Exercise 5: Gymnosperms and Angiosperms
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OBJECTIVES:
1. Describe the distinguishing features of gymnosperms and angiosperms.
2. Understand the evolutionary significance of pollen and seeds.
3. Understand the function of a cone, a flower, and a seed.
4. Relate the life cycle of angiosperms to the other phyla of the plant kingdom.
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INTRODUCTION TO GYMNOSPERMS
Gymnosperms are plants with exposed seeds borne on scale-like structures called cones
(strobili). Like ferns, gymnosperms have a well-developed alternation of generations, but unlike
most ferns, gymnosperms are heterosporous - they produce two types of spores (Fig. 1).
Microspores occur in male cones and form male gametophytes. Megaspores occur in female
cones and form female gametophytes. Gametophytes of gymnosperms are microscopic and
completely
dependent on the
large, free living
sporophyte.
One
advantage of this is
that the delicate
female
gametophytes do not
have to cope with
environmental
stressors - female
gametophytes
and
the embryos they
produce
are
sheltered
from
drought and harmful
UV radiation by
their
enclosure Figure 1: Life cycle of heterosporous vascular plants.
within the moist reproductive tissues of the parental sporophyte generation. Nutrient exchange
also occurs between gametophytes and their parents. In contrast, the free-living gametophytes of
seedless vascular plants must fend for themselves.
In gymnosperms, pollination is the transfer of pollen (Fig. 2) from male cones (where
pollen is produced) to female cones, which house eggs. In these plants, pollen is carried from
male cones to female cones by wind - gymnosperms were the first plants to evolve that did not
need free water to transfer sperm to egg, and were therefore able to thrive in terrestrial habitats.
Pollen grains are also protected by tough coats. After fertilization, seeds are produced with
developing embryos inside. Seeds represent an additional solution to resisting harsh conditions
and increasing dispersal of offspring. In contrast to a spore (in seedless plants), which is singlecelled, a seed is a resistant structure that is multicellular and more complex. A seed consists of a
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sporophyte embryo packaged along with a food supply within a protective coat derived from the
outer covering of the ovule. The seed permits a small, multicellular sporophyte to remain
dormant until conditions are favorable for continued growth.
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Task 1 - CONIFERS (Coniferophyta)
Gymnosperms include four phyla of plants linked by
the presence of vascular tissue and cones that produce seeds
for reproduction. In this lab you will use conifers as the
model for gymnosperms.
Many conifers experience
considerable temporal variability in temperature, and thus
seasonally alter their biochemistry to make them more
resistant to freezing (hardening process). Many conifers also Figure 2: Pinus pollen grains,
have distinctly scented resin, secreted to protect the tree with
enclosed
male
against insect infestation and fungal infection. The leaves of gametophyte - each has a
many conifers are long, thin and have a needle-like small generative cell and a
appearance (Fig. 3). The stomata are in lines or patches on larger tube cell. When the
the leaves, and can be closed when it is very dry or cold. The pollen grain germinates, the
leaves are often dark green in color which can help absorb pollen tube will emerge
energy from weak sunshine at high latitudes or under forest between the two bladdercanopy shade. The cones conifers bear are reproductive shaped wings.
structures of the sporophyte generation that consist of several
scale-like sporophylls that bear pollen. Sporophylls
of male cones are called microsporophylls, and on
the surface of each is a layer of cells called a
microsporangium that produces pollen. Each pollen
grian consist of four nuclei and a pair of wings that
aid in transport to female cones. Male cones
typically form on the lower branches of pine trees
and only live a few weeks - after they release their
pollen they fall off the tree. Sporophylls on female
cones are megasporophylls (scales of cones), each
of which bears two egg-producing megasporangia
on its upper surface. Female cones usually form on
Figure 3: Cross-section of pine needle
upper branches, and in the spring, when the seedbearing cones are small and young, drops of sticky fluid form between scales, and airborne
pollen grains adhere. Pollen grains (Fig. 2) germinate, and slender pollen tubes grow slowly
toward the egg. Non-motile sperm are released, fertilizing the egg and producing a zygote. The
development of the zygote into an embryo (sporophyte) occurs within the ovule, which matures
into a seed, consisting of an embryo, seed coat, and a food supply. Eventually, the seed falls
from the cone and germinates if conditions are right. The embryo grows and becomes a new
pine tree.
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Figure 4: Pine life cycle.
Procedure:
1. Examine the available pine twigs with needles and a terminal bud. Notice which end the
needles grew from.
2. Examine a prepared slide of a cross section of a pine leaf (needle). Locate the vascular
tissue (transports water and nutrients throughout the plant), epidermis (outer covering),
photosynthetic tissue, resin duct, and stoma (gas exchange).
3. Examine a prepared slide of a young staminate cone and note the pine pollen in various
stages of development.
4. Prepare a wet mount of some pine pollen.
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5. If available, examine living or preserved ovulate cones, which will develop and enlarge
before they are mature.
6. Examine a prepared slide of a young ovulate cone ready for pollination, and if available,
an ovulate cone that has been sectioned through an ovule.
7. Examine a prepared slide of a pine seed. Locate the embryo, seed coat, and food supply.
Figure 5: Male (red outline) and female (blue outline) pine cones
Questions:
1. Is the possession of flagellated sperm a primitive or advanced characteristic in the plant
kingdom? Why?
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2. How do the structural features of pine leaves adapt the tree for life in dry environments?
3. Recall that in ferns the antheridia and archegonia on a prothallium mature at different
times to avoid self-fertilization. How might the different locations of male and female
cones on a pine tree help to do the same?
4. What is the probable function of the wings of a pine pollen grain?
5. On which surface of a pine cone scale are the seeds located? Why are they located here
instead of the other side?
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6. What evolutionary advantages might arise from not needing free water for fertilization?
7. Fill in the appropriate information about gymnosperms in your taxa organization chart.
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INTRODUCTION TO ANGIOSPERMS (Anthophyta)
Flowering plants (angiosperms) are the most abundant, diverse, and widespread of all
land plants. They owe their success to several factors, including their structural diversity,
efficient vascular systems, and mutualisms with fungi and animals. As in gymnosperms, the
sporophyte of angiosperms is large and heterosporous - the micrgametophyte (pollen) and
megagametophyte (embryo sac) are completely dependent on the sporophyte. Angiosperms are
commonly divided in monocots and dicots, and their differences are below:
Monocotyledons
1. One cotyledon per embryo
2. Flower parts in sets of three
3. Parallel venation in leaves
4. Multiple rings of vascular bundles in stem
5. Lack a true vascular cambium (tissue between xylem and phloem; lateral meristem)
6. Fibrous root system
Dicotyledons
1. Two cotyledons per embryo
2. Flower parts in sets of four or five
3. Reticulate (i.e. netted) venation in leaves
4. One ring of vascular bundles or cylinder of vascular tissue in stem
5. Have a true vascular cambium (lateral meristem)
6. Tap root system
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Task 2 - STRUCTURE AND FUNCTION OF FLOWERS
Examine the flower model (Fig. 6) and take note of the following structures:
o Receptacle: part of
the flower stalk that
bears
the
floral
organs.
o Sepals:
the
lowermost
or
outermost whorls protect
the
developing flower.
o Petals: whorls located
inside and usually
above the sepals; may
be
large
and
pigmented (in animal
pollinated flowers) or
inconspicuous
(in
wind
pollinated
Figure 6: Basic parts of a flower
plants).
o Stamen: male portion
of the plant that rises above and inside the petals, consisting of filaments with an anther
atop each; inside each anther are pollen grains, which are the micrgametophytes and
contain male gametes.
o Carpel: each made up of an ovary, style, and stigma; the ovary contains ovules that
contain the megagametophyte; the megagametophyte is called the embryo sac and
contains female gametes. During pollination, pollen grains are transferred to the stigma,
where they germinate and grow a tube through the style to the ovary.
Procedure:
1. Obtain a flower and become familiar with its external and internal anatomy.
2. Use your compound light microscope to examine the structure of a pollen grain. To
enhance visibility of the nuclei, stain the pollen with acetocarmine - add one or two drops
to the pollen and gently heat (do not overheat).
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Questions:
1. What are some of the structural advantages of angiosperms over gymnosperms based on
your initial observations?
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Task 3 - POLLINATION, FERTILIZATION, AND SEEDS
Angiosperm
reproduction depends on
pollination.
After the
development
of
a
micrgametophyte (pollen
grain) with sperm and a
megagametophyte (ovule)
with an egg, pollen is
transported to the surface
of the flower’s stigma.
The pollen grain (has
three, haploid nuclei)
germinates and a pollen
tube grows (tube nucleus one of the nuclei of
pollen) through the stigma
and style to the ovary (has
nine nuclei). One sperm
nucleus fuses with the egg
to form a diploid (2n)
zygote, and the other
sperm fuses with the two
polar nuclei to form a
triploid (3n) nucleus. The
Figure 7: Pollination and fertilization. Pollen is transferred from
zygote develops into the
the anther to the stigma of the carpel. Two sperm resulting from
embryo. The triple fusion
division of the generative cell move from the pollen grain through
of the sperm nucleus and
the pollen tube to an opening in the ovule. One sperm fertilizes
two polar nuclei forms the
the egg, and the other fertilizes the polar nuclei.
endosperm that provides
food for the embryo. The integuments of the ovule form the seed coat, and a fruit develops from
the ovary and other parts of the flower.
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A seed is a mature ovule that includes a seed coat, a food supply, and an embryo.
Embryology of the mature zygote occurs within the seed and before germination. This
embryology and its controlling factors are complex, but stages of development are easily
observed. The structures of a dicot seed include (Fig. 8a):
o Micropyle: small opening of the surface of the seed through which the pollen tube grew.
o Hilum: an adjacent, elliptical area at which the ovule was attached to the ovary.
o Cotyledon: food for the embryo.
o Embryo with young root and shoot: develops into new sporophyte
The structures of a monocot seed include (Fig. 8b):
o Endosperm: food for developing embryo.
o Scutellum: helps absorb the endosperm.
o Coleorhiza: sheath enclosing embryonic root of embryo.
o Shoot and root apical meristems
Figure 8: Seed structure of a garden bean (dicot) and corn (monocot). (a) The two cotyledons in
each seed of garden beans absorb the endosperm before germination. (b) Corn has seeds in
kernels (grains); the single cotyledon is an endosperm-absorbing structure called a scutellum.
Procedure:
1. Obtain some beans that have been soaked in water. Peel off the seed coat and separate
the two cotyledons. Examine the open seed and note the presence and location of the
structures in Figure 8a.
2. Add a drop of iodine to the cut surface and observe the staining pattern.
3. Repeat steps 2 and 3 with soaked peas.
4. Examine a prepared slide of a corn grain, and identify the structures in Figure 8b. Using
a razor blade or scalpel, longitudinally split a water-soaked corn grain.
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5. Add a drop of iodine to the cut surface of the corn grain and observe the staining pattern.
Questions:
1. What is the main purpose of the endosperm?
2. How are the seeds of beans, peas, and corn similar? How are they different?
3. What does the staining (iodine) pattern tell you about the content of the endosperm and
the embryo?
4. Do mature seeds of monocots or dicots store most of their food in cotyledons? How can
you tell?
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Task 4 - FRUIT
As observed in gymnosperms, seeds provide an advantage over spores used for offspring
dispersal by plants like bryophytes and ferns - seeds have a developing embryo and a food source
that the embryo can rely on until it reaches suitable conditions. Seeds may be transported by the
wind, but in many angiosperms, an additional transportation vector is present - animals. Many
angiosperms produce fruits that surround and/or carry seeds, and are appetizing meals for a
variety of animals. A fruit is a ripend ovary, and other structures that surround the ovary at
maturity. The wall of the ovary is modified to become the fruit wall (pericarp), which comes in a
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variety of shapes, sizes, and textures. The colors, shapes, aromas, and tastes of fruits are
adaptations to attract certain animals that will eat the fruit and associated seeds, and transport
them to areas away from the parent plant. Thus, angiosperms not only have mutualistic
relationships with animals that enable dispersal of gametes, they also often rely on animals to
disperse developing offspring. After consumption, fruits and their seeds proceed through the
digestive tract of the animal, and the seeds, covered in their protective seed coats, are deposited
along with feces, which have nutrients that may help stimulate growth of the young plant.
Fruits come in a variety of shapes and sizes. Flesh fruits have soft, fleshy pericarps (fruit
walls) at maturity, which include grapes that have a skin-like outer covering, oranges that have a
leathery rind, apples that have papery outer coverings, and watermelons with hard rinds. Other
fruits have pericarps that become dry and hard at maturity, including peas, corn, and nuts. Some
fruits are more complex with clusters of several ripened ovaries produced by a single flower and
born on same receptacle, like a raspberry, or clusters of several ripened ovaries produces by
several flowers in the same inflorescence, like a pineapple.
Procedure:
1. If time permits, observe the variety of fruit available and make predictions about which
parts of each fruit came from their respective flowers.
2. Cut open the available fruit and count the number of seeds each fruit has. Why do you
think some fruits have many seeds (e.g. apple) while others have only one seed (e.g.
walnut)?
Questions
1. Why don’t all plants produce flowers and fruits to aid in reproduction and dispersal?
2. What are some differences in the fruits of seeds that are edible and those that are not?
5. Fill in the appropriate information about angiosperms in your taxa organization chart.
6. Fill in the table noting the major structural and reproductive advancements for each plant.
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Plant group
Green algae
Structural advancement(s)
Reproductive advancement(s)
Seedless, non-vascular
plants
Seedless, vascular
plants
Gymnosperms
Angiosperms
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EXERCISE SUMMARY
You have now examined five different groups of plants, each with unique structural and
reproductive processes. This lab and the previous lab have provided you with the stepwise
progression of evolutionary adaptations in plants: plants were first aquatic, very simple and
small, with all cells requiring proximity to water and nutrients. Vascular and lignified tissues
enabled plants to grow larger and reside in a wider variety of habitats. Seeds increased the
reproductive success of plants through increased dispersal and germination success. Flowers and
fruits increased the efficiency of fertilization and embryo dispersal. In seeded plants, the
quantity of microgametes is considerably greater than the number of megagametes, which will be
a common theme in sexually reproducing animals in the second half of this course. Because
megagametes develop into embryos, they are typically larger and provide additional structures
(e.g. for nourishment, protection) that aid in the success of new plants, and in turn are fewer in
number than microspores, which are typically much smaller. By having many microspores and
large megaspores, it increases the probability that a plant is pollinated and the embryo survives to
become a new plant. Gymnosperms rely on passive methods (i.e. wind) for pollination, while
angiosperms have prominent flowers that attract pollinators, which increases the chances that
pollen will be carried to another flower and/or pollen will be deposited on the stigma and will be
carried through the pollen tube to the ovary.
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