Seed structure: monocot and dicot
Seed structure explained: testa, hilum, micropyle, cotyledons, embryo axis, endosperm, aleurone, scutellum, coleoptile and coleorhiza, bean vs maize.
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A seed is a mature ovule that contains an embryo, a food reserve and a protective coat. It is how a plant carries its next generation through time and space. It can survive drought, cold or years in storage, and then start a new plant when conditions are right. For agriculture, seed structure matters for seed quality, germination testing, sowing depth, seed treatment and storage. It also explains why rice is polished, why wheat bran is nutritious, and why some seedlings push their cotyledons above the soil while others leave them buried.
The fruit develops from the ovary, and the seed develops from the ovule inside it. In cereals the two are fused, so a wheat or maize "seed" is botanically a fruit, the caryopsis.
Parts of a dicot seed: bean and gram
Seed coat
The seed coat develops from the ovule's integuments. Its tough outer layer is the testa, and its thin inner layer is the tegmen. On the surface there are two marks:
- the hilum, a scar where the seed was attached to the fruit by its stalk (funicle);
- the micropyle, a tiny pore next to the hilum through which water enters first during germination.
Embryo
Inside the coat is the embryo, which has three parts:
- two large, fleshy cotyledons (seed leaves), which in bean, gram, pea and groundnut hold almost all the food;
- the embryonal axis, between the cotyledons;
- the plumule (the future shoot) and radicle (the future root) at either end of the axis.
The part of the axis above the cotyledon attachment is the epicotyl; the part below is the hypocotyl. In mature bean and pea seeds the endosperm has been used up during development, so these seeds are exalbuminous (non-endospermic). Castor is an albuminous dicot: it has thin cotyledons and a large endosperm.
Parts of a monocot seed: maize and wheat
A cereal grain is more complex.
- Pericarp fused with the testa forms the thin outer layer, the bran.
- The aleurone layer is one to a few layers of protein-rich cells just inside the bran. During germination it secretes α-amylase and other enzymes in response to the hormone gibberellin.
- The endosperm is the large, starchy food store, and makes up about 80–85% of the grain. Because of it, cereal grains are albuminous (endospermic).
- The embryo (germ) sits on one side near the base and is made up of:
- a single shield-shaped cotyledon called the scutellum, which absorbs food from the endosperm;
- the plumule, enclosed in a protective sheath, the coleoptile;
- the radicle, enclosed in its own sheath, the coleorhiza.
| Feature | Dicot seed (bean) | Monocot grain (maize) |
|---|---|---|
| Cotyledons | Two, fleshy | One (scutellum) |
| Main food store | Cotyledons | Endosperm |
| Endosperm at maturity | Usually absent (exalbuminous) | Present (albuminous) |
| Seed coat | Separate testa | Fused with the pericarp |
| Aleurone layer | Absent | Present |
| Protective sheaths | None | Coleoptile and coleorhiza |
Germination: epigeal and hypogeal
When a seed takes up water (imbibition), enzymes become active, reserves are broken down, and the radicle emerges first. What happens next depends on which part of the axis elongates:
- In epigeal germination the hypocotyl elongates and lifts the cotyledons above the soil, where they often turn green. Examples are bean, cotton, castor, groundnut (partly) and cucurbits.
- In hypogeal germination the epicotyl elongates and the cotyledons stay below ground. Examples are pea, gram, maize, wheat, rice and mango.
This has practical consequences. Epigeal seedlings must push their cotyledons up through the soil, so they are sensitive to deep sowing and soil crusting. Hypogeal seedlings tolerate deeper sowing better, which is why wheat can be drilled deeper than soybean.
How the cereal grain fuels its own germination
When a wheat seed imbibes water, the embryo releases gibberellin. The hormone travels to the aleurone layer, which responds by making α-amylase and proteases. These enzymes break down endosperm starch into sugars and proteins into amino acids. The scutellum absorbs them and passes them to the growing embryo. Maltsters use this sequence when they germinate barley to make malt, and adding gibberellic acid can speed it up.
Seeds and nutrition
Milling separates the parts of the grain, and with them its nutrients:
- Bran (pericarp, testa and aleurone) is rich in fibre, minerals, B vitamins and protein.
- The germ is rich in oil, vitamin E and protein. Its oil goes rancid easily, which is why whole-wheat flour has a shorter shelf life.
- Endosperm is mainly starch with some protein, such as gluten in wheat.
Polishing rice removes bran and germ, so white rice has far less thiamine than brown rice. When people relied on polished rice, this caused beriberi. Parboiling helps: soaking and steaming paddy before milling drives some vitamins into the endosperm.
Key points
- A seed is a mature ovule: seed coat (testa and tegmen), embryo and food reserve.
- The hilum is the scar of the stalk; the micropyle is the pore where water enters.
- Bean is a dicot with two fleshy cotyledons and is exalbuminous. Maize is a monocot with one cotyledon (the scutellum), a large endosperm, and is albuminous.
- In cereals, the coleoptile protects the plumule and the coleorhiza protects the radicle; the aleurone layer secretes amylase when triggered by gibberellin.
- In epigeal germination the cotyledons rise above the soil (bean, cotton); in hypogeal germination they stay below (pea, maize).
- A cereal grain is a caryopsis: the pericarp is fused with the testa.
What the exams ask
- Draw and label a bean seed and a maize grain.
- Distinguish albuminous from exalbuminous seeds, with examples.
- Define coleoptile, coleorhiza, scutellum and aleurone layer.
- Compare epigeal and hypogeal germination, with examples.
- Explain the role of gibberellin and the aleurone layer in germination.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is a groundnut kernel a seed or a fruit?
The shell is the fruit wall (pod), and each kernel inside is a seed. The thin pink or red skin is the testa, and the two halves are the cotyledons, which store the oil and protein. The small "heart" between them is the embryo axis.
Why are onion seeds sown shallow?
Small seeds have little stored food, so the seedling can only grow a short way before it must reach light and start photosynthesising. Sown too deep, it uses up its reserves before emerging. A common rule is to sow seeds at a depth of about two to three times their diameter.
Does a seed breathe in storage?
Yes, slowly. Respiration uses up stored food and produces heat and moisture. Seed moisture and temperature control the rate, which is why seeds are dried before storage. Harrington's rule of thumb says seed life roughly doubles for each 1% fall in moisture, or each 5 °C fall in temperature, within the normal range.
Sources
- Bewley, J. D., Bradford, K. J., Hilhorst, H. W. M. and Nonogaki, H. Seeds: Physiology of Development, Germination and Dormancy, 3rd edition. Springer, 2013.
- Raven, P. H., Evert, R. F. and Eichhorn, S. E. Biology of Plants, 8th edition. W. H. Freeman, 2013.
- Copeland, L. O. and McDonald, M. B. Principles of Seed Science and Technology, 4th edition. Springer, 2001.
- Harrington, J. F. "Seed storage and longevity." In Kozlowski, T. T. (ed.) Seed Biology, vol. III. Academic Press, 1972.
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