Seed & Fruit Anatomy

Overview

The seed and the fruit are both post-fertilization structures — the seed developing from the ovule, the fruit from the ovary wall around it (see Flower Anatomy & Reproductive Structures for the pre-fertilization ovule and ovary structure this page builds on directly). This page covers what each structure looks like once mature, then how fruit types are classified structurally.

Key Concepts

Seed Coat

The ovule’s integuments (one or two layers, see Flower Anatomy & Reproductive Structures) mature into the seed coat: where two integuments were present, the outer hardens into the testa and the inner into the thinner tegmen; where only one integument was present, the mature seed coat is a single layer. The hilum is a scar on the seed coat marking where the funiculus detached at maturity; the micropyle persists as a small pore near the hilum, retaining a functional role in germination by controlling initial water entry into the seed.

The Embryo

The fertilized zygote (see Flower Anatomy & Reproductive Structures) develops into the embryo, organized along a single axis: a radicle (embryonic root) at one end, a plumule (embryonic shoot, itself sometimes called the epicotyl above the cotyledon attachment point) at the other, connected by a hypocotyl (the embryonic axis segment between radicle and cotyledon attachment), and bearing one cotyledon (monocots) or two (dicots) — the same cotyledon-count distinction introduced on Monocot vs Dicot Comparative Anatomy. In many dicot seeds the cotyledons themselves swell to become the seed’s primary nutrient-storage structure (see below); in monocots the single cotyledon (in grasses, specifically modified into a structure called the scutellum) instead functions mainly to absorb and transfer stored nutrient from a separate endosperm rather than storing it directly.

Source: Biology LibreTexts

Endosperm, Perisperm, and Nutrient Storage Strategy

Recall from Flower Anatomy & Reproductive Structures that double fertilization produces both the zygote and a separate triploid primary endosperm nucleus, which develops into endosperm tissue. What happens to that endosperm by seed maturity splits species into two structural categories:

• Endospermic (albuminous) seeds — endosperm persists as a distinct nutrient-storage tissue at maturity, surrounding or adjacent to a comparatively small embryo (e.g. cereal grains, castor bean).
• Non-endospermic (exalbuminous) seeds — the endosperm is fully absorbed and its nutrients transferred into the cotyledons during seed development, so the mature seed shows large, nutrient-packed cotyledons and no separate endosperm tissue at all (e.g. beans, peas).

A separate nutritive tissue, perisperm, occurs in a smaller number of species (e.g. beet, black pepper, coffee): unlike endosperm, perisperm is derived from the diploid nucellus (the ovule tissue surrounding the embryo sac, not a fertilization product), and where it occurs it typically persists alongside a much-reduced or absent endosperm — worth distinguishing from endosperm specifically because the two are sometimes confused despite having entirely different developmental origins (fertilization product vs. unfertilized maternal tissue).

Fruit Wall (Pericarp) Structure

Following fertilization, the ovary wall matures into the pericarp, differentiating into up to three structural layers: exocarp (outermost, often a thin skin), mesocarp (middle, variable — thin and dry, or thick and fleshy, depending on fruit type), and endocarp (innermost, in direct contact with the seed(s), variable — thin and membranous, or hardened into a stone). Not every fruit shows all three layers as structurally distinct; the degree of differentiation is exactly what separates the fruit types below.

Source: Wikimedia Commons, “Drupe fruit diagram” (commons.wikimedia.org)

Fruit Classification

Fruits are classified first by how many ovaries/flowers contributed to the structure, then, for simple fruits, by pericarp texture and dehiscence behavior:

• Simple fruit — develops from a single ovary of a single flower.
  • Fleshy simple fruits — pericarp wholly or partly fleshy at maturity: berry (entire pericarp fleshy, e.g. tomato, grape), drupe (fleshy mesocarp, hardened stony endocarp enclosing the seed, e.g. peach, cherry), pome (fleshy edible tissue derived mostly from the floral receptacle/hypanthium rather than the ovary wall itself, with the true pericarp forming the thin core, e.g. apple, pear).
  • Dry simple fruits, split by whether the pericarp splits open at maturity: dehiscent (splits to release seeds — legume, splitting along two seams, characteristic of Fabaceae; capsule, splitting along multiple seams or pores; silique, a specialized elongated capsule characteristic of Brassicaceae) versus indehiscent (pericarp stays closed, the whole structure dispersing as one unit — achene, small, one seed, pericarp not fused to the seed coat; caryopsis, one seed, pericarp fused tightly to the seed coat, characteristic of grasses/cereals; samara, winged pericarp extension for wind dispersal; nut, one seed, hardened pericarp, usually larger than an achene).
• Aggregate fruit — develops from multiple separate carpels of a single flower’s single gynoecium, each carpel maturing into a small fruitlet, all fruitlets remaining attached to one receptacle (e.g. raspberry, blackberry — each small “bump” is technically a separate small drupe, a drupelet).
• Multiple fruit — develops from the fused ovaries of many separate flowers clustered in one inflorescence, maturing into what appears externally as one unit (e.g. pineapple, fig, mulberry).

Source: Raven, Evert & Eichhorn, Biology of Plants (W.H. Freeman/Macmillan)

Dispersal-Related Structural Adaptations

Several fruit and seed structures are specifically shaped by dispersal mechanism rather than protection alone: winged pericarp extensions (samaras, e.g. maple, ash — structurally an aerodynamic adaptation slowing fall and promoting wind carry), hooked or barbed pericarp surfaces (structurally adapted to catch on animal fur for external transport), fleshy, brightly colored, sugar-rich pericarp layers (structurally an attractant for animals that then disperse the undigested seed after consumption), and buoyant, water-resistant fibrous pericarp layers (e.g. coconut, structurally adapted for water dispersal). These are dispersal-strategy adaptations layered onto the same basic pericarp-layer plan described above, not a different fruit structure altogether.

Comparative Structures

Common Exam Questions

• “Distinguish endosperm from perisperm by developmental origin, and explain why they are sometimes confused.”
• “Explain the structural difference between an achene and a caryopsis.”
• “Distinguish an aggregate fruit from a multiple fruit, referencing how many flowers and ovaries contribute to each.”
• “Trace the exocarp/mesocarp/endocarp layering in a drupe, naming the familiar tissue each layer corresponds to in a peach.”
• “Explain why a pome’s fleshy tissue is mostly not derived from the ovary wall, unlike a berry’s.”
• “Describe two structurally distinct dispersal adaptations and the specific pericarp/seed-coat feature each depends on.”

Visual Reference

Interactive

• Pericarp layer explorer across fruit types (click-through SVG/JS, no new library) — a single cross-section frame that swaps between berry, drupe, and pome, showing how the same three-layer pericarp plan (exocarp/mesocarp/endocarp) is realized completely differently in each, with the pome additionally highlighting the non-ovary-derived fleshy tissue.

• Fruit classification decision tree (click-through SVG/JS, no new library) — starting from “how many ovaries, how many flowers,” branches through simple/aggregate/multiple, then for simple fruits further branches through fleshy/dry and dehiscent/indehiscent, ending at a named fruit type with an example image.

(Static images are placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here. No usable image was found for the legume-pod-dehiscence or samara/achene/caryopsis/nut comparison spec items — dropped from this pass.)

Practice Problems

1. A seed shows large, nutrient-packed cotyledons and no separate endosperm tissue. Classify it as endospermic or non-endospermic and explain what happened to its endosperm during development.
2. Distinguish perisperm from endosperm using ploidy and developmental origin.
3. A dry fruit splits open along two seams to release its seeds. Identify the fruit type and the plant family it is most characteristic of.
4. Explain why a raspberry is classified as an aggregate fruit rather than a simple fruit, referencing the number of ovaries involved.
5. Identify which pericarp layer is hardened into a “pit” or “stone” in a drupe, and name the corresponding layer in an achene.