THE GENERAL LIFE PROCESSES 329 



The materials out of which food is made are carbon 

 dioxide and water. The former passes directly into the 

 leaves from the air, while the latter passes into the roots 

 from the soil, is carried up through the stem, and so reaches 

 the leaves. For this movement of water a special region 

 of the plant is developed. This region is known as the 

 wood. In herbs it appears as fibers. It is continuous from 

 the root, through the stem, and out into the leaves where it 

 takes the form of veins. This forms a very efficient water 

 transporting system, but a satisfactory explanation of the 

 mechanics of this "ascent of sap" is yet lacking. 



The protoplasts of the leaves contain green bodies 

 chiefly composed of protoplasm which are called chloro- 

 plasts. The substance in them which gives them their 

 green color is called chlorophyll, and it is these chloroplasts 

 which do this work of food making. The protoplasts of 

 the leaves receive carbon dioxide and water, and their 

 chloroplasts build these substances into food, such as sugar 

 and starch. 



Thus we see that water and carbon dioxide are not 

 really plant foods as is so often stated, but they are the raw 

 materials out of which the food is manufactured. 



The chloroplasts can work only in the light, and hence 

 leaves seem to seek the light. Food making stops at night. 

 Also, in this process, since there is more oxygen in the raw 

 materials than is needed in the finished product, some of 

 the oxygen is given off into the air as a waste. This has 

 led to a very general misunderstanding of tljis process. 

 Since carbon dioxide is taken in and oxygen given off, and 

 since this is exactly the opposite of what occurs in respira- 

 tion, it was long thought that plant respiration was exactly 



