Action of Injury on Sub-tonic Specimens: Experiment 37.—I will now describe a very interesting experiment which shows how an identical agent may, on account of difference in the tonic condition of the tissue, give rise to diametrically opposite effects. In demonstrating this, I took a specimen in a sub-tonic condition, in which the conduct­ing power of the tissue was so far below par, that the test-stimulus applied at a distance of 15 mm. failed to be transmitted (Fig. 43). The end of the petiole at a distance of 1 cm. beyond the point of application of test-stimulus was now cut off. The after-effect of this injury was found to enhance the conduct­ing power so that the stimulus previously arrested was now effectively transmitted, the velocity being 25 mm. per sec. This enhanced conduct­ing power began slowly to decline, and after half an hour the velocity had declined to 4.1 mm. per sec. The end of the petiole was cut once more, and the effect of injury was again found to enhance the conduct­ing power, the velocity of trans­mission being restored to 25 mm. per sec.

Fig. 43.—Effect of injury in enhancing the conduct­ing power of a sub-normal specimen; (1) Ineffective trans­mission becoming effective at (2) after section; (3) decline after half an hour, and (4) increased conductivity after a fresh cut.

SUMMARY.

There are two different types of propagation of excitation: by convection, and by conduction. In the former the excited cell undergoes deformation and causes mechanical stimulation of the next; example of this type is seen in the stamen of Berberis. The conduction of excitation consists, on the other hand, of propagation of excitatory protoplasmic change. The trans­mission in the petiole of Mimosa is a phenomenon of conduction.

This conduction takes place along vascular elements. The conductivity is very much greater in the longitudinal than in the transverse direction.

Rise of temperature enhances, and fall of temperature lowers, the rate of conduction. Excitation is transmitted in both directions; the centrifugal velocity is greater than the centripetal.

Dessication of conduct­ing tissue by glycerine enhances the conduct­ing power. Local application of cold depresses or arrests the conduction. Application of poison permanently abolishes the power of conduction.

Conductivity is modified by the effect of season, being higher in summer than in winter.