Showing papers on "Water flow published in 1976"

Water Flow in Glaciers: Jökulhlaups, Tunnels and Veins

Abstract: The physics of water flow within and under glacier ice is examined with special reference to the periodic catastrophic outbursts of water (jokulhlaups) from the subglacial lake Grimsvotn, Vatnajokull, Iceland. The lake is sealed until it reaches a critical level which enables it to lift the glacier, helped by a hydrostatic cantilever effect. The differential equations for non-steady water flow in a subglacial tunnel are derived and applied to the 1972 Grimsvotn outburst. The discharge: time relation observed during the growth stage, and the abrupt ending of the flood, are both very well accounted for by a theory which is insensitive to the details of the subglacial tunnel system. The steady state, in which an intergranular vein or tunnel is simultaneously melted open by frictional heat and closed by plastic deformation, may be stable or unstable according to the conditions imposed at the ends. This explains why the flow of water in a vein does not normally increase unstably as in a jokulhlaup. An ice-dammed lake does not drain away through the vein system because the driving force on the vein-water is towards the lake rather than away from it.

The mixing layer at high Reynolds number - Large-structure dynamics and entrainment

Abstract: A turbulent mixing layer in a water channel was observed at Reynolds numbers up to 3 × 10^6. Flow visualization with dyes revealed (once more) large coherent structures and showed their role in the entrainment process; observation of the reaction of a base and an acid indicator injected on the two sides of the layer, respectively, gave some indication of where molecular mixing occurs. Autocorrelations of streamwise velocity fluctuations, using a laser-Doppler velocimeter (LDV) revealed a fundamental periodicity associated with the large structures. The surprisingly long correlation times suggest time scales much longer than had been supposed; it is argued that the mixing-layer dynamics at any point are coupled to the large structure further downstream, and some possible consequences regarding the effects of initial conditions and of the influence of apparatus geometry are discussed.

Boundary layer stability and transition

Abstract: : A review is given of boundary layer stability and transition. The normal modes procedures as they apply to boundary layers are briefly reviewed and the mechanism of instability is discussed. It is shown how normal modes results may be used to give guidance regarding the factors affecting transition. Some remarks are made about the prediction of transition and about the fixing of transition. It is concluded that the process of transition from laminar to turbulent flow remains unsolved. However, significant inroads into inroads into the understanding of transition are now possible because of our ability to do sophisticated theoretical and experimental studies of the stability of laminar boundary layers.

Electrokinetic phenomena associated with earthquakes

Abstract: Electrokinetic phenomena induced by ground water flow associated with earthquakes are proposed to provide a possible means of earthquake prediction. Detectable variations of earth currents, electric potential and geomagnetic field may be caused by diffusion of fluid into a dilatant focal region. The principle of electrokinetic phenomena may be used for earthquake control by controlling the water flow with an artificial electric potential field.

Poiseuille and his law

Abstract: Comparatively little is known of the life of Jean Leonard Marie Poiseuille (1797-1869) of Paris. He made important contributions to the experimental study of circulatory dynamics but it can hardly be said that Poiseuille knowingly described the law which governs laminar flow. He did however establish, in a series of meticulously executed experiments, that at a given temperature water flow through tubes of very fine bore is inversely proportional to the length of the tube and directly proportional to the pressure gradient and to the fourth power of the tube diameter.

Steady wake behind a sphere

Abstract: The characteristics of the steady wake behind a sphere were studied experimentally using dyed water for visualization. This work infers the possibility of the existence of a closed recirculating eddy behind a sphere even at R=10 or lower and shows that the eddy can preserve its steady state up to the case for R=190 (R represents the Reynolds number based on the diameter of the sphere). In addition, a new profile for the variation of the wake separation angle against the steady state Reynolds number is presented.

Experimental Investigation of Flow Over Side Weirs

Abstract: Observations at numerous cross sections in a side weir channel showed that the total energy of flow decreases substantially along the channel, contrary to the assumption that has frequently been made. The decrease in total energy occurs because the longitudinal component of velocity of the spill flow is greater than the longitudinal velocity of the flow remaining in the channel. A satisfactory flow equation is derived from momentum principles and this is further developed into a computer technique for calculating flow profiles and spill discharges for side weirs.

Water and plant life : problems and modern approaches

Abstract: 1 Fundamentals of Plant Water Relations- Preface- A The Structure of Water in the Biological Cell- I Introduction- II Evidence for Structured Aqueous Boundary Layers- III Thermal Anomalies in Biological Tissues- IV Properties of Aqueous Electrolyte Layers- V Conclusions- References- B The States of Water in the Plant-Theoretical Consideration- I Introduction- II Physiological Importance of Processes and Properties Involving Water- III Metabolism and Water Relations- IV Conclusions- References- C The Soil-Plant-Atmosphere Continuum- I Introduction- II Description of the Turgor Pressure as a Function of Environmental Variables- III Water Flow in the SPAC as a Link Between Plant and Environment- IV The Solute-free Transport System- V Effects of Solutes in the SPAC- VI Changes in Resistances or Potential Differences- VII Conclusions- References- D The Water Status in the Plant-Experimental Evidence- I Introduction- II Current Methods for the Determination of Total Water Potential and Its Components- III The Range of Water Potentials Hitherto Determined and the Continuum Conditions Favoring Extreme Values- IV The Component Potentials Adjusting Total Water Potential in the Plant Body: Ranges and Changes- V Why does Water Potential in a Plant Change?- VI Conclusions- References- 2 Water Uptake and Soil Water Relations- Preface- A Root Extension and Water Absorption- I Introduction- II Water Movement Through the Soil-Plant-Atmosphere Continuum: Limitations in the Liquid Phase- III Root Extension and Facilitation of Water Uptake in Unexplored Soil Regions- IV Root Extension Within the Rooted Zone: A Case for Avoidance of Localized Rhizospheric Resistances- V Conclusions- References- B Resistance to Water Flow in the Roots of Cereals- I Introduction- II Anatomy of Cereal Roots- III Zone of Water Absorption- IV Forces Causing Flow of Water- V Resistance to Flow- VI Effect of Root Resistance on Withdrawal of Water from the Soil- VII Conclusions- References- C Soil Water Relations and Water Exchange of Forest Ecosystems- I Introduction- II Water Balance- III Fundamental Equations and Principles- IV Simulation of Evapotranspiration and Percolation- V Conclusions- References- 3 Transpiration and Its Regulation- Preface- A Energy Exchange and Transpiration- I Introduction- II Gas Diffusion- III Energy Balance- IV Transpiration- V Wind Speed Influence- VI Leaf Temperature Affected by Transpiration- VII Conclusions- References- B Water Permeability of Cuticular Membranes- I Introduction- II Cuticular Transpiration-Early Observations and Hypotheses- III The Concept of the Polar Pathway Through Lipid Membranes- IV Conclusions- References- C Physiological Basis of Stomatal Response- I Introduction- II Biochemical Processes Leading to Movement- III Conclusions: Ability of the Mechanism to Explain the Known Facts- References- D Current Perspectives of Steady-state Stomatal Responses to Environment- I Introduction- II Measurement of Stomatal Responses to Environment- III Steady-state Stomatal Responses to Environment- IV Stomatal Responses to Diurnal Changes in Environment- V Conclusions and Future Research Directions- References- E Water Uptake, Storage and Transpiration by Conifers: A Physiological Model- I Introduction- II Description of the Model- III Applications- IV Conclusions- References- 4 Direct and Indirect Water Stress- Preface- A Water Stress, Ultrastructure and Enzymatic Activity- I Introduction- II Effects of Water Stress on Hydrolytic Enzymatic Activity- III Effects of Water Stress on the Ultrastructure of the Cell- IV Relationships of Ultrastructural Alteration and Hydrolytic Enzyme Decompartmentation and Activation, with Alteration of Chloroplasts and Mitochondria Metabolism- V Conclusions- References- B Water Stress and Hormonal Response- I Introduction- II Endogenous Hormonal Changes Due to Water Stress- III The Physiological Significance of Hormonal Effects- IV A Hypothetical Model for the Role of Hormones in Plant Adaptation to Water Stress- V Conclusions- References- C Carbon and Nitrogen Metabolism Under Water Stress- I Introduction- II Carbon Metabolism Under Water Stress- III Nitrogen Metabolism Under Water Stress- IV Biochemical Aspects of Desiccation Resistance- V Conclusions- References- D Water Stress During Freezing- I Introduction- II Frost Injury- III Frost Resistance- IV Conclusions- References- E Cell Permeability and Water Stress- I Introduction- II Principles of Cell Permeability- III Quantitative Determination of Permeability- IV Alterations of Cell Permeability by the Plant Water Deficit- V Possible Mechanisms for Changes in Cell Permeability by Plant Water Stress- VI Conclusions- References- F Water Stress and Dynamics of Growth and Yield of Crop Plants- I Introduction- II Overview of Growth and Yield as Affected by Water- III Some Behavior Observed in the Field- IV Concluding Remarks- References- 5 Water Relations and CO2 Fixation Types- Preface- A Crassulacean Acid Metabolism (CAM): CO2 and Water Economy- I Introduction- II Carbon Metabolism of CAM Plants- III Gas Exchange of CAM Plants- IV Ecological Aspects of CAM- V Conclusions- References- B Balance Between C3 and CAM Pathway of Photosynthesis- I Introduction- II Adaptation to Salinity- III Environmental Control of Photosynthetic Pathways- IV Regulation of the Balance between C3 and CAM- V Ecological Aspects- References- C C4 Pathway and Regulation of the Balance Between C4 and C3 Metabolism- I Introduction- II Carbon Metabolism of C4 Plants- III General Characteristics of C4 Plants- IV Factors Affecting Shift- V Natural C3-C4 Intermediates- VI Ecological Implications- VII Conclusions- References- D Ecophysiology of C4 Grasses- I Introduction- II Environmental Conditions- III Physiological Responses to Environmental Conditions- IV Ecological Implications- V Conclusions: Future Research- References- 6 Water Relations and Productivity- Preface- A The Use of Correlation Models to Predict Primary Productivity from Precipitation or Evapotranspiration- I Introduction- II Construction of Correlation Models and Geographical Patterns (Surfaces)- III Some Examples of Correlation Models of Net Primary Productivity versus Water Factor- IV Accuracy of Correlation Models- V Conclusions- References- B The Use of Simulation Models for Productivity Studies in Arid Regions- I Introduction- II The Structure of the Model- III Description of the Model ARID CROP- IV Validation of the Model- V Application of the Model- VI Conclusions- References- C Irrigation and Water Use Efficiency- I Introduction- II Efficiency of Water Supply- III Transpiration/Photosynthesis Relationships- IV Some Agronomic Aspects- V Conclusions- References- D Estimating Water Status and Biomass of Plant Communities by Remote Sensing- I Introduction- II Water Stress, Reflectance, and Temperature of Single Leaves- III Reflectance and Biomass of Communities- IV Conclusions- References- E Plant Production in Arid and Semi-Arid Areas- I Introduction- II Survey of Phytomass, Net Annual and Relative Annual Production of Some Main Vegetation Units of the Globe- III Phytomass and Production of Some Arid and Semi-Arid Vegetation Units and their Annual Fluctuations- IV Permanent Phytomass- V Potential Production- VI Recovery- VII Conclusions- References- F Water Content and Productivity of Lichens- I Introduction- II Productivity of Lichens- III Water Relations of Lichens- IV Thallus Water Content and Physiological Response- V Conclusions: Water Relations and Productivity-a Synthesis- References- 7 Water and Vegetation Patterns- Preface- A Water Relations and Alpine Timberline- I Introduction- II Water Relations of Trees at the Timberline- III Causes of Winter Desiccation of Trees at Timberline- IV Conclusions: Ecophysiological Analysis of the Alpine Timberline and its Dynamics- References- B The Water Factor and Convergent Evolution in Mediterranean-type Vegetation- I Introduction- II Environmental Stresses in Mediterranean-type Climates- III Ecological Significance of Leaf Structure- IV Seasonal Patterns of Photosynthesis, Water Relations and Productivity- V Evolutionary Consequences of Mediterranean-type Environmental Stresses- VI Conclusions- References- C The Water-Photosynthesis Syndrome and the Geographical Plant Distribution in the Saharan Deserts- I Introduction- II The Floristic and Physiognomic Aspects of the Sahara- III The Water-Photosynthesis Syndrome in the Northern and in the Southern Sahara- IV Holarctic and Palaeotropic Constitution Types- V Conclusions- References- Index of Plant Species

Studies on the Movement of Water Through Apple Trees

Abstract: Resistances to the flow of water through young potted apple trees were estimated by measuring the transpiration rate of trees with and without root systems. Root system resistances were obtained by difference. Whole-plant resistances were of the order 10 x 1013 Pa s m-3 and there was some evidence that root resistances (Rr) varied with transpiration rate; the ratio liT■ Rx (where Rx is resistance to water flow in the stem system) altered from 2:1 at relatively high transpiration rates to 1:1 at lower rates. The trunk of a 9-year-old orchard tree (trunk diameter ~7 cm, height ~2-5 m) was cut under water and estimates of the flow resistances in this tree were obtained. These were much lower than the resistances to flow in the potted trees. Capacitance (defined as the change in stored water content per unit change in plant water potential) values were calculated for the small trees and the large tree from measurements of weight and water potential changes after the trees were removed from water. They were very similar on a weight basis (approx. 2-0 x 10~8 kg kg-1 Pa-1). Leaf capacitance values (~1 x 10~8 kg Pa-1 m-2) were also obtained. Stomatal conductances decreased with water potential and increased with short-wave radiation, but the relationships were not definitive. Estimates of boundary layer conductance in a greenhouse (very low wind speeds) were of the same order (~ 5 mm s-1) as values obtained previously.

Accurate Explicit Equation for Friction Factor

Abstract: The solution of pipe flow problems encountered in engineering practice requires an intermediate step of computation of friction factor. The friction factor of commercial pipes is described by a semi-empirical equation developed by Colebrook. This equation that expresses the friction factor as a function of relative roughness and Reynolds number is an implicit one requiring a trial-and-error procedure for its determination. Wood has pointed out obvious disadvantages of an implicit relationship and recognizing the necessity of an explicit equation proposed one to replace the implicit equation for friction factor. It has, however, been possible to develop a simple and more accurate explicit equation for friction factor, based on the Colebrook-White formula, and is presented herein.

Vortex shedding and resistance in harmonic flow about smooth and rough circular cylinders at high reynolds numbers

Abstract: : This report presents the results on an extensive experimental investigation of the in-line and transverse forces acting on smooth and rough circular cylinders placed in oscillatory water flow at Reynolds numbers up to 700,000, Keulegan-Carpenter numbers up to 150, and relative roughnesses from 0. 002 to 0.02. The drag and inertia coefficients have been determined through the use of the Fourier analysis and the least-squares method. The transverse force (lift) has been analyzed in terms of its maximum, semi-peak-to-peak, and root- mean-square values. In addition, the frequency of vortex shedding and the Strouhal number have been determined.

An analysis of water flow in dry snow

Abstract: The equations describing water movement in a dry snow cover are derived, and examples of flow through ripe, refrozen, and fresh snows are given. The grain size of snow has a large effect on the timing of water discharge. Water is retained by dry snow to raise its temperature and satisfy the irreducible water saturation. These requirements delay and reduce runoff following rain on dry snow.

Time-dependent Linearized Infiltration: III. Strip and Disc Sources

Abstract: Water flow from strip and disc surface sources is analyzed using an approach similar to previous investigations with line and point sources. A line of closely spaced trickle irrigation emitters often wets a surface strip of finite width; similarly, for a single emitter the surface wetted pattern is a disc. Lines of equal matric flux potential are wider and more shallow for these sources than wetting patterns for the line and point. The moisture regime is independent of the source shape and depends only on the total flow rate for regions beyond approximately two times the strip width or disc radius.

Air-Entrainment in High Head Gated Conduits

Abstract: For the successful operation of high head gated conduits, adequate supply of air is of paramount importance. However, no dependable solution is yet available to determine the required quantity of air for different flow types. Based on systematic experiments backed by theoretical considerations, suitable relationships and diagrams have been developed that predict results compatible with prototype measurements. Conduit roughness has no significant effect on air-entrainment. No linear relationship has been found to exist between the head and the maximum air-entraining capacity of the flow. Measurements of air velocity by means of high speed cinematography indicate that the velocity at the interface is much higher than the mean velocity of water flow and that the air velocity distribution is far removed from being logarithmic.

On the form and stability of open hydraulic fractures in the Earth's crust

Abstract: The cross-sectional form and stability of large hydraulic fractures are found to be very sensitive to symmetric gradients in fluid pressure and regional stress. Horizontal sections through vertical fractures may be subjected to symmetric pressure gradients by locating inflow and outflow wells near the fracture center and tips. A dumbbell form, resulting from flow from the tips toward the center, has a limital length beyond which the fracture closes at the center and propagates unstably at the tips. A diamond form, resulting from the opposite sense of flow, has no limital length and a stable propagation. These features of fracture form and stability may play an important role in the design of hydraulic fractures for geothermal energy production. (AIP)

Water Movement Through Root Systems

Abstract: The root system provides a considerable resistance to water flow through it. This resistance is important because it influences the water deficits which can build up in the above-ground parts of the plant even when the roots are in moist soil. The permeability of roots per unit surface area varies between species, and between different parts of the same root (depending on age). It is sensitive to environmental conditions such as temperature, oxygen and salinity; this suggests that a major part of the resistance lies in living material in the 'radial-flow pathway', i.e. between the root surface and entry to the xylem. It is uncertain which is the main pathway of radial flow. Possible alternatives are: (1) the vacuolar pathway, crossing the cortex and other tissues by passing from vacuole to vacuole; (2) the free-space/endodermis pathway, water moving in the cell walls except at the endodermis where it enters the protoplasm; or (3) the symplasm pathway, water moving in the protoplasm and passing from cell to cell by plasmodesmata. Calculations are presented, based on the primary root of maize whose cell dimensions and root permeability are known, to show what permeabilities of membranes, wall material and plasmodesmata would be necessary to allow each of the three pathways to predominate. These calculations predict the symplasm as the most likely pathway. The free-space/endodermis pathway is unlikely because the permeability of cell wall material is too low. Several pieces of experimental evidence also favour the symplasm pathway. There are large variations between species in the permeability of roots for longitudinal flow in the xylem. Usually this xylem flow will present much less resistance than the radial flow pathway. However, there is no adequate evidence on whether the resistance may be greatly increased by bubble formation in vessels under water stress.

Some mechanisms for the control of pumping activity in bivalves

Abstract: 1) Filtration rates of a number of bivalves are compared on a basis of the porosity of their gills. There is no obvious difference in the performance of infaunal, siphonate bivalves and epifaunal, non‐siphonate bivalves. 2) The exhalant aperture may control water flow in three ways : periodic closure of the exhalant aperture; restriction of the exhalant aperture to a small size; rapid and repeated expansion and contraction of the exhalant aperture. 3) In high concentrations of suspension of Phaeodactylum, a reduction in the time spent pumping accounted for reduced filtration rates up to concentrations of 1 million cells/ml in the cases of Mytlius edulis and Mya arenaria, and up to concentrations of ½ million cells/ml in the cases of Cerastoderma edule and Venerupis pullastra. 4) Water flow in a bivalve has been examined from a theoretical standpoint and suggests that an 80 % reduction in the size of the exhalant aperture is required before greatly affecting the total resistance to flow through the system.

Membrane structural and functional responses to vasopressin in toad bladder

Abstract: Freeze-fracture electronmicroscopy demonstrates that vasopressin stimulation of isolated toad bladder results in a striking morphologic alteration of epithelial membrane structure. This alteration is characterized by the aggregation of intramembranous particles in orderly linear arrays at multiple sites in the luminal membranes of granular cells specifically. The size of these aggregates varies considerably, in terms of area, over a range from 0.5 to 70×10−3 μm2. The median aggregate size is about 10.5×10−3 μm2. Since the extent of vasopressin-associated particle aggregation, in terms of frequency of sites per area of membrane or cumulative area of membrane occupied by them, closely correlates with induced changes in transport function, as measured by osmotic water flow, the aggregates themselves appear to be of physiologic significance in the mechanism of action of vasopressin. This hypothesis is supported by the observations that sites of aggregation occur (a) in response to serosal exposure to hormone specifically, (b) independently of an osmotic gradient, and (c) following stimulation with cyclic adenosine monophosphate.

Relative Importance of Soil Resistance and Plant Resistance in Root Water Absorption

Abstract: Soil water potentials, leaf water potentials, and transpiration rates of sweet corn (Zea mays L.), growing in a greenhouse, and grain sorghum (Sorghum bicolor L.), growing in a field, were determined to evaluate the magnitude of the combined soil and plant resistances to water flow in the plant system. Using a theoretical analysis of water absorption by roots, soil resistance was estimated. Plant resistance was inferred by the difference between the measured combined resistances and the estimated soil resistance. A wide range of soil hydraulic conductivity values for the plant rooting media which included nutrient solutions, peat-vermiculite-sand mixture, and a sand and clay soil maintained at various water potentials provided variations in calculated soil resistances. Our results showed that when root density was not unusually low, plant resistance to water transport was much larger than soil resistance, until the threshold soil hydraulic conductivity reached about 10-6 to 10-7 cm/day. This conductivity usually occurred at about -1 and -8 bars for the sandy and clay soils, respectively. These findings emphasize the need to consider plant resistance in water-uptake calculations when using equations that evaluate water potential gradients along the water flow path.

The functioning and interrelationships of blood capillaries and lymphatics.

Abstract: the structure and function of blood capillaries, as related to permeability, depends on tight, close and (in injured vessels) open junctional regions, small vesicles, vacuoles (in injured vessels) and fenestrae. The basement membrane presents a hindrance to the larger macromolecules, at high flow rates, but not to small molecules. The connective tissue channels are probably the paths by which macromolecules, and most of the small ones, pass from the arterial-limbs to the venous ones, and to the lymphatics. In some regions these channels are grouped in special systems: the prelymphatics. The initial lymphatics take up material via open junctions, which close during tissue-compression. The collecting lymphatics retain the lymph because they do not have open junctions. In the close junctional regions the motive force for water flow is the result of Starling's forces; diffusion is very important for other small molecules. The small vesicles transport macromolecules slowly by Brownian motion, as may the vacuoles, but possibly these latter are moved actively. There is much evidence that colloids can develop high effective osmotic pressures even across pores much larger than their molecules, and that proteins can be dragged up a concentration gradient by the resultant fluid flow. On the basis of this, hypotheses have been developed about the functioning of venous-limb fenestrae and the initial lymphatics, for which there is much theoretical, in vitro, and in vivo evidence. Thus, in fenestrae and regions there is held to be a large local circulation through the tissues, of which a quantitatively small, but qualitatively vital, part goes to the lymphatics. Material is considered usually to enter these latter because of the relative concentration of the lymph. It is becoming increasingly evident that in the study of the microvasculature, as with other systems, there is much to be gained by quantifying fine structural observations and by combining and contrasting this data, via physical laws, with that obtained by other methods where the characteristics of whole organs and regions are studied. Thus one can obtain interrelated information, which is not possible by either method alone, and which gives us a vital, comprehensive, perspective of the ways in which whole systems function, and how different systems interact. In this paper I shall show how this approach has yielded much that is new about the functioning of different kinds of blood capillaries, of the tissue channels, of the whole lymphatic system, and of the ways they affect each other.

The Influence of Temperature on the Exudation of Xylem Sap from Detached Root Systems of Rye (Secale cereale) and Barley (Hordeum vulgare)

Abstract: Roots of intact plants of rye and barley which had been growing at 20° were cooled for 12–72 h at 8–14° C while the shoots were kept at 20°. The roots were then excised and placed in solutions at temperatures ranging from 2.5–22.5° C. The rate of exudation of xylem sap and the chemical composition and osmotic potential of the sap were measured and compared with controls which had been kept at 20° C during the pretreatment period. Pre-cooling increased the fluxes of K+, Ca2+ and H2PO 4 - into the xylem sap of both species by factors of two to three; the total volume of exudate rose by larger factors. Thus the concentrations of these ions were lower in the sap exuding from cooled roots than in that from controls. Measurements of the osmotic potential of the sap from barley roots indicated that the osmotic driving force in cooled and control roots was similar even though flow in the former was much greater. The enhancement of exudation was shown to be dependent on the duration and the temperature experienced by the roots during pretreatment, and was lost rapidly when roots of intact plants were returned to 20°. Analysis of the temperature coefficients for exudation and Arrhenius plots revealed very distinct changes in the activation energy for exudation above and below a transition temperature. In control plants of barley and rye this temperature was around 10° C, but in cooled roots of rye there was a significant shift in the transition temperature to 5° C. Activation energies for exudation of control and cooled roots above or below the transition temperature were broadly similar, thus pre-cooling roots did not alter the temperature sensitivity of exudation but merely its rate at a given temperature. The results are discussed in relation to active ion transport, membrane fluidity and the resistance of the root to water flow.

Plastids and Intracellular Transport

Abstract: The chloroplast always seems to be faced with reconciling the irreconcilable. While evolving oxygen, it must simultaneously produce an intermediate more reducing than hydrogen. While reducing NADP it can simultaneously generate ATP, an achievement which, when first reported, seemed almost as remarkable as making water flow uphill. No less striking in their own way are two seemingly conflicting roles in carbon metabolism. On the one hand, the chloroplast must operate its carbon cycle as an autocatalytic breeder reaction, while on the other, it must export elaborated carbon and chemical energy to its cellular environment. In order to export, it must produce more than it uses, but it can only do this by returning newly synthesised intermediates to the cycle. Conversely, in order to satisfy the needs of the cell, it must release newly made products to the cytoplasm. Clearly, these processes could not be efficiently accomplished unless it were possible to strike a delicate balance between recycling, export and internal storage. Precisely how this is done is still largely a matter for speculation, but this Chapter will attempt to show how contemporary work has provided a factual basis for conjecture.

Pressure Reducing Valves in Pipe Network Analysis

Abstract: The three separate systems of equations that are in common use in solving steady flow problems in pipe networks are described and the method for including pressure reducing valves in each of these systems is given The emphasis is on use of efficient computer oriented methods In the method presented the pressure reducing valves are permitted to operate normally in maintaining a constant downstream pressure, or are permitted to act as check valves, preventing reverse flow should downstream pressures exceed valve settings The inclusion of pressure reducing valves as an integral part of the network of pipes, pumps, reservoirs, and minor loss devices causes no appreciable reduction in the performance of computer programs that have been specifically written to carry out the computations for solutions