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An Introduction to Botany

Why should we care about PLANTS?

  • Without plants, most of life on earth as we know it would not exist.
  • Plants comprise about 98% of the earth's biomass!
  • Plants are primarily responsible for creating our oxygen-rich atmosphere via the light reactions of photosynthesis.
  • Plants are the earth's main autotrophs and fixers of carbon and nitrogen.
  • Plants provide the habitat and food upon which almost all other living things ultimately depend.
  • Plants are responsible for most of the products on which you rely to survive (vegetable and animal matter), have a good quality of life (fabric for clothing, medicines), as well as the more frivolous ones (spices, perfumes, dyes, dissolvable sutures, food stabilizers, emulsifiers, Starbuck's, etc.).
  • And don't forget the potential of biofuels (ethanol produced from food crops)
  • Almost every living (terrestrial, and many aquatic) thing interacts with plants in csome way. Plants are involved in every type of symbiotic community interaction known.
  • The plant community--determined by climate and soil conditions--determines what else can live an ecosystem. Like other organisms, plants are involved in an amazing array of symbioses.

    Symbiosis (from the Greek sym, meaning "together" and bios, meaning "life") refers to the members of two different species (i.e., two populations) engaging an ecological interaction that affects both populations. Ecological interactions can drive the coevolution of two interacting populations.

    Plant Symbioses: A few examples

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    obligate mutualism

    +

    +

    both populations benefit and cannot survive without one another

                                                           EXAMPLE: Bucket Orchid and Euglossine Bee

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    protocooperation

    +

    +

    both populations benefit but can survive without one another

                                                           EXAMPLE: Think: Agriculture

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    competition

    -

    -

    populations inhibit one another

                                                            EXAMPLES: Dense Forest; Allelopathic Plants

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    predation

    +

    -

    one population kills and feeds on the other

                                                            EXAMPLES: You Might Know Some of These Guys...

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    parasitism

    +

    -

    one population (parasite) feeds on, but does not kill outright members of the other population (host)

                                                           EXAMPLE: Yes, there are Parasitic Plants, some of them beloved.

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    commensalism

    +

    0

    one population (the commensal) benefits from the other, which is not affected

                                                           EXAMPLE: Epiphytes of many kinds.



    The Plant Clade: Archaeplastida

  • Rhodophyta (red algae)
      At least a billion years ago, an ancestral heterotrophic eukaryote took up a symbiotic relationship with a cyanobacterium-like prokaryote, and the ancestor of Rhodophyta came to be.

      Secondary endosymbiosis gave rise to major groups of algae as well as the ancestors of modern plants.

      Red algae live in deeper waters than most other algae, partly because of their ability to collect light for photosynthesis at great depths.

      They are characterized by phycocyanins (blue) and phycoerythrins (red), photosynthetic pigments unique to the Rhodophyta and to the cyanobacteria.

      • What might be the significance of having a red pigment (phycoerythrin) as a photosynthetic accessory pigment?
      • Hint: which wavelengths would such a pigment (1) absorb and (2) reflect, and which of those wavelengths goes deeper into water?)

  • Green Plants

    Members of the Rhodophyta's sister clade are all around us, providing 98% of the earth's terrestrial biomass. You can recognize them by their characteristic emerald green color, conferred by their photosynthetic pigments.

    Green Plants share the following derived characters:

    • chlorophylls a and b
    • carotenoids as accessory photosynthetic pigments
    • chloroplasts with a double membrane
    • energy stored as starch inside the chloroplast in which it is produced
    • cell walls composed of cellulose (the most abundant organic molecule on earth) and pectin

    Included in the "green plants" are several thousand species of green algae and hundreds of thousands of species of land and aquatic plants.


    Aquatic Cousins of Land Plants

    Green Algae

    Many green algae exhibit a progression of complexity (e.g., the volvocine line of evolution). Multicellularity with true division of labor: adaptive?

    Charales

    The Charales, aslo known as stoneworts or brittleworts, are common in slow-moving, nutrient-rich, freshwater habitats. They are found on every continent except Antarctica.
  • Like land plants, stoneworts have multicellular male and female sex organs (male = antheridium; female = oogonium).

  • Chara is a species of particular interest in southern Florida, as it is a problematic invasive exotic that chokes canals and waterways throughout our area.

    Coleochaetes

    The fifteen extant species of Coleochaete appear to be the closest living relatives of land plants.

    Sporopollenin and Lignin: Tough!

    The Charales and Coleochaetes share several synapomorphies with land plants
    • sporopollenin
    • cell walls contain lignin-like compounds
      • lignin is a structural component of woody plant cell walls
      • lignin is less abundant in herbaceous (soft, non-woody) plants

    Embryophyta: The Land Plants

    Why would terrestrial existence be an advantage for a plant? Why didn't they just stay in the water?
    • air filters less sunlight than water. There's more light for photosynthesis.
    • air has more CO2 than water. There's more fuel for photosynthesis.
    • terrestrial soil is richer in nutrients than aquatic soil (why?)
    • earlist terrestrial habitats were free of pathogens and herbivores.

    Embryophyte synapomorphies

    (Items below in [brackets] are found in all plants, but may also appear in a few highly derived green algae and/or charophytes.)
    • heteromorphic [alternation of generations]
    • multicellular [gametangia] - antheridia in males, archegonia in females
    • heterogamous
      • isogamy - gametes are physically indistinguishable
      • heterogamy - gametes are physically distinguishable
      • oogamy - special case of heterogamy; ovum is large and non-motile; sperm is small and motile
    • [true tissues]
    • waxy cuticle
    • stomates

    • meristems (plant "stem cells" at the tip of every shoot and root)

    • embryo develops inside mother's archegonium
    • secondary metabolites such as alkaloids, terpenes, tannins, phenols, flavonoids (these serve various metabolic functions including deterring herbivores)



    Plant Architecture

    A tissue is an aggregation of cells coordinated to perform a particular function or set of functions.

    Plant tissues

    • meristem (embryonic and totipotent)

    • simple (composed of only one type of cell), such as
      • parenchyma
      • collenchyma
      • sclerenchyma

    • complex (composed of more than one type of cell), such as
      • dermal (protective covering)
      • vascular (conducting tissue)
        • xylem (conducts water and dissolved minerals)
        • phloem (conducts water and dissolved organics)
      • ground (bulk of the body; primarily parenchyma, collenchyma & sclerenchyma)

    A stem cross-section reveals the organization of tissues in a typical plant:

    A stem longitudinal section shows the progression of their development:

    Plants that do not produce wood, remaining soft and pliable all their lives are said to be herbaceous.

    Plants having a vascular cambium become woody.

    Vascular Cambium: Source of Wood

    The vascular cambium is a secondary meristem, meaning it is derived from a primary meristem (the procambium we met before).

    The vascular generates the rings of xylem we know as wood.


    Magical Meristem

    Meristems are regions of undifferentiated, embryonic cells.

    Initially, the cells are totipotent and can differentiate/mature into any other type of cell.

    Recall:

    • totipotent cells have the capacity to develop into any type of cell.
    • pluripotent cells can develop into many, but not all different types of cells.
    • multipotent cells can develop into multiple types of cells, but not as many types as pluripotent cells.

    meristem

  • present throughout the life of the plant
  • apical meristem growth increases plant length
  • lateral meristem growth increases plant girth
    • vascular cambium - gives rise to secondary xylem and phloem (wood)
    • cork cambium - give rise to the bark (replaces epidermis)



    Plant Diversity

    Plants can be broadly classified as

    I. Non-vascular (Bryophytes)
    II. Vascular (Tracheophytes)

      A. Seedless
        1. Club "Mosses"
        2. Ferns and their relatives
      B. Seed-bearing
        1. Gymnosperms (naked seed plants)
        2. Angiosperms (flowering plants)


    The Non-vascular Plants: Amphibians of the Plant World

    The Non-vascular plants, commonly known as bryophytes (from the Greek bryo, meaning "moss" and phyt, meaning "plant") are the descendants of the first true land plants.

    Bryophytes

    • lack xylem and phloem (hence, lack true organs)
    • have a very thin waxy cuticle
    • have stomates fixed in the open position; they cannot be closed
    • release flagellated sperm directly into the environment (and so need water--at least a thin film--for reproduction)
    • have a dominant gametophyte generation, and a short-lived, ephemeral sporophyte

    The liverwort thallus bears a faint resemblance to a certain animal organ that gets very little respect.

    The Doctrine of Signatures is an ancient belief that the shape, color and other properties of plants can tell the herbalist which ones can be used to heal ailments of specific parts of the body. Liverworts were once used to treat liver ailments, though no data exist to suggest that such treatments were (or are) effective.

    The mosses are the most diverse non-vascular plants, and are economically and ecologically important for many reasons.

    Some contain a thin strand of conducting tissue in the center of the erect portion of the plant body: the earliest beginnnings of true stems and leaves (defined by the presence of vascular tissue).


    Vascular Plants

    Most of the plants you see in your day-to-day travels are the sporophytes of vascular plants. The gametophyte is tiny and ephemeral.

    Tracheophytes have several evolutionary advances that made them better suited for a fully terrestrial existence

    • xylem and phloem
    • lignin (not only structural in and of itself, but allows greater turgor pressure)
    • thick waxy cuticle
    • fully functional stomates (open and close with turgor pressure)
    • highly differentiated plant tissues and organs
    Lycopodiophyta - The Club Mosses
    The most primitive seedless vascular plants are the club "mosses" (which are not mosses at all).

    Only two extant genera: Lycopodium and Selaginella


    Pteriodiophyta - Ferns and Horsetails
    This is a diverse group, with more than 11,000 species, second only to the flowering plants in variety. The most familiar pteridophytes are the ferns.

    Fun Fact for Botanical Trivial Pursuit: The tiny fern Ophioglossum reticulatum has more chromosomes--1260--than any other living organism. And yet how tiny!

    Young leaves in most species emerged as coiled structures known as fiddleheads

    Horsetails are very closely related to ferns.


    Spermatopsida - The Seed Plants

    The seed plants are the dominant eukaryotic life forms on the planet.

    There are five phyla of extant seed plants.
    These four produce their seeds in strobili (singular = strobilus), commonly called "cones".

    Gymnosperms:

    Coniferophyta - The Conifers
    The most familiar gymnosperms are the conifers such as pine, spruce, fir, redwoods, cedar, cypress, juniper.
    They bear their seeds in "cones", technically known as strobili (singular = strobilus).


    Anthophyta: Flowering Plants
    The most diverse and successful phylum of seed plants Spermatopsida produces seeds in a whorl of modified leaves comprising a flower.

    Angiosperms/Anthophytes are the most diverse and successful plants, numbering anywhere from 300,000-450,000 species.
    What synapomorphies set angiosperms apart from other plant phyla?

    • flowers
    • fruit

    • double fertilization
      • One gametophyte nucleus (ovum) fuses with one sperm to form the zygote
      • Two gametophyte nuclei (polar nuclei) fuse with one sperm to form nutritive endosperm.


      The Flower

      A typical, generalized flower:

      Like a pine needle fascicle, a flower is actually a shoot that terminates in leaves. But these leaves are all highly specialized for reproduction, and even the sterile portions (stamens, petals) may assist.

      Be sure you remember the meaning and significance of...

      • petal (single) and corolla (collective petals)
      • sepal (single) and calyx (collective sepals)
      • perianth (the petals and sepals together)
      • stamen (the specialized male structure)
        • anther and filament
        • pollen (the male gametophyte (NOT the same as sperm!)
      • pistil
        • stigma and style and ovularly (ovary)


    Double Fertilization

    This process is unique to flowering plants, and provides an "edge" for the growing embryo in the seed: endosperm.

    The Players:

    • pollen - this male gametophyte is haploid, and produces two sperm via mitosis

      Pollen morphology is unique to each species, and pollen is as specific to its species' stigma as a key to a lock. In some cases, even the pollinator is so specifically evolved, that if either the plant or the pollinator were to go extinct, so would its partner. Check out the Bucket Orchid and the Euglossine Bee.

    • 8-nucleate "embryo sac" - this female gametophyte is unique to flowering plants. She develops eight, genetically identical nuclei enclosed in a single mass of cytoplasm.

    Double Fertilization occurs when the two sperm enter the ovule (the female gametophyte surrounded by her mother's tissue).
    • one sperm fuses with the ovum to become the zygote
    • one sperm fuses with both polar nuclei to become the 3n endosperm, which will proliferate and nourish the embryo.
    Endosperm is nutritious! When you eat a seed, the fleshy, fatty/carbohydrate-rich stuff inside is mostly endosperm.
    • the flesh of a pea, lima bean, or peanut
    • the flesh inside a kernel of corn
    • the fatty tissue of a walnut, acorn, or almond
    The list goes on.

    Once fertilization takes place, the ovule matures into a seed: the embryo contained in its bed of nutritive endosperm.



    The Kindergarten Wall by John McCutcheon

    Of all you learn here, remember this the best:
    Don't hurt each other and clean up your mess.
    Take a nap every day, wash before you eat
    Hold hands, stick together, look before you cross the street.
    And remember the seed in the little paper cup:
    First the root goes down, and then the plant grows up.

    Go forth and pollinate.