The many functions of plant stems
What do stems do? Besides holding up the plant (providing support), stems bear the leaves and buds and reproductive structures of plants. They also conduct water throughout the plant and help store food and water, just like leaves.
Stems conduct water and nutrients throughout columns called vascular bundles. Xylem (pronounced zy-lem) is a type of vascular bundle that transports water up the stem from the ground. Phloem (pronounced "flo-em") transports the sugars produced in leaves down the stem. These wonderful vascular bundles are everywhere inside the stem, and are even located in leaves, where they are called veins. Just like in humans, the veins and vascular bundles act as a transport system for nutrients and fluids.
Many commercially important fibers come from stems - like linen, which comes from flax (for those REALLY nice DKNY dresses!). But clearly, the most important fiber to come from stems is wood.