134 PHOTOCONTROL OF GROWTH 



Fig. 4. Dark-grown Red Kidney bean seedlings after 10 days in continu- 

 ous darkness and after being given 2 min of red radiation, 2 min of red 

 and 5 min of far red, or 5 min of far red only (left to right). 



Dark-grown bean plants also respond to red and far red (Fig. 4) 

 (Downs, 1955; Liverman et al., 1955). Unlike light-grown plants, 

 however, the dark-grown ones must first be irradiated with red. Then 

 the far red reverses the potential effect of the red irradiation. This 

 apparent difference in the responses of light-grown and dark-grown 

 beans are in reality fundamental ones that arise from the form of the 

 pigment at the beginning of treatment. Dark-grown material contains 

 the pigment system predominantly in the red-absorbing form prior to 

 treatment, whereas the light-grown material contains the pigment 

 largely in the far-red-absorbing form. Control of internode lengthening 

 thus illustrates that the pigment system is not destroyed by light but is 

 present and functional in the plant at any time whether it is in light or 

 in darkness. 



REFERENCES 



Borthwick, H. A., S. B. Hendricks, and M. W. Parker. 1952. The reaction con- 

 trolling floral initiation. Proc. Natl. Acad. Sci. U. S., 38, 929-34. 



Borthwick, H. A., S. B. Hendricks, M. W. Parker, E. H. Toole, and V. K. 

 Toole. 1952. A reversible photoreaction controlling seed germination. Proc. 

 Natl. Acad. Sci. U. S., 38, 662-66. 



