THE DETERMINING FACTORS IN PHOTOSYNTHESIS 41 



will become green much more quickly than the latter. This is 

 associated with the higher carbohydrate content of wheat leaves. 

 If the bean leaf is floated on a 5-10% sugar solution it also will 

 become green. 



As a final prerequisite for chlorophyll formation, must be men- 

 tioned the presence of chlorophyllogen (also called protochlorophyll) , 

 a pigment which is formed antecedent to chlorophyll and which 

 gives rise to it. Monte verde and Lubimenko (1911) have proposed 

 that chlorophyllogen arises from leucophyll, a colorless chromogen. 

 The chlorophyllogen is a very unstable material very much like 

 chlorophyll, but it arises from the leucophyll quite independently 

 of light. The change from chlorophyllogen to chlorophyll, on the 

 other hand, depends upon light. The green material found in the 

 inner seed coats of melon and lemon seeds and in conifer seedlings 

 grown in the dark is probably chlorophyllogen rather than chloro- 

 phyll, although it is possible that in exceptional cases chemical 

 activity might replace the transformation ordinarily effected only 

 by the light. With the exception of light, none of the other condi- 

 tions mentioned above such as a favorable temperature, presence 

 of carbohydrates, or oxygen is necessary for transforming chloro- 

 phyllogen into chlorophyll. Hence these necessary conditions for 

 greening must be prerequisites for the transformation of the leuco- 

 phyll into chlorophyllogen or for the formation of the leucophyll. 



For the formation of the plastids, as has been shown by Lind- 

 strom (1918), certain hereditary factors must be present. Like- 

 wise, in the virescent forms of kafir and in variegated forms of 

 Pelargonium and Hydrangea, the lack of chloroplasts has been 

 shown to be due to a genetic factor. 



Amount of Carbon Dioxide. — Of all the factors necessary for 

 photosynthesis, the amount of carbon dioxide is generally the 

 limiting factor. The temperature is high enough and there are 

 enough mineral salts, oxygen, etc., present to produce the maximum 

 amount of carbohydrates with the carbon dioxide which is avail- 

 able. This means that at ordinary temperatures of the growing 

 season and on a sunny day, the amount of food manufactured 

 could not be increased by raising the temperature or increasing 

 the illumination but only by increasing the amount of carbon 

 dioxide in the air. The 0.03% of carbon dioxide in the normal air 

 is far below the optimum. The amount of carbon dioxide may be 

 increased up to 10% with benefit to the plant. As the amount 



