350 THE BIOLOGY OF FLOWERING PLANTS 



made to occur in the evening ; in such a flower the second 

 opening can take place only in the following evening. Here 

 we have a rhythm, not indeed causing opening (or only to 

 a very slight extent), but regulating the time at which opening 

 may take place under the influence of the suitable change 

 of external conditions. In the marigold, fully investigated 

 by Stoppel (1910) and by Stoppel and Kniep (1911), 

 opening is caused by illumination. The flowers also open 

 in constant darkness, and in this condition carry out a 

 periodic opening and closing every 24 hours. Illuminated 

 by night and darkened by day the opening is shifted 12 

 hours from the normal time. Illuminated and darkened 

 in 8-hour periods, the rhythm is speeded up to suit the new 

 period of external change. Illuminated and darkened in 

 4-hour periods, the flowers open and close fully only once 

 in the day, but slighter movements show the influence of 

 the external change. Illuminated and darkened in 2-hour 

 periods, only the daily movement takes place, the external 

 change has no visible effect. Here we have an internal 

 rhythm which may be influenced, but only to a limited 

 extent, by external change. In those flowers which open 

 at an approximately definite hour of the day we may suppose 

 that the regularity is partly the effect of internal periodicity 

 and partly due to the regular onset of the external con- 

 ditions, favourable to opening, at a definite time. A factor 

 which may have an important effect in regulating such 

 movements is the diurnal alteration in the conducting 

 power of the atmosphere for electricity (see p. 204). 



The flowers mentioned and many others remain closed 

 through the night, and are thus protected from the deposition 

 of dew on the essential organs. In cool, dull weather 

 opening does not take place through the day, or is only 

 partial, so that protection is again secured. But passing 

 showers on a sunny day, accompanied as they are by lowered 

 temperature and illumination, do not, as a rule, cause closure, 

 and against these the flower is unprotected. The tulip, the 

 crocus, and the pheasant's eye are exceptions which close 

 rapidly on a fall of temperature. It has been suggested, 



