416 PLANT PHYSIOLOGY 



is also dependent upon the conditions of nutrition, nevertheless 

 the time of the transition to reproductive development is not 

 determined by the accumulation of one nutritive substance 

 but may be dependent upon specific factors, probably flower- 

 producing catalysts (Murneek). 



96. Theory of Lysenko on the Stages in Development of 

 Plants. Yarovisation, Its Theoretical Foundation and Practical 

 Significance. — The most important advance in the physiology 

 of development is the method of shortening the vegetative period 

 elaborated by Lysenko, which has become widely known as 

 ''yarovisation." 



The method of yarovisation of plants is very simple and 

 convenient. It is based on the principle that seeds just begin- 

 ning to germinate are subjected to definite external influences 

 that accelerate certain physiological processes causing a transition 

 to the reproductive stage. The earliest case of yarovisation 

 that has been studied is the transformation of winter into spring 

 crops, or rather the stimulation of heading of winter crops during 

 the same season that they were sown, similar to spring crops. 

 In this case, the leading factor of yarovisation is lowering the 

 temperature during a sufficiently prolonged period. 



According to Lysenko, winter crops sown in spring do not 

 fruit because at a definite period of their development they 

 require a low temperature for 20 to 50 days, depending on the 

 variety. The temperature must be below 10°C., but 2 to 0°C. 

 is preferable. Winter crops sown in the field in autumn are 

 subjected under natural conditions to the influence of the 

 temperatures necessary for the process of yarovisation. After 

 they have remained long enough at low temperatures, they do 

 not require any more cold exposure for their further development. 

 When sown in spring, the winter crops do not obtain the cold 

 exposure that is indispensable for their transition to reproduction 

 and are therefore incapable of heading. When sown in spring, 

 they remain vegetative throughout the summer, tillering pro- 

 fusely and producing continuously new leaves but no culms. 

 If they are yaro vised artificially, i.e., if during germination they 

 are kept cold for a sufficiently long period, they will pass through 

 the stage of development that requires a lowered temperature 

 and will head without retardation even when sown in spring 

 (Fig. 131). 



