PREPARATION FOR RESPONSE TO PHOTOPERIOD 95 



growth curve (see Chapter 9 for examples with cocklebur), in which 

 there is an initial rapid period of logarithmic growth followed by a 

 decline in growth rate. Often the decline in growth rate is accom- 

 panied by changes in the plant's morphology, such as an increased 

 number of spur branches on certain trees. This ageing process can 

 be distinguished from maturation, because cuttings taken from old 

 trees, which are rooted or grafted to young root stocks, regain the 

 growth characteristics of youth. There is good preliminary evidence 

 that ageing is mostly a matter of nutrition, although hormonal factors 

 might also be involved. At any rate, there is some reason to think 

 that ageing and flowering might be related, since treatments of trees 

 (mutilations, etc.) which will hasten ageing may also tend to hasten 

 flowering. 



Whatever the relationship might be between maturation, ageing, 

 and ripeness to flower, there are three specific questions which can be 

 asked in relation to flowering : First, which part of the plant must 

 attain ripeness to respond ? In photoperiodism we might ask whether 

 the leaf must become mature enough to respond to day-length or the 

 meristem must become mature enough to respond to the flowering 

 hormone. Second, if a plant requires more than one season to attain 

 ripeness to flower, is it because the plant must be exposed to a given 

 number of cycles in activity and dormancy or because it must simply 

 grow for a given period of time ? Are the cycles counted or the active 

 days added together to reach a total ? Third, what is the physiological 

 nature of ripeness to flower ? 



1. Which Part of the Plant Must Attain Ripeness ? 



Figure 5-4 in the last chapter showed that the sensitivity of the 

 various leaves on a cocklebur changes as the leaves mature. Sensi- 

 tivity of a leaf is dependent upon its size (rate of growth — see 

 Chapter 9) and not age of the plant. In Perilla, however, it is possible 

 to show by inducing detached leaves that the ones produced by 

 mature plants are more sensitive than the ones produced by very 

 young plants. Both observations seem to imply that ripeness to 

 flower must be a matter of the leaf reaching maximum sensitivity to 

 day-length, but we cannot exclude the idea that the meristem must 

 also reach some sort of maturity. 



J. A. D. Zeevaart (80) was able to perform an experiment which 

 has direct bearing on the problem. He took very small Bryophyllum 



