has been critically investigated. It lias been learned that many kinds 

 of plants are much more sensitive to photoperiodic conditions when 

 grown at certain temperatures than at others. Some varieties 

 become more sensitive after they have been subjected to certain tem- 

 peratures for various periods of time. Such exposure is often referred 

 to as preconditioning. This relation to temperature is of great eco- 

 nomic importance, as shown by peach and apricot trees, which leaf 

 out very slowly in the spring or shed their flower buds prematurely 

 unless they have experienced a certain number of hours or days of 

 relatively low temperature during the winter or early spring. Many 

 bulbous plants, some grains, and various other crops grow more 



vigorously after they have been 

 subjected for a time to low tem- 

 peratures. Plants may be pre- 

 conditioned during the seed 

 stage, during periods of relative 

 dormancy, or while they are in 

 active growth. Preconditioning 

 may occur naturally in northern 

 latitudes at certain times of the 

 year, or it may be provided 

 readily wherever facilities for 

 cold storage are available. 



The photoperiodic conditions 

 of a region are the most nearly 

 constant recurrent environ- 

 mental factors to which plants 

 are subjected. The photoperiod 

 and the temperature which pre- 



Some onion varieties when grown 

 where the photoperiod is long produce 

 and mature bulbs, but they fail to form 

 bulbs when grown where the photoperiod 

 is short. 





