LIGHT AND VEGETATION 139 



Obviously such a study is lengthy, but clear and definite 

 results have already been obtained. Here we give a few 

 characteristic examples in which the influence of the day- 

 length is manifested by particular aspects. 



Short-day Plants 



Two pots of chrysanthemums, A and B, are cultivated when 

 the natural day is short and the night long. They are tended 

 in exactly the same way, except that at night A is illuminated 

 with an electric lamp, while B remains in darkness. During 

 the day, both are illuminated normally in natural daylight. 



Pot A receives a nocturnal illumination of about 50 lux — 

 1,000 times lower than the natural light on a rather bright 

 day. This supplementary Ught, too weak to modify ihe 

 nutrition of the chrysanthemum through photosynthesis, 

 reacts profoundly on its development, and no flower buds 

 appear. 



On the other hand, pot B, which is not illuminated during 

 the night, flowers normally. Flowering has been prevented 

 in pot A by a very small quantity of supplementary light 

 given throughout the night. 



If the same experiment is repeated with a stronger lamp 

 for only half an hour in the course of the night, flowering is 

 still suppressed. In order to flower, the chrysanthemum 

 requires long uninterrupted nights of from twelve to sixteen 

 hours, according to the variety. 



The flowering of the chrysanthemum can be retarded at 

 will by night illumination before the formation of the buds, 

 that is, from 15th August. According to some American experi- 

 ments, an illumination of 100 lux is perfectly suitable and can 

 be interrupted every other minute, giving the same result 

 more economically than by continuous lighting. 



Conversely, if the length of the day is shortened in the 

 summer, flowering will be produced earUer, and this process 

 is already being used to advance flowering by one to two weeks. 

 At five or six o'clock in the evening, black cloth, which may 

 be rubberized or not, is stretched on frames surrounding the 



