22 THE ECOLOGY OF ROOTS 



winter wheat is a biennial. Some plants, likewise, that are annuals 

 when grown in periodic climates may be perennials in uniform 

 climates. The castor bean grows in Florida, for example, as a peren- 

 nial and becomes a small tree, while in the northern part of the 

 United States it is an annual. 



11. The Position of Roots in the Soil.— We have already spoken 

 of certain variations in de])th and extent of root systems due to 

 conditions of the environment. The position in the soil of the roots 

 of any particular kind of plant, however, is governed in part also 

 by inheritance. A maple tree, for instance, practically always pro- 

 duces a large number of roots in the surface layer of the soil. This 

 is one of the main reasons why it is so difficult to get lawn grass to 

 grow under a maple tree. On the other hand an oak tree growing 

 under exactly similar conditions has a much smaller number of 

 surface roots. Again, a corn plant, at germination, produces a pri- 

 mary root system from the seed and a more extensive root system 

 is formed later by adventitious roots from the lower part of the 

 stem. This latter root system is always produced at approximately 

 the same depth whether the corn has been planted relatively shallow 

 or very deep. 



In a community of plants where a large number of kinds are 

 growing together it is usually found that they vary greatly in the 

 depths of the main parts of their root systems (Fig. 6). This is 

 distinctly advantageous since it makes it possible for a much greater 

 number of plants to occupy the area than would be the case if they 

 all had their roots at the same level. A similar response to depth 

 is found in underground stems. The underground stem of any par- 

 ticular kind of plant is always at approximately the same distance 

 below the surface of the soil. If soil is piled up above the growing 

 tip of such a stem it grows upward until it reaches the normal dis- 

 tance from the surface. Likewise, if soil is dug away from before it, 

 or if it encounters a natural depression, it grows downward until 

 it reaches its usual depth. This phenomenon, which is not well 

 understood, has been spoken of as the "law of level." 



Of considerable interest in connection with the position of roots 

 in the soil is the phenomenon of root contraction. This again is 

 not well understood but it is rather common nevertheless. A dande- 

 lion is often spoken of as a stemless plant. It, of course, is not stem- 

 less. It has a short stem at the surface of the soil and the stem 

 bears the leaves. This stem increases in length a little each year 

 but, although the plant may live for many years, it never becomes 

 any taller because every year the root contracts enough to pull the 

 stem down as much as it has grown upward. 



