Pollination rigida", this allows the plant to self-pollinate, which is useful when biotic pollinators in the environment have decreased. It is possible for a plant have varying pollination methods, including both biotic and abiotic pollination. The orchid "Oeceoclades maculata" uses both rain and butterflies, depending on its environmental conditions. More commonly, pollination involves pollinators (also called pollen vectors): organisms that carry or move the pollen grains from the anther of one flower to the receptive part of the carpel or pistil (stigma) of another. Between 100,000 and 200,000 species of animal act as pollinators of the world's 250,000 species of flowering
Flower make it a good candidate for causing allergy. It is, however, a relatively rare cause of allergy because the types of allergens in pine pollen appear to make it less allergenic. Instead the allergen is usually the pollen of the contemporary bloom of anemophilous ragweed ("Ambrosia"), which can drift for many miles. Scientists have collected samples of ragweed pollen 400 miles out at sea and 2 miles high in the air. A single ragweed plant can generate a million grains of pollen per day. Among North American plants, weeds are the most prolific producers of allergenic pollen. Ragweed is the