DEVELOPMENT AND STRUCTURE OF THE VASCULAR CONNECTION BETWEEN THE PRIMARY AND SECONDARY ROOT OF GLYCINE MAX (L.) MERR.

The primary vascular connection between primary and secondary root of Glycine max (L.) Merr. was derived from stelar parenchyma and pericycle. Inner stelar parenchyma, associated with the parent metaxylem and outer stelar parenchyma adjacent to the pericycle, were resonsible for the histogenesis of the primary xylem connection. Acropetal maturation of the diarch xylem connection occurred after the lateral root emerged from the parent root. Development of tetrarchy occurred distal to the diarch xylem connection. The concentric primary phloem connection was derived from the pericycle and outer stelar parenchyma. Acropetal maturation of the primary phloem connection occurred prior to lateral root emergence from the parent root. Secondary growth quickly augmented the primary vascular connection. A substantial amount of mature secondary xylem formed prior to maturation of the secondary phloem. The structure of the primary and secondary vascular connections is described.     

DEVELOPMENT AND STRUCTURE OF THE VASCULAR CONNECTION BETWEEN THE PRIMARY AND LATERAL ROOT OF LYCOPERSICON ESCULENTUM

The primary xylem connection between the diarch parent root and the diarch lateral root was derived from the pericycle and stelar parenchyma. Early in lateral root development stelar parenchyma that was positioned between the parent xylem and the primordium divided transversely. These transverse divisions produced a plate of cells, most of which subsequently differentiated into vessel element connectors. After emergence of the lateral root, xylem maturation began in the stelar vessel element connectors and maturation proceeded acropetally into the lateral root. Protoxylem of the lateral root was connected to the metaxylem of the parent root via stelar vessel element connectors. The circular phloem connection was pericyclic in origin. Axial phloem connections which vascularized the lateral root were established with sieve tube elements of both parent phloem poles. Maturation of the phloem connection occurred prior to lateral root emergence. Transaxial phloem, positioned in arches above and below the lateral root vascular cylinder, was derived from the pericycle; and each arch consisted of three to four sieve tube elements. No transfer cells were found in the transaxial phloem.     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. II. INITIATION AND DEVELOPMENT OF SECONDARY ROOTS

In seedlings of Ipomoea purpurea secondary roots are initiated in the primary root pericycle opposite immature protoxylem. Cells derived from immature endodermis, pericycle, and incipient protoxylem and stelar parenchyma contribute to the primordium. The derivatives of the endodermis become a uniseriate covering over the tip and flanks of the primordium and emerged secondary root; the endodermal covering is sloughed off when the lateral root reaches 1–5 mm in length. A series of periclinal and anticlinal divisions in the pericycle and its derivatives gives rise to the main body of the secondary root. The initials for the vascular cylinder, cortex, and rootcap-epidermis complex are established very early during primordium enlargement. After emergence from the primary root, the cortical initials undergo significant structural modifications related to enlargement of the ground meristem and cortex, and the rootcapepidermal initials are partitioned into columellar initials and lateral rootcapepidermal initials. Procambium diameter increases by periclinal divisions in peripheral sectors. The mature vascular cylinder is comprised of several vascular patterns, ranging from diarch to pentarch, that are probably related ontogenetically. Cells derived from incipient protoxylem and stelar parenchyma cells of the primary root form the vascuar connection between primary and secondary roots.     

Mechanisms of Reduction of the Stelar Pattern along Barley Roots

The stelar pattern along the seminal and nodal roots of barley (Hordeum vulgare L.) is gradually simplified due to a decreasing frequency of longitudinal cell division in the apical meristem. The decrease involves the proportion of stelar parenchyma, the number of vascular strands on the periphery of the stele and, in nodal roots with a more complex structure, the number of central metaxylem files. In spite of the fact that the stelar parenchyma is reduced in distal parts of the roots to approximately one half, the discontinuity of central and peripheral metaxylem is preserved. Reduction of the number of central metaxylem files is due to fusion. In the reduction of peripheral xylem and phloem strands, the development of certain xylem strands is discontinued and they are terminated blindly. Two phloem strands that had alternated radially with them, approach each other, coalesce and a single phloem strand continues to develop. In this way the regular alternation of phloem and xylem is re-established. The importance of fusions ensuring reduction of the functional continuity in vascular tissue by formation of a network structure must be stressed. This reduction mechanism is involved not only in files of the wide central metaxylem but also in phloem strands which are thus preferred over blindly terminating peripheral xylem strands.     

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.     

Metabolism of [14C]indole-3-acetic acid by the cortical and stelar tissues of Zea mays L. roots

Reverse-phase high-performance liquid chromatography was used to analyse ¹⁴C-labelled metabolites of indole-3-acetic acid (IAA) formed in the cortical and stelar tissues of Zea mays roots. After a 2-h incubation in [¹⁴C]IAA, stelar segments had metabolised between 1–6% of the methanol-extractable radioactivity compared with 91–92% by the cortical segments. The pattern of metabolites produced by cortical segments was similar to that produced by intact segments bathed in aqueous solutions of [¹⁴C]IAA. In contrast, when IAA was supplied in agar blocks to stelar tissue protruding from the basal ends of segments, negligible metabolism was evident. On the basis of its retention characteristics both before and after methylation, the major metabolite of [¹⁴C]IAA in Zea mays root segments was tentatively identified by high-performance liquid chromatography as oxindole-3-acetic acid.Abbreviations HPLC High-performance liquid chromatography - IAA Indole-3-acetic acid     

Stem anatomy of Dolichos lablab Linn (Fabaceae): Origin of cambium and reverse orientation of vascular bundles

Secondary growth in the stem of Dolichos lablab is achieved by the formation of eccentric successive rings of vascular bundles. The stem is composed of parenchymatous ground tissue and xylem and phloem confined to portions of small cambial segments. However, development of new cambial segments can be observed from the obliterating ray parenchyma, the outermost phloem parenchyma and the secondary cortical parenchyma. Initially cambium develops as small segments, which latter become joined to form a complete cylinder of vascular cambium. Each cambial ring is functionally divided into two distinct regions. The one segment of cambium produces thick-walled lignified xylem derivatives in centripetal direction and phloem elements centrifugally. The other segment produces only thin-walled parenchyma on both xylem and phloem side. In mature stems, some of the axial parenchyma embedded deep inside the xylem acquires meristematic activity and leads to the formation of thick-walled xylem derivatives centrifugally and phloem elements centripetally. The secondary xylem comprises vessel elements, tracheids, fibres and axial parenchyma. Rays are uni-multiseriate in the region of cambium that produces xylem and phloem derivatives, while in some of the regions of cambium large multiseriate, compound, aggregate and polycentric rays can be noticed.     

Resumption of DNA Synthesis and Cell Division in Wheat Roots as Related to Lateral Root Initiation

The arrest of DNA synthesis and termination of cell division in basal meristematic cells as well as the resumption of these processes as related to the initiation of lateral root primordia (LRP) were studied in tissues of Triticum aestivumroots incubated with ³H-thymidine. All cells of the stelar parenchyma and cortex as well as most endodermal and pericycle cells left the mitotic cycle and ceased proliferative activity at the basal end of the meristem and at the beginning of the elongation zone. Some endodermal and pericycle cells started DNA synthesis in the basal part of the meristem and completed it later on during their elongation, but they did not divide. In the cells of these tissues, DNA synthesis resumed above the elongation zone, the cells being located much closer to the root tip than the first newly dividing cells. Thus, the initiation of LRP started much closer to the root tip than it was previously believed judging from the distance of the first dividing pericycle cells from the root tip. DNA synthesizing and dividing cells first appeared in the stelar parenchyma, then, in the pericycle, and later, in the endodermis and cortex. It seems likely that a release from the inhibition of DNA synthesis allows the cells that completed mitotic cycle in the basal part of meristem in the G₁phase to cease the proliferative arrest above the elongation zone and to continue their cycling. The location of the first DNA synthesizing and dividing cells in the stelar parenchyma and pericycle did not strictly correspond to the LRP initiation sites and proximity to the xylem or phloem poles. This indicates that LRP initiation results from the resumption of DNA synthesis in all pericycle and stelar parenchyma cells that retained the ability to synthesize DNA and occurs only in the pericycle sector situated between the two tracheal protoxylem strands, all cells of which terminated their mitotic cycles in the G₁phase.     

Ultrastructure of the haustorial interface and apoplastic continuum between host and the root hemiparasiteOlax phyllanthi (Labill.) R. Br. (Olacaceae)

Summary Structural features of haustorial interface parenchyma of the root hemiparasiteOlax phyllanthi are described. Walls contacting host xylem are thickened non-uniformly with polysaccharides, not lignin, and show only a thin protective wall layer when abutting pits in walls of host xylem vessels or tracheids. Lateral walls of interface parenchyma exhibit an expanded middle layer of open fibrillar appearance, sometimes with, but mostly lacking adjoining layers of dense wall material. Free ribosomes and rough endoplasmic reticulum are prominent and occasional wall ingrowths present. Experiments involving transpirational feeding of the apoplast tracers lanthanum nitrate or uranyl acetate to host roots cut below haustorial connections, indicate effective apoplastic transfer from host to parasite root via the haustorium. Deposits of the tracers suggest a major pathway for water flow through host xylem pits, across the thin protective wall layer, and thence into the haustorium via the electronopaque regions of the terminal and lateral walls of the contact parenchyma. Graniferous tracheary elements and walls of parenchyma cells of the body of the haustorium appear to participate in tracer flow as do walls of cortical cells, stele parenchyma and xylem conducting elements of the parasite root, suggesting that both vascular and non-vascular routes are involved in extracytoplasmic transfer of xylem sap from host to parasite. The Casparian strip of the endodermis and the suberin lamella of the exodermis of theOlax root act as barriers to flow within the system.     

Formation of successive cambia and stem anatomy of Sesuvium sesuvioides (Aizoaceae)

Mature stems of Sesuvium sesuvioides (Fenzl) Verdc. were found to be composed of successive rings of xylem alternating with phloem. Repeated periclinal divisions in the parenchyma outside the primary phloem gave rise to conjunctive tissue and the lateral meristem that differentiate into the vascular cambium on its inner side. After the formation of the vascular cambium, the lateral meristem external to it became indistinct as long as the cambium was functional. As the cambium ceased to divide, the lateral meristem again became apparent prior to the initiation of the next cambial ring. The cambium was exclusively composed of fusiform cambial cells with no rays. In the young saplings, the number of cambial cylinders in the axis varied from the apex to the base, indicating formation of several rings within the year. In each successive ring of the lateral meristem, small segments differentiated into the vascular cambium and gave rise to vessels, axial parenchyma, fibres and fibriform vessels towards the inside, and secondary phloem on the outer side. In the old stems, non‐functional phloem of the innermost rings was replaced by a new set of sieve tube elements formed by periclinal divisions in the cambial segments associated with the non‐functional phloem. In some places the cambial segments completely differentiate into derivatives leaving no cambial cells between the xylem and phloem. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 548–555.     

A histological study of lateral root initiation and development inZea mays

Summary A light microscopic study has been made of the origin and development of lateral roots inZea mays.The initiation of a lateral occurs adjacent to a xylem pole and involves an increase in cytoplasmic basophilia and subsequent divisions of cells of the pericycle and the parenchyma of the stele of the mother root.Cells derived from the parent pericycle form most of the young lateral but its epidermis and root cap are composed of cells of endodermal origin.Two different types of polysaccharides are secreted by cells of the young lateral root. One type which is PAS-positive and non-metachromatic, is produced by the epidermal cells, while the other type, metachromatic and only slightly PAS-positive, is secreted by the root cap cells.     

Consequences of root growth kinetics and vascular structure on the distribution of lateral roots

It has been proposed that the acropetal initiation of lateral roots is a built‐in process specified as part of the general process of cell division and differentiation in the parent root tip. Conversely, it is commonly reported that root branching is essentially a variable feature. In the present study, the interlateral distance along the parent root has been investigated using three banana varieties (Musa spp.) grown in two substrates. The pattern of lateral root initiation was obscured by variations of root growth patterns and vascular structure among roots, genotypes and substrates. A framework model is formulated showing the influence of growth pattern and vascular structure on branching density. The model raises a distinction between growth components which should not affect the branching density (i.e. rate of cell division) and which may affect it (i.e. size of mature cells and number of transverse divisions performed by cells executing their trajectory in the meristem). It appears also that lateral root density and root growth rate might be independently modulated by appropriate changes of root growth patterns, in banana and presumably many other taxa.     

Perfusion of onion root xylem vessels: a method and some evidence of control of the pH of the xylem sap

We describe a method for perfusing the xylem in the stele of excised onion roots with solutions of known composition under a pressure gradient. Tracer studies using [¹⁴C] polyethylene glycol 4000 and the fluorescent dye, Tinopal CBSX, indicated that perfusing solutions passed exclusively through the xylem vessels. The conductance of the xylem was small over the apical 100 mm of the root axis but increased markedly between 100 and 200 mm. Unbuffered perfusion solutions supplied in the range pH 3.7–7.8 emerged after passage through the xylem adjusted to pH 5.2–6.0, indicating the presence of mechanisms for absorbing or releasing protons. This adjustment continued over many hours with net proton fluxes apparently determined by the disparity between the pH of the perfusion solution and the usual xylem sap pH of about 5.5. Mild acidification of the xylem sap by buffered perfusion solutions increased the release of ⁸⁶Rb (K⁺) and ³⁵SO₄ ^2- from the stelar tissue into the xylem stream. The ion-transporting properties of onion roots seemed little changed by excision from the bulbs, or by removal of the apical zones of the root axis. The pH of sap produced by root pressure resembles that found in the outflow solutions of perfused root segments.     

The structure and development of the haustorium in parasitic Scrophulariaceae*

The structure and development of roots and haustoria in 37 species of parasitic Scrophulariaceae was studied using light microscopy. The mature haustorium consists of two regions: the swollen “body” and the parent root, which resembles non-haustorial roots in structure. The body arises from the parent root and is composed of an epidermis, cortex, central region of xylem (the vascular core), a region of parenchyma (the central parenchymatous core), and the portion of the haustorium contained in the host tissue (the endophyte). The xylem of the vascular core is composed predominately of vessel elements. The central parenchymatous core is composed of parenchyma and col-lenchyma. Vessels extend from the vascular core through the central parenchymatous core to the endophyte. The endophyte is composed of parenchyma cells and vessel elements. No phloem is present in the body of the haustorium. Early stages in the development of the haustorium are exogenous. Initial periclinal divisions in the epidermis or outer cortex are followed by hypertrophy of cortical parenchyma. These events are followed by development of the vascular core from the pericycle, attachment of haustorium to the host by a specialized layer of cementing cells or root hairs, and penetration of the host by dissolution of host cells.     

The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure

Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.     

OBSERVATIONS ON STELAR PARENCHYMA IN LEPIDIUM SATIVUM L. ROOTS

Seedlings of Lepidium sativum L. (cress) were germinated and grown under ten different growth conditions to assess the effects of environment on root stelar parenchyma development. Stelar parenchyma did not undergo autolysis in any of the treatments, refuting an earlier suggestion that certain ‘prostelar’ cells (cells adjacent to primary xylem and primary phloem) are replaced by cambial derivatives.     

栽培太子参块根的发育解剖学研究

应用石蜡制片技术研究了栽培太子参纺锤状块根的发育过程。结果表明,栽培太子参的块根是由其不定根发育而成。太子参不定根的初生结构与次生结构的发育可分为4个阶段:原分生组织阶段、初生分生组织阶段、初生结构与次生结构阶段,类似一般草本双子叶植物根的发育。其特点是初生结构的皮层细胞大,仅3 ~4层,内皮层细胞具凯氏带;初生木质部多为三原型,少数为二原型、四原型与五原型。次生结构中次生木质部约占根面积80 %,主要为薄壁组织细胞,导管呈稀疏的放射状分布其中。由不定根发育成块根过程中,根据从根头至根尾不同距离的各组成部分的面积及细胞层数分析,从上向下其维管形成层活动强度不同,从而根的直径大小不同,使根发育成上粗下细的纺锤状肉质块根。高碘酸-Schiff反应显示,在成熟的块根中次生韧皮部的薄壁组织细胞和次生木质部射线间的木薄壁组织细胞内富含淀粉粒,在有些木薄壁组织细胞中还含有草酸钙簇晶。     

Proliferating sieve elements present in bud phloem anastomoses connect sieve tubes of axillary bud traces to stelar vascular bundles in the aquatic monocotyledonPotamogeton natans L. (Potamogetonaceae)

Summary The stem ofPotamogeton natans is characterized by a central stelar vascular system with reduced xylem and abundant phloem. Wide sieve tubes composed of short sieve-tube members joined by simple sieve plates and associated with companion cells establish an effective conduit for assimilates. At each node the phloem forms a network of parallel sieve elements connecting the stem phloem to leaf and bud traces. InP. natans an axillary bud rarely develops into a side branch, its procambial vascular bundles are each connected to the nodal complex via separate anastomoses. Their most unusual components are the anastomosai sieve elements (ANSE), characterized by thin cell walls pitted all over by tiny callose-lined pores resembling plasmodesmata, which can be detected as bright areas by fluorescence microscopy after staining with aniline blue. Several layers of ANSE make up the centre of an anastomosis and link to both the nodal and bud stelar sieve tubes via mediating (MSE) and connecting sieve elements (CSE). The ultrastructural differentiation of ANSE, MSE, and CSE corresponds to that of normal sieve elements, i.e., in the mature stage they are enucleate, evacuolate, and have lost most of their cytoplasm. Their plastids are of form-P2c, containing many cuneate protein crystals, typical of monocotyledonous sieve elements. Quantitative aspects of the pore areas are discussed in relation to the functional significance of bud anastomoses.Abbreviations ANSE anastomosai sieve elements - CSE connecting sieve elements - FM fluorescence microscopy - LM light microscopy - MSE mediating sieve elements - TEM transmission electron microscopy Dedicated to Professor Dr. Rainer Kollmann on the occasion of his retirement     

Evidence for proteins specific for vascular elements in intact and cultured tissues and cells of maize

A group of antigenically distinct proteins characteristic for the tissue complex of the vascular cylinders was found in maize (Zea mays L.) seedlings using an immunofiltration technique. Specific stelar antigens present in the fully developed stele (vascular cylinder) of the primary root were also found in steles extracted from adventitious roots and from the mesocotyl but were absent, within the limits of sensitivity of the immunodiffusion tests employed, in root cortex and epidermis. Some of the stelar antigens were also evident in the meristem of the primary root and were present in traces in the scutellum, the mesocotyl node, and the primary leaves plus coleoptile. The specific stelar antigens could be traced in 13- and 15-day-old developing embryos and were definitely expressed by the 21 st day after pollination. Several stelar-specific antigens were found in embryo-derived callus tissues and in stem-derived cells maintained in serial suspension culture. Higher resolution of the stelar antigens by a modified technique of crossed immunoelectrophoresis was used to demonstrate several minor stelar antigens that were presumably characteristic exclusively of the completely differentiated stele. This technique along with sequential immunoprecipitation of labelled proteins provided a semiquantitative estimate of the specific stelar antigens in the meristem and the stele of the primary root, and in suspension-cultured cells which were devoid of noticeable signs of vascular differentiation.     

掌叶大黄根多糖的积累分布特征

采用组织化学方法和苯酚硫酸比色法研究了掌叶大黄（Rheum palmatum）根中大黄多糖的贮藏分布特征和含量变化规律。结果表明：大黄多糖在根内的贮藏是多位点的,在根周皮的栓内层、次生维管组织的薄壁细胞内不同程度地贮藏和积累了一定数量的大黄多糖,次生木质部的木薄壁细胞和次生韧皮部的韧皮薄壁细胞是主要贮藏和积累的部位;不同发育时期根中大黄多糖含量的变化规律为,随着植物的不断成熟,根及其各组织中大黄多糖的总含量表现为逐渐增高的趋势,但在发育的后期略有下降;韧皮薄壁细胞与木薄壁细胞相比,贮藏大黄多糖的含量相对较多,大黄多糖的贮藏积累方式为逐渐累积的方式。