Phloem Transport - Flow from Source to Sink
The food prepared in the leaves is transported to the different parts of the plant in the soluble form. This process of food transport is referred to as translocation. The excess food materials are stored in the insoluble form in different storage organs.
Direction of translocation
Translocation of food occurs in the downward, upward and lateral directions.
This type of translocation takes place from the leaves downwards to the stem, roots and other storage organs.
In some stages of plant life such as seed germination, emergence of new shoots from underground storage organs and development of buds, flowers and fruits, the food materials are translocated upward.
In certain parts of stem and roots, food is translocated in lateral direction through medullary rays.
The Pressure flow or mass flow hypothesis
The translocation of sugars from source to sink is called the pressure flow hypothesis. In plants, the movement of sugars in the phloem begins at the area of source, where sugars (photosynthates) are loaded due to active transport from the mesophyll tissue. Due to loading of sugars, phloem attains hypertonic condition. Water in the adjacent xylem moves into the phloem by osmosis. Sugars will dissolve in water and are transferred from the area of source (phloem of leaves) to area of sink (phloem of roots) through the sap present in the cytoplasm.
Ringing experiment to demonstrate downward translocation
Take a plant and remove the bark and internal tissues upto phloem in the form of ring leaving the xylem and pith. The portion from where the tissue is removed is sealed with wax. After 7-8 days, the epidermis and cortex of upper portion of the ring become very much swollen and adventitious roots emerge from this part, proving that food materials from the leaves does not pass through the ring and is stored in the upper portion. As we have removed the phloem tissue in the region below the ring, food materials accumulate at the region of cut. This experiment shows that food material is translocated only through phloem cells from leaf (source) to root (sink).