The Structures and Processes of Roots 179 



Accumulation of food in roots. Food accumulates in the 

 roots of many plants, notably in those of biennials like the 

 beet, carrot, turnip, and salsify. The sweet potato and the 

 dahlia are examples of perennials with large storage roots. 

 The most common forms in which carbohydrates accumulate 

 in roots are starch and sugar. Starch as a storage material 

 has the advantages of being insoluble and more concentrated 

 than sugar. When growth begins anew, starch is readily 

 converted (digested) into sugar (page 74). 



Respiration in roots. Respiration must go on in the living 

 cells of the roots just as in the other living parts of the plant. 

 This process requires a constant supply of oxygen. In ob- 

 taining oxygen as well as in obtaining water, the division of 

 the roots into numerous fine branches is an advantage, be- 

 cause it exposes a large surface to the soil air and the soil 

 water. 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 interferes with other root processes besides respiration, 

 and the whole plant suffers. For example, you may have 

 seen yellow, sickly corn in low fields where water has stood 

 for some time. Such plants may recover if the soil is drained. 

 Water plants and swamp plants can grow in poorly aerated 

 soils because the roots are able to secure oxygen through the 

 internal air spaces of the plants. 



Under normal conditions the energy liberated by respira- 

 tion in roots is largely used in growth and in overcoming the 

 resistance of the soil. During the life of a plant the roots, 

 like the stem, continue to develop new branches. 



The growth of roots. In growing through the soil, the tip 

 of a root is continually pushed against and between sharp- 



