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_. [Illustration: FIG. 215.--Diurnal record of _Cassia_ leaf. Note similarity with diurnal record 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). 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. [Illustration: FIG. 216.--Post-maximum after-effect of light on response of leaf of _Cassia_. There is an over-shooting on cessation of light at arrow within a circle.] _Experiment 225._--In _Cassia_, as in _Mimosa_, light acting from above induces at first an erectile movement which reaches a maximum; after this there is a neutralisation and reversal. In the record given in figure 216, light from a small arc lamp acting on the upper half of the pulvinus for 48 minutes gave the maximum positive curvature; this was completely neutralised by further exposure to light for 20 minutes. Light was cut off at neutralisation and there was a sudden fall beyond the equilibrium position, which was more rapid than the movement under light. The after-effect of prolonged exposure is thus an 'over-shooting' beyond the normal position of equilibrium. RESPONSE OF _Mimosa_ TO DARKNESS AT DIFFERENT PARTS OF THE DAY. I now tried the effect of darkness on the movement of _Mimosa_, and was surprised to find that while artificial darkness caused a sudden fall of the leaf in the afternoon, it had no such effect in the forenoon.