PLANT RESPONSE 



37 



FIG. 18. ADDITIVE EFFECT 



(a) A single stimulus of 3 vibration 

 produced little or no effect, but the 

 same stimulus when rapidly super- 

 posed thirty times, produced the 

 large effect (b). (Leaf -stalk of turnip.) 



Additive effect. It is found in animal responses that 

 there is a minimum intensity of stimulus, below which 

 no response can be evoked. But even a sub-minimal 



stimulus will, though singly 

 ineffective, become effective 

 by the summation of seve- 

 ral. In plants, too, we 

 obtain a similar effect, i.e. 

 the summation of single 

 ineffective stimuli produces 

 effective response (fig. 18). 

 Staircase effect Animal 

 tissues sometimes exhibit 

 what is known as the 

 ' staircase effect,' that is to say, the heights of successive 

 responses are gradually increased, though the stimuli 

 are maintained constant. This is exhibited typically 

 by cardiac muscle, though it is not unknown even in 

 nerve. The cause is obscure, but it 

 seems to depend on the condition 

 of the tissue. It appears as if the 

 molecular sluggishness of tissue were 

 in these cases only gradually removed 

 under stimulation, and the increased 

 effects were due to increased mole- 

 cular mobility. Whatever be the 

 explanation, I have sometimes ob- 

 served the same staircase effect in 

 plants (fig. 19). 



Fatigue. It is assumed that in living substances 

 like muscle, fatigue is caused by the break down or 



FIG. 19. ' STAIRCASE 

 EFFECT' IN PLANT 



