THE SYNTHESIS OF FLOWERING HORMONE 



177 



3 6 8 10 13 16 



TIME OF LIGHT INTERRUPTION IN hours after beginning of darkness 



Figure 9-13 

 Effects upon flowering of a one-minute red light interruption (4 photo- 

 flood lamps) appHed at various times during a 16-hr inductive dark 

 period to control plants, and to plants treated before the dark period 

 with ethionine, picolinic acid, or cobaltous ion. Data previously 



unpublished. 



dark period, but they must be applied before the flowering hormone 

 has been translocated out of the leaf. 



The auxins are hormones which are produced in the stem tip, 

 causing elongation of the stem cells below. Since there is more auxin 

 on the dark side of a stem which is illuminated from one side, and 

 on the bottom of a stem laying down, they offer an explanation for 

 why plants grow towards the light and away from gravity. Since 

 their discovery in 1928 by Frits Went, then at the University of 

 Utrecht in Holland, they have revolutionized research in plant 

 physiology, because, as it turns out, there are few plant processes 

 which are not influenced by them. 



In the 1940's it was discovered that applied auxin will inhibit 

 flowering of many plants of all response types. Under certain 

 conditions flowering may even be promoted by auxin. For example, 

 pineapple flowers in response to application of synthetic auxins such 

 as naphthaleneacetic acid. In light of these observations it was 

 theorized that flowering might be a response to changes in the natural 



