NCERT Grounding
Seed structure is covered in Section 1.4.3 (Seed) of NCERT Class 12 Biology, Chapter 1: Sexual Reproduction in Flowering Plants. The text states: "In angiosperms, the seed is the final product of sexual reproduction. It is often described as a fertilised ovule." The NCERT explicitly describes the dual-layer seed coat (integuments hardening into testa and tegmen), the distinction between albuminous and non-albuminous seeds, perisperm in black pepper and beet, and the monocot embryo anatomy including the scutellum. The NIOS Chapter 19 (Reproduction in Plants) supplements this with a comparative table of gram (dicot) versus maize (monocot) seed structure, naming the aleurone layer, coleorhiza, and coleoptile in the monocot grain.
"Seeds are formed inside fruits. A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis."
NCERT Class 12 Biology, Section 1.4.3
Seed: Definition and Development from the Ovule
A seed is the mature ovule after double fertilisation. The developmental transformation from ovule to seed involves three parallel tissue transitions, each tied to a specific precursor structure in the ovule.
Ovule → Seed: Three Parallel Conversions
-
1
Zygote → Embryo
Fertilised egg divides, differentiates into radicle, plumule, hypocotyl, epicotyl, and cotyledon(s).
2n → 2n -
2
PEN → Endosperm
Primary Endosperm Nucleus (3n) divides repeatedly to form triploid nutritive endosperm tissue.
3n tissue -
3
Integuments → Seed Coat
Outer integument hardens into testa; inner integument forms the thinner tegmen. Together = seed coat.
2n tissue
Once the integuments harden into the seed coat, several structural landmarks of the original ovule remain visible as permanent features of the mature seed. The hilum marks the point where the funicle (stalk) was attached — it appears as a scar on the surface of the seed. The micropyle persists as a tiny pore through the seed coat; during germination, water enters the seed primarily through this pore (imbibition). The chalazal region may remain visible as a slightly raised or darkened area opposite the micropyle.
Dicot Seed Structure
The gram seed (Cicer arietinum), or pea or bean, is the standard NEET example of a dicot seed. The two cotyledons are massively enlarged and store all the food reserves, leaving no endosperm in the mature seed.
Figure 1. Cross-section of a gram/pea dicot seed. The two large, fleshy cotyledons store all food; the endosperm is fully absorbed during development (non-albuminous). The seed coat consists of the outer testa and inner tegmen, both derived from the integuments.
Dicot embryo axis terminology: The embryonal axis has two regions relative to the cotyledon attachment point. Above = epicotyl (terminates in the plumule or shoot apex). Below = hypocotyl (terminates in the radicle or root tip, covered by the root cap).
Seed Coat (2 layers)
Testa — outer, hard protective layer; from outer integument
Tegmen — inner, thinner layer; from inner integument
Frequently confused in MCQsExternal Landmarks
Hilum — funicle attachment scar; marks where seed was attached to ovary
Micropyle — pore for water entry during imbibition; remains from ovule
Embryo Axis Parts
Epicotyl → plumule (embryonic shoot); above cotyledon level
Hypocotyl → radicle (embryonic root); below cotyledon level
2 Cotyledons — thick, fleshy; store the entire food reserve
Monocot Seed Structure
In cereals such as maize (Zea mays) and wheat (Triticum aestivum), the "grain" or caryopsis is technically a fruit — the pericarp (fruit wall) is completely fused with the seed coat and cannot be separated. For NEET, it is sufficient to understand that what is commonly called the "maize grain" is a single-seeded fruit.
Figure 2. Longitudinal section of a maize grain (caryopsis). The single cotyledon, the scutellum, lies laterally and absorbs digested nutrients from the endosperm. The epiblast is a small rudimentary outgrowth opposite the scutellum. The plumule is enclosed in the coleoptile; the radicle is enclosed in the coleorhiza.
The aleurone layer is the outermost, protein-rich layer of the endosperm. It is metabolically active during germination: aleurone cells secrete alpha-amylase (triggered by gibberellin from the embryo), which digests the starchy endosperm into soluble sugars that are then absorbed by the scutellum and translocated to the growing embryo axis.
The scutellum functions as an absorptive organ. Its inner surface is lined with secretory epithelium that faces the endosperm. During germination, the scutellum secretes hydrolytic enzymes into the endosperm and absorbs the resulting soluble nutrients — analogous to the digestive role of the intestinal epithelium in animals.
The epiblast is positioned on the side of the embryonal axis opposite the scutellum. It is considered a vestigial second cotyledon — it has no storage or absorptive function. Its presence in certain grass genera has been used to argue that the scutellum is homologous to a cotyledon.
Albuminous vs Non-Albuminous Seeds
The most commonly tested seed classification in NEET is based on whether the endosperm persists at seed maturity.
Albuminous (Endospermic)
Endosperm persists
at seed maturity
- Endosperm NOT completely consumed during embryo development
- Food reserve available to seedling during germination
- Cotyledons thin and leaf-like (not storage organs)
- Examples: wheat, maize, barley, rice, castor, coconut
- Scutellum absorbs endosperm at germination (monocots)
Non-Albuminous (Ex-Albuminous)
No endosperm
at seed maturity
- Endosperm completely absorbed by cotyledons during development
- Food stored in thick, fleshy cotyledons
- Cotyledons large and swollen — the sole food store
- Examples: pea, gram, bean, groundnut, mustard
- Both types DID form endosperm — it is just absent at maturity in this type
Non-albuminous seeds NEVER had endosperm — FALSE
Students often write that non-albuminous seeds "lack endosperm" as if endosperm never formed. This is incorrect. In non-albuminous seeds, the PEN undergoes normal division and endosperm forms, but it is completely consumed by the developing embryo (specifically absorbed into the cotyledons) before the seed matures. The absence of endosperm in the mature seed is the outcome of complete absorption, not an absence of formation.
Rule: All angiosperm seeds form endosperm after triple fusion. Albuminous = endosperm remains at maturity. Non-albuminous = endosperm fully absorbed into cotyledons before maturity.
Perisperm — Persistent Nucellus
In most seeds, the nucellus is completely consumed during embryo and endosperm development. In a few species, residual nucellus tissue persists into the mature seed. This persistent nucellus is called perisperm.
Perisperm ploidy
Perisperm is diploid (2n) because it derives from the nucellus — a sporophytic (maternal) tissue. This contrasts with endosperm (3n, from PEN). Examples: black pepper (Piper nigrum) and beet (Beta vulgaris). Tested in NEET 2019 Q.38.
Perisperm ≠ Endosperm
Both perisperm and endosperm are food-storing tissues found in mature seeds. The critical difference is their origin and ploidy. Endosperm is triploid (3n) — it develops from the primary endosperm nucleus (PEN), which is formed by the fusion of two polar nuclei (2×1n) and one male gamete (1n = 3n). Perisperm is diploid (2n) — it is remnant nucellus, which is sporophytic maternal tissue.
Rule: Perisperm = persistent nucellus = 2n (diploid). Endosperm = from PEN after triple fusion = 3n (triploid). Black pepper is the classic NEET example of perisperm.
Dormancy and Viviparous Germination
As seeds mature, water content drops to 10–15% by mass. Metabolic activity in the embryo slows dramatically. The embryo may enter a state of inactivity called dormancy. Dormancy is primarily controlled by abscisic acid (ABA), which inhibits germination until favourable conditions (adequate moisture, oxygen, and temperature) override it. The adaptive significance is survival through unfavourable seasons.
Years of viable dormancy
Seeds of Lupinus arcticus (Arctic lupine) excavated from frozen Arctic tundra germinated after an estimated 10,000 years — the longest documented seed viability. A more recent record is a 2,000-year-old date palm (Phoenix dactylifera) seed germinated from King Herod's palace excavation near the Dead Sea.
Germination on parent plant
In mangroves (e.g., Rhizophora), seeds germinate while still attached to the mother plant before the fruit drops. The seedling develops a long, pointed hypocotyl (propagule) that pierces the anaerobic mudflat on falling. This is an adaptation to the hostile, saline, anaerobic mangrove environment.
Worked Examples
Which of the following is the correct developmental origin of the seed coat layers in a typical dicot seed?
(1) Testa from inner integument; tegmen from outer integument
(2) Testa from outer integument; tegmen from inner integument
(3) Both testa and tegmen from the outer integument
(4) Testa from the nucellus; tegmen from the integuments
Answer: (2). The ovule has two integuments. The outer integument hardens and differentiates into the testa (the hard outer seed coat layer visible on a dry seed). The inner integument forms the tegmen (thinner, papery inner layer). Both together constitute the seed coat. Recall: outer → testa (T-O alignment; Testa = ouTer). This origin-layer correspondence is a standard NEET one-liner.
In a maize grain, which structure is responsible for enzyme secretion and absorption of digested endosperm nutrients during germination?
(1) Coleoptile (2) Epiblast (3) Scutellum (4) Aleurone layer
Answer: (3) Scutellum. The scutellum is the single cotyledon of maize. It is shield-shaped, positioned laterally against the endosperm, and lined with secretory epithelium that secretes hydrolytic enzymes and absorbs the resulting soluble nutrients. Coleoptile protects the plumule; epiblast is vestigial with no storage/absorptive role; aleurone layer is part of the endosperm itself (it secretes amylase but is not the absorptive organ). NEET 2016 Q.111 tested this directly.
A student observes a mature seed. She finds that the seed contains food stored in a tissue that is diploid (2n) and persists from a structure that surrounded the embryo sac. What is this tissue?
(1) Endosperm (2) Cotyledon (3) Perisperm (4) Aleurone
Answer: (3) Perisperm. The tissue surrounding the embryo sac is the nucellus — sporophytic, diploid (2n). In most seeds, nucellus is consumed. In black pepper and beet, it persists as perisperm. Endosperm is 3n (from PEN). Cotyledon is embryonic (2n but part of the embryo, not the nucellus). Aleurone is the protein-rich outer endosperm layer, also 3n. NEET 2019 Q.38 asked exactly this concept.
Common Confusion and NEET Traps
| Term | What it IS | Common NEET Error | Memory Rule |
|---|---|---|---|
| Scutellum | The entire single cotyledon of monocot (grass) seeds; lateral, shield-shaped; absorbs endosperm nutrients | Calling scutellum a "part of" the cotyledon, or confusing it with the aleurone layer | Scutellum IS the cotyledon — not a piece of it |
| Epiblast | Small rudimentary (vestigial) structure opposite scutellum; considered a second vestigial cotyledon | Attributing food storage or enzyme secretion to epiblast | Epiblast = vestigial only; NO storage, NO absorption |
| Testa | Outer seed coat layer from outer integument | Using "testa" to mean the entire seed coat (both layers) | Testa = outer (T-O); tegmen = inner |
| Perisperm | Persistent nucellus; diploid (2n); from sporophytic maternal tissue | Calling perisperm a type of endosperm or assigning it 3n ploidy | Perisperm = peri (around) + sperm = nucellus remaining; 2n like body cells |
| Non-albuminous | Seeds where endosperm is fully absorbed before maturity (pea, groundnut, gram) | Stating these seeds "never had endosperm" or "no endosperm formed" | Endosperm formed but was consumed; absence = absorption, not non-formation |
| Hilum | Scar of funicle attachment on seed; marks ovule-funicle junction | Confusing hilum with micropyle (both are visible pores/marks on seed surface) | Hilum = where the seed was held (stalk scar); micropyle = pore for water entry |
Coleoptile vs Coleorhiza — both are sheaths, but different ends
In the monocot (grass) embryo, two protective sheaths are present. Students frequently swap them. Coleoptile protects the plumule (shoot) at the upper end of the embryonal axis. Coleorhiza protects the radicle (root) at the lower end. During germination, the coleoptile emerges above ground first; the coleorhiza is ruptured by the radicle below ground.
Rule: Cole-o-ptile → plumule (p = plumule). Cole-o-rhiza → radicle (r = radicle).