CYCADOFILICALES 15 



been observed. Scott (24) remarks that " it is certainly a fact of 

 great interest that the roots of our fossil, which in their ])rimary 

 condition much resemble those of marattiaceous ferns, or of Helmin- 

 Ihostachys among 0])hioglossaceae, should exhibit a mode of second- 

 ary tissue-formation absolutely indistinguishable from that of tyj^ical 

 dicotyledons or gymnosperms." At that time it was not realized 

 that L. Oldhamium was a gymnosperm. There are cases in which 



Fig. 12. — Lyginodendron Oldhamium: part of transverse section of stem showing 

 a primary xylem strand and adjacent tissues; px, protoxylem; x, centripetal primary 

 wood; x", centrifugal primary wood; x^, secondary wood; r, medullary rays; />, pith; 

 ss, secretory sac; Xioo. — After Williamson and Scott (18). 



the growing point has been preserved so perfectly that it is evident 

 there was no such definite apical cell (87) as characterizes most fern 

 roots. 



Megaloxylon. —This stem genus occurs in the British Lower 

 Coal-measures, and has been described by Seward (21). The 

 vascular cylinder only is preserved, being 4 to 5 cm. in diameter, and 

 is a protostele, as in Heterangium; but the secondary wood is more 

 abundant and resembles that of Lyginodendron. The most interest- 

 ing feature, however, is the exarch structure of the primary wood, 

 in contrast with the mesarch structure of both Heterangium and 

 Lyginodendron. The exarch structure occurs in some ferns, as in 

 Lygodium, and Seward remarks that "in anatomical characters the 



