354 LIFE MOVEMENTS IN PLANTS 



discover the important fact that stimulus below the minimal, 

 though ineffective in inducing excitatory contraction, is not 

 below the threshold of perception. The plant not merely 

 perceives such stimulus, bur also responds to it in a definite 

 way, by expansion instead of contraction. I shall designate 

 the stimulus below the minimal, as the sub-minimal. There 

 is a critical point, which demarcates the sub-minimal stimulus 

 with its expansive reaction from the minimal with its re- 

 sponsive contraction. 



The critical stimulus varies in ditterent species of 

 plants . Thus the same intensity of light which induces a 

 retardation of growth in one species of plants will enhance 

 the rate of growth in another. Again, the critical point 

 will vary with the tonic level of the same organ ; in an 

 optimum condition of the tissue, a relatively feeble stimulus 

 will be sufficient to evoke excitatory contraction ; the critical 

 point is therefore low for tissues in tonic condition which 

 may be described as above par. In a sub-tonic condition, 

 on the other nand, strong and long continued stimulation 

 will be necessary to induce the excitatory reaction. The 

 critical point is therefore high, for tissues in a condition 

 below par. Stimulus below the critical point will here 

 induce the opposite physiological reaction, i.e., expansion. 

 The physico-chemical reactions underlying these opposite 

 physiological responses have, for convenience, been distin- 

 guished as the " A " and " D " change (pp. 143, 22'6). The 

 assimilatory 'building up', A change, is associated with an 

 increase of potential energy of the system ; the dissimi- 

 latory ' break down ', D change, on the other hand, is attenil- 

 ed by a run-down of energy. 



Stimulus was shown (p. 225) to give rise to both these 

 reactions, though the A effect is, generally speaking, masked 

 by the predominant D effect. The " A " change is quicker 

 in initiation, while the '"D" effect developes later; again 



