546 hawkes. GEOCHEMICAL PROSPECTING FOR ORES [Ch. 30 



that are absorbed may be concentrated in different parts of the plant, 

 depending in part at least on the circulatory pattern and nutritive 

 requirements of the plant (Rottova, 1947) . The reaction varies from 

 one species to another, and with time of year (McHargue and Roy, 

 1932; Piper and Walkley, 1943). In general it is found that zinc and 

 copper tend to be concentrated in the leaves, twigs, and growing buds 

 (Warren and Howatson, 1947) , whereas lead is commonly precipitated 

 in the root cells (Hammett, 1928) . The zinc content of plants varies 

 widely, depending on the species of plant and the available zinc in 

 the soil (Robinson, Lakin, and Reichen, 1947) , whereas the range for 

 copper is generally much less (Erkama, 1947) . Species of plants that 

 can concentrate relatively large proportions of an element in their 

 living parts are known as "accumulator" plants (Robinson and Edging- 

 ton, 1945). Of the woody plants, willows, aspens, birch, hickory, and 

 pine have been cited as outstanding accumulator plants (Rankama, 

 1940; Robinson and Edgington, 1945; Robinson, Lakin, and Reichen, 

 1947; Thyssen, 1942; Warren and Howatson, 1947; Vogt, Braadlie, 

 and Bergh, 1943; Vogt and Bugge, 1943; Warren and Delavault, 1948; 

 Maliuga, 1947). Accumulator plants are generally regarded as most 

 likely to give good results in prospecting. However, the possibility re- 

 mains that the variation in metal content of a species that does not 

 ordinarily concentrate large amounts of that metal may give a more 

 accurate reflection of the total metal content of the soil. In systematic 

 sampling, it is essential that comparisons be made only with data from 

 the same parts of identical species of plants or trees, preferably all 

 sampled at the same time of year. Successful results have been re- 

 ported by analyzing twigs for both copper and zinc and using the ratio 

 in interpretation rather than the absolute values. Thus an abnormally 

 high Cu:Zn ratio indicates copper in the soil, and a low ratio indicates 

 zinc (Warren and Delavault, 1948). 



In collecting plant tops, care should be taken to avoid contamination 

 of the sample with soil. Tree leaves are relatively free of contamina- 

 tion of this kind. In industrial areas where there are factories or 

 smelters, the plants and trees may become so contaminated with metals 

 from the air that no significant data can be obtained (Dunn and 

 Bloxam, 1932). 



Leaf samples may be collected either by simply picking whole leaves 

 from different parts of one or more trees, or by cutting small discs of 

 standard area from the leaves with a device similar to an ordinary 

 paper punch (Reichen and Lakin, 1949). 



Water. Sampling of stream water as a prospecting method is identi- 

 cal in principle to alluvial prospecting; in both, the trail of increasing 



