Mechanical Architecture and Development in Clematis

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MechanicalBlackwell Publishing Ltd. architecture and development in Clematis: implications for canalised evolution of growth forms

S. Isnard1, T. Speck2 and N. P. Rowe1 1Botanique et Bioinformatique de l’Architecture des Plantes, UMR 5120 CNRS, TA40/PS2, Boulevard de la Lironde, 34398 Montpellier, France; 2Plant Biomechanics Group, Institute for Biology II, Botanical Garden of the Albert-Ludwigs-Universität, Schänzlestrasse 1, D-79104 Freiburg, Germany

Summary

Author for correspondence: • Mechanical architectures of two Clematis species, the herbaceous perennial S. Isnard Clematis recta and the woody liana, Clematis vitalba, were investigated and + Tel: 33 (0) 467617553 compared with the woody rhizomatous sand dune plant Clematis flammula var. Fax: +33 (0) 467615668 Email: [email protected] maritima. • Bending mechanical properties of stems from various developmental stages were Received: 25 September 2002 compared and related to stem geometry and relative proportions of tissues during Accepted: 28 February 2003 development. doi: 10.1046/j.1469-8137.2003.00771.x • Clematis vitalba and C. flammula var. maritima showed mechanical architectures with reductions in structural Young’s modulus of the stem during ontogeny. Irrever- sible loss of stem rigidity was mediated by disruption, separation and eventual loss of primary phloem fibres via secondary growth of the periderm and cambial activity. Each species showed variations of non-self-supporting mechanical architecture relating to specific habitat preferences. In aerial stems of C. recta the structural Young’s modulus remained approximately constant during ontogeny, a mechanical signal characteristic for semi-self-supporting architectures. •Woody aerial plant stems are extremely rare in the Ranunculaceae and seldom, if ever, show self-supporting characteristics. Growth form evolution in the group may have been canalised by evolution of rhizomatous geophytic growth forms with secondary growth confined to underground stems specialized for water conduction, storage and perennation. Variation of this ground plan includes climbing, straggling or rhizomatous architectures but not self-supporting shrubs or trees with secondary growth generating requisite self-supporting mechanical properties. Certain body plan organisations appear to have inbuilt mechanical constraints which may have profound effects on the subsequent evolution of growth forms.

Key words: architecture, biomechanics, canalised evolution, Clematis, growth form. © New Phytologist (2003) 158: 543–559

development and anatomy can be viewed in an evolutionary Introduction context (Rowe & Speck, 2003; Speck et al., 2003). Mechanical properties of plant stems and the changes of these The modiﬁcation in stems of parameters such as ﬂexural properties during stem development are increasingly being stiffness (EI ) and structural Young’s modulus (Estr) between used to investigate the variation and diversity of plant growth young and old stages of development can distinguish different forms (Speck & Rowe, 1999). Such studies can characterize mechanical architectures such as ‘self-supporting’ and ‘non- and distinguish different growth forms and demonstrate how self-supporting’ plants. In self-supporting plants, the struc- various mechanical strategies are deployed by different plants tural Young’s modulus increases during ontogeny from young (Gartner, 1991a, 1991b). More recently such studies have to old stages and in non-self-supporting plants, structural been combined with phylogenetic studies in which trends Young’s modulus decreases (Speck & Rowe, 1999). This in mechanical properties underlying changes in plant approach has detected a third type of growth form in which

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structural Young’s modulus does not change signiﬁcantly annual shoots dominated by primary development. Clematis during development of the stem. This has been termed ‘semi- is positioned along with Ranunculus, Trautvettaria and self-supporting’ in which plants simply lean against the sur- Anemone within the Ranunculoideae ( Johansson & Jansen, rounding vegetation. Biomechanical architectures of living 1993; Hoot, 1995; Johansson, 1995; Hoot et al., 1999), plants are investigated by 2-, 3- or 4-point bending tests. which is possibly sister to a second distal group including These methods are now combined with studies, which inves- genera such as Helleborus, Trollius and Caltha. All of these tigate patterns of tissue development from young to old stages genera are predominantly herbaceous or perennial rhizome- and to correlate primary and secondary tissue development forming plants apart from Clematis, which includes perennial with changes in mechanical properties of the stem system. rhizomatous herbs, climbers and some woody species Recent studies have shown that, within each type of described as shrubs or subshrubs. Species of Clematis mechanical architecture, the exact type of signal can vary. described as shrubs include Clematis davidiana, Clematis stans, Such changes can include global values of mechanical para- Clematis addisonii, Clematis integrifolia and Clematis meters as well as the exact timing and trend of change in a heracleifolia (Walters et al., 1989). Inspection of the precise parameter during ontogeny (Speck & Rowe, 1999). For growth forms of the two latter species indicate that the former example, trees and shrubs show two different types of increase is a perennating rhizomatous species with upright, primary

in Estr during ontogeny and both have self-supporting growth stems; C. heracleifolia is a low, much branched shrub with forms. In trees an increase in structural Estr is seen throughout ﬂexible aerial axes that barely remains upright for lengths of the entire ontogeny of the plant, albeit only slightly towards more than a metre. Further members of the Ranunculaceae

the end of the tree’s growth. In shrubs, Estr levels off although include the genera Aquilegia, Coptis, Isopyrum, Thalictrum stem thickening continues up to the end of growth of the and Xanthorhiza, which form a paraphyletic group, the stem. While secondary growth can play an important role in ‘Thallictroideae’, occurring just above the basal group of changes in stem biomechanics during growth of woody Hydrastis and Glaucidium. These are all perennial herbs with plants, primary differentiation and growth is also important. tuberous, rhizomatous or creeping rhizomatous stems and Young stages of growth of woody plants as well as herbaceous upright annual primary stems. Xanthorhiza is often referred to growth forms lacking secondary growth rely on primary tis- as a woody shrub or subshrub; the single occurring species sues for mechanical support (Niklas, 1990, 1992; Speck & Xanthorhiza simplicissima forms an extensive rhizomatous Vogellehner, 1994; Rowe & Speck, 1997). It is of considerable network producing weakly upright woody stems rarely interest to know what mechanical strategies occur among her- exceeding 60 cm in height. Most members of the Ranun- baceous plants and in what ways these differ from those with culaceae are therefore herbs or tuber/rhizome-forming secondary growth. From an evolutionary perspective, transi- herbs with aerial shoots mostly confined to relatively short- tions between architectures dominated by primary and sec- lived annual stems with primary growth. Secondary growth, ondary growth essentially combine profound changes in body if there is any, is mostly confined to underground organs. The plan organisation with potentially far-reaching effects on the exceptions cited above, may be described as ‘weakly shrubby’ ecology and further evolution of mechanical architectures in view of the fact that the aerial stems are woody, but do not (Niklas, 1997; Bateman et al., 1998; Niklas & Speck, 2001; support strongly self-supporting woody axes above up to a Rowe & Speck, 2003). Are clades that have evolved a lianoid metre in length or height. In the Ranunculaceae, woodiness growth habit capable of returning to a fully self-supporting has been described as secondary ( Judd et al., 2002). habit? Can clades with body plans modified into perennial underground rhizomes and aerial herbaceous axes evolve into Biomechanical approach self-supporting aerial stems with woody axes? Do climbers that have evolved from woody self-supporting ancestors show This study presents new biomechanical data on the archi- different mechanical strategies and properties from those that tecture of two species of Clematis including: Clematis vitalba have evolved from herbaceous or rhizomatous plants? L. and Clematis recta L. and discusses these results in the light of a recent study of Clematis flammula var. maritima L. (Isnard et al., 2003). The study follows previous initial Systematics of the Ranunculaceae and its growth forms reports (Speck, 1991, 1994) in which mechanical bending Clematis is a cosmopolitan genus of the Ranunculaceae tests on C. vitalba and C. recta were used for establishing and showing a range of growth forms mostly confined to per- comparing patterns of mechanical behaviour in non-self- ennial herbs and climbers among its 250 species. Recent supporting and semi-self-supporting plants. phylogenetic analyses indicate that the Ranunculaceae is In this account we compare biomechanical characteristics monophyletic (Hoot, 1995; Hoot et al., 1999) and placed in terms of the overall growth form, and then compare ana- within the Ranunculales among ‘basal tricolpates’. Basal tomical development and quantify changes in geometry and members of the family, Hydrastis and Glaucidium are organisation of mechanically important tissue types. We perennial herbs with underground rhizomes and upright then analyse mechanical and developmental features along

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Research 545 representative branch systems of each species. Finally we were maintained in humid conditions before testing to discuss how developmental and biomechanical features com- conserve the mechanical properties of fresh material. Bending bine to produce different growth forms within Clematis and tests were carried out on a metal frame apparatus on which ask what are the potential evolutionary constraints under- span distances and weight applications could be varied. Tests lying growth form diversity in the group. We discuss how were mostly of the four-point bending type (Vincent, 1990). mechanical architectures might be constrained by the type of Flexural stiffness (EI ) and structural Young’s modulus (Estr) ancestral body plan in which most representatives are were calculated from linear relationships resulting from herbaceous perennials. observed deflections of stem segments after adding a sequence of weights. In this paper we use the term Young’s modulus in the sense of a structural Young’s modulus E (Speck & Rowe, Materials and Methods str 1999), which represents that of an anisotropic material, namely a plant stem. Sampling and bending tests Species of Clematis were selected showing different growth Stem geometry and anatomy forms in different habitats. Clematis vitalba is a woody liana most commonly found as an invasive species in wooded areas Measurements of second moment of area (I ) of tested stem and woodland margins in temperate regions. The species segments of C. vitalba were calculated using a simple elliptical climbs into the host vegetation by sensitive petioles with model using the formula: I = 0.25π a 3b, where a is the radius climbing stems ranging up to 15–20 m in length and up to of the stem in the direction of the applied load and b is the approx. 10 cm in diameter (Fig. 1a,b). radius in the direction perpendicular to the load. I of the Clematis recta shows a growth form which is neither self- hollow stems of C. recta was calculated via the following = π 3 3 supporting nor lianescent (Fig. 1c). Aerial axes are produced formula: I 0.25 (a b – a 1 b1) where a and b represent the from an underground rhizome (Fig. 1d) and rarely reach entire radius of the stem in the direction of the applied load more than 1.5 m in length. The axes are normally upright and and perpendicular to the applied load, respectively; a1 and b1 attached either to each other or to neighbouring plants via represent the radial thickness of the space inside the stem in sensitive petioles. If a support is not available after a certain the direction of applied load and perpendicular to the load. limit of elongation growth, aerial stems become unstable and Dimensional measurements were obtained immediately after arch over and settle on the ground. C. recta has been described bending tests and means were obtained from three to four as having a semi-self-supporting growth form (Rowe et al., positions along the tested stem segment. 1993; Speck, 1994) and shows limited secondary growth at Anatomical analyses included contributions of main the base of aerial stems. Significant secondary growth occurs tissues to axial second moment of area (I ). Tissue measure- in foreshortened rhizomes. ments were carried out on sectors of entire cross-sections We compare mechanical architectures of C. vitalba and C. showing at least two alternating phases of tissue symmetry recta with a recent biomechanical study of C. maritima (Isnard such as two secondary xylem segments and alternating et al., 2003). This species grows on coastal sand dunes in interfascicular rays. The centre of the stem cross section Mediterranean areas. The growth form is best described as represented the theoretical position on the neutral axis of partly prostrate and includes young stages, which form a com- the bending experiment. Surface areas of tissues were traced plex trellis system above the unstable sand substrate. Stems of manually or via automatic thresholds of stained tissues. I was the trellis are interconnected via sensitive petioles and do not calculated via the known centroid of the section and from a generally reach heights of more than 0.4 m. Older stages of macro written for Optimas and kindly provided by Tancred growth are characterized by flexible stems with secondary Almeras. Tissues were stained in HCl (35%) and phloroglu- growth, which are normally buried in the sand dune but are cinol (3%) in 95% ethanol. occasionally exposed by sand movement. Material of C. vitalba was collected from marginal wood- Ontogenetic stages land sites in the Rhine valley in the vicinity of Freiburg (Ger- many) and material of C. recta from the Botanical Gardens of Four ontogenetic stages were defined for the two species the Albert-Ludwigs University, Freiburg. distinguished by specific developmental and anatomical Bending tests were carried out within 3 d of sampling and features (Figs 1 and 2) and external morphology. were confined to straight portions of axis, which tapered < 10% of the mean diameter of each tested segment. Seg- Stage I Young axes essentially comprise primary tissues. ments were pruned from representative positions from all over Primary sclerenchymatous fibres are present as a continuous the plant body. Only aerial stem segments were tested in C. ring in C. vitalba occurring outside the vascular strands recta, as older, foreshortened parts of the rhizome and associ- (Fig. 2a), and as separate bundle sheaths in C. recta (Fig. 2b). ated parts were not testable in bending experiments. Specimens In C. vitalba this stage represents young axes of young lianoid

Fig. 1 Growth forms of two species of Clematis. (a) Mature scandent axes of Clematis vitalba attached via younger stages in the tree crown; (b) young axes of C. vitalba including ‘searchers’ extending from the host plant; (c) erect stems of C. recta leaning into surrounding vegetation, those that do not attach themselves to a support extend beyond their critical buckling length and curve over in a decumbent orientation; (d) horizontal rhizome of C. recta producing erect aerial stems and roots; ontogenetic stages are marked according to the text and the sections in Fig. 2.

‘searchers’, which span the gaps between host plants and this stage is confined to the very basal segments of annual emerge from the host canopy (Fig. 1b). In C. recta this stage aerial stems and is characterised by lignification of paren- incorporates almost the entire aerial part of upright chyma and suberisation of the cortex (Fig. 2f ). herbaceous stems (Fig. 1c). Stage III Centripetal displacement and loss of primary fibres Stage II Axes with secondary tissues and with an initial and development of a sequent periderm in the secondary periderm forming just beneath the primary phloem in C. phloem (Fig. 2c,d). These are represented by older aerial stem vitalba (Fig. 2b) and representing older ‘searchers’. In C. recta segments in C. vitalba (Fig. 1a) and in C. recta by short

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Fig. 2 Transverse sections of four defined ontogenetic stages. (a–d) C. vitalba, (e–h) C. recta. a,e: stage I; b,f: stage II; c,g: stage III; d,h: stage IV. Tissues: 1, collenchyma; 2, primary sclerenchymatous fibres; 3, secondary sclerenchymatous fibres; 4, xylem; 5, pith; 6, vascular cambium; 7, suberised cells; 8, lignified parenchyma; 9, interfascicular ray tissue; 10, periderm. underground segments of stem developing from the rhizome C. vitalba (Fig. 1a) and horizontal underground rhizomes of (Fig. 1g). C. recta (Fig. 1d).

Stage IV Significant secondary growth with stems com- Results prised of secondary xylem with many large vessels, significant interfascicular parenchyma resembling wide rays and Mechanical and developmental variations of the two secondary fibres of the secondary phloem and periderm growth forms (Fig. 2d,h). This final stage of development characterizes older, flexible, and vertically orientated or hanging stems of Size, second moment of area (I ) and geometry The two

Fig. 3 Biomechanical results of the entire data set of C. vitalba and C. recta, The plot includes, for comparison, data based on the sand dune species C. ﬂammula var. maritima (see Isnard et al., 2003). Double logarithmic plot of ﬂexural stiffness (a) and structural Young’s modulus (b) against axial second moment of area for the three species. The neutral line is calculated by using the structural Young’s modulus of the youngest ontogenetic stage as a slope under the assumption of a constant Young’s modulus in all ontogenetic stages (see Speck & Rowe, 1999).

species differ significantly in the size and patterns of change of flexural stiffness between the two species. Axes of C. recta in size and geometry in the aerial stems. Axial second mo- are produced from a rhizome and typically do not exceed ment of area of aerial stems of Clematis recta ranges from 4 1.5 m in height; the stems are hollow and circular to to 120 mm4 (Fig. 3a) and do not reach the large diameters elliptical in cross-section, stems taper only slightly seen in C. vitalba. The second main difference is that aerial (Table 1). C. vitalba shows a significant change in cross- upright stems of C. recta are larger than the young stages of section geometry from octagonal to circular or elliptical C. vitalba (Fig. 4a,b) with values of I for stage I and II of with a crenulated or ribbed outer margin. C. recta also C. vitalba generally lying between 0.7 and 19.5 mm4 and shows generally larger young stems than reported for the those of C. recta between 4 and 120 mm4. This difference trellis-forming young stages of C. maritima (Isnard et al., in size could have an important effect on the relative values 2003).

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Fig. 4 Biomechanical results based on deﬁned ontogenetic stages. Double logarithmic plot of ﬂexural stiffness against axial second moment of area for (a) C. vitalba and (b) C. recta.

Table 1 Comparison of geometrical parameters of the three species Flexural stiffness (EI) (C. vitalba, C. maritima and C. recta) for different ontogenetic stages

Values of EI are plotted against I as a double logarithmic plot (Fig. 3a). Following recent studies on growth form C. recta C. maritima C. vitalba biomechanics this type of graph compares shifts in EI during A (mm2) ontogeny with reference to a ‘neutral line’ for each species OS1 15 ± 3.7 10.2 ± 3.2 6.5 ± 2.8 including data recorded for C. maritima (Isnard et al., 2003). OS2 24.9 ± 6 12.9 ± 3.2 10.8 ± 3 The neutral line represents calculated values of EI for each OS3 – 17.6 ± 4.1 24.7 ± 8 OS4 – 41.6 ± 16.4 154.9 ± 70.7 species in which the value of Estr is ﬁxed as that of the youngest stage of development. If the increase in EI observed from the I (mm4) experimental data was solely caused by the increase in size and OS1 19.8 ± 7.6 8.8 ± 4.9 3.7 ± 3.5 ± ± ± geometry (I ) and if the Young’s modulus (E ) of the plant OS2 44.2 24.4 13.8 6.5 9 5.2 str OS3 – 25.5 ± 11.3 51.8 ± 32.8 stem remained unchanged during ontogeny, the values would OS4 – 152.3 ± 116 2305 ± 2221 be arranged along or scattered equally around the neutral line. This presentation of data allows visual analysis of shifts in A, area (mm2), I, axial second moment of area (mm4). Values EI and I relative to stages of development with a simple represent means expressed as percentages ± SD. –, tissues absent. assumption of the stem’s mechanical properties. In C. vitalba the distribution is typical of non-self- supporting plants where data for larger stages of develop- plant. This trend is very marked in C. vitalba where data ment are situated well below the neutral line (Fig. 3a). This from older stages are well below the neutral line. The data set indicates that older more basal parts of the plant body show a based on developmental stages indicates that both stages I and relatively lower ﬂexural stiffness than younger stages of the II are scattered around the neutral line and that the shift in

Fig. 5 Biomechanical results based on deﬁned ontogenetic stages. Double logarithmic plot of Young’s modulus against axial second moment of area for (a) C. vitalba and (b) C. recta.

mechanical properties occurs immediately after stage II vitalba and C. maritima (Fig. 3a). Shifts in EI for C. maritima (Fig. 4a). differ from those of C. vitalba and occur in the transition from In C. recta the data are distributed around the neutral line stage I to stage II. (Fig. 3a) and this type of mechanical signal during ontogeny is typical of semi-self-supporting plants (Speck & Rowe, Structural Young’s modulus (E ) 1999). The data show a slight tendency of larger stems of stage str

II to be positioned slightly above the neutral line, but this is Consideration of the entire data sets indicates that Estr of hardly marked in all the stems tested. The plot of ontogenetic young aerial stems of C. vitalba is marginally lower than those stages indicates that there is a shift in EI from stage I to higher of C. recta (Fig. 3b). Both show a relatively broad variation values in stage II correlated with the increase in I (Fig. 4b). from observation of the general data set. In aerial stems of Both the neutral lines of C. recta and C. vitalba are rela- C. vitalba there is a general and significant drop in Young’s tively close reflecting similarity in structural Young’s modulus modulus from high levels in young stages to lower values in of young stages (Fig. 3). In C. vitalba there is an initial phase broader stems. A similar drop in Young’s modulus was of increase in EI, followed by a sudden drop around stem axial observed in C. maritima, which shows generally lower values second moment of area of 20–30 mm4 (Fig. 4a). There is then for both young and old stages of development. a gradual increase in EI during ontogeny but this does not Developmental data indicate that in C. vitalba, the Young’s = approach the levels of the neutral line. This indicates an modulus remains relatively stable between stage I (Estr 3074 ± −2 = ± −2 abrupt and irreversible shift in material properties despite an 1083 MN m ) and stage II (Estr 3126 784 MN m ) increase in size during growth and development. A similar (Fig. 5a), and that the drop in Young’s modulus occurs in = ± −2 trend has been observed in the shift in EI for C. maritima stage III (Estr 979 478 MN m ). The drop in Estr during including an irreversible shift in EI (Isnard et al., 2003). Stage ontogeny observed for C. vitalba (Fig. 2b) is characteristic of II segments of C. recta representing the bases of aerial stems other tested non-self-supporting plants and lianas. The rela- are significantly stiffer than the equivalent stages of both C. tively high values of Young’s modulus for young stages of

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= = 4 C. vitalba (mean Estr 3074, max Estr 5315) are comparable 1), second moment of area increases from 3.5 to 4031 mm with those found among other tested lianas (Rowe & Speck, between stages I and IV and the flexural stiffness from 7 × 10−3 2 1996; Speck et al., 1996; Gallenmüller et al., 2001). Estr to 5 N m . These changes occur over 15 m of stem between remains relatively stable in the oldest stage (IV ) compared branches attached in the host canopy and the ground. In C. with the changes observed in preceding stages of development vitalba there is an abrupt increase in second moment of area = ± −2 with a mean value of Estr 772 261 MN m . Interestingly, in all four stems studied where growth stage III corresponds the drop in structural Young’s modulus is very similar between to a point 3–4 m below the highest stages of growth and well C. vitalba (75%) and C. maritima (71%) despite the differ- below the points of attachment to the host vegetation. Trends ences in timing, overall values of material properties, and con- in EI follow those of increasing second moment of area along trast in growth form of the older axes between a scandent stem the stem. and a decumbent or buried stem. Values of Estr show a decrease basally from the points of The pattern of points for the aerial stems of C. recta attachment of young axes with the host. There is an abrupt (Fig. 4b) is consistent with that of other semi-self-supporting decrease between stages II and III approximately 2–4 m from plants as Estr shows little tendency to increase or decrease dur- attachment in the host crowns. In one stem (Fig. 6c, Vit 2) −2 ing ontogeny. Values of Estr and I remain relatively constant values drop from 2685 to 800 MN m in less than 1 m of −2 reflecting the limited change in size and material properties stem. In another (Vit 3), Estr drops from 3160 to 626 MN m during development. The slight increase seen towards larger over only 0.4 m and in Vit 4, there is a drop from 3072 to −2 and older aerial stems is suggestive of a self-supporting 618 MN m over only 0.45 m. In some cases, Estr increases tendency but the change in values seen in C. recta is much proximally in segments of stage II; in Vit 3 (Fig. 6c), for exam- lower than that observed in similarly young phases of growth ple, there is an increase of 1850 MN m−2 in stage I to in many self-supporting trees and shrubs. The structural 3130 MN m−2 in stage II. Young’s modulus of stage I (4521 ± 980 MN m−2) is not sig- In C. recta the increase in second moment of area towards nificantly different from that of stage II (5132 ± 1200 MN m−2) the base of upright stems is only slight. The greatest variation (F = 3.65, P = 0.06). measured corresponds to a shift from 13 to 120 mm4 on axis In C. maritima the drop in Young’s modulus is produced in Rec1 (Fig. 7a). In this species each aerial stem is derived from stage II and continues on into stage III. During stage II, the a single meristem and is unbranched; proximal increase in I is Young’s modulus is in the order of 850 ± 253 MN m−2, which linear and gradual and values for the bases of upright stems are represents a drop of approx. 47% of the value seen in young relatively similar, between 40 and 120 mm4. Values of EI stages of growth. The relative drop in Young’s modulus seen increase towards the base of tested segments. Estr is stable dur- = ± −2 in stage III (Estr 568 142 MN m ) is relatively well cor- ing the relatively limited development seen along aerial axes related with the increase in size (r 2 = 0.56) (Isnard et al., 2003). and values are relatively high compared with the other species never dropping below 2000 MN m−2 (Fig. 7c). Variations of E are independent of ontogeny along aerial stems. The high Mechanical and developmental variation along stems str value of Estr and a slight increase in I (Fig. 7a) influence the In the results detailed above we analysed shifts in bio- flexural stiffness of the stem, which shows a basipetal increase mechanics and development of many segments from (Fig. 7b). Measurements of older stem segments and rhizomes numerous individual plants without reference to the position of C. recta were not possible with standard bending tests on the plant body in terms of distance along the stem or because of the very small span to depth ratios of internodes distance from mechanical continuity with branch points or (Fig. 1d). insertion in the ground. Such data represents what we refer to as ‘general data’ (Rowe & Speck, 1998) and includes a Anatomy, development and mechanical properties wide sample of stems and branches of different individuals. Mechanical, geometrical and developmental features are Mechanical role of primary fibres In C. vitalba, primary described in a general sense in terms of ontogeny of the fibres are organised in a continuous sheath of lignified fibre entire plant where each cloud of points represents a ‘signal’ elements (Fig. 2a) in which denser elements are arranged in of mechanical characteristics for that species and for all its areas opposite the primary vascular bundles. The fibre sheath branch types. The following results investigate mechanical varies widely in contribution to second moment of area from properties on a smaller data set based on measurements taken 31 ± 5.7% (stage I) to 22.4 ± 5.1% (stage II) (Table 2) but is along the stems of each species in relation to the point of not significantly different between stage I and II. Young stages insertion in the soil or from the point of attachment to host of development have relatively high values of Estr and there is plants. a marked drop in stage III when the sheath of fibres is In C. vitalba, both EI and I increase towards the base of the removed by secondary growth processes. In C. recta primary stem, most notably in segments of stages III and IV in the fibres are confined to bundle sheath fibres opposite vascular lower 4 m of the stem (Fig. 6a,b). In one axis (Fig. 6a,b, Vit bundles and do not form a continuous sheath (Fig. 2e).

Fig. 6 Geometrical and biomechanical measurements along four representative stems relative to the distance from the base of C. vitalba. Ontogenetic stages are deﬁned for each level measured. (a) axial second moment of area, (b) ﬂexural stiffness and (c) Young’s modulus.

Contributions to second moment of area are less than in Onset and effect of secondary growth on mechanical C. vitalba varying from 12.4 ± 1.4% (stage I) to 13.3 ± 2.3% properties (stage II). Primary fibres are present in all parts of the aerial stem and are lost only in underground parts of the stem via In C. vitalba an initial (‘primary’) periderm (Esau, 1977) is secondary growth. initiated in the primary phloem tissue in stage II (Fig. 2b) In C. maritima, the contributions of fibres to second isolating outer tissues and causing their disruption and death. moment of area in stage I are significantly less than in C. A sequent periderm is subsequently formed in stage III within vitalba and C. recta and this is reflected in the lower values of the secondary phloem (Fig. 2c). This drastically alters the

Estr (Table 2). mechanical properties of the stem when primary phloem

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Fig. 7 Geometrical and biomechanical measurements along five representative stems of C. recta from the soil to the apex of the stems. Ontogenetic stages are defined for each level measured. (a) axial second moment of area, (b) flexural stiffness and (c) Young’s modulus.

fibres are detached from the outside of the stem along with the basal axes and segments of stage II, where there is an irregular primary cortex, and collenchyma. or slight increase in Estr. Secondary growth is well developed Secondary growth is absent in aerial stems of C. recta apart only in underground stems and rhizomes of C. recta from limited cambial activity at the base of stems and in devel- (Fig. 2g.h) in areas with short internodes (Fig. 1d). It is opment stage II (Fig. 2f ) resulting in a slight increase in diam- similar to that of C. vitalba with the displacement and loss of eter of the stem base. During stage I and II, the epidermis primary fibres by development of the periderms and cambium. persists and is suberised (Fig. 2e,f). Primary fibres remain In both species, the mean contribution of xylem to I during mechanically functional in aerial axes because of limited stages I and II remains relatively constant at approx. 10–13% secondary growth. Minor developmental changes of the stem in C. vitalba and 14–17% in C. recta. Compliant xylem with include lignification of interfascicular parenchyma (Fig. 2f ) in numerous large diameter vessels characterizes the xylem in

Table 2 Relative contributions of tissues to the axial second moment C. recta (Fig. 2e,f ) and C. vitalba (Fig. 2a–d). The contribution of area of the entire stem (I) for different ontogenetic stages of three of secondary xylem to I increases through stage III and IV in Clematis species C. vitalba (Table 2), increasing to 26% by stage IV. In C. Primary ﬁbres Xylem Secondary ﬁbres vitalba, the periderm is produced more and more deeply within the secondary phloem and the external part of the peri- Clematis vitalba derm is detached. The interfascicular cambium forms paren- Stage I 31.0 ± 5.7 9.9 ± 2.6 0.7 ± 0.6 chyma in parallel with the compliant wood and so the wood ± ± ± Stage II 22.4 5.1 12.6 3.2 1.7 0.6 develops within a matrix of compliant parenchymatous tissue. Stage III – 18.8 ± 4.3 1.6 ± 1.1 Stage IV – 26.0 ± 11.8 0.7 ± 0.4 Contribution of xylem tissue to I is independent of obvious mechanical trends in early stages of both species (Fig. 8). In C. Clematis recta ± ± vitalba, values of Estr remain relatively high between approx. Stage I 12.4 1.9 14.2 1.8 – −2 Stage II 13.3 ± 2 .3 16.5 ± 1.6 * 1500 and 5000 MN m and its variation is not strongly correlated with variations in xylem contribution. A similar phe- Clematis maritima Stage I 4.9 ± 1.3 11.2 ± 2.3 1.0 ± 0.7 nomenon is seen in semi-self-supporting stems of C. recta Stage II 2.8 ± 1.2 10.0 ± 3.5 1.9 ± 0.9 where xylem contribution to I does not correlate with changes ± ± Stage III – 14.7 4.1 1.4 0.6 in Estr. The proportion of xylem tissue increases in C. vitalba Stage IV – 19.5 ± 2.2 1.2 ± 0.6 in stage III and notably in stage IV (Table 2). Secondary xylem from stage I on is characterized by non-dense wood Values represent means expressed as percentages ± SD. –, tissues absent; *, negligible values. with wide diameter vessels. The rhizome of C. recta shows a secondary growth very similar in anatomy to that of C. vitalba and C. maritima with

Fig. 8 Variations of Young’s modulus according to the relative contribution to second moment of area (% I) of xylem for C. vitalba (a) and C. recta (b).

www.newphytologist.com © New Phytologist (2003) 158: 543–559 Research 555 segments of wood comprising many wide vessels intersected vertical stems to neighbouring plants forming a mutual by wide areas of ray-like interfascicular parenchyma (Fig. 2f ). support. Secondary growth also includes significant periderm develop- Recent studies on the coastal sand dune plant, Clematis ment of the same kind as observed in C. vitalba and C. mar- flammula var. maritima, indicate a very different habit and itima. Production of secondary phloem fibres continues during mechanical architecture (Isnard et al., 2003). Aerial stems later stages of growth in both species but contributions to I include complex trellises confined to young stages of growth; remain low. The mechanical role of this tissue is probably older stages with secondary growth either straggle across the small because of its very small contribution to second sand surface or become buried in this unstable environment. moment of area and also because the bands of fibres are A comparison of the overall mechanical and geometric prop- separated by compliant tissue. erties of these three species highlights the differences between the three growth habits, which are not entirely self-supporting. Differences in growth habit between these species are not just Discussion a question of shifts in position of modules in terms of aerial or underground ‘rhizomes’ but also a difference in mechanical Mechanical architecture and habit properties. Values of Estr for both young stages of C. maritima In the climber, Clematis vitalba, the aerial stem includes are significantly lower than the woodland climber, C. vitalba, young stems with primary growth as well as older stages with as well as many other tested climbing lianas (Speck et al., significant secondary growth of the vascular cambium and 1996) and those of the erect stems of C. recta. Older portions periderm. The aerial parts of the plant show a drop in Young’s of C. maritima effectively function as ‘rhizomes’ whether modulus (Estr), which is consistent with studies based on a above or below sand level. Older portions of C. vitalba func- wide variety of temperate and tropical lianas (Speck, 1991, tion as scandent compliant lianoid stems with somewhat

1994; Rowe & Speck, 1996; Speck & Rowe, 1999). These higher values of EI and Estr than C. maritima. In C. recta, aerial results are also consistent with the perceived overall mech- stems are only represented by young stages and secondary anical strategy among woody lianas in which young stages growth is largely confined to foreshortened underground stages show a ‘self-supporting’ phase and are adapted to grow across and rhizome but these young stages show higher values of gaps and reach host supports whereas flexible older stages can both EI and Estr than the trellis-forming axes of C. maritima absorb and reduce potentially catastrophic mechanical stresses or the searchers of C. vitalba. resulting from movement of the host plant (Putz, 1984; Fisher & Ewers, 1991; Putz & Holbrook, 1991). Development and biomechanics of the stem In Clematis recta only young stages of growth constitute the aerial body plan and older parts of the plant with secondary In C. vitalba, young stems show high stiffness in axes that growth are almost entirely underground. The mechanical sig- ‘search’ and traverse gaps in the host canopy between sup- nal of the aerial plant body shows a trend in mechanical prop- ports; in this species values of Estr do not drop until late in erties typical of a semi-self-supporting stem where there is stage III. The organisation of fibres in a continuous sheath little significant change in Estr along the aerial axes, which optimises resistance to bending in all directions as well as either lean against or attach themselves to surrounding plants providing young stems with a high flexural stiffness. In this (Speck, 1994; Speck & Rowe, 1999). In C. recta, the global species, sclerenchyma tissue dominates the bending resistance values of Estr are higher than those measured for young stages of young stages to such an extent that the disappearance of of C. vitalba. Because of the limited degree of secondary external collenchyma in stage II has little or no influence growth in aerial stems of this species and despite relatively on the Estr of the stem. This developmental transition is high values of Young’s modulus, stems will become unstable somewhat different in C. maritima where disruption and loss and arch over if they do not find an external support. Never- of collenchyma contributes to a relatively early drop in Estr in theless, C. recta possess a number of advantages, which allow stage II (Isnard et al., 2003). The lower quantity of primary a height (unsupported or supported) of approx. 1 m. First, fibres reported in C. maritima explains the difference in Estr stems are hollow and this almost certainly confers a weight between young stages of the two species and a probable higher advantage for attaining height without self-loading and mechanical importance of collenchyma in C. maritima. This becoming unstable (Niklas, 1992). Second, because of their difference in mechanical architecture is probably linked to lightweight structure and the mechanically effective arrange- differences in function of the long searchers traversing gaps ment of stiff tissues towards the stem periphery, some hollow in the woodland climber and the relatively short and plant stems are able to grow in length for longer periods before spindly stems constituting annual trellis-forming stems in C. reaching critical buckling length than solid stems of the same maritima. In the latter species aerial axes forming erect trellis diameter (Spatz et al., 1997; Spatz et al., 1998). Third, the members show increases in Estr and EI towards sand-level and presence of sensitive petioles in the species allows the plant are thus optimised for a brief self-supporting phase in the to avoid mechanical instability and buckling by attaching formation of the trellis (Isnard et al., 2003). In both the

woody climber and the dune plant, secondary growth The increase in xylem tissue is important in stage IV devel- eliminates primary mechanical tissues and separates them opment in C. vitalba, but varies little during the young

from the rest of the stem. This promotes a drop in Estr, which stages of growth when a large drop in Estr is already observed. irreversibly terminates the self-supporting phase of young The direct contribution by wood tissue (secondary xylem and searcher shoots or erect trellis shoots. interfascicular parenchyma) does not appear to influence the The overall biomechanical signals and anatomical struc- Young’s modulus of relatively young stages of growth of tures of the woody climber (C. vitalba) and the sand dune the stem. However, this type of wood comprising large growth form (C. maritima) are not profoundly different but volumes of interfascicular parenchyma tissue and many ves- differ in detail and are apparently fine-tuned to different sels with large lumens is important during later stages in pro- mechanical constraints in different habitats. In an open eco- ducing compliant older axes. A similar organisation has been logical space, such as a sand dune environment, rigid young observed in rhizomes of C. maritima (Isnard et al., 2003) but stages are capable of climbing on other small-bodied plants bending measurements of old stages is difficult to interpret or forming monospecific trellises. Older more compliant where fissured, partially rotted or senescent stem tissues can stems provide spreading above ground or underground stems also influence mechanical properties. The results on C. vitalba producing branches and trellises in an unstable sand dune indicate that disruption and loss of primary phloem fibres is surface. In C. vitalba the climbing growth form is more char- the principal and irreversible shift in mechanical properties of acteristic of a closed ecosystem with more competition for the stem and a similar case is seen in C. maritima. In C. recta light and thus a higher requirement for a ‘searching’ capability similar changes occur in foreshortened parts of the very basal and forming attachments to host plants. stem and rhizome but play little, if any, part in the resistance Unlike C. vitalba and C. maritima, in C. recta, both primary and stability. Secondary growth in two of these species and phloem fibres and collenchyma remain for the entire life probably in C. recta produces irreversible drops in stem stiff-

span of upright aerial shoots. The Estr varies only slightly and ness and compliant secondary tissues that are not optimized variations observed are not obviously related to positional for self-supporting growth forms. or developmental features of the upright stems. Similar secondary growth processes occur in underground portions Evolution of growth forms of older stages and perennial rhizome. Close inspection of the growth form of C. recta indicates that it has a strictly ‘Woodiness’ and ‘secondary woodiness’ geophytic type of life history like many other Ranuncul- aceae where aerial parts of the plant are annual or relatively The Ranunculaceae comprise a large number of species short-lived. The mechanical architecture observed here of which the majority are herbaceous perennials. Many of underlines this strategy with aerial mechanical tissue com- these include perennial forms with short-lived aerial stems prised of stiff primary tissues and secondary growth processes (Anemone, Ranunculus, Hepatica, Adonis) as well as forms in mainly confined to storage, transport and perennating which herbaceous aerial stems are produced from perennial organs below ground level. Furthermore, the production of underground tubers or rhizomes. Phylogenetic reconstruc- secondary tissues is confined to a series of short segments, tions of the family suggest that basal members are herbaceous which does not interact directly with the bending stability of or herbaceous perennials and that ‘woodiness’ is secondary the aerial shoots. within the family (Judd et al., 2002). The application of the Developmental variation of primary fibres is one of the term ‘woody’ to growth forms is inconsistent in the litera- essential differences between young growth stages of the ture and can lack precision in terms of whether the aerial

three different growth habits. Relatively high values of Estr or subterranean parts of the plant are woody, or both. in C. recta are explained by the presence of a continuous This distinction is important for understanding a range of outer sheath of collenchyma at the periphery of the stem functional aspects of the plant including the mechanical and/ section in addition to conspicuous sclerenchymatous or hydraulic architecture. In the Ranunculaceae, genera bundle sheaths as well as ligniﬁcation of interfascicular referred to as herbaceous perennials contain wood but this segments, parenchyma and cortex. The slight increase of is conﬁned to the underground rhizome (e.g. C. recta). In

Estr in stage II of C. vitalba might be explained by the com- most species of Clematis, woodiness is expressed in the pletion of primary differentiation during stage I, that is, aerial stem and workers refer to such forms as ‘woody’ or as the final lignification of the sclerenchyma fibres (Köhler having evolved ‘secondary woodiness’, for example ( Judd et al., 2000). Further investigation at a finer morphological or et al., 2002). The term ‘secondary woodiness’ suggests that biochemical level are necessary to see how other potential woodiness was lost within a group and then reappeared structural and chemical factors might influence the mechan- during later evolution of the group. The term could also mean ical architectures of young growth stages between species that woodiness was lost from the aerial stem but was retained (Speck et al., 1997; Hoffmann et al., 2000a,b; Hoffman et al., in an underground rhizome and then subsequently expressed 2003). in the aerial stem. A fascinating aspect of the evolution of

www.newphytologist.com © New Phytologist (2003) 158: 543–559 Research 557 growth forms in Ranunculaceae is whether aerial woody mechanical signals demonstrating atypical self-supporting forms in Clematis represent modified development, possibly mechanical signals representing upright growth forms (Speck elongation, of the rhizome ‘module’ or re-expression of et al., 1997; Civeyrel & Rowe, 2001; Rowe & Speck, 2003; secondary cambial activity in aerial modules produced from Speck et al., 2003). These trends suggest that, at least in some the underground rhizome. plant groups and within some levels of evolutionary hierarchy, This study and the comparison with C. maritima indicate specialized lianoid development and organisation exerts a that primary and secondary growth processes and the posi- canalised mechanical architecture, which is difficult to reverse tions in which they are expressed on the plant are crucial in back to truly self-supporting growth forms. This study of influencing the mechanical architectures of the three different Clematis and a consideration of basal members of the Ranun- growth habits. In all three species the ‘type of woodiness’ culaceae suggest a similar kind of canalised development inherited from presumed herbaceous perennial ancestors does constraint where rhizomatous specialization leads to bio- not provide high values of Young’s modulus (Estr) to sustain mechanical constraints on the production of self-supporting woody self-supporting growth forms with woody stems. growth forms via secondary growth. Interestingly this con- Instead, primary development dominates the stiffening straint did not rule out the appearance of lianoid growth mechanical architecture of the young stems, ‘searchers’ of forms. C. vitalba and trellis-forming branches of C. maritima. In Clematis recta, only young stages emerge from the ground and Canalisation, ‘type’ of lianescence and reversibility these are also entirely dependant on primary tissues for of mechanical properties mechanical support. Development constraints and canalisation operating on mechanical architectures might not prevent the appearance of Herbaceous perennials and canalisation lianoid woody plants but they might dictate the type of We propose that the appearance of a herbaceous perennial life lianescence among groups derived from herbaceous perennial history in basal Ranunculaceae meant that secondary wood growth forms. Current biomechanical studies on liana development became specialized for hydraulic conductance, diversity indicate that high stiffness of young stages storage and perennation and not for providing stem stiffness. (‘searchers’) and compliancy in older stages can be produced This resulted in a canalised body plan with extensive by different developmental processes among systematically mechanical constraints on further evolution of woody self- distant groups (Speck et al., 1997; Rowe & Speck, 2003). supporting architectures such as substantial shrubs and trees. One of the main types of variation is stiffness in young stages This might explain why aerial woodiness in the family is rare in which high values of EI are produced either by an outer and represented mostly by climbing growth forms in Clematis band of hypodermal fibres (Rowe & Speck, 2003) as in the or weakly shrubby, partly decumbent woody growth forms case in all Clematis species studies here – or via early cambial such as Clematis heracleifolia and Xanthorhiza. This suggests activity producing dense stiff wood (Rowe & Speck, 1996; that a transition in growth form and architecture from Speck & Rowe, 1999). It is probable that the type of herbaceous perennials to more diverse types of woody mechanical architecture producing lianescence is dependent architectures, particularly self-supporting growth forms, is on the type of primary architecture occurring in the ancestral constrained. group. As a preliminary hypothesis we suggest that in Clematis Studies investigating evolution of plant architectures and the stiff hypodermal architecture characterizing young stages growth forms within phylogenetic contexts are surprisingly of climbing species results from the herbaceous architecture few (Bateman & DiMichele, 1994; Bateman, 1994; Bateman of short-lived aerial stems similar to basal members of the et al., 1998; Bateman, 1999; Civeyrel & Rowe, 2001). Inter- Ranunculaceae. In all three species studied, initial forma- estingly, a number of detailed studies have focussed on tion of wood combines large-diameter vessels and large architectural radiations among island and mountain floras interfascicular parenchyma, which contribute little to stem in which secondary woodiness and/or large body plans have stiffness. This combination of characters might provide appeared among herbaceous groups such as Boraginaceae further constraints on the type of lianoid behaviour after the (Böhle et al., 1996) and Asteraceae (Funk, 1982; Funk & onset of secondary growth. The biomechanical data provided

Brooks, 1990; Knox & Palmer, 1995). Although such archi- here show a sharp and irreversible reduction in Estr and tectures show unusual organisation such as candelabra and relatively lower values of EI, after the loss of the primary rosette growth forms, the mechanical architectures and mechanical tissue in C. vitalba and also in C. maritima (Isnard roles of woody tissues remains unknown. Biomechanical con- et al., 2003). Constraints of mechanical architecture such as straints on growth form evolution have been detected recently irreversibility in mechanical properties probably constrain the among lianoid architectures in the Asclepiadaceae and Aris- type of searcher behaviour, initial climbing dynamics, and tolochiaceae where self-supporting shrubs and trees are rare, trellis formation as well as many other aspects of liana ecology and when non-lianoid forms do occur they show modiﬁed and life history.

Acknowledgements Hoot SB, Magallon S, Crane PR. 1999. Phylogeny of basal eudicots based on three molecular data sets: atpB, rbcL, and 18S nuclear ribosomal DNA We are grateful for the award of a French/German European sequences. Annals of the Missouri Botanical Garden 86: 1–32. exchange grant (PROCOPE) awarded to N. R and T. S. This Isnard S, Rowe NP, Speck T. 2003. Biomechanics, development and growth work was carried out during the receipt of travelling and habit of the mediterranean climber Clematis maritima. Annals of Botany maintenance grants awarded to S. I. by the EGIDE (France) 91: 407–417. Johansson JT. 1995. A revised chloroplast DNA Phylogeny of and the DAAD (Germany). Ranunculaceae. Plant Systematics and Evolution Supplement 9: 253–261. Johansson JT, Jansen RK. 1993. Chloroplast DNA variation and phylogeny References of the Ranunculaceae. Plant Systematics and Evolution 187: 29–49. Judd WS, Cambell CS, Kellog EA, Stevens PF, Donoghue MJ. 2002. Plant Bateman RM. 1994. Evolutionary-developmental change in the systematics a phylogenetic approach. Sunderland, MA, USA: Sinauer growth architecture of fossil rhizomorphic lycopsids: Scenarios Associates. constructed on cladistic foundations. Biological Reviews 69: Knox EB, Palmer JD. 1995. Chloroplast DNA variation and the recent 527–597. radiation of the giant senecios (Asteraceae) on the tall mountains of eastern Bateman RM. 1999. Architectural radiations cannot be optimally Africa. Proceedings of the National Academy of Sciences, USA 92: 10349– interpreted without morphological and molecular phylogenies. In: 10353. Kurmann MH, Hemsley AR, eds. The evolution of plant architecture. Köhler L, Speck T, Spatz H-C. 2000. Micromechanics and anatomical Kew, UK: Royal Botanic Gardens, 221–250. changes during early ontogeny of two lianescent Aristolochia species. Bateman RM, Crane PR, DiMichele WA, Kenrick P, Rowe NP, Speck T, Planta 210: 691–700. Stein WE. 1998. Early evolution of land plants: Phylogeny, physiology Niklas KJ. 1990. Biomechanics of Psilotum nudum and some early Palaeozoic and ecology of the primary terrestrial radiation. Annual Review of Ecology sporophytes. American Journal of Botany 77: 590–606. and Systematics 29: 263–292. Niklas KJ. 1992. Plant biomechanics: an engineering approach to plant form Bateman RM, DiMichele WA. 1994. Saltational evolution of form in and function. Chicago, IL, USA: University of Chicago Press. vascular plants: a neoGoldschmidtian synthesis. In: Ingram DS, Hudson Niklas KJ. 1997. The evolutionary biology of plants. Chicago, IL, USA: The A, eds. Shape and form in plants and fungi. London, UK: Academic Press, University of Chicago Press. 63–102. Niklas KJ, Speck T. 2001. Evolutionary trends in safety factors against Böhle U-R, Hilger HH, Martin WF. 1996. Island colonisation and wind-induced stem failure. American Journal of Botany 88: 1266–1278. evolution of the insular woody habit in Echium L. (Boraginaceae). Putz FE. 1984. The natural history of lianas on Barro Colorado Island, Proceedings of the National Academy of Sciences, USA 93: 11740–11745. Panama. Ecology 65: 1713–1724. Civeyrel L, Rowe NP. 2001. Phylogenetic relationships of Secamonoideae Putz FE, Holbrook NM. 1991. Biomechanical studies of vines. In: Putz FE, based on the plastid gene matK, morphology and biomechanics. Annals of Mooney HA, eds. The biology of vines. Cambridge, UK: Cambridge the Missouri Botanical Garden 88: 583–602. University Press, 73–97. Esau K. 1977. Anatomy of seed plants. New York, USA: John Wiley. Rowe NP, Speck T. 1996. Biomechanical characteristics of the ontogeny and Fisher B, Ewers FW. 1991. Structural responses to stem injury in vines. In: growth habit of the tropical liana Condylocarpon guianense (Apocynaceae). Putz FE, Holbrook NM, eds. The biology of vines. Cambridge, UK: International Journal of Plant Science 157: 406–417. Cambridge University Press, 99–124. Rowe NP, Speck T. 1997. Biomechanics of Lycopodiella cernua and Huperzia Funk VA. 1982. Systematics of Montanoa (Asteraceae: Helianthae). Memoirs squarrosa: implications for inferring growth habits of fossil small-bodied of the New York Botanical Garden 36: 1–135. lycopsids. Mededelingen Nederlands Instituut Voor Toegepaste Funk VA, Brooks DR. 1990. Phylogenetic systematics as the basis of comparative Geowetenschappen TNO 58: 293–302. biology. Washington DC, USA: Smithsonian Institution. Rowe NP, Speck T. 1998. Biomechanics of plant growth forms: The trouble Gallenmüller F, Müller U, Rowe NP, Speck T. 2001. The Growth form of with fossil plants. Review of Palaeobotany and Palynology 102: 43–62. Croton pullei (Euphorbiaceae) – Functional morphology and Rowe NP, Speck T. 2003. Hydraulics and mechanics of plants: novelty, biomechanics of a neotropical liana. Plant Biology 3: 50–61. innovation and evolution. In: Poole I, Hemsley AR, eds. The evolution of Gartner BL. 1991a. Is the climbing habit of poison oak ecotypic? Functional plant physiology. London, UK: Academie Press. Ecology 5: 696–704. Rowe NP, Speck T, Galtier J. 1993. Biomechanical analysis of a Palaeozoic Gartner BL. 1991b. Structural stability and architecture of vines vs. shrubs gymnosperm stem. Proceedings of the Royal Society of London 252: 19–28. of poison oak, Toxicodendron diversilobum. Ecology 1991: 2005–2015. Spatz H-C, Beismann H, Brüchert F, Emanns A, Speck T. 1997. Hoffmann BJ, Chabbert B, Monties B, Speck T. 2000a. Fine-tuning of Biomechanics of the giant reed Arundo donax. Philosophical Transactions of mechanical properties in two tropical lianas. In: Spatz H-C, Speck T, eds. the Royal Society of London B352: 1–10. Plant Biomechanics 2000 – Proceedings of the 3rd International Plant Spatz H-C, Köhler L, Speck T. 1998. Biomechanics and functional anatomy Biomechanics Conference, Freiburg, Badenweiler. Stuttgart, Germany: of hollow stemmed Sphenopsids. I. Equisetum giganteum. American Thieme-Verlag, 10–18. Journal of Botany 85: 305–314. Hoffmann BJ, Chabbert B, Monties B, Speck T. 2000b. Mechanical Speck T. 1991. Changes of the bending mechanics of lianas and properties and chemical cell wall composition of two tropical lianas. In: self-supporting taxa during ontogeny. In: Natural Structures. Principles, Wisser A, Nachtigal W, eds. Proceedings of the Workshops in Saarbrücken Strategies and models in Architecture and Nature, Proceedings of the II 1999, BIONA-Report 14. Mainz, Germany: Akademie der Wissenschaften International Symposium Sonderforschungsbereich 230, part I. Mitteilungen und der Literatur, 10–15. des SFB 230 Heft 6. Stuttgart, Germany: SFB230, 89–95. Hoffman B, Chabbert B, Monties B, Speck T. 2003. Mechanical, chemical Speck T. 1994. Bending stability of plant stems: Ontogenetical, ecological, and X-ray analysis of wood in the two tropical lianas Bauhinia guianense and phylogenetical aspects. Biomimetics 2: 109–128. and Condylocarpon guianensis: variations during ontogeny. Planta 217: Speck T, Neinhuis C, Gallenmüller F, Rowe NP. 1997. Trees and shrubs in (In press.) the mainly lianescent genus. Aristolochia s.l. Secondary evolution of the Hoot SB. 1995. Phylogeny of the Ranunculaceae based on preliminary atpB, self-supporting habit? In: Jeronimidis G, Vincent JFV, eds. Plant rbcL and 18S nuclear ribosomal DNA sequence data. Plant Systematics and biomechanics. Reading, UK: University of Reading, 201–208. Evolutionsupplement 9: 241–251. Speck T, Rowe NP. 1999. A quantitative approach for analytically deﬁning,

growth form and habit in living and fossil plants. In: Kurmann MH, systematics. In: Stuessy T, ed. Deep morphology: Toward a renaissance of Hemsley AR, eds. The evolution of plant architecture. Kew, UK: Royal morphology in plant systematics. Königstein, Germany: Koeltz, (In press.) Botanic Gardens, 447–479. Speck T, Vogellehner D. 1994. Devonische Landpﬂanzen mit und ohne Speck T, Rowe NP, Brüchert F, Haberer W, Gallenmüller F, Spatz H-C. hypodermales Sterom. Eine biomechanische Analyse mit Überlegungen 1996. How plants adjust the ‘material properties’ of their stems according zur Frühevolution des Leit- und Festigungssystems. Palaeontographica B. to differing mechanical constraints during growth: An example of smart 233: 157–227. design in nature. In: Engin AE, ed. Bioengineering. Proceedings of the 1996 Vincent JFV. 1990. Structural biomaterials. Princeton, NJ, USA: Princeton Engineering Systems and Design and Analysis Conference, PD-Vol, 77, Vol. 5. University Press. ASME 1996. New York, USA: The American Society of Mechanical Walters SM, Alexander, JCM, Brady, A, Brickell, CD, Cullen, J, Green, PS, Engineers, 233–241. Heywood, VH, Matthews, VA, Robson, NKB, Yeo, PF, Knees, SG, eds. Speck T, Rowe NP, Civeyrel L, Classen-Bockhoff R, Neinhuis C, Spatz H-C. 1989. The European garden ﬂora. Cambridge, UK: University of 2003. The potential of plant biomechanics in functional biology and Cambridge.