Fig. 73.—A, a club moss (Lycopodium), × ⅓. x, cone. r, root. B, a cone, × 1. C, single scale with sporangium (sp.). D, spores: i, from above; ii, from below, × 325. E, cross section of stem, × 8. f.b. fibro-vascular bundle. F, portion of the fibro-vascular bundle, × 150. G, cells of the ground tissue, × 150.
The xylem (F, xy.) of the fibro-vascular bundle is composed of tracheids, much like those of the ferns; the phloem is composed of narrow cells, pretty much all alike.
The spores (D) are destitute of chlorophyll and have upon the outside a network of ridges, except on one side where three straight lines converge, the spore being slightly flattened between them.
Almost nothing is known of the prothallia of our native species.
The second order (Ligulatæ) is represented by two very distinct families: the smaller club mosses (Selaginelleæ) and the quill-worts (Isoeteæ). Of the former the majority are tropical, but are common in greenhouses where they are prized for their delicate moss-like foliage ([Fig. 74], A).
Fig. 74.—A, one of the smaller club mosses (Selaginella). sp. spore-bearing branch, × 2. B, part of a stem, sending down naked rooting branches (r), × 1. C, longitudinal section of a spike, with a single macrosporangium at the base; the others, microsporangia, × 3. D, a scale and microsporangium, × 5. E, young microsporangium, × 150. The shaded cells are the spore mother cells. F, a young macrospore, × 150. G, section of the stem, × 50. H, a single fibro-vascular bundle, × 150. I, vertical section of the female prothallium of Selaginella, × 50. ar. archegonium. J, section of an open archegonium, × 300. o, the egg cell. K, microspore, with the contained male prothallium, × 300. x, vegetative cell. sp. sperm cells. L, young plant, with the attached macrospore, × 6. r, the first root. l, the first leaves.
The leaves in most species are like those of the larger club mosses, but more delicate. They are arranged in four rows on the upper side of the stem, two being larger than the others. The smaller branches grow out sideways so that the whole branch appears flattened, reminding one of the habit of the higher liverworts. Special leafless branches (B, r) often grow downward from the lower side of the main branches, and on touching the ground develop roots which fork regularly.
The sporangia are much like those of the ground pines, and produced singly at the bases of scale leaves arranged in a spike or cone (A, sp.), but two kinds of spores, large and small, are formed. In the species figured the lower sporangium produces four large spores (macrospores); the others, numerous small spores (microspores).
Even before the spores are ripe the development of the prothallium begins, and this is significant, as it shows an undoubted relationship between these plants and the lowest of the seed plants, as we shall see when we study that group.