40 PLANT PHYSIOLOGY 



but this formation in the dark is less strong and the green material 

 formed in these cases may not actually be chlorophyll. 



Wiesner investigated the effect of light quality upon chlorophyll 

 formation, using the same method which Sachs had used for deter- 

 mining which wave lengths were best for photosynthesis. In weak 

 light the greening is more rapid in red light, but in strong light 

 the plants green more rapidly in the blue end of the spectrum. 

 This has been explained by assuming that red light is more effec- 

 tive both in the formation and destruction of chlorophyll. In the 

 weak light the destructive action does not appear, but in strong 

 light this action is very pronounced so that the plants under the 

 blue become visibly green more rapidly. Another explanation is 

 that the strong red light has an injurious effect upon some chem- 

 ical reaction which precedes the chlorophyll formation. It is only 

 the visible portions of the spectrum which produce greening; 

 neither the infra-red heat waves nor the ultra-violet cause an 

 appreciable increase in chlorophyll. Sayre, using more refined 

 methods, determined the limits as 300 and 680 m/x, at which regions 

 chlorophyll formation stopped rather abruptly. 



Chlorophyll production also depends upon the temperature 

 with a minimum at 2-4°, an optimum about 30°, and the maximum 

 at 40°. The optimum for chlorophyll formation is thus seen to be 

 about the same as for photosynthesis. 



Certain mineral salts especially those of magnesium and iron 

 are necessary for chlorophyll formation. In the absence of either 

 of these elements the plants are yellowish-white or chlorotic. Only 

 a trace of iron and magnesium are sufficient to produce greening. 

 It was formerly thought that iron entered into the composition of 

 chlorophyll; now, however, it is known that the chlorophyll mol- 

 ecule contains magnesium but not iron. According to Fernald, 

 plants grown in a soil rich in magnesium are of a richer green than 

 those grown in ordinary soils, and Zaitseva (1928-1929) experi- 

 mentally increased the chlorophyll yield of plants by suitable 

 additions of magnesium to the nutrient medium. 



Oxygen is needed for greening. Etiolated plants in a chamber 

 with no or little oxygen present will remain etiolated even in the 

 light. 



Palladin (1891) has shown that carbohydrates must be present 

 before greening occurs. If an etiolated leaf of wheat and one of 

 a bean are removed and floated on water in the light, the former 



