Complex tissues are heterogeneous in nature and are composed of different types of cell components. Xylem and phloem are the complex tissues that together form the vascular bundles. The xylem consists of four types of tissues such as tracheids, xylem vessels, xylem fibres and xylem parenchyma. Tracheids, vessels and fibres are dead and elongated cells that help in the transportation of water and minerals and provide mechanical support to the plants. Xylem parenchyma is the living, thin-walled cells that involved in water conduction and act as a storage tissue.

Phloem is a complex tissue of vascular plants that are conducting organic nutrients produced by photosynthesis. Xylem and phloem together form the vascular bundles. It is made up of 4 types of tissues such us sieve tube, companion cells, phloem fibres and phloem parenchyma. Sclereids and various kinds of secretory cells are also found in the phloem. In pteridophytes and many gymnosperms, phloem is composed of sieve cells and phloem parenchyma.

Sclerenchyma is a type of simple permanent tissue consisting of very thick hard and lignified walls with a very low percentage of water. More or less short isodiametric sclerenchyma cells are called sclereids or sclerotic cells. Due to the loss of the living protoplasm at maturity, these cells become dead and performing mechanical function. Generally, simple pits are present in the walls of sclereids. Sclereids are different types based on the difference in shape, size and nature of the cell wall. Osteosclereids, astrosclereids, Brachysclereids or stone cells and macrosclereids are the different types of sclereids.

Sieve tubes are living plant cells without nuclei. These are the highly specialised cells present in the phloem. These are long tube-like protoplasm containing cells whose end walls are profusely perforated forming sieve plates. These are arranged in a longitudinal row and the perforated areas are called sieve areas. Through these pores, the protoplasm of the adjacent elements is connected. Their main characteristic is the absence of nuclei in the mature cytoplasm of the cell. These cells help in the conduction of food materials from leaves to various parts of the plants.

Sieve tubes are living plant cells without nuclei. These are the highly specialised cells present in the phloem. These are arranged in a longitudinal row whose end walls are perforated by a number of pores like a sieve. The perforated areas are called the sieve areas and that part of the wall bearing sieve areas between one sieve tube member and another is called a sieve plate. Through these pores, the cytoplasm of the adjacent sieve elements is connected. Two types of sieve plates are found in plants. These are simple sieve plate and compound sieve plates. A simple sieve plate consists of only one sieve area and more than one sieve areas are there in compound sieve plates.