A Note on the Ontogeny of Cross Veins in Digitaria eriantha

The small cross veins that link the longitudinal bundles arisefrom single files of the ground meristem. The tangential divisionsof the cross vein initial produce two vascular parenchyma cellsand two conducting elements. Digitaria eriantha, ontogeny, cross vein, tracheary element, sieve element     

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.     

A Comparative Study of the Structure-Function Relationship of Cross Veins in Leaves of Digitaria eriantha and Zea mays

Ultrastructural and functional differences between the crossveins of Digitaria eriantha and Zea mays were investigated.Cross veins of both genera possess similar conducting tissues,namely one tracheary element and one sieve cell. In Digitariaeriantha these conducting elements are associated with onlytwo parenchyma cells, and, those in Zea mays are completelysurrounded by chJorenchyma cells. The protein ribulose 1,5-bisphosphatecarboxylase/oxygenase (rubisco) was used as a probe for CO₂fixation sites by comparing its distribution in the varioustissue types in the leaves of the two genera. The protein wasfound to be equally and uniformly distributed in the stromalregions of the chlorenchyma sheath cell chloroplasts of longitudinalveins of both genera. The chlorenchyma sheath cells in crossveins of Zea mays show a similar distribution of the enzymeas the longitudinal bundles. However, this enzyme was shownto be absent in the cross vein parenchyma cells of Digitariaeriantha and in the mesophyll cells of both genera. Cross veins, immuno-gold labelling, ribulose 1,5-bisphosphate carboxylase/oxygenase, Digitaria eriantha, Zea mays     

Cell differentiation in the longitudinal veins and formation of commissural veins in rice (Oryza sativa) and maize (Zea mays)

Vascular development is a central theme in plant science. However, little is known about the mechanism of vascular development in monocotyledons (compared with dicotyledons). Therefore, we investigated sequential processes of differentiation into various different vascular cells by carrying out detailed observations using serial sections of the bases of developing leaves of rice and maize. The developmental process of the longitudinal vascular bundles was divided into six stages in rice and five stages in maize. The initiation of differentiation into procambial progenitor cells forming the commissural vein arose in a circular layer cell that was adjacent to both a metaxylem vessel and one or a few phloem cells in stage V longitudinal vascular bundles. In most cases the differentiation of ground meristem cells into procambial progenitor cells extended in one direction, toward the next longitudinal vascular bundle, and subsequent periclinal divisions and further differentiation produced a vessel element, two companion cells and a sieve element to form a commissural vein. These results suggest the presence of an intercellular signal(s) that induces differentiation of the circular layer cell and the ground meristem cells into procambial progenitor cells, forming a commissural vein sequentially.     

Procambial Development in Some Labiatae in Relation to Vascular Conservatism

A study of procambial development in the flowers of some Labiataehas been made. Each floral appendage initially receives a singleprocambial strand. The direction of procambial differentiationis continuous and acropetal except in Nepeta spicata and Salviasplendens where procambial strands for petals and stamens arisealmost simultaneously. While studying the organogenesis of flowerssome primordia of the floral appendages have been observed whichbecome obscured in mature flowers, but no procambium is differentiatedfor them. Two posterio-lateral sepal primordia in Salvia splendens;one of the primordial of a pair of posterior petals in Pogostemonbenghalensis; and the primordium of fifth posterior stamen inNepeta spicata and Salvia splendens are examples of primordialhumps which become obscured in mature flowers due to the overgrowthof other organs. In the light of these examples the doctrineof vascular conservatism which postulates that the externalform of an organ is less conservative than its vasculature,is not tenable. Labiatae, procambium, vascular conservation     

Sink to Source Transition of Populus Leaves

Development of the Populus leaf is presented as a model system to illustrate the sequence of events that occur during the sink to source transition. A Populus leaf is served by three leaf traces, each of which consists of an original procambial trace bundle that differentiates acropetally and continuously from more mature procambium in the stem and a complement of subsidiary bundles that differentiates bidirectionally from a leaf basal meristem. During development these subsidiary bundles maintain continuity through the meristematic region of the node. The basipetally developing subsidiary bunles form phloem bridges that serve to integrate adjacent leaf traces of the stem vasculature. Distal to the node the acropetally developing bundles from all three leaf traces are reoriented in a precise and orderly sequence to form tiers of petiolar bundles. These tiers of bundles extend into the midrib where bundles diverge at intervals as the major lateral veins. The dorsal-most tier of bundles extends to the lamina tip and each successive tier of bundles contributes to lateral veins situated more proximally in the lamina. Although the midrib and the major vein system differentiate acropetally in the lamina, they mature basipetally. Maturation of the mesophyll and other lamina tissues also mature basipetally. As a consequence of the basi-petal maturation process, the lamina tip matures very early and begins exporting photosynthates while the lamina base is still importing from other leaves. The transition of a leaf from sink to source status must therefore be considered as a progression of structural and functional events that occur in synchrony.     

Histology of Somatic Embryogenesis from Leaf Explants of the Oil Palm Elaeis guineensis

Histological analysis was carried out during the sequence ofevents which lead to the obtaining of somatic embryos of oilpalm. Calluses from the division of perivascular cells formedat the veins of young leaf explants. Subsequent proliferationof histologically similar nodules was by means of a cambium-likezone. Under certain conditions these calluses consisted almostentirely of meristematic cells. They then differentiated rapidly:the cambium-like zone fragmented, leading to protuberance inwhich the cells divide rapidly; epidermal structures were formed,with a network of procambial strands, and synthesis of storagelipids accompanied the formation of these embryo-like structureswhich developed into clumps of true somatic embryos, each witha shoot apex and a root apex. Other structures frequently observedduring in vitro culture are also described and show that alternatepathways do exist. The structure and evolution of somatic embryosare compared to those of zygotic embryos. Storage lipids emergeas an early tracer of the satisfactory development of tissuetowards somatic embryogenesis. Oil palm, Elaeis guineensis, histology, somatic embryogenesis, callogenesis, storage lipids     

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.     

The procambium specification gene Oshox1 promotes polar auxin transport capacity and reduces its sensitivity toward inhibition

The auxin-inducible homeobox gene Oshox1 of rice (Oryza sativa) is a positive regulator of procambial cell fate commitment, and its overexpression reduces the sensitivity of polar auxin transport (PAT) to the PAT inhibitor 1-N-naphthylphthalamic acid (NPA). Here, we show that wild-type rice leaves formed under conditions of PAT inhibition display vein hypertrophy, reduced distance between longitudinal veins, and increased distance between transverse veins, providing experimental evidence for a role of PAT in vascular patterning in a monocot species. Furthermore, we show that Oshox1 overexpression confers insensitivity to these PAT inhibitor-induced vascular-patterning defects. Finally, we show that in the absence of any overt phenotypical change, Oshox1 overexpression specifically reduces the affinity of the NPA-binding protein toward NPA and enhances PAT and its sensitivity toward auxin. These results are consistent with the hypothesis that Oshox1 promotes fate commitment of procambial cells by increasing their auxin conductivity properties and stabilizing this state against modulations of PAT by an endogenous NPA-like molecule.     

Cell cycling frequency and expression of the homeobox gene ATHB-8 during leaf vein development in Arabidopsis

Tissue histogenesis during plant development depends on regulation of cell division plane, timing and frequency to produce cell units of correct size and shape for mature function. Differences among the dermal, ground and vascular tissue systems arise during development, largely through regulation of these aspects of cell cycling in relation to overall tissue expansion. Using a cyclin1At::GUS reporter construct, we demonstrate quantitative differences in cell cycling frequency among tissue systems and among primary, secondary, and tertiary veins; these differences are superimposed upon the more general longitudinal gradient of cell division frequency in developing leaves of Arabidopsis thaliana (L.) Heynh. Patterns of cell cycling frequency coincide almost exactly with those of the earliest known molecular marker of procambial identity, the HD-ZIP III homeobox gene ATHB-8, suggesting that ATHB-8 may play a role in regulating the early events of procambial development, including procambium-specific patterns of cell cycling. Cellular localization of cyc1At::GUS and ATHB-8::GUS within developing vascular strands indicates, however, that ATHB-8 has additional functions related to dorsiventral patterning within veins and cell differentiation events.     

木立芦荟叶的发育解剖学研究

应用植物解剖学方法研究了木立芦荟（Aloe arborescens Mill.)叶的发育过程。研究结果表明,叶原基在发育早期其形态是不对称的,内部为同形细胞组成,但很快分化成原表皮,原形成层束和基本分生组织。以后,原表皮发育成表皮,位于原表皮下的2－5层基本分生组织细胞发民同化薄壁组织,而位于中央的基本分生组织细胞则发育成储水薄壁组织,原形成层束发育成维管束。维管束由维管束鞘、木质部、韧皮部和大型薄壁细胞组成。大型薄壁细胞起源于原形成层束,位于韧皮部内,其发育迟于筛管、伴胞,为芦荟属植物叶的结构特征。     

Transverse Vein Differentiation Associated with Gas Space Formation--Fate of the Middle Cell Layer in Leaf Sheath Development of Rice

In monocotyledons, the leaf vascular network consists of a hierarchicalsequence of vertical vascular bundles and numerous transverseveins that interconnect adjacent vertical veins. In the leafsheath of these species, especially grasses, lysigenous gascavities (gas spaces) are developed into intervascular spacesand provide a gas conducting system to non-aerial parts underflooded conditions. The spatial relationship between gas spaceformation and transverse vein differentiation was investigatedusing the leaf sheath of rice (Oryza sativa L.). Histochemicalobservation showed that patterns of differentiation of the transversevein are distinct from those of vertical vascular bundles. Onthe other hand, gas spaces are formed through the processesof cell death (collapse). Both events are initiated at a specificcell position in the middle layers of the leaf sheath, fromwhich the vascular system of the leaf is derived; this indicatesthat differentiation of transverse veins is associated withgas space formation. The cell-to-cell movement of fluoresceinisothiocyanate-conjugated dextran injected into middle layercells coincided with the area where cell collapse occurred,indicating a close relationship between the middle and adaxialcell layers, but not abaxial cell layers. A uniform cell numberbetween each transverse vein in the leaf sheath suggested theinvolvement of spatial regulation in transverse vein formationregardless of clonal history at the later stage of leaf veincanalization. Copyright 2000 Annals of Botany Company Cell collapse, leaf development, middle cell layer, microinjection, Oryza sativa L., rice, programmed cell death.     

管花肉苁蓉茎异常结构的发育解剖学研究

管花肉苁蓉茎内存在类似于单子叶植物的散生初生维管束。它由散生在基本分生组织中的原形成层束分化而成。在原形成层来分化的过程中,每个原形成层束可通过分离形成2—7个初生维管束,使初生维管束的数目迅速增加。当初生维管束开始正常次生生长时,正常维管束韧皮部外方的薄壁组织细胞或远离维管柬的薄壁组织细胞转变为异常形成层束。异常次生维管束与正常维管束以韧皮部相对或韧皮部并列的方式排列,或异常次生维管束单个存在于薄壁组织中。     

Cell Lineage of Vein Formation in Variegated Leaves of the C4Grass Stenotaphrum secundatum

Clonal analysis of variegated leaves of the C₄grass, Stenotaphrumsecundatum, indicates that invasions among meristematic layersoccur during the organogenetic stage of leaf development, resultingin long, broad white and green stripes. These layer invasionscease prior to the second phase of leaf development when delimitationof leaf regions occurs. Vein precursors mostly arise duringthe second phase, so that procambial strand formation is superimposedon the lineage makeup of earlier-formed tissue. Anatomical evidenceindicates that procambium arises through formative divisionswithin ground tissue of leaf primordia and that each strandis derived from a variable number (one–four) of groundmeristem precursors. If a developing vein straddles the boundarybetween previously-formed green and white sectors, then themature vein is half green and half white, reflecting its mixedcell lineage. In Stenotaphrum, 24.8% of the sectors observedwere bounded by such ‘half veins’. The temporalrelationship of layer invasion and tissue system delimitationin this species supports the view that positional signals aremore important than lineage history in the determination oftissue type. However, analysis of planes of cell division indeveloping veins indicates, that, once formed, procambial strandsare discrete lineage units that extend longitudinally by proliferativedivisions. Thus, lineage restrictions may play an importantrole in the third stage of leaf development, differentiationof tissues and cells, which also includes the maintenance ofcell identity.Copyright 2000 Annals of Botany Company C₄photosynthesis, cell lineage, clonal analysis, leaf development, St. Augustine’s grass,Stenotaphrum secundatum , variegation, vein formation     

Loading and transport of assimilates in different maize leaf bundles

The loading and transport functions of vascular bundles in maize (Zea mays L.) leaf strips were investigated by microautoradiography after application of ¹⁴CO₂. The concentrations of ¹⁴C-contents in thin-walled sieve tubes of individual bundles in the loading and transport regions were determined by digital image analysis of silver-grain density over the sieve tubes and compared. In the loading region, relatively high concentrations of ¹⁴C-contents were found in the thin-walled sieve tubes of small bundles and in the small, thin-walled sieve tubes of the intermediate bundles; the concentration of ¹⁴C-label in large bundles was very low. In the transport region, at a transport distance of 2 cm, all of the small bundles contained ¹⁴C-assimilates, but generally less than the same bundles did in the loading region; by comparison, at that distance intermediate and large bundles contained two-to threefold more ¹⁴C-assimilates than the same bundles in the loading region. The lateral transfer of assimilates from smaller to larger bundles via transverse veins could be demonstrated directly in microautoradiographs. A reverse transport from larger to smaller bundles was not found. At a transport distance of 4 cm, all large and intermediate bundles were ¹⁴C-labeled, but many of the small bundles were not. Although all longitudinal bundles were able to transport ¹⁴C-asimilates longitudinally down the blade, it was the large bundles that were primarily involved with longitudinal transport and the small bundles that were primarily involved with loading.     

Der autoradiographische Nachweis trittierter wasserlöslicher Substanzen in den Siebröhren von Cucurbita und Cucumis

Summary When D-Glucose-6-T is applied to small areas of the leaf blade of Cucumis or Cucurbita, a translocation of labelled material within the veins and the petiole down to the internodes of the axis can be demonstrated by chemical methods and by histoautoradiography. The translocated material was identified by thin layer and paper chromatography as stachyose, raffinose, and sucrose. A dry mounting autoradiographic technique was elaborated which prevented any secondary diffusion of water soluble substances. In autoradiograms of longitudinal and cross sections of veins, petioles and internodes, tritium can be localized within single sieve tubes of exporting bundles. Usually the centre of activity corresponds to the cytoplasmic material or the slime-plugs and to the sieve plates. Since sucrose, stachyose and raffinose are the main labelled translocated substances in these experiments, there is little doubt that with certain precautions such autoradiograms may present a real picture of the long-distance translocation of assimilates.

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Teil einer Dissertation unter der wissenschaftlichen Leitung von Prof. Dr.J. Willenbrink.     

Sieve Elements of Minor Veins in the Leaves of Beta vulgaris L.

End walls of sieve elements of minor veins of the leaves ofBeta vulgaris L. do not contain the multi-perforate sieve plateswhich typically occur on the end walls of sieve-tube membersof major veins. Instead, both end and side walls of the sieveelements of minor veins contain scattered pores which may occursingly or in small numbers. These pores are similar to thosewhich are grouped in sieve plates of major veins in size, possessionof callose and plugs of filaments. In addition to these pores,there are tubular connections 0.1 µ in diameter throughcharacteristically thickened parts of the cell wall betweensieve cells and companion cells. Sieve elements of minor veinsdiffer from those of major veins in structure as well as infunction.     

Formation of the vascular system of developing bean (Phaseolus limensis L.) seeds according to nuclear magnetic resonance microtomography

1H magnetic resonance microtomography imaging was applied to study vascular systems in developing bean (Phaseolus limensis L.) seeds. Using the gradient echo method, we recorded 2D tomographic sections in the sagittal and axial planes of the fruits sampled from a vegetating plant on days 10, 17, 24, and 31 after fertilization. Any vascular connection between the tissues of maternal plant (bean pod and seed coat) and the embryo were undetectable. The embryo has an autonomous branched network of procambial strands in the cotyledons, converging to the embryonic axis. The bean pods are covered with a network of vascular bundles; large vascular strands run along the dorsal and ventral sutures. The seed coat vascular bundles are formed in the process of seed ripening and are represented by a developed vascular system multiply branching in the middle part of the ground parenchyma at the stage of physiological maturity. They are connected with the source of assimilates via the lateral pod veins and a large vascular bundle, entering the seed below the hilum via the placenta. Assimilates enter the external part of the seed coat, which contains no vascular bundles, via the funiculus vascular bundles and hilum tissue.     

Formation of the vascular system of developing bean (<Emphasis Type="Italic">Phaseolus limensis</Emphasis> L.) seeds according to nuclear magnetic resonance microtomography

¹H magnetic resonance microtomography imaging was applied to study vascular systems in developing bean (Phaseolus limensis L.) seeds. Using the gradient echo method, we recorded 2D tomographic sections in the sagittal and axial planes of the fruits sampled from a vegetating plant on days 10, 17, 24, and 31 after fertilization. Any vascular connection between the tissues of maternal plant (bean pod and seed coat) and the embryo were undetectable. The embryo has an autonomous branched network of procambial strands in the cotyledons, converging to the embryonic axis. The bean pods are covered with a network of vascular bundles; large vascular strands run along the dorsal and ventral sutures. The seed coat vascular bundles are formed in the process of seed ripening and are represented by a developed vascular system multiply branching in the middle part of the ground parenchyma at the stage of physiological maturity. They are connected with the source of assimilates via the lateral pod veins and a large vascular bundle, entering the seed below the hilum via the placenta. Assimilates enter the external part of the seed coat, which contains no vascular bundles, via the funiculus vascular bundles and hilum tissue.     

Interrelations between Phyllotaxis, Leaf Development and the Primary-secondary Vascular Transition in Populus deltoides

The procambial system of Populus deltoides Bartr. ex Marsh.plants progressed from phyllotaxy in the cotyledon stage throughthe phyllotactic orders 3/2;5/13. The nodal position at whicheach of these phyllotactic transitions occured was determinedby anatomical analyses; they were found to be remarkably consistentin a large population of young plants. The data were used todiagrammatically reconstruct the procambial system of a typical16 leaf plant. Because all plant parts grew continuously anduninterruptedly, it was not possible to verify the positionsof the phyllotactic transitions by morphological criteria. However,several measured parameters (the number and lengths of primordiawithin the terminal bud, the plastochron interval, and the numberof leaf traces with birefringent xylem elcments) attained constantvalues following establishment of the 5/13 phyllotaxy, suggestingthis to be the stable phyllotactic order for the species. Althoughbud size continued to increase in plants exhibiting 5/13 phyllotaxy,it could be accounted for by the increased number and size ofbasipetal subsidiary bundles in the procambial leaf traces.It was suggested that these phyllotactic transitions in theprocambial system are programmed in the plant to occur at ratherspecific stages of ontogeny. The process is mediated by theolder leaves and it is therefore modified by plant vigour. Locationof the primary-secondary vascular transition zone was also relatedto the order of phyllotaxy. It advanced acropetally in the stemin close association with leaf maturation, but this associationwas further influenced by plant vigour. Populus deltoides Bartr. ex Marsh., cottonwood, vascular anatomy, phyllotaxis, leaf growth, xylem