LIGHT AND VEGETATION 143 



suitable for supplementary lighting? On the gillyflower, a 

 long-day plant, flowering is stimulated by the orange red, the 

 red and the extreme red. The green, blue and violet rays, of 

 shorter wave-length, and the infra-red, have no effect. On a 

 short-day plant, Salvia^ which normally flowers in the 

 autumn, artificial lengthening of the day prevents flowering; 

 there again, it is only the red hght which is effective in sup- 

 pressing the formation of buds when supplementary lighting 

 is given at night. If these observations prove to be of general 

 application, incandescent lamps, rich in red radiations, will 

 be satisfactory. Neon tubes would be even better. 



Treatment by hght can give more complex results con- 

 nected with flowering. For instance, deformed buds will 

 appear on chrysanthemums if the plants are kept in darkness 

 for ten days in July and then left in the Ught for the natural 

 length of the day. 



If the period of darkness is prolonged until the buds 

 appear, their development is no longer subject to the same 

 conditions and they will still be formed. This does not mean 

 that the light has no effect; if the days are lengthened, the 

 capitula will be fuller and the ligules narrower and tubulous ; 

 if the days are short, the flowers will be deeper-set and the 

 ligules flatter and more unfolded. 



But the length of the day affects many other processes 

 besides flowering, and there are many different actions which 

 are still grouped under the name of photoperiodism. 



A late variety of potato, McCormick, cultivated in 

 summer at a rather high average temperature, remained 

 without a tuber when supplementary Ughting was given from 

 nightfall to midnight; a subterranean eye, which would 

 normally have formed a tuber, lengthened into a new stem. 

 From other experiments with various daylengths, the con- 

 clusions were that the best useful crop resulted from a day- 

 length of thirteen hours ; nevertheless, longer daylengths give 

 larger plants and a greater total weight — stems, leaves and 

 tubers included. 



These conclusions obviously relate only to this particular 



