RELATION OF AIR TO FOOD MANUFACTURE 83 



shining upon the plant, it is not able to carry on this work, 

 and the plant factory stops. For that reason all green 

 plants must live where there is light, and they cannot make 

 starch or sugar at night. Artificial light will answer if it 

 is strong enough, but 

 usually it is too weak 

 to have any appreciable 

 effect. 



92. Importance of 

 chlorophyll. It follows 

 from what has been 

 said in the preceding 

 sections that the car- 

 bohydrate manufacture 

 can go on only in the 

 presence of light and 

 chlorophyll. This fact 

 causes us to regard 

 chlorophyll as a very 

 important substance. 

 All plants which have 

 chlorophyll are able to 

 manufacture their own 

 food, while plants which 

 do not (figs. 42 and 43) 

 have it are obliged to FlG - 42 - Indian pipe a dependent plant 



depend upon Securing This P lant is wholly without chlorophyll ; it se- 

 .,/.-,, -, , cures its food from organic substances in the soil 



the food already made. 



We may call those plants which have chlorophyll independent 



plants and those which do not have it dependent plants. 



The process in which the chloroplasts of green plants, with 

 the aid of light, manufacture carbohydrates out of water and 

 carbon dioxide is called photosynthesis. This is the name 

 of the process we have been studying. It is of very great 

 importance in making the food of the world. 



