What are sieve tubes ? Explain.

Explanation

Sieve Elements: Sieve elements are the conducting elements of the pholem. Thtey are of two types, namely sieve cells and sieve tubes.
Sieve Cells: These are primitive type of conducting elements found in Pteridophytes and Gymnosperms. Sieve cells have sieve areas on their lateral walls only. Thtey are not associated with companion cells.
Sieve Tubes: Sieve tubes are long tube like conducting elements in the phloem. These are formed from a series of cells called sieve tube elements. The sieve tube elements are arranged one above the other and form vertical sieve tube. The end wall contains a number of pores and it looks like a sieve. So it is called as sieve plate. The sieve elements show nacreous thickenings on their lateral walls. They may posses simple or sompound sieve plates. The runction of sieve tubes are believed to be controlled by companion cells.
In mature sieve tube. Nucleus is absent. It contains a lining layer of cytoplasm. A special protein (P. Protein= Phloem Protein) called slime body is seen in it. In mature sieve tubes. the pores in the sieve plate are blocked by a substance called callose (callose plug). The conduction of food material takes place through cytoplasmic strands. Sieve tubes occur only in Angiosperms.