Page:Collected Physical Papers.djvu/370

350 So sensitive is the recorder that it shows a change of growth-rate due to slight increase of illumination by the opening of an additional window. The oscillation frequency of the recording plate was once in a second, the magnification being 10,000 times. After taking the first series of record, the second series was obtained after an interval of 15 minutes (fig. 110A). The magnified growth elongation is 9.5 mm. per second; since it is quite easy to measure 0.5 mm., it is possible to record growth for a period as short as 1/20th of a second. It is further seen that the Crescograph enables us to magnify and record a length of 0.0005 mm. which is a fraction of a single wave length of light.

Employing one second, as the unit of time and μ or micron as the unit of length (1/1000 mm.), the absolute rate of growth of S. Kysoor is found as follows:—

If m be the magnifying power, l the average distance between successive dots in mm. at intervals of t seconds,

Effect of Variation of Temperature.

The effect of variation of temperature on growth is shown in fig. 110a. The middle series N was taken at the temperature of the room; the next C was obtained when the temperature was lowered by a few degrees. Finally H was taken when the plant chamber was warmed. The spaces between successive dots are shortened under cooling which induces a diminished rate of growth. The enhancement of the rate of growth