Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 30.3 The Reproductive Adaptations of Angiosperms Include Flowers and Fruits
Angiosperms, or flowering plants, are the most diverse group of seed plants. Their reproductive success is largely due to two defining structures: flowers, which facilitate pollination, and fruits, which aid in seed dispersal. These innovations have made angiosperms the dominant plants on Earth.
Characteristics of Angiosperms
- All angiosperms belong to a single phylum: Anthophyta.
- The name “angiosperm” means “container seed”, referring to seeds enclosed in fruits.
- Angiosperms account for ~90% of all plant species (~290,000 species).
Flower Structure and Function
- Flowers are reproductive structures specialized for sexual reproduction.
- Commonly pollinated by insects or animals, making pollination more targeted than the wind-pollination of gymnosperms.
- A flower can have up to four types of floral organs:
- Sepals: Protect the flower bud.
- Petals: Often brightly colored to attract pollinators.
- Stamens: Male organs (filament + anther), produce pollen.
- Carpels: Female organs (stigma + style + ovary), house ovules.
- Carpels are key to angiosperms—they enclose the ovules, forming fruits after fertilization.
- Flower symmetry can be radial (multiple lines of symmetry) or bilateral (one line), which can influence pollination specificity and speciation rates.
Fruits and Seed Dispersal
- After fertilization, the ovary matures into a fruit, and ovules become seeds.
- Fruit types:
- Fleshy: Tomatoes, plums, grapes (soft pericarp)
- Dry: Nuts, beans, grains (some split open, others don't)
- Fruits aid in seed dispersal:
- Wind: e.g., maples, dandelions
- Water: e.g., coconuts
- Animals: edible fruits, burrs, or digestive transport
Angiosperm Life Cycle & Double Fertilization
- Male gametophytes (in pollen grains) develop in anthers.
- Female gametophytes (embryo sacs) develop in ovules.
- Pollination delivers pollen to the stigma.
- A pollen tube grows through the style, reaching the ovule.
Double fertilization (unique to angiosperms):
- One sperm fertilizes the egg → zygote (2n)
- The other sperm fuses with two polar nuclei → endosperm (3n), a food supply
- Ensures resources are only invested if fertilization is successful
Angiosperm Evolution and Diversity
- Angiosperms likely originated ~140 million years ago in the Cretaceous period.
- Fossils like Archaefructus provide early evidence of flowering plants.
- Early angiosperms were likely woody shrubs, not aquatic as previously thought.
Major Clades of Angiosperms:
- Basal Angiosperms – Amborella, water lilies, star anise
- Magnoliids – Magnolias, laurels, black pepper
- Monocots – Grasses, orchids, palms (~25% of species)
- Eudicots – Roses, legumes, oaks (~70% of species)
In a Nutshell
Flowers and fruits are key reproductive adaptations that enabled angiosperms to dominate Earth’s plant life. With specialized pollination, double fertilization, and diverse seed dispersal methods, angiosperms evolved rapidly and now provide most of our food, medicines, and ecological diversity.