206 



THE FLOWERING PROCESS 



2 4 6 8 10 12 14 !6 18 20 22 24 



hours of light per day 



Figure A-1 

 Flowering response to different day-lengths, plotted as time required for 

 appearance of flowers. Curves are not based on actual data (except for 

 number 6) but are illustrative of the kinds of responses which might be 

 expected. K following the numbers on the ordinate indicates that these 

 numbers might be multiplied by a constant, and are thus arbitrary. The 

 20K time for the earliest appearance of flowers is also arbitrary. Curve 1 

 is a true day-neutral plant; curve 2 would probably also be classed as a 

 day-neutral plant, although it is slightly promoted by long days. 

 Curves 3 and 4 represent quantitative long-day plants, but 4 is more 

 sensitive than 3. Curve 5 is a true absolute long-day plant. Curve 6 is a 

 true absolute short-day plant (the cocklebur). 



to firmly define the day-neutral response (curve 1 or curve 2 ?). To 

 complicate matters even more, a plant may be completely day- 

 neutral when days-to-flowering are considered as in Fig. A-1, but if 

 the number of flowers per plant is measured instead, then there may 

 be a strong promotion by long days (e.g. Leucanthemum cehennense 

 and Saxifraga rotundifolia). 



