The life cycle of a plant consists of three successive stages:
- birth;
- development;
- reproduction.
It can be simple or complex. An example of a simple cycle is chlorella, which reproduces by spores. Developing, this green alga becomes a receptacle for 4–8 autospores, which grow inside the mother's organism and become covered with their own membrane. 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 happens:
- sexual (gametes);
- asexual (spores);
- 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 sex cell with a haploid (ordinary) set of chromosomes. A zygote with a diploid (double) set is formed as a result of the union of two gametes. The zygote develops into a sporophyte whichproduces haploid spores. From the spores - the gametophyte, which is male and female.
For example, we can take an isosporous fern, which has two forms of individuals - the fern itself (sporophyte) and its growth (gametophyte). A sprout is the offspring of adult ferns. It exists for a very short period, but manages to give birth to a single large-leaved individual. The life cycle of a plant due to this feature of reproduction consists of an alternation of generations: from an adult fern to a growth and again to an adult fern.
Methods of reproduction
Most plants reproduce sexually. In this case, a new organism is formed from the zygote after fertilization and the union of gametes (syngamy). Parthenogenesis - reproduction without fertilization - also refers to the sexual method: the daughter organism is formed from an isogamete, which makes isogametes and spores related. Sexual reproduction is almost always combined with other methods - vegetative or asexual, since it itself is characterized by low productivity.
At the same time, this method and asexual reproduction are found in ferns, and in conjunction with the vegetative variant - in some algae. In seed plants, the formation of the germ cell occurs from one daughter zygote, as a result of which this process is more like reproduction than reproduction.
With asexual reproduction, zoospores are formed - cells without a cell wall, which in multicellular plants are in special sporangia, andimmobile cells - aplanospores. Independently, this method of reproduction is very rare in nature. It is usually combined with sexual or vegetative.
There are 2 types of spores: mitospores, which occur during asexual reproduction, and meiospores, which arise during sexual reproduction. Mitospores appear by mitosis, resulting in an individual similar to the mother. Meiospores are formed by meiosis during the germination of the zygote or in sporangia. Most plants are characterized by both methods of reproduction, due to which two different forms of individuals are obtained.
Vegetative propagation method
With the vegetative variant of reproduction, there is a division into akinetes - thick-walled cells. It consists in separating some part of it from the mother liquor - the brood bud or body. Some lower plants reproduce in this way, including sargasso, brown and red algae. Even flowering plants, such as duckweed, reproduce vegetatively. Some of them form brood buds that fall to the ground and take root there. Also, the buds can branch off and separate from the mother plant. In the angiospermous group of plants, the development of shoots underground from the rhizome is very common.
Propagation of plants
One of the final stages of reproduction is the propagation of plants. In nature, there can be 3 options for settling: embryos, spores and seeds. In extremely rare cases, spread can occur with the help of zygotes. More K. Linnaeus associated seed and spore distribution with myogamous and phanerogamous plants. The second type included a group of gymnosperms and angiosperms, and the first type included all other groups, including algae, mosses and ferns.
Methods of plant reproduction have come a long evolutionary path from vegetative to asexual and sexual. Now the division of plants into spore and seed plants is associated not with distribution, but with reproduction. The seed method stands out in a separate group, since it is considered a combination of reproduction by spores and gametes. Seed propagation includes several stages: the formation of zygotes, gametes, spores, embryos and seeds, as well as plant dispersal.
Alternation of generations
The life of plants in the form of two different generations can have different names: change of forms of development, alternation of generations, etc. The change of a large fern and a sprout in the case of an isosporous fern is an example of alternation of generations, marked by the phases of the adult state of individual forms. These two forms are so different in appearance that it is difficult to recognize the same plant in them. Fern growth is very difficult to see with the naked eye. In angiosperms, the analogue of the outgrowth is the embryo sac, which is extremely small and hidden in the depths of the flower. Among some groups of algae, these forms of individuals are similar in appearance, but completely differ in biological characteristics. Alternation of generations occurs 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 the sporophyte takes up most of the life cycle. Bryophyte plants are distinguished by the fact that the sporophyte develops inside the female genital organ and is in continuous connection with the gametophyte. In the case of leafy mosses, it looks like a spore box growing from the top of the gametophyte.
The rest of the higher plants have pronounced sporophytes, which are large and complex multicellular organisms with organs such as foliage, stems and root systems. Most of the plants that a person thinks of when talking about horsetails, 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 predominant form of flowering plants is the heterosporous sporophyte, which is a plant with leaves and a stem. The male gametophyte is represented by a pollen grain, and the female gametophyte is represented by an embryo sac (it develops faster than in gymnosperms). The organ of both sexual and asexual reproduction is a modified shoot - a flower. The rudiments of seeds are protected by the walls of the ovary. The life cycle of the development of plants of this group ends after fertilization and the formation of a seed, the embryo in which has a supply of nutrients and does not depend onexternal factors.
Life cycles of gymnosperms and angiosperms
The group of gymnosperms includes representatives of coniferous trees and shrubs. Most of them have modified needle-like leaves. The life cycle of gymnosperms differs in that microspores (pollen) are formed in small male cones (anthers), and megaspores - in female (ovules). The male gametophyte is formed from the microspores, and the female gametophyte from the megaspore. The life cycle of a plant from this group differs in that fertilization occurs with the help of wind, which delivers pollen to the ovules. After that, an embryo begins to develop inside the ovule, and a seed is formed from it. It lies on the seed scales and is not covered by anything. The seed produces a new sporophyte, from which a new plant grows.
The life cycle of angiosperms differs in that this group has a flower in which spores are formed and fertilization of gametophytes and seed development occur. The peculiarity of this group is in the protection of seeds, which are hidden inside the fruit and protected from the effects of the external environment.
Life cycle of spore plants
Spore plants do not bloom, therefore they are also called non-flowering. They come in two categories:
- higher (ferns, horsetails, mosses, club mosses);
- lower (algae, lichens).
The life cycles of spore plants, depending on the species, can go sexually or asexually. They are notable to reproduce sexually without the participation of the aquatic environment. The gametophyte is used for sexual reproduction, and the sporophyte is used for asexual reproduction. 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 differs 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 - filamentous rhizoids. They grow in marshy, damp places and evaporate moisture very strongly. They reproduce sexually, the sporophyte depends on the gametophyte, the spores are formed in a special box that is located above the gametophyte and is associated with it.
Representatives of ferns have large pinnate leaves (sporangia are located on the underside). The plant has a pronounced root system, and the leaf is actually a branch system called a frond or preshoot. The life cycle of a plant of the fern group consists of two phases: sexual and asexual.
The sexual phase occurs with the participation of gametes, and asexual - spores. The asexual generation begins with a diploid zygote, and the sexual generation begins with a haploid spore. The change of these phases is the major partloop.
