The plant's internal network of tubes (xylem and phloem) that transport water and nutrients from roots to shoots and food from leaves downward.
In plain terms
Plants have plumbing. Xylem tubes transport water and minerals from roots upward. Phloem tubes transport food from leaves downward. Together they're the vascular system. Damage to this system (disease, pest injury, sunscald) blocks transport, weakening the plant. Understanding vascular systems explains why stem damage or girdling is serious.
Why this matters
Understanding vascular systems explains why stem and root diseases are serious—they block transport of essentials throughout the plant.
In practice
Examples
Disease damages xylem; water can't reach leaves; plant wilts.