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They are made up of miniature egg-shaped male particles. The grains travel to the female reproductive system of other plants and so they can breed.
Human beings are not able to see the pollen without a microscope. A single plant may produce thousands of grain. You may see a yellowish dust made up of all the pollen grains together in the inner parts of the plant or in the leaves may be seen.
Which plants cause allergy?
Plants with bright colours (like roses or daisys) normally do not cause allergies. These pollen grains are usually quite big and travel from one flower to another thank to bees and insects.
On the other hand, trees, bushes and small weeds have smaller pollen grains especially apt to be widespread though the wind and therefore cause respiratory allergies. These plants are anemophilous because they breed with the help of the wind.
Usually the pollen season lasts form february or march till october. During spring time (march to june), a high number of different plants bloom at the same time, although the pollen season may delay if the plants are further North located.
During fall, another pollen peaks, mainly due to trees and weeds.
Image:Ignacio Jáuregui Repesa