i6 



MORPHOLOGY OF GYMNOSPERMS 



two genera Lyginodendron and Heterangium approach most nearly 

 to the Osmundaceae and Gleicheniaceae respectively; in Megalo- 

 xylon, on the other hand, the structure of the primary xylem affords 

 evidence that the Lygodium type of stem was also represented in the 

 cycad-fern alliance which played so prominent a part in paleozoic 

 vegetation." Another characteristic feature of Megaloxylon is the 

 structure of the metaxylem, which is usually made up of very short 

 tracheids, often broader than long, interspersed with parenchyma, 



a structure suggesting 

 a storage rather than 

 a conducting region. 

 This feature is shared 

 by the Permian Za- 

 lesskya, which has been 

 shown to be a primi- 

 tive member of the 

 Osmundaceae (87). 



Cycadoxylon.— 

 The stems of this 

 genus have been re- 

 ported from the Lower 

 Coal-measures and 

 from the Permian, and 

 it is evident that the 

 tjrpe was an important 

 one during the Paleo- 

 zoic. Some of the 

 preserved vascular cyl- 

 inders are large enough (14 cm. in diameter) to suggest stems as 

 large as those of small trees. The anatomy resembles that of 

 Lyginodendron, in fact one of the species was described as L. 

 robustum. There is the same siphonostele, with abundant second- 

 ary wood traversed by broad pith rays and containing elements 

 of the same character. Two differences, however, are notable. 

 One is the endarch character of the primary vascular strands. 

 This character is not clear in all the forms, and in some a small 

 amount of centripetal xylem is claimed; but in any event, the 



Fig. 13. — Lyginodendron Oldhamium: part of 

 transverse section of outer portion of stele; iC, second- 

 ary wood; r, medullary rays; cb, cambium; ph, 

 primary phloem; ph^, secondary phloem; ss, secretory 

 sac; pd, periderm; X52. — After Williamson and 

 Scott (18). 



