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 - using a group of somatic cells, and reproduction by specialized haploid cells - spores. The second, more progressive 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 plants capable of seed reproduction have.
From what and how are seeds formed?
Beans are a thermophilic annual culture of the Bean family, it has flowers of a characteristic shape resembling a sailboat or butterfly sitting with folded wings. Inside the flower, in a special part of it called the pestle, there is the seed germ, which hid the embryo sac under its shells. It contains the egg and the diploid structure, called the central cell. Two sperm fertilize them sequentially, 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 also integuments called seed peels.
What are the seeds of dicotyledonous plants
As a result of fertilization, all flowering plants with two cotyledons produce seeds with seeds, in the formation of which all parts of the flower play the main role: a calyx, corolla with petals, androecium, consisting of stamens and, of course, a pistil with seed primordia. 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 seed volume is combined with a relatively small surface area.
This feature minimizes seed contact with the environment. The external structure of the bean seed resembles the main organ of the mammalian excretory system. In human anatomy there is even a definition - bean-shaped kidney. On the inner, concave side, there is a scar - a place by which the bean seed attaches to the dry leaves of the fruit, called a bean. Hence the name of the plant family - Legumes. It has more than 12 thousand species. Most of its representatives are grassy forms, but there are also shrubs and trees. Among legumes, let us name the champions for the content of valuable vegetable protein: this is soy, peas, beans, lentils.
Seed structures and their meaning
We continue to consider the structure of the bean seed . The figure below illustrates the presence of the previously indicated parts, namely: the seed peel, two cotyledons and the embryo located between them.
It was found that the outer part - the skin of the seed - is a derivative of the integument seed cover (integuments). It performs the function of protecting 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 at the next section.
Embryo of a dicotyledonous plant
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, stem and kidney. Let's pay attention to the internal structure of the bean seed. The diagram below demonstrates 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.
Germinal root
The vast majority of terrestrial plants have a well-developed root system: main, lateral or subordinate. Grasses, shrubs and tree species can form two types of underground structures, which in botany are called rod and fibrous 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 plant physiology is the main criterion by which the launch of such a mechanism as seed germination is determined. Beans, tomatoes, peas and other thermophilic and responsive to plentiful watering crops require an optimal combination of various environmental factors in order for this process to end with the emergence of a new young plant.
Cotyledons and their role in the life support of the seedling
In order for the seed to germinate, a supply of nutrient compounds is needed: sugars, amino acids, fats. In dicotyledonous plants, it is accumulated in cotyledons. At the beginning of germination of the embryo, organic substances pass into the dissolved form, the most accessible for absorption by cells. In seedlings of dicotyledonous plants, cotyledons begin to perform the functions of the first terrestrial, the so-called germinal 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 seed life to the stage of active growth of the embryo parts: root and stem with leaves. As a result, a seedling first appears, and then a young plant forms. What kind of bean seed is germinating? The photo presented below clearly shows that the embryo root begins to develop first, then the stem takes cotyledon leaves above the ground. After some time, from the cone of the growth of the stem, consisting of the apical educational tissue - the meristem - the real leaves of the bean plant are formed.
Rest period
After ripening, called beans, the bean seeds collected from the beds are not able to germinate immediately. Not only for legume plants, 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. Breathing of seeds is practically not carried out, the water content is less than one sixth of the total mass.
As a result, the seeds have a minimum germination energy, insufficient for the transition of organic matter of the cotyledons to a soluble form. Both the external and internal structure of the bean seed in the latent period is no different from that in the vegetation interval, 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 resting period of the seeds.
What is needed for germination of the embryo
In agronomy, the following conditions can be distinguished, which ensure high-quality seed germination and the emergence of friendly seedlings: the presence of water, a favorable temperature, the presence of oxygen, and optimal illumination. Consider these factors in more detail. The first place in our list of abiotic conditions is water. It is necessary for cell swelling, which is accompanied by increased respiration.
Studying the structure of the bean seed , we found that the nutrition of the embryo is possible only if the organic substances in the cotyledons go into the dissolved form. This is due to water molecules penetrating their storage parenchyma. Being a heat-loving culture, beans grow well in the soil well warmed by the sun. But lighting does not affect the germination energy. For artificial removal of seeds from a latent state, they are treated with stimulants, for example phytohormones.
The method of scarification is also carried out, that is, mechanically violate the integrity of the peel without damaging the structures included in the internal structure of the bean seed, in particular its cotyledons and embryos. Thus, all of the above agricultural techniques accelerate the process of germination of crops.