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. [Illustration: FIG. 91. _A._ Diagrammatic sketch of a transverse section of a young root of _Calamites_. _x_, xylem; _l_, lacuna. After Hick. _B._ 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.