1094 BIOLOGICAL EFFECTS OF RADIATION 



in darkness if given the proper nutrients, and Sachs too, demonstrated 

 that in seedUngs of Pinus sylvestris and Picea sp. the cotyledons are 

 green in darkness and form functionally active chloroplasts. This 

 research was enlarged upon by Burgerstein (5), taking into consideration 

 the eifect of temperature on the greening in complete darkness. Seed- 

 lings of most of the Coniferae and of the genus Ephedra turn green in the 

 absence of light, the most favorable temperature being 15° to 25°C. 

 Lubimenko found, however, that in the greening of coniferous seedlings 

 in the dark less chlorophyll is produced. It was Pfeffer who first called 

 attention to the fact that the formation of chlorophyll may not necessarily 

 be connected with the presence of light, and its nonformation in darkness 

 in most plants may be due to disturbances of nutrition or pathological 

 conditions. 



Sachs (60) stated that light of moderate intensity is best suited to 

 the formation of chlorophyll in the plant and Famintzin's (18) experi- 

 ments led to the same conclusion. In a rather long series of experiments 

 on sunflowers, tomatoes, tobacco, Geurn, and other plants, Shirley (67) 

 found that chlorophyll concentration increased with decreasing light 

 intensity until the intensity was so low that it hazarded survival. Fur- 

 ther decrease in light intensity caused a decrease in chlorophyll concen- 

 tration. Wiesner (83) believed that this reduced accumulation of 

 chlorophyll in strong light was due to the fact that the decomposition 

 and formation of chlorophyll occurred together, and while the strong 

 light favored rapid formation, it also hastened decomposition. 



Wiesner also investigated qualitatively the effect of wave-length of 

 light on chlorophyll formation and found that in weak hght plants become 

 green more rapidly under the influence of the red, orange, yellow, and a 

 certain region of the green, while in strong light they green more rapidly 

 in the blue rays. Dangeard (7) found that in the seedlings of Lepidium 

 visible greening occurs only in wave-lengths longer than 5100 A. Blanched 

 leaves of spinach turn green between 6800 and 4400 A to a varying 



o 



degree, with a maximum between 6800 and 6560 A and a weaker maxi- 

 mum in the blue-violet. Sayre (63) studied this question more thor- 

 oughly and found that wave-lengths longer than 6800 A are not effective 

 in the formation of chlorophyll in seedUngs of corn, wheat, oats, barley, 

 beans, sunflowers, and radishes. All other regions of the remaining 

 visible and ultra-violet spectrum (to 3000 A) are effective, provided the 

 energy value is sufficient. For approximately equal energy values in 

 these regions, the red rays are more effective than the green, and the 

 green more effective than the blue. The question of the comparative 

 effectiveness of the various wave-lengths of radiation is complicated by 

 the fact that even if equal energy values for each wave-length of light are 

 used, we are still unable to determine how much of any particular wave- 

 length is absorbed and made use of by the reacting compounds which 

 form chlorophyll in the living plant. 



