Plant Anatomy: Practical Course - Panyuta O.O. 2019

Topic 4. Seed Structure
Laboratory Work No. 16. Seed Structure

Theoretical Background. Seeds consist of an embryo and a supply of nutrients. The outer surface of each seed is covered by a seed coat, beneath which lie the embryo and endosperm. Some plants lack an endosperm; therefore, seeds are classified as endospermic (such as in cereals) and non-endospermic (such as in legumes). In certain plants, the nucellus persists and enlarges, becoming filled with reserve nutrients to form the perisperm. Seeds with a perisperm are characteristic of the Chenopodiaceae, Caryophyllaceae, Aceraceae, and others. Both the endosperm and perisperm are seed Tissues that store nutrients. In peas, pumpkins, sunflowers, and other plants, nutrients are stored directly within the PARTS OF THE embryo, specifically in its cotyledons.

The most vital part of the seed is the embryo, from which a new plant develops. The embryo consists of a plumule and a primary ROOT. Between the root and the plumule lie one or two cotyledons, which are the first leaves.

Objective: to examine the Structural Features of seeds.

Materials and Equipment: light microscopes, Glass slides and coverslips, dissecting needles, forceps, glass rods, filter paper, distilled Water, 95% alcohol, iodine in potassium iodide, methylene green with acetic acid, carmine with borax, plant material.

Slide. Structure of a Wheat Grain (Triticum aestivum L.)

Wheat seeds are soaked in water for 24 hours to swell, then transferred to 95% alcohol to kill the grain and fix all its tissues. The material is stored in this state for subsequent study. To prepare sections, a grain is split lengthwise along the groove with a scalpel, and the Location OF THE embryo is identified. These halves are embedded in elderberry pith, and longitudinal sections are made.

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Fig. 102. Cytology/practical/54.html">Longitudinal section of a wheat grain.

A - STRUCTURE OF THE wheat grain: 1 - embryo, 2 - endosperm, 3 - pericarp and seed coats, 4 - aleurone layer.

B - structure of the embryo: 1 - embryonic root, 2 - scutellum, 3 - plumule, 4 - coleoptile, 5 - first embryonic leaf, 6 - second embryonic leaf, 7 - SHOOT apical meristem, 8 - epiblast, 9 - first internode, 10 - coleorhiza, 11 - root cap

In grasses (cereals), which include wheat, the fruit is a caryopsis (Fig. 102, A), consisting of the embryo, endosperm, and pericarp and seed coats. The endosperm makes up the bulk of the caryopsis. The embryo of the caryopsis (Fig. 102, B) comprises the embryonic root, scutellum, and plumule. The scutellum is a modified, single cotyledon of the embryo. It consists of small Cells with delicate walls and is positioned within the grain as if forming a partition between the embryo and the endosperm. The prepared slide also clearly shows the coleoptile and the first embryonic leaf, followed by the second embryonic leaf and the shoot apical meristem. A lateral outgrowth known as the epiblast is visible on the side. In the center of the embryo section, a group of small embryonic cells with very thin and delicate

walls can be seen—this is the first internode. Below these are the coleorhiza and the embryonic root, which terminates in the root cap.

Externally, the seed is covered by a seed coat formed from the integuments of the ovule. Here, the seed coat is fused with the pericarp. A well-developed aleurone layer (Fig. 102, A) lies beneath the seed coat, consisting of large cells packed with protein substances. Two layers can be distinguished in the covering: the upper seed coat and the lower pericarp.

Slide. Structure of a Corn (Maize) Seed (Zea mays L.)

Fig. 103. Structure of a corn seed:

1 - endosperm, 2 - embryo,

3 - aleurone layer, 4 - scutellum, 5 - young root, 6 - root cap, 7 - leaf primordia

Corn seeds are soaked in water until swollen, after which sections are prepared and examined under a Microscope. Externally, the seed is covered by a seed coat fused with the pericarp, effectively forming a single unit. In the seed (Fig. 103), the Cells of the endosperm and embryo are clearly visible. The protein or aleurone layer stands out within the seed coat, consisting of cells filled with protein reserves. The embryo clearly displays the scutellum, young root, root cap, and leaf primordia.

Slide. Structure of the Fruit and Seed of the Oak — Acorn (Quercus robur L.)

The Base of the oak fruit sits within a cupule, which is smooth on the inside and covered with blunt outgrowths on the outside. Microscopic examination of a cross-section reveals that the outer layer of the pericarp is composed of the fruit coat and the seed coat.

Fig. 104. Cross-section of the oak acorn coat:

A - pericarp: 1 - seed coat, 2 - layers of sclereids (stone cells), 3 - thin-walled cells.

B - seed coat: 1 - thin-walled cells with vascular bundles, 2 - endodermis

In the pericarp (Fig. 104, A), a distinct epidermis is visible, consisting of a single layer of cells with heavily thickened walls and devoid of internal contents; the outer surface of the epidermis is covered by a cuticle. Beneath the epidermis lies a layer of sclereids (stone cells) with markedly thickened walls and prominent canals. This is followed by a layer of thin-walled polygonal cells possessing very delicate walls. Interspersed among them are narrow and elongated cells. Cross-sections of the fruit's vascular bundles, as well as isolated Cell clusters and small groups of pitted cells, can be found within this tissue. This layer also contains cells with calcium oxalate crystals, as well as those retaining remnants

of cytoplasmic contents, Tannins, starch, and already bleached METABOLISM/14.html">Chloroplasts.

The inner epidermis, or seed coat (Fig. 104, B), is formed by elongated cells arranged in several layers. The Cell walls of these cells are delicate and thin. Vascular bundles are clearly visible in this tissue. The embryo lies beneath the seed coat, while only a few rows of cells remain of the endosperm. In the embryo, located between the cotyledons, rudimentary leaves and roots are discernible.

As for the cotyledons, in cross-section they appear as semicircles in contact with each other along their upper surfaces. Their surface is covered by an epidermis composed of cubic or slightly elongated cells perpendicular to the surface, which are rich in granular Cytoplasm. Other cells contain various substances, specifically starch, tannins, and oil. This tissue is permeated by vascular bundles.

Slide. Structure of the seed of the small-leaved linden (Tilia cordata Mill.)

Fig. 105. Structure of the linden seed:

A - general view: 1 - hilum, 2 - micropyle;

B - longitudinal section: 1 - seed coat, 2 - endosperm, 3 - embryo, 4 - cotyledons;

C - flattened cotyledon

The linden seed develops inside a fruit known as a nutlet. In a longitudinal section of the linden seed (Fig. 105, A), the hilum is visible at the top, and the micropyle at the bottom. The section also reveals (Fig. 105, B) the seed coat, endosperm, and embryo with folded and crumpled cotyledons. As for the pericarp, it is constructed of thick-walled cells lacking cell contents.

The seed integuments consist of cell layers that form the testa (seed coat). The outer layer bears a cuticle, beneath which lie two or three layers of cells with large intercellular spaces. The cell walls exhibit extensive thickenings that fill the entire cell lumen. The internal mass formed As a result of this thickening is brownish in color and exhibits a reaction characteristic of suberin, which impregnates cork cell walls.

Beneath this layer, cells containing calcium oxalate crystals are visible, followed by a layer of colorless, elongated cells resembling the palisade parenchyma of a leaf. The cells of this layer possess thickenings starting at the base, thereby displacing The Nucleus and cell contents upward. These cells lack a lumen, with only a small slit remaining at the top. A dark band of cytoplasm-filled cells is visible across the cells, with a light band running above it, formed by the thickening of the walls in that region. Above this band, the coat consists of Cellulose.

Below the layer of columnar cells, several rows of brownish parenchymatous cells are noticeable, containing calcium oxalate crystals within the cells themselves. This is followed by the seed coat covered by a cuticle. The cuticle protects the embryo and endosperm from water penetration. In the endosperm itself, oils and protein compounds in the form of aleurone grains are stored as reserve substances, while starch is absent.

Slide. Structure of the seed of the garden pea (Pisum sativum (L.) Cov.)

Pea seeds are soaked in water until swollen. The seed coat, formed from the integuments and remnants of the ovule nucleus (nucellus), is then removed. The surface of the pea seed coat is smooth and composed of elongated cells with thick walls. It is covered externally by the epidermis. The cells of the outer seed coat layer form a palisade epidermis. Pigments are noticeable within it, and even chloroplasts can be found in a young pea, persisting for a long time. Hilums, formed during seed maturation and detachment from the fruit, vary in shape. Near the hilum lies the micropylar opening, adjacent to which is the tip of the embryonic root.

The cotyledons lie beneath the seed coat. Spreading them apart reveals the embryonic root and bud. The cotyledons themselves are attached to the hypocotyl.

After preparing a section of the cotyledon, it is placed on a glass slide and treated with a solution of iodine in potassium iodide. Large starch grains of elliptical, oval, or rounded shape can be observed. The grains exhibit striation and a central cleft. The cotyledons consist of parenchymatous cells that form the storage tissue. The cell walls are heavily thickened. They contain nuclei, which can only be detected using special staining, such as methylene green with acetic acid. These same cells contain an Abundance of aleurone grains. Treatment with iodine stains the aleurone grains golden.

To distinguish between starch and aleurone grains, the slide can also be stained with carmine and brown, followed by rinsing. Afterward, the starch grains remain unstained, while the aleurone grains stain red.

Review Questions

1. Name the main parts of the seed and the Organs of the embryo.

2. What is the structure and Significance of the ovule?

3. What Functions does the seed coat perform?

4. What are the Functions of the cotyledons?

5. In which parts of the seed can reserve nutrients accumulate?

6. What functions are performed by the scutellum, coleoptile, and coleorhiza?

7. How do the seeds of monocotyledonous and dicotyledonous plants differ?