V Movements of Plants 87 



taken, for we have found no exception to the rule that leaves 

 which sleep continue to move during the whole twenty-four 

 hours ; they move, however, more quickly when going 

 to sleep and when awaking than at other times." 



Exceptional Developments of Plant Movement, the 

 Telegraph Plant, Sensitive Plant, etc. — The most re- 

 markable case of continuous movement is that of the Indian 

 Telegraph plant {Desinodiiun or Hedysariim gyrans). Its 

 leaves have three leaflets — a large median one, and two 

 lateral ones which are rudimentary. The main part moves 

 a little during the day and has a remarkable sleep move- 

 ment, but the lateral leaflets which do not sleep move con- 

 stantly, describing with a series of little jerks minute circles. 

 Each revolution sometimes occupies little over a minute. 



The term sleep is often applied, as Linnsus applied it, 

 not only to leaves, but also to the petals of many flowers 

 which close at night. This seems to depend rather upon 

 a difference of temperature determining differences in 

 turgescence than on a difference of illumination, and it has 

 the effect of sheltering the internal parts of the flower 

 from cold winds, rain, and over-radiation. Here certainly 

 Darwin's suggestion of " modified circumnutation " does 

 not apply. 



We have not spoken of one of the most familiar of plant 

 movements — that of the sensitive plant {Mimosa pudica). 

 As is well known, a touch is enough to make the leaves of 

 this plant suddenly assume their sleep position. This is 

 usually referred to a change in a cushion of cells at the 

 base of the leaf. Even Darwin did not think of explaining 

 it as connected with circumnutation, although he justly re- 

 gards it as an extreme case of exaggeration of the sleep 

 movements. Its outward or bionomic utility we cannot 

 fathom, still less its inner mechanism. It is tempting to 

 correlate it with that remarkable " intercellular continuity 



