BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



191 



or more. Zinc-deficient soils are well known in the southern United 

 States, where the lack of zinc causes "rosette" disease of pecan and 

 cherry trees, "mottled leaf" of citrus fruits, and "bronzing" of tung oil 

 trees. The stunting effect of zinc deficiency on grapefruit is shown in 

 Fig. 8-3. 



Courtesy of California Agricultural Kxperiment Station. Reproduced from 

 Hunger Signs in Crops, a publication of the American Society of Agronomy and 

 the National Fertilizer Association, Washington, D. C. 



Fig. 8-3. Grapefruit showing, on the left, the effects of acute zinc defi- 

 ciency; center, fniit from similar tree two months after treatment, showing 

 discolored tough areas impregnated with gum in the thick rind; and, on the 

 right, fruit from a similar tree treated 15 months previously. 



The amount of zinc needed by human beings is not well established, but 

 it has been estimated to be 0.3 mg. per kilogram of body weight for 

 growing children. Adult persons on an average diet in this country 

 probably consume about 12 mg. per day, and this amount appears to be 

 sufficient for all bodily needs. Foods high in zinc include wheat bran, 

 wheat germ, oysters, liver, egg yolk, cocoa, and others (see Appendix, 

 Table A-3). In human tissues the highest concentrations (ca 200 ppm.) 

 are found in the nails and hair. 



Sulfur. Sulfur is present in more organic compounds of biochemical 

 importance than any other mineral element, with the possible exception 

 of phosphorus. A partial list is given in Table 8-2. Plants have the 

 ability to manufacture from inorganic sulfates all the organic sulfur 



