[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 309 



sion of heredity may vary greatly in different environments. The wild 

 ancestors of our cultivated varieties of beets and radishes had compara- 

 tively slender and woody roots. The thickened herbaceous characteris- 

 tics of these cultivated varieties are hereditary, but under certain con- 

 ditions of temperature and length of day the roots fail to thicken; that 

 is, this particular hereditary potentiality is not expressed under certain 

 environmental conditions. It will be helpful to visualize root systems as 

 we do the crowns of trees. Certain specific characteristics are often evi- 

 dent, but in different environments their development may be greatly 

 altered. 



In considering the factors that affect the growth of root systems, one 

 should not forget that sugar, vitamins, and hormones from the leaves are 

 essential factors in root growth, and that any factor that influences the 

 supply of these substances to the roots indirectly affects their growth 

 (Chapter XXI). Gravity affects the direction of growth of roots but 

 cannot be classified as either an atmospheric or a soil factor. Among 

 the factors of the soil environment that affect physiological processes 

 and growth in root systems are compactness or texture of the soil, water, 

 oxygen, carbon dioxide, soil temperature, inorganic salts ( essential, non- 

 essential, and toxic), acidity and alkalinity, and soil fungi and bacteria. 

 Only a few of these factors will be mentioned further in this chapter. 



The compactness of the soil indirectly affects the growth of roots 

 through its effects upon aeration and the rapidity of the movement of 

 water and mineral salts. Directly it affects the penetration of roots. Many 

 clay soils may become so hard during dry seasons that germinating seeds 

 at the surface die through failure of the roots to enter the soil. Root 

 systems in general are less extensive in compact soils, and root growth 

 occurs mainly in the crevices of such soils. Soils of coarse texture are 

 not good soils because, in spite of good aeration, the supply of water 

 and available salts is often deficient under natural conditions. Between 

 tliese two extremes of fineness and coarseness are the soils known as 

 loams, in which roots grow best. 



When roots grow through alternating layers of coarse and fine soils, 

 far more branching and development of lateral roots occur in the layer 

 of loam. Trampling, as it occurs in paths in gardens and greenhouses or 

 on trails and roadways, makes the soil compact. Pioneer roads abandoned 

 for a century or more are still evident in forests because few plants can 

 grow in such compact soils. Excellent virgin soils that were thousands 

 of years in fomiing become more compact through careless farm prac- 



