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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 (dh), 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.

Leaf density links the resistance to mechanical and hydraulic stress in Mediterranean shrubs as it is associated with the water potential at turgor loss and the moduli of elasticity and strength. Understanding the patterns of hydraulic and mechanical trait variation in vascular plants is critical to predicting species’ stress tolerance. Although previous work has shown that hydraulic and mechanical traits are decoupled in stems, there is little information available for leaves, which are organs more diversified in structure, function, and possibly drought tolerance strategies across habitats. We tested for coordination between leaf hydraulic traits related to drought tolerance and the mechanical resistance of leaves, for 17 shrub species from the arid and semiarid vegetation of the California Floristic Province. Bayesian and phylogenetic correlations showed that across species, hydraulic and mechanical traits both had strong associations with the water potential at turgor loss, and with leaf tissue density. However, leaf maximum hydraulic conductance and the water potential at 50% and 80% loss of hydraulic conductance were statistically independent of two key mechanical traits, the leaf modulus of elasticity and leaf structural strength. Our results suggest that leaf biomechanical traits, which reflect construction costs and contribute to leaf longevity, are decoupled from hydraulic capacity and safety. The independence of hydraulic and mechanical protection in leaves enables a wide range of trait combinations in leaves, which would increase their adaptive potential across ecosystems.

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.

Positive root pressure was critical for whole-plant recovery in two resurrection ferns, but foliar water uptake expedited leaf recovery., Desiccation-tolerant (DT) organisms can lose nearly all their water without dying. Desiccation tolerance allows organisms to survive in a nearly completely dehydrated, dormant state. At the cellular level, sugars and proteins stabilize cellular components and protect them from oxidative damage. However, there are few studies of the dynamics and drivers of whole-plant recovery in vascular DT plants. In vascular DT plants, whole-plant desiccation recovery (resurrection) depends not only on cellular rehydration, but also on the recovery of organs with unequal access to water. In this study, in situ natural and artificial irrigation experiments revealed the dynamics of desiccation recovery in two DT fern species. Organ-specific irrigation experiments revealed that the entire plant resurrected when water was supplied to roots, but leaf hydration alone (foliar water uptake) was insufficient to rehydrate the stele and roots. In both species, pressure applied to petioles of excised desiccated fronds resurrected distal leaf tissue, while capillarity alone was insufficient to resurrect distal pinnules. Upon rehydration, sucrose levels in the rhizome and stele dropped dramatically as starch levels rose, consistent with the role of accumulated sucrose as a desiccation protectant. These findings provide insight into traits that facilitate desiccation recovery in dryland ferns associated with chaparral vegetation of southern California.

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.

• Premise of the study: Xylem sap osmolality and salinity is a critical unresolved issue in plant function with impacts on transport efficiency, pressure gradients, and living cell turgor pressure, especially for halophytes such as mangrove trees.• Methods: We collected successive xylem vessel sap samples from stems and shoots of Avicennia germinans and Laguncularia racemosa using vacuum and pressure extraction and measured their osmolality. Following a series of extractions with the pressure chamber, we depressurized the shoot and pressurized again after various equilibration periods (minutes to hours) to test for dynamic control of osmolality. Transpiration and final sap osmolality were measured in shoots perfused with deionized water or different seawater dilutions.• Key results: For both species, the sap osmolality values of consecutive samples collected by vacuum extraction were stable and matched those of the initial samples extracted with the pressure chamber. Further extraction of samples with the pressure chamber decreased sap osmolality, suggesting reverse osmosis occurred. However, sap osmolalities increased when longer equilibration periods after sap extraction were allowed. Analysis of expressed sap with HPLC indicated a 1:1 relation between measured osmolality and the osmolality of the inorganic ions in the sap (mainly Na+, K+, and Cl-), suggesting no contamination by organic compounds. In stems perfused with deionized water, the sap osmolality increased to mimic the native sap osmolality.• Conclusions: Xylem sap osmolality and ionic contents are dynamically adjusted by mangroves and may help modulate turgor pressure, hydraulic conductivity, and water potential, thus being important for mangrove physiology, survival, and distribution.

Pre-dawn xylem pressures were measured with bubble manometers attached near the stem bases of 32 species of vines on Barro Colorado Island, Panama, to determine if pressures were sufficient to allow for possible refilling of embolized vessels. Of 29 dicotyledonous species 26 exhibited only negative xylem pressures, even pre-dawn during the wet season. In contrast, three members of the Dilleniaceae exhibited positive pre-dawn xylem pressures, with a maximum of 64 kPa in Doliocarpusmajor. A pressure of 64 kPa is sufficient to push water to a height of 6.4 m against gravity, but the specimens reached heights of 18 m. Thus, in all 29 dicotyledons examined, the xylem pressures were not sufficient to refill embolized vessels in the upper stems. In contrast, two of the smaller, non-dicotyledonous vines, the climbing fern Lygodiumvenustrum and the viny bamboo Rhipidocladumracemiflorum, had xylem pressures sufficient to push water to the apex of the plants. Therefore, a root pressure mechanism to reverse embolisms in stem xylem could apply to some but not to most of the climbing plants that were studied.

Summary The mechanical resistance of leaves has key ecological implications but its basis has not been well understood, particularly at the tissue scale. We tested the hypotheses that leaf mechanical resistance should be a function of tissue density, increasing from the lamina to the midrib, and higher in drought-tolerant than drought-avoiding species. In a common garden study, we quantified nine leaf biomechanical traits, including measurements of material and structural resistance, and in addition 17 morphological traits, at the tissue and whole-leaf scales, for 21 species from three semi-arid communities of California, USA. The mechanical properties of leaves depended strongly on tissue density. Material resistance was significantly greater in the midrib than in the leaf lamina, and tissue resistances were significantly correlated among tissues, lower in deciduous coastal sage species and higher in evergreen drought-tolerant chaparral species. The proportion of the biomass invested in the midrib was lower in species bearing midribs and laminas of high material resistance. Our results support the hypothesis of a hierarchical partitioning of leaf mechanical resistance among leaf tissues reflecting the investment of dry mass. Also, our data indicated a mechanical compensation in leaf design, where leaves with high material resistance and density deploy a relatively minor proportion of support tissue in the midrib. Finally, our results establish a quantitative basis for differences among communities in leaf biomechanics. Our results supported the classical characterization of the mediterranean-climate flora of California according to the dramatic increase in the mean leaf mechanical resistance from species of coastal sage to chaparral, with diverse leaf types in the Mojave Desert species.

Summary 1. Shrubs in fire prone chaparral communities have evolved different life history types in response to fire. A key to understanding the evolution of life history type differences is to understand how physiological traits are linked to differences in life history type. Vascular adaptations are important for delivering an efficient and stable water supply to evergreen chaparral shrub leaves. This study tested for a link between vascular physiology and life history type in chaparral shrubs. 2. Chaparral shrub species along the south-western coast of North America survive wildfire by three different life histories. Non-sprouters are killed by fire and re-establish exclusively through germination of fire-stimulated seeds, facultative sprouters re-establish by a combination of vegetative sprouting and fire-stimulated seeds, and obligate sprouters re-establish exclusively by vegetative sprouting because their seeds do not survive fire. Non-sprouters and facultative sprouters establish seedlings in the open canopy post fire environment, whereas obligate sprouters establish seedlings in the shady understory of the mature chaparral canopy. 3. Seedlings of nine species (Rhamnaceae) representing three each of the different life history types were grown in deep containers in a common garden under treatments of sun and shade. Hydraulic conductance was measured using a high-pressure flow meter for all organs, and a vacuum technique was used to measure conductance of fine and woody roots. We predicted that non-sprouters would exhibit greater hydraulic efficiency than the sprouting species, and that facultative sprouters would be more efficient than the shade tolerant obligate sprouters. 4. Non-sprouters had the greatest hydraulic conductance per unit leaf and sapwood area at the whole seedling level, whereas facultative and obligate sprouters were not different. Comparing hydraulic conductance across major organs (from fine roots to leaves) showed that the hydraulic system was well coordinated. At the whole seedling level, the root system was more of a bottleneck than the shoot system. This pattern was consistent with high resistance extraxylary pathways in roots and differences in root architecture. 5. The greater hydraulic efficiency of the non-sprouter life history type is attributed to its post-fire pioneering habit and may partially explain the relatively high speciation in the nonsprouters. Lower hydraulic efficiency is associated with a sprouting life history and greater shade tolerance. The seedling root systems represent a hydraulic bottleneck that may place roots under especially intense selection.

Soil moisture is a key limiting resource in arid and semi-arid environments for woody shrub species. We assessed if three arid communities differed in their level of dry season soil moisture content and if the dominant species in these communities differed in their ability to use soil moisture. Water potential of all of the dominant woody plant species occurring in chaparral, coastal sage, and Mojave Desert communities and soil moisture content of these sites were measured seasonally. Species occurring in the Mojave Desert exhibited the most negative water potentials while the coastal sage community displayed the least negative water potentials. Dry season volumetric soil moisture content of the Mojave Desert site was lowest (7%), the chaparral site was intermediate (10%), and the coastal sage scrub site had the moistest dry season soil (20%). These moisture differences developed even though the coastal sage and chaparral communities both received the same annual rainfall and had similar soil cha...

Summary 1. Chaparral shrub species along the southwest coast of North America survive wildfire by three different life history types: non-sprouters ( NS ) are killed by fire and only re-establish through germination of fire-stimulated seeds; facultative sprouters ( FS ) re-establish by a combination of vegetative sprouting and seed germination; and obligate sprouters ( OS ) only re-establish by vegetative sprouting because their seeds do not survive fire. 2. Post-fire seedlings of NS and FS re-establish in open canopy gaps, whereas OS seedlings primarily recruit in deep shade during fire-free intervals. 3. It was hypothesized that these life history differences, compared within the same family, Rhamnaceae, would result in differential seedling resistance to water stress such that NS > FS > OS . 4. Seedlings were grown in large pots in a common garden under treatments of sun, shade, irrigation and water stress. Mortality and physiology of seedlings were monitored for about 1 year. 5. In response to the water stress treatment in full sun, life history types were not different in their dry season predawn water potentials, which were extremely negative (< ‐9 MPa). Water potential of water-stressed shade grown plants were also not different, but were less negative than the sun treatment during the dry season (about ‐6 MPa). 6. For plants grown in full sun under water stress, NS exhibited 23% mortality, FS 60% and OS 35% mortality, while irrigated controls had greatly reduced mortality across all life histories. Among shade grown plants under water stress, NS exhibited 30% mortality, FS 20% and OS 1% mortality. This pattern of differential mortality was the same for irrigated plants indicating that water stress was not the cause of mortality in the shade. 7. Greater stem and root resistance to water stress induced cavitation were strongly correlated with lower percent mortality during the summer dry season suggesting a link between these traits and fitness. 8. Synthesis . This study finds a link between life history type and water stress tolerance. Consistent with open canopy specialization, NS survived best in full sun. The FS and OS life history type had better survival in the shade. The OS was unexpectedly versatile, surviving well in the sun and shade. The evolutionary history of the life history types is discussed as an important factor in linking water stress tolerance and life history.

Plant adaptations to the environment are limited, and therefore plants in similar environments may display similar functional and physiological traits, a pattern termed functional convergence. Evidence was examined for functional convergence among 28 evergreen woody shrubs from three plant communities of the semi-arid winter rainfall region of southern California. Both leaf and water relations traits were examined, including seasonal stomatal conductance (gs), specific leaf area (SLA), leaf specific conductivity (Kl), seasonal water potential (Psi w), stem cavitation resistance (Psi 50), and xylem density. Species display community-specific suites of xylem and leaf traits consistent with different patterns of water use among communities, with coastal sage scrub species utilizing shallow pulses of water, Mojave Desert scrub species relying on deeper water reserves, and chaparral species utilizing both shallow and deep moisture reserves. Communities displayed similar degrees of water stress, with a community-level minimum Psi w (Psi wmin) of c. -4.6 Mpa, similar to other arid communities. Pooled across sites, there was a strong correlation between Psi wmin and xylem density, suggesting that these traits are broadly related and predictive of one another. This comparative community physiology approach may be useful in testing hypotheses of functional convergence across structurally similar semi-arid communities.

Vulnerability to water stress-induced cavitation was measured on 27 woody shrub species from three arid plant communities including chaparral, coastal sage and Mojave Desert scrub. Dry season native embolism and pre-dawn water potential, and both wet and dry season xylem specific hydraulic conductivity (Ks) were measured. Cavitation resistance, estimated as water potential at 50% loss in conductivity (Psi50), was measured on all species during the wet season and on a subset of species during the dry season. Cavitation resistance varied with sampling season, with 8 of 13 sampled species displaying significant seasonal shifts. Native embolism and water potential were useful in identification of species displaying seasonal shifts. The Ks was not different among sites or seasons. The Psi50 varied among species and communities. Within communities, interspecific variation may be partially explained by differences in rooting depth or leaf habit (evergreen, semi-deciduous, deciduous). Communities diverged in their Psi50 with chaparral species displaying the greatest cavitation resistance regardless of sampling season. The greater cavitation resistance of chaparral species is surprising, considering the greater aridity of the Mojave Desert site. Adaptation to arid environments is due to many plant traits, and aridity does not necessarily lead to convergence in cavitation resistance.

Chaparral species of California, USA, exhibit three life history types in response to fire: non-sprouters (NS), facultative sprouters (FS), and obligate sprouters (OS). Adult non-sprouters are killed by fire; thus populations reestablish only through fire- stimulated seed germination and seedling recruitment. Facultative sprouters reestablish by both vegetative sprouting and seed germination. Obligate sprouters reestablish only by vegetative sprouting and do not recruit seedlings post-fire. Previous data suggest that post-fire NS and FS seedlings reestablish as open-canopy gap specialists, whereas OS seedlings primarily reestablish in deep shade during fire-free intervals. Their non-refractory seeds are killed by fire. We hypothesized that these differences in life history, compared within the same taxonomic group, would result in a range of relative resistance to water stress such that NS . FS . OS. To test our hypothesis, we estimated resistance to water stress using resistance to xylem cavitation (the water potential at 50% loss in hydraulic conductivity; W50) for stems and roots in nine species of the family Rhamnaceae: Ceanothus megacarpus, C. crassifolius, and C. cuneatus (NS); C. spinosus, C. oliganthus, and C. leucodermis (FS); and Rhamnus ilicifolia, R. crocea, and R. californica (OS). Stems of NS species displayed greater resistance to cavitation (W50 ¼� 8.38 6 0.47 MPa) compared to both the FS (W50 ¼� 5.07 6 0.55 MPa) and OS species (W50 ¼� 5.99 6 0.38 MPa), whereas FS and OS species were not different. For roots, the general pattern was the same, but roots were generally less cavitation resistant than stems. A hydraulic model predicted that water uptake in OS species was limited by extensive cavitation in vulnerable root xylem, consistent with a reliance on deep soil water. Water uptake in cavitation-resistant NS species was most limited by soil hydraulic resistance, consistent with maximizing extraction of shallow soil water. These results suggest a link between life history and water stress tolerance in chaparral.

Summary • Here, hypotheses about stem and root xylem structure and function were assessed by analyzing xylem in nine chaparral Rhamnaceae species. •T raits characterizing xylem transport efficiency and safety, mechanical strength and storage were analyzed using linear regression, principal components analysis and phylogenetic independent contrasts (PICs). • Stems showed a strong, positive correlation between xylem mechanical strength (xylem density and modulus of rupture) and xylem transport safety (resistance to cavitation and estimated vessel implosion resistance), and this was supported by PICs. Like stems, greater root cavitation resistance was correlated with greater vessel implosion resistance; however, unlike stems, root cavitation resistance was not correlated with xylem density and modulus of rupture. Also different from stems, roots displayed a trade-off between xylem transport safety from cavitation and xylem transport efficiency. Both stems and roots showed a trade-off between xylem transport safety and xylem storage of water and nutrients, respectively. • Stems and roots differ in xylem structural and functional relationships, associated with differences in their local environment (air vs soil) and their primary functions.

Resistance to xylem cavitation depends on the size of xylem pit membrane pores and the strength of vessels to resist collapse or, in the case of freezing-induced cavitation, conduit diameter. Altering these traits may impact plant biomechanics or water transport efficiency. The evergreen sclerophyllous shrub species, collectively referred to as chaparral, which dominate much of the mediterranean-type climate region of southern California, have been shown to display high cavitation resistance (pressure potential at 50% loss of hydraulic conductivity; P50). We examined xylem functional and structural traits associated with more negative P50 in stems of 26 chaparral species. We correlated raw-trait values, without phylogenetic consideration, to examine current relationships between P50 and these xylem traits. Additionally, correlations were examined using phylogenetic independent contrasts (PICs) to determine whether evolutionary changes in these xylem traits correlate with changes in P50. Co-occurring chaparral species widely differ in their P50 (� 0.9 to � 11.0 MPa). Species experiencing the most negative seasonal pressure potential (Pmin) had the highest resistance to xylem cavitation (lowest P50). Decreased P50 was associated with increased xylem density, stem mechanical strength (modulus of rupture), and transverse fiber wall area when both raw values and PICs were analyzed. These results support a functional and evolutionary relationship among these xylem traits and cavitation resistance. Chaparral species that do not sprout following fire but instead recruit post-fire from seed had the greatest resistance to cavitation, presumably because they rely on post-fire survival of seedlings during the summer dry period to persist in the landscape. Raw values of hydraulic vessel diameter (dh), maximum vessel length, and xylem-specific hydraulic conductivity (Ks) were correlated to P50; however, dh, maximum vessel length, and Ks were not correlated to P50 when analyzed using PICs, suggesting that these traits have not undergone correlated evolutionary change. We found no difference in xylem traits between species occurring at freezing vs. nonfreezing sites, although freezing has been shown to affect the survival and distributions of some chaparral species. Stem mechanical strength, fiber properties, and post-fire regeneration type appear to be key factors in the evolution of cavitation resistance among chaparral shrubs.

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.

1 Climate change in South Africa may threaten the sclerophyllous evergreen shrubs of this region. Available data suggest that they are not as tolerant of water stress as chaparral shrubs occurring in climatically similar California, USA. 2 Seventeen species from nine angiosperm families, including both fynbos and succulent karoo species, were studied at a field site in Western Cape Province, South Africa. Minimum seasonal pressure potential (P min ), xylem specific conductivity (K s ), stem strength against breakage (modulus of rupture, MOR), xylem density, theoretical vessel implosion resistance ((t/b) 2 h ) and several fibre and vessel anatomical traits were measured. 3 Species displayed great variability in P min , similar to the range reported for chaparral and karoo shrub species, but in contrast to previous reports for fynbos shrubs. 4 More negative P min was associated with having greater xylem density, MOR and (t/b) 2 h . There was no relationship between P min and traits associated with increased water transport efficiency. 5 Xylem density integrates many xylem traits related to water stress tolerance, including P min , MOR and (t/b) 2 h [ , as well as percentage fibre wall, parenchyma, vessel area and fibre lumen diameter. 6 Xylem density may be an integral trait for predicting the impact of climate change on evergreen shrubs.

Xylem vessel anatomy was examined in tissues surrounding the graft union of sweet cherry (Prunus avium L.) scion (stem) and nondwarfing, semi-dwarfing, or dwarfing rootstock (root) combinations, to characterize potential changes in anatomical features during the initial stages of graft union formation. Vessel element length, frequency, diameter, lumen area (LAV), and mean vessel hydraulic diameter (VDh) were examined in `Rainier' (P. avium) scion wood grafted onto nondwarfing `Colt' (P. pseudocerasus L. × P. avium) or `F 12/1' (P. avium) rootstock and semi-dwarfing `Gisela 6' [`Gi 6' (P. cerasus L. × P. canescens L.)], or dwarfing `Gisela 5' [`Gi 5' (P. cerasus × P. canescens)] rootstock systems in: heterograft combinations (commercial scion-rootstock combinations); homografts (scion and rootstock are the same genetic material); and reciprocal heterografts (rootstock tissue grafted onto scion tissue). Vessel element length was not affected by rootstock, but vessel frequency and lumen area in graft union tissues were smaller in `Rainier'/`Gi 5' (dwarfing combination) than in `Rainier'/`Colt' (nondwarfing combination). The heterograft combination of `Rainier'/`Gi 5' had smaller scion LAV, lower VDh and narrower vessels than homograft or reciprocal heterograft combinations. As callus differentiated into vascular elements, xylem rays in `Rainer'/`Gi 5' tended to develop at an acute angle to the longitudinal axis of the tree and there was an increase in nonfunctional phloem in `Rainer'/`Gi 5' compared to `Rainer'/`F 12/1'. Collectively, the data provides further evidence that a combination of smaller and fewer vessels in the scion and graft union, as well as irregular vessel orientations in the vascular tissue within dwarfing combinations could contribute to hydraulic resistance in the graft union resulting in reduced scion growth (dwarfing).

Dye transport through vascular pathways was examined in tissues surrounding the graft union of second-leaf, field-grown trees of `Lapins'/Gisela 5 (`Gi 5') (dwarfing) and `Lapins'/'Colt' (nondwarfing). Excavated, intact trees were allowed to take up xylemmobile dye via transpiration for 6 h before sectioning the tree into scion, graft union, and rootstock tissue. `Lapins'/'Gi 5' had a significantly larger stem cross-sectional area in the central graft union than did `Lapins'/'Colt'. Per unit cross section, dye transport of both `Lapins'/'Gi 5' and `Lapins'/'Colt' was significantly less in the graft union than in rootstock sections, with still less transported to scion tissues in `Lapins'/'Gi 5'. `Lapins'/'Gi 5' had a tendency to produce vascular elements oriented obliquely to the longitudinal axis of the tree. Dye was distributed more uniformly axially and radially across the graft union in `Lapins'/'Colt' than in `Lapins'/'Gi 5', with an apparent accumulation of dye in `Lapins'/'Gi 5' graft union. Xylem vessel diameters and vessel hydraulic diameters (VDh) were smaller overall in `Lapins'/'Gi 5' than in `Lapins'/'Colt'; however, graft unions in both had smaller VDh than did rootstock sections. These observations suggest reduced transport efficiency of xylem vessels in the graft union in `Lapins'/'Gi 5' may be due to smaller vessels, vascular abnormalities and/or increased amounts of callus and parenchyma tissue.

Xylem sap salinity and conductivity were examined in two mangrove ecosystem tree species . For Avicennia germinans , extracted xylem sap osmotic potentials ranged from 0.24 to - - 1.36 MPa versus - 0.14 to - 0.56 MPa for Conocarpus erectus. Xylem sap of Conocarpus did not vary in osmotic potential between sites nor between predawn and midday. In Avicennia , values were more negative at midday than predawn, and also more negative at hypersaline than hyposaline sites. After removing embolisms, specific conductivity ( K s ) was measured as a function of salinity of the artificial xylem sap perfusion. For both species the lowest K s values, about 70% of the maximum K s , were obtained when stems were perfused with deionized water (0 m M ; 0.0 MPa) or with a 557-m M saline solution ( - - 2.4 MPa). Higher K s values were obtained in the range from - 0.3 to - - 1.2 MPa, with a peak at - 0.82 ± 0.08 MPa for Avicennia and - - 0.75 ± ± ± 0.08 MPa for Conocarpus . The variations in K s values with minima both at very low and very high salt concentrations were consistent with published results for swelling and shrinking of synthetic hydrogels, suggesting native hydrogels in pit membranes of vessels could help regulate conductivity.

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P min), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b)h 2, where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter. Increased cavitation resistance was correlated with increased mechanical strength (r 2 = 0.74 and 0.76 for modulus of elasticity and modulus of rupture, respectively), xylem density (r 2 = 0.88), and P min (r 2 = 0.96). In contrast, cavitation resistance and P min were not correlated with decreased specific conductivity, suggesting no tradeoff between these traits. At the cellular level, increased cavitation resistance was correlated with increased (t/b)h 2 (r 2 = 0.95), increased transverse fiber wall area (r 2 = 0.89), and decreased fiber lumen area (r 2 = 0.76). To our knowledge, the correlation between cavitation resistance and fiber wall area has not been shown previously and suggests a mechanical role for fibers in cavitation resistance. Fiber efficacy in prevention of vessel implosion, defined as inward bending or collapse of vessels, is discussed.

The response to freeze-thaw stress was examined for two co-occurring evergreen species, Malosma laurina and Rhus ovata. Laboratory and field experiments on adults and seedlings were made in the spring and winter in 1996 and again on adults in 2003 and 2004. Laboratory and field results indicated that the stem xylem for adults of M. laurina and R. ovata were similarly susceptible to freezing-induced cavitation (percentage loss of conductivity = 92 ± 2.6% for R. ovata and 90 ± 4.2% for M. laurina at ≤ -6°C). In contrast, leaves of M. laurina were more susceptible to freezing injury than leaves of R. ovata. Among seedlings in the field, leaves of M. laurina exhibited freezing injury at -4°C and total shoot mortality at -7.2°C, whereas co-occurring seedlings of R. ovata were uninjured. Surprisingly, R. ovata tolerates high levels of freezing-induced xylem embolism in the field, an apparently rare condition among evergreen plants. Rhus ovata avoids desiccation when xylem embolism is high by exhibiting low minimum leaf conductance compared to M. laurina. These results suggest a link between minimum leaf conductance and stem hydraulics as a mechanism permitting the persistence of an evergreen leaf habit in freezing environments.

We measured xylem pressure potentials, soil osmotic potentials, hydraulic conductivity and percent loss of conductivity (PLC) due to embolism, and made microscopic observations of perfused dye in the white mangrove tree, Laguncularia racemosa (L.) Gaertn. f., (1) to determine its vulnerability to air embolism compared with published results for the highly salt-tolerant red mangrove tree, Rhizophora mangle L., and (2) to identify possible relationships between air embolism, permanent blockage of vessels and stem diameter. Laguncularia racemosa was more vulnerable to embolism than reported for R. mangle, with 50 PLC at -3.4 MPa. Narrow stems (5-mm diameter) had higher PLC than larger stems (8.4- or 14-mm diameter) of the same plants. Basic fuchsin dye indicated that up to 89% of the vessels, especially in the narrow stems, had permanent blockage that could not be reversed by high pressure perfusion. Air embolism could lead to permanent vessel blockage and eventual stem mortality. Such vulnerability to embolism may restrict the growth of L. racemosa and limit its distribution to less salty areas of mangrove communities.

Confocal Laser Scanning Microscopy (CLSM) was used to observe sclereids from stems of Avicennia germinans and from fruits of two species of pear (Pyrus calleryana qBradfordq and P. communis qRed Bartlettq). The images obtained from thick (25 to 100 mum) free-hand sections were, in certain respects, far superior to those obtained by other, more invasive and time-consuming microscopic techniques upon which previous reports of sclereid morphology were based. The cell wall surfaces, including the qinternalq surfaces of the branched pit canals and cell lumens, were much accentuated with the techniques we describe, resulting in a qfluorescence shellq image, meaning the cell wall did not stain all the way through but instead only at the inner and outer wall surfaces, including the edges of ramiform pits. By controlling the time of staining with 1% aqueous Safranin O, or by changing the number of optical sections used in extended focus images, it was possible to get either a conventional view of the cell wall structure or a unique, three-dimensional view of the elaborate cell interconnections. Similar fluorescence shell images of sclereids were also obtained using a periodic-Schiff (PAS) staining system, but the stain was not as specific to sclereid cell walls as was the Safranin O stain. Particularly with the use of a narrow range band pass emission filter of 505-530 nm, the Safranin O staining may be more specific to lignin than reported in the literature. (C) 2004 Wiley-Liss, Inc.

Progressive diebacks of outer canopy branchlets of Ceanothus crassifolius were repeatedly observed after rainless periods up to 9 mo in duration in the Santa Monica Mountains of southern California. Mean xylem pressures of branchlets near the end of drought were as low as -11.2 MPa (N = 22) with a mean of about 60 dead branchlets per shrub. Inoculation (N = 15) with three species of fungi previously isolated from the same population of C. crassifolius did not promote dieback, suggesting that the observed decline was not fungal induced, as had been proposed. Further, at least 50% of healthy-appearing twigs, without symptoms of dieback, contained isolatible endophytic fungi. We used a centrifugal force method to determine the range of xylem pressure causing cavitation (vulnerability curves) for branchlets (N = 12) and roots (N = 16). We combined vulnerability curves with soil texture data (N = 6) into a water transport model that estimated the critical values (P(Lcrit)) of leaf xylem pressure associated with the loss of water from soil to foliage. Maximum P(Lcrit) was between -10 and -11 MPa and within the range of minimum measured xylem pressures of branchlets during drought and dieback. Branchlet dieback correlated with seasonal declines in xylem pressure in concert with declining safety margins from hydraulic failure. Symptoms of dieback were duplicated in the field by partially severing stem xylem that normally supplied branchlets with water. Taken together, these results indicate that loss of hydraulic conductance to foliage was the probable cause of the observed dieback in C. crassifolius. Partial dieback of peripheral branchlets, and its attendant reduction in evaporative surface area, may be a last-resort mechanism for whole-plant water conservation and drought survival in this species.

Pressure transducers were attached to twigs of orchard trees and potted trees of walnut (Juglans regia L.) to measure winter stem xylem pressures. Experimental potted trees were partially defoliated in the late summer and early autumn to lower the amount of stored carbohydrates. Potted trees were placed in cooling chambers and subjected to various temperature regimes, including freeze-thaw cycles. Xylem pressures were inversely proportional to the previous 48-h air temperature, but positively correlated with the osmolarity of the xylem sap. Defoliated trees had significantly lower concentrations of stored carbohydrates and significantly lower xylem sap osmolarities than controls. Plants kept at 1.5 degrees C developed xylem pressures up to 40 kPa, just 7% of the theoretical osmotic pressure of the xylem sap. However, exposure to low, nonfreezing temperatures followed by freeze-thaw cycles resulted in pressures over 210 kPa, which was 39% of the theoretical osmotic pressure. A simple osmotic model could account for the modest positive winter pressures at low, nonfreezing temperatures, but not for the synergistic effects of freeze-thaw cycles.

Seasonal xylem (wood) conductivity and embolism (air blockage) patterns were monitored in roots vs. stems of two clonal ringporous tree species, Sassafras albidum and Rhus typhina, throughout 1996 and 1997. Stems of both species were 100% embolized in the early spring and became conductive by late June following leaf expansion and maturation of new earlywood vessels. Dyes indicated the stem conduction was restricted almost exclusively to the current year’s growth ring. Stems became totally embolized again by early October, before the first freezing temperatures. In contrast, woody roots of both species maintained low embolism values, many conductive growth rings, and high conductivity values regardless of the season. No positive root pressures were detected in either species. The mean frost depth (204 6 11 mm) was deeper than all sampled roots of Rhus and 47% of sampled roots of Sassafras. The roots that had been in frozen soil either avoided embolism altogether or they were able to reverse embolism by a mechanism other than positive root pressures.

The exotic temperate liana (woody vine) Celastrus orbiculatus has become a weed in Michigan, occurring in many of the same habitats as the native liana Vitis riparia. However, C. orbiculatus frequently develops into extensive monospecific infestations, while V. riparia does not. Freezing-induced embolism may be responsible for limiting liana distribution. Root pressure has been observed in numerous tropical lianas and temperate species of Vitis and has been implicated as vital to the recovery of xylem function in wide vessels following winter freezes. For both of these co-occurring lianas we investigated root pressure and water conductance as possible explanatory factors for their differential spread. According to our hypothesis, C. orbiculatus should have produced greater or more frequent root pressures than V. riparia. However, the reverse proved true, indicating that root pressure is not a prerequisite for weedy proliferation of C. orbiculatus. Additionally, the seasonal patterns of specific conductivity of stem xylem indicate that each species responds differently to environmental constraints. Vitis riparia establishes conductivity early in the growing season, before the leaves emerge, using root pressure to reverse embolism, but loses conductivity with the first freeze in early autumn. Celastrus orbiculatus is slow to establish conductivity, depending on new wood production, but leafs out sooner than V. riparia and maintains green leaves after the first freeze. Vulnerability curves of xylem to cavitation caused by water stress for the two species indicate that they respond similarly to dehydration. These results indicate that root pressures are not responsible for the invasive success of C. orbiculatus and suggest that other factors must be key to its prolific invasion.

summary The hydraulic architecture of the secondary hemiepiphyte Monstera acuminata was examined in native plants from Los Tuxtlas, Veracruz, Mexico, to determine how it compared to better-known growth forms such as trees, shrubs, lianas and primary hemiepiphytes. Monstera acuminata starts its life cycle as a prostrate herb. As it ascends a tree or other vertical support, the stem becomes thicker, produces larger leaves, and may die back from the base upwards until only aerial feeding roots serve to connect the stem to the soil. Unlike the pattern of vessel-size distribution along the stems of woody dicotyledons, M. acuminata has its wider vessels at the top of the stem, decreasing in diameter towards the base. Also peculiar is the fact that Huber values (axis area}distal leaf area) tend to increase exponentially at higher positions within the plant. Based on the hydraulic conductivity (k h ) and leafspecific conductivity (LSC, k h }distal leaf area), the base of the stem potentially acts as a severe hydraulic constriction. This constriction is apparently not limiting, as aerial roots are produced further up the stem. The plants have remarkably strong root pressures, up to 225 kPa, which may contribute to the maintenance of functional vessels by refilling them at night or during periods of very high atmospheric humidity, as in foggy weather and rain. In common with dicotyledonous plants, vessel length, vessel diameter, k h , specific conductivity (k s , k h }axis area) and LSCs were all positively correlated with axis diameter. The features of the hydraulic architecture of M. acuminata may be an evolutionary consequence of an anatomical constraint (lack of vascular cambium and therefore of secondary growth) and the special requirements of the hemiepiphytic growth form.

Chaparral shrub communities of California are often dominated by indigenous species of the genus Ceanothus, which is further divided into two subgenera: Cerastes and Ceanothus. Cerastes is ...

Possible tradeoffs between efficiency of water transport and mechanical strength were examined in stems of two congeneric pairs of co-occurring chaparral shrubs. First, since previously published results indicated that Adenostoma sparsifolium (Rosaceae) had greater specific conductivity (k s or hydraulic conductivity per xylem transverse area) than A. fasciculatum, it was hypothesized that A. sparsifolium would have greater vessel lumen area per square millimeter of xylem area, and less mechanical strength, than A. fasciculatum. Secondly, since Ceanothus megacarpus (Rhamnaceae) is a non-sprouter (unable to sprout from the root crown following fire or other major disturbance) whereas C. spinosus is a sprouter and thus able to form new stems following disturbance, it was hypothesized that C. megacarpus would have greater mechanical strength, but lower k s, than C. spinosus. Both hypotheses were supported. Based upon computer-aided image analyses, A. sparsifolum had significantly higher mean and maximum vessel diameters (16.4, 40.5 vs. 14.6, 35.7 μm), a 34% greater percent vessel lumen area, and a two-fold greater measured and theoretical k s than A. fasciculatum. This corresponded to 14% lower stem density (wet weight/volume) and less mechanical strength, with a 37% lower modulus of elasticity (MOE) and a 30% lower modulus of rupture (MOR) than A. fasciculatum. Similarly, C.␣spinosus had a significantly higher maximum vessel diameter (52.7 vs. 41.8 μm) and a 92% higher theoretical k s (and 43% higher measured k s) than C. megacarpus. This corresponded to a 9% lower stem density and 20% lower MOR than for C. megacarpus. Thus, at least within these two congeneric pairs of chaparral shrubs growing together in the same habitat, there may be tradeoffs between mechanical strength and conductive efficiency of the stem xylem which correspond to differences in transport physiology and life history traits of sprouter versus non-sprouter species.

Leaf death due to freezing was examined for four, co-occurring species of chaparral shrubs from the Santa Monica Mountains of southern California, Rhus laurina ( 5 Malosma laurina), R. ovata, Ceanothus megacarpus , and C. spinosus. Measurements were made on seedlings vs. adults for all species, and for Rhus spp. in winter vs. summer, and at a warm vs. a cold site. We used four methods to determine the temperature for 50% change in activity or cell death (LT50) of leaves: (1) electrical conductivity (electrolyte leakage into a bathing solution), (2) photosynthetic fluorescent capacity (Fv/Fm), (3) percentage of palisade mesophyll cells stained by fluorescein diacetate vital stain, and (4) visual score of leaf color (Munsell color chart). In all four species seedlings were found to be more sensitive to freezing temperatures than were adults by 1 8‐ 38C. For adults the LT50 ranged from 258C for Rhus laurina in the summer to 2168C for Rhus ovata in the winter. The LT50 of R. ovata located at a colder inland site was 4 8 C lower than R. ovata at the warmer coastal site just 4 km apart, suggesting ecotypic differences between R. ovata at the two sites. Both R. laurina and R. ovata underwent significant winter hardening. At the cold site, R. ovata acclimated by 68C on average, while R. laurina acclimated by only 38C. These results were consistent with species distributions and with field observations of differential shoot dieback between these two congeneric species after a natural freeze‐thaw event in the Santa Monica Mountains.

Exotherms and lethal low temperatures were examined in four species of chaparral shrubs that co-occur in the Santa Monica Mountains, southern California: Rhus laurina, Rhus ovata, Ceanothus megacarpus, and Ceanothus spinosus. For all but R. ovata, excised leaves froze at significantly lower temperatures than leaves on intact branches (range 4.3?-6.9?C lower), indicating that only intact shoots should be used for studies of leaf supercooling. Of the four species, only R. laurina showed no leaf supercooling on intact branches. On excised leaves artificially seeded with ice crystals to remove supercooling, the lethal temperature for 50% change in activity or cell death (LT50) was determined by measuring (1) electrical conductivity into a bathing solution (electrolyte leakage), (2) photosynthetic fluorescent capacity (Fv/Fm), (3) the percentage of palisade mesophyll cells stained by fluorescein diacetate (vital stain), and (4) visual score of leaf color (Munsell color chart). In every case, the estimate of LT50 by the electrical conductivity method was significantly higher (less negative) by 2?-5?C than the other three methods, and electrical conductivity results did not correspond with field observations of dieback. Based on the photosynthetic fluorescent technique, which is both reliable and nondestructive, the LT90 ranged from -5.6?C for R. laurina to about -8.5?C for the other three species.

Identification of fossil leaf impressions as Cercis has been questioned based upon the presence or absence of a pulvinus at the base of the lamina (upper pulvinus). In the present study, leaves of Cercis canadensis were examined before and after abscission to explore the degradation processes that could occur prior to fossilization, and the North American record for fossil foliage of Cercis was revised accordingly. Results for C. canadensis indicate that: (1) the pulvinus consists largely of tissues with nonlignified cells (a wide cortex, a nonlignified fiber sheath, phloem, and pith) that degrade rapidly after leaf abscission, (2) the lignified xylem tissue that remains in the pulvinus after degradation is in brittle strands, (3) the pulvinus degrades at a faster rate than the lamina or the petiole, and (4) the degraded pulvinus cushion leaves a semicircular pattern on the lamina. From examination of fossils as well as extant species, we: (1) demonstrated that in fossils, the upper pulvinus can show a greater degree of degradation than the adjoining petiole or lamina tissue, suggesting the degradation of upper pulvinus tissue is similar in modern vs. fossil specimens, (2) defined numerous other laminar characters that can be used in conjunction with, or in the absence of, an upper pulvinus to confirm the presence of Cercis in the fossil record, and (3) showed from those criteria that the earliest known North American fossil leaf record for Cercis, from a specimen newly reported in the present study, is from the middle Miocene Succor Creek flora of Oregon.

The developmental anatomy of root buds and root sprouts was examined in the clonal tree Sassafras albidum. Root samples from 13 clones that varied widely in age and vigor were sectioned and two types of buds were found, "additional" buds and "reparative" buds. Additional buds form during the early growth of uninjured roots and they perennate by growing outwards in concert with the vascular cambium such that bud traces are produced in the secondary xylem. Reparative buds form de novo in response to senescence, injuries, or other types of disturbance. Reparative buds were found on the roots of seven of the clones, whereas additional buds were found on the roots of all 13 clones. The reparative buds had originated in the proliferated pericycle, where they were subtended by sphaeroblasts, or spherical nodules of wood. Few of the reparative buds were vascularized and none were connected with the vasculature of their parent roots. In contrast, most of the additional buds were vascularized, and the leaf traces of several of the additional buds appeared to be contiguous with the conducting xylem of their parent roots. To determine whether both bud types were functional, 82 field-collected root sprouts and 44 incubation-induced sprouts were sectioned at the root-sprout junction and examined for evidence relating to their mode of origin. None of the sprouts were subtended by sphaeroblasts, but 98% were subtended by bud traces, which indicated that they had originated from additional buds. Although reparative buds were more common than additional buds on some of the root samples, they appear to be dysfunctional at sprouting. Additional buds, on the other hand, are able to sprout both as a normal part of clonal spread and from root cuttings.

Stems of Cercis canadensis L. were pruned to three differently aged segments at three different times during the summer of 1992 to determine the effects of treatment on bud fates (vegetative or reproductive). In addition, the development of vegetative and reproductive buds was microscopically examined on untreated stems. In C. canadensis, multiple axillary buds are produced in a linear series at each node. The distal-most buds on the distal-most nodes become vegetative, and the remaining buds sequentially mature into reproductive buds over a 1-5-yr period. Microscopic buds elongate and become macroscopic in the summer prior to maturing the next spring. The number of inflorescences that elongated in 1992 was not affected by the pruning treatment, but of those, the number of inflorescences that aborted in 1993 increased with the number of nodes pruned. Deeper pruning thus reduced the number of mature inflorescences per node. Some buds that would have normally developed into reproductive shoots became vegeta...

Water transport from the roots to leaves in chaparral shrubs of California occurs through xylem vessels and tra-cheids. The formation of gas bubbles in xylem can block water transport (gas embolism), leading to shoot dieback. Two environmental factors that cause gas embolism formation in xylem conduits are drought and freezing air temperatures. We compared the differential vulnerabili-ties of Rhus laurina and Ceanothus megacarpus, co-domi-nant shrub species in the coastal regions of the Santa Monica Mountains of southern California, to both water stress-induced and freezing-induced embolism of their xylem. Rhus laurina has relatively large xylem vessel diameters, a deep root system, and a large basal burl from which it vigorously resprouts after wildfire or freezing injury. In contrast, Ceanothus megacarpus has small-diameter vessels, a shallow root system, no basal burl and is a non-sprouter after shoot removal by wildfire. We found that R. laurina became 50% embolized at a water stress of -3 MPa and 100% embolized by a freeze-thaw cycle at all hydration levels. In contrast, C. megacarpus became 50% embolized at a water stress of -9 MPa and 100% embolized by freeze-thaw events only at water potentials lower than -3 MPa. Reducing thaw rates from 0.8 °C min -1 to 0.08 °C min -1 (the normal thaw rate measured in situ) had no effect on embolism formation in R. laurina but significantly reduced embolism occurrence in well-hydrated C. megacarpus (embolism reduced from 74 to 35%). These results were consistent with the theory of gas bubble formation and dissolution in xylem sap. They also agree with field observations of differential shoot dieback in these two species after a natural freeze-thaw event in the Santa Monica Mountains.

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.

Observations on root pressure were made for 109 species in Costa Rica, Mexico, and Florida during the rainy season. Native and naturalized species, as well as a few outdoor cultivated specimens, were used. Predawn guttation was observed in 15 species. Predawn root pressure, as evidenced by xylem sap exudation from cut stems, was observed in 61 species. Root pressures, measured with a bubble manometer, ranged from 2 kPa to a maximum of 148 kPa in Cissus (Vitaceae). A broad range of climbing monocotyledons, dicotyledons, and one fern (Lygodium) displayed root pressure. Root pressures of selected shoots were followed over consecutive days and were highest in the nighttime, especially at sunrise. The maximum observed root pressure can raise a capillary water column, assuming no resistance, to a height of 14.8 m. Although root pressure alone could not refill embolized xylem vessels in tall trees and vines, the reduction of xylem sap tension would facilitate the dissolving of gas bubbles and the reestablishment...

PREMISE OF THE STUDY Phyllotaxy, the arrangement of leaves on a stem, may impact the mechanical properties of woody stems several years after the leaves have been shed. We explored mechanical properties of a plant with alternate distichous phyllotaxy, with a row of leaves produced on each side of the stem, to determine whether the nodes behave as spring-like joints. METHODS Flexural stiffness of 1 cm diameter woody stems was measured in four directions with an Instron mechanical testing system; the xylem of the stems was then cut into node (former leaf junction) and nonnode segments for measurement of xylem density. KEY RESULTS Stems had 20% greater flexural stiffness in the plane perpendicular to the original leaf placement than in the parallel plane. The xylem in the node region was more flexible, but it had significantly greater tissue density than adjacent regions, contradicting the usual correlation between wood density and stiffness. CONCLUSIONS Nodes can behave as spring-like joints in woody plants. For plagiotropic shoots, distichous phyllotaxy results in stems that resist up-and-down bending more than lateral back-and-forth movement. Thus, they may more effectively absorb applied loads from fruits, animals, wind, rain, and snow and resist stresses due to gravity without cracking and breaking. Under windy conditions, nodes may improve damping by absorbing vibrational energy and thus reducing oscillation damage. The effect of plant nodes also has biomimetic design implications for architects and material engineers.

Pressure probe measurements have been interpreted as showing that xylem pressures below c. -0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below -0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment : (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits ; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms) ; data extended to c. -10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above -0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. -0.5 MPa, cavitation limits its measurement range in situ.

Maximum vessel diameters were examined in the secondary xylem of stems of Gnetum of various sizes. One tree (G. gnemon) and 13 liana species were compared. In three species, vessel length distributions were determined by the latex paint method, and showed many short and fewer long vessels. Latex and compressed air methods, used to find the maximum vessel lengths, showed that maximum vessel lengths were similar for three species of Gnetum. In old stems, mean and maximum vessel diameters tended to be greater in lianas than in the tree species. The skewed distribution of vessel lengths and the trend of wider vessels in lianas as compared to trees were similar to those distributions and trends described previously for angiosperms. In random samples of macerated wood of three species, simple perforation plates were most common in vessel members of all species. Foraminate and modified foraminate perforations were less frequent. Average diameter of vessel members with either foraminate or modified foraminate perforations was less than for those with simple perforations. The resemblance of Gnetum vessels to those of angiosperm trees and vines is most likely a case of convergent evolution (homoplasy) in xylem characteristics.

Cercis is a genus with a geographically disjunct distribution in North America, southern Europe, and eastern Asia. The architecture of cauliflory (flowering from the lower branch and trunk areas of woody plants) was examined in 10 taxa, including 9 of the 11 recognized species in the genus Cercis. In each taxon, a linear series of first-order buds (distinguished as 1) was formed in the axils of vegetative leaves before shoot elongation ceased. The first-order bud developed into either a vegetative shoot or more frequently into an inflorescence. Inflorescences matured and then abscised. Second-order and higher order buds were produced in the axils of the basal bud scales of abscised inflorescences. In addition, vegetative shoots that were from first- or higher order buds produced first order buds (distinguished as 1′) in their leaf axils. Buds from the axils of the basalmost leaves of these vegetative shoots, together with higher order buds from the parent shoot, perpetuated the cauliflorous condition in all taxa studied. Since cauliflory was present and since the architecture of cauliflory was similar in all taxa studied, regardless of geographic distribution, cauliflory probably arose only once in Cercis, before the genus radiated from its site of origin. Key words: cauliflory, Cercis, serial buds, order of buds, geographic distribution, architecture of cauliflory.

 Premise of the study: Vessels are the chief conduit for long-distance water transport in the majority of flplants. Vessel length is a key trait that determines plant hydraulic effi ciency and safety, yet relatively little is known about this xylem feature.  Methods: We used previously published studies to generate a new global data set of vessel length in woody plants. These data were used to examine how evolutionary history, plant habit, environment, and growth ring porosity infl uenced vessel length. We also examined the relationship between mean vessel length and mean vessel diameter and maximum vessel length.  Key results: Data on mean vessel length were available for stems of 130 species and on maximum vessel length for stems of 91 species. A phylogenetic analysis indicated that vessel length did not exhibit signifi cant phylogenetic signal. Liana species had longer vessel lengths than in tree or shrub species. Vessel diameter was not predictive of mean vessel length, but maximum vessel length strongly predicted mean vessel length. Vessel length did not vary between species that differed in growth ring porosity.  Conclusions: Many traits often assumed to be linked to vessel length, including growth ring porosity and vessel diameter, are not associated with vessel length when compared interspecifi cally. Sampling for vessel length has been nonrandom, e.g., there are virtually no data available for roots, and sampling for environment has been confounded with sampling for habit. Increased knowledge of vessel length is key to understanding the structure and function of the plant hydraulic pathway.

 Premise of the Study: California chaparral shrub species have different life history types: Nonsprouters (NS) are killed by fi and persist through a fi re-stimulated seed bank; facultative sprouters (FS) reestablish by a combination of vegetative sprouting and seeding; and obligate sprouters (OS) reestablish exclusively by sprouting. Nonsprouters and FS establish seedlings in opencanopy postfi re environments, whereas OS establish seedlings between fi res in the shady understory. We hypothesized that allocation differences among seedlings of postfi re sprouters and nonsprouters and regeneration niche differences would lead to contrasting patterns in biomass accumulation (NS > FS > OS, in sun; OS > FS > NS, in shade).  Methods: Seedlings of three species from each life history type were grown in sun and 75% shade. We measured net carbon assimilation and biomass accumulation after one year.  Key Results: Biomass accumulation was similar in the sun except FS>OS. In the shade, NS had lower biomass than FS and OS. Assimilation rates, nitrogen relations, and allocation differences could not fully explain biomass accumulation differences. Instead, biomass accumulation was inversely related to water-stress tolerance and shade tolerance. Additionally, OS and FS differed in root/shoot allocation even though both are sprouters.  Conclusions: Seedling growth and carbon assimilation rates were divergent among three life history types and were consistent with differences in tolerance to water stress and shade or sun regeneration niches, but not tradeoffs in sprouting-related allocation differences per se.

SUMMARY The relationship between conduit (vessel and tracheid) diameter and water-stress-induced air embolism was examined using a double staining technique. Comparisons were made between irrigated control plants at water potentials of −13 MPa and water stressed plants at about −8 MPa. Water stress was induced either by natural drought conditions or by laboratory drying of shoots from previously irrigated shrubs. Stem segments were perfused with 0.1 % basic fuchsin to mark the initially conductive conduits, and, following high pressure perfusion of 10 DIM citric acid to remove embolisms, with 0.1% alcian blue to mark the initially embolized conduits. Hydraulic conductance per pressure gradient (kh) was measured before and after embolisms were removed. Diameters of non-embolized and embolized conduits were then measured microscopically in transverse stem sections. In irrigated controls there was little embolism and mean diameters were not significantly different for embolized SM. non-emboltzed conduits. For both artificially dehydrated and naturally droughted plants there was a 91% drop in kh due to embolism, and the mean diameter of embolized conduits was about 30 μm vs. 21 μm vs for non-embolized conduits. With increasing conduit diameter there was an increased probability of embolism. Wider conduits may have larger pores in their pit membranes, thus increasing their vulnerability to water-stress-induced embolism. Alternatively, wider conduits may merely have more pits, thus increasing their statistical chances of having a particularly large pore in an air-exposed pit membrane. Narrow vessels and tracheids provide an interwoven auxiliary transport system that appears to be of importance to transport when many of the wider, more efficient conduits become embolized.