BIO 131, General Botany

Lecture Notes  

Tuesday, Mar. 31, 2009

MEGASPORES
megaspores are never shed from the megasporangium
                WHICH MEANS THAT:
                        female gametophyte develops inside the megasporangium (= nucellus)
                        fertilization occurs inside the megasporangium
                        the young sporophyte (embryo) develops inside the megasporangium
                       (actually only one functional megaspore results from meiosis -- the other 3 disappear)

megasporangium surrounded by integuments = ovule

matured ovule with young sporophyte inside =   seed
                integuments of the ovule produce the seed coat of the mature seed

ovules are protected by the megasporophylls
                in Gymnosperms, the ovules are merely protected,
                but in Angiosperms (= Phylum Magnoliophyta or Anthophyta = the flowering plants)
                the ovules are actually enclosed by the megasporophylls

MICROSPORES
      in seed plants,
microspores develop into pollen grains
        each pollen grain contains a male gametophyte
        after pollination, the pollen grain sprouts a tube (called the pollen tube, of course)

       the sprouted pollen grain with its pollen tube is the mature male gametophyte

        Because of the tube, fertilization in seed plants no longer depends on a film
        of water, as it does in ferns and other non-seed plants

This certainly takes some of the chance out of fertilization! - no sperm depending
        on a film of water

        Because of the pollen tubes, seed plants are sometimes called Siphonogams
            (= male gametes in a tube or siphon!)

        In addition, pollen grains are very nutritious (much protein and lipid)
        and a handy size to gather, which means that many animals take an interest
        in pollen - and, as they take an interest in it, they often spread the pollen around
        - we'll get back to that later.

BOTTOM LINE

SEED PLANTS HAVE MORE EFFICIENT REPRODUCTION
   no free-swimming sperm
   protected ovules (off the ground, protected by megasporophylls)
        rich food source is packaged with or inside the embryo, to provide power for germination
                that also attracts animals, and that helps to disperse seeds

GYMNOSPERMS  the non-flowering seed plants 
        = "naked seeds" - ovules either are completely naked or are somewhat protected by megasporophylls
        no flowers, no fruits, sometimes have cones

ANGIOSPERMS  the flowering seed plants      [all are in Phylum Magnoliophyta or Anthophyta]
        ovules and seeds are completely enclosed by megasporophylls
        have flowers and fruits, no cones

Phylum PROGYMNOSPERMOPHYTA    Progymnosperms
        the first seed plants
      ancestors of seed ferns and modern gymnosperms
        existed from 360 to 300 x 10⁶ yrs BP

Here is a fossil Progymnosperm.

Phylum PTERIDOSPERMOPHYTA        Seed ferns
370 to 145 x 10⁶ yrs BP
Were once abundant, but became extinct during the dinosaurs' heyday.
Could early mammals possibly have played a role in the extinction of this group?  We don't know, but it's possible.
Time of this group's extinction also almost perfectly coincides with the origin of flowering plants.  Hmmmm.

Phylum CYCADEOIDOPHYTA        Fossil cycads
200 - 65 x 10⁶ yrs BP
monoecious cycads (cones of both sexes on the same plant)
some even had bisexual cones, similar to flowers
Apparently become extinct about the time when the asteroid/comet hit the Yucatan Peninsula 65 million yrs ago, and finished off the dinosaurs.  Maybe this group depended on dinosaurs for pollination/dispersal?  An interesting thought, but we really don't have a clue.

Here are some drawings of fossil cycads.

Here are some reproductive structures of fossil cycads.
Note the male and female sporophylls in the same cone in drawing A.

Phylum CONIFEROPHYTA (or Pinophyta)    Conifers
      monoecious or dioecious
        seeds in cones
        non-flagellated sperm
        pollinated by wind
        leaves mostly needle-like or scale-like
        active secondary growth 
        pines, spruces, firs, yews, junipers, etc.
        mostly in northern hemisphere                         
                    

Phylum CYCADOPHYTA        Cycads
        ancient group - goes back at least 320 million yrs
        dioecious
        ovules and microsporangia in cones
        flagellated sperm
        insect facilitated pollination
        leaves palm-like or fern-like
        little secondary growth 
        today's species are mostly remnant and tropical
        The Mesozoic Era (Jurassic, Triassic, Cretaceous) was the Age of Dinosaurs AND Cycads.

        All cycads are considered to be threatened or endangered, and are subject
        to the international Convention on International Trade of Endangered Species (CITES).
        Cycads, however, are also easy to raise from the seed of plants already in cultivation.
        You might want to check out the Cycad Society, an international group of folks dedicated
        to the enjoyment and conservation of cycads.

A female cycad, with a seed cone.

Phylum GINKGOPHYTA  Ginkgo
        dioecious
        naked, paired ovules, sperm with flagella
        wind pollinated
        leaves with unique (dichotomous) venation
        active secondary growth
        The genus Ginkgo is about 80 million yrs old, but very similar fossils go back 280 million yrs.
        Ginkgo is a true living fossil.
        Only one species of Ginkgo exists (= Ginkgo biloba).  It probably last grew wild
        in south-central China, and was transplanted into temple courtyards
        beginning several thousand years ago.  There probably are no wild Ginkgos
        remaining. Ginkgo is one of the toughest, most tolerant trees on Earth,
        very tolerant to compacted soil, air pollution, drought, etc.  Why?  Because, over and over,
        it has seen just about everything Earth's environment can throw at it, and has survived.

        See the Ginkgo pictures in your textbook.