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.
There are at least Four Classification Systems which are in common use:
They are classified by by Seed Structure into those that reproduce through naked seeds, covered seeds, or spores.
They are classified by Tissue Structure into Non-Vascular (mosses) and Vascular Plants (all others).
They are classified by Stature divided into mosses, ferns, shrubs and vines, trees, and herbs.
Plants are classified into Phyla or Divisions based largely on reproductive characteristics.
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.
Monocots (class Monocotyledones) - Monocots include Plants such as the grasses, palm trees, iris, palms, corn and orchids. Most members of this group have one cotyledon, or embryonic leaf, in their seeds. In general, the veins of Monocot leaves are parallel, the flower parts occur in multiples of three, and a fibrous root system is present. Bundles of vascular tissue are scattered throughout the stem instead of appearing in a single ring. It includes Plants like Grasses, Orchids, Lilies, Palms etc.
Dicots (class Dicotyledones) - Dicots include apple trees, magnolia trees, oaks, sunflowers, peas and cacti. Most members of this group have two Cotyledon, or embryonic leaf, in their seeds. In Dicots have branched network of veins in their leaves, the occurrence of their flower parts in groups of four or five, and the presence of a taproot, which is a single main root with tributaries off it. The vascular bundles of dicots are arranged in a tubular pattern in the stem. It includes Plants like Cherry Trees, Maples, Coffee, Daisies etc.
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.
Characteristics and Physical features of Plants
Autotrophs and Photosynthetic - Almost all Plants are Autotrophs which means that they can produce their own food and are not dependant on on other organisms for food. Plants are Photosynthetic, which means that they manufacture their own Food by using energy obtained from light. Plant contain 'Chloroplasts' which are membrane-bound organelles that contain a green pigment which is known as 'Chlorophyll'. Plants utilise this pigment to produce their food in a process called 'Photosynthesis'. In this process Plants use the energy in sunlight to convert carbon dioxide from the atmosphere, plus water, into simple sugars. All Green Plants contain two forms of Chlorophyll, Chlorophyll a and Chlorophyll b. Chlorophyll b pigment is not found in 'Red' or 'Brown Algae' as the almost exclusive mode of nutrition, photosynthetic nutrition, in which chemical energy is produced from water, minerals, and carbon dioxide with the aid of pigments and the sunlight.
Division into Shoots and Roots - Terrestrial Plants have essentially unlimited growth at Meristems (plant tissue especially stems and roots). Plants division into shoots and roots is the most basic shared feature amongst most Plants. The separation between these two portions of the Plants occurred during the evolutionary move from an aqueous environment to a terrestrial one, and each part is essential in its own way to the Plants' ability to survive on land. The Root is that portion of a Plant which is mostly beneath the soil and helps in bringing water and minerals from the soil. It also anchors the Plant to the substrate, providing stability. The Shoot are all the aerial Plant Structures such as stems, leaves, flowers, and fruits. The shoot gathers the carbon dioxide and light energy necessary for Photosynthesis, provides surfaces for gas exchange, and contains the Plant's reproductive organs. Each of these parts, the Root and the Shoot, is dependent on the other, for Roots cannot perform Photosynthesis and Shoots cannot take in water and inorganic nutrients.
Incapable of Movement - Plants do not have any means of movement. They do not have limbs like Animals for travelling. Instead they are dependant on other Organisms do their work like Pollination. They generally remain fixed in one place.
Absence of Sensory and Nervous Systems - Plants have either no or very basic ability to sense as they lacks specialized sense organs and nervous system. Some Plants have sensory organs, for example the Venus Fly Trap, that respond to vibration, light, water, scents, or other specific chemicals. Some Plants sense the location of other Plants and attack and eat part of them.
Alternation of Generations - Plants undergo a Life Cycle that takes them through two distinct Multicellular generations or phases - Haploid and Diploid generations.
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'.
Multicellular and Eukaryotic - Most Plants are Multicellular and Eukaryotic (each cell possesses a membrane-bound nucleus that contains the chromosomes) organisms that develop from an embryo and that have cell walls and Chloroplasts. The rigid cell wall of Plants is made of fibrils of cellulose embedded in a matrix of several other kinds of polymers such as pectin and lignin. Plants have Vacuoles (A tiny cavity filled with fluid in the cytoplasm of a cell) which are surrounded by a single membrane. Young Plant cells often contain many small vacuoles, but as the cells mature, these unite to form a large central vacuole. Vacuoles serve several functions, such as storing foods (e.g., proteins in seeds), storing wastes etc. Plants are distinguished from Algae by a higher degree of multicellular complexity and from Fungi by the ability to photosynthesise.
Tropism - Plants grow in response to the stimuli within their environment. These responses to stimuli are called 'Tropisms' and are controlled by Plant Hormones.
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.
Mostly Terrestrial - Almost all Plants live on land and have adapted to the conditions on land through the development of a waxy cuticle to prevent drying out, structures to absorb and transport water throughout their bodies (the bryophytes are an exception), and rigid internal support to remain erect without the buoyancy available in water. This rigidity is provided in large part by the cell wall, which is composed of cellulose, a complex carbohydrate and lignin, a phenolic compound that stiffens the cellulose fibers.
Plant Respiration - Plants do not have lungs or a blood stream to carry out Respiration. The pores in the leaves are called 'Stomata'. The pores in the branches of trees are called 'Lenticels'. Plant breathing happens in two ways, by gases being absorbed through pores in the Plant and by Photosynthesis. With the absorption of gases through the pores, oxygen is taken in and reacts with glucose. The Plant then releases carbon dioxide. The pores of the Plant are located everywhere, even in the roots. The breathing out is oxygen. The sunlight helps break down water within the Plant and the carbon dioxide and reforms it into oxygen and glucose. Oxygen is then released into the air. At night there is no sunlight for photosynthesis to occur, so the Plants only give off carbon dioxide.
Continuos Growth - Plants grow continually throughout life and have no maximum size or characteristic form in the adult, as compared to Animals.
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.
Geographical Range and Habitat
Plants are often the dominant physical and structural component of 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 Grasslands and Forests. They are found all over the World in varying numbers, be it, the Tundras at the northernmost regions of continental shelves or the southern extremes like Antarctica. Some have even returned to an aquatic Habitat in either fresh or salt water.
History and Evolution of Plants
The earliest fossils of Plants are fossil Green Algae (as evidenced by similarities in pigmentation, cell wall chemistry, biochemistry, and method of cell division) from the Cambrian Period which points towards their evolution. These fossils resemble calcified multicellular members of the Dasycladales. Earlier Precambrian fossils are known which resemble single-cell Green Algae, but definitive identity with that group of Algae is uncertain. The oldest known fossils of Embryophytes date from the Ordovician, though such fossils are fragmentary. By the Silurian, fossils of whole Plants are preserved, including the 'lycophyte Baragwanathia longifolia'. From the Devonian Period, detailed fossils of 'Rhyniophytes' have been found. Early fossils of these ancient Plants show the individual cells within the Plant tissue. The Devonian Period also saw the evolution of supposedly, the first modern tree, 'Archaeopteris'. This fern-like Tree combined a woody trunk with the fronds of a Fern, but produced no seeds.
The first Plants were semi-terrestrial, non-vascular, non-seed-producing Species. Their direct descendants are the Mosses of today. Vascular Plants evolved about 430 million years ago and invaded the land. The first Vascular Plants were similar to the Ferns of today. More recently, Plants with seeds evolved. The Gymnosperms, such as the pines, appeared first. They have naked seeds. Later, the Angiosperms, the flowering Plants, appeared. They had seeds covered by a fruit. Over a period of time different Plant groups become adapted to terrestrial life to varying degrees. Land Plants faced severe environmental threats such as drastic changes in temperature, nutrient availability, gas exchange and reproduction. Thus, many adaptations to land existence evolved in The Plant Kingdom and are reflected among the different major Plant groups. An example is the development of a waxy covering (the cuticle) that covers the Plant body, preventing excess water loss. Specialized tissues and cells (vascular tissue) enabled early Land Plants to absorb and transport water and nutrients to distant parts of the body more effectively and, eventually, to develop a more complex body composed of organs called stems, leaves, and roots. The evolution and incorporation of the substance lignin into the cell walls of Plants provided strength and support. Details of the life history are often a reflection of a Plant’s adaptation to a terrestrial mode of life and may characterize a particular group; for example, the most highly evolved Plants reproduce by means of seeds, and, in the most advanced of all Plants (angiosperms), reproduction is achieved through a flower
Plants can reproduce both asexually and sexually. In asexual production many Plants produce genetically identical offshoots (clones) of themselves, which then develop into independent Plants. This process is called 'Vegetative Propagation'. There are many modes of Vegetative Propagation which include the production of specialized structures such as tubers, runners, and bulbs. Grafting is an artificial form of Vegetative Propagation. In sexual reproduction, Plants undergo a life cycle that takes them through both Haploid and Diploid generations. The multicellular Diploid Plant structure is called the 'Sporophyte', which produces spores through meiotic division. The multicellular Haploid Plant structure is called the 'Gametophyte', which is formed from the spore and gives rise to the Haploid gametes. The fluctuation between these Diploid and Haploid stages that occurs in Plants is called the 'Alternation of Generations'. The way in which the alternation of generations occurs in Plants depends on the type of Plant. In Non-Vascular Plants, the dominant generation is Haploid, so that the Gametophyte constitutes what we think of as the main Plant. The opposite is true for Tracheophytes, in which the Diploid generation is dominant and the Sporophyte constitutes the main Plant. The dominant phase in the Tracheophyte life cycle is the Diploid (sporophyte) stage. The Gametophytes are very small and cannot exist independent of the parent Plant. The reproductive structures of the Sporophyte (cones in gymnosperms and flowers in angiosperms) produce Male and Female Haploid spores: microspores (male) and Megaspores (female). These spores give rise to similarly sexually differentiated Gametophytes, which in turn produce gametes. Fertilization occurs when a Male and Female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new Sporophyte.
Angiosperms are special because they have developed special reproductive systems, which are none other than the flowers. The flower is the reproductive centre of the Angiosperm, containing both the Male and Female parts. The Male reproductive structure is called a 'Stamen' and is usually a slender threadlike filament topped with anthers filled with pollen. The Female reproductive structure is called a 'Pistil' and contains a stigma, style and ovary. The pollination usually occurs through the wind, insect, or other animals. The Male pollen travels down the style to the Female ovary where it then fertilizes the egg. The egg then becomes a seed with an embryo. Under proper conditions this seed can then produce another Plant.