RESPIKATION 



81 



carbon dioxid of the air. To most plants the nitrogen of 

 the air is inert, and serves only to dilute the other ele- 

 ments; but the oxygen is necessary for all life. We know 

 that all animals need this oxygen in order to breathe or 

 respire. In fact, they have become accustomed to it in 

 just the proportions found in the air; and this is now best 

 for them. When animals breathe the air once they make 

 it foul, because they use some of the oxygen and give off 

 carbon dioxid. Likewise, all living parts of the plant 

 must have a constant supply of oxygen. Roots also need 

 it (148). 



173. The oxygen passes into the air-spaces and into the 

 protoplasm, entering into combinations the final products of 

 which are carbon dioxid and water. The air-spaces may be 

 equal in bulk to the tissues (Fig. 115) . As a result of the use 

 of this oxygen alone at night, plants give off carbon dioxid 

 as animals do. Plants respire; but since they are station- 

 ary, and more or less 



inactive, they do not 

 need as much oxygen 

 as animals, and they 

 do not give off so much 

 carbon dioxid. Dur- 

 ing the day plants 

 use so much more 

 carbon dioxid than 



OXVffen that thev are ^^^' ^^^'''°" "^ ^ l^at, showing llie air-spaces. Breatli- 

 ' _ ° , "^ ing pore or stoma at a. The palisade cells -which 



said to purify the air. chiefly contain the chlorophyll are at 6. Epider- 



The carbon dioxid ^^i «eUs at c. 



which plants give off at night is very slight in compari- 

 son with that given off by animals; so that a few plants 

 in a sleeping room need not disturb one more than a family 

 of mice. Plants usually grow most rapidly in darkness. 



174. TRANSPIRATION.— We have found that the plant 

 takes its food from the soil in very dilute solutions. 



