Phloem transport: flow from source to sink
- Sucrose is transported by the vascular tissue phloem from a source to a sink.
- Source is the place which synthesises the food, i.e., the leaf and sink is the part that needs or stores the food.
- Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bi-directional, which contrasts with that of the xylem where the movement is always unidirectional, i.e. upwards.
- Food in phloem sap can be transported in any required direction.
Phloem sap transports water and sucrose, but other sugars, hormones and amino acids are also transported or translocated through phloem
Fig. Source-sink relationship