Leaf Traces Research Articles

Leaf Development and Primary Vascular Organization in Shoots of Anisophyllea disticha

Anisophyllea disticha is characterized by strong shoot dimorphism. Orthotropic shoots with helically arranged scale leaves produce tiers of plagiotropic shoots, while plagiotropic shoots are anisophyllous and bear dorsal scale and ventral foliage leaves arranged in a unique tetrastichous system. In this study we compare the patterns of leaf development and primary vascular organization in the two types of shoots. Orthotropic shoots have an open vascular system with five sympodia. Expansion of orthotropic shoot scale leaves occurs from P1 to P10–12, and leaf tissues mature precociously. Plagiotropic shoots have a closed vascular system with six sympodia. Leaves in ventral and dorsal orthostichies do not differ significantly in size until ca. P15, but ventral leaves are distinct histologically from the second node in an orthostichy, P4–6. Ventral foliage leaves have a diffuse plate meristem, and leaf expansion continues until ca. P30. Differentiation of ventral and dorsal leaf trace procambium parallels the divergent patterns of leaf expansion. These observations demonstrate the strong correlation among shoot symmetry, leaf development, and vascular differentiation within dimorphic shoots of one species.

A Petrified Lycopod from the Uppermost Devonian of Hook Head, County Wexford, Ireland

A collection of pyritic permineralized axes of a new lycopod were made at Hook Head, County Wexford, Ireland, during the summers of 1982 and 1983. The fossil-bearing rocks are Latest Devonian (= Tournaisian 1a). The holotype of Wexfordia hookense gen. et sp. nov. consists of a forked axis of only primary tissues. The vascular tissue is organized into a medullated stele. The center of the axis is parenchymatous and surrounded by a smooth-surfaced cylinder of primary xylem. The tracheids are scalariform with vertical connections between scalariform bars. Protoxylem is peripheral, appearing at times as minute projections of the stelar surface. Presumed primary phloem occurs in a narrow band, 2-4 cells wide, surrounding the xylem. Phloem cells are elongate and thin-walled, and may have a beaded pattern on the walls in longitudinal section. The cortex is two-zoned: an inner cortex of polyhedral parenchymatous cells and an outer cortex of parenchyma cells that are isodiametrical in transverse section and elongate in longitudinal section. Leaf traces originate as terete strands of protoxylem and are associated, immediately, with an abaxial parichnos (lacuna). The parichnos extends into the leaf, becoming a central canal. Leaf bases are circular to oval and persist. Leaves, up to 3.6 mm wide and at least 4 mm long, consist of a thickened midrib with a canal and vascular strand and thinner wings on either side. Leaves are crowded, overlap, and are slightly deflexed at their base, then curve upward to become appressed. The new plant reveals a combination of characters intermediate between the Devonian Protolepidodendrales and the Upper Carboniferous Lepidodendrales.

LEAF DEVELOPMENT AND PRIMARY VASCULAR ORGANIZATION IN SHOOTS OF ANISOPHYLLEA DISTICH A

Anisophyllea disticha is characterized by strong shoot dimorphism. Orthotropic shoots with helically arranged scale leaves produce tiers of plagiotropic shoots, while plagiotropic shoots are anisophyllous and bear dorsal scale and ventral foliage leaves arranged in a unique tetrastichous system. In this study we compare the patterns of leaf development and primary vascular organization in the two types of shoots. Orthotropic shoots have an open vascular system with five sympodia. Expansion of orthotropic shoot scale leaves occurs from P1 to P10–12, and leaf tissues mature precociously. Plagiotropic shoots have a closed vascular system with six sympodia. Leaves in ventral and dorsal orthostichies do not differ significantly in size until ca. P15, but ventral leaves are distinct histologically from the second node in an orthostichy, P4–6. Ventral foliage leaves have a diffuse plate meristem, and leaf expansion continues until ca. P30. Differentiation of ventral and dorsal leaf trace procambium parallels the divergent patterns of leaf expansion. These observations demonstrate the strong correlation among shoot symmetry, leaf development, and vascular differentiation within dimorphic shoots of one species.

Primary Vascular Differentiation in Achyranthes aspera L.

Procambial system of the embryo and the primary vascular differentiation in Achyranthes aspera L. have been studied. Two procambial strands organised near the radicle tip elongate tangentially and bifurcate higher up in the hypocotyl, the daughter strands from opposite side meeting to form two compound strands which give out a minute branch for the epicotyl before going to the cotyledons. During germination the four primary collateral strands ascending the hypocotyl divide and fuse to form first eight then six and ultimately ten strands. Eight of these fuse in opposite groups of four each, to form omega shaped masses which resolve in to the cotyledonary or foliar medians, foliar laterals, three or more peripheral strands and a medullary bundles. The leaf traces departing on the nodes they are formed on, all internodal strands are cauline. The medullary bundles are peripheral in origin. There being no laterales in the cotyledons, the cotyledonary node is unilacunar double trace type, a primitive feature. Diagrams of trace relationship and reconstruction of the primary vasculature have been prepared.

Development of Common Milkweed (Asclepias syriaca) Root Buds Following Emergence from Lateral Roots

In common milkweed, the development of subterranean root buds on excised root segments, following emergence from the parent root, is characterized by development of nodes and internodes followed by internode expansion. Transverse sections of root buds reveal that bicollateral vascular bundles as well as leaf traces and gaps are well developed in buds from 3-month-old plants. Strands of xylem and phloem connect the parent root and root bud in both inhibited and noninhibited root buds. Pitted primary tracheary elements, characteristic of developmentally advanced primary xylem, are present in these traces. The occurrence of a well-developed vascular system throughout the root bud and between the parent root and bud provides evidence that retardation of growth of inhibited root buds in common milkweed is not caused by anatomical constraints.

VASCULAR ARCHITECTURE IN ISOPHYLLOUS AND FACULTATIVELY ANISOPHYLLOUS SPECIES OF PENTADENIA (GESNERIACEAE)

The organization and differentiation of primary vascular tissue in isophyllous shoots of Pentadenia crassicaulis and facultatively anisophyllous shoots of P. orientandina (Gesneriaceae) were compared using serial reconstructions and quantitative methods. Despite clear differences in shoot symmetry, both species are vascularized by four sympodia, with trilacunar, split‐lateral nodal anatomy. Leaf trace tracheary element number and diameter reflect leaf size differences in P. orientandina: these parameters are significantly greater in the large ventral leaves than in the small dorsal leaves. The median and lateral traces of ventral leaves of this species have a similar number of tracheary elements of equal diameter, while there are significantly more tracheary elements in the median than lateral traces of dorsal leaves. The pattern seen in P. crassicaulis is similar to that seen in the dorsal leaves of P. orientandina. In both species, protoxylem development anticipates differences in mature shoot vasculature. Changes in tracheary element number during ontogeny precede or are approximately coincident with changes in leaf size. These results suggest that the facultative expression of leaf size differences in P. orientandina is associated with opportunistic development and differentiation of the lateral trace.

SEQUENCE OF XYLEM DIFFERENTIATION IN LEAVES OF CUPRESSACEAE

Protoxylem differentiates discontinuously within the leaf trace in members of the Cupressaceae sensu stricto, the first tracheid appearing at the apex of the procambial trace more or less concurrently with the differentiation of the first transfusion tracheids. Differentiation of both tissues then proceeds in a basipetal sequence within the lamina, although much more rapidly in the protoxylem than in the adjacent transfusion tissue. Protoxylem also develops acropetally from the proximal end of the procambial trace within the axis and into the decurrent leaf base. The ascending and descending protoxylem strands finally meet opposite or slightly below the resin canal near the point of attachment of the leaf. This distinctive pattern of development has been observed in a range of taxa and leaf types within the family. The taxonomic and adaptive significance of this character is discussed.

MORPHOLOGY AND VASCULATURE OF THE LEAF OF POTATO (SOLANUM TUBEROSUM)

The leaf and stem of the potato plant (Solanum tuberosum L. cv. Russet Burbank) were studied by light microscopy to determine their morphology and vasculature; scanning electron microscopy provided supplemental information on the leaf's morphology. The morphology of the basal leaves of the potato shoot is quite variable, ranging from simple to pinnately compound. The upper leaves of the shoot are more uniform, being odd pinnate with three major pairs of lateral leaflets and a number of folioles. The primary vascular system of the stem is comprised of six bundles, three large and three small ones. The three large bundles form a highly interconnected system through a repeated series of branchings and arch‐producing mergers. Two of the three large bundles give rise to short, lateral leaf traces at each node. Each of the small bundles in the stem is actually a median leaf trace which extends three internodes before diverging into a leaf. The three leaf traces enter the petiole through a single gap; thus the nodel anatomy is three‐trace unilacunar. Upon entering the petiole, each of the laterals splits into an upper and a lower lateral. Whereas the upper laterals diverge entirely into the first pair of leaflets, the lower laterals feed all of the lateral leaflets through a series of bifurcations. Prior to their entering the terminal leaflet, the lower laterals converge on the median bundle to form a single vascular crescent which progresses acropetally into the terminal leaflet as the midvein, or primary vein. In the midrib, portions of the midvein diverge outward and continue as secondaries to the margin on either side of the lamina. Near the tip of the terminal leaflet, the midvein consists of a single vascular bundle which is a continuation of the median bundle. Six to seven orders of veins occur in the terminal leaflet.

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.

Development of Procambium, Xylem, and Phloem in the Shoot Apex of Selaginella

A quantitative study of vascular development in shoot apices of Selaginella rupestris was undertaken, using sample sizes large enough to justify statistical treatment of the data, and with special interest in comparing results from this microphyllous species with results from megaphyllous angiosperms. Selaginella rupestris developed microphyll primordia that initially contained no procambium. When procambium formed in the leaf trace, it was at an isolated locus, as was the later-forming protoxylem and protophloem in the trace. In the stele the first xylem always differentiated closer to the apex than did the phloem; its average distance from the apex of 302 μm in main shoots was highly significant from the 807 μm of the protophloem. The air spaces, which result from the unusual endodermis development in Selaginella, always differentiated even closer to the apex than did the xylem, averaging 101 μm above the protoxylem. A qualitative survey of 15 other species showed a similar pattern in the stelar tissues.

Monograph of the Neotropical Fern Genus Polybotrya (Dryopteridaceae)

The genus Polybotrya (Dryopteridaceae) includes 35 species of neotropical ferns. It is distinguished by 1) strongly dimorphic leaves with fertile leaves that resemble skeletons of the sterile, photosynthetic ones; 2) usually high-climbing stems that are covered with scales; and 3) a unique stem anatomy with 5 to 12 circularly arranged meristeles, each surrounded by a black sclerenchymatous sheath, with numerous tiny leaf traces arching between adjacent meristeles. The center of diversity of the genus is the Andes, where 23 species occur, 12 of which are endemic. The coastal mountains of southeastern Brazil are notable because they contain 5 species, all endemic. The range of Polybotrya is from Chiapas, Mexico, southward through Central America; the West Indies; northern South America southward along the Andes to Bolivia and Paraguay and eastward to the Guiana Highlands; the Amazon River basin and the Matto Grosso; and southeastern Brazil. Species of the genus typically inhabit wet, shaded, primary tropical forests from sea level to 2500 m, most often occurring at middle altitudes between 500 and 2000 m. Polybotrya is divided into three subgenera: 1) Soromanes, leaves simply pinnate and veins anastomosing; 2) Sorhifolia, leaves simply or twice pinnate and veins free, close, and parallel; and 3) Polybotrya, leaves decompound and veins free. Polybotrya cervina, a species usually included in Polybotrya, is removed to the monotypic genus Olfersia (Moran 1986). Polybotrya is related to dryopteroid genera such as Arachniodes, Cyclodium, Maxonia. Olfersia, and Polystichopsis. Carl Christensen, the father of modem fern taxonomy, observed (1916) that Polybotrya may have arisen from Maxonia because both have high-climbing stems and strongly dimorphic leaves. The morphological and anatomical evidence presented here suggests that Polybotrya may have evolved instead from a Cyclodium-like ancestor.
 I chose Polybotrya for study because two aspects of the genus immediately intrigued me: its strongly differentiated sterile and fertile leaves and its long, creeping hemiepiphytic stem (Fig. 1). Since these features evolved separately in unrelated fern genera, studying Polybotrya might well provide insight into broader questions of fern evolution. Polybotrya was suited to monographic study because the number of species (35), all of which are neotropical, was manageable. Finally, no previous monographic work had been done on Polybotrya and many problems of nomenclature and identification remained to be solved.

Variations in Petiolar Structure of Hypodematium crenatum

The importance of leaf traces in fern taxonomy has been discussed in detail by Tansley (1907, 1908), Sinnott (1911), Davie (1918), Ogura (1972), and others. Ching (1936) differentiated between Thelypteris and Dryopteris on the basis of petiolar characters. Bower (1914) used these characters along with others in establishing the relationships between Blechnum and allied genera. Sen (1964), Keating (1968), and Lucansky (1974a, 1974b) also used petiolar characters in working with dennstaedtioid, cyatheoid, dicksonioid, and pteroid ferns as did Bir (1962) and Kato (1972, 1975) in differentiating members of Athyriaceae. Recently, Lin and DeVol (1977, 1978) have given a key, based entirely on petiolar characters, to 170 species of Taiwan ferns and tried to establish relationships between various families of extant ferns. The above literature and many other references show that petiolar characters, particularly vascular pattern, are relatively constant within a species. In order to test constancy in Indian ferns, we have tried to study materials of the same taxon collected from diverse localities (see Khare, 1984; Khare & Shanker, 1984, 1986). In the process we noted that Hypodematium crenatum (Forssk.) Kuhn (Aspidiaceae of Copeland, 1947) showed considerable variation among the petioles of plants collected from different sites. A detailed study of their petioles was then begun.

A new species of Osmundacaulis ( O. jonesii sp. nov.) from Tasmania, Australia

A new petrified osmundaceous axis, Osmundacaulis jonesii sp. nov. is described from alleged Tertiary, but possible mid-Mesozoic strata from southern Tasmania. Distinguishing characters of this fossil axis include an ectophloic dictyoxylic stele composed of up to 65 xylem strands (33 strands by Hewitson's method) surrounding a parenchymatous pith containing small clusters of slerenchymatous strands and medullary traces. The cortex has 15 leaf traces in transverse section. The protoxylem divides into four clusters before the leaf trace separates from the stem xylem. Only one or two cell layers of sclerenchyma intermittently line the leaf gap. These sclerenchyma strands coalesce into one to three fibrous strands in the concavity of the trace. They eventually fuse into one adaxial, crenateshaped strand in the trace as it departs the cortex. One to six groups of sclerenchyma occur in the wings of the stipulate petiole bases. Adventious roots arise in pairs.

Comparison of Shoot Vascular Organization in Isophyllous (Pilea cadieri) and Anisophyllous (Pellionia daveauana) Species of the Urticaceae

Primary vascular organizations in isophyllous shoots of Pilea cadieri and in anisophyllous shoots of Pellionia daveauana were compared using computer-assisted reconstruction of serial cross sections of mature stems and shoot tips. Stems of P. cadieri are characterized by a closed vascular system with six sympodia and trilacunar three-trace nodes. In contrast, shoots of P. daveauana have an intermediate system with four sympodia and trilacunar four-trace nodes on the ventral side of the stem and unilacunar one-trace nodes on the dorsal side of the stem. This highly modified vascular architecture reflects the extreme anisophylly and dorsiventral symmetry of shoots. In P. daveauana, the dorsal leaf trace differs from the ventral leaf traces in phloem cross-sectional area and in tracheary element number and diameter; in P. cadieri, all traces are similar in size. Leaf-trace procambium develops simultaneously into both leaves of a pair in P. cadieri, but in P. daveauana the median trace of the ventral leaf dif...

Role of Stem Growth in Linum usitatissimum Leaf Trace Patterns

Linum usitatissimum stem growth parameters were quantified by computer-assisted analyses of scanning electron micrographs of shoot apical meristems throughout ontogeny. There were progressive decreases in the plastochron and relative plastochron rates of radial and vertical stem growth which resulted in the generation of progressively higher orders of contact parastichy phyllotaxis throughout ontogeny. The change in the relative spacing of primordia initiation on the stem coupled with the iterative differentiation of leaf gap and interfascicular ray parenchyma associated with each leaf primordium resulted in the delimitation of progressively higher orders of leaf trace interconnections throughout ontogeny. A set of developmental rules was generated which should permit simulation of many leaf trace patterns.

ROLE OF STEM GROWTH IN LINUM USITATISSIMUM LEAF TRACE PATTERNS

Linum usitatissimum stem growth parameters were quantified by computer‐assisted analyses of scanning electron micrographs of shoot apical meristems throughout ontogeny. There were progressive decreases in the plastochron and relative plastochron rates of radial and vertical stem growth which resulted in the generation of progressively higher orders of contact parastichy phyllotaxis throughout ontogeny. The change in the relative spacing of primordia initiation on the stem coupled with the iterative differentiation of leaf gap and interfascicular ray parenchyma associated with each leaf primordium resulted in the delimitation of progressively higher orders of leaf trace interconnections throughout ontogeny. A set of developmental rules was generated which should permit simulation of many leaf trace patterns.

AUREALCAULIS CROSSII GEN. ET SP. NOV., AN ARBORESCENT, OSMUNDACEOUS TRUNK FROM THE FORT UNION FORMATION (PALEOCENE), WYOMING

Aurealcaulis crossii gen. et sp. nov., is based on permineralized trunks of an osmundaceous tree fern from the Paleocene Fort Union Formation from near Bitter Creek Station of southwestern Wyoming. This new species is characterized by centripetal (exarch) development of its xylem strands which form part of the leaf traces. Most of the leaf traces depart the stele as two segments that fuse into a single C‐shaped petiole vascular strand outside of the outer cortex. Stipular expansions of the petiole bases of this species lack sclerenchyma, and roots arise from the lateral edges of leaf traces in the inner cortex. The family Osmundaceae and subfamily Osmundoideae are slightly emended to accept genera assignable to this family and subfamily with exarch protoxylem in their steles. Foliage similar to Osmunda greenlandica (Heer) Brown, which is possibly the leaf form of A. crossii, occurred next to an axis of this species which was in growth position. This axis was anchored in a lignite suggesting that this species grew under swampy conditions. Aurealcaulis crossii is the first arborescent member of the Osmundaceae of Tertiary age and the second arborescent form in this family reported from the Northern Hemisphere.

Modifications structurales des faisceaux vasculaires primaires de la tige de Cordyline indivisa (Agavaceae). 1. Analyse du patron de conversion

In Cordyline indivisa Kunth, the primary bundles change from the collateral type (distal part) to the amphivasal type (proximal part). This conversion is related to the pattern of the longitudinal course of the bundles. Ontogenically, this reflects the two stages in the construction of a vascular bundle: the formation of the lower segment, characterized by its indefinite distal end and its subsequent junction with a leaf primordium. That part of the lower segment, which is amphivasal when fully differentiated, is axial in nature; its differentiation proceeds independently of the junction with the primordium. The collateral and intermediate stages are characteristic of the junction strand and that part of the lower segment having a reoriented differentiation. Since their presence depends on the junction with the primordium and is under the influence of the latter, we relate them to the leaf trace. According to our view of vascularization, each "developmental unit" consists of an axial segment which extends in a leaf trace; new axial bundles may be emitted from one part or the other. Consequently, our definition of the leaf trace is opposed to the classical one which states that the leaf traces diverge at intervals from continuous axial bundles.

Galtiera bostonensis, gen. et sp. nov., a protostelic calamopityacean from the New Albany shale of Kentucky

Two parts of a single, anatomically preserved stem fragment form the basis of a new Lower Mississippian taxon that is characterized here in detail. Stelar morphology, leaf trace departure, secondary xylem, outer sparganum cortex, and Kalymma-like leaf bases provide evidence of taxonomic relationship with the Calamopityaceae. The primary vascular column is deeply three ribbed with protoxylem strands restricted to the periphery of the xylem. Trace departure is associated with a dextral shift in the position of primary xylem ribs such that the tip of each rib describes a helical course through the axis. The result is a Fibonacci phyllotactic fraction of at least 2/5. On the basis of our observations, we hypothesize that the protoxylem strands may comprise five sympodia and, further, that although protostelic in gross morphology, the primary vascular system of Galtiera might actually be more similar to typical eusteles of the Calamopityaceae than protosteles of any other major group. Sclerotic clusters and strands in the cortex are of interest because they are unusually numerous and are usually associated with the phloem either of the main stem vascular column or of leaf traces. The evidence is consistent with the hypothesis that this morphology reflects a pathological response by the plant to viral or other infectious agents.

Empirical models of stem growth and vasculature differentiation processes

A new notation for leaf trace patterns was developed which is consistent with contemporary contact paratichy phyllotaxis notation. New computer-aided methods for generating accurate stem tissue maps were developed. Study of the tissue maps for the various leaf trace patterns exhibited by Linum stems through ontogeny generated a set of observations which permits more rigorous definition of the developmental rules for vascular pattern formation. Changes in Linum stem growth through ontogeny was defined by empirical models generated by computer-aided analysis of scanning electron micrographs. Linum stems undergo progressive changes from low order to higher order patterns of leaf trace interconnections and phyllotaxis through ontogeny. Positional relationships between interconnected leaf traces is found predictable using long-known geometric principles of phyllotaxis.