The Processes of Roots 197 



Respiration in roots. Respiration goes on in the hving cells 

 of the roots, and this process requires a constant supply of oxygen. 

 In obtaining oxygen, the division of the roots into numerous fine 

 branches is an advantage, because a large surface is exposed to 

 the soil air. Some plants are easily injured by the lack of oxygen 

 in the soil ; if water stands on the soil and excludes the air, the 

 roots gradually suffocate. Suffocation of a part of the roots inter- 

 feres with absorption and other root processes besides respiration, 

 and the whole plant suffers. For example, corn becomes yellow 

 and sickly in low fields where water has stood for some time. 

 Such plants may recover if the soil is drained as soon as these 

 symptoms appear ; if delayed, the plants will never completely 

 recover. Water plants and swamp plants can grow in poorly 

 aerated soils either because the roots secure sufficient oxygen 

 through the internal air spaces of the plants or because they have 

 a low oxygen requirement. 



The energy liberated in respiration is used in the chemical 

 processes associated with food transformations in the cells of the 

 root. If the aeration of the soil is poor, respiration is slowed 

 down, and instead of carbon dioxide, poisonous substances, such 

 as alcohols and organic acids, are formed. Hence the oxygen 

 content of a soil is one of its important properties. 



A soil is in its most favorable condition for plant growth when 

 there is enough water to balance the loss by transpiration but not 

 enough to interfere with the access of oxygen to the roots. 

 Gardeners determine when it is in this condition by squeezing a 

 handful of the soil. If it barely cHngs together in a ball when 

 the hand is opened, it is about right. In this condition soil has 

 its largest volume, best aeration, is mellowest, and is most easily 

 penetrated by roots. 



Carbon dioxide. A slight increase in the amount of carbon 

 dioxide in the atmosphere increases the rate of photosynthesis 

 and indirectly the growth of many plants. 



When carbon dioxide accumulates in the soil, it becomes toxic 



