THE SLEEP OF PLANTS 197 



When temperature rises, the brass expands more than the 

 steel ; hence the compound strip undergoes a curvature, the 

 brass surface becoming convex. The free end of the strip is 

 attached to a second magnifying lever which thus records the 

 variation of temperature. 



The curves of daily variation of temperature, and the move- 

 ment of the petals, show an astonishing resemblance to each other. 

 There can therefore be no doubt that the cause of the opening 

 and closing of the flower is the diurnal change of temperature. 

 The flower is in a position of ' sleep ' during the day ; a rapid 

 fall of temperature occurs from 6 P.M. and the petals begin to 

 open at first slowly, then very rapidly. The flower becomes 

 completely open and fully expanded by 10 P.M. at night. Though 

 the temperature continues to fall, there is no further possibility of 

 expansion beyond the maximum. At about 6 A.M. the tempera- 

 ture begins to rise, and the reverse movement of closure sets in. 

 The flower continues to close very rapidly till the closure or 

 ' sleep ' movement becomes complete before n A.M. 



It is thus seen that the closure of the flower is brought about 

 by a rise of temperature, the opening being due to a fall of 

 temperature. Both sides of the petals are in a state of growth, 

 but the outer side is the more sensitive to changes of temperature. 

 Thus it happens that during rise of temperature the growth 

 of the outer side is relatively fast ; during cooling it becomes 

 relatively slow. The two opposite reactions give rise to two 

 different curvatures, namely of closure during rise, and of opening 

 during fall of temperature. Other flowers are known, e.g. the 

 Tulip, where the inner side is relatively the more sensitive. 

 Pfeffer has shown that in this flower, rise of temperature brings 

 about an accelerated growth on the inner side of petal. Hence 

 the flower opens during rise and closes during fall of temperature. 



Thus different flowers through their sensitiveness to heat and 

 cold execute the so-called movements- of ' sleep ' and of ' waking.' 

 Some of them have the healthy habit of normal humanity to 

 sleep at night and keep awake in the day-time. Others turn 

 night into day and make up for their long night-watch by 

 sleeping it off in the day-time ! 



The daily movement of the water-lily is thus shown to 

 be due to the predominant effect of variation of temperature 

 on growth. Bose next describes the effect of variation of 

 light and darkness on organs which are sensitive to light. 



