The Forms and Structures of Roots 



i8i 



you have ever seen had only adventitious roots, because seedlings 

 of these plants are either unknown or are rarely grown except by 

 plant breeders. 



Deep and shallow root systems. The root systems of plants 

 are distributed in the soil in a variety of ways. In a soil that is 

 deep, readily penetrated, and sufficiently moist to permit growth, 

 some plants have roots widely distributed just beneath the sur- 

 face, while others have roots penetrating to great depths. There 

 are also plants that combine these two habits ; that is, their 

 roots are spread in a surface group and a deep group (Fig. 107). 



Each of these habits has its advantages. The shallow roots 

 are in contact with water after every rain, and in dry regions 

 where the rainfall of summer showers penetrates but a few inches 

 these plants may be better supplied than others. Deep roots 

 have a distinct advantage in anchoring the stem firmly. Their 

 absorbing surfaces are in contact with the ground water that 

 accumulates from all the rains of the region. This supply 

 is usually a more permanent one in moist regions, but in dry 

 seasons it may fail for longer periods than the supply near the 



J . E. Weaver 



Fig. 107. The principal absorbing root system of a prickly-pear cactus, Opuntia caman- 

 chica, as seen from above. Beneath the stem is a group of vertical anchorage roots not 

 shown in the figure. The diagram is divided into one-foot squares. 



