i86 
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 strobUus of Cycadcoidca, showing are regarded as ab- 
the hairy sheathing bracts, the set of branched stamens normal. The Struc- 
(bent inward so that the backs are towards the ovules), and + ure o f a representa- 
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 
