ROOTS 31 



part, usually more than one-third the total diameter of the 

 root, is xylem or stele through which conduction of water and 

 other substances takes place. The cambium is at the outer 

 edge of the xylem while the more tender outer part consists 

 mostly of cortex with the pericycle and phloem, a narrow 

 band, next to the cambium. The secondary roots can be 

 seen as light-colored threads passing through the cortex 

 region from their origin in the xylem, where they grow when 

 the root is very small. While they push through some tis- 

 sue, much of the cortex grows around the secondary roots 

 after they have grown some distance into the soil. Seeds of 

 plants with tap roots are usually started in permanent beds 

 because tap roots broken in transplanting branch and may 

 weaken the plant. It would be an important problem to 

 determine the effect in several types of soil of transplanting 

 on a wide variety of plants with a pronounced tap root. 



In sharp contrast to the tap root, the fibrous root system 

 results from a slow-growing tap root and fast-growing secon- 

 dary roots, such as in grasses (oats. Fig. 4), or, as in the ad- 

 ventitious roots, from a more or less even rate of root growth, 

 such as is common from the nodes of certain grasses. This 

 type of root system is usually more shallow than a tap-root 

 system and may suffer sooner from drought. Some of the 

 horizontal roots of both systems may be so near the surface 

 that they are easily destroyed in cultivation. These hori- 

 zontal roots may be in dry soil where frequently no water is 

 available. However, since the soil is always well aerated 

 near the surface, these horizontal roots are very important 

 to the plant in its absorption when the deeper soil is so wet 

 that its oxygen content is seriously reduced. 



The root system of a maturing lima bean plant shown in 

 Figure 8 represents a weak tap-root system, with extensive 

 branching near the surface, and illustrates the type of system 

 which suffers from drought because many of the roots are in 

 the upper four inches of soil. This shoot was nineteen inches 



