REPRODUCTION AND DISPERSAL 873 



a crocus flower opens in two to four minutes, when the temperature is 

 raised suddenly fifteen or twenty degrees, and it is sensitive to a change 

 of half a degree. Successive alternations of cold and warmth may induce 

 several successive closings and openings within an hour. It is believed 

 by many investigators that changes in turgor are responsible for some 

 cases of opening and closing, as in the tulip. Possibly some flowers open 

 and close autonomously; the flowers of Calendula arvensis, for example, 

 open in the dark without any change in temperature. 



In some cases light, independently of temperature, has been shown to cause 

 opening (as in the day Uly and in the gentians) . In nature both light and heat prob- 

 ably cooperate, especially in opening; the early closing of far northern flowers, in 

 spite of long daylight, probably is a matter of temperature alone. In a few cases 

 opening and closing are due to moisture changes; so sensitive is the head of Carlina 

 to such changes that it has been used as a hygroscope, closing in moist air and open- 

 ing in dry air. Obviously the explanation of the movements of flowers that do not 

 open rather promptly at sunrise or close at sunset presents certain difficulties. While 

 these have not as yet been solved experimentally, it is likely that certain species 

 require a greater amount of heat or light for opening or a less amount for closing 

 than do others, or that they are less quickly sensitive. Most remarkable are the 

 nocturnal flowers, in which opening is caused by decreased temperature and light 

 instead of by increased temperature and light, as in diurnal flowers. 



The advantages of flower closing. — In general, flowers that close 

 during anthesis are open at the time when their special pollinating in- 

 sects are most active. Most of the latter (especially the bees and butter- 

 flies) are diurnal, and are affected by the same factors that influence 

 flower opening, such as increased heat and light and low atmospheric 

 humidity (or at least absence of rain) . Nocturnal flowers are pollinated 

 almost exclusively by nocturnal insects, such as certain moths. How 

 such a remarkable correlation of flower opening and insect activity may 

 have arisen is altogether unknown. It is probable that closure when 

 the insects are not active is likely to be of considerable protective impor- 

 tance; in diurnal flowers pollen probably is conserved by closure in 

 rainy weather, and in all cases closure for a part of the time would seem 

 to favor the lengthening of the period of stigmatic receptivity, particu- 

 larly in nocturnal flowers, which are freed from the desiccating influence 

 of sunlight. In addition, as previously noted, opening and closing favor 

 close pollination and even geitonogamy in the composites. 



Protective movements other than floral opening and closing. — In some 

 flowers the anther valves are hygroscopic (as in Alchemilla and Laurus), 

 closing in moist weather ; usually they are closed much more quickly 



