Stems are axes that are typically cylindrical, elongated and branching, though many modifications can occur in different species. Shoot apical meristems are present at the tips of all the stem branches; lateral branches are initiated from buds that are borne in the axils of leaves. Stems are most commonly aerial, though some stems occur below ground. Aerial stems are often green and photosynthetic during early growth but subsequently turn brown following radial stem thickening. Some underground stems are modified into storage organs such as corms or rhizomes that allow them to survive a harsh winter or dry season below ground.