Plant life cycle: description, stages, schemes and features
The life cycle of a plant consists of three consecutive stages:
- origin;
- development;
- reproduction.
It can be simple and complex. As an example of a simple cycle, chlorella can be reproduced by spores. Developing, this green alga becomes the receptacle of 4-8 autospores, which grow inside the mother's body and are covered with its own shell. But among plants more often there is a complex development cycle, which consists of 2-3 simple ones.
Peculiarities of Plant Life Cycles
An important property of all living things is the ability to reproduce. The method of reproduction is:
- sexual (gametes);
- asexual (spores);
- vegetative (part of the body).
In complex cycles during sexual reproduction,there are several isolated phases of gametes and zygotes. The gamete is a mature sexual cell with a haploid (ordinary) set of chromosomes. Zygote with a diploid (double) set is formed by combining two gametes. A sporophyte develops from the zygote, which produces haploid spores. Of the dispute - gametophyte, which is male and female.
For example, one can take an equisporous fern,in which there are two forms of individuals - the fern itself (sporophyte) and its adipose (gametophyte). An adolescent is a progeny of adult ferns. He exists a very short period, but manages to produce the only large-leaved specimen. Plant Life Cycle because of this peculiarity of reproduction consists of alternation of generations: from an adult fern to a sprout and again to an adult fern.
Methods of reproduction
Sexually reproduces the majorityplants. This leads to the formation of a new organism from the zygote after fertilization and the combination of gametes (syngamy). Parthenogenesis - reproduction without fertilization - also refers to the sexual method: the daughter organism is formed from the isogamete, which makes relatives of isogametes and spores. Sexual reproduction is almost always combined with other methods - vegetative or asexual, because it itself is characterized by low productivity.
Simultaneously, this method and asexual reproductionoccurs in ferns, and in conjunction with a vegetative variant - in some algae. In seed plants, the formation of the reproductive cell occurs from one daughter zygote, as a result of which this process is more like 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 are aplanospores. This method of reproduction is very rare in nature. Usually it is combined with sexual or vegetative.
There are 2 types of spores: mitospores, which arise from asexual reproduction, and meiospores - arising from sexual reproduction. Mitospores appear with the help of mitosis, resulting in an individual resembling a maternal. Meiospores are formed through meiosis during germination of the zygote or in sporangia. For the majority of plants, both methods of reproduction are characteristic, thanks to which two different forms of individuals are obtained.
Vegetative mode of reproduction
When a vegetative variant of reproduction occursdivision into akinese - thick-walled cells. It consists in the separation from the queen of some part of it - brood kidney or calf. This way, some lower plants reproduce, including sargasso, brown and red algae. Even flowering plants, for example duckweed, grow even vegetatively. Some of them form brood kidneys that fall to the ground and take roots there. Also, the kidneys can branch off and separate from the mother plant. Angiosperm group of plants is very often found the development of shoots under the ground from the rhizome.
Distribution of plants
One of the final stages of reproduction isspread of plants. In nature there may be 3 variants of settlement: embryos, spores and seeds. In extremely rare cases, the spread can occur with the help of zygotes. Still, K. Linnaeus linked seed and spore distribution with cryptic and phanerogamous plants. To the second type belonged the group of gymnosperms and angiosperms, and to the first - all the other groups, including algae, moss and ferns.
The methods of reproduction of plants have passed a longevolutionary path from vegetative to asexual and sexual. Now the division of plants into spore and seminal is associated not with propagation, but with reproduction. The seed method is allocated to a separate group, since it is considered a compound of reproduction by spores and gametes. Seed reproduction includes several stages: the formation of zygotes, gametes, spores, embryos and seeds, as well as plant dispersal.
Alternation of generations
Life of plants in the form of two different generationscan have different names: the change of forms of development, the alternation of generations, etc. The replacement of a large fern and sprout in the case of an equisperse fern is an example of the 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 fern is very difficult to see with the naked eye. In angiosperms, the analogue of the sprout is an embryonic sac, which is extremely small and hidden in the depth of the flower. Among some groups of algae, these forms of individuals are similar in appearance, but completely differ in biological features. Generation alternation occurs in almost all higher plants and evolutionarily developed algae.
Life Cycles of Higher Plants
The life cycle of higher plants, except forbryophytes, is characterized by the fact that the gametophyte is poorly developed, and the sporophyte occupies the most part in the life cycle. Bryophytes differ in that the sporophyte develops within the female sexual organ and is in continuous connection with the gametophyte. In the case of leaf-stalked mosses, it looks like a box with spores growing from the upper part of the gametophyte.
The rest of the higher plants are brightexpressed sporophytes, which are large and complex multicellular organisms with organs such as foliage, stems and root system. Most of the plants that a person imagines when talking about horsetails, ferns or other groups are sporophytes.
Life cycles of flowering plants
The most progressive in terms of evolution areflowering plants. The life cycle of flowering plants is characterized by the fact that often the embryo is able to develop from an unfertilized ovule (apomixis). The predominant form of flowering is the sporophyte sporerophy, which is a plant with leaves and a stem. The male gametophyte is represented by the pollen grains, and the female gametophyte is represented by the embryo sac (it develops faster than the gymnosperms). The organ of both sexual and asexual reproduction is a modified shoot - a flower. The germs of seeds are protected by the walls of the ovary. The life cycle of plant development in this group ends after fertilization and the formation of a seed, the embryo in which has a supply of nutrients and does not depend on external factors.
Life cycles of gymnosperms and angiosperms
To the group of gymnosperms arerepresentatives 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 are formed in female (semyacchatkas). From the microspores a male gametophyte is formed, and from the megaspora - a female gametophyte. The life cycle of a plant from this group is distinguished by the fact that fertilization occurs with the help of wind, which delivers pollen to the ovules. After this, inside the ovule begins to develop the embryo, and from it a seed is formed. It lies on the seed scales and is not covered. The seed gives a new sporophyte, from which a new plant grows.
Life cycle of angiospermsdiffers in that this group has a flower in which spores are formed and the fertilization of gametophytes and the development of seeds take place. The peculiarity of this group is in the protection of seeds that are hidden inside the fetus and are protected from the effects of the external environment.
The life cycle of spore plants
Spore plants do not bloom, because they are also called non-flowering plants. They come in two categories:
- higher (ferns, horsetails, mosses, mosses);
- lower (algae, lichens).
The life cycles of spore plants infrom the species can go sexual or asexual. They can not reproduce sexually without the participation of the aquatic environment. For sexual reproduction is used gametophyte, and for asexual - sporophyte. 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 as a sprouting. The haploid subgroup is distinguished by the fact that the sporophyte has a subordinate status in it.
Life Cycles of Plants: Schemes
Mosses - representatives of a primitive species of higherplants. They have a very conditional division of the organism into a stem and leaves, instead of roots - filiform rhizoids. They grow in marshy, damp places and evaporate moisture very much. They reproduce sexually, sporophyte depends on gametophyte, spores are formed in a special box, which is located above the gametophyte and is associated with it.
Representatives of ferns have largepinnate leaves (sporangia located on the lower side). The plant has a pronounced root system, and the leaf is in fact a system of branches, called vaye or pre-bias. The life cycle of a plant of a group of ferns consists of two phases: sexual and asexual.
