_Determination of the absolute rate of growth: Experiment 51._--For the determination of the absolute rate, I shall interpret the results of a record of growth obtained with a vigorous specimen _S. Kysoor_ on a stationary plate. The oscillation frequency of the plate was once in a second, and the magnification employed was ten thousand times. The magnified growth movement was so rapid that the record consists of a series of short dashes instead of dots (Fig. 59A). For securing regularity in the rate of growth, it is advisable that the plant should be kept in uniform darkness or in uniformly diffused light. So sensitive is the recorder that it shows a change of growth-rate due to the slight increase of illumination by the opening of an additional window. One-sided light, moreover, gives rise to disturbing phototropic curvature. With the precautions described the growth-rate in vigorous specimens is found to be very uniform. [Illustration: FIG. 59.--Crescographic records: (A) successive records of growth at intervals of one second (magnification 10,000 times). (_a_) Effect of temperature on a stationary plate; N, normal rate of growth; C, retarded rate under cold; H, enhanced rate under warmth; (_b_) record on moving plate, where diminished slope of curve denotes retarded rate under cold (Magnification 2,000 times.)] After the completion of the first vertical series, the recording plate was moved 1 cm. to the left; the tip of the recorder was brought once more to the top by the micrometer screw, S, (Fig. 58), and the record taken once more after an interval of 15 minutes. The magnified growth for 4 seconds is 38 mm. in the first record; it is precisely the same in the record taken fifteen minutes after. The successive growth elongations at intervals of 1 second is practically the same throughout, being 9.5 mm. This uniformity in the spacings demonstrates not only the regularity of growth under constant conditions, but also the precision of the apparatus. It also shows that by keeping the external condition constant, the normal growth-rate could be maintained uniform for at least fifteen minutes. The magnified rate of growth is nearly 1 cm. per second, and since it is quite easy to measure 0.5 mm. the Crescograph enables us to magnify and record a length of 0.0005 mm. that is to say, the sixteenth part of a wave of red light. The absolute rate of growth, moreover, can be determined in a period as short as 0.05 of a second. These facts will give some idea of the great possibilities of the Crescograph for future investigations. As the period of experiment is very greatly shortened by the method of high magnification, I shall, in the determination of the absolute rate of growth, adopt a second as the unit of time, and µ, or _micron_, as the unit of length,--the micron, being a millionth part of a metre or a thousandth part of a millimeter.