Seeds of Plants
There are three primary parts of a seed: the embryo, the endosperm, and the seed coat. The embryo refers to the small multicellular organism located inside the seed before it emerges. The endosperm is the food for the embryo (primarily starches) stored inside the seed. The seed coat is one or more protective layers that cover the outside of the seed.
The mature embryo has an embryonic root known as a radicle, an embryonic shoot known as a plumule, and either one or two cotyledons or “seed leaves” depending on whether the plant is a monocot or dicot, respectively. When the seed germinates, the embryo becomes a seedling.
At the tip of both the radical and plumule are apical meristems that trigger the primary growth of the seedling. The embryonic shoot is divided into the epicotyl and hypocotyl. The epicotyl is the portion of the plumule above where it attaches to the cotyledon(s). The hypocotyl is the portion of the plumule below where it attaches to the cotyledon(s).
Monocots store most of their food in the endosperm of the seed. A dicot stores its energy in its cotyledons. The cotyledons expand initially as the plumule and seed leaves emerge from the soil, but as the energy or food stored in them is used up, the cotyledons shrink. The food stored in the endosperm is also used up during germination.
What is Germination?
Germination is the fundamental process by which a seed grows into a young plant or seedling. Seeds are said to be viable if they are capable of germinating. There are four requirements that are essential for germination to occur: water, oxygen, temperature, and light or darkness. All plants require a steady source of water and oxygen but can differ quite a bit in temperature and light requirements necessary for germination to occur. Some seeds may need to reach a certain temperature or remain in a certain temperature range for some minimum period.
Germination begins when the seeds imbibe water rapidly, resulting in the swelling and softening of the seed coat. This starts the growth process by activating certain enzymes. It provides the hydration necessary for the vital activities of its protoplasm and increases the seed permeability. It prepares the seed for rupturing and converts insoluble food (starches, proteins, fats) stored in the seed’s cotyledons or endosperm into soluble form so that the resulting sugars and amino acids can be translocated to the parts of the embryo where it is needed for growth.
The radicle then ruptures the seed coat to form a primary root anchoring the seed in place. Root hairs develop on the new primary root and eventually lateral roots emerge. As the radicle emerges and establishes itself, the embryonic shoot or hypocotyl emerges and starts growing upward with the tip bent downward as it drags the seed leaves through the soil.
Once the shoot and cotyledons breach the surface of the soil, the hypocotyl straightens. The cells of the seed leaves become metabolically active and begin producing food for the plant through photosynthesis, ending its reliance on food stored in the seed’s cotyledons and endosperm.
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