Seasonal Periodicity 225 



cereals, and timothy. If such plants are grown under daylengths 

 shorter than the critical photoperiod, stems tend to be shorter and 

 flowering is suppressed (Fig. 6.21). Other effects may also be pro- 

 duced in the plant, as illustrated by long-day potatoes which produce 

 the best tubers when daylength is below the optimum for shoot 

 growth. 



Fig. 6.21. Control of flowering and of vegetative growth by daylength in 



timothy, a typical long-day plant. The daily exposures (in hours) to light are 



indicated on each container; C = natural daylength. Photo taken in July. 



(Evans and Allard, 1934.) 



In the growing season outside the tropical zone short days occur 

 both in the early spring and in the late summer. Certain short-day 

 plants require a long growing period before they are sufficiently 

 mature to react to the flowering stimulus, and such plants can flower 

 only in the shorter days following the summer solstice. Familiar 

 plants that can bloom naturally only late in the year are tobacco, 

 goldenrod, aster, dahlia, ragweed, cosmos, and chrysanthemum. In 

 daylengths above the critical abnormally great vegetative growth 

 takes place and flowering is much delayed or entirely inhibited (Fig. 

 6.22). Short-day onions and beets develop the largest storage organs 

 under photoperiods that are longer than those best for the growth of 

 the upper part of the plant. 



The plants that bloom in the short days of early spring are mostly 

 perennials in which the flower buds were set the previous autumn. 

 The few annuals blooming at this season either germinated during 

 the winter or are so small that they need little time for vegetative 



