The flowers:

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants. The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs.

The types of pollen that most commonly cause allergic reactions are produced by the plain-looking plants (trees, grasses, and weeds) that do not have showy flowers.

The type of allergens in the pollen is the main factor that determines whether the pollen is likely to cause a fever.

Among North American plants, weeds are the most prolific producers of allergenic pollen.