

r«r 



Pyrethrum produces flowers, which are the source of insect powder, only when grown in 

 localities where the photoperiods are long. Under short photoperiods the plants remain 

 as rosettes. 



vail in any given region arc as important as moisture and soil fertility 

 in determining the specific behavior and economic value of varieties 

 which it may be desired to grow. Although the length of photoperiod 

 in any particular locality changes from day to day throughout the 

 season, the photoperiods of corresponding days in successive years are 

 always of the same length. Therefore, the seasonal variations in this 

 environmental factor can be predicted far more accurately than the 

 variations in temperature, moisture, or soil fertility. 



It is not a difficult task to determine under controlled experimental 

 conditions many of I he special photoperiodic and temperature require- 

 ments of individual varieties. By making such determinations one 

 can readily tell in advance whether certain varieties of soybean-, pea-. 

 onions, barley, and many other plant- arc adapted to the photo- 

 periodic conditions known to prc\ ail in a given locality. Such detailed 

 experiments can be substituted, at least in part . for the far more costly 

 necessity of growing or at tempt ing to grow crops in any given locality 

 to obtain the same information. If a variety were found by experi- 

 ment to be unadapted, it would not be grown in the held. Either an 

 existent variety that was adapted to the locality would be chosen, or il 

 none was known, one or more which would be suitable could be bred. 



18273 i-' 



