Phloem is another type of complex permanent tissue in plants, and it plays a crucial role in the transport of organic compounds, primarily sugars (mainly sucrose), from the site of photosynthesis (usually the leaves) to various parts of the plant where these compounds are needed. Here are the key characteristics and functions of phloem tissue:
- Cell Types: Phloem is composed of several cell types, including:
- Sieve Tube Elements: These are elongated, living cells responsible for the long-distance transport of sugars. They lack a nucleus and have perforated sieve plates at their ends, allowing for the movement of nutrients.
- Companion Cells: Each sieve tube element is closely associated with a companion cell, which has a nucleus and provides metabolic support to the sieve tube element.
- Parenchyma: Phloem parenchyma cells may store and distribute organic compounds within the phloem.
- Fibers: Phloem fibers provide mechanical support to the tissue.
- Functions:
- Transport of Sugars: The primary function of phloem is the transport of sugars (mainly sucrose) produced during photosynthesis in the leaves to other parts of the plant, such as roots, stems, and developing fruits. This process is known as translocation and is essential for providing energy and nutrients to all plant cells.
- Metabolic Support: Companion cells support the metabolic activities of sieve tube elements by providing nutrients and maintaining the functionality of the phloem.
- Bi-directional Flow: Phloem allows for bidirectional flow, meaning it can transport nutrients both upward (source to sink) and downward (sink to source) within the plant.
- Distribution: Phloem tissue is commonly found adjacent to the xylem within vascular bundles in plant stems, leaves, and roots.
- Adaptations: Phloem is adapted to efficiently transport organic compounds. Sieve tube elements are connected end-to-end to form sieve tubes, and the perforated sieve plates enable the flow of nutrients through the phloem.