120 • Age and Flowering 



on a mature plant, as many long days are required to induce as are 

 required by a much younger plant. However, a small portion of 

 the area of one later-produced leaf is sufficient for induction by 

 one long day (Evans, 1960). 



The change in sensitivity of successive leaves, as in Perilla, 

 may be a function of meristem aging. It is also possible that as the 

 meristem itself ages, it becomes more sensitive to the floral stimulus 

 from other parts of the plant; the general question of meristem 

 aging and flowering may also be important for flowering in woody 

 plants (see below) but little is known about it. 



At least in Xanthium , the photoperiodic sensitivity of each leaf 

 varies during its development. Khudairi and Hamner (1954a) 

 studied the flowering responses of plants in which single leaves of 

 different ages and at different stages in expansion were present. 

 Within a wide range of absolute sizes, leaves were most sensitive 

 when they had expanded to about half their final size, being much 

 less so either when very young or when mature. Undoubtedly 

 similar relationships between individual leaf development and 

 photoperiodic sensitivity obtain in other plants as well. 



It is not always true that photoperiodic sensitivity increases 

 with plant age or development. The opposite situation has already 

 been noted in sunflower (Chapter Two). It is an SDP when young 

 but later becomes daylength-indifferent (Dyer et al., 1959); stated 

 otherwise, long days inhibit flowering in the young plant but not 

 in the older. On the other hand, this can still be regarded as an 

 increased sensitivity in the sense that a shorter nightlength is induc- 

 tive in older plants. The mechanism is unknown. 



FLOWERING IN WOODY PLANTS 



It is in the woody plants that the problem of ripeness-to-flower 

 is most obvious. The two major environmental factors affecting 

 flowering in herbaceous plants— photoperiod and temperature— also 

 of course affect woody plants, and by similar mechanisms; however, 

 the dominant factor here, that of maturity, appears to be internal. 

 The lack of flowering in many trees until they have attained a 

 given age is of great practical importance because it affects both 

 food production and breeding programs, and also makes experi- 

 ments slow and costly. Hence the effectiveness of some of the pro- 



