c)
Vascular Tissue
Vascular, or conductive tissue,
is a distinctive feature of the higher plants; one that has made possible
their extensive exploitation of the terrestrial environment.
This tissue includes cells that function as tubes or ducts through
which water and numerous substances in solution move from one part of the
plant body to another. There are
two principle types of vascular tissue: "xylem" and
"phloem". Both of these
are complex tissues (i.e. they consist of more than one kind of cell).
i) Xylem
is a vascular
tissue that functions in the transport of water and dissolved substances upward
in the plant body. It forms a
continuous pathway running through the roots, the stem and appendages of the
stem, such as the leaves. In the
flowering plants (Anthophyta), the xylem usually includes two unique types
of cells: tracheids and vessel elements. It
also includes numerous parenchyma and sclerenchyma cells (especially fibres).
The parenchyma are the only living cells in the mature, functioning
xylem. The cytoplasm and the
nuclei of tracheids, vessel elements and sclerenchyma cells disintegrate at
maturity, leaving the thick cell walls as the functional structures.
Another function of the xylem is one of support, particularly of the
aerial part of the plant. The
numerous fibres in the xylem function almost exclusively in this way, in
addition, the thick-walled tracheids are also important as supportive
elements.
In this diagram A-D represent tracheids, E-G are vessels.
Tracheids of the primary
xylem ( i.e. that which matures first) are stretched during their development
and their secondary walls are usually in the form of rings or spirals.
The secondary walls of
secondary xylem develop after all length-wise growth has ceased and they are
not stretched during their development. Their
secondary walls are, therefore, more continuous.
The tracheids are connected to one another by numerous pits. The pits may
occur anywhere on the cell wall but they are often particularly numerous on
the tapered end of the cell where it abuts with the adjacent cell.
Water and dissolved substances move upwards from tracheid through the
pits.
Note the tracheids and the close-up of the bordered pits in the pictures above.
"Protoxylem" is xylem (tracheids in the above pictures) which matured before the plant organ stopped growing. Therefore the mature xylem was "stretched" as the organ continued to grow. That's why these cells look like they have been stretched like a spring.
Vessel elements
(vessel
cells) are conductive elements which are more highly specialized than
tracheids. They are
characteristic of the flowering plants (Anthophyta) and do not occur in
most gymnosperms (e.g. conifers). In
general, vessel elements are shorter and wider than tracheids.
They have pits and perforations along their sides, through which some
lateral movement of substances may take place, but materials move chiefly
upward or downward through their ends. These
ends are extensively perforated or may be entirely open.
The vessel elements are stacked on top of one another to form a
continuous tube. Such a vertical
series of vessel elements is called a vessel.
Tracheids have only pits (no perforations) which are thin areas in the
in the cell walls.
The large red openings in this vascular bundle are vessels in xs.
In the above picture you can see a vessel cut in long section: note how wide they are.
ii) Phloem is a complex tissue which contains conducting cells as well as supportive fibres and parenchyma.
Sieve elements. The principal vertical conductive elements in phloem are the sieve elements which are arranged in a vertical series to form structure called a sieve tube. Unlike the tracheids and vessels of the xylem, the sieve tubes retain their cytoplasm at maturity but their nuclei disintegrate. The sieve elements are elongated cells with specialized porous areas in their end walls called sieve plates. Strands of protoplasm connect the contents of one cell with those of the next through these pores. Food material located in the cytoplasm, can move from one cell to the next by means of cytoplasmic streaming.
Companion cells. Usually one or more specialized, elongated parenchymatous cells, called companion cells are closely associated with the sieve elements in most flowering plants. The companion cells are derived from the same original cell as the associated sieve element. Mature companion cells retain both their cytoplasm and their nuclei. It has been suggested that the nucleus of the companion cell controls both its own cytoplasm and the cytoplasm of the adjoining sieve element, after its nucleus has disintegrated.
