Plants have stems because the stems store and conduct food and water throughout the plants, keeping them alive. Some stems are woody, as seen in trees, while others are herbaceous, such as the stem of a flower.
Stems transmit food, water and minerals throughout the plant. The roots absorb the water and minerals, and the stem sends it to the leaves and the bud or flower. Meanwhile, the food produced in the leaves is sent through the stem to the bud and roots. According to the Missouri Botanical Garden, stems are the plumbing system of the plant.
Woody stems are reserved for those plants that must survive from year-to-year, like trees. The wood acts as a protective barrier to the more fragile inner tissue of the stem. Plants that only need to last for a year, such as flowers, have herbaceous or soft stems.
Although many plants have stems, not all of them do; for example, mosses have no stems. Other stems are considered modified stems because they do not look or act exactly like regular stems. Some of these modified stems grow underground and are edible, like potatoes. Many stems are edible or make edible products, like the maple syrup of maple trees and the sugar from sugarcane.