PLANT RESPONSE 37 
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, 1.e. 
Ze the summation of single 
saosere | ineffective stimuli produces 
Fra. 18.—Appirive Erect effective response (fig. 18). 
5 Sid rites is etoon tat re Staircase effect.— Animal 
same stimulus when rapidly super- __ « : te 
posed thirty times, produced the tissues sometimes exhibit 
large effect (0). (Leaf-stalk of turnip.) 
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 slugeishness 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  Fyg-,10-> Stumesse 
plants (fig. 19). 
Fatigue.—It is assumed that in living substances 
like muscle, fatigue is caused by the break down or 
