Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/116

106 FERNS The Salviniece include Sahinia and Azolla. Salvinia is a cosmopolitan genus of five or six species of floating plants distributed over the tropical and warm temperate zones. The sporocarps are developed from teeth of the fertile leaves round which in each case a circular wall grows up covering in the tooth on which the sporangia are developed. This arrangement has been compared to the cup-shaped "involucre" of Hymenophyllum. The sporangia are in all the genera trichomes, and their mode of development is very similar to that in Poly- podiacecs. The central mother-cell divides ultimately into eight spore mother-cells. The further progress, how ever, of these varies according as microspores or macrospores are to be formed, each of the eight cells dividing into four in the one case, while in the latter only one does so, and only one of the four daughter-cells attains maturity. The oophore or prothallial generation in RMzocarpeve is dioecious from the first, the macrospore alone producing archegonia. This, however, only carries to an extreme point a differentiation of which there are indications amongst ferns. In Osimmda, all the spores from a sporangium sometimes produce prothallia with antheridia alone. In Rhizocarpeve the differentiation is carried back to the spores themselves. The microspores are male (fig. 14) ; in Salvinia there is a rudimentary prothallium which produces antheridia, and in Marsilea and Salvinia there is a last trace of such a structure. The macrospores also develop a prothallium from a portion of their contents, which, however, is always for the most part included in the parent spore, which ruptures to expose the archegonia (fig. 15), and allow of the fertilization of one of them. Fis. 16. Fie. 18. FIG. 14. Microsporc of Pilularia globulifera burst, and allowing the escape of the antherozoids. FIG. 15. Longitudinal section through the apex of a macrospore of Pilularia globulifera, showing the rudimentary prothallium with an archegonium (a), containing a fertilized oosphere. FIG. 1C. Longitudinal section through rhizome of Equisetum Telmateia, show ing K, septum between two internodul cavities, lt,h; g, g, fibre-vascular bundles; 7, air-canal; S, leaf-sheath. FIG. 17. Union (K) of alternating fibro-vascular bundles (if) of an upper and lower internode in Equisetum Telmateia. FIG. 18. Transverse section of rhizome of Equisetum Telmateia, g, fibro- vascular bundles ; /, air-canals. II. EQUISETIN.E are plants the habit of which in the asexual stage (sporophore) is very singular. The stems are furrowed and jointed, the joints being hollow and closed at each node by a transverse septum (fig. 16). The leaves are reduced to a whorl of membranous teeth which form a sheath at each node, perforated by the slender branches, which also form a verticil. The teeth of each successive node alternate. The fibro-vascular bundles also alternate in each successive internode, since they are arranged in a ring, and correspond to the ridges between the furrows which above are continuous into the teeth of a leaf-sheath (fig. 17). Each fibro-vascular bundle contains an air-canal, and another series of air-canals alternate with these and correspond to the furrows (fig. 18). The spore-bearing shoots are usually a good deal different in appearance from the barren stems; they terminate in a kind of cone (fig. 19), Fi". 21. Fig. 19. FIG. 19. Fructification of Equisetum Tdmatda. FIG. 20. Peltate scale of Egitisetum (s), bearing sporangia (0- FIG. 21. Spore of Equisetum with the four claters, produced by splitting of the exosporc, dry and expanded. FIG. 22. The same with the elaters damp and coiled up. composed of closely approximated whorls of modified leaves, which are peltate in form, and bear from 5 to 1 sporangia on their under surface (fig. 20). The spores are furnished with an external coat which splits up into four strips, the hygro scopic movements of which actively assist their dispersion (figs. 21, 22). The oophore consists of a prothallium, which is curiously lobed and curled, but otherwise differs little from that of ferns. III. LYCOPODIN.E (Dichotomy, Sachs). With a con siderable range of morphological variation the different genera agree in some important points. The leaves are always simple and unbranched, and contain only a single fibro-vascular bundle. The branching of stems and roots is always dichotomous, and the dichotomies, with some exceptions, succeed one another in planes at right angles. In their reproductive processes Lycopodiacece and Ligulatce are related in the same manner as Filices and Rkizocarpece. 1. LT/cojwdiacecp, besides the club-mosses (Lycopodium), includes three other small genera, Psilotum, Tmesipteris, and Pkylloglossum. The first belongs to the warm countries of the new world, and is a singular plant with slender dichotomizing stems, very small leaves, and no true roots. PhyllogloMUin is a minute Australian plant, with a small tuber which produces a rosette of a few long leaves and a stalked spike of sporangia. The species of Lycopodium (fig. 23) attain a more vigorous vegetative development, some almost reaching the size of small shrub?. Their stems contain an axial cylinder, formed by the greater or less coalescence of a number of fibro-vascular bundles forming transverse bands in cross-section. These bands are composed of xylem, with wide scalariform vessels and spiral vessels at each end ; between the xylem plates are the phloem common to each, and containing sieve-tubes. The whole surface is surrounded by a phloem-sheath and a fibro-