EQUISETINEM. 
is caused by layers consisting of from i to 3 strata of parenchyma containing chlorophyll 
(the cells lying transversely). This green tissue lies especially beneath the furrows, cor- 
responding to the stomata, and appears in a transverse section as ribbon-shaped masses 
concave outwardly; in the slender leafy branches, where the ridges sometimes cause 
the transverse section to have a stellate outline [e.g. E. arvense), the tissue containing 
chlorophyll is in excess. The vallecular canals, which correspond to the furrows, arise 
in the fundamental tissue by separation and partially by rupture of the cells ; they may 
be absent from the slender leafy branches. 
The Fibro-'vascular Bundles are arranged, in a transverse section of the internode, 
as in Dicotyledons, in a circle, each corresponding to a ridge of the surface, between 
the cortical canals but somewhat nearer the centre. In the axis of the sporangiferous 
stems, where the diaphragms are wanting, they run in the same manner, and bend out 
singly into the pedicels of the peltate scales (as into the sheath-teeth). The bundles of 
a shoot are all parallel to one another; each bundle is the result of the coalescence 
of two portions ; one of these belongs to the leaf-sheath and developes in the median 
line of one of its teeth from below upwards ; the other portion developes in the internode 
itself from above downwards. At the angle where the two portions meet, the forma- 
tion of tissue begins in both, and thence advances in opposite directions ; the lower 
end of each bundle unites by two lateral commissures with the two next alternate 
bundles of the next lower internode (Fig. 278); the Equisetacese have therefore only 
* common ' bundles. In transverse sections these bundles resemble those of Mono- 
cotyledons, especially of Grasses ; the first-formed annular, spiral, or reticulated vessels 
belonging to the inner side, together with the thin-walled cells which separate them, 
are subsequently destroyed, and a canal (carinal) remains in their place traversing the 
whole length of the fibro-vascular bundle on its inner side. Right and left of this 
lie on the outside a few not very broad vessels thickened reticulately ; external to 
the canal lies the phloem-part of the bundle, formed of a few wide sieve-tubes 
and narrow cambiform cells, and at the circumference of a few thick-walled narrow 
bast-hke cells. A bundle-sheath, as it is termed, sometimes surrounds each bundle 
{E. limosiim), but generally runs continuously outside the circle of all the bundles, as in 
most Phanerogams. 
[Professor W. C. Williamson is led by a study of the internal organisation of Calamiies and 
Calamodendra^ to the conclusion that in England at least we have but one group of these fossil 
plants. When young their vascular zone, separating a medullary from a cortical parenchyma, 
was scarcely more than a thin ring of longitudinal canals, each of which had a few vessels at its 
outer border. In this state the structure of the plant presented a close resemblance to that of a 
recent Equisetum. But as the plant grew in size, new vessels were added to the exterior of the pre- 
existing bundles, so that each of the latter became the starting-point of a woody wedge which con- 
tinued to grow peripherally until it assumed large dimensions. In some specimens these wedges 
measure fully two inches between the canal marking their medullary angle and their peripheral or 
cortical base. P>vch wedge is composed of vertical radiating laminae of barred or reticulated vessels 
separated by cellular rays. The medullary portion became fistular, as in the recent Equisetacece, 
at an early age, and when the fistular cavities became filled with sand or mud, the very thin layer 
of medullary cells which remained did not prevent the sand from moulding itself against the inner 
angles of the vertical woody wedges, which thus produced the longitudinal grooves so characteristic 
of the casts commonly seen in collections. In such specimens most of the vegetable elements dis- 
appeared during fossilisation, and what remained, in the shape of a thin film of coal, moulded itself 
upon the medullary cast, and gave to the specimens the appearance of having had corresponding 
^ [On Fossil Equisetaceae, see Williamson, Mem. Lit. Phil. Soc. Manch, 3rd ser. vol, IV. 
pp. 155-183; Ditto, Trans, Roy. Soc. vol. CLXI, pp. 477-510: also vol. CLXIX. Part 2, 1878.— 
Coemans, Journ. Bot. 1869, pp. 337-340. — Dawson, Ann. Nat. Hist, 4th ser. vol. IV. pp. 272-273. — • 
Grand'Eury, Ann. Nat. Hist. 4th ser. vol. IV. pp. 124 -128 ; Compt. Rend, vol, LXVIII.— M^'Nab, 
Journ. of Bot, 1873, pp. 72-80.] 
