THE MECHANICS OF VARIATION MOVEMENTS 135 



essential for curvature than is a general acceleration of growth for a 

 nutation movement. The automatic variation movements are produced 

 in the same way by an increase of the expansive energy in one-half of 

 the pulvinus and a decrease in the other, for in both cases the rigidity 

 of the pulvinus is unaltered during the movement, whereas a fall or rise of 

 rigidity would inevitably ensue if the movement was due to an increase or 

 decrease of the expansive energy on one side only of the pulvinus. 



These conclusions are mainly attained from estimations of the rigidity 

 of the intact pulvinus under different circumstances, for although no simple 

 relationship exists between the tissue-strains and the weight supported, 

 nevertheless a decreased rigidity indicates a fall, and an increased rigidity 

 a rise of the expansive energy of the active tissues. The original deter- 

 minations were made by Briicke, who noted the angular displacement in the 

 normal and inverted positions with or without the addition of loads. Since 

 the divergences may be from one and a half to two and a half times greater 

 in light than in darkness, it follows that darkness produces a permanent 

 rise of the expansive energy on both sides of the pulvinus ] . 



In the pulvini of P/taseolus, Trifolium, and Desmodium the maximal 

 rigidity is attained at or before the completion of the curvature induced by 

 the withdrawal of light, and since the rigidity is unaltered during the 

 return movement, the latter can only be due to a decrease in the energy of 

 expansion in the contracting half of the pulvinus. If this were not the case, 

 and if, for instance, the partial or complete elimination of the primary 

 curvature were due to a rise in the expansive energy of the compressed 

 half of the pulvinus, then an increase of rigidity would accompany the 

 return movement as well as the original curvature. If, however, the return 

 movement were due solely to a fall of the enhanced expansive energy in 

 the active half of the pulvinus, it would be accompanied by a perceptible 

 decrease of rigidity. Hence there can be no doubt that the expansive 

 energy of the active half of the pulvinus undergoes a transitory increase 

 beyond the stationary value 2 , and the same probably applies even to 

 slowly reacting pulvini. The permanent rise of rigidity after the curvature 

 produced by darkness has been eliminated shows that a permanent rise ot 

 expansive energy is produced in the half of the pulvinus which is at first 

 compressed. 



These facts do not, however, enable us to say whether the darkening 

 does not also produce a certain transitory decrease of expansive energy in the 

 compressed half of the pulvinus, for perceptible changes of rigidity are only 

 produced by pronounced alterations in the expansive energy of the pulvinar 



1 Pfeffer, Period. Bewegungen, 1875, p. 88 seq. 



2 [So that the excess curvature cannot be due to the momentum of the moving leaf. The 

 mechanics of the whole subject require further elucidation and investigation.] 



