Phloem elements, recognisable by the presence of a thickened reticulum enclosing small sieve-areas (fig 133, B, _s_) are fairly abundant, and for the rest this intracambial region is composed of thin-walled parenchyma. In longitudinal section these tissues present an appearance almost identical with that observed in a transverse section. Fig. 133, B represents a longitudinal section, through the intracambial zone and the edge of the stele, of a younger stem than that shown in fig. 133, A. Most of the radially disposed cells internal to the meristematic region are parenchymatous without any distinctive features; a few scattered sieve-tubes (_s_) are recognised by their elliptical sieve-areas and an occasional tracheid can be detected. The cambium cuts off externally a succession of segments which constitute additional cortical tissue (fig. 133, A, _cr_) of homogeneous structure, composed of parenchymatous cells containing starch and rich in intercellular spaces. As the stem grows in thickness the secondary cortex reaches a considerable breadth and the superficial layers are from time to time exfoliated as strips of dead and crushed tissue (fig. 133, A, _b_). The diagrammatic sketch reproduced in fig. 133, A, serves to illustrate the arrangement and relative size of the tissue-regions in an _Isoetes_ stem. In the centre occur numerous spirally or reticulate tracheae scattered in parenchymatous tissue which has been considerably stretched and torn in the peripheral region of the stele; the radiating lines mark the position of the leaf-traces (_lt_) in the more horizontal part of their course. The zone between the cambium (_c_) and the edge of the central cylinder consists of radially disposed secondary tissue of short, and for the most part unlignified, elements including sieve-tubes and parenchyma; the secondary xylem elements consist largely of thin-walled rectangular cells with delicate spiral bands, but discontinuous rows of lignified tracheae (_x²_) occur in certain regions of the intracambial zone. The rest of the stem consists of secondary cortex (_cr_) with patches of dead tissue (_b_) still adhering to the irregularly furrowed surface. The structure of the cambium and its products is shown in the detailed drawing reproduced in fig. 133, D. Many of the elements cut off on the inner side of the cambium exhibit the characters of tracheids: most of these are unlignified, but others have thicker and lignified walls (_tr_).