Seeds & Fruit Notes

Pericarp

  • Pericarp surrounds each seed.
  • The lecture will cover seeds and fruits, presented by Dr. Dylan Phillips.
  • Key structures:
    • Pollen
    • Stigma
    • Anther Tube
    • Petal
    • Style
    • Mesocarp
    • Exocarp
    • Placenta
    • Sepal
    • Ovule
    • Ovary
    • Receptacle
    • Hypanthium
    • Endocarp

Objectives

  • Understand the ecological importance of fruits.
  • Understand the development of the ovary into a fruit.
  • Learn the basic classification of fruit types.
  • Learn about some economically important fruits.

What is a Fruit?

  • A fruit is the seed-bearing structure in flowering plants (angiosperms) that forms from the ovary after flowering.
  • Fruits are divided into two classes: dry fruits and fleshy fruits.
  • Dry Fruits:
    • Predate their fleshy counterparts.
    • Typically dispersed by physical forces (e.g., ejected by pod shattering, swept up by the wind).
    • Can adhere to animal surfaces (fur or feathers) for transport (epizoochory).
  • Fleshy Fruits:
    • Often depend on animals consuming the fruit and dispersing the seeds after ingesting or discarding them.
    • Example: Cleavers (Galium aparine).

Ecological & Evolutionary Importance of Fruits

  • The majority of angiosperm seeds are animal dispersed.
  • Birds are the main frugivores (fruit-eating animals).
  • Fruits play a major role in primate and fruit bat evolution.
  • Endozoochory: seeds are eaten and pass through the animal's digestive tract.

Frugivores

  • Small and large frugivores exist.
  • Approximately 4% of angiosperms attract ants via fleshy structures attached to seeds.
  • Elephants disperse many tropical plants.

Other Dispersal Mechanisms

  • Wind: Example: 'Sea heart' bean of Caribbean liana Entada gigas.
  • Fall or scatter.
  • Water.

From Flower to Seed & Fruit

  • Ovary wall becomes the pericarp.
  • Ovule becomes the seed.
  • Ovary (flower) transforms into the fruit.

Fruits with Dry Pericarp: 'Pod'

  • Example: Pea flower (Pisum sativum).
  • High protein content in cotyledons leading to 'split peas'.

Fruits with Dry Pericarp: 'Grain'

  • Poaceae: grass family.
  • Anthropochory: Human-mediated dispersal.
  • Cereals: oats, barley, wheat.
  • Grain structure:
    • Endosperm
    • 'Germ' (embryo)
    • 'Bran' (pericarp)

Fruits with Dry Pericarp: 'Grain' - Flour

  • White flour is derived from the endosperm, primarily carbohydrate.
  • 'Wholegrain' flour contains bran and germ.
  • Historical reasons for preferring white flour: fatty acids in the 'germ' taste rancid when oxidized during storage which reduces shelf life.

Wheat's 'Dough'

  • Wheat's 'dough' comes from grain endosperm.
  • Endosperm contains starch in a protein matrix.
  • Bonding of glutenins and gliadins yields 'gluten'.
  • The result is a distinctly viscoelastic 'dough'.

Fruits with Dry Pericarp: 'Grain' - Global Food

  • Maize and rice are also in Poaceae.
  • Maize, rice, and wheat contribute to 60% of global food energy intake.

Fruits with Dry Pericarp: 'Nut'

  • Nuts feature a hard, thick pericarp.
  • Synzoochory: Seeds are deliberately carried and stored (e.g., squirrels or jays storing nuts).
  • Example: Oak fruit.

Fruits with Dry Pericarp: 'Samara'

  • Anemochory pertains to the dispersal of seeds, spores, or fruits by wind.
  • Samaras have wings to facilitate wind dispersal.
  • Lightweight seeds or fruits with specialized aerodynamic structures maximize wind transport.
  • Example: Maple fruit, Ash fruit.

Fruits with Dry Pericarp: 'Achene' - Buttercup

  • Example: Buttercup (Ranunculaceae).
  • Achenes ripening from separate carpels.
  • A small, dry, one-seeded fruit where the seed is attached to the pericarp (fruit wall) at only one point.
  • The pericarp is thin and not fused to the seed coat.

Fruits with Dry Pericarp: 'Achene' - Dandelion

  • Example: Dandelion (Asteraceae).
  • Achenes ripening from separate florets.
  • The fruit has a 'pappus' for wind dispersal.
  • One-seed fruits with pericarp free from the seed coat.

Fruits with Dry Pericarp: 'Achene' - Sunflower

  • Example: Sunflower crop (Asteraceae).
  • One-seed fruits with the pericarp free from the seed coat.
  • 'Sunflower hearts' are easy to extract from the black pericarp.
  • Oil-rich cotyledons are used for cooking oil and bird food.

Fruits with Dry Pericarp: 'Capsule' - Iris

  • Type of dry dehiscent fruit that splits open at maturity to release its seeds.
  • Yellow flag iris has three carpels forming a three-chambered ovary that splits along the seams (dehiscence).
  • Seeds have buoyancy tissues for water dispersal (hydrochory).

Fruits with Dry Pericarp: 'Capsule' - Opium Poppy

  • Example: Opium poppy.
  • Ovary with multiple carpels.
  • Seeds shed through pores at the top of the capsule.
  • Has a unique "salt-shaker" dispersal mechanism.
  • Opioids (morphine, codeine, etc.) are found in the latex of the capsule.

Fruits with Dry Pericarp: 'Capsule' - Cotton

  • Example: Cotton.
  • The capsule splits to release seeds.
  • Cotton fibers are single-celled hairs on the seed.
  • Up to 20,000 can form on a fertilized ovule.

From Flower to Seed & Fleshy Fruit: 'Berry'

  • Ovary wall forms the pericarp, which consists of exocarp, mesocarp, and endocarp.
  • Ovule becomes the seed.
  • The entire pericarp is fleshy in a berry.

Fruits with Fleshy Pericarp: 'Berry' - Tomato

  • Example: Tomato.
  • Exocarp and mesocarp are fleshy.
  • Placenta: thick, fleshy tissue inside the tomato that extends from the central axis into the locular cavities (seed-containing chambers).
  • Serves as the attachment point for seeds and provides nutrients to developing seeds.

Fused Carpels

  • Ovaries can consist of fused carpels, resulting in chambers in the ovary.
  • Fruits can develop from an ovary with one carpel or multiple carpels.

Fruits with Fleshy Pericarp: 'Berry' - Citrus Fruits

  • Example: Citrus fruits.
  • Leathery rind = exocarp.
  • Segments = carpels.
  • Juicy tissue = vesicles of endocarp cells.

Fruits with Fleshy Pericarp: 'Berry' - Pomegranate

  • Forms from an inferior ovary.
  • Residual stamens on top.
  • Leathery exocarp.
  • Fleshy mesocarp.
  • Seeds, each with a fleshy outer layer.
  • A many-seeded berry.

Fruits with Fleshy Pericarp: 'Berry' - Cocoa

  • Seeds fermented in heaps, then sun-dried to produce 'cocoa beans'.
  • Key components: Exocarp, Mesocarp, Endocarp, Seed/bean

Fruits with Fleshy Pericarp: 'Berry' - Cocoa Processing

  • Manufacturer roasts and mills the cocoa beans.
  • Natural components create flavors.
  • Sugar, milk, and extra flavors are added to make chocolate.

Fruits with Fleshy Pericarp: 'Berry' - Coffee

  • Coffee berries have two seeds, mostly endosperm.
  • Pericarps are removed, seeds separated, and then roasted to produce 'coffee beans'.

From Flower to Seed & Fleshy Fruit: 'Drupe'

  • Drupe: fleshy exocarp and mesocarp, with seed encased in hard endocarp.

Fruits with Hard Endocarp: 'Drupe' - Peach

  • Example: Peach.
  • Woody endocarp protects the seed in the animal’s gut or is off-putting to consume.

Fruits with Fleshy Pericarp: 'Compound Fruits'

  • Aggregate fruit: from flower with many carpels.
  • Multiple fruit: from many flowers.
    • Pineapple inflorescence segment.
    • Each develops from the carpel of one flower.

'Accessory' or 'False' Fruits

  • Contain tissues not from the ovary.
  • Flesh from the receptacle: base of the flower where all floral parts were attached.
  • Calyx: Consists of sepals that persist from the flower and helps protect the developing fruit.

Summary

  • Frugivores: birds, mammals, ants
  • Ovary wall of flower becomes the 'pericarp' of typical fruits.
  • Fruits with 'dry' pericarps dispersed by various mechanisms.
  • Entirely 'fleshy' fruits = 'berries', or if 'endocarp' is woody = 'drupes'.
  • 'Compound' fruits are from multiple carpels or flowers.
  • All these categories contain major crops.