78 Trees, Stars, and Birds 



bardy poplar, for its branches are spreading. Its leaf- 

 stalks are only slightly flattened, and its leaves are 

 cottony white on the under side. Some people incor- 

 rectly call it the " silver maple." It spreads by means 

 of the roots, the sprouts which it sends up forming within 

 a few years extensive thickets that are difficult to eradi- 

 cate. On this account it is not a desirable tree to plant. 



Root systems. The root system of poplars and most 

 other trees is composed of many slender branches of 

 nearly equal length. Such a system of roots is called 

 a fibrous root system. On the other hand, an oak, a 

 pine, or a walnut or other nut-bearing tree has a tap 

 root; that is, it has one large root going down deep into 

 the ground, with smaller roots branching off from the 

 side. The radish and dandelion are examples of herbs 

 with tap roots. Grasses, corn, and other grains have 

 fibrous root systems. 



The combined length of all the roots of so small a 

 plant as a wheat plant has been found to be more than 



FIG. 46. A cottonwood seedling less than i year old. The roots have 

 reached a length of from 3 to 4 feet. 



