The Measurement of Flowering • 9 



This sort of danger is widely recognized and it is usually avoided 

 by careful workers. One way of doing this depends on the possi- 

 bility, which, as will appear, is often present, of using treatments 

 of short duration followed by a return of all the plants to the 

 same conditions where the same rate of growth will be maintained. 

 Or treatments may be found that have demonstrably little direct 

 effect on vegetative growth rate. Another method, often combined 

 with one of these, is to avoid the use of time as a variable. 



Instead of time, some index of the rate of vegetative growth 

 can be used as an independent variable. The most common such 

 index is simply the number of new leaves or nodes produced in or 

 after treatment before the designated floral stage appears. The node 

 or leaf index can be substituted for the time scale, and systems 

 can be produced that are analogous to those using time. These 

 matters of scale are not trivial. For instance, an experiment on a 

 time scale might show that treatment A caused 45 percent flower- 

 ing and treatment B 95 percent flowering after 20 days; the 

 same results on a nodal scale (also after 20 days) might be: A, 

 100 percent flowering by the third new node; and B, 10 percent 

 flowering by the third new node. Results that "differ" as much as 

 this are not uncommon and require care in interpretation. The 

 reader may find it instructive to invent reasonable data from 

 which such values could arise. 



Naturally, the choice of scale depends on the intention of the 

 experimenter. For practical agricultural or horticultural purposes, 

 emphasis is often placed on flowering time. Investigations on more 

 fundamental questions however, such as the existence or non- 

 existence of flower-inducing hormones, are bound to be concerned 

 with flower initiation or development relative to vegetative growth. 

 In the best practice, results are reported in sufficient detail so that 

 the entire developmental situation can be assessed. Very few factors 

 affect flowering exclusively, without modifying vegetative growth. 

 Whether the changes are brought about indirectly, as a result of 

 flowering, or directly, by the factors causing flowering, a plant 

 which is flowering frequently differs from a vegetative one of the 

 same age in height, branching, leaf shape, or pigmentation (to 

 name only a few characteristics), and not simply in the production 

 of flowers. Such changes may provide clues to the mechanisms 

 underlying flower initiation, or they may be effects of flower 

 development itself; in the cases studied so far, it is not clear which. 



