Two different classes of phenomena arise from variation of temperature—Thermonasty and Thermo-geotropism. In the former the movement is autonomous and determined by the differential growth-activity of the two sides of an anisotropic organ. In the latter the movement is not in relation to the plant but directed by the external stimulus of gravity.
[39] Jost—Ibid, p. 500.
[40] For further information on the subject of Nyctitropism, cf.—
Pfeffer—Ibid, Vol. II (1903), p. 112;
Jost—Ibid, pp. 500, 507;
Vines—Physiology of Plants (1886), pp. 406, 543.
XLVII.—DIURNAL MOVEMENT DUE TO ALTERNATION
OF LIGHT AND DARKNESS
By
Sir J. C. Bose,
Assisted by
Lalit Mohan Mukherji, B.Sc. (Nawroji Scholar).
The nyctitropic movements of the leaflet of Cassia alata and of the terminal leaflet of Desmodium gyrans furnish us with typical examples of the recurrent effects of light and darkness. The petiole of Cassia contains a number of paired leaflets each of which is about 5 cm. long and 2·5 cm. broad. The leaflets are extremely sensitive to light; at night each pair of leaflets fold themselves in a forward direction (see Fig. 150). With the appearance of light they open at first in a lateral direction; later on there is a twist of the pulvinus by which the inner surface of the leaflets faces light coming from above (p. 405). I shall show that the diurnal movements of the leaflets are predominantly due to phototropic action.
Before proceeding further it will be necessary to give a general description of the experimental method employed, and of the apparatus by which diurnal movements are recorded.