Download Structure, Growth and Development

The Plant Kingdom
Plants, also called green plants, are living multicellular organisms of the kingdom Plantae. They
form a clade that includes the flowering plants, conifers and other gymnosperms, ferns, clubmosses,
hornworts, liverworts and mosses, as well as, depending on definition, the green algae. Plants
exclude the red and brown seaweeds such as kelp, the fungi, Archaea and bacteria. Green plants
have cell walls with cellulose and characteristically obtain most of their energy from sunlight via
photosynthesis using chlorophyll contained in chloroplasts, which gives them their green color.
Some plants are parasitic and have lost the ability to produce normal amounts of chlorophyll or to
photosynthesize. Plants are also characterized by sexual reproduction, modular and indeterminate
growth, and an alternation of generations, although asexual reproduction is common.
Precise numbers are difficult to determine, but there are thought to be 300–315 thousand species of
plants, of which the great majority are seed plants Green plants provide most of the world's
molecular oxygen and are the basis of most of the earth's ecologies, especially on land. Botany, also
called plant science(s) or plant biology, is the science of plant life and a branch of biology.
Algae
Algae comprise several different groups of organisms which produce energy through
photosynthesis and for that reason have been included in the plant kingdom in the past. Most
conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble land
plants, but are classified among the brown, red and green algae. Each of these algal groups also
includes various microscopic and single-celled organisms. There is good evidence that some of
these algal groups arose independently from separate non-photosynthetic ancestors, with the result
that many groups of algae are no longer classified within the plant kingdom as it is defined here.
The Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting
of all the descendants of a common ancestor. With a few exceptions among the green algae, all
green plants have many features in common, including cell walls containing cellulose, chloroplasts
containing chlorophylls a and b, and food stores 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.
Fungi
The classification of fungi has been controversial until quite recently in the history of biology, or at
least until 1969, when Robert Whittaker proposed the creation of the kingdom Fungi, on the basis
of the fundamental difference in nutrition between the Fungi and the Plantae. Unlike plants, which
generally gain carbon through photosynthesis, and so are called autotrophic phototrophs, fungi
generally obtain carbon by breaking down and absorbing surrounding materials, and so are called
heterotrophic saprotrophs. In addition, the substructure of multicellular fungi is different from that
of plants, taking the form of many chitinous microscopic strands called hyphae, which may be
further subdivided into cells or may form a syncytium containing many eukaryotic nuclei. Fruiting
bodies, of which mushrooms are most familiar example, are the reproductive structures of fungi,
and are unlike any structures produced by plants.
Embryophytes
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The plants that are likely most familiar to us are the multicellular land plants, called embryophytes.
Embryophytes include the vascular plants, such as ferns, conifers and flowering plants. They also
include the 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, water 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.
Structure, Growth and Development
Most of the solid material in a plant is taken from the atmosphere. Through a process known as
photosynthesis, most plants use the energy in sunlight to convert carbon dioxide from the
atmosphere, plus water, into simple sugars. (Parasitic plants, on the other hand, use the resources of
its host to grow.) These sugars are then used as building blocks and form the main structural
component of the plant. Chlorophyll, a green-colored, magnesium-containing pigment is essential to
this process; it is generally present in plant leaves, and often in other plant parts as well.
Plants usually rely on soil primarily for support and water (in quantitative terms), but also obtain
compounds of nitrogen, phosphorus, potassium, magnesium and other elemental nutrients.
Epiphytic and lithophytic plants depend on air and nearby debris for nutrients, and carnivorous
plants supplement their nutrient requirements with insect prey that they capture. For the majority of
plants to grow successfully they also require oxygen in the atmosphere and around their roots (soil
gas) for respiration. Plants use oxygen and glucose (which may be produced from stored starch) to
provide energy. Some plants grow as submerged aquatics, using oxygen dissolved in the
surrounding water, and a few specialized vascular plants, such as mangroves, can grow with their
roots in anoxic conditions.
Growth is generally determined by environmental factors, such as temperature, available water,
available light, carbon dioxide and available nutrients in the soil. Any change in the availability of
these external conditions will be reflected in the plant's growth. Biotic factors are also capable of
affecting plant growth. Plants compete with other plants for space, water, light and nutrients. Other
plants may be organized according to their seasonal growth pattern: annual plants live and
reproduce within one growing season, biennial plants live for two growing seasons and usually
reproduce in second year, and perennial plants live for many growing seasons and continue to
reproduce once they are mature. These designations often depend on climate and other
environmental factors. 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 of it. 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. Plants
protect themselves from frost and dehydration stress with antifreeze proteins, heat-shock proteins
and sugars.
Plant Anatomy and Morphology
All plants are multicellular eukaryotes, their DNA stored in nuclei. The characteristic features of
plant cells that distinguish them from those of animals and fungi include a primary cell wall
composed of the polysaccharides cellulose, hemicellulose and pectin, larger vacuoles than in animal
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cells and the presence of plastids with unique photosynthetic and biosynthetic functions as in the
chloroplasts. Other plastids contain storage products such as starch (amyloplasts) or lipids
(elaioplasts).
The bodies of vascular plants including clubmosses, ferns and seed plants (gymnosperms and
angiosperms) generally have aerial and subterranean subsystems. The shoots consist of stems
bearing green photosynthesising leaves and reproductive structures. The underground vascularised
roots bear root hairs at their tips and generally lack chlorophyll. Non-vascular plants, the liverworts,
hornworts and mosses do not produce ground-penetrating vascular roots and most of the plant
participates in photosynthesis. The sporophyte generation is nonphotosynthetic in liverworts but
may be able to contribute part of its energy needs by photosynthesis in mosses and hornworts.
The root system and the shoot system are interdependent – the usually nonphotosynthetic root
system depends on the shoot system for food, and the usually photosynthetic shoot system depends
on water and minerals from the root system. Cells in each system are capable of creating cells of the
other and producing adventitious shoots or roots. Stolons and tubers are examples of shoots that can
grow roots. Roots that spread out close to the surface, such as those of willows, can produce shoots
and ultimately new plants. Vascular plants differ from other plants in that they transport nutrients
between different parts through specialized structures, called xylem and phloem. These are the
conductive tissues that transport resources between shoots and roots: the xylem moves water and
minerals from the root to the rest of the plant, and the phloem provides the roots with sugars and
other nutrient produced by the leaves. Roots take up water and minerals from the soil, and are often
adapted to store food such as sugars or starch, as in sugar beets and carrots.
Stems mainly provide support to the leaves and reproductive structures, but can store water in
succulent plants such as cacti, food as in potato tubers, or reproduce vegetatively as in the stolons of
strawberry plants or in the process of layering. Leaves gather sunlight and carry out photosynthesis.
Large, flat, flexible, green leaves are called foliage leaves. Gymnosperms, such as conifers, cycads,
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Ginkgo, and gnetophytes are seed-producing plants with open seeds. Angiosperms are seedproducing plants that produce flowers and have enclosed seeds. Woody plants, such as azaleas and
oaks, undergo a secondary growth phase resulting in two additional types of tissues: wood
(secondary xylem) and bark (secondary phloem and cork). All gymnosperms and many
angiosperms are woody plants. Some plants reproduce sexually, some asexually, and some via both
means.
Plant cells are typically distinguished by their large water-filled central vacuole, chloroplasts, and
rigid cell walls that are made up of cellulose, hemicellulose, and pectin. Cell division is also
characterized by the development of a phragmoplast for the construction of a cell plate in the late
stages of cytokinesis. Just as in animals, plant cells differentiate and develop into multiple cell
types. Totipotent meristematic cells can differentiate into vascular, storage, protective (e.g.
epidermal layer), or reproductive tissues, with more primitive plants lacking some tissue types.
Photosynthesis
Plants are photosynthetic, which means that they manufacture their own food molecules using
energy obtained from light. The primary mechanism plants have for capturing light energy is the
pigment chlorophyll. All green plants contain two forms of chlorophyll, chlorophyll a and
chlorophyll b. The latter of these pigments is not found in red or brown algae. Photosynthesis is a
process used by plants and other organisms to convert light energy, normally from the sun, into
chemical energy that can be used to fuel the organisms' activities. Carbohydrates, such as sugars,
are synthesized from carbon dioxide and water. Oxygen is also released, mostly as a waste product.
Most plants, most algae, and cyanobacteria perform the process of photosynthesis, and are called
photoautotrophs. Photosynthesis maintains atmospheric oxygen levels and supplies all of the
organic compounds and most of the energy necessary for all life on Earth. Although photosynthesis
is performed differently by different species, the process always begins when energy from light is
absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants,
these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells,
while in bacteria they are embedded in the plasma membrane.
Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food
directly from carbon dioxide and water using energy from light. However, not all organisms that
use light as a source of energy carry out photosynthesis. In plants, algae and cyanobacteria,
photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some
differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process
is quite similar in these organisms. However, there are some types of bacteria that carry out
anoxygenic photosynthesis, which consumes carbon dioxide but does not release oxygen.
Carbon dioxide is converted into sugars in a process called carbon fixation. In general outline and in
effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds
are oxidized to produce carbon dioxide and water, and to release exothermic chemical energy to
drive the organism's metabolism. However, the two processes take place through a different
sequence of chemical reactions and in different cellular compartments.
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions
capture the energy of light and use it to make the energy-storage molecules ATP and NADPH.
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During the second stage, the light-independent reactions use these products to capture and reduce
carbon dioxide.
In plants and algae, photosynthesis takes place in organelles called chloroplasts. A typical plant cell
contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane
is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an
intermembrane space between them. Within the membrane is an aqueous fluid called the stroma.
Plants absorb light primarily using the pigment chlorophyll, which is the reason that most plants
have a green color. Although all cells in the green parts of a plant have chloroplasts, most of the
energy is captured in the leaves, except in certain species adapted to conditions of strong sunlight
and aridity, such as Cactus species, whose main photosynthetic organs are their stems.
The main factors affecting photosynthesis are light irradiance and wavelength, carbon dioxide
concentration and temperature.
Ecology
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.
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