The specimens of fossil Equisetums are usually readily recognised by the coherent leaf-segments in the form of nodal sheaths resembling those of recent species. The tissues of the cortex and central cylinder are occasionally represented by a thin layer of coal pressed on to the surface of a sandstone cast, or covering a flattened stem-impression on a piece of shale. It is sometimes possible under the microscope to recognise on the carbonised epidermal tissues the remains of a surface-ornamentation similar to that in recent species, which is due to the occurrence of siliceous patches on the superficial cells. Longitudinal rows of stomata may also be detected under favourable conditions of preservation. The nodal diaphragms of stems have occasionally been preserved apart, but such circular and radially-striated bodies may be misleading if found as isolated objects. Casts of the wide hollow pith of Equisetites, with longitudinal ridges and grooves, and fairly deep nodal constrictions, have often been mistaken for the medullary casts of Calamites.
Several species of Equisetites have been recorded from the Upper Coal-Measures and overlying Permian rocks, but these present special difficulties. In one instance described below, (Equisetites Hemingwayi Kidst.), the species was founded on a cast of what appeared to be a strobilus made up of sporophylls similar to those in an Equisetum cone. In other Permo-Carboniferous species the choice of the generic name Equisetites has been determined by the occurrence of leaf-sheaths either isolated or attached to the node of a stem. The question to consider is, how far may the Equisetum-like leaf-sheath be regarded as a characteristic feature of Equisetites as distinct from Calamites? In the genus Calamites the leaves are generally described as simple linear leaves arranged in a whorl at the nodes, but not coherent in the form of a sheath (fig. 85). The fusion of the segments into a continuous sheath or collar is regarded as a distinguishing characteristic of Equisetites and Equisetum. The typical leaf-sheath of a recent Horse-tail has already been described. In some species we have fairly large and persistent free teeth on the upper margin of the leaf-sheath, but in other Equisetums the rim of the sheath is practically straight and has a truncated appearance, the distal ends of the segments being separated from one another by very slight depressions, as in a portion of the sheath of Equisetum ramosissimum Desf. of fig. 58, C. In other leaf-sheaths of this species there are delicate and pointed teeth adherent to the margin of the coherent segments; the teeth are deciduous, and after they have fallen the sheath presents a truncated appearance. This difference between the sheaths to which the teeth are still attached and those from which they have fallen is illustrated by fig. 58, B and C; it is one which should be borne in mind in the description of fossil species, and has probably been responsible for erroneous specific diagnoses. In some recent Horse-tails the sheath is occasionally divided in one or two places by a slit reaching to the base of the coherent segments[498]; this shows a tendency of the segments towards the free manner of occurrence which is usually considered a Calamitean character. In certain fossils referred to the genus Annularia, the nodes bear whorls of long and narrow leaves which are fused basally into a collar (fig. 58, D). There are good grounds for believing that at least some Annularias were the foliage shoots of true Calamites. Again, in some species of Calamitina, a sub-genus of Calamites, the leaves appear to have been united basally into a narrow sheath. We see, then, that it is a mistake to attach great importance to the separate or coherent character of leaf-segments in attempting to draw a line between the true Calamites and Equisetites. Potonié[499] while pointing out that this distinction does not possess much value as a generic character, retains the genus Equisetites for certain Palaeozoic Equisetum-like leaf-sheaths.
Fig. 56. Calamitean leaf-sheath. From a specimen in the Woodwardian Museum. a, base of leaf-sheath; (very slightly reduced).
Fig. 56 represents a rather faint impression of a leaf-sheath and nodal diaphragm. The specimen is from the Coal-Measures of Ardwick, Manchester. The letter a probably points to the attachment of the sheath to the node of the stem. The flattened sheath is indistinctly divided into segments, and at the middle of the free margin there appears to be a single free tooth. The lower part of the specimen, as seen in the figure, shows the position of the nodal diaphragm. Between the diaphragm and the sheath there are several slight ridges converging towards the nodal line; these agree with the characteristic ridges and grooves of Calamite casts which are described in detail in Chapter X. There is another specimen in the British Museum which illustrates, rather more clearly than that shown in fig. 56, the association of a fused leaf-sheath with a type of cast usually regarded as belonging to a Calamitean stem. Some leaf-sheaths of Permian age described by Zeiller[500] as Equisetites Vaujolyi bear a close resemblance to the sheath in fig. 58 E. The nature of the true Calamite leaves is considered more fully on a later page.
PALAEOZOIC EQUISETITES.
The examples of supposed Equisetites sheaths referred to below may serve to illustrate the kind of evidence on which this genus has been recorded from Upper Palaeozoic rocks. I have retained the name Equisetites in the description of the species, but it would probably be better to speak of such specimens as ‘Calamitean leaf-sheaths’ rather than to describe them as definite species of Equisetites. We have not as yet any thoroughly satisfactory evidence that the Equisetites of Triassic and post-Triassic times existed in the vegetation of earlier periods.
In Grand’Eury’s Flore du Gard[501] a fossil strobilus is figured under the name Calamostachys tenuissima Grand’Eury, which consists of a slender axis bearing series of sporophylls and sporangia apparently resembling those of an Equisetum. There are no sterile appendages or bracts alternating with the sporophylls; and the absence of the former suggests a comparison with Equisetites rather than Calamites. Grand’Eury refers to the fossil as “parfois à peine perceptible,” and a recent examination of the specimen leads me to thoroughly endorse this description. It was impossible to recognise the features represented in Grand’Eury’s drawing. Setting aside this fossil, there are other strobili recorded by Renault[502] and referred by him to the genus Bornia (Archaeocalamites), which also exhibit the Equisetum-like character; the axis bears sporophylls only and no sterile bracts. It would appear then that in the Palaeozoic period the Equisetaceous strobilus, as we know it in Equisetum, was represented in some of the members of the Equisetales.
Fig. 57. A. Equisetites Hemingwayi Kidst. From a specimen in the British Museum. ⅔ nat. size. B. Diaphragm and sheath of an Equisetaceous plant, from the Coal-Measures. ⅔ nat. size. From a specimen in the British Museum.