PHOTOREACTION AND ASSOCIATED CHANGES 425 



the photoreaction to be separated from the complexity of the response 

 which serves as a basis for assay. 



The second simple consequence is that the reaction can both control 

 responses at exceedingly low irradiances by its great sensitivity to 

 radiation as well as in lull sunlight by the reversible balancing. Thus, 

 etiolation phenomena of seedlings are controlled as well as leaf size 

 and internode lengths at high light intensities. These were illustrated 

 for bean seedlings by Dr. Downs. 



The following phenomena have been shown by their photoreversi- 

 bility and by their action spectra to be under the control of the photo- 

 reaction: 



1. Flowering, photoperiodism: (a) Xanthiiim, short day; {b) 

 Hordeiim vulgare, long day 



2. Seed germination, 20 species 



3. Fern spore germination 



4. Elongation (leaf, petiole, stem), 4 species, 14 varieties 



5. Plumular hook unfolding 



6. Pigment formation: {a) Anthocyanin (Brassica oleracea var. 

 rubra), (b) Tomato cuticle 



More limited information indicates that the reversible photocontrol 

 also interacts with: 



7. Epinasty 14. Sex expression 



8. Leaf abscission 15. Root development 



9. Bulb formation 16. Chromosome breakage fol- 



10. Casparian strip formation lowing x-irradiation 



11. Rhizome production 17. Plastid generation 



12. Phylloidy of tracts 18. Protochlorophyll regenera- 



13. Succulency tion 



Any of these responses, accordingly, can be used for study of the 

 photoreaction. 



The pigment system is possibly ubiquitous in seed plants as, to the 

 extent of "present testing, one or the other of the many responses is 

 evident. A plant might fail to show obvious photoperiodic control of 

 flowering but still respond in other ways. This is the case for the 

 tomato, which responds to control of internode elongation and which 

 sometimes has hght-sensitive seed, but has no evident flowering con- 

 trol. 



