Early Development of the Plant Body

Early Development of the Plant Body

Seeds and Seedlings
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 seedling of various ages to learn about the early development and external structure of the angiosperm sporophyte. As you observe seeds, be sure you understand what each structure will become in the seedling.
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), a dicot (text p. 503, 506)
* 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.
* Remove the seed coat of the soaked seed. 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.
* Examine the series of live bean seedlings showing various stages of development. What is the first part of the seedling to emerge from the seed? What is the first part of the bean plant to emerge from the soil?
* 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?
* 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), a dicot (p. 506)
*Examine a soaked pea seed. Identify the seed coat, hilum, and micropyle. Carefully remove the seed coat. Identify the two large cotyledons, the hypocotyl-root axis, and the plumule.
* Examine 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 (Fig 23-11; p. 506).
* 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)
* Examine a soaked grain of corn. NOTE: You are examining a fruit (caryopsis) not just a 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?
* 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 are their functions? Identify the plumule and radicle of the embryo. What will each of them grow into in the seedling?

* (Follow the in-class directions) Cut a soaked corn grain in half longitudinally with a razor blade. Add a drop of toluidine blue. Using a dissecting scope and dissecting needle, find the embryo, scutellum, endosperm, coleoptile, plumule, coleorhiza, and radicle.
*Examine the germinating corn, 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 and first foliage leaves.