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.

Sieve tubes are the components of phloem that 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. In mature sieve tubes, the pores in the sieve plates are often lined with the deposition of colourless insoluble polysaccharide type of carbohydrate called callose. Callose formed a cap-like mass or block the pore passage like a pad on the sieve tube surface. It is called callus pad or simply callus.

The elongated, narrow, tracheid-like phloem cell without prominent sieve areas are called sieve cells. These cells possess unspecialised sieve areas in their lateral and sometimes in their terminal walls. Sieve areas are generally provided with narrow pores and thin connecting strands. Sieve cells are not aggregated in sieve plates. The protoplasm of the cell is lies surrounding the vacuole as a thin layer and it contains starch grains and leucoplastids. The cell wall is made up of cellulose only. These are found in the phloem of pteridophytes and gymnosperms. The main function of these cells is to conduct food materials from leaves to different regions of the plant body.