In the plant world, two forms of reproduction can be distinguished: asexual and sexual. The first type includes such methods of transmitting hereditary information as direct cell division, vegetative - with the help of a group of somatic cells, and reproduction by specialized haploid cells - spores. The second, more advanced form is sexual reproduction, leading to the formation of seeds. It is found in the life cycle of gymnosperms and flowering plants, also called angiosperms. In this work, we will consider the external structure of the bean seed, find out the conditions necessary for its germination, and also determine what advantages the plants capable of seed reproduction have.
From what and how are seeds formed?
Beans are a heat-loving annual crop of the legume family, it has flowers of a characteristic shape resemblinga sailboat or a butterfly sitting with folded wings. Inside the flower, in a special part of it, called the pistil, there is a seed germ, which hid the embryo sac under its shells. It contains the egg and a diploid structure called the central cell. They are successively fertilized by two spermatozoa, as a result of which the bean seed appears. It has an embryo, a supply of organic compounds for growth and development, two cotyledons, and an integument called the seed coat.
What are dicot seeds
In all flowering plants with two cotyledons, as a result of fertilization, fruits with seeds are formed, in the formation of which all parts of the flower play the main role: calyx, corolla with petals, androecium, consisting of stamens and, of course, pistil with seed primordia. The structure of the bean seed is studied in the 6th grade, getting acquainted with such a section of biology as botany. It has an ellipsoidal shape that combines an impressive seed volume with a relatively small surface area.
This feature minimizes the contact of the seed with the environment. The external structure of the bean seed resembles the main organ of the excretory system of mammals. In human anatomy, there is even a definition - a bean-shaped kidney. On the inner, concave side, there is a scar - the place by which the bean seed is attached to the dry leaves of the fruit, called the bean. Hence the name of the plant family - Legumes. It has more than 12 thousand species. Most of itrepresentatives are herbaceous forms, but there are also shrubs and trees. Among legumes, let's name the champions in the content of valuable vegetable protein: these are soybeans, peas, beans, lentils.
Seed structures and their meaning
Let's continue to consider the structure of the bean seed. The figure below clearly shows the presence of the previously indicated parts, namely: the seed coat, two cotyledons and the embryo located between them.
As it was established, the outer part - the skin of the seed - is a derivative of the integuments of the seed germ (integuments). It performs the function of protection against drying, adverse temperatures and other negative abiotic factors. Of course, the main seed structure is the embryo. Why this is so, let's look into the next section.
Dicot embryo
As we remember, in the process of double fertilization, which is inherent only in flowering plants, a multicellular formation develops from a fertilized egg - a zygote. It is called the embryo and has three parts: the germinal root, stalk and kidney. Let's pay attention to the internal structure of the bean seed. The diagram below clearly shows that the embryo, securely hidden between the cotyledons, is not only the most important, but also the most delicate and vulnerable structure. Next, we will answer the question of what functions are inherent in the main components of the embryo.
Fetal root
Overwhelmingthe number of terrestrial plants have a well-developed root system: main, lateral or adventitious. Grasses, shrubs, and woody species can form two types of underground structures, which in botany are called taproot and fibrous root systems. As it turned out, they begin their development from the same part - the germinal root. The beginning of its cell division in plant physiology is the main criterion by which the launch of such a mechanism as seed germination is determined. Beans, tomatoes, peas and other heat-loving and responsive crops require an optimal combination of various environmental factors to complete this process with the emergence of a new young plant.
Cotyledons and their role in the life support of the seedling
In order for a seed to germinate, a supply of nutritional compounds is needed: sugars, amino acids, fats. In dicotyledonous plants, it is accumulated in the cotyledons. At the beginning of embryo germination, organic substances pass into a dissolved form, the most accessible for absorption by cells. In seedlings of dicotyledonous plants, the cotyledons begin to function as the first terrestrial, so-called embryonic leaves. Nevertheless, they are able to carry out photosynthesis and provide the young plant with all the necessary plastic substances.
What is seed germination?
This is a physiological process, which is based on the mechanism of transition from the latent period in the life of the seed to the stage of active growth of parts of the embryo: root and stalk with leaves. As a result, a seedling appears first, andthen a young plant is formed. What is the appearance of a germinating bean seed? The photo below clearly shows that the germinal root begins to develop first, then the stalk brings the cotyledon leaves above the ground. After some time, from the growth cone of the stem, consisting of the apical educational tissue - the meristem - the true leaves of the bean plant are formed.
Resting period
After the fruits called beans ripen, the bean seeds collected from the garden are not able to germinate immediately. Not only for plants of the legume family, but also for representatives of other groups, such as tomatoes, eggplants, cucumbers, it takes time for their seeds to undergo post-harvest ripening. It is characterized primarily by the fact that at this time in the cells of the embryo the metabolism is carried out at a very low level. Seed respiration is practically non-existent, the water content is less than a sixth of the total mass.
As a result, the seeds have a minimum germination energy, insufficient for the transition of the organic substances of the cotyledons into a soluble form. Both the external and internal structure of the bean seed in the latent period is no different from that in the growing season, when they are prepared for sowing in the ground. The differences relate primarily to the rate of metabolic reactions in the cells of the embryo, which is very low during the period of seed dormancy.
What is needed for the germination of the embryo
In agronomy, you canhighlight the following conditions that ensure high-quality germination of the seed and the appearance of friendly seedlings: the presence of water, a favorable temperature, the presence of oxygen, optimal illumination. Let's consider these factors in more detail. First on our list of abiotic conditions is water. It is necessary for the swelling of cells, which is accompanied by an increase in their respiration.
Studying the structure of the bean seed, we found out that the nutrition of the embryo is possible only if the organic substances in the cotyledons pass into a dissolved form. This is due to water molecules penetrating into their storage parenchyma. Being a heat-loving crop, beans sprout well in well-warmed soil. But lighting does not affect the energy of germination. To artificially remove seeds from a latent state, they are treated with stimulants, such as phytohormones.
They also carry out scarification, that is, they mechanically violate the integrity of the peel without damaging the structures that make up the internal structure of the bean seed, especially its cotyledons and embryo. Thus, all of the above agricultural practices accelerate the process of germination of crops.
