CYCADOFILICALES 



19 



rather than intermediate between the Hetcrangium and Lyj^inoden- 

 dron types. 'J'hc polystely of Mcdullosa, howe\-er, is said (87) to 

 differ from that of the ferns (such as Pleris), in that apparently it 

 holds no relation to the presence of leaf gaps. The absence of leaf 

 gaps in Mcdullosa and its allies has suggested that polystely arose 

 within the family, quite independent of polystely among ferns. Occa- 

 sionally vascular strands appear also in the cortex, as among the 

 cycads. 



The leaf trace of Mcdullosa is at first concentric and with secondary 

 wood; but later the secondary tissue disappears, the bundle branches 

 repeatedly, and the branches become collateral and exarch. According 

 to Scott (24), "each leaf stalk re- 

 ceived a large number of vascular 

 bundles, as many as 70 to 80 in all, 

 derived from the repeated ramifi- 

 cation of several of the principal 

 leaf traces. The supply of 

 bundles did not all enter the leaf 

 base from the stem at the same 

 level; they passed in successively 

 in groups corresponding to the 

 various principal leaf traces from 

 which they were derived." 



The later species of Mcdullosa, 

 so far as their structure is known, 

 are more complex anatomically than M. anglica, the vascular bundles 

 being more numerous, and differentiated into an inner group of 

 smaller bundles and a more or less complete peripheral zone of larger 

 ones (fig. 16). All the bundles have the same general structure as 

 those of M. anglica. In the older stems of certain forms, howe\-er, 

 there is an excessive development of secondary wood on the outer side 

 of the peripheral bundles, so that a section of the stem shows a broad 

 zone of secondary wood surrounding the relatively inconspicuous 

 inner region. 



In a specimen of M. stcllata gigantea, whose vascular region is 

 over 45 cm. in diameter, in addition to the large mass of secondary 

 wood developed on the outer side of the peripheral bundles, there 



Fig. 16. — Mcdullosa Solmsii: part of 

 transverse section of stele; st, "star 

 rings"; pla, outer zone of bundles; pU, 

 inner zone of bundles; a and h, bundles 

 which have branched off from the inner 

 zone; X2. — After Weber and Sterzel 

 (17) in POTONIE (20). 



