A review of current knowledge of the distribution of Palaeozoic ferns and fern-like plants is presented. Whilst numerous putative ferns occur in the Devonian, it is not until the Lower Carboniferous that ferns belonging to the Filicales appear. The Namurian represents a major gap in our knowledge. The ecology of late Palaeozoic ferns is reviewed. The association of early fern assemblages with volcanic terrains is considered particularly significant in the relationship between evolution and environment. Extinction of more specialised groups may have been caused by climatic changes.

Matteuccia struthiopteris is distributed throughout most of the boreal region of the northern hemisphere. It has been variously recorded as a vermifuge and an ingredient in beer manufacture as well as a food. Young leaves, alternatively known as croziers or fiddleheads, are picked before they have unfurled and are boiled or steamed and served as a hot vegetable. The market, between Malecite indian and colonists, developed in the Fredericton area of New Brunswick 200 years ago, following a particularly severe winter. The newly arrived United Empire Loyalists, having emigrated from the United States, were so short of food that by the spring of 1784, they were reduced to eating any sort of provender nature could supply. Specific mention is made of fiddleheads, which became a traditional spring vegetable in New Brunswick. This market spread into Maine in the United States, particularly into those areas bordering the St John River. The present market is still predominantly in New Brunswick, where the wild harvest is between 150–200 t/yr, a yield which is approximately four times the harvest in neighbouring Maine. Food companies process about one third of the crop. In Maine, this is done by a single canning company in Wilton, whereas in New Brunswick, tinned fiddleheads have largely been superseded by the frozen product which is the monopoly of a company working in Florenceville. In addition, Canadian companies have recently sprung up which export the fresh spring vegetable in refrigerated lorries to larger centres west of the province. These companies account for less than a quarter of the harvest. The remainder is sold from either roadside stands, or to a wholesaler who distributes them to outlets in the region. The harvest is still predominantly done by natives. Much greater detail of both harvest, food preparation, and economic history is given by von Aderkas (1984). It has recently come to the author's attention that crowns of M. struthiopteris are sold also as a garden ornamental. Over 5000 plants/yr are sold by one Ontario distributor alone. Estimates from other nurseries in West Germany and the United States which do the same trade are presently unavailable.

The effect of photosynthetic photon flux density (PPFD), water, temperature and nutrition on frond emergence, vegetative frond production, fertile frond production, vegetative propagation and dormancy is examined. Ostrich fern plants will not break winter dormancy until they have received a minimum amount of cold exposure below a base temperature which is above 5.8°C and may be as high as 20°C. After the plants have received their chilling requirement, vegetative frond emergence does not occur until temperatures at or above ca. 9.3°C are reached. Percent germination and rate of emergence increases up to ca. 24°C. Photosynthetic photon flux density, water availability and mineral nutrition can affect both vegetative and fertile frond production but water availability appears to be the most critical. A mild water stress of −0.15MPa can have significant effects on water status and gas exchange in fronds. Sporophyte plants for field production have been produced vegetatively from detached meristems occurring naturally on the rhizomes, and through fertilization of axenic gametophytes.

The aquatic fern Azolla has a symbiotic relationship with an N2-fixing cyanobacterium. Fanners in China and Vietnam have used Azolla for centuries as a green manure for rice and as fodder for pigs, ducks and fish. Chinese researchers have been studying Azolla since the early 1950's but many of their achievements are unknown outside China because of language and other communication problems. This article reviews current Chinese Azolla research and focuses on the impact of newly introduced Azolla species in the areas of field application, use of spores, and isolation of the N2-fixing symbiont.

The heterosporous aquatic ferns in the genus Azolla contain a heterocystous cyanobacterium, Anabaena azollae, as a symbiont. The Anabaena occupies cavities formed in the aerial dorsal leaf lobes of the ferns and can provide the symbiotic associations with their total N requirement via the fixation of atmospheric nitrogen. The photosynthetic pigments of the fern and cyanobacterium are complementary. Photosynthesis is of course the source of energy for growth and the ultimate source of the ATP and reductant required for N2 fixation in the light or dark. However, nitrogen fixation is maximal in the light and the phycobili-proteins of the Anabaena are as effective as its chlorophyll in driving this photosystem I-linked process.

The partners exhibit a coordinated pattern of development with the Azolla exerting a control over the Anabaena, affecting both its metabolism and differentiation. Anabaena filaments associated with the fern apices lack heterocysts. As cavities are formed and occupied by the Anabaena, it differentiates a high proportion of heterocysts and exhibits nitrogenase activity. In mature cavities, the Anabaena receives fixed carbon from the Azolla and releases fixed N2 as ammonium. The ammonium is assimilated and/or transported by the Azolla toward its stem apices. Special epidermal cavity trichomes, which are intimately associated with the Anabaena at all stages in the ontogeny of the association, may facilitate metabolite exchange between the fern and cyanobacterium.

The biology of bracken (Pteridium aquilinum L. Kuhn) is reviewed with particular reference to its importance as a troublesome weed of upland grasslands. The problem of control is discussed, particularly with regard to the role of the rhizome system and its associated buds. The modes of action of glyphosate and asulam are described with particular reference to their foliage-absorption, translocation, and the nature of activity in the buds, the ultimate target sites.

Curl-tip disease of bracken (Pteridium aquilinum) is recorded from many sites in Great Britain. Evidence is presented which corroborates the findings of earlier workers that more than one organism is associated with the disease. Three fungi (Phoma aquilina, Ascochyta pteridis and Septoria sp.) are commonly isolated, even from the first visible necroses, suggesting that synergism may operate in pathogenesis. However, Phoma aquilina Sacc. and Penz. is more pathogenic than the others when used independently in inoculations via wounds into bracken raised in growth cabinets or the greenhouse. Field trials of a formulation of spores of P. aquilina, suspended in a dilute solution of the herbicide ioxynil with broth and glycerol, are reported. The potential of such a mycoherbicide for bracken control is considered.

We have studied morphological and biochemical aspects of storage reserves and their degradation in fern spores during the germination process. The results presented here are concerned with the fate of lipid, protein and phytic acid. Depletion of lipid reserves and breakdown of lipid bodies was directly correlated with increased activity of glyoxylate cycle enzymes during early stages of germination. Degradation of protein reserves coincided with the depletion of salt soluble proteins (globulins) from the spores and was related to the time when high activity of aminopeptidase and carboxypeptidase was observed. Lipid and protein appeared to play an important role in germination. Phytic acid reserves were present in the spore and were hydrolysed by phytase after germination occurred. This storage material was implicated in growth of the prothallus.

A high percentage of germination of Onoclea sensibilis L. spores is initiated by a low fluence of red light while imbibed but unirradiated spores exhibit a very low percentage of germination. Onoclea spores contain lipid, protein and sucrose reserves. Sucrose is degraded and starch is synthesised during germination while lipid reserves are mobilised during early gametophytic development. The amount of protein remains constant during both developmental stages. There is very little detectable mobilisation of any of the reserves in the imbibed but ungerminating spores maintained in the dark for up to 18 days. Sucrose degradation and starch synthesis are not interrelated because photoenhanced sucrose degradation can occur without starch synthesis and photoinduction of starch synthesis can occur without sucrose degradation. Evidence is presented to suggest that the mode of action of light in enhancing starch synthesis is to increase the availability of a starch precursor rather than the activities of starch synthesising enzymes. Sucrose may be mobilised after irradiation because it becomes accessible to the sucrose degrading enzymes which are in abundance in the spores.

During germination of Onoclea sensibilis spores, the spore nucleus moves from a central location to one end. Cell division partitions the spore into a small cell which differentiates into a rhizoid, and a larger cell which gives rise, by continued division, to the prothallus. Spore germination is a valuable system in which to study the relationship between asymmetric cell division and the initiation of cell differentiation. It appears that cytoplasmic microtubules and some lipophilic site in the spore are both involved in premitotic nuclear migration. Asymmetric cell division is an obligate step in the initiation of rhizoid differentiation. One hypothesis is that the nucleus must be confined to a small, localised region of the spore for a sufficient time before differentiation can occur, and the confinement is accomplished by the asymmetric cell division. Metal-binding sites are present in the spore coat, specifically on the proximal face. Several types of evidence suggest that the metal-binding region is involved in spore polarity. The sequence of events during germination appears to involve both polarity which is inherent in the spore and polarity which may be imposed by external stimuli. Experiments in which spores are treated with colchicine and polarised red light reveal both types of polarity.

The germination of Onoclea spores is a model system with many advantages for the study of asymmetric cell division and cellular differentiation. Our results suggest that both microtubules and a lipophilic site are important in the nuclear migration to one end of the spore prior to asymmetric cell division. A metalbinding region containing pore-like structures in the proximal face of the spore coat may be a source of the inherent polarity of the spore. The pattern of endogenous metal binding during germination has been characterised using a sulphide-silver stain. Metal-binding sites are described in a differentiating system in which polarity is imposed externally using polarised red light. The possibility of a role of ion gradients in determining the direction of nuclear migration is discussed.

The cell cycle of a protonema of Adiantum capillus-veneris L. cultured under continous red light is controlled by two photoreceptors: phytochrome and a blue and near-ultraviolet light-absorbing pigment (Pb−nuv). Irradiation with blue light, which is absorbed by Pb−nuv located in the nuclear region, shortens the duration of the G1 phase, but does not affect other phases. On the other hand, phytochrome has two roles in cell cycle regulation. Irradiation at the beginning of the G1 phase with far-red light, which is absorbed by phytochrome located throughout the entire protonema, lengthens the duration of the G2 phase. Phytochrome also arrests the cell cycle at the G0 phase under continuous red light, and, after red light irradiation, returns to the G0 phase any cell cycle which has already been started by dark incubation, unless the cell cycle has progressed to a ‘point of no return’ which coincides with the beginning of the S phase. Photoregulation of the cell cycle is summarised in Figure 4.

A brief review of the Apical Cell Theory and its relationship to apical growth in pteridophytes is presented. Currently there are two concepts regarding the importance of the apical cell during development. One group of investigators has presented evidence to demonstrate that the apical cell is mitotically active only during early initiation and organisation of apical meristems. Soon the apical cell becomes essentially inactive in cell division, and may undergo endopolyploidy. The second group has provided evidence in support of the original tenet that the apical cell plays an important and continuing role in shoot and root development and does not undergo endopolyploidy. Curiously enough, evidence for both concepts is based essentially upon the same procedures and techniques: histogenesis, determination of the mitotic index and cell-cycle durations, labelling with 3H-thymidine, and cytophotometric measurements of DNA content of the apical cell and subjacent cells.

Ecological conditions for intergametophytic mating and interspecific hybridisation in the clubmosses are discussed. Several features, including the number of recorded hybrids, records of gametophyte population densities, and conditions for movement of the male gamete in the soil, indicate that intergametophytic mating in Lycopodium is not especially impeded by the subterranean habitat, as has been commonly assumed. The evidence from one site indicates highly effective spore dispersal, and a greater ecological tolerance than expected for the species involved. Two cases are discussed in which hybrids have been formed between ecologically and/or taxonomically very different parent species, in the absence of one parent sporophyte, or with one of the parent species sporophyte poorly adapted to the hybrid habitat. The protected, relatively uniform, subterranean gametophyte habitat is thought to account for successful gametophyte growth and hybridisation between species of widely different sporophyte ecology.

Observations are made on the geographical and ecological distribution of the clubmosses (Lycopodiuni) in India, and their habitats and reproductive abilities.

The requirements of clubmosses in India for conditions of good illumination plus habitats of low vegetational competition pressure are probably the most important factors in restricting the species to either well lit forest canopies in the tropics or to open, treeless hillsides in the temperate zone. In the former, the epiphytic habit has become important. In the latter, the terrestrial habit has been exploited. Reproduction in the epiphytes is by spores, which establish in mossy epiphytic cushions. Reproduction in the terrestrial species is either by spores which establish in mosses or in areas opened by erosion and landslides, or by bulbils. The latter achieve a useful, local, additional means of reproduction in climates where, because of low temperatures, the rates of growth can be very slow.

Species of Isoetes in northeastern North America may have evolved through hybridisation and allopolyploidy. Evidence for natural hybridisation was obtained from overlapping distribution patterns, abortive and polymorphic spore formation, chromosome numbers ranging from diploid to decaploid, and additive enzyme electrophoresis profiles. The ease with which hybrids form in culture supports the hypothesis that natural hybrids occur and that hybridisation could be involved in the evolution of new species.

Hybridisation in Equisetum in Britain and Ireland is examined in respect of the exceptional numbers of hybrids present, their geographic and ecological distribution, their formation and subsequent spread.

It is concluded that the moist oceanic climate and relative paucity of competing species in the British and Irish floras in post-glacial time have created uniquely favourable conditions for the formation and subsequent success of hybrid horsetails, promoting both the short-term persistence of weaker hybrids, and the extremly long-term persistence of others.

The taxonomic inter-relations of parental combinations which have formed ecologically successful hybrids are discussed.

Pteridophytes with subterranean sexual stages exist in five families. Their prothalli grow underground without light, nourished by fungi, and are rounded, massive, and without chlorophyll. Various authors have suggested that such gametophytes may inhibit outbreeding ability and that minor variations may therefore form in colonies through inbreeding.

This paper questions these ideas. The subterranean condition is believed to be suited to fields, grassy meadows, and other habitats subject to seasonal drying. The parenchymatous thallus makes possible water storage. Water can flow between some particles. Sperm may be carried by mass flow in various directions and can evidently swim as much as 2 cm. Close clumping of underground gametophytes appears to be no less common than in surficial gametophytes and evidence exists that hybridisation occurs.

No evidence has been detected that outbreeding is inhibited, and we conclude that it is successful and that cross-fertilisation is no less common among underground gametophytes than among surface gametophytes. Numerous examples are listed from Lycopodiales, Psilotales, and Ophioglossales.

Studies comparing nucleic acids of some pteridophytes with others or with angiosperms are briefly reviewed. Amongst the pteridophytes, the fern genus Osmunda has been most thoroughly investigated by DNA sequence comparisons. O. cinnamomea, O. claytoniana, and O. regalis have equally diverged DNA sequences suggesting that evolutionary lines leading to these three species separated at about the same time; taxonomic placements that link any two of the three species closely together are, therefore, inappropriate. DNAs of the hybrid fern, O. × ruggii, were examined by analysis of restriction fragments of chloroplast DNA and by nuclear DNA hybridisation. The results support the previously proposed parentage.

A review of current knowledge of early fern evolution is presented including new data on age, morphology and anatomy with two diagrams summarising the early diversification of cauline and foliar anatomy and of the reproductive structures. The relationships and even the attribution of some Devonian taxa to the ferns is questioned whilst most of the Carboniferous coenopterids are considered indisputable ferns, some of them being related to modern filicaleans. The discovery, in the Lower Carboniferous of Scotland, of diverse annulate filicalean sporangia suggests an earlier (Tournaisian) radiation of true ferns than was previously assumed.