226 



Light 



Fig. 6.22. Control of flowering and of type of vegetative growth by daylength 

 in garden chrysanthemum, a typical short-day plant. The daily exposures (in 

 hours) to light are indicated on each container; C = natural daylength. Photo 

 taken in July. Flowering occurred as follows: 10 hr, July 6; 12 lir, Oct. 14; 13 

 hr, Oct. 19; 14 hr, Oct. 31; natural day, Oct. 31. (Allard and Garner, 1940.) 



growth before flowering. Early spring flowers growing on the floor 

 of the deciduous forest are able to take advantage of the great amount 

 of light available before the tree leaves are fully developed. 



Since short days occur later in the summer at higher latitudes, the 

 northward distribution of short-day plants may be stopped by the 

 lack of sufficient time for the ripening and hardening of the seed be- 

 tween the occurrene of the critical daylength and the occurrence of 

 a killing frost that terminates the growing season. Ragweed, for 

 example, which flowers when the days have shortened to 14^ hours, 

 will begin its reproductive phase about the first of July in Virginia, 

 and hay-fever sufferers notice its pollen in the air by the middle of 

 August. In northern Vermont the summer days are not reduced to 

 14^ hours until after the first of August, with the result that seed 

 formation is not completed before cold weather, and consequently 

 ragweed is unable to establish itself in abundance at this latitude. 

 The distribution of long-day plants toward the equator is limited by 

 the reverse action of this aspect of the light factor. Species of Sedum 

 that require a daylength of 16 hours or more will flower abundantly 

 in Vermont but will not bloom in Virginia ( Naylor, 1952 ) . 



Plant species in which flowering is not controlled by daylength 

 also occur at all latitudes, but all plants growing at high latitudes must 

 be able to tolerate long days and short nights since these light condi- 

 tions exist during the growing season, and tropical plants must be 



