Pollination
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.
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 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.
The pollen tube elongates and pierces and grows through the megasporangium wall and delivers the sperm cells to the female gametophyte inside.
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 the tube reaches the egg sac, two sperm cells pass through it into the female gametophyte and fertilisation takes place.
),[15][16] and butterflies (e.g., Thymelicus flavus)[17] have been observed to engage in flower constancy, which means they are more likely to transfer pollen to other conspecific plants.
One such syndrome is "buzz pollination" (or "sonication"), where a bee must vibrate at a certain frequency in order to cause pollen to be released from the anthers.
[27] The flowers are borne near the ground, are yeasty smelling, not colourful, and sunbirds reject the nectar with its high xylose content.
Adaptation to a lack of animal food and of predation pressure, might therefore favour reptiles becoming more herbivorous and more inclined to feed on pollen and nectar.
Heavy rain discourages insect pollination and damages unprotected flowers, but can itself disperse pollen of suitably adapted plants, such as Ranunculus flammula, Narthecium ossifragum, and Caltha palustris.
[42] In some orchids ombrophily occurs, and rain water splashes cause the anther cap to be removed, allowing for the pollen to be exposed.
After exposure, raindrops causes the pollen to be shot upward, when the stipe pulls them back, and then fall into the cavity of the stigma.
[44] Pollination can be accomplished by cross-pollination or by self-pollination: An estimated 48.7% of plant species are either dioecious or self-incompatible obligate out-crossers.
Furthermore, the gut contents, wing structures, and mouthpart morphology of fossilized beetles and flies suggest that they acted as early pollinators.
[citation needed] In America, bees are brought to commercial plantings of cucumbers, squash, melons, strawberries, and many other crops.
The vicinity of a forest or wild grasslands with native pollinators near agricultural crops, such as apples, almonds or coffee can improve their yield by about 20%.
[59] The American Institute of Biological Sciences reports that native insect pollination saves the United States agricultural economy nearly an estimated $3.1 billion annually through natural crop production;[60] pollination produces some $40 billion worth of products annually in the United States alone.
The trend to monoculture means that greater concentrations of pollinators are needed at bloom time than ever before, yet the area is forage poor or even deadly to bees for the rest of the season.
Widespread aerial spraying for mosquitoes due to West Nile fears is causing an acceleration of the loss of pollinators.
[62] In some situations, farmers or horticulturists may aim to restrict natural pollination to only permit breeding with the preferred individuals plants.
Additionally, in the past five years there has been a decline in winter managed beehives, which has reached an unprecedented rate of colony losses at near 30%.
ISCA Technologies,[67] from Riverside, California, created a semiochemical formulation called SPLAT Bloom, that modifies the behavior of honey bees, inciting them to visit flowers in every portion of the field.[promotion?]
[69] The more destructive forms of human disturbances are land use changes such as fragmentation, selective logging, and the conversion to secondary forest habitat.
In the first case, species that normally occur in similar seasons or time cycles, now have different responses to environmental changes and therefore no longer interact.
[75] The decrease in populations of the honey bee have been attributed to pesticides, genetically modified crops, fragmentation, parasites and diseases that have been introduced.
[71] Pollinators are also essential because they improve crop quality and increase genetic diversity, which is necessary in producing fruit with nutritional value and various flavors.
[79] Crops that do not depend on animals for pollination but on the wind or self-pollination, like corn and potatoes, have doubled in production and make up a large part of the human diet but do not provide the micronutrients that are needed.
[91] Moreover, California's Central Valley is determined by the World Health Organization as the location of country's worst air pollution.
[94] However, a recent study published in Oxford Academic's Journal of Economic Entomology found that once the costs for maintaining bees specifically for almond pollination, including overwintering, summer management, and the replacement dying bees are considered, almond pollination is barely or not profitable for average beekeepers.
