Imperforate Tracheary Elements Research Articles

The Vascular Cambia of Dodder and its Allies

The vascular cambia of 20 species of the dodder and its allies belonging to the CassythaceaE, Convolvulaceae and Cuscutaceae were surveyed to assess their phenetic and phylogenetic significance. In natural condition all of them contain anomalous cambia. The Cassythaceae bore transitory cambium in the normal position but atypical by asmmetric activity. (The Cambium alternatelt retards accemtuates xylem formation on several segments and there are profuse sehizongenous spaces in wood. Haustoria contains both traheids and vessels).In stems and roots of the Convolvulaceae a series of widely different anomalous cambia were found which sometimes differed in stems and roots of the same species. Stems of arborescent group had bands of interxylary phloem in concentric rings, while in the primitive twiners these are arranged in radial plates. In general, a tendency was to increase the proportion of soft tissue. In Cuscutaceae the specified anomaly was the interfascicular cambium. Ray initials were absent but fusiform initials differentiated into profuse parenchyma cells and a few vessel elements.Haustorium develops only imperforate tracheary elements. The evolutionary history vascular cambium in the Convolvulaceae was mainly one of reduction in activity and area. Thus the vesselless haustorium of Cuscutaceae. may be regarded as a reduced farm in with the vessel bearing haustorium of Cassythaceae. It 1s also concluded that the primitive condition of Cuscutaceae is possibly derived from the general condition of Convolvulaceae.

Evaluating Carlquist’s Law from a physiological perspective

Summary “Carlquist’s Law” is a striking pattern of association between anatomical features in the wood of vessel-bearing plants. It derives from Sherwin Carlquist’s observation that xylem vessels tend to be solitary when embedded in a matrix of imperforate tracheary elements that appear to be conductive, whereas xylem vessels tend to be grouped when surrounded by seemingly non-conductive cells. Vessel-vessel contacts (vessel grouping) allow water to travel between conduits, but also provide pathways for air to propagate from embolized (air-filled) vessels into functional vessels. If the background matrix is conductive, it is conceivable that water could bypass embolized vessels, providing an alternative transport route in species with conductive backgrounds and solitary vessels. Much remains to be tested in this hypothesis, including the topology of the vessel networks in species with solitary versus grouped vessels and how conductive the different imperforate tracheary element types are. Exploring Carlquist’s Law promises to provide key insight into the causes of embolism in plant conduits, the modes of embolism passage between conduits, and how vessels and the cells in which they are imbedded may interact to govern the pathways of water flow through plants.

Fire, frost, and drought constrain the structural diversity of wood within southern African Erica (Ericaceae)

Abstract Erica comprises ~860 species of evergreen shrubs and trees ranged from Europe to southern Africa and Madagascar. Wood structure of the around 20 European species is well studied, but despite its relevance to adaptation across the wider geographic range, it has not yet been explored across the much greater diversity, particularly of southern African lineages. In this study, we examine wood structure of 28 Erica species from southern Africa. In the African Erica clade, loss of scalariform perforation plates could be driven by increased aridity and seasonality in the mid-Miocene, and its re-gain can represent an adaptation to freezing in the high elevation species E. nubigena. As vessels in Erica are mostly solitary, imperforate tracheary elements probably form a subsidiary conduit network instead of vessel groups. Increase of ray frequency in habitats with a prominent dry and hot season probably facilitates refilling of vessels after embolism caused by water stress. Wider rays are ancestral for the lineage comprising African Erica and the Mediterranean E. australis. The negative correlation between ray width and expression of summer drought is consistent with Ojeda’s model explaining the diversification of seeders and resprouters among southern African Erica.

Testing Carlquistian hypotheses on the functional significance of vessel element length

Summary While total vessel length is widely recognized as being of fundamental functional significance, opinion is more divided regarding the potential functional importance of vessel element length, a variable that Sherwin Carlquist regarded as functionally significant. We show that vessel element length can, as Carlquist predicted, affect vessel resistance to deformation. Perforation plates are locally thickened annuli, so tubes with annuli resist deformation better than those without, and tubes with closely-spaced annuli resist deformation better than those with distantly spaced ones. However, there is a tradeoff between deformation resistance and conductance. With a comparative analysis across more than 1000 species of angiosperms, we show that both vessel element length and the areas of individual inter-conduit pits scale positively with vessel diameter. Such covariation is expected if plants are to maintain conductance as they grow taller. Congruent with Carlquist’s thinking, we found that species with vessel elements that are exceptionally short tend to grow in drylands, whereas those with vessel elements that are exceptionally long tend to grow in moist climates. Finally, we show evidence suggesting that selection on vessel element length is an important determinant of imperforate tracheary element length. Conversely, the evidence for selection on imperforate tracheary element length affecting vessel element length appears to be weaker. These results seem sufficient to establish that, whatever the functional importance of total vessel length, vessel element length is a variable of functional significance in its own right, congruent with Sherwin Carlquist’s long-held views.

Imperforate tracheary element classification for studies of xylem structure-function relations

Summary Discovering how xylem function emerges from xylem structure requires approaches that are distinct from those invented for wood identification. I exemplify this need for different approaches by discussing the classification of imperforate tracheary elements, the cells that go by names such as “fibers,” tracheids, fiber-tracheids, and libriform fibers. Currently, there are two classification systems, one for wood identification and one championed by Sherwin Carlquist for research on xylem function. I emphasize that neither is globally “correct” for all applications, but instead each is appropriate for the aims of its field. Wood identification uses an easily-applied classification of imperforate tracheary elements, designed to remain stable over the long term. This stability allows anatomists to build large databases of wood data that are maximally useful for wood identification. In contrast, functional xylem biologists need flexible definitions that serve as hypotheses of imperforate tracheary element function to be tested, modified, and tested again, in an open-ended process of refining knowledge of xylem structure-function relations. I highlight some of the many open questions that the functional classification points to, such as the morphological correlates of the conductive/nonconductive imperforate tracheary element distinction, how imperforate tracheary element features are associated with vessel grouping, whether the libriform fiber-fiber tracheid distinction is arbitrary or not, and other topics. Recognizing that the purpose of a functional classification is to drive research shifts focus away from debates about whether a given cell type classification is “correct” and onto the empirical priorities that need study. While there is a solid tradition within comparative wood anatomy of training in wood identification, there is a need to train functional comparative wood anatomists, who can guide and interact with xylem physiologists to build more robust explanations of xylem structure and function.

The role of imperforate tracheary elements and narrow vessels in wood capacitance of angiosperm trees

Summary There is a broad diversity of imperforate tracheary elements (ITEs) — libriform fibers, fiber-tracheids, true tracheids and vasicentric/vascular tracheids — described thoroughly by Sherwin Carlquist. However, in a quantitative sense, the functional meaning of different ITE types present in the wood of vessel-bearing angiosperms remains unclear because very few structure–function studies measure ITEs’ properties. ITEs with abundant pits and wide pit borders — vascular tracheids, vasicentric tracheids, and true tracheids sensu Carlquist — have been shown to conduct water and, thanks to this conductive ability and the multitude of pits, they could also contribute to wood capacitance. A dataset of 30 temperate angiosperm tree species was reanalysed to record the presence/absence of true, vasicentric, and vascular tracheids including data on conduits15 fraction and vessel-conduit15 contact fraction (conduits15 were defined as cells resembling vessels and with a maximum lumen diameter of 15 μm. They encompassed narrow vessels, vasicentric tracheids, and vessel tails). The presence of tracheids, conduits15 fraction, and contact fraction had no effect on wood capacitance, except, per given wood volumetric lumen water content, species with true tracheids tended to have lower capacitance. These results suggest that the presence of tracheids or conduits15 properties do not limit wood capacitance, but the results do not exclude the potential role these cells may play in internal water dynamics.

Parenchyma is not the sole site of storage: storage in living fibres

Summary Storage of nutrients and water are important functions of secondary xylem that have received much attention lately. In most of these studies, the storage role has been attributed to the fraction and arrangement of ray and axial parenchyma. However, in the current article, we show that in many species, especially those from tropical and subtropical regions (where most of the world’s plant species are found), nutrient and water storage is carried out by living imperforate tracheary elements (ITEs), colloquially termed “living fibres”. The occurrence of living fibres has been long recognized by anatomists, and especially emphasized in the work of Sherwin Carlquist. In spite of this, living fibres have remained largely unacknowledged by most plant physiologists and ecologists. To raise awareness about the existence of living fibres and to celebrate the illuminating work of Sherwin Carlquist, we summarize our current understanding of the structure, function, and occurrence of living fibres and emphasize that they should receive more attention when studying storage in wood.

The vessel wall thickness-vessel diameter relationship across woody angiosperms.

PremiseComparative anatomy is necessary to identify the extremes of combinations of functionally relevant structural traits, to ensure that physiological data cover xylem anatomical diversity adequately, and thus achieve a global understanding of xylem structure–function relations. A key trait relationship is that between xylem vessel diameter and wall thickness of both the single vessel and the double vessel+adjacent imperforate tracheary element (ITE).MethodsWe compiled a comparative data set with 1093 samples, 858 species, 350 genera, 86 families, and 33 orders. We used broken linear regression and an algorithm to explore changes in parameter values from linear regressions using subsets of the data set to identify a threshold, at 90‐µm vessel diameter, in the wall thickness–diameter relationship.ResultsBelow 90 µm diameter for vessels, virtually any wall thickness could be associated with virtually any diameter. Below this threshold, selection is free to favor a very wide array of combinations, such as very thick walls and narrow vessels in ITE‐free herbs, or very thin‐walled, wide vessels in evergreen dryland pioneers. Above 90 µm, there was a moderate positive relationship.ConclusionsOur analysis shows that the space of vessel wall thickness–diameter combinations is very wide, with selection apparently eliminating individuals with vessel walls “too thin” for their diameter. Most importantly, our survey revealed poorly studied plant hydraulic syndromes (functionally significant trait combinations). These data suggest that the full span of trait combinations, and thus the minimal set of hydraulic syndromes requiring study to span woody plant functional diversity adequately, remains to be documented.

Distinctive wood anatomy of early-diverging Asteraceae: Barnadesioideae

Abstract Asteraceae subfamily Barnadesioideae (ten genera, c. 90 species), confined to South America, are sister to the remainder of the family. The relative antiquity of the barnadesioids might lead one to expect that they contain more wood features plesiomorphic for the family, but only one character clearly falls in that category. Pits on imperforate tracheary elements are bordered (except for annuals), whereas simple pits occur in two related families, Calyceraceae (part) and Stylidiaceae (all that have been examined); in Goodeniaceae bordered pits only occur. By attaining fully bordered pits in Chuquiraga, the imperforate tracheary elements qualify as an apomorphy, ‘neotracheids’, valuable for resisting embolism formation in dry and cold South American habitats. Neotracheids are found also in Loricaria (Asteraceae: Inuleae), also from these habitats. Neotracheids, like plesiomorphic tracheids, are conductive, unlike fibre tracheids and libriform fibres. Other barnadesioid wood characters adapted to cold and drought include grouping of vessels, high vessel density, shorter vessel elements and helical sculpture (including helical thickenings on lumen-facing walls) of secondary xylem vessels. In Chuquiraga and Dasyphyllum, these helical thickenings are bordered in some species (new report for angiosperms). Some of the barnadesioid adaptations to cold and drought can be found in North American Artemisia spp. (Asteraceae: Anthemideae), especially in montane and desert areas. Wood features of barnadesioids match their respective habits and habitats: a few trees; shrubs of humid, dry or desert areas; a distinctive rhizomatous succulent in the pampas (Schlechtendalia); a scree/gravel perennial (Huarpea) and two genera of annuals, one with succulent leaves (Duseniella) and one with rayless (at least at first) stems in arid and open soils (Doniophyton). Diversity is unusual considering the small size of the subfamily. Examples of endodermal crystals (Arnaldoa only), pith sclereids and primary xylem fibres are cited.

Narrow vessels cavitate first during a simulated drought in Eucalyptus camaldulensis.

Establishing drying-limits for mortality of different tree species and understanding the anatomical and physiological traits involved is crucial to predict forests' responses to climate change. The xylem of Eucalyptus camaldulensis presents a complex of solitary vessels surrounded by different imperforate tracheary elements and parenchyma that influence, in a poorly known way, its hydraulic functioning. We aimed at describing the dynamics of embolism propagation in this type of xylem, seeking any vessel-size pattern, and unraveling the threshold of xylem embolism leading to nonrecovery after drought in E. camaldulensis. We assigned potted saplings to a protracted water-stress for 70 days. We relied on colorimetric and hydraulic methods to test for links between xylem anatomy and embolism propagation in the main stem. On average, the occurrence of embolism was randomly distributed in the stem xylem, but the probability of embolized vessels was higher than predicted by chance in the narrowest vessels of individuals that experienced low to moderate water-stress. The saplings could recover from severe water-stress if their percentage loss of conductance (PLC) was <77%, but not when the PLC was ˃ 85%. We concluded that, contrary to results reported for most species, the narrowest vessels are the most vulnerable to cavitation in E. camaldulensis, suggesting a lack of tradeoff between xylem efficiency and safety (in response to drought) at the tissue level. These results challenge the well-established paradigm of the effect of vessel size on cavitation, which states that the widest conduits are the most vulnerable to both freeze-thaw and drought-induced cavitation.

Wood anatomy of the crown lineages in Proteoideae (Proteaceae): implications for evolution and adaptive value of bordered pits in imperforate tracheary elements

Abstract As Proteaceae subfamily Proteoideae are richly represented in the Cape Floristic Region of southern Africa and in south-western Australia, it is a convenient group for comparative studies of evolution and adaptation of plants in widely separated areas with a Mediterranean-type climate. Although species of Proteoideae attract considerable attention of researchers, the structural diversity of wood in this group remains under-explored. The wood structure of 51 species of 14 African and Australian genera of Proteoideae (Proteaceae) has been studied. These taxa are uniform in their wood structure; the diversity patterns are more related to plant stature, climatic factors and fire-survival strategies than to the taxonomy or phylogeny of the subfamily. Increases in length and diameter of fibres and diameter of pits in fibre walls are associated with a shift from a semi-arid winter-dry climate to a milder climate with winter rainfall. These trends may imply the transition from non-conducting imperforate tracheary elements to conducting elements in Mediterranean-type climatic regions. This presumable gain of hydraulic function of imperforate tracheary elements could be explained by adaptation to freeze–thaw stress during the late Quaternary, at least in the Cape Region. Vessel grouping shows no correlations with climatic factors. The seeders have a higher vessel frequency and narrower and lower rays than sprouters. We hypothesize that the larger rays in overground stems of sprouters serve as the sites of epicormic buds that provide for resprouting after fire or other damage. The occurrence of sclerified cells and/or irregularly arranged (occasionally circular) tracheary elements in broad rays of some species are thought to be responses to excessive sucrose transport in secondary phloem.

Wood Anatomy of Argophyllaceae (Asterales): Adaptation in a Small Clade

Argophyllaceae (Argophyllum, 14 spp.; Corokia, 6 spp.; Lautea, 1 sp.), are shrubs that occur in the southwestern Pacific and eastern Australia. They occur in habitats where moisture is relatively common but dry days and mild frost may occur. The woods of these genera show enough distinctive features to justify their grouping in a single family: perforation plates with 10–20 bars, vessel elements narrow and numerous per mm2, imperforate tracheary elements about 50% longer than the vessel elements, axial parenchyma scarce, diffuse, multiseriate rays narrow and heterocellular (upright cells common in uniseriate rays), crystals absent, gum deposits common. These features group the genera of Argophyllaceae more closely with each other than with the nearest families in Asterales (Alseuosmiaceae, Phellinaceae). Probable apomorphies of the genera include helical thickenings in vessels and tracheids, together with abundant tracheids and rare septate fiber-tracheids (Corokia); almost total absence of axial parenchyma and tracheids combined with maximal abundance of septate fiber-tracheids and no helical thickenings (Argophyllum, Lautea). Lautea, formerly included within Corokia, has floral and foliar distinctions and is endemic to a single island, Rapa Iti. Woods of Argophyllaceae are alike in their ecological adaptations (perforation plates, vessel diameter and density) but the presence of tracheids and helical thickenings in Corokia suggest adaptations to frost and mild drought. As expected, vessels group more prominently in the tracheid-free species (Argophyllum, Lautea) but very little in the tracheid-rich genus Corokia.

Imperforate tracheary elements and vessels alleviate xylem tension under severe dehydration: insights from water release curves for excised twigs of three tree species.

PremiseWater stored in the xylem of woody plants is important for supporting the transpiration stream under prolonged drought, yet the source of stored water within the xylem during drought remains unclear. Insights into xylem water utilization during drought will uncover the adaptation strategies of the test species to stress.MethodsTo fill the existing knowledge gap, we excised twigs of Abies firma (Japanese fir, conifer), Cercidiphyllum japonicum (katsura tree, diffuse‐porous) and Quercus serrata (konara oak, ring‐porous) to quantify interspecific variation of water transfer in xylem corresponding with increasing cumulative water release (CWR) using micro x‐ray computed tomography and cryo‐SEM.ResultsFor all species studied, the main components of water storage within the operating range of water potential were not living cells but cavitation release and capillaries. Abies firma maintained water in the earlywood‐like cells, for possible maintenance of the transpiration stream. Cercidiphyllum japonicum maintained water in its vessels over 200 kg m‐3 of CWR, while Q. serrata lost most of its water in vessels with increasing CWR up to 100 kg m‐3. Cercidiphyllum japonicum exhibited a higher water storage capacity than Q. serrata. Under high CWR, narrow conduits stored xylem water in C. japonicum and imperforate tracheary elements in Q. serrata.ConclusionsAmong the species examined, increasing CWR appears to indicate differential utilization of stored water in relation to variation of xylem structure, thereby providing insight into the interspecific responses of tree species to drought.

Xylem vessel‐diameter–shoot‐length scaling: ecological significance of porosity types and other traits

AbstractFlowering plants predominantly conduct water in tubes known as vessels, with vessel diameter playing a crucial role in plant adaptation to climate and reactions to climate change. The importance of vessels makes it essential to understand how and why vessel diameter, plant height, and other ecological factors are interrelated. Although shoot length is by far the main driver of variation in mean vessel diameter across angiosperms, much remains to be understood regarding the factors accounting for the abundant variation around the y‐axis in plots of mean species vessel diameter against shoot length. Here, we explore the potential role of porosity types, wood density, leaf phenology, background imperforate tracheary element type, vasicentric tracheids, vascular tracheids, perforation plate type, and successive cambia in causing variation in the y‐intercept or slope of the mean species vessel‐diameter– and vessel‐density–shoot‐length associations at the shoot tip and base. We detected numerous cases of ecologically significant variation. For example, latewood vessels of ring porous species scale with a lower slope than earlywood, i.e., latewood vessels are relatively narrow in taller plants. This pattern is likely the result of selection favoring freezing‐induced embolism resistance via narrow vessels. Wood density was negatively associated with vessel diameter, with low wood density plants having wider vessels for a given height. Species with scalariform perforation plates scale with a lower shoot base vessel‐diameter–shoot‐length slope, likely reflecting selection against scalariform plates in wide vessels. In other cases, functional groups scaled similarly. For example, species with successive cambia did not differ from those with conventional single cambia in their mean vessel‐diameter–shoot‐length scaling, rejecting our prediction that species with successive cambia should have narrower vessels for a given shoot length. They did, however, have fewer vessels per unit shoot cross‐sectional area than plants of similar heights, likely because vessels have longer functional lifespans (and therefore are fewer) in species with successive cambia. Our methods illustrate how vessel diameter can be studied taking shoot length into account to detect ecologically important variation and construct theory regarding plant adaptation via the hydraulic system that includes plant size as a vital element.

Distinctive wood anatomy of the root-parasitic family Lennoaceae (Boraginales)

The four species of Lennoaceae have strands of primary plus secondary xylem in a background of starch-rich parenchyma. These strands constitute a cylinder with large primary rays. The wood within these strands is markedly different from that of other families in the crown group of Boraginales such as Cordiaceae and Ehretiaceae, most of which are woody. Lennoaceae differ because they lack fibrous cells (libriform fibers), lack rays within the vascular strands, and have markedly elliptical vessel-to-vessel pits without vestures. Lennoaceae have secondary xylem with short, wide vessel elements with thick walls, horizontally elongate elliptical pits, simple perforation plates much narrower than the vessel lumen; variously uneven vessel wall thickenings; and axial parenchyma. The wood of Lennoaceae shows resemblances to unrelated succulents such as Kalanchoe (Crassulaceae) and Lithops (Aizoaceae). The vessel features also suggest adaptation to high water tensions as root parasites in desert areas, whereas the lack of imperforate tracheary elements may relate to support of the underground stem portions by sand or rock detritus. Habit and ecology are more important in the architecture of lennoaceous xylem than systematic affinities. The four species of Lennoaceae differ from each other in minor xylary features.

Wood Anatomy of Brassicales: New Information, New Evolutionary Concepts

Wood anatomical data for the 19 families of Brassicales are presented, based on light microscopy and scanning electron microscopy (SEM), arranged according to recent molecular phylogenetic evidence. Because of large species numbers and diversity in ecology and growth form, Brassicales are an ideal case study group for understanding wood evolution. Features newly reported include vestured pits in Cleomaceae, Koeberliniaceae, Pentadiplandraceae, Salvadoraceae, and Setchellanthaceae. Vesturing of primary xylem helices is shown for Raphanus (first report in angiosperms). Fiber dimorphism is newly reported in some genera of the crown group (Capparaceae + Cleomaceae + Brassicaceae). The fiber-tracheid is probably the ancestral imperforate tracheary element type for Brassicales, and from it, libriform fibers, living fibers (including septate fibers), and tracheids have likely been derived. The Baileyan concept of unidirectional evolution from tracheids to libriform fibers must have many exceptions in angiosperms, and tracheids are not uniform. Tracheids occur in Emblingiaceae, Koeberliniaceae, Pentadiplandraceae, Stixaceae, and Tropaeolaceae. Synapomorphies can be identified, as in the Akaniaceae—Tropaeolaceae clade (rays of two sizes, living fibers, scalariform perforation remnants) and the Moringaceae-Caricaceae clade (ground tissue of wood composed of thin-walled fibers or similar parenchymatous cells). Wood of Brassicales is mostly not paedomorphic, although paedomorphic characters suggesting secondary woodiness occur within the families Brassicaceae (abundance of upright ray cells, raylessness), Caricaceae, Cleomaceae, and Moringaceae. Brassicales are probably ancestrally woody, and wood of Sapindales and Malvales has a number of key character states (plesiomorphies) like those in Brassicales, as would be predicted by current molecular phylogenies. Surveys of large taxonomic groupings, such as Brassicales, tend to yield more examples of homoplasies and apomorphies that can be interpreted in terms of adaptation and functional interlinkage (e.g., ray evolution paralleling imperforate tracheary element evolution). In turn, these features can be interpreted in terms of ecology (e.g., xeric habitats) and growth forms (e.g., tree succulents). The assemblages of wood character information in a reasonably well known order of angiosperms permits hypotheses about wood evolution in angiosperms as a whole. Some of the more important hypotheses presented include: (1), that evolution of wood (and other) characters is always progressive, with gene overlays (silencing, modification, etc.) and simultaneous changes in multiple features, so that ancestral conditions are never truly re-attained. (2). Not all characters are of equal value in water economy of any given plant; some (presence of tracheids) may supersede others, and xeromorphic characters can be arranged relative to each other in tiers, although various taxonomic groups have different rosters of conductive safety features. (3). Heterochrony (protracted juvenilism, accelerated adulthood) is extensively represented in angiosperms, and acts as an overlay that is a source of diversity that angiosperms have drawn on since their inception (probably as minimally woody plants). (4). There may be no “purely taxonomic” characters, because genes of an organism relate primarily to changes, ancient and new, that are of adaptive significance, although we may not be able to detect selective value, past or present. Although many families of Brassicales are small and represent occupancy of specialized or extreme habitats (Batis, Koeberlinia, Moringa), active speciation in Brassicaceae and Capparaceae is related to tolerance of drought and cold with mechanisms such as vestured pits, narrow vessels, and abbreviation in life cycle length.

Anatomical features associated with water transport in imperforate tracheary elements of vessel-bearing angiosperms.

Imperforate tracheary elements (ITEs) in wood of vessel-bearing angiosperms may or may not transport water. Despite the significance of hydraulic transport for defining ITE types, the combination of cell structure with water transport visualization in planta has received little attention. This study provides a quantitative analysis of structural features associated with the conductive vs. non-conductive nature of ITEs. Visualization of water transport was studied in 15 angiosperm species by dye injection and cryo-scanning electron microscopy. Structural features of ITEs were examined using light and electron microscopy. ITEs connected to each other by pit pairs with complete pit membranes contributed to water transport, while cells showing pit membranes with perforations up to 2 µm were hydraulically not functional. A close relationship was found between pit diameter and pit density, with both characters significantly higher in conductive than in non-conductive cells. In species with both conductive and non-conductive ITEs, a larger diameter was characteristic of the conductive cells. Water transport showed no apparent relationship with the length of ITEs and vessel grouping. The structure and density of pits between ITEs represent the main anatomical characters determining water transport. The pit membrane structure of ITEs provides a reliable, but practically challenging, criterion to determine their conductive status. It is suggested that the term tracheids should strictly be used for conductive ITEs, while fibre-tracheids and libriform fibres are non-conductive.

What is disjunctive xylem parenchyma? A case study of the African tropical hardwoodOkoubaka aubrevillei(Santalaceae)

The morphological variation and structure-function relationships of xylem parenchyma still remain open to discussion. We analyzed the three-dimensional structure of a poorly known type of xylem parenchyma with disjunctive walls in the tropical hardwood Okoubaka aubrevillei (Santalaceae). Disjunctive cells occurred among the apotracheal parenchyma cells and at connections between axial and ray parenchyma cells. The disjunctive cells were partly detached one from another, but their tubular structures connected them into a continuous network of axial and ray parenchyma. The connecting tubules had thick secondary walls and simple pits with plasmodesmata at the points where one cell contacted a tubule of another cell. The imperforate tracheary elements of the ground tissue were seven times longer than the axial parenchyma strands, a fact that supports a hypothesis that parenchyma cells develop disjunctive walls because they are pulled apart and partly separated during the intrusive growth of fibers. We discuss unresolved details of the formation of disjunctive cell walls and the possible biomechanical advantage of the wood with disjunctive parenchyma: the proportion of tissue that improves mechanical strength is increased by the intrusive elongation of fibers (thick-walled tracheids), whereas the symplastic continuum of the parenchyma is maintained through formation of disjunctive cells.

Non-random Vessel Distribution in Woods: Patterns, Modes, Diversity, Correlations

Vessel grouping is a form of non-random distribution that becomes functionally valuable when the background consists of non-conductive imperforate tracheary elements (fiber-tracheids and libriform fibers); ungrouped vessels, randomly placed, often occur in an all-tracheid background. Types of vessel grouping are described and illustrated: diagonal, tangential, radial, median radial bands, and growth rings. Other non-random distributions considered include degrees and kinds of cable construction, patchy vessel distributions, vessel displacement related to succulence, and patterns involving successive cambia. Non-random vessel distributions inevitably involve non-random placement of imperforate tracheary elements, so that a parallel set of mechanical adaptations is often simultaneously achieved. Correlations between various types of non-random vessel patterns and possible physiological factors are hypothesized. Most correlations involve enhanced conductive safety, but vessel distribution related to water and photosynthate storage, resistance to torsion, and increased longevity of xylem are cited. Non-randomness of vessels is a source of diversity in wood structure that can be achieved readily (as growth rings show) and polyphyletically. These modifications offer numerous ways in which wood histology can be repatterned for probable adaptations in conductive physiology, mechanical strength, and storage capability, perhaps by means of regulatory genes. Grouping of vessels into vascular bundles in primary xylem of stems and leaves in dicots is a form of non-randomness, and the significance of vascular bundles (as opposed to steles) as adaptive forms of organization is considered briefly. Monocots differ from dicots in rarely having division of labor in tracheary elements within an organ, but monocots exhibit tradeoffs in which conductive efficiency (vessel presence in an organ) and conductive safety (tracheids but no vessels in an organ) can be achieved within a single plant.

Wood anatomy and hydraulic architecture of stems and twigs of some Mediterranean trees and shrubs along a mesic-xeric gradient

Woody species populating the major Mediterranean ecosystems in the world are characterized by different levels of adaptation to the seasonal Mediterranean climate conditions. Many species of these ecosystems show wood features that allow high efficiency of transport when water is available, while maintaining hydraulic safety during drought periods. This study focuses on the anatomy of juvenile and mature wood of some species representative of continuous sequences of Mediterranean vegetation formations according to gradients of water availability, from xeric to relatively mesic: Cistus monspeliensis L., Rhamnus alaternus L., Myrtus communis L., Pistacia lentiscus L., Olea europaea L., Quercus ilex L., Fraxinus ornus L. and Ostrya carpinifolia L. Twigwood collected in Southern Italy was anatomically compared with the stemwood of the same species represented in the reference slide collection of the National Herbarium of the Netherlands (Lw). The “hydraulic distance” between the wood of main stems and twigs was estimated on the basis of suites of anatomical features related to water efficiency/safety. Although some attributes (i.e. porosity and type of imperforate tracheary elements) were similar in young twigs and older rings, other traits (i.e. vessel frequency and size) evidenced the different hydraulic properties of twig and stemwood. The difference between juvenile and mature structures was large in the species of the mesic end of the gradient while it was relatively small in those more xeric. This tendency is in agreement with the habit gradient from medium-sized trees to small evergreen/drought deciduous shrubs according to decreasing water availability in Mediterranean vegetation types.