The Essential Nature of Boron to the Growth and Fruiting of the Tomato.

A question that frequently arises in connection with elements regarded necessary for plant growth is whether or not a particular element is an essential plant food or merely a stimulating substance. Is its role that of a catalyzer, or is it actually incorporated into the tissues as an integral part of the plant? These questions are more applicable to such elements as manganese, copper, zinc and boron than to nitrogen, phosphorus and potassium, which have been recognized for many years as essential nutrient units. The elements of both groups have at times been considered toxic, at others as stimulating. Here again confusion exists, for the question of toxicity seems to depend upon circumstances. Thus BRENCHLEY (1) points out that "Typical nutrient salts are toxic when they are applied singly to the plant in certain concentrations, the toxic power decreasing and the nutritive function coming into play more fully on the addition of other nutrient salts." Some elements which are toxic in high concentrations contribute to increased plant growth when their solutions are properly diluted. It is apparent then that no sharp distinction can be drawn between toxic elements, and nutrient elements, or stimulating elements if such exist. The distinction is a question rather of quantity than of kind, which should be considered in connection with other factors such as the presence of other elements. BRENCHLEY (1) has divided the class of elements that are toxic in high concentrations into two groups: " (1) Those that apparently become indifferent in high dilutions and never produce any increase in plant growth; (2) Those that are either essential for growth, or at least cause increased development, when applied in sufficiently small quantities. The former group may be legitimately regarded as toxins; the latter presents more difficulty and even now their function is not settled. It is not clear whether they stimulate the protoplasm or in some way hasten the metabolic processes in the plant, whether they help roots in their absorbent work, or whether they are simple nutrients needed only in infinitesimal quantities. " Boron without a doubt belongs in the second group. Its role in the plant is not known although several investigators have shown that its presence is essential for the normal growth of many plants. WARINGTON (7) states, "The fact that boron can be detected in the stem, leaves, and pods of the broad bean implies that the element becomes distributed throughout