PERIODS OF FLOWEEING. 263 



begins to fade after fecundation has been effected. Many flowers, or 

 heads of flowers, do not open during cloudy or rainy weather, and 

 have been called meteoric. Composite plants frequently exhibit this 

 phenomenon, and it has been remarked in Anagallis arvensis, which 

 has hence been denominated the " poor man's weather-glass." The 

 closing of many flowers in such circumstances protects the pollen from 

 the injurious effects of moisture. 



The opening and closing of flowers is regulated by light and 

 moisture, and also by a certain law of periodicity. A plant accustomed 

 to flower in daylight at a certain time, will continue to expand its 

 flowers at the wonted period, even when kept in a dark room. Decan- 

 doUe made a series of experiments on the flowering of plants kept in 

 darkness, and in a cellar lighted by lamps. He found that the law 

 of periodicity continued to operate for a considerable time, and that 

 in artificial light some flowers opened, while others, such as species 

 of Convolvulus, still followed the clock hours in their opening and 

 closing. 



Light has been said also to have an effect on the position which 

 flowers assume. Some Compositse as Hypochseris radicata and 

 Apargia autumnalis, are stated by Henslow to have been seen in 

 meadows, where they abound, inclining their flowers towards the 

 quarter of the heavens in which the sun is shining. A similar state- 

 ment has been made regarding the Sunflower, but it has not been 

 confirmed in this country at least. Perhaps in its native clime, where 

 the effect of the sun's rays is greater, the phenomenon alluded to may 

 be observable. The effects of light on the direction of the flowers 

 has been noticed in many plants, as Narcissus and certain species of 

 Melampyrum. 



It is of importance, both as regards meteorology and botanical 

 geography, that observations should be made carefully on what are 

 called the annual and diurnal periods of plants : the former being 

 the space of time computed between two successive returns of the 

 leaves, the flowers, and the fruit ; and the latter, the return of the 

 hour of the day at which the flowers of certain species open. The same 

 species should be selected in different localities, and care should be 

 taken that the plants are such as have determinate periods of flower- 

 ing. Eules as to the mode of observing periodical phenomena in 

 plants have been drawn up by a committee of the British Association, 

 and they have published (1.) a list of plants to be observed for the 

 periods of foliation and defoliation ; (2.) a list of plants to be noticed 

 for flowering and ripening of the fruit ; (3.) a list of plants to be 

 observed at the vernal and autumnal equinoxes, and summer solstice, 

 for the hours of opening and closing their flowers. 



