| Summary: | This paper is a brief review of the problem of lead-in the environment, particularly constitutional tolerance to lead about which little is known. Taking Allium cepa L. roots as a model it has been shown that after an initial phase in which lead is toxic to cells, defence processes appear with the results that lead is no longer poisonous. The lead which penetrates into the root symplast is detoxified in vacuoles, cell walls and dictiosomal vesicles. Initial cells of the meristem (quescent centre) which play a basic role in the root regeneration processes are protected against lead penetration. This surprising result is in agreement with the absence of any symptoms of lead poisoning in plants growing in natural conditions, and suggests that there is a defence mechanism specific only for plant cells.
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