The results of many experiments have clearly demonstrated the prime importance of trace elements in the nutrition of plants. The small amounts of these substances required indicate that they have a unique function. Two of the trace elements, iron and manganese, have been given broad consideration because of their association, in one way or another, with the important plant disease or diseases commonly known as chlorosis. The problem of chlorosis in plants has been investigated by many plant scientists because of its urgent agricultural significance.The literature is replete with reports of field tests (2,4,14,19,20) in which iron and manganese have been added to the soil to bring about recovery of plants showing symptoms of the common chlorotic disorders known as ''yellows," "grey speck of grains," "marsh spot of peas," and many other similar diseases. The problem is not a simple one, and frequently the results of analytical data are conflicting and tend to confuse the problem rather than clarify it. For example, several experiments have been reported, (5,6,11,13,14) in which chlorotic plants, when analyzed, were found to have a higher iron content than normal green plants. This suggests that much of the iron present in chlorotic plants is in an insoluble form and is not effective in the metabolic processes.Investigation of the problem of iron availability (14, 20) has led to general acceptance of the theory that it is the soluble form in the plant which plays the important role in iron metabolism. It is evident that this element, by being precipitated within the tissues, may become inactivated and therefore unavailable to metabolic processes of the plant. When this is considered in connection with the work of JOHNSON (8), which shows that ferrous iron is soluble at relatively high pH values whereas ferric iron is precipitated at a relatively low pH value, it is logical to assume that the excessive proportion of inactive iron found in chlorotic plants has been oxidized from the ferrous to the ferric form, which is then precipitated. Just how oxidation may occur is not fully understood, nor is it clear by what mechanism the reduction of ferric iron is suppressed in the plant. The work of KELLEY (10) and JOHNSON (9) is significant since it has shown that certain Hawaiian Island soils contain an abundance of soluble iron, react acid to litmus, and yet pineapple plants grown on them acquired a chlorosis which could be cured by spraying the plants with iron solutions. These soils were found to contain also

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During the past quarter of a century much has been written concerning the place of the so-called trace elements in the economy of plant life. Most of this literature deals with the effects produced upon plants when these elements are absent from the growth substrate and is purely descriptive in nature, portraying the symptoms of deficiency. There is also, in the phvsiological literature of the past decade and a half, much descriptive material portraying the pathological symptoms which appear in plants when these elements oceur in the plant environment in concentrations in excess of those required for growth and development. Thus far these qualitative observations make up practically all of the great mass of the investigational literature upon this subject. So much of this foundation in natural fact has already beeni accomplished, that intensive activity along this line of categorical description can no longer be considered an adequate field of investigation in this particular phase of plant nutrition. It should be remembered, however, that the extension of our knowledge in this phase of nutrition cannot progress without at least some continued activity along these descriptive lines; but our chief interest is no longer in the qualitative pathological symptoms themselves, which develop as the result of trace element malnutrition, but in their relation to those more basic phenomena which can be evaluated only through careful quantitative experimental investigation. From almost the beginning of the present century studies have been directed toward aseertainiing, whether or not certain elements, which are found in trace quantities in both the plant and in its environment, might be included in the category of essential factors for growth and development. There is much contradictory, evidence, as might be expected, relative to this question, which 1 Journial

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