What Is Special About Plectocomia Himalayana Griff. (Calamoideae, Plectocomiinae)?

Blackwell Publishing LtdOxford, UKBOJBotanical Journal of the Linnean Society0024-4074The Linnean Society of London, 2006? 2006 151? 8391 Original Article

MECHANICAL PROPERTIES AND BRANCHING IN PLECTOCOMIA HIMALAYANA S. ISNARD Botanical Journal of the Linnean Society, 2006, 151, 83–91. With 6 figures

The Palms Guest edited by William J. Baker and Scott Zona

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

SANDRINE ISNARD*

Botanique and Bioinformatique de l’Architecture des Plantes, UMR5120 CNRS, TA40/PS,2 Boulevard de la Lironde, F-34398 Montpellier cedex 5, France

Received April 2005; accepted for publication November 2005

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 Calam- oideae. 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, Bota- nical Journal of the Linnean Society, 2006, 151, 83–91.

ADDITIONAL KEYWORDS: adventitious roots – branching – Calamus – climbing palms – Daemonorops – layering – leaf sheath – mechanical properties.

INTRODUCTION (Baker, Dransfield & Hedderson, 2000a; Baker, Hed- derson & Dransfield, 2000b). According to recent phy- Climbing growth forms have appeared several times logenetic studies, the climbing growth form could have during palm evolution and in two of the five currently multiple origins within the Calamoideae, appearing at recognized subfamilies (Dransfield et al., 2005). In the least twice during evolution (Baker et al., 2000a,b). A Arecoideae climbers include Chamaedorea elatior current project on climbing palms aims to characterize Mart. (tribe Chamaedoreeae), Dypsis scandens J. the mechanical properties and development of species Dransf. (tribe Areceae) and seven species of the genus belonging to different subfamilies and subtribes. The Desmoncus (tribe Cocoseae). The Calamoideae objective is to understand: (1) how such relatively includes 560 climbing species in 13 genera and repre- large-bodied plants lacking secondary growth have sents the most diversified subfamily in this respect adopted a scandent habit, (2) whether different origins of the climbing growth form in the family result in different ‘climbing strategies’ and (3) if there are struc- *E-mail: [email protected] tural, mechanical characteristics that can explain the

84 S. ISNARD evolutionary success of the subfamily Calamoideae and unusual in palms in general. Among rattans, and most particularly the subtribe Calaminae. aerial branching has only been described for Korthal- Recent studies have shown that despite overall simi- sia (Calamoideae, Korthalsiinae), which is known to larities, different climbing palms can show variable branch frequently in the canopy (Dransfield, 1978; mechanical architectures linked to the climbing habit Uhl & Dransfield, 1987). Aerial branching is rare in (Rowe, Isnard & Speck, 2004). This variation is based arborescent palms and Laccosperma although some around the potentially variable mechanical properties species of Hyphaene and Nypa do develop dichoto- of the inner stem and external leaf sheath of the entire mous branching (Tomlinson, Zimmermann & Simp- axis, and how such properties may vary during devel- son, 1970; Dransfield, 1978). Branch formation is opment. Species tested within Calamus and Daemon- particularly interesting in terms of the ability of a orops share a broadly comparable mechanical feature species to establish itself in the canopy, survive with dicot lianas represented by a decrease of struc- mechanical or hydraulic failure, and offset the life- tural Young’s modulus, Estr in old parts of the climbing span limitation resulting from hapaxanthy. This axis. Whereas in many dicot lianas tested (Rowe & study investigates the morphology and mechanical Speck, 1996, 1998; Isnard, Speck & Rowe, 2003a), architecture of P. himalayana and its ability to branch such changes result from highly sophisticated devel- in the canopy and produce adventitious roots. opment of the vascular cambium. In the species of Cal- amus and Daemonorops tested, the drop in E along str MATERIAL AND METHODS the axis was a consequence of the senescence, decay and eventual loss of the outer relatively rigid leaf Observations and measurements were carried out in sheath component of the axis. In two climbing species the Xishuangbanna autonomous prefecture, Yunnan of Desmoncus tested, there is an increase in Estr Province, South-west China, during two periods from towards the base of the axis (Isnard, Speck & Rowe, September to January 2003 and 2004. The climate is 2005; Rowe et al., 2004) and this is more similar to the dominated by the south-west monsoon bringing high pattern of change observed in arborescent, self- rainfall from May to October. supporting palms (Rich, 1986, 1987). In one species of Morphological characteristics of the plant stem Desmoncus, loss of the leaf sheath and a resultant loss were observed in the field and measured before the of rigidity is offset by an increase in Estr of the stem. mechanical tests. They include: internode lengths The fact that climbing stems of Desmoncus are not along main axes and branches; positions of buds and especially compliant, instead possessing quite stiff branching, and the state (living or dead) of apical mer- mechanical properties, suggests that there are rather istems and buds. different developmental constraints in climbing architecture within the Arecoideae compared with the Calamoideae, or at least between Desmoncus and spe- MECHANICAL MEASUREMENTS cies belonging to the subtribe Calaminae. This study Young individuals were identifieded by self- on the bending mechanical properties of Plectocomia supporting or unstable branches with a leaf sheath himalayana Griff. (Calamoideae, Plectocomiinae) firmly encircling the stem along the entire length of aims to find out whether the mechanical architecture the axis. Old individuals were identified by climbing described for species of Calamus and Daemonorops axes where the leaf sheath has been lost from a large characterizes other rattan genera of the subfamily, or portion of the axis. Mechanical tests were carried out whether mechanical architecture and climbing strat- on four young individuals and on three older and egies are more diverse. longer climbing axes. Senescence and fragmentation Plectocomia himalayana has a climbing habit and of the leaf sheath were considered integral parts of is remarkable for growing in the Himalayas at alti- axis development and axes with leaf sheaths that tudes up to 2000 m. It has been reported in north-east were senescent or partially so were tested in bending. India, Laos and South Yunnan Province, China Selected axes were cut at the base, just above contact (Evans, 2001). The species grows naturally in the with the main stem (branches) or the rhizome (entire vicinity of Mengsong village, Xishuangbanna (Yun- ramets). The axis was then carefully stripped of nan), at an elevation of 1600 m where this study was leaves before detaching from the surrounding vegeta- carried out. A striking feature of P. himalayana is the tion and stored in humid conditions before mechani- formation of bulb-like shoots in the proximal part of cal tests. lower internodes, a feature that has been described The bending test protocol followed that of recent for the genus in P. elongata Mart. ex Blume (Fisher & analyses for 3-point bending (Rowe & Speck, 1996; Dransfield, 1979; Uhl & Dransfield, 1987). The ability Isnard, Rowe & Speck, 2003b) where consecutive to branch naturally from the aerial stem and form weights are added to a pannier suspended in the exact aerial adventitious roots is rare in climbing palms centre of the stem segment, which is supported in a

MECHANICAL PROPERTIES AND BRANCHING IN PLECTOCOMIA HIMALAYANA 85

RESULTS

MORPHOLOGY AND DEVELOPMENT OF THE CLIMBING HABIT Stems of Plectocomia himalayana are clustering and LS1 can reach at least 40 m in length and probably more. Cane diameters range between 1.5–3 cm without leaf LS2 sheaths and 2–5 cm with leaf sheaths (10 cm maxi- mum recorded, cf. Evans, 2001). The species is hapax- stem anthic, meaning that the shoots die after flowering (Uhl & Dransfield, 1987). Young self-supporting individuals begin to produce reduced cirri (whip-like extensions of the leaf rachis, armed with reflexed spines) on the fourth or fifth leaf produced. At this stage, an intact, green spiny leaf sheath firmly sur- rounds the stem along the entire length of the self- 0.5 cm supporting axis. After reaching c. 1.5–3 m height, the axis becomes unstable and begins to climb on the sur- Figure 1. Plectocomia himalayana, transverse section of rounding vegetation. At this stage, mature leaves are a young axis showing two leaf sheaths surrounding the about 1.6 m in length with cirri about 20–60 cm long. stem. Leaves emerge at the third internode above the depar- ture of each leaf sheath from the stem (i.e. its node). As a consequence, two to three leaf sheaths surround the supporting frame at a precise span distance. Deflec- stem at any given position (Fig. 1). Leaf sheaths tion of the axis was measured via a dissecting micro- senesce and are progressively lost from the base scope and the resulting force/deflection curve used to towards the apex of older climbing axes. In some of the calculate flexural stiffness. The exact protocol and rel- oldest climbing plants observed, the portion of the axis evant formulae as applied to climbing palm stems are with a green and intact leaf sheath is 5–9 m long. available in a recent study of Desmoncus (Isnard et al., Numerous buds are formed several metres below 2005). In this study the term ‘axis’ refers to both the the apex of older climbing stems with lengths of 15– stem and the leaf sheath(s) of tested material. The 20 m (Fig. 2). Bud formation on parts of the stem term ‘stem’ refers only to axes which have shed the where the leaf sheath is still present results in a lon- leaf sheath naturally (i.e. cane) or segments in which gitudinal rupture of the leaf sheath (Fig. 3A). Buds the leaf sheath had been removed experimentally first appear in the proximal parts of internodes (Fig. prior to measuring the mechanical properties of the 3A), but in fact both buds and adventitious roots stem only. develop from the distal parts of internodes (Fig. 3B,C). The principal aim of the bending tests was to mea- This organization has been described for other hapax- sure flexural stiffness (EI, Nmm2), the tangible resis- anthic rattans, including P. elongata, where the inflo- tance of the axis or stem to bending forces, and axial rescence and flagellum are partially adnate to the second moment of area (I, mm4), quantifying the size internode above the original axillary position (Fisher and shape of the tested segment in transverse sec- & Dransfield, 1977). This suggests a similar develop- tion; and to calculate structural Young’s modulus ment of vegetative and inflorescence buds in this spe- 2 (Estr, MN/m ), the inherent stiffness, quantifying the cies. The ability of inflorescence bud sites to produce material properties of the axis or stem. Axes of rat- vegetative shoots has also been described in a species tans are compound structures comprising a stem and of Calamus and appears to be present in this species of one or more layers of leaf sheaths (Fig. 1). This study Plectocomia. Of the numerous buds distributed along investigates the relative mechanical contributions of the stem, some die (Fig. 2). Buds develop on stems the stem and the leaf sheath to the overall mechani- with still living apical meristems and therefore might cal architecture in young to old plants. Stiffness of not necessarily result from injury. Branching can the leaf sheath can be calculated simply from the dif- either replace apical growth (Figs 2A, 4C) after mer- ference between flexural stiffness of the entire axis istem injury, or form additional extensions into the and that measured for the stem proper, after removal canopy. Branches form on climbing axes (Fig. 4A) as of the leaf sheath (Isnard et al., 2005). The diameters well as on stems lying on the forest floor (Fig. 4B). and bending properties of entire axes were measured Adventitious roots are produced up to 5–6 internodes and then re-measured after manually removing the from the branch point on main stems (not shown), and leaf sheaths. branches in contact with the forest floor may become

86 S. ISNARD

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Figure 2. Variation of internode length (cm) as a function of distance from base (m) for two old climbing axes. The height of the forest canopy is approximately 20 m. The position of buds (alive and dead) and branch points are plotted on the axes. A, old climbing axis with dead terminal meristem of the main axis and two sympodial branches, the more distal of which was attached to the forest canopy. B, formation of buds on a long climbing stem with a living meristem (arrows illustrate ﬂuctuations in internode length).

Figure 3. A, appearance of a vegetative bud on the proximal part of an internode and subsequent longitudinal rupture of the leaf sheath. B, formation of aerial adventitious roots at the distal part of an internode. C, later developmental stage of branch emerging from the distal part of an internode and adnate to the internode above. independent after the death of the main stem rooting and (4) height of the forest canopy. The main axis of system or after injuries of the stem and subsequent the ﬁrst individual (Fig. 2A) shows an increase in loss of vascular continuity. We observed this phenom- internode length from the base to 6 m height (21– enon of layering where climbing stems have slipped 41 cm internode length). This is followed by a drop at from their support and come into contact with the soil. 7–9 m (down to 15 cm internode length), and then an Field observations of P. himalayana showed that increase from c. 10–15 m (15–25 cm internode length). internode length can change along the stem and There are a number of marked ﬂuctuations in intern- branches. This part of the study focused on these ode length along this overall pattern (Fig. 2A, arrows). changes in the main stems and branches of two indi- Interestingly bud and branch origins appear to corre- viduals. Patterns of change in internode length were spond to points following drops in internode length. studied in relation to (1) distance from the base of the The main stem apex died at 15 m length, before reach- main axis, (2) juvenile self-supporting to mature ing the surrounding forest canopy, which is approxi- climbing phases, (3) positions of branches and buds mately 20 m in height. The most distal branch (origin

MECHANICAL PROPERTIES AND BRANCHING IN PLECTOCOMIA HIMALAYANA 87

Figure 4. A, branch of a climbing axis, showing development of adventitious roots. B, upright branch derived from a cane lying on the soil. Development of adventitious roots can result in natural layering; the rooted portion of the stem may become an independent plant if connecting tissues with the parent axis decay. C, sympodial branching and rooting of stem in contact with the soil, after injury and death of the apical meristem (black arrows illustrate parent axes and white arrows indicate branches). at 14 m) reached the canopy and showed an increase branches increases distally for most of the length with and then decrease in internode length. The decrease values of the same magnitude as old stages (Fig. 5A). corresponded to the height of the forest canopy (c. The decrease of flexural stiffness in the most apical 20 m), after which internode lengths subsequently parts of three young axes is explained by the relatively decrease for the last 5 m. The branch developing at c. flexible young apical segments, which are composed 3 m on the main axis also showed an increase in inter- mainly of immature tissues. The pattern of flexural node length (c. 20 cm up to 30 cm internode length). A stiffness follows changes in axial second moment of second stem was sampled that was climbing in the area (Fig. 5B). In an older individual where a green understorey, reaching up to 15 m in length. Internode and intact leaf sheath surrounded the stem along a lengths of this second stem show a marked initial large portion of the axis (Fig. 5A, arrow) flexural stiff- increase after which internodes remained long, fluctu- ness also increased distally and this is also concomi- ating between 25 and 45 cm. tant with an increase in axial second moment of area (Fig. 5B). A slight distal increase in flexural stiffness and axial second moment of area also occurs along MECHANICAL ARCHITECTURE AND CHANGES IN canes, i.e. old axes where leaf sheaths have been lost FLEXURAL RIGIDITY (Fig. 5A, B). This is readily explained by the marked Young axes tested are represented by branches with obconical geometry of the canes. The increase in both leaf sheaths, which are intact and green for the entire flexural stiffness and axial second moment of area is length of the branch. Flexural stiffness (EI) along more marked in the distal part of the climbing axis

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l x young stem a e i l x F old stem A 1.0E+06 1.0E+03 02468101214 02468101214 Distance from base (m) Distance from base (m)

Figure 5. Plot of variations of ﬂexural stiffness (EI, Nmm2) (A) and axial second moment of area (I, mm4) (B) as functions of distance from base of the axes (m). Values are plotted for three older axes, which had lost the leaf sheath for all or part of the axis; and four young axes with a green intact leaf sheath along the entire length. Arrow indicates the position at which the leaf sheath has been lost and below which the stem is cane-like. where the leaf sheath is still present, emphasizing the 6000

f o mechanical importance of the leaf sheath to the

s 5000 u mechanics of axes. In P. himalayana, the leaf sheath l u ) d contributes to 72 ± 8.9% (N = 20) of axis stiffness in 2 o 4000 m young branches and apical parts of old climbing axes N/m s ' M g

( 3000 where the leaf sheath was green and intact. n

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2000 e l t a s r

STRUCTURAL YOUNG’S MODULUS ESTR u t

c 1000 young stem u

There is no signiﬁcant change in mean values of Estr r t old stem between young developmental stages such as distal S 0 02468101214 parts of older axes, young individuals with intact leaf Distance from base (m) sheaths, and older basal stages represented by axes

(canes) lacking leaf sheaths [Estr (young axes) = Figure 6. Variation of structural Young’s modulus (E, MN/ 2 2 3250 ± 1030 MN/m (N = 22); Estr (canes) = 4120 ± m ) of the stems as a function of distance from the base 780 MN/m2 (N = 30)]. The structural Young’s modulus (m). Values are plotted for three older axes, which had lost of the stems alone calculated for young individuals is the leaf sheath for all or the basal part of the axis; and four 3391 ± 871 MN/m2 and does not differ significantly young axes with a green intact leaf sheath along the entire from that of entire axes. The mean value of structural length. Arrow indicates position at which the leaf sheath Young’s modulus calculated for leaf sheaths (3509 ± has been lost and below which the stem is cane-like. 1045 MN/m2) is also in the same range of values as that of the stem alone, highlighting the fact that the leaf sheath does not influence the structural Young’s P. himalayana. The presence of bulb-like shoots in the modulus of the stem and leaf sheath combined. proximal part of lower internodes in P. elongata In young axes with intact leaf sheaths, there is a (Dransfield, 1978; Uhl & Dransfield, 1987) suggests decrease of structural Young’s modulus from the base that this feature is perhaps characteristic of the to the apex (Fig. 6). This is also observed along distal genus. The results here suggest that bud formation parts of older climbing axes with intact leaf sheaths, and the subsequent development of branching occur but the pattern is not evident in two older canes with on long climbing axes over 10 m long. Observations senesced leaf sheaths (Fig. 6). Variations in Estr also indicate that branching might not always be linked occur independently from the distance from the base. with flowering. Branch initiation in P. himalayana thus differs from that described for Korthalsia, where DISCUSSION aerial lateral buds are not normally produced until the shoots become reproductive and initiate axillary DISTINCTIVE FEATURES OF PLECTOCOMIA HIMALAYANA inflorescence branches (Fisher & Dransfield, 1979). Morphological novelties such as aerial branching and Branching possibly occurs on axes with a living apical rooting are the most distinctive characteristics of meristem (delayed branching) or after (1) basal stem

© 2006 The Linnean Society of London, Botanical Journal of the Linnean Society, 2006, 151, 83–91 MECHANICAL PROPERTIES AND BRANCHING IN PLECTOCOMIA HIMALAYANA 89 injury and decay of a single rooting system, (2) apical the pattern of internode change observed in the main flowering, or (3) injury of the terminal meristem. The branch of P. himalayana that had reached the canopy. timing and conditions of branching need further inves- However, reduction in internode length might also tigation to understand whether they develop after the correspond to when climbing axes fall entirely or release of apical dominance or on old climbing stems partially from their supports, which could be a result that have reached the canopy. Nevertheless, formation of the ephemeral nature of their attachment device of buds occurring on stems with seemingly living api- (Putz, 1990). Apices of such plants often recommence cal meristems suggests that if branching occurs after vertical growth with reorientation from the horizontal the release of apical dominance, the ability of the plant or downward orientated fallen axis. This organization to survive such events is possibly part of its natural could effectively return the initially upright part of the development and perhaps not to be viewed as some axis to a ‘self-supporting’ organization with short kind of abnormal branching. Inhibition of axillary internodes, possibly observed in the main stem in buds resulting from a strong apical dominance is com- P. himalayana (Fig. 2B). The increase in internode mon in branching monocotyledons (Tomlinson et al., length after about 10 m could correspond to attach- 1970). The basal portion of rattans in which the apex ment and recommencement of the climbing phase. The has died can remain alive for many years (Putz, 1990) second climbing plant showed long internodes, corre- and this can make it difficult to distinguish living, sponding to a climbing phase following a short inter- growing stems from dying stems without examination node at the base (Fig. 2B). This pattern of internode of the terminal meristem. As a consequence, a better length shows frequent fluctuations possibly related to understanding of the initiation and development of variable attachment, but overall the plant retains buds in rattans requires closer examination of the ter- relatively long internodes. Finally, the shortening of minal bud. internode length when the climbing palm reaches the Many individuals were observed with extensive host canopy is comparable with the situation observed branching in the canopy in which branches developed in fallen stems. In a way, such stems might also have on canes, i.e. old stems without leaf sheaths forming redeveloped a self-supporting phase if leaf-bearing loops on the forest floor. When branching occurs on axes have emerged from the surrounding host vegeta- these understorey axes, the development of adventi- tion and have effectively ceased climbing. Further tious roots from several nodes at either side of the studies would be of interest to observe the morphologi- branching, allows layering of the axes, and subsequent cal and mechanical behaviour of leading axes of independence of the branch. Such architectural plas- climbing palms when they emerge from the host veg- ticity could enhance survival from potentially fatal etation, and to ascertain to what extent such stems mechanical and hydraulic trauma, and increase the either anchor the plant below and prevent large-scale vegetative propagation of this hapaxanthic species. slipping, or continue growth either vertically or The phenomenon might also at least partially explain horizontally. the distribution of this species at relatively high alti- tudes and its growth in potentially hostile environ- ments, with snow and frost. Furthermore, the process MECHANICAL ARCHITECTURE has assisted cultivation of this species as the Hani The second part of this study investigated the people from the Mengsong community remove por- mechanical architecture of P. himalayana and focused tions of stems with buds and adventitious roots to on: (1) variations of flexural rigidity and structural propagate the species in plantations. Clearly, further Young’s modulus along axes from young to old stages studies need to be carried out in order to understand of growth and (2) the mechanical contributions of the which processes induce development of buds and why stem and the leaf sheath to the bending properties of and how long the buds could remain dormant. the axis. The results suggest a different mechanical All stems and branches tested show initially short architecture for P. himalayana (Plectocomiinae) from internodes corresponding to a ‘self-supporting’ phase, other climbing palms tested in the subfamily (Calam- i.e. young individuals or young branches, which were oideae), but belonging to a different subtribe (Calam- self-supporting prior to their attachment to surround- inae). Recent studies on five species of Calamus and ing supports. The increase in internode length one of Daemonorops showed that leaf sheath contribu- observed following these juvenile phases characterizes tion to flexural stiffness in young individuals and the climbing behaviour. Decreases in internode length apical parts of older axes is very high and around may be linked with different environmental or life- 90% (Rowe et al., 2004; my unpubl. data). In history factors. It has been shown for several rattan P. himalayana, however, this contribution is signifi- species that internode length in the climbing phase cantly less at about 70%. All climbing palms are char- may be long, but shorter when the crown reaches the acterized by the presence of a thick leaf sheath canopy (Putz, 1990). This pattern is consistent with surrounding the stem (Tomlinson, 1962) that

© 2006 The Linnean Society of London, Botanical Journal of the Linnean Society, 2006, 151, 83–91 90 S. ISNARD increases axis diameter and consequently axis stiff- climbing habit results in a significant modification of ness. In species of Calamus tested, the structural the mechanical architecture observed in arborescent Young’s modulus of the leaf sheath (the inherent stiff- palms. Indeed, in contrast to self-supporting palms, ness of the material, independent of stem size and the stems of at least some species of Calaminae shape) is higher than that of the stem (unpubl. data) (Calamus-Daemonorops) are highly compliant, with and, therefore, contributes significantly to the stiff- values of structural Young’s modulus close to those of ness of compound axes. This mechanical architecture dicotyledonous lianas tested. Moreover the mechani- differs in P. himalayana where values of leaf sheath cal function of the leaf sheath appears to represent a

Estr are in the same range as those of the stem and the major innovation allowing variation of stiffness with contribution of the leaf sheath to the rigidity of the age and in relation to external mechanical stresses. In axes is relatively less than in other tested rattans. P. himalayana, however, material properties of the Senescence and loss of the leaf sheath also reduce stem appear to be less specialized for the climbing stiffness in old stages, but not as drastically as habit in terms of high compliance and for evading observed in the species of Calaminae tested (Rowe high mechanical stresses. Structural Young’s moduli et al., 2004; unpubl. data). Old stems of P. himalayana remain high; and the mechanical function of the leaf are stiffer in terms of material properties (c. 4000 MN/ sheath is reduced. However, this species shows a rare m2) than (1) most dicotyledonous lianas previously ability among climbing palms to branch and produce tested (c. 200–1000 MN/m2) (Speck, 1994; Rowe & aerial roots conferring a higher degree of architectural Speck, 1996, 1998; Isnard et al., 2003a) and (2) stems plasticity. of Calaminae species tested (Calamus and Daemono- rops; Rowe et al. 2004); however, they are less stiff ACKNOWLEDGEMENTS than (3) stems of the South American genus Desmon- cus (Arecoideae subfamily) (c. 10 000–14 000 MN/m2) The author thanks the Xishuangbanna Tropical (Isnard et al., 2005). Botanical Garden (XTBG), Chinese Academy of Sci- Although the mechanical function of the leaf sheath ences for generous support during fieldwork (2004–05) is reduced in P. himalayana compared with other rat- in Yunnan province, China and Hu Jian-Xiang (XTBG) tans tested, the species shares a peculiar mechanical for field assistance and species identification. This design with other rattans, where flexural stiffness of study benefited from helpful discussions with Nick the entire axes decreases towards the base. This pat- Rowe, which are gratefully acknowledged. The tern of geometry and mechanical properties is not research was supported by the ‘Agence Universitaire common among terrestrial plants, even among lianas, de la Francophonie’ (AUF) (France). where there is often an increase in stem diameter towards the base resulting in an increase of flexural REFERENCES stiffness even though the material properties become far more compliant (Speck & Rowe, 1999). This Baker WJ, Dransfield J, Hedderson TA. 2000a. Phylogeny, mechanical organization with a decrease in flexural character evolution, and a new classification of the Calamoid stiffness towards the base is found in some other non- palms. Systematic Botany 25: 297–322. self-supporting plants (Rowe & Speck, 1998). It is Baker WJ, Hedderson TA, Dransfield J. 2000b. Molecular consistent with a nonself-supporting rather than self- phylogenetics of Calamus (Palmae) and related rattan gen- supporting architecture: optimization of axis stiffness era based on 5S nrDNA spacer sequence data. Molecular toward the apex before loss of the leaf sheath is a Phylogenetics and Evolution 14: 218–231. climbing strategy that produces sufficient stiffness to Dransfield J. 1978. Growth forms of rain forest palms. In: Tom- reach and maintain support with the surrounding linson PB, Zimmermann MH, eds. Tropical trees as living sys- vegetation (Speck & Rowe, 1999). tems. Cambridge: Cambridge University Press, 247–268. Dransfield J, Uhl NW, Asmussen CB, Baker WJ, Harley Variations of structural Young’s modulus along the MM, Lewis CE. 2005. A new phylogenetic classification of stem and axis could be linked with variations of inter- the palm family, Arecaceae. Kew Bulletin 60: 559–569. node length during the different phases from self- Evans TD. 2001. A field guide to the rattans of Lao PDR. Kew: supporting to climbing. This analysis has not given Royal Botanic Gardens. unequivocal evidence of this, but further studies Fisher JB, Dransfield J. 1977. Comparative morphology and might elucidate how such morphological features, development of inflorescence adnation in rattan palms. apparently modulated by ontogenetic and environ- Botanical Journal of the Linnean Society 75: 119–140. mental factors, might influence the mechanical prop- Fisher JB, Dransfield J. 1979. Development of axillary and erties of the stem and entire axis. leaf-opposed buds in rattan palms. Annals of Botany 44: 57– In conclusion, the results suggest the evolution of 66. different ‘climbing strategies’ within the subfamily Isnard S, Rowe NP, Speck T. 2003b. Growth habit and Calamoideae. In Calaminae, specialization of the mechanical architecture of the sand dune-adapted climber

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