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:

Xylem	Phloem
1. Is a complex tissue	1. Is a complex tissue
2. Consists of mainly dead elements	2. Consists of living tissue
3. Conducts water and minerals from the roots to the aerial parts of the plant.	3. Conducts prepared food from the leaves to storage organs and growing parts of the body