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Summary X‐ray microanalysis has confirmed, and measured, the phosphorus content (0·03 g cm‐3) of cytochemically‐demonstrated polyphosphate granules in onion mycorrhizas. The volume occupied by the granules measured from stereoscopic high‐voltage electron micrographs of thick sections was 0·8 % of the fungal volume, and cytoplasmic streaming rates of the extra‐matrical mycelium in soil measured in the light microscope were around 12·6 cm h‐1. From these data a flux rate of about 2·7 × 10‐8 mol P cm‐2 s‐1 has been calculated, which fulfils the hypothesis that phosphorus is translocated in the fungus by means of the transport of polyphosphate granules by cytoplasmic streaming.

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Metal localization within and around roots of water hyacinth (Eichhornia crassipes) growing in a wetland receiving urban run-off was studied by energy dispersive X-ray microanalysis of sections from freeze-substituted roots. Sampling randomly from an order of magnitude gradient in metal concentrations (Cu and Pb) allowed us the opportunity to identify general patterns of metal localization. Iron was present at high levels at the root surface, and this may have been a root plaque as described for wetland plants with roots anchored in flooded soils. Iron levels decreased centripetally across the root and were higher in cell walls than within cells. Trace metals (Cu, Zn and Pb) were not localized at the root surface. In contrast with iron, trace metal levels increased centripetally across the root, tended to be higher inside cells and were highest within cells in the stele. Variability of localization was high for all metals analysed. Multivariate statistical analyses (principal components analysis and multidimensional scaling) were useful for identifying overall patterns in elemental distribution. Key-words:Eichhornia crassipes (Mart.) Solms; accumulation; copper; iron; lead; multivariate analysis; root plaque; uptake; X-ray microanalysis; zinc.Abbreviations: PCA, principal components analysis; MDS, multidimensional scaling; STEM, scanning transmission electron microscopy. INTRODUCTIONAquatic plants are known to accumulate metals from their environment (Outridge & Noller 1991) and affect metal fluxes through those ecosystems (Jackson, Rasmussen & Kalff 1994;St-Cyr, Campbell & Guertin 1994). These effects have led scientists, engineers and managers to be interested in metal toxicity and tolerance (van Steveninck et al. 1990a;Ernst, Verkleij & Schat 1992), and the roles of aquatic plants in biogeochemical metal fluxes (Jackson et al. 1994;St-Cyr et al. 1994), as biological filters for polluted waters (Brix & Schierup 1989;Ellis et al. 1994); and as biomonitors of environmental metal levels (Whitton & Kelly 1995;Markert 1995).Metal accumulation is of interest for basic research into the physiology and ecology of plant survival in flooded conditions and under elevated metal levels (Otte et al. 1989). Root plaques, predominantly of Fe, are commonly found on wetland plants with anchored roots and conflicting results have been reported as to their effects on metal uptake, immobilization and tolerance (Otte et al. 1989; StCyr & Campbell 1996;Ye et al. 1997). X-ray microanalysis has been suggested as a way to investigate these questions further (Ye et al. 1997).Despite much work, there are no clear guidelines for biomonitoring of metals using aquatic plants (Langston & Spence 1994;Phillips 1994;Markert 1995). The surface layer of roots may be enriched in metals but not be bioavailable; further it may affect uptake of metals (Markert 1995;St-Cyr & Campbell 1996). There are many instances where wetlands-utilizing aquatic plants are employed for removal of pollutants, including metals, from waters (see Brix & Schierup...

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