599 
Centripetal Xylem in Equisetum . 
rule in the stem. Still another example occurs in the lower Gymnosperms. 
The bundles of the cotyledons of Ginkgo , and also of the vegetative leaves 
to a slight extent, have been shown by Worsdell 1 to be mesarch. Its 
ancestors were probably allied to the primitive Gymnosperms and possessed 
mesarch stem-structure. Further, Jeffrey 2 has recently shown that the 
leaves of a primitive pine-like Gymnosperm, named by him Prepinus , also 
had well-marked centripetal xylem. Thus in many places we find foliar 
organs retaining the typical xylem-structure of the older vascular plants. 
Lately there has been some objection to the division of Vascular 
Cryptogams into the Pteropsida and the Lycopsida, on a basis, in part, 
of the presence or absence, respectively in the two groups, of foliar gaps. 
Equisetum among the Lycopsida has been regarded by some writers as 
varying from other members of the series in possessing leaf-gaps. In the 
examination of leaf-trace structure in this genus, a large number of sections, 
both transverse and radial, through the points of origin of traces, both 
of vegetative and reproductive leaves of several species, has been studied by 
the writer. In no case did a gap exist. The strands of the cone continue 
upward without sign of interruption after giving off the sporophyll-trace. 
Immediately over the origin of the vegetative leaf-trace, and connected 
with all portions of the internodal bundle, lies the unbroken ring of supra- 
nodal wood. Even in the much reduced vascular system of E. Jluviatile , 
this xylem-mass is unbroken ; in the tiny E. scirpoides it likewise com- 
pletely shuts off the leaf-trace from the so-called * gap ’ above. 
Summary and Conclusions. 
In Equisetum the development of the xylem of the vegetative stem 
is centrifugal throughout. 
The internodal bundle does not consist of three united bundles, repre- 
senting different portions of an ancestral stele : it is a unit in structure, and 
represents the much-reduced internodal primary bundle of the Calamites. 
The strobilus shows in its axial bundles conditions intermediate between 
those in the vegetative stems of the Calamites and of Equisetum. 
The vegetative leaf-trace does not arise solely from the protoxylem- 
strand of the internodal bundle ; all three parts of the xylem of the latter 
contribute to its formation. 
Through Dr. Scott’s discovery of an ancient Calamite with centripetal 
wood in the stem-bundle, a link has been formed between the Equisetales 
and the Sphenophyllales, and possibly through the latter with the Lyco- 
podiales. The most primitive Equisetaceous forms doubtless possessed 
well-developed centripetal wood. Early in the history of the series this 
1 Worsdell, W. C. : On Transfusion Tissue. Trans. Linn. Soc. of London. Series 2, vol. v ; 1897. 
2 Jeffrey, E. C. : On the Structure of the Leaf in Cretaceous Pines. Ann. Bot., vol. xxii ; 1908. 
