By
Sir J. C. Bose,
Assisted by
Surendra Chandra Das.
In addition to positive or negative curvatures light induces a responsive torsion. With regard to this Jost says:—
"The mechanics of the torsions have not as yet been fully explained. It has long been believed that these torsions were occasioned only by the action of a series of external factors, such as light, gravity, weight of the organ which individually led to curvatures, but in combination induced torsions; but later investigations have shown that torsions might appear when light only was the functional external factor.... If the torsions cannot generally be regarded as due to the combination of two curvatures, we are completely in the dark as to the mechanics of their production."[23]
A leaf when struck laterally by light undergoes a twist, so that the upper surface is placed, more or less, at right angles to the incident rays; as no explanation was available for this movement, the suggestion has been made that the particular reaction is for the advantage of the plant. I shall presently show, that it is possible to reverse this normal torsion and thus make the upper surface of the leaf move away from light.
The experiments which I shall presently describe will, it is hoped, throw light on the obscure phenomenon. I shall be able to show:
(1) that the torsional response is not dependent on the combination of two curvatures,
(2) that it is also independent of the effect of weight,
(3) that it may be induced not merely by stimulus of light but by all forms of stimulation,
(4) that the direction of the torsional response depends on the direction of the incident stimulus and the differential excitability of the organ, and
(5) that there is a definite law which determines the torsional movement.