Professor Campbell[131] records an additional example of a _Selaginella_—probably _S. Bigelovii_—from the dry region of Southern California in which the spores become completely dried up after the embryo has attained some size, remaining in that state until the more favourable conditions succeeding the dry season induce renewed activity. =Isoetaceae.= The genus _Isoetes_ is peculiar among Pteridophytes both in habit and in anatomical features. In its short and relatively thick tuberous stem, terminating in a crowded rosette of subulate leaves like those of _Juncus_ and bearing numerous adventitious roots, _Isoetes_ presents an appearance similar to that of many monocotyledonous plants. The habit of the genus is well represented by such species as _Isoetes lacustris_ and _I. echinospora_[132] (fig. 132) both of which grow in freshwater lakes in Britain and in other north European countries. The latter species bears leaves reaching a length of 18 cm. The resemblance in habit between this isolated member of the Pteridophytes and certain Flowering plants, although in itself of no morphological significance, is consistent with the view expressed by Campbell that _Isoetes_ may be directly related to the Monocotyledons[133]. [Illustration: FIG. 132. _Isoetes echinospora_ (After Motelay and Vendryès). A. Stem of _I. lacustris._ B. Base of sporophyll: _l_, ligule; _spg_, sporangium partially covered by velum.] There is as a rule little or no difference between the foliage leaves and sporophylls; in _I. lacustris_ the latter are rather larger and in the terrestrial species _I. hystrix_[134] the sterile leaves are represented by the expanded basal portions only, which persist like the leaf-bases of _Lepidodendron_ as dark brown scales to form a protective investment to the older part of the stem. The innermost leaves are usually sterile; next to these are sporophylls bearing megasporangia, and on the outside are the older sporophylls with microsporangia. The long and slender portion of the leaf becomes suddenly expanded close to its attachment to the stem into a broad base of crescentic section which bears a fairly conspicuous ligule (figs. 132, B, _l_, 133, E, _l_) inserted by a foot or glossopodium in a pit near the upper part of the concave inner face. The ligule is usually larger than that of _Selaginella_, though of the same type. The free awl-like lamina contains four large canals bridged across at intervals by transverse diaphragms, and in the axial region a single vascular bundle of collateral structure. Other vascular elements, in the form of numerous short tracheids occur below the base of the transversely elongated ligule.