142 LIGHT, VEGETATION AND CHLOROPHYLL 



grafted on a long-day species, B, which produces in the 

 summer the substances for flowering, they spread to A and 

 cause it to flower. The experiment has been successful with 

 Jerusalem artichoke as the short-day plant A, grafted on a 

 sunflower as the long-day plant B. It was found necessary to 

 strip plant A of its leaves, which shows that the principal 

 action of Ught in preventing the flowering of short-day plants 

 is on the leaves and not on the buds where the transformation 

 is most apparent. 



More direct experiments have proved that the action of 

 light on flowering is eff'ected through the leaves, and through 

 adult leaves, not through those which are too young. We have 

 seen that the chrysanthemum can be brought into flower 

 before the normal date if it is withdrawn from the Ught in the 

 evening and in the morning to lengthen the night artificially 

 but allowed full dayUght in the middle of the day to ensure 

 its normal nutrition. But if the whole plant is not withdrawn 

 and a black cloth is wrapped round the lower part to cover 

 only the adult leaves, which are then subjected to long nights, 

 flowering is stimulated just as effectively although the terminal 

 buds, where the flower buds appear, are still subjected to short 

 nights. Conversely, if only the terminal buds are covered, 

 flowering is not produced. It is therefore the length of the day 

 on the adult leaves which is important and it is in them that 

 the chemical reactions necessary for the production of flowers, 

 and then of seeds, take place. 



Other experiments in which buds have been made to 

 appear on the lower part of the chrysanthemum stem by 

 partial covering of the plant show that, if the adult leaves 

 only are given a long night, flowering is produced as effectively 

 above them as below them. The substances which cause 

 flowering are therefore transported just as well downwards 

 as upwards. 



These substances circulate through the bark and can be 

 arrested by incisions. 



The influence of the composition of the radiation has 

 scarcely been investigated. Are incandescent lamps the most 



