SENSITIVENESS. 6? 



nected with growth, gravitation, or the action of light, 

 which are manifested only during active growth, there 

 are others which occur in the fully-developed leaf, as the 

 periodical night and day movements, the movements af- 

 fected by light and temperature, and lastly, those which 

 are caused by mechanical contact, as by the impact of 

 certain nitrogenous substances, as in the so-called car- 

 nivorous leaves before referred to, and those caused by a 

 touch or other mechanical eifect, as in the leaves of the 

 sensitive plant. Chloroform and ether arrest these move- 

 ments, while they have no effect upon the movements 

 that are due to light and heat. The cause of the move- 

 ments in question is attributed to the sudden contraction 

 of the protoplasm, the expulsion of the watery contents 

 of the cells forming the lower portion of the swelling 

 which leaves endowed with this property possess at the 

 base of their stalks. The cells so emptied become flaccid, 

 and the leaf in consequence falls. The water expelled 

 from the interior of the cells passes into the spaces 

 between them and into the stem, as in the case of frozen 

 leaves (p. 65), and is re-absorbed when the irritation 

 ceases. The balance being restored, the leaf resumes its 

 horizontal position. 



The Action of Gravity on Stems, The cause of the 

 upward growth of stems, though so familiar, is not 

 understood. It is in general exerted in opposition to the 

 direction of gravitation. If a stem be bent downwards, 

 growth takes place much more rapidly on the lower sur- 

 face, tending to make it convex on the lower surface, and 

 consequently to raise its free end (see p. 54). It is this 

 tendency, which, as has been previously stated, permits 

 the stalks of the wheat when laid to recover their erect 

 position. Some stem or portions of stem are, however, 

 directly influenced by gravitation, as in the case of under- 

 ground stems and branches, which burrow in the ground 



