Introduction
The groups of cells that are found to have a similar structure and act together to perform a specific function are called tissues. The word tissue is derived from a verb meaning “to weave” which is a form of an old French language. In animals, there are four different types of tissues such as connective tissue, muscular tissue, nervous tissue, and epithelial tissue.
What is Phloem Tissue?
The complex tissue that acts as a transport system found in the vascular plants for the transport of soluble organic compounds, the above mentioned is the phloem definition.
The food conducting tissue in plants is made up of living tissues that use turgor pressure and energy in the form of ATP to transport sugars to the plant organs such as the fruits, flowers, buds, and roots. The other material that makes up the vascular plant transport system, the xylem, moves water and minerals from the root to various parts of the plant.
Components of Phloem Tissue
The phloem tissue is made up of several various components. Each of these components performs functions by working together, these functions include facilitating the conduction of sugars and the amino acids, from source tissues to the sink tissues where they are consumed or stored. The elements of phloem are as follows;
The Sieve Elements
The sieve elements are elongated and narrow cells that are connected together to form the phloem’s sieve tube structure. They are considered the highly specialized types of cells that are found in plants. These elements lack the nucleus at maturity and are also lacking in organelles such as ribosomes, cytosol, and Golgi apparatus, to maximize the available space for the translocation of materials.
There are two main types of sieve elements: both are derived from a common mother cell form.
Sieve Member: It is found in angiosperms.
Sieve Cells: These are associated with gymnosperms.
Sieve Plates
Sieve plates are located in between the connections of sieve member cells, which are modified plasmodesmata. They are large and thin in structure, these are the areas of pores that help to facilitate the exchange of materials between the element cells.
When the phloem is cut or damaged then in the prevention of loss of sap the sieve plates also act as a barrier, often by an insect or herbivorous animal. After the injury, a unique protein called “Phloem-protein or P-protein”, which is formed within the sieve element is released from its anchor site and accumulates to form a ‘clot’. These clots are present on the pores of the sieve plate that helps in preventing the loss of sap at the damage site.
In gymnosperms, the sieve elements have more primitive features compared to the angiosperms. They have numerous pores at the tapered end of the cell walls instead of sieve plates for material to pass through directly.
The Companion Cells
Each of the sieve element cells is closely associated with a ‘companion cell’ in angiosperms and ‘Strasburger cell’ or an albuminous cell in gymnosperms. Companion cells consist of a nucleus, that is filled with dense cytoplasm. The cytoplasm is made up of numerous ribosomes and mitochondria. Due to this reason the companion cells are responsible for performing several metabolic reactions and other cellular functions. The sieve element is a lack in appropriate organelles due to this these elements cannot involve in the process of metabolic reactions as it lacks the appropriate organelles. For function and survival, the sieve elements are dependent upon or in need of the companion cells.
The sieve tube and companion cells are connected via plasmodesmata, a microscopic channel connecting the cytoplasm of the cells, which allows the transfer of the sucrose, proteins, and other molecules to the sieve elements. The transport of materials around the plant and to the sink tissues is done by companion cells, also helps in the loading of sieve tubes with the products of photosynthesis, and the loaded products get unloaded at the sink tissues.
Phloem Parenchyma
The parenchyma is a collection of cells, which make up the ‘filler’ of plant tissues. They have thin and flexible walls that are made of cellulose. The parenchyma’s main function that is present in the phloem is the storage of starch, fats, and proteins and in the case of some plants, they help in the storage of tannins and resins also.
Phloem Sclerenchyma
The sclerenchyma is the main tissue of the phloem that provides support, stiffness, and strength to the plant. Sclerenchyma comes in two forms: fibers and sclereids; both are usually dead upon reaching maturity and are characterized by a thick secondary cell wall.
The bast fibers provide support to the tensile strength while allowing flexibility of the phloem. They are narrow, elongated cells where the walls consist of thick cellulose, hemicellulose, and lignin, and a narrow lumen.
Sclereids are slightly shorter, irregularly shaped cells that help to add compression strength to the phloem but restrict the flexibility. Sclereids act as protective measures for herbivory by generating a gritty texture when chewed.
Phloem, also known as bast, are plant tissues that transport nourishment from the leaves to the rest of the plant. Phloem tubes, companion cells, phloem fibers, and phloem parenchyma cells are all types of special cells found in the phloem.
The apical meristems (zones of new cell production) of root and shoot tips create primary phloem, which can be either protophloem or metaphloem, depending on whether the cells mature before or after elongation (growth) of the area in which it is found. Protophloem sieve tubes are unable to stretch with the elongating tissues, and as the plant ages, they are torn and destroyed. The phloem's various cell types may be transformed into fibers. In plants with a cambium, the later mature metaphloem is not destroyed and may operate for the rest of the plant's life, but it is replaced by secondary phloem.
Food material flows through a sieve tube, which is a row of sieve tube cells with a sieve-like section with holes in the sidewalls or ends walls.
Metastatic and marginal parenchymal cells, also called phloem parenchyma, are found at the thinnest branches and ends of the vein sieving tubes, where they also play a role in the food supply.
Phloem fibers are long, flexible cells that make up the soft fibers used in commerce (such as flax and hemp).
Phloem is the biological tissue of vascular plants that transports photosynthesis, a soluble organic compound produced during photosynthesis, to various regions of the plant. Translocation is the name of this type of transport. Since phloem is the innermost layer of bark, the name comes from the ancient Greek word o (phloiós), meaning "bark". Carl Nägeli first coined the term in 1858.
FAQs on Phloem
1. What is phloem tissue in plants?
Phloem is a vital complex permanent tissue found in vascular plants. Its primary role is to act as a transport system for soluble organic compounds, mainly sucrose, which is produced during photosynthesis. This process of moving food from the leaves (the source) to other parts of the plant like roots, fruits, and flowers (the sink) is known as translocation.
2. What are the four main components of phloem?
Phloem tissue is composed of four distinct cell types that work together to transport food. These components are:
- Sieve Tubes: These are long, tube-like structures that form a continuous channel for the transport of sugars. They have perforated end walls called sieve plates that allow materials to pass through.
- Companion Cells: Specialised parenchyma cells that are closely associated with sieve tubes. They possess a nucleus and other organelles to regulate the metabolic activities of the sieve tube elements.
- Phloem Parenchyma: These are living cells that store food materials such as starch, fats, and resins. They also assist in the radial conduction of food.
- Phloem Fibres (Bast Fibres): These are non-living, sclerenchymatous cells that provide mechanical strength and structural support to the phloem tissue.
3. How is phloem different from xylem?
Phloem and xylem are both crucial vascular tissues but differ significantly in their function and structure. The main difference is that phloem transports food (sugars) from the leaves, while xylem transports water and minerals from the roots. Furthermore, phloem transport is bidirectional (it can move up and down), whereas xylem transport is always unidirectional (upwards only). Phloem is primarily composed of living cells, whereas xylem consists mostly of dead cells.
4. Why are sieve tubes in phloem considered living cells even though they lack a nucleus at maturity?
Sieve tubes are considered living because they retain an active cell membrane and a living protoplast, which are essential for the active transport of sugars. The loss of the nucleus and other large organelles at maturity is a specialised adaptation that creates an open, unobstructed channel for efficient translocation. They are kept alive and metabolically supported by the adjacent companion cells, which perform all the necessary life-sustaining functions for them.
5. How do companion cells support the function of sieve tubes?
Companion cells are vital for the function of sieve tubes. Since sieve tubes lack a nucleus and most organelles, the companion cells carry out their metabolic processes. Using their own nucleus and mitochondria, they help create the pressure gradients required for loading and unloading sugars into the sieve tube. The two cell types are intricately connected through numerous channels called plasmodesmata, which allow for the direct transfer of ATP, proteins, and signalling molecules.
6. Why is food transport in phloem a bidirectional process?
Food transport in phloem is bidirectional because the direction of flow depends on the plant's needs and the location of the source (where sugar is produced) and the sink (where sugar is used or stored). This source-to-sink relationship can change with the seasons. For instance, in summer, leaves (the source) send sugar down to the roots (the sink). In early spring, the stored sugar in the roots (now the source) is transported upwards to growing buds (now the sink).
7. What would happen to a plant if its phloem tissue was completely removed in a ring around the stem?
If the phloem is removed in a ring around the stem (an experiment called girdling), the transport of sugars from the leaves to the roots would be completely blocked. Sugars would accumulate in the area above the ring, causing it to swell. The roots, being starved of their energy supply, would be unable to perform their functions of absorption and anchorage. Consequently, the roots would die first, which would ultimately lead to the death of the entire plant.
