ABSORPTION OF CARBON 



23 



in form has no direct connection with the absence of chlorophyll 

 but is due to the direct influence of darkness on the growth of cells 

 and tissues. This phenomenon will be discussed in detail in 

 Part IV of this book. 



Another important requirement for chlorophyll formation is 

 the presence of a minimal amount of iron salts in the cells that 



Fig. 7. — Seedlings of horse beans 

 grown in light {from Nathansohn). 



Fig. 8. — Seedlings of horse beans 

 grown in darkness (after Nathansohn) . 



manufacture it. Plants grown in certain water cultures, in solu- 

 tions carefully purified of all traces of iron, are colored pale yellow. 

 Under these circumstances, the process of assimilation goes on 

 feebly, or not at all, and the plants finally die from exhaustion. 

 But, if the leaves of such plants are touched with a solution of an 

 iron salt, green spots are produced in places where the salt has 

 penetrated. Localized green color may also be produced when 

 some roots of a water culture are separately supplied with iron. 



