Download Laboratory: a Dicto Seed

Laboratory: a Dicot Seed
Adapted from “Seeds of Flowering Plants,” © Allyn & Bacon
Introduction
The bean, or bean seed, is a dicot seed – it has two
cotyledons. Beans are produced in a pod, the fruit
of the bean plant.
1. Exterior of a bean:
Each bean is covered by a protective seed coat, or
testa, and was attached to the pod at the hilum. At
one end of the hilum is a small pore called the
micropyle, where the pollen tube entered the ovule
when it was inside the flower. The micropyle is the
only place where water
comes into the seed at
germination.
Examine a soaked bean
seed and find its hilum,
seed coat, and micropyle.
Hilum
2. Testa. Peel the testa (seed coat) from the seed very
carefully. If the two halves of the seed seem to be
coming apart, hold them together. Place the inner
seed on a paper towel.
Use a hand lens or stereoscope to examine the seed coat.
Does the micropyle go all the way through the coat?
Would you expect it to?
long, curved structure. (It might have broken off
back in step 2 when the seed coat was removed.) In
the early growth of bean plants, the hypocotyl forms
the lower stem and root. The radicle is the pointed
tip of the hypocotyl. 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.
5. The plumule. The plumule in the bean seed is the
embryonic shoot. It is part of the epicotyl, covering
its tip. When the epicotyl grows, it will form the
upper stem of the plant, and the plumule will become
the first leaves.
Use the hand lens to locate the epicotyl and the plumule
lying against one of the cotyledons. How many leaves does
the plumule appear to consist of? Do the tiny leaves have
veins? Look closely!
On the sketch at the
right, label: cotyledons,
epicotyl, hypocotyl,
plumule, point of
attachment,
and
radicle.
The seed coat is tough enough to withstand the
digestive juices of animals which eat the seeds, carry
them a distance, and then drop them with their
wastes into a new environment.
6. No endosperm. Mature dicot seeds do not contain
endosperm. The dicot embryo develops much more
than does a monocot embryo during the formation of
the seed, consuming most of the endosperm. The
remainder is converted mainly to starch, a nutrient
stored in the cells of the cotyledons, and available for
the growth of the embryo after the seed germinates.
3. The inner seed. The inner seed of the bean is
made up entirely of embryo. The embryo consists of
two large cotyledons and the primary tissues of a
new plant, the epicotyl and the hypocotyl. The
cotyledons contain stored food, which is transferred
to the epicotyl and hypocotyls at the point where all
are attached together.
To show the presence of starch, 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, verify that it is possible
to see the cells of the cotyledons in the areas that changed
color because of the stained amyloplasts within them.
Slowly and gently pull apart and separate the cotyledons
until you feel the point of attachment break. Then, lay the
two halves flat on a paper towel, like an open book, with
the primary tissues of the embryo at the top.
4. The primary tissues of the embryo. The
hypocotyl, once attached to both cotyledons, is a
7. Describe the germination process in monocots.
Include the following in your answer: aleurone, alpha-amylase,
embryo, gibberellic acid, glucose, root, shoot, starch hydrolysis.
Use bullet points.