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Summary For many organisms, respiratory gas exchange is a vital activity and different types of gas-exchange apparatus have evolved to meet individual needs. They include not only skin, gills, tracheal systems and lungs but also transient structures such as the chorioallantois of avian eggs and the placenta of eutherian mammals. The ability of these structures to allow passage of oxygen by passive diffusion can be expressed as a diffusive conductance (units: cm 3 O 2 min −1 kPa −1 ). Occasionally, the ability to estimate diffusive conductance by physiological techniques is compromised by the difficulty of obtaining O 2 partial pressures on opposite sides of the tissue interface between the delivery medium (air, water, blood) and uptake medium (usually blood). An alternative strategy is to estimate a morphometric diffusive conductance by combining stereological estimates of key structural quantities (volumes, surface areas, membrane thicknesses) with complementary physicochemical data (O 2 -haemoglobin chemical reaction rates and Krogh's permeability coefficients). This approach has proved valuable in a variety of comparative studies on respiratory organs from diverse species. The underlying principles were formulated in pioneering studies on the pulmonary lung but are illustrated here by taking the human placenta as the gas exchanger.

Several species of the stem succulent Caralluma (Apocynaceae) are abundant perennials in arid regions of the Arabian Peninsula. These arid regions have a short wet season with erratic rainfall and are characterized by harsh climatic conditions of high temperature, high evaporation and sand storms. Work presented in this paper aimed at investigating importance of Crassulacean Acid Metabolism (CAM) for survival of three Caralluma species in their natural habitat. Investigations involved studying stomatal characteristics, stomatal diffusive conductance, chlorophyll fluorescence, and CAM in three species of Caralluma, namely C. acutangula (Decne.) N.E.Br., C. edulis (Edgew.) Benth. ex Hook.f., and C. subulata (Forssk.) Decne. Microscopic examination revealed a pattern of stomatal characteristics typical of CAM plants in these three Caralluma species. Results showed that these three Caralluma species were obligate CAM plants exhibiting this mode of photosynthesis during both the wet and the dry seasons. Under protracted water stress during the long dry season very low values of stomatal diffusive conductance and dampening of CAM acidification-deacidification cycles denoted the tendency of these three Caralluma species to shift from the obligate CAM physiotype to CAM-idling mode. Chlorophyll fluorescence measurements indicated that protracted water stress induced a reduction in Photosystem II (PSII) antenna efficiency and quantum yield in the three studied Caralluma species. This reduction of PSII activity occurred in concomitance with a marked rise in non-photochemical quenching of chlorophyll fluorescence denoting operation of non-photochemical energy dissipating mechanisms known to be important for photoprotection of the photosynthetic apparatus.

The objective was to study the leaf temperature (LT) and leaf diffusive vapor conductance (gs) responses to temperature, humidity and incident flux density of photosynthetically active photons (PPFD) of tomato plants grown without water restriction in a plastic greenhouse in Santa Maria, RS, Brazil. The plants were grown in substrate and irrigated daily. The gs was measured using a steady-state null-balance porometer on the abaxial face of the leaves during the daytime. Both leaf surfaces were measured in one day. The PPFD and LT were measured using the porometer. Leaf temperature was determined using an infrared thermometer, and air temperature and humidity were measured using a thermohygrograph. The leaves on the upper layer of the plants had higher gs than the lower layer. The relationship between the gs and PPFD was different for the two layers in the plants. A consistent relationship between the gs and atmospheric water demand was observed only in the lower layer. The LT tended to be lower than the air temperature. The mean value for the gs was 2.88 times higher on the abaxial than adaxial leaf surface.

The intrauterine environment has an important influence on placental development. In pre-eclampsia (PE) and intrauterine growth restriction (IUGR), early remodelling of spiral arteries has repercussions for uteroplacental blood flow. The IUGR placenta exhibits compromised growth of villous trees, a smaller intervillous space and a lower diffusive conductance. Here, we test whether or not term placentas associated with PE or IUGR also exhibit changes in structural quantities (notably, sizes and numbers of intervillous pores and villous domains) pertinent to uteroplacental haemodynamics. Paraffin wax sections were sampled at random locations and orientations and structural quantities obtained by combining design-based stereological estimates of total and star volumes with model-based estimates of pore hydraulic diameters. Total volumes of intervillous pores and villi were estimated by point counting, total villous surface by intersection counting and star volumes by measuring point-sampled intercept lengths. Other quantities were derived secondarily and group estimates compared by two-way analysis of variance. We found significant main effects of IUGR but no main or interaction effects involving PE. In IUGR, there were fewer intervillous pores and these had larger hydraulic diameters. IUGR also produced fewer villous domains but these were constant in star volume and villi had a constant mean diameter and volume fraction. We concluded that IUGR compromises placental development by producing intervillous pores and villous trees different in size and shape from those in control and PE pregnancies. Calculations suggest that Darcian conductances in the intervillous space improve in IUGR but, in reality, placental performance is compromised by other physiological and structural constraints including the known decline in diffusive conductances.

(2007). Non-Hydraulic Factor Controls the Leaf Diffusive Conductance of Rice Grown in Anaerobic Reduced Soils. Plant Production Science: Vol. 10, No. 2, pp. 207-210.

The avian egg and mammalian placenta are transient structures which are discarded after hatching or parturition but, during their lifespan, must adapt to the changing needs of the growing embryo/fetus. The embryo/fetus exerts demands on its environment whether it be the impersonal one in which an egg is laid or the nurturing one within the body of the mother. In the case of the avian egg, nutrients are obtained internally but respiratory oxygen (O2) must be obtained from the environment. In eutherian mammals, nutrients and O2 are obtained from the mother via the placenta. The capacity of the egg and placenta to transfer O2 to the embryo/fetus is expressed as a diffusive conductance (DO2 in the units cm3O2 min−1 kPa−1). For these and other gas exchangers, a morphometric estimate of DO2 can be obtained by combining stereological estimates of relevant microstructural quantities (vascular volumes, exchange surface areas and harmonic mean diffusion distances) with physiological data (O2–haemoglobin reaction rates and Krogh’s permeability coefficients). Here, the avian egg and human placenta are taken as models for gas exchangers, and their structures and functional capacities are compared. Particular focus is accorded to the human haemochorial placenta and its ability to serve the growing needs of the fetus is illustrated using results from normal and compromised pregnancies.

Leaf vapor diffusive conductance (gl) and transpiration (T) measurements in an irrigated orchard of the acid lime "Tahiti" were carried out in a subtropical climatic condition in the State of São Paulo, Brazil. Measurements were made using steady-state null-balance porometers and were taken throughout the day, on several occasions from February to November, 1998. During September and October, measurements were performed on exposed and shaded young and fully expanded leaves, as well as on old leaves inside the tree canopy. The old leaves showed lower values of gl and T when compared to the other groups. In the other months, measurements were taken only with exposed and shaded fully expanded leaves. The highest values of gl and T were obtained from February to April. For exposed leaves gl was higher from early morning to midday, decreasing thereafter with some oscillation in the afternoon. Shaded leaves produced a pattern of increasing gl and T from 8:00-9:00 a.m. to midday, followed by a decrease when values matched those of the exposed leaves in the afternoon. In April, the first measurement in midmorning showed low gl and T values, increasing sharply in the exposed leaves until midday and then decreasing until the end of the afternoon. Between May and November, gl and T decreased sharply compared to the previous months. Boundary line and regression analysis were used to find the mathematical relationships between mean values of gl for the tree and photosynthetic photons flux density, temperature, and vapor pressure deficit of the air measured in the orchard. Based on this analysis, the reasons for the gl and T diurnal and annual variation patterns are discussed, with emphasis on the effects of atmospheric variables upon stomatal regulation. Realizaram-se determinações de condutância foliar à difusão de vapor (gl) e de transpiração (T) em plantas de lima ácida 'Tahiti' com o uso de porômetros de equilíbrio dinâmico e balanço nulo, em pomar irrigado em Piracicaba, SP. O clima da região é do tipo subtropical. Efetuaram-se as medidas ao longo do período diurno, em vários dias entre fevereiro e novembro de 1998, e, em setembro e outubro, em folhas em expansão e naquelas totalmente expandidas, tanto ao sol quanto à sombra, bem como em folhas de lançamentos antigos, no interior da copa. As folhas mais antigas apresentaram valores menores de gl e de T em relação aos outros grupos. Nos outros meses, as medidas foram feitas somente em folhas plenamente expandidas ao sol e à sombra. Os maiores valores de gl e T foram observados de fevereiro a abril. Para folhas ao sol, os valores de gl foram altos desde 8-9h até o meio-dia, decrescendo gradualmente à tarde, com oscilações. Para as folhas à sombra, a variação de gl ao longo do dia apresentou crescente aumento desde a primeira medida até o meio-dia, chegando a valores próximos daqueles obtidos nas folhas ao sol e diminuindo à tarde. Em abril, a primeira medida realizada às 9 h mostrou baixos valores de gl e de T para as folhas ao sol e à sombra, sendo crescentes para aquelas ao sol até o meio-dia, com decréscimo à tarde. De maio a novembro, houve uma diminuição acentuada de gl e de T em relação aos meses anteriores. Realizaram-se análises de regressão para estabelecer relações matemáticas entre os valores médios de gl e de densidade de fluxo de fótons fotossinteticamente ativos, temperatura e déficit de pressão de vapor do ar. Com base nessas análises, discutem-se as causas de ocorrência dos padrões de variação de gl e de T observados ao longo do dia e do período experimental, discutindo-se o efeito de cada variável ambiental sobre a resposta estomática.

The first step of the modelling approach consisted in designing a model of leaf diffusive conductance (gs) adapted from Ball, Woodrow and Berry (1987), expressing gs mainly as a function of Anet. The widely accepted biochemical model of C3 leaf photosynthesis of Farquhar, von Caemmerer and Berry (1980) was parameterized for well-lit, young, mature mango leaves using response curves of net photosynthesis to intercellular CO2 concentration (A-Ci curves). To obtain an analytical solution for leaf photosynthesis, we used the following additional conductance relation : Ci = Ca - Anet / gb - Anet / gs, where Ca stands for the ambient CO2 concentration and gb for the conductance across the laminar boundary layer. The resulting model of leaf photosynthesis was tested against three sets of data from gas exchange measurements obtained with a portable infrared gas analyser on leaves of 12 year old mango trees, cv. Lirfa, at two periods of the year : August, just before flowering, and from November to December, during the fruiting stage. The need to take into account the influence of light availability, leaf age and sink activity on photosynthetic capacity to upgrade the model is discussed.

Grain yield bread wheat (Triticum aestivum L.) has been associated with variation in leaf stomatal conductance. However, breeding programms have been reluctant to exploit this relationship because the equipment required is either too costly or too slow for assessing large breeding populations. This study compared a new hand-held viscous-flow prometer against a steady-state diffusion porometer for leaf conductance measured on progeny derived from wodely varying conductance wheats, 'Quarrion' and 'Genraro 81'. Leaf conductance values from the two instruments were related linearly, adn significant phenotypic (r p = 0.76-0.90**) and genotypic (r g = 0.75-0.98**) correlations were obtained across sampling days, suggesting the new porometer is a robust predictor of leaf diffusive conductance. The enhanced speed of the new viscous-flow porometer in evaluating variation for leaf conductance should enable wheat breeders to screen large breeding populations for leaf conductance more efficiently.

1. Physical characteristics of eggs of the domestic guinea fowl, Numida meleagris galeata, were measured and compared with those of its wild counterpart and with other birds using allometric relationships. 2. The shell thickness increased and the area density of pores decreased from the blunt to the pointed end of the egg. During incubation, shell thickness decreased, but the shell diffusive conductance to water vapour (GH2O) remained constant. 3. Fresh egg mass (m0), length and breadth of the egg, GH2O and specific water vapour conductance, spGH2O (GH2O per g of m0 ), were affected by the age of the laying flock. 4. Eggs of the domestic guinea fowl were bigger and heavier than eggs of the wild one. 5. Allometry showed that guinea fowl eggs differ from those of the other birds by their greater shell thickness and density of pores. However spGH2O was normal, the thickness of the shell being compensated for by a greater density of pores for gas exchanges.

In order to clarify the response of crops to sink restriction, we examined the effects of ear removal on photosynthesis of the flag leaf in wheat. For 11 days there was no difference in CO2 exchange rates between the control and the no-ear plants after ear removal, but after 11 days the rates in the no-ear plants were higher. For 11 days there was no difference in diffusive conductance, apparent quantum yield (AQY) and apparent carboxylation efficiency (ACE) and chlorophyll contents between the plants. Though there was no difference in diffusive conductance, AQY and chlorophyll contents between the plants after 11 days, ACE and ribulose-1, 5-bisphosphate carboxylase/oxygenase contents in the no-ear plants were higher than those in the control. Therefore, it was presumed that activity of CO2 fixation system in the no-ear plants was higher and that flag leaf senescence was delayed due to ear removal.

Our general objective is to design carbon-based models of flowering and fruit quality. The first step of our procedure consisted in modelling carbon gains in mango, cv. Cogshall, using the biochemical model of C3 leaf photosynthesis of Farquhar et al. (1980). Data are presented about the temperature-response of the key parameters of photosynthetic capacity which was assessed in 17-month old mango plants. The temperature-corrected photosynthesis model was then coupled to the model of leaf diffusive conductance (gs) of Ball et al. (1987). Simulation results obtained over one year on recently matured leaves from 13 to 13-year old trees show that our model is apparently robust for leaves from vegetative shoots, in orchard conditions characterized by mild seasonal changes in temperature and non-limiting water supply. However, our model failed to simulate Anet accurately during the flowering period and periods of drought, as well as when the source-sink balance is modified as, for instance, in girdled branches at different fruit loads. Using results obtained over the 5 last years about the effects of flowering, fruiting and waterstress on photosynthesis in mango, we tested three modifications of our model: 1) the fixed parameters of the model of gs were replaced by adapted values for leaves from water-stressed trees and leaves close to inflorescences; 2) the total light-driven photosynthetic electron flux (JT) was corrected to account for the existence of electron fluxes towards alternative sinks in leaves close to inflorescences; and 3) JT was corrected to account for the inhibiting effect of starch accumulation in leaves from girdled branches. Simulation results are presented and discussed in the perspective of our modelling approach. (Resume d'auteur)

Pulmonary gas exchange can be investigated at different scales of interest. Our approach is to couple models of gas exchange to anatomically-detailed models of the airway and pulmonary vascular trees. We are linking a hierarchy of models from the capillary segment up to the whole lung, so that a change in the detailed small-scale behaviour has a flow-on effect to function at a larger scale. The anatomically-based models will be used to understand how regional perturbations to the structure or function of the airway and vascular trees and the state of health of the functional tissue affect gas exchange. We are interested in the degree to which the system can be perturbed before it is detected by standard laboratory measures.

Given that stomatal movement is ultimately a mechanical process and that stomata are morphologically and mechanically diverse, we explored the influence of stomatal mechanical diversity on leaf gas exchange and considered some of the constraints. Mechanical measurements were conducted on the guard cells of four different species exhibiting different stomatal morphologies, including three variants on the classical “kidney” form and one “dumb-bell” type; this information, together with gas-exchange measurements, was used to model and compare their respective operational characteristics. Based on evidence from scanning electron microscope images of cryo-sectioned leaves that were sampled under full sun and high humidity and from pressure probe measurements of the stomatal aperture versus guard cell turgor relationship at maximum and zero epidermal turgor, it was concluded that maximum stomatal apertures (and maximum leaf diffusive conductance) could not be obtained in at least one of the species (the grass Triticum aestivum) without a substantial reduction in subsidiary cell osmotic (and hence turgor) pressure during stomatal opening to overcome the large mechanical advantage of subsidiary cells. A mechanism for this is proposed, with a corollary being greatly accelerated stomatal opening and closure. Gas-exchange measurements on T. aestivum revealed the capability of very rapid stomatal movements, which may be explained by the unique morphology and mechanics of its dumb-bell-shaped stomata coupled with “see-sawing” of osmotic and turgor pressure between guard and subsidiary cells during stomatal opening or closure. Such properties might underlie the success of grasses.

There have been few practical ways of measuring physiological determinants of rice yield. Rapid evaluation of yield determination traits may expedite breeding of high-yielding rice. Here, we report a new remote-sensing technique for the evaluation of canopy ecophysiological status under field conditions developed based on simultaneous measurements of sunlit and suddenly shaded canopy temperatures. This technique has the advantage of instantaneous estimation of aerodynamic resistance (r(a)) and canopy diffusive resistance (r(c) without measuring wind velocity. Canopy diffusive conductance (1 / r(c)) estimated by the remote sensing method was closely related to leaf stomatal conductance (g(s)) measured with a portable gas exchange system. This result supported the validity of this new method for quantitative estimation of canopy physiological characteristics. Significant genotypic differences were obtained in canopy-air temperature difference (Tc-Ta), r(c) and 1 / r(c) during the 2-week period preceding full heading for two years, and 1 / r(c) was highly correlated with crop growth rate (CGR), which was closely related to the final yield. These results suggest that 1 / r(c) can be an effective criterion for the selection of high-yielding rice genotypes, and the remote sensing technique proposed here can be a powerful tool for the rapid evaluation of 1 / r(c) under field conditions.

Maximal VO2 (VO2max) has mostly been the province of exercise physiologists wishing to provide a measure of athletic potential or to characterize subjects in exercise-related research. It is also used clinically to determine a patient's exercise capacity. More recently, it has been recognized that the study of VO2max can provide fundamental insight into O2 transport at all points between inspired air and muscle mitochondria. This review focuses on understanding how VO2max is set and concludes that the more athletic one is, the more VO2max is sensitive to O2 transport conductances in the lungs, circulation, and skeletal muscle. These transport conductances form an integrated system, all components interacting to define VO2max. A particularly important component is diffusive conductance in muscle. This appears to be abnormal in chronic conditions such as obstructive pulmonary disease and heart and renal failure and may well explain why correction of central cardiovascular defects in O2 transport in such patients fails to restore exercise capacity.

To investigate the effects of exercise training and immobilization on structure-function relationships in skeletal muscle, three groups of purpose-bred hounds [control (C), exercised trained (E), and immobilized (I)] were studied. Group E exercised on a treadmill 1 h/day, 5 days/wk for 8 wk, while groups C and I were cage-confined for 8 wk, with group I undergoing left hindlimb immobilization for the last 3 wk. The functional results (Bebout et al. J Appl. Physiol; 74(4): 1697-1703, 1993) showed that exercise training increased peak muscle O2 uptake (VO2) by 38% and estimated diffusive conductance (DO2) by 71% in dog gastrocnemius muscle, while immobilization had no effect on peak VO2/g muscle or DO2/g muscle in spite of a 31% reduction in muscle weight and a 68% decrease in citrate synthase activity.

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On the basis of a simple lung model the limiting effects of stratification on tidal/alveolar transfer of O 2 are calculated using values for diffusive conductance of distal airways previously obtained from analysis of wash-out kinetics of He and SF 6 . In particular it is shown that the alveolar-capillary transfer of O 2 (neglected in a previous study) plays an important role in giving rise to stratificational gradients of O 2 in distal airways. For 10 kg dogs breathing hypoxic mixtures the stratificational component of the alveolar-arterial P o 2 difference is estimated at about O.8 ton- for resting conditions and at about 3.5 torr for medium level exercise.

Maximum diffusive conductance of abaxial epidermis of primary bean leaves was considerably higher than that of adaxial epidermis. While conductances of both epidermes responded parallel to a decrease in leaf water potential (they increased slightly, reached maxima and decreased to very low values), differences in their response to photon flux density were found. The conductance of abaxial epidermis increased rapidly over lower photon flux densities and gradually over higher photon flux densities, on the other hand the conductance of adaxial epidermis increased only gradually over the whole range. The observed parallel or different response of adaxial and abaxial epidermes to leaf water potential or photon flux density did not change with changes in experimental conditions.

The effect of ozone flux density on leaf conductance to ozone in Phaseolus vulgaris was examined. The change in conductance was measured within the first two hours of fumigation for mature, fruiting 6-week-old plants of an ozone sensitive cultivar (Seafarer); for young, 14-day-old plants of the same cultivar; and for an ozone resistant cultivar (Gold Crop). Young Seafarer plants showed no change in conductance to ozone over a wide range of ozone flux densities. Gold Crop showed a decrease in conductance of −3.1 % /(mgO 3 m −2 h −1 ) whereas mature Seafarer plants exhibited a stronger decrease of −7.7% /(mgO 3 m −2 h −1 ). Diffusion porometer measurements taken on fruiting Seafarer plants in the field illustrated that a decrease in leaf diffusive conductance to water is related to visual ozone injury.

Diurnal measurements of leaf water potential and the diffusive conductance of the abaxial surface of two cassava cultivars, M Col 22 and M Mex 59, were made on three occasions on field grown plants during a 10-week period of rainfall exclusion. Conductances of about 10 mm s −1 were observed in the rainfed plots but generally the mean conductance was in the range 3–5 mm s −1 . The minimum water potential of −1.8 MPa was observed in the rainfed plots. Water shortage caused reduction in mean conductance to −1 at which level the control of water loss maintained leaf water potential > −1.5 MPa at all times. Stress plots recovered more slowly during the late afternoon but during the day had higher leaf water potentials than the controls. At the same levels of leaf water potential the conductance of M Mex 59 was less than that of M Col 22 in both control and stress plots. Measurements are also reported of the stomatal distribution, density and pore size for both fully expanded leaves and those whose expansion was seriously restricted by the water shortage.

Plants of five clones of Populus tremuloides Michx. were exposed to 0, 0.2 or 0.5 microliter per liter SO2 for 8 hours in controlled environment chambers. In the absence of the pollutant, two pollution-resistant clones maintained consistently lower daytime diffusive conductance (LDC) than did a highly susceptible clone or two moderately resistant clones. Differences in LDC among the latter three clones were not significant. At 0.2 microliter per liter SO2, LDC decreased in the susceptible clone after 8 hours fumigation while the LDC of the other clones was not affected. Fumigation with 0.5 microliter per liter SO2 decreased LDC of all five clones during the fumigation. Rates of recovery following fumigation varied with the clone, but the LDC of all clones had returned to control values by the beginning of the night following fumigation. Night LDC was higher in the susceptible clone than in the other clones. Fumigation for 16 hours (14 hours day + 2 hours night) with 0.4 microliter per liter SO2 decreased night LDC by half. Sulfur uptake studies generally confirmed the results of the conductance measurements. The results show that stomatal conductance is important in determining relative susceptibility of the clones to pollution stress.

Summary Diffusive conductance increased in a very similar way in wilted and unwilted leaves of A. hastata as external humidity was increased. It was concluded that the humidity response is probably a hydroactive one to water stress within the epidermis, which can be different from bulk leaf water stress.

The O 2 uptake of chicken embryos confined in the eggshell (Ṁ O 2 ) is governed by a shell diffusive conductance (G O 2 ) and P O 2 difference between ambience and air space, suggesting that a relation between G O 2 and air space P O 2 (PA O 2 ) is hyperbolic at constant Ṁ O 2 . Upon wide alteration of G O 2 from the begining of incubation, the Ṁ O 2 was measured on day 16 of incubation and the relation between G O 2 and Pa O 2 examined. The Ṁ O 2 increased hyperbolically with increasing G O 2 , reached maximum at control conductances and decreased with further increase in G O 2 . From these changes in Ṁ O 2 with G O 2 , an equation was derived predicting Pa O 2 as a quadratic function of inverse g O 2 (mass-specific conductance, i.e. , G O 2 standardized by fresh egg mass), and the relation between air space P O 2 and shell conductance was no longer hyperbolic. The arterialized blood P O 2 (Pa O 2 ) of the allantoic vein measured individually was also expressed by a quadratic equation of inverse g O 2 . While for widely altered conductance the Ṁ O 2 was little related to Pa O 2 , the mass (embryo)-specific O 2 uptake increased with Pa O 2 . The excess water loss associated with increased conductance was involved in changes in these variables.

We tested the assumption that preincubation egg dipping in tylosin tartrate plus RCL (disinfectant) solution may affect eggshell gas conductance. This was done by measuring eggshell water vapor diffusive conductance of fertile hatching turkey eggs before and after dipping in: 1) distilled water, 2) tylosin tartrate, 3) RCL, 4) tylosin tartrate and RCL, 5) dry vacuum. A slight, but significant, increase in eggshell conductance was found in all treatments and controls; difference was significant only between all experimental and control groups. This indicates that functional eggshell properties associated with water loss and gas exchange are only marginally affected by the treatment per se and change slightly with age. Both dipping and age changes are not meaningful. No correlation was found between the amounts of solution gained in dipping and egg mass or eggshell conductance.

Summary A comparative study was made of the effect of leaf orientation on photosynthesis, transpiration and diffusive conductance of leaves of Festuca gigantea, F. pratensis, F. arundinacea, F. arundinacea var. atlantigena and F. marei. Conductance was also examined in F. drymeja and F. scariosa. Adaxial leaf surfaces ranged from the relatively flat (F. drymeja and F. gigantea) to the heavily corrugated (F. marei and F. scariosa). Stomatal numbers were greatest on the adaxial surface of all species, whilst the frequency on abaxial surfaces ranged from zero in F. gigantea and F. scariosa to more than 60 mm−2 in F. marei. The photosynthesis/transpiration ratio was slightly greater with the abaxial surface uppermost and greater in those species with relatively flat adaxial surfaces. The small change in the ratio with change in leaf orientation was due to the relatively greater reduction in transpiration than in photosynthesis. Species with heavily corrugated leaves transpired faster than those with flat leaf surfaces. Change in leaf orientation did not affect the ranking of the species in terms of transpiration or of leaf conductance but, because of the contrasting responses of F. gigantea in two separate experiments, ranking was not the same. There was no consistent effect of orientation on total leaf conductance but large differences were found in the relative responses of the two surfaces in some species. In those with very few abaxial stomata the adaxial surface always had greatest conductance, whichever surface was uppermost. Where both leaf surfaces had appreciable numbers of stomata, the upper surface always exhibited the greater conductance, irrespective of whether it was the adaxial or abaxial. F. arundinacea showed the greatest change in the ratio of adaxial leaf conductance in response to change in leaf orientation.