Abstract
Experimental evidence is cited to show that many mineral elements taken from the soil by plants in such minute amounts as to have been considered formerly as only accidental constituents of plants are just as necessary to plant growth as are the so-called "essential elements." Further evidence is held to support the hypothesis that elements such as Cu, Mn, B, and Zn exert a function in plant nutrition comparable to that of the vitamins in animal nutrition. Proposed investigations of the correctness and applications of this hypothesis are outlined.