stele
(Figure 95). A layer of cells known as the
endodermis
separates the stele from the ground tissue
in the outer portion of the root. Only found on roots, the endodermis acts as a barrier for substances
entering the vascular system of the root. The endodermal cells have waxy coatings on their walls
known as suberin. This waxy region, known as the
Casparian strip
, forces water and solutes to cross
the plasma membranes of endodermal cells instead of slipping between the cells. As a result,
pathogens and toxic substances are typically kept out of the endodermis, leaving only the materials
needed by the root. The
pericycle
, the area that can produce lateral roots, is the outermost cell layer
of the root's vascular tissue. In monocot roots, the vascular tissue is arranged in a ring around the pith,
whereas in dicot roots, the xylem and phloem of the stele are alternately arranged in an X shape.
69
(credit: scale-bar data from Matt Russell)
151
Figure 95.
The vascular tissue creates an X in the center of the root in typical dicots (left). Phloem cells and
larger xylem cells form a distinctive ring around the central pith in (right) typical monocots
70
.
6.2 Reading #3
: Root Modifications
Root structures can be changed to serve particular needs. For instance, some roots are starch-
storing bulbs. Two types of aboveground roots—aerial roots and prop roots—offer additional
assistance in securing the plant. Roots that have been altered for food storage include tap roots like
carrot, turnip, and beet (Figure 96).
Figure 96.
Vegetables are frequently modified roots.
A plant can grow on another plant thanks to epiphytic roots. For instance, orchids with
epiphytic roots create spongy tissue to absorb moisture. The banyan tree (
Ficus sp.
) begins as an
epiphyte, growing in the branches of a host tree. Aerial roots eventually emerge from the branches
and reach the ground, offering additional support (Figure 97). In screw pine (
Pandanus
sp.), a palm-
like tree that grows in sandy tropical soils, aboveground prop roots develop from the nodes to provide
additional
71

.
70
30.3 Roots - Biology 2e | OpenStax
71
Clark, M., Choi, J. & Douglas, M. (2020)
Biology 2e
, Chapter 30, -Pp.641-642, OpenStax,
https://openstax.org/details/books/biology-2e
152
Figure 97.
The (a) banyan tree, also known as the strangler fig, begins life as an epiphyte in a host
tree. Aerial roots extend to the ground and support the growing plant, which eventually strangles the host
tree. The (b) screw pine develops aboveground roots that help support the plant in sandy soils
72
.
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