BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VIII. BASICS OF PLANT MORPHOLOGY AND ANATOMY

Features of Reproduction in Angiosperms

The Seed

Seed Formation

A seed develops from an ovule that has enlarged As a result of amphimixis or apomixis.

Seeds of flowering plants vary greatly in shape and size; the latter can range from a few millimeters (orchids, poppies) to tens of centimeters (palms). In shape, a seed may be globose, cylindrical, or oval, and its surface can be variously colored, smooth, rough, covered with spines, hairs, papillae, etc. Sometimes a seed features an aril that facilitates its dispersal.

Externally, the seed reflects to some extent the morphological Features of the ovule from which it was formed. The micropyle, hilum, and raphe are visible on the seed. The micropyle transforms into the seed pore (micropyle), the hilum is formed at the point where the seed detached from the funiculus, and if the ovule was anatropous, the raphe is visible on the seed coat. The seed coat, as is well known, develops from the integuments of the ovule.

An essential part of the seed is the embryo. In shape, it may be straight, curved, spiral, horseshoe-shaped, etc. In some plants, the embryo occupies the central part of the seed and is surrounded by endosperm on all sides; in other plants, the embryo is positioned laterally, but the tip of the embryonic ROOT is always directed toward the micropyle.

Seed Types

The following types of seed Anatomical Structure are distinguished (Fig. 86):

- seed without endosperm and perisperm: the embryo occupies the entire cavity of the embryo sac, and nutrient reserves accumulate in the cotyledons of the embryo. In this case, the seed consists of two parts: the seed coat and the embryo. The embryo is clearly differentiated into the radicle, hypocotyl, and plumule. This seed structure is characteristic of legumes, cucurbits, rosaceous plants, walnuts, fagaceae, etc.;

- seed with endosperm: the seed consists of three parts: the seed coat, endosperm, and embryo. The endosperm is formed from triploid Cells and contains a large amount of reserve nutrients: Proteins, starch, Lipids, etc. Seeds with endosperm are more typical of monocots, but can also occur in dicots such as poppies, solanaceae, and apiaceae;

- seed with perisperm: the endosperm is entirely consumed during The formation of the embryo, while the Cells of the nucellus become filled with nutrients, forming the perisperm. Seeds of this type are characteristic of caryophyllaceae, chenopodiaceae, and nymphaeaceae; - seed with endosperm and perisperm: a rather rare structural type where the seed contains an embryo, endosperm, and perisperm. It is typical of the lotus and nutmeg.

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Fig. 86. STRUCTURE AND TYPES of seeds in angiosperms:

A - seed with endosperm, B - seed with perisperm, C - seed with endosperm and perisperm, D - seed without endosperm and perisperm; 1 - seed coat, 2 - embryo, 3 - endosperm, 4 - perisperm, 5 — pericarp

Since seeds contain abundant nutrients, they serve as a source of food (flour, groats, vegetable oil), medicinal, and industrial products for humans. Depending on which nutrients predominate in the seeds, plants are categorized into cereal crops (grains), protein crops (legumes), oilseeds (sunflower, flax, peanut), and pseudocereal/groat crops (buckwheat, millet).

Seeds of many plants can remain dormant for a long period and germinate only under favorable conditions. Germination is accompanied by profound biochemical and anatomical-physiological changes. Respiration intensifies in the seed, hydrolytic Enzymes are activated, and reserve nutrients are transformed from immobile forms (starch, protein, lipids) into mobile forms (glucose, fructose, Amino Acids, Fatty acids, etc.) accessible for the embryo's Nutrition. Its growth begins with the elongation of the embryonic root, a process in which the hypocotyl participates. Following The Emergence of the root, the cotyledons, SHOOT, and first leaves begin to grow.

In some plants, the hypocotyl is long and lifts the cotyledons above the soil surface, where they function as the first leaves (maple, bean). In other plants (pea, oak, chestnut), the cotyledons remain in the soil and nourish the seedling.

In grasses (cereals), the embryonic bud and leaves are protected by the first embryonic leaf, the coleoptile. During grass seed germination, it protects the delicate, tube-rolled embryonic leaves from damage. The embryonic root is also protected by a multilayered tissue called the coleorhiza. It is ruptured during germination by the primary and adventitious roots and persists around them for some time.