Temporal Variation in Lichen Element Levels
Lichens have certain features which have resulted in their intensive use as indicators of air quality and as monitors of the atmospheric deposition of various elements. These salient features include the lack of roots or structures which have the absorptive function of roots and thus some lichens are dependent for their mineral nutrients to a large extent on material landing on the lichen thallus as the result of wet and dry deposition from the atmosphere. Also, lichens unlike higher plants, do not have a well-developed cuticle and hence there is no comparable physical barrier to impede exchange with the environment. Consequently, lichens can accumulate mineral elements to levels far greater than their expected physio-logical needs. Lichens are perennial and this feature together with the other characteristics has led to the use of these plants as longterm integrators of deposition from the atmosphere of elements originating from both natural and man-made sources.
KeywordsZinc Titanium Dioxide Magnesium Mercury
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