Scalariform Perforation Plates Research Articles

Fossil woods of Cryptocarya (Lauraceae) from the middle Miocene of Southwest China

The structure of lauraceous fossil woods from the middle Miocene deposits of the Dajie Formation of Ninger County, Yunnan Province, China has been studied. The occurrence of wide (> 7 seriate) rays in combination with the presence of scalariform perforation plates, the oil/mucilage cells in rays, axial parenchyma and among fibers as well as some other traits allow us to attribute the fossil woods to the extant genus Cryptocarya. We describe the woods as a new species C. latiradiata R. Zhang, T. Su & A. A. Oskolski sp. nov. (Lauraceae). This is an important improvement in the fossil record of Cryptocarya showing that this genus was widely ranged across southern China in the middle Miocene. The occurrence of the fossil woods of such termophyllous genus as Cryptocarya in the Dajie Formation suggests that this middle Miocene climate in southern Yunnan was frostless. This result is consistent with other evidence for the warm environment caused by complex topographic structures in this region since the middle Miocene linked to the successive uplift of the Tibetan Plateau and the Ailao Mountains. Particularly, it suggests that the Ailao Mountains in the middle Miocene were high enough to weaken the influence of the Asian Winter Monsoon on the territory of Yunnan.

Numerical simulation and structure optimization for hydraulic performance of perforated drip irrigation emitters

ABSTRACT The perforated drip irrigation emitter (PDIE) is a novel drip irrigation emitter incorporating multiple energy dissipation mechanisms, chiefly hedging, deflection, and friction. To characterize the influences of structural parameters and vortices on the hydraulic performance of PDIE, the discharge exponent, discharge rate, and emitter unit constant were analyzed via hydraulic performance experiments and numerical simulation. The key findings are: (1) Discharge exponents of PDIE ranged from 0.478 to 0.515; average high-pressure hydraulic performance exceeded low pressure by 4.882%. (2) The four key structural parameters impacted hydraulic performance from large to small as follows: the width of the perforation (a), the angle of the scalariform perforation plate (α), the distance of the two perforations (b), and the length of the channel cavity (c). Among them, the discharge exponent changes significantly with a and α. (3) Vortex-retaining PDIE exhibited a lower emitter discharge exponent versus vortex-removed versions. However, vortices increase discharge rate and reduce energy consumption of PDIE. After optimization, the emitter discharge exponent of the PDIE with the reserved vortex is 0.461, which is relatively improved by 3.556%. This study can provide a theoretical reference for the structural design of new drip irrigation emitters and functional analysis of vortices.

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.

Water Transport Characteristics of Multiple Structures of Xylem Vessels in Magnolia

The multiple structures of xylem vessels in Magnolia provide stable and efficient water transport channels. The structural parameters of xylem vessels were studied in wood sections and in macerated materials. The results showed that the xylem vessels of Magnolia contained a helical thickening structure and a pit structure of a secondary wall, and the end walls had a scalariform perforation plate. The helical thickening and scalariform perforation plate increased the flow resistance of the vessel, and the pit structure decreased the flow resistance of the vessel. There was a close positive correlation between the flow resistance of the vessels and the helical width, the helical height, the thickness of the scalariform perforation plate, the number of holes in the scalariform perforation plate, the length of the pit canal, and the pit spacing. In addition, there was a negative correlation between the flow resistance of the vessels and the helical spacing, the pit vertical diameter, and the pit domain length. Among these structural parameters, the helical height, the number of holes, and the length of pit canal had a greater influence on the flow resistance. The pit structure caused the vessel to produce radial water transport. The radial transmission efficiency increased with the increase in the pit domain length. With no pit membrane in the pit structure of Magnolia, the radial transmission efficiency would be between 43.99% and 53.21%.

New dicotyledonous woods from the San Carlos Formation (Upper Cretaceous) in Northern Mexico

Summary We describe two new fossil woods from the San Carlos Formation (Upper Cretaceous), Chihuahua State, Mexico. The first wood resembles the fossil genus Metcalfeoxylon in having solitary vessels, scalariform perforation plates, vessel-ray parenchyma pits of similar size as the intervessel pits, axial parenchyma apotracheal diffuse and diffuse in aggregates, and heterocellular multiseriate rays with long, uniseriate tails. The second wood is a new fossil genus, and it is characterized by having diffuse porous wood, vessels predominantly solitary, vessel outlines oval and tending to be of two diameter classes, simple perforation plates, minute alternate intervessel pits, vessel-ray parenchyma pits similar to intervessel pits in size and shape, vasicentric tracheids, non-septate fibers, homocellular rays, and exclusively uniseriate and biseriate rays. This combination of features supports its placement in Myrtales (?Myrtaceae), in a new fossil-genus named Lazarocardenasoxylon. These two new records provide more information about the floristic composition of the Late Cretaceous flora of the San Carlos Formation and its relationship with those from the southern USA. However, a definitive picture of the floristic relationship of these Cretaceous floras of northern Mexico and southern USA remains elusive.

Anatomía de la madera de Magnolia rzedowskianaVázquez, Domínguez & Pedraza, especie endémica de la Sierra Madre Oriental, México

Introduction: Magnolia rzedowskianaVázquez, Domínguez & Pedraza is a tree species endemic to the cloud forest of the Sierra Madre Oriental; its scanty (distributed) and isolated populations are considered at danger of extinction.Objective: To describe the microscopic anatomical characteristics of M. rzedowskiana wood to contribute to the biological knowledge of this species.Materials and methods: Wood samples were determined by a non-destructive method, extracting two growth cores from five adult trees in each of the four sampling sites located in Chilijapa and Zotictla in Hidalgo, La Yesca in Querétaro and ejido La Trinidad in San Luis Potosí. Cores were sectioned into subsamples to elaborate fixed preparations from tangential, transversal and radial cuts. Characteristics were identified using an optical microscopy and described according to IAWA (International Association of Wood Anatomists) criteria.Results and discussion: Growth zones, diffuse porosity, solitary and multiple radial pores in intermediate arrangement between radial and tangential; scalariform perforation plate and libriform fibers; diffuse and scanty paratracheal and apotracheal axial parenchyma; biseriate rays combined with triserial, uniseriate and heterocellular rays composed of procumbent cells with a row above and below of marginal square cells with oil bodies. The wood is distinguished by mesomorphism and resistance to drought.Conclusion: The anatomical description of M. rzedowskiana is a contribution to biological knowledge. From an economic point of view, this wood has potential use to produce pulp for paper.

Anatomical adaptations in aquatic and wetland dicot plants: Disentangling the environmental, morphological and evolutionary signals

Understanding how plants adjust their internal structures to withstand adverse environmental conditions is vital for predicting their responses to ongoing environmental change. Plants have repeatedly evolved small water transporting conduits and large storage parenchyma tissues to cope with anoxia, freezing- or drought‐induced damages. However, how these adaptations evolved in unrelated taxa across hydrological and thermal gradients, remains unclear. Here we show that stem anatomical variations in 212 European aquatic and wetland dicots are driven by thermal and hydrological constraints via control over plant size, growth form, and leaf traits, while phylogenetic constraints have only a weak effect. Phylogenetic comparative analyses controlling for confounding factors showed that both waterlogging (anoxia) and low-temperature promote smaller plants with reduced vessel conduits and limited lignification, but extended parenchyma and hence storage and tissue renewal capacity to secure resilience to biomass loss induced by running water or frost disturbances. Decreasing water depth and anoxia promote larger wetland plants with thick-walled libriform fibers, large vessels with simple perforation plates securing high hydraulic efficiency, and semi-ring porous xylem with wide earlywood vessels in spring and narrow latewood vessels in summer, providing both efficiency and safety in water transport. The aquatic environment promotes plants with a large cortex zone with photosynthetic chlorenchyma and starch-storing parenchyma cells along with extensive air spaces that provide aeration and buoyancy. Low temperatures promote short-stature forbs with smaller vessels, scalariform perforation plate, extended parenchyma, resulting in reduced embolism risk. Although most anatomical variation was explained by differences between aquatic and wet terrestrial growth forms, environmental gradients, plant size, and leaf properties exerted a significant control on plant tissue structures not confounded by phylogenetic inertia. Distinct habitats, spread across broad thermal and hydrological gradients, harbor unrelated species with different evolutionary histories that have converged to similar anatomical and hence morphological structures.

Simulation and Verification of Hydraulic Performance and Energy Dissipation Mechanism of Perforated Drip Irrigation Emitters

Drip irrigation has become an application trend of water-saving irrigation technology due to its excellent water-use efficiency. However, the energy dissipation form of the commonly used labyrinth channel is relatively simple, and the corresponding energy dissipation mechanism research is inadequate. This article proposes a new kind of channel structure of drip irrigation emitters based on the structure of scalariform perforation plates in plant xylem vessels. We establish a total of 16 sets of orthogonal structure schemes. Using numerical simulation and physical experiments, the hydraulic performance and energy dissipation mechanism of the perforated drip irrigation emitters (PDIE) are studied. The results show that the flow index of PDIE is 0.4665–0.5266. The hydraulic performance of PDIE in the high-pressure zone is the best, and the flow index is 0.4665–0.5046. As the pressure increases, the velocity of the flow of the upper perforation increases rapidly, the flow ratio decreases, the flow index decreases, and the hydraulic performance improves. To further verify the energy dissipation mechanism, a lower flow ratio and a better hydraulic performance were obtained through appropriately expanding the upper part of the upper perforation inlet to the channel boundary. The research sheds new insights for optimizing the hydraulic performance of PDIE. Results reported here provide a theoretical basis for the structural design of drip irrigation emitters and the energy dissipation mechanism research.

From Carlquist's ecological wood anatomy to Carlquist's Law: why comparative anatomy is crucial for functional xylem biology.

All students of xylem structure-function relations need to be familiar with the work of Sherwin Carlquist. He studies xylem through the lens of the comparative method, which uses the appearance of similar anatomical features under similar conditions of natural selection to infer function. "Function" in biology implies adaptation; maximally supported adaptation inferences require experimental and comparative xylem scientists to work with one another. Engaging with comparative inferences of xylem function will, more likely sooner rather than later, bring one to the work of Sherwin Carlquist. To mark his 90th birthday, I highlight just a few examples of his extraordinarily perceptive and general comparative insights. One is "Carlquist's Law", the pervasive tendency for vessels to be solitary when background cells are conductive. I cover his pioneering of "ecological" wood anatomy, viewing xylem variation as reflecting the effects of selection across climate and habit variation. Another is the embolism vulnerability-conduit diameter relationship, one of the most widely invoked structure-function relationships in xylem biology. I discuss the inferential richness within the notion of Carlquistian paedomorphosis, including detailed functional inferences regarding ray cell orientation. My final example comes from his very recent work offering the first satisfactory hypothesis accounting for the geographical and histological distribution of scalariform perforation plates as an adaptation, including "Carlquist's Ratchet", why scalariform plates are adaptive but do not re-evolve once lost. This extraordinarily rich production over six decades is filled with comparative inferences that should keep students of xylem function busy testing for decades to come.

Comparative wood anatomy ofAstropanaxandNeocussonia, an Afro-Malagasy lineage of Araliaceae

AbstractAstropanax and Neocussonia are two recently resurrected genera of Araliaceae that had long been considered as an Afro-Malagasy lineage of Schefflera. The wood structure of 11 Neocussonia spp. and eight Astropanax spp. has been studied. Neocussonia shows a higher average length of vessel elements (1319 µm) and number of bars on perforation plates (up to 66) than any other Araliaceae examined to date. Neocussonia is distinct from Astropanax by its smaller diameter and higher frequency of vessels, rare occurrence of simple perforation plates, more numerous bars on scalariform perforation plates and smaller intervessel pits. The interspecific variation in percentage of simple perforation plates, bar number on scalariform perforation plates, vessel frequency and uniseriate ray number is affected by seasonality in temperature and precipitation. The sharp distinction in wood structure between samples of A. abyssinicus from Cameroon and Burundi suggests that the populations from different parts of the disjunct range represent two different species. Our data also suggest that A. goetzenii is an artificial group. Astropanax myrianthus from Madagascar and A. polysciadus from continental Africa, two cryptic species that have long been included in Schefflera myriantha, show significant differences in vessel size and grouping, percentage of simple perforation plates and intervessel pit size.

Comparative wood anatomy of Korean Viburnum L. (Adoxaceae) and its taxonomic implication.

Comparative wood anatomy of Viburnum was carried out to understand the differences in wood features amongst the species which might be useful for taxonomic discrimination in the genus. Altogether, nine taxa belonging to five clades were investigated using a sliding microtome and light microscopy. The growth rings are well represented and earlywood and latewood are distinguishable in cross-section. Some of the important wood features include angular, oval and rounded vessels with scalariform perforation plates, opposite to scalariform inter-vessel pitting, rounded pits with slit-like apertures, thick-walled xylem tracheids with simple, rounded bordered pits, diffuse axial parenchyma, uni- and multiseriate rays, 2–4 cells wide. In general, there is a remarkable uniformity in the qualitative wood features in Viburnum species, although quantitative measurement showed some disparities. The most significant quantitative wood variables which might be useful for taxonomic groupings of the species comprise a frequency of vessels and rays, the diameter of the vessels and tracheids in the radial and tangential planes and height and width of rays in the tangential plane.

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 they‐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 they‐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.

Discerning the Difference Between Lumens and Scalariform Perforation Plates in Impeding Water Flow in Single Xylem Vessels and Vessel Networks in Cotton.

The geometrical structure and spatial arrangement of lumens, bordered pits, and scalariform perforation plates in xylem vessels modulate water flow from roots to leaves. Understanding their respective hydraulic functions is essential to unveil how plants regulate their hydraulic networks to facilitate the ascent of sap under biotic and abiotic stresses but is challenging because of the opaque nature of the vessel networks and water flow within them. We made the first-ever effort to discern the difference between lumens and scalariform perforation plates in cotton in impeding water flow in single vessels and vessel networks using X-ray tomography and pore-scale numerical simulation. Three-dimensional structures of xylem vessels in the stem of two cotton cultivars were acquired non-invasively using X-ray computed tomography (CT) at high spatial resolution, and a lattice Boltzmann model was developed to simulate water flow through the xylem networks at micrometer scale. The detailed water velocity and pressure simulated using the model were used to calculate the hydraulic resistance caused by the lumens and the scalariform perforation plates in individual vessels and the vessel networks of the two cotton cultivars. The results showed that the hydraulic resistance spiked whenever water flowed across a perforation plate and that the overall hydraulic resistance caused by the perforation plates in an individual vessel accounted for approximately 54% of the total resistance of the vessel. We also calculated the hydraulic conductance of individual vessels and vessel networks using the simulated water velocity and pressure at micrometer scale and compared it with those estimated from the Hagen Poiseuille (HP) equation as commonly used in the literature by approximating the xylem vessels in the cotton as isolated tubes. While it was found that the HP equation overestimated the hydraulic conductance by more than 200%, the overestimate was largely due to the incapability of the HP equation to represent the perforation plates rather than its approximation of the irregular vessels by circular tubes.

Functional anatomy and xylem cavitation resistance of three species of monocotyledons grown on flooded substrates.

There are few investigations that analyze the xylem functional anatomy of monocotyledons, as the methods have been developed for woody plants. This study describes the root, rhizome and aerial stem xylem anatomy and functional anatomy of Canna indica, Cyperus papyrus and Phragmites communis grown on flooded substrates; and it aims to evaluate the relationship between the xylem anatomy and its cavitation resistance. To calculate the indexes of vulnerability, mesomorphy, collapse and relative hydraulic conductivity in the three organs mentioned, the diameter, number of vessels per mm2 , thickness of the walls and the length of the tracheal elements were recorded. In addition, the xylem specific conductivity of the aerial stem was measured with the pipette method, and its resistance to cavitation was determined experimentally by the air injection technique. The protoxylem is xeromorphic, it has longer vessel elements, smaller diameters, thin walls and scalariform perforation plates, whereas the metaxylem is mesomorphic, with shorter vessel elements, larger diameters, thicker walls and simple perforation plates. Both present low collapse resistance but have a high relative hydraulic conductivity. P. communis recorded the highest cavitation resistance, and the number of vessels per mm2 was related to xylem cavitation resistance in Canna indica and Cyperus papyrus. The experimental results of this investigation match partially the anatomical indexes and showed that the xylem of these species has a low specific conductivity and is more vulnerable to cavitation than that of other monocots.

KARAKTERISTIK STRUKTUR ANATOMI DINDING SEL JABON MERAH (Anthocephalus macrophyllus) PROVENANSI WAJO

Characteristics of anatomy structure cell walls can be used as a reference to determine the quality of wood, especially in the final product. Variability anatomy of wood is varies greatly between species, parts of the stem and place of growth and provenance. For the purpose of the study, teak wood samples from Wajo provenance. This study aims to identify the structure anatomy characteristics cell wall of jabon merah from Wajo provenance. The structure anatomy cell wall observed were a microfibril angle (MFA), a pit in cell wall, perforation plate, and tylosis. Results showed that the structure anatomy cell wall characteristics jabon merah from Wajo provenance had a MFA values of 22.21°,intervessel pits polygonal alternating, scalariform perforation plates, and have a tylosis in the vessel.

Testing the benefits of early vessel evolution.

This article comments on: Trueba S, Delzon S, Isnard S, and Lens F. 2019. Similar hydraulic efficiency and safety across vesselless angiosperms and vessel-bearing species with scalariform perforation plates. Journal of Experimental Botany 70, 3227–3240.

Similar hydraulic efficiency and safety across vesselless angiosperms and vessel-bearing species with scalariform perforation plates.

The evolution of xylem vessels from tracheids is put forward as a key innovation that boosted hydraulic conductivity and photosynthetic capacities in angiosperms. Yet, the role of xylem anatomy and interconduit pits in hydraulic performance across vesselless and vessel-bearing angiosperms is incompletely known, and there is a lack of functional comparisons of ultrastructural pits between species with different conduit types. We assessed xylem hydraulic conductivity and vulnerability to drought-induced embolism in 12 rain forest species from New Caledonia, including five vesselless species, and seven vessel-bearing species with scalariform perforation plates. We measured xylem conduit traits, along with ultrastructural features of the interconduit pits, to assess the relationships between conduit traits and hydraulic efficiency and safety. In spite of major differences in conduit diameter, conduit density, and the presence/absence of perforation plates, the species studied showed similar hydraulic conductivity and vulnerability to drought-induced embolism, indicating functional similarity between both types of conduits. Interconduit pit membrane thickness (Tm) was the only measured anatomical feature that showed a relationship to significant vulnerability to embolism. Our results suggest that the incidence of drought in rain forest ecosystems can have similar effects on species bearing water-conducting cells with different morphologies.

Early angiosperm woods from the mid-Cretaceous (Turonian) of New Mexico, USA: Paraphyllanthoxylon, two new taxa, and unusual preservation

The fossil record of Cretaceous angiosperm wood is skewed toward the latest part of the period; most described taxa are based on specimens from Campanian or Maastrichtian sediments. The low percentage of pre-Campanian angiosperm woods relative to other flowering plant organs may reflect a taphonomic bias or the existence of relatively few woody angiosperm taxa until the last part of the Cretaceous. The discovery of three fossil angiosperm wood taxa in the Turonian Moreno Hill Formation of New Mexico offers additional data on the occurrence of secondary xylem in early angiosperms. These wood fossils represent a common Cretaceous wood taxon plus two new angiosperm wood types, and increase the number of known pre-Campanian wood types by 10–20%.Analyses of thin sections from a large (>50 cm diameter) silicified log at a locality in the lower Moreno Hill Formation reveal it is Paraphyllanthoxylon arizonense Bailey, a wood taxon known from Cenomanian and Maastrichtian to Paleocene sites in the American Southwest. Paraphyllanthoxylon arizonense represents large trees that may belong to Laurales. Several other sizeable logs in the same area are also likely to be P. arizonense. In contrast, two taxa from a stratigraphically higher site in the Moreno Hill have not been previously described and are each represented by only one specimen. These two new wood types, based upon small, phosphatic axes (5–7 cm in diameter), differ from Paraphyllanthoxylon in their smaller diameter vessels and scalariform perforation plates. The unique combinations of character states of these phosphatic specimens indicate that they are new genera. Although the taxonomic affinities of Herendeenoxylon zuniense gen. et sp. nov. are uncertain, it is possible that it belongs to the Ericales. The affinities of the other new wood type, Vasunum cretaceum gen. et sp. nov. are unknown.The presence of three angiosperm wood taxa in the Moreno Hill Formation is noteworthy because exposures of terrestrial Turonian deposits are uncommon. The large diameter and apparent abundance of P. arizonense in the lower member of the Moreno Hill Formation suggest that these trees were dominant members of woodland or forest habitats of the ancient coastal lowlands. The small diameters and scarcity of the other two wood types suggest that they came from shrubs or small trees that were not common.

Vestured pits and scalariform perforation plate morphology modify the relationships between angiosperm vessel diameter, climate and maximum plant height.

Shared ancestry among species and correlation between vessel diameter and plant height can obscure the mechanisms linking vessel diameter to current climate distributions of angiosperms. Because wood is complex, various traits may interact to influence vessel function. Specifically, pit vesturing (lignified cell wall protuberances associated with bordered pits) and perforation plate morphology could alter the relationships between vessel diameter, climate and plant height. Using phylogenetically informed analyses, we tested for associations between vessel diameter, climate and maximum plant height across angiosperm species with different pit vesturing (presence/absence) and perforation plate morphology (simple/scalariform and quantitative variation). We show significantly larger changes in vessel diameter and maximum plant height across climates for species with vestures and simple perforation plates, compared to nonvestured species and those with scalariform plates. We also found a significantly greater increase in height for a given increase in vessel diameter with lower percentage of scalariform plates. Our study provides novel insights into the evolution of angiosperm xylem by showing that vessel pit vesturing and perforation plate morphologies can modify relationships among xylem vessels, climate and height. Our findings highlight the complexity of xylem adaptations to climate, substantiating an integrative view of xylem function in the study of wood evolution.

Living Cells in Wood 3. Overview; Functional Anatomy of the Parenchyma Network

The very different evolutionary pathways of conifers and angiosperms are very informative precisely because their wood anatomy is so different. New information from anatomy, comparative wood physiology, and comparative ultrastructure can be combined to provide evidence for the role of axial and ray parenchyma in the two groups. Gnetales, which are essentially conifers with vessels, have evolved parallel to angiosperms and show us the value of multiseriate rays and axial parenchyma in a vessel-bearing wood. Gnetales also force us to re-examine optimum anatomical solutions to conduction in vesselless gymnosperms. Axial parenchyma in vessel-bearing woods has diversified to take prominent roles in storage of water and carbohydrates as well as maintenance of conduction in vessels. Axial parenchyma, along with other modifications, has superseded scalariform perforation plates as a safety mechanism and permitted angiosperms to succeed in more seasonal habitats. This diversification has required connection to rays, which have concomitantly become larger and more diverse, acting as pathways for photosynthate passage and storage. Modes of growth such as rapid flushing, vernal leafing-out, drought deciduousness and support of large leaf surfaces become possible, advantaging angiosperms over conifers in various ways. Prominent tracheid-ray pitting (conifers) and axial parenchyma/ray pitting to vessels (angiosperms) are evidence of release of photosynthates into conductive cells; in angiosperms, this system has permitted vessels to survive hydrologic stresses and function in more seasonal habitats. Flow in ray and axial parenchyma cells, suggested by greater length/width ratios of component cells, is confirmed by pitting on end walls of elongate cells: pits are greater in area, more densely placed, and are often bordered. Bordered pit areas and densities on living cells, like those on tracheids and vessels, represent maximal contact areas between cells while minimizing loss of wall strength. Storage cells in rays can be distinguished from flow cells by size and shape, by fewer and smaller pits and by contents. By lacking secondary walls, the entire surfaces of phloem ray and axial phloem parenchyma become conducting areas across which sugars can be translocated. The intercontinuous network of axial parenchyma and ray parenchyma in woods is confirmed; there are no “isolated” living cells in wood when three-dimensional studies are made. Water storage in living cells is reported anatomically and also in the form of percentile quantitative data which reveal degrees and kinds of succulence in angiosperm woods, and norms for “typically woody” species. The diversity in angiosperm axial and ray parenchyma is presented as a series of probable optimal solutions to diverse types of ecology, growth form, and physiology. The numerous homoplasies in these anatomical modes are seen as the informative results of natural experiments and should be considered as evidence along with experimental evidence. Elliptical shape of rays seems governed by mechanical considerations; unusually long (vertically) rays represent a tradeoff in favor of flexibility versus strength. Protracted juvenilism (paedomorphosis) features redirection of flow from horizontal to vertical by means of rays composed predominantly or wholly of upright cells, and the reasons for this anatomical strategy are sought. Protracted juvenilism, still little appreciated, occurs in a sizeable proportion of the world’s plants and is a major source of angiosperm diversification.