Biomechanics and development of the climbing habit in two species of the South American palm genus Desmoncus (Arecaceae)

Mechanical properties are investigated in Desmoncus orthacanthos and D. polyacanthos from French Guiana, South America. Differences in size and axis stiffness are related to different trellis requirements and habitats. The leaf sheath surrounds the stem, increasing stiffness of young self-supporting stages and apical parts of older climbing plants. Senescence of the leaf sheath reduces stiffness of older climbing axes of both species. Its eventual loss in D. orthacanthos facilitates deformation into coils and loops when plants slip from their supports following senescence of leaves bearing attachment organs. In smaller climbing axes of D. polyacanthos, the senescent leaf sheath remains attached and axes rarely form loops and coils below attachment. An increase in stiff mechanical properties toward the base of both species is radically different from that of many dicotyledonous lianas. Besides the presence of attachment organs, stem mechanical properties of Desmoncus are similar to those of erect though not fully self-supporting stems of Bactris major, a sympatric species of the sister group genus to Desmoncus. The climbing habit in Desmoncus may have evolved via (1) heterochronic processes including early elongation of internodes relative to increase in stem diameter (reduction of the establishment phase) and (2) increased persistence of leaf sheaths.     

Biomechanics and development of rattans: what is special about Plectocomia himalayana Griff. (Calamoideae,Plectocomiinae)?

Mechanical and morphological studies of Plectocomia himalayana (subtribe Plectocomiinae) revealed characteristics that differ strongly from species of subtribe Calaminae ( Calamus and Daemonorops ). In species of Calaminae tested previously, the contribution of the leaf sheath drastically increases stiffness in juvenile axes and towards the apex of older plants. In P. himalayana the relative contribution of the leaf sheath to axis stiffness is less and leaf sheath senescence does not strongly reduce axial stiffness as observed in Calamus and Daemonorops . Natural aerial branching, only described in Korthalsia and Laccosperma among rattans, is common in P. himalayana . Aerial branching and adventitious roots occur frequently along old stems allowing autonomy of stems, following mechanical injury and promoting vegetative propagation. The climbing habit is known to have evolved at least twice within the Calamoideae. The results observed here suggest that climbing habits may differ in detail and that different 'climbing strategies' may have evolved within the subfamily Calamoideae resulting from: (1) variable stem flexibility, (2) the variable mechanical role of the leaf sheath ( Calamus–Daemonorops ) and (3) production of branches and aerial roots conferring a higher degree of architectural plasticity ( Plectocomia ).  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 83–91.     

The climbing habit in palms: Biomechanics of the cirrus and flagellum

Climbing palms in the Arecoideae (Desmoncus) and Calamoideae (rattan palms) both evolved cirrate leaves armed with hooks and grapnels for climbing. Some species of Calamoideae develop a different climbing organ known as the flagellum, which also bears hooks. The present study indicates that geometry and mechanical properties of the cirrus vary between species. Cirrate leaves are constructed to optimize bending and torsion in relation to the deployment of recurved hooks. Hook development, size, and strength vary along cirri and flagella and are consistent with observations of these attachment organs functioning as a ratchet mechanism: hooks increase in strength toward the base of attachment organs and always fail before the axis in strength tests. Hook size and strength differ between species and are related to body size and ecological preference. Larger species produce larger hooks, but smaller climbing palms of the understory deploy fine sharp hooks that are effective on small diameter supports as well as large branches and trunks. The ephemeral nature of climbing organs in palms provides a challenge to their life-history development, particularly in terms of mechanical constraints and remaining attached to the host vegetation; these differ significantly from many vines and lianas having more perennial modes of attachment.     

The Mechanical Significance of Clasping Leaf Sheaths in Grasses: Evidence from Two Cultivars of Avena sativa

The elastic (Young's) modulus and flexural rigidity of internodeswith and without their clasping leaf sheaths were determinedfor culms from two cultivars (‘Astro’) and (‘Garry’)of Avena sativa L. Data indicate that early in the developmentof culms, leaf sheaths can have a higher elastic modulus thanthe internodes they envelope, and by virtue of their location,leaf sheaths contribute significantly to the flexural rigidity(hence, resistance to bending) of internodal segments. As culmsmature, the elastic modulus of leaf sheath and internodal tissuesreach parity. However, because of the acropetal pattern by whichnew internodes are produced by shoot apices, sheaths continueto provide mechanical support to distal internodes, particularlythe peduncle. Data for the two cultivars indicate that the elasticmodulus and flexural rigidity of culms can vary significantlywithin the species. Comparisons between the flexural rigidityof the two cultures and the resistance of stems to lodging indicatethat flexural rigidity is not significant to lodging. The engineeringprinciples relevant to the mechanical advantages conferred byclasping leaf sheaths are discussed within the context of grassshoot morphology. Biomechanics, leaf sheath, Avena, elastic modulus     

Diversity of Mechanical Architectures in Climbing Plants: An Evolutionary Perspective

Mechanical architectures of a wide range of climbing plants are reviewed from a wide phylogenetic range and evolutionary contingencies. They include an herbaceous lycopod (Lycopodiaceae) - a woody tropical liana (Apocynaceae), temperate climbers, herbs and shrubs (Ranunculacae), and two representative climbing palms (Arecoideae, Calamoideae). Trends in mechanical properties during development are reviewed and interpreted via changes in anatomical development of the stem and type of connection to host supports. The results indicate that there are some biomechanical features common to diverse climbing plants including (1) phases of relatively rigid stem growth where the climbing stem has to span between supports and (2) a mechanism to achieve greater compliancy towards the base or at points where the slender climbing stem is at risk from excessive mechanical stress. Evolutionary contingencies such as basal plesiomorphic constraint, complexification, simplification and developmental loss can drastically influence ways in which different plants have evolved different biomechanical climbing architectures. Two key developmental features controlling the biomechanics of the climbing stem are (1) the presence/absence of secondary growth and (2) the number, complexity and coordination of development of primary or secondary tissues with varying mechanical properties. Recent research has suggested that evolution of specialized climbing architectures can canalize subsequent evolution of alternative growth forms. The results suggest that the origin and type of climbing architecture can be heavily influenced by ancestral growth forms and architectures. Despite the extremely complex patterns of plant growth form evolution involving escapes to and from more specialized or simpler bauplans, selective pressure towards non-self-supporting growth forms is a remarkably persistent and iterative feature of growth form evolution in land plants.     

Comparative development of the rattan ocrea,a structural innovation that facilitates ant–plant mutualism

The ocrea is an extension of the leaf sheath that occurs in a wide range of angiosperms, including some rattan palms (Arecaceae/Palmae). In some rattan species, the ocrea is an inflated sac-like structure that acts as a domatium. Typically, ants occupy this domatium, tending their young and husbanding aphids, and potentially providing a defence mechanism for the rattan. We present a comparative study of early leaf development in five palm species, including both ocreate and non-ocreate taxa. Early leaf development was examined using SEM in Calamus longipinna, which has a pronounced ocrea, and compared with two other Calamus species with an inconspicuous ocrea, and two non-ocreate palms, Chamaedorea pochutlensis and Rhapis humilis. All three Calamus species examined develop an extension from the top of the leaf sheath, in contrast with the non-ocreate palms examined for comparison. The ocrea is a vascularized outgrowth from the top of the leaf sheath, initiated shortly after differentiation of the lamina and sheath. Calamus longipinna differs from the other two Calamus species in that plications develop on the surface of the ocrea, and persist as folds on the ocrea surface. The highly unusual ocreate plications of C. longipinna form axial folds that permit the sac-like ocrea to become inflated, and give it the properties of a domatium, thus facilitating the ant–plant interaction in this species. The homologies of the ocrea in rattans require further review because the interpretation of leaf-derived appendages such as ligules and stipules remains controversial based on contradictory evidence from location and vasculature.     

Leaf contents differ depending on the position in a rice leaf sheath during sink-source transition.

Carbohydrate contents varied with position in a leaf sheath, and differed between the flag leaf sheath and the second leaf sheath below the flag leaf (-2 leaf sheath) in rice (Oryza sativa L.). In the -2 leaf sheath before heading, light microscopy revealed differences in the distribution of starch granules depending on position. Leaf sheaths were divided into several parts, and the contents of carbohydrates (starch, sucrose, and hexoses) were measured in each part. Before heading, the content of accumulated starch increased linearly from the top to the bottom in -2 leaf sheaths (r2=0.99, P<0.001), as did the contents of accumulated sucrose and hexoses in flag leaf sheaths (r2=0.94, P<0.01). In flag leaf sheaths, the relative content of sucrose synthase (SuS), which plays a central role in the degradation of sucrose into hexoses, increased from the top to the bottom, consistent with hexose contents. After heading, the accumulated carbohydrates were dramatically decreased. In -2 leaf sheaths, the activity of alpha-amylase (EC 3.2.1.1), the rate-limiting step in starch degradation, was consistent with the degree of starch degradation, but in flag leaf sheaths with little starch before heading. These results show that carbohydrate contents differ, depending on the position in a leaf sheath. In addition, there were big differences in leaf contents between flag leaf sheaths and -2 leaf sheaths.     

Wood mechanics, allometry, and life-history variation in a tropical rain forest tree community

Wood density plays a central role in the life-history variation of trees, and has important consequences for mechanical properties of wood, stem and branches, and tree architecture. Wood density, modulus of rupture, modulus of elasticity, and safety factors for buckling and bending were determined for saplings of 30 Bolivian rain forest tree species, and related to two important life-history axes: juvenile light demand and maximum adult stature. Wood density was strongly positively related to wood strength and stiffness. Species safety factor for buckling was positively related to wood density and stiffness, but tree architecture (height : diameter ratio) was the strongest determinant of mechanical safety. Shade-tolerant species had dense and tough wood to enhance survival in the understorey, whereas pioneer species had low-density wood and low safety margins to enhance growth in gaps. Pioneer and shade-tolerant species showed opposite relationships between species traits and adult stature. Light demand and adult stature affect wood properties, tree architecture and plant performance in different ways, contributing to the coexistence of rain forest species.     

Unusual branch development in the palm, Chrysalidocarpus

Vegetative branch buds of C. lutescens are non-axillary and occur within an abaxial, tubular extension of the leaf sheath, either at the base of a shoot or aerially, a position unusual for palms. Buds are initiated on the abaxial surface of a leaf during its first plastochron (the youngest leaf primordium). The foliar origin of the vegetative bud appears to be unique for angiosperms. In contrast, inflorescence buds are axillary and are initiated as an adaxial ridge on the base of a leaf during its third plastochron (the third primordium from the apex). Aerial branches and basal suckers are developmentally identical and changes in their phyllotaxis are described. As far as can be established by comparative morphology, other species of Chrysalidocarpus have the same type of branch development as in C. lutescens. The development of branches is related to the morphogenetic characteristics of arborescent monocotyledons.     

Proteomic analysis of rice seedlings infected by Sinorhizobium meliloti 1021

Rhizobial endophytes infect and colonize not only leguminous plants, but several non‐leguminous species as well. Using green fluorescent protein tagging technique, it has been shown that Rhizobia infect different varieties of rice species and migrate from plant roots to aerial tissues such as leaf sheaths and leaves. The interaction between them was found to promote the growth of rice. The growth promotion is the cumulative result of enhanced photosynthesis and stress resistance. In addition, indole‐3‐acetic acid also contributes to the promotion. Gel‐based comparative proteomic approaches were applied to analyze the protein profiles of three different tissues (root, leaf sheath and leaf) of Sinorhizobium meliloti 1021 inoculated rice in order to get an understanding about the molecular mechanism. Upon the inoculation of rhizobia, proteins involved in nine different functional categories were either up‐regulated or down‐regulated. Photosynthesis related proteins were up‐regulated only in leaf sheath and leaf, while the up‐regulated proteins in root were exclusively defense related. The results implied that there might have been an increase in the import and transport of proteins involved in light and dark reactions to the chloroplast as well as more efficient distribution of nutrients, hence enhanced photosynthesis. Although the initiation of defensive reactions mainly occurred in roots, some different defense mechanisms were also evoked in the aerial tissues.     

Morphometric, AFLP and plastid microsatellite variation in populations of Scalesia divisa and S. incisa (Asteraceae) from the Galápagos Islands

Analysis of the stem vasculature of the American climbing palm Desmoncus reveals structural features differing significantly from the Old World rattan genus Calamus . Desmoncus has a more directly continuous vascular system but nevertheless shows a vessel distribution that makes for high hydraulic resistance in the axial xylem. Desmoncus is like Calamus in having a single very wide metaxylem vessel in each central axial bundle and is also without direct vascular contact between protoxylem and metaxylem tracheary elements. However, in Desmoncus the stem vascular bundle system resembles that in tree palms (as has been described in the model palm Rhapis excelsa ) in having a continuing axial bundle that branches from each outgoing leaf trace together with a large number of bridge connections between leaf traces and peripheral axial bundles. Resistance to axial water transport is, however, evident in the narrowness of the continuing metaxylem elements in the peripheral stem vascular region. Desmoncus has scalariform perforation plates with few thickening bars in the metaxylem vessels, unlike the simple perforation plates found in Calamus . Thus, Desmoncus shows only limited convergence in stem vascular architecture toward the extreme modifications found in Calamus . This is not unexpected since it is clear that the climbing habit evolved independently in the two genera.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 142 , 243−254.     

The role of the sheath tube in the development of expanding leaves in perennial ryegrass

A possible morphogenic effect of leaf sheaths on subsequent leaf development was investigated by varying sheath tube (pseudostem) length in plants of perennial ryegrass (Lolium perenne) cv. Talbot by either incising longitudinally or excising the distal portion of the sheath tube, while leaving the basal length of the tube intact. The tube was maintained at predetermined lengths by incising and excising new growth daily. The youngest rapidly expanding leaf was allowed to grow through the tube and was measured when fully expanded. Reducing tube length by excision or by incision from 60 mm to just above the cowl leaf on the apex reduced lamina length by 87% and 77% respectively. Over all tube lengths, laminae in incised treatments were almost twice as long as those in excised treatments. Sheath length followed a similar pattern. The effect on developing leaves of artificially extending sheath tubes (previously excised to 15 mm) to 30 or 45 mm with foil was similar to that of initially excising sheath tubes to 30 or 45 mm. The shorter the sheath tube (reduced by incision) through which the leaves had to grow, the shorter the cells, especially in the laminae. The estimated cell number per row along the length of the laminae ranged from 190 in tillers (shoots) with a very short tube (just above the cowl leaf) to 454 in intact control tillers. It is concluded that the sheath tube has a morphogenic influence on the development of subsequent leaves due to the change in environment of the leaf lamina on appearance, affecting both cell elongation and cell division.     

Composition of leaf surface waxes of Triticum species: Variation with age and tissue

The composition of cuticular wax from plants of spring wheat (varieties Selkirk and Manitou) and of durum wheat (variety Stewart 63) at various stages of growth, and of wax from different parts of the plants varies considerably. Wax was analysed, without preliminary separation, by GLC using Dexsil 300 as liquid phase. Alcohols are major components of wax from leaf blades and β-diketones are major components of wax from leaf sheaths, especially the flag leaf sheath. Glaucousness of the leaf sheath is due to the high β-diketone content. In the first 50 days after germination, before sheaths and flag leaf are completely developed, the major component is octacosanol (> 50%). At 66 days, when sheath development is complete, β-diketone content is greatest. Hydrocarbon composition differs for wax from leaf blade and leaf sheath and also for different leaf blades and between adaxial and abaxial sides of the flag leaf. From 66 to 100 days ester content of wax increases, especially in Selkirk wheat, apparently due to formation of wax containing high proportions of esters of trans-α,β-unsaturated C₂₂ and C₂₄ acids. The content of these acids in the free fatty acids and of diesters based on these acids also increases during this period.     

Pressure gradients along whole culms and leaf sheaths, and other aspects of humidity-induced gas transport in Phragmites australis

Emergent aquatic macrophytes growing in waterlogged anaerobic sediments overlain by deep water require particularly efficient ventilating systems. In Phragmites australis (Cav.) Trin. ex Steud, pressurized gas flows, generated by humidity-induced diffusion of air into leaf sheaths, enhance oxygen transport to below-ground parts and aid in the removal of respiratory CO2 and sediment-generated CO2 and methane. Although modelling and flow measurements have pointed to the probable involvement of all leaf sheaths in the flow process and the development of pressure gradients along the whole lengths of living culm and leaf sheaths, direct measurements of pressure gradients have never been reported. The aim of this study was to search for pressure gradient development in Phragmites culms and leaf sheaths and to determine their magnitudes and distribution. In addition, dynamic (with gas flow) and static pressures (no flow condition) and their relationship to flows, leaf sheath areas, and living-to-dead culm ratios were further investigated. Dynamic pressures (DeltaPd) recorded in the pith cavities of intact (non-excised) leafy culms, pneumatically isolated from the below-ground parts and venting through an artificial bore-hole near the base, revealed a curvilinear gradient of pressure 'asymptoting' towards the tips of the culms. Similarly, DeltaPd in upper and lower parts of leaf sheaths increased with distance from the base of the culm, with values in the upper parts always being greater. Curvilinear gradients of pressure were also found along pneumatically isolated individual leaf sheaths, but radial channels linking the leaf sheath aerenchyma with the pith cavity of the culm appeared to offer little resistance to flow. In keeping with predictions, static pressure differentials (DeltaPs) achieved in intact and excised culms and single leaf sheaths on intact culms proved to be relatively independent of leaf sheath area, whereas the potential for developing convective flows (pressure-driven flows) increased with increasing leaf sheath area. As measured by the ventilating coefficient [1-(DeltaPd/(DeltaPs)] the old dead (efflux) to living (influx) culm ratio of 1:12 compared with 1:25 raised ventilating efficiency from 31% to 71%, giving flows per tall culm into the rhizome system of c. 2.8 cm3 and 6.5 cm3 min-1, respectively. It was concluded that dynamic pressure gradients probably extend along the whole length of the leafy culms and leaf sheaths of Phragmites and that all leaf sheaths and all exposed points along the leaf sheaths can contribute convective gas-flow to the rhizome system.     

Early Leaf Development and Leaf Sheath Formation of Botrychium strictum and B. virginianum (Ophioglossaceae)

The morphogenesis of the leaf sheath was studied in Botrychiumstrictum and B. virginianum of subgenus Osmundopteris. In thetwo species, the leaf primordium is initiated on the lowestpart of a ridge which is formed by partial growth of the shootapex. The leaf primordium first grows to cover the shoot apexalmost entirely except for a slit-like opening. The openingis formed by the frontal rim of the growing leaf primordium,i.e. the leaf margin, and the rear part of the shoot apex. Asthe leaf grows, the leaf margin elongates and takes a reverseV-shape. On both lateral edges of the leaf margin, marginalgrowth occurs to form the lobes of the leaf sheath. Such marginalgrowth and a small amount of growth on the uppermost portionof the sheath is involved in the leaf sheath formation in B.cirginianum, while only marginal growth takes place in B. strictum.The leaf sheath of Botrychium virginianum, in comparison tothat of B. strictum, has a morphogenesis which is more similarto the completely covering leaf sheath of subgenera Botrychiumand Sceptridhim. Based on the morphogenesis of the leaf sheath,systematic relationships in subgenus Osmundopteris are discussed Botrychium virginianum, B. strictum, subgenus Osmundopteris, leaf ontogeny, leaf sheath formation, scanning electron microscopy, light microscopy     

Freshwater ascomycetes: Natipusillaceae, a new family of tropical fungi, including Natipusilla bellaspora sp. nov. from the Peruvian Amazon

A new ascomycete species, Natipusilla bellaspora, collected from submerged woody debris in a freshwater stream at Los Amigos Biological Station, Madre De Dios in the Peruvian Amazon is described and illustrated. This fungus is characterized by small, globose to subglobose, hyaline ascomata; small, globose to subglobose, eight-spored fissitunicate asci; one-septate, multiguttulate ascospores with two different gelatinous sheaths, an outer amorphous sheath that enlarges in water and an inner sheath that has a distinctive persistent shape and is attached to the ascospore apex. Morphologically N. bellaspora differs from other Natipusilla species in having larger ascospores and two ascospore sheaths. A second Natipusilla species, N. limonensis, is reported for the first time from Peru. Based on the unique morphological characters of taxa in Natipusilla and results of previous molecular phylogenetic analyses with other members of the Dothideomycetes, we establish Natipusillaceae fam. nov. for this unique tropical freshwater clade.     

Changes in the levels and composition of sterols in different tissues of Lolium temulentum plants during floral development

Free, esterified and glycosylated sterols were analysed separately from the shoot apices, leaves, leaf sheaths and stems of Lolium temulentum L. (strain Ceres) plants during floral development. Short-day grown plants (50 days old) were induced to flower by exposure to a single long day. The four major sterols found by GC-MS analysis were sitosterol, cholesterol, campesterol and stigmasterol. The sterol levels in the shoot apex were much higher than those in the leaf, leaf sheath and stem. A much greater proportion of cholesterol was found in the shoot apex than in other tissues and this may reflect a specific association of cholesterol with meristematic and/or reproductive tissues.
During the inductive treatment, the sterol levels decreased in all four tissues. The major effect during early differentiation was the occurrence of transient increases in the free and esterified sterol levels in the leaf and the stem tissues. The steryl ester content peaked 24 h before the appearance of double ridges, followed by a peak in free sterol content at the double ridge stage. Similar changes could not be detected in the shoot apices. This is the first report of the sterol composition of developing shoot apices, and the results emphasize the dynamic nature of sterol metabolism during reproductive growth of L. temulentum.     

The Mechanical Roles of Clasping Leaf Sheaths: Evidence fromArundinaria tecta(Poaceae) Shoots Subjected to Bending and Twisting Forces

Representative shoot segments of the grass speciesArundinariatéctaconsisting of one intact internode and its subtendingnode and clasping leaf sheath were tested to determine the mechanicalinfluence of the leaf sheath on the ability of stems to resistbending and twisting forces. These segments were also used tomeasure shoot morphometry and composite tissue Young's and shearmoduli (EandG,respectively) to simulate the global deformationpatterns attending bending and twisting by means of finite elementanalyses. On average, leaf sheaths contributed 33% of the overallbending stiffness and 43% of the overall torsional stiffnessof stem segments. Comparisons betweenEandGof isolated internodesand leaf sheaths indicated that sheaths were composed of stiffertissues measured either in bending or twisting. Thus, leaf sheathscould act as an external cylindrical brace composed of stiffermaterials than those of the internodes they enveloped. The magnitudesof internodalEandGwere correlated with internodal shape suchthat the ability of internodes to resist twisting relative tothe ability to resist bending forces decreased as internodesbecame more slender or developed thinner walls (both of whichoccur in an acropetal direction from the base to the tip ofshoots). Finite element simulations predicted that, in bending,the leaf sheath laterally braces internodal walls as they tendto ovalize in cross section and push against its inner surfacewhich ovalizes to a lesser extent in the plane normal to thecurvature of shoot flexure. In twisting, the successive ovalizedtransections of internodal walls assumed a helical pattern alongthe length of shoot segments. This helical deformation patternwas attended by an inner lateral contraction of internodal wallsthat was less developed in the leaf sheath that thus provideddecreasing mechanical support to the internode as the lateralcontraction of internodal walls amplified. The twisting of internodesand sheaths was also predicted to concentrate tensile and shearstrains in the nodal diaphragm. Here stress intensities sufficientto produce tissue shear failure were concentrated at two opposingpoints on the surface of the diaphragm. Finite element analysesthus identified a potential weak point in the mechanical constructionof hollow, septate shoots that are, nevertheless, more thanadequately stiff to support their own weight, yet sufficientlyflexible to twist without irreparable damage in normal winds.Copyright1998 Annals of Botany Company Plant stems; nodes; internodes; leaf sheaths; elastic moduli; wind lodging; biomechanics.     

Evolution of lamina anatomy in the palm family (Arecaceae)

The unique properties of tree building in Arecaceae strongly constrain their architectural lability. Potentially compensating for this limitation, the extensive diversification of leaf anatomical structure within palms involves many characters whose alternate states may confer disparate mechanical or physiological capabilities. In the context of a recent global palm phylogeny, we analyzed the evolution of 10 such lamina anatomical characters and leaf morphology of 161 genera, conducting parsimony and maximum likelihood ancestral state reconstructions, as well as tests of correlated evolution. Lamina morphology evolves independently from anatomy. Although many characters do optimize as synapomorphic for major clades, anatomical evolution is highly homoplasious. Nevertheless, it is not random: analyses indicate the recurrent evolution of different cohorts of correlated character states. Notable are two surface layer (epidermis and hypodermis) types: (1) a parallel-laminated type of rectangular epidermal cells with sinuous anticlinal walls, with fibers present in the hypodermis and (2) a cross-laminated type of hexagonal cells in both layers. Correlated with the cross-laminated type is a remarkable decrease in the volume fraction of fibers, accompanied by changes in the architecture and sheath cell type of the transverse veins. We discuss these and other major patterns of anatomical evolution in relation to their biomechanical and ecophysiological significance.     

Compartmentation and fluxes of carbon in leaf blades and leaf sheaths of Poa annua L. and Poa x jemtlandica (Almq.) Richt

Abstract The allocation of photosynthetically fixed carbon in the leaf blades and sheaths of Poa annua (a ruderal grass) and Poa x jemtlandica (a sub-arctic grass) was followed over a light-dark cycle. Labelling with ¹⁴Carbon and gas exchange measurements provide data for an eight-compartment model describing the partitioning of carbon between spatially and chemically separated pools and their rates of turnover. Soluble sugars and fructans were turned over rapidly in the leaf blades of both species. The flux of carbon through pools of storage carbohydrates was higher in the leaves of P. x jemtlandica than in P. annua. The exchange of carbon between pools was slower in the sheath than the blade. Carbohydrates stored in the sheath appeared to have no significant role in metabolism over the light-dark cycle studied here.