_I. hystrix_ appears to be exceptional in retaining its leaf-bases, which form a complete protective investment and prevent the exfoliation of dead cortex. Each leaf-trace consists of a few spiral tracheids accompanied by narrow phloem elements directly continuous with the secondary phloem of the intracambial zone. Dr Scott and Mr Hill have pointed out that a normal cambium is occasionally present in the stem of _I. hystrix_ during the early stages of growth; this gives rise to xylem internally. The few phloem elements observed external to the cambium may be regarded as primary phloem, a tissue not usually represented in an Isoetes stem[144]. The occasional occurrence of this normal cambium, may, as Scott and Hill suggest, be a survival from a former condition in which the secondary thickening followed a less peculiar course. The lower leaf-traces become more or less obliterated as the result of the constant increase in thickness of the broad zone of secondary tissues through which they pass. The adventitious roots are developed acropetally and arranged in parallel series on each side of the median line of the two or three furrows. The three arms of the triangular stele of _I. hystrix_ and the two narrow ends of the long axis of the stele of _I. lacustris_, which in transverse section has the form of a flattened ellipse, are built up of successive root-bases. A root of _Isoetes_ (fig. 133, G) possesses one vascular bundle, _x_, with a single strand of protoxylem, _px_, thus agreeing in its monarch structure with the root-bundle in _Selaginella_ and many species of _Lycopodium_. The cortical region of the root consists of a few layers of outer cortex succeeded by a large space, formed by the breaking down of the inner cortical tissue, into which the vascular bundle projects (fig. 133, F). The peculiarity of the roots in having a hollow cortex and an eccentric vascular bundle was noticed by Von Mohl[145]. In the monarch bundles, as in the fistular cortex and dichotomous branching, the roots of _Isoetes_ present a striking resemblance to the slender rootlets of the Palaeozoic _Stigmaria_ (see Page 246). The longitudinal section through the base of a root of _Isoetes lacustris_ shown in fig. 133, F, affords a further illustration of certain features common to the fossil and recent types. FOSSIL LYCOPODIALES. Isoetaceae