The structure of the seed. External and internal structure of the seed

Even in school in the course of botany (Grade 6)the structure of the seed was quite simple and memorable topic. In fact, this generative plant organ arose as a result of a long evolutionary process and possesses a complex and unique structure. In this article we will consider the features of its structural parts, the structure of the bipartite seed, and also determine the biological role of plant seeds.

The appearance of the seed in the process of evolution

Plants were not always able to formseeds. It is known that life originated in the water, and the first plants were algae. They had a primitive structure and propagated vegetatively - parts of the thallus and with the help of specialized mobile cells - zoospores. The first people on the land were rhinophytes. They, like their future successors - the highest spore plants, multiplied by means of a dispute. But for the development of these specialized cells, water was needed. Therefore, when the environmental conditions changed, their number also decreased.

The next evolutionary stage was the emergence ofseed. This was a huge step forward for the adaptation and distribution 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. So, increase the vitality and species diversity of the planet's flora.

The process of seed formation

Consider this process using the example of a groupplants, which in the modern world is dominant. These are representatives of the Department of Angiosperms. All of them form a flower - the most important generative organ. In his pestle is an ovum, and anthers of stamens contain sperm. After the process of pollination, i.e. transfer of pollen from the anther of stamens to the stigma of the pistil, sperm along the embryonic tube move into the ovary of the stamen, where the process of fusion of gametes - fertilization takes place. As a result, the embryo is formed. When the second sperm merges with the central embryonic cell, a reserve nutrient is formed. It is also called endosperm. The solid outer shell completes the structure of the seed. This structure is the basis for the development of the future plant organism.

External structure of seeds

As already mentioned, the seed is covered from the outsidethe skin. It is dense enough to protect the embryo located inside, from mechanical damage, temperature changes and 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, the color serves to mask. For example, to prevent birds from seeing them in the soil after planting. Other plants, on the contrary, are adapted to the propagation of seeds with the help of various animals. Together with undigested food remnants, they excrete them far beyond the range of the growth of the mother plant.

The internal structure of the seed

The main part of any seed is the fetus. This is the future organism. Therefore, it consists of the same parts as the adult plant. This is the germ of the root, the stem, the leaf and the kidney. The structure of the seed of different plants can differ significantly. In most of them, reserve nutrients accumulate in the endosperm. It is the shell that surrounds the fetus around, protecting and nourishing it throughout the entire period of individual development. But there are cases when during the process of ripening and germination of the seed it completely consumes endosperm substances. Then they accumulate mainly in the fleshy parts of the embryo. They are called cotyledons. This structure is typical, for example, for a pumpkin or beans. But in a shepherd's bag, the stock of substances is concentrated in the tissue of the embryonic root. The seeds of various systematic groups of plants also differ.

Features of seeds of gymnosperms

The external and internal structure of the seed of this grouporganisms is characterized by the fact that the process of formation and development of the embryo occurs on the surface of the seminal rind. In addition to the main parts, the seeds of the gymnosperms have a winglike, membranous growth. It helps spread the seeds of these plants with the help of wind.

Another feature of the seeds of the Gymnospermsis the duration of their formation. So that they become viable, should pass from four months to three years. The process of seed ripening occurs in cones. This is not fruit. They are specialized modifications of the escape. Some seeds of conifers can be stored in cones for dozens of years. All this time they remain viable. To get the seeds into the ground, the scales of the cones open themselves. They are picked up by the wind, sometimes transported to considerable distances. If the cones are soft, outwardly reminiscent of nuts, they are not revealed themselves, but with the help of birds. Especially like to sip on the seeds of various types of sap. This also facilitates the resettlement of the representatives of the Gymnosperms department.

The name of this systematic unitindicates that the embryo of the future plant is poorly protected. Indeed, the presence of endosperm guarantees only the development of the seed. But the cones of many plants are revealed 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 the seeds of flowering plants

In comparison with the Gymnosperms, representativesThe flower department has a number of significant advantages. The formation of their seeds occurs in the ovary of flowers. This is the most expanded part of the pistil, which gives rise to the fruits. As a result, the seeds develop within them. They are twisted by three layers of pericarp, which differ in their properties and functions. Let us consider their structure by the example of the plum blossom. The outer leather layer protects against mechanical damage, providing integrity. Medium is juicy and fleshy. It nourishes and provides the fetus with the necessary moisture. The inner 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 Monocotyledonous plants

The structure of the seed of a monocotyledonvery easy. Their embryo consists of only one cotyledon. These parts are also called germ layers. Monocotyledons are all plants of the families Cereals, Onions and Liliaceae. If you germinate seeds of corn or wheat, soon on the surface of the soil from each grain is formed one leaflet. This is the cotyledon. Tried to divide the grain of rice into several parts? Naturally, this is impossible. All because her fetus is formed by a single cotyledon.

Seeds of Dicots

Seeds of families Rosaceae, Nightshade,The asters, legumes, cabbages and many others differ in structure. Even based on the name, it's easy to guess that their fetus consists of two cotyledons. This is the main systematic feature. The structure of seeds of dicotyledonous plants is easy to consider with the naked eye. For example, a sunflower seed can be easily divided into two equal parts. This is the cotyledons of his embryo. The structure of bipartite seed can be seen from young shoots. Try at home to germinate seeds of ordinary beans. And you will see two carpels that will appear above the surface of the earth.

Seed germination conditions

The structure of seeds of dicotyledonous plants, likerepresentatives 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, this is a certain temperature of the air. For heat-loving plants it is +10 degrees Celsius. But winter wheat begins to develop already at + 1. Water is also needed. Thanks to it, the grain swells, which speeds up the processes of breathing and exchange. Nutrients become a form in which they can be absorbed by the embryo. The presence of air and a sufficient amount of sunlight - two more conditions for the germination of the seed and the development of the entire 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. Distinguish dry and juicy fruits. They differ in the structure of the layers that are located around the seed. In succulent, one of the layers of the pericarp is necessarily fleshy. Plum, peach, apple, raspberry, strawberry ... These delicacies are loved by everyone precisely because they are succulent and sweet. In dry fruits, the pericarp is leathery or ossified. Its layers are usually fused into one, reliably protecting the seeds inside. A capsule of poppies, a pod of mustard, a grain of wheat have exactly this structure.

Biological role of seeds

Most plants on the planet for reproductionuse the seeds. The structure of the seeds of modern plants is the result of a long evolution. These generative organs contain an embryo and a stock of substances, ensuring its growth and development even under adverse conditions. Seeds have adaptations for distribution, which increases their chance of survival and resettlement.

So, the seed is the result of the processFertilization. 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 took the dominant position on the planet.