The above incomplete diagnosis includes only some of the more important structural features of the genus. Thanks to the researches begun by the late Mr Binney of Manchester and considerably extended by Carruthers, Williamson and later investigators, we are now in a position to give a fairly complete account of Calamites. The type of stem most frequently met with in a petrified condition in the English rocks is that to which Göppert applied the name Arthropitys, and it is this subgenus that forms the subject of the following description. Our knowledge of Calamitean anatomy is based on the examination of numerous fragments of petrified twigs and other portions of different specific types of the genus. It is seldom possible to differentiate specifically between the isolated fragments of stems and branches which are met with in calcareous or siliceous nodules. As so frequently happens in fossil-plant material, large specimens showing good surface features and broken fragments with well-preserved internal structure have to be dealt with separately.
YOUNG STEM.
a. Stems.
A transverse section of a young twig, such as is represented in fig. 71, illustrates the chief characteristics of the primary structure of a young branch of Calamites. The figure has been drawn from a section originally described by Hick[605] in 1894. A very young Calamite twig bears an exceedingly close resemblance to the stem of a recent Equisetum. The axial region of the stem may be occupied by parenchymatous cells, or the absence of cells in the centre may indicate the beginning of the gradual formation of the hollow pith, which is one of the characteristics of Calamites. The student of petrified Palaeozoic plants must constantly be on his guard against the possible misinterpretation of Stigmarian ‘rootlets,’ which are frequently found in intimate association with fossil tissues. The intrusion of these rootlets is admirably illustrated by a section of a Calamite stem in the Williamson Collection (No. 1558) in which the hollow pith, 2 cm. broad, contains more than a dozen Stigmarian appendages.
Fig. 71. Transverse section of a young Calamite stem. c, carinal canals; mr, primary medullary rays; a, b, and d, cortex; e, epidermis. From a section in the Manchester Museum, Owens College, × 60.
In the figured specimen of a Calamite twig (fig. 71) there is a clearly marked differentiation into a cortical region and a large stele or central cylinder. The pith-cells are already partially disorganised, but there still remain a few fairly large parenchymatous cells internal to the ring of vascular bundles. The few irregular projections into the cavity of the large pith consist of small fragments of cells, which may be the result of fungal action. Mycelia of fungi are occasionally met with in the tissues of older Calamite stems.
The position of the primary xylem groups is shown by the conspicuous and regularly placed canals, c; these have been formed in precisely the same manner as the corresponding spaces in an Equisetum stem, and they are spoken of in both genera as the carinal canals. Each canal owes its origin to the disorganization and tearing apart of the protoxylem elements and the surrounding cells. This may be occasionally seen in examples of very young Calamites; the canals of a young twig often contain apparently isolated rings which are coils of elongated spiral threads. Fig. 72, B represents the canal of a twig, cut in an oblique direction, in which the remains of spiral tracheids are distinctly seen. In the stem of fig. 71 the development has not advanced far enough to enable us to clearly define the exact limits of each xylem strand. The smaller elements bordering the canals constitute the primary xylem, they are fairly distinct on the outer margin of some of the canals seen in the section. Between the small patches of primary xylem the outward extensions of the parenchyma of the pith constitute the primary medullary rays, mr. The distinct line encircling the canals and primary xylem has been described by Hick as marking the position of the endodermis, but it may possibly owe its existence to the tearing of the tissues along the line where cambial activity is just beginning. This layer of delicate dividing cells would constitute a natural line of weakness. External to this line we have a zone of tissue a, d, containing here and there larger cells with black contents, which are no doubt secretory sacs. It is impossible to distinguish with certainty any definite phloem groups, but in other specimens these have been recognised immediately external to each primary xylem group; the bundles were typically collateral in structure. Towards the periphery of the twig the preservation is much less perfect; the outer portion of the inner cortex, d, consists of rather smaller and thicker-walled cells, but this is succeeded by an ill-defined zone containing a few scattered cells, b, which have been more perfectly preserved. The twig is too young to show any secondary tissue in the cortex; but the tangential walls in some of the cortical cells afford evidence of meristematic activity, which probably represents the beginning of cork-formation. The limiting line, e, possibly represents the cuticularised outer walls of an epidermal layer. The irregularly wavy character of the surface of the specimen is probably the result of shrinking, and does not indicate original surface features.
VASCULAR SYSTEM.
In examining sections of calcareous nodules from the coal seams one meets with numerous fragments of small Calamitean twigs with little or no secondary wood; in some of these there is a small number of carinal canals, in others the canals are much more abundant. The former probably represent the smaller ramifications of a plant, and the latter may be regarded as the young stages of branches capable of developing into stout woody shoots[606]. Longitudinal sections of small branches teach us that the xylem elements next the carinal canals are either spiral or reticulate in character, the older tracheids being for the most part of the scalariform type, with bordered pits on the radial walls. This and other histological characters are admirably shown in the illustrations accompanying Williamson and Scott’s memoir on Calamites. The student should treat the account of the anatomy of Calamites given in these pages as introductory to the much more complete description by these authors. They thus describe the course of the vascular bundles in a Calamitean branch:—