1. Transports sugars:
* Source to Sink: Phloem's main job is to transport sugars produced during photosynthesis (in the leaves, the "source") to other parts of the plant that need them (the "sinks"). These sinks can include:
* Roots: for growth and storage
* Stems: for growth and structural support
* Flowers: for reproduction
* Fruits: for seed development and dispersal
* Storage organs: like tubers or bulbs
2. How it works:
* Phloem Sap: Phloem contains a fluid called phloem sap, which is mainly composed of sugars (mainly sucrose) dissolved in water.
* Pressure Flow: The movement of phloem sap is driven by a pressure gradient. Sugars are actively loaded into phloem cells at the source, increasing the concentration of solutes and water potential. This causes water to move into the phloem from nearby xylem vessels, creating positive pressure. At the sink, sugars are actively unloaded, decreasing the concentration of solutes and water potential. Water then moves out of the phloem, following the gradient and creating a pressure difference.
* Sieve Tubes: Phloem is made up of specialized cells called sieve tubes. They are connected end-to-end and have holes called sieve plates that allow the phloem sap to flow through.
3. Importance:
* Growth & Development: Phloem provides the energy (in the form of sugars) necessary for growth and development of all parts of the plant.
* Survival: The transport of sugars allows the plant to store energy reserves in roots or other organs, which can be used during periods of stress, like drought or winter.
In short, phloem is crucial for the survival and success of green plants, ensuring that vital nutrients reach all parts of the plant where they are needed.
