Understanding Pollination: Self and Cross, Insect, Wind, and Water, Cheat Sheet of Biology

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Page 1 of 19 Table of Contents Table of Contents ................................................................................................... 1 POLLINATION: INTRODUCTION .............................................................................. 2 PROCESS OF POLLINATION ..................................................................................... 3 TYPES OF POLLINATION ......................................................................................... 6 On the Basis of Pollen Source ............................................................................. 6 Self-Pollination ................................................................................................ 6 Cross Pollination .............................................................................................. 6 On the Basis of Pollinating Agent ........................................................................ 6 Abiotic Pollination............................................................................................ 6 Biotic Pollination .............................................................................................. 8 MECHANISM ........................................................................................................ 10 Modes of Cross Pollination: .............................................................................. 10 (1) Anemophily: ............................................................................................. 11 (2) Hydrophily: ............................................................................................... 11 (3) Entomophily: ............................................................................................ 12 (4) Ornithophily: ............................................................................................ 12 (5) Chiropteriphily: ......................................................................................... 12 (6) Malcophily: ............................................................................................... 13 Mechanism of Self Pollination .......................................................................... 13 Cleistogamy ................................................................................................... 13 POLLEN VECTORS ................................................................................................. 14 CONCLUSIONS...................................................................................................... 17 BIBLOGRAPHY ...................................................................................................... 19 Page 2 of 19 INTRODUCTION  Pollination is the process by which pollen is transferred to the female reproductive organs of a plant, thereby enabling fertilization to take place.  Like all living organisms, seed plants have a single major purpose: to pass their genetic information on to the next generation.  The reproductive unit is the seed, and pollination is an essential step in the production of seeds in all spermatophytes (seed plants).  For the process of pollination to be successful, a pollen grain produced by the anther, the male part of a flower, must be transferred to a stigma, the female part of the flower, of a plant of the same species.  The process is rather different in angiosperms (flowering plants) from what it is in gymnosperms (other seed plants).  In angiosperms, after the pollen grain has landed on the stigma, it creates a pollen tube which grows down the style until it reaches the ovary. Sperm cells from the pollen grain then move along the pollen tube enter the egg cell through the micropyle and fertilize it, resulting in the production of a seed.  A successful angiosperm pollen grain (gametophyte) containing the male gamete is transported to the stigma, where it germinates and its pollen tube grows down the style to the ovary. Its two gametes travel down the tube to where the gametophyte(s) containing the female gametes are held within the carpel. One nucleus fuses with the polar bodies to produce the endosperm tissues, and the other with the ovule to produce the embryo, Hence the term: "double fertilization". Page 5 of 19 where fertilization takes place. During this time, the megaspore mother cell divides by meiosis to form four haploid cells, three of which degenerate.  The surviving one develops as a megaspore and divides repeatedly to form an immature female gametophyte (egg sac). Two or three archegonia containing an egg then develop inside the gametophyte.  Meanwhile, in the spring of the second year two sperm cells are produced by mitosis of the body cell of the male gametophyte.  The pollen tube elongates and pierces and grows through the megasporangium wall and delivers the sperm cells to the female gametophyte inside.  Fertilization takes place when the nucleus of one of the sperm cells enters the egg cell in the mega gametophyte’ sarchegonium.  In flowering plants, the anthers of the flower produce microspores by meiosis. These undergo mitosis to form male gametophytes, each of which contains two haploid cells.  Meanwhile, the ovules produce megaspores by meiosis, further division of these form the female gametophytes, which are very strongly reduced, each consisting only of a few cells, one of which is the egg. When a pollen grain adheres to the stigma of a carpel it germinates, developing a pollen tube that grows through the tissues of the style, entering the ovule through the micropyle.  When the tube reaches the egg sac, two sperm cells pass through it into the female gametophyte and fertilization takes place. Page 6 of 19 TYPES OF POLLINATION On the Basis of Pollen Source  Depending on the source of pollen, pollination can be classified into 2 types – o Self-pollination o Cross Pollination (Xenogamy) Self-Pollination  Self-Pollination is the type of Pollination in which pollen grains are transferred from anther to the stigma of the same flower (Autogamy) or pollen grains are transferred from anther to the stigma of different flower of the same plant (Geitonogamy). Cross Pollination  Cross Pollination or Xenogamy is the type of pollination in which pollen grains are transferred from anther to the stigma of a different plant. On the Basis of Pollinating Agent  Depending on agent of Pollination, pollination can be classified into abiotic pollination and biotic pollination Abiotic Pollination • Abiotic pollination refers to situations where pollination is mediated without the involvement of other organisms. • The most common form of abiotic pollination, anemophily, is pollination by wind. Wind pollination is very imprecise, with a minute proportion of pollen grains landing by chance on a suitable receptive stigma, the rest being wasted in the environment. Page 7 of 19 • This form of pollination is used by grasses, most conifers, and many deciduous trees. Hydrophily is pollination by water, and occurs in aquatic plants which release their pollen directly into the surrounding water. • About 80% of all plant pollination is biotic. In gymnosperms, biotic pollination is generally incidental when it occurs, though some gymnosperms and their pollinators are mutually adapted for pollination. • The best-known examples probably are members of the order Cycadales and associated species of beetles. • Of the abiotically pollinated species of plant, 98% are anemophilous and 2% hygrophilous, their pollen being transported by water. • It is thought that among angiosperms, entomophily is the primitive state; this is indicated by the vestigial nectarines in the wind- pollinated Utica and other plants, and the presence of fragrances in some of these plants. • Of the angiosperms, grasses, sedges, rushes and catkin-bearing plants are in general wind pollinated. Other flowering plants are mostly biotic, the pollen being carried by animal vectors. • However, a number of plants in multiple families have secondarily adopted wind pollination in contrast to other members of their groups. Some plants are intermediate between the two pollination methods. • Common heather is regularly pollinated by insects, but produce clouds of pollen and some wind pollination is inevitable, and the hoary plantain is primarily wind pollinated, but is also visited by insects which pollinate it. Page 10 of 19 observed to engage in flower constancy, which means they are more likely to transfer pollen to other conspecific plants.  This can be beneficial for the pollinators, as flower constancy prevents the loss of pollen during interspecific flights and pollinators from clogging stigmas with pollen of other flower species.  It also improves the probability that the pollinator will find productive flowers easily accessible and recognizable by familiar clues. MECHANISM  Pollination can be accomplished by cross-pollination or by self- pollination:  Cross-pollination, also called allogamy, occurs when pollen is delivered from the stamen of one flower to the stigma of a flower on another plant of the same species.  Plants adapted for cross-pollination have several mechanisms to prevent self-pollination; the reproductive organs may be arranged in such a way that self-fertilisation is unlikely, or the stamens and carpels may mature at different times. Modes of Cross Pollination:  The agencies which transfer pollen grains from anthers of one flower to the stigma of different flowers are as follows: WIND (Anemophily), WATER (Hydrophily), INSECTS (Entomophily), BIRDS (Ornithophily)' and BATS (Chiropterophily). Page 11 of 19 (1) Anemophily: • Anemophilous plants produce enormous amount of. Pollen grains: A single plant of Mercurialis annually has been estimated to produce 1,352,000,000 pollen grains. • Anemophilous plants bear small and inconspicuous flower. The pollen grains are small, light, smooth and dry. • Pollen of some plants are said to be blown to 1,300 km. In some plants as Pinus, pollen grains are winged. • The flowers are usually unisexual in some plants e.g. Mulberry is borne in independent catkins which can sway freely and shake off their pollen in air. • The flowers may be borne on long axis (as in grasses) much above the leaves. • The anther is versatile so as to oscillate in all directions at the tip of filament. • In Urticaceae filaments are very long. • Anempohilous flowers have adequate devices to catch the air-borne- pollen grains with utmost efficiency. For this the stigma is usually large and feathery (as in grasses) and brush like as in Typha. (2) Hydrophily:  It is of two types: o Hypohydrogamy: Includes plants which are pollinated inside the water, e.g. Ceratophyllum, Najas. o Epihydrogamy: Vallisneriaspiralis (ribbon weed) is a submerged dioecious plant.  The flowers are borne under water.  When mature, the male flower get detached from the parent plant and float on the surface of water. Page 12 of 19  The pistillate flowers also develop under water, at the time of pollination; they are brought to the surface by their long and slender stalks.  As it arrives on the surface it forms a cuplike depression.  If male flowers floating on water get lodged into the depression, the pollination takes place.  After pollination, the stalk of the pistillate flower undergoes spiral torsion bringing the pollinated flower under water once more. (3) Entomophily:  Some of the insects which help in pollination are bees, flies, wasps, moths and beetles.  Bees, flies and beetles visit flowers which open after sunset. Bees probably carry out 80% of all pollination done by insects.  Bee pollinated flowers are colored, possess special smell and/or produce nectar.  Pollen grains are sticky or with spinousexine.  Also, the stigma is sticky and bees are color blind for red. (4) Ornithophily:  Tiny birds like humming birds and honey thrushes (hardly 1 inch long) feeds on the nectar of flower like Bignonia, Erythrina is visited by crows. (5) Chiropteriphily:  Bauhinia megalandra of Java and Anthocephalus are pollinated by bats. Page 15 of 19  Any kind of animal that often visits or encounters flowers is likely to be a pollen vector to some extent.  For example, a crab spider that stops at one flower for a time and then moves on might carry pollen incidentally, but most pollen vectors of significant interest are those that routinely visit the flowers for some functional activity.  They might feed on pollen, or plant organs, or on plant secretions such as nectar, and carry out acts of pollination on the way. Many plants bear flowers that favor certain types of pollinator over all others.  This need not always be an effective strategy, because some flowers that are of such a shape that they favor pollinators that pass by their anthers and stigmata on the way to the nectar, may get robbed by ants that are small enough to bypass the normal channels, or by short-tongued bees that bite through the bases of deep corolla tubes to extract nectar at the end opposite to the anthers and stigma.  Some pollinator species can show huge variation in pollination effectiveness because their ability to carry pollen is impacted by some morphological trait.  This is the case in the white-lined sphinx moth, in which short- tongued morphs collect pollen on their heads but long-tongued morphs do not carry any pollen.  Some flowers have specialized mechanisms to trap pollinators to increase effectiveness. Other flowers will attract pollinators by odor.  For example, bee species such as Euglossacordata are attracted to orchids this way, and it has been suggested that the bees will Page 16 of 19 become intoxicated during these visits to the orchid flowers, which last up to 90 minutes.  However, in general, plants that rely on pollen vectors tend to be adapted to their particular type of vector, for example day-pollinated species tend to be brightly colored, but if they are pollinated largely by birds or specialist mammals, they tend to be larger and have larger nectar rewards than species that are strictly insect-pollinated.  They also tend to spread their rewards over longer periods, having long flowering seasons; their specialist pollinators would be likely to starve if the pollination season were too short. Page 17 of 19 CONCLUSIONS  Pollination management is a branch of agriculture that seeks to protect and enhance present pollinators and often involves the culture and addition of pollinators in monoculture situations, such as commercial fruit orchards.  The largest managed pollination event in the world is in Californian almond orchards, where nearly half (about one million hives) of the US honey bees are trucked to the almond orchards each spring.  New York's apple crop requires about 30,000 hives; Maine's blueberry crop uses about 50,000 hives each year.  Bees are also brought to commercial plantings of cucumbers, squash, melons, strawberries, and many other crops. Honey bees are not the only managed pollinators: a few other species of bees are also raised as pollinators.  The alfalfa leafcutter bee is an important pollinator for alfalfa seed in western United States and Canada.  Bumblebees are increasingly raised and used extensively for greenhouse tomatoes and other crops.  The ecological and financial importance of natural pollination by insects to agricultural crops, improving their quality and quantity, becomes more and more appreciated and has given rise to new financial opportunities.