LIGHT AND METABOLISM 



indifferent to the alternating change between darkness and light. 

 Moreover, if a plant of one tyjDe is denuded of leaves and the leaf of a 

 plant with a different cyclic character is grafted onto it, the host-plant 

 assumes the periodicity of the grafted leaf.^ 



Two different responses are well recognized. In summer-flowering plants 

 {long-day plants) which bloom when the spring days lengthen, the formation of 



11 



Fig. 7. — The Effect on Flowerixg of Ixtermittent Light during the 



Night. 



On the /eft are two gladioli (long-day plants) grown in a control green- 

 house with a normal solar day-and-night rhythm ; on the right, two similar 

 plants growai with intermittent light during the night (Boyce Thompson 

 Institute for Plant Research). 



flowers is inhibited in darkness while during the periods of light some substance 

 is presumably formed in the leaves which counteracts this inhibition ; in short- 

 day plants which come to flower when the autumn days shorten, both dark and 

 light periods are necessary for the develoiament of the stimulus, each with opposite 

 effects, one depending on light-energy and the other being inhibited by light. In 

 summer-flowering jolants artificial light during the night promotes flowering 

 (Fig. 7) ; in autumn-flowering plants flowering in short days, light during the 

 day promotes flowering but short joeriods of light during the night prevent it. 



1 Melchers (1936-37), Cailahian (1936-47), Loehwing (1938), Borthwick and Parker 

 (1938-40), Hamner and Naylor (1939), Harder and v. Witsch (1940), Withrow et al. 

 (1943), and others. 



