Use of an Optical Planimeter for Measuring Leaf Area.

The rapid, accurate measurement of area in biological materials of varying colors, textures, and shapes is of special concern to the plant scientist. The need for the measurement of leaf area, in particular, has prompted the search for newer and faster methods in recent years. In general, leaf area is an excellent indicator of the photosynthetic capacity of a plant and the measurement of such area is of value in studies in plant nutrition, plant competition, and plant-soil water relations. Leaf size or the total leaf area of a plant may also be an important consideration when studying the response of plants to growth regulators and herbicides. Frear (1) has reviewed the laboratory methods employed in such measurements and described an instrument which he designed for the purpose. Frear's method was a simplified version of the integrating sphere technique used in photometry. His apparatus consisted of a light source and photronic cell at opposite ends of a white painted box. A ground glass plate, masked to a circular opening 18 cm in diameter, was placed midway between the cell and light source. A white, hemispherical baffle was interposed in the direct path of the light below the glass so that the response of the cell was primarily a measure of the diffused illumination from repeated scattering of light within the box. When the leaf material was placed on top of the plate it absorbed light, lowering the level of scattered illumination so that the cell response was correspondingly decreased. Since this decrease was a measure both of the area and of the effective absorption and reflection coefficients of the leaf material, it was necessary to calibrate the instrument with known areas of the particular leaf material to be employed. With highly articulated materials such as carrot leaves it was difficult to obtain known areas for calibration. Furthermore, any variation in leaf color or texture due to difference in species, weather, maturity, or time lag between picking and measurement would cause appreciable error. Accordingly, when a new instrument with larger capacity was needed in this laboratory, a basically different apparatus was developed to overcome these limitations. The new design was made practicable by the present availability of large high-aperture Fresnel type lenses (from Special Product Sales Division, Eastman Kodak Company, 343 State Street, Rochester, New York) molded from a thin plastic sheet; these are virtually free of spherical aberration and are obtainable at low cost. With two of these lenses the light from a projection bulb was collimated into a 16-inch (40 cm) diameter beam, passed through the leaf material carried on a glass plate, and refocused on the photronic cell. Since the effective area of the cell