Plant stalks, also known as stems, are the main support structure of a plant. They are typically composed of cellulose and lignin, and are responsible for transporting water and nutrients from the roots to the leaves. Stems also provide structural support for the plant, allowing it to stand upright and reach for the sun. Stems can be either woody or herbaceous, depending on the type of plant. Woody stems are found in trees and shrubs, while herbaceous stems are found in annuals and perennials.