Phloem
Living vascular tissue that transports sugars and other compounds from source tissues to growing, storing, or metabolically active sink tissues.
Phloem transports products of photosynthesis, primarily sugars, along with amino acids, hormones, signaling molecules, and some mineral nutrients. Mature leaves commonly act as sources, while roots, young leaves, flowers, and developing seeds can act as sinks.
Movement is driven by pressure differences created as sugars are loaded and unloaded. Different phloem pathways can carry material in different directions at the same time, but flow within one conducting tube proceeds from its source toward its sink.
Nutrients vary in phloem mobility. Mobile nutrients can be redistributed from older tissue, while calcium has low phloem mobility and depends heavily on delivery through the xylem Xylem The vascular tissue that carries water and dissolved minerals from root to leaf. Flow is one way and pulled by transpiration, so anything that slows transpiration slows delivery. .