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The extreme vertices design is developed as a procedure for conducting experimen& with mixtures when several factors have constraints placed on them. The constraints so imposed reduce the size of the factor space which would result had the factor levels been restricted to only 0 to 100 per cent. The selection of the vertices and the various centroids of the resulting hyper-polyhedron as the design is a method of determining a unique set of treatment combinations. This selection is motivated by the desire to explore the extremes as well as the center of the factor space.A non-linear programming procedure is used in determining the optimum treatment combination.

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The role of roots in the uptake of nonessential trace metals by aquatic macrophytes was investigated Elodea canadensis. Plants were grown in 2 lakes in which metal concentrations in the water differed. Specimens in each lake were anchored in either control sediment or in sediment contaminated with Cd and Pb. Plants grown in the same water but in sediment from different sources had significantly different concentrations of the 2 metals. Elodea samples rooted in sediments from the same source but grown in water with different levels of metals also accumulated significantly different amounts of Cd and Pb. Thus the importance of both sediment and the surrounding water as sources of metals for aquatic plants was demonstrated. Samples grown in water and sediment containing low levels of metals had minimal concentrations of 0.32 mg/g Cd and 5.2 mg/g Pb, while plants grown in metal—contaminated water and sediment accumulated up to 32.33 mg Cd/g and 160.9 mg Pb/g.

Avoidance reactions of chironomid larvae to contaminated sediment taken from a heavy metal impacted lake were studied . Heavy metal levels in the test sediment ranged from background of o .6 parts per million (ppm) cadmium, 77 ppm zinc and 17 ppm chromium to a maximum of 1,029 ppm cadmium, 17,262 ppm zinc and 2,106 ppm chromium . A linear relationship was established between cadmium and zinc levels in the sediment and avoidance by chironomids .An approximate threshold avoidance of metals in the sediment was determined to be between 213-422 ppm cadmium, 4385-8330 ppm zinc and 799-1513 ppm chromium .