Transportation System in Plants.

 

Plants have a unique transportation system for food, minerals, nutrients and water from sources (roots,leaves) of these components to the sink (top of the trees,branches).
Mainly in vascular plants two different types of tissue are present for transportation.

• XYLEM

• PHLOEM 

The xylem and phloem both are complex tissues. These are composed of more than one type of cells.

XYLEM 

Xylem is one of the transport tissue in vascular plants. The basic function of xylem is to transport water and minerals to shoots and leaves from soil interface(roots) of plants. Xylem also provide mechanical support and storage. Here, transport is unidirectional.

Xylem Structure and their Functions: Its structure is very complex. It is made up of four components tracheids, vessels, xylem parenchyma, xylem fibres.

Tracheids: These are long tube like cells with tapering ends. In these cells protoplast is absent   so these are dead cells or non living cells. These cells are lignified and these walls are perforated with pits. These cells help in transportation of water.

Vessels: These are short tube like cells and without the protoplast i.e,non living or dead cells.They have lignified cell wall.It carried water around the plant.

Xylem parenchyma: These are living cells with cytoplasm. Cell wall of xylem parechyma is cellulosic not lignified. The function are store metabolites and secretion.

Xylem fibres: These are non living cells or dead cells that means there is no protoplast. Their cell walls are very thick lignified and fibres provide support.

 

 PHLOEM 

It is another transport tissue in vascular plants and is also called bast. Its transport the organic soluble compounds formed by photosynthesis in leaves or green parts, compound is mainly sugar sucrose to the other parts where needed. Here, transport is bidirectional.

Phloem Structure and their Functions :It is composed of various specialised cells theses are sieve tubes, companion cells, phloem parenchyma, phloem fibres,

•  Sieve tubes : These are the elongated living cells of phloem which have cytoplasm but nucleus is absent or disappeared. The main function is transport of carbohydrates primarily sucrose.

• Companion cells : These are the thin walled, elongated, parenchyma cells which is closely associated with sieve tubes.These cells have nucleus and other organelles of cells so they provide chemical support for functioning and maintenance of sieve tubes. Both sieve tubes and companion cells connect each other by plasmodesmatas.

• Phloem parenchyma : Parenchyma cells associated with phloem is called phloem parenchyma. These are living in nature. These are help in storage of starch, fats, tannins and resins.

• Phloem fibres : These fibres are sclerenchymatic in nature. These are narrow, vertically elongated cells with thick walls and small lumen. These are the only dead cells in phloem tissues. These cells provide strength and support.

                         

   Difference of xylem and phloem

According to the diagram, tracheids are thin and elongated whereas vessels are thick and smaller than the tracheids. In vessels perforated plates are present. Fibres have small lumen (cavity).

Sieve tubes are perforated and companion cells are attached to them for the connection between both the cells by plasmodesmata.