Phloem | Definition, Types, Location and Functions
Phloem: It is a living tissue that carries food materials, (in particular, sucrose, a sugar), from the place it is available (source) to the places where it is needed (sink). It transports the organic materials, usually from leaves to other parts of the plants like roots, growing tips of stems and leaves, flowers, fruits etc. The food prepared in the leaves through the process of photosynthesis needs to be transported to each and every part of plant body, which is facilitated by this conducting tissue called phloem.
Types of Phloem
Like xylem, phloem is also classified into primary and secondary phloem on the basis of origin.
A. Primary phloem
The phloem which is formed during primary growth of the plant body is called primary phloem. It is further of two types on the basis of relative states of maturity: (i) Protophloem, (ii) Metaphloem.
The first formed primary phloem is called protophloem and later formed primary phloem is called metaphloem.
B. Secondary Phloem
The phloem which is formed during secondary growth of the plant body is called secondary phloem. It is formed by the vascular cambial ring (a lateral meistem).
Elements of Phloem
Phloem is also a complex tissue like xylem and is composed of the following four elements in angiosperms: A. Sieve tube elements, B. Companion cells, C. Phloem fibres, D. Phloem parenchyma.
A. Sieve tube Elements
Structure of Sieve Tube: Like each vessel, sieve tubes are also made up of many cells called sieve tube elements or sieve tube members.
- (a) Sieve tube elements are the long, tube-like structures which are arranged longitudinally. These cells are arranged one above the other in distinct linear rows to form the sieve tubes.
- (b) Sieve tube elements are thin-walled living structures having cellulosic cell walls (made of cellulose). However, their walls are thicker than surrounding parenchyma cells
- (c) A mature sieve tube element possesses a peripheral cytoplasm and a large central vacuole but lacks a nucleus. The cytoplasm occurs in the form of a thin-living layer along the inner side of cell walls.
- (d) The end walls of sieve tube elements are called sieve plates which are generally Oblique. A sieve plate has numerous sieve pores through which the food materials move from one element to another. The end plates are called sieve plates because they are perforated in a sieve-like manner.
- (e) Sieve tube elements are associated with the companion cells (another element of phloem). As mature sieve tube elements lack nuclei, the functions of sieve tubes are controlled by the nucleus of companion cells which are found closely associated with sieve tube elements. Both sieve tube and companion cells are ontogenetically related, hence, they are called sister cells.
- (f) Protophloem consists of narrow sieve tubes and metaphloem has bigger sieve tubes.
Occurrence of Sieve Tube: Sieve tubes are absent in the pteridophytes and gymnosperms. These plants have sieve cells in place of sieve tubes, which are not arranged distinctly in linear rows. Like sieve tubes, the sieve cells also have sieve plates but, not on their end plates rather on their lateral or side walls. Moreover, sieve cells are not much specialised like sieve tubes of angiosperms.
Functions of Sieve Tube: Sieve tube elements are the food conducting elements of plants. The synthesised food translocates through sieve tube elements in the form of a continuous channel.
B. Companion Cells
Structure of Companion Cells: Companion cells are the specialised parenchymatous cells which are closely associated with sieve tube elements, due to which they are called the companion cells.
- (a) These are long, narrow and thin-walled cells, usually attached to the lateral sides of sieve tube elements.
- (b) The common wall present between sieve tube element and companion cell is usually thin and contains pit fields. Pit fields are the wall areas of greatly reduced thickness which look like holes in the walls. The pit fields are present between the common longitudinal walls of sieve tube elements and companion cells as these two are longitudinally attached with each other. The pit fields enable the easy transfer of materials between them. Hence, pit fields maintain close cytoplasmic connections between sieve tube elements and companion cells.
- (c) In contrast to sieve tube elements, the companion cells retain a nucleus throughout their life. They are thus, living cells.
Occurrence of Companion Cells: Companion cells are found in angiosperms only. Gymnosperms and Pteridophytes lack these cells. Gymnosperms possess albuminous cells, in place of Companion cells. The albuminous cells are associated with sieve cells in them.
Functions of Companion cells: The companion cells help in maintaining the pressure gradient in the sieve tubes. The pressure gradient can be defined as the difference in pressure that decides the direction of flow of a fluid.
C. Phloem fibres
They are also called the bast fibres.
- (a) These are the sclerenchymatous fibres.
- (b) These are much elongated fibres which do not have branches (unbranched)
- (c) The fibres have pointed, needie-like apices.
- (d) The cell wall of phloem fibres is quite thick due to which they provide mechanical support to the plant.
- (e) At maturity, these fibres lose their protoplasm and become dead.
Occurrence of Phloem Fibres: These fibres are generally absent in the primary phloem but are found in the secondary phloem.
Functions of Phloem Fibres: Phloem fibres provide the mechanical support to the plant.
Commercial use: Phioem fibres of jute, lax and hemp are used commercially for various purposes.
D. Phloem Parenchyma
It is composed of the living parenchymatous cells. These are the elongated cells which have tapering ends. They are cylindrical in shape.
- (a) These cells have dense cytoplasm and a prominent nucleus
- (b) The cell walls of these cells are made up of cellulose and have pits. With the help of these pits, the adjoining cells get connected in the common walls between them. Plasmodesmata are the microscopic channels which are formed by these pits connecting the cells. Hence Pits form the plasmodesmatal connections between the phloem parenchyma cells.
Occurrence of Phloem Parenchyma: It is found in both primary and secondary phloem. Phloem parenchyma is absent in most of the monocotyledons.
Functions of Phloem Parenchyma:
- The cells of phloem parenchyma perform the main function of storing the food synthesised by the cells carrying out photosynthesis. They also help in the transport of food.
- These cells also store various organic materialslike resins, tannins, mucilage (thick gluey substance), latex (the milky fluid). These can be seen exuding from the plants. The food materials and other organic materials enter in these cells due to the presence of pitted cell walls.