ALBERT

Notes: Two types of garnet porphyroblast occur in the Schneeberg Complex of the Italian Alps. Type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz, decussate mica and small garnet grains, and a margin containing large garnet grains. Orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains. Individual garnet grains without subdomains are small (〈 50 µm), faceted and idioblastic, and have simple zoning profiles with Ca-rich cores and Ca-poor rims. Subdomains of larger garnet grains are similar in size to the individual, small garnet grains. Type 2 porphyroblasts comprise only ellipsoidal garnet, with small subdomains in the centre and larger subdomains at the margin. Each subdomain has its own Ca high, Ca dropping towards subdomain boundaries. Garnet grains, with or without subdomains, all have the same Ca-poor composition at rims in contact with other minerals. The compositional zonation patterns are best explained by simultaneous, multiple nucleation, followed by growth and amalgamation of individual garnet grains. The range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre. The difference between Type 1 and Type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast.Electron backscatter diffraction shows that small, individual garnet grains are randomly oriented. Large marginal garnet grains and subdomain-bearing garnet grains have a strong preferred orientation, clustering around a single garnet orientation. Misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations. The only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other. This process was driven by the reduction of subdomain boundary energy associated with misorientation loss. Rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals (quartz and mica) around the garnet. An analytical model, in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz, suggests that, at the conditions of metamorphism, the rotation required to give a strong preferred orientation can occur on a similar time-scale to that of porphyroblast growth.

Notes: Abstract Porphyroblast textures in a Karakorum phyllite reveal that porphyroblast growth was syn-tectonic with respect to a cleavage forming deformation. During and after porphyroblast growth it partitions the deformation such that zones of intensified cleavage are developed which wrap around the porphyroblast whilst the porphyroblast and its strain shadow undergo little deformation. Porphyroblast strain shadows comprise quartz, calcite and felspar with little mica, and are probably formed by solution transfer during deformation. Unless the deformation is so strongly partitioned that no deformation of the porphyroblasts and their immediate surrounds occurs, inequidimensional porphyroblasts will rotate. Porphyroblasts undergo some dissolution after they have finished growing.

Notes: During plasmolysis of onion epidermal cells, the contracting protoplast remains connected to the cell wall by an intricate, branched system of plasma membrane (PM) ‘Hechtian strands’ which stain strongly with the fluorescent probe DiOC6. In addition, extensive regions of the cortical endoplasmic reticulum (ER) network remain anchored to the cell wall during plasmolysis and do not become incorporated into the contracting protoplast with the other cell organelles. These ER profiles become tightly encased by the PM as the latter contracts towards the centre of the cell. Thus, although the cortical ER is left outside the main protoplast body, it is nonetheless still bound by the PM of the cell. As well as being anchored to the wall, the cortical ER remains intimately linked with plasmodesmata and retains continuity between cells via the central desmotubules which become distended during plasmolysis. The PM also remains in close contact with the plasmodesmatal pore following plasmolysis. It is suggested that plasmodesmata, although sealed, may not be broken during plasmolysis, their substructure being preserved by continuity of both ER and PM through the plasmodesmatal pore. A structural model is presented which links the behaviour of PM, ER and plasmodesmata during plasmolysis.

Notes: Abstract An experimental system has been developed for studying efflux of 14C assimilates in growing potato tubers. Small wells are cut into the phloem-rich perimedulla and filled with trap solutions of varying composition which inhibit or promote assimilate efflux. One well on each tuber acts as the treatment while a second well acts as the control. Movement of 14C into wells occurred at comparable rates to that found in intact tissue, harvested from importing tubers in the form of microcores. Sucrose was the predominant translocated sugar in the stolon and was not hydrolysed in either the wells or the microcores following unloading. Efflux into wells containing agar traps was stimulated 40-fold relative to buffer controls by the addition of 20 mol m−3 EGTA to the agar. This was interpreted as passive efflux to the apoplast due to increased membrane permeability in the pathway between the sieve elements and the collecting wells. The EGTA stimulation was reversed by addition of Ca2+. 14C efflux into buffered solutions was inhibited significantly by both DNP and PCMBS, suggesting the involvement of active and carrier-mediated transport components. However, it was not possible to determine whether these compounds acted at the site of unloading only, or on the short-distance transfer step between phloem and collecting wells. The rate of tracer efflux was not significantly different when 1 mol m−3 and 300 mol m−3 sucrose were applied to the wells, indicating insensitivity of solute movement to low apoplastic solute concentrations. However, raising the solute concentration to 800 mol m−3 caused a severe inhibition of tracer efflux. These results were duplicated with mannitol as the osmoticum. It is suggested that plasmolysis prevented further efflux by disruption of a predominantly symplastic transport pathway between the phloem and collecting wells.

Notes: Abstract Laboratory analysis of clayey silt from the offshore Huanghe Delta reveals thixotropic behavior. Fall cone tests show strength loss on remolding followed by strength regain at virtually constant water contents. The delta-front sediments experience wave loading during storms and widespread mass movement has been detected by acoustic surveying. Measurements from sea floor instruments and repeated sonographic survey during storms indicate repeated episodes of sediment motion consistent with thixotropy.

Notes: Abstract The subaqueous delta of the Huanghe (Yellow River) has been studied using high-resolution acoustic systems. There are many subtle variations in sea floor morphology and sediment geometries; smooth, featureless areas are rare. The main components of the subaqueous delta include broad, shallow channels; moderately disturbed areas with near-surface cut and fill structures; heavily disturbed areas with sea floor depressions, pits, and gullies; and a smooth, gently sloping distal delta apron or “rise.” These features are not directly related to sediment settling from dilute surface plumes but are due to gravity-driven hyperpycnal underflows, submarine mass movements, and silt flows.

Notes: Abstract Studies to date indirectly indicate that only a small percentage of the sediment discharged by the Huanghe (Yellow River) is presently transported from the Gulf of Bohai to the Huanghai (Yellow Sea). Direct measurements in early summer 1985 show low concentrations of suspended sediment east of 119°45′E but high concentrations in Bohai Bay. Stokes drift associated with an amphidrome of the M2 tide may contribute to a northwestward transport of Huanghe sediment.

Notes: Abstract Internal waves with periods of about 5 minutes and trough to crest heights of up to 6.2 m were observed acoustically over the actively accreting delta front of the Huanghe (Yellow River) in the western portion of the Gulf of Bohai, Peoples Republic of China. The radian frequency of the internal waves was close to the locally-observed Brunt-Vaisala frequency. Through the relatively short (∼one hour) duration of an internal wave train, the amplitude was observed to decrease progressively with time. These internal waves may cause resuspension of delta-front sediments.