In our article we will consider the features of generative propagation of plants. This process is the most progressive way of reproducing their own kind, providing a variety of hereditary material of descendants and adaptation.
Breeding methods
The reproduction property of their own kind is inherent in all living things. Vegetative and generative propagation are the main forms of plant reproduction. In the first case, its multicellular part is split off from the whole organism, which eventually acquires the ability to exist independently. This occurs with the help of vegetative propagation organs: root, stem and leaves.
The sexual process involves the union in one cell of the hereditary material of two daughter. This is provided by the generative organs of plant propagation - a flower, a seed and a fruit. Largely thanks to them, angiosperms occupied a dominant position on the planet.
Sexual evolution
For the first time, generative propagation is observed in algae. This happens when adverse conditions occur. At this time, the mother cell forms several reproductive organs. They fall into the water, where they merge in pairs with the formation of a zygote. When environmental conditions normalize, it divides. As a result, mobile spores are formed.
Spore plants are characterized by the alternation of generations - sexual and asexual. Gametangia develop from spores. So-called organs of sexual reproduction, in which female and male germ cells mature. They form an embryo that grows and turns into an individual of the asexual generation, propagated by spores. The process is then repeated.
In gymnosperms, generative propagation occurs in shortened modified shoots - cones.
Flower - Modified Escape
The main organ of generative reproduction is the flower. Its main parts are stamens and pistils. They contain germ cells. Each stamen consists of a thread and anther in which pollen grains - male gametes ripen. Pestle consists of the lower expanded part - the ovary, the elongated middle - the column - and the upper, extended - stigma. It develops a female gamete called an egg.
The remaining parts of the flower provide auxiliary functions. For example, a pedicel is needed to attach to the shoot, a cup is used to protect the internal parts from damage, and a whisk is used to attract insects.
Pollination
In seed plants, generative propagation consists of two successive processes. This is pollination and fertilization. The fact is that genital cells develop in different parts of the flower. Therefore, for their fusion, pollen transfer from the anther of the stamen to the stigma of the pestle is necessary.
In some species, this occurs within the same flower. This process is called self-pollination. Most often, this happens inside the bud, before the flower blooms. Cross-pollination is observed when pollen from the stamens of one flower falls on the pistil of another. Pollen can be carried by wind, insects, birds, water or humans.
Seed formation in flowering plants
The next stage of generative reproduction is fertilization. This is a fusion of germ cells. Getting on the stigma of a pestle, two sperm from pollen grains penetrate into the embryo sac. Each of them merges with two different cells. One sperm combines with the egg. As a result of this, a zygote is formed, from which the embryo develops. Another sperm connects to the central cell. They give rise to endosperm - a reserve nutrient.
The essence of this process in 1898 was discovered by the Soviet scientist Sergei Navashin. Since two sperm take part in fertilization, the scientist called it double.
Seed structure
As a result of pollination and fertilization, another generative organ of reproduction is formed - the seed. It contains a germ protected by a peel and a supply of nutrients. Thanks to this structure, the seed can survive adverse conditions for a long time.
There are times when, even during ripening, nutrients are completely consumed. Then a seed is formed without endosperm. In such plants, the necessary substances are deposited in the first germinal leaves - cotyledons.
In most species, seeds do not germinate even under favorable conditions. This means that they are in a period of rest. It can last from a couple of weeks to several years.
Certain conditions are necessary for seed germination. This is sufficient humidity, the presence of air and light, certain temperature indicators.
What is a fetus?
To protect and spread seeds, angiosperms have another generative reproductive organ. This fruit is a modified flower. It consists of a certain number of seeds protected by pericarp.
The fruits are very diverse. By the amount of moisture in the pericarp they can be dry and juicy. By the number of seeds - single and multi-seeded.
Another important function of the fruit is the resettlement of plants. Water lilies use for this the flow of water, tumbleweed - the wind. And some species spread the seeds themselves. So, a mad cucumber cracks after ripening, and balsam - when touched.
Despite the fact that the sexual process in plants is quite diverse, its essence lies in combining the genetic material of two gametes. As a result, an organism is formed with new features, which significantly increases its adaptive abilities.