Age and Flowering in Herbaceous Plants • 117 



that of "phasic development," has been associated with some studies. 

 It views plant growth as a succession of recognizable phases, each 

 requiring a specific set of environmental conditions for its fulfill- 

 ment, and none of which can be bypassed (see Murneek and Whyte, 

 1948). A concept as unspecific as this is hardly susceptible either to 

 proof or disproof once it is admitted that the characteristics of 

 the phases will not be the same in all plants. Hence, it will not be 

 considered further. Instead, some relationships of age and flowering 

 in some of the familiar herbaceous plants will be discussed first, 

 and will be followed by a consideration of the problems posed by 

 flowering in woody species. 



AGE AND FLOWERING IN HERBACEOUS PLANTS 



Certain plants produce a characteristic minimum leaf number 

 before flower primordia are initiated. In the best-known examples, 

 spring and vernalized winter rye, a minimum of seven leaves 

 appear before the inflorescences no matter what the conditions 

 used, at least in most of the older research with these plants. A 

 partial explanation is that four leaf primordia are already present 

 in the mature embryo, and so precede the inflorescence. However, 

 three more are apparently differentiated during or after germina- 

 tion. Although it is possible to reduce the "minimum leaf number" 

 below 6 by the use of continuous light from germination, or by. 

 starting with prematurely harvested embryos that have differen- 

 tiated fewer leaf primordia, apparently at least one or two leaves 

 in addition to those in the embryo still intervene before flower 

 initiation (Gott et al., 1955). 



Holdsworth (1956) has considered the concept of minimum 

 leaf number extensively, and questions its general usefulness. The 

 number in Xanthium— 8— appears to be accounted for by those 

 leaves present in the embryo plus those developing before induc- 

 tion and the translocation of the floral stimulus have taken place. 

 In certain other plants the number is higher than can be accounted 

 for in such ways. However, both types of observation may depend 

 on differences in the sensitivity of successive leaves to photoperiodic 

 induction, which will be considered below. Other factors affecting 

 minimum leaf number may be the movement of flower-inhibiting 

 or promoting substances from the cotyledons, as observed, for 



