172 Fruits and Seeds. 



they may almost be said to be in perpetual move- 

 ment, though the changes of position are generally 

 so slow that they do not attract attention. This is 

 not, however, always the case. We are all familiar 

 with the Sensitive Plant, which droops its leaves 

 when touched. Another species has leaves like 

 those of an Acacia, and all day the leaflets go 

 slowly up and down. There is a sort of pea living 

 in India which has trifoliate leaves, the lateral leaf- 

 lets being small and narrow ; and these leaflets are 

 perpetually moving round and round, whence the 

 specific name gyrans. In this case the object of 

 the movement is quite unknown to us. In Dion&a, 

 as already mentioned (p. 1 16), the leaves form a 

 regular fly-trap. Directly an insect alights on them 

 they shut up with a snap. 



7. In a great many cases leaves are said to sleep 

 that is to say, at the approach of night they change 

 their position, and sometimes fold themselves up, 

 thus presenting a smaller surface for radiation, and 

 being, in consequence, less exposed to cold. Mr. 

 Darwin has proved experimentally that leaves 

 which were prevented from moving suffered more 

 from cold than those which were allowed to assume 

 their natural position. He has observed with re- 

 ference to the arrowroot plant that, if it has had a 

 severe shock, it cannot get to sleep for the next 

 two or three nights. 



8. The sleep of flowers is also probably a case of 

 the same kind. These motions, indeed, have but 



