Xylem Maturation Research Articles

Stramineopteris aureopilosus gen. et sp. nov.: Reevaluating the Role of Vegetative Anatomy in the Resolution of Leptosporangiate Fern Phylogeny

The discovery of two anatomically preserved rhizomes with attached stipe bases and adventitious roots contributes to our knowledge of filicalean fern diversity and evolution. Specimens are preserved in marine carbonate nodules at the Lower Cretaceous (Valanginian to Hauterivian) Apple Bay locality on Vancouver Island, British Columbia, Canada. They represent a previously unknown filicalean fern with transitional solenostelic‐dictyostelic stelar architecture and are described as Stramineopteris aureopilosus gen. et sp. nov. Rhizomes range from 2.5 to 5.5 mm in diameter and are amphiphloic. Arc‐shaped leaf traces display a slightly asymmetrical divergence pattern and radiate from all sides of the stele. Primary xylem maturation is exarch. Multicellular trichomes are present throughout but are produced in greater numbers at rhizome dichotomies. Both pith and cortex are of a distinctive heterogeneous construction. A comparison of this distinctive combination of characters to previously described fern rhizomes reveals that vegetative anatomy may be more taxonomically diagnostic for a broader range of ferns than is commonly thought. This realization provides the impetus to explore more closely the distribution of several anatomic characters among leptosporangiate ferns and to reexamine the value of vegetative anatomy for developing a clearer understanding of fern systematics.

Spatio-temporal Variations in Axial Conductance of Primary and First-order Lateral Roots of a Maize Crop as Predicted by a Model of the Hydraulic Architecture of Root Systems

Rates at which water can be transported along plant roots (axial pathway) vary through time, in part depending on xylem maturation. Because of experimental constraints, the dynamics of root functional heterogeneity under field conditions remains mostly uncharted territory. Recent advances in mechanistic modelling offer opportunities to bypass such experimental limitations. This paper examines the dynamics of local variations in axial conductance of primary and first-order lateral roots of a maize crop using the architecture-based modelling approach developed by Doussan et al. (Annals of Botany: 81, 213–223, 1998). Specifically, we hypothesised that points of major resistance to long distance water transfers could arise from discrepancies between the hydraulic maturity (or water carrying capacity) of main axes and branch roots. To test this assumption, spatial distributions of root axial conductance were tested after 30, 60 and 100 days at soil depths of 10, 50 and 100 cm under a maize (Zea mays L.) crop sown at a density of 8 plants m−2. As the crop developed, the corresponding root populations encompassed ever increasing amounts of hydraulically mature first-order laterals (branch roots): after a 100-day growth period, the vast majority of laterals had reached their maximum axial conductance at all soil depths down to 100 cm. In contrast, the axial conductance of a large proportion of main axes (primary roots) remained low, even at shallow soil depths and after 100 days of growth. The imbalance between the hydraulic maturity of primary and lateral roots was most conspicuous at soil depths of 100 cm, where ~10% only of the former compared to ~80% of the latter, had reached their maximum axial conductance after a 100-day growth period.

The Anatomy of Rotoxylon dawsonii comb. nov. (Cladoxylon dawsonii) from the Upper Devonian of New York State

Cladoxylon dawsonii (Dawson) Read was reexamined by sectioning and analysis of a portion of Dawson’s original specimen. The primary xylem consists of 18 radially oriented and elongate primary xylem ribs. Toward the center of the system, some xylem ribs are laterally continuous with adjacent ribs, forming elongate xylem segments, whereas others appear discrete. Order of xylem maturation is mesarch, with one to three protoxylem strands located near the tip of each primary xylem rib. Primary phloem immediately surrounds each primary xylem rib and consists of sieve cells. To the outside of the primary phloem, a conspicuous ground tissue is comprised of roughly isodiametric parenchyma with dark contents in their lumina as well as parenchyma cells that are longitudinally elongate with clear lumina. Beyond the vascular column, a fragmentary cortex contains numerous vascular traces embedded in parenchyma. These latter cells are surrounded by conspicuous masses of longitudinally elongate cortical thick‐walled pare...

Role of the gelatinous layer (G-layer) on the origin of the physical properties of the tension wood of Acer sieboldianum

The tension wood (TW) properties of a 70 year-old Acer sieboldianum Miq were analyzed by using the G-fiber model which was proposed in our previous paper. The roles of the G-layer on the origins of (1) a high large tensile growth stress, (2) a large longitudinal Young's modulus, and (3) a high longitudinal drying shrinkage in the tension wood xylem were discussed on the basis of the simulations using the G-fiber model. The results suggest that the G-layer generates a high tensile stress in the longitudinal direction during the xylem maturation; the longitudinal Young's modulus of the green G-layer becomes significantly higher than that of the lignified layer; furthermore, the G-layer tends to shrink extraordinarily higher than that of the lignified layer during the moisture desorption.

Rotafolia songziensis gen. et comb. nov., a sphenopsid from the Late Devonian of Hubei, China

A sphenopsid from the Upper Devonian (Famennian) Xiejingsi Formation, south-western Hubei Province, China, previously named as various species in Sphenophyllum, Hamatophyton, Bowmanites and Sphenophyllostachys, is now reinvestigated and assigned to a new taxon, Rotafolia songziensis gen. et comb. nov. Its ribbed axes are anisotomous and possess slightly expanded nodes. Lateral axes are inserted at nodes on main axes. Whorls of much divided vegetative leaves are attached at nearly right angles to nodes of basal axes, and at acute angles to nodes of terminal axes. There are six leaves per whorl. The terminal strobilus includes a central axis and verticils of fertile units. Each fertile unit consists of a bract and numerous sporangia. The margin of the elongate-cuneate bract bears a distal and many lateral elongate segments. Clusters of elongate sporangia are abaxially attached to the base of the bract at the same level. The axis has an actinostele, composed of a three-ribbed, exarch primary xylem and radial secondary xylem. Although Rotafolia songziensis closely resembles Hamatophyton verticillatum in axis character, leaf morphology and primary xylem type, they are quite different in strobilar structure. Taxonomically, Rotafolia is placed in the order Sphenophyllales by three well-defined characters: 1) whorled appendages; 2) ribbed protosteles; 3) exarch primary xylem maturation.

Hsüa deflexa sp. nov. from the Xujiachong Formation (Lower Devonian) of eastern Yunnan, China

A new vascular plant, Hsua deflexa sp. nov., is documented from the Lower Devonian ((upper) Pragian-lower Emsian) Xujiachong Formation, near Xujiachong village, Qujing District, eastern Yunnan, South China. In three dimensions, the branching system comprises a robust creeping main axis and comparatively slender erect lateral axes inserted oppositely or alternately. The lateral axes depart at right angles from the main axis. Towards the apex, the main axis is perhaps erect with the lateral axes attached at smaller angles. The lateral axes dichotomize equally one to three times in different planes and decrease in diameter and length acropetally. They bear sparse and irregular tiny spines. Apices of a pair of distal lateral axes curve in opposite directions and each terminates in a rounded to reniform sporangium. The sporangium dehisces into two equal valves along an indistinct convex marginal thickening. The xylem is possibly centrarch. This plant fits Hsua in branching pattern, sporangial characters and xylem maturation. Hsua deflexa sp. nov. differs mainly from the type species H. robusta in the presence of axial spines, perpendicular extension of lateral axes from the main axis and curving of distal lateral axes. Based on the possibly centrarch xylem and terminal sporangium, this plant relates to the rhyniophytes (sensu Banks, 1968). In view of the equal sporangial valves with marginal thickenings it resembles the zosterophyllophytes (sensu Banks, 1968). Hsua is now treated as incertae sedis. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 142, 255−271.

Cordaitalean Seed Plants from the Early Permian of North China. I. Delimitation and Reconstruction of the Shanxioxylon sinense Plant

A new whole plant species of cordaitalean coniferophyte is reconstructed from a single Early Permian coal‐ball assemblage in northern China and is named Shanxioxylon sinense after its distinctive stem. This assemblage contains three cordaitalean whole plants reconstructed from evidence, including organs found in organic connection and other examples where two or more organs share distinctive anatomical features. With the exception of the fertile shoot systems, organs of the S. sinense plant may be ascribed to existing cordaitalean morphogenera, although in each case they represent distinct morphospecies. Stems have a septate central pith and have previously been assigned to S. sinense. Primary vascular architecture is sympodial, and primary xylem maturation of the cauline bundles is endarch. Adventitious roots are infrequently observed on smaller stems. Leaf traces are mesarch, diverging from the pith margin as a single bundle and approaching 8/21 phyllotaxis. Leaves are small and needle‐like. Roots are protostelic and ditetrarch, while rootlets are protostelic and diarch, belonging to the morphospecies Amyelon taiyuanense. Both pollen and ovulate cones are assigned to Cathayanthus gen. nov., characterized by helically inserted bract/shoot complexes. Male shoot systems of Cathayanthus ramentrarus comb. nov. have pollen cones with 14–18 scales, of which three to five apical scales may be fertile. Fertile scales possess a ring of approximately five pollen sacs apically, which contain saccate prepollen of the Florinites type. Ovulate cones of Cathayanthus sinensis comb. nov. contain more than 100 scales with the distal half fertile, bearing a single terminal ovule. Ovules are bilaterally symmetrical and have a prominent wing, conforming to the morphospecies Cardiocarpus samaratus. Based on characters of the stem, roots, leaves, male and female shoot systems, and ovules, the S. sinense plant is distinct from other cordaitalean plants, although its features are consistent with models of cordaitalean morphology, anatomy, and architecture, indicating that it belongs to this group. The S. sinense plant shows that coniferophyte fertile shoot systems with vertically arranged rows of bract/shoot complexes can no longer be considered diagnostic of, or synapomorphic to, cordaitalean coniferophytes.

Cellular machinery of wood production: differentiation of secondary xylem in Pinus contorta var. latifolia.

The objectives of this study were to define cell structure during pine secondary xylem development and to integrate this information with current knowledge of the biochemistry and physiology of secondary cell wall biosynthesis in gymnosperms. Lodgepole pine (Pinus contorta var. latifolia Englem.) cambium and secondary xylem were cryofixed using high pressure freezing and freeze-substitution which allowed excellent preservation of the cell structure of developing secondary xylem and enabled high-resolution transmission electron microscopic viewing of these cells for the first time. In contrast to their precursors in the adjacent cambial zone, developing tracheids were active in secondary wall deposition, with abundant cortical microtubules and developing bordered pits. These cells were also characterized by unusual Golgi structures: the trans-Golgi network was highly developed and the associated vesicles were large and darkly stained. These unusual Golgi structures persisted throughout the period of xylem maturation until programmed cell death occurred. Immuno-cytochemistry and enzyme-gold probes were used to investigate the distribution of key secretory products (mannans) and a lignification-associated enzyme (coniferin beta-glucosidase) during xylogenesis. Mannans were localized to the secondary cell wall, the trans-Golgi cisternae and trans-Golgi network vesicles of developing xylem. Coniferin beta-glucosidase was found only in the secondary cell wall. The cell wall localization of coniferin beta-glucosidase, the enzyme responsible for cleaving glucose from coniferin to generate free coniferyl alcohol, provides a mechanism to de-glucosylate monolignols in muro. A two-step model of lignification of conifer tracheids is proposed. First, Golgi-mediated secretion deposits monolignols into the cell wall, where they polymerize in cell corners and middle lamella. Secondly, cell lysis releases stored, vacuolar monolignol glucosides into the wall where they are deglucosylated and their polymerization is influenced by the wall environment including the lignin deposited earlier.

Changes in axial hydraulic conductivity along elongating leaf blades in relation to xylem maturation in tall fescue.

Xylem maturation in elongating leaf blades of tall fescue (Festuca arundinacea) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region in which most of the longitudinal water movements occurred in the LMX. The axial hydraulic conductivity (Kh ) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conductivity (Kt ) computed from the diameter of individual inner vessels. Kt was proportional to Kh throughout the leaf, but Kt was about three times Kh . The changes in Kh and Kt along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, Kh was about 0.30±0.07 mmol s-1 mm MPa-1 . Beyond that region, Kh rapidly increased with metaxylem element maturation to a maximum value of 5.0±0.3 mmol s-1 mm MPa-1 , 105 mm from the leaf base. It then decreased to 3.5±0.2 mmol s-1 mm MPa-1 near the leaf tip. The basal expanding region was observed to restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.

Molecular characterisation of a novel plant homeobox gene expressed in the maturing xylem zone of Populus tremula x tremuloides.

In trees, wood is produced from the vascular cambium. However, little is known about how differentiation of the cambial derivatives is controlled at the molecular level. Hybrid aspen, Populus tremula x tremuloides, is a fast-growing, wood-forming plant from which an efficient transformation and regeneration protocol has been developed. We use this species as a model to study xylem cell differentiation. In a cross-section of the stem, cells in the cambial region at different stages of development can be seen in a radial distribution. We dissected out the cambial region and constructed a cDNA-library from this tissue. The library was screened with a degenerated primer mix complementary to the third helix region of the homeobox. A number of cDNAs hybridising to the probe were isolated, purified, cloned and sequenced. Characterisation of the expression pattern of two such homeobox cDNAs revealed that one of the cDNAs, denoted PttHB1, was specifically expressed in the xylem maturation zone, indicating a role in the regulation of secondary wall formation, while the other, denoted PttHB2 was expressed in differentiating xylem and phloem cells. Structurally, the two hybrid aspen homeobox genes are homologous to each other, but no close relationship could be identified between these genes and any other homeobox sequence described thus far, in plants or other eukaryotes. We therefore conclude that the PttHB1 and PttHB2 genes belong to a novel hitherto undescribed sequence class of homeobox genes. We designate this as PALE.

Water uptake and structural plasticity along roots of a desert succulent during prolonged drought

Desert succulents resume substantial water uptake within 1–2 d of the cessation of drought, but the changes in root structure and hydraulic conductivity underlying such recovery are largely unknown. In the monocotyledonous leaf succulent Agave deserti Engelm. substantial root mortality occurred only for lateral roots near the soil surface; nearly all main roots were alive at 180 d of drought. New main roots were initiated and grew up to 320 mm at soil water potentials lower than – 5·0 MPa, utilizing water from the shoot. The hydraulic conductivity of distal root regions decreased 62% by 45 d of drought and 70% thereafter. After 7 d of rewetting, root hydraulic conductivity was restored following 45 d of drought but not after 90 and 180 d. The production of new lateral roots and the renewed apical elongation of main roots occurred 7–11 d after rewetting following 180 d of drought. Hydraulic conductivity was higher in the distal region than at midroot and often increased again near the root base, where many endodermal cells lacked suberin lamellae. Suberization and xylem maturation were influenced by the availability of moisture, suggesting that developmental plasticity along a root allows A. deserti to capitalize on intermittent or heterogeneous supplies of water.

A laccase-type polyphenol oxidase from lignifying xylem of tobacco

Lignifying xylem from tobacco ( Nicotiana tabacum) expresses oxidase activity capable of oxidizing a range of chromogenic substrates by a non-peroxidative mechanism. These oxidases appear to be ionically bound to the cell wall and can be extracted using 1M NaCl. The extracted oxidases can oxidize the monolignol coniferyl alcohol. Extracts from xylem cell walls contain a number of different oxidase isoforms with isoelectric points in the neutral to mildly acidic range. Xylem from younger, apical tobacco stems yield a different set of oxidase isoforms than xylem from older, basal areas which suggests that oxidase isoforms may be differently expressed during xylem maturation. Non-denaturing SDS-PAGE confirms the presence of a band of oxidase activity ( M r ca 100 kDa) in these extracts that has properties similar to those of the laccase-type polyphenol oxidases ( p-diphenol: O 2 oxidoreductase; EC 1.14.18.1) previously identified in the lignifying tissues of trees. Ion-exchange chromatography on DEAE-Sepharose retained this 100 kDa laccase-like activity and resulted in a ten-fold purification and a six-fold increase in the recovery of oxidase activity, probably as the result of the removal of inhibitors. In contrast, a subsequent hydrophobic interaction chromatography step was unsuccessful, probably as a result of the precipitation of the laccase-like oxidase in the concentrated ammonium sulphate buffers required for this procedure.

Occurrence of plasmodesmata between differentiating vessels and other xylem cells inSorbus torminalis L. Crantz and their fate during xylem maturation

The development of pit-pairs between differentiating xylem cells has been examined by transmission electron microscopy in young shoots ofSorbus torminalis. In some vessel-to-tracheid pits, as well as in previously studied intertracheid pits, a thickening of the pit membrane containing branched plasmodesmata was observed. A secondary wall-like cap was deposited over the thickening prior to cytoplasmic autolysis; some plasmodesmata, parallel to the plane of section, appeared to perforate the cap. At the end of the cell maturation stage, the central part of the primary wall thickening was hydrolysed, while the cap, including plasmodesmata remnants, appeared unaltered. In half-bordered pit-pairs between a parenchyma cell and a vessel or a tracheid, similar structures could be observed beside the conducting elements. When the vessel or tracheid matured, sealing of the pit membrane plasmodesmata resulted from the formation of a protective layer on the parenchyma-side rather than from the deposition of a cap on the conducting cell-side. These observations provide the first information on the presence of symplasmic connections in pits between differentiating vessels and neighbouring xylem cells. InS. torminalis, xylem differentiation is probably highly coordinated within a symplasmic domain; the persistence of such connections may account for the lack of specialization ofSorbus wood.

Vascular Anatomy of the Subterranean Organ of Ophioglossum vulgatum L.

The vascular anatomy of subterranean rhizome and root of Ophioglossum vulgatum was studied. The vascular cylinder of the rhizome which had developed adventitiously is ectophloic siphonostele with endarch primary xylem maturation. In the basal part of the rhizome ectophloic siphonostele reveals a complete cylinder of vascular tissue, whereas an ectophloic siphonostele with overlapping gaps occurs in the upper part of the rhizome. Each meristele is arranged as collateral bundle. The bulk of primary xylary elements in the vascular bundles is radially aligned. The roots are monarch with exarch xylem maturation and the monarch vascular pattern reveals opposite arrangement of xylem and phloem. The relation between stelar and total root diameter shows a low correlation. No signs of secondary thickening are found either in the rhizomes or the monarch roots. The comparison of anatomical features of vascular system of the rhizome and root with other ferns shows significant differences.

RELLIMIA THOMSONII FROM THE GIVETIAN OF NEW YORK: SECONDARY GROWTH IN THREE ORDERS OF BRANCHING

Pyritized axes of the vegetative branching system of Rellimia thomsonii, from the Givetian of New York were described with emphasis on the presence and position of the secondary tissues in relation to the known axis orders. The morphology and anatomy of four axis orders (N, N + 1, N + 2, N + 3) was described using two well‐preserved specimens. One specimen showed the largest known (N) axis order, with spirally attached N + 1 axes. The other specimen, from a more distal portion of the plant, showed the N + 2 axis order with spirally arranged N + 3 axes. The shape of the primary xylem, discernible in three of the axis orders (N + 1, N + 2, N + 3), was a three‐lobed protostele with mesarch maturation of the primary xylem. The protostele of the N + 3 axis order, with only primary growth, was identical to that of fertile N + 3 axes. The other axis orders (N + 2, N + 1, N) had wood surrounding the primary xylem. The wood of the N+2 axis was limited, and occurred only in the basal portion of the axis, but that of the N+1 and N axes was extensively developed with two and three growth layers, respectively. This wood was pycnoxylic with narrow rays similar to other aneurophytalean progymnosperms. Surrounding the secondary xylem was secondary phloem indicating the vascular cambium in Rellimia was bifacial. The presence of growth layers was discussed in relation to the climatic interpretation placed on their presence and how it correlates with paleogeographic reconstructions of the Devonian.

Diversity Among Paleozoic Cordaitales: The Vascular Architecture of Mesoxylon birame Baxter

The cauline vascular architecture of Mesoxylon birame is described from stems collected at the type locality. The type specimen and new specimens differ in several features from M. birame stems from other localities. In stems from the type locality, leaf traces are mesarch, while all other cauline bundles are either endarch or have no recognizable protoxylem tracheids. The specimens display a sympodial architecture of the primary xylem, and there are either one or two cones or vegetative branches in the axils of many leaves. Stems also have reparatory strands that interconnect adjacent sympodia, as in some species of Calamopitys. This contrasts with stems of several other species of Mesoxylon in which there is a discontinuous (nonsympodial) cauline vascular architecture consisting entirely of leaf and branch traces. Using M. priapi as an example, we address the problem of distinguishing primary from secondary xylem. The taxonomic value of xylem maturation and cauline vascular architecture is evaluated for cordaitean stems. Previous descriptions of vascular architecture in cordaitean stems are reviewed, and differences of interpretation are analyzed. The new data indicate that cordaitean stem genera may not reflect genera of whole cordaitean plants and that cordaitean diversity may be significantly greater than currently recognized.

A Reevaluation of the Discontinuity Plate in Psoraleeae and Amorpheae (Leguminosae)

The discontinuity plate, once thought to be a unique feature of the Leguminous tribes Psoraleeae and Amorpheae, has been used as evidence for a close relationship of the two tribes. It is here further studied for taxonomic utility. It is shown that the discontinuity plate comprises four phenomena, distributed in four tribes. Tribe Amorpheae has a discontinuity between the pedicellar and gynoecial traces with a profusion of tracheids at the junction of the gynoecial vascular traces, forming a plate-like structure. This is present and developed at very early stages of the flower and corresponds to the discontinuity plate as originally described. Segregate genera of Psoralea as proposed by Stirton (1981) and Rydberg (1919) also have distinctive vascular features, though these are not the same as the discontinuity plate found in Amorpheae. Psoralea, Otholobium, and all New World genera of Psoraleeae have simple delayed maturation of xylem through the receptacle. Some species of Tephrosia (Tephrosieae) were found to have a similar development. Cullen and Bituminaria (Psoraleeae) have delayed maturation of xylem with an inverted cone-like structure developing in early fruit. A similar structure was found in species of Desmodium (Desmodieae). Because the discontinuity plate is shown to comprise more than one phenomenon, and since these vascular irregularities are now known in four tribes, the discontinuity plate does not necessarily indicate a close relationship between the tribes Psoraleeae and Amorpheae. Furthermore, the phenomena cannot be assumed to be homologous. More complete developmental studies and a larger survey of the Papilionoideae is needed before the vascular features can be confidently used in systematic hypotheses.

A REEVALUATION OF THE DISCONTINUITY PLATE IN PSORALEEAE AND AMORPHEAE (LEGUMINOSAE)

The discontinuity plate, once thought to be a unique feature of the Leguminous tribes Psoraleeae and Amorpheae, has been used as evidence for a close relationship of the two tribes. It is here further studied for taxonomic utility. It is shown that the discontinuity plate comprises four phenomena, distributed in four tribes. Tribe Amorpheae has a discontinuity between the pedicellar and gynoecial traces with a profusion of tracheids at the junction of the gynoecial vascular traces, forming a plate‐like structure. This is present and developed at very early stages of the flower and corresponds to the discontinuity plate as originally described. Segregate genera of Psoralea as proposed by Stirton (1981) and Rydberg (1919) also have distinctive vascular features, though these are not the same as the discontinuity plate found in Amorpheae. Psoralea, Otholobium, and all New World genera of Psoraleeae have simple delayed maturation of xylem through the receptacle. Some species of Tephrosia (Tephrosieae) were found to have a similar development. Cullen and Bituminaria (Psoraleeae) have delayed maturation of xylem with an inverted cone‐like structure developing in early fruit. A similar structure was found in species of Desmodium (Desmodieae). Because the discontinuity plate is shown to comprise more than one phenomenon, and since these vascular irregularities are now known in four tribes, the discontinuity plate does not necessarily indicate a close relationship between the tribes Psoraleeae and Amorpheae. Furthermore, the phenomena cannot be assumed to be homologous. More complete developmental studies and a larger survey of the Papilionoideae is needed before the vascular features can be confidently used in systematic hypotheses.

Vascular Connection Between Lateral Roots and Stem in the Ophioglossaceae

The vascular connection between lateral roots and stem in the Ophioglossaceae and in two leptosporangiate fern species was examined. Two types of connections were found: “gradual” connections, which resemble leaf traces in ontogeny and morphology, and “abrupt” connections, which resemble the connections between lateral roots and their parent roots. Gradual root-stem connections occur in the genera Ophioglossum and Helminthostachys and in Woodwardia virginica. They are initiated in shoot apices distal to the level where cauline xylem elements mature. They resemble leaf traces in being provascular (procambial) strands that connect the cauline stele with the future vasculature of lateral appendages. As with leaf traces, gradual connections are part of the provascular and, later, protoxylem continuity between stems and lateral appendages. Gradual connections have many features in common with leaf traces, and the term root trace is applicable to them. The order of radial maturation of the primary xylem in gradual connections varies in different parts of the connections. It is endarch near the intersection with the cauline stele and exarch where the connections intersect root steles. Gradual connections resemble the transition regions of certain seed plants where protoxylem is also continuous from stem to root and the order of maturation is found to change continuously from stem to root. Abrupt connections occur in Botrychium and Osmunda cinnamomea. They develop in shoot apices at levels where cauline xylem is mature or maturing. The mature xylem does not dedifferentiate, so provascular and protoxylem continuity of the kind found in root traces does not occur. Also, reorientation of the order of maturation does not occur in abrupt connections. Xylem connectors are found in the region where radially oriented elements of the connections abut the longitudinally oriented cauline elements. Abrupt connections resemble the connection of secondary roots with their parent root systems since xylem connectors and the lack of continuity are also features found in these vascular systems. The resemblance of the vascular pattern of the fern root trace to the transition region of seed plants suggests that the radicle is more closely comparable to the cladogenous roots of pteridophytes than hitherto supposed.

VASCULAR CONNECTION BETWEEN LATERAL ROOTS AND STEM IN THE OPHIOGLOSSACEAE

The vascular connection between lateral roots and stem in the Ophioglossaceae and in two leptosporangiate fern species was examined. Two types of connections were found: “gradual” connections, which resemble leaf traces in ontogeny and morphology, and “abrupt” connections, which resemble the connections between lateral roots and their parent roots.Gradual root‐stem connections occur in the genera Ophioglossum and Helminthostachys and in Woodwardia virginica. They are initiated in shoot apices distal to the level where cauline xylem elements mature. They resemble leaf traces in being provascular (procambial) strands that connect the cauline stele with the future vasculature of lateral appendages. As with leaf traces, gradual connections are part of the provascular and, later, protoxylem continuity between stems and lateral appendages. Gradual connections have many features in common with leaf traces, and the term root trace is applicable to them.The order of radial maturation of the primary xylem in gradual connections varies in different parts of the connections. It is endarch near the intersection with the cauline stele and exarch where the connections intersect root steles. Gradual connections resemble the transition regions of certain seed plants where protoxylem is also continuous from stem to root and the order of maturation is found to change continuously from stem to root.Abrupt connections occur in Botrychium and Osmunda cinnamomea. They develop in shoot apices at levels where cauline xylem is mature or maturing. The mature xylem does not dedifferentiate, so provascular and protoxylem continuity of the kind found in root traces does not occur. Also, reorientation of the order of maturation does not occur in abrupt connections. Xylem connectors are found in the region where radially oriented elements of the connections abut the longitudinally oriented cauline elements. Abrupt connections resemble the connection of secondary roots with their parent root systems since xylem connectors and the lack of continuity are also features found in these vascular systems.The resemblance of the vascular pattern of the fern root trace to the transition region of seed plants suggests that the radicle is more closely comparable to the cladogenous roots of pteridophytes than hitherto supposed.