SCIENCE

Scientists think they have the answer to what Charles Darwin called an “abominable mystery”: How flowers evolved and spread to become the dominant plants on Earth. (BBC)

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Discussion Ideas

• New research digs into the genetics of flowering plants. What are flowering plants?
  • Flowering plants, also known as angiosperms, are the most abundant group of plants on Earth. Flowering plants describe not just flowers, but plants such as grains, palm trees, and most trees and shrubs in deciduous forests. Their distinguishing characteristics are:
    • flowers, of course. Flowers are the flashy reproductive organs of flowering plants. They’re a nifty adaptation: “Flowers provided angiosperms with the means to have a more species-specific breeding system, and hence a way to evolve more readily into different species without the risk of crossing back with related species.”
    • modified stamens. Stamens are the plant’s pollen-producing organs. The stamens of many flowering plants are so highly evolved they can only be pollinated by a single species (pollinator).
    • reduced size of gametophytes. A gametophyte describes the very early, sexual stage of plants and some algae. In flowering plants, the reduced size of both the male and female gametophytes helps reduce the time between pollination and fertilization.
      • Pollination describes the time it takes for the pollen grain to reach the (female) pistil.
      • Fertilization describes the next phase of development, in which the male and female gametophytes combine to form a diploid (double-chromosomed) organism called a sporophyte.
    • closed carpels. After fertilization, the seed-bearing part of the flower (carpel or carpels) closes itself off. In many flowering plants, carpels develop into fruit. This is another nifty adaptation: Fruit—so colorful, so sweet-smelling, so delicious—is attractive to animals. After they eat the fruit, these animals are likely to poop out and disperse the fruit’s indigestible seeds.
    • endosperm. Endosperm is the starchy tissue surrounding the seeds of most flowering plants. The plant embryo relies on endosperm for nutrients. Humans like endosperm, too, whether it be ground up as flour (wheat endosperm) or eaten raw (coconut “meat” is endosperm).

• So, that describes about 90% of plants on Earth. What other kinds of plants are there?
  • gymnosperms. Like angiosperms, gymnosperms are seed-bearing plants, but their seeds lack a protective cover (ovary). Gymnosperms include conifers (such as pine trees and cypresses) and ginko.
  • bryophytes. These plants are generally moisture-loving and include mosses and liverworts.
  • ferns. These pretty, ancient plants have neither flowers nor seeds, but reproduce through spores.
  • horsetails. These weird “living fossils” have non-photosynthetic leaves.
  • clubmosses. Clubmosses, like horsetails, are nicknamed “fern allies” because they reproduce through spores.
  • green algae. From microscopic diatoms to large seaweeds, green algae are the most abundant plants in the sea. (Not all botanists think green algae are plants.)

• Millions of years ago, ferns and gymnosperms dominated the Earth. According to the new research, how might angiosperms have crowded them to a smaller ecological niche? Read through the nice BBC article for some help.
  • genome downsizing. An organism’s genome is the set of genes that hold all the inherited characteristics of the organism. Flowering plants, it turns out, have a much smaller genome than gymnosperms, ferns, or bryophytes.
    • According to the BBC, “By shrinking the size of the genome, which is contained within the nucleus of the cell, plants can build smaller cells.
      In turn, this allows greater carbon dioxide uptake and carbon gain from photosynthesis, the process by which plants use light energy to turn carbon dioxide and water into glucose and oxygen.
      Angiosperms can pack more veins and pores into their leaves, maximising their productivity.
      The researchers say genome-downsizing happened only in the angiosperms, and this was ‘a necessary prerequisite for rapid growth rates among land plants’.”

TEACHERS TOOLKIT

BBC: How flowering plants conquered the world

Nat Geo: Game of the Week: Seed Racer