136 MOVEMENTS OF CURVATURE 



tissues, and a decreased expansive energy in the less responsive half of the 

 pulvinus might be masked as regards changes of rigidity by a corresponding 

 increase in the more rapidly reacting half. The behaviour of pulvini, from 

 which one-half has been removed, as well as the analogy with nutation 

 reactions, point against the occurrence of any such transitory decrease 

 of expansive energy 1 . 



The effective energy of expansion is considerable in the case of the 

 primordial leaves of Phaseolus vulgaris, for the pressure required to prevent 

 movement, as measured by a dynamometer or spring-balance, is such as 

 to show that the upper half of the pulvinus generates an energy of expan- 

 sion equalling two to five atmospheres. This is, however, merely the 

 excess pressure over that in the lower half of the pulvinus, so that the 

 pressure in the upper half must be at least from five to seven atmospheres 2 . 

 A pronounced energy of movement is also developed in a mechanically- 

 stimulated leaf of Mimosa pudica, but in this case a pronounced fall of 

 rigidity takes place. 



The tension exerted on the dynamometer shows that the progress 

 of an attempted curvature resembles that of a realized one, while the 

 same increase of expansion in darkness is ultimately shown in the more 

 slowly reacting half of the pulvinus of a fixed leaf, as when a curvature 

 can take place. It remains, however, possible that the realized curvature 

 may act as a retarding stimulus to the expansion of the compressed half 

 of the pulvinus. At the same time the structure of the pulvinus is such 

 that when an attempted curvature is prevented the increased energy of 

 expansion exerts no tension on the opposed half of the pulvinus. This does 

 occur, however, in growing organs, and indeed it is largely by tensions of 

 this kind that growth is regulated and the development of pronounced 

 strains avoided. Hence during nutation curvatures no pronounced rise 

 of rigidity is shown, nor can any pronounced pressure be exercised against 

 a resistance which prevents the attempted movement 3 . 



The behaviour of pulvini from which one of the antagonistic halves 

 has been removed supports the above conclusions 4 . Under these circum- 

 stances the remaining half, whether the upper or under one, shows an 

 increased tendency to expansion when the illumination decreases, and 

 a decrease when it increases, so that in both cases a curvature is produced. 

 The inherent periodicity is shown by the changes in the separate halves of 

 the pulvinus being opposite in character. Hence the leaves of Phaseolus 

 fall at evening, whichever half of the pulvinus is present. If only the under 



1 [The varying mechanical moment of the leaf in its different positions is a factor of the utmost 

 importance in this connexion.] 



2 Pfeffer, Period. Bewegungen, 1875, P- 97 se( l- J Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxni, 

 P- 347- 



3 Pfeffer, 1. c., pp. 92, ill. 4 Pfeffer, 1. c., pp. 7, 84. 



