Different populations of bacteria associated with sheathed and bare regions of roots of field-grown maize

It has been previously shown that soil sheaths cling tightly to some portions of all axile roots and cover all but the growing tips of the young roots of field-grown maize. These sheaths overlie immature regions of the roots which have intact epidermal cells with root hairs, and living, thus non-conducting, late metaxylem elements. Loss of the soil sheath in the proximal region coincides with the opening of these large metaxylem vessels. Now, total, and viable counts have been recorded of bacteria associated with the root surface and adhering soil of sheathed and bare regions. These showed some common features, in that populations of similar size were associated with the two root regions in plants beginning to flower. Each population included about the same numbers of bacteria that were viable on each of three selective media (nitrogen-free, Pseudomonas F or MacConkey). However, more spore-formers capable of growth on nitrogen-free media and more fluorescent bacteria were isolated from the sheathed regions. Actinomycetes were absent from sheathed but plentiful on bare regions.The high numbers of diverse types of bacteria associated with both root surfaces can be related to the previously demonstrated similarity in amounts of organic carbon released from each region. The proliferation of actionomycetes on the bare roots and their exclusion from sheathed roots may in part be due to the lower water status of the bare region, which is related to its greater axial conducting capacity. Thus the distribution of the two types of root surface within an individual root system has important implications for the choice of root and rhizosphere sampling techniques and for root bacterization work.     

Pathways and processes of water and nutrient movement in roots

Recent work in our laboratory provides evidence for a revised view of the functioning of roots of maize, and probably of all the grasses. The development of coherent soil sheaths on the distal 30-cm of these roots, and the loss of the sheaths further back, led us to investigate the differences in surface structure, anatomy, carbon exudation and microflora of the sheathed and bare zones. The significant differences are summarized. But the fact which underlies all these differences is the maturation of the late metaxylem (LMX). In the sheathed zones the LMX elements are still alive and non-conducting; only the early metaxylem (EMX) and protoxylem are open. In the bare zones they are open vessels. This leads directly to the dryness of bare zones and the wetness of sheathed zones, and indirectly to the other differences noted. Branch root junctions are shown to be structures of great significance. Besides connecting the branches to the axile systems, they serve also to connect the EMX and LMX vessels, and contain a tracheid barrier which prevents air embolisms entering the main vessels. These discoveries force us to revise the traditional view of water uptake by the root hair zone, and to suggest that much water must also enter bare roots, possibly via the laterals. There is some published evidence for this. The living LMX elements of the sheathed zone accumulate large concentrations of potassium which must joint the transpiration water at the transition to the bare zone. Calculations suggest that this may be only a tenth of the requirement of a mature plant, and that the balance may enter the bare zones with the transpiration water.     

The water status of the roots of soil-grown maize in relation to the maturity of their xylem

The long delayed maturation of the late metaxylem of maize ( Zea mays ) roots imposes a high-resistance barrier between the immature apices and the negative water potential of the leaves. These apices (20+ cm) bear strongly adhering soil sheaths to within 0.5 to 2 cm of the distal end. It was hypothesized that the sheathed immature apices should show less response to transpiration stress than bare regions. Measurements were made of the relative water content (RWC) of the sheathed and bare zones of the axile roots, both at different ages of the plant, and early and late in the day's transpiration. Sheathed roots maintained a steady RWC of about 83% irrespective of age or transpiration. Bare roots had RWCs of about 63% in the morning, but this fell to 55% in the afternoon. The first-order branches on the bare roots in the morning had still lower values of RWC, near 50%. Plots of RWC against water potential were indistinguishable for the three root types. It is concluded that the immature apices are indeed relatively isolated from the fluctuating tensions in the stem xylem, and that these tensions reduce the water content of bare roots and their branches to low values.     

Accumulation of potassium by differentiating metaxylem elements of maize roots

The previous demonstration that the large late metaxylem vessels of field-grown maize ( Zea mays L. cv. Rosella) roots do not lose their crosswalls until they are 20–30 cm from the tip, and that the presence of a soil sheath outside the root was indicative of immature vessels within, greatly strengthened the hypothesis that ion accumulation into these roots was by uptake into living xylem element vacuoles. Proposals that salt movement into the xylem was by leakage or secretion into dead vessels became much less plausible. Potassium concentration in the vacuoles of late metaxylem elements was measured by X-ray microanalysis in unetched fracture faces of bulk, frozen-hydrated pieces of sheathed roots, and found to be in the range 150–400 m M . Potassium concentration in open vessels of bare roots, measured both with the microprobe and by spectrophotometry of aspirated sap, was in the range of 5 to 25 m M . It is concluded that uptake of potassium (and possibly other ions) is into living xylem elements, and that its release to the transpiration stream occurs by the breakdown of their crosswalls and the addition of their vacuoles to the solution in the vessels above.     

Architecture of Branch-root Junctions in Maize: Structure of the Connecting Xylem and the Porosity of Pit Membranes

The architecture of the connecting xylem network in the vascularplexus linking branch and main root vessels has been examinedusing cryoSEM, and the limiting porosity of the network determinedwith tracers (dye, and particles of known size: latex, polystyreneand gold sols). Dye and water move freely throughout the xylemnetwork, while particles are constrained to follow tortuousvessel-like conduits of irregularly-shaped elements linked bylarge-diameter perforations. These conduits end at special pitmembranes (boundary pit membranes) at the periphery of mainroot vessels. Particles accumulate on the outer side of thesefilters, often filling the terminal elements of these conduitsadjacent to the main root vessels. Some vessel elements withinthe plexus are isolated from the convoluted conduits by normalpit membranes, and often also from each other, by pit membranesand still-intact end walls in otherwise mature elements. Theseextra-conduit elements may be an auxiliary filtering system.The boundary pit membranes filtered out particles with meandiameters as small as 4.9 ± 0.7 nm, indicating a poresize one or two orders of magnitude smaller than most previousmeasurements for pit membranes, but close to pore sizes determinedfor hydrated primary cell walls. It is concluded that boundarypit membranes at branch-root junctions are efficient filtersfor microbes and particulates entering damaged branch roots.They would also restrict entry of air/water interfaces whenmain root xylem tension was less than approx. 100 MPa. Copyright2000 Annals of Botany Company Zea mays, air-seeding, branch-root junction, cryoSEM, embolisms, maize, pit membrane, pore size, xylem, water transport     

Water Retention Capacity in Root Segments Differing in the Degree of Exodermis Development

Water loss from roots back into drying soil is a problem ofpractical importance in plants growing under conditions of verylow substrate water potential, such as dry or saline areas.Root exodermis is relatively impermeable and has been suggestedto play a protective role against water loss. The relative waterretention ability was compared in root segments from exodermal(maize, onion, sunflower, Rhodes grass and sorghum) and non-exodermalspecies (Pisum sativum,Vicia fabaand wheat). Apical and basalsegments from exodermal roots, with different degrees of exodermisdevelopment, were also compared, as were segments from sorghumroots in which the exodermis thickness had been modified bysubjecting the plants to a 30 d water stress treatment. Waterretention was significantly higher in segments from exodermalroots. In each root, water loss was higher in apical than inbasal segments, regardless of the presence of exodermis. Insorghum, prolonged drought treatment increased exodermis thickeningin nodal roots, however, no differences in rates of water losswere observed in segments obtained from control and droughtedplants. Soil sheaths formed around roots of Rhodes grass growingin very dry soil with the epidermis adhering tightly to thesheath. In plants growing in the field, soil sheaths may bemore effective than the exodermis in preventing root water loss.Copyright1999 Annals of Botany Company. Root, exodermis, rhizosheaths, water loss.     

Does Xylem Sap ABA Control the Stomatal Behaviour of Water-Stressed Sycamore (Acer pseudoplatanus L.) Seedlings?

Sycamore seedlings were grown with their root systems dividedequally between two containers. Water was withheld from onecontainer while the other container was kept well-watered. Effectsof soil drying on stomatal behaviour, shoot water status, andabscisic acid (ABA) concentration in roots, xylem sap and leaveswere evaluated. At 3 d, root ABA in the drying container increased significantly,while the root ABA in the unstressed container of the same plantsdid not differ from that of the control. The increase in rootABA was associated with the increase in xylem sap ABA and withthe decrease in stomatal conductance without any significantperturbation in shoot water status. At 7 d, despite the continuous increase in root ABA concentration,xylem sap ABA showed a marked decline when soil water contentwas depleted below 013 g g^–1. This reduction in xylemsap ABA coincided with a partial recovery of stomatal conductance.The results indicate that xylem sap ABA is a function of rootABA as well as the flow rate of water from roots to shoots,and that this ABA can be a sensitive indicator to the shootof the effect of soil drying. Key words: Acer pseudoplatanus L., soil drying, stomatal behaviour, xylem sap ABA     

The Effect of Soil Compaction on Differentiation of Late Metaxylem in Common Bean (Phaseolus vulgaris L.)

Recent studies among several plant species have shown that maturationof the largest vessels in primary xylem of roots occurs muchlater than is commonly assumed. These results have importantimplications for studies of water and nutrient uptake sincethe condition of the vessels, termed late metaxylem (LMX), mighthave a large effect on the potential conductivity of the xylem.To determine whether this phenomenon occurred in common bean(Phaseolus vulgaris L.), patterns of root xylem differentiationwere studied in young bean plants. Soil bulk density was variedin one trial to determine whether differentiation of LMX wassensitive to the growing medium. Vessels of LMX lost cell contentsand-became functional conduits between 100 and 150 mm from theroot apex. Increasing soil bulk density caused the zone of maturationof LMX to shift toward the root apex, but this zone was nevercloser than 67 mm. In the region where the primary root increasedin diameter as it merged with the hypocotyl, a zone was foundwhere vessels increased in number, had a reduced diameter, andwere arranged in a ring, the normal tetrarch arrangement ofthe xylem being lost. Potential conductivity in this zone wasconsiderably less than in zones with conventional large LMXvessels, so the zone appears to present an important restrictionto water transport from the root to the shoot. Thus, while thephenomenon of late maturation of LMX occurs in common bean,its significance in transport of water from roots to shootsis unclear Phaseolus vulgaris L., common bean, metaxylem, soil compaction, roots, anatomy     

Rectifier-like Activities of Roots of Two Desert Succulents

Axial and radial water flows for roots in response to appliedhydrostatic pressure drops, water loss from roots after variousperiods of drying, and development of new roots after rewettingdroughted plants were examined for two sympatric desert succulents.Agave deserti Engelm. and Ferocactus acanthodes (Lemaire) Brittonand Rose. For a 40 kPa hydrostatic pressure drop applied to20 mm long root pieces, radial water flows from the epidermisto the root xylem were 2- to 5-fold greater at the tip thanat midlength and were much less than axial flows along the xylem.Upon drying detached roots in air at 20 °C and a water vapoursaturation deficit of 1.2 kPa (50% relative humidity), radialwater flow decreased more than 10-fold in 3–6 h, and couldrecover to the original level 6 h after rewetting. The rateof water loss from attached roots of plants dried in air at20 °C and a 1.2 kPa saturation deficit decreased about 200-foldin 72 h, which would greatly limit water loss from the plantto a drying soil. At 96 h after rewetting roots of A. desertithat had been exposed to air at 20 °C and a 1.2 kPa saturationdeficit for 120 h, rehydration of existing roots and developmentof new roots contributed about equally to water uptake by thewhole plant. In summary, roots of these desert succulents canreadily take up water from a wet soil but do not lose much waterto a dry soil, thus effectively acting like rectifiers withrespect to plant-soil water movement. Key words: Agave, Cactus, Drought, Root, Water flow, Xylem     

The Developmental Anatomy of the Root System in Yam Bean, Pachyrhizus erosus Urban

Investigations revealed that the anatomy of the primary radicularroot of yam bean (Pachyrhizus erosus L.) was typically dicotyledonousexcept that the xylem was not completely developed centripetally.Most of the roots had tetrarch xylem, although a few triarchand pentarch roots were also observed. In both tuberous andnon-tuberous roots, secondary thickening occurred by the formationof the meristematic vascular cambium which formed secondarytissues in a normal fashion. Subsequently, tuberization wasinitiated in the secondary xylem by the development of anomalous‘secondary’ cambia from parenchyma cells surroundingvessel elements. Anomalous ‘secondary’ cambia alsodeveloped from parenchyma cells not associated with vessels.Subsequently, anomalous ‘tertiary’ cambia differentiatedfrom tissues produced by the anomalous ‘secondary’cambia. Activities of these anomalous cambia resulted in theproduction of parenchyma storage cells and were chiefly responsiblefor the growth of the mature tuber. Pachyrhizus erosus L., yam bean, tuberous root, anatomy, anomalous ‘secondary’ cambia, anomalous ‘tertiary’ cambia, centripetal xylem development     

Anatomy of Seedling Roots of Tropical Maize (Zea mays L.) Cultivars at Low Water Supply

Establishment of maize seedlings can be difficult at low soilmoisture content. Anatomy of root metaxylem vessels may influencethe capacity for water transport and respective genotypic differencesmight be useful for selection purposes. To test this, six tropicalmaize (Zea mays L.) cultivars were grown in large PVC tubescontaining a sandy substrate at 5% (M5) and 10% (M10) moisturecontents for 2 weeks. The percentage changes in root diametersdue to M5 was similar for most cultivars but differed for mainroot types. Root diameters were not consistently related tometaxylem structure, but in a few cases, thin roots had smallerdiameter metaxylem vessels. The M5 treatment reduced the numberof late metaxylem vessels of primary roots by about 0 to 20%,while effects on nodal roots were slight. Generally, the ratioof cross-sectional areas between late and early metaxylem vesselsincreased from primary to seminal and nodal roots. Within thecultivar Tuxpefio this ratio was much reduced by M5. A few cultivarsmaintained the combined cross-sectional areas of metaxylem vesselsat M5 in some main root types, but only one cultivar could achievethis for the total of cross-sectional areas of metaxylem vessels,calculated over all root axes, by increasing the number of seminaland nodal roots. These anatomical traits seemed to be mostlyconstitutive with limited response to an actual environment,but they could be decisive for the suitability of a cultivarto an environment with frequent water shortages during seedlingestablishment. Key words: Metaxylem vessels, water stress, tropical maize     

Water Flow Through Cotton Roots in Relation to Xylem Anatomy

The effect of root anatomy on water flow was studied in 7-d-oldcotton (Gossypium hirsutum L.) seedlings grown in solution culture.The total water flux of the intact root system was measuredusing a pressure chamber. Then successive terminal root sectionswere removed at 2,6,10 and 12 cm behind the root tip and theflux was remeasured after each successive cut was made. Xylemdevelopment at different distances behind the root apex wasstudied with a microscope using sections cut free-hand and stainedwith toluidine blue. Water flux increased with the removal ofsuccessive terminal root sections and this coincided with thedegree of basipetal primary xylem development. The large increasein water flux at 10 to 12 cm was associated with secondary xylemdevelopment and increased xylem vessel number. A comparison of water flow and xylem anatomy between roots withtetrarch (Stoneville 506 and Deltapine 41) and pentarch (T25strain) vascular bundle arrangements showed no significant differencesin the measured values of water flux for the primary root. Waterflux, estimated using Poiseuille's equation and measured xylemdimensions, was greater for the tetrarch roots, primarily becauseof the larger diameter of individual vessel elements. The increasednumber of vessel elements in the pentarch primary root of T25did not result in any apparent decrease in axial resistanceto water flow. Key words: Gossypium hirsutum L., roots, vascular bundle, xylem water flux, xylem     

Effects of Temperature on the Development of Metaxylem in Primary Wheat Roots and Its Hydraulic Consequence

The effects of different temperatures on the development ofmetaxylem were studied in the primary seminal root of winterwheat (Triticum aestivum L.) seedlings. Xylem development wasstudied microscopically at different distances behind the rootapex after safranin staining to reveal lignification. Diameter of the central late metaxylem (LMX) and its proportionto the stele cross-sectional area increased in the acropetaldirection. Diameter of the LMX and stele decreased with an increasein growing temperature. Numbers of early metaxylem (EMX) wereseven, seven and six at 10, 20 and 30 C, respectively. EMXwas lignified much more rapidly than the LMX along the seminalroot axes. Lignification of xylem elements commenced furthertowards the root apex at the higher temperatures. The LMX vesselsof the roots grown at the higher temperature had thicker secondarywalls. The relative conductivity of seminal roots, calculated fromPoiseuille's equation, decreased as growing temperature increased.In a drought-prone environment where wheat plants rely heavilyon stored soil water, a lowered axial conductivity in the rootswould be advantageous. The plants would tend to conserve waterduring vegetative growth for use during the critical periodsof flowering and grain-filling. Breeders selecting wheat plants for altered LMX diameters shouldcontrol temperatures during primary root development, sectionthe roots at the same distance from the tip and be aware thatcross walls may exist in the LMX for up to 30 cm from the tip. Wheat, Triticum aestivum L., roots, xylem development, hydraulic conductivity, temperature     

Growth and Development of Roots of Grapevine (Vitis vinifera L.) in Relation to Water Uptake from Soil

Developmental patterns of lateral roots and their vascular differentiationwere investigated for Vitis vinifera L. cv. Shiraz to assessthe likely contribution of lateral roots to total water uptakeof plants subjected to different irrigation regimes. Correlationanalyses showed a significant positive correlation between mainroot diameter and the diameter of first order lateral rootsof well-watered plants, but in water-stressed plants the twowere not significantly correlated. The correlations betweendiameters of first order lateral roots and the diameters ofmain roots were greater than correlations between the lengthsof first order laterals and the diameters of main roots. Thesuberised surface area of well-watered main roots increasedfrom 4% of total surface area at 0·25 cm to 100% at 10cm from the tip, whereas that of stressed plants increased from15% at 0·25 cm to 100% at 5 cm from the tip. In all treatmentsthe highest linear density of first order laterals was about7 laterals cm^-1 of main root. More than 50% of first order lateralshad diameters less than 0·05 cm, and more than 90% ofthem had lengths less than 5 cm. Calculations of axial resistancesbased on xylem diameter measurements suggest that the axialresistances of root segments may not be uniform along rootsas is often assumed in models of water uptake. Water flow intothe main roots via the lateral root pathway is likely to bemuch smaller than that via the direct radial flow pathway asonly about 1% of surface area of main roots is directly occupiedby lateral roots, leaving the other 99% of main root surfacearea available for the direct radial flow pathway.Copyright1994, 1999 Academic Press Axial resistance, grapevine (Vitis vinifera L. cv. Shiraz) roots, root diameter, root length, xylem vessels     

Drought-induced changes in soil contact and hydraulic conductivity for roots of Opuntia ficus-indica with and without rhizosheaths

Water movement between roots and soil can be limited by incomplete root–soil contact, such as that caused by air gaps due to root shrinkage, and can also be influenced by rhizosheaths, composed of soil particles bound together by root exudates and root hairs. The possible occurrence of air gaps between the roots and the soil and their consequences for the hydraulic conductivity of the root–soil pathway were therefore investigated for the cactus t Opuntia ficus-indica, which has two distinct root regions: a younger, distal region where rhizosheaths occur, and an older, proximal region where roots are bare. Resin-embedded sections of roots in soil were examined microscopically to determine root–soil contact for container-grown plants kept moist for 21 days, kept moist and vibrated to eliminate air gaps, droughted for 21 days, or droughted and vibrated. During drought, roots shrank radially by 30% and root–soil contact in the bare root region of nonvibrated containers was reduced from 81% to 31%. For the sheathed region, the hydraulic conductivity of the rhizosheath was the least limiting factor and the root hydraulic conductivity was the most limiting; for the bare root region, the hydraulic conductivity of the soil was the least limiting factor and the hydraulic conductivity of the root–soil air gap was the most limiting. The rhizosheath, by virtually eliminating root–soil air gaps, facilitated water uptake in moist soil. In the bare root region, the extremely low hydraulic conductivity of the root–soil air gap during drought helped limit water loss from roots to a drier soil.     

A Study of Stelar Ultrastructure in the Heterosporous Water Fern Marsilea quadrifolia L

Sieve tube elements occur in the rhizomes and petioles of Marsileaquadrifolia. These are either thick walled with compound sieveplates in oblique end walls or thin walled with simple sieveplates in transverse end walls. Vessels are restricted to themetaxylem in the roots where the phloem contains sieve cellsonly. The sieve pores are invariably callose lined and as inother pteridophytes, excepting the Lycopsida, refractive spherulesare ubiquitous in the sieve elements of Marsilea. The luminaof the protoxylem tracheary elements in the rhizomes and petiolesare occluded by tyloses but probably remain functional in theroots. Pericycle cells backing on to the root protoxylem armspossess wall ingrowths. Transfer cells are however absent fromthe vascular tissue of the rhizomes and leaves. It is suggestedthat their presence in the root pericycle is related to theretrieval of ions from the xylem sap which may be particularlycritical in water plants. The incidence of transfer cells incryptogams appears to be far more sporadic than in angiosperms.The root endodermis of Marsilea possesses a casparian stripand abundant vacuolar tannin deposits. Plasmalemmasomes arenumerous adjacent to the pericycle transfer cells. vascular ultrastructure, Marsilea quadrifolia L, transfer cells, sieve tube elements, tyloses     

Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes

Sap flows in the xylem of plant roots in response to gradientsin water potential, either between soil and atmosphere (transpiration)or soil layers of different moisture content (termed hydraulicredistribution). The latter has the potential to influence waterbudgets and species interactions, but we lack information forall but a few plant communities. We combined heat pulse measurementsof sap flow with dye and isotope tracing techniques to gaugethe movement of xylem sap within, and exudation from, rootsof Banksia prionotes (Lindley). We demonstrated ‘ hydrauliclift’ during the dry season and provide some evidencethat extremely dry soils limit hydraulic lift. In addition wereport difficulties posed by spiralled xylem tissue in rootsfor the application of heat pulse techniques. Copyright 2000Annals of Botany Company Banksia prionotes, sap flow, hydraulic lift, heat ratio method, deuterium, stable isotopes, root architecture.     

Identification of maize silicon influx transporters

Maize (Zea mays L.) shows a high accumulation of silicon (Si),but transporters involved in the uptake and distribution havenot been identified. In the present study, we isolated two genes(ZmLsi1 and ZmLsi6), which are homologous to rice influx Sitransporter OsLsi1. Heterologous expression in Xenopus laevisoocytes showed that both ZmLsi1 and ZmLsi6 are permeable tosilicic acid. ZmLsi1 was mainly expressed in the roots. By contrast,ZmLsi6 was expressed more in the leaf sheaths and blades. Differentfrom OsLsi1, the expression level of both ZmLsi1 and ZmLsi6was unaffected by Si supply. Immunostaining showed that ZmLsi1was localized on the plasma membrane of the distal side of rootepidermal and hypodermal cells in the seminal and crown roots,and also in cortex cells in lateral roots. In the shoots, ZmLsi6was found in the xylem parenchyma cells that are adjacent tothe vessels in both leaf sheaths and leaf blades. ZmLsi6 inthe leaf sheaths and blades also exhibited polar localizationon the side facing towards the vessel. Taken together, it canbe concluded that ZmLsi1 is an influx transporter of Si, whichis responsible for the transport of Si from the external solutionto the root cells and that ZmLsi6 mainly functions as a Si transporterfor xylem unloading.     

Effects of Flooding and Drought on the Anatomy of Paspalum dilatatum

Paspalum dilatatum occupies different topographic positionsin the Flooding Pampa, Argentina. Populations from differentpositions are subjected to various regimes of flooding and drought,both of which may occur in the same growing season. We investigatedthe constitutive and plastic anatomical traits of P. dilatatumpopulations from habitats with contrasting regimes of floodingand drought. Both events affected root and sheath anatomy, andthese effects were similar for clones from different topographicpositions. Flooding increased the aerenchymatous tissue in theroot cortex and the leaf sheaths and decreased the number ofroot hairs per unit of root length. Drought decreased the diameterof root metaxylem vessels, thus lowering the risk of embolismsand increasing water-flow resistance, and increased the numberof root hairs, thereby increasing water uptake ability. In additionto these plastic responses, all clones showed constitutive characteristicsthat may confer an ability to withstand sudden events of floodingor drought: a high proportion of aerenchyma, which may maintainaeration before plastic responses take place; sclerenchyma,which may prevent root and leaf sheath collapse by soil compaction;and a conspicuous endodermis, which may protect stelar tissuesfrom desiccation. Both constitutive and plastic anatomical characteristicsare likely to contribute to the ability of this species to occupywidely different topographic positions and to resist temporalvariations in water and oxygen availability. Copyright 2001Annals of Botany Company Flooding, drought, aerenchyma, vessels, roots, leaf sheaths, anatomy, Paspalum dilatatum Poir     

The Vascular System of Maize Stems Revisited: Implications for Water Transport and Xylem Safety

The plexus of vascular bundles in the nodes of grasses is notoriouslycomplex, where long axial bundles pass through a network oftransverse bundles. The xylem pathways for water in maize stemshave been investigated anatomically and with dye and particulatetracers, revealing some of the details of this complexity. Onlyapprox. 3% of axial vessels pass through nodes without beinginterrupted by end walls. Axial bundles at nodes differ fromthose in internodes in having the metaxylem and protoxylem vesselsconnected by small tracheary elements. So it is only at nodesthat exchange of sap occurs between the large vessels withina bundle. End walls, acting as filters for particles and gasbubbles, always separate axial vessels from vessels in transversebundles. The high redundancy of bundle connections in the nodalplexus is interpreted as providing alternative water pathwaysto bypass embolisms and damaged or diseased sections of thexylem. The pores in the filters at the base of nodes and betweenaxial and transverse vessels within nodes are <20 nm in diameter.Where axial vessels connect to transverse vessels, a varietyof unusual shapes of vessel elements mediate two- and three-wayconnections within the plexus.Copyright 2000 Annals of BotanyCompany Zea mays, cryoSEM, maize, node, pits, pit membranes, vessel ends, vessels, xylem embolism, xylem pathogens