130 THE FLOWERING PROCESS 



shorter than 24 hr. Thus after a few days they are all out of phase 

 with each other; some are sleeping while others are running. 



K. Insect metamorphosis. A population of fruit flies displays 

 a circadian rhythm in changing from the pupa to the adult stage ; 

 some individuals changing at one time and others at intervals of 

 24 hr thereafter. Colin S. Pittendrigh at Princeton University has 

 made detailed studies of this and many other rhythm phenomena. 



L. Mitosis and the size of the nucleus. It has long been known 

 that cells within certain tissues divide predominantly during certain 

 times of day. Constant environment studies indicate that this is 

 sometimes a manifestation of endogenous rhythms. Biinning and 

 his students have also found that the size of the nucleus may vary 

 according to a circadian rhythm. 



2. Detection of the Season by Biological Time Measurement of the 

 Day and I or the Night 

 This is the phenomenon of photoperiodism. Phytochrome con- 

 trolled examples of photoperiodism in plants were Hsted in Table 7-1 

 in the last chapter. 



A. Initiation and development of reproductive structures. This 

 relates the topic of this book to biological timing in general. 



B. Vegetative growth of many plants. A tomato plant grows 

 very poorly under continuous light and constant temperature. 

 Further, if a normal dark period is interrupted for a short time, 

 growth is not as good as in controls given an uninterrupted dark 

 period. This seems to be very analogous to the short-day flowering 

 response, although other observations are not so analogous. For 

 example, if temperature or humidity are varied, good grov^h may 

 occur in continuous light. Other vegetative responses such as tuber 

 or bulb formation, succulency, etc., are more clear cut. 



C. Entering and breaking dormancy. Many plants in temperate 

 regions go into dormancy as the days shorten and the nights 

 lengthen. Coming out of dormancy is also a photoperiodic 

 response in some species. 



D. Germination of some seeds. Interestingly enough, some 

 seeds depend not just on the presence or absence of light for 

 germination, but upon the duration of light or darkness. 



E. Life cycles of animals. Insects, mites and other animals that 

 undergo stages of metamorphosis from one life-cycle stage to 



