16 Characteristics, Structure, Life of Trees 
itself in change of form of crown, or in lack of vigor. An 
entirely different shape from the expected may develop. 
There are three types of root systems: the tap-root, the 
heart-root, and the tracing-root; in some species the type 
is and remains very pronounced, in others a change and 
adaptation is more easily effected. 
The tap-root, perpendicular like the bole of a conifer and 
penetrating deep into the ground, is adapted to deep soils, 
and the difficulty of maintaining a hickory, a walnut, a fir, 
or some of the pines in a soil which is shallow — due to 
rocks or impenetrable layers, or groundwater close to the 
surface — is often due to the inability of adaptation of their 
tap-root systems. 
The oak, also a tap-root tree, adapts itself more readily 
than those mentioned, splits up its tap-root into several strong 
heart-roots, and develops a considerable quantity of surface 
or tracing-roots, but then the short, stubby stature and rapid 
tapering of the tree above ground shows that it is not in 
its natural condition. Elm, ash, and linden behave in a 
similar manner, hickory and walnut resent curtailment of 
their tap-root more strenuously, and hence are difficult to 
transplant or to grow on shallow soils. The spruce, on the 
other hand, is one of the trees, which, with a typical tracing- 
root system, developing horizontally in the upper soil strata, 
can occupy the shallow soils without effect on its form devel- 
opment. So can willows, poplars, and birches, while beech 
and maple with a regular heart-root system made up of 
several strong roots descending obliquely into the ground, 
stand between the two types first mentioned, both in require- 
ment for soil depth, and in adaptability. 
With the recognition of these characteristics in root 
development we shall not expect the same species of trees 
to produce the same form and remain alike under all 
