PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 427 



but do not mature in time and are therefore injiu'ed by winter 

 frosts. In species of northern origin, the ripening of the shoots 

 is normal with a long exposure. The effect of the length of day 

 upon the inception of dormancy will be discussed in detail in 

 Art. 103. 



Photoperiodic experiments are rather simple. They are based 

 on the practice of covering the plants in morning and evening 

 hours by opaque cloth or paper, thus decreasing the day length 

 and increasing the night. Larger plants are covered with ply- 

 wood covers mounted on wheels. For crops grown in pots, it is 

 convenient to use trucks, which can be pushed in the evening into 

 a dark house and in the morning back into the open. Opaque 

 cloth covers are used in the United States to cover beds of aster, 

 chrysanthemum, etc., grown outdoors or in greenhouses. 



The nature of the photoperiodic effect is as yet unknown. 

 Almost certainly it is not connected directly with the process of 

 photosynthesis. This is proved by the fact that a very weak 

 light, 5 to 10 candle-meters, is sufficient for the production of the 

 photoperiodic effect. At such light intensity below the com- 

 pensation point, photosynthesis is less than respiration. The 

 necessity of excluding the morning and evening hours, i.e., of 

 prolonging the night, also confirms the statement mentioned 

 above. Placing the plants in a dark chamber for several hours 

 in the middle of the day only retards the accumulation of organic 

 substance but does not exert any influence upon the time of 

 blooming. 



According to Lysenko, photoperiodism is the manifestation of 

 definite light and darkness requirements of the plants during the 

 second stage of development, designated by this author as the 

 ''photo stage" and following directly the temperature, or 

 yarovisation, stage. For transition to reproduction, some plants 

 require at this stage a definite period of complete darkness; 

 others, a certain length of day. The first may be called really 

 ''long-night plants''; for the longer the night is, the sooner these 

 plants will obtain the required number of hours of darkness. 

 The second group of plants, which require during their light 

 stage, not darkness, but a definite length of day, belong to the 

 so-called "long-day plants." As this effect of hght and darkness 

 is not connected with photosynthesis, the light stage may be 

 passed through at an early age even in germinating seeds that 



