20 THE ECOLOGY OF ROOTS 



practically no water from the surface layers of the soil. Farther 

 west, on the short grass plains, where, because of low precipitation 

 and high run-off, the soil seldom becomes moist to a depth of more 

 than 2 feet, the roots of the grasses and other plants are confined 

 mostly to the first 2 feet of soil. Still farther to the southwest, in 

 southern Arizona, where precipitation is very low, the roots of 

 annual plants seldom penetrate more than 8 inches, although those 

 of the perennial plants, such as cacti and more especially the 

 shrubs, often penetrate to great depths. 



In the case of several crop plants it has been found that the pres- 

 ence of fertilizers in the soil promotes vigorous growth and profuse 

 branching of the roots. Considerable increases in yield of the 

 potato crop have been obtained in certain regions by working ferti- 

 lizers into the surface layer of soil and then plowing about 9 inches 

 deep before planting the potatoes. The deep placing of the fertilizers 

 causes the roots to develop abundantly in the deeper layers. If, 

 then, there is a dry season while the crop is developing, and the 

 surface layers of soil dry out, the plants are not affected as they 

 would be if the greater numbers of roots were in the surface layers. 



The rate of growth of roots varies greatly under different environ- 

 mental conditions. It is sometimes remarkably rapid. The roots 

 of corn, for example, have been known to grow at a rate of 2 to 

 2.5 inches per day during a period of several weeks. 



9. Aeration and Root Growth.— Roots carry on respiration just 

 as all other parts of the plant do. In this process they use up ox^-gen 

 and excrete carbon dioxide. Therefore, either a deficiency of oxygen 

 or an excessive accumulation of carbon dioxide is injurious to the 

 root system. The commonest condition that results in a deficiency 

 of oxygen is the presence of an excessive amount of water in the 

 soil. When the soil is saturated with water, air cannot get in and 

 a prolonged saturation often causes crop plants actually to drown. 

 These facts need to be taken into consideration wherever irrigation 

 is carried on. Care must be taken not to use too much water. The 

 plants need both water and air and it is necessary for best results 

 to strike a compromise between these two needs. 



In swamps where the soil is constantly saturated with water the 

 majority of roots grow horizontally rather than downward and 

 sometimes even grow vertically upward. This is notably charac- 

 teristic of the bald cypress tree which produces upright branches 

 from horizontal roots which are called cypress knees; but this phe- 

 nomenon is characteristic of many plants besides the cypress. 



