Fig. 92. Tangential section through a node of Calamites, showing a root in organic connection with the stem.
R, R′. Root (Astromyelon) in transverse and oblique section, x, xylem; m, primary medullary ray; t, leaf-trace; s, Stigmarian appendage.
R″, the inner portion of one of the xylem wedges of R′ more highly magnified. Sketched from a section in the Cambridge Botanical Laboratory Collection.
As Renault points out, roots of Calamites have been figured by some writers[698] as examples of stems, but it is usually comparatively easy to distinguish between roots and stems. On examining the xylem groups more closely, one notices that the apex of each is occupied by a triangular group of centripetally-developed primary tracheids, the narrow spiral protoxylem elements occupying the outwardly directed apex. The protoxylem apex is usually followed externally by a ray of one or two radially disposed series of parenchymatous cells. This ray is not distinguished in fig. 92 R″ from the rows of xylem tracheids. Each xylem group is thus formed partly of centripetal xylem and in part of secondary centrifugal xylem; the latter is associated with secondary medullary rays, as in stems, and contains a broader ray (fascicular ray of Williamson and Scott[699]) immediately opposite each protoxylem strand. In the roots of recent plants (e.g. Cucurbita, Phaseolus, &c.) a broad medullary ray is often found opposite the protoxylem, and such an arrangement is a perfectly normal structure in roots[700].
Renault has recently described several species of Calamite roots which he designates by specific names, some of them belonging to stems with the Arthropitys structure, and others to Calamodendron. Some of the roots figured by the French author have an axial strand of xylem with 7–15 projecting angles of protoxylem[701]. These he considers true roots, but the larger specimens with a wide pith he prefers to regard as stolons. In the latter he mentions the union of the primary centripetal with the secondary centrifugal wood as a distinguishing feature. It has been shown, however, that each group of secondary xylem includes a median ray of parenchyma, and that the whole structure is essentially that of a root, and not that of a modified stem or stolon. The organs described by Renault as true roots are probably rootlets, and as Williamson and Scott have demonstrated, there is every gradation between the smaller specimens with a solid xylem axis and those with a large central pith.
It is interesting to note that Renault’s figures of Calamodendron roots show the closest resemblance to those of the subgenus Arthropitys.
d. Cones.
The occurrence of fossil plants in the form of isolated fragments is a constant source of difficulty, and is well illustrated by the numerous examples of strobili which cannot be connected with their parent stems. We are, however, usually able to recognise Calamitean cones if the impressions or petrified specimens are fairly well preserved, but it is seldom possible to correlate particular types of cones with the corresponding species of foliage-shoots or stems. Palaeobotanical literature contains numerous illustrations and descriptions of long and narrow strobili designated by different generic terms such as Volkmannia, Brukmannia, Calamostachys, Macrostachya and others; many of these have since been recognised as the cones of Calamites, while some species of Volkmannia have been identified with Sphenophyllum stems. Before further considering the general question of Calamite cones, a few examples may be described in detail as types of fructification which are known to have been borne by Calamites. The examples selected are species of the two provisional genera Calamostachys and Palaeostachya.
The usual form of a Calamite cone is illustrated in fig. 93, which represents a fertile shoot bearing a few narrow linear leaves of the Calamocladus type; in the axils of some of these are borne the long strobili.
Fig. 93. Calamostachys sp. A fertile Calamitean shoot. From a specimen in the Geological Survey Museum, Jermyn Street, London. From the Upper Coal-Measures of Monmouthshire (No. 5539).
Calamostachys Binneyana (Carr.). Figs. 94 and 95.