312 TEXTBOOK OF BOTANY 



of Stagnant water largely because of the different amounts of oxygen 

 available. When plants are being considered, the expression "good drain- 

 age" should be interpreted as good oxygen supply rather than water 

 movement. 



The well-knov^m fact that the roots of willows, cottonwoods, and elms 

 will develop excessively in drain pipes and stop the flow of water is due 

 to the combined result of constantly available moisture and oxygen. 



The more direct effects of temperature on root development are largely 

 expressions of the fact that higher soil temperatures accelerate and 

 lower ones retard development. In polar regions the soil may be con- 

 stantly frozen a few feet below the surface; only the upper layers thaw 

 in midsummer. Root systems there are usually very shallow. 



The available inorganic salts in a soil may increase or decrease root 

 development. Low concentrations of nitrogen in the form of ammonium 

 compounds and nitrate may restrict it. If nitrogen is present in greater 

 concentration and the manufacture of proteins is not curtailed, average 

 root systems are formed. Excessive amounts of nitrogen may result in 

 larger root systems in some plants and in smaller ones in others, accom- 

 panied in both cases by excessive development of the tops. Conse- 

 quently, one frequently finds greater development of roots in proportion 

 to the shoots in poor soils than in soils rich in nitrogen. 



Large amounts of salts, such as occur in salt and alkali flats in dry 

 regions, become limiting factors to most plants, and only a few species 

 survive these conditions. When salts accumulate to 2 per cent of the 

 dry weight of the soil, practically all flowering plants are killed. 



With the effects of all these factors in mind, one should be able to 

 explain some of the examples of root distribution that he encounters in 

 the field, in road cuts, and in excavations, and the many diagrams of 

 root systems available in botanical literature. 



Root surfaces compared with leaf surfaces. Many studies of the rela- 

 tion between the depth and extent of root penetration and the height 

 and size of the shoots of plants have been made. The relative weights 

 of roots and shoots of many economic plants have been studied at the 

 agricultural experiment stations, particularly following different methods 

 of cultivation, crop rotations, frequency of cutting, and the addition of 

 fertilizers. Few studies have been made of the areas of entire root sys- 

 tems, including the finest roots and root hairs, in relation to the areas of 

 leaves and stems. 



The total areas of tops and roots of winter rye grass have been meas- 



