Flowers, Fruits, and Seeds

Chapter 8: Flowers, Fruits, and Seeds

Introduction to Flowers, Fruits, and Seeds

  • Diversity of Plant Species

    • Nearly 300,000 known species of plants.

    • Example: An underground-flowering orchid discovered accidentally in Australia.

  • Importance of Flowering Plants

    • Provide countless products for humans.

    • 11 species provide 80% of the world's food.

Unique Flowers

1. Corpse Flower

  • Produces an inflorescence that can exceed 2 meters in height.

  • Emits an odor reminiscent of rotting flesh.

  • Fewer than 300 recorded blooms in history.

2. General Characteristics of Flowers

  • May exhibit:

    • Any color

    • Variety of textures

    • Wide variety of odors

Habitats of Flowering Plants

  • Flowering plants thrive in diverse habitats:

    • Epiphytes: grow on other plants, wires, cracks in rocks, fresh and saltwater.

Categories of Plants

  • Annual plants: Complete life cycle in a single season.

  • Biennial plants: Complete life cycle in two growing seasons.

  • Perennial plants: Life cycle spans several to many growing seasons.

    • May produce flowers on new growth while other parts persist indefinitely.

  • Major Classes of flowering plants:

    • Magnoliopsida (Dicots)

    • Liliopsida (Monocots)

Differences Between Dicots and Monocots

  • Cotyledons:

    • Dicots: 2

    • Monocots: 1

  • Flower Parts:

    • Dicots: multiples of 4 or 5

    • Monocots: multiples of 3

Morphological Characteristics

  • Leaf Venation:

    • Dicots: network of veins.

    • Monocots: parallel primary veins.

  • Vascular Structures:

    • Vascular cambium and cork cambium:

    • Dicots: present.

    • Monocots: absent.

  • Vascular Bundles in Stem:

    • Dicots: arranged in a ring.

    • Monocots: scattered.

  • Pollen Grains:

    • Dicots: grains exhibit three apertures.

    • Monocots: grains exhibit one aperture.

Structure of Flowers

  • Development of Flowers:

    • Flowers develop from an embryonic primordium that transitions to a bud.

    • Flowers emerge as specialized branches at the tips of peduncles.

    • May include branchlets called pedicels (the stalk of a single flower).

  • Receptacle:

    • The swollen end of a peduncle or pedicel.

    • Other floral parts attach in whorls:

    • Sepals

    • Petals

    • Stamens

    • Pistil

Sepals and Petals

  • Sepals:

    • Outermost whorl, together known as the calyx.

    • Serves to protect the flower while in bud.

  • Petals:

    • Located inside the sepals, known collectively as the corolla.

    • Often vibrant to attract pollinators.

    • Some plants may have inconspicuous or absent corollas (e.g., trees, weeds, grasses, wind-pollinated plants).

  • Calyx and Corolla form the perianth.

Stamen and Pistil

  • Stamens:

    • Attached at the base of the pistil.

    • Each composed of a filament with an anther at the top, where pollen grains are formed.

  • Pistil:

    • Comprised of stigma, style, and ovary.

    • The ovary is responsible for the development of fruit.

Ovaries

  • Evolution of Ovaries:

    • Derived from carpels with inward-rolled margins.

    • Carpel: a leaf structure with ovules along its edges.

    • Carpels may join to form a compound ovary.

  • Types of Ovaries:

    • Superior Ovary: calyx and corolla attach to the receptacle at the base of the ovary.

    • Inferior Ovary: receptacle grows around the ovary; calyx and corolla appear connected at the top of the ovary.

  • Contents of Ovaries:

    • Contains ovules, which develop into seeds upon fertilization.

Inflorescence

  • Flowers can occur alone or in groups known as inflorescences.

Fruits

  • Defined as the matured ovary and its accessory parts, containing seeds.

  • All fruits arise from flower ovaries and are exclusive to flowering plants.

    • Example: Tomato fruit.

Exocarp, Endocarp, and Mesocarp

  • Exocarp: the outer skin of the fruit.

  • Endocarp: the inner boundary surrounding the seed(s).

  • Mesocarp: the tissue situated between the exocarp and endocarp.

  • Collectively, these parts are termed the pericarp.

    • Example: Peach fruit.

Variability of Fruits

  • Fruits can vary significantly in structures:

    • They may consist of only the ovary and seeds.

    • Can include adjacent flower parts.

    • May be fleshy or dry at maturity.

    • May or may not split upon maturity.

    • Can be derived from one or multiple ovaries.

Kinds of Fruits

Fleshy Fruits

  • Defined by an at least partly fleshy mesocarp at maturity.

    • Simple fleshy fruits develop from a flower with a single pistil.

  • Drupe: a type of simple fleshy fruit with a single seed enclosed by a hard, stony endocarp (pit).

    • Examples include peaches, almonds, olives.

Berries

  • Derived from a compound ovary with multiple seeds and a fleshy pericarp.

  • True Berry: characterized by a thin skin and relatively soft pericarp.

    • Examples: tomatoes, grapes, peppers, blueberries, bananas.

  • Pepo: a berry with a thicker rind.

    • Examples: pumpkins, cucumbers.

  • Hesperidium: special berry with a leathery skin containing oils.

    • Found in members of the citrus family.

Pomes

  • Pome: flesh is derived from an enlarged floral tube that grows around the ovary.

    • Endocarp is papery or leathery.

    • Examples: apples, pears (the core is primarily from the ovary and the remainder from the floral tube).

Dry Fruits

  • Defined as having a dry mesocarp at maturity.

  • Dehiscent Fruits: split open at maturity.

    • Follicle: splits along one side (e.g., larkspur, milkweed, peony).

    • Legume: splits along two sides (common in the legume family, e.g., peas, beans, lentils, peanuts).

Siliques and Silicles

  • Fruits that split open along two sides but have seeds on a central partition, becoming exposed when the two halves separate.

    • Silique: longer than wide (more than three times).

    • Silicle: less than three times longer than wide.

    • Common in the mustard family (e.g., broccoli, cabbage).

Capsules

  • Comprise at least two carpels and split in various ways.

    • Examples include irises, poppies, violets, and snapdragons.

Indehiscent Fruits

  • These fruits are characterized by a single seed that is fused with the pericarp.

    • Achene: base of the seed attached to the pericarp (e.g., sunflower seed, buttercup, buckwheat).

    • Nut: similar to an achene but larger and with a thicker, tougher pericarp (e.g., acorns, hazelnuts).

Other Indehiscent Fruits

  • Grain (Caryopsis): pericarp tightly fused with seed.

    • Common in grasses: corn, wheat, rice, oats, barley.

  • Samara: pericarp extends as wings aiding in dispersal (e.g., maples, ashes, elms).

  • Schizocarp: twin fruit that disintegrates into one-seeded segments called mericarps (common in the parsley family, e.g., carrots).

Aggregate Fruits

  • Aggregate Fruits: arise from a single flower possessing multiple pistils, maturing as a clustered unit on a single receptacle.

    • Examples: raspberries, blackberries, strawberries.

Multiple Fruits

  • Derived from several to many individual flowers within a single inflorescence.

    • Examples: mulberries, Osage orange, pineapples, figs.

Fruit and Seed Dispersal

1. Dispersal by Wind

  • Fruits: possess samaras or plumes/hairs that enhance wind dispersal.

  • Seeds: typically small, lightweight, or equipped with wings.

2. Dispersal by Animals

  • Agents: birds, mammals, and ants assist in seed dispersal.

    • Seeds can pass through the digestive tract of animals.

    • Some fruits and seeds may adhere to fur or feathers for transport.

3. Dispersal via Ants

  • Seeds may have appendages that attract ants, such as oils.

    • Example: Elaiosomes on bleeding hearts serve as food for ants.

4. Dispersal by Water

  • Some fruits possess trapped air, making them suitable for water dispersal.

    • Example: Sedge seeds are surrounded by inflated sacs, and some plants develop thick, spongy pericarps that absorb water slowly, enabling dispersal by ocean currents.

5. Other Dispersal Mechanisms and Agents

  • Certain plants can mechanically eject seeds (e.g., some legumes, touch-me-nots, witch hazel).

  • Geraniums feature fruit carpels that separate and curl back, with seeds reacting to humidity changes to burrow into the ground.

  • Humans inadvertently or deliberately transport seeds.

Seeds

Structure of Seeds

  • Developed from ovules, seeds include:

    • Cotyledons: function as food storage organs, known as "seed leaves".

    • Embryo: consists of cotyledons and the plantlet.

    • Plumule: the shoot part of the embryo.

    • Epicotyl: stem above the attachment point of the cotyledons.

    • Hypocotyl: stem below cotyledon attachment.

    • Radicle: the tip of the embryo that develops into the root.

    • Example: Bean seed.

Germination

  • Germination refers to the initiation or resumption of seed growth.

    • Some seeds require a dormancy period before germination.

    • Triggered by mechanical or physiological factors, such as growth-inhibiting substances present in the seed coat or fruit.

    • Dormancy can be broken through mechanical abrasion, thawing and freezing, bacterial action, or soaking raining.

  • Scarification: involves artificially breaking seed dormancy.

  • After ripening: Seeds must develop embryonically before germination occurs, even when fruits are ripe.

Epigeous and Hypogeous Germination

  • Epigeous Germination:

    • The hypocotyl elongates, bends, and forms a hook shape.

    • The top of the hook emerges first, lifting the cotyledons above the ground.

  • Hypogeous Germination:

    • The hypocotyl remains short, and the cotyledons do not emerge above the soil surface.

Conditions for Germination

  • Favorable environmental conditions are critical for germination:

    • Essential factors include water, oxygen, light (or absence), and a proper temperature range.

    • Following water absorption (imbibition), enzymes in the cytoplasm become active.

Longevity of Seeds

  • Seed viability varies based on species and storage conditions:

    • Viability can be prolonged when stored at low temperatures or kept dry.

  • Vivipary: occurs when seeds grow without a dormancy phase, as seen in red mangroves.