198 



General Botany 



J.E. Weave/' 



Fig. 115. Section showing part of the root system of the bush morning-glory {Ipomoea lep- 

 tophylla), a widely distributed plant of the Great Plains region. The lateral and vertical 

 branches start from a perennial fleshy root, one foot in diameter, and extend into more 

 than 5000 cubic feet of soil. The section is divided into one-foot squares. 



to some plants. Buckwheat, for example, can withstand a low 

 content of oxygen in soils, but is killed by an accumulation of 

 carbon dioxide. The black willow is indifferent to both low 

 oxygen and high carbon dioxide content of soils. 



The distribution of roots in the soil. In addition to the in- 

 herited root habits of plants, an important factor that determines 

 the distribution of roots in soil is the oxygen supply. Various 

 plants have different requirements, but all roots require oxygen 

 for growth. Those who have seen stumps pulled from the land 

 know that the roots go deep in upland sandy soils ; that they 

 do not go so deep in heavy clay soils ; and that they are just 

 beneath the surface in swamp and bog land. The principal 

 reason why one finds the roots near the surface in swamps is that 

 these roots were the only ones that continued to live and grow. 

 The roots that in times of drought penetrated to greater depths 

 were killed off (suffocated) when the water stood at higher levels. 

 The distribution of roots in the soil, therefore, is determined 

 principally by the combined influences of gravity, water, and 

 oxygen. Water and gravity control the direction of growth, and 



