Page:Collected Physical Papers.djvu/341

Rh, suitable marks were made on the scale fixed on the table, so that the intensity of light incident on the leaf was increased in the proportion of 1:3:5:7 by bringing the arc nearer the leaf at the particular distances marked on the scale. The duration of exposure was kept the same.

The responses under increasing intensities of light are seen in figure 95. The resistance is seen to undergo a diminution with the increasing intensity of stimulus.

The responses are appropriately modified by stimulating or depressing agents. Dilute vapour of ether, for example, increases the excitability and enhances the amplitude of response. Strong dose of chloroform, on the other hand, causes depression and death as indicated by gradual diminution and final abolition of response.

The results of other investigations, fully described elsewhere, show that external stimulation gives rise to two definite protoplasmic reactions, which may be described as the A- and the D-effects. The A-effect, usually induced by sub-minimal stimulus, finds outward expression, by induced expansion, increase of turgor, enhancement of the rate of growth, galvanometric positivity and increase of electrical resistance. The D-effect (predominantly induced under stimulus of moderately strong intensity) is outwardly manifested, on the other hand, by contraction, diminution of turgor, diminished rate of growth, galvanometric negativity and diminution of electric resistance. Under very strong stimulation the response tends to become