The stems often show a distinct swelling at the nodes; this may be due, at least in part, to the existence of transverse nodal diaphragms which enabled the dead shoots to resist contraction in the region of the nodes. The leaf-sheaths consist of numerous long and narrow segments often truncated distally, as in fig. 58, B, and as in the sheath of such a recent Horse-tail as E. ramosissimum shown in fig. 58, C. In some specimens one occasionally finds indications of delicate acuminate teeth extending above the limits of a truncated sheath. Brongniart speaks of the existence of caducous acuminate teeth in his diagnosis of the species, and the example represented in fig. 58, B, demonstrates the existence of such deciduous appendages. There is a very close resemblance between the fossil sheath of fig. 58, B, with and without the teeth, and the leaf-sheath of the recent Equisetum in fig. 58, C. In some specimens of E. columnaris in which the cast is covered with a carbonaceous film, each segment in a leaf-sheath is seen to be slightly depressed in the median portion, which is often distinctly marked by numerous small dots, the edges of the segment being flat and smooth. The median region is that in which the stomata are found and on which deposits of silica occur.
6. Equisetites Beani (Bunb.). Figs. 60–62.
Bunbury[518] proposed the name Calamites Beani for some fossil stems from the Lower Oolite beds of the Yorkshire coast, which Bean had previously referred to in unpublished notes as C. giganteus. The latter name was not adopted by Bunbury on account of the possible confusion between this species and the Palaeozoic species Calamites gigas Brongn. The generic name Calamites must be replaced by Equisetites now that we are familiar with more perfect specimens which demonstrate the Equisetean characters of the plant.
Fig. 60. Equisetites Beani (Bunb.). ⅔ nat. size. [After Starkie Gardner (86) Pl. IX. fig. 2.]
Schimper[519] speaks of this species as possibly the pith-cast of Equisetites columnaris, but his opinion cannot be maintained; the species first described by Bunbury has considerably larger stems than those of E. columnaris. It is not impossible, however, that E. columnaris and E. Beani may be portions of the same species. The chief difference between these forms is that of size; but we have not sufficient data to justify the inclusion of both forms under one name. Zigno[520], in his work on the Oolitic Flora, figures an imperfect stem cast of E. Beani under the name of Calamites Beani, but the species has received little attention at the hands of recent writers. In 1886 Starkie Gardner[521] figured a specimen which was identified by Williamson as an example of Bunbury’s species; but the latter pointed out the greater resemblance, as regards the external appearance of the Jurassic stem, to some of the recent arborescent Gramineae[522] than to the Equisetaceae. Williamson, with his usual caution, adds that such appearances have very little taxonomic value. Fig. 60 is reproduced from the block used by Gardner in his memoir on Mesozoic Angiosperms; he quotes the specimen as possibly a Monocotyledonous stem. The fossil is an imperfect cast of a stem showing two clearly marked nodal regions, but no trace of leaf-sheaths. A recent examination of specimens in the museums of Whitby, Scarborough, York and London has convinced me that the plant named by Bunbury Calamites Beani is a large Equisetites. As a rule the specimens do not show any indications of the leaf-sheaths, but in a few cases the sheaths have left fairly distinct impressions.
Fig. 61. Equisetites Beani (Bunb.). From a specimen in the British Museum, ⅔ nat. size. (No. V. 2725.)
In the portion of stem shown in fig. 61 the impressions of the leaf segments are clearly marked. This specimen affords much better evidence of the Equisetaceous character of the plant than those which are simply internal casts. The narrow projecting lines extending upwards from the nodes in the figured specimen probably represent the divisions between the several segments of each leaf-sheath.
In the museums of Whitby and Scarborough there are some long specimens, in one case 44 cm. in length, and 33 cm. in circumference, which are probably casts of the broad pith-cavity. These casts are often transversely broken across at the nodes, so that they consist of three or four separate pieces which fit together by clean-cut faces. This manner of occurrence is most probably due to the existence of large and resistant nodal diaphragms which separated the sand-casts of adjacent internodes. In the York museum there are some large diaphragms, 10 cm. in diameter, preserved separately in a piece of rock containing a cast of Equisetites Beani. The nodal diaphragms of some of the Carboniferous Calamites were the seat of cork development[523], and it may be that the frequent preservation of Equisetaceous diaphragms in Triassic and Jurassic rocks is due to the protection afforded by a corky investment.