226 LEAVES 



oxygen has a great deal to do with respiration , it is through 

 the leaves that carbon dioxide is given off into the air, 

 and carbon dioxide is a waste which results from respira- 

 tion. 



In our own bodies the lungs are the organs whereby 

 oxygen is taken in and carbon dioxide given off. The process 

 whereby these and other gases are inhaled and exhaled is 

 called breathing, but breathing is not respiration any more 

 than eating is assimilation. Breathing simply brings 

 oxygen in contact with cells which can absorb it and carries 

 off the carbon dioxide which they give off as waste. Plants 

 respire, but, strictly speaking, they do not breathe. The 

 leaves are not like our lungs, except that they are the 

 parts in which oxygen is principally received and from 

 which carbon dioxide is principally given off. 



Oxygen enters the intercellular spaces of the leaves and 

 there some of it passes through the permeable cell walls. 

 It becomes a solute in the cell sap, for molecules of gases 

 as well as molecules of solids may become solutes. Oxygen 

 is also found as a gas in the plant body. Bubbles of 

 oxygen mixed with other gases are found in the wood 

 cells. Stems of plants which grow under water often have 

 a special form of structure which provides for aeration, i.e. 

 oxygen supply ; they contain extensive air passages. (See 

 Figures 80 and 81.) Just as water supply is commonly 

 the hardest problem for leaves, so oxygen supply is often 

 the hardest problem for roots and submerged plants. 



Root-hairs absorb oxygen from the air which is present 

 in the soil among the soil grains. The aeration (air supply) 

 of roots is a matter of much importance in agriculture. 

 One of the great advantages of plowing is that it permits 

 more air to enter the soil and thus supply roots with oxygen. 



