ROOTS AND THEIR RELATION TO PLANTS 25 
the root system. Cut off most of the roots of a tree and it 
will die for lack of water. On the other hand, many (though 
not all) kinds of trees may be cut down nearly level with the 
ground and still survive, the stump throwing up a vigorous 
crop of sprouts, which grow into saplings that eventually 
replace the fallen trunk. 
The necessity of roots for anchorage is well shown by figure 4. 
In many cases the power of the roots to hold trees upright is 
greatly increased by the formation of buttresses of wood, which 
extend for some distance up the trunk from the origins of the 
larger roots. In some large tropical trees these buttresses 
attain enormous dimensions. 
22. Earth roots; direction and extent of root system. The 
roots of most common flowering plants in temperate regions 
are earth roots; that is, they grow in and through the soil. 
The nature of the soil greatly influences the extent and posi- 
tion of the root system. Sandy soils favor the development 
of an extensive root system, while clay soils do not. If the 
good soil forms only a shallow layer over shale or sterile clay 
(or in the arctic regions over ice), the roots spread out in a 
flat, mat-like fashion. 
The smaller rootlets are so woven through the soil that it 
is never possible to unravel the entire root system. The roots 
of a single oat plant, if arranged in a straight line, have been 
found to measure over 450 feet in length. Desert shrubs 
sometimes send their roots down as much as 60 feet toward 
the water supply. In parts of California it has been found 
that common farm plants, such as alfalfa and wheat, may have 
roots reaching moist earth at a depth of from 13 to 15 feet. 
Why is deep plowing between hills of half-grown corn 
likely to injure the crop? 
23. Pull of roots due to shortening. Frequently rootlets or 
the taproots of herbaceous plants shorten after they are fully 
grown. This shortening has a tendency to pull the stem and 
leaves of the plant downward. It is interesting to notice how 
some plants with rosettes of leaves, like chicory, dandelion, 
