Biophysical Examinations of the Bending Stability of Various Stele Types and the Upright Axes of Early “Vascular” Land Plants

Biophysical methods allow quantitative deductions as to the “bending effectivity” of various stele types and the contribution of different tissues to the stability of upright plant axes. Considering the first occurrence of the different stele types in geological history, a strong tendency towards a higher “bending effectivity” can be observed. It can be shown that Rhynia gwynne-vaughanii, Aglaophyton major (Rhynia major) and Asteroxylon mackiei as well as probably the vast majority of early “vascular” land plants were turgor systems, i.e. that the parenchyma (when fully turgescent) is by far the most important contributory factor towards the bending stability of the upright axes.     

Endophytic cyanobacteria in a 400-million-yr-old land plant: A scenario for the origin of a symbiosis?

Direct evidence for the origin and evolution of land plant/cyanobacterial symbioses is virtually absent from the fossil record. Here we report on rare occurrences of prostrate mycorrhizal axes of the Early Devonian land plant Aglaophyton major that host a filamentous cyanobacterium, which enters the plant through the stomata and colonizes the substomatal chambers and intercellular spaces in the outer cortex. In dead ends of the intercellular system, the filaments form loops and continue growth in reverse direction. Some filaments penetrate parenchyma cells close to and within the mycorrhizal arbuscule-zone and form intracellular coils. This discovery represents the earliest direct evidence for cyanobacteria growing inside land plants, and offers a model for the types of associations that may have preceded the evolution of mutualistic land plant/cyanobacterial symbioses.     

Novel conducting tissues in Lower Devonian plants

Elongate cells presumed to comprise water-conducting tissues are described from the central regions of short lengths of two naked, stomatiferous, coalified, axial fossils from Lochkovian (Lower Devonian) fluvial rocks in the Welsh Borderland. In one, a discrete central strand is predominantly composed of uniformly thickened cells that are compared with central tissues in coeval plants, e.g. Aglaophyton , and the hydroids of extant mosses. The other has at least two types of cells with pits of plasmodesmata dimensions that perforate only the inner layer of a bilayered wall. These are compared with liverwort and Takakia hydroids and the coeval S-type tracheids that characterize the Rhyniopsida. The affinities of the two axes remain equivocal. The relevance of plasmodesmata-derived pits to the evolution of diversity in water-conducting elements in early cmbryophytes is discussed.     

Studies on Two Palaeozoic Seeds from China

This paper reports two species of the seed order Cardiocarpales: Cardiocarpus (?) minor (Wang) emend and Diplotesta sp. which were found in coal balls. The specimens were collected from Taiyan Formation of Upper Carboniferous near Xiedao Village, West Mountain, 'Taiyan City, Shanxi Province and Wangjiazhai Formation of Upper Permian of Wangjiazhai Mine of Shuicheng district, Guizhou Province, China. The two taxa are described and their taxonomy is discussed in detail. The diagnosis of the species (Cardiocarpus (?) minor (Wang) emend) is amended as follows: Seeds bilaterally symmetric and thinner cardioformed, appoximatly 4–6 mm long, 3–4 mm wide in the primary section and less than 2 mm thick in the secondary section. Integument differentiated into three zones. Outer sarcotesta delimkted into four layers: 1) carbonized cuticle partly preserved; 2) epidermis of thin-walled parenchyma 1–2 rows of small cells; 3) hypodermis of secretory tissue containing large cells, uniform in thickness; and 4) parenchyma layer consisting of thin-walled cells in size varying from 20 to 50 μm middle sclerotesta of vertically arranged fibers is bipartite: outer fibers being smaller in diameter and possessing thicker walls than inner fibers. Sclerotesta 80 μm in thickness commissural laterally to form two lateral ribs. Inner endotesta single rowed of parenchymatous cells. Nucellus free except at base which is attached to the integument by narrow nucellar platform. Pollen chamber not observed. The double vascular system includes integumentary strand beneath secretory layer of sarcotesta and nucellar strand which forms a tracheid mentle; the origin of integumentry and nucellar strands unknown. In addition a computerized three-dimensional reconstuction techniqne is utilized to reconstruct membrane like structure of Diplotesta sp. It is suggested that Cardiocarpus (?) minor is different from other species of the genus Cardiocarpus in the distribution of nucellar vascular tissues. This feature is thonght to be one of the important characters of bilaterally symmetrical seeds in a taxonomic view point. Therefore Cardiocarpus minor wang probally represents a menber of another new seed genus, althongh this species remains in the genus Cardiocarpus.     

STENOKOLEOS BIFIDUS SP. N. IN THE UPPER DEVONIAN OF NEW YORK STATE

Stenokoleos is a genus for petrified axes from the Mississippian New Albany Shale to which an Upper Devonian occurrence in New York is added. Two orders of branching were known and the plant was thought to be related to coenopterid ferns. The new petrified axes from New York reveal three orders of branching. A pair of rachides emerges from one side of the stem at each node. Their position alternates at successive nodes (distichous). Each rachis bears alternately arranged pinnae. The shape of the xylem strand and the number of protoxylem areas are variable. Traces to the pairs of rachides arise either as two separate strands or as a single strand that is presumed to divide while still within the cortex of the stem. Traces to pinnae are ellipsoid or clepsydroid. Tracheids are scalariform and uni- or biseriate, circular-bordered pitted. Peripheral loops are present in all orders of branches. Protoxylem strands are numerous and maturation is mesarch. Cortex is parenchymatous where it is preserved but outer cortex is missing. Stenokoleos and Reimanniopsis are placed in a new family, Stenokoleaceae. This is classified as Incertae Sedis among Pterophytina in Tracheophyta. It is suggested that the plant is related more closely to the Mississippian pteridosperms Tristichia and Tetrastichia than to the coenopterid ferns.     

New information on gametophytes and sporophytes of Aglaophyton major and inferences about possible environmental adaptations

Additional information on gametophytes and sporophytes of Aglaophyton major from the Early Devonian Rhynie chert leads to a much better understanding of many biological and ecological aspects of this early land plant. At least three early ontogenetic stages can be demonstrated, before aerial axes start to develop on the young gametophytes. Only the aerial axes show the essential features of a land plant (stomata, conducting tissue); the initial stages seem more likely adapted to shallow aquatic or edaphic humid than to fully terrestrial environmental conditions. Serial sections of aerial axes of gametophytes and sporophytes of Aglaophyton have yielded new histological and developmental features. These include cuticle structures, stomatal and substomatal features. A hitherto undescribed type of specialized cortical tissue, arrested apices, the formation of second-order axes and bulbil-like organs are documented. New observations on the development of rhizoidal bulges and wound-repair (including conducting) tissues complete this report. The new information provides additional evidence for life (competition) strategies in Aglaophyton, e.g. extensive renewing growth and vegetative propagation. It throws new light upon the adaptive behaviour of Aglaophyton, e.g. protection mechanisms against desiccation of aerial axes. It also illustrates considerable developmental biology of Aglaophyton.     

An alternative model for the earliest evolution of vascular plants

Land plants comprise the bryophytes and the polysporangiophytes. All extant polysporangiophytes are vascular plants (tracheophytes), but to date, some basalmost polysporangiophytes (also called protracheophytes) are considered non‐vascular. Protracheophytes include the Horneophytopsida and Aglaophyton/Teruelia. They are most generally considered phylogenetically intermediate between bryophytes and vascular plants and are therefore essential to elucidate the origins of current vascular floras. Here, we propose an alternative evolutionary framework for the earliest tracheophytes. The supporting evidence comes from the study of the Rhynie chert historical slides from the Natural History Museum of Lille (France). From this, we emphasize that Horneophyton has a particular type of tracheid characterized by narrow, irregular, annular and/or, possibly spiral wall thickenings of putative secondary origin, and hence that it cannot be considered non‐vascular anymore. Accordingly, our phylogenetic analysis resolves Horneophyton and allies (i.e. Horneophytopsida) within tracheophytes, but as sister to eutracheophytes (i.e. extant vascular plants). Together, horneophytes and eutracheophytes form a new clade called herein supereutracheophytes. The thin, irregular, annular to helical thickenings of Horneophyton clearly point to a sequential acquisition of the characters of water‐conducting cells. Because of their simple conducting cells and morphology, the horneophytophytes may be seen as the precursors of all extant vascular plant biodiversity.     

Stem Elongation and Cell Wall Proteins in Flowering Plants

Abstract: The growth of stems (hypocotyls, epicotyls) and stem-like organs (coleoptiles) in developing seedlings is largely due to the elongation of cells in the sub-apical region of the corresponding organ. According to the organismal concept of plant development, the thick outer epidermal wall, which can be traced back to the peripheral cell wall of the zygote, creates a sturdy organ sheath that determines the rate of stem elongation. The cells of the inner tissues are the products of secondary partitioning of one large protoplast; these turgid, thin-walled cells provide the driving force for organ growth. The structural differences between these types of cell walls are described (outer walls: thick, sturdy, helicoidal cellulose architecture; inner walls: thin, extensible, transversely-oriented cellulose microfibrils). On the basis of these facts, current models of cell wall loosening (and wall stiffening) are discussed with special reference to the expansin, enzymatic polymer remodelling and osmiophilic particle hypothesis. It is concluded that the exact biochemical mechanism(s) responsible for the coordinated yielding of the growth-controlling peripheral organ wall(s) have not yet been identified.     

Seed coat development, anatomy and scanning electron microscopy of Harpagophytum procumbens (Devil's Claw), Pedaliaceae

Seed coat development of Harpagophytum procumbens (Devil's Claw) and the possible role of the mature seed coat in seed dormancy were studied by light microscopy (LM), transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Very young ovules of H. procumbens have a single thick integument consisting of densely packed thin-walled parenchyma cells that are uniform in shape and size. During later developmental stages the parenchyma cells differentiate into 4 different zones. Zone 1 is the multi-layered inner epidermis of the single integument that eventually develops into a tough impenetrable covering that tightly encloses the embryo. The inner epidermis is delineated on the inside by a few layers of collapsed remnant endosperm cell wall layers and on the outside by remnant cell wall layers of zone 2, also called the middle layer. Together with the inner epidermis these remnant cell wall layers from collapsed cells may contribute towards seed coat impermeability. Zone 2 underneath the inner epidermis consists of large thin-walled parenchyma cells. Zone 3 is the sub-epidermal layers underneath the outer epidermis referred to as a hypodermis and zone 4 is the single outer seed coat epidermal layer. Both zones 3 and 4 develop unusual secondary wall thickenings. The primary cell walls of the outer epidermis and hypodermis disintegrated during the final stages of seed maturation, leaving only a scaffold of these secondary cell wall thickenings. In the mature seed coat the outer fibrillar seed coat consists of the outer epidermis and hypodermis and separates easily to reveal the dense, smooth inner epidermis of the seed coat. Outer epidermal and hypodermal wall thickenings develop over primary pit fields and arise from the deposition of secondary cell wall material in the form of alternative electron dense and electron lucent layers. ESEM studies showed that the outer epidermal and hypodermal seed coat layers are exceptionally hygroscopic. At 100% relative humidity within the ESEM chamber, drops of water readily condense on the seed surface and react in various ways with the seed coat components, resulting in the swelling and expansion of the wall thickenings. The flexible fibrous outer seed coat epidermis and hypodermis may enhance soil seed contact and retention of water, while the inner seed coat epidermis maintains structural and perhaps chemical seed dormancy due to the possible presence of inhibitors.     

Chemotaxonomy of some Paleozoic vascular plants. Part II: Chemical characterization of major plant groups

Chemical data are given forCooksonia, Rhynia, Zosterophyllum, Pseudosporochnus, Gosslingia,Crenaticaulis, Leclercqia, Tetraxylopteris, Oocampsa, andArchaeopteris, thus extending principal component analysis of multi-state and discrete characters to 27 Paleozoic plant taxa. Ordination patterns of these data suggest that while overlapping of major plant groups occurs, rhyniophytes, zosterophyllophytes, trimerophytes and other supra-generic taxa may be chemically characterized. The effects of heat (thermolysis) on organic constituents is shown to severely alter chemical profiles of plant taxa and is discussed as a thermometric tool. Taxonomic distancemeasures between plant groupings are suggested to be the result of both preand post-fossilization phenomena.     

Anatomy of Stigmaria asiatica Jongmans et Gothan from the Asselian (lowermost Permian) of Wuda Coalfield,Inner Mongolia,North China

Stigmaria asiatica Jongmans et Gothan is a common species from the Permo-Carboniferous of East Asia. It is characterized by a relatively slender rhizomorph and represents the underground rooting system of lowland arborescent lycopsids. This species from the Wuda Coalfield (Asselian, lower Permian) represents the recovery taxon dominating the subsequent peat-forming vegetation after a volcanic eruption destroyed the previous flora, termed the Wuda Tuff Flora (Chinese “vegetational Pompeii”). It is characterized by novel downward axes, probably for access to deeper groundwater. Here, the anatomy of S. asiatica, including that of the axes and rootlets, is reported in detail for the first time. The axis contains stelar tissues, including pith, primary and secondary xylem, and phloem, and both primary and secondary cortical tissues. The primary cortex is tripartite and is divided into inner, middle, and outer zones. Secondary cortex is produced in the interior of the outer cortex splitting the outer cortex into inner and outer portions. The homogeneous pith and mesarch primary xylem maturation of S. asiatica are different from previously recorded species of Stigmaria Brongniart. Rootlets are composed of a central monarch vascular bundle surrounded by inner cortex and a ring of outer cortex. A vacant region is present between the inner and outer cortices. In some cases, connectives between the inner and outer cortices are present. According to the presented statistical analysis, it is proposed that rootlets of S. asiatica were highly branched, with at least for 7 bifurcations. Rootlets, together with the downward penetrating rhizomorph, helped to both anchor the plant and search for and absorb deep groundwater.     

The role of mid-palaeozoic mesofossils in the detection of early bryophytes

Recently discovered Silurian and Devonian coalified mesofossils provide an additional source of data on early embryophytes. Those reviewed in this paper are considered of some relevance to understanding the early history of bryophytes while highlighting the difficulties of recognizing bryophytes in often very fragmentary fossils. The first group comprises sporophytes in which terminal sporangia contain permanent dyads and tetrads. Such spores (cryptospores) are similar to those found dispersed in older Ordovician and Silurian strata, when they are considered evidence for a land vegetation of embryophytes at a bryophyte grade. The phylogenetic significance of plants, where the axes associated with both dyad- and tetrad-containing sporangia are branching, a character state not found in extant bryophytes, is discussed. The second group comprises axial fossils, many with occasional stomata, in which central conducting strands include G-type tracheids and a number of novel types of elongate elements not readily compared with those of any tracheophyte. They include smooth-walled, evenly thickened elongate elements as well as those with numerous branching +/- anastomosing projections into the lumen. Some of the latter bear an additional microporate layer, but the homogenized lateral walls between adjacent cells are never perforate. Such cells, which occur in various combinations in central strands, are compared with the leptoids and hydroids of mosses, hydroids of liverworts and presumed water-conducting cells in coeval Lower Devonian plants such as Aglaophyton. It is concluded that lack of information on the chemistry of their walls hampers sensible assessment of their functions and the affinities of the plants. Finally, a minute fossil, comprising an elongate sporangium in which a central cylindrical cavity containing spores and possible elaters terminates in a complex poral dehiscence apparatus, is used to exemplify problems of identifying early bryophytes. It is concluded that further progress necessitates the discovery of pre-Upper Silurian fossils with well-preserved anatomy, as well as a re-evaluation of criteria used to assess existing and new Devonian fossils for bryophyte affinity.     

DUDRESNAYA PUERTORICENSIS SP. NOV. (DUMONTIACEAE,GIGARTINALES, RHODOPHYTA)1

A new species of Dudresnaya is described from Puerto Rico and Georgia. Gametophytes have cylindrical axes, exserted apical cells, rectangular to hexagonal crystals in the axial cells, ellipsoidal outer assimilatory cells, spermatangial mother cells which are terminal or subterminal cells of the subdichotomously branched outer assimilatory branches and auxiliary cells which are indistinguishable in appearance from adjacent, large, deeply staining cells of the auxiliary cell branch. Tetrasporophytes are unknown.     

Anatomically preserved lepidodendralean plants from lower permian coal balls of northern China: Achlamydocarpon intermedium sp. nov.

Lepidodendralean lycopsids, a dominant component in Late Palaeozoic wetland plant communities, possess a diversity of reproductive structures that are primarily known from the Late Palaeozoic floras of Europe and North America. Here we document an anatomically preserved lepidodendralean lycopsid sporophyll with attached megasporangium from the Lower Permian Taiyuan Formation in Shanxi Province, northern China. The sporophyll has a pedicel onto which the sporangium is attached, and the sporangium is dorsiventrally flattened, proximally dehiscent and longitudinal ridged. The megasporangial wall comprises three zones: an outer uniseriate layer of columnar cells, a middle layer 1–3 cells thick comprising isodiametric parenchymatous cells, and an inner zone 1–3 cells thick of thick-walled cells. The vascular system comprises a single xylem strand surrounded by zone of parenchyma that continues through the pedicel into the lamina. Within the megasporangium a single functional megaspore and three abortive megaspores occur. Features of this specimen conform to Achlamydocarpon Schumacher-Lambry, and comparisons with other species show it shares similarities with A. takhtajanii (Sni.) Schumacher-Lambry and A. varius Taylor and Brack-Hanes. Although the morphology and anatomy of the specimen we describe overlaps with these two species, it is distinct from both leading to the erection of the new species A. intermedium sp. nov. The evolutionary significance of A. intermedium sp. nov. and the identity of its parent plant are considered, and the status and systematic position of “Oriental lepidophytes” from the Cathaysian floras are discussed.     

Nothia aphylla : les clones étaient parmi les premières plantes terrestres !

Nothia aphylla was one of the main plant component in the Early Devonian terrestrial environment of Rhynie (Scotland). The small-sized, herbaceous plant shows a dense, rhizomatous system from which extend dichotomously branched, erected axes. These stems are covered by protuberances, each bearing a single stomatal apparatus. They were clearly chlorophyllous as evidenced by the anatomical structures. Sporangia are arranged laterally and terminally, and characterize the sporophytic stage of the growth cycle of Nothia aphylla. Nothia aphylla also shows a vegetative multiplication, which consists of identical units derived from the branches. Each vegetative unit is constituted by a rhizome that bears rhizoids, whereas its other extremity ends by the aerial stem. This clone can give birth to a population of genetically identical individuals by simple scission between the mother and daughter units. This asexual strategy favours a quick colonisation, but does not allow for long-term maintenance in the environment. It is known amongst several plants coeval to Nothia (e.g. Aglaophyton, Horneophyton, etc.), but also in the modern vegetation. From one hand, it always remains associated to the sexual reproduction in order to optimise the survival of the species. From the other hand, this association is not always sufficient to avoid the disappearance of the poorly competitive species face to new, more efficient forms, as was the fate of Nothia aphylla.     

Fine structure of the thymus in the adult cling fish Sicyases sanguineus (Pisces, Gobiesocidae)

The structure of the thumus in adult specimens of a marine teleost, the cling fish Sicyases sanguineus, has been studied by light and transmission electron microscopy. Most cling fishes have an outer thymus located beneath the opercular epithelium. A few of them, however, have a large inner thymus besides a poorly developed outer thymus. In the well-developed outer thymus of cling fish there are three different zones: outer cortex, inner cortex, and medulla. The inner cortex is similar to the cortical region of the thumus in other vertebrates, whereas the outer cortex is a specialized lympho-epithelial zone containing cystic cells (also present in medullary region) and true Hassall's corpuscles. In accordance with the development of the thymic parenchyma, the medullary or basal region may appear either like a true thymic medulla or like a subcapsular region. In the inner thymus, a subcapsular or peripheral "medullary" region and a central area (inverted cortex) show structural features like those of the medullary (basal) and deep cortical regions of the outer thymus, respectively. In addition to the above regions, sometimes there is a lymphomyeloid perithymic infiltration that often extends along connective tissue septa into the perivascular spaces of the gland. Reticuloepithelial, mesenchymal, and unidentified types of stromal cells within the thymus are described. Some erythrocytes, granulocytes, and monocytoid cells are found, but no plasma cells nor erythropoietic foci are evident. The probable significance of these findings is discussed.     

Vegetative anatomy of subtribe Habenariinae (Orchidaceae)

Leaves of Habenariinae are characterized by anomocytic stomatal apparatuses, homogeneous mesophyll, collateral vascular bundles in a single series, and thin-walled bundle sheath cells. There is no foliar sclerenchyma nor a hypodermis. Cauline cortex consists of thin-walled living cells among which are large and numerous intercellular spaces. The ground tissue is bordered externally by a layer of thick-walled living cells, except in Habenaria repens. Central ground tissue cells are living, and usually thin-walled surrounding intercellular spaces of various dimensions. These are conspicuously large in H. repens. Collateral vascular bundles are scattered across the ground tissue. Sclerenchyma is absent. Absorbing roots are generally velamentous, exodermal dead cells are diin-walled, and passage cells usually have a thickened outer wall. A regular vascular cylinder is present, and vascular tissue is embedded in parenchyma. Root tubers are velamentous, exodermal cells are usually thin-walled, and passage cells frequently have thickened outer walls. Vascular tissue of root tubers is organized into two classes: (1) those with a single vascular cylinder surrounded by a cortex and (2) those with a series of meristeles dispersed throughout the ground tissue. In group (1) cortex is homogeneous either with or without mucilage cells except in Stenoglattis where the cortex is heterogeneous, consisting of water-storage and assimilatory cells, and lacks mucilage cells. In group (2) the ground tissue consists of larger mucilage-containing cells and smaller assimilatory cells.     

Development and Structure of the Haustorium of the Parasite Rhamphicarpa fistulosa (Scrophulariaceae)

Rhamphicarpa fistulosa (Hochst.) Benth. (Scrophulariaceae), a parasite of African cereals, develops secondary haustoria which penetrate the roots of the host plant. Light and electron microscopy have been used to study the structure and development of haustoria in this species, which, until now, have not been well characterized. Haustoria are initiated in the hypodermis of the parasite roots. A meristematic strand is developed between the parasite root stele and the host-parasite interface. From this strand, cells differentiate into xylem elements after penetration of the host root. Xylem differentiation follows an acropetal pattern. Mature haustoria are characterized by a continuous xylem bridge between water conducting elements of parasite and host. A detailed study of the hostparasite interface revealed the presence of collapsed and compressed host cells at the lateral interface (between parasite cells and host cortex), whereas the central interface between parasite cells and the host stele is almost devoid of host cell remnants. Implications of these observations for the penetration mechanisms are discussed.