How does a raspberry develop from the pistils of one flower?

A raspberry does not develop from a single pistil; rather, it is an aggregate fruit that forms from a flower containing numerous separate pistils.12

Each individual "bump" on a raspberry is a drupelet, which is a tiny, fleshy fruit containing a single seed. The development process occurs as follows:341

• Pollination: The flower contains many individual pistils, each consisting of an ovary, a style, and a stigma. During pollination, pollen lands on the stigma of each pistil and travels down its respective style to fertilize the ovule within the ovary.12
• Fruit Formation: Following successful fertilization, each of these numerous, separate ovaries swells and matures into an individual juicy drupelet.1
• Aggregation: These many drupelets cluster together to form the single aggregate fruit known as a raspberry.31
• Remnant Structures: The fine, hair-like projections often visible on the surface of a raspberry are the dried remains of the styles, which originally served to guide pollen to the ovaries.1