404 LECTURE XVI. 



in some cases by a rise of temperature, and that closing is 

 induced by withdrawal of light, and in some cases by a fall of 

 temperature (pages 378 and 400). We can thus readily under- 

 stand how it is that many flowers open in the morning, when 

 the intensity of light is increasing and the temperature rising, 

 and close in the evening when the intensity of light is 

 diminishing and the temperature falling. The daily period is 

 the response of the growing perianth-leaves to the variations 

 in external conditions which accompany the alternation of 

 day and night. 



This induced daily periodicity is impressed more or less 

 deeply upon the organisation of the flower, as is shewn by the 

 fact that the exhibition of it does not at once cease when the 

 flower is kept under nearly constant external conditions, in 

 darkness and at an approximately uniform temperature. 

 Pfeffer observed, for instance, that flowers of the Daisy, when 

 placed one evening in darkness, opened the next morning, 

 though later than usual, and attempted feebly to close in the 

 evening ; on the second day slight opening-and-closing 

 movements were made. The degree in which it has been 

 impressed can also be estimated by comparing the effect of 

 sudden disturbances of the external conditions. For example, 

 Pfeffer found that the flowers of Ficaria and of Anemone 

 open much more quickly than they close when subjected, 

 of course in darkness, to the influence of a rise and then of an 

 equal fall of temperature in the morning when their natural 

 tendency was to open. In Crocus and Tulipa, a rise of tem- 

 perature causes the flowers to open in the afternoon when they 

 are closing. In other cases (Ficaria, Galanthus), a rise of 

 temperature in the afternoon when the flowers are closing, 

 induces only a partial opening. In others again (Nymphcea 

 alba, Oxalis rosea and valdiviana, Mesembryanthemum tricolor 

 and echinatum, Composite) the flowers cannot be made to 

 open in the evening by a rise of temperature which acts 

 rapidly when they are subjected to it, of course in darkness, 

 in the morning-hours. Similarly with regard to the action of 

 light, the effect of withdrawal from light is much less marked 

 in the morning-hours than in the evening-hours. The effect, 



