Drug release from oral pharmaceutical formulations can be modified by applying a polymeric coating film with controlled mass transport properties. Interaction of the coating film with water may crucially influence its composition and permeability to both water and drug. Understanding this interaction between film microstructure, wetting, and mass transport is important for the development of new coatings. We present a novel method for controlled wetting of polymer coating films in an environmental scanning electron microscope, providing direct visual information about the processes occurring as the film goes from dry to wet. Free films made of phase-separated blends of water-insoluble ethyl cellulose (EC) and water-soluble hydroxypropyl cellulose (HPC) were used as a model system, and the blend ratio was varied to study the effect on the water transport properties. Local variations in water transport through the EC/HPC films were directly observed, enabling the immediate analysis of the structure–mass transport relationships. The leaching of HPC could be studied by evaporating water from the films in situ. Significant differences were observed between films of varying composition. The method provides a valuable complement to the current approach of making distinct diffusion and microscopy experiments for studying the dynamic interaction of polymer films with water.

The factual evidence for aquaporins, the constituents of water channels in cell membranes and operation of water channels is reviewed in developing, maturing and germinating seeds.

Environmental scanning electron microscopy has been extensively used for studying the wetting properties of different materials. For some types of investigation, however, the traditional ways of conducting in situ dynamic wetting experiments do not offer sufficient control over the wetting process. Here, we present a novel method for controlled wetting of materials in the environmental scanning electron microscope (ESEM). It offers improved control of the point of interaction between the water and the specimen and renders it more accessible for imaging. It also enables the study of water transport through a material by direct imaging. The method is based on the use of a piezo-driven nanomanipulator to bring a specimen in contact with a water reservoir in the ESEM chamber. The water reservoir is established by local condensation on a Peltier-cooled surface. A fixture was designed to make the experimental setup compatible with the standard Peltier cooling stage of the microscope. The developed technique was successfully applied to individual cellulose fibers, and the absorption and transport of water by individual cellulose fibers were imaged.

À partir d'un modèle de transport d'eau en milieu biphasique et élastique [I. Mrani, J.-C. Bénet, G. Fras, Transport of water in a biconstituant elasticmedium, Appl. Mech. Rev., special issue on Mechanics of swelling 48 (1995) 717–721], on examine dans cette étude les risques de fissurationsinduites par la redistribution d'eau dans un échantillon de forme cylindriqueaprès séchage. La résolution numérique de l'équation de transport de l'eau et del'équation d'équilibre mécanique donnent accès à l'évolution de la teneur eneau, de la déformation et des contraintes dans le milieu. On s'intéressera àl'état du matériau pendant les phases de séchage, de refroidissement et destockage. À l'issue de cette étude, les risques de fissuration sont analyséspour différents scénarios de séchage, qui déterminent l'évolution de l'étathydrique pendant le stockage.

Summary 196

I. INTRODUCTION 196

1. The neglected dimension 196

2. Basic concepts 197

(a) The heuristic notion of vessel-tiers 197

(b) Ohm's law 199

(c) Conductances, resistances and resistivities 199

(d) Lumen and pit resistances 199

(e) The importance of conduit radius and length in conductance 199

II. EVOLUTIONARY TRENDS IN CONDUIT DIMENSIONS 199

1. Nature and origin of xylem conduits 199

2. Increasing hydraulic conductance with increasing diameter and length 200

(a) Evolutionary trends in tracheid dimensions 200

(b) Origin of the vessel 200

3. Functional limitations to increasing vessel length 201

(a) Safety versus efficiency 201

(b) Containment of cavitation and embolism 202

III. MAXIMUM XYLEM TRANSPORT IN THE PRESENCE OF CAVITATION 203

1. Cavitation is linked to the driving force for transport 203

2. Transport models and extreme assumptions about conduit length 203

(a) Unitary cavitation response (n = 1) 203

(b) Infinitely partitioned response (n = ∞) 204

(c) ΔP and cavitation containment 204

IV. INCLUDING VESSEL LENGTH IN A TRANSPORT MODEL 205

1. Framing questions of optimal conduit length 205

2. A numeric model for flow through n conduit tiers 205

(a) Model structure 205

(b) Model solution 206

3. Optimization when f(P) is linear 206

(a) Isolating the effects of n on cavitation containment 206

(b) Optimal conduit tier-length distributions (OCLDs) 207

(c) Abrupt changes in conduit length 208

(d) Optimal frequency of end walls: incorporating Rpit 208

4. Optimization when f(P) is curvilinear 210

V. CONDUIT LENGTH IN MODERN TAXA: IMPLICATIONS FOR TRANSPORT 210

1. Limitations to the concept of conduit tiers 210

(a) Vessel ends are randomly distributed 210

(b) Dispersion around mean length within each ‘tier’ 210

2. Is the xylem optimally partitioned? 211

(a) Optimal number of end walls 211

(b) Conduit length distribution along the pathway 211

3. Hydraulic segmentation 212

(a) Segmentation in hydraulic resistance 212

(b) Segmentation in cavitation vulnerability 212

VI. CONCLUSIONS 212

1. Anatomy 212

2. Modelling flow 213

VII. APPENDIX: ANALYTICAL SOLUTIONS AND PROOFS 213

1. Analytic solution for Qmaxwith a single tier 213

2. The general case for n tiers 214

3. Analytic solution for Qmaxwith two tiers 214

4. Matric flux and n = ∞ 215

5. Rpit, variable pathway resistance and OCLD 215

6. Proof of Eqn 15 describing limited cavitation containment 215

Acknowledgements 216

References 216

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.

Xylem maturation in elongating leaf blades of tall fescue (Festuca arundinacea) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region in which most of the longitudinal water movements occurred in the LMX. The axial hydraulic conductivity (Kh) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conductivity (Kt) computed from the diameter of individual inner vessels. Kt was proportional to Kh throughout the leaf, but Kt was about three times Kh. The changes in Kh and Kt along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, Kh was about 0.30±0.07 mmol s−1 mm MPa−1. Beyond that region, Kh rapidly increased with metaxylem element maturation to a maximum value of 5.0±0.3 mmol s−1 mm MPa−1, 105 mm from the leaf base. It then decreased to 3.5±0.2 mmol s−1 mm MPa−1 near the leaf tip. The basal expanding region was observed to restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.