Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 38.1 Flowers, Double Fertilization, and Fruits Are Key Features of the Angiosperm Life Cycle
Angiosperms dominate land ecosystems thanks to their key reproductive features—flowers, double fertilization, and fruits. These traits promote reproductive efficiency, precise pollination, and effective seed dispersal.
The Angiosperm Life Cycle: Overview
- Alternation of generations with dominant sporophyte (2n).
- Sporophyte produces male and female gametophytes (n) via meiosis.
- Fertilization creates a zygote (2n).
- Gametophytes are highly reduced—only a few cells.
Flower Structure and Function
- Four organs:
- Sepals — protect buds
- Petals — attract pollinators
- Stamens — male; produce pollen (anther + filament)
- Carpels — female; contain ovules (stigma, style, ovary)
- Pistil = single or fused carpels
Gametophyte Development
- Male gametophyte:
- Forms in anther → microsporocyte → 4 microspores → pollen grains (with tube and generative cells)
- Female gametophyte (embryo sac):
- Forms in ovule → megasporocyte → 4 megaspores (1 survives) → 8 nuclei in 7 cells
- Includes egg, synergids, antipodal cells, polar nuclei
Pollination and Fertilization
- Pollination: transfer of pollen to stigma
- Pollen tube guided by synergids toward ovule
- Double fertilization:
- 1 sperm + egg → zygote
- 1 sperm + 2 polar nuclei → endosperm (3n)
- Ensures endosperm forms only if fertilization occurs
Seed Development
- Zygote divides into:
- Terminal cell → embryo
- Basal cell → suspensor
- Cotyledons and apical meristems form
- Endosperm develops (persistent or absorbed)
- Seed coat forms from integuments
- Dormancy ensures germination at optimal conditions
Seed Germination
- Triggered by imbibition (water uptake)
- Nutrients mobilized from endosperm or cotyledons
- Radicle (embryonic root) emerges first
- Shoot follows:
- In dicots: pulled by hypocotyl hook
- In monocots: protected by coleoptile
Fruit Development and Types
- Fruit = mature ovary containing seeds
- Types:
- Simple fruit: from one carpel (e.g., pea)
- Aggregate fruit: from many carpels (e.g., raspberry)
- Multiple fruit: from many flowers (e.g., pineapple)
- Accessory fruit: includes non-ovary tissue (e.g., apple)
- Ripening involves:
- Cell wall digestion (softening)
- Color change, sugar accumulation
- Encourages dispersal by animals
Pollination Strategies and Coevolution
- Biotic pollination (~80%): insects, birds, bats
- Abiotic pollination: wind (20%), water (rare)
- Coevolution shapes floral traits:
- Bees → yellow/blue, UV patterns
- Moths → white, fragrant
- Birds → red/yellow, nectar-rich
In a Nutshell
Angiosperms excel due to innovations like flowers, double fertilization, and fruits. These adaptations enable efficient reproduction and dispersal. With reduced gametophytes and coevolved pollination strategies, they dominate diverse ecosystems and underpin modern agriculture.