GROWTH OF PLANTS 131 



early fall. A temperature too high at the beginning of growth 

 is one of the chief causes of the development of weak plants 

 yielding a poor crop. This commonly results from late sowing. 

 This rule, however, cannot be applied to thermophilous plants 

 of tropical regions. 



Certain plants may show a pronounced need for low tempera- 

 ture. Such are the winter cereals, which are usually sown in 

 autumn and, after a prolonged winter period, produce seed during 

 the following summer. If sown in the spring, they will tiller 

 throughout the summer without reaching the fruiting stage. 

 Formerly it was thought that winter plants require a rest period. 

 Later, however, it was found that if winter cereals were planted 

 after having germinated at a temperature about 0°C., heading 

 could be obtained the same year without any inhibition in devel- 

 opment. Winter cereals, therefore, may be said to require cold 

 in the early stages of their development (Gassner, Lysenko, 

 Maximov). Instead of allowing them to germinate in the cold, 

 it is better to keep seeds of winter cereals that have just started 

 germination for a prolonged period at a low temperature; this 

 method makes it possible to obtain a yield the first year. 



This method has been given the name of ''jarovisation," or 

 ''vernalization," of winter crops. Further investigations of 

 Lysenko have shown that the requirement of a low temperature 

 in the beginning of development applies not only to winter 

 cereals but also to many summer crops. Such crops, in passing 

 the stage preceding reproduction, designated by Lysenko the 

 "stage of jarovisation," require during a definite interval of 

 time a certain temperature. The method of jarovisation 

 elaborated by Lysenko has been given wide application during 

 recent years in agricultural practice in Russia, especially in case 

 it is important to accelerate the development of plants, e.g., in 

 arid and in polar regions. As this method is most intimately 

 connected with ideas of the stages of development of plants, this 

 question will be considered further in Chap. XII, which is devoted 

 to problems of development and reproduction. 



31. Influence of Light on Growth. Daily Periodicity of 

 Growth. Formative Influence of Light. — In contradistinction 

 to temperature, light is not a factor necessary for mere growth. 

 Practically all plants can grow in darkness, while some plants, 

 such as the bacteria and many fungi, can complete their whole 



