KESPIEATION 



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 are 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 

 oxygen that they are 

 said to purify the air. 

 The carbon dioxid 

 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. 



^ ect * on *- a ^ ea ^ > showing the air-spaces. Breath- 

 ing pore or stoma at a. The palisade cells -which 

 chiefly contain the chlorophyll are at 6. Epider- 



mal cells at c - 



