Biology (NTSE/Olympiad)  

2. Tissue

Complex Permanent Tissues :

Complex Permanent Tissues :
A complex tissues can be defined as a collection of different types of cells that help in the performance of a common function.
The important complex tissues in vascular plants are xylem and phloem. Both these together called as vascular tissues.
Both these tissues are an assemblage of living and dead cells and may be primary or secondary, depending upon their mode of origin.
Complex tissue transport water, mineral salts (nutrients) and food material to various parts of plant body.
Complex tissues are of following two types :
Xylem :
Its main function is conduction of water and mineral salts from root to the top of plant.
Primary xylem elements originate from procambuim of apical meristem.
Secondary xylem elements originate from the vascular cambium of lateral meristem.
The xylem elements are of 4 types : xylem tracheids, vessels, fibers and parenchyma.
Xylem Tracheids :
These are lignified and dead cells with bordered pits.
They help in conduction of water in pteridophytes and gymnosperms and provide mechanical support plants.
Xylem Vessels :
The cells are long and tubular with lignified cell wall.
The cross wall (end wall) at both the ends dissolves and form a pipe like channel.
They help in ascent of sap in angiosperms.
Xylem Fibers :
Long and narrow sclerenchymatous fibers with tapering end. The wall is heavily lignified leaving a very narrow Lumen.
It provides tensile strength and mechanical strength.
Xylem Parenchyma :
They are thin walled living cells present in both primary and secondary xylem.
They store food materials.

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