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Comparing Angiosperms: Dicotyledonous and Monocotyledonous Plants

Angiosperms or flowering plants are the most dominant group in the vascular plant world.   These flowering plants have an enclosure of the potential seeds within a hollow ovary. The angiosperms are considered to be advanced as compared with the gymnosperms and other tracheophytes (plants and trees). Flowering plants occur in a wide range of habitats including both salt and fresh water. The basic food supply of the world is derived from the seeds and fruits of angiosperms (rice, wheat, corn) and fibers, wood, drugs, and other products of great economic value.

The studies by John Ray in the 1700's on the structure of seeds led him to discover the difference between monocotyledon (monocots) and dicotyledon (dicots) plants. There are estimated to be about 165,000 different types of dicots and 55,000 types of monocots.  Both monocot and dicot seeds develop in similar ways and have the similar structural arrangements however, monocots start out with one seed leaf, while dicots have two.  The technical word for seed leaf is cotyledon, it is the first leaf to emerge from a developing seed.   Monocots have only one cotyledon, dicots have two cotyledons. A cotyledon contains stored food and serves as a food reservoir. Aside from the difference between the seeds of monocots and dicots there are other different structures that separate monocots and dicots. Monocots have long, narrow leaves with parallel veins (such as grasses.) The parts of monocot flowers are arranged in threes or in multiples of three. Dicots have broad leaves with branched veins. The parts of dicot flowers are arranged in fours and fives or multiples of fours and fives. Although the distinction between monocots and dicots is not always as sharp and clear as once thought, it is a useful taxonomic grouping.

Objective: Outline three differences between the structures of dicotyledonous and monocotyledonous plants

Part A: Plant Sketches

• Draw a sketch of Plant Sample A below and include five or more observations of the plant sample.

• Draw a sketch of Plant Sample B below and include five or more observations of the plant sample.

Part B: Plant Identification

Based on your observations and the background information, hypothesize and justify with evidence with plant samples represent a monocot and dicot.

• Plant Sample A:

• Plant Sample B:

Part C: Monocot & Dicot Seeds

CORN SEED

1. Examine both sides of the corn grain seed that has been soaked overnight.  Identify the following:

1. Corn is covered by the OVARY WALL, which is really an attached FRUIT.
2. At the base is the POINT OF ATTACHMENT where it was in the cob.
3. One side is flattened and shiny. The raised centerline is the EMBRYO.
4. The SILK SCAR is a tiny projection (bump) near the top where the silk or style was attached. It is hard to see. You might feel it.

2. Draw the corn, external view. Label with the capitalized terms from above.

External Sketch

3. Place the corn grain on a paper towel with the embryo side up. Use a pen draw a line on the embryo for your cutting line. Safely use a blade to cut on the line through the corn grain.
4. Find the following internal parts in one half of your corn seed.

1. The outer skin like part is the OVARY WALL.
2. Most of the seed is the watery ENDOSPERM, which stores food.
3. The EMBRYO is pale yellow and surrounded by one white COTYLEDON
4. The EMBRYO’S PLUMULE (top) will be one new leaf.
5. The lower part of the embryo is the RADICLE, which will be the new root.

5. Draw and label the internal view of the corn. Label the terms capitalized above.

Internal Sketch

6. Add dilute iodine solution to one half of the seed. The part, which turns blue-black, is the endosperm; the light purple area is the cotyledon.
7. Can you find the young leaves inside the seed? How many are there? Which part of the seed do you think is the seed coat?

BEAN SEED

1. Examine a bean seed that has been soaked overnight

1. The outer covering is the SEED COAT.
2. On the concave side, the HILUM SCAR is where it was attached to the pod.
3. Find a small PORE (hole) above the hilum scar. Look for a hole, there is a bump on the other end of the scar.
4. Below, make a scientific drawing of the external view, with the helium scar up. Label your drawing with the capitalized terms from above.

External Sketch

2. Loosen and remove the SEED COAT. The 2 big main parts are the COTYLEDONS (seed leaves which store food for germination)
3. Carefully separate the 2 cotyledons.

1. The EMBRYO should remain attached to one of them.
2. The embryo has a PLUMULE (new leaves) and a RADICLE (new root)
3. The plumule has 2 points that will be leaves when the plant grows.
4. Again, apply dilute iodine solution to one half of the seed. The part, which turns blue-black, is the endosperm; the light purple area is the cotyledon.

4. Draw the internal view. Label with capitalized terms from above.
5. Is the corn seed a monocot or dicot?

Internal Sketch

Part D: Analysis Questions

1. Give two examples of plants that are monocots.

2. Give two examples of plants that are dicots.

3. What is a cotyledon?

4. What is the radicle?

5. What is the coleoptile?

6. What is the function of the endosperm?

7. Fill out the table below.  

8. An unknown plant is brought to you and your job is to determine whether it is a monocot or a dicot.  You observe that the plant has 6 petals and its leaves have parallel veins.  Is it a monocot or a dicot?