What are sieve tubes ? Explain.

Explanation

Sieve Elements : Sieve elements are the conducting elements of the phloem. They are two types, namely sieve cells and sieve tubes.
Sieve Cells : These are primitive type of conducting elements found in Pteridophytes and Gymmosperms. Sieve cells have sieve areas on their lateral walls only.
They are not associated with companion cells
Sieve Tubes : Sieve tubes are long tube like conducting elements in the phloem. These are formed form a series of cells called sieve tuve 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 possess simple or compound sieve plates. The function of sieve tubes are belived to be controlled by campanion cells.
In mature sieve, 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