It would appear from the examples selected to illustrate the connection between strobili and vegetative shoots, that the Annularia type of branch usually bears cones which conform to the genus Calamostachys (Stachannularia); while the Asterophyllitean branches—Calamocladus—are associated with Palaeostachya and Macrostachya. But this rule is not constant, and we are not in a position to speak of cones of a particular type as necessarily characteristic of definite types of Calamitean shoots.
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Although it is admitted by the great majority of Palaeobotanists that the Calamites were all true Vascular Cryptogams, the older view that some members of the Calamarieae are gymnospermous has not been given up by Renault[752]. This observer has recently described some seeds which he believes were borne by Calamitean stems; he admits, however, that no undoubted female cones of Calamodendron have so far been found. In view of the unsatisfactory evidence on which Renault’s opinion is based, we need not further discuss the questions which he raises.
[The following specimens in the Williamson Cabinet in the British Museum, may be found useful in illustration of the structure of Calamites.
Stems. (i. Arthropitys.) Young twigs and small branches 1, 2, 6, 10, 14, 19, 116*, 1002, 1007, 1020.
Older stems (transverse sections) 15–17, 62, 77–87, 115 a, 117*, 118*, 120, 122*–124*, 1933 A, 1934, 1941.
(Tangential sections) 20, 24, 26, 37, 38, 49, 90, 91, 130, 138, 1937, 1943.
(Radial sections) 20, 20 A, 21, 22, 48, 65–68, 83–91, 137*, 138*, 1937.
(ii. Arthrodendron) 36, 37, 38, 52, 54.
Roots. 1335, 1347, 1350, 1356.
Strobili. i. Calamostachys Binneyana. 991, 996, 997, 1000, 1003, 1005, 1007, 1008, 1011, 1013, 1016, 1017, 1022, 1023, 1037 A, 1043.
ii. C. Casheana. 1024, 1025, 1587, 1588.
iii. Palaeostachya vera. 110, 1564, 1567, 1569, 1579, 1583.]
III. Pith-casts of Calamites.
A. Calamitina. B. Stylocalamites. C. Eucalamites.
Palaeobotanical literature contains a large number of species of Calamites founded on pith-casts alone. Many of these so-called species are of little or no value botanically, but while we may admit the futility of attempting to recognise specific types in the same sense as in the determination of recent plants, it is necessary to pay attention to such characters as are likely to prove of value for descriptive and comparative purposes. From the nature of the specimens it is clear that many of the differences may be such as are likely to be met with in different branches of the same species, while in others the pith-casts of distinct species or genera may be almost identical.
The most striking differences observable in Calamite casts are in the character of the internodes, the infranodal canals, the number and disposition of branch-scars, and other surface features. Occasionally it is possible to recognise certain anatomical characters in the coaly layer which often surrounds a shale- or sandstone-cast, and the surface of a well preserved cast may give a clue to the nature of the wood in the faint outlines of cells which can sometimes be detected on the cast itself[753]. The breadth of the carbonaceous envelope on a cast has been frequently relied on by some writers as an important character. It has been suggested[754] that we may arrive at the original thickness of the wood of a stem by measuring the coaly layer and multiplying the breadth by 27; the explanation being that a zone of wood 27 mm. in thickness is reduced in the process of carbonisation to a layer 1 mm. thick.
The breadth of the coal on the same form of cast may vary considerably; on this account, and for various other reasons, such a character can have but little value. Our knowledge of anatomy may often help us to interpret certain features of internal casts and to appreciate apparently unimportant details. One occasionally notices that a Calamite pith-cast has large infranodal canals, and in some specimens each internodal ridge may be traversed by a narrow median line or small groove; large infranodal canal casts suggest the type of stem referred to the subgenus Arthrodendron, and the median line on the ridges may be due to bands of hard tissue in each principal medullary ray.
In attempting to identify pith-casts the student must keep in view the probable differences presented by the branching rhizome, the main aerial branches and the finer shoots of the same individual. The long internodal ridges of some casts may be mistaken for the parallel veins of such a leaf as Cordaites, a Palaeozoic Gymnosperm, if there are no nodes visible on the specimen. The fossil figured by Lindley and Hutton[755] as Poacites, and regarded by them as a Monocotyledon, is no doubt a portion of a Calamite with very long internodes. An interesting example of incorrect determination has recently been pointed out by Nathorst[756] in the case of certain casts from Bear Island, originally described by Heer as examples of Calamites; the vertical rows of leaf-trace casts on a Knorria were mistaken for the ribs of a Calamite stem. The specimens in the Stockholm Museum fully bear out Nathorst’s interpretation. The undulating course of internodal ridges and grooves is not in itself a character of specific value. If a Calamite stem were bent slightly, the wood and medullary-ray tissues on the concave side might adapt themselves to the shortening of the stem by becoming more or less folded, and a cast of such a stem would show undulating ridges and grooves on one side and straight ones on the other[757].