Stem

Fern stems vary from the tall, narrow trunks of certain tree ferns that reach 25 metres (80 feet) tall down to clumped or creeping rootstocks, or rhizomes. Rhizomes are the most common stem form. The majority of them grow horizontally upon or just beneath the surface of the soil. Some stems are so narrow as to be threadlike, as in many tropical epiphytic ferns. A few ferns in different parts of the world have evolved radically specialized stems containing chambers in which ants take up residence; the role of the ants in the lives of these ferns is unknown, but it may be for protection against other insects. Vinelike ferns are common, but shrubby ferns are extremely rare.

Stem growth is initiated by one to several large apical cells. These are usually well protected by various types of hairs or scales and by the overarching embryonic leaves. Leaves and leaf bases play a major role in the protection of fern stems, and many stems are said to have a leaf armour. Such stems are densely covered with old sclerified leaf bases, which increase the apparent size of the stem many times. The old leaf bases may serve as protection or as food-storage organs. In most species the stems are indeterminate in growth and thus can theoretically continue to grow indefinitely. Annuals—short-lived species that complete development, shed spores, and die in a single growing season—are exceptional; only a few examples are known.