1 86 



MORPHOLOGY 



occurs in the midst of the xylem (mesarch), a feature characteristic 

 of ferns. The wood of the stem, therefore, has advanced to the 

 endarch condition, while in the leaves the old mesarch character 

 of the ferns remains. The leaf trace is a single and direct vascular 



strand, in striking 

 contrast with the leaf 

 traces of cycads. The 

 tuberous or columnar 

 body bears a crown 

 of huge cycad-like 

 (fernlike) leaves. 



Strobilus. — The 

 strobili of certain 

 European forms have 

 been known for along 

 time, but their real 

 structure was not 

 known until the re- 

 cent study of the 

 American forms. It 

 seems clear now that 

 probably the strobili 

 of the whole group 

 were bisporangiate, a 

 most remarkable con- 

 dition among gymno- 

 sperms, for in all the 

 other groups the 

 strobili are either 

 staminate or ovulate, 

 except in cases that 



Fig. 427. — Diagram of strobilus of Cycadeoj&o, showing ^^^ regarclea aS au- 



the hairy sheathing bracts, the set of branched stamens normal. The Struc- 



(bent inward so that the backs are towards the ovules), and *„..« „f „ rpnresenta- 

 the tip of the axis covered bv ovules. — After Wieland. 



tive strobilus may be 

 described as an illustration of the general condition (figs. 427, 428). 

 The strobilus is sheathed by a series of sterile, overlapping bracts. 

 Within (and above) these there arise ten to twenty microsporophylls 

 (stamens); and within the stamens, covering the rest of the axis of the 



