PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 419 



instance, winter plants not yarovisated may tiller profusely, 

 form numerous leaves, accumulate large amounts of organic 

 substances, and at the same time remain in the same vegetative 

 stage of development. Transition to the following stage will 

 be possible only after the plants have remained for a sufficiently 

 long time under conditions favorable to yarovisation, i.e., after 

 they have been subjected to the influence of low temperatures 

 with a sufficient access of oxygen. 



Not all of the internal stages of development have been 

 sufficiently established as yet. Lysenko describes two: the stage 

 of yarovisation, requiring the conditions that have already been 

 stated above; and the light stage, directly following the pre- 

 ceding one. Winter cereals require for passing the light stage 

 a prolonged diurnal period of illumination, no less than 11 to 

 12 hr., but the best results are obtained with continuous illumina- 

 tion. Low temperature is no longer required. On the contrary, 

 the light stage is accelerated by higher temperatures. But if, 

 after yarovisation, the plants are given an insufficient day 

 length, for instance 10 hr. or less per day, they will not form 

 heads but will continue tillering, like plants that have not been 

 yarovised. 



Conditions necessary for the transition from one stage to 

 another may be different for plants of various physiological 

 types. In yarovising the seeds of millet, for instance, they 

 must be subjected during 5 to 7 days to a temperature of about 

 20 to 25°F., and their light stage proceeds best in a short day 

 or, still better, in complete darkness (Fig. 132). Similar con- 

 ditions are required for the yarovisation of corn, cotton, and 

 other thermophilous plants of southern latitudes. 



According to Lysenko's theory, each stage of development is 

 accompanied by quahtative irreversible changes, which occur 

 in the protoplasm of the plant cells, chiefly in the meristematic 

 region. What exactly these changes are, has not yet been 

 estabhshed with certainty. It has been observed that during 

 the stage of yarovisation changes occur in the oxidation-reduction 

 potential in the meristematic cells (Richter, 1933). Some 

 authors assume that specific hormones of reproduction are 

 accumulated similar to the sex hormones of animals. 



The theory of yarovisation has not been developed completely 

 as yet. Lysenko, as weU as other investigators, is actively 



