Embryos of flowering plants

Now let's take a closer look at the embryo within a seed. The following animation and images give an overview of embryo development within a dicot seed.

animation - development of a dicot embryo

bean seed
This longitudinal section of a seed diagrams a developing embryo of those flowering plants called dicots (for the two cotyledons in the seed). This mature bean seed has been split in half. Note the large cotyledon and the embryonic root and leaves.

Study the following diagram and microscopic images which show development of a typical dicot embryo (Capsella) from a few cells to a relatively mature stage.

basal After fertilization, the diploid zygote divides to form a basal cell and a terminal cell. The basal cell undergoes a few additional divisions to create a suspensor while the terminal cell divides repeatedly to form the embryo.

Microscopic slides - Embryonic development of Capsella

The other major group of flowering plants are called monocots because (as the name implies) they have only one cotyledon. Examine the seed and embryo of the monocot plant below.

wheat seed
The embryo of monocots develop similarly to dicots with one important difference. The embryo absorbs only a small amount of endosperm, so when the embryo is mature the cotyledon is small and most of the seed is filled with endosperm. This diagram is a wheat seed. The main nutrient within the endosperm is starch, but proteins called gluten are also present.

Comparison of dicot and monocot seeds

Working in pairs, obtain a bean (dicot) and 2 corn (monocot) seeds that have been soaked overnight in water to soften the seed coat. Dissect the bean seed as follows:

1. Use your fingers to remove the seed coat (think of a peanut).

2. Gently separate the two halves of the seed (the cotyledons) to reveal the embryo within. Identify the embryonic root and embryonic leaves.

3. Place a few drops of lugol's solution on the cotyledons and embryo. If starch comes in contact with the solution, it will turn dark purple (or black) in color within 5 min.

Dissect the corn seeds as follows:

1. Using your razor blade, cut one seed lengthwise along the wide side of the seed and the other seed lengthwise along its narrow side. Locate the embryo with the seeds.

2.Stain the embryo and endosperm with Lugol's solution as above. Then identify the embryonic root, leaves, and cotyledon (see wheat seed diagram above).

3. Compare the staining of bean vs. corn seeds, then answer question 18.

Starch is the major nutrient within the endosperm of most seeds. Observe the Lugol's test as applied to a thin section of a corn seed.

starch When a thin slice of stained endosperm is examined under the microscope, the starch grains within the endosperm cells are purple in color.

Now you are ready to examine a thin section of a wheat seed by virtual microscopy. Magnify the endosperm region until you can see the individual cells. Capture an image at the highest magnification. Label the cell wall of an endosperm cell and starch grains (unstained) within the cell. Submit your labeled image to WebAssign for question 19.

slide - longitudinal, stained section of a wheat seed viewed by virtual microscopy

Germination and growth of seedlings

You have studied seed germination and emergence of the seedling in the Development of Plants topic. Take a moment to review these processes and note of the difference in early growth of dicot vs. monocot plants. View the demonstrations of early seedling growth.

Once the seedling has emerged, the roots and shoots must elongate. This type of growth can be accomplished by two mechanisms: cell division or increase in cell size. In a growing plant, both processes occur in a coordinated fashion. Observe the root tip below by virtual microscopy. Its tip is covered by large cells that form a protection cap over the growing tip as it pushes through the soil. Just above the root cap, the cells are actively dividing (look for mitotic figures). Above the mitotic zone, the cells cease division and grow, becoming long and thin. This is called the zone of elongation.

slide - root tip viewed by virtual microscopy

Whereas the root of a plant grows down, the shoot grows up. Growth is again accomplished by a combination of mitotic cell division and increase in cell size. The tip of the shoot is called the apical meristem. This is where cells are dividing. Behind this region, the cells stop dividing and become larger (growing cells). The apical meristem also gives rise to pairs of new leaves. Meanwhile, older leaves grow larger and cells within them begin to differentiate into the xylem and phloem of veins.

Observe the shoot tip below by virtual microscopy and locate the above regions. If you need help, review this slide show of primary meristems. Capture two images of the shoot tip at appropriate magnifications. In one image label the apical meristem, growing cells, and new leaves (the smallest leaves in the image). In the second image, examine an older leaf, locate a region where a vein is forming, and label a xylem vessel. Use the highest power to show the xylem clearly. You should recognize xylem vessels from the previous assignment on specialized plant cells. Submit your labeled images to WebAssign for questions 20 and 21.

slide - shoot tip viewed by virtual microscopy

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