PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 429 



conditions when compared with varieties that are slow to bloom 

 but ripen rapidly. 



When plants are placed in conditions of day length under 

 which they cannot bloom at all, e.g., when either short-day 

 plants are cultivated beyond the Arctic Circle, or long-day 

 plants in the tropics, they completely lose their capacity to 

 flower and continue vegetative growth for an indefinite period. 

 They can bloom only when the day reaches its proper length. 

 For short-day plants in the north, this will be in autumn. This 

 leads to the phenomenon, first observed by Garner and Allard. 

 viz., that several varieties of soybeans, especially sensitive to 

 the length of the day, bloom always at the same time of the year 

 during the same autumn month, independent of the time of 

 sowing. The length of their vegetative period is longer, the 

 earlier they are sown. 



The pecuharities of the stage of yarovisation are of no less 

 importance in determining the length of the vegetative period. 

 They are especially noticeable when spring and winter varieties 

 are compared. Spring varieties mature the first summer, while 

 winter varieties are sown in autumn and thus their vegetative 

 period is considerably longer. But if winter crops are yarovised, 

 i.e., are made to pass through the stage of yarovisation in the 

 form of seeds just beginning to germinate, they will head the 

 first summer and frequently even earlier than many spring 

 varieties. Hence, winter and spring varieties differ not in the 

 length of their vegetative period but in their different temperature 

 requirements during the stage of yarovisation. Winter crops 

 need for the passing of this stage a prolonged period at a tem- 

 perature from to 5°C., while spring crops can do without it. 

 Depending upon the time required for passing the stage of yaro- 

 visation, different degrees of ''winterness" may be distinguished, 

 according to Lysenko. Hence, the difference between winter 

 and spring crops is not of a qualitative but only of a quantitative 

 nature, and both groups are connected by a series of transitional 



forms. 



The method of rapid determination of winter and spring 

 seeds that has recently obtained wide use in the practice of seed 

 control stations is also based on the different reaction of varieties 

 to temperature. If unknown seeds are sown in a warm chamber 

 and are further grown under continuous illumination, sprmg 



