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First published online November 7, 2008; doi:10.3732/ajb.0700005 American Journal of Botany 95: 1538-1547 (2008) © 2008 Botanical Society of America, Inc. |
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Biomechanics |
University Montpellier 2, UMR AMAP (botAnique et bioinforMatique de l’Architecture des Plantes), TA A-51/PS2, Bd. de la Lironde, 34398 Montpellier cedex 5, F-34000 France; and CNRS, UMR AMAP Montpellier, F-34000 France
Received for publication 21 December 2007. Accepted for publication 16 September 2008.
ABSTRACT
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.
Key Words: Arecoideae • biomechanics • Calamoideae • cirrus • flagellum • grapnel • hooks • ratchet mechanism
Rattans and other climbing palms are well known for their armed cirri and flagella, which enable them to climb into the forest canopy. In some parts of the world, rattans are known as wait-a-whiles, because anyone caught up in their hooks and grapnels will need time and help to become free—such is the efficiency and strength of the attachment structures. The climbing habit and specialized attachment devices evolved separately in two subfamilies of palms: within the Arecoideae (South and Central America) and the Calamoideae (Southeast Asia and Africa). The parallel origin of climbing traits in the two subfamilies involves a catchy, whiplike extension of the leaf rachis known as the cirrus, that bears sharp, pointed hooks and grapnels. In the Calamoideae or true rattans, some species of Calamus produce a modified inflorescence axis up to several meters in length, bearing hooks and grapnels, known as the flagellum (Dransfield, 1978
). The hooks and grapnels and the cirri and flagella bearing them are highly characteristic of climbing palms. They influence a number of life history processes linked to the climbing habits that differ from vines and lianas that climb via twining stems, hooks, tendrils, or adhesive roots (Corner, 1966; Rowe et al., 2006).
Cirri are found in two palm subfamilies, Arecoideae and Calamoideae (Uhl and Dransfield, 1987). In species of Desmoncus, the principal genus of climbing palms in the Arecoideae, the cirri bear conspicuous acanthophylls, which represent modified leaflets (Fig. 1A–D). Some species bear sharp, recurved hooks produced on either the cirrus rachis, leaf rachis, or both. Cirrate leaves in rattans of Southeast Asia do not produce acanthophylls, only recurved hooks along the cirrus and leaf rachis, that are often grouped into multipointed grapnels (Fig. 2A). Interestingly, cirrate leaves with acanthophylls do occur in the Calamoideae but only in species of the African Laccosperma clade (Corner, 1966; Baker et al., 2000).
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) (Fig. 2B). Nonclimbing species of both subfamilies do not develop cirri or flagella. Furthermore, in all the climbing palm species that we have observed, cirri and flagella do not develop fully in young, self-supporting stages of growth prior to vertical instability and climbing. Instead, cirri and flagella are either absent or are vestigial with few acanthophylls or hooks.
Attachment organs of climbing plants vary greatly in terms of their "working life" up to senescence. Some climbing organs such as twining stems and branches of woody vines and lianas can be functional for the entire lifespan of the plant. Others such as modified leaves or tendrils might be functional for only a brief interval. Organization and strength of the attachment devices can also determine the type of support a climber can attach to (Darwin, 1867; Schenck, 1892; Putz, 1984; Putz and Holbrook, 1991; Rowe et al., 2006) and thus the mechanical constraints that a climber has to survive throughout its life history. Climbers that are loosely fixed via hooks to a dense, shrubby undergrowth are subject to quite different mechanical constraints compared with twining lianas that are fixed tightly to large-bodied forest trees. The type of attachment device and the type of mechanical constraints the stem undergoes once attached, are probably important factors in the ecological and evolutionary success of the climbing habit (Gentry, 1991; Hegarty, 1991; Caballé, 1993; Rowe et al., 2004). Despite their importance for maintaining the climbing habit and thus contributing to ecosystem diversity particularly in the tropics, attachment devices in lianas and vines have been surprisingly little studied.
Climbing palms are reported to be very efficient at latching onto surrounding vegetation (Corner, 1966; Putz, 1990). Although recent biomechanical studies have investigated changes in stiffness before and after loss of the leaf sheath during development (Isnard et al., 2005; Isnard and Rowe, 2008), there have not been any previous studies focusing on the biomechanics of attachment. In this paper we investigate the geometry and mechanical properties of cirri and flagella and attempt to interpret how their organization influences attachment and hook deployment. Second, we demonstrate how the arrangement and mechanical strength of grapnels and hooks are optimized for maintaining attachment to host supports. Our analysis includes mechanical and functional traits from cirri and flagella in two subfamilies of climbing palms, the Calamoideae and Arecoideae. Our comparisons also aim to examine any variation in attachment mechanisms among species and subfamilies and in attachment traits among body sizes and ecological preferences.
MATERIALS AND METHODS
The study includes two cirrate species from the subfamily Arecoideae and two cirrate and one flagellate species from the subfamily Calamoideae (Table 1). Species of Arecoideae were sampled from sites in French Guiana (Isnard et al., 2005) and included two species of Desmoncus. Species of Calamoideae collected from Southeast China included cirrate species of Plectocomia and Daemonorops and a flagellate species of Calamus. A summary of taxonomic affinities, locations, habitats, and mechanical tests are listed in Table 1.
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Bending tests
Three-point bending tests were carried out on segments of cirri and flagella (see Table 1). Cirrate leaves and flagella were harvested and stored in moist conditions before testing. Appropriate lengths of each tested segment were determined for each species, to minimize the influence of shear during three-point bending tests (Vincent, 1990). Bending test protocols followed those used in recent studies on palms and woody plants (Vincent, 1990; Isnard et al., 2003, 2005; Lahaye et al., 2005). Segments were placed on a metal frame apparatus where the appropriate distance between the two supporting points could be adjusted. Consecutive weights were added at the center of the segment, and the deflection was measured via a dissecting microscope. A force-deflection curve was used to calculate flexural stiffness: EI = L3/48R, where L is the distance between the two supports, and R is the slope (mm/N) of the force-deflection curve.
Segments were tested in their natural adaxial/abaxial orientation; some leaf and cirrus rachises were V-shaped in cross-section and required notches to be cut into the supports of the apparatus so that segments would be stable during the test.
The second moment of area (I) of each segment tested was measured after each mechanical test. This parameter quantifies the contribution of cross-sectional area, geometry, and shape of a structure to resist bending (Niklas, 1992). Transverse sections were prepared from each tested axis or rachis at two or three positions along the segment tested. Section outlines were drawn and digitized with reference to the direction of the force applied in the bending test. The axial second moment of area of each segment was then calculated with a macro command kindly provided by T. Almeras for the image analysis software program Optimas. The Young’s modulus, E, was calculated form the two previous measurements of EI and I.
Torsion tests in Desmoncus polyacanthos
Torsion tests were carried out on 12 cirrate leaves from a single plant growing in the forest understory several meters from a treefall gap. Segments were measured via a spring-loaded apparatus detailed fully in (Gallenmüller et al., 2001). Torques were applied to the leaf and cirrus rachis up to a maximum rotation of 120–180°. The resulting rotational angle of the segment was noted from graduations drawn on the external cylinder. Torsional rigidity, GJ, of each segment was calculated using: GJ (N•m2) = l/b, where l (m) is the length of the segment between the two clamps, and b is the slope of the angular deflection and the torque applied (rad/N•m). Results from bending and torsion in D. polyacanthos were used to calculate twist-to-bend ratios EI/GJ (Vogel, 1992) of the cirrus. This describes the relative resistance of a structure to bending vs. torsion. A structure twisting more easily than it will bend will have a ratio EI/GJ greater than one.
Hook and acanthophyll strength
Strength tests were carried out on hooks borne on either the leaf and cirrus rachis or flagellum (Table 1). Segments of cirri and flagella were cut to lengths of 1–3 cm and clamped in a force-measuring device (Fig. 3). A metal hook suspended from the upper mounting was placed in the center of the hook and raised at a rate of 1 mm/s. Force and displacement were measured automatically during the test via transducers. Maximum breaking loads (N) were measured as the maximum value of the force-deflection curve before a significant drop in force indicating failure of the hook (Fig. 3B). Care was taken to avoid the specimen slipping in the clamps during the test and to make sure that the metal hook of the device remained central in the hook or acanthophyll and normal to the force applied vertically (Fig. 3A). The latter was not generally difficult because the hook of the device assumed a constant position soon after the beginning of the experiment. In Desmoncus, acanthophylls occur as opposite or subopposite pairs, and in species of Daemonorops and Calamus tested, spines often form groups of tight clusters around the cirrus and flagellum. Only one acanthophyll of a pair or one spine of a cluster was tested to failure because the strength of neighboring hooks was weakened by fracture surfaces propagated during the initial test.
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Size and development of cirri and flagella
Leaves and cirri are much larger in D. orthacanthos than D. polyacanthos, while cirrus length is about half that of the total leaf length in both species (Table 2). Both species of Desmoncus bear 5–8 pairs of acanthophylls along the cirrus. In early development, they are oriented toward the apex of the cirrus and not at all positioned for attaching to supports. Each acanthophyll becomes reflexed toward the base of the cirrus via its pulvinus (Fig. 1). Reflexed acanthophylls mature and stiffen, finally orientating themselves abaxially at approximately 25°–35° to the cirrus rachis (Fig. 1C). These acanthophylls readily attach themselves to supports positioned below the cirrus. The two species differ in terms of additional hooks and spines available for attachment. In D. polyacanthos small recurved spines (2–3 mm long) are common on the abaxial side of both leaf rachis and cirrus (Fig. 1D), whereas in D. orthacanthos large straight spines are sparsely distributed on the surface of the leaf rachis and leaflets, but not the cirrus. The spines in D. orthacanthos are not at all recurved and were not observed to interlock effectively with adjacent vegetation.
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Flagella of Calamus tetradactylus have a wide reach up to 2 m and a jointed system of elongate primary bracts. Flagella bear recurved spines that are solitary or grouped in whorls of three or four (Fig. 2B). Spines decrease in size or are absent toward the base of each bract. Flagella develop on the third or fourth stem internode whether a support has been contacted or not.
Mechanical properties and geometry of cirrus and flagellum
Cirrate leaves
In Desmoncus polyacanthos, the leaf rachis is V-shaped in cross-section at the base (Fig. 4A) changing to a deep lozenge shape distally along the cirrus. The profile flattens abaxially toward the cirrus apex. Local changes in cross-sectional shape correspond to the departure of leaflets and acanthophylls. Reduction in size along the leaf rachis is accompanied by a great decrease in second moment of area and bending and torsional rigidity (Fig. 4B), occurring concomitantly along the rachis with an increase in Young’s modulus of 4000–12000 MPa. Along the cirrus, cross-sectional geometry and size remain relatively constant and the Young’s modulus decreases slightly toward the cirrus apex. This change in material properties produces a slight decrease in flexural and torsional rigidity along the cirrus, but this drop is much less compared with the loss of rigidity observed along the rachis owing to changes in size. The EI/GJ ratio provides information about the rigidity of a structure in terms of bending vs. torsion. A cross-sectional geometry departing significantly from circular can lead to less rigidity in torsion than in bending. The bilateral symmetry of the cirrus in D. polyacanthos, partly explains the high value of EI/GJ (Fig. 4B); where GJ is about one order of magnitude lower than EI. A relatively high ratio is retained along the leaf and whip-like extension, reflecting the similar relationship in bending and torsion along the leaf and cirrus.
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Geometry and mechanical properties of the cirrus and flagellum
Is there any fundamental difference in geometry and mechanical behavior between leaves of self-supporting palms and climbing palms? Self-supporting palm leaves have a primal mechanical role as self-supporting structures for light capture but cirrate leaves of climbing palms function as self-supporting organs for light capture as well as attachment structures, possibly with quite different mechanical requirements. The bases of cirrate leaves are often V-shaped in cross-section and differ little in terms of geometry from cross-sections typical of many self-supporting palm leaves (Uhl and Dransfield, 1987). This organization promotes resistance to bending in the vertical direction by taking advantage of the high second moment of area (I) of the V-shaped cross-sectional shape. The strategy was evocatively illustrated by Karl Niklas (1992) who demonstrated that a strip of paper pinched to form a V-shape is much more rigid than when flat. This geometrical arrangement would contribute to maintaining relatively high values of flexural rigidity (EI) despite the lower values of Young’s modulus (E) found toward the base of cirrate leaves in both species tested.
Both self-supporting and cirrate leaves taper significantly, so that relatively more material is concentrated near the base of the structure, leading to a higher flexural rigidity where bending moments will be greatest. In terms of both overall geometry and shape, the basal leaf component of cirrate leaves does not differ much in terms of overall organization compared with leaves of self-supporting palms. The cirrus, however, is highly flexible and among the species tested does not incorporate a V-shaped, cross-sectional profile optimized for resisting bending forces vertically. Instead, the cirrus is suspended from the self-supporting part of the cirrate leaf. In terms of overall organization, modifications for attachment and climbing include little modification of the "self-supporting" leaf component but with the addition of a specialized apex, which can locate supports relatively far from the parent plant stem.
The noncircular geometry of the cross-section promotes a higher EI/GJ ratio compared to a circular cross-section (Vogel, 1992). Our results show relatively high bend to twist ratios along cirrate leaves in D. polyacanthos, with values between 6–16; a range higher than values previously measured on tree petioles of 2.2–7.7 (Vogel, 1992). The low torsional rigidity and relatively high bending stiffness is also probably explained by petiole anatomy. Typical anatomical organization of a palm petiole and rachis is illustrated here for a rachis of Plectocomia (Fig. 11), which is characterized by an outer sheath of vascular tissue and thick-walled fibers. This kind of organization is known to promote bending stiffness but low torsional rigidity because of the isolated longitudinal, strengthening elements at the outside of the cross-section (Ennos, 1993; Ennos et al., 2000).
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The organization of the flagellum in Calamus tetradactylus results in a mechanical architecture quite different from that of the cirri. Instead of a drastic increase in flexural rigidity toward the base of the structure, there is a drop in flexural rigidity at the junction of each successive bract where the Young’s modulus systematically drops. Another difference is that more distally positioned attachment organs on the plant stem are more rigid than those below. More apical flagella thus retain a more horizontal arched profile in their position of growth, which presumably increases their probability for contacting potential neighboring supports. Interestingly, this is achieved despite the jointed bract architecture.
The results suggest that different species have different types of hook deployment that might influence the type of "catchiness" and the kinds of vegetation to which they can effectively attach. In Desmoncus polyacanthos, the cross-sectional shape of the cirrus is significantly flattened toward the apex, promoting less bending resistance in a vertical plane and a tendency for upward and downward movement of the cirrus after movement of the plant. In Plectocomia himalayana, the cirrus actually becomes more circular apically; this change promotes a more constant bending rigidity in vertical and horizontal directions resulting in circular movements of the cirrus apex when the leaf and cirrus is moved. In cirri of Desmoncus, the recurved acanthophylls (and spines in D. polyacanthos) are placed abaxially, whereas in P. himalayana, recurved hooks are positioned in whorls. This organization suggests that hook deployment varies between species and is coupled with rachis geometry and resistance to bending. Circular axes bear whorls of spines that can deploy in many directions, while dorsoventrally flattened axes bear hooks only directed in the downward direction. In the flagellate species Calamus tetradactylus, the elliptical base of the flagellum would encourage an up and down movement of the basal part of the flagellum, whereas the circular cross-section toward the apex would promote circular movement. The circular axis of the attachment organ bears recurved spines arranged in partial whorls on the abaxial and lateral surfaces of the axis, suggesting that circular movement is coupled with potential hook attachment in numerous directions around the flagella rather than limited to the vertical.
Hook strength and the ratchet mechanism of climbing palms
Maintaining connection with host vegetation is another crucial aspect of climbing palms, both in young individuals and at the growing apex of mature climbing stems. Putz (1990) proposed that flagellar hooks function as a ratchet mechanism. Once a flagellum is attached via one or more of its recurved hooks, the weight or flexure of the plant provides tension to the point of attachment, and the hook is kept in place. Movement and swaying of the plant or its host would generally disengage and re-engage the hooks toward the base of the flagellum, where a more proximal hook would hold the climbing palm more securely under possibly even more tension. This process of disengaging and re-engaging is readily observed in the field, and climbing palms can effectively tighten and secure their attachment in this way. The ratchet mechanism would also bring the stem closer to its supports, increasing the probability of other free attachment organs to locate and catch supports (Putz, 1990). Is hook strength consistent with a ratchet mechanism? If yes, then hook strength should increase toward the base of the attachment organs and thus be capable of bearing higher loads resulting from increased tension against host vegetation caused by increased flexure and/or weight of the attached palm.
Our tests indicated that hook strength generally does increase toward the base of the flagellum and is probably linked with changes in hook size and the rate at which hooks stiffen with age. A ratchet mechanism is also consistent with the strength measurements and organization of hooks and acanthophylls in cirrate species. Acanthophylls in both species of Desmoncus, as well as hooks in D. polyacanthos, are stronger toward the base of the cirrus. A similar organization is observed in D. jenkinsiana, where (1) hook strength increases basally along the cirrus and (2) hooks on the rachis and petiole are stronger than those distributed on the cirrus. A ratchet mechanism would thus ensure that climbing palms anchor themselves via the more robust acanthophylls and hooks toward the base of attachment organs. A ratchet mechanism and the observed distribution of strength along attachment organs are entirely consistent with the observed strategy whereby stresses would increase with increased ratcheting of the system and more secure attachment.
Attachment and habitat
Our observations of rattans including Calamus, Daemonorops, and Plectocomia indicate that they are extremely efficient at catching onto the surrounding vegetation and that it was impossible to dislodge long and well-established rattan stems without extreme damage to the plant and surrounding vegetation. Other observers have commented on the efficacy of the attachment system of rattans with Putz (1990) stating that as much as a 750 kg force could only partly dislodge an established flagellate species for up to a meter, after which it became fixed again.
In Desmoncus, the strength of the hook system arguably depends on the habitat preference of the species (Table 3). The large-bodied D. orthacanthos bears only several pairs of acanthophylls on robust cirri and can be extremely common in forest margins and disturbed areas. A single acanthophyll of D. orthacanthos can withstand up to 25 kg before breaking, so that a single acanthophyll can suspend an entire individual given a suitable robust branch as a host support. Mean failure loads of acanthophylls in D. orthacanthos are higher than those for hooks of Daemonorops jenkinsiana, which has a comparable stem diameter and length (Table 3). Corner (1966) speculated that acanthophylls of Desmoncus were less effective than the strongly lignified hooks of the Asian Calamoideae but may have been effective in the more open canopy of the South American forest. A difference in neotropical canopy structure and liana growth has also been suggested by Caballé (1993). Larger-bodied, heavier, climbing plants produce larger, stronger hooks (Table 3), and this organization might be correlated with a certain type of habitat offering the relevant kind of supports. The much smaller-bodied D. polyacanthos bears smaller acanthophylls but also many small hooks with the same strength as the acanthophylls. This species occupies a particular understory niche in primary or secondary forest habitats (Isnard et al., 2005). Field observations indicate that hooks of D. polyacanthos were predominantly attached to palm fronds (of other species) and slender branches, twigs, and leaves of the understory. The species is also capable of deploying its fine hooks on broad diameter trunks and branches particularly when the cirrus is under tension or following a slip or fall of the parent plant. Deployment of small sharp hooks is possibly coupled with body size and type of understory habitat, where a few small hooks 2–3 mm in length could support a small plant. Interestingly, this kind of small hook is absent from the larger D. orthacanthos.
DEVELOPMENTAL AND EVOLUTIONARY PERSPECTIVE
Self-supporting and climbing leaves in two palm subfamilies
The overall geometry and tapering of cirrate leaves is not very different from leaves of self-supporting palms. One could argue that the evolution from noncirrate, photosynthetic leaves to cirrate, photosynthetic leaves doubling as attachment organs, would not involve a great deal of developmental change and thus might partly explain the convergent appearance of cirri in both the Arecoideae and Calamoideae. Changes in mechanical traits involved elongation of the distal rachis without a fundamental change in self-supporting organization of the petiole and leaf rachis. Despite the overall similarity of cirri and their mechanical functioning in Arecoideae and Calamoideae, traits do differ between the two groups. Climbing species of the Calamoideae, in particular the Calaminae subtribe, have a swelling of the leaf sheath at the petiole base known as the knee (Tomlinson, 1962; Dransfield, 1978). This enlarged petiole base may enhance leaf strength (Tomlinson, 1962), though the exact mechanical role is difficult to test because of its complex geometry. Further studies might identify whether the structure contributes to bending, torsion, tensile stiffness, and strength or perhaps damping of movement and mechanical stresses. Significantly, the knee is absent from neotropical climbing palms such as Desmoncus. Rattans (Calaminae, Calamoideae) also have fusion of the inflorescence axis to the internode and in some cases, the leaf sheath, which is also absent in Desmoncus. Fisher and Dransfield (1977) suggest that species with the greatest levels of fusion are also the most specialized in high-climbing species. Adnation of highly modified fertile axes to both internode and leaf sheath represents another specialized functional trait found in the Calamoideae. It is a synapomorphy of the genus Calamus (Baker et al., 2000), which is not found in neotropical climbing palms.
Finally, long-term attachment of climbing palm stems is constrained by the ephemeral nature of the leaves, flagella, and leaf sheath. The progressive senescence and loss of these structures toward the base of the plant leaves the remaining stem with no means of attachment (Isnard et al., 2005; Isnard and Rowe, 2008). Long stems of climbing palms often reach 30–40 m in length and are attached to the surrounding vegetation by relatively few cirrate leaves or flagella. To remain attached after the loss of cirri or flagella, the stem has to continue to grow and produce new leaves/flagella and new points of anchorage. Observers have noted that the rate of leaf production roughly corresponds to the amount of downward movement caused by slipping of the stem (Putz, 1990). Slipping from the canopy is common in climbing palms and reflects the ephemeral nature of attachment devices. Once the cirri or flagella senesce and break away, the plant slips downward. New leaves quickly catch on to the vegetation and renew anchorage to the supports. This combination of functional traits in climbing palms results in a peculiar growth pattern centered around their organs of attachment. For this reason, it is common to see rattans and species of Desmoncus forming long loops and coils on the forest floor producing an apparently indeterminate cycle of stem growth, cirrus/flagellum production, attachment, senescence, and slippage.
FOOTNOTES
1 The authors acknowledge grants and financial assistance from the AUF (Agence Universitaire de la Francophonie) to S.I. and funding from an ANR (Agence Nationale de la Recherche) project "Woodiversity" to S.I. and N.R. The authors thank Dr. J. Chen (Xishuangbanna Tropical Botanical Garden, XTBG) for generously affording research facilities and field assistance in China. They also thank the staff and colleagues at ECOFOG on the scientific campus at Kourou, French Guiana for assistance and support. 
2 Author for correspondence (e-mail: sandrine.isnard{at}cirad.fr)
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 L. Menard, D. McKey, and N. Rowe Developmental plasticity and biomechanics of treelets and lianas in Manihot aff. quinquepartita (Euphorbiaceae): a branch-angle climber of French Guiana Ann. Bot., June 1, 2009; 103(8): 1249 - 1259. [Abstract] [Full Text] [PDF]
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