Plant Biology

Plant Classification

Plant Cell Structure

Plant Nutrition & Growth

Plant Reproduction

Plant Classification

Classification of Organisms

Organisms can be classified into six kingdoms: Archebacteria, Eubacteria, Protista, Plantae, Fungi and Animalia.

The Protists are unicellular eukaryotic cells, which resemble animals in internal structure, diet and habit and several groups of plant-like unicellular organisms. Plants are divided into the Fungi, which never photosynthesise and the Plantae, which are autotrophic and photosynthesise aerobically in sunlight. The Plantae kingdom is divided into at least 7 divisions. All multicellular, heterotrophic organisms are placed in the kingdom Animalia, which is split into about 25 phyla. Phyla are subdivided into classes, which are split into orders and orders into families, which can split into genera.

Protistans

The unicellular protistans are abundant as free-living organisms in the ocean, in freshwater, in soil and as symbionts with animals. Most are below 1 mm but a few can be larger. Many forms have one or more flagella (which differ from those in prokaryotes) and groups of protistans possessing at least one flagella in their life cycle are known as flagellates. Relatively short numerous flagella are called cilia and protistans with them are called ciliates. Protistans such as amoeba don't have flagella and move using temporary protrusions, called pseudopodia (false feet). Many protistans have silica skeletons.

Protistans usually reproduce asexually by division of a mature cell into 2 daughter cells. Most species can also reproduce sexually, with two cells exchanging genetic material before cell division. Ciliates have two types of nuclei, meganuclei and micronuclei. Almost all protistans feed on organic material and either absorb small molecules through the outer membrane or through phagocytosis. They are very abundant in the soil, and in water where they feed on cyanobacteria and dead matter. Flagellates are the most common sort in soil but all three kinds occur in water. Protistans are found in the gut of many animals and are the cause of malaria and dysentery. They can survive long periods by secreting materials that form a cyst.

Unicellular Algae

Unicellular algae differ in the chemical composition of their chloroplast pigments, cell walls and food storage materials. Due to this they are placed in different divisions of the animal kingdom. Many algae contain pigments that give them a brown or golden-brown colour. These differing colours affect their photosynthizing performance and they will divide at different water depths and light intensities. They store energy rich compounds into particles. Unicellular algae reproduce mainly by asexual mitotic division of the cells. Some species form colonies and filaments, some live separately. Most of the unicellular algae live in the upper parts of oceans and lakes but some live on submerged or temporally exposed rocks where they provide a source of food for small grazing invertebrates.

Planktonic unicellular algae are abundant in water in which adequate supplies of nutrients, especially phosphates, nitrates, calcium and iron. They are the major food for protistans and small multicullar animals. Photosynthesis by marine unicellular algae and cyanobacteria releases a large part of atmospheric oxygen and takes up a large part of carbon dioxide released from other sources.

Multicellular Algae

Multicellular algae are primarily aquatic though some can survive long periods of air. Their common name is seaweed. They are classified according to colour. An example of multicelluar algae is the sea lettuce which is composed of the holdfast, which grips the rock and the thallus, which is a thin blade of photosynthetic tissue. The life cycle of the sea lettuce is as follows. The sporophyte plant (the mature plant, capable of producing spores) is diploid and has a holdfast and a thallus. Some of the cells of the thallus divide meiotically (splitting in two each being haploidy), drift off and lodge a distance away from the main algae. They then divide mitotically and grow into a gametophyte, which in this case resembles a sporophyte. This gametophyte is still haploid. This gametophyte in turn produces haploid spores which in turn drift away and may fuse with another haploid spore from a similar algae, which then produces a diploid sporophyte, and the cycle restarts. In many species the sporophyte is larger and more complex in structure than the gametophyte.

Classification of Terrestrial Plants

The division Chlorophyta contains all the terrestrial plants, but they differ from algae (and fungi) in that there is an embryonic stage in the life cycle. An embryo is a growing multicellular organism that is surrounded by tissues of the parent plant and receives nutrients from it. The life cycle of most animals is similar to that of protisans, in that two gametes from different parent organisms meet and fuse to form a diploid zygote which may then develop into a new mature organism. In terrestrial plants there is both a multicellular gametophyte (with haploid cells) and a multicellular sporophyte (containing diploid cells). In plants the gametes are produced by mitosis of the haploid gametophyte. This cycle is called the alternation of generations.

Bryophytes

These are mosses and liverworts which lack efficient support and transport structures. Moses do not have roots or flowers. The gametes are aquatic and need standing water to complete their life cycle. They grow in damp shady habitats and avoid extremes of temperatures. What we call a moss is the gametophyte. It has two different gamete producing structures that produce larger non motile female gametes and tiny, flagellated male gametes. After fertilization with a gametes from itself or another plant, the zygote develops into a diploid sporophyte

Tracheophyta

These are the ferns and the fern-like plants. They have special cells that form vessels that transport water and minerals. All of these plants have stout leafy shoots and true roots, which grow from a horizontal underground stem called a rhizome. Like mosses they need water to complete their cycle. The most familiar stage here is the sporophyte. The gametophytes lack the tough outer coating and vascular system of the sporophyte.

Seed Plants

All large terrestrial plants are tracheophytes that bear seeds. They are divided into the Gymnospermidae (naked seeds) which include the conifers and the Angiospermidae (closed, flowering seeds). The flowering plants are split between the monocot and dicots. Monocots have narrow leaves with parallel vessels whereas dicots are much more diverse in structure. The two basic components of the sporophyte of seed plants are the roots and shoots, which includes the stems, leaves and flowers. Many seed bearing plants contain wood, a structural tissue consisting mainly of dead tissue.

The enclosed seeds are resistant to damage from external forces and can nearly all completes their life cycle in the absence of standing water. There is no free living gametophyte generation, the zygote lives on the sporophyte plant and becomes part of an enclosed seed. In angiosperms, the formation of gametes and fertilisation takes place in the flower, which is a modified shoot of the diploid sporophyte that forms the ovary and structures that disperse and collect pollen. The female gametophyte (containing the female gametes) is called the ovule and the flower contains structures that nurture the maturation of the ovule and any result zygote. Pollen is a spore stage that remains dormant until contact with the female structures triggers germination to produce the male gametophyte, one nucleus of which becomes the male gametophyte. A mature flower encloses its pollen in a tough outer coating awaiting dispersal by wind or animals or insects. The male and female reproductive structures may reside on the same flower or on different flowers on the same plant, or rarely, on different plants.

Fertilization begins when pollen falls on the stigma. The pollen grain germinates to form the male gametophyte which elongates to form the pollen tube which grows down the style towards the female gametophyte. The tiny male gametes move down the tube. One male gametes fertilise the ovule to form the diploid embryo while another pollen nucleus fuses with two others and the resulting triploid cell divides to form a storage tissue called the endosperm. The ovary then produces a tough, sometimes woody, coat around the embryo and endosperm. Mitotic cell division of the zygote begins shortly after fertilization and the tissues in the ovary combine to form the seed, the endosperm which will provide nutrients for the growing embryo and one or more layers of protective covering. The ovary may also produce one or more layers of protective or nutritive tissues around each seed or group of seeds, forming fruits or pods. This process may take months (apples) to a few hours (dandelions). Germination is often triggered by a temperature rise or increase in moisture. The gymnopsperm life cycle is similar except the gametophytes and seeds form on seeds instead of flowers.

Fungi

Fungi have some characteristics in common with protistans but also features unique to themselves. In most fungi, the cells form syncytia, consisting of score of nuclei and large quantities of cytoplasm enclosed in the same cell membrane and surrounded by a cell wall. Fungi are non-motile but never contain chlorophyll. They are heterotrophic , but they absorb nutrients from dead plants or animals (saprophytes).

Phycomcytes are structurally simple fungi. They have an aquatic phase in the life cycle and include bread moulds and potato blight. They are unable to break down cellulose or complex carbohydrates. The class Ascomytes, which include the yeasts, consist of single cells or strings of them, and can break down complex carbohydrates. The class Basidiomycetes have complicated life cycles and can often break down wood. This class includes edible mushrooms and also rusts and smuts.

These fungi consist of thread-like hyphae usually white in colour that grow only at the tips and are often variable in form. Under the microscope they appear as transparent tube like structures with rigid walls packed with living material containing numerous tint nuclei. The living material is thus not always divided into discrete cells although many hyphae are divided into interconnecting chambers with partitions called septa. The gametes are single cells with one nucleus. The hyphae secrete enzymes that dissolve outside matter into particles small enough to pass through the tough cell wall.

Most fungi form spores, which are transported by flowing water. The spores of simple fungi are usually diploid but on higher species haploid spores form on sporangia which protrude out of the soil (mushrooms and toadstools are the fruiting bodies). When a spore settles on a suitable food source, it sheds its protective coat and begins to develop hyphae that spread over and through the food source forming a mass called the mycelium.

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Plant Cell Structure

Plant Cell Structure and Function

The tremendous variety of plant species is, in part, a reflection of the many distinct cell types that make up individual plants. Fundamental similarities exist among all these cell types, however, and these similarities indicate the common origin and the interrelationships of the different plant species. Each individual plant cell is at least partly self-sufficient, being isolated from its neighbors by a cell membrane, or plasma membrane, and a cell wall. The membrane and wall allow the individual cell to carry out its functions; at the same time, communication with surrounding cells is made possible through cytoplasmic connections called plasmodesmata.

Cell Wall

The most important feature distinguishing the cells of plants from those of animals is the cell wall. In plants this wall protects the cellular contents and limits cell size. It also has important structural and physiological roles in the life of the plant, being involved in transport, absorption, and secretion.

A plant's cell wall is composed of several chemicals, of which cellulose (made up of molecules of the sugar glucose) is the most important. Cellulose molecules are united into fibrils, which form the structural framework of the wall. Other important constituents of many cell walls are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss from cells. Many plant cells produce both a primary cell wall, while the cell is growing, and a secondary cell wall, laid down inside the primary wall after growth has ceased. Plasmodesmata penetrate both primary and secondary cell walls, providing pathways for transporting substances.

Protoplast

Within the cell wall are the living contents of the cell, called the protoplast. These contents are bounded by a single, three-layered cell membrane. The protoplast contains the cytoplasm, which in turn contains various membrane-bound organelles and vacuoles and the nucleus, which is the hereditary unit of the cell.

Vacuoles

Vacuoles are membrane-bound cavities filled with cell sap, which is made up mostly of water containing various dissolved sugars, salts, and other chemicals. Vacuoles are not found in animal cells.

Plastids

Plastids are organelles-specialized cellular parts that are analogous to organs-bounded by two membranes. Three kinds of plastids are important here. Chloroplasts contain chlorophylls and carotenoid pigments; they are the site of photosynthesis, the process in which light energy from the sun is fixed as chemical energy in the bonds of various carbon compounds. Leucoplasts, which contain no pigments, are involved in the synthesis of starch, oils, and proteins. Chromoplasts manufacture carotenoids.

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Plant Nutrition and Growth

Inorganic Nutrients

The mineral nutrients that are essential for plants are:

Nitrogen, Potassium, Calcium, Phosphorus, Magnesium, Sulphur, (Macronutrients) Iron, Boron (Micronutrients)

To be essential an element must have an identifiable role, no other element can substitute and the plant will die without completing its life cycle. C, H and O are considered essential and make up 96% dry weight. Plants take in Nitrogen as ammonium or nitrate. Beneficial nutrients are required or enhance a plants growth. Hydroponics is a method used to determine nutritional requirements.

Mineral Availability

A root needs to come within millimeters of an ion. The downward movement of minerals is affected by the quantity and pH of water. The lower the pH, the greater the leeching power of water. Soil particles contain negative charges that attract positively charged ions. In acidic soils H+ ions displace other positive ions and in very acidic conditions Al+++ and Fe+++ become available which are toxic (in high levels).

Mineral Absorbion and Distribution

Minerals get into the roots the same way as water does. Plants can concentrate minerals up to 10,000 times greater than the concentration in the surrounding soil. The charged ions used by plants are likely to have difficulty passing through the plasma membrane. Active transport pumps out H+ and allows K+ etc. to enter. Roots of plants such as legumes, soybeans and alfalfa have a symbiotic relationship with Rhizobium bacteria to fix nitrogen. (Plants lack the enzymes necessary to break the Nitrogen gas triple bond. Another symbiotic relationship is that which almost all plants have with the Mycorrhizae fungus, in which the fungus increase the surface area available. Venus fly traps get nitrogen from insects.

Phloem transport

Phloem transports sugars and fairly large amounts for long distances in short times. One model for this behaviour is the pressure-flow model. During the growing season photosynthesising and producing sucrose. This sucrose is carried across the plasma membrane in conjunction with H+ ions which are moving down their concentration gradient. Water now flows into sieve tubes because of their lower osmotic pressure. Transport across sieve-tube membranes is possible because sieve-tube cells have a living plasma membrane. The build-up of water creates a positive pressure potential within the sieve tubes of the leaves compared to the roots. At the roots sucrose is being transported out of the phloem and water follows. This model suggests that phloem sap can rise or fall as appropriate for the plant at a particular time in its cycle.

Plant Growth and Development

Tropisms, nastic movements and thigmomorphogenesis are all ways that plants respond to stimuli by a change of growth pattern.

Tropisms

Various external factors, often acting together with hormones, are also important in plant growth and development. One important class of responses to external stimuli is that of the tropisms-responses that cause a change in the direction of a plant's growth. Examples are phototropism, the bending of a stem toward light, and geotropism, the response of a stem or root to gravity. Thigmotropism is a movement in response to touch. Growth towards a stimulus is a positive tropism and the reverse. Stems are negatively geotropic, growing away from gravity, whereas roots are positively geotropic. Photoperiodism, the response to 24-hour cycles of dark and light, is particularly important in the initiation of flowering. Some plants are short-day, flowering only when periods of light are less than a certain length. Other variables-both internal, such as the age of the plant, and external, such as temperature-are also involved with the complex beginnings of flowering.

Phototropism causes the hormone auxin to migrate from the bright side to the dark side of a plant stem. The cells on that side elongate faster than those on the bright side causing the stem to curve to the light.

Gravitropism is negative when a stem grows upwards against gravity. Roots show positive gravitropism. The auxin hormone moves to the bottom of both root and stem after gravity is detected. it inhibits the growth of cells in the root, so upper surface cells elongate and root curves down and encourages stem cell growth so the lower cells elongate and curve the stem upwards.

Thigmotropism is illustrated by the curling of plants around an object and can be rapid and last for days after the stimulus. It can also be a delayed reaction as leaves touched in the dark will respond after exposure to light.

Thigmoorphogenis is the response of the entire plant to environmental stimuli. A tree growing in a windy location has a shorter, thicker trunk in a more protected location.

In contrast to tropism, nastic movements are independent of direction of stimulus. A Venus flytrap has three hairs which trigger the trap. In response to an insect landing on it, the leaf snaps shut when the hairs are triggered. This is a nerve stimulus type reaction. A sleep movement is a nastic response to light and dark changes.

A circadian rhythm is a biological rhythm with a 24 hour cycle. The internal mechanism by which biorhythm is maintained in absence of environmental stimuli is termed a biological clock, Without environmental stimuli, circadian rhythms continue but the cycle extends. External stimuli may synchronise biological clock.

Hormones

Plant hormones, specialised chemical substances produced by plants, are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. A specific response may depend on a combination of hormones, possibly in a definite ratio. Depending on the target tissue, a given hormone may have different effects. Thus, auxin, one of the most important plant hormones, is produced by growing stem tips and transported to other areas where it may either promote growth or inhibit it. In stems, for example, auxin promotes cell elongation and the differentiation of vascular tissue, whereas in roots it inhibits growth in the main system but promotes the formation of adventitious roots. It also retards the abscission (dropping off) of flowers, fruits, and leaves.

Auxin moves to the shady side of a stem in response to light and binds to cell receptors. this binding activates the proton pump which push H+ out of the cell. The cell walls become acidic, breaking H bonds. Cellulose fibres are weakened and activated enzymes further degrade cell wall. Solutes enter the cell and water follows, causing elongation of the cell. Auxin mediated elongation is observed in younger cells.

Gibberellins are the other important plant-growth hormones; more than 70 kinds are known. They control the elongation of stems, and they cause the germination of some grass seeds by initiating the production of enzymes that break down starch into sugars to nourish the plant embryo. In these cases the endosperm (food tissue for the seed) contain starch. It may be that GA3, one of the gibberlin hormones, attaches to a membrane receptor and then a second messenger inside the cell, namely calcium ions) combines with a protein called calmodulin which activates the gene responsible for amylase, which is the enzyme used for the breaking down of starch,

Cytokinins (cell division hormones) promote the growth of lateral buds, acting in opposition to auxin; they also promote bud formation. The ratio of auxin to cytokinins can control the differential of plant embryo tissue into roots, or flowers or leaves and stems. When a plant ages, it undergoes a process called senescence, during which large molecules are broken down and it has been found that the application of cytokinins may prevent this occurring.

Plants produce the gas ethylene through the partial decomposition of certain hydrocarbons, and ethylene in turn regulates fruit maturation and abscission. Ethylene promotes the active of cellulase, an enzyme that breaks down (hydrolyses) cellulose, soften the cell wall and thus making the fruit ripe. Ethylene is produced at the site of a wound or infection, which is why one apple may spoil the rest in a container.

Absissic acid is sometimes called the stress hormone because it initiates and maintains seed and bud dormancy. Absicissic acid also brings about the closure of stomates.

Photoperiodism

This is a physiological response prompted by changes in day or night length. Plants can be dived into 3 groups:

1. Short-Day Plants which flower when the day length is shorter than a critical length (poinsettia)

2. Long-Day Plants which flower when the day length is longer than a critical length (wheat)

3. Day-Neutral plants in which flowering is not dependent on day length (tomato).

Short day and long day plants can have the same critical length. These changes are related to seasonal changes. It's actually more the length of the dark period which affects the flowering cycle of the plants as brief flashes of light interrupting a dark period can inhibit the flowering of a plant.

Photochrome is used by plants to detect dark and light periods. Its a blue-green leaf pigment that alternatively exists in two forms:

1. Pr (phytochrome red) absorbs red light (of 660 nm) and is converted to Pfr

2. Pfr (phytochrome far-red) absorbs far-red (of 730 nm) and is converted to Pr

Direct sunlight contains red light than far red light therefore Pfr is apt to be present in plant leaves during the day. (As all the Pr is being changed into Pfr). In the shade and at sunset there is more far-red light than red light and the process is reversed. There is also a slow metabolic replacement of Pfr by Pr during the night.

The Pr to Pfr conversion cycle is known to control other growth functions in plants such as the promotion of seed germination and the inhibition of stem elongation.

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Plant Reproduction

Plant Life Cycles

A life cycle is the entire sequence of events from time of fertilization and formation of the zygote to gamete formation once again. In contrast to animals, which have only one type of adult generation in their life cycle, plants have a diploid generation and a haploid generation. These alternate with each other. The diploid generation is called the sporophyte because it produces the spores, which are haploid and which divide to become the haploid generation, which is called gametophyte because it produces gametes. In flowering plants the diploid sporophyte is said to be dominant as this is the plant we most often see. This is the part that produces flowers.

A plant produces two spore types, microspores and megaspores. A microspore develops into a microgametophyte and a megaspore develops into a megagametophyte. The microgametophyte is the pollen grain which is either wind or animal/insect transported. When a pollen grain matures it contains nonflagellated sperm cells which travel by way of a pollen tube to the megagametophyte which is called an embryo sac. Once fertilization occurs, the life cycle begins again.

The sporeophyte contains vascular tissue and other adaptations for land living. Water is noot need for sed dispersal. The pollen tube carries the sperm to the egg and no water is needed here either. Following fertilization the zygote (fertilized egg) develops into an embryo in a seed enclosed by a fruit. Some non-flowering plants produce only kind of spore and this develops into a seperate but water dependent gametophyte.

Flowers

A flower is the reproductive organ in angiospermsdevelops within a bud. In some plants the same shoot apical meristem suddenly stops producing leaves and starts producing flowers. In other plants axillary buds develop directly into flowers. Flower structures are modified leavs attached to a short stem tip called a recepticle. In monocots flower parts occur in threes and multiples of threes. In dicots flower parts are in fours or fives or multiples of them. The sepals are the most leaf like of all the flower parts are usually green and they protect the bud as the flower develops within. When the flower opens there is an outer whorl of sepals and an inner whorl of petals, which gave the flowers its distinctive array of colours. The structure of the flower are attractive to a specific pollinator.

At the centre of the flower is the pistil, which is either simple or compound. A simple pistil contains a single reproductive unit called a carpel. A carpel usually has three parts: the stigma, an enlarged sticky knob, the style, a slender stalk and the ovary, an enlarged base. A compound pistil has multiple carpels which are often fused.

The ovary of apistil has a number of ovules. Grouped around the pistil are a number of stamens, each of which has two parts: the anther, a saclike container and the filament, a slender stalk.

Not all flowers have sepals, petals, stamens and a pistil. Those that do are said to be complete and does that do not are called incomplete. Perfect flowers are those with both stamens and a pistil: those with only stamens are called staminate flowers and those with only pistils are pistillate flowers. If stamenate and pistillate flowers are on seperate plants, the plants is dioecious. Monoecious plants have stameinate and pistillate flowers on one plant.

Calling the pistil the female part and the stamens the male part is not strictly correct as the pistil and stamens produce spores, not sperm and egg (developing into these gametes).

Gametophytes

The overies of a carpel contain one or more ovules which have a mass of parenchyma cells almost completly covered by integuments except where there is an opening, the micropyle. One parenchyma cell enlarges to become a megasporocyte (megaspore mother cell)which undergoes meiosis, producing four haploid megaspores. Three of these distintigrate leaving one functional megaspore, whose nucleus divides mitotically until there are eight nuclei of the embryo sac or megagametophyte. When cells walls form later there are seven cells, one of which is binucleate. The megagametophyte called the embryo sac consists of these seven cells:

1. one egg cell associated with

2. two synergid cells

3. one central cell with two polar cells

4. three antipodal cells

Microgametophytes are produced in the stamens. An anther has four pollen sacs each containing many microsporocytes (microspore mother cells). A microsporocyte undergoes meisos to produce four haploid microspores. A microspore divides mitotically forming two cells enclosed by a wall. This pollen grain is the immature microgametophyte contains a tube cell and a generative cell. Either now or later, the generative cell divides mitotically to produce two sperm cells. The walls seperating the pollen sacs in the anther break down when the pollen grains are ready to be released.

Pollination and Fertilisation

Pollination is the transfer of pollen from the anther to the stigma of a pistil. Fertilisation is the fusion of nuclei as when the sperm nucleus and egg nucleus. Self pollination occurs if the pollen is from the same plant and cross pollination occurs if the pollen is from a different plant.

When a pollen grain lands on the stigma of the same species, it germinates, forming a pollen tube. The germinated pollen grain, containing a tube cell and sperm cells, is the mature microgametophyte. As it grows, the pollen tube passes through the cells of the stigma and the style to reach the micropyle of the ovule. Now double fertisation occurs. One sperm nucleus unites with the egg nucleus, forming a 2n zygote and the other sperm nucleus migrates and unites with the polar nuclei of the central cell, forming a 3n endosperm nucleus. The zygote divides mitotically to become embryo, a young sporophyte and the endosperm nucleus divides mitotically to become the endosperm. Endosperm is the tissue that will nourish the embryo and seedling as they undergo development.

Embryo Development

After double fertisation takes place, the single celled zygote lies beneath the endosperm nucleus. The endosperm nucleus divides to oproduce a mass of endosperm tissue surrounding the embryo. The zygote also divides forming two parts. The upper part is the embryo and and the lower part is the suspensor, which anchors the the embryo and transfers nutrients to it from the sporophyte plant. Soon the cotyledons, seed leaves, can be seen. At this point the dicot embryo is heart shaped. Later, when it becomes torpedo shaped it is possible to distinuish the root apex and the shoot apex. These contain apical meristems, the tissues that bring about primary growth in a plant. The shoot apical meristem is responsible for aboveground growth and the root apical meristem is responsible for underground growth.

Monocots have only one cotyldon. Another important difference between monocots and dicots is the manner in which nutrient molecules are stored in the seed. In a monocot the cotyldon rarely stores food; rather it absorbs food molecules from the endosperm and passes them to the embryo. During the development of a dicot embryo, the cotyldons usually store the nutrient molecules that the embryo uses. The endosperm "disappears" as it has been taken up by the two cotyldons. In a plant embryo the epicotyl is above the cotyledon and contributes to shoot develoment; the hypocotyl is that portion below the cotyldon that contributes to stem development and the radicle contributes to root development. The embryo plus stored food is now contained within a seed.

Fruits

As the zygote develops into an embryo, the integuments of the ovule harden and become the seed coat. A seed is a structure formed formed by the maturation of the ovule; it contains a sporophyte embryo plus stored food. The ovary and sometimes other floral parts develops into a fruit. A fruit is a mature ovary that usually contains seeds.

As fruit develops from an ovary, the ovary wall thickens to become the pericarp. Most fruits are simple fruits and are derived from an individual ovary either simple or compound. Peachs are examples from simple fleshy fruits. In almonds the fleshy part of the pericarp is a husk removed before selling. An apple develops from a compound ovary, but much of the flesh comes from the recepticle, which grows around the ovary. Dry fruits have a dry pericarp, such as peas and beans.

Seed Dispersal

Plants have various means to ensure that dispersal takes place. The hooks and splines of clover attaching to passing taffic. Fruits are eaten andseeds dispersed. Some seeds do not germinate until they have been dormant for some time even though conditions are favourable. Germination requires water, warth and oxygen to sustain growth, requires regulation and both inhibitors and stimulators are known to exist. Some fleshy fruits contain inhibitors so that germination does not occur until the seeds are removed and washed. Mechanical action may also be required. The uptake of water causes the seed coat to burst.

The cotyldons (seed leaves) which supply nutrients to the seedling eventually shrival and disappear. The epicotyl bears young leaves and is called a plumule. As the dicot seedling emerges from the soil, the shoot is hook-shaped to protect the delicate plumule. The hypocotyl becomes the stem and the radicle develops into the roots. When a seed germinates in darkness, it etiolates, the stem is enlongated, the roots and leaves are small and the plant lacks colour and appears spindly.

A corn plant is a monocot that contains a single cotyldon. The endosperm is the food storage tissue in monocots and the cotyldons does not have a storage role. Corn kernels are actually fruits and therefore the outer covering is the pericarp. The plumule and radicle are enclosed in protective sheaths called the coleoptile and coleorhiza, respectively. The plumule and the radicle burst through these coverings when germination occurs.

Asexual Reproduction

Vegatative propagation is asexual reproduction. There is only one parent instead of two as in sexual reproduction.

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