SummaryWe report here that NO 3 ± in the xylem exerts positive feedback on its loading into the xylem through a change in the voltage dependence of the Quickly Activating Anion Conductance, X-QUAC. Properties of this conductance were investigated on xylem-parenchyma protoplasts prepared from roots of Hordeum vulgare by applying the patch-clamp technique. Chord conductances were minimal around ± 40 mV and increased with plasma membrane depolarisation as well as with hyperpolarisation. Two gates with opposite voltage dependences were postulated. When 30 mM Cl ± in the bath was replaced by NO 3 ± , a shift in the midpoint potential of the depolarisation-activated gate by about ± 60 mV from 43 to ± 16 mV occurred (K m = 3.4 mM). No such effect was seen when chloride was replaced by malate. Addition of 10 mM NO 3 ± to the pipette solution and reduction of [Cl ± ] from 124 to 4 mM (to simulate cytoplasmic concentrations) did not interfere with the voltage dependence of X-QUAC activation, nor was it affected by changes in external [K + ]. If only the NO 3 ± effect on gating was considered, an increase of the NO 3 ± concentration in the xylem sap to 5 mM would result in an enhancement of NO 3 ± ef¯ux by about 30%.Although the driving force for NO 3 ± ef¯ux would be reduced simultaneously, NO 3 ± ef¯ux into the xylem through X-QUAC would be maintained with high NO 3 ± concentrations in the xylem sap; a situation which occurs for instance during the night.

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This paper presents the results of experimental studies on the microstructure of clay soils, using scanning electron microscopy. Approximately 300 samples of clay soils of various ages, origin and degree of lithifiation have been analysed. Five main types of microstructure have been identified in clay soils of sedimentary origin, i.e. honeycomb, skeletal, matrix, turbulent and laminar. Among clay soils of sedimentary origin and hydrothermal origin three types of microstructure were distinguished; domain, pseudoglobular and globular.In natural environments, microstructures of mixed features have been found, i.e. honeycomb-matrix, honeycomb-skeletal, turbulent-matrix, etc.The detailed description presented of the different microstructural types comprising morphology and quantitative calculations, shows a close relationship with mineral composition, origin and degree of lithifiation.In addition physical and mechanical properties as well as the resistance and deformation indices of clay soils have been found to be related to microstructural features.Microstructure is one of the most important factors determining the properties of clay soils. Tenaghi (1925) was the first to pay attention to the necessity of studying the clay microstructure. During the ensuing years many investigators were engaged in microstructural studies : and others. However, up to now, clay microstructures have not been adequately studied. One of the main reasons for this is the high dispersity of clay soils, which calls for high magnifications when studies are conducted at the microlevel. Therefore, in spitebf the fact that a number of interesting investigations have been performed with the help of light microscopes, many specific microstructural features of clays remained inaccessible for comprehensive studies. Only the recent advent of the scanning and transmission electron microscopes have made such investigations possible. Beginning in 1968 the

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Thixotropic phenomena in clay soils are accompanied by microstructural changes. The nature of these changes is unclear because of the short duration of the thixotropic processes and the difficulties involved in preparing a specimen at certain stages. However, these technological problems were overcome and SEM photographs were obtained of the thixotropic soil microstructure during deformation of samples in a rotary viscometer with and without vibration. The new data obtained clarify the mechanism of thixotropic phenomena. Soil microstructure was not ruptured by vibration during the shear process. On the contrary, it became more homogeneous over the entire volume and at the same time this induced a decrease in strengthin the system due to a reduction in cohesion at contacts. Disruption of some structural bonds is followed by their rapid restoration—the overall microstructure remaining intact. As a result, in all the samples studied the shear zone disappears. In kaolinite clay and coarser dispersed soils (silty clay, water saturated loess) the local areas of the structural framework (shear planes) become smoother and the orientation of the structural elements along the direction of shear disappears. After the cessation of vibration, the microstructure is rapidly restored to its initial state. Les phénomènes thixotropiques dans les sols argilacés sont accompagnts de changements microstructurels. La nature de ces changements n'est pas clair, à cause de la courte durée des phénombnes thixotropiques et des difficult de la préparation d'un échantillon a` de certaines étapes. Ces problémes technologiques ont été résolus et des images SEM ont été obtenues de la microstructure du sol thixotropique pendant la déformation des échantillons dans un viscométre rotatif avec et sans vibration. Les nouvelles données obtenues clarifient le mécanisme des phénoménes thixotropiques. La microstructure du sol n'a pas été rompue par la vibration pendant le cisaillement. Tout au contraire, elle est devenue plus homogéne sur le volume entier, tandis qu'en même temps ceci a causé une réduction de la résistance du systéme due à une diminution de la cohésion aux contacts. La rupture de quelques liens structurels est suivie de leur rétablissement rapide, tandis que la microstructure entiére reste intacte. Comme résultat, la zone de cisaillement disparaît dans tous les échantillons etudiés. Dans l'argile kaolinite et les sols dispersés plus grossiers (argile limoneuse, loess saturé d'eau) les zones locales du cadre structure1 (plans de cisaillement) deviennent plus lisses, et l'orientation des éléments structuraux le long de la direction de cisaillement disparait. Aprés que la vibration a cessé, la microstructure se rétablit rapidement dans son état initial.