In plant physiology

Plant physiology

Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Closely related fields include plant morphology , plant ecology , phytochemistry , cell biology, and molecular biology.Fundamental processes such as photosynthesis, respiration, plant nutrition,...

, the epicotyl is the embryonic shoot above the cotyledon

Cotyledon

A cotyledon , is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants...

s. In most plants the epicotyl will eventually develop into the leaves of the plant. In dicots, the hypocotyl

Hypocotyl

The hypocotyl is the stem of a germinating seedling, found below the cotyledons and above the radicle .-Dicots:...

 is what appears to be the base stem under the spent withered cotyledons, and the shoot just above that is the epicotyl. In monocot plants, the first shoot that emerges from the ground or from the seed

Seed

A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...

is the epicotyl, from which the first shoots and leaves emerge.

In sunflower seedlings, the difference between epicotyl and hypocotyl can be seen from the parts of the stem divided by the first pair of leaves. Here the stem above the cotyledons is covered in tiny hairs whereas the region of stem below (hypocotyl) is smooth.

Lengthening of the epicotyl is thought to be controlled by the phytochrome photoreceptors.