In 1840 Unger[592] published a note on the structure and affinities of Calamites, and expressed his belief in the close relationship of the Palaeozoic plant and recent Horse-tails.
An important contribution to our knowledge of Calamites was supplied by Petzholdt[593] in 1841. His main contention was the Equisetaceous character of this Palaeozoic genus. The external resemblance between Calamite casts and Equisetum stems had long been recognised, but after Cotta’s account of the internal structure it was believed that the apparent relation between Equisetum and Calamites was not confirmed by the facts of anatomy. Petzholdt based his conclusions on certain partially preserved Permian stems from Plauenscher Grund, near Dresden. Although his account of the fossils is not accurate his general conclusions are correct. The specimens described by Petzholdt differ from the common Calamite casts in having some carbonised remnants of cortical and woody tissue. A transverse section of one of the Plauenscher Grund fossils is shown in fig. 70. The irregular black patches were described by Petzholdt as portions of cortical tissue, while he regarded the spaces as marking the position of canals like the vallecular canals in an Equisetum. Our more complete knowledge of the structure of a Calamite stem enables us to correlate the patches in which no tissue has been preserved with the broad medullary rays, which separated the wedge-shaped groups of xylem elements; the latter being more resistant were converted into a black coaly substance, while the cells of the medullary rays left little or no trace in the sandstone matrix. The thin black line, which forms the limit of the drawing in fig. 70, external to the carbonised wood, no doubt marks the limit of the cortex, and the appendage indicated in the lower part of the figure may possibly be an adventitious root. It is interesting to note that Unger[594] in 1844 expressed the opinion, which we now know to be correct, that the coaly mass in the specimens described by Petzholdt represented the wood, and that there was no proof of the existence of canals in the cortex as Petzholdt believed.
Fig. 70. Transverse section of a Calamite stem, showing carbonised remnants of secondary wood. From a specimen (no. 40934), presented to the British Museum by Dr Petzholdt from Plauenscher Grund, Dresden. ½ nat. size.
Turning to Brongniart’s later work[595] we find an important proposal which led to no little controversy. While retaining the genus Calamites for such specimens as possess a thin bark and a ribbed external surface, showing occasional branch-scars at the nodes, and having such characters as warrant their inclusion in the Equisetaceae, he proposes a second generic name for other specimens which had hitherto been included in Calamites. The fossils assigned to his new genus Calamodendron are described as having a thick woody stem, and as differing from Equisetum in their arborescent nature. Brongniart’s genus Calamodendron is made to include the plants for which Cotta instituted the name Calamitea, and it is placed among the Gymnosperms. This distinction between the Vascular Cryptogam Calamites and the supposed Gymnosperm Calamodendron is based on the presence of secondary wood in the latter type of stem. The prominence formerly assigned to the power of secondary thickening possessed by a plant as a taxonomic feature, is now known to have been the result of imperfect knowledge. The occurrence of a cambium layer and the ability of a plant to increase in girth by the activity of a definite meristem, is a feature which some recent Vascular Cryptogams[596] share with the higher plants; and in former ages many of the Pteridophytes possessed this method of growth in a striking degree.
Although Brongniart’s distinction between Calamites and Calamodendron has not been borne out by subsequent researches, the latter term is still used as a convenient designation for a special type of Calamitean structure. One of the earliest accounts of the anatomy of Calamodendron stems is by Mougeot[597], who published figures and descriptions of two species, Calamodendron striatum and C. bistriatum.
Some years later Göppert[598], who was one of the greatest of the older palaeobotanists, instituted another genus, Arthropitys[599], for certain specimens of silicified stems from the Permian rocks of Chemnitz in Saxony, which Cotta had previously placed in his genus Calamitea under the name of Calamitea bistriata[600]. Göppert rightly decided that the plants so named by Cotta differed in important histological characters from other species of Calamitea. The generic name Arthropitys has been widely adopted for a type of Calamitean stem characterised by definite structural features. The great majority of the petrified Calamite stems found in the English Coal-Measures belong to Göppert’s Arthropitys.
WILLIAMSON.
The next proposal to be noticed is one by Williamson[601] in 1868; he instituted the generic name Calamopitys for a few examples of English stems, which differed in the structure of the wood and primary medullary rays from previously recorded types. We have thus four names which all stand for generic types of Calamitean stems. Of these Calamodendron and Arthropitys are still used as convenient designations for stems with well-defined anatomical characters. The genus Calamitea is no longer in use, and Williamson’s name Calamopitys had previously been made use of by Unger[602] for plants which do not belong to the Calamarieae. As it is convenient to have some term to apply to such stems as those which Williamson made the type of Calamopitys, the name Arthrodendron is suggested by my friend Dr Scott[603] as a substitute for Williamson’s genus.
The twofold division of the Calamites instituted by Brongniart has already been alluded to, and for many years it was generally agreed that both Pteridophytes and Gymnosperms were represented among the Palaeozoic fossils known as Calamites. The work of Prof. Williamson was largely instrumental in proving the unsound basis for this artificial separation; he insisted on the inclusion of all Calamites in the Vascular Cryptogams, irrespective of the presence or absence of secondary wood. By degrees the adherents of Brongniart’s views acknowledged the force of the English botanist’s contention. It is one of the many signs of the value of Williamson’s work that there is now almost complete accord among palaeobotanical writers as to the affinities of Calamitean plants.