6i2 FACTORS AFFECTING GROWTH 



cussion will be limited to two of the more striking examples of such phe- 

 nomena. 



For some years it has been known that exposure of the soaked seeds of 

 winter wheat to temperatures just above the freezing point for a considerable 

 period speeds up the reproductive development of the resulting plants. For 

 example, if soaked seeds of the Turkey Red variety of winter wheat were 

 exposed to a temperature of i° to 30° C. for 9 to 10 weeks before sowing, it 

 was found that the inception of heading was greatly speeded up (Lojkin, 

 1936). At 16-22° C. and in a day-length of 15-16 hours plants from un- 

 treated seeds required about 150 days to reach the heading stage. For treated 

 seeds the total period from the time of beginning the cold treatment to 

 heading was about 1 10-120 days. Such treatments are now generally re- 

 ferred to as the vernalization of seeds. 



With the aid of such treatments winter cereals can be planted in the spring 

 and will produce grain the same growing season. The practical importance 

 of this phenomenon, which is an excellent example of physiological precon- 

 ditioning, has been stressed in recent years by Lyssenko and other Russian 

 workers. Recent investigations have shown that the seeds of a number of 

 other species besides winter wheat can also be vernalized. 



Thompson (1933) has shown that the reproductive development of cer- 

 tain vegetable crop plants can be greatly influenced by pre-treatments at dif- 

 ferent temperatures. This type of physiological preconditioning is therefore 

 in some respects similar to the process of vernalization. For example, when 

 celery plants which had been grown in a greenhouse (60-70° F.) from the 

 middle of February until April i were exposed for 30 days to a temperature 

 of 40-50° F., seed stalks were produced by 74 per cent of the plants after 

 transplanting into the field. In another group of plants from the same 

 planting, set in the field at the same time, but not given a pre-treatment at a 

 lower temperature, not a single seed stalk was produced. 



Discussion Questions 



1. In critical physiological experiments it is often desirable to work with a 



number of plants of identical genetic constitution. How can this be ac- 

 complished? 



2. Tobacco leaves for cigar \vrappers are grown in Connecticut under canvas 



shades. Why are larger leaves developed under these conditions than 

 when the plants are grown in full sunlight? 



3. Flax plants are grown for fiber in moist regions in which days are long, 



and often cloudy. When grown for oil the plants are grown in drier regions 

 in which there is much clear, bright weather. What explanations can you 

 suggest? 



4. Why are north-facing slopes preferred to south-facing slopes as sites for 



orchards? 



