The stem of a plant provides pathways for the distribution of water and nutrients between the roots, leaves, and other parts of the plant. The herbaceous stem of the dandelion (top,center) lacks lignin, the stiffening material in rigid, supportive woody stems. For this reason, herbaceous plants are generally limited in their physical size. Spurges and cacti (bottom, left), their leaves reduced to needles to prevent evaporation in a dry climate, consist entirely of stem material. Tubers, such as potatoes (top, right), are swollen, food-storing, underground stems that nourish growing buds. The stems of some plants are adapted for protection, as in the hawthorn (bottom, left). Others actively compete for sunlight, using touch-sensitive, curling tendrils (top, left) or other structures to climb upwards.