4. Equisetites arenaceus Bronn.
This plant has been found in the Triassic rocks of various parts of Germany and France; it occurs in the Lettenkohl group (Lower Keuper), as well as in the Middle Keuper of Stuttgart and elsewhere. The species may be defined as follows:—
Rhizome from 8–14 cm. in diameter, with short internodes, bearing lateral ovate tubers. Aerial shoots from 4–12 cm. in diameter, bearing whorls of branches, and leaf-sheaths made up of 110–120 coherent uni-nerved linear segments terminating in an apical lanceolate tooth. Strobili oval, consisting of crowded sporangiophores with pentagonal and hexagonal peltate terminations.
The casts of branches, rhizomes, tubers, buds and cones enable us to form a fairly exact estimate of the size and general appearance of this largest fossil Horse-tail. The Strassburg Museum contains many good examples of this species, and a few specimens may be seen in the British Museum. In the École des Mines, Paris, there are some exceptionally clear impressions of cones of this species from a lignite mine in the Vosges.
It is estimated that the plant reached a height of 8 to 10 meters, about equal to that of the tallest recent species of Equisetum, but in the diameter of the stems the Triassic plant far exceeded any existing species.
It is interesting to determine as far as possible, in the absence of petrified specimens, if this Keuper species increased in girth by means of a cambium. There are occasionally found sandstone casts of the pith-cavity which present an appearance very similar to that of Calamitean medullary casts[510]. The nodes are marked by comparatively deep constrictions, which probably represent the projecting nodal wood. The surface of the casts is traversed by regular ridges and grooves as in an ordinary Calamite, and it is probable that in Equisetites arenaceus, as in Calamites, these surface-features are the impression of the inner face of a cylinder of secondary wood (cf. p. 310). Excellent figures of this species of Equisetites are given by Schimper in his Atlas of fossil plants[511], also by Schimper and Koechlin-Schlumberger[512], and by Schoenlein and Schenk[513].
5. Equisetites columnaris Brongn. Figs. 11 and 58, B.
This species, which is by far the best known British Equisetites, was founded by Brongniart[514] on some specimens from the Lower Oolite beds of the Yorkshire coast. Casts of stems are familiar to those who have collected fossils on the coast between Whitby and Scarborough; they are often found in an erect position in the sandstone, and are usually described as occurring in the actual place of growth. As previously pointed out (p. 72), such stems have generally been deposited by water, and have assumed a vertical position (fig. 11). Young and Bird[515] figured a specimen of this species in 1822, and in view of its striking resemblance to the sugar-cane, they regarded the fossil as being of the same family as Saccharum officinarum, if not specifically identical.
A specimen was described by König[516] in 1829, from the Lower Oolite rocks of Brora in the north of Scotland under the name of Oncylogonatum carbonarium, but Brongniart[517] pointed out its identity with the English species Equisetites columnaris.
Our acquaintance with this species is practically limited to the casts of stems. A typical stem of E. columnaris measures 3 to 6 cm. in diameter and has fairly long internodes. The largest stem in the British Museum collection has internodes about 14 cm. long and a diameter of about 5 cm. In some cases the stem casts show irregular lateral projections in the neighbourhood of a node, but there is no evidence that the aerial shoots of this species gave off verticils of branches. In habit E. columnaris probably closely resembled such recent species as Equisetum hiemale L., E. trachyodon A. Br. and others.