Chapter 23:  Angiosperms

In this chapter you will learn about differences between angiosperms and gymnosperms, and then discusses the theoretical origin of flowering plants. The exceptional diversity of form and habit of the flowering plants is reiterated before the chapter continues with a description of the parallel development of the gametophytes in the anthers and ovules. This leads up to pollination, followed by some details of fertilization and the development of a seed.  A discussion of trends of specialization and classification in flowering plants is followed by a section on pollination ecology. Flower preservation, including simple herbarium techniques and practice, is next. The chapter closes with a brief survey on the uses of herbaria and a word of caution to plant collectors concerning unnecessary depletion of native floras.

At the end of this chapter the successful student will be able to

• Recognize the characteristics of Phylum Magnoliophyta
• Recognize gametophyte development 
• Recognize how pollination occurs in angiosperms
• Recognize Fertilization and Seed Development in angiosperms
  • Apomixis and Parthenocarpy
• Recognize and describe Specialization Trends in Flowering Plants
• Recognize Pollination Ecology
• Recognize the process and purposes of Herbaria and Plant Preservation
• Assignments
• Glossary

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Recognize the characteristics of Phylum Magnoliophyta

• Flowering Plants
  • Angiosperms are heterosporous
    • Female gametophytes are wholly enclosed within sporophyte tissue and reduced to only a few cells.
    • At maturity, male gametophytes consist of a germinated pollen grain with three nuclei.

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Gametophyte Development

• Diploid megasporocyte cell differentiates from all other cells in the ovule.
  • Undergoes meiosis and produces four haploid megaspores.
    • Three degenerate
• Integuments differentiate and eventually become the seed coat.
  • Leave micropyle at one end.
• Central Cell Nuclei may become a binucleate cell, or may fuse together, forming a single diploid nucleus.
  • One of the cells functions as the egg.
• Female gametophyte (megagametophyte) consists of a large sac usually containing eight nuclei in seven cells.
• Mature Megasporocyte

• While megagametophyte is developing, a parallel process that leads to the formation of male gametophytes occurs in the anthers.
  • Four patches of tissue differentiate from the main cell mass.
    • Contain diploid microsporocyte cells which undergo meiosis and produce a quartet of microspores.
• Lily Anther Cross Section

• After meiosis, the haploid microspores in the pollen sac undergo several changes.
  • Nucleus in each microspore divides by mitosis.
  • Members of each quartet of microspores separate from one another.
  • A two-layered wall develops around each microspore.
• Microspores are now pollen grains.
  • Outer wall - exine.

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Pollination occurs in angiosperms

• Pollination is the transfer of pollen grains from an anther to a stigma.
• Fertilization involves the union of an egg and a sperm.
  • May not occur until days or weeks or even months after pollination has taken place.

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Fertilization and Seed Development in angiosperms

• After pollination, further development of the male gametophyte may not take place unless the pollen grain is:
  • From a different plant of the same species.
  • From a variety different from that of the receiving flower.
    • Under suitable conditions, a pollen tube grows down between the cells of the stigma and style until it reaches the ovule micropyle.
• When the pollen tube reaches the micropyle, it continues to the female gametophyte and discharges its contents.
  • Double Fertilization (Fusion) Occurs
    • One sperm migrates from the synergid to the egg and forms a zygote.
    • Other sperm cell migrates from the synergid and unites with the central cell nuclei producing a triploid endosperm nucleus.

• Apomixis and Parthenocarpy

  • Apomixis is development or fusion of gametes but with the normal structures otherwise being involved.
    • Fruits that develop from ovaries with unfertilized eggs are parthenocarpic.
      • Seedless (Navel oranges)
    • Seedless watermelon are produced by crossing varieties with different numbers of chromosomes. (Not parthenocarpic)
      • Fertilization and seed formation don’t occur.

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Specialization Trends in Flowering Plants

• Fossil record suggests flowering plants first appeared about 160 million years ago during the late Jurassic period.
  • Most botanists hypothesize primitive flowers had numerous spirally-arranged parts that were not fused together and were variable in number.
    • Flowers were regular and contained both stamens and pistils.
• In advanced flowers, the receptacle has fused to the ovary.
  • When the ovary is embedded in the receptacle, it is said to be inferior.
    • Flower parts attached to to the top of the ovary are epigynous.
  • When the ovary is produced on top of the receptacle, it is said to be superior.
    • Flower parts attached around the base are hypogynous.

• Flowers have tended to become irregular.
• Complete flowers contain a calyx, corolla, stamens, and a pistil.
  • Perfect flowers have stamens and a pistil.
    • Imperfect flowers are missing either stamens or a pistil.
• Monoecious plants have both male and female imperfect flowers.
  • Dioecious plants only have only male or female flowers.

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Pollination Ecology

• Throughout the evolutionary history of flowering plants, the pollinators have evidently coevolved with plants.
  • Twenty thousand bee species are included among current-day pollinators.
    • Many bee-pollinated flowers are delicately sweet and fragrant.
      • Flowers pollinated by beetles tend to have different, stronger odors.
• Moth and butterfly-pollinated flowers also often have sweet fragrances.
  • Night-flying moths tend to visit white or yellow flowers.
  • Butterflies tend to visit bright blue, yellow, or orange flowers.
    • Nectaries are at bottom of corolla tubes.
      • Situated for specialized mouth parts.
• Flowers visited by birds are often bright red or yellow, and usually have little, if any, odor.
  • Typically large flowers.
• Birds are highly active pollinators and tend to burn energy rapidly.
  • Many bird-pollinated flowers produce copious amounts of nectar to assure repeated visits.
• Bats tend to visit flowers that open only at night.

Pollination Observations

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Herbaria and Plant Preservation

• Herbaria are essentially libraries of dried, pressed plants, algae, and fungi, arranged and labeled.
  • Properly prepared and maintained specimens may remain in excellent condition for 300 or more years.

Here is the press we use for preserving plants at KCC

Note the layers of corrugated cardboard and thick blotter like paper.

Here are examples of flowers that we preserved.  Click the image to look at it better.

This student turned his work into something very special.

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Last modified: October 08, 2004 by Cynthia Herbrandson  © Copyright 1999, Kellogg Community College. All rights reserved.