(i) Sieve Elements (Plate and Sieve areas):
There is a presence of sieve plate and sieve areas in sieve elements.
- Groups of pores present in the walls of sieve elements are called sieve areas. In a sieve cell, these areas are generally located in lateral walls whereas in sieve-tube element. These areas are located in the cross-walls (end walls).
- The portion of cross-wall possessing sieve areas is called sieve plate.
- In most of the angiosperms the sieve plate is single and present in the transverse or oblique end wall (eg. Cucurbita.Nicotiana, etc) It is called simple sieve plate.
- In some of the cases, there are many sieve areas in the end walls. These are termed as compound sieve plates.
- The nucleus disappears in mature sieve elements.
Function of Sieve elements:
- Translocation of organic solutes. This is achieved by their special anatomical features. The sieve tubes are syncytes (formed by the fusion of cells) and allow free diffusion of soluble organic substances.
- The callose also plays important role. Usually the perforations in the sieve plates ae surrounded by callose. The callose is soluble and disappears when the solute is dilute so that the solute can pass from one cell to another cell through the pores. The callose reappears and sometimes closes the pores when solute is less dilute stopping the movement.
(ii) Companion cells:
- These are living cells, usually always associated with the sieve tubes.
- In primary phloem the companion cell is formed by unequal longitudinal division or procambial cell.
- Thus single companion cell is usually associated with each sieve tube element. Sometimes, there may be more (e g. Carrot).
- Each companion cell is a living cell with thin cellulose cell wall and active protoplast.
- It possesses all the important cellular contents viz. nucleus, mitochondria, endoplasmic reticulum, dictyosomes, plastids, ribosomes, etc.
- The cytoplasm is compartmentalized due to presence of membrane system. It also contains slime bodies. The common wall between companion cell and sieve tube shows presence of fine pits which are transverse by plasmodesmata.
- The companion cells are absent in pteridophytes and gymnosperms. They are present in angiosperms (both in monocots and dicots).
- They assist the sieve tubes in the process of translocation of solutes.
