Page:Collected Physical Papers.djvu/347

Rh be expected to give rise to a positive response in a very young leaf or in a leaf which is becoming yellow with age.

The above anticipations have been found verified in the following experiments. In figure 98 is seen the positive response of a very young leaf of Musa; the next figure shows similar positive response given by a

very old leaf. There is an additional factor, to be presently described, which also tends to induce a positive response.

The reaction under light, is within limits, proportional to the quantity of light, that is to say, on intensity