Download Laboratory: a Dicto Seed

IB Biology
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Activity: a Seed
Adapted from “Seeds of Flowering Plants,” © Allyn & Bacon
Directions: The seed of a bean plant (or, a bean!)
has two cotyledons. (You will learn what those are
in step 3.) That makes a bean plant a
dicotyledonous plant, or dicot for short. Other
flowering plants’ seeds have only one cotyledon,
so they are nicknamed monocotyledonous plants,
or monocots. Beans are produced in a pod, the
fruit of the bean plant. Check off the tasks below
as you complete them.
1. Exterior of a bean:
Each bean is covered by a protective seed coat. Find
the place it was attached to the pod (formerly ovary)
– it leaves a scar called the hilum. Close to the
hilum is the much smaller micropyle, a pore where
the pollen tube entered
the ovule. Water is
imbibed through the
micropyle at
germination.
On a soaked bean,
find the hilum,
seed coat, and micropyle.
break. Then, lay the two halves flat on a paper towel, like
an open book, with the tissues of the embryo at the top.
4. The tissues of the embryo.
The primary tissues of the embryo may still be
attached to one of the cotyledons, or parts may have
broken off in step 2 when the seed coat was
removed. You should be able to see the plumule, or
embryonic shoot, at one end, and the radicle,
or embryonic root, at the other end. Use Neil
9e figure 38.8 (a) on p. 808, and the third paragraph
on p. 808, to help you!
Find the plumule and the radicle.
5. The radicle.
Inside the intact seed, the radicle is very close to the
micropyle. The radicle is the first tissue to break
through the seed coat when germination occurs.
Identify a germinating seed’s priorities: is it more
important to start making food first, or to start
taking up water?
Hilum
2. Seed coat.
Peel off the seed coat from the seed. If the two
halves of the seed start to come apart, hold them
together. Place these items on a paper towel.
Use a hand lens or stereoscope to examine the seed
coat. Does the micropyle go all the way through?
In some cases the seed coat needs to be tough
enough to withstand the digestive juices of animals
that eat the seeds, carry them a distance, and then
drop them with their wastes into a new environment
(seed dispersal!).
6. The plumule. The plumule or
embryonic shoot contains the
future first leaves of the baby
plant.
Use a stereoscope or hand lens to
examine the plumule. How many
leaves can you see?
This diagram shows a seed from
behind. Label its:
cotyledons
3. The inner seed.
The inner seed of a bean is made up entirely of
embryo. The embryo consists of two large
cotyledons and the primary tissues of the “baby”
plant. The cotyledons contain stored food.
Slowly and gently pull apart and separate the
cotyledons until you feel a point of attachment
plumule
point of attachment
radicle
IB Biology
7. No endosperm. Mature dicot seeds do not
contain endosperm. The endosperm was transferred
to the embryo’s cotyledons as the seed developed.
Scratch the surface of one of the cotyledons with a
dissecting needle. Then, put a drop of Lugol’s
iodine on the scratched cotyledon. What happens?
Under the stereoscope, observe the cotyledon cells –
they are easy to see because of the stained
amyloplasts (starch-containing plastids)
within them.
8. On the back of this handout, outline the
germination process using the three passages from
Neil 9e: p. 809, “Seed germination and seedling
development.” Use bullet points.
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