Download PBIO 115: Fall 2011 Lab 8: Seeds and Seedlings

PBIO 115: Fall 2011
Lab 8: Seeds and Seedlings
INTRODUCTION
Germination of a seed leads to the development of a new sporophyte. In this laboratory you will examine
the structure of angiosperm seeds. You will then compare germinating seeds to seedlings of various ages
to learn more about the early development and external structure of the angiosperm sporophyte (that we
introduced early in the term). As you observe seeds today, be sure you understand what each structure
will become in the seedling.
EXERCISE A. SEEDS AND SEEDLINGS
A seed is a fertilized ovule. A seed consists of an embryo, a seed coat, and nutritive tissues. In
gymnosperms, nutrition for the developing embryo is provided by remnants of the megagametophyte; in
angiosperms, it comes either directly from the endosperm or from the cotyledons, which have absorbed
the endosperm.
Part 1. Bean (Phaseolus vulgaris), a dicot (text p.503, 506)
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Examine a soaked bean seed and one that has been germinated.
The outer covering of the seed is the seed coat. You should see a small hole in the seed coat of the soaked
seed. This is the micropyle. The larger scar near the micropyle is the hilum, which is where the
funiculus was attached to the seed.
From what is the seed coat derived?
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Remove the seed coat of the soaked seed.
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Identify the parts of the embryo: two large cotyledons, the hypocotyl-radicle axis, and the
plumule (the epicotyl and first foliage leaves).
Bean seeds are called exalbuminous because they do not contain endosperm at maturity. Rather, the food
material is absorbed by the fleshy cotyledons and stored there for future use by the embryo and young
seedling.
The hypocotyl is the part of the stem below the cotyledons, while the epicotyl lies above the cotyledons.
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Look at the series of live bean seedlings showing various stages of development.
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What is the first part of the seedling to emerge from the seed?
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What is the first part of the bean plant to emerge from the soil?
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Find the cotyledons. This type of germination, in which the cotyledons are carried above ground,
is called epigeous.
What happens to the cotyledons over time? Why?
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Examine a bean plant from which the soil has been removed. Identify the epicotyl, hypocotyl,
primary root, and lateral roots.
How do the first-formed foliage leaves (not the cotyledons) differ from the later-formed leaves?
Part 2. Pea (Pisum sativum), a dicot (text p. 506)
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Examine a soaked pea seed. Identify the seed coat, hilum, and micropyle.
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Carefully remove the seed coat. Identify the two large cotyledons, the hypocotyl-root axis, and
the plumule.
!
Look at the series of live pea seedlings showing various stages of development and compare them
to similar stages in the bean.
What is the first structure to emerge from the ground?
This type of germination, in which the cotyledons remain below the surface, is called hypogeous.
!
Examine a pea plant from which the soil has been removed. Identify the epicotyl, hypocotyl,
cotyledons, and primary root.
In both epigeous and hypogeous germination, the first part of the seedling to emerge above ground is the
hook. What part of the seedling elongates to form the hook In each case? What protective role does the
hook serve in the germinating seedling?
Part 3. Corn (Zea mays), a monocot (text p. 503, 507, 508)
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Examine a soaked grain of corn.
NOTE: You are examining a fruit (caryopsis) not just the seed. In corn, the seed coat (integument) is first
fused to the surrounding pericarp (ovary wall) and then disintegrates as the fruit develops.
Can you see a hilum or micropyle on the corn fruit? Why or why not?
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Examine (with dissecting and compound scopes) the demonstration slides of a grain of corn in
longitudinal section. Compare to the wheat grain on p. 504 of your text.
Identify the embryo and endosperm. The endosperm visible in the section occupies most of the area
beneath the pericarp.
There is a single massive cotyledon called the scutellum, which has absorbed some of the endosperm.
The corn shoot is surrounded by the coleoptile, and the radicle (embryonic root) is surrounded by the
coleorhiza. Corn seeds (fruits) are considered to be albuminous because endosperm is present when they
are mature.
Identify the coleoptile and coleorhiza. What is their function?
Identify the plumule and radicle of the embryo. What will each of them grow into in the seedling?
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Following your TA's instructions, cut a soaked corn grain (previously stained with toluidine blue)
in half longitudinally with a razor blade. Add a drop of toluidine blue.
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Using a dissecting scope and dissecting needle, find the embryo, scutellum, endosperm,
coleoptile, plumule, coleorhiza, and radicle.
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Examine the germinating corn in the bowl, and then examine the corn seedlings. What portion of
the corn seedling emerges above ground first? Does a corn seedling form a hook? Why or why not?
Identify the coleoptile, first foliage leaves, and adventitious roots (roots that don't arise from the radicle
or from other roots; in corn, they arise from the stems).
EXERCISE B. VASCULAR PLANT BODY (EXTERNAL STRUCTURES)
The vegetative (i.e., non-reproductive) organs of vascular plants are roots, stems and leaves. The roots
collectively form the root system, and the stems and leaves collectively form the shoot system. In the
following excercise, you will get an overview of the external features of the shoot system. The leaves and
stems will be examined in more depth in later labs.
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Examine the more mature bean plants.
Identify the components of the shoot system: stems, nodes (places along the stem where leaves are
attached), internodes (region between two nodes), foliage leaves (not cotyledons), petiole (leaf stalk),
and the leaflets that make up the blade of the compound leaf. The stem cam be thought of as consisting
of repeating units that each consist of an internode and a node that contains both the leaf and axillary bud.
These are called phytomeres.
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Now examine a Coleus plant.
Identify the stems, nodes, internodes, and the petiole and blade of the leaves. Notice that buds
(unexpanded shoots) are produced in the axils of the leaves. These buds may remain dormant or may
develop into new shoots. Identify the first four or five phytomeres at the apex of a branch. Find
examples of axillary branching (shoots developing in the axils of the leaves) in Coleus.
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Reexamine the bean plants. Find a bud or shoot in the axil of each compound leaf. Is there a bud
or shoot at the base of any of the leaflets? This is one way to tell whether a "leaf" is actually a leaf or a
leaflet (and thus part of a compound leaf).