After reading this article, you will learn what a seed germ consists of. In addition, we describe the main stages of its development.
In order to visualize what we are talking about, let us turn to an example. Then we turn to the general characteristics of the embryo and the stages of its development.
What does a wheat seed germ consist of
If you cut the seed of wheat, you can see that most of it is powdery white tissue. It is called an endosperm. Her cells are filled with various nutrients. It is not so easy to find out what the germ of a wheat seed consists of. After all, he is very small. It can only be viewed with a magnifier. Just like a bean embryo, it has a stem, spine, and kidney. However, he has only one cotyledon. This is a thin plate that fits snugly to the endosperm.
This is what a seed germ consists of. Of course, it is different for different types of plants. But on the example of wheat, one can imagine its structure in general terms.
Separation of the germ and suspension
The early stages of embryo development in dicotyledons and monocotyledons are very similar. It begins with the division of the fertilized egg (zygote) in the ovum of the ovum. In most flowering, the plane of the first division passes across (or almost across) its longitudinal axis. In this case, the polarity of the embryo is established: the upper (chalazal) pole is its main growth zone, and the lower (micropilar) pole forms a kind of leg - pendants, or suspensor, anchoring the embryo at the micropile. After several divisions, the final differentiation into the almost spherical embryo proper and the suspensor occurs.
The beginning of the development of tissue systems
What is the seed embryo that has just formed? From the mass of relatively undifferentiated cells. However, changes in its internal structure soon lead to the beginning of the development of plant tissue systems . The future epidermis (protoderm) is formed during the periclinal divisions of the outer cells of the embryo itself. Periclinal divisions are called in which cell plates between two daughter cells are parallel to the surface of that part of the plant where divisions occur.
Separation of procabium and the main meristem
Subsequently, differences in the degree of vacuolization and the density of embryonic cells lead to the isolation of procambium and the main meristem. The latter, more strongly vacuolated and less dense, gives rise to the main tissue surrounding the less vacuolated and more dense procambium, the precursor of conductive tissues - xylem and phloem.
Cotyledon formation
The protoderm, the main meristem and procambium (the so-called primary meristem), without interruption, pass from the cotyledons to the axis of the embryo. The formation of cotyledons can begin either in the process or after laying the primary meristems (the stage of development before the appearance of cotyledons is often called globular). In this case, the globular embryo of dicotyledons gradually takes a bilobate form (this stage is often called heart-shaped). The embryo of a monocotyledon seed forms only one cotyledon. Therefore, it does not have a heart stage.
Suspension cell destruction
Then the cotyledons and the axis of the embryo are lengthened (the so-called torpedo stage) and the primary meristems are distributed along them. Stretching, the embryo remains straight or bends. The only cotyledon of monocotyledons often grows so much that it turns out to be the largest germinal structure. As the embryo continues to grow, the cells of the suspension gradually break down.
Cell division
In the early stages of embryogenesis, cell divisions occur throughout the mass of young sporophyte. However, during the formation of the embryo, the appearance of new cells is gradually limited by the apical shoot and root meristems. In dicotyledons, the first of them is laid between two cotyledons, and in monocotyledons, on one side of the cotyledon, the vagina is completely surrounded by a similar outgrowth of its base. Apical meristems are of great importance, as they are ultimately the source of all new cells that ensure the development of a seedling and an adult plant from the embryo.
Branch ovule
Throughout the entire period of embryo formation, nutrients constantly flow from the parent plant to the ovule tissues. As a result of this, a significant supply of them accumulates in the endosperm, perisperm or in the cotyledons of the developing seed. In the end, the ovule is separated from the ovule, connecting it with the wall of the ovary, and becomes a closed system (in relation to nutrition). The seed dries, giving water to the environment, and the seed peel hardens, as if surrounding the “protective shell” of the embryo, and with it the supply of nutrients.
So, you have learned what the seed germ consists of. As you can see, as it develops, its transformation takes place. Therefore, the structure of the seed germ differs at different stages of its existence.