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Chapter 20 Evolution of the Angiosperms Two classes - Monocotyledons and Dicotyledons Distinctive reproductive feature - carpels

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Chapter 20

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chapter 20
Chapter 20
  • Evolution of the Angiosperms
Two classes - Monocotyledons and Dicotyledons
  • Distinctive reproductive feature - carpels
  • Angiosperms enclose their seeds in structures known as carpels, instead of lying naked on the scales of a strobilus as in gymnosperms. Hence the name "angiosperm" which means "seed in vessel".
review of flower structure
Review of Flower Structure
  • Flowers are reproductive structures that are formed from four sets of modified leaves on a shortened stem. In other words, the flower is a modified strobilus.
  • Sepals - protect floral parts in the bud
  • Petals - attract pollinators
  • Stamens - anthers and filaments
  • Carpels - stigma, style, and ovary (collection of carpels referred to as a pistil)
  • The carpel is a unique structure found only in angiosperms.
Cut into the pistil and you will see one or more tiny chambers, each chamber holding one or more sporangia on tiny stalks.
    • These sporangia are the ovules - each carpel can hold one or several ovules
    • Ovules in the ovary develop into seeds
  • The ovary wall forms a fruit to help disperse the seeds
  • There is an amazing diversity of floral structures. Linnaeus used these differences to classify plants.
evolution of the carpel
Evolution of the Carpel
  • Goethe, German writer, philosopher, and (in his spare time) noted botanist, proposed in 1790 that carpels evolved from leaves.
  • Chambers in the pistil were probably formed from a sporophyll - a fertile leaf bearing ovules.
  • Sporophyll had ovules (modified sporangia) on its outer edges.
  • Edges of the leaf folded over and fused together to form a protective chamber - the carpel.
Pistils probably formed by the fusion of several carpels along the midrib of the modified leaves.
  • Goethe's "foliar theory of the carpel" is still the best hypothesis for explaining the evolution of the carpel.
derived features of angiosperms
Derived Features of Angiosperms
  • Leaves with finely divided venation
  • Complex xylem - incl. vessels and parenchyma
  • Complex phloem - sieve tube elements w/companion cells
  • Herbaceous habit - rapid life cycle (some angios)
  • Ovary to protect ovules ("seeds in vessels")
  • Double fertilization and formation of triploid endosperm
Bisexual Flowers – microsporangia and megasporangia in same strobilus
  • Advanced pollination syndromes - insects, birds, etc.
  • Fruits to protect and diserse seeds
  • Extreme diversity in secondary metabolism
origin of the angiosperms
Origin of the Angiosperms

Darwin called the origin of the angiosperms an "abominable mystery".

  • The evolution of angiosperms remains a mystery to this day, although great progress has been made in recent years solving this mystery using a combination of fossil evidence, molecular data, and the discovery of the primitive angiosperm Amborella.
  • Flowering plants evolved sometime during the Cretaceous, approximately 140 million years ago, while the dinosaurs were at their peak.
However, no fossils showing a transition from gymnosperm to angiosperm have been discovered. This makes the origin of the angiosperms mysterious.
  • Angiosperms quickly became the dominant plants, although gymnosperms continued to rule in cold, dry, or sandy habitats, as they still do today.
  • Regardless of the origin of the angiosperms, by the end of the Cretaceous (65-70 mya) most flowering plant families had evolved.
pollination and seed dispersal
Pollination and Seed Dispersal
  • Coevolution occurs when an evolutionary change in one organism leads to an evolutionary change in another organism that interacts with it.
  • Flowering plants show two great examples of coevolution: evolution of animal pollination and evolution of fruit dispersal.
  • Flowers that rely on wind pollination are tiny and inconspicuous (like oak trees, maple trees, corn, grasses).
  • Flowers that are pollinated by animals have showy petals to attract the pollinators.
Flowers advertise their reward of nectar, sugar water, to attract pollinators.
  • Fruits function to disperse seeds.
    • Animals eat fruit, but don't digest seeds.
    • Tiny hooks and spines to attach to animal.
    • Also dispersed by wind, water (coconuts).
monocots or eudicots
Monocots or Eudicots?
  • Some flowering plants are neither monocots or dicots.
  • Magnolia
evolution of the flower
Evolution of the flower
  • What were the flowers of the earliest angiosperms like?
  • Deduce their nature form what we know of certain living plants and from the fossil record.
  • In general flowers were diverse in the number of floral parts and in their arrangements.
parts of the flower provide clues to evolution
Parts of the flower provide clues to evolution
  • The perianth of early angiosperms did not have distinct sepals and petals
  • Sepals and petals were identical or there was a gradual transition in appearance between these whorls (magnolias and water lilies).
  • i.e. petals can be viewed as modified leaves that have become specialized for attracting pollinators.
in most angiosperms
In most angiosperms
  • Petals were probably derived originally from stamens that lost their sporangia- becoming sterile and modified to new role
  • Most petals like stamens are supplied by one vascular strand
  • In contrast sepals are normally supplied by the same number of vascular strands in a leaf
the stamens
The Stamens
  • Magnoliids- broad, colored, and scented role in attracting floral visitors
  • In others- small greenish, fleshy
  • Many living angiosperm in contrast have thin filaments and thick terminal anthers
  • In stamens of monocots and eudicots are less diverse than Magnoliids
stamens continue
Stamens continue
  • In some specialized flowers the stamens are fused together.
  • Form columnar structure i.e pea, melon and mallow fig 20-8d and sunflower 20-9d
  • Some stamens fused with corolla i.e. snapdragon, phlox, and mint families.
stamens can become nectaries
Stamens can become nectaries
  • In some families stamens become sterile losing their sporangia and becoming specialized nectaries.
  • Nectaries are glands that secrete nectar- sugary fluids tha tattract pollinators and provides food for them.
  • Most nectaries are not modified stamens but arose other ways.
the carpels
The Carpels
  • The carpels of many early angiosperms were unspecialized
  • Carpels with no specialized areas for the entrapment of pollen grains comparable to specialized stigmas of most living andiosperms.
  • Magnoliids- carpels are free from one another unlike most contemporary angiosperms.
four evolutionary trends among flowers are evident
Four evolutionary trends among flowers are evident
  • Evolved toward having few parts that are definite in number
  • Floral whorls have been reduced four to one in more advance ones and the floral parts often have become fused.
  • Ovary has become inferior in position and the perianth has become differentiated into a distinct calyx and corolla
  • The radial symmetry (regularity) or actinomorphy of early flowers has given way to bilateral symmetry (irregularity)or zygomorphy in more advance ones.
the asteraceae and orchidaceae are examples of specialized families
The Asteraceae and Orchidaceae are examples of specialized families

Two largest families of angiosperms

  • Asteraceae- compositae which are eudicots
  • Orchidacea- monocots