RESPIRATION 



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 tiring parts of tli< j>l<mf 

 must have a constant supply of oxygen. Roots also need 

 it (148). 



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

 living protoplasm, performing a function of purification as 

 in animals. The air-spaces in the leaf are equal in bulk to 

 the tissues themselves (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 

 inactivi . thi y do not 

 m i it as much oxygen 

 as animals, and tht y 

 do notgivi off so much 

 carbon dioxid. Dur- 

 ing the day plants 

 use so much more 

 carbon dioxid than 

 oxygen t bat i bey are 

 said to purifj the air. 

 The carbon dioxid 

 which plants give off al nighl is very slight in compari- 

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

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

 of mice. Plants usually grow most rapidlj in darkness. 



171. transpiration. \\Y have found thai the planl 

 takes its food from the soil in vers dilute solutions. 



115 Secti if n leaf, showing the air spaa Breath 



int; pore or stoma .it a The palisad >Ua which 



chieflj contain the chlorophyll are :ii 6. Spider 

 m.-il cells ;it o, 



F 



