8 Plowman . — The Comparative Anatomy and 
superficial plexus of fibro-vascular strands. The phloem-mass of each 
bundle is made up of a few rather large, very thin-walled sieve-tubes, with 
undoubted companion-cells lying in the angles between them. The xylem 
consists of usually two large vessels and a few small tracheids arranged 
in the conventional broad V-shaped section, at the point of which the 
protoxylem-lacuna is often conspicuously developed. At the nodes root- 
strands are attached directly to the superficial bundles of the cylinder, 
while the inconspicuous leaf-trace bundles pass inward to fuse with the 
medullary strands. The course of these bundles may be represented quite 
satisfactorily by Mohl’s original diagram of the bundles of the Palm-stem 
(A, Fig. i). 
The fusion takes place by the strands becoming approximated laterally, 
phloem upon phloem, and xylem upon xylem. The union of the phloem 
elements is accomplished much more promptly than is that of the xylem- 
elements, as a result of which the xylem occasionally forms a semicircular 
zone about the inner side of the phloem. In no instance, however, has 
a true amphivasal bundle been found in the rhizome of this species, either 
at an ordinary node or at the origin of a branch. 
Although the Amphivasae include a vast majority of the Cyperaceae, 
yet there are no great variations from the type in any essential feature 
of the structure of the central cylinder. The most striking irregularity 
appears in the development of the peridesm, which may be almost wholly 
undifferentiated in Eleocharis , uniformly two- or three-layered in Scleria 
and some Carices, or restricted to a few relatively enormous masses in 
certain species of Eriophortnn. The endodermis is always well defined, 
and projected some distance upon outgoing strands, except at the origin of 
aerial shoots, where the cortex and medulla are in free communication. 
The bundles are always relatively numerous, and there is always more 
or less evidence of a superficial plexus, though this is less marked in 
Eleocharis. 
The Centrivasae, though very few, present a range of variability quite 
as wide as the larger group. The medulla varies from cavernous in 
Dulichium and Cladium to densely sclerotic in Scirpus robustus ; the 
peridesm may be centripetally massed in 5. rivularis and Dulichium , 
or wholly undifferentiated in 5. microcarpus ; the endodermis may be 
reinforced to an unusual and unique degree as in A. Jluviatilis , of a quite 
normal type as in Dulichium and robustus , or apparently quite wanting 
in microcarpus ; finally, there may be a well-developed superficial fibro- 
vascular plexus, as in S. jluviatilis, or there may be nothing of the kind, as 
in Cladium. There is free communication between cortex and medulla 
through all foliar and ramular gaps in this group except in 5. Jluviatilis , 
in which the peculiar sclerotic jacket mentioned above closely encases the 
central cylinder of the rhizome in every part, even cutting directly across 
