Even in the school course in botany (grade 6), the structure of the seed was a fairly simple and memorable topic. In fact, this generative organ of the plant arose as a result of a long evolutionary process and has a complex and unique structure. In our article, we will consider the features of its structural parts, the structure of the dicotyledonous seed, and also determine the biological role of plant seeds.
The emergence of seed during evolution
Plants were not always capable of forming seeds. It is known that life arose in water, and the first plants were algae. They had a primitive structure and propagated vegetatively - by parts of the thallus and with the help of specialized mobile cells - zoospores. The first natives of the land were riniophytes. They, like their future successors - the highest spore plants, multiplied by spores. But for the development of these specialized cells, water was needed. Therefore, when environmental conditions changed, their numbers decreased.
The next evolutionary stage was the emergence of seed. This was a huge step forward for the adaptation and spread of many plant species. The external and internal structure of the seed determines the reliable protection of the embryo, surrounded by a supply of water and nutrients. That means they increase the vitality and species diversity of the planet's flora.
Seed formation process
Consider this process as an example of a group of plants, which in the modern world is dominant. These are representatives of the Angiosperms department. All of them form a flower - the most important generative organ. The egg is located in its pistil, and the anthers of the stamens contain sperm. After the pollination process, i.e. Pollen transfer from the anther of the stamens to the stigma of the pestle, sperm along the embryonic tube advance into the ovary of the stamens, where the process of gamete fusion occurs - fertilization. As a result, an embryo is formed. When the second sperm merges with the central germ cell, a reserve nutrient is formed. It is also called endosperm. A strong outer shell completes the seed structure. Such a structure is the basis for the development of the future plant organism.
External structure of seeds
As already mentioned, outside the seed is covered with a peel. It is dense enough to protect the nucleus inside, from mechanical damage, temperature changes and the penetration of harmful microorganisms. But the color of the seeds varies widely: from black to bright red. This structure of the seed is easy to explain. In some plants, color is used to mask. For example, so that the birds could not see them in the soil after planting. Other plants, on the contrary, are adapted to the distribution of seeds through various animals. Together with undigested food debris, they secrete them far beyond the area of growth of the mother plant.
The internal structure of the seed
The main part of any seed is the germ. This is the future organism. Therefore, it consists of the same parts as an adult plant. This is the germinal root, stem, leaf and kidney. The structure of the seeds of different plants can vary significantly. In most of them, reserve nutrients accumulate in the endosperm. This is the shell that surrounds the embryo around, protecting and nourishing it throughout the entire period of individual development. But there are times when, during the process of ripening and seed germination, it completely consumes endosperm substances. Then they accumulate mainly in the fleshy parts of the embryo. They are called cotyledons. Such a structure is typical, for example, for pumpkins or beans. But in the shepherd’s bag, the supply of substances is concentrated in the tissue of the germinal root. The seeds of various systematic plant groups are also different.
Features of seeds of gymnosperms
The external and internal structure of the seed of this group of organisms is characterized by the fact that the process of formation and development of the embryo takes place on the surface of the seed coat. In addition to the main parts, gymnospermous seeds have a pterygoid membranous outgrowth. It helps the seeds of these plants spread through the wind.
Another feature of gymnospermous seeds is the duration of their formation. For them to become viable, four months to three years must pass. The process of seed maturation occurs in cones. These are not at all fruits. They are specialized modifications of the escape. Some coniferous seeds can be stored in cones for decades. All this time they remain viable. To seed hit the ground, cone flakes open independently. The wind picks them up, sometimes moving over considerable distances. If the cones are soft, resembling nuts outwardly, they are not opened by themselves, but with the help of birds. Various kinds of jays are especially fond of eating seeds. It also contributes to the resettlement of gymnosperms.
The very name of this systematic unit indicates that the embryo of the future plant is poorly protected. Indeed, the presence of an endosperm guarantees only the development of the seed. But the cones of many plants open during unfavorable conditions of development. Once on the surface of the soil, the seeds are exposed to low temperatures and lack of moisture, so not all of them germinate and give rise to a new plant.
Features of Flowering Plant Seeds
Compared to Gymnosperms, representatives of the Flowering department have a number of significant advantages. The formation of their seeds occurs in the ovary of flowers. This is the most expanded part of the pestle, which gives rise to the fruit. As a result, seeds develop within them. They are surrounded by three layers of pericarp, which differ in their properties and functions. Consider their structure on the example of plum drupe. The outer leathery layer protects against mechanical damage, ensuring integrity. The medium is juicy and fleshy. It nourishes and provides the fetus with the necessary moisture. The internal ossified layer is an additional protection. As a result, the seeds have all the necessary conditions for development and germination, even under adverse circumstances.
Seeds of Monocotyledonous Plants
The structure of the seed of a monocotyledonous plant is very easy to determine. Their embryo consists of only one cotyledon. These parts are also called germinal leaves. Monocotyledonous are all plants of the Cereal, Onion, and Liliaceae families . If you germinate the seeds of corn or wheat, soon on the surface of the soil from each grain one leaf is formed. This is the cotyledon. Have you tried to split a grain of rice into several parts? Naturally, this is not possible. All because her embryo is formed by a single cotyledon.
Dicotyledonous Seeds
The seeds of the families Rosaceae, Solanaceae , Aster, Legumes, Cabbage and many others are somewhat different in structure. Even based on the name, it is easy to guess that their embryo consists of two cotyledons. This is a major systematic feature. The structure of the seeds of dicotyledonous plants is easy to see with the naked eye. For example, a sunflower seed is easily divided into two equal parts. This is the cotyledon of his embryo. The structure of the dicotyledonous seed is also visible by young seedlings. Try sprouting ordinary bean seeds at home . And you will see two carpels that appear above the surface of the earth.
Seed germination conditions
The structure of the seeds of dicotyledonous plants, as well as representatives of other systematic units of this kingdom of living nature, determines the presence of all the necessary substances for the development of the embryo. But other conditions are necessary for germination. For each plant, they are completely different. Firstly, it is a certain air temperature. For thermophilic plants, this is +10 degrees Celsius. But winter wheat begins to develop already at + 1. Water is also needed. Thanks to her, the grain swells, which accelerates the processes of respiration and metabolism. Nutrients go into a form in which they can be absorbed by the fetus. The presence of air and a sufficient amount of sunlight are two more conditions for seed germination and the development of the whole plant, since photosynthesis is impossible without them.
Seeds and Fruits
Each fruit contains seeds. The structure of the seeds of higher plants is almost identical. But the fruits are more diverse. Dry and juicy fruits are distinguished. They differ in the structure of the layers that are located around the seed. In juicy, one of the layers of the pericarp is necessarily meaty. Plum, peach, apple, raspberry, strawberry ... These delicacies are loved by everyone precisely because they are juicy and sweet. In dry fruits, the pericarp is leathery or ossified. Its layers usually grow together into one, reliably protecting the seeds inside. A box of poppies, a mustard pod, a grain of wheat have just such a structure.
The biological role of seeds
Most plants on the planet use seeds for propagation. The seed structure of modern plants is the result of a long evolution. These generative organs contain the embryo and a supply of substances, ensuring its growth and development even in adverse conditions. Seeds have devices for distribution, which increases their chance of survival and resettlement.
So, the seed is the result of the fertilization process. It is a structure consisting of an embryo, reserve substances and a protective peel. All its elements perform certain functions, thanks to which a group of seed plants has taken a dominant position on the planet.