597 
Centripetal Xylem in Equisetum . 
xylem-system is much reduced, particularly in the leaves. Yet indications 
of the mesarch structure of the leaf-trace were seen even in these two last- 
named species. 
A brief description of the course of the trace will aid in understanding 
the conditions presented. As shown in the first part of this paper, the leaf- 
trace is formed by a continuation of the various elements of the internodal 
bundle. The carinal canal becomes discontinuous as it approaches the node. 
The protoxylem occupying that space enlarges, extending radially, then 
passes upward and forms the innermost tracheides of the supranodal wood. 
Just before that xylem-mass is reached, a ‘branch’ to the leaf is sent off 
more or less horizontally. Each of the metaxylem-groups enlarges some- 
what, and gives off a small strand which passes upward and slightly outward, 
joining the protoxylem of the leaf-trace a short distance outside the point 
where the latter is first seen clearly. These three strands fuse more or 
less, and form the leaf-trace. Fig. 4 shows the structure of such a trace in 
E. hyemale var. robustum as it passes through the cortex. (The leaf lies 
above to the right.) Centrally are seen the scattered rings of protoxylem ; 
to the right is metaxylem that is clearly formed centrifugally, its outermost 
elements being pitted. Another group of metaxylem is seen upon the inner 
side, centripetally developed. Thus the trace is mesarch. Parenchyma 
cells occur scattered through the xylem. Fig. 3 of the same plant does not 
show centripetal wood for the reason that it is not strictly median as to the 
bundle, but passes to one side of the centre, so as to show the attachment 
of the metaxylem. E. maximum (Fig. 5) shows very similar conditions — 
disorganized protoxylem with metaxylem both external and internal 
thereto. The strongly sclerified cells are those in the axil of the leaf- 
sheath. Mesarch structure in these two species appears close to the origin 
of the trace, and continues nearly to the end of its xylem. E. hyemale 
resembles its variety robustum very closely in the organization of the 
leaf-trace. In E. arvense no distinction was noticed among the 
tracheides until the trace had left the stem, but throughout the leaf 
proper mesarch conditions are well defined. Even in that most reduced 
species, E. scirpoides i with stems only a few millimetres in diameter, the 
foliar bundle is distinctly mesarch. 
The study of transverse sections of these bundles in the larger species 
completes the understanding of their structure. The leaf-trace of E. hyemale 
var. robustum just before it leaves the cortex has a strong resemblance to the 
axial bundle of the cone. It is made up as follows : centrally, or often 
a little to one side, is a protoxylem-lacuna ; external to this are the 
centrifugal tracheides arranged largely in two groups upon its flanks ; 
directly inward from the centre are the centripetal elements, from three 
to seven in number. E. maximum presents much the same conditions. 
The protoxylem-lacuna is well developed in the proximal portion of the 
S.s 2 
