Page:EB1911 - Volume 22.djvu/627

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611
PTERIDOPHYTA

leptophylla is perennial, the sporophyte being annually borne on it. The phenomena of apogamy and apospory which have now been observed in a number of Ferns, may be mentioned here. In the former the prothallus produces one or more fern-plants vegetatively, the projection which develops into the sporophyte in many cases occupying the position of an archegonium. In some apogamous Ferns sporangia may occur on the prothallus and the vegetative organs of the sporophyte may also occur singly. In apospory the converse phenomenon is seen, the gametophyte springing vegetatively from the sporangium, receptacle of the sorus, or leaf-margin of the fern-plant. In a number of cases, though not in all, apospory appears to be correlated with a failure of the sporangia to develop.

EB1911 Pteridophyta - Nephrodium filix-mas.jpg
(From Strasburger's Lehrbuch der Botanik.)
Fig. 7.—Nephrodium filix-mas.
A, Prothallus viewed from the lower surface; ar, archegonia; an, antheridia; rh, rhizoids (much enlarged).
B, Prothallus bearing a young fern plant; b, first leaf; w, primary root.
EB1911 Pteridophyta - Polypodium vulgare - antheridia and spermatozoids.jpg
(From Strasburger's Lehrbuch der Botanik.)
Fig. 8.—Polypodium vulgare.
A, Mature antheridium.
B, Empty antheridium; p, prothallial cell; 1, 2, cells of antheridial walls; 3, cap cell.
C, D, Spermatozoids.

The adaptations in the vegetative organs of the sporophyte are similar to those in the Flowering Plants. Thus there are a few Ferns which climb, others are water plants, while many, especially those which live as epiphytes, are more or less xerophytic. Some of the epiphytic forms (Polypodium quercifolium, Platycerium) have strongly dimorphic leaves, the sterile leaves serving in some cases to catch falling débris, and thus to provide the plant with soil. Lastly, the symbiotic relation between the plant and ants is found in Ferns, the rhizome of Polypodium carnosum containing cavities inhabited by these insects. The existence of these myrmecophilous Ferns suggests a possible explanation of the nectaries on the leaves of some other species, such as the Common Bracken.

The main existing groups of the Filicaceae may now be briefiy described, with special reference to the characters of gametophyte and sporophyte, which have been found of value in determining affinities.

Marattiaceae.—These are ferns of considerable size, the large leaves of which are borne on a short, erect, swollen stem (Angiopteris, Marattia), or arise from a more or less horizontal rhizome (Danaea, Kaulfussia). The leaves, at the base of which are two large stipule-like outgrowths, have a thick leaf-stalk, and are simple or simply pinnate in Danaea, pinnate in Archangiopteris, bi- to tri-pinnate in Marattia and Angiopteris, and digitately lobed in Kaulfussia. The stem, from the ground tissue of which sclerenchyma is absent, has a complicated system of steles arranged in concentric circles; the thick roots, the central cylinders of which have several alternating groups of xylem and phloem, arise in relation to these. The pinnae, except in a few filmy forms, are thick; in Kaulfussia large pores derived from stomata occur in the epidermis. The sori are borne on the under surface of the pinnae, usually in a single row on either side of the midrib, but in Kaulfussia dotted over the expanded lamina. The large sporangia, each of which originates from a number of superficial cells, are here incompletely separated from one another and arranged in a single circle forming a synangium. The association is closest in Danaea, where the individual sporangia of the elongated sorus, which is sunk in a depression of the leaf, open by pores; in Marattia and Kaulfussia (fig. 2, e) they dehisce by slits on the inner face; while in Angiopteris (fig. 2, f) they are almost free from one another. The spores produce a green prothallus of large size, the sexual organs of which hardly project from the surface. The cotyledon and stem grow up vertically through the prothallus, the root turning downwards into the soil.

EB1911 Pteridophyta - Polypodium vulgare - archegonia.jpg
(From Strasburger's Lehrbuch der Botanik.)
Fig. 9.—Polypodium vulgare.
A, Unopened archegonium; o, ovum; k″, ventral canal cell; k′ neck-canal-cell.
B, Mature opened archegonium.

Osmundaceae.—The two genera of this group, Osmunda and Todea, have thick erect stems, covered with the closely crowded leaf bases. The stem is monostelic, the vascular tissues being separated into curved groups comparable with collateral vascular bundles, which surround the pith. The somewhat thick roots are diarch. The leaves are large and pinnate; their, lamina is usually thick, though filmy species of Todea occur. The leaf-base shows indications of stipular outgrowths. In Todea the sori, each of which consists of a single circle of bulky sporangia, are borne on the under surface of the pinnae. In Osmunda the region of the leaf which bears the sporangia has its lamina little developed; the leaf thus bears sterile and fertile pinnae, or, as in O. cinnamomea, sterile and fertile leaves may be present. The sporangia originate from single cells, though surrounding cells may contribute to the formation of the stalk. The latter is thick and short, and the wall of the sporangium, which opens by a median slit, has a group of thick-walled cells at the summit, forming the annulus. The prothalli are similar to those of the other Filicaceae, but more massive; the same may be said of the archegonia and antheridia, which, however, project more than in the preceding group.

Schizaeaceae.—The anatomy of the stem differs in the four recent genera of this order, and presents a series possibly illustrating the origin of a number of concentric steles from a solid stele, the intermediate step being represented by those forms in which the central cylinder is tubular. The sporangia are borne singly or in sori of two or three on the margin or under surface of leaves, the fertile pinnae of which differ more or less from the sterile segments. The sporangium is of considerable size, and dehisces by a median slit, the annulus being a more or less definitely limited horizontal ring of cells near the apex. The prothallus and sexual organs may resemble those of the Polypodiaceae; in Aneimia and Mohria the prothallus, though flattened, is not bilaterally symmetrical, the growing point being on one side; a filamentous type of prothallus is known in Schizaea.

Gleicheniaceae.—These forms have a horizontal rhizome, from which simply pinnate leaves arise in Platyzoma, while Gleichenia bears compound pinnate leaves with continued apical growth. The rhizome usually has a solid central cylinder in Gleichenia, while that of Platyzoma is tubular. The sporangia arise simultaneously in the sorus, which is borne on the under surface of the ordinary pinna; in those species with large sporangia the latter form a single circle, in others sporangia may also arise from the central part of the receptacle. The annulus is horizontal and the dehiscence median. The prothalli, while resembling those of the Polypodiaceae, have points of similarity with those of the preceding groups.

Matoniaceae.—This contains the single genus Matonia, two species of which are known from the eastern tropics. They are of special interest, since they have been shown to be the surviving forms of a group species which have been identified from Jurassic and Cretaceous rocks. The living species have a long rhizome, from the upper surface of which the large leaves arise; these are branched in a pedate manner, each branch being pinnate. The structure of the rhizome is complicated, a transverse section showing that the centre may be occupied by a solid stele, outside of which are two tubular steles. The sori are borne on the under surface of the pinnae,