BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland


Three recently discovered new kinds of flowering plants found in the
Sierra Madre del Sur, Guerrero, Mexico, by Maryland botanists

I. General information about flowering plants (angiosperms = covered seed)

  1. Possess flowers and fruits with seeds
  2. Habitat- everywhere; dominant plants of earth's flora

II. Significance of flowering plants

  1. Extent of flowering plant success
    1. Area
      1. Dominant plants in almost every terrestrial environment
      2. Exception - northern coniferous forests where dominant plants are conifers
    2. Number of species
      1. Estimated at 260,000
      2. This is 88 percent of the species estimated to be in the Plant Kingdom (295,000)
  2. Evolutionary significance of flowering plants
    1. Most advanced plants on earth
    2. Advancements over gymnosperms
      1. Flowers- many use animal pollinators- assures greater reproductive success
      2. Fruits and seeds adapted for dispersal
      3. Double fertilization -> endosperm in seed
      4. Other structural advancements that will be discussed in later lectures
  3. Ecological roles of flowering plants
    1. Food, shelter, and concealment for animals
    2. Help form and improve soil; prevent soil erosion

-- © Ross Konig at Eastern Connecticut State University

III. Sexual reproduction in flowering plants

  1. Floral structure
    1. Male sex organs
      1. The stamens are the male sex organs composed of
      2. The stamens may have derived from a leaf blade with a sporangia near its tip (apex) or from slender branch systems bearing terminal sporangia.
    2. Female sex organs
      1. The carpel is the evolutionary female structure (a modified leaf).
      2. One or more carpels forms a pistil (the central organ of flowers that is composed of an ovary, style, and stigma)
        • 1) The ovary is the enlarged basal portion of a carpel or fused carpels that contain the ovules. A mature ovary, sometimes with other adherent parts, is a fruit.
          2) The ovules are structures within the ovary containing the female gametophyte with egg cell. When mature, the ovule becomes a seed.
          3) The stigma is the region of a carpel that serves as a receptive surface for pollen grains and on which they germinate to form pollen tubes. The movement of pollen to the stigma is termed pollination
          4) The style is a slender column of tissue that arises from the top of the ovary. Pollen tubes grow through the style.
          5) An example of a pistil composed of several carpels is the orange (each section in the fruit is derived from a single carpel)
          6) Ovaries may be "superior" (located above the sepals and petals) or "inferior" (located below the sepals and petals)

Non-reproductive whorls in flowers

  1. Consist of an outer whorl of leaf-like bracts called sepals
  2. And an inner whorl of leaf-like bracts called petals
  3. Sepals tend to be green while petals tend to be variously colored and usually much less leaf-like
  4. The sepals and petals can be variously shaped such as free and simply arranged (actinomorphic) or variously fused and of dissimilar shaped and size (zygomorphic)

IV. Life cycle of flowering plants

  1. Heterosporous
    1. male - diploid sporophyte anther with microsporangium containing microspore mother cells; through meiosis they produce haploid microspores which germinate within the anther sac to produce immature male gametophytes (pollen grains) containing tube nucleus and generative cell (2 sperm nuclei)
    2. female - diploid sporophyte ovule within ovary has megasporangium containing megaspore mother cells; through meiosis they produce megaspores which germinate into female gametophyte containing egg and 2 polar nuclei
  2. Double fertilization
    1. pollen grain germinates on stigma forming pollen tube growing to the ovary; the two haploid sperm travel down the tube to the ovary where one sperm nucleus fertilizes the haploid egg forming a diploid zygote and the other sperm nucleus unites with the two polar nuclei forming a triploid endosperm nucleus which grows into the endosperm
  3. Seeds
    1. the fertilized egg becomes a zygote that grows into a plant embryo.
    2. the endosperm nucleus grows into the endosperm, a tissue containing stored food that will be used by the growing embryo (most of these storage tissues are absorbed by the developing embryo before the seed becomes dormant)
    3. the mature ovule becomes a seed coat
  4. Flowering plants (technically called Magnoliophyta) are divided into two groups, dicotyledons (Magnoliopsida) and monocotyledons (Liliopsida); each of these is divided further into additional smaller, technical groups - see examples of each group of dicots and each of the smaller monocots groups
    1. As the zygote grows into the embryo, the first leaves of the young sporophyte develop and are referred to as cotyledons (seed leaves).
      1. These fleshy modified leaves store food materials that are used by the germinating seed (include carbohydrates, proteins, and lipids).
      2. The origin of the stored food is provided, at least initially, by the endosperm.
    2. There are two groups of flowering plants that are differentiated by the number of cotyledons that develop in the young embryo: those with one cotyledon (monocots) and those with two (dicots).
      Monocots (monocotyledons)- corn, lily
    3. Dicots (dicotyledons)- bean, oak
    4. Comparison of monocots and dicots
          FEATURE                      MONOCOTS       DICOTS
          cotyledons                    1              2
          leaf venation                 parallel       net
          woody or herbaceous           herbaceous     either
          vascular bundles in stem      scattered      arranged in circle
          root system                   fibrous        tap
          number of floral parts        in 3's         in 4's or 5's
          endosperm in mature seed      yes            seldom

Other sites of interest:

All about flowers
Review of flower parts with definition of terms: from Texas A&M
Review of the life cycles of a flower - 1
Review of the life cycles of a flower - 2
Review of the life cycles of a flower - 3
Review of the life cycles of a flower - 4
Review of the life cycles of a flower - 5
Pollen allergy
Seeds of life: A good review of seeds

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Last revised: 23 Aug 1998 - Reveal