Generally, plant stems are thought to be vertical structures, but they may also lie on the ground, climb on rocks and other objects, or be completely subterranean. However, no matter what their shape or size, the stems of all plants carry out similar functions.

The principal occupation of the stem is to sustain the leaves by conducting water, food, and minerals to them. Through photosynthesis, the leaves transform the materials into a more utilizable form, which the stem then transports to the roots and other areas of the plant. In order to conduct the substances, the plant stem utilizes the vascular tissues xylem and phloem.

Many plants have modified stems that are used for additional purposes. Some stems, such as tubers and rhizomes, store large amounts of food and water. The potatoes that are eaten by humans are actually underground plant stems swollen with stored starch. Water storage is particularly important to the stems of cacti since they must endure long periods with minimal amounts of rainfall. Cacti stems are also primarily responsible for photosynthesis, since most species do not bear leaves.