12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



Light within certain wave length and intensity Hmits is generally 

 considered essential to chlorophyll formation, although it is true that 

 certain pine seedlings and a few algae become green in darkness. 

 As early as 1874 Wiesner (21) found that chlorophyll was formed 

 in plants when illuminated by light passed through solutions of 

 potassium bichromate and copper sulphate. These filters divided the 

 visible spectrum into two parts. He also showed that no greening 

 occurred in the nonluminous heat rays. 



Sayre (15) studied the development of chlorophyll in seedlings of 

 several varieties of plants by growing them under Corning glass ray 

 filters and noting the relative greenness as compared with seedlings 

 grown in full daylight. No greening was observed in wave lengths 

 longer than 6,800 A. In the visible spectrum he found that for ap- 

 proximately equal energy values the red wave lengths were more 

 effective for the development of chlorophyll than the green and the 

 green more effective than the blue. The effectiveness apparently in- 

 creased with increasing wave length up to 6,800 A, where it ended 

 abruptly. 



Shirley (16) working with several types of plants grown in the 

 spectral greenhouses at the Boyce Thompson Institute for Plant 

 Research found an increase in chlorophyll concentration with de- 

 creasing intensity to a point so low as to hazard the health of the 

 plant. At approximately 10 per cent of full sunlight intensity the 

 chlorophyll content was practically the same for wave lengths 3,890- 

 7,200, 3,740-5,850 and 4,720-7,200 A. 



Plants grown at high altitudes were found by Henrici (9) to con- 

 tain less chlorophyll than similar ones grown at lower altitudes. As 

 noted by Spoehr (17) "this is presumably due to the greater 

 intensity of light at higher altitudes. However, whether the lower 

 chlorophyll-content of plants grown under high illumination intensity 

 can be directly ascribed to the destructive action of such light (espe- 

 cially the red-yellow rays) on chlorophyll, does not seem entirely 

 established." 



Tomato plants grown under ordinary greenhouse conditions and 

 then placed under continuous artificial illumination were found by 

 Guthrie (8) to show a marked decrease in their chlorophyll content in 

 a few days. The leaves turned yellow and later showed necrotic areas. 

 By analysis the chlorophyll decrease was greater on the dry-weight 

 than the fresh-weight basis, due to a very large increase in carbo- 

 hydrates. It is interesting to note that this author found a consistent 

 lowering of the chlorophyll a/chlorophyll b ratio. Both a and b de- 

 creased under the effect of the light, but a decreased faster. 



