7 
Phytogeny of the Cyperaceae . 
type. The majority of them are grouped near the peripheral portion 
of the central cylinder, while each of those lying nearer the centre is 
characterized by the possession of a large centripetal mass of scleren- 
chymatous tissue. This is the greatly exaggerated peridesm of the bundle, 
which in the peripheral bundles is either more uniformly distributed or even 
wholly undifferentiated. 
The phloem-mass is composed of very numerous, small, thin-walled 
elements of fairly uniform size, not readily distinguishable as sieve-tubes 
and companion-cells. However, in closely related species of less compact 
habit of growth, such as 5 . atrovirens and S. sylv aliens , it is often possible 
to make out these different elements of the phloem quite definitely. The 
xylem-elements are rather small but of various sizes, crowded in a dense 
ring around the phloem. Often there may be more than a single ring 
of xylem, or an irregular massing on one side. In some of the bundles, 
particularly those lying centrally, there is a small but unmistakable 
protoxylem-lacuna on the central margin. This is an indication of the 
original collateral nature of these bundles. As Chrysler ( 9 ) and other 
observers have shown from developmental studies, amphivasal bundles are 
derived from collateral bundles by excessive development of the xylem 
elements, so that the original broad V-shaped cross-section of the xylem 
takes on a U-shape, and finally closes completely into an O-shape. The 
same result is often to be observed in the fusion of collateral bundles in the 
nodal anastomoses of the aerial stems. A fuller consideration of the origin 
and phylogenetic significance of the amphivasal bundles is reserved for 
a later page. 
Very commonly in rhizomes of this type there is a dense plexus of 
transverse and oblique fibro-vascular strands in the surface of the central 
cylinder, just inside the endodermis. Fig. io represents a radial section 
through this region. This superficial plexus in many cases undoubtedly 
affords the chief mechanical support to the central cylinder, while to it are 
attached all of the root-strands and many of the smaller leaf-trace bundles. 
The larger leaf-trace bundles, on the other hand, like the ramular bundles, 
pass inward to the medulla and then outward to the superficial plexus, 
following a course very similar to that figured by De Bary for the Palm- 
stem (B, Fig. i). It should be noted in passing that fully 90 per cent, 
of the branching and anastomosing of bundles in the rhizome takes place 
in the superficial plexus. 
As an example of the Centrivasae we may examine more in detail the 
root-stock of Scirpus americanus , Pers. (S.puugens,V ahl), a form in which 
the internodes are long and the leaves reduced to the merest brown scales. 
Here the central cylinder (Fig. 16) is less compact ; the bundles are much 
less numerous, and collateral throughout ; the peridesm is only slightly 
developed, as is also true of the endodermal sheath ; there is no dense 
