The Growth of Epicotyls & Leaves of Bean Plants

The Hypocotyl

• The hypocotyl is the main, earliest stem of the plant that pushes two bean-shaped capsules called the cotyledons up through the soil. Enclosed in the center of these cotyledons is a second, undeveloped stem called the epicotyl. The hypocotyl appears as a sturdy green arch topped by matching pods and begins to straighten as it reaches the surface and sunlight. As the hypocotyl pushes up it forces the cotyledons into the light.

The Cotyledons

• The cotyledons are seed-leaves that are tightly wrapped inside the bean before germination. They are the plant's earliest food source and are pushed up through the soil by the hypocotyl, a strong, short green stem, until they can unfold in the light. Once opened, the cotyledons begin photosynthesis to support the continued development of the plant. As their nutrients are used up by the epicotyl and leaves, they wither and drop away.

The Epicotyl

• The epicotyl emerges from the cotyledons and heads upward as the cotyledons begin photosynthesis. It is the true stem of the plant and tightly furled around it are the first true leaves which begin to unwrap as the stem shoots upward.

The True Leaves

• True leaves begin as a pair of real green leaves that unfurl from the epicotyl after it emerges from the cotyledons. There is one on each side, opposite each other. Subsequent leaves will form as trifoliolates, each leaf will have three leaflets. There is also a top bud on the epicotyl called the apical meristem. It is always hidden in the newest leaf before the leaf unfolds or unwinds from the stem and is the point from which new leaves grow as the plant develops.