480 The Physiology of Plants BOOK in 



questions of sensitiveness and response to stimulation 

 later. 



The behaviour of the leaves of Mimosa and other sensi- 

 tive plants has been shown to be affected by various 

 influences, among which contact stimulation must be in- 

 cluded. Others are disturbance caused by shaking, and 

 modifications of the tonic action of light. The mechanism 

 of the pulvinus which is concerned in the movement was 

 investigated by Pfeffer in 1873. He showed that the 

 stimulus of contact with some portion of the leaf blade, 

 or some other part of the plant, causes a change in the 

 turgid tissue of the lower half of the pulvinus, bringing 

 about a diminution of its turgor, water passing from the cells 

 into the intercellular spaces. The result is a different con- 

 dition of equilibrium of the tissue tensions in the pulvinus and 

 the weight of the leaf then causes it to fall. The original 

 position and its accompanying equilibrium of strain is repro- 

 duced as the turgor gradually returns. Vines put forward 

 in 1878 the view that the disturbance of turgor and con- 

 sequent movement are aided by an active contraction of the 

 cells on the lower side of the pulvinus, and Gardiner, as the 

 result of independent investigations, supported him in 1887. 

 Pfeffer, however, did not accept the view, though in the latest 

 edition of his Pflanzenphysiologie he said it is uncertain. 



The movements of the stamens of the Cynareae, of 

 Berberis and other flowers, received little notice. In 

 1861-3 they were investigated by Cohn, who held that 

 their movement is produced by a change in the shape of 

 the filaments without any escape of water. He compared 

 the contracting cells to muscle fibres. Unger put for- 

 ward similar views in 1862-3. Hofmeister opposed the 

 muscular hypothesis in 1867 and attributed the movement 

 to the properties of the cell wall. In 1873 Pfeffer claimed 

 that the movement has as its cause a change similar to that 

 in the pulvinus of Mimosa. 



