The life cycle of a plant consists of three successive stages:
- origin;
- development;
- reproduction.
It can be simple and complex. An example of a simple cycle is chlorella, which is propagated by spores. Developing, this green alga becomes a receptacle of 4-8 autospores that grow inside the mother's body and become covered by its own shell. But among plants, a complex development cycle is more common, which consists of 2-3 simple ones.
Features of plant life cycles
An important property of all living things is the ability to reproduce. The method of reproduction is:
- genital (gametes);
- asexual (by disputes);
- vegetative (part of the body).
In complex cycles during sexual reproduction, there are always several separate phases of the gamete and zygote. A gamete is a mature reproductive cell with a haploid (ordinary) set of chromosomes. A zygote with a diploid (double) set is formed as a result of combining two gametes. A sporophyte develops from the zygote, which produces haploid spores. Of the spores - the gametophyte, which is male and female.
For example, you can take an equipore fern, which has two forms of individuals - the fern itself (sporophyte) and its growth (gametophyte). Growth is the offspring of adult ferns. It exists for a very short period, but manages to produce the only large-leaved individual. Plant life cycle because of this feature of reproduction, it consists of an alternation of generations: from an adult fern to a seedling and again to an adult fern.
Breeding methods
Most plants reproduce sexually. In this case, a new organism is formed from the zygote after fertilization and the unification of gametes (syngamy). Parthenogenesis - reproduction without fertilization - also refers to the sexual method: the daughter organism is formed from isogametes, which makes relatives of isogametes and spores. Sexual reproduction is almost always combined with other methods - vegetative or asexual, as it is characterized by low productivity.
At the same time, this method and asexual reproduction occurs in ferns, and in conjunction with the vegetative variant, in some algae. In seed plants, the formation of germ cells occurs from one daughter zygote, as a result of which this process more closely resembles reproduction rather than reproduction.
With asexual reproduction, zoospores are formed - cells without a cell wall, which in multicellular plants are in special sporangia, and immobile cells - aplanospores. Independently, this method of reproduction is very rare in nature. Usually it is combined with sexual or vegetative.
There are 2 types of spores: mitospores that occur during asexual reproduction, and meiospores that arise during sexual intercourse. Mitospores appear through mitosis, resulting in an individual that is similar to the mother. Meiospores are formed by meiosis during germination of the zygote or in sporangia. For most plants, both methods of reproduction are characteristic, due to which two different forms of individuals are obtained.
Vegetative propagation method
In the vegetative variant of reproduction, there is a division into akinets - thick-walled cells. It consists in separating from the mother liquor some part of it - the brood kidney or body. In this way, some lower plants propagate, including sargassum, brown and red algae. Even flowering plants, such as duckweed, vegetatively propagate. Some of them form brood buds that fall to the ground and take root there. Also, the buds can branch and separate from the mother plant. In the angiosperms group of plants, the development of shoots underground from the rhizome is very common.
Plant distribution
One of the final stages of reproduction is the spread of plants. In nature, there can be 3 options for settlement: embryos, spores and seeds. In extremely rare cases, the spread can occur with the help of zygotes. C. Linnaeus associated seed and spore distribution with secretive and cohabitating plants. The second type was the group of gymnosperms and angiosperms, and the first was all the other groups, including algae, moss and ferns.
Plant propagation methods have come a long evolutionary path from the vegetative to asexual and sexual. Now the division of plants into spore and seed is associated not with distribution, but with reproduction. The seed method is allocated in a separate group, since it is considered a compound of reproduction by spores and gametes. Seed propagation includes several stages: the formation of zygotes, gametes, spores, embryos and seeds, as well as the resettlement of the plant.
Intergenerational
The life of plants in the form of two different generations can have different names: change of development forms, alternation of generations, etc. The changeability of a large fern and sprout in the case of an equispore fern is an example of alternation of generations, marked by the phases of the adult state of the forms of individuals. These two forms are so different in appearance that it is difficult to recognize the same plant in them. The growth of fern is very difficult to notice with the naked eye. In angiosperms, an analogue of a seedling is an embryo sac, which is extremely small and hidden deep in the flower. Among some groups of algae, these forms of individuals are similar in appearance, but completely differ in biological characters. Alternation of generations is found in almost all higher plants and evolutionarily developed algae.
Life cycles of higher plants
The life cycle of higher plants, except for bryophytes, is characterized by the fact that the gametophyte is poorly developed, and in the life cycle, sporophyte occupies most of the life cycle. Bryophytes are distinguished by the fact that sporophyte develops inside the female genital organ and is in continuous communication with the gametophyte. In the case of leafy mosses, it looks like a spore box growing from the top of the gametophyte.
The remaining higher plants have pronounced sporophytes, which are large and complex multicellular organisms with organs such as foliage, stems and root system. Most plants that a person imagines when talking about horsetail, ferns or other groups are sporophytes.
Life Cycles of Flowering Plants
The most progressive in terms of evolution are flowering plants. The life cycle of flowering plants is characterized by the fact that often the embryo is able to develop from an unfertilized egg (apomixis). The prevailing form of flowering is a diverse sporophyte, which is a plant with leaves and a stem. Male gametophyte is represented by pollen grains, and female gametes are represented by the embryo sac (it develops faster than gymnosperms). The organ of both sexual and asexual reproduction is a modified shoot - a flower. The seed buds are protected by the walls of the ovary. The life cycle of the development of plants of this group ends after fertilization and seed formation, in which the embryo has a supply of nutrients and is independent of external factors.
Life cycles of gymnosperms and angiosperms
The group of gymnosperms includes representatives of conifers and shrubs. Most of them have modified needle leaves. The life cycle of gymnosperms is characterized in that microspores (pollen) are formed in small male cones (anthers), and megaspores in female (ovules). The male gametophyte is formed from microspores, and the female from megaspores. The life cycle of a plant from this group is characterized by the fact that fertilization occurs with the help of wind, which delivers pollen to the ovules. After that, the embryo begins to develop inside the ovule, and the seed is formed from it. It lies on the seed scales and is not covered by anything. Seed gives a new sporophyte from which a new plant grows.
The life cycle of angiosperms is characterized by the fact that this group has a flower in which spores form and gametophytes are fertilized and seeds develop. The peculiarity of this group is in the protection of seeds that are hidden inside the fetus and protected from environmental influences.
Spore Plant Life Cycle
Spore plants do not bloom, because they are also called non-flowering. They come in two categories:
- higher (ferns, horsetails, mosses, crowns);
- lower (algae, lichens).
Life cycles of spore plants, depending on the species, can be sexual or asexual. They are not able to reproduce sexually without the participation of the aquatic environment. Gametophyte is used for sexual reproduction, and sporophyte is used for asexual. There are two subgroups of spore plants: haploid and diploid. The haploid subgroup includes mosses, horsetails and ferns, in which the gametophyte is more developed, and the sporophyte is formed in the form of a growth. The haploid subgroup is characterized in that the sporophyte has a subordinate status in it.
Plant Life Cycles: Schemes
Mosses are representatives of a primitive species of higher plants. They have a very conditional division of the body into a stem and leaves, instead of roots - threadlike rhizoids. They grow in marshy, humid places and evaporate moisture very much. They reproduce sexually, sporophyte depends on the gametophyte, spores form in a special box, which is located above the gametophyte and connected with it.
Representatives of ferns have large cirrus leaves (sporangia are located on the underside). The plant has a pronounced root system, and the leaf is actually a branch system called the vaya or pre-run. The life cycle of a plant of a group of ferns consists of two phases: sexual and asexual.
The sexual phase occurs with the participation of gametes, and asexual - dispute. The asexual generation begins with a diploid zygote, and the sexual one begins with a haploid spore. The interchangeability of these phases makes up most of the cycle.