162 GREEN PLANTS AS FOOD MAKERS 



Proteins can be manufactured in any of the cells of green plants 

 where starches or sugars and certain salts are found. The presence 

 of light does not seem to be a necessary factor for the process. 

 How they are manufactured is a matter of conjecture. The 

 minerals, nitrates, sulphates, and phosphates in the soil water 

 give nitrogen, sulphur, and phosphorus, and the sugar or starch 

 gives carbon, hydrogen, and oxygen, all of which elements are 

 found in proteins. Proteins are probably not made directly into 

 protoplasm in the leaf, but are transported to other parts of the 

 plant, stored there and used when needed, either to form new 

 cells or to repair waste. 



Enzymes and their work. It is a matter of common knowledge 

 that starch food is stored in fruits, seeds, roots, and stems. We 

 also know that starches cannot pass from one part of the plant to 

 another because they are insoluble substances. The particles of 

 which they are formed cannot go through the membranes which 

 surround each cell in the plant. To make possible the circulation 

 of food from one part of the plant to another insoluble foods must 

 be made soluble. This is done by means of substances called 

 enzymes. We have little knowledge of their actual composition, 

 but we do know that they have the power to speed up chemical 

 action in the cells so as to cause certain insoluble substances to 

 become soluble. Each nutrient requires a specific enzyme to 

 change it from an insoluble to a soluble form. This process which 

 seems to go on in almost all plant cells as well in the darkness as 

 in the daylight, is called digestion. 



Functions of food. While plants and animals obtain their 

 food in different ways, they probably make it into living sub- 

 stance {assimilate it) in the same manner. Foods serve exactly 

 the same purposes in plants and in animals ; they either are used 

 to build living matter 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 hard rocks. 



Relation of carbohydrate-making 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 



