

 R. H. Compton, "The Anatomy of Dioon edule" New Phyiologist, vol. 

 vii. 1908, p. 222. 



