Page:Life Movements in Plants.djvu/195

Rh (Fig. 60) with a magnification of two thousand times shows

that there is an expansion dur- ing the rise of temperature, and that the variable period lasted for a minute, after which there was a cessation of physical movement, the record becoming once more horizontal. The obvious pre- cautions to be taken in such a case, is to wait for several minutes for the attainment of steady temperature. The movement caused by physical change abates in a short time whereas the change of rate of growth brought about by phy- siological reaction is persistent.

Fig. 60. — Horizontal record shows absence of growth in a dead branch ; physical expansion on application of warmth at arrow followed by hori- zontal record on attainment of steady temperature. (Magnification 2,000 times.)

Experiment 55. — In the determination of time-relations of responsive change in growth under external stimulus, I shall take the typical case of the effect of electric shock from a secondary coil of one second's duration. Two electrodes were applied, one above and the other below the growing region of a bud of Grinum. The record was taken on a moving plate, the magnification employed being two thousand times, and successive dots made at intervals of two seconds. It was a matter of surprise to me to find that the growth of the plant was affected by an intensity of stimulus far below the limit of our own per- ception. As regards the relative sensitiveness ol plant and animal, some of my experiments show that the leaf of Mimosa piidica in a favourable condition responds to an electric stimulus which is one-tenth the minimum intensity