Bean Plant Seed Structure

From the Outside

• A lima bean seed is a good specimen to examine because of its relatively large size. Rub the seed carefully between thumb and fingers and a filmy layer will crack and slip easily off the seed. This is the seed coat. Look for a small hole along the edge of the seed. This tiny pore is the micropyle that allows water to be absorbed into the seed. Just below the micropyle is a scar (hilum) that marks the place that the seed was attached to the bean stalk during the seed's development.

A Look Inside

• Using your fingernail, pry the bean seed open along the slit going around the bean seed's edge. The bean will separate into two equal oval-shaped halves. These are the cotyledons that feed the seed until its leaves and roots develop, allowing the new plant to draw nutrients from the soil and water and energy from the sun. Since there are two coyyledons, the bean is classified as a dicotyledon or dicot.

The Makings of a New Bean Plant

• Inside the split bean seed, you will see the tiny beginnings of a new bean plant. This is the embryo. The leaf like structure (plumule) at one end of the embryo will become the leaves of a new bean plant, while the other end contains the radicule that will form the roots. For growth to begin, the seed must germinate.

Germination

• Not all bean seeds will germinate. The proper conditions must be met for growth to occur. The seed itself must be alive and not dormant. An overly wet seed will rot while one too dry will crack and deteriorate. The seed needs warmth, moisture, oxygen and sunlight for germination to occur. The micropyle allows water inside the bean. The cotyledons feed the embryo allowing it to split the seed coat and emerge above the ground and grow roots beneath. The cotyledons shrivel as the embryo grows and eventually drop off as the leaves and roots take over to carry nutrition to the growing plant.

Watch Germination Occur

• After examining the bean seed structure, more of the seeds may be placed in various circumstances to see what conditions are needed for germination to occur. Place a seed 1 inch deep in potting soil and keep the soil slightly damp. Place other seeds directly into water, into dry soil with no water, and into water soaked soil. Observe the seeds daily for two weeks. Record observations and make conclusions. The students will have learned how a bean seed grows into a new bean plant.