THE AMERICAN NATURALIST [Vol. XLIV 



One well known is the interference of stimuli in tendrils. 

 As Fitting showed, friction on the dorsal side of tendrils 

 which usually curve after irritation of the ventral side, 

 inhibits the curvature, not by setting up a countervailing 

 rate of growth, for it apparently has no power to influ- 

 ence growth, but by interfering with the reaction some- 

 where between perception and the growth response. 

 Here, if the stimulus were a source of energy, or limited 

 the reaction, the double excitation should produce a 

 doubly strong response. Nor can the well-known inter- 

 ference of light or sound waves in unlike phases be cited 

 as an analogy of the inhibition in this case, because no 

 such superposition is possible. It must be admitted that 

 in cases of this kind the energy of the stimulus is not 

 directly related to the response. 



Of course, in urging this I must not be understood as 

 denying that the excitation is often proportional to the 

 intensity and duration of the stimulus, and that the final 

 response may be influenced by the amount of excitation ; 

 the point is only that potential energy is released in 

 amounts not at present referable quantitatively to that of 

 the stimulus. 



On analyzing the varied reactions to stimuli, it will be 

 found in many cases that it is possible to recognize two 

 well-marked phases, which may be designated primary 

 and secondary. In other cases no such distinction can be 

 made. One of the primary phases of response is known 

 as perception, a word which, as used by plant physiolo- 

 gists, is entirely without psychological implications. 

 Whatever change in the protoplasm is connoted by that 

 term takes place almost instantly, as shown by the phe- 

 nomena of summation. Stimuli of extremely brief dura- 

 tion—say a small fraction of a second— so transient that 

 they produce no observable effect, nevertheless, if re- 

 peated at proper intervals, finally give rise to a reaction. 

 The brevity of each period of stimulation gives indica- 

 tion of the speed of the perception change; for if no 

 effect were produced by a single stimulus, repetition 

 could have no effect. 



