88 BOTANICAL GAZETTE [FEBRUARY 
that appears to separate the xylem of the outgoing leaf trace from 
the stele cannot be lightly dismissed, for the early appearance of 
this tissue related to more than one leaf is probably the origin of 
the pith in the Marattiaceae. 
The obscurity of this transition is due to the telescoping of the 
dictyostelic into the protostelic stage. A slight extension of this 
process would result in the simultaneous origin of two or three 
leaves at the cotyledonary node, from which a dicotyledonous 
habit may have been derived in some offshoot of the family. 
Rarely the solenostele forms a complete cylinder. It is gener- 
ally broken by two or more leaf gaps, whose large size and close 
arrangement cause them to overlap. At this stage the bundle 
arrangement is like that in the Dicksonieae, some of the Poly- 
podiaceae, and Ophioglossum. The last differs from the others in 
the collateral structure of the bundles. In some stems of Marattia 
the tendency toward a reduction of internal phloem and endo- 
dermis, referred to by JEFFREY (15), appears. This is not uniform, 
however, and is more likely to appear in the medium sized than in 
the larger stems. 
THE MEDULLARY SYSTEM 
When the stele reaches a diameter of 2-4 mm., a medullary 
strand appears and initiates the final stage in the development of 
the stele. It may originate in three different ways: (1) Most 
commonly it branches from the inner surface of the solenostele. Pro- 
tophloem, otherwise absent on the inner surface of the stele, 
appears in a patch, which is bulged outward by an increase in 
parenchyma (fig. 23, pp), and later xylem elements increase at that 
point. Usually a root is given off from the external surface of the 
bundle opposite this branch (fig. 24). This is the origin described 
by FARMER and Hitt (9) and BREBNER (3) as a “local hypertrophy 
of the internal phloem.” (2) It may arise by a branch which 
runs into the pith from the upper part of the margin of a leaf gap. 
(3) The pith may develop a cambium which gives rise to a strand 
of phloem in the center of which xylem then forms. The xylem 
may run 0.3 mm. before a commissure connects it with the main 
vascular system. 
