PLANT LIFE 245 



The green parts of plants (whether leaves or stems), 

 whenever the light is shining on them, and whenever the 

 necessary materials are present, keep working away at 

 their task of food-making. Food-making is an elaborate 

 process. The molecules of the substances out of which 

 food is made begin in the leaf a long series of changes. 

 Presently we find them changed to starch or sugar, and 

 these are true foods of the carbohydrate class, as you have 

 learned in physiology. It is this work of carbohydrate 

 manufacture that requires light. Other kinds of food 

 (fats and proteins) may be made out of carbohydrates in 

 the absence of light. To make them out of carbohydrates 

 does not require the energy that is necessary to make car- 

 bohydrates out of carbon dioxide and water. 



Now you can understand how a plant lives. You know 

 what roots, stems, and leaves are for. You know that 

 they work together in the great process of food-making. 

 After the food is made it may pass in liquid form to any 

 part of the plant. Thus, in the case of corn, much of the 

 food passes into the ear and fills the grains. The most 

 important food plants in the world are the cereals wheat, 

 rice, corn, barley, oats, etc. In all these plants the food 

 used by man is starch in the seeds. But how do the seeds 

 help the plant in its life? This is another story, quite dif- 

 ferent from the story of root, stem, and leaf. 



QUESTIONS 



1. Explain the work of roots. 



2. What are root-hairs, and of what advantage are they to the 



plant ? 



3. Describe the exchange of gases through the leaves. 



4. What things are necessary for food-making by green plants? 



5. What are some of the dangers to our crops ? 



