Individual Conduits Research Articles

Sap flow through partially embolized xylem vessel networks.

Sap is transported through numerous conduits in the xylem of woody plants along the path from the soil to the leaves. When all conduits are functional, vessel lumen diameter is a strong predictor of hydraulic conductivity. As vessels become embolized, sap movement becomes increasingly affected by factors operating at scales beyond individual conduits, creating resistances that result in hydraulic conductivity diverging from diameter-based estimates. These effects include pit resistances, connectivity, path length, network topology, and vessel or sector isolation. The impact of these factors varies with the level and distribution of emboli within the network, and manifest as alterations in the relationship between the number and diameter of embolized vessels with measured declines in hydraulic conductivity across vulnerability to embolism curves. Divergences between measured conductivity and diameter-based estimates reveal functional differences that arise because of species- and tissue-specific vessel network structures. Such divergences are not uniform, and xylem tissues may diverge in different ways and to differing degrees. Plants regularly operate under nonoptimal conditions and contain numerous embolized conduits. Understanding the hydraulic implications of emboli within a network and the function of partially embolized networks are critical gaps in our understanding of plants occurring within natural environments.

Dynamic Manipulation of Chiral Domain Wall Spacing for Advanced Spintronic Memory and Logic Devices.

Nanoscopic magnetic domain walls (DWs), via their absence or presence, enable highly interesting binary data bits. The current-controlled, high-speed, synchronous motion of sequences of chiral DWs in magnetic nanoconduits induced by current pulses makes possible high-performance spintronic memory and logic devices. The closer the spacing between neighboring DWs in an individual conduit or nanowire, the higher the data density of the device, but at the same time, the more difficult it is to read the bits. Here, we show how the DW spacing can be dynamically varied to facilitate reading for otherwise closely packed bits. In the first method, the current density is increased in portions of the conduit that, thereby, locally speeds up the DWs, decompressing them and making them easier to read. In the second method, a localized bias current is used to compress and decompress the DW spacing. Both of these methods are demonstrated experimentally and validated by micromagnetic simulations. DW compression and decompression rates as high as 88% are shown. These methods can increase the density with which DWs can be packed in future DW-based spintronic devices by more than an order of magnitude.

Negative allometry of leaf xylem conduit diameter and double-wall thickness: implications for implosion safety.

Xylem conduits have lignified walls to resist crushing pressures. The thicker the double-wall (T) relative to its diameter (D), the greater the implosion safety. Having safer conduits may incur higher costs and reduced flow, while having less resistant xylem may lead to catastrophic collapse under drought. Although recent studies have shown that conduit implosion commonly occurs in leaves, little is known about how leaf xylem scales T vs D to trade off safety, flow efficiency, mechanical support, and cost. We measured T and D in > 7000 conduits of 122 species to investigate how T vs D scaling varies across clades, habitats, growth forms, leaf, and vein sizes. As conduits become wider, their double-cell walls become proportionally thinner, resulting in a negative allometry between T and D. That is, narrower conduits, which are usually subjected to more negative pressures, are proportionally safer than wider ones. Higher implosion safety (i.e. higher T/D ratios) was found in asterids, arid habitats, shrubs, small leaves, and minor veins. Despite the strong allometry, implosion safety does not clearly trade off with other measured leaf functions, suggesting that implosion safety at whole-leaf level cannot be easily predicted solely by individual conduits' anatomy.

Lipid concentration and composition in xylem sap of woody angiosperms from a tropical savanna and a seasonal rainforest

Lipids may play an important role in preventing gas embolisms by coating nanobubbles in xylem sap. Few studies on xylem sap lipids have been reported for temperate plants, and it remain unclear whether sap lipids have adaptational significance in tropical plants. In this study, we quantify the lipid composition of xylem sap for angiosperm species from a tropical savanna (seven species) and a seasonal rainforest (five species) using mass spectrometry. We found that all twelve species studied contained lipids in their xylem sap, including galactolipids, phospholipids and triacylglycerol, with a total lipid concentration ranging from 0.09 to 0.26 nmol/L. There was no difference in lipid concentration or composition between plants from the two sites, and the lipid concentration was negatively related to species’ open vessel volume. Furthermore, savanna species showed little variation in lipid composition between the dry and the rainy season. These results support the hypothesis that xylem sap lipids are derived from the cytoplasm of individual conduit cells, remain trapped inside individual conduits, and undergo few changes in composition over consecutive seasons. A xylem sap lipidomic data set, which includes 12 tropical tree species from this study and 11 temperate tree species from literature, revealed no phylogenetic signals in lipid composition for these species. This study fills a knowledge gap in the lipid content of xylem sap in tropical trees and provides additional support for their common distribution in xylem sap of woody angiosperms. It appears that xylem sap lipids have no adaptive significance.

Karst genesis and wormhole formation in carbonate joint networks: A comparison between 3D and 2D modeling

Accurate modeling of karst genesis is an important issue to understand the spatial and temporal evolutions of transport properties in karstified carbonate rocks. The previous modeling about karst genesis in fracture networks is mainly based on the simplified 2D conduit/fracture networks without considering wormhole formation on the scale of individual conduits or fractures. In this study, we simulated the karst genesis and wormhole formation (i.e., the emergence of preferential flow pathways within individual fractures) in carbonate joint networks using a DFN model fully-discretized in 3D and compared its difference with the 2D models. The results show that, compared to the 2D modeling, the breakthrough time (time of the abrupt rise in flow rate) in the 3D models may be much less sensitive to the network connectivity because of the flow focusing during wormhole formation on the scale of individual joints with a relatively low flow rate. Also, the dimensions of dissolving networks as well as the position and the number of developed preferential pathways in the well-connected networks may be altered when switching from 2D to 3D modeling. We further provide detailed insights into the underlying mechanism governing the dynamics of wormhole formation based on two representative networks (a critically-connected network and a well-connected network). We observed that the generated incipient karst patterns are similar between 2D and 3D models if the initial flow rate is sufficiently large (i.e., no flow focusing inside individual joints before breakthrough) or small (i.e., wormhole pathway is extremely localized, but the breakthrough time is much overestimated). If the initial flow rate falls within the transition range, the dissolution dynamics are complicated and dominated by the combined effect of flow migration over the network and flow focusing on the scale of individual joints, the magnitude of which depends on the fracture roughness. This study highlights the importance of incorporating the third dimension in DFN for accurate predictions of karst network evolution.

Xylem embolism spread is largely prevented by interconduit pit membranes until the majority of conduits are gas-filled.

Xylem embolism resistance varies across species influencing drought tolerance, yet little is known about the determinants of the embolism resistance of an individual conduit. Here we conducted an experiment using the optical vulnerability method to test whether individual conduits have a specific water potential threshold for embolism formation and whether pre-existing embolism in neighbouring conduits alters this threshold. Observations were made on a diverse sample of angiosperm and conifer species through a cycle of dehydration, rehydration and subsequent dehydration to death. Upon rehydration after the formation of embolism, no refilling was observed. When little pre-existing embolism was present, xylem conduits had a conserved, individual embolism-resistance threshold that varied across the population of conduits. The consequence of a variable conduit-specific embolism threshold is that a small degree of pre-existing embolism in the xylem results in apparently more resistant xylem in subsequent dehydrations, particularly in angiosperms with vessels. While our results suggest that pit membranes separating xylem conduits are critical for maintaining a conserved individual conduit threshold for embolism when little pre-existing embolism is present, as the percentage of embolized conduits increases, gas movement, local pressure differencesand connectivity between conduits increasingly contribute to embolism spread.

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

Processes occurring in volcanic conduits, the pathways through which magma travels from its storage region to the surface, have a fundamental control on the nature of eruptions and associated phenomena. It has been well established that magma flows, crystallizes, degasses, and fragments in conduits, that fluids migrate in and out of conduits, and that seismic and acoustic waves are generated and travel within conduits. A better understanding of volcanic conduits and related processes is of paramount importance for improving eruption forecasting, volcanic hazard assessment and risk mitigation. However, despite escalating advances in the characterization of individual conduit processes, our understanding of their mutual interactions and the consequent control on volcanic activity is still limited. With the purpose of addressing this topic, a multidisciplinary workshop led by a group of international scientists was hosted from 25 to 27 October 2014 by the Pisa branch of the Istituto Nazionale di Geofisica e Vulcanologia under the sponsorship of the MeMoVolc Research Networking Programme of the European Science Foundation. The workshop brought together the experimental, theoretical, and observational communities devoted to volcanological research. After 3 days of oral and poster presentations, breakout sessions, and plenary discussions, the participants identified three main outstanding issues common to experimental, analytical, numerical, and observational volcanology: unsteadiness (or transience), disequilibrium, and uncertainty. A key outcome of the workshop was to identify the specific knowledge areas in which exchange of information among the sub-disciplines would lead to efficient progress in addressing these three main outstanding issues. It was clear that multidisciplinary collaboration of this sort is essential for progressing the state of the art in understanding of conduit magma dynamics and eruption behavior. This holistic approach has the ultimate aim to deliver fundamental improvements in understanding the underlying processes generating and controlling volcanic activity.

Measurements and simulations of turbines on common grid

Speed droop control is of basic importance for the primary governing in the Nordic grid. The speed droop control. a mandatory and build-in regulatory loop on all larger units. is automatically changing the produced power on synchronous units as the grid frequency changes. This part of the governor allows a certain deviance from the nominal 50 Hz grid frequency. If the grid frequency is decreasing this means that the load on the grid is greater than the power delivered into the grid. and the local speed droop regulatory loop on each unit then autonomously increases the production to obtain a new balance between load and production. which will be at a lower frequency than 50 Hz. If the power delivered into the grid is greater than the load. the rotating masses will be accelerated (thus increasing the grid frequency) and the speed droop operation will act to reduce the power produced to obtain a new balance. this time at a higher frequency than 50 Hz. The frequency in the Nordic power grid has in recent years for increasing duration been outside the allowed steady state frequency band of 50 ± 0.1 Hz. In order to study the behaviour of a turbine operating on a common grid, measurements have been done at site. The measurements performed are the generator power, main servo motor position, the rotational speed of the unit and the grid frequency. The purpose of the measurements was to see if it is possible to observe the behaviour of the machine as it is linked together with all the other machines on a synchronous grid. It is interesting to observe the response to deviations in the frequency due to the speed droop operation. In order to better understand the behaviour, a simulation model of two power plants, complete with individual conduit system, turbine and generator, connected to the same grid was used.

Conduits for Coronary Bypass: Strategies

Strategic planning is integral to any operation but complexity varies immensely and therefore the effort necessary to create the optimal plan. The previous three reports have discussed individual conduits and herein is an attempt to present approaches to common situations which the author favors. Although much has been learned over 45 years about use and subsequent behavior of venous and arterial grafts we continue to learn and, as a result, evolve new strategies or modify those now popular. Thus the reader must recognize that in spite of trying to be balanced and inclusive all surgeons have personal opinions and also prejudices which influence the approach taken and which may not be the optimal one for others or for the patient.

Visualization of embolism formation in the xylem of liana stems using neutron radiography.

Cold neutron radiography was applied to directly observe embolism in conduits of liana stems with the aim to evaluate the suitability of this method for studying embolism formation and repair. Potential advantages of this method are a principally non-invasive imaging approach with low energy dose compared with synchrotron X-ray radiation, a good spatial and temporal resolution, and the possibility to observe the entire volume of stem portions with a length of several centimetres at one time. Complete and cut stems of Adenia lobata, Aristolochia macrophylla and Parthenocissus tricuspidata were radiographed at the neutron imaging facility CONRAD at the Helmholtz-Zentrum Berlin für Materialien und Energie, with each measurement cycle lasting several hours. Low attenuation gas spaces were separated from the high attenuation (water-containing) plant tissue using image processing. Severe cuts into the stem were necessary to induce embolism. The formation and temporal course of an embolism event could then be successfully observed in individual conduits. It was found that complete emptying of a vessel with a diameter of 100 µm required a time interval of 4 min. Furthermore, dehydration of the whole stem section could be monitored via decreasing attenuation of the neutrons. The results suggest that cold neutron radiography represents a useful tool for studying water relations in plant stems that has the potential to complement other non-invasive methods.

Gas Permeability of Shale

Summary Permeability is one of the most fundamental properties of any reservoir rock required for modeling hydrocarbon production. However, shale permeability has not yet been understood fully because of the complexities involved in modeling flow through nanoscale throats. In this paper, we analyze the effects of adsorbed layers of methane (CH4) and of gas slippage at pore walls on the flow behavior in individual conduits of simple geometry and in networks of such conduits. The network is based on scanning elecron microscope (SEM) images and a drainage experiment in shale. To represent the effect of adsorbed gas, the effective size of each throat in the network depends on the pressure. The hydraulic conductance of each throat is determined on the basis of the Knudsen number (Kn) criterion (Knudsen 1909). The combined effects of adsorption and slip depend strongly on pressure and on conduit diameter. The results indicate that laboratory measurements made with N2 at ambient temperature and 5-MPa pressure, which is typical for a transient pulse-decay (TPD) method, overestimate the gas permeability at early life of production by a factor of four. This ratio increases if the measurement is run at ambient condition because the low pressure enhances the slippage and reduces the thickness of the adsorbed layer. Moreover, the permeability increases nonlinearly as the in-situ pressure decreases during production. This effect contributes to mitigating the decline in the production rate of shale-gas wells. Laboratory data available in the literature for CH4 permeability at pressures below 7 MPa agree with model predictions of the effect of pressure.

A Leaky-Conduit Model of Transient Flow in Karstic Aquifers

Karst Flow Model (KFM) simulates transient flow in an unconfined karstic aquifer having a well-developed conduit system. KFM treats the springshed as a two-dimensional porous matrix containing a triangulated irregular network of leaky conduits. The number and location of conduits can be specified arbitrarily, perhaps using field information as a guide, or generated automatically. Conduit networks can be tree-like or braided. Rainwater that has infiltrated down from the surface leaks into the conduits from the adjacent porous matrix at a rate dictated by Darcy’s law, then flows turbulently to the spring via the conduits. KFM is calibrated using the known steady state; geometry and recharge determine the steady fluxes in the conduits, and the head distribution determines conduit gradients and sizes. Spring flow can vary with time due to spatially and temporally variable recharge and due to prescribed variations in the elevation of the spring. KFM is illustrated by four examples run on a test aquifer consisting of 27 nodes, 42 elements, and 26 conduits. Three examples (drought, uniform rainstorm, storm-water input to one element) are simulations, while the fourth uses data from a spring-basin flooding event. The qualitative fit between the predicted and observed spring discharge in the fourth example provides support of the hypothesis that the dynamic behavior of a karst conduit system is an emergent property of a self-organized system, largely independent of the locations and properties of individual conduits.

A tissue-engineered bioabsorbable nerve conduit created by three-dimensional culture of induced pluripotent stem cell-derived neurospheres

We previously reported a bioabsorbable nerve conduit coated with Schwann cells for the treatment of peripheral nerve defects. Since there have been dramatic developments in induced pluripotent stem (iPS) cells in recent years, the purpose of the present study was to create a tissue-engineered nerve conduit coated with iPS cell-derived neurospheres. Such a conduit was constructed by three-dimensional (3D)-culture of these cells using a bioabsorbable polymer conduit as a scaffold. The nerve conduit was composed of a mesh of poly L-lactide, and a porous sponge of 50% poly L-lactide and 50% poly ε-caprolactone. The primary and secondary neurospheres (PNS and SNS, respectively) induced from iPS cells were suspended in individual conduits. The conduits were incubated for 7 or 14 days in vitro and then evaluated using immunohistochemistry. All of the 7- and 14-day differentiated PNS and SNS were observed to have adhered to the inner surface of the conduits and to have migrated into the inner porous sponge. The engrafted cells were positive for anti-Tuj1, -S-100 and -GFAP antibodies, indicating that their pluripotent ability to form neural or glial cells was maintained. These findings indicate the feasibility of creating nerve conduits coated with a 3D-culture of iPS cell-derived neurospheres for the treatment of peripheral nerve defects.

Formation of smooth terrains on Comet Tempel 1

Abstract We suggest that the regions of smooth terrain which were observed on Comet 9P/Tempel 1 by the Deep Impact spacecraft were formed by blowing ice grains in an outburst of gas from the comet interior. When gas is released from 10 to 20 m deep layers which were heated to 135 K, it is released quiescently onto the surface by individual conduits. If large amounts of gas are released, the drainage system cannot release them fast enough and wider interconnected channels are formed, leading to sudden outburst of gas. Instability triggering a sudden shift of flow is well known in subglacial drainage of water. The ballistic trajectory of the ice particles reach a distance of 3 km in the atmosphereless comet, whose gravity is 0.034 cm s −1 , if ejected at an angle of 45° at a speed of 95 cm s −1 . This speed is close to the speeds measured in laboratory experiments: 167, 140 × sin i and 167 cm s −1 , for particles of 0.3, 1000 and 14–650 μm, respectively. Blowing of ice grains can overcome the 1650 m long horizontal section of smooth terrain i1 (Fig. 1), whereas simple flow of material downhill would stop close to the foot of the hill. The ice particles at the end of their trajectory have a horizontal velocity component and this low velocity ballistic sedimentation would lead to formation of lineaments on the smooth terrain, like in solid-particulate volcanic eruptions.

The relevance of xylem network structure for plant hydraulic efficiency and safety

The xylem is one of the two long distance transport tissues in plants, providing a low resistance pathway for water movement from roots to leaves. Its properties determine how much water can be transported and transpired and, at the same time, the plant's vulnerability to transport dysfunctions (the formation and propagation of emboli) associated to important stress factors, such as droughts and frost. Both maximum transport efficiency and safety against embolism have classically been attributed to the properties of individual conduits or of the pit membrane connecting them. But this approach overlooks the fact that the conduits of the xylem constitute a network. The topology of this network is likely to affect its overall transport properties, as well as the propagation of embolism through the xylem, since, according to the air-seeding hypothesis, drought-induced embolism propagates as a contact process (i.e., between neighbouring conduits). Here we present a model of the xylem that takes into account its system-level properties, including the connectivity of the xylem network. With the tools of graph theory and assuming steady state and Darcy's flow we calculated the hydraulic conductivity of idealized wood segments at different water potentials. A Monte Carlo approach was adopted, varying the anatomical and topological properties of the segments within biologically reasonable ranges, based on data available from the literature. Our results showed that maximum hydraulic conductivity and vulnerability to embolism increase with the connectivity of the xylem network. This can be explained by the fact that connectivity determines the fraction of all the potential paths or conduits actually available for water transport and spread of embolism. It is concluded that the xylem can no longer be interpreted as the mere sum of its conduits, because the spatial arrangement of those conduits in the xylem network influences the main functional properties of this tissue. This brings new arguments into the long-standing discussion on the efficiency vs. safety trade-off in the plants’ xylem.

Numerical simulation of cardiovascular dynamics with different types of VAD assistance

A variety of methods by which mechanical circulatory support (MCS) can be provided have been described. However, the haemodynamic benefits of the different methods have not been adequately quantified. The aim of this paper is to compare the haemodynamic effects of six forms of MCS by numerical simulation. Three types of ventricular assist device (VAD) are studied: positive displacement; impeller and a novel reciprocating-valve design. Similarly, three pumping modes are modelled: constant flow; counterpulsation and copulsation. The cardiovascular system is modelled using an approach developed previously, using the concentrated parameter method by considering flow resistance, vessel elasticity and inertial effects of blood in individual conduit segments. The dynamic modelling of displacement and impeller pumps is represented by VAD inlet/outlet flow-rate changes. The dynamics of the reciprocating-valve pump is modelled with a specified displacement profile. Results show that in each simulation, the physiological variables of mean arterial pressure and systemic flow are adequately maintained. Modulation of the impeller pump flow profile produces a small (5 mmHg) oscillatory component to arterial pressure, whereas the displacement and reciprocating-valve pumps generate substantial arterial pressure and flow pulsatility. The impeller pump requires the least power input, the reciprocating valve pump slightly more, and the displacement pump the most. The in parallel configuration of the impeller and displacement pump designs with respect to the left ventricle provides near complete unloading and can cause the aortic valve to remain closed throughout the entire cardiac cycle with the attendant risk of aortic valve leaflet fusion following prolonged support. The in series configuration of the reciprocating-valve pump avoids this shortcoming but activation must be carefully synchronized to the cardiac cycle to allow adequate coronary perfusion. The reciprocating-valve pump is associated with haemodynamic advantages and a favourable power consumption.

Mid-Term Angiographic Comparison of Sequential and Individual Anastomosis Techniques for Diagonal Artery

The mid-term patency rates for individual and sequential grafts as coronary bypass conduits for diagonal arteries were angiographically compared; the impact of native coronary vessel and type of the conduit characteristics are investigated. Between March 1992 and April 2000, we performed a total number of 811 distal anastomosis on diagonal arteries of left anterior descending (LAD) artery in 296 patients who underwent coronary artery bypass surgery (CABG) distal anastomosis in our clinic. The patients were divided into two groups in this prospective study. In group A (n = 195) individual anastomosis technique, in group B (n = 101) sequential anastomosis technique was chosen as the myocardial revascularization strategy. At an average of 49.4 +/- 13.2 months after coronary revascularization procedure coronary angiographies were evaluated. Individual and sequential grafting techniques were compared by graft patency rates. The patency rates of sequential conduits were markedly higher than those of individual conduits (66.7% vs. 89.2%, p = 0.0001). This difference was also clear in coronary arteries with poor quality and small (<1.5 mm) diameter (49.1% vs. 66.6%, p = 0.032). Also, the patency rates of sequential radial artery conduits were higher than sequential saphenous vein graft (SVG) conduits (sequential radial artery; 94.1%, sequential SVG; 85.3%, p = 0.043). Sequential grafting for diagonal artery is technically more demanding but the mid-term results are better than individual grafting especially in coronary arteries with poor quality. Using radial artery as a sequential graft increases the mid-term graft patency rates.

Do Angiographic Results From Symptom-Directed Studies Reflect True Graft Patency?

Coronary artery graft patency results have been obtained from repeat angiograms in patients who presented with evidence of ischemia. The purpose of this study is to compare protocol-directed angiographic results from a randomized clinical trial with symptom-directed angiography in nontrial patients. Repeat angiography after primary isolated coronary artery bypass grafting was performed in 337 of 2,259 patients between July 1996 and September 2004. Patients were divided into two groups: 596 graft angiograms in 192 trial patients were compared with 389 graft angiograms in 142 nontrial patients. The mean interval from surgery was 1,306 +/- 800 days versus 1,119 +/- 777 days, respectively. Grafting techniques were similar in both groups except that the right internal thoracic artery was used almost exclusively as a free graft in the trial patients. Angiographic outcomes were defined as patent (stenosis <80%) or failure (stenosis > or =80%, occlusion, or the string sign). Comparisons of trial versus nontrial grafts were made using a generalized linear mixed model. Five-year estimates of graft patency were made using survival analyses accounting for interval censoring. The odds ratio for graft failure for nontrial compared with trial patient grafts was 2.6 (95% confidence interval, 1.6 to 4.3; p < 0.001). Cumulative patency estimates for all grafts at 5 years were trial 91% versus nontrial 83%, p = 0.004. Five-year estimates for individual conduits were left internal thoracic artery, 99% versus 92%, p = 0.002; right internal thoracic artery, 86% versus 87%, p = 0.8; radial artery, 87% versus 86%, p = 0.6; and saphenous vein, 86% versus 56%, p = 0.003. Graft patency rates were superior in the trial compared with nontrial patients. Symptom-directed graft failure rates were approximately double those of trial patients.

Tansley Review No. 119

Vascular plants have shown a strong evolutionary trend towards increasing length in xylem conduits. Increasing conduit length affects water transport in two opposing ways, creating a compromise that should ultimately define an optimal conduit length. The most obvious effect of increased length is to decrease the sequential number of separate conduits needed to traverse the entire pathway, and thereby to reduce the number of wall‐crossings and the hydraulic resistance to flow within the xylem. This is an essential evolutionary pressure towards the development of the vessel, a conduit of multicellular origin whose length is not restricted by developmental constraints. The vessel has been an essential component in all plant lineages, achieving transport tissues with very high specific conductivity. A countering effect, however, arises from the partitioning of the cavitation response, a process whereby individual xylem conduits drain of water and lose conducting capacity. Flow in the xylem is down a gradient of negative pressure, which is necessarily most negative in the distal regions (i.e. near the foliage). Cavitation can be caused directly by negative pressures, and results in a total loss of the hydraulic conductance of the individual conduits within which it occurs. If cavitation is triggered by low pressure experienced only at the very distal end of a long conduit, the conduit nevertheless loses its conducting capacity along its entire length. Pathways composed of long conduits will therefore suffer greater total conductance loss for equivalent pressure gradients, because the effects of cavitation are not effectively restricted to the tissue regions within which the cavitation events are generated. By contrast, short conduits can restrict cavitation to distal regions, leaving trunk and root tissues less seriously affected. The increased total conductance loss of a system made entirely of very long conduits translates into a lower maximum rate of water transport in the xylem. The loss in hydraulic capacity associated with failure to partition the flow pathway fully, and locally contain the effects of cavitation, theoretically reaches a maximum of 50% for the extreme case in which a single set of conduits traverses the entire pathway. Shorter conduits confine individual cavitation events to smaller regions and permit the pathway as a whole to have a more gradual conductance loss in conjunction with the pressure gradient. A compromise exists between (1) minimizing total conductance loss from cavitation via fine partitioning of the pathway with many tiers of short conduits, and (2) reducing total wall resistance via coarse partitioning with a few tiers of long conduits. An analysis is presented of the optimal number of end walls (i.e. mean conduit length relative to total pathway length) to maximize transport capacity. The principle of optimal containment of cavitation also predicts that conduits should not be of equal length in all portions of the pathway. The frequency of end walls should rather be proportional to the magnitude of the water‐potential gradient at each point, and conduits should be longest in the basal portion (roots) and progressively shortened as they move up the stems to the foliage. These concepts have implications for our understanding of the contrasting xylem anatomies of roots and shoots, as well as the limits to evolution for increased hydraulic conductance per xylem cross‐sectional area. They also indicate that to model the hydraulic behaviour of plants accurately it is necessary to know the conduit length distribution in the water flux pathway associated with species‐specific xylem anatomy. CONTENTS Summary 196 I. INTRODUCTION 196 II. EVOLUTIONARY TRENDS IN CONDUIT DIMENSIONS 199 III. MAXIMUM XYLEM TRANSPORT IN THE PRESENCE OF CAVITATION 203 IV. INCLUDING VESSEL LENGTH IN A TRANSPORT MODEL 205 V. CONDUIT LENGTH IN MODERN TAXA: IMPLICATIONS FOR TRANSPORT 210 VI. CONCLUSIONS 212 VII. APPENDIX: ANALYTICAL SOLUTIONS AND PROOFS 213 Acknowledgements 216

Use of a Multiple Lumen Cuff for Nerve Regeneration

AbstractInitial studies using the rat sciatic nerve demonstrated the ability to adapt a multiple lumen cuff face to a nerve stump repair site. The neurons in the proximal stump grew through the individual conduits of the silicone rubber cuff, crossed a 5 mm gap, and continued into the distal stump. The effect of the cuff design on axonal regeneration was studied by comparing macroscopic and microstructural results for experimental groups of Sprague-Dawley rats with controls at 8, 12, 16, and 24 weeks post-implantation. The several individual nerve bundles which formed within the cuff lumens during these periods maintained their alignment on the distal side of the gap. The use of the multiple lumen system provided suitable scaffolding support and control of orientation and direction for fibers and established a sized, controlled environment for regeneration within each of the separate nerve cuff compartments.