differenciate between sieve tube, sieve plate and companion cells

Also tell about sieve cells

Sieve Tubes Vs Companion Cells In Tabular Form
BASIS OF COMPARISON SIEVE TUBES COMPANION CELLS
Description Sieve Tubes are a series of cells placed end to end to form a continuous tube. Companion cells are specialized parenchyma cell, located in the phloem of flowering plants and closely associated in development and function with sieve tube elements.
Nature Of Cells They are usually living cells with an elongated structure. They are living and elongated in structure with thin walls.
Mitochondria & Other Organelles They contain fewer mitochondria. They contain a large number of mitochondria and ribosomes as they are very metabolically active.
Function They are responsible for the transport of food throughout the plant body. They are responsible for regulating the activity of the sieve element.
Prominence Of Nucleus In sieve tubes, nucleus may be prominent in young stage of the plant but later disappears. Companion cells have a prominent elongated nucleus, which is retained throughout the life of the cell.
Presence The sieve cells of gymnosperms and pteridophytes have no companion cells. The companion cells only exist in angiosperms.
Other Structures The connecting strands in a sieve area remains encased in a cylinder of substance referred to as callose. Structures like callose, are not found in companion cells.
Metabolic Activity They show low rate of metabolic activity. They show high rate of metabolic activity.
Transverse Wall The transverse wall of sieve tubes contains pores. The transverse wall of companion cells does not contain pores.
Size Of Vacuole Sieve tubes are comparatively large cells with fairly large vacuoles. There are vacuoles in the companion cells but they remain very small and also contain nucleus.

Similarities between Sieve tubes and Companion cells



Below are the similarities between the two:
(a) Both are two different types of cells that are found on the phloem of angiosperms.
(b) Transportation of food throughout the plant is done by these two cells.
(c) Both the cells have an elongated shape.
(d) Both the cells emerge from the meristem.

Sieve plates are the connection sites between sieve elements. During early development of young sieve tubes, sieve plates resemble normal cell walls. As in other cell walls, plasmodesmata connect the cytoplast of adjacent sieve elements.