A capitulum is a dense head of flowers that several plant families have developed their equivalent of. One plant family can truly lay claim to it though and have taken it to such an extreme in certain species that they often looks like a single flower like the Sunflower but is of course made of many individual flowers often with the individuals around the edge becoming specialized and more elongated and flattened to look like individual petals. This flower form must be successful as the Daisy family is the largest on Earth by number of total species and occupy virtually every known habitat except the sea.