XXIX] 



HETERANGIUM 



73 



with little primary centrifugal wood, by the abundant secretory 

 sacs of the stele, and by the rather scattered leaves.' 



Heterangium Grievii has a radially symmetrical stem bearing 

 compound leaves with decurrent petioles which give to the other- 

 wise cylindrical axis an angular outline as seen in transverse 

 section (fig. 411, A). The phyllotaxis appears to be f. The 



FIG. 411. Heterangium Grievii. It, a, b, c, leaf -traces ; p, pericycle with periderm ; 

 ph, phloem ; x l , x 2 , primary and secondary xylem ; px, protoxylem ; r, root ; 

 s, sclerous tissue. 



stem rarely exceeds 1-5 cm. in diameter: in the centre is a com- 

 paratively large stele consisting in young stems of primary xylem 

 and phloem, but in older stems these are separated by a cylinder 

 of secondary vascular tissue which in this species is always narrower 

 than in Lyginopteris oldhamia and, as Williamson pointed out, 

 often of unequal thickness on different radii. The medullated stele 



