ROOTS AND THEIR USES 177 



ample, are usually annuals. A radish may form a com- 

 paratively large root in a few weeks; if it is left in the 

 ground, it goes on to produce flowers and seeds during the 

 same season. The horseradish and the dandelion, on the 

 other hand, are perennials, whose thick tap roots, containing 

 storage food, live and grow for many years. The reserve 

 food of the dahlia, another perennial, is stored, not in a tap 

 root, but in a cluster of thick branch roots. The lower part 

 of the stem with the roots attached is dug up and kept over 

 the winter. In the spring it may be divided into as many 

 parts as there are resting buds at the base of the stem, and 

 each bud will grow into an upright, flower-bearing shoot. 

 Roots like those of the dahlia are often called tuberous roots ; 

 of course they must be distinguished from true tubers like 

 those of the potato, which as we know are branches, not roots. 

 The sweet potato, very differently from the ordinary potato, 

 is a thickened branch root ; it is, therefore, much like one of 

 the tuberous roots of the dahlia. Tapioca is a starchy food 

 obtained from the tuberous roots of cassava. Many orchids 

 have rounded or branched tuberous roots, as well as other 

 slender absorbing roots. The tuberous root, with a single 

 bud attached, is the part of the orchid plant that lives over the 

 winter and develops into a new plant the next spring. Some 

 desert plants have thick roots which serve for the storage 

 of water. 



199. Respiration. We must not overlook the fact that 

 respiration goes on in roots as well as in all other parts of the 

 plant where there are living cells. Roots are usually not in 

 so good a position to obtain oxygen as are stems and leaves. 

 Porous soils, if they are not water-soaked, contain a great 

 deal of air in their crevices, and since oxygen makes up about 

 one-fifth of the volume of ordinary air, roots living in such 

 soils find enough oxygen. A hard, compact soil, or one that 

 is water-soaked, may contain far too little air to meet the 

 needs of roots. One important reason for the plowing and 



