THE DETERMINING FACTORS IN PHOTOSYNTHESIS 39 



Soil Salts. — An excess of salts in the soil retards photosyn- 

 thesis probably due to its success in checking the intake of water 

 as will be explained in Chapter XIX. Halophytes (plants living 

 in salty or brackish water) grow, though slowly, even in salty 

 soils because their stomata do not close like those of normal 

 plants and the entrance of carbon dioxide is not prohibited. 



Chlorophyll. — Not only is chlorophyll absolutely essential for 

 photosynthesis, but the amount of food manufactured by green 

 plants varies almost directly with the amount of chlorophyll 

 present (Emerson, 1929). Grapes have larger fruits when many 

 leaves are present, and there is a definite correlation between the 

 amount of chlorophyll and the dry weight of tops in corn (Sprague 

 and Shive, 1929). The effect of hail on a corn crop, the damage 

 done by locusts, and the results of defoliation (either accidental or 

 experimental) all attest the importance of the quantitative rela- 

 tion between manufactured food and chlorophyll. As one passes 

 from a country where there is plenty of chlorophyll to a desert 

 region where this substance is much rarer, one is much impressed 

 by the fact that lower animals, the human inhabitants, and even 

 the cities all grow progressively rarer, paralleling the decrease in 

 chlorophyll. 



Among the factors necessary for the formation of chlorophyll, 

 light is of the highest importance. Dangeard claims to have grown 

 the alga, Scenedesmus, in the dark for eight years, at the end of 

 which time the plant was as green as ever, with chlorophyll that 

 showed the normal spectrum. This is rather astonishing if true, 

 but practically all angiosperm leaves are yellow in the dark, as 

 anyone who has ever turned over a board which has been lying 

 upon the grass for some time may attest. Such yellow leaves 

 (said to be etiolated) soon become green when exposed to the 

 light. Even a weak light is sufficient and is, in fact, better than 

 very strong light. Wiesner (1874) explained this by showing that 

 strong light decomposes chlorophyll. The two processes of forma- 

 tion and decomposition go on simultaneously, and in strong light 

 the decomposition is more rapid than the formation, while in 

 weak light the reverse is the case, the decomposition being nearly 

 absent. Attention has previously been called to the fact that 

 shade plants are greener than sun plants and we have here the 

 explanation. Some plant organs such as pine seedlings and young 

 fern fronds may become green even in the dark (Lubimenko, 1910) 



