BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland


I. Moving onto the land

  1. Similar challenges faced by both plants and animals, but plants and animals evolved different ways to meet the challenges
    1. Animals - the first invertebrate animals on land were probably crustaceans (insects, spiders); the first vertebrates were amphibians
  2. Plants evolved from algae
    1. Algae cannot survive on land except in moist environments
    2. Clearly plants had to adapt (or evolve) characters that would enable them to survive on dry land
    3. Cooksonia: The earliest known land plant are non-vascular "nematophytes" related to byrophytes (early Silurian); vascular land plants appeared first in the mid Silurian; near-shore plants from marine depoits in Saudi Arabia from the Ordovician
  3. The actual ancestor of plants was probably a green alga, hundreds of millions of years ago (probably something like the modern algal genus Coleochaete)
    1. It has the same photosynthetic pigments of plants (chlorophylls a and b, carotenes (orange), and xanthophylls (pale yellow)
    2. Details of mitosis (cell plate) are similar in green algae and plants and unknown in other organisms
    3. Has same photosynthetic storage product (starch)
    4. Both have cellulose in cell walls
    5. Both have alternation of generations

II. Environmental challenges of living on land [REQUIRED READING] (these evolved characteristics define plants)

  1. Obtaining enough water
    1. Animals - move to water source; requires nervous system (to be able o move and sense water's presence)
    2. Plants - evolved roots to anchor the plant in place and absorb water (and dissolved minerals)
      1. Tap root system - allows deeper penetration of water table underneath the surface
        1. example - mesquite
      2. Fibrous root system - covers large, shallow area (very effective when it rains
        1. example - large trees can be blown over by wind
  2. Preventing excessive water loss
    1. Animals - scales of reptile skin vs. moist skin of amphibians
    2. Plants
      1. Evolution of cuticle, a waxy layer covering the surface of all plant parts exposed to air
      2. Evolution of multi-cellular gametangia (sex organs) - helped protect the delicate gametes from drying out
      3. Evolution of a resistant coat on spores that prevents drying out
  3. Getting enough energy
    1. Animals - eat plants; this means that animals did not become permanent colonizers of land until plants were established
    2. Plants - getting enough light for photosynthesis (light wasn't a problem on land at first); several different strategies
      1. Grow tall, above other plants that are causing shade (requires evolution of support cells such as fibers - like iron bars in concrete)
          1. OR
      2. Adapt to a lower light intensity
  4. The photosynthesis/water trade-off
    1. Problem - plants need pores (stomata) to allow exchange of gasses for photosynthesis, but the pores allow a lot of water to leave by transpiration (loss of water vapor from land plants); 95-97% water taken through roots is lost by transpiration
    2. One solution - stomata are open during the day (to allow for gas exchange for photosynthesis) and closed at night (to allow plant to recover water lost during the day

III. Other features that were encouraged by living on land, features that algae have, but developed by land plants

  1. Multicellularity
    1. Multicellularity evolved in the oceans before plants and animals colonized the land
    2. However, multicellularity clearly gives plants an advantage on land
      1. Enables plants to root into the ground to obtain water
      2. Enables plants to cover and protect delicate cells such as gametes
      3. Enables plants to grow tall and shade out competitors
    3. Multicellularity creates a new challenge in plants - getting water, essential minerals, and food to all its cells
      1. Algae get water and minerals by absorption from their surroundings (immersed in water)
      2. Most algal cells photosynthesize, so no need to conduct food
      3. Plants evolved vascular tissues - xylem (conducts always up water and dissolved minerals) and phloem (conducts food) to get all these materials to all parts of the plant
  2. Sexual reproduction
    1. Sexual reproduction evolved in the oceans before plants and animals colonized the land
    2. However, sexual reproduction gave plant offspring the genetic variability that enables them to adapt to changes in the environment
    3. This is more crucial on land than in the ocean because the land environment has much greater changes (wet/dry, hot/cold)
  3. . Alternation of generations
    1. Example - humans = 46 chromosomes (2n=46); 2 - 23 chromosome sets (n=23)
                               --- adult human --
                             /                   \
                   (embryo in uterus)             \
                       mitosis)                    \
                          |                         \                           
                          |                          \
                        zygote                        \
                          |           2n=46        (in sex organs)
                   (fertilization) ----------------- (meiosis)      
                         |  |                           |  |
                         |  |          n=23             |  |
                         |  \                           /  |
                         |   \<---- egg ---------------/ / \ / \ <------ sperm --------------/ 
  4. Plant life cycle
    1. Alternation of generations - plants (and some protists and fungi) spend part of their life cycle in the haploid stage and part in the diploid stage
                        /-----> adult plant ->----\
                       /                           \
                   (mitosis)                        \
                     /                               \                    
                  embryo                      (in sex organs only)
                   /                                  |           \
                (mitosis)                           female         male
                 /                                sporocyte     sporocyte
              zygote                         (megasporangium) (microsporangium)
        (microsporangium)   Sporophyte generation (2n) |           |
               |                                       |           |
         (fertilization)------------------------------ | (meiosis) |
               |       \                               |           |
               |        \          Gametophyte     megaspores  microspores
               |         \        generation (n)       |           |
               \          \                           /         (mitosis) 
               sperm cell  \ egg <- (mitosis) <- female / nucleus gametophyte / \ / \ <---------------- male <--------------/ gametophyte (pollen grain) 
    1. Some algae have alternation of generations, but the gametophyte (haploid) generation is dominant
      1. This makes sense because gametophyte organisms reproduce by gametes (which can swim in an aquatic medium)
      2. Actually, both gametes and spores are dispersed by water in algae

    2. The sporophyte generation becomes progressively more dominant in plants
      1. Spores (produced by a sporophyte) can be carried by air current OR remain within the protected reproductive tissue of the plant
      2. In contrast, swimming gametes are a real disadvantage on land

    3. Eventually, plants evolved gametes (pollen grain sperm cells) that are transported by wind or other organisms

IV. Bryophytes: Mosses, liverworts, hornworts (non-vascular plants; 16,000 species)

  1. Bryophytes include the mosses and liverworts. They do not have a well developed vascular system. In the bryophyte life cycle, the smaller dependent sporophyte lives on the gametophyte.
  2. Evolution - Not in direct path to higher plants; a sideline developed from green algae or maybe even from higher plants; Advancements over algae- cuticle; multicellular gametangia; stomata
  3. Habitat - Require moist environment; some are tolerant of dry areas (need moisture for active growth and sexual reproduction)
  4. Life cycle
    1. Gametophytes
      1. In bryophytes, the gametophyte generation is dominant.
      2. Gametophytes of mosses have green "leafy stems" and structures that resemble roots (rhizoids). The rhizoids only hold the plant to the ground.
      3. They possess stomata and cuticle (adaptations to live on land).
      4. The lack of vascular (conducting) tissues prevents the mosses from having true roots, stems, or leaves. Gametophytes may grow either erect or flat on the ground. Their size is restricted by lack of vascular tissue.
      5. Gametophyte plants produce multicellular sex organs: archegonia, which produce eggs, and antheridia, which produce motile sperm.
      6. The sterile layer that comprises the outer part of archegonia and antheridia helps protect the delicate gametes by preventing them from drying out.
      7. The unprotected motile sperm must swim or splash from the antheridia to the archegonia.
    2. Sporophyte
      1. After an egg is fertilized by a sperm, a zygote forms in the archegonium which grows into a multicellular embryo.
      2. A sporophyte develops from an embryo which is still enclosed in the archegonium of the gametophyte plant; therefore, the sporophyte is permanently attached to the gametophyte. Because the sporophyte not photosynthesize, it is dependent upon the gametophyte for food.
      3. The mature sporophyte consists of a foot (the attachment point of the sporophyte to the gametophyte), a seta (stalk), and a capsule (spore case).
      4. Sporocytes within the sporophyte undergo meiosis to produce a single kind (homosporous) of haploid spore. If a spore lands in a suitable place, it can germinate into a protonema, the initial filamentous stage of a gametophyte plant.
  5. Significance of mosses; Inconspicuous but important part of biosphere
    1. Food for mammals, birds
    2. Important in soil formation and reduction of soil erosion, particularly along stream banks
    3. Commercial moss- peat moss (Sphagnum)
      1. Fuel (dried).
      2. Soil conditioner- helps retain moisture.
      3. Also used by florists to hold moisture.

V. Ferns [REQUIRED READING] - Polypodiophyta (Other scientific names for the ferns are "Filicophyta" - an incorrect name - and Pterophyta)

  1. Ferns are a graceful and attractive group of plants with more than 10,000 living species. Unlike the bryophytes with their small dependent sporophyte generation, seedless vascular plants like ferns have an independent sporophyte generation that is larger and more conspicuous than the independent gametophyte generation. The gametophyte plant is small and short-lived.
  2. Evolution
    1. Ancient group; one of oldest group of vascular plants (possess conducting tissues- xylem and phloem).
    2. Fossils 390 million years old.
    3. Dominant plants 300 million years ago.
    4. Advancements over the mosses (remember that they did not evolve from the mosses)- vascular tissue and dominant sporophyte.
  3. Habitat - terrestrial (mostly) from tropics to Arctic Circle.
    1. Most species in moist tropics.
    2. Found in woodlands and along stream banks here.
  4. Life cycle
    1. Spore germination
    2. Gametophyte development
    3. Sperm migration
    4. Sporophyte development
  5. Sporophyte:
    1. Life cycle of ferns- sporophyte generation is dominant.
    2. Most are relatively small plants.
    3. Tree ferns in tropics- 20-60 feet tall, 6-12 foot leaves.
    4. The sporophyte generation in ferns is a perennial plant with large complex leaves called fronds.
  6. Structure of the sporophyte and vascular tissue:
    1. There are several reasons why vascular plants are better adapted for terrestrial life than the non-vascular liverworts and mosses.
    2. Vascular plants often have a well-developed cuticle, a waxy layer that covers the epidermis of those plant parts exposed to the air. A cuticle helps reduce water loss, which is a significant environmental challenge for land plants. (Mosses have a rudimentary cuticle on their sporophytes.)
    3. Vascular plants possess tissues that are specialized for internal transport of water (xylem) and food (phloem). The water conducting cells in the xylem are dead at maturity; only their cell walls remain. The water conducting cells in ferns and gymnosperms (including pine trees) are called tracheids. The tracheid cells look like hollow toothpicks with many small holes at each end. An important chemical, lignin, occurs in the xylem cell walls. Lignin is a very rigid substance that provides the strength and support that allows plants to grow larger.
    4. Sugars are transported either up or down the plant through the living phloem cells called sieve cells in ferns and gymnosperms. A sugar molecule passes through the plasma membrane of a phloem cell, travels to the opposite side of the cell by cytoplasmic streaming, and leaves the cell through the plasma membrane. Here it enters the next cell and is passed along.
    5. Two other important cell types are the fiber cells and the parenchyma cells. The fiber cells are dead at maturity and have very thick walls. They help support the plant. The parenchyma cells are living thin-walled cells that generally function as storage cells.
  7. Sporophyte sexual reproduction:
    1. Besides photosynthesizing, fronds play a role in reproduction. It is on the fronds that spores are produced in sporangia that often occur in clusters called sori (singular, sorus).
    2. Within the sporangia, meiosis occurs producing the haploid spores.
  8. Gametophyte:
    1. Each independent heart-shaped gametophyte plant, called a prothallus (plural, prothalli), grows from a single spore and is quite small and inconspicuous.
    2. Archegonia and antheridia are produced on the prothalli. Generally, when spores germinate they produce female gametophytes. These produce chemicals that influence the newly germinating spores to produce male gametophytes. Occasionally, when only a few spores germinate, the prothalli will form both sexes.
    3. The prothallus needs a saturated environment and lives for a very brief time. When a sperm from an antheridium swims to an archegonium and then fuses with an egg inside, the diploid sporophyte generation begins. The zygote develops into an embryo that eventually grows into a mature, multicellular sporophyte plant.
  9. Significance of ferns:
    1. Ecological importance:
      1. Hold and form soil and prevent soil erosion.
    2. Commercial importance:
      1. Food- fern fiddleheads.
      2. Ornamental plants.
      3. Coal- formed 300 million years ago from partly decomposed and consolidated plant material; swampy = oxygen depletion and incomplete decomposition = peat accumulation -> -> -> pressure and heat -> coal.
      4. Coal formation and fossilization.

VI. Other Lower Vascular Plants. [REQUIRED READING]

  1. About the Carboniferous. and its life
  2. Rhyniophyta - Devonian plants
    1. Features.
  3. Zosterophyllophyta
    1. Features
    2. Zosterophyllophyta: Devonian plant; probably origin of Lypopodiophyta ("club mosses")
  4. Trimerophytophya
    1. Features
    2. Trimerophyta: Devonian plants; probably origin of ferns
  5. Psilotophyta: Wisk ferns (often incorrectly called "Psilophyta") - no known fossil record; related to ferns?
    1. Features
    2. Life cycle
  6. Lycopodiophyta: Club mosses (often incorrectly called "Lycophyta")
    1. Features
    2. Life cycle
    3. Fossil lycopods
    4. Fossil record
  7. Equisetophyta: Horse tails (sometimes called Sphenophyta; one modern genus, Equisetum) and numerous fossil representatives
    1. Features
    2. Life cycle
    3. General morphology

Important Resources Critical to this Lecture:
Explanation of Geologic Time
Life through geologic time
A complete time scale

Other Sites of Interest:
Excellent Review
Major review: Now that you are this far along in class, this essay brings much of it all together
Review of early land plant evolution
Review of early land plants - Illustrated!
Review of the first vascular land plants - Illustrated!
A review and preview
Plants onto land - algae to gymnosperms
An excellent review on the ferns and fern allies
Plant Kingdom: a detailed over-view
Illustration and classification of bryophytes
Alternation of generations: A review
Fossil plants: from Mazon Creek, Illinois

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Last revised: 15 Aug 1998 - Reveal