There are certain external characters by which one may often recognise a Calamitean root. There is no division into nodes and internodes as in stems, and as the pith of the root was usually solid the parallel ribs and grooves of stem-casts are not present. In smaller flattened roots there may sometimes be seen a central or excentric black line representing the stele, and the surface of the root presents a curious wrinkled or shagreen texture, probably due to the shrinkage of the loose lacunar cortex. The occasional excentric position of the stele is no doubt due to the displacement of the vascular cylinder as a result of the rapid decay of the cortical tissues. In the Bergakademie of Berlin there are some unusually good examples of Calamite casts bearing well-preserved root-impressions; these include the original specimens figured by Weiss[691].
No doubt some of the roots figured by various writers under the names Pinnularia[692] and Hydatica[693] belong to Calamites, but it is often impossible to identify detached specimens with any certainty.
The section figured diagrammatically in fig. 91 A shows the characteristic single series of large lacunae, l, in the middle cortical region. In the centre there is a wide solid pith surrounded by a ring of vascular tissue, x. The appearance of the middle cortex is very like that of the stem of a water-plant such as Myriophyllum, the Water Milfoil; it shows that the Calamite roots grew either in water or swampy ground. In fig. 91 B, the root characters are clearly seen; the centre of the stele is occupied by large parenchymatous cells which are rather longer than broad in longitudinal view; at the periphery there are four protoxylem groups px, alternating with four groups of phloem, ph, the latter being situated a little further from the centre of the stele. The structure is therefore that of a typical tetrarch root. In the example represented in the figure secondary thickening has begun, and the cambial cells internal to each phloem group have given rise to a few radially disposed tracheids, x2. Beyond the phloem there are two layers of parenchyma representing, as regards position, a pericycle and an endodermis. In the ordinary pericycle and endodermis of the roots of most plants the cells of the two layers are on alternate radii, but in the Calamite root, as in Equisetum roots, the cells of these layers are placed on the same radii, as seen in the neighbourhood of x2 in the figure. This correspondence of the radial walls of the endodermal and pericyclic cells points to the development of both layers from one mother-layer, and suggests the ‘double endodermis’ or phloeoterma of Equisetum (p. 254). The cells in the outer of these two layers have slight thickenings on the radial walls recalling the usual character of endodermal cells. The phloeoterma is succeeded by a few layers of parenchyma, constituting the inner cortex, and beyond this we have the large lacunae separated from one another by slender trabeculae of cells. The outer cortex is limited by a well-defined layer of thick-walled cells, which may be spoken of as the epidermoidal[694] layer. Roots possessing this superficial layer of thicker cells have no doubt lost the original surface-layer which produced the absorptive root-hairs.
Fig. 91.
- Diagrammatic sketch of a transverse section of a young root of Calamites. x, xylem; l, lacuna. After Hick.
- Central cylinder (stele) of root, px, protoxylem; ph, phloem; x2, secondary xylem; l, phloeoterma. × 75. After Williamson and Scott.
The xylem elements have the form of spiral, reticulate and scalariform tracheids.
In roots or rootlets smaller than that shown in fig. 91 B, the primary xylem may extend to the centre of the stele, and form a continuous axial strand; in such examples the structure may be diarch, triarch or tetrarch. The origin of the cambium agrees with that in recent roots, the cells immediately external to the protoxylem tracheids become meristematic, as also those internal to the phloem. Another root-character is seen in the endogenous origin of lateral members. Good examples of branching roots are figured by Williamson[695] and by Williamson and Scott[696].
Older roots[697] are usually found in a decorticated condition. A transverse section of root in which secondary thickening has been active for some time presents on a superficial view a close resemblance to a stem of Calamites, but a careful comparison at once reveals important points of difference. The specimen diagrammatically sketched in fig. 92 illustrates very clearly the origin of a root from the node of a Calamite stem. The section has passed through a stem in a tangential direction, showing the characteristic arrangement of the vascular bundles x, and principal medullary rays m. The small leaf-traces, t, t, afford another feature characteristic of a Calamite stem. The portion of stem to the right of the figure has been slightly displaced, and between this piece and the root R, one of the ubiquitous Stigmarian appendages, s, has inserted itself. At R a fairly thick and decorticated root is seen in oblique transverse section; at the upper end the root tracheids are seen in direct continuity with the xylem of the stem. In the centre of the root is the large solid pith surrounded by twelve bluntly pointed xylem groups, composed in the main of radially disposed scalariform elements with narrow secondary medullary rays like those in a stem. Between each xylem group there is a broad medullary ray, which tapers rapidly towards the outside, and is soon obliterated by the formation of interfascicular secondary xylem. At R′ a portion of another root is seen in transverse section, and R″ the inner part of a single xylem group is shown more clearly. The solid pith and the absence of carinal canals are the two most obvious distinguishing features of the roots.
