THE EFFECT OF LENGTH OF DAY 



121 



I>ength of day is by no means the only factor concerned in de- 

 termining the time of flowering and frniting. In many cases it has 

 been shown that the critical length of day for reproductive activities 

 may be altered by changing the temperature. Plants have also 

 been caused to bloom early by a process called vernalization, which 

 consists of soaking the seeds and then subjecting them to chilling 

 temperatures above freezing for several days. 



Many plants, such as buckwheat, chickweed, and dandelion, are 

 neither long-day nor short-day plants but belong to an intermediate 



Fig. 57. —Aster linariifoUus L. Plants in box on left exposed to light from 

 9 A.M. to 4 P.M. daily. In full bloom when photographed, June 21. Plants in 

 box on right left out of doors during the test. Showed no indications of flower 

 heads when photographed, June 24. (After Garner and Allard.) 



or neutral group which is little, if any, affected by length of day. In 

 fact, the number of species belonging to this group is probably far 

 larger than that of either of the other two groups. The group to 

 which any species belongs can usually be determined quite readily 

 by growing it under daily light exposures of ten and eighteen hours 

 and observing under which, if either, its reproductive phenomena are 

 accelerated. 



Usually vegetative growth is favored by a length of day that is 

 just the reverse of that most favorable for flowering and fruiting. 



