CHAPTER IV 

 ADJUSTMENT TO LIGHT 



87. Relation of the plant to light. As the source of energy 

 or the food-making activities of the plant, light is scarcely second- 

 ry to water in importance. It is unnecessary only in the case 

 if molds, mushrooms, etc., which do not make their own food, 

 lut use that made by other organisms. The primary response 

 f green plants to light is the production of chlorophyll by the 

 lastid. Under the influence of light, the chloroplast is able to 

 ecompose carbon dioxide and water to form sugars or elaborated 

 3ods. A large amount of the light absorbed is converted into 

 eat in the plastid, and produces evaporation. The number and 

 rrangement of the chloroplasts are determined by the intensity 

 f the light. The latter also regulates the movement of the guard- 

 slls, causing the stoma to open in strong light and to close in 

 arkness. The direction of the light produces a turning or bend- 

 ig of the plant, which is termed phototropism. The normal position 

 [ leaves is due in large measure to the action of light, and the 

 ime is true of the day and night positions taken by many of 

 lem. Finally, the form of the leaf is probably due more to light 

 lan to any other factor. The effects of light may be summarized 

 3 follows: 



1. Production of chlorophyll. 



2. Decomposition of carbon dioxide and water to form sugar. 



3. Loss of water from the chloroplast. 



4. Changes in the number and position of chloroplasts. 



5. Daily opening and closing of the stomata. 



6. Turning of stems and leaves. 



7. Day and night position of leaves. 



8. Changes in the form and structure of the leaf. 



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