ROOTS. 415 



surface holds back practically all the rain which is then evap- 

 orated into the air and lost to the root systems. 3d, In heavy 

 and long-continued rains the water breaks through the leaf 

 system to such an extent that roots under the tree would be as 

 well supplied as those outside, and the ground outside being 

 saturated anyway, the roots do not need the small additional 

 water which may have been shed outward. 4th, It is the habit 

 of plants where left undisturbed (except in rare cases), to grow 

 in more or less dense formations or societies. Here there is no 

 opportunity for any appreciable centrifugal distribution of rain- 

 fall and yet the root distribution is practically the same, except 

 that the root systems of adjacent plants are interlaced. 



II. Kinds of Roots. 



793. The root system. From the foregoing, it will be under- 

 stood that the roots of a plant taken together form the root sys- 

 tem of that plant. In soil roots in general we usually recognize 

 two kinds of root systems. 



794. The fibrous-root system. Roots which are composed of 

 numerous slender branching roots resembling "fibers," are 

 termed fibrous, or the plant is said to have a fibrous-root system. 

 The bean, corn, most grasses, and many other plants have fibrous- 

 root systems. 



795. The tap-root system. Plants with a recognizable cen- 

 tral shaft-like root, more or less thickened and considerably 

 stouter than the lateral roots, are said to have tap roots, or they 

 have a tap-root system. The dandelion, beet, carrot (see crown 

 tuber) are examples. The hickory, walnut, and some other 

 trees have very prominent tap-roots when young. The tap-root 

 is maintained in old age, but the lateral roots often become 

 finally as large as the tap-root. Besides tap-roots and fibrous- 

 roots, which include the larger number, several other kinds of 

 roots are to be enumerated. 



796. Aerial roots. Aerial roots are most abundantly devel- 

 oped in certain tropical plants, especially in the orchids and 

 aroids. Many examples of these nlants are grown in conserva- 



