The Food of Plants 



Nearly all of the other four fifths is nitrogen. A very 

 small part is carbon dioxid. (Exp. 5.) All air contains 

 some water, and this, also, is in an invisible gaseous form 

 called water vapor. (Exp. 6.) 



Oxygen. In some respects oxygen is the most im- The process 

 portant of the gases that compose the air. It is the f breaihm 9 

 breath of life for animals and man. When we inhale 

 air, oxygen is absorbed from i into the blood passing 

 through the lungs. The oxygen of the air combines 

 chemically with the impurities of the blood, slow com- 

 bustion takes place, and carbon dioxid is formed all 

 through the body. This gas is collected in the lungs 

 and is breathed out as we exhale. If for any reason the 

 lungs do not get a sufficient amount of oxygen, or if 

 breathing ceases for even a few minutes, the system 

 is rapidly poisoned, and death results. Plenty of air 

 burns up the poisons that 

 would otherwise accumulate 

 in the blood. (Exp. 7.) 



Plant life,* also, depends 

 on oxygen (Fig. 14 and Exp. 

 8). The leaves absorb it, 

 and the roots will not do 

 well unless plenty of air fills 

 the spaces among the soil 

 particles. We know that a 

 plant would not be suffocated 

 by the exclusion of air for a 

 few minutes, as an animal w. T. 



WOUld be ; but if a plant is Fl - ^ There air enough in 



either of these bottles for a few seeds, 

 Unable tO Obtain OXygen for but not enough for many. 



The effect 

 of lack of 

 air upon 

 plants 



