Small mysteries

A couple of weeks into our walk-to-school routine I started to worry that our little urban environment was quite limited and we'd already started to exhaust its possibilities for discovery. Now I'm embarrassed to report that of course it was only my observational abilities that were limited; fortunately they're starting to improve with exercise.

For example, I've idly wondered in the past why I invariably see these two things lying together under the same tree:

They look completely different, right? So today I stopped to look a little more closely and discovered some intermediate steps:

My guess is that the unfertilized flowers end up as the puny relics at bottom right, while their cousins who get lucky grow into the fine fat seedpods above. Is there anybody out there with actual botanical knowledge who can confirm it for me?