Complex tissues are of two types:
- Xylem
- Phloem (bast)
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
Phloem (Bast)
Phloem or bast is also a conducting tissue and is composed of elements of different kinds.
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.
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 |
