418 PLANT PHYSIOLOGY 



Lysenko's method of artificial yarovisation is based on the 

 interesting feature that the plants are subjected to the influence 

 of the complex of external factors indispensable for the transi- 

 tion to reproduction in their earliest germination period, and 

 not in a mature state when it is difficult to reproduce the neces- 

 sary complex even under laboratory conditions, not to mention 

 in the field, where it is almost impossible. In order to retard 

 growth during the comparatively prolonged period of yarovisa- 

 tion, 20 to 50 days, the seeds are given only 50 per cent of the 

 total amount of water necessary for germination. With such a 

 degree of moistening, most of the biochemical processes obtaining 

 in the cell during the active state continue their course, while 

 growth is considerably retarded. Such a curtailment of growth 

 is necessary for the sake of convenience in the transport and 

 sowing of yaro vised seeds; for otherwise the seedlings with 

 elongated rootlets and plumules might be injured. 



Lysenko bases his method of yarovisation on the following 

 theoretical concepts: 



1. Growth and development are not identical phenomena. 



2. The development of an annual seed plant consists of separate stages. 



3. The stages always proceed in strict sequence, and the successive stage 

 cannot begin so long as the preceding stage has not terminated. 



4. The different stages of development of the same plant require various 

 combinations of external conditions. 



Lysenko lays greatest emphasis upon the idea that growth 

 and development are two different phenomena. Growth is the 

 increase in size of a plant without deep qualitative changes of 

 the growing parts. Development according to him is the 

 alteration of the separate stages that advance the plant toward 

 reproduction. The transition from one stage to the other is 

 accompanied by profound qualitative changes, but these may 

 not be accompanied by any apparent external morphological 

 evidence. 



Hence, the physiological stages of development differ sharply 

 from the external well-marked phases of development, which are 

 commonly noted in phenological observations, when the vege- 

 tation of a plant is described. Such phases as the appearance of 

 the seedlings, the opening of the first and second leaf, tillering, 

 shooting, heading — all these denote the separate phases of 

 growth independently of the stages of development. For 



