Typical plant cells are eukaryotic (they have a nucleus), autotrophic (they produce their own food) and photosynthetic (they use light to make food). Plant cells also have chloroplasts and a cell wall (a structure exterior to the plasma membrane) made of cellulose.
While plant cells are eukaryotic, autotrophic, photosynthetic and have chloroplasts and a cell wall, animal cells are eukaryotic, heterotrophic and do not have chloroplasts nor a cell wall.
Plants have specialized organs (such as reproductive organs, roots, branches, leaves) and differentiated tissues (vascular tissue in tracheophytes, support tissue, parenchyma, etc.)
The kingdom Plantae is divided into two large subkingdoms: bryophytes and tracheophytes (pteridophytes, gymnosperms and angiosperms). The criterion for the division is the presence or not of conducting (vascular) tissue.
Bryophytes are nonvascular plants (mosses, liverworts, hornworts), meaning that they do not have a conducting system for the transport of sugar, water and nutrients. Tracheophyte plants are vascular plants, meaning that they have conducting structures.
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In botany, the plant kingdom is divided into bryophytes, pteridophytes, gymnosperms and angiosperms.
Cryptogamic (hidden sex organs) plants do not present flowers or seeds. They include the bryophytes and pteridophytes.
Phanerogamic plants have seeds. They include the gymnosperms and angiosperms.
Angiosperms are divided into monocotyledonous and dicotyledonous angiosperms. (These categories are explained later in the angiosperms section.)
Sexual reproduction may take place through three different types of life cycles: the haplontic haplobiontic cycle (the organism is haploid, a single type of organism) ; the diplontic haplobiontic cycle (the organism is diploid , a single type of organism); and the diplobiontic cycle (two types of organisms, one haploid and the other diploid). The diplobiontic cycle is known as the alternation of generations, or metagenesis. In humans, the cycle is diplontic haplobiontic (a single diploid organism).
Zygotic meiosis occurs in the haplontic haplobiontic life cycle. Gametes from adult haploid individuals unite to form the diploid zygote. The zygote undergoes meiosis and generates four haploid cells that develop into adult individuals via mitosis. Therefore, in zygotic meiosis, the cell that undergoes meiosis is the zygote and the gametes are formed by mitosis.
Gametic meiosis is when meiosis produces gametes, or rather, haploid cells which can each unite with another gamete to form the zygote. It occurs in the diplontic haplobiontic life cycle (in humans, for example), in which the individual is diploid and meiosis forms gametes.
Sporic meiosis happens in metagenesis (the alternation of generations, or diplobiontic life cycle). In this life cycle, cells from the diploid individual (called a sporophyte) undergo meiosis, producing haploid spores that do not unite with others but instead develop by mitosis into haploid individuals (called gametophytes). In this life cycle, the gametes are produced via mitosis from cells of the gametophyte.
In the plant life cycle (diplobiontic life cycle) and in the haplontic haplobiontic life cycle, gametes are made via mitosis and not via meiosis. Obviously, in some stage of these sexual life cycles, meiosis must occur.
The plant life cycle is known as the alternation of generations because, in this cycle, there are two different forms of living organisms that alternate with each other, one of which is haploid and the other of which is diploid. The alternation of generations is also called the diplobiontic cycle or metagenesis, and it does not just occur in plants. Other living organisms, such as cnidarians, go through this cycle.
In the haplontic haplobiontic life cycle, the single and lasting form is haploid. In the diplontic haplobiontic life cycle, it is diploid. In the diplobiontic life cycle, the lasting individual, which alternates with the intermediate form, may be the haploid gametophyte (as in bryophytes) or the diploid sporophyte (as in pteridophytes).
There are asexual forms of reproduction in plants. Naturally detached pieces of roots, limbs or leaves may develop into another adult specimen. The artificial asexual reproduction of plants can be carried out by means of grafting or cutting.
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