Tissues in Plants and Animals


   
 
Complex Tissues
Complex tissues are made up of more than one type of cells and they work together as a unit. They transport water, salt and prepared food material to various parts of the plant body. Complex tissues are of two types.
 
Xylem
 
Phloem (bast)
 
Xylem and Phloem are also called vascular tissue and together they constitute vascular bundles.
 
Xylem
Xylem or wood is a conducting tissue and is composed of elements of different kinds. They are:
 
Tracheids
 
Vessels or tracheae
 
Xylem parenchyma
 
Xylem sclerenchyma
 
Nature
 
Tracheids
 
These are elongated cells with sloping walls enclosing a large empty cavity. Cell walls are thickened with lignin. The cells are dead.
 
Vessels or trachea
 
These are cylindrical tube-like structures formed by a row of cells placed end to end. The transverse walls between the cells are almost dissolved forming a continuous channel or water-pipe. The cells are dead.
 
Xylem parenchyma
 
These are made up of parenchymatous cells. They are thin walled and are living.
 
Xylem sclerenchyma
 
These are sclerenchymatous cells and form the wood in older plants. The cells are dead.
 
Occurrence
 
Found in the root, stem and leaves. They occur along with phloem to form the vascular bundle.
 
Function:
 
Conduct water and minerals upwards from the roots to the stem and leaves.
 
Cells that are lignified to give mechanical strength to the plant.
 
                           
 
                            Tracheids with scalariform pits
 
Phloem (bast)
Phloem or bast is also a conducting tissue and is composed of elements of different kinds.
 
Elements of phloem
 
 
Nature
 
Sieve Tubes
 
The cell walls are thin and the cells elongated, placed end-to-end forming a slender tubular structure. Transverse walls are perforated forming a sieve plate. Cytoplasm of one sieve element is continuous with those of the sieve elements above and below by cytoplasmic connections passing through the pores of the sieve plate.
 
Companion cells
 
Associated with the sieve tube is a small cell containing dense cytoplasm and a large elongated nucleus.
 
Phloem Parenchyma
 
These are ordinary parenchymatous cells and are living. They are absent in monocot stems.
 
Bast Fibres
 
These are sclerenchymatous cells. The cells are dead.
 
Occurrence
 
Phloem is found in all parts of the plantlike roots, stem and leaves. They occur together with the xylem to form the vascular bundle.
 
Function
 
The function of the phloem is to conduct prepared food from the leaves to the growing parts of the plant and the storage organs.
 
                        
 
                                         Sieve-tube in transection
 
In a nutshell Xylem and Phloem can be summarized like this:
 
 
 
     
   
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