PROTECTION OF POLLEN 349 



peduncles droop in the evening so that the flower, erect by 

 the day, is pendent at night when the danger of wetting by 

 dew is greatest. More general and striking are the move- 

 ments carried out in the flowers of many plants (by the whole 

 flower head of the Compositas) which result in the corolla 

 closing at night and opening, for a shorter or longer period, 

 through the day. The tulip, the crocus, the daisy, the 

 dandelion, the goat's-beard, the flax, the ice plant, are com- 

 mon and familiar examples. These movements are classed 

 along with the " sleep " or nyctinastic movements of 

 leaves. Unlike the majority of leaf movements they are 

 due to differential growth rates on the two sides of the 

 petals. Depending on the life of the flower, they occur 

 only once or several times. The factor which usually 

 causes opening is rise of temperature. This is certainly 

 eflFective in the tulip, crocus, flax, scarlet pimpernel, and 

 many others. In the daisy and marigold {Calendula 

 officinalis), change from dark to light is the causal factor. 

 Closure may take place as the result of a fall in temperature, 

 as in the crocus and tulip ; it is probable that it is more 

 generally due to an automatic reverse at a definite interval 

 after opening. Such a reverse is seen even in the tulip and 

 crocus, though in the former it does not lead to complete 

 closure. This automatic reverse leads to the early closure 

 of such flowers as the goat's-beard, which shuts when the 

 sun is at its zenith. 



In the majority of cases, just as with leaf movements, 

 a rhythm underlies the movement, which is induced, accentu- 

 ated, or modifled by the external factor. This rhythm may 

 be due to a summing up of the effects of successive changes 

 in external conditions, or it may be inherent ; in different 

 flowers its nature may differ. At present it is not possible 

 to decide definitely which explanation is correct. The 

 scarlet pimpernel opens for the second time, only if the 

 temperature is raised about 24 hours after the first opening ; 

 a suitable rise of temperature before this is quite without 

 effect. Normally the opening occurs about 9 to 10 a.m. 

 By suitable treatment the first opening may, however, be 



