Plants are composed of leaves, stems, and roots. The primary function of a leaf is photosynthesis, while the stem lends support and the root anchors the plant allowing the absorption of water and minerals. Each plant organ comprises meristematic or actively dividing tissues that specialize to become non-dividing permanent tissues. Meristems differentiate into three main types. Dermal tissue covers the plant in a waxy cuticle that prevents water loss through evaporation. Specialized pores in the cuticle, called the stomata, allow the exchange of gasses. Vascular tissue such as the xylem transports water and dissolved nutrients from the root throughout the plant. In contrast, phloem distributes organic compounds from sites of photosynthesis to other plant parts. Ground tissue performs different metabolic functions depending on the cell type and location. For example, parenchyma, collenchyma, and sclerenchyma provide a supporting matrix to the vascular tissue, help store water, and dissolved sugars, and provide structural support to the stem.