erection of the leaf; gradual lowering of temperature, on the other hand, inducing progressive depression of the leaf. Thus the effect of temperature, as such, is expansion with rise and contraction with fall. These opposite effects of erection and fall are progressive and slow. Excitatory reaction, on the other hand, is sudden and always attended by the contractile fall of the leaf.
The Mimosa used for experiment may be an entire plant; or, if more convenient, a cut branch containing a leaf may be employed. The result obtained is the same in both cases. It is found that during continuous rise of
Fig. 57.—Excitatory response of Mimosa induced by sudden application of either cold water (C) or hot water (H).
temperature the leaf is erected till it reaches a critical temperature at which the expansion is converted into a spasmodic excitatory contraction. The curve is thus v-shaped, the turning-point of the thermo-mechanical curve being very sharp and definite. Under constant conditions, the critical point of inversion is also very definite. The sudden inversion marks the initiation of the death-change.
Here it is necessary to bear in mind certain conditions for the securing of definite results. It is obvious that death will ensue if a plant be placed in an unfavourable environment as regards temperature for a prolonged period. But as such a temperature would only cause the death of the plant by indirect and cumulative action, it cannot be said to
