ROOTS 21 



water-soaked soil, the air spaces are filled with water. Our 

 ordinary crop plants require a well-aired soil in order to de- 

 velop root hairs in abundance. One of the chief objects of 

 stirring the soil is to admit air to the roots. Orchard trees 

 have been known to die as a result of the "puddling" of the 

 soil. Trees are also sometimes killed by cattle tramping and 

 packing the ground about them, such that the air supply to 

 the roots is largely cut off. Root-hair development is often 

 inhibited by a concentrated soil solution. High tempera- 

 tures, and low temperatures, are inimical to root-hair growth. 

 Root hairs develop in the light and dark about equally well, 

 providing there is ample moisture. 



Length of Life of Roots. — Roots that live but one vegeta- 

 tive period, that is, one season, are annual. All of our com- 

 mon cereals, such as wheat, oats, barley, rye, com, rice, 

 sorghum, and also such common crop plants as buckwheat, 

 beans, peas, tomatoes, melons, etc., have annual roots. 

 Biennial plants live two vegetative periods. Common 

 biennials are beet, cabbage, carrot, and parsnip. From the 

 seed of beet, for example, there is developed the first season 

 a large fleshy tap root, and a short crown from which the 

 leaves arise. This fleshy structure ("beet"), stored with 

 food, rests over the winter, and the next growing period 

 sends up stout, branching stems to a height of 3 or 4 feet, 

 which give rise to flowers and seed (Fig. 119). At the end 

 of the second season of growth, after seed production, the 

 entire plant dies. Under our cultural conditions winter 

 wheat is a biennial. The roots of trees and shrubs and some 

 herbs Uve from year to year, increasing in size each season. 

 Such plants are perennial in habit. In most cases the length 

 of life of roots is the same as that of the shoot system. How- 

 ever, underground perennial stems, such as are possessed 

 by quackgrass, Canada thistle, false Solomon's seal, etc., 

 may have annual roots. 



