A number of species of Taraxacum are regarded as seed dispersed weeds or ruderals, especially the Common dandelion (T. officinale), which has been introduced over much of the temperate world as a lawn weed. After pollination and flowering is finished, the dandelion flower dries out for a day or two and then the seed-bearing parachutes expand and lift out of the dried flower head. The dried part of the flower drops off and the parachute ball opens into a full sphere. The parachute drops off when the seed strikes an obstacle. Often dandelions are observed growing in crevices near a wall; when the blowing fruits hit the wall, the feathery pappi comes off, dropping the dandelion seeds to the base of the wall or into a crevice. After the seed is released, the parachutes lose their feathered structure and take on a fuzzy, cotton-like appearance, often called "dandelion snow".