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Arguments based on their technical advantages, undoubted antiquity and lack of evolutionary extravagances are advanced for promoting the study of the Pteridophyta. Sound interpretations of fundamental aspects of the land plants, together with useful knowledge, are likely to follow, and grievous errors arising from too great a concentration on the flowering plants can be avoided.
Certain aspects of the cytology of germination of spores of Anemia phyllitidis and Pteris vittata, with particular reference to the role of nucleic acid synthesis, are reviewed. As studied by autoradiography of 3H-thymidine or 3H-uridine incorporation, spore germination resulting in the formation of the rhizoid and protonemal cell is accompanied by the synthesis of DNA and RNA. Experiments using inhibitors of RNA synthesis on gibberellic acid-induced germination of spores of A. phyllitidis have indicated that dry spores contain conserved mRNA and that gibberellic acid-induced polyadenylation of mRNA occurs during the early phase of germination of spores. Recent demonstrations of the presence of polyadenylated mRNA in dry spores of A. phyllitidis suggest specific roles for both functional, conserved mRNA containing polyadenylic acid, as well as for gibberellic acid-induced polyadenylation of mRNA during the early phase of spore germination.
Spore germination and the subsequent processes of development in fern gametophytes such as the induction, swelling and phototropism and protonemal growth and the progression of component phases in the cell cycle are antagonistically regulated by phytochrome and chemically unknown, blue and near-ultraviolet light-absorbing pigment(s). The physiological capacity to adjust these developmental processes to the seasonal and other environmental changes by wavelength, timing and direction of irradiations with near-ultraviolet, visible and far-red lights and also the electric vector of polarised light, is crucial for fern development. The diversity of Pfr requirement and of recovery time from blue light-induced inhibition in a spore population results in variations in the timing of spore germination, so that wild fern species may find a chance for survival under natural conditions. Photocontrolled processes are differentiated temporally and spatially in the course of fern morphogenesis, especially in the haploid generation. Using this material, the premitotic positioning of the nucleus and protein synthesis were studied.
Fern gametophytes provide unique opportunities for the investigation of various aspects of physiology, genetics and development. Past genetic studies of gametophytes have primarily involved investigations of variegation and radiation-induced plastid aberrations. Two studies have utilised the gametophyte generation to develop screens for specific types of mutations. The considerable natural variation that exists in gametophyte development has not been investigated genetically. Genetic studies can provide needed information regarding basic aspects of development. A selection system with broad applicability has been developed to isolate gametophyte mutations that confer resistance to growth regulators. An example of the use of the screen with abscisic acid is given and preliminary characterisations of some of the mutants are presented.
Developmental ultrastructural studies have led to major advances in our understanding of key questions ranging from the causal basis for the alternation of generations and the role of the cytoskeleton in cellshaping processes to the phylogeny of archegoniate plants. Oogenesis is characterised by profound nuclearcytoplasmic interactions accompanied by striking changes in the egg mitochondria and plastids. At fertilization, egg penetration is a physical process and the plastids from the spermatozoids are excluded from the egg. Considerable dissimilarity between the shapes of the biflagellate spermatozoids of Lycopodium and Selaginella underlines their ancient separation. Equally striking differences in spermatozoid architecture argue against any direct phyletic link between heterosporous and homosporous ferns. Taxonomic variations between the spermatozoids of homosporous ferns suggests blepharoplast morphology to be a potentially rich source of new systematic data. Whilst there is general agreement that the multilayered structure is a cytoskeletal alignment system, a proposed shape-generating system situated near the nuclear envelope, which provides the force necessary for spermatozoid morphogensis, has not yet been identified. New fixation procedures have revealed hitherto overlooked filamentous elements associated with the nucleus. Whereas tests for actin were negative, immunoblotting suggests that these contain the intermediate filament protein antigens.
The biochemical characteristics of pteridophytes firmly ally them with other Tracheophyta, with Bryophyta and with the class Charophyceae of the algal division Chlorophyta. Pteridophyte sporophytes, like the sporophytes of other terrestrial Tracheophyta, generally have the attributes of homoiohydric plants. All gametophytes are poikilohydric. Many gametophytes, and some sporophytes, are desiccation tolerant.
Quantitative comparisons between pteridophyte sporophytes and the sporophytes of other tracheophytes show that there are probably no systematic differences between the efficiency of important processes: examples are the quantum yield of photosynthesis, and the water use efficiency of organic matter accumulation, in the (mainly) C3 pteridophytes relative to other C3 tracheophytes. By contrast, the potential rales of physiological processes, as indicated by the conductance of photosynthetic (C3) carbon assimilation, and of water movement in the xylem, are generally towards the low end of the range for terrestrial tracheophytes. These low conductances restrict the maximum specific growth rate of pteridophyte sporophytes to rates lower than those found in annual angiosperms. Constraints imposed by the pteridophyte life cycle may have limited the capacity of pteridophytes to function in nature as annuals, and thus have reduced the selection pressure for high conductances (e.g. by a more widespread occurrence of vesseles in xylem of pteridophytes).
Low temperature SEM of juvenile leaves of Pteridium aquilinum (L.) Kuhn revealed that outgrowths arise from leaf cells as soon as three days after their detachment from the plant. Such outgrowths matured into fully functional gametophytes, so it can be said that the phase change from sporophyte to gametophyte occurred during the three day interval between detachment and outgrowth protrusion. Measurement of the incorporation of tritiated uridine into leaves detached for 0–5 days indicated that RNA levels rise during the first two days of culture and subsequently decline. The significance of this finding is discussed in relation to theories concerning changes in phase, in particular the suggestion that RNA connected with sporophytic growth must be expunged and that concerned with gametophytic growth synthesised during phase change.
Male gamete development in the heterosporous water fern Marsilea vestita is a highly synchronous process which is completed within 5 to 6 hours at 30°C. The morphogenetic and ultrastructural changes which take place during spermatogenesis have been well characterised in light and electron microscope investigations (Sharp 1914; Hepler 1976) but few studies exist on any biochemical aspects of spermatogenesis in this system. To obtain a more comprehensive overview of the molecular events occurring during microspore development, we have investigated the pattern of some of the biochemical processes by a variety of methods.
Results from experiments in which inhibitors of RNA, DNA and protein synthesis and microtubule function were applied to developing microspores at various times during the 6 hour period have been described previously (Hyams et al. 1983). They have enabled us, with certain limitations, to determine at what times during spermatogenesis specific molecular syntheses or functions are important. Protein synthesis has been studied further in some detail. Polyacrylamide gel electrophoresis of extracts of Marsilea microspores during development reveals that there is an overall increase in the amount of protein present as spermatogenesis proceeds. In addition to tubulin, the major flagellar protein and constituent of the microtubule ribbon, major polypeptides are detected at 15 k, 26 k, 31 k and 90 k on Coomassie Blue stained gels. Few qualitative changes over the 6 hour period can be detected, even when very sensitive staining methods are used to detect minor proteins. This would appear to show that few proteins are synthesised specifically at particular stages of microspore development. We have investigated this question further by labelling the microspores with 14C leucine and 35S methionine, and performing fluorography of SDS polyacrylamide gels run with the radiolabelled samples. Fluorographs reveal more detail than it is possible to resolve with ordinary staining methods, and several polypeptides have been detected which appear to be heavily labelled at the onset of spermatogenesis but which incorporate progressively less radioactive amino acid as development proceeds. As yet, the identity of these proteins is unknown.
We have initiated a study into the presence of individual proteins during spermatogenesis, using specific antibodies reacted against Western blots of polyacrylamide gels. A monoclonal antibody against tubulin (Kilmartin et al. 1982) reveals that this protein is present in small amounts from the onset of microspore development but that the amount increases greatly between 3 and 4 hours, the time when sperm flagella are observed to assemble. We have recently raised a polyclonal serum in rabbit which cross reacts with a high molecular weight polypeptide on Western blots of Marsilea microspore extracts. This protein appears to be present in large amounts at the beginning of the 6 hour period but decreases slightly as development proceeds. The identity of this protein is currently under investigation.
Microsporogenesis in cultured material of Azolla microphylla was studied with the light and transmission electron microscopes. The first formed sporangium, a megasporangium, aborts and several microsporangia develop below. Initially, a single sporogenous cell is present, surrounded by a single layered tapetum and the microsporangial wall. Subsequently, several sporogenous cells are connected by plasmodesmata. The microspore mother cells are less densely cytoplasmic than the tapetal cells. Callose-like material is deposited around the microspore mother cells, but disappears before meiosis. The tetrads of microspores contain well defined organelles but less dense cytoplasm than the surrounding periplasmodium. Electron dense material deposited on the plasma membrane of the microspores eventually forms the endospore. The unornamented exospore develops by continued deposition of electron dense material. Degeneration of the periplasmodium gives rise to membranous material which appears to form a template for the massulae.
The Braithwaite system of agamospory is reviewed. The few ferns having this system are very diverse, both systematically and in their behaviour. It is concluded that this type of agamospory has arisen independently several times by different routes.
The fixation or loss of somatic mutations is considered in terms of apical initials and apical meristem organisation. Two general kinds of apical meristems are mathematically modelled: structured and stochastic. Structured apices have permanent apical initials whereas stochastic apices are more dynamic systems with apical initials randomly selected from an apical cell pool. With regard to neutral or completely recessive mutations, the probabilities of fixation or loss of mutations are identical for both structured and stochastic apices. For mutations which have a disadvantageous effect in heterozygous cells, diplontic selection occurs. In such cases, stochastic apices are purged of mutations more effectively than structured apices.
The majority of ferns have highly structured apices based upon single apical cells. Since such apices are the least able to be purged of disadvantages mutations, mutational load may be a significant aspect of the biology of these plants. Ferns that exhibit primarily asexual reproduction (either apogamy or vegetative reproduction) may also exhibit strong linkage disequilibrium between loci due to mutation.
Evolution of new forms of organisms must be accompanied by evolution of the informational processes which regulate the development of these new forms. During plant phylogeny, products of metabolism have become phytohormones through the evolution of receptor molecules. Although nothing is known about these receptor molecules, it is suggested that the schizaeaceous ferns are the most primitive group in which a gibberellin-like substance acts as a signal for morphogenesis, and that their antheridiogen pheromones, which stimulate antheridium formation and spore germination, are the ancestors of the gibberellin hormones which influence seed plant development. Chemical and biological evidence for this suggestion is discussed.
The entire soil-plant-atmosphere continuum must be analysed to elucidate how xylem anatomy relates to water flow in plants. Measurements of water potential gradients and volume of water flow per unit time are needed to obtain values of hydraulic conductance per unit length. By comparing values of hydraulic conductance per unit length along the plant, the regions where xylem structure restricts water flow can be determined. Previous studies of fern water relations demonstrated that very large water potential gradients occurring in stipes of certain ferns were closely correlated with reduced conducting area of stipe xylem. A new study on Cyrtomium falcatum showed that the water potential gradient was relatively small and constant along the stipe and rachis; however, a much larger gradient occurred from the rachies into the pinnae. Hydraulic conductance per unit length varied with the leaf area to be supplied, leading to the fairly constant water potential gradient along the rachis.. The measured hydraulic conductance per unit length was only half the value predicted from the Hagen-Poiseuille equation. Although the Hagen-Poiseuille equation overestimated the measured value by a factor of 2, it did support the assumption that conduit number and lumen diameter are the principal determinants of water conductance in the xylem.
This assessment of selected literature on pteridophyte edaphic adaptation is presented in three parts: (1) pH and general soil nutrient background, (2) growth responses to specific inorganic ions and frond mineral content, and (3) collected works on Pteridium aquilinum edaphic adaptations. In this report, a case is made for the overriding effect of both soil pH and inorganic ion content in determining pteridophyte occurrence. Recommendations are made for more field and laboratory co-ordinated studies, especially as they pertain to the determination of species-specific amplitudes of adaptation, and for the initiation of physiological and metabolic investigations into the basis of these edaphic adaptations.
The structure and diversity of fern spores, based on SEM studies, are assessed in relation to general evolutionary trends and systematics. A review of spores of myrmecophytic ferns includes SEM figures of eight species and the ant Iridomyrmex cordataus. The diversity, especially in Lecanopteris spores, is correlated with other characters indicating systematic differences of the species. Convergence is shown in the echinate spores of the neotropical Solanopteris and those of the paleotropical Drynaria and Selliguea. Evidence for adaptation of spore structure to transport by ants is especially clear in the development of filamentous elements, unique among pteridophytes, in Lecanopteris mirabilis. These demonstrate an unusual labile nature of the wall, and provide insight into the functional role of intricate surface formation of spores.
Spores of the ant-fern Lecanopteris mirabilis (C.Chr.) Ching are discharged in groups of 16 spores per sporangium and are held together by long filamentous outgrowths of their surface. It is contended that the filaments are not primarily an adaptation to ant-dispersal, as has been suggested, but maintain genetic variability by promoting intergametophytic mating of the prothallial populations produced as an outcome of the spores being held together in clusters.
The fungal parasites of ferns and their allies have been largely neglected, both by pteridologists and mycologists. Greater interest is advocated in the light of recent studies of patterns of host specificity of such fungi on angiosperms, which have provided valuable insights into the systematics of these hosts. Our knowledge of the major groups of fungi reported as parasites of pteridophytes is reviewed, and their potential contribution to host classification and phylogeny discussed. The relationship of the Uredinales (rust fungi) and their fern hosts is examined in detail.
Bracken in Britain is a host for 27 species of insect herbivores, with a further 11 species that either feed below ground (and are poorly studied), or appear to be only rarely associated with the plant. A typical site in northern England has an average of 15–16 of these species in any one year. Compared with perennial herbaceous angiosperms with similar wide distributions, bracken is not noticeably depauperate in the number of insect species that feed upon it. Bracken in others parts of the world is attacked by a wide variety of insects, with more species present in the geographical areas where bracken is most common.
The ‘feeding niches’ of some of these insects are reviewed. Most are very rare relative to the biomass of their host plants, probably because of the impact of natural enemies; the effect of most of the insects upon their host-plant is consequently negligible.
Reverse effects, of host-plant upon the insects, are subtle but poorly understood. Experiments to elucidate these effects are briefly outlined.
The past decade has seen the emergence of significant fossil evidence of a history of pterodophytearthropod interaction extending back to the Devonian period. Such fossils include plant tissue showing lesions, bites and borings with associated features implicating arthropods as causal agents. Gut contents of Carboniferous arthropods, which include lycopod xylem elements and spores, are a tangible demonstration of phytophagy. Pteridophyte spores in fossil droppings (coprolites) indicate the prevalence of arthropod spore-eating in the Palaeozoic. This may have had compensations for the source plant and evidently represented the start of the co–evolution which culminated in the elaborate adaptations shown by flowering plants and their insect pollination vectors.