Download Lab 6: Plant Reproduction

Lab #6: Plant Reproduction
Objectives:
Observe the parts of the flower and their variations.
Learn the functions of the parts of a flower (petal, sepal, carpel, stamen, anther, filament,
stigma, style, ovary, ovule).
Identify types of fruits and how they are dispersed, and observe how they are formed
Identify the difference between gymnosperm seed cones and pollen cones
Identify different parts of a seed.
General Information
Angiosperms have two characteristics that set
them apart from the other plants: flowers and
fruits. Flowers are structure specialized for
sexual reproduction. Fruits are structures that
help protect the seed and aid in dispersal of the
seed.
Seed dispersal is the movement of a seed to a
new location. Plants that are able to disperse
their seeds over a broad area have a greater
chance of any one seed finding a good location
to grow.
Flower Structure
Flowers consist of 4 whorls (or layers) of modified leaves: sepals, petals, stamens and carpels.
Sepals are the outer whorl. They cover the flower before it is open, helping to protect it. They
are often green, but may also be brightly colored, similar to the petals.
Petals are the next whorl. They are often brightly colored. The coloration of both the petals and
sepals functions to attract pollinators.
Pollinators are organisms (typically insects, but some mammals) that move pollen from one
flower to another flower. Flowers that are wind pollinated, like grasses, typically are not brightly
colored.
Stamens are the third whorl in, and are the male reproductive structures. They produce the
pollen, which contains the male gametophyte which will produce the sperm. The pollen is
formed in the anthers, and the filament attaches the anthers to the flower.
The carpel is the female reproductive structure, found at the center of the flower, or the
innermost whorl. There may be one or many carpels. (This structure is also referred to as the
pistol.). The top of the carpel is the stigma. This is the structure that receives the pollen. The
style connects the stigma to the ovary. The ovary contains the ovules, which contain the female
gametophyte, which produce the eggs. An ovary may have one or many ovules (so one or many
eggs).
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When pollen lands on the stigma, the process is called pollination. Once the pollen lands, the
sperm matures and burrows down the style, into the ovary and ovule to reach the egg. When the
sperm and egg fuse, the process is called fertilization.
Once the egg has been fertilized, the egg will go through several rounds of mitosis, forming the
multicellular embryo, which is the developing plant.
The seed will then begin to develop. A seed is the embryo and a food supply wrapped in a
protective covering called a seed coat. The seed protects the embryo from drying out and
provides food until the embryo is able to grow its first true leaves and start photosynthesizing to
produce its own food.
In angiosperms, the seed is enclosed in a fruit. A fruit is the ripened ovary of a flower. It
functions to protect the seed and to aid in dispersal of the seed. Remember that gymnosperms
also form seeds, but they are found naked on cones, not in a fruit.
Part I: Flower Observations
In order to identify angiosperms, botanists focus on flower and fruit structure. Leaf morphology
can display plasticity and may change in response to environmental conditions, whereas flowers
and fruits are more conservative. In order to understand the structure of a flower it helps to take
it apart and examine each of its structures individually.
Procedure:
You will use your fingers and a razor blade to dissect the flowers. Use a watch glass to hold
your specimen and examine it using the dissecting microscope. You will begin by removing the
outermost segments and work your way inward. Make sure you examine each part of the flower
under the dissecting scope. Record your observations in your notebook.
Materials:
• “Model” flower – Alstroemeria sp.
• Second flower – your choice from the various specimens
• Dissecting needle, razor blade, watch glass, dissecting scope
A. Make a quick sketch of the intact flower, showing each part and their spatial relationships to
one another. Identify each of the whorls (layers).
B. Now observe each part separately under the dissecting scope. Use your fingers to remove the
outermost layer (4th whorl) - this will be the sepals. After recording your observations, continue
removing the layers (whorls) until you are done. Make sure you examine each part of the
flower under the dissecting scope.
You may place the table below in your notebook and use it to make a drawing and answer the
questions.
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Drawings/descriptions
Sepals: Remove the sepals. How many are there? Color? Markings? Hairs?
Function
Petals: Remove the petals. How many are there? Color? Markings? Hairs?
Stamens: Examine the stamens. How many are there? Shape of the anther?
Where are the anthers located in relation to the stigma (higher or lower)? Is
there pollen visible on the anthers? If yes, what color is the pollen?
Carpels: Examine the carpel. Does the flower have a single carpel or multiple
carpels? Describe the stigma. Is it branched? If so, how many branches?
Ovary: Make a cross-section of the ovary at its widest point and exam it under
the dissecting scope. How many chambers are present? Are there ovules
present? How many?
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Questions:
1. Identify each specimen as either a monocot or eudicot and explain how you determined that
designation.
2. What do you think are the selective pressures for flower morphology? Hypothesize how
these selective pressures may have affected the different flower types observed in lab (or on the
greenhouse tour).
Part 2: Demonstration stations
Procedure: There are several demonstrations set up around the room. Go to each station and
examine the specimens and answer the questions in your lab notebook. Make any additional
drawings or notes necessary for you to remember the material and to be able to answer the
questions.
Part 3: Gymnosperm Reproduction
After finishing with this station you should be familiar with:
• Determining the difference between a mature seed cone and a mature pollen cone.
• Identifying pollen, sporophylls, microsporangium and microspores.
Demonstration slides:
Pinus staminate cones
Pinus pollen grains
Identify sporophyll, microsporangium, microspores
Identify pollen
Questions:
3. How would you describe the difference between a seed cone and a pollen cone of a typical
conifer?
4. How are seeds of conifers dispersed?
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