PLANT DEVELOPMENT

We've talked about plants' metabolic functions, and how they participate in ecosystems and biogeochemical cycles. Time now to learn more about the plants themselves, and how they grow.

Let's start at the very beginning, with the GENERALIZED LIFE CYCLE OF A PLANT.

Note that ALL plants, no matter how primitive or derived, go through this cycle, and have the following life cycle phases:

  • zygote - the diploid "fertilized ovum": fusion of ovum & sperm
  • sporophyte - the diploid individual that grows from the zygote (via mitosis) and eventually produces spores (via meiosis)
  • spore - the haploid propagule produced by the sporophyte (via meiosis)
  • gametophyte - the haploid individual that grows from the spore (via mitosis)
  • gamete - the haploid propagule produced by the gametophyte (via mitosis), which will eventually fuse with a complementary gamete to form the zygote.

    ...and we've come full circle.

    (Note: Any little package (spore, seed, or whatever) that contains all the necessary materials to make a new plant is called a PROPAGULE.)

    The only differences in the above life cycle across plant species are what the various life cycle stages LOOK LIKE. They all have them, and all in the same life cycle order.

    Note that not all plants reproduce by means of seeds. Seeds are found only in the most recently evolved plants, known as the GYMNOSPERMS ("naked seed" plants) and the ANGIOSPERMS ("boxed seed" plants, also known as flowering plants), and are basically a protective "case" for the developing zygote.

    However, since we've been talking mainly about seed plants (the dominant plants on earth) so far, we will continue to use them as our example for generalized plant development and growth.

    Why have seeds? What functions do they serve?

  • a way for offspring to be transported away from the parent (dispersal)
  • seeds can remain dormant until environmental conditions are right for developement
  • seeds, unlike spores, provide food for the plant embryo, giving it a "head start" in life that plants without seeds don't get.

    Seed dispersal:

    1. wind dispersal

    2. water dispersal (what types of seeds wash up on beaches?)

    3. animal dispersal (or, why does ripe fruit taste good?)

    Here's a closeup look at what's inside a generalized SEED. (You should be able to recognize the parts of a seed and the parts of the plant embryo inside a mature seed.)

    Germination depends upon many internal and external factors:

  • water
  • O2
  • temperature
  • light (in some species with very little endosperm)

    Ways in which water may help induce germination:

  • imbibition
  • washes off germination-inhibiting hormone

    (Note: most seeds must undergo a period of dormancy before germination, either going through a dry season or a cold season before they are even able to germinate.)

    Plant development:

    First, the primary root appears. Then the hypocotyl (dicots) or coleoptile (monocots) and the rest of the shoot. Further development is mediated by hormones.

    What happens?

    GROWTH

    What is growth?

    Growth in living organisms refers to growth of cells.

    Cells may increase in size (volume or length) or number (according to your book: "it may be defined simply as 'an increase in mass due to the division and enlargement of cells' and may be applied to an organism as a whole or to any of its parts").

    Growth consists of:

    1. increase in cell number

    2. increase in cell volume

    3. increase in cell length

    4. differentiation of cells for various functions

    Growth can be indeterminate or determinate.

    Indeterminate growth: growth occurs throughout the life of the organism

    Determinate growth: growth occurs until the structure reaches a certain size (or for a given juvenile period), then stops.

    What type of growth do humans exhibit? Redwood tree stems?

    Plant growth factors and regulators...

    1. genes - are responsible for determining the types and amounts of enzymes and hormones which are produced
    2. enzymes - speed up the rate of chemical reactions in biological systems without being changed themselves and are generally produced in the cells where they're used
    3. hormones - (from the Greek 'horman,' "to set in motion") have specific physical and/or physiological effects on cell growth and are generally not produced in the cells where they're used (they are manufactured in one place, and transported to another place where they have their effect).
    4. vitamins - function as coenzymes (i.e., enzyme "activators") and are synthesized in the membranes and cytoplasm of cells

    Basic types of plant movements:

    1. TROPISMS - permanent, directed movement (growth) in response to an external stimulus. Tropisms may be positive (going towards a stimulus) or negative (going away from a stimulus).

    Examples:

  • phototropism: growth in response to light
  • geo/gravitropism: growth in response to gravity (e.g., roots are positively gravitropic, shoots are negatively gravitropic)
  • thigmotropism: " " mechanical contact
  • hydrotropism: movement in response to water
    1. TURGOR MOVEMENTS - result from changes in internal water pressures

    1. CIRCADIAN RHYTHMS - movements associated with diurnal cycles (night/day).

    Examples:

    Opening/closing of leaves, flowers, etc.