Life Movements in Plants, Volume II

by Jagadis Chandra Bose

(1) 18 strokes per minute caused a drift of + 266 cm. per minute.

(2) 18·53 strokes per minute caused a drift of - 530 cm. per minute.

Hence by interpolation the exact balance is found to correspond to 18·177 strokes per minute.

Therefore the absolute rate of growth

= 18·177 × 0·025 µ per second.
= 0·45 µ per second.
= 0·000018 inch per second.

We learn further from (1) and (2) that a variation of ^{(18·53 - 18)}⁄_18·177 produces a change of drift of the spot of light from + 266 to - 530 cm., i.e., of 796 cm. per minute. As it is easy to detect a drift of 1 cm. per minute a variation of ^0·53⁄_{(18·177 × 796)}, or 1 part in 27,000 may thus be detected by the Method of Balance. The spectroscopic method enabled us, as we saw, to detect change of wave length 1 part in a thousand. The sensibility of the Balanced Crescograph is thus seen to rival, if not surpass that of the spectroscope.

For obtaining a general idea of the sensitiveness, the absolute of growth in the instance given above was 0·00018 inch per second, and the Balanced Crescograph was shown capable of discriminating a variation of 1 part in 27,000; hence it is possible to detect by this means a variation of ¹⁄_1,500 millionth of an inch per second.

This method of unprecedented delicacy opens out a new field of investigation on the effect of changes of environment in modification of growth; instances of this will be found in subsequent chapters. I give below accounts of certain demonstrations which will no doubt appear as very striking.

After obtaining the exact balance a match was struck in the neighbourhood of the plant. This produced a marked movement of the hitherto quiescent spot of light, thus indicating the perception of such an extremely feeble stimulus by the plant.

Breathing on the plant causes an enhancement of growth due to the joint effects of warmth and carbonic acid gas. A more striking experiment is to fill a small jar with carbonic acid and empty it over the plant. A violent movement of the spot of light to the right demonstrates the stimulating effect of this gas on growth.