the Anatomy of the Cone and Fei'tile Stem of Equisetum . 429 
be more or less circular or even oval with the slightly longer axis directed 
radially. But, though the radial extent of the xylem is often considerable, 
Compared with its width, the actual number of tracheides in the metaxylem 
of the small bundle is not great and a comparison with the bundles in 
younger, immature cones shows that the disorganized protoxylem elements, 
replaced by relatively large carinal canals, were few in number. Protoxylem 
and metaxylem are usually continuous, but may be locally separated by 
parenchymatous cells. 
The individual strands of the axis of the cone are closely surrounded 
by a sheath, often very distinct, the cells of which do not show typical 
endodermal markings. They are much larger than the cells of the bundle and 
are often radially elongated with reference to the latter (cf. PI. XXI, Fig. 7). 
The sheath always follows closely the shape of the bundle, and when there 
is a band of xylem, formed by the coalescence of several strands, the cells 
of the sheath disappear at the points of junction of the strands, their place 
being taken by tracheides mixed with small parenchymatous cells. Thus 
the laterally elongated bands of vascular tissue are surrounded by sheaths 
exactly resembling those round the narrower bundles. 
In the cone of E. variegatum the xylem also consists of tracheides 
of the usual type. The protoxylem is usually in contact with the metaxylem, 
and though parenchymatous cells are mingled with the tracheides they are 
not, as a rule, as numerous, even relatively to the size of the bundle, as in 
E. sylvaticum or E. maximum. The tracheides of metaxylem and proto- 
xylem are often very similar ; the latter may be irregularly scattered along 
the inner edge of the former and not distributed in definite groups. The 
amount and distribution of the metaxylem varies much, even in neighbour- 
ing bundles of a section. Perhaps the commonest form of strand is that in 
which the xylem forms an oval band, as seen in a transverse section of the 
axis, wider than deep, broken up by a few parenchymatous cells, but often 
attaining locally a depth of three to four cells, excluding the protoxylem 
(cf. PL XXI, Fig. 5). 
Sometimes certain relatively large tracheides at the edge of the 
bundle appear to be more deeply seated than the protoxylem (PI. XXI, Fig. 5, 
the bundle near the top of the figure on the readers left). Such xylem is, 
ontogenetically speaking, centripetal, since its differentiation as tracheides 
occurred after that of the smaller celled protoxylem outside it. These 
relatively large tracheides abut on the large cells of the sheaths that 
surround the separate bundles or bands of vascular tissue and usually line 
one of the sides of this sheath. It is obvious from the early appearance 
round the bundle of a definite sheath that the cells of the latter soon attain 
too great a size for them to develop as ordinary metaxylem tracheides. It 
is, therefore, possible that when the conditions in the developing bundle are 
favourable to a considerable differentiation of metaxylem, the extension of 
F f 
