Inflorescence is a flowering shoot that bears more than one flower. There are two types of inflorescence Cymose and Racemose. Of the two types of inflorescence Cymose inflorescence is more primitive and Racemose is derived. (Parkin 1914)
In this type of inflorescence the main axis does not end in a flower, but it grows continuously and develops flowers on its lateral sides in acropetal succession (acropetal (adj.) - of leaves or flowers; developing or opening in succession from base to apex ).
Acropetal sucession is observed in raceme, the simplest racemose
Now Capitulum or Head, it is another type of racemose inflorescence in which
the main axis or receptacle becomes suppressed, and almost flat, and the flowers (also known as florets) are sessile (without stalk) so that they become crowded together on the flat surface of the receptacle. The florets are arranged in a centripetal manner on the receptacle, i.e., the outer flowers are older and open earlier than the inner ones.
In the following L.S. of sunflower you can see the disc florets developing/opening in centripetal manner.
P.S. I'm not a botanist but a candidate of B.Sc Zoology. I had Botany as my additional subject so I answered it from my knowledge.