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PLANT PHYSIOLOGY AND ECOLOGY 



chiefly falls. A few species, such as the dandelion, bloom through- 

 out the growing period, and are termed aianthous or ever-blooming. 

 By far the greater number of plants complete their flowering within 

 a certain period. Consequently, flowers may be grouped as 

 prevernal, vernal, sestival, and serotinal, corresponding to early 

 spring, spring, summer, and autumn. Prevernal flowers are the 

 first few that appear before spring has really begun. Vernal bloom- 

 ers flower from about the middle of April to the middle of June. 

 The sestival period closes about the middle of August, and the 

 serotinal period lasts until the time of killing frosts. There is 

 naturally no fixed limit for each period, but during each the 

 general nature of the vegetation is characteristic. 



151. Time of daily flowering. The time of day at which the 

 flowers of each species open, and the life period of a single flower. 



Fig. 36. Head of an aster, Machwranthera aspera, showing the position of 

 the rays during the day, and at night or in cloudy weather. 



are habits that are more definitely fixed than the seasonal flowering. 

 Flowers begin to open as early as 3 a.m. The majority of day- 

 bloomers open before 8 a.m., and practically all are open before 

 10 a.m. Night-bloomers open between 4 p.m. and 8 p.m., the 

 latest usually blooming just at twilight. Many species do not 

 close their flowers at all, the latter merely withering and d3dng 

 at the end of the life period. Hemeranthous and nyctanthous 

 flowers open and close daily, with the exception of ephemeral 

 ones in which the life period is less than one day. These move- 

 ments, which are controlled by temperature, ordinarily occur at 

 stated times. The dependence upon temperature is so absolute 

 that opening or closing may be hastened or delayed by artificial 



