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Plants are Living Organisms which are characterised by absence of voluntary movement. Plants are also characterised as Multicellular, Eukaryotic life forms with Six Fundamental Characteristics (mentioned below). Plants are however not restricted to these Characteristics, as some Plants are not green and thus do not produce their own food by Photosynthesis rather they are Parasitic on other living Plants. There are about 350,000 extant Species of Plants. The earliest Plants evolved from an aquatic, Green Algal ancestor. There are two groups of Green Algae, which are the closest relatives of Land Plants (embryophytes). The first of these groups is the Charophyta (desmids and stoneworts), from which the Embryophytes developed. The sister group to the combined Embryophytes and Charophytes is the other group of Green Algae, Chlorophyta, and this more inclusive group is collectively referred to as the Green Plants or Viridiplantae. At one time the Photosynthetic aquatic organisms commonly referred to as Algae were considered members of the Plant kingdom. The various major Algal groups, such as the Green Algae, Brown Algae, and Red Algae, are now placed in the kingdom Protista because they lack one or more of the features that are characteristic of Plants. The organisms known as Fungi were also once considered to be Plants since they reproduce by spores and possess a cell wall. The Fungi, however, uniformly lack Chlorophyll (heterotrophic) and are chemically distinct from the Plants; they are now placed in a separate kingdom, Fungi. Having evolved from aquatic ancestors, Plants have subsequently migrated over the entire surface of the Earth. Plants are a source of food for all the Animals and other Plants either directly or indirectly. Over a period of time Humans learned to grow Plants and soon they became a major part of their diet. Plant crops like maize, wheat and rice are major source of diet for Human populations all over the World. Other Human food includes vegetables, spices, fruits, nuts, herbs, and edible flowers. Beverages like coffee, tea, wine, beer and alcohol are also produced from Plants. Even other Animals like cows, pigs, sheep, goats etc. are dependant on Plants for food. Plants also provide wood which is used for various purposes.
Scientific classification
Domain - Eukaryota.
(unranked) - Archaeplastida.
Kingdom - Plantae.
There are at least Four Classification Systems which are in common use:
However, there is another classification which is based on Habitat but is less commonly used
5. Plants are also classified according to the habitat in which they are found
1. Classification according to Seed Structure
I) Seed Plants - The Plants that produce seeds are known as Seed Plants. They are also known as Spermatophytes or Phanerogams.
Seed Plants are further divided into:
1. Gymnosperms - Gymnosperm usually produces both a Female or seed cone and a Male or pollen cone. The seeds develop in the exposed upper surface of the Female cone and are fertilized by pollen that is released from the Male cone and carried by the wind. It consists of Plants that produce a Cone, such as Pines, Firs, Cedars and Redwoods. These are the Plants of the class Gymnospermae whose seeds are not enclosed in an ovary or fruit.
2. Angiosperms - Angiosperms are the Plants which have flowers and their seeds are enclosed in a protective shell called a 'Fruit'. Angiosperms are also known as 'Flowering Plants'. The Angiosperm group consists of over 250,000 Species of Flowering Plants, making it the most diverse of all Plant Phyla.
Angiosperms are divided into 2 categories. These terms derive from the number of seed leaves, or Cotyledons the Plants have upon germination.
II) Seedless Plants - The Plants that do not produce seeds are known as Seedless Plants. This Plant group reproduces by Spores. Plants such as Ferns have spores on the underside of their fronds. Ferns, which reproduce without seeds, contain approximately 13,000 Species. Other Plants include Ferns, Horsetails, Club Mosses etc.
2. Classification according to Tissue Structure
I) Vascular Plants - Vascular plants first appeared during the Silurian period. These Plants possess specialized supporting and water-conducting tissue, called Xylem, and food-conducting tissue, called Phloem. The Xylem is composed of non-living cells (tracheids and vessel elements) that are stiffened by the presence of Lignin, a hardening substance that reinforces the cellulose cell wall. The living sieve elements that comprise the Phloem are not lignified. Xylem and Phloem are collectively called Vascular Tissue and form a central column (stele) through the Plant axis. It is because they have well-developed Vascular System, they are also known as 'Vascular Plants'. The Ferns, Gymnosperms, and Flowering Plants are all Vascular Plants. Because they possess vascular tissues, these Plants have true stems, leaves, and roots. They are also known as Tracheophytes.
II) Non-Vascular Plants - These Plants lack specialized vascular tissue (xylem and phloem) for internal water and food conduction and support. Bryophytes are, therefore, known as Non-Vascular Plants as they possess no true roots, stems, or leaves. They are the most primitive group of Plants and comprise about 24,000 Species. Non-Vascular Plants include Moss, Liverworts, Hornworts etc.
3. Classification according to Stature
I) Cacti (Cactus) - Cactus Plants are well adapted to hot and dry weather by storing water in their succulent stems. They are also known for their spines, for which they are famous.
II) Biennials - A Plant that takes two years to grow from seed to fruition, bloom and die or a Plant that lasts two years is a Biennial Plant.
III) Perennials - Perennials are those Flowering Plants that last longer and keep on flowering. Most take two years until they are old enough to bloom. Examples are Bleeding Heart, Water Lily, Gerberas, Geraniums, Anthuriums etc. Once you Plant them there is no need for replanting year after year.
IV) Flowers - Flowers are the reproductive part of Angiosperms, also known as Flowering Plants.
V) Herbs - Herbs are used for culinary, medicinal and spiritual uses. In cuisine, the leaves of the herb are normally the only part used. All parts of herbs are used in various medical or spiritual practices.
VI) Shrubs and Bushes - Usually under 6 m tall, shrubs and bushes are categorized as woody Plants. Shrubs have multiple stems and many are covered with flowers of all shapes and sizes.
VII) Trees - Trees are generally tall and large Plants.
VIII) Creepers and Climbers - Creepers are those Plants that can grow along the top of the compound wall or as a cover for walls, or at the entrance as an arch. There are varieties of creepers available like, Bignonia venusta, allamanda, passiflora (fashion flower), Jacquemontia. Climbers are the Plants with soft stems that grow only with a support. They rely on something else for support; another Plant, a wall or trellis. Examples are Clerodendrum thomsoniae (Bleeding Heart), Cissus rhombifolia (Grape Ivy), Jasminum multipartitum (Starry Wild Jasmine), Clematis montana (Clematis) etc.
IX) Vegetables - The term 'vegetable' is not actually a scientific classification of a Plant, but rather strictly a culinary term. Vegetables are parts of Plants (flower buds, seeds, stems, fruits, etc) that are edible and used in culinary dishes.
4. Classification according to Phylum
I) Bryophyta - These are the most primitive group, lacking vascular tissues for the transport of water. Bryophytes have no root, leaves or stems. These are commonly known as Liverworts, Hornworts, Mosses. There are about 24,000 Species of Bryophyta.
II) Psilophyta - These are simple Vascular Plants lacking true roots and, in some Species, leaves. They reproduce by spores. They are commonly known as Whiskferns. There are about 12 Species.
III) Sphenophyta or Equisetophyta - These are found in moist, muddy habitats; stems creeping underground and producing erect annual or perennial stems with tiny leaves whorled into sheaves around stem. They are characterised by having jointed hollow stems and rough ribbed texture caused by the mineral silica. They are commonly known as Horsetails and Scouring Rushes. They also reproduce by spores. There are about 20 Species of Sphenophyta.
IV) Filicinophyta or Pterydophyta - These are Vascular Plants which reproduce by spores; stems mostly creeping, large leaves (megaphylls) with branching veins. The most complex, diverse and abundant of the Plant Phyla that do not produce seeds. They are commonly known as Ferns. There are about 12,000 Species of Filicinophyta.
V) Cycadophyta - Commonly known as Cycads, these are evergreen perennial Shrubs or Trees with stems that are usually unbranched but thickened by some secondary growth. Palm-like or fern-like compound leaves; they contain symbiotic cyanobacteria in special roots. There are about 100 Species of Cycads.
VI) Ginkgophyta - Gingko is a tall Tree with deciduous fan-shaped leaves; the only living descendant of a once-large group. It is commonly known as Ginkgo and Maidenhair Tree. There is only one Species of Ginkgophyta, known as 'Ginkgo biloba'.
VII) Coniferophyta or Pinatae - It is a Gymnosperm (plants having naked seeds) usually having evergreen Shrubs or Trees with simple needle-like leaves, spirally arranged. These are commonly known as Conifers. There are about 550 Species of Conifers.
VIII) Gnetophyta - These are cone-bearing desert Plants and are commonly known as Gnetophytes. They resemble flowering Plants in many ways and were once thought to be link between Conifers and Angiosperms. There are about 70 Species of Gnetophytes.
IX) Angiospermophyta or Magnoliophyta - It is the dominant land vegetation of the Earth, including nearly every familiar tree or shrub that produces flowers and seeds. These are characterised by the aggregation of sexual reproductive structures with specialized shoots (flowers), which typically comprise four kinds of modified leaves; sepals, petals, stamens (male organs) and carpels (female organs). These are commonly known as Angiosperms, flowering Plants. There are about 230000 Species of this type.
5. Classification according to Habitat
1) Terrestrial Plants - Plants that grow on land are called Terrestrial Plants or Land Plants. They are called terrestrial because of the initial words ‘terra’ which means land. Most of the Terrestrial plants have stomata on the bottoms of their leaves, this is because sufficient supply of air is easily available to them. These plants have long roots because of the need for water. Some of the examples include bamboo, mango etc.
2) Aquatic Plants - Plants that grow in water are called Aquatic Plants. They are called Aquatic because they live in or under water and the term aquatic itself means ‘water’. Aquatic plants have their leaves near or under the water. Because of this they have stomata on top of their leaves which allow access to air from the surface of the water. Plants that live completely under water obtain carbon dioxide from the water. When they release oxygen tiny air bubbles can be gathering around them. These plants have short but branched out roots which allows them to float under or over the water. Some of the examples include lotus, water hyacinth
3) Aerial Plants - Plants that grow above the ground level are called Aerial Plants. They are called aerial because they are located high up the ground. They are attached to other tall plants by their short roots. They are common in rainforests and are dependent on tall plants for their survival as most of the plants have evolved to grow tall to get maximum sunlight. Some of the examples include bromeliads, orchids, stag-horn fern and tillansias.
i) Haploid Phase - In this phase of Plants chromosomes are present only as single copies. The Haploid organism produces gametes that fuse to form an embryo, which develops into a Diploid organism. The Haploid generation is known as the Gametophyte which produces the sex cells, or gametes.
ii) Diploid Phase - In this phase of Plants the nuclei of the Plant cells contain two sets of chromosomes (chromosomes are present in pairs). The Diploid organism produces Haploid spores that germinate to form the Haploid organism. This 'Alternation of Generations' is found only in Plants and some Algae. The Diploid generation is known as the 'Sporophyte', which literally means 'Spore-Producing Plant'.
There are Three Main Types of Tropisms:
i) Phototropism - The tendency of a Plant to move toward light is known as Phototropism. It results from the rapid elongation of cells on the dark side of the Plant, which causes the Plant to bend in the opposite direction.
ii) Gravitropism - The tendency of a Plant to grow toward or against gravity. A Plant that displays Positive Gravitropism will grow downwards, toward the earth. A Plant that displays Negative Gravitropism will grow upward, away from the earth. Most Plants are Negatively Gravitropic. Gravitropism is also controlled by auxin (plant hormone). In a horizontal root or stem, auxin is concentrated in the lower half, pulled by gravity. In a Positively Gravitropic Plant, this auxin concentration will inhibit cell growth on that lower side, causing the stem to bend downward. In a Negatively Gravitropic Plant, this auxin concentration will inspire cell growth on that lower side, causing the stem to bend upward.
iii) Thigmotropism - It is a reaction to touch, causes parts of the Plant to thicken or coil as they touch or are touched by environmental entities. Tree trunks, for instance, grow thicker when exposed to strong winds and vines tend to grow straight until they encounter a substrate to wrap around.
Size - Plants range in size from diminutive duckweeds only a few millimetres in length to the giant sequoias of California that reach 90 metres (300 feet) or more in height.
Diet - Majority of Plant manufacture their own food. During Photosynthesis, Plants use the energy in sunlight to convert carbon dioxide from the atmosphere, plus water, into simple sugars. However there are few Species of Plants which are Carnivores like the 'Pitcher Plant' of Borneo and tropical Asia, 'Sundew Plant' and 'Venus's-Flytrap' eat Insects and digests them by using their digestive chemicals. There are some Parasitic Plants as well, like the 'Western Australian Christmas Tree', 'Yellow Rattle' etc. Parasitic Plants have a modified root, the 'Haustorium', that penetrates the host Plant and connects to the Xylem (vascular tissue), Phloem (tissue that conducts synthesized food substances to parts where needed), or both.
