Morphology of Flowering Plants · Lesson 9 of 11
Dicotyledonous and Monocotyledonous Seeds
“Compare embryo, storage tissues and protective coverings in dicot and maize seeds.”
• Describe the seed coat and embryo of a typical dicot seed. • Distinguish testa, tegmen, hilum and micropyle. • Explain endospermic and non-endospermic seeds. • Label scutellum, aleurone, coleoptile and coleorhiza in a maize grain.
A seed is a protected young plant
After fertilisation, an ovule generally develops into a seed. Its embryo contains a young plant body: a radicle that will form the primary root, a plumule that develops into the shoot, and an embryonal axis connecting these regions. Cotyledons are embryonic seed leaves that can store or transfer nutrients. A protective seed coat surrounds the embryo, allowing it to survive through drying, movement and a period before suitable conditions trigger germination.
A common introductory comparison separates seeds with two cotyledons from those with one. Dicotyledonous embryos generally bear two cotyledons, as in gram and pea, while monocotyledonous embryos have one, as in wheat and maize. However, the number of cotyledons does not by itself reveal whether storage food lies inside the cotyledons or in a separate tissue called the endosperm.
External and internal features of a dicot seed
A typical dicot seed is surrounded by a seed coat with two recognised layers: the outer testa and the inner tegmen. The hilum is a scar marking the point at which the seed was attached during development, and a small nearby opening called the micropyle is visible in many seeds. These are not the root and shoot tips: the radicle and plumule are parts of the embryo inside the coat.
The outer and inner protective layers, respectively, of the seed coat.
A scar indicating the earlier attachment site of the developing seed.
A small opening in the seed coverings associated with the original ovule opening.
When the coat is removed from a soaked gram or pea seed, two substantial cotyledons can be separated. They often contain stored food used during early growth. Between them is a small embryonal axis with the plumule at the shoot end and the radicle at the root end. A correct labelled sketch should show those endpoints, rather than using the general word embryo as a substitute for every individual part.
What happens to the endosperm?
The endosperm forms during the reproductive events leading to seed development and often nourishes the embryo. In castor the mature seed retains substantial endosperm, making it endospermic. By contrast, in gram, pea and bean, food reserves are largely accumulated in cotyledons and mature seeds usually lack a substantial separate endosperm; these are non-endospermous or exalbuminous seeds.
Endospermic is not synonymous with monocotyledonous. Many monocot grains retain endosperm, yet castor is a dicot with endosperm; orchids can have tiny monocot seeds without a persistent endosperm. The more accurate comparison asks where the food reserve is retained at maturity. Two cotyledons and non-endospermy commonly occur together in pea but are logically separate characters.
The maize grain has one special cotyledon
In cereal grains such as maize, the membranous seed coat is usually fused with the fruit wall; the grain is technically a caryopsis. Much of the grain interior is bulky endosperm containing reserve food. A distinct protein-rich outermost endosperm layer is called the aleurone layer. A comparatively small embryo lies to one side near the grain base. Its single large shield-shaped cotyledon, the scutellum, lies against the endosperm and helps supply nutrients to the growing embryo.
The embryonal axis of maize carries a plumule and a radicle. Each is protected by a sheath: the coleoptile surrounds the shoot tip, while the coleorhiza surrounds the root tip. These names are often confused because both start with 'coleo-'. Their function becomes clearer when associated with the organ protected: 'optile' with the emerging shoot and 'rhiza' with the embryonic root. Both belong to the embryo, not to the thick endosperm.
| Character | Common dicot seed such as gram | Maize monocot grain |
|---|---|---|
| Cotyledons | Two prominent cotyledons | One scutellum |
| Main food reserve at maturity | Often cotyledons | Large endosperm |
| Coverings | Testa and tegmen in seed coat | Thin seed coat usually fused to fruit wall |
| Protective embryo sheaths | No cereal-type coleoptile or coleorhiza | Coleoptile over shoot, coleorhiza over root |
| Embryo placement | Embryonal axis between cotyledons | Small embryo at one side of endosperm |
Build a labelled seed diagram logically
For gram, begin with the outer seed outline, mark hilum and micropyle, then show two cotyledons and the embryonal axis with opposite radicle and plumule ends. For maize, draw the larger endosperm first, add the surrounding aleurone layer, place the embryo to one side and identify the scutellum. Finally label the shoot and root sheaths separately. A side-by-side sketch helps prevent confusing seed coat with fruit wall and scutellum with endosperm.
Soaking a gram seed softens its coat and makes the embryo easier to see. Cutting a maize grain lengthwise is useful for viewing the embryo beside the large endosperm. In every case, preserve orientation before labelling structures.
Quick Quiz
Quiz
How many cotyledons are typical of a dicot embryo?
Which part of maize is the single cotyledon?
Which structure protects the maize plumule?
What does endospermic mean?
Which feature is usually a scar marking an earlier attachment?
Practice Problems
- Draw or describe the labelled parts of a dicot seed.
- Compare endospermic and non-endospermic seeds with examples.
- Name the unique monocot seed structures of maize and their functions.
- Why is it incorrect to say all dicot seeds lack endosperm?
- Explain the distinction between hilum and micropyle.
Model Answers
1. Include outer testa, inner tegmen, hilum, micropyle, two cotyledons, embryonal axis, plumule and radicle. Explain that cotyledons commonly store food in gram or pea.
2. Endospermic seeds retain nutritive endosperm at maturity, for example castor and maize. Non-endospermic seeds lack substantial endosperm at maturity, for example gram, pea and bean.
3. Scutellum is the single cotyledon and transfers nutrients from endosperm; aleurone is the outermost protein-rich layer of endosperm; coleoptile protects plumule; coleorhiza protects radicle.
4. Castor is a dicot seed that retains endosperm; endosperm presence is distinct from the number of cotyledons.
5. Hilum is the seed attachment scar; micropyle is a small opening in seed coverings near the hilum.
Key Takeaways
A seed normally has an embryo and protective seed coat. Dicots have two cotyledons; monocots have one, but endosperm retention is a separate trait. Dicot coverings include testa and tegmen; hilum marks attachment and micropyle an opening. Maize has large endosperm and one scutellum; aleurone is a distinct outer endosperm layer. Coleoptile protects the embryonic shoot and coleorhiza protects the embryonic root.