422 



MOVEMENTS. 



tion is alwa^'S slightly cooler than the rest of the petiole, but 



upon the movement from irritation it rises in temperature ; not 



enough, however, 

 to account for the 

 raising of so con- 

 siderable a weight 

 as that of the leaf. 

 1091. Some 

 physiologists have 

 regarded the sen- 

 sitiveness of the 

 pulviuus of the 

 Sensitive plant 

 and of other motile 

 parts as residing 

 chiefly if not whol- 

 * 192 h" in the cell-wall, 



while others have 



thought that it resided in the contractile protoplasm. It is now 



generallj- held to be due to some sudden variation in the osmotic 



power of the proto- 

 plasm, particularh' in 



its peripheral portion 



in contact with the 



cell-wall, by which the 



turgescence of the cell 



is suddenly changed.^ 

 1092. If a plant 



with motile leaves is 



kept in darkness for 



a day or so, even if 



the temperature is fav- 

 orable to motion, its 



power of movement is 



either greatly impaired 



or for a time wholly 



lost. A dimiuislied 



amount of light is sufficient to produce the same effect in the 



case of the Sensitive plant. 



1 Compare Hofineister : Die Lehre von der Pflanzenzelle, 1867, p. 300; 

 Briicke : Archiv fiir Anatomie, Physiologie, mid wiss. Medicin, 1848, p. 434 ; 

 Unger : Botanische Zeitung, 1862, p.' 113 ; 1863, p. 349. 



Fig. 192 Traiisversesectionof themotileorganof aleiifletofOxaliscarnea. (Sachs.) 

 Fio. 193. Vertical section tlirougli the niotilo organ of a leaflet of OxaliB carnea. 

 (SndiB.1 



