Page:EB1911 - Volume 22.djvu/628

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612
PTERIDOPHYTA


each consisting of a single series of large sporangia covered by a coriaceous indusium, which is attached to the central part of the receptacle. The sporangium, which corresponds on the whole to that of the Gleicheniaceae, has a somewhat oblique annulus; the dehiscence also is not truly median. The gametophyte is unknown.

Loxsomaceae.—The single genus Loxsoma has a tubular stele in its rhizome, which bears leaves resembling those of some Davallias. The elongated receptacle of the marginal sori is surrounded by a basal cup-shaped indusium. The sporangia, which arise in basipetal succession on the receptacle, dehisce by a median slit, though the annulus is somewhat oblique; they have resemblances to the Gleicheniaceae. When mature, the sporangia are raised above the margin of the indusium by the elongation of the receptacle, thus facilitating the dispersion of the spores. The gametophyte is unknown.

Hymenophyllaceae.—This group, which contains the two genera Hymenophyllum and Trichomanes, is characterized by the prevalent “filmy” texture of the leaves. Many of the species inhabit situations in which the air is constantly moist, especially in the tropics; some are terrestrial; others, some of which are very minute, are epiphytic on tree-stems. A single solid central cylinder is found in the rhizome. The sori, which are marginal, have a long receptacle, bearing the sporangia in basipetal succession, and are surrounded by a cup-shaped indusium. The sporangia present a considerable range in size, the largest being found in species of Hymenophyllum, the smallest in Trichomanes. Each has an almost horizontal annulus resembling that of Gleichenia, but the dehiscence is lateral. The gametophyte in Hymenophyllum is flat and variously lobed; that of Trichomanes may be similar, but in other species is filamentous. The archegonia and antheridia present points of similarity to those of the Gleicheniaceae.

Cyatheaceae.—This order includes the majority of existing tree-ferns, as well as some of smaller size. The stem has a ring of flattened steles. The sorus has a somewhat elongated receptacle, on which the sporangia arise basipetally; the indusium may be cup-shaped, bivalve or wanting. The dehiscence of the sporangium is almost transverse, as in the Polypodiaceae, but the annulus is slightly oblique. The prothalli correspond to those of the next group.

Polypodiaceae.—This group, which contains the remaining ferns, includes a number of distinct lines of descent and will doubtless require subdivision as our knowledge of the morphology of the genera classed in it becomes extended. Space will not allow of an account of the progress already made in this direction. The stem in the more primitive forms has a tubular stele (solenostele); for the most part two to many steles, arranged in a ring (dictyostele). In a number of genera, which there is reason to regard as relatively primitive, the sporangia show the same regular basipetal succession as in some of the preceding groups; in the great majority, however, the succession is not regular, but those of various ages are intermixed in the sorus (fig. 2, g). The sporangia dehisce by a transverse slit, the annulus being truly vertical or, in some of the genera in which they are regularly arranged, very slightly oblique. The structure of the prothallus and sexual organs will be evident from figs. 7, 8 and 9; some of the more interesting modifications have been referred to above.

Our knowledge of the extinct Filicales cannot be readily summarized, since it is in a transition state, owing to the recent evidence which has shown that many of the fern-like plants of the Palaeozoic period belonged to a group of seed-bearing plants derived from a filicineous ancestry. There is, however, abundant evidence that the Ferns were represented in the most ancient floras known, though they were not such a dominant group as has hitherto been supposed. The best known of these ancient Ferns belong to the Botryopterideae; the characters of this group point to its having been the starting-point of several series of existing Ferns (see Palaeobotany: Palaeozoic).

A consideration of the Filicaceae as arranged above will show that the several sub-orders may in general terms be said to form a series between those in which the sorus consists of a single circle of bulky sporangia and those Polypodiaceae in which the numerous small sporangia appear to be grouped without order in the sorus. When the survey is extended to the extinct Ferns of which the fructification is known, many of those from the more ancient rocks are found to group themselves with the existing sub-orders with large sporangia, such as the Marattiaceae, Gleicheniaceae and Schizaeaceae; the Polypodiaceae, on the other hand, do not appear until much later. The extinct forms cannot be dealt with in detail here; but it may be pointed out that their order of appearance affords a certain amount of direct evidence that the existing Ferns with a single circle of large sporangia in the sorus are relatively primitive. The series which can be constructed from a study of the sorus is in general supported by the anatomy of the sporophyte, and by the structure and sexual organs of the gametophyte. A more detailed investigation of all the characters of the Ferns will be needed before the course of evolution thus broadly indicated can be traced, but the results obtained afford a deeper insight into the general method of progression and the selective factors in the process. On the ground mainly of an examination of the sorus and sporangium, Bower has shown that the Filicaceae may be divided into three groups—the Simplices, Gradatae and Mixtae—in which the sporangia arise simultaneously, in basipetal succession, or irregularly in the sorus respectively. The first includes the Marattiaceae, Osmundaceae, Schizaeaceae, Gleicheniaceae and Matoniaceae; the second the Loxsomaceae, Hymenophyllaceae, Cyatheaceae and the Dennstaedtineae (a group including species placed in the Synopsis Filicum in Dicksonia and Davallia); while the remaining Polypodiaceae constitute the Mixtae. The change from the one type of sorus to the other may have taken place in several different lines of descent, some of which have been traced. A consideration of the biology of the sorus gives an insight into the advantages obtained by the one type over the preceding, as regards protection, spore production and the dispersal of the spores, and thus indicates the way in which natural selection may have acted. The differences in the form and mode of dehiscence of the sporangia (those of the Simplices having median dehiscence and a horizontal annulus, those of the Gradatae a more or less oblique position of the annulus and of the plane of dehiscence, while in the Mixtae the annulus is vertical and the dehiscence transverse) stand in relation to the position of the sporangia in the sorus relatively to one another. The application of the important criteria which Bower has thus pointed out toithe construction of a strictly phylogenetic classification of the Filicaceae cannot be made until the anatomy, the sexual generation and the palaeobotanical evidence have been further examined from this point of view. Though on this account and because the subdivisions Simplices, Gradatae and Mixtae do not correspond to definite phylogenetic groups, they have not been used in classifying the Ferns above; they are of great importance as an advance towards a natural classification.

Hydropterideae.—Two very distinct orders of heterosporous Filicales, the Salviniaceae and the Marsiliaceae, are included in this group. The difficulty of determining their exact relationship to the other orders of Ferns is increased by the more or less completely aquatic habit of the plants and the modifications and reductions in structure associated with this. The absence of an annulus from their indehiscent sporangia makes it impossible to compare them with the other Ferns in respect of this important character. It has been suggested with considerable probability that the Marsiliaceae are allied to the Schizaeaceae, while the Salviniaceae may possibly be related to the Hymenophyllaceae or to some other family of the Gradatae. Space will only permit of a brief general account of the more obvious features of the several genera, the structure and life-history of which are known in great detail. Unlike as they are in many respects, the two orders agree in being heterosporous. The microspores on germination produce a small, greatly reduced male prothallus bearing one or two antheridia which give rise to a number of spirally coiled, multiciliate spermatozoids. The single large megaspore contained in each megasporangium produces a small prothallus, which bears one or a few archegonia; these are exposed on the surface of the prothallus at the summit of the germinated mega spore (fig. 1, i).

1. The Salviniaceae include the two genera Salvinia (fig. 10) and Azolla. The small dorsiventral plants are in both cases floating aquatics. Azolla has roots depending from the lower surface of the stem into the water, while these organs are completely wanting in Salvinia, their place being taken functionally by highly divided leaves borne on the ventral surface of the stem. Nostoc colonies are constantly present in a special cavity of the dorsal lobe of the leaf in Azolla. The sporangia in both genera are associated in sori enclosed by indusia springing from the base of the receptacle. In Salvinia (fig. 2, h) the sori are borne towards the base of the submerged leaves, in Azolla on the reduced ventral lobe of the leaf. They consist either of microsporangia or megasporangia, which are arranged in basipetal succession on the receptacle. In the megasorus of Azolla