Each “flower head” is actually an inflorescence. The individual flower has bilateral symmetry: two large petals and two tiny ones. Apparently, this is bilateral symmetry is a big deal in flower evolution because it gives the flower an advantage in pollination (?). See the image: http://www.pnas.org/content/104/42/16714/F1.expansion.html
madame scientist's not-so-mad musings 
Each “flower head” is actually an inflorescence. The individual flower has bilateral symmetry: two large petals and two tiny ones. Apparently, this is bilateral symmetry is a big deal in flower evolution because it gives the flower an advantage in pollination (?). See the image: http://www.pnas.org/content/104/42/16714/F1.expansion.html