Early Vascular Plants Research Articles

Aarabia, a new Early Devonian vascular plant from Africa (Morocco)

Aarabia brevicaulis Meyer-Berthaud and Gerrienne, gen. et sp. nov. is described from the Early Emsian of Central Morocco. The preserved parts of the aerial system are interpreted as erect shoots bearing branches in a planar-alternate arrangement. Branching is pseudomonopodial in all orders of axes. Distinction is made between short and long branches. Short branches consist of a foreshortened axis dichotomising in two terminal segments recurved abaxially and adaxially towards the axis of previous order. Arrangement of short laterals on main stem is unpredictable. It is typically proximal on long branches. Long branches of all orders are characterised by short internodes proximally, that increase in length distally. Fertile branches display few anisotomous divisions and bear a small number of non-paired elongated sporangia. Associated large spores in the vicinity of the fertile appendages suggest that the plant might have been heterosporous. Aarabia is compared to Early Devonian euphyllophytes. A preliminary cladistic analysis places it as sister-group of the Euphyllophytina.

A new species of vascular plants from the Xujiachong Formation (Lower Devonian) of Yunnan Province, China

A new species of early land vascular plants, Huia gracilis, is described from the Lower Devonian (Pragian–early Emsian) Xujiachong Formation, Qujing district, Yunnan Province, southwestern China. The plant possesses K- or H-type branching in the rhizome and a dichotomously branched erect system. An axillary tubercle is sometimes present at the branching position. Fertile axes terminate in loose spikes comprising terminal and lateral sporangia arranged helically. Sporangia with long stalks are ovate or elongate–ovate and reflex adaxially. The sporangium dehisces longitudinally in the radial plane of the fertile axis into two parts. The xylem is probably centrarch. Tracheids of G-type are characterised by annular secondary thickenings, between which is a sheet with irregular simple perforations. There may be some ‘pores’ in the enclosing wall of the perforation. H. gracilis differs from the type species H. recurvata mainly in the branching pattern, more slender morphology of the stem, sporangium and stalk. It is suggested that Huia may have originated from the primitive Cooksonia-like plants.

Scoyenia burrows from Ordovician palaeosols of the Juniata Formation in Pennsylvania

Scoyenia beerboweri is a new ichnospecies of burrow from the late Ordovician (Ashgill) Juniata Formation in central Pennsylvania, USA. The burrows are abundant in red calcareous palaeosols, and were created by animals living at the time of soil formation, because they are filled with red sediment like that of the palaeosol matrix, and both cut across, and are cut by, nodules of pedogenic carbonate. The isotopically light carbon and oxygen of carbonate in the palaeosols indicate a terrestrial ecosystem of well‐drained floodplains in a tropical seasonally‐dry semi‐arid palaeoclimate. Backfill layering within the burrows is evidence of a bilaterally symmetrical animal. Size distribution of the burrows reveals discontinuous growth, as found in arthropods. Ferruginized faecal pellets in the burrows indicate that they ingested sediment. For these reasons the burrows of Scoyenia beerboweri are most likely to be the work of millipedes. The nature of vegetation supporting them is unknown, although a single problematic plant‐like fossil cast was found, and liverwort spores are widespread in rocks of this age. Vegetative biomass was limited judging from the degree of chemical weathering, extent of burial gleization and isotopic composition of carbon in the palaeosols. These distinctive respiration‐dominated liverwort‐millipede polsterlands lived at a time of global greenhouse climate, following Precambrian–Cambrian lichen‐algal microbial earths and supplanted by Silurian brakelands of early vascular land plants.

Roots: evolutionary origins and biogeochemical significance.

Roots, as organs distinguishable developmentally and anatomically from shoots (other than by occurrence of stomata and sporangia on above-ground organs), evolved in the sporophytes of at least two distinct lineages of early vascular plants during their initial major radiation on land in Early Devonian times (c. 410-395 million years ago). This was some 15 million years after the appearance of tracheophytes and c. 50 million years after the earliest embryophytes of presumed bryophyte affinity. Both groups are known initially only from spores, but from comparative anatomy of extant bryophytes and later Lower Devonian fossils it is assumed that, during these times, below-ground structures (if any) other than true roots fulfilled the functions of anchorage and of water and nutrient acquisition, despite lacking an endodermis (as do the roots of extant Lycopodium spp.). By 375 million years ago root-like structures penetrated almost a metre into the substratum, greatly increasing the volume of mineral matter subject to weathering by the higher than atmospheric CO(2) levels generated by plant and microbial respiration in material with restricted diffusive contact with the atmosphere. Chemical weathering consumes CO(2) in converting silicates into bicarbonate and Si(OH)(4). The CO(2) consumed in weathering ultimately came from atmospheric CO(2) via photosynthesis and respiration; this use of CO(2) probably accounts for most of the postulated 10-fold decrease in atmospheric CO(2) from 400-350 million years ago, with significant effects on shoot evolution. Subsequent evolution of roots has yielded much-branched axes down to 40 microm diameter, a lower limit set by long-distance transport constraints. Finer structures involved in the uptake of nutrients of low diffusivity in soil evolved at least 400 million years ago as arbuscular mycorrhizas or as evaginations of "roots" ("root hairs").

Modeling fossil plant form-function relationships: a critique

Attempts to model form-function relationships for fossil plants rely on the facts that the physiological and structural requirements for plant growth, survival, and reproductive success are remarkably similar for the majority of extant and extinct species regardless of phyletic affiliation and that most of these requirements can be quantified by means of comparatively simple mathematical expressions drawn directly from the physical and engineering sciences. Owing in part to the advent and rapid expansion of computer technologies, the number of fossil plant form-function models has burgeoned in the last two decades and encompasses every level of biological organization ranging from molecular self-assembly to ecological and evolutionary dynamics. This recent and expansive interest in modeling fossil plant form-function relationships is discussed in the context of the general philosophy of modeling past biological systems and how the reliability of models can be examined (i.e., direct experimental manipulation or observation of the system being modeled). This philosophy is illustrated and methods of validating models are critiqued in terms of four models drawn from the author's work (the quantification of wind-induced stem bending stresses, wind pollination efficiency of early Paleozoic ovulate reproductive structures, population dynamics and species extinction in monotypic and “mixed” communities, and the adaptive radiation of early vascular land plants). The assumptions and logical (mathematical) consequences (predictions) of each model are broadly outlined, and, in each case, the model is shown to be overly simplistic despite its ability to predict the general or particular behavior or operation of the system modeled. Nonetheless, these four models, which illustrate some of pros and cons of modeling fossil form-function relationships, are argued to be pedagogically useful because, like all models, they expose the internal logical consistency of our basic assumptions about how organic form and function interrelate.

Late Silurian trilete spores from northern Jiangsu, China

The Late Silurian is generally considered to a particular significant key period in the study of early land vascular plants. A trilete spore assemblage of the Upper Silurian is described from northern Jiangsu, China. This assemblage comprises 11 genera and 20 species of trilete spores (including laevigate, apiculate, perinotrilite, patinate, rarely distally murornate and equatorially crassitate, and three indeterminate trilete miospores forms). It has similarities to those described from coeval assemblages from around the world (e.g., England and South Wales; Tripolitania, Libya; Cornwallis Island, Canadian Arctic; Northwest Spain). The rare cryptospore, only one specimen ( Tetrahedraletes sp.) had been found to be associated with the Chinese trilete spore assemblage. The discovery of the trilete spores from Late Silurian rocks indicates the existence of early land plants, some possibly vascular, at that time in northern Jiangsu, China.

The origin and early evolution of tracheids in vascular plants: integration of palaeobotanical and neobotanical data

Although there is clear evidence for the establishment of terrestrial plant life by the end of the Ordovician, the fossil record indicates that land plants remained extremely small and structurally simple until the Late Silurian. Among the events associated with this first major radiation of land plants is the evolution of tracheids, complex water-conducting cells defined by the presence of lignified secondary cell wall thickenings. Recent palaeobotanical analyses indicate that Early Devonian tracheids appear to possess secondary cell wall thickenings composed of two distinct layers: a degradation-prone layer adjacent to the primary cell wall and a degradation-resistant (possibly lignified) layer next to the cell lumen. In order to understand better the early evolution of tracheids, developmental and comparative studies of key basal (and potentially plesiomorphic) extant vascular plants have been initiated. Ultrastructural analysis and enzyme degradation studies of wall structure (to approximate diagenetic alterations of fossil tracheid structure) have been conducted on basal members of each of the two major clades of extant vascular plants: Huperzia (Lycophytina) and Equisetum (Euphyllophytina. This research demonstrates that secondary cell walls of extant basal vascular plants include a degradation-prone layer ('template layer') and a degradation-resistant layer ('resistant layer'). This pattern of secondary cell wall formation in the water-conducting cells of extant vascular plants matches the pattern of wall thickenings in the tracheids of early fossil vascular plants and provides a key evolutionary link between tracheids of living vascular plants and those of their earliest fossil ancestors. Further studies of tracheid development and structure among basal extant vascular plants will lead to a more precise reconstruction of the early evolution of water-conducting tissues in land plants, and will add to the current limited knowledge of spatial, temporal and cytochemical aspects of cell wall formation in tracheary elements of vascular plants.

Modeling fossil plant form-function relationships: A critique

Attempts to model form-function relationships for fossil plants rely on the facts that the physiological and structural requirements for plant growth, survival, and reproductive success are remarkably similar for the majority of extant and extinct species regardless of phyletic affiliation and that most of these requirements can be quantified by means of comparatively simple mathematical expressions drawn directly from the physical and engineering sciences. Owing in part to the advent and rapid expansion of computer technologies, the number of fossil plant form-function models has burgeoned in the last two decades and encompasses every level of biological organization ranging from molecular self-assembly to ecological and evolutionary dynamics. This recent and expansive interest in modeling fossil plant form-function relationships is discussed in the context of the general philosophy of modeling past biological systems and how the reliability of models can be examined (i.e., direct experimental manipulation or observation of the system being modeled). This philosophy is illustrated and methods of validating models are critiqued in terms of four models drawn from the author's work (the quantification of wind-induced stem bending stresses, wind pollination efficiency of early Paleozoic ovulate reproductive structures, population dynamics and species extinction in monotypic and “mixed” communities, and the adaptive radiation of early vascular land plants). The assumptions and logical (mathematical) consequences (predictions) of each model are broadly outlined, and, in each case, the model is shown to be overly simplistic despite its ability to predict the general or particular behavior or operation of the system modeled. Nonetheless, these four models, which illustrate some of pros and cons of modeling fossil form-function relationships, are argued to be pedagogically useful because, like all models, they expose the internal logical consistency of our basic assumptions about how organic form and function interrelate.

The minimum size of seeds and spores in relation to the ontogeny of homoiohydric plants

Homoiohydry may be defined as the capacity of nondormant terrestrial plants to remain hydrated, albeit without the capacity for net CO2 fixation, when the soil can supply water to the plant less rapidly than the atmosphere can potentially remove water in transpiration. Homoiohydry is an attribute of the sporophytes of most extant vascular plants and involves significant differentiation with roots (or functionally analogous structures), xylem, cuticle, stomata and intercellular gas spaces (Raven 1977; Edwards, Abbott & Raven 1996; Bell & Mooers 1997). Homoiohydry was a major physiological innovation in the evolution of embryophytes and has enabled plants 1-100 m high to grow in terrestrial habitats with fluctuating soil water availability and evaporative demand of the atmosphere (Raven 1977, 1984a, 1993, 1997a, 1998). The possession of homoiohydric characteristics permits non-dormant sporophytic structures to be desiccation-intolerant, i.e. unable to survive equilibration with an atmospheric relative humidity of less than c. 50% (i.e. a water potential of c. -100 MPa). Whatever the mechanistic basis for this height restriction of desiccation-tolerant growing vegetative structures of land plants to less than 1 m, it appears that the occurrence of terrestrial plants in the 1-100 m height range requires desiccation-intolerance in the growing sporophyte combined with homoiohydry. It must be remembered that desiccation-tolerance (as defined above) is common in dormant structures (e.g. seeds) of those plants which are desiccation-intolerant when growing and apparently relates to the widespread occurrence in vascular plants of a number of closely related genes encoding 'late-embryogenesisabundant' proteins, one group of which is termed the dehydrins (Ingrams & Bartels 1996). Major reproductive innovations in the evolution of embryophytes were the polyphyletic evolution of heterospory and, in a few of these parallel heterosporous lines of evolution, the seed habit, and culminated in the plant supplying all of the water needed to its reproductive structures, so that above-ground sexual reproductive processes relied only on soil water supply to the

Tracheid Structure in a Primitive Extant Plant Provides an Evolutionary Link to Earliest Fossil Tracheids

Most attempts to understand the early evolution of tracheids have centered on fossil Silurian and Devonian vascular plants, and these efforts have led to a wealth of new information on early water‐conducting cells. All of these early tracheids appear to possess secondary cell wall thickenings composed of two distinct layers: a layer adjacent to the primary cell wall that is prone to degradation (presumably during the process of fossilization) and a degradation‐resistant (possibly lignified) layer next to the cell lumen. Developmental studies of secondary wall formation in tracheary elements of extant vascular plants have been confined to highly derived seed plants, and it is evident that the basic structure of these secondary cell wall thickenings does not correspond well to those of tracheids of the Late Silurian and Early Devonian. Significantly, secondary cell wall thickenings of tracheary elements of seed plants are not known to display the coupled degradation‐prone and degradation‐resistant layers characteristic of tracheids in early tracheophytes. We report a previously unknown pattern of cell wall formation in the tracheids of a living plant. We show that in Huperzia, one of the most primitive extant vascular plants, secondary cell wall deposition in tracheids includes a first‐formed layer of wall material that is degradation‐prone (“template layer”) and a later‐formed degradation‐resistant layer (“resistant layer”). These layers match precisely the pattern of wall thickenings in the tracheids of early fossil vascular plants and provide an evolutionary link between tracheids of living vascular plants and those of their earliest fossil ancestors. Moreover, our developmental data provide the essential information for an explicit model of the early evolution of tracheid secondary wall thickenings. Finally, congruence of tracheid structure in extant Huperzia and Late Silurian and Early Devonian vascular plants supports the hypothesis of a single origin of tracheids in land plants.

The gain of three mitochondrial introns identifies liverworts as the earliest land plants.

The first evidence for the emergence of land plants (embryophytes) consists of mid-Ordovician spore tetrads (approximately 476 Myr old). The identity of the early plants that produced these spores is unclear; they are sometimes claimed to be liverworts, but there are no associated megafossils, and similar spores can be produced by a diversity of plants. Indeed, the earliest unequivocal megafossils of land plants consist of early vascular plants and various plants of uncertain affinity. Different phylogenetic analyses have identified liverworts, hornworts and bryophytes as each being the first lineage of land plants; the consensus of these conflicting topologies yields an unresolved polychotomy at the base of land plants. Here we survey 352 diverse land plants and find that three mitochondrial group II introns are present, with occasional losses, in mosses, hornworts and all major lineages of vascular plants, but are entirely absent from liverworts, green algae and all other eukaryotes. These results indicate that liverworts are the earliest land plants, with the three introns having been acquired in a common ancestor of all other land plants, and have important implications concerning the early stages of plant evolution.

Developmental logic: establishing a relationship between developmental process and phylogenetic pattern in primitive vascular plants

Cladistic methods serve as a powerful example of the current shift in epistemology from a largely inductivist stance toward more `hypothetico-deductive' or `strong inference' approaches. However, in the area of character analysis, especially in addressing the roles of developmental and evolutionary processes influencing our understanding of homology, synapomorphy, and evolutionary pattern, it appears that little has changed. This omission has the very real potential of substantially limiting resolution and reliability of cladistic hypotheses, and points to the need for an expanded methodology that simultaneously addresses evolutionary pattern and underlying causal factors. Due to unique aspects of both plant development and evolutionary history, early vascular plants offer attractive features that make them highly suitable as a test case along these lines. Expanding on the Zimmermann's `telome theory' of a strong inference approach to developmental process is suggested by employing an analogy with logic gates and logic circuits in programming theory. Hypothesized units of developmental dynamic, termed `evolutionary developmental gates' (EDG) define a relationship between external and internal `environmental' factors, and developmental outcome, using a logical conditional. EDGs are then assembled into networks, specifying a relationship between developmental processes and resultant morphological structure, that may be tested. Testing utilizes several lines of evidence including comparing EDGs with specific developmental data, computer modelling, and evolutionary comparisons based on fossil and living taxa. To illustrate this approach, and to initiate analysis of what constitutes a set of developmental processes necessary for describing primitive vascular plant form, four developmental modules comprised of networks of EDGs are proposed. These include: (1) establishment of self-recognition at the shoot apex along with initiation of a hormone (auxin) gradient, (2) normal cell division modeled as an iterative and recursive process, (3) developmental cascades leading to tissue differentiation of epidermis, cortex, and vascular tissues, and (4) establishment of new apices either by bifurcation or de novo formation. Although clearly more detail is required to enable important evolutionary comparisons, it is anticipated that networks of EDGs can comprise a more realistic perspective with which to estimate homology in evolutionary studies.

Very large plant and root traces from the Early to Middle Devonian: Implications for early terrestrial ecosystems and atmospheric p(CO2)

Plant and root traces from the Fort Prevel Member of the Battery Point Formation (late Early Devonian, Emsian), Gaspe Bay, Quebec (Canada), are larger and more complex than previously postulated for land plants of this time. The traces are preserved as clay- and silt-lined casts in or near growth position and provide evidence that early vascular land plants achieved substantial stature (2–3 m) and were capable of deep rooting (to nearly 1 m). The root traces and alluvial deposits in which they occur suggest increased landscape stabilization and root system and paleosol morphologies that were influenced by a water-stressed, episodically energetic environment. Early Devonian plants of such large stature may have been partly responsible for initiation of a steep decline in atmospheric p (CO 2 ), through organic carbon burial and accelerated terrestrial weathering.

Adaptive walks through fitness landscapes for early vascular land plants

Hypothetical adaptive walks (i. e., morphological transformation series gaining increasing relative fitness) were simulated through a computer‐generated domain for early vascular land plant morphologies to examine the relationship between the dynamics of adaptive walks and the topologies of fitness landscapes. A total of 15 hypothetical adaptive walks were simulated, assuming that relative fitness was based on performing one or more of four biological tasks: maximizing light interception, mechanical stability, and reproductive success, and minimizing total surface area. Morphologies occupying fitness peaks typically were similar to some early vascular land plant remains. The most stringent task (the minimization of total surface area) resulted in a few, comparatively small Y‐shaped morphologies. Based on the 15 walks, the number of fitness peaks increased and their heights decreased as the number of tasks simultaneously performed increased. These results (which are consistent with prior computer‐simulated walks treating light interception, mechanical stability, and reproductive success) suggest that the biological requirement to conserve water reduced the number of phenotypic options available to the earliest land plants, and that, once this adaptive hurtle was overcome, the simultaneous performance of two or more tasks, increased the number of phenotypic options with equivalent relative fitnesses that could be rapidly reached due to the comparatively small fitness differential between derived and ancestral morphologies.

Effects of hypothetical developmental barriers and abrupt environmental changes on adaptive walks in a computer-generated domain for early vascular land plants

Computer-generated searches through hypothetical fitness landscapes for progressively more fit variants were used to assess the effects of developmental barriers (mimicked by barring specific types of morphological transformations) and abruptly shifting environmental conditions (mimicked by sudden shifts in how fitness was defined) on the number and accessibility of optimal phenotypes. Relative fitness was defined in terms of maximizing light interception, mechanical stability, or reproductive success, or minimizing surface area, or optimizing the performance of various combinations of these tasks. Developmentally obstructed and unobstructed walks located, on average, equivalent numbers of phenotypic optima. The number of optima identified by both kinds of walks increased in proportion to the number of simultaneously performed tasks used to measure fitness. Walks passing from more complex to less complex fitness landscapes located more optima than walks passing through unchanging, stable landscapes. The model thus suggests that there are no a priori reasons to assume that (1) the morphological options available to adaptive evolution become more restrictive as biological complexity increases, (2) “developmental barriers” necessarily restrain a lineage from evolving well-adapted morphologies, and (3) generalist organisms are less successful than specialists. Also, because the number and accessibility of fitness peaks were proportional to the complexity of fitness landscapes, the model predicts that the probability of cladogenesis will increase as landscape complexity increases, while anagenesis will be encouraged when fitness is defined in terms of performing one or a few tasks simultaneously.

Chemical characterization of Lower Devonian vascular plants

Vegetative remains of three coalified Lower Devonian vascular plants ( Zosterophyllum, Psilophyton, Renalia) were analyzed using flash pyrolysis-gas chromatography-mass spectrometry. The distributions of pyrolysis products are compared with those from younger vascular plant fossil xylem ( Cordaixylon, Callixylon) and cuticle ( Pachypteris). The likelihood of the chemical preservation of characteristic higher plant macromolecules (e.g., lignin and cutan) in the Lower Devonian plant fossils is considered in light of this comparison and associated thermal maturity assessments. Reflectance values from vitrinite-like macerals, which may not be vitrinite sensu stricto in the Lower Devonian host rocks for the fossils selected for this study, are shown to provide a reasonable assessment of the thermal maturity of these early vascular plant fossils. Although lignin altered through burial maturation is the most likely source of the prominent alkylphenols and aromatic hydrocarbons in the Lower Devonian tracheophyte flash pyrolysates, a contribution from thermally modified tannins cannot be ruled out. Comparison of the highly aliphatic pyrolysates from the Zosterophyllum and Psilophyton axes with that of a thermally mature fossil gymnosperm leaf revealed that cutan was an important component in the Devonian plant remains. This is the earliest chemical evidence for the presence of cutan in vascular plants.

An Early Silurian vascular plant

An Early Silurian vascular plant

ALTERNATION OF GENERATIONS IN LAND PLANTS: NEW PHYLOGENETIC AND PALAEOBOTANICAL EVIDENCE

SummaryCurrent ideas on the evolution of alternation of generations in land plants are reviewed in the context of important recent advances in plant systematics and the discovery of remarkable new palaeobotanical evidence on early embryophyte life cycles. An overview of relationships in major groups of green plants is presented together with a brief review of the early fossil record as a prelude to discussing hypotheses of life cycle evolution. Recent discoveries of life cycles in the early fossil record are described and assessed. The newly discovered gametophyte and sporophyte associations are based on exceptionally well‐preserved material from the Rhynie Chert, Scotland (Middle Devonian: 380–408 Myr) and compression fossils from other Devonian localities. These data document diplobiontic life cycles in plants at the ‘protracheophyte’ and early tracheophyte level of organization. Furthermore, the early fossils have a more or less isomorphic alternation of generations, a striking departure from life cycles in extant embryophytes. This unexpected similarity between gametophyte and sporophyte calls for a cautious approach in identifying ploidy level in early groups. Viewed in a systematic context, the neontological and palaeontological data contribute towards the formulation of a coherent hypothesis of life cycle evolution in major, early embryophyte groups. Evidence from extant groups strongly supports a single direct origin of the diplobiontic life cycles of land plants from haploid, haplobiontic life cycles in ancestral ‘charophycean algae’. The interest of the new palaeobotanical data lies in its relevance to life cycle evolution at the restricted level of vascular plants rather than at the more general level of embryophytes (vascular plants plus ‘bryophytes’). The occurrence of morphologically complex, axial gametophytes in early vascular plants is consistent with the moss sister‐group proposed in some cladistic analyses. Similarities of moss gametophytes to fossils in the vascular plant stem‐group are discussed, and it is argued that the late appearance of mosses in the macrofossil record may be due to the problem of recognizing stem‐group taxa. The new palaeobotanical evidence conflicts with previous hypotheses based on extant groups that interpret morphological simplicity as the plesiomorphic condition in the gametophytes of vascular plants. These new data indicate that a significant elaboration of both gametophyte and sporophyte occurred early in the tracheophyte lineage, and that the gametophytes of extant ‘pteridophytes’ are highly reduced compared to those of some of the earliest ‘protracheophytes’. Vestiges of this early morphological complexity may remain in the gametophytes of some extant groups such as Lycopodiaceae.

Efficiency and evolution of water transport systems in higher plants: a modelling approach. I. The earliest land plants

The evolution of the stele was studied under the functional aspect of water transport problems by using a numerical approach. The underlying mathematical model describes the behaviour of a fluid-filled porous medium and is based on the coupling of Hooke’s law and Darcy’s law including a dynamic permeability approach which leads to a self-organization of the considered structure according to the resulting fluid- pressure field. Calculations dealing with two problems were performed. The essential demand of a water conducting system for a plant was demonstrated quantitatively. As soon as the plant shows an upright habit, the need for efficient water transport occurring through a highly porous apoplastic pathway becomes evident. In a second approach, the evolution of the protostele was simulated using the concept of dynamic permeability. The simulations of structures with self-regulating hydraulic conductivity yielded two strategies according to the pressure-permeability relationship. Increasing hydraulic conductivity with increasing negative fluid pressure results in peripheral layers of the conducting tissues, whereas the inverse pressure-permeability relationship yields a central position of the conducting tissues. The latter arrangement corresponds to the protostelar construction of early vascular plants.

Physiological analyses of aspects of the functioning of vascular tissue in early plants

Summary Comparison of the vascular anatomy of the earliest known vascular plants (Upper Silurian/ Lower Devonian) with the functional anatomy of extant vascular plants permits the following conclusions (excluding previous analyses of xylem conductivity and biomechanics). The absence of an endodermis from early vascular plants may have hindered the regulation of the supply of soil-derived nutrients to the shoot. The absence of intraxylary parenchyma in early vascular plants may have interfered with the differential distribution of soil-derived nutrients among aerial branches. The earliest vascular plants, like extant plants, lacked gas spaces in vascular tissues, so that the early plants could have shared with extant plants some protection from the impact of embolism of xylem and, less importantly, reduce O2 damage to ‘phloem’. Early vascular plants also lacked the lignified UV-B screen found around the phloem in some extant plants, although this was probably not a serious lack in their relatively short-lived, nucleate phloem cells. An early vascular plant with a main axis and laterals shares with extant plants a xylem constriction at the base of lateral branches, which favours survival of the main axis during water shortage.