Page 88 - Biology Form Two

Biology for Secondary Schools

Dicotyledonous roots
Dicots have tap roots which are single
thick roots that grow deep into the soil Ground tissue
and have smaller lateral roots. The Vascular bundle
vascular bundles of dicot roots are
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located in the middle of the root. The
xylem is lobed, centrally positioned Epidermis
and star-shaped. The phloem is found
between the extensions of the xylem, Figure 4.8: Cross-section of a monocot stem
as shown in Figure 4.7.
Dicotyledonous stems
In a dicot stem, the vascular bundles
are arranged in a ring around the central
Endodermis pith, as seen in Figure 4.9. In each
Cortex
vascular bundle, the xylem and phloem
Xylem are separated by a vascular cambium
Phloem
Epidermis that contains cells which divide to
Pericycle expand the girth of the stem. The
Root hair vascular cambium also replaces the old

xylem and phloem tissues. Within the
Figure 4.7: Cross-section of a dicot root vascular bundles, the xylem is located
interior to the vascular cambium ring,
Monocotyledonous stems and the phloem is exterior. In between
The arrangement of vascular bundles
in roots and stems differs significantly. the vascular bundles and the epidermis,
there are parenchyma cells forming the
In monocot stems, the arrangement cortex region.
of vascular bundles is random or
scattered throughout the ground tissue, Epidermis
as indicated in Figure 4.8. The ground Cortex
tissue is not differentiated into distinct Pith
region like cortex and pith. Most of Xylem
the vascular bundles are concentrated Vascular
towards the periphery of the stem and cambium Vascular bundle
are more scattered towards the centre. Phloem
These vascular bundles are surrounded
by parenchyma cells in the ground Figure 4.9: Cross-section of a dicot stem
tissue region.

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