No. 522] PHYSIOLOGICAL RESPONSE 



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not of synthesis, and so of necessity proceeds in the other 

 direction. We may not believe it possible to account for 

 the behavior of living things by considering them as 

 mechanisms whose actions are to be described in terms of 

 matter and energy; and certainly it is not possible to do 

 this now for any but the simplest actions. Yet it must 

 not be forgotten that all the progress that has been made 

 has been made in this direction; so that it commends 

 itself to us both by the a priori and the inductive method. 



The phenomena of response are probably the most 

 complicated to be seen in the plant, and as yet we are far 

 from being able to describe them completely in terms of 

 physics and chemistry. I purpose only to present some 

 suggestions on the nature of these phenomena, as a con- 

 tribution toward the mechanistic conception of the plant, 

 in the hope that this presentation may help to rid us of 

 some of the subconscious vitalistic notions that are apt 

 to cling so persistently about our thought and speech. 



First let us consider the relation of the phenomena of 

 response in living and non-living matter. To speak of 

 response by non-living matter can occasion no surprise. 

 Sachs, long ago, in his " Lectures on the Physiology of 

 Plants," pointed out the fact that response is not pecu- 

 liar to living things, citing as an example the crystals of 

 the yellow iodide of mercury, which change their con- 

 stitution and color when stimulated by various agents - 

 a scratch sufficing to initiate a change which spreads 

 gradually through the whole. Many other substances 

 behave similarly. To explain such a change we predicate 

 a state of metastable molecular equilibrium, the upset- 

 ting of which at one point incites a spreading disturb- 

 ance, as a row of properly spaced blocks fall. 



which is conditioned by external factors. In moist air 

 many anhydrous salts and oxids unite with water, and in 

 some cases in proportions differing with the amount of 

 the vapor pressure. Thus, anhydrous copper sulfate, at 

 a temperature of 50° C. with the vapor pressure gradu- 

 allv increasing from zero, would take up one molecule of 



