Editing Pollination (section)

==Process of pollination==
[[File:Pollen grains observed in aeroplankton of South Europe.png|thumb|upright=1.5| {{center|Pollen grains observed in [[aeroplankton]]<br />of South Europe<ref>Denisow, B. and Weryszko-Chmielewska, E. (2015) "Pollen grains as airborne allergenic particles". ''Acta Agrobotanica'', '''68'''(4). {{doi|10.5586/aa.2015.045}}.</ref>}}]]
[[File:Pollinated Tomato Pistil.jpg|thumb|Decolorized aniline blue fluorescence image showing growing pollen tubes in a tomato pistil]]
Pollen [[germination]] has three stages; hydration, activation and pollen tube emergence. The pollen grain is severely dehydrated so that its mass is reduced, enabling it to be more easily transported from flower to flower. Germination only takes place after rehydration, ensuring that premature germination does not take place in the anther. Hydration allows the plasma membrane of the pollen grain to reform into its normal bilayer organization providing an effective osmotic membrane. Activation involves the development of [[actin]] filaments throughout the cytoplasm of the cell, which eventually become concentrated at the point from which the pollen tube will emerge. Hydration and activation continue as the pollen tube begins to grow.<ref name=Raghavan>{{cite book | last = Raghavan | first = Valayamghat | name-list-style = vanc |title=Molecular Embryology of Flowering Plants |url=https://books.google.com/books?id=LQfaytKsOu4C&pg=PA210 |year=1997 |publisher=Cambridge University Press |isbn=978-0-521-55246-2 |pages=210–216}}</ref>
In conifers, the reproductive structures are borne on cones. The cones are either pollen cones (male) or ovulate cones (female), but some species are [[monoecious]] and others [[dioecious]].  A pollen cone contains hundreds of microsporangia carried on (or borne on) reproductive structures called sporophylls. Spore mother cells in the microsporangia divide by [[meiosis]] to form haploid microspores that  develop further by two mitotic divisions into immature male gametophytes (pollen grains).  The four resulting cells consist of a large tube cell that forms the [[pollen tube]], a generative cell that will produce two sperm by [[mitosis]], and two prothallial cells that degenerate.  These cells comprise a very reduced [[microgametophyte]], that is contained within the resistant.

The pollen grains are dispersed by the wind to the female, ovulate cone that is made up of many overlapping scales (sporophylls, and thus megasporophylls), each protecting two ovules, each of which consists of a megasporangium  (the nucellus) wrapped in  two layers of tissue, the integument and the cupule, that were derived from highly modified branches of ancestral gymnosperms.  When a pollen grain lands close enough to the tip of an ovule, it is drawn in through the micropyle ( a pore in the integuments covering the tip of the ovule) often by means of a drop of liquid known as a pollination drop. The pollen enters a  pollen chamber close to the nucellus, and there  it may wait for a year before it germinates and forms a pollen tube that grows through the wall of the megasporangium (=nucellus) where fertilisation 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. Fertilisation takes place when the nucleus of one of the sperm cells enters the egg cell in the megagametophyte's archegonium.<ref name=Runions>{{cite journal |title=Sexual reproduction of interior spruce (Pinaceae). I. Pollen germination to archegonial maturation |last1=Runions |first1=C. John |last2=Owens  |first2=John N. | name-list-style = vanc |journal=International Journal of Plant Sciences |volume=160 |issue=4 |date=1999 |pages=631–640 |doi=10.1086/314170|s2cid=2766822 }}</ref>

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 fertilisation takes place.<ref name=Campbell/>