Return to: Biology of Horticulture


Diversity - Gymnosperms


What does the name "gymnosperm" mean or signify?

"naked seed" on surface of sporophyll

What is the important difference between gametophytes of this group and the ferns?

Gymnosperms are heterosporous and the female gametophyte is retained by the parent plant.

What is the important difference in early sporophyte development?

Sporophyte begins in the seed and does not need to grow into a plant immediately.

What advantage does this give the gymnosperms?

Their life cycle is not so vulnerable as lower plants.

When were the gymnosperms the dominant land plants?

in the Jurassic and Cretaceous period (age of dinosaurs)

What are two important differences between stems of conifers and dicot trees?

conifers do not have vessels and have smaller rays

What adaptations to xeric (dry) conditions do we see in conifer leaves?

small surface area (but still a megaphyll), sunken stomata, thick cuticle

What are the two different kinds of cone on a pine tree?

male and female

What structures are present in these cones and to what do they give rise?

Sporophylls of male have sporangia which produce microspores (pollen) by meiosis. Sporophylls of female strobilus carry sporangia (ovules) in which a single cell undergoes meiosis to produce 4 megaspores.

Where does meiosis occur in pine trees?

The sporangia produce microspores (pollen) by meiosis.

What structures represent the mega- and microgametophytes?

Mitosis in the pollen grain produces the male gametophyte, two prothallial cells, a tube cell and a generative cell (which later produces a stalk cell and two sperm). One of the megaspores of the 4 in the ovule divides many time to produce the female gametophyte which contains 2 to 3 archegonia.

How does fertilization occur and to what does it give rise?

  • Pollen lands on the female cone, gets drawn inside and germinates.
  • Sperm nuclei pass down tube and one fertilizes one (usually) egg in one archegonium.
  • The zygote grows into an embryo (sporophyte) still inside the female gametophyte which is inside the seed coat and nucellus derived from the parent sporophyte.

What kinds of habitats are gymnosperms found?

They are adapted to dry and/or cold situations.

Why do we regard Cycas and Ginkgo trees as survivors?

Both were much more abundant in earlier times. Ginkgo biloba was only known in the west as a fossil until the last century, and is the only surviving species of its division; it is a useful street tree.

A pine tree represents which phase of the life cycle:

  1. gametophyte
  2. sporophyte
  3. prothallus

  1. Gametophytes are an inconspicuous phase of the life cycle in most of the plant kingdom (the Hepaticophyta and Bryophyta are exceptions)

     

  2. As in most of the plant kingdom the dominant phase of the life cycle is the sporophyte.

     

  3. There is no prothallus in the conifer life cycle (although a large gametophyte stage does form in the female cone, this term is reserved for the gametophyte phase of seedless vascular plants)

So what is a pine tree's level of ploidy:

  1. 2n
  2. 1n
  3. 4n

  1. Yes the sporophyte is diploid (as always).

     

  2. No the sporophyte is never haploid.

     

  3. There is no tetraploid stage in the life cycle.

Is a pine tree:

  1. homosporous
  2. heterosporous
  3. asporous

  1. This would make it like a fern which produces only one kind of spore.

     

  2. Yes pine trees produce two kinds of spores: the microspores are commonly called pollen and readily seen; the megaspores are hidden away in the female cone.

     

  3. This would mean it produced no spores; it does although we may not always refer to them as such.

How does a microspore develop:

  1. by mitosis only
  2. by meiosis only
  3. by meiosis and mitosis

  1. Remember the sporophyte is diploid and spores are always haploid.

     

  2. Is there going to be no gametophyte generation?

     

  3. Yes cells in the male cones of the sporophyte undergo meiosis and then the haploid microspores develop a much reduced male gametophyte with just four cells at the time that they are shed.

How does a megaspore originate and develop:

  1. by mitosis only
  2. by meiosis only
  3. by meiosis and mitosis

  1. The haploid megaspore develops from a diploid sporophyte

     

  2. Is there going to be no gametophyte generation?

     

  3. Yes a cell on an ovuliferous scale in the female cone of the sporophyte undergoes meiosis. One out of four (haploid) megaspores develops into the female gametophyte.

What develops on the gametophyte?

  1. archegonia
  2. antheridia
  3. antheridia and archegonia

  1. Yes, the female gametophyte develops more fully than the male and does form archegonia.

     

  2. Aren't we talking about a female gametophyte here? (They are not formed on the male gametophyte either)

     

  3. Bisexual gametophyte are formed by the spores of homosporous plants, but we agreed that the pine is heterosporous

How does fertilization occur?

  1. sperm swim to egg
  2. pollen lands on egg
  3. nuclei move down tube

  1. The male gametophyte lacks antheridia and free-living sperm are not formed.

     

  2. Pollen cannot get that close to the egg

     

  3. Yes, the pollen grain germinates after landing on the female cone and a tube grows towards the archegonium.

What kinds of tissue are present in the pine seed?

  1. gametophyte only
  2. sporophyte only
  3. gametophyte and sporophyte

  1. The female gametophyte makes up a large part of the seed, but fertilization has occurred by the time that the seed is shed. (and where does the seed coat come from?)

     

  2. The seed coat and embryo are certainly sporophytic tissue, but what is in between?

     

  3. Yes, the seed coat is from one sporophyte generation, and the embryo is the beginning of the next. The female gametophyte remains to provide a store of food for the germinating seed.

Apart from seeds, what do the conifers have that is missing from the seedless vascular plants alive today?

  1. megaphylls
  2. woody stems
  3. strobili

  1. Megaphylls are present in the Pterophyta (ferns)

     

  2. Although there were woody plants among extinct seedless vascular plants, none are truly woody today, so the conifers are different in this respect.

     

  3. Strobili (cones) are present in the Lycophyta and Sphenophyta

What kinds of habitat are conifers better adapted to than angiosperm or broadleaved trees?

  1. wetter
  2. warmer
  3. drier

  1. What do you know about the leaf and stem anatomy of conifers relative to that of broadleaved trees?

     

  2. As you go north or climb a mountain which group of trees becomes more common?

     

  3. Yes the leaf and stem anatomy of many conifers is better adapted than those of broadleaved trees to deal with water shortage.

Which group of gymnosperms is most useful for street trees in Ohio?

  1. Coniferophyta
  2. Cycadophyta
  3. Ginkgophyta

  1. Although they are cold-hardy, many conifers are intolerant of the pollutants in the urban atmosphere

     

  2. There are no cold-hardy cycads, but they might be good street trees in Florida!

     

  3. Yes, although there is only a single species in the division, it turns out to be very urban-tolerant. (You can see it on High Street in downtown Columbus)

Which groups of gymnosperms have at least one deciduous species?

  1. Coniferophyta and Ginkgophyta
  2. Cycadophyta and Ginkgophyta
  3. Only Ginkgophyta

  1. Yes, in addition to Ginkgo there are deciduous conifers such as Taxodium distichum (bald cypress) and Larix decidua (larch)

     

  2. There are no deciduous cycads

     

  3. Ginkgo biloba is certainly deciduous, but so are some other gymnosperms

Return to: Biology of Horticulture