Phloem is a complex tissue of a plant which was first introduced by a scientist Nageli in the year 1853. It is a part of the vascular system in a plant cell which involves the translocation of organic molecules from the leaves to the different parts of plants like stem, flowers, fruits and roots.
Therefore phloem is a cell which is made of specialized tissue known as “Vascular tissue” that allows conductance of food in the vascular plants. It consists of living cells like sieve cells, companion cells, phloem parenchyma and the only dead cell which is phloem fibres.
- Definition of Phloem
- Structure of Phloem
- Important facts
Definition of Phloem
Phloem can define as the specialized tissue of the plant cell which anchors the conductance of food from the photosynthesized part (leaf) to the non-photosynthesized parts (like stem, flowers, buds, fruits, roots). Therefore it carries out the translocation of the soluble organic molecules like amino acids, sugars etc. as a source of food and energy to the plant.
The phloem can be classified into two types that are given below:
Primary phloem: It has originated from the procambium which develops as a result of the primary growth of the plant. It is again subdivided into protophloem and metaphloem. Protophloem is the type, which has formed first whereas the metaphloem is a type, which has formed later. The chief difference between protophloem and metaphloem is:
- Protophloem consists of narrow sieve tubes whereas metaphloem consist of broad sieve tubes.
In most of the plant, the existence of phloem in both stem and roots is found external to that of the xylem.
But, the phloem can be present either internal or external to the xylem. Therefore, when the phloem is internal to the xylem, then it will refer as “Internal Phloem” and when present external to the xylem then it will refer as “External phloem”.
In leaves, the occurrence of phloem is on the lower side or abaxial surface of the leaf. But, in certain plants species which belong to the Cucurbitaceae and Convolvulaceae family, the phloem is present on both the adaxial and abaxial surface of the leaf.
Structure of Phloem
The structural components of the phloem are as follows:
It is the essential element which is composed of living cells. Sieve elements comprise of sieve tube and sieve cells.
- Sieve tube: Sieve tubes are the long and slender tube which are joined to each other from end to end. At the end of the sieve tube, a sieve plate is present which consist of many sieves or sieve pores.
Sieve tubes lack nucleus and cell organelles like the Golgi body, ribosomes, endoplasmic reticulum etc. which is a peculiar feature. On maturity of the plant, sieve tube contains a large vacuole due to which the cytoplasm will move towards the periphery and will present in the form of a thin layer.
- Sieve cells: These are long, thin-walled with tapering ends. Sieve pores are present all over the surface of the sieve cell in contrast to sieve tubes.
Functions of sieve elements:
- Sieve tubes are involved in food conductance.
- Cellulose microfibrils which are the major constituent of the sieve tube provide the mechanical strength to the sieve tube.
- Sieve plate also allows back and forth movement of sap through the holes.
These are also the living components of the plant cell which are associated with each sieve tubes. Companion cells contain its own nucleus by which they not only control their activity but also controls the activity of the neighbouring sieve tubes. It can refer as modified parenchyma which comprises of dense cytoplasm, cell organelles. Due to the presence of cell organelles, the companion cells are metabolically very active. Companion cells release ATP for the translocation of food and thus helps in food conductance.
Plasmodesmata: These are the cytoplasmic strands which link the sieve tube and the companion cells.
Lumen: It is the empty space of the tube or has no organelles.
Cell wall: It is the outermost covering of the sieve tube. The cell wall provides resistance to the sieve tube from the high pressure.
Cell membrane: It is the phospholipid bilayer membrane which is present interior to the cell wall. Cell membrane holds the sap inside the tube and pumps sucrose in and out of the sieve tube.
Phloem parenchyma: It is also a living component which is composed of parenchyma cells. It is responsible for the radial conduction of food and also acts as storage cells which store latex, resins etc.
Phloem fibre: It is the only dead element, which is composed of sclerenchyma. The fibre is thick walled with a narrow cavity and tapering ends. Phloem fibres provide mechanical strength to the cell.
- Presence of sieve tube is the identifying feature that is found in Angiosperms.
- In winter, the sieve tube which consists of sieve pores is blocked by the callose carbohydrate also known as “Callose plugs”. The callose plug will block the passage of sap through the sieve pores or stops the conductance of food. But as the spring season comes, the callose that has blocked the sieve pores will dissolve due to the rise in temperature and will open the sieve pores.
- The presence of sieve cells is the identifying feature that occurs in Gymnosperms and Pteridophytes.
- Sieve tube and the companion cells are also known as “Sister Cells” because both of them have originated from a single mother cell.
- In gymnosperms and pteridophytes, companion cells are absent.
- Gymnosperms (Particularly conifers) consist of albuminous cells in place of the companion cells.
- Phloem parenchyma is absent in monocots.
- Phloem helps in the food conductance like sugar, amino acids etc. from leaves to the other parts of plants.
- It can also help in the transportation of proteins and mRNAs.
- The sieve tubes of phloem give strength to the plant against cell bursting.
- Sieve plate allows the continuous movement of sap through the sieve pores both in a forward and backward direction that keeps the cell hydrated.
- Phloem parenchyma can store ergastic materials like resin, latex etc.
Therefore, phloem helps in the translocation of food from the source (Leaves) which carry out the process of photosynthesis and sinks it to the other parts which are used up for the growth and other activities of the plant.