What is the germ of the seed of a plant? The structure of the seed embryo

After reading this article, you will find out what the seed germ consists of. In addition, we describe the main stages of its development.

In order to visualize what we are talking about, let's look at an example. Then we proceed to the general characterization of the embryo and the stages of its development.

What is the germ of wheat seed

If you cut a seed of wheat, you can see that most of it is a powdery white cloth. It is called endosperm. Its cells are filled with various nutrients. It is not so easy to find out what the germ of the seed of wheat consists of. After all, he is very small. It can only be viewed with a magnifying glass. Just like the bean embryo, it has a stalk, a root and a kidney. However, he has only one cotyledon. It is a thin plate that fits snugly into the endosperm.

This is what the germ of the seed consists of. Of course, different types of plants, it is different. But on the example of wheat, you can imagine its structure in general terms.

Separation of the embryo and the suspension

The early stages of embryonic development in dicots and monocots are very similar. It begins with the division of a fertilized egg (zygote) in the embryo sac of the ovule. In most flowering planes, the first division plane runs across (or almost across) its longitudinal axis. At the same time, the polarity of the embryo is established: the upper (halazal) pole is its main growth zone, and the lower (micropilar) pole forms a peculiar leg-suspension, or suspension, anchoring the embryo at the micropyle. After several divisions, the final differentiation into an almost spherical germ and a suspenor occurs.

The beginning of the development of tissue systems

What is the germ of the seed that has just formed? From the mass of relatively undifferentiated cells. However, soon changes in its internal structure lead to the beginning of development of systems of plant tissues. Future epidermis (protoderma) is formed during periklinnyh divisions of the outer cells of the embryo proper. Periclinal is called fission, in which the cell plates between two daughter cells are parallel to the surface of that part of the plant where fission occurs.

Separation of procalibium and basic meristem

Further differences in the degree of vacuolization and density of embryonic cells lead to the isolation of procambium and the basic meristem. The latter, more strongly vacuolated and less dense, gives rise to the main tissue surrounding the less vacuolized and denser procambium, the precursor of conducting tissues - xylem and phloem.

Cotyledon formation

Protoderm, the main meristem and procambium (the so-called primary meristems), without interruption, pass from the cotyledons to the axis of the embryo. The formation of cotyledons can begin either in the process or after the primary meristem has been laid (the stage of development before the appearance of cotyledons is often called globular). The globular embryo of dicotyledons gradually assumes a two-bladed shape (this stage is often called heart-shaped). The seed of monocotyledon seed forms only one cotyledon. Therefore, it does not have a heart-shaped stage.

Destruction of cells of the suspension

Then the cotyledons and the axis of the embryo are elongated (the so-called torpedo stage) and the primary meristems are distributed along them. Stretching, the embryo remains straight or curved. The single cotyledon of monocots often grows so much that it turns out to be the largest embryonic structure. As the embryo continues to grow, the cells of the suspension gradually break down.

Cell division

In the early stages of embryogenesis, cell division occurs throughout the mass of the young sporophyte. However, during the formation of the embryo, the appearance of new cells is gradually limited to the apical meristems of shoot and root. In dicotyledons, the first of them is laid between two cotyledons, and in monocotyledons with one side of the cotyledon and completely surrounded by the vagina - a similar outgrowth of its base. Apical meristems are of great importance, since they are ultimately the source of all new cells providing development from the embryo of a germ and an adult plant.

Branch of oviduct

During the entire period of embryo formation, nutrients constantly flow from the parent plant to the tissues of the ovule. As a result, a significant stock of them accumulates in the endosperm, perisperm or in the cotyledons of the developing seed. In the end, the ovules are separated from the stem, which binds it to the ovary wall, and becomes a closed system (with respect to nutrition). The seed dries up, giving water to the environment, and the seed peel hardens, as if encircling the embryo with the "protective shell", and with it a supply of nutrients.

So, you have learned what the germ of the seed consists of. As you can see, as it develops, its transformation takes place. Therefore, the structure of the seed embryo differs at different stages of its existence.