In the plant world, two forms can be distinguishedreproduction: asexual and sexual. The first type includes such methods of transmission of 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 progressive form is sexual reproduction, which leads to the formation of seeds. It occurs in the life cycle of gymnosperms and flowering plants, called angiosperms. In this paper we will look at the external structure of the bean seed, find out the conditions necessary for its germination, and also determine the advantages of plants capable of seed reproduction.
From what and how are the seeds formed?
Beans - heat-loving annual culturefamily Legumes, it has flowers of characteristic shape, reminiscent of a sailboat or a butterfly sitting with folded wings. Inside the flower, in a special part called the pestle, there is a seed rudiment, hidden under its shells is the embryo sac. It contains an ovum and a diploid structure, called the central cell. They are consistently impregnated with two sperm, as a result of which the seed of the bean appears. It has an embryo, a stock of organic compounds for growth and development, two cotyledons, and covers, called seed skin.
What are the seeds of dicotyledonous plants
All flowering plants with two cotyledons inThe result of fertilization is the formation of fruits with seeds, in the formation of which the main role is played by all parts of the flower: a cup, a corolla with petals, androtsa, consisting of stamens and, of course, a pestle with seed germs. The structure of the bean seed is studied in the 6th grade, getting acquainted with such a branch of biology as botany. It has an ellipsoidal shape, due to which an impressive volume of seed is combined with a relatively small surface area.
This feature minimizes seed contactwith the environment. The external structure of the bean seed resembles the main organ of the excretory system of mammals. In the anatomy of man there is even a definition - the bean-shaped form of the kidney. On the inner, concave side 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 family of plants - Legumes. It has more than 12 thousand species. Most of its representatives are herbaceous forms, but there are also shrubs and trees. Among the legumes we call the record holders for the content of valuable vegetable protein: soybeans, peas, beans, lentils.
Seed structures and their meaning
We continue to consider the structure of the bean seed.The figure below shows clearly the presence of the previously mentioned parts, namely: the seed coat, the two cotyledons and the embryo located between them.
As it was established, the outer part - the skinseed - is a derivative of the covers of the seed rudiment (integuments). It performs the function of protecting against drying out, unfavorable temperatures and other negative abiotic factors. Of course, the main seed structure is the fetus. Why this is so, let's look at the next section.
The embryo of a dicot plant
As we recall, in the process of doublefertilization, inherent only to flowering plants, from a fertilized egg - the zygote - develops a multicellular formation. It has received the name of an embryo and has three parts: an embryonic root, a stem and a kidney. Let's pay attention to the internal structure of the bean seed. The diagram below shows clearly that the embryo, safely hidden between the cotyledons, is not only the most important, but also the most delicate and vulnerable structure. Next, we answer the question, what are the functions that are inherent in the main components of the embryo.
Embryonic root
The overwhelming number of terrestrial plants havea well-developed system of roots: the main, lateral, or accessory. Grasses, shrubs and tree species can form two types of underground structures, which in botany are called rod and friable root systems. As it turned out, they begin their development from the same part - the germinal root. The beginning of the division of its cells in the physiology of plants is the main criterion by which the triggering of a mechanism such as the germination of a seed is determined. Beans, tomatoes, peas and other thermophilic and responsive to abundant watering crops require an optimal combination of various environmental factors to this process resulted in the emergence of a new young plant.
Cotyledons and their role in the life-support of the sprout
In order for the seed to germinate, it is necessarystock of nutritional compounds: sugars, amino acids, fats. In dicotyledonous plants it is accumulated in cotyledons. At the beginning of the germination of the embryo, organic substances pass into the dissolved form, the most accessible for assimilation by cells. In seedlings of dicotyledonous plants, the cotyledons begin to perform the functions of the first terrestrial, the so-called embryonic leaves. Nevertheless, they are able to perform photosynthesis and provide the young plant with all the necessary plastic substances.
What is seed germination?
This is a physiological process, in the basis of whichlies the mechanism of the transition from the latent period in the life of the seed to the stage of active growth of parts of the embryo: the root and stem with leaves. As a result, a seedling appears first, and then a young plant is formed. What kind of seed does the seed of the bean have? The photo presented below clearly shows that the germinal root begins to develop first, then the stalk leaves the cotyledon leaves above the surface of the earth. After a while, the real leaves of the bean plant are formed from the cone of the growth of the stem, consisting of the apical educational tissue - meristems.
The rest period
After the ripening of the fruits, called beans, the seedsbeans harvested from the garden are not able to germinate immediately. Not only for legume plants, but also for representatives of other groups, for example tomatoes, aubergines, 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. Breath of seeds is practically not carried out, the water content is less than a sixth of the total mass.
As a consequence, seeds have minimal energygermination, insufficient for the transition of organic substances of cotyledons into a soluble form. Both the external and internal structure of the bean seed in the latent period does not differ from that in the vegetation interval when they are prepared for sowing in the soil. The differences relate primarily to the rate of metabolic reactions in the cells of the embryo, which is very low during the rest period of the seeds.
What is necessary for the germination of the embryo
In agronomy, the following conditions can be distinguished:providing quality germination of seed and the emergence of friendly shoots: the presence of water, a favorable temperature, the presence of oxygen, optimal illumination. Let's consider these factors in more detail. The first place in our list of abiotic conditions is occupied by water. It is necessary for the swelling of cells, which is accompanied by increased breathing.
Studying the structure of the bean seed, we found out thatthe nutrition of the embryo is possible only if the organic substances in the cotyledons go into a dissolved form. This is due to the water molecules that penetrate their store parenchyma. Being a heat-loving crop, the bean crops well in well-warmed sun. But the lighting does not affect the energy of germination. For artificial removal of seeds from the latent state, they are treated with stimulants, for example phytohormones.
Also receive scarification, that ismechanically break the integrity of the peel without damaging the structures entering the internal structure of the bean seed, especially its cotyledons and embryo. Thus, all the above agrotechnical methods accelerate the process of germination of crops.
