ROOTS 43 
nodes, and such secondary roots may be the only roots of 
many plants (Fig. 46). 
In propagating plants by layering 
(§ 23) or by cuttings, the roots are necessarily all secondary 
roots. Even erect stems 
sometimes send down 
secondary roots into the 
soil from the lower joints 
(Fig. 77), as is very com- 
mon in corn. 
35. Root-cap. — The 
growing tip of each root 
and rootlet is protected 
by a cap of cells called 
the root-cap (Fig. 70). 
This root-cap consists of 
several layers of cells, 
the outer ones gradually 
dying or being wornaway 
as the tip of the root 
pushes through the soil, 
and being replaced by 
new layers which are 
continually forming be- 
neath. In some plants 
the root-cap is very 
easily seen as a conical 
thickening at the tip of 
the root; in others it 
can be demonstrated 
only by examining un- 
der the microscope lon- 
Sy <\ iu H 
Va 
Fic. 70.—Longitudinal section through root- 
tip of spiderwort, showing central vascular 
axis (pl), cortex (p), epidermis (e), and 
root-cap (c). 
gitudinal sections through the root-tip. The presence of 
such a protective cap in the root is in strong contrast with 
the stem, whose growing tips are protected by overlapping 
leaves. 
