Structure-function constraints of tracheid-based xylem: a comparison of conifers and ferns

The ferns comprise one of the most ancient tracheophytic plant lineages, and occupy habitats ranging from tundra to deserts and the equatorial tropics. Like their nearest relatives the conifers, modern ferns possess tracheid-based xylem but the structure-function relationships of fern xylem are poorly understood. Here, we sampled the fronds (megaphylls) of 16 species across the fern phylogeny, and examined the relationships among hydraulic transport, drought-induced cavitation resistance, the xylem anatomy of the stipe, and the gas-exchange response of the pinnae. For comparison, the results are presented alongside a similar suite of conifer data. Fern xylem is as resistant to cavitation as conifer xylem, but exhibits none of the hydraulic or structural trade-offs associated with resistance to cavitation. On a conduit diameter basis, fern xylem can exhibit greater hydraulic efficiency than conifer and angiosperm xylem. In ferns, wide and long tracheids compensate in part for the lack of secondary xylem and allow ferns to exhibit transport rates on a par with those of conifers. We suspect that it is the arrangement of the primary xylem, in addition to the intrinsic traits of the conduits themselves, that may help explain the broad range of cavitation resistance in ferns.     

The relationship between xylem conduit diameter and cavitation caused by freezing

The centrifuge method for measuring the resistance of xylem to cavitation by water stress was modified to also account for any additional cavitation that might occur from a freeze-thaw cycle. A strong correlation was found between cavitation by freezing and mean conduit diameter. On the one extreme, a tracheid-bearing conifer and diffuse-porous angiosperms with small-diameter vessels (mean diameter <30 μm) showed no freezing-induced cavitation under modest water stress (xylem pressure = −0.5 MPa), whereas species with larger diameter vessels (mean >40 μm) were nearly completely cavitated under the same conditions. Species with intermediate mean diameters (30–40 μm) showed partial cavitation by freezing. These results are consistent with a critical diameter of 44 μm at or above which cavitation would occur by a freeze–thaw cycle at −0.5 MPa. As expected, vulnerability to cavitation by freezing was correlated with the hydraulic conductivity per stem transverse area. The results confirm and extend previous reports that small-diameter conduits are relatively resistant to cavitation by freezing. It appears that the centrifuge method, modified to include freeze–thaw cycles, may be useful in separating the interactive effects of xylem pressure and freezing on cavitation.     

Water stress induced cavitation and embolism in some woody plants

A comparison was made of the relative vulnerability of xylem conduits to cavitation and embolism in three species [ Thuja occidentalis L., Tsuga canadensis (L.) Carr. and Acer saccharum Marsh.]. Waterlogged samples of wood were air dehydrated while measuring relative water loss, loss of hydraulic conductance, cumulative acoustic emissions (= cavitations) and xylem water potential. Most cavitation events and loss of hydraulic conductance occurred while water potential declined from – 1 to –6 MPa. There were differences in vulnerability between species. Other people have hypothesized that large xylem conduits (e.g. vessels) should be more vulnerable to cavitations than small conduits (e.g. tracheids). Our findings are contrary to this hypothesis. Under water stress, the vessel bearing wood retained water better than tracheid bearing wood. However, within a species large conduits were more prone to cavitation than small conduits.     

Hydraulic traits are influenced by phylogenetic history in the drought-resistant, invasive genus Juniperus (Cupressaceae)

In the conifer genus Juniperus (Cupressaceae), many species are increasing rapidly in distribution, abundance, and dominance in arid and semiarid regions. To help understand the success of junipers in drier habitats, we studied hydraulic traits associated with their water stress resistance, including vulnerability to xylem cavitation, specific conductivity (K(S)), tracheid diameter, conduit reinforcement, and wood density in stems and roots, as well as specific leaf area (SLA) of 14 species from the United States and the Caribbean. A new phylogeny based on DNA sequences tested the relationships between vulnerability to cavitation and other traits using both traditional cross-species correlations and independent contrast correlations. All species were moderately to highly resistant to water-stress-induced cavitation in both roots and shoots. We found strong phylogenetic support for two clades previously based on leaf margin serration (serrate and smooth). Species in the serrate clade were 34-39% more resistant to xylem cavitation in stems and roots than were species in the smooth clade and had ～35% lower K(S) and 39% lower SLA. Root and stem resistance to cavitation and SLA were all highly conserved traits. A high degree of conservation within clades suggests that hydraulic traits of Juniperus species strongly reflect phylogenetic history. The high resistance to cavitation observed may help explain the survival of junipers during recent extreme droughts in the southwestern United States and their expansion into arid habitats across the western and central United States.     

Inter-tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection

Plant xylem must balance efficient delivery of water to the canopy against protection from air entry into the conduits via air-seeding. We investigated the relationship between tracheid allometry, end wall pitting, safety from air-seeding, and the hydraulic efficiency of conifer wood in order to better understand the trade-offs between effective transport and protection against air entry. Root and stem wood were sampled from conifers belonging to the Pinaceae, Cupressaceae, Podocarpaceae, and Araucariaceae. Hydraulic resistivity of tracheids decreased with increasing tracheid diameter and width, with 64 ± 4% residing in the end wall pitting regardless of tracheid size or phylogenetic affinity. This end-wall percentage was consistent with a near-optimal scaling between tracheid diameter and length that minimized flow resistance for a given tracheid length. There was no evidence that tracheid size and hydraulic efficiency were constrained by the role of the pits in protecting against cavitation by air-seeding. An increase in pit area resistance with safety from cavitation was observed only for species of the northern hemisphere (Pinaceae and Cupressaceae), but this variable was independent of tracheid size, and the increase in pit resistance did not significantly influence tracheid resistance. In contrast to recent work on angiosperm vessels, protection against air-seeding in conifer tracheids appears to be uncoupled from conduit size and conducting efficiency.     

Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function

Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.     

散孔材与环孔材树种枝干、叶水力学特性的比较研究

为揭示散孔材与环孔材树种树木水分生理特性的差异,选取了常见的3种散孔材落叶树种(毛白杨、法国梧桐和樱花)和3种环孔材落叶树种(刺槐、合欢和白蜡),研究了其枝干与叶水力学性质的差异及其协调性。结果表明:3种环孔材树种枝干横截面积基础上的最大比导水率(Ks-max)大于3种散孔材树种,但其木质部对空穴化的脆弱性(P50branch)高于散孔材树种,6种树木枝干的水分传输能力和抵抗空穴化能力之间存在一种相互制约的权衡关系。3种散孔材与3种环孔材树种的叶最大水力导度(Kl-max)和水力脆弱性(P50leaf)并无显著差异;对于3种散孔材树种,叶的水力脆弱性要高于枝干,但对3种环孔材树种而言,枝干的水力脆弱性要高于叶。6种树木枝干和叶的水力学性质(Kmax、P50)之间并无相关关系。这些结果表明:散孔材与环孔材树种的枝干水力学特性有明显差异,但叶水力学特性无差异;枝干与叶水力学性质之间是相互独立的。     

Size and function in conifer tracheids and angiosperm vessels

The wide size range of conifer tracheids and angiosperm vessels has important consequences for function. In both conduit types, bigger is better for conducting efficiency. The gain in efficiency with size is maximized by the control of conduit shape, which balances end-wall and lumen resistances. Although vessels are an order of magnitude longer than tracheids of the same diameter, they are not necessarily more efficient because they lack the low end-wall resistance of tracheids with torus-margo pits. Instead, vessels gain conducting efficiency over tracheids by achieving wider maximum diameters. End-walls contributed 56-64% to total xylem resistance in both conduit types, indicating that length limits conducting efficiency. Tracheid dimensions may be more limited by unicellularity and the need to supply strength to homoxylous wood than by the need to protect against cavitation. In contrast, the greater size of the multicellular vessel is facilitated by fibers that strengthen heteroxylous wood. Vessel dimensions may be most limited by the need to restrict intervessel pitting and cavitation by air-seeding. Stressful habitats that promote narrow vessels should favor coexistence of conifers and angiosperms. The evolution of vessels in angiosperm wood may have required early angiosperms to survive a phase of mechanic and hydraulic instability.     

Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current‐year growth

Xylem vessel structure changes as trees grow and mature. Age‐ and development‐related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water‐stress‐induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current‐year growth of young (1–4 years), intermediate (2–7 years), and older (3–10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple‐aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.     

Vulnerability to xylem cavitation and the distribution of Sonoran Desert vegetation

We studied 15 riparian and upland Sonoran desert species to evaluate how the limitation of xylem pressure (Ψ(x)) by cavitation corresponded with plant distribution along a moisture gradient. Riparian species were obligate riparian trees (Fraxinus velutina, Populus fremontii, and Salix gooddingii), native shrubs (Baccharis spp.), and an exotic shrub (Tamarix ramosissima). Upland species were evergreen (Juniperus monosperma, Larrea tridentata), drought-deciduous (Ambrosia dumosa, Encelia farinosa, Fouquieria splendens, Cercidium microphyllum), and winter-deciduous (Acacia spp., Prosopis velutina) trees and shrubs. For each species, we measured the "vulnerability curve" of stem xylem, which shows the decrease in hydraulic conductance from cavitation as a function of Ψ(x) and the Ψ(crit) representing the pressure at complete loss of transport. We also measured minimum in situ Ψ(x)(Ψ(xmin)) during the summer drought. Species in desert upland sites were uniformly less vulnerable to cavitation and exhibited lower Ψ(xmin) than riparian species. Values of Ψ(crit) were correlated with minimum Ψ(x). Safety margins (Ψ(xmin)-Ψ(crit)) tended to increase with decreasing Ψ(xmin) and were small enough that the relatively vulnerable riparian species could not have conducted water at the Ψ(x) experienced in upland habitats (-4 to -10 MPa). Maintenance of positive safety margins in riparian and upland habitats was associated with minimal to no increase in stem cavitation during the summer drought. The absence of less vulnerable species from the riparian zone may have resulted in part from a weak but significant trade-off between decreasing vulnerability to cavitation and conducting efficiency. These data suggest that cavitation vulnerability limits plant distribution by defining maximum drought tolerance across habitats and influencing competitive ability of drought tolerant species in mesic habitats.     

Comparative community physiology: nonconvergence in water relations among three semi-arid shrub communities

Plant adaptations to the environment are limited, and therefore plants in similar environments may display similar functional and physiological traits, a pattern termed functional convergence. Evidence was examined for functional convergence among 28 evergreen woody shrubs from three plant communities of the semi-arid winter rainfall region of southern California. Both leaf and water relations traits were examined, including seasonal stomatal conductance (gs), specific leaf area (SLA), leaf specific conductivity (Kl), seasonal water potential (Psi w), stem cavitation resistance (Psi 50), and xylem density. Species display community-specific suites of xylem and leaf traits consistent with different patterns of water use among communities, with coastal sage scrub species utilizing shallow pulses of water, Mojave Desert scrub species relying on deeper water reserves, and chaparral species utilizing both shallow and deep moisture reserves. Communities displayed similar degrees of water stress, with a community-level minimum Psi w (Psi wmin) of c. -4.6 Mpa, similar to other arid communities. Pooled across sites, there was a strong correlation between Psi wmin and xylem density, suggesting that these traits are broadly related and predictive of one another. This comparative community physiology approach may be useful in testing hypotheses of functional convergence across structurally similar semi-arid communities.     

Hydraulic vulnerability, vessel refilling, and seasonal courses of stem water potential of Sorbus aucuparia L. and Sambucus nigra L.

Differences in the seasonal variation in stem water potential between the two shrub species Sorbus aucuparia and Sambucus nigra were related with their vulnerability to xylem cavitation. It was also demonstrated indirectly that the two species differ in the extent to which they reverse cavitation. Seasonal variation in stem water potential was investigated during three growing seasons with in situ stem psychrometers. Sorbus experienced wide water potential variations and reached a minimum of -4.2 MPa during drought. Under the same microclimatic conditions, Sambucus experienced consistent stem water potentials with a minimum of -1.7 MPa. The relationship between percentage loss in hydraulic conductivity (PLC) and water potential (hydraulic vulnerability curve) of the two species differed in shape: a flat curve with nearly total loss of conductivity at -6 MPa was found for SORBUS: Sambucus showed a steep vulnerability curve with 90% loss conductivity at -2.2 MPa. Thus, Sambucus is extremely vulnerable to cavitation, but Sorbus is an almost invulnerable species. This different cavitation resistance adjusted the ranges of field stem water potential that the species experienced. Finally, seasonal courses of naturally occurring (native) embolism were compared with calculated PLC courses. This comparison indicates that Sorbus did not refill embolized xylem vessels whereas Sambucus reversed embolism. It was concluded that species which are highly vulnerable to cavitation and drought-induced embolism need refilling of embolized vessels as well as isohydric water potential patterns as two strategies of survival.     

Comparative axial widening of phloem and xylem conduits in small woody plants

Key message

Along the stem axis phloem’s sieve elements increase in diameter basally at rates comparable to those of xylem conduits and in agreement with principles of hydraulic optimization.

Abstract

Plant physiology relies on the efficiency of the two long-distance transport systems of xylem and phloem. Xylem architecture comprises conduits of small dimensions towards the stem apex, where transpiration-induced tensions are the highest along the root-to-leaves hydraulic pathway, and widen basally to minimize the path length resistance to water flow. Instead, information on phloem anatomy and allometry is extremely scarce, although potentially relevant for the efficiency of sugar transportation. We measured the hydraulic diameter (Dh) of both xylem conduits and phloem sieve elements in parallel at different heights along the stem of a small tree of Picea abies, Fraxinus excelsior and Salix eleagnos. Dh increased from the stem apex to base in both xylem and phloem, with a higher scaling exponent (b) of sieve elements than that of tracheids in the conifer (0.19 vs. 0.14) and lower than that of vessels in the angiosperms (0.14–0.22 vs. 0.19–0.40). In addition, sieve elements were larger than tracheids in P. abies and narrower than angiosperms vessels at any height along the stem. In conclusion, axial conduit widening would seem to be a key feature of both xylem and phloem long-distance transport architectures.     

Xylem function of arid-land shrubs from California, USA: an ecological and evolutionary analysis

Xylem traits were examined among 22 arid-land shrub species, including measures of vessel dimensions and pit area. These structural measures were compared with the xylem functional traits of transport efficiency and safety from cavitation. The influence of evolution on trait relationships was examined using phylogenetic independent contrasts (PICs). A trade-off between xylem safety and efficiency was supported by a negative correlation between vessel dimensions and cavitation resistance. Pit area was correlated with cavitation resistance when cross species data were examined, but PICs suggest that these traits have evolved independently of one another. Differences in cavitation resistance that are not explained by pit area may be related to differences in pit membrane properties or the prevalence of tracheids, the latter of which may alter pit area through the addition of vessel-to-tracheid pits or through changes in xylem conduit connectivity. Some trait relationships were robust regardless of species ecology or evolutionary history. These trait relationships are likely to be the most valuable in predictive models that seek to examine anatomical and functional trait relationships among extant and fossil woods and include the relationship among hydraulic conductivity and vessel diameter, between vessel diameter and vessel length, and between hydraulic conductivity and wood density.     

Structural determinants of increased susceptibility to dehydration‐induced cavitation in post‐fire resprouting chaparral shrubs

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem‐specific hydraulic conductivity (K_s) and resistance to cavitation (P₅₀) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co‐occurring post‐fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in K_s. Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter‐vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants.     

Hydraulic conductance and vulnerability to cavitation in corn (Zea mays L.) hybrids of differing drought resistance

Water transport through the xylem is essential for replacing water loss during transpiration, thus preventing desiccation and permitting photosynthesis. The occurrence of cavitation and embolism due to drought impairs transport to the transpiring leaves. Most research in this discipline has been conducted on woody plants. Less attention has been given to cavitation of crops and its physiological significance for understanding crop water relations. In this paper, hydraulic conductance and vulnerability of xylem to cavitation were studied in corn hybrids with different drought resistances. The results indicated that stems of drought-resistant ‘Pioneer 3902’ not only had a higher conductivity on both a stem area and leaf area basis but also had a greater resistance to cavitation. The estimated xylem pressure at 63.2% loss of conductivity (Weibull fitting parameter b) and at 50% loss of conductivity (P₅₀) in ‘Pioneer 3902’ were about 0.2 MPa lower than in ‘Pride 5’. Higher conductivity in ‘Pioneer 3902’ was mainly attributed to more vascular bundles per stem area rather than greater vessel diameter. The central bundles and peripheral vascular bundles showed the same degree of cavitation although the vessels of central bundles were generally larger than in peripheral bundles.     

Relationships among xylem transport, biomechanics and storage in stems and roots of nine Rhamnaceae species of the California chaparral

Here, hypotheses about stem and root xylem structure and function were assessed by analyzing xylem in nine chaparral Rhamnaceae species. Traits characterizing xylem transport efficiency and safety, mechanical strength and storage were analyzed using linear regression, principal components analysis and phylogenetic independent contrasts (PICs). Stems showed a strong, positive correlation between xylem mechanical strength (xylem density and modulus of rupture) and xylem transport safety (resistance to cavitation and estimated vessel implosion resistance), and this was supported by PICs. Like stems, greater root cavitation resistance was correlated with greater vessel implosion resistance; however, unlike stems, root cavitation resistance was not correlated with xylem density and modulus of rupture. Also different from stems, roots displayed a trade-off between xylem transport safety from cavitation and xylem transport efficiency. Both stems and roots showed a trade-off between xylem transport safety and xylem storage of water and nutrients, respectively. Stems and roots differ in xylem structural and functional relationships, associated with differences in their local environment (air vs soil) and their primary functions.     

Duration and extension of anatomical changes in wood structure after cambial injury

Cambial injury has been reported to alter wood structure in broad-leaved trees. However, the duration and extension of associated anatomical changes have rarely been analysed thoroughly. A total of 18 young European ash (Fraxinus excelsior L.) trees injured on the stem by a spring flood were sampled with the aim of comparing earlywood vessels and rays formed prior to and after the scarring event. Anatomical and hydraulic parameters were measured in five successive rings over one-quarter of the stem circumference. The results demonstrate that mechanical damage induces a decrease in vessel lumen size (up to 77%) and an increase in vessel number (up to 475%) and ray number (up to 115%). The presence of more earlywood vessels and rays was observed over at least three years after stem scarring. By contrast, abnormally narrow earlywood vessels mainly developed in the first ring formed after the event, increasing the thickness-to-span ratio of vessels by 94% and reducing both xylem relative conductivity and the index for xylem vulnerability to cavitation by 54% and 32%, respectively. These vessels accumulated in radial groups in a 30° sector immediately adjacent to the wound, raising the vessel grouping index by 28%. The wound-induced anatomical changes in wood structure express the functional need of trees to improve xylem hydraulic safety and mechanical strength at the expense of water transport. Xylem hydraulic efficiency was restored in one year, while xylem mechanical reinforcement and resistance to cavitation and decay lasted over several years.     

Hardly a relict: freezing and the evolution of vesselless wood in Winteraceae

The Winteraceae are traditionally regarded as the least-specialized descendents of the first flowering plants, based largely on their lack of xylem vessels. Since vessels have been viewed as a key innovation for angiosperm diversification, Winteraceae have been portrayed as declining relicts, limited to wet forest habitats where their tracheid-based wood does not impose a significant hydraulic constraints. In contrast, phylogenetic analyses place Winteraceae among angiosperm clades with vessels, indicating that their vesselless wood is derived rather than primitive, whereas extension of the Winteraceae fossil record into the Early Cretaceous suggests a more complex ecological history than has been deduced from their current distribution. However, the selective regime and ecological events underlying the possible loss of vessels in Winteraceae have remained enigmatic. Here we examine the hypothesis that vessels were lost as an adaptation to freezing-prone environments in Winteraceae by measuring the responses of xylem water transport to freezing for a diverse group of Winteraceae taxa as compared to Canella winterana (Canellaceae, a close relative with vessels) and sympatric conifer taxa. We found that mean percent loss of xylem water transport capacity following freeze-thaw varied from 0% to 6% for Winteraceae species from freezing-prone temperate climates and approximately 20% in those taxa from tropical (nonfreezing) climates. Similarly, conifers exhibit almost no decrease in xylem hydraulic conductivity following freezing. In contrast, water transport in Canella stems is nearly 85% blocked after freeze-thaw. Although vessel-bearing wood of Canella possesses considerably greaterhydraulic capacity than Winteraceae, nearly 20% of xylem hydraulic conductance remains, a value that is comparable to the hydraulic capacity of vesselless Winteraceae xylem, if the proportion of hydraulic flow through vessels (modeled as ideal capillaries) is removed. Thus, the evolutionary removal of vessels may not necessarily require a deleterious shift to an ineffective vascular system. By integrating Winteraceae's phylogenetic relationships and fossil history with physiological and ecological observations, we suggest that, as ancestors of modern Winteraceae passed through temperate conditions present in Southern Gondwana during the Early Cretaceous, they were exposed to selective pressures against vessel-possession and returned to a vascular system relying on tracheids. These results suggest that the vesselless condition is advantageous in freezing-prone areas, which is supported by the strong bias in the ecological abundance of Winteraceae to wet temperate and tropical alpine habitats, rather than a retained feature from the first vesselless angiosperms. We believe that vesselless wood plays an important role in the ecological abundance of Winteraceae in Southern Hemisphere temperate environments by enabling the retention of leaves and photosynthesis in the face of frequent freeze-thaw events.     

Stem vascular architecture in the rattan palm Calamus (Arecaceae-Calamoideae-Calaminae)

Climbing stems in the rattan genus Calamus can reach lengths of well over 100 m, are long-lived, and yet their vascular tissue is entirely primary. Such a combination suggests that stem vasculature is efficient and resistant to hydraulic disruption. By means of an optical shuttle and video recording of sequential images we analyzed the stem of a cultivated species. The stem has vascular features that are unusual compared with those in arborescent palms and seemingly inefficient in terms of long-distance water transport. Axial bundles are discontinuous basally because leaf traces, when followed downwards, always end blindly below. Furthermore, there is no regular distal branching of each leaf trace at its level of departure into a leaf, so that neither a continuing axial bundle nor bridges to adjacent axial bundles are produced as in the standard palm construction. Instead, the axial bundles in the stem periphery are connected to leaf traces and to each other by narrow and irregular transverse or oblique commissures that are not the developmental homologues of bridges. As in other palms, metaxylem within a leaf trace is not continuous into the leaf so that the only connection to a leaf is via protoxylem. Within the stem, protoxylem (tracheids) and metaxylem (vessels) are never contiguous, unlike in other palms, which suggests that water can only move from metaxylem to protoxylem, and hence into the leaf, across a hydraulic resistance. We suggest that this minimizes cavitation of vessels and/or may be associated with an unknown mechanism that refills embolized vessels. Also, the metaxylem can be significant in stem water storage in the absence of abundant ground parenchyma.