_Experiment 57._--A scale 3 metres long divided into cm. is placed against the screen. A metronome beating half seconds is started at the moment when the spot of light transits across the zero division; the number of beats is counted till the index traverses the 300 cm. At the normal temperature of the room (30°C.), the index traversed 300 cm. in five seconds. The plant chamber was next cooled to 26°C. by the blowing in of cooled water vapour; the time taken by the spot of light to traverse the scale was now 20 seconds, _i.e._, the growth-rate was depressed to a fourth. Under continuous lowering of temperature the growth-rate became slowed down till at 21°C. there was an arrest of growth. Warm vapour was next introduced, gradually raising the temperature of the chamber to 35°C. The spot of light now rushed across the scale in a second and a half, _i.e._, the growth was enhanced to more than three times the normal rate. The entire series of the above experiments, on the effect of temperature on growth, was thus completed in the course of 15 minutes. SUMMARY. A description is given of the High Magnification Crescograph, which enables an automatic record of growth magnified ten thousand times. The absolute rate of growth can be easily determined from the data given in the record. A magnification of a million times is obtained by the employment of Magnetic amplification. An increment of growth so minute as a millionth part of a mm. or 0.00000004 inch may thus be detected. It is also possible to detect the growth of a plant for a period shorter than a hundredth part of a second. The influence of external conditions on variation of rate of growth is obtained by two methods of record. In STATIONARY METHOD, the increase or diminution of the distance between successive dots representing magnified rate of growth, demonstrates the stimulating or depressing nature of the changed condition. In the second, or MOVING PLATE METHOD, a curve is obtained, the ordinate representing growth elongation, and the abscissa, time. A stimulating agent causes an upward flexure of the normal curve; a depressing agent, on the other hand, lessens the slope of the curve. The action of external stimulus induces a variation of the rate of growth, the time relations of which are found from the automatic record of the growth. The latent period is shortened with the intensity of the stimulus. A responsive variation of growth is induced by an intensity of stimulus which is below human perception. It is often possible to obtain record of the pulsatory nature of growth-elongation. Thus, with the growing peduncle of _Zephyranthes_, the growth pulse commences with a sudden elongation, the maximum rate being 0.0004 mm. per second. The pulse exhausts itself in 15 seconds, after which there is a partial recovery in course of 13 seconds, the period of complete pulse being 28 seconds. The resultant growth in each pulse is the difference between elongation and recovery.