The Growth Form of Croton pullei (Euphorbiaceae) - Functional Morphology and Biomechanics of a Neotropical Liana

Abstract: Croton pullei (Euphorbiaceae) is a woody climber of the lowland rainforest in French Guyana and Surinam. During ontogeny, a shift from a juvenile free-standing growth phase to an older supported growth phase is observed. The following biomechanical parameters were studied: structural Young's modulus, structural torsional modulus, flexural stiffness and bend to twist ratios. Changes in anatomical development were also analysed for different stages of development of C. pullei which differ significantly in their mechanical properties. Free-standing plants show a nearly constant structural Young's modulus and structural torsional modulus during ontogeny, with flexural stiffness increasing proportionally with the axial second moment of area. These patterns are typical for “semi-self-supporting plants". In contrast, supported plants show a significant decrease in structural Young's modulus in older stem parts, as well as a decrease in structural torsional modulus. Due to the decrease in structural Young's modulus, flexural stiffness does not increase proportionally with the axial second moment of area. These patterns are typical for non-self-supporting lianas. In all supported plants, a sudden transition occurs from early dense wood to a wood type with a much higher proportion of large diameter vessels. In contrast, only the dense wood type is present in all tested free-standing plants. The data are compared with results from other climbing species of the same study area and discussed with reference to observed features characterizing the growth form and life history of C. pullei.     

阿拉善干旱荒漠区不同植被类型土壤种子库研究

[1]兰州大学草地农业科技学院,兰州730020 [2]甘肃草原生态研究所,兰州730020 [3]内蒙古阿拉善盟草原站,巴彦浩特750360     

Safety and streamlining of woody shoots in wind: an empirical study across 39 species in tropical Australia

? Wind is a key mechanical stress for woody plants, so how do shoot traits affect performance in wind? ? We used a vehicle mounted apparatus to measure drag, streamlining and mechanical safety in 127 vertical lead-shoots, 1.2 m long, across 39 species in tropical Australia. ? Shoot dimensions and stem tissue properties were closely coupled so that shoots with low stem specific gravity or larger projected area had thicker stems. Thicker stems provide larger second moment of area (I), which increased shoot safety and bending stiffness but impeded shoot reconfiguration in strong winds, including frontal area reduction. Nonetheless, increasing I also improved streamlining. Streamlining was unrelated to traits except I. Stem tissue material properties only had small effects. Higher modulus of rupture increased shoot safety and higher Young's modulus impeded shoot reconfiguration. ? We found no conflict between bending stiffness and streamlining for woody shoots. Stiffness might help streamlining by increasing damping and stability, thereby reducing flagging in wind. Tissue-level traits did influence shoot-level mechanical safety and behaviour, but shoot geometry was much more important. Variable shoot and stem traits, which all influenced shoot biomechanics, were integrated in shoots to yield a relatively narrow range of outcomes in wind.     

Morphology and transverse stiffness of Drosophila myofibrils measured by atomic force microscopy

Atomic force microscopy was used to investigate the surface morphology and transverse stiffness of myofibrils from Drosophila indirect flight muscle exposed to different physiologic solutions. I- and A-bands were clearly observed, and thick filaments were resolved along the periphery of the myofibril. Interfilament spacings correlated well with estimates from previous x-ray diffraction studies. Transverse stiffness was measured by using a blunt tip to indent a small section of the myofibrillar surface in the region of myofilament overlap. At 10 nm indention, the effective transverse stiffness (K( perpendicular)) of myofibrils in rigor solution (ATP-free, pCa 4.5) was 10.3 +/- 5.0 pN nm(-1) (mean +/- SEM, n = 8); in activating solution (pCa 4.5), 5.9 +/- 3.1 pN nm(-1); and in relaxing solution (pCa 8), 4.4 +/- 2.0 pN nm(-1). The apparent transverse Young's modulus (E( perpendicular)) was 94 +/- 41 kPa in the rigor state and 40 +/- 17 kPa in the relaxed state. The value of E( perpendicular) for calcium-activated myofibrils (55 +/- 29 kPa) was approximately a tenth that of Young's modulus in the longitudinal direction, a difference that at least partly reflects the transverse flexibility of the myosin molecule.     

Do partially buried dune plants grow in optimal trajectories?

Plant adaptations minimising costs of burial responses are vital in mobile dune ecosystems. Conventionally, the burial responses of dune plants have been measured as vertical growth. However, a model developed here shows that growth normal to accumulating non-horizontal dune surfaces requires up to 18% less stem production than vertical growth. To determine whether dune plants grow with this optimal geometry a field survey of growth trajectories was made for three coastal plants, and a greenhouse experiment tested whether plants could actively change growth trajectories away from vertical. In the field and greenhouse partial burial resulted in shifts towards vertical growth, indicating that negative gravitropism is the major response to burial, and reduces the amount of stem needed to respond to burial relative to unburied growth trajectories. However, for two species in the field many buried stems grew closer to optimal (shorter) than vertical, mostly on northward, sun-facing dunes. Thus phototropism is the major stimulus for non-vertical growth, and coincides with optimal trajectories on north-facing dunes. Arctotheca populifolia also displayed non-vertical growth after burial on steep south-facing dunes, but responded to burial in the greenhouse with vertical growth. Measurements of the pressure of sand avalanches occurring on dune slipfaces and stem elasticity indicated that deep avalanches were sufficient to orient the flexible shoots of A. populifolia towards optimal trajectories. Thus, dune plants respond to burial by actively modulating growth towards vertical via negative gravitropism, and passively towards non-vertical—more optimal—trajectories via phototropic growth and the influence of sand avalanches.     

Ethylene sensitivity affects changes in growth patterns, but not stem properties, in response to mechanical stress in tobacco

Plant responses to mechanical stress (e.g. wind or touch) involve a suite of physiologic and developmental changes, collectively known as thigmomorphogenesis, including reductions in height increment, Young's modulus of stems, shoot growth, and seed production, and increased stem girth and root growth. A role of the phytohormone ethylene in thigmomorphogenesis has been proposed but the extent of this involvement is not entirely clear. To address this issue, wild-type (WT) and ethylene-insensitive transgenic (Tetr) tobacco ( Nicotianum tabacum ) plants were subjected to three levels of mechanical stress: 0, 25 and 75 daily flexures. Flexed plants produced shorter, thicker stems with a lower Young's modulus than non-flexed ones, and these responses occurred independently of genotype. This suggests that ethylene does not play a role in thigmomorphogenesis-related changes in stem characteristics in tobacco. The effect of mechanical stress on dry mass increment (growth), on the other hand, differed between the genotypes: in the WT plants, shoot growth but not root growth was reduced under mechanical stress, resulting in reduced total growth and increased root mass fractions. In the Tetr plants, neither shoot nor root growth were affected. This suggests that ethylene is involved in the inhibition of tobacco shoot growth under mechanical stress.     

Mechanical properties of DNA films

The Young's dynamical modulus (E) and the DNA film logarithmic decrement (theta) at frequencies from 50 Hz to 20 kHz are measured. These values are investigated as functions of the degree of hydration and temperature. Isotherms of DNA film hydration at 25 degrees C are measured. The process of film hydration changing with temperature is studied. It is shown that the Young's modulus for wet DNA films (E = 0.02-0.025 GN m-2) strongly increases with decreasing hydration and makes E = 0.5-0.7 GN m-2. Dependence of E on hydration is of a complex character. Young's modulus of denatured DNA films is larger than that of native ones. All peculiarities of changing of E and theta of native DNA films (observed at variation of hydration) vanish in the case of denatured ones. The native and denatured DNA films isotherms are different and depend on the technique of denaturation. The Young's modulus of DNA films containing greater than 1 g H2O/g dry DNA is found to decrease with increasing temperature, undergoing a number of step-like changes accompanied by changes in the film hydration. At low water content (less than 0.3 g H2O/g dry DNA), changing of E with increasing temperature takes place smoothly. The denaturation temperature is a function of the water content.     

Intrinsic mechanical properties of trabecular calcaneus determined by finite-element models using 3D synchrotron microtomography

To determine intrinsic mechanical properties (elastic and failure) of trabecular calcaneus bone, chosen as a good predictor of hip fracture, we looked for the influence of image's size on a numerical simulation. One cubic sample of cancellous bone (9 x 9 x 9 mm(3)) was removed from the body of the calcaneus (6 females, 6 males, 79+/-9 yr). These samples were tested under compressive loading. Before compressive testing, these samples were imaged at 10.13 microm resolution using a 3D microcomputed tomography (muCT) (ESRF, France). The muCT images were converted to finite-element models. Depending on the bone density values (BV/TV), we compared two different finite element models: a linear hexahedral and a linear beam finite element models. Apparent experimental Young's modulus (E(app)(exp)) and maximum apparent experimental compressive stress (sigma(max)(exp)) were significantly correlated with bone density obtained by Archimedes's test (E(app)(exp)=236+/-231 MPa [19-742 MPa], sigma(max)(exp)=2.61+/-1.97 MPa [0.28-5.81 MPa], r>0.80, p<0.001). Under threshold at 40 microm, the size of the numerical samples (5.18(3) and 6.68(3)mm(3)) seems to be an important parameter on the accuracy of the results. The numerical trabecular Young's modulus was widely higher (E(trabecular)(num)=34,182+/-22,830 MPa [9700-87,211 MPa]) for the larger numerical samples and high BV/TV than those found classically by other techniques (4700-15,000 MPa). For rod-like bone samples (BV/TV<12%, n=7), Young's modulus, using linear beam element (E(trabecular)(num-skeleton): 10,305+/-5500 MPa), were closer to the Young's modulus found by other techniques. Those results show the limitation of hexahedral finite elements at 40 microm, mostly used, for thin trabecular structures.     

Computer simulation of a model network for the erythrocyte cytoskeleton.

The geometry and mechanical properties of the human erythrocyte membrane cytoskeleton are investigated by a computer simulation in which the cytoskeleton is represented by a network of polymer chains. Four elastic moduli as well as the area and thickness are predicted for the chain network as a function of temperature and the number of segments in each chain. Comparisons are made with mean field arguments to examine the importance of steric interactions in determining network properties. Applied to the red blood cell, the simulation predicts that in the bilayer plane the membrane cytoskeleton has a shear modulus of 10 +/- 2 x 10(-6) J/m2 and an areal compression modulus of 17 +/- 2 x 10(-6) J/m2. The volume compression modulus and the transverse Young's modulus of the cytoskeleton are predicted to be 1.2 +/- 0.1 x 10(3) J/m3 and 2.0 +/- 0.1 x 10(3) J/m3, respectively. Elements of the cytoskeleton are predicted to have a mean displacement from the bilayer plane of 15 nm. The simulation agrees with some, but not all, of the shear modulus measurements. The other predicted moduli have not been measured.     

降水量下限引种区沙地樟子松幼林种群树高分布偏斜度和不整齐性

对降水量下限引种区沙地樟子松自然稀疏前人工林种群树高分布偏斜度和不整齐性的研究表明,种群高生长的不整齐性和分布的不对称性在林分自疏前随着林分发育而下降,造成高生长事整齐性变化的因素主要有年龄,初始生长状况,种群密度,对土壤水分的竞争和沙丘部位的差异;沙丘部位对林分不整齐性的影响作用要大于密度。     

科尔沁沙地盐蒿种群的空间分布格局及关联性

以科尔沁沙地半固定沙丘和流动沙丘两种不同沙丘上的盐蒿(Artemisia halodendron)种群为研究对象,采用空间点格局方法研究了 0～20 m尺度上盐蒿种群的空间分布格局及其关联性.结果表明,半固定沙丘上盐蒿种群的数量远大于流动沙丘,不同生长发育阶段的种群结构呈偏正态分布,属于稳定型种群;流动沙丘上不同生长发...     

Three-axial strain controlled testing applied to bone specimens from the proximal tibial epiphysis

Reproducibility of the determination of Young's modulus and energy absorption along the three axes of trabecular bone cubes was analysed by non-destructive compression to 0.5% strain using different testing protocols. These protocols included testing with and without pre-conditioning to a viscoelastic steady state, and different orders of test directions. Reproducibility of conditioned tests was generally better than that of non-conditioned tests. No major effect of changing the order of the test direction was found. Three-axial conditioned testing of cubes from the proximal tibial epiphysis of five humans revealed a global transverse isotrophy while most cubes showed orthotropy. The ratio between stiffness along the long axis of the tibia and the stiffness in the transverse plane was 3.7 +/- 0.4 (mean +/- 2 SE). The corresponding ratios for elastic energy storage and viscoelastic energy dissipation were 2.5 +/- 0.2. There was no difference between the relative energy loss during a testing cycle (loss tangent) in the three axes.     

Comparison of the equilibrium response of articular cartilage in unconfined compression,confined compression and indentation

At mechanical equilibrium, articular cartilage is usually characterized as an isotropic elastic material with no interstitial fluid flow. In this study, the equilibrium properties (Young's modulus, aggregate modulus and Poisson's ratio) of bovine humeral, patellar and femoral cartilage specimens (n=26) were investigated using unconfined compression, confined compression, and indentation tests. Optical measurements of the Poisson's ratio of cartilage were also carried out. Mean values of the Young's modulus (assessed from the unconfined compression test) were 0.80+/-0.33, 0.57+/-0.17 and 0.31+/-0.18MPa and of the Poisson's ratio (assessed from the optical test) 0.15+/-0.06, 0.16+/-0.05 and 0.21+/-0.05 for humeral, patellar, and femoral cartilages, respectively. The indentation tests showed 30-79% (p<0.01) higher Young's modulus values than the unconfined compression tests. In indentation, values of the Young's modulus were independent of the indenter diameter only in the humeral cartilage. The mean values of the Poisson's ratio, obtained indirectly using the mathematical relation between the Young's modulus and the aggregate modulus in isotropic material, were 0.16+/-0.06, 0.21+/-0.05, and 0.26+/-0.08 for humeral, patellar, and femoral cartilages, respectively. We conclude that the values of the elastic parameters of the cartilage are dependent on the measurement technique in use. Based on the similar values of Poisson's ratios, as determined directly or indirectly, the equilibrium response of articular cartilage under unconfined and confined compression is satisfactorily described by the isotropic elastic model. However, values of the isotropic Young's modulus obtained from the in situ indentation tests are higher than those obtained from the in vitro unconfined or confined compression tests and may depend on the indenter size in use.     

Climbing strategy in herbs does not necessarily lead to lower investments into stem biomass

Herbaceous climbers (vines) represent a growth strategy in which the stem lacks most of its supporting function. This has led to the hypothesis that herbaceous climbers are structural parasites that invest less into stems than self-supporting plants. So far, the support for this idea has been ambiguous, as woody and herbaceous plants have been discussed jointly and evidence is often based on young plants in pot experiments. We collected in wild fully grown temperate herbaceous climbers and self-supporting herbs to examine the idea. We made a phylogenetically informed comparison of biomass allocation into stems and leaves of 16 climber species and 74 self-supporting herbs. Furthermore, we compared our results with those published for woody climbers to gain insight into different biomass allocation between herbaceous and woody growth forms. We found that herbaceous climbers and self-supporting herbs do not differ in their proportion of stem biomass to leaf biomass. Herbaceous climbers reach much higher in the canopy thanks to their climbing habit and in average more than seven times longer stems, but contrary to the expectation and unlike their woody counterparts, they do not save on investment into the stem. Herbaceous climbers and self-supporting herbs represent a study system which provides insight into biomass scaling with versus without supporting function where both stems as well as leaves are seasonal.

     

Xylem pathways in liana stems with variant secondary growth

FISHER, J. B. & EWERS, F. W., 1992. Xylem pathways in liana stems with variant secondary growth. The three-dimensional construction of stem xylem in tropical lianas (woody vines) was studied using several approaches: 1. observations of the xylem surface in stems with bark removed after NaOH treatment or natural retting; 2. reconstructions from serial transverse sections; 3. movement of dye solutions up isolated xylem sectors in intact plants, and 4. flow of dye solutions down branches and xylem sectors in isolated stem segments. Long distance (up to several metres) xylem pathways in unbranched stems and connections between lateral branches and main stems are described for !5 species in eight families which represented seven differnt patterns of secondary growth. The xylem in even the most complex stems is integrated by three-dimensional interconnections of xylem regions which may appear isolated in transverse section. Interconnections are most common at leaf and branch nodes. Some old stems have peripheral xylem that remains isolated over long distances in unbranched stems, but even these had structural and physiological interconnections between xylem regions at branch nodes.     

干旱区荒漠景观的植被自然更新机制初探

根据实地调在和参阅资料．依山前洪积扇、河流沿岸阶地、沙丘及丘间低地、戈壁、盐沼的景观斑块,初步探讨干旱区植被的自然更新机制。干旱区的高温干燥、温差强烈、风蚀沙埋已成为植物繁衍后代的动力;萌蘖、劈裂、地下茎萌生等多种繁殖方式．使荒漠植物在严酷的条件下实现自然更新,形成以母株为中心的母系居群。地貌形态和种子传播动力决定了群落外形。在干旱区,以种子繁殖的植物之种子成熟期与年内的丰水期相吻合,有利于植被更新．只有在连续2年降水较大时（超过多年平均降水量）,沙丘和丘间地,戈壁植被的自然更新才可能发生;河流沿岸植被的自然更新受地下水影响最明显;湖盆盐碱地植被带有水生植被的痕迹,自然更新相对较难;景观斑块镶嵌分布．形成微环境均异的廊道．有利于植被的自然更新。     

沙丘坡面异质性小生境中准噶尔无叶豆对水分条件变化的响应

以多年生克隆植物准噶尔无叶豆(Eremosparton songoricum(Litv.)Vass.)为材料,选择河边(A种群)和沙漠腹地(B种群)两个沙丘,研究从沙丘底部至顶部,沿着水分条件连续变化的梯度,准噶尔无叶豆在分株种群和克隆片段水平的形态变化特征,以期能揭示其在异质性小生境内利用水分资源的对策,并为准噶尔无叶豆的资源保护、培育和利用提供有意义的参考.研究发现:①在分株种群水平,A种群分株高度及地上部生物量显著高于B种群,而B种群地下部(根)的生物量则显著高于A种群;②在克隆片段水平,随着沙丘底部至顶部,A种群与B种群克隆片段高度和地上生物量都减小,而分株密度都增加,但升高或降低的强度不同;A种群根的生物量和长度增加,主要是水平的位于地下0～10 cm层面的直径10mm以下的根长度增加,而B种群根的生物量减小,但长度却在增加,主要是水平的位于地下0～10 cm层面的直径6mm以下的细根长度增加.水平细根的长度增加,更利于无性系进行广泛觅食,同时促进无性系尽快越过不利生境斑块和提高分株在有利生境中的生长概率.结果表明,准噶尔无叶豆对沙丘坡面水分条件连续变化的异质性小生境存在分株种群及克隆片段两个等级的可塑性响应,并通过可塑性变化适应了沙丘坡面水分条件的分异.     

不同沙丘生境主要植物比叶面积和叶干物质含量的比较

研究了生长在不同沙丘生境中 (流动沙丘 ,半固定沙丘和固定沙丘 ) 2 0个植物种 (10个 1年生植物种和 10个多年生植物种 )的比叶面积 (SL A)和叶干物质含量 (L DMC)的变化 ,并且分析了各个沙丘生境的土壤养分特征。结果表明 ,各个植物种的平均 SL A和 L DMC在植物种之间差异显著 ;多数在两种或 3种沙丘生境均有分布的植物其 SL A在不同沙丘生境之间差异显著 ,但是仅有 6个植物种的 L DMC在不同沙丘生境之间表现出差异 (p<0 .0 5 )。与许多研究结果类似 ,1年生植物的 SL A显著大于多年生植物的 SL A,而且两者之间 L DMC存在一定的差异。 1年生植物 SL A和 L DMC之间相关性不显著 ,但多年生植物SL A和 L DMC之间呈显著负相关。综合所有 2 0个植物种可以发现 ,SL A增大时 ,L DMC有下降的趋势     

Comparison of mechanical properties of four large, wave-exposed seaweeds

Seaweeds have a simple structural design compared to most terrestrial plants. Nonetheless, some species have adapted to the severe mechanical conditions of the surf zone. The material properties of either tissue sections or the whole stipe of four wave-exposed seaweeds, Durvillaea antarctica, D. willana, Laminaria digitata, and L. hyperborea, were tested in tension, bending, and torsion. Durvillaea has a very low modulus of elasticity in tension (E(tension) = 3-7 MN·m(-2)) and in bending (E(bending) = 9-12 MN · m(-2)), torsion modulus (G = 0.3 MN · m(-2)) and strength (σ(b)rk = 1-2 MN · m(-2)), combining a compliable and twistable stipe "material" with a comparatively high breaking strain (ε(brk) = 0.4-0.6). In comparison, the smaller stipes of Laminaria have a higher modulus of elasticity in tension (E(tension) = 6-28 MN·m(-2)) and in bending (E(bending) = 84-109 MN·m(-2)), similar strength (σ(brk) = 1-3 MN·m(-2)), and a higher torsion modulus (G = 0.7-10 MN·m(-2)), combined with a lower breaking strain (ε(brk) = 0.2-0.3) than Durvillaea. Time-dependent, viscoelastic reactions were investigated with cycling tests. The tested species dissipated 42-52% of the loading energy in tension through plastic-viscoelastic processes, a finding that bears important ecological implications. Overall, there seems to be no correlation between single material properties and the size or habitat position of the tested seaweed species.     

科尔沁沙地农田-沙丘交错区白草无性系的生态适应性分析

对生长在不同沙地生境下的白草无性系进行了个体生长与形态以及种群生物量等指标的比较研究,结果表明,生长在不同生境下的白草无性系表现出显著的形态可塑性和不同的生物量分配模式,生长在农田-沙丘过滤带上的白草无性系与生长在半固定沙丘上的白草无性系相比,具有更大的生物量,更快的叶片延伸速率,根茎总长度和节间长度更长,不定芽更多,地下生物量所占比例更大且分布更深;这些特征的出现与农田-沙丘过渡带上土壤表层比较疏松而下层水分状况相对优越有着密切关系,因而生长在农田-沙丘过渡带上的白草无性系能更快地拓展其生存空间,在固沙和沙生植被演替中起重要作用。