How Seeds Start New Plants: From Structure to Germination

A dicotyledonous seed, like a bean or gram, has a tough outer seed coat, a scar called the hilum, a tiny pore called the micropyle, two large cotyledons, and an embryo made up of the plumule and radicle. The outer seed coat is actually two layers: the hard outer testa and the thin inner tegmen. The hilum is the spot where the seed was attached to the fruit, while the micropyle is a small opening through which water can enter during germination. The two cotyledons store food for the embryo and make up most of the seed’s bulk. The embryo has two main parts: the plumule, which will grow into the shoot, and the radicle, which will form the root. For example, if you soak a dried bean seed in water overnight and then gently remove the seed coat, you can see the two cotyledons and the tiny embryo between them. Many students mix up the hilum and micropyle, but the hilum is larger and more visible, while the micropyle is a tiny dot near it.

A dicot seed (like bean or gram) has two cotyledons, while a monocot seed (like maize) has only one cotyledon and a large endosperm. In dicot seeds, the cotyledons store food and nourish the embryo during germination. In monocot seeds, the endosperm acts as the main food store, and the single cotyledon (called the scutellum) helps absorb food from the endosperm. The seed coat in monocots is often fused with the fruit wall, making it harder to separate. For example, if you split open a soaked bean, you see two thick cotyledons, but in maize, you see a single cotyledon and a starchy endosperm. Students sometimes think all seeds have two cotyledons, but monocot seeds like maize and rice have only one.

Germination is the process by which a seed develops into a new plant when it gets the right conditions. For a seed to germinate, it needs water, air (oxygen), and a suitable temperature. Water softens the seed coat and activates enzymes that help break down stored food. Oxygen is needed for respiration, which gives the embryo energy to grow. The right temperature ensures the enzymes work properly; too cold or too hot, and the seed may not germinate. For example, if you plant a bean seed in moist soil and keep it warm, it will sprout, but if the soil is dry or too cold, nothing happens. Some students think seeds need sunlight to germinate, but most seeds can germinate in darkness as long as they have water, air, and the right temperature.

Epigeal germination is when the cotyledons are pushed above the soil, while in hypogeal germination, the cotyledons stay below the soil. In epigeal germination (seen in beans and castor), the hypocotyl (the part below the seed leaves) grows and lifts the cotyledons up, so they become visible above the ground and often turn green. In hypogeal germination (seen in pea, gram, and maize), the epicotyl (the part above the seed leaves) grows, so the cotyledons remain underground and only the shoot emerges. For example, if you plant beans and peas side by side, you’ll see bean cotyledons above the soil but pea cotyledons stay below. Students often confuse the two types, but looking at where the cotyledons end up after sprouting helps you tell them apart.