a flower
Chrysanthemums, are a genus of about 30 species of perennial flowering plants in the family Asteraceae
they dont grow at all.
'Florist' (or 'exhibition') chrysanthemums are specially bred for beauty, not hardiness. They can't take cold winters. Unless you live where it's warm all year, stick to the 'garden' (or 'hardy')...
Not all flowers have them. I think it is the thing sticking out from the center on those that do.