100 RESEARCHES ON IRRITABILITY OF PLANTS 



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 environ- 

 ment 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 



