SEED GERMINATION 537 



they show a decrease of protein nitrogen accompanied by an accumulation 

 of nitrogen in the germination medium. This, according to these authors, 

 indicates that the desensibihzation caused by prolonged sojourn in dark- 

 ness may possibly be connected with the activity of proteolytic enzymes. 



5. CONCLUSIONS 

 5-1. SUMMARY OF FACTS 



Before discussing the theories of photoblastism set forth by differ- 

 ent authors, we would like to summarize those facts which are well 

 established: 



1. The photoblastic reaction, photosensitivdty, and photore((uirement 

 are highly dependent upon temperature. 



2. The visible spectrum contains bands of stimulating and inhibiting 

 light. The action spectra of positively and negatively photoblastic seeds 

 appear to be very similar. 



3. Treatment with different chemicals, foremost among them being 

 potassium nitrate and thiourea, brings about full germination of most 

 positively photoblastic seeds in darkness, abohshing the photorequire- 

 ment of the treated seeds. 



4. Other compounds, like coumarin, bring about positive photoblastism 

 in certain seeds that are normally indifferent to light. 



5. The photoblastic reaction depends upon the oxygen and carbon 

 dioxide contents of the surrounding atmosphere. 



6. Photoblastic seeds are made dormant by keeping them when soaked 

 for some time under Hght conditions adverse to their germination. 



7. Dry seeds are not photosensitive. Photosensitivity becomes appar- 

 ent some time after imbibition of water begins, reaches a peak, and then 

 decreases. 



8. When photoblastic seeds are exposed to light when wet and then 

 dried, the light effect is preserved. 



9. By the removal of fruit coats, seed coats, endosperm, or hulls, photo- 

 blastism of many photoblastic seeds is aboHshed; they then germinate 

 well under otherwise adverse light conditions. After the removal of the 

 coats, embryos do not develop thermo-, skoto-, and photodormancy. 



10. With afterripening the photoblastism of most photoblastic seeds 

 changes. Generally photoreciuirement decreases and photosensitivity 

 increases with increasing afterripening. 



5-2. THEORIES 



As point of departure we take Gassner's theory (191 la, b) of photo- 

 blastism, which he developed in his basic research on Chloris ciliata. The 

 hulls render the entry of oxygen into the germinating grain more difficult. 

 This germination inhibition is counteracted by light; i.e., the hulled grains 



