Question
Compare the structure of a monocot seed (e.g., maize) and a dicot seed (e.g., gram/bean). What are the functions of cotyledons, endosperm, and embryo axis?
(CBSE Class 6 and 11, NEET — a straightforward comparison tested regularly)
Solution — Step by Step
A typical dicot seed has:
- Seed coat (testa + tegmen) — protective outer covering
- Two cotyledons — store food (in non-endospermic seeds like gram) or absorb food from endosperm (in endospermic seeds like castor)
- Embryo axis with:
- Plumule — develops into the shoot
- Radicle — develops into the root
- Hypocotyl — region below cotyledon attachment
- Epicotyl — region above cotyledon attachment
A maize grain is actually a fruit (caryopsis) with fused seed coat and fruit wall:
- Single cotyledon called scutellum — shield-shaped, absorbs nutrients from endosperm
- Large endosperm — stores starch (the main food reserve)
- Coleoptile — sheath protecting the plumule
- Coleorhiza — sheath protecting the radicle
- Aleurone layer — protein-rich outer layer of endosperm (produces enzymes during germination)
| Feature | Dicot Seed (Gram) | Monocot Seed (Maize) |
|---|---|---|
| Cotyledons | Two | One (scutellum) |
| Endosperm | Often absent (food in cotyledons) | Large, persistent |
| Seed coat | Distinct testa + tegmen | Fused with fruit wall |
| Plumule protection | None specific | Coleoptile |
| Radicle protection | None specific | Coleorhiza |
graph TD
A[Seed Types] --> B[Dicot - Gram/Bean]
A --> C[Monocot - Maize]
B --> D["2 Cotyledons"]
B --> E["Plumule + Radicle"]
B --> F["Endosperm may be absent"]
C --> G["1 Cotyledon = Scutellum"]
C --> H["Coleoptile + Coleorhiza"]
C --> I["Large endosperm present"]Why This Works
The number of cotyledons is the fundamental distinction between monocots and dicots, and it reflects a deep evolutionary divergence. Dicot seeds often store food directly in the thick cotyledons (like in gram, where the two halves you see are the cotyledons). Monocot seeds keep food in the endosperm and use the single cotyledon (scutellum) as an absorptive interface.
During germination, the dicot cotyledons either emerge above ground (epigeal — bean) or remain below (hypogeal — gram). In maize, the coleoptile pushes through the soil first, protecting the delicate plumule inside.
Alternative Method
For NEET, remember scutellum = cotyledon of monocots and aleurone layer = enzyme source during germination. These two terms are NEET favourites. If a question asks “which part of the maize seed produces amylase during germination?” — the answer is the aleurone layer.
Common Mistake
Students often say “maize seed has no cotyledon.” Maize has one cotyledon — the scutellum. It is not absent, just modified into a thin, shield-like structure that absorbs digested food from the endosperm. Saying “no cotyledon” is factually wrong and will cost marks.
Another trap: confusing endospermic vs non-endospermic seeds with monocot vs dicot. While most monocots are endospermic, some dicots are too (castor, coconut). And some dicots are non-endospermic (gram, pea). The presence of endosperm is independent of the monocot-dicot classification.