2023-05-01 00:50:07
One day, more than 375 million years ago, something spectacular happened: pollen grains and seeds evolved in a lineage of plants. And since then nothing in nature has ever been the same.
When we think of pollen, we then think of allergies. However, it is estimated that of the more than 6,000 species of plants that exist in the Iberian Peninsula, less than a hundred play an important role in the development of allergy to pollen. Almost all of them belong to eight specific plant families: Graminaceae, Oleaceae, Cupressaceae, Urticaceae, Amaranthaceae, Asteraceae, Platanaceae, and Betulaceae.
In general, the pollen grain is very small, from 10 to 150 microns, which allows it to be blown away by the wind without difficulty, a very effective dispersal method with low energy cost, since it is not necessary to reward pollen pollinators for their work.
The pollen grain basically consists of a receptacle that contains the male genome and whose objective is the naked ovule of another plant of the same species. To guarantee reproductive success, each pollen grain has two covers, one external and one internal, the latter is responsible for enveloping two sperm cells and one vegetative cell.
Pollen grains are produced in the anthers of plants, which when opened disseminate it. A task that they usually do between the early hours of the morning and noon, after the dew has dried.
This propagation process is not continuous, it stops when the anthers are either too wet or too dry, and it starts again when the humidity and temperature are favorable again.
Pollen grains in the air are generally measured with the Rotorod technique, which basically consists of a sampler with greased silicone rods that collect air particles for more than 24 hours. If the pollen level is less than 50 it is considered low, while if it is higher than a thousand it is classified as very high.
Last stop: the stigma
Throughout evolution pollen has diversified, of the 300,000 species of plants that have pollen, there are 300,000 different forms of pollen, of different colors, shapes and textures, which have evolved according to the biological idiosyncrasy of each plant.
Despite the fact that pollen sometimes makes a mistake in its fertilization attempt and lands in another moist medium, such as the nasal mucosa of Homo sapiens, triggering a salvo of sneezes, the success of its journey, in biological terms, is very high.
The objective of the pollen is to reach the stigma, the gateway to flowering plants (angiosperms), a tissue located at the end of the pistils and which allows the pollination of the seeds.
In a way, the stigma acts as a gatekeeper, since it allows the passage of pollen of the correct species, thanks to the coating of a very fine and sticky hair that acts as an anchor, and prevents the entry of unwanted pollen.
If the landing has been correct, the first thing that occurs is a hydration process, since pollen is generally very dry and needs water to grow. Once the cells have been hydrated they become activated and form tubes that extend through the stigma towards the ovule of the plant. These pollen cells know in which direction the tubes should form: the one from which the water comes.
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