HOW PLANTS DO THEIR WORK 297 



In some way it succeeds in hitching them together so that 

 they may do their work. 



We have only to think back to our study of soil chem- 

 istry to remember that starch is made out of oxygen and 

 hydrogen from the water, and carbon from carbon dioxide 

 of the air, and that proteids contain these three elements 

 with the addition of nitrogen from nitrates, sulphur, 

 phosphorus, etc., from the soil. The leaves take in the 

 carbon dioxide probably through their stomata. What 

 must be done with this compound before the plant can use 

 the carbon? If the protoplasm decomposes the carbon 

 dioxide and uses the carbon, what becomes of the oxygen? 

 It is thrown out of the leaf into the air. Therefore, 

 when a plant is actively engaged in manufacturing starch 

 it is taking carbon dioxide from the air and giving out 

 oxygen. Could this process be carried on during the 

 night? Why? 



The question now arises, what do the plants do with the 

 starch proteids and oils that they make ? By discussion the 

 facts may be brought out that the starch by a process 

 something like digestion in our bodies is changed into 

 sugar, and that this and the other foods are conveyed in 

 liquid form from the leaves to all parts of the plants when 

 they are used in the growth of these parts. Some of the 

 food is stored for future use. Recall the study of potato, 

 corn seeds, biennial roots, etc. 



Dig up some corn plants that have been growing two 

 or three weeks. Examine the grains. What has become 

 of their contents? 



But plants need something besides food in order to live 

 and grow. They are like animals in this respect. They 



