ONTOGENY OF THE OPEN DICHOTOMOUS VENATION IN THE PINNA OF THE FERN NEPHROLEPIS

Pray , Thomas r . (U. South. California, Los Angeles.) Ontogeny of the open dichotomous venation in the pinna of the fern Nephrolepis. Amer. Jour. Bot. 47(5) : 319—328. Illus. 1960.–The venation of the pinna of Nephrolepis consists of a midvein and 2 lateral series of dichotomizing veins all of which terminate freely near the margins. The development of the pinna is analyzed with particular attention to the nature of the marginal meristem and the organization of the embryonic pinna as it appears in paradermal section. The arrangement of cells in pinna wings during the period of marginal growth displays a pattern which foreshadows the pattern of the mature venation. In contrast with the development of the leaves of angiosperms, marginal growth continues into a relatively late phase of pinna ontogeny and apparently is active throughout the phases of ontogeny concerned with blocking-out the pathways of procambial differentiation. Thus the pattern of venation appears to be correlated with the manner of activity of the marginal meristem and subsequent orientation of its derivatives. The theoretical aspects of the result of this investigation are discussed in relation to other studies of foliar venation ontogeny.     

QUANTITATIVE STUDIES OF PINNULE DEVELOPMENT IN THE FERNS ADIANTUM RADDIANUM AND CHEILANTHES VIRIDIS

Pinnule development was investigated in two fern species, Adiantum raddianum Presl cv. Decorum and Cheilanthes viridis (Forsk.) Swartz, by using clearings to facilitate the recording of mitotic divisions. Both species were found to possess a marginal meristem. This meristem consists of both a marginal row of large initials and a submarginal meristematic zone. The marginal meristem in these ferns is responsible for establishing the layers of the lamina, providing new cells which by enlargement will expand the pinnule, establishing general pinnule form, initiating the procambial stands, and forming the false indusia. The cells of the submarginal meristem were found to divide parallel to the pinnule margin more frequently if they were to become ground tissue, while dividing perpendicular to the margin more frequently if they were to become procambial. Details of vein dichotomies were also studied. Perimeter expansion was found to be associated with dichotomy of the veins, and venation pattern was found to be correlated with leaf form. The marginal meristem is active from the time of pinnule initiation until the pinnule reaches about 50% of its final length or width. Leaf development in leptosporangiate ferns resembles the traditional concept of development in angiosperms somewhat more than it does the more recent concepts. It is clear, though, that there is not a high degree of convergence in the marginal growth of fern and angiosperm leaves.     

Developmental anatomy of foliage leaves,bracts, calyx and corolla inPharbitis nil

The structure and ontogeny of the foliage leaves, bracts, bracteoles, calyx and corolla ofPharbitis nil were investigated, with special reference to the development of the lamina and the procambium. Reproductive organs used are those of a terminal inflorescence and axillary flowers induced by a single 16 hr dark period given to the seedling. The foliage leaf consists of the petiole and the broad lamina. Bracts show various forms and structures, which fluctuate from a lower leafy bract to an upper scaly one in a terminal inflorescence. The sepal is scaly. The corolla is funnel-shaped, and composed of five wedge-shaped petals. In the lamina of the foliage leaf primordium, marginal growth is followed by active growth by the plate meristem, and procambial strands of lateral veins differentiate from the residual meristem. The primordium of the lowest bract of the terminal inflorescence has already been initiated before the dark period, and develops into the bract, the residual meristem disappearing after the treatment. The leafy bract shows marginal growth and growth by the plate meristem similar to that of the foliage leaf, but of short duration. The activity of marginal growth of the scaly bract and the sepal decreases rapidly and procambial strands of lateral veins differentiate acropetally from highly vacuolated cells. The activity of marginal growth of the petal decreases gradually, and derivatives of the marginal meristem divide as a plate meristem. The corolla tube is initiated by co-operation of interprimordial growth and marginal growth of petal primordia.     

ONTOGENY OF FOLIAR VENATION IN EUPHORBIA FORBESII

Six species of Euphorbia endemic to the Hawaiian Islands have disjunct veins as a normal component of their foliar anatomy. An ontogenic study of the foliar venation of one of these species, E. forbesii, showed a normal development of the foliar procambium as determined by previous studies of dicotyledonous leaves. The disjunct veinlets are isolated early in the histogenesis of the intersecondary veins when certain procambial cells fail to differentiate into vascular tissue. It appears that these cells develop into normal parenchymatous cells of the ground tissue. It is suggested that these cells are physiologically distinct from the rest of the procambial cells. In no instance was a tracheary element seen which appeared to have arisen independently of the normal procambial reticulum.     

MORPHOLOGY AND DICHOTOMOUS VASCULATURE OF THE LEAF OF KINGDONIA UNIFLORA

Foster , Adriance S. (U. California, Berkeley), and Howard J. Arnott . Morphology and dichotomous vasculature of the leaf of Kingdonia uniflora. Amer. Jour. Bot. 47 (8): 684–698. Illus. 1960.—An intensive study of the nodal anatomy, petiolar vasculature and open dichotomous venation of the leaf of Kingdonia has revealed a type of foliar vascular system of unusual morphological and phylogenetic interest. The vascular supply at the nodal level consists of 4 collateral traces which diverge from a single gap into the sheathing leaf base. This type of nodal anatomy is perhaps primitive, and comparisons are made with the unilacunar nodes and the 2- and 4-parted leaf trace systems characteristic of many angiospermous cotyledons and the foliage leaves of certain woody ranalian genera. The petiole of Kingdonia is vascularized by 2 pairs of bundles which represent the upward continuation of the 4 leaf traces. A transition from an even (4) to an odd (3) number of strands occurs near the point of attachment of the 5, lobed, cuneiform lamina segments to the petiole. Each of the 2 abaxial bundles dichotomizes and the central derivative branches fuse to form a double bundle which enters the base of the median lamina segment. The 2 adaxial petiolar bundles diverge right and left into the bases of the paired lateral segments of the lamina. An analogous type of transition from an even to an odd number of veins occurs in many angiospermous cotyledons which develop a definable mid-vein. But, in Kingdonia, the bundles which enter the bases of the lamina segments give rise to systems of dichotomizing veinlets devoid of “mid-veins.” Although the majority of the terminal veinlets enter the marginal teeth of the lamina segments, “blind” endings, unrelated to the dentations, occur in all the leaves studied. Typically, all of the vein endings in a given lobule of a lamina segment are derived from the same dichotomous vein system. However, in some leaves, a veinlet dichotomizes directly below a sinus and the branches diverge into the marginal regions of 2 separate lobules. The phylogenetic significance of the occurrence of open dichotomous venation in such an herbaceous angiosperm as Kingdonia is briefly discussed. From a purely morphological viewpoint, the Kingdonia type of venation invites direct comparison with the venation of Sphenophyllum, certain ferns or Ginkgo rather than with any of the known reticulate venation patterns of modern angiosperms. Although the foliar venation of Kingdonia may represent the result of evolutionary reversion, the very rare anastomoses which occur seem primitive in type rather than “vestiges” of a former system of closed venation.     

ONTOGENY OF MAJOR VEINS IN THE LAMINA OF POPULUS DELTOIDES BARTR

The ontogeny of the major venation in the lamina of Populus deltoides Bartr. leaves was investigated in relation to the development of original procambial bundles, subsidiary bundles, and their derivatives. Serial sections and clearings were used to show that the midrib region is a composite structure consisting of several independent vascular bundles, each of which eventually diverges into the lamina to become a secondary vein. The sequence of events in the ontogeny of major secondary veins is: (1) an original procambial strand develops acropetally and becomes the precursor of the first vascular bundle of the midrib region of the lamina, (2) ground tissue at the forefront of acropetally developing subsidiary procambial bundles differentiates in a wavelike continuum; meristematic regions precede the acropetally developing procambial bundles, (3) discrete subsidiary bundles differentiate in the meristematic regions as they advance acropetally, (4) subsidiary bundles diverge obliquely in the lamina margin giving rise to the secondary veins in a basipetal fashion, and (5) subsequent differentiation and maturation of the secondary veins occurs within the lamina. The original procambial bundles and first-formed subsidiary bundles become the secondary veins of the uppermost portions of the lamina, the next-formed subsidiary bundles become the secondary veins of the middle portions of the lamina, and the last-formed subsidiary bundles become the secondary veins of the lowermost portion of the lamina.     

TENDRILS OF PASSIFLORA FOETIDA: HISTOGENESIS AND MORPHOLOGY

Passiflora foetida bears an unbranched tendril, one or two laterally situated flowers, and one accessory vegetative bud in the axil of each leaf. The vegetative shoot apex has a single-layered tunica and an inner corpus. The degree of stratification in the peripheral meristem, the discreteness of the central meristem, and its centric and acentric position in the shoot apex are important plastochronic features. The procambium of the lateral leaf trace is close to the site of stipule initiation. The main axillary bud differentiates at the second node below the shoot apex. Adaxial to the bud 1–3 layers of cells form a shell-zone delimiting the bud meristem from the surrounding cells. A group of cells of the bud meristem adjacent to the axis later differentiates as an accessory bud. A second accessory bud also develops from the main bud opposite the previous one. A bud complex then consists of two laterally placed accessory bud primordia and a centrally-situated tendril bud primordium. The two accessory bud primordia differentiate into floral branches. During this development the initiation of a third vegetative accessory bud occurs on the axis just above the insertion of the tendril. This accessory bud develops into a vegetative branch and does not arise from the tissue of the tendril and adjacent two floral buds. The trace of the tendril bud consists of two procambial strands. There is a single strand for the floral branch trace. The tendril primordium grows by marked meristematic activity of its apical region and general intercalary growth.     

Stage-specific markers define early steps of procambium development in Arabidopsis leaves and correlate termination of vein formation with mesophyll differentiation

During leaf development, ground meristem cells along continuous lines undergo coordinated oriented cell divisions and differentiate to form procambial cells, the precursors of all vascular cells. The molecular genetic dissection of early procambial development suffers from the lack of easily identifiable markers, especially of cell states preceding procambium formation. In this study, we have identified and characterized three reporter gene expression markers that reflect three distinct preprocambial stages, as well as one marker whose expression seems to be perfectly congruent with the appearance of procambial cells. All four markers are invariably expressed in continuous domains connected to pre-existing vasculature and their expression profiles reveal a common spatiotemporal pattern of early vein formation. We observed progressive extension of vascular strands at the preprocambial stage, suggesting that veins are initiated as freely ending preprocambial domains and that network formation occurs through subsequent fusion of these domains. Consistent with this interpretation, we demonstrate that veins are generally not programmed to become freely ending or interconnected network elements. Instead, we found that the progressive extension of preprocambial domains can be interrupted experimentally and that this leads to less complex vein patterns consisting of fewer vein orders, in which even lower-order veins become freely ending. Mesophyll differentiation turned out to be strictly correlated with the termination of preprocambial domain extension. These findings suggest that Arabidopsis vein pattern is not inherently determinate, but arises through reiterative initiation of new preprocambial branches until this process becomes terminated by the differentiation of mesophyll.     

Auxin transport-dependent,stage-specific dynamics of leaf vein formation

For centuries, the formation of vein patterns in the leaf has intrigued biologists, mathematicians and philosophers. In leaf development, files of vein-forming procambial cells emerge from seemingly homogeneous subepidermal tissue through the selection of anatomically inconspicuous preprocambial cells. Although the molecular details underlying the orderly differentiation of veins in the leaf remain elusive, gradually restricted transport paths of the plant hormone auxin have long been implicated in defining sites of vein formation. Several recent advances now appear to converge on a more precise definition of the role of auxin flow at different stages of vascular development. The picture that emerges is that of vein formation as a self-organizing, reiterative, auxin transport-dependent process.Key words: arabidopsis, leaf development, polar auxin transport, procambium, vascular patterningThe vascular system of plants is a branching array of cell files extending through all organs.¹ In dicot leaves, these vascular strands, or ‘veins’, are arranged in a ramified pattern that largely reflects the shape of the leaf (Fig. 1A).^2,3 ‘Lateral veins’ branch from a conspicuous central vein (‘midvein’) that is continuous with the stem vasculature. In many species, lateral veins extend along the leaf edge to form ‘marginal veins’, which connect to adjacent lateral veins to form prominent closed loops. Finally, a series of ‘higher-order veins’ branch from midvein and loops and can either terminate in the lamina (‘free-ending veins’) or join two veins (‘connected veins’).Open in a separate windowFigure 1Conceptual summary of dicot leaf vein formation. (A) Schematics of a simplified mature leaf illustrating midvein (M), first, second and third loops (L1, L2 and L3, respectively)—each derived from corresponding lateral (LV) and marginal (MV) veins—free-ending (FV) and connected (CV) higher-order veins, hydathodes (H) and middle-to-margin positions (decreasing green gradient) as used in the text. (B) State transitions in leaf subepidermal cell differentiation. Available evidence suggests that the vein patterning process is limited to ground meristem cells (white), while subepidermal cells that have begun to acquire mesophyll characteristics are incapable of responding to vein-inducing signals.^11,13,19,38 Expression of preprocambial (blue) and mesophyll emergence markers seem to identify two mutually exclusive and typically irreversible cell states, one leading to procambium (pink) and the other to mature mesophyll (green) formation. The transition from ground meristem to differentiated mesophyll could conceivably occur through a cell state that is formally equivalent to the preprocambial state in vascular differentiation. However, the existence of such a ‘premesophyll’ state (faded gray), the extent of its stability, its mutual exclusivity or competition with the preprocambial state and its responsiveness to vein-inducing signals still remain open questions. (C) Stage-specific dynamics of leaf vein patterning and their dependency on auxin levels and transport as exemplified for loop formation, but in general equally applicable to all veins. Upper series: PIN1-labeled auxin transport paths corresponding to preprocambial cell selection zones (yellow). Note how loops are composed of a lateral PIN1 expression domain (LD) and an initially free-ending marginal PIN1 expression domain (MD). Further, note slightly expanded PIN1 expression domains in a fraction of hydathode-associated third loops during normal development, broad PIN1 domains on the side of local auxin application (arrowhead) and nearly ubiquitous PIN1 expression upon systemic auxin transport inhibition. Middle series: directions of Athb8/J1721-marked preprocambial strand formation (blue arrows). Note middle-to-margin progression of preprocambial strand formation during normal loop development. Further, note margin-to-middle preprocambial strand extension in a fraction of third loops during normal development and in all loops forming on the side of auxin application. Finally, note co-existence of middle-to-margin and margin-to-middle polarities of preprocambial strand extension during the formation of individual loops in response to auxin transport inhibition. Lower series: gradual appearance of procambial cell identity acquisition (pink to magenta). Note simultaneous differentiation of lateral and marginal procambial strands in normal loop development. Further, note successive formation of lateral and marginal procambial strands in a fraction of third loops during normal development and in all loops formed on the side of auxin application and under conditions of reduced auxin transport. Arrows temporally connect successive stages of vein formation. See text for additional details.Vascular cells mature from procambial cells: narrow, cytoplasmdense cells, characteristically arranged in continuous strands.⁴ Leaf procambial strands differentiate from files of isodiametric preprocambial cells, which are selected from the anatomically homogeneous subepidermal tissue of the leaf primordium, the ground meristem (Fig. 1B).^5,6 The mechanism by which ground meristem cells are specified to procambial cell fate is unknown, but an instrumental role for auxin transport and resulting auxin distribution patterns in this process has increasingly gained support.^7–13 This brief essay summarizes a recent group of articles that emphasizes the importance of auxin transport in leaf vein formation.     

ORIGIN AND EARLY DEVELOPMENT OF NONARTICULATED LATICIFERS IN EMBRYOS OF EUPHORBIA MARGINATA

In E. marginata 12 nonarticulated laticifer initials arise in the cotyledonary node of the young embryo during the early heart stage. The initials arise progressively in the developing embryo, the first laticifers differentiating simultaneously with or shortly before the elements of the pro-cambium. The laticifers occupy a position lateral to the six procambial strands which are formed in the embryo. Upon subsequent growth each laticifer becomes vacuolated and nuclear division unaccompanied by cytokinesis results in the formation of a coenocytic protoplast. The enlarging laticifer produces several branches, one growing into the cotyledon, another growing down along the hypocotyl penetrating toward the root meristem, and one or several growing along intercellular spaces of adjacent cells. No fusion of these branches with one another or adjoining parenchyma cells was observed.     

INITIATION OF THE VASCULAR SYSTEM IN THE FERTILE FLORET OF ANTHOXANTHUM ODORATUM (GRAMINEAE)

Procambium was initially isolated near the insertions of lemma and stamen primordia in the grass Anthoxanthum. The palea was initiated before its procambium. The acropetal, continuous differentiation of procambium involved in the siting of leaves on shoots of many other megaphyllous plants, does not occur in the rachilla of this grass. A portion of the vascular system of the fertile floret of Anthoxanthum became connected with the vascular system of the rest of the spikelet by basipetal differentiation of the procambial trace of the fertile lemma. A core of residual meristem persisted in the fertile floret above the procambial trace to the fertile lemma. Vascular continuity between the procambial trace to the fertile lemma and the procambial traces of the stamens was achieved by the differentiation of procambium from this core of residual meristem.     

STUDIES OF SPHENOPHYLLUM SHOOTS: SPECIES DELIMITATION WITHIN THE TAXON SPHENOPHYLLUM

Structurally preserved ultimate vegetative shoots and attached foliage of Sphenophyllum multirame and two additional taxa from Upper and Middle Pennsylvanian coal balls are described. Shoot axes of all taxa are delimited by small cube-shaped epidermal cells and contain three isolated strands of protoxylem tracheids separated by undifferentiated procambial cells. Metaxylem maturation is delayed for some distance below the apical meristem. Branches originate from a single protoxylem strand and are contiguous with the subjacent leaf trace for a short distance. Sphenophyllum multirame is considered a valid taxon. It is suggested that shoot and foliar anatomical characteristics may form valid criteria for species delimitation within the genus, and S. reedae is delimited by such characteristics. An additional form is tentatively suggested as a possible new species. Anatomical similarities between 5. reedae and Peltastrobus reedae suggest that these taxa represent vegetative and reproductive structures of the same plant.     

ACTIVITY OF MARGINAL AND PLATE MERISTEMS DURING LEAF DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

The mitotic and biosynthetic activities of the marginal and plate meristems were studied during the entire course of leaf development of Xanthium pennsylvanicum. In contrast to statements in the literature, marginal meristem activity is long in duration, as assayed by the mitotic counts and H³-thymidine incorporation. This me istem is active 23 days. The plate meristem is active for an additional 3 days after cessation of cell division in the marginal meristem, but the total duration of its mitotic activity is also approximately 23 days. Numerous periclinal cell divisions of the plate meristem form additional cell layers and contribute to the growth of the lamina in thickness. Incorporation of H³-thymidine increased during the course of leaf development. Cells between plastochronic ages 0 and 2.0 incorporated more of the radioisotopic precursor than those of younger leaf primordia. The uptake and incorporation of H³-thymidine into nuclear DNA was more sluggish during the early stages of development than in the more expanded leaves. No DNA synthesis was demonstrated after cessation of cell division in the leaf lamina. Metabolic or endomitotic DNA synthesis after leaf plastochron index (LPI) 3.0 seems improbable. No significant differences in the incorporation of H³-thymidine could be demonstrated between the marginal and plate meristems. This would indicate no distinct biosynthetic differences between the two meristems. The definitions of the marginal and plate meristems of Xanthium leaves were formulated in view of the above findings.     

STRUCTURE AND DEVELOPMENT OF THE PERIANTH IN DOWNINGIA BACIGALUPII

The structure and ontogeny of the calyx and corolla of Downingia bacigalupii Weiler (Campanulaceae; Lobelioideae) were investigated for the purpose of comparing perianth development with previous observations on the floral bract, as well as elucidating the mechanism of development of the zygomorphic, sympetalous corolla. Sepals are uni-traced with a palmate, reticulate venation. They have basal and apical hydathodes, as well as storage tracheids. Sepals show a reduction in size, venation and hydathode number when compared to the bract. The pentamerous, zygomorphic corolla is bilabiate, consisting of a three-lobed adaxial lip and a two-lobed abaxial lip connected by a short tubular region. The constituent petal lobes are also uni-traced and have a reticulate venation, resembling that of the sepal and bract, but lack storage tracheids and hydathodes. Sepals arise in an adaxial to abaxial succession and are initiated in the outer corpus layer of the floral apex. Expansion of the floral apex follows and is accompanied by the establishment of a second tunica layer. Sepals undergo apical, marginal, and intercalary growth accompanied by acropetal differentiation of procambium. The petals arise simultaneously and are initiated in the second tunica layer and the outer corpus cells. After initiation, the petals exhibit a period of apical and marginal growth followed by intercalary growth. Apical growth in petals is less protracted than in sepals, but plate meristem activity is more extensive. The free petal lobes become temporarily fused by an interlocking of marginal epidermal layers, but they separate at anthesis. Zonal growth beneath the originally free lobes forms the tube and lip regions of the sympetalous corolla. Zygomorphy is evident from the time of initiation of petals and is accentuated by later differential growth. Comparative observations of corolla ontogeny in autogamous species of Doumingia indicate that the reduced corollas in these taxa are derived by a simple process of neoteny.     

MITOTIC ACTIVITY IN THE ROOT APEX OF THE WATER FERN MARSILEA VESTITA HOOK. AND GREV.

Roots of Marsilea vestita ranging from 1–120 mm in length, as well as root primordia, were analyzed to determine mitotic activity and ploidy levels in the apical cell, five well-defined regions of the root proper, and two regions in the root cap. The mitotic index of the apical cell tended to be above the overall mean mitotic index for the entire apical meristem. No diurnal rhythm in mitotic index was apparent. The cell-cycle duration of the apical cell ranged from 12.1–25.2 hr, that of other regions of the root from 16.1–41.5 hr. There was no indication of polyploidy in any part of the apical meristem except in a few procambial cells. Thus, the results support the classical concept that the apical cell is the ultimate source of cells in the root.     

ONTOGENY OF THE STEM-NODE-LEAF VASCULAR CONTINUUM OF AUSTROBAILEYA

Developmental study of the stem-node-leaf vascular continuum of Austrobaileya scandens White reveals that the vasculature within each leaf originates from a single procambial strand, that becomes separated into two strands only at the junction of leaf and stem. At lower levels in the stem the two strands become incorporated into independent portions of the stele. At later stages of development the solitary vascular bundle within the young leaf undergoes considerable lateral growth, resulting in an essentially continuous arc of vascular tissue. Ontogenetic evidence indicates that the vascular bundle in the midrib of the lamina should be regarded as a fundamentally single bundle and not interpreted as two bundles that have undergone various degrees of secondary fusion. A condition of two totally separate bundles extending the entire length of the leaf was not encountered. Our observations confirm the characterization of Austrobaileya as an example of “second rank” level of leaf vasculature. Nodal anatomy emphasizes the extremely isolated taxonomic position of Austrobaileya within the primitive dicotyledons.     

The Morphology and Development of Cross Veins in the Leaves of Bread Wheat (Triticum aestivum L.)

In the leaves of bread wheat Triticum aestivum L. the longitudinalvascular bundles are linked by small transverse bundles Pairsof similar small vascular bundles also link the upper ends ofminor longitudinal bundles to their neighbours in a Y-shapedarrangement The cross-vein procambial strands arise from unexpanded cellsof one layer of the mesophyll tissue. Lines of these cells connectone longitudinal procambial strand to the next The procambialcells subsequently undergo two tangential divisions to producecells which differentiate to form the conducting and parenchymatouselements of the mature cross veins. Anomalous cross veins are sometimes found. possible modes oforigin of these anomalous cross veins are considered.     

STUDIES OF PALEOZOIC FERNS: THE FROND OF ANKYROPTERIS GLABRA

Eggert , Donald A. (Southern Illinois U., Carbondale.) Studies of Palerzoic ferns: The frond of Ankyropteris glabra. Amer. Jour. Bot. 50(4): 379–387. Illus. 1963—The major features of the frond of A. glabra are described on the basis of preserved parts found in Middle Pennsylvanian coal ball material from Illinois. The frond is planated and has well-developed foliar laminae. Primary pinnae arise from the petiole in 2 alternating series, and secondary pinnae arise in a similar fashion from the primary pinnae. Foliar laminae occur on the secondary pinnae and have dichotomous venation. The xylem of the petiole has a diupsilon configuration in the lower part of the axis, while higher in the petiole the xylem forms a strand resembling that of the European species A. westfaliensis. The xylem strands of the primary pinnae arise from the adaxial antennae of the petiolar vascular strand as somewhat C-shapcd bodies and develop antennae and become H-shaped at higher levels. A gap occurs in the antenna of the petiole vascular system above the level of departure of the primary pinna trace. Terete vascular strands occur in the secondary pinna axes which arise from the adaxial antennae of the xylem of the primary pinnae. The foliar laminae are relatively thin, have an irregular outline, and their histology is like that found in many living ferns. The frond of A. glabra illustrates that leaf evolution had progressed in at least one species of the coenopterid family Zygopteridaceae to the extent that an essentially 2-dimensional frond of modern aspect, and with well-developed foliar laminae, was present by Middle Pennsylvanian time.     

VH1, a provascular cell-specific receptor kinase that influences leaf cell patterns in Arabidopsis

The formation of the venation pattern in leaves is ideal for examining signaling pathways that recognize and respond to spatial and temporal information, because the pattern is two-dimensional and heritable and the resulting veins influence the three-dimensional spatial organization of the surrounding differentiating leaf cell types. We identified a provascular/procambial cell-specific gene that encodes a Leu-rich repeat receptor kinase, which we named VASCULAR HIGHWAY1 (VH1). A change in the expression domain and level of VH1 marks the transition from an uncommitted provascular state to a committed procambial state in early vascular development. The coding sequence, expression pattern, and transgenic phenotypes together suggest that VH1 transduces extracellular spatial and temporal signals into downstream cell differentiation responses in provascular/procambial cells.