CHLOROPHYLL FORMATION 421 



Of 16 species examined by Trebitz (1905), the cereals Hordeum distichum, 

 Triticum sativum, and Avena sativa required the lowest air pressure, 

 10 mm Hg, and Lepidium sativum, the highest, 60 mm Hg. 



Experiments of Friedel (1904) indicated that the quantity of oxygen 

 rather than the pressure was the determining factor in producing greening, 

 but Liro (1908) concluded from his experiments that not only the abso- 

 lute amount of oxygen but also its pressure affects the rate of greening. 



The action of oxygen is local, because etiolated leaves half submerged 

 in water green only in the section freely exposed to air (Trebitz, 1905). 



Oxygen is involved in the thermochemical and not the photochemical 

 part of the greening process. Liro (1908, pp. 103 ff.) demonstrated this 

 by alternately placing etiolated leaves in the light at low air pressures 

 and in the dark at normal air pressures. These leaves became green. 

 In a comparable experiment he illuminated etiolated leaves continuously 

 at low air pressures. Those leaves scarcely greened, if they greened at all. 

 During the sojourn in the dark at normal air pressure, the leaves form 

 chlorophyll precursor, which they convert to chlorophyll when brought 

 into the light. But at the lower air pressures they are unable to form 

 the precursor, and consequently they do not green. 



Although oxygen is necessary for the greening of etiolated leaves, it is 

 not necessary for the transformation of protochlorophyll to chlorophyll. 

 Liro (1908) demonstrated that traces of chlorophyll are formed in etio- 

 lated leaves illuminated in an atmosphere of hydrogen from which all the 

 oxygen had been removed by treatment with pyrogallol, and Smith (195Q- 

 1951) obtained an 80 per cent conversion of protochlorophyll to chloro- 

 phyll a in a hydrogen atmosphere containing less than 10~^ part oxygen. 

 But Scharfnagel (1931) got only the barest trace of transformation in an 

 atmosphere of nitrogen rigorously freed of oxygen. This finding needs 

 to be reexamined. 



Frenkel and Rieger (1951) have evidence that the pigments of Porphy- 

 ridium cruentum increase under anaerobic conditions. In lower photo- 

 synthetic forms, e.g., Athiorhodoceae, the characteristic chlorophylls are 

 also formed in the absence of oxygen (cf. Van Niel, 1944). 



Carbon Dioxide. Chlorophyll does not accumulate in etiolated plants 

 illuminated in an atmosphere of pure carbon dioxide (Boehm, 1865; 

 Correns, 1892). Trebitz (1905) found that at 1 per cent carbon dioxide 

 neither an acceleration nor an inhibition of greening was apparent, but 

 that retardation was evident at 2-5 per cent. The minimum carbon 

 dioxide concentration that stops greening varies with the plant species: 

 for Lupinus albiis it lies between 7 and 17 per cent, and for the cereals 

 Hordeum distichum, Avena sativa, and Triticum sativum it lies between 

 50 and 70 per cent. 



Carbon dioxide is not essential for the formation of chlorophyll, as 

 Rudolph (1934) and Rombeck (1943) have shown. In some cases, how- 



