III.] MOVEMENTS OF PLANTS. 49 



the outer envelopes form an exquisite little cage. 

 Another case, perhaps, is that of Nigella, the " Devil- 

 in-a-bush," or, as it is sometimes more prettily called, 

 " Love-in-a-mist," of old English gardens. 



Again, the protection of the seed is in many cases 

 attained by curious movements of the plant itself. 

 In fact, plants move much more than is generally 

 supposed. So far from being motionless, they may 

 almost be said to be in perpetual movement, 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 (Averrkoa bilimbt) has leaves like those of an 

 Acacia, and all day the leaflets go slowly up and down. 

 Desmodium gyrans, a species of pea living in India 

 has trifoliate leaves, the lateral leaflets being small and 

 narrow ; and these leaflets, as was first observed by 

 Lady Monson, are perpetually moving round and 

 round, whence the specific name gyrans. In these two 

 cases the object of the movement is quite unknown 

 to us. In Dioncea, on the other hand, the leaves form 

 a regular fly-trap. Directly an insect alights on them 

 they shut up with a snap. 



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 

 he prevented from moving suff"ered more from cold 

 than those which were allowed to assume their natural 



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