The Dormant Embryo
The humble yet remarkable bean seed holds all the genetic information it needs to produce a new plant that, in turn, will flower, fruit and produce more seeds to carry on the reproduction process. Each bean seed is a small, seemingly lifeless capsule that awaits just the right conditions--water, sunlight and nutrients--before it will germinate. The seed is indeed alive, but in a dormant state. The tough outer layer of the seed coat, or testa, protects a tiny embryo inside the seed. Enough moisture exists within the seed to keep the dormant baby plant alive.
The Right Conditions for Germination
Warm soil and the right planting season are critical to bean seed germination. Once the bean seed is planted in soil warmed by the sun, the waiting embryo begins to receive one of the first elements it needs to grow--water. Water enters through a small hole in the seed coat called the micropyle. The micropyle not only allows water to enter inside the seed, but it will later serve as the exit point for the radicle, the first root produced by the embryo. At this point, the embryo takes its energy from the two large lobes called cotyledons that surround it.
The interior of the seed begins to swell as it takes in water. It continues to swell, until the first root pushes its way through the hole, or micropyle, causing the seed coat and the two lobes to begin to split apart. The first root continues its downward push, reaching gradually deeper into the soil. An amazing feature of the radicle is that it will always grow down into the soil, no matter which way the seed was planted. Like a slow-moving, intricate ballet, the radicle pushes downward, at the same time forcing the lobes upward toward the surface of the soil. Just under the cotyledon, the part of the bean seed that will eventually become the stem emerges (the hypocotyl), and continues to bend slowly and gracefully above the soil.
Life Above the Soil
While the hypocotyl is pushing upward, and the radicle is pushing downward, secondary roots have begun to form along the first root. Once the hypocotyl is above the soil and has straightened, stretching toward the sunlight, the seed coat falls off, and two very tiny leaves called plumule appear on the new stem above the two lobes.
In a bean, the plumule has already formed inside the seed. The emergent plumule forms the first real leaves of the new bean plant. The stem continues to grow, new leaves sprout, flowers bloom and beans with new embryonic capsules develop.