Botany - B.Ye. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter IX. Division Angiosperms, or Flowering Plants (Angiospermae, Magnoliophyta)
Laboratory Session Topic 14.40. Structure and Types of Seeds. Structure of Monocotyledonous Seeds

General Remarks. A seed is the reproductive and dissemination organ of angiosperms. Depending on the storage Location of nutrient reserves, seeds are classified into five types: those storing reserves in the endosperm, nucellus, embryo, both endosperm and nucellus, or both endosperm and embryo. Seeds with and without an endosperm are the most widespread and widely used in agricultural production. Therefore, special attention should be paid to their study, particularly their Structure and origin.

Objects: Grass family, cereals (Poaceae, Gramineae)

1. Caryopsis of common wheat (Triticum aestivum L.)

2. Caryopsis of maize (Zea mays L.)

Tasks:

1. Examine The structure of the endospermic seed in various plant species.

2. Sketch the STRUCTURE OF THE endospermic seed in your laboratory notebook and label its component parts.

Equipment and Materials: MBR-1 or Biolam microscopes, magnifiers, scalpels, Reagents, slide preparations, charts, and other laboratory accessories.

Microscopic examination of a Cytology/practical/54.html">Longitudinal section of a wheat caryopsis. Examine the slide preparation under high magnification of the Microscope and compare it with the illustration in Figure 107. Even under low magnification, distinct tissue regions are clearly visible. Take a pencil and sketch the layout of the pericarp, seed coat, endosperm, and embryo. Carefully examine each part in detail under high magnification. The preparation clearly reveals a fairly robust pericarp layer, which itself consists of a series of layers already examined in Topic 2.4. Internally, the pericarp fuses with the seed coat of the caryopsis. The pericarp develops from the Ovary wall Cells, whereas the seed coat originates from the ovule integuments. The aleurone layer lies beneath the protective covering of the seed coat and pericarp. It consists of a single layer of cells packed with aleurone grains, from which its name derives. Aleurone grains are proteinaceous in nature. The starchy endosperm forms the third tissue block. Its characteristic feature is large, polygonal cells that do not coalesce at the corners, thus forming intercellular spaces. The cells themselves are packed with large starch grains. The embryo is located in the lower part of the caryopsis and occupies a significantly smaller area than the endosperm. This is because reserve nutrients are stored in the endosperm rather than the embryo.

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Figure 107. Structure of the endospermic seed in a wheat caryopsis:

1 — pericarp; 2 — aleurone layer; 3 — endosperm cells; 4 — absorptive layer;

5 — scutellum (first cotyledon); 6 — coleoptile; 7 — embryonic leaves;

8 — plumule; 9 — epiblast; 10 — embryonic stem; 11 — radicle;

12 — coleorhiza; 13 — embryo

Let us examine the structure of the embryo in greater detail. It consists primarily of primary meristem, which gives rise to its various parts. Most notably, the scutellum stands out in the embryo; it fuses with the endosperm and utilizes Cells of the absorptive layer (outer layer) to draw nutrients from it that are essential for caryopsis germination. The scutellum is a modified cotyledon. In its upper region, the SHOOT apex is prominent, covered above by embryonic (primordial) leaves enclosed within a continuous, thin sheath—the coleoptile. This sheath not only protects the young plant parts but also helps them push upward through fine soil particles. The embryonic stem is located in the middle. Extending downward from it is the radicle, which is reliably protected by the coleorhiza, ensuring its emergence through the caryopsis Tissues. Attached laterally to the embryo is the epiblast, which represents a vestige of the second cotyledon.

Method for preparing a longitudinal section of a maize caryopsis. Take a soaked maize caryopsis and, using a sharp scalpel or razor blade, make a longitudinal cut through its broader section. Moisten it additionally with Water or apply a drop of iodine in potassium iodide. Prepare a series of thin longitudinal sections and place them on a Glass microscope slide. Using a magnifying glass, select a few of the best sections and place them in a drop of iodine in potassium iodide on a pre-prepared microscope slide. Moisten the section further and cover it with a coverslip. Secure the preparation with stage clips on the microscope stage.

Microscopic examination of the slide preparation. Under low magnification, one can observe that the preparation clearly distinguishes three parts: the seed coat, endosperm, and embryo. At the same time, a significant portion of the section appears stained violet. This is the endosperm, packed with starch grains. The seed coat, consisting of the seed coat and pericarp, remains unstained. Treatment with iodine stains the embryo yellow. Within the embryo, one can distinguish a closed sheath—the coleoptile—attached beneath the young embryonic leaves and the plumule. The plumule transitions at the base into the embryonic stem, which extends into the radicle. The latter is surrounded by the coleorhiza. At the level of the embryonic stem, one can observe the epiblast, which is a vestige of the second cotyledon. Thus, the structure of the maize caryopsis, much like that of the wheat caryopsis, comprises three main blocks: the pericarp, endosperm, and embryo (Fig. 108).

Figure 108. Structure of the endospermic seed in a maize caryopsis:

1 — pericarp; 2 — aleurone layer;

3 — endosperm; 4 — absorptive layer;

5 — scutellum (first cotyledon);

6 — radicle; 7 — coleorhiza;

8 — embryonic stem; 9 — plumule;

10 — germ layers; 11 — coleoptile

Conclusion. The seed is a vital generative organ that ensures the preservation and Propagation of the species. Using the caryopsis as an example, one can observe The formation of the three aforementioned tissue blocks, which serve three main Functions: protecting the seed from environmental factors; storing nutrients; and supporting The Development of a new Organism.

Self-Assessment Questions

1. From which part of the plant is the seed formed?

2. Explain The Essence of double Fertilization. Who discovered it?

3. Explain the origin and structure of the endosperm.

4. Which parts represent the cotyledons in a seed with endosperm?

5. Name the Structural components of the cereal embryo.

6. From which part does the pericarp develop?

7. Which part of the embryo performs the function of protecting the shoot apical meristem?

8. Name the component of the embryo that protects the embryonic ROOT.

9. In which part of the seed are aleurone grains located?

10. Name the MAIN TYPES OF seeds based on the storage location of reserve nutrients. Which of them are used in agricultural production?