Kingdom Plantae

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Plants are predominantly photosynthetic eukaryotes, forming the kingdom Plantae. Plantae is the kingdom of living things. In different biological boundary systems, the concept of plants and the groups they include are also different. For example, when organisms are divided into two kingdoms, plants and animals, the plant kingdom includes algae, lichens, mosses, ferns and seed plants; In the kingdom system, the plant kingdom includes only multicellular photoautotrophic groups, while fungi, lichens and unicellular algae and prokaryotic cyanobacteria are not included. (Note: Although some saprophytic plants such as Gastrodia elata and crystal orchid cannot photosynthesize and belong to heterotrophs, they still belong to the plant kingdom.)

The main difference between the plant kingdom and other biological groups is that it contains chlorophyll, can carry out photosynthesis, and can produce organic matter by itself; in addition, most of them live in a fixed environment and cannot move freely (except for a small number of lower algae). With a cell wall; the cells are totipotent, that is, a plant cell can be cultivated into a plant body, etc.

Plants cover most of the earth's land surface, and do so in oceans, lakes, rivers, and ponds. Their size and lifespan vary greatly, from tiny algae invisible to the naked eye to giant algae in the ocean and the huge "world master" (North American redwood) on land that has lived for thousands of years. Plants are almost the only primary producers in various large and small ecosystems in the natural biosphere. The relationship between plants and humans is extremely close, and it is the basis for the survival of humans and other living things.

There are about 380,000 known species of plants, of which the majority, some 260,000, produce seeds. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems. Grain, fruit, and vegetables are basic human foods and have been domesticated for millennia. Plants have many cultural and other uses, such as ornaments, building materials, writing materials, and, in great variety, they have been the source of medicines. The scientific study of plants is known as botany, a branch of biology.

noun

A taxonomic group comprised of plants, particularly land plants and green algae

Supplement

Plantae is a taxonomic group that includes land plants and green algae. In the older classification of organisms, there are basically five kingdoms according to Robert Whittaker: Animalia, Plantae, Fungi, Protista, and Monera. Kingdom Plantae includes multicellular, (mostly) autotrophic eukaryotes that (usually) conduct photosynthesis. Kingdom is formerly the highest taxonomic rank or the most general taxon used in classifying organisms. Newer classification scheme though such as that introduced by Carl Woese includes three domains. In this scheme, domain is the most general taxon and kingdom is only next to the hierarchy. Recent classification scheme also placed Plantae as a subkingdom to the more inclusive, Kingdom Archaeplastida. Other subgroups under Kingdom Archaeplastida are red algae and glaucophytes.

All living things were traditionally placed into one of two groups, plants and animals. This classification dates from Aristotle (384–322 BC), who distinguished different levels of beings in his biology, based on whether living things had a "sensitive soul" or like plants only a "vegetative soul". Theophrastus, Aristotle's student, continued his work in plant taxonomy and classification. Much later, Linnaeus (1707–1778) created the basis of the modern system of scientific classification, but retained the animal and plant kingdoms.

plants are thought to have evolved from an aquatic green alga protist. Later, they evolved important adaptations for land, including vascular tissues, seeds, and flowers. Each of these major adaptations made plants better suited for life on dry land and much more successful.

The Earliest Plants

The earliest plants were probably similar to the stonewort, an aquatic algae pictured in Figure below. Unlike most modern plants, stoneworts have stalks rather than stiff stems, and they have hair-like structures called rhizoids instead of roots. On the other hand, stoneworts have distinct male and female reproductive structures, which is a plant characteristic. For fertilization to occur, sperm need at least a thin film of moisture to swim to eggs. In all these ways, the first plants may have resembled stoneworts.

Life on Land

By the time the earliest plants evolved, animals were already the dominant organisms in the ocean. Plants were also constrained to the upper layer of water that received enough sunlight for photosynthesis. Therefore, plants never became dominant marine organisms. But when plants moved onto land, everything was wide open. Why was the land devoid of other life? Without plants growing on land, there was nothing for other organisms to feed on. Land could not be colonized by other organisms until land plants became established.

Plants may have colonized the land as early as 700 million years ago. The oldest fossils of land plants date back about 470 million years. The first land plants probably resembled modern plants called liverworts.

Colonization of the land was a huge step in plant evolution. Until then, virtually all life had evolved in the ocean. Dry land was a very different kind of place. The biggest problem was the dryness. Simply absorbing enough water to stay alive was a huge challenge. It kept early plants small and low to the ground. Water was also needed for sexual reproduction, so sperm could swim to eggs. In addition, temperatures on land were extreme and always changing. Sunlight was also strong and dangerous. It put land organisms at high risk of mutations.

Vascular Plants Evolve

Plants evolved a number of adaptations that helped them cope with these problems on dry land. One of the earliest and most important was the evolution of vascular tissues. Vascular tissues form a plant’s “plumbing system.” They carry water and minerals from soil to leaves for photosynthesis. They also carry food (sugar dissolved in water) from photosynthetic cells to other cells in the plant for growth or storage. The evolution of vascular tissues revolutionized the plant kingdom. The tissues allowed plants to grow large and endure periods of drought in harsh land environments. 

In addition to vascular tissues, these early plants evolved other adaptations to life on land, including lignin, leaves, roots, and a change in their life cycle.

• Lignin is a tough carbohydrate molecule that is hydrophobic (“water fearing”). It adds support to vascular tissues in stems. It also waterproofs the tissues so they don’t leak, which makes them more efficient at transporting fluids. Because most other organisms cannot break down lignin, it helps protect plants from herbivores and parasites.
• Leaves are rich in chloroplasts that function as solar collectors and food factories. The first leaves were very small, but leaves became larger over time.
• Roots are vascular organs that can penetrate soil and even rock. They absorb water and minerals from soil and carry them to leaves. They also anchor a plant in the soil. Roots evolved from rhizoids, which nonvascular plants had used for absorption.
• Land plants evolved a dominant diploid sporophyte generation. This was adaptive because diploid individuals are less likely to suffer harmful effects of mutations. They have two copies of each gene, so if a mutation occurs in one gene, they have a backup copy. This is extremely important on land, where there’s a lot of solar radiation.

With all these advantages, it’s easy to see why vascular plants spread quickly and widely on land. Many nonvascular plants went extinct as vascular plants became more numerous. Vascular plants are now the dominant land plants on Earth.

Kingdom Plantae is broadly composed of four evolutionarily related groups: bryophytes (mosses), (seedless vascular plants), gymnosperms (cone bearing seed plants), and angiosperms (flowering seed plants). These groups share features such as the production of embryos, photosynthetic chloroplasts, and cell walls primarily composed of cellulose. A variety of reproductive strategies can be found in plants, both sexual and asexual, often with more than one strategy utilized by a single species. While the aforementioned groups are primarily divided by differences in reproductive strategy, all land plants share a reproductive phenomenon named the Alternation of Generations. This is characterized by a reproductive cycle that has both a multicellular diploid (2 n) as well as a multicellular haploid (1 n) stage. In this cycle the sporophyte (multicellular diploid) undergoes meiosis to produce haploid cells called spores. These spores then divide by mitosis to form the multicellular haploid gametophyte, which will in turn produce the male and female gametes. Unlike in animals, gametes are produced by mitosis in plants since the gametophyte is already haploid. These gametes fuse to produce a diploid zygote, which will then undergo mitotic divisions leading back to the sporophyte stage.

The ancestors of land plants were aquatic green algae and the transition to life in terrestrial environments presented a number of challenges not least in regard to reproduction. Indeed novel reproductive structures that enhance desiccation tolerance and remove the need for water in gamete transfer are predominant characteristics in the classification of these organisms. In addition, throughout the evolution of land plants the haploid gametophyte stage of the life cycle has been progressively reduced, from being the dominant life stage in bryophytes, to only 3–9 cells in angiosperms (flowering plants). This trend towards dominance of the diploid sporophyte and reduction of the haploid gametophyte life stages is often interpreted as being an adaptation to the substantially higher levels of mutagenic UV light in terrestrial than aquatic environments, favoring diploids which inherently possess a “back-up” set of genetic material.View chapterPurchase book

Dioecy is much less prevalent in plants than in animals. Approximately 96% of species within the kingdom Plantae are flowering seed plants (angiosperms; Roskov et al., 2014) and only about 6% of angiosperm species are dioecious (Renner and Ricklefs, 1995; Barrett, 2002; Vamosi et al., 2003). Among the remaining seed plants (gymnosperms) 50% of species are dioecious, including 37% of conifers (Givnish, 1980; Bateman et al., 2011). Among spore-forming plants, dioecy is very rare in ferns (Jesson and Garnock-Jones, 2012) but relatively common in bryophytes (mosses, liverworts, and hornworts) where it occurs in 50–60% of species (Hedenas and Bisang, 2011; Jesson and Garnock-Jones, 2012; McDaniel et al., 2013).

In most dioecious plants, only reproductive structures distinguish the sexes. These consist of primary reproductive organs (e.g., the stamens and pistils of angiosperm flowers) plus surrounding somatic tissues, often formed from modified stems or leaves (e.g., the calyx and corolla of angiosperms). Sexual dimorphisms in these somatic tissues are generally interpreted as adaptations in males for dispersing pollen or sperm and in females for capturing pollen or sperm and for protecting and provisioning embryos (Givnish, 1980; Eckhart, 1999; Geber, 1999; Barrett and Hough, 2013; McDaniel et al., 2013). These are the predominant secondary sexual dimorphisms in plants, and are most pronounced in species that depend on wind or water for pollen dispersal but produce large seeds or fruit that are dispersed by animals (Givnish, 1980; Renner and Ricklefs, 1995; Vamosi et al., 2003; Biernaskie, 2010; Bateman et al., 2011).

The secondary sexual trait most likely to differ between male and female reproductive structures is size. Flower size dimorphisms occur in more than 80% of diecious angiosperm species, with male flowers somewhat more likely to be larger than the reverse (Eckhart, 1999). In contrast, in dioecious gymnosperms the female reproductive organs (strobili) are larger and only the female strobili develop into the large, long-lived cones typical of conifers and cycads. In angiosperms, the number of flowers and the sizes of inflorescences (flower clusters) may also differ between sexes, with males more commonly having more flowers or larger flower clusters than females. In a few families, differences in flower shape and in the position or orientation of the flowers on the plant have also been described (Eckhart, 1999; Barrett and Hough, 2013).

Other than reproductive structures, the most prevalent SDM in plants is overall size. In both angiosperms and gymnosperms, males tend to be larger than females in large, long-lived, woody species (e.g., trees and shrubs), whereas females tend to be larger in small, short-lived, herbaceous species (Obeso, 2002; Barrett and Hough, 2013). Size dimorphism is most extreme in pleurocarpous mosses (class Bryophyta) where, in about a third of species, dwarf males develop from spores that land on the stems or leaves of females and remain attached to the female throughout their lives (Hedenas and Bisang, 2011).

Sexual dimorphisms in other aspects of plant vegetative morphology are much less common but have been noted in at least a few species. Examples include leaf size (females more commonly larger), leaf shape, stem size (females more commonly thicker), and branching architecture (males usually more branched) (Dawson and Geber, 1999; Kavanagh et al., 2011; Barrett and Hough, 2013). As in animals, male and female plants also often differ in ecological and life-history traits (Dudley, 2006; Geber et al., 1999; Barrett and Hough, 2013). The sexes are sometimes partially segregated by habitat, with females more restricted to sites with more water or nutrients, and males often exceed females in their capacity for clonal reproduction, reproduce at an earlier age, and senesce earlier than females. These differences reflect sex-specific trade-offs between growth and reproduction and, as in animals, are interpreted as adaptations for maximizing fitness through male or female reproductive functions.View chapterPurchase book

Distribution

Plants are distributed almost worldwide. While they inhabit several biomes which can be divided into a multitude of ecoregions, only the hardy plants of the Antarctic flora, consisting of algae, mosses, liverworts, lichens, and just two flowering plants, have adapted to the prevailing conditions on that southern continent.

Plants are often the dominant physical and structural component of the habitats where they occur. Many of the Earth's biomes are named for the type of vegetation because plants are the dominant organisms in those biomes, such as grassland, savanna, and tropical rainforest.

Primary producers

The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis, at first by cyanobacteria and later by photosynthetic eukaryotes, radically changed the composition of the early Earth's anoxic atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems. Plants form about 80% of the world biomass at about 450 gigatonnes (4.4×1011 long tons; 5.0×1011 short tons) of carbon.

Ecological relationships

Numerous animals have coevolved with plants; flowering plants have evolved pollination syndromes, suites of flower traits that favour their reproduction. Many, including insect and bird partners, are pollinators, visiting flowers and accidentally transferring pollen in exchange for food in the form of pollen or nectar.

Many animals disperse seeds that are adapted for such dispersal. Various mechanisms of dispersal have evolved. Some fruits offer nutritious outer layers attractive to animals, while the seeds are adapted to survive the passage through the animal's gut; others have hooks that enable them to attach to a mammal's fur. Myrmecophytes are plants that have coevolved with ants. The plant provides a home, and sometimes food, for the ants. In exchange, the ants defend the plant from herbivores and sometimes competing plants. Ant wastes serve as organic fertilizer.

The majority of plant species have fungi associated with their root systems in a mutualistic symbiosis known as mycorrhiza. The fungi help the plants gain water and mineral nutrients from the soil, while the plant gives the fungi carbohydrates manufactured in photosynthesis. Some plants serve as homes for endophytic fungi that protect the plant from herbivores by producing toxins. The fungal endophyte Neotyphodium coenophialum in tall fescue grass has pest status in the American cattle industry.

Many legumes have Rhizobium nitrogen-fixing bacteria in nodules of their roots, which fix nitrogen from the air for the plant to use; in return, the plants supply sugars to the bacteria. Nitrogen fixed in this way can become available to other plants, and is important in agriculture; for example, farmers may grow a crop rotation of a legume such as beans, followed by a cereal such as wheat, to provide cash crops with a reduced input of nitrogen fertilizer.

Some 1% of plants are parasitic. They range from the semi-parasitic mistletoe that merely takes some nutrients from its host, but still has photosynthetic leaves, to the fully-parasitic broomrape and toothwort that acquire all their nutrients through connections to the roots of other plants, and so have no chlorophyll. Full parasites can be extremely harmful to their plant hosts.

Plants that grow on other plants, usually trees, without parasitizing them, are called epiphytes. These may support diverse arboreal ecosystems. Some may indirectly harm their host plant, such as by intercepting light. Hemiepiphytes like the strangler fig begin as epiphytes, but eventually set their own roots and overpower and kill their host. Many orchids, bromeliads, ferns, and mosses grow as epiphytes. Among the epiphytes, the bromeliads accumulate water in their leaf axils; these water-filled cavities can support complex aquatic food webs.

Some 630 species of plants are carnivorous, such as the Venus flytrap (Dionaea muscipula) and sundew (Drosera species). They trap small animals and digest them to obtain mineral nutrients, especially nitrogen and phosphorus.

Competition

Competition for shared resources reduces a plant's growth. Shared resources include sunlight, water and nutrients. Light is a critical resource because it is necessary for photosynthesis. Plants use their leaves to shade other plants from sunlight and grow quickly to maximize their own expose. Water too is essential for photosynthesis; roots compete to maximize water uptake from soil. Some plants have deep roots that are able to locate water stored deep underground, and others have shallower roots that are capable of extending longer distances to collect recent rainwater. Minerals are important for plant growth and development. Common nutrients competed for amongst plants include nitrogen, phosphorus, and potassium.

1. ability to make its own food by photosynthesis, i.e. capable of capturing energy via the green pigment (chlorophyll) inside the chloroplast, and of using carbon dioxide and water to produce sugars as food and oxygen as byproduct
2. foods are stored in forms of sugars and starch.
3. presence of rigid cell walls apart from the cell membrane.
4. eukaryotic cells, i.e. the presence of a distinct nucleus surrounded by a membrane
5. mostly are multicellular, i.e. made up of many cells organized to perform a specific function as a unit
6. unlimited growth at meristems (when present).
7. organs are specialized for anchorage, support, and photosynthesis (e.g. roots, stems, leaves, etc.)
8. response to stimuli is rather slow due to the absence of sensory organs and nervous systems, as do animals
9. limited movements due to a lack of organs for mobility, as do animals
10. life cycle that involves both sporophytic and gametophytic phases (alternation of generation)