84 



HOW GREEN PLANTS MAKE FOOD 



protoplasm. It is present in the leaf and is found also in the 

 stem and root. Proteins can be manufactm^ed in any of the cells 

 of green plants where starches or sugars and certain salts are 

 found, the presence of light not seeming to be a necessary factor. 

 How they are manufactured is a matter of conjecture. The 

 minerals brought up in the soil water form part of their composition, 



and starch or sugar gives three 

 elements (C, H, and 0). The 

 element nitrogen is taken up 

 by the roots as a nitrate 

 (nitrogen in combination with 

 oxygen and some other 

 element, usually with lime or 

 potash) . Proteins are probably 

 not made directly into proto- 

 plasm in the leaf, but are stored 

 by the cells of the plant and 

 used when needed, either to 

 form new cells in growth or 

 to repair waste. ^\Tiile plants 

 and animals obtain their food 

 in different ways, they prob- 

 ably make it into living sub- 

 stance {assimilate it) in the 

 same manner. 

 Foods serve exactly the same purposes in 

 they either are used to build living mattei 



A rock split by a growing tiee 



Functions of Food. 



plants and in animals 

 or they are burned (oxidized) to furnish energy (power to do work) . 

 If you doubt that a plant exerts energy, note how the roots of a 

 tree bore their way through the hardest soil, and how^ stems or roots 

 of trees often split the hardest rocks, as illustrated in the figure. 

 Starch Making and its Relation to Human Welfare. — Leaves 

 which have been in darkness show starch to be present soon after 

 exposure to light. A corn plant may send almost half an ounce of 

 reserve food into the ears in a single day. The formation of fruit 

 and the growth of grain, potatoes, and other food crops, show the 

 economic importance of the work of green leaves. Not only do 

 plants make their own food and store it away, but they m^ke food 



