Biology XII Content
They are the tissues that have lost the potentiality of cell division after gaining the maturity. They are of various types as described below:
A) . Simple permanent tissue:
They are composed of similar types of cells having certain .This tissue consists of only one type of cells, e.g. parenchyma, collenchymas and sclerenchyma.
B) . Complex permanent tissue:
They are composed of more than one type of cells but have common functions. They are of following types:
It is a complex tissue composed of several types of cells. The various constituent of xylem are :(a) tracheids, (b) vessels, (c) fibres,(d) parenchyma cells. Tracheids and vessels are known as tracheary elements. Primary xylem develops from procambium during the development of plant. Secondary xylem is produced later by the activity of vascular cambium during secondary growth. The detailed structures of various elements of xylem are described below.
These are elongated cells pointed at both ends. The end walls are imperforate. Tracheids are present in all groups of vascular plants.
The vessels are shorter and broader than tracheids. Their length varies from one to many cells. They are joined end-to-end form a long chain of cells. Vessels are also found in some pteridophytes.
Different types if lignin depositions are found in tracheary elements. Accordingly, they show the following types of thickenings: Annual: Both are found in protoxylem elements, Spiral Scalariform: Both are found in metaxylem., Reticulate, The tracheids show simple pit or bordered pit.
The main function of tracheids and vessels is conduction of water.
- Xylem fibres (Wood Fibres):
The fibres are long cells with lignified secondary walls. They provide mechanical strength while and sometimes conduct organic food.
- Xylem Parenchyma:
They are only living cells in xylem. These cells store stratch, oils and many ergastic substances. Some of the parenchyma becomes sclerified forming sclereids cells.
Phloem is composed of variety cell types like xylem. The constituent cells are: (a) Sieve tube elements (b) Companion cells (c) Phloem fibres (d) Phloem parenchyma. Primary phloem is produced by the procambium while the secondary phloem develops from the vascular cambium. It develops from the vascular cambium. The detailed structure of various phloem elements are given below.
- Seive tube elements:
It includes sieve tubes with sieve plates and sieve cells. Both of these are thin-walled living cells. Sieve tube members are long slender tube-like joined end to end to form long tubular to form a long tubular channel. Seive plates are present at the end of the tube. Seive cells:
These are elongated narrow cells with tapering ends. In these cells sieve areas occur all over the wall.
- Companion cells:
All angiosperms have specialized parenchyma cells associated with the sieve tube elements. Usually, a single companion cell extends through the whole length of the sieve tube. These are living cells and its function is associated with sieve tube.
- Phloem fibres:
It is the only dead tissue in xylem. These are found both in primary and secondary phloem. Fibres are invariably long and the walls are thick.. The main function is mechanical support.
- Phloem parenchyma:
These are living cells contain various substances, such as starch, tannins, crystal, etc. these cells are associated with fibres. The main functions are storage and translocation food.