ROOTS AND ROOT SYSTEMS 267 



the surface checks or at least greatly hinders the invasion of the lower soil 

 layers by roots. If a water table is close to the soil surface downward growth 

 of roots of most species is retarded because of the deficient aeration of satur- 

 ated soils. Only the roots of hydrophytes, as a rule, can penetrate very far 

 into saturated soils. In dry climates, as for example the western plains area 

 of the United States, the lower limit of root growth is determined by the 

 depth of infiltration into the soil of the scanty rainfall, as generally speaking, 

 roots cannot grow into dry soils. In deep, moist, well-drained soils, on the 

 other hand, the depth of penetration of roots is limited not by soil conditions 

 but by factors inherent within the plant. 



Extensive investigations have been conducted by Weaver (1926) and 

 Weaver and Bruner (1927) on the depth of penetration and general distribu- 

 tion in the soil of the roots of many crop plants as well as of some species 

 in their natural habitats. Formerly the concept was prevalent that the roots 

 of crop plants do not penetrate greatly below the depth to which the soil is 

 plowed. Weaver's observations completely refute this idea. He showed that 

 in well-drained soils that the bulk of roots of most crop plants is located in a 

 zone between the surface and a depth of three to five feet. Some individual 

 roots penetrate to greater distances; in most crop plants a few roots were 

 found to reach depths of six or eight feet depending upon the species. 



Contrary to popular opinion the roots of trees do not usually penetrate for 

 very great distances into the soil. As a rule most of the root system of the 

 vast majority of trees will be found in the top few feet of the soil. If soil 

 conditions permit a few roots penetrate to greater depths, but growth of tree 

 roots to a depth of more than ten feet beneath the soil surface is uncommon. 

 Trees growing in deep, well-drained soils, especially if sandy or gravelly, may 

 prove exceptions to this statement. Under such conditions the roots of some 

 species, such as Cottonwood, may penetrate into the soil for twenty or more 

 feet. 



The lateral extent as well as the depth of penetration is an important 

 gross feature of any root system. In general the lateral roots lying close to 

 the soil surface attain the greatest horizontal spread. This varies greatly ac- 

 cording to the environmental conditions to which the root system of a plant is 

 subjected. In the more arid climates of the world it is a common observa- 

 tion that the scantier the rainfall the more extensive the lateral development 

 of the root system of many species. Corresponding with this increased lateral 

 development there is usually, under such conditions, a decrease in the depth 

 of penetration. 



The density of the vegetation is also a factor of importance in determining 

 the lateral spread of roots. The influence of this factor has been observed 



