THE MECHANICS OF VARIATION MOVEMENTS 137 



half is present sudden darkening during the evening produces a reaction 

 opposed to the normal periodic one, whereas in the intact pulvinus the 

 photonastic and periodic reactions would coincide. 



The general agreement of the facts observed points to the conclusion 

 that each half of the pulvinus when freed from its counterpart reacts in 

 the same way as it did in the intact pulvinus. The behaviour of pulvini 

 which have been operated upon does not, however, indicate with certainty 

 what goes out in the pulvinus as a whole, for it is well known that 

 mechanical or other injurious agencies often very strongly modify the power 

 of reaction. Hence, although after operation the remaining under half of 

 the pulvinus of Pliaseolus shows a rapid increase of expansive energy, it 

 does not follow that this half of the pulvinus reacts equally rapidly in the 

 intact pulvinus. 



Similar observations indicate that darkening also causes an expansion 

 in the halves of the pulvinus, but since it takes the same progress on both 

 sides no curvature results under normal conditions *. The same applies to 

 the upper and under halves when the plant is rotated on a klinostat. The 

 primary similarity can be removed by exposure to the action of gravity, 

 and it depends upon the normal or inverted position of the plant whether 

 the dorsal or ventral half of the pulvinus is compressed when a curvature 

 follows darkening. This fact points to the conclusion that the photonastic 

 curvature of this plant involves a quantitatively but not a qualitatively 

 dissimilar reaction in the antagonistic halves of the pulvinus. 



Historical, Dassen 2 distinguished between curvatures with and without pulvini, 

 but did not recognize that in the one case the movement is one of variation, and in 

 the other is due to growth. Pfeffer 3 showed that the opening and closing movements 

 of flowers were due to growth ; and the same was observed by Batalin * in a few foliage- 

 leaves, but this author erroneously supposed that the movements of pulvini were also 

 due to unequal growth. The true condition of affairs was revealed by Pfeffer's 

 investigations on periodic movement 5 . Burgerstein's 6 statement that the opening of 

 flowers is not due to growth, but to stretching by turgor, is either based on error or 

 on an incorrect grasp of the facts. It is difficult to see how this author in his later 

 work is able to deny that growth is responsible for the movements of the perianth- 

 segments of Crocus and Tulipa, for growth always occurs when a permanent elonga- 

 tion takes place. How the growth is produced is naturally another matter. 



A fact of great importance was that observed by Briicke 7 , who found the rigidity 



1 Pfeffer, Physiol. Unters., 1873, p. u. 



2 Dassen, "Wiegmann's Archiv f. Naturgeschichte, 1838, iv. Jahrg., Bd. I, p. 214 ; iv. 2, p. 159. 

 For additional literature se*e Pfeffer, Period. Bewegungen, 1875, p. 163. 



5 Pfeffer, 1. c., p. 161. * Batalin, Flora, 1873, p. 450. 



5 Pfeffer, Period. Bewegnngen, 1875. 



6 Burgerstein, Oesterreich. Bot. Zeitschrift, 1901, Nr. 6; Ueber die Bewegungserscheinnngen 

 der Perigonblatter von Tulipa und Crocus, 1902. 



7 Briicke, Miiller's Archiv f. Anatomic u. Physiologic, 1848, p. 440. 



