Ragweed pollen. Colored scanning electron micrograph (SEM) of pollen grains (pink) on the stigma (green) of a ragweed plant (Ambrosia sp.). These spiky pollen grains contain the male sex cells of the plant. The multi-lobed stigma, the surface of the flower's female sex organ, the carpel, is where the pollen germinates. Ambrosia sp. is a daisy-like flower which grows in bare and waste ground. Its pollen is wind-borne and released in great quantities for cross-pollination. Ambrosia sp. pollen acts as an allergen causing hay fever (allergic rhinitis or pollinosis) in susceptible individuals. Magnification: x150 at 5x7cm size. Magnification: x75 at 35mm size.