Page:Life Movements in Plants.djvu/202

172 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 :1 mm. 01' 000000004 inch may thus be detected. It is also possi- ble 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 I'illl' 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 llexure 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 Zerhyranthes, 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 be- tween elongation and recovery.

The Magnetic Creseograph enables demonstration of principal phenomena of growth and its variation before a large audience.