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• 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. more...

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). more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

 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. more...

 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. more...

Possible trade-offs between hydraulic conductivity and mechanical properties of woody stems from five species were assessed. Acer negundo is a ruderal tree, A. saccharinum, and A. rubrum are fast-growing and shade-intolerant soft maples, whereas A. nigrum and A. saccharum are slow-growing and shade-tolerant hard maples. It was hypothesized that the ruderal and soft maples would have lower modulus of elasticity (MOE) and modulus of rupture (MOR), but higher maximum specific conductivity (K(s max)) than hard maples. Many anatomical and general morphological characteristics were measured in an attempt to correlate them to water transport and/or mechanical strength differences between species. No difference was found between species in vessel diameter, fiber wall thickness, initial hydraulic conductivity (K(h initial)), specific conductivity (K(s max)), native percent embolism, or Huber value. Similarly, no trade-off was found between K(s max) and MOE or MOR across the genus. However, fiber lumen diameter was inversely correlated to MOE and MOR. Surprisingly, percentage of ray parenchyma was positively related to MOE. The results suggest transport/mechanical trade-offs do not occur in Acer and differences in mechanical properties may be due to fiber lumen differences that do not influence the efficient transport of water. more...

The influence of junctions between main roots and lateral roots on water flow was investigated for the desert succulents Agave deserti and Ferocactus acanthodes during 21 d of drying in soil. Under wet conditions, the junctions did not restrict xylem water flow from lateral roots to main roots, consistent with predictions of axial conductance based on vessel diameters. Embolism caused by drying reduced such axial conductance more at the junctions than in adjoining root regions. Connective tracheary elements at the junctions were abundantly pitted and had large areas of unlignified primary wall, apparently making them more susceptible to embolism than vessels or tracheids elsewhere in the roots. Unlike the decrease in axial conductance, the overall hydraulic conductivity of the junction increased during drying because of an increase in the conductivity of the radial pathway. Despite such increases, main roots may not lose substantial amounts of water to a dry soil during drought, initially because embolism at the junctions can limit xylem flow and later because soil hydraulic conductivity decreases. Moreover, the increased root hydraulic conductivity and a potentially rapid recovery from embolism by connective tracheary elements may favor water uptake near the junctions upon soil rewetting. more...

Fertilized stands of Pseudotsuga menziesii were found to have glaucous needles. We investigated the morphological and quantitative characteristics of the epicuticular waxes of needles of fertilized and control trees. Glaucousness was caused by ornate tubular epicuticular wax. Dipping needles in chloroform, which dissolves waxes, eliminated the glaucous appearance. Based on cryostage scanning electron microscopic observations, the epicuticular waxes in the nonstomatal region were much more ornate on the needles of the fertilized trees (experimental needles) than in unfertilized trees (control needles). The stomatal region in both experimental and control needles showed similarly ornate waxes. Quantities of waxes were similar in experimental and control needles. The glaucousness was not the result of greater quantities of wax; rather, fertilization altered wax morphology in the nonstomatal regions. The correlation of leaf glaucousness with the morphology of epicuticular wax has been documented in many more...

Chestnut blight has commonly been regarded as a phloem disease due to conspicuous stem cankers that result from infection by the fungal pathogen Cryphonectria (Endothia) parasitica . Stomatal conductance (g s ), transpiration rate (E) and leaf water potential were measured throughout the day on leaves distal to naturally-occurring virulent (sunken bark with abundant stromata) or hypovirulent (swollen bark lacking stromata) cankers and cankers induced by inoculation with virulent or hypovirulent strains of C. parasitica . Relative to control stems, hydraulic conductivity (K h ), g s and E were reduced significantly (α = 0·05) for both natural virulent cankers, and cankers that were induced by a virulent strain. These effects were less pronounced for both natural and induced hypovirulent cankers. Isolation experiments revealed that the percentage of xylem tissue chips yielding C. parasitica was greater for virulent than for hypovirulent cankers. The data provide evidence that the localized presence of C. parasitica in cankers of American chestnut results in stomatal closure, possibly as a direct result of xylem dysfunction. more...

Needle leaves of Pinus longaeva can be accurately dated and remain alive on branches for 30 or more yrs, making this species ideal to study secondary growth in leaves. Field observations and regression analysis of needle age versus mean needle length both indicate primary (elongation) growth of needles is completed in the first year. Statistical analysis of cell counts for one- to 33-yr-old needles indicate production along the entire length of needles of 1.0-1.7 cell layers per year of secondary phloem, but no secondary xylem. Microscopic measurements and cell counts reveal that with advancing needle age phloem increases radially and transfusion tissue buckles, but the number of endodermal cells and xylem width do not change. Living phloem remains constant in amount (ca. 9 layers) with advancing needle age, indicating yearly replacement of old by new phloem. For comparison to P. longaeva, needle leaves were also studied for ten other conifer taxa with maximum needle longevities ranging from 3 to 19 yrs. In needles of each taxon no secondary xylem is produced, but secondary phloem production occurs throughout the post-elongation lifespan of the needles regardless of maximum needle longevity. more...

Correlations entre la longueur et le diametre des vaisseaux de six especes de lianes. Variations inter- et intraspecifiques. Implications fonctionnelles et ecologiques

A B S T R A C T The fate of the pine dwarf shoot (DS) apex after needle initiation has been controversial. Dwarf shoot primordia of Pinus longaeva were examined to determine the developmental basis for DS with and without interfoliar buds. Interfoliar buds are microscopic buds derived from the original terminal apex of the DS. In October, all the DS primordia are similar in size and appearance. However, as the needles elongate in the following June the apices of more proximal DS decrease in size7 such that by July there is a clear diminishing size gradient of apical domes in going from the most distal to the most proximal positions. The distal DSs start to form bud scales in July and have fully formed interfoliar buds by mid-August. In contrast, those DS apices lacking protective bud scales at needle maturity become suberized and can never proliferate into long shoots. The distal placement of interfoliar buds may be due to a group effect where each developing DS inhibits the more proximal DSs in the long shoot terminal bud. more...

Leaf-specific conductivities (LSCs – hydraulic conductivity per dry weight of supplied leaves). Huber values (transverse sapwood area per dry weight of supplied leaves), specific conductivity (hydraulic conductivity per transverse sapwood area) and tracheid diameters were measured throughout the trunk and crown of 20-year-old trees of Abies balsamca (L.) Mill. Measured specific conductivity was proportional to the radius to the fourth power of tracheids. LSCs, which indicate the relative water availability to different plant parts, are much higher in the trunk than in first order branches, and lowest in second order branches. The structural basis for this “hydraulic hierarchy” lies both in Huber values and in tracheid diameters. For similar diameter stem segments, there was no statistically significant difference for trunks versus branches in specific conductivity. However, in old parts of the tree, trunks are wider than supported branches and producer wider tracheids resulting in greater specific conductivities than in branches. In vigorous trees with strong apical control, Huber values were 12.0 times greater in the trunk than in similar diameter branch segments. In slow-growing trees with weak apical control, Huber values were 2.2 times greater in the trunk versus similar branch segments. more...

Mature needles and elongating current year's needles of Pinus strobus growing in Massachusetts and P. brutia growing in Israel were collected monthly or bimonthly for seasonal analysis of leaf cambial activity. Mature needles produced secondary phloem but no xylem, and, regardless of the season, had a cambial zone from 2 to 3 cell layers wide. In the current year's needles maturation was basipetal and the procambium differentiated into primary xylem, primary phloem, and the phloem-producing vascular cambium before needle maturity. Oneand 2-year-old needles of Pinus strobus produced slightly over 4 cell layers of phloem between April 15 and September 1 of 1983, with a peak production rate of about 2 cell layers per month in May and early June. One-year-old needles of P. brutia produced about 6 phloem cell layers in 1983, with phloem being produced throughout the year except in midsummer. This was contrasted by fall and winter dormancy in needles of P. strobus. THERE HAVE BEEN many seasonal time-course studies on cambial activity in stems of plants (Esau, 1965; Philipson, Ward and Butterfield, 1971; Fahn, 1982). These include studies of xylem and phloem production in Pinus strobus (Wilson, 1966; Alfieri and Evert, 1968). Most studies involve temperate plants, but plants from Mediterranean (e.g., Fahn, 1962; Liphschitz, Lev-Yadun and Waisel, 1981; Liphschitz et al., 1984; Liphschitz, Lev-Yadun and Waisel, 1985; Liphschitz and Lev-Yadun, 1986) and tropical regions (e.g., Paliwal and Prasad, 1970; Amboni, 1974; Dave and Rao, 1982; Fahn, 1982) have also been examined in this way. Cambial activity in evergreen leaves has received little attention, mostly because it has not been widely appreciated that evergreen leaves commonly undergo secondary growth (Ewers, 1982a, b). Our previous approach to studying secondary growth in leaves was to compare leaves of different known ages (Ewers, 1 982a, b). For example, we examined 1 to 33year-old needles of Pinus longaeva and found that the needle vascular cambium remained active throughout the life of the needle. In each of twelve taxa of conifers that we have examined, including P. strobus and P. brutia, the I Received for publication 17 July 1986; revision accepted 11 November 1986. This work was initiated while Ewers was a Cabot PostDoctoral Fellow and Aloni was Bullard Fellow at Harvard Forest in Petersham, Massachusetts. We thank Ms. Monica Mattmuller for technical assistance and Drs. N. Liphschitz, R. Schmid, and P. Baas for reviewing the manuscript. mature needles produce secondary phloem each year but no secondary xylem (Ewers, 1982a, b; Ewers and Aloni, 1984, 1985). The secondary phloem arises from a unidirectional phloem-producing needle vascular cambium (Ewers, 1982a). In the present study we examined the current year's needles, 1-year-old, and 2-year-old needles of Pinus strobus and P. brutia many times throughout the year for a more detailed seasonal analysis of vascular differentiation. The objectives were to determine the time of onset of secondary phloem production and the time of peak phloem production in developing as well as mature needles. We also examined the width of the needle vascular cambium throughout the year. The two species were chosen to represent two very different climatic regions. P. strobus is from temperate Massachusetts and P. brutia is from Mediterranean Israel. Taxonomically, these two species are from different subgenera. P. brutia is a diploxylon pine whereas P. strobus is a haploxylon species (Mirov, 1967). Coniferous leaves grow by means of basal meristem; the tip matures while the needle base is still elongating (Doak, 1935; Mirov, 1967; Ewers, 1982a, b). To follow vascular development in elongating current year needles, we compared over time the vascular tissue at specific distances from the needle tip (Fig. 6). We define secondary growth as growth (i.e., cell production) after elongation of the plant part has been completed. This is consistent with the classical definitions strongly advocated by Esau (1943, 1965). Due to the basipetal maturation more...

Results were compared between the latex paint and compressed air methods for determining total vessel lengths, and between the sectioning and maceration methods for determining vessel diameters. The minimum, mean, median, and maximum vessel diameters were less with the sectioning method than with the maceration technique. Vessel diameter distributions were always nonnormal and had roughly similar patterns with the two techniques, but were statistically different from one another. In all six species where the paint and air methods for determining vessel length were compared, both methods showed a similar skewed vessel length distribution, with many short vessels and few long ones. Although there was no consistent pattern to the difference in results with these two methods, the vessel length frequency distributions were statistically different from one another. With the paint method, many vessels, especially many of the narrowest ones, were not paint-filled at the paint infusion port. The air method utilized the paint method, in part, and, in addition, is based upon the incorrect assumption that all vessels in the stem are the same diameter. Both techniques tended to exclude vessel lengths of the narrowest vessels. However, the narrow vessels, although numerous, contributed an insignificant amount to the total theoretical hydraulic conductance in stems. more...

To determine the efficiency of xylem conductance in the liana (woody vine) Bauhinia fassoglensis Kotschy ex Schweinf., we measured hydraulic conductance per unit stem length (measured K(h)), leaf-specific conductivity (LSC = K(h)/distal leaf area), transpiration rate (E), xylem water potential (epsilon), vessel number, and vessel diameter. The measured K(h) was 49% (se = 7%) of the predicted K(h) from Poiseuille's law. The mean LSC for unbranched stem segments was 1.10 x 10(-8) square meters per megapascal per second (se = 0.07). LSCs were much lower (about 0.2) at branch junctions. At midday, with E at 7 x 10(-8) meters per second, the measured drop in epsilon was about 0.08 megapascal per meter along the stems and branches and about 0.27 megapascal in going from stem to leaf. In addition, there was a drop of about 0.20 megapascal at branch junctions as predicted by E/LSC. In diurnal measurements leaf epsilon never dropped below about -1.2 megapascal. For long (e.g. 16 meters) stems, the predicted mid-day drop in epsilon through the xylem transport system might be great enough to have substantial physiological impact. more...