30 Effects of Toxicants (pollutants) on a Biological Species - Some Mathematical Models J B SHUKLA, A K AGRAWAL & B DUBEY INTRODUCTION It is well recognized that the deterioration of our environment by various kinds of industrial discharges (toxic effluents, pollutants) to the environment has very undesirable consequences on all living beings including plants. There are many examples of how air pollutants (primary and secondary) can destroy the character and productivity of vast areas of forests, agricultural crops and vegetation (Kozlowski, 1975,1980; Davis, 1972; Manning, 1975; Pack & Sulzback, 1976; Constantinidou & Kozlowski, 1979a,b; Garsed et al., 1981; Henriksson & Pearson, 1981; Norby & Kozlowski, 1981; Reinert & Gray, 1981; Smith, 1981; Stan & Schicker, 1982; McLaughlin, 1985; Singh et al., 1985, 1988; Yunus et al., 1985; Singh & Rai, 1991; Yunus & Iqbal, 1996; Dekok & Stolen, 1998; Agrawal & Agrawal, 2000). Primary pollutants originate in a toxic form at the source (e.g., S02' HF), secondary pollutants develop as a result of interactions and chemical reactions between pollutants sometimes in presence of sunlight or water vapour in the atmosphere. Examples of latter are the photochemical oxidants, viz., 0 3 and PAN (peroxyacyl nitrate), which are formed by sunlight acting on products offuel combustion, especially nitrogen dioxide and hydrocarbons that are emitted by motor vehicles (Kozlowski, 1975, 1980). A review of atmospheric deposition of gases and particles from the atmosphere induced by plant assimilation has been presented by Environmental Stress: Indication, Mitigation & Eco-conservation. Edited by Yunus et al. © 2000 Kluwer Academic Publishers, The Netherlands.