Omar Bennett
The Kingdom Plantae is a large and varied group of organisms, with over 300,000 species of plants, including mosses, ferns, conifers, and flowering plants. It is the second-highest taxonomic rank, just below domain, and is contained within the supergroup Archaeplastida, which also includes red and green algae. The kingdom has been classified into five subgroups, with some debate over whether all algae should be considered plants. Modern angiosperms (flowering plants) appear to be a monophyletic group, meaning they originate from a single ancestor, but there is no consensus on how many kingdoms exist, and some traditional kingdoms are not monophyletic.
| Characteristics | Values |
|---|---|
| Number of catalogued species | 300,000+ |
| Number of seed plants | 260,000+ |
| Members | Mosses, ferns, conifers, flowering plants, etc. |
| Type of organisms | Large and varied |
| Type of cells | Eukaryotic |
| Number of cells | Multicellular |
| Body structure | Differentiated tissues |
| Photosynthesis | Yes |
| Chlorophyll | Yes |
| Chloroplasts | Yes |
| Cell walls | Yes, primarily composed of cellulose |
| Reproduction | Sexual and asexual |
| Growth | Indeterminate |
| Ancestor | Single |
| Classification | Five or six subgroups |
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What You'll Learn
The kingdom Plantae is composed of four evolutionarily related groups
Bryophytes are the closest extant relatives of early terrestrial plants. They include mosses, hornworts, and liverworts, and require water for sexual reproduction. They generally lack lignin and do not have tracheids (xylem cells specialised for forming conduits inside the plant to conduct water).
Seedless vascular plants include ferns, horsetails, club mosses, and whisk ferns. They possess a vascular system and a well-differentiated body structure.
Gymnosperms are cone-bearing seed plants that include conifers, cycads, Ginkgo, and gnetophytes. Like angiosperms, they bear seeds, but these are not encased within a fruit.
Angiosperms are flowering seed plants that include grasses, lilies, magnolia trees, laurels, and water lilies. They are classified in a single phylum: the Anthophyta. Modern angiosperms appear to be a monophyletic group, meaning they originate from a single ancestor.
While the four groups are primarily divided by differences in reproductive strategy, all land plants share a reproductive phenomenon known as the Alternation of Generations, characterised by a reproductive cycle with both multicellular diploid and haploid stages.
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Angiosperms are a monophyletic group
Angiosperms are distinct from other plants in that they enclose their ovules in carpellary tissue, and pollen tubes grow through this tissue to the ovules. They also have a more complex set of conducting tissues than gymnosperms. The water-conducting tissue (xylem) in angiosperms includes long tubes called vessels, which are only found in one small group of gymnosperms, the Gnetophyta. Angiosperms also have companion cells in their food-conducting tissue (phloem) that are directly related to the sieve tubes through which conduction occurs. This is in contrast to gymnosperms, which have less specialized sieve cells and lack companion cells.
The female gametophyte of angiosperms, called the embryo sac, is tiny and contains only a few nuclei. During sexual reproduction, one of these nuclei serves as the egg, uniting with one of the two sperm nuclei delivered by the pollen tube. This process, known as double fertilization, is unique to angiosperms and is a key feature of the group.
The evolution of angiosperms included the development of seeds covered with a carpel, a reduced female gametophyte, double fertilization, and the presence of vessels as a water-conducting system. Angiosperms did not become widespread until the early Tertiary period, coinciding with the radiation of modern birds and mammals. The evolution of large, colourful fruits and seeds is associated with this group. Angiosperms are also characterized by the presence of syringyl lignins, which play a key role in plant physiology.
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Bryophytes are the closest relatives of early terrestrial plants
The kingdom Plantae constitutes a large and varied group of organisms. All are multicellular, photosynthetic, and have differentiated tissues. The kingdom is composed of four evolutionarily related groups: bryophytes (mosses), seedless vascular plants, gymnosperms (cone-bearing seed plants), and angiosperms (flowering seed plants).
Bryophytes are a group of non-vascular land plants that include liverworts, hornworts, and mosses. They are the closest living relatives of early terrestrial plants. The first bryophytes (liverworts) likely appeared in the Ordovician period, around 450 million years ago. Bryophytes are characteristically limited in size and prefer moist habitats, although some species can survive in drier environments. They constitute a major part of the biodiversity in moist forests, wetlands, mountains, and tundra ecosystems.
Bryophytes are significant components of the vegetation in many regions of the world. They offer microhabitats that are critical to the survival of a diverse range of organisms, including single-celled eukaryotes, protozoa, and numerous groups of invertebrates. These plants are also important environmental indicators and have been used to predict past climate change, validate climate models, and indicate potential global warming.
Bryophytes are distinguished by their lack of vascular structure, although this distinction is problematic as some early non-bryophytes lacked true vascular tissue, and some mosses have well-developed water-conducting vessels. A more useful distinction lies in the structure of their sporophytes. In bryophytes, the sporophyte is a simple, unbranched structure with a single spore-forming organ, while in all other land plants, the sporophyte is branched and carries many sporangia.
Molecular phylogenetic studies have concluded that bryophytes are the earliest diverging lineages of extant land plants, providing insights into the migration of plants from aquatic environments to land. Several physical features link bryophytes to both land and aquatic plants, including the presence of chlorophyll a and b, and similar chloroplast structures.
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The kingdom Plantae is contained within the supergroup Archaeplastida
The Archaeplastida is a major group of eukaryotes, comprising photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. The name "Plantae" is considered ambiguous, as it has also been applied to less inclusive clades such as Viridiplantae and embryophytes. To distinguish the larger group, it is sometimes referred to as Plantae sensu lato, meaning "plants in the broad sense".
The kingdom Plantae constitutes a large and varied group of organisms, all of which are eukaryotic, multicellular with differentiated tissues, and photosynthetic. There are more than 300,000 species of cataloged plants, with over 260,000 being seed plants. Mosses, ferns, conifers, and flowering plants are all members of the plant kingdom.
The plant kingdom, as currently conceived, contains 10 clades recognized at the rank of phylum. The first five phyla have "free-sporing" life cycles, while the second five divisions are seed plants in which the spores are retained and develop into gametophytes within the sporophyte tissue.
The concept of "what is a plant" has changed significantly over the last century. By the mid-twentieth century, the algal groups ("thallophytes"), fungi, and bacteria were placed elsewhere, with the remaining group, the embryophytes, being the sole recipients of the title "plant." For the past five decades, the embryophytes have been placed in their own kingdom, Plantae, and nest within the Viridiplantae clade of the Archaeplastida.
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The kingdom Plantae is a large and varied group of organisms
The plant kingdom is composed of four evolutionarily related groups: bryophytes (mosses), seedless vascular plants, gymnosperms (cone-bearing seed plants), and angiosperms (flowering seed plants). Bryophytes are the closest extant relatives of early terrestrial plants, and they first appeared in the Ordovician period, about 450-490 million years ago. By the Silurian period, vascular plants had spread through the continents, indicating that non-vascular plants must have preceded this period. Today, more than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They are the major flora of inhospitable environments like the tundra, where their small size and tolerance to desiccation offer distinct advantages.
Gymnosperms include plants that possess a vascular system and a well-differentiated body structure. They bear seeds like angiosperms, but they are not encased within a fruit. Angiosperms are classified in a single phylum: the Anthophyta. Modern angiosperms appear to be a monophyletic group, which means that they originate from a single ancestor. Flowering plants are divided into two major groups: monocots, which include grasses and lilies, and eudicots or dicots, which form a polyphyletic group. Basal angiosperms are believed to have branched off before the separation into monocots and eudicots, as they exhibit traits from both groups.
There are differing opinions on whether all algae should be considered plants. Some scientists consider all algae to be plants, while others argue that only the Charophytes belong in the kingdom Plantae. This is due to the different evolutionary paths to photosynthesis in different types of algae. While all algae are photosynthetic, they did not all become photosynthetic via the same path. The ancestors of green algae became photosynthetic by engulfing a photosynthetic bacterium about 1.65 billion years ago. This algal line evolved into the Charophytes, and eventually into modern mosses, ferns, gymnosperms, and angiosperms. Their evolutionary trajectory was relatively straight and monophyletic. In contrast, other types of algae became photosynthetic through secondary or tertiary endosymbiotic events.
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Frequently asked questions
Kingdom Plantae is the second-highest taxonomic rank, just below domain. It is a large and varied group of organisms, with more than 300,000 species of catalogued plants, of which about 260,000 are plants that produce seeds. Mosses, ferns, conifers, and flowering plants are all members of the plant kingdom.
Kingdom Plantae is composed of four evolutionarily related groups: bryophytes (mosses), gymnosperms (cone-bearing seed plants), angiosperms (flowering seed plants), and thallophytes. Bryophytes are non-vascular plants and the closest extant relatives of early terrestrial plants. Gymnosperms include plants with a vascular system and a well-differentiated body structure. Angiosperms are flowering plants that are classified in a single phylum: the Anthophyta. Thallophytes lack a well-differentiated body structure and have a plant body that is thallus-like.
Modern angiosperms appear to be a monophyletic group, meaning they originate from a single ancestor. However, there is debate on how to classify the various groups within the plant kingdom, and some sources state that not all traditional kingdoms are monophyletic.
