FLOWERS
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FLOWERS
FRUITS AND SEEDS
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FLOWERSThere are two purposes associated with reproduction -
1) Maintenance of genetic diversity. Production of new combinations of parental characteristics through cross-fertilization = fuel for evolution.
2) Multiplication and dispersal of the organism.
These two activities (cross-fertilization and dispersal) overlap. Sexual reproduction requires movement or dispersal of sperm between the two parent plants. However, its not ideal in terms of dispersal - sperm are small and delicate so can't travel very far, so there has to be another stage where dispersal over larger distances can take place. These two distinct requirements have led to the development of two distinct kinds of plant, which usually look rather different, each undertaking one phase in the life cycle and alternating with the other - leading to the term alternation of generations.
1) The mature gametophyte produces gametes. Because a gametophyte plant is, by definition, a haploid, its cells only have to divide by mitosis to produce haploid gametes. This process = gametogenesis. Two different types of gametes are formed. The female gamete is the egg and is produced in the archegonia. The male gamete is the sperm, produced in the antheridia.
2) Fertilization of the egg by the sperm produces a diploid zygote - it has two sets of chromosomes, so we've moved into the second generation. Mitotic division of the embryo - the process of embryogenesis, then produces the mature sporophyte plant.
3) The mature sporophyte then produces spores through meiosis - process of SPOROGENESIS. These spores are haploid, so we're back to the first generation or plant type. Spores have the role of dispersing the species as they can resist desiccation. The spores then go on to divide and produce the mature gametophyte plant.
4) Both the sporophyte and gametophyte mature plants are able to divide asexually (depending on the species concerned).
There is some evidence that the first land plants had homomorphic generations, so the mature gametophyte looked the same as the mature sporophyte. Most land plants (all tracheophytes) today have specialized so the sporophyte is the dominant phase in the process. This can be explained with a number of reasons:
1) Gametophytes must remain small because of the need for water as the vector in fertilization (sperm is motile and must be able to swim to reach the egg).
2) Having a big sporophyte plant means that dispersal and multiplication is likely to be more efficient - the plant is able to produce large numbers of spores and they may be high above the ground.
3) Meiosis is occurring in the big plants and it is meiosis where there is the greatest potential for mutation to occur. Because big plants can produce many sporangia there are more meioses per plant per generation, hence there are greater numbers of mutations and therefore much greater evolutionary potential. It doesn't matter if some mutations are disadvantageous when there are so many spores produced.
Angiosperms are the flowering plants, the most structurally complex and diverse of all living plants. The conifers only consist of 500 species, whereas one family within the Angiosperms - the Asteraceae - daisy family contains 20,000 species. They show the greatest diversity in the structure of their reproductive structures and they have the most highly modified cell types of all the vascular plants. Angiosperms classified as a single division Magnolipsida divided into Magnoliidae (dicots) and Lilliidae (monocots). The nature of the flower has been basic to all systems of classification of the Angiosperms. The classifications also relate to the evolutionary history of the groups. Two major subclasses within the angiosperms - monocotyledons and dicotyledons. The names refer to the number of seed leaves produced, but there are other characteristics to look for:
1. In monocots flower parts are basically in multiples of 3 - those of dicots are in 4s or 5s.
2. Monocot leaves show parallel venation, leaves usually long, narrow and less divided. Dicots exhibit great variety in leaf shape and venation is reticulate (net like).
3. Monocots -vascular bundles are scattered in the stem; those of dicots form a cylinder.
4. Vascular cambium absent in monocots.
5. Pollen grain germination pores - various numbers in dicots, only 1 in monocots.
Examples of dicots - oak, daisy, sunflowers, cactus. Examples of monocots - grasses, (cereals - wheat, barley, maize, rice), palms, lilies, orchids, pineapples.
1. Carpel - ovules and seeds are protected within an enclosed structure. From this the name is derived, as angiosperm means "enclosed seed".
2. Reduced male gametophyte - produces only three haploid nuclei.
3. Reduced female gametophyte - embryosac is very reduced generally 8 nuclei at maturity.
4. Double fertilization - to produce a diploid zygote and triploid endosperm nucleus - non-motile gametes are delivered to the embryosac by a pollen tube.
5. Flower - specialized for pollination by various agents (see diagram below). We find sepals and petals surrounding the male stamens, which produce pollen, and these in turn, surround the female pistil. The pistil is made up of 3 parts; the stigma where pollen is 'caught', the style and the ovary or carpel which contains the ovule. After fertilization of the ovule, part of all of the carpel undergoes changes to produce a fruit so that most angiosperm seeds are enclosed by the fruit.
Sepals - collectively the calyx (outermost appendages) are modified leaves - protect flower bud from bacterial and fungal spores - usually thickest most waxy part of flower. May be green, or colourful (petalloid) to help attract pollinators. Petals - collectively the corolla - also leaf-like in shape, have pigments other than chlorophyll, few or no fibres, tend to be thin, delicate. Function is attraction of pollinators. Sepals and petals together are the perianth. Stamens - collectively the androecium. Carpels - collectively the gynoecium. Pedicel - the flower stalk. Receptacle - the stem of the flower, to which the other parts are attached. When the other flower parts are below the ovary, the ovary is superior, the flower is termed hypogynous, when the flower parts arise from the top of the ovary, the ovary is inferior and the flower epigynous. When flower parts are surrounding the ovary on the bowl like structure, the flower is perigynous. Flowers can be complete (all flower parts) or incomplete (one or more parts absent) eg. dioecious flowers. In terms of shape flowers can be divided into two main types. Those which are radially symmetrical are termed actinomorphic, bilaterally symmetrical flowers are zygomorphic. There are also some irregular flowers are those which are essentially asymmetrical - very unusual.
The stamen generally consists of a filament (the stalk) at the top of which is the anther composed of pollen sacs (microsporangia). Can see in cross-section that the microsporangia are fused together and they dehisce simply along longitudinal lines. Microspore mother cells undergo meiosis to give four microspores. The gametophyte phase of the life cycle (haploid) therefore occurs within the spore/pollen wall - development is endosporic. The gametophyte is so reduced it only consists of a few cells. In angiosperms, the first cell division into a vegetative nucleus and generative nucleus occurs while the pollen is still in the anther. It is the vegetative cell which then produces the pollen tube while the generative cells divides into the two sperm cells. As the grains are formed in the anthers, they become coated by a material called sporopollenin and this forms a pattern, often characteristic to the particular species. Because sporopollenin is extremely resistant to decay, pollen grains partially fossilized in peats and lake sediments are often fully identifiable and much of our vegetation history has been discovered through looking at the pollen
The megasporangium is enclosed by the carpel or ovary. This has a stigma, often elevated on a long style. The female gametophyte is called the embryo sac in angiosperms, but its formation is initially the same as in the gymnosperms. Firstly a spore mother cell is differentiated, and this divided into a tetrad of haploid megaspores only one of which survives. The nucleus of this single large megaspore divides and eventually, amongst the mass of cytoplasm 8 cells emerge, in a pattern that is seem through all the flowering plants. At the chalazal end of the sac there are 3 cells known as antipodal cells. At the centre there are 2 cells - the polar cells, and these actually fuse to form a single diploid cell. Finally, at the micropylar end there are 2 synergid cells and a single egg cell.
Double Fertilization
Pollen lands on stigma. Germinates by producing a pollen tube that penetrates into tissue of stigma - grows down absorbing nutrients from stigma and style. Almost all pollen cytoplasm is at end of pollen tube, the rest of the tube and pollen grain filled with a giant vacuole. The end of the pollen tube breaks down and the two male sperm nuclei move into the embryo sac. One gamete fuses with the egg cell while the other fuses with the central cell (the polar cells). The new zygote is diploid, and central cell is now triploid. This triploid nucleus divides and produces the endosperm which acts as the nutrient source for the growing embryo. Initially nucleus replicates many times then later cell walls are created.
Growth of the Embryo
The embryo grows in a very precise way, eventually differentiating into a young root (radicle) young shoot (plumule), with the hard seed coat (testa) providing protection.
The evolution of the carpel and flower - last major event in the evolution of vascular plants. Subsequent evolution of the angiosperms over 110 - 130 Myr has led to their dominance. Most numerous in terms of numbers of taxa and numbers of individuals. Very little known about their origins, their phylogeny and the evolutionary patterns that have accompanied their history. Major reason for this is the lack of angiosperm fossils. Despite much searching, the fossil record is very poor. Charles Darwin called the origin of angiosperms an Abominable Mystery. Ancestors probably from an ancient group of gymnosperms, which in turn, probably arose from the seed ferns.
Debate - polyphyletic - more than 1 ancestral source evidence - an extraordinary degree of morphological and ecological diversity or monophyletic - all from common ancestor? Evidence for monophyletic origin - common morphological structures, e.g. most flowers have the same general structure, and great similarities in reproductive systems. Improbable that same structure for an ovule could have evolved separately in different lines..
Where? consensus that first angiosperms evolved in West Gondwana - ancient continent equivalent to modern South America plus Africa.. The habitat was montane and tropical or subtropical. This may account for the absence of fossils, so the areas where the plants were growing was far from areas where there is natural accumulation of sediments, bogs, swamps etc.
Fundamentally, the angiosperm flower differs little from a gymnosperm strobilus. The main difference is that in angiosperms, the megasporophylls (which formed the major structure in the cone) are reduced and have enclosed the ovule. The megasporophylls have therefore formed the carpel. The structural difference may not be much, but the biological difference is great. Inside the carpels, new environmental conditions have been created for the growth of the ovules and for the process of fertilization. As a result many fundamental changes have taken place in the structure of the female gametophyte and the process of fertilization itself.
Evolution of the flower:
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FRUITS AND SEEDS As the embryo matures, the nucellus and integuments harden to form a seed coat which is often very impervious to water. The ovary or carpel becomes the fruit and sometimes additional layers outside become involved e.g.- the fleshy part of a strawberry is the receptacle of the flower and the scales of the inflorescences remain attached to pineapple. Technically, the origin of the tissue leads to fruit being classified as either false or true fruits, but this is not a very useful way to distinguish between fruits as there are such large changes in the tissue structures of fruits at maturity, it is often very difficult to identify their origin. The outer wall either becomes hard - forming a nut, or fleshy eg tomatoes, cherries.
Fruits have the primary function of protecting the seed as it matures, but also allow seed release or germination. In more primitive angiosperm families the fruit opens while it is still on the plant; with the seed itself being the unit of dispersal, but in many of the more advanced groups, the fruit also takes on the function of dispersal.
1. To move to better environmental conditions - because parent or other individuals reducing light, nutrients, moisture etc. - or because conditions of parent poor - stressed plants often produce more seeds or better fruit.
2. Increase opportunity to cross with genetically different individuals- increase diversity
3. Proximity to parent may increase chance of predation.
There is some sort of dispersal for almost every seed plant, although the efficiency and distance vary enormously. The giant seed of the double coconut has no dispersal mechanism - it just falls off the tree and if the tree is on a slope it will roll downhill. Unlike the ordinary coconut, it cannot survive transport by the sea. Those species which have developed dispersal mechanisms either have fruits which are adapted to interact with the wind, water or animals or possess an internal mechanism - called ballistic dispersal.
The first function of the fruit is to protect the seed against the environment and attack by seed predators. This would be easily accomplished by making the fruit as sclerenchymatous as possible. eg. a thick shell around a coconut - a thick layer of sclereids. However, there is a problem with this solution, as the embryos grow to form the seed, the fruit will often have to expand to accommodate it and they must allow water and O2 into the embryo even after it is mature - too much sclerenchymous material is inappropriate. When the seed is ready to germinate the fruit also has to let the embryo out. There are 2 solutions to this -
1. The fruit opens naturally, as part of its development, to let the embryo emerge (known as dehiscent fruits).
2. The fruit is wet enough to allow the embryo to break out. These are called indehiscent fruits generally the fruit wall is thinner and weaker so the embryo can crack it by absorbing water and swelling. If the fruit wall is not thin enough when it is dispersed, the cells may be prone to decay through bacteria or fungi until the fruit is both permeable and weak enough to allow germination.
In the case of fruits which are fleshy and edible the seeds must be protected from the grinding or crushing of an animal's chewing actions and then must be isolated from digestive enzymes. This is either accomplished mechanically with a sclerenchymatous inner layer - as in the cherry, or a tough seed coat, or chemically, by having the innermost fruit layers so bitter that they cannot be eaten.
The primary method employed to attract animals is being fleshy. Many mature fleshy fruits have three layers: The exocarp = the "skin", the mesocarp = the "flesh" and the inner endocarp. These layers can take different forms as you can see from the Handout table provided, with the berry and the drupe being most common. Fruits that are eaten must generally be more complex than those dispersed in some other way. While immature, they deter consumption by being astringent (sour), hard or bitter. Then when the embryo/seed is mature, the fruit must undergo rapid metabolism (ripening) that involves the production of free sugars, the softening of walls and a change of colour and odour that signals the right animal agent. These changes usually occur in the outer exocarp and mesocarp while the endocarp becomes sclerified to protect the seed. The outer layers become softer while the starches and acids are converted to sugars. In some fruits they may be changed to oils - ripe olives contain 20% oil and 25% of an avocado is oil.
Fruits will of course be "opened" by being eaten or through rapid "rotting of the outer layers. The tough inner layer will usually have a specific dehiscence mechanism. The manner in which a fruit presents itself to the animal is also of interest. In many, the fruit is abscised and allowed to fall. For birds, the fruit must be close to a branch that is stout enough to support the bird while it eats. Those fruits eaten by bats, such as mangoes, hang down, clear of the foliage but not too close to the ground. Many fruits are distributed by animals without being eaten - they have claws or hooks to catch in the animals fur or feathers. Another unusual example is mistletoe epiphytic parasitic on trees. Fruit are initially eaten by birds but then regurgitated. They contain some very sticky substances so the fruit then sticks to the birds beak and the bird wipes its beak on the branch to clean itself - so depositing the seed in an ideal position.
Fruits dispersed by other agents are dry seeds. Wind dispersal is the most common agent and there are all sorts of adaptations for this. Wings such as those of sycamore and "parachutes" are perhaps the most usual examples - with fruits generally only containing a single seed. The parachutes such as those seen in dandelions originate from the sepals of the flower and not the carpel alone. So this can be termed a false fruit.
Dispersal by water is much less common but it is used in marshes and on oceanic islands. Buoyancy can be provided by having a large cavity in the fruit or a corky, fibrous wall.
A small number of fruits disperse their seeds by explosive mechanisms often through a same form gradual build up of pressure e.g. twisting until the fruit suddenly splits (explodes) along a line of which casts seeds in all directions.
Once the fruit has been dispersed the seed must germinate. The germinating seed takes in water it swells until the seed coat is ruptured and the radicle and plumule emerge. The cotyledons may remain within the seed at or below soil level - hypogeal germination e.g. broad bean, or become elevated and photosynthetic epigeal germination e.g. mustard.
Many seeds do not germinate immediately they leave the parent plant, but become dormant for a period of time. The length of time they remain dormant is variable depending upon the species, and often the environmental conditions. The most obvious environmental condition is temperature, with many species in temperate climates remaining dormant over the winter period. In arid conditions, availability of water may be the critical factor. Finally, light is often an important factor required for germination to occur. If you take a sample of soil from beneath the surface and spread it out in moist trays in a glasshouse, an enormous number of seedlings will appear, an indication of the large number of seeds that are held within the soil layer. Seeds end up here either because they are washed through by rain water, or through effects of soil movement over time. The seeds which remain dormant in the substrata are known as the seed bank. This represents a resource of the plant kingdom and fulfills a number of functions through allowing the dispersal of species in time. Functions include acting as a refuge and a buffering effect on genetic variation:
A refuge allows plants which are inferior competitors, and are therefore prone to be outcompeted in a usual situation, to survive and maintain their numbers. The seed bank is one of these refuges for species which have long-lived seeds. Other refuges include tolerance of extreme conditions which no other species can survive or resistance to herbivory.
A long-lived seed bank from which germinating individuals are drawn causes extensive overlapping of generations which may have important genetic consequences. For those genes which show constant fitness effects - changing to fit the environment, the individuals from the seed bank retard / slow down the annual rate of gene frequency change. The seed bank can thus provide a buffering, reducing sudden changes in a species which have been generated from random events.
In addition to the use of seed banks to maintain species, the frequency of reproduction is often used as a survival tactic. Most tree species, particularly in temperate regions, have 'mast' years in which seed production is very high indeed. Such a year is followed by a group of years in which the tree behaves as if it is exhausted by this effort and produces relatively few seeds. This feature also appears to have its motivation in ecology. the size of a population of predators is unlikely to be able to adjust quickly to such changes in winter food supply. In mast years a particularly high proportion of seed rain may escape predation. In fact,, mast years are usually synchronized between individuals of the same species in a forest and consequently seed crops of glut proportions are interspersed with famine years. It may be only in these mast years that a parent is able to produce descendants, and as a result we often see even-aged batches of tree seedlings.