36 ELEMENTARY STUDIES IN BOTANY 



hydrates are multiples of 6. In examining the second hall 

 of the equation, it becomes evident (1) that the carbohydrate 

 contains hydrogen and oxygen in the same proportion as in 

 water, a fact which gives name to the compound (" carbohy- 

 drate " means carbon and water) ; and (2) that oxygen is 

 freed as a waste product (or by-product) in the same propor- 

 tion as it exists in carbon dioxide. The total result is to get 

 the carbon out of the carbon dioxide and combine it with 

 water, and therefore the process is often called the " fixation " 

 of carbon. Hydrogen and oxygen are gases, so that carbon 

 is the only solid that enters into the fabric of the plant, and 

 this solid is obtained from a gas that exists in the air. 



The carbohydrates thus formed in the plant are usually 

 starches or sugars, and they are freely transformed into one 

 another. It is often stated that green plants form starch, 

 but the fact is that starch is only the visible form of the carbo- 

 hydrate. It is visible because it does not dissolve in the cell 

 sap, while sugar is invisible because it does dissolve. When 

 more carbohydrate is manufactured than is being used, it 

 becomes stored up in the form of starch, and therefore starch 

 is spoken of as the storage form of a carbohydrate. On the 

 other hand, when the carbohydrate is being used and is 

 being carried around through the plant, it is in the form of 

 sugar, for a substance must be in solution to be carried about, 

 and therefore sugar is spoken of as the transfer form of a 

 carbohydrate. 



28. The by-product. It has been noted that during 

 photosynthesis oxygen is given off as a by-product. Nothing 

 more than a statement of this fact would be needed if it were 

 not connected with a persistent misconception in reference 

 to photosynthesis. When it was first observed that green 

 plants take in carbon dioxide and give out oxygen, it was 

 natural to suppose that this gas exchange represented the 

 respiration of plants. Since the gas exchange in the respira- 

 tion of animals is just the reverse (taking in oxygen and giving 



