80 



PLA^'T PHYSIOLOGY AND ECOLOGY 



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in autumn leaves. Chlorophyll is a product of protoplasmic ac- 

 tivity, formed during the complex processes of nutrition. Appa- 

 rentl}' it is not a part of the protoplasm, but is held in solution 

 in it. It is produced most actively in white light, but it has been 

 found that the yellow rays are the most active 

 in its formation. The function of chlorophyll is 

 not definitely known. The view commonly ac- 

 cepted is that it absorbs certain rays of light, 

 particularly the red and the blue, converting them 

 into a form of energy capable of decomposing 

 carbon dioxide and water. It has also been sug- 

 gested that the rays absorbed are harmful to the 

 activity of the plastid, and are eliminated in order 

 to promote the action of the other rays. What- 

 ever the exact control exerted by the chlorophyll 

 may be, there is no question of its necessity for 



the formation of elabo- 

 rated food by the decom- 

 position of carbon dioxide 

 and water. 



100. The influence of 

 darkness. Disregarding 

 the exceptions already 

 noted, seeds germinated 

 in the dark produceplants 

 that are whitish or yellow- 

 ish in color. The same 

 result occurs in green 

 plants which are placed 

 in darkness. An exam- 

 ination of the plastids 

 shows that they are more 

 or less yellow in color, due 

 to the presence of a pig- 

 ment called etiolin. The 

 latter is apparently con- 

 vertible into chlorophyll, since a yellowish plastid becomes green 

 upon exposure to light. Plastids which occur regularly in the 

 dark, viz., leucoplasts, are also capable of turning green in light, 

 as is well known in the case of the potato tuber, but the existence 



Fig. 2.3. Potato-plants grown in darkness 

 (A) and in light (B). The elongation of 

 corresponding internodes may be com- 

 pared by reference to the figures. (After 

 Pfeffer.) 



