Natural History 625 



cools and reopens in the sun's warmth. By alternately raising 

 and lowering the temperature it may be made to open and close 

 several times in one day. The daisy and the marigold open in 

 the light and close in the dark ; the night stock opens in the dark 

 and closes in the light. The marigold, too, may be made to open 

 and close as many as three times in the twenty-four hours ; it may 

 be made to open at night and close through the day by suitable 

 changes in illumination. But its case is not so simple as that of 

 the tulip, for, when kept in continuous dark, it opens and closes 

 day and night. Even a flower which has opened for the first time 

 in the dark, and has been kept thereafter in continuous dark, 

 which has never experienced the daily change, carries out the 

 periodic movement regularly. 



In the case of leaves the movement is induced by the change 

 from light to dark. But in leaves, too, it may take place in con- 

 tinuous light or continuous dark. A scarlet runner may be raised 

 from the seed in darkness and an even temperature, and under 

 these conditions the leaves rise and fall day and night. 



In such cases there is evidently a periodic movement inde- 

 pendent of external changes in illumination and temperature, 

 though these changes certainly strengthen and emphasise the 

 normal daily swing. In some cases the periodic movement may 

 be due to an after-effect of the induced movement. But in the 

 scarlet runner raised from seed, in the marigold brought to flower 

 in continuous dark, this cannot be so. It is likely that the effective 

 influence is some change in the electrical condition of the atmos- 

 phere ; but the possibility of an inherited periodicity must be con- 

 sidered. 



The significance of these sleep movements is still a matter 

 of discussion. Darwin tried to show that they benefited the plant 

 by saving it from excessive radiation and cooling on clear cold 

 nights. Other investigators see a use in the avoidance of the 

 deposit of dew. Flowers may save their pollen from being spoiled 

 by rain by closing in dull weather. 



