This is not of a single flower, but an inflorescence, or a bunch of individuals flowers all crammed together. This plant is in the sunflower family (also called the Asteraceae) and most species in this family (all greater 20,000 of them!) have flowers more or less like this. The green bits in the middle are individual flowers (called disc florets) that have not opened yet. Further out are open flowers with small red petals and sexy anthers covered in pollen. The flowers around the very edge are called ray florets, they are otherwise basically the same as disc florets except they have a single large, in this case, yellow petal. These ray florets give the flower the distinctive ‘sunflower’ look and provide the illusion of this being just a single flower, but like so many things in nature, close inspection reveals greater complexity.