The Manufacture of Food 39 



Insects and plant diseases are often serious hindrances to photo- 

 synthesis. When insects eat the leaves of plants, they decrease 

 the supply of carbohydrates in proportion to the amount of leaves 

 they destroy. If the plant happens to be a crop plant, the injury 

 done by insects may result in the failure of the plant to manu- 

 facture sufficient food for filling out the fruit, grain, or seed for 

 which it was grown. Diseases of plants caused by fungi or bac- 

 teria also greatly interfere with the power of the plant to manu- 

 facture carbohydrates. 



Carbohydrates as storehouses for energy. When the carbon 

 dioxide and water are converted into carbohydrates by photo- 

 synthesis, the energy supplied by the sunlight in doing this work 

 is stored as potential energy in the new substances formed. Then, 

 when these carbohydrates are oxidized or burned (or in other 

 words, when they are changed back into carbon dioxide and 

 water), the exact amount of energy that was stored is set free. 

 Thus the plant acts as a storehouse from which we can draw energy 

 at any time. 



The importance of photosynthesis as a life process. Photo- 

 synthesis is not only important to the plant itself, but, broadly 

 speaking, it is the most important of all life processes. The sun 

 pours a constant flood of energy on the earth, and this energy 

 warms the earth, causes the winds and rains, and in general 

 furnishes the power for the work that we see going on in nature 

 about us. From running water, winds, and direct sunKght man 

 obtains a certain amount of energy for his own use, but the great 

 source of the energy that we use for heating purposes and for 

 power is wood, coal, petroleum, or gas. The energy stored in 

 these was accumulated through photosynthesis. It came origi- 

 nally from the sun, and but for the plants would have radiated 

 off into space as heat waves from the earth. But through the 

 work of green plants it was locked up in the molecules of the wood 

 and coal, and by burning these fuels man can release the energy 

 that is stored in them and use it for his own purposes. We may, 



