220 



BOTANICAL GAZETTE 



[SEPl EMBER 



of the secondary wood, exhibits no sign whatever of having* 

 given rise to the more or less distinctly individualized curved 

 bundles of the cylinder of Lygiiiodendron Oldhammtn, The 

 writer, on the contrary, believes it to be the homologue of a 

 single stele of Medullosa, and can trace a gradual reduction in 



Efes: # 





..a.- — ,x 



2 



the number of steles in these 

 plants, from the very numerous 

 ones oi MediUlosa Solmsii, ^rsily, 

 to the very few steles (two or 

 three in number) of M. anglica 



i'-pith <^r Colpoxylon, and finally, to 



^:y- X 



ence letters as before. 



the single large stele of Heter- 

 angium or of Megaloxylon. He 

 may add, moreover, that the 

 derivation of the tubular from 



Fig. 6.-The endarch type : refer- ^j^^ g^^j^ g^-^j^ jg ^^ be SOUght 



for, not in these semi-gymnos- 

 permous forms, but much farther back, viz., among the ferns. 



When we arrive at the stage of our modern cycads, which are 

 certainly descended from some of these cycadofilicinean plants, 

 a further modification meets our eyes, for here we find the 

 mesarch bundle of the stem cylinder almost entirely replaced 

 by the e?uiarch type, in which the innermost primary or cen- 

 tripetal group of xylem has vanished, so that the protoxylem 

 which, once upon a time in the ancient concentric bundle, occu- 

 pied the center of the latter, now constitutes the most internal 

 portion of the bundle, viz., that nearest the pith [fig. 6), But 

 the older mesarch bundle has not become completely extinct 

 throughout the group, for in that most primitive of axial organs, 

 the peduncle of the cone, the centripetal xylem in part persists,, 

 as can be well seen in Stangeria and Bowenia. Those cycads, 

 such as Cycas, Encephalartos, Macrozamia, and Bowenla, which 

 possess more than one vascular cylinder in the stem, one withm 

 another to the number sometimes of a dozen (a truly striking 

 character), are descended from such types as those Medulloseae 

 haWng more than one cylinder or ring of concentric bundles. 



