Plant tissues part II

1. Plant tissues (part II)
Complex Tissues

2.  A complex plant tissue is made up of more than one type of cells. There are two
types of complex plant tissues, viz. xylem and phloem.
 Xylem is vascular tissue that transports water and dissolved minerals from roots to
to stems and leaves. This type of tissue consists of dead cells that lack end walls
between adjacent cells. The side walls are thick and reinforced with lignin, which
makes them stiff and water proof.
 Phloem is vascular tissue that transports food (sugar dissolved in water) from
photosynthetic cells to other parts of the plant for growth or storage. This type of
tissue consists of living cells that are separated by end walls with tiny perforations,
or holes

3. Xylem:
Xylem is composed of four elements, viz.
tracheids,
vessels,
xylem fibres and
xylem parenchyma.

4.  Tracheids: These are elongated or tube-like cells. They have thick and lignified
walls and tapering ends. The tracheid cells are dead and protoplasm is absent.
Tracheids and vessels are the main conducting elements in flowering plants.
 Vessel: These are long, cylindrical tube-like structures. These are made up of
many cells which are called vessel members. Each vessel member has lignified
and a large central cavity. Protoplasm is absent in vessel cells. The vessel
are interconnected through perforations in their common walls. The presence of
vessels is a characteristic feature of angiosperms.

5.  Xylem Fibres: The xylem fibres have highly thickened walls and obliterated central
lumens. Septa may be present or absent in xylem fibres.
 Xylem Parenchyma: The xylem parenchyma is composed of living cells which are
thin-walled. The cell wall of xylem parenchyma is made up of cellulose. Xylem
parenchyma stores food in the form of starch or fat, and some other substances
like tannins. Parenchymatous cells facilitate the radial conduction of water in

9.  The phloem; in angiosperms; is composed of four elements, viz.
 sieve tube,
 companion cells,
 phloem parenchyma and
 phloem fibres.
 In case of gymnosperms, the phloem has albuminous cells and sieve cells. Sieve
tubes and companion cells are absent in the phloem in gymnosperms.

10.  Sieve Tube: The sieve tube elements are long, tube-like structures. They are arranged
longitudinally. The end walls of sieve tubes are perforated to form sieve plates. A
mature sieve element has a peripheral cytoplasm and a large vacuole but no nucleus.
The nucleus of the companion cell controls the function of sieve element.
 Companion Cells: These are specialized parenchymatous cells. The companion cells
are closely associated with sieve tube elements. There is a common pit field in the
common longitudinal wall between the sieve tube element and companion cell. These
piths connect both of them. The companion cell helps in maintaining the pressure
gradient in the sieve tubes.
 Phloem Parenchyma: The cells of phloem parenchyma are elongated, tapering
cylindrical cells. The cells have dense cytoplasm and nucleus. Cell wall is composed of
cellulose and has pits. Plasmodesmatal connections exist between the cells through
these pits. Phloem parenchyma stores food and other substances like resin, latex and
mucilage. Phloem parenchyma is usually absent in monocotyledonous.

11.  Phloem Fibre: These are also called bast fibres and are made up of sclerenchyma.
Phloem fibres are usually absent in primary phloem but present in secondary
phloem. The phloem fibres are much elongated, unbranched and have pointed,
needle-like apices. Phloem fibres of jute, flax and hemp are commercially used.
 The primary phloem which is first formed consists of narrow sieve tubes and is
called protophloem. The later formed phloem has bigger sieve tubes and is
metaphloem.

14. Source and sink mechanism

15.  References
 https://www.ck12.org/
 Principles of Botany by Tanveer Ahmad Malik