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Coniferous trees, dicotyledonous trees, and dicotyledonous lianas (woody vines) form interesting morphological contrasts in their xylem structure and function. Lianas have among the largest (up to 8 metres or more) and widest (up to 500 µm) vessels in the plant kingdom. In conifers the water transport occurs through tracheids, which are relatively inefficient in transport. We can compare disparate growth forms in terms of leaf-specific. conductivity (LSC), which is hydraulic conductivity per surface area of leaves supplied by a stem. LSC is inversely proportional to localised pressure potential gradients. LSC is equal to the Huber value (sapwood area per leaf area supplied) times the specific conductivity (hydraulic conductivity per sapwood area). Lianas are similar to dicot trees and conifers in having hydraulic constrictions (low LSCs) at branch junctions. However, lianas generally have greater LSCs and specific conductivities but lower Huber values than do conifers. Dicot trees are intermediate in these values. The narrow but efficient stems of lianas are possible partly because lianas are not self-supporting; the mechanical requirements are reduced. Secondly, the wide and efficient vessels of lianas remain conductive for much longer than might be expected (two to several years, versus one year for similar wide vessels in dicots). Based upon experiments with glass capillary tubes and with living stem tissue, larger vessels are more susceptible to freezinginduced embolism than are small ones. However, in lianas, root pressures might serve to refill cavitated vessels on a daily or seasonal basis.
The notion that most xylem transport in stems of ring-porous trees occurs in the outermost growth ring requires experimental support. Significance of this ring is challenged by workers who find tracer dyes appearing in 4 to 8 growth rings rather than in only the outermost increment. We test the hypothesis that the outermost growth ring is of overriding significance in fluid transport through stems of Ulmus, a ring-porous tree. Fluid flow through the outermost ring was quantified by removing that ring, calculating gravity flow rates (hydraulic conductivity at 10.13 kPa m-1 ), and by tracing the transport pathway through control and experimental stem segments. From measurements corroborating theoretical calculations based on Poiseuille's law, over 90% of fluid flow through the stem occurs through the outermost ring. Remaining rings combine to account for less than 10% of xylem transport. As a result of dependence upon transport in the most superficial xylem, ring-porous trees such as elm, oak, ash, and chestnut are particularly susceptible to xylem pathogens entering from the bark.
We examined the concept that high vessel number provides xylem safety and also show that under certain circumstances high vessel number may increase rather than decrease the probab ility of mortality. The independent variable was the number of vessels per organ (redundancy). The dependent variable was the probab ility of organ death for which we set three thresho1ds for catastrophic runaway embolism (50, 75 and 90% embolisrn). Results were calculated based upon the probability that any particu1arvesse1would become embolized (P). When the modeled p was below the runaway embolism thresho1d, the safety benefits (decreased probability of organ death) increased dramatically in going from one to ten vessels and approached maximum levels of safety in organs with lOO or more vessels.Vessel redundancy conferred the greatest advantage when p approached, but was less than, the runaway embolism threshold of the organ. However, when p exceeded the runaway embolism threshold the redund ancy relationship was reversed and safety was greatest in organs with lower vessel numbers. Having greater vessel redundancy increased the likelihood of an "average" result, i.e., mortality if p is above the threshold, and survival when p is below the threshold. Model predictions are discussed in terms of redundancy segmentation, stern splitting and various other ecological and evolutionary strategies for plants exposed to different environmental conditions.
Gelatinous fibres (g-fibres) differ from most fibres in that the innermost layer of their secondary cell wall is rich in cellulose and poor in lignin. G-fibres are often produced in response to gravitational and mechanical stresses in the roots, stems, and leaves of angiosperms, with their main function being the reorientation or contraction of these organs. G-fibres also occur in the three genera (Ephedra, Gnetum, and Welwitschia) of the Gnetales, making them the only known gymnosperms with g-fibres in their shoots. The shrubby species E. aspera and E. viridis were studied to determine the function and cues for production of g-fibres in the genus. It was hypothesized that E. aspera and E. viridis would produce g-fibres as a response to gravitational and internal stresses due to downward displacement (bending). Total number of g-fibres and number of g-fibres per area did not differ between displaced and untreated (control) stems of E. aspera. For the younger stems of E. viridis, control stems had more g-fibres than displaced stems, indicating that the production of additional g-fibres in control stems may be a response to wind or other perturbations. For both species, the oldest stems studied had the lowest g-fibre frequency, suggesting that little to no new g-fibres were produced as the stems aged, regardless of treatment. Furthermore, there were no other indications of reaction anatomy (asymmetry of phloem, compression wood, etc.) for E. aspera or E. viridis. These results and the cell wall composition of the fibres, especially those in the cortex, call into question whether the fibres of shrubby Ephedra are typical g-fibres.
ABSTRACT
A node is the point of attachment of the leaf to the stem of a plant; gaps associated with nodes have been viewed as discontinuities of the stem vascular system. We tested the hypothesis that the node/gap is a spring-like joint that impacts stem flexibility even well after the leaves have been shed, with some stems specialized for elongation and others for flexibility. Four-point bending tests were done using an Instron Mechanical Testing Device with the independent variable being the number of nodes in the stem segment and dependent variables being Modulus of Elasticity (MOE), Modulus of Rupture (MOR), and xylem density. Node anatomy was examined microscopically to assess structure and function. The stiffness of the stem was inversely proportional to the frequency of leaf nodes. Surprisingly, xylem density was inversely proportional to the frequency of leaf nodes in stems of adult trees. The tissue around nodes/gaps consisted of twisted and contorted cells that may be effective at absorbing compressive and tensile stresses. Because nodes behave as spring-like joints, the frequency of nodes relates to function, with some stems specialized for vertical expansion and others for light capture and damping of wind stress. The ultimate stems on a tree are the most bendable, which may allow the trees to avoid breakage.
Summary
Sherwin Carlquist established xylem vessel indices and parameters to quantify the degree of mesomorphy or xeromorphy exhibited by plants. These indices were developed as part of efforts to establish a quantitative approach to plant anatomy and the developing fields of functional and ecological anatomy. In this paper, we discuss the origin of such parameters and their merits and demerits in light of current theory and practice. Vessel diameter, vessel element length, and vessel density (vessels/mm2) remain relevant anatomical characters that describe and quantify plant function and ecology. From a functional perspective, mean diameter can be replaced by hydraulic mean diameter (d h), inspired by the Hagen–Poiseuille Law. Vessel density is presumably linked to hydraulic safety through redundancy and embolism resistance and is an essential feature of xeromorphic woods that tend to have many narrow vessels. Although vessel element length strongly correlates to xeromorphy, the reasons for the link between element length and xeromorphy are unclear. The use of anatomical indices, such as those developed and proposed by Carlquist, helped shape our understanding of plant hydraulic strategies and will continue to be important as we connect plant anatomy to plant function.
For trees and shrubs it is well known that vessels tend to be wider in roots than in stems. It is also well known that vines have narrow stems with wide vessels, but roots of vines have been little studied. It was hypothesized that the evolution of the vine habit involved greater changes in stems than in terrestrial roots, and thus vessels in stems of vines would tend to be as wide, or wider, than in roots. Radial vessel diameters were compared in roots versus stems of 62 taxa from 20 families of plants based upon collections made at Barro Colorado Island (BCI) in Panama and Fairchild Tropical Garden (FTG) in Miami, FL, USA. As expected, for Fabaceae trees + shrubs, mean and maximum vessel diameters were significantly greater in roots than in stems. Tbe reverse was true for Fabaceae lianas (woody vines), where vessel diameters were significantly greater in stems. When comparing stems of all c1imbing species (n = 51) to non-c1imbing species (n = 11), the c1imbing species had significantly greater mean and maximum vessel diameters. In contrast, for root vessels differences between growth forms were not statistically significant.