The Light Relation 



421 



By the use of the protected electric arc during half of the 

 night Bailey was able to hasten lettuce two weeks. The 



FIG. 115. Lettuce of the same age under normal sunlight (above) and 

 with electric arc a part of the night in addition (below). [After Bailey.] 



naked electric arc yields light distinctly injurious to the 

 majority of crops. This injury is due to the richness in 

 ultraviolet rays, which, as already shown, are destructive 

 to protoplasm. When screened by glass, clear or opales- 



