From source to sink, phloem transports food (mainly sucrose)

Source: Who prepares food

Sink: Part that needs and stores food

• Phloem is bidirectional as source-sink relationship is variable, the direction of movement in phloem can be downwards or upwards.
• Xylem is unidirectional as movement is always upward.
• Water and sucrose mainly formed phloem sap. But hormones, amino acids and sugars can also transported by phloem.

Mass flow hypothesis/Pressure flow:

• It is mechanism used for translocation of sugars from source to sink.
• Glucose is converted to sucrose at source.
• By passing through companion cells, sucrose reached to sieve tube cells by active transport. This produce hypertonic conditions in phloem.
• By osmosis, water moves from xylem to adjacent phloem.
• Osmotic pressure must be reduced.
•    Sucrose is converted into sugar, cellulose or energy by active transport to move out from phloem.

As osmotic pressure decrease, water moves out form phloem.

• Sieve tubes of phloem form long columns with holes in their end cells called sieve plates.
• Continuous filaments are formed when cytoplasmic strands pass through these holes.
• Sap moves through phloem when pressure inside phloem sieve tube increase and pressure flow begins.
• Sugar is removed as complex carbohydrates, at sink.
• This loss of solute produces high water potential in phloem, and water return to xylem.

Girdling:- Experiment used to identify the tissues responsible for food transportation.

• Carefully removal of the ring on bark of tree up to phloem layer depth is done.
• After few weeds, ring on stem becomes swollen in the absence of downward movement of food.

Result of Experiment:-  Phloem is responsible for translocation of food.

Transportation is unidirectional i.e. towards roots.