318 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



The main body of this organ 1 consists of parenchymatous tissue containing 

 many intercellular spaces. The cell walls are markedly thinner in the lower 

 half than in the upper half of the pulvinus. A vascular bundle transverses the 

 central part. It can be shown that a very great tissue strain normally exists 

 in the pulvinus, the exterior portion being under strong pressure while the inner 

 part is stretched. This may be observed easily if the pulvinus or some part of it 

 is cut out and placed in water, when the outer part expands, while the inner part 

 contracts. The direct inference may be drawn from these facts, that the falling 

 of the stimulated leaf is the result of a change in turgidity in the cells of the 

 upper or lower half of the pulvinus. 



If the lower half of the pulvinus is cut away as far as the vascular bundle, 

 the petiole falls and remains in this position without again rising. If the upper 

 half of the pulvinus is similarly removed, the petiole also falls subsequently, 

 but it afterward erects itself and assumes a higher position than before. It 

 therefore follows that the falling of the leaf is produced by a decrease in turgidity 

 of the cells in the under half of the pulvinus, while the opposite movement is the 

 result of a return of turgidity in these cells. That the leaf finally takes a higher 

 position when the upper half of the pulvinus is removed is due to the fact that 

 the cells of the lower half are now able to expand to a much greater degree than 

 before the operation, since they encounter no resistance from the turgidity of the 

 opposite portion. If a stimulus is applied to an inverted Mimosa plant, the 

 leaves do not sink but begin to rise instead; that is, they move in the same direc- 

 tion, with reference to the stem and roots, as they did when the plant was up- 

 right. This rise is a result of the removal of resistance on the usually lower 

 (now upper) side. 



The decrease in turgidity of the cells of the normally lower half of the pul- 

 vinus is accompanied by a decrease in their circumference, a part of the water 

 contained in these cells therefore migrates elsewhere. This water does not 

 escape to the outside, for the surface of the cushion is dry after the response. 

 It may be observed, however, that the pulvinus is dark-colored after the leaf 

 falls, appearing as though injected with water. Brticke concluded that the 

 water escaping from the cells passes into the intercellular spaces, displacing the 

 air; when the stimulus is removed this water soon re-enters the cells and the inter- 

 cellular spaces become refilled with air, thus rendering the pulvinus again light- 

 colored. 



The cause of the temporary extrusion of water by the cells of the lower half 

 of the pulvinus is naturally to be attributed to changes in the properties of 

 their protoplasmic membranes, brought about as a result of the stimulus. The 

 exact nature of these changes is still unknown. The response of the Mimosa 

 leaf, which is one of the indications that the plant is alive, occurs only under 

 conditions that are favorable to the life-processes in general; there must be the 

 proper kind and intensity of the temperature and moisture conditions, and 

 oxygen must be supplied from the surrounding atmosphere. Chloroform anes- 



1 The pulvinus of the primary petiole is best for this kind of investigation and all the experiments here 

 described have reference to this organ. 



