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Correlations between placental size and fetal mass during gestation fail to account for changes in composition that accompany placental growth and maturation. This study uses stereological data on the sizes of different tissue compartments in human placentas from 10 weeks of gestation to term and relates them to placental volume and to fetal mass by means of allometric analysis. In addition, tissue dimensions are used to calculate a physiological transport measure (diffusive conductance) for the villous membrane. Histological sections randomly sampled from placentas and analysed stereologically provided estimates of structural quantities (volumes, exchange surface areas, lengths, numbers of nuclei, diffusion distances). These data were combined with a physicochemical quantity (Krogh's diffusion coefficient) in order to estimate oxygen diffusive conductances for the villous membrane and its two components (trophoblast and stroma). Allometric relationships between these quantities and placental volume or fetal mass were obtained by linear regression analyses after log-transformation. Placental tissues had different growth trajectories: most grew more rapidly than placental volume and all grew more slowly than fetal mass. Diffusion distances were inversely related to placental and fetal size. Differential growth impacted on diffusive conductances, which, again, did not improve commensurately with placental volume but did match exactly growth of the fetus. Findings show that successful integration between supply and demand can be achieved by differential tissue growth. Allometric analysis of results from recent studies on the murine placenta suggest further that diffusive conductances may also be matched to fetal mass during gestation and to fetal mass at term across species. [ABSTRACT FROM AUTHOR]

Stomatal density and diffusive conductance were characterized for both leaf surfaces in three amphistomatous hybrid cultivars of poplar. Consistent differences in stomatal density were observed between cultivars and between leaf surfaces within a cultivar. Mean stomatal density in the three cultivars ranged from 150 to 330 stomata mm-2 for abaxial leaf surfaces and from 75 to 100 stomata mm-2 for adaxial surfaces. The density of stomata on the abaxial versus adaxial surface was related to the spatial orientation of leaves with respect to the horizon and this stomatal ratio ranged from 1.4 to 4.0 in the three clones. Also, stomatal density was greater in leaves at higher rather than lower nodal positions. Differences in diffusive conductance between cultivars and leaf surfaces were observed on intact and detached leaves in the light and dark. Within each cultivar mean abaxial conductance (kab) was greater than adaxial conductance (kad). Mean conductances in the light for the three cultivars ranged from 0.22 to 0.62 cm s-1 for abaxial, and from 0.15 to 0.17 cm s-1 for adaxial surfaces, and in the dark they were between 0.06 and 0.26 cm s-1 for abaxial, and from 0.04 to 0.06 for adaxial surfaces. The differences in conductance between cultivars and between leaf surfaces were correlated with their respective stomatal densities. Stomatal response to light and to leaf excision also varied between cultivars and between the two leaf surfaces. [ABSTRACT FROM AUTHOR]

Leaf surfaces of Festuca arundinacea Schreb. were subjected to controlled polishing and abrasion on a test-bed designed to stimulate components of abrasive wind damage. Both treatments substantially increased the leaf surface conductance, particularly the polishing treatment. Scanning electron micrographs of cryo-fixed leaf surfaces showed displacement and smoothing of the epicuticular waxes, damage to the cuticle, collapse of epidermal cells and fracture of trichomes. The importance of the epicuticular waxes in determining leaf diffusivity and permeability is discussed. [ABSTRACT FROM AUTHOR]