The above explanations having proved unsatisfactory we have to search for other factors to account for the fall of the leaf on the cessation of light. In this connection I was struck by the extraordinary similarity of the diurnal curve of the petiole of Cassia alata with that of Mimosa.
DIURNAL CURVE OF THE PETIOLE OF Cassia alata.
Experiment 224.—The leaf of Cassia exhibits as in the leaf of Mimosa a slight erectile movement after the thermal-noon at 2 p.m., there is next a sudden fall after 5 p.m., which continues about 9 p.m.; after this the leaf exhibits a continuous rise with the fall of temperature, till the climax is reached about 6 a.m. in the morning; the leaf then undergoes a fall with rise of temperature, there being a number of pulsatory movements in the forenoon, evidently due to unstable balance under the opposing effects of light and of rise of temperature (Fig. 215).
Fig. 215.—Diurnal record of Cassia leaf. Note similarity with diurnal record of Mimosa.
The reason of this similarity between the records of Cassia and Mimosa was found in the fact:
(1) That the main pulvinus of the leaf of Cassia is, like the pulvinus of Mimosa, differentially excitable, the lower half being more excitable than the upper. This is demonstrated by sending a diffuse electric shock through the leaf, the response being by a fall of the leaf due to the greater contraction of the lower half of the pulvinus. The leaf recovered after an interval of 20 minutes, the curve of response being similar to that of Mimosa. The only difference between the two organs is in the lesser excitability of the pulvinus of Cassia, on account of which a greater intensity of shock is necessary for producing the responsive fall.
(2) The responses to light are the same in both as will be seen in the following experiment.
