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Aluminium (Al) toxicity is a very important factor limiting the growth of plants on acidic soils. Recently, a number of workers have shown that, under certain conditions, silicon (Si) can ameliorate the toxic effects of A1 in hydroponic culture. The mechanism of the amelioration is unclear, but three suggestions have been put forward: Si-induced increase in solution pH during the preparation of hydroponic solutions; reduced availability of Al due to the formation of hydroxyaluminosilicate (HAS) species in those solutions during plant growth; or in planta detoxification. It is now known that it is possible to make up Al and Si solutions in an order in which pH is lowered prior to Al addition; in these cases amelioration has still been observed. Amelioration has also been noted in experiments where HAS formation is minimal. These observations would suggest that, at least under some circumstances, there is an in planta component to the amelioration phenomenon. Several microanalytical investigations have noted codeposition of Al and Si in root cell walls. We propose a model in which root cell walls are the main internal sites of aluminosilicate (AS) and/or HAS formation and of Al detoxification. Factors promoting AS/HAS formation in this compartment include: high apoplastic pH; the presence of organic substances (e.g. malate); and the presence of suitable local concentrations of reactive forms of Al and Si, on or within the surfaces of the wall matrix. All these are likely to be important in the amelioration of Al toxicity.