Specimens of pith-casts occasionally present the appearance of a curved and rapidly tapered ram’s horn, and the narrow end of such a cast is sometimes found in contact with the node of another cast. This juxtaposition of casts is shown unusually well in fig. 82. In some of the published restorations of Calamites the plant is represented as having thick branches attached to the main stem by little more than a point. Williamson[624] clearly explained this apparently unusual and indeed physically impossible method of branching, by means of sections of petrified stems. The branches seen in fig. 82 are of course pith-casts, and in the living plant the pith of each branch was surrounded by a mass of secondary wood developed from as many primary groups of xylem as there are grooves on the surface of the cast, each of the grooves on an internode corresponding to the projecting edge of a xylem group. At the junction of one branch with another the pith was much narrower and the enclosing wood thicker, so that the tapered ends of the cast merely show the continuity by a narrow union between the pith-cavities of different branches. Most probably the casts of fig. 82 are those of a branched rhizome which grew underground, giving off aerial shoots and adventitious roots. There is a fairly close resemblance between the Calamite casts of fig. 82 and a stout branching rhizome of a Bamboo, e.g. Bambusa arundinacea Willd.; it is not surprising that the earlier writers looked upon the Calamite as a reed-like plant.
Before leaving the consideration of stem structures there is another feature to which attention must be drawn. On the casts shown in fig. 82 there is a circle of small oval scars situated just below the nodes, these are clearly shown at c, c, c. Each of the scars is in reality a slight projection from the upper end of an internodal ridge. As the ridges correspond to the broad inner faces of medullary rays, the small projection at the upper end of each ridge is a cast of a depression or canal which existed in the medullary tissue of the living plant. There have been various suggestions as to the meaning of these oval projections; several writers have referred to them as the points of attachments of roots or other appendages, but Williamson proved them to be the casts of canal-like gaps which traversed the upper ends of principal medullary rays in a horizontal direction. In a tangential section of a Calamite stem the summit of each primary medullary ray often contains a group of smaller elements which are in process of disorganisation, and in some cases these cells give place to an oval and somewhat irregular canal. In the diagrammatic tangential section represented in fig. 83, A the upper end of each ray is perforated by a large oval space, which has been formed as the result of the breaking down of a horizontal band of cells. Williamson designated these spaces infranodal canals. While proving that they had nothing to do with the attachment of lateral members, he suggested that they might be concerned with secretion; but their physiological significance is still a matter of speculation. The casts of infranodal canals are especially large and conspicuous in the subgenus Arthrodendron, a form of Calamite characterised by certain histological features to be referred to later. Williamson[625] originally regarded the presence of infranodal canals as one of the distinguishing features of Arthrodendron, but they occur also in the casts of the commoner type Arthropitys. As a rule we have only the cast of the inner ends of the infranodal canals preserved as slight projections like those in fig. 83, A; but in one exceptionally interesting pith-cast described by Williamson, these casts of the infranodal canals have been preserved as slender spoke-like columns radiating from the upper ends of the ridges of the infranodal region of a pith-cast.
Fig. 82. Branched rhizome of Calamites. ½ nat. size. C, C, nodes showing casts of infranodal canals. From a specimen in the Manchester Museum, Owens College.
This specimen, which was figured by Williamson[626] in two of his papers, and by Lyell[627] in the fifth edition of his Elementary Geology, is historically interesting as being one of the first important plants obtained by Williamson early in the fifties, when he began his researches into the structure of Carboniferous plants. A joiner, who was employed by Williamson to make a piece of machinery for grinding fossils, brought a number of sandstone fragments as an offering to his employer, whom he found to be interested in stones. The specimens “were in the main the merest rubbish, but amongst them,” writes Williamson, “I detected a fragment which was equally elegant and remarkable.... In later days, when the specimen so oddly and accidentally obtained, came to be intelligently studied, its history became clear enough, and the priceless fragment is now one of the most precious gems in my cabinet[628].”
Comparison of three types of structure met with in Calamitean stems,—Arthropitys, Arthrodendron, and Calamodendron.
ARTHROPITYS.
The anatomical features which have so far been described as characteristic of Calamites represent the common type met with in the English Coal-Measures. The same type occurs also in France, Germany and elsewhere. It is that form of stem known as Arthropitys, a sub-genus of Calamites.
Arthropitys may be briefly diagnosed as follows,—confining our attention to the structure of the stem: A ring of collateral bundles surrounds a large hollow pith, each primary xylem strand terminates internally in a more or less bluntly rounded apex traversed by a longitudinally carinal canal. The principal medullary rays consist of large-celled parenchyma, of which the individual elements are usually tangentially elongated as seen in transverse section, and four or five times longer than broad as seen in a tangential longitudinal section. The secondary xylem consists of scalariform and reticulately pitted tracheids; the interfascicular xylem may be formed completely across each primary ray at an early stage in the growth of the stem[629], or it may be developed more gradually so as to leave a tapering principal ray of parenchyma between each primary xylem bundle. In the latter case the principal rays present the characteristic appearance shown in figs. 71, 74, A, 75 and 78, a type of stem which we may refer to as Calamites (Arthropitys) communis. In the former case the stem presents the appearance shown in fig. 83, D[630]. A third variety of Arthropitys stem is one which was originally named by Göppert Arthropitys bistriata; in this form the principal rays retain their individuality as bands of parenchyma throughout the whole thickness of the wood[631]. Such stems as those of figs. 73 and 74, B, may be young examples of Arthropitys communis or possibly of A. bistriata. The narrow secondary medullary rays of Arthropitys usually consist of a single row of cells which are three to five times higher than broad, as seen in tangential longitudinal section. Infranodal canals occur in some examples of Arthropitys.
ARTHRODENDRON.