128 MOVEMENTS OF CURVATURE 



the position of equilibrium results from the co-operation of photonastic 

 and geotropic reactions. It is also evident that a physiologically radial 

 organ will no longer respond on a klinostat to changes of illumination, 

 independently of whether the action of gravity renders possible a photo- 

 nastic response by inducing a labile physiological dorsiventrality, or whether 

 the geotropic irritability alters according to the illumination. 



No safe argument can be drawn by analogy, since the same result and 

 purpose may be obtained in various ways. Even if in a particular case 

 a labile induction, responsible for a photonastic reaction, persisted for a time 

 after the removal of the inducing external agent, its detection would not 

 show that the conjoint action of tropic and diffuse stimuli always takes 

 place in this way. It is, in fact, well known that both the geotropic and 

 phototropic irritabilities are capable not only of autonomic modification, 

 but may also be affected by various external factors. 



The production of a power of photonastic response in the pulvini of 

 Phaseolus appears to be due to geotropic induction, for the photonastic 

 irritability is acquired or modified in conjunction with the performance of 

 a pronounced geotropic curvature, and for this reaction illumination is not 

 essential. Noll's 1 view, according to which we are here dealing with a 

 modification of the geotropic irritability of illumination, is the result of 

 a biased and incomplete comprehension of the problem. In any case, 

 however, it is still necessary to determine whether the changed reaction is 

 due to a modification of the photonastic irritability or to an altered power 

 of movement in the antagonistic halves of the pulvinus. That the latter is 

 possible is shown by the fact that the geotropic curvature considerably 

 modifies the expansive energy of the opposed halves, that in the lower side 

 after reversal increasing, and that in the side now facing upwards decreasing. 

 Since in general the existent mechanical considerations influence the pro- 

 gress and in some cases the activity of the response, it is not inconceivable 

 that on darkening the increased expansion which produces the photonastic 

 curvature should always take place more rapidly in the less expanded half 

 of the pulvinus than in the more expanded one. 



SECTION 28. The Mechanics of Nutation Movements. 



We must confine ourselves to photonastic and thermonastic movements, 

 since no researches have as yet been performed upon the mechanics of 

 hydronastic curvature. It is evident that whenever a rise or fall of tempera- 

 ture or illumination affects the growth of the two sides of an organ unequally 

 a curvature will result, which will continue until a position of equilibrium 



Noll, Die heterogene Induction, 1892, p. 12. 



