BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

THE SEED

A seed is a generative organ in seed plants that develops from an ovule. It serves as the primary organ for the reproduction and dissemination of flowering plants and, typically, consists of an embryo, stored nutrients, and a seed coat (spermoderm). At the point where the seed detaches from the funiculus, a hilum is formed, near which the micropyle (micropylar pore) is located.

The embryo forms after the completion of double Fertilization from the zygote. The endosperm develops from the fertilized central Nucleus. The seed coat is formed As a result of the transformation of the ovule integuments.

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Fig. 18. Seed types: A - with endosperm surrounding the embryo (in poppy); B - with endosperm lying adjacent to the embryo (in wheat); C - with perisperm (in corncockle); D - with endosperm surrounding the embryo and a robust perisperm (in pepper); E - with nutrient reserves stored in the embryonic cotyledons (in pea); F - with endosperm and nutrient reserves stored in the embryonic cotyledons (in flax): 1 - spermoderm, 2 - endosperm, 3 - radicle, 4 - hypocotyl, 5 - plumule, 6 - cotyledon (3-6 - embryo), 7 - pericarp, 8 - perisperm.

The Development of the embryo and endosperm varies among different plants. In most cases, the zygote (an egg Cell fertilized by a single sperm) initially becomes encased in a relatively thick wall, after which a dormancy period begins. The duration of this dormancy period varies among plants, lasting from 3-4 hours to several months. Following the dormancy period (or without it), the zygote divides to form two Cells. One of these, facing the micropyle, forms a suspensor that anchors the embryo to the wall of the embryo sac. The suspensor cells generally divide transversely. The embryo develops from the other cell. This cell divides by two mutually perpendicular walls to form four cells. Each of these divides once more, resulting in eight cells. Subsequent divisions produce a spherical, multi-cellular embryonic body. Next, the apex of the embryo flattens, and two protuberances initiate on opposite sides. In dicotyledons, these protuberances develop symmetrically to form two cotyledons. In monocotyledons, one embryonic cotyledon develops more vigorously, while the other lags in growth. The asymmetrically developed cotyledon continues to grow and assumes a terminal position, whereas the other remains rudimentary. In dicotyledons, the stem apex (SHOOT apical meristem) is established between the two cotyledons, whereas in monocotyledons it is displaced. It represents the apical bud of the seed embryo. The hypocotyl and embryonic ROOT are formed between the cotyledons and the suspensor. The endosperm develops within the embryo sac. The fertilized central Nucleus of the embryo sac bypasses the dormancy period, divides, and gives rise to the triploid endosperm. Based on its formation pattern, Three types of endosperm are distinguished: nuclear, cellular, and intermediate.

In the nuclear type, the fertilized central nucleus of the embryo sac undergoes multiple divisions, forming numerous triploid nuclei within the Cytoplasm of the embryo sac cavity. Initially, these nuclei line the inner wall of the embryo sac. By this time, a large quantity of nutrient reserves accumulates within it. Cell walls appear at a later stage. The nuclear type of endosperm is characteristic of monocotyledons and certain dicotyledons.

The cellular type of endosperm differs in that cell walls (partitions) form successively with each division. The cellular type of endosperm is found in the most highly evolved dicotyledons.

The intermediate type differs from the nuclear and cellular types in that upon the initial division of the zygote into two cells, a wall immediately forms between the two nuclei, dividing the embryo sac into two asymmetrical halves. Subsequent nuclear divisions do not involve the immediate formation of cell walls between them. Walls are formed only after a sufficient number of nuclei have accumulated. The intermediate type is considered primitive and occurs in Ranunculaceae, Nymphaeaceae, and certain representatives of Rosaceae.

In some plants, the endosperm does not form after fertilization. Its function is performed by a rapidly developing specialized tissue originating from the nucellus, the cells of which accumulate nutrient reserves. This tissue is referred to as the perisperm.

In many plants, neither endosperm nor perisperm develops, and nutrient reserves are stored in the enlarging cotyledons. Seeds with endosperm are produced by many monocotyledons (Poaceae, Liliaceae). Among dicotyledons, seeds with endosperm are found in persimmon, tomato, carrot, hemp, linden, grape, and others. Seeds with perisperm occur in Chenopodiaceae, tropical Nymphaeaceae, Piperaceae, and others. Seeds lacking endosperm are characteristic of Fabaceae, Asteraceae, Brassicaceae, Rosaceae, and others. Among monocotyledons, seeds without endosperm are found in Alismataceae, Sagittaria, and others. Seeds possessing both endosperm and perisperm include black pepper, yellow Water-lily, and others. Seeds with an underdeveloped embryo occur in lesser celandine, as well as certain species within the Magnoliaceae, Lauraceae, Arecaceae, and Liliaceae families.

Depending on the nutrient reserves accumulated within the seed, scientists distinguish proteinaceous seeds (pea, bean), starchy seeds (wheat, rye), and oily seeds (sunflower, cotton, flax, etc.).

Structure of seeds with endosperm. This type of seed consists of three parts: the seed coat, the embryo, and the endosperm. Here, the seed coat fuses tightly with the Ovary wall, which forms the pericarp, resulting in a caryopsis fruit.

The embryo consists of the plumule, from which the shoot develops upon seed germination. Unlike most monocotyledons, the plumule of grasses is well-developed and typically possesses 2-3, or occasionally more, embryonic leaves. These are enveloped by an outer, hood-like leaf of the plumule known as the coleoptile, which protects them and plays a vital role during seed germination.

The grass embryo contains another crucial structure — the scutellum. It is situated laterally to the plumule, occupying an intermediate position between the embryo and the endosperm. Morphologically, the scutellum represents the first leaf of the plumule, i.e., the cotyledon. The physiological function of the scutellum is the absorption of nutrients from the endosperm during seed germination. On the side of the embryo opposite the scutellum lies a horn-shaped, scale-like outgrowth known as the epiblast. There are various viewpoints regarding the origin and Significance of the epiblast:

✵ The epiblast represents a remnant of the second cotyledon (if the scutellum is considered the first);

✵ The epiblast represents the first and only cotyledon, whereas the scutellum belongs to axial structures;

✵ The epiblast is a tissue fold formed as a result of the bending of the embryonic axis.

The hypocotyl in grasses is underdeveloped, while the radicle (sometimes 2-3 or many) is surrounded by a specialized multi-layered protective sheath known as the coleorhiza, which swells during germination and produces absorbing hairs on its surface. The radicle pierces the coleorhiza tissue and emerges into the soil. The main root of the plant develops from the embryonic radicle.

There is also a perspective stating that the coleorhiza is an underdeveloped primary root, whereas the endogenous rootlets piercing it are adventitious in origin.

The endosperm typically serves as the storage site for nutrient reserves. The composition and ratio of nutrients within it vary. For instance, in wheat, the coarse-celled layer of the endosperm stores starch and protein (gluten), whereas the fine-celled layer of the endosperm, located just beneath the seed coat, stores protein (the aleurone layer).

Structure of seeds lacking endosperm and perisperm. Examples include the seeds of beans, peas, apple trees, sunflowers, and others. Such seeds consist exclusively of a seed coat and an embryo. Nutrient reserves are stored within the embryo itself, most frequently in the cotyledons.

Seed germination begins with the activation of enzymatic processes within its Tissues, which involve The conversion of complex compounds into simpler ones that can be readily assimilated by the growing embryo. Enzymes are activated under specific moisture and Temperature regimes, along with an adequate supply of oxygen.

STRUCTURE OF THE seedling. During seed germination, a seedling develops from the embryo. The root of the seedling originates from the embryonic radicle and transforms into the primary root. The transition zone between the root and the stem is marked by the root collar. The root collar is distinguished by a thickening and a darker coloration of the cortex. The stem region extending from the root collar to the cotyledons is termed the hypocotyl.

Fig. 19. Seed germination and seedling structure: A (a-d); B (a-c) - oat; 1 - primary root; 2 - hypocotyl; 3 - epicotyl; 4 - root collar; 5 - cotyledons; 6 - apical bud, 7 - first leaf (leaves); 8 - coleoptile; 9 - seminal roots.

The attachment point of the cotyledon to the stem represents the first node. The stem section from the cotyledons to the node of the first leaves is called the epicotyl (or the first internode of the shoot). In many dicotyledonous plants (such as peas, beans, oak, and walnut), the seed cotyledons are very large and fleshy, and they are not brought above ground during germination. Remaining in the soil, they serve as a source of nutrients for the seedling until roots and true leaves develop. In other dicotyledonous plants (such as beans, soybeans, lupines, and clover), the cotyledons are pushed above ground during germination, turn green, and for some time—prior to The formation of true leaves—not only supply the seedling with nutrients but also function as green leaves. Seeds of plants whose cotyledons emerge above ground should not be sown too deeply, as the seedlings will be unable to break through a thick layer of soil and will perish.