,152 A TEXTBOOK OF THEORETICAL BOTANY 



particularly at the base of the members, whereby their angle with the axis 

 is greatly increased. In some cases, especially in sepals, this may go so far 

 that the parts are completely bent backwards against the axis below the 

 flower, when they are said to be reflexed. Salisbury has shown that the 

 reflexed sepals in some species of Ranunculus are sensitive to light, and that 

 in light of diminished intensity the epinastic curvature is greatly lessened 

 and the sepals do not become reflexed. The same is true of the epinastic 

 growth at the bases of vegetative leaves. These growth movements must be 

 distinguished from the variable movements of parts in the expanded flower, 

 such as the movements of night-closing, which are usually due to variations 

 of turgor in the tissues, associated with changes either of light or of tempera- 

 ture, or else follow mechanical shock. 



The range of the size of flowers is very great. Minute annuals may have 

 flowers only i mm. across. At the other end of the scale are the immense 

 flowers of Rafflesia arnoldi (see Fig. 1576) and Arhtolochia gigas which may 

 be nearly i metre across. 



Relatively few flowers are more than 10 cm. in diameter, and most 

 large flowers are tropical. Conspicuousness is otherwise attained by mass- 

 ing small flowers together, by which means the chances of cross-pollination 

 are increased. 



Generally speaking the size of flowers is in inverse relation to the 

 numbers produced, the giant flowers being always solitary and the largest 

 inflorescences producing, on the other hand, numerous small flowers. 



The duration of flowers is also very variable. Many last for less than a 

 day, opening only once and closing after a definite number of hours. Obser- 

 vation of such ephemeral flowers has enabled gardeners to construct " floral 

 clocks " which show the time of day by the opening and closing of the 

 various flowers of which they are composed. The most ephemeral flower- 

 ing on record is that of Hibiscus trionum, which opens about 9 a.m. and closes 

 by noonday, a period of only three hours. 



According to Kerner there is an inverse relationship between the 

 number of flowers produced and their longevity. If a plant produces but 

 one flower annually, as do many Monocotyledons, the flower usually lasts 

 for many days, thus increasing the chances of insect pollination. Conversely 

 flowers of large inflorescences and, moreover, flowers with numerous 

 stamens and abundant pollen are usually short lived. Longest lived of all 

 are the flowers of some Orchids, which may last from one month to three 

 months or more. There is a definite relation of longevity to pollination and 

 m most flowers fading follows rapidly after poUination, even in long-lived 

 flowers. See also p. 1480. 



The physiology of the fading of flowers is a complex problem. In some 

 cases the evanescent parts, especially petals and stamens, are cut off by 

 abscission layers at the base and dropped while they are still fresh. In 

 others the abscission layer is formed but the parts are not dropped. They 

 wither because their water supply is interrupted. Many form no abscission 

 layer but the water content of the petals is apparently withdrawn by neigh- 



