TOPIC 1.CLASSIFICATIO KINGDOM FUNGI
Phloem
The phloem tissue is made up of
sieve-tube elements and companion cells.
Phloem
tissue
Like xylem vessels, sieve-tube
elements are made of cells that are joined end to end. However, the end walls
of these cells are not completely broken down. They have perforations or pores
that form sieve plates. These cells contain cytoplasm but they have no
nucleus. Fibres run through the pores thereby connecting adjacent sieve-tube
cells.
Each sieve-tube element has a
companion cell; they are separated by a thin wall made up of parenchyma
cells with pores called plasmodesmatawhich allow exchange of materials
between them.
Companion cells have a high
concentration of mitochondria. They provide the sieve-tube elements with
energy.
The movement of substances in the
phloem is by translocation. It can be in any direction.
The distribution of vascular bundles
in plants
The way the vascular bundles are
arranged in the roots, stems and leaves of monocots and dicots differ. This
arrangement also differs in the roots and stems of the two categories of
plants.
Monocotyledonous root
The arrangement of vascular bundles
is as shown
Dicotyledonous root
The xylem is centrally positioned
and star-shaped. The phloem is found between the extensions of the xylem as
shown in Figure below
