In contrast with the above is the fact that the growing region of the shoot is both sensitive and responsive to geotropic stimulus.
As the effects of direct and indirect stimulation on growth are antithetic, the responses of shoot and root to the direct and indirect stimulus must be of opposite signs.
There is no necessity for postulating two different irritabilities for the shoot and the root, since tissues in general exhibit positive or negative curvatures according as the stimulus is direct or indirect.
[37] "This view has been the subject of a considerable amount of controversy. Wiesner denies the localisation of geotropic sensitiveness. Czapek, on the other hand, supports Darwin's theory. Recently Picard has attacked the problem in a new way (and) concludes that not only the root tip but also the entire growing zone is capable of perceiving gravitational stimuli.... As both Picard's experimental method and his interpretation are open to criticism, the author has repeated his experiments with a more satisfactory apparatus. He finds that in Vicia Faba, Phaseolus multeflorus and Lupinus albus, both apex and growing zone are geotropically sensitive, the former being by far the more sensitive of the two, and the curvature of the growing zone being without a doubt largely induced by secondary stimuli transmitted from the apical region. Charles Darwin's views were therefore in the main correct."—Haberlandt—Ibid, p. 748.
XLIII.—LOCALISATION OF GEO-PERCEPTIVE LAYER
BY MEANS OF THE ELECTRIC PROBE
By
Sir J. C. Bose,
Assisted by
Satyendra Chandra Guha.
The obscurities which surround the phenomenon of geotropism arise: (1) from the invisibility of the stimulating agent, (2) from want of definite knowledge as to whether the fundamental reaction is contractile or expansive, and (3) from the peculiar characteristic that the stimulus is only effective when the external force of gravity reacts internally through the mass of contents of the sensitive cells.
The experiments that have been detailed in the foregoing chapters will have removed most of the difficulties. But beyond these is the question of that power possessed by plants of perceiving geotropic stimulus by means of certain localised sense organs, which send out impulses in response to which neighbouring cells carry out the movement of orientation in a definite direction. Are the sensitive cells diffusely distributed in the organ or do they form a definite layer? Could we by the well established method of physiological response localise the sensitive cells in the interior of the organ? As the internal cells are not accessible, the problem would appear to be beyond the reach of experimental investigation.