SENSITIVENESS. C7 



dismissed with a few words only, as it is not, so far as at 

 present known, of much practical importance to agricul- 

 turists. In .addition to the movements immediately con- 

 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 peri- 

 odical night and day movements, the movements affected 

 by light and temperature, and, lastly, those which are caused 

 by mechanical contact, as by the impact of certain nitro- 

 genous substances, as in the so-called carnivorous leaves 

 before referred to, and those caused by a touch or other 

 mechanical effect, as in the leaves of the sensitive plant. 

 Chloroform and ether arrest these movements, while they 

 have no effect upon the movements that are due to light 

 and heat. The cause of the movements 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 conse- 

 quence 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 under- 

 stood. 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 

 surface, tending to make it convex on the lower surface 



F 2 



