. A handbook of cryptogamic botany. Cryptogams. SELA GINELLA CEM SI the fovea, and separated from it by a ridge called the saddle, is a smaller depression, thefoveola, the lower margin of which forms a lip-like struc- ture, the labium, and from its base rises a narrow membranous acuminate structure, the ligule, with a cordate base, and usually projecting above the foveola. The sheath passes above into the lamina, which is narrow and thick, almost cylindrical, but flattened in front, contains chlorophyll, and is traversed by four wide longitudinal air-cavities, segmented by transverse septa. A
. A handbook of cryptogamic botany. Cryptogams. SELA GINELLA CEM SI the fovea, and separated from it by a ridge called the saddle, is a smaller depression, thefoveola, the lower margin of which forms a lip-like struc- ture, the labium, and from its base rises a narrow membranous acuminate structure, the ligule, with a cordate base, and usually projecting above the foveola. The sheath passes above into the lamina, which is narrow and thick, almost cylindrical, but flattened in front, contains chlorophyll, and is traversed by four wide longitudinal air-cavities, segmented by transverse septa. A rosette of these fertile leaves or sporophylls is pro- duced annually, but between these whorls are alternate whorls of phyl- lades, or imperfect leaves, consisting, in the submerged species, of only a small lamina with no sheath, while in the terrestrial species they are reduced to mere scales. Stomates occur in the paludose and terrestrial, but not in the submerged species. Scattered spiral tracheides are found in the parenchymatous base of the leaf. The fundamental tissue, which. Fig. 33.—Development, of microsporange of T. lacustris. t, tapetal cells ; Tr, trabecules : the archespore and sporogenous cells derived from it are shaded. (After Goebel, magnified,) is not separated from the single ' vascular' bundle by a bundle-sheath, has a strong tendency to become sclerenchymatous, especially beneath the epiderm and in the sheath. The very simple bundle which occurs in each leaf is stated by Russow to be collateral, the xylem and phloem lying side by side. The roots spring from the furrows of the stem, and resemble, in structure and mode of branching, those of Selaginella. There is no rhizophore. The sporanges of Isoetes do not make their appearance until the third year after germination. Each sporophyll bears only a single sporange, which is undoubtedly a product of the leaf, and is situated below the ligule in the fovea, to which it is attached by a narrow base. The outer
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