A stem in this stage of development is seen in fig. 105, where only the wood and internal tissues are preserved. The very characteristic canals associated with the primary bundles are clearly shown. The amount of secondary wood steadily increased as the stems grew (there appear to have been no “annual rings”) till there was a very large quantity of secondary tissue of regular texture, through which ran small medullary rays, so that the stems became increasingly like those of the higher plants as they grew older. It is the primary structure which is the important factor in considering their affinity, and that is essentially the same as in the other members of the family in which secondary thickening is not developed. As we have seen already in other groups of fossils, secondary wood appears to develop on similar lines whenever it is needed in any group, and therefore has but little value as an indication of systematic position. This important fact is one, however, which has only been realized as a result of the study of fossil plants. [Illustration: Fig. 106.—Diagram of the Arrangement of the Bundles at the Node of a _Calamite_, showing how those of consecutive internodes alternate _n_, Region of node] [Illustration: Fig. 107.—Leaf of _Calamites_ in Cross Section _v_, Vascular bundles; _s_, cells of sheath, filled with blackened contents; _p_, palisade cells; _e_, epidermis.] The longitudinal section of the stems, when cut tangentially, is very characteristic, as the bundles run straight down to each node and there divide, the neighbouring halves joining so that the bundles of each node alternate with those of the ones above and below it (see fig. 106). The leaves which were attached at the nodes were naturally much larger than those of the present Equisetums, though they were similarly simple and undivided. Their anatomy is preserved in a number of cases (see fig. 107), and was simple, with a single small strand of vascular tissue lying in the centre. They had certain large cells, sometimes very black in the fossils, which may have been filled with mucilage. [Illustration: Fig. 108.—Transverse Section of Young Root of _Calamites_ _w_, Wood of axis; _l_, spaces in the lacunar cortex, whose radiating strands _r_ are somewhat crushed; _ex_, outermost cells of cortex with thickened wall.] [Illustration: Fig. 109.—Diagram of Cone of _Calamites_ A, Main axis; _br_, sterile bracts; _sp_, sporophylls with four sporangia S attached to each, of which two only are seen.]