GROWTH HORMONE IN TERMINAL SHOOTS OF NICOTIANA IN RELATION TO LIGHT

THE INFLUENCE of light on plant morphogenesis is becoming more generally recognized as a major problem dealing with the growing points of plants. To influence growing points is to influence the whole course of plant development, because it is at these meristematic centers, these centers of continued embryogeny, that new cells and new organs are constantly being formed. Specific effects of light on meristems are not known, but it is becoming increasingly clear that substances which regulate growth and development are being formed in them, and that these substances subsequently move downward in the axis, influencing cell elongation, development of lateral buds, etc. Hence, any effect of light on the shoot tip is translated into effect on growth and form. While light favors the formation of growth promoting substances in plants, darkness favors their physiological activity (Thimann and Skoog, 1934). Higher concentrations of growth hormone have been found in young illuminated plants than in darkened ones (Navez, 1933; van Overbeek, 1933; Thimann and Skoog, 1934; Avery, 1935; Went, 1935), but the relationship of intensity and quality of light to the hormone content of plants is not yet understood. Studies on the growth depressing effect of light in phototropism and in the normal growth of plants have pointed to various possible explanations of light effects through influence upon movement, lowered activity, or destruction of growth hormones. The purpose of this paper is to present the results of studies on certain effects of darkness and of continuous illumination of varied intensity, duration, and quality upon the growth hormone concentration in terminal shoots of Nicotiana tabacum L. var. Turkish. 'IATERIALS AND METHODS.-Turkish tobacco plants were grown from seed planted in rich soil in flats or in pots in a greenhouse. In a few instances plants were grown also in complete nutrient sand cultures according to the methods previously reported (Avery, Burkholder, and Creighton, 1937b). When the plants had attained suitable size for experimental purposes, they were selected for uniformity and transferred into a special room maintained at 800F. and 90 per cent humidity. In this room light or dark treatment was given as stated in the detailed descriptions of the separate series of experiments. Mazda lamps of 100, 300, or 1000 watts operated at 110 volts A.C. were employed in different numbers and at varied distances from the plants in order to obtain a series of different intensities. In a given experiment only lamps of the same wattage were employed, so that the emission spectra might be comparable. For any one experiment the different intensities were run simultaneously in order to have the