3] EFFECT OF EXTERNAL AGENTS UPON GROWTH 487 



present time, be hypothetical. The attempt to picture to our- 

 selves the probable processes which bring about this condition 

 should, nevertheless, be undertaken. It is generally accepted 

 that a stimulus of any kind produces a chemical change in the 

 protoplasm, and this leads to a change in the activity of the 

 protoplasm the response. The different kinds of stimuli 

 usually induce different kinds of responses, and this is probably 

 because each kind of stimulus affects a particular kind of mole- 

 cule a kind capable of being transformed by the stimulus in 

 question. If we assume that the reception of any stimulus is 

 due to the transformation of a specific kind of molecule capable 

 of being acted upon by the stimulus in question, then it is not 

 difficult to see how, by repeated, violent stimulations of the 

 same kind, nearly all of the molecules upon which sensation 

 depends should become transformed, so that, thenceforth, the 

 protoplasm should be incapable of receiving that kind of 

 stimulus until such time as the sensitive substance shall have 

 been reproduced. Let us imagine 100 molecules (a v a 2 , 3 , a 

 a 98 , 99 , a 100 ) in the root tips which are capable of being 

 decomposed by daylight and as a result setting in motion a 

 series of changes resulting in the phototropic response. Let 

 us imagine that the irritation of light during the first half hour 

 decomposes 50 of them ; during the second, 25 of the remain- 

 der ; during the third, 12 of the remainder, and so on ; then, 

 eventually, the sensation will grow weaker if the substance is 

 not renewed, so that the response will diminish in intensity 

 and finally fail altogether. Then we say the organ is accli- 

 mated to the reagent, for the application of the agent produces 

 no response. 



This explanation of acclimatization to violent agents may 

 now be easily extended to cover the case of attunement. Let 

 us imagine a plant which, living in the dark, is negatively pho- 

 totropic to ordinary daylight. It is attuned to a low intensity 

 of light. If, however, the plant comes gradually to change its 

 habitat so that it is repeatedly subjected to the sunlight, then, 

 as a result of repeated stimulation, those sensitive molecules 

 which are affected by the light are destroyed, so that the plant 

 no longer turns from the light. It is attuned to it. The same 

 theory, mutatis mutandis, will account for attunement to tern- 



