Download Lab 5: Plants: Nontracheophytes and Seedless Vascular Plants Part 2

Name: _________________________
Lab 5: Plants: Nontracheophytes and
Seedless Vascular Plants
Part 2
Objectives:
• Describe the relationship between microsporophyll, microsporangium, microspore,
microgametophyte, and pollen in conifers
• Describe the relationship between megasporophyll, megasporangium, megaspore,
megagametophyte, ovule, and seed in conifers
• Describe the structures of the “male” and “female” cones
• Differentiate between pollination and fertilization
General Introduction:
Plants are generally defined as multicellular, photosynthetic eukaryotes. Plant cells have cell walls composed of
cellulose, and store surplus carbohydrates as starch. They utilize two photosystems in photosynthesis with two forms of
chlorophyll (a and b). This list of characteristics is not mutually exclusive to the Plant Kingdom however as several phyla of
algae (Kingdom Protista) also fit the description. Therefore, the definition of plants can be refined to include the fact that
plants enclose their embryos in parental tissue. (Whether looking at moss or ferns where the embryos grow directly on the
parental (haploid) gametophyte, or the seed plants where the embryos are enclosed by parental tissue in the seed.) The plants
are sometimes referred to as “embryophytes” due to this. The plants also show adaptations to a terrestrial life cycle as most
live on land. Aquatic plants are generally viewed as secondarily aquatic.
Plants live a life cycle referred to as “alternation of generations”. The key to this life cycle is that both the haploid
stage (gametophyte) and the diploid stage (sporophyte) are multicellular. The gametophyte produces haploid gametes
through mitosis and the sporophyte stage produces haploid spores through meiosis. As we examine the Plant Kingdom, you
should notice a shift in the alternation of generations from a gametophyte dominant life cycle (mosses and liverworts) to a
sporophyte dominant life cycle (ferns, conifers, and flowering plants). Importantly, remember that all the plants still exhibit
a true alternation of generations.
We will examine the Plant Kingdom through a series of 4 evolutionary stages. The nontracheophytes (mosses and
liverworts) exhibit the first stage: embryos wrapped in parental tissue. The second stage, formation of vascular tissue is
displayed in the vascular seedless plants (ferns and horsetails). Evolution of seeds is examined in the conifers, and the
evolution of flowers will be studied later this quarter in the anthophytes.
The evolution of the seed plants involved several major changes in the life cycle and life cycle structures from
those of the seedless plants. We will look at these changes in lab including: the reduction of the gametophyte stages (and
subsequently a sporophyte dominant alternation of generations), a heterosporous life cycle, and the evolution of the seed and
pollen. The seed plants, unlike the seedless plants, have a gametophyte stage that is nutritionally dependent on the
sporophyte generation. The reduced gametophyte stages are enclosed their entire lives by parental sporophyte tissue
(whether on the plant itself or wrapped in a seed coat).
The gametophytes are the result of mitotic divisions of the haploid spores. In the heterosporous life cycle,
microspores (produced through meiosis in the microsporangium) divide mitotically to become the multicellular
microgametophytes (pollen grains). Within a pollen grain is at least one sperm nucleus. In that same heterosporous life
cycle, megaspores (produced through meiosis in the megasporangium) divide mitotically to become the multicellular
megagametophytes (ovules). Within an ovule is an egg nucleus. Transfer of the pollen grains to the ovule is termed
pollination and union of a sperm nucleus and an egg nucleus is termed fertilization, with a resulting diploid zygote. The
zygote will ultimately develop into the new sporophyte generation.
The zygote initially develops into an embryo within the megagametophyte. The embryo, the megagametophyte, and
a seed coat (derived from diploid tissue surrounding the megagametophyte) are packaged together by the plant as a seed.
The seed can be released from the plant to germinate in appropriate environmental conditions. Seeds provide several
reproductive advantages for these plants. First, they can increase dispersal of the next diploid generation as the seed can be
carried by the wind, water, or another organism. Second, the megagametophyte tissue serves as a carbon energy source for
the developing embryo giving it an energy boost early in its life. With this energy boost, the embryo will produce leaves that
allow for photosynthesis and metabolic independence. Third, the seed coat provides protection for the embryo while it waits
to germinate until conditions are suitable.
Bio 203: Lab 5: Plant Kingdom (Nonvascular and Seedless Vascular Plants)
General
1. Work in a group (the size of the groups will be determined by the size of the class and by the amount of equipment
available
2. Examine the slides and the plant specimens provided in the lab.
3. Please do not hoard all slides at your lab station, and allow other students to have access to all slides.
4. Use the pictures in the photo atlas and your textbook to guide you through the slides and specimens.
Note: Some of these slides are difficult to interpret. Do your best and ask me if you have questions.
1) Phylum Coniferophyta
Background: The Phylum Coniferophyta represents a larger group of organisms known as gymnosperms. The
gymnosperms bear naked seeds on sporophylls. The conifers are the largest of 4 phyla within this group. The
conifers (“cone-bearing plants”) are represented in lab by several species found on campus (pines, cedars, etc.) and
by prepared slides.
Almost all the conifers are “evergreen” holding their needle-like or scale-like leaves year round. This
allows for growth year round, although this growth is reduced in the seasons of less sunlight. The reduced leaves
are adapted to colder, drier climates with a thick cuticle, and stomata located in pits. Commercially, most of our
lumber and paper pulp comes from the wood of conifers.
We will examine the pine tree structures in lab, and will focus on the life cycle stages discussed above.
The diploid pine sporophyte (the tree) contains two types of cones. Staminate (or pollen) cones contain the
microsporangia that give rise to the microgametophytes, pollen grains, containing the sperm nuclei. Ovulate cones
contain the megasporangia that give rise to the megagametophytes, ovules, containing the egg nuclei.
Define and/or recognize: see structures listed below
Slides:
Pinus, ovulate cone, l.s. Observe megasporophyll, ovule
Pinus, ovule, l.s. Observe megasporophyll, ovule
Pinus, megaspore, l.s. Find megaspore mother cell and micropyle.
Pinus, archegonia, with egg, l.s. Find egg and archegonium.
Pinus, pollen (staminate) cone, l.s. and c.s. Observe microsporangium, microsporophyll, pollen.
Pinus, pollen grains, w.m. Observe pollen grain and haploid nuclei; note shape.
Pinus, proembryo, l.s. Find proembryo.
Pinus, mature embryo, l.s. Observe mature embryo and megagametophyte.
Live specimens: Branches and cones (staminate and ovulate) from a conifer
Questions to answer:
1.
What are the evolutionary “advances” you observe in the seed plants?
2.
Are conifers heterosporous or homosporous?
3.
How does pollination occur in the conifers? (Who is the pollinator?) Are there any significant disadvantages to this
pollination strategy? Explain.
2
Bio 203: Lab 5: Plant Kingdom (Nonvascular and Seedless Vascular Plants)
4.
Is the life cycle of a conifer sporophyte dominant or gametophyte dominant? Support your anwer.
5.
In the table below indicate whether the structure is haploid, diploid, or triploid, whether it is unicellular or multicellular,
and if multicellular, whether it is the gametophyte or the sporophyte.
Structure
Haploid, diploid, or
triploid?
Unicellular or
multicellular?
Gametophyte or
sporophyte?
Conifer Plant Body
Microsporangium
Microspore
Pollen grain
Megasporangium
Megaspore
Ovule
Egg
Zygote
Embryo
Endosperm of a seed
Seed coat
To Hand In
1. Labeled drawings of microscope slides
2. Answers to questions
3