The structure of the seeds. The structure of seeds of monocotyledonous and dicotyledonous plants

All plants can be divided into spore and seed. Disputed species include mosses, crowns, ferns and horsetails. Their life cycle is divided into sporophyte and gametophyte. The sporophyte reproduces asexually, forming spores. Gametophyte is characterized by sexual reproduction, in which the plant forms gametes - germ cells - male and female. When they combine, a zygote forms, from which a new individual grows, which, in turn, will already form spores. In seed plants, everything is more complicated, since they form seeds from the zygote.

What it is?

Seed is a special multicellular structure necessary for a plant to reproduce. They are studied by the science of plants - botany, which includes biology. The structure of the seeds can be complex and depends on the department and class to which the plant belongs.

Classification of seed plants

All of them are divided into two departments: gymnosperms and angiosperms. The determining factor in the separation is the structure of the seeds, namely the presence or absence of additional protection.

Gymnosperms

This department consists of about 700 plant species. They are divided into four classes: coniferous, ginkgoic, cypress and opiate.

Class opaque

It is represented by three families: coniferous, oppressive and velvichievy. The last family consists of a single species - Velvichia amazing. The family of oppressive is represented by about 40 types of oppression, and coniferous - by 67 species of conifers, or ephedra, including Rough Ephedra, Ephedra Mountain, and others.

Ginkgo

Only one species of plants belongs to it - Ginkgo bilobate. This is a relict organism that has been preserved since the times of the Perm period.

Class Cycas

It consists of the family of the same name, including 90 species of plants. These include, but are not limited to, Cygnus crestiform, Cycas, drooping, Cygnus Tuara, etc.

Conifers

This is the most numerous gymnosperm department class. Previously, this class was divided into three orders, representatives of two of which are currently extinct. Today, conifers consist of one order - pine. It, in turn, includes seven families: pine, yew, araucaria, cypress, podocarp, sciadopitis and capitate.

Angiosperms Department

These plants are more numerous than gymnosperms. This is the dominant department in our time. It is divided into two large classes: monocotyledonous and dicotyledonous. The decisive factor in this division was the structure of plant seeds.

Monocotyledonous

This class is represented by 60 families, including lily, onion and cereal. In total, this class has about 60 thousand plant species.

Dicotyledon class

Consists of about 350 families. The most famous of them are cruciferous, rosaceae, legumes, asteraceae and nightshade.

The structure of gymnosperm seeds

Consider the seeds of coniferous, ginkgo, cygnix and opiate. These are the first plants that received seed during evolution.

Its external structure provides for the presence of a dense peel. Additional outgrowths may be present on it, which contribute to better protection and distribution of seeds. So, in pine seeds have pterygoid appendages that help them spread.

Since gymnosperms do not have a fetus, their peel has a complex structure. So, in the Cycas and Ginkgo, it consists of three layers. The topmost is called a sarcotest. It is soft and fleshy. The middle layer is the hardest, it protects the seed. It is called a sclerotest. The inner layer at the time of seed ripening becomes filmy, it is called the endotest. Basically, these seeds are distributed through animals that eat a tasty meaty sarcotest without damaging the hard sarcotest. As you can see, the peel of seeds of gymnosperms is practically an analogue of the fruit of angiosperms.

Inside contains the embryo and endosperm.

A germ is essentially a small plant. It has a germinal root and shoot, consisting of a stem, leaflets (their number may vary) and apical kidney.

Endosperm is the nutrient that is needed for seed germination.

The structure of monocotyledonous seeds

In angiosperms, the seed structure is slightly more complex than in gymnosperms. In addition, they are further protected by the fetus. A striking example of monocotyledonous plants is cereals. Therefore, we consider the structure of the seed of wheat. They, like gymnosperm seeds, are built from the peel, endosperm and embryo, consisting of a root, leaf and kidney, but they also contain a cotyledon (in this case, one). A cotyledon is a thick leaf that, when a seed germinates, becomes the first leaf. Grain, including wheat, is not a seed, but a fruit (grain), consisting of a seed and pericarp, which is tightly fused with the skin. Endosperm occupies most of the internal space of a monocotyledon seed - a combination of nutrients (starch, fats, proteins, etc.). The cotyledon separates the embryo from the endosperm.

The seed structure of all monocotyledons resembles that of wheat seed . But there are some exceptions. For example, there is no endosperm in the seeds of the arrowhead, and the nutrient chemical compounds necessary for germination are already in the bud itself. And in onions and lily of the valley, the endosperm is located around the embryo.

Dicotyledonous

The structure of the seed of a dicotyledonous plant is largely similar to that of monocotyledons. However, they have differences. The main thing that distinguishes the structure of the seeds of monocotyledonous and dicotyledonous plants is the number of cotyledons. The plants under consideration now have two of them. They are located on both sides of the embryo. The stem, spine and bud are located between the cotyledons.

As a typical example, you can take the structure of bean seeds. This is a typical representative of the class of dicotyledons, belonging to the legume family. The structure of the bean seeds provides for the presence of a thick, shiny peel that reliably protects the embryo. On the concave side of the seed is a hem. This is the place to which the placenta is attached, necessary to connect the ovule with the wall of the ovary. Next to it is a small hole - the vas deferens. The structure of the bean seeds also provides for the presence of nutrients in the cotyledons. This is observed in many dicotyledonous plants, so the seeds of many of them do not contain endosperm at all.

However, there are dicotyledonous plants, the embryos of which receive organic chemical compounds for germination only from the endosperm. This, for example, lilac, sweet pepper, linden, poppy. There are plants in whose seeds nutrients are present in both the endosperm and cotyledons. This, for example, ash.

Additional Angiosperm Seed Protection

This is the fruit. It serves to preserve the seed from mechanical and thermal damage. In addition, it is necessary to ensure the spread of seeds over long distances.

Fruits are simple and complex. The simple ones are single fruits, and the complex ones are harvested from several fused fruits. Complex fruits are also called apocarpies.

The fruit of angiosperms is formed from the ovary of a flower. The remaining parts in most cases wither, but sometimes additional shells may form from them.

What is formed from the ovary is called the pericarp. It consists of three shells: endocarp, mesocarp, and exocarp, or epicarp. The first layer is the inner, the second is the middle, and the third is the outer. These three layers are easy to identify with the naked eye. For example, consider the peach fruit. Its peel is exocarpy, its flesh is mesocarpy, and the ligneous shell, which reliably protects the only seed in the fruit, is endocarpy. Everything looks like an apple: the peel is the exocarp, the flesh is the mesocarp, and the transparent plates surrounding the seeds are the exocarp. Basically, all the fruits of mesocarp are represented by pulp, but there are exceptions. For example, in citrus exocarpies it is the skin, mesocarpium is the white or yellowish layer between the skin and the pulp, and the pulp is the endocarpy.

Seed distribution

This is very important for plants, since in this way they can spread over as large an area as possible. Seeds, especially flowering plants, can spread much further than spores. This is one of the significant advantages of seed plants over spore plants.

There are four main types of seed distribution:

• by air;
• on water;
• with the help of animals;
• with the help of people.

Depending on the type of distribution, the seeds and their fruits have various additional devices, for example, dandelion parachutes for air flight, clinging needles of a burdock for spreading on animal hair, etc. Juicy attractive mesocarpy, which we eat, is also needed by plants for effective seed distribution through animals and humans.

What is the advantage of seeds over spores?

Firstly, such a structure has a good chance of germination, as it has a sufficient amount of nutrients in the form of endosperm and a skin with which the seed can survive adverse conditions and germinate later.

In addition, the presence of water is not necessary for their distribution, as is the case with spores. They can also spread much farther than spores, which ensures the development of new territories by gymnosperms and angiosperms.

And the third advantage is that the seeds, unlike spores, are the result of sexual reproduction, which allows us to diversify the genotype of plants and provide their best adaptation to environmental conditions.

Conclusion: table

Now you know how seeds are arranged, why they are needed and why they are better than spores.