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82 [Fig. 31 (2)]. We may describe the whole cycle of change as follows: by the shock of operation the isolated preparation is rendered insensitive for nearly an hour, the excitability is then gradually restored almost to its normal value before operation. Under constant external conditions, this excitability remains fairly constant for about 24 hours after which depression sets in. The rate of fall of excitability becomes very rapid 40 hours after the operation, being finally abolished after the fiftieth hour. It is probable that in a colder climate the fall of excitability would be much slower. The most important outcome of this inquiry is the demonstration of the possibility of obtaining persistent and uniform sensibility in isolated preparations. On account of this, not only is the difficulty of supply of material entirely removed but a very high degree of accuracy secured for the investigation itself.

Experiment 24.—The determination of the rôle played by different parts of the pulvinus in response and recovery is of much theoretical importance. Our knowledge on this subject is unfortunately very scanty. The generally accepted view is that on excitation "the actual downward curvature of the pulvinus is partly due to a contraction of the walls of the motor cells consequent upon the decrease of turgor, but is accentuated by expansion of the insensitive adaxial half of the pulvinus—which was strongly compressed in the unstimulated condition of the organ—and also by the weight of the leaf." According to Pfeffer, after excitation of the organ, "the original condition of turgor is gradually reproduced in the lower half of the pulvinus, which expands, raising the leaf and producing compression of the