THE AMERICAN BOTANIST 123 



use of food, however, plants are like animals and digest, assimi- 

 late and break it down in practically the same way. 



It is not true that plants differ from animals by giving off 

 oxygen in breathing while animals give off carbon-dioxide. In 

 breathing, plants and animals are exactly alike so far as the 

 gaseous exchange is concerned. Both take in oxygen and give 

 off carbon-dioxide. Where plants really differ from animals 

 is in possessing another function, peculiarly their own, known 

 as photosynthesis, by means of which food is formed. In this 

 process, carbon-dioxide is taken in and oxygen given off — just 

 the reverse of breathing. This latter exchange rather over- 

 shadows that which results from breathing and has probably 

 given rise to the popular misconception regarding it. 



Plant food is formed in the leaves and other green parts 

 of plants from the carbon-dioxide absorbed from the air in 

 photosynthesis and soil water taken up by the roots. The 

 energy necessary to' combine these into food is secured by a 

 multitude of green bodies called chloroplasts in the cells. These 

 chloroplasts stop some of the light rays and probably turn 

 them into some form of electrical energy. When it is known 

 that plants get their energy from sunlight, it is easy to under- 

 stand why all green plants must have light and also why plants 

 without chlorophyll, as the green coloring matter is called. 

 cannot form food even in the light. Fungi and the colorless 

 flowering plants have to depend upon food already formed and 

 must live on other plant or animal parts, living or dead. They 

 can also grow as well in darkness as in light. 



The question of the harmfulness of plants in the sickroom 

 is also connected with this subject. When daylight ends, 

 photosynthesis ceases and with it the exhaling of oxygen. The 

 carbon-dioxide given off in breathing, however, continues, 

 though in such small amounts that it requires a rather carefully 

 conducted experiment to detect it. Under these circumstances 

 a whole roomful of plants would not -use up as much oxygen 



