photoperiodic conditions in any area. If no such photoperiodically 

 adapted varieties arc available, they may be developed. 



Experiments of the United States Bureau of Plant Industry are 

 now in progress toward the development of new strains of onions 

 resistant to diseases and insects and adapted to the photoperiodic 

 conditions in various sections of the United Slates. By making the 

 desired crosses and then growing the seedlings under experimental 

 conditions in which the temperature and photoperiod are carefully 

 controlled, those which are unfit are being eliminated. Only those 

 thai are most likely to succeed under the specific natural conditions 

 prevailing in those sections where it is desirable to produce them are 

 retained. After selection these plants serve as parent- for further 

 crosses that provide the seeds for use in the tesl plot- to be grown in 

 the different chosen regions. In simplest term-, what this procedure 

 provides is a more ready means of selection and increase of new 

 photoperiodically adapted varieties which may combine the desirable 

 qualities of varieties now grown. 



More than hall' the time required to establish the foundations of 

 new strains of onions may he saved by planting the seed under con- 

 trolled environment a- soon a-- it mature- in the summer instead of 

 waiting until the following year to plant it in the field. Also, such 

 seed stocks are of known higher quality and greater uniformity than 

 if there ha- been only genera] selection under held conditions. 



This same general procedure may he applied to testing ami im- 

 proving many vegetables, especially those classed a- biennials. By 

 combining these methods with those followed in the etlieient produc- 

 tion of sugar-beel seed, the problem of growing Large crops of irood 

 seed in much less time may he more readily solved. 



Many plants grown principally for their flowers are similar in 

 photoperiodic behavior to food and forage crops. In many instances 

 seasons of bloom arc directly related to the seasonal temperatures 

 and particularly to the Length of the periods of daylighl prevailing 

 at different seasons. Tin- i- especially noticeable in northern lati- 

 tude-. In many plants photoperiodic adaptation i- a definitely fixed 

 character, and their period of blooming is regulated by it. Hut as 

 flowers are generally cultivated on a less extensive scale than are 

 ino-t food crop-, it i- commercially feasible to give them much more 

 detailed attention. In greenhouses and to some extenl out oi door-. 

 artificial regulation of the temperature and photoperiodic condition- 

 may he done with prolii Any prevailing natural or seasonal photo- 

 period can he shortened by completelj darkening the plants each 

 day with black cloth or l>\ placing them in dark chamlier-. ll ma\ 

 he Lengthened In the use of electric Light. Both practice- arc now 

 widely used by florist s to hasten or to dela\ the blooming oi chrysan- 

 themums and some other crop- and llm- to gain advantage oi market 

 demands. 



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