Von Mohl[141], and a few years later Hofmeister[142], were the first botanists to give a satisfactory account of the anatomy of _Isoetes_ but it is only recently[143] that fresh light has been thrown upon the structural features of the genus the interest of which is enhanced by the many points of resemblance between the recent type and the Palaeozoic Lepidodendreae. A striking anatomical feature is the power of the stem to produce secondary vascular and non-vascular tissue; the genus is also characterised by the early appearance of secondary meristematic activity which renders it practically impossible to draw any distinct line between primary and secondary growth. A cylinder of thin-walled tissue (fig. 133, A, _a_) surrounds the primary central cylinder and in this a cambial zone, _c_, is recognised even close to the stem-apex; this zone of dividing cells is separated from the xylem by a few layers of rectangular cells to which the term prismatic zone has been applied. The early appearance of the cambial activity on the edge of the vascular cylinder is shown in fig. 133, C, which represents part of a transverse section of a young stem. A leaf-trace, _lt_, is in connexion with the primary xylem, _x′_, which consists of short tracheids, often represented only by their spiral or reticulately thickened bands of lignified wall, and scattered parenchyma. Some of the radially elongated cells on the sides of the leaf-trace are seen to be in continuity on the outer edge of the stele, at _st_, with flattened elements, some of which are sieve-tubes. The position of a second leaf-trace is shown at _lt′_. External to the sieve-tubes the tissue consists of radially arranged series of rectangular cells, some of which have already assumed the function of a cambium (_c_). The tissue produced by the cambium on its inner edge consists of a varying amount of secondary xylem composed of very short spiral tracheids; a few of these may be lignified (fig. 133, A, _x²_) while others remain thin.