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The pollen wall protects the sperm nucleus while the pollen grain is moving from the anther to the stigma, it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall prevents the pollen grain from shrinking and crushing the genetic material during desiccation and it is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin. The pollen tube passes through the wall by way of structures called apertures.<ref>http://www.geo.arizona.edu/palynology/polkey.html#key</ref>

 

 

Pollen grains may have furrows, the orientation of which (relative to the original tetrad of microspores) classify the pollen as '''colpate''' or '''sulcate'''. The number of furrows or pores helps classify the [[flowering plant]]s, with [[eudicots]] having three colpi ('''tricolpate'''), and other groups having one sulcus.<ref>{{cite journal | url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1469-8137.1972.tb04826.x | author=Kenneth R. Sporne | year=1972 | title=Some Observations on the Evolution of Pollen Types in Dicotyledons | journal=New Phytologist | volume= 71 | issue=1 | pages=181–185 | doi=10.1111/j.1469-8137.1972.tb04826.x}}</ref><ref>{{cite journal|author = Walter S. Judd and Richard G. Olmstead|year = 2004|title = A survey of tricolpate (eudicot) phylogenetic relationships|journal = American Journal of Botany| volume = 91|pages = 1627–1644|doi = 10.3732/ajb.91.10.1627}} ([http://www.amjbot.org/cgi/content/full/91/10/1627 full text])</ref>

When placed on the [[Carpel|stigma]] of a flowering plant, under favorable circumstances, a pollen grain puts forth a [[pollen tube]] which grows down the tissue of the style to the [[ovary (plants)|ovary]], and makes its way along the [[placenta]], guided by projections or hairs, to the micropyle of an [[ovule]]. The nucleus of the tube cell has meanwhile passed into the tube, as does also the generative nucleus which divides (if it hasn't already) to form two [[sperm]] cells. The sperm cells are carried to their destination in the tip of the pollen-tube.

 

A Russian theoretical biologist, Vigen Geodakyan (Geodakian), has suggested that the quantity of pollen reaching a [[Carpel|pistillate]] flower can transmit [[Ecology|ecological]] information and also regulate [[phenotypic plasticity|evolutionary plasticity]] in cross-pollinating plants. Plentiful pollen indicates optimum environmental conditions (for example a plant that is situated at the center of its natural range, in ideal growing conditions, with a large number of male plants nearby, and favorable weather conditions), whereas a small amount of pollen indicates extreme conditions (at the borders of its range, with a deficiency of male plants, and adverse weather conditions). Geodakian believes that the quantity of pollen reaching a pistillate flower defines the [[sex ratio]], dispersion and [[sexual dimorphism]] of a plant population. High pollen quantity leads to a reduction of these characteristics and stabilization of a [[population]]. Small quantity leads to their increase and destabilization of a population.<ref> Geodakyan V. A. (1977). The Amount of Pollen as a Regulator of Evolutionary Plasticity of Cross-Pollinating Plants. “Doklady Biological Sciences” '''234''' N 1-6, 193–196.</ref>

 

== Lihat juga ==

{{commonscat|Pollen}}

* [[Evolution of sex]]

* [[Palynivore]]

* [[Sex ratio]]

 

* [http://www.flmnh.ufl.edu/natsci/paleobotany/paleobotany.htm Paleobotany and Palynology at the Florida Museum of Natural History]

* [http://www.geo.arizona.edu/palynology/ Palynology at the University of Arizona]

* [http://www.shef.ac.uk/uni/academic/N-Q/palysc/palyshef.html Palynology at the University of Sheffield]

* [http://www.albertlleal.com/Nature%20photography%20reports.html SEM coloured and confocal microscope images of pollen]

{{1911}}

* [http://www.bio.uu.nl/~palaeo/Engels/engels.html Palynology in Utrecht, the Netherlands]

 

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