The gametophyte of some Jamaican thelypteroid ferns

A study of the development of the gametophyte of 16 thelypteroid ferns from Jamaica reveals similarities and some differences among them which may have some bearing on their taxonomic position. They all possess a hairy, relatively long-lived thallus which at maturity is cordate, relatively delicate for its size, with a wide cushion, broad wings, abundant colourless or pale tan rhizoids and sex organs of the advanced type. Although spore ornamentation, length of gametophytic simple hair and form and dehiscence of the antheridium are features which differ among the species, two gametophytic characteristics set apart the goniopteroid representatives: a branched hair and the development of the cell plate from subterminal cells of the germination filament when the filament ends in a hair. The presence of these with other minor and inconsistent differences suggests a not very close relationship of Goniopteris with Meniscium and supports those authors who prefer generic distinction.     

Antheridial dehiscence in ferns

We investigated the mechanism of antheridial dehiscence in ferns for the first time using fluorescence microscopy as well as scanning and transmission electron microscopy. The mechanism leading to antheridial dehiscence in Polystichum setiferum, Asplenium trichomanes and A. onopteris was found to depend on the different cellulose contents of the inner and outer walls of the ring cells detected with calcofluor white stain and the Thiéry test. The extremely low cellulose content of the ring cell walls facing spermatozoids made them less mechanically resilient than external wall cells. When the ring cells absorbed water they expanded only into the antheridial cavity, pushing the gametes against the cap cell, which detached from the ring cell below and enabled spermatozoid release. The newly released spermatozoids were spherical bodies covered in cellulose fibrils. The significance of cellulose fibrils could be to isolate the gametes from each other, to reinforce the electron transparent material and to protect the gamete from pressure created by the ring cells during release.     

The occurrence of axillary antheridia in dioecious mosses

Abstract

Antheridia that are undoubtedly axillary occur in species of Bryum, Mnium and Aulacomnium. It is suggested that this is a primitive feature and that the typical moss antheridial head, with all the antheridia apparently terminal, has been derived from species with axillary antheridia, either by restriction of the antheridia to the axils of the uppermost bracts, as in the liverwort Haplomitrium blumei, or by the suppression of the upper bracts.     

Biogeography of Mesozoic leptosporangiate ferns related to extant ferns

The paleobiogeography of the extant leptosporangiate fern families present in the Mesozoic is reviewed based mainly on data from megafossils that possess enough characters to be assigned to a family. Times of first appearances are discussed for each family, followed by their distribution throughout the Mesozoic. Paleolatitudes and climatic conditions indicate that first appearances of many fern families occur about or above 30° and in conditions of higher moisture. Much of fern distribution in the past was likely due to long-distance dispersal.     

Synaptonemal complexes in antheridia of Achlya ambisexualis E87

This is the first report of longitudinal sections of synaptonemal complexes in oömycetous fungi. These indicators of meiosis were observed in antheridial nuclei of Achlya ambisexualis E87. They were attached to a platelike structure at the inner membrane of the nuclear envelope. The lateral elements were separated from each other by an average distance of 160 nm. These results provide new ultrastructural evidence for gametangial meiosis in Oömycetes.     

Dehiscence of the moss capsule

Summary The initials of the anulus cells show apical growth. The cell tip is therefore dome-shaped and thin-walled. Eventually this terminal foramen is sealed off by a last layer—the swelling layer of the cell — when the apices finally cease to grow. The swelling layer of the mature cell is thus locally in contact with what seems to be the primary wall. Separation of the tissue occurs between this latter wall region and the moderately incrusted secondary wall.Two conclusions were drawn: (1) pressure is exerted through the foramina on lid and urn mouth, and (2) the observed asymmetry of the anulus provides the structural basis for generating shearing stress.Annulus is an erroneous spelling of anulus (see Oxford Dictionary 1933, vol.I, 344).     

Evolutionary mechanisms in tropical ferns

Cytotaxonomic work on tropical fern floras has now reached the stage at which it is possible to compare different geographical regions meaningfully. It is also possible, although the evidence is on a more restricted scale, to compare the taxonomic and geographical incidence of different evolutionary mechanisms of the kind which can be investigated by the experimental techniques of biosystematics. Autopolyploidy, allopolyploidy, genetical speciation without visible chromosome changes, and speciation involving visible structural chromosome changes, are all now known to have taken place in tropical examples some of which will be illustrated. Certain types of change are restricted to certain fern genera being apparently absent from others. Such restrictions do not explain the marked regional peculiarities exhibited by Africa versus India, or by Malaya when compared with Ceylon or Jamaica. Interpretation of the results requires attention to be directed to factors in the past history of the various areas which might not otherwise be recognized.     

The classification of ferns

Intensive morphological studies have been devoted to the more primitive ferns, which represent a small minority of living species, but too little is yet known about the vast majority of other ferns, with the result that recent attempts at a natural classification show considerable differences of treatment.
The problem is complicated by convergent evolution in the characters of almost all parts of a fern plant. Not only similar soral form, but also similar frond form, types of venation, scales, etc. have been developed on different evolutionary lines.
To illustrate the nature of the problem an attempt has been made to state the probable characters of a primitive leptosporangiate fern, and the kinds of ways in which existing ferns have developed from this condition. Evolutionary change in different parts of the plant has proceeded in different ways and to different degrees in the many genera of existing ferns. Primitive characters of one kind or another are shown by a great number of ferns, along with highly advanced characters of other kinds.
Recent schemes of classification are briefly compared, and a summary is given of the author's own scheme, with notes on evolutionary trends in the various groups as he sees them.
Much more information is needed on which to establish a really satisfactory scheme. The present one is put forward in the hope that others will take up the work. With modern facilities for travel, it is to be hoped that more botanists will come to the tropics and see ferns and other too-little-known plants in their native habitats. Morphological study needs to be undertaken with an understanding of the living plant and of its environment.     

The taxonomic significance of xanthones in ferns

The occurrence of xanthone O- and C-glycosides in ferns is reviewed. The C-glycosides have a restricted distribution within four families: Hymenophyllaceae, Aspleniaceae s.l., Davalliaceae and Marsileaceae. The compounds are of little taxonomic value at higher levels because they appear to have evolved on several separate occasions and are, therefore, an interesting example of the parallel evolution of a chemical feature. Their major value appears to be in the identification of allopolyploidy.