Biomechanics and functional anatomy of hollow-stemmed sphenopsids. I. Equisetum giganteum (Equisetaceae)

The hollow stem of Equisetum giganteum owes its mechanical stability to an outer ring of strengthening tissue, which provides stiffness and strength in the longitudinal direction, but also to an inner lining of turgid parenchyma, which lends resistance to local buckling. With a height >2.5 m isolated stems are mechanically unstable. However, in dense stands individual stems support each other by interlacing with their side branches, the typical growth habit of semi-self-supporters.     

Wood biomechanics and anatomy of PACHYCEREUS PRINGLEI

We report the longitudinal, biomechanical, and anatomical trends observed for tissue samples drawn from the parallel aligned, prismatic woody vascular bundles running the length of a Pachycereus pringlei plant measuring 5.22 m in height. The main vertical stem of this plant was cut into five segments (labeled A through E in the acropetal direction) measuring ～1.02 m in length. Four of the 14 vascular bundles in each segment were surgically removed to obtain 20 vascular bundle segments that were tested in bending to determine their stiffness measured in the radial E(R) and tangential E(T) direction. We also determined the lignin content of representative samples of wood.A nonlinear trend in stiffness was observed: E(R) and E(T) were highest in segments B or C (1.67 GN/m and 1.09 GN/m, respectively), lower in segment A (E(R) = 1.18 GN/m and E(T) = 0.35 GN/m), and lowest in segment E (E(R) = 0.03 GN/m and E(T) = 0.20 GN/m). Similar longitudinal trends were seen for axial tissue volume fraction and fiber wall thickness, which achieved their highest values in segment B (69.8% and 6.59 μm, respectively). Wood stiffness also correlated significantly with cell wall lignin content: with respect to segment B (which had the highest lignin content, and was thus used as the standard reference for percent lignin content), lignin content, was 15, 60, 85, and 43% in segments E, D, C, and A, respectively. Fiber cell length, which increased toward the base of the stem and toward the vascular cambium in the most proximal vascular bundle segment, did not correlate with E(R) or E(T).Basic engineering principles were used to calculate stem stresses resulting from self-loading and any wind-induced bending moment (produced by drag forces). Calculations indicated that the less stiff wood produced in segment A eliminates a rapid and potentially dangerous increase in stresses that would otherwise occur in segments B or C. The less stiff wood in segment A also reduces the probability of shear failure at the cellular interface between the wood and surrounding tissues in this portion of the stem.We conclude that P. pringlei wood stiffness is dependent on the volume fraction and lignification of axial tissues, less so on fiber wall thickness, and that wood development in this species is adaptively responsive to self-loading and differentially applied external mechanical forces.     

Tetrahydroisoquinoline alkaloids of the Mexican columnar cactus Pachycereus Weberi

Eight tetrahydroisoquinoline alkaloids have been crystallized and identified from the nonphenolic and phenolic extracts of the giant Mexican cereoid cactus, Pachycereus weberi (Coult.) Br. and R. The identities were established as 5,6,7-trimethoxy-1,2,3,4- tetrahydroisoquinoline (nortehuanine) 1; 7,8-dimethoxy-1,2,3,4-tetrahydroisoquinoline (lemaireocercine) 2; 7-methoxy-1, 2,3,4-tetrahydroisoquinoline (weberidine) 3; 5,6,7,8-tetramethoxy-1,2,3,4-tetrahydroisoquinoline (weberine) 4; 6,7- dimethoxy-1,2,3,4-tetrahydroisoquinoline (heliamine) 5; 2-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (N- methylheliamine or oxymethyl-corypalline) 6; 2-methyl-5,6,7-trimethoxy-1,2,3,4-tetrahydroisoquinoline (tehuanine) 7; and 1,2-dimethyl-6,7-dimethoxy-8-hydroxy-1,2,3,4-tetrahydroisoquinoline (pellotine) 8. Compounds 1- 4 have not been identified previously as natural compounds, while compounds 5-8 are previously known cactus alkaloids.     

Biomechanics of Isolated Plant Cuticles

The mechanical properties of isolated cuticles of leaves (Yucca aloifolia, Clusia fluminensis, Nerium oleander, Hedera helix) and one fruit (Lycopersicon esculentum) were investigated by means of a tensile test. Samples of the leaves and the fruit were cut to identical size (12.5 × 50 mm) and the cuticles were enzymatically isolated, cleaned, and air dried. The morphology of the isolated cuticular membranes (CM) was investigated by scanning electron microscopy (SEM) and showed considerable differences. The thickness of the CM was determined by a digital image analysis system and ranged between 2.4 and 13.4 μm. The CM were subjected to a tensile test and the results are presented as stress-strain diagrams. From the latter, Young's moduli were calculated, a measure for the stiffness which allows the direct comparison of different materials. The obtained values ranged between 0.1 and 1.3 CPa. Hydration of CM caused a decrease of Young's moduli of about 35–50%. A possible role of the cuticle as a factor for the stabilization of plant organs is discussed.     

Patterns of abundance and population structure of Pachycereus pringlei (Cactaceae), a columnar cactus of the Sonoran Desert

Understanding the mechanisms that determine the distribution and abundance of plants is a major problem in ecology. However, very few studies have explored the factors controlling the abundance of columnar cacti throughout their range of distribution. In this paper, we describe the density and size structure of 26 populations of Pachycereus pringlei throughout its distribution range in the Sonoran Desert. Major differences in abundance were detected between island and mainland and peninsular areas, with islands sustaining significantly larger densities than mainland and peninsular populations. Within peninsular populations, the abundance was negatively associated with latitude and positively associated with annual and seasonal rainfall. In contrast, the abundance in mainland populations showed neither latitudinal trend nor an association with rainfall. In peninsular populations, mean height and basal diameter of branched plants showed a negative association with population density whereas mainland populations showed no significant association. None of the populations exhibited a population structure that fitted the log-normal distribution expected for young, growing populations with constant recruitment. Insular, peninsular and␣mainland populations showed a population structure with an uneven size distribution typical of populations experiencing regeneration pulses.     

The role of the epidermis as a stiffening agent in Tulipa (Liliaceae) stems

We investigated the hypothesis that the epidermis is a tension-stressed "skin' whose contribution to stem stiffness depends on the turgor pressure exerted on it by an hydrostatically inflated inner "core' of tissues. This hypothesis was tested by relying on the intensities of bending stresses due to stem flexure, which must reach their maximum levels at the outer surface of epidermis such that damage to the surface of the stem should produce the most significant decrease in overall flexural stiffness. We discerned whether the principal tension supporting members at the stem surface (cellulosic microfibrils) were oriented parallel or normal to stem length by comparing the bending stiffness of stems before and after their surface cells first received three parallel longitudinal incisions followed by one helical incision, and by comparing the bending stiffness of stems for which the sequence of cuts was reversed. The same protocol was also applied to stems with various water potentials to determine the effect of hydrostatic pressure on stem stiffness contributed by the surface. Based on the behavior of 82 turgid Tulipa stems, parallel cuts reduced, on average, stem stiffness by 8%, whereas a subsequent helical incision further reduced stiffness by 42%. In contrast, an initial helical incision reduced stem stiffness by 50%, while three subsequent parallel cuts through the same stems did not significantly further reduce stiffness. These results suggested that the net orientation of cellulose microfibrils in the outer epidermal walls was parallel to stem length. This was confirmed by microscopic observations of cells with dichroic staining and polarized light. The responses to surgical damage were directly proportional to stem water potential. We thus conclude that the epidermis, probably in conjunction with a single layer of subepidermal collenchyma cells, acts as a tension-stiffening agent that can contribute as much as 50% to overall stem stiffness We present a simple mechanical model that can account for all our observations.     

Hydrodynamics and Biomechanics of the Submerged Water Moss Fontinalis antipyretica - a Comparison of Specimens from Habitats with Different Flow Velocities

Flow velocity has an influence on the hydrodynamic and biomechanical properties, as well as on the morphology and the anatomy of the submerged water moss Fontinalis antipyretica Hedw. Cross-sections of the plant stems show two main types of tissues. The strengthening tissue in the outer part is characterized by thick-walled cells with a small lumen, the parenchyma in the centre by thin-walled cells with a large lumen. The specimens from habitats of different flow velocities differ in the proportions of the strengthening tissue and the branching angle of the leaves. A flow tank with a special sensitive two-component balance inserted into the bottom of the flume was used to measure the hydrodynamic drag, which acts on the plant stems at different flow velocities. The drag forces increase with the length of the plant. Mechanical properties such as elasticity and ultimate strength of the plant stems were tested in tension. Relating the data to the relative proportions of the strengthening tissue results in different estimates of Young's moduli for the strengthening tissue of plants from the different sites. The critical strains to which the stems can be extended are remarkably high. Loading and unloading cycles reveal viscoelastic behaviour of the stem tissues. In the first cycle plastic deformation is also observed, but only to a lesser degree in subsequent cycles.     

Brillouin microscopy for the evaluation of hair micromechanics and effect of bleaching

Brillouin microscopy is a new form of optical elastography and an emerging technique in mechanobiology and biomedical physics. It was applied here to map the viscoelastic properties of human hair and to determine the effect of bleaching on hair properties. For hair samples, longitudinal measurements (i.e. along the fibre axis) revealed peaks at 18.7 and 20.7 GHz at the location of the cuticle and cortex, respectively. For hair treated with a bleaching agent, the frequency shifts for the cuticle and cortex were 19.7 and 21.0 GHz, respectively, suggesting that bleaching increases the cuticle modulus and—to a minor extent—the cortex modulus. These results demonstrate the capability of Brillouin spectroscopy to address questions on micromechanical properties of hair and to validate the effect of applied treatments.     

Geographic variation in the breeding system and the evolutionary stability of trioecy in Pachycereus pringlei (Cactaceae)

The Sonoran Desert columnar cactus Pachycereus pringlei has a geographically variable, non-hermaphroditic breeding system. It is trioecious (separate males, females and hermaphrodites) in the northern two-thirds of its range in Sonora, Mexico, and in the southern three-quarters of its range in Baja California, Mexico, and is gynodioecious (separate females and hermaphrodites) elsewhere. Trioecy occurs near known maternity roosts of its major pollinator, the nectar-feeding bat Leptonycteris curasoae; gynodioecy occurs>50km from known bat roosts. The observed geographic patterns cannot be explained by limited gene flow or by the geographic distributions of diurnal avian pollinators. Our field observations plus a theoretical analysis suggest that the abundance of chiropteran pollinators plays an important role in the maintenance of trioecy in this plant. Under pollinator limitation, trioecy can be a stable breeding system in this species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variations in the mechanical properties of Alouatta palliata molar enamel

Teeth have provided insights into many topics including primate diet, paleobiology, and evolution, due to the fact that they are largely composed of inorganic materials and may remain intact long after an animal is deceased. Previous studies have reported that the mechanical properties, chemistry, and microstructure of human enamel vary with location. This study uses nanoindentation to map out the mechanical properties of Alouatta palliata molar enamel on an axial cross‐section of an unworn permanent third molar, a worn permanent first molar, and a worn deciduous first molar. Variations were then correlated with changes in microstructure and chemistry using scanning electron microscopy and electron microprobe techniques. The hardness and Young's modulus varied with location throughout the cross‐sections from the occlusal surface to the dentin‐enamel junction (DEJ), from the buccal to lingual sides, and also from one tooth to another. These changes in mechanical properties correlated with changes in the organic content of the tooth, which was shown to increase from ～6% near the occlusal surface to ～20% just before the DEJ. Compared to human enamel, the Alouatta enamel showed similar microstructures, chemical constituents, and magnitudes of mechanical properties, but showed less variation in hardness and Young's modulus, despite the very different diet of this species. Am J Phys Anthropol 2010. © 2009 Wiley‐Liss, Inc.     

Stem biomechanics of three columnar cacti from the Sonoran Desert

The allometric relationship of stem length L with respect to mean stem diameter D was determined for 80 shoots of each of three columnar cactus species (Stenocereus thurberi, Lophocereus schottii, and S. gummosus) to determine whether this relationship accords with that predicted by each of three contending models purporting to describe the mechanical architecture of vertical shoots (i.e., geometric, stress, and elastic similitude, which predict L proportional to D(alpha), with alpha = 1/1, 1/2, and 2/3, respectively). In addition, anatomical, physical, and biomechanical stem properties were measured to determine how the stems of these three species maintain their elastic stability as they increase in size. Reduced major axis regression of L with respect to D showed that alpha = 2.82 ± 0.14 for S. thurberi, 2.32 ± 0.19 for L. schottii, and 4.21 ± 0.31 for S. gummosus. Thus, the scaling exponents for the allometry of L differed significantly from that predicted by each of the three biomechanical models. In contrast, these exponents were similar to that for the allometry previously reported for saguaro. Analyses of biomechanical data derived from bending tests performed on 30 stems selected from each of the three species indicated that the bulk stem tissue stiffness was roughly proportional to L2, while stem flexural rigidity (i.e., the ability to resist a bending force) scaled roughly as L3. Stem length was significantly and positively correlated with the volume fraction of wood, while regression analysis of the pooled data from the three species (i.e., 90 stems) indicated that bulk tissue stiffness scaled roughly as the 5/3-power of the volume fraction of wood in stems. These data were interpreted to indicate that wood served as the major stiffening agent in stems and that this tissue accumulates at a sufficient rate to afford unusually high scaling exponents tot stem length with respect to stem diameter (i.e., disproportionately large increments of stem length with respect to increments in stem diameter). Nevertheless, the safety factor against the elastic failure of stems (computed on the basis of the critical buckling height divided by actual stem length) decreased with increasing stem size tot each species, even though each species maintained an average safety factor equal to two. We speculate that the apparent upper limit to plant height calculated for each species may serve as a biomechanical mechanism for vegetative propagation and the establishment of dense plant colonies by means of extreme stem flexure and ultimate breakage, especially for S. gummosus.     

A rotating bed system bioreactor enables cultivation of primary osteoblasts on well‐characterized sponceram® regarding structural and flow properties

The development of bone tissue engineering depends on the availability of suitable biomaterials, a well‐defined and controlled bioreactor system, and on the use of adequate cells. The biomaterial must fulfill chemical, biological, and mechanical requirements. Besides biocompatibility, the structural and flow characteristics of the biomaterial are of utmost importance for a successful dynamic cultivation of osteoblasts, since fluid percolation within the microstructure must be assured to supply to cells nutrients and waste removal. Therefore, the biomaterial must consist of a three‐dimensional structure, exhibit high porosity and present an interconnected porous network. Sponceram®, a ZrO₂ based porous ceramic, is characterized in the presented work with regard to its microstructural design. Intrinsic permeability is obtained through a standard Darcy's experiment, while Young's modulus is derived from a two plates stress–strain test in the linear range. Furthermore, the material is applied for the dynamic cultivation of primary osteoblasts in a newly developed rotating bed bioreactor. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Quantitative mechanical evaluation and analysis of Drosophila embryos through the stages of embryogenesis

The fruit fly Drosophila embryo is one of the most important model organisms in genetics and developmental biology research. To better understand the biomechanical properties involved in Drosophila embryo research, this work presents a mechanical characterization of living Drosophila embryos through the stages of embryogenesis. Measurements of the mechanical forces of Drosophila embryos are implemented using a novel, in situ, and minimally invasive force sensing tool with a resolution in the range of microN. The measurements offer an essential understanding of penetration force profiles during the microinjection of Drosophila embryos. Sequentially quantitative evaluation and analysis of the mechanical properties, such as Young's modulus, stiffness, and mechanical impedance of living Drosophila embryos are performed by extracting the force measurements throughout the stages of embryogenesis. Experimental results illustrate the changing mechanical properties of Drosophila embryos during development, and thus mathematical models are proposed. The evaluation provides a critical step toward better understanding of the biomechanical properties of Drosophila embryos during embryogenesis, and could contribute to more efficient and significant genetic and embryonic development research on Drosophila.     

Demography of the columnar cactus Neobuxbaumia macrocephala: a comparative approach using population projection matrices

Neobuxbaumia macrocephala is a long-lived columnar cactus endemic to the Tehuacán-Cuicatlán Valley in south-central Mexico. This plant has a very restricted distribution and few recruitment events have been detected in its populations. In this study, we analyze the N. macrocephala demographic pattern using a projection matrix in order to determine the main limiting factors of this species. To accomplish this goal, we compare our results with those obtained for another species of the same genus, N. tetetzo. Considering that both species inhabit the same valley, we believe that this comparative study will offer insights into the main demographic limitations of N. macrocephala. Results showed that these species of columnar cacti have similar demographic patterns in which survival is the process with the highest relative contribution to λ, followed by growth and reproduction. Of all the life cycle stages, seeds and seedlings have the lowest survival probabilities due to a high mortality caused by seed predation and effects of direct solar radiation on germinated seeds. The estimated growth rates indicate that populations of these species of Neobuxbaumia are in a numerical equilibrium. With respect to reproduction, N. macrocephala produce a lower number of seeds per plant than N. tetetzo. This low level of sexual reproduction may decrease the probability of establishment of new individuals in N. macrocephala populations. It is suggested that pollen limitation and pre-dispersal seed predation could be some factors that limit the distribution and abundance of this columnar cactus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Molecular investigation of the mechanical properties of single actin filaments based on vibration analyses

The mechanical vibration properties of single actin filaments from 50 to 288 nm are investigated by the molecular dynamics simulation in this study. The natural frequencies obtained from the molecular simulations agree with those obtained from the analytical solution of the equivalent Euler–Bernoulli beam model. Through the convergence study of the mechanical properties with respect to the filament length, it was found that the Euler–Bernoulli beam model can only be reliably used when the single actin filament is of the order of hundreds of nanometre scale. This molecular investigation not only provides the evidence for the use of the continuum beam model in characterising the mechanical properties of single actin filaments, but also clarifies the criteria for the effective use of the Euler–Bernoulli beam model.     

Mechanical properties of desiccated ragweed pollen grains determined by micromanipulation and theoretical modelling

The mechanical properties of desiccated ragweed pollen grains were determined using a micromanipulation technique and a theoretical model. Single pollen grains with a diameter of approximately 20 microm were compressed and held, compressed and released, and compressed to rupture at different speeds between two parallel surfaces. Simultaneously, the force being imposed on the pollen grains was measured. It has been found that the rupture force of pollen grains increased linearly with their displacement at rupture on average, but was independent of their diameter. The mean rupture force was 1.20 +/- 0.03 mN, and mean deformation (the ratio between the displacement and diameter) at rupture was 22 +/- 0.6%. Single pollen grains were modeled as a capsule with a core full of air and a non permeable wall. A constitutive equation based on Hookean law was used to determine the mechanical property parameters Eh (product of the Young's modulus and wall thickness), and the mean value of Eh of desiccated pollen gains was estimated to be 1653 +/- 36 N/m.     

Preferential states of longitudinal tension in the outer tissues of Taraxcum officinale (Asteraceae) peduncles

We tested Wilhelm Hofmeister's hypothesis that the outer layers of herbaceous stem tissues are held in a preferential state of longitudinal tension by more internal stem tissues that are held in a reciprocal state of compression. We measured (1) the biaxial stiffness of dandelion peduncles that were barometrically inflated with a Scholander pressure bomb, and (2) the stiffness and mechanical behavior of different layers of tissues that were surgically manipulated as longitudinal strips placed in uniaxial tension. Hofmeister's hypothesis predicts that stems will shorten and expand in girth as their volume transiently increases (due to barometric or hydrostatic inflation), that they will longitudinally rupture when excessively inflated, and that the principal stiffening agents in their outer tissues will be aligned in the longitudinal direction with respect to stem length. Our experiments confirmed these predictions: (1) the longitudinal strains observed for inflated peduncles were negative and smaller than the circumferential strains such that stems contracted in length and expanded in girth, (2) peduncles longitudinally ruptured when excessively inflated, (3) surgical experiments indicated that the epidermis was stiffer in longitudinal tension than any other immature peduncle tissue and was as stiff as any other tissue region in mature stems, and (4) microscopic analyses showed that the net orientation of cellulose microfibrils in the cell walls of the outer region of stem tissues was parallel to stem length. A strong positive correlation existed between the tensile stiffness of tissues and the net orientation of cell wall microfibrils.