In our article we will consider the features of generative propagation of plants. It is this process that is the most progressive way to reproduce their own kind, providing a variety of hereditary material for descendants and adaptation.
Methods of reproduction
The property of reproducing one's own kind is inherent in all living beings. 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 happens with the help of organs of vegetative propagation: root, stem and leaves.
The sexual process involves the union in one cell of the hereditary material of two daughters. This is provided by the generative organs of plant reproduction - a flower, a seed and a fruit. Largely thanks to them, angiosperms have taken a dominant position on the planet.
Evolution of the sexual process
For the first time, generative reproduction has been observed inalgae. This happens when adverse conditions occur. At this time, the mother cell forms several sex cells. They enter the water, where they merge in pairs to form a zygote. When environmental conditions normalize, it divides. As a result, mobile spores are formed.
Spore plants are characterized by alternation of generations - sexual and asexual. The spores develop into gametangia. This is the name of the organs of sexual reproduction, in which female and male germ cells mature. They form an embryo that germinates and turns into an individual of an asexual generation that reproduces by spores. The process then repeats.
In gymnosperms, generative reproduction occurs in shortened modified shoots - cones.
Flower - modified shoot
The main organ of generative reproduction is a flower. Its main parts are stamens and pistils. They contain the sex cells. Each stamen consists of a filament and an anther, in which pollen grains - male gametes - mature. The pistil consists of the lower expanded part - the ovary, the elongated middle part - the style - and the upper, expanded part - the stigma. It develops a female gamete called an egg.
The rest of the flower provides auxiliary functions. For example, the pedicel is needed to attach to the shoot, the calyx is needed to protect the internal parts from damage, the corolla is needed to attract insects.
Pollination
In seed plants generativereproduction consists of two successive processes. This is pollination and fertilization. The fact is that sex cells develop in different parts of the flower. Therefore, their fusion requires the transfer of pollen from the anther of the stamen to the stigma of the pistil.
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 occurs when pollen from the stamen 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 the fusion of sex cells. Getting on the stigma of the pistil, two sperm from the pollen grain penetrate the embryo sac. Each of them merges with two different cells. One sperm connects with an egg. As a result, a zygote is formed, from which the embryo develops. Another sperm connects to the central cell. They give rise to the endosperm, a reserve nutrient.
The essence of this process was discovered in 1898 by the Soviet scientist Sergei Navashin. Since two sperm are involved in fertilization, the scientist called it double.
The structure of the seed
As a result of pollination and fertilization, another generative reproductive organ is formed - the seed. It contains the germ, protected by the peel, and a supply of nutrients. Thanks to this structure, the seed can survive for a long time.adverse conditions.
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 dormant period. It can last from a couple of weeks to several years.
Seeds need certain conditions to germinate. This is sufficient humidity, the presence of air and light, certain temperature indicators.
What is a fetus?
To protect and distribute seeds, angiosperms have another organ of generative reproduction. This fruit is a modified flower. It consists of a certain number of seeds protected by the 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 the flow of water for this, tumbleweeds use the wind. And some species scatter their seeds themselves. So, a mad cucumber cracks after ripening, and a balsam - when touched.
Despite the fact that the sexual process in plants is quite diverse, its essence lies in the combination of the genetic material of two gametes. As a result, an organism is formed that has new features,which greatly increases its adaptive capacity.
