ASSIMILATION OF CARBON BY PLANTS 173 



of vitamin A. Carotene can be regarded as provitamin A, and 

 this explains the high dietetic vahie of vegetables, especially of 

 carrots, which contain much carotene, as do also lettuce, spinach, 

 and other leafy vegetables. 



40. Conditions of Chlorophyll Formation. Etiolation and 

 Chlorosis. — The formation of chlorophyll is dependent upon 

 several definite requirements. Of these, one of the most impor- 

 tant is the presence in the cells of plastids, in which chlorophyll, 

 a green pigment, is synthesized. Chlorophyll is not formed in 

 roots, except in the aerial roots of plants such as the tropical 

 orchids. Another important requirement for the formation of 

 chlorophyll is light exposure. Leaves and stems developed in 

 darkness remain almost white or are colored yellow by carotenes 

 and xanthophylls. Such plants are said to be etiolated. They 

 turn green rapidly when exposed to light. It is supposed that 

 they contain a special substance called 'Ueucophyll," which is 

 colorless but readily changes into chlorophyll under the influence 

 of light. The nature of this ''chromogen/' as substances easily 

 turning into pigments are called, has not been investigated. In 

 some cases, e.g., in the germinating seeds of conifers, chlorophyll 

 may form also in the dark. 



Besides their pale hue, etiolated plants differ from normal ones 

 also in respect to their form. The aerial organs usually grow 

 out very long, while the leaves, on the contrary, are undeveloped 

 (Figs. 50 and 51). This modification in form has no direct con- 

 nection with the absence of chlorophyll but is due to the growth 

 of cells and tissues in the absence of light. 



Another important requirement for chlorophyll formation is 

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

 synthesize it. Plants grown in water cultures, in solutions care- 

 fully purified of all traces of iron, have a pale yellow color. 

 Under these circumstances, the pro^cess of assimilation goes on 

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

 But if the leaves of such plants are sprayed with a solution of 

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

 penetrated. Locahzed green color may also be produced when 

 part of the roots of a water culture are separately supplied with 

 iron. When thus treated, parts of the plant drawing directly 

 on these roots will turn green. 



