Movement of substances in xylem is unidirectional, while in phloem it is bidirectional. Explain.

(N/A) $\Rightarrow$ $Xylem$ transport is unidirectional: It carries water and minerals from the roots to the aerial parts of the plant. This movement is always upwards due to transpiration pull and root pressure.
$\Rightarrow$ $Phloem$ transport is bidirectional: It transports food (sucrose) from the source to the sink.
$\Rightarrow$ $Source$: The part of the plant that synthesizes food, typically the leaves.
$\Rightarrow$ $Sink$: The part of the plant that needs or stores food, such as roots, fruits, or developing buds.
$\Rightarrow$ The source-sink relationship is dynamic and can change based on the season or the plant's physiological needs.
$\Rightarrow$ For example, in early spring, sugar stored in the roots (acting as a source) is mobilized to the developing buds (acting as a sink) for growth.
$\Rightarrow$ Because the source and sink can reverse roles, the movement of organic solutes in the phloem can occur in any direction (upwards or downwards), making it bidirectional.