872 



ECOLOGY 



flowers are more ephemeral than are diurnal flowers (as in the night- 

 blooming cereus), but the flowers of evening primroses, although opening 

 but once, remain open for some hours after sunrise; the flowers of some 

 species of Silene open for at least three successive nights. Flowers 



often have a longer period of anthesis in 

 spring and autumn than in summer; even 



f> ^ **THS355^ fill suc ^ e P nemer al flowers as those of Hibiscus 



Trionum and Hemerocattis fulva may open 

 toward the beginning or the end of the 

 flowering period on three successive days. 



Flowers cannot be classed simply as diurnal or 

 nocturnal, since most hours of the day and night 

 are marked by the opening or closing of the flowers 

 of some species. The opening and the closing 

 hours of a given species vary widely with the lati- 

 tude and the season, but in general the earlier 

 diurnal flowers open about sunrise (as in the 

 chicory and the morning glory) and the later about 

 noon (as in Mesembrianthemum). The nocturnal 

 series begins about dusk (as in Silene) and closes 

 about ten (as in Cereus). The times of closing 

 are much less definite than are those of opening, 

 and they extend over most of the twenty-four 

 hours; this is partly because flowers open much 

 more rapidly than they close, the former process 

 sometimes being sudden, as in Oenothera. Begin- 

 ning with the early forenoon (as in salsify and 

 chicory) each hour until after sunset is marked by 



FIGS. 1193, 1194. Floral 

 opening and closing in the dan- 

 delion (Taraxacum officinale): 

 1193, an open head as seen in 

 full sunlight; 1194, the same 

 head as seen at night; the invo- 

 lucre (*) is double, consisting of 

 short outer bracts (b) and a 

 single row of long inner bracts 

 (b') ; opening and closing are due 

 chiefly to the movements of the 

 inner bracts, the position of the 

 outer bracts shifting but slightly. 



the closing of some diurnal flowers. Similarly 

 nocturnal flowers may close at any time from midnight (as in Cereus) to sunrise, or 

 even during the following forenoon (as in the evening primrose). The entire 

 scheme, as above outlined, may be disarranged in cloudy weather. 



The factors involved in the opening and the closing of flowers. The 

 mechanism of opening and closing and the factors controlling this mech- 

 anism are in part unknown. It has been shown in a few cases, and it is 

 believed to be true in others, that these movements are true movements of 

 growth, opening being due to epinasty, and closing to hyponasty, in the 

 segments of the perianth or involucre. Probably the chief single factor 

 causing epinasty, and hence opening, is an increase of temperature. 

 Opening as a result of an increase of temperature has been proven ex- 

 perimentally in a number of cases, notably in the tulip and the crocus; 



