2 BOTANICAL GAZETTE [JANUARY 
Avena barbata.—At a short distance above a node the stem pre- 
sents a simple structure compared with that of many grasses, showing 
just two circles of collateral bundles surrounding a central cavity. 
The inner circle of bundles lies at the periphery of the central cylinder, 
which in this genus is not clearly marked off from the cortex. The 
bundles of the outer circle lie between groups of cells, which above 
the leaf-sheath bear chlorophyll; they are considerably smaller than 
the bundles of the inner ring, and from comparison with correspond- | 
ing bundles in other genera must be regarded as cortical bundles. 
In the upper part of a node these assume an amphivasal structure 
and immediately anastomose with one another and with bundles 
of the inner circle. But just at this level the structure of the stem 
is further complicated by the entrance of bundles of the leaf-trace, 
the course of which must now be described. The leaf-sheath in 
this species extends a little more than 360° around the stem, and 
contains, as do most of the genera examined, bundles of two distinct 
sizes: larger ones, alternating with others which are less than Nalf 
as great in diameter. These two kinds of leaf-trace bundles have 
a different course in the stem. The larger bundles undergo a pro- 
found modification as they enter the stem, as may be seen by com- 
paring figs. r and 2, in which the magnification is the same. As 
such a bundle enters the stem it rapidly increases in size, owing to 
increase in the number of xylem elements. Most of the added 
elements are tracheids with more or less suberized scalariform or 
reticulate thickenings, but some parenchyma cells are also present. 
These elements gradually extend around the sides of the phlcem 
until this is surrounded by xylem, except a small area on its outer 
side, which is generally occupied by sclerotic fibres belonging to the 
group which is so well marked im the sheath (fig. 1). In certain 
species, e. g., Lolium perenne, these fibres disappear, and the phlcem 
is completely surrounded by xylem. These bundles evidently must 
be placed in the amphivasal class. As fig. 2 shows, the phloem — 
enlarges very little, but the xylem increases so much that the bundle : 
may be five or SIX times as broad in the middle of the node as it is ~ 
in the leaf-sheath. These bundles are by far the most conspicuous — 
objects in a cross-section of the stem at this level. They have 2 
spindle shape which is not due to their oblique course, for they slant — 
