Encyclopedia
Plants are a major group of living things including familiar
organisms such as
trees,
flowers,
herbs,
ferns, and
mosses. About 350,000 species of plants, defined as
seed plants,
bryophytes,
ferns and fern allies, have been estimated to exist. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering and 15,000 bryophytes.
Aristotle divided all living things between plants, which generally do not move, and animals. In
Linnaeus' system, these became the Kingdoms Vegetabilia and
Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the
fungi and several groups of
algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Indeed, any attempt to match "plant" with a single taxon is doomed to fail, because plant is a vaguely defined concept unrelated to the presumed phylogenic concepts on which modern taxonomy is based.
Embryophytes
- Main article: Embryophytes
Most familiar are the multicellular land plants, called
embryophytes. They include the vascular plants, plants with full systems of
leaves, stems, and
roots. They also include a few of their close relatives, often called
bryophytes, of which
mosses and liverworts are the most common.
All of these plants have
eukaryotic cells with
cell walls composed of
cellulose, and most obtain their energy through
photosynthesis, using
light and
carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Plants are distinguished from
green algae, which represent a mode of photosynthetic life similar to the kind modern plants are believed to have evolved from, by having specialized reproductive organs protected by non-reproductive tissues.
Bryophytes first appeared during the early Palaeozoic. They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte.
Vascular plants first appeared during the Silurian period, and by the
Devonian had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small.
The first primitive seed plants, Pteridosperms and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a
seed, which develops while on the parent plant, and with fertilisation by means of
pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions.
Early seed plants are referred to as gymnosperms , as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the
conifers, which are dominant
trees in several biomes. The angiosperms, comprising the
flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic and diversifying rapidly during the Cretaceous. These differ in that the seed embryo is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today.
Algae and fungi
The
algae comprise several different groups of organisms that produce energy through photosynthesis. However, they are not classified within the Kingdom Plantae but mostly in the Kingdom Protista. Most conspicuous are the
seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the
green,
red, and
brown algae. These and other algal groups also include various single-celled organisms.
The embryophytes developed from green algae; the two groups are collectively referred to as the green plants or Viridiplantae. The Kingdom Plantae is often taken to mean this
monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing
cellulose and
chloroplasts containing
chlorophylls
a and
b, and store food in the form of starch. They undergo closed
mitosis without
centrioles, and typically have
mitochondria with flat cristae.
The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of the
red algae, and the two groups are generally believed to have a common origin . In contrast, most other algae have chloroplasts with three or four membranes. They are not close relatives of the green plants, presumably in origin acquiring chloroplasts separately from ingested or symbiotic green and red algae.
Unlike embryophytes and algae,
fungi are not photosynthetic, but are saprotrophs: obtaining food by breaking down and absorbing surrounding materials. Most fungi are formed by microscopic structures called hyphae, which may or may not be divided into cells but contain eukaryotic
nuclei. Fruiting bodies, of which
mushrooms are most familiar, are the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of
animals. Therefore, the
fungi are in a kingdom of their own.
Importance
The study of plant uses by people is termed economic botany or ethnobotany. They are often used as synonyms but some consider economic botany to focus mainly on uses of modern cultivated plants, while ethnobotany studies uses of indigenous plants by native peoples. Human cultivation of plants is part of
agriculture, which is the basis of human civilization. Plant agriculture is subdivided into agronomy, horticulture and
forestry.
Food
Virtually all human nutrition depends on land plants, directly or indirectly. The animals people eat are mainly
herbivores. Much of human nutrition depends on
cereals, especially
corn,
wheat and
rice or other staple crops such as
potato,
cassava, and
legumes. Other plants that are eaten include
fruits,
vegetables, nuts,
herbs,
spices and
edible flowers. Beverages from plants include
coffee,
tea,
wine,
beer and
alcohol.
Sugar is obtained mainly from
sugar cane and
sugar beet. Cooking oils and
margarine come from from corn,
soybean,
canola,
safflower,
sunflower,
olive and others. Food additives include
gum arabic, guar gum,
locust bean gum, starch and pectin.
Nonfood products
Wood is used for buildings, furniture, paper, cardboard, musical instruments and sports equipment. Cloth is often made from
cotton,
flax or synthetic fibers derived from
cellulose, such as
rayon and acetate. Renewable fuels from plants include
firewood,
peat and many other
biofuels.
Coal and
petroleum are fossil fuels derived from plants. Medicines derived from plants include
aspirin,
taxol,
morphine,
quinine,
reserpine,
colchicine,
digitalis and
vincristine. There are hundreds of herbal supplements such as
ginkgo,
Echinacea,
feverfew, and
Saint John's wort. Pesticides derived from plants include
nicotine,
rotenone,
strychnine and
pyrethrins. Illegal drugs from plants include
opium,
cocaine and
marijuana. Poisons from plants include
ricin,
hemlock and
curare. Plants are the source of many natural products such as fibers, essential oils, dyes, pigments, waxes, tannins, latex, gums, resins, alkaloids, amber and cork. Products derived from plants include soaps, paints, shampoos, perfumes, cosmetics, turpentine, rubber, varnish, lubricants, linoleum, plastics, inks, chewing gum and hemp rope. Plants are also a primary source of basic chemicals for the industrial synthesis of a vast array of organic chemicals.
Aesthetic uses
Thousands of plant species are cultivated to beautify the human environment as well as to provide shade, modify temperatures, reduce windspeed, abate noise, provide privacy and prevent soil erosion. People use cut flowers, dried flowers and house plants indoors. Outdoors, they use lawngrasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials and bedding plants. Images of plants are often used in art, architecture, humor,
language and photography and on textiles, money, stamps, flags and coats of arms. Living plant art forms include
topiary,
bonsai,
ikebana and
espalier. Ornamental plants have sometimes changed the course of history, as in
tulipomania. Plants are the basis of a multi-billion dollar per year tourism industry which includes travel to
arboretums,
botanical gardens, historic gardens,
national parks,
tulip festivals,
rainforests,
forests with colorful autumn leaves and the
National Cherry Blossom Festival. Venus flytrap,
sensitive plant and resurrection plant are examples of plants sold as novelties.
Scientific and cultural uses
Tree rings are an important method of dating in archeology and serve as a record of past climates. Basic biological research has often been done with plants, such as the pea plants used to derive
Gregor Mendel's laws of genetics. Space stations or space colonies may one day rely on plants for life support. Plants are used as national and state emblems, including
state trees and
state flowers. Ancient trees are revered and many are famous. Numerous world records are held by plants. Plants are often used as memorials, gifts and to mark special occasions such as births, deaths, weddings and holidays. Plants figure prominently in mythology, religion and literature. The field of ethnobotany studies plant use by indigenous cultures which helps to conserve endangered species as well as discover new
medicinal plants.
Gardening is the most popular leisure activity in the U.S. Working with plants or horticulture therapy is beneficial for rehabilitating people with disabilities.
Negative effects
Weeds are plants growing where people do not want them. They cause billions of dollars in crop losses annually. People have spread plants far beyond their native ranges. Many introduced plants are
invasive in areas where they are not native. Invasive plants are often major weeds. They also crowd out native species and damage or destroy existing ecosystems. Many people suffer from hay fever caused by pollen allergies. A wide variety of plants are poisonous. Several plants cause skin irritations when touched, especially
poison ivy. Use of
tobacco products are a major cause of several diseases such as
lung cancer and heart disease.
Growth
It is a common misconception that most of the solid material in a plant is taken from the soil, when in fact almost all of it is actually taken from the atmosphere. Through a process known as
photosynthesis, plants use the energy in
sunlight to convert carbon dioxide from the atmosphere into simple
sugars. These sugars are then used as building blocks and form the main structural component of the plant. Plants rely on soil primarily for water , but also obtain
nitrogen,
phosphorus and other crucial elemental nutrients. For the majority of plants to grow successfully they also require oxygen in the atmosphere and oxygen around their roots. A few specialised vascular plants, such as
Mangroves, can however grow with their roots in anoxic conditions
Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Other plants may be organized according to their seasonal growth pattern:
- Annual: live and reproduce within one growing season.
- Biennial: live for two growing seasons; usually reproduce in second year.
- Perennial: live for many growing seasons; continue to reproduce once mature.
Among the vascular plants, perennials include both
evergreens that keep their leaves the entire year, and
deciduous plants which lose their leaves for some part. In temperate and boreal climates, they generally lose their leaves during the winter; many
tropical plants lose their leaves during the dry season.
The growth rate of plants is extremely variable. Some mosses grow less than 0.001 mm/h, while most trees grow 0.025-0.250 mm/h. Some climbing species, such as
kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h.
Plants protect themselves from
frost and dehydration stress with
antifreeze proteins, heat-shock proteins and sugars . LEA protein expression is induced by stresses and protects other proteins from aggregation as a result of
desiccation and freezing.
Ecological relationships
The
photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis radically changed the composition of the early Earth's 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. Many animals rely on plants for shelter as well as oxygen and food.
Land plants are key components of the
water cycle and several other
biogeochemical cycles. Some plants have
coevolved with
nitrogen fixing bacteria, making plants an important part of the
nitrogen cycle. Plant roots play an essential role in
soil development and prevention of
soil erosion. The Earth's biomes are named for the type of vegetation because plants are the dominant organisms in biomes.
Numerous animals have coevolved with plants. Many animals
pollinate flowers in exchange for food in the form of pollen or
nectar. Many animals
disperse seeds, often by eating
fruit and passing the seeds in their feces. 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 provide organic
fertilizer.
The majority of plant species have various kinds of fungi associated with their root systems in a kind of
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 does tremendous economic damage to the cattle industry in the U.S.
Various forms of parasitism are also fairly common among plants, 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, so have no chlorophyll. Some plants, known as
myco-heterotrophs, parasitize mycorrhizal fungi, and hence act as
epiparasites on other plants.
Many plants are
epiphytes, meaning they grow on other plants, usually trees, without parasitizing them. Epiphytes may indirectly harm their host plant by intercepting mineral nutrients and light that the host would otherwise receive. The weight of large numbers of epiphytes may break tree limbs. Many
orchids,
bromeliads,
ferns and
mosses often grow as epiphytes. Bromeliad epiphytes accumulate water in leaf axils to form phytotelmata, complex aquatic food webs.
A few plants are
carnivorous, such as
Venus flytrap and
sundew. They trap small animals and digest them to obtain mineral nutrients, especially nitrogen.
Fossils
Plant
fossils include roots, wood, leaves, seeds, fruit,
pollen,
spores, phytoliths, and
amber . Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments.
