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PHYSIOLOGICAL BOTAXV. PART II. 



some are diurnal, others nocturnal. " Meteoric" 

 flowers are such as are influenced by the state of the 

 atmosphere. A few of these as the Calendula pluvialis 

 close at the approach of rain ; others as the Campanula 

 glomerata when the sky is clouded. 



(-."'!.) St'nniilnnts to L'.t/t'iitxion. Light and not 

 heat appears to be the chief stimulus which regulates 

 the expansion of the blossom ; and the influences of 

 moisture alone do not seem to affect it greatly; at least 

 plants when wholly immersed in water expand as 

 freely as in the open air. The phenomenon of their 

 alternately expanding and closing, is allied to the sleep 

 of the leaves (art. 155.), and like the periodic returns of 

 flowering, appears to be regulated by the joint operation 

 of several causes, among which we must allow that the 

 peculiar idiosyncracy of each individual plays its part. 

 For independently of the effect produced by the external 

 stimulus of light, if a plant aci-mstomed to flower at a 

 given period of the day be removed to a dark room it 

 will still make an effort to expand its flowers at the 

 wonted hour. De Candolle proved this by shutting up 

 some of these equinoctial plants, as Linnfeus termed 

 them, in a dark chamber by day and exposing them by 

 night to strong lamp-light. This treatment occasioned 

 for a while the greatest irregularity in their periods of 

 expanding ; but at length they became accustomed to the 

 change, and closed their petals by day and opened them 

 by night. In some cases the expansion of the flower 

 is evidently influenced by the effects of light, heat, and 

 moisture. The common dandelion (Leontodon tarax- 

 acum"), when closed on a cloudy day, upon being 

 brought into the stove will immediately expand its blos- 

 soms, though it may now be exposed to less light and 

 more moisture than before. On the other hand, if the 

 same plant be exposed to the light of the sun, it will also 

 expand though the temperature may be lower than 

 on a cloudy day, when it would continue shut. It 

 has been often asserted and as frequently denied, that 

 the common sunflower will continue to turn its bios- 



