SUMMARYAn apparatus is described for precise measurement of growth rate of an alga as a function of intensity and intermittency of illumination. Dilution of a culture is controlled by a photometric device to just such a rate as will maintain constant density of population and balance the growth rate. Maintenance of constant volume by an automatic siphon allows direct application of the differential form of the growth equation in evaluating specific growth rate. The relationships of the new apparatus to other steady-state growth devices are discussed. periment. Cells harvested from a continuous-culture apparatus were centrifuged out and resuspended in a 1/4-strength EDTA Knops solution (13) to a population density which gave a 50 % transmission in an Evelyn colorimeter with 600 m,u glter. In the growth chamber this suspension had a transmission of 81 % as measured with a fully-illuminated photocell placed behind the suspension. Constancy of the photometric control was checked duringf and at the end of each experiment by readings of transmission of the suspension in the Evelyn colorimeter; all such readings fell within the range of 48-52 %. In each experiment the specific growth rate, k, was evaluated in loge units per day as described in the preceding paper. The temperature used throughout was 25°C.INTENSITY MEASUREMENT: Measurements of light intensity at the position of the growth chamber were made with a large surface Moll thermopile with a compensated Aryton shunt and Rubicon Type T galvanometer. It was calibrated without a shielding window against an NBS standard lamp. For subsequent measurements the thermopile was used with a thin glass shielding window. Estimate of the intensity of visible radiation was obtained by use of a Jena RG8 filter which transmits the near infrared and has a sharp cut-off at 7000 A (fig 1). Readings with the thermopile were taken alternately with the system open, with the RG8 filter, and with the system closed by a shutter. (For positions of the shutter and filter in the optical system see figure 2 of the preceding paper.) Total deflection with the system open, minus deflection with the RGS filter was taken as the measure of visible radiation. The procedure is similar to that used by Kok (8)

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The action spectrum for the transformation of protochlorophyll to chlorophyll a in normal and albino corn seedlings. Arch. Biochem. and Biophys. 31: 1-17. 1951. 13. KRASNOVSKII, A. A. and KOSOBUTSKAYA, L. M. Spectral analysis of the constitution of chlorophyll during its formation in plants and in colloidal solutions of the substance of etiolated leaves.

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